Internet Engineering Task Force (IETF)                           F. Gont
Request for Comments: 9098                                  SI6 Networks
Category: Informational                                      N. Hilliard
ISSN: 2070-1721                                                     INEX
                                                              G. Doering
                                                             SpaceNet AG
                                                               W. Kumari
                                                                  Google
                                                               G. Huston
                                                                   APNIC
                                                                  W. Liu
                                                     Huawei Technologies
                                                          September 2021


    Operational Implications of IPv6 Packets with Extension Headers

Abstract

   This document summarizes the operational implications of IPv6
   extension headers specified in the IPv6 protocol specification (RFC
   8200) and attempts to analyze reasons why packets with IPv6 extension
   headers are often dropped in the public Internet.

Status of This Memo

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

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9098.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Disclaimer
   4.  Background Information
   5.  Previous Work on IPv6 Extension Headers
   6.  Packet-Forwarding Engine Constraints
     6.1.  Recirculation
   7.  Requirement to Process Layer 3 / Layer 4 Information in
           Intermediate Systems
     7.1.  ECMP and Hash-Based Load Sharing
     7.2.  Enforcing Infrastructure ACLs
     7.3.  DDoS Management and Customer Requests for Filtering
     7.4.  Network Intrusion Detection and Prevention
     7.5.  Firewalling
   8.  Operational and Security Implications
     8.1.  Inability to Find Layer 4 Information
     8.2.  Route-Processor Protection
     8.3.  Inability to Perform Fine-Grained Filtering
     8.4.  Security Concerns Associated with IPv6 Extension Headers
   9.  IANA Considerations
   10. Security Considerations
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   IPv6 extension headers (EHs) allow for the extension of the IPv6
   protocol and provide support for core functionality such as IPv6
   fragmentation.  However, common implementation limitations suggest
   that EHs present a challenge for IPv6 packet routing equipment and
   middleboxes, and evidence exists that IPv6 packets with EHs are
   intentionally dropped in the public Internet in some circumstances.

   This document has the following goals:

   *  Raise awareness about the operational and security implications of
      IPv6 extension headers specified in [RFC8200] and present reasons
      why some networks resort to intentionally dropping packets
      containing IPv6 extension headers.

   *  Highlight areas where current IPv6 support by networking devices
      may be suboptimal, such that the aforementioned support is
      improved.

   *  Highlight operational issues associated with IPv6 extension
      headers, such that those issues are considered in IETF
      standardization efforts.

   Section 4 of this document provides background information about the
   IPv6 packet structure and associated implications.  Section 5
   summarizes previous work that has been carried out in the area of
   IPv6 extension headers.  Section 6 discusses packet-forwarding engine
   constraints in contemporary routers.  Section 7 discusses why
   intermediate systems may need to access Layer 4 information to make a
   forwarding decision.  Finally, Section 8 discusses operational
   implications of IPv6 EHs.

2.  Terminology

   This document uses the term "intermediate system" to describe both
   routers and middleboxes when there is no need to distinguish between
   the two and where the important issue is that the device being
   discussed forwards packets.

3.  Disclaimer

   This document analyzes the operational challenges represented by
   packets that employ IPv6 extension headers and documents some of the
   operational reasons why these packets are often dropped in the public
   Internet.  This document is not a recommendation to drop such
   packets, but rather an analysis of why they are currently dropped.

4.  Background Information

   It is useful to compare the basic structure of IPv6 packets against
   that of IPv4 packets and analyze the implications of the two
   different packet structures.

   IPv4 packets have a variable-length header size that allows for the
   use of IPv4 "options" -- optional information that may be of use to
   nodes processing IPv4 packets.  The IPv4 header length is specified
   in the "Internet Header Length" (IHL) field of the mandatory IPv4
   header and must be in the range of 20 octets (the minimum IPv4 header
   size) to 60 octets, accommodating at most 40 octets of options.  The
   upper-layer protocol type is specified via the "Protocol" field of
   the mandatory IPv4 header.

                  Protocol, IHL
                       +--------+
                       |        |
                       |        v
                  +------//-----+------------------------+
                  |             |                        |
                  |    IPv4     |       Upper-Layer      |
                  |    Header   |       Protocol         |
                  |             |                        |
                  +-----//------+------------------------+

                  variable length
                  <------------->

                      Figure 1: IPv4 Packet Structure

   IPv6 took a different approach to the IPv6 packet structure.  Rather
   than employing a variable-length header as IPv4 does, IPv6 employs a
   packet structure similar to a linked list, where a mandatory fixed-
   length IPv6 header is followed by an arbitrary number of optional
   extension headers, with the upper-layer header being the last header
   in the IPv6 header chain.  Each extension header typically specifies
   its length (unless it is implicit from the extension header type) and
   the "next header" (NH) type that follows in the IPv6 header chain.

          NH          NH, EH-length      NH, EH-length
           +-------+      +------+            +-------+
           |       |      |      |            |       |
           |       v      |      v            |       v
     +-------------+-------------+-//-+---------------+--------------+
     |             |             |    |               |              |
     |    IPv6     |    Ext.     |    |     Ext.      |  Upper-Layer |
     |    header   |    Header   |    |     Header    |  Protocol    |
     |             |             |    |               |              |
     +-------------+-------------+-//-+---------------+--------------+

      fixed length    variable number of EHs & length
     <------------> <-------------------------------->

                      Figure 2: IPv6 Packet Structure

   This packet structure has the following implications:

   *  [RFC8200] requires the entire IPv6 header chain to be contained in
      the first fragment of a packet, therefore limiting the IPv6 header
      chain to the size of the path MTU.

   *  Other than the path MTU constraints, there are no other limits to
      the number of IPv6 EHs that may be present in a packet.
      Therefore, there is no upper limit regarding how deep into the
      IPv6 packet the upper-layer protocol header may be found.

   *  The only way for a node to obtain the upper-layer protocol type or
      find the upper-layer protocol header is to parse and process the
      entire IPv6 header chain, in sequence, starting from the mandatory
      IPv6 header until the last header in the IPv6 header chain is
      found.

5.  Previous Work on IPv6 Extension Headers

   Some of the operational and security implications of IPv6 extension
   headers have been discussed in the IETF:

   *  [OPERATORS] discusses a rationale for which operators drop IPv6
      fragments.

   *  [HEADERS] discusses possible issues arising from "long" IPv6
      header chains.

   *  [PARSING] describes how inconsistencies in the way IPv6 packets
      with extension headers are parsed by different implementations
      could result in evasion of security controls and presents
      guidelines for parsing IPv6 extension headers, with the goal of
      providing a common and consistent parsing methodology for IPv6
      implementations.

   *  [IPV6-EH] analyzes the security implications of IPv6 EHs, as well
      as the operational implications of dropping packets that employ
      IPv6 EHs and associated options.

   *  [RFC7113] discusses how some popular Router Advertisement Guard
      (RA-Guard) implementations are subject to evasion by means of IPv6
      extension headers.

   *  [RFC8900] analyzes the fragility introduced by IP fragmentation.

   A number of recent RFCs have discussed issues related to IPv6
   extension headers and have specified updates to RFC 2460 [RFC2460]
   (an earlier version of the IPv6 standard).  Many of these updates
   have now been incorporated into the current IPv6 core standard
   [RFC8200] or the IPv6 node requirements [RFC8504].  Namely,

   *  [RFC5095] discusses the security implications of Routing Header
      Type 0 (RHT0) and deprecates it.

   *  [RFC5722] analyzes the security implications of overlapping
      fragments and provides recommendations in this area.

   *  [RFC7045] clarifies how intermediate nodes should deal with IPv6
      extension headers.

   *  [RFC7112] discusses the issues arising in a specific fragmentation
      case where the IPv6 header chain is fragmented into two or more
      fragments and formally forbids such fragmentation.

   *  [RFC6946] discusses a flawed (but common) processing of the so-
      called IPv6 "atomic fragments" and specifies improved processing
      of such packets.

   *  [RFC8021] deprecates the generation of IPv6 atomic fragments.

   *  [RFC8504] clarifies processing rules for packets with extension
      headers and also allows hosts to enforce limits on the number of
      options included in IPv6 EHs.

   *  [RFC7739] discusses the security implications of predictable
      fragment Identification values and provides recommendations for
      the generation of these values.

   *  [RFC6980] analyzes the security implications of employing IPv6
      fragmentation with Neighbor Discovery for IPv6 and formally
      recommends against such usage.

   Additionally, [RFC8200] has relaxed the requirement that "all nodes
   must examine and process the Hop-by-Hop Options header" from
   [RFC2460], by specifying that only nodes that have been explicitly
   configured to process the Hop-by-Hop Options header are required to
   do so.

   A number of studies have measured the extent to which packets
   employing IPv6 extension headers are dropped in the public Internet:

   *  [PMTUD-Blackholes] and [Linkova-Gont-IEPG90] present some
      preliminary measurements regarding the extent to which packets
      containing IPv6 EHs are dropped in the public Internet.

   *  [RFC7872] presents more comprehensive results and documents the
      methodology used to obtain these results.

   *  [Huston-2017] and [Huston-2020] measure packet drops resulting
      from IPv6 fragmentation when communicating with DNS servers.

6.  Packet-Forwarding Engine Constraints

   Most contemporary carrier-grade routers use dedicated hardware, e.g.,
   Application-Specific Integrated Circuits (ASICs) or Network
   Processing Units (NPUs), to determine how to forward packets across
   their internal fabrics (see [IEPG94-Scudder] and [APNIC-Scudder] for
   details).  One common method of handling next-hop lookups is to send
   a small portion of the ingress packet to a lookup engine with
   specialized hardware, e.g., ternary content-addressable memory (TCAM)
   or reduced latency dynamic random-access memory (RLDRAM), to
   determine the packet's next hop.  Technical constraints mean that
   there is a trade-off between the amount of data sent to the lookup
   engine and the overall packet-forwarding rate of the lookup engine.
   If more data is sent, the lookup engine can inspect further into the
   packet, but the overall packet-forwarding rate of the system will be
   reduced.  If less data is sent, the overall packet-forwarding rate of
   the router will be increased, but the packet lookup engine may not be
   able to inspect far enough into a packet to determine how it should
   be handled.

      |  NOTE:
      |
      |     For example, some contemporary high-end routers are known to
      |     inspect up to 192 bytes, while others are known to parse up
      |     to 384 bytes of header.

   If a hardware-forwarding engine on a contemporary router cannot make
   a forwarding decision about a packet because critical information is
   not sent to the lookup engine, then the router will normally drop the
   packet.  Section 7 discusses some of the reasons for which a
   contemporary router might need to access Layer 4 information to make
   a forwarding decision.

   Historically, some packet-forwarding engines punted packets of this
   kind to the control plane for more in-depth analysis, but this is
   unfeasible on most contemporary router architectures as a result of
   the vast difference between the hardware-based forwarding capacity of
   the router and the processing capacity of the control plane and the
   size of the management link that connects the control plane to the
   forwarding plane.  Other platforms may have a separate software-based
   forwarding plane that is distinct both from the hardware-based
   forwarding plane and the control plane.  However, the limited CPU
   resources of this software-based forwarding plane, as well as the
   limited bandwidth of the associated link, results in similar
   throughput constraints.

   If an IPv6 header chain is sufficiently long such that it exceeds the
   packet lookup capacity of the router, the router might be unable to
   determine how the packet should be handled and thus could resort to
   dropping the packet.

6.1.  Recirculation

   Although type-length-value (TLV) chains are amenable to iterative
   processing on architectures that have packet lookup engines with deep
   inspection capabilities, some packet-forwarding engines manage IPv6
   header chains using recirculation.  This approach processes extension
   headers one at a time: when processing on one extension header is
   completed, the packet is looped back through the processing engine
   again.  This recirculation process continues repeatedly until there
   are no more extension headers left to be processed.

   Recirculation is typically used on packet-forwarding engines with
   limited lookup capability, because it allows arbitrarily long header
   chains to be processed without the complexity and cost associated
   with packet-forwarding engines, which have deep lookup capabilities.
   However, recirculation can impact the forwarding capacity of
   hardware, as each packet will pass through the processing engine
   multiple times.  Depending on configuration, the type of packets
   being processed, and the hardware capabilities of the packet-
   forwarding engine, the data-plane throughput performance on the
   router might be negatively affected.

7.  Requirement to Process Layer 3 / Layer 4 Information in Intermediate
    Systems

   The following subsections discuss some of the reasons for which
   intermediate systems may need to process Layer 3 / Layer 4
   information to make a forwarding decision.

7.1.  ECMP and Hash-Based Load Sharing

   In the case of Equal Cost Multipath (ECMP) load sharing, the
   intermediate system needs to make a decision regarding which of its
   interfaces to use to forward a given packet.  Since round-robin usage
   of the links is usually avoided to prevent packet reordering,
   forwarding engines need to use a mechanism that will consistently
   forward the same data streams down the same forwarding paths.  Most
   forwarding engines achieve this by calculating a simple hash using an
   n-tuple gleaned from a combination of Layer 2 through to Layer 4
   protocol header information.  This n-tuple will typically use the
   src/dst Media Access Control (MAC) addresses, src/dst IP addresses,
   and, if possible, further Layer 4 src/dst port information.

   In the IPv6 world, flows are expected to be identified by means of
   the IPv6 "Flow Label" [RFC6437].  Thus, ECMP and hash-based load
   sharing should be possible without the need to process the entire
   IPv6 header chain to obtain upper-layer information to identify
   flows.  [RFC7098] discusses how the IPv6 Flow Label can be used to
   enhance Layer 3/4 load distribution and balancing for large server
   farms.

   Historically, many IPv6 implementations failed to set the Flow Label,
   and hash-based ECMP/load-sharing devices also did not employ the Flow
   Label for performing their task.  While support of [RFC6437] is
   currently widespread for current versions of all popular host
   implementations, there is still only marginal usage of the IPv6 Flow
   Label for ECMP and load balancing [Almeida-2020].  A contributing
   factor could be the issues that have been found in host
   implementations and middleboxes [Jaeggli-2018].

   Clearly, widespread support of [RFC6437] would relieve intermediate
   systems from having to process the entire IPv6 header chain, making
   Flow Label-based ECMP and load sharing [RFC6438] feasible.

   If an intermediate system cannot determine consistent n-tuples for
   calculating flow hashes, data streams are more likely to end up being
   distributed unequally across ECMP and load-shared links.  This may
   lead to packet drops or reduced performance.

7.2.  Enforcing Infrastructure ACLs

   Infrastructure Access Control Lists (iACLs) drop unwanted packets
   destined to a network's infrastructure.  Typically, iACLs are
   deployed because external direct access to a network's infrastructure
   addresses is operationally unnecessary and can be used for attacks of
   different sorts against router control planes.  To this end, traffic
   usually needs to be differentiated on the basis of Layer 3 or Layer 4
   criteria to achieve a useful balance of protection and functionality.
   For example, an infrastructure may be configured with the following
   policy:

   *  Permit some amount of ICMP echo (ping) traffic towards a router's
      addresses for troubleshooting.

   *  Permit BGP sessions on the shared network of an exchange point
      (potentially differentiating between the amount of packets/second
      permitted for established sessions and for connection
      establishment), but do not permit other traffic from the same peer
      IP addresses.

   If a forwarding router cannot determine consistent n-tuples for
   calculating flow hashes, data streams are more likely to end up being
   distributed unequally across ECMP and load-shared links.  This may
   lead to packet drops or reduced performance.

   If a network cannot deploy infrastructure ACLs, then the security of
   the network may be compromised as a result of the increased attack
   surface.

7.3.  DDoS Management and Customer Requests for Filtering

   The case of customer Distributed Denial-of-Service (DDoS) protection
   and edge-to-core customer protection filters is similar in nature to
   the iACL protection.  Similar to iACL protection, Layer 4 ACLs
   generally need to be applied as close to the edge of the network as
   possible, even though the intent is usually to protect the customer
   edge rather than the provider core.  Application of Layer 4 DDoS
   protection to a network edge is often automated using BGP Flowspec
   [RFC8955] [RFC8956].

   For example, a website that normally only handles traffic on TCP
   ports 80 and 443 could be subject to a volumetric DDoS attack using
   NTP and DNS packets with a randomized source IP address, thereby
   rendering source-based remote triggered black hole [RFC5635]
   mechanisms useless.  In this situation, ACLs that provide DDoS
   protection could be configured to block all UDP traffic at the
   network edge without impairing the web server functionality in any
   way.  Thus, being able to block arbitrary protocols at the network
   edge can avoid DDoS-related problems both in the provider network and
   on the customer edge link.

7.4.  Network Intrusion Detection and Prevention

   Network Intrusion Detection Systems (NIDS) examine network traffic
   and try to identify traffic patterns that can be correlated to
   network-based attacks.  These systems generally attempt to inspect
   application-layer traffic (if possible) but, at the bare minimum,
   inspect Layer 4 flows.  When attack activity is inferred, the
   operator is notified of the potential intrusion attempt.

   Network Intrusion Prevention Systems (IPS) operate similarly to
   NIDSs, but they can also prevent intrusions by reacting to detected
   attack attempts by e.g., triggering packet filtering policies at
   firewalls and other devices.

   Use of extension headers can be problematic for NIDS/IPS, since:

   *  Extension headers increase the complexity of resulting traffic and
      the associated work and system requirements to process it.

   *  Use of unknown extension headers can prevent a NIDS or IPS from
      processing Layer 4 information.

   *  Use of IPv6 fragmentation requires a stateful fragment-reassembly
      operation, even for decoy traffic employing forged source
      addresses (see, e.g., [nmap]).

   As a result, in order to increase the efficiency or effectiveness of
   these systems, packets employing IPv6 extension headers are often
   dropped at the network ingress point(s) of networks that deploy these
   systems.

7.5.  Firewalling

   Firewalls enforce security policies by means of packet filtering.
   These systems usually inspect Layer 3 and Layer 4 traffic but can
   often also examine application-layer traffic flows.

   As with a NIDS or IPS (Section 7.4), use of IPv6 extension headers
   can represent a challenge to network firewalls, since:

   *  Extension headers increase the complexity of resulting traffic and
      the associated work and system requirements to process it, as
      outlined in [Zack-FW-Benchmark].

   *  Use of unknown extension headers can prevent firewalls from
      processing Layer 4 information.

   *  Use of IPv6 fragmentation requires a stateful fragment-reassembly
      operation, even for decoy traffic employing forged source
      addresses (see, e.g., [nmap]).

   Additionally, a common firewall filtering policy is the so-called
   "default deny", where all traffic is blocked (by default), and only
   expected traffic is added to an "allow/accept list".

   As a result, packets employing IPv6 extension headers are often
   dropped by network firewalls, either because of the challenges
   represented by extension headers or because the use of IPv6 extension
   headers has not been explicitly allowed.

   Note that although the data presented in [Zack-FW-Benchmark] was
   several years old at the time of publication of this document, many
   contemporary firewalls use comparable hardware and software
   architectures; consequently, the conclusions of this benchmark are
   still relevant, despite its age.

8.  Operational and Security Implications

8.1.  Inability to Find Layer 4 Information

   As discussed in Section 7, intermediate systems that need to find the
   Layer 4 header must process the entire IPv6 header chain.  When such
   devices are unable to obtain the required information, the forwarding
   device has the option to drop the packet unconditionally, forward the
   packet unconditionally, or process the packet outside the normal
   forwarding path.  Forwarding packets unconditionally will usually
   allow for the circumvention of security controls (see, e.g.,
   Section 7.5), while processing packets outside of the normal
   forwarding path will usually open the door to Denial-of-Service (DoS)
   attacks (see, e.g., Section 6).  Thus, in these scenarios, devices
   often simply resort to dropping such packets unconditionally.

8.2.  Route-Processor Protection

   Most contemporary carrier-grade routers have a fast hardware-assisted
   forwarding plane and a loosely coupled control plane, connected
   together with a link that has much less capacity than the forwarding
   plane could handle.  Traffic differentiation cannot be performed by
   the control plane because this would overload the internal link
   connecting the forwarding plane to the control plane.

   The Hop-by-Hop Options header has been particularly challenging
   since, in most circumstances, the corresponding packet is punted to
   the control plane for processing.  As a result, many operators drop
   IPv6 packets containing this extension header [RFC7872].  [RFC6192]
   provides advice regarding protection of a router's control plane.

8.3.  Inability to Perform Fine-Grained Filtering

   Some intermediate systems do not have support for fine-grained
   filtering of IPv6 extension headers.  For example, an operator that
   wishes to drop packets containing RHT0 may only be able to filter on
   the extension header type (Routing Header).  This could result in an
   operator enforcing a coarser filtering policy (e.g., "drop all
   packets containing a Routing Header" vs. "only drop packets that
   contain a Routing Header Type 0").

8.4.  Security Concerns Associated with IPv6 Extension Headers

   The security implications of IPv6 extension headers generally fall
   into one or more of these categories:

   *  Evasion of security controls

   *  DoS due to processing requirements

   *  DoS due to implementation errors

   *  Issues specific to the extension header type

   Unlike IPv4 packets where the upper-layer protocol can be trivially
   found by means of the IHL field of the IPv4 header, the structure of
   IPv6 packets is more flexible and complex.  This can represent a
   challenge for devices that need to find this information, since
   locating upper-layer protocol information requires that all IPv6
   extension headers be examined.  In turn, this presents implementation
   difficulties, since some packet-filtering mechanisms that require
   upper-layer information (even if just the upper-layer protocol type)
   can be trivially circumvented by inserting IPv6 extension headers
   between the main IPv6 header and the upper-layer protocol header.
   [RFC7113] describes this issue for the RA-Guard case, but the same
   techniques could be employed to circumvent other IPv6 firewall and
   packet-filtering mechanisms.  Additionally, implementation
   inconsistencies in packet-forwarding engines can result in evasion of
   security controls [PARSING] [Atlasis2014] [BH-EU-2014].

   Sometimes, packets with IPv6 extension headers can impact throughput
   performance on intermediate systems.  Unless appropriate mitigations
   are put in place (e.g., packet dropping and/or rate limiting), an
   attacker could simply send a large amount of IPv6 traffic employing
   IPv6 extension headers with the purpose of performing a DoS attack
   (see Sections 6.1 and 8 for further details).  The extent to which
   performance is affected on these devices is implementation dependent.

      |  NOTE:
      |
      |     In the most trivial case, a packet that includes a Hop-by-
      |     Hop Options header might go through the slow forwarding
      |     path, to be processed by the router's CPU.  Alternatively, a
      |     router configured to enforce an ACL based on upper-layer
      |     information (e.g., upper-layer protocol type or TCP
      |     Destination Port) may need to process the entire IPv6 header
      |     chain in order to find the required information, thereby
      |     causing the packet to be processed in the slow path
      |     [Cisco-EH-Cons].  We note that, for obvious reasons, the
      |     aforementioned performance issues can affect devices such as
      |     firewalls, NIDSs, etc.  [Zack-FW-Benchmark].

   IPv6 implementations, like all other software, tend to mature with
   time and wide-scale deployment.  While the IPv6 protocol itself has
   existed for over 20 years, serious bugs related to IPv6 extension
   header processing continue to be discovered (see, e.g., [Cisco-Frag],
   [Microsoft-SA], and [FreeBSD-SA]).  Because there is currently little
   operational reliance on IPv6 extension headers, the corresponding
   code paths are rarely exercised, and there is the potential for bugs
   that still remain to be discovered in some implementations.

   The IPv6 Fragment Header is employed for the fragmentation and
   reassembly of IPv6 packets.  While many of the security implications
   of the fragmentation/reassembly mechanism are known from the IPv4
   world, several related issues have crept into IPv6 implementations.
   These range from DoS attacks to information leakages, as discussed in
   [RFC7739], [Bonica-NANOG58], and [Atlasis2012].

9.  IANA Considerations

   This document has no IANA actions.

10.  Security Considerations

   The security implications of IPv6 extension headers are discussed in
   Section 8.4.  This document does not introduce any new security
   issues.

11.  References

11.1.  Normative References

   [RFC5095]  Abley, J., Savola, P., and G. Neville-Neil, "Deprecation
              of Type 0 Routing Headers in IPv6", RFC 5095,
              DOI 10.17487/RFC5095, December 2007,
              <https://www.rfc-editor.org/info/rfc5095>.

   [RFC5722]  Krishnan, S., "Handling of Overlapping IPv6 Fragments",
              RFC 5722, DOI 10.17487/RFC5722, December 2009,
              <https://www.rfc-editor.org/info/rfc5722>.

   [RFC6946]  Gont, F., "Processing of IPv6 "Atomic" Fragments",
              RFC 6946, DOI 10.17487/RFC6946, May 2013,
              <https://www.rfc-editor.org/info/rfc6946>.

   [RFC6980]  Gont, F., "Security Implications of IPv6 Fragmentation
              with IPv6 Neighbor Discovery", RFC 6980,
              DOI 10.17487/RFC6980, August 2013,
              <https://www.rfc-editor.org/info/rfc6980>.

   [RFC7112]  Gont, F., Manral, V., and R. Bonica, "Implications of
              Oversized IPv6 Header Chains", RFC 7112,
              DOI 10.17487/RFC7112, January 2014,
              <https://www.rfc-editor.org/info/rfc7112>.

   [RFC8021]  Gont, F., Liu, W., and T. Anderson, "Generation of IPv6
              Atomic Fragments Considered Harmful", RFC 8021,
              DOI 10.17487/RFC8021, January 2017,
              <https://www.rfc-editor.org/info/rfc8021>.

   [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>.

11.2.  Informative References

   [Almeida-2020]
              Almeida, R., Cunha, I., Teixeira, R., Veitch, D., and C.
              Diot, "Classification of Load Balancing in the Internet",
              IEEE INFOCOM 2020, DOI 10.1109/INFOCOM41043.2020.9155387,
              July 2020, <https://homepages.dcc.ufmg.br/~cunha/papers/
              almeida20infocom-mca.pdf>.

   [APNIC-Scudder]
              Scudder, J., "Modern router architecture and IPv6", APNIC
              Blog, June 2020, <https://blog.apnic.net/2020/06/04/
              modern-router-architecture-and-ipv6/>.

   [Atlasis2012]
              Atlasis, A., "Attacking IPv6 Implementation Using
              Fragmentation", Black Hat Europe 2012, March 2012,
              <https://void.gr/kargig/ipv6/bh-eu-12-Atlasis-
              Attacking_IPv6-Slides.pdf>.

   [Atlasis2014]
              Atlasis, A., "A Novel Way of Abusing IPv6 Extension
              Headers to Evade IPv6 Security Devices", May 2014,
              <http://www.insinuator.net/2014/05/a-novel-way-of-abusing-
              ipv6-extension-headers-to-evade-ipv6-security-devices/>.

   [BH-EU-2014]
              Atlasis, A., Rey, E., and R. Schaefer, "Evasion of High-
              End IDPS Devices at the IPv6 Era", Black Hat Europe 2014,
              2014, <https://www.ernw.de/download/eu-14-Atlasis-Rey-
              Schaefer-briefings-Evasion-of-HighEnd-IPS-Devices-wp.pdf>.

   [Bonica-NANOG58]
              Bonica, R., "IPv6 Fragmentation: The Case For
              Deprecation", NANOG 58, June 2013,
              <https://www.nanog.org/sites/default/files/
              mon.general.fragmentation.bonica.pdf>.

   [Cisco-EH-Cons]
              Cisco, "IPv6 Extension Headers Review and Considerations",
              October 2006,
              <http://www.cisco.com/en/US/technologies/tk648/tk872/
              technologies_white_paper0900aecd8054d37d.pdf>.

   [Cisco-Frag]
              Cisco, "Cisco IOS XR Software Crafted IPv6 Packet Denial
              of Service Vulnerability", June 2015,
              <http://tools.cisco.com/security/center/content/
              CiscoSecurityAdvisory/cisco-sa-20150611-iosxr>.

   [FreeBSD-SA]
              The FreeBSD Project, "IPv6 Hop-by-Hop options use-after-
              free bug", September 2020,
              <https://www.freebsd.org/security/advisories/FreeBSD-SA-
              20:24.ipv6.asc>.

   [HEADERS]  Kumari, W., Jaeggli, J., Bonica, R. P., and J. Linkova,
              "Operational Issues Associated With Long IPv6 Header
              Chains", Work in Progress, Internet-Draft, draft-wkumari-
              long-headers-03, 16 June 2015,
              <https://datatracker.ietf.org/doc/html/draft-wkumari-long-
              headers-03>.

   [Huston-2017]
              Huston, G., "Dealing with IPv6 fragmentation in the DNS",
              APNIC Blog, August 2017,
              <https://blog.apnic.net/2017/08/22/dealing-ipv6-
              fragmentation-dns/>.

   [Huston-2020]
              Huston, G., "Measurement of IPv6 Extension Header
              Support", NPS/CAIDA 2020 Virtual IPv6 Workshop, June 2020,
              <https://www.cmand.org/workshops/202006-v6/
              slides/2020-06-16-xtn-hdrs.pdf>.

   [IEPG94-Scudder]
              Petersen, B. and J. Scudder, "Modern Router Architecture
              for Protocol Designers", IEPG 94, November 2015,
              <http://www.iepg.org/2015-11-01-ietf94/IEPG-
              RouterArchitecture-jgs.pdf>.

   [IPV6-EH]  Gont, F. and W. Liu, "Recommendations on the Filtering of
              IPv6 Packets Containing IPv6 Extension Headers at Transit
              Routers", Work in Progress, Internet-Draft, draft-ietf-
              opsec-ipv6-eh-filtering-08, 3 June 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-opsec-
              ipv6-eh-filtering-08>.

   [Jaeggli-2018]
              Jaeggli, J., "IPv6 flow label: misuse in hashing", APNIC
              Blog, January 2018, <https://blog.apnic.net/2018/01/11/
              ipv6-flow-label-misuse-hashing/>.

   [Linkova-Gont-IEPG90]
              Linkova, J. and F. Gont, "IPv6 Extension Headers in the
              Real World v2.0", IEPG 90, July 2014,
              <http://www.iepg.org/2014-07-20-ietf90/iepg-ietf90-ipv6-
              ehs-in-the-real-world-v2.0.pdf>.

   [Microsoft-SA]
              Microsoft, "Windows TCP/IP Remote Code Execution
              Vulnerability", CVE-2021-24094, February 2021,
              <https://msrc.microsoft.com/update-guide/vulnerability/
              CVE-2021-24094>.

   [nmap]     Lyon, G., "Firewall/IDS Evasion and Spoofing", Chapter 15.
              Nmap Reference Guide,
              <https://nmap.org/book/man-bypass-firewalls-ids.html>.

   [OPERATORS]
              Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
              M., and T. Taylor, Ed., "Why Operators Filter Fragments
              and What It Implies", Work in Progress, Internet-Draft,
              draft-taylor-v6ops-fragdrop-02, 3 December 2013,
              <https://datatracker.ietf.org/doc/html/draft-taylor-v6ops-
              fragdrop-02>.

   [PARSING]  Kampanakis, P., "Implementation Guidelines for Parsing
              IPv6 Extension Headers", Work in Progress, Internet-Draft,
              draft-kampanakis-6man-ipv6-eh-parsing-01, 5 August 2014,
              <https://datatracker.ietf.org/doc/html/draft-kampanakis-
              6man-ipv6-eh-parsing-01>.

   [PMTUD-Blackholes]
              De Boer, M. and J. Bosma, "Discovering Path MTU black
              holes on the Internet using RIPE Atlas", University of
              Amsterdam, MSc. Systems & Network Engineering, July 2012,
              <http://www.nlnetlabs.nl/downloads/publications/pmtu-
              black-holes-msc-thesis.pdf>.

   [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>.

   [RFC5635]  Kumari, W. and D. McPherson, "Remote Triggered Black Hole
              Filtering with Unicast Reverse Path Forwarding (uRPF)",
              RFC 5635, DOI 10.17487/RFC5635, August 2009,
              <https://www.rfc-editor.org/info/rfc5635>.

   [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>.

   [RFC6437]  Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
              "IPv6 Flow Label Specification", RFC 6437,
              DOI 10.17487/RFC6437, November 2011,
              <https://www.rfc-editor.org/info/rfc6437>.

   [RFC6438]  Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
              for Equal Cost Multipath Routing and Link Aggregation in
              Tunnels", RFC 6438, DOI 10.17487/RFC6438, November 2011,
              <https://www.rfc-editor.org/info/rfc6438>.

   [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>.

   [RFC7098]  Carpenter, B., Jiang, S., and W. Tarreau, "Using the IPv6
              Flow Label for Load Balancing in Server Farms", RFC 7098,
              DOI 10.17487/RFC7098, January 2014,
              <https://www.rfc-editor.org/info/rfc7098>.

   [RFC7113]  Gont, F., "Implementation Advice for IPv6 Router
              Advertisement Guard (RA-Guard)", RFC 7113,
              DOI 10.17487/RFC7113, February 2014,
              <https://www.rfc-editor.org/info/rfc7113>.

   [RFC7739]  Gont, F., "Security Implications of Predictable Fragment
              Identification Values", RFC 7739, DOI 10.17487/RFC7739,
              February 2016, <https://www.rfc-editor.org/info/rfc7739>.

   [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>.

   [RFC8900]  Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
              and F. Gont, "IP Fragmentation Considered Fragile",
              BCP 230, RFC 8900, DOI 10.17487/RFC8900, September 2020,
              <https://www.rfc-editor.org/info/rfc8900>.

   [RFC8955]  Loibl, C., Hares, S., Raszuk, R., McPherson, D., and M.
              Bacher, "Dissemination of Flow Specification Rules",
              RFC 8955, DOI 10.17487/RFC8955, December 2020,
              <https://www.rfc-editor.org/info/rfc8955>.

   [RFC8956]  Loibl, C., Ed., Raszuk, R., Ed., and S. Hares, Ed.,
              "Dissemination of Flow Specification Rules for IPv6",
              RFC 8956, DOI 10.17487/RFC8956, December 2020,
              <https://www.rfc-editor.org/info/rfc8956>.

   [Zack-FW-Benchmark]
              Zack, E., "Firewall Security Assessment and Benchmarking
              IPv6 Firewall Load Tests", IPv6 Hackers Meeting #1, June
              2013, <https://www.ipv6hackers.org/files/meetings/ipv6-
              hackers-1/zack-ipv6hackers1-firewall-security-assessment-
              and-benchmarking.pdf>.

Acknowledgements

   The authors would like to thank (in alphabetical order) Mikael
   Abrahamsson, Fred Baker, Dale W. Carder, Brian Carpenter, Tim Chown,
   Owen DeLong, Gorry Fairhurst, Guillermo Gont, Tom Herbert, Lee
   Howard, Tom Petch, Sander Steffann, Eduard Vasilenko, √Čric Vyncke,
   Rob Wilton, Jingrong Xie, and Andrew Yourtchenko for providing
   valuable comments on earlier draft versions of this document.

   Fernando Gont would like to thank Jan Zorz / Go6 Lab
   <https://go6lab.si/>, Jared Mauch, and Sander Steffann
   <https://steffann.nl/> for providing access to systems and networks
   that were employed to perform experiments and measurements involving
   packets with IPv6 extension headers.

Authors' Addresses

   Fernando Gont
   SI6 Networks
   Segurola y Habana 4310, 7mo Piso
   Villa Devoto
   Ciudad Autonoma de Buenos Aires
   Argentina

   Email: fgont@si6networks.com
   URI:   https://www.si6networks.com


   Nick Hilliard
   INEX
   4027 Kingswood Road
   Dublin
   24
   Ireland

   Email: nick@inex.ie


   Gert Doering
   SpaceNet AG
   Joseph-Dollinger-Bogen 14
   D-80807 Muenchen
   Germany

   Email: gert@space.net


   Warren Kumari
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   United States of America

   Email: warren@kumari.net


   Geoff Huston

   Email: gih@apnic.net
   URI:   https://www.apnic.net


   Will (Shucheng) Liu
   Huawei Technologies
   Bantian, Longgang District
   Shenzhen
   518129
   China

   Email: liushucheng@huawei.com