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Versions: 00 01 02

SPRING                                                       C. Filsfils
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                                J. Leddy
Expires: May 3, 2018                                             Comcast
                                                                D. Voyer
                                                              D. Bernier
                                                             Bell Canada
                                                            D. Steinberg
                                                    Steinberg Consulting
                                                               R. Raszuk
                                                            Bloomberg LP
                                                           S. Matsushima
                                                                SoftBank
                                                               D. Lebrun
                                        Universite catholique de Louvain
                                                             B. Decraene
                                                                  Orange
                                                              B. Peirens
                                                                Proximus
                                                              S. Salsano
                                        Universita di Roma "Tor Vergata"
                                                                 G. Naik
                                                       Drexel University
                                                              H. Elmalky
                                                                Ericsson
                                                         P. Jonnalagadda
                                                               M. Sharif
                                                       Barefoot Networks
                                                             A. Ayyangar
                                                                  Arista
                                                                S. Mynam
                                                   Dell Force10 Networks
                                                           W. Henderickx
                                                                   Nokia
                                                             A. Bashandy
                                                                 K. Raza
                                                                D. Dukes
                                                                 F. Clad
                                                       P. Camarillo, Ed.
                                                     Cisco Systems, Inc.
                                                        October 30, 2017


                        SRv6 Network Programming
           draft-filsfils-spring-srv6-network-programming-02






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Abstract

   This document describes the SRv6 network programming concept and its
   most basic functions.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

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

   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 May 3, 2018.

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
   (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.









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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  SRv6 Segment  . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Functions associated with a Local SID . . . . . . . . . . . .   8
     4.1.  End: Endpoint . . . . . . . . . . . . . . . . . . . . . .   9
     4.2.  End.X: Endpoint with Layer-3 cross-connect  . . . . . . .   9
     4.3.  End.T: Endpoint with specific IPv6 table lookup . . . . .  10
     4.4.  End.DX2: Endpoint with decapsulation and Layer-2 cross-
           connect . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.5.  End.DX2V: Endpoint with decapsulation and VLAN L2 table
           lookup  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.6.  End.DT2U: Endpoint with decapsulation and unicast MAC L2
           table lookup  . . . . . . . . . . . . . . . . . . . . . .  12
     4.7.  End.DT2M: Endpoint with decapsulation and L2 table
           flooding  . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.8.  End.DX6: Endpoint with decapsulation and IPv6 cross-
           connect . . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.9.  End.DX4: Endpoint with decapsulation and IPv4 cross-
           connect . . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.10. End.DT6: Endpoint with decapsulation and specific IPv6
           table lookup  . . . . . . . . . . . . . . . . . . . . . .  15
     4.11. End.DT4: Endpoint with decapsulation and specific IPv4
           table lookup  . . . . . . . . . . . . . . . . . . . . . .  16
     4.12. End.DT46: Endpoint with decapsulation and specific IP
           table lookup  . . . . . . . . . . . . . . . . . . . . . .  16
     4.13. End.B6: Endpoint bound to an SRv6 policy  . . . . . . . .  17
     4.14. End.B6.Encaps: Endpoint bound to an SRv6 encapsulation
           policy  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     4.15. End.BM: Endpoint bound to an SR-MPLS policy . . . . . . .  18
     4.16. End.S: Endpoint in search of a target in table T  . . . .  19
     4.17. SR-aware application  . . . . . . . . . . . . . . . . . .  19
     4.18. Non SR-aware application  . . . . . . . . . . . . . . . .  20
     4.19. Flavours  . . . . . . . . . . . . . . . . . . . . . . . .  20
       4.19.1.  PSP: Penultimate Segment Pop of the SRH  . . . . . .  20
       4.19.2.  USP: Ultimate Segment Pop of the SRH . . . . . . . .  20
   5.  Transit behaviors . . . . . . . . . . . . . . . . . . . . . .  21
     5.1.  T: Transit behavior . . . . . . . . . . . . . . . . . . .  21
     5.2.  T.Insert: Transit with insertion of an SRv6 Policy  . . .  21
     5.3.  T.Encaps: Transit with encapsulation in an SRv6 Policy  .  22
     5.4.  T.Encaps.L2: Transit with encapsulation of L2 frames  . .  22
   6.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .  23
     6.1.  Reserved FUNC opcodes . . . . . . . . . . . . . . . . . .  23
     6.2.  Counters  . . . . . . . . . . . . . . . . . . . . . . . .  23
     6.3.  Flow-based hash computation . . . . . . . . . . . . . . .  24
   7.  Basic security for intra-domain deployment  . . . . . . . . .  24
     7.1.  SEC 1 . . . . . . . . . . . . . . . . . . . . . . . . . .  25



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     7.2.  SEC 2 . . . . . . . . . . . . . . . . . . . . . . . . . .  25
     7.3.  SEC 3 . . . . . . . . . . . . . . . . . . . . . . . . . .  25
     7.4.  SEC 4 . . . . . . . . . . . . . . . . . . . . . . . . . .  26
   8.  Control Plane . . . . . . . . . . . . . . . . . . . . . . . .  26
     8.1.  IGP . . . . . . . . . . . . . . . . . . . . . . . . . . .  26
     8.2.  BGP-LS  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     8.3.  BGP IP/VPN  . . . . . . . . . . . . . . . . . . . . . . .  27
     8.4.  Summary . . . . . . . . . . . . . . . . . . . . . . . . .  27
   9.  Illustration  . . . . . . . . . . . . . . . . . . . . . . . .  28
     9.1.  Simplified SID allocation . . . . . . . . . . . . . . . .  28
     9.2.  Reference diagram . . . . . . . . . . . . . . . . . . . .  29
     9.3.  Basic security  . . . . . . . . . . . . . . . . . . . . .  29
     9.4.  SR-IPVPN  . . . . . . . . . . . . . . . . . . . . . . . .  30
     9.5.  SR-Ethernet-VPWS  . . . . . . . . . . . . . . . . . . . .  31
     9.6.  SR-EVPN-FXC . . . . . . . . . . . . . . . . . . . . . . .  32
     9.7.  SR-EVPN . . . . . . . . . . . . . . . . . . . . . . . . .  33
       9.7.1.  EVPN Bridging . . . . . . . . . . . . . . . . . . . .  33
       9.7.2.  EVPN Multi-homing with ESI filtering  . . . . . . . .  35
       9.7.3.  EVPN Layer-3  . . . . . . . . . . . . . . . . . . . .  36
       9.7.4.  EVPN Integrated Routing Bridging (IRB)  . . . . . . .  36
     9.8.  SR TE for Underlay SLA  . . . . . . . . . . . . . . . . .  37
       9.8.1.  SR policy from the Ingress PE . . . . . . . . . . . .  37
       9.8.2.  SR policy at a midpoint . . . . . . . . . . . . . . .  38
     9.9.  End-to-End policy with intermediate BSID  . . . . . . . .  39
     9.10. TI-LFA  . . . . . . . . . . . . . . . . . . . . . . . . .  40
     9.11. SR TE for Service chaining  . . . . . . . . . . . . . . .  41
   10. Benefits  . . . . . . . . . . . . . . . . . . . . . . . . . .  42
     10.1.  Seamless deployment  . . . . . . . . . . . . . . . . . .  42
     10.2.  Integration  . . . . . . . . . . . . . . . . . . . . . .  43
     10.3.  Security . . . . . . . . . . . . . . . . . . . . . . . .  43
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  43
   12. Work in progress  . . . . . . . . . . . . . . . . . . . . . .  43
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  43
   14. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  44
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  44
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  44
     15.2.  Informative References . . . . . . . . . . . . . . . . .  44
   Appendix A.  Additional Contributors  . . . . . . . . . . . . . .  46
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  46

1.  Introduction

   Segment Routing leverages the source routing paradigm.  An ingress
   node steers a packet through a ordered list of instructions, called
   segments.  Each one of these instructions represents a function to be
   called at a specific location in the network.  A function is locally
   defined on the node where it is executed and may range from simply
   moving forward in the segment list to any complex user-defined



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   behavior.  The network programming consists in combining segment
   routing functions, both simple and complex, to achieve a networking
   objective that goes beyond mere packet routing.

   This document illustrates the SRv6 Network Programming concept and
   aims at standardizing the main segment routing functions to enable
   the creation of interoperable overlays with underlay optimization and
   service chaining.

   Familiarity with the Segment Routing Header
   [I-D.ietf-6man-segment-routing-header] is assumed.

2.  Terminology

   SRH is the abbreviation for the Segment Routing Header.  We assume
   that the SRH may be present multiple times inside each packet.

   NH is the abbreviation of the IPv6 next-header field.

   NH=SRH means that the next-header field is 43 with routing type 4.

   When there are multiple SRHs, they must follow each other: the next-
   header field of all SRH except the last one must be SRH.

   The effective next-header (ENH) is the next-header field of the IP
   header when no SRH is present, or is the next-header field of the
   last SRH.

   In this version of the document, we assume that there is no other
   extension header than the SRH.  These will be lifted in future
   versions of the document.

   SID: A Segment Identifier which represents a specific segment in
   segment routing domain.  The SID type used in this document is IPv6
   address (also referenced as SRv6 Segment or SRv6 SID).

   A SID list is represented as <S1, S2, S3> where S1 is the first SID
   to visit, S2 is the second SID to visit and S3 is the last SID to
   visit along the SR path.

   (SA,DA) (S3, S2, S1; SL) represents an IPv6 packet with:

   - IPv6 header with source and destination addresses respectively SA
     and DA and next-header is SRH

   - SRH with SID list <S1, S2, S3> with SegmentsLeft = SL





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   - Note the difference between the <> and () symbols: <S1, S2, S3>
     represents a SID list where S1 is the first SID and S3 is the last
     SID.  (S3, S2, S1; SL) represents the same SID list but encoded in
     the SRH format where the rightmost SID in the SRH is the first SID
     and the leftmost SID in the SRH is the last SID.  When referring to
     an SR policy in a high-level use-case, it is simpler to use the
     <S1, S2, S3> notation.  When referring to an illustration of the
     detailed behavior, the (S3, S2, S1; SL) is more convenient.

   - The payload of the packet is omitted.

   SRH[SL] represents the SID pointed by the SL field in the first SRH.
   In our example, SRH[2] represents S1, SRH[1] represents S2 and SRH[0]
   represents S3.

   FIB is the abbreviation for the forwarding table.  A FIB lookup is a
   lookup in the forwarding table.  When a packet is intercepted on a
   wire, it is possible that SRH[SL] is different from the DA.

3.  SRv6 Segment

   An SRv6 Segment is a 128-bit value.  "SID" (abbreviation for Segment
   Identifier) is often used as a shorter reference for "SRv6 Segment".

   An SRv6-capable node N maintains a "My Local SID Table".  This table
   contains all the local SRv6 segments explicitly instantiated at node
   N.  N is the parent node for these SIDs.

   A local SID of N can be an IPv6 address associated to a local
   interface of N but it is not mandatory.  Nor is the My Local SID
   table populated by default with all IPv6 addresses defined on node N.

   In most use-cases, a local SID will NOT be an address associated to a
   local interface of N.

   A local SID of N could be routed to N but it does not have to be.
   Most often, it is routed to N via a shorter-mask prefix.


   Let's provide a classic illustration.

   Node N is configured with a loopback0 interface address of C1::1/40
   originated in its IGP.  Node N is configured with two SIDs: C1::100
   and C2::101.

   The entry C1::1 is not defined explicitly as an SRv6 SID and hence
   does not appear in the "My Local SID Table".  The entries C1::100 and




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   C2::101 are defined explicitly as SRv6 SIDs and hence appear in the
   "My Local SID Table".

   The network learns about a path to C1::/40 via the IGP and hence a
   packet destined to C1::100 would be routed up to N.  The network does
   not learn about a path to C2::/40 via the IGP and hence a packet
   destined to C2::101 would not be routed up to N.

   A packet could be steered to a non-routed SID C2::101 by using a SID
   list <...,C1::100,C2::101,...> where the non-routed SID is preceded
   by a routed SID to the same node.  This is similar to the local vs
   global segments in SR-MPLS.


   Every SRv6 local SID instantiated has a specific instruction bound to
   it.  This information is stored in the "My Local SID Table".  The "My
   Local SID Table" has three main purposes:

   - Define which local SIDs are explicitly instantiated

   - Specify which instruction is bound to each of the instantiated SIDs

   - Store the parameters associated with such instruction (i.e.  OIF,
     NextHop,...)

   We represent an SRv6 local SID as LOC:FUNCT where LOC is the L most
   significant bits and FUNCT is the 128-L least significant bits.  L is
   called the locator length and is flexible.  Each operator is free to
   use the locator length it chooses.  Most often the LOC part of the
   SID is routable and leads to the node which owns that SID.

   Often, for simplicity of illustration, we will use a locator length
   of 64 bits.  This is just an example.  Implementations must not
   assume any a priori prefix length.

   The FUNCT part of the SID is an opaque identification of a local
   function bound to the SID.  Hence the name SRv6 Local SID.

   A function may require additional arguments that would be placed in
   the rightmost-bits of the 128-bit space.  In such case, the SRv6
   Local SID will have the form LOC:FUNCT:ARGS.

   These arguments may vary on a per-packet basis and may contain
   information related to the flow, service, or any other information
   required by the function associated to the SRv6 Local SID.

   For to this reason, the "My Local SID Table" matches on a per
   longest-prefix-match basis.



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   A node may receive a packet with an SRv6 SID in the DA without an
   SRH.  In such case the packet should still be processed by the
   Segment Routing engine.

4.  Functions associated with a Local SID

   Each entry of the "My Local SID Table" indicates the function
   associated with the local SID.

   We define hereafter a set of well-known functions that can be
   associated with a SID.

 End            Endpoint function
                The SRv6 instantiation of a prefix SID
 End.X          Endpoint function with Layer-3 cross-connect
                The SRv6 instantiation of a Adj SID
 End.T          Endpoint function with specific IPv6 table lookup
 End.DX2        Endpoint with decapsulation and Layer-2 cross-connect
                L2VPN use-case
 End.DX2V       Endpoint with decapsulation and VLAN L2 table lookup
                EVPN Flexible cross-connect use-cases
 End.DT2U       Endpoint with decapsulation and unicast MAC L2 table lookup
                EVPN Bridging unicast use-cases
 End.DT2M       Endpoint with decapsulation and L2 table flooding
                EVPN Bridging BUM use-cases with ESI filtering
 End.DX6        Endpoint with decapsulation and IPv6 cross-connect
                IPv6 L3VPN use (equivalent of a per-CE VPN label)
 End.DX4        Endpoint with decapsulation and IPv4 cross-connect
                IPv4 L3VPN use (equivalent of a per-CE VPN label)
 End.DT6        Endpoint with decapsulation and IPv6 table lookup
                IPv6 L3VPN use (equivalent of a per-VRF VPN label)
 End.DT4        Endpoint with decapsulation and IPv4 table lookup
                IPv4 L3VPN use (equivalent of a per-VRF VPN label)
 End.DT46       Endpoint with decapsulation and IP table lookup
                IP L3VPN use (equivalent of a per-VRF VPN label)
 End.B6         Endpoint bound to an SRv6 policy
                SRv6 instantiation of a Binding SID
 End.B6.Encaps  Endpoint bound to an SRv6 encapsulation Policy
                SRv6 instantiation of a Binding SID
 End.BM         Endpoint bound to an SR-MPLS Policy
                SRv6/SR-MPLS instantiation of a Binding SID
 End.S          Endpoint in search of a target in table T

   The list is not exhaustive.  In practice, any function can be
   attached to a local SID: e.g. a node N can bind a SID to a local VM
   or container which can apply any complex function on the packet.





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   We call N the node who has an explicitly defined local SID S and we
   detail the function that N binds to S.

   At the end of this section we also present some flavours of these
   well-known functions.

4.1.  End: Endpoint

   The Endpoint function ("End" for short) is the most basic function.

   When N receives a packet whose IPv6 DA is S and S is a local End SID,
   N does:

 1.   IF NH=SRH and SL > 0
 2.      decrement SL
 3.      update the IPv6 DA with SRH[SL]
 4.      FIB lookup on updated DA                                ;; Ref1
 5.      forward accordingly to the matched entry                ;; Ref2
 6.   ELSE
 7.      drop the packet                                         ;; Ref3

   Ref1: The End function performs the FIB lookup in the forwarding
   table associated to the ingress interface

   Ref2: If the FIB lookup matches a multicast state, then the related
   RPF check must be considered successful

   Ref3: a local SID could be bound to a function which authorizes the
   decapsulation of an outer header (e.g.  IPinIP) or the punting of the
   packet to TCP, UDP or any other protocol.  This however needs to be
   explicitly defined in the function bound to the local SID.  By
   default, a local SID bound to the well-known function "End" only
   allows the punting to OAM protocols and neither allows the
   decapsulation of an outer header nor the cleanup of an SRH.  As a
   consequence, an End SID cannot be the last SID of an SRH and cannot
   be the DA of a packet without SRH.

   This is the SRv6 instantiation of a Prefix SID
   [I-D.ietf-spring-segment-routing].

4.2.  End.X: Endpoint with Layer-3 cross-connect

   The "Endpoint with cross-connect to an array of layer-3 adjacencies"
   function (End.X for short) is a variant of the End function.

   When N receives a packet destined to S and S is a local End.X SID, N
   does:




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 1.   IF NH=SRH and SL > 0
 2.      decrement SL
 3.      update the IPv6 DA with SRH[SL]
 4.      forward to layer-3 adjacency bound to the SID S         ;; Ref1
 5.   ELSE
 6.      drop the packet                                         ;; Ref2

   Ref1: If an array of adjacencies is bound to the End.X SID, then one
   entry of the array is selected based on a hash of the packet's
   header.

   Ref2: An End.X function only allows punting to OAM and does not allow
   decaps.  An End.X SID cannot be the last SID of an SRH and cannot be
   the DA of a packet without SRH.

   The End.X function is required to express any traffic-engineering
   policy.

   This is the SRv6 instantiation of an Adjacency SID
   [I-D.ietf-spring-segment-routing].

   If a node N has 30 outgoing interfaces to 30 neighbors, usually the
   operator would explicitly instantiate 30 End.X SIDs at N: one per
   layer-3 adjacency to a neighbor.  Potentially, more End.X could be
   explicitly defined (groups of layer-3 adjacencies to the same
   neighbor or to different neighbors).

   Note that with SR-MPLS, an AdjSID is typically preceded by a
   PrefixSID.  This is unlikely in SRv6 as most likely an End.X SID is
   globally routed to N.

   Note that if N has an outgoing interface bundle I to a neighbor Q
   made of 10 member links, N may allocate up to 11 End.X local SIDs for
   that bundle: one for the bundle itself and then up to one for each
   member link.  This is the equivalent of the L2-Link Adj SID in SR-
   MPLS [I-D.ietf-isis-l2bundles].

4.3.  End.T: Endpoint with specific IPv6 table lookup

   The "Endpoint with specific IPv6 table lookup" function (End.T for
   short) is a variant of the End function.

   When N receives a packet destined to S and S is a local End.T SID, N
   does:







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 1.   IF NH=SRH and SL > 0                                       ;; Ref1
 2.      decrement SL
 3.      update the IPv6 DA with SRH[SL]
 4.      lookup the next segment in IPv6 table T associated with the SID
 5.      forward via the matched table entry
 6.   ELSE
 7.      drop the packet

   Ref1: The End.T SID must not be the last SID

   The End.T is used for multi-table operation in the core.

4.4.  End.DX2: Endpoint with decapsulation and Layer-2 cross-connect

   The "Endpoint with decapsulation and Layer-2 cross-connect to OIF"
   function (End.DX2 for short) is a variant of the endpoint function.

   When N receives a packet destined to S and S is a local End.DX2 SID,
   N does:

 1.   IF NH=SRH and SL > 0
 2.      drop the packet                                         ;; Ref1
 3.   ELSE IF ENH  = 59                                          ;; Ref2
 4.      pop the (outer) IPv6 header and its extension headers
 5.      forward the resulting frame via OIF associated to the SID
 6.   ELSE
 7.      drop the packet

   Ref1: An End.DX2 SID must always be the last SID, or it can be the
   Destination Address of an IPv6 packet with no SRH header.

   Ref2: We conveniently reuse the next-header value 59 allocated to
   IPv6 No Next Header [RFC2460].  When the SID is of function End.DX2
   and the Next-Header=59, we know that an Ethernet frame is in the
   payload without any further header.

   An End.DX2 function could be customized to expect a specific VLAN
   format and rewrite the egress VLAN header before forwarding on the
   outgoing interface.

   One of the applications of the End.DX2 function is the L2VPN use-
   case.

4.5.  End.DX2V: Endpoint with decapsulation and VLAN L2 table lookup

   The "Endpoint with decapsulation and specific VLAN L2 table lookup"
   function (End.DX2V for short) is a variant of the endpoint function.




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   When N receives a packet destined to S and S is a local End.DX2V SID,
   N does:

 1.   IF NH=SRH and SL > 0
 2.      drop the packet                                         ;; Ref1
 3.   ELSE IF ENH  = 59                                          ;; Ref2
 4.      pop the (outer) IPv6 header and its extension headers
 5.      lookup the exposed inner VLANs in L2 table T
 6.      forward via the matched table entry
 7.   ELSE
 8.      drop the packet

   Ref1: An End.DX2V SID must always be the last SID, or it can be the
   Destination Address of an IPv6 packet with no SRH header.

   Ref2: We conveniently reuse the next-header value 59 allocated to
   IPv6 No Next Header [RFC2460].  When the SID is of function End.DX2V
   and the Next-Header=59, we know that an Ethernet frame is in the
   payload without any further header.

   An End.DX2V function could be customized to expect a specific VLAN
   format and rewrite the egress VLAN header before forwarding on the
   outgoing interface.

   The End.DX2V is used for EVPN Flexible cross-connect use-cases.

4.6.  End.DT2U: Endpoint with decapsulation and unicast MAC L2 table
      lookup

   The "Endpoint with decapsulation and specific unicast MAC L2 table
   lookup" function (End.DT2U for short) is a variant of the endpoint
   function.

   When N receives a packet destined to S and S is a local End.DT2U SID,
   N does:

 1.   IF NH=SRH and SL > 0
 2.      drop the packet                                         ;; Ref1
 3.   ELSE IF ENH  = 59                                          ;; Ref2
 4.      pop the (outer) IPv6 header and its extension headers
 5.      learn he exposed inner MAC SA in L2 table T             ;; Ref3
 6.      lookup the exposed inner MAC DA in L2 table T
 7.      forward via the matched T entry else to all L2OIF in T
 8.   ELSE
 9.      drop the packet

   Ref1: An End.DT2U SID must always be the last SID, or it can be the
   Destination Address of an IPv6 packet with no SRH header.



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   Ref2: We conveniently reuse the next-header value 59 allocated to
   IPv6 No Next Header [RFC2460].  When the SID is of function End.DT2U
   and the Next-Header=59, we know that an Ethernet frame is in the
   payload without any further header.

   Ref3: In EVPN, the learning of the exposed inner MAC SA is done via
   control plane.

   The End.DT2U is used for EVPN Bridging unicast use cases.

4.7.  End.DT2M: Endpoint with decapsulation and L2 table flooding

   The "Endpoint with decapsulation and specific L2 table flooding"
   function (End.DT2M for short) is a variant of the endpoint function.

   This function may take an argument: "Arg.FE2".  It is an argument
   specific to EVPN ESI filtering.  It is used to exclude a specific OIF
   from L2 table T flooding.  The Arg.FE2 SID is merged with an End.DT2M
   function by bit ORing operation to form an End.DT2M(FE2)single SID.

   When N receives a packet destined to S and S is a local End.DT2M SID,
   N does:

 1.   IF NH=SRH and SL > 0
 2.      drop the packet                                         ;; Ref1
 3.   ELSE IF ENH  = 59                                          ;; Ref2
 4.      pop the (outer) IPv6 header and its extension headers
 5.      learn the exposed inner MAC SA in L2 table T            ;; Ref3
 6.      forward on all L2OIF excluding the one specified in Arg.FE2
 7.   ELSE
 8.      drop the packet

   Ref1: An End.DT2M SID must always be the last SID, or it can be the
   Destination Address of an IPv6 packet with no SRH header.

   Ref2: We conveniently reuse the next-header value 59 allocated to
   IPv6 No Next Header [RFC2460].  When the SID is of function End.DT2M
   and the Next-Header=59, we know that an Ethernet frame is in the
   payload without any further header.

   Ref3: In EVPN, the learning of the exposed inner MAC SA is done via
   control plane

   The End.DT2M is used for EVPN Bridging BUM use case with ESI
   filtering capability.






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4.8.  End.DX6: Endpoint with decapsulation and IPv6 cross-connect

   The "Endpoint with decapsulation and cross-connect to an array of
   IPv6 adjacencies" function (End.DX6 for short) is a variant of the
   End and End.X functions.

   When N receives a packet destined to S and S is a local End.DX6 SID,
   N does:

 1.   IF NH=SRH and SL > 0
 2.      drop the packet                                         ;; Ref1
 3.   ELSE IF ENH = 41                                           ;; Ref2
 4.      pop the (outer) IPv6 header and its extension headers
 5.      forward to layer-3 adjacency bound to the SID S         ;; Ref3
 6.   ELSE
 7.      drop the packet

   Ref1: The End.DX6 SID must always be the last SID, or it can be the
   Destination Address of an IPv6 packet with no SRH header.

   Ref2: 41 refers to IPv6 encapsulation as defined by IANA allocation
   for Internet Protocol Numbers

   Ref3: Selected based on a hash of the packet's header (at least SA,
   DA, Flow Label)

   One of the applications of the End.DX6 function is the L3VPN use-case
   where a FIB lookup in a specific tenant table at the egress PE is not
   required.  This would be equivalent to the per-CE VPN label in
   MPLS[RFC4364].

4.9.  End.DX4: Endpoint with decapsulation and IPv4 cross-connect

   The "Endpoint with decapsulation and cross-connect to an array of
   IPv4 adjacencies" function (End.DX4 for short) is a variant of the
   End and End.X functions.

   When N receives a packet destined to S and S is a local End.DX4 SID,
   N does:

 1.   IF NH=SRH and SL > 0
 2.      drop the packet                                         ;; Ref1
 3.   ELSE IF ENH  =  4                                          ;; Ref2
 4.      pop the (outer) IPv6 header and its extension headers
 5.      forward to layer-3 adjacency bound to the SID S         ;; Ref3
 6.   ELSE
 7.      drop the packet




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   Ref1: The End.DX4 SID must always be the last SID, or it can be the
   Destination Address of an IPv6 packet with no SRH header.

   Ref2: 4 refers to IPv4 encapsulation as defined by IANA allocation
   for Internet Protocol Numbers

   Ref3: Selected based on a hash of the packet's header (at least SA,
   DA, Flow Label)

   One of the applications of the End.DX4 function is the L3VPN use-case
   where a FIB lookup in a specific tenant table at the egress PE is not
   required.  This would be equivalent to the per-CE VPN label in
   MPLS[RFC4364].

4.10.  End.DT6: Endpoint with decapsulation and specific IPv6 table
       lookup

   The "Endpoint with decapsulation and specific IPv6 table lookup"
   function (End.DT6 for short) is a variant of the End function.

   When N receives a packet destined to S and S is a local End.DT6 SID,
   N does:

 1.   IF NH=SRH and SL > 0
 2.      drop the packet                                         ;; Ref1
 3.   ELSE IF ENH = 41                                           ;; Ref2
 4.      pop the (outer) IPv6 header and its extension headers
 5.      lookup the exposed inner IPv6 DA in IPv6 table T
 6.      forward via the matched table entry
 7.   ELSE
 8.      drop the packet

   Ref1: the End.DT6 SID must always be the last SID, or it can be the
   Destination Address of an IPv6 packet with no SRH header.

   Ref2: 41 refers to IPv6 encapsulation as defined by IANA allocation
   for Internet Protocol Numbers

   One of the applications of the End.DT6 function is the L3VPN use-case
   where a FIB lookup in a specific tenant table at the egress PE is
   required.  This would be equivalent to the per-VRF VPN label in
   MPLS[RFC4364].

   Note that an End.DT6 may be defined for the main IPv6 table in which
   case and End.DT6 supports the equivalent of an IPv6inIPv6 decaps
   (without VPN/tenant implication).





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4.11.  End.DT4: Endpoint with decapsulation and specific IPv4 table
       lookup

   The "Endpoint with decapsulation and specific IPv4 table lookup"
   function (End.DT4 for short) is a variant of the End function.

   When N receives a packet destined to S and S is a local End.DT4 SID,
   N does:

 1.   IF NH=SRH and SL > 0
 2.      drop the packet                                         ;; Ref1
 3.   ELSE IF ENH = 4                                            ;; Ref2
 4.      pop the (outer) IPv6 header and its extension headers
 5.      lookup the exposed inner IPv4 DA in IPv4 table T
 6.      forward via the matched table entry
 7.   ELSE
 8.      drop the packet

   Ref1: the End.DT4 SID must always be the last SID, or it can be the
   Destination Address of an IPv6 packet with no SRH header.

   Ref2: 4 refers to IPv4 encapsulation as defined by IANA allocation
   for Internet Protocol Numbers

   One of the applications of the End.DT4 is the L3VPN use-case where a
   FIB lookup in a specific tenant table at the egress PE is required.
   This would be equivalent to the per-VRF VPN label in MPLS[RFC4364].

   Note that an End.DT4 may be defined for the main IPv4 table in which
   case and End.DT4 supports the equivalent of an IPv4inIPv6 decaps
   (without VPN/tenant implication).

4.12.  End.DT46: Endpoint with decapsulation and specific IP table
       lookup

   The "Endpoint with decapsulation and specific IP table lookup"
   function (End.DT46 for short) is a variant of the End function.

   When N receives a packet destined to S and S is a local End.DT46 SID,
   N does:











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  1.   IF NH=SRH and SL > 0
  2.      drop the packet                                        ;; Ref1
  3.   ELSE IF ENH  =  4                                         ;; Ref2
  4.      pop the (outer) IPv6 header and its extension headers
  5.      lookup the exposed inner IPv4 DA in IPv4 table T
  6.      forward via the matched table entry
  7.   ELSE IF ENH = 41                                          ;; Ref2
  8.      pop the (outer) IPv6 header and its extension headers
  9.      lookup the exposed inner IPv6 DA in IPv6 table T
 10.      forward via the matched table entry
 11.   ELSE
 12.      drop the packet

   Ref1: the End.DT46 SID must always be the last SID, or it can be the
   Destination Address of an IPv6 packet with no SRH header.

   Ref2: 4 and 41 refer to IPv4 and IPv6 encapsulation respectively as
   defined by IANA allocation for Internet Protocol Numbers

   One of the applications of the End.DT46 is the L3VPN use-case where a
   FIB lookup in a specific tenant table at the egress PE is required.
   This would be equivalent to the per-VRF VPN label in MPLS[RFC4364].

   Note that an End.DT46 may be defined for the main IP table in which
   case and End.DT46 supports the equivalent of an IPinIPv6 decaps
   (without VPN/tenant implication).

4.13.  End.B6: Endpoint bound to an SRv6 policy

   The "Endpoint bound to an SRv6 Policy" is a variant of the End
   function.

   When N receives a packet destined to S and S is a local End.B6 SID, N
   does:

 1.   IF NH=SRH and SL > 0                                       ;; Ref1
 2.      do not decrement SL nor update the IPv6 DA with SRH[SL]
 3.      insert a new SRH                                        ;; Ref2
 4.      set the IPv6 DA to the first segment of the SRv6 Policy
 5.      forward according to the first segment of the SRv6 Policy
 6.   ELSE
 7.      drop the packet

   Ref1: An End.B6 SID, by definition, is never the last SID.

   Ref2: In case that an SRH already exists, the new SRH is inserted in
   between the IPv6 header and the received SRH




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   Note: Instead of the term "insert", "push" may also be used.

   The End.B6 function is required to express scalable traffic-
   engineering policies across multiple domains.  This is the SRv6
   instantiation of a Binding SID [I-D.ietf-spring-segment-routing].

4.14.  End.B6.Encaps: Endpoint bound to an SRv6 encapsulation policy

   This is a variation of the End.B6 behavior where the SRv6 Policy also
   includes an IPv6 Source Address A.

   When N receives a packet destined to S and S is a local End.B6.Encaps
   SID, N does:

   1.   IF NH=SRH and SL > 0
   2.      decrement SL and update the IPv6 DA with SRH[SL]
   3.      push an outer IPv6 header with its own SRH
   4.      set the outer IPv6 SA to A
   5.      set the outer IPv6 DA to the first segment of the SRv6 Policy
   6.      forward according to the first segment of the SRv6 Policy
   7.   ELSE
   8.      drop the packet

   Instead of simply inserting an SRH with the policy (End.B6), this
   behavior also adds an outer IPv6 header.  The source address defined
   for the outer header does not have to be a local SID of the node.

4.15.  End.BM: Endpoint bound to an SR-MPLS policy

   The "Endpoint bound to an SR-MPLS Policy" is a variant of the End.B6
   function.

   When N receives a packet destined to S and S is a local End.BM SID, N
   does:

 1.   IF NH=SRH and SL > 0                                       ;; Ref1
 2.      decrement SL and update the IPv6 DA with SRH[SL]
 3.      push an MPLS label stack <L1, L2, L3> on the received packet
 4.      forward according to L1
 5.   ELSE
 6.      drop the packet

   Ref1: an End.BM SID, by definition, is never the last SID.

   The End.BM function is required to express scalable traffic-
   engineering policies across multiple domains where some domains
   support the MPLS instantiation of Segment Routing.




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   This is an SRv6 instantiation of a SR-MPLS Binding SID
   [I-D.ietf-spring-segment-routing].

4.16.  End.S: Endpoint in search of a target in table T

   The "Endpoint in search of a target in Table T" function (End.S for
   short) is a variant of the End function.

   When N receives a packet destined to S and S is a local End.S SID, N
   does:

 1.   IF NH=SRH and SL = 0                                       ;; Ref1
 2.      drop the packet
 3.   ELSE IF match(last SID) in specified table T
 4.      forward accordingly
 5.   ELSE
 6.      apply the End behavior

   Ref1: By definition, an End.S SID cannot be the last SID, as the last
   SID is the targeted object.

   The End.S function is required in information-centric networking
   (ICN) use-cases where the last SID in the SRv6 SID list represents a
   targeted object.  If the identification of the object would require
   more than 128 bits, then obvious customization of the End.S function
   may either use multiple SIDs or a TLV of the SR header to encode the
   searched object ID.

4.17.  SR-aware application

   Generally, any SR-aware application can be bound to an SRv6 SID.
   This application could represent anything from a small piece of code
   focused on topological/tenant function to a much larger process
   focusing on higher-level applications (e.g. video compression,
   transcoding etc.).

   The ways in which an SR-aware application can binds itself on a local
   SID depends on the operating system.  Let us consider an SR-aware
   application running on a Linux operating system.  A possible approach
   is to associate an SRv6 SID to a target (virtual) interface, so that
   packets with IP DA corresponding to the SID will be sent to the
   target interface.  In this approach, the SR-aware application can
   simply listen to all packets received on the interface.

   A different approach for the SR-aware app is to listen to packets
   received with a specific SRv6 SID as IPv6 DA on a given transport
   port (i.e. corresponding to a TCP or UDP socket).  In this case, the
   app can read the SRH information with a getsockopt Linux system call



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   and can set the SRH information to be added to the outgoing packets
   with a setsocksopt system call.

4.18.  Non SR-aware application

   [I-D.clad-spring-segment-routing-service-chaining] defines a set of
   additional functions in order to enable non SR-aware applications to
   be associated with a SRv6 Local SID.

4.19.  Flavours

   We present the PSP and USP variants of the functions End, End.X and
   End.T.  For each of these functions these variants can be enabled or
   disabled either individually or together.

4.19.1.  PSP: Penultimate Segment Pop of the SRH

   After the instruction 'update the IPv6 DA with SRH[SL]' is executed,
   the following instructions must be added:

 1.   IF updated SL = 0 & PSP is TRUE
 2.      pop the top SRH                                         ;; Ref1

   Ref1: The received SRH had SL=1.  When the last SID is written in the
   DA, the End, End.X and End.T functions with the PSP flavour pop the
   first (top-most) SRH.  Subsequent stacked SRH's may be present but
   are not processed as part of the function.

4.19.2.  USP: Ultimate Segment Pop of the SRH

   We insert at the beginning of the pseudo-code the following
   instructions:

 1.   IF SL = 0 & NH=SRH & USP=TRUE                              ;; Ref1
 2.      pop the top SRH
 3.      restart the function processing on the modified packet  ;; Ref2

   Ref1: The next header is an SRH header

   Ref2: Typically SL of the exposed SRH is > 0 and hence the restarting
   of the complete function would lead to decrement SL, update the IPv6
   DA with SRH[SL], FIB lookup on updated DA and forward accordingly to
   the matched entry.








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5.  Transit behaviors

   We define hereafter the set of basic transit behaviors.

   T            Transit behavior
   T.Insert     Transit behavior with insertion of an SRv6 Policy
   T.Encaps     Transit behavior with encapsulation in an SRv6 policy
   T.Encaps.L2  T.Encaps behavior of the received L2 frame

   This list can be expanded in case any new functionality requires it.

5.1.  T: Transit behavior

   As per [RFC2460], if a node N receives a packet (A, S2)(S3, S2, S1;
   SL=2) and S2 is neither a local address nor a local SID of N then N
   forwards the packet without inspecting the SRH.

   This means that N treats the following two packets with the same
   performance:

   - (A, S2)

   - (A, S2)(S3, S2, S1; SL=2)

   A transit node does not need to count by default the amount of
   transit traffic with an SRH extension header.  This accounting might
   be enabled as an optional behavior leveraging SEC4 behavior described
   later in this document.Section 7.4

   A transit node MUST include the outer flow label in its ECMP
   hash[RFC6437].

5.2.  T.Insert: Transit with insertion of an SRv6 Policy

   Node N receives two packets P1=(A, B2) and P2=(A,B2)(B3, B2, B1;
   SL=1).  B2 is neither a local address nor SID of N.

   N steers the transit packets P1 and P2 into an SRv6 Policy with one
   SID list <S1, S2, S3>.

   The "T.Insert" transit insertion behavior is defined as follows:

 1.   insert the SRH (B2, S3, S2, S1; SL=3)             ;; Ref1, Ref1bis
 2.   set the IPv6 DA = S1
 3.   forward along the shortest path to S1

   Ref1: The received IPv6 DA is placed as last SID of the inserted SRH.




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   Ref1bis: The SRH is inserted before any other IPv6 Routing Extension
   Header.

   After the T.Insert behavior, P1 and P2 respectively look like:

   - (A, S1) (B2, S3, S2, S1; SL=3)

   - (A, S1) (B2, S3, S2, S1; SL=3) (B3, B2, B1; SL=1)

5.3.  T.Encaps: Transit with encapsulation in an SRv6 Policy

   Node N receives two packets P1=(A, B2) and P2=(A,B2)(B3, B2, B1;
   SL=1).  B2 is neither a local address nor SID of N.

   N steers the transit packets P1 and P2 into an SR Encapsulation
   Policy with a Source Address T and a Segment list <S1, S2, S3>.

   The T.Encaps transit encapsulation behavior is defined as follows:

1.   push an IPv6 header with its own SRH (S3, S2, S1; SL=2)
2.   set outer IPv6 SA = T and outer IPv6 DA = S1
3.   set outer payload length, traffic class and flow label     ;; Ref 1
4.   update the next_header value                               ;; Ref 1
5.   decrement inner Hop Limit or TTL                           ;; Ref 1
6.   forward along the shortest path to S1

   After the T.Encaps behavior, P1 and P2 respectively look like:

   - (T, S1) (S3, S2, S1; SL=2) (A, B2)

   - (T, S1) (S3, S2, S1; SL=2) (A, B2) (B3, B2, B1; SL=1)

   The T.Encaps behavior is valid for any kind of Layer-3 traffic.  This
   behavior is commonly used for L3VPN with IPv4 and IPv6 deployements.

   The SRH MAY be omitted when the SRv6 Policy only contains one segment
   and there is no need to use any flag, tag or TLV.

   Ref 1: As described in [RFC2473] (Generic Packet Tunneling in IPv6
   Specification)

5.4.  T.Encaps.L2: Transit with encapsulation of L2 frames

   While T.Encaps encapsulates the received IP packet, T.Encaps.L2
   encapsulates the received L2 frame (i.e. the received ethernet header
   and its optional VLAN header is in the payload of the outer packet).





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   If the outer header is pushed without SRH then the DA must be a SID
   of type End.DX2, End.DX2V, End.DT2U or End.DT2M and the next-header
   must be 59 (IPv6 NoNextHeader).  The received Ethernet frame follows
   the IPv6 header and its extension headers.

   Else, if the outer header is pushed with an SRH, then the last SID of
   the SRH must be of type End.DX2, End.DX2V, End.DT2U or End.DT2M and
   the next-header of the SRH must be 59 (IPv6 NoNextHeader).  The
   received Ethernet frame follows the IPv6 header and its extension
   headers.

6.  Operation

6.1.  Reserved FUNC opcodes

   The following SRv6 LocalSID function opcodes are reserved:

   - Opcode 0: Invalid

   - Opcode 1-63: Reserved

     - Opcode 1: End with PSP

     - Opcode 2: End with USP

   - Opcode ~0 (all 1s): Wildcard

   The SRv6 LocalSID argument value "0" means "No argument".

6.2.  Counters

   Any SRv6 capable node SHOULD implement the following set of combined
   counters (packets and bytes):

   - CNT1: Per entry of the "My Local SID Table", traffic that matched
     that SID and was processed correctly.

   - CNT2: Per SRv6 Policy, traffic steered into it and processed
     correctly.

   Furthermore, an SRv6 capable node maintains an aggregate counter CNT0
   tracking the IPv6 traffic that was received with a destination
   address matching a local interface address that is not a local SID
   and the next-header is SRH with SL>0.  We remind that this traffic is
   dropped as an interface address is not a local SID by default.  A SID
   must be explicitly instantiated.





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6.3.  Flow-based hash computation

   When a flow-based selection within a set needs to be performed, the
   source address, the destination address and the flow-label MUST be
   included in the flow-based hash.

   This occurs when the destination address is updated and a FIB lookup
   is performed and multiple ECMP paths exist to the updated destination
   address.

   This occurs when End.X is bound to an array of adjacencies.

   This occurs when the packet is steered in an SR policy whose selected
   path has multiple SID lists
   [I-D.filsfils-spring-segment-routing-policy].

7.  Basic security for intra-domain deployment

   We use the following terminology:

      An internal node is a node part of the domain of trust.

      A border router is an internal node at the edge of the domain of
      trust.

      An external interface is an interface of a border router towards
      another domain.

      An internal interface is an interface entirely within the domain
      of trust.

      The internal address space is the IP address block dedicated to
      internal interfaces.

      An internal SID is a SID instantiated on an internal node.

      The internal SID space is the IP address block dedicated to
      internal SIDs.

      External traffic is traffic received from an external interface to
      the domain of trust.

      Internal traffic is traffic the originates and ends within the
      domain of trust.

   The purpose of this section is to document how a domain of trust can
   operate SRv6-based services for internal traffic while preventing any
   external traffic from accessing the internal SRv6-based services.



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   It is expected that future documents will detail enhanced security
   mechanisms for SRv6 (e.g. how to allow external traffic to leverage
   internal SRv6 services).

7.1.  SEC 1

   An SRv6 router MUST support an ACL on the external interface that
   drops any traffic with SA or DA in the internal SID space.

   A provider would generally do this for its internal address space to
   prevent access to internal addresses and in order to prevent
   spoofing.  The technique is extended to the local SID space.

   The typical counters of an ACL are expected.

7.2.  SEC 2

   An SRv6 router MUST support an ACL with the following behavior:

   1.   IF (DA == LocalSID) && (SA != internal address or SID space)
   2.      drop

   This prevents access to local SIDs from outside the operator's
   infrastructure.  Note that this ACL may not be enabled in all cases.
   For example, specific SIDs can be used to provide resources to
   devices that are outside of the operator's infrastructure.

   When an SID is in the form of LOC:FUNCT:ARGS the DA match should be
   implemented as a prefix match covering the argument space of the
   specific SID i.s.o. a host route.

   The typical counters of an ACL are expected.

7.3.  SEC 3

   As per the End definition, an SRv6 router MUST only implement the End
   behavior on a local IPv6 address if that address has been explicitly
   enabled as a segment.

   This address may or may not be associated with an interface.  This
   address may or may not be routed.  The only thing that matters is
   that the local SID must be explicitly instantiated and explicitly
   bound to a function (the default function is the End function).








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7.4.  SEC 4

   An SRv6 router should support Unicast-RPF on source address on
   external interface.

   This is a generic provider technique applied to the internal address
   space.  It is extended to the internal SID space.

   The typical counters to validate such filtering are expected.

8.  Control Plane

   In an SDN environment, one expects the controller to explicitly
   provision the SIDs and/or discover them as part of a service
   discovery function.  Applications residing on top of the controller
   could then discover the required SIDs and combine them to form a
   distributed network program.

   The concept of "SRv6 network programming" refers to the capability
   for an application to encode any complex program as a set of
   individual functions distributed through the network.  Some functions
   relate to underlay SLA others to overlay/tenant, others to complex
   applications residing in VM and containers.

   The specification of the SRv6 control-plane is outside the scope of
   this document.

   We limit ourselves to a few important observations.

8.1.  IGP

   The End and End.X SIDs express topological functions and hence are
   expected to be signaled in the IGP together with the flavours PSP and
   USP [I-D.bashandy-isis-srv6-extensions].

   The presence of SIDs in the IGP do not imply any routing semantics to
   the addresses represented by these SIDs.  The routing reachability to
   an IPv6 address is solely governed by the classic, non-SID-related,
   IGP information.  Routing is not governed neither influenced in any
   way by a SID advertisement in the IGP.

   These two SIDs provide important topological functions for the IGP to
   build FRR/TI-LFA solution and for TE processes relying on IGP LSDB to
   build SR policies.







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8.2.  BGP-LS

   BGP-LS is expected to be the key service discovery protocol.  Every
   node is expected to advertise via BGP-LS its SRv6 capabilities (e.g.
   how many SIDs in can insert as part of an T.Insert behavior) and any
   locally instantiated SID[I-D.ietf-idr-bgp-ls-segment-routing-ext][I-D
   .ietf-idr-te-lsp-distribution].

8.3.  BGP IP/VPN

   The End.DX46, End.DT46 and End.DX2 SIDs are expected to be signaled
   in BGP[I-D.dawra-idr-srv6-vpn].

8.4.  Summary

   The following table summarizes which SID would be signaled in which
   signaling protocol.

             +------------------+-----+--------+------------+
             |                  | IGP | BGP-LS | BGP IP/VPN |
             +------------------+-----+--------+------------+
             | End   (PSP, USP) |  X  |   X    |            |
             | End.X (PSP, USP) |  X  |   X    |            |
             | End.T (PSP, USP) |  X  |   X    |            |
             | End.DX2          |     |   X    |     X      |
             | End.DX2V         |     |   X    |     X      |
             | End.DT2U         |     |   X    |     X      |
             | End.DT2M         |     |   X    |     X      |
             | End.DX6          |  X  |   X    |     X      |
             | End.DX4          |     |   X    |     X      |
             | End.DT6          |  X  |   X    |     X      |
             | End.DT4          |     |   X    |     X      |
             | End.DT46         |     |   X    |     X      |
             | End.B6           |     |   X    |            |
             | End.B6.Encaps    |     |   X    |            |
             | End.B6.BM        |     |   X    |            |
             | End.S            |     |   X    |            |
             +------------------+-----+--------+------------+

                     Table 1: SRv6 LocalSID signaling

   The following table summarizes which transit capability would be
   signaled in which signaling protocol.








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                +-------------+-----+--------+------------+
                |             | IGP | BGP-LS | BGP IP/VPN |
                +-------------+-----+--------+------------+
                | T           |     |   X    |            |
                | T.Insert    |     |   X    |            |
                | T.Encaps    |     |   X    |            |
                | T.Encaps.L2 |     |   X    |            |
                +-------------+-----+--------+------------+

                 Table 2: SRv6 transit behaviors signaling

   The previous table describes generic capabilities.  It does not
   describe specific instantiated SID.

   For example, a BGP-LS advertisement of the T capability of node N
   would indicate that node N supports the basic transit behavior.  The
   T.Insert behavior would describe the capability of node N to
   instantiation a T.Insert behavior, specifically it would describe how
   many SIDs could be inserted by N without significant performance
   degradation.  Same for T.Encaps (the number potentially lower as the
   overhead of the additional outer IP header is accounted).


   The reader should also remember that any specific instantiated SR
   policy (via T.Insert or T.Encaps) is always assigned a Binding SID.
   He should remember that BSIDs are advertised in BGP-LS as shown in
   Table 1.  Hence, it is normal that Table 2 only focuses on the
   generic capabilities related to T.Insert and T.Encaps as Table 1
   advertises the specific instantiated BSID properties.

9.  Illustration

   We introduce a simplified SID allocation technique to ease the
   reading of the text.  We document the reference diagram.  We then
   illustrate the network programming concept through different use-
   cases.  These use-cases have been thought to allow straightforward
   combination between each other.

9.1.  Simplified SID allocation

   To simplify the illustration, we assume:

      A::/4 is dedicated to the internal SRv6 SID space

      B::/4 is dedicated to the internal address space

      We assume a location expressed in 48 bits and a function expressed
      in 80 bits



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      Node k has a classic IPv6 loopback address Bk::/128 which is
      advertised in the IGP

      Node k has Ak::/48 for its local SID space.  Its SIDs will be
      explicitly allocated from that block

      Node k advertises Ak::/48 in its IGP

      Function 0:0:0:0:1 (function 1, for short) represents the End
      function with PSP support

      Function 0:0:0:0:C2 (function C2, for short) represents the End.X
      function towards neighbor 2

   Each node K has:

      An explicit SID instantiation Ak::1/128 bound to an End function
      with additional support for PSP

      An explicit SID instantiation Ak::Cj/128 bound to an End.X
      function to neighbor J with additional support for PSP

9.2.  Reference diagram

   Let us assume the following topology where all the links have IGP
   metric 10 except the link 23 which is 100.

   Nodes A, 1 to 8 and B are considered within the network domain while
   nodes CE-A and CE-B are outside the domain.

                                4------5---9
                              / |       \ /
                             3  |        6
                              \ |       /
                        A--1--- 2------7---8--B
                          /                 \
                       CE-A                 CE-B
                    Tenant100            Tenant100 with
                                           IPv4 20/8

                       Figure 1: Reference topology

9.3.  Basic security

   Any edge node such as 1 would be configured with an ACL on any of its
   external interface (e.g. from CE-A) which drops any traffic with SA
   or DA in A::/4.  See SEC 1 (Section 7.1).




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   Any core node such as 6 could be configured with an ACL with the SEC2
   (Section 7.2) behavior "IF (DA == LocalSID) && (SA is not in A::/4 or
   B::/4) THEN drop".

   SEC 3 (Section 7.3) protection is a default property of SRv6.  A SID
   must be explicitly instantiated.  In our illustration, the only
   available SIDs are those explicitly instantiated.

   Any edge node such as 1 would be configured with Unicast-RPF on
   source address on external interface (e.g. from CE-A).  See SEC 4
   (Section 7.4).

9.4.  SR-IPVPN

   Let us illustrate the SR-IPVPN use-case applied to IPv4.

   Nodes 1 and 8 are configured with a tenant 100, each respectively
   connected to CE-A and CE-B.

   Node 8 is configured with a local SID A8::D100 of function End.DT4
   bound to tenant IPv4 table 100.

   Via BGP signaling or an SDN-based controller, Node 1's tenant-100
   IPv4 table is programmed with an IPv4 SR-VPN route 20/8 via SRv6
   policy <A8::D100>.

   When 1 receives a packet P from CE-A destined to 20.20.20.20, P looks
   up its tenant-100 IPv4 table and finds an SR-VPN entry 20/8 via SRv6
   policy <A8::D100>.  As a consequence, 1 pushes an outer IPv6 header
   with SA=A1::0, DA=A8::D100 and NH=4. 1 then forwards the resulting
   packet on the shortest path to A8::/40.

   When 8 receives the packet, 8 matches the DA in its My LocalSID
   table, finds the bound function End.DT4(100) and confirms NH=4.  As a
   result, 8 decaps the outer header, looks up the inner IPv4 DA in
   tenant-100 IPv4 table, and forward the (inner) IPv4 packet towards
   CE-B.

   The reader can easily infer all the other SR-IPVPN IP instantiations:












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  +---------------------------------+----------------------------------+
  | Route at ingress PE(1)          | SR-VPN Egress SID of egress PE(8)|
  +---------------------------------+----------------------------------+
  | IPv4 tenant route with egress   | End.DT4 function bound to        |
  | tenant table lookup             | IPv4-tenant-100 table            |
  +---------------------------------+----------------------------------+
  | IPv4 tenant route without egress| End.DX4 function bound to        |
  | tenant table lookup             | CE-B (IPv4)                      |
  +---------------------------------+----------------------------------+
  | IPv6 tenant route with egress   | End.DT6 function bound to        |
  | tenant table lookup             | IPv6-tenant-100 table            |
  +---------------------------------+----------------------------------+
  | IPv6 tenant route without egress| End.DX6 function bound to        |
  | tenant table lookup             | CE-B (IPv6)                      |
  +---------------------------------+----------------------------------+

9.5.  SR-Ethernet-VPWS

   Let us illustrate the SR-Ethernet-VPWS use-case.

   Node 1 is configured with an ethernet VPWS service:

   - Local attachment circuit: Ethernet interface from CE-A

   - Local End.DX2 bound to the local attachment circuit: A1::DC2A

   - Remote End.DX2 SID: A8::DC2B

   Node 8 is configured with an ethernet VPWS service:

   - Local attachment circuit: Ethernet interface from CE-B

   - Local End.DX2 bound to the local attachment circuit: A8::DC2B

   - Remote End.DX2 SID: A1::DC2A

   These configurations can either be programmed by an SDN controller or
   partially derived from a BGP-based signaling and discovery service.

   When 1 receives a packet P from CE-A, 1 pushes an outer IPv6 header
   with SA=A1::0, DA=A8::DC2B and NH=59.  Note that no additional header
   is pushed. 1 then forwards the resulting packet on the shortest path
   to A8::/40.

   When 8 receives the packet, 8 matches the DA in its My LocalSID table
   and finds the bound function End.DX2.  After confirming that the
   next-header=59, 8 decaps the outer IPv6 header and forwards the inner
   Ethernet frame towards CE-B.



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   The reader can easily infer the Ethernet VPWS use-case:

      +------------------------+-----------------------------------+
      | Route at ingress PE(1) | SR-VPN Egress SID of egress PE(8) |
      +------------------------+-----------------------------------+
      | Ethernet VPWS          | End.DX2 function bound to         |
      |                        | CE-B (Ethernet)                   |
      +------------------------+-----------------------------------+

9.6.  SR-EVPN-FXC

   Let us illustrate the SR-EVPN-FXC use-case (Flexible cross-connect
   service).

   Node 1 is configured with an EVPN-FXC service:

   - Local attachment circuit: Ethernet interface from CE1-A over VLAN
     100

   - Local attachment circuit: Ethernet interface from CE2-A over VLAN
     200

   - Local End.DX2 bound to the local attachment circuit: A1::DC2A

   - Remote End.DX2 SID: A8::DC2B

   Node 8 is configured with an EVPN-FXC service:

   - Local attachment circuit: Ethernet interface from CE1-B over VLAN
     100

   - Local attachment circuit: Ethernet interface from CE2-B over VLAN
     200

   - Local End.DX2 bound to the local attachment circuit: A8::DC2B

   - Remote End.DX2 SID: A1::DC2A

   These configurations can either be programmed by an SDN controller or
   derived from a BGP-based EVPN-FXC service.  EVPN route Type-1 is used
   for that purpose.

   When node 1 receives a packet P from CE-A, it pushes an outer IPv6
   header with SA=A1::0, DA=A8::DC2B and NH=59.  Note that no additional
   header is pushed.  Node 1 then forwards the resulting packet on the
   shortest path to A8::/40.





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   When node 8 receives the packet, it matches the IP DA in its My
   LocalSID table and finds the bound function End.DX2V.  After
   confirming that the next-header=59, node 8 decaps the outer IPv6
   header, performs a VLAN loopkup in table T1 and forwards the inner
   Ethernet frame to matching interface e.g. for VLAN 100, packet is
   forwarded to CE1-B and for VLAN 200, packet is forwarded to CE2-B.

   The reader can easily infer the Ethernet FXC use-case:

+------------------------------------+------------------------------------+
| Route at ingress PE (1)            | SR-VPN Egress SID of egress PE (8) |
+------------------------------------+------------------------------------+
| EVPN-FXC                           | End.DX2V function bound to         |
|                                    | CE1-B / CE2-B (Ethernet)           |
+------------------------------------+------------------------------------+

9.7.  SR-EVPN

   There are few use cases to illustrate under SR-EVPN: bridging
   (unicast and multicast), multi-homing ESI filtering, EVPN L3, EVPN-
   IRB.

9.7.1.  EVPN Bridging

   Node 1 is configured with an EVPN bridging service (E-LAN service):

   - Local attachment circuit: Ethernet interface from CE-A

   - Local End.DT2U bound to a local layer2 table T1 where EVPN is
     enable: A1::D2AA.  That SID is used to attract unicast traffic

   - Local End.DT2M bound to the same local layer2 table T1 where EVPN
     is enable: A1::D2AF:0.  That SID is used to attract BUM traffic

   Node 4 is configured with an EVPN bridging service:

   - Local attachment circuit: Ethernet interface from CE-B

   - Local End.DT2U bound to a local layer2 table T1 where EVPN is
     enable: A4::D2BA.  That SID is used to attract unicast traffic

   - Local End.DT2M bound to the same local layer2 Table T1 where EVPN
     is enable: A4::D2BF:0.  That SID is used to attract BUM traffic

   Node 8 is configured with an EVPN bridging service:

   - Local attachment circuit: Ethernet interface from CE-C




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   - Local End.DT2U bound to a local layer2 table T1 where EVPN is
     enable: A8::D2CA.  That SID is used to attract unicast traffic

   - Local End.DT2M bound to the same local layer2 Table T1 where EVPN
     is enable: A8::D2CF:0/112.  That SID is used to attract BUM traffic

   The End.DT2M SID are exchanged between nodes via BGP-based EVPN
   route-3.

   Upon reception of EVPN Type-3 routes, each node build its own
   replication list per layer2 table T1.

   On node 1, the replication list looks like: A4::D2BF:0, A8::D2CF:0.
   On node 4, the replication list looks like: A1::D2AF:0, A8::D2CF:0.
   On node 8, the replication list looks like: A1:D2AF:0, A4:D2BF:0.  In
   the case of ingress replication, Ingress PE transmitting the BUM
   traffic stream replicates the traffic using that list.

   When node 1 receives a BUM packet P from CE-A, it replicates that
   packet to remote nodes.  For node 4, it pushes an outer IPv6 header
   with SA=A1::0, DA=A4::D2BF:0 and NH=59.  For node 8, it performs the
   same operation but DA=A8::D2CF:0.  Note that no additional header is
   pushed.  Node 1 then forwards the resulting packet on the shortest
   path for each replication e.g.  A4::D2BF:0/112 and A8::D2CF:0/112.

   When node 4 receives the packet, it matches the DA in its My LocalSID
   table and finds the bound function End.DT2M and its related layer2
   table T1.  After confirming that the next-header=59, node 4 decaps
   the outer IPv6 header and forwards the inner Ethernet frame to all
   layer-2 output interface found to table T1.  Similar processing is
   also performed by node 8 upon packet reception.  This example is the
   same for any BUM stream coming from CE-B and CE-C.

   Node 1,4 and 8 are also performing software MAC learning to exchange
   MAC reachability information (unicast traffic) via BGP among
   themselves.

   Each MAC being learned in software are exchanged using BGP-based EVPN
   route type-2.

   When node 1 receives an unicast packet P from CE-A, it learns its
   MAC-SA=CEA in software.  Node 1 transmits that MAC and its associated
   SID A1::D2AA using BGP-based EVPN route-type 2 to all remote nodes.

   When node 4 receives an unicast packet P from CE-B destinated to MAC-
   DA=CEA, it performs a L2 table T1 MAC-DA lookup to find the
   associated SID.  It pushes an outer IPv6 header with SA=A4::0,
   DA=A1::D2AA and NH=59.  Note that no additional header is pushed.



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   Node 4 then forwards the resulting packet on the shortest path to
   A1::/40.  Similar processing is also performed by node 8.

9.7.2.  EVPN Multi-homing with ESI filtering

   In L2 network, traffic loop avoidance is a MUST.  In EVPN all-active
   multi-homing scenario, ESI filtering feature enforce that
   requirement.

   Node 1 and node 2 are peering partners of a redundancy group where
   the access CE-A is connected in an all-active multi-homing way with
   these two nodes.

   Node 1 is configured with an EVPN bridging service (E-LAN service):

   - Local attachment circuit: Ethernet interface from CE-A

   - Local Arg.FE2 bound to the attachment circuit: 0xC1

   - Local End.DT2M bound to the same local layer2 table T1 where EVPN
     is enable: A1::D2AF:0/112.  That SID is used to attract BUM traffic

   Node 2 is configured with an EVPN bridging service:

   - Local attachment circuit: Ethernet interface from CE-A

   - Local Arg.FE2 bound to the attachment circuit: 0xC2

   - Local End.DT2M bound to the same local layer2 Table T1 where EVPN
     is enable: A2::D2BF:0.  That SID is used to attract BUM traffic

   The End.DT2M SID are exchanged between nodes via BGP-based EVPN route
   type-3.

   Upon reception of EVPN Type-3 routes, each node build its own
   replication list per layer2 table T1.

   The Arg.FE2 SID are exchange between nodes via BGP ESI-filtering
   extended community attached to BGP-based EVPN route type-1.

   Upon reception of EVPN route type-1 and type-3, node 1 merges the
   End.DT2M SID and the Arg.FE2 SID from node 2; its peering partner.
   Its replication list looks like A2::D2BF:C1.  Similar procedure is
   performed by node 2.

   When node 1 receives a BUM packet P from CE-A, it replicates that
   packet to remote nodes.  For node 2, it pushes an outer IPv6 header
   with SA=A1::0, DA=A2::D2BF:C1 and NH=59.  Note that no additional



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   header is pushed.  Node 1 then forwards the resulting packet on the
   shortest path for each replication e.g.  A2::D2BF:00/112.  Again,
   similar processing is also performed by node 8 upon packet reception

9.7.3.  EVPN Layer-3

   EVPN layer-3 works exactly in the same way of IPVPN.  Please refer to
   SR-IPVPN section

9.7.4.  EVPN Integrated Routing Bridging (IRB)

   EVPN IRB brings Layer-2 and Layer-3 together.  It uses BGP-based EVPN
   route type-2 to achieve Layer-2 intra-subnet and Layer-3 inter-subnet
   forwarding.  The EVPN route type-2 maintain the associated of a MAC/
   IP association.

   Node 1 is configured with an EVPN IRB service:

   - Local attachment circuit: Ethernet interface from CE-A

   - Local End.DT2U bound to a local layer2 table T1 where EVPN is
     enable: SID = A1::D2AA.  That SID is used to attract unicast L2
     traffic

   - Local End.DT2 bound to tenant IPv4 table 100: SID = A1::D3AA.  That
     SID is used to attract L3 traffic

   Node 8 is configured with an EVPN IRB service:

   - Local attachment circuit: Ethernet interface from CE-C

   - Local End.DT2U bound to a local layer2 table T1 where EVPN is
     enable: SID = A8::D2CB.  That SID is used to attract unicast L2
     traffic

   - Local End.DT2 bound to tenant IPv4 table 100: SID = A8::D3CB.  That
     SID is used to attract L3 traffic

   Each ARP/ND request learned by each node are exchanged using BGP-
   based EVPN route type-2.

   When node 1 receives an ARP/ND packet P from a host (10.10.10.10) on
   CE-A destined to 20.20.20.20, it learns its MAC-SA=CEA in software.
   It also learns the ARP/ND entry (IP SA=10.10.10.10) in its cache.
   Node 1 transmits that MAC/IP and its associated L2 SID A1::D2AA and
   L3 SID A1::D3AA using BGP-based EVPN route-type 2 to all remote
   nodes.




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   When node 8 receives a packet P from CE-C destined to 10.10.10.10
   from a host (20.20.20.20), P looks up its tenant-100 IPv4 table and
   finds an SR-VPN entry for that prefix.  As a consequence, node 8
   pushes an outer IPv6 header with SA=A8::0, DA=A1::D3AA and NH=4.
   Node 8 then forwards the resulting packet on the shortest path to
   A1::/40.  EVPN inter-subnet forwarding is then achieved.

   When node 8 receives a packet P from CE-C destined to 10.10.10.10
   from a host (10.10.10.11), P looks up its L2 table T1 MAC-DA lookup
   to find the associated SID.  It pushes an outer IPv6 header with
   SA=A8::0, DA=A1::D2AA and NH=59.  Note that no additional header is
   pushed.  Node 8 then forwards the resulting packet on the shortest
   path to A1::/40.  EVPN intra-subnet forwarding is then achieved.

9.8.  SR TE for Underlay SLA

9.8.1.  SR policy from the Ingress PE

   Let's assume that node 1's tenant-100 IPv4 route "20/8 via A8::D100"
   is programmed with a color/community that requires low-latency
   underlay optimization [I-D.filsfils-spring-segment-routing-policy].

   In such case, node 1 either computes the low-latency path to the
   egress node itself or delegates the computation to a PCE.

   In either case, the location of the egress PE can easily be found by
   looking for who originates the SID block comprising the SID A8::D100.
   This can be found in the IGP's LSDB for a single domain case, and in
   the BGP-LS LSDB for a multi-domain case.

   Let us assume that the TE metric encodes the per-link propagation
   latency.  Let us assume that all the links have a TE metric of 10,
   except link 27 which has TE metric 100.

   The low-latency path from 1 to 8 is thus 1245678.

   This path is encoded in a SID list as: first a hop through A4::C5 and
   then a hop to 8.

   As a consequence the SR-VPN entry 20/8 installed in the Node1's
   Tenant-100 IPv4 table is: T.Encaps with SRv6 Policy <A4::C5,
   A8::D100>.

   When 1 receives a packet P from CE-A destined to 20.20.20.20, P looks
   up its tenant-100 IPv4 table and finds an SR-VPN entry 20/8.  As a
   consequence, 1 pushes an outer header with SA=A1::0, DA=A4::C5,
   NH=SRH followed by SRH (A8::D100, A4::C5; SL=1; NH=4). 1 then
   forwards the resulting packet on the interface to 2.



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   2 forwards to 4 along the path to A4::/40.

   When 4 receives the packet, 4 matches the DA in its My LocalSID table
   and finds the bound function End.X to neighbor 5. 4 notes the PSP
   capability of the SID A4::C5. 4 sets the DA to the next SID A8::D100.
   As 4 is the penultimate segment hop, it performs PSP and pops the
   SRH. 4 forwards the resulting packet to 5.

   5, 6 and 7 forwards along the path to A8::/40.

   When 8 receives the packet, 8 matches the DA in its My LocalSID
   Table and finds the bound function End.DT(100).  As a result, 8
   decaps the outer header, looks up the inner IPv4 DA in tenant-100
   IPv4 table, and forward the (inner) IPv4 packet towards CE-B.

9.8.2.  SR policy at a midpoint

   Let us analyze a policy applied at a midpoint on a packet without
   SRH.

   Packet P1 is (A1::, A8::D100).

   Let us consider P1 when it is received by node 2 and let us assume
   that that node 2 is configured to steer A8::/40 in a transit behavior
   T.Insert associated with SR policy <A4::C5>.

   In such a case, node 2 would send the following modified packet P1 on
   the link to 4:

   (A1::, A4::C5)(A8::D100, A4::C5; SL=1).

   The rest of the processing is similar to the previous section.


   Let us analyze a policy applied at a midpoint on a packet with an
   SRH.

   Packet P2 is (A1::, A7::1)(A8::D100, A7::1; SL=1).

   Let us consider P2 when it is received by node 2 and let us assume
   that node 2 is configured to steer A7::/40 in a transit behavior
   T.Insert associated with SR policy <A4::C5, A9::1>.

   In such a case, node 2 would send the following modified packet P2 on
   the link to 4:

   (A1::, A4::C5)(A7::1, A9::1, A4::C5; SL=2)(A8::D100, A7::1; SL=1)




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   Node 4 would send the following packet to 5: (A1::, A9::1)(A7::1,
   A9::1, A4::C5; SL=1)(A8::D100, A7::; SL=1)

   Node 5 would send the following packet to 9: (A1::, A9::1)(A7::1,
   A9::1, A4::C5; SL=1)(A8::D100, A7::1; SL=1)

   Node 9 would send the following packet to 6: (A1::, A7::1)(A8::D100,
   A7::1; SL=1)

   Node 6 would send the following packet to 7: (A1::, A7::1)(A8::D100,
   A7::1; SL=1)

   Node 7 would send the following packet to 8: (A1::, A8::D100)

9.9.  End-to-End policy with intermediate BSID

   Let us now describe a case where the ingress VPN edge node steers the
   packet destined to 20.20.20.20 towards the egress edge node connected
   to the tenant100 site with 20/8, but via an intermediate SR Policy
   represented by a single routable Binding SID.  Let us illustrate this
   case with an intermediate policy which both encodes underlay
   optimization for low-latency and the service chaining via two SR-
   aware container-based apps.

   Let us assume that the End.B6 SID A2::B1 is configured at node 2 and
   is associated with midpoint T.Insert policy <A4::C5, A9::A1, A6::A2>.

   A4::C5 realizes the low-latency path from the ingress PE to the
   egress PE.  This is the underlay optimization part of the
   intermediate policy.

   A9::A1 and A6::A2 represent two SR-aware NFV applications residing in
   containers respectively connected to node 9 and 6.

   Let us assume the following ingress VPN policy for 20/8 in tenant 100
   IPv4 table of node 1: T.Encaps with SRv6 Policy <A2::B1, A8::D100>.

   This ingress policy will steer the 20/8 tenant-100 traffic towards
   the correct egress PE and via the required intermediate policy that
   realizes the SLA and NFV requirements of this tenant customer.

   Node 1 sends the following packet to 2: (A1::, A2::B1) (A8::D100,
   A2::B1; SL=1)

   Node 2 sends the following packet to 4: (A1::, A4::C5) (A6::A2,
   A9::A1, A4::C5; SL=2)(A8::D100, A2::B1; SL=1)





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   Node 4 sends the following packet to 5: (A1::, A9::A1) (A6::A2,
   A9::A1, A4::C5; SL=1)(A8::D100, A2::B1; SL=1)

   Node 5 sends the following packet to 9: (A1::, A9::A1) (A6::A2,
   A9::A1, A4::C5; SL=1)(A8::D100, A2::B1; SL=1)

   Node 9 sends the following packet to 6: (A1::, A6::A2) (A8::D100,
   A2::B1; SL=1)

   Node 6 sends the following packet to 7: (A1::, A8::D100)

   Node 7 sends the following packet to 8: (A1::, A8::D100) which decaps
   and forwards to CE-B.

   The benefits of using an intermediate Binding SID are well-known and
   key to the Segment Routing architecture: the ingress edge node needs
   to push fewer SIDs, the ingress edge node does not need to change its
   SR policy upon change of the core topology or re-homing of the
   container-based apps on different servers.  Conversely, the core and
   service organizations do not need to share details on how they
   realize underlay SLA's or where they home their NFV apps.

9.10.  TI-LFA

   Let us assume two packets P1 and P2 received by node 2 exactly when
   the failure of link 27 is detected.

      P1: (A1::, A7::1)

      P2: (A1::, A7::1)(A8::D100, A7::1; SL=1)

   Node 2's pre-computed TI-LFA backup path for the destination A7:: is
   <A4::C5>.  It is installed as a T.Insert transit behavior.

   Node 2 protects the two packets P1 and P2 according to the pre-
   computed TI-LFA backup path and send the following modified packets
   on the link to 4:

      P1: (A1::, A4::C5)(A7::1, A4::C5; SL=1)

      P2: (A1::, A4::C5)(A7::1, A4::C5; SL=1) (A8::D100, A7::1; SL=1)

   Node 4 then sends the following modified packets to 5:

      P1: (A1::, A7::1)

      P2: (A1::, A7::1)(A8::D100, A7::1; SL=1)




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   Then these packets follow the rest of their post-convergence path
   towards node 7 and then go to node 8 for the VPN decaps.

9.11.  SR TE for Service chaining

   We have illustrated the service chaining through SR-aware apps in a
   previous section.

   We illustrate the use of End.AS function
   [I-D.clad-spring-segment-routing-service-chaining] to service chain
   an IP flow bound to the internet through two SR-unaware applications
   hosted in containers.

   Let us assume that servers 20 and 70 are respectively connected to
   nodes 2 and 7.  They are respectively configured with SID spaces
   A020::/40 and A070::/40.  Their connected routers advertise the
   related prefixes in the IGP.  Two SR-unaware container-based
   applications App2 and App7 are respectively hosted on server 20 and
   70.  Server 20 (70) is configured explicitly with an End.AS SID
   A020::2 for App2 (A070::7 for App7).

   Let us assume a broadband customer with a home gateway CE-A connected
   to edge router 1.  Router 1 is configured with an SR policy which
   encapsulates all the traffic received from CE-A into a T.Encaps
   policy <A020::2, A070::7, A8::D0> where A8::D0 is an End.DT4 SID
   instantiated at node 8.

   P1 is a packet sent by the broadband customer to 1: (X, Y) where X
   and Y are two IPv4 addresses.

   1 sends the following packet to 2: (A1::0, A020::2)(A8::D0, A070::7,
   A020::2; SL=2; NH=4)(X, Y).

   2 forwards the packet to server 20.

   20 receives the packet (A1::0, A020::2)(A8::D0, A070::7, A020::2;
   SL=2; NH=4)(X, Y) and forwards the inner IPv4 packet (X,Y) to App2.
   App2 works on the packet and forwards it back to 20. 20 pushes the
   outer IPv6 header with SRH (A1::0, A070::7)(A8::D0, A070::7, A020::2;
   SL=1; NH=4) and sends the (whole) IPv6 packet with the encapsulated
   IPv4 packet back to 2.

   2 and 7 forward to server 70.

   70 receives the packet (A1::0, A070::7)(A8::D0, A070::7, A020::2;
   SL=1; NH=4)(X, Y) and forwards the inner IPv4 packet (X,Y) to App7.
   App7 works on the packet and forwards it back to 70. 70 pushes the
   outer IPv6 header with SRH (A1::0, A8::D0)(A8::D0, A070::7, A020::2;



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   SL=0; NH=4) and sends the (whole) IPv6 packet with the encapsulated
   IPv4 packet back to 7.

   7 forwards to 8.

   8 receives (A1::0, A8::D0)(A8::D0, A070::7, A020::2; SL=0; NH=4)(X,
   Y) and performs the End.DT4 function and sends the IP packet (X, Y)
   towards its internet destination.

10.  Benefits

10.1.  Seamless deployment

   The VPN use-case can be realized with SRv6 capability deployed solely
   at the ingress and egress PE's.

      All the nodes in between these PE's act as transit routers as per
      [RFC2460].  No software/hardware upgrade is required on all these
      nodes.  They just need to support IPv6 per [RFC2460].

   The SRTE/underlay-SLA use-case can be realized with SRv6 capability
   deployed at few strategic nodes.

      It is well-known from the experience deploying SR-MPLS that
      underlay SLA optimization requires few SIDs placed at strategic
      locations.  This was illustrated in our example with the low-
      latency optimization which required the operator to enable one
      single core node with SRv6 (node 4) where one single and End.X SID
      towards node 5 was instantiated.  This single SID is sufficient to
      force the end-to-end traffic via the low-latency path.

   The TI-LFA benefits are collected incrementally as SRv6 capabilities
   are deployed.

      It is well-know that TI-LFA is an incremental node-by-node
      deployment.  When a node N is enabled for TI-LFA, it computes TI-
      LFA backup paths for each primary path to each IGP destination.
      In more than 50% of the case, the post-convergence path is loop-
      free and does not depend on the presence of any remote SRv6 SID.
      In the vast majority of cases, a single segment is enough to
      encode the post-convergence path in a loop-free manner.  If the
      required segment is available (that node has been upgraded) then
      the related back-up path is installed in FIB, else the pre-
      existing situation (no backup) continues.  Hence, as the SRv6
      deployment progresses, the coverage incrementally increases.
      Eventually, when the core network is SRv6 capable, the TI-LFA
      coverage is complete.




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   The service chaining use-case can be realized with SRv6 capability
   deployed at few strategic nodes.

      The service-chaining deployment is again incremental and does not
      require any pre-deployment of SRv6 in the network.  When an NFV
      app A1 needs to be enabled for inclusion in an SRv6 service chain,
      all what is required is to install that app in a container or VM
      on an SRv6-capable server (Linux 4.10 or FD.io 17.04 release).
      The app can either be SR-aware or not, leveraging the proxy
      functions described in this document.

      By leveraging the various END functions it can also be used to
      support any current PNF/VNF implementations and their forwarding
      methods (e.g.  Layer 2).

      The ability to leverage SR TE policies and BSIDs also permits
      building scalable, hierarchical service-chains.

10.2.  Integration

   The SRv6 network programming concept allows integrating all the
   application and service requirements: multi-domain underlay SLA
   optimization with scale, overlay VPN/Tenant, sub-50msec automated
   FRR, security and service chaining.

10.3.  Security

   The combination of well-known techniques (SEC1, SEC2, SEC4) and
   carefully chosen architectural rules (SEC3) ensure a secure
   deployment of SRv6 inside a multi-domain network managed by a single
   organization.

   Inter-domain security will be described in a companion document.

11.  IANA Considerations

   This document has no actions for IANA.

12.  Work in progress

   We are working on a extension of this document to provide Yang
   modelling for all the functionality described in this document.

13.  Acknowledgements

   TBD.





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14.  Contributors

   Stefano Previdi, Dave Barach, Mark Townsley, Peter Psenak, Paul
   Wells, Robert Hanzl, Dan Ye, Patrice Brissette, Gaurav Dawra, Faisal
   Iqbal, Zafar Ali, Jaganbabu Rajamanickam, David Toscano, Asif Islam,
   Jianda Liu, Yunpeng Zhang, Jiaoming Li, Narendra A.K, Mike Mc Gourty,
   Bhupendra Yadav, Sherif Toulan, Satish Damodaran, John Bettink,
   Kishore Nandyala Veera Venk, Jisu Bhattacharya and Saleem Hafeez
   substantially contributed to the content of this document.

15.  References

15.1.  Normative References

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

15.2.  Informative References

   [I-D.bashandy-isis-srv6-extensions]
              Ginsberg, L., Bashandy, A., Filsfils, C., and B. Decraene,
              "IS-IS Extensions to Support Routing over IPv6 Dataplane",
              draft-bashandy-isis-srv6-extensions-01 (work in progress),
              September 2017.

   [I-D.clad-spring-segment-routing-service-chaining]
              Clad, F., Filsfils, C., Camarillo, P.,
              daniel.bernier@bell.ca, d., Decraene, B., Peirens, B.,
              Yadlapalli, C., Xu, X., Salsano, S., Abdelsalam, A., and
              G. Dawra, "Segment Routing for Service Chaining", draft-
              clad-spring-segment-routing-service-chaining-00 (work in
              progress), October 2017.

   [I-D.dawra-idr-srv6-vpn]
              Dawra, G., Filsfils, C., Dukes, D., Brissette, P.,
              Camarillo, P., Leddy, J., daniel.voyer@bell.ca, d.,
              daniel.bernier@bell.ca, d., Steinberg, D., Raszuk, R.,
              Decraene, B., and S. Matsushima, "BGP Signaling of IPv6-
              Segment-Routing-based VPN Networks", draft-dawra-idr-
              srv6-vpn-02 (work in progress), October 2017.









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   [I-D.filsfils-spring-segment-routing-policy]
              Filsfils, C., Sivabalan, S., Raza, K., Liste, J., Clad,
              F., Lin, S., bogdanov@google.com, b., Horneffer, M.,
              Steinberg, D., Decraene, B., and S. Litkowski, "Segment
              Routing Policy for Traffic Engineering", draft-filsfils-
              spring-segment-routing-policy-01 (work in progress), July
              2017.

   [I-D.ietf-6man-segment-routing-header]
              Previdi, S., Filsfils, C., Raza, K., Leddy, J., Field, B.,
              daniel.voyer@bell.ca, d., daniel.bernier@bell.ca, d.,
              Matsushima, S., Leung, I., Linkova, J., Aries, E., Kosugi,
              T., Vyncke, E., Lebrun, D., Steinberg, D., and R. Raszuk,
              "IPv6 Segment Routing Header (SRH)", draft-ietf-6man-
              segment-routing-header-07 (work in progress), July 2017.

   [I-D.ietf-idr-bgp-ls-segment-routing-ext]
              Previdi, S., Psenak, P., Filsfils, C., Gredler, H., and M.
              Chen, "BGP Link-State extensions for Segment Routing",
              draft-ietf-idr-bgp-ls-segment-routing-ext-03 (work in
              progress), July 2017.

   [I-D.ietf-idr-te-lsp-distribution]
              Previdi, S., Dong, J., Chen, M., Gredler, H., and j.
              jefftant@gmail.com, "Distribution of Traffic Engineering
              (TE) Policies and State using BGP-LS", draft-ietf-idr-te-
              lsp-distribution-07 (work in progress), July 2017.

   [I-D.ietf-isis-l2bundles]
              Ginsberg, L., Bashandy, A., Filsfils, C., Nanduri, M., and
              E. Aries, "Advertising L2 Bundle Member Link Attributes in
              IS-IS", draft-ietf-isis-l2bundles-07 (work in progress),
              May 2017.

   [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-12 (work in progress), June 2017.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <https://www.rfc-editor.org/info/rfc2460>.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
              December 1998, <https://www.rfc-editor.org/info/rfc2473>.





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   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437,
              DOI 10.17487/RFC6437, November 2011,
              <https://www.rfc-editor.org/info/rfc6437>.

Appendix A.  Additional Contributors

   Patrice Brissete
   Cisco Systems, Inc.
   Canada

   Email: pbrisset@cisco.com

Authors' Addresses

   Clarence Filsfils
   Cisco Systems, Inc.
   Belgium

   Email: cf@cisco.com


   John Leddy
   Comcast
   United States of America

   Email: john_leddy@cable.comcast.com


   Daniel Voyer
   Bell Canada
   Canada

   Email: daniel.voyer@bell.ca


   Daniel Bernier
   Bell Canada
   Canada

   Email: daniel.bernier@bell.ca






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   Dirk Steinberg
   Steinberg Consulting
   Germany

   Email: dws@dirksteinberg.de


   Robert Raszuk
   Bloomberg LP
   United States of America

   Email: robert@raszuk.net


   Satoru Matsushima
   SoftBank
   1-9-1,Higashi-Shimbashi,Minato-Ku
   Tokyo  105-7322
   Japan

   Email: satoru.matsushima@g.softbank.co.jp


   David Lebrun
   Universite catholique de Louvain
   Belgium

   Email: david.lebrun@uclouvain.be


   Bruno Decraene
   Orange
   France

   Email: bruno.decraene@orange.com


   Bart Peirens
   Proximus
   Belgium

   Email: bart.peirens@proximus.com









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   Stefano Salsano
   Universita di Roma "Tor Vergata"
   Italy

   Email: stefano.salsano@uniroma2.it


   Gaurav Naik
   Drexel University
   United States of America

   Email: gn@drexel.edu


   Hani Elmalky
   Ericsson
   United States of America

   Email: hani.elmalky@gmail.com


   Prem Jonnalagadda
   Barefoot Networks
   United States of America

   Email: prem@barefootnetworks.com


   Milad Sharif
   Barefoot Networks
   United States of America

   Email: msharif@barefootnetworks.com


   Arthi Ayyangar
   Arista
   United States of America

   Email: arthi@arista.com


   Satish Mynam
   Dell Force10 Networks
   United States of America

   Email: satish_mynam@dell.com




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   Wim Henderickx
   Nokia
   Belgium

   Email: wim.henderickx@nokia.com


   Ahmed Bashandy
   Cisco Systems, Inc.
   United States of America

   Email: bashandy@cisco.com


   Kamran Raza
   Cisco Systems, Inc.
   Canada

   Email: skraza@cisco.com


   Darren Dukes
   Cisco Systems, Inc.
   Canada

   Email: ddukes@cisco.com


   Francois Clad
   Cisco Systems, Inc.
   France

   Email: fclad@cisco.com


   Pablo Camarillo Garvia (editor)
   Cisco Systems, Inc.
   Spain

   Email: pcamaril@cisco.com











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