Internet Engineering Task Force                                   PIM WG
INTERNET-DRAFT                                          Bill Fenner/AT&T
draft-ietf-pim-sm-bsr-03.txt                           Mark Handley/ACIRI Handley/ICIR
                                                  Roger Kermode/Motorola
                                                  David Thaler/Microsoft
                                                        21 November 2001
                                                        25 February 2003
                                                    Expires: May 2002 August 2003

          Bootstrap Router (BSR) Mechanism for PIM Sparse Mode

Status of this Document

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

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

The list of Internet-Draft Shadow Directories can be accessed at

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


     This document specifies the Bootstrap Router (BSR) mechanism
     for Protocol Independent Multicast - Sparse Mode (PIM-SM).
     BSR is one way that a PIM-SM 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.

                           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. . . . . .   7
     2. BSR State and Timers. . . . . . . . . . . . . . . . . .   9
     3. Bootstrap Router Election and RP-Set
     Distribution . . . . . . . . . . . . . . . . . . . . . . .  10
      3.1. Sending Candidate-RP-Advertisements. . . . . . . . .  17  18
      3.2. Creating the RP-Set at the BSR . . . . . . . . . . .  18  19
      3.3. Forwarding Bootstrap Messages. . . . . . . . . . . .  19  20
      3.4. Receiving and Using the RP-Set . . . . . . . . . . .  19  21
     4. Message Formats . . . . . . . . . . . . . . . . . . . .  20  21
      4.1. Bootstrap Message Format . . . . . . . . . . . . . .  22  23
       4.1.1. Semantic Fragmentation of BSMs. . . . . . . . . .  25  26
      4.2. Candidate-RP-Advertisement Format. . . . . . . . . .  27  28
     5. Default Values for Timers . . . . . . . . . . . . . . .  28  29
     6. Security Considerations . . . . . . . . . . . . . . . .  29  30
      6.1. Possible Threats . . . . . . . . . . . . . . . . . .  29  30
      6.2. Limiting Third-Party DoS Attacks . . . . . . . . . .  30  31
      6.3. BS Message Security. . . . . . . . . . . . . . . . .  30  31
      6.4. C-RP-Advertisement Security. . . . . . . . . . . . .  32  33
      6.5. Denial of Service using IPsec. . . . . . . . . . . .  32  33
     7. Authors' Addresses. . . . . . . . . . . . . . . . . . .  33  34
     8. References. . . . . . . . . . . . . . . . . . . . . . .  34  35
     9. Acknowledgments . . . . . . . . . . . . . . . . . . . .  34  35

                            List of Figures

     Figure 1. Per-Scope-Zone State-machine for a candi-
     date BSR . . . . . . . . . . . . . . . . . . . . . . . . .  10
     Figure 2. Per-Scope-Zone State-machine for a router
     not configured as C-BSR. . . . . . . . . . . . . . . . . .  12

1.  Introduction

Note: this document assumes familiarity with the workings of Protocol
Independent Multicast - Sparse Mode, as defined in [3], and with
Administratively Scoped Multicast, as described in [6].

For correct operation, every PIM Sparse-mode router within a PIM domain
must be able to map a particular global-scope 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 domain in
this context 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.

A PIM domain may also 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.
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-SM specification does 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 [2], 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.

1.1.  General Overview and Background

Every PIM-SM multicast group needs to be associated with the IP address
of a Rendezvous-Point (RP).  When a new multicast sender starts sending,
its local Designated Router (DR) will encapsulate these data packets in
a PIM Register message and send them to the RP for that multicast group.
When a new multicast receiver joins, its local DR will send a PIM Join
message towards the RP for that multicast group.  When any PIM router
sends a (*,G) Join message, it needs to know which is the next how
router towards the RP for G to send the message to.  Also when a PIM
router is forwarding data packets using (*,G) state, it needs to know
which is the correct incoming interface for packets destined for G, as
it needs to reject any packets that arrive on other interfaces.  Thus it
is important for all the PIM routers in a domain to be able to map each
multicast group to the correct RP address.

There are a number of ways that group-to-RP mapping can be done; the
simplest solution is for all the routers in the domain to be 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.

Before we discuss the BSR mechanism itself, we should understand the
rules a PIM-SM router will apply to the mapping information.
Irrespective of how it obtains the mapping information, a PIM-SM router
will have a mapping table containing multiple entries, each of which has
the following form:

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

o RP Priority.

o IP address of RP.

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 (specified in the PIM-SM protocol
specification) so that all routers in the domain make the same choice of
entry and hence apply the same group-to-RP mapping.

The BSR mechanism provides a way in which viable group-to-RP mappings
can be created and distributed to all the PIM-SM 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.

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 them report their candidacy
to the elected BSR, which will choose a subset of the C-RPs to form the
actual set of RPs to the used.  This RP-Set will then be distributed out
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-SM 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 by 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 heard 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.  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.

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 a 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.

All the PIM routers in the domain receive and store Bootstrap messages
originated by the BSR.  When a DR receives an indication of local
membership (typically from IGMP [4] or MLD [1]) or a data packet from a
directly connected host, for a group for which it has no forwarding
state, the DR uses a hash function to map the group address to one of
the C-RPs from the RP-Set whose group-prefix includes the group (see
[3]). The DR then sends a Join message towards that RP if the local host
joined the group, or it Register-encapsulates and unicasts the data
packet to the RP if the local host sent a packet to the group.

A Bootstrap message indicates liveness of the RPs included therein.  If
an RP is included in the message, then it is tagged as `up' at the
routers; RPs not included in the message are removed from the list of
RPs over which the hash algorithm acts. Each router continues to use the
contents of the most recently received Bootstrap message from the BSR
until it accepts a new Bootstrap message.

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.

1.3.  Administratively Scoped Multicast and BSR

Administratively Scoped IP Multicast, as defined in RFC 2365, 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 PIM-SM using 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

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
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 RPset 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

2.  BSR State and Timers

A PIM-SM router implementing BSR holds the following state in addition
to the state needed for PIM-SM operation:

     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 List of Scope Group-Ranges for this BSR

          RP Set

     At a Candidate BSR:

          Per Scope Zone:

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

     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 Candidate RP (C):

                    C-RP Expiry Timer: CET(C,Z)

     At the C-RPs only:

          C-RP Advertisement Timer: CRPT

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.

Per-Scope-Zone Candidate-BSR State Machine

                 | Figures omitted from text version |

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

In in tabular form this state machine is:

|                         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


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.

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

                 | Figures omitted from text version |

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

In C-
                          BSR in tabular form this state machine is:

|                        When in No Info 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             |     -> No Info state    |
|               |     Forward BSM; Store      |     cancel timers;      |
|   Action      |     RP-Set; Set BS          |     clear state         |
|               |     Timer to BS             |                         |
|               |     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;    |                   |                  |
| Action     |   Store RP-Set;   |                   |                  |
|            |   Set BS Timer    |                   |                  |
|            |   to BS Timeout   |                   |                  |

A router that is not a candidate-BSR may be in one of three states:

No Info
     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

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.  If the router has an RP-Set cached from an
     obsolete bootstrap message, it continues to use it.

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

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 No Info; 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:

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 (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

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.  In 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
          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 BSM 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: following for IPv4:

          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

     SZ Period 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 1500 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 multicast-
          capable interfaces, except interfaces with PIM
          neighbors (including the interface it was is received on,
          or on), except
          where this would cause the BSM to cross an admin-scope
          boundary for the scope zone indicated in the message.  The
          source IP address of the message is the forwarding 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.

          As an optimation, 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 MUST 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.

          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 and 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.

     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.

     Store RP Set
          The RP-Set from the received bootstrap message is stored and
          used by the router to decide the RP for each group that the
          router has state for.  Storing this RP Set may cause other
          state-transitions to occur in the router.  The BSR's IP
          address and priority from the received bootstrap message are
          also stored to be used to decide if future bootstrap messages
          are preferred.

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.

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

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 RPset.

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, then it adds the RP address to its local pool
of candidate RPs.  For each C-RP, the BSR holds the following

     IP address
          The IP address of the C-RP.

     Group Prefix and Mask list
          The list of group prefixes and group masks from the C-RP

          The HoldTime from the C-RP-Adv message.  This is included
          later in the RP-set information in the Bootstrap Message.

     C-RP Expiry Timer
          The C-RP-Expiry Timer is used to time out the state associated
          with a C-RP when the BSR fails to receive C-RP-Advertisements
          from it.  The expiry timer is initialized to the HoldTime from
          the RP's C-RP-Adv, and is reset to the HoldTime whenever a C-
          RP-Adv is received from that C-RP.

     C-RP Priority
          The C-RP Priority is used to determine the subset of possible
          RPs to use in the RP-Set. Smaller values are deemed to be of
          higher priority than large ones.

When the C-RP Expiry Timer expires, the C-RP is removed from the pool of
available C-RPs.

The BSR uses the pool of C-RPs to construct the RP-Set which is included
in Bootstrap Messages and sent to all the routers in the PIM domain.
The BSR may apply a local policy to limit the number of Candidate RPs
included in the Bootstrap message.  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.

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

3.4.  Receiving and Using the RP-Set

When a router receives and stores a new RP-Set, it checks if each of the
RPs referred to by existing state (i.e., by (*,G), (*,*,RP), or
(S,G,rpt) entries) is in the new RP-Set.

If an RP is not in the new RP-Set, that RP is considered unreachable and
the hash algorithm (see PIM-SM specification) is re-performed for each
group with locally active state that previously hashed to that RP. This
will cause those groups to be distributed among the remaining RPs.

If the new RP-Set contains a RP that was not previously in the RP-Set,
the hash value of the new RP is calculated for each group covered by the
new C-RP's Group-prefix.  Any group for which the new RP's hash value is
greater than hash value of the group's previous RP is switched over to
the new RP.

4.  Message Formats

BSR messages are PIM messages, as defined in [3]. The values of the PIM
message Type field for BSR messages are:

4    Bootstrap Message

8    Candidate-RP-Advertisement

In this section we use the following terms defined in the PIM-SM [3]:

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

     Values of 0-127 are as assigned by the IANA for Internet Address
     Families in [5]. 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

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 |   Reserved  |Z|  Mask Len     |
|                Group multicast Address

Addr Family
     described above.

Encoding Type
     described above.

     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
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:

 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    |
|                               .                               |
|                               .                               |

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

PIM Version, Reserved, Checksum
     Described in [3].

Type PIM Message Type.  Value is 8 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

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 priority is
     255 (the higher the better), whereas the highest RP Priority (see
     below) is 0 (the lower the better).

Unicast BSR Address
     The address of the bootstrap router for the domain.  The format for
     this address is given in the Encoded-Unicast address in [3].

Group Address 1..n
     The group prefix (address and mask) with which the Candidate RPs
     are associated.  Format described in [3]. 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.

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 [3].

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

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

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.  In 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

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 Cnt    |   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 [3].

Type PIM Message Type.  Value is 4 8 for a Candidate-RP-Advertisement

Prefix Cnt
     The number of encoded group addresses included in the message;
     indicating the group prefixes for which the C-RP is advertising. A
     Prefix Cnt of `0' implies all multicast groups, e.g. for IPv4 a
     prefix of 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.

     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.

     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

Group Address-1..n
     The group prefixes for which the C-RP is advertising.  Format
     described in Encoded-Group-Address in [3].

5.  Default Values for Timers

Timer Name: Bootstrap Timer (BST)

|  Value Name       |   Value                  |   Explanation          |
|  BS Period        |   Default: 60 secs       |   Period between       |
|                   |                          |   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: C-RP Expiry Timer (CET(R))

| Value Name     |  Value           |  Explanation                      |
| C-RP 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
specified in a C-RP-Adv message should be set to (2.5 * C-RP-Adv-Period


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                          | 1500 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
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

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

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 implementator 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
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

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 principle 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.

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 [3], and so we
do not discuss the details further here.  The same security mechanisms
than 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 [3]. Note that the use of cryptographic security for C-RP-
Advs 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 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.  Authors' Addresses

     Bill Fenner
     AT&T Labs - Research
     75 Willow Road
     Menlo Park, CA 94025

     Mark Handley
     1947 Center St, Suite 600
     Berkeley, CA 94708

     Roger Kermode
     Motorola Australian Research Centre
     Locked Bag 5028
     Botany  NSW  1455,

     David Thaler
     Microsoft Corporation
     One Microsoft Way
     Redmond, WA 98052

8.  References

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

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

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

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

[5] IANA, "Address Family Numbers", linked from

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

9.  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.