Multicast in Virtual Private LAN Service (VPLS)

Abstract

RFCs 4761 and 4762 describe a solution for Virtual Private LAN Service (VPLS) multicast that relies on the use of point-to-point or multipoint-to-point unicast Label Switched Paths (LSPs) for carrying multicast traffic. This solution has certain limitations for certain VPLS multicast traffic profiles. For example, it may result in highly non-optimal bandwidth utilization when a large amount of multicast traffic is to be transported.

This document describes solutions for overcoming a subset of the limitations of the existing VPLS multicast solution. It describes procedures for VPLS multicast that utilize multicast trees in the service provider (SP) network. The solution described in this document allows sharing of one such multicast tree among multiple VPLS instances. Furthermore, the solution described in this document allows a single multicast tree in the SP network to carry traffic belonging only to a specified set of one or more IP multicast streams from one or more VPLS instances.

Status of This Memo

This is an Internet Standards Track document.

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). Further information on Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7117.

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Table of Contents

   1. Introduction ....................................................4
   2. Terminology .....................................................5
      2.1. Specification of Requirements ..............................6
   3. Overview ........................................................6
      3.1. Inclusive and Selective Multicast Trees ....................7
      3.2. BGP-Based VPLS Membership Auto-discovery ...................8
      3.3. IP Multicast Group Membership Discovery ....................8
      3.4. Advertising P-Multicast Tree to VPLS/C-Multicast Binding ...9
      3.5. Aggregation ...............................................10
      3.6. Inter-AS VPLS Multicast ...................................11
   4. Intra-AS Inclusive P-Multicast Tree Auto-discovery/Binding .....12
      4.1. Originating Intra-AS VPLS A-D Routes ......................13
      4.2. Receiving Intra-AS VPLS A-D Routes ........................14
   5. Demultiplexing P-Multicast Tree Traffic ........................15
      5.1. One P-Multicast Tree - One VPLS Mapping ...................15
      5.2. One P-Multicast Tree - Many VPLS Mapping ..................15
   6. Establishing P-Multicast Trees .................................16
      6.1. Common Procedures .........................................16
      6.2. RSVP-TE P2MP LSPs .........................................16
           6.2.1. P2MP TE LSP - VPLS Mapping .........................17
      6.3. Receiver-Initiated P2MP LSP ...............................18
           6.3.1. P2MP LSP - VPLS Mapping ............................18
      6.4. Encapsulation of Aggregate P-Multicast Trees ..............18
   7. Inter-AS Inclusive P-Multicast Tree A-D/Binding ................18
      7.1. VSIs on the ASBRs .........................................19
           7.1.1. Option (a): VSIs on the ASBRs ......................19
           7.1.2. Option (e): VSIs on the ASBRs ......................20
      7.2. Option (b) - Segmented Inter-AS Trees .....................20
           7.2.1. Segmented Inter-AS Trees VPLS Inter-AS
                  A-D/Binding ........................................20
           7.2.2. Propagating BGP VPLS A-D Routes to Other
                  ASes: Overview .....................................21
                  7.2.2.1. Propagating Intra-AS VPLS A-D
                           Routes in EBGP ............................23
                  7.2.2.2. Inter-AS A-D Route Received via EBGP ......23
                  7.2.2.3. Leaf A-D Route Received via EBGP ..........25
                  7.2.2.4. Inter-AS A-D Route Received via IBGP ......25
      7.3. Option (c): Non-segmented Tunnels .........................26
   8. Optimizing Multicast Distribution via Selective Trees ..........27
      8.1. Protocol for Switching to Selective Trees .................29
      8.2. Advertising (C-S, C-G) Binding to a Selective Tree ........30
      8.3. Receiving S-PMSI A-D Routes by PEs ........................32
      8.4. Inter-AS Selective Tree ...................................34
           8.4.1. VSIs on the ASBRs ..................................35
                  8.4.1.1. VPLS Inter-AS Selective Tree A-D Binding ..35
   
           8.4.2. Inter-AS Segmented Selective Trees .................35
                  8.4.2.1. Handling S-PMSI A-D Routes by ASBRs .......36
                           8.4.2.1.1. Merging Selective Tree
                                      into an Inclusive Tree .........37
           8.4.3. Inter-AS Non-segmented Selective Trees .............38
   9. BGP Extensions .................................................38
      9.1. Inclusive Tree/Selective Tree Identifier ..................38
      9.2. MCAST-VPLS NLRI ...........................................39
           9.2.1. S-PMSI A-D Route ...................................40
           9.2.2. Leaf A-D Route .....................................41
   10. Aggregation Considerations ....................................41
   11. Data Forwarding ...............................................43
      11.1. MPLS Tree Encapsulation ..................................43
           11.1.1. Mapping Multiple VPLS Instances to a P2MP LSP .....43
           11.1.2. Mapping One VPLS Instance to a P2MP LSP ...........44
   12. VPLS Data Packet Treatment ....................................45
   13. Security Considerations .......................................46
   14. IANA Considerations ...........................................47
   15. References ....................................................47
      15.1. Normative References .....................................47
      15.2. Informative References ...................................48
   16. Acknowledgments ...............................................50

1. Introduction

[RFC4761] and [RFC4762] describe a solution for VPLS multicast/broadcast that relies on the use of pseudowires transported over unicast point-to-point (P2P) RSVP Traffic Engineering (RSVP-TE) or multipoint-to-point (MP2P) LDP Label Switched Paths (LSPs) ([RFC3209] [RFC5036]). In this document, we refer to this solution as "ingress replication".

With ingress replication, when an ingress Provider Edge (PE) of a given VPLS instance receives a multicast/broadcast packet from one of the Customer Edges (CEs) that belong to that instance, the ingress PE replicates the packet for each egress PE that belong to that instance, and it sends the packet to each such egress PE using unicast tunnels.

The solution based on ingress replication has certain limitations for certain VPLS multicast/broadcast traffic profiles. For example, it may result in highly non-optimal bandwidth utilization in the MPLS network when a large amount of multicast/broadcast traffic is to be transported (for more see [RFC5501]).

Ingress replication may be an acceptable model when the bandwidth of the multicast/broadcast traffic is low and/or there is a small number of replications performed on each outgoing interface for a particular VPLS customer multicast stream. If this is not the case, it is desirable to utilize multicast trees in the SP network to transmit VPLS multicast and/or broadcast packets [RFC5501].

This document describes procedures for overcoming the limitations of existing VPLS multicast solutions. It describes procedures for using MPLS point-to-multipoint (P2MP) LSPs in the SP network to transport VPLS multicast and/or broadcast packets, where these LSPs are signaled by either P2MP RSVP-TE [RFC4875] or Multipoint LDP (mLDP) [RFC6388].

The procedures described in this document are applicable to both [RFC4761] and [RFC4762].

2. Terminology

This document uses terminology described in [RFC4761] and [RFC4762].

In this document, we refer to various auto-discovery routes, as "A-D routes".

This document uses the prefix 'C' to refer to the customer control or data packets and 'P' to refer to the provider control or data packets. An IP (multicast source, multicast group) tuple is abbreviated to (S, G).

An "Inclusive tree" is a single multicast distribution tree in the SP network that carries all the multicast traffic from one VPLS instance on a given PE.

An "Aggregate Inclusive tree" is a single multicast distribution tree in the SP network that carries all the multicast traffic from more than one VPLS instance on a given PE.

A "Selective tree" is a single multicast distribution tree in the SP network that carries multicast traffic belonging only to a specified set of IP multicast streams, and all these streams belong to the same VPLS instance on a given PE. A Selective tree differs from an Inclusive tree in that it may reach a subset of the PEs reached by an Inclusive tree.

An "Aggregate Selective tree" is a single multicast distribution tree in the SP network that carries multicast traffic belonging only to a specified set of IP multicast streams, and all these streams belong to more than one VPLS instance on a given PE.

2.1. Specification of Requirements

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].

3. Overview

Procedures described in this document provide mechanisms that allow a single multicast distribution tree in the SP network to carry all the multicast traffic from one or more VPLS sites connected to a given PE, irrespective of whether these sites belong to the same or different VPLS instances. We refer to such a tree as an "Inclusive tree" if it carries multicast traffic from one VPLS instance on a given PE. We refer to such a tree as an "Aggregate Inclusive tree" if it carries multicast traffic from more than one VPLS instance on a given PE. See the "Inclusive and Selective Multicast Trees" section for further discussion on Inclusive trees.

To further improve bandwidth utilization for IP multicast streams, this document also provides procedures by which a single multicast distribution tree in the SP network can be used to carry traffic belonging only to a specified set of IP multicast streams, originated in one or more VPLS sites connected to a given PE, irrespective of whether these sites belong to the same or different VPLS instances. We refer to such a tree as a "Selective tree" if it carries the IP multicast stream(s) that belongs to the same VPLS instance on a given PE. We refer to such a tree as an "Aggregate Selective tree" if it carries the IP multicast streams that belong to different VPLS instances on a given PE. Use of Selective and/or Aggregate Selective trees allows multicast traffic, by default, to be carried on an Inclusive tree, while traffic from some specific IP multicast streams, e.g., high-bandwidth streams, could be carried on one of the Selective trees. See the "Inclusive and Selective Multicast Trees" section for further discussion on Selective trees.

Note that this document covers the use of Selective trees only for carrying IP multicast streams. Any other use of such trees is outside the scope of this document.

Unicast packets destined to unknown Media Access Control (MAC) addresses (i.e., not learned yet at the ingress PE) in a given VPLS instance are flooded to remote PEs participating in the same VPLS instance. This flooding MAY still use ingress replication (as specified in [RFC4761] and [RFC4762]), or MAY use the procedures defined in this document to optimize flooding across the SP core.

While the use of multicast trees in the SP network can be beneficial when the bandwidth of the multicast traffic is high, or when it is desirable to optimize the number of copies of a multicast packet transmitted on a given link, this benefit comes at a cost of state in the SP network to build multicast trees and overhead to maintain this state.

3.1. Inclusive and Selective Multicast Trees

Multicast trees used for VPLS can be of two types:

• Inclusive trees. This option supports the use of a single multicast distribution tree, referred to as an "Inclusive P-multicast tree", in the SP network to carry all the multicast traffic from a specified set of VPLS sites connected to a given PE. There is no assumption made with respect to whether or not this traffic is IP encapsulated. A particular P-multicast tree can be set up to carry the traffic originated by sites belonging to a single VPLS instance or to carry the traffic originated by sites belonging to different VPLS instances. In the context of this document, the ability to carry the traffic of more than one VPLS instance on the same P-multicast tree is called "aggregation". The tree includes every PE that is a member of any of the VPLS instances that are using the tree. This implies that a PE may receive multicast traffic for a multicast stream even if it doesn't have any receivers that are interested in receiving traffic for that stream.

An Inclusive P-multicast tree, as defined in this document, is a P2MP tree. Thus, a P2MP tree is used to carry traffic only from VPLS sites that are connected to the PE that is the root of the tree.

• Selective trees. A Selective P-multicast tree is used by a PE to send IP multicast traffic for one or more specific IP multicast streams, received by the PE over PE-CE interfaces that belong to the same or different VPLS instances, to a subset of the PEs that belong to those VPLS instances. Each of the PEs in the subset should be on the path to a receiver of one or more multicast streams that are mapped onto the tree. In the context of this document, the ability to use the same P-multicast tree for multicast streams that belong to different VPLS instances is called "aggregation". The reason for having Selective P-multicast trees is to provide a PE the ability to create separate SP multicast trees for specific multicast streams, e.g., high-bandwidth multicast streams. This allows traffic for these multicast streams to reach only those PE routers that have receivers for these streams. This avoids flooding other PE routers in the VPLS instance.

An SP can use both Inclusive P-multicast trees and Selective P-multicast trees or either of them for a given VPLS on a PE, based on local configuration. Inclusive P-multicast trees can be used for both IP and non-IP data multicast traffic, while Selective P-multicast trees, as previously stated, must be used only for IP multicast data traffic. The use of Selective P-multicast trees for non-IP multicast traffic is outside the scope of this document.

P-multicast trees in the SP network can be realized via a variety of technologies. For both Inclusive and Selective P-multicast trees, these technologies include P2MP LSPs created by RSVP-TE or mLDP. This document also describes the data plane encapsulations for supporting these technologies. Other technologies for realizing P-multicast trees are outside the scope of this document.

3.2. BGP-Based VPLS Membership Auto-discovery

Inclusive P-multicast trees may be established for one or more VPLS instances. In this case, aggregation can be performed (using either mLDP or P2MP RSVP-TE as the tunneling technology) or simple tunneling can be performed (using P2MP RSVP-TE tunneling). If either of these approaches is used, the PE acting as the root of a P2MP LSP must be able to discover the other PEs that have membership of each of the VPLS instances. Once the root PE discovers these other PEs, it includes them as leaves in the P-multicast tree (i.e., P2MP LSP). This document uses the BGP-based procedures described in [RFC4761] and [RFC6074] for discovering the VPLS membership of all PEs. For more on aggregation, see the "Aggregation Considerations" section. When no aggregation is performed and the tunneling technology is mLDP, then the root of the P2MP LSP need not discover the other PEs that are the leaves of that LSP tree.

The leaves of the Inclusive P-multicast tree must also be able to auto-discover the identifier of the tree (note that this applies when the tree is established by either mLDP or P2MP RSVP-TE). Procedures to accomplish this are described in the "Advertising P-Multicast Tree to VPLS/C-Multicast Binding" section.

3.3. IP Multicast Group Membership Discovery

The setup of a Selective P-multicast tree for one or more IP multicast (C-S, C-G)s, requires the ingress PE to learn the PEs that have receivers in one or more of these (C-S, C-G)s, in the following cases:

• When aggregation is used (with either mLDP or P2MP RSVP-TE as the tunneling technology), OR

• When the tunneling technology is P2MP RSVP-TE

• If ingress replication is used and the ingress PE wants to send traffic for (C-S, C-G)s to only those PEs that are on the path to receivers for the (C-S, C-G)s.

For more on aggregation, see the "Aggregation Considerations" section.

For discovering the IP multicast group membership, this document describes procedures that allow an ingress PE to enable explicit tracking of IP multicast membership. Thus, an ingress PE can request the IP multicast membership from egress PEs for one or more C-multicast streams. These procedures are described in the "Optimizing Multicast Distribution via Selective Trees" section.

These procedures are applicable when IGMP ([RFC2236] [RFC3376]) or MLD ([RFC2710] [RFC3810]) is used as the multicast signaling protocol between the VPLS CEs. They are also applicable when PIM ([RFC4601]) in either the Any-Source Multicast (ASM) or the Source-Specific Multicast (SSM) service model is used as the multicast routing protocol between the VPLS CEs, and PIM join suppression is disabled on all the CEs.

However, these procedures do not apply when PIM is used as the multicast routing protocol between the VPLS CEs and PIM join suppression is not disabled on all the CEs. This is because when PIM join suppression is not disabled on all the CEs, PEs connected to these CEs can not rely on PIM to determine IP multicast membership of the receivers behind these CEs. Procedures for this case are outside the scope of this document.

The leaves of the Selective P-multicast trees must also be able to discover the identifier of these trees. Procedures to accomplish this are described in the "Advertising P-Multicast Tree to VPLS/C-Multicast Binding" section.

3.4. Advertising P-Multicast Tree to VPLS/C-Multicast Binding

This document describes procedures based on BGP VPLS Auto-Discovery (A-D) routes ([RFC4761] [RFC6074]) that are used by the root of an Aggregate P-multicast tree to advertise the Inclusive or Selective P-multicast tree binding and the demultiplexing information to the leaves of the tree. This document uses the Provider Multicast Service Interface (PMSI) Tunnel attribute defined [RFC6514] for this purpose.

Once an ingress PE decides to bind a set of VPLS instances or customer multicast groups to an Inclusive P-multicast tree or a Selective P-multicast tree, the PE needs to announce this binding to other PEs in the network. This procedure is referred to as "Inclusive P-multicast tree binding distribution" or "Selective P-multicast tree binding distribution" and is performed using BGP. The decision to bind a set of VPLS instances or customer multicast groups is a local matter to the ingress, and is controlled via provisioning/configuration on that PE.

When an Aggregated Inclusive P-multicast tree is used by an ingress PE, this binding distribution implies that (a) an ingress PE MUST announce the binding of all VPLS instances bound to the Inclusive P-multicast tree and (b) other PEs that have these instances receive these announcements. The inner label assigned by the ingress PE for each VPLS MUST be included if more than one VPLS is bound to the same P-multicast tree. The Inclusive P-multicast tree Identifier MUST be included.

For a Selective P-multicast tree, this binding distribution implies announcing all the specific <C-S, C-G> entries bound to this P-multicast tree along with the Selective P-multicast tree Identifier. The inner label assigned for each <C-S, C-G> MUST be included if <C-S, C-G> from different VPLS instances are bound to the same P-multicast tree. The labels MUST be distinct on a per-VPLS basis and MAY be distinct per <C-S, C-G> entry. The Selective P-multicast tree Identifier MUST be included.

3.5. Aggregation

As described earlier in this document, the ability to carry the traffic of more than one VPLS on the same P-multicast tree is called aggregation.

Aggregation enables the SP to place a bound on the amount of multicast tree forwarding and control plane state that the P-routers must have. Let us call the number of VPLS instances aggregated onto a single P-multicast tree the "Aggregation Factor". When Inclusive source P-multicast trees are used, the number of trees that a PE is the root of is proportional to the number of VPLS instances on the PE divided by the Aggregation Factor.

In this case, the state maintained by a P-router is proportional to:

        AveVPLS            NPE
        -------    X    --------
          Aggr          AvePTree

Where:

AveVPLS is the average number of VPLS instances on a PE

Aggr is the Aggregation Factor

NPE is the number of PEs

AvePTree is the average number of P-multicast that transit a given P-router

Thus, the state does not grow linearly with the number of VPLS instances.

Aggregation requires a mechanism for the egresses of the P-multicast tree to demultiplex the multicast traffic received over the P-multicast tree. To enable the egress nodes to perform this demultiplexing, upstream-assigned labels [RFC5331] MUST be assigned and distributed by the root of the aggregate P-multicast tree.

Aggregation procedures would require two MPLS labels in the label stack. This does not introduce any new implications on MTU, as even VPLS multicast supported by ingress replication requires two MPLS labels in the label stack.

3.6. Inter-AS VPLS Multicast

This document defines four models of inter-AS (Autonomous System) VPLS service, referred here as options (a), (b), (c), and (e). Options (a), (b), and (c) defined in this document are very similar to methods (a), (b), and (c), described in the "Multi-AS VPLS" section of [RFC4761], which in turn extends the concepts of [RFC4364] to inter-AS VPLS.

For option (a) and option (b) support, this document specifies a model where inter-AS VPLS service can be offered without requiring a single P-multicast tree to span multiple ASes. There are two variants of this model, and they are described in the "Inter-AS Inclusive P-Multicast Tree A-D/Binding" section.

For option (c) support, this document specifies a model where inter- AS VPLS service is offered by requiring a single P-multicast tree to span multiple ASes. This is because in the case of option (c), the Autonomous System Border Routers (ASBRs) do not exchange BGP-VPLS Network Layer Reachability Information (NLRI) or A-D routes.

In addition to options (a), (b), and (c), this document also specifies option (e), which one may think of as a variant of option (a).

For more on these inter-AS options, see the "Inter-AS Inclusive P-Multicast Tree A-D/Binding" section.

4. Intra-AS Inclusive P-Multicast Tree Auto-discovery/Binding

This section specifies procedures for the intra-AS auto-discovery of VPLS membership and the distribution of information used to instantiate P-multicast Tunnels.

VPLS auto-discovery/binding consists of two components: intra-AS and inter-AS. The former provides VPLS auto-discovery/binding within a single AS. The latter provides VPLS auto-discovery/binding across multiple ASes. Inter-AS auto-discovery/binding is described in the "Inter-AS Inclusive P-Multicast Tree A-D/Binding" section.

VPLS auto-discovery using BGP, as described in [RFC4761] and [RFC6074], enables a PE to learn the VPLS instance membership of other PEs. A PE that belongs to a particular VPLS instance announces a BGP NLRI that identifies the Virtual Switch Instance (VSI). This NLRI is constructed from the <Route Distinguisher (RD), VPLS Edge Device Identifier (VE-ID)> tuple. The NLRI defined in [RFC4761] comprises the <RD, VE-ID> tuple and label blocks for pseudowire (PW) signaling. The VE-ID in this case is a two-octet number encoded in the VE-ID of NLRI defined in [RFC4761]. The NLRI defined in [RFC6074] comprises only the <RD, PE_addr>. The VE-ID in this case is a four-octet number encoded in the PE_addr of the NLRI defined in [RFC6074].

The procedures for constructing Inclusive Intra-AS and Inter-AS trees, as specified in this document, require the BGP A-D NLRI to carry only the <RD, VE-ID>. Hence, these procedures can be used for both BGP-VPLS and LDP-VPLS with BGP A-D.

It is to be noted that BGP A-D is an inherent feature of BGP-VPLS. However, it is not an inherent feature of LDP-VPLS. In fact, there are deployments and/or implementations of LDP-VPLS that require configuration to enable a PE in a particular VPLS to determine other PEs in the VPLS and exchange PW labels using Forwarding Equivalence

Class (FEC) 128 (PWid FEC) [RFC4447]. The use of BGP A-D for LDP- VPLS [RFC6074], to enable automatic setup of PWs, requires FEC 129 (Generalized PWid FEC) [RFC4447]. However, FEC 129 is not required in order to use procedures in this document for LDP-VPLS. An LDP- VPLS implementation that supports this document MUST support the BGP A-D procedures to set up P-multicast trees, as described here, and it MAY support FEC 129 to automate the signaling of PWs.

4.1. Originating Intra-AS VPLS A-D Routes

To participate in the VPLS auto-discovery/binding, a PE router that has a given VSI of a given VPLS instance originates a BGP VPLS Intra- AS A-D route and advertises this route in Multiprotocol (MP) IBGP. The route is constructed as described in [RFC4761] and [RFC6074].

The route carries a single Layer 2 Virtual Private Network (L2VPN) NLRI with the RD set to the RD of the VSI and the VE-ID set to the VE-ID of the VSI. The route also carries one or more Route Targets (RTs), as specified in [RFC4761] and [RFC6074].

If an Inclusive P-multicast tree is used to instantiate the provider tunnel for VPLS multicast on the PE, the advertising PE MUST advertise the type and the identity of the P-multicast tree in the PMSI Tunnel attribute. This attribute is described in the "Inclusive Tree/Selective Tree Identifier" section.

A PE that uses an Inclusive P-multicast tree to instantiate the provider tunnel MAY aggregate two or more VPLS instances present on the PE onto the same tree. If the PE decides to perform aggregation after it has already advertised the intra-AS VPLS A-D routes for these VPLS instances, then aggregation requires the PE to re-advertise these routes. The re-advertised routes MUST be the same as the original ones, except for the PMSI Tunnel attribute (the re-advertised route will replace the previously advertised route). If the PE has not previously advertised Intra-AS A-D routes for these VPLS instances, then the aggregation requires the PE to advertise (new) Intra-AS A-D routes for these VPLS instances. The PMSI Tunnel attribute in the newly advertised/re-advertised routes MUST carry the identity of the P-multicast tree that aggregates the VPLS instances as well as an MPLS upstream-assigned label [RFC5331]. Each re-advertised or newly advertised route MUST have a label that is distinct within the scope of the PE that advertises the route.

Discovery of PE capabilities in terms of what tunnel types they support is outside the scope of this document. Within a given AS, PEs participating in a VPLS are expected to advertise tunnel bindings whose tunnel types are supported by all other PEs that are participating in this VPLS and are part of the same AS.

4.2. Receiving Intra-AS VPLS A-D Routes

When a PE receives a BGP Update message that carries an Intra-AS A-D route such that (a) the route was originated by some other PE within the same AS as the local PE, (b) at least one of the RTs of the route matches one of the import RTs configured for a particular VSI on the local PE, (c) the BGP route selection determines that this is the best route with respect to the NLRI carried by the route, and (d) the route carries the PMSI Tunnel attribute, the PE performs the following:

• If the Tunnel Type in the PMSI Tunnel attribute is set to LDP P2MP LSP, the PE SHOULD join the P-multicast tree whose identity is carried in the PMSI Tunnel attribute.

• If the Tunnel Type in the PMSI Tunnel attribute is set to RSVP-TE P2MP LSP, the receiving PE has to establish the appropriate state to properly handle the traffic received over that LSP. The PE that originated the route MUST establish an RSVP-TE P2MP LSP with the local PE as a leaf. This LSP MAY have been established before the local PE receives the route.

• If the PMSI Tunnel attribute does not carry a label, then all packets that are received on the P-multicast tree, as identified by the PMSI Tunnel attribute, are forwarded using the VSIs that have at least one of their import RTs that matches one of the RTs of the received A-D route.

• If the PMSI Tunnel attribute has the Tunnel Type set to LDP P2MP LSP or RSVP-TE P2MP LSP, and the attribute also carries an MPLS label, then the egress PE MUST treat this as an upstream-assigned label, and all packets that are received on the P-multicast tree, as identified by the PMSI Tunnel attribute, with that upstream label are forwarded using the VSIs that have at least one of their import RTs that matches one of the RTs of the received Intra-AS A-D route.

Furthermore, if the local PE uses RSVP-TE P2MP LSP for sending (multicast) traffic, originated by VPLS sites connected to the PE, to the sites attached to other PEs, then the local PE MUST use the Originating Router's IP Address information carried in the Intra-AS A-D route to add the PE, that originated the route, as a leaf node to the LSP. This MUST be done irrespective of whether or not the received Intra-AS A-D route carries the PMSI Tunnel attribute.

5. Demultiplexing P-Multicast Tree Traffic

Demultiplexing received VPLS traffic requires the receiving PE to determine the VPLS instance to which the packet belongs. The egress PE can then perform a VPLS lookup to further forward the packet. It also requires the egress PE to determine the identity of the ingress PE for MAC learning, as described in the "VPLS Data Packet Treatment" section.

5.1. One P-Multicast Tree - One VPLS Mapping

When a P-multicast tree is mapped to only one VPLS, determining the tree on which the packet is received is sufficient to determine the VPLS instance on which the packet is received. The tree is determined based on the tree encapsulation. If MPLS encapsulation is used, e.g., RSVP-TE P2MP LSPs, the outer MPLS label is used to determine the tree. Penultimate Hop Popping (PHP) MUST be disabled on the MPLS LSP (RSVP-TE P2MP LSP or mLDP P2MP LSP).

5.2. One P-Multicast Tree - Many VPLS Mapping

As traffic belonging to multiple VPLS instances can be carried over the same tree, there is a need to identify the VPLS to which the packet belongs. This is done by using an inner label that determines the VPLS for which the packet is intended. The ingress PE uses this label as the inner label while encapsulating a customer multicast data packet. Each of the egress PEs must be able to associate this inner label with the same VPLS and use it to demultiplex the traffic received over the Aggregate Inclusive tree or the Aggregate Selective tree.

If traffic from multiple VPLS instances is carried on a single tree, upstream-assigned labels [RFC5331] MUST be used. Hence, the inner label is assigned by the ingress PE. When the egress PE receives a packet over an Aggregate tree, the outer encapsulation (in the case of MPLS P2MP LSPs, the outer MPLS label) specifies the label space to perform the inner-label lookup. The same label space MUST be used by the egress PE for all P-multicast trees that have the same root [RFC5331].

If the tree uses MPLS encapsulation, as in RSVP-TE P2MP LSPs, the outer MPLS label and, optionally, the incoming interface provide the label space of the label beneath it. This assumes that PHP is disabled. The egress PE MUST NOT advertise IMPLICIT NULL or EXPLICIT NULL for that tree once it is known to the egress PE that the tree is bound to one or more VPLS instances. Once the label representing the tree is popped off the MPLS label stack, the next label is the demultiplexing information that allows the proper VPLS instance to be determined.

The ingress PE informs the egress PEs about the inner label as part of the tree binding procedures described in the "BGP Extensions" section.

6. Establishing P-Multicast Trees

This document supports only P2MP P-multicast trees wherein it is possible for egress PEs to identify the ingress PE to perform MAC learning. Specific procedures are identified only for RSVP-TE P2MP LSPs and mLDP P2MP LSPs. An implementation that supports this document MUST support RSVP-TE P2MP LSPs and mLDP P2MP LSPs.

6.1. Common Procedures

The following procedures apply to both RSVP-TE P2MP and mLDP P2MP LSPs.

Demultiplexing the C-multicast data packets at the egress PE requires that the PE must be able to determine the P2MP LSP on which the packets are received. This enables the egress PE to determine the VPLS instances to which the packet belongs. To achieve this, the LSP MUST be signaled with PHP off and a non-special purpose MPLS label off as described in the "Demultiplexing P-Multicast Tree Traffic" section. In other words, an egress PE MUST NOT advertise IMPLICIT NULL or EXPLICIT NULL for a P2MP LSP that is carrying traffic for one or more VPLS instances. This is because the egress PE needs to rely on the MPLS label, that it advertises to its upstream neighbor, to determine the P2MP LSP on which a C-multicast data packet is received.

The egress PE also needs to identify the ingress PE to perform MAC learning. When P2MP LSPs are used as P2MP trees, determining the P2MP LSP on which the packets are received is sufficient to determine the ingress PE. This is because the ingress PE is the root of the P2MP LSP.

The egress PE relies on receiving the PMSI Tunnel attribute in BGP to determine the VPLS instance to P2MP LSP mapping.

6.2. RSVP-TE P2MP LSPs

This section describes procedures that are specific to the usage of RSVP-TE P2MP LSPs for instantiating a P-multicast tree. Procedures in [RFC4875] are used to signal the P2MP LSP. The LSP is signaled as the root of the P2MP LSP discovers the leaves. The egress PEs are discovered using the procedures described in the "Intra-AS Inclusive P-Multicast Tree Auto-discovery/Binding" section. Aggregation, as described in this document, is supported.

6.2.1. P2MP TE LSP - VPLS Mapping

P2MP TE LSP to VPLS mapping is learned at the egress PEs using BGP- based advertisements of the P2MP TE LSP - VPLS mapping. They require that the root of the tree include in the BGP advertisements the P2MP TE LSP identifier as the P-multicast tree identifier. This P-multicast tree identifier contains the following information elements:

- The type of the tunnel is set to RSVP-TE P2MP LSP - RSVP-TE P2MP LSP's SESSION Object

See the "Inclusive Tree/Selective Tree Identifier" section for more details on how this tree identifier is carried in BGP advertisements.

Once the egress PE receives the P2MP TE LSP to VPLS mapping:

• If the egress PE already has RSVP-TE state for the P2MP TE LSP, it MUST begin to assign an MPLS label from the non-special purpose label range, for the P2MP TE LSP and signal this to the previous hop of the P2MP TE LSP. Further, it MUST create forwarding state to forward packets received on the P2MP LSP.

• If the egress PE does not have RSVP-TE state for the P2MP TE LSP, it MUST retain this mapping. Subsequently, when the egress PE receives the RSVP-TE P2MP signaling message, it creates the RSVP- TE P2MP LSP state. It MUST then assign an MPLS label from the non-reserved label range, for the P2MP TE LSP, and signal this to the previous hop of the P2MP TE LSP.

Note that if the signaling to set up an RSVP-TE P2MP LSP is completed before a given egress PE learns, via a PMSI Tunnel attribute, of the VPLS or set of VPLS instances to which the LSP is bound, the PE MUST discard any traffic received on that LSP until the binding is received. In order for the egress PE to be able to discard such traffic, it needs to know that the LSP is associated with one or more VPLS instances and that the VPLS A-D route that binds the LSP to a VPLS has not yet been received. This is provided by extending [RFC4875] with [RFC6511].

6.3. Receiver-Initiated P2MP LSP

Receiver-initiated P2MP LSPs can also be used. The mLDP procedures ([RFC6388]) MUST be used to signal such LSPs. The LSP is signaled once the leaves receive the LDP FEC for the tree from the root, as described in the "Intra-AS Inclusive P-Multicast Tree Auto- discovery/Binding" section. When aggregation is used, an ingress PE is required to discover the egress PEs (see the "Aggregation Considerations" section for the rationale), and this is achieved using the procedures in the "Intra-AS Inclusive P-Multicast Tree Auto-discovery/Binding" section.

6.3.1. P2MP LSP - VPLS Mapping

P2MP LSP to VPLS mapping is learned at the egress PEs using BGP-based advertisements of the P2MP LSP - VPLS mapping. They require that the root of the tree include in the BGP advertisements the P2MP LSP identifier as the P-multicast tree identifier. This P-multicast tree identifier contains the following information elements:

- The type of the tunnel is set to LDP P2MP LSP
- LDP P2MP FEC that includes an identifier generated by the root.

See the "Inclusive Tree/Selective Tree Identifier" section for more details on how this tree identifier is carried in BGP advertisements.

Each egress PE SHOULD "join" the P2MP MPLS tree by sending LDP label mapping messages for the LDP P2MP FEC, that was learned in the BGP advertisement, using procedures described in [RFC6388].

6.4. Encapsulation of Aggregate P-multicast Trees

An Aggregate Inclusive P-multicast tree or an Aggregate Selective P-multicast tree MUST use MPLS encapsulation, as described in [RFC5332].

7. Inter-AS Inclusive P-Multicast Tree A-D/Binding

As stated earlier, this document defines four models of inter-AS VPLS service, referred here as option (a), (b), (c), and (e). This section contains procedures to support these models.

For supporting option (a), (b), and (e), this section specifies a model where inter-AS VPLS service can be offered without requiring a single P-multicast tree to span multiple ASes. This allows individual ASes to potentially use different P-tunneling technologies. There are two variants of this model. One that requires MAC lookup on the ASBRs and applies to option (a) and (e). The other is one that does not require MAC lookup on the ASBRs, and instead it builds segmented Inter-AS Inclusive or Selective trees. This applies only to option (b).

For supporting option (c), this section specifies a model where Inter-AS VPLS service is offered by requiring a single Inclusive P-multicast tree to span multiple ASes. This is referred to as a "non-segmented P-multicast tree". This is because in the case of option (c), the ASBRs do not exchange BGP-VPLS NLRIs or VPLS A-D routes. Support for Inter-AS Selective trees for option (c) may be segmented or non-segmented.

An implementation MUST support options (a), (b), and (c), and MAY support option (e). When there are multiple ways for implementing one of these options, this section specifies which one is mandatory.

7.1. VSIs on the ASBRs

When VSIs are configured on ASBRs, the ASBRs MUST perform a MAC lookup, in addition to any MPLS lookups, to determine the forwarding decision on a VPLS packet. The P-multicast trees are confined to an AS. An ASBR on receiving a VPLS packet from another ASBR is required to perform a MAC lookup to determine how to forward the packet. Thus, an ASBR is required to keep a VSI for the VPLS instance and MUST be configured with its own VE-ID for the VPLS instance. The BGP VPLS A-D routes generated by PEs in an AS MUST NOT be propagated outside the AS.

7.1.1. Option (a): VSIs on the ASBRs

In option (a), an ASBR acts as a PE for the VPLSs that span the AS of the ASBR and an AS to which the ASBR is connected. The local ASBR views the ASBR in the neighboring AS as a CE connected to it by a link with separate VLAN sub-interfaces for each such VPLS. Similarly, the ASBR in the neighboring AS acts as a PE for such VPLS from the neighboring AS's point of view, and views the local ASBR as a CE.

The local ASBR uses a combination of the incoming link and a particular VLAN sub-interface on that link to determine the VSI for the packets it receives from the ASBR in the neighboring AS.

In option (a), the ASBRs do not exchange VPLS A-D routes.

An implementation MUST support option (a).

7.1.2. Option (e): VSIs on the ASBRs

The VSIs on the ASBRs scheme can be used such that the interconnect between the ASBRs is a PW and MPLS encapsulation is used between the ASBRs. An ASBR in one AS determines the VSI for packets received from an adjoining ASBR in another AS based on the incoming MPLS PW label. This is referred to as "option (e)". The only VPLS A-D routes that are propagated outside the AS are the ones originated by ASBRs. This MPLS PW connects the VSIs on the ASBRs and MUST be signaled using the procedures defined in [RFC4761] or [RFC4762].

The P-multicast trees for a VPLS are confined to each AS and the VPLS auto-discovery/binding MUST follow the intra-AS procedures described in the "Demultiplexing P-Multicast Tree Traffic" section.

An implementation MAY support option (e).

7.2. Option (b) - Segmented Inter-AS Trees

In this model, an inter-AS P-multicast tree, rooted at a particular PE for a particular VPLS instance, consists of a number of "segments", one per AS, which are stitched together at ASBRs. These are known as "segmented inter-AS trees". Each segment of a segmented inter-AS tree may use a different multicast transport technology. In this model, an ASBR is not required to keep a VSI for the VPLS instance, and is not required to perform a MAC lookup in order to forward the VPLS packet. This implies that an ASBR is not required to be configured with a VE-ID for the VPLS.

An implementation MUST support option (b) using this model.

The construction of segmented inter-AS trees requires the BGP-VPLS A-D NLRI described in [RFC4761] and [RFC6074]. A BGP VPLS A-D route for an <RD, VE-ID> tuple advertised outside the AS, to which the originating PE belongs, will be referred to as an "Inter-AS VPLS A-D route" (though this route is originated by a PE as an intra-AS route, and is referred to as an "inter-AS route outside the AS").

In addition to this, segmented inter-AS trees require support for the PMSI Tunnel attribute described in the "Inclusive Tree/Selective Tree Identifier" section. They also require additional procedures in BGP to signal leaf A-D routes between ASBRs as explained in subsequent sections.

7.2.1. Segmented Inter-AS Trees VPLS Inter-AS A-D/Binding

This section specifies the procedures for inter-AS VPLS A-D/binding for segmented Inter-AS trees.

An ASBR must be configured to support a particular VPLS as follows:

• An ASBR MUST be configured with a set of (import) RTs that specify the set of VPLS instances supported by the ASBR. These RTs control acceptance of BGP VPLS auto-discovery routes by the ASBR. Note that instead of being configured, the ASBR MAY obtain this set of (import) RTs by using Route Target Constrain [RFC4684].

• The ASBR MUST be configured with the tunnel types for the intra- AS segments of the VPLS instances supported by the ASBR, as well as (depending on the tunnel type) the information needed to create the PMSI Tunnel attribute for these tunnel types. Note that instead of being configured, the ASBR MAY derive the tunnel types from the Intra-AS A-D routes received by the ASBR from the PEs in its own AS.

If an ASBR is configured to support a particular VPLS instance, the ASBR MUST participate in the intra-AS VPLS auto-discovery/binding procedures for that VPLS instance within the ASBR's own AS, as defined in this document.

Moreover, in addition to the above, the ASBR performs procedures specified in the "Propagating BGP VPLS A-D Routes to Other ASes: Overview" section.

7.2.2. Propagating BGP VPLS A-D Routes to Other ASes: Overview

A BGP VPLS A-D route for a given VPLS, originated by a PE within a given AS, is propagated via BGP to other ASes. The precise rules for distributing and processing the Inter-AS A-D routes are given in subsequent sections.

Suppose that an ASBR "A" receives and installs a BGP VPLS A-D route for VPLS "X" and VE-ID "V" that originated at a particular PE "PE1" that is in the same AS as A. The BGP next hop of that received route becomes A's "upstream neighbor" on a multicast distribution tree for (X, V) that is rooted at PE1. Likewise, when A re-advertises this route to ASBRs in A's neighboring ASes, from the perspective of these ASBRs A becomes their "upstream neighbor" on the multicast distribution tree for (X, V) that is rooted at PE1.

When the BGP VPLS A-D routes have been distributed to all the necessary ASes, they define a "reverse path" from any AS that supports VPLS X and VE-ID V back to PE1. For instance, if AS2 supports VPLS X, then there will be a reverse path for VPLS X and VE

ID V from AS2 to AS1. This path is a sequence of ASBRs, the first of which is in AS2 and the last of which is in AS1. Each ASBR in the sequence is the BGP next hop of the previous ASBR in the sequence.

This reverse path information can be used to construct a unidirectional multicast distribution tree for VPLS X and VE-ID V, containing all the ASes that support X, and having PE1 at the root. We call such a tree an "inter-AS tree". Multicast data originating in VPLS sites for VPLS X connected to PE1 will travel downstream along the tree which is rooted at PE1.

The path along an inter-AS tree is a sequence of ASBRs. It is still necessary to specify how the multicast data gets from a given ASBR to the set of ASBRs that are immediately downstream of the given ASBR along the tree. This is done by creating "segments". ASBRs in adjacent ASes will be connected by inter-AS segments; ASBRs in the same AS will be connected by "intra-AS segments".

For a given inter-AS tree and a given AS, there MUST be only one ASBR within that AS that accepts traffic flowing on that tree. Further, for a given inter-AS tree and a given AS, there MUST be only one ASBR in that AS that sends the traffic flowing on that tree to a particular adjacent AS. The precise rules for accomplishing this are given in subsequent sections.

An ASBR initiates creation of an intra-AS segment when the ASBR receives an Inter-AS A-D route from an External BGP (EBGP) neighbor. Creation of the segment is completed as a result of distributing, via IBGP, this route within the ASBR's own AS.

For a given inter-AS tunnel, each of its intra-AS segments could be constructed by its own independent mechanism. Moreover, by using upstream-assigned labels within a given AS, multiple intra-AS segments of different inter-AS tunnels of either the same or different VPLS instances may share the same P-multicast tree.

If the P-multicast tree instantiating a particular segment of an inter-AS tunnel is created by a multicast control protocol that uses receiver-initiated joins (e.g, mLDP), and this P-multicast tree does not aggregate multiple segments, then all the information needed to create that segment will be present in the Inter-AS A-D routes received by the ASBR from the neighboring ASBR. But if the P-multicast tree instantiating the segment is created by a protocol that does not use receiver-initiated joins (e.g., RSVP-TE, ingress unicast replication), or if this P-multicast tree aggregates multiple segments (irrespective of the multicast control protocol used to create the tree), then the ASBR needs to learn the leaves of the segment. These leaves are learned from A-D routes received from other PEs in the AS, for the same VPLS as the one to which the segment belongs.

The following sections specify procedures for propagation of Inter-AS A-D routes across ASes in order to construct inter-AS segmented trees.

7.2.2.1. Propagating Intra-AS VPLS A-D Routes in EBGP

For a given VPLS configured on an ASBR when the ASBR receives Intra- AS A-D routes originated by PEs in its own AS, the ASBR MUST propagate each of these route in EBGP. This procedure MUST be performed for each of the VPLS instances configured on the ASBR. Each of these routes is constructed as follows:

• The route carries a single BGP VPLS A-D NLRI with the RD and VE-ID being the same as the NLRI in the received Intra-AS A-D route.

• The Next Hop field of the MP_REACH_NLRI attribute is set to a routable IP address of the ASBR.

• The route carries the PMSI Tunnel attribute with the Tunnel Type set to Ingress Replication; the attribute carries no MPLS labels.

• The route MUST carry the export RT used by the VPLS.

7.2.2.2. Inter-AS A-D Route Received via EBGP

When an ASBR receives from one of its EBGP neighbors a BGP Update message that carries an Inter-AS A-D route, if (a) at least one of the RTs carried in the message matches one of the import RTs configured on the ASBR, and (b) the ASBR determines that the received route is the best route to the destination carried in the NLRI of the route, the ASBR re-advertises this Inter-AS A-D route to other PEs and ASBRs within its own AS. The best route selection procedures MUST ensure that for the same destination, all ASBRs in an AS pick the same route as the best route. The best route selection procedures are specified in [RFC4761] and clarified in [MULTI-HOMING]. The best route procedures ensure that if multiple ASBRs, in an AS, receive the same Inter-AS A-D route from their EBGP neighbors, only one of these ASBRs propagates this route in Internal BGP (IBGP). This ASBR becomes the root of the intra-AS segment of the inter-AS tree and ensures that this is the only ASBR that accepts traffic into this AS from the inter-AS tree.

When re-advertising an Inter-AS A-D route, the ASBR MUST set the Next Hop field of the MP_REACH_NLRI attribute to a routable IP address of the ASBR.

Depending on the type of a P-multicast tunnel used to instantiate the intra-AS segment of the inter-AS tunnel, the PMSI Tunnel attribute of the re-advertised Inter-AS A-D route is constructed as follows:

• If the ASBR uses ingress replication to instantiate the intra-AS segment of the inter-AS tunnel, the re-advertised route MUST NOT carry the PMSI Tunnel attribute.

• If the ASBR uses a P-multicast tree to instantiate the intra-AS segment of the inter-AS tunnel, the PMSI Tunnel attribute MUST contain the identity of the tree that is used to instantiate the segment (note that the ASBR could create the identity of the tree prior to the actual instantiation of the segment). If, in order to instantiate the segment, the ASBR needs to know the leaves of the tree, then the ASBR obtains this information from the A-D routes received from other PEs/ASBRs in the ASBR's own AS.

• An ASBR that uses a P-multicast tree to instantiate the intra-AS segment of the inter-AS tunnel MAY aggregate two or more VPLS instances present on the ASBR onto the same tree. If the ASBR already advertises Inter-AS A-D routes for these VPLS instances, then aggregation requires the ASBR to re-advertise these routes.

The re-advertised routes MUST be the same as the original ones, except for the PMSI Tunnel attribute. If the ASBR has not previously advertised Inter-AS A-D routes for these VPLS instances, then the aggregation requires the ASBR to advertise (new) Inter-AS A-D routes for these VPLS instances. The PMSI Tunnel attribute in the newly advertised/re-advertised routes MUST carry the identity of the P-multicast tree that aggregates the VPLS instances, as well as an MPLS upstream-assigned label [RFC5331]. Each newly advertised or re-advertised route MUST have a label that is distinct within the scope of the ASBR.

In addition, the ASBR MUST send to the EBGP neighbor, from whom it receives the Inter-AS A-D route, a BGP Update message that carries a leaf A-D route. The exact encoding of this route is described in the "BGP Extensions" section. This route contains the following information elements:

• The route carries a single NLRI with the Route Key field set to the <RD, VE-ID> tuple of the BGP VPLS A-D NLRI of the Inter-AS A-D route received from the EBGP neighbor. The NLRI also carries the IP address of the ASBR (this MUST be a routable IP address).

• The leaf A-D route MUST include the PMSI Tunnel attribute with the Tunnel Type set to Ingress Replication, and the Tunnel Identifier set to a routable address of the advertising router. The PMSI Tunnel attribute MUST carry a downstream-assigned MPLS label that is used to demultiplex the VPLS traffic received over a unicast tunnel by the advertising router.

• The Next Hop field of the MP_REACH_NLRI attribute of the route SHOULD be set to the same IP address as the one carried in the Originating Router's IP Address field of the route.

• To constrain the distribution scope of this route, the route MUST carry the NO_ADVERTISE BGP Community ([RFC1997]).

• The ASBR constructs an IP-address-specific RT by placing the IP address carried in the Next Hop field of the received Inter-AS VPLS A-D route in the Global Administrator field of the community, with the Local Administrator field of this community set to 0. It also sets the Extended Communities attribute of the leaf A-D route to that community. Note that this RT is the same as the ASBR Import RT of the EBGP neighbor from which the ASBR received the Inter-AS VPLS A-D route.

7.2.2.3. Leaf A-D Route Received via EBGP

When an ASBR receives, via EBGP, a leaf A-D route, the ASBR accepts the route only if (a) at least one of the RTs carried in the message matches one of the import RTs configured on the ASBR and (b) the ASBR determines that the received route is the best route to the destination carried in the NLRI of the route.

If the ASBR accepts the leaf A-D route, the ASBR looks for an existing A-D route whose BGP-VPLS A-D NLRI has the same value as the <RD, VE-ID> field of the leaf A-D route just accepted. If such an A-D route is found, then the MPLS label carried in the PMSI Tunnel attribute of the leaf A-D route is used to stitch a one hop ASBR-ASBR LSP to the tail of the intra-AS tunnel segment associated with the found A-D route.

7.2.2.4. Inter-AS A-D Route Received via IBGP

In the context of this section, we use the term "PE/ASBR router" to denote either a PE or an ASBR router.

Note that a given Inter-AS A-D route is advertised within a given AS by only one ASBR, as described above.

When a PE/ASBR router receives, from one of its IBGP neighbors, a BGP Update message that carries an Inter-AS A-D route, if (a) at least one of the RTs carried in the message matches one of the import RTs configured on the PE/ASBR and (b) the PE/ASBR determines that the received route is the best route to the destination carried in the NLRI of the route, the PE/ASBR performs the following operations. The best route determination is as described in [RFC4761] and clarified in [MULTI-HOMING].

If the router is an ASBR, then the ASBR propagates the route to its EBGP neighbors. When propagating the route to the EBGP neighbors, the ASBR MUST set the Next Hop field of the MP_REACH_NLRI attribute to a routable IP address of the ASBR.

If the received Inter-AS A-D route carries the PMSI Tunnel attribute with the Tunnel Type set to LDP P2MP LSP, the PE/ASBR SHOULD join the P-multicast tree whose identity is carried in the PMSI Tunnel attribute.

If the received Inter-AS A-D route carries the PMSI Tunnel attribute with the Tunnel Identifier set to RSVP-TE P2MP LSP, then the ASBR that originated the route MUST establish an RSVP-TE P2MP LSP with the local PE/ASBR as a leaf. This LSP MAY have been established before the local PE/ASBR receives the route, or it MAY be established after the local PE receives the route.

If the received Inter-AS A-D route carries the PMSI Tunnel attribute with the Tunnel Type set to LDP P2MP LSP, or RSVP-TE P2MP LSP, but the attribute does not carry a label, then the P-multicast tree, as identified by the PMSI Tunnel attribute, is an intra-AS LSP segment that is part of the inter-AS tunnel for the <VPLS, VE-ID> advertised by the Inter-AS A-D route and rooted at the PE that originated the A-D route. If the PMSI Tunnel attribute carries a (upstream- assigned) label, then a combination of this tree and the label identifies the intra-AS segment. If the receiving router is an ASBR, this intra-AS segment may further be stitched to ASBR-ASBR inter-AS segment of the inter-AS tunnel. If the PE/ASBR has local receivers in the VPLS, packets received over the intra-AS segment must be forwarded to the local receivers using the local VSI.

7.3. Option (c): Non-segmented Tunnels

In this model, there is a multi-hop EBGP peering between the PEs (or BGP Route Reflector) in one AS and the PEs (or BGP Route Reflector) in another AS. The PEs exchange BGP-VPLS NLRI or BGP-VPLS A-D NLRI, along with the PMSI Tunnel attribute, as in the intra-AS case described in the "Demultiplexing P-Multicast Tree Traffic" section.

The PEs in different ASes use a non-segmented inter-AS P2MP tunnel for VPLS multicast. A non-segmented inter-AS tunnel is a single tunnel that spans AS boundaries. The tunnel technology cannot change from one point in the tunnel to the next, so all ASes through which the tunnel passes must support that technology. In essence, AS boundaries are of no significance to a non-segmented inter-AS P2MP tunnel.

This model requires no VPLS A-D routes in the control plane or VPLS MAC address learning in the data plane on the ASBRs. The ASBRs only need to participate in the non-segmented P2MP tunnel setup in the control plane and do MPLS label forwarding in the data plane.

When the tunneling technology is P2MP LSP signaled with mLDP, and one does not use [RFC6512], the setup of non-segmented inter-AS P2MP tunnels requires the P-routers in one AS to have IP reachability to the loopback addresses of the PE routers in another AS. That is, the reachability to the loopback addresses of PE routers in one AS MUST be present in the IGP in another AS.

The data forwarding in this model is the same as in the intra-AS case described in the "Demultiplexing P-Multicast Tree Traffic" section.

An implementation MUST support this model.

8. Optimizing Multicast Distribution via Selective Trees

Whenever a particular multicast stream is being sent on an Inclusive P-multicast tree, it is likely that the data of that stream is being sent to PEs that do not require it, as the sites connected to these PEs may have no receivers for the stream. If a particular stream has a significant amount of traffic, it may be beneficial to move it to a Selective P-multicast tree that has, at its leaves, only those PEs, connected to sites that have receivers for the multicast stream (or at least includes fewer PEs that are attached to sites with no receivers compared to an Inclusive tree).

A PE connected to the multicast source of a particular multicast stream may be performing explicit tracking; that is, it may know the PEs that have receivers in the multicast stream. The "Receiving S-PMSI A-D Routes by PEs" section describes procedures that enable explicit tracking. If this is the case, Selective P-multicast trees can also be triggered on other criteria. For instance, there could be a "pseudo-wasted bandwidth" criterion: switching to a Selective tree would be done if the bandwidth multiplied by the number of "uninterested" PEs (PEs that are receiving the stream but have no receivers) is above a specified threshold. The motivation is that (a) the total bandwidth wasted by many sparsely subscribed low- bandwidth groups may be large and (b) there's no point to moving a high-bandwidth group to a Selective tree if all the PEs have receivers for it.

Switching a (C-S, C-G) stream to a Selective P-multicast tree may require the root of the tree to determine the egress PEs that need to receive the (C-S, C-G) traffic. This is true in the following cases:

• If the tunnel is a P2MP tree, such as an RSVP-TE P2MP Tunnel, the PE needs to know the leaves of the tree before it can instantiate the Selective tree.

• If a PE decides to send traffic for multicast streams, belonging to different VPLS instances, using one P-multicast Selective tree, such a tree is called an "Aggregate tree with a selective mapping". The setting up of such an Aggregate tree requires the ingress PE to know all the other PEs that have receivers for multicast groups that are mapped onto the tree (see the "Aggregation Considerations" section for the rationale).

• If ingress replication is used and the ingress PE wants to send traffic for (C-S, C-G)s to only those PEs that are on the path to receivers to the (C-S, C-G)s.

For discovering the IP multicast group membership, for the above cases, this document describes procedures that allow an ingress PE to enable explicit tracking. Thus, an ingress PE can request the IP multicast membership from egress PEs for one or more C-multicast streams. These procedures are described in the "Receiving S-PMSI A-D Routes by PEs" section.

The root of the Selective P-multicast tree MAY decide to do explicit tracking of the IP multicast stream only after it has decided to move the stream to a Selective tree, or it MAY have been doing explicit tracking all along. This document also describes explicit tracking for a wildcard source and/or group in the "Receiving S-PMSI A-D Routes by PEs" section, which facilitates a Selective P-multicast tree only mode in which IP multicast streams are always carried on a Selective P-multicast tree. In the description on Selective P-multicast trees, the notation C-S is intended to represent either a specific source address or a wildcard. Similarly, C-G is intended to represent either a specific group address or a wildcard.

The PE at the root of the tree MUST signal the leaves of the tree that the (C-S, C-G) stream is now bound to the Selective tree. Note that the PE could create the identity of the P-multicast tree prior to the actual instantiation of the tunnel.

If the Selective tree is instantiated by an RSVP-TE P2MP LSP, the PE at the root of the tree MUST establish the P2MP RSVP-TE LSP to the leaves. This LSP MAY have been established before the leaves receive the Selective tree binding, or it MAY be established after the leaves receive the binding. A leaf MUST NOT switch to the Selective tree until it receives the binding and the RSVP-TE P2MP LSP is set up to the leaf.

8.1. Protocol for Switching to Selective Trees

Selective trees provide a PE the ability to create separate P-multicast trees for certain (C-S, C-G) streams. The source PE, which originates the Selective tree, and the egress PEs MUST use the Selective tree for the (C-S, C-G) streams that are mapped to it. This may require the source and egress PEs to switch to the Selective tree from an Inclusive tree if they were already using an Inclusive tree for the (C-S, C-G) streams mapped to the Selective tree.

Once a source PE decides to set up a Selective tree, it MUST announce the mapping of the (C-S, C-G) streams (which may be in different VPLS instances) that are mapped to the tree to the other PEs using BGP. After the egress PEs receive the announcement, they set up their forwarding path to receive traffic on the Selective tree if they have one or more receivers interested in the (C-S, C-G) streams mapped to the tree. Setting up the forwarding path requires setting up the demultiplexing forwarding entries based on the top MPLS label (if there is no inner label) or the inner label (if present) as described in the "Establishing P-Multicast Trees" section.

When the P2MP LSP is established using mLDP, the egress PEs MAY perform this switch to the Selective tree once the announcement from the ingress PE is received, or they MAY wait for a preconfigured timer to do so after receiving the announcement.

When the P2MP LSP protocol is P2MP RSVP-TE, an egress PE MUST perform this switch to the Selective tree only after the announcement from the ingress PE is received and the RSVP-TE P2MP LSP has been set up to the egress PE. This switch MAY be done after waiting for a preconfigured timer after these two steps have been accomplished.

A source PE MUST use the following approach to decide when to start transmitting data on the Selective tree, if it is currently using an Inclusive tree. After announcing the (C-S, C-G) stream mapping to a Selective tree, the source PE MUST wait for a "switchover" delay before sending (C-S, C-G) stream on the Selective tree. It is RECOMMENDED to allow this delay to be configurable. Once the

"switchover" delay has elapsed, the source PE MUST send (C-S, C-G) stream on the Selective tree. In no case is any (C-S, C-G) packet sent on both Selective and Inclusive trees.

When a (C-S, C-G) stream is switched from an Inclusive to a Selective tree, the purpose of running a switchover timer is to minimize packet loss without introducing packet duplication. However, jitter may be introduced due to the difference in transit delays between the Inclusive and Selective trees.

For best effect, the switchover timer should be configured to a value that is "just long enough" (a) to allow all the PEs to learn about the new binding of (C-S, C-G) to a Selective tree and (b) to allow the PEs to construct the P-tunnel associated with the Selective tree, if it doesn't already exist.

8.2. Advertising (C-S, C-G) Binding to a Selective Tree

The ingress PE informs all the PEs that are on the path to receivers of the (C-S, C-G) of the binding of the Selective tree to the (C-S, C-G), using BGP. The BGP announcement is done by sending update for the MCAST-VPLS address family using what we referred to as an "S-PMSI A-D route". The format of the NLRI of this route is described in the "Inclusive Tree/Selective Tree Identifier" section. The NLRI MUST be constructed as follows:

• The Route Distinguisher (RD) MUST be set to the RD configured locally for the VPLS. This is required to uniquely identify the <C-S, C-G> as the addresses could overlap between different VPLS instances. This MUST be the same RD value used in the VPLS auto- discovery process.

• The Multicast Source field MUST contain the source address associated with the C-multicast stream, and the Multicast Source Length field is set appropriately to reflect this. If the source address is a wildcard, the source address is set to 0.

• The Multicast Group field MUST contain the group address associated with the C-multicast stream, and the Multicast Group Length field is set appropriately to reflect this. If the group address is a wildcard, the group address is set to 0.

• The Originating Router's IP Address field MUST be set to the IP address that the (local) PE places in the BGP Next Hop of the BGP-VPLS A-D routes. Note that the <RD, Originating Router's IP Address> tuple uniquely identifies a given VPLS instance on a PE.

The PE constructs the rest of the Selective A-D route as follows.

Depending on the type of a P-multicast tree used for the P-tunnel, the PMSI Tunnel attribute of the S-PMSI A-D route is constructed as follows:

• The PMSI Tunnel attribute MUST contain the identity of the P-multicast tree (note that the PE could create the identity of the tree prior to the actual instantiation of the tree).

• If, in order to establish the P-multicast tree, the PE needs to know the leaves of the tree within its own AS, then the PE obtains this information from the leaf A-D routes received from other PEs/ASBRs within its own AS (as other PEs/ASBRs originate leaf A-D routes in response to receiving the S-PMSI A-D route) by setting the Leaf Information Required flag in the PMSI Tunnel attribute to 1. This enables explicit tracking for the multicast stream(s) advertised by the S-PMSI A-D route.

• If a PE originates S-PMSI A-D routes with the Leaf Information Required flag in the PMSI Tunnel attribute set to 1, then the PE MUST be (auto-)configured with an import RT, which controls acceptance of leaf A-D routes by the PE. (Procedures for originating leaf A-D routes by the PEs that receive the S-PMSI A-D route are described in the "Receiving S-PMSI A-D Routes by PEs" section.)

This RT is IP address specific. The Global Administrator field of this RT MUST be set to the IP address carried in the Next Hop field of all the S-PMSI A-D routes advertised by this PE (if the PE uses different Next Hop fields, then the PE MUST be (auto-)configured with multiple import RTs, one per each such Next Hop field). The Local Administrator field of this Route Target MUST be set to 0.

If the PE supports Route Target Constrain [RFC4684], the PE SHOULD advertise this import RT within its own AS using Route Target Constrain. To constrain distribution of the Route Target Constrain routes to the AS of the advertising PE these routes SHOULD carry the NO_EXPORT Community ([RFC1997]).

• A PE MAY aggregate two or more S-PMSIs originated by the PE onto the same P-multicast tree. If the PE already advertises S-PMSI A-D routes for these S-PMSIs, then aggregation requires the PE to re-advertise these routes. The re-advertised routes MUST be the same as the original ones, except for the PMSI Tunnel attribute. If the PE has not previously advertised S-PMSI A-D routes for these S-PMSIs, then the aggregation requires the PE to advertise

(new) S-PMSI A-D routes for these S-PMSIs. The PMSI Tunnel attribute in the newly advertised/re-advertised routes MUST carry the identity of the P-multicast tree that aggregates the S-PMSIs. If at least some of the S-PMSIs aggregated onto the same P-multicast tree belong to different VPLS instances, then all these routes MUST carry an MPLS upstream-assigned label [RFC5331]. If all these aggregated S-PMSIs belong to the same VPLS, then the routes MAY carry an MPLS upstream-assigned label [RFC5331]. The labels MUST be distinct on a per-VPLS-instance basis, and they MAY be distinct on a per-route basis.

The Next Hop field of the MP_REACH_NLRI attribute of the route SHOULD be set to the same IP address as the one carried in the Originating Router's IP Address field.

By default, the set of RTs carried by the route MUST be the same as the RTs carried in the BGP-VPLS A-D route originated from the VSI. The default could be modified via configuration.

8.3. Receiving S-PMSI A-D Routes by PEs

Consider a PE that receives an S-PMSI A-D route. If one or more of the VSIs on the PE have their import RTs that contain one or more of the RTs carried by the received S-PMSI A-D route, then for each such VSI, the PE performs the following.

Procedures for receiving an S-PMSI A-D route by a PE (both within and outside of the AS of the PE that originates the route) are the same as specified in the "Inter-AS A-D Route Received via IBGP" section, except that (a) instead of Inter-AS A-D routes the procedures apply to S-PMSI A-D routes, (b) the rules for determining whether the received S-PMSI A-D route is the best route to the destination carried in the NLRI of the route are the same as BGP path selection rules and may be modified by policy, and (c) a PE performs procedures specified in that section only if in addition to the criteria specified in that section the following is true:

• If, as a result of multicast state snooping on the PE-CE interfaces, the PE has snooped state for at least one multicast join that matches the multicast source and group advertised in the S-PMSI A-D route. Further, the oifs (outgoing interfaces) for this state contain one or more interfaces to the locally attached CEs. When the multicast signaling protocol among the CEs is IGMP, then snooping and associated procedures are defined in [RFC4541]. The snooped state is determined using these procedures. When the multicast signaling protocol among the CEs is PIM, the procedures in [RFC4541] are not sufficient to determine the snooped state. The additional details required to determine the snooped state when CE-CE protocol is PIM are for further study. When such procedures are defined, it is expected that the procedures in this section will apply to the snooped state created as a result of PIM as PE-CE protocol.

The snooped state is said to "match" the S-PMSI A-D route if any of the following is true:

     + The S-PMSI A-D route carries (C-S, C-G) and the snooped state is
     
       for (C-S, C-G) or for (C-*, C-G), OR
     
     + The S-PMSI A-D route carries (C-*, C-G) and (a) the snooped state
       is for (C-*, C-G) OR (b) the snooped state is for at least one
       multicast join with the multicast group address equal to C-G and
       there doesn't exist another S-PMSI A-D route that carries (C-S,
       C-G) where C-S is the source address of the snooped state.
     
     + The S-PMSI A-D route carries (C-S, C-*) and (a) the snooped state
       is for at least one multicast join with the multicast source
       address equal to C-S, and (b) there doesn't exist another S-PMSI
       A-D route that carries (C-S, C-G) where C-G is the group address
       of the snooped state.
     
     + The S-PMSI A-D route carries (C-*, C-*) and there is no other
       S-PMSI A-D route that matches the snooped state as per the above
       conditions.

Note if the above conditions are true, and if the received S-PMSI A-D route has a PMSI Tunnel attribute with the Leaf Information Required flag set to 1, then the PE originates a leaf A-D route, constructed as follows:

• The route carries a single MCAST-VPLS NLRI with the Route Key field set to the MCAST-VPLS NLRI of the received S-PMSI A-D route.

• The Originating Router's IP Address set to the IP address of the PE (this MUST be a routable IP address).

• The PE constructs an IP-address-specific RT by placing the IP address carried in the Next Hop field of the received S-PMSI A-D route in the Global Administrator field of the Community, with the Local Administrator field of this Community set to 0 and setting the Extended Communities attribute of the leaf A-D route to that Community.

• The Next Hop field of the MP_REACH_NLRI attribute of the route MUST be set to the same IP address as the one carried in the Originating Router's IP Address field of the route.

• To constrain the distribution scope of this route, the route MUST carry the NO_EXPORT Community [RFC1997], except for the inter-AS scenario with option (c).

Once the leaf A-D route is constructed, the PE advertises this route into IBGP.

In addition to the procedures specified in the "Inter-AS A-D Route Received via IBGP" section, the PE MUST set up its forwarding path to receive traffic, for each multicast stream in the matching snooped state, from the tunnel advertised by the S-PMSI A-D route (the PE MUST switch to the Selective tree).

When a new snooped state is created by a PE, then the PE MUST first determine if there is an S-PMSI A-D route that matches the snooped state as per the conditions described above. If such an S-PMSI A-D route is found, then the PE MUST follow the procedures described in this section, for that particular S-PMSI A-D route. If later on the snooped state ages out and is deleted from the PE, the PE SHOULD withdraw the leaf A-D route that it had originated in response to the S-PMSI A-D route.

8.4. Inter-AS Selective Tree

Inter-AS Selective trees support all three options of inter-AS VPLS service, option (a), (b), and (c), that are supported by Inter-AS Inclusive trees. They are constructed in a manner that is very similar to Inter-AS Inclusive trees.

For option (a) and option (b), support Inter-AS Selective trees are constructed without requiring a single P-multicast tree to span multiple ASes. This allows individual ASes to potentially use different P-tunneling technologies. There are two variants of this. One that requires MAC and IP multicast lookup on the ASBRs and another that does not require MAC/IP multicast lookup on the ASBRs and instead builds segmented Inter-AS Selective trees.

Segmented Inter-AS Selective trees can also be used with option (c), unlike Segmented Inter-AS Inclusive trees. This is because the S-PMSI A-D routes can be exchanged via ASBRs (even though BGP VPLS A-D routes are not exchanged via ASBRs).

In the case of Option (c), an Inter-AS Selective tree may also be a non-segmented P-multicast tree that spans multiple ASes.

8.4.1. VSIs on the ASBRs

The requirements on ASBRs, when VSIs are present on the ABSRs, include the requirements presented in the "Inter-AS Inclusive P-Multicast Tree A-D/Binding" section. The source ASBR (that receives traffic from another AS) may independently decide whether or not it wishes to use Selective trees. If it uses Selective trees, the source ASBR MUST perform a MAC lookup to determine the Selective tree to forward the VPLS packet on.

8.4.1.1. VPLS Inter-AS Selective Tree A-D Binding

The mechanisms for propagating S-PMSI A-D routes are the same as the intra-AS case described in the "MCAST-VPLS NLRI" section. The BGP Selective tree A-D routes generated by PEs in an AS MUST NOT be propagated outside the AS.

8.4.2. Inter-AS Segmented Selective Trees

Inter-AS Segmented Selective trees MUST be implemented when option (b) is used to provide the inter-AS VPLS service. They MAY be used when option (c) is implemented to provide the inter-AS VPLS service.

A Segmented inter-AS Selective Tunnel is constructed similar to an inter-AS Segmented Inclusive Tunnel. Namely, such a tunnel is constructed as a concatenation of tunnel segments. There are two types of tunnel segments: an intra-AS tunnel segment (a segment that spans ASBRs within the same AS) and inter-AS tunnel segment (a segment that spans adjacent ASBRs in adjacent ASes). ASes that are spanned by a tunnel are not required to use the same tunneling mechanism to construct the tunnel -- each AS may pick up a tunneling mechanism to construct the intra-AS tunnel segment of the tunnel, in its AS.

The PE that decides to set up a Selective tree advertises the Selective tree to multicast stream binding using an S-PMSI A-D route, as per procedures in the "Advertising (C-S, C-G) Binding to a Selective Tree" section, to the routers in its own AS.

An S-PMSI A-D route advertised outside the AS, to which the originating PE belongs, will be referred to as an Inter-AS S-PMSI tree A-D route (although this route is originated by a PE as an intra-AS S-PMSI A-D route, it is referred to as an Inter-AS route outside the AS).

8.4.2.1. Handling S-PMSI A-D Routes by ASBRs

Procedures for handling an S-PMSI A-D route by ASBRs (both within and outside of the AS of the PE that originates the route) are the same as specified in the "Propagating BGP VPLS A-D Routes to Other ASes" section, except that instead of Inter-AS A-D routes and their NLRI, these procedures apply to S-PMSI A-D routes and their NLRI.

In addition to these procedures, an ASBR advertises a leaf A-D route in response to an S-PMSI A-D route only if:

• The S-PMSI A-D route was received via EBGP from another ASBR and the ASBR merges the S-PMSI A-D route into an Inter-AS BGP VPLS A-D route as described in the next section. OR

• The ASBR receives a leaf A-D route from a downstream PE or ASBR in response to the S-PMSI A-D route, received from an upstream PE or ASBR, that the ASBR propagated inter-AS to downstream ASBRs and PEs.

• The ASBR has snooped state from local CEs that matches the NLRI carried in the S-PMSI A-D route as per the following rules:

      i) The NLRI encodes (C-S, C-G), which is the same as the snooped
         (C-S, C-G)
     
     ii) The NLRI encodes (*, C-G), there is snooped state for at least
         one (C-S, C-G), and there is no other matching S-PMSI A-D route
         for (C-S, C-G) OR there is snooped state for (*, C-G)
    
    iii) The NLRI encodes (*, *), there is snooped state for at least
         one (C-S, C-G) or (*, C-G), and there is no other matching
         S-PMSI A-D route for that (C-S, C-G) or (*, C-G), respectively.

The C-multicast data traffic is sent on the Selective tree by the originating PE. When it reaches an ASBR that is on the inter-AS segmented tree, it is delivered to local receivers, if any. It is then forwarded on any inter-AS or intra-AS segments that exist on the Inter-AS Selective segmented tree. If the Inter-AS Selective segmented tree is merged onto an Inclusive tree, as described in the next section, the data traffic is forwarded onto the Inclusive tree.

8.4.2.1.1. Merging Selective Tree into an Inclusive Tree

Consider the situation where:

• An ASBR is receiving (or expecting to receive) inter-AS (C-S, C-G) data from upstream via a Selective tree.

• The ASBR is sending (or expecting to send) the inter-AS (C-S, C-G) data downstream via an Inclusive tree.

This situation may arise if the upstream providers have a policy of using Selective trees but the downstream providers have a policy of using Inclusive trees. To support this situation, an ASBR MAY, under certain conditions, merge one or more upstream Selective trees into a downstream Inclusive tree. Note that this can be the case only for option (b) and not for option (c) as, for option (c), the ASBRs do not have Inclusive tree state.

A Selective tree (corresponding to a particular S-PMSI A-D route) MAY be merged by a particular ASBR into an Inclusive tree (corresponding to a particular Inter-AS BGP VPLS A-D route) if and only if the following conditions all hold:

• The S-PMSI A-D route and the Inter-AS BGP VPLS A-D route originate in the same AS. The Inter-AS BGP VPLS A-D route carries the originating AS in the AS_PATH attribute of the route. The S-PMSI A-D route carries the originating AS in the AS_PATH attribute of the route.

• The S-PMSI A-D route and the Inter-AS BGP VPLS A-D route have exactly the same set of RTs.

An ASBR performs merging by stitching the tail end of the P-tunnel, as specified in the PMSI Tunnel attribute of the S-PMSI A-D route received by the ASBR, to the head of the P-tunnel, as specified in the PMSI Tunnel attribute of the Inter-AS BGP VPLS A-D route re-advertised by the ASBR.

An ASBR that merges an S-PMSI A-D route into an Inter-AS BGP VPLS A-D route MUST NOT re-advertise the S-PMSI A-D route.

8.4.3. Inter-AS Non-segmented Selective Trees

Inter-AS Non-segmented Selective trees MAY be used in the case of option (c).

In this method, there is a multi-hop EBGP peering between the PEs (or a Route Reflector) in one AS and the PEs (or Route Reflector) in another AS. The PEs exchange BGP Selective tree A-D routes, along with PMSI Tunnel attribute, as in the intra-AS case described in the "Option (c): Non-segmented Tunnels" section.

The PEs in different ASes use a non-segmented Selective inter-AS P2MP tunnel for VPLS multicast.

This method requires no VPLS information (in either the control or the data plane) on the ASBRs. The ASBRs only need to participate in the non-segmented P2MP tunnel setup in the control plane and do MPLS label forwarding in the data plane.

The data forwarding in this model is the same as in the intra-AS case described in the "Establishing P-Multicast Trees" section.

9. BGP Extensions

This section describes the encoding of the BGP extensions required by this document.

9.1. Inclusive Tree/Selective Tree Identifier

Inclusive P-multicast tree and Selective P-multicast tree advertisements carry the P-multicast tree identifier. For the purpose of carrying this identifier, this document reuses the BGP attribute, called "PMSI_TUNNEL" that is defined in [RFC6514].

This document supports only the following Tunnel Types when the PMSI Tunnel attribute is carried in VPLS A-D or VPLS S-PMSI A-D routes:

+ 0 - No tunnel information present
+ 1 - RSVP-TE P2MP LSP
+ 2 - LDP P2MP LSP
+ 6 - Ingress Replication

9.2. MCAST-VPLS NLRI

This document defines a new BGP NLRI, called the "MCAST-VPLS NLRI".

Following is the format of the MCAST-VPLS NLRI:

                +-----------------------------------+
                |    Route Type (1 octet)           |
                +-----------------------------------+
                |     Length (1 octet)              |
                +-----------------------------------+
                |    Route Type specific (variable) |
                +-----------------------------------+

The Route Type field defines encoding of the Route Type specific field of MCAST-VPLS NLRI.

The Length field indicates the length in octets of the Route Type specific field of MCAST-VPLS NLRI.

This document defines the following route types for A-D routes:

     + 3 - Selective Tree A-D route;
     + 4 - Leaf A-D route.

The MCAST-VPLS NLRI is carried in BGP using BGP Multiprotocol Extensions [RFC4760] with an Address Family Identifier (AFI) of 25 (L2VPN AFI), and a Subsequent Address Family Identifier (SAFI) of MCAST-VPLS. The NLRI field in the MP_REACH_NLRI/MP_UNREACH_NLRI attribute contains the MCAST-VPLS NLRI (encoded as specified above).

In order for two BGP speakers to exchange labeled MCAST-VPLS NLRI, they must use BGP Capabilities Advertisement to ensure that they both are capable of properly processing such NLRI. This is done as specified in [RFC4760], by using capability code 1 (multiprotocol BGP) with an AFI of 25 and a SAFI of MCAST-VPLS.

The following describes the format of the Route Type specific field of MCAST-VPLS NLRI for various route types defined in this document.

9.2.1. S-PMSI A-D Route

The Route Type specific field of MCAST-VPLS NLRI of an S-PMSI A-D route consists of the following:

                +-----------------------------------+
                |      RD   (8 octets)              |
                +-----------------------------------+
                | Multicast Source Length (1 octet) |
                +-----------------------------------+
                |  Multicast Source (Variable)      |
                +-----------------------------------+
                |  Multicast Group Length (1 octet) |
                +-----------------------------------+
                |  Multicast Group   (Variable)     |
                +-----------------------------------+
                |   Originating Router's IP Addr    |
                +-----------------------------------+

The RD is encoded as described in [RFC4364].

The Multicast Source field contains the C-S address, i.e., the address of the multicast source. If the Multicast Source field contains an IPv4 address, then the value of the Multicast Source Length field is 32. If the Multicast Source field contains an IPv6 address, then the value of the Multicast Source Length field is 128. The value of the Multicast Source Length field may be set to 0 to indicate a wildcard.

The Multicast Group field contains the C-G address, i.e., the address of the multicast group. If the Multicast Group field contains an IPv4 address, then the value of the Multicast Group Length field is 32. If the Multicast Group field contains an IPv6 address, then the value of the Multicast Group Length field is 128. The Multicast Group Length field may be set to 0 to indicate a wildcard.

Whether the Originating Router's IP Address field carries an IPv4 or IPv6 address is determined by the value of the Length field of the MCAST-VPLS NLRI. If the Multicast Source field contains an IPv4 address and the Multicast Group field contains an IPv4 address, then the value of the Length field is 22 bytes if the Originating Router's IP Address carries an IPv4 address and 34 bytes if it is an IPv6 address. If the Multicast Source and Multicast Group fields contain IPv6 addresses, then the value of the Length field is 46 bytes if the Originating Router's IP Address carries an IPv4 address and 58 bytes if it is an IPv6 address. The following table summarizes the above.

      Multicast   Multicast                Originating Router's   Length
      Source      Group                       IP Address
      
        IPv4      IPv4                        IPv4                  22
        IPv4      IPv4                        IPv6                  34
        IPv6      IPv6                        IPv4                  46
        IPv6      IPv6                        IPv6                  58

Usage of Selective Tree A-D routes is described in the "Optimizing Multicast Distribution via Selective Trees" section.

9.2.2. Leaf A-D Route

The Route Type specific field of MCAST-VPLS NLRI of a leaf A-D route consists of the following:

                +-----------------------------------+
                |      Route Key (variable)         |
                +-----------------------------------+
                |   Originating Router's IP Addr    |
                +-----------------------------------+

Whether the Originating Router's IP Address field carries an IPv4 or IPv6 address is determined by the Length field of the MCAST-VPLS NLRI and the length of the Route Key field. From these two length fields, one can compute the length of the Originating Router's IP Address. If this computed length is 4, then the address is an IPv4 address; if its 16, then the address is an IPv6 address.

Usage of leaf A-D routes is described in the "Inter-AS Inclusive P-Multicast Tree A-D/Binding" and "Optimizing Multicast Distribution via Selective Trees" sections.

10. Aggregation Considerations

This document does not specify the mandatory implementation of any particular set of rules for determining whether or not the Inclusive or Selective trees of two particular VPLS instances are to be instantiated by the same Aggregate Inclusive/Selective tree. This determination can be made by implementation-specific heuristics, by configuration, or even perhaps by the use of offline tools.

This section discusses potential methodologies with respect to aggregation.

In general, the heuristic used to decide which VPLS instances or <C-S, C-G> entries to aggregate is implementation dependent. It is also conceivable that offline tools can be used for this purpose. This section discusses some trade-offs with respect to aggregation.

The "congruency" of aggregation is defined by the amount of overlap in the leaves of the client trees that are aggregated on an SP tree. For Aggregate Inclusive trees, the congruency depends on the overlap in the membership of the VPLS instances that are aggregated on the Aggregate Inclusive tree. If there is complete overlap, aggregation is perfectly congruent. As the overlap between the VPLS instances that are aggregated reduces, the congruency reduces.

From the above definition of "congruency", it follows that in order for a given PE to determine the congruency of the client trees that this PE could aggregate, the PE has to know the leaves of these client trees. This is irrespective of whether the aggregated SP tree is established using mLDP or RSVP-TE.

If aggregation is done such that it is not perfectly congruent, a PE may receive traffic for VPLS instances to which it doesn't belong. As the amount of multicast traffic in these unwanted VPLS instances increases, aggregation becomes less optimal with respect to delivered traffic. Hence, there is a trade-off between reducing multicast state in the core and delivering unwanted traffic.

An implementation should provide knobs to control aggregation based on the congruency of the tree to be aggregated. This will allow an SP to deploy aggregation depending on the VPLS membership and traffic profiles in its network. If different PEs are setting up Aggregate Inclusive trees, this will also allow an SP to engineer the maximum amount of unwanted VPLS instances for which a particular PE may receive traffic.

The state/bandwidth optimality trade-off can be further improved by having a versatile many-to-many association between client trees and provider trees. Thus, a VPLS instance can be mapped to multiple Aggregate trees. The mechanisms for achieving this are for further study. Also, it may be possible to use both ingress replication and an Aggregate tree for a particular VPLS. Mechanisms for achieving this are also for further study.

11. Data Forwarding

11.1. MPLS Tree Encapsulation

11.1.1. Mapping Multiple VPLS Instances to a P2MP LSP

The following diagram shows the progression of the VPLS multicast packet as it enters and leaves the SP network when MPLS trees are being used for multiple VPLS instances. RSVP-TE P2MP LSPs are examples of such trees.

      Packets received        Packets in transit      Packets forwarded
      at ingress PE           in the service          by egress PEs
                              provider network
      
                              +---------------+
                              |MPLS Tree Label|
                              +---------------+
                              | VPLS Label    |
      ++=============++       ++=============++       ++=============++
      ||C-Ether Hdr  ||       || C-Ether Hdr ||       || C-Ether Hdr ||
      ++=============++ >>>>> ++=============++ >>>>> ++=============++
      || C-IP Header ||       || C-IP Header ||       || C-IP Header ||
      ++=============++ >>>>> ++=============++ >>>>> ++=============++
      || C-Payload   ||       || C-Payload   ||       || C-Payload   ||
      ++=============++       ++=============++       ++=============++

When an ingress PE receives a packet, the ingress PE using the procedures defined in [RFC4761] and [RFC4762] determines the VPLS instance associated with the packet. If the packet is an IP multicast packet, and the ingress PE uses an Aggregate Selective tree for the (C-S, C-G) carried in the packet, then the ingress PE pushes the VPLS Label associated with the VPLS instance on the ingress PE and the MPLS Tree Label associated with the Aggregate Selective tree, and it sends the packet over the P2MP LSP associated with the Aggregate Selective tree. Otherwise, if the ingress PE does not use an Aggregate Selective tree for the (C-S, C-G), or the packet is either non-IP multicast or broadcast, the ingress PE pushes the VPLS label associated with the VPLS instance on the ingress PE and the MPLS Tree Label associated with the Aggregate Inclusive tree, and it sends the packet over the P2MP LSP associated with the Aggregate Inclusive tree.

The egress PE does a lookup on the outer MPLS tree label, and determines the MPLS forwarding table in which to look up the inner MPLS label (VPLS label). This table is specific to the tree label space (as identified by the MPLS Tree Label). The inner label (VPLS label) is unique within the context of the root of the tree (as it is assigned by the root of the tree, without any coordination with any other nodes). Thus, it is not unique across multiple roots. So, to unambiguously identify a particular VPLS, one has to know the VPLS label, and the context within which that label is unique. The context is provided by the outer MPLS label (MPLS Tree Label) [RFC5331].

The outer MPLS label is popped. The lookup of the resulting MPLS label determines the VSI in which the egress PE needs to do the C-multicast data packet lookup. It then pops the inner MPLS label and sends the packet to the VSI for multicast data forwarding.

11.1.2. Mapping One VPLS Instance to a P2MP LSP

The following diagram shows the progression of the VPLS multicast packet as it enters and leaves the SP network when a given MPLS tree is being used for a single VPLS instance. RSVP-TE P2MP LSPs are examples of such trees.

      Packets received        Packets in transit      Packets forwarded
      at ingress PE           in the service          by egress PEs
                              provider network
      
                              +---------------+
                              |MPLS Tree Label|
      ++=============++       ++=============++       ++=============++
      ||C-Ether Hdr  ||       || C-Ether Hdr ||       || C-Ether Hdr ||
      ++=============++ >>>>> ++=============++ >>>>> ++=============++
      || C-IP Header ||       || C-IP Header ||       || C-IP Header ||
      ++=============++ >>>>> ++=============++ >>>>> ++=============++
      || C-Payload   ||       || C-Payload   ||       || C-Payload   ||
      ++=============++       ++=============++       ++=============++

When an ingress PE receives a packet, the ingress PE using the procedures defined in [RFC4761] and [RFC4762] determines the VPLS instance associated with the packet. If the packet is an IP multicast packet, and the ingress PE uses a Selective tree for the (C-S, C-G) carried in the packet, then the ingress PE pushes the MPLS Tree Label associated with the Selective tree, and it sends the packet over the P2MP LSP associated with the Selective tree. Otherwise, if the ingress PE does not use a Selective tree for the (C-S, C-G), or the packet is either non-IP multicast or broadcast, the ingress PE pushes the MPLS Tree Label associated with the Inclusive tree, and it sends the packet over the P2MP LSP associated with the Inclusive tree.

The egress PE does a lookup on the MPLS tree label and determines the VSI in which the receiver PE needs to do the C-multicast data packet lookup. It then pops the MPLS label and sends the packet to the VSI for multicast data forwarding.

12. VPLS Data Packet Treatment

If the destination MAC address of a VPLS packet received by an ingress PE from a VPLS site is a multicast address, a P-multicast tree SHOULD be used to transport the packet, if possible. If the packet is an IP multicast packet and a Selective tree exists for that multicast stream, the Selective tree MUST be used. Else, if a (C-*, C-*) Selective tree exists for the VPLS it SHOULD be used. Else, if an Inclusive tree exists for the VPLS, it SHOULD be used.

If the destination MAC address of a VPLS packet is a broadcast address, it is flooded. If a (C-*, C-*) Selective tree exists for the VPLS, the PE SHOULD flood over it. Else, if an Inclusive tree exists for the VPLS, the PE SHOULD flood over it. Else, the PE MUST flood the packet using the procedures in [RFC4761] or [RFC4762].

If the destination MAC address of a packet is a unicast address and it has not been learned, the packet MUST be sent to all PEs in the VPLS. Inclusive P-multicast trees or a Selective P-multicast tree bound to (C-*, C-*) SHOULD be used for sending unknown unicast MAC packets to all PEs. When this is the case, the receiving PEs MUST support the ability to perform MAC address learning for packets received on a multicast tree. In order to perform such learning, the receiver PE MUST be able to determine the sender PE when a VPLS packet is received on a P-multicast tree. This further implies that the MPLS P-multicast tree technology MUST allow the egress PE to determine the sender PE from the received MPLS packet.

When a receiver PE receives a VPLS packet with a source MAC address, which has not yet been learned, on a P-multicast tree, the receiver PE determines the PW to the sender PE. The receiver PE then creates forwarding state in the VPLS instance with a destination MAC address being the same as the source MAC address being learned, and the PW being the PW to the sender PE.

It should be noted that when a sender PE that is sending packets destined to an unknown unicast MAC address over a P-multicast tree learns the PW to use for forwarding packets destined to this unicast MAC address, it might immediately switch to transport such packets over this particular PW. Since the packets were initially being forwarded using a P-multicast tree, this could lead to packet reordering. This constraint should be taken into consideration if unknown unicast frames are forwarded using a P-multicast tree, instead of multiple PWs based on [RFC4761] or [RFC4762].

An implementation SHOULD support the ability to transport unknown unicast traffic over Inclusive P-multicast trees. Furthermore, an implementation MUST support the ability to perform MAC address learning for packets received on a P-multicast tree.

13. Security Considerations

Security considerations discussed in [RFC4761] and [RFC4762] apply to this document. This section describes additional considerations.

As mentioned in [RFC4761], there are two aspects to achieving data privacy and protecting against denial-of-service attacks in a VPLS: securing the control plane and protecting the forwarding path. Compromise of the control plane could result in a PE sending multicast data belonging to some VPLS to another VPLS, or black- holing VPLS multicast data, or even sending it to an eavesdropper; none of which are acceptable from a data privacy point of view. In addition, compromise of the control plane could result in black- holing VPLS multicast data and could provide opportunities for unauthorized VPLS multicast usage (e.g., exploiting traffic replication within a multicast tree to amplify a denial-of-service attack based on sending large amounts of traffic).

The mechanisms in this document use BGP for the control plane. Hence, techniques such as in [RFC5925] help authenticate BGP messages, making it harder to spoof updates (which can be used to divert VPLS traffic to the wrong VPLS) or withdrawals (denial-of- service attacks). In the multi-AS methods (b) and (c) described in the "Inter-AS Inclusive P-Multicast Tree A-D/Binding" section, this also means protecting the inter-AS BGP sessions, between the ASBRs, the PEs, or the Route Reflectors.

Note that [RFC5925] will not help in keeping MPLS labels, associated with P2MP LSPs or the upstream MPLS labels used for aggregation, private -- knowing the labels, one can eavesdrop on VPLS traffic. However, this requires access to the data path within an SP network, which is assumed to be composed of trusted nodes/links.

One of the requirements for protecting the data plane is that the MPLS labels be accepted only from valid interfaces. This applies both to MPLS labels associated with P2MP LSPs and to the upstream- assigned MPLS labels. For a PE, valid interfaces comprise links from other routers in the PE's own AS. For an ASBR, valid interfaces comprise links from other routers in the ASBR's own AS, and links from other ASBRs in ASes that have instances of a given VPLS. It is especially important in the case of multi-AS VPLS instances that one accept VPLS packets only from valid interfaces.

14. IANA Considerations

This document defines a new NLRI, called "MCAST-VPLS", to be carried in BGP using multiprotocol extensions. IANA has assigned it a SAFI value of 8.

This document defines a BGP-optional transitive attribute called "PMSI_TUNNEL". This is the same attribute as the one defined in [RFC6514] and the code point for this attribute has already been assigned by IANA as 22 [BGP-IANA]. Hence, no further action is required from IANA regarding this attribute.

15. References

15.1. Normative References

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC3209]   Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
               and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
               Tunnels", RFC 3209, December 2001.
   
   [RFC4760]   Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
               "Multiprotocol Extensions for BGP-4", RFC 4760, January
               2007.
   
   [RFC4761]   Kompella, K., Ed., and Y. Rekhter, Ed., "Virtual Private
               LAN Service (VPLS) Using BGP for Auto-Discovery and
               Signaling", RFC 4761, January 2007.
   
   [RFC4762]   Lasserre, M., Ed., and V. Kompella, Ed., "Virtual Private
               LAN Service (VPLS) Using Label Distribution Protocol
               (LDP) Signaling", RFC 4762, January 2007.
   
   [RFC5036]   Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
               "LDP Specification", RFC 5036, October 2007.
   
   [RFC5331]   Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
               Label Assignment and Context-Specific Label Space", RFC
               5331, August 2008.
   
   [RFC6511]   Ali, Z., Swallow, G., and R. Aggarwal, "Non-Penultimate
               Hop Popping Behavior and Out-of-Band Mapping for RSVP-TE
               Label Switched Paths", RFC 6511, February 2012.
   
   [RFC6512]   Wijnands, IJ., Rosen, E., Napierala, M., and N. Leymann,
               "Using Multipoint LDP When the Backbone Has No Route to
               the Root", RFC 6512, February 2012.

15.2. Informative References

   [RFC6514]   Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
               Encodings and Procedures for Multicast in MPLS/BGP IP
               VPNs", RFC 6514, February 2012.
   
   [RFC6513]   Rosen, E., Ed., and R. Aggarwal, Ed., "Multicast in
               MPLS/BGP IP VPNs", RFC 6513, February 2012.
   
   [RFC6388]   Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B.
               Thomas, "Label Distribution Protocol Extensions for
               Point-to-Multipoint and Multipoint-to-Multipoint Label
               Switched Paths", RFC 6388, November 2011.
   
   [RFC6074]   Rosen, E., Davie, B., Radoaca, V., and W. Luo,
               "Provisioning, Auto-Discovery, and Signaling in Layer 2
               Virtual Private Networks (L2VPNs)", RFC 6074, January
               2011.
   
   [RFC5925]   Touch, J., Mankin, A., and R. Bonica, "The TCP
               Authentication Option", RFC 5925, June 2010.
   
   [RFC5501]   Kamite, Y., Ed., Wada, Y., Serbest, Y., Morin, T., and L.
               Fang, "Requirements for Multicast Support in Virtual
               Private LAN Services", RFC 5501, March 2009.
   
   [RFC5332]   Eckert, T., Rosen, E., Ed., Aggarwal, R., and Y. Rekhter,
               "MPLS Multicast Encapsulations", RFC 5332, August 2008.
   
   [RFC4684]   Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
               R., Patel, K., and J. Guichard, "Constrained Route
               Distribution for Border Gateway Protocol/MultiProtocol
               Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual
               Private Networks (VPNs)", RFC 4684, November 2006.
   
   [RFC4875]   Aggarwal, R., Ed., Papadimitriou, D., Ed., and S.
               Yasukawa, Ed., "Extensions to Resource Reservation
               Protocol - Traffic Engineering (RSVP-TE) for Point-to-
               Multipoint TE Label Switched Paths (LSPs)", RFC 4875, May
               2007.
   
   [RFC4601]   Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
               "Protocol Independent Multicast - Sparse Mode (PIM-SM):
               Protocol Specification (Revised)", RFC 4601, August 2006.
   
   [RFC4541]   Christensen, M., Kimball, K., and F. Solensky,
               "Considerations for Internet Group Management Protocol
               (IGMP) and Multicast Listener Discovery (MLD) Snooping
               Switches", RFC 4541, May 2006.
   
   [RFC4447]   Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T., and
               G. Heron, "Pseudowire Setup and Maintenance Using the
               Label Distribution Protocol (LDP)", RFC 4447, April 2006.
   
   [RFC4364]   Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
               Networks (VPNs)", RFC 4364, February 2006.
   
   [RFC3810]   Vida, R., Ed., and L. Costa, Ed., "Multicast Listener
               Discovery Version 2 (MLDv2) for IPv6", RFC 3810, June
               2004.
   
   [RFC3376]   Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
               Thyagarajan, "Internet Group Management Protocol, Version
               3", RFC 3376, October 2002.
   
   [RFC2710]   Deering, S., Fenner, W., and B. Haberman, "Multicast
               Listener Discovery (MLD) for IPv6", RFC 2710, October
               1999.
   
   [RFC2236]   Fenner, W., "Internet Group Management Protocol, Version
               2", RFC 2236, November 1997.
   
   [RFC1997]   Chandra, R., Traina, P., and T. Li, "BGP Communities
               Attribute", RFC 1997, August 1996.

[MULTI-HOMING]

               Kothari, B., Kompella, K., Henderickx, W., Balus, F.,
               Uttaro, J., Palislamovic, S., and W. Lin, "BGP based
               Multi-homing in Virtual Private LAN Service", Work in
               Progress, July 2013.
   
   [BGP-IANA]  IANA, "Border Gateway Protocol (BGP) Parameters",
               <http://www.iana.org/assignments/bgp-parameters>.

16. Acknowledgments

Many thanks to Thomas Morin for his support of this work.

We would also like to thank authors of [RFC6514] and [RFC6513], as the details of the inter-AS segmented tree procedures in this document, as well as some text that describes these procedures have benefited from those in [RFC6514] and [RFC6513]. The same applies to the notion of Inclusive and Selective trees, as well as the procedures for switching from Inclusive to Selective trees.

We would also like to thank Nabil Bitar, Stewart Bryant, Wim Henderickx, and Eric Rosen for their review and comments.

Authors' Addresses

   Rahul Aggarwal
   998 Lucky Avenue
   Menlo Park, CA 94025
   USA
   Phone: +1-415-806-5527
   EMail: raggarwa_1@yahoo.com
   
   Yuji Kamite
   NTT Communications Corporation
   Granpark Tower
   3-4-1 Shibaura, Minato-ku
   Tokyo  108-8118
   Japan
   EMail: y.kamite@ntt.com
   
   Luyuan Fang
   Microsoft
   EMail: lufang@microsoft.com

Yakov Rekhter
Juniper Networks
1194 North Mathilda Ave.
Sunnyvale, CA 94089
USA
EMail: yakov@juniper.net

Chaitanya Kodeboniya
EMail: chaitk@yahoo.com