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Versions: (draft-filsfils-spring-segment-routing-policy) 00 01

SPRING Working Group                                         C. Filsfils
Internet-Draft                                         S. Sivabalan, Ed.
Intended status: Standards Track                     Cisco Systems, Inc.
Expires: December 13, 2018                                      D. Voyer
                                                             Bell Canada
                                                             A. Bogdanov
                                                            Google, Inc.
                                                               P. Mattes
                                                               Microsoft
                                                           June 11, 2018


                  Segment Routing Policy Architecture
            draft-ietf-spring-segment-routing-policy-01.txt

Abstract

   Segment Routing (SR) allows a headend node to steer a packet flow
   along any path.  Intermediate per-flow states are eliminated thanks
   to source routing.  The headend node steers a flow into an SR Policy.
   The header of a packet steered in an SR Policy is augmented with an
   ordered list of segments associated with that SR Policy.  This
   document details the concepts of SR Policy and steering into an SR
   Policy.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://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
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 13, 2018.




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Copyright Notice

   Copyright (c) 2018 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  SR Policy . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Identification of an SR Policy  . . . . . . . . . . . . .   4
     2.2.  Candidate Path and Segment List . . . . . . . . . . . . .   4
     2.3.  Protocol-Origin of a Candidate Path . . . . . . . . . . .   5
     2.4.  Originator of a Candidate Path  . . . . . . . . . . . . .   6
     2.5.  Discriminator of a Candidate Path . . . . . . . . . . . .   6
     2.6.  Identification of a Candidate Path  . . . . . . . . . . .   7
     2.7.  Preference of a Candidate Path  . . . . . . . . . . . . .   7
     2.8.  Validity of a Candidate Path  . . . . . . . . . . . . . .   8
     2.9.  Active Candidate Path . . . . . . . . . . . . . . . . . .   8
     2.10. Validity of an SR Policy  . . . . . . . . . . . . . . . .   9
     2.11. Instantiation of an SR Policy in the Forwarding Plane . .   9
     2.12. Priority of an SR Policy  . . . . . . . . . . . . . . . .   9
     2.13. Summary . . . . . . . . . . . . . . . . . . . . . . . . .  10
   3.  Segment Routing Database  . . . . . . . . . . . . . . . . . .  10
   4.  Segment Types . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Explicit Null . . . . . . . . . . . . . . . . . . . . . .  15
   5.  Validity of a Candidate Path  . . . . . . . . . . . . . . . .  15
     5.1.  Explicit Candidate Path . . . . . . . . . . . . . . . . .  15
     5.2.  Dynamic Candidate Path  . . . . . . . . . . . . . . . . .  16
   6.  Binding SID . . . . . . . . . . . . . . . . . . . . . . . . .  17
     6.1.  BSID of a candidate path  . . . . . . . . . . . . . . . .  17
     6.2.  BSID of an SR Policy  . . . . . . . . . . . . . . . . . .  17
     6.3.  Forwarding Plane  . . . . . . . . . . . . . . . . . . . .  19
     6.4.  Non-SR usage of Binding SID . . . . . . . . . . . . . . .  19
   7.  SR Policy State . . . . . . . . . . . . . . . . . . . . . . .  19
   8.  Steering into an SR Policy  . . . . . . . . . . . . . . . . .  20
     8.1.  Validity of an SR Policy  . . . . . . . . . . . . . . . .  20
     8.2.  Drop upon invalid SR Policy . . . . . . . . . . . . . . .  20
     8.3.  Incoming Active SID is a BSID . . . . . . . . . . . . . .  21



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     8.4.  Per-Destination Steering  . . . . . . . . . . . . . . . .  21
     8.5.  Recursion on an on-demand dynamic BSID  . . . . . . . . .  22
     8.6.  Per-Flow Steering . . . . . . . . . . . . . . . . . . . .  23
     8.7.  Policy-based Routing  . . . . . . . . . . . . . . . . . .  24
     8.8.  Optional Steering Modes for BGP Destinations  . . . . . .  24
   9.  Protection  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     9.1.  Leveraging TI-LFA local protection of the constituent IGP
           segments  . . . . . . . . . . . . . . . . . . . . . . . .  26
     9.2.  Using an SR Policy to locally protect a link  . . . . . .  27
     9.3.  Using a Candidate Path for Path Protection  . . . . . . .  27
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  27
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   12. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  28
   13. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  28
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  29
     14.2.  Informative References . . . . . . . . . . . . . . . . .  29
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   Segment Routing (SR) allows a headend node to steer a packet flow
   along any path.  Intermediate per-flow states are eliminated thanks
   to source routing [I-D.ietf-spring-segment-routing].

   The headend node is said to steer a flow into an Segment Routing
   Policy (SR Policy).

   The header of a packet steered into an SR Policy is augmented with an
   ordered list of segments associated with that SR Policy.

   This document details the concepts of SR Policy and steering packets
   into an SR Policy.  These apply equally to the MPLS and SRv6
   instantiations of segment routing.

   For reading simplicity, the illustrations are provided for the MPLS
   instantiations.

2.  SR Policy

   An SR Policy is a framework that enables instantiation of an ordered
   list of segments on a node for implementing a source routing policy
   with a specific intent for traffic steering from that node.

   The Segment Routing architecture [I-D.ietf-spring-segment-routing]
   specifies that any instruction can be bound to a segment.  Thus, an
   SR Policy can be built using any type of Segment Identifier (SID)
   including those associated with topological or service instructions.



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   This section defines the key aspects and constituents of an SR
   Policy.

2.1.  Identification of an SR Policy

   An SR Policy is identified through the tuple <headend, color,
   endpoint>.  In the context of a specific headend, one may identify an
   SR policy by the <color, endpoint> tuple.

   The headend is the node where the policy is instantiated/implemented.
   The headend is specified as an IPv4 or IPv6 address.

   The endpoint indicates the destination of the policy.  The endpoint
   is specified as an IPv4 or IPv6 address.  In a specific case (refer
   to Section 8.8.1), the endpoint can be the null address (0.0.0.0 for
   IPv4, ::0 for IPv6).

   The color is a 32-bit numerical value that associates the SR Policy
   with an intent (e.g. low-latency).

   The endpoint and the color are used to automate the steering of
   service or transport routes on SR Policies (refer to Section 8).

   An implementation MAY allow assignment of a symbolic name comprising
   of printable ASCII characters to an SR Policy to serve as a user-
   friendly attribute for debug and troubleshooting purposes.  Such
   symbolic names MUST NOT be considered as identifiers for an SR
   Policy.

2.2.  Candidate Path and Segment List

   An SR Policy is associated with one or more candidate paths.  A
   candidate path is the unit for signaling of an SR Policy to a headend
   via protocols like Path Computation Element (PCE) Communication
   Protocol (PCEP) [RFC8281] or BGP SR Policy
   [I-D.ietf-idr-segment-routing-te-policy].

   A candidate path is itself associated with a Segment-List (SID-List)
   or a set of SID-Lists.

   A SID-List represents a specific source-routed path to send traffic
   from the headend to the endpoint of the corresponding SR policy.

   A candidate path is either dynamic or explicit.

   An explicit candidate path is associated with a SID-List or a set of
   SID-Lists.




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   A dynamic candidate path expresses an optimization objective and a
   set of constraints.  The headend (potentially with the help of a PCE)
   computes the solution SID-List (or set of SID-Lists) that solves the
   optimization problem.

   If a candidate path is associated with a set of SID-Lists, each SID-
   List is associated with a weight for weighted load balancing (refer
   Section 2.11 for details).  The default weight is 1.

   A variation of SR Policy can be used for packet replication.  A
   candidate path could comprise multiple SID-Lists; one for each
   replication path.  In such a scenario, packets are actually
   replicated through each SID List of the SR Policy to realize a point-
   to-multipoint service delivery.  The weight of each SID-List does not
   come into the picture in this case since there is no load-balancing.
   The details of this and other such mechanisms for use of SR Policy
   for point-to-multipoint delivery are outside the scope of this
   document.

2.3.  Protocol-Origin of a Candidate Path

   A headend may be informed about a candidate path for an SR Policy
   <color, endpoint> by various means including: via configuration, PCEP
   [RFC8281] or BGP [I-D.ietf-idr-segment-routing-te-policy].

   Protocol-Origin of a candidate path is an 8-bit value which
   identifies the component or protocol that originates or signals the
   candidate path.

   The table below specifies the RECOMMENDED default values:

    +-------+---------------------------------------------------------+
    | Value | Protocol-Origin                                         |
    +-------+---------------------------------------------------------+
    |   10  | PCEP                                                    |
    |   20  | BGP SR Policy                                           |
    |   30  | Local (via CLI, Yang model through NETCONF, gRPC, etc.) |
    +-------+---------------------------------------------------------+

                    Table 1: Protocol-origin Identifier

   Implementations MAY allow modifications of these default values
   assigned to protocols on the headend along similar lines as a routing
   administrative distance.  Its application in the candidate path
   selection is described in Section 2.9.






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2.4.  Originator of a Candidate Path

   Originator identifies the node which provisioned or signalled the
   candidate path on the headend.  The originator is expressed in the
   form of a 160 bit numerical value formed by the concatenation of the
   fields of the tuple <ASN, node-address> as below:

   o  ASN : represented as a 4 byte number.

   o  Node Address : represented as a 128 bit value.  IPv4 addresses are
      encoded in the lowest 32 bits.

   Its application in the candidate path selection is described in
   Section 2.9.

   When Protocol-Origin is Local, the ASN and node address MAY be set to
   either the headend or the provisioning controller/node ASN and
   address.  Default value is 0 for both AS and node address.

   When Protocol-Origin is PCEP, it is the IPv4 or IPv6 address of the
   PCE and the AS number SHOULD be set to 0 by default when not
   available or known.

   Protocol-Origin is BGP SR Policy, it is provided by the BGP component
   on the headend and is:

   o  the BGP Router ID and ASN of the node/controller signalling the
      candidate path when it has a BGP session to the headend, OR

   o  the BGP Router ID of the eBGP peer signalling the candidate path
      along with ASN of origin when the signalling is done via one or
      more intermediate eBGP routers, OR

   o  the BGP Originator ID [RFC4456] and the ASN of the node/controller
      when the signalling is done via one or more route-reflectors over
      iBGP session.

2.5.  Discriminator of a Candidate Path

   The Discriminator is a 32 bit value associated with a candidate path
   that uniquely identifies it within the context of an SR Policy from a
   specific Protocol-Origin as specified below:

   When Protocol-Origin is Local, this is an implementation's
   configuration model specific unique identifier for a candidate path.
   Default value is 0.





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   When PCEP is the Protocol-Origin, the method to uniquely identify
   signalled path will be specified in a future PCEP document.  Default
   value is 0.

   When BGP SR Policy is the Protocol-Origin, it is the distinguisher
   specified in Section 2.1 of [I-D.ietf-idr-segment-routing-te-policy].

   Its application in the candidate path selection is described in
   Section 2.9.

2.6.  Identification of a Candidate Path

   A candidate path is identified in the context of a single SR Policy.

   A candidate path is not shared across SR Policies.

   A candidate path is not identified by its SID-List(s).

      If CP1 is a candidate path of SR Policy Pol1 and CP2 is a
      candidate path of SR Policy Pol2, then these two candidate paths
      are independent, even if they happen to have the same SID-List.
      The SID-List does not identify a candidate path.  The SID-List is
      an attribute of a candidate path.

   The identity of a candidate path MUST be uniquely established in the
   context of an SR Policy <headend, color, endpoint> in order to handle
   add, delete or modify operations on them in an unambiguous manner
   regardless of their source(s).

   The tuple <Protocol-Origin, originator, discriminator> uniquely
   identifies a candidate path.

   Candidate paths MAY also be assigned or signaled with a symbolic name
   comprising printable ASCII characters to serve as a user-friendly
   attribute for debug and troubleshooting purposes.  Such symbolic
   names MUST NOT be considered as identifiers for a candidate path.

2.7.  Preference of a Candidate Path

   The preference of the candidate path is used to select the best
   candidate path for an SR Policy.  The default preference is 100.

   It is RECOMMENDED that each candidate path of a given SR policy has a
   different preference.







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2.8.  Validity of a Candidate Path

   A candidate path is valid if it is usable.  A common path validity
   criterion is the reachability of its constituent SIDs.  The
   validation rules are specified in Section 5.

2.9.  Active Candidate Path

   A candidate path is selected when it is valid and it is determined to
   be the best path of the SR Policy.  The selected path is referred to
   as the "active path" of the SR policy in this document.

   Whenever a new path is learned or an active path is deleted, the
   validity of an existing path changes or an existing path is changed,
   the selection process MUST be re-executed.

   The candidate path selection process operates on the candidate path
   Preference.  A candidate path is selected when it is valid and it has
   the highest preference value among all the candidate paths of the SR
   Policy.

   In the case of multiple valid candidate paths of the same preference,
   the tie-breaking rules are evaluated on the identification tuple in
   the following order until only one valid best path is selected:

   1.  Higher value of Protocol-Origin is selected.

   2.  Lower value of originator is selected.

   3.  Finally, the higher value of discriminator is selected.

   An implementation MAY choose to override any of the tie-breaking
   rules above and maintain the already selected candidate path as
   active path.

   The rules are framed with multiple protocols and sources in mind and
   hence may not follow the logic of a single protocol (e.g.  BGP best
   path selection).  The motivation behind these rules are as follows:

   o  The Protocol-Origin allows an operator to setup a default
      selection mechanism across protocol sources, e.g., to prefer
      locally provisioned over paths signalled via BGP SR Policy or
      PCEP.

   o  The preference, being the first tiebreaker, allows an operator to
      influence selection across paths thus allowing provisioning of
      multiple path options, e.g., CP1 is preferred and if it becomes
      invalid then fall-back to CP2 and so on.  Since preference works



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      across protocol sources it also enables (where necessary)
      selective override of the default protocol-origin preference,
      e.g., to prefer a path signalled via BGP SR Policy over what is
      locally provisioned.

   o  The originator allows an operator to have multiple redundant
      controllers and still maintain a deterministic behaviour over
      which of them are preferred even if they are providing the same
      candidate paths for the same SR policies to the headend.

   o  The discriminator performs the final tiebreaking step to ensure a
      deterministic outcome of selection regardless of the order in
      which candidate paths are signalled across multiple transport
      channels or sessions.

   [I-D.filsfils-spring-sr-policy-considerations] provides a set of
   examples to illustrate the active candidate path selection rules.

2.10.  Validity of an SR Policy

   An SR Policy is valid when it has at least one valid candidate path.

2.11.  Instantiation of an SR Policy in the Forwarding Plane

   A valid SR Policy is instantiated in the forwarding plane.

   Only the active candidate path is used for forwarding traffic that is
   being steered onto that policy.

   If a set of SID-Lists is associated with the active path of the
   policy, then the steering is per flow and W-ECMP based according to
   the relative weight of each SID-List.

   The fraction of the flows associated with a given SID-List is w/Sw
   where w is the weight of the SID-List and Sw is the sum of the
   weights of the SID-Lists of the selected path of the SR Policy.

   The accuracy of the weighted load-balancing depends on the platform
   implementation.

2.12.  Priority of an SR Policy

   Upon topological change, many policies could be recomputed.  An
   implementation MAY provide a per-policy priority configuration.  The
   operator MAY set this field to indicate order in which the policies
   should be re-computed.  Such a priority is represented by an integer
   in the range (0, 255) where the lowest value is the highest priority.
   The default value of priority is 128.



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   An SR Policy may comprise multiple Candidate Paths received from the
   same or different sources.  A candidate path MAY be signaled with a
   priority value.  When an SR Policy has multiple candidate paths with
   distinct signaled non-default priority values, the SR Policy as a
   whole takes the lowest value (i.e. the highest priority) amongst
   these signaled priority values.

2.13.  Summary

   In summary, the information model is the following:

            SR policy POL1 <headend, color, endpoint>
              Candidate-path CP1 <protocol-origin = 20, originator =
   100:1.1.1.1, discriminator = 1>
                Preference 200
                Weight W1, SID-List1 <SID11...SID1i>
                Weight W2, SID-List2 <SID21...SID2j>
              Candidate-path CP2 <protocol-origin = 20, originator =
   100:2.2.2.2, discriminator = 2>
                Preference 100
                Weight W3, SID-List3 <SID31...SID3i>
                Weight W4, SID-List4 <SID41...SID4j>

   The SR Policy POL1 is identified by the tuple <headend, color,
   endpoint>.  It has two candidate paths CP1 and CP2.  Each is
   identified by a tuple <protocol-origin, originator, discriminator>.
   CP1 is the active candidate path (it is valid and it has the highest
   preference).  The two SID-Lists of CP1 are installed as the
   forwarding instantiation of SR policy Pol1.  Traffic steered on Pol1
   is flow-based hashed on SID-List <SID11...SID1i> with a ratio
   W1/(W1+W2).

3.  Segment Routing Database

   An SR headend maintains the Segment Routing Database (SR-DB).

   An SR headend leverages the SR-DB to validate explicit candidate
   paths and compute dynamic candidate paths.

   The information in the SR-DB MAY include:

   o  IGP information (topology, IGP metrics based on ISIS [RFC1195] and
      OSPF [RFC2328] [RFC5340])
   o  Segment Routing information (such as SRGB, SRLB, Prefix-SIDs, Adj-
      SIDs, BGP Peering SID, SRv6 SIDs)
      [I-D.ietf-spring-segment-routing]
      [I-D.ietf-idr-bgpls-segment-routing-epe]
      [I-D.filsfils-spring-srv6-network-programming]



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   o  TE Link Attributes (such as TE metric, SRLG, attribute-flag,
      extended admin group) [RFC5305] [RFC3630].
   o  Extended TE Link attributes (such as latency, loss) [RFC7810]
      [RFC7471]
   o  Inter-AS Topology information
      [I-D.ietf-idr-bgpls-segment-routing-epe].

   The attached domain topology MAY be learned via IGP, BGP-LS or
   NETCONF.

   A non-attached (remote) domain topology MAY be learned via BGP-LS or
   NETCONF.

   In some use-cases, the SR-DB may only contain the attached domain
   topology while in others, the SR-DB may contain the topology of
   multiple domains and in this case it is multi-domain capable.

   The SR-DB MAY also contain the SR Policies instantiated in the
   network.  This can be collected via BGP-LS
   [I-D.ietf-idr-te-lsp-distribution] or PCEP [RFC8231] and
   [I-D.sivabalan-pce-binding-label-sid].  This information allows to
   build an end-to-end policy on the basis of intermediate SR policies
   (see Section 6 for further details).

   The SR-DB MAY also contain the Maximum SID Depth (MSD) capability of
   nodes in the topology.  This can be collected via ISIS
   [I-D.ietf-isis-segment-routing-msd], OSPF
   [I-D.ietf-ospf-segment-routing-msd], BGP-LS
   [I-D.ietf-idr-bgp-ls-segment-routing-msd] or PCEP
   [I-D.ietf-pce-segment-routing].

   The use of the SR-DB for computation and validation of SR Policies is
   outside the scope of this document.  Some implementation aspects
   related to this are covered in
   [I-D.filsfils-spring-sr-policy-considerations].

4.  Segment Types

   A SID-List is an ordered set of segments represented as <S1, S2, ...
   Sn> where S1 is the first segment.

   Based on the desired dataplane, either the MPLS label stack or the
   SRv6 SRH is built from the SID-List.  However, the SID-List itself
   can be specified using different segment-descriptor types and the
   following are currently defined:

   Type 1: SR-MPLS Label:




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         A MPLS label corresponding to any of the segment types defined
         for SR-MPLS (as defined in [I-D.ietf-spring-segment-routing] or
         other SR-MPLS specifications) can be used.  Additionally,
         reserved labels like explicit-null or in general any MPLS label
         may also be used.  e.g. this type can be used to specify a
         label representation which maps to an optical transport path on
         a packet transport node.  This type does not require the
         headend to perform SID resolution.

   Type 2: SRv6 SID:
         An IPv6 address corresponding to any of the segment types
         defined for SRv6 (as defined in
         [I-D.filsfils-spring-srv6-network-programming] or other SRv6
         specifications) can be used.  This type does not require the
         headend to perform SID resolution.

   Type 3: IPv4 Prefix with optional SR Algorithm:
         The headend is required to resolve the specified IPv4 Prefix
         Address to the SR-MPLS label corresponding to a Prefix SID
         segment (as defined in [I-D.ietf-spring-segment-routing]).  The
         SR algorithm (refer to Section 3.1.1 of
         [I-D.ietf-spring-segment-routing]) to be used MAY also be
         provided.

   Type 4: IPv6 Global Prefix with optional SR Algorithm for SR-MPLS:
         In this case the headend is required to resolve the specified
         IPv6 Global Prefix Address to the SR-MPLS label corresponding
         to its Prefix SID segment (as defined in
         [I-D.ietf-spring-segment-routing]).  The SR Algorithm (refer to
         Section 3.1.1 of [I-D.ietf-spring-segment-routing]) to be used
         MAY also be provided.

   Type 5: IPv4 Prefix with Local Interface ID:
         This type allows identification of Adjacency SID (as defined in
         [I-D.ietf-spring-segment-routing]) or BGP EPE Peering SID (as
         defined in [I-D.ietf-idr-bgpls-segment-routing-epe]) label for
         point-to-point links including IP unnumbered links.  The
         headend is required to resolve the specified IPv4 Prefix
         Address to the Node originating it and then use the Local
         Interface ID to identify the point-to-point link whose
         adjacency is being referred to.  The Local Interface ID link
         descriptor follows semantics as specified in [RFC7752].  This
         type can also be used to indicate indirection into a layer 2
         interface (i.e. without IP address) like a representation of an
         optical transport path or a layer 2 Ethernet port or circuit at
         the specified node.

   Type 6: IPv4 Addresses for link endpoints as Local, Remote pair:



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         This type allows identification of Adjacency SID (as defined in
         [I-D.ietf-spring-segment-routing]) or BGP EPE Peering SID (as
         defined in [I-D.ietf-idr-bgpls-segment-routing-epe]) label for
         links.  The headend is required to resolve the specified IPv4
         Local Address to the Node originating it and then use the IPv4
         Remote Address to identify the link adjacency being referred
         to.  The Local and Remote Address pair link descriptors follows
         semantics as specified in [RFC7752].

   Type 7: IPv6 Prefix and Interface ID for link endpoints as Local,
   Remote pair for SR-MPLS:
         This type allows identification of Adjacency SID (as defined in
         [I-D.ietf-spring-segment-routing]) or BGP EPE Peering SID (as
         defined in [I-D.ietf-idr-bgpls-segment-routing-epe]) label for
         links including those with only Link Local IPv6 addresses.  The
         headend is required to resolve the specified IPv6 Prefix
         Address to the Node originating it and then use the Local
         Interface ID to identify the point-to-point link whose
         adjacency is being referred to.  For other than point-to-point
         links, additionally the specific adjacency over the link needs
         to be resolved using the Remote Prefix and Interface ID.  The
         Local and Remote pair of Prefix and Interface ID link
         descriptor follows semantics as specified in [RFC7752].  This
         type can also be used to indicate indirection into a layer 2
         interface (i.e. without IP address) like a representation of an
         optical transport path or a layer 2 Ethernet port or circuit at
         the specified node.

   Type 8: IPv6 Addresses for link endpoints as Local, Remote pair for
   SR-MPLS:
         This type allows identification of Adjacency SID (as defined in
         [I-D.ietf-spring-segment-routing]) or BGP EPE Peering SID (as
         defined in [I-D.ietf-idr-bgpls-segment-routing-epe]) label for
         links with Global IPv6 addresses.  The headend is required to
         resolve the specified Local IPv6 Address to the Node
         originating it and then use the Remote IPv6 Address to identify
         the link adjacency being referred to.  The Local and Remote
         Address pair link descriptors follows semantics as specified in
         [RFC7752].

   Type 9: IPv6 Global Prefix with optional SR Algorithm for SRv6:
         The headend is required to resolve the specified IPv6 Global
         Prefix Address to the SRv6 END function SID (as defined in
         [I-D.filsfils-spring-srv6-network-programming]) corresponding
         to the node which is originating the prefix.  The SR Algorithm
         (refer to Section 3.1.1 of [I-D.ietf-spring-segment-routing])
         to be used MAY also be provided.




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   Type 10: IPv6 Prefix and Interface ID for link endpoints as Local,
   Remote pair for SRv6:
         This type allows identification of SRv6 END.X SID (as defined
         in [I-D.filsfils-spring-srv6-network-programming]) for links
         with only Link Local IPv6 addresses.  The headend is required
         to resolve the specified IPv6 Prefix Address to the Node
         originating it and then use the Local Interface ID to identify
         the point-to-point link whose adjacency is being referred to.
         For other than point-to-point links, additionally the specific
         adjacency needs to be resolved using the Remote Prefix and
         Interface ID.  The Local and Remote pair of Prefix and
         Interface ID link descriptor follows semantics as specified in
         [RFC7752].

   Type 11: IPv6 Addresses for link endpoints as Local, Remote pair for
   SRv6:
         This type allows identification of SRv6 END.X SID (as defined
         in [I-D.filsfils-spring-srv6-network-programming]) for links
         with Global IPv6 addresses.  The headend is required to resolve
         the specified Local IPv6 Address to the Node originating it and
         then use the Remote IPv6 Address to identify the link adjacency
         being referred to.  The Local and Remote Address pair link
         descriptors follows semantics as specified in [RFC7752].


   When the algorithm is not specified for the SID types above which
   optionally allow for it, the headend SHOULD use the Strict Shortest
   Path algorithm if available; otherwise it SHOULD use the default
   Shortest Path algorithm.  The specification of algorithm enables the
   use of IGP Flex Algorithm [I-D.ietf-lsr-flex-algo] specific SIDs in
   SR Policy.

   For SID types 3-through-11, a SID value may also be optionally
   provided to the headend for verification purposes.  Section 5.1.
   describes the resolution and verification of the SIDs and Segment
   Lists on the headend.

   When building the MPLS label stack or the IPv6 Segment list from the
   Segment List, the node instantiating the policy MUST interpret the
   set of Segments as follows:

   o  The first Segment represents the topmost label or the first IPv6
      segment.  It identifies the first segment the traffic will be
      directed toward along the explicit SR path.
   o  The last Segment represents the bottommost label or the last IPv6
      segment the traffic will be directed toward along the explicit SR
      path.




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4.1.  Explicit Null

   A Type 1 SID may be any MPLS label, including reserved labels.

   For example, assuming that the desired traffic-engineered path from a
   headend 1 to an endpoint 4 can be expressed by the SID-List <16002,
   16003, 16004> where 16002, 16003 and 16004 respectively refer to the
   IPv4 Prefix SIDs bound to node 2, 3 and 4, then IPv6 traffic can be
   traffic-engineered from nodes 1 to 4 via the previously described
   path using an SR Policy with SID-List <16002, 16003, 16004, 2> where
   mpls label value of 2 represents the "IPv6 Explicit NULL Label".

   The penultimate node before node 4 will pop 16004 and will forward
   the frame on its directly connected interface to node 4.

   The endpoint receives the traffic with top label "2" which indicates
   that the payload is an IPv6 packet.

   When steering unlabeled IPv6 BGP destination traffic using an SR
   policy composed of SID-List(s) based on IPv4 SIDs, the Explicit Null
   Label Policy is processed as specified in
   [I-D.ietf-idr-segment-routing-te-policy]) Section 2.4.4.  When an
   "IPv6 Explicit NULL label" is not present as the bottom label, the
   headend SHOULD automatically impose one.  Refer to Section 8 for more
   details.

5.  Validity of a Candidate Path

5.1.  Explicit Candidate Path

   An explicit candidate path is associated with a SID-List or a set of
   SID-Lists.

   An explicit candidate path is provisioned by the operator directly or
   via a controller.

   The computation/logic that leads to the choice of the SID list is
   external to the SR Policy headend.  The SR Policy headend does not
   compute the SID list.  The SR Policy headend only confirms its
   validity.

   A SID-List of an explicit candidate path MUST be declared invalid
   when:

   o  It is empty.
   o  Its weight is 0.
   o  The headend is unable to resolve the first SID into one or more
      outgoing interface(s) and next-hop(s).



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   o  The headend is unable to resolve any non-first SID of type
      3-through-11 into an MPLS label or an SRv6 SID.
   o  The headend verification fails for any SID for which verification
      has been explicitly requested.

   "Unable to resolve" means that the headend has no path to the SID in
   its SR database.

   SID verification is performed when the headend is explicitly
   requested to verify SID(s) by the controller via the signaling
   protocol used.  Implementations MAY provide a local configuration
   option to enable verification on a global or per policy or per
   candidate path basis.

   "Verification fails" for a SID means any of the following:

   o  The headend is unable to find the SID in its SR DB
   o  The headend detects mis-match between the SID value and its
      context provided for SIDs of type 3-through-11 in its SR DB.
   o  The headend is unable to resolve any non-first SID of type
      3-through-11 into an MPLS label or an SRv6 SID.

   In multi-domain deployments, it is expected that the headend be
   unable to verify the reachability of the SIDs in remote domains.
   Types 1 or 2 MUST be used for the SIDs for which the reachability
   cannot be verified.  Note that the first SID MUST always be reachable
   regardless of its type.

   In addition, a SID-List MAY be declared invalid when:

   o  Its last segment is not a Prefix SID (including BGP Peer Node-SID)
      advertised by the node specified as the endpoint of the
      corresponding SR policy.
   o  Its last segment is not an Adjacency SID (including BGP Peer
      Adjacency SID) of any of the links present on neighbor nodes and
      that terminate on the node specified as the endpoint of the
      corresponding SR policy.

   An explicit candidate path is invalid as soon as it has no valid SID-
   List.

5.2.  Dynamic Candidate Path

   A dynamic candidate path is specified as an optimization objective
   and constraints.

   The headend of the policy leverages its SR database to compute a SID-
   List ("solution SID-List") that solves this optimization problem.



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   The headend re-computes the solution SID-List any time the inputs to
   the problem change (e.g., topology changes).

   When local computation is not possible (e.g., a policy's tailend is
   outside the topology known to the headend) or not desired, the
   headend MAY send path computation request to a PCE supporting PCEP
   extension specified in [I-D.ietf-pce-segment-routing].

   If no solution is found to the optimization objective and
   constraints, then the dynamic candidate path MUST be declared
   invalid.

   [I-D.filsfils-spring-sr-policy-considerations] discusses some of the
   optimization objectives and constraints that may be considered by a
   dynamic candidate path.  It illustrates some of the desirable
   properties of the computation of the solution SID list.

6.  Binding SID

   The Binding SID (BSID) is fundamental to Segment Routing
   [I-D.ietf-spring-segment-routing].  It provides scaling, network
   opacity and service independence.
   [I-D.filsfils-spring-sr-policy-considerations] illustrates some of
   these benefits.

6.1.  BSID of a candidate path

   Each candidate path MAY be defined with a BSID.

   Candidate Paths of the same SR policy SHOULD have the same BSID.

   Candidate Paths of different SR policies MUST NOT have the same BSID.

6.2.  BSID of an SR Policy

   The BSID of an SR policy is the BSID of its active candidate path.

   When the active candidate path has a specified BSID, the SR Policy
   uses that BSID if this value (label in MPLS, IPv6 address in SRv6) is
   available (i.e., not associated with any other usage: e.g. to another
   MPLS client, to another SID, to another SR Policy).

   Optionally, instead of only checking that the BSID of the active path
   is available, a headend MAY check that it is available within a given
   SID range i.e., Segment Routing Local Block (SRLB) as specified in
   [I-D.ietf-spring-segment-routing].





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   When the specified BSID is not available (optionally is not in the
   SRLB), an alert message is generated.

   In the cases (as described above) where SR Policy does not have a
   BSID available, then the SR Policy MAY dynamically bind a BSID to
   itself.  Dynamically bound BSID SHOULD use an available SID outside
   the SRLB.

   Assuming that at time t the BSID of the SR Policy is B1, if at time
   t+dt a different candidate path becomes active and this new active
   path does not have a specified BSID or its BSID is specified but is
   not available (e.g. it is in use by something else), then the SR
   Policy keeps the previous BSID B1.

   The association of an SR Policy with a BSID thus MAY change over the
   life of the SR policy (e.g., upon active path change).  Hence, the
   BSID SHOULD NOT be used as an identification of an SR Policy.

6.2.1.  Frequent use-cases : unspecified BSID

   All the candidate paths of the same SR Policy can have an unspecified
   BSID.

   In such a case, a BSID MAY be dynamically bound to the SR Policy as
   soon as the first valid candidate path is received.  That BSID is
   kept along all the life of the SR Policy and across changes of active
   candidate path.

6.2.2.  Frequent use-case: all specified to the same BSID

   All the paths of the SR Policy can have the same specified BSID.

6.2.3.  Specified-BSID-only

   An implementation MAY support the configuration of the Specified-
   BSID-only restrictive behavior on the headend for all SR Policies or
   individual SR Policies.  Further, this restrictive behavior MAY also
   be signaled on a per SR Policy basis to the headend.

   When this restrictive behavior is enabled, if the candidate path has
   an unspecified BSID or if the specified BSID is not available when
   the candidate path becomes active then no BSID is bound to it and it
   is considered invalid.  An alert is triggered.  Other candidate paths
   MUST then be evaluated for becoming the active candidate path.







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6.3.  Forwarding Plane

   A valid SR Policy installs a BSID-keyed entry in the forwarding plane
   with the action of steering the packets matching this entry to the
   selected path of the SR Policy.

   If the Specified-BSID-only restrictive behavior is enabled and the
   BSID of the active path is not available (optionally not in the
   SRLB), then the SR Policy does not install any entry indexed by a
   BSID in the forwarding plane.

6.4.  Non-SR usage of Binding SID

   An implementation MAY choose to associate a Binding SID with any type
   of interface (e.g. a layer 3 termination of an Optical Circuit) or a
   tunnel (e.g.  IP tunnel, GRE tunnel, IP/UDP tunnel, MPLS RSVP-TE
   tunnel, etc).  This enables the use of other non-SR enabled
   interfaces and tunnels as segments in an SR Policy SID-List without
   the need of forming routing protocol adjacencies over them.

   The details of this kind of usage are beyond the scope of this
   document.  A specific packet optical integration use case is
   described in [I-D.anand-spring-poi-sr]

7.  SR Policy State

   The SR Policy State is maintained on the headend to represent the
   state of the policy and its candidate paths.  This is to provide an
   accurate representation of whether the SR Policy is being
   instantiated in the forwarding plane and which of its candidate paths
   and segment-list(s) are active.  The SR Policy state MUST also
   reflect the reason when a policy and/or its candidate path is not
   active due to validation errors or not being preferred.

   The SR Policy state can be reported by the headend node via BGP-LS
   [I-D.ietf-idr-te-lsp-distribution] or PCEP [RFC8231] and
   [I-D.sivabalan-pce-binding-label-sid].

   SR Policy state on the headend also includes traffic accounting
   information for the flows being steered via the policies.  The
   details of the SR Policy accounting are beyond the scope of this
   document.  The aspects related to the SR traffic counters and their
   usage in the broader context of traffic accounting in a SR network
   are covered in [I-D.filsfils-spring-sr-traffic-counters] and
   [I-D.ali-spring-sr-traffic-accounting] respectively.

   Implementations MAY support an administrative state to control
   locally provisioned policies via mechanisms like CLI or NETCONF.



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8.  Steering into an SR Policy

   A headend can steer a packet flow into a valid SR Policy in various
   ways:

   o  Incoming packets have an active SID matching a local BSID at the
      headend.
   o  Per-destination Steering: incoming packets match a BGP/Service
      route which recurses on an SR policy.
   o  Per-flow Steering: incoming packets match or recurse on a
      forwarding array of where some of the entries are SR Policies.
   o  Policy-based Steering: incoming packets match a routing policy
      which directs them on an SR policy.

   For simplicity of illustration, this document uses the SR-MPLS
   example.

8.1.  Validity of an SR Policy

   An SR Policy is invalid when all its candidate paths are invalid.

   By default, upon transitioning to the invalid state,

   o  an SR Policy and its BSID are removed from the forwarding plane.
   o  any steering of a service (PW), destination (BGP-VPN), flow or
      packet on the related SR policy is disabled and the related
      service, destination, flow or packet is routed per the classic
      forwarding table (e.g. longest-match to the destination or the
      recursing next-hop).

8.2.  Drop upon invalid SR Policy

   An SR Policy MAY be enabled for the Drop-Upon-Invalid behavior:

   o  an invalid SR Policy and its BSID is kept in the forwarding plane
      with an action to drop.
   o  any steering of a service (PW), destination (BGP-VPN), flow or
      packet on the related SR policy is maintained with the action to
      drop all of this traffic.

   The drop-upon-invalid behavior has been deployed in use-cases where
   the operator wants some PW to only be transported on a path with
   specific constraints.  When these constraints are no longer met, the
   operator wants the PW traffic to be dropped.  Specifically, the
   operator does not want the PW to be routed according to the IGP
   shortest-path to the PW endpoint.





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8.3.  Incoming Active SID is a BSID

   Let us assume that headend H has a valid SR Policy P of SID-List <S1,
   S2, S3> and BSID B.

   When H receives a packet K with label stack <B, L2, L3>, H pops B and
   pushes <S1, S2, S3> and forwards the resulting packet according to
   SID S1.

      "Forwarding the resulting packet according to S1" means: If S1 is
      an Adj SID or a PHP-enabled prefix SID advertised by a neighbor, H
      sends the resulting packet with label stack <S2, S3, L2, L3> on
      the outgoing interface associated with S1; Else H sends the
      resulting packet with label stack <S1, S2, S3, L2, L3> along the
      path of S1.

   H has steered the packet into the SR policy P.

   H did not have to classify the packet.  The classification was done
   by a node upstream of H (e.g., the source of the packet or an
   intermediate ingress edge node of the SR domain) and the result of
   this classification was efficiently encoded in the packet header as a
   BSID.

   This is another key benefit of the segment routing in general and the
   binding SID in particular: the ability to encode a classification and
   the resulting steering in the packet header to better scale and
   simplify intermediate aggregation nodes.

   If the SR Policy P is invalid, the BSID B is not in the forwarding
   plane and hence the packet K is dropped by H.

8.4.  Per-Destination Steering

   Let us assume that headend H:

   o  learns a BGP route R/r via next-hop N, extended-color community C
      and VPN label V.
   o  has a valid SR Policy P to (endpoint = N, color = C) of SID-List
      <S1, S2, S3> and BSID B.
   o  has a BGP policy which matches on the extended-color community C
      and allows its usage as SLA steering information.

   If all these conditions are met, H installs R/r in RIB/FIB with next-
   hop = SR Policy P of BSID B instead of via N.

   Indeed, H's local BGP policy and the received BGP route indicate that
   the headend should associate R/r with an SR Policy path to endpoint N



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   with the SLA associated with color C.  The headend therefore installs
   the BGP route on that policy.

   This can be implemented by using the BSID as a generalized next-hop
   and installing the BGP route on that generalized next-hop.

   When H receives a packet K with a destination matching R/r, H pushes
   the label stack <S1, S2, S3, V> and sends the resulting packet along
   the path to S1.

   Note that any SID associated with the BGP route is inserted after the
   SID-List of the SR Policy (i.e., <S1, S2, S3, V>).

   The same behavior is applicable to any type of service route: any
   AFI/SAFI of BGP [RFC4760] any AFI/SAFI of LISP [RFC6830].

8.4.1.  Multiple Colors

   When a BGP route has multiple extended-color communities each with a
   valid SR Policy NLRI, the BGP process installs the route on the SR
   policy whose color is of highest numerical value.

   Let us assume that headend H:

   o  learns a BGP route R/r via next-hop N, extended-color communities
      C1 and C2 and VPN label V.
   o  has a valid SR Policy P1 to (endpoint = N, color = C1) of SID list
      <S1, S2, S3> and BSID B1.
   o  has a valid SR Policy P2 to (endpoint = N, color = C2) of SID list
      <S4, S5, S6> and BSID B2.
   o  has a BGP policy which matches on the extended-color communities
      C1 and C2 and allows their usage as SLA steering information

   If all these conditions are met, H installs R/r in RIB/FIB with next-
   hop = SR Policy P2 of BSID=B2 (instead of N) because C2 > C1.

8.5.  Recursion on an on-demand dynamic BSID

   In the previous section, it was assumed that H had a pre-established
   "explicit" SR Policy (endpoint N, color C).

   In this section, independently to the a-priori existence of any
   explicit candidate path of the SR policy (N, C), it is to be noted
   that the BGP process at headend node H triggers the instantiation of
   a dynamic candidate path for the SR policy (N, C) as soon as:

   o  the BGP process learns of a route R/r via N and with color C.




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   o  a local policy at node H authorizes the on-demand SR Policy path
      instantiation and maps the color to a dynamic SR Policy path
      optimization template.

8.5.1.  Multiple Colors

   When a BGP route R/r via N has multiple extended-color communities Ci
   (with i=1 ... n), an individual on-demand SR Policy dynamic path
   request (endpoint N, color Ci) is triggered for each color Ci.

8.6.  Per-Flow Steering

   Let us assume that headend H:

   o  has a valid SR Policy P1 to (endpoint = N, color = C1) of SID-List
      <S1, S2, S3> and BSID B1.
   o  has a valid SR Policy P2 to (endpoint = N, color = C2) of SID-List
      <S4, S5, S6> and BSID B2.
   o  is configured to instantiate an array of paths to N where the
      entry 0 is the IGP path to N, color C1 is the first entry and
      Color C2 is the second entry.  The index into the array is called
      a Forwarding Class (FC).  The index can have values 0 to 7.
   o  is configured to match flows in its ingress interfaces (upon any
      field such as Ethernet destination/source/vlan/tos or IP
      destination/source/DSCP or transport ports etc.) and color them
      with an internal per-packet forwarding-class variable (0, 1 or 2
      in this example).

   If all these conditions are met, H installs in RIB/FIB:

   o  N via a recursion on an array A (instead of the immediate outgoing
      link associated with the IGP shortest-path to N).
   o  Entry A(0) set to the immediate outgoing link of the IGP shortest-
      path to N.
   o  Entry A(1) set to SR Policy P1 of BSID=B1.
   o  Entry A(2) set to SR Policy P2 of BSID=B2.

   H receives three packets K, K1 and K2 on its incoming interface.
   These three packets either longest-match on N or more likely on a
   BGP/service route which recurses on N.  H colors these 3 packets
   respectively with forwarding-class 0, 1 and 2.  As a result:

   o  H forwards K along the shortest-path to N (which in SR-MPLS
      results in the pushing of the prefix-SID of N).
   o  H pushes <S1, S2, S3> on packet K1 and forwards the resulting
      frame along the shortest-path to S1.
   o  H pushes <S4, S5, S6> on packet K2 and forwards the resulting
      frame along the shortest-path to S4.



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   If the local configuration does not specify any explicit forwarding
   information for an entry of the array, then this entry is filled with
   the same information as entry 0 (i.e. the IGP shortest-path).

   If the SR Policy mapped to an entry of the array becomes invalid,
   then this entry is filled with the same information as entry 0.  When
   all the array entries have the same information as entry0, the
   forwarding entry for N is updated to bypass the array and point
   directly to its outgoing interface and next-hop.

   The array index values (e.g. 0, 1 and 2) and the notion of
   forwarding-class are implementation specific and only meant to
   describe the desired behavior.  The same can be realized by other
   mechanisms.

   This realizes per-flow steering: different flows bound to the same
   BGP endpoint are steered on different IGP or SR Policy paths.

8.7.  Policy-based Routing

   Finally, headend H may be configured with a local routing policy
   which overrides any BGP/IGP path and steer a specified packet on an
   SR Policy.  This includes the use of mechanisms like IGP Shortcut for
   automatic routing of IGP prefixes over SR Policies intended for such
   purpose.

8.8.  Optional Steering Modes for BGP Destinations

8.8.1.  Color-Only BGP Destination Steering

   In the previous section, it is seen that the steering on an SR Policy
   is governed by the matching of the BGP route's next-hop N and the
   authorized color C with an SR Policy defined by the tuple (N, C).

   This is the most likely form of BGP destination steering and the one
   recommended for most use-cases.

   This section defines an alternative steering mechanism based only on
   the color.

   This color-only steering variation is governed by two new flags "C"
   and "O" defined in the color extended community [ref draft-ietf-idr-
   segment-routing-te-policy section 3].

   The Color-Only flags "CO" are set to 00 by default.

   When 00, the BGP destination is steered as follows:




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       IF there is a valid SR Policy (N, C) where N is the IPv4 or IPv6

               endpoint address and C is a color;
           Steer into SR Policy (N, C);
       ELSE;
           Steer on the IGP path to the next-hop N.

   This is the classic case described in this document previously and
   what is recommended in most scenarios.

   When 01, the BGP destination is steered as follows:

       IF there is a valid SR Policy (N, C) where N is the IPv4 or IPv6

               endpoint address and C is a color;
           Steer into SR Policy (N, C);
       ELSE IF there is a valid SR Policy (null endpoint, C) of the
               same address-family of N;
           Steer into SR Policy (null endpoint, C);
       ELSE IF there is any valid SR Policy
               (any address-family null endpoint, C);
           Steer into SR Policy (any null endpoint, C);
       ELSE;
           Steer on the IGP path to the next-hop N.

   When 10, the BGP destination is steered as follows:

       IF there is a valid SR Policy (N, C) where N is an IPv4 or IPv6
               endpoint address and C is a color;
           Steer into SR Policy (N, C);
       ELSE IF there is a valid SR Policy (null endpoint, C)
               of the same address-family of N;
           Steer into SR Policy (null endpoint, C);
       ELSE IF there is any valid SR Policy
               (any address-family null endpoint, C);
           Steer into SR Policy (any null endpoint, C);
       ELSE IF there is any valid SR Policy (any endpoint, C)
               of the same address-family of N;
           Steer into SR Policy (any endpoint, C);
       ELSE IF there is any valid SR Policy
               (any address-family endpoint, C);
           Steer into SR Policy (any address-family endpoint, C);
       ELSE;
           Steer on the IGP path to the next-hop N.

   The null endpoint is 0.0.0.0 for IPv4 and ::0 for IPv6 (all bits set
   to the 0 value).




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   The value 11 is reserved for future use and SHOULD NOT be used.  Upon
   reception, an implementations MUST treat it like 00.

8.8.2.  Multiple Colors and CO flags

   The steering preference is first based on highest color value and
   then CO-dependent for the color.  Assuming a Prefix via (NH,
   C1(CO=01), C2(CO=01)); C1>C2 The steering preference order is:

   o  SR policy (NH, C1).
   o  SR policy (null, C1).
   o  SR policy (NH, C2).
   o  SR policy (null, C2).
   o  IGP to NH.

8.8.3.  Drop upon Invalid

   This document defined earlier that when all the following conditions
   are met, H installs R/r in RIB/FIB with next-hop = SR Policy P of
   BSID B instead of via N.

   o  H learns a BGP route R/r via next-hop N, extended-color community
      C and VPN label V.
   o  H has a valid SR Policy P to (endpoint = N, color = C) of SID-List
      <S1, S2, S3> and BSID B.
   o  H has a BGP policy which matches on the extended-color community C
      and allows its usage as SLA steering information.

   This behavior is extended by noting that the BGP policy may require
   the BGP steering to always stay on the SR policy whatever its
   validity.

   This is the "drop upon invalid" option described in Section 8.2
   applied to BGP-based steering.

9.  Protection

9.1.  Leveraging TI-LFA local protection of the constituent IGP segments

   In any topology, Topology-Independent Loop Free Alternate (TI-LFA)
   [I-D.bashandy-rtgwg-segment-routing-ti-lfa] provides a 50msec local
   protection technique for IGP SIDs.  The backup path is computed on a
   per IGP SID basis along the post-convergence path.

   In a network that has deployed TI-LFA, an SR Policy built on the
   basis of TI-LFA protected IGP segments leverages the local protection
   of the constituent segments.




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   In a network that has deployed TI-LFA, an SR Policy instantiated only
   with non-protected Adj SIDs does not benefit from any local
   protection.

9.2.  Using an SR Policy to locally protect a link


                                       1----2-----6----7
                                       |    |     |    |
                                       4----3-----9----8


                Figure 1: Local protection using SR Policy

   An SR Policy can be instantiated at node 2 to protect the link 2to6.
   A typical explicit SID list would be <3, 9, 6>.

   A typical use-case occurs for links outside an IGP domain: e.g. 1, 2,
   3 and 4 are part of IGP/SR sub-domain 1 while 6, 7, 8 and 9 are part
   of IGP/SR sub-domain 2.  In such a case, links 2to6 and 3to9 cannot
   benefit from TI-LFA automated local protection.

9.3.  Using a Candidate Path for Path Protection

   An SR Policy allows for multiple candidate paths, of which at any
   point in time there is a single active candidate path that is
   provisioned in the forwarding plane and used for traffic steering.
   However, another (lower preference) candidate path MAY be designated
   as the backup for a specific or all (active) candidate path(s).  Such
   a backup candidate path is generally disjoint from the active
   candidate path.

   The headend MAY compute a-priori and validate such backup candidate
   paths as well as provision them into forwarding plane as backup for
   the active path.  A fast re-route mechanism MAY then be used to
   trigger sub 50msec switchover from the active to the backup candidate
   path in the forwarding plane.  Mechanisms like BFD MAY be used for
   fast detection of such failures.

10.  Security Considerations

   This document does not define any new protocol extensions and does
   not impose any additional security challenges.








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11.  IANA Considerations

   This document has no actions for IANA.

12.  Acknowledgement

   The authors would like to thank Tarek Saad, Dhanendra Jain and
   Ruediger Geib for their valuable comments and suggestions.

13.  Contributors

   The following people have contributed to this document:

   Ketan Talaulikar
   Cisco Systems
   Email: ketant@cisco.com

   Zafar Ali
   Cisco Systems
   Email: zali@cisco.com

   Jose Liste
   Cisco Systems
   Email: jliste@cisco.com

   Francois Clad
   Cisco Systems
   Email: fclad@cisco.com

   Kamran Raza
   Cisco Systems
   Email: skraza@cisco.com

   Shraddha Hegde
   Juniper Networks
   Email: shraddha@juniper.net

   Steven Lin
   Google, Inc.
   Email: stevenlin@google.com

   Przemyslaw Krol
   Google, Inc.
   Email: pkrol@google.com

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



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   Dirk Steinberg
   Steinberg Consulting
   Email: dws@steinbergnet.net

   Bruno Decraene
   Orange Business Services
   Email: bruno.decraene@orange.com

   Stephane Litkowski
   Orange Business Services
   Email: stephane.litkowski@orange.com

   Luay Jalil
   Verizon
   Email: luay.jalil@verizon.com

14.  References

14.1.  Normative References

   [I-D.ietf-spring-segment-routing]
              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-15 (work
              in progress), January 2018.

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

14.2.  Informative References

   [I-D.ali-spring-sr-traffic-accounting]
              Ali, Z., Filsfils, C., Talaulikar, K., Sivabalan, S.,
              Horneffer, M., Raszuk, R., Litkowski, S., and d.
              daniel.voyer@bell.ca, "Traffic Accounting in Segment
              Routing Networks", draft-ali-spring-sr-traffic-
              accounting-02 (work in progress), June 2018.

   [I-D.anand-spring-poi-sr]
              Anand, M., Bardhan, S., Subrahmaniam, R., Tantsura, J.,
              Mukhopadhyaya, U., and C. Filsfils, "Packet-Optical
              Integration in Segment Routing", draft-anand-spring-poi-
              sr-05 (work in progress), February 2018.






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   [I-D.bashandy-rtgwg-segment-routing-ti-lfa]
              Bashandy, A., Filsfils, C., Decraene, B., Litkowski, S.,
              Francois, P., and d. daniel.voyer@bell.ca, "Topology
              Independent Fast Reroute using Segment Routing", draft-
              bashandy-rtgwg-segment-routing-ti-lfa-04 (work in
              progress), April 2018.

   [I-D.filsfils-spring-sr-policy-considerations]
              Filsfils, C., Talaulikar, K., Krol, P., Horneffer, M., and
              P. Mattes, "SR Policy Implementation and Deployment
              Considerations", draft-filsfils-spring-sr-policy-
              considerations-01 (work in progress), June 2018.

   [I-D.filsfils-spring-sr-traffic-counters]
              Filsfils, C., Ali, Z., Horneffer, M.,
              daniel.voyer@bell.ca, d., Durrani, M., and R. Raszuk,
              "Segment Routing Traffic Accounting Counters", draft-
              filsfils-spring-sr-traffic-counters-00 (work in progress),
              June 2018.

   [I-D.filsfils-spring-srv6-network-programming]
              Filsfils, C., Li, Z., Leddy, J., daniel.voyer@bell.ca, d.,
              daniel.bernier@bell.ca, d., Steinberg, D., Raszuk, R.,
              Matsushima, S., Lebrun, D., Decraene, B., Peirens, B.,
              Salsano, S., Naik, G., Elmalky, H., Jonnalagadda, P., and
              M. Sharif, "SRv6 Network Programming", draft-filsfils-
              spring-srv6-network-programming-04 (work in progress),
              March 2018.

   [I-D.ietf-idr-bgp-ls-segment-routing-msd]
              Tantsura, J., Chunduri, U., Mirsky, G., and S. Sivabalan,
              "Signaling Maximum SID Depth using Border Gateway Protocol
              Link-State", draft-ietf-idr-bgp-ls-segment-routing-msd-01
              (work in progress), October 2017.

   [I-D.ietf-idr-bgpls-segment-routing-epe]
              Previdi, S., Filsfils, C., Patel, K., Ray, S., and J.
              Dong, "BGP-LS extensions for Segment Routing BGP Egress
              Peer Engineering", draft-ietf-idr-bgpls-segment-routing-
              epe-15 (work in progress), March 2018.

   [I-D.ietf-idr-segment-routing-te-policy]
              Previdi, S., Filsfils, C., Jain, D., Mattes, P., Rosen,
              E., and S. Lin, "Advertising Segment Routing Policies in
              BGP", draft-ietf-idr-segment-routing-te-policy-03 (work in
              progress), May 2018.





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   [I-D.ietf-idr-te-lsp-distribution]
              Previdi, S., Dong, J., Chen, M., Gredler, H., and J.
              Tantsura, "Distribution of Traffic Engineering (TE)
              Policies and State using BGP-LS", draft-ietf-idr-te-lsp-
              distribution-08 (work in progress), December 2017.

   [I-D.ietf-isis-segment-routing-msd]
              Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
              "Signaling MSD (Maximum SID Depth) using IS-IS", draft-
              ietf-isis-segment-routing-msd-12 (work in progress), May
              2018.

   [I-D.ietf-lsr-flex-algo]
              Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
              A. Gulko, "IGP Flexible Algorithm", draft-ietf-lsr-flex-
              algo-00 (work in progress), May 2018.

   [I-D.ietf-ospf-segment-routing-msd]
              Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
              "Signaling MSD (Maximum SID Depth) using OSPF", draft-
              ietf-ospf-segment-routing-msd-14 (work in progress), May
              2018.

   [I-D.ietf-pce-segment-routing]
              Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "PCEP Extensions for Segment Routing",
              draft-ietf-pce-segment-routing-11 (work in progress),
              November 2017.

   [I-D.sivabalan-pce-binding-label-sid]
              Sivabalan, S., Tantsura, J., Filsfils, C., Previdi, S.,
              Hardwick, J., and D. Dhody, "Carrying Binding Label/
              Segment-ID in PCE-based Networks.", draft-sivabalan-pce-
              binding-label-sid-04 (work in progress), March 2018.

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, DOI 10.17487/RFC1195,
              December 1990, <https://www.rfc-editor.org/info/rfc1195>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              DOI 10.17487/RFC3630, September 2003,
              <https://www.rfc-editor.org/info/rfc3630>.




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   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
              <https://www.rfc-editor.org/info/rfc4456>.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,
              <https://www.rfc-editor.org/info/rfc4760>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [RFC6830]  Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
              Locator/ID Separation Protocol (LISP)", RFC 6830,
              DOI 10.17487/RFC6830, January 2013,
              <https://www.rfc-editor.org/info/rfc6830>.

   [RFC7471]  Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
              Previdi, "OSPF Traffic Engineering (TE) Metric
              Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,
              <https://www.rfc-editor.org/info/rfc7471>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [RFC7810]  Previdi, S., Ed., Giacalone, S., Ward, D., Drake, J., and
              Q. Wu, "IS-IS Traffic Engineering (TE) Metric Extensions",
              RFC 7810, DOI 10.17487/RFC7810, May 2016,
              <https://www.rfc-editor.org/info/rfc7810>.

   [RFC8231]  Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for Stateful PCE", RFC 8231,
              DOI 10.17487/RFC8231, September 2017,
              <https://www.rfc-editor.org/info/rfc8231>.







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   [RFC8281]  Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for PCE-Initiated LSP Setup in a Stateful PCE
              Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
              <https://www.rfc-editor.org/info/rfc8281>.

Authors' Addresses

   Clarence Filsfils
   Cisco Systems, Inc.
   Pegasus Parc
   De kleetlaan 6a, DIEGEM  BRABANT 1831
   BELGIUM

   Email: cfilsfil@cisco.com


   Siva Sivabalan (editor)
   Cisco Systems, Inc.
   2000 Innovation Drive
   Kanata, Ontario  K2K 3E8
   Canada

   Email: msiva@cisco.com


   Daniel Voyer
   Bell Canada
   671 de la gauchetiere W
   Montreal, Quebec  H3B 2M8
   Canada

   Email: daniel.voyer@bell.ca


   Alex Bogdanov
   Google, Inc.

   Email: bogdanov@google.com


   Paul Mattes
   Microsoft
   One Microsoft Way
   Redmond, WA  98052-6399
   USA

   Email: pamattes@microsoft.com



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