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Versions: 00 01 draft-filsfils-spring-segment-routing

Network Working Group                                   C. Filsfils, Ed.
Internet-Draft                                           S. Previdi, Ed.
Intended status: Standards Track                             A. Bashandy
Expires: December 30, 2013                           Cisco Systems, Inc.
                                                             B. Decraene
                                                            S. Litkowski
                                                                  Orange
                                                            M. Horneffer
                                                        Deutsche Telekom
                                                            I. Milojevic
                                                          Telekom Srbija
                                                               R. Shakir
                                                         British Telecom
                                                                 S. Ytti
                                                                  TDC Oy
                                                           W. Henderickx
                                                          Alcatel-Lucent
                                                             J. Tantsura
                                                                Ericsson
                                                               E. Crabbe
                                                            Google, Inc.
                                                           June 28, 2013


                      Segment Routing Architecture
                draft-filsfils-rtgwg-segment-routing-00

Abstract

   Segment Routing (SR) leverages the source routing and tunneling
   paradigms.  A node steers a packet through a controlled set of
   instructions, called segments, by prepending the packet with an SR
   header.  A segment can represent any instruction, topological or
   service-based.  A segment can have a local semantic to an SR node or
   global within an SR domain.  SR allows to enforce a flow through any
   topological path and service chain while maintaining per-flow state
   only at the ingress node to the SR domain.

   The Segment Routing architecture can be directly applied to the MPLS
   dataplane with no change on the forwarding plane.  It requires minor
   extension to the existing link-state routing protocols.  Segment
   Routing can also be applied to IPv6 with a new type of routing
   extension header.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this



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   document are to be interpreted as described in RFC 2119 [RFC2119].

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on December 30, 2013.

Copyright Notice

   Copyright (c) 2013 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Illustration . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  7
     1.3.  Properties . . . . . . . . . . . . . . . . . . . . . . . .  8
     1.4.  Companion Documents  . . . . . . . . . . . . . . . . . . .  9
     1.5.  Relationship with  MPLS and IPv6 . . . . . . . . . . . . .  9
   2.  Abstract Routing Model . . . . . . . . . . . . . . . . . . . . 10
     2.1.  Traffic Engineering with SR  . . . . . . . . . . . . . . . 12
     2.2.  Segment Routing Database . . . . . . . . . . . . . . . . . 13
   3.  Link-State IGP Segments  . . . . . . . . . . . . . . . . . . . 13
     3.1.  Illustration . . . . . . . . . . . . . . . . . . . . . . . 13
       3.1.1.  Example 1  . . . . . . . . . . . . . . . . . . . . . . 14
       3.1.2.  Example 2  . . . . . . . . . . . . . . . . . . . . . . 15
       3.1.3.  Example 3  . . . . . . . . . . . . . . . . . . . . . . 15
       3.1.4.  Example 4  . . . . . . . . . . . . . . . . . . . . . . 15
       3.1.5.  Example 5  . . . . . . . . . . . . . . . . . . . . . . 16
     3.2.  IGP Segment Terminology  . . . . . . . . . . . . . . . . . 16
       3.2.1.  IGP Segment, IGP SID . . . . . . . . . . . . . . . . . 16
       3.2.2.  IGP-Prefix Segment, Prefix-SID . . . . . . . . . . . . 17
       3.2.3.  IGP-Node Segment, Node-SID . . . . . . . . . . . . . . 17
       3.2.4.  IGP-Anycast Segment, Anycast SID . . . . . . . . . . . 17
       3.2.5.  IGP-Adjacency Segment, Adj-SID . . . . . . . . . . . . 18
       3.2.6.  Finally  . . . . . . . . . . . . . . . . . . . . . . . 18
     3.3.  IGP Segment Allocation, Advertisement and SRDB
           Maintenance  . . . . . . . . . . . . . . . . . . . . . . . 19
       3.3.1.  Prefix-SID . . . . . . . . . . . . . . . . . . . . . . 19
       3.3.2.  Adj-SID  . . . . . . . . . . . . . . . . . . . . . . . 20
     3.4.  Inter-Area Considerations  . . . . . . . . . . . . . . . . 22
     3.5.  IGP Mirroring Context  Segment . . . . . . . . . . . . . . 22
   4.  Service Segments . . . . . . . . . . . . . . . . . . . . . . . 23
   5.  MPLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
   6.  IPv6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   7.  OAM  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   8.  Multicast  . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   10. Manageability Considerations . . . . . . . . . . . . . . . . . 25
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 25
     13.2. Informative References . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26







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1.  Introduction

   In this section, we illustrate the key properties of the SR
   architecture, introduce the companion documents to this note and
   relate SR to the MPLS and IPv6 architectures.

   Section 2 defines the SR abstract routing model.  Section 3 defines
   the IGP-based segments.  Section 4 defines the Service Segments.
   Section 5 and Section 6 define the instantiations of SR in MPLS and
   IPv6.

1.1.  Illustration

   In the context of Figure 1 where all the links have the same IGP
   cost, let us assume that a packet P enters the SR domain at an
   ingress edge router I and that the operator requests the following
   requirements for packet P:

      The local service S offered by node B must be applied to packet P.

      The links AB and CE cannot be used to transport the packet P.

      Any node N along the journey of the packet should be able to
      determine where the packet P entered the SR domain and where it
      will exit.  The intermediate node should be able to determine the
      paths from the ingress edge router to itself, and from itsef to
      the egress edge router.

      Per-flow State for packet P should only be created at the ingress
      edge router.

      State for packet P can only be created at the ingress edge router.

      The operator can forbid, for security reasons, anyone outside the
      operator domain to exploit its intra-domain SR capabilities.

                             I---A---B---C---E
                                  \  |  / \ /
                                   \ | /   F
                                    \|/
                                     D

                Figure 1: An illustration of SR properties

   All these properties may be realized by instructing the ingress SR
   edge router I to push the following SR header on the packet P.





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   +---------------------------------------------------------------+
   |                                   |                           |
   |           SR Header               |                           |
   |                                   |                           |
   | {SD, SB, SS, SF, SE}, Ptr, SI, SE |        Transported        |
   |  ^                     |          |           Packet          |
   |  |                     |          |             P             |
   |  +---------------------+          |                           |
   |                                   |                           |
   +---------------------------------------------------------------+

                       Figure 2: Packet P at node I

   The SR header contains a source route encoded as a list of segments
   {SD, SB, SS, SF, SE}, a pointer (Ptr) and the identification of the
   ingress and egress SR edge routers (segments SI and SE).

   A segment is a 32-bit identification either for a topological
   instruction or a service instruction.  A segment can either be global
   or local.  The instruction associated with a global segment is
   recognized and executed by any SR-capable node in the domain.  The
   instruction associated with a local segment is only supported by the
   specific node that originates it.

   Let us assume some ISIS/OSPF extensions to define a "Node Segment" as
   a global instruction within the IGP domain to forward a packet along
   the shortest path to the specified node.  Let us further assume that
   within the SR domain illustrated in Figure 1, segments SI, SD, SB, SE
   and SF respectively identify IGP node segments to I, D, B, E and F.

   Let us assume that node B identifies its local service S with local
   segment SS.

   With all of this in mind, let us describe the journey of the packet
   P.

   The packet P reaches the ingress SR edge router.  I pushes the SR
   header illustrated in Figure 2 and sets the pointer to the first
   segment of the list (SD).

   SD is an instruction recognized by all the nodes in the SR domain
   which causes the packet to be forwarded along the shortest path to D.

   Once at D, the pointer is incremented and the next segment is
   executed (SB).

   SB is an instruction recognized by all the nodes in the SR domain
   which causes the packet to be forwarded along the shortest path to B.



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   Once at B, the pointer is incremented and the next segment is
   executed (SS).

   SS is an instruction only recognized by node B which causes the
   packet to receive service S.

   Once the service applied, the next segment is executed (SF) which
   causes the packet to be forwarded along the shortest path to F.

   Once at F, the pointer is incremented and the next segment is
   executed (SE).

   SE is an instruction recognized by all the nodes in the SR domain
   which causes the packet to be forwarded along the shortest path to E.

   E then removes the SR header and the packet continues its journey
   outside the SR domain.

   All of the requirements are met.

   First, the packet P has not used links AB and CE: the shortest-path
   from I to D is I-A-D, the shortest-path from D to B is D-B, the
   shortest-path from B to F is B-C-F and the shortest-path from F to E
   is F-E, hence the packet path through the SR domain is I-A-D-B-C-F-E
   and the links AB and CE have been avoided.

   Second, the service S supported by B has been applied on packet P.

   Third, any node along the packet path is able to identify the service
   and topological journey of the packet within the SR domain.  For
   example, node C receives the packet illustrated in Figure 3 and hence
   is able to infer where the packet entered the SR domain (SI), how it
   got up to itself {SD, SB, SS, SE}, where it will exit the SR domain
   (SE) and how it will do so {SF, SE}.
   +---------------------------------------------------------------+
   |                                   |                           |
   |           SR Header               |                           |
   |                                   |                           |
   | {SD, SB, SS, SF, SE}, Ptr, SI, SE |        Transported        |
   |               ^        |          |           Packet          |
   |               |        |          |             P             |
   |               +--------+          |                           |
   |                                   |                           |
   +---------------------------------------------------------------+

                       Figure 3: Packet P at node C

   Fourth, only node I maintains per-flow state for packet P. The entire



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   program of topological and service instructions to be executed by the
   SR domain on packet P is encoded by the ingress edge router I in the
   SR header in the form of a list of segments where each segment
   identifies a specific instruction.  No further per-flow state is
   required along the packet path.  The per-flow state is in the SR
   header and travels with the packet.  Intermediate nodes only hold
   states related to the IGP global node segments and the local IGP
   adjacency segments.  These segments are not per-flow specific and
   hence scale very well.  Typically, an intermediate node would
   maintain in the order of 100's to 1000's global node segments and in
   the order of 10's to 100 of local adjacency segments.  Typically the
   SR IGP forwarding table is expected to be much less than 10000
   entries.

   Fifth, the SR header is inserted at the entrance to the domain and
   removed at the exit of the operator domain.  For security reasons,
   the operator can forbid anyone outside its domain to use its intra-
   domain SR capability.

1.2.  Terminology

   The following terminology is defined:

   +---------------+---------------------------------------------------+
   |      Term     | Definition                                        |
   +---------------+---------------------------------------------------+
   |    Segment    | A segment that identifies an instruction          |
   +---------------+---------------------------------------------------+
   |      SID      | A 32-bit identification for a segment             |
   +---------------+---------------------------------------------------+
   |  Segment List | Ordered list of segments encoding the topological |
   |               | and service source route of the packet            |
   +---------------+---------------------------------------------------+
   |     Active    | The segment that MUST be used by the receiving    |
   |    Segment    | router to process the packet.  It is identified   |
   |               | by the pointer                                    |
   +---------------+---------------------------------------------------+
   | SR-Pointer or | In the SR header, it indicates the active segment |
   |    pointer    | in the segment list                               |
   +---------------+---------------------------------------------------+
   |     Global    | The related instruction is supported by all the   |
   |    Segment    | SR-capable nodes in the local domain              |
   +---------------+---------------------------------------------------+
   |      SRGB     | SR Global Block: the set of global segments in    |
   |               | the local SR domain                               |
   +---------------+---------------------------------------------------+
   | Local Segment | The related instruction is supported only by the  |
   |               | node originating it                               |



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   |  IGP Segment  | The generic names for a segment attached to a     |
   |   or IGP SID  | piece of information advertised by a link-state   |
   |               | IGP, e.g. an IGP prefix or an IGP adjacency       |
   +---------------+---------------------------------------------------+
   |   IGP-Prefix  | An IGP-Prefix Segment is an IGP segment attached  |
   |   Segment or  | to an IGP prefix.  An IGP-Prefix Segment is       |
   |   Prefix-SID  | always global within the SR/IGP domain and        |
   |               | identifies the ECMP-aware shortest-path computed  |
   |               | by the IGP to the related prefix.  The Prefix-SID |
   |               | is the SID of the IGP-Prefix Segment              |
   +---------------+---------------------------------------------------+
   |    IGP-Node   | An IGP-Node Segment is a an IGP-Prefix Segment    |
   |   Segment or  | which identifies a specific router (e.g. a        |
   |  Node Segment | loopback).  The terms "Node Segment" or Node-SID" |
   |  or Node-SID  | are often used as an abbreviation                 |
   +---------------+---------------------------------------------------+
   |  IGP-Anycast  | An IGP-Anycast Segment is an IGP-prefix segment   |
   |   Segment or  | which does not identify a specific router, but a  |
   |    Anycast    | set of routers.  The terms "Anycast Segment" or   |
   |   Segment or  | "Anycast-SID" are often used as an abbreviation   |
   |  Anycast-SID  |                                                   |
   +---------------+---------------------------------------------------+
   | IGP-Adjacency | An IGP-Adjacency Segment is an IGP segment        |
   |   Segment or  | attached to an unidirectional adjacency or a set  |
   |   Adjacency   | of unidirectional adjacencies.  An IGP-Adjacency  |
   |   Segment or  | Segment is local to the node which advertises it  |
   |    Adj-SID    |                                                   |
   +---------------+---------------------------------------------------+
   |      SRDB     | The SR Database.  Each entry is indexed by a      |
   |               | segment value.  Each entry must list the SR       |
   |               | header operation to apply and the next-hop to     |
   |               | forward the packet to                             |
   +---------------+---------------------------------------------------+
   |   SR Header   | Push, Continue and Next are operations applied on |
   |   Operation   | the SR segment list                               |
   +---------------+---------------------------------------------------+

                   Table 1: Segment Routing Terminology

1.3.  Properties

   Assuming a packet flow F entering an SR domain at ingress SR edge
   router I, the properties offered by the SR architecture are:

      Per-Flow state for F is only maintained by node I.






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      Any topological path through the SR domain can be enforced.

      Any chain of services through the SR domain can be enforced.

      Any mix of topological paths and chain of services can be
      enforced.

      Any node along the flow path can determine where flow entered the
      SR domain, how it got up to that node, where it will exit the SR
      domain and how it will get there.

1.4.  Companion Documents

   This document defines the SR architecture, its routing model, the
   IGP-based segments and the service segments.

   Use cases are described in
   [draft-filsfils-rtgwg-segment-routing-use-cases-00].

   IS-IS protocol extensions for Segment Routing are described in
   [draft-previdi-isis-segment-routing-extensions-00].

   OSPF protocol extensions for Segment Routing are defined in
   [draft-psenak-ospf-segment-routing-extensions-00].

   The PCEP protocol extensions for Segment Routing are defined in
   [draft-msiva-pce-pcep-segment-routing-extensions-00].

   In the future, it is expected that Section 5 and Section 6 of this
   document be removed and submitted as independent documents,
   respectively as instantiations of SR in MPLS and IPv6.

   In the future, we will submit a SR-FRR specific document.

1.5.  Relationship with  MPLS and IPv6

   The source routing model is inherited from the one proposed by
   [RFC2460].

   The notion of abstract segment identifier which can represent any
   instruction is inherited from MPLS ([RFC3031]).

   Deployment experiences has shown the need to limit the number of per-
   flow states maintained in the network while preserving information on
   the topological and service journey of a packet (e.g. the ingress to
   the domain for accounting/billing purpose).

   The main differences from the IPv6 source route model are:



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      The source route is encoded as an ordered list of segments instead
      of IP addresses.

      A segment can represent any instruction either a service or a
      topological path.  Topologically, the path to an IP address is
      often limited to the shortest-path to that address.  A segment can
      represent any path (e.g. an adjacency segment forces a packet to a
      nexthop through a specific adjacency even if the shortest-path to
      the next-hop does not use that adjacency).

      The ingress and egress egde routers are identified and always
      available, allowing for interesting accounting and policy
      applications.

      The source route functionality cannot be controlled from outside
      the SR domain.

   The main differences from the MPLS model are:

      Global segments are introduced (e.g.  IGP node segments).

      LDP and RSVP MPLS signaling protocols are not required.  If
      present, SR can coexist and interwork with LDP and
      RSVP.[draft-filsfils-rtgwg-segment-routing-use-cases-00].

      Per-flow states are only maintained at the ingress edge router.

   SR can be instantiated on the IPv6 dataplane.  Section 6 details the
   new routing extension header which carry all the elements of the
   abstract SR header.  All the SR properties are preserved.

   SR can be instantiated on the MPLS dataplane.  In Section 5, we
   explain that the information present in the SR abstract header is
   encoded as a stack of labels.  The notion of pointer and full segment
   list containing the full history of the path from ingress to egress
   edge routers is thus lost in the MPLS instantiation of SR.  However
   all the other SR properties are preserved and especially the MPLS
   dataplane can be reused without any change.


2.  Abstract Routing Model

   Segment Routing (SR) leverages the source routing and tunneling
   paradigms.

   At the entrance of the SR domain, the ingress SR edge router pushes
   the SR header on top of the packet.  At the exit of the SR domain,
   the egress SR edge router removes the SR header.



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   The SR header contains an ordered list of segments, a pointer
   identifying the next segment to process and the identifications of
   the ingress and egress SR edge routers on the path of this packet.
   The pointer identifies the segment that MUST be used by the receiving
   router to process the packet.  This segment is called the active
   segment.

   A property of the architecture is that the entire source route of the
   packet, including the identity of the ingress and egress edge routers
   is always available with the packet.  This allows for interesting
   accounting and service applications.

   We define three SR-header operations:

      "PUSH": an SR header is pushed on an IP packet, or additional
      segments are added at the head of the segment list.  The pointer
      is moved to the first entry of the added segments.

      "NEXT": the active segment is completed, the pointer is moved to
      the next segment in the list.

      "CONTINUE": the active segment is not completed, the pointer is
      left unchanged.

   In the future, other SR-header management operations may be defined.

   As the packet travels through the SR domain, the pointer is
   incremented through the ordered list of segments and the source route
   encoded by the SR ingress edge node is executed.

   A node processes an incoming packet according to the instruction
   associated with the active segment.

   Any instruction might be associated with a segment: for example, an
   intra or inter-domain topological strict or loose forwarding
   instruction, a service instruction, etc.

   At minimum, a segment instruction must define two elements: the
   identity of the next-hop to forward the packet to (this could be the
   same node or a context within the node) and which SR-header
   management operation to execute.

   Each segment is known in the network through a Segment Identifier
   (SID), a value allocated from the 32-bit Segment IDentifier space.
   The first 16 values are reserved.  The terms "segment" and "SID" are
   interchangeable.

   Within an SR domain, all the SR-capable nodes are configured with the



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   Segment Routing Global Block (SRGB).  The SRGB is a subset of the 32-
   bit SID space.  SRGB can be a non-contiguous set of segments.

   All global segments must be allocated from the SRGB.  Any SR capable
   node MUST be able to process any global segment advertised by any
   other node within the SR domain.

   Any segment outside the SRGB has a local significance and is called a
   "local segment".  An SR-capable node MUST be able to process the
   local segments it originates.  An SR-capable node MUST NOT support
   the instruction associated with a local segment originated by a
   remote node.

2.1.  Traffic Engineering with SR

   An SR Traffic Engineering policy is composed of two elements: a flow
   classification and a segment-list to prepend on the packets of the
   flow.

   In the SR architecture, this per-flow state only exists at the
   ingress egde router whether the policy is defined and the SR header
   is pushed.

   It is outside the scope of the document to define the process that
   leads to the instantiation at a node N of an SR Traffic Engineering
   policy.

   [draft-filsfils-rtgwg-segment-routing-use-cases-00] illustrates
   various alternatives:

      N is deriving this policy automatically (e.g.  FRR).

      N is provisioned explicitly by the operator.

      N is provisioned by a stateful PCE server.

      N is provisioned by the operator with a high-level policy which is
      mapped into a path thanks to a local CSPF-based computation (e.g.
      affinity/SRLG exclusion).

   Any architecture that involves the insertion of information onto a
   packet involves performance consideration.
   [draft-filsfils-rtgwg-segment-routing-use-cases-00]explains why the
   majority of use-cases require very short segment-lists.

   A stateful PCE server, which desires to instantiate at node N an SR
   Traffic Engineering policy, collects the SR capability of node N such
   as to ensure that the policy meets its capability



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   [draft-msiva-pce-pcep-segment-routing-extensions-00].

2.2.  Segment Routing Database

   The Segment routing Database (SRDB) is a set of entries where each
   entry is identified by a segment value.  The instruction associated
   with each entry at least defines the identity of the next-hop to
   which the packet should be forwarded and what operation should be
   performed on the SR header (PUSH, CONTINUE, NEXT).
   +---------+-----------+---------------------------------+
   | Segment |  Next-Hop |  SR Header operation            |
   +---------+-----------+---------------------------------+
   |   Sk    |     M     | CONTINUE                        |
   |   Sj    |     N     | NEXT                            |
   |   Sl    | NAT Srvc  | NEXT                            |
   |   Sm    |  FW srvc  | NEXT                            |
   |   Sn    |     Q     | NEXT                            |
   |  etc.   |   etc.    | etc.                            |
   +---------+-----------+---------------------------------+

                           Figure 4: SR Database

   Each SR-capable node maintains its local SRDB.  SRDB entries can
   either derive from local policy or or from protocol segment
   advertisement.  The next section will detail segment advertisement by
   IGP protocols."


3.  Link-State IGP Segments

   Within a link-state IGP domain, an SR-capable IGP node advertises
   segments for its attached prefixes and adjacencies.  These segments
   are called IGP segments or IGP SIDs.  They play a key role in the
   Segment Routing architecture and use-cases
   [draft-filsfils-rtgwg-segment-routing-use-cases-00] as they enable
   the expression of any topological path throughout the IGP domain.
   Such a topological path is either expressed as a single IGP segment
   or a list of multiple IGP segments.

   In the first sub-section, we introduce a terminology for a set of IGP
   segments which are very frequently seen in the SR use-cases.  The
   second sub-section details the IGP segment allocation and SRDB
   construction rules.

3.1.  Illustration

   Assuming the network diagram of Figure 5 and the IP address and IGP
   Segment allocation of Figure 6, the following examples can be



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   constructed.
                                 +--------+
                                /          \
                        R1-----R2----------R3-----R8
                               | \        / |
                               |  +--R4--+  |
                               |            |
                               +-----R5-----+

                   Figure 5: IGP Segments - Illustration

   +-----------------------------------------------------------+
   | IP address allocated by the operator:                     |
   |                      192.0.2.1/32 as a loopback of R1     |
   |                      192.0.2.2/32 as a loopback of R2     |
   |                      192.0.2.3/32 as a loopback of R3     |
   |                      192.0.2.4/32 as a loopback of R4     |
   |                      192.0.2.5/32 as a loopback of R5     |
   |                      192.0.2.8/32 as a loopback of R8     |
   |              198.51.100.9/32 as an anycast loopback of R4 |
   |              198.51.100.9/32 as an anycast loopback of R5 |
   |                                                           |
   | SRGB defined by the operator as 1000-5000                 |
   |                                                           |
   | Global IGP SID allocated by the operator:                 |
   |                      1001 allocated to 192.0.2.1/32       |
   |                      1002 allocated to 192.0.2.2/32       |
   |                      1003 allocated to 192.0.2.3/32       |
   |                      1004 allocated to 192.0.2.4/32       |
   |                      1008 allocated to 192.0.2.8/32       |
   |                      2009 allocated to 198.51.100.9/32    |
   |                                                           |
   | Local IGP SID allocated dynamically by R2                 |
   |                     for its "north" adjacency to R3: 9001 |
   |                     for its "north" adjacency to R3: 9003 |
   |                     for its "south" adjacency to R3: 9002 |
   |                     for its "south" adjacency to R3: 9003 |
   +-----------------------------------------------------------+

        Figure 6: IGP Address and Segment Allocation - Illustration

3.1.1.  Example 1

   R1 may send a packet P1 to R8 simply by pushing an SR header with
   segment list {1008}.

   1008 is a global IGP segment attached to the IP prefix 192.0.2.8/32.
   Its semantic is global within the IGP domain: any router forwards a



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   packet received with active segment 1008 to the next-hop along the
   ECMP-aware shortest-path to the related prefix.

   In conclusion, the path followed by P1 is R1-R2--R3-R8.  The ECMP-
   awareness ensures that the traffic be load-shared between any ECMP
   path, in this case the two north and south links between R2 and R3.

3.1.2.  Example 2

   R1 may send a packet P2 to R8 by pushing an SR header with segment
   list {1002, 9001, 1008}.

   1002 is a global IGP segment attached to the IP prefix 192.0.2.2/32.
   Its semantic is global within the IGP domain: any router forwards a
   packet received with active segment 1002 to the next-hop along the
   shortest-path to the related prefix.

   9001 is a local IGP segment attached by node R2 to its north link to
   R3.  Its semantic is local to node R2: R2 switches a packet received
   with active segment 9001 towards the north link to R3.

   In conclusion, the path followed by P2 is R1-R2-north-link-R3-R8.

3.1.3.  Example 3

   R1 may send a packet P3 along the same exact path as P1 using a
   different segment list {1002, 9003, 1008}.

   9003 is a local IGP segment attached by node R2 to both its north and
   south links to R3.  Its semantic is local to node R2: R2 switches a
   packet received with active segment 9003 towards either the north or
   south links to R3 (e.g. per-flow loadbalancing decision).

   In conclusion, the path followed by P3 is R1-R2-any-link-R3-R8.

3.1.4.  Example 4

   R1 may send a packet P4 to R8 while avoiding the links between R2 and
   R3 by pushing an SR header with segment list {1004, 1008}.

   1004 is a global IGP segment attached to the IP prefix 192.0.2.4/32.
   Its semantic is global within the IGP domain: any router forwards a
   packet received with active segment 1004 to the next-hop along the
   shortest-path to the related prefix.

   In conclusion, the path followed by P4 is R1-R2-R4-R3-R8.





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3.1.5.  Example 5

   R1 may send a packet P5 to R8 while avoiding the links between R2 and
   R3 while still benefitting from all the remaining shortest paths (via
   R4 and R5) by pushing an SR header with segment list {2009, 1008}.

   2009 is a global IGP segment attached to the anycast IP prefix
   198.51.100.9/32.  Its semantic is global within the IGP domain: any
   router forwards a packet received with active segment 2009 to the
   next-hop along the shortest-path to the related prefix.

   In conclusion, the path followed by P5 is either R1-R2-R4-R3-R8 or
   R1-R2-R5-R3-R8 .

3.2.  IGP Segment Terminology

3.2.1.  IGP Segment, IGP SID

   The terms "IGP Segment" and "IGP SID" are the generic names for a
   segment attached to a piece of information advertised by a link-state
   IGP, e.g. an IGP prefix or an IGP adjacency.

   The IGP signaling extension to advertise an IGP segment includes the
   G-Flag indicating whether the IGP segment is global or local.
                                       IGP-SID
                                       +--+--+
                                      /   |   \
                             Prefix-SID   x   Adj-SID
                             +----+---+
                            /     |    \
                      Node-SID    y   Anycast-SID

                       Figure 7: IGP SID Terminology

   The IGP Segment terminology is introduced to ease the documentation
   of SR use-cases and hence does not propose a name for any possible
   variation of IGP segment supported by the architecture.  For example,
   y in Figure 7 could represent a local IGP segment attached to an IGP
   Prefix.  This variation, while supported by the SR architecture is
   not seen in the SR use-cases and hence does not receive a specific
   name.

   In Figure 5 and Figure 6, SIDs 1001, 1002, 1003, 1004, 1008, 2009,
   9001, 9002 and 9003 are called IGP SIDs.







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3.2.2.  IGP-Prefix Segment, Prefix-SID

   An IGP-Prefix Segment is an IGP segment attached to an IGP prefix.
   An IGP-Prefix Segment is always global within the SR/IGP domain and
   identifies the ECMP-aware shortest-path computed by the IGP to the
   related prefix.  The G-Flag MUST be set.  The Prefix-SID is the SID
   of the IGP-Prefix Segment.

   A packet injected anywhere within the SR/IGP domain with an active
   Prefix-SID will be forwarded along the shortest-path to that prefix.

   The IGP signaling extension for IGP-Prefix segment includes the
   P-Flag.  A Node N advertising a Prefix-SID SID-R for its attached
   prefix R resets the P-Flag to allow its connected neighbors to
   perform the NEXT operation while processing SID-R.  This behavior is
   equivalent to Pen-ultimate Hop Popping in MPLS.  When set, the
   neighbors of N must perform the CONTINUE operation while processing
   SID-R.

   While the architecture allows to attach a local segment to an IGP
   prefix, we specifically assume that when the terms "IGP-Prefix
   Segment" and "Prefix-SID" are used then the segment is global (the
   SID is allocated from the SRGB).  This is consistent with
   [draft-filsfils-rtgwg-segment-routing-use-cases-00] as all the
   described use-cases require global segments attached to IGP prefix.

   In Figure 5 and Figure 6, SIDs 1001, 1002, 1003, 1004, 1008, 2009 are
   called Prefix-SIDs.

3.2.3.  IGP-Node Segment, Node-SID

   An IGP-Node Segment is a an IGP-Prefix Segment which identifies a
   specific router (e.g. a loopback).  The terms "Node Segment" or
   "Node-SID" are often used as an abbreviation.

   A "Node Segment" or "Node-SID" is fundamental to the SR architecture.
   From anywhere in the network, it enforces the ECMP-aware shortest-
   path forwarding of the packet towards the related node as explained
   in [draft-filsfils-rtgwg-segment-routing-use-cases-00].

   In Figure 5 and Figure 6, SIDs 1001, 1002, 1003, 1004 and 1008 are
   called Node-SIDs.

3.2.4.  IGP-Anycast Segment, Anycast SID

   An IGP-Anycast Segment is an IGP-prefix segment which does not
   identify a specific router, but a set of routers.  The terms "Anycast
   Segment" or "Anycast-SID" are often used as an abbreviation.



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   An "Anycast Segment" or "Anycast SID" enforces the ECMP-aware
   shortest-path forwarding towards the closest node of the anycast set.
   This is useful to express macro-engineering policies as described in
   [draft-filsfils-rtgwg-segment-routing-use-cases-00].

   In Figure 5 and Figure 6, SID 2009 is called Anycast SID.

3.2.5.  IGP-Adjacency Segment, Adj-SID

   An IGP-Adjacency Segment is an IGP segment attached to an
   unidirectional adjacency or a set of unidirectional adjacencies.  An
   IGP-Adjacency Segment is local to the node which advertises it.  The
   SID of the IGP-Adjacency Segment is called the Adj-SID.  The G-Flag
   must be reset.

   The adjacency is formed by the local node (i.e.: the node advertising
   the adjacency in the IGP) and the remote node (i.e.: the other end of
   the adjacency).  The local node MUST be an IGP node.  The remote node
   MAY be:

      An adjacent IGP node (i.e.: an IGP neighbor).

      A non-adjacent neighbor (e.g.: a Forwarding Adjacency, [RFC4206]).

      A virtual neighbor outside the IGP domain (e.g.: an interface
      connecting another AS) as defined in [RFC5316].

   A packet injected anywhere within the SR/IGP domain with a segment
   list {SN, SNL}, where SN is the Node-SID of node N and SNL is an Adj-
   Sid attached by node N to its adjacency over link L, will be
   forwarded along the shortest-path to N and then be switched by N,
   without any IP shortest-path consideration, towards link L. If the
   Adj-Sid identifies a set of adjacencies, then the node N load-
   balances the traffic along the various members of the set.

   An "IGP Adjacency Segment" or "Adj-SID" enforces the switching of the
   packet from a note towards a defined interface or set of interfaces.
   This is key to theoretically prove that any path can be expressed as
   a list of segments as explained in
   [draft-filsfils-rtgwg-segment-routing-use-cases-00].

   In Figure 5 and Figure 6, SIDs 9001, 9002 and 9003 are called Adj-
   SIDs.

3.2.6.  Finally

   Figure 8 summarizes the different terms that can be used to refer to
   the SID's used in the example illustrated by Figure 5 and Figure 6.



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   "Y" means that the term can be used to refer to the SID, "N" means
   that the term cannot be used to refer to the SID.
   +---------------------------------------------------------------+
   |  SID  | IGP SID | Prefix-SID | Node-SID| Anycast SID| Adj-SID |
   | Value |         |            |         |            |         |
   +---------------------------------------------------------------+
   | 1001  |    Y    |      Y     |    Y    |      N     |    N    |
   | 1002  |    Y    |      Y     |    Y    |      N     |    N    |
   | 1003  |    Y    |      Y     |    Y    |      N     |    N    |
   | 1004  |    Y    |      Y     |    Y    |      N     |    N    |
   | 1005  |    Y    |      Y     |    Y    |      N     |    N    |
   | 1008  |    Y    |      Y     |    Y    |      N     |    N    |
   | 2009  |    Y    |      Y     |    N    |      Y     |    N    |
   | 9001  |    Y    |      N     |    N    |      N     |    Y    |
   | 9002  |    Y    |      N     |    N    |      N     |    Y    |
   | 9003  |    Y    |      N     |    N    |      N     |    Y    |
   +---------------------------------------------------------------+

                       Figure 8: Terminology Example

3.3.  IGP Segment Allocation, Advertisement and SRDB Maintenance

3.3.1.  Prefix-SID

   Multiple Prefix-SID's may be allocated to the same IGP Prefix (e.g.
   for class of service purpose).  Typically a single Prefix-SID is
   allocated to an IGP Prefix.

   A Prefix-SID is allocated from the SRGB according to a similar
   process to IP address allocation.  Typically the Prefix-SID is
   allocated by policy by the operator (or NMS) and the SID very rarely
   changes.

   The allocation process MUST NOT allocate the same Prefix-SID to
   different IP prefixes.

   If a node learns a Prefix-SID having a value that falls outside the
   locally configured SRGB range, then the node MUST NOT use the Prefix-
   SID and SHOULD issue an error log warning for misconfiguration.

   The required IGP protocol extensions are defined in
   [draft-previdi-isis-segment-routing-extensions-00] and
   [draft-psenak-ospf-segment-routing-extensions-00].

   A node N attaching a Prefix-SID SID-R to its attached prefix R MUST
   maintain the following SRDB entry:





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   Incoming Active Segment: SID-R
   Ingress Operation: NEXT
   Egress interface: NULL

   A remote node M MUST maintain the following SRDB entry for any
   learned Prefix-SID SID-R attached to IP prefix R:
   Incoming Active Segment: SID-R
   Ingress Operation:
            If the next-hop of R is the originator of R
            and instructed to remove the active segment: NEXT
            Else: CONTINUE
   Egress interface: the interface towards the next-hop along
            the shortest-path to prefix R.

3.3.2.  Adj-SID

   The Adjacency Segment SID (Adj-SID) identifies a unidirectional
   adjacency or a set of unidirectional adjacencies.
   A node SHOULD allocate one Adj-SIDs for each of its adjacencies.
   A node MAY allocate multiple Adj-SIDs to the same adjacency.
   A node MAY allocate the same Adj-SID to multiple adjacencies.

   Adjacency suppression MUST NOT be performed by the IGP.

   A node MUST install an SRDB entry for any Adj-SID of value V attached
   to data-link L:
   Incoming Active Segment: V
   Operation: NEXT
   Egress Interface: L

   When associated to a Forwarding Adjacency ([RFC4206]), the Adj-SID
   MAY also include the necessary information in order to describe the
   path to the remote end of the Forwarding Adjacency in the form of an
   Explicit Route Object.

   The Adj-SID implies, from the router advertising it, the forwarding
   of the packet through the adjacency identified by the Adj-SID,
   regardless its IGP/SPF cost.  In other words, the use of Adjacency
   Segments overrides the routing decision made by SPF algorithm.

3.3.2.1.  Parallel Adjacencies

   Adj-SIDs can be used in order to represent a set of parallel
   interfaces between two adjacent routers.  For example, SID 9003 in
   figures 5 and 6 identify the set of interfaces between R2 and R3.

   A node MUST install an SRDB entry for any locally originated
   Adjacency Segment (Adj-SID) of value W attached to a set of link B



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   with:
   Incoming Active Segment: W
   Ingress Operation: NEXT
   Egress interface: loadbalance between any data-link within set B

3.3.2.2.  LAN Adjacency Segments

   In LAN subnetworks, link-state protocols define the concept of
   Designated Router (DR, in OSPF) or Designated Intermediate System
   (DIS, in IS-IS) that conduct flooding in broadcast subnetworks and
   that describe the LAN topology in a special routing update (OSPF
   Type2 LSA or IS-IS Pseudonode LSP).

   The difficulty with LANs is that each router only advertises its
   connectivity to the DR/DIS and not to each other individual nodes in
   the LAN.  Therefore, additional protocol mechanisms (IS-IS and OSPF)
   are necessary in order for each router in the LAN to advertise an
   Adj-SID associated to each neighbor in the LAN.  These extensions are
   defined in [draft-previdi-isis-segment-routing-extensions-00] and
   [draft-psenak-ospf-segment-routing-extensions-00].

3.3.2.3.  External Adjacencies Considerations

   IGPs have been extended in order to advertise virtual adjacencies
   that represent external links ([RFC5316]).

   Segment Routing allows to allocate an Adj-SID to these external
   links.
                           AS1     )       (    AS2
                          IGP-1    ) eBGP  (   IGP-2
                                   )       (
                         B------C--)-------(--F-----G
                        /       |  )       (  |     |
                  S---A/        |  )       (  |     |
                       \        |  )       (  |     |
                        \D------E--)-------(--H-----I----Z
                                   )       (
                                   )       (

                   Figure 9: External Adjacency Example

   In the diagram above, C advertises in the IGP an adjacency to peer F
   of AS2 together with an associated Adj-SID.  When S wants to force an
   inter-domain path to Z via the peering link CF, S encapsulates the
   packets with the list {Prefix-SID(C), Adj-SID(C,F, AS2)}.

   [draft-filsfils-rtgwg-segment-routing-use-cases-00] provides an
   external-adjacency use-case.



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3.4.  Inter-Area Considerations

   In the following example diagram we assume an IGP deployed using
   areas and where SR has been deployed.
                  !          !
                  !          !
           B------C-----F----G-----K
          /       |          |     |
    S---A/        |          |     |
         \        |          |     |
          \D------I----------J-----L----Z (192.0.2.1/32, Node-SID: 150)
                  !          !
          Area-1  ! Backbone ! Area 2
                  !   area   !

                  Figure 10: Inter-Area Topology Example

   In area 2, node Z allocates Node-SID 150 to his local prefix
   192.0.2.1/32.  ABRs G and J will propagate the prefix into the
   backbone area by creating a new instance of the prefix according to
   normal inter-area/level IGP propagation rules.

   Nodes C and I will apply the same behavior when leaking prefixes from
   the backbone area down to area 1.  Therefore, node S will see prefix
   192.0.2.1/32 with Prefix-SID 150 and advertised by nodes C and I.

   It therefore results that a Prefix-SID remains attached to its
   related IGP Prefix through the inter-area process.

   When node S sends traffic to 192.0.2.1/32, it pushes Node-SID(150) as
   active segment and forward it to A.

   When packet arrives at ABR I (or C), the ABR forwards the packet
   according to the active segment (Node-SID(150)).  Forwarding
   continues across area borders, using the same Node-SID(150), until
   the packet reaches its destination.

   When an ABR propagates a prefix from one area to another it MUST set
   the R-Flag.

3.5.  IGP Mirroring Context  Segment

   It is beneficial for an IGP node to be able to advertise its ability
   to process traffic originally destined to another IGP node, called
   the Mirrored node and identified by an IP address or a Node-SID,
   provided that a "Mirroring Context" segment be inserted in the
   segment list prior to any service segment local to the mirrored node.




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   [draft-filsfils-rtgwg-segment-routing-use-cases-00] illustrates such
   a use-case where two IGP nodes offer the same set of services (e.g.
   BGP VPN) and mirror each other upon their failure.  A similar
   behavior is described in [I-D.minto-rsvp-lsp-egress-fast-protection].

   IS-IS and OSPF Router Capability extensions are described in
   [draft-previdi-isis-segment-routing-extensions-00] and
   [draft-psenak-ospf-segment-routing-extensions-00].


4.  Service Segments

   A service segment refers to a service offered by a node (e.g.
   firewall, vpn, etc.).

   Further informations will be included in future revisions.


5.  MPLS

   The 20 right-most bits of the segment are encoded as a label.  This
   implies that, in the MPLS instantiation, the SID values are allocated
   within a reduced 20-bit space out of the 32-bit SID space.

   A list of segments is represented as a stack of labels.

   The active segment is the top label.

   The CONTINUE operation is implemented as an MPLS swap operation where
   the outgoing label value is equal to the incoming label value.

   The NEXT operation is implemented as an MPLS pop operation.

   The PUSH operation is implemented as an MPLS push of a label stack.

   The SRGB label space is allocated to Segment Routing.  The SR
   operator manages the SRGB space as a registry and ensures the unique
   allocation of the global resource.

   A local segment is a locally allocated label.

   In conclusion:

      There are no changes in the operations of the data-plane currently
      used in MPLS networks.






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      The SR solution can co-exist and interwork with other MPLS
      control-plane protocols, see
      [draft-filsfils-rtgwg-segment-routing-use-cases-00] for more
      details.

      In the MPLS instantiation, as the packet travels through the SR
      domain, the stack is depleted and the segment list history is
      gradually lost.


6.  IPv6

   The text will be added in future revision.


7.  OAM

   SR offers an interesting capability to monitor SR domains:

      Any path can be monitored by setting the segment list accordingly.

      A path can be expressed with ECMP-awareness or not.

      The probe travels along the desired path while staying at the
      forwarding level.

      A monitoring system is able to check any element of the entire SR
      domain, even if it located multiple hops away.

   Some elements of the SR/OAM functionality will require
   standardization and a related independent draft will eventually be
   submitted.

   SR/OAM use-cases are described in
   [draft-filsfils-rtgwg-segment-routing-use-cases-00].


8.  Multicast

   The text will be added in future revision.


9.  IANA Considerations

   TBD






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10.  Manageability Considerations

   TBD


11.  Security Considerations

   TBD


12.  Acknowledgements

   We would like to thank Dave Ward, Dan Frost, Stewart Bryant, Pierre
   Francois, Thomas Telkamp, Les Ginsberg, Ruediger Geib and Hannes
   Gredler for their contribution to the content of this document.


13.  References

13.1.  Normative References

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

   [RFC4206]  Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
              Hierarchy with Generalized Multi-Protocol Label Switching
              (GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005.

   [RFC5316]  Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
              Support of Inter-Autonomous System (AS) MPLS and GMPLS
              Traffic Engineering", RFC 5316, December 2008.

13.2.  Informative References

   [I-D.minto-rsvp-lsp-egress-fast-protection]
              Jeganathan, J., Gredler, H., and Y. Shen, "RSVP-TE LSP
              egress fast-protection",
              draft-minto-rsvp-lsp-egress-fast-protection-02 (work in
              progress), April 2013.

   [draft-filsfils-rtgwg-segment-routing-use-cases-00]
              Filsfils, C., "Segment Routing Use Cases", May 2013.



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   [draft-msiva-pce-pcep-segment-routing-extensions-00]
              Filsfils, C. and S. Sivabalan, "PCEP Extensions for
              Segment Routing", May 2013.

   [draft-previdi-isis-segment-routing-extensions-00]
              Previdi, S., Filsfils, C., and A. Bashandy, "IS-IS Segment
              Routing Extensions", May 2013.

   [draft-psenak-ospf-segment-routing-extensions-00]
              Psenak, P. and S. Previdi, "OSPF Segment Routing
              Extensions", May 2013.


Authors' Addresses

   Clarence Filsfils (editor)
   Cisco Systems, Inc.
   Brussels,
   BE

   Email: cfilsfil@cisco.com


   Stefano Previdi (editor)
   Cisco Systems, Inc.
   Via Del Serafico, 200
   Rome  00142
   Italy

   Email: sprevidi@cisco.com


   Ahmed Bashandy
   Cisco Systems, Inc.
   170, West Tasman Drive
   San Jose, CA  95134
   US

   Email: bashandy@cisco.com


   Bruno Decraene
   Orange
   FR

   Email: bruno.decraene@orange.com





Filsfils, et al.        Expires December 30, 2013              [Page 26]


Internet-Draft               Segment Routing                   June 2013


   Stephane Litkowski
   Orange
   FR

   Email: stephane.litkowski@orange.com


   Martin Horneffer
   Deutsche Telekom
   Hammer Str. 216-226
   Muenster  48153
   DE

   Email: Martin.Horneffer@telekom.de


   Igor Milojevic
   Telekom Srbija
   Takovska 2
   Belgrade
   RS

   Email: igormilojevic@telekom.rs


   Rob Shakir
   British Telecom
   London
   UK

   Email: rob.shakir@bt.com


   Saku Ytti
   TDC Oy
   Mechelininkatu 1a
   TDC  00094
   FI

   Email: saku@ytti.fi











Filsfils, et al.        Expires December 30, 2013              [Page 27]


Internet-Draft               Segment Routing                   June 2013


   Wim Henderickx
   Alcatel-Lucent
   Copernicuslaan 50
   Antwerp  2018
   BE

   Email: wim.henderickx@alcatel-lucent.com


   Jeff Tantsura
   Ericsson
   300 Holger Way
   San Jose, CA  95134
   US

   Email: Jeff.Tantsura@ericsson.com


   Edward Crabbe
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   Email: edc@google.com


























Filsfils, et al.        Expires December 30, 2013              [Page 28]


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