IPv6 Hop-by-Hop Options Processing Procedures
draft-ietf-6man-hbh-processing-10

Network Working Group                                          R. Hinden
Internet-Draft                                      Check Point Software
Updates: 8200 (if approved)                                 G. Fairhurst
Intended status: Standards Track                  University of Aberdeen
Expires: 29 March 2024                                 26 September 2023

             IPv6 Hop-by-Hop Options Processing Procedures
                   draft-ietf-6man-hbh-processing-10

Abstract

   This document specifies procedures for how IPv6 Hop-by-Hop options
   are processed in IPv6 routers and hosts.  It modifies the procedures
   specified in the IPv6 Protocol Specification (RFC8200) to make
   processing of the IPv6 Hop-by-Hop Options header 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
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   This Internet-Draft will expire on 29 March 2024.

Copyright Notice

   Copyright (c) 2023 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
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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 . . . . . . . . . . .   7
     5.1.  Processing the Extension Header Carrying Hop-by-Hop
           Options . . . . . . . . . . . . . . . . . . . . . . . . .   7
       5.1.1.  Configuration Enabling Hop-by-Hop Header
               Processing  . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Hop-by-Hop Options Processing . . . . . . . . . . . . . .   8
       5.2.1.  Router Alert Option . . . . . . . . . . . . . . . . .  10
       5.2.2.  Configuration of Hop-by-Hop Option Processing . . . .  11
   6.  Defining New Hop-by-Hop Options . . . . . . . . . . . . . . .  11
     6.1.  Example of Robust Usage . . . . . . . . . . . . . . . . .  12
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  15
   10. Change log [RFC Editor: Please remove]  . . . . . . . . . . .  15
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  17
   12. Informative References  . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   This document specifies procedures for processing IPv6 Hop-by-Hop
   options in IPv6 routers and hosts.  It modifies the procedures
   specified in the IPv6 Protocol Specification [RFC8200] to make
   processing of IPv6 Hop-by-Hop Options header practical with the goal
   of making IPv6 Hop-by-Hop options useful to deploy and use at IPv6
   routers and hosts.

   The current list of defined Hop-by-Hop options can be found at
   [IANA-HBH].  The focus for this document is to set a lower bound for
   the minimum number of Hop-by-Hop options that ought to be processed.
   This document does not discuss an upper bound.  That topic is
   discussed in [I-D.ietf-6man-eh-limits].

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   When published, this document updates [RFC8200].

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 routers exchange user data through the
      forwarding plane.  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.  They also exchange routing information with one
      another.  Management and routing information are processed by its
      recipient.  Management and control information can be forwarded by
      a router that process fields contained in packet headers.

   *  Fast Path: A path through a router that is optimized for
      forwarding packets.  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.

   *  Full Forwarding Rate: This is the rate that a router can forward
      packets without adversely impacting the aggregate forwarding rate.
      For example, a router could process packets with Hop-by-Hop
      options at a rate that allows it to maintain the full speed on its
      outgoing interfaces, which is sometimes called "wire speed".

   NOTE: [RFC6192] is an example of how designs can separate control
   plane and forwarding plane functions.  The separation between
   hardware and software processing described in [RFC6398] does not

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   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 for Hop-by-Hop options.

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, as observed in Section 2.2 of RFC7045: "it is to
   be expected that high-performance routers will either ignore it or
   assign packets containing it to a slow processing path".  The reason
   behind this includes:

   *  Inability to process Hop-by-Hop options at the full forwarding
      rate can result in issues.  In some cases, Hop-by-Hop options
      would be sent to the Control Plane.  This could degrade a router's
      performance and also its ability to process critical control
      traffic.  Both of which could be exploited as a Denial of Service
      attack against the router.

   *  If a subset of packets in a flow were to include Hop-by-Hop
      options, this could introduce a potential for packets to be
      delivered out of order to the destination.  This might result when
      the Extension Header was included in only some packets, or if a
      specific Hop-by-Hop option required different processing for some
      packets in a flow.  Significant reordering of the packets
      belonging to a flow can impact the performance of upper layer
      protocols and needs to be avoided.

   *  Packets could include multiple Hop-by-Hop options.  Too many
      options could make the previous issues worse by increasing the
      resources required to process them.  The total size of the options
      determines the number of header bytes that might need to be
      processed.

   [RFC6564] specified a uniform format for new IPv6 Extension Headers.
   It updated [RFC2460] and this update was incorporated into
   Section 4.8 of [RFC8200].

   When the IPv6 Specification was updated and published in July 2017 as
   [RFC8200], the procedures relating to Hop-by-Hop options were
   specified ([RFC8200], Section 4) as follows:

      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

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      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 of Hop-by-Hop options in [RFC8200] was
   not intended to change the processing of these options.  It
   documented how they were being used in the Internet at the time
   RFC8200 was published (see Appendix B of [RFC8200]).  This was a
   constraint on publishing the IPv6 specification as an IETF Standard.

   The main issues remain:

   *  Routers can be configured to drop transit packets containing Hop-
      by-Hop Options that would have required processing in the Control
      Plane.  This could protect against a denial of service attack on
      the router [RFC9098].

   *  IPv6 Packets that include a Hop-by-Hop Options header are dropped
      by some Internet paths.  A survey in 2015 reported a high loss
      rate in transit ASs for packets with Hop-by-Hop options [RFC7872].
      The operational implications of IPv6 Packets that include
      Extension Headers are discussed in [RFC9098].

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

   *  Including larger or multiple Hop-by-Hop options in a Hop-by-Hop
      Options header increases the number of bytes that need to be
      processed in forwarding, which can in some designs impact the cost
      of processing a packet, and in turn could increase the probability
      of drop [RFC7872].  A larger Extension Header could also reduce
      the probability that a router can locate all the header bytes
      required to successfully process an access control list operating
      on fields after the Hop-by-Hop Options header.

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   *  Any option that can be used to force packets into the router's
      Control Plane 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 adversely impact router
      operation.  This is an issue for the Router Alert option, which
      intentionally forward packets to the Control Plane, and is
      discussed in [RFC6398].  This impact could be mitigated by
      limiting the use of control plane resources by a specific packet,
      and/or by the use of per-function rate-limiters for packets
      processed by the control plane.

   Section 3 of RFC 6398 includes a summary of processing the IP Router
   Alert Option:

      "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."

   This is an example of the need to limit the resources that can be
   consumed when a particular function is executed and to avoid
   consuming control-plane resources where support for a function has
   not been configured.

   There has been research that has 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".  Operational aspects of the topics discussed in this
   section are further 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 clarifies the description in Section 2.2 of
   [RFC7045], using the language of BCP 14 [RFC2119] [RFC8174].

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   The document defines a set of procedures for the Hop-by-Hop Options
   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 within a limited domain (e.g.,
   where a specific service is 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].  Section 5.1
   describes processing of the Hop-by-Hop option Extension Header, and
   Section 5.1.1 describes processing of individual Hop-by-Hop Options.

5.1.  Processing the Extension Header Carrying Hop-by-Hop Options

   When a packet includes one or more Extension Headers, the Next Header
   field of the IPv6 Header does not identify the transport protocol.

   The Extension Header used to carry Hop-by-Hop options is defined in
   Section 4.3 of [RFC8200] and is identified by a Next Header value of
   0 in the IPv6 header.  Section 4.1 of [RFC8200] requires this 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]:

   Routers SHOULD process the Hop-by-Hop Options header using the method
   defined in this document.  If a router does not process the Hop-by-
   Hop Options header, it MUST forward the packet normally based on the
   remaining Extension Header after the Hop-by-Hop option (i.e., a
   router must not drop a packet solely because it contains an Extension
   Header carrying Hop-by-Hop options).  A configuration could control
   that normal processing skips any or all of the Hop-by-Hop options
   carried in the Hop-by-Hop Options header.

   It is expected that the Hop-by-Hop Options header will be processed
   by the Destination.  Hosts SHOULD process the Hop-by-Hop Options
   header in received packets.  If a Destination does not process the
   Hop-by-Hop Options header, it MUST process the remainder of the
   packet normally.  Further details on requirements for host processing
   are described in [I-D.ietf-6man-eh-limits].

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5.1.1.  Configuration Enabling Hop-by-Hop Header Processing

   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 the path only examine and process the Hop-by-Hop
      Options header if explicitly configured to do so.

   A configuration could control whether processing skips any specific
   Hop-by-Hop options carried in the Hop-by-Hop Options header.  A
   router that does not process the contents of the Hop-by-Hop Options
   header does not therefore process the identifiers of individual
   Option Types to perform any specified action.

5.2.  Hop-by-Hop Options Processing

   A Source creating packets with a Hop-by-Hop Options header SHOULD use
   a method that is robust to network nodes processing only the first
   Hop-by-Hop option that is included in the packet, or that forward
   packets without the option being processed (see Section 6.1).  A
   Source MAY, based on local configuration, include more than one Hop-
   by-Hop option [I-D.ietf-6man-eh-limits], but might wish to restrict
   the size to increase the likelihood of successful transfer across a
   network path.  This motivates Sources to order the placement of Hop-
   by-Hop options within the Hop-by-Hop Options header in decreasing
   order of importance for their processing by nodes on the path.

   A router MUST NOT be configured to support the Hop-by-Hop Option
   header if it cannot process the first Hop-by-Hop option at full
   forwarding rate.  If configured to do so, a router SHOULD process
   additional Hop-by-Hop options providing that these also can be
   processed at the full forwarding rate.

   If a router is unable to process any Hop-by-Hop option (or is not
   configured to do so), it SHOULD behave in the way specified for an
   unrecognized Option Type when the action bits were set to "00".

   If a router is unable to process further Hop-by-Hop options (or is
   not configured to do so), the router SHOULD skip the remaining
   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.  After skipping an option, the router continues
   processing the remaining options in the header.  Skipped options do
   not need to be verified.

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   The Router Alert option [RFC2711] is an exception that can result in
   processing in the Control Plane, see Section 5.2.1.

   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 "01" value to relax the
   requirement to drop a packet.  It also modifies the 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 "01","10" and
   "11" values is:

       01 - MAY discard the packet. Nodes should not rely on routers
            dropping these unrecognized Option Types.

       10 - MAY 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 - MAY 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.

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   When a node sends an ICMP message in response to a multicast packet,
   this could be exploited as an amplification attack.  This is
   particularly problematic when the source address is not valid (which
   can be mitigated when using a reverse path forwarding (RPF) check).
   A node SHOULD only send ICMP messages in response to a multicast
   address when this is enabled for the specific source and/or the group
   destination address.

   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.  Generating an ICMP message
   incurs additional router processing.  Reception of this message is
   not guaranteed, routers might be unable to be configured so that they
   do not generate these messages, and they are not always forwarded to
   the Source.  The motivation here is to loosen the requirement to send
   an ICMPv6 Parameter Problem message when a router forwards a packet
   without processing the list of all options.

5.2.1.  Router Alert Option

   The purpose of the Router Alert option [RFC2711] is to tell a router
   that the packet needs additional processing in the Control Plane.

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

   DISCUSSION

      The function of a Router Alert option results in the processing
      that this specification is proposing to eliminate, that is, to
      instruct a router to process the packet in the Control Plane.  The
      function of a Router Alert option is an instruction to a router to
      process the packet in the Control Plane that results in the
      concerns discussed in section 4.  One approach would be to
      deprecate this, because current usage beyond the local network
      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 Router Alert option could have a potential for use with new
      functions that have to be processed in the Control Plane.  Keeping
      this as the single exception for processing in the Control Plane
      with the following restrictions is a reasonable compromise to
      allow future flexibility.  These are compatible with Section 5 of
      [RFC6398].

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   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 MUST
   protect itself from infrastructure attack that could result from
   processing in the Control Plane.  This might include some combination
   of an access control list to only permit this from trusted nodes,
   rate limiting of processing, or other methods [RFC6398].

   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.  An
   implementation SHOULD verify that a Router Alert option 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 to the Control Plane for further
   processing [RFC6398].  Otherwise, the router implementation SHOULD
   forward this packet subject to all normal policies and forwarding
   rules).

5.2.2.  Configuration of Hop-by-Hop Option Processing

   A router can be configured to process a specific Option.  A possible
   approach to implementing this is to maintain a lookup table based on
   Option Type of the IPv6 options that can be processed at full
   forwarding rate.  This would allow a router to quickly determine if
   an option is supported and can be processed.  If the option is not
   supported, then the router processes the option as described in
   Section 5.1 of this document.

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

6.  Defining New Hop-by-Hop Options

   This section updates Section 4.8 of [RFC8200].

   Any new IPv6 Hop-by-Hop option designed in the future should be
   designed to be processed at full forwarding rate (e.g., without
   slowing processing of other packets).  New Hop-by-Hop options should
   have the following characteristics:

   *  New Hop-by-Hop options SHOULD be designed to ensure the router can
      process the options at the full forwarding rate.  That is, they
      should be simple to process, see Section 5.2.

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   *  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 router does not recognize the
      option.

   *  The size of a Hop-by-Hop option SHOULD NOT extend beyond what can
      be expected to be executed at full forwarding rate.  A larger Hop-
      by-Hop Option header can increase the likelihood that that a
      packet will be dropped.

   *  New Hop-by-Hop options SHOULD be designed expecting that a router
      might be configured to only process a subset of Hop-by-Hop options
      (e.g., the first) in the Hop-by-Hop Options header.

   *  New Hop-by-Hop options SHOULD be designed expecting that a router
      may drop packets containing the new Hop-by-Hop option.

   Any new Hop-by-Hop option that is standardized that does not meet
   these criteria MUST include in the specification a detailed
   explanation why this can not be accomplished and to show that there
   is a reasonable expectation that the option can be proceed at full
   forwarding rate.  This is consistent with [RFC6564].

   The general issue of robust operation of packets with new Hop-by-Hop
   options is described in Section 6.1 below.

6.1.  Example of Robust Usage

   Recent measurement surveys (e.g., [Cus23]) show that packets that
   include Extension Headers can cause the packets to be dropped by some
   Internet paths.  In a controlled domain, routers can be configured or
   updated to provide support for any required Hop-by-Hop options.

   The primary motivation of this document is to make it more
   practicable to use Hop-by-Hop options beyond such a single domain,
   with the expectation that applications can improve the quality of or
   add new features to their offered service when the path successfully
   forwards packets with the required Hop-by-Hop options(s), and
   otherwise refrains from using these option(s).  The focus is on
   incremental deployability.  A protocol feature (such as using Hop-by-
   Hop options) is incrementally deployable if early adopters gain some
   benefit over the paths being used, even though other paths do not
   support the protocol feature.  A Source ought to order the Hop-by-Hop
   options that are carried in the Hop-by-Hop Options header in
   decreasing order of importance for processing by nodes on the path.

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   Methods can be developed that do not rely upon all routers to
   implement a specific Hop-by-Hop option (e.g., [RFC9268] , and that
   are robust when the current path drops packets that contain a Hop-by-
   Hop option (e.g., [RFC9098]).

   For example, an application can be designed to first send a test
   packet(s) that includes the required option, or combination of
   options, and sends other packets without including the option.  The
   application then does not send additional packets that include this
   option (or set of options) until the test packet(s) is acknowledged.
   The need for potential loss recovery when a path drops these test
   packets can be avoided by choosing packets that do not carry
   application data that needs to be reliably delivered.

   Since the set of nodes forming a path can change with time, this
   discovery process ought to be repeated from time-to-time.  The
   process of sending packets both with and without a specific header to
   discover whether a path can support a specific header is sometimes
   called "racing" (e.g., transport protocol racing is explained in
   [I-D.ietf-taps-arch], or "A/B protocol feature testing" is described
   in [Tram17]).

7.  IANA Considerations

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

8.  Security Considerations

   Security issues with including 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 Control Plane can be exploited as a denial of
   service attack on a transit router by saturating the resources needed
   for router management (e.g., routing protocols, network management
   protocols, etc.) that may cause the router to fail or perform sub-
   optimally.  This is because it has been observed that certain routers
   drop packets that include a Hop-by-Hop Options header.

   While Hop-by-Hop options are not required to be processed in the
   Control Plane, 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 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 the Hop-by-Hop Options header is
   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 Hop-by-Hop option MUST be
      processed and additional Hop-by-Hop options MAY be processed based
      on local configuration.

   *  It adds criteria for the Router Alert option Section 5.2.1 to
      allow control over how the Router Alert option is processed and
      that a node configured to support these options must protect
      itself from attacks using the Router Alert option.

   *  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
      option that limit their size and computational requirements.

   The authors intent is that these changes significantly reduce the
   security issues relating to processing the IPv6 Hop-by-Hop Options
   header and to enable Hop-by-Hop options to be safely used 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, Dave Thaler, Ana
   Custura, Tim Winters, Fernando Gont, Jingrong Xie, Lorenzo Colitti,
   Toerless Eckert, Suresh Krishnan, Mikael Abrahamsson, and other
   members of the 6MAN working group.

10.  Change log [RFC Editor: Please remove]

   draft-ietf-6man-hbh-processing-10, 2023-September-26:

   *  Clarifying changes based on comments received during the IPv6 w.g.
      session at IETF117 from Lorenzo Colitti, Toerless Eckert, and
      others.
   *  Clarifying changes based on comments received after the first w.g.
      last call.
   *  Editorial Changes.

   draft-ietf-6man-hbh-processing-09, 2023-July-4:

   *  Revised text in Section 3 relating to fast/slow path and
      processing
   *  Restructured Section 5 to separate Hop-by-Hop Options header and
      Hop-by-Hop options processing and configuration.
   *  Revised MUST/SHOULD language in Section 5.2.
   *  Revised text to use consistant names for Hop-by-Hop Options header
      and Hop-by-Hop options.
   *  Revised Section 5.2 regarding the modified behaviour of the action
      bits "01", "02", and 03" to be a MAY to be consistant with text
      earlier in that section.
   *  Added to Section 6 that new Hop-by-Hop options SHOULD be designed
      expecting that routers may drop packets with the new option.
   *  Added new Section 6.1 on "Example of Robust Usage".
   *  Other editorial changes to improve readability and clarity.

   draft-ietf-6man-hbh-processing-08, 2023-April-30:

   *  Changed document that is no longer updates [RFC7045], it now
      clarifies it using the language of BCP 14.
   *  Added additional clarification to Section 4.
   *  Editorial changes

   draft-ietf-6man-hbh-processing-07, 2023-April-6:

   *  Changed text to clarify how hosts and routers process the Hop-by-
      Hop Options header based on comments at 6MAN session at IETF 116.

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   *  Editorial changes

   draft-ietf-6man-hbh-processing-06, 2023-March-11:

   *  Added reference to RFC6564.
   *  Editorial changes

   draft-ietf-6man-hbh-processing-05, 2023-February-23:

   *  Clarified text in Section 6 about processing complexity and time
      to process.
   *  Added a definition to Section 3 for "Full Forwarding Rate".
   *  Added text to Section 5.1 about nodes that do not process the Hop-
      by-Hop Options header.
   *  Added text to Section 4 about slow path processing can cause
      packets to be deliver out of order to the destination.
   *  Editorial changes

   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 5.1 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 5.1.1 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 5.1
   *  Revised text in Section 4 to reflect constraints on publishing
      RFC8200.

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   *  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 minimum 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.

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

   *  6MAN Working Group Draft
   *  Reworked text to talk about processing Hop-by-Hop options at full
      forwarding rates, instead of "fast path"
   *  Revised Section 6 "New Hop-by-Hop options" to allow variable sized
      Hop-by-Hop 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 Hop-by-Hop option MUST be processed
      and additional Hop-by-Hop options MAY be processed based on local
      configuration.
   *  Clarified that all Hop-by-Hop options (with one exception) must be
      processed on the Fast Path.
   *  Kept the Router Alert option as the single exception for Slow Path
      processing.
   *  Rewrote and expanded section on New Hop-by-Hop options.
   *  Removed requirement for Hop-by-Hop option size and alignment.
   *  Removed sections evaluating currently defined Hop-by-Hop 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

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   [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>.

   [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

   [Cus23]    Custura, A. and G. Fairhurst, "Internet Measurements: IPv6
              Extension Header Edition", IEPG, IETF-116 , March 2023,
              <http://www.iepg.org/2023-03-26-ietf116/eh.pdf>.

   [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-05, 25 September 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-6man-eh-
              limits-05>.

   [I-D.ietf-taps-arch]
              Pauly, T., Trammell, B., Brunstrom, A., Fairhurst, G., and
              C. Perkins, "An Architecture for Transport Services", Work
              in Progress, Internet-Draft, draft-ietf-taps-arch-18, 30
              May 2023, <https://datatracker.ietf.org/doc/html/draft-
              ietf-taps-arch-18>.

   [I-D.ietf-v6ops-hbh]
              Peng, S., Li, Z., Xie, C., Qin, Z., and G. S. Mishra,
              "Operational Issues with Processing of the Hop-by-Hop
              Options Header", Work in Progress, Internet-Draft, draft-

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              ietf-v6ops-hbh-05, 10 September 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-v6ops-
              hbh-05>.

   [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>.

   [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>.

   [RFC6564]  Krishnan, S., Woodyatt, J., Kline, E., Hoagland, J., and
              M. Bhatia, "A Uniform Format for IPv6 Extension Headers",
              RFC 6564, DOI 10.17487/RFC6564, April 2012,
              <https://www.rfc-editor.org/info/rfc6564>.

   [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>.

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   [RFC9268]  Hinden, R. and G. Fairhurst, "IPv6 Minimum Path MTU Hop-
              by-Hop Option", RFC 9268, DOI 10.17487/RFC9268, August
              2022, <https://www.rfc-editor.org/info/rfc9268>.

   [Tram17]   Trammell, B., Kuehlewind, M., De Vaere, P., Learmonth,
              IR., and G. Fairhurst, "Tracking Transport-Layer Evolution
              with PATH Spider", ANRW , July 2017,
              <https://irtf.org/anrw/2017/anrw17-final16.pdf>.

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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