Limits on Sending and Processing IPv6 Extension Headers
draft-ietf-6man-eh-limits-04

Network Working Group                                         T. Herbert
Internet-Draft                                                   SiPanda
Updates: 8200, 8504 (if approved)                             6 May 2023
Intended status: Standards Track                                        
Expires: 7 November 2023

        Limits on Sending and Processing IPv6 Extension Headers
                      draft-ietf-6man-eh-limits-04

Abstract

   This specification defines various limits that may be applied to
   receiving, sending, and otherwise processing packets that contain
   IPv6 extension headers.  The need for such limits is pragmatic to
   facilitate interoperability amongst hosts and routers in the presence
   of extension headers and thereby increasing the feasibility of
   deployment of extension headers.  The limits described herein
   establish the minimum baseline of support for use of extension
   headers in the Internet.  If it is known that all communicating
   parties for a particular communication, including end hosts and any
   intermediate nodes in the path, are capable of supporting more than
   the baseline then these default limits may be freely exceeded.  When
   published, this document updates [RFC8200] and [RFC8504].

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 7 November 2023.

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
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Related work  . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Overview and motivation of extension header limits  . . . . .   4
     2.1.  Types of nodes  . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Types of limits . . . . . . . . . . . . . . . . . . . . .   5
       2.2.1.  Limits on extension header length . . . . . . . . . .   5
       2.2.2.  Limits on option length . . . . . . . . . . . . . . .   5
       2.2.3.  Limits on number of extension headers . . . . . . . .   6
       2.2.4.  Limits on number of options . . . . . . . . . . . . .   6
       2.2.5.  Limits on padding options . . . . . . . . . . . . . .   7
       2.2.6.  Limit on IPv6 header chain length . . . . . . . . . .   8
     2.3.  Action when limit is exceeded . . . . . . . . . . . . . .  10
     2.4.  Design Philosophy . . . . . . . . . . . . . . . . . . . .  11
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.1.  List of limits  . . . . . . . . . . . . . . . . . . . . .  12
     3.2.  Host requirements . . . . . . . . . . . . . . . . . . . .  13
       3.2.1.  Sending extension headers . . . . . . . . . . . . . .  13
       3.2.2.  Receiving extension headers . . . . . . . . . . . . .  14
     3.3.  Intermediate node and intermediate destination
           requirements  . . . . . . . . . . . . . . . . . . . . . .  16
     3.4.  Intermediate destination requirements . . . . . . . . . .  17
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   Extension headers are a core component of the IPv6 protocol as
   specified in [RFC8200].  IPv6 extension headers were originally
   defined with few restrictions.  For instance, there is no specified
   limit on the number of extension headers a packet may have, nor is
   there a limit on the length in bytes of extension headers in a packet
   (other than being limited by the MTU).  Similarly, variable length
   extension headers typically do not have prescribed limits such as

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   limits on the number of Hop-by-Hop or Destination options in a
   packet.  The lack of limits essentially requires implementations to
   handle every conceivable usage of the protocol, including a myriad of
   use cases those are obviously outside the realm of ever being
   realistic or useful in real world deployment.

   The lack of limits and the requirements for supporting a virtually
   open-ended protocol have led to a significant lack of support and
   deployment of extension headers [RFC7872].  Instead of attempting to
   satisfy the protocol requirements concerning extension headers, some
   router and middlebox vendors have opted to either invent and apply
   their own ad hoc limits, relegate packets with extension headers to
   slow path processing, or have gone so far as to summarily discard all
   packets with extension headers [RFC9098].  The net result of this
   situation is that deployment and use of extension headers is
   underwhelming to the extent that they are sometimes considered
   unusable on the Internet, and hence IPv6 extension headers have not
   lived up to their potential as the extensibility mechanism of IPv6.

   As an example, consider that there is no limit on how many Hop-by-Hop
   or Destination options may be in an extension header in a packet, nor
   any limits as to how many options a receiver must process.  A single
   1500 byte MTU sized packet could legally contain a Hop-by-Hop Options
   header with over seven hundred two byte options.  There is no use
   case for this other than a Denial of Service attack where an attacker
   simply creates packets with hundreds of small unknown Hop-by-Hop
   options with the two high order bits in the option type set to 00
   meaning to skip the unknown option.  Any node in the path that
   attempts to dutifully process all these options per the requirements
   of [RFC2460] would be easily overwhelmed by the processing needed to
   parse these options (this is true for both hardware or software
   implementations).

   This specification describes various limits that hosts and
   intermediate nodes may apply to the processing of extension headers.
   The goal of establishing limits is to narrow the requirements to
   better match reasonable use cases thereby facilitating practical
   implementation.  Subsequently, this increases the viability of
   extension headers as the extensibility mechanism of IPv6.

   When published, this document updates requirements pertaining to
   extension header processing in [RFC8200] and [RFC8504].

1.1.  Related work

   Some of the problems of unlimited extension headers have been
   addressed in certain aspects.

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   [RFC8200] relaxed the requirement that all nodes in the path must
   process all Hop-by-Hop options in a packet to be:

      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.

   Section 5.3 of [RFC8504] defines a number of limits that hosts may
   apply to processing extension headers.  For instance:

      A host MAY set a limit on the maximum number of non-padding
      options allowed in a Destination Options header or Hop-by-Hop
      Options header.  If this feature is supported, the maximum number
      SHOULD be configurable, and the default value SHOULD be set to 8.

   [RFC8883] defines a set of ICMP errors that my be sent if a limit
   concerning extension headers is exceeded and a node discards a packet
   as a result.  This RFC allows both hosts and routers to send such
   messages (effectively acknowledging that some routers drop packets
   with extension headers even though such behavior is non-conformant
   with [RFC8200]).

   [RFC7872] presents real-world data regarding the extent to which
   packets with IPv6 Extension Headers (EHs) are dropped in the
   Internet, and [RFC9098] summarizes the operational implications of
   IPv6 extension headers, and attempts to analyze reasons why packets
   with IPv6 extension headers are often dropped in the public Internet.

   This document sets the upper bounds on the number of Hop-by-Hop
   options that a node should process.  The lower bound is discussed in
   [I-D.ietf-6man-hbh-processing].

2.  Overview and motivation of extension header limits

   This specification considers extension header limits in three
   dimensions: 1) The types of nodes that may process extension headers
   and the requirements specific to each type, 2) The types of limits
   that may be applied, 3) The action taken when a limit is exceeded.

2.1.  Types of nodes

   For the purposes of describing handling of extension headers this
   specification considers three types of node in an IPv6 network:

   *  Hosts: The source of an IPv6 packet, as addressed by the source

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      address; or the final destination node of a packet as addressed by
      the destination address in a packet with no Routing header or as
      addressed by last segment in a Routing header

   *  Intermediate destination: An intermediate destination node in a
      Routing header as addressed by the destination address of a packet
      with a Routing header where the address is not the address of the
      last segment in the Routing header

   *  Intermediate nodes: A router on the path that is not addressed by
      a packet's destination address

2.2.  Types of limits

   The limits and requirements for handling extension headers defined in
   this specification fall in the following categories:

   *  Limits on extension header length

   *  Limits on option length

   *  Limits on number of extension headers

   *  Limits on number of options

   *  Limits on padding for extension headers with options

   *  Limits on the length of the IPv6 header chain

2.2.1.  Limits on extension header length

   [RFC8504] defines limits that may be defined for the length of an
   extension header.  Those limits are extended to be applicable to
   intermediate nodes.  [RFC8883] defines ICMP Parameter Problem codes
   that may be sent when an extension header is exceeded.

2.2.2.  Limits on option length

   A node may establish a limit on the size of individual Hop-by-Hop or
   Destination options.  Conceivably, such a limit could apply to all
   option types, or length limits may be specific to individual options.
   [RFC8883] defines ICMP Parameter Problem codes that may be sent when
   an option length limit is exceeded.

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2.2.3.  Limits on number of extension headers

   A node may define a limit on the number of extension headers it will
   process.  Although [RFC8200] only defines four types of extension
   headers, it does not preclude the same type of extension header being
   present multiple times.  A limit on the number of extension headers
   could be useful to disallow packets that contain multiple instances
   of the same extension header.

2.2.4.  Limits on number of options

   Limits may be established for the number of options sent or received
   (specifically applicable to Hop-by-Hop Options headers and
   Destination Options headers).  The need for this limit arises from
   the fact that [RFC8200] does not specify a limit.  Requiring nodes to
   process packets with tens or hundreds of options has no foreseeable
   use cases in deployment except as a denial of service attack.
   [RFC8504] has proposed such a limit for host processing of a Hop-by-
   Hop Options header or Destination Options header with a default of
   eight options.  This specification extends that limit to be
   applicable to intermediate nodes.  Specific limits may be established
   for the number of non-padding options or the number of all options
   including padding.

   To derive a limit for the total number options in an extension
   header, one can assume that at most one padding option is used
   between two non-padding options (an explicit limit on consecutive
   padding options is described below).  With this assumption, we can
   extrapolate a reasonable limit on the number of all options that
   should be twice the limit of the number of non-padding options.  Per
   [RFC8504], the recommended default limit for the number of non-
   padding options is eight, so this specification establishes a default
   limit of sixteen options including padding options.  The choice of
   sixteen options as a default limit attempts to strikes a balance
   between allowing extensibility and maintaining reasonable
   expectations for node processing requirements.

   With regards to extensibility, it is observed that in the almost
   thirty year history of IPv6 there are only thirteen defined non-
   deprecated Destination options and Hop-by-Hop options and three
   temporary assigned options.  Current evidence suggests that having
   more than one Destination option or Hop-by-Hop option in a extension
   header is rare, and extrapolating that point with the rate of new
   options being defined suggests a limit of eight non-padding options
   allows for sufficient extensibility in the foreseeable future.

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   With regards to processing requirements, TLVs, such as Hop-by-Hop
   options and Destination options, have historically been considered
   difficult to process efficiently due to their serial processing
   requirements and combinatorial nature.  TLV processing has been a
   particularly acute problem for ASIC based hardware devices.
   Recently, there is a strong trend in programmable implementation,
   even in high performance routers, of using emerging programming
   frameworks such as PANDA and P4.  Programmable implementations are
   better equipped to handle TLVs, at least for a reasonably small
   number of them.  It might also be pointed out that the need to
   efficiently process TLVs exists in other protocols, for instance
   processing TCP requires processing of TLVs in the form of TCP options
   which are an intrinsic part of the protocol.

2.2.5.  Limits on padding options

   [RFC8200] defines PAD1 and PADN options that respectively provide one
   byte or N bytes of padding in a Hop-by-Hop Options or Destination
   Options header.  The purpose of padding is to properly align the
   following non-padding option to its expected alignment, or to add
   padding after the last Destination or Hop-by-Hop option so that the
   length of the extension header is a multiple of eight bytes as
   required by [RFC8200].  [RFC8504] defines limits on number of bytes
   used for consecutive padding where the amount of padding between
   options or at the end of the extension header is no more than seven
   bytes; this limit is sufficient to align any following data after the
   padding to eight bytes.  These limits are extended to be applicable
   to intermediate nodes.

   This specification allows a receiving node to set a requirement that
   consecutive padding options are not present in a packet; which in
   turn requires a sender not to place consecutive padding options in a
   packet.  The rationale for this limit is that a PAD1 or PADN option
   is able to provide one to 257 bytes of padding, so a single padding
   option is sufficient for expected use cases of padding.  When the
   sender creates options, it can compute the amount of padding
   necessary to satisfy the alignment requirements of the following
   data.  If one byte of padding is needed a PAD1 option is used, if
   more than one byte of padding is needed then an appropriate PADN
   option is used.

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2.2.6.  Limit on IPv6 header chain length

   Intermediate nodes often perform deep packet inspection (DPI) in
   order to implement various functions in the network.  Routers perform
   DPI when they inspect packets beyond the IPv6 header or beyond the
   Hop-by-Hop Options header if present.  Some router implementations
   must inspect the transport layer headers in order to process and
   forward the packet, and if the transport layer headers are not
   readable a packet might be dropped.  Even if a transport layer header
   is in plain text within a packet, some devices may not be capable of
   reading it if the header is too deep in the packet.

   Hardware devices often have constraints on how much of the headers in
   a packet can be parsed for DPI.  A typical design is that some
   portion of the beginning of a received packet is loaded into a memory
   buffer for header parsing (i.e. the parsing buffer).  The size of
   this parsing buffer is often fixed per device or line cards installed
   in a chassis.

   To derive a size limit for the IPv6 header chain, we need to take
   into account headers in a packet that might be subject to DPI which
   include the link layer header through at least the pertinent fields
   of the transport layer header.  The most common required information
   is the transport layer port numbers which typically occupy the first
   four bytes of the transport headers (in TCP, UDP, SCTP, DCCP, etc.).
   Inspection of port numbers may be needed for stateless load balancing
   as well as port filtering.  There are middleboxes that may need to
   inspect more of transport layer headers or the transport payload,
   however those can be considered specialized devices that perform work
   beyond simple packet forwarding and filtering and hence should have
   more capabilities for DPI.

   In addition to limits on the length of the IP header chain, it is
   conceivable that there could be a limit on the length of the whole
   header chain.  The whole header chain would comprise the IPv6 header
   chain as well as any headers that are part of network encapsulation
   that precede the innermost transport layer.  The definition of such a
   limit is out of scope for this document, however [RFC8883] defines an
   ICMP error to send when a limit on size of an aggregate header chain
   is exceeded.

   This document specifies that the minimum supported limit for IPv6
   header chains is 104 bytes.  The value is derived by assuming that
   nodes have the ability to process at least the first 128 bytes of a
   packet (that is they have a parsing buffer that can contain at least
   128 bytes).  The 128 byte parsing buffer would be expected to at
   least contain:

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   *  16 bytes for a Layer 2 header (for instance an Ethernet header)

   *  40 bytes for the IPv6 header

   *  64 bytes for the extension headers

   *  8 bytes for the transport layer (i.e the first eight bytes of the
      transport layer header

   This scheme thus establishes a requirement that all Internet devices
   are capable of correctly processing packets with up to sixty-four
   bytes of extension headers, and subsequently it establishes a
   requirement that a host shouldn't send packets with more than sixty-
   four bytes of extension headers.  Note that this establishes a global
   baseline requirement across the Internet; within a limited domain
   higher limits could be applied.

   128 bytes is likely the minimal useful parsing buffer size in
   deployment today.  Devices performing a very narrow DPI could
   conceptually use a smaller parsing buffer, for instance that could be
   as small as sixty-four bytes which accommodates an L2 header, IPv6
   header, and eight bytes of transport header; however, such a device
   would be extremely limited in capabilities and if they do exist they
   are likely legacy devices that will eventually be decommissioned.
   Many routers now have the capability to perform DPI into
   encapsulation headers which implies they already have a larger
   parsing buffer than this baseline minimum.

   Similar to limiting the number of options allowing in a packet,
   setting a limit for IP header length chain is a tradeoff between
   extensibility and feasible implementation.

   For extensibility, the pertinent extension headers contributing to
   the sixty-four byte limit are mostly the Hop-by-Hop Options header
   and Destination Options header.  The Routing header is really
   intended for limited domains and not the Internet (for instance, the
   SRv6 Routing header is confined to a Segment Routing Domain) and
   therefore would be subject to a domain specific limit for IP header
   chain length.  The Encryption header may be used on the Internet,
   however encryption obfuscates the encapsulated transport headers such
   that such that intermediate nodes can't inspect them regardless of
   their position in a packet.  Fragmentation may be used in the
   Internet, however only the first fragment of a fragmented packet
   might contain transport layer headers that could be read by an
   Intermediate node.  In any case, the Fragment header is only four
   bytes so that would not be a particularly large portion of a sixty-
   four byte limit.

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   The Authentication header is usable on the Internet and does allow
   the transport layer headers to be in readable in plain text.  The
   Authentication header is relatively large, typically thirty-two bytes
   or more, so it would contribute significantly to a limit on IP header
   chain length; however, the use of the Authentication header without
   encryption is currently rare on the Internet.

   Individual Hop-by-Hop or Destination options may also be categorized
   as being intended for use over the Internet or just in limited
   domains.  For instance, the IOAM Hop-by-Hop option is intended for
   use in limited domains.

   Paring this down, the types of extension headers and Destination and
   Hop-by-Hop options that might be used outside of limited domains are
   fairly limited.  Options that are intended for use over the public
   Internet could be defined to be small and compact to promote not
   exceeding a sixty-four byte limit on extension headers, whereas
   options constrained to a limited domain could be larger since larger
   limits might be assumed.

2.3.  Action when limit is exceeded

   For each limit that is defined, an action is specified for when the
   limit is exceeded.  The appropriate action depends on whether the
   processing node is the destination host, an intermediate destination,
   or an intermediate node.  For a destination host, the typical action
   to take when a limit is exceeded is to discard the packet.  This is
   appropriate since the destination host is required to process all of
   the headers in a packet, and if a limit is exceeded then it cannot
   process the packet so there is no other alternative but to discard.

   For intermediate nodes, the typical action to take when a limit is
   exceeded is to stop processing headers at the point the limit is
   reached and to forward the packet on.  If an intermediate node needs
   to access transport layer information it may continue inspecting
   extension headers, but not processing them, after a limit has been
   reached for the purposes of locating the transport layer header.
   [RFC8200] allows that an intermediate node may not process the Hop-
   by-Hop Options headers, therefore an intermediate node may ignore all
   of the Hop-by-Hop options in a packet.  This specification expands on
   that requirement to allow an intermediate node to process some
   arbitrary subset of consecutive Hop-by-Hop options in the TLV list
   and to ignore the following ones.  In the case of an egregious
   violation of a limit, for instance an attacker sends three hundred
   options in a packet, the destination host can decide if the
   appropriate response is to drop (the destination host must process
   all options).  Note that this provision motivates the sender to place
   Hop-by-Hop options in the extension header so that those considered

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   more important are placed first.  It should also be noted that
   [RFC8504] sets a default limit of eight; this specification adds a
   counterpart for sending hosts that they shouldn't send more than
   eight Hop-by-Hop options by default.

   Intermediate destinations have characteristics of both hosts and
   intermediate modes.  If a limit is exceeded related to Hop-by-Hop
   options then the suggested action in this specification is to assume
   the same processing of limits as intermediate nodes.  If limits are
   exceeded that affect the processing specific to an intermediate
   destination, such as limits on a Destination Options header before
   the Routing header, then the action should be to discard packet.

2.4.  Design Philosophy

   The limits defined in this document are applicable to both senders
   and receivers.  With a few exceptions as described below, the limits
   described herein are optional to configure and enforce.  If a limit
   is configurable there is a suggested default value.

   A sender of extension headers should generally be conservative in its
   use of extension headers to maximize the chances of packets being
   delivered to their destination.  Default values for sending limits
   are assumed to be useful in arbitrary environment such as the public
   Internet, that is they can be considered "baseline limits".  These
   limits may be relaxed if a sender has a priori information that all
   possible nodes in path will properly handle packets that exceed the
   baseline limits.  In particular, if a sender is sending in a limited
   domain, it might be known that all nodes in the limited domain have
   sufficient capabilities to handle packets exceeding the baseline
   limits.

   Specific mechanisms for a host to determine that baseline limits for
   extension headers may be exceeded are out of scope for this document.
   Conceivably, this determination could be done by configuration,
   capabilities probing, or applying historical knowledge that all
   intermediate nodes in the path and the destination node are capable
   of handling packets that exceed the baseline limits.

   Receivers of extension headers should be liberal in accepting packets
   with extension headers, however per this document they may ignore
   extension headers or options within extension headers (in accordance
   with [RFC8200]).  In particular, the philosophy of this specification
   is that intermediate nodes should not drop packets with extension
   headers solely on the basis that they don't have sufficient
   capabilities to process all the headers in a packet.  As such,
   intermediate nodes may define arbitrarily restrictive limits on what
   they process with regards to extension headers as long as the action

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   taken when those limits are exceeded is to ignore items beyond the
   limit.  Hosts are more constrained in this regard since they
   generally can't correctly process a packet without processing all the
   headers, so when limits are exceeded on a host, packets should be
   discarded.  It should be noted that hosts stacks inherently have more
   processing capabilities than intermediate nodes, so it is expected
   that they should be able to support higher limits for processing
   extension headers.

   This specification does specify one hard requirement for receiving
   nodes, namely nodes must be able to properly handle packets having an
   IPv6 header chain length up to 104 bytes.  This requirement
   acknowledges that some intermediate nodes perform deep packet
   inspection to extract information from transport layer headers
   [RFC9098].  Often a node that requires parsing transport layer
   information will have a fixed sized "parsing buffer" to contain
   packet headers.  If the transport layer headers within a packet are
   beyond the extent of the parsing buffer then an implementation might
   take some detrimental action such as arbitrarily dropping packets.
   To this end, this specification requires that any intermediate node
   that requires access to to transport layer header must minimally be
   able to parse at least 128 bytes of headers, from which the 104 byte
   limit for the IP header chain is derived.

3.  Requirements

   This section lists the normative requirements related to sending and
   processing extension headers.

   The requirements in this section update the processing requirements
   specified in Section 4 of [RFC8200]; in particular, requirements for
   how many Hop-by-Hop options an intermediate node must process are
   updated.

   The requirements in this section update section 5.3 of [RFC8504] by
   extending the limits applicable to end host nodes to be applicable to
   intermediate nodes as well.

3.1.  List of limits

   The set of limits that a node may apply when processing extension
   headers include:

   *  Too many non-padding or padding options

   *  Extension header too big

   *  Option too big

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   *  Too many consecutive padding options

   *  Too many consecutive bytes of padding

   *  Extension header chain too long

   *  Aggregate header chain too long

   *  Too many extension headers

3.2.  Host requirements

3.2.1.  Sending extension headers

   The requirements are:

   *  A host MUST NOT send more than 8 non-padding options in a
      Destination Options header unless it has explicit knowledge that
      the destination, and all intermediate destinations in the case of
      a Destination Options header before the routing header, are able
      to process a greater number of options.

   *  A host MUST NOT send more than 8 non-padding options in a Hop-by-
      Hop Options header unless it has explicit knowledge that the final
      destination host is able to process a greater number of options.

   *  A host SHOULD NOT send more than 8 non-padding options in a Hop-
      by-Hop Options header unless it has explicit knowledge that all
      possible intermediate nodes are able to process a greater number
      of options or will ignore options that exceeds their limit.

   *  A host MUST NOT send a packet with an extension header larger than
      64 bytes unless it has explicit knowledge that all nodes that
      might process the extension header are capable of processing a
      larger header.

   *  A host MUST NOT send a packet with a Destination option or Hop-by-
      Hop option with Data Length greater than 60 bytes unless it has
      explicit knowledge that all nodes that might process the option
      are capable of processing ones with a larger Data Length.

   *  A host node MUST NOT send a packet with an IPv6 header chain
      larger than 104 bytes unless it has explicit knowledge that all
      nodes in the path are capable of properly handling packets with
      larger header chains.  This requirement is equivalently stated as
      a host MUST NOT send a packet with more than 64 bytes of aggregate
      extension headers.

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   *  A host MUST NOT set more than one consecutive pad option, either
      PAD1 or PADN, in a Destination Options header or Hop-by-Hop
      Options header.

   *  A host MUST NOT send a PadN option in a Hop-by-Hop Options header
      or Destination Options header with total length of more than seven
      bytes.

   *  A host node MUST NOT send more than 16 (padding or non-padding)
      options in a Destination Options header unless it has explicit
      knowledge that the destination, and all intermediate destinations
      in the case of a Destination Options header before the Routing
      header, are able to process a greater number of options.  Note
      that if the above requirements on a host sending non-padding
      Destination options and requirements on option padding are met,
      then this requirement is implicitly satisfied.

   *  A host node MUST NOT send more than 16 options (padding or non-
      padding) in a Hop-by-Hop Options header unless it has explicit
      knowledge that the final destination host is able to process a
      greater number of options.  Note that if the above requirements on
      a host sending non-padding Hop-by-Hop options and requirements on
      padding are met, then this requirement is implicitly satisfied.

3.2.2.  Receiving extension headers

   Per [RFC8200], a host node that receives a packet with extension
   headers must process all the extension headers in the packet before
   accepting the payload and processing the payload.

   As described in [RFC8504] a host may establish limits on the
   processing of extension headers.  This specification reiterates and
   updates those requirements to allow for a host to send an RFC8883
   error if a limit has been exceeded.

   *  A host MAY set a limit on the maximum number of non-padding
      options allowed in a Destination Options header or Hop-by-Hop
      Options header.  If this limit is supported then the maximum
      number SHOULD be configurable, the limit MUST be greater than or
      equal to 8, and the default value SHOULD be set to 8.  The limits
      for Destination Options headers and Hop-by-Hop Options headers MAY
      be separately configurable.  If a packet is received and the
      number of Destination or Hop-by-Hop options exceeds the limit,
      then the packet SHOULD be discarded and and an ICMP Parameter
      Problem with code 9 MAY be sent to the packet's source address.

   *  A host MAY set a limit on the maximum number of options (padding

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      or non-padding) allowed in a Destination Options header or Hop-by-
      Hop Options header.  If this limit is supported then the maximum
      number SHOULD be configurable and the limit MUST be greater than
      or equal to 16.  The limits for Destination Options headers and
      Hop-by-Hop Options headers MAY be separately configurable.  If a
      packet is received and the number of Destination or Hop-by-Hop
      options exceeds the limit, then the packet SHOULD be discarded and
      and an ICMP Parameter Problem with code 9 MAY be sent to the
      packet's source address

   *  A host node MAY set a limit on the length of an extension header.
      If this limit is supported then the limit SHOULD be configurable
      and the limit MUST be greater than or equal to 64 bytes.  The
      length limits for different extension headers MAY be separately
      configurable.

   *  A host node MAY set a limit on the Data Length of a Hop-by-Hop or
      Destination option.  If this limit is supported then the limit
      SHOULD be configurable, and the limit MUST be greater than or
      equal to 60 bytes.  The limits for Destination options and Hop-by-
      Hop options MAY be separately configurable.  If a packet is
      received and a Hop-by-Hop or Destination option has a length that
      exceeds the limit, then the packet SHOULD be discarded and an ICMP
      Parameter Problem with code 10 MAY be sent to the packet's source
      address.

   *  A host MAY limit the number of consecutive PAD1 options in a
      Destination Options header or Hop-by-Hop Options header to 7.  In
      this case, if there are more than 7 consecutive PAD1 options
      present, the packet SHOULD be discarded and an ICMP Parameter
      Problem with code 10 MAY be sent to the packet's source address

   *  A host MAY limit the number of bytes in a PADN option to be less
      than 8.  In such a case, if a PADN option is present that has a
      length greater than 7, the packet SHOULD be discarded and an ICMP
      Parameter Problem with code 10 MAY be sent to the packet's source
      address.

   *  A host MAY set a limit on the maximum length of a Destination
      Options header or Hop-by-Hop Options header.  This value SHOULD be
      configurable, and if the limit is used then the limit MUST be
      greater than or equal to 64 bytes.  If a packet is received and
      the length of the Destination Options header or Hop-by-Hop Options
      header exceeds the length limit, then the packet SHOULD be
      discarded and an ICMP Parameter Problem with code 6 MAY be sent to
      the packet's source address.

   *  A host node MAY set a limit on the maximum length of the IPv6

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      header chain, or equivalently a host MAY set a limit on the
      aggregate length of extension headers in a packet.  If the limit
      is used then it MUST be greater than or equal to 104 bytes, or,
      equivalently, the limit on aggregate header extension length MUST
      be greater than or equal to 64 bytes.  If a packet is received and
      the aggregate length of the IPv6 header chain exceeds the limit
      then the packet SHOULD be discarded and an ICMP Parameter Problem
      with code 7 MAY be sent to the packet's source address.

   *  A host MAY disallow consecutive padding options, either PAD1 or
      PADN, to be present in a packet.  If consecutive padding options
      are received and disallowed by the host, then packet SHOULD be
      discarded and an ICMP Parameter Problem with code 9 MAY be sent to
      the packet's source address.

3.3.  Intermediate node and intermediate destination requirements

   The following common requirements are established for intermediate
   nodes and intermediate destination nodes that receive and process
   packets with extension headers.

   *  An intermediate node MUST be able to correctly forward packets
      that contain an IPv6 header chain of 104 or fewer bytes, or
      equivalently an intermediate node MUST be able to process a packet
      with an aggregate length of extension headers less than or equal
      to 64 bytes.

   *  Per [RFC8200] an intermediate node MAY be configured not to
      process Hop-by-Hop Options headers.  If a node is configured as
      such and a packet with a Hop-by-Hop Options header is received,
      the extension header MUST be be skipped and the packet MUST
      otherwise be properly processed and forwarded.

   *  An intermediate node MAY limit the number of non-padding Hop-by-
      Hop options that it processes.  If a limit is exceeded, that is a
      Hop-by-Hop Options header contains more non-padding options than
      are configured to process, the intermediate node SHOULD stop
      processing the Hop-by-Hop Option header and ignore any Hop-by-Hop
      options beyond the limit.  It is NOT RECOMMENDED that an
      intermediate node discards the packet because the limit is
      exceeded, however if it does so then the intermediate node MAY
      send an ICMP Parameter Problem with code 10 to the packet's source
      address.

   *  An intermediate node MAY limit the number of Hop-by-Hop options
      (padding or non-padding) that it processes.  If a limit is
      exceeded, that is a Hop-by-Hop Options header contains more non-
      padding options than are configured to process, the intermediate

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      node SHOULD stop processing the Hop-by-Hop Options header and
      ignore any Hop-by-Hop options beyond the limit.  It is NOT
      RECOMMENDED that the intermediate node discards the packet because
      the limit is exceeded, however if it does so then the intermediate
      node MAY send an ICMP Parameter Problem with code 10 to the
      packet's source address.

   *  If an intermediate node encounters an unknown Hop-by-Hop option
      and the two high order bits are not 00 then the node SHOULD
      immediately stop processing the Hop-by-Hop Options header and
      ignore any Hop-by-Hop options beyond the unknown option.  An
      intermediate node MAY either elect to discard the packet and MAY
      send an ICMP Parameter Problem per the requirements of [RFC8200]
      and [I-D.ietf-6man-hbh-processing]; or the intermediate node MAY
      forward the packet and effectively disregard the high order two
      bits in the option type.  The motivation for this requirement is
      to simplify processing at intermediate nodes.  Note, that if the
      high order two bits are non-zero for an option that is unknown to
      the destination host then the packet will be discarded since the
      destination host is required to process all Hop-by-Hop options in
      a packet or to discard a packet if its limit for maximum number of
      options to process is exceeded.

   *  An intermediate node MAY set a limit on the maximum length of a
      Hop-by-Hop Options header.  This value SHOULD be configurable.  If
      this limit is exceeded, that is a packet has an extension header
      larger then the limit, then the intermediate node SHOULD stop
      processing the Hop-by-Hop Option header and ignore any Hop-by-Hop
      options beyond the limit.  It is NOT RECOMMENDED that the
      intermediate node discards the packet because the limit is
      exceeded, however if it does so then the intermediate node MAY
      send an ICMP Parameter Problem with code 10 to the packet's source
      address.

3.4.  Intermediate destination requirements

   The following are requirements specific to intermediate destinations
   pertaining to the processing of a Destination Options header before
   the Routing header.  For processing a Hop-by-Hop Options header at an
   intermediate destination, the requirements for processing them at an
   intermediate node are assumed.

   *  An intermediate destination MAY limit the maximum length of a

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      Destination Options header before the Routing header.  This value
      SHOULD be configurable, and the default is to accept options of
      any length.  If a limit is defined is MUST be at least 64 bytes.
      If the limit is exceeded then the intermediate destination SHOULD
      discard the packet and MAY send an ICMP Parameter Problem with
      code 6 to the packet's source address.

   *  An intermediate destination node MAY limit the number of non-
      padding options in a Destination Options header before the Routing
      header.  If this limit is supported then the maximum number SHOULD
      be configurable and the limit MUST be greater than or equal to 8.
      If a limit is exceeded, that is a packet contains more non-padding
      options than are configured to process, the intermediate
      destination node SHOULD discard the packet and MAY send an ICMP
      Parameter Problem with code 10 to the packet's source address.

   *  An intermediate destination node MAY limit the number of options
      (padding or non-padding) in a Destination Options header before
      the Routing header.  If this limit is supported then the maximum
      number SHOULD be configurable and the limit MUST be greater than
      or equal to 16.  If a limit is exceeded, that is a packet contains
      more options than are configured to process, the intermediate
      destination node SHOULD discard the packet and MAY send an ICMP
      Parameter Problem with code 10 to the packet's source address.

   *  An intermediate destination MAY limit the total number bytes in
      consecutive PAD1 options in a Destination Options header before
      the Routing header to 7.  If the limit is exceeded, that is there
      are more than seven bytes in consecutive PAD1 or PADN options
      present, the intermediate destination node SHOULD discard the
      packet and MAY send an ICMP Parameter Problem with code 10 to the
      packet's source address.

   *  An intermediate destination MAY limit the number of bytes in a
      PADN option in a Destination Option header before the Routing
      header to be less than 8.  In such a case, if a PADN option is
      present that has a length greater than 7, the packet SHOULD be
      discarded and the intermediate destination node SHOULD discard the
      packet and MAY send an ICMP Parameter Problem with code 10 to the
      packet's source address.

   *  An intermediate MAY limit the maximum length of a Destination

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      Options header before the Routing header.  If this limit is
      supported then the limit SHOULD be configurable and the limit MUST
      be greater than or equal to 64 bytes.  If a packet is received and
      the length of the a Destination Options header before the Routing
      header exceeds the length limit, the intermediate destination node
      SHOULD discard the packet and MAY send an ICMP Parameter Problem
      with code 10 to the packet's source address.

4.  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].

   Of particular concern is a Distributed Denial-of-Service attack
   (DDOS) wherein an attacker sends many Hop-by-Hop options or
   Destination options in a packet for the purposes of forcing receivers
   to consume inordinate resources processing packets.  Since there is
   no hard limit on the number of options in an extension header, it is
   conceivable that an attacker could craft MTU sized packets with
   hundreds of small Hop-by-Hop or Destination options where the option
   type is chosen to be one that will be unknown to the receiver and the
   higher order type bits are set to 00 to indicate that an unknown
   option is ignored.  A receiver attempting to process all the options
   in such packet would require a lot of resources as TLV processing is
   notoriously hard to do efficiently (in either hardware or software).

   This document addresses the DDOS concern of extension headers and
   options in extension headers by allowing receivers to configure
   limits the number of extension headers or options that they process.
   Such limits cap the amount of processing needed for extension headers
   and hence mitigate the DDOS concerns of extension headers.

   This document does not otherwise introduce any new security concerns.

5.  Acknowledgments

   The author would like to thank Brian Carpenter, Bob Hinden, Nick
   Hilliard, Gorry Fairhurst, Darren Dukes, and Vasilenko Eduard for
   their comments and suggestions that improved this document.

6.  References

6.1.  Normative References

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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>.

   [RFC8504]  Chown, T., Loughney, J., and T. Winters, "IPv6 Node
              Requirements", BCP 220, RFC 8504, DOI 10.17487/RFC8504,
              January 2019, <https://www.rfc-editor.org/info/rfc8504>.

   [RFC8883]  Herbert, T., "ICMPv6 Errors for Discarding Packets Due to
              Processing Limits", RFC 8883, DOI 10.17487/RFC8883,
              September 2020, <https://www.rfc-editor.org/info/rfc8883>.

6.2.  Informative References

   [I-D.ietf-6man-hbh-processing]
              Hinden, R. M. and G. Fairhurst, "IPv6 Hop-by-Hop Options
              Processing Procedures", Work in Progress, Internet-Draft,
              draft-ietf-6man-hbh-processing-08, 30 April 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-6man-
              hbh-processing-08>.

   [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>.

   [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>.

   [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>.

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   [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>.

Author's Address

   Tom Herbert
   SiPanda
   Santa Clara, CA,
   United States of America
   Email: tom@herbertland.com

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