Network Working Group M. Handley
Request for Comments: 2776 ACIRI
Category: Standards Track D. Thaler
Microsoft
R. Kermode
Motorola
February 2000
Multicast-Scope Zone Announcement Protocol (MZAP)
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 defines a protocol, the Multicast-Scope Zone
Announcement Protocol (MZAP), for discovering the multicast
administrative scope zones that are relevant at a particular
location. MZAP also provides mechanisms whereby common
misconfigurations of administrative scope zones can be discovered.
Table of Contents
1 IntrodUCtion ................................................ 2
2 Terminology ................................................. 4
3 Overview .................................................... 5
3.1 Scope Nesting ............................................. 6
3.2 Other Messages ............................................ 7
3.3 Zone IDs .................................................. 7
4 Detecting Router Misconfigurations .......................... 8
4.1 Detecting non-convex scope zones .......................... 8
4.2 Detecting leaky boundaries for non-local scopes ........... 9
4.3 Detecting a leaky Local Scope zone ........................ 10
4.4 Detecting conflicting scope zones ......................... 10
5 Packet Formats .............................................. 11
5.1 Zone Announcement Message ................................. 14
5.2 Zone Limit Exceeded (ZLE) ................................. 15
5.3 Zone Convexity Message .................................... 15
5.4 Not-Inside Message ........................................ 16
6 Message Processing Rules .................................... 17
6.1 Internal entities listening to MZAP messages .............. 17
6.2 Sending ZAMs .............................................. 18
6.3 Receiving ZAMs ............................................ 18
6.4 Sending ZLEs .............................................. 20
6.5 Receiving ZLEs ............................................ 20
6.6 Sending ZCMs .............................................. 21
6.7 Receiving ZCMs ............................................ 21
6.8 Sending NIMs .............................................. 21
6.9 Receiving NIMs ............................................ 22
7 Constants ................................................... 22
8 Security Considerations ..................................... 23
9 Acknowledgements ............................................ 24
10 References ................................................. 25
11 Authors' Addresses ......................................... 26
12 Full Copyright Statement ................................... 27
1. Introduction
The use of administratively-scoped IP multicast, as defined in RFC
2365 [1], allows packets to be addressed to a specific range of
multicast addresses (e.g., 239.0.0.0 to 239.255.255.255 for IPv4)
such that the packets will not cross configured administrative
boundaries, and also allows such addresses to be locally assigned and
hence are not required to be unique across administrative boundaries.
This property of logical naming both allows for address reuse, as
well as provides the capability for infrastructure services such as
address allocation, session advertisement, and service location to
use well-known addresses which are guaranteed to have local
significance within every organization.
The range of administratively-scoped addresses can be subdivided by
administrators so that multiple levels of administrative boundaries
can be simultaneously supported. As a result, a "multicast scope" is
defined as a particular range of addresses which has been given some
topological meaning.
To support such usage, a router at an administrative boundary is
configured with one or more per-interface filters, or "multicast
scope boundaries". Having such a boundary on an interface means that
it will not forward packets matching a configured range of multicast
addresses in either direction on the interface.
A specific area of the network topology which is within a boundary
for a given scope is known as a "multicast scope zone". Since the
same ranges can be reused within disjoint areas of the network, there
may be many "multicast scope zones" for any given multicast scope. A
scope zone may have zero or more textual names (in different
languages) for the scope, for human convenience. For example, if the
range 239.192/14 were assigned to span an entire corporate network,
it might be given (internally) the name "BigCo Private Scope".
Administrative scope zones may be of any size, and a particular host
may be within many administrative scope zones (for different scopes,
i.e., for non-overlapping ranges of addresses) of various sizes, as
long as scope zones that intersect topologically do not intersect in
address range.
Applications and services are interested in various ASPects of the
scopes within which they reside:
o Applications which present users with a choice of which scope in
which to operate (e.g., when creating a new session, whether it is
to be confined to a corporate intranet, or whether it should go
out over the public Internet) are interested in the textual names
which have significance to users.
o Services which use "relative" multicast addresses (as defined in
[1]) in every scope are interested in the range of addresses used
by each scope, so that they can apply a constant offset and
compute which address to use in each scope.
o Address allocators are interested in the address range, and
whether they are allowed to allocate addresses within the entire
range or not.
o Some applications and services may also be interested in the
nesting relationships among scopes. For example, knowledge of the
nesting relationships can be used to perform "eXPanding-scope"
searches in a similar, but better behaved, manner to the well-
known expanding ring search where the TTL of a query is steadily
increased until a replier can be found. Studies have also shown
that nested scopes can be useful in localizing multicast repair
traffic [8].
Two barriers currently make administrative scoping difficult to
deploy and use:
o Applications have no way to dynamically discover information on
scopes that are relevant to them. This makes it difficult to use
administrative scope zones, and hence reduces the incentive to
deploy them.
o Misconfiguration is easy. It is difficult to detect scope zones
that have been configured so as to not be convex (the shortest
path between two nodes within the zone passes outside the zone),
or to leak (one or more boundary routers were not configured
correctly), or to intersect in both area and address range.
These two barriers are addressed by this document. In particular,
this document defines the Multicast Scope Zone Announcement Protocol
(MZAP) which allows an entity to learn what scope zones it is within.
Typically servers will cache the information learned from MZAP and
can then provide this information to applications in a timely fashion
upon request using other means, e.g., via MADCAP [9]. MZAP also
provides diagnostic information to the boundary routers themselves
that enables misconfigured scope zones to be detected.
2. Terminology
The "Local Scope" is defined in RFC2365 [1] and represents the
smallest administrative scope larger than link-local, and the
associated address range is defined as 239.255.0.0 to 239.255.255.255
inclusive (for IPv4, FF03::/16 for IPv6). RFC2365 specifies:
"239.255.0.0/16 is defined to be the IPv4 Local Scope. The Local
Scope is the minimal enclosing scope, and hence is not further
divisible. Although the exact extent of a Local Scope is site
dependent, locally scoped regions must obey certain topological
constraints. In particular, a Local Scope must not span any other
scope boundary. Further, a Local Scope must be completely
contained within or equal to any larger scope. In the event that
scope regions overlap in area, the area of overlap must be in its
own Local Scope. This implies that any scope boundary is also a
boundary for the Local Scope."
A multicast scope Zone Boundary Router (ZBR) is a router that is
configured with a boundary for a particular multicast scope on one or
more of its interfaces. Any interface that is configured with a
boundary for any administrative scope zone MUST also have a boundary
for the Local Scope zone, as described above.
Such routers SHOULD be configured so that the router itself is within
the scope zone. This is shown in Figure 1(a), where router A is
inside the scope zone and has the boundary configuration.
............ ................
. . +B+--> . *B+-->
. . / . / .
. * . + .
. <---+A*---+C+-> . <---+A+---*C+->
. + . . + .
. / . . / .
. zone X <-- . . zone X <-- .
.............. ..................
A,B,C - routers * - boundary interface + - interface
(a) Correct zone boundary (b) Incorrect zone boundary
Figure 1: Administrative scope zone boundary placement
It is possible for the first router outside the scope zone to be
configured with the boundary, as illustrated in Figure 1(b) where
routers B and C are outside the zone and have the boundary
configuration, whereas A does not, but this is NOT RECOMMENDED. This
rule does not apply for Local Scope boundaries, but applies for all
other boundary routers.
We next define the term "Zone ID" to mean the lowest IP address used
by any ZBR for a particular zone for sourcing MZAP messages into that
scope zone. The combination of this IP address and the first
multicast address in the scope range serve to uniquely identify the
scope zone. Each ZBR listens for messages from other ZBRs for the
same boundary, and can determine the Zone ID based on the source
addresses seen. The Zone ID may change over time as ZBRs come up and
down.
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 [2].
Constants used by this protocol are shown as [NAME-OF-CONSTANT], and
summarized in section 7.
3. Overview
When a ZBR is configured correctly, it can deduce which side of the
boundary is inside the scope zone and which side is outside it.
Such a ZBR then sends periodic Zone Announcement Messages (ZAMs) for
each zone for which it is configured as a boundary into that scope
zone, containing information on the scope zone's address range, Zone
ID, and textual names. These messages are multicast to the well-
known address [MZAP-LOCAL-GROUP] in the Local Scope, and are relayed
across Local Scope boundaries into all Local Scope zones within the
scope zone referred to by the ZAM message, as shown in Figure 2.
###########################
# Zone1 = Zone2 # ##### = large scope zone boundary
*E-----+--->A*-----+-x #
# = v # ===== = Local Scope boundaries
# ======*===*==#
# = B F # ----> = path of ZAM originated by E
G*<-----+--->C*-> ^ #
# v = <-+---+ # ABCDE = ZBRs
# D = Zone3 #
#######*################### * = boundary interface
Figure 2: ZAM Flooding Example
Any entity can thus listen on a single well-known group address and
learn about all scopes in which it resides.
3.1. Scope Nesting
MZAP also provides the ability to discover the nesting relationships
between scope zones. Two zones are nested if one is comprised of a
subset of the routers in the other, as shown in Figure 3.
+-----------+ +-----------+ +-------------+
Zone 1 Zone 3 Zone 5
+------+ +------+ ...........
Zone 2 Zone 4 : Zone 6 :
+--A---+ C D :
+-----------+ +----+--B---+ +--------E----+ :
:..........:
(a) "Contained" (b) "Common Border" (c) "Overlap"
Zone 2 nests Zone 4 nests Zones 5 and 6
inside Zone 1 inside Zone 3 do not nest
Figure 3: Zone nesting examples
A ZBR cannot independently determine whether one zone is nested
inside another. However, it can determine that one zone does NOT
nest inside another. For example, in Figure 3:
o ZBR A will pass ZAMs for zone 1 but will prevent ZAMs from zone 2
from leaving zone 2. When ZBR A first receives a ZAM for zone 1,
it then knows that zone 1 does not nest within zone 2, but it
cannot, however, determine whether zone 2 nests within zone 1.
o ZBR B acts as ZBR for both zones 3 and 4, and hence cannot
determine if one is nested inside the other. However, ZBR C can
determine that zone 3 does not nest inside zone 4 when it receives
a ZAM for zone 3, since it is a ZBR for zone 4 but not zone 3.
o ZBR D only acts as ZBR zone 6 and not 5, hence ZBR D can deduce
that zone 5 does not nest inside zone 6 upon hearing a ZAM for
zone 5. Similarly, ZBR E only acts as ZBR zone 5 and not 6, hence
ZBR E can deduce that zone 6 does not nest inside zone 5 upon
hearing a ZAM for zone 6.
The fact that ZBRs can determine that one zone does not nest inside
another, but not that a zone does nest inside another, means that
nesting must be determined in a distributed fashion. This is done by
sending Not-Inside Messages (NIMs) which express the fact that a zone
X is not inside a zone Y. Such messages are sent to the well-known
[MZAP-LOCAL-GROUP] and are thus seen by the same entities listening
to ZAM messages (e.g., MADCAP servers). Such entities can then
determine the nesting relationship between two scopes based on a
sustained absence of any evidence to the contrary.
3.2. Other Messages
Two other message types, Zone Convexity Messages (ZCMs) and Zone
Limit Exceeded (ZLE) messages, are used only by routers, and enable
them to compare their configurations for consistency and detect
misconfigurations. These messages are sent to MZAP's relative
address within the scope range associated with the scope zone to
which they refer, and hence are typically not seen by entities other
than routers. Their use in detecting specific misconfiguration
scenarios will be covered in the next section.
Packet formats for all messages are described in Section 5.
3.3. Zone IDs
When a boundary router first starts up, it uses its lowest IP address
which it considers to be inside a given zone, and which is routable
everywhere within the zone (for example, not a link-local address),
as the Zone ID for that zone. It then schedules ZCM (and ZAM)
messages to be sent in the future (it does not send them
immediately). When a ZCM is received for the given scope, the sender
is added to the local list of ZBRs (including itself) for that scope,
and the Zone ID is updated to be the lowest IP address in the list.
Entries in the list are eventually timed out if no further messages
are received from that ZBR, such that the Zone ID will converge to
the lowest address of any active ZBR for the scope.
Note that the sender of ZAM messages MUST NOT be used in this way.
This is because the procedure for detecting a leaky Local scope
described in Section 4.3 below relies on two disjoint zones for the
same scope range having different Zone IDs. If ZAMs are used to
compute Zone IDs, then ZAMs leaking across a Local Scope boundary
will cause the two zones to converge to the same Zone ID.
4. Detecting Router Misconfigurations
In this section, we cover how to detect various error conditions. If
any error is detected, the router should attempt to alert a network
administrator to the nature of the misconfiguration. The means to do
this lies outside the scope of MZAP.
4.1. Detecting non-convex scope zones
Zone Convexity Messages (ZCMs) are used by routers to detect non-
convex administrative scope zones, which are one possible
misconfiguration. Non-convex scope zones can cause problems for
applications since a receiver may never see administratively-scoped
packets from a sender within the same scope zone, since packets
travelling between them may be dropped at the boundary.
In the example illustrated in Figure 4, the path between B and D goes
outside the scope (through A and E). Here, Router B and Router C
send ZCMs within a given scope zone for which they each have a
boundary, with each reporting the other boundary routers of the zone
from which they have heard. In Figure 4, Router D cannot see Router
B's messages, but can see C's report of B, and so can conclude the
zone is not convex.
#####*####========
# B # = ##### = non-convex scope boundary
# ->A* =
# # = ===== = other scope boundaries
# ####*####
# E # ----> = path of B's ZCM
# v D*
# C # * = boundary interface
#####*############
Figure 4: Non-convexity detection
Non-convex scope zones can be detected via three methods:
(1) If a ZBR is listed in ZCMs received, but the next-hop interface
(according to the multicast RIB) towards that ZBR is outside the
scope zone,
(2) If a ZBR is listed in ZCMs received, but no ZCM is received from
that ZBR for [ZCM-HOLDTIME] seconds, as illustrated in Figure 4,
or
(3) ZAM messages can also be used in a manner similar to that for
ZCMs in (1) above, as follows: if a ZAM is received from a ZBR on
an interface inside a given scope zone, and the next-hop
interface (according to the multicast RIB) towards that ZBR is
outside the scope zone.
Zone Convexity Messages MAY also be sent and received by correctly
configured ordinary hosts within a scope region, which may be a
useful diagnostic facility that does not require privileged Access.
4.2. Detecting leaky boundaries for non-local scopes
A "leaky" boundary is one which logically has a "hole" due to some
router not having a boundary applied on an interface where one ought
to exist. Hence, the boundary does not completely surround a piece
of the network, resulting in scoped data leaking outside.
Leaky scope boundaries can be detected via two methods:
(1) If it receives ZAMs originating inside the scope boundary on an
interface that points outside the zone boundary. Such a ZAM
message must have escaped the zone through a leak, and flooded
back around behind the boundary. This is illustrated in Figure
5.
=============#####*########
= Zone1 # A Zone2 # C = misconfigured router
= +---->*E v #
= # B # ##### = leaky scope boundary
=======*=====#====*=======#
= D # # ===== = other scope boundaries
= ^-----*C<--+ #
= Zone4 # Zone3 # ----> = path of ZAMs
=============##############
Figure 5: ZAM Leaking
(2) If a Zone Length Exceeded (ZLE) message is received. The ZAM
packet also contains a Zones Traveled Limit. If the number of
Local Scope zones traversed becomes equal to the Zones Traveled
Limit, a ZLE message is generated (the suppression mechanism for
preventing implosion is described later in the Processing Rules
section). ZLEs detect leaks where packets do not return to
another part of the same scope zone, but instead reach other
Local Scope zones far away from the ZAM originator.
In either case, the misconfigured router will be either the message
origin, or one of the routers in the ZBR path list which is included
in the message received (or perhaps a router on the path between two
such ZBRs which ought to have been a ZBR itself).
4.3. Detecting a leaky Local Scope zone
A local scope is leaky if a router has an administrative scope
boundary on some interface, but does not have a Local Scope boundary
on that interface as specified in RFC2365. This can be detected via
the following method:
o If a ZAM for a given scope is received by a ZBR which is a
boundary for that scope, it compares the Origin's Scope Zone ID in
the ZAM with its own Zone ID for the given scope. If the two do
not match, this is evidence of a misconfiguration. Since a
temporary mismatch may result immediately after a recent change in
the reachability of the lowest-addressed ZBR, misconfiguration
should be assumed only if the mismatch is persistent.
The exact location of the problem can be found by doing an mtrace [5]
from the router detecting the problem, back to the ZAM origin, for
any group within the address range identified by the ZAM. The router
at fault will be the one reporting that a boundary was reached.
4.4. Detecting conflicting scope zones
Conflicting address ranges can be detected via the following method:
o If a ZBR receives a ZAM for a given scope, and the included start
and end addresses overlap with, but are not identical to, the
start and end addresses of a locally-configured scope.
Conflicting scope names can be detected via the following method:
o If a ZBR is configured with a textual name for a given scope and
language, and it receives a ZAM or ZCM with a name for the same
scope and language, but the scope names do not match.
Detecting either type of conflict above indicates that either the
local router or the router originating the message is misconfigured.
Configuration tools SHOULD strip white space from the beginning and
end of each name to avoid accidental misconfiguration.
5. Packet Formats
All MZAP messages are sent over UDP, with a destination port of
[MZAP-PORT] and an IPv4 TTL or IPv6 Hop Limit of 255.
When sending an MZAP message referring to a given scope zone, a ZBR
MUST use a source address which will have significance everywhere
within the scope zone to which the message refers. For example,
link-local addresses MUST NOT be used.
The common MZAP message header (which follows the UDP header), is
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version B PTYPE Address Family NameCount
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Message Origin
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Zone ID Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Zone Start Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Zone End Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Encoded Zone Name-1 (variable length)
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. . . Encoded Zone Name-N (variable length)
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Padding (if needed)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version:
The version defined in this document is version 0.
"Big" scope bit (B):
If clear, indicates that the addresses in the scoped range are not
subdividable, and that address allocators may utilize the entire
range. If set, address allocators should not use the entire
range, but should learn an appropriate sub-range via another
mechanism (e.g., AAP [7]).
Packet Type (PTYPE):
The packet types defined in this document are:
0: Zone Announcement Message (ZAM)
1: Zone Limit Exceeded (ZLE)
2: Zone Convexity Message (ZCM)
3: Not-Inside Message (NIM)
Address Family:
The IANA-assigned address family number [10,11] identifying the
address family for all addresses in the packet. The families
defined for IP are:
1: IPv4
2: IPv6
Name Count:
The number of encoded zone name blocks in this packet. The count
may be zero.
Zone Start Address: 32 bits (IPv4) or 128 bits (IPv6)
This gives the start address for the scope zone boundary. For
example, if the zone is a boundary for 239.1.0.0 to 239.1.0.255,
then Zone Start Address is 239.1.0.0.
Zone End Address: 32 bits (IPv4) or 128 bits (IPv6)
This gives the ending address for the scope zone boundary. For
example, if the zone is a boundary for 239.1.0.0 to 239.1.0.255,
then Zone End Address is 239.1.0.255.
Message Origin: 32 bits (IPv4) or 128 bits (IPv6)
This gives the IP address of the interface that originated the
message.
Zone ID Address: 32 bits (IPv4) or 128 bits (IPv6)
This gives the lowest IP address of a boundary router that has
been observed in the zone originating the message. Together with
Zone Start Address and Zone End Address, it forms a unique ID for
the zone. Note that this ID is usually different from the ID of
the Local Scope zone in which the origin resides.
Encoded Zone Name:
+--------------------+
D Reserved (7 bits)
+--------------------+
LangLen (1 byte)
+--------------------+-----------+
Language Tag (variable size)
+--------------------+-----------+
NameLen (1 byte)
+--------------------+-----------+
Zone Name (variable size)
+--------------------------------+
The first byte contains flags, of which only the high bit is
defined. The other bits are reserved (sent as 0, ignored on
receipt).
"Default Language" (D) bit:
If set, indicates a preference that the name in the following
language should be used if no name is available in a desired
language.
Language tag length (LangLen): 1 byte
The length, in bytes, of the language tag.
Language Tag: (variable size)
The language tag, such as "en-US", indicating the language of the
zone name. Language tags are described in [6].
Name Len:
The length, in bytes, of the Zone Name field. The length MUST NOT
be zero.
Zone Name: multiple of 8 bits
The Zone Name is an ISO 10646 character string in UTF-8 encoding
[4] indicating the name given to the scope zone (eg: "ISI-West
Site"). It should be relatively short and MUST be less than 256
bytes in length. White space SHOULD be stripped from the
beginning and end of each name before encoding, to avoid
accidental conflicts.
Padding (if needed):
The end of the MZAP header is padded with null bytes until it is
4-byte aligned.
5.1. Zone Announcement Message
A Zone Announcement Message has PTYPE=0, and is periodically sent by
a ZBR for each scope for which it is a boundary, EXCEPT:
o the Local Scope
o the Link-local scope
The format of a Zone Announcement Message is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MZAP Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ZT ZTL Hold Time
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Local Zone ID Address 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Router Address 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Local Zone ID Address 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Router Address N
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Local Zone ID Address N
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are defined as follows:
Zones Traveled (ZT): 8 bits
This gives the number of Local Zone IDs contained in this message
path.
Zones Traveled Limit (ZTL): 8 bits
This gives the limit on number of local zones that the packet can
traverse before it MUST be dropped. A value of 0 indicates that
no limit exists.
Hold Time:
The time, in seconds, after which the receiver should assume the
scope no longer exists, if no subsequent ZAM is received. This
should be set to [ZAM-HOLDTIME].
Zone Path: multiple of 64 bits (IPv4) or 256 bits (IPv6)
The zone path is a list of Local Zone ID Addresses (the Zone ID
Address of a local zone) through which the ZAM has passed, and IP
addresses of the router that forwarded the packet. The origin
router fills in the "Local Zone ID Address 0" field when sending
the ZAM. Every Local Scope router that forwards the ZAM across a
Local Scope boundary MUST add the Local Zone ID Address of the
local zone that the packet of the zone into which the message is
being forwarded, and its own IP address to the end of this list,
and increment ZT accordingly. The zone path is empty which the
ZAM is first sent.
5.2. Zone Limit Exceeded (ZLE)
The format of a ZLE is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MZAP Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ZT ZTL Hold Time
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Local Zone ID Address 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Router Address 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Local Zone ID Address 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Router Address N
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Local Zone ID Address N
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
All fields are copied from the ZAM, except PTYPE which is set to one.
5.3. Zone Convexity Message
A Zone Announcement Message has PTYPE=2, and is periodically sent by
a ZBR for each scope for which it is a boundary (except the Link-
local scope). Note that ZCM's ARE sent in the Local Scope.
Unlike Zone Announcement Messages which are sent to the [MZAP-LOCAL-
GROUP], Zone Convexity Messages are sent to the [ZCM-RELATIVE-GROUP]
in the scope zone itself. The format of a ZCM is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MZAP Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ZNUM unused Hold Time
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ZBR Address 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
ZBR Address N
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are as follows:
Number of ZBR addresses (ZNUM): 8 bits
This field gives the number of ZBR Addresses contained in this
message.
Hold Time:
The time, in seconds, after which the receiver should assume the
sender is no longer reachable, if no subsequent ZCM is received.
This should be set to [ZCM-HOLDTIME].
ZBR Address: 32 bits (IPv4) or 128 bits (IPv6)
These fields give the addresses of the other ZBRs from which the
Message Origin ZBR has received ZCMs but whose hold time has not
expired. The router should include all such addresses which fit
in the packet, preferring those which it has not included recently
if all do not fit.
5.4. Not-Inside Message
A Not-Inside Message (NIM) has PTYPE=3, and is periodically sent by a
ZBR which knows that a scope X does not nest within another scope Y
("X not inside Y"):
The format of a Not-Inside Message is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MZAP Header
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Not-Inside Zone Start Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are as follows:
MZAP Header: Header fields identifying the scope X. The Name Count
may be 0.
Not-Inside Zone Start Address: 32 bits (IPv4) or 128 bits (IPv6) This
gives the start address for the scope Y.
6. Message Processing Rules
6.1. Internal entities listening to MZAP messages
Any host or application may join the [MZAP-LOCAL-GROUP] to listen for
Zone Announcement Messages to build up a list of the scope zones that
are relevant locally, and for Not-Inside Messages if it wishes to
learn nesting information. However, listening to such messages is
not the recommended method for regular applications to discover this
information. These applications will normally query a local
Multicast Address Allocation Server (MAAS) [3], which in turn listens
to Zone Announcement Messages and Not-Inside Messages to maintain
scope information, and can be queried by clients via MADCAP messages.
An entity (including a MAAS) lacking any such information can only
assume that it is within the Global Scope, and the Local Scope, both
of which have well-known address ranges defined in [1].
An internal entity (e.g., an MAAS) receiving a ZAM will parse the
information that is relevant to it, such as the address range, and
the names. An address allocator receiving such information MUST also
use the "B" bit to determine whether it can add the address range to
the set of ranges from which it may allocate addresses (specifically,
it may add them only if the bit is zero). Even if the bit is zero,
an MAAS SHOULD still store the range information so that clients who
use relative- addresses can still oBTain the ranges by requesting
them from the MAAS.
An internal entity (e.g., an MAAS) should assume that X nests within
Y if:
a) it first heard ZAMs for both X and Y at least [NIM-HOLDTIME]
seconds ago, AND
b) it has not heard a NIM indicating that "X not inside Y" for at
least [NIM-HOLDTIME] seconds.
6.2. Sending ZAMs
Each ZBR should send a Zone Announcement Message for each scope zone
for which it is a boundary every [ZAM-INTERVAL] seconds, +/- 30% of
[ZAM-INTERVAL] each time to avoid message synchronisation.
The ZAM packet also contains a Zones Traveled Limit (ZTL). If the
number of Local Zone IDs in the ZAM path becomes equal to the Zones
Traveled Limit, the packet will be dropped. The ZTL field is set
when the packet is first sent, and defaults to 32, but can be set to
a lower value if a network administrator knows the expected size of
the zone.
6.3. Receiving ZAMs
When a ZBR receives a ZAM for some scope zone X, it uses the
following rules.
If the local ZBR does NOT have any configuration for scope X:
(1) Check to see if the included start and end addresses overlap
with, but are not identical to, the start and end addresses of
any locally-configured scope Y, and if so, signal an address
range conflict to a local administrator.
(2) Create a local "X not inside" state entry, if such an entry does
not already exist. The ZBR then restarts the entry's timer at
[ZAM-HOLDTIME]. Existence of this state indicates that the ZBR
knows that X does not nest inside any scope for which it is a
boundary. If the entry's timer expires (because no more ZAMs for
X are heard for [ZAM-HOLDTIME]), the entry is deleted.
If the local ZBR does have configuration for scope X:
(1) If the ZAM originated from OUTSIDE the scope (i.e., received over
a boundary interface for scope X):
a) If the Scope Zone ID in the ZAM matches the ZBR's own Scope
Zone ID, then signal a leaky scope misconfiguration.
b) Drop the ZAM (perform no further processing below). For
example, router G in Figure 2 will not forward the ZAM. This
rule is primarily a safety measure, since the placement of G in
Figure 2 is not a recommended configuration, as discussed
earlier.
2) If the ZAM originated from INSIDE the scope:
a) If the next-hop interface (according to the multicast RIB)
towards the Origin is outside the scope zone, then signal a
non- convexity problem.
b) If the Origin's Scope Zone ID in the ZAM does not match the
Scope Zone ID kept by the local ZBR, and this mismatch
continues to occur, then signal a possible leaky scope warning.
c) For each textual name in the ZAM, see if a name for the same
scope and language is locally-configured; if so, but the scope
names do not match, signal a scope name conflict to a local
administrator.
d) If the ZAM was received on an interface which is NOT a Local
Scope boundary, and the last Local Zone ID Address in the path
list is 0, the ZBR fills in the Local Zone ID Address of the
local zone from which the ZAM was received.
If a ZAM for the same scope (as identified by the origin Zone ID and
first multicast address) was received in the last [ZAM-DUP-TIME]
seconds, the ZAM is then discarded. Otherwise, the ZAM is cached for
at least [ZAM-DUP-TIME] seconds. For example, when router C in
Figure 2 receives the ZAM via B, it will not be forwarded, since it
has just forwarded the ZAM from E.
The Zones Travelled count in the message is then incremented, and if
the updated count is equal to or greater than the ZTL field, schedule
a ZLE to be sent as described in the next subsection and perform no
further processing below.
If the Zone ID of the Local Scope zone in which the ZBR resides is
not already in the ZAM's path list, then the ZAM is immediately re-
originated within the Local Scope zone. It adds its own address and
the Zone ID of the Local Scope zone into which the message is being
forwarded to the ZAM path list before doing so. A ZBR receiving a
ZAM with a non-null path list MUST NOT forward that ZAM back into a
Local Scope zone that is contained in the path list. For example, in
Figure 2, router F, which did not get the ZAM via A due to packet
loss, will not forward the ZAM from B back into Zone 2 since the path
list has { (E,1), (A,2), (B,3) } and hence Zone 2 already appears.
In addition, the ZBR re-originates the ZAM out each interface with a
Local Scope boundary (except that it is not sent back out the
interface over which it was received, nor is it sent into any local
scope zone whose ID is known and appears in the path list). In each
such ZAM re-originated, the ZBR adds its own IP address to the path
list, as well as the Zone ID Address of the Local Scope Zone into
which the ZAM is being sent, or 0 if the ID is unknown. (For
example, if the other end of a point-to-point link also has a
boundary on the interface, then the link has no Local Scope Zone ID.)
6.4. Sending ZLEs
This packet is sent by a local-zone boundary router that would have
exceeded the Zone Traveled Limit if it had forwarded a ZAM packet.
To avoid ZLE implosion, ZLEs are multicast with a random delay and
suppressed by other ZLEs. It is only scheduled if at least [ZLE-
MIN-INTERVAL] seconds have elapsed since it previously sent a ZLE to
any destination. To schedule a ZLE, the router sets a random delay
timer within the interval [ZLE-SUPPRESSION-INTERVAL], and listens to
the [MZAP-RELATIVE-GROUP] within the included scope for other ZLEs.
If any are received before the random delay timer expires, the timer
is cleared and the ZLE is not sent. If the timer expires, the router
sends a ZLE to the [MZAP-RELATIVE-GROUP] within the indicated scope.
The method used to choose a random delay (T) is as follows:
Choose a random value X from the uniform random interval [0:1]
Let C = 256
Set T = [ZLE-SUPPRESSION-INTERVAL] log( C*X + 1) / log(C)
This equation results in an exponential random distribution which
ensures that close to one ZBR will respond. Using a purely uniform
distribution would begin to exhibit scaling problems as the number of
ZBRs rose. Since ZLEs are only suppressed if a duplicate ZLE arrives
before the time chosen, two routers choosing delays which differ by
an amount less than the propagation delay between them will both send
messages, consuming excess bandwidth. Hence it is desirable to
minimize the number of routers choosing a delay close to the lowest
delay chosen, and an exponential distribution is suitable for this
purpose.
A router SHOULD NOT send more than one Zone Limit Exceeded message
every [ZLE-MIN-INTERVAL] regardless of destination.
6.5. Receiving ZLEs
When a router receives a ZLE, it performs the following actions:
(1) If the router has a duplicate ZLE message scheduled to be sent,
it unschedules its own message so another one will not be sent.
(2) If the ZLE contains the router's own address in the Origin field,
it signals a leaky scope misconfiguration.
6.6. Sending ZCMs
Each ZBR should send a Zone Convexity Message (ZCM) for each scope
zone for which it is a boundary every [ZCM-INTERVAL] seconds, +/- 30%
of [ZCM-INTERVAL] each time to avoid message synchronisation.
ZCMs are sent to the [ZCM-RELATIVE-GROUP] in the scoped range itself.
(For example, if the scope range is 239.1.0.0 to 239.1.0.255, then
these messages should be sent to 239.1.0.252.) As these are not
Locally-Scoped packets, they are simply multicast across the scope
zone itself, and require no path to be built up, nor any special
processing by intermediate Local Scope ZBRs.
6.7. Receiving ZCMs
When a ZCM is received for a given scope X, on an interface which is
inside the scope, it follows the rules below:
(1) The Origin is added to the local list of ZBRs (including itself)
for that scope, and the Zone ID is updated to be the lowest IP
address in the list. The new entry is scheduled to be timed out
after [ZCM-HOLDTIME] if no further messages are received from
that ZBR, so that the Zone ID will converge to the lowest address
of any active ZBR for the scope.
(2) If a ZBR is listed in ZCMs received, but the next-hop interface
(according to the multicast RIB) towards that ZBR is outside the
scope zone, or if no ZCM is received from that ZBR for [ZCM-
HOLDTIME] seconds, as in the example in Figure 4, then signal a
non-convexity problem.
(3) For each textual name in the ZCM, see if a name for the same
scope and language is locally-configured; if so, but the scope
names do not match, signal a scope name conflict to a local
administrator.
6.8. Sending NIMs
Periodically, for each scope zone Y for which it is a boundary, a
router originates a Not-Inside Message (NIM) for each "X not inside"
entry it has created when receiving ZAMs. Like a ZAM, this message
is multicast to the address [MZAP-LOCAL-GROUP] from one of its
interfaces inside Y.
Each ZBR should send such a Not-Inside Message every [NIM-INTERVAL]
seconds, +/- 30% of [NIM-INTERVAL] to avoid message synchronization.
6.9. Receiving NIMs
When a ZBR receives a NIM saying that "X is not inside Y", it is
forwarded, unmodified, in a manner similar to ZAMs:
(1) If the NIM was received on an interface with a boundary for
either X or Y, the NIM is discarded.
(2) Unlike ZAMs, if the NIM was not received on the interface towards
the message origin (according to the Multicast RIB), the NIM is
discarded.
(3) If a NIM for the same X and Y (where each is identified by its
first multicast address) was received in the last [ZAM-DUP-TIME]
seconds, the NIM is not forwarded.
(4) Otherwise, the NIM is cached for at least [ZAM-DUP-TIME] seconds.
(5) The ZBR then re-originates the NIM (i.e., with the original UDP
payload) into each local scope zone in which it has interfaces,
except that it is not sent back into the local scope zone from
which the message was received, nor is it sent out any interface
with a boundary for either X or Y.
7. Constants
[MZAP-PORT]: The well-known UDP port to which all MZAP messages are
sent. Value: 2106.
[MZAP-LOCAL-GROUP]: The well-known group in the Local Scope to which
ZAMs are sent. All Multicast Address Allocation servers and Zone
Boundary Routers listen to this group. Value: 239.255.255.252 for
IPv4.
[ZCM-RELATIVE-GROUP]: The relative group in each scope zone, to
which ZCMs are sent. A Zone Boundary Router listens to the relative
group in each scope for which it is a boundary. Value: (last IP
address in scope range) - 3. For example, in the Local Scope, the
relative group is the same as the [MZAP-LOCAL-GROUP] address.
[ZAM-INTERVAL]: The interval at which a Zone Boundary Router
originates Zone Announcement Messages. Default value: 600 seconds
(10 minutes).
[ZAM-HOLDTIME]: The holdtime to include in a ZAM. This SHOULD be
set to at least 3 * [ZAM-INTERVAL]. Default value: 1860 seconds (31
minutes).
[ZAM-DUP-TIME]: The time interval after forwarding a ZAM, during
which ZAMs for the same scope will not be forwarded. Default value:
30 seconds.
[ZCM-INTERVAL]: The interval at which a Zone Boundary Router
originates Zone Convexity Messages. Default value: 600 seconds (10
minutes).
[ZCM-HOLDTIME]: The holdtime to include in a ZCM. This SHOULD be
set to at least 3 * [ZCM-INTERVAL]. Default value: 1860 seconds (31
minutes).
[ZLE-SUPPRESSION-INTERVAL]: The interval over which to choose a
random delay before sending a ZLE message. Default value: 300
seconds (5 minutes).
[ZLE-MIN-INTERVAL]: The minimum interval between sending ZLE
messages, regardless of destination. Default value: 300 seconds (5
minutes).
[NIM-INTERVAL]: The interval at which a Zone Boundary Router
originates Not-Inside Messages. Default value: 1800 seconds (30
minutes).
[NIM-HOLDTIME]: The holdtime to include the state within a NIM.
This SHOULD be set to at least 3 * [NIM-INTERVAL]. Default value:
5460 (91 minutes)
8. Security Considerations
While unauthorized reading of MZAP messages is relatively innocuous
(so encryption is generally not an issue), accepting unauthenticated
MZAP messages can be problematic. Authentication of MZAP messages
can be provided by using the IPsec Authentication Header (AH) [12].
In the case of ZCMs and ZLEs, an attacker can cause false logging of
convexity and leakage problems. It is likely that is would be purely
an annoyance, and not cause any significant problem. (Such messages
could be authenticated, but since they may be sent within large
scopes, the receiver may not be able to authenticate a non-malicious
sender.)
ZAMs and NIMs, on the other hand, are sent within the Local Scope,
where assuming a security relationship between senders and receivers
is more practical.
In the case of NIMs, accepting unauthenticated messages can cause the
false cancellation of nesting relationships. This would cause a
section of the hierarchy of zones to flatten. Such a flattening
would lessen the efficiency benefits afforded by the hierarchy but
would not cause it to become unusable.
Accepting unauthenticated ZAM messages, however, could cause
applications to believe that a scope zone exists when it does not.
If these were believed, then applications may choose to use this
non-existent administrative scope for their uses. Such applications
would be able to communicate successfully, but would be unaware that
their traffic may be traveling further than they expected. As a
result, any application accepting unauthenticated ZAMs MUST only take
scope names as a guideline, and SHOULD assume that their traffic sent
to non-local scope zones might travel anywhere. The confidentiality
of such traffic CANNOT be assumed from the fact that it was sent to a
scoped address that was discovered using MZAP.
In addition, ZAMs are used to inform Multicast Address Allocation
Servers (MAASs) of names and address ranges of scopes, and accepting
unauthenticated ZAMs could result in false names being presented to
users, and in wrong addresses being allocated to users. To counter
this, MAAS's authenticate ZAMs as follows:
(1) A ZBR signs all ZAMs it originates (using an AH).
(2) A ZBR signs a ZAM it relays if and only if it can authenticate
the previous sender. A ZBR MUST still forward un-authenticated
ZAMs (to provide leak detection), but should propagate an
authenticated ZAM even if an un-authenticated one was received
with the last [ZAM-DUP-TIME] seconds.
(3) A MAAS SHOULD be configured with the public key of the local zone
in which it resides. A MAAS thus configured SHOULD ignore an
unauthenticated ZAM if an authenticated one for the same scope
has been received, and MAY ignore all unauthenticated ZAMs.
9. Acknowledgements
This document is a product of the MBone Deployment Working Group,
whose members provided many helpful comments and suggestions, Van
Jacobson provided some of the original ideas that led to this
protocol. The Multicast Address Allocation Working Group also
provided useful feedback regarding scope names and interactions with
applications.
10. References
[1] Meyer, D., "Administratively Scoped IP Multicast", BCP 23, RFC
2365, July 1998.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
[3] Thaler, D., Handley, M. and D. Estrin, "The Internet Multicast
Address Allocation Architecture", Work in Progress.
[4] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.
[5] Fenner, W. and S. Casner, "A `traceroute' facility for IP
Multicast", Work in Progress.
[6] Alvestrand, H., "Tags for the Identification of Languages", RFC
1766, March 1995.
[7] Handley, M. and S. Hanna. "Multicast Address Allocation
Protocol (AAP)", Work in Progress.
[8] Kermode, R. "Scoped Hybrid Automatic Repeat reQuest with Forward
Error Correction (SHARQFEC)", ACM SIGCOMM 98, September 1998,
Vancouver, Canada.
[9] Hanna, S., Patel, B., and M. Shah. "Multicast Address Dynamic
Client Allocation Protocol (MADCAP)", RFC2730, December 1999.
[10] Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC1700,
October 1994.
[11] IANA, "Address Family Numbers", http://www.isi.edu/in-
notes/iana/assignments/address-family-numbers
[12] Kent, S. and R. Atkinson, "IP Authentication Header", RFC2402,
November 1998.
11. Authors' Addresses
Mark Handley
AT&T Center for Internet Research at ICSI
1947 Center St, Suite 600
Berkely, CA 94704
USA
EMail: mjh@aciri.org
David Thaler
Microsoft
One Microsoft Way
Redmond, WA 98052
USA
EMail: dthaler@microsoft.com
Roger Kermode
Motorola Australian Research Centre
12 Lord St,
Botany, NSW 2019
Australia
EMail: Roger.Kermode@motorola.com
12. Full Copyright Statement
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included on all such copies and derivative works. However, this
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