Network Working Group C. Perkins, Ed.
Request for Comments: 3344 Nokia Research Center
Obsoletes: 3220 August 2002
Category: Standards Track
IP Mobility Support for IPv4
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document specifies protocol enhancements that allow transparent
routing of IP datagrams to mobile nodes in the Internet. Each mobile
node is always identified by its home address, regardless of its
current point of attachment to the Internet. While situated away
from its home, a mobile node is also associated with a care-of
address, which provides information about its current point of
attachment to the Internet. The protocol provides for registering
the care-of address with a home agent. The home agent sends
datagrams destined for the mobile node through a tunnel to the care-
of address. After arriving at the end of the tunnel, each datagram
is then delivered to the mobile node.
Contents
1. IntrodUCtion 3
1.1. Protocol Requirements . . . . . . . . . . . . . . . . . 4
1.2. Goals . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Assumptions . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Applicability . . . . . . . . . . . . . . . . . . . . . 5
1.5. New Architectural Entities . . . . . . . . . . . . . . 5
1.6. Terminology . . . . . . . . . . . . . . . . . . . . . . 6
1.7. Protocol Overview . . . . . . . . . . . . . . . . . . . 9
1.8. Message Format and Protocol Extensibility . . . . . . . 13
1.9. Type-Length-Value Extension Format for Mobile IP
Extensions . . . . . . . . . . . . . . . . . . . . . 15
1.10. Long Extension Format . . . . . . . . . . . . . . . . . 16
1.11. Short Extension Format . . . . . . . . . . . . . . . . 16
2. Agent Discovery 17
2.1. Agent Advertisement . . . . . . . . . . . . . . . . . . 18
2.1.1. Mobility Agent Advertisement Extension . . . . 20
2.1.2. Prefix-Lengths Extension . . . . . . . . . . . 22
2.1.3. One-byte Padding Extension . . . . . . . . . . 22
2.2. Agent Solicitation . . . . . . . . . . . . . . . . . . 23
2.3. Foreign Agent and Home Agent Considerations . . . . . . 23
2.3.1. Advertised Router Addresses . . . . . . . . . . 24
2.3.2. Sequence Numbers and Rollover Handling . . . . 24
2.4. Mobile Node Considerations . . . . . . . . . . . . . . 25
2.4.1. Registration Required . . . . . . . . . . . . . 26
2.4.2. Move Detection . . . . . . . . . . . . . . . . 26
2.4.3. Returning Home . . . . . . . . . . . . . . . . 27
2.4.4. Sequence Numbers and Rollover Handling . . . . 28
3. Registration 28
3.1. Registration Overview . . . . . . . . . . . . . . . . . 29
3.2. Authentication . . . . . . . . . . . . . . . . . . . . 30
3.3. Registration Request . . . . . . . . . . . . . . . . . 30
3.4. Registration Reply . . . . . . . . . . . . . . . . . . 33
3.5. Registration Extensions . . . . . . . . . . . . . . . . 36
3.5.1. Computing Authentication Extension Values . . . 36
3.5.2. Mobile-Home Authentication Extension . . . . . 37
3.5.3. Mobile-Foreign Authentication Extension . . . . 37
3.5.4. Foreign-Home Authentication Extension . . . . . 38
3.6. Mobile Node Considerations . . . . . . . . . . . . . . 38
3.6.1. Sending Registration Requests . . . . . . . . . 40
3.6.2. Receiving Registration Replies . . . . . . . . 44
3.6.3. Registration Retransmission . . . . . . . . . . 47
3.7. Foreign Agent Considerations . . . . . . . . . . . . . 47
3.7.1. Configuration and Registration Tables . . . . . 48
3.7.2. Receiving Registration Requests . . . . . . . . 49
3.7.3. Receiving Registration Replies . . . . . . . . 52
3.8. Home Agent Considerations . . . . . . . . . . . . . . . 54
3.8.1. Configuration and Registration Tables . . . . . 55
3.8.2. Receiving Registration Requests . . . . . . . . 56
3.8.3. Sending Registration Replies . . . . . . . . . 59
4. Routing Considerations 62
4.1. Encapsulation Types . . . . . . . . . . . . . . . . . . 62
4.2. Unicast Datagram Routing . . . . . . . . . . . . . . . 62
4.2.1. Mobile Node Considerations . . . . . . . . . . 62
4.2.2. Foreign Agent Considerations . . . . . . . . . 63
4.2.3. Home Agent Considerations . . . . . . . . . . . 64
4.3. Broadcast Datagrams . . . . . . . . . . . . . . . . . . 66
4.4. Multicast Datagram Routing . . . . . . . . . . . . . . 66
4.5. Mobile Routers . . . . . . . . . . . . . . . . . . . . 67
4.6. ARP, Proxy ARP, and Gratuitous ARP . . . . . . . . . . 69
5. Security Considerations 73
5.1. Message Authentication Codes . . . . . . . . . . . . . 73
5.2. Areas of Security Concern in this Protocol . . . . . . 73
5.3. Key Management . . . . . . . . . . . . . . . . . . . . 74
5.4. Picking Good Random Numbers . . . . . . . . . . . . . . 74
5.5. Privacy . . . . . . . . . . . . . . . . . . . . . . . . 74
5.6. Ingress Filtering . . . . . . . . . . . . . . . . . . . 75
5.7. Replay Protection for Registration Requests . . . . . . 75
5.7.1. Replay Protection using Timestamps . . . . . . 75
5.7.2. Replay Protection using Nonces . . . . . . . . 77
6. IANA Considerations 77
6.1. Mobile IP Message Types . . . . . . . . . . . . . . . . 78
6.2. Extensions to RFC1256 Router Advertisement . . . . . . 78
6.3. Extensions to Mobile IP Registration Messages . . . . . 79
6.4. Code Values for Mobile IP Registration Reply
Messages. . . . . . . . . . . . . . . . . . . . . . 79
7. Acknowledgments 80
A. Patent Issues 82
B. Link-Layer Considerations 82
C. TCP Considerations 83
C.1. TCP Timers . . . . . . . . . . . . . . . . . . . . . . 83
C.2. TCP Congestion Management . . . . . . . . . . . . . . . 83
D. Example Scenarios 84
D.1. Registering with a Foreign Agent Care-of Address . . . 84
D.2. Registering with a Co-Located Care-of Address . . . . . 84
D.3. Deregistration . . . . . . . . . . . . . . . . . . . . 85
E. Applicability of Prefix-Lengths Extension 86
F. Interoperability Considerations 86
G. Changes since RFC2002 87
G.1. Major Changes . . . . . . . . . . . . . . . . . . . . . 87
G.2. Minor Changes . . . . . . . . . . . . . . . . . . . . . 89
G.3. Changes since revision 04 of RFC2002bis . . . . . . . . 91
H. Example Messages 92
H.1. Example ICMP Agent Advertisement Message Format . . . . 92
H.2. Example Registration Request Message Format . . . . . . 93
H.3. Example Registration Reply Message Format . . . . . . . 94
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 98
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 99
1. Introduction
IP version 4 assumes that a node's IP address uniquely identifies the
node's point of attachment to the Internet. Therefore, a node must
be located on the network indicated by its IP address in order to
receive datagrams destined to it; otherwise, datagrams destined to
the node would be undeliverable. For a node to change its point of
attachment without losing its ability to communicate, currently one
of the two following mechanisms must typically be employed:
a) the node must change its IP address whenever it changes its
point of attachment, or
b) host-specific routes must be propagated throughout much of the
Internet routing fabric.
Both of these alternatives are often unacceptable. The first makes
it impossible for a node to maintain transport and higher-layer
connections when the node changes location. The second has obvious
and severe scaling problems, especially relevant considering the
eXPlosive growth in sales of notebook (mobile) computers.
A new, scalable, mechanism is required for accommodating node
mobility within the Internet. This document defines such a
mechanism, which enables nodes to change their point of attachment to
the Internet without changing their IP address.
Changes between this revised specification for Mobile IP and the
original specifications (see [33, 32, 34, 43, 8]) are detailed in the
appendix section G.
1.1. Protocol Requirements
A mobile node must be able to communicate with other nodes after
changing its link-layer point of attachment to the Internet, yet
without changing its IP address.
A mobile node must be able to communicate with other nodes that do
not implement these mobility functions. No protocol enhancements are
required in hosts or routers that are not acting as any of the new
architectural entities introduced in Section 1.5.
All messages used to update another node as to the location of a
mobile node must be authenticated in order to protect against remote
redirection attacks.
1.2. Goals
The link by which a mobile node is directly attached to the Internet
may often be a wireless link. This link may thus have a
substantially lower bandwidth and higher error rate than traditional
wired networks. Moreover, mobile nodes are likely to be battery
powered, and minimizing power consumption is important. Therefore,
the number of administrative messages sent over the link by which a
mobile node is directly attached to the Internet should be minimized,
and the size of these messages should be kept as small as is
reasonably possible.
1.3. Assumptions
The protocols defined in this document place no additional
constraints on the assignment of IP addresses. That is, a mobile
node can be assigned an IP address by the organization that owns the
machine.
This protocol assumes that mobile nodes will generally not change
their point of attachment to the Internet more frequently than once
per second.
This protocol assumes that IP unicast datagrams are routed based on
the destination address in the datagram header (and not, for example,
by source address).
1.4. Applicability
Mobile IP is intended to enable nodes to move from one IP subnet to
another. It is just as suitable for mobility across homogeneous
media as it is for mobility across heterogeneous media. That is,
Mobile IP facilitates node movement from one Ethernet segment to
another as well as it accommodates node movement from an Ethernet
segment to a wireless LAN, as long as the mobile node's IP address
remains the same after such a movement.
One can think of Mobile IP as solving the "macro" mobility management
problem. It is less well suited for more "micro" mobility management
applications -- for example, handoff amongst wireless transceivers,
each of which covers only a very small geographic area. As long as
node movement does not occur between points of attachment on
different IP subnets, link-layer mechanisms for mobility (i.e.,
link-layer handoff) may offer faster convergence and far less
overhead than Mobile IP.
1.5. New Architectural Entities
Mobile IP introduces the following new functional entities:
Mobile Node
A host or router that changes its point of attachment from one
network or subnetwork to another. A mobile node may change its
location without changing its IP address; it may continue to
communicate with other Internet nodes at any location using its
(constant) IP address, assuming link-layer connectivity to a
point of attachment is available.
Home Agent
A router on a mobile node's home network which tunnels
datagrams for delivery to the mobile node when it is away from
home, and maintains current location information for the mobile
node.
Foreign Agent
A router on a mobile node's visited network which provides
routing services to the mobile node while registered. The
foreign agent detunnels and delivers datagrams to the mobile
node that were tunneled by the mobile node's home agent. For
datagrams sent by a mobile node, the foreign agent may serve as
a default router for registered mobile nodes.
A mobile node is given a long-term IP address on a home network.
This home address is administered in the same way as a "permanent" IP
address is provided to a stationary host. When away from its home
network, a "care-of address" is associated with the mobile node and
reflects the mobile node's current point of attachment. The mobile
node uses its home address as the source address of all IP datagrams
that it sends, except where otherwise described in this document for
datagrams sent for certain mobility management functions (e.g., as in
Section 3.6.1.1).
1.6. Terminology
The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [4].
In addition, this document frequently uses the following terms:
Authorization-enabling extension
An authentication which makes a (registration) message
acceptable to the ultimate recipient of the registration
message. An authorization-enabling extension MUST contain
an SPI.
In this document, all uses of authorization-enabling
extension refer to authentication extensions that enable the
Registration Request message to be acceptable to the home
agent. Using additional protocol structures specified
outside of this document, it may be possible for the mobile
node to provide authentication of its registration to the
home agent, by way of another authenticating entity within
the network that is acceptable to the home agent (for
example, see RFC2794 [6]).
Agent Advertisement
An advertisement message constructed by attaching a special
Extension to a router advertisement [10] message.
Authentication
The process of verifying (using cryptographic techniques,
for all applications in this specification) the identity of
the originator of a message.
Care-of Address
The termination point of a tunnel toward a mobile node, for
datagrams forwarded to the mobile node while it is away from
home. The protocol can use two different types of care-of
address: a "foreign agent care-of address" is an address of
a foreign agent with which the mobile node is registered,
and a "co-located care-of address" is an externally oBTained
local address which the mobile node has associated with one
of its own network interfaces.
Correspondent Node
A peer with which a mobile node is communicating. A
correspondent node may be either mobile or stationary.
Foreign Network
Any network other than the mobile node's Home Network.
Gratuitous ARP
An ARP packet sent by a node in order to spontaneously cause
other nodes to update an entry in their ARP cache [45]. See
section 4.6.
Home Address
An IP address that is assigned for an extended period of
time to a mobile node. It remains unchanged regardless of
where the node is attached to the Internet.
Home Network
A network, possibly virtual, having a network prefix
matching that of a mobile node's home address. Note that
standard IP routing mechanisms will deliver datagrams
destined to a mobile node's Home Address to the mobile
node's Home Network.
Link
A facility or medium over which nodes can communicate at the
link layer. A link underlies the network layer.
Link-Layer Address
The address used to identify an endpoint of some
communication over a physical link. Typically, the Link-
Layer address is an interface's Media Access Control (MAC)
address.
Mobility Agent
Either a home agent or a foreign agent.
Mobility Binding
The association of a home address with a care-of address,
along with the remaining lifetime of that association.
Mobility Security Association
A collection of security contexts, between a pair of nodes,
which may be applied to Mobile IP protocol messages
exchanged between them. Each context indicates an
authentication algorithm and mode (Section 5.1), a secret (a
shared key, or appropriate public/private key pair), and a
style of replay protection in use (Section 5.7).
Node
A host or a router.
Nonce
A randomly chosen value, different from previous choices,
inserted in a message to protect against replays.
Security Parameter Index (SPI)
An index identifying a security context between a pair of
nodes among the contexts available in the Mobility Security
Association. SPI values 0 through 255 are reserved and MUST
NOT be used in any Mobility Security Association.
Tunnel
The path followed by a datagram while it is encapsulated.
The model is that, while it is encapsulated, a datagram is
routed to a knowledgeable decapsulating agent, which
decapsulates the datagram and then correctly delivers it to
its ultimate destination.
Virtual Network
A network with no physical instantiation beyond a router
(with a physical network interface on another network). The
router (e.g., a home agent) generally advertises
reachability to the virtual network using conventional
routing protocols.
Visited Network
A network other than a mobile node's Home Network, to which
the mobile node is currently connected.
Visitor List
The list of mobile nodes visiting a foreign agent.
1.7. Protocol Overview
The following support services are defined for Mobile IP:
Agent Discovery
Home agents and foreign agents may advertise their
availability on each link for which they provide service. A
newly arrived mobile node can send a solicitation on the
link to learn if any prospective agents are present.
Registration
When the mobile node is away from home, it registers its
care-of address with its home agent. Depending on its
method of attachment, the mobile node will register either
directly with its home agent, or through a foreign agent
which forwards the registration to the home agent.
silently discard
The implementation discards the datagram without further
processing, and without indicating an error to the sender.
The implementation SHOULD provide the capability of logging
the error, including the contents of the discarded datagram,
and SHOULD record the event in a statistics counter.
The following steps provide a rough outline of operation of the
Mobile IP protocol:
- Mobility agents (i.e., foreign agents and home agents)
advertise their presence via Agent Advertisement messages
(Section 2). A mobile node may optionally solicit an Agent
Advertisement message from any locally attached mobility agents
through an Agent Solicitation message.
- A mobile node receives these Agent Advertisements and
determines whether it is on its home network or a foreign
network.
- When the mobile node detects that it is located on its home
network, it operates without mobility services. If returning
to its home network from being registered elsewhere, the mobile
node deregisters with its home agent, through exchange of a
Registration Request and Registration Reply message with it.
- When a mobile node detects that it has moved to a foreign
network, it obtains a care-of address on the foreign network.
The care-of address can either be determined from a foreign
agent's advertisements (a foreign agent care-of address), or by
some external assignment mechanism such as DHCP [13] (a co-
located care-of address).
- The mobile node operating away from home then registers its new
care-of address with its home agent through exchange of a
Registration Request and Registration Reply message with it,
possibly via a foreign agent (Section 3).
- Datagrams sent to the mobile node's home address are
intercepted by its home agent, tunneled by the home agent to
the mobile node's care-of address, received at the tunnel
endpoint (either at a foreign agent or at the mobile node
itself), and finally delivered to the mobile node (Section
4.2.3).
- In the reverse direction, datagrams sent by the mobile node are
generally delivered to their destination using standard IP
routing mechanisms, not necessarily passing through the home
agent.
When away from home, Mobile IP uses protocol tunneling to hide a
mobile node's home address from intervening routers between its home
network and its current location. The tunnel terminates at the
mobile node's care-of address. The care-of address must be an
address to which datagrams can be delivered via conventional IP
routing. At the care-of address, the original datagram is removed
from the tunnel and delivered to the mobile node.
Mobile IP provides two alternative modes for the acquisition of a
care-of address:
a) A "foreign agent care-of address" is a care-of address provided
by a foreign agent through its Agent Advertisement messages.
In this case, the care-of address is an IP address of the
foreign agent. In this mode, the foreign agent is the endpoint
of the tunnel and, upon receiving tunneled datagrams,
decapsulates them and delivers the inner datagram to the mobile
node. This mode of acquisition is preferred because it allows
many mobile nodes to share the same care-of address and
therefore does not place unnecessary demands on the already
limited IPv4 address space.
b) A "co-located care-of address" is a care-of address acquired by
the mobile node as a local IP address through some external
means, which the mobile node then associates with one of its
own network interfaces. The address may be dynamically
acquired as a temporary address by the mobile node such as
through DHCP [13], or may be owned by the mobile node as a
long-term address for its use only while visiting some foreign
network. Specific external methods of acquiring a local IP
address for use as a co-located care-of address are beyond the
scope of this document. When using a co-located care-of
address, the mobile node serves as the endpoint of the tunnel
and itself performs decapsulation of the datagrams tunneled to
it.
The mode of using a co-located care-of address has the advantage that
it allows a mobile node to function without a foreign agent, for
example, in networks that have not yet deployed a foreign agent. It
does, however, place additional burden on the IPv4 address space
because it requires a pool of addresses within the foreign network to
be made available to visiting mobile nodes. It is difficult to
efficiently maintain pools of addresses for each subnet that may
permit mobile nodes to visit.
It is important to understand the distinction between the care-of
address and the foreign agent functions. The care-of address is
simply the endpoint of the tunnel. It might indeed be an address of
a foreign agent (a foreign agent care-of address), but it might
instead be an address temporarily acquired by the mobile node (a co-
located care-of address). A foreign agent, on the other hand, is a
mobility agent that provides services to mobile nodes. See Sections
3.7 and 4.2.2 for additional details.
For example, figure 1 illustrates the routing of datagrams to and
from a mobile node away from home, once the mobile node has
registered with its home agent. In figure 1, the mobile node is
using a foreign agent care-of address, not a co-located care-of
address.
2) Datagram is intercepted 3) Datagram is
by home agent and detunneled and
is tunneled to the delivered to the
care-of address. mobile node.
+-----+ +-------+ +------+
home =======> foreign ------> mobile
agent agent <------ node
+-----+ +-------+ +------+
1) Datagram to /\ /
mobile node / 4) For datagrams sent by the
arrives on / mobile node, standard IP
home network / routing delivers each to its
via standard _ destination. In this figure,
IP routing. +----+ the foreign agent is the
host mobile node's default router.
+----+
Figure 1: Operation of Mobile IPv4
A home agent MUST be able to attract and intercept datagrams that are
destined to the home address of any of its registered mobile nodes.
Using the proxy and gratuitous ARP mechanisms described in Section
4.6, this requirement can be satisfied if the home agent has a
network interface on the link indicated by the mobile node's home
address. Other placements of the home agent relative to the mobile
node's home location MAY also be possible using other mechanisms for
intercepting datagrams destined to the mobile node's home address.
Such placements are beyond the scope of this document.
Similarly, a mobile node and a prospective or current foreign agent
MUST be able to exchange datagrams without relying on standard IP
routing mechanisms; that is, those mechanisms which make forwarding
decisions based upon the network-prefix of the destination address in
the IP header. This requirement can be satisfied if the foreign
agent and the visiting mobile node have an interface on the same
link. In this case, the mobile node and foreign agent simply bypass
their normal IP routing mechanism when sending datagrams to each
other, addressing the underlying link-layer packets to their
respective link-layer addresses. Other placements of the foreign
agent relative to the mobile node MAY also be possible using other
mechanisms to exchange datagrams between these nodes, but such
placements are beyond the scope of this document.
If a mobile node is using a co-located care-of address (as described
in (b) above), the mobile node MUST be located on the link identified
by the network prefix of this care-of address. Otherwise, datagrams
destined to the care-of address would be undeliverable.
1.8. Message Format and Protocol Extensibility
Mobile IP defines a set of new control messages, sent with UDP [37]
using well-known port number 434. The following two message types
are defined in this document:
1 Registration Request
3 Registration Reply
Up-to-date values for the message types for Mobile IP control
messages are specified in the most recent "Assigned Numbers" [40].
In addition, for Agent Discovery, Mobile IP makes use of the
existing Router Advertisement and Router Solicitation messages
defined for ICMP Router Discovery [10].
Mobile IP defines a general Extension mechanism to allow optional
information to be carried by Mobile IP control messages or by ICMP
Router Discovery messages. Some extensions have been specified to
be encoded in the simple Type-Length-Value format described in
Section 1.9.
Extensions allow variable amounts of information to be carried
within each datagram. The end of the list of Extensions is
indicated by the total length of the IP datagram.
Two separately maintained sets of numbering spaces, from which
Extension Type values are allocated, are used in Mobile IP:
- The first set consists of those Extensions which may appear
only in Mobile IP control messages (those sent to and from UDP
port number 434). In this document, the following Types are
defined for Extensions appearing in Mobile IP control messages:
32 Mobile-Home Authentication
33 Mobile-Foreign Authentication
34 Foreign-Home Authentication
- The second set consists of those extensions which may appear
only in ICMP Router Discovery messages [10]. In this document,
the following Types are defined for Extensions appearing in
ICMP Router Discovery messages:
0 One-byte Padding (encoded with no Length nor Data field)
16 Mobility Agent Advertisement
19 Prefix-Lengths
Each individual Extension is described in detail in a separate
section later in this document. Up-to-date values for these
Extension Type numbers are specified in the most recent "Assigned
Numbers" [40].
Due to the separation (orthogonality) of these sets, it is
conceivable that two Extensions that are defined at a later date
could have identical Type values, so long as one of the Extensions
may be used only in Mobile IP control messages and the other may be
used only in ICMP Router Discovery messages.
The type field in the Mobile IP extension structure can support up to
255 (skippable and not skippable) uniquely identifiable extensions.
When an Extension numbered in either of these sets within the range 0
through 127 is encountered but not recognized, the message containing
that Extension MUST be silently discarded. When an Extension
numbered in the range 128 through 255 is encountered which is not
recognized, that particular Extension is ignored, but the rest of the
Extensions and message data MUST still be processed. The Length
field of the Extension is used to skip the Data field in searching
for the next Extension.
Unless additional structure is utilized for the extension types, new
developments or additions to Mobile IP might require so many new
extensions that the available space for extension types might run
out. Two new extension structures are proposed to solve this
problem. Certain types of extensions can be aggregated, using
subtypes to identify the precise extension, for example as has been
done with the Generic Authentication Keys extensions [35]. In many
cases, this may reduce the rate of allocation for new values of the
type field.
Since the new extension structures will cause an efficient usage of
the extension type space, it is recommended that new Mobile IP
extensions follow one of the two new extension formats whenever there
may be the possibility to group related extensions together.
The following subsections provide details about three distinct
structures for Mobile IP extensions:
- The simple extension format
- The long extension format
- The short extension format
1.9. Type-Length-Value Extension Format for Mobile IP Extensions
The Type-Length-Value format illustrated in figure 2 is used for
extensions which are specified in this document. Since this simple
extension structure does not encourage the most efficient usage of
the extension type space, it is recommended that new Mobile IP
extensions follow one of the two new extension formats specified in
sections 1.10 or 1.11 whenever there may be the possibility to group
related extensions together.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Type Length Data ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Figure 2: Type-Length-Value extension format for Mobile IPv4
Type Indicates the particular type of Extension.
Length Indicates the length (in bytes) of the data field within
this Extension. The length does NOT include the Type and
Length bytes.
Data The particular data associated with this Extension. This
field may be zero or more bytes in length. The format
and length of the data field is determined by the type
and length fields.
1.10. Long Extension Format
This format is applicable for non-skippable extensions which carry
information more than 256 bytes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Sub-Type Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Data .....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Long Extension format requires that the following fields be
specified as the first fields of the extension.
Type is the type, which describes a collection of extensions
having a common data type.
Sub-Type is a unique number given to each member in the aggregated
type.
Length indicates the length (in bytes) of the data field within
this Extension. It does NOT include the Type, Length and
Sub-Type bytes.
Data is the data associated with the subtype of this
extension. This specification does not place any
additional structure on the subtype data.
Since the length field is 16 bits wide, a the extension data can
exceed 256 bytes in length.
1.11. Short Extension Format
This format is compatible with the skippable extensions defined in
section 1.9. It is not applicable for extensions which require more
than 256 bytes of data; for such extensions, use the format described
in section 1.10.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Length Sub-Type Data ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Short Extension format requires that the following fields be
specified as the first fields of the extension:
Type is the type, which describes a collection of extensions
having a common data type.
Sub-Type is a unique number given to each member in the aggregated
type.
Length 8-bit unsigned integer. Length of the extension, in
bytes, excluding the extension Type and the extension
Length fields. This field MUST be set to 1 plus the
total length of the data field.
Data is the data associated with this extension. This
specification does not place any additional structure on
the subtype data.
2. Agent Discovery
Agent Discovery is the method by which a mobile node determines
whether it is currently connected to its home network or to a foreign
network, and by which a mobile node can detect when it has moved from
one network to another. When connected to a foreign network, the
methods specified in this section also allow the mobile node to
determine the foreign agent care-of address being offered by each
foreign agent on that network.
Mobile IP extends ICMP Router Discovery [10] as its primary mechanism
for Agent Discovery. An Agent Advertisement is formed by including a
Mobility Agent Advertisement Extension in an ICMP Router
Advertisement message (Section 2.1). An Agent Solicitation message
is identical to an ICMP Router Solicitation, except that its IP TTL
MUST be set to 1 (Section 2.2). This section describes the message
formats and procedures by which mobile nodes, foreign agents, and
home agents cooperate to realize Agent Discovery.
Agent Advertisement and Agent Solicitation may not be necessary for
link layers that already provide this functionality. The method by
which mobile nodes establish link-layer connections with prospective
agents is outside the scope of this document (but see Appendix B).
The procedures described below assume that such link-layer
connectivity has already been established.
No authentication is required for Agent Advertisement and Agent
Solicitation messages. They MAY be authenticated using the IP
Authentication Header [22], which is unrelated to the messages
described in this document. Further specification of the way in
which Advertisement and Solicitation messages may be authenticated is
outside of the scope of this document.
2.1. Agent Advertisement
Agent Advertisements are transmitted by a mobility agent to advertise
its services on a link. Mobile nodes use these advertisements to
determine their current point of attachment to the Internet. An
Agent Advertisement is an ICMP Router Advertisement that has been
extended to also carry an Mobility Agent Advertisement Extension
(Section 2.1.1) and, optionally, a Prefix-Lengths Extension (Section
2.1.2), One-byte Padding Extension (Section 2.1.3), or other
Extensions that might be defined in the future.
Within an Agent Advertisement message, ICMP Router Advertisement
fields of the message are required to conform to the following
additional specifications:
- Link-Layer Fields
Destination Address
The link-layer destination address of a unicast Agent
Advertisement MUST be the same as the source link-layer
address of the Agent Solicitation which prompted the
Advertisement.
- IP Fields
TTL The TTL for all Agent Advertisements MUST be set
to 1.
Destination Address
As specified for ICMP Router Discovery [10], the IP
destination address of an multicast Agent Advertisement
MUST be either the "all systems on this link" multicast
address (224.0.0.1) [11] or the "limited broadcast"
address (255.255.255.255). The subnet-directed broadcast
address of the form <prefix>.<-1> cannot be used since
mobile nodes will not generally know the prefix of the
foreign network. When the Agent Advertisement is unicast
to a mobile node, the IP home address of the mobile node
SHOULD be used as the Destination Address.
- ICMP Fields
Code The Code field of the agent advertisement is
interpreted as follows:
0 The mobility agent handles common traffic -- that
is, it acts as a router for IP datagrams not
necessarily related to mobile nodes.
16 The mobility agent does not route common traffic.
However, all foreign agents MUST (minimally)
forward to a default router any datagrams received
from a registered mobile node (Section 4.2.2).
Lifetime
The maximum length of time that the Advertisement is
considered valid in the absence of further
Advertisements.
Router Address(es)
See Section 2.3.1 for a discussion of the addresses that
may appear in this portion of the Agent Advertisement.
Num Addrs
The number of Router Addresses advertised in this
message. Note that in an Agent Advertisement message,
the number of router addresses specified in the ICMP
Router Advertisement portion of the message MAY be set to
0. See Section 2.3.1 for details.
If sent periodically, the nominal interval at which Agent
Advertisements are sent SHOULD be no longer than 1/3 of the
advertisement Lifetime given in the ICMP header. This interval MAY
be shorter than 1/3 the advertised Lifetime. This allows a mobile
node to miss three successive advertisements before deleting the
agent from its list of valid agents. The actual transmission time
for each advertisement SHOULD be slightly randomized [10] in order to
avoid synchronization and subsequent collisions with other Agent
Advertisements that may be sent by other agents (or with other Router
Advertisements sent by other routers). Note that this field has no
relation to the "Registration Lifetime" field within the Mobility
Agent Advertisement Extension defined below.
2.1.1. Mobility Agent Advertisement Extension
The Mobility Agent Advertisement Extension follows the ICMP Router
Advertisement fields. It is used to indicate that an ICMP Router
Advertisement message is also an Agent Advertisement being sent by a
mobility agent. The Mobility Agent Advertisement Extension is
defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Length Sequence Number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Registration Lifetime RBHFMGrT reserved
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
zero or more Care-of Addresses
...
Type 16
Length (6 + 4*N), where 6 accounts for the number of bytes in
the Sequence Number, Registration Lifetime, flags, and
reserved fields, and N is the number of care-of addresses
advertised.
Sequence Number
The count of Agent Advertisement messages sent since the
agent was initialized (Section 2.3.2).
Registration Lifetime
The longest lifetime (measured in seconds) that this
agent is willing to accept in any Registration Request.
A value of 0xffff indicates infinity. This field has no
relation to the "Lifetime" field within the ICMP Router
Advertisement portion of the Agent Advertisement.
R Registration required. Registration with this foreign
agent (or another foreign agent on this link) is required
even when using a co-located care-of address.
B Busy. The foreign agent will not accept registrations
from additional mobile nodes.
H Home agent. This agent offers service as a home agent on
the link on which this Agent Advertisement message is
sent.
F Foreign agent. This agent offers service as a foreign
agent on the link on which this Agent Advertisement
message is sent.
M Minimal encapsulation. This agent implements receiving
tunneled datagrams that use minimal encapsulation [34].
G GRE encapsulation. This agent implements receiving
tunneled datagrams that use GRE encapsulation [16].
r Sent as zero; ignored on reception. SHOULD NOT be
allocated for any other uses.
T Foreign agent supports reverse tunneling [27].
reserved
Sent as zero; ignored on reception.
Care-of Address(es)
The advertised foreign agent care-of address(es) provided
by this foreign agent. An Agent Advertisement MUST
include at least one care-of address if the 'F' bit is
set. The number of care-of addresses present is
determined by the Length field in the Extension.
A home agent MUST always be prepared to serve the mobile nodes for
which it is the home agent. A foreign agent may at times be too busy
to serve additional mobile nodes; even so, it must continue to send
Agent Advertisements, so that any mobile nodes already registered
with it will know that they have not moved out of range of the
foreign agent and that the foreign agent has not failed. A foreign
agent may indicate that it is "too busy" to allow new mobile nodes to
register with it, by setting the 'B' bit in its Agent Advertisements.
An Agent Advertisement message MUST NOT have the 'B' bit set if the
'F' bit is not also set. Furthermore, at least one of the 'F' bit
and the 'H' bit MUST be set in any Agent Advertisement message sent.
When a foreign agent wishes to require registration even from those
mobile nodes which have acquired a co-located care-of address, it
sets the 'R' bit to one. Because this bit applies only to foreign
agents, an agent MUST NOT set the 'R' bit to one unless the 'F' bit
is also set to one.
2.1.2. Prefix-Lengths Extension
The Prefix-Lengths Extension MAY follow the Mobility Agent
Advertisement Extension. It is used to indicate the number of bits
of network prefix that applies to each Router Address listed in the
ICMP Router Advertisement portion of the Agent Advertisement. Note
that the prefix lengths given DO NOT apply to care-of address(es)
listed in the Mobility Agent Advertisement Extension. The Prefix-
Lengths Extension is defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Length Prefix Length ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 19 (Prefix-Lengths Extension)
Length N, where N is the value (possibly zero) of the Num Addrs
field in the ICMP Router Advertisement portion of the
Agent Advertisement.
Prefix Length(s)
The number of leading bits that define the network number
of the corresponding Router Address listed in the ICMP
Router Advertisement portion of the message. The prefix
length for each Router Address is encoded as a separate
byte, in the order that the Router Addresses are listed
in the ICMP Router Advertisement portion of the message.
See Section 2.4.2 for information about how the Prefix-Lengths
Extension MAY be used by a mobile node when determining whether it
has moved. See Appendix E for implementation details about the use
of this Extension.
2.1.3. One-byte Padding Extension
Some IP protocol implementations insist upon padding ICMP messages to
an even number of bytes. If the ICMP length of an Agent
Advertisement is odd, this Extension MAY be included in order to make
the ICMP length even. Note that this Extension is NOT intended to be
a general-purpose Extension to be included in order to word- or
long-align the various fields of the Agent Advertisement. An Agent
Advertisement SHOULD NOT include more than one One-byte Padding
Extension and if present, this Extension SHOULD be the last Extension
in the Agent Advertisement.
Note that unlike other Extensions used in Mobile IP, the One-byte
Padding Extension is encoded as a single byte, with no "Length" nor
"Data" field present. The One-byte Padding Extension is defined as
follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
Type
+-+-+-+-+-+-+-+-+
Type 0 (One-byte Padding Extension)
2.2. Agent Solicitation
An Agent Solicitation is identical to an ICMP Router Solicitation
with the further restriction that the IP TTL Field MUST be set to 1.
2.3. Foreign Agent and Home Agent Considerations
Any mobility agent which cannot be discovered by a link-layer
protocol MUST send Agent Advertisements. An agent which can be
discovered by a link-layer protocol SHOULD also implement Agent
Advertisements. However, the Advertisements need not be sent, except
when the site policy requires registration with the agent (i.e., when
the 'R' bit is set), or as a response to a specific Agent
Solicitation. All mobility agents MUST process packets that they
receive addressed to the Mobile-Agents multicast group, at address
224.0.0.11. A mobile node MAY send an Agent Solicitation to
224.0.0.11. All mobility agents SHOULD respond to Agent
Solicitations.
The same procedures, defaults, and constants are used in Agent
Advertisement messages and Agent Solicitation messages as specified
for ICMP Router Discovery [10], except that:
- a mobility agent MUST limit the rate at which it sends broadcast
or multicast Agent Advertisements; the maximum rate SHOULD be
chosen so that the Advertisements do not consume a significant
amount of network bandwidth, AND
- a mobility agent that receives a Router Solicitation MUST NOT
require that the IP Source Address is the address of a neighbor
(i.e., an address that matches one of the router's own addresses
on the arrival interface, under the subnet mask associated with
that address of the router).
- a mobility agent MAY be configured to send Agent Advertisements
only in response to an Agent Solicitation message.
If the home network is not a virtual network, then the home agent for
any mobile node SHOULD be located on the link identified by the
mobile node's home address, and Agent Advertisement messages sent by
the home agent on this link MUST have the 'H' bit set. In this way,
mobile nodes on their own home network will be able to determine that
they are indeed at home. Any Agent Advertisement messages sent by
the home agent on another link to which it may be attached (if it is
a mobility agent serving more than one link), MUST NOT have the 'H'
bit set, unless the home agent also serves as a home agent (to other
mobile nodes) on that other link. A mobility agent MAY use different
settings for each of the 'R', 'H', and 'F' bits on different network
interfaces.
If the home network is a virtual network, the home network has no
physical realization external to the home agent itself. In this
case, there is no physical network link on which to send Agent
Advertisement messages advertising the home agent. Mobile nodes for
which this is the home network are always treated as being away from
home.
On a particular subnet, either all mobility agents MUST include the
Prefix-Lengths Extension or all of them MUST NOT include this
Extension. Equivalently, it is prohibited for some agents on a given
subnet to include the Extension but for others not to include it.
Otherwise, one of the move detection algorithms designed for mobile
nodes will not function properly (Section 2.4.2).
2.3.1. Advertised Router Addresses
The ICMP Router Advertisement portion of the Agent Advertisement MAY
contain one or more router addresses. An agent SHOULD only put its
own addresses, if any, in the advertisement. Whether or not its own
address appears in the Router Addresses, a foreign agent MUST route
datagrams it receives from registered mobile nodes (Section 4.2.2).
2.3.2. Sequence Numbers and Rollover Handling
The sequence number in Agent Advertisements ranges from 0 to 0xffff.
After booting, an agent MUST use the number 0 for its first
advertisement. Each subsequent advertisement MUST use the sequence
number one greater, with the exception that the sequence number
0xffff MUST be followed by sequence number 256. In this way, mobile
nodes can distinguish a reduction in the sequence number that occurs
after a reboot from a reduction that results in rollover of the
sequence number after it attains the value 0xffff.
2.4. Mobile Node Considerations
Every mobile node MUST implement Agent Solicitation. Solicitations
SHOULD only be sent in the absence of Agent Advertisements and when a
care-of address has not been determined through a link-layer protocol
or other means. The mobile node uses the same procedures, defaults,
and constants for Agent Solicitation as specified for ICMP Router
Solicitation messages [10], except that the mobile node MAY solicit
more often than once every three seconds, and that a mobile node that
is currently not connected to any foreign agent MAY solicit more
times than MAX_SOLICITATIONS.
The rate at which a mobile node sends Solicitations MUST be limited
by the mobile node. The mobile node MAY send three initial
Solicitations at a maximum rate of one per second while searching for
an agent. After this, the rate at which Solicitations are sent MUST
be reduced so as to limit the overhead on the local link. Subsequent
Solicitations MUST be sent using a binary exponential bacKOFf
mechanism, doubling the interval between consecutive Solicitations,
up to a maximum interval. The maximum interval SHOULD be chosen
appropriately based upon the characteristics of the media over which
the mobile node is soliciting. This maximum interval SHOULD be at
least one minute between Solicitations.
While still searching for an agent, the mobile node MUST NOT increase
the rate at which it sends Solicitations unless it has received a
positive indication that it has moved to a new link. After
successfully registering with an agent, the mobile node SHOULD also
increase the rate at which it will send Solicitations when it next
begins searching for a new agent with which to register. The
increased solicitation rate MAY revert to the maximum rate, but then
MUST be limited in the manner described above. In all cases, the
recommended solicitation intervals are nominal values. Mobile nodes
MUST randomize their solicitation times around these nominal values
as specified for ICMP Router Discovery [10].
Mobile nodes MUST process received Agent Advertisements. A mobile
node can distinguish an Agent Advertisement message from other uses
of the ICMP Router Advertisement message by examining the number of
advertised addresses and the IP Total Length field. When the IP
total length indicates that the ICMP message is longer than needed
for the number of advertised addresses, the remaining data is
interpreted as one or more Extensions. The presence of a Mobility
Agent Advertisement Extension identifies the advertisement as an
Agent Advertisement.
If there is more than one advertised address, the mobile node SHOULD
pick the first address for its initial registration attempt. If the
registration attempt fails with a status Code indicating rejection by
the foreign agent, the mobile node MAY retry the attempt with each
subsequent advertised address in turn.
When multiple methods of agent discovery are in use, the mobile node
SHOULD first attempt registration with agents including Mobility
Agent Advertisement Extensions in their advertisements, in preference
to those discovered by other means. This preference maximizes the
likelihood that the registration will be recognized, thereby
minimizing the number of registration attempts.
A mobile node MUST ignore reserved bits in Agent Advertisements, as
opposed to discarding such advertisements. In this way, new bits can
be defined later, without affecting the ability for mobile nodes to
use the advertisements even when the newly defined bits are not
understood.
2.4.1. Registration Required
When the mobile node receives an Agent Advertisement with the 'R' bit
set, the mobile node SHOULD register through the foreign agent, even
when the mobile node might be able to acquire its own co-located
care-of address. This feature is intended to allow sites to enforce
visiting policies (such as accounting) which require exchanges of
authorization.
If formerly reserved bits require some kind of monitoring/enforcement
at the foreign link, foreign agents implementing the new
specification for the formerly reserved bits can set the 'R' bit.
This has the effect of forcing the mobile node to register through
the foreign agent, so the foreign agent could then monitor/enforce
the policy.
2.4.2. Move Detection
Two primary mechanisms are provided for mobile nodes to detect when
they have moved from one subnet to another. Other mechanisms MAY
also be used. When the mobile node detects that it has moved, it
SHOULD register (Section 3) with a suitable care-of address on the
new foreign network. However, the mobile node MUST NOT register more
frequently than once per second on average, as specified in Section
3.6.3.
2.4.2.1. Algorithm 1
The first method of move detection is based upon the Lifetime field
within the main body of the ICMP Router Advertisement portion of the
Agent Advertisement. A mobile node SHOULD record the Lifetime
received in any Agent Advertisements, until that Lifetime expires.
If the mobile node fails to receive another advertisement from the
same agent within the specified Lifetime, it SHOULD assume that it
has lost contact with that agent. If the mobile node has previously
received an Agent Advertisement from another agent for which the
Lifetime field has not yet expired, the mobile node MAY immediately
attempt registration with that other agent. Otherwise, the mobile
node SHOULD attempt to discover a new agent with which to register.
2.4.2.2. Algorithm 2
The second method uses network prefixes. The Prefix-Lengths
Extension MAY be used in some cases by a mobile node to determine
whether or not a newly received Agent Advertisement was received on
the same subnet as the mobile node's current care-of address. If the
prefixes differ, the mobile node MAY assume that it has moved. If a
mobile node is currently using a foreign agent care-of address, the
mobile node SHOULD NOT use this method of move detection unless both
the current agent and the new agent include the Prefix-Lengths
Extension in their respective Agent Advertisements; if this Extension
is missing from one or both of the advertisements, this method of
move detection SHOULD NOT be used. Similarly, if a mobile node is
using a co-located care-of address, it SHOULD not use this method of
move detection unless the new agent includes the Prefix-Lengths
Extension in its Advertisement and the mobile node knows the network
prefix of its current co-located care-of address. On the expiration
of its current registration, if this method indicates that the mobile
node has moved, rather than re-registering with its current care-of
address, a mobile node MAY choose instead to register with a the
foreign agent sending the new Advertisement with the different
network prefix. The Agent Advertisement on which the new
registration is based MUST NOT have expired according to its Lifetime
field.
2.4.3. Returning Home
A mobile node can detect that it has returned to its home network
when it receives an Agent Advertisement from its own home agent. If
so, it SHOULD deregister with its home agent (Section 3). Before
attempting to deregister, the mobile node SHOULD configure its
routing table appropriately for its home network (Section 4.2.1). In
addition, if the home network is using ARP [36], the mobile node MUST
follow the procedures described in Section 4.6 with regard to ARP,
proxy ARP, and gratuitous ARP.
2.4.4. Sequence Numbers and Rollover Handling
If a mobile node detects two successive values of the sequence number
in the Agent Advertisements from the foreign agent with which it is
registered, the second of which is less than the first and inside the
range 0 to 255, the mobile node SHOULD register again. If the second
value is less than the first but is greater than or equal to 256, the
mobile node SHOULD assume that the sequence number has rolled over
past its maximum value (0xffff), and that reregistration is not
necessary (Section 2.3).
3. Registration
Mobile IP registration provides a flexible mechanism for mobile nodes
to communicate their current reachability information to their home
agent. It is the method by which mobile nodes:
- request forwarding services when visiting a foreign network,
- inform their home agent of their current care-of address,
- renew a registration which is due to expire, and/or
- deregister when they return home.
Registration messages exchange information between a mobile node,
(optionally) a foreign agent, and the home agent. Registration
creates or modifies a mobility binding at the home agent, associating
the mobile node's home address with its care-of address for the
specified Lifetime.
Several other (optional) capabilities are available through the
registration procedure, which enable a mobile node to:
- discover its home address, if the mobile node is not configured
with this information.
- maintain multiple simultaneous registrations, so that a copy of
each datagram will be tunneled to each active care-of address
- deregister specific care-of addresses while retaining other
mobility bindings, and
- discover the address of a home agent if the mobile node is not
configured with this information.
3.1. Registration Overview
Mobile IP defines two different registration procedures, one via a
foreign agent that relays the registration to the mobile node's home
agent, and one directly with the mobile node's home agent. The
following rules determine which of these two registration procedures
to use in any particular circumstance:
- If a mobile node is registering a foreign agent care-of
address, the mobile node MUST register via that foreign agent.
- If a mobile node is using a co-located care-of address, and
receives an Agent Advertisement from a foreign agent on the
link on which it is using this care-of address, the mobile node
SHOULD register via that foreign agent (or via another foreign
agent on this link) if the 'R' bit is set in the received Agent
Advertisement message.
- If a mobile node is otherwise using a co-located care-of
address, the mobile node MUST register directly with its home
agent.
- If a mobile node has returned to its home network and is
(de)registering with its home agent, the mobile node MUST
register directly with its home agent.
Both registration procedures involve the exchange of Registration
Request and Registration Reply messages (Sections 3.3 and 3.4). When
registering via a foreign agent, the registration procedure requires
the following four messages:
a) The mobile node sends a Registration Request to the prospective
foreign agent to begin the registration process.
b) The foreign agent processes the Registration Request and then
relays it to the home agent.
c) The home agent sends a Registration Reply to the foreign agent
to grant or deny the Request.
d) The foreign agent processes the Registration Reply and then
relays it to the mobile node to inform it of the disposition of
its Request.
When the mobile node instead registers directly with its home agent,
the registration procedure requires only the following two messages:
a) The mobile node sends a Registration Request to the home agent.
b) The home agent sends a Registration Reply to the mobile node,
granting or denying the Request.
The registration messages defined in Sections 3.3 and 3.4 use the
User Datagram Protocol (UDP) [37]. A nonzero UDP checksum SHOULD be
included in the header, and MUST be checked by the recipient. A zero
UDP checksum SHOULD be accepted by the recipient. The behavior of
the mobile node and the home agent with respect to their mutual
acceptance of packets with zero UDP checksums SHOULD be defined as
part of the mobility security association which exists between them.
3.2. Authentication
Each mobile node, foreign agent, and home agent MUST be able to
support a mobility security association for mobile entities, indexed
by their SPI and IP address. In the case of the mobile node, this
must be its Home Address. See Section 5.1 for requirements for
support of authentication algorithms. Registration messages between
a mobile node and its home agent MUST be authenticated with an
authorization-enabling extension, e.g. the Mobile-Home Authentication
Extension (Section 3.5.2). This extension MUST be the first
authentication extension; other foreign agent-specific extensions MAY
be added to the message after the mobile node computes the
authentication.
3.3. Registration Request
A mobile node registers with its home agent using a Registration
Request message so that its home agent can create or modify a
mobility binding for that mobile node (e.g., with a new lifetime).
The Request may be relayed to the home agent by the foreign agent
through which the mobile node is registering, or it may be sent
directly to the home agent in the case in which the mobile node is
registering a co-located care-of address.
IP fields:
Source Address Typically the interface address from which the
message is sent.
Destination Address Typically that of the foreign agent or the
home agent.
See Sections 3.6.1.1 and 3.7.2.2 for details. UDP fields:
Source Port variable
Destination Port 434
The UDP header is followed by the Mobile IP fields shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type SBDMGrTx Lifetime
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Home Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Home Agent
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Care-of Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ Identification +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Extensions ...
+-+-+-+-+-+-+-+-
Type 1 (Registration Request)
S Simultaneous bindings. If the 'S' bit is set, the mobile
node is requesting that the home agent retain its prior
mobility bindings, as described in Section 3.6.1.2.
B Broadcast datagrams. If the 'B' bit is set, the mobile
node requests that the home agent tunnel to it any
broadcast datagrams that it receives on the home network,
as described in Section 4.3.
D Decapsulation by mobile node. If the 'D' bit is set, the
mobile node will itself decapsulate datagrams which are
sent to the care-of address. That is, the mobile node is
using a co-located care-of address.
M Minimal encapsulation. If the 'M' bit is set, the mobile
node requests that its home agent use minimal
encapsulation [34] for datagrams tunneled to the mobile
node.
G GRE encapsulation. If the 'G' bit is set, the mobile
node requests that its home agent use GRE encapsulation
[16] for datagrams tunneled to the mobile node.
r Sent as zero; ignored on reception. SHOULD NOT be
allocated for any other uses.
T Reverse Tunneling requested; see [27].
x Sent as zero; ignored on reception.
Lifetime
The number of seconds remaining before the registration
is considered expired. A value of zero indicates a
request for deregistration. A value of 0xffff indicates
infinity.
Home Address
The IP address of the mobile node.
Home Agent
The IP address of the mobile node's home agent.
Care-of Address
The IP address for the end of the tunnel.
Identification
A 64-bit number, constructed by the mobile node, used for
matching Registration Requests with Registration Replies,
and for protecting against replay attacks of registration
messages. See Sections 5.4 and 5.7.
Extensions
The fixed portion of the Registration Request is followed
by one or more of the Extensions listed in Section 3.5.
An authorization-enabling extension MUST be included in
all Registration Requests. See Sections 3.6.1.3 and
3.7.2.2 for information on the relative order in which
different extensions, when present, MUST be placed in a
Registration Request message.
3.4. Registration Reply
A mobility agent returns a Registration Reply message to a mobile
node which has sent a Registration Request (Section 3.3) message. If
the mobile node is requesting service from a foreign agent, that
foreign agent will receive the Reply from the home agent and
subsequently relay it to the mobile node. The Reply message contains
the necessary codes to inform the mobile node about the status of its
Request, along with the lifetime granted by the home agent, which MAY
be smaller than the original Request.
The foreign agent MUST NOT increase the Lifetime selected by the
mobile node in the Registration Request, since the Lifetime is
covered by an authentication extension which enables authorization by
the home agent. Such an extension contains authentication data which
cannot be correctly (re)computed by the foreign agent. The home
agent MUST NOT increase the Lifetime selected by the mobile node in
the Registration Request, since doing so could increase it beyond the
maximum Registration Lifetime allowed by the foreign agent. If the
Lifetime received in the Registration Reply is greater than that in
the Registration Request, the Lifetime in the Request MUST be used.
When the Lifetime received in the Registration Reply is less than
that in the Registration Request, the Lifetime in the Reply MUST be
used.
IP fields:
Source Address Typically copied from the destination address
of the Registration Request to which the
agent is replying. See Sections 3.7.2.3 and
3.8.3.1 for complete details.
Destination Address Copied from the source address of the
Registration Request to which the agent is
replying
UDP fields:
Source Port <variable>
Destination Port Copied from the source port of the
corresponding Registration Request (Section
3.7.1).
The UDP header is followed by the Mobile IP fields shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Code Lifetime
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Home Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Home Agent
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ Identification +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Extensions ...
+-+-+-+-+-+-+-+-
Type 3 (Registration Reply)
Code A value indicating the result of the Registration
Request. See below for a list of currently defined Code
values.
Lifetime
If the Code field indicates that the registration was
accepted, the Lifetime field is set to the number of
seconds remaining before the registration is considered
expired. A value of zero indicates that the mobile node
has been deregistered. A value of 0xffff indicates
infinity. If the Code field indicates that the
registration was denied, the contents of the Lifetime
field are unspecified and MUST be ignored on reception.
Home Address
The IP address of the mobile node.
Home Agent
The IP address of the mobile node's home agent.
Identification
A 64-bit number used for matching Registration Requests
with Registration Replies, and for protecting against
replay attacks of registration messages. The value is
based on the Identification field from the Registration
Request message from the mobile node, and on the style of
replay protection used in the security context between
the mobile node and its home agent (defined by the
mobility security association between them, and SPI value
in the authorization-enabling extension). See Sections
5.4 and 5.7.
Extensions
The fixed portion of the Registration Reply is followed
by one or more of the Extensions listed in Section 3.5.
An authorization-enabling extension MUST be included in
all Registration Replies returned by the home agent. See
Sections 3.7.2.2 and 3.8.3.3 for rules on placement of
extensions to Reply messages.
The following values are defined for use within the Code field.
Registration successful:
0 registration accepted
1 registration accepted, but simultaneous mobility
bindings unsupported
Registration denied by the foreign agent:
64 reason unspecified
65 administratively prohibited
66 insufficient resources
67 mobile node failed authentication
68 home agent failed authentication
69 requested Lifetime too long
70 poorly formed Request
71 poorly formed Reply
72 requested encapsulation unavailable
73 reserved and unavailable
77 invalid care-of address
78 registration timeout
80 home network unreachable (ICMP error received)
81 home agent host unreachable (ICMP error received)
82 home agent port unreachable (ICMP error received)
88 home agent unreachable (other ICMP error received)
Registration denied by the home agent:
128 reason unspecified
129 administratively prohibited
130 insufficient resources
131 mobile node failed authentication
132 foreign agent failed authentication
133 registration Identification mismatch
134 poorly formed Request
135 too many simultaneous mobility bindings
136 unknown home agent address
Up-to-date values of the Code field are specified in the most recent
"Assigned Numbers" [40].
3.5. Registration Extensions
3.5.1. Computing Authentication Extension Values
The Authenticator value computed for each authentication Extension
MUST protect the following fields from the registration message:
- the UDP payload (that is, the Registration Request or
Registration Reply data),
- all prior Extensions in their entirety, and
- the Type, Length, and SPI of this Extension.
The default authentication algorithm uses HMAC-MD5 [23] to compute a
128-bit "message digest" of the registration message. The data over
which the HMAC is computed is defined as:
- the UDP payload (that is, the Registration Request or
Registration Reply data),
- all prior Extensions in their entirety, and
- the Type, Length, and SPI of this Extension.
Note that the Authenticator field itself and the UDP header are NOT
included in the computation of the default Authenticator value. See
Section 5.1 for information about support requirements for message
authentication codes, which are to be used with the various
authentication Extensions.
The Security Parameter Index (SPI) within any of the authentication
Extensions defines the security context which is used to compute the
Authenticator value and which MUST be used by the receiver to check
that value. In particular, the SPI selects the authentication
algorithm and mode (Section 5.1) and secret (a shared key, or
appropriate public/private key pair) used in computing the
Authenticator. In order to ensure interoperability between different
implementations of the Mobile IP protocol, an implementation MUST be
able to associate any SPI value with any authentication algorithm and
mode which it implements. In addition, all implementations of Mobile
IP MUST implement the default authentication algorithm (HMAC-MD5)
specified above.
3.5.2. Mobile-Home Authentication Extension
Exactly one authorization-enabling extension MUST be present in all
Registration Requests, and also in all Registration Replies generated
by the Home Agent. The Mobile-Home Authentication Extension is
always an authorization-enabling for registration messages specified
in this document. This requirement is intended to eliminate problems
[2] which result from the uncontrolled propagation of remote
redirects in the Internet. The location of the extension marks the
end of the authenticated data.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Length SPI ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... SPI (cont.) Authenticator ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 32
Length 4 plus the number of bytes in the Authenticator.
SPI Security Parameter Index (4 bytes). An opaque
identifier (see Section 1.6).
Authenticator (variable length) (See Section 3.5.1.)
3.5.3. Mobile-Foreign Authentication Extension
This Extension MAY be included in Registration Requests and Replies
in cases in which a mobility security association exists between the
mobile node and the foreign agent. See Section 5.1 for information
about support requirements for message authentication codes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Length SPI ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... SPI (cont.) Authenticator ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 33
Length 4 plus the number of bytes in the Authenticator.
SPI Security Parameter Index (4 bytes). An opaque
identifier (see Section 1.6).
Authenticator (variable length) (See Section 3.5.1.)
3.5.4. Foreign-Home Authentication Extension
This Extension MAY be included in Registration Requests and Replies
in cases in which a mobility security association exists between the
foreign agent and the home agent. See Section 5.1 for information
about support requirements for message authentication codes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Length SPI ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
... SPI (cont.) Authenticator ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type 34
Length 4 plus the number of bytes in the Authenticator.
SPI Security Parameter Index (4 bytes). An opaque
identifier (see Section 1.6).
Authenticator (variable length) (See Section 3.5.1.)
3.6. Mobile Node Considerations
A mobile node MUST be configured with a netmask and a mobility
security association for each of its home agents. In addition, a
mobile node MAY be configured with its home address, and the IP
address of one or more of its home agents; otherwise, the mobile node
MAY discover a home agent using the procedures described in Section
3.6.1.2.
If the mobile node is not configured with a home address, it MAY use
the Mobile Node NAI extension [6] to identify itself, and set the
Home Address field of the Registration Request to 0.0.0.0. In this
case, the mobile node MUST be able to assign its home address after
extracting this information from the Registration Reply from the home
agent.
For each pending registration, the mobile node maintains the
following information:
- the link-layer address of the foreign agent to which the
Registration Request was sent, if applicable,
- the IP destination address of the Registration Request,
- the care-of address used in the registration,
- the Identification value sent in the registration,
- the originally requested Lifetime, and
- the remaining Lifetime of the pending registration.
A mobile node SHOULD initiate a registration whenever it detects a
change in its network connectivity. See Section 2.4.2 for methods by
which mobile nodes MAY make such a determination. When it is away
from home, the mobile node's Registration Request allows its home
agent to create or modify a mobility binding for it. When it is at
home, the mobile node's (de)Registration Request allows its home
agent to delete any previous mobility binding(s) for it. A mobile
node operates without the support of mobility functions when it is at
home.
There are other conditions under which the mobile node SHOULD
(re)register with its foreign agent, such as when the mobile node
detects that the foreign agent has rebooted (as specified in Section
2.4.4) and when the current registration's Lifetime is near
expiration.
In the absence of link-layer indications of changes in point of
attachment, Agent Advertisements from new agents SHOULD NOT cause a
mobile node to attempt a new registration, if its current
registration has not expired and it is still also receiving Agent
Advertisements from the foreign agent with which it is currently
registered. In the absence of link-layer indications, a mobile node
MUST NOT attempt to register more often than once per second.
A mobile node MAY register with a different agent when transport-
layer protocols indicate excessive retransmissions. A mobile node
MUST NOT consider reception of an ICMP Redirect from a foreign agent
that is currently providing service to it as reason to register with
a new foreign agent. Within these constraints, the mobile node MAY
register again at any time.
Appendix D shows some examples of how the fields in registration
messages would be set up in some typical registration scenarios.
3.6.1. Sending Registration Requests
The following sections specify details for the values the mobile node
MUST supply in the fields of Registration Request messages.
3.6.1.1. IP Fields
This section provides the specific rules by which mobile nodes pick
values for the IP header fields of a Registration Request.
IP Source Address:
- When registering on a foreign network with a co-located care-of
address, the IP source address MUST be the care-of address.
- Otherwise, if the mobile node does not have a home address, the
IP source address MUST be 0.0.0.0.
- In all other circumstances, the IP source address MUST be the
mobile node's home address.
IP Destination Address:
- When the mobile node has discovered the agent with which it is
registering, through some means (e.g., link-layer) that does
not provide the IP address of the agent (the IP address of the
agent is unknown to the mobile node), then the "All Mobility
Agents" multicast address (224.0.0.11) MUST be used. In this
case, the mobile node MUST use the agent's link-layer unicast
address in order to deliver the datagram to the correct agent.
- When registering with a foreign agent, the address of the agent
as learned from the IP source address of the corresponding
Agent Advertisement MUST be used. This MAY be an address which
does not appear as an advertised care-of address in the Agent
Advertisement. In addition, when transmitting this
Registration Request message, the mobile node MUST use a link-
layer destination address copied from the link-layer source
address of the Agent Advertisement message in which it learned
this foreign agent's IP address.
- When the mobile node is registering directly with its home
agent and knows the (unicast) IP address of its home agent, the
destination address MUST be set to this address.
- If the mobile node is registering directly with its home agent,
but does not know the IP address of its home agent, the mobile
node may use dynamic home agent address resolution to
automatically determine the IP address of its home agent
(Section 3.6.1.2). In this case, the IP destination address is
set to the subnet-directed broadcast address of the mobile
node's home network. This address MUST NOT be used as the
destination IP address if the mobile node is registering via a
foreign agent, although it MAY be used as the Home Agent
address in the body of the Registration Request when
registering via a foreign agent.
IP Time to Live:
- The IP TTL field MUST be set to 1 if the IP destination address
is set to the "All Mobility Agents" multicast address as
described above. Otherwise a suitable value should be chosen
in accordance with standard IP practice [38].
3.6.1.2. Registration Request Fields
This section provides specific rules by which mobile nodes pick
values for the fields within the fixed portion of a Registration
Request.
A mobile node MAY set the 'S' bit in order to request that the home
agent maintain prior mobility binding(s). Otherwise, the home agent
deletes any previous binding(s) and replaces them with the new
binding specified in the Registration Request. Multiple simultaneous
mobility bindings are likely to be useful when a mobile node using at
least one wireless network interface moves within wireless
transmission range of more than one foreign agent. IP explicitly
allows duplication of datagrams. When the home agent allows
simultaneous bindings, it will tunnel a separate copy of each
arriving datagram to each care-of address, and the mobile node will
receive multiple copies of datagrams destined to it.
The mobile node SHOULD set the 'D' bit if it is registering with a
co-located care-of address. Otherwise, the 'D' bit MUST NOT be set.
A mobile node MAY set the 'B' bit to request its home agent to
forward to it, a copy of broadcast datagrams received by its home
agent from the home network. The method used by the home agent to
forward broadcast datagrams depends on the type of care-of address
registered by the mobile node, as determined by the 'D' bit in the
mobile node's Registration Request:
- If the 'D' bit is set, then the mobile node has indicated that
it will decapsulate any datagrams tunneled to this care-of
address itself (the mobile node is using a co-located care-of
address). In this case, to forward such a received broadcast
datagram to the mobile node, the home agent MUST tunnel it to
this care-of address. The mobile node de-tunnels the received
datagram in the same way as any other datagram tunneled
directly to it.
- If the 'D' bit is NOT set, then the mobile node has indicated
that it is using a foreign agent care-of address, and that the
foreign agent will thus decapsulate arriving datagrams before
forwarding them to the mobile node. In this case, to forward
such a received broadcast datagram to the mobile node, the home
agent MUST first encapsulate the broadcast datagram in a
unicast datagram addressed to the mobile node's home address,
and then MUST tunnel this resulting datagram to the mobile
node's care-of address.
When decapsulated by the foreign agent, the inner datagram will
thus be a unicast IP datagram addressed to the mobile node,
identifying to the foreign agent the intended destination of
the encapsulated broadcast datagram, and will be delivered to
the mobile node in the same way as any tunneled datagram
arriving for the mobile node. The foreign agent MUST NOT
decapsulate the encapsulated broadcast datagram and MUST NOT
use a local network broadcast to transmit it to the mobile
node. The mobile node thus MUST decapsulate the encapsulated
broadcast datagram itself, and thus MUST NOT set the 'B' bit in
its Registration Request in this case unless it is capable of
decapsulating datagrams.
The mobile node MAY request alternative forms of encapsulation by
setting the 'M' bit and/or the 'G' bit, but only if the mobile node
is decapsulating its own datagrams (the mobile node is using a co-
located care-of address) or if its foreign agent has indicated
support for these forms of encapsulation by setting the corresponding
bits in the Mobility Agent Advertisement Extension of an Agent
Advertisement received by the mobile node. Otherwise, the mobile
node MUST NOT set these bits.
The Lifetime field is chosen as follows:
- If the mobile node is registering with a foreign agent, the
Lifetime SHOULD NOT exceed the value in the Registration
Lifetime field of the Agent Advertisement message received from
the foreign agent.
When the method by which the care-of address is learned does
not include a Lifetime, the default ICMP Router Advertisement
Lifetime (1800 seconds) MAY be used.
- The mobile node MAY ask a home agent to delete a particular
mobility binding, by sending a Registration Request with the
care-of address for this binding, with the Lifetime field set
to zero (Section 3.8.2).
- Similarly, a Lifetime of zero is used when the mobile node
deregisters all care-of addresses, such as upon returning home.
The Home Address field MUST be set to the mobile node's home address,
if this information is known. Otherwise, the Home Address MUST be
set to zeroes.
The Home Agent field MUST be set to the address of the mobile node's
home agent, if the mobile node knows this address. Otherwise, the
mobile node MAY use dynamic home agent address resolution to learn
the address of its home agent. In this case, the mobile node MUST
set the Home Agent field to the subnet-directed broadcast address of
the mobile node's home network. Each home agent receiving such a
Registration Request with a broadcast destination address MUST reject
the mobile node's registration and SHOULD return a rejection
Registration Reply indicating its unicast IP address for use by the
mobile node in a future registration attempt.
The Care-of Address field MUST be set to the value of the particular
care-of address that the mobile node wishes to (de)register. In the
special case in which a mobile node wishes to deregister all care-of
addresses, it MUST set this field to its home address.
The mobile node chooses the Identification field in accordance with
the style of replay protection it uses with its home agent. This is
part of the mobility security association the mobile node shares with
its home agent. See Section 5.7 for the method by which the mobile
node computes the Identification field.
3.6.1.3. Extensions
This section describes the ordering of any mandatory and any optional
Extensions that a mobile node appends to a Registration Request.
This following ordering MUST be followed:
a) The IP header, followed by the UDP header, followed by the
fixed-length portion of the Registration Request, followed by
b) If present, any non-authentication Extensions expected to be
used by the home agent (which may or may not also be useful to
the foreign agent), followed by
c) An authorization-enabling extension, followed by
d) If present, any non-authentication Extensions used only by the
foreign agent, followed by
e) The Mobile-Foreign Authentication Extension, if present.
Note that items (a) and (c) MUST appear in every Registration Request
sent by the mobile node. Items (b), (d), and (e) are optional.
However, item (e) MUST be included when the mobile node and the
foreign agent share a mobility security association.
3.6.2. Receiving Registration Replies
Registration Replies will be received by the mobile node in response
to its Registration Requests. Registration Replies generally fall
into three categories:
- the registration was accepted,
- the registration was denied by the foreign agent, or
- the registration was denied by the home agent.
The remainder of this section describes the Registration Reply
handling by a mobile node in each of these three categories.
3.6.2.1. Validity Checks
Registration Replies with an invalid, non-zero UDP checksum MUST be
silently discarded.
In addition, the low-order 32 bits of the Identification field in the
Registration Reply MUST be compared to the low-order 32 bits of the
Identification field in the most recent Registration Request sent to
the replying agent. If they do not match, the Reply MUST be silently
discarded.
Also, the Registration Reply MUST be checked for presence of an
authorization-enabling extension. For all Registration Reply
messages containing a Status Code indicating status from the Home
Agent, the mobile node MUST check for the presence of an
authorization-enabling extension, acting in accordance with the Code
field in the Reply. The rules are as follows:
a) If the mobile node and the foreign agent share a mobility
security association, exactly one Mobile-Foreign Authentication
Extension MUST be present in the Registration Reply, and the
mobile node MUST check the Authenticator value in the
Extension. If no Mobile-Foreign Authentication Extension is
found, or if more than one Mobile-Foreign Authentication
Extension is found, or if the Authenticator is invalid, the
mobile node MUST silently discard the Reply and SHOULD log the
event as a security exception.
b) If the Code field indicates that service is denied by the home
agent, or if the Code field indicates that the registration was
accepted by the home agent, exactly one Mobile-Home
Authentication Extension MUST be present in the Registration
Reply, and the mobile node MUST check the Authenticator value
in the Extension. If the Registration Reply was generated by
the home agent but no Mobile-Home Authentication Extension is
found, or if more than one Mobile-Home Authentication Extension
is found, or if the Authenticator is invalid, the mobile node
MUST silently discard the Reply and SHOULD log the event as a
security exception.
If the Code field indicates an authentication failure, either at the
foreign agent or the home agent, then it is quite possible that any
authenticators in the Registration Reply will also be in error. This
could happen, for example, if the shared secret between the mobile
node and home agent was erroneously configured. The mobile node
SHOULD log such errors as security exceptions.
3.6.2.2. Registration Request Accepted
If the Code field indicates that the request has been accepted, the
mobile node SHOULD configure its routing table appropriately for its
current point of attachment (Section 4.2.1).
If the mobile node is returning to its home network and that network
is one which implements ARP, the mobile node MUST follow the
procedures described in Section 4.6 with regard to ARP, proxy ARP,
and gratuitous ARP.
If the mobile node has registered on a foreign network, it SHOULD
re-register before the expiration of the Lifetime of its
registration. As described in Section 3.6, for each pending
Registration Request, the mobile node MUST maintain the remaining
lifetime of this pending registration, as well as the original
Lifetime from the Registration Request. When the mobile node
receives a valid Registration Reply, the mobile node MUST decrease
its view of the remaining lifetime of the registration by the amount
by which the home agent decreased the originally requested Lifetime.
This procedure is equivalent to the mobile node starting a timer for
the granted Lifetime at the time it sent the Registration Request,
even though the granted Lifetime is not known to the mobile node
until the Registration Reply is received. Since the Registration
Request is certainly sent before the home agent begins timing the
registration Lifetime (also based on the granted Lifetime), this
procedure ensures that the mobile node will re-register before the
home agent expires and deletes the registration, in spite of possibly
non-negligible transmission delays for the original Registration
Request and Reply that started the timing of the Lifetime at the
mobile node and its home agent.
3.6.2.3. Registration Request Denied
If the Code field indicates that service is being denied, the mobile
node SHOULD log the error. In certain cases the mobile node may be
able to "repair" the error. These include:
Code 69: (Denied by foreign agent, Lifetime too long)
In this case, the Lifetime field in the Registration Reply will
contain the maximum Lifetime value which that foreign agent is
willing to accept in any Registration Request. The mobile node
MAY attempt to register with this same agent, using a Lifetime
in the Registration Request that MUST be less than or equal to
the value specified in the Reply.
Code 133: (Denied by home agent, Identification mismatch)
In this case, the Identification field in the Registration
Reply will contain a value that allows the mobile node to
synchronize with the home agent, based upon the style of replay
protection in effect (Section 5.7). The mobile node MUST
adjust the parameters it uses to compute the Identification
field based upon the information in the Registration Reply,
before issuing any future Registration Requests.
Code 136: (Denied by home agent, Unknown home agent address)
This code is returned by a home agent when the mobile node is
performing dynamic home agent address resolution as described
in Sections 3.6.1.1 and 3.6.1.2. In this case, the Home Agent
field within the Reply will contain the unicast IP address of
the home agent returning the Reply. The mobile node MAY then
attempt to register with this home agent in future Registration
Requests. In addition, the mobile node SHOULD adjust the
parameters it uses to compute the Identification field based
upon the corresponding field in the Registration Reply, before
issuing any future Registration Requests.
3.6.3. Registration Retransmission
When no Registration Reply has been received within a reasonable
time, another Registration Request MAY be transmitted. When
timestamps are used, a new registration Identification is chosen for
each retransmission; thus it counts as a new registration. When
nonces are used, the unanswered Request is retransmitted unchanged;
thus the retransmission does not count as a new registration (Section
5.7). In this way a retransmission will not require the home agent
to resynchronize with the mobile node by issuing another nonce in the
case in which the original Registration Request (rather than its
Registration Reply) was lost by the network.
The maximum time until a new Registration Request is sent SHOULD be
no greater than the requested Lifetime of the Registration Request.
The minimum value SHOULD be large enough to account for the size of
the messages, twice the round trip time for transmission to the home
agent, and at least an additional 100 milliseconds to allow for
processing the messages before responding. The round trip time for
transmission to the home agent will be at least as large as the time
required to transmit the messages at the link speed of the mobile
node's current point of attachment. Some circuits add another 200
milliseconds of satellite delay in the total round trip time to the
home agent. The minimum time between Registration Requests MUST NOT
be less than 1 second. Each successive retransmission timeout period
SHOULD be at least twice the previous period, as long as that is less
than the maximum as specified above.
3.7. Foreign Agent Considerations
The foreign agent plays a mostly passive role in Mobile IP
registration. It relays Registration Requests between mobile nodes
and home agents, and, when it provides the care-of address,
decapsulates datagrams for delivery to the mobile node. It SHOULD
also send periodic Agent Advertisement messages to advertise its
presence as described in Section 2.3, if not detectable by link-layer
means.
A foreign agent MUST NOT transmit a Registration Request except when
relaying a Registration Request received from a mobile node, to the
mobile node's home agent. A foreign agent MUST NOT transmit a
Registration Reply except when relaying a Registration Reply received
from a mobile node's home agent, or when replying to a Registration
Request received from a mobile node in the case in which the foreign
agent is denying service to the mobile node. In particular, a
foreign agent MUST NOT generate a Registration Request or Reply
because a mobile node's registration Lifetime has expired. A foreign
agent also MUST NOT originate a Registration Request message that
asks for deregistration of a mobile node; however, it MUST relay
valid (de)Registration Requests originated by a mobile node.
3.7.1. Configuration and Registration Tables
Each foreign agent MUST be configured with a care-of address. In
addition, for each pending or current registration the foreign agent
MUST maintain a visitor list entry containing the following
information obtained from the mobile node's Registration Request:
- the link-layer source address of the mobile node
- the IP Source Address (the mobile node's Home Address) or its
co-located care-of address (see description of the 'R' bit in
section 2.1.1)
- the IP Destination Address (as specified in 3.6.1.1)
- the UDP Source Port
- the Home Agent address
- the Identification field
- the requested registration Lifetime, and
- the remaining Lifetime of the pending or current registration.
If the mobile node's Home Address is zero in the Registration Request
message, then the foreign agent MUST follow the procedures specified
in RFC2794 [6]. In particular, if the foreign agent cannot manage
pending registration request records with such a zero Home Address
for the mobile node, the foreign agent MUST return a Registration
Reply with Code indicating NONZERO_HOMEADDR_REQD (see [6]).
The foreign agent MAY configure a maximum number of pending
registrations that it is willing to maintain (typically 5).
Additional registrations SHOULD then be rejected by the foreign agent
with code 66. The foreign agent MAY delete any pending Registration
Request after the request has been pending for more than 7 seconds;
in this case, the foreign agent SHOULD reject the Request with code
78 (registration timeout).
As with any node on the Internet, a foreign agent MAY also share
mobility security associations with any other nodes. When relaying a
Registration Request from a mobile node to its home agent, if the
foreign agent shares a mobility security association with the home
agent, it MUST add a Foreign-Home Authentication Extension to the
Request and MUST check the required Foreign-Home Authentication
Extension in the Registration Reply from the home agent (Sections 3.3
and 3.4). Similarly, when receiving a Registration Request from a
mobile node, if the foreign agent shares a mobility security
association with the mobile node, it MUST check the required Mobile-
Foreign Authentication Extension in the Request and MUST add a
Mobile-Foreign Authentication Extension to the Registration Reply to
the mobile node.
3.7.2. Receiving Registration Requests
If the foreign agent accepts a Registration Request from a mobile
node, it checks to make sure that the indicated home agent address
does not belong to any network interface of the foreign agent. If
not, the foreign agent then MUST relay the Request to the indicated
home agent. Otherwise, if the foreign agent denies the Request, it
MUST send a Registration Reply to the mobile node with an appropriate
denial Code, except in cases where the foreign agent would be
required to send out more than one such denial per second to the same
mobile node. The following sections describe this behavior in more
detail.
If the foreign agent has configured one of its network interfaces
with the IP address specified by the mobile node as its home agent
address, the foreign agent MUST NOT forward the request again. If
the foreign agent serves the mobile node as a home agent, the foreign
agent follows the procedures specified in section 3.8.2. Otherwise,
if the foreign agent does not serve the mobile node as a home agent,
the foreign agent rejects the Registration Request with code 136
(unknown home agent address).
If a foreign agent receives a Registration Request from a mobile node
in its visitor list, the existing visitor list entry for the mobile
node SHOULD NOT be deleted or modified until the foreign agent
receives a valid Registration Reply from the home agent with a Code
indicating success. The foreign agent MUST record the new pending
Request as a separate part of the existing visitor list entry for the
mobile node. If the Registration Request requests deregistration,
the existing visitor list entry for the mobile node SHOULD NOT be
deleted until the foreign agent has received a successful
Registration Reply. If the Registration Reply indicates that the
Request (for registration or deregistration) was denied by the home
agent, the existing visitor list entry for the mobile node MUST NOT
be modified as a result of receiving the Registration Reply.
3.7.2.1. Validity Checks
Registration Requests with an invalid, non-zero UDP checksum MUST be
silently discarded. Requests with non-zero bits in reserved fields
MUST be rejected with code 70 (poorly formed request). Requests with
the 'D' bit set to 0, and specifying a care-of address not offered by
the foreign agent, MUST be rejected with code 77 (invalid care-of
address).
Also, the authentication in the Registration Request MUST be checked.
If the foreign agent and the mobile node share a mobility security
association, exactly one Mobile-Foreign Authentication Extension MUST
be present in the Registration Request, and the foreign agent MUST
check the Authenticator value in the Extension. If no Mobile-Foreign
Authentication Extension is found, or if more than one Mobile-Foreign
Authentication Extension is found, or if the Authenticator is
invalid, the foreign agent MUST silently discard the Request and
SHOULD log the event as a security exception. The foreign agent also
SHOULD send a Registration Reply to the mobile node with Code 67.
3.7.2.2. Forwarding a Valid Request to the Home Agent
If the foreign agent accepts the mobile node's Registration Request,
it MUST relay the Request to the mobile node's home agent as
specified in the Home Agent field of the Registration Request. The
foreign agent MUST NOT modify any of the fields beginning with the
fixed portion of the Registration Request up through and including
the Mobile-Home Authentication Extension or other authentication
extension supplied by the mobile node as an authorization-enabling
extension for the home agent. Otherwise, an authentication failure
is very likely to occur at the home agent. In addition, the foreign
agent proceeds as follows:
- It MUST process and remove any Extensions following the
Mobile-Home Authentication Extension,
- It MAY append any of its own non-authentication Extensions of
relevance to the home agent, if applicable, and
- It MUST append the Foreign-Home Authentication Extension, if
the foreign agent shares a mobility security association with
the home agent.
Specific fields within the IP header and the UDP header of the
relayed Registration Request MUST be set as follows:
IP Source Address
The foreign agent's address on the interface from which
the message will be sent.
IP Destination Address
Copied from the Home Agent field within the Registration
Request.
UDP Source Port
<variable>
UDP Destination Port
434
After forwarding a valid Registration Request to the home agent, the
foreign agent MUST begin timing the remaining lifetime of the pending
registration based on the Lifetime in the Registration Request. If
this lifetime expires before receiving a valid Registration Reply,
the foreign agent MUST delete its visitor list entry for this pending
registration.
3.7.2.3. Denying Invalid Requests
If the foreign agent denies the mobile node's Registration Request
for any reason, it SHOULD send the mobile node a Registration Reply
with a suitable denial Code. In such a case, the Home Address, Home
Agent, and Identification fields within the Registration Reply are
copied from the corresponding fields of the Registration Request.
If the Reserved field is nonzero, the foreign agent MUST deny the
Request and SHOULD return a Registration Reply with status code 70 to
the mobile node. If the Request is being denied because the
requested Lifetime is too long, the foreign agent sets the Lifetime
in the Reply to the maximum Lifetime value it is willing to accept in
any Registration Request, and sets the Code field to 69. Otherwise,
the Lifetime SHOULD be copied from the Lifetime field in the Request.
Specific fields within the IP header and the UDP header of the
Registration Reply MUST be set as follows:
IP Source Address
Copied from the IP Destination Address of Registration
Request, unless the "All Agents Multicast" address was
used. In this case, the foreign agent's address (on the
interface from which the message will be sent) MUST be
used.
IP Destination Address
If the Registration Reply is generated by the Foreign
Agent in order to reject a mobile node's Registration
Request, and the Registration Request contains a Home
Address which is not 0.0.0.0, then the IP Destination
Address is copied from the Home Address field of the
Registration Request. Otherwise, if the Registration
Reply is received from the Home Agent, and contains a
Home Address which is not 0.0.0.0, then the IP
Destination Address is copied from the Home Address field
of the Registration Reply. Otherwise, the IP Destination
Address of the Registration Reply is set to be
255.255.255.255.
UDP Source Port
434
UDP Destination Port
Copied from the UDP Source Port of the Registration
Request.
3.7.3. Receiving Registration Replies
The foreign agent updates its visitor list when it receives a valid
Registration Reply from a home agent. It then relays the
Registration Reply to the mobile node. The following sections
describe this behavior in more detail.
If upon relaying a Registration Request to a home agent, the foreign
agent receives an ICMP error message instead of a Registration Reply,
then the foreign agent SHOULD send to the mobile node a Registration
Reply with an appropriate "Home Agent Unreachable" failure Code
(within the range 80-95, inclusive). See Section 3.7.2.3 for details
on building the Registration Reply.
3.7.3.1. Validity Checks
Registration Replies with an invalid, non-zero UDP checksum MUST be
silently discarded.
When a foreign agent receives a Registration Reply message, it MUST
search its visitor list for a pending Registration Request with the
same mobile node home address as indicated in the Reply. If no such
pending Request is found, and if the Registration Reply does not
correspond with any pending Registration Request with a zero mobile
node home address (see section 3.7.1), the foreign agent MUST
silently discard the Reply. The foreign agent MUST also silently
discard the Reply if the low-order 32 bits of the Identification
field in the Reply do not match those in the Request.
Also, the authentication in the Registration Reply MUST be checked.
If the foreign agent and the home agent share a mobility security
association, exactly one Foreign-Home Authentication Extension MUST
be present in the Registration Reply, and the foreign agent MUST
check the Authenticator value in the Extension. If no Foreign-Home
Authentication Extension is found, or if more than one Foreign-Home
Authentication Extension is found, or if the Authenticator is
invalid, the foreign agent MUST silently discard the Reply and SHOULD
log the event as a security exception. The foreign agent also MUST
reject the mobile node's registration and SHOULD send a Registration
Reply to the mobile node with Code 68.
3.7.3.2. Forwarding Replies to the Mobile Node
A Registration Reply which satisfies the validity checks of Section
3.8.2.1 is relayed to the mobile node. The foreign agent MUST also
update its visitor list entry for the mobile node to reflect the
results of the Registration Request, as indicated by the Code field
in the Reply. If the Code indicates that the home agent has accepted
the registration and the Lifetime field is nonzero, the foreign agent
SHOULD set the Lifetime in the visitor list entry to the minimum of
the following two values:
- the value specified in the Lifetime field of the Registration
Reply, and
- the foreign agent's own maximum value for allowable
registration lifetime.
If, instead, the Code indicates that the Lifetime field is zero, the
foreign agent MUST delete its visitor list entry for the mobile node.
Finally, if the Code indicates that the registration was denied by
the home agent, the foreign agent MUST delete its pending
registration list entry, but not its visitor list entry, for the
mobile node.
The foreign agent MUST NOT modify any of the fields beginning with
the fixed portion of the Registration Reply up through and including
the Mobile-Home Authentication Extension. Otherwise, an
authentication failure is very likely to occur at the mobile node.
In addition, the foreign agent SHOULD perform the following
additional procedures:
- It MUST process and remove any Extensions following the
Mobile-Home Authentication Extension,
- It MAY append its own non-authentication Extensions of
relevance to the mobile node, if applicable, and
- It MUST append the Mobile-Foreign Authentication Extension, if
the foreign agent shares a mobility security association with
the mobile node.
Specific fields within the IP header and the UDP header of the
relayed Registration Reply are set according to the same rules
specified in Section 3.7.2.3.
After forwarding a valid Registration Reply to the mobile node, the
foreign agent MUST update its visitor list entry for this
registration as follows. If the Registration Reply indicates that
the registration was accepted by the home agent, the foreign agent
resets its timer of the lifetime of the registration to the Lifetime
granted in the Registration Reply; unlike the mobile node's timing of
the registration lifetime as described in Section 3.6.2.2, the
foreign agent considers this lifetime to begin when it forwards the
Registration Reply message, ensuring that the foreign agent will not
expire the registration before the mobile node does. On the other
hand, if the Registration Reply indicates that the registration was
rejected by the home agent, the foreign agent deletes its visitor
list entry for this attempted registration.
3.8. Home Agent Considerations
Home agents play a reactive role in the registration process. The
home agent receives Registration Requests from the mobile node
(perhaps relayed by a foreign agent), updates its record of the
mobility bindings for this mobile node, and issues a suitable
Registration Reply in response to each.
A home agent MUST NOT transmit a Registration Reply except when
replying to a Registration Request received from a mobile node. In
particular, the home agent MUST NOT generate a Registration Reply to
indicate that the Lifetime has expired.
3.8.1. Configuration and Registration Tables
Each home agent MUST be configured with an IP address and with the
prefix size for the home network. The home agent MUST be configured
with the mobility security association of each authorized mobile node
that it is serving as a home agent.
When the home agent accepts a valid Registration Request from a
mobile node that it serves as a home agent, the home agent MUST
create or modify the entry for this mobile node in its mobility
binding list containing:
- the mobile node's home address
- the mobile node's care-of address
- the Identification field from the Registration Reply
- the remaining Lifetime of the registration
The home agent MAY optionally offer the capability to dynamically
associate a home address to a mobile node upon receiving a
Registration Request from that mobile node. The method by which a
home address is allocated to the mobile node is beyond the scope of
this document, but see [6]. After the home agent makes the
association of the home address to the mobile node, the home agent
MUST put that home address into the Home Address field of the
Registration Reply.
The home agent MAY also maintain mobility security associations with
various foreign agents. When receiving a Registration Request from a
foreign agent, if the home agent shares a mobility security
association with the foreign agent, the home agent MUST check the
Authenticator in the required Foreign-Home Authentication Extension
in the message, based on this mobility security association.
Similarly, when sending a Registration Reply to a foreign agent, if
the home agent shares a mobility security association with the
foreign agent, the home agent MUST include a Foreign-Home
Authentication Extension in the message, based on this mobility
security association.
3.8.2. Receiving Registration Requests
If the home agent accepts an incoming Registration Request, it MUST
update its record of the the mobile node's mobility binding(s) and
SHOULD send a Registration Reply with a suitable Code. Otherwise
(the home agent denies the Request), it SHOULD send a Registration
Reply with an appropriate Code specifying the reason the Request was
denied. The following sections describe this behavior in more
detail. If the home agent does not support broadcasts (see section
4.3), it MUST ignore the 'B' bit (as opposed to rejecting the
Registration Request).
3.8.2.1. Validity Checks
Registration Requests with an invalid, non-zero UDP checksum MUST be
silently discarded by the home agent.
The authentication in the Registration Request MUST be checked. This
involves the following operations:
a) The home agent MUST check for the presence of an
authorization-enabling extension, and perform the indicated
authentication. Exactly one authorization-enabling extension
MUST be present in the Registration Request; and the home agent
MUST either check the Authenticator value in the extension or
verify that the authenticator value has been checked by another
agent with which it has a security association. If no
authorization-enabling extension is found, or if more than one
authorization-enabling extension is found, or if the
Authenticator is invalid, the home agent MUST reject the mobile
node's registration and SHOULD send a Registration Reply to the
mobile node with Code 131. The home agent MUST then discard
the Request and SHOULD log the error as a security exception.
b) The home agent MUST check that the registration Identification
field is correct using the context selected by the SPI within
the authorization-enabling extension. See Section 5.7 for a
description of how this is performed. If incorrect, the home
agent MUST reject the Request and SHOULD send a Registration
Reply to the mobile node with Code 133, including an
Identification field computed in accordance with the rules
specified in Section 5.7. The home agent MUST do no further
processing with such a Request, though it SHOULD log the error
as a security exception.
c) If the home agent shares a mobility security association with
the foreign agent, the home agent MUST check for the presence
of a valid Foreign-Home Authentication Extension. Exactly one
Foreign-Home Authentication Extension MUST be present in the
Registration Request in this case, and the home agent MUST
check the Authenticator value in the Extension. If no
Foreign-Home Authentication Extension is found, or if more than
one Foreign-Home Authentication Extension is found, or if the
Authenticator is invalid, the home agent MUST reject the mobile
node's registration and SHOULD send a Registration Reply to the
mobile node with Code 132. The home agent MUST then discard
the Request and SHOULD log the error as a security exception.
In addition to checking the authentication in the Registration
Request, home agents MUST deny Registration Requests that are sent to
the subnet-directed broadcast address of the home network (as opposed
to being unicast to the home agent). The home agent MUST discard the
Request and SHOULD returning a Registration Reply with a Code of 136.
In this case, the Registration Reply will contain the home agent's
unicast address, so that the mobile node can re-issue the
Registration Request with the correct home agent address.
Note that some routers change the IP destination address of a
datagram from a subnet-directed broadcast address to 255.255.255.255
before injecting it into the destination subnet. In this case, home
agents that attempt to pick up dynamic home agent discovery requests
by binding a socket explicitly to the subnet-directed broadcast
address will not see such packets. Home agent implementors should be
prepared for both the subnet-directed broadcast address and
255.255.255.255 if they wish to support dynamic home agent discovery.
3.8.2.2. Accepting a Valid Request
If the Registration Request satisfies the validity checks in Section
3.8.2.1, and the home agent is able to accommodate the Request, the
home agent MUST update its mobility binding list for the requesting
mobile node and MUST return a Registration Reply to the mobile node.
In this case, the Reply Code will be either 0 if the home agent
supports simultaneous mobility bindings, or 1 if it does not. See
Section 3.8.3 for details on building the Registration Reply message.
The home agent updates its record of the mobile node's mobility
bindings as follows, based on the fields in the Registration Request:
- If the Lifetime is zero and the Care-of Address equals the
mobile node's home address, the home agent deletes all of the
entries in the mobility binding list for the requesting mobile
node. This is how a mobile node requests that its home agent
cease providing mobility services.
- If the Lifetime is zero and the Care-of Address does not equal
the mobile node's home address, the home agent deletes only the
entry containing the specified Care-of Address from the
mobility binding list for the requesting mobile node. Any
other active entries containing other care-of addresses will
remain active.
- If the Lifetime is nonzero, the home agent adds an entry
containing the requested Care-of Address to the mobility
binding list for the mobile node. If the 'S' bit is set and
the home agent supports simultaneous mobility bindings, the
previous mobility binding entries are retained. Otherwise, the
home agent removes all previous entries in the mobility binding
list for the mobile node.
In all cases, the home agent MUST send a Registration Reply to the
source of the Registration Request, which might indeed be a different
foreign agent than that whose care-of address is being
(de)registered. If the home agent shares a mobility security
association with the foreign agent whose care-of address is being
deregistered, and that foreign agent is different from the one which
relayed the Registration Request, the home agent MAY additionally
send a Registration Reply to the foreign agent whose care-of address
is being deregistered. The home agent MUST NOT send such a Reply if
it does not share a mobility security association with the foreign
agent. If no Reply is sent, the foreign agent's visitor list will
expire naturally when the original Lifetime expires.
The home agent MUST NOT increase the Lifetime above that specified by
the mobile node in the Registration Request. However, it is not an
error for the mobile node to request a Lifetime longer than the home
agent is willing to accept. In this case, the home agent simply
reduces the Lifetime to a permissible value and returns this value in
the Registration Reply. The Lifetime value in the Registration Reply
informs the mobile node of the granted lifetime of the registration,
indicating when it SHOULD re-register in order to maintain continued
service. After the expiration of this registration lifetime, the
home agent MUST delete its entry for this registration in its
mobility binding list.
If the Registration Request duplicates an accepted current
Registration Request, the new Lifetime MUST NOT extend beyond the
Lifetime originally granted. A Registration Request is a duplicate
if the home address, care-of address, and Identification fields all
equal those of an accepted current registration.
In addition, if the home network implements ARP [36], and the
Registration Request asks the home agent to create a mobility binding
for a mobile node which previously had no binding (the mobile node
was previously assumed to be at home), then the home agent MUST
follow the procedures described in Section 4.6 with regard to ARP,
proxy ARP, and gratuitous ARP. If the mobile node already had a
previous mobility binding, the home agent MUST continue to follow the
rules for proxy ARP described in Section 4.6.
3.8.2.3. Denying an Invalid Request
If the Registration Reply does not satisfy all of the validity checks
in Section 3.8.2.1, or the home agent is unable to accommodate the
Request, the home agent SHOULD return a Registration Reply to the
mobile node with a Code that indicates the reason for the error. If
a foreign agent was involved in relaying the Request, this allows the
foreign agent to delete its pending visitor list entry. Also, this
informs the mobile node of the reason for the error such that it may
attempt to fix the error and issue another Request.
This section lists a number of reasons the home agent might reject a
Request, and provides the Code value it should use in each instance.
See Section 3.8.3 for additional details on building the Registration
Reply message.
Many reasons for rejecting a registration are administrative in
nature. For example, a home agent can limit the number of
simultaneous registrations for a mobile node, by rejecting any
registrations that would cause its limit to be exceeded, and
returning a Registration Reply with error code 135. Similarly, a
home agent may refuse to grant service to mobile nodes which have
entered unauthorized service areas by returning a Registration Reply
with a Code of 129.
Requests with non-zero bits in reserved fields MUST be rejected with
code 134 (poorly formed request).
3.8.3. Sending Registration Replies
If the home agent accepts a Registration Request, it then MUST update
its record of the mobile node's mobility binding(s) and SHOULD send a
Registration Reply with a suitable Code. Otherwise (the home agent
has denied the Request), it SHOULD send a Registration Reply with an
appropriate Code specifying the reason the Request was denied. The
following sections provide additional detail for the values the home
agent MUST supply in the fields of Registration Reply messages.
3.8.3.1. IP/UDP Fields
This section provides the specific rules by which home agents pick
values for the IP and UDP header fields of a Registration Reply.
IP Source Address
Copied from the IP Destination Address of Registration
Request, unless a multicast or broadcast address was
used. If the IP Destination Address of the Registration
Request was a broadcast or multicast address, the IP
Source Address of the Registration Reply MUST be set to
the home agent's (unicast) IP address.
IP Destination Address
Copied from the IP Source Address of the Registration
Request.
UDP Source Port
Copied from the UDP Destination Port of the Registration
Request.
UDP Destination Port
Copied from the UDP Source Port of the Registration
Request.
When sending a Registration Reply in response to a Registration
Request that requested deregistration of the mobile node (the
Lifetime is zero and the Care-of Address equals the mobile node's
home address) and in which the IP Source Address was also set to the
mobile node's home address (this is the normal method used by a
mobile node to deregister when it returns to its home network), the
IP Destination Address in the Registration Reply will be set to the
mobile node's home address, as copied from the IP Source Address of
the Request.
In this case, when transmitting the Registration Reply, the home
agent MUST transmit the Reply directly onto the home network as if
the mobile node were at home, bypassing any mobility binding list
entry that may still exist at the home agent for the destination
mobile node. In particular, for a mobile node returning home after
being registered with a care-of address, if the mobile node's new
Registration Request is not accepted by the home agent, the mobility
binding list entry for the mobile node will still indicate that
datagrams addressed to the mobile node should be tunneled to the
mobile node's registered care-of address; when sending the
Registration Reply indicating the rejection of this Request, this
existing binding list entry MUST be ignored, and the home agent MUST
transmit this Reply as if the mobile node were at home.
3.8.3.2. Registration Reply Fields
This section provides the specific rules by which home agents pick
values for the fields within the fixed portion of a Registration
Reply.
The Code field of the Registration Reply is chosen in accordance with
the rules specified in the previous sections. When replying to an
accepted registration, a home agent SHOULD respond with Code 1 if it
does not support simultaneous registrations.
The Lifetime field MUST be copied from the corresponding field in the
Registration Request, unless the requested value is greater than the
maximum length of time the home agent is willing to provide the
requested service. In such a case, the Lifetime MUST be set to the
length of time that service will actually be provided by the home
agent. This reduced Lifetime SHOULD be the maximum Lifetime allowed
by the home agent (for this mobile node and care-of address).
If the Home Address field of the Registration Request is nonzero, it
MUST be copied into the Home Address field of the Registration Reply
message. Otherwise, if the Home Address field of the Registration
Request is zero as specified in section 3.6, the home agent SHOULD
arrange for the selection of a home address for the mobile node, and
insert the selected address into the Home Address field of the
Registration Reply message. See [6] for further relevant details in
the case where mobile nodes identify themselves using an NAI instead
of their IP home address.
If the Home Agent field in the Registration Request contains a
unicast address of this home agent, then that field MUST be copied
into the Home Agent field of the Registration Reply. Otherwise, the
home agent MUST set the Home Agent field in the Registration Reply to
its unicast address. In this latter case, the home agent MUST reject
the registration with a suitable code (e.g., Code 136) to prevent the
mobile node from possibly being simultaneously registered with two or
more home agents.
3.8.3.3. Extensions
This section describes the ordering of any required and any optional
Mobile IP Extensions that a home agent appends to a Registration
Reply. The following ordering MUST be followed:
a) The IP header, followed by the UDP header, followed by the
fixed-length portion of the Registration Reply,
b) If present, any non-authentication Extensions used by the
mobile node (which may or may not also be used by the foreign
agent),
c) The Mobile-Home Authentication Extension,
d) If present, any non-authentication Extensions used only by the
foreign agent, and
e) The Foreign-Home Authentication Extension, if present.
Note that items (a) and (c) MUST appear in every Registration Reply
sent by the home agent. Items (b), (d), and (e) are optional.
However, item (e) MUST be included when the home agent and the
foreign agent share a mobility security association.
4. Routing Considerations
This section describes how mobile nodes, home agents, and (possibly)
foreign agents cooperate to route datagrams to/from mobile nodes that
are connected to a foreign network. The mobile node informs its home
agent of its current location using the registration procedure
described in Section 3. See the protocol overview in Section 1.7 for
the relative locations of the mobile node's home address with respect
to its home agent, and the mobile node itself with respect to any
foreign agent with which it might attempt to register.
4.1. Encapsulation Types
Home agents and foreign agents MUST support tunneling datagrams using
IP in IP encapsulation [32]. Any mobile node that uses a co-located
care-of address MUST support receiving datagrams tunneled using IP in
IP encapsulation. Minimal encapsulation [34] and GRE encapsulation
[16] are alternate encapsulation methods which MAY optionally be
supported by mobility agents and mobile nodes. The use of these
alternative forms of encapsulation, when requested by the mobile
node, is otherwise at the discretion of the home agent.
4.2. Unicast Datagram Routing
4.2.1. Mobile Node Considerations
When connected to its home network, a mobile node operates without
the support of mobility services. That is, it operates in the same
way as any other (fixed) host or router. The method by which a
mobile node selects a default router when connected to its home
network, or when away from home and using a co-located care-of
address, is outside the scope of this document. ICMP Router
Advertisement [10] is one such method.
When registered on a foreign network, the mobile node chooses a
default router by the following rules:
- If the mobile node is registered using a foreign agent care-of
address, it MAY use its foreign agent as a first-hop router.
The foreign agent's MAC address can be learned from Agent
Advertisement. Otherwise, the mobile node MUST choose its
default router from among the Router Addresses advertised in
the ICMP Router Advertisement portion of that Agent
Advertisement message.
- If the mobile node is registered directly with its home agent
using a co-located care-of address, then the mobile node SHOULD
choose its default router from among those advertised in any
ICMP Router Advertisement message that it receives for which
its externally obtained care-of address and the Router Address
match under the network prefix. If the mobile node's
externally obtained care-of address matches the IP source
address of the Agent Advertisement under the network prefix,
the mobile node MAY also consider that IP source address as
another possible choice for the IP address of a default router.
The network prefix MAY be obtained from the Prefix-Lengths
Extension in the Router Advertisement, if present. The prefix
MAY also be obtained through other mechanisms beyond the scope
of this document.
While they are away from the home network, mobile nodes MUST NOT
broadcast ARP packets to find the MAC address of another Internet
node. Thus, the (possibly empty) list of Router Addresses from the
ICMP Router Advertisement portion of the message is not useful for
selecting a default router, unless the mobile node has some means not
involving broadcast ARP and not specified within this document for
obtaining the MAC address of one of the routers in the list.
Similarly, in the absence of unspecified mechanisms for obtaining MAC
addresses on foreign networks, the mobile node MUST ignore redirects
to other routers on foreign networks.
4.2.2. Foreign Agent Considerations
Upon receipt of an encapsulated datagram sent to its advertised
care-of address, a foreign agent MUST compare the inner destination
address to those entries in its visitor list. When the destination
does not match the address of any mobile node currently in the
visitor list, the foreign agent MUST NOT forward the datagram without
modifications to the original IP header, because otherwise a routing
loop is likely to result. The datagram SHOULD be silently discarded.
ICMP Destination Unreachable MUST NOT be sent when a foreign agent is
unable to forward an incoming tunneled datagram. Otherwise, the
foreign agent forwards the decapsulated datagram to the mobile node.
The foreign agent MUST NOT advertise to other routers in its routing
domain, nor to any other mobile node, the presence of a mobile router
(Section 4.5) or mobile node in its visitor list.
The foreign agent MUST route datagrams it receives from registered
mobile nodes. At a minimum, this means that the foreign agent must
verify the IP Header Checksum, decrement the IP Time To Live,
recompute the IP Header Checksum, and forward such datagrams to a
default router.
A foreign agent MUST NOT use broadcast ARP for a mobile node's MAC
address on a foreign network. It may obtain the MAC address by
copying the information from an Agent Solicitation or a Registration
Request transmitted from a mobile node. A foreign agent's ARP cache
for the mobile node's IP address MUST NOT be allowed to expire before
the mobile node's visitor list entry expires, unless the foreign
agent has some way other than broadcast ARP to refresh its MAC
address associated with the mobile node's IP address.
Each foreign agent SHOULD support the mandatory features for reverse
tunneling [27].
4.2.3. Home Agent Considerations
The home agent MUST be able to intercept any datagrams on the home
network addressed to the mobile node while the mobile node is
registered away from home. Proxy and gratuitous ARP MAY be used in
enabling this interception, as specified in Section 4.6.
The home agent must examine the IP Destination Address of all
arriving datagrams to see if it is equal to the home address of any
of its mobile nodes registered away from home. If so, the home agent
tunnels the datagram to the mobile node's currently registered care-
of address or addresses. If the home agent supports the optional
capability of multiple simultaneous mobility bindings, it tunnels a
copy to each care-of address in the mobile node's mobility binding
list. If the mobile node has no current mobility bindings, the home
agent MUST NOT attempt to intercept datagrams destined for the mobile
node, and thus will not in general receive such datagrams. However,
if the home agent is also a router handling common IP traffic, it is
possible that it will receive such datagrams for forwarding onto the
home network. In this case, the home agent MUST assume the mobile
node is at home and simply forward the datagram directly onto the
home network.
For multihomed home agents, the source address in the outer IP header
of the encapsulated datagram MUST be the address sent to the mobile
node in the home agent field of the registration reply. That is, the
home agent cannot use the the address of some other network interface
as the source address.
See Section 4.1 regarding methods of encapsulation that may be used
for tunneling. Nodes implementing tunneling SHOULD also implement
the "tunnel soft state" mechanism [32], which allows ICMP error
messages returned from the tunnel to correctly be reflected back to
the original senders of the tunneled datagrams.
Home agents MUST decapsulate packets addressed to themselves, sent by
a mobile node for the purpose of maintaining location privacy, as
described in Section 5.5. This feature is also required for support
of reverse tunneling [27].
If the Lifetime for a given mobility binding expires before the home
agent has received another valid Registration Request for that mobile
node, then that binding is deleted from the mobility binding list.
The home agent MUST NOT send any Registration Reply message simply
because the mobile node's binding has expired. The entry in the
visitor list of the mobile node's current foreign agent will expire
naturally, probably at the same time as the binding expired at the
home agent. When a mobility binding's lifetime expires, the home
agent MUST delete the binding, but it MUST retain any other (non-
expired) simultaneous mobility bindings that it holds for the mobile
node.
When a home agent receives a datagram, intercepted for one of its
mobile nodes registered away from home, the home agent MUST examine
the datagram to check if it is already encapsulated. If so, special
rules apply in the forwarding of that datagram to the mobile node:
- If the inner (encapsulated) Destination Address is the same as
the outer Destination Address (the mobile node), then the home
agent MUST also examine the outer Source Address of the
encapsulated datagram (the source address of the tunnel). If
this outer Source Address is the same as the mobile node's
current care-of address, the home agent MUST silently discard
that datagram in order to prevent a likely routing loop. If,
instead, the outer Source Address is NOT the same as the mobile
node's current care-of address, then the home agent SHOULD
forward the datagram to the mobile node. In order to forward
the datagram in this case, the home agent MAY simply alter the
outer Destination Address to the care-of address, rather than
re-encapsulating the datagram.
- Otherwise (the inner Destination Address is NOT the same as the
outer Destination Address), the home agent SHOULD encapsulate
the datagram again (nested encapsulation), with the new outer
Destination Address set equal to the mobile node's care-of
address. That is, the home agent forwards the entire datagram
to the mobile node in the same way as any other datagram
(encapsulated already or not).
4.3. Broadcast Datagrams
When a home agent receives a broadcast datagram, it MUST NOT forward
the datagram to any mobile nodes in its mobility binding list other
than those that have requested forwarding of broadcast datagrams. A
mobile node MAY request forwarding of broadcast datagrams by setting
the 'B' bit in its Registration Request message (Section 3.3). For
each such registered mobile node, the home agent SHOULD forward
received broadcast datagrams to the mobile node, although it is a
matter of configuration at the home agent as to which specific
categories of broadcast datagrams will be forwarded to such mobile
nodes.
If the 'D' bit was set in the mobile node's Registration Request
message, indicating that the mobile node is using a co-located care-
of address, the home agent simply tunnels appropriate broadcast IP
datagrams to the mobile node's care-of address. Otherwise (the 'D'
bit was NOT set), the home agent first encapsulates the broadcast
datagram in a unicast datagram addressed to the mobile node's home
address, and then tunnels this encapsulated datagram to the foreign
agent. This extra level of encapsulation is required so that the
foreign agent can determine which mobile node should receive the
datagram after it is decapsulated. When received by the foreign
agent, the unicast encapsulated datagram is detunneled and delivered
to the mobile node in the same way as any other datagram. In either
case, the mobile node must decapsulate the datagram it receives in
order to recover the original broadcast datagram.
4.4. Multicast Datagram Routing
As mentioned previously, a mobile node that is connected to its home
network functions in the same way as any other (fixed) host or
router. Thus, when it is at home, a mobile node functions
identically to other multicast senders and receivers. This section
therefore describes the behavior of a mobile node that is visiting a
foreign network.
In order to receive multicasts, a mobile node MUST join the multicast
group in one of two ways. First, a mobile node MAY join the group
via a (local) multicast router on the visited subnet. This option
assumes that there is a multicast router present on the visited
subnet. If the mobile node is using a co-located care-of address, it
SHOULD use this address as the source IP address of its IGMP [11]
messages. Otherwise, it MAY use its home address.
Alternatively, a mobile node which wishes to receive multicasts MAY
join groups via a bi-directional tunnel to its home agent, assuming
that its home agent is a multicast router. The mobile node tunnels
IGMP messages to its home agent and the home agent forwards multicast
datagrams down the tunnel to the mobile node. For packets tunneled
to the home agent, the source address in the IP header SHOULD be the
mobile node's home address.
The rules for multicast datagram delivery to mobile nodes in this
case are identical to those for broadcast datagrams (Section 4.3).
Namely, if the mobile node is using a co-located care-of address (the
'D' bit was set in the mobile node's Registration Request), then the
home agent SHOULD tunnel the datagram to this care-of address;
otherwise, the home agent MUST first encapsulate the datagram in a
unicast datagram addressed to the mobile node's home address and then
MUST tunnel the resulting datagram (nested tunneling) to the mobile
node's care-of address. For this reason, the mobile node MUST be
capable of decapsulating packets sent to its home address in order to
receive multicast datagrams using this method.
A mobile node that wishes to send datagrams to a multicast group also
has two options: (1) send directly on the visited network; or (2)
send via a tunnel to its home agent. Because multicast routing in
general depends upon the IP source address, a mobile node which sends
multicast datagrams directly on the visited network MUST use a co-
located care-of address as the IP source address. Similarly, a
mobile node which tunnels a multicast datagram to its home agent MUST
use its home address as the IP source address of both the (inner)
multicast datagram and the (outer) encapsulating datagram. This
second option assumes that the home agent is a multicast router.
4.5. Mobile Routers
A mobile node can be a router that is responsible for the mobility of
one or more entire networks moving together, perhaps on an airplane,
a ship, a train, an automobile, a bicycle, or a kayak. The nodes
connected to a network served by the mobile router may themselves be
fixed nodes or mobile nodes or routers. In this document, such
networks are called "mobile networks".
A mobile router MAY act as a foreign agent and provide a foreign
agent care-of address to mobile nodes connected to the mobile
network. Typical routing to a mobile node via a mobile router in
this case is illustrated by the following example:
a) A laptop computer is disconnected from its home network and
later attached to a network port in the seat back of an
aircraft. The laptop computer uses Mobile IP to register on
this foreign network, using a foreign agent care-of address
discovered through an Agent Advertisement from the aircraft's
foreign agent.
b) The aircraft network is itself mobile. Suppose the node
serving as the foreign agent on the aircraft also serves as the
default router that connects the aircraft network to the rest
of the Internet. When the aircraft is at home, this router is
attached to some fixed network at the airline's headquarters,
which is the router's home network. While the aircraft is in
flight, this router registers from time to time over its radio
link with a series of foreign agents below it on the ground.
This router's home agent is a node on the fixed network at the
airline's headquarters.
c) Some correspondent node sends a datagram to the laptop
computer, addressing the datagram to the laptop's home address.
This datagram is initially routed to the laptop's home network.
d) The laptop's home agent intercepts the datagram on the home
network and tunnels it to the laptop's care-of address, which
in this example is an address of the node serving as router and
foreign agent on the aircraft. Normal IP routing will route
the datagram to the fixed network at the airline's
headquarters.
e) The aircraft router and foreign agent's home agent there
intercepts the datagram and tunnels it to its current care-of
address, which in this example is some foreign agent on the
ground below the aircraft. The original datagram from the
correspondent node has now been encapsulated twice: once by
the laptop's home agent and again by the aircraft's home agent.
f) The foreign agent on the ground decapsulates the datagram,
yielding a datagram still encapsulated by the laptop's home
agent, with a destination address of the laptop's care-of
address. The ground foreign agent sends the resulting datagram
over its radio link to the aircraft.
g) The foreign agent on the aircraft decapsulates the datagram,
yielding the original datagram from the correspondent node,
with a destination address of the laptop's home address. The
aircraft foreign agent delivers the datagram over the aircraft
network to the laptop's link-layer address.
This example illustrated the case in which a mobile node is attached
to a mobile network. That is, the mobile node is mobile with respect
to the network, which itself is also mobile (here with respect to the
ground). If, instead, the node is fixed with respect to the mobile
network (the mobile network is the fixed node's home network), then
either of two methods may be used to cause datagrams from
correspondent nodes to be routed to the fixed node.
A home agent MAY be configured to have a permanent registration for
the fixed node, that indicates the mobile router's address as the
fixed host's care-of address. The mobile router's home agent will
usually be used for this purpose. The home agent is then responsible
for advertising connectivity using normal routing protocols to the
fixed node. Any datagrams sent to the fixed node will thus use
nested tunneling as described above.
Alternatively, the mobile router MAY advertise connectivity to the
entire mobile network using normal IP routing protocols through a
bi-directional tunnel to its own home agent. This method avoids the
need for nested tunneling of datagrams.
4.6. ARP, Proxy ARP, and Gratuitous ARP
The use of ARP [36] requires special rules for correct operation when
wireless or mobile nodes are involved. The requirements specified in
this section apply to all home networks in which ARP is used for
address resolution.
In addition to the normal use of ARP for resolving a target node's
link-layer address from its IP address, this document distinguishes
two special uses of ARP:
- A Proxy ARP [39] is an ARP Reply sent by one node on behalf of
another node which is either unable or unwilling to answer its
own ARP Requests. The sender of a Proxy ARP reverses the
Sender and Target Protocol Address fields as described in [36],
but supplies some configured link-layer address (generally, its
own) in the Sender Hardware Address field. The node receiving
the Reply will then associate this link-layer address with the
IP address of the original target node, causing it to transmit
future datagrams for this target node to the node with that
link-layer address.
- A Gratuitous ARP [45] is an ARP packet sent by a node in order
to spontaneously cause other nodes to update an entry in their
ARP cache. A gratuitous ARP MAY use either an ARP Request or
an ARP Reply packet. In either case, the ARP Sender Protocol
Address and ARP Target Protocol Address are both set to the IP
address of the cache entry to be updated, and the ARP Sender
Hardware Address is set to the link-layer address to which this
cache entry should be updated. When using an ARP Reply packet,
the Target Hardware Address is also set to the link-layer
address to which this cache entry should be updated (this field
is not used in an ARP Request packet).
In either case, for a gratuitous ARP, the ARP packet MUST be
transmitted as a local broadcast packet on the local link. As
specified in [36], any node receiving any ARP packet (Request
or Reply) MUST update its local ARP cache with the Sender
Protocol and Hardware Addresses in the ARP packet, if the
receiving node has an entry for that IP address already in its
ARP cache. This requirement in the ARP protocol applies even
for ARP Request packets, and for ARP Reply packets that do not
match any ARP Request transmitted by the receiving node [36].
While a mobile node is registered on a foreign network, its home
agent uses proxy ARP [39] to reply to ARP Requests it receives that
seek the mobile node's link-layer address. When receiving an ARP
Request, the home agent MUST examine the target IP address of the
Request, and if this IP address matches the home address of any
mobile node for which it has a registered mobility binding, the home
agent MUST transmit an ARP Reply on behalf of the mobile node. After
exchanging the sender and target addresses in the packet [39], the
home agent MUST set the sender link-layer address in the packet to
the link-layer address of its own interface over which the Reply will
be sent.
When a mobile node leaves its home network and registers a binding on
a foreign network, its home agent uses gratuitous ARP to update the
ARP caches of nodes on the home network. This causes such nodes to
associate the link-layer address of the home agent with the mobile
node's home (IP) address. When registering a binding for a mobile
node for which the home agent previously had no binding (the mobile
node was assumed to be at home), the home agent MUST transmit a
gratuitous ARP on behalf of the mobile node. This gratuitous ARP
packet MUST be transmitted as a broadcast packet on the link on which
the mobile node's home address is located. Since broadcasts on the
local link (such as Ethernet) are typically not guaranteed to be
reliable, the gratuitous ARP packet SHOULD be retransmitted a small
number of times to increase its reliability.
When a mobile node returns to its home network, the mobile node and
its home agent use gratuitous ARP to cause all nodes on the mobile
node's home network to update their ARP caches to once again
associate the mobile node's own link-layer address with the mobile
node's home (IP) address. Before transmitting the (de)Registration
Request message to its home agent, the mobile node MUST transmit this
gratuitous ARP on its home network as a local broadcast on this link.
The gratuitous ARP packet SHOULD be retransmitted a small number of
times to increase its reliability, but these retransmissions SHOULD
proceed in parallel with the transmission and processing of its
(de)Registration Request.
When the mobile node's home agent receives and accepts this
(de)Registration Request, the home agent MUST also transmit a
gratuitous ARP on the mobile node's home network. This gratuitous
ARP also is used to associate the mobile node's home address with the
mobile node's own link-layer address. A gratuitous ARP is
transmitted by both the mobile node and its home agent, since in the
case of wireless network interfaces, the area within transmission
range of the mobile node will likely differ from that within range of
its home agent. The ARP packet from the home agent MUST be
transmitted as a local broadcast on the mobile node's home link, and
SHOULD be retransmitted a small number of times to increase its
reliability; these retransmissions, however, SHOULD proceed in
parallel with the transmission and processing of its (de)Registration
Reply.
While the mobile node is away from home, it MUST NOT transmit any
broadcast ARP Request or ARP Reply messages. Finally, while the
mobile node is away from home, it MUST NOT reply to ARP Requests in
which the target IP address is its own home address, unless the ARP
Request is unicast by a foreign agent with which the mobile node is
attempting to register or a foreign agent with which the mobile node
has an unexpired registration. In the latter case, the mobile node
MUST use a unicast ARP Reply to respond to the foreign agent. Note
that if the mobile node is using a co-located care-of address and
receives an ARP Request in which the target IP address is this care-
of address, then the mobile node SHOULD reply to this ARP Request.
Note also that, when transmitting a Registration Request on a foreign
network, a mobile node may discover the link-layer address of a
foreign agent by storing the address as it is received from the Agent
Advertisement from that foreign agent, but not by transmitting a
broadcast ARP Request message.
The specific order in which each of the above requirements for the
use of ARP, proxy ARP, and gratuitous ARP are applied, relative to
the transmission and processing of the mobile node's Registration
Request and Registration Reply messages when leaving home or
returning home, are important to the correct operation of the
protocol.
To summarize the above requirements, when a mobile node leaves its
home network, the following steps, in this order, MUST be performed:
- The mobile node decides to register away from home, perhaps
because it has received an Agent Advertisement from a foreign
agent and has not recently received one from its home agent.
- Before transmitting the Registration Request, the mobile node
disables its own future processing of any ARP Requests it may
subsequently receive requesting the link-layer address
corresponding to its home address, except insofar as necessary
to communicate with foreign agents on visited networks.
- The mobile node transmits its Registration Request.
- When the mobile node's home agent receives and accepts the
Registration Request, it performs a gratuitous ARP on behalf of
the mobile node, and begins using proxy ARP to reply to ARP
Requests that it receives requesting the mobile node's link-
layer address. In the gratuitous ARP, the ARP Sender Hardware
Address is set to the link-layer address of the home agent.
If, instead, the home agent rejects the Registration Request,
no ARP processing (gratuitous nor proxy) is performed by the
home agent.
When a mobile node later returns to its home network, the following
steps, in this order, MUST be performed:
- The mobile node decides to register at home, perhaps because it
has received an Agent Advertisement from its home agent.
- Before transmitting the Registration Request, the mobile node
re-enables its own future processing of any ARP Requests it may
subsequently receive requesting its link-layer address.
- The mobile node performs a gratuitous ARP for itself. In this
gratuitous ARP, the ARP Sender Hardware Address is set to the
link-layer address of the mobile node.
- The mobile node transmits its Registration Request.
- When the mobile node's home agent receives and accepts the
Registration Request, it stops using proxy ARP to reply to ARP
Requests that it receives requesting the mobile node's link-
layer address, and then performs a gratuitous ARP on behalf of
the mobile node. In this gratuitous ARP, the ARP Sender
Hardware Address is set to the link-layer address of the mobile
node. If, instead, the home agent rejects the Registration
Request, the home agent MUST NOT make any change to the way it
performs ARP processing (gratuitous nor proxy) for the mobile
node. In this latter case, the home agent should operate as if
the mobile node has not returned home, and continue to perform
proxy ARP on behalf of the mobile node.
5. Security Considerations
The mobile computing environment is potentially very different from
the ordinary computing environment. In many cases, mobile computers
will be connected to the network via wireless links. Such links are
particularly vulnerable to passive eavesdropping, active replay
attacks, and other active attacks.
5.1. Message Authentication Codes
Home agents and mobile nodes MUST be able to perform authentication.
The default algorithm is HMAC-MD5 [23], with a key size of 128 bits.
The foreign agent MUST also support authentication using HMAC-MD5 and
key sizes of 128 bits or greater, with manual key distribution. Keys
with arbitrary binary values MUST be supported.
The "prefix+suffix" use of MD5 to protect data and a shared secret is
considered vulnerable to attack by the cryptographic community.
Where backward compatibility with existing Mobile IP implementations
that use this mode is needed, new implementations SHOULD include
keyed MD5 [41] as one of the additional authentication algorithms for
use when producing and verifying the authentication data that is
supplied with Mobile IP registration messages, for instance in the
extensions specified in sections 3.5.2, 3.5.3, and 3.5.4.
More authentication algorithms, algorithm modes, key distribution
methods, and key sizes MAY also be supported for all of these
extensions.
5.2. Areas of Security Concern in this Protocol
The registration protocol described in this document will result in a
mobile node's traffic being tunneled to its care-of address. This
tunneling feature could be a significant vulnerability if the
registration were not authenticated. Such remote redirection, for
instance as performed by the mobile registration protocol, is widely
understood to be a security problem in the current Internet if not
authenticated [2]. Moreover, the Address Resolution Protocol (ARP)
is not authenticated, and can potentially be used to steal another
host's traffic. The use of "Gratuitous ARP" (Section 4.6) brings
with it all of the risks associated with the use of ARP.
5.3. Key Management
This specification requires a strong authentication mechanism (keyed
MD5) which precludes many potential attacks based on the Mobile IP
registration protocol. However, because key distribution is
difficult in the absence of a network key management protocol,
messages with the foreign agent are not all required to be
authenticated. In a commercial environment it might be important to
authenticate all messages between the foreign agent and the home
agent, so that billing is possible, and service providers do not
provide service to users that are not legitimate customers of that
service provider.
5.4. Picking Good Random Numbers
The strength of any authentication mechanism depends on several
factors, including the innate strength of the authentication
algorithm, the secrecy of the key used, the strength of the key used,
and the quality of the particular implementation. This specification
requires implementation of keyed MD5 for authentication, but does not
preclude the use of other authentication algorithms and modes. For
keyed MD5 authentication to be useful, the 128-bit key must be both
secret (that is, known only to authorized parties) and pseudo-random.
If nonces are used in connection with replay protection, they must
also be selected carefully. Eastlake, et al. [14] provides more
information on generating pseudo-random numbers.
5.5. Privacy
Users who have sensitive data that they do not wish others to see
should use mechanisms outside the scope of this document (such as
encryption) to provide appropriate protection. Users concerned about
traffic analysis should consider appropriate use of link encryption.
If absolute location privacy is desired, the mobile node can create a
tunnel to its home agent. Then, datagrams destined for correspondent
nodes will appear to emanate from the home network, and it may be
more difficult to pinpoint the location of the mobile node. Such
mechanisms are all beyond the scope of this document.
5.6. Ingress Filtering
Many routers implement security policies such as "ingress filtering"
[15] that do not allow forwarding of packets that have a Source
Address which appears topologically incorrect. In environments where
this is a problem, mobile nodes may use reverse tunneling [27] with
the foreign agent supplied care-of address as the Source Address.
Reverse tunneled packets will be able to pass normally through such
routers, while ingress filtering rules will still be able to locate
the true topological source of the packet in the same way as packets
from non-mobile nodes.
5.7. Replay Protection for Registration Requests
The Identification field is used to let the home agent verify that a
registration message has been freshly generated by the mobile node,
not replayed by an attacker from some previous registration. Two
methods are described in this section: timestamps (mandatory) and
"nonces" (optional). All mobile nodes and home agents MUST implement
timestamp-based replay protection. These nodes MAY also implement
nonce-based replay protection (but see Appendix A).
The style of replay protection in effect between a mobile node and
its home agent is part of the mobile security association. A mobile
node and its home agent MUST agree on which method of replay
protection will be used. The interpretation of the Identification
field depends on the method of replay protection as described in the
subsequent subsections.
Whatever method is used, the low-order 32 bits of the Identification
MUST be copied unchanged from the Registration Request to the Reply.
The foreign agent uses those bits (and the mobile node's home
address) to match Registration Requests with corresponding replies.
The mobile node MUST verify that the low-order 32 bits of any
Registration Reply are identical to the bits it sent in the
Registration Request.
The Identification in a new Registration Request MUST NOT be the same
as in an immediately preceding Request, and SHOULD NOT repeat while
the same security context is being used between the mobile node and
the home agent. Retransmission as in Section 3.6.3 is allowed.
5.7.1. Replay Protection using Timestamps
The basic principle of timestamp replay protection is that the node
generating a message inserts the current time of day, and the node
receiving the message checks that this timestamp is sufficiently
close to its own time of day. Unless specified differently in the
security association between the nodes, a default value of 7 seconds
MAY be used to limit the time difference. This value SHOULD be
greater than 3 seconds. Obviously the two nodes must have adequately
synchronized time-of-day clocks. As with any messages, time
synchronization messages may be protected against tampering by an
authentication mechanism determined by the security context between
the two nodes.
If timestamps are used, the mobile node MUST set the Identification
field to a 64-bit value formatted as specified by the Network Time
Protocol [26]. The low-order 32 bits of the NTP format represent
fractional seconds, and those bits which are not available from a
time source SHOULD be generated from a good source of randomness.
Note, however, that when using timestamps, the 64-bit Identification
used in a Registration Request from the mobile node MUST be greater
than that used in any previous Registration Request, as the home
agent uses this field also as a sequence number. Without such a
sequence number, it would be possible for a delayed duplicate of an
earlier Registration Request to arrive at the home agent (within the
clock synchronization required by the home agent), and thus be
applied out of order, mistakenly altering the mobile node's current
registered care-of address.
Upon receipt of a Registration Request with an authorization-enabling
extension, the home agent MUST check the Identification field for
validity. In order to be valid, the timestamp contained in the
Identification field MUST be close enough to the home agent's time of
day clock and the timestamp MUST be greater than all previously
accepted timestamps for the requesting mobile node. Time tolerances
and resynchronization details are specific to a particular mobility
security association.
If the timestamp is valid, the home agent copies the entire
Identification field into the Registration Reply it returns the Reply
to the mobile node. If the timestamp is not valid, the home agent
copies only the low-order 32 bits into the Registration Reply, and
supplies the high-order 32 bits from its own time of day. In this
latter case, the home agent MUST reject the registration by returning
Code 133 (identification mismatch) in the Registration Reply.
As described in Section 3.6.2.1, the mobile node MUST verify that the
low-order 32 bits of the Identification in the Registration Reply are
identical to those in the rejected registration attempt, before using
the high-order bits for clock resynchronization.
5.7.2. Replay Protection using Nonces
The basic principle of nonce replay protection is that node A
includes a new random number in every message to node B, and checks
that node B returns that same number in its next message to node A.
Both messages use an authentication code to protect against
alteration by an attacker. At the same time node B can send its own
nonces in all messages to node A (to be echoed by node A), so that it
too can verify that it is receiving fresh messages.
The home agent may be expected to have resources for computing
pseudo-random numbers useful as nonces [14]. It inserts a new nonce
as the high-order 32 bits of the identification field of every
Registration Reply. The home agent copies the low-order 32 bits of
the Identification from the Registration Request message into the
low-order 32 bits of the Identification in the Registration Reply.
When the mobile node receives an authenticated Registration Reply
from the home agent, it saves the high-order 32 bits of the
identification for use as the high-order 32 bits of its next
Registration Request.
The mobile node is responsible for generating the low-order 32 bits
of the Identification in each Registration Request. Ideally it
should generate its own random nonces. However it may use any
expedient method, including duplication of the random value sent by
the home agent. The method chosen is of concern only to the mobile
node, because it is the node that checks for valid values in the
Registration Reply. The high-order and low-order 32 bits of the
identification chosen SHOULD both differ from their previous values.
The home agent uses a new high-order value and the mobile node uses a
new low-order value for each registration message. The foreign agent
uses the low-order value (and the mobile host's home address) to
correctly match registration replies with pending Requests (Section
3.7.1).
If a registration message is rejected because of an invalid nonce,
the Reply always provides the mobile node with a new nonce to be used
in the next registration. Thus the nonce protocol is self-
synchronizing.
6. IANA Considerations
Mobile IP specifies several new number spaces for values to be used
in various message fields. These number spaces include the
following:
- Mobile IP message types sent to UDP port 434, as defined in
section 1.8.
- types of extensions to Registration Request and Registration
Reply messages (see sections 3.3 and 3.4, and also consult [27,
29, 6, 7, 12])
- values for the Code in the Registration Reply message (see
section 3.4, and also consult [27, 29, 6, 7, 12])
- Mobile IP defines so-called Agent Solicitation and Agent
Advertisement messages. These messages are in fact Router
Discovery messages [10] augmented with mobile-IP specific
extensions. Thus, they do not define a new name space, but do
define additional Router Discovery extensions as described
below in Section 6.2. Also see Section 2.1 and consult [7,
12].
There are additional Mobile IP numbering spaces specified in [7].
Information about assignment of mobile-ip numbers derived from
specifications external to this document is given by IANA at
http://www.iana.org/numbers.Html. From that URL, follow the
hyperlinks to [M] within the "Directory of General Assigned Numbers",
and subsequently to the specific section for "Mobile IP Numbers".
6.1. Mobile IP Message Types
Mobile IP messages are defined to be those that are sent to a message
recipient at port 434 (UDP or TCP). The number space for Mobile IP
messages is specified in Section 1.8. Approval of new extension
numbers is subject to Expert Review, and a specification is required
[30]. The currently standardized message types have the following
numbers, and are specified in the indicated sections.
Type Name Section
---- -------------------------------------------- ---------
1 Registration Request 3.3
3 Registration Reply 3.4
6.2. Extensions to RFC1256 Router Advertisement
RFC1256 defines two ICMP message types, Router Advertisement and
Router Solicitation. Mobile IP defines a number space for extensions
to Router Advertisement, which could be used by protocols other than
Mobile IP. The extension types currently standardized for use with
Mobile IP have the following numbers.
Type Name Reference
---- -------------------------------------------- ---------
0 One-byte Padding 2.1.3
16 Mobility Agent Advertisement 2.1.1
19 Prefix-Lengths 2.1.2
Approval of new extension numbers for use with Mobile IP is subject
to Expert Review, and a specification is required [30].
6.3. Extensions to Mobile IP Registration Messages
The Mobile IP messages, specified within this document, and listed in
sections 1.8 and 6.1, may have extensions. Mobile IP message
extensions all share the same number space, even if they are to be
applied to different Mobile IP messages. The number space for Mobile
IP message extensions is specified within this document. Approval of
new extension numbers is subject to Expert Review, and a
specification is required [30].
Type Name Reference
---- -------------------------------------------- ---------
0 One-byte Padding
32 Mobile-Home Authentication 3.5.2
33 Mobile-Foreign Authentication 3.5.3
34 Foreign-Home Authentication 3.5.4
6.4. Code Values for Mobile IP Registration Reply Messages
The Mobile IP Registration Reply message, specified in section 3.4,
has a Code field. The number space for the Code field values is also
specified in Section 3.4. The Code number space is structured
according to whether the registration was successful, or whether the
foreign agent denied the registration request, or lastly whether the
home agent denied the registration request, as follows:
0-8 Success Codes
9-63 No allocation guidelines currently exist
64-127 Error Codes from the Foreign Agent
128-192 Error Codes from the Home Agent
193-255 No allocation guidelines currently exist
Approval of new Code values requires Expert Review [30].
7. Acknowledgments
Special thanks to Steve Deering (Xerox PARC), along with Dan Duchamp
and John Ioannidis (JI) (Columbia University), for forming the
working group, chairing it, and putting so much effort into its early
development. Columbia's early Mobile IP work can be found in [18,
19, 17].
Thanks also to Kannan Alaggapan, Greg Minshall, Tony Li, Jim Solomon,
Erik Nordmark, Basavaraj Patil, and Phil Roberts for their
contributions to the group while performing the duties of
chairperson, as well as for their many useful comments.
Thanks to the active members of the Mobile IP Working Group,
particularly those who contributed text, including (in alphabetical
order)
- Ran Atkinson (Naval Research Lab),
- Samita Chakrabarti (Sun Microsystems)
- Ken Imboden (Candlestick Networks, Inc.)
- Dave Johnson (Carnegie Mellon University),
- Frank Kastenholz (FTP Software),
- Anders Klemets (KTH),
- Chip Maguire (KTH),
- Alison Mankin (ISI)
- Andrew Myles (Macquarie University),
- Thomas Narten (IBM)
- Al Quirt (Bell Northern Research),
- Yakov Rekhter (IBM), and
- Fumio Teraoka (Sony).
- Alper Yegin (NTT DoCoMo)
Thanks to Charlie Kunzinger and to Bill Simpson, the editors who
produced the first drafts for of this document, reflecting the
discussions of the Working Group. Much of the new text in the later
revisions preceding RFC2002 is due to Jim Solomon and Dave Johnson.
Thanks to Greg Minshall (Novell), Phil Karn (Qualcomm), Frank
Kastenholz (FTP Software), and Pat Calhoun (Sun Microsystems) for
their geNerous support in hosting interim Working Group meetings.
Sections 1.10 and 1.11, which specify new extension formats to be
used with aggregatable extension types, were included from a
specification document (entitled "Mobile IP Extensions
Rationalization (MIER)", which was written by
- Mohamed M.Khalil, Nortel Networks
- Raja Narayanan, nVisible Networks
- Haseeb Akhtar, Nortel Networks
- Emad Qaddoura, Nortel Networks
Thanks to these authors, and also for the additional work on
MIER, which was contributed by Basavaraj Patil, Pat Calhoun, Neil
Justusson, N. Asokan, and Jouni Malinen.
A. Patent Issues
The IETF has been notified of intellectual property rights claimed
in regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on
the IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances
of licenses to be made available, or the result of an attempt
made to obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
B. Link-Layer Considerations
The mobile node MAY use link-layer mechanisms to decide that its
point of attachment has changed. Such indications include the
Down/Testing/Up interface status [24], and changes in cell or
administration. The mechanisms will be specific to the particular
link-layer technology, and are outside the scope of this document.
The Point-to-Point-Protocol (PPP) [42] and its Internet Protocol
Control Protocol (IPCP) [25], negotiates the use of IP addresses.
The mobile node SHOULD first attempt to specify its home address,
so that if the mobile node is attaching to its home network, the
unrouted link will function correctly. When the home address is
not accepted by the peer, but a transient IP address is dynamically
assigned to the mobile node, and the mobile node is capable of
supporting a co-located care-of address, the mobile node MAY
register that address as a co-located care-of address. When the peer
specifies its own IP address, that address MUST NOT be assumed to be
a foreign agent care-of address or the IP address of a home agent.
PPP extensions for Mobile IP have been specified in RFC2290 [44].
Please consult that document for additional details for how to handle
care-of address assignment from PPP in a more efficient manner.
C. TCP Considerations
C.1. TCP Timers
When high-delay (e.g. SATCOM) or low-bandwidth (e.g. High-Frequency
Radio) links are in use, some TCP stacks may have insufficiently
adaptive (non-standard) retransmission timeouts. There may be
spurious retransmission timeouts, even when the link and network
are actually operating properly, but just with a high delay because
of the medium in use. This can cause an inability to create or
maintain TCP connections over such links, and can also cause unneeded
retransmissions which consume already scarce bandwidth. Vendors
are encouraged to follow the algorithms in RFC2988 [31] when
implementing TCP retransmission timers. Vendors of systems designed
for low-bandwidth, high-delay links should consult RFCs 2757 and
2488 [28, 1]. Designers of applications targeted to operate on
mobile nodes should be sensitive to the possibility of timer-related
difficulties.
C.2. TCP Congestion Management
Mobile nodes often use media which are more likely to introduce
errors, effectively causing more packets to be dropped. This
introduces a conflict with the mechanisms for congestion management
found in modern versions of TCP [21]. Now, when a packet is dropped,
the correspondent node's TCP implementation is likely to react as
if there were a source of network congestion, and initiate the
slow-start mechanisms [21] designed for controlling that problem.
However, those mechanisms are inappropriate for overcoming errors
introduced by the links themselves, and have the effect of magnifying
the discontinuity introduced by the dropped packet. This problem has
been analyzed by Caceres, et al. [5]. TCP approaches to the problem
of handling errors that might interfere with congestion management
are discussed in documents from the [pilc] working group [3, 9].
While such approaches are beyond the scope of this document,
they illustrate that providing performance transparency to mobile
nodes involves understanding mechanisms outside the network layer.
Problems introduced by higher media error rates also indicate the
need to avoid designs which systematically drop packets; such designs
might otherwise be considered favorably when making engineering
tradeoffs.
D. Example Scenarios
This section shows example Registration Requests for several common
scenarios.
D.1. Registering with a Foreign Agent Care-of Address
The mobile node receives an Agent Advertisement from a foreign
agent and wishes to register with that agent using the advertised
foreign agent care-of address. The mobile node wishes only
IP-in-IP encapsulation, does not want broadcasts, and does not want
simultaneous mobility bindings:
IP fields:
Source Address = mobile node's home address
Destination Address = copied from the IP source address of the
Agent Advertisement
Time to Live = 1
UDP fields:
Source Port = <any>
Destination Port = 434
Registration Request fields:
Type = 1
S=0,B=0,D=0,M=0,G=0
Lifetime = the Registration Lifetime copied from the
Mobility Agent Advertisement Extension of the
Router Advertisement message
Home Address = the mobile node's home address
Home Agent = IP address of mobile node's home agent
Care-of Address = the Care-of Address copied from the
Mobility Agent Advertisement Extension of the
Router Advertisement message
Identification = Network Time Protocol timestamp or Nonce
Extensions:
An authorization-enabling extension (e.g., the
Mobile-Home Authentication Extension)
D.2. Registering with a Co-Located Care-of Address
The mobile node enters a foreign network that contains no foreign
agents. The mobile node obtains an address from a DHCP server [13]
for use as a co-located care-of address. The mobile node supports
all forms of encapsulation (IP-in-IP, minimal encapsulation, and
GRE), desires a copy of broadcast datagrams on the home network, and
does not want simultaneous mobility bindings:
IP fields:
Source Address = care-of address obtained from DHCP server
Destination Address = IP address of home agent
Time to Live = 64
UDP fields:
Source Port = <any>
Destination Port = 434
Registration Request fields:
Type = 1
S=0,B=1,D=1,M=1,G=1
Lifetime = 1800 (seconds)
Home Address = the mobile node's home address
Home Agent = IP address of mobile node's home agent
Care-of Address = care-of address obtained from DHCP server
Identification = Network Time Protocol timestamp or Nonce
Extensions:
The Mobile-Home Authentication Extension
D.3. Deregistration
The mobile node returns home and wishes to deregister all care-of
addresses with its home agent.
IP fields:
Source Address = mobile node's home address
Destination Address = IP address of home agent
Time to Live = 1
UDP fields:
Source Port = <any>
Destination Port = 434
Registration Request fields:
Type = 1
S=0,B=0,D=0,M=0,G=0
Lifetime = 0
Home Address = the mobile node's home address
Home Agent = IP address of mobile node's home agent
Care-of Address = the mobile node's home address
Identification = Network Time Protocol timestamp or Nonce
Extensions:
An authorization-enabling extension (e.g., the
Mobile-Home Authentication Extension)
E. Applicability of Prefix-Lengths Extension
Caution is indicated with the use of the Prefix-Lengths Extension
over wireless links, due to the irregular coverage areas provided by
wireless transmitters. As a result, it is possible that two foreign
agents advertising the same prefix might indeed provide different
connectivity to prospective mobile nodes. The Prefix-Lengths
Extension SHOULD NOT be included in the advertisements sent by agents
in such a configuration.
Foreign agents using different wireless interfaces would have to
cooperate using special protocols to provide identical coverage in
space, and thus be able to claim to have wireless interfaces situated
on the same subnetwork. In the case of wired interfaces, a mobile
node disconnecting and subsequently connecting to a new point of
attachment, may well send in a new Registration Request no matter
whether the new advertisement is on the same medium as the last
recorded advertisement. And, finally, in areas with dense
populations of foreign agents it would seem unwise to require the
propagation via routing protocols of the subnet prefixes associated
with each individual wireless foreign agent; such a strategy could
lead to quick depletion of available space for routing tables,
unwarranted increases in the time required for processing routing
updates, and longer decision times for route selection if routes
(which are almost always unnecessary) are stored for wireless
"subnets".
F. Interoperability Considerations
This document specifies revisions to RFC2002 that are intended to
improve interoperability by resolving ambiguities contained in the
earlier text. Implementations that perform authentication according
to the new more precisely specified algorithm would be interoperable
with earlier implementations that did what was originally expected
for producing authentication data. That was a major source of non-
interoperability before.
However, this specification does have new features that, if used,
would cause interoperability problems with older implementations.
All features specified in RFC2002 will work with the new
implementations, except for V-J compression [20]. The following list
details some of the possible areas of compatibility problems that may
be experienced by nodes conforming to this revised specification,
when attempting to interoperate with nodes obeying RFC2002.
- A client that expects some of the newly mandatory features
(like reverse tunneling) from a foreign agent would still be
interoperable as long as it pays attention to the `T' bit.
- Mobile nodes that use the NAI extension to identify themselves
would not work with old mobility agents.
- Mobile nodes that use a zero home address and expect to receive
their home address in the Registration Reply would not work
with old mobility agents.
- Mobile nodes that attempt to authenticate themselves without
using the Mobile-Home authentication extension will be unable
to successful register with their home agent.
In all of these cases, a robust and well-configured mobile node is
very likely to be able to recover if it takes reasonable actions upon
receipt of a Registration Reply with an error code indicating the
cause for rejection. For instance, if a mobile node sends a
registration request that is rejected because it contains the wrong
kind of authentication extension, then the mobile node could retry
the registration with a mobile-home authentication extension, since
the foreign agent and/or home agent in this case will not be
configured to demand the alternative authentication data.
G. Changes since RFC2002
This section details differences between the original Mobile IP base
specification (RFC2002 and ff.) that have been made as part of this
revised protocol specification for Mobile IP.
G.1. Major Changes
- Specification for Destination IP address of Registration Reply
transmitted from Foreign Agent, to avoid any possible
transmission to IP address 0.0.0.0.
- Specification of two new formats for Mobile IP extensions,
according to the ideas contained in MIER.
- Specification that the SPI of the MN-HA authentication
extension is to be used as part of the data over which the
authentication algorithm must be computed.
- Eliminated Van-Jacobson Compression feature
- Specification that foreign agents MAY send advertisements at a
rate faster than once per second, but chosen so that the
advertisements do not burden the capacity of the local link.
For simplicity, the foreign agent now MAY send advertisements
at an interval less than 1/3 the advertised ICMP Lifetime.
- Specification that foreign agents SHOULD support reverse
tunneling, and home agents MUST support decapsulation of
reverse tunnels.
- Changed the preconfiguration requirements in section 3.6 for
the mobile node to reflect the capability, specified in RFC
2794 [6], for the mobile node to identify itself by using its
NAI, and then getting a home address from the Registration
Reply.
- Changed section 3.7.3.1 so that a foreign agent is not required
to discard Registration Replies that have a Home Address field
that does not match any pending Registration Request.
- Allowed registrations to be authenticated by use of a security
association between the mobile node and a suitable
authentication entity acceptable to the home agent. Defined
"Authorization-enabling Extension" to be an authentication
extension that makes a registration message acceptable to the
recipient. This is needed according to specification in [6].
- Mandated that HMAC-MD5 be used instead of the "prefix+suffix"
mode of MD5 as originally mandated in RFC2002.
- Specified that the mobile node SHOULD take the first care-of
address in a list offered by a foreign agent, and MAY try each
subsequent advertised address in turn if the attempted
registrations are rejected by the foreign agent
- Clarification that a mobility agent SHOULD only put its own
addresses into the initial (i.e., not mobility-related) list of
routers in the mobility advertisement. RFC2002 suggests that
a mobility agent might advertise other default routers.
- Specification that a mobile node MUST ignore reserved bits in
Agent Advertisements, as opposed to discarding such
advertisements. In this way, new bits can be defined later,
without affecting the ability for mobile nodes to use the
advertisements even when the newly defined bits are not
understood. Furthermore, foreign agents can set the `R' bit to
make sure that new bits are handled by themselves instead of
some legacy mobility agent.
- Specification that the foreign agent checks to make sure that
the indicated home agent address does not belong to any of its
network interfaces before relaying a Registration Request. If
the check fails, and the foreign agent is not the mobile node's
home agent, then the foreign agent rejects the request with
code 136 (unknown home agent address).
- Specification that, while they are away from the home network,
mobile nodes MUST NOT broadcast ARP packets to find the MAC
address of another Internet node. Thus, the (possibly empty)
list of Router Addresses from the ICMP Router Advertisement
portion of the message is not useful for selecting a default
router, unless the mobile node has some means not involving
broadcast ARP and not specified within this document for
obtaining the MAC address of one of the routers in the list.
Similarly, in the absence of unspecified mechanisms for
obtaining MAC addresses on foreign networks, the mobile node
MUST ignore redirects to other routers on foreign networks.
- Specification that a foreign agent MUST NOT use broadcast ARP
for a mobile node's MAC address on a foreign network. It may
obtain the MAC address by copying the information from an Agent
Solicitation or a Registration Request transmitted from a
mobile node.
- Specification that a foreign agent's ARP cache for the mobile
node's IP address MUST NOT be allowed to expire before the
mobile node's visitor list entry expires, unless the foreign
agent has some way other than broadcast ARP to refresh its MAC
address associated to the mobile node's IP address.
- At the end of section 4.6, clarified that a home agent MUST NOT
make any changes to the way it performs proxy ARP after it
rejects an invalid deregistration request.
- In section 4.2.3, specification that multihomed home agents
MUST use the the address sent to the mobile node in the home
agent field of the registration reply as the source address in
the outer IP header of the encapsulated datagram.
- Inserted 'T' bit into its proper place in the Registration
Request message format (section 3.3).
G.2. Minor Changes
- Allowed registration replies to be processed by the mobile
node, even in the absence of any Mobile-Home Authentication
extension, when containing rejection code by the foreign agent.
- Specification that the foreign agent MAY configure a maximum
number of pending registrations that it is willing to maintain
(typically 5). Additional registrations SHOULD then be
rejected by the foreign agent with code 66. The foreign agent
MAY delete any pending Registration Request after the request
has been pending for more than 7 seconds; in this case, the
foreign agent SHOULD reject the Request with code 78
(registration timeout).
- Relaxation of the requirement that, when a mobile node has
joined a multicast group at the router on the foreign network,
the mobile node MUST use its home address as the source IP
address for multicast packets,
- Clarification that a mobility agent MAY use different settings
for each of the 'R', 'H', and 'F' bits on different network
interfaces.
- Replacement of the terminology "recursive tunneling" by the
terminology "nested tunneling".
- Specification that the mobile node MAY use the IP source
address of an agent advertisement as its default router
address.
- Clarification that keys with arbitrary binary values MUST be
supported as part of mobility security associations.
- Specification that the default value may be chosen as 7
seconds, for allowable time skews between a home agent and
mobile node using timestamps for replay protection. Further
specification that this value SHOULD be greater than 3 seconds.
- Specification that Registration Requests with the 'D' bit set
to 0, and specifying a care-of address not offered by the
foreign agent, MUST be rejected with code 77 (invalid care-of
address).
- Clarification that the foreign agent SHOULD consider its own
maximum value when handling the Lifetime field of the
Registration Reply.
- Clarification that the home agent MUST ignore the 'B' bit (as
opposed to rejecting the Registration Request) if it does not
support broadcasts.
- Advice about the impossibility of using dynamic home agent
discovery in the case when routers change the IP destination
address of a datagram from a subnet-directed broadcast address
to 255.255.255.255 before injecting it into the destination
subnet.
- Clarified that when an Agent Advertisement is unicast to a
mobile node, the specific IP home address of a mobile node MAY
be used as the destination IP address.
- Included a reference to RFC2290 within appendix B, which deals
with PPP operation.
- Created IANA Considerations section
- In section 3.8.3, clarified that a home agent SHOULD arrange
the selection of a home address for a mobile node when the
Registration Reply contains a zero Home Address.
G.3. Changes since revision 04 of RFC2002bis
This section lists the changes between this version (...-06.txt) and
the previous version (...-04.txt) of the document. This section can
be deleted by the RFCeditor.
- Noted that HMAC-MD5 should be considered for use in place of
the "prefix+suffix" mode of MD5 as originally mandated in RFC
2002.
- Included a reference to RFC2290 within appendix B, which deals
with PPP operation.
- Revamped IANA Considerations section
- Revamped Changes section
- Replaced Patents section with wording mandated from RFC2026.
- Updated citations.
H. Example Messages
H.1. Example ICMP Agent Advertisement Message Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type Code Checksum
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Num Addrs Addr Entry Size Lifetime
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Router Address[1]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Preference Level[1]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Router Address[2]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Preference Level[2]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type = 16 Length Sequence Number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Registration Lifetime RBHFMGrT reserved
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Care-of Address[1]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Care-of Address[2]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Optional Extensions :
: .... ...... ...... :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
H.2. Example Registration Request Message Format
The UDP header is followed by the Mobile IP fields shown below:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type = 1 SBDMGrTx Lifetime
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Home Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Home Agent
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Care-of Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ Identification +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Optional Non-Auth Extensions for HA ...
( variable length )
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type =32 Length SPI
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SPI (cont..)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: MN-HA Authenticator ( variable length ) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Optional Non-Auth Extensions for FA .........
: Optional MN-FA Authentication Extension...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
H.3. Example Registration Reply Message Format
The UDP header is followed by the Mobile IP fields shown below:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type = 3 Code Lifetime
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Home Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Home Agent
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ Identification +
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Optional HA Non-Auth Extensions ...
( variable length )
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type =32 Length SPI
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SPI (cont...)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: MN-HA Authenticator ( variable length ) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: Optional Extensions used by FA.........
: Optional MN-FA Authentication Extension...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
References
[1] Allman, M., Glover, D. and L. Sanchez, "Enhancing TCP Over
Satellite Channels using Standard Mechanisms", BCP 28, RFC
2488, January 1999.
[2] S. M. Bellovin. Security Problems in the TCP/IP Protocol
Suite. ACM Computer Communications Review, 19(2), March 1989.
[3] Border, J., Kojo, M., Griner, J., Montenegro, G. and Z. Shelby,
"Performance Enhancing Proxies", RFC3135, June 2001.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
[5] Ramon Caceres and Liviu Iftode. Improving the Performance of
Reliable Transport Protocols in Mobile Computing Environments.
IEEE Journal on Selected Areas in Communications, 13(5):850--
857, June 1995.
[6] Calhoun P. and C. Perkins, "Mobile IP Network Access Identifier
Extension for IPv4", RFC2794, January 2000.
[7] Calhoun, P. and C. Perkins, "Mobile IP Foreign Agent
Challenge/Response Extension", RFC3012, December 2000.
[8] Cong, D., Hamlen, M. and C. Perkins, "The Definitions of
Managed Objects for IP Mobility Support using SMIv2", RFC2006,
October 1996.
[9] Dawkins, S., Montenegro, G., Kojo, M., Magret, V. and N.
Vaidya, "End-to-end Performance Implications of Links with
Errors", BCP 50, RFC3155, August 2001.
[10] Deering, S., "ICMP Router Discovery Messages", RFC1256,
September 1991.
[11] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC
1112, August 1989.
[12] Dommety, G. and K. Leung, "Mobile IP Vendor/Organization-
Specific Extensions", RFC3115, April 2001.
[13] Droms, R., "Dynamic Host Configuration Protocol", RFC2131,
March 1997.
[14] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC1750, December 1994.
[15] Ferguson P. and D. Senie, "Network Ingress Filtering: Defeating
Denial of Service Attacks which employ IP Source Address
Spoofing", BCP 38, RFC2827, May 2000.
[16] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic
Routing Encapsulation (GRE)", RFC1701, October 1994.
[17] J. Ioannidis. Protocols for Mobile Internetworking. PhD
Dissertation - Columbia University in the City of New York,
July 1993.
[18] John Ioannidis, Dan Duchamp, and Gerald Q. Maguire Jr. IP-
Based Protocols for Mobile Internetworking. In Proceedings of
the SIGCOMM '91 Conference: Communications Architectures &
Protocols, pages 235--245, September 1991.
[19] John Ioannidis and Gerald Q. Maguire Jr. The Design and
Implementation of a Mobile Internetworking Architecture. In
Proceedings of the Winter USENIX Technical Conference, pages
489--500, January 1993.
[20] Jacobson, V., "Compressing TCP/IP headers for low-speed serial
links", RFC1144, February 1990.
[21] Jacobson, V., "Congestion Avoidance and Control. In
Proceedings, SIGCOMM '88 Workshop, pages 314--329. ACM Press,
August 1988. Stanford, CA.
[22] Kent, S. and R. Atkinson, "IP Authentication Header", RFC2402,
November 1998.
[23] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
for Message Authentication", RFC2104, February 1997.
[24] McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB",
RFC2863, June 2000.
[25] McGregor, G., "The PPP Internet Protocol Control Protocol
(IPCP)", RFC1332, May 1992.
[26] Mills, D., "Network Time Protocol (Version 3) Specification,
Implementation", RFC1305, March 1992.
[27] Montenegro, G., "Reverse Tunneling for Mobile IP (revised)",
RFC3024, January 2001.
[28] Montenegro, G., Dawkins, S., Kojo, M., Magret, V. and N.
Vaidya, "Long Thin Networks", RFC2757, January 2000.
[29] Montenegro, G. and V. Gupta, "Sun's SKIP Firewall Traversal for
Mobile IP", RFC2356, June 1998.
[30] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC2434, October 1998.
[31] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC2988, November 2000.
[32] Perkins, C., "IP Encapsulation within IP", RFC2003, October
1996.
[33] Perkins, C., "IP Mobility Support", RFC2002, October 1996.
[34] Perkins, C., "Minimal Encapsulation within IP", RFC2004,
October 1996.
[35] Perkins, C. and P. Calhoun, "AAA Registration Keys for Mobile
IP", Work in Progress, July 2001.
[36] Plummer, D., "Ethernet Address Resolution Protocol: Or
converting network protocol addresses to 48.bit Ethernet
address for transmission on Ethernet hardware", STD 37, RFC
826, November 1982.
[37] Postel, J., "User Datagram Protocol", STD 6, RFC768, August
1980.
[38] Postel, J., "Internet Protocol", STD 5, RFC791, September
1981.
[39] Postel, J., "Multi-LAN Address Resolution", RFC925, October
1984.
[40] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
1700, October 1994.
[41] Rivest, R., "The MD5 Message-Digest Algorithm", RFC1321, April
1992.
[42] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC
1661, July 1994.
[43] Solomon, J., "Applicability Statement for IP Mobility Support"
RFC2005, October 1996.
[44] Solomon J. and S. Glass, "Mobile-IPv4 Configuration Option for
PPP IPCP", RFC2290, February 1998.
[45] Stevens, W., "TCP/IP Illustrated, Volume 1: The Protocols"
Addison-Wesley, Reading, Massachusetts, 1994.
Authors' Addresses
The working group can be contacted via the current chairs:
Basavaraj Patil
Nokia
6000 Connection Dr.
Irving, TX. 75039
USA
Phone: +1 972-894-6709
EMail: Basavaraj.Patil@nokia.com
Phil Roberts
Megisto Corp. Suite 120
20251 Century Blvd
Germantown MD 20874
USA
Phone: +1 847-202-9314
EMail: PRoberts@MEGISTO.com
Questions about this memo can also be directed to the editor:
Charles E. Perkins
Communications Systems Lab
Nokia Research Center
313 Fairchild Drive
Mountain View, California 94043
USA
Phone: +1-650 625-2986
EMail: charliep@iprg.nokia.com
Fax: +1 650 625-2502
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