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Network Working Group                                     Anil Kumar S N
INTERNET-DRAFT                                            Gaurav Agrawal
Intended Status: Standard Track                            Vinod Kumar S
Expires: April 11, 2016                        Huawei Technologies India
                                                         October 9, 2015


             Maximally Redundant Trees in Segment Routing
             draft-agv-rtgwg-spring-segment-routing-mrt-01


Abstract

      This document presents a Fast Reroute (FRR) approach aimed at
   providing link and node protection of node and adjacency segments
   within the Segment Routing (SR) framework.  This FRR behavior builds
   on Maximally Redundant Trees (MRT) FRR algorithm [I-D.atlas-rtgwg-
   mrt-mc-arch].

      Fast-Reroute with Maximally Redundant Trees (MRT-FRR) using
   Segment routing is a technology that gives link-protection and node-
   protection with 100% coverage in any network topology that is still
   connected after the failure. MRT is computational efficient. For any
   router in the network, the MRT computation is less than the LFA
   computation for a node with three or more neighbors in SR domain (Ex:
   Topology Independent Fast Reroute).

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html




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Copyright and License 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
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   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 Standard Terminology . . . . . . . . . . . . . . . . . . . .  3
   2. Draft Specific Terminology  . . . . . . . . . . . . . . . . . .  3
   3. Overview of SR Signaling for MRT  . . . . . . . . . . . . . . .  5
   4. Protecting segments . . . . . . . . . . . . . . . . . . . . . .  5
     4.1. The top segment is a node segment . . . . . . . . . . . . .  5
     4.1. The top segment is an adjacency segment . . . . . . . . . .  5
   5. Requirements for SR MRT implementation  . . . . . . . . . . . .  6
   3  Security Considerations . . . . . . . . . . . . . . . . . . . .  6
   4  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .  6
   5  References  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     5.1  Normative References  . . . . . . . . . . . . . . . . . . .  7
     5.2.  Informative References . . . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  8


















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1  Introduction

      SR MRT FRR is one among the local repair mechanisms for Segment
   routing network. Another well known local repair mechanism for SR is
   Topology Independent Fast Reroute which is also capable of restoring
   end-to-end connectivity in case of a failure of a link or a node,
   with guaranteed coverage properties. MRT also provides 100% network
   failure coverage. The advantage of MRT over TI-LFA would be the
   computation complexities involved in MRT is much lesser then TI-LFA.
   MRT is best suited for access/aggregate ring network or low end
   devices which has low computing capacity.

1.1 Standard Terminology

   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].


2. Draft Specific Terminology

   For ease of reading, some of the terminology defined in [I-D.ietf-
   rtgwg-mrt-frr-architecture] is repeated here.

   Redundant Trees (RT):   A pair of trees where the path from any node
   X to the root R along the first tree is node-disjoint with the path
   from the same node X to the root along the second tree. These can be
   computed in 2-connected graphs.

   Maximally Redundant Trees (MRT):   A pair of trees where the path
   from any node X to the root R along the first tree and the path from
   the same node X to the root along the second tree share the minimum
   number of nodes and the minimum number of links. Each such shared
   node is a cut-vertex.  Any shared links are cut-links. Any RT is an
   MRT but many MRTs are not RTs.  The two MRTs are referred to as MRT-
   Blue and MRT-Red.

   MRT-Red:   MRT-Red is used to describe one of the two MRTs; it is
   used to described the associated forwarding topology and MT-ID.
   Specifically, MRT-Red is the decreasing MRT where links in the GADAG
   are taken in the direction from a higher topologically ordered node
   to a lower one.

   MRT-Blue:   MRT-Blue is used to describe one of the two MRTs; it is
   used to described the associated forwarding topology and MT-ID.
   Specifically, MRT-Blue is the increasing MRT where links in the GADAG
   are taken in the direction from a lower topologically ordered node to
   a higher one.



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   Rainbow MRT MT-ID:   It is useful to have an MT-ID that refers to the
   multiple MRT topologies and to the default topology.  This is
   referred to as the Rainbow MRT MT-ID and is used by LDP to reduce
   signaling and permit the same label to always be advertised to all
   peers for the same (MT-ID, Prefix).

   MRT Island:   From the computing router, the set of routers that
   support a particular MRT profile and are connected via MRT- eligible
   links.

   Island Border Router (IBR):   A router in the MRT Island that is
   connected to a router not in the MRT Island and both routers are in a
   common area or level.

   Island Neighbor (IN):   A router that is not in the MRT Island but is
   adjacent to an IBR and in the same area/level as the IBR..



































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3. Overview of SR Signaling for MRT

      To extend MRT support to Segment routing following requirement
   need to be achieved :

   1.  SR MRT Capabilities must be advertised using IGP extension for SR
   MRT.  Also SR MRT capabilities must be in sync with IGP specific MRT
   capabilities advertisement. If the peer has not advertised the SR MRT
   capability, then it indicates that LSR does not support MRT
   procedures.

   2.  As specified in MRT Architecture draft-ietf-rtgwg-mrt-frr-
   architecture, both Option 1A and Option 1B can be used for the
   implementation of SR MRT.

   For Option 1A, two additional Prefix SID's/Label for RED and BLUE MT
   must be advertised in addition to default prefix SID/Label. The IGP
   extension carrying prefix SID for RED and BLUE MT must have
   corresponding MT-ID allocated by IANA for default MRT profile.

   For Option 1B, Global Unique Context SID/Label for Red & Blue as
   topology identifier must be used.

4. Protecting segments

   In this section, we explain how a protecting router processes the
   label stack of a packet upon the failure of its interface.

   The behavior depends on the type of top segment in SR label stack to
   be protected. Its assumed the MRT algorithm is run prior to interface
   failure and alternate is selected as per the algorithm. The alternate
   segment could be MRT-RED label or MRT Blue label of Next/Next-to-Next
   Hop depending on the scenario.


4.1. The top segment is a node segment The current SR Label Stack is
   kept intact. The alternate segment is pushed at the top of SR label
   stack and continue forwarding the packet as per updated label stack.

4.1. The top segment is an adjacency segment The top adjacency label in
   the SR label stack representing the failed interface is popped & the
   alternate segment is pushed at the top of SR Label stack and contineu
   forwarding the packet as per updated label stack.








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5. Requirements for SR MRT implementation

   REQ1 : IGP Extension to carry the Segment Routing Node MRT Capability
   in addition to exiting IGP extension carrying IGP MRT Capability

   REQ2 : IGP Extension to carry Red & Blue MRT SR Segments in addition
   to existing Default SR Segment

3  Security Considerations

   None of the security consideration are identified


4  IANA Considerations

   None of the IANA consideration are identified



































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5  References

5.1  Normative References

   [I-D.ietf-rtgwg-mrt-frr-algorithm]

              Envedi, G., Csaszar, A., Atlas, A., Bowers, C., and A.
              Gopalan, "Algorithms for computing Maximally Redundant
              Trees for IP/LDP Fast- Reroute", draft-ietf-rtgwg-mrt-frr-
              algorithm-05 (work in progress), July 2015.

   [I-D.ietf-rtgwg-mrt-frr-architecture]
              Atlas, A., Kebler, R., Bowers, C., Envedi, G., Csaszar,
              A., Tantsura, J., and R. White, "An Architecture for IP/
              LDP Fast-Reroute Using Maximally Redundant Trees", draft-
              ietf-rtgwg-mrt-frr-architecture-05 (work in progress),
              January 2015.

   [RFC5036]
               Andersson, L., Ed., Minei, I., Ed., and B. Thomas, Ed.,
              "LDP Specification", RFC 5036, DOI 10.17487/RFC5036,
              October 2007, <http://www.rfc-editor.org/info/rfc5036>.

   [RFC5561]
              Thomas, B., Raza, K., Aggarwal, S., Aggarwal, R., and JL.
              Le Roux, "LDP Capabilities", RFC 5561, DOI
              10.17487/RFC5561, July 2009,              <http://www.rfc-
              editor.org/info/rfc5561>.
   [RFC6420]
              Cai, Y. and H. Ou, "PIM Multi-Topology ID (MT-ID) Join
              Attribute", RFC 6420, DOI 10.17487/RFC6420, November 2011,
              <http://www.rfc-editor.org/info/rfc6420>.

   [RFC7307]
              Zhao, Q., Raza, K., Zhou, C., Fang, L., Li, L., and D.
              King, "LDP Extensions for Multi-Topology", RFC 7307, DOI
              10.17487/RFC7307, July 2014, <http://www.rfc-
              editor.org/info/rfc7307>.













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5.2.  Informative References

   [I-D.atlas-rtgwg-mrt-mc-arch]
              Atlas, A., Kebler, R., Wijnands, I., Csaszar, A., and G.
              Envedi, "An Architecture for Multicast Protection Using
              Maximally Redundant Trees", draft-atlas-rtgwg-mrt-mc-
              arch-02 (work in progress), July 2013.

   [I-D.ietf-isis-mrt]
              Li, Z., Wu, N., Zhao, Q., Atlas, A., Bowers, C., and J.
              Tantsura, "Intermediate System to Intermediate System (IS-
              IS) Extensions for Maximally Redundant Trees (MRT)",
              draft-ietf-isis-mrt-00 (work in progress), February 2015.

   [I-D.ietf-ospf-mrt]
              Atlas, A., Hegde, S., Bowers, C., Tantsura, J., and Z. Li,
              "OSPF Extensions to Support Maximally Redundant Trees",
              draft-ietf-ospf-mrt-00 (work in progress), January 2015.

   [I-D.wijnands-mpls-mldp-node-protection]
              Wijnands, I., Rosen, E., Raza, K., Tantsura, J., Atlas,
              A., and Q. Zhao, "mLDP Node Protection", draft-wijnands-
              mpls-mldp-node-protection-04 (work in progress), June
              2013.

   [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>.


Authors' Addresses



              Anil Kumar S N
              Huawei Technologies India Pvt. Ltd,
              Near EPIP Industrial Area,
              Kundalahalli Village,
              Whitefield,
              Bangalore - 560037

              EMail: anil.sn@huawei.com



              Gaurav Agrawal



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              Huawei Technologies India Pvt. Ltd,
              Near EPIP Industrial Area,
              Kundalahalli Village,
              Whitefield,
              Bangalore - 560037

              EMail: gaurav.agrawal@huawei.com




              Vinod Kumar S
              Huawei Technologies India Pvt. Ltd,
              Near EPIP Industrial Area,
              Kundalahalli Village,
              Whitefield,
              Bangalore - 560037

              EMail: vinods.kumar@huawei.com
































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