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

Network Working Group                                   C. Filsfils, Ed.
Internet-Draft                                           S. Previdi, Ed.
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: December 22, 2017                                   B. Decraene
                                                            S. Litkowski
                                                                  Orange
                                                               R. Shakir
                                                            Google, Inc.
                                                           June 20, 2017


                      Segment Routing Architecture
                  draft-ietf-spring-segment-routing-12

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 semantic local 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 nodes 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) 2017 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 . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Link-State IGP Segments . . . . . . . . . . . . . . . . . . .   7
     3.1.  IGP-Prefix Segment, Prefix-SID  . . . . . . . . . . . . .   7
       3.1.1.  Prefix-SID Algorithm  . . . . . . . . . . . . . . . .   7
       3.1.2.  MPLS Dataplane  . . . . . . . . . . . . . . . . . . .   8
       3.1.3.  IPv6 Dataplane  . . . . . . . . . . . . . . . . . . .   9
     3.2.  IGP-Node Segment, Node-SID  . . . . . . . . . . . . . . .  10
     3.3.  IGP-Anycast Segment, Anycast SID  . . . . . . . . . . . .  10
     3.4.  IGP-Adjacency Segment, Adj-SID  . . . . . . . . . . . . .  13
       3.4.1.  Parallel Adjacencies  . . . . . . . . . . . . . . . .  14
       3.4.2.  LAN Adjacency Segments  . . . . . . . . . . . . . . .  15
     3.5.  Binding Segment . . . . . . . . . . . . . . . . . . . . .  15
       3.5.1.  Mapping Server  . . . . . . . . . . . . . . . . . . .  15
       3.5.2.  Tunnel Head-end . . . . . . . . . . . . . . . . . . .  16
     3.6.  Inter-Area Considerations . . . . . . . . . . . . . . . .  16
   4.  BGP Peering Segments  . . . . . . . . . . . . . . . . . . . .  17
   5.  IGP Mirroring Context  Segment  . . . . . . . . . . . . . . .  18



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   6.  Multicast . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     8.1.  MPLS Data Plane . . . . . . . . . . . . . . . . . . . . .  19
     8.2.  IPv6 Data Plane . . . . . . . . . . . . . . . . . . . . .  20
   9.  Manageability Considerations  . . . . . . . . . . . . . . . .  21
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  23
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  23
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     12.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

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
   semantic local 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 segment,

   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



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   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.  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
   ([I-D.ietf-spring-oam-usecase]).

   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.

   The problem statement and requirements are described in [RFC7855].

   Use cases are described in .[I-D.ietf-spring-ipv6-use-cases],
   [I-D.ietf-spring-resiliency-use-cases] and
   [I-D.ietf-spring-oam-usecase].

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: an MPLS label, an
   index value in an MPLS label space, an IPv6 address.  Other types of
   SIDs can be defined in the future.

   Segment List: ordered list of SIDs encoding the ordered set of
   instructions to be applied to the packet as it traverses the SR
   domain.  For example, the topological and service source route of the
   packet.  The Segment List is instantiated as a stack of labels in the
   MPLS architecture and as 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



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   network).  They may as well be remotely connected to each other
   (e.g.: an enterprise VPN or an overlay).  Note that an 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 it is the top label.
   In the IPv6 dataplane it is the destination address of a packet
   having the Segment Routing Header (SRH) as defined in
   [I-D.ietf-6man-segment-routing-header].

   PUSH: the instruction consisting of the insertion of a segment at the
   top of the segment list.  In the MPLS dataplane the top of the
   segment list is the topmost (outer) label of the label stack.  In the
   IPv6 dataplane, the top of the segment list is represented by the
   first segment in the Segment Routing Header as defined in
   [I-D.ietf-6man-segment-routing-header].

   NEXT: when the active segment is completed, NEXT is the instruction
   consisting of the inspection of the next segment.  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
   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 is represented by 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



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   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: an IGP-Prefix Segment is an IGP Segment
   representing 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 routing algorithm specified in the algorithm
   field, in the topology and the IGP instance where it is advertised.
   Also referred to as Prefix Segment.

   Prefix SID: the SID of the IGP-Prefix Segment.

   IGP-Anycast Segment: an IGP-Anycast Segment is an IGP-Prefix Segment
   which identify an anycast prefix advertised by a set of routers.

   Anycast-SID: the SID of the IGP-Anycast Segment.

   IGP-Adjacency Segment: 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 (unless
   explicitly advertised otherwise) to the node that advertises it.
   Also referred to as Adjacency Segment.

   Adj-SID: the SID of the IGP-Adjacency Segment.

   IGP-Node Segment: an IGP-Node Segment is an IGP-Prefix Segment which
   identifies a specific router (e.g., a loopback).  Also referred to as
   Node Segment.

   Node-SID: the SID of the IGP-Node Segment.

   Note that for all of the above, the SID is often used to refer to the
   Segment itself.  For example, Prefix-SID is sometimes used to refer
   to Prefix Segment.

   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 path
   throughout the IGP domain.  Such a path is either expressed as a
   single IGP segment or a list of multiple IGP segments.

   IGP segments require extensions in link-state IGP protocols.  IGP
   extensions are required in order to advertise the IGP segments.

3.1.  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.

3.1.1.  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:

   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.



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   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,
   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.

   A router MUST drop any SR traffic associated with the SR algorithm to
   the adjacent router, if the adjacent router has not advertised
   support for such SR algorithm.

   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.  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.

   It has to be noted that Fast Reroute (FRR) mechanisms are still
   compliant with the Strict-SPF.  In other words, a packet received
   with a Strict-SPF SID may be rerouted through a FRR mechanism.

   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].

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



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      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, 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.

3.1.3.  IPv6 Dataplane

   When SR is used over the IPv6 dataplane:





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   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.

3.2.  IGP-Node Segment, Node-SID

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

3.3.  IGP-Anycast Segment, Anycast SID

   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.







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                               +--------------+
                               |   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   |
                               +--------------+

                      Figure 1: 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}.
   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.





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   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)|
                         |                         |
                         +-------------------------+

                Figure 2: Transit paths via anycast group A

   Considering an 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 an MPLS payload with the following
   stack of labels.

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

   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.




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   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 an MPLS
   VPN deployment, some traffic may leak between VPNs).

3.4.  IGP-Adjacency Segment, 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
   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 does load-
   balance 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 a set of flags among which
   there is 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 also include the L-flag.  When set, the
   Adj-SID has local significance.  By default, the L-flag is set.



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   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.

   Obviously, in order to be able to advertise in the IGP all the Adj-
   SIDs representing the IGP adjacencies between two nodes, parallel
   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
      Ingress 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 the SPF algorithm.

3.4.1.  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: load-balance 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 load-balancing factor over the parallel
   adjacencies.  As shown in Figure 3, A and B are connected through two
   parallel adjacencies








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

                   Figure 3: 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 load-balance 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.

3.4.2.  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.

3.5.  Binding Segment

3.5.1.  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].



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

3.5.2.  Tunnel Head-end

   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.

3.6.  Inter-Area Considerations

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

   The example here below assumes the IPv6 control plane with the MPLS
   dataplane.

               !          !
               !          !
        B------C-----F----G-----K
       /       |          |     |
 S---A/        |          |     |
      \        |          |     |
       \D------I----------J-----L----Z (2001:DB8::2:1/128, Node-SID 150)
               !          !
       Area-1  ! Backbone ! Area 2
               !   area   !

                   Figure 4: Inter-Area Topology Example

   In area 2, node Z allocates Node-SID 150 to his local IPv6 prefix
   2001:DB8::2:1/128.

   ABRs G and J will propagate the prefix and its SIDs 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
   2001:DB8::2:1/128 with Prefix-SID 150 and advertised by nodes C and
   I.



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   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 2001:DB8::2:1/128, 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
   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/SIDs:
   PeerNode SID, PeerAdj SID and PeerSet SID.

   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, the semantic is:

      *  SR header operation: NEXT.

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



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

      *  SR header operation: NEXT.

      *  Next-Hop: load-balance 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.

   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 processes 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.








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7.  IANA Considerations

   This document does not require any action from IANA.

8.  Security Considerations

   Segment Routing is applicable to both MPLS and IPv6 data planes.

   Segment Routing adds some meta-data (instructions) on the packet,
   with the list of forwarding path elements (e.g.: nodes, links,
   services, etc.) that the packet must traverse.  It has to be noted
   that the complete source routed path may be represented by a single
   segment.  This is the case of the Binding SID.

8.1.  MPLS Data Plane

   When applied to the MPLS data plane, Segment Routing does not
   introduce any new behavior or any change in the way MPLS data plane
   works.  Therefore, from a security standpoint, this document does not
   define any additional mechanism in the MPLS data plane.

   SR allows the expression of a source routed path using a single
   segment (the Binding SID).  Compared to RSVP-TE which also provides
   explicit routing capability, there are no fundamental differences in
   term of information provided.  Both RSVP-TE and Segment Routing may
   express a source routed path using a single segment.

   When a path is expressed using a single label, the syntax of the
   meta-data is equivalent between RSVP-TE and SR.

   When a source routed path is expressed with a list of segments
   additional meta-data is added to the packet consisting of the source
   routed path the packet must follow expressed as a segment list.

   When a path is expressed using a label stack, if one has access to
   the meaning (i.e.: the Forwarding Equivalence Class) of the labels,
   one has the knowledge of the explicit path.  For the MPLS data plane,
   as no data plane modification is required, there is no fundamental
   change of capability.  Yet, the occurrence of label stacking will
   increase.

   From a network protection standpoint, there is an assumed trust model
   such that any node imposing a label stack on a packet is assumed to
   be allowed to do so.  This is a significant change compared to plain
   IP offering shortest path routing but not fundamentally different
   compared to existing techniques providing explicit routing capability
   such as RSVP-TE.  By default, the explicit routing information MUST
   NOT be leaked through the boundaries of the administered domain.



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   Segment Routing extensions that have been defined in various
   protocols, leverage the security mechanisms of these protocols such
   as encryption, authentication, filtering, etc.

   In the general case, a segment routing capable router accepts and
   install labels, only if these labels have been previously advertised
   by a trusted source.  The received information is validated using
   existing control plane protocols providing authentication and
   security mechanisms.  Segment Routing does not define any additional
   security mechanism in existing control plane protocols.

   Segment Routing does not introduce signaling between the source and
   the mid points of a source routed path.  With SR, the source routed
   path is computed using SIDs previously advertised in the IP control
   plane.  Therefore, in addition to filtering and controlled
   advertisement of SIDs at the boundaries of the SR domain, filtering
   in the data plane is also required.  Filtering MUST be performed on
   the forwarding plane at the boundaries of the SR domain and may
   require looking at multiple labels/instruction.

   For the MPLS data plane, there are no new requirement as the existing
   MPLS architecture already allows such source routing by stacking
   multiple labels.  And for security protection, [RFC4381] section 2.4
   and [RFC5920] section 8.2 already calls for the filtering of MPLS
   packets on trust boundaries.

8.2.  IPv6 Data Plane

   When applied to the IPv6 data plane, Segment Routing does introduce
   the Segment Routing Header (SRH,
   [I-D.ietf-6man-segment-routing-header]) which is a type of Routing
   Extension header as defined in [RFC2460].

   The SRH adds some meta-data on the IPv6 packet, with the list of
   forwarding path elements (e.g.: nodes, links, services, etc.) that
   the packet must traverse and that are represented by IPv6 addresses.
   A complete source routed path may be encoded in the packet using a
   single segment (single IPv6 address).

   From a network protection standpoint, there is an assumed trust model
   such that any node adding an SRH to the packet is assumed to be
   allowed to do so.  Therefore, by default, the explicit routing
   information MUST NOT be leaked through the boundaries of the
   administered domain.  Segment Routing extensions that have been
   defined in various protocols, leverage the security mechanisms of
   these protocols such as encryption, authentication, filtering, etc.





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   In the general case, an SR IPv6 router accepts and install segments
   identifiers (in the form of IPv6 addresses), only if these SIDs are
   advertised by a trusted source.  The received information is
   validated using existing control plane protocols providing
   authentication and security mechanisms.  Segment Routing does not
   define any additional security mechanism in existing control plane
   protocols.

   In addition, SR domain boundary routers, by default, MUST apply data
   plane filters so to only accept packets whose DA and SRH (if any)
   contain addresses previously advertised as SIDs.

   There are a number of security concerns with source routing at the
   IPv6 data plane [RFC5095].  The new IPv6-based segment routing header
   is defined in [I-D.ietf-6man-segment-routing-header].  This document
   also discusses the above security concerns.

9.  Manageability Considerations

   In SR enabled networks, the path the packet takes is encoded in the
   header.  As the path is not signaled through a protocol, OAM
   mechanisms are necessary in order for the network operator to
   validate the effectiveness of a path as well as to check and monitor
   its liveness and performance.  However, it has to be noted that SR
   allows to reduce substantially the number of states in transit nodes
   and hence the number of elements that a transit node has to manage is
   smaller.

   SR OAM use cases and requirements for the MPLS data plane are defined
   in [I-D.ietf-spring-oam-usecase] and
   [I-D.ietf-spring-sr-oam-requirement].  SR OAM procedures for the MPLS
   data plane are defined in [I-D.ietf-mpls-spring-lsp-ping].

   SR routers receive advertisements of SIDs (index, label or IPv6
   address) from the different routing protocols being extended for SR.
   Each of these protocols have monitoring and troubleshooting
   mechanisms to provide operation and management functions for IP
   addresses that MUST be extended in order to include troubleshooting
   and monitoring functions of the SID.

   SR architecture introduces the usage of global segments.  Each global
   segment must be bound to a globally-unique index or address.  The
   management of the allocation of such index or address by the operator
   is critical for the network behavior to avoid situations like mis-
   routing.  In addition to the allocation policy/tooling that the
   operator will have in place, an implementation SHOULD protect the
   network in case of conflict detection by providing a deterministic
   resolution approach.



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   An operator may implement tools in order to audit the network and
   ensure the good allocation of indexes, SIDs or IP addresses.
   Conflict detection between SIDs, including Mapping Server binding
   SIDs, and their resolution are addressed in
   [I-D.ietf-spring-conflict-resolution].

   SR with the MPLS data plane, can be gracefully introduced in an
   existing LDP [RFC5036] network.  This is described in
   [I-D.ietf-spring-segment-routing-ldp-interop].  SR and LDP may also
   inter-work.  In this case, the introduction of mapping-server may
   introduce some additional manageability considerations that are
   discussed in [I-D.ietf-spring-segment-routing-ldp-interop].

   When a path is expressed using a label stack, the occurrence of label
   stacking will increase.  A node may want to signal in the control
   plane its ability in terms of size of the label stack it can support.

   A YANG data model [RFC6020] for segment routing configuration and
   operations has been defined in [I-D.ietf-spring-sr-yang].

   When Segment Routing is applied to the IPv6 data plane, segments are
   identified through IPv6 addresses.  The allocation, management and
   troubleshooting of segment identifiers is no different than the
   existing mechanisms applied to the allocation and management of IPv6
   addresses.

   The DA of the packet gives the active segment address.  The segment
   list in the SRH gives the entire path of the packet.  The validation
   of the source routed path is done through inspection of DA and SRH
   present in the packet header matched to the equivalent routing table
   entries.

   In the context of SR over the IPv6 data plane, the source routed path
   is encoded in the SRH as described in
   [I-D.ietf-6man-segment-routing-header].  The SR IPv6 source routed
   path is instantiated into the SRH as a list of IPv6 address where the
   active segment is in the Destination Address (DA) field of the IPv6
   packet header.  Typically, by inspecting in any node the packet
   header, it is possible to derive the source routed path it belongs
   to.  Similar to the context of SR over MPLS data plane, an
   implementation may originate path control and monitoring packets
   where the source routed path is inserted in the SRH and where each
   segment of the path inserts in the packet the relevant data in order
   to measure the end to end path and performance.







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10.  Contributors

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

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

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

   Wim Henderickx
   Nokia
   Email: wim.henderickx@nokia.com

   Jeff Tantsura
   Email: jefftant@gmail.com

   Edward Crabbe
   Email: edward.crabbe@gmail.com

   Igor Milojevic
   Email: milojevicigor@gmail.com

   Saku Ytti
   TDC
   Email: saku@ytti.fi

11.  Acknowledgements

   We would like to thank Dave Ward, Peter Psenak, 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.

12.  References

12.1.  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>.





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   [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>.

   [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>.

12.2.  Informative References

   [I-D.ietf-6man-segment-routing-header]
              Previdi, S., Filsfils, C., Raza, K., Leddy, J., Field, B.,
              daniel.voyer@bell.ca, d., daniel.bernier@bell.ca, d.,
              Matsushima, S., Leung, I., Linkova, J., Aries, E., Kosugi,
              T., Vyncke, E., Lebrun, D., Steinberg, D., and R. Raszuk,
              "IPv6 Segment Routing Header (SRH)", draft-ietf-6man-
              segment-routing-header-06 (work in progress), March 2017.

   [I-D.ietf-isis-segment-routing-extensions]
              Previdi, S., Filsfils, C., Bashandy, A., Gredler, H.,
              Litkowski, S., Decraene, B., and j. jefftant@gmail.com,
              "IS-IS Extensions for Segment Routing", draft-ietf-isis-
              segment-routing-extensions-13 (work in progress), June
              2017.

   [I-D.ietf-mpls-spring-lsp-ping]
              Kumar, N., Swallow, G., Pignataro, C., Akiya, N., Kini,
              S., Gredler, H., and M. Chen, "Label Switched Path (LSP)
              Ping/Traceroute for Segment Routing Networks with MPLS
              Dataplane", draft-ietf-mpls-spring-lsp-ping-03 (work in
              progress), June 2017.

   [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-09 (work in progress), March
              2017.






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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-16 (work in progress), May 2017.

   [I-D.ietf-pce-segment-routing]
              Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "PCEP Extensions for Segment Routing",
              draft-ietf-pce-segment-routing-09 (work in progress),
              April 2017.

   [I-D.ietf-spring-conflict-resolution]
              Ginsberg, L., Psenak, P., Previdi, S., and M. Pilka,
              "Segment Routing MPLS Conflict Resolution", draft-ietf-
              spring-conflict-resolution-04 (work in progress), May
              2017.

   [I-D.ietf-spring-ipv6-use-cases]
              Brzozowski, J., Leddy, J., Filsfils, C., Maglione, R., and
              M. Townsley, "IPv6 SPRING Use Cases", draft-ietf-spring-
              ipv6-use-cases-11 (work in progress), June 2017.

   [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-06 (work in
              progress), February 2017.

   [I-D.ietf-spring-resiliency-use-cases]
              Filsfils, C., Previdi, S., Decraene, B., and R. Shakir,
              "Resiliency use cases in SPRING networks", draft-ietf-
              spring-resiliency-use-cases-11 (work in progress), May
              2017.

   [I-D.ietf-spring-segment-routing-central-epe]
              Filsfils, C., Previdi, S., Aries, E., and D. Afanasiev,
              "Segment Routing Centralized BGP Egress Peer Engineering",
              draft-ietf-spring-segment-routing-central-epe-05 (work in
              progress), March 2017.

   [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-08 (work in
              progress), June 2017.





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   [I-D.ietf-spring-segment-routing-mpls]
              Filsfils, C., Previdi, S., Bashandy, A., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing with MPLS
              data plane", draft-ietf-spring-segment-routing-mpls-09
              (work in progress), June 2017.

   [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-03 (work in
              progress), January 2017.

   [I-D.ietf-spring-sr-yang]
              Litkowski, S., Qu, Y., Sarkar, P., and J. Tantsura, "YANG
              Data Model for Segment Routing", draft-ietf-spring-sr-
              yang-06 (work in progress), March 2017.

   [RFC4381]  Behringer, M., "Analysis of the Security of BGP/MPLS IP
              Virtual Private Networks (VPNs)", RFC 4381,
              DOI 10.17487/RFC4381, February 2006,
              <http://www.rfc-editor.org/info/rfc4381>.

   [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>.

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

   [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>.

   [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS
              Networks", RFC 5920, DOI 10.17487/RFC5920, July 2010,
              <http://www.rfc-editor.org/info/rfc5920>.






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   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <http://www.rfc-editor.org/info/rfc6020>.

   [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>.

   [RFC7855]  Previdi, S., Ed., Filsfils, C., Ed., Decraene, B.,
              Litkowski, S., Horneffer, M., and R. Shakir, "Source
              Packet Routing in Networking (SPRING) Problem Statement
              and Requirements", RFC 7855, DOI 10.17487/RFC7855, May
              2016, <http://www.rfc-editor.org/info/rfc7855>.

Authors' Addresses

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

   Email: cfilsfil@cisco.com


   Stefano Previdi (editor)
   Cisco Systems, Inc.
   Italy

   Email: stefano@previdi.net


   Bruno Decraene
   Orange
   FR

   Email: bruno.decraene@orange.com




Filsfils, et al.        Expires December 22, 2017              [Page 27]


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   Stephane Litkowski
   Orange
   FR

   Email: stephane.litkowski@orange.com


   Rob Shakir
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   Email: robjs@google.com





































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