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SPRING Working Group                                        S. Litkowski
Internet-Draft                                                    Orange
Intended status: Standards Track                             M. Aissaoui
Expires: July 31, 2017                                             Nokia
                                                        January 27, 2017


             Implementing non protected paths using SPRING
             draft-litkowski-spring-non-protected-paths-01

Abstract

   Segment Routing (SR) leverages the source routing paradigm.  A node
   can steer a packet on a specific path by prepending the packet with
   an SR header.  In the framework of traffic-engineering use cases, a
   customer may request its service provider to implement some non
   protected paths.  This means that in case of a failure within the
   network, fast-reroute (or similar) techniques should not be activated
   for those paths.  This document analyzes the different options to
   implement a non protected path with Segment Routing and in a future
   release will provide a recommandation on the best option.

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 [RFC2119].

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 31, 2017.







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Copyright Notice

   Copyright (c) 2017 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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   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.

Table of Contents

   1.  Problem statement . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements for a non protected LSP  . . . . . . . . . . . .   6
     2.1.  ECMP considerations . . . . . . . . . . . . . . . . . . .   7
   3.  Options to create a non protected path with Segment Routing .   7
     3.1.  Using only non protected adjacency segments . . . . . . .   7
     3.2.  Using a combination of node segments and adjacency
           segments  . . . . . . . . . . . . . . . . . . . . . . . .   8
       3.2.1.  Adding a protection flag in the Node SID  . . . . . .   8
       3.2.2.  Using Strict SPF Node SID . . . . . . . . . . . . . .   9
       3.2.3.  Using two Node-SIDs with different local policies . .   9
       3.2.4.  Advantages and drawbacks  . . . . . . . . . . . . . .   9
     3.3.  Using a combination of adjacency segments and binding-SID  10
   4.  Comparison  . . . . . . . . . . . . . . . . . . . . . . . . .  11
   5.  Recommended option(s) . . . . . . . . . . . . . . . . . . . .  13
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Problem statement

   In some cases, a customer may prefer to react on network failures
   using its own mechanism.  In such cases, the customer usually has two
   disjoint paths, so a path can take over the traffic in case of
   failure of the other.  The disjoint paths can be provided by a single
   provider or by multihoming to different providers as displayed in the
   figure below.






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            _________________________________________
           /                                         \
          /                                           \
         |                                             |
         |                                             |
         |                                             |
         |          ***********************>           |
         | +------+           10             +------+  |
   CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2
         | +------+       |        |         +------+  |
         |                |        |                   |
         |                |        |                   |
         | +------+       |        |         +------+  |
   CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4
         | +------+ ***********************> +------+  |
         |                                             |
          \                                           /
           \_________________________________________/



          Figure 1 - Disjoint paths provided by a single provider





























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            _________________________________________
           /                                         \
          /                                           \
         |                                             |
         |                                             |
         |          ***********************>           |
         | +------+           10             +------+  |
   CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2
         | +------+                          +------+  |
         |                                             |
          \                                           /
           \_________________________________________/

            _________________________________________
           /                                         \
          /                                           \
         |                                             |
         |                                             |
         | +------+                          +------+  |
   CE3 ****| PE 1 | ----- R3 ---- R4 ------- | PE 2 |**** CE4
         | +------+ ***********************> +------+  |
         |                                             |
          \                                           /
           \_________________________________________/



         Figure 2 - Disjoint paths provided by using two providers

   As the traffic protection is ensured by an end-to-end mechanism at
   the customer level, the customer requests the service provider to not
   protect the paths.  This is particularly required to avoid both
   protection mechanisms (customer level and provider level) to be
   activated at the same time which may lead to unpredictable side
   effects.  However the service provider is allowed to restore the end-
   to-end path automatically when the primary path is failing by
   computing and installing a new primary path at the head-end.  How the
   end-to-end protection is handled is out of scope of this document and
   will be under the customer responsibility.

   Another use case could be a service provider selling the traffic
   protection as a service option.  So by default, the provided IP/MPLS
   path is not protected by any fast-reroute mechanism but the customer
   can subscribe to an option to activate fast-reroute for its traffic.
   In the figure 3, the Customer1 service between PE1 and PE2 is
   protected, in case of failure between R1 and R2, the LSP can use a
   bypass through R3-R4 nodes until the convergence occurs.  The
   Customer2 did not subscribe to the traffic protection option.  If



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   R3-R4 fails, the traffic between CE3 and CE4 will be disrupted until
   the convergence occurs.

          _________________________________________
         /                                         \
        /                                           \
       |             Protected LSP                   |
       |          ***********************>           |
       | +------+           10             +------+  |
 CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2    Customer1
       | +------+       |*      *|         +------+  |
       |                |*      *|                   |
       |                |*      *|                   |
       | +------+       |********|         +------+  |
 CE3 ****| PE 3 | ----- R3 ---- R4 ------- | PE 4 |**** CE4    Customer2
       | +------+ ***********************> +------+  |
       |              Non protected LSP              |
        \                                           /
         \_________________________________________/



        Figure 3 - Provider selling traffic protection as an option

   A service provider may also propose a traffic protection service
   based on path protection rather than local repair on each transit
   node.  In the figure 4, on PE1, two LSPs were created to ensure the
   customer traffic protection between PE1 and PE2.  The primary LSP is
   used to carry the traffic in the nominal situation.  The protection
   LSP is built as disjoint from the primary LSP and may be
   preestablished (from controlplane and/or dataplane point of view).
   When the primary LSP fails, PE1 is responsible to switch the traffic
   to the protection LSP.  As the protection is provided by PE1, both
   primary and protection LSPs should be setup as non protected so
   transit nodes will not activate any local-repair mechanism for those
   LSPs.















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          _________________________________________
         /                                         \
        /                                           \
       |             Primary LSP                     |
       |          ***********************>           |
       | +------+           10             +------+  |
 CE1 ****| PE 1 | ----- R1 ---- R2 ------- | PE 2 |**** CE2    Customer1
       | +------+       |        |         +------+  |
       |         *\     |        |        />         |
       |          *\    |        |       /*          |
       |           *\   |        |      /*           |
       |            *+- R3 ---- R4 ----+*               |
       |             *******************             |
       |               Protection LSP                |
        \                                           /
         \_________________________________________/



        Figure 4 - Provider selling traffic protection as an option

   A segment-routing path is expressed as a list of segment identifiers
   (SID) from different types (Node-SID, Adj-SID, Binding-SID ...).  In
   order to ensure that the segment routing path is not protected, we
   need to ensure that it does not contain any segment representing a
   protected path.  As an example, in the Figure 1, we consider a path
   from PE1 to PE2 expressed with the following segment list:
   {Adj_R1R3,Node_R2,Adj_R2PE2}. If we want to ensure that this path is
   not protected, we need to ensure that the segment represented by
   Adj_R1R3 represents a non protected segment, as well as the segments
   Node_R2 and Adj_R2PE2.

   The segment routing path may be computed by a Path Computation
   Element (PCE).  In order to fulfil the non protected path constraint,
   the PCE needs to be aware of the available SIDs in the network and
   their protection status.

   Several techniques may be used to represent a non protected path with
   a segment identifier.  We propose to analyze the different options.

2.  Requirements for a non protected LSP

   o  A non protected LSP SHOULD follow a primary path defined based on
      the constraints of the LSP.  This path can be the shortest path
      (as per the IGP metric) or a more constrained path (explicit path)
      to fulfil for example a bandwidth, latency or disjointness
      requirement.




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   o  Upon a failure, a non protected LSP SHOULD be reestablished over a
      new suitable non-protected path that still fulfils the constraints
      of the LSP.

   o  Upon a failure (link, node, srlg...), the traffic of a non
      protected LSP MUST NOT use any local-repair or any local-rerouting
      mechanism on transit nodes.

   o  The computation of a new primary path for the LSP will be handled
      by the computation node responsible of this LSP (it could be the
      head-end or a PCE).

   o  Upon any other traffic-engineering topology change (metric change,
      overload status change, bandwidth change, latency change...), the
      non protected LSP MAY be reoptimized to a better path.

2.1.  ECMP considerations

   When equal cost paths are available within the end-to-end path,
   implementations may reuse a fast-reroute like mechanism in the
   dataplane, so when one of the outgoing interface fails, the dataplane
   switches traffic immediately to the remaining outgoing interfaces in
   the ECMP set.  This behavior is usually hardcoded and cannot be
   disabled.  Based on this assumption, a non protected LSP SHOULD avoid
   ECMPs.

3.  Options to create a non protected path with Segment Routing

3.1.  Using only non protected adjacency segments

   A node can advertise multiple adjacency segments for a particular
   link with different properties.  The non-protected property is
   already defined as part of the protocol encodings
   ([I-D.ietf-isis-segment-routing-extensions],
   [I-D.ietf-ospf-segment-routing-extensions] and
   [I-D.gredler-idr-bgp-ls-segment-routing-extension]) through the B
   flag.  However, from an implementation perspective, advertising a
   protected adjacency segment, a non protected adjacency segment or
   both for each link is optional.

   It is important to note that even if an adjacency segment has the B
   flag set (protected), it remains up to a local policy of the
   advertising router to implement the protection or not.

   If both protected and non protected Adj-SID are advertised, every
   node in the network (including PCEs) can be aware of the adjacency
   segments protection property.  When a non protected path is




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   requested, the path computation module can choose to encode the path
   with a list a non protected adjacency segments only.

   One of the advantage of using only adjacency segments is the
   insurance that the traffic will never go transiently outside the path
   defined by the computation module responsible of the path.  This
   solution is fully compliant with the requirements sets in Section 2.

   One of the drawbacks of using only adjacency segments is the
   resulting label stack depth as each hop should require a segment in
   the stack: crossing 15 nodes, means stacking 15 labels to encode the
   SR tunnel.  Having such a deep stack may be a problem for current
   hardwares and softwares for either pushing the stack (because the
   head end is limited in the number of labels it can push) or
   loadbalancing flows on transit nodes (as deep packet inspection or
   entropy label look up may be difficult with a deep label stack).
   Another drawback of advertising both protected and non protected
   adjacency segments is the additional controlplane and dataplane
   resource consumption used in the network.  As the adjacency SIDs have
   a local significance, this resource consumption can be considered as
   negligeable from a data plane point of view.  From a control plane
   point of view, this can also be considered as negligeable with the
   current CPU and memory usually available on routers.

3.2.  Using a combination of node segments and adjacency segments

   Using a combination of node segments and adjacency segments is the
   usual way of creating a segment routing path.  However the well known
   Node-SID (algorithm type Shortest Path) may be protected by a local-
   repair mechanism by any transit node or may use ECMPs which may be a
   problem when used for a non protected path.  Protecting a particular
   Node-SID is a matter of a local policy configuration on every node.
   The following discusses a number of possible approaches.

3.2.1.  Adding a protection flag in the Node SID

   As for adjacency segments, a new flag may be added in the Prefix-SID
   to encode the willingness of protection.  Each node will then
   advertise two Node-SIDs (using SPF algorithm), one with the
   protection flag set, the other without the protection flag set.  The
   same discussion regarding ECMP is also applicable here.

   The remaining flag space in the Prefix-SID is small, so adding a new
   flag requires analysis but this should not be considered as a
   showstopper.






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3.2.2.  Using Strict SPF Node SID

   [I-D.ietf-spring-segment-routing] defines a Strict Shortest Path
   algorithm which mandates that the packet is forwarded according to
   ECMP-aware SPF algorithm and instructs any router in the path to
   ignore any possible local policy overriding SPF decision.  The use of
   a local-repair for a strict SPF Node-SID is allowed as long as the
   FRR mechanism enforces the post convergence path to the destination.

   This solution does not bring any benefit compared to the regular
   Node-SID (as it has similar properties).

3.2.3.  Using two Node-SIDs with different local policies

   Having two instances of the Node-SID (protected and not protected) is
   a requirement when using Node-SID in protected and non protected
   paths.  The protection of a Node-SID is a matter of a local policy
   configuration on every node in the network.  A service provider may
   configure two Node-SIDs per node and may adjust the local-repair on
   every node to protect one Node-SID but not the other.  As the
   protection of the Node-SID is inherited from the protection of the
   associated prefix, the service provider will need to deploy a new set
   of prefixes to all nodes to deploy the new set of Node-SIDs.  Then it
   will need to maintain the local-repair policy on every node to ensure
   that the prefixes associated to the non protected Node-SID are not
   using the local-repair.

   The path computation engine (head-end or PCE) must be aware of the
   policy defined by the service provider so it can select the right
   SIDs/prefixes when computing a path.

3.2.4.  Advantages and drawbacks

   One advantage of combining adjacency and node segments is the
   reduction of the label stack size.

   The drawbacks are the increase of the controlplane and dataplane
   resource consumption.  Whereas having two adjacency SIDs introduces a
   negligeable impact, having two nodes SIDs increases controlplane and
   dataplane processing as each node in the network will have to install
   an MPLS->MPLS and IP->MPLS entry for each additional Node-SID.  The
   regular IP convergence time of the network may be doubled in the
   worst case while the newly deployed node-SIDs are only used for
   traffic-engineering applications.  One of the other drawback is that
   a Node-SID may be transiently rerouted on a path that does not fit
   the constraints anymore if a transit node converges faster than the
   head-end: this concern is not new and applies to all traffic-
   engineering use cases.  Note that there is a high chance for a



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   transit node to reroute faster than the head-end as it has usually
   less computations to run (SPF+CSPFs) and less prefixes to rewrite; it
   may also run less features leaving more CPU slots for IGP
   reconvergence.  The transient rerouting of the Node-SID may lead to
   microloops in the network that may impact the customer traffic.
   Node-SIDs are subject to ECMP and a local-repair mechanism may be
   implemented for equal cost paths with no way to disable it.  If the
   requirement of preventing any local-repair or ECMP is strict, the
   path computation engine needs to prevent the usage of all Node-SIDs
   or needs to detect that a particular Node-SID will be subject to ECMP
   and enforce the usage of additional adjacency SIDs to break the ECMP.
   In any case, more adjacency-SIDs will be required in the stack to
   avoid the ECMP, leading to a deeper label stack.

3.3.  Using a combination of adjacency segments and binding-SID

   [I-D.ietf-spring-segment-routing] defines the binding segment with
   multiple use cases.  One of the use case of the binding segment is to
   advertise a tunnel as a segment.  When a computation engine computes
   a non protected path and if the resulting label stack using only non
   protected adjacency segments is too deep for the network, an external
   component may create shortcuts in the network by creating a binding
   segment representing a list of non protected adjacency segments.


   PE1--P1     P6       P10--PE2
         \    /  \     /
          P4-P5   P7  P9
                   \ /
                    P8


                       Figure 3 - Use of Binding SID

   In the example above, the path from PE1 to PE2 must be expressed with
   the stack: {Adj_P1P4,Adj_P4P5,Adj_P5P6,Adj_P6P7,Adj_P7P8,Adj_P8P9,Adj
   _P9P10,Adj_P10PE2}.  This stack is too deep due to the limitations of
   the network.  An external component may create a binding Binding1 on
   P5 that represents the non protected path (P5->P6->P7->P8->P9->P10).
   When the binding is created and advertised in the topology, the
   computation engine can use this binding SID in a path, resulting for
   a PE1 to PE2 path to the stack:
   {Adj_P1P4,Adj_P4P5,Binding1,Adj_P10PE2}.  The usage of the binding
   SID in the stack allowed to reduce its size to an acceptable value.

   One advantage of combining adjacency and binding segments is the
   reduction of the label stack size.  The label stack size can be




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   reduced to a small amount of labels at some price (creating some
   states on transit nodes).

   The drawbacks are the increase of the controlplane and dataplane
   resource consumption.  This controlplane and dataplane resource
   consumption are variable and will be linked to the intelligence of
   the external controller and computation engines and especially how
   the placement of the bindings is done to maximize the sharing between
   LSPs.  Moreover any optimization try in the binding segment may
   introduce churn in the network controlplane (Make Before Break can be
   used to ensure that dataplane is not affected).  Programming a
   binding-SID on a transit node is feasible only if the programming
   node has the necessary protocol sessions to do so.  When a head-end
   router is performing a path computation, it is usually not the case.
   When a controller (PCE) is used, it may not have a session to all
   LSRs in the network, as only edge nodes may require a path
   computation.  The controller may be limited for the placement of the
   binding SID to the nodes it has a protocol session with (it cannot
   setup a PCEP session by itself).  A full deployment of protocol
   sessions with the controller may not be feasible for technical
   reasons (scaling, ...) or economical reasons.  A potential mitigation
   could be to allow protocol sessions to be setup dynamically (when
   requirement comes) to an authorized subset of nodes in the network:
   some protocol modifications may be necessary to allow this behavior.

4.  Comparison

   The following table tries to summarize the various solution pros/cons
   within a comparison table:

   o  Solution 1: using adjacency-SIDs only

   o  Solution 2: using adjacency-SIDs + Node-SIDs with strict SPF
      algorithm

   o  Solution 3: using adjacency-SIDs + Node-SIDs with new protection
      flag

   o  Solution 4: using adjacency-SIDs + two regular Node-SIDs with a
      different policy

   o  Solution 5: using adjacency-SIDs + Binding-SIDs

   We consider a network with N nodes and L links, with an average of l
   links per node.

   +----------+--------+-----------+------------+------------+---------+
   | Criteria | Soluti |  Solution | Solution 3 | Solution 4 | Solutio |



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   |          |  on 1  |     2     |            |            |   n 5   |
   +----------+--------+-----------+------------+------------+---------+
   |  Label   |  One   |  Reduced  |  Reduced   |  Reduced   | Reduced |
   |  stack   | label  |           |            |            |         |
   |   size   |  per   |           |            |            |         |
   |          |  hop   |           |            |            |         |
   |          |        |           |            |            |         |
   | Controlp | Neglig | Potential |   + 2*N    |   + 2*N    |   Adds  |
   |   lane   |  ible  | additiona | entries in | entries in |  states |
   |          |        | l computa |    RIB     |    RIB     |  in the |
   |          |        |   tion +  |            |            |   LSRs  |
   |          |        |    2*N    |            |            |         |
   |          |        |  entries  |            |            |         |
   |          |        |   in RIB  |            |            |         |
   |          |        |           |            |            |         |
   | Dataplan | +l ent |    +2*N   |    +2*N    |    +2*N    | Variabl |
   |    e     |  ries  |  entries  |  entries   |  entries   |    e    |
   |          |        |           |            |            |         |
   | IP conve |  None  |   Double  |   Double   |   Double   |   None  |
   |  rgence  |        |           |            |            |         |
   |   time   |        |           |            |            |         |
   |          |        |           |            |            |         |
   | Computat | Needs  |  Needs to |  Needs to  |  Needs to  |  Needs  |
   |   ion    |   to   |   select  |   select   |   select   |    to   |
   |  engine  | select |  Adj-SIDs |  Adj-SIDs  |  Adj-SIDs  |  select |
   |          |  Adj-  |  with B=0 |  with B=0  |  with B=0  |   Adj-  |
   |          |  SIDs  | and Node- | and Node-  | and needs  |   SIDs  |
   |          |  with  | SIDs with | SIDs with  |     to     |   with  |
   |          |  B=0   |   strict  |    B=0     | understand | B=0 and |
   |          |        |    SPF    |            |   policy   |  place  |
   |          |        |           |            |  from the  |   the   |
   |          |        |           |            |   SP to    | binding |
   |          |        |           |            | select the |  SID in |
   |          |        |           |            |   right    | a smart |
   |          |        |           |            | Node-SIDs  |   way   |
   |          |        |           |            |            |         |
   | Protocol |  None  |    None   | Need a new |    None    |   None  |
   |          |        |           |    flag    |            |         |
   |          |        |           |            |            |         |
   | ECMP avo | Suppor | Supported | Supported  | Supported  | Support |
   |  idance  |  ted   |   at the  |   at the   |   at the   |    ed   |
   |          |        | price of  |  price of  |  price of  |         |
   |          |        | increasin | increasing | increasing |         |
   |          |        |   g the   | the label  | the label  |         |
   |          |        |   label   |   stack    |   stack    |         |
   |          |        |   stack   |            |            |         |
   |          |        |           |            |            |         |
   | Requirem |  Yes   | Partially | Partially  | Partially  |   Yes   |



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   | ents ful |        | (allows E | (allows EC | (allows EC |         |
   | filment  |        | CMP+trans | MP+transie | MP+transie |         |
   |          |        | ient rero |     nt     |     nt     |         |
   |          |        |   uting)  | rerouting) | rerouting) |         |
   |  Others  |  None  |    None   |    None    |    None    | Require |
   |          |        |           |            |            | s a con |
   |          |        |           |            |            | troller |
   |          |        |           |            |            | with se |
   |          |        |           |            |            |  ssions |
   |          |        |           |            |            |  to all |
   |          |        |           |            |            |  nodes  |
   |          |        |           |            |            | (even t |
   |          |        |           |            |            | ransit) |
   +----------+--------+-----------+------------+------------+---------+

                          Comparison of solutions

5.  Recommended option(s)

   Based on the analysis in Section 4, we only have two solutions that
   fulfill the requirements expressed in Section 2: usage of adjacency-
   SIDs only, usage of a combination of adjacency SIDs and binding SIDs.

   As using only Adjacency-SIDs may reduce today the possibility of
   creating a path (due to the hardware/software limitations), authors
   would like to encourage the usage of a combination of adjacency-SIDs
   and binding-SIDs (Section 3.3) as a short-term solution.

   However this approach has also several drawbacks, but authors think
   that these drawbacks can be reduced by enhancing existing protocols.

   As a long term solution, authors would like to encourage vendors to
   support the ability for a node to push a significant number of
   labels, up to the full network diameter.

6.  Security Considerations

   TBD.

7.  Acknowledgements

   Authors would like to thank Bruno Decraene for his valuable comments.

8.  IANA Considerations

   N/A





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9.  Normative References

   [I-D.gredler-idr-bgp-ls-segment-routing-extension]
              Gredler, H., Ray, S., Previdi, S., Filsfils, C., Chen, M.,
              and J. Tantsura, "BGP Link-State extensions for Segment
              Routing", draft-gredler-idr-bgp-ls-segment-routing-
              extension-02 (work in progress), October 2014.

   [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-09 (work in progress), October
              2016.

   [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-10 (work in progress), October 2016.

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Decraene, B., Litkowski, S.,
              and R. Shakir, "Segment Routing Architecture", draft-ietf-
              spring-segment-routing-10 (work in progress), November
              2016.

   [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>.

Authors' Addresses

   Stephane Litkowski
   Orange

   Email: stephane.litkowski@orange.com


   Mustapha Aissaoui
   Nokia

   Email: mustapha.aissaoui@nokia.com







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