wdiff draft-ietf-msdp-spec-03.txt draft-ietf-msdp-spec-04.txt

Network Working Group Dino Farinacci
INTERNET DRAFT Procket Networks
Yakov Rekhter
David Meyer
Cisco Systems
Peter Lothberg
Sprint
Hank Kilmer
Jeremy Hall
UUnet
Category Standards Track
January,
February, 2000

Multicast Source Discovery Protocol (MSDP)
<draft-ietf-msdp-spec-03.txt>
<draft-ietf-msdp-spec-04.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.

o Global Source State

Global source state is not required, since a router need not
cache Source Active (SA) messages (see below). MSDP is a
periodic protocol.

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

An RP (or other MSDP SA originator) in a PIM-SM [RFC2362] domain will
have a MSDP peering relationship with a MSDP peers in another domain.
The peering relationship will be made up of a TCP connection in which
control information is exchanged. Each domain will have one or more
connections to this virtual topology.

The purpose of this topology is to have domains 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 can be realized by
the underlying BGP routing system.

6. Procedure

A source 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 a PIM Register message to the RP in the domain.

The RP will construct a "Source-Active" (SA) message and send 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 is examined to determine which peer is the NEXT_HOP towards the
originating RP of the SA message. 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.

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 (See
[RFC2362] Section 3.2.2). 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, and it is
caching SAs, then 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. Controlling State

While RPs which receive SA messages are not required to keep MSDP
(S,G) state, an RP SHOULD cache SA messages by default. The advantage
of caching is that newly formed MSDP peers can get MSDP (S,G) state
sooner and therefore reduce join latency for new joiners. 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 will not send more than one
periodic SA message for a given (S,G) within an SA Advertisement
interval. Originating periodic SA messages is important so that new
receivers who join after a source has been active can get data
quickly via the receiver's own RP when it is not caching SA state.
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 starts the SA-Advertisement-Timer when the MSDP process is
configured. When the timer expires, an RP advertises any candidate
internal sources to its peers and resets the timer to [SA-
Advertisement-Period] seconds. The timer is deleted when the MSDP
process is deconfigured. Note that a caching implementation may also
wish to check the SA-Cache on this timer event.

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 caching 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

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

8.5. KeepAlive Timer

The KeepAlive timer is used by the active connect side of the MSDP
connection to track the state of the passive-connect side of the
connection. In particular, the KeepAlive timer is be used to reset the
TCP connection when the passive-connect side of the connection goes
down. The KeepAlive timer is set to [KeepAlive-Period] when
passive-connect passive-
connect peer comes up. [KeepAlive-Period] SHOULD NOT be less that 75
seconds. The timer is reset to [KeepAlive-Period] upon receipt of
data from peer, and deleted when the timer expires or the
passive-connect passive-
connect peer closes the connection.

8.6. 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 is reset to [ConnectRetry-Period]. It is
deleted 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 connection 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 either a KeepAlive or any SA message is received.

9. Intermediate MSDP Peers

Intermediate RPs do not originate periodic SA messages on behalf of
sources in other domains. In general, an RP MUST only originate an SA
for its own sources.

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 will 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

If an MSDP peer decides to cache SA state, it MAY accept SA-Requests
from other MSDP peers. When an MSDP peer 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 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 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, the packet is forwarded appropriately.
Note that when doing data encapsulation, an implementation MUST bound
the time during which the source 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 different 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. Peer-RPF Forwarding Rules

An SA message originated by an MSDP originator R and received by a
MSDP router from MSDP peer N is accepted if N is the appropriate RPF
neighbor for originator R, and discarded otherwise.

The RPF neighbor is chosen using the first of the following rules
that matches:

(i). R is the RPF neighbor if we have an MSDP peering with R.

(ii). The external MBGP neighbor towards which we are
poison-reversing the MBGP route towards R is the RPF neighbor
if we have an MSDP peering with it.

(iii). If we have any MSDP peerings with neighbors in the first
AS along the AS_PATH (the AS from which we learned this
route), but no external MBGP peerings with them,
pick one via a deterministic rule.

(vi). The internal MBGP advertiser of the router towards R is
the RPF neighbor if we have an MSDP peering with it.

(v). If none of the above match, and we have 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 a default route. It is intended to
be used in those cases where a stub network isn't running BGP or
MBGP. An MSDP peer configured with a default-peer accepts all SA
messages from the default-peer. Note that a router running BGP or
MBGP SHOULD NOT allow configuration of default peers, since this
allows the possibility for SA looping or black-holes to occur.

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 on 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 |
+-----|--------->+----------+ |
| | +->| 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 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. The
minimum length required is 4 octets. The total length
of the TLV should be a multiple of 2 octets.

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.

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 messages. The 1400 octet
size does not include the TCP, IP, layer-2 headers.

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 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 caching
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, and it has been configured to peer with an SA-state caching
peer, it may send an SA-Request message for the group.

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 | Gprefix Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address Prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type
IPv4 Source-Active Request TLV is type 2.

Gprefix Len
The route prefix length associated with the group address prefix.

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 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 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 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 be cleared.

Message Header Error subcodes:

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

SA-Request Error subcodes:

0 - Unspecific (MC)
1 - Does not cache SA (MC)
2 - 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:

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 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 non-caching MSDP peer, or at a caching MSDP peer when an
invalid group address requested.

When a non-caching MSDP peer receives an SA-Request, it returns the
following notification:

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| 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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| 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

17.3.2. Invalid RP Address

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 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 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)

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

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 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 with contained in the
data field of the Notification message.

17.3.8. Adminstrative 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 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 |O| 3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 8 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Peer IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

17.4. Hold Time Expired

If a system does not receive successive KEEPALIVE or any SA Message
and/or Notification messages within the period specified in the Hold
Timer, then the notification message with Hold Timer Expired Error
Code 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 TCPC TCP encapsulation of SA data.
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 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 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. GRE 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 [GRE]. 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. Security Considerations

An MSDP implementation MAY use IPsec [RFC1825] or keyed MD5 [RFC1828]
to secure control messages. When encapsulating SA data 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.

20. Acknowledgments

The authors would like to thank Bill Nickless, John Meylor, Liming
Wei, Manoj Leelanivas, Mark Turner, John Zwiebel, and Cristina
Radulescu-Banu for their 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, and Bill Fenner helped to clarify the
Peer-RPF rules.

21. Author's Address:

Dino Farinacci
Procket Networks
3850 No. First St., Ste. C
San Jose, CA 95134
Email: dino@procket.com

Yakov Rehkter
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
Email: yakov@cisco.com

Peter Lothberg
Sprint
VARESA0104
12502 Sunrise Valley Drive
Reston VA, 20196
Email: roll@sprint.net

Hank Kilmer
Email: hank@rem.com

Jeremy Hall
UUnet Technologies
3060 Williams Drive
Fairfax, VA 22031
Email: jhall@uu.net

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

22. REFERENCES

[GRE] Farinacci, D., et al., "Generic Routing Encapsulation
(GRE)", draft-meyer-gre-update-02.txt, draft-meyer-gre-update-03.txt, January,
2000. Work in Progress.

[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] Atkinson, R., "Security Architecture for the Internet
Protocol", RFC 1825, August, 1995.

[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.