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RFC2746 - RSVP Operation Over IP Tunnels

王朝other·作者佚名  2008-05-31
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Network Working Group A. Terzis

Request for Comments: 2746 UCLA

Category: Standards Track J. Krawczyk

ArrowPoint Communications

J. Wroclawski

MIT LCS

L. Zhang

UCLA

January 2000

RSVP Operation Over IP Tunnels

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 (2000). All Rights Reserved.

Abstract

This document describes an approach for providing RSVP protocol

services over IP tunnels. We briefly describe the problem, the

characteristics of possible solutions, and the design goals of our

approach. We then present the details of an implementation which

meets our design goals.

1. Introduction

IP-in-IP "tunnels" have become a widespread mechanism to transport

datagrams in the Internet. Typically, a tunnel is used to route

packets through portions of the network which do not directly

implement the desired service (e.g. IPv6), or to augment and modify

the behavior of the deployed routing architecture (e.g. multicast

routing, mobile IP, Virtual Private Net).

Many IP-in-IP tunneling protocols exist today. [IP4INIP4] details a

method of tunneling using an additional IPv4 header. [MINENC]

describes a way to reduce the size of the "inner" IP header used in

[IP4INIP4] when the original datagram is not fragmented. The generic

tunneling method in [IPV6GEN] can be used to tunnel either IPv4 or

IPv6 packets within IPv6. [RFC1933] describes how to tunnel IPv6

datagrams through IPv4 networks. [RFC1701] describes a generic

routing encapsulation, while [RFC1702] applies this encapsulation to

IPv4. Finally, [ESP] describes a mechanism that can be used to

tunnel an encrypted IP datagram.

From the perspective of traditional best-effort IP packet delivery, a

tunnel behaves as any other link. Packets enter one end of the

tunnel, and are delivered to the other end unless resource overload

or error causes them to be lost.

The RSVP setup protocol [RFC2205] is one component of a framework

designed to extend IP to support multiple, controlled classes of

service over a wide variety of link-level technologies. To deploy

this technology with maximum flexibility, it is desirable for tunnels

to act as RSVP-controllable links within the network.

A tunnel, and in fact any sort of link, may participate in an RSVP-

aware network in one of three ways, depending on the capabilities of

the equipment from which the tunnel is constructed and the desires of

the operator.

1. The (logical) link may not support resource reservation or QoS

control at all. This is a best-effort link. We refer to this as

a best-effort or type 1 tunnel in this note.

2. The (logical) link may be able to promise that some overall

level of resources is available to carry traffic, but not to

allocate resources specifically to individual data flows. A

configured resource allocation over a tunnel is an example of

this. We refer to this case as a type 2 tunnel in this note.

3. The (logical) link may be able to make reservations for

individual end-to-end data flows. We refer to this case as a

type 3 tunnel. Note that the key feature that distinguishes

type 3 tunnels from type 2 tunnels is that in the type 3 tunnel

new tunnel reservations are created and torn down dynamically

as end-to-end reservations come and go.

Type 1 tunnels exist when at least one of the routers comprising the

tunnel endpoints does not support the scheme we describe here. In

this case, the tunnel acts as a best-effort link. Our goal is simply

to make sure that RSVP messages traverse the link correctly, and the

presence of the non-controlled link is detected, as required by the

integrated services framework.

When the two end points of the tunnel are capable of supporting RSVP

over tunnels, we would like to have proper resources reserved along

the tunnel. Depending on the requirements of the situation, this

might mean that one client's data flow is placed into a larger

aggregate reservation (type 2 tunnels) or that possibly a new,

separate reservation is made for the data flow (type 3 tunnels).

Note that an RSVP reservation between the two tunnel end points does

not necessarily mean that all the intermediate routers along the

tunnel path support RSVP, this is equivalent to the case of an

existing end-to-end RSVP session transparently passing through non-

RSVP cloud.

Currently, however, RSVP signaling over tunnels is not possible.

RSVP packets entering the tunnel are encapsulated with an outer IP

header that has a protocol number other than 46 (e.g. it is 4 for

IP-in-IP encapsulation) and do not carry the Router-Alert option,

making them virtually "invisible" to RSVP routers between the two

tunnel endpoints. Moreover, the current IP-in-IP encapsulation

scheme adds only an IP header as the external wrapper. It is

impossible to distinguish between packets that use reservations and

those that don't, or to differentiate packets belonging to different

RSVP Sessions while they are in the tunnel, because no distinguishing

information such as a UDP port is available in the encapsulation.

This document describes an IP tunneling enhancement mechanism that

allows RSVP to make reservations across all IP-in-IP tunnels. This

mechanism is capable of supporting both type 2 and type 3 tunnels, as

described above, and requires minimal changes to both RSVP and other

parts of the integrated services framework.

2. The Design

2.1. Design Goals

Our design choices are motivated by several goals.

* Co-existing with most, if not all, current IP-in-IP tunneling

schemes.

* Limiting the changes to the RSVP spec to the minimum possible.

* Limiting the necessary changes to only the two end points of a

tunnel. This requirement leads to simpler deployment, lower

overhead in the intermediate routers, and less chance of failure

when the set of intermediate routers is modified due to routing

changes.

* Supporting correct inter-operation with RSVP routers that have

not been upgraded to handle RSVP over tunnels and with non-RSVP

tunnel endpoint routers. In these cases, the tunnel behaves as a

non-RSVP link.

2.2. Basic Approach

The basic idea of the method described in this document is to

recursively apply RSVP over the tunnel portion of the path. In this

new session, the tunnel entry point Rentry sends PATH messages and

the tunnel exit point Rexit sends RESV messages to reserve resources

for the end-to-end sessions over the tunnel.

We discuss next two different ASPects of the design: how to enhance

an IP-in-IP tunnel with RSVP capability, and how to map end-to-end

RSVP sessions to a tunnel session.

2.2.1. Design Decisions

To establish a RSVP reservation over a unicast IP-in-IP tunnel, we

made the following design decisions:

One or more Fixed-Filter style unicast reservations between the two

end points of the tunnel will be used to reserve resources for

packets traversing the tunnel. In the type 2 case, these reservations

will be configured statically by a management interface. In the type

3 case, these reservations will be created and torn down on demand,

as end-to-end reservation requests come and go.

Packets that do not require reservations are encapsulated in the

normal way, e. g. being wrapped with an IP header only, specifying

the tunnel entry point as source and the exit point as destination.

Data packets that require resource reservations within a tunnel must

have some attribute other than the IP addresses visible to the

intermediate routers, so that the routers may map the packet to an

appropriate reservation. To allow intermediate routers to use

standard RSVP filterspec handling, we choose to encapsulate such data

packets by prepending an IP and a UDP header, and to use UDP port

numbers to distinguish packets of different RSVP sessions. The

protocol number in the outer IP header in this case will be UDP.

Figure 1 shows RSVP operating over a tunnel. Rentry is the tunnel

entry router which encapsulates data into the tunnel. Some number of

intermediate routers forward the data across the network based upon

the encapsulating IP header added by Rentry. Rexit is the endpoint

of the tunnel. It decapsulates the data and forwards it based upon

the original, "inner" IP header.

........... ............... .............

: _______ : : _____ :

: : : :

Intranet :-- Rentry===================Rexit___:Intranet

: _______ : : _____ :

..........: : Internet : :...........

:..............

___________________

Figure 1. An example IP Tunnel

2.2.2. Mapping between End-to-End and Tunnel Sessions

Figure 2 shows a simple topology with a tunnel and a few hosts. The

sending hosts H1 and H3 may be one or multiple IP hops away from

Rentry; the receiving hosts H2 and H4 may also be either one or

multiple IP hops away from Rexit.

H1 H2

: :

: :

+--------+ +---+ +---+ +---+ +-------+

H3... Rentry =================================== Rexit ..... H4

+--------+ +---+ +---+ +---+ +-------+

Figure 2: An example end-to-end path with

a tunnel in the middle.

An RSVP session may be in place between endpoints at hosts H1 and H2.

We refer to this session as the "end-to-end" (E2E for short) or

"original" session, and to its PATH and RESV messages as the end-to-

end messages. One or more RSVP sessions may be in place between

Rentry and Rexit to provide resource reservation over the tunnel. We

refer to these as the tunnel RSVP sessions, and to their PATH and

RESV messages as the tunnel or tunneling messages. A tunnel RSVP

session may exist independently from any end-to-end sessions. For

example through network management interface one may create a RSVP

session over the tunnel to provide QoS support for data flow from H3

to H4, although there is no end-to-end RSVP session between H3 and

H4.

When an end-to-end RSVP session crosses a RSVP-capable tunnel, there

are two cases to consider in designing mechanisms to support an end-

to-end reservation over the tunnel: mapping the E2E session to an

existing tunnel RSVP session (type 2 tunnel), and dynamically

creating a new tunnel RSVP session for each end-to-end session (type

3 tunnel). In either case, the picture looks like a recursive

application of RSVP. The tunnel RSVP session views the two tunnel

endpoints as two end hosts with a unicast Fixed-Filter style

reservation in between. The original, end-to-end RSVP session views

the tunnel as a single (logical) link on the path between the

source(s) and destination(s).

Note that in practice a tunnel may combine type 2 and type 3

characteristics. Some end-to-end RSVP sessions may trigger the

creation of new tunnel sessions, while others may be mapped into an

existing tunnel RSVP session. The choice of how an end-to-end session

is treated at the tunnel is a matter of local policy.

When an end-to-end RSVP session crosses a RSVP-capable tunnel, it is

necessary to coordinate the actions of the two RSVP sessions, to

determine whether or when the tunnel RSVP session should be created

and torn down, and to correctly transfer error and ADSPEC information

between the two RSVP sessions. We made the following design

decision:

* End-to-end RSVP control messages being forwarded through a

tunnel are encapsulated in the same way as normal IP packets,

e.g. being wrapped with the tunnel IP header only, specifying

the tunnel entry point as source and the exit point as

destination.

2.3. Major Issues

As IP-in-IP tunnels are being used more widely for network traffic

management purposes, it is clear we must support type 2 tunnels

(tunnel reservation for aggregate end-to-end sessions). Furthermore,

these type 2 tunnels should allow more than one (configurable,

static) reservation to be used at once, to support different traffic

classes within the tunnel. Whether it is necessary to support type 3

tunnels (dynamic per end-to-end session tunnel reservation) is a

policy issue that should be left open. Our design supports both

cases.

If there is only one RSVP session configured over a tunnel, then all

the end-to-end RSVP sessions (that are allowed to use this tunnel

session) will be bound to this configured tunnel session. However

when more than one RSVP session is in use over an IP tunnel, a second

design issue is how the association, or binding, between an original

RSVP reservation and a tunnel reservation is created and conveyed

from one end of the tunnel to the other. The entry router Rentry and

the exit router Rexit must agree on these associations so that

changes in the original reservation state can be correctly mapped

into changes in the tunnel reservation state, and that errors

reported by intermediate routers to the tunnel end points can be

correctly transformed into errors reported by the tunnel endpoints to

the end-to-end RSVP session.

We require that this same association mechanism work for both the

case of bundled reservation over a tunnel (type 2 tunnel), and the

case of one-to-one mapping between original and tunnel reservations

(type 3 tunnel). In our scheme the association is created when a

tunnel entry point first sees an end-to-end session's RESV message

and either sets up a new tunnel session, or adds to an existing

tunnel session. This new association must be conveyed to Rexit, so

that Rexit can reserve resources for the end-to-end sessions inside

the tunnel. This information includes the identifier and certain

parameters of the tunnel session, and the identifier of the end-to-

end session to which the tunnel session is being bound. In our

scheme, all RSVP sessions between the same two routers Rentry and

Rexit will have identical values for source IP address, destination

IP address, and destination UDP port number. An individual session is

identified primarily by the source port value.

We identified three possible choices for a binding mechanism:

1. Define a new RSVP message that is exchanged only between two

tunnel end points to convey the binding information.

2. Define a new RSVP object to be attached to end-to-end PATH

messages at Rentry, associating the end-to-end session with one

of the tunnel sessions. This new object is interpreted by Rexit

associating the end-to-end session with one of the tunnel

sessions generated at Rentry.

3. Apply the same UDP encapsulation to the end-to-end PATH

messages as to data packets of the session. When Rexit

decapsulates the PATH message, it deduces the relation between

the source UDP port used in the encapsulation and the RSVP

session that is specified in the original PATH message.

The last approach above does not require any new design. However it

requires additional resources to be reserved for PATH messages (since

they are now subject to the tunnel reservation). It also requires a

priori knowledge of whether Rexit supports RSVP over tunnels by UDP

encapsulation. If Rentry encapsulates all the end-to-end PATH

messages with the UDP encapsulation, but Rexit does not understand

this encapsulation, then the encapsulated PATH messages will be lost

at Rexit.

On the other hand, options (1) and (2) can handle this case

transparently. They allow Rexit to pass on end-to-end PATHs received

via the tunnel (because they are decapsulated normally), while

throwing away the tunnel PATHs, all without any additional

configuration. We chose Option (2) because it is simpler. We

describe this object in the following section.

Packet exchanges must follow the following constraints:

1. Rentry encapsulates and sends end-to-end PATH messages over the

tunnel to Rexit where they get decapsulated and forwarded

downstream.

2. When a corresponding end-to-end RESV message arrives at Rexit,

Rexit encapsulates it and sends it to Rentry.

3. Based on some or all of the information in the end-to-end PATH

messages, the flowspec in the end-to-end RESV message and local

policies, Rentry decides if and how to map the end-to-end

session to a tunnel session.

4. If the end-to-end session should be mapped to a tunnel session,

Rentry either sends a PATH message for a new tunnel session or

updates an existing one.

5. Rentry sends a E2E Path containing a SESSION_ASSOC object

associating the end-to-end session with the tunnel session

above. Rexit records the association and removes the object

before forwarding the Path message further.

6. Rexit responds to the tunnel PATH message by sending a tunnel

RESV message, reserving resources inside the tunnel.

7. Rentry UDP-encapsulates arriving packets only if a

corresponding tunnel session reservation is actually in place

for the packets.

2.3.1. SESSION_ASSOC Object

The new object, called SESSION_ASSOC, is defined with the following

format:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

length class c-type

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

SESSION object (for the end-to-end session)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Sender FILTER-SPEC (for the tunnel session)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

SESSION_ASSOC Object

Length

This field contains the size of the SESSION_ASSOC object in bytes.

Class

Should be 192.

Ctype

Should be sent as zero and ignored on receipt.

SESSION object

The end-to-end SESSION contained in the object is to be mapped to

the tunnel session described by the Sender FILTER-SPEC defined

below.

Sender FILTER-SPEC

This is the tunnel session that the above mentioned end-to-end

session maps to over the tunnel. As we mentioned above, a tunnel

session is identified primarily by source port. This is why we use

a Sender Filter-Spec for the tunnel session, in the place of a

SESSION object.

2.3.2. NODE_CHAR Object

There has to be a way (other than through configuration) for Rexit to

communicate to Rentry the fact that it is a tunnel endpoint

supporting the scheme described in this document. We have defined for

this reason a new object, called NODE_CHAR, carrying this

information. If a node receives this object but does not understand

it, it should drop it without producing any error report. Objects

with Class-Num = 10bbbbbb (`b' represents a bit), as defined in the

RSVP specification [RFC2205], have the characteristics we need. While

for now this object only carries one bit of information, it can be

used in the future to describe other characteristics of an RSVP

capable node that are not part of the original RSVP specification.

The object NODE_CHAR has the following format:

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

length class c-type

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Reserved T

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Length

This field contains the size of the NODE_CHAR object in bytes. It

should be set to eight.

Class

An appropriate value should be assigned by the IANA. We propose

this value to be 128.

Ctype

Should be sent as zero and ignored on receipt.

T bit

This bit shows that the node is a RSVP-tunnel capable node.

When Rexit receives an end-to-end reservation, it appends a NODE_CHAR

object with the T bit set, to the RESV object, it encapsulates it and

sends it to Rentry. When Rentry receives this RESV message it deduces

that Rexit implements the mechanism described here and so it creates

or adjusts a tunnel session and associates the tunnel session to the

end-to-end session via a SESSION_ASSOC object. Rentry should remove

the NODE_CHAR object, before forwarding the RESV message upstream. If

on the other hand, Rentry does not support the RSVP Tunnels mechanism

it would simply ignore the NODE_CHAR object and not forward it

further upstream.

3. Implementation

In this section we discuss several cases separately, starting from

the simplest scenario and moving to the more complex ones.

3.1. Single Configured RSVP Session over an IP-in-IP Tunnel

Treating the two tunnel endpoints as a source and destination host,

one easily sets up a FF-style reservation in between. Now the

question is what kind of filterspec to use for the tunnel

reservation, which directly relates to how packets get encapsulated

over the tunnel. We discuss two cases below.

3.1.1. In the Absence of End-to-End RSVP Session

In the case where all the packets traversing a tunnel use the

reserved resources, the current IP-in-IP encapsulation could be used.

The RSVP session over the tunnel would simply specify a FF style

reservation (with zero port number) with Rentry as the source address

and Rexit as the destination address.

However if only some of the packets traversing the tunnel should

benefit from the reservation, we must encapsulate the qualified

packets in IP and UDP. This allows intermediate routers to use

standard RSVP filterspec handling, without having to know about the

existence of tunnels.

Rather than supporting both cases we choose to simplify

implementations by requiring all data packets using reservations to

be encapsulated with an outer IP and UDP header. This reduces special

case checking and handling.

3.1.2. In the Presence of End-to-End RSVP Session(s)

According to the tunnel control policies, installed through some

management interface, some or all end-to-end RSVP sessions may be

allowed to map to the single RSVP session over the tunnel. In this

case there is no need to provide dynamic binding information between

end-to-end sessions and the tunnel session, given that the tunnel

session is unique and pre-configured, and therefore well-known.

Binding multiple end-to-end sessions to one tunnel session, however,

raises a new question of when and how the size of the tunnel

reservation should be adjusted to accommodate the end-to-end sessions

mapped onto it. Again the tunnel manager makes such policy decision.

Several scenarios are possible. In the first, the tunnel reservation

is never adjusted. This makes the tunnel the rough equivalent of a

fixed-capacity hardware link. In the second, the tunnel reservation

is adjusted whenever a new end-to-end reservation arrives or an old

one is torn down. In the third, the tunnel reservation is adjusted

upwards or downwards occasionally, whenever the end-to-end

reservation level has changed enough to warrant the adjustment. This

trades off extra resource usage in the tunnel for reduced control

traffic and overhead.

We call a tunnel whose reservation cannot be adjusted a "hard pipe",

as opposed to a "soft pipe" where the amount of resources allocated

is adjustable. Section 5.2 eXPlains how the adjustment can be carried

out for soft pipes.

3.2. Multiple Configured RSVP Sessions over an IP-in-IP Tunnel

It is straightforward to build on the case of a single configured

RSVP session over a tunnel by setting up multiple FF-style

reservations between the two tunnel endpoints using a management

interface. In this case Rentry must carefully encapsulate data

packets with the proper UDP port numbers, so that packets belonging

to different tunnel sessions will be distinguished by the

intermediate RSVP routers. Note that this case and the one described

before describe what we call type 2 tunnels.

3.2.1. In the Absence of End-to-End RSVP Session

Nothing more needs to be said in this case. Rentry classifies the

packets and encapsulates them accordingly. Packets with no

reservations are encapsulated with an outer IP header only, while

packets qualified for reservations are encapsulated with a UDP header

as well as an IP header. The UDP source port value should be properly

set to map to the corresponding tunnel reservation the packet is

supposed to use.

3.2.2. In the Presence of End-to-End RSVP Session(s)

Since in this case, there is more than one RSVP session operating

over the tunnel, one must explicitly bind each end-to-end RSVP

session to its corresponding tunnel session. As discussed

previously, this binding will be provided by the new SESSION_ASSOC

object carried by the end-to-end PATH messages.

3.3. Dynamically Created Tunnel RSVP Sessions

This is the case of a type 3 tunnel. The only differences between

this case and that of Section 4.2 are that:

- The tunnel session is created when a new end-to-end session

shows up.

- There is a one-to-one mapping between the end-to-end and tunnel

RSVP sessions, as opposed to possibly many-to-one mapping that

is allowed in the case described in Section 4.2.

4. RSVP Messages handling over an IP-in-IP Tunnel

4.1. RSVP Messages for Configured Session(s) Over A Tunnel

Here one or more RSVP sessions are set up over a tunnel through a

management interface. The session reservation parameters never

change for a "hard pipe" tunnel. The reservation parameters may

change for a "soft pipe" tunnel. Tunnel session PATH messages

generated by Rentry are addressed to Rexit, where they are processed

and deleted.

4.2. Handling of RSVP Messages at Tunnel Endpoints

4.2.1. Handling End-to-End PATH Messages at Rentry

When forwarding an end-to-end PATH message, a router acting as the

tunnel entry point, Rentry, takes the following actions depending on

the end-to-end session mentioned in the PATH message. There are two

possible cases:

1. The end-to-end PATH message is a refresh of a previously known

end-to-end session.

2. The end-to-end PATH message is from a new end-to-end session.

If the PATH message is a refresh of a previously known end-to-end

session, then Rentry refreshes the Path state of the end-to-end

session and checks to see if this session is mapped to a tunnel

session. If this is the case, then when Rentry refreshes the end-to-

end session, it includes in the end-to-end PATH message a

SESSION_ASSOC object linking this session to its corresponding tunnel

session It then encapsulates the end-to-end PATH message and sends it

over the tunnel to Rexit. If the tunnel session was dynamically

created, the end-to-end PATH message serves as a refresh for the

local tunnel state at Rentry as well as for the end-to-end session.

Otherwise, if the PATH message is from a new end-to-end session that

has not yet been mapped to a tunnel session, Rentry creates Path

state for this new session setting the outgoing interface to be the

tunnel interface. After that, Rentry encapsulates the PATH message

and sends it to Rexit without adding a SESSION_ASSOC message.

When an end-to-end PATH TEAR is received by Rentry, this node

encapsulates and forwards the message to Rexit. If this end-to-end

session has a one-to-one mapping to a tunnel session or if this is

the last one of the many end-to-end sessions mapping to a tunnel

session, Rentry tears down the tunnel session by sending a PATH TEAR

for that session to Rexit. If, on the other hand, there are remaining

end-to-end sessions mapping to the tunnel session, then Rentry sends

a tunnel PATH message adjusting the Tspec of the tunnel session.

4.2.2. Handling End-to-End PATH Messages at Rexit

Encapsulated end-to-end PATH messages are decapsulated and processed

at Rexit. Depending on whether the end-to-end PATH message contains a

SESSION_ASSOC object or not, Rexit takes the following steps:

1. If the end-to-end PATH message does not contain a SESSION_ASSOC

object, then Rentry sets the Non_RSVP flag at the Path state

stored for this end-to-end sender, sets the global break bit in

the ADSPEC and forwards the packets downstream. Alternatively,

if tunnel sessions exist and none of them has the Non_RSVP flag

set, Rexit can pick the worst-case Path ADSPEC params from the

existing tunnel sessions and update the end-to-end ADSPEC using

these values. This is a conservative estimation of the composed

ADSPEC but it has the benefit of avoiding to set the break bit

in the end-to-end ADSPEC before mapping information is

available. In this case the Non_RSVP flag at the end-to-end

Path state is not set.

2. If the PATH message contains a SESSION_ASSOC object and no

association for this end-to-end session already exists, then

Rexit records the association between the end-to-end session

and the tunnel session described by the object. If the end-to-

end PATH arrives early before the tunnel PATH message arrives

then it creates PATH state at Rexit for the tunnel session.

When the actual PATH message for the tunnel session arrives it

is treated as an update of the existing PATH state and it

updates any information missing. We believe that this situation

is another transient along with the others existing in RSVP and

that it does not have any long-term effects on the correct

operation of the mechanism described here.

Before further forwarding the message to the next hop along the

path to the destination, Rexit finds the corresponding tunnel

session's recorded state and turns on Non_RSVP flag in the

end-to-end Path state if the Non_RSVP bit was turned on for the

tunnel session. If the end-to-end PATH message carries an

ADSPEC object, Rexit performs composition of the

characterization parameters contained in the ADSPEC. It does

this by considering the tunnel session's overall (composed)

characterization parameters as the local parameters for the

logical link implemented by the tunnel, and composing these

parameters with those in the end-to-end ADSPEC by executing

each parameter's defined composition function. In the logical

link's characterization parameters, the minimum path latency

may take into account the encapsulation/decapsulation delay and

the bandwidth estimate can represent the decrease in available

bandwidth caused by the addition of the extra UDP header.

ADSPECs and composition functions are discussed in great detail

in [RFC2210].

If the end-to-end session has reservation state, while no

reservation state for the matching tunnel session exists, Rexit

send a tunnel RESV message to Rentry matching the reservation

in the end-to-end session.

If Rentry does not support RSVP tunneling, then Rexit will have no

PATH state for the tunnel. In this case Rexit simply turns on the

global break bit in the decapsulated end-to-end PATH message and

forwards it.

4.2.3. Handling End-to-End RESV Messages at Rexit

When forwarding a RESV message upstream, a router serving as the exit

router, Rexit, may discover that one of the upstream interfaces is a

tunnel. In this case the router performs a number of tests.

Step 1: Rexit must determine if there is a tunnel session bound to

the end-to-end session given in the RESV message. If not, the tunnel

is treated as a non-RSVP link, Rexit appends a NODE_CHAR object with

the T bit set, to the RESV message and forwards it over the tunnel

interface (where it is encapsulated as a normal IP datagram and

forwarded towards Rentry).

Step 2: If a bound tunnel session is found, Rexit checks to see if a

reservation is already in place for the tunnel session bound to the

end-to-end session given in the RESV message. If the arriving end-

to-end RESV message is a refresh of existing RESV state, then Rexit

sends the original RESV through tunnel interface (after adding the

NODE_CHAR object). For dynamic tunnel sessions, the end-to-end RESV

message acts as a refresh for the tunnel session reservation state,

while for configured tunnel sessions, reservation state never

expires.

If the arriving end-to-end RESV message causes a change in the end-

to-end RESV flowspec parameters, it may also trigger an attempt to

change the tunnel session's flowspec parameters. In this case Rexit

sends a tunnel session RESV, including a RESV_CONFIRM object.

In the case of a "hard pipe" tunnel, a new end-to-end reservation or

change in the level of resources requested by an existing reservation

may cause the total resource level needed by the end-to-end

reservations to exceed the level of resources reserved by the tunnel

reservation. This event should be treated as an admission control

failure, identically to the case where RSVP requests exceed the level

of resources available over a hardware link. A RESV_ERR message with

Error Code set to 01 (Admission Control failure), should be sent back

to the originator of the end-to-end RESV message.

If a RESV CONFIRM response arrives, the original RESV is encapsulated

and sent through the tunnel. If the updated tunnel reservation fails,

Rexit must send a RESV ERR to the originator of the end-to-end RESV

message, using the error code and value fields from the ERROR_SPEC

object of the received tunnel session RESV ERR message. Note that the

pre-existing reservations through the tunnel stay in place. Rexit

continues refreshing the tunnel RESV using the old flowspec.

Tunnel session state for a "soft pipe" may also be adjusted when an

end-to-end reservation is deleted. The tunnel session gets reduced

whenever one of the end-to-end sessions using the tunnel goes away

(or gets reduced itself). However even when the last end-to-end

session bound to that tunnel goes away, the configured tunnel session

remains active, perhaps with a configured minimal flowspec.

Note that it will often be appropriate to use some hysteresis in the

adjustment of the tunnel reservation parameters, rather than

adjusting the tunnel reservation up and down with each arriving or

departing end-to-end reservation. Doing this will require the tunnel

exit router to keep track of the resources allocated to the tunnel

(the tunnel flowspec) and the resources actually in use by end-to-end

reservations (the sum or statistical sum of the end-to-end

reservation flowspecs) separately.

When an end-to-end RESV TEAR is received by Rexit, it encapsulates

and forwards the message to Rentry. If the end-to-end session had

created a dynamic tunnel session, then a RESV TEAR for the

corresponding tunnel session is send by Rexit.

4.2.4. Handling of End-to-End RESV Messages at Rentry.

If the RESV message received is a refresh of an existing reservation

then Rentry updates the reservation state and forwards the message

upstream. On the other hand, if this is the first RESV message for

this end-to-end session and a NODE_CHAR object with the T bit set is

present, Rentry should initiate the mapping between this end-to-end

session and some (possibly new) tunnel session. This mapping is based

on some or all of the contents of the end-to-end PATH message, the

contents of the end-to-end RESV message, and local policies. For

example, there could be different tunnel sessions based on the

bandwidth or delay requirements of end-to-end sessions)

If Rentry decides that this end-to-end session should be mapped to an

existing configured tunnel session, it binds this end-to-end session

to that tunnel session.

If this end-to-end RSVP session is allowed to set up a new tunnel

session, Rentry sets up tunnel session PATH state as if it were a

source of data by starting to send tunnel-session PATH messages to

Rexit, which is treated as the unicast destination of the data. The

Tspec in this new PATH message is computed from the original PATH

message by adjusting the Tspec parameters to include the tunnel

overhead of the encapsulation of data packets. In this case Rentry

should also send a PATH message from the end-to-end session this time

containing the SESSION_ASSOC object linking the two sessions. The

receipt of this PATH message by Rexit will trigger an update of the

end-to-end Path state which in turn will have the effect of Rexit

sending a tunnel RESV message, allocating resources inside the

tunnel.

The last case is when the end-to-end session is not allowed to use

the tunnel resources. In this case no association is created between

this end-to-end session and a tunnel session and no new tunnel

session is created.

One limitation of our scheme is that the first RESV message of an

end-to-end session determines the mapping between that end-to-end

session and its corresponding session over the tunnel. Moreover as

long as the reservation is active this mapping cannot change.

5. Forwarding Data

When data packets arrive at the tunnel entry point Rentry, Rentry

must decide whether to forward the packets using the normal IP-in-IP

tunnel encapsulation or the IP+UDP encapsulation expected by the

tunnel session. This decision is made by determining whether there

is a resource reservation (not just PATH state) actually in place for

the tunnel session bound to the arriving packet, that is, whether the

packet matches any active filterspec.

If a reservation is in place, it means that both Rentry and Rexit are

RSVP-tunneling aware routers, and the data will be correctly

decapsulated at Rexit.

If no tunnel session reservation is in place, the data should be

encapsulated in the tunnel's normal format, regardless of whether

end-to-end PATH state covering the data is present.

6. Details

6.1. Selecting UDP port numbers

There may be multiple end-to-end RSVP sessions between the two end

points Rentry and Rexit. These sessions are distinguished by the

source UDP port. Other components of the session ID, the source and

destination IP addresses and the destination UDP port, are identical

for all such sessions.

The source UDP port is chosen by the tunnel entry point Rentry when

it establishes the initial PATH state for a new tunnel session. The

source UDP port associated with the new session is then conveyed to

Rexit by the SESSION_ASSOC object.

The destination UDP port used in tunnel sessions should the one

assigned by IANA (363).

6.2. Error Reporting

When a tunnel session PATH message encounters an error, it is

reported back to Rentry. Rentry must relay the error report back to

the original source of the end-to-end session.

When a tunnel session RESV request fails, an error message is

returned to Rexit. Rexit must treat this as an error in crossing the

logical link (the tunnel) and forward the error message back to the

end host.

6.3. MTU Discovery

Since the UDP encapsulated packets should not be fragmented, tunnel

entry routers must support tunnel MTU discovery as discussed in

section 5.1 of [IP4INIP4]. Alternatively, the Path MTU Discovery

mechanism discussed in RFC2210 [RFC2210] can be used.

6.4. Tspec and Flowspec Calculations

As multiple End-to-End sessions can be mapped to a single tunnel

session, there is the need to compute the aggregate Tspec of all the

senders of those End-to-End sessions. This aggregate Tspec will the

Tspec of the representative tunnel session. The same operation needs

to be performed for flowspecs of End-to-End reservations arriving at

Rexit.

The semantics of these operations are not addressed here. The

simplest way to do them is to compute a sum of the end-to-end Tspecs,

as is defined in the specifications of the Controlled-Load and

Guaranteed services (found at [RFC2211] and [RFC2212] respectively).

However, it may also be appropriate to compute the aggregate

reservation level for the tunnel using a more sophisticated

statistical or measurement-based computation.

7. IPSEC Tunnels

In the case where the IP-in-IP tunnel supports IPSEC (especially ESP

in Tunnel-Mode with or without AH) then the Tunnel Session uses the

GPI SESSION and GPI SENDER_TEMPLATE/FILTER_SPEC as defined in

[RSVPESP] for the PATH and RESV messages.

Data packets are not encapsulated with a UDP header since the SPI can

be used by the intermediate nodes for classification purposes.

Notice that user oriented keying must be used between Rentry and

Rexit, so that different SPIs are assigned to data packets that have

reservation and "best effort" packets, as well as packets that belong

to different Tunnel Sessions if those are supported.

8. RSVP Support for Multicast and Multipoint Tunnels

The mechanisms described above are useful for unicast tunnels.

Unicast tunnels provide logical point-to-point links in the IP

infrastructure, though they may encapsulate and carry either unicast

or multicast traffic between those points.

Two other types of tunnels may be imagined. The first of these is a

"multicast" tunnel. In this type of tunnel, packets arriving at an

entry point are encapsulated and transported (multicast) to -all- of

the exit points. This sort of tunnel might prove useful for

implementing a hierarchical multicast distribution network, or for

emulating efficiently some portion of a native multicast distribution

tree.

A second possible type of tunnel is the "multipoint" tunnel. In this

type of tunnel, packets arriving at an entry point are normally

encapsulated and transported to -one- of the exit points, according

to some route selection algorithm.

This type of tunnel differs from all previous types in that the '

shape' of the usual data distribution path does not match the 'shape'

of the tunnel. The topology of the tunnel does not by itself define

the data transmission function that the tunnel performs. Instead,

the tunnel becomes a way to express some shared property of the set

of connected tunnel endpoints. For example, the "tunnel" may be used

to create and embed a logical shared broadcast network within some

larger network. In this case the tunnel endpoints are the nodes

connected to the logical shared broadcast network. Data traffic may

be unicast between two such nodes, broadcast to all connected nodes,

or multicast between some subset of the connected nodes. The tunnel

itself is used to define a domain in which to manage routing and

resource management - essentially a virtual private network.

Note that while a VPN of this form can always be implemented using a

multicast tunnel to emulate the broadcast medium, this approach will

be very inefficient in the case of wide area VPNs, and a multipoint

tunnel with appropriate control mechanisms will be preferable.

The following paragraphs provide some brief commentary on the use of

RSVP in these situations. Future versions of this note will provide

more concrete details and specifications.

Using RSVP to provide resource management over a multicast tunnel is

relatively straightforward. As in the unicast case, one or more RSVP

sessions may be used, and end-to-end RSVP sessions may be mapped onto

tunnel RSVP sessions on a many-to-one or one-to-one basis. Unlike the

unicast, case, however, the mapping is complicated by RSVP's

heterogeneity semantics. If different receivers have made different

reservation requests, it may be that the RESV messages arriving at

the tunnel would logically map the receiver's requests to different

tunnel sessions. Since the data can actually be placed into only one

session, the choice of session must be reconciled (merged) to select

the one that will meet the needs of all applications. This requires a

relatively simple extension to the session mapping mechanism.

Use of RSVP to support multipoint tunnels is somewhat more difficult.

In this case, the goal is to give the tunnel as a whole a specific

level of resources. For example, we may wish to emulate a "logical

shared 10 megabit Ethernet" rather than a "logical shared Ethernet".

However, the problem is complicated by the fact that in this type of

tunnel the data does not always go to all tunnel endpoints. This

implies that we cannot use the destination address of the

encapsulated packets as part of the packet classification filter,

because the destination address will vary for different packets

within the tunnel.

This implies the need for an extension to current RSVP session

semantics in which the Session ID (destination IP address) is used

-only- to identify the session state within network nodes, but is not

used to classify packets. Other than this, the use of RSVP for

multipoint tunnels follows that of multicast tunnels. A multicast

group is created to represent the set of nodes that are tunnel

endpoints, and one or more tunnel RSVP sessions are created to

reserve resources for the encapsulated packets. In the case of a

tunnel implementing a simple VPN, it is most likely that there will

be one session to reserve resources for the whole VPN. Each tunnel

endpoint will participate both as a source of PATH messages and a

source of (FF or SE) RESV messages for this single session,

effectively creating a single shared reservation for the entire

logical shared medium. Tunnel endpoints MUST NOT make wildcard

reservations over multipoint tunnels.

9. Extensions to the RSVP/Routing Interface

The RSVP specification [RFC2205] states that through the RSVP/Routing

Interface, the RSVP daemon must be able to learn the list of local

interfaces along with their IP addresses. In the RSVP Tunnels case,

the RSVP daemon needs also to learn which of the local interface(s)

is (are) IP-in-IP tunnel(s) having the capabilities described here.

The RSVP daemon can acquire this information, either by directly

querying the underlying network and physical layers or by using any

existing interface between RSVP and the routing protocol properly

extended to provide this information.

10. Security Considerations

The introduction of RSVP Tunnels raises no new security issues other

than those associated with the use of RSVP and tunnels. Regarding

RSVP, the major issue is the need to control and authenticate Access

to enhanced qualities of service. This requirement is discussed

further in [RFC2205]. [RSVPCRYPTO] describes the mechanism used to

protect the integrity of RSVP messages carrying the information

described here. The security issues associated with IP-in-IP tunnels

are discussed in [IPINIP4] and [IPV6GEN].

11. IANA Considerations

IANA should assign a Class number for the NODE_CHAR object defined in

Section 3.3.2. This number should be in the 10bbbbbb range. The

suggested value is 128.

12. Acknowledgments

We thank Bob Braden for his insightful comments that helped us to

produce this updated version of the document.

13. References

[ESP] Atkinson, R., "IP Encapsulating Security Payload (ESP)",

RFC1827, August 1995.

[IP4INIP4] Perkins, C., "IP Encapsulation within IP", RFC2003,

October 1996.

[IPV6GEN] Conta, A. and S. Deering, "Generic Packet Tunneling in

IPv6 Specification", RFC2473, December 1998.

[MINENC] Perkins, C., "Minimal Encapsulation within IP", RFC

2004, October 1996.

[RFC1701] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic

Routing Encapsulation (GRE)", RFC1701, October 1994.

[RFC1702] Hanks, S., Li, T., Farinacci, D. and P. Traina, "Generic

Routing Encapsulation over IPv4 Networks", RFC1702,

October 1994.

[RFC1933] Gilligan, R. and E. Nordmark, "Transition Mechanisms for

IPv6 Hosts and Routers", RFC1933, April 1996.

[RFC2210] Wroclawski, J., "The Use of RSVP with IETF Integrated

Services", RFC2210, September 1997.

[RFC2211] Wroclawski, J., "Specification of the Controlled-Load

Network Element Service", RFC2211, September 1997.

[RFC2212] Shenker, S., Partridge, C. and R. Guerin, "Specification

of the Guaranteed Quality of Service", RFC2212,

September 1997.

[RFC2205] Braden, R., Zhang, L., Berson, S., Herzog, S. and S.

Jamin, "Resource ReSerVation Protocol (RSVP) -- Version

1 Functional Specification", RFC2205, September 1997.

[RSVPESP] Berger, L. and T. O'Malley, "RSVP Extensions for IPSEC

Data Flows", RFC2207, September 1997.

[RSVPCRYPTO] Baker, F., Lindell, B. and M. Talwar, "RSVP

Cryptographic Authentication", RFC2747, January 2000.

14. Authors' Addresses

John Krawczyk

ArrowPoint Communications

50 Nagog Park

Acton, MA 01720

Phone: 978-206-3027

EMail: jj@arrowpoint.com

John Wroclawski

MIT Laboratory for Computer Science

545 Technology Sq.

Cambridge, MA 02139

Phone: 617-253-7885

Fax: 617-253-2673

EMail: jtw@lcs.mit.edu

Lixia Zhang

UCLA

4531G Boelter Hall

Los Angeles, CA 90095

Phone: 310-825-2695

EMail: lixia@cs.ucla.edu

Andreas Terzis

UCLA

4677 Boelter Hall

Los Angeles, CA 90095

Phone: 310-267-2190

EMail: terzis@cs.ucla.edu

15. Full Copyright Statement

Copyright (C) The Internet Society (2000). All Rights Reserved.

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFCEditor function is currently provided by the

Internet Society.

 
 
 
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