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Versions: (draft-filsfils-spring-segment-routing) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 8402

Network Working Group                                   C. Filsfils, Ed.
Internet-Draft                                           S. Previdi, Ed.
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: November 12, 2016                                   B. Decraene
                                                            S. Litkowski
                                                                  Orange
                                                               R. Shakir
                                                     Jive Communications
                                                            May 11, 2016


                      Segment Routing Architecture
                  draft-ietf-spring-segment-routing-08

Abstract

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

   Segment Routing can be directly applied to the MPLS architecture with
   no change on the forwarding plane.  A segment is encoded as an MPLS
   label.  An ordered list of segments is encoded as a stack of labels.
   The segment to process is on the top of the stack.  Upon completion
   of a segment, the related label is popped from the stack.

   Segment Routing can be applied to the IPv6 architecture, with a new
   type of routing header.  A segment is encoded as an IPv6 address.  An
   ordered list of segments is encoded as an ordered list of IPv6
   addresses in the routing header.  The active segment is indicated by
   the Destination Address of the packet.  The next active segment is
   indicated by a pointer in the new routing header.

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.




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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Companion Documents . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Link-State IGP Segments . . . . . . . . . . . . . . . . . . .   7
     3.1.  IGP Segment, IGP SID  . . . . . . . . . . . . . . . . . .   7
     3.2.  IGP-Prefix Segment, Prefix-SID  . . . . . . . . . . . . .   7
       3.2.1.  Prefix-SID Algorithm  . . . . . . . . . . . . . . . .   7
       3.2.2.  MPLS Dataplane  . . . . . . . . . . . . . . . . . . .   8
       3.2.3.  IPv6 Dataplane  . . . . . . . . . . . . . . . . . . .  10
     3.3.  IGP-Node Segment, Node-SID  . . . . . . . . . . . . . . .  10
     3.4.  IGP-Anycast Segment, Anycast SID  . . . . . . . . . . . .  11
     3.5.  IGP-Adjacency Segment, Adj-SID  . . . . . . . . . . . . .  13
       3.5.1.  Parallel Adjacencies  . . . . . . . . . . . . . . . .  15
       3.5.2.  LAN Adjacency Segments  . . . . . . . . . . . . . . .  16
     3.6.  Binding Segment . . . . . . . . . . . . . . . . . . . . .  16
       3.6.1.  Mapping Server  . . . . . . . . . . . . . . . . . . .  16
       3.6.2.  Tunnel Headend  . . . . . . . . . . . . . . . . . . .  16
     3.7.  Inter-Area Considerations . . . . . . . . . . . . . . . .  16
   4.  BGP Peering Segments  . . . . . . . . . . . . . . . . . . . .  17



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   5.  IGP Mirroring Context  Segment  . . . . . . . . . . . . . . .  18
   6.  Multicast . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  19
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     11.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   With Segment Routing (SR), a node steers a packet through an ordered
   list of instructions, called segments.  A segment can represent any
   instruction, topological or service-based.  A segment can have a
   local semantic to an SR node or global within an SR domain.  SR
   allows to enforce a flow through any path and service chain while
   maintaining per-flow state only at the ingress node of the SR domain.

   Segment Routing can be directly applied to the MPLS architecture
   ([RFC3031]) with no change on the forwarding plane.  A segment is
   encoded as an MPLS label.  An ordered list of segments is encoded as
   a stack of labels.  The active segment is on the top of the stack.  A
   completed segment is popped off the stack.  The addition of a segment
   is performed with a push.

   In the Segment Routing MPLS instantiation, a segment could be of
   several types:

   o  an IGP segment,

   o  a BGP Peering segments,

   o  an LDP LSP segment,

   o  an RSVP-TE LSP segment,

   o  a BGP LSP segment.

   The first two (IGP and BGP Peering segments) types of segments are
   defined in this document.  The use of the last three types of
   segments is illustrated in [I-D.ietf-spring-segment-routing-mpls].

   Segment Routing can be applied to the IPv6 architecture ([RFC2460]),
   with a new type of routing header.  A segment is encoded as an IPv6
   address.  An ordered list of segments is encoded as an ordered list
   of IPv6 addresses in the routing header.  The active segment is



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   indicated by the Destination Address of the packet.  Upon completion
   of a segment, a pointer in the new routing header is incremented and
   indicates the next segment.

   Numerous use-cases illustrate the benefits of source routing either
   for FRR, OAM or Traffic Engineering reasons.

   This document defines a set of instructions (called segments) that
   are required to fulfill the described use-cases.  These segments can
   either be used in isolation (one single segment defines the source
   route of the packet) or in combination (these segments are part of an
   ordered list of segments that define the source route of the packet).

1.1.  Companion Documents

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

   Use cases are described in [I-D.ietf-spring-problem-statement],
   [I-D.ietf-spring-segment-routing-central-epe],
   [I-D.ietf-spring-segment-routing-msdc],
   [I-D.filsfils-spring-large-scale-interconnect],
   [I-D.ietf-spring-ipv6-use-cases],
   [I-D.ietf-spring-resiliency-use-cases], [I-D.ietf-spring-oam-usecase]
   and [I-D.ietf-spring-sr-oam-requirement].

   Segment Routing for MPLS dataplane is documented in
   [I-D.ietf-spring-segment-routing-mpls].

   Segment Routing for IPv6 dataplane is documented in
   [I-D.ietf-6man-segment-routing-header].

   IGP protocol extensions for Segment Routing are described in
   [I-D.ietf-isis-segment-routing-extensions],
   [I-D.ietf-ospf-segment-routing-extensions] and
   [I-D.ietf-ospf-ospfv3-segment-routing-extensions] referred in this
   document as "IGP SR extensions documents".

   The FRR solution for SR is documented in
   [I-D.francois-rtgwg-segment-routing-ti-lfa].

   The PCEP protocol extensions for Segment Routing are defined in
   [I-D.ietf-pce-segment-routing].

   The interaction between SR/MPLS with other MPLS Signaling planes is
   documented in [I-D.ietf-spring-segment-routing-ldp-interop].





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2.  Terminology

   Segment: an instruction a node executes on the incoming packet (e.g.:
   forward packet according to shortest path to destination, or, forward
   packet through a specific interface, or, deliver the packet to a
   given application/service instance).

   SID: a Segment Identifier.  Examples of SIDs are: a MPLS label, an
   index value in a MPLS label space, an IPv6 address.  Other types of
   SIDs can be defined in the future.

   Segment List: ordered list of SID's encoding the topological and
   service source route of the packet.  It is a stack of labels in the
   MPLS architecture.  It is an ordered list of IPv6 addresses in the
   IPv6 architecture.

   Segment Routing Domain (SR Domain): the set of nodes participating
   into the source based routing model.  These nodes may be connected to
   the same physical infrastructure (e.g.: a Service Provider's network)
   as well as nodes remotely connected to each other (e.g.: an
   enterprise VPN or an overlay).  Note that a SR domain may also be
   confined within an IGP instance, in which case it is named SR-IGP
   Domain.

   Active segment: the segment that MUST be used by the receiving router
   to process the packet.  In the MPLS dataplane is the top label.  In
   the IPv6 dataplane is the destination address of a packet having the
   Segment Routing Header as defined in
   [I-D.ietf-6man-segment-routing-header].

   PUSH: the insertion of a segment at the head of the Segment list.

   NEXT: the active segment is completed, the next segment becomes
   active.

   CONTINUE: the active segment is not completed and hence remains
   active.  The CONTINUE instruction is implemented as the SWAP
   instruction in the MPLS dataplane.  In IPv6, this is the plain IPv6
   forwarding action of a regular IPv6 packet according to its
   Destination Address.

   SR Global Block (SRGB): local property of an SR node.  In the MPLS
   architecture, SRGB is the set of local labels reserved for global
   segments.  Using the same SRGB on all nodes within the SR domain ease
   operations and troubleshooting and is expected to be a deployment
   guideline.  In the IPv6 architecture, the equivalent of the SRGB is
   in fact the set of addresses used as global segments.  Since there
   are no restrictions on which IPv6 address can be used, the concept of



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   the SRGB includes all IPv6 global address space used within the SR
   domain.

   Global Segment: the related instruction is supported by all the SR-
   capable nodes in the domain.  In the MPLS architecture, a Global
   Segment has a globally-unique index.  The related local label at a
   given node N is found by adding the globally-unique index to the SRGB
   of node N.  In the IPv6 architecture, a global segment is a globally-
   unique IPv6 address.

   Local Segment: the related instruction is supported only by the node
   originating it.  In the MPLS architecture, this is a local label
   outside the SRGB.  In the IPv6 architecture, this can be any IPv6
   address whose reachability is not advertised in any routing protocol
   (hence, the segment is known only by the local node).

   IGP Segment: the generic name for a segment attached to a piece of
   information advertised by a link-state IGP, e.g. an IGP prefix or an
   IGP adjacency.

   IGP-prefix Segment, Prefix-SID: an IGP-Prefix Segment is an IGP
   segment attached to an IGP prefix.  An IGP-Prefix Segment is global
   (unless explicitly advertised otherwise) within the SR IGP instance/
   topology and identifies an instruction to forward the packet along
   the path computed using the algorithm field, in the topology and the
   IGP instance where it is advertised.  The Prefix-SID is the SID of
   the IGP-Prefix Segment.

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

   IGP-Adjacency: an IGP-Adjacency Segment is an IGP segment attached to
   an unidirectional adjacency or a set of unidirectional adjacencies.
   By default, an IGP-Adjacency Segment is local (unless explicitly
   advertised otherwise) to the node that advertises it.

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

   SR Tunnel: a list of segments to be pushed on the packets directed on
   the tunnel.  The list of segments can be specified explicitly or
   implicitly via a set of abstract constraints (latency, affinity,
   SRLG, ...).  In the latter case, a constraint-based path computation
   is used to determine the list of segments associated with the tunnel.
   The computation can be local or delegated to a PCE server.  An SR
   tunnel can be configured by the operator, provisioned via netconf or



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   provisioned via PCEP.  An SR tunnel can be used for traffic-
   engineering, OAM or FRR reasons.

   Segment List Depth: the number of segments of an SR tunnel.  The
   entity instantiating an SR Tunnel at a node N should be able to
   discover the depth insertion capability of the node N.  The PCEP
   discovery capability is described in [I-D.ietf-pce-segment-routing].

3.  Link-State IGP Segments

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

3.1.  IGP Segment, IGP SID

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

3.2.  IGP-Prefix Segment, Prefix-SID

   An IGP-Prefix Segment is an IGP segment attached to an IGP prefix.
   An IGP-Prefix Segment is global (unless explicitly advertised
   otherwise) within the SR/IGP domain.

   The required IGP protocol extensions are defined in IGP SR extensions
   documents.

3.2.1.  Prefix-SID Algorithm

   The IGP protocol extensions for Segment Routing define the Prefix-SID
   advertisement which includes a set of flags and the algorithm field.
   The algorithm field has the purpose of associating a given Prefix-SID
   to a routing algorithm.

   In the context of an instance and a topology, multiple Prefix-SID's
   MAY be allocated to the same IGP Prefix as long as the algorithm
   value is different in each one.

   Multiple instances and topologies are defined in IS-IS and OSPF in:
   [RFC5120], [RFC6822], [RFC6549] and [RFC4915].

   Initially, two "algorithms" have been defined:



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   o  "Shortest Path": this algorithm is the default behavior.  The
      packet is forwarded along the well known ECMP-aware SPF algorithm
      however it is explicitly allowed for a midpoint to implement
      another forwarding based on local policy.. The "Shortest Path"
      algorithm is in fact the default and current behavior of most of
      the networks where local policies may override the SPF decision.

   o  "Strict Shortest Path": This algorithm mandates that the packet is
      forwarded according to ECMP-aware SPF algorithm and instruct any
      router in the path to ignore any possible local policy overriding
      SPF decision.  The SID advertised with "Strict Shortest Path"
      algorithm ensures that the path the packet is going to take is the
      expected, and not altered, SPF path.

   An IGP-Prefix Segment identifies the path, to the related prefix,
   along the path computed as per the algorithm field.

   A packet injected anywhere within the SR/IGP domain with an active
   Prefix-SID will be forwarded along path computed by the algorithm
   expressed in the algorithm field.

   The ingress node of an SR domain validates that the path to a prefix,
   advertised with a given algorithm, includes nodes all supporting the
   advertised algorithm.  In other words, when computing paths for a
   given algorithm, the transit nodes MUST compute the algorithm X on
   the IGP topology, regardless of the support of the algorithm X by the
   nodes in that topology.  As a consequence, if a node on the path does
   not support algorithm X, the IGP-Prefix segment will be interrupted
   and will drop packet on that node.  It's the responsibility of the
   ingress node using a segment to check that all downstream nodes
   support the algorithm of the segment.

   Details of the two defined algorithms are defined in
   [I-D.ietf-isis-segment-routing-extensions],
   [I-D.ietf-ospf-segment-routing-extensions] and
   [I-D.ietf-ospf-ospfv3-segment-routing-extensions].

3.2.2.  MPLS Dataplane

   When SR is used over the MPLS dataplane:

   o  the IGP signaling extension for IGP-Prefix segment includes the
      P-Flag ([I-D.ietf-isis-segment-routing-extensions]) or the NP-Flag
      ([I-D.ietf-ospf-segment-routing-extensions]).  A Node N
      advertising a Prefix-SID SID-R for its attached prefix R unset the
      P-Flag (or NP-Flag) in order to instruct its connected neighbors
      to perform the NEXT operation while processing SID-R.  This
      behavior is equivalent to Penultimate Hop Popping in MPLS.  When



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      the flag is unset, the neighbors of N MUST perform the NEXT
      operation while processing SID-R.  When the flag is set, the
      neighbors of N MUST perform the CONTINUE operation while
      processing SID-R.

   o  A Prefix-SID is allocated in the form of an index in the SRGB (or
      as a local MPLS label) according to a process similar to IP
      address allocation.  Typically the Prefix-SID is allocated by
      policy by the operator (or NMS) and the SID very rarely changes.

   o  While SR allows to attach a local segment to an IGP prefix (using
      the L-Flag), we specifically assume that when the terms "IGP-
      Prefix Segment" and "Prefix-SID" are used, the segment is global
      (the SID is allocated from the SRGB or as an index).  This is
      consistent with all the described use-cases that require global
      segments attached to IGP prefixes.

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

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

   o  If a node N advertises Prefix-SID SID-R for a prefix R that is
      attached to N, N MUST either clear the P-Flag in the advertisement
      of SID-R, or else maintain the following FIB entry:

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

   o  A remote node M MUST maintain the following FIB entry for any
      learned Prefix-SID SID-R attached to IP prefix R:

     Incoming Active Segment: SID-R
     Ingress Operation:
        If the next-hop of R is the originator of R
        and instructed to remove the active segment: NEXT
        Else: CONTINUE
     Egress interface: the interface towards the next-hop along the
                       path computed using the algorithm advertised with
                       the SID toward prefix R.







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3.2.3.  IPv6 Dataplane

   When SR is used over the IPv6 dataplane:

   o  The Prefix-SID is the prefix itself.  No additional identifier is
      needed for Segment Routing over IPv6.

   o  Any address belonging to any of the node's prefixes can be used as
      Prefix-SIDs.

   o  An operator may want to explicitly indicate which of the node's
      prefixes can be used as Prefix-SIDs through the setting of a flag
      (e.g.: using the IGP prefix attribute defined in [RFC7794]) in the
      routing protocol used for advertising the prefix.

   o  A global SID is instantiated through any globally advertised IPv6
      address.

   o  A local SID is instantiated through a local IPv6 prefix not being
      advertised and therefore known only by the local node.

   A node N advertising an IPv6 address R usable as a segment identifier
   MUST maintain the following FIB entry:

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

   Regardless Segment Routing, any remote IPv6 node will maintain a
   plain IPv6 FIB entry for any prefix, no matter if they represent a
   segment or not.

3.3.  IGP-Node Segment, Node-SID

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

   A "Node Segment" or "Node-SID" is fundamental to SR.  From anywhere
   in the network, it enforces the ECMP-aware shortest-path forwarding
   of the packet towards the related node.

   An IGP Node-SID MUST NOT be associated with a prefix that is owned by
   more than one router within the same routing domain.






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3.4.  IGP-Anycast Segment, Anycast SID

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

   An "Anycast Segment" or "Anycast SID" enforces the ECMP-aware
   shortest-path forwarding towards the closest node of the anycast set.
   This is useful to express macro-engineering policies or protection
   mechanisms.

   An IGP-Anycast Segment MUST NOT reference a particular node.

   Within an anycast group, all routers MUST advertise the same prefix
   with the same SID value.

                               +--------------+
                               |   Group A    |
                               |192.0.2.10/32 |
                               |    SID:100   |
                               |              |
                        +-----------A1---A3----------+
                        |      |    | \ / |   |      |
             SID:10     |      |    |  /  |   |      |     SID:30
       203.0.113.1/32   |      |    | / \ |   |      |  203.0.113.3/32
               PE1------R1----------A2---A4---------R3------PE3
                 \     /|      |              |      |\     /
                  \   / |      +--------------+      | \   /
                   \ /  |                            |  \ /
                    /   |                            |   /
                   / \  |                            |  / \
                  /   \ |      +--------------+      | /   \
                 /     \|      |              |      |/     \
               PE2------R2----------B1---B3----+----R4------PE4
       203.0.113.2/32   |      |    | \ / |   |      | 203.0.113.4/32
             SID:20     |      |    |  /  |   |      |     SID:40
                        |      |    | / \ |   |      |
                        +-----+-----B2---B4----+-----+
                               |              |
                               |   Group B    |
                               | 192.0.2.1/32 |
                               |    SID:200   |
                               +--------------+

                           Transit device groups

   The figure above describes a network example with two groups of
   transit devices.  Group A consists of devices {A1, A2, A3 and A4}.



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   They are all provisioned with the anycast address 192.0.2.10/32 and
   the anycast SID 100.

   Similarly, group B consists of devices {B1, B2, B3 and B4} and are
   all provisioned with the anycast address 192.0.2.1/32, anycast SID
   200.  In the above network topology, each PE device is connected to
   two routers in each of the groups A and B.

   PE1 can choose a particular transit device group when sending traffic
   to PE3 or PE4.  This will be done by pushing the anycast SID of the
   group in the stack.

   Processing the anycast, and subsequent segments, requires special
   care.

   Obviously, the value of the SID following the anycast SID MUST be
   understood by all nodes advertising the same anycast segment.

                         +-------------------------+
                         |       Group A           |
                         |     192.0.2.10/32       |
                         |        SID:100          |
                         |-------------------------|
                         |                         |
                         |   SRGB:         SRGB:   |
      SID:10             |(1000-2000)   (3000-4000)|             SID:30
        PE1---+       +-------A1-------------A3-------+       +---PE3
               \     /   |    | \           / |    |   \     /
                \   /    |    |  +-----+   /  |    |    \   /
         SRGB:   \ /     |    |         \ /   |    |     \ /   SRGB:
      (7000-8000) R1     |    |          \    |    |      R3 (6000-7000)
                 / \     |    |         / \   |    |     / \
                /   \    |    |  +-----+   \  |    |    /   \
               /     \   |    | /           \ |    |   /     \
        PE2---+       +-------A2-------------A4-------+       +---PE4
      SID:20             |   SRGB:         SRGB:   |             SID:40
                         |(2000-3000)   (4000-5000)|
                         |                         |
                         +-------------------------+

                     Transit paths via anycast group A

   Considering a MPLS deployment, in the above topology, if device PE1
   (or PE2) requires to send a packet to the device PE3 (or PE4) it
   needs to encapsulate the packet in a MPLS payload with the following
   stack of labels.

   o  Label allocated by R1 for anycast SID 100 (outer label).



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   o  Label allocated by the nearest router in group A for SID 30 (for
      destination PE3).

   While the first label is easy to compute, in this case since there
   are more than one topologically nearest devices (A1 and A2), unless
   A1 and A2 allocated the same label value to the same prefix,
   determining the second label is impossible.  Devices A1 and A2 may be
   devices from different hardware vendors.  If both don't allocate the
   same label value for SID 30, it is impossible to use the anycast
   group "A" as a transit anycast group towards PE3.  Hence, PE1 (or
   PE2) cannot compute an appropriate label stack to steer the packet
   exclusively through the group A devices.  Same holds true for devices
   PE3 and PE4 when trying to send a packet to PE1 or PE2.

   To ease the use of anycast segment in a short term, it is recommended
   to configure the same SRGB on all nodes of a particular anycast
   group.  Using this method, as mentioned above, computation of the
   label following the anycast segment is straightforward.

   Using anycast segment without configuring the same SRGB on nodes
   belonging to the same device group may lead to misrouting (in a MPLS
   VPN deployment, some traffic may leak between VPNs).

3.5.  IGP-Adjacency Segment, Adj-SID

   An IGP-Adjacency Segment is an IGP segment attached to a
   unidirectional adjacency or a set of unidirectional adjacencies.  By
   default, an IGP-Adjacency Segment is local to the node which
   advertises it.  However, an Adjacency Segment can be global if
   advertised by the IGP as such.  The SID of the IGP-Adjacency Segment
   is called the Adj-SID.

   The adjacency is formed by the local node (i.e., the node advertising
   the adjacency in the IGP) and the remote node (i.e., the other end of
   the adjacency).  The local node MUST be an IGP node.  The remote node
   MAY be an adjacent IGP neighbor or a non-adjacent neighbor (e.g.: a
   Forwarding Adjacency, [RFC4206]).

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

   Similarly, when using a global Adj-SID, a packet injected anywhere
   within the SR domain with a segment list {SNL}, where SNL is a global



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   Adj-SID attached by node N to its adjacency over link L, will be
   forwarded along the shortest-path to N and then be switched by N,
   without any IP shortest-path consideration, towards link L.  If the
   Adj-SID identifies a set of adjacencies, then the node N load-
   balances the traffic among the various members of the set.  The use
   of global Adj-SID allows to reduce the size of the segment list when
   expressing a path at the cost of additional state (i.e.: the global
   Adj-SID will be inserted by all routers within the area in their
   forwarding table).

   An "IGP Adjacency Segment" or "Adj-SID" enforces the switching of the
   packet from a node towards a defined interface or set of interfaces.
   This is key to theoretically prove that any path can be expressed as
   a list of segments.

   The encodings of the Adj-SID include the B-flag.  When set, the Adj-
   SID refers to an adjacency that is eligible for protection (e.g.:
   using IPFRR or MPLS-FRR).

   The encodings of the Adj-SID include the L-flag.  When set, the Adj-
   SID has local significance.  By default the L-flag is set.

   A node SHOULD allocate one Adj-SIDs for each of its adjacencies.

   A node MAY allocate multiple Adj-SIDs to the same adjacency.  An
   example is where the adjacency is established over a bundle
   interface.  Each bundle member MAY have its own Adj-SID.

   A node MAY allocate the same Adj-SID to multiple adjacencies.

   Adjacency suppression MUST NOT be performed by the IGP.

   A node MUST install a FIB entry for any Adj-SID of value V attached
   to data-link L:

      Incoming Active Segment: V
      Operation: NEXT
      Egress Interface: L

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








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3.5.1.  Parallel Adjacencies

   Adj-SIDs can be used in order to represent a set of parallel
   interfaces between two adjacent routers.

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

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

   When parallel adjacencies are used and associated to the same Adj-
   SID, and in order to optimize the load balancing function, a "weight"
   factor can be associated to the Adj-SID advertised with each
   adjacency.  The weight tells the ingress (or a SDN/orchestration
   system) about the loadbalancing factor over the parallel adjacencies.
   As shown in Figure 1, A and B are connected through two parallel
   adjacencies

                                  link-1
                                +--------+
                                |        |
                            S---A        B---C
                                |        |
                                +--------+
                                  link-2

                   Figure 1: Parallel Links and Adj-SIDs

   Node A advertises following Adj-SIDs and weights:

   o  Link-1: Adj-SID 1000, weight: 1

   o  Link-2: Adj-SID 1000, weight: 2

   Node S receives the advertisements of the parallel adjacencies and
   understands that by using Adj-SID 1000 node A will loadbalance the
   traffic across the parallel links (link-1 and link-2) according to a
   1:2 ratio.

   The weight value is advertised with the Adj-SID as defined in IGP SR
   extensions documents.








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3.5.2.  LAN Adjacency Segments

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

   The difficulty with LANs is that each router only advertises its
   connectivity to the DR/DIS and not to each other individual nodes in
   the LAN.  Therefore, additional protocol mechanisms (IS-IS and OSPF)
   are necessary in order for each router in the LAN to advertise an
   Adj-SID associated to each neighbor in the LAN.  These extensions are
   defined in IGP SR extensions documents.

3.6.  Binding Segment

3.6.1.  Mapping Server

   A Remote-Binding SID S advertised by the mapping server M for remote
   prefix R attached to non-SR-capable node N signals the same
   information as if N had advertised S as a Prefix-SID.  Further
   details are described in the SR/LDP interworking procedures
   ([I-D.ietf-spring-segment-routing-ldp-interop].

   The segment allocation and SRGB Maintenance rules are the same as
   those defined for Prefix-SID.

3.6.2.  Tunnel Headend

   The segment allocation and SRGB Maintenance rules are the same as
   those defined for Adj-SID.  A tunnel attached to a head-end H acts as
   an adjacency attached to H.

   Note: an alternative consists of representing tunnels as forwarding-
   adjacencies ( [RFC4206]).  In such case, the tunnel is presented to
   the routing area as a routing adjacency and is considered as such by
   all area routers.  The Remote-Binding SID is preferred as it allows
   to advertise the presence of a tunnel without influencing the LSDB
   and the SPF computation.

3.7.  Inter-Area Considerations

   In the following example diagram we assume an IGP deployed using
   areas and where SR has been deployed.






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                 !          !
                 !          !
          B------C-----F----G-----K
         /       |          |     |
   S---A/        |          |     |
        \        |          |     |
         \D------I----------J-----L----Z (192.0.2.1/32, Node-SID: 150)
                 !          !
         Area-1  ! Backbone ! Area 2
                 !   area   !

                   Figure 2: Inter-Area Topology Example

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

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

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

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

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

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

4.  BGP Peering Segments

   In the context of BGP Egress Peer Engineering (EPE), as described in
   [I-D.ietf-spring-segment-routing-central-epe], an EPE enabled Egress
   PE node MAY advertise segments corresponding to its attached peers.
   These segments are called BGP peering segments or BGP Peering SIDs.
   They enable the expression of source-routed inter-domain paths.

   An ingress border router of an AS may compose a list of segments to
   steer a flow along a selected path within the AS, towards a selected
   egress border router C of the AS and through a specific peer.  At
   minimum, a BGP Peering Engineering policy applied at an ingress PE



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   involves two segments: the Node SID of the chosen egress PE and then
   the BGP Peering Segment for the chosen egress PE peer or peering
   interface.

   Hereafter, we will define three types of BGP peering segments/SID's:
   PeerNodeSID, PeerAdjSID and PeerSetSID.

   o  PeerNode SID.  A BGP PeerNode segment/SID is a local segment.  At
      the BGP node advertising it, its semantics is:

      *  SR header operation: NEXT.

      *  Next-Hop: the connected peering node to which the segment is
         related.

   o  PeerAdj SID: A BGP PeerAdj segment/SID is a local segment.  At the
      BGP node advertising it, its semantics is:

      *  SR header operation: NEXT.

      *  Next-Hop: the peer connected through the interface to which the
         segment is related.

   o  PeerSet SID.  A BGP PeerSet segment/SID is a local segment.  At
      the BGP node advertising it, its semantics is:

      *  SR header operation: NEXT.

      *  Next-Hop: loadbalance across any connected interface to any
         peer in the related group.

      A peer set could be all the connected peers from the same AS or a
      subset of these.  A group could also span across AS.  The group
      definition is a policy set by the operator.

   The BGP extensions necessary in order to signal these BGP peering
   segments will be defined in a separate document.

5.  IGP Mirroring Context Segment

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






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   When a given node B wants to provide egress node A protection, it
   advertises a segment identifying node's A context.  Such segment is
   called "Mirror Context Segment" and identified by the Mirror SID.

   The Mirror SID is advertised using the Binding Segment defined in SR
   IGP protocol extensions ( [I-D.ietf-isis-segment-routing-extensions],
   [I-D.ietf-ospf-segment-routing-extensions] and
   [I-D.ietf-ospf-ospfv3-segment-routing-extensions]).

   In the event of a failure, a point of local repair (PLR) diverting
   traffic from A to B does a PUSH of the Mirror SID on the protected
   traffic.  B, when receiving the traffic with the Mirror SID as the
   active segment, uses that segment and process underlying segments in
   the context of A.

6.  Multicast

   Segment Routing is defined for unicast.  The application of the
   source-route concept to Multicast is not in the scope of this
   document.

7.  IANA Considerations

   This document does not require any action from IANA.

8.  Security Considerations

   This document doesn't introduce new security considerations when
   applied to the MPLS dataplane.

   There are a number of security concerns with source routing at the
   IPv6 dataplane [RFC5095].  The new IPv6-based segment routing header
   defined in [I-D.ietf-6man-segment-routing-header] and its associated
   security measures address these concerns.  The IPv6 Segment Routing
   Header is defined in a way that blind attacks are never possible,
   i.e., attackers will be unable to send source routed packets that get
   successfully processed, without being part of the negations for
   setting up the source routes or being able to eavesdrop legitimate
   source routed packets.  In some networks this base level security may
   be complemented with other mechanisms, such as packet filtering,
   cryptographic security, etc.

9.  Contributors

   The following people have substantially contributed to the definition
   of the Segment Routing architecture and to the editing of this
   document:




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   Ahmed Bashandy
   Cisco Systems, Inc.
   Email: bashandy@cisco.com

   Martin Horneffer
   Deutsche Telekom
   Email: Martin.Horneffer@telekom.de

   Wim Henderickx
   Alcatel-Lucent
   Email: wim.henderickx@alcatel-lucent.com

   Jeff Tantsura
   Ericsson
   Email: Jeff.Tantsura@ericsson.com

   Edward Crabbe
   Individual
   Email: edward.crabbe@gmail.com

   Igor Milojevic
   Email: milojevicigor@gmail.com

   Saku Ytti
   TDC
   Email: saku@ytti.fi

10.  Acknowledgements

   We would like to thank Dave Ward, Dan Frost, Stewart Bryant, Pierre
   Francois, Thomas Telkamp, Les Ginsberg, Ruediger Geib, Hannes
   Gredler, Pushpasis Sarkar, Eric Rosen and Chris Bowers for their
   comments and review of this document.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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>.





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   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <http://www.rfc-editor.org/info/rfc3031>.

   [RFC4206]  Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
              Hierarchy with Generalized Multi-Protocol Label Switching
              (GMPLS) Traffic Engineering (TE)", RFC 4206,
              DOI 10.17487/RFC4206, October 2005,
              <http://www.rfc-editor.org/info/rfc4206>.

11.2.  Informative References

   [I-D.filsfils-spring-large-scale-interconnect]
              Filsfils, C., Cai, D., Previdi, S., Henderickx, W.,
              Shakir, R., Cooper, D., Ferguson, F., Lin, S., Laberge,
              T., Decraene, B., Jalil, L., and J. Tantsura,
              "Interconnecting Millions Of Endpoints With Segment
              Routing", draft-filsfils-spring-large-scale-
              interconnect-02 (work in progress), April 2016.

   [I-D.francois-rtgwg-segment-routing-ti-lfa]
              Francois, P., Filsfils, C., Bashandy, A., and B. Decraene,
              "Topology Independent Fast Reroute using Segment Routing",
              draft-francois-rtgwg-segment-routing-ti-lfa-00 (work in
              progress), August 2015.

   [I-D.ietf-6man-segment-routing-header]
              Previdi, S., Filsfils, C., Field, B., Leung, I., Linkova,
              J., Kosugi, T., Vyncke, E., and D. Lebrun, "IPv6 Segment
              Routing Header (SRH)", draft-ietf-6man-segment-routing-
              header-01 (work in progress), March 2016.

   [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-06 (work in progress), December 2015.

   [I-D.ietf-ospf-ospfv3-segment-routing-extensions]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPFv3
              Extensions for Segment Routing", draft-ietf-ospf-ospfv3-
              segment-routing-extensions-05 (work in progress), March
              2016.






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   [I-D.ietf-ospf-segment-routing-extensions]
              Psenak, P., Previdi, S., Filsfils, C., Gredler, H.,
              Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", draft-ietf-ospf-segment-
              routing-extensions-08 (work in progress), April 2016.

   [I-D.ietf-pce-segment-routing]
              Sivabalan, S., Medved, J., Filsfils, C., Crabbe, E.,
              Lopez, V., Tantsura, J., Henderickx, W., and J. Hardwick,
              "PCEP Extensions for Segment Routing", draft-ietf-pce-
              segment-routing-07 (work in progress), March 2016.

   [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-06 (work in progress), March 2016.

   [I-D.ietf-spring-oam-usecase]
              Geib, R., Filsfils, C., Pignataro, C., and N. Kumar, "A
              Scalable and Topology-Aware MPLS Dataplane Monitoring
              System", draft-ietf-spring-oam-usecase-03 (work in
              progress), April 2016.

   [I-D.ietf-spring-problem-statement]
              Previdi, S., Filsfils, C., Decraene, B., Litkowski, S.,
              Horneffer, M., and R. Shakir, "SPRING Problem Statement
              and Requirements", draft-ietf-spring-problem-statement-08
              (work in progress), April 2016.

   [I-D.ietf-spring-resiliency-use-cases]
              Francois, P., Filsfils, C., Decraene, B., and R. Shakir,
              "Use-cases for Resiliency in SPRING", draft-ietf-spring-
              resiliency-use-cases-03 (work in progress), April 2016.

   [I-D.ietf-spring-segment-routing-central-epe]
              Filsfils, C., Previdi, S., Ginsburg, D., and D. Afanasiev,
              "Segment Routing Centralized BGP Peer Engineering", draft-
              ietf-spring-segment-routing-central-epe-01 (work in
              progress), March 2016.

   [I-D.ietf-spring-segment-routing-ldp-interop]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B., and
              S. Litkowski, "Segment Routing interworking with LDP",
              draft-ietf-spring-segment-routing-ldp-interop-01 (work in
              progress), April 2016.





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   [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-04 (work in
              progress), March 2016.

   [I-D.ietf-spring-segment-routing-msdc]
              Filsfils, C., Previdi, S., Mitchell, J., and P. Lapukhov,
              "BGP-Prefix Segment in large-scale data centers", draft-
              ietf-spring-segment-routing-msdc-01 (work in progress),
              April 2016.

   [I-D.ietf-spring-sr-oam-requirement]
              Kumar, N., Pignataro, C., Akiya, N., Geib, R., Mirsky, G.,
              and S. Litkowski, "OAM Requirements for Segment Routing
              Network", draft-ietf-spring-sr-oam-requirement-01 (work in
              progress), December 2015.

   [RFC4915]  Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, DOI 10.17487/RFC4915, June 2007,
              <http://www.rfc-editor.org/info/rfc4915>.

   [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>.

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,
              <http://www.rfc-editor.org/info/rfc5120>.

   [RFC6549]  Lindem, A., Roy, A., and S. Mirtorabi, "OSPFv2 Multi-
              Instance Extensions", RFC 6549, DOI 10.17487/RFC6549,
              March 2012, <http://www.rfc-editor.org/info/rfc6549>.

   [RFC6822]  Previdi, S., Ed., Ginsberg, L., Shand, M., Roy, A., and D.
              Ward, "IS-IS Multi-Instance", RFC 6822,
              DOI 10.17487/RFC6822, December 2012,
              <http://www.rfc-editor.org/info/rfc6822>.

   [RFC7794]  Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
              U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
              and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794,
              March 2016, <http://www.rfc-editor.org/info/rfc7794>.



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Authors' Addresses

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

   Email: cfilsfil@cisco.com


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

   Email: sprevidi@cisco.com


   Bruno Decraene
   Orange
   FR

   Email: bruno.decraene@orange.com


   Stephane Litkowski
   Orange
   FR

   Email: stephane.litkowski@orange.com


   Rob Shakir
   Jive Communications, Inc.
   1275 West 1600 North, Suite 100
   Orem, UT  84057

   Email: rjs@rob.sh












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