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Versions: 00 01 02

BESS                                                            Z. Zhang
Internet-Draft                                               L. Giuliano
Intended status: Standards Track                        Juniper Networks
Expires: June 28, 2019                                          K. Patel
                                                                  Arrcus
                                                             I. Wijnands
                                                               M. Mishra
                                                           Cisco Systems
                                                                A. Gulko
                                                               Refinitiv
                                                       December 25, 2018


                          BGP Based Multicast
                   draft-zzhang-bess-bgp-multicast-02

Abstract

   This document specifies a BGP address family and related procedures
   that allow BGP to be used for setting up multicast distribution
   trees.  This document also specifies procedures that enable BGP to be
   used for multicast source discovery, and for showing interest in
   receiving particular multicast flows.  Taken together, these
   procedures allow BGP to be used as a replacement for other multicast
   routing protocols, such as PIM or mLDP.  The BGP procedures specified
   here are based on the BGP multicast procedures that were originally
   designed for use by providers of Multicast Virtual Private Network
   service.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC2119.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any




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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 28, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
       1.1.1.  Native/unlabeled Multicast  . . . . . . . . . . . . .   3
       1.1.2.  Labeled Multicast . . . . . . . . . . . . . . . . . .   4
     1.2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   4
       1.2.1.  (x,g) Multicast . . . . . . . . . . . . . . . . . . .   5
         1.2.1.1.  Source Discovery for ASM  . . . . . . . . . . . .   5
         1.2.1.2.  ASM Shared-tree-only Mode . . . . . . . . . . . .   6
         1.2.1.3.  Integration with BGP-MVPN . . . . . . . . . . . .   7
       1.2.2.  BGP Inband Signaling for mLDP Tunnel  . . . . . . . .   7
       1.2.3.  BGP Sessions  . . . . . . . . . . . . . . . . . . . .   7
       1.2.4.  LAN and Parallel Links  . . . . . . . . . . . . . . .   8
       1.2.5.  Transition  . . . . . . . . . . . . . . . . . . . . .   9
   2.  Specification . . . . . . . . . . . . . . . . . . . . . . . .  10
     2.1.  BGP NLRIs and Attributes  . . . . . . . . . . . . . . . .  10
       2.1.1.  S-PMSI A-D Route  . . . . . . . . . . . . . . . . . .  11
       2.1.2.  Leaf A-D Route  . . . . . . . . . . . . . . . . . . .  11
       2.1.3.  Source Active A-D Route . . . . . . . . . . . . . . .  12
       2.1.4.  S-PMSI A-D Route for C-multicast mLDP . . . . . . . .  13
       2.1.5.  Session Address Extended Community  . . . . . . . . .  13
     2.2.  Procedures  . . . . . . . . . . . . . . . . . . . . . . .  14
       2.2.1.  Source Discovery for ASM  . . . . . . . . . . . . . .  14
       2.2.2.  Originating Tree Join Routes  . . . . . . . . . . . .  14
         2.2.2.1.  (x,g) Multicast Tree  . . . . . . . . . . . . . .  14
         2.2.2.2.  BGP Inband Signaling for mLDP Tunnel  . . . . . .  15
       2.2.3.  Receiving Tree Join Routes  . . . . . . . . . . . . .  15



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       2.2.4.  Withdrawl of Tree Join Routes . . . . . . . . . . . .  16
       2.2.5.  LAN procedures for (x,g) Unidirectional Tree  . . . .  16
         2.2.5.1.  Originating S-PMSI A-D Routes . . . . . . . . . .  16
         2.2.5.2.  Receiving S-PMSI A-D Routes . . . . . . . . . . .  17
       2.2.6.  Distributing Label for Upstream Traffic for
               Bidirectional Tree/Tunnel . . . . . . . . . . . . . .  17
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     5.2.  Informative References  . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

1.1.  Motivation

   This section provides some motivation for BGP signaling for native
   and labeld multicast.  One target deployment would be a Data Center
   that requires multicast but uses BGP as its only routing protocol
   [RFC7938].  In such a deployment, it would be desirable to support
   multicast by extending the deployed routing protocol, without
   requiring the deployment of tree building protocols such as PIM,
   mLDP, RSVP-TE P2MP, and without requiring an IGP.

   Additionally, compared to PIM, BGP based signaling has several
   advantage as described in the following section, and may be desired
   in non-DC deployment scenarios as well.

1.1.1.  Native/unlabeled Multicast

   Protocol Independent Multicast (PIM) has been the prevailing
   multicast protocol for many years.  Despite its success, it has two
   drawbacks:

   o  The ASM model, which is prevalent, introduces complexity in the
      following areas: source discovery procedures, need for Rendezvous
      Points (RPs) and group-to-RP mappings, need to switch between RP-
      rooted trees and source-rooted trees, etc.

   o  Periodical protocol state refreshes due to soft state nature.

   While PIM-SSM removes the complexity of PIM-ASM, it requires that
   multicast sources are known apriori.  There have not been a good way
   of discovering sources, so its deployment has been limited.  PIM-Port
   (PIM over Reliable Transport) solves the soft state issue, though its
   deployment has also been limited for two reasons:




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   o  It does not remove the ASM complexities.

   o  In many of the scenarios where reliable transport is deemed
      important, BGP-based multicast (e.g.  BGP-MVPN) has been used
      instead of PORT.

   Partly because of the above mentioned problems, some Data Center
   operators have been avoiding deploying multicast in their networks.

   BGP-MVPN [RFC6514] uses BGP to signal VPN customer multicast state
   over provider networks.  It removes the above mentioned problems from
   the SP environment, and the deployment experiences have been
   encouraging.  While RFC 6514 makes it possible for an SP to provide
   MVPN service without running PIM on its backbone, that RFC still
   assumes that PIM (or mLDP) runs on the PE-CE links. [draft-ietf-bess-
   mvpn-pe-ce] adapts the concept of BGP-MVPN to PE-CE links so that the
   use of PIM on the PE-CE links can be eliminated (though the PIM-ASM
   complexities still remains in the customer network), and this
   document extends it further to general topologies, so that they can
   be run on any router, as a replacement for PIM or mLDP.

   With that, PIM can be completely eliminated from the network.  PIM
   soft state is replaced by BGP hard state.  For ASM, source specific
   trees are set up directly after simpler source discovery (data driven
   on FHRs and control driven elsewhere), all based on BGP.  All the
   complexities related to source discovery and shared/source tree
   switch are also eliminated.  Additionally, the trees can be setup
   with MPLS labels, with just minor enhancements in the signaling.

1.1.2.  Labeled Multicast

   There could be two forms of labeled multicast signaled by BGP.  The
   first one is labeled (x,g) multicast where 'x' stands for either 's'
   or '*'.  Basically, it is for BGP-signaled multicast tree as
   described in previous section but with labels.  The second one is for
   mLDP tunnels with BGP signaling in part or whole through a BGP
   domain.

   For both cases, BGP is used because other label distribution
   mechanisms like mLDP may not be desired by some operators.  For
   example, a DC operator may prefer to have a BGP-only deployment.

1.2.  Overview








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1.2.1.  (x,g) Multicast

   PIM-like functionality is provided, using BGP-based join/prune
   signaling and BGP-based source discovery for ASM.  The BGP-based join
   signaling supports both labeled multicast and IP multicast.

   The same RPF procedures as in PIM are used for each router to
   determine the RPF neighbor for a particular source or RPA (in case of
   Bidirectional Tree).  Except in the Bidirectional Tree case and a
   special case described in Section 1.2.1.2, no (*,G) join is used -
   LHR routers discover the sources for ASM and then join towards the
   sources directly.  Data driven mechanisms like PIM Assert is replaced
   by control driven mechanisms (Section 1.2.4).

   The joins are carried in BGP Updates with MCAST-TREE SAFI and S-PMSI/
   Leaf A-D routes defined in this document.  The updates are targeted
   at the upstream neighbor by use of Route Targets.  [Note - earlier
   version of this draft uses C-multicast route to send joins.  We're
   now switching to S-PMSI/Leaf routes for three reasons. a) when the
   routes go through RRs, we have to distinguish different routes based
   on upstream router and downstream router.  This leads to Leaf routes.
   b) for labeled bidirectional trees, we need to signal "upstream fec".
   S-PMSI suits this very well. c) we may want to allow the option of
   setting up trees from the roots instead of from the leaves.  S-PMSI
   suits that very well.]

   If the BGP updates carry labels (via Tunnel Encapsulation Attribute
   [I-D.ietf-idr-tunnel-encaps]), then (s,g) multicast traffic can use
   the labels.  This is very similar to mLDP Inband Signaling [RFC6826],
   except that there are no corresponding "mLDP tunnels" for the PIM
   trees.  Similar to mLDP, labeled traffic on transit LANs are point to
   point.  Of course, traffic sent to receivers on a LAN by a LHR is
   native multicast.

   For labeled bidirectional (*,g) trees, downstream traffic (away from
   the RPA) can be forwarded as in the (s,g) case.  For upstream traffic
   (towards RPA), the upstream neighbor needs to advertise a label for
   its downstream neighbors.  The same label that the upstream neighbor
   advertises to its upstream is the same one that it advertises to its
   downstreams, using an S-PMSI A-D route.

1.2.1.1.  Source Discovery for ASM

   This document does not support ASM via shared trees (aka RP Tree, or
   RPT) with one exception discussed in the next section.  Instead,
   FHRs, LHRs, and optionally RRs work together to propagate/discover
   source information via control plane and LHRs join source specific
   Shortest Path Trees (SPT) directly.



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   A FHR originates Source Active A-D routes upon discovering sources
   for particular flows and advertise them to its peers.  It is desired
   that the SA routes only reach LHRs that are interested in receiving
   the traffic.  To achieve that, the SA routes carry an IPv4 or IPv6
   address specific Route Target.  The Global Administrator field is set
   the group address of the flow, and the Local Administrator field is
   set to 0.  An LHR advertises Route Target Membership routes, with the
   Route Target field in the NLRI set according to the groups it wants
   to receive traffic for, as how a FHR encode the Route Target in its
   Source Active routes.  The propagation of the SA routes is subject to
   cooperative export filtering as specified in [RFC4684] and referred
   to as RTC mechanism in this document.  That way, the LHR only
   receives Source Active routes for groups that it is interested in.

   Typically, a set of RRs are used and they maintains all Source Active
   routes but only distribute to interested LHRs on demand (upon
   receiving corresponding Route Target Memberhip routes, which are
   triggered on LHRs when they receive IGMP/MLD membership routes).  The
   rest of the document assumes that RRs are used, even though that is
   not required.

1.2.1.2.  ASM Shared-tree-only Mode

   It may be desired that only a shared tree is used to distribute all
   traffic for a particular ASM group from its RP to all LHRs, as
   described in Section 4.1 "PIM Shared Tree Forwarding" of [RFC7438].
   This will significantly cut down the number of trees and works out
   very well in certain deployment scenarios.  For example, all the
   sources could be connected to the RP, or clustered close the to RP.
   In the latter case, either the path from FHRs to the RP do not
   intersect the shared tree so natvie forwarding can be used between
   the FHRs and the RP, or other means outside of this document could be
   used to forward traffic from FHRs to the RP.

   For native forwading from FHRs to the RP, SA routes may be used to
   announce the sources so that the RP can join source specific trees to
   pull traffic, but the group specific Route Target is not needed.  The
   LHRs do not advertise the group specific Route Target Membership
   routes as they do not need the SA routes.

   To establish the shared tree, (*,g) Leaf A-D routes are used as in
   the bidirectional tree case, though no forwarding state is
   established to forward traffic from downstream neighbors.








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1.2.1.3.  Integration with BGP-MVPN

   For each VPN, the Source Active routes distribution in that VPN do
   not have to invlove PEs at all unless there are sources/receivers
   directly connected to some PEs and they are independent of MVPN SA
   routes.  For example, FHRs and LHRs establish BGP sessions with RRs
   of that particular VPN for the purpose of SA distribution.

   After source discovery, BGP multicast signaling is done from LHRs
   towards the sources.  When the signaling reaches an egress PE, BGP-
   MVPN signaling takes over, as if a PIM (s,g) join/prune was received
   on the PE-CE interface.  When the BGP-MVPN signaling reaches the
   ingress PE, BGP multicast signaling as specified in this document
   takes over, similar to how BGP-MVPN triggers PIM (s,g) join/prune on
   PE-CE interfaces.

1.2.2.  BGP Inband Signaling for mLDP Tunnel

   Part of an (or the whole) mLDP tunnel can also be signaled via BGP
   and seamlessly integrated with the rest of mLDP tunnel signaled
   natively via mLDP.  All the procedures are similar to mLDP except
   that the signaling is done via BGP.  The mLDP FEC is encoded as the
   BGP NLRI, with MCAST-TREE SAFI and S-PMSI/Leaf A-D Routes for
   C-multicast mLDP defined in this document.  The Leaf A-D routes
   correspond to mLDP Label Mapping messages, and the S-PMSI A-D routes
   are used to signal upstream FEC for MP2MP mLDP tunnels, similar to
   the bidirection (*,g) case.

1.2.3.  BGP Sessions

   In order for two BGP speakers to exchange MCAST-TREE NLRI, they must
   use BGP Capabilities Advertisement [RFC5492] to ensure that they both
   are capable of properly processing the MCAST-TREE NLRI.  This is done
   as specified in [RFC4760], by using a capability code 1
   (multiprotocol BGP) with an AFI of IPv4 (1) or IPv6 (2) and a SAFI of
   MCAST-TREE with a value to be assigned by IANA.

   How the BGP peer sessions are provisioned, whether EBGP or IBGP,
   whether statically, automatically (e.g., based on IGP neighbor
   discovery), or programmably via an external controller, is outside
   the scope of this document.

   In case of IBGP, it could be that every router peering with Route
   Reflectors, or hop by hop IBGP sessions could be used to exchange
   MCAST-TREE NLRIs for joins.  In the latter case, unless desired
   otherwise for reasons outside of the scope of this document, the hop
   by hop IBGP sessions SHOULD only be used to exchange MCAST-TREE
   NLRIs.



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   When multihop BGP is used, a router advertises its local interface
   addresses, for the same purposes that the Address List TLV in LDP
   serves.  This is achieved by advertising the interface address as
   host prefixes with IPv4/v6 Adderss Specific ECs corresponding to the
   router's local addresses used for its BGP sessions (Section 2.1.5).

   Because the BGP Capability Advertisement is only between two peers,
   when the sessions are only via RRs, a router needs another way to
   determine if its neighbor is capable of signaling multicast via BGP.
   The interface address advertisement can be used for that purpose -
   the inclusion of a Session Address EC indicates that the BGP speaker
   identified in the EC supports the C-Multicast NLRI.

   FHRs and LHRs may also establish BGP sessions to some Route
   Reflectors for source discovery purpose (Section 1.2.1.1).

   With the traditional PIM, the FHRs and LHRs refer to the PIM DRs on
   the source or receiver networks.  With BGP based multicast, PIM may
   not be running at all, and the FHRs and LHRs refer to the IGMP/MLD
   queriers, or the DF elected per [I-D.wijnands-bier-mld-lan-election].
   Alternatively, if it is known that a network only has senders then no
   IGMP/MLD or DF election is needed - any router may generate SA
   routes.  That will not cause any issue other than redundnant SA
   routes being originated.

1.2.4.  LAN and Parallel Links

   There could be parallel links between two BGP peers.  A single multi-
   hop session, whether IBGP or EBGP, between loopback addresses may be
   used.  Except for LAN interfaces in case of unlabeled (x,g)
   unidirectional trees (note that transit LAN interface is not
   supported for BGP signaled (*,g) bidirectional tree and for mLDP
   tunnels, traffic on transit LAN is point to point between neighbors),
   any link between the two peers can be automatically used by a
   downstream peer to receive traffic from the upstream peer, and it is
   for the upstream peer to decide which link to use.  If one of the
   links goes down, the upstream peer switches to a different link and
   there is no change needed on the downstream peer.

   For unlabeled (x,g) unidirectional trees, the upstream peer MAY
   prefer LAN interfaces to send traffic, since multiple downstream
   peers may be reached simultaneously, or it may make a decision based
   on local policy, e.g., for load balancing purpose.  Because different
   downstream peers might choose different upstream peers for RPF, when
   an upstream peer decides to use a LAN interface to send traffic, it
   originates an S-PMSI A-D route indicating that one or more LAN
   interface will be used.  The route carries Route Targets specific to
   the LANs so that all the peers on the LANs import the route.  If more



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   than one router originate the route specifying the same LAN for the
   same (s,g) or (*,g) flow, then assert procedure based on the S-PMSI
   A-D routes happens and assert losers will stop sending traffic to the
   LAN.

1.2.5.  Transition

   A network currently running PIM can be incrementally transitioned to
   BGP based multicast.  At any time, a router supporting BGP based
   multicast can use PIM with some neighbors (upstream or downstream)
   and BGP with some other neighbors.  PIM and BGP MUST not be used
   simultaneously between two neighbors for multicast purpose, and
   routers connected to the same LAN MUST be transitioned during the
   same maintenance window.

   In case of PIM-SSM, any router can be transitioned at any time
   (except on a LAN all routers must be transitioned together).  It may
   receive source tree joins from a mixed set of BGP and PIM downstream
   neighbors and send source tree joins to its upstream neighbor using
   either PIM or BGP signaling.

   In case of PIM-ASM, the RPs are first upgraded to support BGP based
   multicast.  They learn sources either via PIM procedures from PIM
   FHRs, or via Source Active A-D routes from BGP FHRs.  In the former
   case, the RPs can originate proxy Source Active A-D routes.  There
   may be a mixed set of RPs/RRs - some capable of both traditional PIM
   RP functionalities while some only redistribute SA routes.

   Then any routers can be transitioned incrementally.  A transitioned
   LHR router will pull Source Active A-D routes from the RPs/RRs when
   they receive IGMP/MLD (*,G) joins for ASM groups, and may send either
   PIM (s,g) joins or BGP Source Tree Join routes.  A transitioned
   transit router may receive (*,g) PIM joins but only send source tree
   joins after pulling Source Active A-D routes from RPs/RRs.

   Similarly, a network currently running mLDP can be incrementally
   transitioned to BGP signaling.  Without the complication of ASM, any
   router can be transitioned at any time, even without the restriction
   of coordinated transition on a LAN.  It may receive mixed mLDP label
   mapping or BGP updates from different downstream neighbors, and may
   exchange either mLDP label mapping or BGP updates with its upstream
   neighbors, depending on if the neighbor is using BGP based signaling
   or not.








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2.  Specification

2.1.  BGP NLRIs and Attributes

   The BGP Multiprotocol Extensions [RFC4760] allow BGP to carry routes
   from multiple different "AFI/SAFIs".  This document defines a new a
   new SAFI known as a MCAST-TREE SAFI with a value to be assigned by
   the IANA.  This SAFI is used along with the AFI of IPv4 (1) or IPv6
   (2).

   The MCAST-TREE NLRI defined below is carried in the BGP UPDATE
   messages [RFC4271] using the BGP multiprotocol extensions [RFC4760]
   with a AFI of IPv4 (1) or IPv6 (2) assigned by IANA and a MCAST-TREE
   SAFI with a value to be assigned by the IANA.

   The Next hop field of MP_REACH_NLRI attribute SHALL be interpreted as
   an IPv4 adress whenever the length of the Next Hop address is 4
   octets, and as an IPv6 address whenever the length of the Next Hop is
   address is 16 octets.

   The NLRI field in the MP_REACH_NLRI and MP_UNREACH_NLRI is a prefix
   with a maximum length of 12 octers for IPv4 AFI and 36 octets for
   IPv6 AFI.  The following is the format of the MCAST-TREE NLRI:

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

   The Route Type field defines encoding of the rest of the MCAST-TREE
   NLRI.  (Route Type specific MCAST-TREE NLRI).

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

   The following new route types are defined:

          3 -  S-PMSI A-D Route for (x,g)
          4 -  Leaf A-D Route
          5 -  Source Active A-D Route
       0x43 -  S-PMSI A-D Route for C-multicast mLDP

   Except for the Source Active A-D routes, the routes are to be
   consumed by targeted upstream/downstream neighbors, and are not




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   propagated further.  This can be achieved by outbound filtering based
   on the RTs that lead to the importation of the routes.

   The Type-3/4 routes MAY carry a Tunnel Encapsulation Attribute (TEA)
   [I-D.ietf-idr-tunnel-encaps].  The Type-0x43 route MUST carry a TEA.
   When used for mLDP, the Type-4 route MUST carry a TEA.  Only the MPLS
   tunnel type for the TEA is considered.  Others are outside the scope
   of this document.

2.1.1.  S-PMSI A-D Route

   Similar to defined in RFC 6514, an S-PMSI A-D Route Type specific
   MCAST-TREE NLRI consists of the following, though it does not have an
   RD:


         +-----------------------------------+
         | Multicast Source Length (1 octet) |
         +-----------------------------------+
         |  Multicast Source (variable)      |
         +-----------------------------------+
         |  Multicast Group Length (1 octet) |
         +-----------------------------------+
         |  Multicast Group   (variable)     |
         +-----------------------------------+
         |  Upstream Router's IP Address     |
         +-----------------------------------+

   If the Multicast Source (or Group) field contains an IPv4 address,
   then the value of the Multicast Source (or Group) Length field is 32.
   If the Multicast Source (or Group) field contains an IPv6 address,
   then the value of the Multicast Source (or Group) Length field is
   128.

   Usage of other values of the Multicast Source Length and Multicast
   Group Length fields is outside the scope of this document.

   There are two usages for S-PMSI A-D route.  They're described in
   Section 2.2.5 and Section 2.2.6 respectively.

2.1.2.  Leaf A-D Route

   Similar to the Leaf A-D route in [RFC6514], a MCAST-TREE Leaf A-D
   route's route key includes the corresponding S-PMSI NLRI, plus the
   Originating Router's IP Addr.  The difference is that there is no RD.






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         +-----------------------------------+
         |  S-PMSI NLRI                      |
         +-----------------------------------+
         |  Originating Router's IP Addrress |
         +-----------------------------------+

   For example, the entire NLRI of a Leaf A-D route for (x,g) tree is as
   following:


         +-     +-----------------------------------+
         |      |    Route Type - 4 (Leaf A-D)      |
         |      +-----------------------------------+
         |      |     Length (1 octet)              |
         |   +- +-----------------------------------+ --+
         |   |  |    Route Type - 3 (S-PMSI A-D)    |   |
       L | L |  +-----------------------------------+   | S
       E | E |  |     Length (1 octet)              |   | |
       A | A |  +-----------------------------------+   | P
       F | F |  | Multicast Source Length (1 octet) |   | M
         |   |  +-----------------------------------+   | S
       N | R |  |  Multicast Source (variable)      |   | I
       L | O |  +-----------------------------------+   |
       R | U |  |  Multicast Group Length (1 octet) |   | N
       I | T |  +-----------------------------------+   | L
         | E |  |  Multicast Group   (variable)     |   | R
         |   |  +-----------------------------------+   | I
         | K |  |  Upstream Router's IP Address     |   |
         | E |  +-----------------------------------+ --+
         | Y |  |  Originating Router's IP Addrress |
         +-  +- +-----------------------------------+

   Even though the MCAST-TREE Leaf A-D route is unsolicited, unlike the
   Leaf A-D route for GTM in [RFC7524], it is encoded as if a
   corresponding S-PMSI A-D route had been received.

   When used for signaling mLDP tunnels, even though the Leaf A-D route
   is unsolicited, unlike the "Route-type 0x44 Leaf A-D route for
   C-multicast mLDP" as in [RFC7441], it is Route-type 4 and encoded as
   if a corresponding S-PMSI A-D route had been received.

2.1.3.  Source Active A-D Route

   Similar to defined in RFC 6514, a Source Active A-D Route Type
   specific MCAST NLRI consists of the following:






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         +-----------------------------------+
         | Multicast Source Length (1 octet) |
         +-----------------------------------+
         |   Multicast Source (variable)     |
         +-----------------------------------+
         |  Multicast Group Length (1 octet) |
         +-----------------------------------+
         |  Multicast Group (variable)       |
         +-----------------------------------+

   The definition of the source/length and group/length fields are the
   same as in the S-PMSI A-D routes.

   Usage of Source Active A-D routes is described in Section 1.2.1.1.

2.1.4.  S-PMSI A-D Route for C-multicast mLDP

   The route is used to signal upstream FEC for an MP2MP mLDP tunnel.
   The route key include the mLDP FEC and the Upstream Router's IP
   Address field.  The encoding is similar to the same route in
   [RFC7441], though there is no RD.

2.1.5.  Session Address Extended Community

   For two BGP speakers to determine if they are directly connected,
   each will advertise their local interface addresses, with an Session
   Address Extended Community.  This is an Address Specific EC, with the
   Global Admin Field set to the local address used for its multihop
   sessions and the Local Admin Field set to the prefix length
   corresponding to the interface's network mask.

   For example, if a router has two interfaces with address
   10.10.10.1/24 and 10.12.0.1/16 respectively (notice the different
   network mask), and a loopback address 11.11.11.1/32 that is used for
   BGP sessions, then it will advertise prefix 10.10.10.1/32 with a
   Session Address EC 11.11.11.1:24 and 10.12.0.1/32 with a Session
   Address EC 11.11.11.1:16.  If it also uses another loopback address
   11.11.11.11/32 for other BGP sessions, then the routes will
   additionally carry Session Address EC 11.11.11.11:24 and
   11.11.11.11:16 respectively.

   This achieves what the Address List TLV in LDP Address Messages
   achieves, and can also be used to indicate that a router supports the
   BGP multicast signaling procedures specified in this document.

   Only those interface addresses that will be used as resolved nexthops
   in the RIB need to be advertised with the Session Address EC.  For
   example, the RPF lookup may say that the resolved nexthop address is



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   A1, so the router needs to find out the corresponding BGP speaker
   with address A1 through the (interface address, session address)
   mapping built according to the interface address NLRI with the
   Session Address EC.  For comparison with LDP, this is done via the
   (interface address, session address) mapping that is built by the LDP
   Address Messages.

2.2.  Procedures

2.2.1.  Source Discovery for ASM

   When a FHR first receives a multicast packet addressed to an ASM
   group, it originates a Source Active route.  It carries a IP/IPv6
   Address Specific RT, with the Global Admin Field set to the group
   address and the Local Admin Field set to 0.  The route is advertised
   to its peers, who will re-advertise further based on the RTC
   mechanisms.  Note that typically the route is advertised only to the
   RRs.

   The FHRs withdraws the Source Active route after a certain amount of
   time since it last received a packet of an (s,g) flow.  The amount of
   time to wait is a local matter.

2.2.2.  Originating Tree Join Routes

   Note that in this document, tree join routes are S-PMSI/Leaf A-D
   routes.

2.2.2.1.  (x,g) Multicast Tree

   When a router learns from IGMP/MLD or a downstream PIM/BGP peer that
   it needs to join a particular (s,g) tree, it determines the RPF
   nexthop address wrt the source, following the same RPF procedures as
   defined for PIM.  It further finds the BGP router that advertised the
   nexthop address as one of its local addresses.

   If the RPF neighbor supports MCAST-TREE SAFI, this router originates
   a Leaf A-D route.  Although it is unsolicited, it is constructed as
   if there was a corresponding S-PMSI A-D route.  The Upstream Router's
   IP Address field is set to the RPF neighbor's session address (learnt
   via the EC attached to the host route for the RPF nexthop address).
   An Address Specific RT corresponding to the session address is
   attached to the route, with the Global Administrative Field set to
   the session address and the local administrative field set to 0.

   Similarly, when a router learns that it needs to join a bi-
   directional tree for a particular group, it determines the RPF
   neighbor wrt the RPA.  If the neighbor supports MCAST-TREE SAFI, it



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   originates a Leaf A-D Route and advertises the route to the RPF
   neighbor (in case of EBGP or hop-by-hop IBGP), or one or more RRs.

   When a router first learns that it needs to receive traffic for an
   ASM group, either because of a local (*,g) IGMP/MLD report or a
   downstream PIM (*,g) join, it originates a RTC route with the NLRI's
   AS field set to its AS number and the Route Target field set to an
   address based RT, with the Global Administrator field set to group
   address and the Local Administrator field set to 0.  The route is
   advertised to its peers (most practically some RRs), so that the
   router can receive matching Source Active A-D routes.  Upon the
   receiving of the Source Active A-D routes, the router originates Leaf
   A-D routes as described above, as long as it still needs to receive
   traffic for the flows (i.e., the corresponding IGMP/MLD membership
   exists or join from downstream PIM/BGP neighbor exists).

   When a Leaf A-D route is originated by this router, it sets up
   corresponding forwarding state such that the expected incoming
   interface list includes all non-LAN interfaces directly connecting to
   the upstream neighbor.  LAN interfaces are added upon receiving
   corresponding S-PMSI A-D route (Section 2.2.5.2).  If the upstream
   neighbor is not directly connected, tunnels may be used - details to
   be included in future revisions.

   When the upstream nbr changes, the previously advertised Leaf A-D
   route is withdrawn.  If there is a new upstream neighbor, a new Leaf
   A-D route is originated, corresponding to the new neighbor.  Because
   NLRIs are different for the old and new Leaf A-D routes, make-before-
   break can be achieved, so can MoFRR [RFC7431].

2.2.2.2.  BGP Inband Signaling for mLDP Tunnel

   The same mLDP procedures as defined in [RFC6388] are followed, except
   that where a label mapping message is sent in [RFC6388], a Leaf A-D
   route is sent if the the upstream neighbor supports BGP based
   signaling.

2.2.3.  Receiving Tree Join Routes

   A router (auto-)configures Import RTs matching itself so that it can
   import tree join routes from their peers.  Note that in this
   document, tree join routes are S-PMSI/Leaf A-D routes.

   When a router receives a tree join route and imports it, it
   determines if it needs to originate its own corresponding route and
   advertise further upstream wrt the source/RPA or mLDP tunnel root.
   If itself is the FHR or is on the RPL or is the tunnel root, then it




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   does not need to.  Otherwise the procedures in Section 2.2.2 are
   followed.

   Additionally, the router sets up its corresponding forwarding state
   such that traffic will be sent to the downstream neighbor, and
   received from the downstream neighbor in case of birectional tree/
   tunnel.  If the downstream neighbor is not directly connected,
   tunnels may be used - details to be included in future revisions.

2.2.4.  Withdrawl of Tree Join Routes

   For a particular tree or tunnel, if a downstream neighbor withdraw
   its Leaf A-D route, the neighbor is removed from the corresponding
   forwarding state.  If all downstream neighbors withdraw their tree
   join routes and this router no longer has local receivers, it
   withdraws the tree join routes that it previously originated.

   As mentioned earlier, when the upstream neighbor changes, the
   previously advertised Leaf A-D route is also withdrawn.  The
   corresponding incoming interfaces are also removed from the
   corresponding forwarding state.

2.2.5.  LAN procedures for (x,g) Unidirectional Tree

   For a unidirectional (x,g) multicast tree, if there is a LAN
   interface connecting to the downstream neighbor, it MAY be preferred
   over non-LAN interfaces, but an S-PMSI A-D route MUST be originated
   to facilitate the analog of the Assert process (Section 2.2.5.1).

2.2.5.1.  Originating S-PMSI A-D Routes

   If this router chooses to use a LAN interface to send traffic to its
   neighbors for a particular (s,g) or (*,g) flow, it MUST announce that
   by originating a corresponding S-PMSI A-D route.  The Tunnel Type in
   the PMSI Tunnel Attribute (PTA) is set to 0 (no tunnel information
   Present).  The LAN interface is identified by an IP address specific
   RT, with the Global Administrative Field set to the LAN interface's
   address prefix and the Local Administrative Field set to the prefix
   length.  The RT also serves the purpose of restricting the importing
   of the route by all routers on the LAN.  An operator MUST ensure that
   RTs encoded as above are not used for other purposes.  Practically
   that should not be unreasonable.

   If multiple LAN interfaces are to be used (to reach different sets of
   neighbors), then the route will include multiple RTs, one for each
   used LAN interface as described above.





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   The S-PMSI A-D routes may also be used to announce tunnels that could
   be used to send traffic to downstream neighbors that are not directly
   connected.  Details may be added in future revisions.

2.2.5.2.  Receiving S-PMSI A-D Routes

   A router (auto-)configures an Import RT for each of its LAN
   interfaces over which BGP is used for multicast signaling.  The
   construction of the RT is described in the previous section.

   When a router R1 imports an S-PMSI A-D route for flow (x,g) from
   router R2, R1 checks to see if it also originating an S-PMSI A-D
   route with the same NLRI except the Upstream Router's IP Address
   field.  When a router R1 originates an S-PMSI A-D route, it checks to
   see if it also has installed an S-PMSI A-D route, from some other
   router R2, with the same NLRI except the Upstream Router's IP Address
   field.  In either case, R1 checks to see if the two routes have an RT
   in common and the RT is encoded as in Section 2.2.5.1.  If so, then
   there is a LAN attached to both R1 and R2, and both routers are
   prepared to send (S,G) traffic onto that LAN.  This kicks off the
   assert procedure to elect a winner - the one with the highest
   Upstream Router's IP Address in the NLRI wins.  An assert loser will
   not include the corresponding LAN interface in its outgoing interface
   list, but it keeps the S-PMSI A-D route that it originates.

   If this router does not have a matching S-PMSI route of its own with
   some common RTs, and the originator of the received S-PMSI route is a
   chosen upstream neighbor for the corresponding flow, then this router
   updates its forwarding state to include the LAN interface in the
   incoming interface list.  When the last S-PMSI route with a RT
   matching the LAN is withdrawn later, the LAN interface is removed
   from the incoming interface list.

   Note that a downstream router on the LAN does not participate in the
   assert procedure.  It adds/keeps the LAN interface in the expected
   incoming interfaces as long as its chosen upstream peer originates
   the S-PMSI AD route.  It does not switch to the assert winner as its
   upstream.  An assert loser MAY keep sending joins upstream based on
   local policy even if it has no other downstream neighbors (this could
   be used for fast switch over in case the assert winner would fail).

2.2.6.  Distributing Label for Upstream Traffic for Bidirectional Tree/
        Tunnel

   For MP2MP mLDP tunnels or labeled (*,g) bidirectional trees, an
   upstream router needs to advertise a label to all its downstream
   neighbors so that the downstream neighbors can send traffic to
   itself.



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   For MP2MP mLDP tunnels, the same procedures for mLDP are followed
   except that instead of MP2MP-U Label Mapping messages, S-PMSI A-D
   Routes for C-Multicast mLDP are used.

   For labeled (*,g) bidirectional trees, for a Leaf A-D route received
   from a downstream neighbor, a corresponding S-PMSI A-D route is sent
   back to the downstream router.

   In both cases, a single S-PMSI A-D route is originated for each tree
   from this router, but with multiple RTs (one for each downstream
   neighbor on the tree).  A TEA specifies a label allocated by the
   upstream router for its downstream neighbors to send traffic with.
   Note that this is still a "downstream allocated" label (the upstream
   router is "downstream" from traffic direction point of view).

   The S-PMSI routes do not carry a PTA, unless a P2MP tunnel is used to
   reach downstream neighbors.  Such use case is out of scope of this
   document for now and may be specified in the future.

3.  Security Considerations

   This document does not introduce new security risks?

4.  Acknowledgements

   The authors thank Marco Rodrigues for his initial idea/ask of using
   BGP for multicast signaling beyond MVPN.  We thank Eric Rosen for his
   questions, suggestions, and help finding solutions to some issues.
   We also thank Luay Jalil and James Uttaro for their comments and
   support for the work.

5.  References

5.1.  Normative References

   [I-D.ietf-idr-tunnel-encaps]
              Rosen, E., Patel, K., and G. Velde, "The BGP Tunnel
              Encapsulation Attribute", draft-ietf-idr-tunnel-encaps-10
              (work in progress), August 2018.

   [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>.







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   [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,
              <https://www.rfc-editor.org/info/rfc4601>.

   [RFC4684]  Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
              R., Patel, K., and J. Guichard, "Constrained Route
              Distribution for Border Gateway Protocol/MultiProtocol
              Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual
              Private Networks (VPNs)", RFC 4684, DOI 10.17487/RFC4684,
              November 2006, <https://www.rfc-editor.org/info/rfc4684>.

   [RFC5015]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
              "Bidirectional Protocol Independent Multicast (BIDIR-
              PIM)", RFC 5015, DOI 10.17487/RFC5015, October 2007,
              <https://www.rfc-editor.org/info/rfc5015>.

   [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>.

   [RFC7441]  Wijnands, IJ., Rosen, E., and U. Joorde, "Encoding
              Multipoint LDP (mLDP) Forwarding Equivalence Classes
              (FECs) in the NLRI of BGP MCAST-VPN Routes", RFC 7441,
              DOI 10.17487/RFC7441, January 2015,
              <https://www.rfc-editor.org/info/rfc7441>.

5.2.  Informative References

   [I-D.ietf-bess-mvpn-pe-ce]
              Patel, K., Rosen, E., and Y. Rekhter, "BGP as an MVPN PE-
              CE Protocol", draft-ietf-bess-mvpn-pe-ce-01 (work in
              progress), October 2015.

   [I-D.wijnands-bier-mld-lan-election]
              Wijnands, I., Pfister, P., and Z. Zhang, "Generic
              Multicast Router Election on LAN's", draft-wijnands-bier-
              mld-lan-election-01 (work in progress), July 2016.

   [RFC6826]  Wijnands, IJ., Ed., Eckert, T., Leymann, N., and M.
              Napierala, "Multipoint LDP In-Band Signaling for Point-to-
              Multipoint and Multipoint-to-Multipoint Label Switched
              Paths", RFC 6826, DOI 10.17487/RFC6826, January 2013,
              <https://www.rfc-editor.org/info/rfc6826>.





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   [RFC7431]  Karan, A., Filsfils, C., Wijnands, IJ., Ed., and B.
              Decraene, "Multicast-Only Fast Reroute", RFC 7431,
              DOI 10.17487/RFC7431, August 2015,
              <https://www.rfc-editor.org/info/rfc7431>.

   [RFC7938]  Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
              BGP for Routing in Large-Scale Data Centers", RFC 7938,
              DOI 10.17487/RFC7938, August 2016,
              <https://www.rfc-editor.org/info/rfc7938>.

Authors' Addresses

   Zhaohui Zhang
   Juniper Networks

   EMail: zzhang@juniper.net


   Lenny Giuliano
   Juniper Networks

   EMail: lenny@juniper.net


   Keyur Patel
   Arrcus

   EMail: keyur@arrcus.com


   IJsbrand Wijnands
   Cisco Systems

   EMail: ice@cisco.com


   Mankamana Mishra
   Cisco Systems

   EMail: mankamis@cisco.com


   Arkadiy Gulko
   Refinitiv

   EMail: arkadiy.gulko@refinitiv.com





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