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Versions: 00 01 02 draft-filsfils-spring-segment-routing-use-cases

Network Working Group                                   C. Filsfils, Ed.
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                        P. Francois, Ed.
Expires: January 15, 2014                                 IMDEA Networks
                                                              S. Previdi
                                                     Cisco Systems, Inc.
                                                             B. Decraene
                                                            S. Litkowski
                                                                  Orange
                                                            M. Horneffer
                                                        Deutsche Telekom
                                                            I. Milojevic
                                                          Telekom Srbija
                                                               R. Shakir
                                                         British Telecom
                                                                 S. Ytti
                                                                  TDC Oy
                                                           W. Henderickx
                                                          Alcatel-Lucent
                                                             J. Tantsura
                                                                Ericsson
                                                               E. Crabbe
                                                            Google, Inc.
                                                           July 14, 2013


                       Segment Routing Use Cases
           draft-filsfils-rtgwg-segment-routing-use-cases-01

Abstract

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

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

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",



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   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on January 15, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Companion Documents  . . . . . . . . . . . . . . . . . . .  5
     1.2.  Editorial simplification . . . . . . . . . . . . . . . . .  6
   2.  IGP-based MPLS Tunneling . . . . . . . . . . . . . . . . . . .  6
   3.  FRR  . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Protecting a resource along the path of a Node Segment . .  8
     3.2.  Protecting an adjacency segment  . . . . . . . . . . . . .  9
     3.3.  Protecting a node segment upon the failure of its
           advertising node . . . . . . . . . . . . . . . . . . . . . 10
       3.3.1.  Advertisement of the Mirroring Capability  . . . . . . 11
       3.3.2.  Mirroring Table  . . . . . . . . . . . . . . . . . . . 11
       3.3.3.  LFA FRR at the Point of Local Repair . . . . . . . . . 11
       3.3.4.  Modified IGP Convergence upon Node deletion  . . . . . 12
       3.3.5.  Conclusions  . . . . . . . . . . . . . . . . . . . . . 12
   4.  Traffic Engineering without Bandwidth Admission Control  . . . 13
     4.1.  Anycast Node Segment . . . . . . . . . . . . . . . . . . . 14
       4.1.1.  Disjointness in dual-plane networks  . . . . . . . . . 14
       4.1.2.  CoS-based Traffic Engineering  . . . . . . . . . . . . 16
     4.2.  Distributed CSPF-based Traffic Engineering . . . . . . . . 18
     4.3.  Egress Peering Traffic Engineering . . . . . . . . . . . . 19
     4.4.  Deterministic non-ECMP Path  . . . . . . . . . . . . . . . 21
       4.4.1.  Node Segment . . . . . . . . . . . . . . . . . . . . . 22
       4.4.2.  Forwarding Adjacency . . . . . . . . . . . . . . . . . 22
     4.5.  Load-balancing among non-parallel links  . . . . . . . . . 22
   5.  Traffic Engineering with Bandwidth Admission Control . . . . . 23
     5.1.  Capacity Planning Process  . . . . . . . . . . . . . . . . 24
     5.2.  SDN /SR use-case . . . . . . . . . . . . . . . . . . . . . 26
       5.2.1.  Illustration . . . . . . . . . . . . . . . . . . . . . 27
       5.2.2.  Benefits . . . . . . . . . . . . . . . . . . . . . . . 28
       5.2.3.  Dataset analysis . . . . . . . . . . . . . . . . . . . 29
     5.3.  Residual Bandwidth . . . . . . . . . . . . . . . . . . . . 30
   6.  SR co-existence and interworking with other MPLS Control
       Plane  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     6.1.  Ship-in-the-night coexistence  . . . . . . . . . . . . . . 30
       6.1.1.  MPLS2MPLS co-existence . . . . . . . . . . . . . . . . 33
       6.1.2.  IP2MPLS co-existence . . . . . . . . . . . . . . . . . 33
     6.2.  Migration from LDP to SR . . . . . . . . . . . . . . . . . 33
     6.3.  SR and LDP Interworking  . . . . . . . . . . . . . . . . . 34
       6.3.1.  LDP to SR  . . . . . . . . . . . . . . . . . . . . . . 35
       6.3.2.  SR to LDP  . . . . . . . . . . . . . . . . . . . . . . 35
     6.4.  Leveraging SR benefits for LDP-based traffic . . . . . . . 36
       6.4.1.  Eliminating Directed LDP Session . . . . . . . . . . . 38
       6.4.2.  Guaranteed FRR coverage  . . . . . . . . . . . . . . . 39
     6.5.  Inter-AS Option C, Carrier's Carrier and Seamless MPLS . . 40
   7.  OAM  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     7.1.  Monitoring a remote bundle . . . . . . . . . . . . . . . . 40



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     7.2.  Monitoring a remote peering link . . . . . . . . . . . . . 41
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 41
   9.  Manageability Considerations . . . . . . . . . . . . . . . . . 41
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 41
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 41
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 42
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 42
     12.2. Informative References . . . . . . . . . . . . . . . . . . 42
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 44










































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1.  Introduction

   In this document, we document various SR use-cases.

   Section 2 illustrates the ability to tunnel traffic towards remote
   service points without any other protocol than the IGP.

   Section 3 reports various FRR use-cases leveraging the SR
   functionality.

   Section 4 and Section 5 document traffic-engineering use-cases,
   respectively without and with a notion of bandwidth admission
   control.

   Section 6 documents the co-existence and interworking with MPLS
   Signaling protocols.

   Section 7 illustrates OAM use-cases.

   The objective of this document is to illustrate the properties and
   benefits of the SR architecture.

1.1.  Companion Documents

   The main reference for this document is the SR architecture defined
   in [draft-filsfils-rtgwg-segment-routing-00].

   The SR instantiation in the MPLS dataplane is described in
   [I-D.gredler-isis-label-advertisement].

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

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

   Fast-Reroute for Segment Routing is described in
   [I-D.francois-sr-frr].

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

   The SR instantiation in the IPv6 dataplane will be described in a
   future draft.







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1.2.  Editorial simplification

   A unique index is allocated to each IGP Prefix Segment.  The related
   absolute segment associated to an IGP Prefix SID is determined by
   summing the index and the base of the SRSB.  In the SR architecture,
   each node can be configured with a different SRSB and hence the
   absolute SID associated to an IGP Prefix Segment can change from node
   to node.

   We have described the first use-case of this document in the most
   generic way, i.e. with different SRSB at each node in the SR IGP
   domain.  We have detailed the packet path highlighting that the SID
   of a Prefix Segment may change hop by hop.

   For editorial simplification purpose, we will assume for all the
   other use cases that the operator ensures a single consistent SRSB
   across all the nodes in the SR IGP domain.  In this specific context,
   we call the SRSB the SRGB and we use global absolute SID's for the
   IGP Prefix SID's.  Indeed, when all the nodes have the same SRSB, all
   the nodes associate the same absolute SID with the same index and
   hence one can use the absolute SID value instead of the index.

   Several operators have indicated that they would deploy the SR
   technology in this way: with a single consistent SRGB across all the
   nodes.  They motivated their choice based on operational simplicity
   (e.g. troubleshooting across different nodes).

   While this document notes this operator feedback and we use this
   deployment model to simplify the text, we highlight that the SR
   architecture is not limited to this specific deployment use-case.
   Shared segments are always advertised in the IGP extensions as
   indexed values.


2.  IGP-based MPLS Tunneling

   SR, applied to the MPLS dataplane, offers the ability to tunnel
   services (VPN, VPLS, VPWS) from an ingress PE to an egress PE,
   without any other protocol than ISIS or OSPF.  LDP and RSVP-TE
   signaling protocols are not required.

   The operator only needs to allocate one node segment per PE and the
   SR IGP control-plane automatically builds the required MPLS
   forwarding constructs from any PE to any PE.







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                                  P1---P2
                                 /       \
                    A---CE1---PE1         PE2---CE2---Z
                                 \       /
                                  P4---P4

                    Figure 1: IGP-based MPLS Tunneling

   In Figure 1 above, the four nodes A, CE1, CE2 and Z are part of the
   same VPN.  CE2 advertises to PE2 a route to Z. PE2 binds a local
   label LZ to that route and propagates the route and its label via
   MPBGP to PE1 with nhop 192.168.0.2.  PE1 installs the VPN prefix Z in
   the appropriate VRF and resolves the next-hop onto the node segment
   associated with PE2.  Upon receiving a packet from A destined to Z,
   PE1 pushes two labels onto the packet: the top label is the Prefix
   SID attached to 192.168.0.2/32, the bottom label is the VPN label LZ
   attached to the VPN route Z.

   The Prefix-SID attached to prefix 192.168.0.2 is a shared segment
   within the IGP domain, as such it is indexed.

   Let us assume that:

      - the operator allocated the index 2 to the prefix 192.168.0.2/32

      - the operator allocated SRSB [100, 199] at PE1

      - the operator allocated SRSB [200, 299] at P1

      - the operator allocated SRSB [300, 399] at P2

      - the operator allocated SRSB [400, 499] at P3

      - the operator allocated SRSB [500, 599] at P4

      - the operator allocated SRSB [600, 699] at PE2

   Thanks to this context, any SR-capable IGP node in the domain can
   determine what is the segment associated with the Prefix-SID attached
   to prefix 192.168.0.2/32:

      - PE1's SID is 100+2=102

      - P1's SID is 200+2=202







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      - P2's SID is 300+2=302

      - P3's SID is 400+2=402

      - P4's SID is 500+2=502

      - PE2's SID is 600+2=602

   Specifically to our example this means that PE1 load-balance the
   traffic to VPN route Z between P1 and P4.  The packets sent to P1
   have a top label 202 while the packets sent to P4 have a top label
   502.  P1 swaps 202 for 302 and forwards to P2.  P2 pops 302 and
   forwards to PE2.  The packets sent to P4 had label 502.  P4 swaps 502
   for 402 and forwards the packets to P3.  P3 pops the top label and
   forwards the packets to PE2.  Eventually all the packets reached PE2
   with one single lable: LZ, the VPN label attached to VPN route Z.

   This scenario illustrates how supporting MPLS services (VPN, VPLS,
   VPWS) with SR has the following benefits:

      - Simple operation: one single intra-domain protocol to operate:
      the IGP.  No need to support IGP synchronization extensions as
      described in [RFC5443] and [RFC6138].

      - Excellent scaling: one Node-SID per PE.


3.  FRR

3.1.  Protecting a resource along the path of a Node Segment

   SR leverages the technologies stemming from the IPFRR framework to
   provide fast recovery of end-to-end connectivity upon failures.  This
   section assumes familiarity with Remote-LFA concepts described in
   [I-D.ietf-rtgwg-remote-lfa].

   Consider an arbitrary protected link S-E.  In LFA FRR, if a path from
   a neighbor N of S towards the destination does not cause packets to
   loop back over the link S-E (i.e.  N is a loop-free alternate (LFA)),
   then S can forward packets to N and packets will be delivered to the
   destination using the pre-failure LFA forwarding information.

   If there is no such LFA neighbor, then S may be able to create a
   virtual LFA by using a tunnel to carry the packet to a point in the
   network which is not a direct neighbor of S and from which the packet
   will be delivered to the destination without looping back to S.
   Remote LFA (RLFA, [I-D.ietf-rtgwg-remote-lfa]) calls such a tunnel a
   repair tunnel.  The tail-end of this tunnel is called a "remote LFA"



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   or a "PQ node".  We refer to RLFA for the definitions of the P and Q
   sets.

   In networks with symmetric IGP metrics (the metric of a link AB is
   the same as the metric of the reverse link BA), we can prove that P
   and the Q sets intersect or there is at least one P node that is
   adjacent to a Q node.

   If the P and Q sets do not intersect (i.e. there is no RLFA PQ node),
   we propose to use a Directed LFA (DLFA) repair tunnel from S to a Q
   node that is adjacent to the P space ([I-D.ietf-rtgwg-remote-lfa]).
   The LFA repair tunnel only requires two segments: a node segment to a
   P node which is adjacent to the Q node and an adjacency segment from
   the P node to its adjacent Q node.

   Thanks to the DLFA extension, we thus have a guaranteed LFA-based FRR
   technique for any network with symmetric IGP metrics.  Future
   versions of the document will describe the solutions leveraging SR
   capabilities to provided guaranteed FRR applicability in any IGP
   topology.

   Resolving FRR with SR has the following benefits:

      Preservation of the simplicity properties of LFA FRR ([RFC6571]).

      Preservation of the capacity planning properties (unlike SDH and
      other FRR solutions, the repaired packet does not go back to the
      next-hop or next-next-hop but uses shortest-path forwarding from a
      much closer release point, [RFC6571]).

      Simplification of the RLFA operation: no dynamically-established
      directed LDP sessions to the repair nodes.

      No requirement for any extra computation on top of the one
      required for RLFA.

      Guaranteed coverage for symmetric networks,

      The repair tunnel in symmetric network can be encoded efficiently
      with only two segments.

3.2.  Protecting an adjacency segment

   More details will be provided in a future version.







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3.3.  Protecting a node segment upon the failure of its advertising node

   Referring to the below figure, let us assume:

      A is identified by IP address 192.0.2.1/32 to which Node-SID 101
      is attached.

      B is identified by IP address 192.0.2.2/32 to which Node-SID 102
      is attached

      A and B host the same set of services.

      Each service is identified by a local segment at each node: i.e.
      node A allocates a local service segment 9001 to identify a
      specific service S while the same service is identified by a local
      service segment 9002 at B. Specifically, for the sake of this
      illustration, let us assume that service S is a BGP-VPN service
      where A announces a VPN route V with BGP nhop 192.0.2.1/32 and
      local VPN label 9001 and B announces the same VPN route V with BGP
      nhop 192.0.2.2/32 and local VPN label 9002.

      A generic mesh interconnects the three nodes M, Q and B.

      N prefers to use the service S offered by A and hence sends its
      S-destined traffic with segment list {101, 9001}.

      Q is a node connected to A.

      Q has a method to detect the loss of node A within a few 10's of
      msec.

                               __
                              {  }---Q---A(service S)
                       N--M--{    }
                              {__}---B(service S)

                        Figure 2: Service Mirroring

   In that context, we would like to protect the traffic destined to
   service S upon the failure of node A.

   The solution is built upon several components:









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   1. B advertises its mirroring capability for mirrored Node-SID 101
   2. B pre-installs a mirroring table in order to process the
      packets originally destined to 101.
   3. Q and any neighbor of A pre-install the Mirror_FRR LFA
      extension
   4. All nodes implements a modified SRDB convergence upon Node-SID
      101 deletion

3.3.1.  Advertisement of the Mirroring Capability

   B advertises a MIRROR sub-TLV in its IGP Link-State Router Capability
   TLV with the values (TTT=000, MIRRORED_OBJECT=101,
   CONTEXT_SEGMENT=10002),[draft-filsfils-rtgwg-segment-routing-00],
   [draft-previdi-isis-segment-routing-extensions-00] and
   [draft-psenak-ospf-segment-routing-extensions-00] for more details in
   the encodings.

   Doing so, B advertises within the routing domain that it is willing
   to backup any traffic originally sent to Node-SID 101 provided that
   this rerouted traffic gets to B with the context segment 10002
   directly preceding any local service segment advertised by A. 10002
   is a local context segment allocated by B to identify traffic that
   was originally meant for A. This allows B to match the subsequent
   service segment (e.g. 9001) correctly.

3.3.2.  Mirroring Table

   We assume that B is able to discover all the local service segments
   allocated by A (e.g.  BGP route reflection and add-path).  B maps all
   the services advertised by A to its similar service representations.
   For example, service 9001 advertised by A is mapped to service 9002
   advertised by B as both relate to the same service S (the same VPN
   route V).  For example, B applies the same service treatment to a
   packet received with top segments {102, 10002, 9001} or with top
   segments {102, 9002}.  Basically, B treats {10002, 9001} as a synonym
   of {9002}.

3.3.3.  LFA FRR at the Point of Local Repair

   In advance of any failure of A, Q (and any other node connected to A)
   learns the identity of the IGP Mirroring node for each Node-SID
   advertised by A (MIRROR_TLV advertised by B) and pre-installs the
   following new MIRROR_FRR entry:








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   - Trigger condition: the loss of nhop A
   - Incoming active segment: 101 (a Node-SID advertised by A)
   - Primary Segment processing: pop 101
      - Backup Segment processing: pop 101, push {102, 10002}
   - Primary nhop: A
      - Backup nhop: primary path to node B

   Upon detecting the loss of node A, Q intercepts any traffic destined
   to Node-SID 101, pops the segment to A (101) and push a repair tunnel
   {102, 10002}.  Node-SID 102 steers the repaired traffic to B while
   context segment 10002 allows B to process the following service
   segment {9001} in the right context table.

3.3.4.  Modified IGP Convergence upon Node deletion

   Upon the failure of A, all the neighbors of A will flood the loss of
   their adjacency to A and eventually every node within the IGP domain
   will delete 192.0.2.1/32 from their RIB.

   The RIB deletion of 192.0.2.1/32 at N is beneficial as it triggers
   the BGP FRR Protection onto the precomputed backup next-hop
   [draft-rtgwg-bgp-pic-01.txt].

   The RIB deletion at node M, if it occurs before the RIB deletion at
   N, would be disastrous as it would lead to the loss of the traffic
   from N to A before Q is able to apply the Mirroring protection.

   The solution consists in delaying the deletion of the SRDB entry for
   101 by 2 seconds while still deleting the IP RIB 192.0.2.1/32 entry
   immediately.

   The RIB deletion triggers the BGP FRR and BGP Convergence.  This is
   beneficial and must occur without delay.

   The deletion of the SRDB entry to Node-SID101 is delayed to ensure
   that the traffic still in transit towards Node-SID 101 is not
   dropped.

   The delay timer should be long enough to ensure that either the BGP
   FRR or the BGP Convergence has taken place at N.

3.3.5.  Conclusions

   In our reference figure, N sends its packets towards A with the
   segment list {101, 9001}.  The shortest-path from S to A transits via
   M and Q.

   Within a few msec of the loss of A, Q activates its pre-installed



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   Mirror_FRR entry and reroutes the traffic to B with the following
   segment list {102, 10002, 9001}.

   Within a few 100's of msec, any IGP node deletes its RIB entry to A
   but keeps its SRDB entry to Node-SID 101 for an extra 2 seconds.

   Upon deleting its RIB entry to 192.0.2.1/32, N activates its BGP FRR
   entry and reroutes its S destined traffic towards B with segment list
   {102, 9002}.

   By the time any IGP node deletes the SRDB entry to Node-SID 101, N no
   longer sends any traffic with Node-SID 101.

   The deletion of the SRDB entry to Node-SID101 is delayed to ensure
   that the traffic still in transit towards Node-SID 101 is not
   dropped.

   In conclusion, the traffic loss only depends on the ability of Q to
   detect the node failure of its adjacent node A.


4.  Traffic Engineering without Bandwidth Admission Control

   This section describes traffic-engineering use-cases which do not
   require bandwidth admission control.

   The first sub-section illustrates the use of anycast segments to
   express macro policies.  Two examples are provided: one involving a
   disjointness enforcement within a so-called dual-plane network, and
   the other involving CoS-based policies.

   The second sub-section illustrate how a head-end router can combine a
   distributed CSPF computation with SR.  Various examples are provided
   where the CSPF constraint or objective is either a TE affinity, an
   SRLG or a latency metric.

   The third sub-section illustrates how SR can help traffic-engineer
   outbound traffic among different external peers, overriding the best
   installed IP path at the egress border routers.

   The fourth sub-section describes how SR can be used to express
   deterministic non-ECMP path.  Several techniques to compress the
   related segment lists are also introduced.

   The fifth sub-section describes a use-case where a node attaches an
   Adj-SID to a set of its interfaces however not sharing the same
   neighbor.  The illustrated benefit relates to loadbalancing.




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4.1.  Anycast Node Segment

   The SR architecture defines an anycast segment as a segment attached
   to an anycast IP prefix ([RFC4786]).

   The anycast node segment is an interesting tool for traffic
   engineering:

      Macro-policy support: anycast segments allow to express policies
      such as "go via plane1 of a dual-plane network" (Section 4.1.1) or
      "go via Region3" (Section 4.3).

      Implicit node resiliency: the traffic-engineering policy is not
      anchored to a specific node whose failure could impact the
      service.  It is anchored to an anycast address/Anycast-SID and
      hence the flow automatically reroutes on any ECMP-aware shortest-
      path to any other router part of the anycast set.

   The two following sub-sections illustrate to traffic-engineering use-
   cases leveraging Anycast-SID.

4.1.1.  Disjointness in dual-plane networks

   Many networks are built according to the dual-plane design:
      Each access region k is connected to the core by two C routers
      (C(1,k) and C(2,k)).

      C(1,k) is part of plane 1 and aggregation region K

      C(2,k) is part of plane 2 and aggregation region K

      C(1,k) has a link to C(2, j) iff k = j.

         The core nodes of a given region are directly connected.
         Inter-region links only connect core nodes of the same plane.

      {C(1,k) has a link to C(1, j)} iff {C(2,k) has a link to C(2, j)}.

         The distribution of these links depends on the topological
         properties of the core of the AS. The design rule presented
         above specifies that these links appear in both core planes.

   We assume a common design rule found in such deployments: the inter-
   plane link costs (Cik-Cjk where i<>j) are set such that the route to
   an edge destination from a given plane stays within the plane unless
   the plane is partitioned.





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                             Edge Router A
                                 /  \
                                /    \
                               /      \  Agg Region A
                              /        \
                             /          \
                            C1A----------C2A
                            | \         | \
                            |  \        |  \
                            |   C1B----------C2B
                  Plane1    |    |      |    |     Plane2
                            |    |      |    |
                            C1C--|-----C2C   |
                              \  |        \  |
                               \ |         \ |
                               C1Z----------C2Z
                                  \        /
                                   \      /  Agg Region Z
                                    \    /
                                     \  /
                                 Edge Router Z

               Figure 3: Dual-Plane Network and Disjointness

   In the above network diagram, let us that the operator configures:

      The four routers (C1A, C1B, C1C, C1Z) with an anycast loopback
      address 192.0.2.1/32 and an Anycast-SID 101.

      The four routers (C2A, C2B, C2C, C2Z) with an anycast loopback
      address 192.0.2.2/32 and an Anycast-SID 102.

      Edge router Z with Node-SID 109.

   A can then use the three following segment lists to control its
   Z-destined traffic:

      {109}: the traffic is load-balanced across any ECMP path through
      the network.

      {101, 109}: the traffic is load-balanced across any ECMP path
      within the Plane1 of the network.

      {102, 109}: the traffic is load-balanced across any ECMP path
      within the Plane2 of the network.

   Most of the data traffic to Z would use the first segment list, such
   as to exploit the capacity efficiently.  The operator would use the



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   two other segment lists for specific premium traffic that has
   requested disjoint transport.

   For example, let us assume a bank or a government customer has
   requested that the two flows F1 and F2 injected at A and destined to
   Z should be transported across disjoint paths.  The operator could
   classify F1 (F2) at A and impose and SR header with the second
   (third) segment list.  Focusing on F1 for the sake of illustration, A
   would route the packets based on the active segment, Anycast-SID 101,
   which steers the traffic along the ECMP-aware shortest-path to the
   closest router part of the Anycast-SID 101, C1A is this example.
   Once the packets have reached C1A, the second segment becomes active,
   Node-SID 109, which steers the traffic on the ECMP-aware shortest-
   path to Z. C1A load-balances the traffic between C1B-C1Z and C1C-C1Z
   and then C1Z forwards to Z.

   This SR use-case has the following benefits:

      Zero per-service state and signaling on midpoint and tail-end
      routers.

      Only two additional node segments (one Anycast-SID per plane).

      ECMP-awareness.

      Node resiliency property: the traffic-engineering policy is not
      anchored to a specific core node whose failure could impact the
      service.

4.1.2.  CoS-based Traffic Engineering

   Frequently, different classes of service need different path
   characteristics.

   In the example below, a single-area international network with
   presence in four different regions of the world has lots of cheap
   network capacity from Region4 to Region1 via Region2 and some scarce
   expensive capacity via Region3.
                         +-------[Region2]-------+
                         |                       |
                A----[Region4]               [Region1]----Z
                         |                       |
                         +-------[Region3]-------+

                 Figure 4: International Topology Example

   In such case, the IGP metrics would be tuned to have a shortest-path
   from A to Z via Region2.



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   This would provide efficient capacity planning usage while fulfilling
   the requirements of most of the traffic demands.  However, it may not
   suite the latency requirements of the voice traffic between the two
   cities.

   Let us illustrate how this can be solved with Segment Routing.

   The operator would configure:
   - All the core routers in Region3 with an anycast loopback
     192.0.2.3/32 to which Anycast-SID 333 is attached.
   - A loopback 192.0.2.9/32 on Z and would attach Node-SID 109
     to it.
   - The IGP metrics such that the shortest-path from Region4 to
     Region1 is via Region2, from Region4 to Region3 is directly
     to Region3, the shortest-path from Region3 to Region1 is not
     back via Region4 and Region2 but straight to Region1.

   With this in mind, the operator would instruct A to apply the
   following policy for its Z-destined traffic:
   - Voice traffic: impose segment-list {333, 109}
      - Anycast-SID 333 steers the Voice traffic along the
        ECMP-aware shortest-path to the closest core router in
        Region3, then Node-SID 109 steers the Voice traffic along
        the ECMP-aware shortest-path to Z. Hence the Voice traffic
        reaches Z from A via the low-latency path through Region3.

   - Any other traffic: impose segment-list {109}: Node-SID 109
     steers the Voice traffic along the ECMP-aware shortest-path
     to Z. Hence the bulk traffic reaches Z from A via the cheapest
     path for the operator.

   This SR use-case has the following benefits:

      Zero per-service state and signaling at midpoint and tailend
      nodes.

      One additional anycast segment per region.

      ECMP-awareness.

      Node resiliency property: the traffic-engineering policy is not
      anchored to a specific core node whose failure could impact the
      service.








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4.2.  Distributed CSPF-based Traffic Engineering

   In this section, we illustrate how a head-end router can map the
   result of its distributed CSPF computation into an SR segment list.
                                 +---E---+
                                 |       |
                           A-----B-------C-----Z
                                 |       |
                                 +---D---+

                         Figure 5: SRLG-based CSPF

   Let us assume that in the above network diagram:

      The operator configures a policy on A such that its Z-destined
      traffic must avoid SRLG1.

      The operator configures SRLG1 on the link BC (or is learned
      dynamically from the IP/Optical interaction with the DWDM
      network).

      The SRLG's are flooded in the link-state IGP.

      The operator respectively configures the Node-SIDs 101, 102, 103,
      104, 105 and 109 at nodes A, B, C, D, E and Z.

   In that context, A can apply the following CSPF behavior:
























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   - It prunes all the links affected by the SRLG1, computes an SPF
     on the remaining topology and picks one of the SPF paths.
      - In our example, A finds two possible paths ABECZ and ABDCZ
        and let's assume it takes the ABDCZ path.

   - It translates the path as a list of segments
      - In our example, ABDCZ can be expressed as {104, 109}: a
        shortest path to node D, followed by a shortest-path to
        node Z.

   - It monitors the status of the LSDB and upon any change
     impacting the policy, it either recomputes a path meeting the
     policy or update its translation as a list of segments.
     - For example, upon the loss of the link DC, the shortest-path
       to Z from D (Node-SID 109) goes via the undesired link BC.
       After a transient time immediately following such failure,
       the node A would figure out that the chosen path is no longer
       valid and instead select ABECZ which is translated as
       {103, 109}.

   - This behavior is a local matter at node A and hence the details
     are outside the scope of this document.

   The same use-case can be derived from any other C-SPF objective or
   constraint (TE affinity, TE latency, SRLG, etc.) as defined in
   [RFC5305] and [I-D.previdi-isis-te-metric-extensions].  Note that the
   bandwidth case is specific and hence is treated in Section 5.

4.3.  Egress Peering Traffic Engineering
                                     +------+
                                     |      |
                                 +---D      F
                    +---------+ /    | AS 2 |\ +------+
                    |         |/     +------+ \|   Z  |
                    A         C                |      |
                    |         |\     +------+ /| AS 4 |
                    B   AS1   | \    |      |/ +------+
                    |         |  +---E      G
                    +---------+      | AS 3 |
                                     +------+\

               Figure 6: Egress peering traffic engineering

   Let us assume that:







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      C in AS1 learns about destination Z of AS 4 via two BGP paths
      (AS2, AS4) and (AS3, AS4).

      C sets next-hop-self before propagating the paths within AS1.

      C propagates all the paths to Z within AS1 (add-path).

      C only installs the path via AS2 in its RIB.

   In that context, the operator of AS1 cannot apply the following
   traffic-engineering policy:

      Steer 60% of the Z-destined traffic received at A via AS2 and 40%
      via AS3.

      Steer 80% of the Z-destined traffic received at B via AS2 and 20%
      via AS3.

   This traffic-engineering policy can be supported thanks to the
   following SR configuration.

   The operator configures:

      C with a loopback 192.0.2.1/32 and attach the Node-SID 101 to it.

      C to bind an external adjacency segment
      ([draft-filsfils-rtgwg-segment-routing-00]) to each of its peering
      interface.

   For the sake of this illustration, let us assume that the external
   adjacency segments bound by C for its peering interfaces to (D, AS2)
   and (E, AS3) are respectively 9001 and 9002.

   These external adjacencies (and their attached segments) are flooded
   within the IGP domain of AS1 [RFC5316].

   As a result, the following information is available within AS1:
   ISIS Link State Database:

   - Node-SID 101 is attached to IP address 192.0.2.1/32 advertised
     by C.
   - C is connected to a peer D with external adjacency segment 9001.
   - C is connected to a peer E with external adjacency segment 9002.
   BGP Database:

   - Z is reachable via 192.0.2.1 with AS Path {AS2, AS4}.
   - Z is reachable via 192.0.2.1 with AS Path {AS3, AS4}.




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   The operator of AS1 can thus meet its traffic-engineering objective
   by enforcing the following policies:

      A should apply the segment list {101, 9001} to 60% of the
      Z-destined traffic and the segment list {101, 9002} to the rest.

      B should apply the segment list {101, 9001} to 80% of the
      Z-destined traffic and the segment list {101, 9002} to the rest.

   Node segment 101 steers the traffic to C.

   External adjacency segment 9001 forces the traffic from C to (D,
   AS2), without any IP lookup at C.

   External adjacency segment 9002 forces the traffic from C to (E,
   AS3), without any IP lookup at C.

   A and B can also use the described segments to assess the liveness of
   the remote peering links, see OAM section.

4.4.  Deterministic non-ECMP Path

   The previous sections have illustrated the ability to steer traffic
   along ECMP-aware shortest-paths.  SR is also able to express
   deterministic non-ECMP path: i.e. as a list of adjacency segments.
   We illustrate such an use-case in this section.
                             A-B-C-D-E-F-G-H-Z
                               |           |
                               +-I-J-K-L-M-+

                   Figure 7: Non-ECMP deterministic path

   In the above figure, it is assumed all nodes are SR capable and only
   the following SIDs are advertised:
     - A advertises Adj-SID 9001 for its adjacency to B
     - B advertises Adj-SID 9002 for its adjacency to C
     - C advertises Adj-SID 9003 for its adjacency to D
     - D advertises Adj-SID 9004 for its adjacency to E
     - E advertises Adj-SID 9001 for its adjacency to F
     - F advertises Adj-SID 9002 for its adjacency to G
     - G advertises Adj-SID 9003 for its adjacency to H
     - H advertises Adj-SID 9004 for its adjacency to Z
     - E advertises Node-SID 101
     - Z advertises Node-SID 109

   The operator can steer the traffic from A to Z via a specific non-
   ECMP path ABCDEFGHZ by imposing the segment list {9001, 9002, 9003,
   9004, 9001, 9002, 9003, 9004}.



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   The following sub-sections illustrate how the segment list can be
   compressed.

4.4.1.  Node Segment

   Clearly the same exact path can be expressed with a two-entry segment
   list {101, 109}.

   This example illustrates that a Node Segment can also be used to
   express deterministic non-ECMP path.

4.4.2.  Forwarding Adjacency

   The operator can configure Node B to create a forwarding-adjacency to
   node H along an explicit path BCDEFGH.  The following behaviors can
   then be automated by B:

      B attaches an Adj-SID (e.g. 9007) to that forwarding adjacency
      together with an ERO sub-sub-TLV which describes the explicit path
      BCDEFGH.

      B installs in its Segment Routing Database the following entry:

         Active segment: 9007.

         Operation: NEXT and PUSH {9002, 9003, 9004, 9001, 9002, 9003}

   As a result, the operator can configure node A with the following
   compressed segment list {9001, 9007, 9004}.

4.5.  Load-balancing among non-parallel links

   A given node may assign the same Adj-SID to multiple of its
   adjacencies, even if these ones lead to different neighbors.  This
   may be useful to support traffic engineering policies.

                                 +---C---D---+
                                 |           |
                       PE1---A---B-----F-----E---PE2

         Figure 8: Adj-SID For Multiple (non-parallel) Adjacencies

   In the above example, let us assume that the operator:

      Requires PE1 to load-balance its PE2-destined traffic between the
      ABCDE and ABFE paths.





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      Configures B with Node-SID 102 and E with Node-SID 202.

      Configures B to advertise an individual Adj-SID per adjacency
      (e.g. 9001 for BC and 9002 for BF) and, in addition, an Adj-SID
      for the adjacency set (BC, BF) (e.g. 9003).

   With this context in mind, the operator achieves its objective by
   configuring the following traffic-engineering policy at PE1 for the
   PE2-destined traffic: {102, 9003, 202}:

      Node-SID 102 steers the traffic to B.

      Adj-SID 9003 load-balances the traffic to C or F.

      From either C or F, Node-SID 202 steers the traffic to PE2.

      In conclusion, the traffic is load-balanced between the ABCDE and
      ABFE paths, as desired.


5.  Traffic Engineering with Bandwidth Admission Control

   The implementation of bandwidth admission control within a network
   (and its possible routing consequence which consists in routing along
   explicit paths where the bandwidth is available) requires a capacity
   planning process.

   The spreading of load among ECMP paths is a key attribute of the
   capacity planning processes applied to packet-based networks.

   The first sub-section details the capacity planning process and the
   role of ECMP load-balancing.  We highlight the relevance of SR in
   that context.

   The next two sub-sections document two use-cases of SR-based traffic
   engineering with bandwidth admission control.

   The second sub-section documents a concrete SR applicability
   involving centralized-based admission control.  This is often
   referred to as the "SDN/SR use-case".

   The third sub-section introduces a future research topic involving
   the notion of residual bandwidth introduced in
   [I-D.atlas-mpls-te-express-path].







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5.1.  Capacity Planning Process

   Capacity Planning anticipates the routing of the traffic matrix onto
   the network topology, for a set of expected traffic and topology
   variations.  The heart of the process consists in simulating the
   placement of the traffic along ECMP-aware shortest-paths and
   accounting for the resulting bandwidth usage.

   The bandwidth accounting of a demand along its shortest-path is a
   basic capability of any planning tool or PCE server.

   For example, in the network topology described below, and assuming a
   default IGP metric of 1 and IGP metric of 2 for link GF, a 1600Mbps
   A-to-Z flow is accounted as consuming 1600Mbps on links AB and FZ,
   800Mbps on links BC, BG and GF, and 400Mbps on links CD, DF, CE and
   EF.
                                   C-----D
                                 /  \     \
                            A---B    +--E--F--Z
                                 \        /
                                  G------+

             Figure 9: Capacity Planning an ECMP-based demand

   ECMP is extremely frequent in SP, Enterprise and DC architectures and
   it is not rare to see as much as 128 different ECMP paths between a
   source and a destination within a single network domain.  It is a key
   efficiency objective to spread the traffic among as many ECMP paths
   as possible.

   This is illustrated in the below network diagram which consists of a
   subset of a network where already 5 ECMP paths are observed from A to
   M.
                                    C
                                   / \
                                  B-D-L--
                                 / \ /   \
                                A   E     \
                                 \         M
                                  \   G   /
                                   \ / \ /
                                    F   K
                                     \ /
                                      I

                     Figure 10: ECMP Topology Example

   Segment Routing offers a simple support for such ECMP-based shortest-



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   path placement: a node segment.  A single node segment enumerates all
   the ECMP paths along the shortest-path.

   When the capacity planning process detects that a traffic growth
   scenario and topology variation would lead to congestion, a capacity
   increase is triggered and if it cannot be deployed in due time, a
   traffic engineering solution is activated within the network.

   A basic traffic engineering objective consists of finding the
   smallest set of demands that need to be routed off their shortest
   path to eliminate the congestion, then to compute an explicit path
   for each of them and instantiating these traffic-engineered policies
   in the network.

   Segment Routing offers a simple support for explicit path policy.
   Let us provide two examples based on Figure 10.

   First example: let us assume that the process has selected the flow
   AM for traffic-engineering away from its ECMP-enabled shortest path
   and flow AM must avoid consuming resources on the LM and the FG
   links.

   The solution is straightforward: A sends its M-destined traffic
   towards the nhop F with a two-label stack where the top label is the
   adjacent segment FI and the next label is the node segment to M.
   Alternatively, a three-label stack with adjacency segments FI, IK and
   KM could have been used.

   Second example: let us assume that AM is still the selected flow but
   the constraint is relaxed to only avoid using resources from the LM
   link.

   The solution is straightforward: A sends its M-destined traffic
   towards the nhop F with a one-label stack where the label is the node
   segment to M. Note that while the AM flow has been traffic-engineered
   away from its natural shortest-path (ECMP across three paths), the
   traffic-engineered path is still ECMP-aware and leverages two of the
   three initial paths.  This is accomplished with a single-label stack
   and without the enumeration of one tunnel per path.

   Under the light of these examples, Segment Routing offers an
   interesting solution for Capacity Planning because:

      One node segment represents the set of ECMP-aware shortest paths.







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      Adjacency segments allow to express any explicit path.

      The combination of node and adjacency segment allows to express
      any path without having to enumerate all the ECMP options.

      The capacity planning process ensures that the majority of the
      traffic rides on node segments (ECMP-based shortest path), while a
      minority of the traffic is routed off its shortest-path.

      The explicitly-engineered traffic (which is a minority) still
      benefits from the ECMP-awareness of the node segments within their
      segment list.

      Only the head-end of a traffic-engineering policy maintains state.
      The midpoints and tail-ends do not maintain any state.

5.2.  SDN /SR use-case

   The heart of the application of SR to the SDN use-case lies in the
   SDN controller, also called Stateful PCE
   ([I-D.ietf-pce-stateful-pce]).

   The SDN controller is responsible to control the evolution of the
   traffic matrix and topology.  It accepts or denies the addition of
   new traffic into the network.  It decides how to route the accepted
   traffic.  It monitors the topology and upon failure, determines the
   minimum traffic that should be rerouted on an alternate path to
   alleviate a bandwidth congestion issue.

   The algorithms supporting this behavior are a local matter of the SDN
   controller and are outside the scope of this document.

   The means of collecting traffic and topology information are the same
   as what would be used with other SDN-based traffic-engineering
   solutions (e.g.  [RFC5101] and [I-D.ietf-idr-ls-distribution].

   The means of instantiating policy information at a traffic-
   engineering head-end are the same as what would be used with other
   SDN-based traffic-engineering solutions (e.g.:
   [I-D.ward-i2rs-framework], [I-D.crabbe-pce-pce-initiated-lsp] and
   [draft-msiva-pce-pcep-segment-routing-extensions-00]).










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5.2.1.  Illustration
                                        _______________
                                       {               }
                      +--C--+    V    {  SDN Controller }
                      |/   \|   /      {_______________}
                  A===B--G--D==F--Y
                      |\   /|   \
                      +--E--+    Z

                              SDN/SR use-case

   Let us assume that in the above network diagram:

      An SDN Controller (SC) is connected to the network and is able to
      retrieve the topology and traffic information, as well as set
      traffic-engineering policies on the network nodes.

      The operator (likely via the SDN Controller) as provisioned the
      Node-SIDs 101, 102, 103, 104, 105, 106, 107, 201, 202 and 203
      respectively at nodes A, B, C, D, E, F, G, V, Y and Z.

      All the links have the same BW (e.g. 10G) and IGP cost (e.g. 10)
      except the links BG and GD which have IGP cost 50.

      Each described node connectivity is formed as a bundle of two
      links, except (B, G) and (G, D) which are formed by a single link
      each.

      Flow FV is traveling from A to destinations behind V.

      Flow FY is traveling from A to destinations behind Y.

      Flow FZ is traveling from A to destinations behind Z.

      The SDN Controller has admitted all these flows and has let A
      apply the default SR policy: "map a flow onto its ECMP-aware
      shortest-path".

         In this example, this means that A respectively maps the flows
         FV onto segment list {201}, FY onto segment list {202} and FZ
         onto segment list {203}.

         In this example, the reader should note that the SDN Controller
         knows what A would do and hence knows and controls that none of
         these flows are mapped through G.

   Let us describe what happens upon the failure of one of the two links
   E-D.



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   The SDN Controller monitors the link-state database and detects a
   congestion risk due to the reduced capacity between E and D.
   Specifically, SC updates its simulation of the traffic according to
   the policies he instructed the network to use and discovers that too
   much traffic is mapped on the remaining link E-D.

   The SDN Controller then computes the minimum number of flows that
   should be deviated from their existing path.  For example, let us
   assume that the flow FZ is selected.

   The SDN controller then computes an explicit path for this flow.  For
   example, let us assume that the chosen path is ABGDFZ.

   The SDN controller then maps the chosen path into an SR-based policy.
   In our example, the path ABGDFZ is translated into a segment list
   {107, 203}.  Node-SID steers the traffic along ABG and then Node-SID
   203 steers the traffic along GDFZ.

   The SDN controller then applies the following traffic-engineering
   policy at A: "map any packet of the classified flow FZ onto segment-
   list {107, 203}".  The SDN Controller uses PCEP extensions to
   instantiate that policy at A
   ([draft-msiva-pce-pcep-segment-routing-extensions-00]).

   As soon as A receives the PCEP message, it enforces the policy and
   the traffic classified as FZ is immediately mapped onto segment list
   {107, 203}.

   This immediately eliminate the congestion risk.  Flows FV and FY were
   untouched and keep using the ECMP-aware shortest-path.  The minimum
   amount of traffic was rerouted (FZ).  No signaling hop-by-hop through
   the network from A to Z is required.  No admission control hop-by-hop
   is required.  No state needs to be maintained by B, G, D, F or Z. The
   only maintained state is within the SDN controller and the head-end
   node (A).

5.2.2.  Benefits

   In the context of Centralized-Based Optimization and the SDN use-
   case, here are the benefits provided by the SR architecture:

      Explicit routing capability with or without ECMP-awareness.

      No signaling hop-by-hop through the network.







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      State is only maintained at the policy head-end.  No state is
      maintained at mid-points and tail-ends.

      Automated guaranteed FRR for any topology (Section 3.

      Optimum virtualization: the policy state is in the packet header
      and not in the intermediate node along the policy.  The policy is
      completely virtualized away from midpoints and tail-ends.

      Highly responsive to change: the SDN Controller only needs to
      apply a policy change at the head-end.  No delay is lost
      programming the midpoints and tail-end along the policy.

5.2.3.  Dataset analysis

   A future version of this document will report some analysis of the
   application of the SDN/SR use-case to real operator data sets.

   A first, incomplete, report is available here below.

5.2.3.1.  Example 1

   The first data-set consists in a full-mesh of 12000 explicitly-routed
   tunnels observed on a real network.  These tunnels resulted from
   distributed headend-based CSPF computation.

   We measured that only 65% of the traffic is riding on its shortest
   path.

   Three well-known defects are illustrated in this data set:

      The lack of ECMP support in explicitly--routed tunnels: ATM-alike
      traffic-steering mechanisms steer the traffic along a non-ECMP
      path.

      The increase of the number of explicitly-routed non-ECMP tunnels
      to enumerate all the ECMP options.

      The inefficiency of distributed optimization: too much traffic is
      riding off its shortest path.

   We applied the SDN/SR use-case to this dataset.  This means that:

      The distributed CSPF computation is replaced by centralized
      optimization and BW admission control, supported by the SDN
      Controller.





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         As part of the optimization, we also optimized the IGP-metrics
         such as to get a maximum of traffic load-spread among ECMP-
         paths by default.

      The traffic-engineering policies are supported by SR segment-
      lists.

   As a result, we measured that 98% of the traffic would be kept on its
   normal policy (ride shortest-path) and only 2% of the traffic
   requires a path away from the shortest-path.

   Let us highlight a few benefits:

      98% of the traffic-engineering head-end policies are eliminated.

         Indeed, by default, an SR-capable ingress edge node maps the
         traffic on a single Node-ID to the egress edge node.  No
         configuration or policy needs to be maintained at the ingress
         edge node to realize this.

      100% of the states at mid/tail nodes are eliminated.

5.3.  Residual Bandwidth

   The notion of Residual Bandwidth (RBW) is introduced by
   [I-D.atlas-mpls-te-express-path].

   A future version of this document will describe the SR/RBW research
   opportunity.


6.  SR co-existence and interworking with other MPLS Control Plane

   The first section describes the co-existence of SR with other MPLS
   Control Plane.  The second section documents a method to migrate from
   LDP to SR-based MPLS tunneling.  The third section documents the
   interworking of LDP and SR in the case of non-homogenous deployment.
   The fourth use-case describes how a partial SR deployment can be used
   to provide SR benefits to LDP-based traffic.  The fifth section
   describes a possible application of SR in the context of inter-domain
   MPLS use-cases.

6.1.  Ship-in-the-night coexistence

   We call "MPLS Control Plane Client (MCC)" any control-plane protocol
   installing forwarding entries in the MPLS dataplane.  SR, LDP,
   RSVP-TE, BGP 3107, VPNv4, etc. are examples of MCCs.




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   An MCC, operating at node N, must ensure that the incoming label it
   installs in the MPLS dataplane of Node N has been uniquely allocated
   to himself.

   Thanks to the defined segment allocation rule and specifically the
   notion of the SRGB, SR can co-exist with any other MCC.

   This is clearly the case for the adjacency segment: it is a local
   label allocated by the label manager, as for any MCC.

   This is clearly the case for the prefix segment: the label manager
   allocates the SRGB set of labels to the SR MCC client and the
   operator ensures the unique allocation of each global prefix segment/
   label within the allocated SRGB set.

   Note that this static label allocation capability of the label
   manager has been existing for many years across several vendors and
   hence is not new.  Furthermore, note that the label-manager ability
   to statically allocate a range of labels to a specific application is
   not new either.  This is required for MPLS-TP operation.  In this
   case, the range is reserved by the label manager and it is the
   MPLS-TP NMS (acting as an MCC) that ensures the unique allocation of
   any label within the allocated range and the creation of the related
   MPLS forwarding entry.

   Let us illustrate an example of ship-in-the-night (SIN) coexistence.
                              PE2          PE4
                                \          /
                          PE1----A----B---C---PE3

                        Figure 11: SIN coexistence

   The EVEN VPN service is supported by PE2 and PE4 while the ODD VPN
   service is supported by PE1 and PE3.  The operator wants to tunnel
   the ODD service via LDP and the EVEN service via SR.

   This can be achieved in the following manner:

      The operator configures PE1, PE2, PE3, PE4 with respective
      loopbacks 192.0.2.201/32, 192.0.2.202/32, 192.0.2.203/32,
      192.0.2.204/32.  These PE's advertised their VPN routes with next-
      hop set on their respective loopback address.

      The operator configures A, B, C with respective loopbacks
      192.0.2.1/32, 192.0.2.2/32, 192.0.2.3/32.






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      The operator configures PE2, A, B, C and PE4 with SRGB {100, 300}.

      The operator attaches the respective Node-SIDs 202, 101, 102, 103
      and 204 to the loopbacks of nodes PE2, A, B, C and PE4.  The Node-
      SID's are configured to request penultimate-hop-popping.

      PE1, A, B, C and PE3 are LDP capable.

      PE1 and PE3 are not SR capable.

   PE3 sends an ODD VPN route to PE1 with next-hop 192.0.2.203 and VPN
   label 10001.

   From an LDP viewpoint: PE1 received an LDP label binding (1037) for
   FEC 192.0.2.203/32 from its nhop A. A received an LDP label binding
   (2048) for that FEC from its nhop B. B received an LDP label binding
   (3059) for that FEC from its nhop C. C received implicit-null LDP
   binding from its next-hop PE3.

   As a result, PE1 sends its traffic to the ODD service route
   advertised by PE3 to next-hop A with two labels: the top label is
   1037 and the bottom label is 10001.  A swaps 1037 with 2048 and
   forwards to B. B swaps 2048 with 3059 and forwards to C. C pops 3059
   and forwards to PE3.

   PE4 sends an EVEN VPN route to PE2 with next-hop 192.0.2.204 and VPN
   label 10002.

   From an SR viewpoint: PE1 maps the IGP route 192.0.2.204/32 onto
   Node-SID 204; A swaps 204 with 204 and forwards to B; B swaps 204
   with 204 and forwards to C; C pops 204 and forwards to PE4.

   As a result, PE2 sends its traffic to the VPN service route
   advertised by PE4 to next-hop A with two labels: the top label is 204
   and the bottom label is 10002.  A swaps 204 with 204 and forwards to
   B. B swaps 204 with 204 and forwards to C. C pops 204 and forwards to
   PE4.

   The two modes of MPLS tunneling co-exist.

      The ODD service is tunneled from PE1 to PE3 through a continuous
      LDP LSP traversing A, B and C.

      The EVEN service is tunneled from PE2 to PE4 through a continuous
      SR node segment traversing A, B and C.






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6.1.1.  MPLS2MPLS co-existence

   We want to highlight that several MPLS2MPLS entries can be installed
   in the dataplane for the same prefix.

   Let us examine A's MPLS forwarding table as an example:

      Incoming label: 1037
          - outgoing label: 2048
          - outgoing nhop: B
          - Note: this entry is programmed by LDP for 192.0.2.203/32

      Incoming label: 203
          - outgoing label: 203
          - outgoing nhop: B
          - Note: this entry is programmed by SR for 192.0.2.203/32

   These two entries can co-exist because their incoming label is
   unique.  The uniqueness is guaranteed by the label manager allocation
   rules.

   The same applies for the MPLS2IP forwarding entries.

6.1.2.  IP2MPLS co-existence

   By default, we propose that if both LDP and SR propose an IP2MPLS
   entry for the same IP prefix, then the LDP route is selected.

   A local policy on a router MUST allow to prefer the SR-provided
   IP2MPLS entry.

6.2.  Migration from LDP to SR
                               PE2        PE4
                                 \        /
                          PE1----P5--P6--P7---PE3

                           Figure 12: Migration

   Several migration techniques are possible.  We describe one technique
   inspired by the commonly used method to migrate from one IGP to
   another.

   T0: all the routers run LDP.  Any service is tunneled from an ingress
   PE to an egress PE over a continuous LDP LSP.

   T1: all the routers are upgraded to SR.  They are configured with the
   SRGB range (100, 200).  PE1, PE2, PE3, PE4, P5, P6 and P7 are
   respectively configured with the node segments 101, 102, 103, 104,



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   105, 106 and 107 (attached to their service-recursing loopback).

      At this time, the service traffic is still tunneled over LDP LSP.
      For example, PE1 has an SR node segment to PE3 and an LDP LSP to
      PE3 but by default, as seen earlier, the LDP IP2MPLS encapsulation
      is preferred.

   T2: the operator enables the local policy at PE1 to prefer SR IP2MPLS
   encapsulation over LDP IP2MPLS.

      The service from PE1 to any other PE is now riding over SR.  All
      other service traffic is still transported over LDP LSP.

   T3: gradually, the operator enables the preference for SR IP2MPLS
   encapsulation across all the edge routers.

      All the service traffic is now transported over SR.  LDP is still
      operational and services could be reverted to LDP.

   T4: LDP is unconfigured from all routers.

6.3.  SR and LDP Interworking

   In this section, we analyze a use-case where SR is available in one
   part of the network and LDP is available in another part.  We
   describe how a continuous MPLS tunnel can be built throughout the
   network.
                             PE2            PE4
                               \            /
                        PE1----P5--P6--P7--P8---PE3

                    Figure 13: SR and LDP Interworking

   Let us analyze the following example:

      P6, P7, P8, PE4 and PE3 are LDP capable.

      PE1, PE2, P5 and P6 are SR capable.  PE1, PE2, P5 and P6 are
      configured with SRGB (100, 200) and respectively with node
      segments 101, 102, 105 and 106.

      A service flow must be tunneled from PE1 to PE3 over a continuous
      MPLS tunnel encapsulation.  We need SR and LDP to interwork.








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6.3.1.  LDP to SR

   In this section, we analyze a right-to-left traffic flow.

   PE3 has learned a service route whose nhop is PE1.  PE3 has an LDP
   label binding from the nhop P8 for the FEC "PE1".  Hence PE3 sends
   its service packet to P8 as per classic LDP behavior.

   P8 has an LDP label binding from its nhop P7 for the FEC "PE1" and
   hence P8 forwards to P7 as per classic LDP behavior.

   P7 has an LDP label binding from its nhop P6 for the FEC "PE1" and
   hence P7 forwards to P6 as per classic LDP behavior.

   P6 does not have an LDP binding from its nhop P5 for the FEC "PE1".
   However P6 has an SR node segment to the IGP route "PE1".  Hence, P6
   forwards the packet to P5 and swaps its local LDP-label for FEC "PE1"
   by the equivalent node segment (i.e. 101).

   P5 pops 101 (assuming PE1 advertised its node segment 101 with the
   penultimate-pop flag set) and forwards to PE1.

   PE1 receives the tunneled packet and processes the service label.

   The end-to-end MPLS tunnel is built from an LDP LSP from PE3 to P6
   and the related node segment from P6 to PE1.

6.3.2.  SR to LDP

   In this section, we analyze the left-to-right traffic flow.

   We assume that the operator configures P5 to act as a Segment Routing
   Mapping Server (SRMS) and advertise the following mappings: (P7,
   107), (P8, 108), (PE3, 103) and (PE4, 104).

   The mappings advertised by an SR mapping server result from local
   policy information configured by the operator.  IF PE3 had been SR
   capable, the operator would have configured PE3 with node segment
   103.  Instead, as PE3 is not SR capable, the operator configures that
   policy at the SRMS and it is the latter which advertises the mapping.
   Multiple SRMS servers can be provisioned in a network for redundancy.

   The mapping server advertisements are only understood by the SR
   capable routers.  The SR capable routers install the related node
   segments in the MPLS dataplane exactly like if the node segments had
   been advertised by the nodes themselves.

   For example, PE1 installs the node segment 103 with nhop P5 exactly



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   as if PE3 had advertised node segment 103.

   PE1 has a service route whose nhop is PE3.  PE1 has a node segment
   for that IGP route: 103 with nhop P5.  Hence PE1 sends its service
   packet to P5 with two labels: the bottom label is the service label
   and the top label is 103.

   P5 swaps 103 for 103 and forwards to P6.

   P6's next-hop for the IGP route "PE3" is not SR capable (P7 does not
   advertise the SR capability).  However, P6 has an LDP label binding
   from that next-hop for the same FEC (e.g.  LDP label 1037).  Hence,
   P6 swaps 103 for 1037 and forwards to P7.

   P7 swaps this label with the LDP-label received from P8 and forwards
   to P8.

   P8 pops the LDP label and forwards to PE3.

   PE3 receives the tunneled packet and processes the service label.

   The end-to-end MPLS tunnel is built from an SR node segment from PE1
   to P6 and an LDP LSP from P6 to PE3.

6.4.  Leveraging SR benefits for LDP-based traffic

   SR can be deployed such as to enhance LDP transport.  The SR
   deployment can be limited to the network region where the SR benefits
   are most desired.

   In Figure 14, let us assume:

      All link costs are 10 except FG which is 30.

      All routers are LDP capable.

      X, Y and Z are PE's participating to an important service S.

      The operator requires 50msec link-based FRR for service S.

      A, B, C, D, E, F and G are SR capable.

      X, Y, Z are not SR capable, e.g. as part of a staged migration
      from LDP to SR, the operator deploys SR first in a sub-part of the
      network and then everywhere.






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                                     X
                                     |
                              Y--A---B---E--Z
                                 |   |    \
                                 D---C--F--G
                                         30

          Figure 14: Leveraging SR benefits for LDP-based-traffic

   The operator would like to resolve the following issues:

      To protect the link BA along the shortest-path of the important
      flow XY, B requires an RLFA repair tunnel to D and hence a
      directed LDP session from B to D. The operator does not like these
      dynamically established multi-hop LDP sessions and would seek to
      eliminate them.

      There is no LFA/RLFA solution to protect the link BE along the
      shortest path of the important flow XZ.  The operator wants a
      guaranteed link-based FRR solution.

   The operator can meet these objectives by deploying SR only on A, B,
   C, D, E and F:

      The operator configures A, B, C, D, E, F and G with SRGB (100,
      200) and respective node segments 101, 102, 103, 104, 105, 106 and
      107.

      The operator configures D as an SR Mapping Server with the
      following policy mapping: (X, 201), (Y, 202), (Z, 203}.

      Each SR node automatically advertises local adjacency segment for
      its IGP adjacencies.  Specifically, F advertises adjacency segment
      9001 for its adjacency FG.

   A, B, C, D, E, F and G keep their LDP capability and hence the flows
   XY and XZ are transported over end-to-end LDP LSP's.

   For example, LDP at B installs the following MPLS dataplane entries:
   Incoming label: local LDB label bound by B for FEC Y
      Outgoing label: LDP label bound by A for FEC Y
      Outgoing nhop: A
   Incoming label: local LDB label bound by B for FEC Z
     Outgoing label: LDP label bound by E for FEC Z
     Outgoing nhop: E

   The novelty comes from how the backup chains are computed for these
   LDP-based entries.  While LDP labels are used for the primary nhop



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   and outgoing labels, SR information is used for the FRR construction.
   In steady state, the traffic is transported over LDP LSP.  In
   transient FRR state, the traffic is backup thanks to the SR enhanced
   capabilities.

   This helps meet the requirements of the operator:

      Eliminate directed LDP session.

      Guaranteed FRR coverage.

      Keep the traffic over LDP LSP in steady state.

      Partial SR deployment only where needed.

6.4.1.  Eliminating Directed LDP Session

   B's MPLS entry to Y becomes:
   - Incoming label: local LDB label bound by B for FEC Y
       Outgoing label: LDP label bound by A for FEC Y
       Backup outgoing label: SR node segment for Y {202}
       Outgoing nhop: A
       Backup nhop: repair tunnel: node segment to D {104}
        with outgoing nhop: C

   In steady-state, X sends its Y-destined traffic to B with a top label
   which is the LDP label bound by B for FEC Y. B swaps that top label
   for the LDP label bound by A for FEC Y and forwards to A. A pops the
   LDP label and forwards to Y.

   Upon failure of the link BA, B swaps the incoming top-label with the
   node segment for Y (202) and sends the packet onto a repair tunnel to
   D (node segment 104).  Thus, B sends the packet to C with the label
   stack {104, 202}.  C pops the node segment 104 and forwards to D. D
   swaps 202 for 202 and forwards to A. A's nhop to Y is not SR capable
   and hence A swaps the incoming node segment 202 to the LDP label
   announced by its next-hop (in this case, implicit null).

   After IGP convergence, B's MPLS entry to Y will become:
   - Incoming label: local LDB label bound by B for FEC Y
       Outgoing label: LDP label bound by C for FEC Y
       Outgoing nhop: C

   And the traffic XY travels again over the LDP LSP.

   Conclusion: the operator has eliminated its first problem: directed
   LDP sessions are no longer required and the steady-state traffic is
   still transported over LDP.  The SR deployment is confined to the



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   area where these benefits are required.

6.4.2.  Guaranteed FRR coverage

   B's MPLS entry to Z becomes:
   - Incoming label: local LDB label bound by B for FEC Z
       Outgoing label: LDP label bound by E for FEC Z
       Backup outgoing label: SR node segment for Z {203}
       Outgoing nhop: E
       Backup nhop: repair tunnel to G: {106, 9001}

           G is reachable from B via the combination of a
           node segment to F {106} and an adjacency segment
           FG {9001}

           Note that {106, 107} would have equally work.
           Indeed, in many case, P's shortest path to Q is
           over the link PQ. The adjacency segment from P to
           Q is required only in very rare topologies where
           the shortest-path from P to Q is not via the link
           PQ.

   In steady-state, X sends its Z-destined traffic to B with a top label
   which is the LDP label bound by B for FEC Z. B swaps that top label
   for the LDP label bound by E for FEC Z and forwards to E. E pops the
   LDP label and forwards to Z.

   Upon failure of the link BE, B swaps the incoming top-label with the
   node segment for Z (203) and sends the packet onto a repair tunnel to
   G (node segment 106 followed by adjacency segment 9001).  Thus, B
   sends the packet to C with the label stack {106, 9001, 203}.  C pops
   the node segment 106 and forwards to F. F pops the adjacency segment
   9001 and forwards to G. G swaps 203 for 203 and forwards to E. E's
   nhop to Z is not SR capable and hence E swaps the incoming node
   segment 203 for the LDP label announced by its next-hop (in this
   case, implicit null).

   After IGP convergence, B's MPLS entry to Z will become:
   - Incoming label: local LDB label bound by B for FEC Z
       Outgoing label: LDP label bound by C for FEC Z
       Outgoing nhop: C

   And the traffic XZ travels again over the LDP LSP.

   Conclusion: the operator has eliminated its second problem:
   guaranteed FRR coverage is provided.  The steady-state traffic is
   still transported over LDP.  The SR deployment is confined to the
   area where these benefits are required.



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6.5.  Inter-AS Option C, Carrier's Carrier and Seamless MPLS
                       PE1---R1---B1---B2---R2---PE2
                       <----------->   <----------->
                            AS1            AS2

                       Figure 15: Inter-AS Option C

   In Inter-AS Option C [RFC4364], B2 advertises to B1 a BGP3107 route
   for PE2 and B1 reflects it to its internal peers, such as PE1.  PE1
   learns from a service route reflector a service route whose nhop is
   PE2.  PE1 resolves that service route on the BGP3107 route to PE2.
   That BGP3107 route to PE2 is itself resolved on the AS1 IGP route to
   B1.

   If AS1 operates SR, then the tunnel from PE1 to B1 is provided by the
   node segment from PE1 to B1.

   PE1 sends a service packet with three labels: the top one is the node
   segment to B1, the next-one is the BGP3107 label provided by B1 for
   the route "PE2" and the bottom one is the service label allocated by
   PE2.

   The same straightforward SR applicability is derived for CsC and
   Seamless MPLS ([I-D.ietf-mpls-seamless-mpls]).


7.  OAM

7.1.  Monitoring a remote bundle

   This section documents a few representative SR/OAM use-cases.
               +--+    _   +--+                    +-------+
               |  |   { }  |  |---991---L1---662---|       |
               |MS|--{   }-|R1|---992---L2---663---|R2 (72)|
               |  |   {_}  |  |---993---L3---664---|       |
               +--+        +--+                    +-------+

            Figure 16: Probing all the links of a remote bundle

   In the above figure, a monitoring system (MS) needs to assess the
   dataplane availability of all the links within a remote bundle
   connected to routers R1 and R2.

   The monitoring system retrieves the segment information from the IGP
   LSDB and appends the following segment list: {72, 662, 992, 664} on
   its IP probe (whose source and destination addresses are the address
   of AA).




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   MS sends the probe to its connected router.  If the connected router
   is not SR compliant, a tunneling technique can be used to tunnel the
   SR-based probe to the first SR router.  The SR domain forwards the
   probe to R2 (72 is the node segment of R2).  R2 forwards the probe to
   R1 over link L1 (adjacency segment 662).  R1 forwards the probe to R2
   over link L2 (adjacency segment 992).  R2 forwards the probe to R1
   over link L3 (adjacency segment 664).  R1 then forwards the IP probe
   to AA as per classic IP forwarding.

7.2.  Monitoring a remote peering link

   In Figure 6, node A can monitor the dataplane liveness of the
   unidirectional peering link from C to D of AS2 by sending an IP probe
   with destination address A and segment list {101, 9001}.  Node-SID
   101 steers the probe to C and External Adj-SID 9001 steers the probe
   from C over the desired peering link to D of AS2.  The SR header is
   removed by C and D receives a plain IP packet with destination
   address A. D returns the probe to A through classic IP forwarding.
   BFD Echo mode ([RFC5880]) would support such liveliness
   unidirectional link probing application.


8.  IANA Considerations

   TBD


9.  Manageability Considerations

   TBD


10.  Security Considerations

   TBD


11.  Acknowledgements

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


12.  References






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12.1.  Normative References

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

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, February 2006.

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, December 2006.

   [RFC5101]  Claise, B., "Specification of the IP Flow Information
              Export (IPFIX) Protocol for the Exchange of IP Traffic
              Flow Information", RFC 5101, January 2008.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, October 2008.

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

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

12.2.  Informative References

   [I-D.atlas-mpls-te-express-path]
              Atlas, A., Drake, J., Giacalone, S., Ward, D., Previdi,
              S., and C. Filsfils, "Performance-based Path Selection for
              Explicitly Routed LSPs",
              draft-atlas-mpls-te-express-path-02 (work in progress),
              February 2013.

   [I-D.crabbe-pce-pce-initiated-lsp]
              Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "PCEP
              Extensions for PCE-initiated LSP Setup in a Stateful PCE
              Model", draft-crabbe-pce-pce-initiated-lsp-01 (work in
              progress), April 2013.

   [I-D.francois-sr-frr]
              Francois, P., Filsfils, C., Bashandy, A., Previdi, S., and
              B. Decraene, "Segment Routing Fast Reroute",
              draft-francois-sr-frr-00 (work in progress), July 2013.

   [I-D.gredler-isis-label-advertisement]
              Gredler, H., Amante, S., Scholl, T., and L. Jalil,
              "Advertising MPLS labels in IS-IS",



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              draft-gredler-isis-label-advertisement-03 (work in
              progress), May 2013.

   [I-D.ietf-idr-ls-distribution]
              Gredler, H., Medved, J., Previdi, S., Farrel, A., and S.
              Ray, "North-Bound Distribution of Link-State and TE
              Information using BGP", draft-ietf-idr-ls-distribution-03
              (work in progress), May 2013.

   [I-D.ietf-mpls-seamless-mpls]
              Leymann, N., Decraene, B., Filsfils, C., Konstantynowicz,
              M., and D. Steinberg, "Seamless MPLS Architecture",
              draft-ietf-mpls-seamless-mpls-03 (work in progress),
              May 2013.

   [I-D.ietf-pce-stateful-pce]
              Crabbe, E., Medved, J., Minei, I., and R. Varga, "PCEP
              Extensions for Stateful PCE",
              draft-ietf-pce-stateful-pce-04 (work in progress),
              May 2013.

   [I-D.ietf-rtgwg-remote-lfa]
              Bryant, S., Filsfils, C., Previdi, S., Shand, M., and S.
              Ning, "Remote LFA FRR", draft-ietf-rtgwg-remote-lfa-02
              (work in progress), May 2013.

   [I-D.previdi-isis-te-metric-extensions]
              Previdi, S., Giacalone, S., Ward, D., Drake, J., Atlas,
              A., and C. Filsfils, "IS-IS Traffic Engineering (TE)
              Metric Extensions",
              draft-previdi-isis-te-metric-extensions-03 (work in
              progress), February 2013.

   [I-D.ward-i2rs-framework]
              Atlas, A., Nadeau, T., and D. Ward, "Interface to the
              Routing System Framework", draft-ward-i2rs-framework-00
              (work in progress), February 2013.

   [RFC5443]  Jork, M., Atlas, A., and L. Fang, "LDP IGP
              Synchronization", RFC 5443, March 2009.

   [RFC6138]  Kini, S. and W. Lu, "LDP IGP Synchronization for Broadcast
              Networks", RFC 6138, February 2011.

   [RFC6571]  Filsfils, C., Francois, P., Shand, M., Decraene, B.,
              Uttaro, J., Leymann, N., and M. Horneffer, "Loop-Free
              Alternate (LFA) Applicability in Service Provider (SP)
              Networks", RFC 6571, June 2012.



Filsfils, et al.        Expires January 15, 2014               [Page 43]


Internet-Draft          Segment Routing Use Cases              July 2013


   [draft-filsfils-rtgwg-segment-routing-00]
              Filsfils, C. and S. Previdi, "Segment Routing
              Architecture", June 2013.

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

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

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

   [draft-rtgwg-bgp-pic-01.txt]
              Filsfils, C., Bashandy, A., and P. Mohapatra, "BGP Prefix
              Independent Convergence", March 2013.


Authors' Addresses

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

   Email: cfilsfil@cisco.com


   Pierre Francois (editor)
   IMDEA Networks
   Leganes,
   ES

   Email: pifranco@cisco.com


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

   Email: sprevidi@cisco.com





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   Bruno Decraene
   Orange
   FR

   Email: bruno.decraene@orange.com


   Stephane Litkowski
   Orange
   FR

   Email: stephane.litkowski@orange.com


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

   Email: Martin.Horneffer@telekom.de


   Igor Milojevic
   Telekom Srbija
   Takovska 2
   Belgrade
   RS

   Email: igormilojevic@telekom.rs


   Rob Shakir
   British Telecom
   London
   UK

   Email: rob.shakir@bt.com


   Saku Ytti
   TDC Oy
   Mechelininkatu 1a
   TDC  00094
   FI

   Email: saku@ytti.fi




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Internet-Draft          Segment Routing Use Cases              July 2013


   Wim Henderickx
   Alcatel-Lucent
   Copernicuslaan 50
   Antwerp  2018
   BE

   Email: wim.henderickx@alcatel-lucent.com


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

   Email: Jeff.Tantsura@ericsson.com


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

   Email: edc@google.com


























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