💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc5059.txt captured on 2023-09-28 at 19:57:37.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
Network Working Group N. Bhaskar
Request for Comments: 5059 Arastra
Obsoletes: 2362 A. Gall
Updates: 4601 SWITCH
Category: Standards Track J. Lingard
Arastra
S. Venaas
UNINETT
January 2008
Bootstrap Router (BSR) Mechanism
for Protocol Independent Multicast (PIM)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document specifies the Bootstrap Router (BSR) mechanism for the
class of multicast routing protocols in the PIM (Protocol Independent
Multicast) family that use the concept of a Rendezvous Point as a
means for receivers to discover the sources that send to a particular
multicast group. BSR is one way that a multicast router can learn
the set of group-to-RP mappings required in order to function. The
mechanism is dynamic, largely self-configuring, and robust to router
failure.
Bhaskar, et al. Standards Track [Page 1]
RFC 5059 BSR Mechanism for PIM January 2008
Table of Contents
1. Introduction ....................................................3
1.1. Background .................................................3
1.2. Protocol Overview ..........................................5
1.3. Administrative Scoping and BSR .............................6
2. BSR State and Timers ............................................8
3. Bootstrap Router Election and RP-Set Distribution ...............9
3.1. Bootstrap Router Election ..................................9
3.1.1. Per-Scope-Zone Candidate-BSR State Machine .........10
3.1.2. Per-Scope-Zone State Machine for
Non-Candidate-BSR Routers ..........................11
3.1.3. Bootstrap Message Processing Checks ................13
3.1.4. State Machine Transition Events ....................14
3.1.5. State Machine Actions ..............................15
3.2. Sending Candidate-RP-Advertisement Messages ...............17
3.3. Creating the RP-Set at the BSR ............................18
3.4. Forwarding Bootstrap Messages .............................21
3.5. Bootstrap Messages to New and Rebooting Routers ...........22
3.5.1. No-Forward Bootstrap Messages ......................23
3.5.2. Unicasting Bootstrap Messages ......................23
3.6. Receiving and Using the RP-Set ............................23
4. Message Formats ................................................24
4.1. Bootstrap Message Format ..................................26
4.1.1. Semantic Fragmentation of BSMs .....................30
4.2. Candidate-RP-Advertisement Message Format .................31
5. Timers and Timer Values ........................................33
6. Security Considerations ........................................36
6.1. Possible Threats ..........................................36
6.2. Limiting Third-Party DoS Attacks ..........................36
6.3. Bootstrap Message Security ................................37
6.3.1. Unicast Bootstrap Messages .........................37
6.3.2. Multi-Access Subnets ...............................38
6.4. Candidate-RP-Advertisement Message Security ...............38
6.4.1. Non-Cryptographic Security of C-RP-Adv Messages ....38
6.4.2. Cryptographic Security of C-RP-Adv Messages ........39
6.5. Denial of Service using IPsec .............................39
7. Contributors ...................................................40
8. Acknowledgments ................................................40
9. Normative References ...........................................40
10. Informative References ........................................41
Bhaskar, et al. Standards Track [Page 2]
RFC 5059 BSR Mechanism for PIM January 2008
1. Introduction
This document assumes some familiarity with the concepts of Protocol
Independent Multicast - Sparse Mode (PIM-SM) [1] and Bidirectional
Protocol Independent Multicast (BIDIR-PIM) [2], as well as with
Administratively Scoped IP Multicast [3] and the IPv6 Scoped Address
Architecture [4].
For correct operation, every multicast router within a PIM domain
must be able to map a particular multicast group address to the same
Rendezvous Point (RP). The PIM specifications do not mandate the use
of a single mechanism to provide routers with the information to
perform this group-to-RP mapping.
This document describes the PIM Bootstrap Router (BSR) mechanism.
BSR is one way that a multicast router can learn the information
required to perform the group-to-RP mapping. The mechanism is
dynamic, largely self-configuring, and robust to router failure.
BSR was first defined in RFC 2362 [7] as part of the original PIM-SM
specification, which has been obsoleted by RFC 4601 [1]. 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 flavors of routing protocols.
Throughout the document, any reference to the PIM protocol family is
restricted to the subset of RP-based protocols, namely PIM-SM and
BIDIR-PIM, unless stated otherwise.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [6].
1.1. Background
A PIM domain is a contiguous set of routers that all implement PIM
and are configured to operate within a common boundary defined by PIM
Multicast Border Routers (PMBRs). PMBRs connect each PIM domain to
the rest of the Internet.
Every PIM multicast group needs to be associated with the IP address
of a Rendezvous Point (RP). This address is used as the root of a
group-specific distribution tree whose branches extend to all nodes
in the domain that want to receive traffic sent to the group.
Senders inject packets into the tree in such a manner that they reach
all connected receivers. How this is done and how the packets are
forwarded along the distribution tree depends on the particular
routing protocol.
Bhaskar, et al. Standards Track [Page 3]
RFC 5059 BSR Mechanism for PIM January 2008
For all senders to reach all receivers, it is crucial that all
routers in the domain use the same mappings of group addresses to RP
addresses.
An exception to the above is where a PIM domain has been broken up
into multiple administrative scope regions. These are regions where
a border has been configured so that a set of multicast groups will
not be forwarded across that border. In this case, all PIM routers
within the same scope region must map a particular scoped group to
the same RP within that region.
In order to determine the RP for a multicast group, a PIM router
maintains a collection of group-to-RP mappings, called the RP-Set. A
group-to-RP mapping contains the following elements.
o Multicast group range, expressed as an address and prefix
length
o RP priority
o RP address
o Hash mask length
o SM / BIDIR flag
In general, the group ranges of these group-to-RP mappings may
overlap in arbitrary ways; hence, a particular multicast group may be
covered by multiple group-to-RP mappings. When this is the case, the
router chooses only one of the RPs by applying a deterministic
algorithm so that all routers in the domain make the same choice. It
is important to note that this algorithm is part of the specification
of the individual routing protocols (and may differ among them), not
of the BSR specification. For example, PIM-SM [1] defines one such
algorithm. It makes use of a hash function for the case where a
group range has multiple RPs with the same priority. The hash mask
length is used by this function.
There are a number of ways in which such group-to-RP mappings can be
established. The simplest solution is for all the routers in the
domain to be statically configured with the same information.
However, static configuration generally doesn't scale well, and,
except when used in conjunction with Anycast-RP (see [8] and [9]),
does not dynamically adapt to route around router or link failures.
The BSR mechanism provides a way in which viable group-to-RP mappings
can be created and rapidly distributed to all the PIM routers in a
domain. It is adaptive, in that if an RP becomes unreachable, this
Bhaskar, et al. Standards Track [Page 4]
RFC 5059 BSR Mechanism for PIM January 2008
will be detected and the RP-Sets will be modified so that the
unreachable RP is no longer used.
1.2. Protocol Overview
In this section we give an informal and non-definitive overview of
the BSR mechanism. The definitive specification begins in section 2.
The general idea behind the BSR mechanism is that some of the PIM
routers within a PIM domain are configured to be potential RPs for
the domain. These are known as Candidate-RPs (C-RPs). A subset of
the C-RPs will eventually be used as the actual RPs for the domain.
In addition, some of the PIM routers in the domain are configured to
be candidate bootstrap routers, or Candidate-BSRs (C-BSRs). One of
these C-BSRs will be elected to be the bootstrap router (BSR) for the
domain, and all the PIM routers in the domain will learn the result
of this election through Bootstrap messages. The C-RPs will then
report their candidacy to the elected BSR, which chooses a subset of
these C-RPs and distributes corresponding group-to-RP mappings to all
the routers in the domain through Bootstrap messages.
In more detail, the BSR mechanism works as follows. There are four
basic phases (although in practice, all phases may be occurring
simultaneously):
1. BSR Election. Each Candidate-BSR originates Bootstrap messages
(BSMs). Every BSM contains a BSR Priority field. Routers within
the domain flood the BSMs throughout the domain. A C-BSR that
hears about a higher-priority C-BSR than itself suppresses its
sending of further BSMs for some period of time. The single
remaining C-BSR becomes the elected BSR, and its BSMs inform all
the other routers in the domain that it is the elected BSR.
2. C-RP Advertisement. Each Candidate-RP within a domain sends
periodic Candidate-RP-Advertisement (C-RP-Adv) messages to the
elected BSR. A C-RP-Adv message includes the priority of the
advertising C-RP, as well as a list of group ranges for which the
candidacy is advertised. In this way, the BSR learns about
possible RPs that are currently up and reachable.
3. RP-Set Formation. The BSR selects a subset of the C-RPs that it
has received C-RP-Adv messages from to form the RP-Set. In
general, it should do this in such a way that the RP-Set is
neither so large that all the routers in the domain cannot be
informed about it, nor so small that the load is overly
concentrated on some RPs. It should also attempt to produce an
RP-Set that does not change frequently.
Bhaskar, et al. Standards Track [Page 5]
RFC 5059 BSR Mechanism for PIM January 2008
4. RP-Set Flooding. In future Bootstrap messages, the BSR includes
the RP-Set information. Bootstrap messages are flooded through
the domain, which ensures that the RP-Set rapidly reaches all the
routers in the domain. BSMs are originated periodically to
ensure consistency after failure restoration.
When a PIM router receives a Bootstrap message, it adds the
group-to-RP mappings contained therein to its pool of mappings
obtained from other sources (e.g., static configuration). It
calculates the final mappings of group addresses to RP addresses
from this pool according to rules specific to the particular
routing protocol and uses that information to construct multicast
distribution trees.
If a PIM domain becomes partitioned, each area separated from the old
BSR will elect its own BSR, which will distribute an RP-Set
containing RPs that are reachable within that partition. When the
partition heals, another election will occur automatically and only
one of the BSRs will continue to send out Bootstrap messages. As is
expected at the time of a partition or healing, some disruption in
packet delivery may occur. The duration of the disruption period
will be on the order of the region's round-trip time and the
BS_Timeout value.
1.3. Administrative Scoping and BSR
The mechanism described in the previous section does not work when
the PIM domain is divided into administratively scoped regions. To
handle this situation, we use the protocol modifications described in
this section.
In the remainder of this document, we will use the term scope zone,
or simply zone, when we are talking about a connected region of
topology of a given scope. For a more precise definition of scope
zones, see [4], which emphasizes that the scope zones are
administratively configured.
Administrative scoping permits a PIM domain to be divided into
multiple admin-scope zones. Each admin-scope zone is a convex
connected set of PIM routers and is associated with a set of group
addresses. The boundary of the admin-scope zone is formed by Zone
Border Routers (ZBRs). ZBRs are configured not to forward traffic
for any of the scoped group addresses into or out of the scoped zone.
It is important to note that a given scope boundary always creates at
least two scoped zones: one on either side of the boundary.
Bhaskar, et al. Standards Track [Page 6]
RFC 5059 BSR Mechanism for PIM January 2008
In IPv4, administratively scoped zones are associated with a set of
addresses given by an address and a prefix length. In IPv6,
administratively scoped zones are associated with a set of addresses
given by a single scope ID value. The set of addresses corresponding
to a given scope ID value is defined in [5]. For example, a scope ID
of 5 maps to the 16 IPv6 address ranges ff[0-f]5::/16.
There are certain topological restrictions on admin-scope zones. The
scope zone border must be complete and convex. By this we mean that
there must be no path from the inside to the outside of the scoped
zone 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.
Administrative scoping complicates BSR because we do not want a PIM
router within the scoped zone to use an RP outside the scoped zone.
Thus we need to modify the basic mechanism to ensure that this
doesn't happen.
This is done by running a separate copy of the basic BSR mechanism,
as described in the previous section, within each admin-scope zone of
a PIM domain. Thus a separate BSR election takes place for each
admin-scope zone, a C-RP typically registers to the BSR of every
admin-scope zone it is in, and every PIM router receives Bootstrap
messages for every scope zone it is in. The Bootstrap messages sent
by the BSR for a particular scope zone contain information about the
RPs that should be used for the set of addresses associated with that
scope zone.
Bootstrap messages are marked to indicate which scope zone they
belong to. Such admin-scoped Bootstrap messages are flooded in the
normal way, but will not be forwarded by a ZBR across the boundary
for that scope zone.
For the BSR mechanism to function correctly with admin scoping, there
must be at least one C-BSR within each admin-scope zone, and there
must be at least one C-RP that is configured to be a C-RP for the set
of group addresses associated with the scoped zone.
Even when administrative scoping is used, a copy of the BSR mechanism
is still used across the entire PIM domain in order to distribute RP
information for groups that are not administratively scoped. We call
this copy of the mechanism non-scoped BSR. The copies of the
mechanism run for each admin-scope zone are called scoped BSR.
Only the C-BSRs and the ZBRs need to be configured to know about the
existence of the scope zones. Other routers, including the C-RPs,
learn of their existence from Bootstrap messages.
Bhaskar, et al. Standards Track [Page 7]
RFC 5059 BSR Mechanism for PIM January 2008
All PIM routers within a PIM bootstrap domain where admin-scope
ranges are in use must be capable of receiving Bootstrap messages and
storing the winning BSR and RP-Set for all admin-scope zones that
apply. Thus, PIM routers that only implement RFC 2362 or non-scoped
BSR (which only allows one BSR per domain) cannot be used within the
admin-scope zones of a PIM domain.
2. BSR State and Timers
A PIM router implementing BSR holds the following state.
RP-Set
Per Configured or Learned Scope Zone (Z):
At all routers:
Current Bootstrap Router's IP Address
Current Bootstrap Router's BSR Priority
Last BSM received from current BSR
Bootstrap Timer (BST(Z))
Per group-to-RP mapping (M):
Group-to-RP mapping Expiry Timer (GET(M,Z))
At a Candidate-BSR for Z:
My state: One of "Candidate-BSR", "Pending-BSR",
"Elected-BSR"
At a router that is not a Candidate-BSR for Z:
My state: One of "Accept Any", "Accept Preferred"
Scope-Zone Expiry Timer (SZT(Z))
At the current Bootstrap Router for Z only:
Per group-to-C-RP mapping (M):
Group-to-C-RP mapping Expiry Timer (CGET(M,Z))
Bhaskar, et al. Standards Track [Page 8]
RFC 5059 BSR Mechanism for PIM January 2008
At a C-RP only:
C-RP Advertisement Timer (CRPT)
3. Bootstrap Router Election and RP-Set Distribution
3.1. Bootstrap Router Election
For simplicity, Bootstrap messages are used in both the BSR election
and the RP-Set distribution mechanisms.
Each Bootstrap message indicates the scope to which it belongs. If
the Admin Scope Zone bit is set in the first group range in the
Bootstrap message, the message is called a scoped BSM. If the Admin
Scope Zone bit is not set in the first group range in the Bootstrap
message, the message is called a non-scoped BSM.
In a scoped IPv4 BSM, the scope of the message is given by the first
group range in the message, which can be any sub-range of 224/4. In
a scoped IPv6 BSM, the scope of the message is given by the scope ID
of the first group range in the message, which must have a mask
length of at least 16. For example, a group range of ff05::/16 with
the Admin Scope Zone bit set indicates that the Bootstrap message is
for the scope with scope ID 5. If the mask length of the first group
range in a scoped IPv6 BSM is less than 16, the message MUST be
dropped and a warning SHOULD be logged.
The state machine for Bootstrap messages depends on whether or not a
router has been configured to be a Candidate-BSR for a particular
scope zone. The per-scope-zone state machine for a C-BSR is given
below, followed by the state machine for a router that is not
configured to be a C-BSR.
A key part of the election mechanism is that we associate a weight
with each BSR. The weight of a BSR is defined to be the
concatenation in fixed-precision unsigned arithmetic of the BSR
Priority field from the Bootstrap message and the IP address of the
BSR from the Bootstrap message (with the BSR Priority taking the
most-significant bits and the IP address taking the least-significant
bits).
Bhaskar, et al. Standards Track [Page 9]
RFC 5059 BSR Mechanism for PIM January 2008
3.1.1. Per-Scope-Zone Candidate-BSR State Machine
+-------------------------------------------------------------------+
| When in C-BSR state |
+----------+-----------------+-------------------+------------------+
| Event | Receive | Bootstrap | Receive Non- |
| | Preferred BSM | Timer Expires | preferred BSM |
| | | | from Elected |
| | | | BSR |
+----------+-----------------+-------------------+------------------+
| | -> C-BSR state | -> P-BSR state | -> P-BSR state |
| | Forward BSM; | Set Bootstrap | Forward BSM; |
| Action | Store RP-Set; | Timer to | Set Bootstrap |
| | Set Bootstrap | BS_Rand_Override | Timer to |
| | Timer to | | BS_Rand_Override |
| | BS_Timeout | | |
+----------+-----------------+-------------------+------------------+
+-------------------------------------------------------------------+
| When in P-BSR state |
+-----------+------------------+------------------+-----------------+
| Event | Receive | Bootstrap | Receive Non- |
| | Preferred BSM | Timer Expires | preferred BSM |
+-----------+------------------+------------------+-----------------+
| | -> C-BSR state | -> E-BSR state | -> P-BSR state |
| | Forward BSM; | Originate BSM; | Forward BSM |
| Action | Store RP-Set; | Set Bootstrap | |
| | Set Bootstrap | Timer to | |
| | Timer to | BS_Period | |
| | BS_Timeout | | |
+-----------+------------------+------------------+-----------------+
+-------------------------------------------------------------------+
| When in E-BSR state |
+-----------+------------------+------------------+-----------------+
| Event | Receive | Bootstrap | Receive Non- |
| | Preferred BSM | Timer Expires | preferred BSM |
+-----------+------------------+------------------+-----------------+
| | -> C-BSR state | -> E-BSR state | -> E-BSR state |
| | Forward BSM; | Originate BSM; | Originate BSM; |
| Action | Store RP-Set; | Set Bootstrap | Set Bootstrap |
| | Set Bootstrap | Timer to | Timer to |
| | Timer to | BS_Period | BS_Period |
| | BS_Timeout | | |
+-----------+------------------+------------------+-----------------+
Bhaskar, et al. Standards Track [Page 10]
RFC 5059 BSR Mechanism for PIM January 2008
A Candidate-BSR may be in one of three states for a particular scope
zone:
Candidate-BSR (C-BSR)
The router is a candidate to be the BSR for the scope zone, but
currently another router is the preferred BSR.
Pending-BSR (P-BSR)
The router is a candidate to be the BSR for the scope zone.
Currently, no other router is the preferred BSR, but this router
is not yet the elected BSR. This is a temporary state that
prevents rapid thrashing of the choice of BSR during BSR
election.
Elected-BSR (E-BSR)
The router is the elected BSR for the scope zone and it must
perform all the BSR functions.
In addition to the three states, there is one timer:
o The Bootstrap Timer (BST) - used to time out old bootstrap router
information, and used in the election process to terminate P-BSR
state.
The initial state for this configured scope zone is "Pending-BSR";
the Bootstrap Timer is initialized to BS_Rand_Override. This is the
case both if the router is a Candidate-BSR at startup, and if it is
reconfigured to become one later.
3.1.2. Per-Scope-Zone State Machine for Non-Candidate-BSR Routers
The following state machine is used for scope zones that are
discovered by the router from bootstrap messages. A simplified state
machine is used for scope zones that are explicitly configured on the
router and for the global zone. The differences are listed at the
end of this section.
+-------------------------------------------------------------------+
| When in NoInfo state |
+--------------+----------------------------------------------------+
| Event | Receive BSM |
+--------------+----------------------------------------------------+
| | -> AP state |
| Action | Forward BSM; Store RP-Set; |
| | Set Bootstrap Timer to BS_Timeout |
+--------------+----------------------------------------------------+
Bhaskar, et al. Standards Track [Page 11]
RFC 5059 BSR Mechanism for PIM January 2008
+-------------------------------------------------------------------+
| When in Accept Any state |
+-------------+---------------------------+-------------------------+
| Event | Receive BSM | Scope-Zone Expiry |
| | | Timer Expires |
+-------------+---------------------------+-------------------------+
| | -> AP state | -> NoInfo state |
| | Forward BSM; Store | Remove scope zone |
| Action | RP-Set; Set | state |
| | Bootstrap Timer to | |
| | BS_Timeout | |
+-------------+---------------------------+-------------------------+
+-------------------------------------------------------------------+
| When in Accept Preferred state |
+---------+---------------------+------------------+----------------+
| Event | Receive Preferred | Bootstrap | Receive Non- |
| | BSM | Timer Expires | preferred BSM |
+---------+---------------------+------------------+----------------+
| | -> AP state | -> AA state | -> AP state |
| | Forward BSM; Store | Refresh RP- | |
| Action | RP-Set; Set | Set; Remove | |
| | Bootstrap Timer to | BSR state; Set | |
| | BS_Timeout | SZT to | |
| | | SZ_Timeout | |
+---------+---------------------+------------------+----------------+
A router that is not a Candidate-BSR may be in one of three states:
NoInfo
The router has no information about this scope zone. When in
this state, no state information is held and no timers (that
refer to this scope zone) run. Conceptually, the state machine
is only instantiated when the router receives a scoped BSM for a
scope about which it has no prior knowledge. However, because
the router immediately transitions to the AA state
unconditionally, the NoInfo state can be considered to be
virtual in a certain sense. For this reason, it is omitted from
the description in section 2.
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.
Bhaskar, et al. Standards Track [Page 12]
RFC 5059 BSR Mechanism for PIM January 2008
Accept Preferred (AP)
The router knows the identity of the current BSR, and is using
the RP-Set provided by that BSR. Only Bootstrap messages from
that BSR or from a C-BSR with higher weight than the current BSR
will be accepted.
In addition to the three states, there are two timers:
o The Bootstrap Timer (BST) - used to time out old bootstrap router
information.
o The Scope-Zone Expiry Timer (SZT) - used to time out the scope
zone itself if Bootstrap messages specifying this scope zone stop
arriving.
The initial state for scope zones about which the router has no
knowledge is "NoInfo".
The state machine used for scopes that have been configured
explicitly on the router and for the global scope (which always
exists) differs from the state machine above as follows.
o The "NoInfo" state doesn't exist.
o No SZT is maintained. Hence, the event "Scope-Zone Expiry Timer
Expires" does not exist and no actions with regard to this timer
are executed.
The initial state for this state machine is "Accept Any".
3.1.3. Bootstrap Message Processing Checks
When a Bootstrap message is received, the following initial checks
must be performed:
if ((DirectlyConnected(BSM.src_ip_address) == FALSE) OR
(we have no Hello state for BSM.src_ip_address)) {
drop the Bootstrap message silently
}
if (BSM.dst_ip_address == ALL-PIM-ROUTERS) {
if (BSM.no_forward_bit == 0) {
if (BSM.src_ip_address != RPF_neighbor(BSM.BSR_ip_address)) {
drop the Bootstrap message silently
}
} else if ((any previous BSM for this scope has been accepted) OR
(more than BS_Period has elapsed since startup)) {
Bhaskar, et al. Standards Track [Page 13]
RFC 5059 BSR Mechanism for PIM January 2008
#only accept no-forward BSM if quick refresh on startup
drop the Bootstrap message silently
}
} else if ((Unicast BSM support enabled) AND
(BSM.dst_ip_address is one of my addresses)) {
if ((any previous BSM for this scope has been accepted) OR
(more than BS_Period has elapsed since startup)) {
#the packet was unicast, but this wasn't
#a quick refresh on startup
drop the Bootstrap message silently
}
} else {
drop the Bootstrap message silently
}
if (the interface the message arrived on is an admin scope
border for the BSM.first_group_address) {
drop the Bootstrap message silently
}
Basically, the packet must have come from a directly connected
neighbor for which we have active Hello state. It must have been
sent to the ALL-PIM-ROUTERS group, and unless it is a No-Forward BSM,
it must have been sent by the correct upstream router towards the BSR
that originated the Bootstrap message; or, if it is a No-Forward BSM,
we must have recently restarted and have no BSR state for that admin
scope. Also, if unicast BSM support is enabled, a unicast BSM is
accepted if it is addressed to us, we have recently restarted, and we
have no BSR state for that admin scope. In addition, it must not
have arrived on an interface that is a configured admin-scope border
for the first group address contained in the Bootstrap message.
3.1.4. State Machine Transition Events
If the Bootstrap message passes the initial checks above without
being discarded, then it may cause a state transition event in one of
the above state machines. For both candidate and non-candidate BSRs,
the following transition events are defined:
Receive Preferred BSM
A Bootstrap message is received from a BSR that has weight
higher than or equal to that of the current BSR. If a router
is in P-BSR state, then it uses its own weight as that of the
current BSR.
A Bootstrap message is also preferred if it is from the
current BSR with a lower weight than the previous BSM it sent,
provided that if the router is a Candidate-BSR the current BSR
Bhaskar, et al. Standards Track [Page 14]
RFC 5059 BSR Mechanism for PIM January 2008
still has a weight higher than or equal to that of the router
itself. In this case, the "Current Bootstrap Router's BSR
Priority" state must be updated. (For lower weight, see Non-
preferred BSM from Elected BSR case.)
Receive Non-preferred BSM
A Bootstrap message is received from a BSR other than the
current BSR that has lower weight than that of the current
BSR. If a router is in P-BSR state, then it uses its own
weight as that of the current BSR.
Receive Non-preferred BSM from Elected BSR
A Bootstrap message is received from the elected BSR, but the
BSR Priority field in the received message has changed, so
that now the currently elected BSR has lower weight than that
of the router itself.
Receive BSM
A Bootstrap message is received, regardless of BSR weight.
In addition to state machine transitions caused by the receipt of
Bootstrap messages, a state machine transition takes place each time
the Bootstrap Timer or Scope-Zone Expiry Timer expires.
3.1.5. State Machine Actions
The state machines specify actions that include setting the Bootstrap
Timer and the Scope-Zone Expiry Timer to various values. These
values are defined in section 5.
In addition to setting and cancelling the timers, the following
actions may be triggered by state changes in the state machines:
Forward BSM
A multicast Bootstrap message with No-Forward bit cleared 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 3.4.
Originate BSM
A new Bootstrap message is constructed by the BSR, giving the
BSR's address and BSR priority, and containing the BSR's
chosen RP-Set. The message is forwarded out of all interfaces
on which PIM neighbors exist, except where this would cause
the BSM to cross an admin-scope boundary for the scope zone
indicated in the message.
Bhaskar, et al. Standards Track [Page 15]
RFC 5059 BSR Mechanism for PIM January 2008
Store RP-Set
The router uses the group-to-RP mappings contained in a BSM to
update its local RP-Set.
This action is skipped for an empty BSM. A BSM is empty if it
contains no group ranges, or if it only contains a single
group range where that group range has the Admin Scope Zone
bit set (a scoped BSM) and an RP count of zero.
If a mapping does not yet exist, it is created and the
associated Group-to-RP mapping Expiry Timer (GET) is
initialized with the holdtime from the BSM.
If a mapping already exists, its GET is set to the holdtime
from the BSM. If the holdtime is zero, the mapping is removed
immediately. Note that for an existing mapping, the RP
priority must be updated if changed.
Mappings for a group range are also to be immediately removed
if they are not present in the received group range. This
means that if there are any existing group-to-RP mappings for
a range where the respective RPs are not in the received
range, then those mappings must be removed.
All RP mappings associated with the scope zone of the BSM are
updated with the new hash mask length from the received BSM.
This includes RP mappings for all group ranges learned for
this zone, not just the ranges in this particular BSM.
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 Bootstrap Timer expires, the router uses the copy of
the last BSM that it has received to refresh its RP-Set
according to the action Store RP-Set as if it had just
received it. This will increase the chance that the group-to-
RP mappings will not expire during the election of the new
BSR.
Remove BSR state
When the Bootstrap Timer expires, all state associated with
the current BSR is removed (address, priority, BST, and saved
last BSM; see section 2). Note that this does not include any
group-to-RP mappings.
Bhaskar, et al. Standards Track [Page 16]
RFC 5059 BSR Mechanism for PIM January 2008
Remove scope zone state
When the Scope-Zone Expiry Timer expires, all state associated
with the scope zone is removed (see section 2).
3.2. Sending Candidate-RP-Advertisement Messages
Every C-RP periodically unicasts a C-RP-Adv message to the BSR for
each scope zone for which it has state, to inform the BSR of the
C-RP's willingness to function as an RP. These messages are sent
with an interval of C_RP_Adv_Period, except when a new BSR is
elected; see below.
When a new BSR is elected, the C-RP MUST send one to three C-RP-Adv
messages and wait a small randomized period C_RP_Adv_Backoff before
sending each message. We recommend sending three messages because it
is important that the BSR quickly learns which RPs are active, and
some packet loss may occur when a new BSR is elected due to changes
in the network. One way of implementing this is to set the CRPT to
C_RP_Adv_Backoff when the new BSR is elected, as well as setting a
counter to 2. Whenever the CRPT expires, we first send a C-RP-Adv
message as usual. Next, if the counter is non-zero, it is
decremented and the CRPT is again set to C_RP_Adv_Backoff instead of
C_RP_Adv_Period.
The Priority field in these messages is used by the BSR to select
which C-RPs to include in the RP-Set. Note that lower values of this
field indicate higher priorities, so that a value of zero is the
highest possible priority. C-RPs should, by default, send C-RP-Adv
messages with the Priority field set to 192.
When a C-RP is being shut down, it SHOULD immediately send a C-RP-Adv
message to the BSR for each scope zone for which it is currently
serving as an RP; the Holdtime in this C-RP-Adv message should be
zero. The BSR will then immediately time out the C-RP and generate a
new Bootstrap message with the shut down RP holdtime set to 0.
A C-RP-Adv message carries a list of group address and group mask
field pairs. This enables the C-RP to specify the group ranges for
which it is willing to be the RP. If the C-RP becomes an RP, it may
enforce this scope acceptance when receiving Register or Join/Prune
messages.
A C-RP is configured with a list of group ranges for which it should
advertise itself as the C-RP. A C-RP uses the following algorithm to
determine which ranges to send to a given BSR.
Bhaskar, et al. Standards Track [Page 17]
RFC 5059 BSR Mechanism for PIM January 2008
For each group range R in the list, the C-RP advertises that range to
the scoped BSR for the smallest scope that "contains" R. For IPv6,
the containing scope is determined by matching the scope identifier
of the group range with the scope of the BSR. For IPv4, it is the
longest-prefix match for R, amongst the known admin-scope ranges. If
no scope is found to contain the group range, the C-RP includes it in
the C-RP-Adv sent to the non-scoped BSR. If a non-scoped BSR is not
known, the range is not included in any C-RP-Adv.
In addition, for each IPv4 group range R in the list, for each scoped
BSR whose scope range is strictly contained within R, the C-RP SHOULD
by default advertise that BSR's scope range to that BSR. And for
each IPv6 group range R in the list with prefix length < 16, the C-RP
SHOULD by default advertise each sub-range of prefix length 16 to the
scoped BSR with the corresponding scope ID. An implementation MAY
supply a configuration option to prevent the behavior described in
this paragraph, but such an option SHOULD be disabled by default.
For IPv6, the mask length of all group ranges included in the
C-RP-Adv message sent to a scoped BSR MUST be >= 16.
If the above algorithm determines that there are no group ranges to
advertise to the BSR for a particular scope zone, a C-RP-Adv message
MUST NOT be sent to that BSR. A C-RP MUST NOT send a C-RP-Adv
message with no group ranges in it.
If the same router is the BSR for more than one scope zone, the
C-RP-Adv messages for these scope zones MAY be combined into a single
message.
If the C-RP is a ZBR for an admin-scope zone, then the Admin Scope
Zone bit MUST be set in the C-RP-Adv messages it sends for that scope
zone; otherwise this bit MUST NOT be set. This information is
currently only used for logging purposes by the BSR, but might allow
for future extensions of the protocol.
3.3. Creating the RP-Set at the BSR
Upon receiving a C-RP-Adv message, the router needs to decide whether
or not to accept each of the group ranges included in the message.
For each group range in the message, the router checks to see if it
is the elected BSR for any scope zone that contains the group range,
or if it is elected as the non-scoped BSR. If so, the group range is
accepted; if not, the group range is ignored.
Bhaskar, et al. Standards Track [Page 18]
RFC 5059 BSR Mechanism for PIM January 2008
For security reasons, we recommend that implementations have a way of
restricting which IP addresses the BSR accepts C-RP-Adv messages
from, e.g., access lists. For use of scoped BSR, it may also be
useful to specify which group ranges should be accepted.
If the group range is accepted, a group-to-C-RP mapping is created
for this group range and the RP Address from the C-RP-Adv message.
If the mapping is not already part of the C-RP-Set, it is added to
the C-RP-Set and the associated Group-to-C-RP mapping Expiry Timer
(CGET) is initialized to the holdtime from the C-RP-Adv message. Its
priority is set to the Priority from the C-RP-Adv message.
If the mapping is already part of the C-RP-Set, it is updated with
the Priority from the C-RP-Adv message, and its associated CGET is
reset to the holdtime from the C-RP-Adv message. If the holdtime is
zero, the mapping is immediately removed from the C-RP-Set.
The hash mask length is a global property of the BSR and is therefore
the same for all mappings managed by the BSR.
For compatibility with the previous version of the BSR specification,
a C-RP-Adv message with no group ranges SHOULD be treated as though
it contained the single group range ff00::/8 or 224/4. Therefore,
according to the rule above, this group range will be accepted if and
only if the router is elected as the non-scoped BSR.
When a CGET expires, the corresponding group-to-C-RP mapping is
removed from the C-RP-Set.
The BSR constructs the RP-Set from the C-RP-Set. It may apply a
local policy to limit the number of Candidate-RPs included in the
RP-Set. The BSR may override the range indicated in a C-RP-Adv
message unless the 'Priority' field from the C-RP-Adv message is less
than 128.
If the BSR learns of both BIDIR and PIM-SM Candidate-RPs for the same
group range, the BSR MUST only include RPs for one of the protocols
in the BSMs. The default behavior SHOULD be to prefer BIDIR.
For inclusion in a BSM, the RP-Set is subdivided into sets of {group-
range, RP-Count, RP-addresses}. For each RP-address, the
"RP-Holdtime" field is set to the Holdtime from the C-RP-Set, subject
to the constraint that it MUST be larger than BS_Period and SHOULD be
larger than 2.5 times BS_Period to allow for some Bootstrap messages
getting lost. If some holdtimes from the C-RP-Sets do not satisfy
Bhaskar, et al. Standards Track [Page 19]
RFC 5059 BSR Mechanism for PIM January 2008
this constraint, the BSR MUST replace those holdtimes with a value
satisfying the constraint. An exception to this is the holdtime of
zero, which is used to immediately withdraw mappings.
The format of the Bootstrap message allows 'semantic fragmentation',
if the length of the original Bootstrap message exceeds the packet
maximum boundaries. However, to reduce the semantic fragmentation
required, we recommend against configuring a large number of routers
as C-RPs.
In general, BSMs are originated at regular intervals according to the
BS_Period timer. We do recommend that a BSM is also originated
whenever the RP-set to be announced in the BSMs changes. This will
usually happen when receiving C-RP advertisements from a new C-RP, or
when a C-RP is shut down (C-RP advertisement with a holdtime of
zero). There MUST however be a minimum of BS_Min_Interval between
each time a BSM is sent. In particular, when a new BSR is elected,
it will first send one BSM (which is likely to be empty since it has
not yet received any C-RP advertisements), and then wait at least
BS_Min_Interval before sending a new one. During that time, it is
likely to have received C-RP advertisements from all usable C-RPs
(since we say that a C-RP should send one or more advertisements with
small random delays of C_RP_Adv_Backoff when a new BSR is elected).
For this case in particular, where routers may not have a usable RP-
set, we recommend originating a BSM as soon as BS_Min_Interval has
passed. We suggest though that a BSR can do this in general. One
way of implementing this, is to decrease the Bootstrap Timer to
BS_Min_Interval whenever the RP-set changes, while not changing the
timer if it is less than or equal to BS_Min_Interval.
A BSR originates separate scoped BSMs for each scope zone for which
it is the elected BSR, as well as originating non-scoped BSMs if it
is the elected non-scoped BSR.
Each group-to-C-RP mapping is included in precisely one of these BSMs
-- namely, the scoped BSM for the narrowest scope containing the
group range of the mapping, if any, or the non-scoped BSM otherwise.
A scoped BSM MUST have at least one group range, and the first group
range in a scoped BSM MUST have the Admin Scope Zone bit set. This
group range identifies the scope of the BSM. In a scoped IPv4 BSM,
the first group range is the range corresponding to the scope of the
BSM. In a scoped IPv6 BSM, the first group range may be any group
range subject to the general condition that all the group ranges in
such a BSM MUST have a mask length of at least 16 and MUST have the
same scope ID as the scope of the BSM.
Bhaskar, et al. Standards Track [Page 20]
RFC 5059 BSR Mechanism for PIM January 2008
Apart from identifying the scope, the first group range in a scoped
BSM is treated like any other range with respect to RP mappings.
That is, all mappings in the RP-set for this group range, if any,
must be included in this first group range in the BSM. After this
group range, other group ranges in this scope (for which there are RP
mappings) appear in any order.
The Admin Scope Zone bit of all group ranges other than the first
SHOULD be set to 0 on origination, and MUST be ignored on receipt.
When an elected BSR is being shut down, it should immediately
originate a Bootstrap message listing its current RP-Set, but with
the BSR Priority field set to the lowest priority value possible.
This will cause the election of a new BSR to happen more quickly.
3.4. Forwarding Bootstrap Messages
Generally, bootstrap messages originate at the BSR, and are hop-by-
hop forwarded by intermediate routers if they pass the Bootstrap
Message Processing Checks. There are two exceptions to this. One is
that a bootstrap message is not forwarded if its No-Forward bit is
set; see section 3.5.1. The other is that unicast BSMs (see section
3.5.2) are usually not forwarded. Implementers MAY, however, at
their own discretion choose to re-send a No-Forward or unicast BSM in
a multicast BSM, which MUST have the No-Forward bit cleared. It is
essential that the No-Forward bit is cleared, since no Reverse Path
Forwarding (RPF) check is performed by the receiver when it is set.
By hop-by-hop forwarding, we mean that the Bootstrap message itself
is forwarded, not the entire IP packet. Each hop constructs an IP
packet for each of the interfaces the BSM is to be forwarded out of;
each packet contains the entire BSM that was received.
When a Bootstrap message is forwarded, it is forwarded out of every
multicast-capable interface that has PIM neighbors (including the one
over which the message was received). The exception to this is if
the interface is an admin-scope boundary for the admin-scope zone
indicated in the first group range in the Bootstrap message packet.
As an optimization, a router MAY choose not to forward a BSM out of
the interface the message was received on if that interface is a
point-to-point interface. On interfaces with multiple PIM neighbors,
a router SHOULD forward an accepted BSM out of 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 out of that interface by a
small randomized interval to prevent message implosion. A
Bhaskar, et al. Standards Track [Page 21]
RFC 5059 BSR Mechanism for PIM January 2008
configuration option MAY be provided to disable forwarding out of the
interface a message was received on, but we recommend that the
default behavior is to forward out of that interface.
Rationale: A BSM needs to be forwarded out of 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 out of the incoming interface
may safely be disabled.
A ZBR constrains all BSMs that are of equal or smaller scope than the
configured boundary. That is, the BSMs are not accepted from,
originated, or forwarded on the interfaces on which the boundary is
configured. For IPv6, the check is a comparison between the scope of
the first range in the scoped BSM and the scope of the configured
boundary. For IPv4, the first range in the scoped BSM is checked to
see if it is contained in or is the same as the range of the
configured boundary.
3.5. Bootstrap Messages to New and Rebooting Routers
When a Hello message is received from a new neighbor, or a Hello
message with a new GenID is received from an existing neighbor, one
router on the LAN sends a stored copy of the Bootstrap message for
each admin-scope zone to the new or rebooting router. This allows
new or rebooting routers to learn the RP-Set quickly.
This message SHOULD be sent as a No-Forward Bootstrap message; see
section 3.5.1. For backwards compatibility, this message MAY instead
or in addition be sent as a unicast Bootstrap message; see section
3.5.2. These messages MUST only be accepted at startup; see section
3.1.3.
The router that does this is the Designated Router (DR) on the LAN,
or, if the new or rebooting router is the DR, the router that would
be the DR if the new or rebooting router were excluded from the DR
election process.
Before sending a Bootstrap message in this manner, the router must
wait until it has sent a triggered Hello message on this interface;
otherwise, the new neighbor will discard the Bootstrap message.
Bhaskar, et al. Standards Track [Page 22]
RFC 5059 BSR Mechanism for PIM January 2008
3.5.1. No-Forward Bootstrap Messages
A No-Forward Bootstrap message, is a bootstrap message that has the
No-Forward bit set. All implementations SHOULD support sending of
No-Forward Bootstrap messages, and SHOULD also accept them. The RPF
check MUST NOT be performed in the BSM processing check for a No-
Forward BSM; see section 3.1.3. The messages have the same source
and destination addresses as the usual multicast Bootstrap messages.
3.5.2. Unicasting Bootstrap Messages
For backwards compatibility, implementations MAY support unicast
Bootstrap messages. Whether to send unicast Bootstrap messages
instead of or in addition to No-Forward Bootstrap messages, and also
whether to accept such messages, SHOULD be configurable. This
message is unicast to the neighbor.
3.6. Receiving and Using the RP-Set
The RP-Set maintained by BSR is used by RP-based multicast routing
protocols like PIM-SM and BIDIR-PIM. These protocols may obtain RP-
Sets from other sources as well. How the final group-to-RP mappings
are obtained from these RP-Sets is not part of the BSR specification.
In general, the routing protocols need to re-calculate the mappings
when any of their RP-Sets change. How such a change is signalled to
the routing protocol is also not part of the present specification.
Some group-to-RP mappings in the RP-Set indicate group ranges for
which PIM-SM should be used; others indicate group ranges for use
with BIDIR-PIM. Routers that support only one of these protocols
MUST NOT ignore ranges indicated as being for the other protocol.
They MUST NOT treat them as being for the protocol they support.
If a mapping is not already part of the RP-Set, it is added to the
RP-Set and the associated Group-to-RP mapping Expiry Timer (GET) is
initialized to the holdtime from the Bootstrap message. Its priority
is set to the Priority from the Bootstrap message.
If a mapping is already part of the RP-Set, it is updated with the
Priority from the Bootstrap message and its associated GET is reset
to the holdtime from the Bootstrap message. If the holdtime is zero,
the mapping is removed from the RP-Set immediately.
Bhaskar, et al. Standards Track [Page 23]
RFC 5059 BSR Mechanism for PIM January 2008
4. Message Formats
BSR messages are PIM messages, as defined in [1]. The values of the
PIM Message Type field for BSR messages are:
4 Bootstrap
8 Candidate-RP-Advertisement
As with all other PIM control messages, BSR messages have IP protocol
number 103.
Candidate-RP-Advertisement messages are unicast to a BSR. Usually,
Bootstrap messages are multicast with TTL 1 to the ALL-PIM-ROUTERS
group, but in some circumstances (described in section 3.5.2)
Bootstrap messages may be unicast to a specific PIM neighbor.
The IP source address used for Candidate-RP-Advertisement messages is
a domain-wide reachable address. The IP source address used for
Bootstrap messages (regardless of whether they are being originated
or forwarded) is the link-local address of the interface on which the
message is being sent (i.e., the same source address that the router
uses for the Hello messages that it sends out that interface).
The IPv4 ALL-PIM-ROUTERS group is 224.0.0.13. The IPv6 ALL-PIM-
ROUTERS group is ff02::d.
In this section, we use the following terms defined in the PIM-SM
specification [1]:
o Encoded-Unicast format
o Encoded-Group format
We repeat these here to aid readability.
Encoded-Unicast address
An Encoded-Unicast address takes the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Addr Family | Encoding Type | Unicast Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
Bhaskar, et al. Standards Track [Page 24]
RFC 5059 BSR Mechanism for PIM January 2008
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 [11]. 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 |B| Reserved |Z| Mask Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group multicast Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...
Addr Family
Described above.
Encoding Type
Described above.
[B]IDIR bit
When set, all BIDIR-capable PIM routers will operate the
protocol described in [2] for the specified group range.
Reserved
Transmitted as zero. Ignored upon receipt.
Admin Scope [Z]one
When set, this bit indicates that this group range is an
administratively scoped range.
Bhaskar, et al. Standards Track [Page 25]
RFC 5059 BSR Mechanism for PIM January 2008
Mask Len
The Mask length field is 8 bits. The value is the number of
contiguous one bits that are left justified and used as a mask;
when combined with the group address, it 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 may be divided up into 'semantic fragments' if
the resulting IP datagram would exceed the maximum packet size
boundaries. Basically, a single Bootstrap message can be sent as
multiple semantic fragments (each in a separate IP datagram), so long
as the fragment tags of all the semantic fragments comprising the
message are the same. The format of a single non-fragmented message
is the same as the one used for semantic fragments.
The format of a single 'fragment' is given below:
Bhaskar, et al. Standards Track [Page 26]
RFC 5059 BSR Mechanism for PIM January 2008
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 |N| Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fragment Tag | Hash Mask Len | BSR Priority |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BSR Address (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address 1 (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP Count 1 | Frag RP Cnt 1 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP Address 1 (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP1 Holdtime | RP1 Priority | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP Address 2 (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP2 Holdtime | RP2 Priority | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP Address m (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPm Holdtime | RPm Priority | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address 2 (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address n (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP Count n | Frag RP Cnt n | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP Address 1 (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP1 Holdtime | RP1 Priority | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP Address 2 (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP2 Holdtime | RP2 Priority | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
Bhaskar, et al. Standards Track [Page 27]
RFC 5059 BSR Mechanism for PIM January 2008
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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.
[N]o-Forward bit
When set, this bit means that the Bootstrap message fragment is
not to be forwarded.
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.
BSRs should by default set this field to 64. 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].
Bhaskar, et al. Standards Track [Page 28]
RFC 5059 BSR Mechanism for PIM January 2008
Group Address 1..n
The group ranges (address and mask) with which the Candidate-RPs
are associated. Format described in [1]. In a fragment
containing admin-scope ranges, the first group range in the
fragment MUST satisfy the following conditions:
o it MUST have the Admin Scope Zone bit set;
o for IPv4, it MUST be the group range for the entire admin-
scope range (this is required 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);
o for IPv6, the Mask Len MUST be at least 16 and have the scope
ID of the admin-scope range.
RP Count 1..n
The number of Candidate-RP addresses included in the whole
Bootstrap message for the corresponding group range. A router
does not replace its old RP-Set for a given group range
until/unless it receives 'RP-Count' addresses for that range;
the addresses could be carried over several fragments. If only
part of the RP-Set for a given group range was received, the
router discards it without updating that specific group range'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 range.
The 'Frag RP Cnt' field facilitates parsing of the RP-Set for a
given group range, when carried over more than one fragment.
RP address 1..m
The address of the Candidate-RPs, for the corresponding group
range. The format for these addresses is given in the Encoded-
Unicast address in [1].
RP1..m Holdtime
The Holdtime (in seconds) for the corresponding RP. This field
is copied from the 'Holdtime' field of the associated RP stored
at the BSR.
RP1..m Priority
The 'Priority' of the corresponding RP and Encoded-Group
Address. This field is copied from the 'Priority' field stored
at the BSR when receiving a C-RP-Adv message. The highest
priority is '0' (i.e., unlike BSR priority, the lower the value
of the 'Priority' field, the better). Note that the priority is
per RP and per Group Address.
Bhaskar, et al. Standards Track [Page 29]
RFC 5059 BSR Mechanism for PIM January 2008
Within a Bootstrap message, the BSR Address, all the Group Addresses,
and all the RP Addresses MUST be of the same address family. In
addition, the address family of the fields in the message MUST be the
same as the IP source and destination addresses of the packet. This
permits maximum implementation flexibility for dual-stack IPv4/IPv6
routers.
4.1.1. Semantic Fragmentation of BSMs
Bootstrap messages may be split over several PIM Bootstrap Message
Fragments (BSMFs); this is known as semantic fragmentation. Each of
these must follow the above format. All fragments of a given
Bootstrap message MUST have identical values for the Type, No-Forward
bit, Fragment Tag, Hash Mask Len, BSR Priority, and BSR Address
fields. That is, only the group-to-RP mappings may differ between
fragments.
This is useful if the BSM would otherwise exceed the MTU of the link
the message will be forwarded over. If one relies purely on IP
fragmentation, one would lose the entire message if a single fragment
is lost. By use of semantic fragmentation, a single lost IP fragment
will only cause the loss of the semantic fragment that the IP
fragment was part of. As described below, a router only needs to
receive all the RPs for a specific group range to update that range.
This means that loss of a semantic fragment, due to an IP fragment
getting lost, only affects the group ranges for which the lost
semantic fragment contains information.
If the BSR can split up the BSM so that each group range (and all of
its RP information) can fit entirely inside one BSMF, then it should
do so. If a BSMF is lost, the state from the previous BSM for the
group ranges from the missing BSMF will be retained. Each fragment
that does arrive will update the RP information for the group ranges
contained in that fragment, and the new group-to-RP mappings for
those can be used immediately. The information from the missing
fragment will be obtained when the next BSM is transmitted.
If the list of RPs for a single group range is long, one may split
the information across multiple BSMFs to avoid IP fragmentation. In
this case, all the BSMFs comprising the information for that group
range 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 BSMFs from the same BSM (i.e., that have the same fragment
tag) must wait until BSMFs providing RP Count RPs for that group
range have been received before the new group-to-RP mapping can be
used for that group range. If a single BSMF from such a large group
Bhaskar, et al. Standards Track [Page 30]
RFC 5059 BSR Mechanism for PIM January 2008
range is lost, then that entire group range will have to wait until
the next BSM is originated. Hence, in this case, the benefit of
using semantic fragmentation is dubious.
Next we need to consider how a BSR would remove group ranges. A
router receiving a set of BSMFs cannot tell if a group range is
missing. If it has seen a group range before, it must assume that
that group range still exists, and that the BSMF describing that
group range has been lost. The router should retain this information
for BS_Timeout. Thus, for a BSR to remove a group range, it should
include that group range, but with an RP Count of zero, and it should
resend this information in each BSM for BS_Timeout.
4.2. Candidate-RP-Advertisement Message Format
Candidate-RP-Advertisement messages are periodically unicast from the
C-RPs to the BSR.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|PIM Ver| Type | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Count | Priority | Holdtime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RP Address (Encoded-Unicast format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address 1 (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address n (Encoded-Group format) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
PIM Version, Reserved, Checksum
Described in [1].
Type
PIM Message Type. Value is 8 for a Candidate-RP-Advertisement
message.
Prefix Count
The number of Encoded-Group Addresses included in the message;
indicating the group range for which the C-RP is advertising.
C-RPs MUST NOT send C-RP-Adv messages with a Prefix Count of
'0'.
Bhaskar, et al. Standards Track [Page 31]
RFC 5059 BSR Mechanism for PIM January 2008
Priority
The 'Priority' of the included RP, for the corresponding
Encoded- Group Address (if any). The highest priority is '0'
(i.e., the lower the value of the 'Priority' field, the higher
the priority). This field is stored at the BSR upon receipt
along with the RP address and corresponding Encoded-Group
Address.
Holdtime
The amount of time (in seconds) the advertisement is valid.
This field allows advertisements to be aged out. This field
should be set to 2.5 times C_RP_Adv_Period.
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 ranges for which the C-RP is advertising. Format
described in Encoded-Group-Address in [1].
Within a Candidate-RP-Advertisement message, the RP Address and all
the Group Addresses MUST be of the same address family. In addition,
the address family of the fields in the message MUST be the same as
the IP source and destination addresses of the packet. This permits
maximum implementation flexibility for dual-stack IPv4/IPv6 routers.
Bhaskar, et al. Standards Track [Page 32]
RFC 5059 BSR Mechanism for PIM January 2008
5. Timers and Timer Values
Timer Name: Bootstrap Timer (BST(Z))
+------------------+-------------------------+----------------------+
| Value Name | Value | Explanation |
+------------------+-------------------------+----------------------+
| BS_Period | Default: 60 seconds | Periodic interval |
| | | with which BSMs |
| | | are normally |
| | | originated |
+------------------+-------------------------+----------------------+
| BS_Timeout | Default: 130 seconds | Interval after |
| | | which a BSR is |
| | | timed out if no |
| | | BSM is received |
| | | from that BSR |
+------------------+-------------------------+----------------------+
| BS_Min_Interval | Default: 10 seconds | Minimum interval |
| | | with which BSMs |
| | | may be originated |
+------------------+-------------------------+----------------------+
| BS_Rand_Override | see below | Randomized |
| | | interval used to |
| | | reduce control |
| | | message overhead |
| | | during BSR |
| | | election |
+------------------+-------------------------+----------------------+
Note that BS_Timeout MUST be larger than BS_Period, even if their
values are changed from the defaults. We recommend that BS_Timeout
is set to 2 times BS_Period plus 10 seconds.
BS_Rand_Override is calculated using the following pseudocode, in
which all values are in units of seconds. The values of
BS_Rand_Override generated by this pseudocode are between 5 and 23
seconds, with smaller values generated if the C-BSR has a high
bootstrap weight, and larger values generated if the C-BSR has a low
bootstrap weight.
BS_Rand_Override = 5 + priorityDelay + addrDelay
where priorityDelay is given by:
priorityDelay = 2 * log_2(1 + bestPriority - myPriority)
and addrDelay is given by the following for IPv4:
Bhaskar, et al. Standards Track [Page 33]
RFC 5059 BSR Mechanism for PIM January 2008
if (bestPriority == myPriority) {
addrDelay = log_2(1 + bestAddr - myAddr) / 16
} else {
addrDelay = 2 - (myAddr / 2^31)
}
and addrDelay is given by the following for IPv6:
if (bestPriority == myPriority) {
addrDelay = log_2(1 + bestAddr - myAddr) / 64
} else {
addrDelay = 2 - (myAddr / 2^127)
}
and bestPriority is given by:
bestPriority = max(storedPriority, myPriority)
and bestAddr is given by:
bestAddr = max(storedAddr, myAddr)
and where myAddr is the Candidate-BSR's address, storedAddr is the
stored BSR's address, myPriority is the Candidate-BSR's configured
priority, and storedPriority is the stored BSR's priority.
Timer Name: Scope Zone Expiry Timer (SZT(Z))
+---------------+---------------------------+-----------------------+
| Value Name | Value | Explanation |
+---------------+---------------------------+-----------------------+
| SZ_Timeout | Default: 1300 seconds | Interval after |
| | | which a scope zone |
| | | is timed out if no |
| | | BSM is received |
| | | for that scope |
| | | zone |
+---------------+---------------------------+-----------------------+
Note that SZ_Timeout MUST be larger than BS_Timeout, even if their
values are changed from the defaults. We recommend that SZ_Timeout
is set to 10 times BS_Timeout.
Bhaskar, et al. Standards Track [Page 34]
RFC 5059 BSR Mechanism for PIM January 2008
Timer Name: Group-to-C-RP mapping Expiry Timer (CGET(M,Z))
+------------------------+-------------------+----------------------+
| Value Name | Value | Explanation |
+------------------------+-------------------+----------------------+
| C-RP Mapping Timeout | from message | Holdtime from C- |
| | | RP-Adv message |
+------------------------+-------------------+----------------------+
Timer Name: Group-to-RP mapping Expiry Timer (GET(M,Z))
+-----------------------+-------------------+-----------------------+
| Value Name | Value | Explanation |
+-----------------------+-------------------+-----------------------+
| RP Mapping Timeout | from message | Holdtime from BSM |
+-----------------------+-------------------+-----------------------+
Timer Name: C-RP Advertisement Timer (CRPT)
+-------------------+------------------------+----------------------+
| Value Name | Value | Explanation |
+-------------------+------------------------+----------------------+
| C_RP_Adv_Period | Default: 60 seconds | Periodic interval |
| | | with which C-RP- |
| | | Adv messages are |
| | | sent to a BSR |
+-------------------+------------------------+----------------------+
| C_RP_Adv_Backoff | Default: 0-3 seconds | Whenever a |
| | | triggered C_RP_Adv |
| | | is sent, a new |
| | | randomized value |
| | | between 0 and 3 |
| | | is used |
+-------------------+------------------------+----------------------+
Bhaskar, et al. Standards Track [Page 35]
RFC 5059 BSR Mechanism for PIM January 2008
6. Security Considerations
6.1. Possible Threats
Threats affecting the PIM BSR mechanism are primarily of two forms:
denial-of-service (DoS) attacks and traffic-diversion attacks. An
attacker that subverts the BSR mechanism can prevent multicast
traffic from reaching the intended recipients, can divert multicast
traffic to a place where they can monitor it, and can potentially
flood third parties with traffic.
Traffic can be prevented from reaching the intended recipients by one
of two mechanisms:
o Subverting a BSM, and specifying RPs that won't actually forward
traffic.
o Registering with the BSR as a C-RP, and then not forwarding
traffic.
Traffic can be diverted to a place where it can be monitored by both
of the above mechanisms; in this case, the RPs would forward the
traffic, but are located so as to aid monitoring or man-in-the-middle
attacks on the multicast traffic.
A third party can be flooded by either of the above two mechanisms by
specifying the third party as the RP, and register traffic will then
be forwarded to the third party.
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 implementer to decide. However, we note that a router
implementing such rate limiting must only do so if the ICMP packet
correctly echoes part of a Register packet that was sent to the RP.
If this check were not made, then simply sending ICMP Unreachable
packets to the DR with the source address of the RP spoofed would be
sufficient to cause a denial-of-service attack on the multicast
traffic originating from that DR.
Bhaskar, et al. Standards Track [Page 36]
RFC 5059 BSR Mechanism for PIM January 2008
6.3. Bootstrap Message Security
If a legitimate PIM router in a domain is compromised, there is
little any security mechanism can do to prevent that router from
subverting PIM traffic in that domain.
Implementations SHOULD provide a per-interface configuration option
where one can specify that no Bootstrap messages are to be sent out
of or accepted on the interface. This should generally be configured
on all PMBRs in order not to receive messages from neighboring
domains. This avoids receiving legitimate messages with conflicting
BSR information from other domains, and also prevents BSR attacks
from neighboring domains. This option is also useful on leaf
interfaces where there are only hosts present. However, the Security
Considerations section of [1] states that there should be a mechanism
for not accepting PIM Hello messages on leaf interfaces and that
messages should only be accepted from valid PIM neighbors. There may
however be additional issues with unicast Bootstrap messages; see
below. In addition to dropping all multicast Bootstrap messages on
PMBRs, we also recommend configuring PMBRs (both towards other
domains and on leaf interfaces) to drop all unicast PIM messages
(Bootstrap message, Candidate-RP Advertisement, PIM register, and PIM
register stop).
6.3.1. Unicast Bootstrap Messages
There are some possible security issues with unicast Bootstrap
messages. The Bootstrap Message Processing Checks prevent a router
from accepting a Bootstrap message from outside of the PIM Domain, as
the source address on Bootstrap messages must be an immediate PIM
neighbor. There is however a small window of time after a reboot
where a PIM router will accept a bad Bootstrap message that is
unicast from an immediate neighbor, and it might be possible to
unicast a Bootstrap message to a router during this interval from
outside the domain, using the spoofed source address of a neighbor.
The best way to protect against this is to use the above-mentioned
mechanism of configuring border and leaf interfaces to drop all
bootstrap messages, including unicast messages. This can also 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 use of unicast Bootstrap messages is for backwards compatibility
only. Due to the possible security implications, implementations
supporting unicast Bootstrap messages SHOULD provide a configuration
option for whether they are to be used.
Bhaskar, et al. Standards Track [Page 37]
RFC 5059 BSR Mechanism for PIM January 2008
6.3.2. Multi-Access Subnets
As mentioned above, implementations SHOULD provide a per-interface
configuration option so that leaf interfaces and interfaces facing
other domains can be configured to drop all Bootstrap messages. In
this section, we will consider multi-access subnets where there are
both multiple PIM routers in a PIM domain and PIM routers outside the
PIM domain or non-trusted hosts. On such subnets, one should (if
possible) configure the PMBRs to drop Bootstrap messages. This is
possible provided that the routers in the PIM domain receive
Bootstrap messages on other internal subnets. That is, for each of
the routers on the multi-access subnet that are in our domain, the
RPF interface for each of the Candidate-BSR addresses must be an
internal interface (an interface not on a multi-access subnet).
There are however network topologies where this is not possible. For
such topologies, we recommend that IPsec Authentication Header (AH)
is used to protect communication between the PIM routers in the
domain, and that such routers are configured to drop and log
communication attempts from any nodes 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. In order to prevent replay attacks, one will need to
have one security association (SA) per sender and use the sender
address for SA lookup. The securing of interactions between PIM
neighbors is discussed in more detail in the Security Considerations
section of [1], and so we do not discuss the details further here.
The same security mechanisms that can be used to secure PIM Join,
Prune, and Assert messages should also be used to secure Bootstrap
messages. How exactly to secure PIM link-local messages is still
being worked on by the PIM working group; see [10].
6.4. Candidate-RP-Advertisement Message Security
Even if it is not possible to subvert Bootstrap messages, an attacker
might be able to perform most of the same attacks by simply sending
C-RP-Adv messages to the BSR specifying the attacker's choice of RPs.
Thus, it is necessary to control the sending of C-RP-Adv messages in
essentially the same ways that we control Bootstrap messages.
However, C-RP-Adv messages are unicast and normally travel multiple
hops, so controlling them is more difficult.
6.4.1. Non-Cryptographic Security of C-RP-Adv Messages
We recommend that PMBRs are configured to drop C-RP-Adv messages.
One might configure the PMBRs to drop all unicast PIM messages
(Bootstrap message, Candidate-RP Advertisement, PIM register, and PIM
register stop). PMBRs may also perform source-address filtering to
prevent packets entering the PIM domain with IP source addresses that
Bhaskar, et al. Standards Track [Page 38]
RFC 5059 BSR Mechanism for PIM January 2008
are infrastructure addresses in the PIM domain. We also recommend
that implementations have a way of restricting which IP addresses the
BSR accepts C-RP-Adv messages from. The BSR can then be configured
to only accept C-RP-Adv messages from infrastructure addresses or the
subset used for Candidate-RPs.
If the unicast and multicast topologies are known to be congruent,
the following checks should be made. On interfaces that are
configured to be leaf subnets, all C-RP-Adv messages should be
dropped. On multi- access subnets with multiple PIM routers and
hosts that are not trusted, the router can at least check that the
source Media Access Control (MAC) address is that of a valid PIM
neighbor.
6.4.2. Cryptographic Security of C-RP-Adv Messages
For true security, we recommend that all C-RPs are configured to use
IPsec authentication. The authentication process for a C-RP-Adv
message between a C-RP and the BSR is identical to the authentication
process for PIM Register messages between a DR and the relevant RP,
except that there will normally be fewer C-RPs in a domain than there
are DRs, so key management is a little simpler. We do not describe
the details of this process further here, but refer to the Security
Considerations section of [1]. Note that the use of cryptographic
security for C-RP-Adv messages does not remove the need for the non-
cryptographic mechanisms, as explained above.
6.5. Denial of Service using IPsec
An additional concern is that of denial-of-service attacks caused by
sending high volumes of Bootstrap messages or C-RP-Adv messages with
invalid IPsec authentication information. It is possible that these
messages could overwhelm the CPU resources of the recipient.
The non-cryptographic security mechanisms above restrict from where
unicast Bootstrap messages and C-RP-Adv messages are accepted. In
addition, we recommend that rate-limiting mechanisms can be
configured, to be applied on receipt of unicast PIM packets. The
rate-limiter MUST independently rate-limit different types of PIM
packets -- for example, a flood of C-RP-Adv messages MUST NOT cause a
rate limiter to drop low- rate Bootstrap messages. Such a rate-
limiter might itself be used to cause a denial-of-service attack by
causing valid packets to be dropped, but in practice this is more
likely to constrain bad PIM messages. The rate-limiter will prevent
attacks on PIM from affecting other activity on the receiving router,
such as unicast routing.
Bhaskar, et al. Standards Track [Page 39]
RFC 5059 BSR Mechanism for PIM January 2008
7. Contributors
Bill Fenner, Mark Handley, Roger Kermode, and David Thaler have
contributed greatly to this document. They were authors of this
document up to version 03, and much of the current text comes from
version 03.
8. Acknowledgments
PIM-SM was designed over many years by a large group of people,
including ideas from Deborah Estrin, Dino Farinacci, Ahmed Helmy,
Steve Deering, Van Jacobson, C. Liu, Puneet Sharma, Liming Wei, Tom
Pusateri, Tony Ballardie, Scott Brim, Jon Crowcroft, Paul Francis,
Joel Halpern, Horst Hodel, Polly Huang, Stephen Ostrowski, Lixia
Zhang, Girish Chandranmenon, Pavlin Radoslavov, John Zwiebel, Isidor
Kouvelas, and Hugh Holbrook. This BSR specification draws heavily on
text from RFC 2362.
Many members of the PIM Working Group have contributed comments and
corrections for this document, including Christopher Thomas Brown,
Ardas Cilingiroglu, Murthy Esakonu, Venugopal Hemige, Prashant
Jhingran, Rishabh Parekh, and Katta Sambasivarao.
9. Normative References
[1] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol
Specification (Revised)", RFC 4601, August 2006.
[2] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-PIM)", RFC
5015, October 2007.
[3] Meyer, D., "Administratively Scoped IP Multicast", BCP 23, RFC
2365, July 1998.
[4] Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and B.
Zill, "IPv6 Scoped Address Architecture", RFC 4007, March 2005.
[5] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
Bhaskar, et al. Standards Track [Page 40]
RFC 5059 BSR Mechanism for PIM January 2008
10. Informative References
[7] Estrin, D., et al., "Protocol Independent Multicast-Sparse Mode
(PIM-SM): Protocol Specification", RFC 2362, June 1998.
[8] Kim, D., Meyer, D., Kilmer, H., and D. Farinacci, "Anycast
Rendevous Point (RP) mechanism using Protocol Independent
Multicast (PIM) and Multicast Source Discovery Protocol (MSDP)",
RFC 3446, January 2003.
[9] Farinacci, D. and Y. Cai, "Anycast-RP Using Protocol Independent
Multicast (PIM)", RFC 4610, August 2006.
[10] Atwood, W. and S. Islam, "Security Issues in PIM-SM Link-local
Messages", Work in Progress, July 2007.
[11] IANA, "Address Family Numbers",
<http://www.iana.org/assignments/address-family-numbers>.
Authors' Addresses
Nidhi Bhaskar
Arastra, Inc.
P.O. Box 10905
Palo Alto, CA 94303
USA
EMail: nidhi@arastra.com
Alexander Gall
SWITCH
P.O. Box
CH-8021 Zurich
Switzerland
EMail: alexander.gall@switch.ch
James Lingard
Arastra, Inc.
P.O. Box 10905
Palo Alto, CA 94303
USA
EMail: jchl@arastra.com
Stig Venaas
UNINETT
NO-7465 Trondheim
Norway
EMail: venaas@uninett.no
Bhaskar, et al. Standards Track [Page 41]
RFC 5059 BSR Mechanism for PIM January 2008
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Bhaskar, et al. Standards Track [Page 42]