Network Working Group                                           A. Karan
Internet-Draft                                               C. Filsfils
Intended status: Informational                              D. Farinacci
Expires: September 14, 2011                          Cisco Systems, Inc.
                                                             B. Decraene
                                                          France Telecom
                                                              N. Leymann
                                                               U. Joorde
                                                        Deutsche Telekom
                                                              T. Telkamp
                                              Cariden Technologies, Inc.
                                                          March 13, 2011

                      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.

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
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   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 September 14, 2011.

Copyright Notice

   Copyright (c) 2011 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.  Topologies for MoFRR . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Dual-Plane Topology  . . . . . . . . . . . . . . . . . . .  4
   4.  Detecting Failures . . . . . . . . . . . . . . . . . . . . . .  7
   5.  ECMP-mode MoFRR  . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Non-ECMP-mode MoFRR  . . . . . . . . . . . . . . . . . . . . .  8
     6.1.  Variation  . . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  Keep It Simple Principle . . . . . . . . . . . . . . . . . . . 10
   8.  Capacity Planning for MoFRR  . . . . . . . . . . . . . . . . . 10
   9.  Other Applications . . . . . . . . . . . . . . . . . . . . . . 11
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 12
     12.2. Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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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.  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.

   MoFRR involves transmitting a multicast join message from a receiver
   towards a source on a primary path and transmitting a secondary
   multicast join message from the receiver towards the source on a
   backup path.  Data packets are received from the primary and
   secondary paths.  The redundant packets are discarded at topology
   merge points using RPF checks.  When a failure is detected on the
   primary path, the repair occurs by changing the interface on which
   packets are accepted to the secondary interface.  Since the repair is
   local, it is fast - greatly improving convergence times in the event
   of node or link failures on the primary path.

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.

   Primary Join :  Multicast join message sent from receiver towards the
      source on the primary path.

   Secondary Join :  Multicast join message sent from receiver towards
      the source on the secondary path.

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2.  Basic Overview

   MoFRR uses standard PIM JOIN/PRUNE messages to set up a primary and a
   secondary multicast forwarding path by establishing a primary and a
   secondary RPF interface on each router that receives a PIM join.  The
   outgoing interface list remains the same.

   Data packets are received from the primary and backup paths.
   Redundant packets received on the secondary RPF interface are
   discarded because of an RPF failure.  When the router detects a
   forwarding failure in the primary path, it changes RPF to the
   secondary path and immediately has packets available to forward out
   each outgoing interface.

   The primary and secondary MoFRR forwarding paths should not use the
   same nodes or links.  This may be configured or determined by
   computations described in this document.

   Note, the impact of additional amount of data on the network is
   mitigated when group membership is densely populated.  When a part of
   the network has redundant data flowing, join latency for new joining
   members is reduced because joins don't have to propagate far to get
   to on-tree routers.

3.  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.

3.1.  Dual-Plane Topology

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                   PJ   / \ PJ
                       /   \
                ^    G1     R1  ^
               PJ    /       \  PJ
                    /         \
                   G2----------R2   ^
                   | \         | \  PJ
               ^   |  \        |  \
               PJ  |   G3----------R3
                   |    |      |    |
                   |    |      |    | ^
                   G4---|------R4   | PJ
                 ^   \  |        \  |
                 PJ   \ |         \ |
                   ^   |           | ^
                   PJ  |           | PJ
                       |           |
                      Gi           Ri
                       \ \__    ^  /|
                        \   \  SJ1/ | ^
                       ^ \  ^\   /  |PJ2
                      PJ1 \SJ2\_/__ |
                           \   /   \|
                            PE1     PE2
   PJ = Primary Join
   SJ = Secondary Join

         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 SJ where
   the PE routers send a secondary PIM joins to their ECMP neighbor
   towards the source.  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 multicast RPF checks
   the multicast stream received over the primary path is accepted and
   forwarded to the downstream links.  The copy of the stream received

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   on the secondary path is discarded.

   When a router detects a routing failure on its primary RPF interface,
   it will switch to the secondary RPF interface and accept packets on
   that stream.  If the failure is repaired the router may switch back.
   The primary and secondary path 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 send the secondary PIM joins.

   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 is no PIM 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
       PIM trees corresponding to IPTV channels cover the backbone and
       distribution topology in a very dense manner.  As a consequence,
       the secondary joins graft into the normal PIM trees (ie. trees
       signaled by PIM 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 PIM join is sent on an ECMP disjoint
       path.  By definition, the neighbor receiving this secondary PIM
       join is closer to the source and hence will not send a PIM join

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   The topology we just analyzed is very frequent and can be modelled as
   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

4.  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

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   A first 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 second 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 third 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 RPF interface is changed to the secondary RPF interface.
   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.

   A fourth option consists in leveraging connected link failure.  This
   option makes sense when MoFRR is deployed across the network (not
   only at PE).

5.  ECMP-mode MoFRR

   If the IGP installs two ECMP paths to the source and if the (S, G)
   PIM state is enabled for ECMP-Mode MoFRR, the router installs them as
   primary RPF and secondary RPF.  It sends a PIM join to both RPF
   entries.  Only packets receive from the primary RPF entry are
   processed.  Packets received from the secondary RPF are dropped
   (equivalent to an RPF failure).

   The selected primary RPF interface 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 RPF interfaces.  Information from the IGP link-state
   topology could be leveraged to optimize this selection.

   Note, MoFRR does not restrict the number of paths on which joins are
   sent.  Implementations may use as many paths as are configured.

6.  Non-ECMP-mode MoFRR

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                    /    \
                   /      \
                  |        |
                  |        |
                  |        |

           Fig5. Non-ECMP-Mode MoFRR

   X is configured for MoFRR for state (S, G)
   R(X) is Xs RPF to S
   N is a neighbor of X
   R(N) is Ns RPF to S
   xs represents the IGP metric from X to S
   ns represents the IGP metric from N to S
   xn represents the IGP metric from X to N

   A router X configured for non-ECMP-mode MoFRR for (S, G) sends a
   primary PIM join to its primary RPF R(X) and a secondary PIM Join to
   a neighbor N if the following three conditions are met.

   C1: xs < xn + ns
   C2: ns < nx + xs
   C3: X cannot send a secondary join to N if N is the only member of the OIF list

   The first condition ensures that N is not on the primary branch from
   X to S.

   The second condition ensures that X is not on the primary branch from
   N to S.

   These two conditions ensure that at least locally the two paths are

   The third condition is required to break control-plane loops which
   could occur in some scenarios.

   For example in FIG3, if PE1 and PE2 have received an igmp request for
   (S, G), they will both send a primary PIM join on their plane and a
   secondary PIM join to the neighbor PE.  If their receivers would
   leave at the same time, it could be possible for the (S, G) states on
   PE1 and PE2 to never get deleted as each PE refresh each other via
   the secondary PIM joins (remember that a secondary PIM join is not
   distinguishable from a primary PIM join.  MoFRR does not require any
   PIM protocol modification).

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   A control-plane loop occurs when two nodes keep a state forever due
   to the secondary joins they send to each other.  This forever
   condition is not acceptable as no real receiver is connected to the
   nodes (directly via IGMP or indirectly via PIM).  Rule 3 prevents
   this case as it prevents the mutual refresh of secondary joins and it
   applies it in the specific case where there is no real receiver

6.1.  Variation

   Rule R3 can be removed if Rule 2 is restricted as follows:

   R2p: ns < xs

   This ensures that X only sends a secondary join to a neighbor N who
   is strictly closer to the source than X is.  By reciprocity, N will
   thus never be able to send an sedondary join to the same source via
   X. The strictly smaller than is key here.

   Note that this non-ECMP-mode MoFRR variation does not support the
   square topology and hence is less preferred.

7.  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 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
   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.

8.  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.

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   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

9.  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.

   The MoFRR behavior described for PIM are immediately applicable to
   MLDP.  Alternate methods to detect failures such as MPLS-OAM or BFD
   may be considered.

   The MoFRR principle may be applied to MVPNs.

   The MoFRR principle may be applied to mLDP [I-D.ietf-mpls-ldp-p2mp].
   The reader may simply switch the term secondary-PIM-Join by
   secondary-Label-Map message.

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10.  Security Considerations

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

11.  Acknowledgments

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

12.  References

12.1.  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.

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, August 2006.

              Minei, I., Wijnands, I., Kompella, K., and B. Thomas,
              "Label Distribution Protocol Extensions for Point-to-
              Multipoint and Multipoint-to-Multipoint Label Switched
              Paths", draft-ietf-mpls-ldp-p2mp-12 (work in progress),
              February 2011.

Authors' Addresses

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


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


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


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


   Nicolai Leymann
   Deutsche Telekom
   Winterfeldtstrasse 21
   Berlin  10781


   Uwe Joorde
   Deutsche Telekom
   Winterfeldtstrasse 21
   Berlin  10781


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   Thomas Telkamp
   Cariden Technologies, Inc.
   888 Villa Street, Suite 500
   Mountain View  CA, 94041


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