RFC 2362
Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification
3 Detailed Protocol Description This section describes the protocol operations from the perspective of an individual router implementation. In particular, for each message type we describe how it is generated and processed.
3.1 Hello Hello messages are sent so neighboring routers can discover each other. 3.1.1 Sending Hellos Hello messages are sent periodically between PIM neighbors, every [Hello-Period] seconds. This informs routers what interfaces have PIM neighbors. Hello messages are multicast using address 224.0.0.13 (ALL-PIM-ROUTERS group). The packet includes a Holdtime, set to [Hello-Holdtime], for neighbors to keep the information valid. Hellos are sent on all types of communication links. 3.1.2 Receiving Hellos When a router receives a Hello message, it stores the network layer address for that neighbor, sets its Neighbor-timer for the Hello sender to the Holdtime included in the Hello, and determines the Designated Router (DR) for that interface. The highest addressed system is elected DR. Each Hello received causes the DR's address to be updated. When a router that is the active DR receives a Hello from a new neighbor (i.e., from an address that is not yet in the DRs neighbor table), the DR unicasts its most recent RP-set information to the new neighbor. 3.1.3 Timing out neighbor entries A periodic process is run to time out PIM neighbors that have not sent Hellos. If the DR has gone down, a new DR is chosen by scanning all neighbors on the interface and selecting the new DR to be the one with the highest network layer address. If an interface has gone down, the router may optionally time out all PIM neighbors associated with the interface. 3.2 Join/Prune Join/Prune messages are sent to join or prune a branch off of the multicast distribution tree. A single message contains both a join and prune list, either one of which may be null. Each list contains a set of source addresses, indicating the source-specific trees or shared tree that the router wants to join or prune.
3.2.1 Sending Join/Prune Messages Join/Prune messages are merged such that a message sent to a particular upstream neighbor, N, includes all of the current joined and pruned sources that are reached via N; according to unicast routing Join/Prune messages are multicast to all routers on multi- access networks with the target address set to the next hop router towards S or RP. Join/Prune messages are sent every [Join/Prune- Period] seconds. In the future we will introduce mechanisms to rate- limit this control traffic on a hop by hop basis, in order to avoid excessive overhead on small links. In addition, certain events cause triggered Join/Prune messages to be sent. Periodic Join/Prune Messages: A router sends a periodic Join/Prune message to each distinct RPF neighbor associated with each (S,G), (*,G) and (*,*,RP) entry. Join/Prune messages are only sent if the RPF neighbor is a PIM neighbor. A periodic Join/Prune message sent to a particular RPF neighbor is constructed as follows: 1 Each router determines the RP for a (*,G) entry by using the hash function described. The RP address (with RPT and WC bits set) is included in the join list of a periodic Join/Prune message under the following conditions: 1 The Join/Prune message is being sent to the RPF neighbor toward the RP for an active (*,G) or (*,*,RP) entry, and 2 The outgoing interface list in the (*,G) or (*,*,RP) entry is non-NULL, or the router is the DR on the same interface as the RPF neighbor. 2 A particular source address, S, is included in the join list with the RPT and WC bits cleared under the following conditions: 1 The Join/Prune message is being sent to the RPF neighbor toward S, and 2 There exists an active (S,G) entry with the RPT-bit flag cleared, and 3 The oif list in the (S,G) entry is not null.
3 A particular source address, S, is included in the prune
list with the RPT and WC bits cleared under the following
conditions:
1 The Join/Prune message is being sent to the RPF
neighbor toward S, and
2 There exists an active (S,G) entry with the RPT-bit
flag cleared, and
3 The oif list in the (S,G) entry is null.
4 A particular source address, S, is included in the prune
list with the RPT-bit set and the WC bit cleared under the
following conditions:
1 The Join/Prune message is being sent to the RPF
neighbor toward the RP and there exists a (S,G) entry with
the RPT-bit flag set and null oif list, or
2 The Join/Prune message is being sent to the RPF
neighbor toward the RP, there exists a (S,G) entry with the
RPT-bit flag cleared and SPT-bit set, and the incoming
interface toward S is different than the incoming interface
toward the RP, or
3 The Join/Prune message is being sent to the RPF
neighbor toward the RP, and there exists a (*,G) entry and
(S,G) entry for a directly connected source.
5 The RP address (with RPT and WC bits set) is included in
the prune list if:
1 The Join/Prune message is being sent to the RPF
neighbor toward the RP and there exists a (*,G) entry with
a null oif list (see Section 3.5.2).
Triggered Join/Prune Messages:
In addition to periodic messages, the following events will
trigger Join/Prune messages if as a result, a) a new entry is
created, or b) the oif list changes from null to non-null or non-
null to null. The contents of triggered messages are the same as
the periodic, described above.
1 Receipt of an indication from IGMP that the state of
directly-connected-membership has changed (i.e., new members
have just joined `membership indication' or all members have
left), for a group G, may cause the last-hop router to build or
modify corresponding (*,G) state. When IGMP indicates that
there are no longer directly connected members, the oif is
removed from the oif list if the oif-timer is not running. A
Join/Prune message is triggered if and only if a) a new entry is
created, or b) the oif list changes from null to non-null or
non-null to null, as follows:
1 If the receiving router does not have a route entry
for G the router creates a (*,G) entry, copies the oif list
from the corresponding (*,*,RP) entry (if it exists), and
includes the interface included in the IGMP membership
indication in the oif list; as always, the router never
includes the entry's iif in the oif list. The router sends
a Join/Prune message towards the RP with the RP address and
RPT-bit and WC-bits set in the join list. Or,
2 If a (S,G)RPT-bit or (*,G) entry already exists, the
interface included in the IGMP membership indication is
added to the oif list (if it was not included already).
2 Receipt of a Join/Prune message for (S,G), (*,G) or
(*,*,RP) will cause building or modifying corresponding state,
and subsequent triggering of upstream Join/Prune messages, in
the following cases:
1 When there is no current route entry, the RP address
included in the Join/Prune message is checked against the
local RP-Set information. If it matches, an entry will be
created and the new entry will in turn trigger an upstream
Join/Prune message. If the router has no RP-Set information
it may discard the message, or optionally use the RP
address included in the message.
2 When the outgoing interface list of an (S,G)RPT-bit
entry becomes null, the triggered Join/Prune message will
contain S in the prune list.
3 When there exists a (S,G)RPT-bit with null oif list,
and an (*,G) Join/Prune message is received, the arriving
interface is added to the oif list and a (*,G) Join/Prune
message is triggered upstream.
4 When there exists a (*,G) with null oif list, and a
(*,*,RP) Join/Prune message is received, the receiving
interface is added to the oif list and a (*,*,RP)
Join/Prune message is triggered upstream.
3 Receipt of a packet that matches on a (S,G) entry whose
SPT-bit is cleared triggers the following if the packet arrived
on the correct incoming interface and there is a (*,G) or
(*,*,RP) entry with a different incoming interface: a) the
router sets the SPT-bit on the (S,G) entry, and b) the router
sends a Join/Prune message towards the RP with S in the prune
list and the RPT-bit set.
4 Receipt of a packet at the DR from a directly connected
source S, on the subnet containing the address S, triggers a
Join/Prune message towards the RP with S in the prune list and
the RPT-bit set under the following conditions: a) there is no
matching (S,G) state, and b) there exists a (*,G) or (*,*,RP)
for which the DR is not the RP.
5 When a Join/Prune message is received for a group G, the
prune list is checked. If the prune list contains a source or RP
for which the receiving router has a corresponding active (S,G),
(*,G) or (*,*,RP) entry, and whose iif is that on which the
Join/Prune was received, then a join for (S,G), (*,G) or
(*,*,RP) is triggered to override the prune, respectively. (This
is necessary in the case of parallel downstream routers
connected to a multi-access network.)
6 When the RP fails, the RP will not be included in the
Bootstrap messages sent to all routers in that domain. This
triggers the DRs to send (*,G) Join/Prune messages towards the
new RP for the group, as determined by the RP-Set and the hash
function. As described earlier, PMBRs trigger (*,*,RP) joins
towards each RP in the RP-Set.
7 When an entry's Join/Prune-Suppression timer expires, a
Join/Prune message is triggered upstream corresponding to that
entry, even if the outgoing interface has not transitioned
between null and non-null states.
8 When the RPF neighbor changes (whether due to an Assert or
changes in unicast routing), the router sets a random delay
timer (the Random-Delay-Join-Timer) whose expiration triggers
sending of a Join/Prune message for the asserted route entry to
the Assert winner (if the Join/Prune Suppression timer has
expired.)
We do not trigger prunes onto interfaces based on data packets. Data
packets that arrive on the wrong incoming interface are silently
dropped. However, on point-to-point interfaces triggered prunes may
be sent as an optimization.
aragraphFragmentation It is possible that a Join/Prune message constructed according to the preceding rules could exceed the MTU of a network. In this case, the message can undergo semantic fragmentation whereby information corresponding to different groups can be sent in different messages. However, if a Join/Prune message must be fragmented the complete prune list corresponding to a group G must be included in the same Join/Prune message as the associated RP-tree Join for G. If such semantic fragmentation is not possible, IP fragmentation should be used between the two neighboring hops. 3.2.2 Receiving Join/Prune Messages When a router receives Join/Prune message, it processes it as follows. The receiver of the Join/Prune notes the interface on which the PIM message arrived, call it I. The receiver then checks to see if the Join/Prune message was addressed to the receiving router itself (i.e., the router's address appears in the Unicast Upstream Neighbor Router field of the Join/Prune message). (If the router is connected to a multiaccess LAN, the message could be intended for a different router.) If the Join/Prune is for this router the following actions are taken. For each group address G, in the Join/Prune message, the associated join list is processed as follows. We refer to each address in the join list as Sj; Sj refers to the RP if the RPT-bit and WC-bit are both set. For each Sj in the join list of the Join/Prune message: 1 If an address, Sj, in the join list of the Join/Prune message has the RPT-bit and WC-bit set, then Sj is the RP address used by the downstream router(s) and the following actions are taken: 1 If Sj is not the same as the receiving router's RP mapping for G, the receiving router may ignore the Join/Prune message with respect to that group entry. If the router does not have any RP-Set information, it may use the address Sj included in the Join/Prune message as the RP for the group. 2 If Sj is the same as the receiving router's RP mapping for G, the receiving router adds I to the outgoing interface list of the (*,G) route entry (if there is no (*,G) entry, the router creates one first) and sets the Oif-timer for that interface to the Holdtime specified in the Join/Prune message. In addition, the Oif-Deletion-Delay for that interface is set to 1/3rd the Holdtime specified
in the Join/Prune message. If a (*,*,RP) entry exists, for
the RP associated with G, then the oif list of the newly
created (*,G) entry is copied from that (*,*,RP) entry.
3 For each (Si,G) entry associated with group G: i) if
Si is not included in the prune list, ii) if I is not on
the same subnet as the address Si, and iii) if I is not the
iif, then interface I is added to the oif list and the
Oif-timer for that interface in each affected entry is
increased (never decreased) to the Holdtime included in the
Join/Prune message. In addition, if the Oif-timer for that
interface is increased, the Oif-Deletion-Delay for that
interface is set to 1/3rd the Holdtime specified in the
Join/Prune message.
If the group address in the Join/Prune message is `*' then
every (*,G) and (S,G) entry, whose group address hashes to
the RP indicated in the (*,*,RP) Join/Prune message, is
updated accordingly. A `*' in the group field of the
Join/Prune is represented by a group address 224.0.0.0 and
a group mask length of 4, indicating a (*,*,RP) Join.
4 If the (Si,G) entry has its RPT-bit flag set to 1, and
its oif list is the same as the (*,G) oif list, then the
(Si,G)RPT-bit entry is deleted,
5 The incoming interface is set to the interface used to
send unicast packets to the RP in the (*,G) route entry,
i.e., RPF interface toward the RP.
2 For each address, Sj, in the join list whose RPT-bit and
WC-bit are not set, and for which there is no existing (Sj,G)
route entry, the router initiates one. The router creates a
(S,G) entry and copies all outgoing interfaces from the
(S,G)RPT-bit entry, if it exists. If there is no (S,G) entry,
the oif list is copied from the (*,G) entry; and if there is no
(*,G) entry, the oif list is copied from the (*,*,RP) entry, if
it exists. In all cases, the iif of the (S,G) entry is always
excluded from the oif list.
1 The outgoing interface for (Sj,G) is set to I. The
incoming interface for (Sj,G) is set to the interface used
to send unicast packets to Sj (i.e., the RPF neighbor).
2 If the interface used to reach Sj, is the same as I,
this represents an error (or a unicast routing change) and
the Join/Prune must not be processed.
3 For each address, Sj, in the join list of the Join/Prune
message, for which there is an existing (Sj,G) route entry,
1 If the RPT-bit is not set for Sj listed in the
Join/Prune message, but the RPT-bit flag is set on the
existing (Sj,G) entry, the router clears the RPT-bit flag
on the (Sj,G) entry, sets the incoming interface to point
towards Sj for that (Sj,G) entry, and sends a Join/Prune
message corresponding to that entry through the new
incoming interface; and
2 If I is not the same as the existing incoming
interface, the router adds I to the list of outgoing
interfaces.
3 The Oif-timer for I is increased (never decreased) to
the Holdtime included in the Join/Prune message. In
addition, if the Oif-timer for that interface is increased,
the Oif-Deletion-Delay for that interface is set to 1/3rd
the Holdtime specified in the Join/Prune message.
4 The (Sj,G) entry's SPT bit is cleared until data comes
down the shortest path tree.
For each group address G, in the Join/Prune message, the
associated prune list is processed as follows. We refer to each
address in the prune list as Sp; Sp refers to the RP if the RPT-
bit and WC-bit are both set. For each Sp in the prune list of the
Join/Prune message:
1 For each address, Sp, in the prune list whose RPT-bit and
WC-bit are cleared:
1 If there is an existing (Sp,G) route entry, the router
lowers the entry's Oif-timer for I to its Oif-Deletion-
Delay, allowing for other downstream routers on a multi-
access LAN to override the prune. However, on point-to-
point links, the oif-timer is expired immediately.
2 If the router has a current (*,G), or (*,*,RP), route
entry, and if the existing (Sp,G) entry has its RPT-bit
flag set to 1, then this (Sp,G)RPT-bit entry is maintained
(not deleted) even if its outgoing interface list is null.
2 For each address, Sp, in the prune list whose RPT-bit is
set and whose WC-bit cleared:
1 If there is an existing (Sp,G) route entry, the router
lowers the entry's Oif-timer for I to its Oif-Deletion-
Delay, allowing for other downstream routers on a multi-
access LAN to override the prune. However, on point-to-
point links, the oif-timer is expired immediately.
2 If the router has a current (*,G), or (*,*,RP), route
entry, and if the existing (Sp,G) entry has its RPT-bit
flag set to 1, then this (Sp,G)RPT-bit entry is not
deleted, and the Entry-timer is restarted, even if its
outgoing interface list is null.
3 If (*,G), or corresponding (*,*,RP), state exists, but
there is no (Sp,G) entry, an (Sp,G)RPT-bit entry is created
. The outgoing interface list is copied from the (*,G), or
(*,*,RP), entry, with the interface, I, on which the prune
was received, is deleted. Packets from the pruned source,
Sp, match on this state and are not forwarded toward the
pruned receivers.
4 If there exists a (Sp,G) entry, with or without the
RPT-bit set, the oif-timer for I is expired, and the
Entry-timer is restarted.
3 For each address, Sp, in the prune list whose RPT-bit and
WC-bit are both set:
1 If there is an existing (*,G) entry, with Sp as the RP
for G, the router lowers the entry's Oif-timer for I to its
Oif-Deletion-Delay, allowing for other downstream routers
on a multi-access LAN to override the prune. However, on
point-to-point links, the oif-timer is expired immediately.
2 If the corresponding (*,*,RP) state exists, but there
is no (*,G) entry, a (*,G) entry is created. The outgoing
interface list is copied from (*,*,RP) entry, with the
interface, I, on which the prune was received, deleted.
For any new (S,G), (*,G) or (*,*,RP) entry created by an
incoming Join/Prune message, the SPT-bit is cleared (and if a
Join/Prune-Suppression timer is used, it is left off.)
If the entry has a Join/Prune-Suppression timer associated with it,
and if the received Join/Prune does not indicate the router as its
target, then the receiving router examines the join and prune lists
to see if any addresses in the list `completely-match' existing
(S,G), (*,G), or (*,*,RP) state for which the receiving router
currently schedules Join/Prune messages. An element on the join or
prune list `completely-matches' a route entry only if both the addresses and RPT-bit flag are the same. If the incoming Join/Prune message completely matches an existing (S,G), (*,G), or (*,*,RP) entry and the Join/Prune arrived on the iif for that entry, then the router compares the Holdtime included in the Join/Prune message, to its own [Join/Prune-Holdtime]. If its own [Join/Prune-Holdtime] is lower, the Join/Prune-Suppression-timer is started at the [Join/Prune-Suppression-Timeout]. If the [Join/Prune-Holdtime] is equal, the tie is resolved in favor of the Join/Prune Message originator that has the higher network layer address. When the Join/Prune timer expires, the router triggers a Join/Prune message for the corresponding entry(ies). 3.3 Register and Register-Stop When a source first starts sending to a group its packets are encapsulated in Register messages and sent to the RP. If the data rate warrants source-specific paths, the RP sets up source specific state and starts sending (S,G) Join/Prune messages toward the source, with S in the join list. 3.3.1 Sending Registers and Receiving Register-Stops Register messages are sent as follows: 1 When a DR receives a packet from a directly connected source, S, on the subnet containing the address S, 1 If there is no corresponding (S,G) entry, and the router has RP-Set information, and the DR is not the RP for G, the DR creates an (S,G) entry with the Register- Suppression-timer turned off and the RP address set according to the hash function mapping for the corresponding group. The oif list is copied from existing (*,G) or (*,*,RP) entries, if they exist. The iif of the (S,G) entry is always excluded from the oif list. If there exists a (*,G) or (*,*,RP) entry, the DR sends a Join/Prune message towards the RP with S in the prune list and the RPT-bit set. 2 If there is a (S,G) entry in existence, the DR simply restarts the corresponding Entry-timer. When a PMBR (e.g., a router that connects the PIM-SM region to a dense mode region running DVMRP or PIM-DM) receives a packet from a source in the dense mode region, the router
treats the packet as if it were from a directly connected
source. A separate document will describe the details of
interoperability.
2 If the new or previously-existing (S,G) entry's Register-
Suppression-timer is not running, the data packet is
encapsulated in a Register message and unicast to the RP for
that group. The data packet is also forwarded according to (S,G)
state in the DR if the oif list is not null; since a receiver
may join the SP-tree while the DR is still registering to the
RP.
3 If the (S,G) entry's Register-Suppression-timer is running,
the data packet is not sent in a Register message, it is just
forwarded according to the (S,G) oif list.
When the DR receives a Register-Stop message, it restarts the
Register-Suppression-timer in the corresponding (S,G) entry(ies) at
[Register-Suppression-Timeout] seconds. If there is data to be
registered, the DR may send a null Register (a Register message with
a zero-length data portion in the inner packet) to the RP, [Probe-
Time] seconds before the Register-Suppression-timer expires, to avoid
sending occasional bursts of traffic to an RP unnecessarily.
3.3.2 Receiving Register Messages and Sending Register-Stops
When a router (i.e., the RP) receives a Register message, the router
does the following:
1 Decapsulates the data packet, and checks for a
corresponding (S,G) entry.
1 If a (S,G) entry with cleared (0) SPT bit exists, and
the received Register does not have the Null-Register-Bit
set to 1, the packet is forwarded; and the SPT bit is left
cleared (0). If the SPT bit is 1, the packet is dropped,
and Register-Stop messages are triggered. Register-Stops
should be rate-limited (in an implementation-specific
manner) so that no more than a few are sent per round trip
time. This prevents a high datarate stream of packets from
triggering a large number of Register-Stop messages between
the time that the first packet is received and the time
when the source receives the first Register-Stop.
2 If there is no (S,G) entry, but there is a (*,G)
entry, and the received Register does not have the Null-
Register-Bit set to 1, the packet is forwarded according to
the (*,G) entry.
3 If there is a (*,*,RP) entry but no (*,G) entry, and
the Register received does not have the Null-Register-Bit
set to 1, a (*,G) or (S,G) entry is created and the oif
list is copied from the (*,*,RP) entry to the new entry.
The packet is forwarded according to the created entry.
4 If there is no G or (*,*,RP) entry corresponding to G,
the packet is dropped, and a Register-Stop is triggered.
5 A "Border bit" bit is added to the Register message,
to facilitate interoperability mechanisms. PMBRs set this
bit when registering for external sources (see Section
2.7). If the "Border bit" is set in the Register,
the RP does the following:
1 If there is no matching (S,G) state, but there
exists (*,G) or (*,*,RP) entry, the RP creates a (S,G)
entry, with a `PMBR' field. This field holds the
source of the Register (i.e. the outer network layer
address of the register packet). The RP triggers a
(S,G) join towards the source of the data packet, and
clears the SPT bit for the (S,G) entry. If the
received Register is not a `null Register' the packet
is forwarded according to the created state. Else,
2 If the `PMBR' field for the corresponding (S,G)
entry matches the source of the Register packet, and
the received Register is not a `null Register', the
decapsulated packet is forwarded to the oif list of
that entry. Else,
3 If the `PMBR' field for the corresponding (S,G)
entry matches the source of the Register packet, the
decapsulated packet is forwarded to the oif list of
that entry, else
4 The packet is dropped, and a Register-stop is
triggered towards the source of the Register.
The (S,G) Entry-timer is restarted by Registers arriving from
that source to that group.
2 If the matching (S,G) or (*,G) state contains a null oif
list, the RP unicasts a Register-Stop message to the source of
the Register message; in the latter case, the source-address
field, within the Register-Stop message, is set to the wildcard
value (all 0's). This message is not processed by intermediate
routers, hence no (S,G) state is constructed between the RP and
the source.
3 If the Register message arrival rate warrants it and there
is no existing (S,G) entry, the RP sets up a (S,G) route entry
with the outgoing interface list, excluding iif(S,G), copied
from the (*,G) outgoing interface list, its SPT-bit is
initialized to 0. If a (*,G) entry does not exist, but there
exists a (*,*,RP) entry with the RP corresponding to G , the oif
list for (S,G) is copied - excluding the iif - from that
(*,*,RP) entry.
A timer (Entry-timer) is set for the (S,G) entry and this timer
is restarted by receipt of data packets for (S,G). The (S,G)
entry causes the RP to send a Join/Prune message for the
indicated group towards the source of the register message.
If the (S,G) oif list becomes null, Join/Prune messages will not
be sent towards the source, S.
3.4 Multicast Data Packet Forwarding
Processing a multicast data packet involves the following steps:
1 Lookup route state based on a longest match of the source
address, and an exact match of the destination address in the
data packet. If neither S, nor G, find a longest match entry,
and the RP for the packet's destination group address has a
corresponding (*,*,RP) entry, then the longest match does not
require an exact match on the destination group address. In
summary, the longest match is performed in the following order:
(1) (S,G), (2) (*,G). If neither is matched, then a lookup is
performed on (*,*,RP) entries.
2 If the packet arrived on the interface found in the
matching-entry's iif field, and the oif list is not null:
1 Forward the packet to the oif list for that entry,
excluding the subnet containing S, and restart the Entry-
timer if the matching entry is (S,G). Optionally, the
(S,G) Entry-timer may be restarted by periodic checking of
the matching packet count.
2 If the entry is a (S,G) entry with a cleared SPT-bit,
and a (*,G) or associated (*,*,RP) also exists whose
incoming interface is different than that for (S,G), set
the SPT-bit for the (S,G) entry and trigger an (S,G) RPT-
bit prune towards the RP.
3 If the source of the packet is a directly-connected
host and the router is the DR on the receiving interface,
check the Register-Suppression-timer associated with the
(S,G) entry. If it is not running, then the router
encapsulates the data packet in a register message and
sends it to the RP.
This covers the common case of a packet arriving on the RPF
interface to the source or RP and being forwarded to all
joined branches. It also detects when packets arrive on the
SP-tree, and triggers their pruning from the RP-tree. If it
is the DR for the source, it sends data packets
encapsulated in Registers to the RPs.
3 If the packet matches to an entry but did not arrive on the
interface found in the entry's iif field, check the SPT-bit
of the entry. If the SPT-bit is set, drop the packet. If
the SPT-bit is cleared, then lookup the (*,G), or (*,*,RP),
entry for G. If the packet arrived on the iif found in
(*,G), or the corresponding (*,*,RP), forward the packet to
the oif list of the matching entry. This covers the case
when a data packet matches on a (S,G) entry for which the
SP-tree has not yet been completely established upstream.
4 If the packet does not match any entry, but the source of
the data packet is a local, directly-connected host, and
the router is the DR on a multi-access LAN and has RP-Set
information, the DR uses the hash function to determine the
RP associated with the destination group, G. The DR creates
a (S,G) entry, with the Register-Suppression-timer not
running, encapsulates the data packet in a Register message
and unicasts it to the RP.
5 If the packet does not match to any entry, and it is not a
local host or the router is not the DR, drop the packet.
3.4.1 Data triggered switch to shortest path tree (SP-tree)
Different criteria can be applied to trigger switching over from the
RP-based shared tree to source-specific, shortest path trees.
One proposed example is to do so based on data rate. For example, when a (*,G), or corresponding (*,*,RP), entry is created, a data rate counter may be initiated at the last-hop routers. The counter is incremented with every data packet received for directly connected members of an SM group, if the longest match is (*,G) or (*,*,RP). If and when the data rate for the group exceeds a certain configured threshold (t1), the router initiates `source-specific' data rate counters for the following data packets. Then, each counter for a source, is incremented when packets matching on (*,G), or (*,*,RP), are received from that source. If the data rate from the particular source exceeds a configured threshold (t2), a (S,G) entry is created and a Join/Prune message is sent towards the source. If the RPF interface for (S,G) is not the same as that for (*,G) -or (*,*,RP), then the SPT-bit is cleared in the (S,G) entry. Other configured rules may be enforced to cause or prevent establishment of (S,G) state. 3.5 Assert Asserts are used to resolve which of the parallel routers connected to a multi-access LAN is responsible for forwarding packets onto the LAN. 3.5.1 Sending Asserts The following Assert rules are provided when a multicast packet is received on an outgoing multi-access interface "I" of an existing active (S,G), (*,G) or (*,*,RP) entry: 1 Do unicast routing table lookup on source address from data packet, and send assert on interface "I" for source address in data packet; include metric preference of routing protocol and metric from routing table lookup. 2 If route is not found, use metric preference of 0x7fffffff and metric 0xffffffff. When an assert is sent for a (*,G) entry, the first bit in the metric preference (the RPT-bit) is set to 1, indicating the data packet is routed down the RP-tree. Asserts should be rate-limited in an implementation-specific manner.
3.5.2 Receiving Asserts When an Assert is received the router performs a longest match on the source and group address in the Assert message, only active entries -- that have packet forwarding state -- are matched. The router checks the first bit of the metric preference (RPT-bit). 1 If the RPT-bit is set, the router first does a match on (*,G), or (*,*,RP), entries; if no matching entry is found, it ignores the Assert. 2 If the RPT-bit is not set in the Assert, the router first does a match on (S,G) entries; if no matching entry is found, the router matches (*,G) or (*,*,RP) entries. Receiving Asserts on an entry's outgoing interface: If the interface that received the Assert message is in the oif list of the matched entry, then this Assert is processed by this router as follows: 1 If the Assert's RPT-bit is set and the matching entry is (*,*,RP), the router creates a (*,G) entry. If the Assert's RPT-bit is cleared and the matching entry is (*,G), or (*,*,RP), the router creates a (S,G)RPT-bit entry. Otherwise, no new entry is created in response to the Assert. 2 The router then compares the metric values received in the Assert with the metric values associated with the matched entry. The RPT-bit and metric preference (in that order) are treated as the high-order part of an Assert metric comparison. If the value in the Assert is less than the router's value (with ties broken by the IP address, where higher network layer address wins), delete the interface from the entry. When the deletion occurs for a (*,G) or (*,*,RP) entry , the interface is also deleted from any associated (S,G)RPT-bit or (*,G) entries, respectively. The Entry-timer for the affected entries is restarted. 3 If the router has won the election the router keeps the interface in its outgoing interface list. It acts as the forwarder for the LAN. The winning router sends an Assert message containing its own metric to that outgoing interface. This will cause other routers on the LAN to prune that interface from their route entries. The winning router sets the RPT-bit in the Assert message if a (*,G) or (S,G)RPT-bit entry was matched.
Receiving Asserts on an entry's incoming interface
If the Assert arrived on the incoming interface of an existing (S,G),
(*,G), or (*,*,RP) entry, the Assert is processed as follows. If the
Assert message does not match the entry, exactly, it is ignored; i.e,
longest-match is not used in this case. If the Assert message does
match exactly, then:
1 Downstream routers will select the upstream router with the
smallest metric preference and metric as their RPF neighbor. If
two metrics are the same, the highest network layer address is
chosen to break the tie. This is important so that downstream
routers send subsequent Joins/Prunes (in SM) to the correct
neighbor. An Assert-timer is initiated when changing the RPF
neighbor to the Assert winner. When the timer expires, the
router resets its RPF neighbor according to its unicast routing
tables to capture network dynamics and router failures.
2 If the downstream routers have downstream members, and if
the Assert caused the RPF neighbor to change, the downstream
routers must trigger a Join/Prune message to inform the upstream
router that packets are to be forwarded on the multi-access
network.
3.6 Candidate-RP-Advertisements and Bootstrap messages
Candidate-RP-Advertisements (C-RP-Advs) are periodic PIM messages
unicast to the BSR by those routers that are configured as
Candidate-RPs (C-RPs).
Bootstrap messages are periodic PIM messages originated by the
Bootstrap router (BSR) within a domain, and forwarded hop-by-hop to
distribute the current RP-set to all routers in that domain.
The Bootstrap messages also support a simple mechanism by which the
Candidate BSR (C-BSR) with the highest BSR-priority and address
(referred to as the preferred BSR) is elected as the BSR for the
domain. We recommend that each router configured as a C-RP also be
configured as a C-BSR. Sections 3.6.2 and 3.6.3 describe the combined
function of Bootstrap messages as the vehicle for BSR election and
RP-Set distribution.
A Finite State Machine description of the BSR election and RP-Set
distribution mechanisms is included in Appendix II.
3.6.1 Sending Candidate-RP-Advertisements C-RPs periodically unicast C-RP-Advs to the BSR for that domain. The interval for sending these messages is subject to local configuration at the C-RP. Candidate-RP-Advertisements carry group address and group mask fields. This enables the advertising router to limit the advertisement to certain prefixes or scopes of groups. The advertising router may enforce this scope acceptance when receiving Registers or Join/Prune messages. C-RPs should send C-RP-Adv messages with the `Priority' field set to `0'. 3.6.2 Receiving C-RP-Advs and Originating Bootstrap Upon receiving a C-RP-Adv, a router does the following: 1 If the router is not the elected BSR, it ignores the message, else 2 The BSR adds the RP address to its local pool of candidate RPs, according to the associated group prefix(es) in the C-RP- Adv message. The Holdtime in the C-RP-Adv message is also stored with the corresponding RP, to be included later in the Bootstrap message. The BSR may apply a local policy to limit the number of Candidate RPs included in the Bootstrap message. The BSR may override the prefix indicated in a C-RP-Adv unless the `Priority' field is not zero. The BSR keeps an RP-timer per RP in its local RP-set. The RP-timer is initialized to the Holdtime in the RP's C-RP-Adv. When the timer expires, the corresponding RP is removed from the RP-set. The RP- timer is restarted by the C-RP-Advs from the corresponding RP. The BSR also uses its Bootstrap-timer to periodically send Bootstrap messages. In particular, when the Bootstrap-timer expires, the BSR originates a Bootstrap message on each of its PIM interfaces. To reduce the bootstrap message overhead during partition healing, the BSR should set a random time (as a function of the priority and address) after which the Bootstrap message is originated only if no other preferred Bootstrap message is received. For details see appendix 6.2. The message is sent with a TTL of 1 to the `ALL-PIM- ROUTERS' group. In steady state, the BSR originates Bootstrap messages periodically. At startup, the Bootstrap-timer is initialized to [Bootstrap-Timeout], causing the first Bootstrap message to be originated only when and if the timer expires. For
timer details, see Section 3.6.3. A DR unicasts a Bootstrap message to each new PIM neighbor, i.e., after the DR receives the neighbor's Hello message (it does so even if the new neighbor becomes the DR). The Bootstrap message is subdivided into sets of group-prefix,RP- Count,RP-addresses. For each RP-address, the corresponding Holdtime is included in the "RP-Holdtime" field. The format of the Bootstrap message allows `semantic fragmentation', if the length of the original Bootstrap message exceeds the packet maximum boundaries (see Section 4). However, we recommend against configuring a large number of routers as C-RPs, to reduce the semantic fragmentation required. 3.6.3 Receiving and Forwarding Bootstrap Each router keeps a Bootstrap-timer, initialized to [Bootstrap- Timeout] at startup. When a router receives Bootstrap message sent to `ALL-PIM-ROUTERS' group, it performs the following: 1 If the message was not sent by the RPF neighbor towards the BSR address included, the message is dropped. Else 2 If the included BSR is not preferred over, and not equal to, the currently active BSR: 1 If the Bootstrap-timer has not yet expired, or if the receiving router is a C-BSR, then the Bootstrap message is dropped. Else 2 If the Bootstrap-timer has expired and the receiving router is not a C-BSR, the receiving router stores the RP- Set and BSR address and priority found in the message, and restarts the timer by setting it to [Bootstrap-Timeout]. The Bootstrap message is then forwarded out all PIM interfaces, excluding the one over which the message arrived, to `ALL-PIM-ROUTERS' group, with a TTL of 1. 3 If the Bootstrap message includes a BSR address that is preferred over, or equal to, the currently active BSR, the router restarts its Bootstrap-timer at [Bootstrap-Timeout] seconds. and stores the BSR address and RP-Set information. The Bootstrap message is then forwarded out all PIM interfaces, excluding the one over which the message arrived, to `ALL-PIM- ROUTERS' group, with a TTL of 1.
4 If the receiving router has no current RP set information
and the Bootstrap was unicast to it from a directly connected
neighbor, the router stores the information as its new RP-set.
This covers the startup condition when a newly booted router
obtains the RP-Set and BSR address from its DR.
When a router receives a new RP-Set, it checks if each of the RPs
referred to by existing state (i.e., by (*,G), (*,*,RP), or
(S,G)RPT-bit entries) is in the new RP-Set. If an RP is not in the
new RP-set, that RP is considered unreachable and the hash algorithm
(see below) is re-performed for each group with locally active state
that previously hashed to that RP. This will cause those groups to be
distributed among the remaining RPs. When the new RP-Set contains a
new RP, the value of the new RP is calculated for each group covered
by that C-RP's Group-prefix. Any group for which the new RP's value
is greater than the previously active RP's value is switched over to
the new RP.
3.7 Hash Function
The hash function is used by all routers within a domain, to map a
group to one of the C-RPs from the RP-Set. For a particular group, G,
the hash function uses only those C-RPs whose Group-prefix covers G.
The algorithm takes as input the group address, and the addresses of
the Candidate RPs, and gives as output one RP address to be used.
The protocol requires that all routers hash to the same RP within a
domain (except for transients). The following hash function must be
used in each router:
1 For RP addresses in the RP-Set, whose Group-prefix covers
G, select the RPs with the highest priority (i.e. lowest
`Priority' value), and compute a value:
Value(G,M,C(i))=
(1103515245 * ((1103515245 * (G&M)+12345) XOR C(i)) + 12345) mod 2^31
where C_i is the RP address and M is a hash-mask included in
Bootstrap messages. The hash-mask allows a small number of
consecutive groups (e.g., 4) to always hash to the same RP. For
instance, hierarchically-encoded data can be sent on consecutive
group addresses to get the same delay and fate-sharing
characteristics.
For address families other than IPv4, a 32-bit digest to be used
as C_i must first be derived from the actual RP address. Such a
digest method must be used consistently throughout the PIM
domain. For IPv6 addresses, we recommend using the equivalent
IPv4 address for an IPv4-compatible address, and the CRC-32
checksum [7] of all other IPv6 addresses.
2 From the RPs with the highest priority (i.e. lowest
`Priority' value), the candidate with the highest resulting
value is then chosen as the RP for that group, and its identity
and hash value are stored with the entry created.
Ties between RPs having the same hash value and priority, are
broken in advantage of the highest address.
The hash function algorithm is invoked by a DR, upon reception of a
packet, or IGMP membership indication, for a group, for which the DR
has no entry. It is invoked by any router that has (*,*,RP) state
when a packet is received for which there is no corresponding (S,G)
or (*,G) entry. Furthermore, the hash function is invoked by all
routers upon receiving a (*,G) or (*,*,RP) Join/Prune message.
3.8 Processing Timer Events
In this subsection, we enumerate all timers that have been discussed
or implied. Since some critical timer events are not associated with
the receipt or sending of messages, they are not fully covered by
earlier subsections.
Timers are implemented in an implementation-specific manner. For
example, a timer may count up or down, or may simply expire at a
specific time. Setting a timer to a value T means that it will expire
after T seconds.
3.8.1 Timers related to tree maintenance
Each (S,G), (*,G), and (*,*,RP) route entry has multiple timers
associated with it: one for each interface in the outgoing interface
list, one for the multicast routing entry itself, and one optional
Join/Prune-Suppression-Timer. Each (S,G) and (*,G) entry also has an
Assert-timer and a Random-Delay-Join-Timer for use with Asserts. In
addition, DR's have a Register-Suppression-timer for each (S,G) entry
and every router has a single Join/Prune-timer. (A router may
optionally keep separate Join/Prune-timers for different interfaces
or route entries if different Join/Prune periods are desired.)
* [Join/Prune-Timer] This timer is used for periodically
sending aggregate Join/Prune messages. To avoid
synchronization among routers booting simultaneously, it is
initially set to a random value between 1 and [Join/Prune-
Period]. When it expires, the timer is immediately restarted
to [Join/Prune-Period]. A Join/Prune message is then sent out
each interface. This timer should not be restarted by other
events.
* [Join/Prune-Suppression-Timer (kept per route entry)] A
route entry's (optional) Join/Prune-Suppression-Timer may be
used to suppress duplicate joins from multiple downstream
routers on the same LAN. When a Join message is received from
a neighbor on the entry's incoming interface in which the
included Holdtime is higher than the router's own
[Join/Prune-Holdtime] (with ties broken by higher network
layer address), the timer is set to [Join/Prune-Suppression-
Timeout], with some random jitter introduced to avoid
synchronization of triggered Join/Prune messages on
expiration. (The random timeout value must be < 1.5 *
[Join/Prune-Period] to prevent losing data after 2 dropped
Join/Prunes.) The timer is restarted every time a subsequent
Join/Prune message (with higher Holdtime/IP address) for the
entry is received on its incoming interface. While the timer
is running, Join/Prune messages for the entry are not sent.
This timer is idle (not running) for point-to-point links.
* [Oif-Timer (kept per oif for each route entry)] A timer for
each oif of a route entry is used to time out that oif.
Because some of the outgoing interfaces in an (S,G) entry are
copied from the (*,G) outgoing interface list, they may not
have explicit (S,G) join messages from some of the downstream
routers (i.e., where members are joining to the (*,G) tree
only). Thus, when an Oif-timer is restarted in a (*,G) entry,
the Oif-timer is restarted for that interface in each existing
(S,G) entry whose oif list contains that interface. The same
rule applies to (*,G) and (S,G) entries when restarting an
Oif-timer on a (*,*,RP) entry.
The following table shows its usage when first adding the oif
to the entry's oiflist, when it should be restarted (unless it
is already higher), and when it should be decreased (unless it
is already lower).
Set to | When | Applies to
included Holdtime | adding oif off Join/Prune | (S,G) (*,G)
| | (*,*,RP)
Increased (only) to | When | Applies to
included Holdtime | received Join/Prune | (S,G) (*,G)
| | (*,*,RP)
(*,*,RP) oif-timer value | (*,*,RP) oif-timer restarted | (S,G) (*,G)
(*,G) oif-timer value | (*,G) oif-timer restarted | (S,G)
When the timer expires, the oif is removed from the oiflist if
there are no directly-connected members. When deleted, the oif
is also removed in any associated (S,G) or (*,G) entries.
* [Entry-Timer (kept per route entry)] A timer for each route
entry is used to time out that entry. The following table
summarizes its usage when first adding the oif to the entry's
oiflist, and when it should be restarted (unless it is already
higher).
Set to | When | Applies to
[Data-Timeout] | created off data packet | (S,G)
included Holdtime | created off Join/Prune | (S,G) (*,G) (*,*,RP)
Increased (only) to | When | Applies to
[Data-Timeout] | receiving data packets | (S,G)no RPT-bit
oif-timer value | any oif-timer restarted | (S,G)RPT-bit (*,G)
| | (*,*,RP)
[Assert-Timeout] | assert received | (S,G)RPT-bit (*,G)
| | w/null oif
When the timer expires, the route entry is deleted; if the
entry is a (*,G) or (*,*,RP) entry, all associated (S,G)RPT-
bit entries are also deleted.
* [Register-Suppression-Timer (kept per (S,G) route entry)]
An (S,G) route entry's Register-Suppression-Timer is used to
suppress registers when the RP is receiving data packets
natively. When a Register-Stop message for the entry is
received from the RP, the timer is set to a random value in
the range 0.5 * [Register-Suppression-Timeout] to 1.5 *
[Register-Suppression-Timeout]. While the timer is running,
Registers for that entry will be suppressed. If null
registers are used, a null register is sent [Probe-Time]
seconds before the timer expires.
* [Assert-Timer (per (S,G) or (*,G) route entry)] The
Assert-Timer for an (S,G) or (*,G) route entry is used for
timing out Asserts received. When an Assert is received and
the RPF neighbor is changed to the Assert winner, the Assert-
Timer is set to [Assert-Timeout], and is restarted to this
value every time a subsequent Assert for the entry is received
on its incoming interface. When the timer expires, the router
resets its RPF neighbor according to its unicast routing
table.
* [Random-Delay-Join-Timer (per (S,G) or (*,G) route entry)]
The Random-Delay-Join-Timer for an (S,G) or (*,G) route entry
is used to prevent synchronization among downstream routers on
a LAN when their RPF neighbor changes. When the RPF neighbor
changes, this timer is set to a random value between 0 and
[Random-Delay-Join-Timeout] seconds. When the timer expires, a
triggered Join/Prune message is sent for the entry unless its
Join/Prune-Suppression-Timer is running.
3.8.2 Timers relating to neighbor discovery
* [Hello-Timer] This timer is used to periodically send Hello
messages. To avoid synchronization among routers booting
simultaneously, it is initially set to a random value between
1 and [Hello-Period]. When it expires, the timer is
immediately restarted to [Hello-Period]. A Hello message is
then sent out each interface. This timer should not be
restarted by other events.
* [Neighbor-Timer (kept per neighbor)] A Neighbor-Timer for
each neighbor is used to time out the neighbor state. When a
Hello message is received from a new neighbor, the timer is
initially set to the Holdtime included in the Hello message
(which is equal to the neighbor's value of [Hello-Holdtime]).
Every time a subsequent Hello is received from that neighbor,
the timer is restarted to the Holdtime in the Hello. When the
timer expires, the neighbor state is removed.
3.8.3 Timers relating to RP information
* [C-RP-Adv-Timer (C-RP's only)] Routers configured as
candidate RP's use this timer to periodically send C-RP-Adv
messages. To avoid synchronization among routers booting
simultaneously, the timer is initially set to a random value
between 1 and [C-RP-Adv-Period]. When it expires, the timer is
immediately restarted to [C-RP-Adv-Period]. A C-RP-Adv message
is then sent to the elected BSR. This timer should not be
restarted by other events.
* [RP-Timer (BSR only, kept per RP in RP-Set)] The BSR uses a
timer per RP in the RP-Set to monitor liveness. When a C-RP is
added to the RP-Set, its timer is set to the Holdtime included
in the C-RP-Adv message from that C-RP (which is equal to the
C-RP's value of [RP-Holdtime]). Every time a subsequent C-RP-
Adv is received from that RP, its timer is restarted to the
Holdtime in the C-RP-Adv. When the timer expires, the RP is
removed from the RP-Set included in Bootstrap messages.
* [Bootstrap-Timer] This timer is used by the BSR to
periodically originate Bootstrap messages, and by other
routers to time out the BSR (see 3.6.3). This timer is
initially set to [Bootstrap-Timeout]. A C-BSR restarts this
timer to [Bootstrap-Timeout] upon receiving a Bootstrap
message from a preferred router, and originates a Bootstrap
message and restarts the timer to [Bootstrap-Period] when it
expires. Routers not configured as C-BSR's restart this timer
to [Bootstrap-Timeout] upon receiving a Bootstrap message from
the elected or a more preferred BSR, and ignore Bootstrap
messages from non-preferred C-BSRs while it is running.
3.8.4 Default timer values
Most of the default timeout values for state information are 3.5
times the refresh period. For example, Hellos refresh Neighbor state
and the default Hello-timer period is 30 seconds, so a default
Neighbor-timer duration of 105 seconds is included in the Holdtime
field of the Hellos. In order to improve convergence, however, the
default timeout value for information related to RP liveness and
Bootstrap messages is 2.5 times the refresh period.
In this version of the spec, we suggest particular numerical timer
settings. A future version of the specification will specify a
mechanism for timer values to be scaled based upon observed network
parameters.
* [Join/Prune-Period] This is the interval between
sending Join/Prune messages. Default: 60 seconds. This value
may be set to take into account such things as the configured
bandwidth and expected average number of multicast route
entries for the attached network or link (e.g., the period
would be longer for lower-speed links, or for routers in the
center of the network that expect to have a larger number of
entries). In addition, a router could modify this value (and
corresponding Join/Prune-Holdtime value) if the number of
route entries changes significantly (e.g., by an order of
magnitude). For example, given a default minimum Join/Prune-
Period value, if the number of route entries with a particular
iif increases from N to N*100, the router could increase its
Join/Prune-Period (and Join/Prune-Holdtime), for that
interface, by a factor of 10; and if/when the number of
entries decreases back to N, the Join/Prune-Period (and
Join/Prune-Holdtime) could be decreased to its previous value.
If the Join/Prune-Period is modified, these changes should be
made relatively infrequently and the router should continue to
refresh at its previous Join/Prune-Period for at least
Join/Prune-Holdtime, in order to allow the upstream router to
adapt.
* [Join-Prune Holdtime] This is the Holdtime specified in
Join/Prune messages, and is used to time out oifs. This should
be set to 3.5 * [Join/Prune-Period]. Default: 210 seconds.
* [Join/Prune-Suppression-Timeout] This is the mean
interval between receiving a Join/Prune with a higher Holdtime
(with ties broken by higher network layer address) and
allowing duplicate Join/Prunes to be sent again. This should
be set to approximately 1.25 * [Join/Prune-Period]. Default:
75 seconds.
* [Data-Timeout] This is the time after which (S,G) state
for a silent source will be deleted. Default: 210 seconds.
* [Register-Suppression-Timeout] This is the mean
interval between receiving a Register-Stop and allowing
Registers to be sent again. A lower value means more frequent
register bursts at RP, while a higher value means longer join
latency for new receivers. Default: 60 seconds. (Note that
if null Registers are sent [Probe-Time] seconds before the
timeout, register bursts are prevents, and [Register-
Suppression-Timeout] may be lowered to decrease join latency.)
* [Probe-Time] When null Registers are used, this is the
time between sending a null Register and the Register-
Suppression-Timer expiring unless it is restarted by receiving
a Register-Stop. Thus, a null Register would be sent when the
Register-Suppression-Timer reaches this value. Default: 5
seconds.
* [Assert-Timeout] This is the interval between the last
time an Assert is received, and the time at which the assert
is timed out. Default: 180 seconds.
* [Random-Delay-Join-Timeout] This is the maximum
interval between the time when the RPF neighbor changes, and
the time at which a triggered Join/Prune message is sent.
Default: 4.5 seconds.
* [Hello-Period] This is the interval between sending
Hello messages. Default: 30 seconds.
* [Hello-Holdtime] This is the Holdtime specified in
Hello messages, after which neighbors will time out their
neighbor entries for the router. This should be set to 3.5 *
[Hello-Period]. Default: 105 seconds.
* [C-RP-Adv-Period] For C-RPs, this is the interval
between sending C-RP-Adv messages. Default: 60 seconds.
* [RP-Holdtime] For C-RPs, this is the Holdtime specified
in C-RP-Adv messages, and is used by the BSR to time out RPs.
This should be set to 2.5 * [C-RP-Adv-Period]. Default: 150
seconds.
* [Bootstrap-Period] At the elected BSR, this is the
interval between originating Bootstrap messages, and should be
equal to 60 seconds.
* [Bootstrap-Timeout] This is the time after which the
elected BSR will be assumed unreachable when Bootstrap
messages are not received from it. This should be set to `2 *
[Bootstrap-Period] + 10'. Default: 130 seconds.
3.9 Summary of flags used
Following is a summary of all the flags used in our scheme.
Bit | Used in | Definition
Border | Register | Register for external sources is coming
from PIM multicast border router
Null | Register | Register sent as Probe of RP, the
encapsulated IP data packet should not
be forwarded
RPT | Route entry | Entry represents state on the RP-tree
RPT | Join/Prune | Join is associated with the shared tree and
therefore the Join/Prune message is
propagated along the RP-tree (source
encoded is an RP address)
RPT | Assert | The data packet was routed down the shared
tree; thus, the path indicated corresponds
to the RP tree
SPT | (S,G) entry | Packets have arrived on the iif towards
S, and the iif is different from the
(*,G) iif
WC |Join | The receiver expects to receive packets
from all sources via this (shared tree)
path. Thus, the Join/Prune applies to a
(*,G) entry
WC | Route entry | Wildcard entry; if there is no more
specific match for a particular source,
packets will be forwarded according to
this entry
3.10 Security All PIM control messages may use IPsec [6] to address security concerns.
(page 41 continued on part 3)
