INTERNET DRAFT Venu Hemige
Alcatel-Lucent
Internet Engineering Task Force Yetik Serbest
Document: AT&T
draft-ietf-l2vpn-vpls-pim-snooping-01.txt Ray Qiu
Suresh Boddapati
Alcatel-Lucent
March 2007
Category: Informational
Expires: September 2007
PIM Snooping over VPLS
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Abstract
In Virtual Private LAN Service (VPLS), as also in IEEE Bridged
Networks, the switches simply flood multicast traffic on all ports in
the LAN by default. IGMP Snooping is commonly deployed to ensure
multicast traffic is not forwarded on ports without IGMP receivers.
The procedures and recommendations for IGMP Snooping are defined in
[IGMP-SNOOP]. But when any protocol other than IGMP is used, the
common practice is to simply flood multicast traffic to all ports.
PIM-SM, PIM-SSM, PIM-BIDIR are widely deployed routing protocols. PIM
Snooping procedures are important to restrict multicast traffic to
only the routers interested in receiving such traffic.
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While most of the PIM Snooping procedures defined here also apply to
IEEE Bridged Networks, VPLS demands certain special procedures due to
the split-horizon rules that require the Provider Edge (PE) devices
to co-operate. This document describes the procedures and
recommendations for PIM-Snooping in VPLS to facilitate replication to
only those ports behind which there are interested PIM routers and/or
IGMP hosts.
This document also describes procedures for PIM Proxy. PIM Proxy is
required on PEs for VPLS Multicast to work correctly when Join
suppression is enabled in the VPLS. PIM Proxy also helps scale VPLS
Multicast much better than just PIM Snooping.
Conventions used in this document
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 [RFC 2119].
Table of Contents
1. Introduction .............................................3
1.1. Assumptions...............................................4
1.2. PIM Snooping and PIM Proxy Complexity.....................4
1.3. Definitions...............................................5
2. Multicast Traffic over VPLS...............................6
2.1. Constraining of IP Multicast in a VPLS....................6
2.2. IPv6 Considerations.......................................7
2.3. PIM-SM (*,*,RP) Considerations............................7
2.4. PIM Packet Types to Snoop.................................8
2.5. PIM Snooping vs PIM Proxy.................................8
2.5.1. Differences between PIM Snooping and PIM Proxy............9
2.5.2. PIM Control Message Latency...............................9
2.5.3. When to Snoop and When to Proxy......................... 10
3. PIM Snooping for VPLS................................... 10
3.1. General Rules for PIM Snooping in VPLS.................. 11
3.1.1. Snooping PIM Packets ................................... 11
3.2. Discovering PIM Routers................................. 11
3.3. PIM-SM and PIM-SSM...................................... 12
3.3.1. Building PIM-SM Snooping States......................... 13
3.3.2. Explanation for per (S,G,N) states...................... 15
3.3.3. Receiving (*,G) PIM-SM Join/Prune Messages.............. 15
3.3.4. Receiving (S,G) PIM-SM Join/Prune Messages.............. 18
3.3.5. Receiving (S,G,rpt) Join/Prune Messages................. 19
3.3.6. Sending (*,G) Join/Prune Messages....................... 19
3.3.7. Sending (S,G) Join/Prune Messages....................... 20
3.3.8. Sending PIM Join/Prune message upstream................. 20
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3.3.8.1. Sending Triggered vs Refresh Join/Prune messages....... 20
3.3.9. Triggering ASSERT Election in PIM-SM.................... 21
3.4. Bidirectional-PIM (PIM-BIDIR)........................... 21
3.4.1. Building PIM-BIDIR Snooping States...................... 22
3.5. PIM-DM.................................................. 22
3.5.1. Building PIM-DM Snooping States......................... 23
3.5.2. PIM-DM Downstream Per-Port PIM(S,G,N) State Machine .... 23
3.5.3. Triggering ASSERT election in PIM-DM.................... 23
3.6. PIM Proxy............................................... 24
3.6.1. Downstream PIM Proxy behavior........................... 24
3.6.2. Upstream PIM Proxy behavior............................. 25
3.6.3. Source IP Address in Proxy PIM Join/Prune Packets....... 25
3.7. Directly Connected Multicast Source .................... 25
3.8. Data Forwarding Rules................................... 26
3.8.1. PIM-SM Data Forwarding Rules ........................... 26
3.8.2. PIM-BIDIR Data Forwarding Rules......................... 27
3.8.3. PIM-DM Data Forwarding Rules............................ 28
4. IANA Considerations..................................... 28
5. Security Considerations................................. 29
6. Acknowledgements........................................ 29
7. References.............................................. 29
7.1. Normative References ................................... 29
7.2. Informative References.................................. 29
Appendix A. Example Network Scenario............................ 31
Appendix A.1 PIM-Snooping Example............................... 31
Appendix A.2 PIM Proxy Example with (S,G) / (*,G) interaction... 33
1. Introduction
In Virtual Private LAN Service (VPLS), the Provider Edge (PE) devices
provide a logical interconnect such that Customer Edge (CE) devices
belonging to a specific VPLS instance appear to be connected by a
single LAN. Forwarding information base for particular VPLS instance
is populated dynamically by source MAC address learning. This is a
straightforward solution to support unicast traffic, with reasonable
flooding for unicast unknown traffic. Since a VPLS provides LAN
emulation for IEEE bridges as wells as for routers, the unicast and
multicast traffic need to follow the same path for layer-2 protocols
to work properly. As such, multicast traffic is treated as broadcast
traffic and is flooded to every site in the VPLS instance.
VPLS solutions (i.e., [VPLS-LDP] and [VPLS-BGP]) perform replication
for multicast traffic at the ingress PE devices. As stated in the
VPLS Multicast Requirements draft [VPLS-MCAST-REQ], there are two
issues with VPLS Multicast today:
A. Multicast traffic is replicated to non-member sites.
B. Replication of PWs on shared physical path.
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This document solves Issue A of [VPLS-MCAST-REQ] by ensuring that IP
multicast traffic is not forwarded to non-member sites. Issue B is
outside the scope of this document. The different mechanisms to
tunnel IP multicast traffic in a VPLS from the ingress PE to the
egress PEs are discussed in [VPLS-MCAST-TREES]. The solution in this
document when combined with the solutions proposed in the working
group to solve Issue B will provide a complete VPLS Multicast
solution set.
Using IGMP/PIM Snooping in VPLS has the following advantages:
- It improves IP Multicast bandwidth usage in the VPLS core by
ensuring traffic is replicated only to PEs with member sites.
Note that this is not necessarily optimum, as there can still be
bandwidth waste if traffic from a PE to other PE(s) is not
forwarded along a minimum cost spanning tree.
- It prevents sending multicast traffic to non-member sites.
Procedures for IGMP Snooping are specified in [IGMP-SNOOP]. This
document describes the procedures for Protocol Independent Multicast
(PIM) snooping over VPLS for efficient distribution of IP multicast
traffic. It also describes the rules when both IGMP and PIM are
active in a VPLS instance.
This document also describes procedures for PIM Proxy. PIM Proxy is
required on PEs for VPLS Multicast to work correctly when Join
suppression is enabled in the VPLS. PIM Proxy also helps scale VPLS
Multicast much better than just PIM Snooping.
1.1. Assumptions
Since this draft describes the procedures for PIM Snooping and PIM
Proxy, the draft assumes that the reader has a good understanding of
the PIM protocols. The text in this draft is written in the same
style as the PIM RFCs to help correlate the concepts and to make it
easier to follow. In order to avoid replicating the text relating to
PIM protocol handling here, this draft assumes that the user will
infer such detail from the PIM RFC referenced in this document.
Deviations in protocol handling specific to PIM Snooping and PIM
Proxy are specified in this draft. There could be cross references
into definitions of macros and procedures from the PIM RFCs.
1.2. PIM Snooping and PIM Proxy Complexity
The PIM Snooping and PIM Proxy solutions described here requires a
switch to examine and operate on only PIM Hello and PIM Join/Prune
packets. The switch does not need to examine any other PIM packets.
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The switch does not need to have any routing tables like is required
in PIM Multicast Routing. It knows how to forward Join/Prunes by
looking at the Upstream Neighbor field in the Join/Prune packets.
The switch does not need to know about Rendezvous Points (RP) and
does not have to maintain any RP Set. All that is transparent to a
PIM Snooping switch.
Most of the procedures in PIM Snooping and PIM Proxy in the handling
of PIM Hellos and PIM Join/Prune packets are very similar to that of
a PIM Router.
The solutions described here provide complete separation of control
and data planes.
A PIM Proxy solution minimizes the control plane messages received at
CE routers by proxying one message upstream on behalf of a large
number of downstream CEs. As such control plane messaging is very
similar to that of a PIM Router.
1.3. Definitions
There are several definitions referenced in this document that are
well described in the PIM RFCs [PIM-SM, PIM-BIDIR, PIM-DM].
The following definitions and abbreviations are used throughout this
document:
- A port is defined as either an attachment circuit (AC) or a
Pseudo-Wire (PW).
- When we say a PIM message is 'received' on a port, it means
any one of the following:
o that a PIM Snooping switch snooped the PIM message.
o that a PIM message was received via LDP on a PW if LDP (as
defined in [VPLS-MCAST-LDP]) is used for propogating
multicast states among the PEs.
Abbreviations used in the document:
- S: IP Address of the Multicast Source.
- G: IP Address of the Multicast Group.
- N: Upstream Neighbor field in a Join/Prune/Graft message.
- Rport(N): Port on which neighbor N is learnt
Other definitions are explained in the sections where they are
introduced.
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2. Multicast Traffic over VPLS
In VPLS, if a PE receives a frame from an Attachment Circuit (AC)
with no matching entry in the forwarding information base for that
particular VPLS instance, it floods the frame to all other PEs (which
are part of this VPLS instance) and to directly connected ACs (other
than the one that the frame is received from). The flooding of a
frame occurs when:
- The destination MAC address has not been learned,
- The destination MAC address is a broadcast address,
- The destination MAC address is a multicast address.
Malicious attacks (e.g., receiving unknown frames constantly) aside,
the first situation is handled by VPLS solutions as long as
destination MAC address can be learned. After that point on, the
frames will not be flooded. A PE is REQUIRED to have safeguards,
such as unknown unicast limiting and MAC table limiting, against
malicious unknown unicast attacks.
There is no way around flooding broadcast frames. To prevent runaway
broadcast traffic from adversely affecting the VPLS service and the
SP network, a PE is REQUIRED to have tools to rate limit the
broadcast traffic as well.
Similar to broadcast frames, multicast frames are flooded as well, as
a PE cannot know where multicast members reside. Rate limiting
multicast traffic, while possible, should be should be done carefully
since several network control protocols relies on multicast. For one
thing, layer-2 and layer-3 protocols utilize multicast for their
operation. For instance, Bridge Protocol Data Units (BPDUs) use an
IEEE assigned all bridges multicast MAC address, and OSPF is
multicast to all OSPF routers multicast MAC address. If the rate-
limiting of multicast traffic is not done properly, the customer
network will experience instability and poor performance. For the
other, it is not straightforward to determine the right rate limiting
parameters for multicast.
A VPLS solution MUST NOT affect the operation of customer layer-2
protocols (e.g., BPDUs). Additionally, a VPLS solution MUST NOT
affect the operation of layer-3 protocols.
In the following section, we describe procedures to constrain the
flooding of IP multicast traffic in a VPLS.
2.1. Constraining of IP Multicast in a VPLS
For a PE in a VPLS (a layer-2 device) to constrain IP multicast
traffic, it needs to be able to learn which CEs are interested in
receiving multicast traffic for what flows.
The most obvious solution is to snoop IP multicast control traffic at
the PEs. Snooping as a solution to constrain multicast traffic makes
sense under the following circumstances:
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- The CE-CE protocol the PEs snoop is a popular and widely
deployed protocol.
- It does not require any changes on the CEs and it should be
completely transparent to the CEs.
IGMP/MLD and PIM are the popular IP Multicast Routing protocols
today. Other routing protocols such as DVMRP or MOSPF are outside the
scope of this document.
This document describes the guidelines for PIM Snooping and PIM Proxy
in VPLS. The specifications in this document could be used for either
PIM Snooping or PIM Proxy. The PIM Proxy solution is described in
section 3.6 Differences that need to be observed while implementing
one or the other and recommendations on which method to employ in
different scenarios are noted in section 2.5We will largely refer to
PIM "Snooping" in this document. Unless specifically specified, the
same procedures should apply to a Proxy solution as well.
In the following sub-sections, we provide some guidelines for the
implementation of PIM snooping in VPLS. Snooping techniques need to
be employed on ACs at the downstream PEs. Snooping techniques can
also be employed on PWs at the upstream PEs. This may work well for
small to medium scale deployments. However, if there are a large
number of VPLS instances with a large number of PEs per instances,
then the amount of snooping required at the upstream PEs can
overwhelm the upstream PEs. In [VPLS-MCAST-LDP] and [VPLS-MCAST-BGP],
procedures are defined to exchange multicast membership information
between the PEs using LDP or BGP. Using a reliable mechanism like LDP
or BGP allows the upstream PEs to eliminate the requirement to snoop
on PWs. It also eliminates the need to refresh multicast states on
the upstream PEs.
2.2. IPv6 Considerations
In VPLS, PEs forward Ethernet frames received from CEs and as such
are agnostic of the layer-3 protocol used by the CEs. However, as an
IGMP and PIM snooping switch, the PE would have to look deeper into
the IP and IGMP/PIM packets and build snooping state based on that.
The PIM Protocol specifications handle both IPv4 and IPv6. The
specification for PIM Snooping in this draft can be applied to both
IPv4 and IPv6 payloads.
2.3. PIM-SM (*,*,RP) Considerations
This draft does not address (*,*,RP) states in the VPLS network.
Although [PIM-SM] specifies that routers MUST support (*,*,RP)
states, there are very few implementations that actually support it
in actual deployments. Given the complexity of supporting (*,*,RP)
states and knowing that there is little to no use to supporting it,
this draft omits the specification relating to (*,*,RP) support.
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2.4. PIM Packet Types to Snoop
A PIM Snooping switch need only snoop on PIM Hellos and PIM
Join/Prune packets. All other PIM packets can be transparently
flooded unexamined.
2.5. PIM Snooping vs PIM Proxy
PIM Snooping switches simply snoop on PIM packets as they are being
forwarded in the VPLS. As such it truly provides transparent LAN
services since no customer packets are modified or consumed or new
packets introduced in the VPLS. It is also slightly simpler to
implement than PIM Proxy. However for PIM Snooping to work correctly,
it is a requirement that CE routers MUST disable Join suppression in
the VPLS.
Given that a large number of existing CE deployments do not support
disabling of Join suppression and given the operational complexity
for a provider to manage disabling of Join suppression in the VPLS,
it becomes a difficult solution to deploy. Another disadvantage of
PIM Snooping as a solution is that it does not scale as well as PIM
Proxy. If there are a large number of CEs in a VPLS, then every CE
will see every other CE's Join/Prune messages.
PIM Proxy on the PEs has the advantage that it does not require Join
suppression to be disabled in the VPLS. Multicast as a VPLS service
can be very easily be provided without requiring any changes on the
CE routers. It also helps scale VPLS Multicast very well since the
PEs intelligently forward only one Join/Prune message for a given
flow and only to the upstream CE.
PIM Proxy as a solution however loses the transparency argument since
Join/Prunes could get modified or even consumed at a PE. Also, new
packets could get introduced in the VPLS. However, this loss of
transparency is limited to PIM Join/Prune packets. It is in the
interest of optimizing multicast in the VPLS and helping a VPLS
network scale much better. Data traffic will still be completely
transparent.
Both PIM Snooping and PIM Proxy procedures can be used in conjunction
with [VPLS-MCAST-LDP] for propogating multicast states among the PEs.
If [VPLS-MCAST-LDP] is used for propogating multicast states among
the PEs, then both PIM Snooping and PIM Proxy switches do not process
any PIM packets arriving on a PW.
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2.5.1. Differences between PIM Snooping and PIM Proxy
For PIM-SM and PIM-BIDIR, a PIM Snooping/Proxy Switch only needs to
examine PIM Hello and Join/Prune messages. PIM Proxy for PIM-DM is
for future study and is not currently specified in this draft.
The proxy proposal is to perform proxy of only the Join/Prune
messages. PIM Hello messages are snooped by both PIM Snooping and PIM
Proxy switches.
Details on the PIM Proxy solution are discussed in section 3.6 This
section is presented here to say that most of the procedures to
follow (unless explicitly specified) are common to both PIM Snooping
and PIM Proxy.
Differences between a PIM Snooping switch and a PIM Proxy switch can
be summarized as the following:
+------------------------------|--------------------------------+
| PIM Snooping | PIM Proxy |
+==============================|================================+
| 1. PIM Snooping switches | 1. PIM Proxy switches also |
| snoop Hello and Join/Prune| snoop PIM Hello messages |
| messages while they are | while they are transparently|
| transparently flooded in | flooded in the VPLS. But |
| the VPLS. | they consume PIM Join/Prune |
| | messages and do not flood |
| | them as is in the VPLS. |
+------------------------------|--------------------------------+
| 2. PIM Snooping switches do | 2. PIM Proxy switches may |
| not originate any PIM | originate new or modified |
| packets. They may however | PIM Hello and Join/Prune |
| originate PIM messages to | packets. |
| be sent via LDP on PWs. | |
+------------------------------|--------------------------------+
Other than the above simple differences, most of the procedures are
common to PIM Snooping and PIM Proxy. There are additional
simplifications to PIM Snooping that can be made if [VPLS-MCAST-LDP]
is not used for PE-PE communication, but otherwise the procedures for
PIM Snooping and PIM Proxy are mostly the same. In the text to
follow, we describe the procedures for PIM "Snooping". Unless
specifically stated otherwise, such procedures apply to PIM Proxy as
well.
2.5.2. PIM Control Message Latency
A PIM Snooping or PIM Proxy switch snoops on PIM Hello packets while
transparently flooding it in the VPLS. As such there is no latency
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introduced by the VPLS in the delivery of PIM Hello packets to remote
CEs in the VPLS.
A PIM Proxy switch consumes PIM Join/Prune packets and generates
proxy Join/Prune packets to be sent upstream. This can result in
additional latency for a downstream CE to receive multicast traffic
after it has sent a Join. When a downstream CE prunes a multicast
stream, the traffic should stop flowing to the CE with no additional
latency introduced by the VPLS.
A PIM Snooping switch snoops on PIM Join/Prune packets while
transparently flooding them in the VPLS. There is no latency
introduced by the VPLS in the delivery of PIM Join/Prune packets when
PIM Snooping is employed.
2.5.3. When to Snoop and When to Proxy
Explicit Tracking (ET) is enabled in a VPLS when all PIM CE Routers
in the VPLS advertise Tracking Support in their PIM Hello messages.
If even one does not advertise Tracking Support, then all PIM CE
routers disable ET in the VPLS. When ET is enabled, it implies that
Join Suppression is disabled and vice versa.
PIM Snooping PEs can determine if ET is enabled or disabled in a VPLS
by examining PIM Hellos. If ET is disabled, PIM Proxy MUST be used.
If ET is enabled, PIM Snooping SHOULD be used.
3. PIM Snooping for VPLS
IGMP snooping procedures described in [IGMP-SNOOP] provide efficient
delivery of IP multicast traffic in a given VPLS service when end
stations are connected to the VPLS. However, when VPLS is offered as
a WAN service it is likely that the CE devices are routers and would
run PIM between them. To provide efficient IP multicasting in such
cases, it is necessary that the PE routers offering the VPLS service
do PIM snooping.
PIM is a multicast routing protocol, which runs exclusively between
routers. PIM shares many of the common characteristics of a routing
protocol, such as discovery messages (e.g., neighbor discovery using
Hello messages), topology information (e.g., multicast tree), and
error detection and notification (e.g., dead timer and designated
router election). On the other hand, PIM does not participate in any
kind of exchange of databases, as it uses the unicast routing table
to provide reverse path information for building multicast trees.
There are a few variants of PIM. In PIM-DM ([PIM-DM]), multicast
data is pushed towards the members similar to broadcast mechanism.
PIM-DM constructs a separate delivery tree for each multicast group.
As opposed to PIM-DM, other PIM flavors (PIM-SM [PIM-SM], PIM-SSM
[PIM-SSM], and PIM-BIDIR [PIM-BIDIR]) invoke a pull methodology
instead of push technique.
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PIM routers periodically exchange Hello messages to discover and
maintain stateful sessions with neighbors. After neighbors are
discovered, PIM routers can signal their intentions to join or prune
specific multicast groups. This is accomplished by having downstream
routers send an explicit Join/Prune message (for the sake of
generalization, consider Graft messages for PIM-DM as Join messages)
to the upstream routers. The Join/Prune message can be group
specific (*,G) or group and source specific (S,G).
In PIM snooping, a PE snoops on the PIM message exchanged between
routers, and builds its multicast states.
Based on the multicast states, it forwards IP multicast traffic
accordingly to avoid unnecessary flooding.
In the following sub-sections, snooping mechanisms for each variety
of PIM are specified.
3.1. General Rules for PIM Snooping in VPLS
The following rules for the correct operation of PIM snooping MUST be
followed.
- PIM messages and multicast data traffic forwarded by PEs MUST
follow the split-horizon rule for mesh PWs as defined in
[VPLS-LDP].
- PIM snooping states in a PE MUST be per VPLS instance.
- Multicast traffic MUST be replicated per PW and AC basis,
i.e., even if there are more than one PIM neighbor behind a
PW/AC, only one replication MUST be sent to that PW/AC.
3.1.1. Snooping PIM Packets
PIM-SM and PIM-BIDIR snooping PEs need to snoop on just the PIM Hello
and PIM Join/Prune messages to build its multicast states.
- PIM-DM snooping PEs have to also snoop on PIM Graft and PIM
State Refresh messages.
3.2. Discovering PIM Routers
A PIM Snooping PE MUST snoop on PIM Hellos received on ACs and PWs.
i.e. the PE transparently floods the PIM Hello while snooping on it.
PIM Hellos are used by the snooping switch to discover PIM routers
and their characteristics.
For each neighbor discovered by a PE, it includes an entry in the PIM
Neighbor Database with the following fields:
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- Layer 2 encapsulation for the Router sending the PIM Hello.
- IP Address and address family of the Router sending the PIM
Hello.
- Port (AC / PW) on which the PIM Hello was received.
- Hello TLVs
The PE should be able to interpret and act on Hello TLVs currently
defined in the PIM RFCs. The TLVs of particular interest in this
document are:
- Hello-Hold-Time
- Tracking Support
- DR Priority
Please refer to [PIM-SM] for a list of the Hello TLVs.
When a PIM Hello is received, the PE MUST reset the neighbor-expiry-
timer to Hello-Hold-Time. If a PE does not receive a Hello message
from a router within Hello-Hold-Time, the PE MUST remove that
neighbor from its PIM Neighbor Database. If a PE receives a Hello
message from a router with Hello-Hold-Time value set to zero, the PE
MUST remove that router from the PIM snooping state immediately.
From the PIM Neighbor Database, a PE MUST be able to use the
procedures defined in [PIM-SM] to identify the PIM Designated Router
in the VPLS instance. It should also be able to determine if Tracking
Support is active in the VPLS instance.
3.3. PIM-SM and PIM-SSM
The key characteristic of PIM-SM and PIM-SSM is explicit join
behavior. In this model, multicast traffic is only forwarded to
locations that specifically request it. The root node of a tree is
the Rendezvous Point (RP) in case of a shared tree (PIM-SM only) or
the first hop router that is directly connected to the multicast
source in the case of a shortest path tree. All the procedures
described in this section apply to both PIM-SM and PIM-SSM, except
for the fact that there is no (*,G) state in PIM-SSM.
The procedures to discover PIM-SM routers in a VPLS instance are as
described in section 3.2
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3.3.1. Building PIM-SM Snooping States
PIM-SM and PIM-SSM Snooping states are built by snooping on the PIM-
SM Join/Prune messages received on AC/PWs.
The downstream state machine of a PIM-SM snooping switch very closely
resembles the downstream state machine of PIM-SM routers. The
downstream state consists of:
Per downstream (Port, *, G):
- DownstreamJPState: One of { "NoInfo" (NI), "Join" (J), "Prune
Pending" (PP) }
Per downstream (Port, *, G, N):
- Prune Pending Timer (PPT(N))
- Join Expiry Timer (ET(N))
Per downstream (Port, S, G):
- DownstreamJPState: One of { "NoInfo" (NI), "Join" (J), "Prune
Pending" (PP) }
Per downstream (Port, S, G, N):
- Prune Pending Timer (PPT(N))
- Join Expiry Timer (ET(N))
Per downstream (Port, S, G, rpt):
- DownstreamJPRptState: One of { "NoInfo" (NI), "Pruned" (P),
"Prune Pending" (PP) }
Per downstream (Port, S, G, rpt, N):
- Prune Pending Timer (PPT(N))
- Join Expiry Timer (ET(N))
Where S is the address of the multicast source, G is the Group
address and N is the upstream neighbor field in the Join/Prune
message. Notice that unlike on PIM-SM routers where PPT and ET are
per (Interface, S, G), PIM Snooping switches have to maintain PPT and
ET per (Port, S, G, N). The reasons for this are explained in section
3.3.2
Apart from the above states, we define the following state
summarization macros.
UpstreamNeighbors(*,G): If there is one or more Join(*,G) received on
any port with upstream neighbor N and ET(N) is active, then N is
added to UpstreamNeighbors(*,G). This set is used to determine if a
Join(*,G) or a Prune(*,G) with upstream neighbor N needs to be sent
upstream.
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UpstreamNeighbors(S,G): If there is one or more Join(S,G) received on
any port with upstream neighbor N and ET(N) is active, then N is
added to UpstreamNeighbors(S,G). This set is used to determine if a
Join(S,G) or a Prune(S,G) with upstream neighbor N needs to be sent
upstream.
UpstreamPorts(*,G): This is the set of all Rport(N) ports where N is
in the set UpstreamNeighbors(*,G). Multicast Streams forwarded using
a (*,G) match MUST be forwarded to these ports in addition to
downstream ports. So UpstreamPorts(*,G) MUST be added to
OutgoingPortList(*,G).
UpstreamPorts(S,G): This is the set of all Rport(N) ports where N is
in the set UpstreamNeighbors(S,G). UpstreamPorts(S,G) MUST be added
to OutgoingPortList(S,G).
UpstreamPorts(S,G,rpt): If PruneDesired(S,G,rpt) becomes true, then
this set is set to UpstreamPorts(*,G). Otherwise, this set is empty.
UpstreamPorts(*,G) (-) UpstreamPorts(S,G,rpt) MUST be added to
OutgoingPortList(S,G).
See section 3.8.1 on Data Forwarding Rules for the specification on
how OutgoingPortList(S,G) is calculated.
UpstreamPorts(G): This set is the union of all the UpstreamPorts(S,G)
and UpstreamPorts(*,G) for a given G. Proxy (S,G) Join/Prune and
(*,G) Join/Prune messages MUST be sent to a subset of
UpstreamPorts(G) as specified in section 3.3.8.
PWPorts: This is the set of all PWs.
OutgoingPortList(*,G): This is the set of all ports to which traffic
needs to be forwarded on a (*,G) match. Split Horizon rules apply as
noted in section 3.8
OutgoingPortList(S,G): This is the set of all ports to which traffic
needs to be forwarded on an (S,G) match. Split Horizon rules apply as
noted in section 3.8
NumETsActive(Port,*,G): Number of (Port,*,G,N) entries that have
Expiry Timer running. This macro keeps track of the number of
Join(*,G)s that are received on this Port with different upstream
neighbors.
NumETsActive(Port,S,G): Number of (Port,S,G,N) entries that have
Expiry Timer running. This macro keeps track of the number of
Join(*,G)s that are received on this Port with different upstream
neighbors.
RpfVectorTlvs(*,G): RPF Vectors [RPF-VECTOR] are TLVs that may be
present in received Join(*,G) messages. If present, they must be
copied to RpfVectorTlvs(*,G).
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RpfVectorTlvs(S,G): RPF Vectors [RPF-VECTOR] are TLVs that may be
present in received Join(S,G) messages. If present, they must be
copied to RpfVectorTlvs(S,G).
Since there are a few differences between the downstream state
machines of PIM-SM Routers and PIM-SM snooping switches, we specify
the details of the downstream state machine of PIM-SM snooping
switches at the risk of repeating most of the text documented in
[PIM-SM].
3.3.2. Explanation for per (S,G,N) states
In PIM Routing protocols, states are built per (S,G). On a router, an
(S,G) has only one RPF-Neighbor. However, a PIM Snooping switch does
not have the Layer 3 routing information available to the routers in
order to determine the RPF-Neighbor for a multicast flow. It merely
discovers it by snooping the Join/Prune message. A PE could have
snooped on two or more different Join/Prune messages for the same
(S,G) that could have carried different Upstream-Neighbor fields.
This could happen during transient network conditions or due to dual-
homed sources. A PE cannot make assumptions on which one to pick, but
instead must facilitate the CE routers decide which Upstream Neighbor
gets elected the RPF-Neighbor. And for this purpose, the PE will have
to track downstream and upstream Join/Prune states per (S,G,N).
3.3.3. Receiving (*,G) PIM-SM Join/Prune Messages
A Join(*,G) or Prune(*,G) is "received" when the port on which it was
received is not also the port on which the upstream-neighbor N of the
Join/Prune(*,G) was learnt.
When a router receives a Join(*,G) or a Prune(*,G) with upstream
neighbor N, it must process the message as defined in the state
machine below. Note that the macro computations of the various macros
resulting from this state machine transition is exactly as specified
in the PIM-SM RFC [PIM-SM].
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We define the following per-port (*,G,N) macro to help with the state
machine below.
Figure 1: Downstream per-port (*,G) state machine in tabular form
+---------------++----------------------------------------+
| || Previous State |
| ++------------+--------------+------------+
| Event ||NoInfo (NI) | Join (J) | Prune-Pend |
+---------------++------------+--------------+------------+
| Receive ||-> J state | -> J state | -> J state |
| Join(*,G) || Action | Action | Action |
| || RxJoin(N) | RxJoin(N) | RxJoin(N) |
+---------------++------------+--------------+------------+
|Receive || - | -> PP state | -> PP state|
|Prune(*,G) and || | Start PPT(N) | |
|NumETsActive<=1|| | | |
+---------------++------------+--------------+------------+
|Receive || - | -> J state | - |
|Prune(*,G) and || | Start PPT(N) | |
NumETsActive>1 || | | |
+---------------++------------+--------------+------------+
|PPT(N) expires || - | -> J state | -> NI state|
| || | Action | Action |
| || | PPTExpiry(N) |PPTExpiry(N)|
+---------------++------------+--------------+------------+
|ET(N) expires || - | -> NI state | -> NI state|
|and || | Action | Action |
|NumETsActive<=1|| | ETExpiry(N) | ETExpiry(N)|
+---------------++------------+--------------+------------+
|ET(N) expires || - | -> J state | -> NI state|
|and || | Action | Action |
|NumETsActive>1 || | ETExpiry(N) | ETExpiry(N)|
+---------------++------------+--------------+------------+
Action RxJoin(N):
If ET(N) is not already running, then start ET(N). Otherwise restart
ET(N).
If N is not already in UpstreamNeighbors(*,G), then add N to
UpstreamNeighbors(*,G) and trigger a Join(*,G) with upstream neighbor
N to be forwarded upstream as specified in section 3.3.8
Record N as RPF_Neighbor(*,G).
If there are RPF Vector TLVs in the received (S,G) message and if
they are different from the recorded RpfVectorTlvs(S,G), then copy
them into RpfVectorTlvs(S,G). Also trigger a Join(S,G) with upstream
neighbor N to be forwarded upstream as specified in section 3.3.8
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Action PPTExpiry(N):
Disable timers ET(N) and PPT(N). If there are no other (Port,*,G)
states with NumETsActive(Port,*,G) > 0, then trigger a Prune(*,G)
with upstream neighbor N to be forwarded upstream as specified in
section 3.3.8 Then delete N from UpstreamNeighbors(*,G).
Send a Prune-Echo(*,G) with upstream-neighbor N on the downstream
port.
Action ETExpiry(N):
Disable timers ET(N) and PPT(N). If there are no other (Port,*,G)
states with NumETsActive(Port,*,G) > 0, then trigger a Prune(*,G)
with upstream neighbor N to be forwarded upstream as specified in
section 3.3.8 Then delete N from UpstreamNeighbors(*,G).
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3.3.4. Receiving (S,G) PIM-SM Join/Prune Messages
A Join(S,G) or Prune(S,G) is "received" when the port on which it was
received is not also the port on which the upstream-neighbor N of the
Join/Prune(S,G) was learnt.
When a router receives a Join(S,G) or a Prune(S,G) with upstream
neighbor N, it must process the message as defined in the state
machine below. Note that the macro computations of the various macros
resulting from this state machine transition is exactly as specified
in the PIM-SM RFC [PIM-SM].
Figure 2: Downstream per-port (S,G) state machine in tabular form
+---------------++----------------------------------------+
| || Previous State |
| ++------------+--------------+------------+
| Event ||NoInfo (NI) | Join (J) | Prune-Pend |
+---------------++------------+--------------+------------+
| Receive ||-> J state | -> J state | -> J state |
| Join(S,G) || Action | Action | Action |
| || RxJoin(N) | RxJoin(N) | RxJoin(N) |
+---------------++------------+--------------+------------+
|Receive || - | -> PP state | -> PP state|
|Prune (S,G) and|| | Start PPT(N) | |
|NumETsActive<=1|| | | |
+---------------++------------+--------------+------------+
|Receive || - | -> J state | - |
|Prune(S,G) and || | Start PPT(N) | |
NumETsActive>1 || | | |
+---------------++------------+--------------+------------+
|PPT(N) expires || - | -> J state | -> NI state|
| || | Action | Action |
| || | PPTExpiry(N) |PPTExpiry(N)|
+---------------++------------+--------------+------------+
|ET(N) expires || - | -> NI state | -> NI state|
|and || | Action | Action |
|NumETsActive<=1|| | ETExpiry(N) | ETExpiry(N)|
+---------------++------------+--------------+------------+
|ET(N) expires || - | -> J state | -> NI state|
|and || | Action | Action |
|NumETsActive>1 || | ETExpiry(N) | ETExpiry(N)|
+---------------++------------+--------------+------------+
Action RxJoin(N):
If ET(N) is not already running, then start ET(N). Otherwise, restart
ET(N).
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If N is not already in UpstreamNeighbors(S,G), then add N to
UpstreamNeighbors(S,G) and trigger a Join(S,G) with upstream neighbor
N to be forwarded upstream as specified in section 3.3.8
Record N as RPF_Neighbor(S,G).
If there are RPF Vector TLVs in the received (S,G) message and if
they are different from the recorded RpfVectorTlvs(S,G), then copy
them into RpfVectorTlvs(S,G). Also trigger a Join(S,G) with upstream
neighbor N to be forwarded upstream as specified in section 3.3.8
Action PPTExpiry(N):
Disable timers ET(N) and PPT(N). If there are no other (Port,S,G)
states with NumETsActive(Port,S,G) > 0, then trigger a Prune(S,G)
with upstream neighbor N to be forwarded upstream as specified in
section 3.3.8Then delete N from UpstreamNeighbors(S,G).
Send a Prune-Echo(S,G) with upstream-neighbor N on the downstream
port.
Action ETExpiry(N):
Disable timers ET(N) and PPT(N). If there are no other (Port,S,G)
states with NumETsActive(Port,S,G) > 0, then trigger a Prune(S,G)
with upstream neighbor N to be forwarded upstream as specified in
section 3.3.8 Then delete N from UpstreamNeighbors(S,G).
3.3.5. Receiving (S,G,rpt) Join/Prune Messages
A Join(S,G,rpt) or Prune(S,G,rpt) is "received" when the port on which
it was received is not also the port on which the upstream-neighbor N
of the Join/Prune(S,G,rpt) was learnt.
While it is important to ensure that the (S,G) and (*,G) state machines
allow for handling per (S,G,N) states, it is not as important for
(S,G,rpt) states. It suffices to say that the downstream (S,G,rpt)
state machine is the same as what is defined in section 4.5.4 of the
PIM-SM RFC [PIM-SM].
3.3.6. Sending (*,G) Join/Prune Messages
A PIM Proxy PE MUST implement the Upstream (*,G) state machine for
which the procedures are similar to what is defined in section 4.5.6
of [PIM-SM]. Section 3.3.8of this draft specifies how the message
should be sent.
For the purposes of the Upstream (*,G) state machine, a Join(*,G) or
Prune(*,G) message with upstream neighbor N is "seen" on a PIM
Snooping switch if the port on which the message was received is also
the port on which the upstream neighbor N was learnt.
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3.3.7. Sending (S,G) Join/Prune Messages
A PIM Proxy PE MUST implement the Upstream (S,G) state machine for
which the procedures are similar to what is defined in section 4.5.6
of [PIM-SM]. Section 3.3.8of this draft specifies how the message
should be sent.
For the purposes of the Upstream (S,G) state machine, a Join(*,G) or
Prune(*,G) message with upstream neighbor N is "seen" on a PIM
Snooping switch if the port on which the message was received is also
the port on which the upstream neighbor N was learnt.
3.3.8. Sending PIM Join/Prune message upstream.
Sending of PIM Join/Prune messages upstream is only required on a PIM
Proxy Switch and not on a PIM Snooping Switch. This section applies
only to a PIM Proxy Switch.
The downstream Join/Prune state machines above describe when PIM
Join/Prune packets must be forwarded upstream and with what upstream
neighbor field. In order to correctly facilitate assert among the CE
routers, such Join/Prunes need to sent not only towards the upstream
neighbor, but also on certain PWs as described below. It is important
to note that Join/Prune packets are sent to a subset of the ports in
UpstreamPorts(G) and is not simply flooded to all PWs.
If RpfVectorTlvs(*,G) is not empty, then it must be encoded in a
Join(*,G) message sent upstream.
If RpfVectorTlvs(S,G) is not empty, then it must be encoded in a
Join(S,G) message sent upstream.
3.3.8.1. Sending Triggered vs Refresh Join/Prune messages
A Join is a refresh join if it is being sent as a result of upstream
join timer expiry. If the join is being sent because there was a
change in the downstream join/prune state machine, then it is a
triggered join.
If the Join/Prune message being sent out is a refresh Join(*,G)
message, then send the refresh Join(*,G) on all ports in
UpstreamPorts(G). The Upstream Neighbor field should be the recorded
RPF_Neighbor(*,G).
If the Join/Prune message being sent out is a refresh Join(S,G)
message, then send the refresh Join(S,G) on all ports in
UpstreamPorts(G). The Upstream Neighbor field should be the recorded
RPF_Neighbor(S,G).
If the Join/Prune message being sent out is a triggered Join/Prune
message (due to an event in the downstream Join/Prune state machine),
then the following rules apply. These rules apply to both (S,G) and
(*,G) Join/Prune messages to be sent out:
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- The upstream neighbor field N in the Join/Prune to be sent is
dictated by the downstream Join/Prune state machine
transition.
- If the downstream Join/Prune event was on an AC port, then
send the upstream Join/Prune message to all PWs in
UpstreamPorts(G). Send the Join/Prune message to Rport(N)
also.
- If the downstream Join/Prune event was on a PW port and if
Rport(N) is a PW, then silently discard the Join/Prune
message without sending it. If Rport(N) is an AC, then send
the Join/Prune message on that AC.
3.3.9. Triggering ASSERT Election in PIM-SM
In PIM-SM, there are scenarios where multiple routers could be
forwarding the same multicast traffic on a LAN. When this happens,
using PIM Assert Election process by sending PIM Assert Messages,
routers ensure that only the Assert Winner forwards traffic on the
LAN. In a typical LAN, the Assert Election is a data driven event and
happens only if a router sees traffic on the interface to which it
should be forwarding the traffic. Therefore, in the case of VPLS, in
order to trigger Assert Election and stop duplicate traffic, it is
necessary that two routers that are forwarding duplicate traffic for
an (S,G)/(*,G) see each other's traffic.
PIM Snooping switches must hence ensure that they not only forward
multicast traffic for an (S,G) on the ports on which they snooped
Joins(S,G)/Joins(*,G), but also on the ports on which such Joins were
forwarded (i.e. towards the upstream neighbor(s)). So if two or more
Joins(S,G) each carrying a different upstream neighbor field were
snooped at a PE, then the ports on which such Joins were snooped
along with the ports on which the upstream neighbors were learnt must
be added to the outgoing port list.
The above logic needs to be facilitated without breaking VPLS Split
Horizon Rules. i.e. traffic should not be forwarded on the port on
which it was received. And traffic arriving on a PW MUST NOT be
forwarded onto other PW(s). The rules specified above in calculating
the outgoing port list ensures this.
An example network scenario is discussed in Appendix A with possible
ASSERT Election scenarios.
3.4. Bidirectional-PIM (PIM-BIDIR)
PIM-BIDIR is a variation of PIM-SM. The main differences between
PIM-SM and Bidirectional-PIM are as follows:
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- There are no source-based trees, and source-specific
multicast is not supported (i.e., no (S,G) states) in PIM-
BIDIR.
- Multicast traffic can flow up the shared tree in PIM-BIDIR.
- To avoid forwarding loops, one router on each link is elected
as the Designated Forwarder (DF) for each RP in PIM-BIDIR.
The main advantage of PIM-BIDIR is that it scales well for many-to-
many applications. However, the lack of source-based trees means
that multicast traffic is forced to remain on the shared tree.
The procedures to discover PIM-SM routers in a VPLS instance are as
described in section 3.2 For PIM-BIDIR to work properly, all routers
within the domain must know the address of the RP. During RP
discovery time, PIM routers elect DF per subnet for each RP. The
algorithm to elect the DF is as follows: all PIM neighbors in a
subnet advertise their unicast route to elect the RP and the router
with the best route is elected.
Snooping for PIM-BIDIR is much simpler than it is for PIM-SM. The
complexity resulting from various combinations of (S,G), (*,G), IGMP
and assert states makes PIM-SM procedures fairly complex. PIM-BIDIR
has none of those issues since PIM-BIDIR builds only (*,G) states and
all routers on a LAN agree on who the upstream neighbor, i.e. DF(RP)
is. So the snooping procedures for PIM-BIDIR is very much like that
on a PIM-BIDIR router [PIM-BIDIR].
3.4.1. Building PIM-BIDIR Snooping States
The PEs MUST snoop on PIM Hello and PIM-BIDIR Join/Prune packets and
build states as described in [PIM-BIDIR]. The PEs SHOULD simply
flood all other PIM packet types without examining them.
PIM Proxy Rules specified in section 2.5can be applied to PIM-BIDR
also. Only additional requirement is that if the Upstream Port of a
PIM-BIDIR group is a PW, then the proxy PIM Join/Prune packet MUST
be sent on all PWs.
3.5. PIM-DM
The characteristics of PIM-DM is flood and prune behavior. Shortest
path trees are built as a multicast source starts transmitting.
The procedures to discover PIM-DM routers are as explained in section
3.2
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3.5.1. Building PIM-DM Snooping States
PIM-DM Snooping states are built by snooping on the PIM-DM Join,
Prune, Graft and State Refresh messages received on AC/PWs and State-
Refresh Messages sent on AC/PWs. By snooping on these PIM-DM
messages, a PE builds the following states per (S,G,N) where S is the
address of the multicast source, G is the Group address and N is the
upstream neighbor to which Prunes/Grafts are sent by downstream CEs:
Per PIM (S,G,N):
Per Port PIM (S,G,N) Prune State:
- DownstreamPState(S,G,N,Port): One of {"NoInfo" (NI), "Pruned"
(P), "PrunePending" (PP)}
- Prune Pending Timer (PPT)
- Prune Timer (PT)
- Upstream Port (valid if the PIM(S,G,N) Prune State is
"Pruned").
3.5.2. PIM-DM Downstream Per-Port PIM(S,G,N) State Machine
The downstream per-port PIM(S,G,N) state machine is as defined in
section 4.4.2 of [PIM-DM] with a few changes relevant to PIM
Snooping. When reading section 4.4.2 of [PIM-DM] for the purposes of
PIM-Snooping please be aware that the downstream states are built per
(S, G, N, Downstream-Port} in PIM-Snooping and not per {Downstream-
Interface, S, G} as in a PIM-DM router. As noted in the previous
section 3.5.1, the states (DownstreamPState) and timers (PPT and PT)
are per (S,G,N,P).
3.5.3. Triggering ASSERT election in PIM-DM
Since PIM-DM is a flood-and-prune protocol, traffic is flooded to all
routers unless explicitly pruned. Since PIM-DM routers do not prune
on non-RPF interfaces, PEs should typically not receive Prunes on
Rport(RPF-neighbor). So the asserting routers should typically be in
pim_oiflist(S,G). In most cases, assert election should occur
naturally without any special handling since data traffic will be
forwarded to the asserting routers.
However, there are some scenarios where a prune might be received on
a port which is also an upstream port (UP). If we prune the port from
pim_oiflist(S,G), then it would not be possible for the asserting
routers to determine if traffic arrived on their downstream port.
This can be fixed by adding pim_iifs(S,G) to pim_oiflist(S,G) so that
data traffic flows to the UP ports.
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3.6. PIM Proxy
As noted earlier in section 2.5, PIM Snooping will work correctly
only if Join Suppression is disabled in the VPLS. If Join Suppression
is enabled in the VPLS, then PEs MUST do PIM Proxy for VPLS Multicast
to work correctly.
A PIM Proxy switch behaves like a PIM Router by doing most of the
functionality of a PIM Router. The complexity however is much lesser
on a switch since many of the issues that a PIM Router has to deal
with are not relevant on a switch. A PIM Router needs to be able to
build and maintain RP-Sets. They also have to deal with the Register
and Assert State Machines. There are other complexities for a PIM
Router resulting from inter-domain multicast. A PIM Snooping or PIM
Proxy switch can be agnostic of all of this. All that a PIM Proxy
switch cares about is building multicast states using PIM Hellos and
PIM Join/Prune message. As such it's complexity is greatly reduced.
Other than the procedures defined here, the rest of the procedures
that apply to PIM Snooping apply to PIM Proxy as well.
3.6.1. Downstream PIM Proxy behavior
A PIM-SM or PIM-BIDIR Proxy PE is interested in the Hello and
Join/Prune messages. The proposed PIM Proxy solution for PIM-SM and
PIM-BIDIR is to proxy only Join/Prune messages. PIM Proxy for PIM-DM
is for future study.
PIM Hellos MUST be snooped while being flooded in the VPLS. i.e. PIM
Hellos MUST NOT be consumed at a PE and regenerated.
PIM Join/Prune messages arriving at an AC MUST be consumed. If [VPLS-
MCAST-LDP] is not used to distribute multicast states among the PEs,
then PIM Join/Prune messages arriving at a PW MUST also be consumed.
All other PIM packet types are flooded in the VPLS without needing
observation.
Performing only proxy of Join/Prune messages keeps the switch
behavior very similar to that of a PIM router without introducing too
much additional complexity. It keeps the PIM Proxy solution fairly
simple. Since Join/Prunes are forwarded by a PE along the slow-path
and all other PIM packet types are forwarded along the fast-path, it
is very likely that packets forwarded along the fast-path will arrive
"ahead" of Join/Prune packets at a CE router (note the stress on the
fact that fast-path messages will never arrive after Join/Prunes). Of
particular importance are Hello packets sent along the fast-path. We
can construct a variety of scenarios resulting in out of order
delivery of Hellos and Join/Prune messages. However, there should be
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no deviation from normal expected behavior observed at the CE router
receiving these messages out of order.
The other option for a PIM Proxy solution is to proxy both Hello and
Join/Prune messages that a PE is interested in building states for.
If Hellos are being proxied, then it becomes necessary that the PE
proxy all other PIM packet types also. Because if Hellos are received
after other packet types are received at a CE router, then bad things
will happen. That means every PIM packet has to be sent along the
slow-path. This greatly increases the complexity on the CE router, it
is very compute intensive and does not scale well. Also, proxying
Hellos will result in added latency to delivery of Hello messages to
a CE and that affects multicast convergence in the VPLS.
3.6.2. Upstream PIM Proxy behavior
Since a PIM Proxy switch consumes Join/Prune messages, it must also
originate PIM Join/Prune messages to be sent upstream. If [VPLS-
MCAST-LDP] is employed, then triggered Join/Prune messages are sent
via LDP to forward PIM Join/Prunes on PWs. Join/Prune messages need
not be refreshed on PWs when [VPLS-MCAST-LDP] is employed. On ACs,
both triggered and refresh Join/Prunes are forwarded as PIM packets.
3.6.3. Source IP Address in Proxy PIM Join/Prune Packets
The source IP address in PIM packets sent upstream SHOULD be the
address of a PIM neighbor in the VPLS. The address picked MUST NOT be
the upstream neighbor field to be encoded in the packet. The layer 2
encapsulation for the selected source IP address MUST be the
encapsulation recorded in the PIM Neighbor database for that IP
address.
If Explicit Tracking (ET) is disabled in the VPLS, then it does not
matter what Source IP Address is picked in the packets sent upstream
as long as we adhere to the rule in the previous paragraph.
If ET is enabled, it means that a CE router is interested in tracking
every CE that wishes to join a stream. If a PE determines that ET is
enabled, then it SHOULD use PIM Snooping procedures instead of PIM
Proxy.
3.7. Directly Connected Multicast Source
If there is a source in the CE network that connects directly into
the VPLS instance, then multicast traffic from that source MUST be
sent to all PIM routers on the VPLS instance apart from the igmp
receivers in the VPLS. If there is already (S,G) or (*,G) snooping
state that is formed on any PE, this will not happen per the current
forwarding rules and guidelines. So, in order to determine if
traffic needs to be flooded to all routers, a PE must be able to
determine if the traffic came from a host on that LAN. There are
three ways to address this problem:
[Page 25]
draft-ietf-l2vpn-vpls-pim-snooping-01.txt March, 2007
- The PE would have to do ARP snooping to determine if a source
is directly connected.
- Another option is to have configuration on all PEs to say
there are CE sources that are directly connected to the VPLS
instance and disallow snooping for the groups for which the
source is going to send traffic. This way traffic from that
source to those groups will always be flooded within the
provider network.
- A third option is to require that sources of CE multicast
routers must appear behind a router.
3.8. Data Forwarding Rules
First we define the rules that are common to PIM-SM, PIM-BIDIR and
PIM-DM PEs. Forwarding rules for each protocol type is specified in
the sub-sections.
If there is no matching forwarding state, then the PE MAY either
discard the packet or send it towards all the snooped PIM CE routers
or to a configured set of ports. How this is determined is outside
the scope of this document.
The following rules MUST be followed when forwarding multicast
traffic in a VPLS:
- Traffic arriving on a port MUST NOT be forwarded back onto
the same port.
- Due to VPLS Split-Horizon rules, traffic ingressing on a PW
MUST NOT be forwarded to any other PW.
3.8.1. PIM-SM Data Forwarding Rules
Per the rules in [PIM-SM] and per the additional rules specified in
this document,
OutgoingPortList(*,G) = inherited_olist(*,G) (+) UpstreamPorts(*,G)
(+) Rport(PimDR)
OutgoingPortList(S,G) = inherited_olist(S,G) (+) UpstreamPorts(S,G)
(+) (UpstreamPorts(*,G) (-)
UpstreamPorts(S,G,rpt))
(+) Rport(PimDR)
[PIM-SM] specifies how inherited_olist(*,G) and inherited_olist(S,G)
are built. PimDR is the IP address of the PIM DR in the VPLS.
The PIM-SM Snooping forwarding rules are defined below in pseudocode:
BEGIN
[Page 26]
draft-ietf-l2vpn-vpls-pim-snooping-01.txt March, 2007
iif is the incoming port of the multicast packet.
S is the Source IP Address of the multicast packet.
G is the Destination IP Address of the multicast packet.
If there is (S,G) state on the PE
Then
OutgoingPortList = OutgoingPortList(S,G)
Else if there is (*,G) state on the PE
Then
OutgoingPortList = OutgoingPortList(*,G)
Else
OutgoingPortList = UserDefinedPortList
Endif
If iif is an AC
Then
OutgoingPortList = OutgoingPortList (-) iif
Else
## iif is a PW
OutgoingPortList = OutgoingPortList (-) PWPorts
Endif
Forward the packet to OutgoingPortList.
END
First if there is (S,G) state on the PE, then the set of outgoing
ports is OutgoingPortList(S,G).
Otherwise if there is (*,G) state on the PE, the set of outgoing
ports is OutgoingPortList(*,G).
The packet is forwarded to the selected set of outgoing ports while
observing the rules above in section 3.8
3.8.2. PIM-BIDIR Data Forwarding Rules
The PIM-BIDIR Snooping forwarding rules are defined below in
pseudocode:
BEGIN
iif is the incoming port of the multicast packet.
G is the Destination IP Address of the multicast packet.
If there is forwarding state for G
Then
OutgoingPortList = olist(G)
Else
OutgoingPortList = UserDefinedPortList
Endif
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draft-ietf-l2vpn-vpls-pim-snooping-01.txt March, 2007
If iif is an AC
Then
OutgoingPortList = OutgoingPortList (-) iif
Else
## iif is a PW
OutgoingPortList = OutgoingPortList (-) PWPorts
Endif
Forward the packet to OutgoingPortList.
END
If there is forwarding state for G, then forward the packet to
olist(G) while observing the rules above in section 3.8
[PIM-BIDIR] specifies how olist(G) is contructed.
3.8.3. PIM-DM Data Forwarding Rules
The PIM-DM Snooping data forwarding rules are defined below in
pseudocode:
BEGIN
iif is the incoming port of the multicast packet.
S is the Source IP Address of the multicast packet.
G is the Destination IP Address of the multicast packet.
If there is (S,G) state on the PE
Then
OutgoingPortList = olist(S,G)
Else
OutgoingPortList = UserDefinedPortList
Endif
If iif is an AC
Then
OutgoingPortList = OutgoingPortList (-) iif
Else
## iif is a PW
OutgoingPortList = OutgoingPortList (-) PWPorts
Endif
Forward the packet to OutgoingPortList.
END
If there is forwarding state for (S,G), then forward the packet to
olist(S,G) while observing the rules above in section 3.8
[PIM-DM] specifies how olist(S,G) is contructed.
4. IANA Considerations
This document does not require any IANA assignments or action.
[Page 28]
draft-ietf-l2vpn-vpls-pim-snooping-01.txt March, 2007
5. Security Considerations
Security considerations provided in VPLS solution documents (i.e.,
[VPLS-LDP] and [VPLS-BGP) apply to this document as well.
6. Acknowledgements
Many members of the L2VPN and PIM working groups have contributed to
and provided valuable comments and feedback to this draft, including
Vach Kompella, Shane Amante, Sunil Khandekar, Rob Nath, Marc Lassere,
Yuji Kamite, Yiqun Cai, Ali Sajassi, Jayant Kotalwar, Jozef Raets,
Himanshu Shah (Ciena), Himanshu Shah (Alcatel-Lucent).
7. References
7.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[PIM-DM] Deering, S., et al. "Protocol Independent Multicast
Version 2 - Dense Mode Specification", RFC 3973,
January 2005.
[PIM-SM] Fenner, W, et al. "Protocol Independent Multicast-
Sparse Mode (PIM-SM): Protocol Specification
(Revised)", RFC 4601, August 2006.
[PIM-SSM] Holbrook, H., et al. "Source-Specific Multicast for
IP", RFC 4607, August 2006
[PIM-BIDIR] Handley, M., et al. "Bi-directional Protocol
Independent Multicast (BIDIR-PIM)", work in
progress
[RPF-VECTOR] IJ Wijnands, et al, "The RPF Vector TLV",
draft-ietf-pim-rpf-vector-03, Work in progress
7.2. Informative References
[VPLS-LDP] Lasserre, M, et al. "Virtual Private LAN Services
using LDP Signaling", RFC 4762, January 2007
[VPLS-BGP] Kompella, K, et al. "Virtual Private LAN Service
using BGP for Auto-Discovery and Signaling", RFC
4761, January 2007
[IGMP-SNOOP] Christensen, M., et al. "Considerations for IGMP
and MLD Snooping Switches", RFC 4541, May 2006
[VPLS-MCAST-REQ] Kamite, Y, et al, "Requirements for Multicast
Support in Virtual Private LAN Services",
draft-ietf-l2vpn-vpls-mcast-reqts-03,
Work in Progress
[VPLS-MCAST-LDP] Qui, R, Serbest, Y, et al, "Using LDP for VPLS
Multicast", draft-qiu-serbest-vpls-mcast-ldp-00.txt,
Work in progress
[Page 29]
draft-ietf-l2vpn-vpls-pim-snooping-01.txt March, 2007
[VPLS-MCAST-BGP] Aggarwal, R, et al, "Propagation of VPLS IP
Multicast Group Membership Information", draft-
raggarwa-l2vpn-vpls-mcast-ctrl-00.txt, Work in
progress
[VPLS-MCAST-TREES] Aggarwal, R, et al. "Multicast in VPLS",
draft-raggarwa-l2vpn-vpls-mcast-01.txt,
Work in progress.
[Page 30]
draft-ietf-l2vpn-vpls-pim-snooping-01.txt March, 2007
Appendix A. Example Network Scenario
Let us consider the scenario in Figure 3.
Figure 3: An Example Scenario for Triggering Assert
+------+ AC3 +------+
| PE2 |-----| CE3 |
/| | | |
/ +------+ +------+
/ | |
/ | |
/PW12 | |
/ | +-----+
/ |PW23 | S |
/ | +-----+
/ | |
/ | |
/ | |
+------+ +------+ / +------+ +------+
| CE1 | | PE1 |/ PW13 | PE3 | | CE4 |
| |-----| |-------------| |-----| |
+------+ AC1 +------+ +------+ AC4 +------+
|
|AC2
+------+
| CE2 |
| |
+------+
In the scenario depicted in Figure 3, S is the source of a multicast
stream (S,G). CE1 and CE2 both have two ECMP routes to reach the
source.
In the examples below, JT(Port,S,G,N) is the downstream Join Expiry
Timer on the specified Port for the (S,G) with upstream neighbor N.
Appendix A.1 PIM-Snooping Example
1. CE1 Sends a Join(S,G) with Upstream Neighbor(S,G) = CE3.
2. PE1 snoops on the Join(S,G) while flooding it in the VPLS. PE2
and PE3 also snoop on the Join(S,G) while flooding it in the
VPLS.
The resulting states at the PEs is as follows:
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draft-ietf-l2vpn-vpls-pim-snooping-01.txt March, 2007
At PE1:
JT(AC1,S,G,CE3) = JP_HoldTime
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { PW12 }
OutgoingPortList(S,G) = { AC1, PW12 }
At PE2:
JT(PW12,S,G,CE3) = JP_HoldTime
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { AC3 }
OutgoingPortList(S,G) = { PW12, AC3 }
At PE3: PE3 ignores the Join(S,G) for the following
reasons:
. It does not already have (S,G) state.
. The Join(S,G) was received on a PW and the Upstream
RPort is also a PW.
3. The multicast stream (S,G) flows along CE3 -> PE2 -> PE1 -> CE1
4. Now CE2 sends a Join(S,G) with Upstream Neighbor(S,G) = CE4.
5. All PEs snoop on the Join(S,G).
The resulting states at the PEs:
At PE1:
JT(AC1,S,G,CE3) = active
JT(AC2,S,G,CE4) = JP_HoldTime.
UpstreamNeighbors(S,G) = { CE3, CE4 }
UpstreamPorts(S,G) = { PW12, PW13 }
OutgoingPortList(S,G) = { AC1, PW12, AC2, PW13 }
At PE2: Note: Since PE2 already has (S,G) state, it does not
ignore the Join(S,G) even though it received the
Join(S,G) on a PW and the Upstream Rport is a PW.
JT(PW12,S,G,CE4) = JP_HoldTime
JT(PW12,S,G,CE3) = active
UpstreamNeighbors(S,G) = { CE3, CE4 }
UpstreamPorts(S,G) = { AC3, PW23 }
OutgoingPortList(S,G) = { PW12, AC3, PW23 }
At PE3:
JT(PW13,S,G,CE4) = JP_HoldTime
UpstreamNeighbors(S,G) = { CE4 }
UpstreamPorts(S,G) = { AC4 }
OutgoingPortList(S,G) = { PW13, AC4 }
6. The multicast stream (S,G) flows into the VPLS from the two CEs
CE3 and CE4. PE2 forwards the stream received from CE3 to PW23
and PE3 forwards the stream to AC4. This facilitates the CE
routers to trigger assert election. Let us say CE3 becomes the
assert winner.
[Page 32]
draft-ietf-l2vpn-vpls-pim-snooping-01.txt March, 2007
7. CE3 sends an Assert message to the VPLS. The PEs flood the
Assert message without examining it.
8. CE4 stops sending the multicast stream to the VPLS.
9. CE2 notices an RPF change due to Assert and sends a Prune(S,G)
with Upstream Neighbor = CE4. CE2 also sends a Join(S,G) with
Upstream Neighbor = CE3.
10. All the PEs start a prune-pend timer on the ports on which
they received the Prune(S,G). When the prune-pend timer expires,
all PEs will remove the downstream (S,G,CE4) states.
Resulting states at the PEs:
At PE1:
JT(AC1,S,G,CE3) = active
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { PW12 }
OutgoingPortList(S,G) = { AC1, AC2, PW12 }
At PE2:
JT(PW12,S,G,CE3) = active
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { AC3 }
OutgoingPortList(S,G) = { PW12, AC3 }
At PE3: no (S,G) state.
Note that at the end of the assert election, there should be no
duplicate traffic forwarded downstream and traffic should flow only
on the desired path. Also note that there are no unnecessary (S,G)
states on PE3 after the assert election.
Appendix A.2 PIM Proxy Example with (S,G) / (*,G) interaction
In the same network, let us assume CE4 is the Upstream Neighbor
towards the RP for G.
1. CE1 Sends a Join(S,G) with Upstream Neighbor(S,G) = CE3.
2. PE1 consumes the Join(S,G). PE1 looks up the neighbor database
and determines CE3 was learnt on PW12. PE1 sends a Proxy
Join(S,G) to the resulting UpstreamPorts(G). i.e. it sends the
proxy Join(S,G) on PW12.
3. Likewise, PE2 consumes the Join(S,G) and sends a proxy Join(S,G)
on AC3 with Upstream Neighbor = CE3.
The resulting states at the PEs is as follows:
At PE1:
JT(AC1,S,G,CE3) = JP_HoldTime
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { PW12 }
[Page 33]
draft-ietf-l2vpn-vpls-pim-snooping-01.txt March, 2007
OutgoingPortList(S,G) = { AC1, PW12 }
At PE2:
JT(PW12,S,G,CE3) = JP_HoldTime
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { AC3 }
OutgoingPortList(S,G) = { PW12, AC3 }
At PE3: PE3 did not receive any PIM Join(S,G). So it has
no (S,G) state.
4. The multicast stream (S,G) flows along CE3 -> PE2 -> PE1 -> CE1.
5. Now let us say CE1 sends a Join(*,G) towards CE4.
6. PE1 consumes the Join(*,G). PE1 sends a Proxy Join(*,G) to the
resulting UpstreamPorts(G). Since UpstreamPorts(G) now has both
PW12 and PW13, the Join(*,G) gets sent on both PW12 and PW13.
Note that the UpstreamPorts(S,G) and OutgoingPortList(S,G)
inherit the corresponding (*,G) sets, but not vice versa.
7. PE2 and PE3 perform a similar function. PE2 received the
Join(*,G) on a PW and the Upstream Neighbor is also on a PW.
Hence PE2 only adds UpstreamPorts(*,G) to OutgoingPortList(*,G)
and not the downstream port PW12.
At PE1:
JT(AC1,S,G,CE3) = active
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { PW12, PW13 }
OutgoingPortList(S,G) = { AC1, PW12, PW13 }
JT(AC1,*,G,CE4) = JP_HoldTime.
UpstreamNeighbors(*,G) = { CE4 }
UpstreamPorts(*,G) = { PW13 }
OutgoingPortList(*,G) = { AC1, PW13 }
UpstreamPorts(G) = { PW12, PW13 }
At PE2:
JT(PW12,S,G,CE3) = active
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { AC3, PW23 }
OutgoingPortList(S,G) = { PW12, AC3, PW23 }
JT(PW12,*,G,CE4) = JP_HoldTime
UpstreamNeighbors(*,G) = { CE4 }
UpstreamPorts(G) = { PW23 }
OutgoingPortList(*,G) = { PW23 }
At PE3:
JT(PW13,*,G,CE4) = JP_HoldTime
UpstreamNeighbors(*,G) = { CE4 }
UpstreamPorts(*,G) = { AC4 }
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draft-ietf-l2vpn-vpls-pim-snooping-01.txt March, 2007
OutgoingPortList(*,G) = { PW13, AC4 }
8. The above state results in both (S,G) and (*,G) streams to be
forwarded to AC1. The above state also results in the (S,G)
stream to be forwarded from CE3 to CE4 resulting in an (S,G)
assert election. Following the assert election, CE3 becomes the
(S,G) assert winner. CE4 stops sending (S,G) stream down the
RPT.
9. CE1 notices an RPF change due to assert. It sends a
Prune(S,G,rpt) with Upstream Neighbor = CE4.
10. PE1 consumes the Prune(S,G,rpt) and forwards the
Prune(S,G,rpt) to both PW12 and PW13. PE2 consumes the
Prune(S,G,rpt) and updates its states. PE3 updates its states
and forwards the Prune(S,G,rpt) on AC4.
At PE1:
JT(AC1,S,G,CE3) = active
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { PW12 }
OutgoingPortList(S,G) = { AC1, PW12 }
JT(AC1,*,G,CE4) = active.
UpstreamNeighbors(*,G) = { CE4 }
UpstreamPorts(*,G) = { PW13 }
OutgoingPortList(*,G) = { AC1, PW13 }
At PE2:
JT(PW12,S,G,CE3) = active
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(*,G) = { AC3 }
OutgoingPortList(S,G) = { PW12, AC3 }
JT(PW12,*,G,CE4) = JP_HoldTime
UpstreamNeighbors(*,G) = { CE4 }
UpstreamPorts(*,G) = { PW23 }
OutgoingPortList(*,G) = { PW23 }
At PE3:
JT(PW13,*,G,CE4) = JP_HoldTime
UpstreamNeighbors(*,G) = { CE4 }
UpstreamPorts(G) = { AC4 }
OutgoingPortList(*,G) = { PW13, AC4 }
Even in this example, at the end of the (S,G) / (*,G) assert
election, there should be no duplicate traffic forwarded downstream
and traffic should flow only to the desired CEs.
Other more complex scenarios exist. This draft should addressin PIM-
SM and the rules specified in this draft should ensure that assert is
triggered among the CEs in all scenarios.
[Page 35]
draft-ietf-l2vpn-vpls-pim-snooping-01.txt March, 2007
Authors' Addresses
Venu Hemige
Alcatel-Lucent
701 East Middlefield Rd.
Mountain View, CA 94043
Venu.hemige@alcatel-lucent.com
Yetik Serbest
AT&T Labs
9505 Arboretum Blvd.
Austin, TX 78759
Yetik_serbest@labs.att.com
Ray Qiu
Alcatel-Lucent
701 East Middlefield Rd.
Mountain View, CA 94043
Ray.Qiu@alcatel-lucent.com
Suresh Boddapati
Alcatel-Lucent
701 East Middlefield Rd.
Mountain View, CA 94043
Suresh.boddapati@alcatel-lucent.com
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[Page 36]
draft-ietf-l2vpn-vpls-pim-snooping-01.txt March, 2007
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