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Internet Engineering Task Force (IETF)
Request for Comments: 7582
Updates: 6513, 6514, 6625
Category: Standards Track
ISSN: 2070-1721
E. Rosen
Juniper Networks, Inc.
IJ. Wijnands
Cisco Systems, Inc.
Y. Cai
Microsoft
A. Boers
July 2015

Multicast Virtual Private Network (MVPN):

Using Bidirectional P-Tunnels

Abstract

A set of prior RFCs specify procedures for supporting multicast in BGP/MPLS IP VPNs. These procedures allow customer multicast data to travel across a service provider's backbone network through a set of multicast tunnels. The tunnels are advertised in certain BGP multicast auto-discovery routes, by means of a BGP attribute known as the "Provider Multicast Service Interface (PMSI) Tunnel" attribute. Encodings have been defined that allow the PMSI Tunnel attribute to identify bidirectional (multipoint-to-multipoint) multicast distribution trees. However, the prior RFCs do not provide all the necessary procedures for using bidirectional tunnels to support multicast VPNs. This document updates RFCs 6513, 6514, and 6625 by specifying those procedures. In particular, it specifies the procedures for assigning customer multicast flows (unidirectional or bidirectional) to specific bidirectional tunnels in the provider backbone, for advertising such assignments, and for determining which flows have been assigned to which tunnels.

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/rfc7582.

Copyright Notice

Copyright © 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................4
      1.1. Terminology ................................................4
      1.2. Overview ...................................................9
           1.2.1. Bidirectional P-Tunnel Technologies ................10
           1.2.2. Reasons for Using Bidirectional P-Tunnels ..........11
           1.2.3. Knowledge of Group-to-RP and/or
                  Group-to-RPA Mappings ..............................12
           1.2.4. PMSI Instantiation Methods .........................12
   2. The All BIDIR-PIM Wildcard .....................................15
   3. Using Bidirectional P-Tunnels ..................................15
      3.1. Procedures Specific to the Tunneling Technology ...........15
           3.1.1. BIDIR-PIM P-Tunnels ................................16
           3.1.2. MP2MP LSPs .........................................17
      3.2. Procedures Specific to the PMSI Instantiation Method ......17
           3.2.1. Flat Partitioning ..................................17
                  3.2.1.1. When an S-PMSI Is a 'Match for
                           Transmission' .............................19
                  3.2.1.2. When an I-PMSI Is a 'Match for
                           Transmission' .............................20
                  3.2.1.3. When an S-PMSI Is a 'Match for Reception' .21
                  3.2.1.4. When an I-PMSI Is a 'Match for Reception' .22
           3.2.2. Hierarchical Partitioning ..........................23
                  3.2.2.1. Advertisement of PE Distinguisher Labels ..24
                  3.2.2.2. When an S-PMSI Is a 'Match for
                           Transmission' .............................25
                  3.2.2.3. When an I-PMSI Is a 'Match for
                           Transmission' .............................26
                  3.2.2.4. When an S-PMSI Is a 'Match for Reception' .27
                  3.2.2.5. When an I-PMSI Is a 'Match for Reception' .27
           3.2.3. Unpartitioned ......................................28
                  3.2.3.1. When an S-PMSI Is a 'Match for
                           Transmission' .............................30
                  3.2.3.2. When an S-PMSI Is a 'Match for Reception' .30
           3.2.4. Minimal Feature Set for Compliance .................31
   4. Security Considerations ........................................32
   5. References .....................................................32
      5.1. Normative References ......................................32
      5.2. Informative References ....................................33
   Acknowledgments ...................................................34
   Authors' Addresses ................................................34

1. Introduction

The RFCs that specify multicast support for BGP/MPLS IP VPNs ([RFC6513], [RFC6514], and [RFC6625]) allow customer multicast data to be transported across a service provider's network though a set of multicast tunnels. These tunnels are advertised in BGP multicast auto-discovery (A-D) routes, by means of a BGP attribute known as the "Provider Multicast Service Interface (PMSI) Tunnel" attribute. The base specifications allow the use of bidirectional (multipoint-to- multipoint) multicast distribution trees and describe how to encode the identifiers for bidirectional trees into the PMSI Tunnel attribute. However, those specifications do not provide all the necessary detailed procedures for using bidirectional tunnels; the full specification of these procedures was considered to be outside the scope of those documents. The purpose of this document is to provide all the necessary procedures for using bidirectional trees in a service provider's network to carry the multicast data of VPN customers.

1.1. Terminology

This document uses terminology from [RFC6513] and, in particular, uses the prefixes "C-" and "P-", as specified in Section 3.1 of [RFC6513], to distinguish addresses in the "customer address space" from addresses in the "provider address space". The following terminology and acronyms are particularly important in this document:

  • MVPN

Multicast Virtual Private Network -- a VPN [RFC4364] in which multicast service is offered.

  • VRF

VPN Routing and Forwarding table [RFC4364].

  • PE

A Provider Edge router, as defined in [RFC4364].

  • SP

Service Provider.

  • LSP

An MPLS Label Switched Path.

  • P2MP

Point-to-Multipoint.

  • MP2MP

Multipoint-to-multipoint.

  • Unidirectional

Adjective for a multicast distribution tree in which all traffic travels downstream from the root of the tree. Traffic can enter a unidirectional tree only at the root. A P2MP LSP is one type of unidirectional tree. Multicast distribution trees set up by Protocol Independent Multicast - Sparse Mode (PIM-SM) [RFC4601] are also unidirectional trees. Data traffic traveling along a unidirectional multicast distribution tree is sometimes referred to in this document as "unidirectional traffic".

  • Bidirectional

Adjective for a multicast distribution tree in which traffic may travel both upstream (towards the root) and downstream (away from the root). Traffic may enter a bidirectional tree at any node. An MP2MP LSP is one type of bidirectional tree. Multicast distribution trees created by Bidirectional Protocol Independent Multicast (BIDIR-PIM) [RFC5015] are also bidirectional trees.

Data traffic traveling along a bidirectional multicast distribution tree is sometimes referred to in this document as "bidirectional traffic".

  • P-tunnel

A tunnel through the network of one or more SPs. In this document, the P-tunnels we speak of are instantiated as bidirectional multicast distribution trees.

  • SSM
      Source-Specific Multicast.   When SSM is being used, a multicast
      distribution tree carries traffic from only a single source.
  • ASM

Any Source Multicast. When ASM is being used, some multicast distribution trees ("share trees") carry traffic from multiple sources.

  • C-S

Multicast Source. A multicast source address, in the address space of a customer network.

  • C-G

Multicast Group. A multicast group address (destination address) in the address space of a customer network. When used without qualification, "C-G" may refer to either a unidirectional group address or a bidirectional group address.

  • C-G-BIDIR

A bidirectional multicast group address (i.e., a group address whose IP multicast distribution tree is built by BIDIR-PIM). 

   o  C-multicast flow or C-flow

A customer multicast flow. A C-flow travels through VPN customer sites on a multicast distribution tree set up by the customer. These trees may be unidirectional or bidirectional, depending upon the multicast routing protocol used by the customer. A C-flow travels between VPN customer sites by traveling through P-tunnels.

A C-flow from a particular customer source is identified by the ordered pair (source address, group address), where each address is in the customer's address space. The identifier of such a C-flow is usually written as (C-S,C-G).

If a customer uses the ASM model, then some or all of the customer's C-flows may be traveling along the same "shared tree". In this case, we will speak of a "(C-*,C-G)" flow to refer to a set of C-flows that travel along the same shared tree in the customer sites.

  • C-BIDIR flow or bidirectional C-flow

A C-flow that, in the VPN customer sites, travels along a bidirectional multicast distribution tree. The term "C-BIDIR flow" indicates that the customer's bidirectional tree has been set up by BIDIR-PIM.

  • RP

A Rendezvous Point, as defined in [RFC4601].

  • C-RP

A Rendezvous Point whose address is in the customer's address space.

  • RPA

A Rendezvous Point Address, as defined in [RFC5015].

  • C-RPA

An RPA in the customer's address space.

  • P-RPA

An RPA in the SP's address space.

  • Selective P-tunnel

A P-tunnel that is joined only by PE routers that need to receive one or more of the C-flows that are traveling through that P-tunnel.

  • Inclusive P-tunnel

A P-tunnel that is joined by all PE routers that attach to sites of a given MVPN.

  • PMSI

Provider Multicast Service Interface. A PMSI is a conceptual overlay on a Service Provider backbone, allowing a PE in a given MVPN to multicast to other PEs in the MVPN. PMSIs are instantiated by P-tunnels.

  • I-PMSI

Inclusive PMSI. Traffic multicast by a PE on an I-PMSI is received by all other PEs in the MVPN. I-PMSIs are instantiated by Inclusive P-tunnels.

  • S-PMSI

Selective PMSI. Traffic multicast by a PE on an S-PMSI is received by some (but not necessarily all) of the other PEs in the MVPN. S-PMSIs are instantiated by Selective P-tunnels. 

   o  Intra-AS I-PMSI A-D route

Intra-AS (Autonomous System) Inclusive Provider Multicast Service Interface Auto-Discovery route. Carried in BGP Update messages, these routes can be used to advertise the use of Inclusive P-tunnels. See [RFC6514], Section 4.1.

  • S-PMSI A-D route

Selective Provider Multicast Service Interface Auto-Discovery route. Carried in BGP Update messages, these routes are used to advertise the fact that a particular C-flow or a particular set of C-flows is bound to (i.e., is traveling through) a particular P-tunnel. See [RFC6514], Section 4.3. 

   o  (C-S,C-G) S-PMSI A-D route

An S-PMSI A-D route whose NLRI (Network Layer Reachability Information) contains C-S in its "Multicast Source" field and C-G in its "Multicast Group" field. 

   o  (C-*,C-G) S-PMSI A-D route

An S-PMSI A-D route whose NLRI contains the wildcard (C-*) in its "Multicast Source" field and C-G in its "Multicast Group" field. See [RFC6625]. 

   o  (C-*,C-G-BIDIR) S-PMSI A-D route

An S-PMSI A-D route whose NLRI contains the wildcard (C-*) in its "Multicast Source" field and C-G-BIDIR in its "Multicast Group" field. See [RFC6625]. 

   o  (C-*,C-*) S-PMSI A-D route

An S-PMSI A-D route whose NLRI contains the wildcard C-* in its "Multicast Source" field and the wildcard C-* in its "Multicast Group" field. See [RFC6625]. 

   o  (C-*,C-*-BIDIR) S-PMSI A-D route

An S-PMSI A-D route whose NLRI contains the wildcard C-* in its "Multicast Source" field and the wildcard "C-*-BIDIR" in its "Multicast Group" field. See Section 2 of this document. 

   o  (C-S,C-*) S-PMSI A-D route

An S-PMSI A-D route whose NLRI contains C-S in its "Multicast Source" field and the wildcard C-* in its "Multicast Group" field. See [RFC6625]. 

   o  Wildcard S-PMSI A-D route
   
      A (C-*,C-G) S-PMSI A-D route, a (C-*,C-*) S-PMSI A-D route, a
      (C-S,C-*) S-PMSI A-D route, or a (C-*,C-*-BIDIR) S-PMSI A-D route.
  • PTA

PMSI Tunnel attribute, a BGP attribute that identifies a P-tunnel. See [RFC6514], Section 8.

The terminology used for categorizing S-PMSI A-D routes will also be used for categorizing the S-PMSIs advertised by those routes. For example, the S-PMSI advertised by a (C-*,C-G) S-PMSI A-D route will be known as a "(C-*,C-G) S-PMSI".

Familiarity with multicast concepts and terminology [RFC4601] is also presupposed.

This specification uses the terms "match for transmission" and "match for reception" as they are defined in [RFC6625]. When it is clear from the context whether we are talking of transmission or reception, we will sometimes talk simply of a C-flow "matching" an I-PMSI or S-PMSI A-D route.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document, when and only when appearing in all caps, are to be interpreted as described in [RFC2119].

1.2. Overview

The base documents for MVPN ([RFC6513] and [RFC6514]) define a "PMSI Tunnel attribute" (PTA). This is a BGP Path attribute that may be attached to the BGP "I-PMSI A-D routes" and "S-PMSI A-D routes" that are defined in those documents. The base documents define the way in which the identifier of a bidirectional P-tunnel is to be encoded in the PTA. However, those documents do not contain the full set of specifications governing the use of bidirectional P-tunnels; rather, those documents declare the full set of specifications for using bidirectional P-tunnels to be outside their scope. Similarly, the use of bidirectional P-tunnels advertised in wildcard S-PMSI A-D routes is declared by [RFC6625] to be "outside the scope" of that document.

This document provides the specifications governing the use of bidirectional P-tunnels to provide MVPN support. This includes the procedures for assigning C-flows to specific bidirectional P-tunnels, for advertising the fact that a particular C-flow has been assigned to a particular bidirectional P-tunnel, and for determining the bidirectional P-tunnel on which a given C-flow may be expected.

The C-flows carried on bidirectional P-tunnels may, themselves, be either unidirectional or bidirectional. Procedures are provided for both cases.

This document does not specify any new data encapsulations for bidirectional P-tunnels. Section 12 ("Encapsulations") of [RFC6513] applies unchanged.

With regard to the procedures for using bidirectional P-tunnels to instantiate PMSIs, if there is any conflict between the procedures specified in this document and the procedures of [RFC6513], [RFC6514], or [RFC6625], the procedures of this document take precedence.

The use of bidirectional P-tunnels to support extranets [MVPN-XNET] is outside the scope of this document. The use of bidirectional P-tunnels as "segmented P-tunnels" (see Section 8 of [RFC6513] and various sections of [RFC6514]) is also outside the scope of this document.

1.2.1. Bidirectional P-Tunnel Technologies

This document supports two different technologies for creating and maintaining bidirectional P-tunnels:

  • Multipoint-to-multipoint Label Switched Paths (MP2MP LSPs) that are created through the use of the Label Distribution Protocol (LDP) Multipoint-to-Multipoint extensions [RFC6388].
  • Multicast distribution trees that are created through the use of BIDIR-PIM [RFC5015].

Other bidirectional tunnel technologies are outside the scope of this document.

1.2.2. Reasons for Using Bidirectional P-Tunnels

Bidirectional P-tunnels can be used to instantiate I-PMSIs and/or S-PMSIs.

An SP may decide to use bidirectional P-tunnels to instantiate certain I-PMSIs and/or S-PMSIs in order to provide its customers with C-BIDIR support, using the "Partitioned Set of PEs" technique discussed in Section 11.2 of [RFC6513] and Section 3.6 of [RFC6517]. This technique can be used whether the C-BIDIR flows are being carried on an I-PMSI or an S-PMSI.

Even if an SP does not need to provide C-BIDIR support, it may still decide to use bidirectional P-tunnels, in order to save state in the network's transit nodes. For example, if an MVPN has n PEs attached to sites with multicast sources, and there is an I-PMSI for that MVPN, instantiating the I-PMSI with unidirectional P-tunnels (i.e., with P2MP multicast distribution trees) requires n multicast distribution trees, each one rooted at a different PE. If the I-PMSI is instantiated by a bidirectional P-tunnel, a single multicast distribution tree can be used, assuming appropriate support by the provisioning system.

An SP may decide to use bidirectional P-tunnels for either or both of these reasons. Note that even if the reason for using bidirectional P-tunnels is to provide C-BIDIR support, the same P-tunnels can also be used to carry unidirectional C-flows, if that is the choice of the SP.

These two reasons for using bidirectional P-tunnels may appear to be somewhat in conflict with each other, since (as will be seen in subsequent sections) the use of bidirectional P-tunnels for C-BIDIR support may require multiple bidirectional P-tunnels per VPN. Each such P-tunnel is associated with a particular "distinguished PE", and can only carry those C-BIDIR flows whose C-RPAs are reachable through its distinguished PE. However, on platforms that support MPLS upstream-assigned labels ([RFC5331]), PE Distinguisher Labels (Section 4 of [RFC6513] and Section 8 of [RFC6514]) can be used to aggregate multiple bidirectional P-tunnels onto a single outer bidirectional P-tunnel, thereby allowing one to provide C-BIDIR support with minimal state at the transit nodes.

Since there are two fundamentally different reasons for using bidirectional P-tunnels, and since many deployed router platforms do not support upstream-assigned labels at the current time, this document specifies several different methods of using bidirectional P-tunnels to instantiate PMSIs. We refer to these as "PMSI Instantiation Methods". The method or methods deployed by any particular SP will depend upon that SP's goals and engineering trade- offs and upon the set of platforms deployed by that SP.

The rules for using bidirectional P-tunnels in I-PMSI or S-PMSI A-D routes are not exactly the same as the rules for using unidirectional P-tunnels, and the rules are also different for the different PMSI instantiation methods. Subsequent sections of this document specify the rules in detail.

1.2.3. Knowledge of Group-to-RP and/or Group-to-RPA Mappings

If a VPN customer is making use of a particular ASM group address, the PEs of that VPN generally need to know the group-to-RP mappings that are used within the VPN. If a VPN customer is making use of BIDIR-PIM group addresses, the PEs need to know the group-to-RPA mappings that are used within the VPN. Commonly, the PEs obtain this knowledge either through provisioning or by participating in a dynamic "group-to-RP(A) mapping discovery protocol" that runs within the VPN. However, the way in which this knowledge is obtained is outside the scope of this document.

The PEs also need to be able to forward traffic towards the C-RPs and/or C-RPAs and to determine whether the next-hop interface of the route to a particular C-RP(A) is a VRF interface or a PMSI. This is done by applying the procedures of [RFC6513], Section 5.1.

1.2.4. PMSI Instantiation Methods

This document specifies three methods for using bidirectional P-tunnels to instantiate PMSIs: two partitioned methods (the Flat Partitioned Method and the Hierarchical Partitioned Method) and the Unpartitioned Method.

  • Partitioned Methods

In the Partitioned Methods, a particular PMSI is instantiated by a set of bidirectional P-tunnels. These P-tunnels may be aggregated (as inner P-tunnels) into a single outer bidirectional P-tunnel ("Hierarchical Partitioning"), or they may be unaggregated ("Flat Partitioning"). Any PE that joins one of these P-tunnels can transmit a packet on it, and the packet will be received by all the other PEs that have joined the P-tunnel. For each such P-tunnel (each inner P-tunnel, in the case of Hierarchical Partitioning) there is one PE that is its distinguished PE. When a PE receives a packet from a given P-tunnel, the PE can determine from the packet's encapsulation the P-tunnel it has arrived on, and it can thus infer the identity of the distinguished PE associated with the packet. This association plays an important role in the treatment of the packet, as specified later on in this document.

The number of P-tunnels needed (the number of inner P-tunnels needed, if Hierarchical Partitioning is used) depends upon a number of factors that are described later in this document.

The Hierarchical Partitioned Method requires the use of upstream- assigned MPLS labels (PE Distinguisher Labels) and requires the use of the PE Distinguisher Labels attribute in BGP. The Flat Partitioned Method requires neither of these.

The Partitioned Method (either Flat or Hierarchical) is a prerequisite for implementing the "Partitioned Sets of PEs" technique of supporting C-BIDIR, as discussed in [RFC6513], Section 11.2. The Partitioned Method (either Flat or Hierarchical) is also a prerequisite for applying the "Discarding Packets from Wrong PE" technique, discussed in [RFC6513], Section 9.1.1, to a PMSI that is instantiated by a bidirectional P-tunnel.

The Flat Partitioned Method is a prerequisite for implementing the "Partial Mesh of MP2MP P-Tunnels" technique for carrying customer bidirectional (C-BIDIR) traffic, as discussed in [RFC6513], Section 11.2.3.

The Hierarchical Partitioned Method is a prerequisite for implementing the "Using PE Distinguisher Labels" technique of carrying customer bidirectional (C-BIDIR) traffic, as discussed in [RFC6513], Section 11.2.2.

Note that a particular deployment may choose to use the Partitioned Methods for carrying the C-BIDIR traffic on bidirectional P-tunnels, while carrying other traffic either on unidirectional P-tunnels or on bidirectional P-tunnels using the Unpartitioned Method. Routers in a given deployment must be provisioned to know which PMSI instantiation method to use for which PMSIs.

There may be ways of implementing the Partitioned Methods with PMSIs that are instantiated by unidirectional P-tunnels. (See, e.g., [MVPN-BIDIR-IR].) However, that is outside the scope of the current document.

  • Unpartitioned Method

In the Unpartitioned Method, a particular PMSI can be instantiated by a single bidirectional P-tunnel. Any PE that joins the tunnel can transmit a packet on it, and the packet will be received by all the other PEs that have joined the tunnel. The receiving PEs can determine the tunnel on which the packet was transmitted, but they cannot determine which PE transmitted the packet, nor can they associate the packet with any particular distinguished PE.

When the Unpartitioned Method is used, this document does not mandate that only one bidirectional P-tunnel be used to instantiate each PMSI. It allows for the case where more than one P-tunnel is used. In this case, the transmitting PEs will have a choice of which such P-tunnel to use when transmitting, and the receiving PEs must be prepared to receive from any of those P-tunnels. The use of multiple P-tunnels in this case provides additional robustness, but it does not provide additional functionality.

If bidirectional P-tunnels are being used to instantiate the PMSIs of a given MVPN, one of these methods must be chosen for that MVPN. All the PEs of that MVPN must be provisioned to know the method that is being used for that MVPN.

I-PMSIs may be instantiated by bidirectional P-tunnels using either the Partitioned (either Flat or Hierarchical) Methods or the Unpartitioned Method. The method used for a given MVPN is determined by provisioning. It SHOULD be possible to provision this on a per- MVPN basis, but all the VRFs of a single MVPN MUST be provisioned to use the same method for the given MVPN's I-PMSI.

If a bidirectional P-tunnel is used to instantiate an S-PMSI (including the case of a (C-*,C-*) S-PMSI), either the Partitioned Methods (either Flat or Hierarchical) or the Unpartitioned Method may be used. The method used by a given VRF is determined by provisioning. It is desirable to be able to provision this on a per- MVPN basis. All the VRFs of a single MVPN MUST be provisioned to use the same method for those of their S-PMSIs that are instantiated by bidirectional P-tunnels.

If one of the Partitioned Methods is used, all the VRFs of a single MVPN MUST be provisioned to use the same variant of the Partitioned Methods, i.e., either they must all use the Flat Partitioned Method or they must all use the Hierarchical Partitioned Method.

It is valid to use the Unpartitioned Method to instantiate the I-PMSIs, while using one of the Partitioned Methods to instantiate the S-PMSIs.

It is valid to instantiate some S-PMSIs by unidirectional P-tunnels and others by bidirectional P-tunnels.

The procedures for the use of bidirectional P-tunnels, specified in subsequent sections of this document, depend on both the tunnel technology and the PMSI instantiation method. Note that this document does not specify procedures for every possible combination of tunnel technology and PMSI instantiation method.

2. The All BIDIR-PIM Wildcard

[RFC6514] specifies the method of encoding C-multicast source and group addresses into the NLRI of certain BGP routes. [RFC6625] extends that specification by allowing the source and/or group address to be replaced by a wildcard. When an MVPN customer is using BIDIR-PIM, it is useful to be able to advertise an S-PMSI A-D route whose semantics are "by default, all BIDIR-PIM C-multicast traffic (within a given VPN) that has not been bound to any other P-tunnel is bound to the bidirectional P-tunnel identified by the PTA of this route". This can be especially useful if one is using a bidirectional P-tunnel to carry the C-BIDIR flows while using unidirectional P-tunnels to carry other C-flows. To do this, it is necessary to have a way to encode a (C-*,C-*) wildcard that is restricted to BIDIR-PIM C-groups.

Therefore, we define a special value of the group wildcard, whose meaning is "all BIDIR-PIM groups". The "BIDIR-PIM groups wildcard" is encoded as a group field whose length is 8 bits and whose value is zero. That is, the "multicast group length" field contains the value 0x08, and the "multicast group" field is a single octet containing the value 0x00. (This encoding is distinct from the group wildcard encoding defined in [RFC6625]). We will use the notation (C-*,C-*-BIDIR) to refer to the "all BIDIR-PIM groups" wildcard.

3. Using Bidirectional P-Tunnels

A bidirectional P-tunnel may be advertised in the PTA of an Intra-AS I-PMSI A-D route or in the PTA of an S-PMSI A-D route. The advertisement of a bidirectional P-tunnel in the PTA of an Inter-AS I-PMSI A-D route is outside the scope of this document.

3.1. Procedures Specific to the Tunneling Technology

This section discusses the procedures that are specific to a given tunneling technology (BIDIR-PIM or the MP2MP procedures of mLDP (Multipoint LDP)) but that are independent of the method (Unpartitioned, Flat Partitioned, or Hierarchical Partitioned) used to instantiate a PMSI.

3.1.1. BIDIR-PIM P-Tunnels

Each BIDIR-PIM P-tunnel is identified by a unique P-group address ([RFC6513], Section 3.1). (The P-group address is called a "P-Multicast Group" in [RFC6514]). Section 5 of [RFC6514] specifies the way to identify a particular BIDIR-PIM P-tunnel in the PTA of an I-PMSI or S-PMSI A-D route.

Ordinary BIDIR-PIM procedures are used to set up the BIDIR-PIM P-tunnels. A BIDIR-PIM P-group address is always associated with a unique Rendezvous Point Address (RPA) in the SP's address space. We will refer to this as the "P-RPA". Every PE needing to join a particular BIDIR-PIM P-tunnel must be able to determine the P-RPA that corresponds to the P-tunnel's P-group address. To construct the P-tunnel, PIM Join/Prune messages are sent along the path from the PE to the P-RPA. Any P routers along that path must also be able to determine the P-RPA, so that they too can send PIM Join/Prune messages towards it. The method of mapping a P-group address to an RPA may be static configuration, or some automated means of RPA discovery that is outside the scope of this specification.

If a BIDIR-PIM P-tunnel is used to instantiate an I-PMSI or an S-PMSI, it is RECOMMENDED that the path from each PE in the tunnel to the RPA consist entirely of point-to-point links. On a point-to- point link, there is no ambiguity in determining which router is upstream towards a particular RPA, so the BIDIR-PIM "Designated Forwarder Election" is very quick and simple. Use of a BIDIR-PIM P-tunnel containing multiaccess links is possible, but considerably more complex.

The use of BIDIR-PIM P-tunnels to support the Hierarchical Partitioned Method is outside the scope of this document.

When the PTA of an Intra-AS I-PMSI A-D route or an S-PMSI A-D route identifies a BIDIR-PIM tunnel, the originator of the route SHOULD NOT include a PE Distinguisher Labels attribute. If it does, that attribute MUST be ignored. When we say the attribute is "ignored", we do not mean that its normal BGP processing is not done, but that the attribute has no effect on the data plane. However, it MUST be treated by BGP as if it were an unsupported optional transitive attribute. (PE Distinguisher Labels are used for the Hierarchical Partitioning Method, but this document does not provide support for the Hierarchical Partitioning Method with BIDIR-PIM P-tunnels.)

3.1.2. MP2MP LSPs

Each MP2MP LSP is identified by a unique "MP2MP FEC (Forwarding Equivalence Class) element" [RFC6388]. The FEC element contains the IP address of the root node, followed by an opaque value that identifies the MP2MP LSP uniquely in the context of the root node's IP address. This opaque value may be configured or autogenerated; there is no need for different root nodes to use the same opaque value for a given MVPN.

The mLDP specification supports the use of several different ways of constructing the tunnel identifiers. The current specification does not place any restriction on the type or types of tunnel identifier that is used in a given deployment. A given implementation is not expected to be able to advertise (in the PTAs of I-PMSI or S-PMSI A-D routes) tunnel identifiers of every possible type. However, an implementation SHOULD be able to accept and properly process a PTA that uses any legal type of tunnel identifier.

Section 5 of [RFC6514] specifies the way to identify a particular MP2MP P-tunnel in the PTA of an I-PMSI or S-PMSI A-D route.

Ordinary mLDP procedures for MP2MP LSPs are used to set up the MP2MP LSP.

3.2. Procedures Specific to the PMSI Instantiation Method

   When either the Flat Partitioned Method or the Hierarchical
   Partitioned Method is used to implement the "Partitioned Sets of PEs"
   method of supporting C-BIDIR, as discussed in Section 11.2 of
   [RFC6513] and Section 3.6 of [RFC6517], a C-BIDIR flow MUST be
   carried only on an I-PMSI or on a (C-*,C-G-BIDIR), (C-*,C-*-BIDIR),
   or (C-*,C-*) S-PMSI.  A PE MUST NOT originate any (C-S,C-G-BIDIR)
   S-PMSI A-D routes.  (Though it may, of course, originate (C-S,C-G)
   S-PMSI A-D routes for C-G's that are not C-BIDIR groups.)  Packets of
   a C-BIDIR flow MUST NOT be carried on a (C-S,C-*) S-PMSI.

Sections 3.2.1 and 3.2.2 specify additional details of the two Partitioned Methods.

3.2.1. Flat Partitioning

The procedures of this section and its subsections apply when (and only when) the Flat Partitioned Method is used. This method is introduced in [RFC6513], Section 11.2.3, where it is called "Partial Mesh of MP2MP P-Tunnels". This method can be used with MP2MP LSPs or with BIDIR-PIM P-tunnels.

When a PE originates an I-PMSI or S-PMSI A-D route whose PTA specifies a bidirectional P-tunnel, the PE MUST be the root node of the specified P-tunnel.

If BIDIR-PIM P-tunnels are used, each advertised P-tunnel MUST have a distinct P-group address. The PE advertising the tunnel will be considered to be the root node of the tunnel. Note that this creates a unique mapping from P-group address to root node. The assignment of P-group addresses to MVPNs is by provisioning.

If MP2MP LSPs are used, each P-tunnel MUST have a distinct MP2MP FEC (i.e., a distinct combination of root node and opaque value). The PE advertising the tunnel MUST be the same PE identified in the root node field of the MP2MP FEC that is encoded in the PTA.

It follows that two different PEs may not advertise the same bidirectional P-tunnel. Any PE that receives a packet from the P-tunnel can infer the identity of the P-tunnel from the packet's encapsulation. Once the identity of the P-tunnel is known, the root node of the P-tunnel is also known. The root node of the P-tunnel on which the packet arrived is treated as the distinguished PE for that packet.

The Flat Partitioned Method does not use upstream-assigned labels in the data plane, and hence does not use the BGP PE Distinguisher Labels attribute. When this method is used, I-PMSI and/or S-PMSI A-D routes SHOULD NOT contain a PE Distinguisher Labels attribute; if such an attribute is present in a received I-PMSI or S-PMSI A-D route, it MUST be ignored. (When we say the attribute is "ignored", we do not mean that its normal BGP processing is not done, but that the attribute has no effect on the data plane. It MUST, however, be treated by BGP as if it were an unsupported optional transitive attribute.)

When the Flat Partitioned Method is used to instantiate the I-PMSIs of a given MVPN, every PE in that MVPN that originates an Intra-AS I-PMSI A-D route MUST include a PTA that specifies a bidirectional P-tunnel. If the intention is to carry C-BIDIR traffic on the I-PMSI, a PE MUST originate an Intra-AS I-PMSI A-D route if one of its VRF interfaces is the next-hop interface on its best path to the C-RPA of any bidirectional C-group of the MVPN. 

   When the Flat Partitioned Method is used to instantiate a (C-*,C-*)
   S-PMSI, a (C-*,C-*-BIDIR) S-PMSI, or a (C-*,C-G-BIDIR) S-PMSI, a PE
   that originates the corresponding S-PMSI A-D route MUST include in
   that route a PTA specifying a bidirectional P-tunnel.  Per the
   procedures of [RFC6513] and [RFC6514], a PE will originate such an
   S-PMSI A-D route only if one of the PE's VRF interfaces is the next-

hop interface of the PE's best path to the C-RPA of a C-BIDIR group that is to be carried on the specified S-PMSI.

PMSIs that are instantiated via the Flat Partitioned Method may carry customer bidirectional traffic AND customer unidirectional traffic. The rules of Sections 3.2.1.1 and 3.2.1.2 determine when a given customer multicast packet is a match for transmission to a given PMSI. However, if the "Partitioned Set of PEs" method of supporting C-BIDIR traffic is being used for a given MVPN, the PEs must be provisioned in such a way that packets from a C-BIDIR flow of that MVPN never match any PMSI that is not instantiated by a bidirectional P-tunnel. (For example, if the given MVPN's (C-*,C-*) S-PMSI were not instantiated by a bidirectional P-tunnel, one could meet this requirement by carrying all C-BIDIR traffic of that MVPN on a (C-*,C-*-BIDIR) S-PMSI.)

When a PE receives a customer multicast data packet from a bidirectional P-tunnel, it associates that packet with a distinguished PE. The distinguished PE for a given packet is the root node of the tunnel from which the packet is received. The rules of Sections 3.2.1.1 and 3.2.1.2 ensure that:

  • If the received packet is part of a unidirectional C-flow, its distinguished PE is the PE that transmitted the packet onto the P-tunnel.
  • If the received packet is part of a bidirectional C-flow, its distinguished PE is not necessarily the PE that transmitted it, but rather the transmitter's upstream PE [RFC6513] for the C-RPA of the bidirectional C-group.

The rules of Sections 3.2.1.3 and 3.2.1.4 allow the receiving PEs to determine the expected distinguished PE for each C-flow, and ensure that a packet will be discarded if its distinguished PE is not the expected distinguished PE for the C-flow to which the packet belongs. This prevents duplication of data for both bidirectional and unidirectional C-flows.

3.2.1.1. When an S-PMSI Is a 'Match for Transmission'

Suppose a given PE, say PE1, needs to transmit multicast data packets of a particular C-flow. Section 3.1 of [RFC6625] gives a four-step algorithm for determining the S-PMSI A-D route, if any, that matches that C-flow for transmission.

If the C-flow is not a BIDIR-PIM C-flow, those rules apply unchanged; the remainder of this section applies only to C-BIDIR flows. If a C-BIDIR flow has group address C-G-BIDIR, the rules applied by PE1 are given below:

  • If the C-RPA for C-G-BIDIR is a C-address of PE1, or if PE1's route to the C-RPA is via a VRF interface, then:
      *  If there is a (C-*,C-G-BIDIR) S-PMSI A-D route currently
         originated by PE1, then the C-flow matches that route.
      
      *  Otherwise, if there is a (C-*,C-*-BIDIR) S-PMSI A-D route
         currently originated by PE1, then the C-flow matches that
         route.
      
      *  Otherwise, if there is a (C-*,C-*) S-PMSI A-D route currently
         originated by PE1, then the C-flow matches that route.
  • If PE1 determines the upstream PE for C-G-BIDIR's C-RPA to be some other PE, say PE2, then:
      *  If there is an installed (C-*,C-G-BIDIR) S-PMSI A-D route
         originated by PE2, then the C-flow matches that route.
      
      *  Otherwise, if there is an installed (C-*,C-*-BIDIR) S-PMSI A-D
         route originated by PE2, then the C-flow matches that route.
      
      *  Otherwise, if there is an installed (C-*,C-*) S-PMSI A-D route
         originated by PE2, then the C-flow matches that route.

If there is an S-PMSI A-D route that matches a given C-flow, and if PE1 needs to transmit packets of that C-flow or other PEs, then it MUST transmit those packets on the bidirectional P-tunnel identified in the PTA of the matching S-PMSI A-D route.

3.2.1.2. When an I-PMSI Is a 'Match for Transmission'

Suppose a given PE, say PE1, needs to transmit packets of a given C-flow (of a given MVPN) to other PEs, but according to the conditions of Section 3.2.1.1 and/or Section 3.1 of [RFC6625], that C-flow does not match any S-PMSI A-D route. Then, the packets of the C-flow need to be transmitted on the MVPN's I-PMSI.

If the C-flow is not a BIDIR-PIM C-flow, the P-tunnel on which the C-flow MUST be transmitted is the one identified in the PTA of the Intra-AS I-PMSI A-D route originated by PE1 for the given MVPN.

If the C-flow is a BIDIR-PIM C-flow with group address C-G-BIDIR, the rules applied by PE1 are:

  • Suppose that the C-RPA for C-G-BIDIR is a C-address of PE1, or that PE1's route to the C-RPA is via a VRF interface. Then, if there is an I-PMSI A-D route currently originated by PE1, the C-flow MUST be transmitted on the P-tunnel identified in the PTA of that I-PMSI A-D route.
  • If PE1 determines the upstream PE for C-G-BIDIR's C-RPA to be some other PE, say PE2, then if there is an installed I-PMSI A-D route originated by PE2, the C-flow MUST be transmitted on the P-tunnel identified in the PTA of that route.

If there is no I-PMSI A-D route meeting the above conditions, the C-flow MUST NOT be transmitted.

3.2.1.3. When an S-PMSI Is a 'Match for Reception'

Suppose a given PE, say PE1, needs to receive multicast data packets of a particular C-flow. Section 3.2 of [RFC6625] specifies procedures for determining the S-PMSI A-D route, if any, that matches that C-flow for reception. Those rules apply unchanged for C-flows that are not BIDIR-PIM C-flows. The remainder of this section applies only to C-BIDIR flows.

The rules of [RFC6625], Section 3.2.1, are not applicable to C-BIDIR flows. The rules of [RFC6625], Section 3.2.2, are replaced by the following rules.

Suppose PE1 needs to receive (C-*,C-G-BIDIR) traffic. Suppose also that PE1 has determined that PE2 is the upstream PE [RFC6513] for the C-RPA of C-G-BIDIR. Then: 

   o  If PE1 is not the same as PE2, and PE1 has an installed (C-*,C-G-
   
      BIDIR) S-PMSI A-D route originated by PE2, then (C-*,C-G-BIDIR)
      matches this route.
  • Otherwise, if PE1 is the same as PE2, and PE1 has currently originated a (C-*,C-G-BIDIR) S-PMSI A-D route, then (C-*,C-G-BIDIR) matches this route.
  • Otherwise, if PE1 is not the same as PE2, and PE1 has an installed (C-*,C-*-BIDIR) S-PMSI A-D route originated by PE2, then (C-*,C-G-BIDIR) matches this route.
  • Otherwise, if PE1 is the same as PE2, and PE1 has currently originated a (C-*,C-*-BIDIR) S-PMSI A-D route, then (C-*,C-G-BIDIR) matches this route.
  • Otherwise, if PE1 is not the same as PE2, and PE1 has an installed (C-*,C-*) S-PMSI A-D route originated by PE2, then (C-*,C-G-BIDIR) matches this route.
  • Otherwise, if PE1 is the same as PE2, and PE1 has currently originated a (C-*,C-*) S-PMSI A-D route, then (C-*,C-G-BIDIR) matches this route.

If there is an S-PMSI A-D route matching (C-*,C-G-BIDIR), according to these rules, the root node of that P-tunnel is considered to be the distinguished PE for that (C-*,C-G-BIDIR) flow. If a (C-*,C-G-BIDIR) packet is received on a P-tunnel whose root node is not the distinguished PE for the C-flow, the packet MUST be discarded.

3.2.1.4. When an I-PMSI Is a 'Match for Reception'

Suppose a given PE, say PE1, needs to receive packets of a given C-flow (of a given MVPN) from another PE, but according to the conditions of Section 3.2.1.3 and/or Section 3.2 of [RFC6625], that C-flow does not match any S-PMSI A-D route. Then, the packets of the C-flow need to be received on the MVPN's I-PMSI.

If the C-flow is not a BIDIR-PIM C-flow, the rules for determining the P-tunnel on which packets of the C-flow are expected are given in [RFC6513]. The remainder of this section applies only to C-BIDIR flows.

Suppose that PE1 needs to receive (C-*,C-G-BIDIR) traffic from other PEs. Suppose also that PE1 has determined that PE2 is the upstream PE [RFC6513] for the C-RPA of C-G-BIDIR. Then, PE1 considers PE2 to be the distinguished PE for (C-*,C-G-BIDIR). If PE1 has an installed Intra-AS I-PMSI A-D route originated by PE2, PE1 will expect to receive packets of the C-flow from the tunnel specified in that route's PTA. (If all VRFs of the MVPN have been properly provisioned to use the Flat Partitioned Method for the I-PMSI, the PTA will specify a bidirectional P-tunnel.) Note that if PE1 is the same as PE2, then the relevant Intra-AS I-PMSI A-D route is the one currently originated by PE1.

If a (C-*,C-G-BIDIR) packet is received on a P-tunnel other than the expected one, the packet MUST be discarded.

3.2.2. Hierarchical Partitioning

The procedures of this section and its subsections apply when (and only when) the Hierarchical Partitioned Method is used. This method is introduced in [RFC6513], Section 11.2.2. This document only provides procedures for using this method when using MP2MP LSPs as the P-tunnels.

The Hierarchical Partitioned Method provides the same functionality as the Flat Partitioned Method, but it requires a smaller amount of state to be maintained in the core of the network. However, it requires the use of upstream-assigned MPLS labels ("PE Distinguisher Labels"), which are not necessarily supported by all hardware platforms. The upstream-assigned labels are used to provide an LSP hierarchy, in which an outer MP2MP LSP carries multiple inner MP2MP LSPs. Transit routers along the path between PE routers then only need to maintain state for the outer MP2MP LSP.

When this method is used to instantiate a particular PMSI, the bidirectional P-tunnel advertised in the PTA of the corresponding I-PMSI or S-PMSI A-D route is the outer P-tunnel. When a packet is received from a P-tunnel, the PE that receives it can infer the identity of the outer P-tunnel from the MPLS label that has risen to the top of the packet's label stack. However, the packet's distinguished PE is not necessarily the root node of the outer P-tunnel. Rather, the identity of the packet's distinguished PE is inferred from the PE Distinguisher Label further down in the label stack. (See [RFC6513], Section 12.3.) The PE Distinguisher Label may be thought of as identifying an inner MP2MP LSP whose root is the PE corresponding to that label.

In the context of a given MVPN, if it is desired to use the Hierarchical Partitioned Method to instantiate an I-PMSI, a (C-*,C-*) S-PMSI, or a (C-*,C-*-BIDIR) S-PMSI, the corresponding A-D routes MUST be originated by some of the PEs that attach to that MVPN. The PEs that are REQUIRED to originate these routes are those that satisfy one of the following conditions:

  • There is a C-BIDIR group for which the best path from the PE to the C-RPA of that C-group is via a VRF interface.
  • The PE might have to transmit unidirectional customer multicast traffic on the PMSI identified in the route (of course this condition does not apply to (C-*,C-*-BIDIR) or to (C-*,C-G-BIDIR) S-PMSIs).
   o  The PE is the root node of the MP2MP LSP that is used to
      instantiate the PMSI.

When the Hierarchical Partitioned method is used to instantiate a (C-*,C-G-BIDIR) S-PMSI, the corresponding (C-*,C-G-BIDIR) S-PMSI route MUST NOT be originated by a given PE unless either (a) that PE's best path to the C-RPA for C-G-BIDIR is via a VRF interface, or (b) the C-RPA is a C-address of the PE. Further, that PE MUST be the root node of the MP2MP LSP identified in the PTA of the S-PMSI A-D route.

If any VRF of a given MVPN uses this method to instantiate an S-PMSI with a bidirectional P-tunnel, all VRFs of that MVPN must use this method.

Suppose that for a given MVPN, the Hierarchical Partitioned Method is used to instantiate the I-PMSI. In general, more than one of the PEs in the MVPN will originate an Intra-AS I-PMSI A-D route for that MVPN. This document allows the PTAs of those routes to all specify the same MP2MP LSP as the "outer tunnel". However, it does not require that those PTAs all specify the same MP2MP LSP as the outer tunnel. By having all the PEs specify the same outer tunnel for the I-PMSI, one can minimize the amount of state in the transit nodes. By allowing them to specify different outer tunnels, one uses more state, but may increase the robustness of the system.

The considerations of the previous paragraph apply as well when the Hierarchical Partitioned Method is used to instantiate an S-PMSI.

3.2.2.1. Advertisement of PE Distinguisher Labels

A PE Distinguisher Label is an upstream-assigned MPLS label [RFC5331] that can be used, in the context of an MP2MP LSP, to denote a particular PE that either has joined or may in the future join that LSP.

In order to use upstream-assigned MPLS labels in the context of an outer MP2MP LSP, there must be a convention that identifies a particular router as the router that is responsible for allocating the labels and for advertising the labels to the PEs that may join the MP2MP LSP. This document REQUIRES that the PE Distinguisher Labels used in the context of a given MP2MP LSP be allocated and advertised by the router that is the root node of the LSP.

This convention accords with the rules of Section 7 of [RFC5331]. Note that according to Section 7 of [RFC5331], upstream-assigned labels are unique in the context of the IP address of the root node; if two MP2MP LSPs have the same root node IP address, the upstream- assigned labels used within the two LSPs come from the same label space.

This document assumes that the root node address of an MP2MP LSP is an IP address that is uniquely assigned to the node. The use of an "anycast address" as the root node address is outside the scope of this document.

A PE Distinguisher Labels attribute SHOULD NOT be attached to an I-PMSI or S-PMSI A-D route unless that route also contains a PTA that specifies an MP2MP LSP. (While PE Distinguisher Labels could in theory also be used if the PTA specifies a BIDIR-PIM P-tunnel, such use is outside the scope of this document.)

The PE Distinguisher Labels attribute specifies a set of <MPLS label, IP address> bindings. Within a given PE Distinguisher Labels attribute, each such IP address MUST appear at most once, and each MPLS label MUST appear only once. Otherwise, the attribute is considered to be malformed, and the "treat-as-withdraw" error- handling approach described in Section 2 of [BGP-ERROR] MUST be used.

When a PE Distinguisher Labels attribute is included in a given I-PMSI or S-PMSI A-D route, it MUST assign a label to the IP address of each of the following PEs:

  • The root node of the MP2MP LSP identified in the PTA of the route.
  • Any PE that is possibly the ingress PE for a C-RPA of any C-BIDIR group.
  • Any PE that may need to transmit non-C-BIDIR traffic on the MP2MP LSP identified in the PTA of the route.

One simple way to meet these requirements is to assign a PE Distinguisher label to every PE that has originated an Intra-AS I-PMSI A-D route.

3.2.2.2. When an S-PMSI Is a 'Match for Transmission'

Suppose a given PE, say PE1, needs to transmit multicast data packets of a particular C-flow. Section 3.1 of [RFC6625] gives a four-step algorithm for determining the S-PMSI A-D route, if any, that matches that C-flow for transmission.

If the C-flow is not a BIDIR-PIM C-flow, those rules apply unchanged. If there is a matching S-PMSI A-D route, the P-tunnel on which the C-flow MUST be transmitted is the one identified in the PTA of the matching route. Each packet of the C-flow MUST carry the PE Distinguisher Label assigned by the root node of that P-tunnel to the IP address of PE1. See Section 12.3 of [RFC6513] for encapsulation details.

The remainder of this section applies only to C-BIDIR flows. If a C-BIDIR flow has group address C-G-BIDIR, the rules applied by PE1 are the same as the rules given in Section 3.2.1.1.

If there is a matching S-PMSI A-D route, PE1 MUST transmit the C-flow on the P-tunnel identified in its PTA. Suppose PE1 has determined that PE2 is the upstream PE for the C-RPA of the given C-flow. In constructing the packet's MPLS label stack, PE1 must use the PE Distinguisher Label that was assigned by the P-tunnel's root node to the IP address of "PE2", not the label assigned to the IP address of "PE1" (unless, of course, PE1 is the same as PE2). See Section 12.3 of [RFC6513] for encapsulation details. Note that the root of the P-tunnel might be a PE other than PE1 or PE2.

3.2.2.3. When an I-PMSI Is a 'Match for Transmission'

Suppose a given PE, say PE1, needs to transmit packets of a given C-flow (of a given MVPN) to other PEs, but according to the conditions of Section 3.2.2.2 and/or Section 3.1 of [RFC6625], that C-flow does not match any S-PMSI A-D route. Then the packets of the C-flow need to be transmitted on the MVPN's I-PMSI.

If the C-flow is not a BIDIR-PIM C-flow, the P-tunnel on which the C-flow MUST be transmitted is the one identified in the PTA of the Intra-AS I-PMSI A-D route originated by PE1 for the given MVPN. Each packet of the C-flow MUST carry the PE Distinguisher Label assigned by the root node of that P-tunnel to the IP address of PE1.

If the C-flow is a BIDIR-PIM C-flow with group address C-G-BIDIR, the rules as applied by PE1 are the same as those given in Section 3.2.1.2.

If there is a matching I-PMSI A-D route, PE1 MUST transmit the C-flow on the P-tunnel identified in its PTA. In constructing the packet's MPLS label stack, it must use the PE Distinguisher Label that was assigned by the P-tunnel's root node to the IP address of "PE2", not the label assigned to the IP address of "PE1" (unless, of course, PE1 is the same as PE2). (Section 3.2.1.2 specifies the difference between PE1 and PE2.) See Section 12.3 of [RFC6513] for encapsulation details. Note that the root of the P-tunnel might be a PE other than PE1 or PE2.

If, for a packet of a particular C-flow, there is no S-PMSI A-D route or I-PMSI A-D route that is a match for transmission, the packet MUST NOT be transmitted.

3.2.2.4. When an S-PMSI Is a 'Match for Reception'

Suppose a given PE, say PE1, needs to receive multicast data packets of a particular C-flow. Section 3.2 of [RFC6625] specifies procedures for determining the S-PMSI A-D route, if any, that matches that C-flow for reception. Those rules require that the matching S-PMSI A-D route has been originated by the upstream PE for the C-flow. The rules are modified in this section, as follows:

Consider a particular C-flow. Suppose either:

  • the C-flow is unidirectional, and PE1 determines that its upstream PE is PE2, or
  • the C-flow is bidirectional, and PE1 determines that the upstream PE for its C-RPA is PE2

Then, the C-flow may match an installed S-PMSI A-D route that was not originated by PE2, as long as:

  1. the PTA of that A-D route identifies an MP2MP LSP,
  1. there is an installed S-PMSI A-D route originated by the root node of that LSP, or PE1 itself is the root node of the LSP and there is a currently originated S-PMSI A-D route from PE1 whose PTA identifies that LSP, and
  1. the latter S-PMSI A-D route (the one identified in 2 just above) contains a PE Distinguisher Labels attribute that assigned an MPLS label to the IP address of PE2.
   However, a bidirectional C-flow never matches an S-PMSI A-D route
   whose NLRI contains (C-S,C-G).

If a multicast data packet is received over a matching P-tunnel, but does not carry the value of the PE Distinguisher Label that has been assigned to the upstream PE for its C-flow, then the packet MUST be discarded.

3.2.2.5. When an I-PMSI Is a 'Match for Reception'

If a PE needs to receive packets of a given C-flow (of a given MVPN) from another PE, and if, according to the conditions of Section 3.2.2.4, that C-flow does not match any S-PMSI A-D route, then the packets of the C-flow need to be received on the MVPN's I-PMSI. The P-tunnel on which the packets are expected to arrive is determined by the Intra-AS I-PMSI A-D route originated by the distinguished PE for the given C-flow. The PTA of that route specifies the "outer

P-tunnel" and thus determines the top label that packets of that C-flow will be carrying when received. A PE that needs to receive packets of a given C-flow must determine the expected value of the second label for packets of that C-flow. This will be the value of a PE Distinguisher Label, taken from the PE Distinguisher Labels attribute of the Intra-AS I-PMSI A-D route of the root node of that outer tunnel. The expected value of the second label on received packets (corresponding to the "inner tunnel") of a given C-flow is determined according to the following rules.

First, the distinguished PE for the C-flow is determined:

  • If the C-flow is not a BIDIR-PIM C-flow, the distinguished PE for the C-flow is its upstream PE, as determined by the rules of [RFC6513].
  • If the C-flow is a BIDIR-PIM C-flow, the distinguished PE for the C-flow is its upstream PE of the C-flow's C-RPA, as determined by the rules of [RFC6513].

The expected value of the second label is the value that the root PE of the outer tunnel has assigned, in the PE Distinguisher Labels attribute of its Intra-AS I-PMSI A-D route, to the IP address of the distinguished PE.

Packets addressed to C-G that arrive on other than the expected inner and outer P-tunnels (i.e., that arrive with unexpected values of the top two labels) MUST be discarded.

3.2.3. Unpartitioned

When a particular MVPN uses the Unpartitioned Method of instantiating an I-PMSI with a bidirectional P-tunnel, it MUST be the case that at least one VRF of that MVPN originates an Intra-AS I-PMSI A-D route that includes a PTA specifying a bidirectional P-tunnel. The conditions under which an Intra-AS I-PMSI A-D route must be originated from a given VRF are as specified in [RFC6514]. This document allows all but one of such routes to omit the PTA. However, each such route MAY contain a PTA. If the PTA is present, it MUST specify a bidirectional P-tunnel. As specified in [RFC6513] and [RFC6514], every PE that imports such an Intra-AS I-PMSI A-D route into one of its VRFs MUST, if the route has a PTA, join the P-tunnel specified in the route's PTA.

Packets received on any of these P-tunnels are treated as having been received over the I-PMSI. The disposition of a received packet MUST NOT depend upon the particular P-tunnel over which it has been received.

When a PE needs to transmit a packet on such an I-PMSI, then if that PE advertised a P-tunnel in the PTA of an Intra-AS I-PMSI A-D route that it originated, the PE SHOULD transmit the on that P-tunnel. However, any PE that transmits a packet on the I-PMSI MAY transmit it on any of the P-tunnels advertised in any of the currently installed Intra-AS I-PMSI A-D routes for its VPN.

This allows a single bidirectional P-tunnel to be used to instantiate the I-PMSI, but also allows the use of multiple bidirectional P-tunnels. There may be a robustness advantage in having multiple P-tunnels available for use, but the number of P-tunnels used does not impact the functionality in any way. If there are, e.g., two P-tunnels available, these procedures allow each P-tunnel to be advertised by a single PE, but they also allow each P-tunnel to be advertised by multiple PEs. Note that the PE advertising a given P-tunnel does not have to be the root node of the tunnel. The root node might not even be a PE router, and it might not originate any BGP routes at all.

In the Unpartitioned Method, packets received on the I-PMSI cannot be associated with a distinguished PE, so duplicate detection using the techniques of Section 9.1.1 of [RFC6513] is not possible; the techniques of Sections 9.1.2 or 9.1.3 of [RFC6513] would have to be used instead. Support for C-BIDIR using the "Partitioned set of PEs" technique (Section 11.2 of [RFC6513] and Section 3.6 of [RFC6517]) is not possible when the Unpartitioned Method is used. If it is desired to use that technique to support C-BIDIR, but also to use the Unpartitioned Method to instantiate the I-PMSI, then all the C-BIDIR traffic would have to be carried on an S-PMSI, where the S-PMSI is instantiated using one of the Partitioned Methods.

When a PE, say PE1, needs to transmit multicast data packets of a particular C-flow to other PEs, and PE1 does not have an S-PMSI that is a match for transmission for that C-flow (see Section 3.2.3.1), PE1 transmits the packets on one of the P-tunnel(s) that instantiates the I-PMSI. When a PE, say PE1, needs to receive multicast data packets of a particular C-flow from another PE, and PE1 does not have an S-PMSI that is a match for reception for that C-flow (see Section 3.2.3.2), PE1 expects to receive the packets on any of the P-tunnels that instantiate the I-PMSI.

When a particular MVPN uses the Unpartitioned Method to instantiate a (C-*,C-*) S-PMSI or a (C-*,C-*-BIDIR) S-PMSI using a bidirectional P-tunnel, the same conditions apply as when an I-PMSI is instantiated via the Unpartitioned Method. The only difference is that a PE need not join a P-tunnel that instantiates the S-PMSI unless that PE needs to receive multicast packets on the S-PMSI.

When a particular MVPN uses bidirectional P-tunnels to instantiate other S-PMSIs, different S-PMSI A-D routes that do not contain (C-*,C-*) or (C-*,C-*-BIDIR), originated by the same or by different PEs, MAY have PTAs that identify the same bidirectional tunnel, and they MAY have PTAs that do not identify the same bidirectional tunnel.

While the Unpartitioned Method MAY be used to instantiate an S-PMSI to which one or more C-BIDIR flows are bound, it must be noted that the "Partitioned Set of PEs" method discussed in Section 11.2 of [RFC6513] and Section 3.6 of [RFC6517] cannot be supported using the Unpartitioned Method. C-BIDIR support would have to be provided by the procedures of [RFC6513], Section 11.1.

3.2.3.1. When an S-PMSI Is a 'Match for Transmission'

Suppose a PE needs to transmit multicast data packets of a particular customer C-flow. [RFC6625], Section 3.1, gives a four-step algorithm for determining the S-PMSI A-D route, if any, that matches that C-flow for transmission. When referring to that section, please recall that BIDIR-PIM groups are also ASM groups.

When bidirectional P-tunnels are used in the Unpartitioned Method, the same algorithm applies, with one modification, when the PTA of an S-PMSI A-D route identifies a bidirectional P-tunnel. One additional step is added to the algorithm. This new step occurs before the fourth step of the algorithm, and is as follows: 

   o  Otherwise, if there is a (C-*,C-*-BIDIR) S-PMSI A-D route
      currently originated by PE1, and if C-G is a BIDIR group, the
      C-flow matches that route.

When the Unpartitioned Method is used, the PE SHOULD transmit the C-flow on the P-tunnel advertised in the in the matching S-PMSI A-D route, but it MAY transmit the C-flow on any P-tunnel that is advertised in the PTA of any installed S-PMSI A-D route that contains the same (C-S,C-G) as the matching S-PMSI A-D route.

3.2.3.2. When an S-PMSI Is a 'Match for Reception'

Suppose a PE needs to receive multicast data packets of a particular customer C-flow. Section 3.2 of [RFC6625] specifies the procedures for determining the S-PMSI A-D route, if any, that advertised the P-tunnel on which the PE should expect to receive that C-flow.

When bidirectional P-tunnels are used in the Unpartitioned Method, the same procedures apply, with one modification.

The last paragraph of Section 3.2.2 of [RFC6625] begins: 

      If (C-*,C-G) does not match a (C-*,C-G) S-PMSI A-D route from PE2,
      but PE1 has an installed (C-*,C-*) S-PMSI A-D route from PE2, then
      (C-*,C-G) matches the (C-*,C-*) route if one of the following
      conditions holds:

This is changed to: 

      If (C-*,C-G) does not match a (C-*,C-G) S-PMSI A-D route from PE2,
      but C-G is a BIDIR group and PE1 has an installed (C-*,C-*-BIDIR)
      S-PMSI A-D route, then (C-*,C-G) matches that route.  Otherwise,
      if PE1 has an installed (C-*,C-*) S-PMSI A-D route from PE2, then
      (C-*,C-G) matches the (C-*,C-*) route if one of the following
      conditions holds:

When the Unpartitioned Method is used, the PE MUST join the P-tunnel that is advertised in the matching S-PMSI A-D route, and it MUST also join the P-tunnels that are advertised in other installed S-PMSI A-D routes that contain the same (C-S,C-G) as the matching S-PMSI A-D route.

3.2.4. Minimal Feature Set for Compliance

Implementation of bidirectional P-tunnels is OPTIONAL. If bidirectional P-tunnels are not implemented, the issue of compliance to this specification does not arise. However, for the case where bidirectional P-tunnels ARE implemented, this section specifies the minimal set of features that MUST be implemented in order to claim compliance to this specification.

In order to be compliant with this specification, an implementation that provides bidirectional P-tunnels MUST support at least one of the two P-tunnel technologies mentioned in Section 1.2.1.

A PE that does not provide C-BIDIR support using the "partitioned set of PEs" method is deemed compliant to this specification if it supports the Unpartitioned Method, using either MP2MP LSPs or BIDIR- PIM multicast distribution trees as P-tunnels.

A PE that does provide C-BIDIR support using the "partitioned set of PEs" method MUST, at a minimum, be able to provide C-BIDIR support using the "Partial Mesh of MP2MP P-tunnels" variant of this method (see Section 11.2 of [RFC6513]). An implementation will be deemed compliant to this minimum requirement if it can carry all of a VPN's C-BIDIR traffic on a (C-*,C-*-BIDIR) S-PMSI that is instantiated by a bidirectional P-tunnel, using the Flat Partitioned Method.

4. Security Considerations

There are no additional security considerations beyond those of [RFC6513] and [RFC6514], or any that may apply to the particular protocol used to set up the bidirectional tunnels ([RFC5015], [RFC6388]).

5. References

5.1. Normative References

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119,
               DOI 10.17487/RFC2119, March 1997,
               <http://www.rfc-editor.org/info/rfc2119>.
   
   [RFC4364]   Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
               Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364,
               February 2006, <http://www.rfc-editor.org/info/rfc4364>.
   
   [RFC4601]   Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
               "Protocol Independent Multicast - Sparse Mode (PIM-SM):
               Protocol Specification (Revised)", RFC 4601,
               DOI 10.17487/RFC4601, August 2006,
               <http://www.rfc-editor.org/info/rfc4601>.
   
   [RFC5015]   Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
               "Bidirectional Protocol Independent Multicast (BIDIR-
               PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,
               <http://www.rfc-editor.org/info/rfc5015>.
   
   [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, DOI 10.17487/RFC6388, November
               2011, <http://www.rfc-editor.org/info/rfc6388>.
   
   [RFC6513]   Rosen, E., Ed., and R. Aggarwal, Ed., "Multicast in
               MPLS/BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513,
               February 2012, <http://www.rfc-editor.org/info/rfc6513>.
   
   [RFC6514]   Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
               Encodings and Procedures for Multicast in MPLS/BGP IP
               VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
               <http://www.rfc-editor.org/info/rfc6514>.
   
   [RFC6625]   Rosen, E., Ed., Rekhter, Y., Ed., Hendrickx, W., and R.
               Qiu, "Wildcards in Multicast VPN Auto-Discovery Routes",
               RFC 6625, DOI 10.17487/RFC6625, May 2012,
               <http://www.rfc-editor.org/info/rfc6625>.

5.2. Informative References

[BGP-ERROR] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K. 

               Patel, "Revised Error Handling for BGP UPDATE Messages",
               Work in Progress, draft-ietf-idr-error-handling-19, April
               2015.

[MVPN-BIDIR-IR] 

               Zhang, Z., Rekhter, Y., and A. Dolganow, "Simulating
               'Partial Mesh of MP2MP P-Tunnels' with Ingress
               Replication", Work in Progress,
               draft-ietf-bess-mvpn-bidir-ingress-replication-00,
               January 2015.
   
   [MVPN-XNET] Rekhter, Y., Ed., Rosen, E., Ed., Aggarwal, R., Cai, Y.,
               and T. Morin, "Extranet Multicast in BGP/IP MPLS VPNs",
               Work in Progress, draft-ietf-bess-mvpn-extranet-02, May
               2015.
   
   [RFC5331]   Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream
               Label Assignment and Context-Specific Label Space", RFC
               5331, DOI 10.17487/RFC5331, August 2008,
               <http://www.rfc-editor.org/info/rfc5331>.
   
   [RFC6517]   Morin, T., Ed., Niven-Jenkins, B., Ed., Kamite, Y.,
               Zhang, R., Leymann, N., and N. Bitar, "Mandatory Features
               in a Layer 3 Multicast BGP/MPLS VPN Solution", RFC 6517,
               DOI 10.17487/RFC6517, February 2012,
               <http://www.rfc-editor.org/info/rfc6517>.

Acknowledgments

The authors wish to thank Karthik Subramanian, Rajesh Sharma, and Apoorva Karan for their input. We also thank Yakov Rekhter for his valuable critique.

Special thanks go to Jeffrey (Zhaohui) Zhang for his careful review, probing questions, and useful suggestions.

Authors' Addresses

   Eric C. Rosen
   Juniper Networks, Inc.
   10 Technology Park Drive
   Westford, MA  01886
   United States

Email: 

          erosen@juniper.net
   
   IJsbrand Wijnands
   Cisco Systems, Inc.
   De kleetlaan 6a
   Diegem  1831
   Belgium

Email: 

          ice@cisco.com
   
   Yiqun Cai
   Microsoft
   1065 La Avenida
   Mountain View, CA  94043
   United States

Email: 

          yiqunc@microsoft.com
   
   Arjen Boers

Email: 

          arjen@boers.com