MVPN Explicit Tracking: Explicit Tracking with Wildcard Routes in Multicast VPN

Internet Engineering Task Force (IETF)                       A. Dolganow
Request for Comments: 8534                                   J. Kotalwar
Updates: 6514, 6625, 7524, 7582, 7900                              Nokia
Category: Standards Track                                  E. Rosen, Ed.
ISSN: 2070-1721                                                 Z. Zhang
                                                  Juniper Networks, Inc.
                                                           February 2019


        Explicit Tracking with Wildcard Routes in Multicast VPN

Abstract

   The base Multicast VPN (MVPN) specifications (RFCs 6513 and 6514)
   provide procedures to allow a multicast ingress node to invoke
   "explicit tracking" for a multicast flow or set of flows, thus
   learning the egress nodes for that flow or set of flows.  However,
   the specifications are not completely clear about how the explicit
   tracking procedures work in certain scenarios.  This document
   provides the necessary clarifications.  It also specifies a new,
   optimized explicit-tracking procedure.  This new procedure allows an
   ingress node, by sending a single message, to request explicit
   tracking of each of a set of flows, where the set of flows is
   specified using a wildcard mechanism.  This document updates RFCs
   6514, 6625, 7524, 7582, and 7900.

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8534.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  The Explicit-Tracking Flags . . . . . . . . . . . . . . . . .   5
   3.  Match for Tracking versus Match for Reception . . . . . . . .   7
   4.  Ingress Node Initiation of Tracking . . . . . . . . . . . . .   9
   5.  Egress Node Response to the Match for Tracking  . . . . . . .  11
     5.1.  General Egress Node Procedures  . . . . . . . . . . . . .  11
     5.2.  Responding to the LIR-pF Flag . . . . . . . . . . . . . .  12
     5.3.  When the Egress Node Is an ABR or ASBR  . . . . . . . . .  15
   6.  Ingress Node Handling of Received Leaf A-D Routes with
       LIR-pF Set  . . . . . . . . . . . . . . . . . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   The base Multicast VPN (MVPN) specifications, [RFC6513] and
   [RFC6514], define the "Selective Provider Multicast Service Interface
   Auto-Discovery route" (S-PMSI A-D route).

   Per those RFCs, the S-PMSI A-D route contains a Network Layer
   Reachability Information (NLRI) field that identifies a particular
   multicast flow.  In the terminology of those RFCs, each flow is
   denoted by (C-S,C-G), where C-S is an IP source address and C-G is an
   IP multicast address, both in the address space of a VPN customer.
   The (C-S,C-G) of the multicast flow is encoded into the NLRI field.

   An S-PMSI A-D route also carries a PMSI Tunnel attribute (PTA).
   Typically, the PTA is used to identify a tunnel through the provider
   backbone network (a "P-tunnel").

   By originating an S-PMSI A-D route identifying a particular multicast
   flow and a particular P-tunnel, a node is advertising the following:

      If the node has to transmit packets of the identified flow over
      the backbone, it will transmit them through the identified tunnel.

   [RFC6513] and [RFC6514] also define a procedure that allows an
   ingress node of a particular multicast flow to determine the set of
   egress nodes that have requested to receive that flow from that
   ingress node.  The ability of an ingress node to identify the egress
   nodes for a particular flow is known as "explicit tracking".  An
   ingress node requests explicit tracking by setting a flag (the "Leaf
   Information Required" flag, or LIR flag) in the PTA.  When an egress
   node receives an S-PMSI A-D route with the LIR flag set, the egress
   node originates a Leaf A-D route whose NLRI field contains the NLRI
   from the corresponding S-PMSI A-D route.  In this way, the egress
   node advertises that it has requested to receive the particular flow
   identified in the NLRI of that S-PMSI A-D route.

   [RFC6513] and [RFC6514] also allow an ingress node to originate an
   S-PMSI A-D route whose PTA has the LIR flag set but that does not
   identify any P-tunnel.  This mechanism can be used when desired to do
   explicit tracking of a flow without at the same time binding that
   flow to a particular P-tunnel.

   [RFC6625] (and other RFCs that update it) extends the specification
   of S-PMSI A-D routes and allows an S-PMSI A-D route to encode a
   wildcard in its NLRI.  Either the C-S or the C-G or both can be
   replaced by wildcards.  These routes are known as (C-*,C-S) S-PMSI
   A-D routes, (C-S,C-*) S-PMSI A-D routes, or (C-*,C-*) S-PMSI A-D
   routes, depending on whether the C-S or C-G or both have been
   replaced by wildcards.  These routes are known jointly as "wildcard
   S-PMSI A-D routes".

   One purpose of this document is to clarify the way that the explicit
   tracking procedures of [RFC6513] and [RFC6514] are applied when
   wildcard S-PMSI A-D routes are used.

   In addition, this document addresses the following scenario, which is
   not addressed in [RFC6513], [RFC6514], or [RFC6625].  Suppose an
   ingress node originates an S-PMSI A-D route whose NLRI specifies, for
   example, (C-*,C-*) (i.e., both C-S and C-G are replaced by wildcards)
   and whose PTA identifies a particular P-tunnel.  Now suppose that the
   ingress node wants explicit tracking for each individual flow that it
   transmits (following the procedures of [RFC6625]) on that P-tunnel.

   In this example, if the ingress node sets the LIR flag in the PTA of
   the wildcard S-PMSI A-D route, each egress node that needs to receive
   a flow from the ingress node will respond with a Leaf A-D route whose
   NLRI contains the (C-*,C-*) wildcard.  This allows the ingress node
   to determine the set of egress nodes that are interested in receiving
   flows from the ingress node.  However, it does not allow the ingress
   node to determine exactly which flows are of interest to which egress
   nodes.

   If the ingress node needs to determine which egress nodes are
   interested in receiving which flows, it needs to originate an S-PMSI
   A-D route for each individual (C-S,C-G) flow that it is transmitting,
   and it needs to set the LIR flag in the PTA of each such route.
   However, since all the flows are being sent through the tunnel
   identified in the (C-*,C-*) S-PMSI A-D route, there is no need to
   identify a tunnel in the PTA of each (C-S,C-G) S-PMSI A-D route.  Per
   Section 5 of [RFC6514], the PTA of the (C-S,C-G) S-PMSI A-D routes
   can specify "no tunnel information present".  This procedure allows
   explicit tracking of individual flows, even though all those flows
   are assigned to tunnels by wildcard S-PMSI A-D routes.

   However, this procedure requires several clarifications:

   o  The procedures of [RFC6625] do not address the case of an S-PMSI
      A-D route whose NLRI contains wildcards but whose PTA specifies
      "no tunnel information present".

   o  If it is desired to send a set of flows through the same tunnel
      (where that tunnel is advertised in a wildcard S-PMSI A-D route),
      but it is also desired to explicitly track each individual flow
      transmitted over that tunnel, one has to send an S-PMSI A-D route
      (with the LIR flag set in the PTA) for each individual flow.  It
      would be more optimal if the ingress node could just send a single
      wildcard S-PMSI A-D route binding the set of flows to a particular
      tunnel and have the egress nodes respond with Leaf A-D routes for
      each individual flow.

   o  [RFC6513] and [RFC6514] support the notion of "segmented
      P-tunnels", where "segmentation" occurs at Autonomous System
      Border Routers (ASBRs); [RFC7524] extends the notion of segmented
      P-tunnels so that segmentation can occur at Area Border Routers
      (ABRs).  One can think of a segmented P-tunnel as passing through
      a number of "segmentation domains".  In each segmentation domain,
      a given P-tunnel has an ingress node and a set of egress nodes.

      The explicit tracking procedures allow an ingress node of a
      particular segmentation domain to determine, for a particular flow
      or set of flows, the egress nodes of that segmentation domain.
      This has given rise to two further problems:

      *  The explicit tracking procedures do not allow an ingress node
         to "see" past the boundaries of the segmentation domain.

      *  The prior specifications do not make it very clear whether a
         segmented tunnel egress node, upon receiving an S-PMSI A-D
         route whose PTA specifies "no tunnel information present", is
         expected to forward the S-PMSI A-D route, with the same PTA, to
         the next segmentation domain.

      These problems are addressed in Section 5.3.

   This document clarifies the procedures for originating and receiving
   S-PMSI A-D routes and Leaf A-D routes.  This document also adds new
   procedures to allow more efficient explicit tracking.  The procedures
   being clarified and/or extended are discussed in multiple places in
   the documents being updated.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  The Explicit-Tracking Flags

   [RFC6514] defines one flag in the PTA, the "Leaf Information
   Required" (LIR) flag, that is used for explicit tracking.

   This document defines a new flag in the Flags field of the PMSI
   Tunnel attribute.  This new flag is known as the "Leaf Information
   Required per Flow" flag (LIR-pF).  This flag may be set in the PTA of
   a (C-*,C-*), (C-*,C-G), or (C-S,C-*) S-PMSI A-D route.  The
   conditions under which it should be set by the originator of the
   route are discussed in Section 4.  The significance of the flag in a
   received wildcard S-PMSI A-D route is discussed in Sections 5 and
   5.2.

   The LIR-pF flag may also be set in the PTA of a Leaf A-D route.  The
   conditions under which it should be set by the originator of the
   route are discussed in Section 5.2.  The significance of the flag in
   a received Leaf A-D route is discussed in Section 6.

   Note that support for the LIR-pF flag is OPTIONAL.  This flag SHOULD
   NOT be set in a route's PTA unless it is known that the flag is
   supported by all the Provider Edge (PE) routers that are to receive
   that route.  Typically, this might mean that the ability to set this
   flag would be controlled by a configuration knob, and operators would
   not set this knob unless they know that all the relevant PEs support
   this feature.  How this is known is outside the scope of this
   document.

   This document only defines procedures for the LIR-pF flag when that
   flag is in the PTA of a wildcard S-PMSI A-D route or a Leaf A-D
   route.  In all other cases, the flag SHOULD be clear, and its value
   SHOULD be ignored.  Use of the flag in these other cases is outside
   the scope of this document.

   Section 5 of [RFC6514] lists a number of tunnel types.  We will refer
   to these as "6514-tunnel-types".  Other tunnel types will be referred
   to as "non-6514-tunnel-types".  This document specifies procedures
   for using the LIR-pF flag with 6514-tunnel-types.  Procedures for
   using the LIR-pF flag with non-6514-tunnel-types are outside the
   scope of this document.

   If it is desired to use a particular non-6514-tunnel-type in MVPN,
   there needs to be a specification for how that tunnel type is used in
   MVPN.  If it is desired to use that tunnel type along with the LIR-pF
   flag, that specification (or a follow-on specification) will have to
   specify the rules for using the LIR-pF flag with that tunnel type.
   As an example, see [BIER-MVPN].  In the absence of such a
   specification (or in the absence of support for such a
   specification):

   o  the originator of a route that carries a PTA SHOULD NOT set the
      LIR-pF flag in any PTA that specifies that tunnel type, and

   o  the receiver of a route that carries a PTA specifying that tunnel
      type SHOULD treat the LIR-pF flag as if it were not set.

   If the LIR-pF flag is set in the PTA of an S-PMSI A-D route, the
   originator of that route MUST also set the LIR flag.  If the PTA of a
   received wildcard S-PMSI A-D route has the LIR-pF flag set but does
   not have the LIR flag set, the receiver MUST log the fact that the
   flags appear to have been improperly set.  However, the route MUST
   then be processed normally (as if both flags were set), as specified
   in this document.

   It is worth noting what will happen if the LIR-pF flag is set in the
   PTA of, for example, a (C-*,C-*) S-PMSI A-D route originated by an
   ingress node, but one or more of the egress nodes do not support the
   LIR-pF flag:

   1.  The ingress node will not be able to determine the complete set
       of egress nodes that are expecting a particular multicast flow
       from that ingress node.

   2.  Depending upon the tunnel type, the ingress node may send a
       particular multicast flow only to the egress nodes that do
       support the LIR-pF flag.  From the perspective of egress nodes
       that do not support the LIR-pF flag, certain flows may appear to
       be "blackholed".

   It is also worth noting that it is possible for an ingress node that
   sets the LIR-pF flag in an S-PMSI A-D route to detect the presence of
   egress nodes that do not support this flag.

   Since an ingress node that sets the LIR-pF flag is also required to
   set the LIR flag, egress nodes that do not support the LIR-pF flag
   will respond, as specified in [RFC6514], to the ingress node's
   (C-*,C-*) S-PMSI A-D route with a Leaf A-D route.

   As discussed in Section 5.2, any Leaf A-D route originated in
   response to an S-PMSI A-D route that has the LIR-pF flag set will
   carry a PTA whose LIR-pF flag is set.  If an ingress node receives a
   Leaf A-D route whose Route Key field corresponds to the NLRI of an
   S-PMSI A-D route whose PTA has the LIR-pF flag set, but the Leaf A-D
   route lacks a PTA or has a PTA where the LIR-pF flag is clear, the
   ingress node can infer that the egress node originating that Leaf A-D
   route does not support the LIR-pF flag.  The software at the ingress
   node MUST detect this and MUST have a way of alerting the operator
   that the deployment is not properly configured.

3.  Match for Tracking versus Match for Reception

   Section 3.2 of [RFC6625] specifies a set of rules for finding the
   S-PMSI A-D route that is the "match for data reception" (or more
   simply, the "match for reception") of a given (C-S,C-G) or (C-*,C-G)
   state.  These rules do not take into account the fact that some
   S-PMSI A-D routes may not be carrying PTAs at all or may be carrying
   PTAs that do not identify any P-tunnel.  (A PTA that does not
   identify any P-tunnel is one whose Tunnel Type field has been set to
   "no tunnel information present", as specified in Section 5 of
   [RFC6514].)

   The rules for finding a "match for reception" in [RFC6625] are hereby
   modified as follows:

      When applying the rules of Sections 3.2.1 or 3.2.2 of [RFC6625],
      it is REQUIRED to ignore any S-PMSI A-D route that has no PTA, or
      whose PTA specifies "no tunnel information present".

   There are other RFCs that update [RFC6625] and modify the rules for
   finding a "match for reception".  See, e.g., [RFC7582] and [RFC7900].
   When applying those modified rules, it is REQUIRED to ignore any
   S-PMSI A-D route that has no PTA, or whose PTA specifies "no tunnel
   information present".

   We also introduce a new notion, the "match for tracking":

      For a given C-flow ((C-S,C-G) or (C-*,C-G)), the "match for
      tracking" is chosen as follows.  Ignore any S-PMSI A-D route that
      has no PTA.  Also ignore any S-PMSI A-D route whose PTA specifies
      "no tunnel information present" and has neither the LIR flag nor
      the LIR-pF flag set.  (That is, *do not* ignore an S-PMSI A-D
      route that has a PTA specifying "no tunnel information present"
      unless its LIR and LIR-pF flags are both clear).  Then apply the
      rules (from [RFC6625] and other documents that update it) for
      finding the "match for reception".  The result (if any) is the
      "match for tracking".

      Note that the procedure for finding the match for tracking takes
      into account S-PMSI A-D routes whose PTAs specify "no tunnel
      information present" and that have either the LIR or LIR-pf flag
      set.  The procedure for finding the match for reception ignores
      such routes.

   We will clarify this with a few examples.  In these examples, we
   assume that there is only one segmentation domain.  In this case, the
   ingress and egress nodes are PE routers.

   Suppose a given PE router, PE1, has chosen PE2 as the "upstream PE"
   [RFC6513] for a given flow (C-S1,C-G1).  And suppose PE1 has
   installed the following two routes that were originated by PE2:

   o  Route1: A (C-*,C-*) S-PMSI A-D route whose PTA specifies a tunnel.

   o  Route2: A (C-S1,C-G1) S-PMSI A-D route whose PTA specifies "no
      tunnel information present" and has the LIR flag set.

   Route1 is the match of (C-S1,C-G1) for reception, and Route2 is the
   match of (C-S1,C-G1) for tracking.

   Continuing this example, suppose:

   o  PE1 has chosen PE2 as the upstream PE for a different flow,
      (C-S2,C-G2).

   o  PE2 has not originated an S-PMSI A-D route for (C-S2,C-G2).

   In this case, PE1 would consider Route1 to be the match of
   (C-S2,C-G2) for tracking as well as its match for reception.

   Also note that if a match for tracking does not have the LIR flag or
   the LIR-pF flag set, no explicit tracking information will be
   generated.  See Section 5.

   As another example, suppose PE1 has installed the following two
   routes that were originated by PE2:

   o  Route1: A (C-*,C-*) S-PMSI A-D route (irrespective of whether the
      PTA specifies a tunnel).

   o  Route2: A (C-S1,C-G1) S-PMSI A-D route whose PTA specifies a
      tunnel.

   In this case, Route2 is both the "match for reception" and the "match
   for tracking" for (C-S1,C-G1).

   Note that for a particular C-flow, PE1's match for reception might be
   the same route as its match for tracking, or its match for reception
   might be a "less specific" route than its match for tracking.  But
   its match for reception can never be a "more specific" route than its
   match for tracking.

4.  Ingress Node Initiation of Tracking

   An ingress node that needs to initiate explicit tracking for a
   particular flow or set of flows can do so by performing one of the
   following procedures:

   1.  An ingress node can initiate explicit tracking for (C-S1,C-G1) by
       originating an S-PMSI A-D route that identifies (C-S1,C-G1) in
       its NLRI, including a PTA in that route, and setting the LIR flag
       in that PTA.  The PTA may specify either a particular tunnel or
       "no tunnel information present".

       However, the PTA of the (C-S1,C-G1) S-PMSI A-D route SHOULD NOT
       specify "no tunnel information present" unless the ingress node
       also originates an A-D route carrying a PTA that specifies the
       tunnel to be used for carrying (C-S1,C-G1) traffic.  Such a route
       could be an "Inclusive Provider Multicast Service Interface Auto-
       Discovery route" (I-PMSI A-D route), a (C-*,C-G1) S-PMSI A-D
       route, a (C-S1,C-*) S-PMSI A-D route, or a (C-*,C-*) S-PMSI A-D
       route.  (There is no point in requesting explicit tracking for a
       given flow if there is no tunnel on which the flow is being
       carried.)

       Note that if the ingress node originates a wildcard S-PMSI A-D
       route carrying a PTA specifying the tunnel to be used for
       carrying (C-S1,C-G1) traffic, and if that PTA has the LIR-pF flag
       set, then explicit tracking for (C-S1,C-G1) is requested by that
       S-PMSI A-D route.  In that case, the ingress node SHOULD NOT
       originate a (C-S1,C-G1) S-PMSI A-D route whose PTA specifies "no
       tunnel information present"; such a route would not provide any
       additional functionality.

       To terminate explicit tracking that has been initiated by an
       S-PMSI A-D route whose PTA specifies "no tunnel information
       present", the ingress node withdraws the route.

       To terminate explicit tracking that has been initiated by an
       S-PMSI A-D route whose PTA specifies a tunnel, the ingress node
       re-originates the route without the LIR flag set.

   2.  The following procedure can be used if and only if it is known
       that the egress nodes support the optional LIR-pF flag.  If the
       ingress node originates a wildcard S-PMSI A-D route, it can
       initiate explicit tracking for the individual flows that match
       the wildcard route by setting the LIR-pF flag in the PTA of the
       wildcard route.  If an egress node needs to receive one or more
       flows for which that wildcard route is a match for tracking, the
       egress node will originate a Leaf A-D route for each such flow,
       as specified in Section 5.2).

       When following this procedure, the PTA of the S-PMSI A-D route
       may specify either a tunnel or "no tunnel information present".
       The choice between these two options is determined by
       considerations that are outside the scope of this document.

       To terminate explicit tracking that has been initiated by an
       S-PMSI A-D route whose PTA specifies "no tunnel information
       present", the ingress node withdraws the route.

       To terminate explicit tracking that has been initiated by an
       S-PMSI A-D route whose PTA specifies a tunnel, the ingress node
       re-originates the route without either the LIR or LIR-pF flags
       set.

       Note that this procedure (Procedure 2 of Section 4) may not yield
       the expected results if there are egress nodes that do not
       support the LIR-pF flag; hence, it SHOULD NOT be used in that
       case.

5.  Egress Node Response to the Match for Tracking

5.1.  General Egress Node Procedures

   There are four cases to consider:

   1.  With regard to a particular (C-S,C-G) or (C-*,C-G) multicast
       state, the egress node's match for tracking is the same as its
       match for reception, and neither the LIR nor the LIR-pF flags are
       set.

       In this case, the egress node does not originate a Leaf A-D route
       in response to the match for reception/tracking, and there is no
       explicit tracking of the flow.  This document specifies no new
       procedures for this case.

   2.  With regard to a particular (C-S,C-G) or (C-*,C-G) multicast
       state, the egress node's match for tracking is the same as its
       match for reception, and the LIR flag is set, but the LIR-pF flag
       is not set.

       In this case, a Leaf A-D route is originated by the egress node,
       corresponding to the S-PMSI A-D route that is the match for
       reception/tracking.  Construction of the Leaf A-D route is as
       specified in [RFC6514]; this document specifies no new procedures
       for this case.

   3.  With regard to a particular (C-S,C-G) or (C-*,C-G) multicast
       state, the egress node's match for tracking is the same as its
       match for reception, and LIR-pF is set.  The egress node follows
       whatever procedures are required by other specifications, based
       on the match for reception.  However, any Leaf A-D route
       originated by the egress node as a result MUST have the LIR-pF
       flag set in its PTA.  The egress node MUST also follow the
       procedures of Section 5.2.

   4.  With regard to a particular (C-S,C-G) or (C-*,C-G) multicast
       state, the egress node's match for tracking is *not* the same as
       its match for reception.  This can only happen if the match for
       tracking has a PTA specifying "no tunnel information present",
       with either the LIR flag or the LIR-pF flag set.  In this case,
       the egress node MUST respond, separately, to *both* the match for
       tracking and the match for reception.

       If a Leaf A-D route is originated in response to the match for
       reception, the LIR-pF flag in the Leaf A-D route's PTA MUST have
       the same value as the LIR-pF flag in the match for reception's
       PTA.  In all other respects, the procedures for responding to the
       match for reception are not affected by this document.

       If the match for tracking has the LIR flag set but the LIR-pF
       flag is not set, then the behavior of the egress node is not
       affected by the procedures of this document.

       If the match for tracking has the LIR-pF flag set, the egress
       node MUST follow the procedures of Section 5.2.

       Note that if the LIR flag is set in the PTA of the match for
       reception, the egress node may need to originate one or more Leaf
       A-D routes corresponding to the match for tracking, as well as
       originating a Leaf A-D route corresponding to the match for
       reception.

5.2.  Responding to the LIR-pF Flag

   To respond to a match for tracking that has the LIR-pF flag set, an
   egress node originates one or more Leaf A-D routes.

   Suppose the egress node has multicast state for a (C-S,C-G) or a
   (C-*,C-G) flow and has determined a particular S-PMSI A-D route,
   which has the LIR-pF flag set, to be the match for tracking for that
   flow.  Then if the egress node supports the LIR-pF flag, it MUST
   originate a Leaf A-D route whose NLRI identifies that particular
   flow.  Note that if a single S-PMSI A-D route (with wildcards) is the
   match for tracking for multiple flows, the egress node may need to
   originate multiple Leaf A-D routes, one for each such flow.  We say
   that, from the perspective of a given egress node, a given S-PMSI A-D
   route tracks the set of flows for which it is the match for tracking.
   Each of the Leaf A-D routes originated in response to that S-PMSI A-D
   route tracks a single such flow.

   The NLRI of each Leaf A-D route that tracks a particular flow is
   constructed as follows.  The Route Key field of the NLRI will have
   the format shown in Figure 1 (as defined in Sections 4.3 and 4.4 of
   [RFC6514]).

                  +------------------------------------+
                  |      RD   (8 octets)               |
                  +------------------------------------+
                  |  Multicast Source Length (1 octet) |
                  +------------------------------------+
                  |  Multicast Source (Variable)       |
                  +------------------------------------+
                  |  Multicast Group Length (1 octet)  |
                  +------------------------------------+
                  |  Multicast Group (Variable)        |
                  +------------------------------------+
                  |  Ingress PE's IP Address           |
                  +------------------------------------+

                    Figure 1: NLRI of S-PMSI A-D Route

   o  The "ingress PE" address is taken from the Originating Router
      field of the NLRI of the S-PMSI A-D route that is the match for
      tracking.  Section 2 of [RFC6515] explains how the receiver of a
      Leaf A-D route determines the length of this field and the address
      family of the PE's IP address.

   o  The Multicast Source and Multicast Group fields respectively
      specify a source address (S) and a group address(G) that together
      identify the flow or flows being tracked by this Leaf A-D route.
      If a (C-*,C-G) is being tracked by this Leaf A-D route, the
      Multicast Source field is omitted, and the Multicast Source Length
      field is set to 0.  In this case, the Leaf A-D route is known as a
      "wildcard Leaf A-D route".

   o  The Route Distinguisher (RD) field is set to the value of the RD
      field from the NLRI of the S-PMSI A-D route.

   The encoding of these Leaf A-D routes is similar to the encoding of
   the Leaf A-D routes described in Section 6.2.2 of [RFC7524], which
   were designed for the support of "global table multicast".  However,
   that document sets the RD to either 0 or -1; following the procedures
   of the present document, the RD will never be 0 or -1.  Therefore,
   Leaf A-D routes constructed according to the procedures of this
   section can always be distinguished from the Leaf A-D routes
   constructed according to the procedures of Section 6.2.2 of
   [RFC7524].  Also, Leaf A-D routes constructed according to the
   procedures of this section are VPN-specific routes and will always
   carry an IP-address-specific Route Target, as specified in [RFC6514].

   If a Leaf A-D route is originated as a response to a match for
   tracking whose PTA specifies "no tunnel information present", the
   Leaf A-D route MUST carry a PTA that specifies "no tunnel information
   present".  The LIR-pF flag in this PTA MUST be set.

   If an egress node originates multiple Leaf A-D routes in response to
   a single S-PMSI A-D route, and that S-PMSI A-D route is later
   withdrawn, then those Leaf A-D routes MUST also be withdrawn.

   Similarly, a Leaf A-D route needs to be withdrawn (either implicitly
   or explicitly) if the egress node changes its Upstream Multicast Hop
   (UMH) [RFC6513] for the flow that is identified in the Leaf A-D
   route's NLRI, or if the egress node that originated the route no
   longer needs to receive that flow.

   It is possible that an egress node will acquire (C-S,C-G) state or
   (C-*,C-G) state after it has already received the S-PMSI A-D that is
   the match for tracking for that state.  In this case, a Leaf A-D
   route needs to be originated at that time, and the egress node must
   remember that the new Leaf A-D route corresponds to that match for
   tracking.

   If a particular S-PMSI A-D route is a match for tracking but not a
   match for reception, the LIR flag in its PTA is ignored if the LIR-pF
   flag is set.

   When the match for tracking is the same as the match for reception,
   the PTA of the match for tracking/reception will have specified a
   tunnel type.  Some of the rules for constructing the PTA of the Leaf
   A-D route depend on the tunnel type, and some are independent of the
   tunnel type.  No matter what the tunnel type is, the LIR-pF flag MUST
   be set.

   If the match for tracking/reception is a wildcard S-PMSI A-D route,
   the egress node may originate a wildcard Leaf A-D route in response,
   as well as originating one or more non-wildcard Leaf A-D routes.
   Note that the LIR-pF flag MUST be set in the wildcard Leaf A-D route
   as well as in the non-wildcard Leaf A-D routes.

   This document provides additional rules for constructing the PTA when
   the tunnel type is a 6514-tunnel-type (see Section 2).

   As discussed in Section 2, if a non-6514-tunnel-type is being used,
   then presumably there is a specification for how that tunnel type is
   used in MVPN.  If it is desired to use that tunnel type along with
   the LIR-pF flag, that specification (or a follow-on specification)
   will have to specify the additional rules for constructing the PTA.
   As an example, see [BIER-MVPN].

   For 6514-tunnel-types, additional rules for constructing the PTA are
   as follows:

   o  If the tunnel type of the PTA attached to the match for tracking/
      reception is Ingress Replication, the Leaf A-D route's PTA MAY
      specify Ingress Replication.  In this case, the MPLS Label field
      of the PTA MAY be a non-zero value.  If so, this label value will
      be used by the ingress PE when it transmits, to the egress PE,
      packets of the flow identified in the Leaf A-D route's NLRI.

      Alternatively, the egress PE MAY specify an MPLS label value of
      zero, or it MAY specify a tunnel type of "no tunnel information
      present".  In either of these cases, when the ingress PE transmits
      packets of the identified flow to the egress PE, it will use the
      label that the egress PE specified in the PTA of the Leaf A-D
      route that it originated in response to the LIR flag of the match
      for reception.

   o  If the tunnel type of the PTA attached to the match for tracking/
      reception is any of the other 6514-tunnel-types, the PTA attached
      to the Leaf A-D route MUST specify a tunnel type of "no tunnel
      information present".

   It may happen that the tunnel type is a non-6514-tunnel type, but
   either (a) there is no specification for how to use that tunnel type
   with the LIR-pF flag or (b) there is such a specification, but the
   egress node does not support it.  In that case, the egress node MUST
   treat the match for tracking/reception as if it had the LIR-pF flag
   clear.

5.3.  When the Egress Node Is an ABR or ASBR

   When segmented P-tunnels are used, the ingress and egress nodes may
   be ABRs or ASBRs.  An egress ABR/ASBR that receives and installs an
   S-PMSI A-D route also forwards that route.  If the received PTA of an
   installed S-PMSI A-D route specifies a tunnel, the egress ABR/ASBR
   MAY change the PTA before forwarding the route, in order to specify a
   different tunnel type (as discussed in [RFC6514] and/or [RFC7524]).
   The egress ABR/ASBR may also need to originate a Leaf A-D route, as
   specified in [RFC6514] and/or [RFC7524].

   Suppose the S-PMSI A-D route as received has a PTA specifying a
   tunnel and also has the LIR-pF flag set.  The egress ABR/ASBR
   originates a corresponding Leaf A-D route for a given (C-S,C-G) only
   if it knows that it needs to receive that flow.  It will know this by
   virtue of receiving a corresponding Leaf A-D route from downstream.
   (In the case where the PTA specifies a tunnel but the LIR-pF flag is
   not set, this document does not specify any new procedures.)

   The procedures in the remainder of this section apply only when an
   egress ABR/ASBR has installed an S-PMSI A-D route whose PTA as
   received specifies "no tunnel information present" but has the LIR
   flag or the LIR-pF flag set.

   If the received PTA of the installed S-PMSI A-D route specifies "no
   tunnel information present", the egress ABR/ASBR MUST pass the PTA
   along unchanged when it forwards the S-PMSI A-D route.  (That is, a
   PTA specifying "no tunnel information present" MUST NOT be changed
   into a PTA specifying a tunnel.)  Furthermore, if the PTA specifies
   "no tunnel information present", the LIR and LIR-pF flags in the PTA
   MUST be passed along unchanged.

   As a result of propagating such an S-PMSI A-D route, the egress ABR/
   ASBR may receive one or more Leaf A-D routes that correspond to that
   S-PMSI A-D route.  These routes will be received carrying an
   IP-address-specific Route Target (RT) Extended Community that
   specifies the address of the egress ABR/ASBR.  The egress ABR/ASBR
   will propagate these Leaf A-D routes after changing the RT as
   follows.  The Global Administrator field of the modified RT will be
   set to the IP address taken either from the S-PMSI A-D route's
   Next-Hop field [RFC6514] or its Segmented Point-to-Multipoint (P2MP)
   Next-Hop Extended Community [RFC7524].  The address from the
   Segmented P2MP Next-Hop Extended Community is used if that Extended
   Community is present; otherwise, the address from the Next-Hop field
   is used.

   This procedure enables the ingress PE to explicitly track the egress
   PEs for a given flow, even if segmented tunnels are being used.
   However, cross-domain explicit tracking utilizes S-PMSI A-D routes
   that do not specify tunnel information; therefore, it can only be
   done when the S-PMSI A-D route that is a flow's match for tracking is
   different from the S-PMSI A-D route that is that flow's match for
   reception.

6.  Ingress Node Handling of Received Leaf A-D Routes with LIR-pF Set

   Consider the following situation:

   o  An ingress node, call it N, receives a Leaf A-D route, call it L.

   o  L carries an IP-address-specific RT identifying N.

   o  The Route Key field of L's NLRI is not identical to the NLRI of
      any current I-PMSI or S-PMSI A-D route originated by N.

   Per the procedures of [RFC6514] and [RFC7524], such a Leaf A-D route
   does not cause any MVPN-specific action to be taken by N.

   This document modifies those procedures in the case where there is a
   current wildcard S-PMSI A-D route, originated by N, to which L is a
   valid response according to the procedures of Section 5.2.  In this
   case, L MUST be processed by N.

   Suppose that L's PTA specifies a tunnel type of Ingress Replication
   and that it also specifies a non-zero MPLS label.  Then if N needs to
   send to L a packet belonging to the multicast flow or flows
   identified in L's NLRI, N MUST use the specified label.

   If L's PTA meets any of the following conditions:

   o  It specifies a tunnel type of "no tunnel information present", or

   o  It specifies a tunnel type of Ingress Replication, but specifies
      an MPLS label of zero, or

   o  It specifies any other 6514-tunnel-type,

   then the action taken by N when it receives L is a local matter.  In
   this case, the Leaf A-D route L provides N with explicit tracking
   information for the flow identified by L's NLRI.  However, that
   information is for management/monitoring purposes and does not have
   any direct effect on the flow of multicast traffic.

   If L's PTA specifies a non-6514-tunnel-type not mentioned above,
   presumably there is a specification for how MVPN uses that tunnel
   type.  If the LIR-pF flag is to be used with that tunnel type, that
   specification must specify the actions that N is to take upon
   receiving L.  As an example, see [BIER-MVPN].  In the absence of such
   a specification, the LIR-pF flag SHOULD BE ignored.  See Section 2
   for further discussion of non-6514-tunnel-types.

7.  IANA Considerations

   IANA has added the following entry to the "P-Multicast Service
   Interface (PMSI) Tunnel Attribute Flags" registry under the "Border
   Gateway Protocol (BGP) Parameters" registry.  This registry is
   defined in [RFC7902].  The entry appears as:

   o  Value: 2

   o  Name: Leaf Information Required per-Flow (LIR-pF)

   o  Reference: RFC 8534

8.  Security Considerations

   The Security Considerations of [RFC6513] and [RFC6514] apply.

   By setting the LIR-pF flag in a single wildcard S-PMSI A-D route, a
   large number of Leaf A-D routes can be elicited.  If this flag is set
   when not desired (through either error or malfeasance), a significant
   increase in control plane overhead can result.  Properly protecting
   the control plane should prevent this kind of attack.

   In the event such an attack occurs, mitigating it is unfortunately
   not very straightforward.  The ingress node can take note of the fact
   that it is getting, in response to an S-PMSI A-D route that has
   LIR-pF clear, one or more Leaf A-D routes that have LIR-pF set.  By
   default, the reception of such a route MUST be logged.  However, it
   is possible for such log entries to be "false positives" that
   generate a lot of "noise" in the log; therefore, implementations
   SHOULD have a knob to disable this logging.

   In theory, if one or more Leaf A-D routes with LIR-pF set arrive in
   response to an S-PMSI A-D route with LIR-pF clear, withdrawing the
   S-PMSI A-D route could put a stop to the attack.  In practice, that
   is not likely to be a very good strategy, because:

   o  Under normal operating conditions, there are some race conditions
      that may cause the ingress node to think it is being attacked,
      when in fact it is not.

   o  If some egress nodes have a bug that causes them to set LIR-pF
      when it should be clear, withdrawing the S-PMSI A-D route will
      stop the flow of multicast data traffic to all the egress nodes,
      causing an unnecessary customer-visible disruption.

   o  The same situation that caused the S-PMSI A-D route to be
      originated in the first place will still exist after the S-PMSI
      A-D route is withdrawn, so the route will just be re-originated.

   In other words, any action that would ameliorate the effects of this
   sort of attack would likely have a negative effect during normal
   operation.  Therefore, it is really better to rely on security
   mechanisms that protect the control plane generally than to have a
   mechanism that is focused on this one particular type of attack.

9.  References

9.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC6513]  Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
              BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
              2012, <https://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,
              <https://www.rfc-editor.org/info/rfc6514>.

   [RFC6515]  Aggarwal, R. and E. Rosen, "IPv4 and IPv6 Infrastructure
              Addresses in BGP Updates for Multicast VPN", RFC 6515,
              DOI 10.17487/RFC6515, February 2012,
              <https://www.rfc-editor.org/info/rfc6515>.

   [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,
              <https://www.rfc-editor.org/info/rfc6625>.

   [RFC7524]  Rekhter, Y., Rosen, E., Aggarwal, R., Morin, T.,
              Grosclaude, I., Leymann, N., and S. Saad, "Inter-Area
              Point-to-Multipoint (P2MP) Segmented Label Switched Paths
              (LSPs)", RFC 7524, DOI 10.17487/RFC7524, May 2015,
              <https://www.rfc-editor.org/info/rfc7524>.

   [RFC7902]  Rosen, E. and T. Morin, "Registry and Extensions for
              P-Multicast Service Interface Tunnel Attribute Flags",
              RFC 7902, DOI 10.17487/RFC7902, June 2016,
              <https://www.rfc-editor.org/info/rfc7902>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [BIER-MVPN]
              Rosen, E., Sivakumar, M., Aldrin, S., Dolganow, A., and
              T. Przygienda, "Multicast VPN Using BIER", Work in
              Progress, draft-ietf-bier-mvpn-11, March 2018.

   [RFC7582]  Rosen, E., Wijnands, IJ., Cai, Y., and A. Boers,
              "Multicast Virtual Private Network (MVPN): Using
              Bidirectional P-Tunnels", RFC 7582, DOI 10.17487/RFC7582,
              July 2015, <https://www.rfc-editor.org/info/rfc7582>.

   [RFC7900]  Rekhter, Y., Ed., Rosen, E., Ed., Aggarwal, R., Cai, Y.,
              and T. Morin, "Extranet Multicast in BGP/IP MPLS VPNs",
              RFC 7900, DOI 10.17487/RFC7900, June 2016,
              <https://www.rfc-editor.org/info/rfc7900>.

Acknowledgments

   The authors wish to thank Robert Kebler for his ideas and comments
   and Stephane Litkowski and Benjamin Kaduk for their thorough reviews
   and useful suggestions.  We would also like to thank Mirja Kuhlewind
   for her attention to the Security Considerations section.

Authors' Addresses

   Andrew Dolganow
   Nokia
   438B Alexandra Rd #08-07/10
   Alexandra Technopark
   119968
   Singapore

   Email: andrew.dolganow@nokia.com


   Jayant Kotalwar
   Nokia
   701 East Middlefield Rd
   Mountain View, California  94043
   United States of America

   Email: jayant.kotalwar@nokia.com


   Eric C. Rosen (editor)
   Juniper Networks, Inc.
   10 Technology Park Drive
   Westford, Massachusetts  01886
   United States of America

   Email: erosen52@gmail.com


   Zhaohui Zhang
   Juniper Networks, Inc.
   10 Technology Park Drive
   Westford, Massachusetts  01886
   United States of America

   Email: zzhang@juniper.net