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Versions: (draft-morin-bess-mvpn-fast-failover) 00 01 02 03 04

Network Working Group                                      T. Morin, Ed.
Internet-Draft                                                    Orange
Intended status: Standards Track                          R. Kebler, Ed.
Expires: May 10, 2019                                   Juniper Networks
                                                          G. Mirsky, Ed.
                                                               ZTE Corp.
                                                        November 6, 2018


                  Multicast VPN fast upstream failover
                 draft-ietf-bess-mvpn-fast-failover-04

Abstract

   This document defines multicast VPN extensions and procedures that
   allow fast failover for upstream failures, by allowing downstream PEs
   to take into account the status of Provider-Tunnels (P-tunnels) when
   selecting the upstream PE for a VPN multicast flow, and extending BGP
   MVPN routing so that a C-multicast route can be advertised toward a
   standby upstream PE.

Requirements Language

   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.

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
   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 May 10, 2019.






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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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  UMH Selection based on tunnel status  . . . . . . . . . . . .   3
     3.1.  Determining the status of a tunnel  . . . . . . . . . . .   4
       3.1.1.  mVPN tunnel root tracking . . . . . . . . . . . . . .   5
       3.1.2.  PE-P Upstream link status . . . . . . . . . . . . . .   5
       3.1.3.  P2MP RSVP-TE tunnels  . . . . . . . . . . . . . . . .   5
       3.1.4.  Leaf-initiated P-tunnels  . . . . . . . . . . . . . .   6
       3.1.5.  ((S, G)) counter information  . . . . . . . . . . . .   6
       3.1.6.  BFD Discriminator . . . . . . . . . . . . . . . . . .   6
       3.1.7.  Per PE-CE link BFD Discriminator  . . . . . . . . . .   9
   4.  Standby C-multicast route . . . . . . . . . . . . . . . . . .  10
     4.1.  Downstream PE behavior  . . . . . . . . . . . . . . . . .  11
     4.2.  Upstream PE behavior  . . . . . . . . . . . . . . . . . .  12
     4.3.  Reachability determination  . . . . . . . . . . . . . . .  13
     4.4.  Inter-AS  . . . . . . . . . . . . . . . . . . . . . . . .  13
       4.4.1.  Inter-AS procedures for downstream PEs, ASBR fast
               failover  . . . . . . . . . . . . . . . . . . . . . .  14
       4.4.2.  Inter-AS procedures for ASBRs . . . . . . . . . . . .  14
   5.  Hot leaf standby  . . . . . . . . . . . . . . . . . . . . . .  14
   6.  Duplicate packets . . . . . . . . . . . . . . . . . . . . . .  15
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   10. Contributor Addresses . . . . . . . . . . . . . . . . . . . .  16
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     11.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19





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1.  Introduction

   In the context of multicast in BGP/MPLS VPNs, it is desirable to
   provide mechanisms allowing fast recovery of connectivity on
   different types of failures.  This document addresses failures of
   elements in the provider network that are upstream of PEs connected
   to VPN sites with receivers.

   Section 3 describes local procedures allowing an egress PE (a PE
   connected to a receiver site) to take into account the status of
   P-tunnels to determine the Upstream Multicast Hop (UMH) for a given
   (C-S, C-G).  This method does not provide a "fast failover" solution
   when used alone, but can be used with the following sections for a
   "fast failover" solution.

   Section 4 describes protocol extensions that can speed up failover by
   not requiring any multicast VPN routing message exchange at recovery
   time.

   Moreover, section 5 describes a "hot leaf standby" mechanism, that
   uses a combination of these two mechanisms.  This approach has
   similarities with the solution described in [RFC7431] to improve
   failover times when PIM routing is used in a network given some
   topology and metric constraints.

2.  Terminology

   The terminology used in this document is the terminology defined in
   [RFC6513] and [RFC6514].

   x-PMSI: I-PMSI or S-PMSI

3.  UMH Selection based on tunnel status

   Current multicast VPN specifications [RFC6513], section 5.1, describe
   the procedures used by a multicast VPN downstream PE to determine
   what the upstream multicast hop (UMH) is for a given (C-S,C-G).

   The procedure described here is an OPTIONAL procedure that consists
   of having a downstream PE take into account the status of P-tunnels
   rooted at each possible upstream PEs, for including or not including
   each given PE in the list of candidate UMHs for a given (C-S,C-G)
   state.  The result is that, if a P-tunnel is "down" (see
   Section 3.1), the PE that is the root of the P-tunnel will not be
   considered for UMH selection, which will result in the downstream PE
   to failover to the upstream PE which is next in the list of
   candidates.




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   A downstream PE monitors the status of the tunnels of UMHs that are
   ahead of the current one.  Whenever the downstream PE determines that
   one of these tunnels is no longer "known to down", the PE selects the
   UMH corresponding to that as the new UMH.

   More precisely, UMH determination for a given (C-S,C-G) will consider
   the UMH candidates in the following order:

   o  first, the UMH candidates that either (a) advertise a PMSI bound
      to a tunnel, where the specified tunnel is not known to be down or
      (b) do not advertise any x-PMSI applicable to the given (C-S,C-G)
      but have associated a VRF Route Import BGP attribute to the
      unicast VPN route for S (this is necessary to avoid incorrectly
      invalidating an UMH PE that would use a policy where no I-PMSI is
      advertised for a given VRF and where only S-PMSI are used, the
      S-PMSI advertisement being possibly done only after the upstream
      PE receives a C-multicast route for (C-S, C-G)/(C-*, C-G) to be
      carried over the advertised S-PMSI)

   o  second, the UMH candidates that advertise a PMSI bound to a tunnel
      that is "down" -- these will thus be used as a last resort to
      ensure a graceful fallback to the basic MVPN UMH selection
      procedures in the hypothetical case where a false negative would
      occur when determining the status of all tunnels

   For a given downstream PE and a given VRF, the P-tunnel corresponding
   to a given upstream PE for a given (C-S,C-G) state is the S-PMSI
   tunnel advertised by that upstream PE for this (C-S,C-G) and imported
   into that VRF, or if there isn't any such S-PMSI, the I-PMSI tunnel
   advertised by that PE and imported into that VRF.

   Note that this document assumes that if a site of a given MVPN that
   contains C-S is dual-homed to two PEs, then all the other sites of
   that MVPN would have two unicast VPN routes (VPN-IPv4 or VPN-IPv6)
   routes to C-S, each with its own RD.

3.1.  Determining the status of a tunnel

   Different factors can be considered to determine the "status" of a
   P-tunnel and are described in the following sub-sections.  The
   procedure proposed here also allows that all downstream PEs don't
   apply the same rules to define what the status of a P-tunnel is
   (please see Section 6), and some of them will produce a result that
   may be different for different downstream PEs.  Thus what is called
   the "status" of a P-tunnel in this section, is not a characteristic
   of the tunnel in itself, but is the status of the tunnel, *as seen
   from a particular downstream PE*.  Additionally, some of the
   following methods determine the ability of downstream PE to receive



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   traffic on the P-tunnel and not specifically on the status of the
   P-tunnel itself.  This could be referred to as "P-tunnel reception
   status", but for simplicity, we will use the terminology of P-tunnel
   "status" for all of these methods.

   Depending on the criteria used to determine the status of a P-tunnel,
   there may be an interaction with another resiliency mechanism used
   for the P-tunnel itself, and the UMH update may happen immediately or
   may need to be delayed.  Each particular case is covered in each
   separate sub-section below.

3.1.1.  mVPN tunnel root tracking

   A condition to consider that the status of a P-tunnel is up is that
   the root of the tunnel, as determined in the PMSI tunnel attribute,
   is reachable through unicast routing tables.  In this case, the
   downstream PE can immediately update its UMH when the reachability
   condition changes.

   This is similar to BGP next-hop tracking for VPN routes, except that
   the address considered is not the BGP next-hop address, but the root
   address in the PMSI tunnel attribute.

   If BGP next-hop tracking is done for VPN routes and the root address
   of a given tunnel happens to be the same as the next-hop address in
   the BGP auto-discovery route advertising the tunnel, then this
   mechanisms may be omitted for this tunnel, as it will not bring any
   specific benefit.

3.1.2.  PE-P Upstream link status

   A condition to consider a tunnel status as Up can be that the last-
   hop link of the P-tunnel is up.

   This method should not be used when there is a fast restoration
   mechanism (such as MPLS FRR [RFC4090]) in place for the link.

3.1.3.  P2MP RSVP-TE tunnels

   For P-tunnels of type P2MP MPLS-TE, the status of the P-tunnel is
   considered up if one or more of the P2MP RSVP-TE LSPs, identified by
   the P-tunnel Attribute, are in Up state.  The determination of
   whether a P2MP RSVP-TE LSP is in Up state requires Path and Resv
   state for the LSP and is based on procedures in [RFC4875].  In this
   case, the downstream PE can immediately update its UMH when the
   reachability condition changes.





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   When signaling state for a P2MP TE LSP is removed (e.g. if the
   ingress of the P2MP TE LSP sends a PathTear message) or the P2MP TE
   LSP changes state from Up to Down as determined by procedures in
   [RFC4875], the status of the corresponding P-tunnel SHOULD be re-
   evaluated.  If the P-tunnel transitions from up to Down state, the
   upstream PE, that is the ingress of the P-tunnel, SHOULD NOT be
   considered a valid UMH.

3.1.4.  Leaf-initiated P-tunnels

   A PE can be removed from the UMH candidate list for a given ((S, G))
   if the P-tunnel for this (S, G) (I or S , depending) is leaf
   triggered (PIM, mLDP), but for some reason internal to the protocol
   the upstream one-hop branch of the tunnel from P to PE cannot be
   built.  In this case, the downstream PE can immediately update its
   UMH when the reachability condition changes.

3.1.5.  ((S, G)) counter information

   In cases, where the downstream node can be configured so that the
   maximum inter-packet time is known for all the multicast flows mapped
   on a P-tunnel, the local per-(C-S,C-G) traffic counter information
   for traffic received on this P-tunnel can be used to determine the
   status of the P-tunnel.

   When such a procedure is used, in the context where fast restoration
   mechanisms are used for the P-tunnels, downstream PEs should be
   configured to wait before updating the UMH, to let the P-tunnel
   restoration mechanism happen.  A configurable timer MUST be provided
   for this purpose, and it is recommended to provide a reasonable
   default value for this timer.

   This method can be applicable, for instance, when a ((S, G)) flow is
   mapped on an S-PMSI.

   In cases where this mechanism is used in conjunction with
   Hot leaf standby, then no prior knowledge of the rate of the
   multicast streams is required; downstream PEs can compare reception
   on the two P-tunnels to determine when one of them is down.

3.1.6.  BFD Discriminator

   P-tunnel status can be derived from the status of a multipoint BFD
   session [I-D.ietf-bfd-multipoint] whose discriminator is advertised
   along with an x-PMSI A-D route.

   This document defines the format and ways of usingr a new BGP
   attribute called the "BGP- BFD attribute".  This is an optional



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   transitive BGP attribute.  The format of this attribute is defined as
   follows:


              +-------------------------------+
              |       Flags (1 octet)         |
              +-------------------------------+
              |  BFD Discriminator (4 octets) |
              +-------------------------------+



   The Flags field has the following format:



                    0 1 2 3 4 5 6 7
                    +-+-+-+-+-+-+-+-+
                    |   reserved    |
                    +-+-+-+-+-+-+-+-+



3.1.6.1.  Upstream PE Procedures

   When it is desired to track the P-tunnel status using p2mp BFD
   session, the Upstream PE:

   o  MUST initiate BFD session and set bfd.SessionType = MultipointHead
      as described in [I-D.ietf-bfd-multipoint];

   o  MUST use [Ed.note] address as destination IP address when
      transmitting BFD control packets;

   o  MUST use the IP address of the Upstream PE as source IP address
      when transmitting BFD control packets;

   o  MUST include the BGP-BFD Attribute in the x-PMSI A-D Route with
      BFD Discriminator value set to My Discriminator value.

   If tracking of the P-tunnel by using a p2mp BFD session is to be
   enabled after the P-tunnel has been already signaled, the the
   procedure described above MUST be followed.  Note that x-PMSI A-D
   Route MUST be re-sent with exactly the same attributes as before and
   the BGP-BFD Attribute included.






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   If P-tunnel is already signaled, and P-tunnel status tracked using
   the p2mp BFD session and it is desired to stop tracking P-tunnel
   status using BFD, then:

   o  x-PMSI A-D Route MUST be re-sent with exactly the same attributes
      as before, but the BGP-BFD Attribute MUST be excluded;

   o  the p2mp BFD session SHOULD be deleted.

3.1.6.2.  Downstream PE Procedures

   On receiving the BGP-BFD Attribute in the x-PMSI A-D Route, the
   Downstream PE:

   o  MUST associate the received BFD discriminator value with the
      P-tunnel originating from the Root PE;

   o  MUST create p2mp BFD session and set bfd.SessionType =
      MultipointTail as described in [I-D.ietf-bfd-multipoint];

   o  MUST use the source IP address of a BFD control packet, the value
      of BFD Discriminator from the BGP-BFD Attribute to properly
      demultiplex BFD sessions;

   After the state of the p2mp BFD session is up, i.e. bfd.SessionState
   = Up, the session state will then be used to track the health of the
   P-tunnel.

   According to [I-D.ietf-bfd-multipoint], if the Downstream PE receives
   Down or AdminDown in the State field of the BFD control packet or
   associated with the BFD session Detection Timer expires, the BFD
   session state is down, i.e. bfd.SessionState = Down.  When the BFD
   session state is Down, then the P-tunnel associated with the BFD
   session as down MUST be declared down.  Then The Downstream PE MAY
   initiate a switchover of the traffic from the Primary Upstream PE to
   the Standby Upstream PE only if the Standby Upstream PE deemed
   available.  The dedicated p2mp BFD session MAY monitor the state of
   the Standby Upstream PE.

   If the Downstream PE's P-tunnel is already up when the Downstream PE
   receives the new x-PMSI A-D Route with BGP-BFD Attribute, the
   Downstream PE MUST accept the x-PMSI A-D Route and associate the
   value of BFD Discriminator field with the P-tunnel.  The Upstream PE
   MUST follow procedures listed above in this section to bring the p2mp
   BFD session up and use it to monitor the state of the associated
   P-tunnel.





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   If the Downstream PE's P-tunnel is already up, its state being
   monitored by the p2mp BFD session, and the Downstream PE receives the
   new x-PMSI A-D Route without the BGP-BFD Attribute, the Downstream
   PE:

   o  MUST accept the x-PMSI A-D Route;

   o  MUST stop receiving BFD control packets for this p2mp BFD session;

   o  SHOULD delete the p2mp BFD session associated with the P-tunnel;

   o  SHOULD NOT switch the traffic to the Standby Upstream PE.

   When such a procedure is used, in the context where fast restoration
   mechanisms are used for the P-tunnels, leaf PEs should be configured
   to wait before updating the UMH, to let the P-tunnel restoration
   mechanism happen.  A configurable timer MUST be provided for this
   purpose, and it is recommended to provide a reasonable default value
   for this timer.

3.1.7.  Per PE-CE link BFD Discriminator

   The following approach is defined for the fast failover in response
   to the detection of PE-CE link failures, in which UMH selection for a
   given C-multicast route takes into account the state of the BFD
   session associated with the state of the upstream PE-CE link.

3.1.7.1.  Upstream PE Procedures

   For each protected PE-CE link, the upstream PE initiates a multipoint
   BFD session [I-D.ietf-bfd-multipoint] as MultipointHead toward
   downstream PEs.  A downstream PE monitors the state of the p2mp
   session as MultipointTail and MAY interpret transition of the BFD
   session into Down state as the indication of the associated PE-CE
   link being down.

   For SSM groups, the upstream PE advertises an ((S, G)) S-PMSI A-D
   route or wildcard (S,*) S-PMSI A-D route for each received SSM ((S,
   G)) C-multicast route for which protection is desired.  For each ASM
   ((S, G)) C-multicast route for which protection is desired, the
   upstream PE advertises a ((S, G)) S-PMSI A-D route.  For each ASM
   (*,G) C-Multicast route for which protection is desired, the upstream
   PE advertises a wildcard (*,G) S-PMSI A-D route.  Note that all
   S-PMSI A-D routes can signal the same P-tunnel, so there is no need
   for a new P-tunnel for each S-PMSI A-D route.  Multicast flows for
   which protection is desired is controlled by configuration/policy on
   the upstream PE.  The protected link is the RPF PE-CE interface
   towards the src/RP.  The upstream PE advertises the BFD discriminator



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   of the protected link in the S-PMSI A-D route.  If the route to the
   src/RP changes such that the RPF interface is changed to be a new PE-
   CE interface, then the upstream PE will update the S-PMSI A-D route
   with included BGP-BFD Attribute so that value of the BFD
   Discriminator is associated with the new RPF link.

3.1.7.2.  Downstream PE Procedures

   If an S-PMSI A-D route bound to a given C-multicast is signaled with
   a multipoint BFD session, then the upstream PE is considered during
   UMH selection for the C-multicast if and only if the corresponding
   BFD session is not in state Down, i.e bfd.SessionState != Down.
   Whenever the state of the BFD session changes to Down the Provider
   Tunnel will be considered down, and the downstream PE MAY switch to
   the backup Provider Tunnel only if the backup Provider Tunnel deemed
   available.  The dedicated p2mp BFD session MAY monitor the state of
   the backup Provider Tunnel.  Note that the Provider Tunnel is
   considered down only for the C-multicast states that match to an
   S-PMSI A-D route which included BGP-BFD Attribute with the BFD
   Discriminator of the p2mp BFD session which is down.

4.  Standby C-multicast route

   The procedures described below are limited to the case where the site
   that contains C-S is connected to exactly two PEs.  The procedures
   require all the PEs of that MVPN to follow the single forwarder PE
   selection, as specified in [RFC6513].  The procedures assume that if
   a site of a given MVPN that contains C-S is dual-homed to two PEs,
   then all the other sites of that MVPN would have two unicast VPN
   routes (VPN-IPv4 or VPN-IPv6) routes to C-S, each with its own RD.

   As long as C-S is reachable via both PEs, a given downstream PE will
   select one of the PEs connected to C-S as its Upstream PE with
   respect to C-S.  We will refer to the other PE connected to C-S as
   the "Standby Upstream PE".  Note that if the connectivity to C-S
   through the Primary Upstream PE becomes unavailable, then the PE will
   select the Standby Upstream PE as its Upstream PE with respect to
   C-S.  When the Primary PE later becomes available, then the PE will
   select the Primary Upstream PE again as its Upstream PE.  This is
   referred to as "revertive" behavior and MUST be supported.  Non-
   revertive behavior would refer to the behavior of continuing to
   select the backup PE as the UMH even after the Primary has come up.
   This non-revertive behavior can also be optionally supported by an
   implementation and would be enabled through some configuration.

   For readability, in the following sub-sections, the procedures are
   described for BGP C-multicast Source Tree Join routes, but they apply
   equally to BGP C-multicast Shared Tree Join routes failover for the



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   case where the customer RP is dual-homed (substitute "C-RP" to
   "C-S").

4.1.  Downstream PE behavior

   When a (downstream) PE connected to some site of an MVPN needs to
   send a C-multicast route (C-S, C-G), then following the procedures
   specified in Section "Originating C-multicast routes by a PE" of
   [RFC6514] the PE sends the C-multicast route with RT that identifies
   the Upstream PE selected by the PE originating the route.  As long as
   C-S is reachable via the Primary Upstream PE, the Upstream PE is the
   Primary Upstream PE.  If C-S is reachable only via the Standby
   Upstream PE, then the Upstream PE is the Standby Upstream PE.

   If C-S is reachable via both the Primary and the Standby Upstream PE,
   then in addition to sending the C-multicast route with an RT that
   identifies the Primary Upstream PE, the PE also originates and sends
   a C-multicast route with an RT that identifies the Standby Upstream
   PE.  This route, that has the semantics of being a 'standby'
   C-multicast route, is further called a "Standby BGP C-multicast
   route", and is constructed as follows:

   o  the NLRI is constructed as the original C-multicast route, except
      that the RD is the same as if the C-multicast route was built
      using the standby PE as the UMH (it will carry the RD associated
      to the unicast VPN route advertised by the standby PE for S)

   o  SHOULD carry the "Standby PE" BGP Community (this is a new BGP
      Community, see Section 7)

   The normal and the standby C-multicast routes must have their Local
   Preference attribute adjusted so that, if two C-multicast routes with
   same NLRI are received by a BGP peer, one carrying the "Standby PE"
   attribute and the other one *not* carrying the "Standby PE"
   community, then preference is given to the one *not* carrying the
   "Standby PE" attribute.  Such a situation can happen when, for
   instance, due to transient unicast routing inconsistencies, two
   different downstream PEs consider different upstream PEs to be the
   primary one; in that case, without any precaution taken, both
   upstream PEs would process a standby C-multicast route and possibly
   stop forwarding at the same time.  For this purpose, routes that
   carry the "Standby PE" BGP Community MUST have the LOCAL_PREF
   attribute set to zero.

   Note that, when a PE advertises such a Standby C-multicast join for
   an ((S, G)) it must join the corresponding P-tunnel.





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   If at some later point the local PE determines that C-S is no longer
   reachable through the Primary Upstream PE, the Standby Upstream PE
   becomes the Upstream PE, and the local PE re-sends the C-multicast
   route with RT that identifies the Standby Upstream PE, except that
   now the route does not carry the Standby PE BGP Community (which
   results in replacing the old route with a new route, with the only
   difference between these routes being the presence/absence of the
   Standby PE BGP Community).

4.2.  Upstream PE behavior

   When a PE receives a C-multicast route for a particular (C-S, C-G),
   and the RT carried in the route results in importing the route into a
   particular VRF on the PE, if the route carries the Standby PE BGP
   Community, then the PE performs as follows:

      when the PE determines that C-S is not reachable through some
      other PE, the PE SHOULD install VRF PIM state corresponding to
      this Standby BGP C-multicast route (the result will be that a PIM
      Join message will be sent to the CE towards C-S, and that the PE
      will receive (C-S,C-G) traffic), and the PE SHOULD forward (C-S,
      C-G) traffic received by the PE to other PEs through a P-tunnel
      rooted at the PE.

   Furthermore, irrespective of whether C-S carried in that route is
   reachable through some other PE:

   a) based on local policy, as soon as the PE receives this Standby BGP
      C-multicast route, the PE MAY install VRF PIM state corresponding
      to this BGP Source Tree Join route (the result will be that Join
      messages will be sent to the CE toward C-S, and that the PE will
      receive (C-S,C-G) traffic)

   b) based on local policy, as soon as the PE receives this Standby BGP
      C-multicast route, the PE MAY forward (C-S, C-G) traffic to other
      PEs through a P-tunnel independently of the reachability of C-S
      through some other PE. [note that this implies also doing (a)]

   Doing neither (a) or (b) for a given (C-S,C-G) is called "cold root
   standby".

   Doing (a) but not (b) for a given (C-S,C-G) is called "warm root
   standby".

   Doing (b) (which implies also doing (a)) for a given (C-S,C-G) is
   called "hot root standby".





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   Note that, if an upstream PE uses an S-PMSI only policy, it shall
   advertise an S-PMSI for an ((S, G)) as soon as it receives a
   C-multicast route for ((S, G)), normal or Standby; i.e. it shall not
   wait for receiving a non-Standby C-multicast route before advertising
   the corresponding S-PMSI.

   Section 9.3.2 of [RFC6514], describes the procedures of sending a
   Source-Active A-D result as a result of receiving the C-multicast
   route.  These procedures should be followed for both the normal and
   Standby C-multicast routes.

4.3.  Reachability determination

   The standby PE can use the following information to determine that
   C-S can or cannot be reached through the primary PE:

   o  presence/absence of a unicast VPN route toward C-S

   o  supposing that the standby PE is an egress of the tunnel rooted at
      the Primary PE, the standby PE can determine the reachability of
      C-S through the Primary PE based on the status of this tunnel,
      determined thanks to the same criteria as the ones described in
      Section 3.1 (without using the UMH selection procedures of
      Section 3)

   o  other mechanisms MAY be used

4.4.  Inter-AS

   If the non-segmented inter-AS approach is used, the procedures in
   section 4 can be applied.

   When multicast VPNs are used in an inter-AS context with the
   segmented inter-AS approach described in section 8.2 of [RFC6514],
   the procedures in this section can be applied.

   A pre-requisite for the procedures described below to be applied for
   a source of a given MVPN is:

   o  that any PE of this MVPN receives two Inter-AS I-PMSI auto-
      discovery routes advertised by the AS of the source (or more)

   o  that these Inter-AS I-PMSI auto-discovery routes have distinct
      Route Distinguishers (as described in item "(2)" of section 9.2 of
      [RFC6514]).






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   As an example, these conditions will be satisfied when the source is
   dual-homed to an AS that connects to the receiver AS through two ASBR
   using auto-configured RDs.

4.4.1.  Inter-AS procedures for downstream PEs, ASBR fast failover

   The following procedure is applied by downstream PEs of an AS, for a
   source S in a remote AS.

   Additionally, to choosing an Inter-AS I-PMSI auto-discovery route
   advertised from the AS of the source to construct a C-multicast
   route, as described in section 11.1.3 [RFC6514] a downstream PE will
   choose a second Inter-AS I-PMSI auto-discovery route advertised from
   the AS of the source and use this route to construct and advertise a
   Standby C-multicast route (C-multicast route carrying the Standby
   extended community) as described in Section 4.1.

4.4.2.  Inter-AS procedures for ASBRs

   When an upstream ASBR receives a C-multicast route, and at least one
   of the RTs of the route matches one of the ASBR Import RT, the ASBR
   locates an Inter-AS I-PMSI A-D route whose RD and Source AS matches
   the RD and Source AS carried in the C-multicast route.  If the match
   is found, and C-multicast route carries the Standby PE BGP Community,
   then the ASBR performs as follows:

   o  if the route was received over iBGP; the route is expected to have
      a LOCAL_PREF attribute set to zero and it should be re-advertised
      in eBGP with a MED attribute (MULTI_EXIT_DISC) set to the highest
      possible value (0xffff)

   o  if the route was received over eBGP; the route is expected to have
      a MED attribute set of 0xffff and should be re-advertised in iBGP
      with a LOCAL_PREF attribute set to zero

   Other ASBR procedures are applied without modification.

5.  Hot leaf standby

   The mechanisms defined in sections Section 4 and Section 3 can be
   used together as follows.

   The principle is that, for a given VRF (or possibly only for a given
   C-S,C-G):

   o  downstream PEs advertise a Standby BGP C-multicast route (based on
      Section 4)




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   o  upstream PEs use the "hot standby" optional behavior and thus will
      forward traffic for a given multicast state as soon as they have
      whether a (primary) BGP C-multicast route or a Standby BGP
      C-multicast route for that state (or both)

   o  downstream PEs accept traffic from the primary or standby tunnel,
      based on the status of the tunnel (based on Section 3)

   Other combinations of the mechanisms proposed in Section 4) and
   Section 3 are for further study.

   Note that the same level of protection would be achievable with a
   simple C-multicast Source Tree Join route advertised to both the
   primary and secondary upstream PEs (carrying as Route Target extended
   communities, the values of the VRF Route Import attribute of each VPN
   route from each upstream PEs).  The advantage of using the Standby
   semantic for is that, supposing that downstream PEs always advertise
   a Standby C-multicast route to the secondary upstream PE, it allows
   to choose the protection level through a change of configuration on
   the secondary upstream PE, without requiring any reconfiguration of
   all the downstream PEs.

6.  Duplicate packets

   Multicast VPN specifications [RFC6513] impose that a PE only forwards
   to CEs the packets coming from the expected upstream PE
   (Section 9.1).

   We highlight the reader's attention to the fact that the respect of
   this part of multicast VPN specifications is especially important
   when two distinct upstream PEs are susceptible to forward the same
   traffic on P-tunnels at the same time in the steady state.  This will
   be the case when "hot root standby" mode is used (Section 4), and
   which can also be the case if procedures of Section 3 are used and
   (a) the rules determining the status of a tree are not the same on
   two distinct downstream PEs or (b) the rule determining the status of
   a tree depend on conditions local to a PE (e.g. the PE-P upstream
   link being up).

7.  IANA Considerations

   Allocation is expected from IANA for the BGP "Standby PE" community.
   (TBC)

   [Note to RFC Editor: this section may be removed on publication as an
   RFC.]





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8.  Security Considerations

9.  Acknowledgments

   The authors want to thank Greg Reaume, Eric Rosen, and Jeffrey Zhang
   for their review and useful feedback.

10.  Contributor Addresses

   Below is a list of other contributing authors in alphabetical order:

      Rahul Aggarwal
      Arktan

      Email: raggarwa_1@yahoo.com



      Nehal Bhau
      Alcatel-Lucent, Inc.
      701 E Middlefield Rd
      Mountain View, CA  94043
      USA

      Email: Nehal.Bhau@alcatel-lucent.com



      Clayton Hassen
      Bell Canada
      2955 Virtual Way
      Vancouver
      CANADA

      Email: Clayton.Hassen@bell.ca



      Wim Henderickx
      Alcatel-Lucent
      Copernicuslaan 50
      Antwerp  2018
      Belgium

      Email: wim.henderickx@alcatel-lucent.com






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      Pradeep Jain
      Alcatel-Lucent, Inc.
      701 E Middlefield Rd
      Mountain View, CA  94043
      USA

      Email: pradeep.jain@alcatel-lucent.com



      Jayant Kotalwar
      Alcatel-Lucent, Inc.
      701 E Middlefield Rd
      Mountain View, CA  94043
      USA

      Email: Jayant.Kotalwar@alcatel-lucent.com


      Praveen Muley
      Alcatel-Lucent
      701 East Middlefield Rd
      Mountain View, CA  94043
      U.S.A.

      Email: praveen.muley@alcatel-lucent.com



      Ray (Lei) Qiu
      Juniper Networks
      1194 North Mathilda Ave.
      Sunnyvale, CA  94089
      U.S.A.

      Email: rqiu@juniper.net



      Yakov Rekhter
      Juniper Networks
      1194 North Mathilda Ave.
      Sunnyvale, CA  94089
      U.S.A.

      Email: yakov@juniper.net





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      Kanwar Singh
      Alcatel-Lucent, Inc.
      701 E Middlefield Rd
      Mountain View, CA  94043
      USA

      Email: kanwar.singh@alcatel-lucent.com



11.  References

11.1.  Normative References

   [I-D.ietf-bfd-multipoint]
              Katz, D., Ward, D., Networks, J., and G. Mirsky, "BFD for
              Multipoint Networks", draft-ietf-bfd-multipoint-18 (work
              in progress), June 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>.

   [RFC4875]  Aggarwal, R., Ed., Papadimitriou, D., Ed., and S.
              Yasukawa, Ed., "Extensions to Resource Reservation
              Protocol - Traffic Engineering (RSVP-TE) for Point-to-
              Multipoint TE Label Switched Paths (LSPs)", RFC 4875,
              DOI 10.17487/RFC4875, May 2007,
              <https://www.rfc-editor.org/info/rfc4875>.

   [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>.

   [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>.








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11.2.  Informative References

   [RFC4090]  Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
              Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
              DOI 10.17487/RFC4090, May 2005,
              <https://www.rfc-editor.org/info/rfc4090>.

   [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>.

Authors' Addresses

   Thomas Morin (editor)
   Orange
   2, avenue Pierre Marzin
   Lannion  22307
   France

   Email: thomas.morin@orange-ftgroup.com


   Robert Kebler (editor)
   Juniper Networks
   1194 North Mathilda Ave.
   Sunnyvale, CA  94089
   U.S.A.

   Email: rkebler@juniper.net


   Greg Mirsky (editor)
   ZTE Corp.

   Email: gregimirsky@gmail.com















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