draft-ietf-pim-sm-bsr-03.txt

BCAST Implementation for NS2

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.



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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
scoped.

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



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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
configured.


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",
                 "Elected-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:



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     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.




























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Per-Scope-Zone Candidate-BSR State Machine


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

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


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


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



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zone:

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

Pending-BSR (P-BSR)
     The router is a candidate to be the BSR for the scope zone.
     Currently no other router is the preferred BSR, but this router is
     not yet the BSR.  For comparisons with incoming BS messages, the
     router treats itself as the BSR.  This is a temporary state that
     prevents rapid thrashing of the choice of BSR during BSR election.

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

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

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

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


























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