| PALS Workgroup | O. Dornon |
| Internet-Draft | J. Kotalwar |
| Intended status: Informational | Alcatel-Lucent |
| Expires: October 23, 2015 | V. Hemige |
| R. Qiu | |
| Z. Zhang | |
| Juniper Networks, Inc. | |
| April 21, 2015 |
Protocol Independent Multicast (PIM) over Virtual Private LAN Service (VPLS)
draft-ietf-pals-vpls-pim-snooping-00
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 Protocol Independent Multicast (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, Provider Edges (PEs) do not flood PIM Join/Prune messages but only generate their own and send 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 Equipment (CE) devices and useful to reduce PIM control traffic in a VPLS domain.
The document also describes PIM relay, which can be viewed as light-weight proxying, where all downstream Join/Prune messages are simply forwarded out of certain ports but not flooded to avoid triggering PIM Join suppression on CE devices.
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 [RFC2119].
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 23, 2015.
Copyright (c) 2015 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 (http://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.
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 a VPLS instance is populated dynamically by source 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 Multicast) traffic:
Multicast frames are flooded because a PE cannot know where corresponding multicast group members reside. 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:
Issue A 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 B is outside the scope of this document and discussed in [VPLS-MCAST].
While this document is in the context of VPLS, the procedures apply to regular layer-2 switches interconnected by physical connections as well, albeit this is outside of the scope of this document. In that case, the PW related concept/procedures are not applicable and that's all.
IGMP snooping procedures described in [IGMP-SNOOP] make sure that IP multicast traffic is only sent on the following:
Notice that traffic is always sent on ports that have point-to-point connections to routers ot that are attached to a LAN on which there is a router, even those on which there are no snooped group memberships, because IGMP snooping alone can not determine if there are interested receivers beyond those routers. To further restrict traffic sent to those routers, PIM snooping can be used. This document describes the procedures for PIM snooping, including the rules when both IGMP and PIM snooping are enabled in a VPLS instance, which are elaborated in sections Section 2.8 and Section 2.11.
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 how the control messages are handled (transparently flooded, selectively forwarded, or consumed and then regenerated), the procedure/process may be called Snooping or proxy in different contexts.
Unless explicitly noted, the procedures in this document are used for either PIM snooping or PIM proxying, and we will largely refer to PIM snooping in this document. The PIM proxying specific procedures 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 Section 2.4.
This document also describes PIM relay, which can be viewed as light-weight PIM proxying. Unless explicitly noted, in the rest of the document proxying implicitly includes relay as well.
This document assumes that the reader has good understanding of the PIM protocols. This document 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 text related to PIM protocol handling from the PIM RFCs, this document cross references corresponding definitions and procedures in these RFCs. Deviations in protocol handling specific to PIM snooping are specified in this document.
There are several definitions referenced in this document that are well described in the PIM RFCs [PIM-SM], [BIDIR-PIM], [PIM-DM]. The following definitions and abbreviations are used throughout this document:
Abbreviations used in the document:
Other definitions are explained in the sections where they are introduced.
PIM is a multicast routing protocol running between routers, which are CE devices in a VPLS. 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). PIM does not participate in exchange of unicast routing databases, but 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], 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 (PIM-SM [PIM-SM], PIM-SSM [PIM-SSM], and BIDIR-PIM [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 the upstream routers. The Join/Prune message can be group specific (*,G) or group and source specific (S,G).
The following rules for the correct operation of PIM snooping MUST be followed.
In PIM-SM/DM, 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. 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 VPLS with PIM snooping, two routers may forward the same multicast datagrams at the same time but each copy may reach different set of PEs, and that 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/SM snooping MUSTforward multicast traffic for an (S,G)/(*,G) not only on the ports on which they snooped Joins(S,G)/Joins(*,G), but also towards the upstream neighbor(s)). In other words, the ports on which the upstream neighbors are learnt must be added to the outgoing port list along with the ports on which Joins are snooped.
Similarly, PIM-DM snooping SHOULD make sure that asserts can be triggered (Section 2.9.3).
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).
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 as required in PIM multicast routing. It knows how to forward Join/Prunes only by looking at the Upstream Neighbor field in the Join/Prune packets.
The PE 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 PE.
In the following sub-sections, we list some considerations and observations for the implementation of PIM snooping in VPLS.
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 to 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 PEs signal the snooped states to other PEs out of band via BGP, as described in [VPLS-MCAST]. In this document, it is assumed that snooping is performed on PWs.
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 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 draft can be applied to both IPv4 and IPv6 payloads.
This document 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, and it is being removed from the PIM protocol in its ongoing advancement process in IETF. Given that, this document omits the specification relating to (*,*,RP) support.
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.
Differences between PIM snooping and relay/proxying can be summarized as the following:
+--------------------+---------------------+-----------------------+ | PIM snooping | PIM relay | PIM proxying | +====================|=====================|=======================+ | Join/Prune messages| Join/Prune messages | Join/Prune messages | | snooped and flooded| snooped; forwarded | consumed. Regenerated | | everywhere | as is out of certain| ones sent out of | | | upstream ports | certain upstream ports| +--------------------+---------------------+-----------------------+ | 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 | +--------------------+---------------------+-----------------------+
Note that the differences apply only to PIM Join/Prune messages. PIM Hello messages are snooped and flooded in all cases.
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 or 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.
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 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 a VPLS service 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 case 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/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.
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.
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 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/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 no deviation from normal expected behavior observed at the CE router receiving these messages out of order.
From the above descriptions, factors that affect the choice of snooping/relay/proxying include:
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.
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, Explicit Tracking is possible, allowing an upstream router to track all the downstream neighbors that have Join states for any (S,G) or (*,G). That has two benefits:
While full proxying can be used with or without Join Suppression on CEs and does not interfere with an upstream CE's bypass of PrunePending process, it does proxy all its downstream CEs as a single one to the upstream, removing the second benefit mentioned above.
Therefore, the general rule is that if Join Suppression is enabled on CEs then proxying or relay MUST be used and if Suppression is known to be disabled on all CEs then either snooping, relay, or proxying MAY be used while snooping or relay SHOULD be used.
An implementation MAY choose dynamic determination of 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.
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 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:
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:
[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.
Please refer to
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.
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.
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 PE very closely resembles the downstream state machine of PIM-SM routers. The downstream state consists of:
Per downstream (Port, *, G):
Per downstream (Port, *, G, N):
Per downstream (Port, S, G):
Per downstream (Port, S, G, N):
Per downstream (Port, S, G, rpt):
Per downstream (Port, S, G, rpt, 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 PEs have to maintain 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 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.
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 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.
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 on Data Forwarding Rules for the specification on how OutgoingPortList is calculated.
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(S,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).
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 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 [PIM-SM].
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 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 per (S,G,N).
A Join(*,G) or Prune(*,G) is considered "received" if the following conditions are met:
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 Assert, but that is only needed for those states with AC UpstreamPort. Note that in 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 is exactly as specified in the PIM-SM RFC [PIM-SM].
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 | - | |and || | Action | | |NumETsActive>1 || | ETExpiry(N) | | +---------------++------------+--------------+------------+
Action RxJoin(N):
Action PPTExpiry(N):
Action ETExpiry(N):
A Join(S,G) or Prune(S,G) is considered "received" if the following conditions are met:
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 Assert, but that is only needed for those states with AC UpstreamPort. See Appendix B for examples.
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 [PIM-SM][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 | - | |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) | | +---------------++------------+--------------+------------+
Action RxJoin(N):
Action PPTExpiry(N):
Action ETExpiry(N):
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].
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 for which the procedures are similar to what is defined in section 4.5.6 of [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 assert among the CE routers, such Join/Prunes need to send not only towards the upstream neighbor, but also on certain PWs as described below.
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.
The following rules apply, to both forwarded (in case of PIM relay), refresh and triggered (in case of PIM proxying) (S,G)/(*,G) Join/Prune messages.
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 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 they'll receive from each other, triggering assert.
BIDIR-PIM is a variation of PIM-SM. The main differences between PIM-SM and Bidirectional-PIM are as follows:
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 [BIDIR-PIM], parts of a BIDIR-PIM enabled network may forward traffic without exchanging Join/Prune messages, for instance between DF's 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 are gathered in Appendix A.
Whenever IGMP snooping is enabled in conjunction with PIM snooping in the same VPLS instance the PE SHOULD follow these rules:
The characteristics of PIM-DM is flood and prune behavior. Shortest path trees are built as a multicast source starts transmitting.
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):
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 2.9.1, the states (DownstreamPState) and timers (PPT and PT) are per (S,G,N,P).
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 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.
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 the full proxying case and not relay.
A PIM proxying PE consumes Join/Prune messages and regenerates PIM Join/Prune messages to be sent upstream by implementing Upstream FSM as specified in the PIM RFC. 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 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 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:
This document recommends the third option - sources traffic must be behind a router.
First we define the rules that are common to PIM-SM 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 SHOULD discard the packet, i.e., the UserDefinedPortList below SHOULD be empty.
The following general rules MUST be followed when forwarding multicast traffic in a VPLS:
Per the rules in [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)
[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 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
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 general rules above in Section 2.12
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 Section 2.12
[PIM-DM] specifies how olist(S,G) is constructed.
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an RFC.
Security considerations provided in VPLS solution documents (i.e., [VPLS-LDP] and [VPLS-BGP]) apply to this document as well.
Yetik Serbest, Suresh Boddapati co-authored earlier versions.
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.
Many members of the former L2VPN and PIM working groups have contributed to and provided valuable comments and feedback to this document, including Vach Kompella, Shane Amante, Sunil Khandekar, Rob Nath, Marc Lassere, Yuji Kamite, Yiqun Cai, Ali Sajassi, Jozef Raets, Himanshu Shah (Ciena), Himanshu Shah (Alcatel-Lucent).
| [BIDIR-PIM] | Handley, M., Kouvelas, I., Speakman, T. and L. Vicisano, "Bidirectional Protocol Independent Multicast (BIDIR-PIM)", RFC 5015, 2007. |
| [PIM-DM] | Adams, A., Nicholas, J. and W. Siadak, "Protocol Independent Multicast Version 2 - Dense Mode Specification", RFC 3973, 2005. |
| [PIM-SM] | Fenner, B., Handley, M., Holbrook, H. and I. Kouvelas, "Protocol Independent Multicast- Sparse Mode (PIM-SM): Protocol Specification (Revised)", RFC 4601, 2006. |
| [PIM-SSM] | Holbrook, H. and B. Cain, "Source-Specific Multicast for IP", RFC 4607, 2006. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, 1997. |
| [RPF-VECTOR] | Wijnands, I., Boers, A. and E. Rosen, "The Reverse Path Forwarding (RPF) Vector TLV", RFC 5496, 2009. |
| [IGMP-SNOOP] | Christensen, M., Kimball, K. and F. Solensky, "Considerations for IGMP and MLD snooping PEs", RFC 4541, 2006. |
| [VPLS-BGP] | Kompella, K. and Y. Rekhter, "Virtual Private LAN Service using BGP for Auto-Discovery and Signaling", RFC 4761, 2007. |
| [VPLS-LDP] | Lasserre, M. and V. Kompella, "Virtual Private LAN Services using LDP Signaling", RFC 4762, 2007. |
| [VPLS-MCAST] | Aggarwal, R., Kamite, Y., Fang, L. and Y. Rekhter, "Multicast in Virtual Private LAN Servoce (VPLS)", RFC 7117, 2014. |
| [VPLS-MCAST-REQ] | Kamite, Y., Wada, Y., Serbest, Y., Morin, T. and L. Fang, "Requirements for Multicast Support in Virtual Private LAN Services", RFC 5501, 2009. |
This section describes some guidelines that may be used to preserve BIDIR-PIM functionality in combination with Pim snooping.
In order to preserve BIDIR-PIM Pim snooping routers need to set up forwarding states so that :
The information needed to setup these states may be obtained by :
The BIDIR-PIM 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
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 general rules above in Section 2.12
[BIDIR-PIM] specifies how olist(G) is constructed.
Let us consider the scenario in Figure 3.
An Example Network for Triggering Assert
+------+ AC3 +------+
| PE2 |-----| CE3 |
/| | +------+
/ +------+ |
/ | |
/ | |
/PW12 | |
/ | /---\
/ |PW23 | S |
/ | \---/
/ | |
/ | |
/ | |
+------+ / +------+ |
+------+ | PE1 |/ PW13 | PE3 | +------+
| CE1 |-----| |-------------| |-----| CE4 |
+------+ AC1 +------+ +------+ AC4 +------+
|
|AC2
+------+
| CE2 |
+------+
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.
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 Upstream Neighbor(S,G) = CE3.
2. PE1 snoops on the Join(S,G) and builds forwarding states 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 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 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 is as follows:
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:
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 Upstream Neighbor(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 existing (S,G) state with an AC in
UpstreamPorts(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:
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.
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:
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:
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.
In the same network, let us assume 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 Upstream Neighbor(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
existing (*,G) state with an AC in UpstreamPorts(*,G) nor an
existing (S,G) state with an AC in UpstreamPorts(S,G)
The resulting states at the PEs is 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 Joins(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 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 CE2 sends a Join(*,G) with
UpstreamNeighbor(*,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) and though this is a PW-only join,
forwarding state is build on this Join(*,G) since PE2 has
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 OutgoingPortsList(*,G)
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 is 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.
6. In case traffic is not flowing yet (i.e. step 3 is delayed to
come 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 timer 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 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 the two
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.
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 facilitates 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 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 FLAG sgrpt prune
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 }
At PE2:
JT(PW12,*,G,CE4) = active
UpstreamNeighbors(*,G) = { CE4 }
UpstreamPorts(*,G) = { PW23 }
OutgoingPortList(*,G) = { PW23 }
JT(PW12,S,G,CE4) = JP_Holdtime with FLAG sgrpt prune
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 }
At PE3:
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 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, the reason we don't have duplicate traffic is because one of the CEs stops sending traffic due to assert, not because we don't have any forwarding state in the PEs to do this forwarding.