



Internet Engineering Task Force (IETF)                       S. Krishnan
Request for Comments: 9602                                         Cisco
Category: Informational                                     October 2024
ISSN: 2070-1721


    Segment Routing over IPv6 (SRv6) Segment Identifiers in the IPv6
                        Addressing Architecture

Abstract

   Segment Routing over IPv6 (SRv6) uses IPv6 as the underlying data
   plane.  Thus, Segment Identifiers (SIDs) used by SRv6 can resemble
   IPv6 addresses and behave like them while exhibiting slightly
   different behaviors in some situations.  This document explores the
   characteristics of SRv6 SIDs and focuses on the relationship of SRv6
   SIDs to the IPv6 Addressing Architecture.  This document allocates
   and makes a dedicated prefix available for SRv6 SIDs.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see 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/rfc9602.

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Table of Contents

   1.  Introduction
   2.  Terminology
   3.  SRv6 SIDs and the IPv6 Addressing Architecture
   4.  Special Considerations for Compressed SIDs
   5.  Allocation of a Prefix for SIDs
   6.  IANA Considerations
   7.  Security Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Acknowledgments
   Author's Address

1.  Introduction

   Segment Routing over IPv6 (SRv6) [RFC8754] uses IPv6 as the
   underlying data plane.  In SRv6, SR source nodes initiate packets
   with a Segment Identifier (SID) in the Destination Address of the
   IPv6 header, and SR segment endpoint nodes process a local segment
   present in the Destination Address of an IPv6 header.  Thus, SIDs in
   SRv6 can, and do, appear in the Destination Address of IPv6 datagrams
   by design.  This document explores the characteristics of SRv6 SIDs
   and focuses on the relationship of SRv6 SIDs to the IPv6 Addressing
   Architecture [RFC4291].  This document allocates and makes a
   dedicated prefix available for SRv6 SIDs.

2.  Terminology

   The following terms are used as defined in [RFC8402].

   *  Segment Routing (SR)

   *  SR Domain

   *  Segment

   *  Segment Identifier (SID)

   *  SRv6

   *  SRv6 SID

   The following terms are used as defined in [RFC8754].

   *  Segment Routing Header (SRH)

   *  SR Source Node

   *  Transit Node

   *  SR Segment Endpoint Node

   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 SIDs and the IPv6 Addressing Architecture

   [RFC8754] defines the Segment List of the SRH as a contiguous array
   of 128-bit IPv6 addresses; further, it states that each of the
   elements in this list are SIDs.  But all of these elements are not
   necessarily made equal.  Some of these elements may represent a local
   interface as described in Section 4.3 of [RFC8754] as "A FIB entry
   that represents a local interface, not locally instantiated as an
   SRv6 SID".  It follows that not all the SIDs that appear in the SRH
   are SRv6 SIDs as defined by [RFC8402].

   As stated above, the non-SRv6-SID elements that appear in the SRH SID
   list are simply IPv6 addresses assigned to local interfaces, and they
   need to conform to [RFC4291].  So, the following discussions are
   applicable solely to SRv6 SIDs that are not assigned to local
   interfaces.

   One of the key questions to address is how these SRv6 SIDs appearing
   as IPv6 Destination Addresses are perceived and treated by "transit
   nodes" (that are not required to be capable of processing a Segment
   or the Segment Routing Header).

   Section 3.1 of [RFC8986] describes the format of an SRv6 SID as being
   composed of three parts, 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).  If L+F+A < 128,
   the ARG is followed by enough zero bits to fill the 128-bit SID.
   Such an SRv6 SID is assigned to a node within a prefix defined as a
   Locator of length L.  When an SRv6 SID occurs in the IPv6 Destination
   Address of an IPv6 header, only the longest matching prefix
   corresponding to the Locator [BCP198] is used by the transit node to
   forward the packet to the node identified by the Locator.

   It is clear that this format for SRv6 SIDs is not compliant with the
   requirements set forth in [RFC4291] for IPv6 addresses, but it is
   also clear that SRv6 SIDs are not intended for assignment onto
   interfaces on end hosts.  They look and act like other mechanisms
   that use IPv6 addresses with different formats, such as those
   described in "IPv6 Addressing of IPv4/IPv6 Translators" [RFC6052] and
   "An IPv6 Prefix for Overlay Routable Cryptographic Hash Identifiers
   Version 2 (ORCHIDv2)" [RFC7343].

   While looking at the transit nodes, it becomes apparent that these
   addresses are used purely for forwarding and not for packet delivery
   to end hosts.  Hence, the relevant specification to apply here is
   [BCP198], which requires implementations to support the use of
   variable-length prefixes in forwarding while explicitly decoupling
   IPv6 routing and forwarding from the IPv6 address/prefix semantics
   described in [RFC4291].  Please note that [BCP198] does not override
   the rules in [RFC4291]: it merely limits where their impact is
   observed.

   Furthermore, in the SRv6 specifications, all SIDs assigned within a
   given Locator prefix are located inside the node identified by
   Locator.  Therefore, there does not appear to be a conflict with
   Section 2.6.1 of [RFC4291] since subnet-router anycast addresses are
   neither required nor useful within a node.

4.  Special Considerations for Compressed SIDs

   [CSID] introduces an encoding for Compressed-SIDs (C-SIDs), and
   describes how to use a single entry in the Segment List as a
   container for multiple SIDs.  A node taking part in this mechanism
   accomplishes this by using the ARG part [RFC8986] of the Destination
   Address of the IPv6 header to derive a new Destination Address.  That
   is, the Destination Address field of the packet changes at a segment
   endpoint in a way similar to how the address changes as the result of
   processing a segment in the SRH.

   One key thing to note here is that the Locator Block at the beginning
   of the address does not get modified by the operations needed for
   supporting C-SIDs.  As we have established that the SRv6 SIDs are
   being treated simply as routing prefixes on transit nodes within the
   SR Domain, this does not constitute a modification to the IPv6 data
   plane on such transit nodes: any changes are restricted to SR-aware
   nodes.

5.  Allocation of a Prefix for SIDs

   All of the SRv6-related specifications discussed above are intended
   to be applicable to a contained SR Domain or between collaborating SR
   Domains.  Nodes either inside or outside the SR Domains that are not
   SR-aware will not perform any special behavior for SRv6 SIDs and will
   treat them solely as IPv6 routing prefixes.

   As an added factor of security, it is desirable to allocate some
   address space that explicitly signals that the addresses within that
   space cannot be expected to comply with [RFC4291].  As described in
   Section 3, there is precedent for mechanisms that use IPv6 addresses
   in a manner different from that specified in [RFC4291].  This would
   be useful in identifying and potentially filtering packets at the
   edges of the SR Domains to make it simpler for the SR Domain to fail
   closed.

   At the time of writing, global DNS [RFC9499] SHOULD NOT reference
   addresses assigned from this block.  Further specifications are
   needed to describe the conventions and guidelines for the use of this
   newly allocated address block.  The SRv6 operational community, which
   is the first intended user of this block, is requested to come up
   with such conventions and guidelines in line with their requirements.

6.  IANA Considerations

   IANA has assigned the following /16 address block for the purposes
   described in Section 5 and recorded the allocation in the "IANA IPv6
   Special-Purpose Address Registry" [SPECIAL] as follows:

   Address Block:
      5f00::/16

   Name:
      Segment Routing (SRv6) SIDs

   RFC:
      RFC 9602

   Allocation Date:
      2024-04

   Termination Date:
      N/A

   Source:
      True

   Destination:
      True

   Forwardable:
      True

   Globally Reachable:
      False

   Reserved-by-Protocol:
      False

7.  Security Considerations

   The security considerations for the use of Segment Routing [RFC8402],
   SRv6 [RFC8754], and SRv6 network programming [RFC8986] apply to the
   use of these addresses.  The use of IPv6 tunneling mechanisms
   (including SRv6) also brings up additional concerns such as those
   described in [RFC6169].  The usage of the prefix allocated by this
   document improves security by making it simpler to filter traffic at
   the edge of the SR Domains.

   In case the deployments do not use this allocated prefix, additional
   care needs to be exercised at network ingress and egress points so
   that SRv6 packets do not leak out of SR Domains and do not
   accidentally enter SR-unaware Domains.  Similarly, as stated in
   Section 5.1 of [RFC8754], the SR Domain needs to be configured to
   filter out packets entering that use the selected prefix.

8.  References

8.1.  Normative References

   [BCP198]   Best Current Practice 198,
              <https://www.rfc-editor.org/info/bcp198>.
              At the time of writing, this BCP comprises the following:

              Boucadair, M., Petrescu, A., and F. Baker, "IPv6 Prefix
              Length Recommendation for Forwarding", BCP 198, RFC 7608,
              DOI 10.17487/RFC7608, July 2015,
              <https://www.rfc-editor.org/info/rfc7608>.

   [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>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [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>.

   [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>.

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

8.2.  Informative References

   [CSID]     Cheng, W., Filsfils, C., Li, Z., Decraene, B., and F.
              Clad, "Compressed SRv6 Segment List Encoding", Work in
              Progress, Internet-Draft, draft-ietf-spring-srv6-srh-
              compression-18, 22 July 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              srv6-srh-compression-18>.

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              DOI 10.17487/RFC6052, October 2010,
              <https://www.rfc-editor.org/info/rfc6052>.

   [RFC6169]  Krishnan, S., Thaler, D., and J. Hoagland, "Security
              Concerns with IP Tunneling", RFC 6169,
              DOI 10.17487/RFC6169, April 2011,
              <https://www.rfc-editor.org/info/rfc6169>.

   [RFC7343]  Laganier, J. and F. Dupont, "An IPv6 Prefix for Overlay
              Routable Cryptographic Hash Identifiers Version 2
              (ORCHIDv2)", RFC 7343, DOI 10.17487/RFC7343, September
              2014, <https://www.rfc-editor.org/info/rfc7343>.

   [RFC9499]  Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
              RFC 9499, DOI 10.17487/RFC9499, March 2024,
              <https://www.rfc-editor.org/info/rfc9499>.

   [SPECIAL]  IANA, "IANA IPv6 Special-Purpose Address Registry",
              <https://www.iana.org/assignments/iana-ipv6-special-
              registry>.

Acknowledgments

   The author would like to extend a special note of thanks to Brian
   Carpenter and Erik Kline for their precisely summarized thoughts on
   this topic that provided the seed of this document.  The author would
   also like to thank Andrew Alston, Fred Baker, Ron Bonica, Nick
   Buraglio, Bruno Decraene, Dhruv Dhody, Darren Dukes, Linda Dunbar,
   Reese Enghardt, Adrian Farrel, Clarence Filsfils, Jim Guichard, Joel
   Halpern, Ted Hardie, Bob Hinden, Murray Kucherawy, Cheng Li, Acee
   Lindem, Jen Linkova, Gyan Mishra, Yingzhen Qu, Robert Raszuk, Alvaro
   Retana, Michael Richardson, John Scudder, Petr Spacek, Mark Smith,
   Dirk Steinberg, Ole Troan, Eduard Vasilenko, Éric Vyncke, Rob Wilton,
   Jingrong Xie, Chongfeng Xie, and Juan Carlos Zuniga for their ideas
   and comments to improve this document.

Author's Address

   Suresh Krishnan
   Cisco
   Email: suresh.krishnan@gmail.com
