IPv6 Segment Routing Header (SRH)
RFC 8754

Internet Engineering Task Force (IETF)                  C. Filsfils, Ed.
Request for Comments: 8754                                 D. Dukes, Ed.
Category: Standards Track                            Cisco Systems, Inc.
ISSN: 2070-1721                                               S. Previdi
                                                                  Huawei
                                                                J. Leddy
                                                              Individual
                                                           S. Matsushima
                                                                SoftBank
                                                                D. Voyer
                                                             Bell Canada
                                                              March 2020


                   IPv6 Segment Routing Header (SRH)

Abstract

   Segment Routing can be applied to the IPv6 data plane using a new
   type of Routing Extension Header called the Segment Routing Header
   (SRH).  This document describes the SRH and how it is used by nodes
   that are Segment Routing (SR) capable.

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/rfc8754.

Copyright Notice

   Copyright (c) 2020 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
     1.1.  Terminology
     1.2.  Requirements Language
   2.  Segment Routing Header
     2.1.  SRH TLVs
       2.1.1.  Padding TLVs
       2.1.2.  HMAC TLV
   3.  SR Nodes
     3.1.  SR Source Node
     3.2.  Transit Node
     3.3.  SR Segment Endpoint Node
   4.  Packet Processing
     4.1.  SR Source Node
       4.1.1.  Reduced SRH
     4.2.  Transit Node
     4.3.  SR Segment Endpoint Node
       4.3.1.  FIB Entry Is a Locally Instantiated SRv6 SID
       4.3.2.  FIB Entry Is a Local Interface
       4.3.3.  FIB Entry Is a Nonlocal Route
       4.3.4.  FIB Entry Is a No Match
   5.  Intra-SR-Domain Deployment Model
     5.1.  Securing the SR Domain
     5.2.  SR Domain as a Single System with Delegation among
           Components
     5.3.  MTU Considerations
     5.4.  ICMP Error Processing
     5.5.  Load Balancing and ECMP
     5.6.  Other Deployments
   6.  Illustrations
     6.1.  Abstract Representation of an SRH
     6.2.  Example Topology
     6.3.  SR Source Node
       6.3.1.  Intra-SR-Domain Packet
       6.3.2.  Inter-SR-Domain Packet -- Transit
       6.3.3.  Inter-SR-Domain Packet -- Internal to External
     6.4.  Transit Node
     6.5.  SR Segment Endpoint Node
     6.6.  Delegation of Function with HMAC Verification
       6.6.1.  SID List Verification
   7.  Security Considerations
     7.1.  SR Attacks
     7.2.  Service Theft
     7.3.  Topology Disclosure
     7.4.  ICMP Generation
     7.5.  Applicability of AH
   8.  IANA Considerations
     8.1.  Segment Routing Header Flags Registry
     8.2.  Segment Routing Header TLVs Registry
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   Segment Routing (SR) can be applied to the IPv6 data plane using a
   new type of routing header called the Segment Routing Header (SRH).
   This document describes the SRH and how it is used by nodes that are
   SR capable.

   "Segment Routing Architecture" [RFC8402] describes Segment Routing
   and its instantiation in two data planes: MPLS and IPv6.

   The encoding of IPv6 segments in the SRH is defined in this document.

1.1.  Terminology

   This document uses the terms Segment Routing (SR), SR domain, SR over
   IPv6 (SRv6), Segment Identifier (SID), SRv6 SID, Active Segment, and
   SR Policy as defined in [RFC8402].

1.2.  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.

2.  Segment Routing Header

   Routing headers are defined in [RFC8200].  The Segment Routing Header
   (SRH) has a new Routing Type (4).

   The SRH is defined as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Next Header   |  Hdr Ext Len  | Routing Type  | Segments Left |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Last Entry   |     Flags     |              Tag              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Segment List[0] (128-bit IPv6 address)             |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
                                  ...
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |            Segment List[n] (128-bit IPv6 address)             |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    //                                                             //
    //         Optional Type Length Value objects (variable)       //
    //                                                             //
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   Next Header:  Defined in [RFC8200], Section 4.4.

   Hdr Ext Len:  Defined in [RFC8200], Section 4.4.

   Routing Type:  4.

   Segments Left:  Defined in [RFC8200], Section 4.4.

   Last Entry:  contains the index (zero based), in the Segment List, of
      the last element of the Segment List.

   Flags:  8 bits of flags.  Section 8.1 creates an IANA registry for
      new flags to be defined.  The following flags are defined:

          0 1 2 3 4 5 6 7
         +-+-+-+-+-+-+-+-+
         |U U U U U U U U|
         +-+-+-+-+-+-+-+-+

      U: Unused and for future use.  MUST be 0 on transmission and
      ignored on receipt.

   Tag:  Tag a packet as part of a class or group of packets -- e.g.,
      packets sharing the same set of properties.  When Tag is not used
      at the source, it MUST be set to zero on transmission.  When Tag
      is not used during SRH processing, it SHOULD be ignored.  Tag is
      not used when processing the SID defined in Section 4.3.1.  It may
      be used when processing other SIDs that are not defined in this
      document.  The allocation and use of tag is outside the scope of
      this document.

   Segment List[0..n]:  128-bit IPv6 addresses representing the nth
      segment in the Segment List.  The Segment List is encoded starting
      from the last segment of the SR Policy.  That is, the first
      element of the Segment List (Segment List[0]) contains the last
      segment of the SR Policy, the second element contains the
      penultimate segment of the SR Policy, and so on.

   TLV:  Type Length Value (TLV) is described in Section 2.1.

   In the SRH, the Next Header, Hdr Ext Len, Routing Type, and Segments
   Left fields are defined in Section 4.4 of [RFC8200].  Based on the
   constraints in that section, Next Header, Header Ext Len, and Routing
   Type are not mutable while Segments Left is mutable.

   The mutability of the TLV value is defined by the most significant
   bit in the type, as specified in Section 2.1.

   Section 4.3 defines the mutability of the remaining fields in the SRH
   (Flags, Tag, Segment List) in the context of the SID defined in this
   document.

   New SIDs defined in the future MUST specify the mutability properties
   of the Flags, Tag, and Segment List and indicate how the Hashed
   Message Authentication Code (HMAC) TLV (Section 2.1.2) verification
   works.  Note that, in effect, these fields are mutable.

   Consistent with the SR model, the source of the SRH always knows how
   to set the Segment List, Flags, Tag, and TLVs of the SRH for use
   within the SR domain.  How it achieves this is outside the scope of
   this document but may be based on topology, available SIDs and their
   mutability properties, the SRH mutability requirements of the
   destination, or any other information.

2.1.  SRH TLVs

   This section defines TLVs of the Segment Routing Header.

   A TLV provides metadata for segment processing.  The only TLVs
   defined in this document are the HMAC (Section 2.1.2) and padding
   TLVs (Section 2.1.1).  While processing the SID defined in
   Section 4.3.1, all TLVs are ignored unless local configuration
   indicates otherwise (Section 4.3.1.1.1).  Thus, TLV and HMAC support
   is optional for any implementation; however, an implementation adding
   or parsing TLVs MUST support PAD TLVs.  Other documents may define
   additional TLVs and processing rules for them.

   TLVs are present when the Hdr Ext Len is greater than (Last
   Entry+1)*2.

   While processing TLVs at a segment endpoint, TLVs MUST be fully
   contained within the SRH as determined by the Hdr Ext Len.  Detection
   of TLVs exceeding the boundary of the SRH Hdr Ext Len results in an
   ICMP Parameter Problem, Code 0, message to the Source Address,
   pointing to the Hdr Ext Len field of the SRH, and the packet being
   discarded.

   An implementation MAY limit the number and/or length of TLVs it
   processes based on local configuration.  It MAY limit:

   *  the number of consecutive Pad1 (Section 2.1.1.1) options to 1.  If
      padding of more than one byte is required, then PadN
      (Section 2.1.1.2) should be used.

   *  The length in PadN to 5.

   *  The maximum number of non-Pad TLVs to be processed.

   *  The maximum length of all TLVs to be processed.

   The implementation MAY stop processing additional TLVs in the SRH
   when these configured limits are exceeded.

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-----------------------
   |     Type      |    Length     | Variable-length data
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-----------------------

   Type:  An 8-bit codepoint from the "Segment Routing Header TLVs"
      [IANA-SRHTLV].  Unrecognized Types MUST be ignored on receipt.

   Length:  The length of the variable-length data field in bytes.

   Variable-length data:  data that is specific to the Type.

   Type Length Value (TLV) entries contain OPTIONAL information that may
   be used by the node identified in the Destination Address (DA) of the
   packet.

   Each TLV has its own length, format, and semantic.  The codepoint
   allocated (by IANA) to each TLV Type defines both the format and the
   semantic of the information carried in the TLV.  Multiple TLVs may be
   encoded in the same SRH.

   The highest-order bit of the TLV type (bit 0) specifies whether or
   not the TLV data of that type can change en route to the packet's
   final destination:

      0: TLV data does not change en route

      1: TLV data does change en route

   All TLVs specify their alignment requirements using an xn+y format.
   The xn+y format is defined as per [RFC8200].  The SR source nodes use
   the xn+y alignment requirements of TLVs and Padding TLVs when
   constructing an SRH.

   The Length field of the TLV is used to skip the TLV while inspecting
   the SRH in case the node doesn't support or recognize the Type.  The
   Length defines the TLV length in octets, not including the Type and
   Length fields.

   The following TLVs are defined in this document:

      Padding TLVs

      HMAC TLV

   Additional TLVs may be defined in the future.

2.1.1.  Padding TLVs

   There are two types of Padding TLVs, Pad1 and PadN, and the following
   applies to both:

      Padding TLVs are used for meeting the alignment requirement of the
      subsequent TLVs.

      Padding TLVs are used to pad the SRH to a multiple of 8 octets.

      Padding TLVs are ignored by a node processing the SRH TLV.

      Multiple Padding TLVs MAY be used in one SRH.

2.1.1.1.  Pad1

   Alignment requirement: none

      0 1 2 3 4 5 6 7
     +-+-+-+-+-+-+-+-+
     |     Type      |
     +-+-+-+-+-+-+-+-+

   Type:  0

   A single Pad1 TLV MUST be used when a single byte of padding is
   required.  A Pad1 TLV MUST NOT be used if more than one consecutive
   byte of padding is required.

2.1.1.2.  PadN

   Alignment requirement: none

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |      Padding (variable)       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   //                    Padding (variable)                       //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type:  4

   Length:  0 to 5.  The length of the Padding field in bytes.

   Padding:  Padding bits have no semantic.  They MUST be set to 0 on
      transmission and ignored on receipt.

   The PadN TLV MUST be used when more than one byte of padding is
   required.

2.1.2.  HMAC TLV

   Alignment requirement: 8n

   The keyed Hashed Message Authentication Code (HMAC) TLV is OPTIONAL
   and has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Type     |     Length    |D|        RESERVED             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      HMAC Key ID (4 octets)                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                              //
   |                      HMAC (variable)                         //
   |                                                              //
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   where:

   Type:  5.

   Length:  The length of the variable-length data in bytes.

   D:  1 bit. 1 indicates that the Destination Address verification is
      disabled due to use of a reduced Segment List (see Section 4.1.1).

   RESERVED:  15 bits.  MUST be 0 on transmission.

   HMAC Key ID:  A 4-octet opaque number that uniquely identifies the
      pre-shared key and algorithm used to generate the HMAC.

   HMAC:  Keyed HMAC, in multiples of 8 octets, at most 32 octets.

   The HMAC TLV is used 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.  This also allows for verification
   that the current segment (by virtue of being in the authorized
   Segment List) is authorized for use.  The SR domain ensures that the
   source node is permitted to use the source address in the packet via
   ingress filtering mechanisms as defined in BCP 84 [RFC3704] or other
   strategies as appropriate.

2.1.2.1.  HMAC Generation and Verification

   Local configuration determines when to check for an HMAC.  This local
   configuration is outside the scope of this document.  It may be based
   on the active segment at an SR Segment endpoint node, the result of
   an Access Control List (ACL) that considers incoming interface, HMAC
   Key ID, or other packet fields.

   An implementation that supports the generation and verification of
   the HMAC supports the following default behavior, as defined in the
   remainder of this section.

   The HMAC verification begins by checking that the current segment is
   equal to the destination address of the IPv6 header.  The check is
   successful when either:

   *  HMAC D bit is 1 and Segments Left is greater than Last Entry, or

   *  HMAC Segments Left is less than or equal to Last Entry, and the
      destination address is equal to Segment List[Segments Left].

   The HMAC field is the output of the HMAC computation as defined in
   [RFC2104], using:

   *  key: The pre-shared key identified by HMAC Key ID

   *  HMAC algorithm: Identified by the HMAC Key ID

   *  Text: A concatenation of the following fields from the IPv6 header
      and the SRH, as it would be received at the node verifying the
      HMAC:

      -  IPv6 header: Source address (16 octets)

      -  SRH: Last Entry (1 octet)

      -  SRH: Flags (1 octet)

      -  SRH: HMAC 16 bits following Length

      -  SRH: HMAC Key ID (4 octets)

      -  SRH: All addresses in the Segment List (variable octets)

   The HMAC digest is truncated to 32 octets and placed in the HMAC
   field of the HMAC TLV.

   For HMAC algorithms producing digests less than 32 octets long, the
   digest is placed in the lowest-order octets of the HMAC field.
   Subsequent octets MUST be set to zero such that the HMAC length is a
   multiple of 8 octets.

   If HMAC verification is successful, processing proceeds as normal.

   If HMAC verification fails, an ICMP error message (parameter problem,
   error code 0, pointing to the HMAC TLV) SHOULD be generated (but rate
   limited) and logged, and the packet SHOULD be discarded.

2.1.2.2.  HMAC Pre-shared Key Algorithm

   The HMAC Key ID field allows for the simultaneous existence of
   several hash algorithms (SHA-256, SHA3-256 ... or future ones) as
   well as pre-shared keys.

   The HMAC Key ID field is opaque -- i.e., it has neither syntax nor
   semantic except as an identifier of the right combination of pre-
   shared key and hash algorithm.

   At the HMAC TLV generating and verification nodes, the Key ID
   uniquely identifies the pre-shared key and HMAC algorithm.

   At the HMAC TLV generating node, the Text for the HMAC computation is
   set to the IPv6 header fields and SRH fields as they would appear at
   the verification node(s), not necessarily the same as the source node
   sending a packet with the HMAC TLV.

   Pre-Shared key rollover is supported by having two key IDs in use
   while the HMAC TLV generating node and verifying node converge to a
   new key.

   The HMAC TLV generating node may need to revoke an SRH for which it
   previously generated an HMAC.  Revocation is achieved by allocating a
   new key and key ID, then rolling over the key ID associated with the
   SRH to be revoked.  The HMAC TLV verifying node drops packets with
   the revoked SRH.

   An implementation supporting HMAC can support multiple hash
   functions.  An implementation supporting HMAC MUST implement SHA-2
   [FIPS180-4] in its SHA-256 variant.

   The selection of pre-shared key and algorithm and their distribution
   is outside the scope of this document.  Some options may include:

   *  setting these items in the configuration of the HMAC generating or
      verifying nodes, either by static configuration or any SDN-
      oriented approach

   *  dynamically using a trusted key distribution protocol such as
      [RFC6407]

   While key management is outside the scope of this document, the
   recommendations of BCP 107 [RFC4107] should be considered when
   choosing the key management system.

3.  SR Nodes

   There are different types of nodes that may be involved in segment
   routing networks: SR source nodes that originate packets with a
   segment in the destination address of the IPv6 header, transit nodes
   that forward packets destined to a remote segment, and SR segment
   endpoint nodes that process a local segment in the destination
   address of an IPv6 header.

3.1.  SR Source Node

   A SR source node is any node that originates an IPv6 packet with a
   segment (i.e., SRv6 SID) in the destination address of the IPv6
   header.  The packet leaving the SR source node may or may not contain
   an SRH.  This includes either:

   *  A host originating an IPv6 packet, or

   *  An SR domain ingress router encapsulating a received packet in an
      outer IPv6 header, followed by an optional SRH.

   It is out of the scope of this document to describe the mechanism
   through which a segment in the destination address of the IPv6 header
   and the Segment List in the SRH are derived.

3.2.  Transit Node

   A transit node is any node forwarding an IPv6 packet where the
   destination address of that packet is not locally configured as a
   segment or a local interface.  A transit node is not required to be
   capable of processing a segment or SRH.

3.3.  SR Segment Endpoint Node

   An SR segment endpoint node is any node receiving an IPv6 packet
   where the destination address of that packet is locally configured as
   a segment or local interface.

4.  Packet Processing

   This section describes SRv6 packet processing at the SR source,
   Transit, and SR segment endpoint nodes.

4.1.  SR Source Node

   A source node steers a packet into an SR Policy.  If the SR Policy
   results in a Segment List containing a single segment, and there is
   no need to add information to the SRH flag or add TLV; the DA is set
   to the single Segment List entry, and the SRH MAY be omitted.

   When needed, the SRH is created as follows:

      The Next Header and Hdr Ext Len fields are set as specified in
      [RFC8200].

      The Routing Type field is set to 4.

      The DA of the packet is set with the value of the first segment.

      The first element of the SRH Segment List is the ultimate segment.
      The second element is the penultimate segment, and so on.

      The Segments Left field is set to n-1, where n is the number of
      elements in the SR Policy.

      The Last Entry field is set to n-1, where n is the number of
      elements in the SR Policy.

      TLVs (including HMAC) may be set according to their specification.

      The packet is forwarded toward the packet's Destination Address
      (the first segment).

4.1.1.  Reduced SRH

   When a source does not require the entire SID list to be preserved in
   the SRH, a reduced SRH may be used.

   A reduced SRH does not contain the first segment of the related SR
   Policy (the first segment is the one already in the DA of the IPv6
   header), and the Last Entry field is set to n-2, where n is the
   number of elements in the SR Policy.

4.2.  Transit Node

   As specified in [RFC8200], the only node allowed to inspect the
   Routing Extension Header (and therefore the SRH) is the node
   corresponding to the DA of the packet.  Any other transit node MUST
   NOT inspect the underneath routing header and MUST forward the packet
   toward the DA according to its IPv6 routing table.

   When a SID is in the destination address of an IPv6 header of a
   packet, it's routed through an IPv6 network as an IPv6 address.
   SIDs, or the prefix(es) covering SIDs, and their reachability may be
   distributed by means outside the scope of this document.  For
   example, [RFC5308] or [RFC5340] may be used to advertise a prefix
   covering the SIDs on a node.

4.3.  SR Segment Endpoint Node

   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

4.3.1.  FIB Entry Is a Locally Instantiated SRv6 SID

   This document and section define a single SRv6 SID.  Future documents
   may define additional SRv6 SIDs.  In such a case, the entire content
   of this section will be defined in that document.

   If the FIB entry represents a locally instantiated SRv6 SID, process
   the next header chain of the IPv6 header as defined in Section 4 of
   [RFC8200].  Section 4.3.1.1 describes how to process an SRH;
   Section 4.3.1.2 describes how to process an upper-layer header or the
   absence of a Next Header.

   Processing this SID modifies the Segments Left and, if configured to
   process TLVs, it may modify the "variable-length data" of TLV types
   that change en route.  Therefore, Segments Left is mutable, and TLVs
   that change en route are mutable.  The remainder of the SRH (Flags,
   Tag, Segment List, and TLVs that do not change en route) are
   immutable while processing this SID.

4.3.1.1.  SRH Processing

   S01. When an SRH is processed {
   S02.   If Segments Left is equal to zero {
   S03.     Proceed to process the next header in the packet,
            whose type is identified by the Next Header field in
            the routing header.
   S04.   }
   S05.   Else {
   S06.     If local configuration requires TLV processing {
   S07.       Perform TLV processing (see TLV Processing)
   S08.     }
   S09.     max_last_entry  =  ( Hdr Ext Len /  2 ) - 1
   S10.     If  ((Last Entry > max_last_entry) or
   S11.          (Segments Left is greater than (Last Entry+1)) {
   S12.       Send an ICMP Parameter Problem, Code 0, message to
              the Source Address, pointing to the Segments Left
              field, and discard the packet.
   S13.     }
   S14.     Else {
   S15.       Decrement Segments Left by 1.
   S16.       Copy Segment List[Segments Left] from the SRH to the
              destination address of the IPv6 header.
   S17.       If the IPv6 Hop Limit is less than or equal to 1 {
   S18.         Send an ICMP Time Exceeded -- Hop Limit Exceeded in
                Transit message to the Source Address and discard
                the packet.
   S19.       }
   S20.       Else {
   S21.         Decrement the Hop Limit by 1
   S22.         Resubmit the packet to the IPv6 module for transmission
                to the new destination.
   S23.       }
   S24.     }
   S25.   }
   S26. }

4.3.1.1.1.  TLV Processing

   Local configuration determines how TLVs are to be processed when the
   Active Segment is a local SID defined in this document.  The
   definition of local configuration is outside the scope of this
   document.

   For illustration purposes only, two example local configurations that
   may be associated with a SID are provided below.

   Example 1:
   For any packet received from interface I2
     Skip TLV processing

   Example 2:
   For any packet received from interface I1
     If first TLV is HMAC {
       Process the HMAC TLV
     }
     Else {
       Discard the packet
     }

4.3.1.2.  Upper-Layer Header or No Next Header

   When processing the upper-layer header of a packet matching a FIB
   entry locally instantiated as an SRv6 SID defined in this document:

   IF (Upper-layer Header is IPv4 or IPv6) and
       local configuration permits {
     Perform IPv6 decapsulation
     Resubmit the decapsulated packet to the IPv4 or IPv6 module
   }
   ELSE {
     Send an ICMP parameter problem message to the Source Address and
     discard the packet.  Error code (4) "SR Upper-layer
     Header Error", pointer set to the offset of the upper-layer
     header.
   }

   A unique error code allows an SR source node to recognize an error in
   SID processing at an endpoint.

4.3.2.  FIB Entry Is a Local Interface

   If the FIB entry represents a local interface and is not locally
   instantiated as an SRv6 SID, the SRH is processed as follows:

      If Segments Left is zero, the node must ignore the routing header
      and proceed to process the next header in the packet, whose type
      is identified by the Next Header field in the routing header.

      If Segments Left is non-zero, the node must discard the packet and
      send an ICMP Parameter Problem, Code 0, message to the packet's
      Source Address, pointing to the unrecognized Routing Type.

4.3.3.  FIB Entry Is a Nonlocal Route

   Processing is not changed by this document.

4.3.4.  FIB Entry Is a No Match

   Processing is not changed by this document.

5.  Intra-SR-Domain Deployment Model

   The use of the SIDs exclusively within the SR domain and solely for
   packets of the SR domain is an important deployment model.

   This enables the SR domain to act as a single routing system.

   This section covers:

   *  securing the SR domain from external attempts to use its SIDs

   *  using the SR domain as a single system with delegation between
      components

   *  handling packets of the SR domain

5.1.  Securing the SR Domain

   Nodes outside the SR domain are not trusted: they cannot directly use
   the SIDs of the domain.  This is enforced by two levels of access
   control lists:

   1.  Any packet entering the SR domain and destined to a SID within
       the SR domain is dropped.  This may be realized with the
       following logic.  Other methods with equivalent outcome are
       considered compliant:

       *  Allocate all the SIDs from a block S/s

       *  Configure each external interface of each edge node of the
          domain with an inbound infrastructure access list (IACL) that
          drops any incoming packet with a destination address in S/s

       *  Failure to implement this method of ingress filtering exposes
          the SR domain to source-routing attacks, as described and
          referenced in [RFC5095]

   2.  The distributed protection in #1 is complemented with per-node
       protection, dropping packets to SIDs from source addresses
       outside the SR domain.  This may be realized with the following
       logic.  Other methods with equivalent outcome are considered
       compliant:

       *  Assign all interface addresses from prefix A/a

       *  At node k, all SIDs local to k are assigned from prefix Sk/sk

       *  Configure each internal interface of each SR node k in the SR
          domain with an inbound IACL that drops any incoming packet
          with a destination address in Sk/sk if the source address is
          not in A/a.

5.2.  SR Domain as a Single System with Delegation among Components

   All intra-SR-domain packets are of the SR domain.  The IPv6 header is
   originated by a node of the SR domain and is destined to a node of
   the SR domain.

   All interdomain packets are encapsulated for the part of the packet
   journey that is within the SR domain.  The outer IPv6 header is
   originated by a node of the SR domain and is destined to a node of
   the SR domain.

   As a consequence, any packet within the SR domain is of the SR
   domain.

   The SR domain is a system in which the operator may want to
   distribute or delegate different operations of the outermost header
   to different nodes within the system.

   An operator of an SR domain may choose to delegate SRH addition to a
   host node within the SR domain and delegate validation of the
   contents of any SRH to a more trusted router or switch attached to
   the host.  Consider a top-of-rack switch T connected to host H via
   interface I.  H receives an SRH (SRH1) with a computed HMAC via some
   SDN method outside the scope of this document.  H classifies traffic
   it sources and adds SRH1 to traffic requiring a specific Service
   Level Agreement (SLA).  T is configured with an IACL on I requiring
   verification of the SRH for any packet destined to the SID block of
   the SR domain (S/s).  T checks and verifies that SRH1 is valid and
   contains an HMAC TLV; T then verifies the HMAC.

   An operator of the SR domain may choose to have all segments in the
   SR domain verify the HMAC.  This mechanism would verify that the SRH
   Segment List is not modified while traversing the SR domain.

5.3.  MTU Considerations

   An SR domain ingress edge node encapsulates packets traversing the SR
   domain and needs to consider the MTU of the SR domain.  Within the SR
   domain, well-known mitigation techniques are RECOMMENDED, such as
   deploying a greater MTU value within the SR domain than at the
   ingress edges.

   Encapsulation with an outer IPv6 header and SRH shares the same MTU
   and fragmentation considerations as IPv6 tunnels described in
   [RFC2473].  Further investigation on the limitation of various
   tunneling methods (including IPv6 tunnels) is discussed in
   [INTAREA-TUNNELS] and SHOULD be considered by operators when
   considering MTU within the SR domain.

5.4.  ICMP Error Processing

   ICMP error packets generated within the SR domain are sent to source
   nodes within the SR domain.  The invoking packet in the ICMP error
   message may contain an SRH.  Since the destination address of a
   packet with an SRH changes as each segment is processed, it may not
   be the destination used by the socket or application that generated
   the invoking packet.

   For the source of an invoking packet to process the ICMP error
   message, the ultimate destination address of the IPv6 header may be
   required.  The following logic is used to determine the destination
   address for use by protocol-error handlers.

   *  Walk all extension headers of the invoking IPv6 packet to the
      routing extension header preceding the upper-layer header.

      -  If routing header is type 4 Segment Routing Header (SRH)

         o  The SID at Segment List[0] may be used as the destination
            address of the invoking packet.

   ICMP errors are then processed by upper-layer transports as defined
   in [RFC4443].

   For IP packets encapsulated in an outer IPv6 header, ICMP error
   handling is as defined in [RFC2473].

5.5.  Load Balancing and ECMP

   For any interdomain packet, the SR source node MUST impose a flow
   label computed based on the inner packet.  The computation of the
   flow label is as recommended in [RFC6438] for the sending Tunnel End
   Point.

   For any intradomain packet, the SR source node SHOULD impose a flow
   label computed as described in [RFC6437] to assist ECMP load
   balancing at transit nodes incapable of computing a 5-tuple beyond
   the SRH.

   At any transit node within an SR domain, the flow label MUST be used
   as defined in [RFC6438] to calculate the ECMP hash toward the
   destination address.  If a flow label is not used, the transit node
   would likely hash all packets between a pair of SR Edge nodes to the
   same link.

   At an SR segment endpoint node, the flow label MUST be used as
   defined in [RFC6438] to calculate any ECMP hash used to forward the
   processed packet to the next segment.

5.6.  Other Deployments

   Other deployment models and their implications on security, MTU,
   HMAC, ICMP error processing, and interaction with other extension
   headers are outside the scope of this document.

6.  Illustrations

   This section provides illustrations of SRv6 packet processing at SR
   source, transit, and SR segment endpoint nodes.

6.1.  Abstract Representation of an SRH

   For a node k, its IPv6 address is represented as Ak, and its SRv6 SID
   is represented as Sk.

   IPv6 headers are represented as the tuple of (source,destination).
   For example, a packet with source address A1 and destination address
   A2 is represented as (A1,A2).  The payload of the packet is omitted.

   An SR Policy is a list of segments.  A list of segments 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.

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

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

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

   *  Note the difference between the <> and () symbols.  <S1,S2,S3>
      represents a SID list where the leftmost segment is the first
      segment.  In contrast, (S3,S2,S1; SL) represents the same SID list
      but encoded in the SRH Segment List format where the leftmost
      segment is the last segment.  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 detailed behavior, the
      (S3,S2,S1; SL) notation is more convenient.

   At its SR Policy headend, the Segment List <S1,S2,S3> results in SRH
   (S3,S2,S1; SL=2) represented fully as:

       Segments Left=2
       Last Entry=2
       Flags=0
       Tag=0
       Segment List[0]=S3
       Segment List[1]=S2
       Segment List[2]=S1

6.2.  Example Topology

   The following topology is used in examples below:

           + * * * * * * * * * * * * * * * * * * * * +

           *         [8]                [9]          *
                      |                  |
           *          |                  |           *
   [1]----[3]--------[5]----------------[6]---------[4]---[2]
           *          |                  |           *
                      |                  |
           *          |                  |           *
                      +--------[7]-------+
           *                                         *

           + * * * * * * *  SR domain  * * * * * * * +

                                  Figure 1

   *  3 and 4 are SR domain edge routers

   *  5, 6, and 7 are all SR domain routers

   *  8 and 9 are hosts within the SR domain

   *  1 and 2 are hosts outside the SR domain

   *  The SR domain implements ingress filtering as per Section 5.1 and
      no external packet can enter the domain with a destination address
      equal to a segment of the domain.

6.3.  SR Source Node

6.3.1.  Intra-SR-Domain Packet

   When host 8 sends a packet to host 9 via an SR Policy <S7,A9> the
   packet is

   P1: (A8,S7)(A9,S7; SL=1)

6.3.1.1.  Reduced Variant

   When host 8 sends a packet to host 9 via an SR Policy <S7,A9> and it
   wants to use a reduced SRH, the packet is

   P2: (A8,S7)(A9; SL=1)

6.3.2.  Inter-SR-Domain Packet -- Transit

   When host 1 sends a packet to host 2, the packet is

   P3: (A1,A2)

   The SR domain ingress router 3 receives P3 and steers it to SR domain
   egress router 4 via an SR Policy <S7,S4>.  Router 3 encapsulates the
   received packet P3 in an outer header with an SRH.  The packet is

   P4: (A3,S7)(S4,S7; SL=1)(A1,A2)

   If the SR Policy contains only one segment (the egress router 4), the
   ingress router 3 encapsulates P3 into an outer header (A3,S4) without
   SRH.  The packet is

   P5: (A3,S4)(A1,A2)

6.3.2.1.  Reduced Variant

   The SR domain ingress router 3 receives P3 and steers it to SR domain
   egress router 4 via an SR Policy <S7,S4>.  If router 3 wants to use a
   reduced SRH, it encapsulates the received packet P3 in an outer
   header with a reduced SRH.  The packet is

   P6: (A3,S7)(S4; SL=1)(A1,A2)

6.3.3.  Inter-SR-Domain Packet -- Internal to External

   When host 8 sends a packet to host 1, the packet is encapsulated for
   the portion of its journey within the SR domain.  From 8 to 3 the
   packet is

   P7: (A8,S3)(A8,A1)

   In the opposite direction, the packet generated from 1 to 8 is

   P8: (A1,A8)

   At node 3, P8 is encapsulated for the portion of its journey within
   the SR domain, with the outer header destined to segment S8.  This
   results in

   P9: (A3,S8)(A1,A8)

   At node 8, the outer IPv6 header is removed by S8 processing, then
   processed again when received by A8.

6.4.  Transit Node

   Node 5 acts as transit node for packet P1 and sends packet

   P1: (A8,S7)(A9,S7;SL=1)

   on the interface toward node 7.

6.5.  SR Segment Endpoint Node

   Node 7 receives packet P1 and, using the logic in Section 4.3.1,
   sends packet

   P7: (A8,A9)(A9,S7; SL=0)

   on the interface toward router 6.

6.6.  Delegation of Function with HMAC Verification

   This section describes how a function may be delegated within the SR
   domain.  In the following sections, consider a host 8 connected to a
   top of rack 5.

6.6.1.  SID List Verification

   An operator may prefer to apply the SRH at source 8, while 5 verifies
   that the SID list is valid.

   For illustration purposes, an SDN controller provides 8 an SRH
   terminating at node 9, with Segment List <S5,S7,S6,A9>, and HMAC TLV
   computed for the SRH.  The HMAC key ID and key associated with the
   HMAC TLV is shared with 5.  Node 8 does not know the key.  Node 5 is
   configured with an IACL applied to the interface connected to 8,
   requiring HMAC verification for any packet destined to S/s.

   Node 8 originates packets with the received SRH, including HMAC TLV.

   P15: (A8,S5)(A9,S6,S7,S5;SL=3;HMAC)

   Node 5 receives and verifies the HMAC for the SRH, then forwards the
   packet to the next segment

   P16: (A8,S7)(A9,S6,S7,S5;SL=2;HMAC)

   Node 6 receives

   P17: (A8,S6)(A9,S6,S7,S5;SL=1;HMAC)

   Node 9 receives

   P18: (A8,A9)(A9,S6,S7,S5;SL=0;HMAC)

   This use of an HMAC is particularly valuable within an enterprise-
   based SR domain [SRN].

7.  Security Considerations

   This section reviews security considerations related to the SRH,
   given the SRH processing and deployment models discussed in this
   document.

   As described in Section 5, it is necessary to filter packets' ingress
   to the SR domain, destined to SIDs within the SR domain (i.e.,
   bearing a SID in the destination address).  This ingress filtering is
   via an IACL at SR domain ingress border nodes.  Additional protection
   is applied via an IACL at each SR Segment Endpoint node, filtering
   packets not from within the SR domain, destined to SIDs in the SR
   domain.  ACLs are easily supported for small numbers of seldom
   changing prefixes, making summarization important.

   Additionally, ingress filtering of IPv6 source addresses as
   recommended in BCP 38 [RFC2827] SHOULD be used.

7.1.  SR Attacks

   An SR domain implements distributed and per-node protection as
   described in Section 5.1.  Additionally, domains deny traffic with
   spoofed addresses by implementing the recommendations in BCP 84
   [RFC3704].

   Full implementation of the recommended protection blocks the attacks
   documented in [RFC5095] from outside the SR domain, including
   bypassing filtering devices, reaching otherwise-unreachable Internet
   systems, network topology discovery, bandwidth exhaustion, and
   defeating anycast.

   Failure to implement distributed and per-node protection allows
   attackers to bypass filtering devices and exposes the SR domain to
   these attacks.

   Compromised nodes within the SR domain may mount the attacks listed
   above along with other known attacks on IP networks (e.g., DoS/DDoS,
   topology discovery, man-in-the-middle, traffic interception/
   siphoning).

7.2.  Service Theft

   Service theft is defined as the use of a service offered by the SR
   domain by a node not authorized to use the service.

   Service theft is not a concern within the SR domain, as all SR source
   nodes and SR segment endpoint nodes within the domain are able to
   utilize the services of the domain.  If a node outside the SR domain
   learns of segments or a topological service within the SR domain,
   IACL filtering denies access to those segments.

7.3.  Topology Disclosure

   The SRH is unencrypted and may contain SIDs of some intermediate SR
   nodes in the path towards the destination within the SR domain.  If
   packets can be snooped within the SR domain, the SRH may reveal
   topology, traffic flows, and service usage.

   This is applicable within an SR domain, but the disclosure is less
   relevant as an attacker has other means of learning topology, flows,
   and service usage.

7.4.  ICMP Generation

   The generation of ICMPv6 error messages may be used to attempt
   denial-of-service attacks by sending an error-causing destination
   address or SRH in back-to-back packets.  An implementation that
   correctly follows Section 2.4 of [RFC4443] would be protected by the
   ICMPv6 rate-limiting mechanism.

7.5.  Applicability of AH

   The SR domain is a trusted domain, as defined in [RFC8402], Sections
   2 and 8.2.  The SR source is trusted to add an SRH (optionally
   verified as having been generated by a trusted source via the HMAC
   TLV in this document), and segments advertised within the domain are
   trusted to be accurate and advertised by trusted sources via a secure
   control plane.  As such, the SR domain does not rely on the
   Authentication Header (AH) as defined in [RFC4302] to secure the SRH.

   The use of SRH with AH by an SR source node and its processing at an
   SR segment endpoint node are not defined in this document.  Future
   documents may define use of SRH with AH and its processing.

8.  IANA Considerations

   This document makes the following registrations in the "Internet
   Protocol Version 6 (IPv6) Parameters" "Routing Types" subregistry
   maintained by IANA:

         +-------+------------------------------+---------------+
         | Value | Description                  | Reference     |
         +=======+==============================+===============+
         | 4     | Segment Routing Header (SRH) | This document |
         +-------+------------------------------+---------------+

                        Table 1: SRH Registration

   This document makes the following registrations in the "Type 4 -
   Parameter Problem" message of the "Internet Control Message Protocol
   version 6 (ICMPv6) Parameters" registry maintained by IANA:

                  +------+-----------------------------+
                  | Code | Name                        |
                  +======+=============================+
                  | 4    | SR Upper-layer Header Error |
                  +------+-----------------------------+

                      Table 2: SR Upper-layer Header
                            Error Registration

8.1.  Segment Routing Header Flags Registry

   This document describes a new IANA-managed registry to identify SRH
   Flags Bits.  The registration procedure is "IETF Review" [RFC8126].
   The registry name is "Segment Routing Header Flags".  Flags are 8
   bits.

8.2.  Segment Routing Header TLVs Registry

   This document describes a new IANA-managed registry to identify SRH
   TLVs.  The registration procedure is "IETF Review".  The registry
   name is "Segment Routing Header TLVs".  A TLV is identified through
   an unsigned 8-bit codepoint value, with assigned values 0-127 for
   TLVs that do not change en route and 128-255 for TLVs that may change
   en route.  The following codepoints are defined in this document:

          +---------+--------------------------+---------------+
          | Value   | Description              | Reference     |
          +=========+==========================+===============+
          | 0       | Pad1 TLV                 | This document |
          +---------+--------------------------+---------------+
          | 1       | Reserved                 | This document |
          +---------+--------------------------+---------------+
          | 2       | Reserved                 | This document |
          +---------+--------------------------+---------------+
          | 3       | Reserved                 | This document |
          +---------+--------------------------+---------------+
          | 4       | PadN TLV                 | This document |
          +---------+--------------------------+---------------+
          | 5       | HMAC TLV                 | This document |
          +---------+--------------------------+---------------+
          | 6       | Reserved                 | This document |
          +---------+--------------------------+---------------+
          | 124-126 | Experimentation and Test | This document |
          +---------+--------------------------+---------------+
          | 127     | Reserved                 | This document |
          +---------+--------------------------+---------------+
          | 252-254 | Experimentation and Test | This document |
          +---------+--------------------------+---------------+
          | 255     | Reserved                 | This document |
          +---------+--------------------------+---------------+

              Table 3: Segment Routing Header TLVs Registry

   Values 1, 2, 3, and 6 were defined in draft versions of this
   specification and are Reserved for backwards compatibility with early
   implementations and should not be reassigned.  Values 127 and 255 are
   Reserved to allow for expansion of the Type field in future
   specifications, if needed.

9.  References

9.1.  Normative References

   [FIPS180-4]
              National Institute of Standards and Technology (NIST),
              "Secure Hash Standard (SHS)", FIPS PUB 180-4, DOI 10.6028/
              NIST.FIPS.180-4, August 2015,
              <http://csrc.nist.gov/publications/fips/fips180-4/fips-
              180-4.pdf>.

   [IANA-SRHTLV]
              IANA, "Segment Routing Header TLVs",
              <https://www.iana.org/assignments/ipv6-parameters/>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/info/rfc2104>.

   [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>.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <https://www.rfc-editor.org/info/rfc2827>.

   [RFC3704]  Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", BCP 84, RFC 3704, DOI 10.17487/RFC3704, March
              2004, <https://www.rfc-editor.org/info/rfc3704>.

   [RFC4107]  Bellovin, S. and R. Housley, "Guidelines for Cryptographic
              Key Management", BCP 107, RFC 4107, DOI 10.17487/RFC4107,
              June 2005, <https://www.rfc-editor.org/info/rfc4107>.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,
              <https://www.rfc-editor.org/info/rfc4302>.

   [RFC5095]  Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
              of Type 0 Routing Headers in IPv6", RFC 5095,
              DOI 10.17487/RFC5095, December 2007,
              <https://www.rfc-editor.org/info/rfc5095>.

   [RFC6407]  Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
              of Interpretation", RFC 6407, DOI 10.17487/RFC6407,
              October 2011, <https://www.rfc-editor.org/info/rfc6407>.

   [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>.

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
              <https://www.rfc-editor.org/info/rfc6438>.

   [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>.

9.2.  Informative References

   [INTAREA-TUNNELS]
              Touch, J. and M. Townsley, "IP Tunnels in the Internet
              Architecture", Work in Progress, Internet-Draft, draft-
              ietf-intarea-tunnels-10, 12 September 2019,
              <https://tools.ietf.org/html/draft-ietf-intarea-tunnels-
              10>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
              DOI 10.17487/RFC5308, October 2008,
              <https://www.rfc-editor.org/info/rfc5308>.

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

   [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>.

   [SRN]      Lebrun, D., Jadin, M., Clad, F., Filsfils, C., and O.
              Bonaventure, "Software Resolved Networks: Rethinking
              Enterprise Networks with IPv6 Segment Routing", 2018,
              <https://inl.info.ucl.ac.be/system/files/
              sosr18-final15-embedfonts.pdf>.

Acknowledgements

   The authors would like to thank Ole Troan, Bob Hinden, Ron Bonica,
   Fred Baker, Brian Carpenter, Alexandru Petrescu, Punit Kumar Jaiswal,
   David Lebrun, Benjamin Kaduk, Frank Xialiang, Mirja Kühlewind, Roman
   Danyliw, Joe Touch, and Magnus Westerlund for their comments to this
   document.

Contributors

   Kamran Raza, Zafar Ali, Brian Field, Daniel Bernier, Ida Leung, Jen
   Linkova, Ebben Aries, Tomoya Kosugi, Éric Vyncke, David Lebrun, Dirk
   Steinberg, Robert Raszuk, Dave Barach, John Brzozowski, Pierre
   Francois, Nagendra Kumar, Mark Townsley, Christian Martin, Roberta
   Maglione, James Connolly, and Aloys Augustin contributed to the
   content of this document.

Authors' Addresses

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

   Email: cfilsfil@cisco.com


   Darren Dukes (editor)
   Cisco Systems, Inc.
   Ottawa
   Canada

   Email: ddukes@cisco.com


   Stefano Previdi
   Huawei
   Italy

   Email: stefano@previdi.net


   John Leddy
   Individual
   United States of America

   Email: john@leddy.net


   Satoru Matsushima
   SoftBank

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


   Daniel Voyer
   Bell Canada

   Email: daniel.voyer@bell.ca