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Internet Engineering Task Force                                   PIM WG
INTERNET-DRAFT                                       Nidhi Bhaskar/Cisco
draft-ietf-pim-sm-bsr-04.txt                       Alexander Gall/SWITCH
                                                     Stig Venaas/UNINETT
                                                            18 July 2004
                                                   Expires: January 2005



                Bootstrap Router (BSR) Mechanism for PIM




Status of this Document


By submitting this Internet-Draft, I certify that any applicable patent
or other IPR claims of which I am aware have been disclosed, or will be
disclosed, and any of which I become aware will be disclosed, in
accordance with RFC 3668.


Internet-Drafts are working documents of the Internet Engineering Task
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Copyright Notice


Copyright (C) The Internet Society (2004). All Rights Reserved.







Bhaskar/Gall/Venaas                                             [Page 1]

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                                Abstract



     This document specifies the Bootstrap Router (BSR) mechanism
     for the class of multicast routing protocols in the PIM
     (Protocol Independent Multicast protocol) 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/Venaas                                             [Page 2]

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                           Table of Contents



     1. Introduction. . . . . . . . . . . . . . . . . . . . . .   4
      1.1. General Overview and Background. . . . . . . . . . .   4
      1.2. Overview of Bootstrap and RP Discovery for
      Global Scope. . . . . . . . . . . . . . . . . . . . . . .   7
      1.3. Administratively Scoped Multicast and BSR. . . . . .   8
      1.4. Bi-directional PIM . . . . . . . . . . . . . . . . .   9
     2. BSR State and Timers. . . . . . . . . . . . . . . . . .   9
     3. Bootstrap Router Election and RP-Set
     Distribution . . . . . . . . . . . . . . . . . . . . . . .  10
      3.1. Sending Candidate-RP-Advertisements. . . . . . . . .  19
      3.2. Creating the RP-Set at the BSR . . . . . . . . . . .  20
      3.3. Forwarding Bootstrap Messages. . . . . . . . . . . .  21
      3.4. Receiving and Using the RP-Set . . . . . . . . . . .  22
     4. Message Formats . . . . . . . . . . . . . . . . . . . .  22
      4.1. Bootstrap Message Format . . . . . . . . . . . . . .  25
       4.1.1. Semantic Fragmentation of BSMs. . . . . . . . . .  28
      4.2. Candidate-RP-Advertisement Format. . . . . . . . . .  30
     5. Default Values for Timers . . . . . . . . . . . . . . .  31
     6. Security Considerations . . . . . . . . . . . . . . . .  32
      6.1. Possible Threats . . . . . . . . . . . . . . . . . .  32
      6.2. Limiting Third-Party DoS Attacks . . . . . . . . . .  33
      6.3. BS Message Security. . . . . . . . . . . . . . . . .  33
      6.4. C-RP-Advertisement Security. . . . . . . . . . . . .  35
      6.5. Denial of Service using IPsec. . . . . . . . . . . .  36
     7. Contributors. . . . . . . . . . . . . . . . . . . . . .  36
     8. Acknowledgments . . . . . . . . . . . . . . . . . . . .  36
     9. IANA Considerations . . . . . . . . . . . . . . . . . .  37
     10. Normative References . . . . . . . . . . . . . . . . .  37
     11. Informative References . . . . . . . . . . . . . . . .  37
     12. Authors' Addresses . . . . . . . . . . . . . . . . . .  37




                            List of Figures



     Figure 1. Per-Scope-Zone State-machine for a candi-
     date BSR . . . . . . . . . . . . . . . . . . . . . . . . .  11
     Figure 2. Per-Scope-Zone State-machine for a router
     not configured as C-BSR. . . . . . . . . . . . . . . . . .  13









Bhaskar/Gall/Venaas                                             [Page 3]

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1.  Introduction


This document assumes familiarity with the workings of Rendezvous Point-
based multicast routing protocols in the PIM protocol family, in
particular with Protocol Independent Multicast - Sparse Mode (PIM-SM),
as defined in [1], and Bi-directional Protocol Independent Multicast
(BIDIR-PIM), as defined in [3], as well as with Administratively Scoped
Multicast, as described in [2].


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


For correct operation, every multicast router within a PIM domain must
be able to map a particular multicast group address to the same RP.  If
this is not the case then black holes may appear, where some receivers
in the domain cannot receive some groups.  A PIM domain in this context
is a contiguous set of routers that all implement the multicast routing
protocol 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.


A PIM domain may also be broken up into multiple administrative scope
regions - these are regions where a border has been configured so that a
range of multicast groups will not be forwarded across that border.  For
more information on Administratively Scoped IP Multicast, see RFC 2365
[2]. The modified criteria for admin-scoped regions are that the region
is convex with respect to forwarding based on the MRIB, and that all PIM
routers within the same scope region map a particular scoped group to
the same RP within that region.


The PIM specifications do not mandate the use of a single mechanism to
provide routers with the information to perform the group-to-RP mapping.
This document describes the Bootstrap Router (BSR) mechanism.  BSR was
first defined in RFC 2362 [5], 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.


1.1.  General Overview and Background


Every PIM multicast group needs to be associated with the IP address of
a Rendezvous Point (RP).  This address is used to build a group-specific
distribution tree rooted at that address 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/Venaas                               Section 1.1.  [Page 4]

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For all senders to reach all receivers, it is crucial that there exists
a single distribution tree.  This can only be achieved when all routers
in the domain use the same mappings of group addresses to RP addresses.


There are a number of ways how 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 also does not
dynamically adapt to route around router or link failures.  The
mechanism specified in this document is known as the PIM BootStrap
Router mechanism, or BSR for short, and provides a dynamic, adaptive
mechanism to distribute group-to-RP mapping information rapidly
throughout a domain.


A group-to-RP mapping contains the following elements.


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


o RP Priority


o <<<<<<< bsr.nrf IP address of RP.


o RP Holdtime ======= IP address of RP


o RP Holdtime >>>>>>> 1.49


o Hash mask length


The collection of all group-to-RP mappings known to a router at any
point in time is called the RP-set.  The router that assumes the role of
the BSR maintains a logically distinct RP-set called the C-RP-set, from
which it constructs the actual RP-set as explained below.


In general, these mapping entries may overlap in arbitrary ways; a
particular multicast group may be covered by multiple mapping entries.
When this is the case, the router chooses only one of the entries by
applying a deterministic algorithm so that all routers in the domain
make the same choice and hence apply the same group-to-RP mapping.  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.


The BSR mechanism provides a way in which viable group-to-RP mappings
can be created and 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 mapping tables will be modified so that the unreachable RP is no
longer used, and the new tables will be rapidly distributed throughout
the domain.




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

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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 as
candidate bootstrap routers (C-BSRs).  One of these C-BSRs will be
elected to serve as 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 stores the group-to-C-RP mappings in its C-RP-
set.  It will chose a subset of these mappings as the actual RP-set and
distribute it to all the routers in the domain through Bootstrap
messages.


The mechanism is complicated slightly by the presence of
administratively-scoped multicast regions within the PIM domain.  An
admin-scope region is a convex connected set of PIM routers surrounded
by an admin-scope boundary.  The boundary specifies a range of multicast
addresses that will not be forwarded into or out of the scoped region.
This complicates BSR because we do not want a PIM router within the
scoped region to use an RP outside the scoped region (or vice-versa).
Thus we need to modify the basic mechanism to ensure that this doesn't
happen - this is done by electing a BSR for every admin-scope region
within a PIM domain, and also a global BSR for the whole PIM domain.  C-
RPs typically register multiple times; once to the BSR of every admin
scope zone the C-RP is in.  For the remainder of this overview we will
ignore admin-scope regions, and concentrate on the global BSR and its
role.  Within each scope zone, the BSR for that zone acts in a similar
manner to how the global BSR acts for the whole domain.


There are four basic phases to the BSR mechanism (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.  In this way, the BSR learns about possible RPs that
     are currently up and reachable.


3.   C-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




Bhaskar/Gall/Venaas                               Section 1.1.  [Page 6]

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     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.  As bootstrap messages are flooded rapidly
     through the domain, this ensures that the RP-Set rapidly reaches
     all the routers in the domain.  BSMs are originated periodically to
     ensure 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.


In the following sections we discuss more details about BSR for global
scope and for admin scoping, before specifying the protocol starting in
section 2.


1.2.  Overview of Bootstrap and RP Discovery for Global Scope


A small set of routers from a domain are configured as candidate
bootstrap routers (C-BSRs) and, through a simple election mechanism, a
single BSR is selected for that domain.  A set of routers within a
domain are also configured as candidate RPs (C-RPs); typically these
will be the same routers that are configured as C-BSRs.  Candidate RPs
periodically unicast Candidate-RP-Advertisement messages (C-RP-Advs) to
the BSR of that domain, advertising their willingness to be an RP.  A C-
RP-Adv message includes the address of the advertising C-RP, as well as
an optional list of group addresses and mask length fields, indicating
the group prefix(es) for which the candidacy is advertised.  The BSR
then includes a set of these Candidate-RPs (the RP-Set), along with
their corresponding group prefixes, in Bootstrap messages it
periodically originates.  Bootstrap messages are distributed hop-by-hop
throughout the domain.


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
bootstrap router timeout value.





Bhaskar/Gall/Venaas                               Section 1.2.  [Page 7]

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1.3.  Administratively Scoped Multicast and BSR


Administratively Scoped IP Multicast, as defined in RFC 2365 [2],
permits a network provider to configure scope boundaries at multicast
routers.  Such a scope boundary consists of a range of multicast
addresses (expressed as an address and mask) that the router will not
forward across the boundary.  For correct operation, such a 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.


For BSR to function correctly with admin scoping, there must be a BSR
and at least one C-RP within every admin scope region.  Admin scope zone
boundaries must be configured at the Zone Border Routers (ZBRs), as they
need to filter PIM Join messages that might inadvertently cross the
border due to error conditions.  In addition, at least one C-BSR within
the admin scope zone must be configured to be a C-BSR for the admin
scope zone's address range.


A separate BSR election will then take place (using bootstrap messages)
for every admin scope range (plus one for the global range).  Admin
scope ranges are identified in the bootstrap message because the group
range is marked (using the "Admin Scope" bit, previously a "Reserved"
bit) to indicate that this is an administrative scope range, and not
just a range that a particular set of RPs are configured to handle.


Such admin scoped bootstrap message packets are flooded in the normal
way, but will not be forwarded by another ZBR across the boundary for
that scope zone (see Section 3.3 for the specifics of this).


We do not require that C-RPs within the scope zone be configured to know
about the scope zone, as they can learn of its existence from bootstrap
messages.  However, we recommend that router vendors implement
configuration options that allow a C-RP to be configured to be a C-RP
for global scope only, for one of more admin scopes only, or for all
scopes, both global and admin scoped.  We also recommend that the
default be that a C-RP is a C-RP for all scopes, both global and admin
scoped.


Unless configured otherwise, C-RPs discover the existence of the admin
scope zone and its group range from receiving a bootstrap message from
the scope zone's elected BSR containing the scope zone's group-range,
marked using the "Admin Scope" bit.  A C-RP stores each elected BSR's
address and the admin scope range contained in its bootstrap message.
It separately unicasts Candidate-RP-Advertisement messages to the
appropriate BSR for every admin scope range within which it is willing




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

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to serve as an RP.


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 (which only allows one BSR per
domain) cannot be used in PIM domains where admin scope zones are
configured.



1.4.  Bi-directional PIM


Routers doing Bi-directional PIM [3] need group-to-RP mappings similar
to PIM-SM.  Group ranges for use by Bi-directional PIM are identified in
bootstrap messages by a "Bi-dir" bit (previously a "Reserved" bit).
Routers that don't support Bi-directional PIM MUST NOT ignore these
ranges.  They MUST NOT treat them as having the default mode, i.e. PIM-
SM.



2.  BSR State and Timers


A PIM router implementing BSR holds the following state


     At all routers:


          List of Active Scope Zones


          Per Scope Zone:


               Bootstrap State:


                    o Bootstrap Router's IP Address


                    o BSR Priority


                    o Bootstrap Timer (BST)


                    o Last BSM received from this BSR


          RP-Set


     At a Candidate BSR:


          Per Scope Zone:


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




Bhaskar/Gall/Venaas                                 Section 2.  [Page 9]

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     At a router that is not a Candidate BSR:


          Per Scope Zone:


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


               Scope-Zone Expiry Timer: SZT(Z)


Bootstrap state is described in section 3, and the RP-Set is described
in section 3.4.


The following timers are also required:


     At the Bootstrap Router only:


          Per Scope Zone (Z):


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


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


     At the C-RPs only:


          C-RP Advertisement Timer: CRPT


     At all routers:


          Per Scope Zone (Z):


               Per group-to-RP mapping (M):


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



3.  Bootstrap Router Election and RP-Set Distribution


For simplicity, bootstrap messages (BSMs) are used in both the BSR
election and the RP-Set distribution mechanisms.


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.








Bhaskar/Gall/Venaas                                Section 3.  [Page 10]

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Per-Scope-Zone Candidate-BSR State Machine



Figure 1: Per-Scope-Zone State-machine for a candidate BSR in tabular form


+-----------------------------------------------------------------------+
|                         When in C-BSR state                           |
+------------+-------------------+-------------------+------------------+
| Event      |  Receive          |  BS Timer         |  Receive non-    |
|            |  Preferred BSM    |  Expires          |  preferred BSM   |
|            |                   |                   |  from Elected    |
|            |                   |                   |  BSR             |
+------------+-------------------+-------------------+------------------+
|            |  -> C-BSR state   |  -> P-BSR state   |  -> P-BSR state  |
|            |  Forward BSM;     |  Set BS Timer     |  Set BS Timer    |
| Action     |  Store RP-Set;    |  to               |  to              |
|            |  Set BS Timer     |  rand_override    |  rand_override   |
|            |  to BS Timeout    |                   |                  |
+------------+-------------------+-------------------+------------------+



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



+-----------------------------------------------------------------------+
|               When in E-BSR state                                     |
+-------------+-------------------+------------------+------------------+
| Event       |  Receive          | BS Timer         |  Receive Non-    |
|             |  Preferred BSM    | Expires          |  preferred BSM   |
+-------------+-------------------+------------------+------------------+
|             |  -> C-BSR state   | -> E-BSR state   |  -> E-BSR state  |
|             |  Forward BSM;     | Originate BSM;   |  Originate BSM;  |
| Action      |  Store RP-Set;    | Set BS Timer     |  Set BS Timer    |
|             |  Set BS Timer     | to BS Period     |  to BS Period    |
|             |  to BS Timeout    |                  |                  |
+-------------+-------------------+------------------+------------------+
A candidate-BSR may be in one of three states for a particular scope




Bhaskar/Gall/Venaas                                Section 3.  [Page 11]

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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 BSR.  For comparisons with incoming BS messages, the
     router treats itself as the 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 bootstrap router for the scope zone and
     it must perform all the BSR functions.


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


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


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



























Bhaskar/Gall/Venaas                                Section 3.  [Page 12]

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Per-Scope-Zone State-machine for Non-Candidate-BSR Routers



Figure 2: Per-Scope-Zone State-machine for a router not configured as C-
                          BSR in tabular form


+-----------------------------------------------------------------------+
|                         When in NoInfo state                          |
+--------------------+--------------------------------------------------+
|     Event          |         Receive BSM for unknown Admin Scope      |
+--------------------+--------------------------------------------------+
|                    |         -> AP State                              |
|     Action         |         Forward BSM; Store RP-Set;               |
|                    |         Set BS Timer to BS Timeout;              |
|                    |         Set SZ Timer to SZ Timeout               |
+--------------------+--------------------------------------------------+



+-----------------------------------------------------------------------+
|                       When in Accept Any state                        |
+---------------+-----------------------------+-------------------------+
|   Event       |     Receive BSM             |     SZ Timer Expires    |
+---------------+-----------------------------+-------------------------+
|               |     -> AP State             |     -> NoInfo state     |
|               |     Forward BSM; Store      |     cancel timers;      |
|               |     RP-Set; Set BS          |     clear state         |
|   Action      |     Timer to BS             |                         |
|               |     Timeout; Set SZ         |                         |
|               |     Timer to SZ             |                         |
|               |     Timeout                 |                         |
+---------------+-----------------------------+-------------------------+



+-----------------------------------------------------------------------+
|         When in Accept Preferred state                                |
+------------+--------------------+------------------+------------------+
| Event      |  Receive           |   BS Timer       |  Receive Non-    |
|            |  Preferred BSM     |   Expires        |  preferred BSM   |
+------------+--------------------+------------------+------------------+
|            |  -> AP State       |   -> AA State    |  -> AP State     |
|            |  Forward BSM;      |   Refresh RP-    |                  |
|            |  Store RP-Set;     |   set; Remove    |                  |
| Action     |  Set BS Timer      |   BSR state      |                  |
|            |  to BS Timeout;    |                  |                  |
|            |  Set SZ Timer      |                  |                  |
|            |  to SZ Timeout     |                  |                  |
+------------+--------------------+------------------+------------------+





Bhaskar/Gall/Venaas                                Section 3.  [Page 13]

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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.


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 SZ
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.


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


o The bootstrap timer (BS Timer) - that is used to time out old
  bootstrap router information.


o The scope zone timer (SZ Timer) - that is used to time out the scope
  zone itself if BS messages specifying this scope zone stop arriving.


Bootstrap Message Processing Checks


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













Bhaskar/Gall/Venaas                                Section 3.  [Page 14]

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if ( (DirectlyConnected(BSM.src_ip_address) == FALSE)
     OR (we have no Hello state for BSM.src_ip_address)) {
  drop the BS message silently
}
if (BSM.dst_ip_address == ALL-PIM-ROUTERS group) {
  if ( BSM.src_ip_address != RPF_neighbor(BSM.BSR_ip_address) ) {
     drop the BS message silently
  }
} else if (BSM.dst_ip_address is one of my addresses) {
  if (Any previous BSM for this scope has been accepted) {
     #the packet was unicast, but this wasn't
     #a quick refresh on startup
     drop the BS message silently
  }
} else {
  drop the BS message silently
}
if (the interface the message arrived on is an Admin Scope
    border for the BSM.first_group_address) {
  drop the BS 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 BS message, or the router must have no BSR state for
that admin scope (it just restarted) and have received the BS 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 BS message.



BS 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 greater
          than or 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.


          The weighting for a BSR is 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




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          bootstrap message (with the BSR priority taking the most-
          significant bits and the IP address taking the least
          significant bits).


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


A non-candidate BSR also has the following transition event defined:


     Receive BSM for unknown Admin Scope
          As "Receive BSM", except that the admin scope zone indicated
          in the BSM was not previously known at this router.


BS State-machine Actions


The state-machines specify actions that include setting the BS timer to
the following values:


     BS Period
          The periodic interval with which bootstrap messages are
          normally sent.  The default value is 60 seconds.


     BS Timeout
          The interval after which bootstrap router state is timed out
          if no bootstrap message from that router has been heard.  The
          default value is 2 times the BS Period plus 10 seconds, which
          is 130 seconds.


     Randomized Override Interval
          The randomized interval during which a router avoids sending a
          bootstrap message while it waits to see if another router has
          a higher bootstrap weight.  This interval is to reduce control
          message overhead during BSR election.  The following
          pseudocode is proposed as an efficient implementation of this
          "randomized" value:


          Delay = 5 + 2 * log_2(1 + bestPriority - myPriority)
                  + AddrDelay



          where myPriority is the Candidate-BSR's configured priority,
          and bestPriority equals:


          bestPriority = Max(storedPriority, myPriority)


          and AddrDelay is given by the following for IPv4:






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          if ( bestPriority == myPriority) {
              AddrDelay = log_2(storedAddr - myAddr) / 16
          } else {
              AddrDelay = 2 - (myAddr / 2^31)
          }


          and AddrDelay is given by the following for IPv6:


          if ( bestPriority == myPriority) {
              AddrDelay = log_2(storedAddr - myAddr) / 64
          } else {
              AddrDelay = 2 - (myAddr / 2^127)
          }



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


     SZ Timeout
          The interval after which a router will time out an Admin Scope
          zone that it has dynamically learned.  The interval MUST be
          larger than the BS Timeout.  The default value is ten times
          the BS Timeout, which is 1300 seconds.


In addition to setting 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
          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 3.3.


     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 multicast-
          capable interfaces, except where this would cause the BSM to
          cross an admin-scope boundary for the scope zone indicated in
          the message.  The IP source address of the message is the
          originating router's IP address on the interface the message
          is being forwarded from, the destination address is ALL-PIM-
          ROUTERS, and the TTL of the message is set to 1.






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


          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.


          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.


     Refresh RP-Set
          When the BS 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 BS 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.


In addition to the above state-machine actions, a DR also unicasts a
stored copy of the Bootstrap message to each new PIM neighbor, i.e.,
after the DR receives the neighbor's first Hello message, and sends a
Hello message in response.  It does so even if the new neighbor becomes
the DR.


3.1.  Sending Candidate-RP-Advertisements


Every C-RP periodically unicasts a C-RP-Adv to the BSR for that scope
zone to inform the BSR of the C-RP's willingness to function as an RP.
Unless configured otherwise, it does this for every Admin Scope zone for
which it has state, and for the global scope zone.  If the same router
is the BSR for more than one scope zone, the C-RP-Adv 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 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.




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The interval for sending these messages is subject to local
configuration at the C-RP, but must be smaller than the HoldTime in the
C-RP-Adv.


[NOTE: possible optimization: prime the CRPT with a small random value
when a new BSR is elected.  This will allow the newly elected BSR to
learn group mappings fast.]


A Candidate-RP-Advertisement carries a list of group address and group
mask field pairs.  This enables the C-RP router to restrict the
advertisement to certain prefixes or scopes of groups.  If the C-RP
becomes an RP, it may enforce this scope acceptance when receiving
Registers or Join/Prune messages.


The C-RP priority field determines which C-RPs get selected by the BSR
to be in the RP-Set.  Note 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 to
the BSR for each scope 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 BSR message removing
the shut down RP from the RP-set.


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


[NOTE: what happens if the message becomes too large to fit in a single
packet?  With the per-mapping timers, it's not a problem to send several
advertisements.  We need to say something about this.]



3.2.  Creating the RP-Set at the BSR


Upon receiving a C-RP-Adv, if the router is not the elected BSR, it
silently ignores the message.


If the router is the BSR, it creates a group-to-C-RP mapping for every
group range in the C-RP-Adv.  These mappings have identical values for
the C-RP address, C-RP priority and holdtime.  The hash mask length is a
global property of the BSR and is therefore the same for all mappings
managed by the BSR.


[NOTE: This is substantially different from version 03, where state was
kept per C-RP.  The behaviour is the same if a C-RP always sends all
advertisements in a single message.  However, there seems to be no
requirement for this (and the case where all advertisements don't fit in




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a single message seems not to be covered).  Keeping state per group
mapping allows a C-RP to send C-RP-Advs in chunks and even allows for
different holdtimes for different group ranges (which may or may not be
useful) while being compatible with the old version.]


Every mapping that is not part of the C-RP-set is added to the C-RP-set
and the associated group-to-C-RP mapping expiry timer (CGET) is
initialized to the holdtime.


If a mapping already exists (i.e. the C-RP-set contains a mapping with
identical group-range and C-RP-address), it is updated with the
information from the BSM and its associated CGET is reset to the
holdtime from the BSM.  If the holdtime is zero, the mapping is
immediately removed from the C-RP-set.


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 unless the
`Priority' field from the C-RP-Adv 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.


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.



3.3.  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
Check.  Bootstrap messages are multicast to the `ALL-PIM-ROUTERS' group.
When a BS 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 BS message packet.  The IP source address
on the bootstrap message should be set to the forwarding router's IP




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address on the interface the message is being forwarded from.  Bootstrap
messages are always originated or forwarded with an IP TTL value of 1.


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.



3.4.  Receiving and Using the RP-Set


[NOTE: what should go into this section?  It doesn't make sense to
repeat what is said in the "Store RP-set" action.]


The RP-set maintained by BSR is used by RP-based multicast routing
protocols like PIM-SM and Bi-directional 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.


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 Message


8    Candidate-RP-Advertisement





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In this section we use the following terms defined in the PIM-SM [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 [7]. 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.



Encoded-Group address











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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 [3] 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.






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


If the bootstrap message contains information about more than one admin
scope zone, each different scope zone MUST occupy a different semantic
fragment.  This allows Zone Border Routers for an admin scope zone to
not forward only those fragments that should not traverse the admin
scope boundary.


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




































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 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    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                               .                               |
|                               .                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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|             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.



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 be
     the group range for the entire admin scope range, and this MUST
     have the Admin Scope bit set.  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.







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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 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 Candidate-RP-Advertisement.  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.



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.  There
are two reasons for semantic fragmentation:


o    The BSM would exceed the link MTU the packet will be forwarded
     over.


o    The BSM includes information about more than one admin scope zone.






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Let us initially consider only the former case; 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 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 seconds.  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 seconds.


Finally, we come to the case of fragments for the purpose of conveying
admin scope group-prefixes.  In general, the information for each admin
scope range is independent of information about other admin scope
ranges.  As no BSMF is allowed to convey information for more than one
admin scope range, then the procedure above also applies to BSMs that
are fragmented due to admin scoping.  However, to time out all the state
for an entire admin scope zone requires waiting SZ Timeout rather than
BS Timeout, as admin scope zones are not expected to come and go
frequently.




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4.2.  Candidate-RP-Advertisement Format


Candidate-RP-Advertisements 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. A
     Prefix Count of `0' implies all multicast groups, e.g. for IPv4 a
     prefix of 224.0.0.0 with mask length of 4.  If the C-RP is not
     configured with Group-prefix information, the C-RP puts a default
     value of `0' in this field.



Priority
     The `Priority' of the included RP, for the corresponding Encoded-
     Group Address (if any).  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.






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Holdtime
     The amount of time the advertisement is valid. This field allows
     advertisements to be aged out.



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].


5.  Default Values for Timers


Timer Name: Bootstrap Timer (BST)



+-------------------+--------------------------+------------------------+
|  Value Name       |   Value                  |   Explanation          |
+-------------------+--------------------------+------------------------+
|  BS Period        |   Default: 60 secs       |   Period between       |
|                   |                          |   originating          |
|                   |                          |   bootstrap messages   |
+-------------------+--------------------------+------------------------+
|  BS Timeout       |   2 * BS_Period + 10     |   Period after last    |
|                   |   seconds                |   BS message before    |
|                   |                          |   BSR is timed out     |
|                   |                          |   and election         |
|                   |                          |   begins               |
+-------------------+--------------------------+------------------------+
|  rand_override    |   rand(0, 5.0 secs)      |   Suppression period   |
|                   |                          |   in BSR election to   |
|                   |                          |   prevent thrashing    |
+-------------------+--------------------------+------------------------+


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



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


C-RP Advertisement messages are sent periodically with period C-RP-Adv-
Period.  C-RP-Adv-Period defaults to 60 seconds.  The holdtime to be




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specified in a C-RP-Adv message should be set to (2.5 * C-RP-Adv-Period
).  This timer is used for group-to-C-RP mappings in the C-RP-set at the
BSR.


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



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


This timer is identical to CGET, except that it applies to the mappings
in the RP-set rather than the C-RP-set.


Timer Name: C-RP Advertisement Timer (CRPT)



+--------------------+--------------------------+-----------------------+
| Value Name         |   Value                  |  Explanation          |
+--------------------+--------------------------+-----------------------+
| C-RP-Adv-Period    |   Default: 60 seconds    |  Period with which    |
|                    |                          |  periodic C-RP        |
|                    |                          |  Advertisements are   |
|                    |                          |  sent to BSR          |
+--------------------+--------------------------+-----------------------+


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



+------------------------------------+--------------+--------------------+
|Value Name      Value   Explanation |              |                    |
+------------------------------------+--------------+--------------------+
|SZ Timeout                          | 1300 seconds | Interval after     |
|                                    |              | which a scope zone |
|                                    |              | will be timed out  |
|                                    |              | if the state is    |
|                                    |              | not refreshed      |
+------------------------------------+--------------+--------------------+



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




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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.


6.3.  BS 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




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IP addresses of valid BSR routers, and that any BS 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 BS message from outside of the PIM Domain, as the source address on BS
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 BS
Message unicast from an immediate neighbor, and it might be possible to
unicast a BS 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 BS 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 BS message to another PIM router during the brief interval
after it has restarted.



All BS Messages SHOULD carry the Router Alert IP option.  If a PIM
router receives a BS 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 BS message to travel
multiple IP hops.



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.






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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 BS messages.


6.4.  C-RP-Advertisement Security


Even if it is not possible to subvert BS 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 BS Messages.  However, C-RP-Adv messages are
unicast and normally travel multiple hops, so controlling them is a
little harder.



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.  All PIM routers forwarding such a packet are then
capable of checking whether the packet came from a valid neighbor.  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, 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.



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-
Advs does not remove the need for the non-cryptographic mechanisms, as
explained below.





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6.5.  Denial of Service using IPsec


An additional concern is that of Denial-of-Service attacks caused by
sending high volumes of BS 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 BS
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 BS 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.



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.








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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-09.ps


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


[3] M. Handley, I. Kouvelas, T. Speakman, L. Vicisano, "Bi-directional
     Protocol Independent Multicast (BIDIR-PIM)", Internet Draft draft-
     ietf-pim-bidir-06.txt



11.  Informative References


[4] S. Deering , W. Fenner , B. Haberman, "Multicast Listener Discovery
     (MLD) for IPv6", RFC 2710, Oct 1999.


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


[6] W. Fenner, "Internet Group Management Protocol, Version 2", RFC
     2236, Nov 1997.


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



12.  Authors' Addresses


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








Bhaskar/Gall/Venaas                               Section 12.  [Page 36]

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



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



Copyright Statement


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set forth therein, the authors retain all their rights.


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Bhaskar/Gall/Venaas                               Section 12.  [Page 37]


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