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Multicast-only Fast Reroute Based on Topology Independent Loop-free Alternate Fast Reroute
draft-liu-pim-mofrr-tilfa-05

Document Type Active Internet-Draft (individual)
Authors Yisong Liu , Mike McBride , Zheng Zhang , Jingrong Xie , Changwang Lin
Last updated 2022-03-04
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draft-liu-pim-mofrr-tilfa-05
PIM Working Group                                          Yisong Liu
Internet Draft                                            China Mobile
Intended status: Informational                               M. McBride
Expires: September 4, 2022                                    Futurewei
                                                               Z. Zhang
                                                                    ZTE
                                                                 J. Xie
                                                                 Huawei
                                                                 C. Lin
                                                   New H3C Technologies
                                                            Mar 4, 2022

    Multicast-only Fast Reroute Based on Topology Independent Loop-free
                          Alternate Fast Reroute
                       draft-liu-pim-mofrr-tilfa-05

Status of this Memo

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   This Internet-Draft will expire on September 4, 2022.

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   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
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Abstract

   Multicast-only Fast Reroute (MoFRR) has been defined in [RFC7431],
   but the selection of the secondary multicast next hop only according
   to the loop-free alternate fast reroute, which has restrictions in
   multicast deployments. This document describes a mechanism for
   Multicast-only Fast Reroute by using Topology Independent Loop-free
   Alternate fast reroute, which is independent of network topology and
   can achieve covering more network environments.

Table of Contents

   1. Introduction...................................................2
      1.1. Requirements Language.....................................3
      1.2. Terminology...............................................3
   2. Problem Statement..............................................3
   3. Solution.......................................................4
   4. IANA Considerations............................................8
   5. Security Considerations........................................8
   6. References.....................................................8
      6.1. Normative References......................................8
      6.2. Informative References....................................9
   7. Acknowledgments................................................9
   Authors' Addresses...............................................10

1. Introduction

   As the deployment of video services, operators are paying more and
   more attention to solutions that minimize the service disruption due
   to faults in the IP network carrying the packets for these services.
   Multicast-only Fast Reroute (MoFRR) has been defined in [RFC7431],
   which can minimize multicast packet loss in a network when node or
   link failures occur by making simple enhancements to multicast
   routing protocols such as Protocol Independent Multicast (PIM). But
   the selection of the secondary multicast next hop only according to
   the loop-free alternate fast reroute in [RFC7431], and there are
   limitations in multicast deployments for this mechanism. This

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   document describes a new mechanism for Multicast-only Fast Reroute
   using Topology Independent Loop-free Alternate (TILFA) fast reroute,
   which is independent of network topology and can achieve covering
   more network environments.

1.1. 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 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2. Terminology

   This document use the terms defined in [RFC7431], and also use the
   concepts defined in [RFC7490]. The specific content of each term is
   not described in this document.

2. Problem Statement

   In [RFC7431] section 3, the secondary Upstream Multicast Hop (UMH)
   of PIM for MoFRR is a loop-free alternate (LFA). However, the
   traditional LFA mechanism needs to satisfy at least one neighbor
   whose next hop to the destination node is an acyclic next hop,
   existing limitations in network deployments, and can only cover part
   of the network topology environments. In some network topology, the
   corresponding secondary UMH cannot be calculated, so PIM cannot
   establish a standby multicast tree and cannot implement MoFRR
   protection. Therefore, the current MoFRR of PIM is only available in
   the network topology applicable to LFA.

   The remote loop-free alternate (RLFA) defined in [RFC7490] is
   extended from the LFA and can cover more network deployment
   scenarios through the tunnel as an alternate path. The RLFA
   mechanism needs to satisfy at least one node assumed to be N in the
   network that the fault node is neither on the path from the source
   node to the N node, nor on the path from the N node to the
   destination node. RLFA only has enhancement compared to LFA but
   still has limitations in network deployments.

   [I-D.ietf-rtgwg-segment-routing-ti-lfa] defined a unicast FRR
   solution based on the TILFA mechanism. The TILFA mechanism can
   express the backup path with an explicit path, and has no constraint
   on the topology, providing a more reliable FRR mechanism. The
   unicast traffic can be forwarded according to the explicit path list
   as an alternate path to implement unicast traffic protection, and
   can achieve full coverage of various networking environments.

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   The alternate path provided by the TILFA mechanism is actually a
   Segment List, including one or more Adjacency SIDs of one or more
   links between the P space and the Q space, and the NodeSID of P
   space node. PIM can look up the corresponding node IP address in the
   unicast route according to the NodeSID, and the IP addresses of the
   two endpoints of the corresponding link in the unicast route
   according to the Adjacency SIDs, but the multicast protocol packets
   cannot be directly sent along the path of the Segment List.

   PIM join message need to be sent hop-by-hop to establish a standby
   multicast tree. However, not all of the nodes and links on the
   unicast alternate path are included in the Segment List. If the PIM
   protocol packets are transmitted only in unicast mode, then
   equivalently the PIM packets are transmitted through the unicast
   tunnel like unicast traffic, and cannot pass through the
   intermediate nodes of the tunnel. The intermediate nodes of the
   alternate path cannot forward multicast traffic because there are no
   PIM state entries on the nodes. PIM needs to create entries on the
   device hop-by-hop and generate an incoming interface and an outgoing
   interface list. So it can form an end-to-end complete multicast tree
   for forwarding multicast traffic. Therefore, it is not possible to
   send PIM packets like unicast traffic according to the Segment List
   path and can only establish a standby multicast tree.

3. Solution

   A secondary Upstream Multicast Hop (UMH) is a candidate next-hop
   that can be used to reach the root of the tree.  In This document
   the secondary UMH is based on unicast routing to find the Segment
   List calculated by TILFA.

   It is available in principle that the path information of the
   Segment List is added to the PIM packets to guide the hop-by-hop RPF
   selection. The IP address of the node corresponding to the NodeSID
   can be used as the segmented root node, and the IP addresses of the
   interfaces at both endpoints of the link corresponding to the
   Adjacency SID can be used directly as the local upstream interface
   and upstream neighbor.

   For the PIM protocol, the PIM RPF Vector attribute was defined in
   [RFC5496], which can carry the node IP address corresponding to the
   NodeSID. The explicit RPF Vector was defined in [RFC7891], which can
   carry the peer IP address corresponding to the Adjacency SID.

   This document can use the above two RPF Vector standards and does
   not need to extend the PIM protocol, to establish the standby
   multicast tree according to the Segment List calculated by TILFA,

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   and can achieve full coverage of various networking environments for
   MoFRR protection of multicast services.

   Assume that the Segment List calculated by TILFA is (NodeSID(A),
   AdjSID(A-B)). Node A belongs to the P Space, and node B belongs to
   the Q space. The IP address corresponding to NodeSID(A) can be
   looked up in the local link state database of the IGP protocol, and
   can be assumed to be IP-a. The IP addresses of the two endpoints of
   the link corresponding to AdjSID(A-B) can also be looked up in the
   local link state database of the IGP protocol, and can be assumed to
   be IP-La and IP-Lb.

   In the procedure of PIM, IP-a can be looked as the normal RPF vector
   attribute and added to the PIM join packet. IP-La can be looked as
   the local address of the incoming interface, and IP-Lb can be
   looked as the address of the upstream neighbor. So IP-Lb can be
   added to the PIM join packet as the explicit RPF Vector attribute.

   The PIM protocol firstly can select the RPF incoming interface and
   upstream towards IP-a, and can join hop-by-hop to establish the PIM
   standby multicast tree until the node A. On the node A, IP-Lb can be
   looked as the PIM upstream neighbor. The node A can find the
   incoming interface in the unicast routing table according to the IP-
   Lb  and IP-Lb is used as the RPF upstream address of the PIM join
   packet to the node B.

   After the PIM join packet is received on the node B, the PIM
   protocol can find no more RPF Vector attributes and select the RPF
   incoming interface and upstream towards the multicast source
   directly, and then can continue to join hop-by-hop to establish the
   PIM standby multicast tree until the router directly connected the
   source.

4. Illustration

   This section provides an illustration of MoFRR based on TI-LFA. The
   example topology is shown in Figure 1. The metric of R3-R4 link is
   100, and the metrics of other links are 10. All the link metrics are
   bidirectional.

           <-----Priamry Path--- (S,G) Join

   [S]---(R1)---(R2)******(R6)-------[R]
                  |        |
           <---   |        |   |
              |   |        |   |
              |   |       (R5) |
              |   |        |   |

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              |   |        |   |
              |   |        |   |
              | (R3)------(R4) |
              |                |
              --Secondary Path--

   Figure 1:  Sample Topology

   The IP addresses and MPLS SIDs, which may be involved in the MoFRR
   calculation, are configured as following:

   Node    IP Address   Node SID
   R4      4.4.4.4/32   16004

   Link    IP Address   Adjacency SID
   R3->R4  14.0.0.1/24  24001
   R4->R3  14.0.0.2/24  24002

   The primary path of the PIM join packet is R6->R2->R1, and the
   secondary path is R6->R5->R4->R3->R2->R1. However, the traditional
   LFA does not work properly for the secondary path, because the
   shortest path to R2 from R5 (or from R4) still goes through R6-R2
   link. In this case, R6 needs to calculate the secondary UMH using
   the proposed MoFRR method based on TI-LFA.

   According to the TI-LFA algorithm, P-Space and Q-Space are shown in
   Figure 2. The TI-LFA repair path consists of the Node SID of R4 and
   the Adjacency SID of R4->R3. The repair segment list is (16004,
   24002).

              ........
              .      .
   [S]---(R1)---(R2)******(R6)---[R]
              .   |  .     |
              .   |  .  +++|++++
              .   |  .  +  |   +
              .   |  .  + (R5) +
              .   |  .  +  |   +
              .   |  .  +  |   +
              .   |  .  +  |   +
              . (R3)------(R4) +
              .      .  +      +
              ........  ++++++++
              Q-Space    P-Space

   Figure 2:  P-Space and Q-Space

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   In the procedure of PIM, the IP addresses associated with the repair
   segment list are looked up in the IGP link state database. The Node
   SID 16004 corresponds to 4.4.4.4, which will be carried in the RPF
   Vector Attribute. The Adjacency SID 24002 corresponds to local
   address 14.0.0.2 and remote peer address 14.0.0.1, and 14.0.0.1 will
   be carried in the Explicit RPF Vector Attribute. Therefore, R6
   installs the secondary UMH with these RPF Vectors.

   The forwarding of PIM join packet along the secondary path is shown
   in Figure 3.

      +--------+
      |Type = 0|
      |4.4.4.4 |
      +--------+     +--------+
      |Type = 4|     |Type = 4|
      |14.0.0.1|     |14.0.0.1|
      +--------+     +--------+    No RPF Vector

   R6----->R5---->R4------------>R3----->R2---->R1

   Figure 3:  Forwarding PIM Join Packet along Secondary Path

   R6 inserts two RPF Vector Attributes in the PIM join packet, which
   are 4.4.4.4 of Type 0 (RPF Vector Attribute) and 14.0.0.1 of Type 4
   (Explicit RPF Vector Attribute). Then R6 forwards the packet along
   the secondary path.

   When R5 receives the packet, R5 performs a unicast route lookup of
   the first RPF Vector 4.4.4.4 and sends the packet to R4.

   R4 is the owner of 4.4.4.4, so it removes the first RPF Vector from
   the packet and forwards according to the following RPF Vector. R4
   sends the packet to R3 according to the next RPF Vector 14.0.0.1,
   since its PIM neighbor R3 corresponds to 14.0.0.1.

   When R3 receives the packet, as the owner of 14.0.0.1, it removes
   the RPF Vector. Then the packet has no RPF Vector, and will be
   forwarded to the source through R3->R2->R1 according to unicast
   routes.

   After the PIM join packet reaches R1, a secondary multicast tree,
   R1->R2->R3->R4->R5->R6, is established hop-by-hop for (S, G) by
   MoFRR based on TI-LFA.

   The above procedures can also work in IPv6 data plane. The TI-LFA
   path computation algorithm in the SRv6 data plane is the same as in
   the SR-MPLS data plane. Instead of MPLS labels, SRv6 SIDs are used

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   to build repair list. Similarly, the RPF Vector Attributes and the
   Explicit RPF Vector Attributes will contain IPv6 addresses instead
   of IPv4 addresses.

5. IANA Considerations

   No IANA actions are required for this document.

6. Security Considerations

   This document does not change the security properties of PIM. For
   general PIM-SM protocol Security Considerations, see [RFC7761].

7. References

7.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5384] Boers, A., Wijnands, I., and E. Rosen, "The Protocol
             Independent Multicast (PIM) Join Attribute Format",
             RFC 5384, November 2008

   [RFC5496] Wijnands, IJ., Boers, A., and E. Rosen, "The Reverse Path
             Forwarding (RPF) Vector TLV", RFC 5496, March 2009

    [RFC7431] Karan, A., Filsfils, C., Wijnands, IJ., Ed., and B.
             Decraene, "Multicast-Only Fast Reroute", RFC 7431, August
             2015

   [RFC7490] Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
             So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
             RFC 7490, April 2015

   [RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas,
             I.,Parekh, R., Zhang, Z., and L. Zheng, "Protocol
             IndependentMulticast - Sparse Mode (PIM-SM): Protocol
             Specification(Revised)", RFC 7761, March 2016

   [RFC7891] Asghar, J., Wijnands, IJ., Ed., Krishnaswamy, S., Karan,
             A., and V. Arya, "Explicit Reverse Path Forwarding (RPF)
             Vector", RFC 7891, June 2016

   [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, May 2017

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   [I-D.ietf-rtgwg-segment-routing-ti-lfa] Litkowski, S., Bashandy, A.,
             Filsfils, C., Francois, P., Decraene, B., and D. Voyer,
             "Topology Independent Fast Reroute using Segment Routing",
             draft-ietf-rtgwg-segment-routing-ti-lfa-08, work-in-
             progress, January 2022

7.2. Informative References

   TBD

8. Acknowledgments

   The authors would like to thank the following for their valuable
   contributions of this document:

   TBD

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Authors' Addresses

   Yisong Liu
   China Mobile
   China
   Email: liuyisong@chinamobile.com

   Mike McBride
   Futurewei Inc.
   USA
   Email: michael.mcbride@futurewei.com

   Zheng(Sandy) Zhang
   ZTE Corporation
   China
   Email: zzhang_ietf@hotmail.com

   Jingrong Xie
   Huawei Technologies
   China
   Email: xiejingrong@huawei.com

   Changwang Lin
   New H3C Technologies
   China
   Email: linchangwang.04414@h3c.com

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