Multicast VPN Fast Upstream Failover
draft-ietf-bess-mvpn-fast-failover-12

The information below is for an old version of the document
Document Type Active Internet-Draft (bess WG)
Authors Thomas Morin  , Robert Kebler  , Greg Mirsky 
Last updated 2020-10-28 (latest revision 2020-10-02)
Replaces draft-morin-bess-mvpn-fast-failover
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Network Working Group                                      T. Morin, Ed.
Internet-Draft                                                    Orange
Intended status: Standards Track                          R. Kebler, Ed.
Expires: May 1, 2021                                    Juniper Networks
                                                          G. Mirsky, Ed.
                                                               ZTE Corp.
                                                        October 28, 2020

                  Multicast VPN Fast Upstream Failover
                 draft-ietf-bess-mvpn-fast-failover-12

Abstract

   This document defines multicast VPN extensions and procedures that
   allow fast failover for upstream failures by allowing downstream PEs
   to consider the status of Provider-Tunnels (P-tunnels) when selecting
   the upstream PE for a VPN multicast flow.  The fast failover is
   enabled by using RFC 8562 BFD for Multipoint Networks and the new BGP
   Attribute - BFD Discriminator.  Also, the document introduces a new
   BGP Community, Standby PE, extending BGP MVPN routing so that a
   C-multicast route can be advertised toward a Standby Upstream PE.

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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on May 1, 2021.

Copyright Notice

   Copyright (c) 2020 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

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   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.  Conventions used in this document . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     2.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     2.3.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  UMH Selection Based on Tunnel Status  . . . . . . . . . . . .   5
     3.1.  Determining the Status of a Tunnel  . . . . . . . . . . .   6
       3.1.1.  mVPN Tunnel Root Tracking . . . . . . . . . . . . . .   6
       3.1.2.  PE-P Upstream Link Status . . . . . . . . . . . . . .   7
       3.1.3.  P2MP RSVP-TE Tunnels  . . . . . . . . . . . . . . . .   7
       3.1.4.  Leaf-initiated P-tunnels  . . . . . . . . . . . . . .   8
       3.1.5.  (C-S, C-G) Counter Information  . . . . . . . . . . .   8
       3.1.6.  BFD Discriminator Attribute . . . . . . . . . . . . .   8
       3.1.7.  Per PE-CE Link BFD Discriminator  . . . . . . . . . .  12
   4.  Standby C-multicast Route . . . . . . . . . . . . . . . . . .  12
     4.1.  Downstream PE Behavior  . . . . . . . . . . . . . . . . .  13
     4.2.  Upstream PE Behavior  . . . . . . . . . . . . . . . . . .  14
     4.3.  Reachability Determination  . . . . . . . . . . . . . . .  15
     4.4.  Inter-AS  . . . . . . . . . . . . . . . . . . . . . . . .  15
       4.4.1.  Inter-AS Procedures for downstream PEs, ASBR Fast
               Failover  . . . . . . . . . . . . . . . . . . . . . .  16
       4.4.2.  Inter-AS Procedures for ASBRs . . . . . . . . . . . .  16
   5.  Hot Root Standby  . . . . . . . . . . . . . . . . . . . . . .  16
   6.  Duplicate Packets . . . . . . . . . . . . . . . . . . . . . .  17
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     7.1.  Standby PE Community  . . . . . . . . . . . . . . . . . .  18
     7.2.  BFD Discriminator . . . . . . . . . . . . . . . . . . . .  18
     7.3.  BFD Discriminator Optional Sub-TLV Type . . . . . . . . .  19
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  20
   10. Contributor Addresses . . . . . . . . . . . . . . . . . . . .  20
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     11.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

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1.  Introduction

   It is assumed that the reader is familiar with the workings of
   multicast MPLS/BGP IP VPNs as described in [RFC6513] and [RFC6514].

   In the context of multicast in BGP/MPLS VPNs [RFC6513], 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).  One of the optional methods uses [RFC8562] and the new
   BGP Attribute - BFD Discriminator.  None of these methods provide a
   "fast failover" solution when used alone, but can be used together
   with the mechanism described in Section 4 for a "fast failover"
   solution.

   Section 4 describes an optional BGP extension, a new Standby PE
   Community. that can speed up failover by not requiring any multicast
   VPN routing message exchange at recovery time.

   Section 5 describes a "hot leaf standby" mechanism that can be used
   to improve failover time in MVPN.  The approach combines mechanisms
   defined in Section 3 and Section 4 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.

   The procedures described in this document are optional to enable an
   operator to provide protection for multicast services in BGP/MPLS IP
   VPNs.  An operator would enable these mechanisms using a method
   discussed in Section 3 in combination with the redundancy provided by
   a standby PE connected to the source of the multicast flow, and it is
   assumed that all PEs in the network would support these mechanisms
   for the procedures to work.  In the case that a BGP implementation
   does not recognize or is configured to not support the extensions
   defined in this document, it will continue to provide the multicast
   service, as described in [RFC6513].

2.  Conventions used in this document

2.1.  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

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   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.2.  Terminology

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

   The term 'upstream' (lower case) throughout this document refers to
   links and nodes that are upstream to a PE connected to VPN sites with
   receivers of a multicast flow.

   The term 'Upstream' (capitalized) throughout this document refers to
   a PE or an Autonomous System Border Router (ASBR) at which (S,G) or
   (*,G) data packets enter the VPN backbone or the local AS when
   traveling through the VPN backbone.

2.3.  Acronyms

   PMSI: P-Multicast Service Interface

   I-PMSI: Inclusive PMSI

   S-PMSI: Selective PMSI

   x-PMSI: Either an I-PMSI or an S-PMSI

   P-tunnel: Provider-Tunnels

   UMH: Upstream Multicast Hop

   VPN: Virtual Private Network

   MVPN: Multicast VPN

   RD: Route Distinguisher

   RP: Rendezvous Point

   NLRI: Network Layer Reachability Information

   VRF: VPN Routing and Forwarding Table

   MED: Multi-Exit Discriminator

   P2MP: Point-to-Multipoint

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3.  UMH Selection Based on Tunnel Status

   Section 5.1 of [RFC6513] describes procedures used by a multicast VPN
   downstream PE to determine the Upstream Multicast Hop (UMH) for a
   given (C-S, C-G).

   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.

   The procedure described here is an OPTIONAL procedure that is based
   on a downstream PE taking into account the status of P-tunnels rooted
   at each possible Upstream PE, for including or not including each
   given PE in the list of candidate UMHs for a given (C-S, C-G) state.
   If it is not possible to determine whether a P-tunnel's current
   status is Up, the state shall be considered "not known to be Down",
   and it may be treated as if it is Up so that attempts to use the
   tunnel are acceptable.  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.  This will result in the downstream
   PE failing over to use the next Upstream PE in the list of
   candidates.  Some downstream PEs could arrive at a different
   conclusion regarding the tunnel's state because the failure impacts
   only a subset of branches.  Because of that, the procedures of
   Section 9.1.1 of [RFC6513] are applicable when using I-PMSI
   P-tunnels.  That document is a foundation for this document, and its
   processes all apply here.  Section 9.1.1 mandates the use of specific
   procedures for sending intra-AS I-PMSI A-D Routes.

   There are three options specified in Section 5.1 of [RFC6513] for a
   downstream PE to select an Upstream PE.

   o  The first two options select the Upstream PE from a candidate PE
      set either based on an IP address or a hashing algorithm.  When
      used together with the optional procedure of considering the
      P-tunnel status as in this document, a candidate Upstream PE is
      included in the set if it either:

      A.  advertises an x-PMSI bound to a tunnel, where the specified
          tunnel's state is not known to be Down, or,

      B.  does not advertise any x-PMSI applicable to the given (C-S,
          C-G) but has associated a VRF Route Import BGP attribute to
          the unicast VPN route for S.  That is necessary to avoid
          incorrectly invalidating a UMH PE that would use a policy
          where no I-PMSI is advertised for a given VRF and where only

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          S-PMSI are used.  The S-PMSI can be advertised 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.

      If the resulting candidate set is empty, then the procedure is
      repeated without considering the P-tunnel status.

   o  The third option uses the installed UMH Route (i.e., the "best"
      route towards the C-root) as the Selected UMH Route, and its
      originating PE is the selected Upstream PE.  With the optional
      procedure of considering P-tunnel status as in this document, the
      Selected UMH Route is the best one among those whose originating
      PE's P-tunnel is not "down".  If that does not exist, the
      installed UMH Route is selected regardless of the P-tunnel status.

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
   optional procedures described in this section also handle the case
   the downstream PEs do not all 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, the "status" of a P-tunnel in this section is not a
   characteristic of the tunnel in itself, but is the tunnel status, as
   seen from a particular downstream PE.  Additionally, some of the
   following methods determine the ability of a downstream PE to receive
   traffic on the P-tunnel and not specifically on the status of the
   P-tunnel itself.  That 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.

   An implementation may support any combination of the methods
   described in this section and provide a network operator with control
   to choose which one to use in the particular deployment.

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 x-PMSI Tunnel attribute,
   is reachable through unicast routing tables.  In this case, the

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   downstream PE can immediately update its UMH when the reachability
   condition changes.

   That 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 x-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 A-D Route advertising the tunnel, then checking, in unicast
   routing tables, whether the tunnel root is reachable, will be
   unnecessary duplication and thus 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.  Conversely, if the last-hop link of
   the P-tunnel is Down then this can be taken as an indication that the
   P-tunnel is Down.

   Using this method when a fast restoration mechanism (such as MPLS FRR
   [RFC4090]) is in place for the link requires careful consideration
   and coordination of defect detection intervals for the link and the
   tunnel.  In many cases, it is not practical to use both protection
   methods at the same time because uncorrelated timers might cause
   unnecessary switchovers and destabilize the network.

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 the sub-LSP to this downstream PE is in the Up
   state.  The determination of whether a P2MP RSVP-TE LSP is in the Up
   state requires Path and Resv state for the LSP and is based on
   procedures specified in [RFC4875].  As a result, the downstream PE
   can immediately update its UMH when the reachability condition
   changes.

   When using this method and if the 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 MUST be re-evaluated.  If the P-tunnel
   transitions from Up to Down state, the Upstream PE that is the
   ingress of the P-tunnel MUST NOT be considered a valid UMH.

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3.1.4.  Leaf-initiated P-tunnels

   An Upstream PE SHOULD be removed from the UMH candidate list for a
   given (C-S, C-G) if the P-tunnel (I-PMSI or S-PMSI) for this (S, G)
   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.  As a result, the downstream PE can immediately
   update its UMH when the reachability condition changes.

3.1.5.  (C-S, C-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, a configurable timer MUST be
   set on the downstream PE to wait before updating the UMH, to let the
   P-tunnel restoration mechanism to execute its actions.  An
   implementation SHOULD use three seconds as the default value for this
   timer.

   In cases where this mechanism is used in conjunction with the method
   described in Section 5, 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 Attribute

   P-tunnel status may be derived from the status of a multipoint BFD
   session [RFC8562] whose discriminator is advertised along with an
   x-PMSI A-D Route.

   This document defines the format and ways of using a new BGP
   attribute called the "BFD Discriminator".  It is an optional
   transitive BGP attribute.  An implementation that does not recognize
   or is configured not to support this attribute MUST follow procedures
   defined for optional transitive path attributes in Section 5 of
   [RFC4271].  In Section 7.2, IANA is requested to allocate the
   codepoint value (TBA2).  The format of this attribute is shown in
   Figure 1.

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    BFD Mode   |                  Reserved                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       BFD Discriminator                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                         Optional TLVs                         ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 1: Format of the BFD Discriminator Attribute

   Where:

      BFD Mode field is the one octet long.  This specification defines
      the P2MP BFD Session as value 1 Section 7.2.

      Reserved field is three octets long, and the value MUST be zeroed
      on transmission and ignored on receipt.

      BFD Discriminator field is four octets long.

      Optional TLVs is the optional variable-length field that MAY be
      used in the BFD Discriminator attribute for future extensions.
      TLVs MAY be included in a sequential or nested manner.  To allow
      for TLV nesting, it is advised to define a new TLV as a variable-
      length object.  Figure 2 presents the Optional TLV format TLV that
      consists of:

      *  one octet-long field of TLV's Type value (Section 7.3)

      *  one octet-long field of the length of the Value field in octets

      *  variable length Value field.

      The length of a TLV MUST be multiple of four octets.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Type     |     Length    |           Value             ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 2: Format of the Optional TLV

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   The BFD Discriminator attribute MUST be considered malformed if its
   length is not a non-zero multiple of four.  If the attribute
   considered malformed, the UPDATE message SHALL be handled using the
   approach of Attribute Discard per [RFC7606].

3.1.6.1.  Upstream PE Procedures

   To enable downstream PEs to track the P-tunnel status using a point-
   to-multipoint (P2MP) BFD session the Upstream PE:

   o  MUST initiate the BFD session and set bfd.SessionType =
      MultipointHead as described in [RFC8562];

   o  MUST set the IP destination address of the inner IP header to one
      of the internal loopback addresses from 127/8 range for IPv4 or
      one of IPv4-mapped IPv6 addresses from ::ffff:127.0.0.0/104 range
      for IPv6 when transmitting BFD Control packets;

   o  MUST use its IP address as the source IP address when transmitting
      BFD Control packets;

   o  MUST include the BFD Discriminator attribute in the x-PMSI A-D
      Route with the value set to My Discriminator value;

   o  MUST periodically transmit BFD Control packets over the x-PMSI
      P-tunnel after the P-tunnel is considered established.  Note that
      the methods to declare a P-tunnel has been established are outside
      the scope of this specification.

   If the tracking of the P-tunnel by using a P2MP BFD session is
   enabled after the x-PMSI A-D Route has been already advertised, the
   x-PMSI A-D Route MUST be re-sent with precisely the same attributes
   as before and the BFD Discriminator attribute included.

   If the x-PMSI A-D Route is advertised with 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 precisely the same
      attributes as before, but the BFD Discriminator attribute MUST be
      excluded;

   o  the P2MP BFD session SHOULD be deleted.

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3.1.6.2.  Downstream PE Procedures

   Upon receiving the BFD Discriminator 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 Upstream PE and the IP address of
      the Upstream PE;

   o  MUST create a P2MP BFD session and set bfd.SessionType =
      MultipointTail as described in [RFC8562];

   o  MUST use the source IP address of the BFD Control packet, the
      value of the BFD Discriminator field, and the x-PMSI Tunnel
      Identifier [RFC6514] the BFD Control packet was received 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 [RFC8562], 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 is
   down, i.e., bfd.SessionState == Down.  When the BFD session state is
   Down, then the P-tunnel associated with the BFD session MUST be
   considered down.  If the site that contains C-S is connected to two
   or more PEs, a downstream PE will select one as its Primary Upstream
   PE, while others are considered as Standby Upstream PEs.  In such a
   scenario, when the P-tunnel is considered down, 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 is deemed
   available.

   If the downstream PE's P-tunnel is already established when the
   downstream PE receives the new x-PMSI A-D Route with BFD
   Discriminator attribute, the downstream PE MUST associate the value
   of BFD Discriminator field with the P-tunnel and follow procedures
   listed above in this section if and only if the x-PMSI A-D Route was
   properly processed as per [RFC6514], and the BFD Discriminator
   attribute was validated.

   If the downstream PE's P-tunnel is already established, its state
   being monitored by the P2MP BFD session, and the downstream PE
   receives the new x-PMSI A-D Route without the BFD Discriminator
   attribute, and the x-PMSI A-D Route was processed without any error
   as per the relevant specifications, the downstream PE:

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   o  MUST stop processing 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.

3.1.7.  Per PE-CE Link BFD Discriminator

   The following approach is defined in response to the detection by the
   Upstream PE of a PE-CE link failure.  Even though the provider tunnel
   is still up, it is desired for the downstream PEs to switch to a
   backup Upstream PE.  To achieve that, if the Upstream PE detects that
   its PE-CE link fails, it SHOULD set the bfd.LocalDiag of the P2MP BFD
   session to Concatenated Path Down and/or Reverse Concatenated Path
   Down (per Section 6.8.17 [RFC5880]), unless it switches to a new PE-
   CE link within the time of bfd.DesiredMinTxInterval for the P2MP BFD
   session (in that case, the Upstream PE will start tracking the status
   of the new PE-CE link).  When a downstream PE receives that
   bfd.LocalDiag code, it treats it as if the tunnel itself failed and
   tries to switch to a backup PE.

4.  Standby C-multicast Route

   The procedures described below are limited to the case where the site
   that contains C-S is connected to two or more PEs though, to simplify
   the description, the case of dual-homing is described.  The
   procedures require all the PEs of that MVPN to follow the same UMH
   selection procedure, as specified in [RFC6513], whether the PE
   selected based on its IP address, hashing algorithm described in
   section 5.1.3 of [RFC6513], or Installed UMH Route.  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) to C-S, each with its 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 for 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 for C-S.  When the Primary PE
   later becomes available, then the PE will select the Primary Upstream
   PE again as its Upstream PE.  Such behavior is referred to as
   "revertive" behavior and MUST be supported.  Non-revertive behavior
   refers to the behavior of continuing to select the backup PE as the
   UMH even after the Primary has come up.  This non-revertive behavior
   MAY also be supported by an implementation and would be enabled
   through some configuration.

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   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 for the 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 11.1 of [RFC6514], the PE sends the C-multicast
   route with an 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 downstream PE also originates
   and sends a C-multicast route with an RT that identifies the Standby
   Upstream PE.  The 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 C-multicast route with an RT that
      identifies the Primary Upstream PE, except that the RD is the same
      as if the C-multicast route was built using the Standby Upstream
      PE as the UMH (it will carry the RD associated to the unicast VPN
      route advertised by the Standby Upstream PE for S and a Route
      Target derived from the Standby Upstream PE's UMH route's VRF RT
      Import EC);

   o  MUST carry the "Standby PE" BGP Community (this is a new BGP
      Community.  Section 7.1 requested IANA to allocate value TBA1).

   The Local Preference attribute of the normal and the standby
   C-multicast route needs to be adjusted. so that, if a BGP peer
   receives two C-multicast routes with the same NLRI, one carrying the
   "Standby PE" community and the other one not carrying the "Standby
   PE" community, then preference is given to the one not carrying the
   "Standby PE" community.  Such a situation can happen when, for
   instance, due to transient unicast routing inconsistencies or lack of
   support of the Standby PE community, 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.

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

   If at some later point, the PE determines that C-S is no longer
   reachable through the Primary Upstream PE, the Standby Upstream PE
   becomes the Upstream PE, and the 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).  The LOCAL_PREF attribute MUST be set to zero.

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 (the use of the particular method to detect
      the failure is outside the scope of this document) 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".

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

   Note that, if an Upstream PE uses an S-PMSI only policy, it shall
   advertise an S-PMSI for a (C-S, C-G) as soon as it receives a
   C-multicast route for (C-S, C-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 Route as a result of receiving the C-multicast
   route.  These procedures MUST be followed for both the normal and
   Standby C-multicast routes.

4.3.  Reachability Determination

   The Standby Upstream PE can use the following information to
   determine that C-S can or cannot be reached through the Primary
   Upstream PE:

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

   o  supposing that the Standby Upstream PE is the egress of the tunnel
      rooted at the Primary Upstream PE, the Standby Upstream PE can
      determine the reachability of C-S through the Primary Upstream 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
   described in Section 4.1 through Section 4.3 can be applied.

   When multicast VPNs are used in an inter-AS context with the
   segmented inter-AS approach described in Section 9.2 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 or more Inter-AS I-PMSI A-D
      Routes advertised by the AS of the source

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

   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 A-D 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 A-D 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,
   that supports this specification, MUST try to locate an Inter-AS
   I-PMSI A-D Route whose RD and Source AS respectively match the RD and
   Source AS carried in the C-multicast route.  If the match is found,
   and the C-multicast route carries the Standby PE BGP Community, then
   the ASBR MUST perform as follows:

   o  if the route was received over iBGP and its LOCAL_PREF attribute
      is set to zero, then it MUST 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 and its MED attribute set to
      0xffff, then it MUST be re-advertised in iBGP with a LOCAL_PREF
      attribute set to zero

   Other ASBR procedures are applied without modification.

5.  Hot Root Standby

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

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   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)

   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 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
   of [RFC6513]).

   We draw 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.  That 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
   depends on conditions local to a PE (e.g., the PE-P upstream link
   being up).

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7.  IANA Considerations

7.1.  Standby PE Community

   IANA is requested to allocate the BGP "Standby PE" community value
   (TBA1) from the Border Gateway Protocol (BGP) Well-known Communities
   registry using the First Come First Served registration policy.

7.2.  BFD Discriminator

   This document defines a new BGP optional transitive attribute, called
   "BFD Discriminator".  IANA is requested to allocate a codepoint
   (TBA2) in the "BGP Path Attributes" registry to the BFD Discriminator
   attribute.

   IANA is requested to create a new BFD Mode sub-registry in the Border
   Gateway Protocol (BGP) Parameters registry.  The registration
   policies, per [RFC8126], for this sub-registry are according to
   Table 1.

                  +-----------+-------------------------+
                  | Value     |          Policy         |
                  +-----------+-------------------------+
                  | 0- 175    |       IETF Review       |
                  | 176 - 249 | First Come First Served |
                  | 250 - 254 |     Experimental Use    |
                  | 255       |       IETF Review       |
                  +-----------+-------------------------+

           Table 1: BFD Mode Sub-registry Registration Policies

   IANA is requested to make initial assignments according to Table 2.

             +-----------+------------------+---------------+
             | Value     |   Description    | Reference     |
             +-----------+------------------+---------------+
             | 0         |     Reserved     | This document |
             | 1         | P2MP BFD Session | This document |
             | 2- 175    |    Unassigned    | This document |
             | 176 - 249 |    Unassigned    | This document |
             | 250 - 254 | Experimental Use | This document |
             | 255       |     Reserved     | This document |
             +-----------+------------------+---------------+

                      Table 2: BFD Mode Sub-registry

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7.3.  BFD Discriminator Optional Sub-TLV Type

   IANA is requested to create a new BFD Discriminator Optional sub-TLV
   Type sub-registry in Border Gateway Protocol (BGP).  The registration
   policies, per [RFC8126], for this sub-registry are according to
   Table 3.

                  +-----------+-------------------------+
                  | Value     |          Policy         |
                  +-----------+-------------------------+
                  | 0- 175    |       IETF Review       |
                  | 176 - 249 | First Come First Served |
                  | 250 - 254 |     Experimental Use    |
                  | 255       |       IETF Review       |
                  +-----------+-------------------------+

       Table 3: BFD Discriminator Optional Sub-TLV Type Sub-registry
                           Registration Policies

   IANA is requested to make initial assignments according to Table 4.

             +-----------+------------------+---------------+
             | Value     |   Description    | Reference     |
             +-----------+------------------+---------------+
             | 0         |     Reserved     | This document |
             | 1- 175    |    Unassigned    | This document |
             | 176 - 249 |    Unassigned    | This document |
             | 250 - 254 | Experimental Use | This document |
             | 255       |     Reserved     | This document |
             +-----------+------------------+---------------+

       Table 4: BFD Discriminator Optional Sub-TLV Type Sub-registry

8.  Security Considerations

   This document describes procedures based on [RFC6513] and [RFC6514]
   and hence shares the security considerations respectively represented
   in these specifications.

   This document uses P2MP BFD, as defined in [RFC8562], which, in turn,
   is based on [RFC5880].  Security considerations relevant to each
   protocol are discussed in the respective protocol specifications.  An
   implementation that supports this specification MUST use a mechanism
   to control the maximum number of P2MP BFD sessions that can be active
   at the same time.

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9.  Acknowledgments

   The authors want to thank Greg Reaume, Eric Rosen, Jeffrey Zhang,
   Martin Vigoureux, Adrian Farrel, and Zheng (Sandy) Zhang for their
   reviews, useful comments, and helpful suggestions.

10.  Contributor Addresses

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

      Rahul Aggarwal
      Arktan

      Email: raggarwa_1@yahoo.com

      Nehal Bhau
      Cisco

      Email: NBhau@cisco.com

      Clayton Hassen
      Bell Canada
      2955 Virtual Way
      Vancouver
      CANADA

      Email: Clayton.Hassen@bell.ca

      Wim Henderickx
      Nokia
      Copernicuslaan 50
      Antwerp  2018
      Belgium

      Email: wim.henderickx@nokia.com

      Pradeep Jain
      Nokia
      701 E Middlefield Rd
      Mountain View, CA  94043

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      USA

      Email: pradeep.jain@nokia.com

      Jayant Kotalwar
      Nokia
      701 E Middlefield Rd
      Mountain View, CA  94043
      USA

      Email: Jayant.Kotalwar@nokia.com

      Praveen Muley
      Nokia
      701 East Middlefield Rd
      Mountain View, CA  94043
      U.S.A.

      Email: praveen.muley@nokia.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

      Kanwar Singh
      Nokia
      701 E Middlefield Rd

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      Mountain View, CA  94043
      USA

      Email: kanwar.singh@nokia.com

11.  References

11.1.  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>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [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>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <https://www.rfc-editor.org/info/rfc5880>.

   [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>.

   [RFC7606]  Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
              Patel, "Revised Error Handling for BGP UPDATE Messages",
              RFC 7606, DOI 10.17487/RFC7606, August 2015,
              <https://www.rfc-editor.org/info/rfc7606>.

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   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

   [RFC8562]  Katz, D., Ward, D., Pallagatti, S., Ed., and G. Mirsky,
              Ed., "Bidirectional Forwarding Detection (BFD) for
              Multipoint Networks", RFC 8562, DOI 10.17487/RFC8562,
              April 2019, <https://www.rfc-editor.org/info/rfc8562>.

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

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   Greg Mirsky (editor)
   ZTE Corp.

   Email: gregimirsky@gmail.com

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