Network Working Group                                           A. Karan
Internet-Draft                                               C. Filsfils
Intended status: Informational                              D. Farinacci
Expires: December 21, 2013                             IJ. Wijnands, Ed.
                                                     Cisco Systems, Inc.
                                                             B. Decraene
                                                          France Telecom
                                                               U. Joorde
                                                        Deutsche Telekom
                                                           W. Henderickx
                                                           June 19, 2013

                      Multicast only Fast Re-Route


   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 draft describes a mechanism for minimizing packet loss in a
   network when node or link failures occur.  Multicast only Fast Re-
   Route (MoFRR) works by making simple enhancements to multicast
   routing protocols such as PIM and mLDP.

Status of this Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 21, 2013.

Copyright Notice

   Copyright (c) 2013 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
   Provisions Relating to IETF Documents
   ( 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.  Upstream Multicast Hop Selection . . . . . . . . . . . . . . .  4
     3.1.  PIM  . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  mLDP . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Topologies for MoFRR . . . . . . . . . . . . . . . . . . . . .  5
     4.1.  Dual-Plane Topology  . . . . . . . . . . . . . . . . . . .  5
   5.  Detecting Failures . . . . . . . . . . . . . . . . . . . . . .  8
   6.  ECMP-mode MoFRR  . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  Non-ECMP-mode MoFRR  . . . . . . . . . . . . . . . . . . . . .  9
   8.  Keep It Simple Principle . . . . . . . . . . . . . . . . . . . 10
   9.  Capacity Planning for MoFRR  . . . . . . . . . . . . . . . . . 11
   10. Other Applications . . . . . . . . . . . . . . . . . . . . . . 11
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   13. Contributor Addresses  . . . . . . . . . . . . . . . . . . . . 12
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 12
     14.2. Informative References . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

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1.  Introduction

   Multiple techniques have been developed and deployed to improve
   service guarantees, both for multicast video traffic and Video on
   Demand traffic.  Most existing solutions are geared towards finding
   an alternate path around one or more failed network elements (link,
   node, path failures).

   This draft describes a mechanism for minimizing packet loss in a
   network when node or link failures occur.  Multicast only Fast Re-
   Route (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.

   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 later in this document.  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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2.  Terminology

   MoFRR :  Multicast only Fast Re-Route.

   ECMP :  Equal Cost Multi-Path.

   mLDP :  Multi-point 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.

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   tree :  Either a PIM (S,G)/(*,G) tree or a mLDP P2MP or 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 a mLDP P2MP or MP2MP LSP.

   LFA :  Loop Free Alternate, a candidate UMH that can be used for the
      secondary MoFRR path.

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 two divergent paths SHOULD never merge
   upstream, otherwise the maximal redundancy is compromised.  Sometimes
   the topology guarantees maximal redundancy, other times additional
   configuration or techniques are needed to enforce it.  See later in
   this document.

   A merge point router should only accept and forward on one of the
   upstream paths at the time in order to avoid duplicate packet
   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.

   Note, the impact of 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 its likely a tree merge point is not far

3.  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.  See below;

3.1.  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

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   used to pass or fail the RPF check.  If packets are received on an
   interface that was not selected by the RPF procedure, or not the
   primary, the packets are discarded.

3.2.  mLDP

   The UMH selection in mLDP also depends on unicast routing, but the
   difference with PIM is that the acceptance of multicast packets is
   based on MPLS labels and independent on the interface the packet is
   received on.  Using the procedures as defined in [RFC6388] an
   upstream Label Switched 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 LSPs
   (primary) upstream LSR.

4.  Topologies for MoFRR

   MoFRR works best in topologies illustrated in the figure 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.

4.1.  Dual-Plane Topology

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                   P    / \ P
                       /   \
                ^    G1     R1  ^
                P    /       \  P
                    /         \
                   G2----------R2   ^
                   | \         | \  P
               ^   |  \        |  \
               P   |   G3----------R3
                   |    |      |    |
                   |    |      |    | ^
                   G4---|------R4   | P
                 ^   \  |        \  |
                 P    \ |         \ |
                   ^   |           | ^
                   P   |           | P
                       |           |
                      Gi           Ri
                       \ \__    ^  /|
                        \   \   S1/ | ^
                       ^ \  ^\   /  |P2
                       P1 \ S2\_/__ |
                           \   /   \|
                            PE1     PE2
   P = Primary path
   S = Secondary path

         FIG1. 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 PJ indicate the normal 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 anyways.

   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
   received over the primary path is accepted and forwarded to the
   downstream receivers.  The copy of the stream received from the

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   secondary UNH is discarded.

   When a router detects a routing failure on the path to its 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.  PEs not enabled for MoFRR do not see any
       change or degradation.  Inter-operability testing is not required
       as there are no PIM or mLDP protocol change.

   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.

   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 graft into the normal Multicast
       trees (ie. trees signaled by PIM or mLDP without MoFRR extension)
       at the aggregation level and hence do not demand any extra
       capacity either on the distribution links or in the backbone.
       They simply use the capacity that is normally used, without any
       duplication.  This is different from conventional FRR mechanisms
       which often duplicate the capacity requirements (the backup path
       crosses links/nodes which already carry the primary/normal tree
       and hence twice as much capacity is required).

   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

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   per Fig2.  The PE has two ECMP disjoint paths to the source.  Each
   ECMP path uses a disjoint plane of the network.

                            /    \
                        Plane1  Plane2
                           |      |
                           A1    A2
                             \  /

           FIG2. PE is dual-homed to Dual-Plane Backbone

   Another frequent topology is described in Fig 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 Fig 2.  However, there is clearly a
   way to provide MoFRR benefits as each PE can offer a disjoint
   secondary path to the other plane PE (via the disjoint path).

   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
   extension and is described later in the document.

                                /    \
                            Plane1  Plane2
                               |      |
                               A1    A2
                               |      |

           FIG3. PEs are connected in pairs to Dual-Plane Backbone

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

   The first (and simplest) option to detect a path failure is if a

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   directly connected link that is used as MoFRR UMH goes down.  This
   option can be used in combination with the other options as
   documented below. 50msec switchover is possible.

   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. 50msec switchover is possible.

   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, 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.  Realistic availability
   requirements (sub-second to sub-200msec) should be possible.

6.  ECMP-mode MoFRR

   If the IGP installs two ECMP paths to the source and if the Multicast
   tree is enabled for ECMP-Mode MoFRR, the router installs them as
   primary and secondary UMH.  Only packets received from the primary
   UMH path are processed.  Packets received from the secondary UMH are

   The selected primary UMH should be the same as if MoFRR extension was
   not enabled.

   If more than two ECMP paths exist, two are selected as primary and
   secondary UMH.  Information from the IGP link-state topology could be
   leveraged to optimize this selection.

   Note, MoFRR does not restrict the number of UMH paths that are
   joined.  Implementations may use as many paths as are configured.

7.  Non-ECMP-mode MoFRR

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                    /    \
                   /      \
                  |        |
                  |        |
                  |        |

           Fig5. Non-ECMP-Mode MoFRR

   X is configured for MoFRR for a Multicast tree
   R(X) is the primary UMH to S for X
   N is a neighbor of X
   R(N) is the LFA UMH to S for X

   Router X in FIG5 has one primary path R(X) and one secondary LFA path
   R(N) to reach the source.  How it is determined that N is a LFA path
   from X to S follows the procedures as documented in [RFC5286].  A
   router X configured for non-ECMP-mode MoFRR for a Multicast tree
   joins a primary path to its primary UMH R(X) and a secondary path to
   LFA UMH N. Router X MUST stop joining the seconday path if the
   following as described below occurs;

   Consider the example in FIG3, if 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 would leave at the same time, it could be possible for the
   Multicast tree on PE1 and PE2 to never get deleted as each PE refresh
   each other via the secondary path joins (remember that a secondary
   path join is not distinguishable from a primary join).  In order to
   prevent control-plane loops a router MUST never setup a secondary
   path to a LFA UMH if this UMH is the only member in the OIF list.

8.  Keep It Simple Principle

   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.  PEs not enabled for MoFRR
   do not see any change or degradation.

   Multicast streams with Tight SLA requirements are often characterized
   by a continuous high packet rate (SD video has a continuous

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   interpacket gap of ~ 3msec).  MoFRR simply leverages the stream
   characteristic to detect any failures along the primary branch and
   switch-over on the secondary branch in a few 10s of msec.

9.  Capacity Planning for MoFRR

   As for LFA FRR (draft-ietf-rtgwg-lfa-applicability-00), MoFRR
   applicability is topology dependent.

   In this document, we have described two very frequent designs (Fig 2
   and Fig 3) which provide maximum MoFRR benefits.

   Designers with topologies different than Fig2 and 3 can still benefit
   from MoFRR benefits 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 for hundreds of PEs and
   hundreds of sources.

   This allows to assess 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 tool to optimize the topology (add links, change igp

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
   3-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.

   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.

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   Alternate methods to detect failures such as MPLS-OAM or BFD may be

   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.  Acknowledgments

   The authors would like to thank John Zwiebel, Greg Shepherd and Dave
   Oran for their review of the draft.

13.  Contributor Addresses

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

   Nicolai Leymann
   Deutsche Telekom
   Winterfeldtstrasse 21
   Berlin  10781

   Jeff Tantsura
   300 Holger Way
   San Jose CA 95134

14.  References

14.1.  Normative References

   [RFC5036]  Andersson, L., Minei, I., and B. Thomas, "LDP
              Specification", RFC 5036, October 2007.

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

   [RFC5286]  Atlas, A. and A. Zinin, "Basic Specification for IP Fast

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              Reroute: Loop-Free Alternates", RFC 5286, September 2008.

14.2.  Informative References

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601, August 2006.

   [RFC6388]  Wijnands, IJ., Minei, I., Kompella, K., and B. Thomas,
              "Label Distribution Protocol Extensions for Point-to-
              Multipoint and Multipoint-to-Multipoint Label Switched
              Paths", RFC 6388, November 2011.

Authors' Addresses

   Apoorva Karan
   Cisco Systems, Inc.
   3750 Cisco Way
   San Jose  CA, 95134


   Clarence Filsfils
   Cisco Systems, Inc.
   De kleetlaan 6a
   Diegem  BRABANT 1831


   Dino Farinacci
   Cisco Systems, Inc.
   425 East Tasman Drive
   San Jose  CA, 95134


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   IJsbrand Wijnands (editor)
   Cisco Systems, Inc.
   De Kleetlaan 6a
   Diegem  1831


   Bruno Decraene
   France Telecom
   38-40 rue du General Leclerc
   Issy Moulineaux  cedex 9, 92794


   Uwe Joorde
   Deutsche Telekom
   Hammer Str. 216-226
   Muenster  D-48153


   Wim Henderickx
   Copernicuslaan 50
   Antwerp  2018


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