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The IPv6 Compact Routing Header (CRH)
draft-ietf-6man-comp-rtg-hdr-06

Document Type Active Internet-Draft (6man WG)
Authors Ron Bonica , Yuji Kamite , Andrew Alston , Daniam Henriques , Luay Jalil
Last updated 2024-05-03
Replaces draft-bonica-6man-comp-rtg-hdr
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IANA IANA review state Version Changed - Review Needed
draft-ietf-6man-comp-rtg-hdr-06
6man                                                           R. Bonica
Internet-Draft                                          Juniper Networks
Intended status: Experimental                                  Y. Kamite
Expires: 4 November 2024                  NTT Communications Corporation
                                                               A. Alston
                                                            D. Henriques
                                                          Liquid Telecom
                                                                L. Jalil
                                                                 Verizon
                                                              3 May 2024

                 The IPv6 Compact Routing Header (CRH)
                    draft-ietf-6man-comp-rtg-hdr-06

Abstract

   This document describes an experiment in which two new IPv6 Routing
   headers are implemented and deployed.  Collectively, they are called
   the Compact Routing Headers (CRH).  Individually, they are called
   CRH-16 and CRH-32.

   One purpose of this experiment is to demonstrate that the CRH can be
   implemented and deployed in a production network.  Another purpose is
   to demonstrate that the security considerations, described in this
   document, can be addressed with access control lists.  Finally, this
   document encourages replication of the experiment.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 4 November 2024.

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Copyright Notice

   Copyright (c) 2024 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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  The Compact Routing Headers (CRH) . . . . . . . . . . . . . .   3
   4.  The CRH Forwarding Information Base (CRH-FIB) . . . . . . . .   5
   5.  Processing Rules  . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Computing Minimum CRH Length  . . . . . . . . . . . . . .   7
   6.  Mutability  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Destination Address Transparency  . . . . . . . . . . . . . .   8
   8.  Applications And SIDs . . . . . . . . . . . . . . . . . . . .   8
   9.  Management Considerations . . . . . . . . . . . . . . . . . .   8
   10. ICMPv6 Considerations . . . . . . . . . . . . . . . . . . . .   8
   11. Textual Representation  . . . . . . . . . . . . . . . . . . .   9
   12. Security Considerations . . . . . . . . . . . . . . . . . . .   9
   13. Implementation and Deployment Status  . . . . . . . . . . . .  10
   14. Experimental Results  . . . . . . . . . . . . . . . . . . . .  11
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   16. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   17. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  12
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     18.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     18.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  CRH Processing Examples  . . . . . . . . . . . . . .  14
     A.1.  The SID List Contains One Entry For Each Segment In The
           Path  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     A.2.  The SID List Omits The First Entry In The Path  . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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1.  Introduction

   IPv6 [RFC8200] source nodes use Routing headers to specify the path
   that a packet takes to its destination.  The IETF has defined several
   Routing header types [IANA-RH].  This document defines two new
   Routing header types.  Collectively, they are called the Compact
   Routing Headers (CRH).  Individually, they are called CRH-16 and CRH-
   32.

   The CRH allows IPv6 source nodes to specify the path that a packet
   takes to its destination.  The CRH can be encoded in relatively few
   bytes.  The following are reasons for encoding the CRH in as few
   bytes as possible:

   *  Many ASIC-based forwarders copy headers from buffer memory to on-
      chip memory.  As header sizes increase, so does the cost of this
      copy.

   *  Because Path MTU Discovery (PMTUD) [RFC8201] is not entirely
      reliable, many IPv6 hosts refrain from sending packets larger than
      the IPv6 minimum link MTU (i.e., 1280 bytes).  When packets are
      small, the overhead imposed by large Routing Headers is excessive.

   This document describes an experiment whose purposes are:

   *  To demonstrate that the CRH can be implemented and deployed.

   *  To demonstrate that the security considerations, described in this
      document, can be addressed with access control lists.

   *  To encourage replication of the experiment.

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.

3.  The Compact Routing Headers (CRH)

   Both CRH versions (i.e., CRH-16 and CRH-32) contain the following
   fields:

   *  Next Header - Defined in [RFC8200].

   *  Hdr Ext Len - Defined in [RFC8200].

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   *  Routing Type - Defined in [RFC8200].  (CRH-16 value is 5.  CRH-32
      value is 6).

   *  Segments Left - Defined in [RFC8200].

   *  Type-specific Data - Described in [RFC8200].

   In the CRH, the Type-specific data field contains a list of Segment
   Identifiers (SIDs).  Each SID identifies an entry in the CRH
   Forwarding Information Base (CRH-FIB) (Section 4).  Each CRH-FIB
   entry identifies an interface on the path that the packet takes to
   its destination.

   SIDs are listed in reverse order.  So, the first SID in the list
   represents the final interface in the path.  Because segments are
   listed in reverse order, the Segments Left field can be used as an
   index into the SID list.  In this document, the "current SID" is the
   SID list entry referenced by the Segments Left field.

   The first segment in the path can be omitted from the list.  See
   Appendix A for an example.

   In the CRH-16 (Figure 1), each SID is encoded in 16-bits.  In the
   CRH-32 (Figure 2), each SID is encoded in 32-bits.

   In all cases, the CRH MUST end on a 64-bit boundary.  So, the Type-
   specific data field MUST be padded with zeros if the CRH would
   otherwise not end on a 64-bit boundary.

      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 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             SID[0]            |          SID[1]               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
     |                          .........
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                              Figure 1: CRH-16

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      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 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +                             SID[0]                            +
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +                             SID[1]                            +
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     //                                                              //
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     +                             SID[n]                            +
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                              Figure 2: CRH-32

4.  The CRH Forwarding Information Base (CRH-FIB)

   Each SID identifies a CRH-FIB entry.

   Each CRH-FIB entry contains:

   *  An IPv6 address.

   *  A topological function.

   *  Arguments for the topological function.  (Optional).

   The topological function specifies how the processing node forwards
   the packet to the interface identified by the IPv6 address.  The
   following are examples:

   *  Forward the packet through the least-cost path to the interface
      identified by the IPv6 address (i.e., loose source routing).

   *  Forward the packet through a specified interface to the interface
      identified by the IPv6 address (i.e.,strict source routing)

   Some topological functions require parameters.  For example, a
   topological function might require a parameter that identifies the
   interface through which the packet is forwarded.

   The CRH-FIB can be populated:

   *  By an operator, using a Command Line Interface (CLI).

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   *  By a controller, using the Path Computation Element (PCE)
      Communication Protocol (PCEP) [RFC5440] or the Network
      Configuration Protocol (NETCONF) [RFC6241].

   *  By a distributed routing protocol [ISO10589-Second-Edition],
      [RFC5340], [RFC4271].

   The above-mentoned mechanisms are not defined here and are beyond the
   scope of this document

5.  Processing Rules

   The following rules describe CRH processing:

   *  If Segments Left equals 0, skip over the CRH and process the next
      header in the packet.  The IPv6 address in the IPv6 Header's
      Destination Address field is that of the ultimate recipient.

   *  If Hdr Ext Len indicates that the CRH is larger than the
      implementation can process, discard the packet and send an ICMPv6
      [RFC4443] Parameter Problem, Code 0, message to the Source
      Address, pointing to the Hdr Ext Len field.

   *  Compute L, the minimum CRH length ( Section 5.1).

   *  If L is greater than Hdr Ext Len, discard the packet and send an
      ICMPv6 Parameter Problem, Code 0, message to the Source Address,
      pointing to the Segments Left field.

   *  Decrement Segments Left.

   *  Search for the current SID in the CRH-FIB.  In this document, the
      "current SID" is the SID list entry referenced by the Segments
      Left field.

   *  If the search does not return a CRH-FIB entry, discard the packet
      and send an ICMPv6 Parameter Problem, Code 0, message to the
      Source Address, pointing to the current SID.

   *  If Segments Left is greater than 0 and the CRH-FIB entry contains
      a multicast address, discard the packet and send an ICMPv6
      Parameter Problem, Code 0, message to the Source Address, pointing
      to the current SID.

   *  Copy the IPv6 address from the CRH-FIB entry to the Destination
      Address field in the IPv6 header.

   *  Decrement the IPv6 Hop Limit.

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   *  Submit the packet, its topological function and its parameters to
      the IPv6 module.  See NOTE.

   NOTE: By default, the IPv6 module determines the next-hop and
   forwards the packet.  However, the topological function may elicit
   another behavior.  For example, the IPv6 module may forward the
   packet through a specified interface.

5.1.  Computing Minimum CRH Length

   The algorithm described in this section accepts the following CRH
   fields as its input parameters:

   *  Routing Type (i.e., CRH-16 or CRH-32).

   *  Segments Left.

   It yields L, the minimum CRH length.  The minimum CRH length is
   measured in 8-octet units, not including the first 8 octets.

   <CODE BEGINS>
   switch(Routing Type) {
       case CRH-16:
           if (Segments Left <= 2)
               return(0)
           sidsBeyondFirstWord = Segments Left - 2;
           sidPerWord = 4;
       case CRH-32:
           if (Segments Left <= 1)
               return(0)
           sidsBeyondFirstWord = Segments Left - 1;
           sidsPerWord = 2;
       case default:
           return(0xFF);
       }

   words = sidsBeyondFirstWord div sidsPerWord;
   if (sidsBeyondFirstWord mod sidsPerWord)
       words++;

   return(words)
   <CODE ENDS>

6.  Mutability

   In the CRH, the Segments Left field is mutable.  All remaining fields
   are immutable.

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7.  Destination Address Transparency

   When a packet containing the CRH header leaves its source, it does
   not include its final destination address.  The final destination
   address is not added to the packet until the final SID is resolved.

   While destination address transparency enhances privacy, it prevents
   intermediate nodes from verifying transport layer checksums.

8.  Applications And SIDs

   A CRH contains one or more SIDs.  Each SID is processed by exactly
   one node.

   Therefore, a SID is not required to have domain-wide significance.
   Applications can:

   *  Allocate SIDs so that they have domain-wide significance.

   *  Allocate SIDs so that they have node-local significance.

9.  Management Considerations

   PING and TRACEROUTE [RFC2151] both operate correctly in the presence
   of the CRH.  TCPDUMP and Wireshark have been extended to support the
   CRH.

   PING and TRACEROUTE report 16-bit SIDS for CRH-16, and 32-bit SIDS
   for CRH-32.  It is recommended that the experimental versions of PING
   use the text representations described herein.

10.  ICMPv6 Considerations

   A node can emit ICMPv6 messages when processing a packet that
   contains the CRH.  The following are ICMPv6 considerations:

   *  ICMPv6 messages are subject to rate limits.

   *  ICMPv6 message delivery is not reliable.

   *  ICMPv6 messages are easily forged.

   *  Most ICMPv6 implementations process all ICMPv6 Parameter Problem
      messages identically, regardless of the pointer value.

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11.  Textual Representation

   A 16-bit SID can be represented by four hexadecimal digits.  Leading
   zeros SHOULD be omitted.  However, the all-zeros SID MUST be
   represented by a single 0.  The following are examples:

   *  beef

   *  eef

   *  0

   A 16-bit SID also can be represented in dotted-decimal notation.  The
   following are examples:

   *  192.0

   *  192.51

   A 32-bit SID can be represented by four hexadecimal digits, a colon
   (:), and another four hexadecimal digits.  Leading zeros MUST be
   omitted.  The following are examples:

   *  dead:beef

   *  ead:eef

   *  :beef

   *  beef:

   *  :

   A 32-bit SID can also be represent in dotted-decimal notation.  The
   following are examples:

   *  192.0.2.1

   *  192.0.2.2

   *  192.0.2.3

12.  Security Considerations

   In this document, one node trusts another only if both nodes are
   operated by the same party.

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   A node can encounter security vulnerabilities by indiscriminately
   processing packets that contain Routing Headers [RFC5095].
   Therefore, nodes MUST discard packets containing the CRH when both of
   the following conditions are true:

   *  The Source Address does not identify an interface on a trusted
      node.

   *  The Destination Address identifies an interface on the local node.

   The above-state rule does not protect the node from attack packets
   that contain a forged (i.e., spoofed) Source Address.  In order to
   mitigate this risk, nodes MAY also discard packets containing the CRH
   when all of the following conditions are true:

   *  The Source Address identifies an interface on a trusted node.

   *  The Destination Address identifies an interface on the local node.

   *  The packet does not pass an Enhanced Feasible-Path Unicast Reverse
      Path Forwarding (RPF) [RFC8704],

   The RPF check eliminates some, but not all packets with forged source
   addresses.  Therefore, a network operator that deploys CRH MUST
   implement Access Control Lists (ACL) on each of its edge nodes.  The
   ACL discards packets whose source address identifies an interface on
   a trusted node.

   The CRH is compatible with end-to-end IPv6 Authentication Header (AH)
   [RFC4302] processing.  This is becasue the source node MUST calculate
   the Integrity Check Value (ICV) over the packet as it arrives at the
   destination node.  The CRH is not compatibile with AH processing at
   intermediate nodes.

13.  Implementation and Deployment Status

   Juniper Networks has produced experimental implementations of the CRH
   on the MX-series (ASIC-based) router

   Liquid Telecom has produced experimental implementations of the CRH
   on software based routers.

   The CRH has carried non-production traffic in CERNET and Liquid
   Telecom.

   Interoperability among these implementations has not yet been
   demonstrated.

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14.  Experimental Results

   Parties participating in this experiment should publish experimental
   results within one year of the publication of this document.
   Experimental results should address the following:

   *  Effort required to deploy

      -  Was deployment incremental or network-wide?

      -  Was there a need to synchronize configurations at each node or
         could nodes be configured independently

      -  Did the deployment require hardware upgrade?

      -  Did SIDs have domain-wide or node-local significance?

   *  Effort required to secure

   *  Performance impact

   *  Effectiveness of risk mitigation with ACLs

   *  Cost of risk mitigation with ACLs

   *  Mechanism used to populate the FIB

   *  Scale of deployment

   *  Interoperability

      -  Did you deploy two inter-operable implementations?

      -  Did you experience interoperability problems?

      -  Did implementations generally implement the same topological
         functions with identical arguments?

      -  Were topological function semantics identical on each
         implementation?

   *  Effectiveness and sufficiency of OAM mechanism

      -  Did PING work?

      -  Did TRACEROUTE work?

      -  Did Wireshark work?

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      -  Did TCPDUMP work?

15.  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     |
            +=======+==============================+===============+
            | 5     | CRH-16                       | This document |
            +-------+------------------------------+---------------+
            | 6     | CRH-32                       | This document |
            +-------+------------------------------+---------------+

16.  Acknowledgements

   Thanks to Dr. Vanessa Ameen, Dale Carder, Brian Carpenter, Adrian
   Farrel, Fernando Gont, Naveen Kottapalli, Joel Halpern, Mark Smith,
   Reji Thomas, Tony Li, Xing Li, Gerald Schmidt, Nancy Shaw, Ketan
   Talaulikar, and Chandra Venkatraman for their contributions to this
   document.

17.  Contributors

      Gang Chen

      Baidu

      No.10 Xibeiwang East Road Haidian District

      Beijing 100193 P.R.  China

      Email: phdgang@gmail.com

      Yifeng Zhou

      ByteDance

      Building 1, AVIC Plaza, 43 N 3rd Ring W Rd Haidian District

      Beijing 100000 P.R.  China

      Email: yifeng.zhou@bytedance.com

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      Gyan Mishra

      Verizon

      Silver Spring, Maryland, USA

      Email: hayabusagsm@gmail.com

18.  References

18.1.  Normative References

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

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

   [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>.

   [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>.

   [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>.

18.2.  Informative References

   [IANA-RH]  IANA, "Routing Headers",
              <https://www.iana.org/assignments/ipv6-parameters/
              ipv6-parameters.xhtml#ipv6-parameters-3>.

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   [ISO10589-Second-Edition]
              International Organization for Standardization,
              ""Intermediate system to Intermediate system intra-domain
              routeing information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473)", ISO/IEC
              10589:2002, Second Edition,", November 2001.

   [RFC2151]  Kessler, G. and S. Shepard, "A Primer On Internet and TCP/
              IP Tools and Utilities", FYI 30, RFC 2151,
              DOI 10.17487/RFC2151, June 1997,
              <https://www.rfc-editor.org/info/rfc2151>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [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>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC8201]  McCann, J., Deering, S., Mogul, J., and R. Hinden, Ed.,
              "Path MTU Discovery for IP version 6", STD 87, RFC 8201,
              DOI 10.17487/RFC8201, July 2017,
              <https://www.rfc-editor.org/info/rfc8201>.

   [RFC8704]  Sriram, K., Montgomery, D., and J. Haas, "Enhanced
              Feasible-Path Unicast Reverse Path Forwarding", BCP 84,
              RFC 8704, DOI 10.17487/RFC8704, February 2020,
              <https://www.rfc-editor.org/info/rfc8704>.

Appendix A.  CRH Processing Examples

   This appendix demonstrates CRH processing in the following scenarios:

   *  The SID list contains one entry for each segment in the path
      (Appendix A.1).

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   *  The SID list omits the first entry in the path (Appendix A.2).

    -----------                 -----------                 -----------
   |Node: S    |               |Node: I1   |               |Node: I2   |
   |Loopback:  |---------------|Loopback:  |---------------|Loopback:  |
   |2001:db8::a|               |2001:db8::1|               |2001:db8::2|
    -----------                 -----------                 -----------
         |                                                       |
         |                      -----------                      |
         |                     |Node: D    |                     |
          ---------------------|Loopback:  |---------------------
                               |2001:db8::b|
                                -----------

                        Figure 3: Reference Topology

   Figure 3 provides a reference topology that is used in all examples.

                +=====+==============+===================+
                | SID | IPv6 Address | Forwarding Method |
                +=====+==============+===================+
                | 2   | 2001:db8::2  | Least-cost path   |
                +-----+--------------+-------------------+
                | 11  | 2001:db8::b  | Least-cost path   |
                +-----+--------------+-------------------+

                            Table 1: Node SIDs

   Table 1 describes two entries that appear in each node's CRH-FIB.

A.1.  The SID List Contains One Entry For Each Segment In The Path

   In this example, Node S sends a packet to Node D, via I2.  In this
   example, I2 appears in the CRH segment list.

        +=====================================+===================+
        | As the packet travels from S to I2: |                   |
        +=====================================+===================+
        | Source Address = 2001:db8::a        | Segments Left = 1 |
        +-------------------------------------+-------------------+
        | Destination Address = 2001:db8::2   | SID[0] = 11       |
        +-------------------------------------+-------------------+
        |                                     | SID[1] = 2        |
        +-------------------------------------+-------------------+

                                  Table 2

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        +=====================================+===================+
        | As the packet travels from I2 to D: |                   |
        +=====================================+===================+
        | Source Address = 2001:db8::a        | Segments Left = 0 |
        +-------------------------------------+-------------------+
        | Destination Address = 2001:db8::b   | SID[0] = 11       |
        +-------------------------------------+-------------------+
        |                                     | SID[1] = 2        |
        +-------------------------------------+-------------------+

                                  Table 3

A.2.  The SID List Omits The First Entry In The Path

   In this example, Node S sends a packet to Node D, via I2.  In this
   example, I2 does not appear in the CRH segment list.

        +=====================================+===================+
        | As the packet travels from S to I2: |                   |
        +=====================================+===================+
        | Source Address = 2001:db8::a        | Segments Left = 1 |
        +-------------------------------------+-------------------+
        | Destination Address = 2001:db8::2   | SID[0] = 11       |
        +-------------------------------------+-------------------+

                                  Table 4

        +=====================================+===================+
        | As the packet travels from I2 to D: |                   |
        +=====================================+===================+
        | Source Address = 2001:db8::a        | Segments Left = 0 |
        +-------------------------------------+-------------------+
        | Destination Address = 2001:db8::b   | SID[0] = 11       |
        +-------------------------------------+-------------------+

                                  Table 5

Authors' Addresses

   Ron Bonica
   Juniper Networks
   2251 Corporate Park Drive
   Herndon, Virginia 20171
   United States of America
   Email: rbonica@juniper.net

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   Yuji Kamite
   NTT Communications Corporation
   3-4-1 Shibaura, Minato-ku,
   108-8118
   Japan
   Email: y.kamite@ntt.com

   Andrew Alston
   Liquid Telecom
   Nairobi
   Kenya
   Email: Andrew.Alston@liquidtelecom.com

   Daniam Henriques
   Liquid Telecom
   Johannesburg
   South Africa
   Email: daniam.henriques@liquidtelecom.com

   Luay Jalil
   Verizon
   Richardson, Texas
   United States of America
   Email: luay.jalil@one.verizon.com

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