[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 5059

Internet Engineering Task Force                                   PIM WG
INTERNET-DRAFT                                       Nidhi Bhaskar/Cisco
draft-ietf-pim-sm-bsr-06.txt                       Alexander Gall/SWITCH
                                                           James Lingard
                                                     Stig Venaas/UNINETT
                                                         23 October 2005
                                                     Expires: April 2006


                Bootstrap Router (BSR) Mechanism for PIM



Status of this Document

By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware have
been or will be disclosed, and any of which he or she becomes aware will
be disclosed, in accordance with Section 6 of BCP 79.

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/1id-abstracts.html

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

This document is a product of the IETF PIM WG.  Comments should be
addressed to the authors, or the WG's mailing list at pim@ietf.org.

Copyright Notice

Copyright (C) The Internet Society (2005).





Bhaskar/Gall/Lingard/Venaas                                     [Page 1]

INTERNET-DRAFT             Expires: April 2006              October 2005


                                Abstract


     This document specifies the Bootstrap Router (BSR) mechanism
     for the class of multicast routing protocols in the PIM
     (Protocol Independent Multicast) family that use the concept
     of a Rendezvous Point as a means for receivers to discover the
     sources that send to a particular multicast group.  BSR is one
     way that a multicast router can learn the set of group-to-RP
     mappings required in order to function.  The mechanism is
     dynamic, largely self-configuring, and robust to router
     failure.







































Bhaskar/Gall/Lingard/Venaas                                     [Page 2]

INTERNET-DRAFT             Expires: April 2006              October 2005


                           Table of Contents


     1. Introduction. . . . . . . . . . . . . . . . . . . . . .   5
      1.1. Background . . . . . . . . . . . . . . . . . . . . .   5
      1.2. Protocol Overview. . . . . . . . . . . . . . . . . .   7
      1.3. Administrative Scoping and BSR . . . . . . . . . . .   8
     2. BSR State and Timers. . . . . . . . . . . . . . . . . .  10
     3. Bootstrap Router Election and RP-Set
        Distribution. . . . . . . . . . . . . . . . . . . . . .  10
      3.1. Bootstrap Router Election. . . . . . . . . . . . . .  10
       3.1.1. Per-Scope-Zone Candidate-BSR State
              Machine . . . . . . . . . . . . . . . . . . . . .  11
       3.1.2. Per-Scope-Zone State Machine for Non-
              Candidate-BSR Routers . . . . . . . . . . . . . .  13
       3.1.3. Bootstrap Message Processing Checks . . . . . . .  15
       3.1.4. State Machine Transition Events . . . . . . . . .  15
       3.1.5. State Machine Actions . . . . . . . . . . . . . .  16
      3.2. Sending Candidate-RP-Advertisement Messages. . . . .  18
      3.3. Creating the RP-Set at the BSR . . . . . . . . . . .  19
      3.4. Forwarding Bootstrap Messages. . . . . . . . . . . .  22
      3.5. Unicasting Bootstrap Messages to New and
           Rebooting Routers. . . . . . . . . . . . . . . . . .  22
      3.6. Receiving and Using the RP-Set . . . . . . . . . . .  23
     4. Message Formats . . . . . . . . . . . . . . . . . . . .  23
      4.1. Bootstrap Message Format . . . . . . . . . . . . . .  25
       4.1.1. Semantic Fragmentation of BSMs. . . . . . . . . .  29
      4.2. Candidate-RP-Advertisement Message Format. . . . . .  30
     5. Timers and Timer Values . . . . . . . . . . . . . . . .  32
     6. Security Considerations . . . . . . . . . . . . . . . .  35
      6.1. Possible Threats . . . . . . . . . . . . . . . . . .  35
      6.2. Limiting Third-Party DoS Attacks . . . . . . . . . .  35
      6.3. Bootstrap Message Security . . . . . . . . . . . . .  36
       6.3.1. Rejecting Unicast Bootstrap Messages. . . . . . .  36
       6.3.2. Rejecting Bootstrap Messages from Invalid
              Neighbors . . . . . . . . . . . . . . . . . . . .  37
      6.4. Candidate-RP-Advertisement Message Security. . . . .  37
       6.4.1. Non-Cryptographic Security of C-RP-Adv
              Messages. . . . . . . . . . . . . . . . . . . . .  37
       6.4.2. Cryptographic Security of C-RP-Adv
              Messages. . . . . . . . . . . . . . . . . . . . .  38
      6.5. Denial of Service using IPsec. . . . . . . . . . . .  38
     7. Contributors. . . . . . . . . . . . . . . . . . . . . .  39
     8. Acknowledgments . . . . . . . . . . . . . . . . . . . .  39
     9. IANA Considerations . . . . . . . . . . . . . . . . . .  39
     10. Normative References . . . . . . . . . . . . . . . . .  39
     11. Informative References . . . . . . . . . . . . . . . .  40




Bhaskar/Gall/Lingard/Venaas                                     [Page 3]

INTERNET-DRAFT             Expires: April 2006              October 2005


1.  Introduction

This document assumes some familiarity with the concepts of Protocol
Independent Multicast - Sparse Mode (PIM-SM), as defined in [1], and Bi-
directional Protocol Independent Multicast (BIDIR-PIM), as defined in
[2], as well as with Administratively Scoped IP Multicast, as described
in [3], and the IPv6 Scoped Address Architecture, described in [4].

For correct operation, every multicast router within a PIM domain must
be able to map a particular multicast group address to the same
Rendezvous Point (RP).  The PIM specifications do not mandate the use of
a single mechanism to provide routers with the information to perform
this group-to-RP mapping.

This document describes the PIM Bootstrap Router (BSR) mechanism.  BSR
is one way that a multicast router can learn the information required to
perform the group-to-RP mapping.  The mechanism is dynamic, largely
self-configuring, and robust to router failure.

BSR was first defined in RFC 2362 [7], which has since been obsoleted.
This document provides an updated specification of the BSR mechanism
from RFC 2362, and also extends it to cope with administratively scoped
region boundaries and different flavours of routing protocols.

Throughout the document, any reference to the PIM protocol family is
restricted to the subset of RP-based protocols, namely PIM-SM and BIDIR-
PIM, unless stated otherwise.

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 RFC 2119 [6].

1.1.  Background

A PIM domain is a contiguous set of routers that all implement PIM and
are configured to operate within a common boundary defined by PIM
Multicast Border Routers (PMBRs).  PMBRs connect each PIM domain to the
rest of the internet.

Every PIM multicast group needs to be associated with the IP address of
a Rendezvous Point (RP).  This address is used as the root of a group-
specific distribution tree whose branches extend to all nodes in the
domain that want to receive traffic sent to the group.  Senders inject
packets into the tree in such a manner that they reach all connected
receivers.  How this is done and how the packets are forwarded along the
distribution tree depends on the particular routing protocol.





Bhaskar/Gall/Lingard/Venaas                       Section 1.1.  [Page 4]

INTERNET-DRAFT             Expires: April 2006              October 2005


For all senders to reach all receivers, it is crucial that all routers
in the domain use the same mappings of group addresses to RP addresses.

An exception to the above is where a PIM domain has been broken up into
multiple administrative scope regions.  These are regions where a border
has been configured so that a set of multicast groups will not be
forwarded across that border.  In this case, all PIM routers within the
same scope region must map a particular scoped group to the same RP
within that region.

In order to determine the RP for a multicast group, a PIM router
maintains a collection of group-to-RP mappings, called the RP-Set.  A
group-to-RP mapping contains the following elements.

o Multicast group range, expressed as an address and prefix length

o RP priority

o RP address

o Hash mask length

o SM / BIDIR flag

In general, the group ranges of these group-to-RP mappings may overlap
in arbitrary ways; hence a particular multicast group may be covered by
multiple group-to-RP mappings.  When this is the case, the router
chooses only one of the RPs by applying a deterministic algorithm so
that all routers in the domain make the same choice.  It is important to
note that this algorithm is part of the specification of the individual
routing protocols (and may differ among them), not of the BSR
specification.

There are a number of ways in which such group-to-RP mappings can be
established.  The simplest solution is for all the routers in the domain
to be statically configured with the same information.  However, static
configuration generally doesn't scale well, and, except when used in
conjunction with Anycast-RP (see [8] and [9]), does not dynamically
adapt to route around router or link failures.

The BSR mechanism provides a way in which viable group-to-RP mappings
can be created and rapidly distributed to all the PIM routers in a
domain.  It is adaptive, in that if an RP becomes unreachable, this will
be detected and the RP-Sets will be modified so that the unreachable RP
is no longer used.






Bhaskar/Gall/Lingard/Venaas                       Section 1.1.  [Page 5]

INTERNET-DRAFT             Expires: April 2006              October 2005


1.2.  Protocol Overview

In this section we give an informal and non-definitive overview of the
BSR mechanism.  The definitive specification begins in section 2.

The general idea behind the BSR mechanism is that some of the PIM
routers within a PIM domain are configured to be potential RPs for the
domain.  These are known as Candidate-RPs (C-RPs).  A subset of the C-
RPs will eventually be used as the actual RPs for the domain.  In
addition, some of the PIM routers in the domain are configured to be
candidate bootstrap routers, or Candidate-BSRs (C-BSRs).  One of these
C-BSRs will be elected to be the bootstrap router (BSR) for the domain,
and all the PIM routers in the domain will learn the result of this
election through Bootstrap messages.  The C-RPs will then report their
candidacy to the elected BSR, which chooses a subset of these C-RPs and
distributes corresponding group-to-RP mappings to all the routers in the
domain through Bootstrap messages.

In more detail, the BSR mechanism works as follows.  There are four
basic phases (although in practice all phases may be occurring
simultaneously):

1.   BSR Election.  Each Candidate-BSR originates Bootstrap messages
     (BSMs).  Every BSM contains a BSR Priority field.  Routers within
     the domain flood the BSMs throughout the domain.  A C-BSR that
     hears about a higher-priority C-BSR than itself then suppresses its
     sending of further BSMs for some period of time.  The single
     remaining C-BSR becomes the elected BSR, and its BSMs inform all
     the other routers in the domain that it is the elected BSR.

2.   C-RP Advertisement.  Each Candidate-RP within a domain sends
     periodic Candidate-RP-Advertisement (C-RP-Adv) messages to the
     elected BSR.  A C-RP-Adv message includes the priority of the
     advertising C-RP, as well as a list of group ranges for which the
     candidacy is advertised.  In this way, the BSR learns about
     possible RPs that are currently up and reachable.

3.   RP-Set Formation.  The BSR selects a subset of the C-RPs that it
     has received C-RP-Adv messages from to form the RP-Set.  In general
     it should do this in such a way that the RP-Set is neither too
     large to inform all the routers in the domain about, nor too small
     so that load is overly concentrated on some RPs.  It should also
     attempt to produce an RP-Set that does not change frequently.

4.   RP-Set Flooding.  In future Bootstrap messages, the BSR includes
     the RP-Set information.  Bootstrap messages are flooded through the
     domain, which ensures that the RP-Set rapidly reaches all the
     routers in the domain.  BSMs are originated periodically to ensure



Bhaskar/Gall/Lingard/Venaas                       Section 1.2.  [Page 6]

INTERNET-DRAFT             Expires: April 2006              October 2005


     consistency after failure restoration.

     When a PIM router receives a Bootstrap message, it adds the group-
     to-RP mappings contained therein to its pool of mappings obtained
     from other sources (e.g. static configuration).  It calculates the
     final mappings of group addresses to RP addresses from this pool
     according to rules specific to the particular routing protocol and
     uses that information to construct multicast distribution trees.

If a PIM domain becomes partitioned, each area separated from the old
BSR will elect its own BSR, which will distribute an RP-Set containing
RPs that are reachable within that partition.  When the partition heals,
another election will occur automatically and only one of the BSRs will
continue to send out Bootstrap messages.  As is expected at the time of
a partition or healing, some disruption in packet delivery may occur.
This time will be on the order of the region's round-trip time and the
BS_Timeout value.

1.3.  Administrative Scoping and BSR

The mechanism described in the previous section does not work when the
PIM domain is divided into administratively scoped regions.  To handle
this situation, we use the protocol modifications described in this
section.

Administrative scoping permits a PIM domain to be divided into multiple
admin-scope regions.  Each admin-scope region is a convex connected set
of PIM routers, and is associated with a set of group addresses.  The
boundary of the admin-scope region is formed by Zone Border Routers
(ZBRs).  ZBRs are configured not to forward traffic for any of the
scoped group addresses into or out of the scoped region.  It is
important to note that a given scope boundary always creates at least
two scoped regions: one on either side of the boundary.

In IPv4, administratively scoped regions are associated with a set of
addresses given by an address and a prefix length.  In IPv6,
administratively scoped regions are associated with a set of addresses
given by a single scope ID value.  The set of addresses corresponding to
a given scope ID value is defined in [5].  For example, a scope ID of 5
maps to the 16 IPv6 address ranges ff[0-f]5::/16.

There are certain topological restrictions on admin-scope regions.
Firstly, the scope zone border must be complete and convex.  By this we
mean that there must be no path from inside the scoped zone to outside
it that does not pass through a configured scope border router, and that
the multicast capable path between any arbitrary pair of multicast
routers in the scope zone must remain in the zone.  In addition, a
boundary for one scope must always be a boundary for all smaller scopes,



Bhaskar/Gall/Lingard/Venaas                       Section 1.3.  [Page 7]

INTERNET-DRAFT             Expires: April 2006              October 2005


where a smaller scope for IPv4 is one whose address range is contained
within the other address range, and for IPv6 is one whose scope ID is
less than the other scope ID.

Administrative scoping complicates BSR because we do not want a PIM
router within the scoped region to use an RP outside the scoped region.
Thus we need to modify the basic mechanism to ensure that this doesn't
happen.

This is done by running a separate copy of the basic BSR mechanism, as
described in the previous section, within each admin scope region of a
PIM domain.  Thus a separate BSR election takes place for each admin-
scope region, a C-RP typically registers to the BSR of every admin scope
zone it is in, and every PIM router receives Bootstrap messages for
every scope zone it is in.  The Bootstrap messages sent by the BSR for a
particular scope zone contain information about the RPs that should be
used for the set of addresses associated with that scope zone.

Bootstrap messages are marked to indicate which scope zone they belong
to.  Such admin scoped Bootstrap messages are flooded in the normal way,
but will not be forwarded by a ZBR across the boundary for that scope
zone.

For the BSR mechanism to function correctly with admin scoping, within
each admin scope region there must be at least one C-BSR, and at least
one C-RP that is configured to be a C-RP for the set of group addresses
associated with the scoped region.

Even when administrative scoping is used, a copy of the BSR mechanism is
still used across the entire PIM domain, in order to distribute RP
information for groups that are not administratively scoped.  We call
this copy of the mechanism Non-Scoped BSR.  The copies of the mechanism
run for each admin-scope region are called Scoped BSR.

Only the C-BSRs and the ZBRs need to be configured to know about the
existence of the scope zones.  Other routers, including the C-RPs, learn
of their existence from Bootstrap messages.

All PIM routers within a PIM bootstrap domain where admin scope ranges
are in use must be capable of receiving Bootstrap messages and storing
the winning BSR and RP-Set for all admin scope zones that apply.  Thus
PIM routers that only implement RFC 2362 or Non-Scoped BSR (which only
allows one BSR per domain) cannot be used within the admin-scope regions
of a PIM domain.







Bhaskar/Gall/Lingard/Venaas                       Section 1.3.  [Page 8]

INTERNET-DRAFT             Expires: April 2006              October 2005


2.  BSR State and Timers

A PIM router implementing BSR holds the following state.

     RP-Set

     Per Configured or Learned Scope Zone (Z):

          At all routers:

               Current Bootstrap Router's IP Address

               Current Bootstrap Router's BSR Priority

               Last BSM received from current BSR

               Bootstrap Timer (BST(Z))

               Per group-to-RP mapping (M):

                    Group-to-RP mapping Expiry Timer (GET(M,Z))

          At a Candidate-BSR for Z:

               My state: One of "Candidate-BSR", "Pending-BSR",
                    "Elected-BSR"

          At a router that is not a Candidate-BSR for Z:

               My state: One of "Accept Any", "Accept Preferred"

               Scope-Zone Expiry Timer (SZT(Z))

          At the current Bootstrap Router for Z only:

               Per group-to-C-RP mapping (M):

                    Group-to-C-RP mapping Expiry Timer (CGET(M,Z))

     At a C-RP only:

          C-RP Advertisement Timer (CRPT)









Bhaskar/Gall/Lingard/Venaas                       Section 3.1.  [Page 9]

INTERNET-DRAFT             Expires: April 2006              October 2005


3.  Bootstrap Router Election and RP-Set Distribution

3.1.  Bootstrap Router Election

For simplicity, Bootstrap messages are used in both the BSR election and
the RP-Set distribution mechanisms.

Each Bootstrap message indicates the scope that it belongs to.  If the
Admin Scope Zone bit is set in the first group range in the Bootstrap
message, the message is called a scoped BSM.  If the Admin Scope Zone
bit is not set in the first group range in the Bootstrap message, the
message is called a non-scoped BSM.

In a scoped IPv4 BSM, the scope of the message is given by the first
group range in the message, which can be any sub-range of 224/4.  In a
scoped IPv6 BSM, the scope of the message is given by the scope ID of
the first group range in the message, which must have a mask length of
at least 16.  For example, a group range of ff05::/16 with the Admin
Scope Zone bit set indicates that the Bootstrap message is for the scope
with scope ID 5.  If the mask length of the first group range in a
scoped IPv6 BSM is less than 16, the message MUST be dropped and a
warning SHOULD be logged.

The state machine for Bootstrap messages depends on whether or not a
router has been configured to be a Candidate-BSR for a particular scope
zone.  The per-scope-zone state machine for a C-BSR is given below,
followed by the state machine for a router that is not configured to be
a C-BSR.

3.1.1.  Per-Scope-Zone Candidate-BSR State Machine


+-----------------------------------------------------------------------+
|                         When in C-BSR state                           |
+-----------+------------------+--------------------+-------------------+
| Event     |  Receive         |  Bootstrap         | Receive Non-      |
|           |  Preferred BSM   |  Timer Expires     | preferred BSM     |
|           |                  |                    | from Elected      |
|           |                  |                    | BSR               |
+-----------+------------------+--------------------+-------------------+
|           |  -> C-BSR state  |  -> P-BSR state    | -> P-BSR state    |
|           |  Forward BSM;    |  Set Bootstrap     | Set Bootstrap     |
| Action    |  Store RP-Set;   |  Timer to          | Timer to          |
|           |  Set Bootstrap   |  BS_Rand_Override  | BS_Rand_Override  |
|           |  Timer to        |                    |                   |
|           |  BS_Timeout      |                    |                   |
+-----------+------------------+--------------------+-------------------+




Bhaskar/Gall/Lingard/Venaas                    Section 3.1.1.  [Page 10]

INTERNET-DRAFT             Expires: April 2006              October 2005


+-----------------------------------------------------------------------+
|                         When in P-BSR state                           |
+------------+-------------------+-------------------+------------------+
| Event      |  Receive          |  Bootstrap        |  Receive Non-    |
|            |  Preferred BSM    |  Timer Expires    |  preferred BSM   |
+------------+-------------------+-------------------+------------------+
|            |  -> C-BSR state   |  -> E-BSR state   |  -> P-BSR state  |
|            |  Forward BSM;     |  Originate BSM;   |                  |
| Action     |  Store RP-Set;    |  Set Bootstrap    |                  |
|            |  Set Bootstrap    |  Timer to         |                  |
|            |  Timer to         |  BS_Period        |                  |
|            |  BS_Timeout       |                   |                  |
+------------+-------------------+-------------------+------------------+


+-----------------------------------------------------------------------+
|                         When in E-BSR state                           |
+------------+-------------------+-------------------+------------------+
| Event      |  Receive          |  Bootstrap        |  Receive Non-    |
|            |  Preferred BSM    |  Timer Expires    |  preferred BSM   |
+------------+-------------------+-------------------+------------------+
|            |  -> C-BSR state   |  -> E-BSR state   |  -> E-BSR state  |
|            |  Forward BSM;     |  Originate BSM;   |  Originate BSM;  |
| Action     |  Store RP-Set;    |  Set Bootstrap    |  Set Bootstrap   |
|            |  Set Bootstrap    |  Timer to         |  Timer to        |
|            |  Timer to         |  BS_Period        |  BS_Period       |
|            |  BS_Timeout       |                   |                  |
+------------+-------------------+-------------------+------------------+


A Candidate-BSR may be in one of three states for a particular scope
zone:

Candidate-BSR (C-BSR)
     The router is a candidate to be the BSR for the scope zone, but
     currently another router is the preferred BSR.

Pending-BSR (P-BSR)
     The router is a candidate to be the BSR for the scope zone.
     Currently no other router is the preferred BSR, but this router is
     not yet the elected BSR.  This is a temporary state that prevents
     rapid thrashing of the choice of BSR during BSR election.

Elected-BSR (E-BSR)
     The router is the elected BSR for the scope zone and it must
     perform all the BSR functions.





Bhaskar/Gall/Lingard/Venaas                    Section 3.1.1.  [Page 11]

INTERNET-DRAFT             Expires: April 2006              October 2005


In addition to the three states, there is one timer:

o The Bootstrap Timer (BST) - used to time out old bootstrap router
  information, and used in the election process to terminate P-BSR
  state.

On startup, the initial state for this configured scope zone is
"Pending-BSR"; the Bootstrap Timer is initialized to BS_Rand_Override.


3.1.2.  Per-Scope-Zone State Machine for Non-Candidate-BSR Routers


+-----------------------------------------------------------------------+
|                        When in NoInfo state                           |
+---------------------+-------------------------------------------------+
|     Event           |        Receive BSM                              |
+---------------------+-------------------------------------------------+
|                     |        -> AP state                              |
|     Action          |        Forward BSM; Store RP-Set;               |
|                     |        Set Bootstrap Timer to BS_Timeout;       |
|                     |        Set SZT to SZ_Timeout                    |
+---------------------+-------------------------------------------------+


+-----------------------------------------------------------------------+
|                       When in Accept Any state                        |
+---------------+----------------------------+--------------------------+
|   Event       |    Receive BSM             |     Scope-Zone Expiry    |
|               |                            |     Timer Expires        |
+---------------+----------------------------+--------------------------+
|               |    -> AP state             |     -> NoInfo state      |
|               |    Forward BSM; Store      |     Cancel timers;       |
|   Action      |    RP-Set; Set             |     Clear state          |
|               |    Bootstrap Timer to      |                          |
|               |    BS_Timeout; Set         |                          |
|               |    SZT to SZ_Timeout       |                          |
+---------------+----------------------------+--------------------------+













Bhaskar/Gall/Lingard/Venaas                    Section 3.1.2.  [Page 12]

INTERNET-DRAFT             Expires: April 2006              October 2005


+-----------------------------------------------------------------------+
|                    When in Accept Preferred state                     |
+----------+-----------------------+------------------+-----------------+
| Event    | Receive Preferred     |  Bootstrap       |  Receive Non-   |
|          | BSM                   |  Timer Expires   |  preferred BSM  |
+----------+-----------------------+------------------+-----------------+
|          | -> AP state           |  -> AA state     |  -> AP state    |
|          | Forward BSM; Store    |  Refresh RP-     |                 |
| Action   | RP-Set; Set           |  Set; Remove     |                 |
|          | Bootstrap Timer to    |  BSR state       |                 |
|          | BS_Timeout; Set SZT   |                  |                 |
|          | to SZ_Timeout         |                  |                 |
+----------+-----------------------+------------------+-----------------+
A router that is not a Candidate-BSR may be in one of three states:

NoInfo
     The router has no information about this scope zone.  This state
     does not apply if the router is configured to know about this scope
     zone, or for the global scope zone.  When in this state, no state
     information is held and no timers run that refer to this scope
     zone.

Accept Any (AA)
     The router does not know of an active BSR, and will accept the
     first Bootstrap message it sees as giving the new BSR's identity
     and the RP-Set.

Accept Preferred (AP)
     The router knows the identity of the current BSR, and is using the
     RP-Set provided by that BSR.  Only Bootstrap messages from that BSR
     or from a C-BSR with higher weight than the current BSR will be
     accepted.

In addition to the three states, there are two timers:

o The Bootstrap Timer (BST) - used to time out old bootstrap router
  information.

o The Scope-Zone Expiry Timer (SZT) - used to time out the scope zone
  itself if Bootstrap messages specifying this scope zone stop arriving.

On startup, the initial state for this scope zone is "Accept Any" for
routers that know about this scope zone, either through configuration or
because the scope zone is the global scope which always exists; the
Scope-Zone Expiry Timer is considered to be always running for such
scope zones.  For routers that do not know about a particular scope
zone, the initial state is NoInfo; no timers exist for the scope zone.




Bhaskar/Gall/Lingard/Venaas                    Section 3.1.2.  [Page 13]

INTERNET-DRAFT             Expires: April 2006              October 2005


3.1.3.  Bootstrap Message Processing Checks

When a Bootstrap message is received, the following initial checks must
be performed:

if ((DirectlyConnected(BSM.src_ip_address) == FALSE) OR
     (we have no Hello state for BSM.src_ip_address)) {
  drop the Bootstrap message silently
}

if (BSM.dst_ip_address == ALL-PIM-ROUTERS) {
  if (BSM.src_ip_address != RPF_neighbor(BSM.BSR_ip_address)) {
    drop the Bootstrap message silently
  }
} else if (BSM.dst_ip_address is one of my addresses) {
  if ((any previous BSM for this scope has been accepted) OR
      (more than BS_Period has elapsed since startup)) {
    #the packet was unicast, but this wasn't
    #a quick refresh on startup
    drop the Bootstrap message silently
  }
} else {
  drop the Bootstrap message silently
}

if (the interface the message arrived on is an Admin Scope
    border for the BSM.first_group_address) {
  drop the Bootstrap message silently
}

Basically, the packet must have come from a directly connected neighbor
for which we have active Hello state.  It must have been sent to the
ALL-PIM-ROUTERS group by the correct upstream router towards the BSR
that originated the Bootstrap message, or the router must have recently
restarted, have no BSR state for that admin scope and have received the
Bootstrap message by unicast.  In addition it must not have arrived on
an interface that is a configured admin scope border for the first group
address contained in the Bootstrap message.

3.1.4.  State Machine Transition Events

If the Bootstrap message passes the initial checks above without being
discarded, then it may cause a state transition event in one of the
above state machines.  For both candidate and non-candidate BSRs, the
following transition events are defined:

     Receive Preferred BSM
          A Bootstrap message is received from a BSR that has higher or



Bhaskar/Gall/Lingard/Venaas                    Section 3.1.4.  [Page 14]

INTERNET-DRAFT             Expires: April 2006              October 2005


          equal weight than the current BSR.  If a router is in P-BSR
          state, then it uses its own weight as that of the current BSR.

          A Bootstrap message is also preferred if it is from the
          current BSR with a lower weight than the previous BSM it sent,
          provided that if the router is a Candidate BSR the current BSR
          still has a weight higher or equal than the router itself.  In
          this case, the "Current Bootstrap Router's BSR Priority" state
          must be updated. (For lower weight, see Non-preferred BSM from
          Elected BSR case.)

          The weight of a BSR is defined to be the concatenation in
          fixed-precision unsigned arithmetic of the BSR Priority field
          from the Bootstrap message and the IP address of the BSR from
          the Bootstrap message (with the BSR Priority taking the most-
          significant bits and the IP address taking the least
          significant bits).

     Receive Non-preferred BSM
          A Bootstrap message is received from a BSR that has lower
          weight than the current BSR.  If a router is in P-BSR state,
          then it uses its own weight as that of the current BSR.

     Receive Non-preferred BSM from Elected BSR
          A Bootstrap message is received from the elected BSR, but the
          BSR Priority field in the received message has changed, so
          that now the currently elected BSR has lower weight that the
          router itself.

     Receive BSM
          A Bootstrap message is received, regardless of BSR weight.

In addition to state machine transitions caused by the receipt of
Bootstrap messages, a state machine transition takes place each time the
Bootstrap Timer or Scope-Zone Expiry Timer expires.

3.1.5.  State Machine Actions

The state machines specify actions that include setting the Bootstrap
Timer and the Scope-Zone Expiry Timer to various values.  These values
are defined in Section 5.

In addition to setting and cancelling the timers, the following actions
may be triggered by state changes in the state machines:

     Forward BSM
          A Bootstrap message that passes the Bootstrap Message
          Processing Checks is forwarded out of all interfaces with PIM



Bhaskar/Gall/Lingard/Venaas                    Section 3.1.5.  [Page 15]

INTERNET-DRAFT             Expires: April 2006              October 2005


          neighbors (including the interface it is received on), except
          where this would cause the BSM to cross an admin-scope
          boundary for the scope zone indicated in the message.  For
          details, see section 3.4.

     Originate BSM
          A new Bootstrap message is constructed by the BSR, giving the
          BSR's address and BSR priority, and containing the BSR's
          chosen RP-Set.  The message is forwarded out of all interfaces
          on which PIM neighbors exist, except where this would cause
          the BSM to cross an admin-scope boundary for the scope zone
          indicated in the message.

     Store RP-Set
          The router uses the group-to-RP mappings contained in a BSM to
          update its local RP-Set.

          This action is skipped for an empty BSM.  A BSM is empty if it
          contains no group ranges, or if it only contains a single
          group range where that group range has the Admin Scope Zone
          bit set (a scoped BSM) and an RP count of zero.

          If a mapping does not yet exist, it is created and the
          associated Group-to-RP mapping Expiry Timer (GET) is
          initialized with the holdtime from the BSM.

          If a mapping already exists, its GET is set to the holdtime
          from the BSM.  If the holdtime is zero, the mapping is removed
          immediately.  Note that for an existing mapping, the RP
          priority must be updated if changed.

          Mappings for a group range are also to be immediately removed
          if they are not present in the received group range.  This
          means that if there are any existing Group-to-RP mappings for
          a range where the respective RPs are not in the received
          range, then those mappings must be removed.

          All RP mappings associated with the scope zone of the BSM are
          updated with the new hash mask length from the received BSM.
          This includes any RP mappings learned from the current BSR but
          not contained in the received BSM, as well as any RP mappings
          learned from any previous BSR for the scope zone.

          In addition, the entire BSM is stored for use in the action
          Refresh RP-Set and to prime a new PIM neighbor as described
          below.





Bhaskar/Gall/Lingard/Venaas                    Section 3.1.5.  [Page 16]

INTERNET-DRAFT             Expires: April 2006              October 2005


     Refresh RP-Set
          When the Bootstrap Timer expires, the router uses the copy of
          the last BSM that it has received to refresh its RP-Set
          according to the action Store RP-Set as if it had just
          received it.  This will increase the chance that the group-to-
          RP mappings will not expire during the election of the new
          BSR.

     Remove BSR state
          When the Bootstrap Timer expires, all state associated with
          the current BSR is removed (see section 2).  Note that this
          does not include any group-to-RP mappings.

3.2.  Sending Candidate-RP-Advertisement Messages

Every C-RP periodically unicasts a C-RP-Adv message to the BSR for each
scope zone for which it has state, to inform the BSR of the C-RP's
willingness to function as an RP.  These messages are sent with an
interval of C_RP_Adv_Period, except when a new BSR is elected, see
below.

When a new BSR is elected, the C-RP SHOULD send one to three C-RP-Adv
messages, waiting a randomized amount of 0-3 seconds before sending each
message.  We recommend sending three messages because it is important
that the BSR quickly learns which RPs are active, and some packet loss
may occur when a new BSR is elected due to changes in the network.  One
way of implementing this is to set the CRPT to 0-3 seconds when the new
BSR is elected, as well as setting a counter to 2.  Whenever the CRPT
expires, we first send a C-RP-Adv message as usual.  Next, if the
counter is non-zero, it is decremented and the CRPT is again set to 0-3
seconds instead of C_RP_Adv_Period.

[NOTE: Add a name for this timer and counter?]

The Priority field in these messages is used by the BSR to select which
C-RPs to include in the RP-Set.  Note that lower values of this field
indicate higher priorities, so that a value of zero is the highest
possible priority.  C-RPs should by default send C-RP-Adv messages with
the Priority field set to 192.

When a C-RP is being shut down, it SHOULD immediately send a C-RP-Adv
message to the BSR for each scope zone for which it is currently serving
as an RP; the Holdtime in this C-RP-Adv message should be zero.  The BSR
will then immediately time out the C-RP and generate a new Bootstrap
message removing the shut down RP from the RP-Set.

[NOTE: Should a new BSM be sent immediately when a C-RP-Adv message with
Holdtime of 0 is received?  Need to clarify.]



Bhaskar/Gall/Lingard/Venaas                      Section 3.2.  [Page 17]

INTERNET-DRAFT             Expires: April 2006              October 2005


A C-RP-Adv message carries a list of group address and group mask field
pairs.  This enables the C-RP to specify the group prefixes for which it
is willing to be the RP.  If the C-RP becomes an RP, it may enforce this
scope acceptance when receiving Register or Join/Prune messages.

A C-RP is configured with a list of group ranges for which it should
advertise itself as the C-RP.  A C-RP uses the following algorithm to
determine which ranges to send to a given BSR.

For each group range R in the list, the C-RP advertises that range to
the scoped BSR for the smallest scope that "contains" R.  For IPv6, the
containing scope is determined by matching the scope identifier of the
group range with the scope of the BSR.  For IPv4, it is the longest-
prefix match for R, amongst the known admin-scope ranges.  If no scope
is found to contain the group range the C-RP includes it in the C-RP-Adv
sent to the non-scoped BSR.  If a non-scoped BSR is not known, the range
is not included in any C-RP-Adv.

In addition, for each IPv4 group range R in the list, for each scoped
BSR whose scope range is strictly contained within R, the C-RP SHOULD by
default advertise that BSR's scope range to that BSR.  And for each IPv6
group range R in the list with prefix length < 16, the C-RP SHOULD by
default advertise each sub-range of prefix length 16 to the scoped BSR
with the corresponding scope ID.  An implementation MAY supply a
configuration option to prevent the behavior described in this
paragraph, but such an option SHOULD be disabled by default.

For IPv6, the mask length of all group ranges included in the C-RP-Adv
message sent to a scoped BSR MUST be >= 16.

If the above algorithm determines that there are no group ranges to
advertise to the BSR for a particular scope zone, a C-RP-Adv message
MUST NOT be sent to that BSR.  A C-RP MUST NOT send a C-RP-Adv message
with no group ranges in it.

If the same router is the BSR for more than one scope zone, the C-RP-Adv
messages for these scope zones MAY be combined into a single message.

If the C-RP is a ZBR for an admin scope zone, then the Admin Scope Zone
bit MUST be set in the C-RP-Adv messages it sends for that scope zone;
otherwise this bit MUST NOT be set.  This information is currently only
used for logging purposes by the BSR, but might allow for future
extensions of the protocol.

3.3.  Creating the RP-Set at the BSR

Upon receiving a C-RP-Adv message, the router needs to decide whether or
not to accept each of the group ranges included in the message.  For



Bhaskar/Gall/Lingard/Venaas                      Section 3.3.  [Page 18]

INTERNET-DRAFT             Expires: April 2006              October 2005


each group range in the message, the router checks to see if it is the
elected BSR for any scope zone that contains the group range, or if it
is elected as the non-scoped BSR.  If so, the group range is accepted;
if not, the group range is ignored.

If the group range is accepted, a group-to-C-RP mapping is created for
this group range and the RP Address from the C-RP-Adv message.

If the mapping is not already part of the C-RP-Set, it is added to the
C-RP-Set and the associated Group-to-C-RP mapping Expiry Timer (CGET) is
initialized to the holdtime from the C-RP-Adv message.  Its priority is
set to the Priority from the C-RP-Adv message.

If the mapping is already part of the C-RP-Set, it is updated with the
Priority from the C-RP-Adv message and its associated CGET is reset to
the holdtime from the C-RP-Adv message.  If the holdtime is zero, the
mapping is immediately removed from the C-RP-Set.

The hash mask length is a global property of the BSR and is therefore
the same for all mappings managed by the BSR.

For compatibility with the previous version of the BSR specification, a
C-RP-Adv message with no group ranges SHOULD be treated as though it
contained the single group range ff00::/8 or 224/4.  Therefore,
according to the rule above, this group range will be accepted if and
only if the router is elected as the non-scoped BSR.

When a CGET expires, the corresponding group-to-C-RP mapping is removed
from the C-RP-Set.

The BSR constructs the RP-Set from the C-RP-Set.  It may apply a local
policy to limit the number of Candidate-RPs included in the RP-Set.  The
BSR may override the prefix indicated in a C-RP-Adv message unless the
`Priority' field from the C-RP-Adv message is less than 128.

For inclusion in a BSM, the RP-Set is subdivided into sets of {group-
prefix, RP-Count, RP-addresses}.  For each RP-address, the corresponding
Holdtime is included in the "RP-Holdtime" field.  The format of the
Bootstrap message allows `semantic fragmentation', if the length of the
original Bootstrap message exceeds the packet maximum boundaries.
However, we recommend against configuring a large number of routers as
C-RPs, to reduce the semantic fragmentation required.

In general BSMs are originated at regular intervals according to the
BS_Period timer.  We do recommend that a BSM is also originated whenever
the RP-set to be announced in the BSMs changes.  This will usually
happen when receiving C-RP advertisements from a new C-RP, or when a C-
RP is shut down (C-RP advertisement with a holdtime of zero).  There



Bhaskar/Gall/Lingard/Venaas                      Section 3.3.  [Page 19]

INTERNET-DRAFT             Expires: April 2006              October 2005


MUST however be a minimum of 10 seconds between each time a BSM is sent.
In particular, when a new BSR is elected, it will first send one BSM
(which is likely to be empty since it has not yet received any C-RP
advertisements), and then wait at least 10 seconds before sending a new
one.  During those 10 seconds, it is likely to have received C-RP
advertisements from all usable C-RPs (since we say that a C-RP should
send one or more advertisements with small random delays of 0-3 seconds
when a new BSR is elected).  For this case in particular, where routers
may not have a usable RP-set, we recommend originating a BSM as soon as
those 10 seconds have passed.  We suggest though that a BSR can do this
in general.  One way of implementing this, is to decrease the Bootstrap
Timer to 10 seconds whenever the RP-set changes, while not changing the
timer if it is less or equal to 10.

[NOTE: Add a name for this 10s value as a function of the 0-3s random
delay?]

A BSR originates separate scoped BSMs for each scope zone for which it
is the elected BSR, as well as originating non-scoped BSMs if it is the
elected non-scoped BSR.

Each group-to-C-RP mapping is included in precisely one of these BSM,
namely the scoped BSM for the narrowest scope containing the group range
of the mapping, if any, or the non-scoped BSM otherwise.

A scoped BSM MUST have at least one group range, and the first group
range in a scoped BSM MUST have the "Admin Scope Zone" bit set.  This
group range identifies the scope of the BSM.  In a scoped IPv4 BSM, the
first group range is the range corresponding to the scope of the BSM.
In a scoped IPv6 BSM, the first group range may be any group range
subject to the general condition that all the group ranges in such a BSM
MUST have a mask length of at least 16 and MUST have the same scope ID
as the scope of the BSM.

RP mappings may be included in the first group range of a BSM, just as
for any other group range.  After this group range, other group ranges
for which there are RP mappings appear in any order.

The "Admin Scope Zone" bit of all group ranges other than the first
SHOULD be set to 0 on origination, and MUST be ignored on receipt.

When an elected BSR is being shut down, it should immediately originate
a Bootstrap message listing its current RP-Set, but with the BSR
Priority field set to the lowest priority value possible.  This will
cause the election of a new BSR to happen more quickly.






Bhaskar/Gall/Lingard/Venaas                      Section 3.3.  [Page 20]

INTERNET-DRAFT             Expires: April 2006              October 2005


3.4.  Forwarding Bootstrap Messages

Bootstrap messages originate at the BSR, and are hop-by-hop forwarded by
intermediate routers if they pass the Bootstrap Message Processing
Checks.  When a Bootstrap message is forwarded, it is forwarded out of
every multicast-capable interface which has PIM neighbors (including the
one over which the message was received).  The exception to this is if
the interface is an administrative scope boundary for the admin scope
zone indicated in the first group address in the Bootstrap message
packet.

As an optimization, a router MAY choose not to forward a BSM out of the
interface the message was received on if that interface is a point-to-
point interface.  On interfaces with multiple PIM neighbors, a router
SHOULD forward an accepted BSM onto the interface that BSM was received
on, but if the number of PIM neighbors on that interface is large, it
MAY delay forwarding a BSM onto that interface by a small randomized
interval to prevent message implosion.  A configuration option MAY be
provided to disable forwarding onto the interface a message was received
on, but we recommend that the default behavior is to forward onto that
interface.

Rationale: A BSM needs to be forwarded onto the interface the message
was received on (in addition to the other interfaces) because the
routers on a LAN may not have consistent routing information.  If three
routers on a LAN are A, B, and C, and at router B RPF(BSR)==A and at
router C RPF(BSR)==B, then router A originally forwards the BSM onto the
LAN, but router C will only accept it when router B re-forwards the
message onto the LAN.  If the underlying routing protocol configuration
guarantees that the routers have consistent routing information, then
forwarding onto the incoming interface may safely be disabled.

A ZBR constrains all BSMs which are of equal or smaller scope than the
configured boundary.  That is, the BSMs are not accepted from,
originated or forwarded on the interfaces on which the boundary is
configured.  For IPv6 the check is a comparison between the scope of the
first range in the scoped BSM and the scope of the configured boundary.
For IPv4, the first range in the scoped BSM is checked to see if it is
contained in or is the same as the range of the configured boundary.

3.5.  Unicasting Bootstrap Messages to New and Rebooting Routers

To allow new or rebooting routers to learn the RP-Set quickly, when a
Hello message is received from a new neighbor, or a Hello message with a
new GenID is received from an existing neighbor, one router on the LAN
unicasts a stored copy of the Bootstrap message for each admin scope
zone to the new or rebooting router.




Bhaskar/Gall/Lingard/Venaas                      Section 3.5.  [Page 21]

INTERNET-DRAFT             Expires: April 2006              October 2005


The router that does this is the Designated Router (DR) on the LAN, or,
if the new or rebooting router is the DR, the router that would be the
DR if the new or rebooting router were excluded from the DR election
process.

Before unicasting a Bootstrap message in this manner, the DR must wait
until it has sent a triggered Hello message on this interface;
otherwise, the new neighbor will discard the Bootstrap message.

3.6.  Receiving and Using the RP-Set

The RP-Set maintained by BSR is used by RP-based multicast routing
protocols like PIM-SM and BIDIR-PIM.  These protocols may obtain RP-Sets
from other sources as well.  How the final group-to-RP mappings are
obtained from these RP-Sets is not part of the BSR specification.  In
general, the routing protocols need to re-calculate the mappings when
any of their RP-Sets change.  How such a change is signalled to the
routing protocol is also not part of the present specification.

Some group-to-RP mappings in the RP-Set indicate group ranges for which
PIM-SM should be used; others indicate group ranges for use with BIDIR-
PIM.  Routers that only support one of these protocols MUST NOT ignore
ranges indicated as being for the other protocol.  They MUST NOT treat
them as being for the protocol they support.

4.  Message Formats

BSR messages are PIM messages, as defined in [1].  The values of the PIM
Message Type field for BSR messages are:

4    Bootstrap

8    Candidate-RP-Advertisement

As with all other PIM control messages, BSR messages have IP protocol
number 103.

Candidate-RP-Advertisement messages are unicast to a BSR.  Usually,
Bootstrap messages are multicast with TTL 1 to the ALL-PIM-ROUTERS
group, but in some circumstances (described in section 3.5) Bootstrap
messages are unicast to a specific PIM neighbor.

The IP source address used for Candidate-RP-Advertisement messages is a
domain-wide reachable address.  The IP source address used for Bootstrap
messages (regardless of whether they are being originated or forwarded)
is the link-local address of the interface on which the message is being
sent (that is, the same source address that the router uses for the
Hello messages it sends out that interface).



Bhaskar/Gall/Lingard/Venaas                        Section 4.  [Page 22]

INTERNET-DRAFT             Expires: April 2006              October 2005


All Bootstrap and Candidate-RP-Advertisement messages SHOULD carry the
Router Alert IP option.  See section 6 for information about the way in
which the Router Alert option is checked by receving routers.

The IPv4 ALL-PIM-ROUTERS group is 224.0.0.13.  The IPv6 ALL-PIM-ROUTERS
group is ff02::d.

In this section we use the following terms defined in the PIM-SM
specification [1]:

o    Encoded-Unicast format

o    Encoded-Group format

We repeat these here to aid readability.

Encoded-Unicast address

An Encoded-Unicast address takes the following 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Addr Family  | Encoding Type |     Unicast Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...


Addr Family
     The PIM address family of the `Unicast Address' field of this
     address.

     Values of 0-127 are as assigned by the IANA for Internet Address
     Families in [10].  Values 128-250 are reserved to be assigned by
     the IANA for PIM-specific Address Families.  Values 251 though 255
     are designated for private use.  As there is no assignment
     authority for this space, collisions should be expected.

Encoding Type
     The type of encoding used within a specific Address Family.  The
     value `0' is reserved for this field, and represents the native
     encoding of the Address Family.

Unicast Address
     The unicast address as represented by the given Address Family and
     Encoding Type.






Bhaskar/Gall/Lingard/Venaas                        Section 4.  [Page 23]

INTERNET-DRAFT             Expires: April 2006              October 2005


Encoded-Group address

Encoded-Group addresses take the following 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Addr Family  | Encoding Type |B| Reserved  |Z|  Mask Len     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                Group multicast Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...


Addr Family
     described above.

Encoding Type
     described above.

[B]IDIR bit
     When set, all BIDIR capable PIM routers will operate the protocol
     described in [2] for the specified group range.

Reserved
     Transmitted as zero.  Ignored upon receipt.

Admin Scope [Z]one
     When set, this bit indicates that this group address range is an
     administratively scoped range.

Mask Len
     The Mask length field is 8 bits.  The value is the number of
     contiguous one bits left justified used as a mask which, combined
     with the group address, describes a range of groups.  It is less
     than or equal to the address length in bits for the given Address
     Family and Encoding Type.  If the message is sent for a single
     group then the Mask length must equal the address length in bits
     for the given Address Family and Encoding Type.  (e.g. 32 for IPv4
     native encoding and 128 for IPv6 native encoding).

Group multicast Address
     Contains the group address.

4.1.  Bootstrap Message Format

A Bootstrap message is divided up into `semantic fragments' if the
original message exceeds the maximum packet size boundaries.  Basically,
a single Bootstrap message can be sent as multiple packets (semantic



Bhaskar/Gall/Lingard/Venaas                      Section 4.1.  [Page 24]

INTERNET-DRAFT             Expires: April 2006              October 2005


fragments), so long as the fragment tags of all the packets comprising
the message is the same.

The format of a single `fragment' is given below:















































Bhaskar/Gall/Lingard/Venaas                      Section 4.1.  [Page 25]

INTERNET-DRAFT             Expires: April 2006              October 2005


 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type  |   Reserved    |           Checksum            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         Fragment Tag          | Hash Mask Len | BSR Priority  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             BSR Address (Encoded-Unicast format)              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Group Address 1 (Encoded-Group format)             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP Count 1    | Frag RP Cnt 1 |         Reserved              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             RP Address 1 (Encoded-Unicast format)             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          RP1 Holdtime         | RP1 Priority  |   Reserved    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             RP Address 2 (Encoded-Unicast format)             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          RP2 Holdtime         | RP2 Priority  |   Reserved    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               .                               |
|                               .                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             RP Address m (Encoded-Unicast format)             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          RPm Holdtime         | RPm Priority  |   Reserved    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Group Address 2 (Encoded-Group format)             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               .                               |
|                               .                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Group Address n (Encoded-Group format)             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP Count n    | Frag RP Cnt n |          Reserved             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             RP Address 1 (Encoded-Unicast format)             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          RP1 Holdtime         | RP1 Priority  |   Reserved    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             RP Address 2 (Encoded-Unicast format)             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          RP2 Holdtime         | RP2 Priority  |   Reserved    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               .                               |
|                               .                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



Bhaskar/Gall/Lingard/Venaas                      Section 4.1.  [Page 26]

INTERNET-DRAFT             Expires: April 2006              October 2005


|             RP Address m (Encoded-Unicast format)             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          RPm Holdtime         | RPm Priority  |   Reserved    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


PIM Version, Reserved, Checksum
     Described in [1].

Type
     PIM Message Type.  Value is 4 for a Bootstrap message.

Fragment Tag
     A randomly generated number, acts to distinguish the fragments
     belonging to different Bootstrap messages; fragments belonging to
     same Bootstrap message carry the same `Fragment Tag'.

Hash Mask Len
     The length (in bits) of the mask to use in the hash function.  For
     IPv4 we recommend a value of 30.  For IPv6 we recommend a value of
     126.  This field SHOULD be the same for all fragments belonging to
     the same Bootstrap message.

BSR Priority
     Contains the BSR priority value of the included BSR.  This field is
     considered as a high order byte when comparing BSR addresses.  Note
     that for historical reasons, the highest BSR priority is 255 (the
     higher the better), whereas the highest RP Priority (see below) is
     0 (the lower the better).

BSR Address
     The address of the bootstrap router for the domain.  The format for
     this address is given in the Encoded-Unicast address in [1].

Group Address 1..n
     The group prefix (address and mask) with which the Candidate-RPs
     are associated.  Format described in [1]. In a fragment containing
     admin scope ranges, the first group address in the fragment MUST
     satisfy the following conditions: it MUST have the Admin Scope bit
     set; for IPv4 it MUST be the group range for the entire admin scope
     range (this is the case even if there are no RPs in the RP-Set for
     the entire admin scope range - in this case the sub-ranges for the
     RP-Set are specified later in the fragment along with their RPs);
     for IPv6 the Mask Len MUST be at least 16 and have the scope ID of
     the admin scope range.






Bhaskar/Gall/Lingard/Venaas                      Section 4.1.  [Page 27]

INTERNET-DRAFT             Expires: April 2006              October 2005


RP Count 1..n
     The number of Candidate-RP addresses included in the whole
     Bootstrap message for the corresponding group prefix.  A router
     does not replace its old RP-Set for a given group prefix
     until/unless it receives `RP-Count' addresses for that prefix; the
     addresses could be carried over several fragments.  If only part of
     the RP-Set for a given group prefix was received, the router
     discards it, without updating that specific group prefix's RP-Set.

Frag RP Cnt 1..m
     The number of Candidate-RP addresses included in this fragment of
     the Bootstrap message, for the corresponding group prefix.  The
     `Frag RP Cnt' field facilitates parsing of the RP-Set for a given
     group prefix, when carried over more than one fragment.

RP address 1..m
     The address of the Candidate-RPs, for the corresponding group
     prefix.  The format for these addresses is given in the Encoded-
     Unicast address in [1].

RP1..m Holdtime
     The Holdtime (in seconds) for the corresponding RP.  This field is
     copied from the `Holdtime' field of the associated RP stored at the
     BSR.

RP1..m Priority
     The `Priority' of the corresponding RP and Encoded-Group Address.
     This field is copied from the `Priority' field stored at the BSR
     when receiving a C-RP-Adv message.  The highest priority is `0'
     (i.e. unlike BSR priority, the lower the value of the `Priority'
     field, the better).  Note that the priority is per RP per Group
     Address.

Within a Bootstrap message, the BSR Address, all the Group Addresses and
all the RP Addresses MUST be of the same address family.  In addition,
the address family of the fields in the message MUST be the same as the
IP source and destination addresses of the packet.  This permits maximum
implementation flexibility for dual-stack IPv4/IPv6 routers.

4.1.1.  Semantic Fragmentation of BSMs

Bootstrap messages may be split over several PIM Bootstrap Message
Fragment (BSMF) packets; this is known as semantic fragmentation.  It is
needed when the BSM would exceed the MTU of the link the packet will be
forwarded over.

The packet would be too large because the set of group prefixes and the
RPs for each group prefix are too long to fit in one packet.  The BSR



Bhaskar/Gall/Lingard/Venaas                    Section 4.1.1.  [Page 28]

INTERNET-DRAFT             Expires: April 2006              October 2005


will then split the BSM across several BSMF packets; each of these must
be a well-formed BSMF packet in its own right.

If the BSR can split up the BSM so that different group prefixes (and
their RP information) can fit in different fragments, then it should do
so.  If one of these BSMF packets is then lost, the state from the
previous BSM for the group-prefix from the missing packet will be
retained.  Each fragment that does arrive will update the RP information
for the group-prefixes contained in that fragment, and the new group-to-
RP mapping for those can be used immediately.  The information from the
missing fragment will be obtained when the BSM is next transmitted.  In
this case, whilst the Fragment Tag must be set to the same value for all
BSMFs comprising a single BSM, the tag is not actually used by routers
receiving the BSM.

If the list of RPs for a single group-prefix is too long to fit in a
single BSMF packet, then that information must be split across multiple
BSMF packets.  In this case, all the BSMF packets comprising the
information for that group-prefix must be received before the group-to-
RP mapping in use can be modified.  This is the purpose of the RP Count
field - a router receiving BSMF packets from the same BSM (ie that have
the same fragment tag) must wait until the BSMFs providing RP Count RPs
for that group-prefix have been received before the new group-to-RP
mapping can be used for that group-prefix.  If a single BSMF from such a
large group-prefix is lost, then that entire group-prefix will have to
wait until the next BSM is originated.

Next we need to consider how a BSR would remove group-prefixes from the
BSM.  A router receiving a set of BSMFs cannot tell if a group-prefix is
missing.  If it has seen a group-prefix before, it must assume that that
group-prefix still exists, and that the BSMF describing it has been
lost.  It should retain this information for BS_Timeout.  Thus for a BSR
to remove a group-prefix from the BSR, it should include that group-
prefix, but with a RP Count of zero, and it should resend this
information in each BSM for BS_Timeout.
















Bhaskar/Gall/Lingard/Venaas                      Section 4.2.  [Page 29]

INTERNET-DRAFT             Expires: April 2006              October 2005


4.2.  Candidate-RP-Advertisement Message Format

Candidate-RP-Advertisement messages are periodically unicast from the C-
RPs to the BSR.

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type  |   Reserved    |           Checksum            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Count  |   Priority    |           Holdtime            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|             RP Address (Encoded-Unicast format)               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Group Address 1 (Encoded-Group format)             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               .                               |
|                               .                               |
|                               .                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Group Address n (Encoded-Group format)             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


PIM Version, Reserved, Checksum
     Described in [1].

Type
     PIM Message Type.  Value is 8 for a Candidate-RP-Advertisement
     message.

Prefix Count
     The number of encoded group addresses included in the message;
     indicating the group prefixes for which the C-RP is advertising.
     C-RPs MUST NOT send C-RP-Adv messages with a Prefix Count of `0'.

Priority
     The `Priority' of the included RP, for the corresponding Encoded-
     Group Address (if any).  The highest priority is `0' (i.e. the
     lower the value of the `Priority' field, the higher the priority).
     This field is stored at the BSR upon receipt along with the RP
     address and corresponding Encoded-Group Address.

Holdtime
     The amount of time (in seconds) the advertisement is valid.  This
     field allows advertisements to be aged out.  This field should be
     set to 2.5 times C_RP_Adv_Period.




Bhaskar/Gall/Lingard/Venaas                      Section 4.2.  [Page 30]

INTERNET-DRAFT             Expires: April 2006              October 2005


RP Address
     The address of the interface to advertise as a Candidate RP.  The
     format for this address is given in the Encoded-Unicast address in
     [1].

Group Address-1..n
     The group prefixes for which the C-RP is advertising.  Format
     described in Encoded-Group-Address in [1].

Within a Candidate-RP-Advertisement message, the RP Address and all the
Group Addresses MUST be of the same address family.  In addition, the
address family of the fields in the message MUST be the same as the IP
source and destination addresses of the packet.  This permits maximum
implementation flexibility for dual-stack IPv4/IPv6 routers.

5.  Timers and Timer Values

Timer Name: Bootstrap Timer (BST(Z))

+---------------------+--------------------------+----------------------+
| Value Name          |  Value                   |   Explanation        |
+---------------------+--------------------------+----------------------+
| BS_Period           |  Default: 60 seconds     |   Periodic interval  |
|                     |                          |   with which BSMs    |
|                     |                          |   are normally       |
|                     |                          |   originated         |
+---------------------+--------------------------+----------------------+
| BS_Timeout          |  Default: 130 seconds    |   Interval after     |
|                     |                          |   which a BSR is     |
|                     |                          |   timed out if no    |
|                     |                          |   BSM is received    |
|                     |                          |   from that BSR      |
+---------------------+--------------------------+----------------------+
| BS_Rand_Override    |  see below               |   Randomized         |
|                     |                          |   interval used to   |
|                     |                          |   reduce control     |
|                     |                          |   message overhead   |
|                     |                          |   during BSR         |
|                     |                          |   election           |
+---------------------+--------------------------+----------------------+

Note that BS_Timeout MUST be larger than BS_Period, even if their values
are changed from the defaults.  We recommend that BS_Timeout is set to 2
times BS_Period plus 10 seconds.







Bhaskar/Gall/Lingard/Venaas                        Section 5.  [Page 31]

INTERNET-DRAFT             Expires: April 2006              October 2005


BS_Rand_Override is calculated using the following pseudocode, in which
all values are in units of seconds.  The values of BS_Rand_Override
generated by this pseudocode are between 5 and 23 seconds, with smaller
values generated if the C-BSR has a high bootstrap weight, and larger
values generated if the C-BSR has a low bootstrap weight.

     BS_Rand_Override = 5 + priorityDelay + addrDelay

where priorityDelay is given by:

     priorityDelay = 2 * log_2(1 + bestPriority - myPriority)

and addrDelay is given by the following for IPv4:

     if (bestPriority == myPriority) {
         addrDelay = log_2(1 + bestAddr - myAddr) / 16
     } else {
         addrDelay = 2 - (myAddr / 2^31)
     }

and addrDelay is given by the following for IPv6:

     if (bestPriority == myPriority) {
         addrDelay = log_2(1 + bestAddr - myAddr) / 64
     } else {
         addrDelay = 2 - (myAddr / 2^127)
     }

and bestPriority is given by:

     bestPriority = max(storedPriority, myPriority)

and bestAddr is given by:

     bestAddr = max(storedAddr, myAddr)

and where myAddr is the Candidate-BSR's address, storedAddr is the
stored BSR's address, myPriority is the Candidate-BSR's configured
priority, and storedPriority is the stored BSR's priority.












Bhaskar/Gall/Lingard/Venaas                        Section 5.  [Page 32]

INTERNET-DRAFT             Expires: April 2006              October 2005


Timer Name: Scope Zone Expiry Timer (SZT(Z))

+----------------+-----------------------------+------------------------+
|  Value Name    |   Value                     |   Explanation          |
+----------------+-----------------------------+------------------------+
|  SZ_Timeout    |   Default: 1300 seconds     |   Interval after       |
|                |                             |   which a scope zone   |
|                |                             |   is timed out if no   |
|                |                             |   BSM is received      |
|                |                             |   for that scope       |
|                |                             |   zone                 |
+----------------+-----------------------------+------------------------+

Note that SZ_Timeout MUST be larger than BS_Timeout, even if their
values are changed from the defaults.  We recommend that SZ_Timeout is
set to 10 times BS_Timeout.

Timer Name: Group-to-C-RP mapping Expiry Timer (CGET(M,Z))

+--------------------------+--------------------+-----------------------+
|  Value Name              |    Value           |    Explanation        |
+--------------------------+--------------------+-----------------------+
|  C-RP Mapping Timeout    |    from message    |    Holdtime from C-   |
|                          |                    |    RP-Adv message     |
+--------------------------+--------------------+-----------------------+

Timer Name: Group-to-RP mapping Expiry Timer (GET(M,Z))

+-------------------------+--------------------+------------------------+
|  Value Name             |   Value            |    Explanation         |
+-------------------------+--------------------+------------------------+
|  RP Mapping Timeout     |   from message     |    Holdtime from BSM   |
+-------------------------+--------------------+------------------------+

Timer Name: C-RP Advertisement Timer (CRPT)

+---------------------+-------------------------+-----------------------+
|  Value Name         |  Value                  |   Explanation         |
+---------------------+-------------------------+-----------------------+
|  C_RP_Adv_Period    |  Default: 60 seconds    |   Periodic interval   |
|                     |                         |   with which C-RP-    |
|                     |                         |   Adv messages are    |
|                     |                         |   sent to a BSR       |
+---------------------+-------------------------+-----------------------+







Bhaskar/Gall/Lingard/Venaas                        Section 5.  [Page 33]

INTERNET-DRAFT             Expires: April 2006              October 2005


6.  Security Considerations

6.1.  Possible Threats

Threats affecting the PIM BSR mechanism are primarily of two forms:
denial of service attacks, and traffic diversion attacks.  An attacker
that subverts the BSR mechanism can prevent multicast traffic from
reaching the intended recipients, can divert multicast traffic to a
place where they can monitor it, and can potentially flood third parties
with traffic.

Traffic can be prevented from reaching the intended recipients by one of
two mechanisms:

o    Subverting a BSM, and specifying RPs that won't actually forward
     traffic.

o    Registering with the BSR as a C-RP, and then not forwarding
     traffic.

Traffic can be diverted to a place where it can be monitored by both of
the above mechanisms; in this case the RPs would forward the traffic,
but are located so as to aid monitoring or man-in-the-middle attacks on
the multicast traffic.

A third party can be flooded by either of the above two mechanisms by
specifying the third party as the RP, and register-encapsulated traffic
will then be forwarded to them.

6.2.  Limiting Third-Party DoS Attacks

The third party DoS attack above can be greatly reduced if PIM routers
acting as DR do not continue to forward Register traffic to the RP in
the presence of ICMP Protocol Unreachable or ICMP Host Unreachable
responses.  If a PIM router sending Register packets to an RP receives
one of these responses to a data packet it has sent, it should rate-
limit the transmission of future Register packets to that RP for a short
period of time.

As this does not affect interoperability, the precise details are left
to the implementor to decide.  However we note that a router
implementing such rate limiting must only do so if the ICMP packet
correctly echoes part of a Register packet that was sent to the RP.  If
this check were not made, then simply sending ICMP Unreachable packets
to the DR with the source address of the RP spoofed would be sufficient
to cause a denial-of-service attack on the multicast traffic originating
from that DR.




Bhaskar/Gall/Lingard/Venaas                      Section 6.2.  [Page 34]

INTERNET-DRAFT             Expires: April 2006              October 2005


6.3.  Bootstrap Message Security

If a legitimate PIM router is compromised, there is little any security
mechanism can do to prevent that router subverting PIM traffic in that
domain.  However we recommend that implementors provide a mechanism
whereby a PIM router using the BSR mechanisms can be configured with the
IP addresses of valid BSR routers, and that any Bootstrap message from
any other BSR should then be dropped and logged as a security issue.  We
also recommend that this not be enabled by default, as it makes the
initial configuration of a PIM domain problematic - it is the sort of
feature that might be enabled once the configuration of a domain has
stabilized.

The primary security requirement for BSR (as for PIM) is that it is
possible to prevent hosts that are not legitimate PIM routers, either
within or outside the domain, from subverting the BSR mechanism.

The Bootstrap Message Processing Checks prevent a router from accepting
a Bootstrap message from outside of the PIM Domain, as the source
address on Bootstrap messages must be an immediate PIM neighbor.  There
is however a small window of time after a reboot where a PIM router will
accept a bad Bootstrap message unicast from an immediate neighbor, and
it might be possible to unicast a Bootstrap message to a router during
this interval from outside the domain, using the spoofed source address
of a neighbor.  This can be prevented if PMBRs perform source-address
filtering to prevent packets entering the PIM domain with IP source
addresses that are infrastructure addresses in the PIM domain.

The principal threat to Bootstrap message security comes from hosts
within the PIM domain that attempt to subvert the BSR mechanism.  They
may be able to do this by sending PIM messages to their local router, or
by unicasting a Bootstrap message to another PIM router during the brief
interval after it has restarted.

6.3.1.  Rejecting Unicast Bootstrap Messages

All Bootstrap messages SHOULD carry the Router Alert IP option.  If a
PIM router receives a Bootstrap message that does not carry the Router
Alert option, it SHOULD drop it (a configuration option should also be
provided to disable this check on a per-interface basic for backward
compatibility with older PIM routers).  The Router Alert option allows a
PIM router to perform checks on unicast packets it would otherwise
blindly forward.  All PIM routers should check that packets with Router
Alert that are not destined for the router itself are not PIM Bootstrap
messages.  Any such packets should be dropped and logged as a possible
security issue - it is never acceptable for a PIM Bootstrap message to
travel multiple IP hops.




Bhaskar/Gall/Lingard/Venaas                    Section 6.3.1.  [Page 35]

INTERNET-DRAFT             Expires: April 2006              October 2005


6.3.2.  Rejecting Bootstrap Messages from Invalid Neighbors

Most hosts that are likely to attempt to subvert PIM BSR are likely to
be located on leaf subnets.  We recommend that implementors provide a
configuration option that specifies an interface is a leaf subnet, and
that no PIM packets are accepted on such interfaces.

On multi-access subnets with multiple PIM routers and hosts that are not
trusted, we recommend that IPsec AH is used to protect communication
between PIM routers, and that such routers are configured to drop and
log communication attempts from any host that do not pass the
authentication check.  When all the PIM routers are under the same
administrative control, this authentication may use a configured shared
secret.  The securing of interactions between PIM neighbors is discussed
in more detail in the Security Considerations section of [1], and so we
do not discuss the details further here.  The same security mechanisms
that can be used to secure PIM Join, Prune and Assert messages should
also be used to secure Bootstrap messages.

6.4.  Candidate-RP-Advertisement Message Security

Even if it is not possible to subvert Bootstrap messages, an attacker
might be able to perform most of the same attacks by simply sending C-
RP-Adv messages to the BSR specifying the attacker's choice of RPs.
Thus it is necessary to control the sending of C-RP-Adv messages in
essentially the same ways that we control Bootstrap messages.  However,
C-RP-Adv messages are unicast and normally travel multiple hops, so
controlling them is more difficult.

6.4.1.  Non-Cryptographic Security of C-RP-Adv Messages

We specify that C-RP-Adv messages SHOULD also carry the Router Alert IP
option, and that the BSR SHOULD by default drop and log C-RP-Adv
messages that do not carry this option.  Setting Router Alert on these
packets is practical because the rate of C-RP-Adv messages should be
very low, so the extra load on routers forwarding these packets will be
insignificant.  PIM routers forwarding such a packet may then be capable
of checking whether the packet came from a valid PIM neighbor, although
note that such checks are only possible if the unicast and multicast
topologies in the network are congruent.  If this is not the case, it is
legitimate to receive a C-RP-Adv message from a router which is not a
valid PIM neighbor, and therefore in this situation a PIM router MUST
NOT drop C-RP-Adv messages that do not come from a valid PIM neighbor.

If the unicast and multicast topologies are known to be congruent, the
following checks should be made.  On interfaces that are configured to
be leaf subnets, all C-RP-Adv messages should be dropped.  On multi-
access subnets with multiple PIM routers and hosts that are not trusted,



Bhaskar/Gall/Lingard/Venaas                    Section 6.4.1.  [Page 36]

INTERNET-DRAFT             Expires: April 2006              October 2005


the router can at least check that the source MAC address is that of a
valid PIM neighbor.  PMBRs should ensure that no C-RP-Adv messages enter
the domain from an external neighbor.

6.4.2.  Cryptographic Security of C-RP-Adv Messages

For true security, we recommend that all C-RPs are configured to use
IPsec authentication.  The authentication process for a C-RP-Adv message
between a C-RP and the BSR is identical to the authentication process
for PIM Register messages between a DR and the relevant RP, except that
there will normally be fewer C-RPs in a domain than there are DRs, so
key management is a little simpler.  We do not describe the details of
this process further here, but refer to the Security Considerations
section of [1].  Note that the use of cryptographic security for C-RP-
Adv messages does not remove the need for the non-cryptographic
mechanisms, as explained below.

6.5.  Denial of Service using IPsec

An additional concern is that of Denial-of-Service attacks caused by
sending high volumes of Bootstrap messages or C-RP-Adv messages with
invalid IPsec authentication information.  It is possible that these
messages could overwhelm the CPU resources of the recipient.

The non-cryptographic security mechanisms above prevent unicast
Bootstrap messages from traveling multiple hops, and constrain who can
originate such messages.  However, it is obviously important that PIM
messages that are required to have Router Alert checked are checked for
this option before the IPsec AH is checked.  Thus the remaining
vulnerability primarily exists for hosts on multi-access subnets
containing more than one PIM router.  A PIM router receiving PIM packets
with Router Alert set from such a subnet should already be checking that
the source MAC address is that of a valid PIM neighbor, but this is
hardly strong security.  In addition, we recommend that rate-limiting
mechanisms can be configured, to be applied to the forwarding of unicast
PIM packets containing Router Alert options.  The rate-limiter MUST
independently rate-limit different types of PIM packets - for example a
flood of C-RP-Adv messages MUST NOT cause a rate limiter to drop low-
rate Bootstrap messages.  Such a rate-limiter might itself be used to
cause a denial of service attack by causing valid packets to be dropped,
but in practice this is more likely to constrain bad PIM messages close
to their origin.  In addition, the rate limiter will prevent attacks on
PIM from affecting other activity on the destination router, such as
unicast routing.







Bhaskar/Gall/Lingard/Venaas                      Section 6.5.  [Page 37]

INTERNET-DRAFT             Expires: April 2006              October 2005


7.  Contributors

Bill Fenner, Mark Handley, Roger Kermode and David Thaler have
contributed greatly to this draft.  They were authors of this draft up
to version 03.  Most of the current text is identical to 03.

8.  Acknowledgments

PIM-SM was designed over many years by a large group of people,
including ideas from Deborah Estrin, Dino Farinacci, Ahmed Helmy, Steve
Deering, Van Jacobson, C. Liu, Puneet Sharma, Liming Wei, Tom Pusateri,
Tony Ballardie, Scott Brim, Jon Crowcroft, Paul Francis, Joel Halpern,
Horst Hodel, Polly Huang, Stephen Ostrowski, Lixia Zhang, Girish
Chandranmenon, Pavlin Radoslavov, John Zwiebel, Isidor Kouvelas and Hugh
Holbrook.  This BSR specification draws heavily on text from RFC 2362.

Many members of the PIM Working Group have contributed comments and
corrections for this document, including Christopher Thomas Brown, Ardas
Cilingiroglu, Murthy Esakonu, Venugopal Hemige, Prashant Jhingran,
Rishabh Parekh and Katta Sambasivarao.

9.  IANA Considerations

This document has no actions for IANA.

10.  Normative References

[1] W. Fenner, M. Handley, H. Holbrook, I. Kouvelas, "Protocol
     Independent Multicast - Sparse Mode (PIM-SM): Protocol
     Specification (Revised)", Internet Draft draft-ietf-pim-sm-
     v2-new-11.txt

[2] M. Handley, I. Kouvelas, T. Speakman, L. Vicisano, "Bi-directional
     Protocol Independent Multicast (BIDIR-PIM)", Internet Draft draft-
     ietf-pim-bidir-07.txt

[3] D. Meyer, "Administratively Scoped IP Multicast", RFC 2365, Jul
     1998.

[4] S. Deering, B. Haberman, T. Jinmei, E. Nordmark, B. Zill, "IPv6
     Scoped Address Architecture", RFC 4007, Mar 2005.

[5] R. Hinden, S. Deering, "Internet Protocol Version 6 (IPv6)
     Addressing Architecture", RFC 3513, Apr 2003.

[6] S. Bradner, "Key words for use in RFCs to Indicate Requirement
     Levels", BCP 14, RFC 2119, Mar 1997.




Bhaskar/Gall/Lingard/Venaas                       Section 10.  [Page 38]

INTERNET-DRAFT             Expires: April 2006              October 2005


11.  Informative References

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

[8] D. Kim, D. Meyer, H. Kilmer, D. Farinacci, "Anycast Rendevous Point
     (RP) mechanism using Protocol Independent Multicast (PIM) and
     Multicast Source Discovery Protocol (MSDP)", RFC 3446, Jan 2003.

[9] D. Farinacci, Y. Cai, "Anycast-RP using PIM", Internet Draft draft-
     ietf-pim-anycast-rp-04.txt

[10] IANA, "Address Family Numbers", linked from
     http://www.iana.org/numbers.html

Authors' Addresses

     Nidhi Bhaskar
     Cisco Systems
     170 W. Tasman Drive
     San Jose, CA 95134
     USA
     nbhaskar@cisco.com


     Alexander Gall
     SWITCH
     Limmatquai 138
     P.O. Box
     CH-8021 Zurich
     Switzerland
     gall@switch.ch


     James Lingard
     Data Connection Ltd
     100 Church Street
     Enfield
     EN2 6BQ
     United Kingdom
     james@lingard.com









Bhaskar/Gall/Lingard/Venaas                       Section 11.  [Page 39]

INTERNET-DRAFT             Expires: April 2006              October 2005


     Stig Venaas
     UNINETT
     NO-7465 Trondheim
     Norway
     venaas@uninett.no


Copyright Statement

Copyright (C) The Internet Society (2005).  This document is subject to
the rights, licenses and restrictions contained in BCP 78, and except as
set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR
IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in this
document or the extent to which any license under such rights might or
might not be available; nor does it represent that it has made any
independent effort to identify any such rights.  Information on the
procedures with respect to rights in RFC documents can be found in BCP
78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an attempt
made to obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification can be
obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary rights
that may cover technology that may be required to implement this
standard.  Please address the information to the IETF at ietf-
ipr@ietf.org.







Bhaskar/Gall/Lingard/Venaas                       Section 11.  [Page 40]


Html markup produced by rfcmarkup 1.108, available from http://tools.ietf.org/tools/rfcmarkup/