



Internet Engineering Task Force (IETF)                  C. Filsfils, Ed.
Request for Comments: 8986                             P. Camarillo, Ed.
Category: Standards Track                            Cisco Systems, Inc.
ISSN: 2070-1721                                                 J. Leddy
                                                     Akamai Technologies
                                                                D. Voyer
                                                             Bell Canada
                                                           S. Matsushima
                                                                SoftBank
                                                                   Z. Li
                                                     Huawei Technologies
                                                           February 2021


          Segment Routing over IPv6 (SRv6) Network Programming

Abstract

   The Segment Routing over IPv6 (SRv6) Network Programming framework
   enables a network operator or an application to specify a packet
   processing program by encoding a sequence of instructions in the IPv6
   packet header.

   Each instruction is implemented on one or several nodes in the
   network and identified by an SRv6 Segment Identifier in the packet.

   This document defines the SRv6 Network Programming concept and
   specifies the base set of SRv6 behaviors that enables the creation of
   interoperable overlays with underlay optimization.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8986.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction
   2.  Terminology
     2.1.  Requirements Language
   3.  SRv6 SID
     3.1.  SID Format
     3.2.  SID Allocation within an SR Domain
     3.3.  SID Reachability
   4.  SR Endpoint Behaviors
     4.1.  End: Endpoint
       4.1.1.  Upper-Layer Header
     4.2.  End.X: L3 Cross-Connect
     4.3.  End.T: Specific IPv6 Table Lookup
     4.4.  End.DX6: Decapsulation and IPv6 Cross-Connect
     4.5.  End.DX4: Decapsulation and IPv4 Cross-Connect
     4.6.  End.DT6: Decapsulation and Specific IPv6 Table Lookup
     4.7.  End.DT4: Decapsulation and Specific IPv4 Table Lookup
     4.8.  End.DT46: Decapsulation and Specific IP Table Lookup
     4.9.  End.DX2: Decapsulation and L2 Cross-Connect
     4.10. End.DX2V: Decapsulation and VLAN L2 Table Lookup
     4.11. End.DT2U: Decapsulation and Unicast MAC L2 Table Lookup
     4.12. End.DT2M: Decapsulation and L2 Table Flooding
     4.13. End.B6.Encaps: Endpoint Bound to an SRv6 Policy with
            Encapsulation
     4.14. End.B6.Encaps.Red: End.B6.Encaps with Reduced SRH
     4.15. End.BM: Endpoint Bound to an SR-MPLS Policy
     4.16. Flavors
       4.16.1.  PSP: Penultimate Segment Pop of the SRH
       4.16.2.  USP: Ultimate Segment Pop of the SRH
       4.16.3.  USD: Ultimate Segment Decapsulation
   5.  SR Policy Headend Behaviors
     5.1.  H.Encaps: SR Headend with Encapsulation in an SR Policy
     5.2.  H.Encaps.Red: H.Encaps with Reduced Encapsulation
     5.3.  H.Encaps.L2: H.Encaps Applied to Received L2 Frames
     5.4.  H.Encaps.L2.Red: H.Encaps.Red Applied to Received L2 Frames
   6.  Counters
   7.  Flow-Based Hash Computation
   8.  Control Plane
     8.1.  IGP
     8.2.  BGP-LS
     8.3.  BGP IP/VPN/EVPN
     8.4.  Summary
   9.  Security Considerations
   10. IANA Considerations
     10.1.  Ethernet Next Header Type
     10.2.  SRv6 Endpoint Behaviors Registry
       10.2.1.  Registration Procedures
       10.2.2.  Initial Registrations
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   Segment Routing [RFC8402] leverages the source routing paradigm.  An
   ingress node steers a packet through an 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 behavior.  Network Programming combines Segment Routing
   functions, both simple and complex, to achieve a networking objective
   that goes beyond mere packet routing.

   This document defines the SRv6 Network Programming concept and
   specifies the main Segment Routing behaviors to enable the creation
   of interoperable overlays with underlay optimization.

   [SRV6-NET-PGM-ILLUST] illustrates the concepts defined in this
   document.

   Familiarity with the Segment Routing Header [RFC8754] is expected.

2.  Terminology

   The following terms used within this document are defined in
   [RFC8402]: Segment Routing (SR), SR Domain, Segment ID (SID), SRv6,
   SRv6 SID, SR Policy, Prefix-SID, and Adj-SID.

   The following terms used within this document are defined in
   [RFC8754]: Segment Routing Header (SRH), SR source node, transit
   node, SR Segment Endpoint Node, Reduced SRH, Segments Left, and Last
   Entry.

   The following terms are used in this document as defined below:

   FIB:  Forwarding Information Base.  A FIB lookup is a lookup in the
      forwarding table.

   SA:  Source Address

   DA:  Destination Address

   L3:  Layer 3

   L2:  Layer 2

   MAC:  Media Access Control

   EVPN:  Ethernet VPN

   ESI:  Ethernet Segment Identifier

   Per-CE VPN label:  A single label for each attachment circuit that is
      shared by all routes with the same "outgoing attachment circuit"
      (Section 4.3.2 of [RFC4364])

   Per-VRF VPN label:  A single label for the entire VPN Routing and
      Forwarding (VRF) table that is shared by all routes from that VRF
      (Section 4.3.2 of [RFC4364])

   SL:  The Segments Left field of the SRH

   SRv6 SID function:  The function part of the SID is an opaque
      identification of a local behavior bound to the SID.  It is
      formally defined in Section 3.1 of this document.

   SRv6 Endpoint behavior:  A packet processing behavior executed at an
      SRv6 Segment Endpoint Node.  Section 4 of this document defines
      SRv6 Endpoint behaviors related to traffic-engineering and overlay
      use cases.  Other behaviors (e.g., service programming) are
      outside the scope of this document.

   An SR Policy is resolved to a SID list.  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:

   *  Source Address (SA), Destination Address (DA), and next header
      (SRH).

   *  SRH with SID list <S1, S2, S3> with Segments Left = SL.

      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 to traverse.  (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 packet behavior, the (S3, S2, S1; SL)
      notation is more convenient.

   *  The payload of the packet is omitted.

2.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  SRv6 SID

   [RFC8402] defines an SRv6 Segment Identifier as an IPv6 address
   explicitly associated with the segment.

   When an SRv6 SID is in the Destination Address field of an IPv6
   header of a packet, it is routed through transit nodes in an IPv6
   network as an IPv6 address.

   Its processing is defined in Section 4.3 of [RFC8754] and reproduced
   here as a reminder:

   |  Without constraining the details of an implementation, the SR
   |  segment endpoint node creates Forwarding Information Base (FIB)
   |  entries for its local SIDs.
   |  
   |  When an SRv6-capable node receives an IPv6 packet, it performs a
   |  longest-prefix-match lookup on the packet's destination address.
   |  This lookup can return any of the following:
   |  *  A FIB entry that represents a locally instantiated SRv6 SID
   |  
   |  *  A FIB entry that represents a local interface, not locally
   |     instantiated as an SRv6 SID
   |  
   |  *  A FIB entry that represents a nonlocal route
   |  
   |  *  No Match

   Section 4 of this document defines a new set of SRv6 SID behaviors in
   addition to that defined in Section 4.3.1 of [RFC8754].

3.1.  SID Format

   This document defines an SRv6 SID as consisting of LOC:FUNCT:ARG,
   where a locator (LOC) is encoded in the L most significant bits of
   the SID, followed by F bits of function (FUNCT) and A bits of
   arguments (ARG).  L, the locator length, is flexible, and an operator
   is free to use the locator length of their choice.  F and A may be
   any value as long as L+F+A <= 128.  When L+F+A is less than 128, then
   the remaining bits of the SID MUST be zero.

   A locator may be represented as B:N where B is the SRv6 SID block
   (IPv6 prefix allocated for SRv6 SIDs by the operator) and N is the
   identifier of the parent node instantiating the SID.

   When the LOC part of the SRv6 SIDs is routable, it leads to the node,
   which instantiates the SID.

   The FUNCT is an opaque identification of a local behavior bound to
   the SID.

   The term "function" refers to the bit string in the SRv6 SID.  The
   term "behavior" identifies the behavior bound to the SID.  Some
   behaviors are defined in Section 4 of this document.

   An SRv6 Endpoint behavior may require additional information for its
   processing (e.g., related to the flow or service).  This information
   may be encoded in the ARG bits of the SID.

   In such a case, the semantics and format of the ARG bits are defined
   as part of the SRv6 Endpoint behavior specification.

   The ARG value of a routed SID SHOULD remain constant among packets in
   a given flow.  Varying ARG values among packets in a flow may result
   in different ECMP hashing and cause reordering.

3.2.  SID Allocation within an SR Domain

   Locators are assigned consistent with IPv6 infrastructure allocation.
   For example, a network operator may:

   *  Assign block B::/48 to the SR domain

   *  Assign a unique B:N::/64 block to each SRv6-enabled node in the
      domain

   As an example, one mobile service provider has commercially deployed
   SRv6 across more than 1000 commercial routers and 1800 whitebox
   routers.  All these devices are enabled for SRv6 and advertise SRv6
   SIDs.  The provider historically deployed IPv6 and assigned
   infrastructure addresses from the Unique Local Address (ULA) space
   [RFC4193].  They specifically allocated three /48 prefixes (Country
   X, Country Y, Country Z) to support their SRv6 infrastructure.  From
   those /48 prefixes, each router was assigned a /64 prefix from which
   all SIDs of that router are allocated.

   In another example, a large mobile and fixed-line service provider
   has commercially deployed SRv6 in their country-wide network.  This
   provider is assigned a /20 prefix by a Regional Internet Registry
   (RIR).  They sub-allocated a few /48 prefixes to their infrastructure
   to deploy SRv6.  Each router is assigned a /64 prefix from which all
   SIDs of that router are allocated.

   IPv6 address consumption in both these examples is minimal,
   representing less than one billionth and one millionth of the
   available address space, respectively.

   A service provider receiving the current minimum allocation of a /32
   prefix from an RIR may assign a /48 prefix to their infrastructure
   deploying SRv6 and subsequently allocate /64 prefixes for SIDs at
   each SRv6 node.  The /48 assignment is one sixty-five thousandth
   (1/2^16) of the usable IPv6 address space available for assignment by
   the provider.

   When an operator instantiates a SID at a node, they specify a SID
   value B:N:FUNCT and the behavior bound to the SID using one of the
   SRv6 Endpoint Behavior codepoints of the registry defined in this
   document (see Table 6).

   The node advertises the SID, B:N:FUNCT, in the control plane (see
   Section 8) together with the SRv6 Endpoint Behavior codepoint
   identifying the behavior of the SID.

   An SR source node cannot infer the behavior by examination of the
   FUNCT value of a SID.

   Therefore, the SRv6 Endpoint Behavior codepoint is advertised along
   with the SID in the control plane.

   An SR source node uses the SRv6 Endpoint Behavior codepoint to map
   the received SID (B:N:FUNCT) to a behavior.

   An SR source node selects a desired behavior at an advertising node
   by selecting the SID (B:N:FUNCT) advertised with the desired
   behavior.

   As an example:

   *  A network operator may assign an SRv6 SID block 2001:db8:bbbb::/48
      from their in-house operation block for their SRv6 infrastructure.

   *  A network operator may assign an SRv6 Locator 2001:db8:bbbb:3::/64
      to one particular router, for example Router 3, in their SR
      Domain.

   *  At Router 3, within the locator 2001:db8:bbbb:3::/64, the network
      operator or the router performs dynamic assignment for:

      -  Function 0x0100 associated with the behavior End.X (Endpoint
         with L3 cross-connect) between router 3 and its connected
         neighbor router (e.g., Router 4).  This function is encoded as
         a 16-bit value and has no arguments (F=16, A=0).

         This SID is advertised in the control plane as
         2001:db8:bbbb:3:100:: with an SRv6 Endpoint Behavior codepoint
         value of 5.

      -  Function 0x0101 associated with the behavior End.X (Endpoint
         with L3 cross-connect) between router 3 and its connected
         neighbor router (e.g., Router 2).  This function is encoded as
         a 16-bit value and has no arguments (F=16, A=0).

         This SID is advertised in the control plane as
         2001:db8:bbbb:3:101:: with an SRv6 Endpoint Behavior codepoint
         value of 5.

   These examples do not preclude any other IPv6 addressing allocation
   scheme.

3.3.  SID Reachability

   Most often, the node N would advertise IPv6 prefix(es) matching the
   LOC parts covering its SIDs or shorter-mask prefix.  The distribution
   of these advertisements and calculation of their reachability are
   specific to the routing protocol and are outside of the scope of this
   document.

   An SRv6 SID is said to be routed if its SID belongs to an IPv6 prefix
   advertised via a routing protocol.  An SRv6 SID that does not fulfill
   this condition is non-routed.

   Let's provide a classic illustration:

   Node N is configured explicitly with two SIDs: 2001:db8:b:1:100:: and
   2001:db8:b:2:101::.

   The network learns about a path to 2001:db8:b:1::/64 via the IGP;
   hence, a packet destined to 2001:db8:b:1:100:: would be routed up to
   N.  The network does not learn about a path to 2001:db8:b:2::/64 via
   the IGP; hence, a packet destined to 2001:db8:b:2:101:: would not be
   routed up to N.

   A packet could be steered through a non-routed SID 2001:db8:b:2:101::
   by using a SID list <...,2001:db8:b:1:100::,2001:db8:b:2:101::,...>
   where the non-routed SID is preceded by a routed SID to the same
   node.  A packet could also be steered to a node instantiating a non-
   routed SID by preceding it in the SID list with an Adj-SID to that
   node.  Routed and non-routed SRv6 SIDs are the SRv6 instantiation of
   global and local segments, respectively [RFC8402].

4.  SR Endpoint Behaviors

   The following is a set of well-known behaviors that can be associated
   with a SID.

    +-------------------+---------------------------------------------+
    | End               | Endpoint                                    |
    |                   |                                             |
    |                   | The SRv6 instantiation of a Prefix-SID      |
    |                   | [RFC8402]                                   |
    +-------------------+---------------------------------------------+
    | End.X             | Endpoint with L3 cross-connect              |
    |                   |                                             |
    |                   | The SRv6 instantiation of an Adj-SID        |
    |                   | [RFC8402]                                   |
    +-------------------+---------------------------------------------+
    | End.T             | Endpoint with specific IPv6 table lookup    |
    +-------------------+---------------------------------------------+
    | End.DX6           | Endpoint with decapsulation and IPv6 cross- |
    |                   | connect                                     |
    |                   |                                             |
    |                   | e.g., IPv6-L3VPN (equivalent to per-CE VPN  |
    |                   | label)                                      |
    +-------------------+---------------------------------------------+
    | End.DX4           | Endpoint with decapsulation and IPv4 cross- |
    |                   | connect                                     |
    |                   |                                             |
    |                   | e.g., IPv4-L3VPN (equivalent to per-CE VPN  |
    |                   | label)                                      |
    +-------------------+---------------------------------------------+
    | End.DT6           | Endpoint with decapsulation and specific    |
    |                   | IPv6 table lookup                           |
    |                   |                                             |
    |                   | e.g., IPv6-L3VPN (equivalent to per-VRF VPN |
    |                   | label)                                      |
    +-------------------+---------------------------------------------+
    | End.DT4           | Endpoint with decapsulation and specific    |
    |                   | IPv4 table lookup                           |
    |                   |                                             |
    |                   | e.g., IPv4-L3VPN (equivalent to per-VRF VPN |
    |                   | label)                                      |
    +-------------------+---------------------------------------------+
    | End.DT46          | Endpoint with decapsulation and specific IP |
    |                   | table lookup                                |
    |                   |                                             |
    |                   | e.g., IP-L3VPN (equivalent to per-VRF VPN   |
    |                   | label)                                      |
    +-------------------+---------------------------------------------+
    | End.DX2           | Endpoint with decapsulation and L2 cross-   |
    |                   | connect                                     |
    |                   |                                             |
    |                   | e.g., L2VPN use case                        |
    +-------------------+---------------------------------------------+
    | End.DX2V          | Endpoint with decapsulation and VLAN L2     |
    |                   | table lookup                                |
    |                   |                                             |
    |                   | e.g., EVPN Flexible Cross-connect use case  |
    +-------------------+---------------------------------------------+
    | End.DT2U          | Endpoint with decapsulation and unicast MAC |
    |                   | L2 table lookup                             |
    |                   |                                             |
    |                   | e.g., EVPN Bridging Unicast use case        |
    +-------------------+---------------------------------------------+
    | End.DT2M          | Endpoint with decapsulation and L2 table    |
    |                   | flooding                                    |
    |                   |                                             |
    |                   | e.g., EVPN Bridging Broadcast, Unknown      |
    |                   | Unicast, and Multicast (BUM) use case with  |
    |                   | Ethernet Segment Identifier (ESI) filtering |
    +-------------------+---------------------------------------------+
    | End.B6.Encaps     | Endpoint bound to an SRv6 Policy with       |
    |                   | encapsulation                               |
    |                   |                                             |
    |                   | SRv6 instantiation of a Binding SID         |
    +-------------------+---------------------------------------------+
    | End.B6.Encaps.Red | End.B6.Encaps with reduced SRH              |
    |                   |                                             |
    |                   | SRv6 instantiation of a Binding SID         |
    +-------------------+---------------------------------------------+
    | End.BM            | Endpoint bound to an SR-MPLS Policy         |
    |                   |                                             |
    |                   | SRv6 instantiation of an SR-MPLS Binding    |
    |                   | SID                                         |
    +-------------------+---------------------------------------------+

                        Table 1: Endpoint Behaviors

   The list is not exhaustive.  In practice, any behavior can be
   attached to a local SID; for example, a node N can bind a SID to a
   local Virtual Machine (VM) or container that can apply any complex
   processing on the packet, provided there is an SRv6 Endpoint Behavior
   codepoint allocated for the processing.

   When an SRv6-capable node (N) receives an IPv6 packet whose
   destination address matches a FIB entry that represents a locally
   instantiated SRv6 SID (S), the IPv6 header chain is processed as
   defined in Section 4 of [RFC8200].  For SRv6 SIDs associated with an
   Endpoint behavior defined in this document, the SRH and Upper-Layer
   header are processed as defined in the following subsections.

   The pseudocode describing these behaviors details local processing at
   a node.  An implementation of the pseudocode is compliant as long as
   the externally observable wire protocol is as described by the
   pseudocode.

   Section 4.16 defines flavors of some of these behaviors.

   Section 10.2 of this document defines the IANA registry used to
   maintain all these behaviors as well as future ones defined in other
   documents.

4.1.  End: Endpoint

   The Endpoint behavior ("End" for short) is the most basic behavior.
   It is the instantiation of a Prefix-SID [RFC8402].

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

   S01. When an SRH is processed {
   S02.   If (Segments Left == 0) {
   S03.      Stop processing the SRH, and proceed to process the next
                header in the packet, whose type is identified by
                the Next Header field in the routing header.
   S04.   }
   S05.   If (IPv6 Hop Limit <= 1) {
   S06.      Send an ICMP Time Exceeded message to the Source Address
                with Code 0 (Hop limit exceeded in transit),
                interrupt packet processing, and discard the packet.
   S07.   }
   S08.   max_LE = (Hdr Ext Len / 2) - 1
   S09.   If ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) {
   S10.      Send an ICMP Parameter Problem to the Source Address
                with Code 0 (Erroneous header field encountered)
                and Pointer set to the Segments Left field,
                interrupt packet processing, and discard the packet.

   S11.   }
   S12.   Decrement IPv6 Hop Limit by 1
   S13.   Decrement Segments Left by 1
   S14.   Update IPv6 DA with Segment List[Segments Left]
   S15.   Submit the packet to the egress IPv6 FIB lookup for
             transmission to the new destination
   S16. }

      |  Note:
      |  
      |  The End behavior operates on the same FIB table (i.e.,
      |  identified by VRF or L3 relay ID) associated to the packet.
      |  Hence, the FIB lookup on line S15 is done in the same FIB table
      |  as the ingress interface.

4.1.1.  Upper-Layer Header

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End SID, N does the following:

   S01. If (Upper-Layer header type is allowed by local configuration) {
   S02.   Proceed to process the Upper-Layer header
   S03. } Else {
   S04.   Send an ICMP Parameter Problem to the Source Address
             with Code 4 (SR Upper-layer Header Error)
             and Pointer set to the offset of the Upper-Layer header,
             interrupt packet processing, and discard the packet.
   S05  }

   Allowing the processing of specific Upper-Layer header types is
   useful for Operations, Administration, and Maintenance (OAM).  As an
   example, an operator might permit pinging of SIDs.  To do this, they
   may enable local configuration to allow Upper-Layer header type 58
   (ICMPv6).

   It is RECOMMENDED that an implementation of local configuration only
   allows Upper-Layer header processing of types that do not result in
   the packet being forwarded (e.g., ICMPv6).

4.2.  End.X: L3 Cross-Connect

   The "Endpoint with L3 cross-connect" behavior ("End.X" for short) is
   a variant of the End behavior.

   It is the SRv6 instantiation of an Adj-SID [RFC8402], and its main
   use is for traffic-engineering policies.

   Any SID instance of this behavior is associated with a set, J, of one
   or more L3 adjacencies.

   When N receives a packet destined to S and S is a local End.X SID,
   the line S15 from the End processing is replaced by the following:

   S15.   Submit the packet to the IPv6 module for transmission
             to the new destination via a member of J

      |  Note:
      |  
      |  S15.  If the set J contains several L3 adjacencies, then one
      |  element of the set is selected based on a hash of the packet's
      |  header (see Section 7).

   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 L3
   adjacency to a neighbor.  Potentially, more End.X could be explicitly
   defined (groups of L3 adjacencies to the same neighbor or to
   different neighbors).

   Note that if N has an outgoing interface bundle I to a neighbor Q
   made of 10 member links, N might allocate up to 11 End.X local SIDs:
   one for the bundle itself and then up to one for each L2 member link.
   The flows steered using the End.X SID corresponding to the bundle
   itself get load-balanced across the member links via hashing while
   the flows steered using the End.X SID corresponding to a member link
   get steered over that specific member link alone.

   When the End.X behavior is associated with a BGP Next-Hop, it is the
   SRv6 instantiation of the BGP peering segments [RFC8402].

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.X SID, process the packet as per
   Section 4.1.1.

4.3.  End.T: Specific IPv6 Table Lookup

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

   The End.T behavior is used for multi-table operation in the core.
   For this reason, an instance of the End.T behavior is associated with
   an IPv6 FIB table T.

   When N receives a packet destined to S and S is a local End.T SID,
   the line S15 from the End processing is replaced by the following:

   S15.1.   Set the packet's associated FIB table to T
   S15.2.   Submit the packet to the egress IPv6 FIB lookup for
              transmission to the new destination

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.T SID, process the packet as per
   Section 4.1.1.

4.4.  End.DX6: Decapsulation and IPv6 Cross-Connect

   The "Endpoint with decapsulation and IPv6 cross-connect" behavior
   ("End.DX6" for short) is a variant of the End.X behavior.

   One of the applications of the End.DX6 behavior is the L3VPNv6 use
   case where a FIB lookup in a specific tenant table at the egress
   Provider Edge (PE) is not required.  This is equivalent to the per-CE
   VPN label in MPLS [RFC4364].

   The End.DX6 SID MUST be the last segment in an SR Policy, and it is
   associated with one or more L3 IPv6 adjacencies J.

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

   S01. When an SRH is processed {
   S02.   If (Segments Left != 0) {
   S03.      Send an ICMP Parameter Problem to the Source Address
                with Code 0 (Erroneous header field encountered)
                and Pointer set to the Segments Left field,
                interrupt packet processing, and discard the packet.
   S04.   }
   S05.   Proceed to process the next header in the packet
   S06. }

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.DX6 SID, N does the following:

   S01. If (Upper-Layer header type == 41(IPv6) ) {
   S02.    Remove the outer IPv6 header with all its extension headers
   S03.    Forward the exposed IPv6 packet to the L3 adjacency J
   S04. } Else {
   S05.    Process as per Section 4.1.1
   S06. }

      |  Note:
      |  
      |  S01. "41" refers to "IPv6 encapsulation" as defined in the IANA
      |  "Assigned Internet Protocol Numbers" registry.
      |  
      |  S03.  If the End.DX6 SID is bound to an array of L3
      |  adjacencies, then one entry of the array is selected based on
      |  the hash of the packet's header (see Section 7).

4.5.  End.DX4: Decapsulation and IPv4 Cross-Connect

   The "Endpoint with decapsulation and IPv4 cross-connect" behavior
   ("End.DX4" for short) is a variant of the End.X behavior.

   One of the applications of the End.DX4 behavior is the L3VPNv4 use
   case where a FIB lookup in a specific tenant table at the egress PE
   is not required.  This is equivalent to the per-CE VPN label in MPLS
   [RFC4364].

   The End.DX4 SID MUST be the last segment in an SR Policy, and it is
   associated with one or more L3 IPv4 adjacencies J.

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

   S01. When an SRH is processed {
   S02.   If (Segments Left != 0) {
   S03.      Send an ICMP Parameter Problem to the Source Address
                with Code 0 (Erroneous header field encountered)
                and Pointer set to the Segments Left field,
                interrupt packet processing, and discard the packet.
   S04.   }
   S05.   Proceed to process the next header in the packet
   S06. }

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.DX4 SID, N does the following:

   S01. If (Upper-Layer header type == 4(IPv4) ) {
   S02.    Remove the outer IPv6 header with all its extension headers
   S03.    Forward the exposed IPv4 packet to the L3 adjacency J
   S04. } Else {
   S05.    Process as per Section 4.1.1
   S06. }

      |  Note:
      |  
      |  S01. "4" refers to "IPv4 encapsulation" as defined in the IANA
      |  "Assigned Internet Protocol Numbers" registry.
      |  
      |  S03.  If the End.DX4 SID is bound to an array of L3
      |  adjacencies, then one entry of the array is selected based on
      |  the hash of the packet's header (see Section 7).

4.6.  End.DT6: Decapsulation and Specific IPv6 Table Lookup

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

   One of the applications of the End.DT6 behavior is the L3VPNv6 use
   case where a FIB lookup in a specific tenant table at the egress PE
   is required.  This is 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 an End.DT6 supports the equivalent of an IPv6-in-IPv6
   decapsulation (without VPN/tenant implication).

   The End.DT6 SID MUST be the last segment in an SR Policy, and a SID
   instance is associated with an IPv6 FIB table T.

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

   S01. When an SRH is processed {
   S02.   If (Segments Left != 0) {
   S03.      Send an ICMP Parameter Problem to the Source Address
                with Code 0 (Erroneous header field encountered)
                and Pointer set to the Segments Left field,
                interrupt packet processing, and discard the packet.
   S04.   }
   S05.   Proceed to process the next header in the packet
   S06. }

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.DT6 SID, N does the following:

   S01. If (Upper-Layer header type == 41(IPv6) ) {
   S02.    Remove the outer IPv6 header with all its extension headers
   S03.    Set the packet's associated FIB table to T
   S04.    Submit the packet to the egress IPv6 FIB lookup for
              transmission to the new destination
   S05. } Else {
   S06.    Process as per Section 4.1.1
   S07. }

4.7.  End.DT4: Decapsulation and Specific IPv4 Table Lookup

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

   One of the applications of the End.DT4 behavior is the L3VPNv4 use
   case where a FIB lookup in a specific tenant table at the egress PE
   is required.  This is 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 an End.DT4 supports the equivalent of an IPv4-in-IPv6
   decapsulation (without VPN/tenant implication).

   The End.DT4 SID MUST be the last segment in an SR Policy, and a SID
   instance is associated with an IPv4 FIB table T.

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

   S01. When an SRH is processed {
   S02.   If (Segments Left != 0) {
   S03.      Send an ICMP Parameter Problem to the Source Address
                with Code 0 (Erroneous header field encountered)
                and Pointer set to the Segments Left field,
                interrupt packet processing, and discard the packet.
   S04.   }
   S05.   Proceed to process the next header in the packet
   S06. }

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.DT4 SID, N does the following:

   S01. If (Upper-Layer header type == 4(IPv4) ) {
   S02.    Remove the outer IPv6 header with all its extension headers
   S03.    Set the packet's associated FIB table to T
   S04.    Submit the packet to the egress IPv4 FIB lookup for
              transmission to the new destination
   S05. } Else {
   S06.    Process as per Section 4.1.1
   S07. }

4.8.  End.DT46: Decapsulation and Specific IP Table Lookup

   The "Endpoint with decapsulation and specific IP table lookup"
   behavior ("End.DT46" for short) is a variant of the End.DT4 and
   End.DT6 behavior.

   One of the applications of the End.DT46 behavior is the L3VPN use
   case where a FIB lookup in a specific IP tenant table at the egress
   PE is required.  This is equivalent to the single per-VRF VPN label
   (for IPv4 and IPv6) in MPLS [RFC4364].

   Note that an End.DT46 may be defined for the main IP table, in which
   case an End.DT46 supports the equivalent of an IP-in-IPv6
   decapsulation (without VPN/tenant implication).

   The End.DT46 SID MUST be the last segment in an SR Policy, and a SID
   instance is associated with an IPv4 FIB table T4 and an IPv6 FIB
   table T6.

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

   S01. When an SRH is processed {
   S02.   If (Segments Left != 0) {
   S03.      Send an ICMP Parameter Problem to the Source Address
                with Code 0 (Erroneous header field encountered)
                and Pointer set to the Segments Left field,
                interrupt packet processing, and discard the packet.
   S04.   }
   S05.   Proceed to process the next header in the packet
   S06. }

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.DT46 SID, N does the following:

   S01. If (Upper-Layer header type == 4(IPv4) ) {
   S02.    Remove the outer IPv6 header with all its extension headers
   S03.    Set the packet's associated FIB table to T4
   S04.    Submit the packet to the egress IPv4 FIB lookup for
              transmission to the new destination
   S05. } Else if (Upper-Layer header type == 41(IPv6) ) {
   S06.    Remove the outer IPv6 header with all its extension headers
   S07.    Set the packet's associated FIB table to T6
   S08.    Submit the packet to the egress IPv6 FIB lookup for
              transmission to the new destination
   S09. } Else {
   S10.    Process as per Section 4.1.1
   S11. }

4.9.  End.DX2: Decapsulation and L2 Cross-Connect

   The "Endpoint with decapsulation and L2 cross-connect" behavior
   ("End.DX2" for short) is a variant of the Endpoint behavior.

   One of the applications of the End.DX2 behavior is the L2VPN
   [RFC4664] / EVPN Virtual Private Wire Service (VPWS) [RFC7432]
   [RFC8214] use case.

   The End.DX2 SID MUST be the last segment in an SR Policy, and it is
   associated with one outgoing interface I.

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

   S01. When an SRH is processed {
   S02.   If (Segments Left != 0) {
   S03.      Send an ICMP Parameter Problem to the Source Address
                with Code 0 (Erroneous header field encountered)
                and Pointer set to the Segments Left field,
                interrupt packet processing, and discard the packet.
   S04.   }
   S05.   Proceed to process the next header in the packet
   S06. }

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.DX2 SID, N does the following:

   S01. If (Upper-Layer header type == 143(Ethernet) ) {
   S02.    Remove the outer IPv6 header with all its extension headers
   S03.    Forward the Ethernet frame to the OIF I
   S04. } Else {
   S05.    Process as per Section 4.1.1
   S06. }

      |  Note:
      |  
      |  S01.  IANA has allocated value "143" for "Ethernet"
      |  [IEEE.802.3_2018] in the "Assigned Internet Protocol Numbers"
      |  registry (see Section 10.1).
      |  
      |  S03.  An End.DX2 behavior could be customized to expect a
      |  specific IEEE header (e.g., VLAN tag) and rewrite the egress
      |  IEEE header before forwarding on the outgoing interface.

   Note that an End.DX2 SID may also be associated with a bundle of
   outgoing interfaces.

4.10.  End.DX2V: Decapsulation and VLAN L2 Table Lookup

   The "Endpoint with decapsulation and VLAN L2 table lookup" behavior
   ("End.DX2V" for short) is a variant of the End.DX2 behavior.

   One of the applications of the End.DX2V behavior is the EVPN Flexible
   Cross-connect use case.  The End.DX2V behavior is used to perform a
   lookup of the Ethernet frame VLANs in a particular L2 table.  Any SID
   instance of this behavior is associated with an L2 table T.

   When N receives a packet whose IPv6 DA is S and S is a local End.DX2
   SID, the processing is identical to the End.DX2 behavior except for
   the Upper-Layer header processing, which is modified as follows:

   S03. Look up the exposed VLANs in L2 table T, and forward
           via the matched table entry.

      |  Note:
      |  
      |  S03.  An End.DX2V behavior could be customized to expect a
      |  specific VLAN format and rewrite the egress VLAN header before
      |  forwarding on the outgoing interface.

4.11.  End.DT2U: Decapsulation and Unicast MAC L2 Table Lookup

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

   One of the applications of the End.DT2U behavior is the EVPN Bridging
   Unicast [RFC7432].  Any SID instance of the End.DT2U behavior is
   associated with an L2 table T.

   When N receives a packet whose IPv6 DA is S and S is a local End.DT2U
   SID, the processing is identical to the End.DX2 behavior except for
   the Upper-Layer header processing, which is as follows:

   S01. If (Upper-Layer header type == 143(Ethernet) ) {
   S02.    Remove the outer IPv6 header with all its extension headers
   S03.    Learn the exposed MAC Source Address in L2 table T
   S04.    Look up the exposed MAC Destination Address in L2 table T
   S05.    If (matched entry in T) {
   S06.       Forward via the matched table T entry
   S07.    } Else {
   S08.       Forward via all L2 OIFs in table T
   S09.    }
   S10. } Else {
   S11.    Process as per Section 4.1.1
   S12. }

      |  Note:
      |  
      |  S01.  IANA has allocated value "143" for "Ethernet" in the
      |  "Assigned Internet Protocol Numbers" registry (see
      |  Section 10.1).
      |  
      |  S03.  In EVPN [RFC7432], the learning of the exposed MAC Source
      |  Address is done via the control plane.  In L2VPN Virtual
      |  Private LAN Service (VPLS) [RFC4761] [RFC4762], reachability is
      |  obtained by standard learning bridge functions in the data
      |  plane.

4.12.  End.DT2M: Decapsulation and L2 Table Flooding

   The "Endpoint with decapsulation and L2 table flooding" behavior
   ("End.DT2M" for short) is a variant of the End.DT2U behavior.

   Two of the applications of the End.DT2M behavior are the EVPN
   Bridging of Broadcast, Unknown Unicast, and Multicast (BUM) traffic
   with Ethernet Segment Identifier (ESI) filtering [RFC7432] and the
   EVPN Ethernet-Tree (E-Tree) [RFC8317] use cases.

   Any SID instance of this behavior is associated with an L2 table T.
   The behavior also takes an argument: "Arg.FE2".  This argument
   provides a local mapping to ESI for split-horizon filtering of the
   received traffic to exclude a specific OIF (or set of OIFs) from L2
   table T flooding.  The allocation of the argument values is local to
   the SR Segment Endpoint Node instantiating this behavior, and the
   signaling of the argument to other nodes for the EVPN functionality
   occurs via the control plane.

   When N receives a packet whose IPv6 DA is S and S is a local End.DT2M
   SID, the processing is identical to the End.DX2 behavior except for
   the Upper-Layer header processing, which is as follows:

   S01. If (Upper-Layer header type == 143(Ethernet) ) {
   S02.    Remove the outer IPv6 header with all its extension headers
   S03.    Learn the exposed MAC Source Address in L2 table T
   S04.    Forward via all L2 OIFs excluding those associated with the
              identifier Arg.FE2
   S05. } Else {
   S06.    Process as per Section 4.1.1
   S07. }

      |  Note:
      |  
      |  S01.  IANA has allocated value "143" for "Ethernet" in the
      |  "Assigned Internet Protocol Numbers" registry (see
      |  Section 10.1).
      |  
      |  S03.  In EVPN [RFC7432], the learning of the exposed MAC Source
      |  Address is done via the control plane.  In L2VPN VPLS [RFC4761]
      |  [RFC4762], reachability is obtained by standard learning bridge
      |  functions in the data plane.

4.13.  End.B6.Encaps: Endpoint Bound to an SRv6 Policy with
       Encapsulation

   This is a variation of the End behavior.

   One of its applications is to express scalable traffic-engineering
   policies across multiple domains.  It is one of the SRv6
   instantiations of a Binding SID [RFC8402].

   Any SID instance of this behavior is associated with an SR Policy B
   and a source address A.

   When N receives a packet whose IPv6 DA is S and S is a local
   End.B6.Encaps SID, N does the following:

   S01. When an SRH is processed {
   S02.   If (Segments Left == 0) {
   S03.      Stop processing the SRH, and proceed to process the next
                header in the packet, whose type is identified by
                the Next Header field in the routing header.
   S04.   }
   S05.   If (IPv6 Hop Limit <= 1) {
   S06.      Send an ICMP Time Exceeded message to the Source Address
                with Code 0 (Hop limit exceeded in transit),
                interrupt packet processing, and discard the packet.
   S07.   }
   S08.   max_LE = (Hdr Ext Len / 2) - 1
   S09.   If ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) {
   S10.      Send an ICMP Parameter Problem to the Source Address
                with Code 0 (Erroneous header field encountered)
                and Pointer set to the Segments Left field,
                interrupt packet processing, and discard the packet.
   S11.   }
   S12.   Decrement IPv6 Hop Limit by 1
   S13.   Decrement Segments Left by 1
   S14.   Update IPv6 DA with Segment List[Segments Left]
   S15.   Push a new IPv6 header with its own SRH containing B
   S16.   Set the outer IPv6 SA to A
   S17.   Set the outer IPv6 DA to the first SID of B
   S18.   Set the outer Payload Length, Traffic Class, Flow Label,
             Hop Limit, and Next Header fields
   S19.   Submit the packet to the egress IPv6 FIB lookup for
             transmission to the new destination
   S20. }

      |  Note:
      |  
      |  S15.  The SRH MAY be omitted when the SRv6 Policy B only
      |  contains one SID and there is no need to use any flag, tag, or
      |  TLV.
      |  
      |  S18.  The Payload Length, Traffic Class, Hop Limit, and Next
      |  Header fields are set as per [RFC2473].  The Flow Label is
      |  computed as per [RFC6437].

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.B6.Encaps SID, process the
   packet as per Section 4.1.1.

4.14.  End.B6.Encaps.Red: End.B6.Encaps with Reduced SRH

   This is an optimization of the End.B6.Encaps behavior.

   End.B6.Encaps.Red reduces the size of the SRH by one SID by excluding
   the first SID in the SRH of the new IPv6 header.  Thus, the first
   segment is only placed in the IPv6 Destination Address of the new
   IPv6 header, and the packet is forwarded according to it.

   The SRH Last Entry field is set as defined in Section 4.1.1 of
   [RFC8754].

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

4.15.  End.BM: Endpoint Bound to an SR-MPLS Policy

   The "Endpoint bound to an SR-MPLS Policy" behavior ("End.BM" for
   short) is a variant of the End behavior.

   The End.BM behavior is required to express scalable traffic-
   engineering policies across multiple domains where some domains
   support the MPLS instantiation of Segment Routing.  This is an SRv6
   instantiation of an SR-MPLS Binding SID [RFC8402].

   Any SID instance of this behavior is associated with an SR-MPLS
   Policy B.

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

   S01. When an SRH is processed {
   S02.   If (Segments Left == 0) {
   S03.      Stop processing the SRH, and proceed to process the next
                header in the packet, whose type is identified by
                the Next Header field in the routing header.
   S04.   }
   S05.   If (IPv6 Hop Limit <= 1) {
   S06.      Send an ICMP Time Exceeded message to the Source Address
                with Code 0 (Hop limit exceeded in transit),
                interrupt packet processing, and discard the packet.
   S07.   }
   S08.   max_LE = (Hdr Ext Len / 2) - 1
   S09.   If ((Last Entry > max_LE) or (Segments Left > Last Entry+1)) {
   S10.      Send an ICMP Parameter Problem to the Source Address
                with Code 0 (Erroneous header field encountered)
                and Pointer set to the Segments Left field,
                interrupt packet processing, and discard the packet.

   S11.   }
   S12.   Decrement IPv6 Hop Limit by 1
   S13.   Decrement Segments Left by 1
   S14.   Update IPv6 DA with Segment List[Segments Left]
   S15.   Push the MPLS label stack for B
   S16.   Submit the packet to the MPLS engine for transmission
   S17. }

   When processing the Upper-Layer header of a packet matching a FIB
   entry locally instantiated as an End.BM SID, process the packet as
   per Section 4.1.1.

4.16.  Flavors

   The Penultimate Segment Pop (PSP) of the SRH, Ultimate Segment Pop
   (USP) of the SRH, and Ultimate Segment Decapsulation (USD) flavors
   are variants of the End, End.X, and End.T behaviors.  The End, End.X,
   and End.T behaviors can support these flavors either individually or
   in combinations.

4.16.1.  PSP: Penultimate Segment Pop of the SRH

4.16.1.1.  Guidelines

   SR Segment Endpoint Nodes advertise the SIDs instantiated on them via
   control-plane protocols as described in Section 8.  Different
   behavior IDs are allocated for flavored and unflavored SIDs (see
   Table 6).

   An SR Segment Endpoint Node that offers both PSP- and non-PSP-
   flavored behavior advertises them as two different SIDs.

   The SR Segment Endpoint Node only advertises the PSP flavor if the
   operator enables this capability at the node.

   The PSP operation is deterministically controlled by the SR source
   node.

   A PSP-flavored SID is used by the SR source node when it needs to
   instruct the penultimate SR Segment Endpoint Node listed in the SRH
   to remove the SRH from the IPv6 header.

4.16.1.2.  Definition

   SR Segment Endpoint Nodes receive the IPv6 packet with the
   Destination Address field of the IPv6 header equal to its SID
   address.

   A penultimate SR Segment Endpoint Node is one that, as part of the
   SID processing, copies the last SID from the SRH into the IPv6
   Destination Address and decrements the Segments Left value from one
   to zero.

   The PSP operation only takes place at a penultimate SR Segment
   Endpoint Node and does not happen at any transit node.  When a SID of
   PSP flavor is processed at a non-penultimate SR Segment Endpoint
   Node, the PSP behavior is not performed as described in the
   pseudocode below since Segments Left would not be zero.

   The SRH processing of the End, End.X, and End.T behaviors are
   modified: after the instruction "S14.  Update IPv6 DA with Segment
   List[Segments Left]" is executed, the following instructions must be
   executed as well:

   S14.1.   If (Segments Left == 0) {
   S14.2.      Update the Next Header field in the preceding header to
                  the Next Header value from the SRH
   S14.3.      Decrease the IPv6 header Payload Length by
                  8*(Hdr Ext Len+1)
   S14.4.      Remove the SRH from the IPv6 extension header chain
   S14.5.   }

   The usage of PSP does not increase the MTU of the IPv6 packet and
   hence does not have any impact on the Path MTU (PMTU) discovery
   mechanism.

   As a reminder, Section 5 of [RFC8754] defines the SR Deployment Model
   within the SR Domain [RFC8402].  Within this framework, the
   Authentication Header (AH) is not used to secure the SRH as described
   in Section 7.5 of [RFC8754].  Hence, the discussion of applicability
   of PSP along with AH usage is beyond the scope of this document.

   In the context of this specification, the End, End.X, and End.T
   behaviors with PSP do not contravene Section 4 of [RFC8200] because
   the destination address of the incoming packet is the address of the
   node executing the behavior.

4.16.1.3.  Use Case

   One use case for the PSP functionality is streamlining the operation
   of an egress border router.

     +----------------------------------------------------+
     |                                                    |
   +-+-+         +--+         +--+         +--+         +-+-+
   |iPE+-------->+R2+-------->+R3+-------->+R4+-------->+ePE|
   | R1|         +--+         +--+         +--+         |R5 |
   +-+-+ +-----+      +-----+      +-----+      +-----+ +-+-+
     |   |IPv6 |      |IPv6 |      |IPv6 |      |IPv6 |   |
     |   |DA=R3|      |DA=R3|      |DA=R5|      |DA=R5|   |
     |   +-----+      +-----+      +-----+      +-----+   |
     |   | SRH |      | SRH |      | IP  |      | IP  |   |
     |   |SL=1 |      |SL=1 |      +-----+      +-----+   |
     |   | R5  |      | R5  |                             |
     |   +-----+      +-----+                             |
     |   | IP  |      | IP  |                             |
     |   +-----+      +-----+                             |
     |                                                    |
     +----------------------------------------------------+

                      Figure 1: PSP Use Case Topology

   In the above illustration, for a packet sent from the ingress
   provider edge (iPE) to the egress provider edge (ePE), node R3 is an
   intermediate traffic-engineering waypoint and is the penultimate
   segment endpoint router; this node copies the last segment from the
   SRH into the IPv6 Destination Address and decrements Segments Left to
   0.  The Software-Defined Networking (SDN) controller knows that no
   other node after R3 needs to inspect the SRH, and it instructs R3 to
   remove the exhausted SRH from the packet by using a PSP-flavored SID.

   The benefits for the egress PE are straightforward:

   *  As part of the decapsulation process, the egress PE is required to
      parse and remove fewer bytes from the packet.

   *  If a lookup on an upper-layer IP header is required (e.g., per-VRF
      VPN), the header is more likely to be within the memory accessible
      to the lookup engine in the forwarding ASIC (Application-Specific
      Integrated Circuit).

4.16.2.  USP: Ultimate Segment Pop of the SRH

   The SRH processing of the End, End.X, and End.T behaviors are
   modified; the instructions S02-S04 are substituted by the following
   ones:

   S02.     If (Segments Left == 0) {
   S03.1.      Update the Next Header field in the preceding header to
                  the Next Header value of the SRH
   S03.2.      Decrease the IPv6 header Payload Length by
                  8*(Hdr Ext Len+1)
   S03.3.      Remove the SRH from the IPv6 extension header chain
   S03.4.      Proceed to process the next header in the packet
   S04.     }

   One of the applications of the USP flavor is when a packet with an
   SRH is destined to an application on hosts with smartNICs ("Smart
   Network Interface Cards") implementing SRv6.  The USP flavor is used
   to remove the consumed SRH from the extension header chain before
   sending the packet to the host.

4.16.3.  USD: Ultimate Segment Decapsulation

   The Upper-Layer header processing of the End, End.X, and End.T
   behaviors are modified as follows:

   End:

   S01. If (Upper-Layer header type == 41(IPv6) ) {
   S02.    Remove the outer IPv6 header with all its extension headers
   S03.    Submit the packet to the egress IPv6 FIB lookup for
              transmission to the new destination
   S04. } Else if (Upper-Layer header type == 4(IPv4) ) {
   S05.    Remove the outer IPv6 header with all its extension headers
   S06.    Submit the packet to the egress IPv4 FIB lookup for
              transmission to the new destination
   S07. Else {
   S08.    Process as per Section 4.1.1
   S09. }

   End.T:

   S01. If (Upper-Layer header type == 41(IPv6) ) {
   S02.    Remove the outer IPv6 header with all its extension headers
   S03.    Set the packet's associated FIB table to T
   S04.    Submit the packet to the egress IPv6 FIB lookup for
              transmission to the new destination
   S05. } Else if (Upper-Layer header type == 4(IPv4) ) {
   S06.    Remove the outer IPv6 header with all its extension headers
   S07.    Set the packet's associated FIB table to T
   S08.    Submit the packet to the egress IPv4 FIB lookup for
              transmission to the new destination
   S09. Else {
   S10.    Process as per Section 4.1.1
   S11. }

   End.X:

   S01. If (Upper-Layer header type == 41(IPv6) ||
             Upper-Layer header type == 4(IPv4) ) {
   S02.    Remove the outer IPv6 header with all its extension headers
   S03.    Forward the exposed IP packet to the L3 adjacency J
   S04. } Else {
   S05.    Process as per Section 4.1.1
   S06. }

   One of the applications of the USD flavor is the case of a Topology
   Independent Loop-Free Alternate (TI-LFA) in P routers with
   encapsulation.  The USD flavor allows the last SR Segment Endpoint
   Node in the repair path list to decapsulate the IPv6 header added at
   the TI-LFA Point of Local Repair and forward the inner packet.

5.  SR Policy Headend Behaviors

   This section describes a set of SRv6 Policy Headend [RFC8402]
   behaviors.

    +-----------------+-----------------------------------------------+
    | H.Encaps        | SR Headend with Encapsulation in an SR Policy |
    +-----------------+-----------------------------------------------+
    | H.Encaps.Red    | H.Encaps with Reduced Encapsulation           |
    +-----------------+-----------------------------------------------+
    | H.Encaps.L2     | H.Encaps Applied to Received L2 Frames        |
    +-----------------+-----------------------------------------------+
    | H.Encaps.L2.Red | H.Encaps.Red Applied to Received L2 Frames    |
    +-----------------+-----------------------------------------------+

                    Table 2: SR Policy Headend Behaviors

   This list is not exhaustive, and future documents may define
   additional behaviors.

5.1.  H.Encaps: SR Headend with Encapsulation in an SR 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.

   Node N is configured with an IPv6 address T (e.g., assigned to its
   loopback).

   N steers the transit packets P1 and P2 into an SRv6 Policy with a
   Source Address T and a segment list <S1, S2, S3>.

   The H.Encaps encapsulation behavior is defined as follows:

   S01.   Push an IPv6 header with its own SRH
   S02.   Set outer IPv6 SA = T and outer IPv6 DA to the first SID
             in the segment list
   S03.   Set outer Payload Length, Traffic Class, Hop Limit, and
             Flow Label fields
   S04.   Set the outer Next Header value
   S05.   Decrement inner IPv6 Hop Limit or IPv4 TTL
   S06.   Submit the packet to the IPv6 module for transmission to S1

      |  Note:
      |  
      |  S03: As described in [RFC2473] and [RFC6437].

   After the H.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 received packet is encapsulated unmodified (with the exception of
   the IPv4 TTL or IPv6 Hop Limit that is decremented as described in
   [RFC2473]).

   The H.Encaps behavior is valid for any kind of L3 traffic.  This
   behavior is commonly used for L3VPN with IPv4 and IPv6 deployments.
   It may be also used for TI-LFA [SR-TI-LFA] at the Point of Local
   Repair.

   The push of 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.

5.2.  H.Encaps.Red: H.Encaps with Reduced Encapsulation

   The H.Encaps.Red behavior is an optimization of the H.Encaps
   behavior.

   H.Encaps.Red reduces the length of the SRH by excluding the first SID
   in the SRH of the pushed IPv6 header.  The first SID is only placed
   in the Destination Address field of the pushed IPv6 header.

   After the H.Encaps.Red behavior, P1' and P2' respectively look like:

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

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

   The push of 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.

5.3.  H.Encaps.L2: H.Encaps Applied to Received L2 Frames

   The H.Encaps.L2 behavior encapsulates a received Ethernet
   [IEEE.802.3_2018] frame and its attached VLAN header, if present, in
   an IPv6 packet with an SRH.  The Ethernet frame becomes the payload
   of the new IPv6 packet.

   The Next Header field of the SRH MUST be set to 143.

   The push of 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.

   The encapsulating node MUST remove the preamble (if any) and frame
   check sequence (FCS) from the Ethernet frame upon encapsulation, and
   the decapsulating node MUST regenerate, as required, the preamble and
   FCS before forwarding the Ethernet frame.

5.4.  H.Encaps.L2.Red: H.Encaps.Red Applied to Received L2 Frames

   The H.Encaps.L2.Red behavior is an optimization of the H.Encaps.L2
   behavior.

   H.Encaps.L2.Red reduces the length of the SRH by excluding the first
   SID in the SRH of the pushed IPv6 header.  The first SID is only
   placed in the Destination Address field of the pushed IPv6 header.

   The push of 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.

6.  Counters

   A node supporting this document SHOULD implement a pair of traffic
   counters (one for packets and one for bytes) per local SID entry, for
   traffic that matched that SID and was processed successfully (i.e.,
   packets that generate ICMP Error Messages or are dropped are not
   counted).  The retrieval of these counters from MIB, NETCONF/YANG, or
   any other data structure is outside the scope of this document.

7.  Flow-Based Hash Computation

   When a flow-based selection within a set needs to be performed, the
   IPv6 Source Address, the IPv6 Destination Address, and the IPv6 Flow
   Label of the outer IPv6 header MUST be included in the flow-based
   hash.

   This may occur in any of the following scenarios:

   *  A FIB lookup is performed and multiple ECMP paths exist to the
      updated destination address.

   *  End.X, End.DX4, or End.DX6 is bound to an array of adjacencies.

   *  The packet is steered in an SR Policy whose selected path has
      multiple SID lists.

   Additionally, any transit router in an SRv6 domain includes the outer
   flow label in its ECMP flow-based hash [RFC6437].

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 of
   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, and others to complex
   applications residing in VMs and containers.

   While not necessary for an SDN control plane, the remainder of this
   section provides a high-level illustrative overview of how control-
   plane protocols may be involved with SRv6.  Their specification is
   outside the scope of this document.

8.1.  IGP

   The End, End.T, and End.X SIDs express topological behaviors and
   hence are expected to be signaled in the IGP together with the
   flavors PSP, USP, and USD.  The IGP should also advertise the Maximum
   SID Depth (MSD) capability of the node for each type of SRv6
   operation -- in particular, the SR source (e.g., H.Encaps),
   intermediate endpoint (e.g., End and End.X), and final endpoint
   (e.g., End.DX4 and End.DT6) behaviors.  These capabilities are
   factored in by an SR source node (or a controller) during the SR
   Policy computation.

   The presence of SIDs in the IGP does not imply any routing semantics
   to the addresses represented by these SIDs.  The routing reachability
   to an IPv6 address is solely governed by the non-SID-related IGP
   prefix reachability information that includes locators.  Routing is
   neither governed nor influenced in any way by a SID advertisement in
   the IGP.

   These SIDs provide important topological behaviors for the IGP to
   build Fast Reroute (FRR) solutions based on TI-LFA [SR-TI-LFA] and
   for TE processes relying on an IGP topology database to build SR
   Policies.

8.2.  BGP-LS

   BGP-LS provides the functionality for topology discovery that
   includes the SRv6 capabilities of the nodes, their locators, and
   locally instantiated SIDs.  This enables controllers or applications
   to build an inter-domain topology that can be used for computation of
   SR Policies using the SRv6 SIDs.

8.3.  BGP IP/VPN/EVPN

   The End.DX4, End.DX6, End.DT4, End.DT6, End.DT46, End.DX2, End.DX2V,
   End.DT2U, and End.DT2M SIDs can be signaled in BGP.

   In some scenarios, an egress PE advertising a VPN route might wish to
   abstract the specific behavior bound to the SID from the ingress PE
   and other routers in the network.  In such case, the SID may be
   advertised using the Opaque SRv6 Endpoint Behavior codepoint defined
   in Table 6.  The details of such control-plane signaling mechanisms
   are out of the scope of this document.

8.4.  Summary

   The following table summarizes which SID behaviors may be signaled in
   which control-plane protocol.

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

            Table 3: SRv6 Locally Instantiated SIDs Signaling

   The following table summarizes which SR Policy Headend capabilities
   may be signaled in which control-plane protocol.

           +=================+=====+========+=================+
           |                 | IGP | BGP-LS | BGP IP/VPN/EVPN |
           +=================+=====+========+=================+
           | H.Encaps        |  X  |   X    |                 |
           +-----------------+-----+--------+-----------------+
           | H.Encaps.Red    |  X  |   X    |                 |
           +-----------------+-----+--------+-----------------+
           | H.Encaps.L2     |     |   X    |                 |
           +-----------------+-----+--------+-----------------+
           | H.Encaps.L2.Red |     |   X    |                 |
           +-----------------+-----+--------+-----------------+

             Table 4: SRv6 Policy Headend Behaviors Signaling

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

   For example, a BGP-LS advertisement of H.Encaps behavior would
   describe the capability of node N to perform H.Encaps behavior.
   Specifically, it would describe how many SIDs could be pushed by N
   without significant performance degradation.


   As a reminder, an SR Policy is always assigned a Binding SID
   [RFC8402].  Binding SIDs are also advertised in BGP-LS as shown in
   Table 3.  Hence, Table 4 only focuses on the generic capabilities
   related to H.Encaps.

9.  Security Considerations

   The security considerations for Segment Routing are discussed in
   [RFC8402].  Section 5 of [RFC8754] describes the SR Deployment Model
   and the requirements for securing the SR Domain.  The security
   considerations of [RFC8754] also cover topics such as attack vectors
   and their mitigation mechanisms that also apply the behaviors
   introduced in this document.  Together, they describe the required
   security mechanisms that allow establishment of an SR domain of
   trust.  Having such a well-defined trust boundary is necessary in
   order to operate SRv6-based services for internal traffic while
   preventing any external traffic from accessing or exploiting the
   SRv6-based services.  Care and rigor in IPv6 address allocation for
   use for SRv6 SID allocations and network infrastructure addresses, as
   distinct from IPv6 addresses allocated for end users and systems (as
   illustrated in Section 5.1 of [RFC8754]), can provide the clear
   distinction between internal and external address space that is
   required to maintain the integrity and security of the SRv6 Domain.
   Additionally, [RFC8754] defines a Hashed Message Authentication Code
   (HMAC) TLV permitting SR Segment Endpoint Nodes in the SR domain to
   verify that the SRH applied to a packet was selected by an authorized
   party and to ensure that the segment list is not modified after
   generation, regardless of the number of segments in the segment list.
   When enabled by local configuration, HMAC processing occurs at the
   beginning of SRH processing as defined in Section 2.1.2.1 of
   [RFC8754].

   This document introduces SRv6 Endpoint and SR Policy Headend
   behaviors for implementation on SRv6-capable nodes in the network.
   The definition of the SR Policy Headend should be consistent with the
   specific behavior used and any local configuration (as specified in
   Section 4.1.1).  As such, this document does not introduce any new
   security considerations.

   The SID behaviors specified in this document have the same HMAC TLV
   handling and mutability properties with regard to the Flags, Tag, and
   Segment List field as the SID behavior specified in [RFC8754].

10.  IANA Considerations

10.1.  Ethernet Next Header Type

   IANA has allocated "Ethernet" (value 143) in the "Assigned Internet
   Protocol Numbers" registry (see <https://www.iana.org/assignments/
   protocol-numbers/>).  Value 143 in the Next Header field of an IPv6
   header or any extension header indicates that the payload is an
   Ethernet frame [IEEE.802.3_2018].

10.2.  SRv6 Endpoint Behaviors Registry

   IANA has created a new top-level registry called "Segment Routing"
   (see <https://www.iana.org/assignments/segment-routing/>).  This
   registry serves as a top-level registry for all Segment Routing
   subregistries.

   Additionally, IANA has created a new subregistry called "SRv6
   Endpoint Behaviors" under the top-level "Segment Routing" registry.
   This subregistry maintains 16-bit identifiers for the SRv6 Endpoint
   behaviors.  This registry is established to provide consistency for
   control-plane protocols that need to refer to these behaviors.  These
   values are not encoded in the function bits within a SID.

10.2.1.  Registration Procedures

   The range of the registry is 0-65535 (0x0000-0xFFFF).  The table
   below contains the allocation ranges and registration policies
   [RFC8126] for each:

   +=============+===============+=========================+===========+
   | Range       |  Range (Hex)  |       Registration      |    Note   |
   |             |               |        Procedures       |           |
   +=============+===============+=========================+===========+
   | 0           |     0x0000    |         Reserved        | Not to be |
   |             |               |                         | allocated |
   +-------------+---------------+-------------------------+-----------+
   | 1-32767     | 0x0001-0x7FFF |        First Come       |           |
   |             |               |       First Served      |           |
   +-------------+---------------+-------------------------+-----------+
   | 32768-34815 | 0x8000-0x87FF |       Private Use       |           |
   +-------------+---------------+-------------------------+-----------+
   | 34816-65534 | 0x8800-0xFFFE |         Reserved        |           |
   +-------------+---------------+-------------------------+-----------+
   | 65535       |     0xFFFF    |         Reserved        |   Opaque  |
   +-------------+---------------+-------------------------+-----------+

                      Table 5: Registration Procedures

10.2.2.  Initial Registrations

   The initial registrations for the subregistry are as follows:

   +=============+===============+=========================+===========+
   | Value       |      Hex      |    Endpoint Behavior    | Reference |
   +=============+===============+=========================+===========+
   | 0           |     0x0000    |         Reserved        |           |
   +-------------+---------------+-------------------------+-----------+
   | 1           |     0x0001    |           End           |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 2           |     0x0002    |       End with PSP      |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 3           |     0x0003    |       End with USP      |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 4           |     0x0004    |    End with PSP & USP   |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 5           |     0x0005    |          End.X          |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 6           |     0x0006    |      End.X with PSP     |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 7           |     0x0007    |      End.X with USP     |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 8           |     0x0008    |   End.X with PSP & USP  |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 9           |     0x0009    |          End.T          |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 10          |     0x000A    |      End.T with PSP     |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 11          |     0x000B    |      End.T with USP     |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 12          |     0x000C    |   End.T with PSP & USP  |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 13          |     0x000D    |        Unassigned       |           |
   +-------------+---------------+-------------------------+-----------+
   | 14          |     0x000E    |      End.B6.Encaps      |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 15          |     0x000F    |          End.BM         |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 16          |     0x0010    |         End.DX6         |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 17          |     0x0011    |         End.DX4         |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 18          |     0x0012    |         End.DT6         |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 19          |     0x0013    |         End.DT4         |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 20          |     0x0014    |         End.DT46        |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 21          |     0x0015    |         End.DX2         |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 22          |     0x0016    |         End.DX2V        |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 23          |     0x0017    |         End.DT2U        |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 24          |     0x0018    |         End.DT2M        |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 25          |     0x0019    |         Reserved        |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 26          |     0x001A    |        Unassigned       |           |
   +-------------+---------------+-------------------------+-----------+
   | 27          |     0x001B    |    End.B6.Encaps.Red    |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 28          |     0x001C    |       End with USD      |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 29          |     0x001D    |    End with PSP & USD   |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 30          |     0x001E    |    End with USP & USD   |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 31          |     0x001F    |   End with PSP, USP &   |  RFC 8986 |
   |             |               |           USD           |           |
   +-------------+---------------+-------------------------+-----------+
   | 32          |     0x0020    |      End.X with USD     |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 33          |     0x0021    |   End.X with PSP & USD  |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 34          |     0x0022    |   End.X with USP & USD  |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 35          |     0x0023    |   End.X with PSP, USP   |  RFC 8986 |
   |             |               |          & USD          |           |
   +-------------+---------------+-------------------------+-----------+
   | 36          |     0x0024    |      End.T with USD     |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 37          |     0x0025    |   End.T with PSP & USD  |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 38          |     0x0026    |   End.T with USP & USD  |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 39          |     0x0027    |   End.T with PSP, USP   |  RFC 8986 |
   |             |               |          & USD          |           |
   +-------------+---------------+-------------------------+-----------+
   | 40-32766    | 0x0028-0x7FFE |        Unassigned       |           |
   +-------------+---------------+-------------------------+-----------+
   | 32767       |     0x7FFF    |    The SID defined in   | RFC 8986, |
   |             |               |         RFC 8754        |  RFC 8754 |
   +-------------+---------------+-------------------------+-----------+
   | 32768-34815 | 0x8000-0x87FF |   Reserved for Private  |  RFC 8986 |
   |             |               |           Use           |           |
   +-------------+---------------+-------------------------+-----------+
   | 34816-65534 | 0x8800-0xFFFE |         Reserved        |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+
   | 65535       |     0xFFFF    |          Opaque         |  RFC 8986 |
   +-------------+---------------+-------------------------+-----------+

                       Table 6: Initial Registrations

11.  References

11.1.  Normative References

   [IEEE.802.3_2018]
              IEEE, "IEEE Standard for Ethernet", IEEE 802.3-2018,
              DOI 10.1109/IEEESTD.2018.8457469, 31 August 2018,
              <https://ieeexplore.ieee.org/document/8457469>.

   [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>.

   [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>.

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,
              <https://www.rfc-editor.org/info/rfc8200>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

11.2.  Informative References

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [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>.

   [RFC4664]  Andersson, L., Ed. and E. Rosen, Ed., "Framework for Layer
              2 Virtual Private Networks (L2VPNs)", RFC 4664,
              DOI 10.17487/RFC4664, September 2006,
              <https://www.rfc-editor.org/info/rfc4664>.

   [RFC4761]  Kompella, K., Ed. and Y. Rekhter, Ed., "Virtual Private
              LAN Service (VPLS) Using BGP for Auto-Discovery and
              Signaling", RFC 4761, DOI 10.17487/RFC4761, January 2007,
              <https://www.rfc-editor.org/info/rfc4761>.

   [RFC4762]  Lasserre, M., Ed. and V. Kompella, Ed., "Virtual Private
              LAN Service (VPLS) Using Label Distribution Protocol (LDP)
              Signaling", RFC 4762, DOI 10.17487/RFC4762, January 2007,
              <https://www.rfc-editor.org/info/rfc4762>.

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8214]  Boutros, S., Sajassi, A., Salam, S., Drake, J., and J.
              Rabadan, "Virtual Private Wire Service Support in Ethernet
              VPN", RFC 8214, DOI 10.17487/RFC8214, August 2017,
              <https://www.rfc-editor.org/info/rfc8214>.

   [RFC8317]  Sajassi, A., Ed., Salam, S., Drake, J., Uttaro, J.,
              Boutros, S., and J. Rabadan, "Ethernet-Tree (E-Tree)
              Support in Ethernet VPN (EVPN) and Provider Backbone
              Bridging EVPN (PBB-EVPN)", RFC 8317, DOI 10.17487/RFC8317,
              January 2018, <https://www.rfc-editor.org/info/rfc8317>.

   [SR-TI-LFA]
              Litkowski, S., Bashandy, A., Filsfils, C., Francois, P.,
              Decraene, B., and D. Voyer, "Topology Independent Fast
              Reroute using Segment Routing", Work in Progress,
              Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
              06, 1 February 2021, <https://tools.ietf.org/html/draft-
              ietf-rtgwg-segment-routing-ti-lfa-06>.

   [SRV6-NET-PGM-ILLUST]
              Filsfils, C., Camarillo, P., Ed., Li, Z., Matsushima, S.,
              Decraene, B., Steinberg, D., Lebrun, D., Raszuk, R., and
              J. Leddy, "Illustrations for SRv6 Network Programming",
              Work in Progress, Internet-Draft, draft-filsfils-spring-
              srv6-net-pgm-illustration-03, 25 September 2020,
              <https://tools.ietf.org/html/draft-filsfils-spring-srv6-
              net-pgm-illustration-03>.

Acknowledgements

   The authors would like to acknowledge Stefano Previdi, Dave Barach,
   Mark Townsley, Peter Psenak, Thierry Couture, Kris Michielsen, Paul
   Wells, Robert Hanzl, Dan Ye, Gaurav Dawra, Faisal Iqbal, 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, Saleem Hafeez, and Brian Carpenter.

Contributors

   Daniel Bernier
   Bell Canada
   Canada

   Email: daniel.bernier@bell.ca


   Dirk Steinberg
   Lapishills Consulting Limited
   Cyprus

   Email: dirk@lapishills.com


   Robert Raszuk
   Bloomberg LP
   United States of America

   Email: robert@raszuk.net


   Bruno Decraene
   Orange
   France

   Email: bruno.decraene@orange.com


   Bart Peirens
   Proximus
   Belgium

   Email: bart.peirens@proximus.com


   Hani Elmalky
   Google
   United States of America

   Email: helmalky@google.com


   Prem Jonnalagadda
   Barefoot Networks
   United States of America

   Email: prem@barefootnetworks.com


   Milad Sharif
   SambaNova Systems
   United States of America

   Email: milad.sharif@sambanova.ai


   David Lebrun
   Google
   Belgium

   Email: dlebrun@google.com


   Stefano Salsano
   Universita di Roma "Tor Vergata"
   Italy

   Email: stefano.salsano@uniroma2.it


   Ahmed AbdelSalam
   Gran Sasso Science Institute
   Italy

   Email: ahmed.abdelsalam@gssi.it


   Gaurav Naik
   Drexel University
   United States of America

   Email: gn@drexel.edu


   Arthi Ayyangar
   Arrcus, Inc
   United States of America

   Email: arthi@arrcus.com


   Satish Mynam
   Arrcus, Inc
   United States of America

   Email: satishm@arrcus.com


   Wim Henderickx
   Nokia
   Belgium

   Email: wim.henderickx@nokia.com


   Shaowen Ma
   Juniper
   Singapore

   Email: mashao@juniper.net


   Ahmed Bashandy
   Individual
   United States of America

   Email: abashandy.ietf@gmail.com


   Francois Clad
   Cisco Systems, Inc.
   France

   Email: fclad@cisco.com


   Kamran Raza
   Cisco Systems, Inc.
   Canada

   Email: skraza@cisco.com


   Darren Dukes
   Cisco Systems, Inc.
   Canada

   Email: ddukes@cisco.com


   Patrice Brissete
   Cisco Systems, Inc.
   Canada

   Email: pbrisset@cisco.com


   Zafar Ali
   Cisco Systems, Inc.
   United States of America

   Email: zali@cisco.com


   Ketan Talaulikar
   Cisco Systems, Inc.
   India

   Email: ketant@cisco.com


Authors' Addresses

   Clarence Filsfils (editor)
   Cisco Systems, Inc.
   Belgium

   Email: cf@cisco.com


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

   Email: pcamaril@cisco.com


   John Leddy
   Akamai Technologies
   United States of America

   Email: john@leddy.net


   Daniel Voyer
   Bell Canada
   Canada

   Email: daniel.voyer@bell.ca


   Satoru Matsushima
   SoftBank
   Japan

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


   Zhenbin Li
   Huawei Technologies
   China

   Email: lizhenbin@huawei.com
