IPv6 Hop-by-Hop Options Processing Procedures
draft-ietf-6man-hbh-processing-04

Network Working Group                                          R. Hinden
Internet-Draft                                      Check Point Software
Updates: 8200, 7405 (if approved)                           G. Fairhurst
Intended status: Standards Track                  University of Aberdeen
Expires: 24 April 2023                                   21 October 2022

             IPv6 Hop-by-Hop Options Processing Procedures
                   draft-ietf-6man-hbh-processing-04

Abstract

   This document specifies procedures for how IPv6 Hop-by-Hop options
   are processed.  It modifies the procedures specified in the IPv6
   Protocol Specification (RFC8200) to make processing of IPv6 Hop-by-
   Hop options practical with the goal of making IPv6 Hop-by-Hop options
   useful to deploy and use in the Internet.  When published, this
   document updates RFC8200 and RFC7045.

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
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   This Internet-Draft will expire on 24 April 2023.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Hop-by-Hop Header Processing Procedures . . . . . . . . . . .   6
     5.1.  Hop-by-Hop Options Per Packet . . . . . . . . . . . . . .   6
     5.2.  Hop-by-Hop Options Headers Processing . . . . . . . . . .   7
     5.3.  Router Alert Option . . . . . . . . . . . . . . . . . . .   8
     5.4.  Configuration . . . . . . . . . . . . . . . . . . . . . .   9
   6.  New Hop-by-Hop Options  . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   10. Change log [RFC Editor: Please remove]  . . . . . . . . . . .  12
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  13
   12. Informative References  . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   This document specifies procedures for how IPv6 Hop-by-Hop options
   are processed.  It modifies the procedures specified in the IPv6
   Protocol Specification (RFC8200) to make processing of IPv6 Hop-by-
   Hop options practical with the goal of making IPv6 Hop-by-Hop options
   useful to deploy and use in the Internet.

   The editors focus for this document is to set a lower bound for the
   minimum number of hop-by-hop options that a node should process.
   This document does not discuss an upper bound.  That topic is
   discussed in [I-D.ietf-6man-eh-limits].

   When published this document updates [RFC8200] and updates section
   2.2 of [RFC7045].

   The current list of defined Hop-by-Hop options can be found at
   [IANA-HBH].

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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.  Terminology

   This document uses the following loosely defined terms:

   *  Forwarding Plane: IPv6 hosts exchange user data through the
      forwarding plane.  User data is processed by its recipient (i.e.,
      an IPv6 host).  User data can traverse routers between its source
      and its destination.  These routers process fields contained in
      packet headers.  However, they do not process information
      contained in packet payloads.

   *  Control Plane: IPv6 routers exchange management and routing
      information with controllers.  They also exchange routing
      information with one another.  Management and routing information
      is processed by its recipient (i.e., an IPv6 router or
      controller).  Management and control information can traverse
      router that process fields contained in packet headers.  However,
      they do not process information contained in packet payloads.

   *  Fast Path: A path through a router that is optimized for
      forwarding packets without processing their payloads.  The Fast
      Path might be supported by Application Specific Integrated
      Circuits (ASICS), Network Processor (NP), or other special purpose
      hardware.  This is the usual processing path within a router taken
      by the forwarding plane.

   *  Slow Path: A path through a router that is capable of general
      purpose processing and is not optimized for any particular
      function.  This processing path is used for packets that require
      special processing or differ from assumptions made in Fast Path
      heuristics or to process router control protocols used by the
      control plane.

   NOTE: [RFC6192] is an example of how designs can separate control
   plane (Slow Path) and forwarding plane (Fast Path) functions.  The
   separation between hardware and software processing described in
   [RFC6398] does not apply to all router architectures.  However, a
   router that performs all or most processing in software might still
   incur more processing cost when providing special processing (aka
   Slow Path).

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4.  Background

   In the first versions of the IPv6 specification [RFC1883] and
   [RFC2460], Hop-by-Hop options were required to be processed by all
   nodes: routers and hosts.  This proved to not be practical in current
   high speed routers due to several factors, including:

   *  Inability to process the hop-by-hop options at the full forwarding
      rate (e.g., routers with no support on the Fast Path).

   *  Hop-by-Hop options would be sent to the Slow Path.  This could
      degrade a router's performance and its ability to process
      important control traffic.

   *  It was recognised that a mechanism that forces packets from any
      source to the routers "Slow Path" could be exploited as a Denial
      of Service attack against the router.

   *  Packets could contain multiple Hop-by-Hop options making the
      previous issues worse by increasing the complexity required to
      process them, or requiring access to more bytes of protocol
      header.

   When the IPv6 Specification was updated and published in July 2017 as
   [RFC8200], the procedures relating to hop-by-hop options were as
   follows:

      Extension headers (except for the Hop-by-Hop Options header) are
      not processed, inserted, or deleted by any node along a packet's
      delivery path, until the packet reaches the node (or each of the
      set of nodes, in the case of multicast) identified in the
      Destination Address field of the IPv6 header.

      The Hop-by-Hop Options header is not inserted or deleted, but may
      be examined or processed by any node along a packet's delivery
      path, until the packet reaches the node (or each of the set of
      nodes, in the case of multicast) identified in the Destination
      Address field of the IPv6 header.  The Hop-by-Hop Options header,
      when present, must immediately follow the IPv6 header.  Its
      presence is indicated by the value zero in the Next Header field
      of the IPv6 header.

      NOTE: While [RFC2460] required that all nodes must examine and
      process the Hop-by-Hop Options header, it is now expected that
      nodes along a packet's delivery path only examine and process the
      Hop-by-Hop Options header if explicitly configured to do so.

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   The changes meant that an implementation complied with the IPv6
   specification even if it did not process hop-by-hop options, and that
   it was expected that routers would add configuration information to
   control which hop-by-hop options they would process.

   The text regarding processing Hop-by Hop Options in [RFC8200] was not
   intended to change the processing of Hop-by-Hop options.  It only
   documented how they were being used in the Internet at the time
   RFC8200 was published.  This was a constraint on publishing the IPv6
   specification as an IETF Standard.

   The main issues remain:

   *  Routers are commonly configured to drop transit packets containing
      hop-by-hop options that would have required processing in the Slow
      Path.  This could be to protect against a denial of service attack
      on the router.

   *  A survey in 2015 reported a high loss rate in transit ASs for
      packets with HBH options [RFC7872].  The operational implications
      of IPv6 Packets that set extension headers are discussed in
      [RFC9098].

   *  Allowing multiple hop-by-hop options in a single packet in some
      cases consumes more router resources to process these packets.  It
      also adds complexity to the number of permutations that might need
      to be processed/configured.

   *  Any mechanism that can be used to force packets into the router's
      Slow Path can be exploited as a denial of service attack on a
      transit router by saturating the resources needed for router
      management protocols (e.g., routing protocols, network management
      protocols, etc.) that could cause the router to fail.  This is an
      issue for the Router Alert option, which intentionally places
      packets on the Slow Path, is discussed in [RFC6398].  Section 3 of
      that RFC includes a good summary:

      "In a nutshell, the IP Router Alert Option does not provide a
      convenient universal mechanism to accurately and reliably
      distinguish between IP Router Alert packets of interest and
      unwanted IP Router Alert packets.  This, in turn, creates a
      security concern when the IP Router Alert Option is used, because,
      short of appropriate router-implementation-specific mechanisms,
      the router Slow Path is at risk of being flooded by unwanted
      traffic."

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   There has been research that discussed the general problem with
   dropping packets containing IPv6 extension headers, including the
   Hop-by-Hop Options header.  For example [Hendriks] states that
   "dropping all packets with Extension Headers, is a bad practice", and
   that "The share of traffic containing more than one EH however, is
   very small.  For the design of hardware able to handle the dynamic
   nature of Extension Headers we therefore recommend to support at
   least one EH".

   The topic discussed in this section is also discussed in
   [I-D.ietf-v6ops-hbh].

   "Transmission and Processing of IPv6 Extension Headers" [RFC7045]
   clarified how intermediate nodes should process extension headers.
   This document is generally consistent with [RFC7045], and was
   considered when [RFC2460] was updated and was itself replaced by
   [RFC8200].  This document updates [RFC8200] as described in the next
   section and consequently updates the description in Section 2.2 of
   [RFC7045].

   This document defines a set of procedures for the Hop-by-Hop Option
   header that are intended to make the processing of hop-by-hop options
   practical in modern transit routers.  The authors expectations are
   that some hop-by-hop options will be processed across the Internet
   while others will only be processed in a limited domain (e.g., where
   there is a specific service made available in that network segment
   that relies on one or more hop-by-hop options).

5.  Hop-by-Hop Header Processing Procedures

   This section describes several changes to [RFC8200].

5.1.  Hop-by-Hop Options Per Packet

   The Hop-by-Hop Option Header as defined in Section 4.3 of [RFC8200]
   is identified by a Next Header value of 0 in the IPv6 header.
   Section 4.1 of [RFC8200] requires a Hop-by-Hop Options header to
   appear immediately after the IPv6 header.  [RFC8200] also requires
   that a Hop-by-Hop Options header can only appear once in a packet.

   The Hop-by-Hop Options Header as defined in [RFC8200] can contain one
   or more Hop-by-Hop options.  This document updates [RFC8200] to
   specify that a node MUST process the first Option in the Hop-by-Hop
   Header at full forwarding rate (e.g. on the router's Fast Path) and
   MAY process additional Hop-by-Hop Options if configured to do so.
   The motivation for this change is to simplify the processing of Hop-
   by-Hop options as a part of normal forwarding.

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   Nodes creating packets with a Hop-by-Hop option header SHOULD by
   default include a single Hop-by-Hop Option in the packet and based on
   local configuration MAY include more Hop-by-Hop Options.

5.2.  Hop-by-Hop Options Headers Processing

   Nodes MUST process all Hop-by-Hop options at full forwarding rates.
   The one exception to this is the Router Alert Option [RFC2711].  See
   Section 5.3 for discussion of the Router Alert Option.

   If the node is unable to process an option at the full forwarding
   rate, it MUST behave in the way specified for an unrecognized Option
   Type when the action bits were set to "00".  That is, it must skip
   over this option and continue processing the header (as described in
   the next paragraph).

   If there are more than one Hop-by-Hop options in the Hop-by-Hop
   Options header, the node MAY skip the rest of the options without
   having to examine these options using the "Hdr Ext Len" field in the
   Hop-by-Hop Options header.  This field specifies the length of the
   Option Header in 8-octet units.  The additional options do not need
   to be processed or verified.

   Section 4.2 of [RFC8200] defines the Option Type identifiers as
   internally encoded such that their highest-order 2 bits specify the
   action that must be taken if the processing IPv6 node does not
   recognize the Option Type.  The text is:

      00 - skip over this option and continue processing the header.

      01 - discard the packet.

      10 - discard the packet and, regardless of whether or not the
           packet's Destination Address was a multicast address, send an
           ICMPv6 Parameter Problem, Code 2, message to the packet's
           Source Address, pointing to the unrecognized Option Type.

      11 - discard the packet and, only if the packet's Destination
           Address was not a multicast address, send an ICMPv6 Parameter
           Problem, Code 2, message to the packet's Source Address,
           pointing to the unrecognized Option Type.

   This document modifies this behaviour for the "10" and "11" values
   that the node MAY send an ICMP Parameter Problem, Code 2, message to
   the packet's Source Address, pointing to the unrecognized Option
   Type.  The modified text for "10" and 11" values is:

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      10 - discard the packet and, regardless of whether or not the
           packet's Destination Address was a multicast address, MAY
           send an ICMP Parameter Problem, Code 2, message to the
           packet's Source Address, pointing to the unrecognized Option
           Type.

      11 - discard the packet and, only if the packet's Destination
           Address was not a multicast address, MAY send an ICMP
           Parameter Problem, Code 2, message to the packet's Source
           Address, pointing to the unrecognized Option Type.

   The motivation for this change is to loosen the requirement to send
   ICMPv6 Parameter Problem messages by simplifying what the router
   needs to do when it performs forwarding of an Option Type it does not
   recognize.

   When an ICMP Parameter Problem, Code 2, message is delivered to the
   source, the source can become aware that at least one node on the
   path has failed to recognize the option.

5.3.  Router Alert Option

   The Router Alert option [RFC2711] purpose is to tell the node that
   the packet needs additional processing on the Slow Path.

   The Router Alert option includes a two-octet Value field that
   describes the protocol that is carried in the packet.  The current
   values can be found in the IANA Router Alert Value registry
   [IANA-RA].

   DISCUSSION

      The Router Alert Option is a problem since its function is to do
      what this specification is proposing to eliminate, that is, to
      instruct a router to process the packet in the Slow Path.  One
      approach would be to deprecate because current usage appears to be
      limited and packets containing Hop-by-Hop options are frequently
      dropped.  Deprecation would allow current implementations to
      continue and its use could be phased out over time.

      The authors' current thinking is that the Router Alert function
      may have reasonable potential use for new functions that have to
      be processed in the Slow Path.  We think that keeping it as the
      single exception for Slow Path processing with the following
      restrictions is a reasonable compromise to allow future
      flexibility.  These are compatible with Section 5 of [RFC6398].

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   As specified in [RFC2711] the top two bits of Option Type for the
   Router Alert option are always set to "00" indicating the node should
   skip over this option and continue processing the header in this
   case.  A Fast Path implementation SHOULD verify that a Router Alert
   contains a protocol, as indicated by the Value field in the Router
   Alert option, that is configured as a protocol of interest to that
   router.  A verified packet SHOULD be sent on the Slow Path for
   processing [RFC6398].  Otherwise, the router implementation SHOULD
   forward within the Fast Path (subject to all normal policies and
   forwarding rules).

   Implementations of the IP Router Alert Option SHOULD offer the
   configuration option to simply ignore the presence of "IP Router
   Alert" in IPv4 and IPv6 packets" [RFC6398].

   A node that is configured to process a Router Alert option using the
   Slow Path MUST protect itself from infrastructure attack that could
   result from processing on the Slow Path.  This might include some
   combination of access control list to only permit from trusted nodes,
   rate limiting of processing, or other methods [RFC6398].

5.4.  Configuration

   Section 4 of [RFC8200] allows a router to control its processing of
   IPv6 Hop-by-Hop options by local configuration.  The text is:

      NOTE: While [RFC2460] required that all nodes must examine and
      process the Hop-by-Hop Options header, it is now expected that
      nodes along a packet's delivery path only examine and process the
      Hop-by-Hop Options header if explicitly configured to do so.

   A possible approach to implementing this is to maintain a lookup
   table based on Option Type of the IPv6 options that are supported in
   the Fast Path.  This would allow for a node to quickly determine if
   an option is supported and can be processed.  If the option is not
   supported, then the node processes it as described in Section 5.2 of
   this document.

   This requires the node to examine the first two bits of the option
   even if it does not support the specific option.  A node MUST drop
   the packet if the top two bits of the Option Type field of the first
   HBH option is non-zero as specified in Section 5.2.

   The actions of the lookup table SHOULD be configurable by the
   operator of the router.

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6.  New Hop-by-Hop Options

   Any new IPv6 Hop-by-Hop option designed in the future should be
   designed to be processed at full forwarding rate (e.g., on a router's
   Fast Path, or at least without slowing processing of other packets).
   New options SHOULD NOT be defined that are not expected to be
   executed at full forwarding rate.  New Hop-by-Hop options should have
   the following characteristics:

   *  Straight forward to process.  That is, Hop-by-Hop options SHOULD
      be designed to keep the time to process low.

   *  New options SHOULD be defined with the Action type (highest-order
      2 bits of the Option Type) set to 00 to skip over this option and
      continue processing the header if a node does not recognize the
      option.

   *  New Hop-by-Hop options SHOULD be designed to be the first option
      in a Hop-by-Hop options header.

   *  The size of an option SHOULD NOT extend beyond what can be
      reasonably expected to be executed at full forwarding rate (e.g.,
      forwarded on a router's fast path).

   Any new Hop-by-Hop option that is standardized that does not meet
   these criteria needs to explain in detail in its specification why
   this can not be accomplished and that there is a reasonable
   expectation that it can be proceed at full forwarding rate.

7.  IANA Considerations

   There are no actions required for IANA defined in this document.

8.  Security Considerations

   Security issues with IPv6 Hop-by-Hop options are well known and have
   been documented in several places, including [RFC6398], [RFC6192],
   [RFC7045] and [RFC9098].  The main issue, as noted in Section 4, is
   that any mechanism that can be used to force packets into the
   router's Slow Path can be exploited as a denial of service attack on
   a transit router by saturating the resources needed for router
   management protocols (e.g., routing protocols, network management
   protocols, etc.) that may cause the router to fail or perform sub-
   optimally.  Due to this it's common for transit routers to drop
   packets with a Hop-by-Hop options header.

   While Hop-by-Hop options are not required to be processed in the Slow
   Path, the Router Alert option is designed to do just that.

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   Some IPv6 nodes implement features that access more of the protocol
   information than a typical IPv6 router (e.g.  [RFC9098]).  Examples
   are nodes that provide virus-scanning, DDOS mitigation, Firewall/
   access control, traffic engineering, or traffic normalization.  These
   nodes could be configured to drop packets when they are unable to
   access and process all extension headers, or are unable to locate and
   process the higher-layer packet information.  This document provides
   guidance on the requirements concerning Hop-by-Hop Options.

   Finally, the document notes that Internet protocol processing needs
   to be robust to malformed/malicious protocol fields.  This
   requirement is not specific to Hop-by-Hop Options.  It is important
   that implementations fail gracefully when a malformed or malicious
   Hop-by-Hop Option is encountered.

   This document changes the way Hop-by-Hop options are processed in
   several ways that significantly reduce the attack surface.  These
   changes include:

   *  All Hop-by-Hop options (with one exception) must be processed at
      full forwarding rate.  Only one HBH Option MUST be processed and
      additional HBH Options MAY be processed based on local
      configuration.

   *  Only the Router Alert option can be processed in the Slow Path,
      and the router MUST be configured to do so.

   *  The document added criteria to allow control over how Router Alert
      options are processed and that a node configured to support these
      options must protect itself from attacks using the Router Alert.

   *  The document limited the default number of Hop-by-Hop options that
      can be included in a packet to a single Hop-by-Hop option.

   *  Additional Hop-by-Hop options MAY be included, based on local
      configuration.  Although nodes only process these additional Hop-
      by-Hop Options if configured to do so.

   *  The document added restrictions to any future new Hop-by-Hop
      options that limit their size and computational requirements.

   The authors intent is that these changes significantly reduce the
   security issues relating to IPv6 Hop-by-Hop options and will enable
   them to be used safely in the Internet.

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9.  Acknowledgments

   Helpful comments were received from Brian Carpenter, Ron Bonica, Ole
   Troan, Mark Heard, Tom Herbert, Cheng Li, Eric Vyncke, Greg Mirksy,
   Xiao Min, Fernando Gont, Darren Dukes, Peng Shuping, [your name
   here], and other members of the 6MAN working group.

10.  Change log [RFC Editor: Please remove]

   draft-ietf-6man-hbh-processing-04, 2022-October-21:

   *  Add a paragraph to Section 4 that describes the relationship to
      [RFC7045] "Transmission and Processing of IPv6 Extension Headers".
   *  Change that this draft updates section 2.2 of [RFC7045].

   draft-ietf-6man-hbh-processing-03, 2022-October-12:

   *  Changed in Section 5.2 to have router skip over options if can't
      process at full forwarding rate.
   *  Added to Section 6 that new options should be defined with action
      type set to 00.

   draft-ietf-6man-hbh-processing-02, 2022-August-23:

   *  Several clarification and editorial changes suggested by a review
      by Peng Shuping.
   *  Editorial changes.
   *  Revised text relating to fast/slow path and processing rates.
   *  Revised the third paragraph in Section 5.4 to be clearer.
   *  Revised text in Security section based on comments from Fernando
      Gont.

   draft-ietf-6man-hbh-processing-01, 2022-June-15:

   *  Fixed typo in last paragraph of Section 5.2
   *  Revised text in Section 4 to reflect constraints on publishing
      RFC8200.
   *  Changed text in Section 6 that new options SHOULD NOT (from MUST
      NOT) be defined that require that are not expected to be excepted
      at full forwarding rates.
   *  Added reference to RFC7872 in Section 4.
   *  Added text to Section 1 that the focus of this document is to set
      a minium bound on the number of Hop-by-Hop Options a node should
      process.
   *  Added text to Section 4 that the authors some Hop-by-Hop options
      will be supported Internet wide, and others only in limited
      domains.
   *  Editorial changes.

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   draft-ietf-6man-hbh-processing-00, 2022-January-29:

   *  6MAN Working Group Draft
   *  Reworked text to talk about processing HBH options at full
      forwarding rates, instead of "fast path"
   *  Revised Section 6 "New Hop-by-Hop Options" to allow variable sized
      HBH options, remove specific length requirements, and other
      clarifications.
   *  Editorial changes.

   draft-hinden-6man-hbh-processing-01, 2021-June-2:

   *  Expanded terminology section to include Forwarding Plane and
      Control Plane.
   *  Changed draft that only one HBH Option MUST be processed and
      additional HBH Options MAY be processed based on local
      configuration.
   *  Clarified that all HBH options (with one exception) must be
      processed on the Fast Path.
   *  Kept the Router Alert options as the single exception for Slow
      Path processing.
   *  Rewrote and expanded section on New Hop-by-Hop Options.
   *  Removed requirement for HBH Option size and alignment.
   *  Removed sections evaluating currently defined HBH Options.
   *  Added content to the Security Considerations section.
   *  Added people to the acknowledgements section.
   *  Numerous editorial changes

   draft-hinden-6man-hbh-processing-00, 2020-Nov-29:

   *  Initial draft.

11.  Normative References

   [IANA-HBH] "Destination Options and Hop-by-Hop Options",
              <https://www.iana.org/assignments/ipv6-parameters/
              ipv6-parameters.xhtml#ipv6-parameters-2>.

   [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>.

   [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>.

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   [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>.

12.  Informative References

   [Hendriks] Hendriks, L., Velan, P., Schmidt, RO., Boer, P., and A.
              Aiko, "Threats and Surprises behind IPv6 Extension
              Headers",  , August 2017,
              <http://dl.ifip.org/db/conf/tma/tma2017/
              tma2017_paper22.pdf>.

   [I-D.ietf-6man-eh-limits]
              Herbert, T., "Limits on Sending and Processing IPv6
              Extension Headers", Work in Progress, Internet-Draft,
              draft-ietf-6man-eh-limits-00, 31 January 2022,
              <https://www.ietf.org/archive/id/draft-ietf-6man-eh-
              limits-00.txt>.

   [I-D.ietf-v6ops-hbh]
              Peng, S., Li, Z., Xie, C., Qin, Z., and G. Mishra,
              "Operational Issues with Processing of the Hop-by-Hop
              Options Header", Work in Progress, Internet-Draft, draft-
              ietf-v6ops-hbh-01, 18 April 2022,
              <https://www.ietf.org/archive/id/draft-ietf-v6ops-hbh-
              01.txt>.

   [IANA-RA]  "IPv6 Router Alert Option Values",
              <https://www.iana.org/assignments/ipv6-routeralert-values/
              ipv6-routeralert-values>.

   [RFC1883]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 1883, DOI 10.17487/RFC1883,
              December 1995, <https://www.rfc-editor.org/info/rfc1883>.

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

   [RFC2711]  Partridge, C. and A. Jackson, "IPv6 Router Alert Option",
              RFC 2711, DOI 10.17487/RFC2711, October 1999,
              <https://www.rfc-editor.org/info/rfc2711>.

   [RFC6192]  Dugal, D., Pignataro, C., and R. Dunn, "Protecting the
              Router Control Plane", RFC 6192, DOI 10.17487/RFC6192,
              March 2011, <https://www.rfc-editor.org/info/rfc6192>.

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   [RFC6398]  Le Faucheur, F., Ed., "IP Router Alert Considerations and
              Usage", BCP 168, RFC 6398, DOI 10.17487/RFC6398, October
              2011, <https://www.rfc-editor.org/info/rfc6398>.

   [RFC7045]  Carpenter, B. and S. Jiang, "Transmission and Processing
              of IPv6 Extension Headers", RFC 7045,
              DOI 10.17487/RFC7045, December 2013,
              <https://www.rfc-editor.org/info/rfc7045>.

   [RFC7872]  Gont, F., Linkova, J., Chown, T., and W. Liu,
              "Observations on the Dropping of Packets with IPv6
              Extension Headers in the Real World", RFC 7872,
              DOI 10.17487/RFC7872, June 2016,
              <https://www.rfc-editor.org/info/rfc7872>.

   [RFC9098]  Gont, F., Hilliard, N., Doering, G., Kumari, W., Huston,
              G., and W. Liu, "Operational Implications of IPv6 Packets
              with Extension Headers", RFC 9098, DOI 10.17487/RFC9098,
              September 2021, <https://www.rfc-editor.org/info/rfc9098>.

Authors' Addresses

   Robert M. Hinden
   Check Point Software
   959 Skyway Road
   San Carlos, CA 94070
   United States of America
   Email: bob.hinden@gmail.com

   Godred Fairhurst
   University of Aberdeen
   School of Engineering
   Fraser Noble Building
   Aberdeen
   AB24 3UE
   United Kingdom
   Email: gorry@erg.abdn.ac.uk

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