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Internet Engineering Task Force (IETF) O. Dornon
Request for Comments: 8220 J. Kotalwar
Category: Informational V. Hemige
ISSN: 2070-1721 Nokia
R. Qiu
mistnet.io
Z. Zhang
Juniper Networks, Inc.
September 2017
Protocol Independent Multicast (PIM)
over Virtual Private LAN Service (VPLS)
Abstract
This document describes the procedures and recommendations for
Virtual Private LAN Service (VPLS) Provider Edges (PEs) to facilitate
replication of multicast traffic to only certain ports (behind which
there are interested Protocol Independent Multicast (PIM) routers
and/or Internet Group Management Protocol (IGMP) hosts) via PIM
snooping and proxying.
With PIM snooping, PEs passively listen to certain PIM control
messages to build control and forwarding states while transparently
flooding those messages. With PIM proxying, PEs do not flood PIM
Join/Prune messages but only generate their own and send them out of
certain ports, based on the control states built from downstream
Join/Prune messages. PIM proxying is required when PIM Join
suppression is enabled on the Customer Edge (CE) devices and is
useful for reducing PIM control traffic in a VPLS domain.
This document also describes PIM relay, which can be viewed as
lightweight proxying, where all downstream Join/Prune messages are
simply forwarded out of certain ports and are not flooded, thereby
avoiding the triggering of PIM Join suppression on CE devices.
Dornon, et al. Informational [Page 1]
RFC 8220 PIM Snooping September 2017
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8220.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................4
1.1. Multicast Snooping in VPLS .................................5
1.2. Assumptions ................................................6
1.3. Definitions ................................................6
1.4. Requirements Language ......................................7
2. PIM Snooping for VPLS ...........................................7
2.1. PIM Protocol Background ....................................7
2.2. General Rules for PIM Snooping in VPLS .....................8
2.2.1. Preserving Assert Triggers ..........................8
2.3. Some Considerations for PIM Snooping .......................9
2.3.1. Scaling .............................................9
2.3.2. IPv4 and IPv6 ......................................10
2.3.3. PIM-SM (*,*,RP) ....................................10
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RFC 8220 PIM Snooping September 2017
2.4. PIM Snooping vs. PIM Proxying .............................10
2.4.1. Differences between PIM Snooping, Relay,
and Proxying .......................................10
2.4.2. PIM Control Message Latency ........................11
2.4.3. When to Snoop and When to Proxy ....................12
2.5. Discovering PIM Routers ...................................13
2.6. PIM-SM and PIM-SSM ........................................14
2.6.1. Building PIM-SM States .............................15
2.6.2. Explanation for Per-(S,G,N) States .................17
2.6.3. Receiving (*,G) PIM-SM Join/Prune Messages .........18
2.6.4. Receiving (S,G) PIM-SM Join/Prune Messages .........20
2.6.5. Receiving (S,G,rpt) Join/Prune Messages ............22
2.6.6. Sending Join/Prune Messages Upstream ...............23
2.7. Bidirectional PIM (BIDIR-PIM) .............................24
2.8. Interaction with IGMP Snooping ............................24
2.9. PIM-DM ....................................................25
2.9.1. Building PIM-DM States .............................25
2.9.2. PIM-DM Downstream Per-Port PIM(S,G,N) State
Machine ............................................25
2.9.3. Triggering Assert Election in PIM-DM ...............26
2.10. PIM Proxy ................................................26
2.10.1. Upstream PIM Proxy Behavior .......................26
2.11. Directly Connected Multicast Source ......................26
2.12. Data-Forwarding Rules ....................................27
2.12.1. PIM-SM Data-Forwarding Rules ......................28
2.12.2. PIM-DM Data-Forwarding Rules ......................29
3. IANA Considerations ............................................29
4. Security Considerations ........................................30
5. References .....................................................30
5.1. Normative References ......................................30
5.2. Informative References ....................................31
Appendix A. BIDIR-PIM Considerations ..............................32
A.1. BIDIR-PIM Data-Forwarding Rules ............................32
Appendix B. Example Network Scenario ..............................33
B.1. PIM Snooping Example .......................................33
B.2. PIM Proxy Example with (S,G) / (*,G) Interaction ...........36
Acknowledgements ..................................................42
Contributors ......................................................42
Authors' Addresses ................................................43
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RFC 8220 PIM Snooping September 2017
1. Introduction
In the 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. The Forwarding Information Base (FIB) for a VPLS
instance is populated dynamically by Media Access Control (MAC)
address learning. Once a unicast MAC address is learned and
associated with a particular Attachment Circuit (AC) or pseudowire
(PW), a frame destined to that MAC address only needs to be sent on
that AC or PW.
For a frame not addressed to a known unicast MAC address, flooding
has to be used. This happens with the following so-called "BUM"
(Broadcast, Unknown Unicast, and Multicast) traffic:
o B: The destination MAC address is a broadcast address.
o U: The destination MAC address is unknown (has not been learned).
o M: The destination MAC address is a multicast address.
Multicast frames are flooded because a PE cannot know where
corresponding multicast group members reside. VPLS solutions
(RFC 4762 [VPLS-LDP] and RFC 4761 [VPLS-BGP]) perform replication for
multicast traffic at the ingress PE devices. As stated in the VPLS
Multicast Requirements document (RFC 5501 [VPLS-MCAST-REQ]), there
are two issues with VPLS multicast today:
1. Multicast traffic is replicated to non-member sites.
2. Multicast traffic may be replicated when several PWs share a
physical path.
Issue 1 can be solved by multicast snooping -- PEs learn sites with
multicast group members by snooping multicast protocol control
messages on ACs and forward IP multicast traffic only to member
sites. This document describes the procedures to achieve this when
CE devices are PIM adjacencies of each other. Issue 2 is outside the
scope of this document and is discussed in RFC 7117 [VPLS-MCAST].
While descriptions in this document are in the context of the VPLS,
the procedures also apply to regular Layer 2 switches interconnected
by physical connections, except that the PW-related concepts and
procedures do not apply in that case.
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1.1. Multicast Snooping in VPLS
IGMP snooping procedures described in RFC 4541 [IGMP-SNOOP] make sure
that IP multicast traffic is only sent on the following:
o ACs connecting to hosts that report related group membership
o ACs connecting to routers that join related multicast groups
o PWs connecting to remote PEs that have the above-described ACs
Note that traffic is always sent on ports that have point-to-point
connections to routers that are attached to a LAN on which there is
at least one other router. Because IGMP snooping alone cannot
determine if there are interested receivers beyond those routers, we
always need to send traffic to these ports, even if there are no
snooped group memberships. To further restrict traffic sent to those
routers, PIM snooping can be used. This document describes the
procedures for PIM snooping, including rules for when both IGMP and
PIM snooping are enabled in a VPLS instance; see Sections 2.8 and
2.11 for details.
Note that for both IGMP and PIM, the term "snooping" is used loosely,
referring to the fact that a Layer 2 device peeks into Layer 3
routing protocol messages to build relevant control and forwarding
states. Depending on whether the control messages are transparently
flooded, selectively forwarded, or aggregated, the processing may be
called "snooping" or "proxying" in different contexts.
We will use the term "PIM snooping" in this document; however, unless
explicitly noted otherwise, the procedures apply equally to PIM
snooping and PIM proxying. The procedures specific to PIM proxying
are described in Section 2.6.6. 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 2.4.
This document also describes PIM relay, which can be viewed as
lightweight PIM proxying. Unless explicitly noted otherwise, in the
rest of this document proxying implicitly includes relay as well.
Please refer to Section 2.4.1 for an overview of the differences
between snooping, proxying, and relay.
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1.2. Assumptions
This document assumes that the reader has a good understanding of the
PIM protocols. To help correlate the concepts and make the text
easier to follow, this document is written in the same style as the
following PIM RFCs:
o RFC 3973 [PIM-DM]
o RFC 4607 [PIM-SSM]
o RFC 5015 [BIDIR-PIM]
o RFC 5384 [JOIN-ATTR]
o RFC 7761 [PIM-SM]
In order to avoid replicating text related to PIM protocol handling
from the PIM RFCs, this document cross-references corresponding
definitions and procedures in those RFCs. Deviations in protocol
handling specific to PIM snooping are specified in this document.
1.3. Definitions
There are several definitions referenced in this document that are
well described in the following PIM RFCs: RFC 3973 [PIM-DM], RFC 5015
[BIDIR-PIM], and RFC 7761 [PIM-SM]. The following definitions and
abbreviations are used throughout this document:
o A port is defined as either an AC or a PW.
o When we say that a PIM message is received on a PE port, it means
that the PE is processing the message for snooping/proxying or
relaying.
Abbreviations used in this document:
o S: IP address of the multicast source.
o G: IP address of the multicast group.
o N: Upstream Neighbor field in a Join/Prune/Graft message.
o Port(N): Port on which neighbor N is learned, i.e., the port on
which N's Hellos are received.
o rpt: Rendezvous Point Tree.
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RFC 8220 PIM Snooping September 2017
o PIM-DM: Protocol Independent Multicast - Dense Mode.
o PIM-SM: Protocol Independent Multicast - Sparse Mode.
o PIM-SSM: Protocol Independent Multicast - Source-Specific
Multicast.
1.4. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. PIM Snooping for VPLS
2.1. PIM Protocol Background
PIM is a multicast routing protocol running between routers, which
are CE devices in a VPLS. It uses the unicast routing table to
provide reverse-path information for building multicast trees. There
are a few variants of PIM. As described in RFC 3973 [PIM-DM],
multicast datagrams are pushed towards downstream neighbors, similar
to a broadcast mechanism, but in areas of the network where there are
no group members, routers prune back branches of the multicast tree
towards the source. Unlike PIM-DM, other PIM flavors (RFC 7761
[PIM-SM], RFC 4607 [PIM-SSM], and RFC 5015 [BIDIR-PIM]) employ a pull
methodology via explicit Joins instead of the push-and-prune
technique.
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 their corresponding upstream router. The Join/Prune message can
be group specific (*,G) or group and source specific (S,G).
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2.2. General Rules for PIM Snooping in VPLS
The following rules for the correct operation of PIM snooping MUST be
followed.
o PIM snooping MUST NOT affect the operation of customer Layer 2
protocols or Layer 3 protocols.
o PIM messages and multicast data traffic forwarded by PEs MUST
follow the split-horizon rule for mesh PWs, as defined in RFC 4762
[VPLS-LDP].
o PIM states in a PE MUST be per VPLS instance.
o PIM Assert triggers MUST be preserved to the extent necessary to
avoid sending duplicate traffic to the same PE (see
Section 2.2.1).
2.2.1. Preserving Assert Triggers
In PIM-SM / PIM-DM, there are scenarios where multiple routers could
be forwarding the same multicast traffic on a LAN. When this
happens, these routers start the PIM Assert election process by
sending PIM Assert messages, to ensure that only the Assert winner
forwards multicast traffic on the 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. In the case
of a VPLS with PIM snooping, two routers may forward the same
multicast datagrams at the same time, but each copy may reach a
different set of PEs; this is acceptable from the point of view of
avoiding duplicate traffic. If the two copies may reach the same PE,
then the sending routers must be able to see each other's traffic, in
order to trigger Assert election and stop duplicate traffic. To
achieve that, PEs enabled with PIM-SSM / PIM-SM snooping MUST forward
multicast traffic for an (S,G) / (*,G) not only on the ports on which
they snooped Join(S,G) / Join(*,G) but also towards the upstream
neighbor(s). In other words, the ports on which the upstream
neighbors are learned must be added to the outgoing port list, along
with the ports on which Joins are snooped. Please refer to
Section 2.6.1 for the rules that determine the set of upstream
neighbors for a particular (x,G).
Similarly, PIM-DM snooping SHOULD make sure that Asserts can be
triggered (Section 2.9.3).
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The above logic needs to be facilitated without breaking VPLS
split-horizon forwarding rules. That is, 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).
2.3. Some Considerations for PIM Snooping
The PIM snooping solution described here requires a PE to examine and
operate on only PIM Hello and PIM Join/Prune packets. The PE
does not need to examine any other PIM packets.
Most of the PIM snooping procedures for handling Hello/Join/Prune
messages are very similar to those executed in a PIM router.
However, the PE does not need to have any routing tables like those
required in PIM routing. It knows how to forward Join/Prune messages
only by looking at the Upstream Neighbor field in the Join/Prune
packets, as described in Section 2.12.
The PE does not need to know about Rendezvous Points (RPs) and
does not have to maintain any RP Set. All of that is transparent to
a PIM snooping PE.
In the following subsections, we list some considerations and
observations for the implementation of PIM snooping in the VPLS.
2.3.1. Scaling
PIM snooping needs to be employed on ACs at the downstream PEs (PEs
receiving multicast traffic across the VPLS core) to prevent traffic
from being sent out of ACs unnecessarily. PIM snooping techniques
can also be employed on PWs at the upstream PEs (PEs receiving
traffic from local ACs in a hierarchical VPLS) to prevent traffic
from being sent to PEs unnecessarily. This may work well for
small-scale or medium-scale deployments. However, if there are a
large number of VPLS instances with a large number of PEs per
instance, then the amount of snooping required at the upstream PEs
can overwhelm the upstream PEs.
There are two methods to reduce the burden on the upstream PEs. One
is to use PIM proxying, as described in Section 2.6.6, to reduce the
control messages forwarded by a PE. The other is not to snoop on the
PWs at all but to have PEs signal the snooped states to other PEs out
of band via BGP, as described in RFC 7117 [VPLS-MCAST]. In this
document, it is assumed that snooping is performed on PWs.
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2.3.2. IPv4 and IPv6
In the VPLS, PEs forward Ethernet frames received from CEs and as
such are agnostic of the Layer 3 protocol used by the CEs. However,
as a PIM snooping PE, the PE would have to look deeper into the IP
and 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 document can be applied to both IPv4 and
IPv6 payloads.
2.3.3. PIM-SM (*,*,RP)
This document does not address (*,*,RP) states in the VPLS network,
as they have been removed from the PIM protocol as described in
RFC 7761 [PIM-SM].
2.4. PIM Snooping vs. PIM Proxying
This document has previously alluded to PIM snooping/relay/proxying.
Details on the PIM relay/proxying solution are discussed in
Section 2.6.6. In this section, a brief description and comparison
are given.
2.4.1. Differences between PIM Snooping, Relay, and Proxying
Differences between PIM snooping and relay/proxying can be summarized
as follows:
+--------------------+---------------------+-----------------------+
| PIM snooping | PIM relay | PIM proxying |
+====================|=====================|=======================+
| Join/Prune messages| Join/Prune messages | Join/Prune messages |
| snooped and flooded| snooped; forwarded | consumed. Regenerated|
| according to VPLS | as is out of certain| ones sent out of |
| flooding procedures| upstream ports | certain upstream ports|
+--------------------+---------------------+-----------------------+
| Hello messages | Hello messages | Hello messages |
| snooped and flooded| snooped and flooded | snooped and flooded |
| according to VPLS | according to VPLS | according to VPLS |
| flooding procedures| flooding procedures | flooding procedures |
+--------------------+---------------------+-----------------------+
| No PIM packets | No PIM packets | New Join/Prune |
| generated | generated | messages generated |
+--------------------+---------------------+-----------------------+
| CE Join suppression| CE Join suppression | CE Join suppression |
| not allowed | allowed | allowed |
+--------------------+---------------------+-----------------------+
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Other than the above differences, most of the procedures are common
to PIM snooping and PIM relay/proxying, unless specifically stated
otherwise.
Pure PIM snooping PEs simply snoop on PIM packets as they are being
forwarded in the VPLS. As such, they truly provide transparent LAN
services, since no customer packets are modified or consumed nor are
new packets introduced in the VPLS. It is also simpler to implement
than PIM proxying. However, for PIM snooping to work correctly, it
is a requirement that CE routers MUST disable Join suppression in the
VPLS. Otherwise, most of the CE routers with interest in a given
multicast data stream will fail to send Join/Prune messages for that
stream, and the PEs will not be able to tell which ACs and/or PWs
have listeners for that stream.
Given that a large number of existing CE deployments do not support
the disabling of Join suppression and given the operational
complexity for a provider to manage the disabling of Join suppression
in the VPLS, it becomes a difficult solution to deploy. Another
disadvantage of PIM snooping is that it does not scale as well as PIM
proxying. If there are a large number of CEs in a VPLS, then every
CE will see every other CE's Join/Prune messages.
PIM relay/proxying has the advantage that it does not require Join
suppression to be disabled in the VPLS. Multicast as part of a VPLS
can be very easily provided without requiring any changes on the CE
routers. PIM relay/proxying helps scale VPLS multicast, since
Join/Prune messages are only sent to certain upstream ports instead
of flooded, and in cases of full proxying (vs. relay), the PEs
intelligently generate only one Join/Prune message for a given
multicast stream.
PIM proxying, however, loses the transparency argument, since
Join/Prune packets 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, for both the provider and the customer.
Data traffic will still be completely transparent.
2.4.2. PIM Control Message Latency
A PIM snooping/relay/proxying PE snoops on PIM Hello packets while
transparently flooding them in the VPLS. As such, there is no
latency introduced by the VPLS in the delivery of PIM Hello packets
to remote CEs in the VPLS.
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A PIM snooping PE 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.
A PIM relay/proxying PE does not simply flood PIM Join/Prune packets.
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.
Performing only proxying of Join/Prune and not Hello messages keeps
the PE's behavior very similar to that of a PIM router, without
introducing too much additional complexity. It keeps the PIM
proxying solution fairly simple. Since Join/Prune messages 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/Prune packets). 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 no
deviation from normal expected behavior observed at the CE router
receiving these messages out of order.
2.4.3. When to Snoop and When to Proxy
From the above descriptions, factors that affect the choice of
snooping/relay/proxying include:
o Whether CEs do Join suppression or not
o Whether Join/Prune latency is critical or not
o Whether the scale of PIM protocol messages/states in a VPLS
requires the scaling benefit of proxying
Of the above factors, Join suppression is the hard one -- pure
snooping can only be used when Join suppression is disabled on all
CEs. The latency associated with relay/proxying is implementation
dependent and may not be a concern at all with a particular
implementation. The scaling benefit may not be important either,
in that on a real LAN with Explicit Tracking (ET) a PIM router will
need to receive and process all PIM Join/Prune messages as well.
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A PIM router indicates that Join suppression is disabled if the T-bit
is set in the LAN Prune Delay option of its Hello message. If all
PIM routers on a LAN set the T-bit, ET is possible, allowing an
upstream router to track all the downstream neighbors that have Join
states for any (S,G) or (*,G). This has two benefits:
o No need for the Prune-Pending process -- the upstream router may
immediately stop forwarding data when it receives a Prune from the
last downstream neighbor and immediately prune to its upstream
neighbor.
o For management purposes, the upstream router knows exactly which
downstream routers exist for a particular Join state.
While full proxying can be used with or without Join suppression on
CEs and does not interfere with an upstream CE's bypass of the
Prune-Pending process, it does proxy all its downstream CEs as a
single one to the upstream neighbors, removing the second benefit
mentioned above.
Therefore, the general rule is that if Join suppression is enabled on
one or more CEs, then proxying or relay MUST be used, but if Join
suppression is known to be disabled on all CEs, then snooping, relay,
or proxying MAY be used, while snooping or relay SHOULD be used.
An implementation MAY choose to dynamically determine which mode to
use, through the tracking of the above-mentioned T-bit in all snooped
PIM Hello messages, or MAY simply require static provisioning.
2.5. Discovering PIM Routers
A PIM snooping PE MUST snoop on PIM Hellos received on ACs and PWs.
That is, the PE transparently floods the PIM Hello while snooping on
it. PIM Hellos are used by the snooping PE 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:
o Layer 2 encapsulation for the router sending the PIM Hello.
o IP address and address family of the router sending the PIM Hello.
o Port (AC/PW) on which the PIM Hello was received.
o Hello Option fields.
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The PE should be able to interpret and act on Hello Option fields as
currently defined in RFC 7761 [PIM-SM]. The Option fields of
particular interest in this document are:
o Hello-Hold-Time
o Tracking Support
o Designated Router (DR) Priority
Please refer to RFC 7761 [PIM-SM] for a list of the Hello Option
fields. 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 the 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 RFC 7761 [PIM-SM] to identify the PIM DR in the
VPLS instance. It should also be able to determine if tracking
support is active in the VPLS instance.
2.6. 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. 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.
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2.6.1. Building PIM-SM States
PIM-SM and PIM-SSM states are built by snooping on the PIM-SM
Join/Prune messages received on ACs/PWs.
The downstream state machine of a PIM-SM snooping PE very closely
resembles the downstream state machine of PIM-SM routers. The
downstream state consists of:
Per downstream (Port,*,G):
o DownstreamJPState: One of {"NoInfo" (NI), "Join" (J),
"Prune-Pending" (PP)}
Per downstream (Port,*,G,N):
o Prune-Pending Timer (PPT(N))
o Join Expiry Timer (ET(N))
Per downstream (Port,S,G):
o DownstreamJPState: One of {"NoInfo" (NI), "Join" (J),
"Prune-Pending" (PP)}
Per downstream (Port,S,G,N):
o Prune-Pending Timer (PPT(N))
o Join Expiry Timer (ET(N))
Per downstream (Port,S,G,rpt):
o DownstreamJPRptState: One of {"NoInfo" (NI), "Pruned" (P),
"Prune-Pending" (PP)}
Per downstream (Port,S,G,rpt,N):
o Prune-Pending Timer (PPT(N))
o 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.
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Note that unlike the case of PIM-SM routers, where the PPT and ET are
per (Interface,S,G), PIM snooping PEs have to maintain the PPT and ET
per (Port,S,G,N). The reasons for this are explained in
Section 2.6.2.
Apart from the above states, we define the following state
summarization macros:
UpstreamNeighbors(*,G): If there are one or more Join(*,G)s 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.
UpstreamNeighbors(S,G): If there are one or more Join(S,G)s 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 Port(N) ports where N is
in the set UpstreamNeighbors(*,G). Multicast streams forwarded
using a (*,G) match MUST be forwarded to these ports. So,
UpstreamPorts(*,G) MUST be added to OutgoingPortList(*,G).
UpstreamPorts(S,G): This is the set of all Port(N) ports where N is
in the set UpstreamNeighbors(S,G). UpstreamPorts(S,G) MUST be
added to OutgoingPortList(S,G).
InheritedUpstreamPorts(S,G): This is the union of UpstreamPorts(S,G)
and UpstreamPorts(*,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).
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 2.6.6.1.
PWPorts: This is the set of all PWs.
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OutgoingPortList(*,G): This is the set of all ports to which traffic
needs to be forwarded on a (*,G) match.
OutgoingPortList(S,G): This is the set of all ports to which traffic
needs to be forwarded on an (S,G) match.
See Section 2.12 ("Data-Forwarding Rules") for the specification on
how OutgoingPortList is calculated.
NumETsActive(Port,*,G): This is the number of (Port,*,G,N) entries
that have the 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): This is the number of (Port,S,G,N) entries
that have the Expiry Timer running. This macro keeps track of the
number of Join(S,G)s that are received on this Port with different
upstream neighbors.
JoinAttributeTlvs(*,G): Join Attributes (RFC 5384 [JOIN-ATTR]) are
TLVs that may be present in received Join(*,G) messages. An
example would be Reverse Path Forwarding (RPF) Vectors (RFC 5496
[RPF-VECTOR]). If present, they must be copied to
JoinAttributeTlvs(*,G).
JoinAttributeTlvs(S,G): Join Attributes (RFC 5384 [JOIN-ATTR]) are
TLVs that may be present in received Join(S,G) messages. If
present, they must be copied to JoinAttributeTlvs(S,G).
Since there are a few differences between the downstream state
machines of PIM-SM routers and PIM-SM snooping PEs, we specify the
details of the downstream state machine of PIM-SM snooping PEs, at
the risk of repeating most of the text documented in RFC 7761
[PIM-SM].
2.6.2. 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 PE
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 allow the CE routers to decide which upstream
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neighbor gets elected as the RPF-Neighbor. And for this purpose,
the PE will have to track downstream and upstream Joins and Prunes
per (S,G,N).
2.6.3. Receiving (*,G) PIM-SM Join/Prune Messages
A Join(*,G) or Prune(*,G) is considered "received" if one of the
following conditions is met:
o The port on which it arrived is not Port(N) where N is the
upstream neighbor N of the Join/Prune(*,G).
o If both Port(N) and the arrival port are PWs, then there exists at
least one other (*,G,Nx) or (Sx,G,Nx) state with an AC
UpstreamPort.
For simplicity, the case where both Port(N) and the arrival port are
PWs is referred to as "PW-only Join/Prune" in this document. The
PW-only Join/Prune handling is so that the Port(N) PW can be added to
the related forwarding entries' OutgoingPortList to trigger an
Assert, but that is only needed for those states with AC
UpstreamPorts. Note that in the PW-only case, it is OK for the
arrival port and Port(N) to be the same. See Appendix B for
examples.
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 are exactly as
specified in RFC 7761 [PIM-SM].
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We define the following per-port (*,G,N) macro to help with the state
machine below.
+---------------++-------------------------------------------------+
| || Previous State |
| ++-------------+--------------+--------------------+
| Event || NoInfo (NI) | Join (J) | Prune-Pending (PP) |
+---------------++-------------+--------------+--------------------+
| 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 | - |
|and || | Action | |
|NumETsActive>1 || | ETExpiry(N) | |
+---------------++-------------+--------------+--------------------+
Figure 1: Downstream Per-Port (*,G) State Machine in Tabular Form
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. If there are Join
Attribute TLVs in the received (*,G) message and if they are
different from the recorded JoinAttributeTlvs(*,G), then copy them
into JoinAttributeTlvs(*,G). In the case of conflicting
attributes, the PE will need to perform conflict resolution per
(N) as described in RFC 5384 [JOIN-ATTR].
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Action PPTExpiry(N):
Same as Action ETExpiry(N) below, plus send a Prune-Echo(*,G) with
upstream neighbor N on the downstream port.
Action ETExpiry(N):
Disable timers ET(N) and PPT(N). Delete Neighbor state
(Port,*,G,N). If there are no other (Port,*,G) states with
NumETsActive(Port,*,G) > 0, transition DownstreamJPState (RFC 7761
[PIM-SM]) to NoInfo. If there are no other (Port,*,G,N) states
(different ports but for the same N), remove N from
UpstreamPorts(*,G) -- this will also trigger the Upstream Finite
State Machine (FSM) with "JoinDesired(*,G,N) to FALSE".
2.6.4. Receiving (S,G) PIM-SM Join/Prune Messages
A Join(S,G) or Prune(S,G) is considered "received" if one of the
following conditions is met:
o The port on which it arrived is not Port(N) where N is the
upstream neighbor N of the Join/Prune(S,G).
o If both Port(N) and the arrival port are PWs, then there exists at
least one other (*,G,Nx) or (S,G,Nx) state with an AC
UpstreamPort.
For simplicity, the case where both Port(N) and the arrival port are
PWs is referred to as "PW-only Join/Prune" in this document. The
PW-only Join/Prune handling is so that the Port(N) PW can be added to
the related forwarding entries' OutgoingPortList to trigger an
Assert, but that is only needed for those states with AC
UpstreamPorts. Note that in the PW-only case, it is OK for the
arrival port and Port(N) to be the same. See Appendix B for
examples.
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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 are exactly as
specified in RFC 7761 [PIM-SM].
+---------------++-------------------------------------------------+
| || Previous State |
| ++-------------+--------------+--------------------+
| Event || NoInfo (NI) | Join (J) | Prune-Pending (PP) |
+---------------++-------------+--------------+--------------------+
| Receive || -> J state | -> J state | -> J state |
| Join(S,G) || Action | Action | Action |
| || RxJoin(N) | RxJoin(N) | RxJoin(N) |
+---------------++-------------+--------------+--------------------+
|Receive || - | -> 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 | - |
|and || | Action | |
|NumETsActive>1 || | ETExpiry(N) | |
+---------------++-------------+--------------+--------------------+
Figure 2: Downstream Per-Port (S,G) State Machine in Tabular Form
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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(S,G), then add N to
UpstreamNeighbors(S,G) and trigger a Join(S,G) with upstream
neighbor N to be forwarded upstream. If there are Join Attribute
TLVs in the received (S,G) message and if they are different from
the recorded JoinAttributeTlvs(S,G), then copy them into
JoinAttributeTlvs(S,G). In cases of conflicting attributes, the
PE will need to perform conflict resolution per (N) as described
in RFC 5384 [JOIN-ATTR].
Action PPTExpiry(N):
Same as Action ETExpiry(N) below, plus send a Prune-Echo(S,G) with
upstream neighbor N on the downstream port.
Action ETExpiry(N):
Disable timers ET(N) and PPT(N). Delete Neighbor state
(Port,S,G,N). If there are no other (Port,S,G) states with
NumETsActive(Port,S,G) > 0, transition DownstreamJPState to
NoInfo. If there are no other (Port,S,G,N) states (different
ports but for the same N), remove N from UpstreamPorts(S,G) --
this will also trigger the Upstream FSM with "JoinDesired(S,G,N)
to FALSE".
2.6.5. 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 learned.
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 4.5.3 of RFC 7761 [PIM-SM].
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2.6.6. Sending Join/Prune Messages Upstream
This section applies only to a PIM relay/proxying PE and not to a PIM
snooping PE.
A full PIM proxying (not relay) PE MUST implement the Upstream FSM
along the lines of the procedure described in Section 4.5.4 of
RFC 7761 [PIM-SM].
For the purposes of the Upstream FSM, a Join or Prune message with
upstream neighbor N is "seen" on a PIM relay/proxying PE if the port
on which the message was received is also Port(N) and the port is an
AC. The AC requirement is needed because a Join received on the
Port(N) PW must not suppress this PE's Join on that PW.
A PIM relay PE does not implement the Upstream FSM. It simply
forwards received Join/Prune messages out of the same set of upstream
ports as in the PIM proxying case.
In order to correctly facilitate Asserts among the CE routers, such
Join/Prune messages need to send not only towards the upstream
neighbor but also on certain PWs, as described below.
If JoinAttributeTlvs(*,G) is not empty, then it must be encoded in a
Join(*,G) message sent upstream.
If JoinAttributeTlvs(S,G) is not empty, then it must be encoded in a
Join(S,G) message sent upstream.
2.6.6.1. Where to Send Join/Prune Messages
The following rules apply to both (1) forwarded (in the case of PIM
relay) and (2) refreshed and triggered (in the case of PIM proxying)
(S,G) / (*,G) Join/Prune messages.
o The Upstream Neighbor field in the Join/Prune to be sent is set to
the N in the corresponding Upstream FSM.
o If Port(N) is an AC, send the message to Port(N).
o Additionally, if OutgoingPortList(x,G,N) contains at least one AC,
then the message MUST be sent to at least all the PWs in
UpstreamPorts(G) (for (*,G)) or InheritedUpstreamPorts(S,G) (for
(S,G)). Alternatively, the message MAY be sent to all PWs.
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Sending to a subset of PWs as described above guarantees that if
traffic (of the same flow) from two upstream routers were to reach
this PE, then the two routers will receive from each other,
triggering an Assert.
Sending to all PWs guarantees that if two upstream routers both send
traffic for the same flow (even if it is to different sets of
downstream PEs), then the two routers will receive from each other,
triggering an Assert.
2.7. Bidirectional PIM (BIDIR-PIM)
Bidirectional PIM (BIDIR-PIM) is a variation of PIM-SM. The main
differences between PIM-SM and BIDIR-PIM are as follows:
o There are no source-based trees, and SSM is not supported (i.e.,
no (S,G) states) in BIDIR-PIM.
o Multicast traffic can flow up the shared tree in BIDIR-PIM.
o To avoid forwarding loops, one router on each link is elected as
the Designated Forwarder (DF) for each RP in BIDIR-PIM.
The main advantage of BIDIR-PIM 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.
As described in RFC 5015 [BIDIR-PIM], parts of a BIDIR-PIM-enabled
network may forward traffic without exchanging Join/Prune messages --
for instance, between DFs and the Rendezvous Point Link (RPL).
As the described procedures for PIM snooping rely on the presence of
Join/Prune messages, enabling PIM snooping on BIDIR-PIM networks
could break the BIDIR-PIM functionality. Deploying PIM snooping on
BIDIR-PIM-enabled networks will require some further study. Some
thoughts on this topic are discussed in Appendix A.
2.8. Interaction with IGMP Snooping
Whenever IGMP snooping is enabled in conjunction with PIM snooping in
the same VPLS instance, the PE SHOULD follow these rules:
o To maintain the list of multicast routers and ports on which they
are attached, the PE SHOULD NOT use the rules described in
RFC 4541 [IGMP-SNOOP] but SHOULD rely on the neighbors discovered
by PIM snooping. This list SHOULD then be used to apply the first
forwarding rule (rule 1) listed in Section 2.1.1 of RFC 4541
[IGMP-SNOOP].
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o If the PE supports proxy reporting, an IGMP membership learned
only on a port to which a PIM neighbor is attached (i.e., not
learned elsewhere) SHOULD NOT be included in the summarized
upstream report sent to that port.
2.9. PIM-DM
The key characteristic of PIM-DM is flood-and-prune behavior.
Shortest-path trees are built as a multicast source starts
transmitting.
2.9.1. Building PIM-DM States
PIM-DM states are built by snooping on the PIM-DM Join, Prune, Graft,
and State Refresh messages received on ACs/PWs and State Refresh
messages sent on ACs/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):
Port PIM(S,G,N) Prune State:
* DownstreamPState(S,G,N,Port): One of {"NoInfo" (NI),
"Pruned" (P), "Prune-Pending" (PP)}
* Prune-Pending Timer (PPT)
* Prune Timer (PT)
* Upstream Port (valid if the PIM(S,G,N) Prune state is "Pruned")
2.9.2. 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 RFC 3973 [PIM-DM], with a few changes relevant to
PIM snooping. When reading Section 4.4.2 of RFC 3973 [PIM-DM],
please be aware that, for the purposes of PIM snooping, 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 Section 2.9.1, the states (DownstreamPState) and
timers (PPT and PT) are per (S,G,N,Port).
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2.9.3. 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
Port(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 that is also an upstream port. 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 upstream ports.
2.10. PIM Proxy
As noted earlier, 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 relay/proxying for VPLS multicast
to work correctly. This section applies specifically to full
proxying and not to relay.
2.10.1. Upstream PIM Proxy Behavior
A PIM proxying PE consumes Join/Prune messages and regenerates PIM
Join/Prune messages to be sent upstream by implementing the Upstream
FSM as specified in Section 4.5.4 of RFC 7761 [PIM-SM]. This is the
only difference from PIM relay.
The source IP address in PIM packets sent upstream SHOULD be the
address of a PIM downstream neighbor in the corresponding Join/Prune
state. The chosen address 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.
2.11. Directly Connected Multicast Source
PIM snooping/relay/proxying could be enabled on a LAN that connects a
multicast source and a PIM First-Hop Router (FHR). As the FHR
will not send any downstream Join/Prune messages, we will not be able
to establish any forwarding states for that source. Therefore, 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 in addition to the IGMP
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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:
o The PE would have to do IPv4 ARP snooping and/or IPv6 Neighbor
Discovery snooping to determine if a source is directly connected.
o Another option is to configure all PEs to indicate that 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.
o A third option is to require that sources of CE multicast traffic
must be behind a router.
This document recommends the third option -- sources of traffic must
be behind a router.
2.12. Data-Forwarding Rules
First, we define the rules that are common to PIM-SM and PIM-DM PEs.
Forwarding rules for each protocol type are specified in the
subsections below.
If there is no matching forwarding state, then the PE SHOULD discard
the packet, i.e., the UserDefinedPortList (Sections 2.12.1 and
2.12.2) SHOULD be empty.
The following general rules MUST be followed when forwarding
multicast traffic in a VPLS:
o Traffic arriving on a port MUST NOT be forwarded back onto the
same port.
o Due to VPLS split-horizon rules, traffic ingressing on a PW
MUST NOT be forwarded to any other PW.
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2.12.1. PIM-SM Data-Forwarding Rules
Per the rules in RFC 7761 [PIM-SM] and per the additional rules
specified in this document,
OutgoingPortList(*,G) = immediate_olist(*,G) (+)
UpstreamPorts(*,G) (+)
Port(PimDR)
OutgoingPortList(S,G) = inherited_olist(S,G) (+)
UpstreamPorts(S,G) (+)
(UpstreamPorts(*,G) (-)
UpstreamPorts(S,G,rpt)) (+)
Port(PimDR)
RFC 7761 [PIM-SM] specifies how immediate_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 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 = 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
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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, then the set of
outgoing ports is OutgoingPortList(*,G).
The packet is forwarded to the selected set of outgoing ports while
observing the general rules above in Section 2.12.
2.12.2. 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 general rules above in Section 2.12.
RFC 3973 [PIM-DM] specifies how olist(S,G) is constructed.
3. IANA Considerations
This document does not require any IANA actions.
Dornon, et al. Informational [Page 29]
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4. Security Considerations
Security considerations provided in the VPLS solution documents
(i.e., RFC 4762 [VPLS-LDP] and RFC 4761 [VPLS-BGP]) apply to this
document as well.
5. References
5.1. Normative References
[BIDIR-PIM]
Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast
(BIDIR-PIM)", RFC 5015, DOI 10.17487/RFC5015,
October 2007, <https://www.rfc-editor.org/info/rfc5015>.
[JOIN-ATTR]
Boers, A., Wijnands, I., and E. Rosen, "The Protocol
Independent Multicast (PIM) Join Attribute Format",
RFC 5384, DOI 10.17487/RFC5384, November 2008,
<https://www.rfc-editor.org/info/rfc5384>.
[PIM-DM] Adams, A., Nicholas, J., and W. Siadak, "Protocol
Independent Multicast - Dense Mode (PIM-DM): Protocol
Specification (Revised)", RFC 3973, DOI 10.17487/RFC3973,
January 2005, <https://www.rfc-editor.org/info/rfc3973>.
[PIM-SM] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
Multicast - Sparse Mode (PIM-SM): Protocol Specification
(Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761,
March 2016, <https://www.rfc-editor.org/info/rfc7761>.
[PIM-SSM] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
<https://www.rfc-editor.org/info/rfc4607>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in
RFC 2119 Key Words", BCP 14, RFC 8174,
DOI 10.17487/RFC8174, May 2017,
<https://www.rfc-editor.org/info/rfc8174>.
Dornon, et al. Informational [Page 30]
RFC 8220 PIM Snooping September 2017
[RPF-VECTOR]
Wijnands, IJ., Boers, A., and E. Rosen, "The Reverse Path
Forwarding (RPF) Vector TLV", RFC 5496,
DOI 10.17487/RFC5496, March 2009,
<https://www.rfc-editor.org/info/rfc5496>.
5.2. Informative References
[IGMP-SNOOP]
Christensen, M., Kimball, K., and F. Solensky,
"Considerations for Internet Group Management Protocol
(IGMP) and Multicast Listener Discovery (MLD) Snooping
Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006,
<https://www.rfc-editor.org/info/rfc4541>.
[VPLS-BGP]
Kompella, K., Ed., and Y. Rekhter, Ed., "Virtual Private
LAN Service (VPLS) Using BGP for Auto-Discovery and
Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
<https://www.rfc-editor.org/info/rfc4761>.
[VPLS-LDP]
Lasserre, M., Ed., and V. Kompella, Ed., "Virtual Private
LAN Service (VPLS) Using Label Distribution Protocol (LDP)
Signaling", RFC 4762, DOI 10.17487/RFC4762, January 2007,
<https://www.rfc-editor.org/info/rfc4762>.
[VPLS-MCAST]
Aggarwal, R., Ed., Kamite, Y., Fang, L., Rekhter, Y., and
C. Kodeboniya, "Multicast in Virtual Private LAN Service
(VPLS)", RFC 7117, DOI 10.17487/RFC7117, February 2014,
<https://www.rfc-editor.org/info/rfc7117>.
[VPLS-MCAST-REQ]
Kamite, Y., Ed., Wada, Y., Serbest, Y., Morin, T., and L.
Fang, "Requirements for Multicast Support in Virtual
Private LAN Services", RFC 5501, DOI 10.17487/RFC5501,
March 2009, <https://www.rfc-editor.org/info/rfc5501>.
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Appendix A. BIDIR-PIM Considerations
This appendix describes some guidelines that may be used to preserve
BIDIR-PIM functionality in combination with PIM snooping.
In order to preserve BIDIR-PIM snooping, routers need to set up
forwarding states so that:
o on the RPL, all traffic is forwarded to all Port(N) ports.
o on any other interface, traffic is always forwarded to the DF.
The information needed to set up these states may be obtained by:
o determining the mapping between the group (range) and the RP.
o snooping and storing DF election information.
o determining where the RPL is. This could be achieved by static
configuration or by combining the information mentioned in the two
bullet items above.
A.1. BIDIR-PIM Data-Forwarding Rules
The BIDIR-PIM snooping data-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
If iif is an AC
Then
OutgoingPortList = OutgoingPortList (-) iif
Else
## iif is a PW
OutgoingPortList = OutgoingPortList (-) PWPorts
Endif
Forward the packet to OutgoingPortList.
END
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If there is forwarding state for G, then forward the packet to
olist(G) while observing the general rules above in Section 2.12.
RFC 5015 [BIDIR-PIM] specifies how olist(G) is constructed.
Appendix B. Example Network Scenario
Let us consider the scenario in Figure 3.
+------+ AC3 +------+
| PE2 |-----| CE3 |
/| | +------+
/ +------+ |
/ | |
/ | |
/PW12 | |
/ | /---\
/ |PW23 | S |
/ | \---/
/ | |
/ | |
/ | |
+------+ / +------+ |
+------+ | PE1 |/ PW13 | PE3 | +------+
| CE1 |-----| |-------------| |-----| CE4 |
+------+ AC1 +------+ +------+ AC4 +------+
|
|AC2
+------+
| CE2 |
+------+
Figure 3: An Example Network for Triggering an Assert
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.
B.1. PIM Snooping Example
In the network 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.
1. CE1 sends a Join(S,G) with UpstreamNeighbors(S,G) = CE3.
2. PE1 snoops on the Join(S,G) and builds forwarding state, since it
is received on an AC. It also floods the Join(S,G) in the VPLS.
PE2 snoops on the Join(S,G) and builds forwarding state, since
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the Join(S,G)is targeting a neighbor residing on an AC. PE3
does not create forwarding state for (S,G) because this is a
PW-only Join and there is neither an existing (*,G) state with an
AC in UpstreamPorts(*,G) nor an existing (S,G) state with an AC
in UpstreamPorts(S,G). Both PE2 and PE3 will also flood the
Join(S,G) in the VPLS.
The resulting states at the PEs are as follows:
PE1 states:
JT(AC1,S,G,CE3) = JP_HoldTime
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { PW12 }
OutgoingPortList(S,G) = { AC1, PW12 }
PE2 states:
JT(PW12,S,G,CE3) = JP_HoldTime
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { AC3 }
OutgoingPortList(S,G) = { PW12, AC3 }
PE3 states:
No (S,G) state
3. The multicast stream (S,G) flows along CE3 -> PE2 -> PE1 -> CE1.
4. Now CE2 sends a Join(S,G) with UpstreamNeighbors(S,G) = CE4.
5. All PEs snoop on the Join(S,G), build forwarding state, and flood
the Join(S,G) in the VPLS. Note that for PE2, even though this
is a PW-only Join, forwarding state is built on this Join(S,G),
since PE2 has an existing (S,G) state with an AC in
UpstreamPorts(S,G).
The resulting states at the PEs are as follows:
PE1 states:
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 }
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PE2 states:
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 }
PE3 states:
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 two of the
CEs -- CE3 and CE4. PE2 forwards the stream received from CE3 to
PW23, and PE3 forwards the stream to AC4. This helps the CE
routers to trigger Assert election. Let us say that CE3 becomes
the Assert winner.
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 the 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-Pending timer on the ports on which
they received the Prune(S,G). When the Prune-Pending timer
expires, all PEs will remove the downstream (S,G,CE4) states.
The resulting states at the PEs are as follows:
PE1 states:
JT(AC1,S,G,CE3) = active
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { PW12 }
OutgoingPortList(S,G) = { AC1, AC2, PW12 }
PE2 states:
JT(PW12,S,G,CE3) = active
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { AC3 }
OutgoingPortList(S,G) = { PW12, AC3 }
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PE3 states:
JT(PW13,S,G,CE3) = JP_HoldTime
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { PW23 }
OutgoingPortList(S,G) = { PW13, PW23 }
Note that at this point at PE3, since there is no AC in
OutgoingPortList(S,G) and no (*,G) or (S,G) state with an AC in
UpstreamPorts(*,G) or UpstreamPorts(S,G), respectively, the
existing (S,G) state at PE3 can also be removed. So, finally:
PE3 states:
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.
B.2. PIM Proxy Example with (S,G) / (*,G) Interaction
In the same network, let us assume that CE4 is the upstream neighbor
towards the RP for G.
JPST(S,G,N) is the JP sending timer for the (S,G) with upstream
neighbor N.
1. CE1 sends a Join(S,G) with UpstreamNeighbors(S,G) = CE3.
2. PE1 consumes the Join(S,G) and builds forwarding state, since the
Join(S,G) is received on an AC.
PE2 consumes the Join(S,G) and builds forwarding state, since the
Join(S,G) is targeting a neighbor residing on an AC.
PE3 consumes the Join(S,G) but does not create forwarding state
for (S,G), since this is a PW-only Join and there is neither an
existing (*,G) state with an AC in UpstreamPorts(*,G) nor an
existing (S,G) state with an AC in UpstreamPorts(S,G).
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The resulting states at the PEs are as follows:
PE1 states:
JT(AC1,S,G,CE3) = JP_HoldTime
JPST(S,G,CE3) = t_periodic
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { PW12 }
OutgoingPortList(S,G) = { AC1, PW12 }
PE2 states:
JT(PW12,S,G,CE3) = JP_HoldTime
JPST(S,G,CE3) = t_periodic
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { AC3 }
OutgoingPortList(S,G) = { PW12, AC3 }
PE3 states:
No (S,G) state
Joins are triggered as follows:
PE1 triggers a Join(S,G) targeting CE3. Since the Join(S,G) was
received on an AC and is targeting a neighbor that is residing
across a PW, the triggered Join(S,G) is sent on all PWs.
PE2 triggers a Join(S,G) targeting CE3. Since the Join(S,G) is
targeting a neighbor residing on an AC, it only sends the Join
on AC3.
PE3 ignores the Join(S,G), since this is a PW-only Join and there
is neither an existing (*,G) state with an AC in
UpstreamPorts(*,G) nor an existing (S,G) state with an AC in
UpstreamPorts(S,G).
3. The multicast stream (S,G) flows along CE3 -> PE2 -> PE1 -> CE1.
4. Now let us say that CE2 sends a Join(*,G) with
UpstreamNeighbors(*,G) = CE4.
5. PE1 consumes the Join(*,G) and builds forwarding state, since the
Join(*,G) is received on an AC.
PE2 consumes the Join(*,G); although this is a PW-only Join,
forwarding state is built on this Join(*,G), since PE2 has an
existing (S,G) state with an AC in UpstreamPorts(S,G). However,
since this is a PW-only Join, PE2 only adds the PW towards PE3
(PW23) into UpstreamPorts(*,G) and hence into
OutgoingPortList(*,G). It does not add the PW towards PE1 (PW12)
into OutgoingPortList(*,G).
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PE3 consumes the Join(*,G) and builds forwarding state, since the
Join(*,G) is targeting a neighbor residing on an AC.
The resulting states at the PEs are as follows:
PE1 states:
JT(AC1,*,G,CE4) = JP_HoldTime
JPST(*,G,CE4) = t_periodic
UpstreamNeighbors(*,G) = { CE4 }
UpstreamPorts(*,G) = { PW13 }
OutgoingPortList(*,G) = { AC2, PW13 }
JT(AC1,S,G,CE3) = active
JPST(S,G,CE3) = active
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { PW12 }
OutgoingPortList(S,G) = { AC1, PW12, PW13 }
PE2 states:
JT(PW12,*,G,CE4) = JP_HoldTime
UpstreamNeighbors(*,G) = { CE4 }
UpstreamPorts(G) = { PW23 }
OutgoingPortList(*,G) = { PW23 }
JT(PW12,S,G,CE3) = active
JPST(S,G,CE3) = active
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { AC3 }
OutgoingPortList(S,G) = { PW12, AC3, PW23 }
PE3 states:
JT(PW13,*,G,CE4) = JP_HoldTime
JPST(*,G,CE4) = t_periodic
UpstreamNeighbors(*,G) = { CE4 }
UpstreamPorts(*,G) = { AC4 }
OutgoingPortList(*,G) = { PW13, AC4 }
Joins are triggered as follows:
PE1 triggers a Join(*,G) targeting CE4. Since the Join(*,G) was
received on an AC and is targeting a neighbor that is residing
across a PW, the triggered Join(S,G) is sent on all PWs.
PE2 does not trigger a Join(*,G) based on this Join, since this
is a PW-only Join.
PE3 triggers a Join(*,G) targeting CE4. Since the Join(*,G) is
targeting a neighbor residing on an AC, it only sends the Join
on AC4.
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6. If traffic is not flowing yet (i.e., step 3 is delayed so that it
occurs after step 6) and in the interim JPST(S,G,CE3) on PE1
expires, causing it to send a refresh Join(S,G) targeting CE3,
since the refresh Join(S,G) is targeting a neighbor that is
residing across a PW, the refresh Join(S,G) is sent on all PWs.
7. Note that PE1 refreshes its JT based on reception of refresh
Joins from CE1 and CE2.
PE2 consumes the Join(S,G) and refreshes the JT(PW12,S,G,CE3)
timer.
PE3 consumes the Join(S,G). It also builds forwarding state on
this Join(S,G), even though this is a PW-only Join, since now PE2
has an existing (*,G) state with an AC in UpstreamPorts(*,G).
However, since this is a PW-only Join, PE3 only adds the PW
towards PE2 (PW23) into UpstreamPorts(S,G) and hence into
OutgoingPortList(S,G). It does not add the PW towards PE1 (PW13)
into OutgoingPortList(S,G).
PE3 states:
JT(PW13,*,G,CE4) = active
JPST(S,G,CE4) = active
UpstreamNeighbors(*,G) = { CE4 }
UpstreamPorts(*,G) = { AC4 }
OutgoingPortList(*,G) = { PW13, AC4 }
JT(PW13,S,G,CE3) = JP_HoldTime
UpstreamNeighbors(*,G) = { CE3 }
UpstreamPorts(*,G) = { PW23 }
OutgoingPortList(*,G) = { PW13, AC4, PW23 }
Joins are triggered as follows:
PE2 already has (S,G) state, so it does not trigger a Join(S,G)
based on reception of this refresh Join.
PE3 does not trigger a Join(S,G) based on this Join, since this
is a PW-only Join.
8. The multicast stream (S,G) flows into the VPLS from two of the
CEs -- CE3 and CE4. PE2 forwards the stream received from CE3 to
PW12 and PW23. At the same time, PE3 forwards the stream
received from CE4 to PW13 and PW23.
The stream received over PW12 and PW13 is forwarded by PE1 to AC1
and AC2.
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The stream received by PE3 over PW23 is forwarded to AC4. The
stream received by PE2 over PW23 is forwarded to AC3. Either of
these helps the CE routers to trigger Assert election.
9. CE3 and/or CE4 send(s) Assert message(s) to the VPLS. The PEs
flood the Assert message(s) without examining it.
10. CE3 becomes the (S,G) Assert winner, and CE4 stops sending the
multicast stream to the VPLS.
11. CE2 notices an RPF change due to the Assert and sends a
Prune(S,G,rpt) with upstream neighbor = CE4.
12. PE1 consumes the Prune(S,G,rpt), and since
PruneDesired(S,G,Rpt,CE4) is TRUE, it triggers a Prune(S,G,rpt)
to CE4. Since the Prune is targeting a neighbor across a PW, it
is sent on all PWs.
PE2 consumes the Prune(S,G,rpt) and does not trigger any Prune
based on this Prune(S,G,rpt), since this was a PW-only Prune.
PE3 consumes the Prune(S,G,rpt), and since
PruneDesired(S,G,rpt,CE4) is TRUE, it sends the Prune(S,G,rpt)
on AC4.
PE1 states:
JT(AC2,*,G,CE4) = active
JPST(*,G,CE4) = active
UpstreamNeighbors(*,G) = { CE4 }
UpstreamPorts(*,G) = { PW13 }
OutgoingPortList(*,G) = { AC2, PW13 }
JT(AC2,S,G,CE4) = JP_HoldTime with S,G,rpt prune flag
JPST(S,G,CE4) = none, since this is sent along
with the Join(*,G) to CE4 based
on JPST(*,G,CE4) expiry
UpstreamPorts(S,G,rpt) = { PW13 }
UpstreamNeighbors(S,G,rpt) = { CE4 }
JT(AC1,S,G,CE3) = active
JPST(S,G,CE3) = active
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { PW12 }
OutgoingPortList(S,G) = { AC1, PW12, AC2 }
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PE2 states:
JT(PW12,*,G,CE4) = active
UpstreamNeighbors(*,G) = { CE4 }
UpstreamPorts(*,G) = { PW23 }
OutgoingPortList(*,G) = { PW23 }
JT(PW12,S,G,CE4) = JP_HoldTime with S,G,rpt prune flag
JPST(S,G,CE4) = none, since this was created
off a PW-only Prune
UpstreamPorts(S,G,rpt) = { PW23 }
UpstreamNeighbors(S,G,rpt) = { CE4 }
JT(PW12,S,G,CE3) = active
JPST(S,G,CE3) = active
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { AC3 }
OutgoingPortList(*,G) = { PW12, AC3 }
PE3 states:
JT(PW13,*,G,CE4) = active
JPST(*,G,CE4) = active
UpstreamNeighbors(*,G) = { CE4 }
UpstreamPorts(*,G) = { AC4 }
OutgoingPortList(*,G) = { PW13, AC4 }
JT(PW13,S,G,CE4) = JP_HoldTime with S,G,rpt prune flag
JPST(S,G,CE4) = none, since this is sent along
with the Join(*,G) to CE4 based
on JPST(*,G,CE4) expiry
UpstreamNeighbors(S,G,rpt) = { CE4 }
UpstreamPorts(S,G,rpt) = { AC4 }
JT(PW13,S,G,CE3) = active
JPST(S,G,CE3) = none, since this state is
created by a PW-only Join
UpstreamNeighbors(S,G) = { CE3 }
UpstreamPorts(S,G) = { PW23 }
OutgoingPortList(S,G) = { PW23 }
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.
However, we don't have duplicate traffic because one of the CEs stops
sending traffic due to the Assert, not because we don't have any
forwarding state in the PEs to do this forwarding.
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Acknowledgements
Many members of the former L2VPN and PIM working groups have
contributed to, and provided valuable comments and feedback on, this
document, including Vach Kompella, Shane Amante, Sunil Khandekar, Rob
Nath, Marc Lasserre, Yuji Kamite, Yiqun Cai, Ali Sajassi, Jozef
Raets, Himanshu Shah (Ciena), and Himanshu Shah (Cisco).
Contributors
Yetik Serbest and Suresh Boddapati coauthored earlier draft versions
of this document.
Karl (Xiangrong) Cai and Princy Elizabeth made significant
contributions to bring the specification to its current state,
especially in the area of Join forwarding rules.
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Authors' Addresses
Olivier Dornon
Nokia
Copernicuslaan 50
B-2018 Antwerp
Belgium
Email: olivier.dornon@nokia.com
Jayant Kotalwar
Nokia
701 East Middlefield Rd.
Mountain View, CA 94043
United States of America
Email: jayant.kotalwar@nokia.com
Venu Hemige
Nokia
Email: vhemige@gmail.com
Ray Qiu
mistnet.io
Email: ray@mistnet.io
Jeffrey Zhang
Juniper Networks, Inc.
10 Technology Park Drive
Westford, MA 01886
United States of America
Email: zzhang@juniper.net
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