Network Working Group                                          R. Hinden
Internet-Draft                                      Check Point Software
Updates: 8200 (if approved)                                 G. Fairhurst
Intended status: Standards Track                  University of Aberdeen
Expires: 8 January 2023                                      7 July 2022


             IPv6 Hop-by-Hop Options Processing Procedures
                   draft-ietf-6man-hbh-processing-01

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.

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 8 January 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
   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  . . . . . . . . . . . . . . . . . . . . . . . .   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 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 . . . . . . . . . . . . . . . . . . . . . . .  11
   10. Change log [RFC Editor: Please remove]  . . . . . . . . . . .  11
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  12
   12. Informative References  . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

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
   expectation for the minimum number of hop-by-hop options that a node
   supports.  This document does not discuss an upper bound.  This topic
   is discussed in [I-D.ietf-6man-eh-limits].

   When published this document updates [RFC8200].

   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 intermediate nodes (i.e.,
      routers) between its source and its destination.  These
      intermediate nodes process metadata 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
      intermediate nodes (i.e., routers) between its source and its
      destination.  These intermediate nodes process metadata contained
      in packet headers.  However, they do not process information
      contained in packet payloads.  So, from their perspective, this
      information is user data.

   *  Fast Path: A path through a router that is optimized for
      forwarding packets without processing their payloads.  The Fast
      Path may 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 is 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.







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   NOTE: This distinct separation between hardware and software
   processing from [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).

   [RFC6192] is an example of how designs can separate control plane
   (Slow Path) and forwarding plane (Fast Path) functions.

4.  Background

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

   *  Inability to process the hop-by-hop options at full the 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
      could degrade the a router's performance and it's ability to
      process important control traffic.

   *  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.

   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.











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      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.

   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 be processed in the Slow Path.
      This behavior is seen as protecting 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 is discussed in [RFC9098].

   *  Allowing multiple hop-by-hop options in a single packet makes it
      even more expensive in router resources to process these packets.
      It adds complexity to the number of permutations that might need
      to be processed.

   *  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.  This 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:



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      "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."

   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 Extenstion Headers we therefore recommend to support at
   least one EH".

   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).

   This document defines a set of procedures for the hop-by-hop option
   header that make the processing of hop-by-hop options practical in
   modern transit routers.

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] that a
   node MUST process the first Option in the Hop-by-Hop Header at full
   forwarding rate the (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 headers SHOULD
   include a single Hop-by-Hop Option in the packet and MAY include more
   based on local configuration.

   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.

5.2.  Hop-by-Hop Headers Processing

   Nodes that implement a differentiation between a Fast Path and a Slow
   Path MUST process all (with one exception noted below) Hop-by-Hop
   options in the Fast Path.  The one exception to this is the Router
   Alert Option [RFC2711].  See Section 5.3 for discussion of the Router
   Alert.

   If the node can not process an option at the full forwarding rate, it
   MUST behave as if it does not recognize the Option Type (as described
   in the next paragraph).

   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
           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, send an ICMP 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 it's function is to do
      what this specification is proposing to eliminate, that is,
      process the packet in the Slow Path.  One approach would be to
      deprecate it as it's usage appears to be limited and packets
      containing Hop-by-Hop options are frequently dropped.  Deprecation
      would allow current implementations to continue and it's 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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   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).  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.

   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 for a router to control it's 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.

   A node configured not to process HBH options, MUST drop the packet if
   the top two bits of the Option Type field of the first HBH option is
   non-zero.

   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).  New options SHOULD NOT be defined that are not expected
   to be executed at full forwarding rates.  New Hop-by-Hop options
   SHOULD have the following characteristics:

   *  Straight forward to process.  That is, they should be designed to
      keep the time to process low.

   *  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],
   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 need for router management
   protocols (e.g., routing protocols, network management protocols,
   etc.) that may cause the router to fail.  Due to this it's common for
   transit routers to drop packets with Hop-by-Hop options headers.

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

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






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   *  All Hop-by-Hop options (with one exception) must be processed in
      the Fast Path.  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.

   *  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.

   *  Limited the default number of Hop-by-Hop options that that can be
      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.

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

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

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, [your name here], and other
   members of the 6MAN working group.

10.  Change log [RFC Editor: Please remove]

   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.




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   *  Added text to Section 4 that the authors some Hop-by-Hop optoions
      will be supported Internet wide, and others only in limited
      domains.
   *  Editorial changes.

   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>.




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   [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>.

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://datatracker.ietf.org/doc/html/draft-ietf-6man-eh-
              limits-00>.

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

   [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>.

   [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>.

   [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>.






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Internet-Draft           HBH Options Processing                July 2022


   [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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