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rfc:rfc7431

Internet Engineering Task Force (IETF) A. Karan Request for Comments: 7431 C. Filsfils Category: Informational IJ. Wijnands, Ed. ISSN: 2070-1721 Cisco Systems, Inc.

                                                           B. Decraene
                                                                Orange
                                                           August 2015
                    Multicast-Only Fast Reroute

Abstract

 As IPTV deployments grow in number and size, service providers are
 looking for solutions that minimize the service disruption due to
 faults in the IP network carrying the packets for these services.
 This document describes a mechanism for minimizing packet loss in a
 network when node or link failures occur.  Multicast-only Fast
 Reroute (MoFRR) works by making simple enhancements to multicast
 routing protocols such as Protocol Independent Multicast (PIM) and
 Multipoint LDP (mLDP).

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7431.

Karan et al. Informational [Page 1] RFC 7431 MoFRR August 2015

Copyright Notice

 Copyright (c) 2015 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
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Introduction ....................................................3
    1.1. Conventions Used in This Document ..........................3
    1.2. Terminology ................................................3
 2. Basic Overview ..................................................4
 3. Determination of the Secondary UMH ..............................5
    3.1. ECMP-Mode MoFRR ............................................5
    3.2. Non-ECMP-Mode MoFRR ........................................5
 4. Upstream Multicast Hop Selection ................................6
    4.1. PIM ........................................................6
    4.2. mLDP .......................................................6
 5. Detecting Failures ..............................................6
 6. MoFRR Applicability to Dual-Plane Topology ......................7
 7. Other Topologies ...............................................10
 8. Capacity Planning for MoFRR ....................................11
 9. PE Nodes .......................................................11
 10. Other Applications ............................................11
 11. Security Considerations .......................................12
 12. References ....................................................12
    12.1. Normative References .....................................12
    12.2. Informative References ...................................12
 Acknowledgments ...................................................13
 Contributors ......................................................13
 Authors' Addresses ................................................14

Karan et al. Informational [Page 2] RFC 7431 MoFRR August 2015

1. Introduction

 Different solutions have been developed and deployed to improve
 service guarantees, both for multicast video traffic and Video on
 Demand traffic.  Most of these solutions are geared towards finding
 an alternate path around one or more failed network elements (link,
 node, or path failures).
 This document describes a mechanism for minimizing packet loss in a
 network when node or link failures occur.  Multicast-only Fast
 Reroute (MoFRR) works by making simple changes to the way selected
 routers use multicast protocols such as PIM and mLDP.  No changes to
 the protocols themselves are required.  With MoFRR, in many cases,
 multicast routing protocols don't necessarily have to depend on or
 have to wait on unicast routing protocols to detect network failures;
 see Section 5.
 On a Merge Point, MoFRR logic determines a primary Upstream Multicast
 Hop (UMH) and a secondary UMH and joins the tree via both
 simultaneously.  Data packets are received over the primary and
 secondary paths.  Only the packets from the primary UMH are accepted
 and forwarded down the tree; the packets from the secondary UMH are
 discarded.  The UMH determination is different for PIM and mLDP and
 explained in Section 4.  When a failure is detected on the path to
 the primary UMH, the repair occurs by changing the secondary UMH into
 the primary and the primary into the secondary.  Since the repair is
 local, it is fast -- greatly improving convergence times in the event
 of node or link failures on the path to the primary UMH.

1.1. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].

1.2. Terminology

 MoFRR: Multicast-only Fast Reroute.
 ECMP: Equal-Cost Multipath.
 mLDP: Multipoint Label Distribution Protocol.
 PIM: Protocol Independent Multicast.
 UMH: Upstream Multicast Hop.  A candidate next-hop that can be used
    to reach the root of the tree.

Karan et al. Informational [Page 3] RFC 7431 MoFRR August 2015

 tree: Either a PIM (S,G)/(*,G) tree or an mLDP Point-to-Multipoint
    (P2MP) or Multipoint-to-Multipoint (MP2MP) LSP.
 OIF: Outgoing interface.  An interface used to forward multicast
    packets down the tree towards the receivers.  Either a PIM
    (S,G)/(*,G) tree or an mLDP P2MP or MP2MP LSP.
 LFA: Loop-Free Alternate as defined in [RFC5286].  In unicast Fast
    Reroute, this is an alternate next-hop that can be used to reach a
    unicast destination without using the protected link or node.
 Merge Point: A router that joins a multicast stream via two divergent
    upstream paths.
 RPF: Reverse Path Forwarding.
 RP: Rendezvous Point.
 LSP: Label Switched Path.
 LSR: Label Switching Router.
 BFD: Bidirectional Forwarding Detection.
 IGP: Interior Gateway Protocol.
 MVPN: Multicast Virtual Private Network.
 POP: Point Of Presence, an access point into the network.

2. Basic Overview

 The basic idea of MoFRR is for a Merge Point router to join a
 multicast tree via two divergent upstream paths in order to get
 maximum redundancy.  The determination of this alternate upstream is
 defined in Section 3.
 In order to maximize robustness against any failure, the two paths
 should be as diverse as possible.  Ideally, they should not merge
 upstream.  Sometimes the topology guarantees maximal redundancy;
 other times additional configuration or techniques are needed to
 enforce it.  See Section 6 for more discussion on the applicability
 of MoFRR depending on the network topology.
 A Merge Point router should only accept and forward on one of the
 upstream paths at a time in order to avoid duplicate packet

Karan et al. Informational [Page 4] RFC 7431 MoFRR August 2015

 forwarding.  The selection of the primary and secondary UMH is done
 by the MoFRR logic and normally based on unicast routing to find
 loop-free candidates.  This is described in Section 4.
 Note, the impact of an additional amount of data on the network is
 mitigated when tree membership is densely populated.  When a part of
 the network has redundant data flowing, join latency for new joining
 members is reduced because it's likely a tree Merge Point is not far
 away.

3. Determination of the Secondary UMH

 The secondary UMH is a Loop-Free Alternate (LFA) as per [RFC5286].

3.1. ECMP-Mode MoFRR

 If the IGP installs two ECMP paths to the source, then as per
 [RFC5286] the LFA is a primary next-hop.  If the multicast tree is
 enabled for ECMP-mode MoFRR, the router installs the paths as primary
 and secondary UMHs.  Before the failure, only packets received from
 the primary UMH path are processed, while packets received from the
 secondary UMH are dropped.
 The selected primary UMH SHOULD be the same as if the MoFRR extension
 were not enabled.
 If more than two ECMP paths exist, one is selected as primary and
 another as secondary UMH.  The selection of the primary and secondary
 is a local decision.  Information from the IGP link-state topology
 could be leveraged to optimize this selection such that the primary
 and secondary paths are maximal divergent and don't lead to the same
 upstream node.  Note that MoFRR does not restrict the number of UMH
 paths that are joined.  Implementations may use as many paths as are
 configured.

3.2. Non-ECMP-Mode MoFRR

 A router X configured for non-ECMP-mode MoFRR for a multicast tree
 joins a primary path to its primary UMH and a secondary path to its
 LFA UMH.  In order to prevent control-plane loops, a router MUST stop
 joining the secondary UMH if this UMH is the only member in the OIF
 list.
 To illustrate the reason for this rule, let's consider the example in
 Figure 3.  If two Provider Edge routers, PE1 and PE2, have received
 an IGMP request for a multicast tree, they will both join the primary
 path on their plane and a secondary path to the neighbor PE.  If
 their receivers leave at the same time, it's possible for the

Karan et al. Informational [Page 5] RFC 7431 MoFRR August 2015

 multicast tree on PE1 and PE2 to never get deleted, as the PEs
 refresh each other via the secondary path joins (remember that a
 secondary path join is not distinguishable from a primary join).

4. Upstream Multicast Hop Selection

 An Upstream Multicast Hop (UMH) is a candidate next-hop that can be
 used to reach the root of the tree.  This is normally based on
 unicast routing to find loop-free candidate(s).  With MoFRR
 procedures, we select a primary and a backup UMH.  The procedures for
 determining the UMH are different for PIM and mLDP.

4.1. PIM

 The UMH selection in PIM is also known as the Reverse Path Forwarding
 (RPF) procedure.  Based on a unicast route lookup on either the
 source address or Rendezvous Point (RP) [RFC4601], an upstream
 interface is selected for sending the PIM Joins/Prunes AND accepting
 the multicast packets.  The interface the packets are received on is
 used to pass or fail the RPF check.  If packets are received on an
 interface that was not selected as the primary by the RPF procedure,
 the packets are discarded.

4.2. mLDP

 The UMH selection in mLDP also depends on unicast routing, but the
 difference from PIM is that the acceptance of multicast packets is
 based on MPLS labels and is independent of the interface on which the
 packet is received.  Using the procedures as defined in [RFC6388], an
 upstream Label Switching Router (LSR) is elected.  The upstream LSR
 that was elected for a Label Switched Path (LSP) gets a unique local
 MPLS label allocated.  Multicast packets are only forwarded if the
 MPLS label matches the MPLS label that was allocated for that LSP's
 (primary) upstream LSR.

5. Detecting Failures

 Once the two paths are established, the next step is detecting a
 failure on the primary path to know when to switch to the backup
 path.  This is a local issue, but this section explores some
 possibilities.
 The first (and simplest) option is to detect the failure of the local
 interface as it's done for unicast Fast Reroute.  Detection can be
 performed using the loss of signal or the loss of probing packets
 (e.g., BFD).  This option can be used in combination with the other
 options as documented below.  Just like for unicast fast reroute,
 50 msec switchover is possible.

Karan et al. Informational [Page 6] RFC 7431 MoFRR August 2015

 A second option consists of comparing the packets received on the
 primary and secondary streams but only forwarding one of them -- the
 first one received, no matter which interface it is received on.
 Zero packet loss is possible for RTP-based streams.
 A third option assumes a minimum known packet rate for a given data
 stream.  If a packet is not received on the primary RPF within this
 time frame, the router assumes primary path failure and switches to
 the secondary RPF interface. 50 msec switchover may be possible for
 high-rate streams (e.g., IPTV where SD video has a continuous inter-
 packet gap of about 3 msec), but in general the delay is dependent on
 the rate of the multicast stream.
 A fourth option leverages the significant improvements of the IGP
 convergence speed.  When the primary path to the source is withdrawn
 by the IGP, the MoFRR-enabled router switches over to the backup
 path, and the UMH is changed to the secondary UMH.  Since the
 secondary path is already in place, and assuming it is disjoint from
 the primary path, convergence times would not include the time
 required to build a new tree and hence are smaller.  Sub-second to
 sub-200 msec switchover should be possible.

6. MoFRR Applicability to Dual-Plane Topology

 MoFRR applicability is topology dependent.  The applicability is the
 same as LFA FRR, which is discussed in [RFC6571].
 The following section will discuss MoFRR applicability to dual-plane
 network topologies.
 MoFRR works best in dual-planes topologies as illustrated in the
 figures below.  MoFRR may be enabled on any router in the network.
 In the figures below, MoFRR is shown enabled on the Provider Edge
 (PE) routers to illustrate one way in which the technology may be
 deployed.

Karan et al. Informational [Page 7] RFC 7431 MoFRR August 2015

                          S
                    P    / \ P
                        /   \
                 ^    G1     R1  ^
                 P    /       \  P
                     /         \
                    G2----------R2   ^
                    | \         | \  P
                ^   |  \        |  \
                P   |   G3----------R3
                    |    |      |    |
                    |    |      |    | ^
                    G4---|------R4   | P
                  ^   \  |        \  |
                  P    \ |         \ |
                        G5----------R5
                    ^   |           | ^
                    P   |           | P
                        |           |
                       Gi           Ri
                        \ \__    ^  /|
                         \   \   S1/ | ^
                        ^ \  ^\   /  |P2
                        P1 \ S2\_/__ |
                            \   /   \|
                             PE1     PE2
     P = Primary path
     S = Secondary path
         Figure 1: Two-Plane Network Design
 The topology has two planes, a primary plane and a secondary plane
 that are fully disjoint from each other all the way into the POPs.
 This two-plane design is common in service provider networks as it
 eliminates single point of failures in their core network.  The links
 marked P indicate the normal (primary) path of how the PIM Joins flow
 from the POPs towards the source of the network.  Multicast streams,
 especially for the densely watched channels, typically flow along
 both the planes in the network anyway.
 The only change MoFRR adds to this is on the links marked S where the
 PE routers join a secondary path to their secondary ECMP UMH.  As a
 result of this, each PE router receives two copies of the same
 stream, one from the primary plane and the other from the secondary
 plane.  As a result of normal UMH behavior, the multicast stream

Karan et al. Informational [Page 8] RFC 7431 MoFRR August 2015

 received over the primary path is accepted and forwarded to the
 downstream receivers.  The copy of the stream received from the
 secondary UMH is discarded.
 When a router detects a routing failure on the path to its primary
 UMH, it will switch to the secondary UMH and accept packets for that
 stream.  If the failure is repaired, the router may switch back.  The
 primary and secondary UMHs have only local context and not end-to-end
 context.
 As one can see, MoFRR achieves the faster convergence by pre-building
 the secondary multicast tree and receiving the traffic on that
 secondary path.  The example discussed above is a simple case where
 there are two ECMP paths from each PE device towards the source, one
 along the primary plane and one along the secondary.  In cases where
 the topology is asymmetric or is a ring, this ECMP nature does not
 hold, and additional rules have to be taken into account to choose
 when and where to join the secondary path.
 MoFRR is appealing in such topologies for the following reasons:
 1.  Ease of deployment and simplicity: the functionality is only
     required on the PE devices, although it may be configured on all
     routers in the topology.  Furthermore, each PE device can be
     enabled separately; there is no need for network-wide
     coordination in order to deploy MoFRR.  Interoperability testing
     is not required as there are no PIM or mLDP protocol changes.
 2.  End-to-end failure detection and recovery: any failure along the
     path from the source to the PE can be detected and repaired with
     the secondary disjoint stream.  (See the second, third, and
     fourth options in Section 5.)
 3.  Capacity efficiency: as illustrated in the previous example, the
     multicast trees corresponding to IPTV channels cover the backbone
     and distribution topology in a very dense manner.  As a
     consequence, the secondary path grafts onto the normal multicast
     trees (i.e., trees signaled by PIM or mLDP without the MoFRR
     extension) at the aggregation level and hence does not demand any
     extra capacity either on the distribution links or in the
     backbone.  The secondary path simply uses the capacity that is
     normally used, without any duplication.  This is different from
     conventional FRR mechanisms that often duplicate the capacity
     requirements when the backup path crosses links/nodes that
     already carry the primary/normal tree, and thus twice as much
     capacity is required.

Karan et al. Informational [Page 9] RFC 7431 MoFRR August 2015

 4.  Loop-free: the secondary path join is sent on an ECMP disjoint
     path.  By definition, the neighbor receiving this request is
     closer to the source and hence will not cause a loop.
 The topology we just analyzed is very frequent and can be modeled as
 per Figure 2.  The PE has two ECMP disjoint paths to the source.
 Each ECMP path uses a disjoint plane of the network.
                          Source
                          /    \
                      Plane1  Plane2
                         |      |
                         A1    A2
                           \  /
                            PE
     Figure 2: PE is Dual-Homed to Dual-Plane Backbone
 Another frequent topology is described in Figure 3.  PEs are grouped
 by pairs.  In each pair, each PE is connected to a different plane.
 Each PE has one single shortest-path to a source (via its connected
 plane).  There is no ECMP like in Figure 2.  However, there is
 clearly a way to provide MoFRR benefits as each PE can offer a
 disjoint secondary path to the PE in the other plane (via the
 disjoint path).
 The MoFRR secondary neighbor selection process needs to be extended
 in this case as one cannot simply rely on using an ECMP path as
 secondary neighbor.  This extension is referred to as non-ECMP-mode
 MoFRR and is described in Section 3.2.
                          Source
                          /    \
                      Plane1  Plane2
                         |      |
                         A1    A2
                         |      |
                        PE1----PE2
    Figure 3: PEs Are Connected in Pairs to Dual-Plane Backbone

7. Other Topologies

 As mentioned in Section 6, MoFRR works best in dual-plane topologies.
 If MoFRR is applied to non-dual-plane networks, it's possible that
 the secondary path is affected by the same failure that affected the

Karan et al. Informational [Page 10] RFC 7431 MoFRR August 2015

 primary path.  In that case, there is no guarantee that the backup
 path will provide an uninterrupted traffic flow of packets without
 loss or duplication.

8. Capacity Planning for MoFRR

 The previous section has described two very frequent designs (Figures
 2 and 3) which provide maximum MoFRR benefits.
 Designers with topologies different than Figures 2 and 3 can still
 benefit from MoFRR, thanks to the use of capacity planning tools.
 Such tools are able to simulate the ability of each PE to build two
 disjoint branches of the same tree.  This simulation could be for
 hundreds of PEs and hundreds of sources.
 This allows an assessment of the MoFRR protection coverage of a given
 network, for a set of sources.
 If the protection coverage is deemed insufficient, the designer can
 use such a tool to optimize the topology (add links, change IGP
 metrics).

9. PE Nodes

 Many Service Providers devise their topology such that PEs have
 disjoint paths to the multicast sources.  MoFRR leverages the
 existence of these disjoint paths without any PIM or mLDP protocol
 modification.  Interoperability testing is thus not required.  In
 such topologies, MoFRR only needs to be deployed on the PE devices.
 Each PE device can be enabled one by one.

10. Other Applications

 While all the examples in this document show the MoFRR applicability
 on PE devices, it is clear that MoFRR could be enabled on aggregation
 or core routers.
 MoFRR can be popular in data center network configurations.  With the
 advent of lower-cost Ethernet and increasing port density in routers,
 there is more meshed connectivity than ever before.  When using a
 three-level access, distribution, and core layers in a data center,
 there is a lot of inexpensive bandwidth connecting the layers.  This
 will lend itself to more opportunities for ECMP paths at multiple
 layers.  This allows for multiple layers of redundancy protecting
 link and node failure at each layer with minimal redundancy cost.

Karan et al. Informational [Page 11] RFC 7431 MoFRR August 2015

 Redundancy costs are reduced because only one packet is forwarded at
 every link along the primary and secondary data paths so there is no
 duplication of data on any link thereby providing make-before-break
 protection at a very small cost.
 A MoFRR router only accepts packets from the primary path and
 discards packets from the secondary path.  For that reason,
 management applications (like ping and mtrace) will not work when
 verifying the secondary path.
 The MoFRR principle may be applied to MVPNs.

11. Security Considerations

 There are no security considerations for this design other than what
 is already in the main PIM specification [RFC4601] and mLDP
 specification [RFC6388].

12. References

12.1. 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,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC5286]  Atlas, A., Ed., and A. Zinin, Ed., "Basic Specification
            for IP Fast Reroute: Loop-Free Alternates", RFC 5286,
            DOI 10.17487/RFC5286, September 2008,
            <http://www.rfc-editor.org/info/rfc5286>.

12.2. Informative References

 [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
            "Protocol Independent Multicast - Sparse Mode (PIM-SM):
            Protocol Specification (Revised)", RFC 4601,
            DOI 10.17487/RFC4601, August 2006,
            <http://www.rfc-editor.org/info/rfc4601>.
 [RFC6388]  Wijnands, IJ., Ed., Minei, I., Ed., Kompella, K., and B.
            Thomas, "Label Distribution Protocol Extensions for Point-
            to-Multipoint and Multipoint-to-Multipoint Label Switched
            Paths", RFC 6388, DOI 10.17487/RFC6388, November 2011,
            <http://www.rfc-editor.org/info/rfc6388>.

Karan et al. Informational [Page 12] RFC 7431 MoFRR August 2015

 [RFC6571]  Filsfils, C., Ed., Francois, P., Ed., Shand, M., Decraene,
            B., Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
            Alternate (LFA) Applicability in Service Provider (SP)
            Networks", RFC 6571, DOI 10.17487/RFC6571, June 2012,
            <http://www.rfc-editor.org/info/rfc6571>.

Acknowledgments

 Thanks to Dave Oran and Alvaro Retana for their review and comments
 on this document.
 The authors would like to especially acknowledge Dino Farinacci, John
 Zwiebel, and Greg Shepherd for the genesis of the MoFRR concept.

Contributors

 Below is a list of the contributors in alphabetical order:
 Dino Farinacci
 Email: farinacci@gmail.com
 Wim Henderickx
 Alcatel-Lucent
 Copernicuslaan 50
 Antwerp  2018
 Belgium
 Email: wim.henderickx@alcatel-lucent.com
 Uwe Joorde
 Deutsche Telekom
 Dahlweg 100
 D-48153 Muenster
 Germany
 Email: Uwe.Joorde@telekom.de
 Nicolai Leymann
 Deutsche Telekom
 Winterfeldtstrasse 21
 Berlin  10781
 Germany
 Email: N.Leymann@telekom.de
 Jeff Tantsura
 Ericsson
 300 Holger Way
 San Jose, CA  95134
 United States
 Email: jeff.tantsura@ericsson.com

Karan et al. Informational [Page 13] RFC 7431 MoFRR August 2015

Authors' Addresses

 Apoorva Karan
 Cisco Systems, Inc.
 3750 Cisco Way
 San Jose, CA  95134
 United States
 Email: apoorva@cisco.com
 Clarence Filsfils
 Cisco Systems, Inc.
 De kleetlaan 6a
 Diegem  BRABANT 1831
 Belgium
 Email: cfilsfil@cisco.com
 IJsbrand Wijnands (editor)
 Cisco Systems, Inc.
 De Kleetlaan 6a
 Diegem  1831
 Belgium
 Email: ice@cisco.com
 Bruno Decraene
 Orange
 38-40 rue du General Leclerc
 Issy Moulineaux  Cedex 9, 92794
 France
 Email: bruno.decraene@orange.com

Karan et al. Informational [Page 14]

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