IPv6 Segment Routing Header (SRH)
draft-previdi-6man-segment-routing-header-07

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Network Working Group                                    S. Previdi, Ed.
Internet-Draft                                               C. Filsfils
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: January 21, 2016                                       B. Field
                                                                 Comcast
                                                                I. Leung
                                                   Rogers Communications
                                                                E. Aries
                                                                Facebook
                                                               E. Vyncke
                                                     Cisco Systems, Inc.
                                                               D. Lebrun
                                        Universite Catholique de Louvain
                                                           July 20, 2015

                   IPv6 Segment Routing Header (SRH)
              draft-previdi-6man-segment-routing-header-07

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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Table of Contents

   1.  Structure of this document  . . . . . . . . . . . . . . . . .   3
   2.  Segment Routing Documents . . . . . . . . . . . . . . . . . .   3
   3.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Data Planes supporting Segment Routing  . . . . . . . . .   4
     3.2.  Illustration  . . . . . . . . . . . . . . . . . . . . . .   5
   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 . . . . . . . . . . . .  11
     5.1.  Segment Identifiers (SIDs) and SRGB . . . . . . . . . . .  11
       5.1.1.  Node-SID  . . . . . . . . . . . . . . . . . . . . . .  11
       5.1.2.  Adjacency-SID . . . . . . . . . . . . . . . . . . . .  11
     5.2.  Segment Routing Extension Header (SRH)  . . . . . . . . .  12
       5.2.1.  SRH and RFC2460 behavior  . . . . . . . . . . . . . .  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 . . . . . . . . . . . .  19
       6.2.4.  SR Segment Endpoint Node  . . . . . . . . . . . . . .  19
     6.3.  FRR Flag Settings . . . . . . . . . . . . . . . . . . . .  19
   7.  SR and Tunneling  . . . . . . . . . . . . . . . . . . . . . .  19

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   8.  Example Use Case  . . . . . . . . . . . . . . . . . . . . . .  20
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
     9.1.  Threat model  . . . . . . . . . . . . . . . . . . . . . .  22
       9.1.1.  Source routing threats  . . . . . . . . . . . . . . .  22
       9.1.2.  Applicability of RFC 5095 to SRH  . . . . . . . . . .  23
       9.1.3.  Service stealing threat . . . . . . . . . . . . . . .  24
       9.1.4.  Topology disclosure . . . . . . . . . . . . . . . . .  24
       9.1.5.  ICMP Generation . . . . . . . . . . . . . . . . . . .  24
     9.2.  Security fields in SRH  . . . . . . . . . . . . . . . . .  25
       9.2.1.  Selecting a hash algorithm  . . . . . . . . . . . . .  26
       9.2.2.  Performance impact of HMAC  . . . . . . . . . . . . .  26
       9.2.3.  Pre-shared key management . . . . . . . . . . . . . .  27
     9.3.  Deployment Models . . . . . . . . . . . . . . . . . . . .  27
       9.3.1.  Nodes within the SR domain  . . . . . . . . . . . . .  27
       9.3.2.  Nodes outside of the SR domain  . . . . . . . . . . .  28
       9.3.3.  SR path exposure  . . . . . . . . . . . . . . . . . .  28
       9.3.4.  Impact of BCP-38  . . . . . . . . . . . . . . . . . .  29
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
   11. Manageability Considerations  . . . . . . . . . . . . . . . .  29
   12. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  29
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  30
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     14.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32

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.

2.  Segment Routing Documents

   Segment Routing terminology is defined in
   [I-D.ietf-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].

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

   The security mechanisms of the Segment Routing Header (SRH) are
   described in [I-D.vyncke-6man-segment-routing-security].

3.  Introduction

   Segment Routing (SR), defined in [I-D.ietf-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 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.ietf-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.ietf-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.ietf-spring-segment-routing].  Mechanisms through which segment
   are known and advertised are outside the scope of this document.

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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
      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

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   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
   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).

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   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 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

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   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.

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.

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   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.

   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.

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   [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).

      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.

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5.  IPv6 Instantiation of Segment Routing

5.1.  Segment Identifiers (SIDs) and SRGB

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

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

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.

   Node SIDs are IPv6 addresses part of the SRGB (i.e.: addresses of
   global scope).

5.1.2.  Adjacency-SID

   Adjacency-SIDs can be either globally scoped IPv6 addresses or any
   128-bit identifier representing the adjacency.  Obviously, in the
   latter case, the scope of the Adjacency-SID is local to the router
   and any packet with the a such Adjacency-SID would need first to
   reach the node through the node's Node-SID prior for the node to
   process the Adjacency-SID.  In other wrods, two segments (SIDs) would
   then be required: the first is the node's Node-SID that brings the
   packet to the node and the second is the Adjacency-SID that will make
   the node to forward the packet through the interface the Adjacency-
   SID is allocated to.

   In the SR architecture defined in [I-D.ietf-spring-segment-routing]
   the Adjacency-SID (or Adj-SID) is the segment identifier associated
   with the instruction of forwarding the packet through the interface
   the Adj-SID is assigned to.  Adj-SIDs can be either globally scoped
   IPv6 addresses or any 128-bit identifier representing the adjacency.
   Obviously, in the latter case, the scope of the Adj-SID is local to
   the router and any packet with the a such Adj-SID would need first to
   reach the node through the node's Node-SID prior for the node to
   process the Adj-SID.  In other wrods, two segments (SIDs) would then
   be required: the first is the node's Node-SID that brings the packet
   to the node and the second is the Adj-SID that will make the node to
   forward the packet through the interface the Adj-SID is allocated to.

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   A node may advertise one (or more) Adj-SIDs allocated to the same
   interface as well as a node can advertise the same Adj-SID for
   multiple interfaces.  Use cases of Adj-SID advertisements are
   described in [I-D.ietf-spring-segment-routing]The semantic of the
   Adj-SID is:

      Send out the packet to the interface this Adj-SID is allocated to.

   When SR is applied to IPv6, Node-SIDs are a global IPv6 addresses and
   therefore, an Adj-SID has a global significance (i.e.: 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, (suggested value 4) to be assigned by IANA.

   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  | Segments Left |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | First Segment |             Flags             |  HMAC Key ID  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Segment List[0] (128 bits ipv6 address)            |

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    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
                                  ...
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Segment List[n] (128 bits ipv6 address)            |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Policy List[0] (optional)                          |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Policy List[1] (optional)                          |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Policy List[2] (optional)                          |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Policy List[3] (optional)                          |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
    |                                                               |
    |                       HMAC (256 bits)                         |
    |                        (optional)                             |
    |                                                               |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

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

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   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 (suggested value: 4).

   o  Segments Left.  Defined in [RFC2460], it contains the index, in
      the Segment List, of the next segment to inspect.  Segments Left
      is decremented at each segment and it is used as an index in the
      segment list.

   o  First Segment: offset in the SRH, not including the first 8 octets
      and expressed in 16-octet units, pointing to the last element of
      the segment list, which is in fact the first segment of the
      segment routing path.

   o  Flags: 16 bits of flags.  Following flags are defined:

                              1
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |C|P|R|R|    Policy Flags       |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

         R-flags.  Reserved and for future use.

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

            Bits 4-6: determine the type of the first element after the
            segment list.

            Bits 7-9: determine the type of the second element.

            Bits 10-12: determine the type of the third element.

            Bits 13-15: determine the type of the fourth element.

         The following values are used for the type:

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

            0x1: SR Ingress.

            0x2: SR Egress.

            0x3: Original Source Address.

   o  HMAC Key ID and HMAC field, and their use are defined in
      [I-D.vyncke-6man-segment-routing-security].

   o  Segment List[n]: 128 bit IPv6 addresses representing the nth
      segment in the Segment List.  The Segment List is encoded starting
      from the last segment of the path.  I.e., the first element of the
      segment list (Segment List [0]) contains the last segment of the
      path while the last segment of the Segment List (Segment List[n])
      contains the first segment of the path.  The index contained in
      "Segments Left" identifies the current active segment.

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

         SR Ingress: a 128 bit generic identifier representing the
         ingress in the SR domain (i.e.: it needs not to be a valid IPv6
         address).

         SR Egress: a 128 bit generic identifier representing the egress
         in the SR domain (i.e.: it needs not to be a valid IPv6
         address).

         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.

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.ietf-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 active segment.

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

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   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.

      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).

   The following are the steps of the creation of the SRH:

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

      Routing Type field is set as TBD (SRH).

      The Segment List is built with the FIRST segment of the path
      encoded in the LAST element of the Segment List.  Subsequent
      segments are encoded on top of the first segment.  Finally, the
      LAST segment of the path is encoded in the FIRST element of the
      Segment List.  In other words, the Segment List is encoded in the
      reverse order of the path.

      The original DA of the packet is encoded as the last segment of
      the path (encoded in the first element of the Segment List).

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      The DA of the packet is set with the value of the first segment
      (found in the last element of the segment list).

      The Segments Left field is set to n-1 where n is the number of
      elements in the Segment List.

      The First Segment field is set to n-1 where n is the number of
      elements in the Segment List.

      The packet is sent out towards the first segment (i.e.:
      represented in the packet DA).

6.2.1.1.  Security at Ingress

   The procedures related to the Segment Routing security are detailed
   in [I-D.vyncke-6man-segment-routing-security].

   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
   [I-D.vyncke-6man-segment-routing-security] 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.

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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:

   1.   IF DA = myself (segment endpoint)
   2.      IF Segments Left > 0 THEN
              decrement Segments Left
              update DA with Segment List[Segments Left]
   3.         IF Segments Left == 0 THEN
                 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.  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, 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 SA in the Policy List and in the original
   DA in the Segment List.

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8.  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.

   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: X, J, F, C
         . Segments Left: 3 (i.e.: Segment List size - 1)
         . PL: C (ingress), J (egress)
        Note that X is the last segment and C is the
        first segment (i.e.: the SL is encoded in the reverse
        path order).
      . HMAC

   . When packet arrives in C (first segment), C does:
      . Validate the HMAC of the SRH.
      . Decrement Segments Left by one: 2
      . Update the DA with the next segment found in
        Segment List[2]. DA is set to F.
      . Forward the packet to F.

   . When packet arrives in F (second segment), F does:
      . Decrement Segments Left by one: 1
      . Update the DA with the next segment found in
        Segment List[1]. DA is set to J.
      . Forward the packet 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.

   . When packet arrives in J (third segment), J does:
      . Decrement Segments Left by one: 0
      . Update the DA with the next segment found in
        Segment List[0]. DA is set to X.
      . If the cleanup bit is set, then node J will strip out
        the SRH from the packet.
      . Forward the packet to X.

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   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.

9.  Security Considerations

   This section analyzes the security threat model, the security issues
   and proposed solutions related to the new Segment Routing Header.

   The Segment Routing Header (SRH) is simply another type of the
   routing header as described in RFC 2460 [RFC2460] and is:

   o  inserted by a SR edge router when entering the segment routing
      domain or by the originating host itself.  The source host can
      even be outside the SR domain;

   o  inspected and acted upon when reaching the destination address of
      the IP header per RFC 2460 [RFC2460].

   Per RFC2460 [RFC2460], 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 content of SRH.  Routers whose one
   interface IPv6 address equals the destination address field of the
   IPv6 packet MUST to parse the SRH and, if supported and if the local
   configuration allows it, MUST act accordingly to the SRH content.

   According to RFC2460 [RFC2460], the default behavior of a non SR-
   capable router upon receipt of an IPv6 packet with SRH destined to an
   address of its, is to:

   o  ignore the SRH completely if the Segment Left field is 0 and
      proceed to process the next header in the IPv6 packet;

   o  discard the IPv6 packet if Segment Left field is greater than 0,
      it MAY send a Parameter Problem ICMP message back to the Source
      Address.

9.1.  Threat model

9.1.1.  Source routing threats

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

   o  amplification attacks: where a packet could be forged in such a
      way to cause looping among a set of SR-enabled routers causing

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      unnecessary traffic, hence a Denial of Service (DoS) 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 De-Militarized Zone) to attack
      another host (for example in the datacenter or any back-end
      server).

9.1.2.  Applicability of RFC 5095 to SRH

   First of all, the reader must remember this specific part of section
   1 of RFC5095 [RFC5095], "A side effect is that this also eliminates
   benign RH0 use-cases; however, such applications may be facilitated
   by future Routing Header specifications.".  In short, it is not
   forbidden to create new secure type of Routing Header; for example,
   RFC 6554 (RPL) [RFC6554] also creates a new Routing Header type for a
   specific application confined in a single network.

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

   o  nodes within the SR 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, man-in-the-middle
      attacks, more server DoS by dropping packets, and so on.

   o  nodes outside of the SR 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.

   The main use case for SR consists of the single administrative domain
   where only trusted nodes with SR enabled and configured participate
   in SR: this is the same model as in RFC6554 [RFC6554].  All non-
   trusted nodes do not participate as either SR processing is not
   enabled by default or because they only process SRH from nodes within
   their domain.

   Moreover, all SR nodes ignore SRH created by outsiders based on
   topology information (received on a peering or internal interface) or
   on presence and validity of the HMAC field.  Therefore, if

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   intermediate nodes ONLY act on valid and authorized SRH (such as
   within a single administrative domain), then there is no security
   threat similar to RH-0.  Hence, the RFC 5095 [RFC5095] attacks are
   not applicable.

9.1.3.  Service stealing threat

   Segment routing 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'.

9.1.4.  Topology disclosure

   The SRH may also contains IPv6 addresses of some intermediate SR-
   nodes in the path towards the destination, this obviously reveals
   those addresses to the potentially hostile attackers if those
   attackers are able to intercept packets containing SRH.  On the other
   hand, if the attacker can do a traceroute whose probes will be
   forwarded along the SR path, then there is little learned by
   intercepting the SRH itself.  Also the clean-bit of SRH can help by
   removing the SRH before forwarding the packet to potentially a non-
   trusted part of the network.

9.1.5.  ICMP Generation

   Per section 4.4 of RFC2460 [RFC2460], when destination nodes (i.e.
   where the destination address is one of theirs) receive a Routing
   Header with unsupported Routing Type, the required behavior is:

   o  If Segments Left is zero, the node must ignore the Routing header
      and proceed to process the next header in the packet.

   o  If Segments Left is non-zero, the node must discard the packet and
      send an ICMP Parameter Problem, Code 0, message to the packet's
      Source Address, pointing to the unrecognized Routing Type.

   This required behavior could be used by an attacker to force the
   generation of ICMP message by any node.  The attacker could send
   packets with SRH (with Segment Left set to 0) destined to a node not
   supporting SRH.  Per RFC2460 [RFC2460], the destination node could
   generate an ICMP message, causing a local CPU utilization and if the
   source of the offending packet with SRH was spoofed could lead to a
   reflection attack without any amplification.

   It must be noted that this is a required behavior for any unsupported
   Routing Type and not limited to SRH packets.  So, it is not specific
   to SRH and the usual rate limiting for ICMP generation is required

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   anyway for any IPv6 implementation and has been implemented and
   deployed for many years.

9.2.  Security fields in SRH

   This section summarizes the use of specific fields in the SRH.  They
   are based on a key-hashed message authentication code (HMAC).

   The security-related fields in SRH are:

   o  HMAC Key-id, 8 bits wide;

   o  HMAC, 256 bits wide (optional, exists only if HMAC Key-id is not
      0).

   The HMAC field is the output of the HMAC computation (per RFC 2104
   [RFC2104]) using a pre-shared key identified by HMAC Key-id and of
   the text which consists of the concatenation of:

   o  the source IPv6 address;

   o  First Segment field;

   o  an octet whose bit-0 is the clean-up bit flag and others are 0;

   o  HMAC Key-id;

   o  all addresses in the Segment List.

   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
   trust the upstream router.  This is to speed up forwarding operations
   because SR routers which do not validate the SRH do not need to parse
   the SRH until the end.

   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 field is

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   opaque, i.e., it has neither syntax not semantic except as an index
   to the right combination of pre-shared key and hash algorithm and
   except that a value of 0 means that there is no HMAC field.  It could
   also allow for interoperation among different SR domains if allowed
   by local policy and assuming a collision-free Key Id allocation.

   When a specific SRH is linked to a time-related service (such as
   turbo-QoS for a 1-hour period) where the DA, Segment ID (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 shared-secret expires.

9.2.1.  Selecting a hash algorithm

   The HMAC field in the SRH is 256 bit wide.  Therefore, the HMAC MUST
   be based on a hash function whose output is at least 256 bits.  If
   the output of the hash function is 256, then this output is simply
   inserted in the HMAC field.  If the output of the hash function is
   larger than 256 bits, then the output value is truncated to 256 by
   taking the least-significant 256 bits and inserting them in the HMAC
   field.

   SRH implementations can support multiple hash functions but MUST
   implement SHA-2 [FIPS180-4] in its SHA-256 variant.

   NOTE: SHA-1 is currently used by some early implementations used for
   quick interoperations testing, the 160-bit hash value must then be
   right-hand padded with 96 bits set to 0.  The authors understand that
   this is not secure but is ok for limited tests.

9.2.2.  Performance impact of HMAC

   While adding a HMAC to each and every SR packet increases the
   security, it has a performance impact.  Nevertheless, it must be
   noted that:

   o  the HMAC field is used only when SRH is inserted by a device (such
      as a home set-up box) which is outside of the segment routing
      domain.  If the SRH is added by a router in the trusted segment
      routing domain, then, there is no need for a HMAC field, hence no
      performance impact.

   o  when present, the HMAC field MUST only be checked and validated by
      the first router of the segment routing domain, this router is
      named 'validating SR router'.  Downstream routers may not inspect
      the HMAC field.

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   o  this validating router can also have a cache of <IPv6 header +
      SRH, HMAC field value> to improve the performance.  It is not the
      same use case as in IPsec where HMAC value was unique per packet,
      in SRH, the HMAC value is unique per flow.

   o  Last point, hash functions such as SHA-2 have been optmized for
      security and performance and there are multiple implementations
      with good performance.

   With the above points in mind, the performance impact of using HMAC
   is minimized.

9.2.3.  Pre-shared key management

   The field HMAC Key-id allows for:

   o  key roll-over: when there is a need to change the key (the hash
      pre-shared secret), then multiple pre-shared keys can be used
      simultaneously.  The validating routing can have a table of <HMAC
      Key-id, pre-shared secret> for the currently active and future
      keys.

   o  different algorithm: by extending the previous table to <HMAC Key-
      id, hash function, pre-shared secret>, the validating router can
      also support simultaneously several hash algorithms (see section
      Section 9.2.1)

   The pre-shared secret distribution can be done:

   o  in the configuration of the validating routers, either by static
      configuration or any SDN oriented approach;

   o  dynamically using a trusted key distribution such as [RFC6407]

   The intent of this document is NOT to define yet-another-key-
   distribution-protocol.

9.3.  Deployment Models

9.3.1.  Nodes within the SR domain

   A SR domain is defined as a set of interconnected routers where all
   routers at the perimeter are configured to insert and act on SRH.
   Some routers inside the SR domain can also act on SRH or simply
   forward IPv6 packets.

   The routers inside a SR domain can be trusted to generate SRH and to
   process SRH received on interfaces that are part of the SR domain.

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   These nodes MUST drop all SRH 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.

9.3.2.  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 trusted means (outside
   of the scope of this document) a complete SRH for each new connection
   (i.e. new destination address).  The received SRH MUST include a HMAC
   Key-id and HMAC field which is computed correctly (see Section 9.2).

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

   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.

9.3.3.  SR path exposure

   As the intermediate SR nodes addresses appears in the SRH, if this
   SRH is visible to an outsider then he/she could reuse this knowledge
   to launch an attack on the intermediate SR nodes or get some insider
   knowledge on the topology.  This is especially applicable when the
   path between the source node and the first SR domain ingress router
   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

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   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
   [RFC4303] cannot be use to protect the SRH as per RFC4303 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.

9.3.4.  Impact of BCP-38

   BCP-38 [RFC2827], also known as "Network Ingress Filtering", checks
   whether the source address of packets received on an interface is
   valid for this interface.  The use of loose source routing such as
   SRH forces packets to follow a path which differs from the expected
   routing.  Therefore, if BCP-38 was implemented in all routers inside
   the SR domain, then SR packets could be received by an interface
   which is not expected one and the packets could be dropped.

   As a SR domain is usually a subset of one administrative domain, and
   as BCP-38 is only deployed at the ingress routers of this
   administrative domain and as packets arriving at those ingress
   routers have been normally forwarded using the normal routing
   information, then there is no reason why this ingress router should
   drop the SRH packet based on BCP-38.  Routers inside the domain
   commonly do not apply BCP-38; so, this is not a problem.

10.  IANA Considerations

   TBD

11.  Manageability Considerations

   TBD

12.  Contributors

   The authors would like to thank Dave Barach, John Leddy, John
   Brzozowski, Pierre Francois, Nagendra Kumar, Mark Townsley, Christian
   Martin, Roberta Maglione, Eric Vyncke, James Connolly, David Lebrun,
   Aloys Augustin and Fred Baker for their contribution to this
   document.

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13.  Acknowledgements

   TBD

14.  References

14.1.  Normative References

   [FIPS180-4]
              National Institute of Standards and Technology, "FIPS
              180-4 Secure Hash Standard (SHS)", March 2012,
              <http://csrc.nist.gov/publications/fips/fips180-4/
              fips-180-4.pdf>.

   [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-05 (work in progress), June 2015.

   [I-D.psenak-ospf-segment-routing-ospfv3-extension]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3
              Extensions for Segment Routing", draft-psenak-ospf-
              segment-routing-ospfv3-extension-02 (work in progress),
              July 2014.

   [I-D.vyncke-6man-segment-routing-security]
              Vyncke, E., Previdi, S., and D. Lebrun, "IPv6 Segment
              Routing Security Considerations", draft-vyncke-6man-
              segment-routing-security-02 (work in progress), February
              2015.

   [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, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <http://www.rfc-editor.org/info/rfc4303>.

   [RFC5095]  Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
              of Type 0 Routing Headers in IPv6", RFC 5095,
              DOI 10.17487/RFC5095, December 2007,
              <http://www.rfc-editor.org/info/rfc5095>.

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   [RFC6407]  Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
              of Interpretation", RFC 6407, DOI 10.17487/RFC6407,
              October 2011, <http://www.rfc-editor.org/info/rfc6407>.

14.2.  Informative References

   [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-01 (work in progress),
              October 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-04 (work in progress), March 2015.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
              and R. Shakir, "Segment Routing Architecture", draft-ietf-
              spring-segment-routing-03 (work in progress), May 2015.

   [I-D.ietf-spring-segment-routing-mpls]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., Horneffer, M., Shakir, R., Tantsura, J.,
              and E. Crabbe, "Segment Routing with MPLS data plane",
              draft-ietf-spring-segment-routing-mpls-01 (work in
              progress), May 2015.

   [RFC1940]  Estrin, D., Li, T., Rekhter, Y., Varadhan, K., and D.
              Zappala, "Source Demand Routing: Packet Format and
              Forwarding Specification (Version 1)", RFC 1940,
              DOI 10.17487/RFC1940, May 1996,
              <http://www.rfc-editor.org/info/rfc1940>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February
              1997.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <http://www.rfc-editor.org/info/rfc2827>.

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   [RFC4942]  Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/
              Co-existence Security Considerations", RFC 4942,
              DOI 10.17487/RFC4942, September 2007,
              <http://www.rfc-editor.org/info/rfc4942>.

   [RFC6554]  Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
              Routing Header for Source Routes with the Routing Protocol
              for Low-Power and Lossy Networks (RPL)", RFC 6554, March
              2012.

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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   Ebben Aries
   Facebook
   US

   Email: exa@fb.com

   Eric Vyncke
   Cisco Systems, Inc.
   De Kleetlaann 6A
   Diegem  1831
   Belgium

   Email: evyncke@cisco.com

   David Lebrun
   Universite Catholique de Louvain
   Place Ste Barbe, 2
   Louvain-la-Neuve, 1348
   Belgium

   Email: david.lebrun@uclouvain.be

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