wdiff draft-ietf-msdp-spec-08.txt draft-ietf-msdp-spec-09.txt

Network Working Group David Meyer (Editor)
INTERNET DRAFT Bill Fenner (Editor)
Category Standards Track
April,
May, 2001

Multicast Source Discovery Protocol (MSDP)
<draft-ietf-msdp-spec-08.txt>
<draft-ietf-msdp-spec-09.txt>

1. Status of this Memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.

Internet Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

2. Abstract

The Multicast Source Discovery Protocol, MSDP, describes a mechanism
to connect multiple PIM-SM domains together. Each PIM-SM domain uses
its own independent RP(s) and does not have to depend on RPs in other
domains.

3. Copyright Notice

4. Introduction

o No Third-party resource dependencies on RP

PIM-SM domains can rely on their own RPs only.

o Receiver only Domains

Domains with only receivers get data without globally
advertising group membership.

Note that MSDP may be used with protocols other than PIM-SM, but such
usage is not specified in this memo.

The keywords MUST, MUST NOT, MAY, OPTIONAL, REQUIRED, RECOMMENDED,
SHALL, SHALL NOT, SHOULD, SHOULD NOT are to be interpreted as defined
in RFC 2119 [RFC2119].

5. Overview

MSDP-speaking routers in a PIM-SM [RFC2362] domain will have a MSDP
peering relationship with MSDP peers in another domain. The peering
relationship will be is made up of a TCP connection in which control
information is exchanged. Each domain will have has one or more connections to
this virtual topology.

The purpose of this topology is to allow domains to discover
multicast sources from other domains. If the multicast sources are of
interest to a domain which has receivers, the normal source-tree
building mechanism in PIM-SM will be used to deliver multicast data
over an inter-domain distribution tree.

We envision this virtual topology will essentially be congruent to
the existing BGP topology used in the unicast-based Internet today.
That is, the TCP connections between MSDP peers are likely to be
congruent to the connections in the BGP routing system.

6. Procedure

A source

When an RP in a PIM-SM domain originates traffic to a multicast group.
The PIM DR which is directly connected to the source sends the data
encapsulated in first learns of a new sender, e.g. via
PIM Register message to the RP in the domain.

The RP will construct register messages, it constructs a "Source-Active" (SA) message
and send sends it to its MSDP peers. The SA message contains the following
fields:

o Source address of the data source.
o Group address the data source sends to.
o IP address of the RP.

Each MSDP peer receives and forwards the message away from the RP
address in a "peer-RPF flooding" fashion. The notion of peer-RPF
flooding is with respect to forwarding SA messages. The BGP routing
table Multicast RPF
Routing Information Base (MRIB) is examined to determine which peer is the NEXT_HOP
towards the originating RP of the SA message. message is selected. Such a peer
is called an "RPF peer". See section 14 below for the details of
peer-RPF forwarding.

If the MSDP peer receives the SA from a non-RPF peer towards the
originating RP, it will drop the message. Otherwise, it forwards the
message to all its MSDP peers (except the one from which it received
the SA message).

The flooding can be further constrained to children of the peer by
interrogating BGP reachability information. That is, if a BGP peer
advertises a route (back to you) and you are the next to last AS in
the AS_PATH, the peer is using you as the NEXT_HOP. This is known in
other circles as Split-Horizon with Poison Reverse. An implementation
SHOULD NOT forward SA messages (which were originated from the RP
address covered by a route) to peers which have not Poison Reversed
that route.

When an MSDP peer which is also an RP for its own domain receives a
new SA message, it determines if it has any group members interested
in the group which the SA message describes. That is, the RP checks
for a (*,G) entry with a non-empty outgoing interface list; this
implies that the domain is interested in the group. In this case, the
RP triggers a (S,G) join event towards the data source as if a
Join/Prune message was received addressed to the RP itself. This sets
up a branch of the source-tree to this domain. Subsequent data
packets arrive at the RP which are forwarded down the shared-tree
inside the domain. If leaf routers choose to join the source-tree
they have the option to do so according to existing PIM-SM
conventions. Finally, if an RP in a domain receives a PIM Join
message for a new group G, the RP SHOULD trigger a (S,G) join event
for each SA for that group in its cache.

This procedure has been affectionately named flood-and-join because
if any RP is not interested in the group, they can ignore the SA
message. Otherwise, they join a distribution tree.

7. Caching

A MSDP speaker MUST SHOULD cache SA messages. Caching allows pacing of
MSDP messages as well as reducing join latency for new receivers of a
group G at an orginating originating RP which has existing MSDP (S,G) state. In
addition, caching greatly aids in diagnosis and debugging of various
problems.

8. Timers

The main timers for MSDP are: SA-Advertisement-Timer, SA-Hold-Down-
Timer, SA Cache Entry timer, KeepAlive timer, and ConnectRetry and Peer
Hold Timer. Each is considered below.

8.1. SA-Advertisement-Timer

RPs which originate SA messages do it periodically as long as there
is data being sent by the source. There is one SA-Advertisement-Timer
covering the sources that an RP may advertise. [SA-Advertisement-
Period] MUST be 60 seconds. An RP MUST not send more than one
periodic SA message for a given (S,G) within an SA Advertisement
interval. Originating periodic SA messages is important required to keep
announcements alive in caches, and so that new receivers who join
after a source has been active can get data quickly via the receiver's own a non-caching
RP. Finally, an originating RP SHOULD trigger the transmission of an
SA message as soon as it receives data from an internal source for
the first time.

8.2. SA-Advertisement-Timer Processing

An RP MUST spread the generation of periodic SA messages over its
reporting interval (i.e. SA-Advertisement-Period). An RP starts the
SA-Advertisement-Timer when the MSDP process is configured. When the
timer expires, an RP resets the timer to [SA-Advertisement-Period]
seconds, and begins the advertisement of its active sources. Active
sources are advertised in the following manner: An RP packs its
active sources into an SA message until the largest MSDP packet that
can be sent is built or there are no more sources, and then sends the
message. This process is repeated periodically within the SA-
Advertisement-Period in such a way that all of the RP's sources are
advertised. Note that the largest MSDP packet that can be sent has
size that is the minimum of MTU of outgoing link minus size of TCP
and IP headers, and 1400 (largest MSDP packet). Finally, the timer is
deleted when the MSDP process is deconfigured.

8.3. SA Cache Timeout (SA-State-Timer)

Each entry in an SA Cache has an associated SA-State-Timer. A
(S,G)-SA-State-Timer is started when an (S,G)-SA message is initially
received by a MSDP peer. The timer is reset to [SA-State-Period] if
another (S,G)-SA message is received before the (S,G)-SA-State-Timer
expires. [SA-State-Period] MUST NOT be less than 90 seconds.

8.4. SA-Hold-Down-Timer

The per-(S,G) timer is set to [SA-Hold-Down-Period] when forwarding
an SA message, and a SA message MUST only be forwarded when it's its
associated timer is not running. [SA-Hold-Down-Period] SHOULD be set
to 30 seconds. A MSDP peer MUST NOT forward a (S,G)-SA message it has
received in during the previous [SA-Hold-Down-Period] seconds.
Finally, the timer is deleted when the SA cache entry is deleted.

8.5. KeepAlive Peer Hold Timer

If a system has not received any MSDP message within the period
specified by the Hold Timer, then a Notification message with Hold
Timer Expired Error Code MUST be sent and the MSDP connection MUST be
closed. [Hold-Time-Period] MUST be at least three seconds. A
suggested value for [Hold-Time-Period] is 90 seconds.

The KeepAlive timer contols Hold Timer is initialized to [Hold-Time-Period] when the peer's
transport connection is established, and is reset to send [Hold-Time-
Period] when any MSDP KeepAlive messages. In
particular, message is received. Finally, the KeepAlive timer is used to reset the TCP connection
deleted when the passive-connect peer's transport connection is closed.

8.6. KeepAlive Timer

Once an MSDP transport connection is established, each side of the
connection goes down. sends a KeepAlive message and sets a KeepAlive timer. If
the KeepAlive timer expires, the local system sends a KeepAlive
message and restarts its KeepAlive timer.

The KeepAlive timer is set to [KeepAlive-Period] when the passive-connect peer comes
up. [KeepAlive-Period] SHOULD NOT be less that than 75 seconds. seconds, and MUST
NOT be less than [Hold-Time-Period]. The timer is reset to
[KeepAlive-Period] upon receipt of each time an MSDP message from is sent to peer, and deleted
reset when the timer expires or expires. Finally, the
passive-connect peer closes KeepAlive timer is deleted
when the connection.

8.6. peer's transport connection is closed.

8.7. ConnectRetry Timer

The ConnectRetry timer is used by an MSDP peer to transition from
INACTIVE to CONNECTING states. There is one timer per peer, and the
[ConnectRetry-Period] SHOULD be set to 30 seconds. The timer is
initialized to [ConnectRetry-Period] when an MSDP peer's active
connect attempt fails. When the timer expires, the peer retries the
connection and the timer is reset to [ConnectRetry-Period]. It is
deleted if either the connection transitions into ESTABLISHED state
or the peer is deconfigured.

8.7. Peer Hold Timer

If a system does not receive successive KeepAlive messages (or any SA
message) within the period specified by the Hold Timer, then a
Notification message with Hold Timer Expired Error Code MUST be sent
and the MSDP connection MUST be closed. [Hold-Time-Period] MUST be at
least three seconds. A suggested value for [Hold-Time-Period] is 90
seconds.

The Hold Timer is initialized to [Hold-Time-Period] when the peer's
transport connection is established, and is reset to [Hold-Time-
Period] when any MSDP message is received.

9. Intermediate MSDP Peers

Intermediate RPs MSDP speakers do not originate periodic SA messages on
behalf of sources in other domains. In general, an RP MUST only
originate an SA for a source which would register to it. it, and ONLY RPs
may originate SA messages.

10. SA Filtering and Policy

As the number of (S,G) pairs increases in the Internet, an RP may
want to filter which sources it describes in SA messages. Also,
filtering may be used as a matter of policy which at the same time
can reduce state. Only the RP co-located in the same domain as the
source can restrict SA messages. Note, however, that MSDP peers in
transit domains should not filter SA messages or the flood-and-join
model can not guarantee that sources will be known throughout the
Internet (i.e., SA filtering by transit domains can cause undesired
lack of connectivity). In general, policy should be expressed using
MBGP [RFC2283]. This will cause MSDP messages to flow in the desired
direction and peer-RPF fail otherwise. An exception occurs at an
administrative scope [RFC2365] boundary. In particular, a SA message
for a (S,G) MUST NOT be sent to peers which are on the other side of
an administrative scope boundary for G.

11. SA Requests

A MSDP speaker MAY accept SA-Requests from other MSDP peers. When an
MSDP speaker receives an SA-Request for a group range, it will
respond to the peer with a set of SA entries, in an SA-Response
message, for all active sources in its SA cache sending to the group range
requested in the SA-Request message. The peer that sends the request
will not flood the responding SA-Response message to other peers. See
section 17 for discussion of error handling relating to SA requests
and responses.

12. Encapsulated Data Packets

For bursty sources, the

The RP may encapsulate multicast data from the source. An interested
RP may decapsulate the packet, which SHOULD be forwarded as if a PIM
register encapsulated packet was received. That is, if packets are
already arriving over the interface toward the source, then the
packet is dropped. Otherwise, if the outgoing interface list is non-null, non-
null, the packet is forwarded appropriately. Note that when doing
data encapsulation, an implementation MUST bound the time during
which packets are encapsulated.

This allows for small bursts to be received before the multicast tree
is built back toward the source's domain. For example, an
implementation SHOULD encapsulate at least the first packet to
provide service to bursty sources.

13. Other Scenarios

MSDP is not limited to deployment across different routing domains.
It can be used within a routing domain when it is desired to deploy
multiple RPs for the same group ranges. As long as all RPs have a
interconnected MSDP topology, each can learn about active sources as
well as RPs in other domains.

14. MSDP Peer-RPF Forwarding

The MSDP Peer-RPF Forwarding rules are used for forwarding SA
messages throughout an MSDP enabled internet. Unlike the RPF check
used when forwarding data packets, the Peer-RPF check is against the
RP address carried in the SA message.

14.1. Definitions

The following definitions are used in the description of the Peer-RPF
Forwarding Rules:

14.1.1. Multicast RPF Routing Information Base (MRIB)

The MRIB is the multicast topology table. It is typically derived
from the unicast routing table or from other routing protocols such
as multi-protocol BGP [RFC2283].

14.1.2. RPF Route

The RPF route is the route that the MRIB chooses for a given address.
The RPF route for a SA's originating RP is used to select the peer
from which the SA is accepted.

14.2. Peer-RPF Forwarding Rules

An SA message originated by R and received by X from N is
accepted if N is the peer-RPF neighbor for R, X, and is discarded
otherwise.

MP(R,N)

MPP(R,N) MP(N,X)
R ---------....-------> N ------------------> X
SA(S,G,R) SA(S,G,R)

Where MP(R,N) MPP(R,N) is an MSDP peering path (one (zero or more MSDP
peers) between R and N, and N. SA(S,G,R) is an SA message for source
S on group G orignated originated by an RP R. MP(N,X) is an MSDP
peering between N and X.

The peer-RPF neighbor is chosen deterministically, using the
first of the following rules that matches.

X accepts the SA from R forwarded by In particular,
N if :

(i). R is the RPF neighbor of X with respect to R if we have

(i). N == R (X has an MSDP peering with R (e.g. N == R).

(ii). N is the RPF neighbor BGP NEXT_HOP of X if N the active RPF route
for R.

(iii). The active RPF route for R is learned through a MSDP peer of
X
distance-vector or path-vector routing protocol
(e.g. BGP, RIP, DVMRP) and N is the next hop toward R.

(iii) N is neighbor that
advertised the active RPF neighbor of X if route for R.

(iv). N resides in the first an AS
towards R that is in the AS_PATH of the active
RPF route for R, and N has a higher the highest IP address than any other among
the MSDP peer of X peers that resides reside in first AS towards R.

(iv). ASs in that AS_PATH.

(v). N is configured as the RPF neighbor of X if (intra-domain case):

(a). N == R (i.e. N originated the SA), or

(b). X and N are part of a static RPF-peer for R.

14.3. MSDP Mesh Group. Note that in
this case every member of mesh group is an peer-RPF
neighbor of X.

(v). static RPF-peer semantics

If none of the above match, and we have rules (i) - (iv) are able to determine an
MSDP default-peer configured, the MSDP
default-peer is the RPF neighbor.

14.2. MSDP default-peer semantics

An MSDP default-peer is much like peer for
R, a default route. It longest-match lookup is intended to
be used performed in those cases where a stub network isn't running BGP. An
MSDP peer configured with a default-peer accepts all SA messages from the default-peer. Note that static RPF peer table.
This table MUST be able to contain a router running BGP SHOULD NOT allow
configuration of default entry, and SHOULD be
able to contain prefix or per-host (RP) entries. This table
statically maps RP addresses to peers, since this and allows configuration of
topology that is e.g. unknown to the MRIB.

The result of the longest-match lookup of an RP address R in the possibility
static RPF peer table is an MSDP peer, which is the RPF neighbor for
SA looping or black-holes to occur.

14.3.
R.

14.4. MSDP mesh-group semantics

A MSDP mesh-group is a operational mechanism for reducing SA
flooding, typically in an intra-domain setting. In particular, when
some subset of a domain's MSDP speakers are fully meshed, then can be
configured into a mesh-group.

Note that mesh-groups assume that a member doesn't have to forward an
SA to other members of the mesh-group because the originator will
forward to all members. To be able for the originator to forward to
all members (and to have each member also be a potential originator),
the mesh-group must be a full mesh of MSDP peering among all members.

The semantics of the mesh-group are as follows:

(i). If a member R of a mesh-group M receives a SA message from an
MSDP peer that is also a member of mesh-group M, R accepts the
SA message and forwards it to all of it's its peers that are not
part of any mesh-group. R MUST NOT forward the SA message to
other members of mesh-group M.

(ii). If a member R of a mesh-group M receives a SA message from an
MSDP peer that is not a member of mesh-group M, and the SA
message passes the peer-RPF check, then R forwards the SA
message to all members of mesh-group M.

(iii). Cross mesh-group forwarding

If a member R of a mesh-groups M and N receives an SA
message from an MSDP peer in mesh-group M, R forwards the SA
to its MSDP peers in mesh-group N if it receives that SA
message from a peer that is in the same mesh-group as its
peer-RPF neighbor for that SA.

For example, consider the case in which three routers (R1, R2,
and R3) and three mesh-groups (A, B, and C) are arranged in a
triangle, e.g.,

[R2] {A,B}
/ \
/ \
/ \
/ \
{A,C} [R1]--------[R3] {B,C}

Now, when R1 receives an SA message from R2 and R1's
peer-RPF neighbor for this SA lies in mesh-group A, R1
forwards the SA message its peers in other mesh-groups
(in particular, R3 in mesh-group C). Similarly, if R3's
peer-RPF neighbor lies in mesh-group B, R3 will forward an
SA message from R2. In this case, both R1 and R3 will send
SA messages to each other (because they share common mesh-group
C), but neither of them will forward any further the SA messages
received from each other (as their peer-RPF neighbors do
not lie in mesh-group C).

Note that since mesh-groups suspend peer-RPF checking of SAs received
from a mesh-group member ((i). above), they allow for mis-
configuration to cause SA looping.

15. MSDP Connection Establishment

MSDP messages will be encapsulated in a TCP connection. An MSDP peer
listens for new TCP connections on port 639. One side of the MSDP
peering relationship will listen on the well-known port and the other
side will do an active connect to the well-known port. The side with
the higher peer IP address will do the listen. This connection
establishment algorithm avoids call collision. Therefore, there is no
need for a call collision procedure. It should be noted, however,
that the disadvantage of this approach is that it may result in
longer startup times at the passive end.

An MSDP peer starts in the INACTIVE state. MSDP peers establish
peering sessions according to the following state machine:

De-configured or
disabled
+-------------------------------------------+
| |
| |
Enable |
+-----|--------->+----------+ Connect Retry Timer |
| | +->| INACTIVE |----------------+ |
| | | +----------+ | |
Deconf'ed | | | /|\ /|\ | | Lower Address
or | | | | | | |
disabled | | | | | \|/ |
| | | | | | +-------------+
| | | | | +---------------| CONNECTING |
| | | | | Timeout or +-------------+
| | | | | Local Address Change |
\|/ \|/ | | | |
+----------+ | | | |
| DISABLED | | | +---------------------+ | TCP Established
+----------+ | | | |
/|\ /|\ | | Connection Timeout, | |
| | | | Local Address change, | |
| | | | Authorization Failure | |
| | | | | |
| | | | | \|/
| | | | +-------------+
| | Local | | | ESTABLISHED |
| | Address | | Higher Address +-------------+
| | Change | \|/ /|\ |
| | | +--------+ | |
| | +--| LISTEN |--------------------+ |
| | +--------+ TCP Accept |
| | | |
| | | |
| +---------------+ |
| De-configured or |
| disabled |
| |
+------------------------------------------------------+
De-configured or
disabled

16. Packet Formats

MSDP messages will be encoded in TLV format. If an implementation
receives a TLV that has length that is longer than expected, the TLV
SHOULD be accepted. Any additional data SHOULD be ignored.

16.1. MSDP TLV format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type (8 bits)
Describes the format of the Value field.

Length (16 bits)
Length of Type, Length, and Value fields in octets.
minimum length required is 4 octets, except for
Keepalive messages. The maximum TLV length is 1400.

Value (variable length)
Format is based on the Type value. See below. The length of
the value field is Length field minus 3. All reserved fields
in the Value field MUST be transmitted as zeros and ignored on
receipt.

16.2. Defined TLVs

The following TLV Types are defined:

Code Type
===========================================================
1 IPv4 Source-Active
2 IPv4 Source-Active Request
3 IPv4 Source-Active Response
4 KeepAlive
5 Notification

Each TLV is described below.

In addition, the following TLV Types are assigned but not described
in this memo:

Code Type
===========================================================
6 MSDP traceroute in progress
7 MSDP traceroute reply

16.2.1. IPv4 Source-Active TLV

The maximum size SA message that can be sent is 1400 octets. If an
MSDP peer needs to originate a message with information greater than
1400 octets, it sends successive 1400 octet or smaller messages. The
1400 octet size does not include the TCP, IP, layer-2 headers.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | x + y | Entry Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Sprefix Len | \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ \
| Group Address | ) z
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ /
| Source Address | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type
IPv4 Source-Active TLV is type 1.

Length x
Is the length of the control information in the message. x is
8 octets (for the first two 32-bit quantities) plus 12 times
Entry Count octets.

Length y
If 0, then there is no data encapsulated. Otherwise an IPv4
packet follows and y is the length of the total length field
of the IPv4 header encapsulated. If there are multiple SA TLVs
in a message, and data is also included, y must be 0 in all SA
TLVs except the last one and the last SA TLV must reflect the
source and destination addresses in the IP header of the
encapsulated data.

Entry Count
Is the count of z entries (note above) which follow the RP
address field. This is so multiple (S,G)s from the same domain
can be encoded efficiently for the same RP address.

RP Address
The address of the RP in the domain the source has become
active in.

Reserved
The Reserved field MUST be transmitted as zeros and MUST be
ignored by a receiver.

Sprefix Len
The route prefix length associated with source address.
This field MUST be transmitted as 32 (/32). An Invalid
Sprefix Len Notification SHOULD be sent upon receipt
of any other value.

Group Address
The group address the active source has sent data to.

Source Address
The IP address of the active source.

Multiple SA TLVs MAY appear in the same message and can be batched
for efficiency at the expense of data latency. This would typically
occur on intermediate forwarding of SA messages.

16.2.2. IPv4 Source-Active Request TLV

The Source-Active Request is used to request SA-state from a MSDP
peer. If an RP in a domain receives a PIM Join message for a group,
creates (*,G) state and wants to know all active sources for group G,
it may send an SA-Request message for the group.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 8 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address Prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type
IPv4 Source-Active Request TLV is type 2.

Reserved
Must be transmitted as zero and ignored on receipt.

Group Address
The group address the MSDP peer is requesting.

16.2.3. IPv4 Source-Active Response TLV

The Source-Active Response is sent in response to a Source-Active
Request message. The Source-Active Response message has the same
format as a Source-Active message but does not allow encapsulation of
multicast data.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | x | .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type
IPv4 Source-Active Response TLV is type 3.

Length x
Is the length of the control information in the message. x is 8
octets (for the first two 32-bit quantities) plus 12 times Entry
Count octets.

16.2.4. KeepAlive TLV

A KeepAlive TLV is sent to an MSDP peer if and only if there were no
MSDP messages sent to the peer after a period of time. This message
is necessary for the active connect side of the MSDP connection. The
passive connect side of the connection knows that the connection will
be reestablished when a TCP SYN packet is sent from the active
connect side. However, the active connect side will not know when the
passive connect side goes down. Therefore, the KeepAlive timeout will
be used to reset the TCP connection.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The length of the message is 3 octets which encompasses the one octet
Type field and the two octet Length field.

16.2.5. Notification TLV

A Notification message is sent when an error condition is detected,
and has the following form:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | x + 5 |O| Error Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error subcode | ... |
+-+-+-+-+-+-+-+-+ |
| Data |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type
The Notification TLV is type 5.

Length
Length is a two octet field with value x + 5, where x is
the length of the notification data field.

O-bit
Open-bit. If clear, the connection will be closed.

Error code
This 7-bit unsigned integer indicates the type of Notification.
The following Error Codes have been defined:

Error Code Symbolic Name Reference

1 Message Header Error Section 17.1
2 SA-Request Error Section 17.2
3 SA-Message/SA-Response Error Section 17.3
4 Hold Timer Expired Section 17.4
5 Finite State Machine Error Section 17.5
6 Notification Section 17.6
7 Cease Section 17.7

Error subcode:
This one-octet unsigned integer provides more specific information
about the reported error. Each Error Code may have one or more
Error
Subcodes associated with it. If no appropriate Error Subcode is
defined, then a zero (Unspecific) value is used for the Error

Subcode
field, and the O-bit must be cleared (i.e. the connection will be
closed). The used notation in the error description below is: MC =
Must Close connection = O-bit clear; CC = Can Close connection =
O-bit might MAY be cleared.

Message Header Error subcodes:

0 - Unspecific (MC)
2 - Bad Message Length (MC)
3 - Bad Message Type (CC)

SA-Request Error subcodes: subcodes (the O-bit is always clear):

0 - Unspecific (MC)
1 - Invalid Group (MC)

SA-Message/SA-Response Error subcodes

0 - Unspecific (MC)
1 - Invalid Entry Count (CC)
2 - Invalid RP Address (MC)
3 - Invalid Group Address (MC)
4 - Invalid Source Address (MC)
5 - Invalid Sprefix Length (MC)
6 - Looping SA (Self is RP) (MC)
7 - Unknown Encapsulation (MC)
8 - Administrative Scope Boundary Violated (MC)

Hold Timer Expired subcodes (the O-bit is always clear):

0 - Unspecific (MC)

Finite State Machine Error subcodes: subcodes (the O-bit is always clear):

0 - Unspecific (MC)
1 - Unexpected Message Type FSM Error (MC)

Notification subcodes (the O-bit is always clear):

0 - Unspecific (MC)

Cease subcodes (the O-bit is always clear):

0 - Unspecific (MC)

17. MSDP Error Handling

This section describes actions to be taken when errors are detected
while processing MSDP messages. MSDP Error Handling is similar to
that of BGP [RFC1771].

When any of the conditions described here are detected, a
Notification message with the indicated Error Code, Error Subcode,
and Data fields is sent. In addition, the MSDP connection might MAY be
closed. If no Error Subcode is specified, then a zero (Unspecific)
must be used.

The phrase "the MSDP connection is closed" means that the transport
protocol connection has been closed and that all resources for that
MSDP connection have been deallocated.

17.1. Message Header Error Handling

All errors detected while processing the Message Header are indicated
by sending the Notification message with Error Code Message Header
Error. The Error Subcode describes the specific nature of the error.
The Data field contains the erroneous Message (including the message
header).

If the Length field of the message header is less than 4 or greater
than 1400, or the length of a KeepAlive message is not equal to 3,
then the Error Subcode is set to Bad Message Length.

If the Type field of the message header is not recognized, then the
Error Subcode is set to Bad Message Type.

17.2. SA-Request Error Handling

The SA-Request Error code is used to signal the receipt of a SA
request at a MSDP peer when an invalid group address requested.

When a MSDP peer receives a request for an invalid group, it returns
the following notification:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | 16 12 |O| 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | Reserved | Gprefix Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gprefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Invalid Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

17.3. SA-Message/SA-Response Error Handling

The SA-Message/SA-Response Error code is used to signal the receipt
of a erroneous SA Message at an MSDP peer, or the receipt of an SA-
Response Message by a peer that did not issue a SA-Request. It has
the following form:

17.3.1. Invalid Entry Count (IEC)

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | 6 |O| 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | IEC Entry Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

17.3.2. Invalid RP Address

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | 12 |O| 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Invalid RP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

17.3.3. Invalid Group Address

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | 12 |O| 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Invalid Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

17.3.4. Invalid Source Address

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | 12 |O| 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Invalid Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

17.3.5. Invalid Sprefix Length (ISL)

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | 6 |O| 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | ISL Sprefix Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

17.3.6. Looping SAs (Self is RP in received SA)

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | x + 5 |O| 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 6 | Looping SA Message ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Length x
x is the length of the looping SA message contained in the data
field of the Notification message.

17.3.7. Unknown Encapsulation

This notification is sent on receipt of SA data that is encapsulated
in an unknown encapsulation type. See section 18 for known
encapsulations.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | x + 5 |O| 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 7 | SA Message ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Length x
x is the length of the SA message (which contained data which
was encapsulated in some unknown way) that is contained in the
data field of the Notification message.

17.3.8. Administrative Scope Boundary Violated

This notification is used when an SA message is received for a group
G from a peer which is across an administrative scope boundary for G.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | 16 12 |O| 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 8 | Reserved | Gprefix Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Gprefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

17.4. Hold Time Expired

If a system does has not receive successive KeepAlive or received any SA Message
and/or Notification messages MSDP message within the period
specified in the Hold Timer, the notification message with Hold Timer
Expired Error Code and no additional data MUST be sent and the MSDP
connection closed.

17.5. Finite State Machine Error Handling

Any error detected by the MSDP Finite State Machine (e.g., receipt of
an unexpected event) is indicated by sending the Notification message
with Error Code Finite State Machine Error.

17.6. Notification Message Error Handling

If a node sends a Notification message, and there is an error in that
message, and the O-bit of that message is not clear, a Notification
with O-bit clear, Error Code of Notification Error, and subcode
Unspecific must be sent. In addition, the Data field must include
the Notification message that triggered the error. However, if the
erroneous Notification message had the O-bit clear, then any error,
such as an unrecognized Error Code or Error Subcode, should be
noticed, logged locally, and brought to the attention of the
administrator of the remote node.

17.7. Cease

In absence of any fatal errors (that are indicated in this section),
an MSDP node may choose at any given time to close its MSDP
connection by sending the Notification message with Error Code Cease.
However, the Cease Notification message MUST NOT be used when a fatal
error indicated by this section does exist.

18. SA Data Encapsulation

This section describes UDP, GRE, and TCP encapsulation of data
packets to be included with SA data. messages. Encapsulation type is a
configuration option.

18.1. UDP Data Encapsulation

Data packets MAY be encapsulated in UDP. In this case, the UDP
pseudo-header has the following form:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Origin RP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Source port, Destination Port, Length, and Checksum are used
according to RFC 768. Source and Destination ports are known via an
implementation-specific method (e.g. per-peer configuration).

Checksum
The checksum is computed according to RFC 768 [RFC768].

Originating RP Address
The Originating RP Address is the address of the RP sending
the encapsulated data.

18.2. GRE Encapsulation

MSDP SA-data MAY be encapsulated in GRE using protocol type [MSDP-
GRE-ProtocolType]. The GRE header and payload packet have the
following form:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| Reserved0 | Ver | [MSDP-GRE-ProtocolType] |\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ GRE Header
| Checksum (optional) | Reserved1 |/
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originating RP IPv4 Address |\
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Payload
| (S,G) Data Packet .... /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

18.2.1. Encapsulation and Path MTU Discovery [RFC1191]

Existing implementations of GRE, when using IPv4 as the Delivery
Header, do not implement Path MTU discovery and do not set the Don't
Fragment bit in the Delivery Header. This can cause large packets to
become fragmented within the tunnel and reassembled at the tunnel
exit (independent of whether the payload packet is using PMTU). If a
tunnel entry point were to use Path MTU discovery, however, that
tunnel entry point would also need to relay ICMP unreachable error
messages (in particular the "fragmentation needed and DF set" code)
back to the originator of the packet, which is not required by the
GRE specification [RFC2784]. Failure to properly relay Path MTU
information to an originator can result in the following behavior:
the originator sets the don't fragment bit, the packet gets dropped
within the tunnel, but since the originator doesn't receive proper
feedback, it retransmits with the same PMTU, causing subsequently
transmitted packets to be dropped.

18.3. TCP Data Encapsulation

As discussed earlier, encapsulation of data in SA messages MAY be
supported for backwards compatibility with legacy MSDP peers.

19. IANA Considerations

The IANA should assigne assign 0x0009 from the IANA SNAP Protocol IDs [IANA]
to MSDP-GRE-ProtocolType.

20. Security Considerations

An MSDP implementation MAY use IPsec [RFC1825] [RFC2401] or keyed MD5 [RFC1828]
to secure control messages. When encapsulating SA data packets in GRE,
security should be relatively similar to security in a normal IPv4
network, as routing using GRE follows the same routing that IPv4 uses
natively. Route filtering will remain unchanged. However packet
filtering at a firewall requires either that a firewall look inside
the GRE packet or that the filtering is done on the GRE tunnel
endpoints. In those environments in which this is considered to be a
security issue it may be desirable to terminate the tunnel at the
firewall.

21. Acknowledgments

The editor would like to thank the original authors, Dino Farinacci,
Yakov Rehkter, Peter Lothberg, Hank Kilmer, and Jermey Hall for their
orginal contribution to the MSDP specification. In addition, Bill
Fenner, Bill Nickless, John Meylor, Liming Wei, Manoj Leelanivas,
Mark Turner, John Zwiebel, Cristina Radulescu-Banu Radulescu-Banu, Brian Edwards and
IJsbrand Wijnands provided useful and productive design feedback and
comments. In addition to many other contributions, Tom Pusateri
helped to clarify the connection state machine, Dave Thaler helped to
clarify the Notification message types, types. Ravi Shekhar helped clarify
the semantics of mesh-groups, and countless others helped to clarify
the Peer-RPF rules.

22. Editor's Editors' Address:

David Meyer
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
Email: dmm@cisco.com

Bill Fenner
AT&T Labs -- Research
75 Willow Road
Menlo Park, CA 94025
Email: fenner@research.att.com

23. REFERENCES

[IANA] www.iana.org

[RFC1700] J. Reynolds and J. Postel, "Assigned Numbers", RFC 1700,
October, 1994.

[RFC2784] Farinacci, D., et al., "Generic Routing Encapsulation
(GRE)", RFC 2784, March 2000.

[RFC768] Postel, J. "User Datagram Protocol", RFC 768, August,
1980.

[RFC1191] Mogul, J., and S. Deering, "Path MTU Discovery",
RFC 1191, November 1990.

[RFC1771] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4
(BGP-4)", RFC 1771, March 1995.

[RFC1825]

[RFC2401] Kent, S. and R. Atkinson, R., "Security Architecture for
the Internet Protocol", RFC 1825, August, 1995. 2401, November 1998.

[RFC1828] P. Metzger and W. Simpson, "IP Authentication using
Keyed MD5", RFC 1828, August, 1995.

[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March, 1997.

[RFC2283] Bates, T., Chandra, R., Katz, D., and Y. Rekhter.,
"Multiprotocol Extensions for BGP-4", RFC 2283,
February 1998.

[RFC2362] Estrin D., et al., "Protocol Independent Multicast -
Sparse Mode (PIM-SM): Protocol Specification", RFC
2362, June 1998.

[RFC2365] Meyer, D. "Administratively Scoped IP Multicast", RFC
2365, July, 1998.

24. Full Copyright Statement

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