IPv6 Segment Routing Header (SRH)
draft-previdi-6man-segment-routing-header-01

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Network Working Group                                    S. Previdi, Ed.
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: December 11, 2014                                      B. Field
                                                                 Comcast
                                                                I. Leung
                                                   Rogers Communications
                                                            June 9, 2014

                   IPv6 Segment Routing Header (SRH)
              draft-previdi-6man-segment-routing-header-01

Abstract

   Segment Routing (SR) allows a node to steer a packet through a
   controlled set of instructions, called segments, by prepending a SR
   header to the packet.  A segment can represent any instruction,
   topological or service-based.  SR allows to enforce a flow through
   any path (topological, or application/service based) while
   maintaining per-flow state only at the ingress node to the SR domain.

   Segment Routing can be applied to the IPv6 data plane with the
   addition of a new type of Routing Extension Header.  This draft
   describes the Segment Routing Extension Header Type and how it is
   used by SR capable nodes.

Requirements Language

   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].

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
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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."

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   This Internet-Draft will expire on December 11, 2014.

Copyright Notice

   Copyright (c) 2014 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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   described in the Simplified BSD License.

Table of Contents

   1.  Structure of this document  . . . . . . . . . . . . . . . . .   3
   2.  Segment Routing Documents . . . . . . . . . . . . . . . . . .   3
   3.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Data Planes supporting Segment Routing  . . . . . . . . .   4
     3.2.  Illustration  . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Abstract Routing Model  . . . . . . . . . . . . . . . . . . .   8
     4.1.  Segment Routing Global Block (SRGB) . . . . . . . . . . .   9
     4.2.  Traffic Engineering with SR . . . . . . . . . . . . . . .   9
     4.3.  Segment Routing Database  . . . . . . . . . . . . . . . .  10
   5.  IPv6 Instantiation of Segment Routing . . . . . . . . . . . .  10
     5.1.  Segment Identifiers (SIDs) and SRGB . . . . . . . . . . .  10
       5.1.1.  Node-SID  . . . . . . . . . . . . . . . . . . . . . .  11
       5.1.2.  Adjacency-SID . . . . . . . . . . . . . . . . . . . .  11
     5.2.  Segment Routing Extension Header (SRH)  . . . . . . . . .  11
       5.2.1.  SRH and RFC2460 behavior  . . . . . . . . . . . . . .  14
       5.2.2.  SRH Optimization  . . . . . . . . . . . . . . . . . .  15
   6.  SRH Procedures  . . . . . . . . . . . . . . . . . . . . . . .  16
     6.1.  Segment Routing Operations  . . . . . . . . . . . . . . .  16
     6.2.  Segment Routing Node Functions  . . . . . . . . . . . . .  16
       6.2.1.  Ingress SR Node . . . . . . . . . . . . . . . . . . .  17
       6.2.2.  Transit Non-SR Capable Node . . . . . . . . . . . . .  18
       6.2.3.  SR Intra Segment Transit Node . . . . . . . . . . . .  18
       6.2.4.  SR Segment Endpoint Node  . . . . . . . . . . . . . .  18
     6.3.  FRR Flag Settings . . . . . . . . . . . . . . . . . . . .  19
   7.  SRH Security  . . . . . . . . . . . . . . . . . . . . . . . .  19
     7.1.  Threat model  . . . . . . . . . . . . . . . . . . . . . .  19
     7.2.  Applicability of RFC 5095 to SRH  . . . . . . . . . . . .  20
     7.3.  Security fields in SRH  . . . . . . . . . . . . . . . . .  21
     7.4.  Nodes within the SR domain  . . . . . . . . . . . . . . .  22

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     7.5.  Nodes outside of the SR domain  . . . . . . . . . . . . .  22
     7.6.  SR path exposure  . . . . . . . . . . . . . . . . . . . .  22
   8.  SR and Tunneling  . . . . . . . . . . . . . . . . . . . . . .  23
   9.  Example Use Case  . . . . . . . . . . . . . . . . . . . . . .  23
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   11. Manageability Considerations  . . . . . . . . . . . . . . . .  26
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  26
   13. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  26
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     15.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

1.  Structure of this document

   Section 3 gives an introduction on SR for IPv6 networks.

   Section 4 describes the Segment Routing abstract model.

   Section 5 defines the Segment Routing Header (SRH) allowing
   instantiation of SR over IPv6 dataplane.

   Section 6 details the procedures of the Segment Routing Header.

   Section 7 describes the security aspect of SR-IPv6.

2.  Segment Routing Documents

   Segment Routing terminology is defined in
   [I-D.filsfils-spring-segment-routing].

   Segment Routing use cases are described in
   [I-D.filsfils-spring-segment-routing-use-cases].

   Segment Routing IPv6 use cases are described in
   [I-D.ietf-spring-ipv6-use-cases].

   Segment Routing protocol extensions are defined in
   [I-D.ietf-isis-segment-routing-extensions], and
   [I-D.psenak-ospf-segment-routing-ospfv3-extension].

3.  Introduction

   Segment Routing (SR), defined in
   [I-D.filsfils-spring-segment-routing], allows a node to steer a
   packet through a controlled set of instructions, called segments, by
   prepending a SR header to the packet.  A segment can represent any

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   instruction, topological or service-based.  SR allows to enforce a
   flow through any path (topological or service/application based)
   while maintaining per-flow state only at the ingress node to the SR
   domain.  Segments can be derived from different components: IGP, BGP,
   Services, Contexts, Locators, etc.  The list of segment forming the
   path is called the Segment List and is encoded in the packet header.

   SR allows the use of strict and loose source based routing paradigms
   without requiring any additional signaling protocols in the
   infrastructure hence delivering an excellent scalability property.

   The source based routing model described in
   [I-D.filsfils-spring-segment-routing] is inherited from the ones
   proposed by [RFC1940] and [RFC2460].  The source based routing model
   offers the support for explicit routing capability.

3.1.  Data Planes supporting Segment Routing

   Segment Routing (SR), can be instantiated over MPLS
   ([I-D.filsfils-spring-segment-routing-mpls]) and IPv6.  This document
   defines its instantiation over the IPv6 data-plane based on the use-
   cases defined in [I-D.ietf-spring-ipv6-use-cases].

   Segment Routing for IPv6 (SR-IPv6) is required in networks where MPLS
   data-plane is not used or, when combined with SR-MPLS, in networks
   where MPLS is used in the core and IPv6 is used at the edge (home
   networks, datacenters).

   This document defines a new type of Routing Header (originally
   defined in [RFC2460]) called the Segment Routing Header (SRH) in
   order to convey the Segment List in the packet header as defined in
   [I-D.filsfils-spring-segment-routing].  Mechanisms through which
   segment are known and advertised are outside the scope of this
   document.

3.2.  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

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      will exit.  The intermediate node should be able to determine the
      paths from the ingress edge router to itself, and from itself to
      the egress edge router.

      Per-flow State for packet P should 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 abstract SR header on the packet
   P.

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

                       Figure 2: Packet P at node I

   The abstract 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 identifies 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 IGP (i.e.: ISIS and OSPF) extensions to define a
   "Node Segment" as a global instruction within the IGP domain to

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   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.

   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

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   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 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.

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4.  Abstract Routing Model

   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.

   The abstract 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 SR 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-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.

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   Each segment is known in the network through a Segment Identifier
   (SID).  The terms "segment" and "SID" are interchangeable.

4.1.  Segment Routing Global Block (SRGB)

   In the SR abstract model, a segment is identified by a Segment
   Routing Identifier (SID).  The SR abstract model doesn't mandate a
   specific format for the SID (IPv6 address or other formats).

   In Segment Routing IPv6 the SID is an IPv6 address.  Therefore, the
   SRGB is materialized by the global IPv6 address space which
   represents the set of IPv6 routable addresses in the SR domain.  The
   following rules apply:

   o  Each node of the SR domain MUST be configured with the Segment
      Routing Global Block (SRGB).

   o  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.

   o  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.

4.2.  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 SR, this per-flow state only exists at the ingress edge node where
   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.

   [I-D.filsfils-spring-segment-routing-use-cases] illustrates various
   alternatives:

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

      N is provisioned explicitly by the operator.

      N is provisioned by a controller or server (e.g.: SDN Controller).

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      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).

      N could also be provisioned by other means.

   [I-D.filsfils-spring-segment-routing-use-cases] explains why the
   majority of use-cases require very short segment-lists, hence
   minimizing the performance impact, if any, of inserting and
   transporting the segment list.

   A SDN controller, 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.

4.3.  Segment Routing Database

   The Segment routing Database (SRDB) is a set of entries where each
   entry is identified by a SID.  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 from protocol segment
   advertisement.

5.  IPv6 Instantiation of Segment Routing

5.1.  Segment Identifiers (SIDs) and SRGB

   Segment Routing, as described in
   [I-D.filsfils-spring-segment-routing], defines Node-SID and
   Adjacency-SID.  When SR is used over IPv6 data-plane the following
   applies.

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   The SRGB is the global IPv6 address space which represents the set of
   IPv6 routable addresses in the SR domain.

   Node SIDs are IPv6 addresses part of the SRGB (i.e.: routable
   addresses).  Adjacency-SIDs are IPv6 addresses which may not be part
   of the global IPv6 address space.

5.1.1.  Node-SID

   The Node-SID identifies a node.  With SR-IPv6 the Node-SID is an IPv6
   prefix that the operator configured on the node and that is used as
   the node identifier.  Typically, in case of a router, this is the
   IPv6 address of the node loopback interface.  Therefore, SR-IPv6 does
   not require any additional SID advertisement for the Node Segment.
   The Node-SID is in fact the IPv6 address of the node.

5.1.2.  Adjacency-SID

   In the SR architecture defined in
   [I-D.filsfils-spring-segment-routing] the Adjacency-SID (or Adj-SID)
   identifies a given interface and is a local segment (i.e.: the value
   has significance only to the node advertising the Adj-SID).  A node
   may advertise one (or more) Adj-SIDs allocated to a given interface
   so to force the forwarding of the packet (when received with that
   particular Adj-SID) into the interface regardless the routing entry
   for the packet destination.  The semantic of the Adj-SID is:

      Send out the packet to the interface this prefix is allocated to.

   When SR is applied to IPv6, any SID is in an IPv6 address and
   therefore, an Adj-SID may have a global significance (i.e.: when the
   IPv6 address representing the SID is a global address).  In other
   words, a node that advertises the Adj-SID in the form of a global
   IPv6 address representing the link/adjacency the packet has to be
   forwarded to, will apply to the Adj-SID a global significance.

   Advertisement of Adj-SID may be done using multiple mechanisms among
   which the ones described in ISIS and OSPF protocol extensions:
   [I-D.ietf-isis-segment-routing-extensions] and
   [I-D.psenak-ospf-segment-routing-ospfv3-extension].  The distinction
   between local and global significance of the Adj-SID is given in the
   encoding of the Adj-SID advertisement.

5.2.  Segment Routing Extension Header (SRH)

   A new type of the Routing Header (originally defined in [RFC2460]) is
   defined: the Segment Routing Header (SRH) which has a new Routing
   Type, to be assigned by IANA.

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   As an example, if an explicit path is to be constructed across a core
   network running ISIS or OSPF, the segment list will contain SIDs
   representing the nodes across the path (loose or strict) which,
   usually, are the IPv6 loopback interface address of each node.  If
   the path is across service or application entities, the segment list
   contains the IPv6 addresses of these services or application
   instances.

   The Segment Routing Header (SRH) is defined as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Next Header   |  Hdr Ext Len  | Routing Type  | Next Segment  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Last Segment  | Flags |  HMAC Key ID  | Policy List Flags     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Segment List[0] (128 bits ipv6 address)            |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
                                  ...
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Segment List[n] (128 bits ipv6 address)            |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Policy List[0] (128 bits ipv6 address)             |
    |                        (optional)                             |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Policy List[1] (128 bits ipv6 address)             |
    |                        (optional)                             |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Policy List[2] (128 bits ipv6 address)             |
    |                        (optional)                             |
    |                                                               |

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    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
    |                                                               |
    |                       HMAC (256 bits)                         |
    |                        (optional)                             |
    |                                                               |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   o  Next Header: 8-bit selector.  Identifies the type of header
      immediately following the SRH.

   o  Hdr Ext Len: 8-bit unsigned integer, is the length of the SRH
      header in 8-octet units, not including the first 8 octets.

   o  Routing Type: TBD, to be assigned by IANA.

   o  Next Segment (originally defined as "Segments Left" in [RFC2460]):
      offset (in multiple of 8 octets not including the first 8 octets)
      of the next active segment (according to terminology defined in
      [I-D.filsfils-spring-segment-routing]) in the SRH.  Note that this
      differs from the semantic defined in the Routing Header
      specification ([RFC2460] defines it as "Segments Left").
      Therefore, in the Segment Routing context, the "Segments Left"
      field is renamed as "Next Segment".

   o  Last Segment: offset (in multiple of 8 octets not including the
      first 8 octets) of the last segment of the path in the SRH.

   o  Flags: 4 bits of flags.  Two flags are defined:

         Bit-0: Clean-up Bit. Set when the SRH has to be removed from
         the packet when packet reaches the last segment.

         Bit-1: Protected Bit. Set when the packet has been rerouted
         through FRR mechanism by a SR endpoint node.  See Section 6.3
         for more details.

   o  HMAC Key ID and HMAC field are defined in Section 7.

   o  Policy List flags.  Define the type of the IPv6 addresses encoded
      into the Policy List (see below).  The following have been
      defined:

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         Bits 0-2: determine the type of the first element after the
         segment list.

         Bits 3-5: determine the type of the second element.

         Bits 6-8: determine the type of the third element.

         Bits 9-11: determine the type of the fourth element.

      The following values are used for the type:

         0x0: Not present.  If value is set to 0x0, it means the element
         represented by these bits is not present.

         0x1: Ingress SR PE address.

         0x2: Egress SR PE address.

         0x3: Original Source Address.

   o  Segment List[n]: 128 bit IPv6 addresses representing the nth
      segment of the path.

   o  Policy List.  Optional addresses representing specific nodes in
      the SR path such as:

         Ingress SR PE: IPv6 address representing the SR node which has
         imposed the SRH (SR domain ingress).

         Egress SR PE: IPv6 address representing the egress SR domain
         node.

         Original Source Address: IPv6 address originally present in the
         SA field of the packet.

      The segments in the Policy List are encoded after the segment list
      and they are optional.  If none are in the SRH, all bits of the
      Policy List Flags MUST be set to 0x0.

5.2.1.  SRH and RFC2460 behavior

   The SRH being a new type of the Routing Header, it also has the same
   properties:

      SHOULD only appear once in the packet.

      Only the router whose address is in the DA field of the packet
      header MUST inspect the SRH.

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   Therefore, Segment Routing in IPv6 networks implies that the segment
   identifier (i.e.: the IPv6 address of the segment) is moved into the
   DA of the packet.

   The DA of the packet changes at each segment termination/completion
   and therefore the original DA of the packet MUST be encoded as the
   last segment of the path.

   As illustrated in Section 3.2, nodes that are within the path of a
   segment will forward packets based on the DA of the packet without
   inspecting the SRH.  This ensures full interoperability between SR-
   capable and non-SR-capable nodes.

5.2.2.  SRH Optimization

   In order to optimize the way the SRH and, more precisely, the Segment
   List is processed by SR nodes, it is desirable that most of the
   necessary information of the SL is placed at the top of the list so
   to avoid reading the whole content of the SRH prior to make
   forwarding decisions.

   With this in mind, when the SRH is created and the segment list is
   inserted, the order of the segments in the segment list is as
   follows:

   o  The Next Segment field points to the next segment to be examined
      (offset within the SRH).

   o  The first segment being encoded in the DA by the ingress node, it
      doesn't need to sit in the first position of the list.

   o  Hence, the first element of the segment list is the second segment
      of the path so that, when the packet reaches the end of the first
      segment, the node inspecting the SRH will find the second segment
      at the beginning of the segment list.

   o  The other segments of the path are encoded sequentially after the
      second segment.

   o  The last segment of the path is the original DA address.

   o  The last segment in the Segment List is used to encode the first
      segment.  This segment will never be inspected anyway (at least
      not for forwarding purposes).

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6.  SRH Procedures

   In this section we describe the different procedures on the SRH.

6.1.  Segment Routing Operations

   When Segment Routing is instantiated over the IPv6 data plane the
   following applies:

   o  The segment list is encoded in the SRH.

   o  The active segment is in the destination address of the packet.

   o  The Segment Routing CONTINUE operation (as described in
      [I-D.filsfils-spring-segment-routing]) is implemented as a
      regular/plain IPv6 operation consisting of DA based forwarding.

   o  The NEXT operation is implemented through the update of the DA
      with the value represented by the Next Segment field in the SRH.

   o  The PUSH operation is implemented through the insertion of the SRH
      or the insertion of additional segments in the SRH segment list.

6.2.  Segment Routing Node Functions

   SR packets are forwarded to segments endpoints (i.e.: nodes whose
   address is in the DA field of the packet).  The segment endpoint,
   when receiving a SR packet destined to itself, does:

   o  Inspect the SRH.

   o  Determine the next segment.

   o  Update the SRH (or, if requested, remove the SRH from the packet).

   o  Update the DA.

   o  Send the packet to the next segment.

   The procedures applied to the SRH are related to the node function.
   Following nodes functions are defined:

      Ingress SR Node.

      Transit Non-SR Node.

      Transit SR Intra Segment Node.

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      SR Endpoint Node.

6.2.1.  Ingress SR Node

   Ingress Node can be a router at the edge of the SR domain or a SR-
   capable host.  The ingress SR node may obtain the segment list by
   either:

      Local path computation.

      Local configuration.

      Interaction with an SDN controller delivering the path as a
      complete SRH.

      Any other mechanism (mechanisms through which the path is acquired
      are outside the scope of this document).

   When creating the SRH (either at ingress node or in the SDN
   controller) the following is done:

      Next Header and Hdr Ext Len fields are set according to [RFC2460].

      Routing Type field is set as TBD (SRH).

      The DA of the packet is set with the address of the FIRST segment
      of the path.

      Next Segment field contains the offset of the SECOND segment of
      the path which is encoded in the FIRST position of the segment
      list.  The segment list is encoded as follows:

         The first element of the list contains the second segment (as
         stated above).

         All subsequent segments are encoded following the second
         segment.

         The original DA of the packet is encoded as the last segment of
         the path (which is NOT the last segment of the segment list).

         The last segment of the segment list is the FIRST segment of
         the path.

      Last Segment field contains the offset of the last segment of the
      path (i.e.: the original DA of the packet).

      The packet is sent out to the first segment.

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6.2.1.1.  Security at Ingress

   The procedures related to the Segment Routing security are detailed
   in Section 7.

   In the case where the SR domain boundaries are not under control of
   the network operator (e.g.: when the SR domain edge is in a home
   network), it is important to authenticate and validate the content of
   any SRH being received by the network operator.  In such case, the
   security procedure described in Section 7 is to be used.

   The ingress node (e.g.: the host in the home network) requests the
   SRH from a control system (e.g.: an SDN controller) which delivers
   the SRH with its HMAC signature on it.

   Then, the home network host can send out SR packets (with an SRH on
   it) that will be validated at the ingress of the network operator
   infrastructure.

   The ingress node of the network operator infrastructure, is
   configured in order to validate the incoming SRH HMACs in order to
   allow only packets having correct SRH according to their SA/DA
   addresses.

6.2.2.  Transit Non-SR Capable Node

   SR is interoperable with plain IPv6 forwarding.  Any non SR-capable
   node will forward SR packets solely based on the DA.  There's no SRH
   inspection.  This ensures full interoperability between SR and non-SR
   nodes.

6.2.3.  SR Intra Segment Transit Node

   Only the node whose address is in DA inspects and processes the SRH
   (according to [RFC2460]).  An intra segment transit node is not in
   the DA and its forwarding is based on DA and its SR-IPv6 FIB.

6.2.4.  SR Segment Endpoint Node

   The SR segment endpoint node is the node whose address is in the DA.
   The segment endpoint node inspects the SRH and does:

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   1.   IF DA = myself (segment endpoint)
   2.      IF Next Segment <> Last Segment THEN
              update DA with Next Segment
              increment Next Segment
   3.      ELSE IF Last Segment <> DA THEN
              update DA with Next Segment
              IF Clean-up bit is set THEN remove the SRH
   4.      ELSE give the packet to next PID (application)
                End of processing.
   5.   Forward the packet out

6.3.  FRR Flag Settings

   A node supporting SR and doing Fast Reroute (as described in
   [I-D.filsfils-spring-segment-routing-use-cases], when rerouting
   packets through FRR mechanisms, SHOULD inspect the rerouted packet
   header and look for the SRH.  If the SRH is present, the rerouting
   node SHOULD set the Protected bit on all rerouted packets.

7.  SRH Security

   This section analyses the security threat model as well as the
   security issues and proposed solutions related to the new routing
   header for segment routing.

   The SRH is simply another version of the routing header as described
   in [RFC2460] and is:

   o  inserted when entering the segment routing domain which could be
      done by a node or by a router;

   o  inspected and acted upon when reaching the destination address of
      the IP header.

   Routers on the path that simply forward an IPv6 packet (i.e. the IPv6
   destination address is none of theirs) will never inspect and process
   the SRH.  Routers whose one interface IPv6 address equals the
   destination address field of the SRH will have to parse the SRH and,
   if supported and if the local configuration allows it, will act on
   the SRH.

7.1.  Threat model

   Using a SRH, which is basically source routing, has some well-known
   security issues as described in [RFC4942] section 2.1.1 and
   [RFC5095]:

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   o  amplification attacks: where a packet could be forged in such a
      way to cause looping among a set of SR-enabled routers causing
      unnecessary traffic, hence a denial of service against bandwidth;

   o  reflection attack: where a hacker could force an intermediate node
      to appear as the immediate attacker, hence hiding the real
      attacker from naive forensic;

   o  bypass attack: where an intermediate node could be used as a
      stepping stone (for example in a DMZ) to attack another host (for
      example in the datacenter or any back-end server.

   These security issues did lead to obsoleting the routing-header type
   0, RH-0, with [RFC5095] because:

   o  it was assumed to be inspected and acted upon by default by each
      and every router on the Internet;

   o  it contained multiple segments in the payload.

   Therefore, if intermediate nodes ONLY act on valid and authorized
   SRH, then there is no security threat similar to RH-0.

   But as SR is used for added value services, there is also a need to
   prevent non-participating nodes to use those services; this is called
   'service stealing prevention'.  The SRH also contains all IPv6
   addresses of intermediate SR-nodes, this obviously reveals those
   addresses to the potentially hostile attackers if those attackers are
   on the path.

7.2.  Applicability of RFC 5095 to SRH

   In the segment routing architecture described in
   [I-D.filsfils-spring-segment-routing] there are basically two kinds
   of nodes (routers and hosts):

   o  nodes within the segment routing domain, which is within one
      single administrative domain, i.e., where all nodes are trusted
      anyway else the damage caused by those nodes could be worse than
      amplification attacks: traffic interception and man-in-the-middle
      attacks, more server DoS by dropping packets, and so on.

   o  Nodes outside of the segment routing domain, which is outside of
      the administrative segment routing domain hence they cannot be
      trusted because there is no physical security for those nodes,
      i.e., they can be replaced by hostile nodes or can be coerced in
      wrong behaviors.

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7.3.  Security fields in SRH

   The security-related fields in SRH are:

   o  HMAC Key-id, 8 bits wide, if HMAC key-id is null, then there is no
      HMAC field;

   o  HMAC, 256 bits wide.

   The HMAC field is the output of the hash of the concatenation of:

   o  the source IPv6 address;

   o  last segment field, an octet whose bit-0 is the clean-up bit flag
      and others are 0, HMAC key-id, all addresses in the Segment List;

   o  a pre-shared secret between SR nodes in the SR domain (routers,
      controllers, ...);

   o  if required by the hash algorithm a pad field filled with 0.

   The purpose of the HMAC field is to verify the validity, the
   integrity and the authorization of the SRH itself.  If an outsider of
   the SR domain does not have access to a current pre-shared secret,
   then it cannot compute the right HMAC field and the first SR router
   on the path processing the SRH and configured to check the validity
   of the HMAC will simply reject the packet.

   The HMAC field is located at the end of the SRH simply because only
   the router on the ingress of the SR domain needs to process it, then
   all other SR nodes can ignore it (based on local policy) because they
   can trust the upstream router.  This is to speed up forwarding
   operations because some hardware platforms can only parse in hardware
   so many bytes.

   The HMAC Key-id field allows for the simultaneous existence of
   several hash algorithms (SHA-256, SHA3-256 ... or future ones) as
   well as pre-shared keys.  This allows for pre-shared key roll-over
   when two pre-shared keys are supported for a while when all SR nodes
   converged to a fresher pre-shared key.  The HMAC key-id is opaque,
   i.e., it has no syntax except as an index to the right combination of
   pre-shared key and hash algorithm.  It also allows for interoperation
   among different SR domains if allowed by local policy.

   When a specific SRH is linked to a time-related service (such as
   turbo-QoS for a 1-hour period) where the DA, SID are identical, then
   it is important to refresh the shared-secret frequently as the HMAC
   validity period expires only when the HMAC key-id and its associated

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   shared-secret expires.  How HMAC key-id and pre-shared secret are
   synchronized between participating nodes in the SR domain is outside
   of the scope of this document ([RFC6407] GDOI could be a basis).

7.4.  Nodes within the SR domain

   Those nodes can be trusted to generate SRH and to process SRH
   received on interfaces that are part of the SR domain.  These nodes
   MUST drop all packets received on an interface that is not part of
   the SR domain and containing a SRH whose HMAC field cannot be
   validated by local policies.  This includes obviously packet with a
   SRH generated by a non-cooperative SR domain.

   If the validation fails, then these packets MUST be dropped, ICMP
   error messages (parameter problem) SHOULD be generated (but rate
   limited) and SHOULD be logged.

7.5.  Nodes outside of the SR domain

   Nodes outside of the SR domain cannot be trusted for physical
   security; hence, they need to request by some means (outside of the
   scope of this document) a complete SRH for each new connection (i.e.
   new destination address).  The SRH MUST include a HMAC key-id and
   HMAC field which is computed correctly (see Section 7.3).

   When an outside node sends a packet with an SRH and towards a SR
   ingress node, the packet MUST contain the HMAC key-id and HMAC field
   and the SR ingress node MUST be the destination address.

   The ingress SR router, i.e., the router with an interface address
   equals to the destination address, MUST verify the HMAC field with
   respect to the HMAC key-id.

   If the validation is successful, then the packet is simply forwarded
   as usual for a SR packet.  As long as the packet travels within the
   SR domain, no further HMAC check needs to be done.  Subsequent
   routers in the SR domain MAY verify the HMAC field when they process
   the SRH (i.e. when they are the destination).

   If the validation fails, then this packet MUST be dropped, an ICMP
   error message (parameter problem) SHOULD be generated (but rate
   limited) and SHOULD be logged.

7.6.  SR path exposure

   As the intermediate SR nodes addresses appears in the SRH, if this
   SRH is visible to an outside then he/she could reuse this knowledge
   to launch an attack on the intermediate SR nodes or get some insider

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   knowledge on the topology.  This is especially applicable when the
   path between the source node and the first SR-node in the domain is
   on the public Internet.

   The first remark is to state that 'security by obscurity' is never
   enough; in other words, the security policy of the SR domain MUST
   assume that the internal topology and addressing is known by the
   attacker.  A simple traceroute will also give the same information
   (with even more information as all intermediate nodes between SID
   will also be exposed).  IPsec Encapsulating Security Payload (RFC
   4303) cannot be use to protect the SRH as per RFC 4303 the ESP header
   must appear after any routing header (including SRH).

   To prevent a user to leverage the gained knowledge by intercepting
   SRH, it it recommended to apply an infrastructure Access Control List
   (iACL) at the edge of the SR domain.  This iACL will drop all packets
   from outside the SR-domain whose destination is any address of any
   router inside the domain.  This security policy should be tuned for
   local operations.

8.  SR and Tunneling

   Encapsulation can be realized in two different ways with SR-IPv6:

      Outer encapsulation.

      SRH with SA/DA original addresses.

   Outer encapsulation tunneling is the traditional method where an
   additional IPv6 header is prepended to the packet.  The original IPv6
   header being encapsulated, everything is preserved and the packet is
   switched/routed according to the outer header (that could contain a
   SRH).

   SRH allows encoding both original SA and DA and therefore, hence an
   operator may decide to change the SA/DA at ingress and restore them
   at egress.  This can be achieved without outer encapsulation, by
   changing SA/DA and encoding the original values in the Segment List
   (the last segment of the path being the original DA) and in the
   Policy List (original SA).

9.  Example Use Case

   A more detailed description of use cases are available in
   [I-D.ietf-spring-ipv6-use-cases].  In this section, a simple SR-IPv6
   example is illustrated.

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   In the topology described in Figure 6 it is assumed an end-to-end SR
   deployment.  Therefore SR is supported by all nodes from A to J.

    Home Network |          Backbone         |    Datacenter
                 |                           |
                 |   +---+   +---+   +---+   |   +---+   |
             +---|---| C |---| D |---| E |---|---| I |---|
             |   |   +---+   +---+   +---+   |   +---+   |
             |   |     |       |       |     |     |     |  +---+
   +---+   +---+ |     |       |       |     |     |     |--| X |
   | A |---| B | |   +---+   +---+   +---+   |   +---+   |  +---+
   +---+   +---+ |   | F |---| G |---| H |---|---| J |---|
                 |   +---+   +---+   +---+   |   +---+   |
                 |                           |
                 |        +-----------+
                          |    SDN    |
                          | Orch/Ctlr |
                          +-----------+

                       Figure 6: Sample SR topology

   The following workflow applies to packets sent by host A and destined
   to server X.

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   . Host A sends a request for a path to server X to the SDN
     controller or orchestration system.

   . The SDN controller/orchestrator builds a SRH with:
      . Segment List: C, F, J, X
      . HMAC
     that satisfies the requirements expressed in the request
     by host A and based on policies applicable to host A.

   . Host A receives the SRH and insert it into the packet.
     The packet has now:
      . SA: A
      . DA: C
      . SRH with
         . SL: F,J,X,C
         . PL: C (ingress), J (egress)
        Note that X is the last segment and C is the
        first segment (encoded at the end of the SL).

   . When packet arrives in C (first segment), C does:
      . Validate the HMAC of the SRH.

      . Update the DA with the next segment (found in SRH):
        DA is set to F.
      . Forward the packet to F.

   . Packet arrives in F which inspects the SRH and find the
     next segment:
      . DA is set to J.

   . Packet travels across G and H nodes which do plain
     IPv6 forwarding based on DA. No inspection of SRH needs
     to be done in these nodes. However, any SR capable node
     is allowed to set the Protected bit in case of FRR
     protection.

   . Packet arrives in J where two options are available
     depending on the settings of the cleanup bit set in the
     SRH:
      . If the cleanup bit is set, then node J will strip out
        the SRH from the packet, set the DA as X and send
        the packet out.
      . If the clean-up bit is not set, the DA is set to X
        and the packet is sent out with the SRH.

   The packet arrives in the server that may or may not support SR.  The
   return traffic, from server to host, may be sent using the same
   procedures.

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10.  IANA Considerations

   TBD

11.  Manageability Considerations

   TBD

12.  Security Considerations

   Security mechanisms applied to Segment Routing over IPv6 networks are
   detailed in Section 7.

13.  Contributors

   Eric Vyncke contributed to this document through the writings of
   Section 7.

14.  Acknowledgements

   The authors would like to thank John Leddy, John Brzozowski, Pierre
   Francois, Nagendra Kumar, Mark Townsley, Christian Martin, Roberta
   Maglione and James Connolly for their contribution to this document.

15.  References

15.1.  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.

   [RFC5095]  Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
              of Type 0 Routing Headers in IPv6", RFC 5095, December
              2007.

   [RFC6407]  Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
              of Interpretation", RFC 6407, October 2011.

15.2.  Informative References

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   [I-D.filsfils-spring-segment-routing]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
              "Segment Routing Architecture", draft-filsfils-spring-
              segment-routing-03 (work in progress), June 2014.

   [I-D.filsfils-spring-segment-routing-mpls]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., Tantsura, J., and E. Crabbe,
              "Segment Routing with MPLS data plane", draft-filsfils-
              spring-segment-routing-mpls-02 (work in progress), June
              2014.

   [I-D.filsfils-spring-segment-routing-use-cases]
              Filsfils, C., Francois, P., Previdi, S., Decraene, B.,
              Litkowski, S., Horneffer, M., Milojevic, I., Shakir, R.,
              Ytti, S., Henderickx, W., Tantsura, J., Kini, S., and E.
              Crabbe, "Segment Routing Use Cases", draft-filsfils-
              spring-segment-routing-use-cases-00 (work in progress),
              March 2014.

   [I-D.ietf-isis-segment-routing-extensions]
              Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
              Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS
              Extensions for Segment Routing", draft-ietf-isis-segment-
              routing-extensions-01 (work in progress), June 2014.

   [I-D.ietf-spring-ipv6-use-cases]
              Brzozowski, J., Leddy, J., Leung, I., Previdi, S.,
              Townsley, W., Martin, C., Filsfils, C., and R. Maglione,
              "IPv6 SPRING Use Cases", draft-ietf-spring-ipv6-use-
              cases-00 (work in progress), May 2014.

   [I-D.psenak-ospf-segment-routing-ospfv3-extension]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., and W. Henderickx, "OSPFv3 Extensions for
              Segment Routing", draft-psenak-ospf-segment-routing-
              ospfv3-extension-01 (work in progress), February 2014.

   [RFC1940]  Estrin, D., Li, T., Rekhter, Y., Varadhan, K., and D.
              Zappala, "Source Demand Routing: Packet Format and
              Forwarding Specification (Version 1)", RFC 1940, May 1996.

   [RFC4942]  Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/
              Co-existence Security Considerations", RFC 4942, September
              2007.

Previdi, et al.         Expires December 11, 2014              [Page 27]
Internet-Draft      IPv6 Segment Routing Header (SRH)          June 2014

Authors' Addresses

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

   Email: sprevidi@cisco.com

   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels
   BE

   Email: cfilsfil@cisco.com

   Brian Field
   Comcast
   4100 East Dry Creek Road
   Centennial, CO  80122
   US

   Email: Brian_Field@cable.comcast.com

   Ida Leung
   Rogers Communications
   8200 Dixie Road
   Brampton, ON  L6T 0C1
   CA

   Email: Ida.Leung@rci.rogers.com

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