IPv6 Operations Working Group (v6ops)                            F. Gont
Internet-Draft                                              SI6 Networks
Intended status: Informational                               N. Hilliard
Expires: October 10, December 11, 2021                                          INEX
                                                              G. Doering
                                                             SpaceNet AG
                                                               W. Kumari
                                                                  Google
                                                               G. Huston
                                                                   APNIC
                                                                  W. Liu
                                                     Huawei Technologies
                                                           April 8,
                                                            June 9, 2021

    Operational Implications of IPv6 Packets with Extension Headers
               draft-ietf-v6ops-ipv6-ehs-packet-drops-06
               draft-ietf-v6ops-ipv6-ehs-packet-drops-07

Abstract

   This document summarizes the operational implications of IPv6
   extension headers specified in the IPv6 protocol specification
   (RFC8200), and attempts to analyze reasons why packets with IPv6
   extension headers are often dropped in the public Internet.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 10, December 11, 2021.

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
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Disclaimer  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Disclaimer  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Background Information  . . . . . . . . . . . . . . . . . . .   3
   4.
   5.  Previous Work on IPv6 Extension Headers . . . . . . . . . . .   5
   5.
   6.  Packet Forwarding Engine Constraints  . . . . . . . . . . . .   7
     5.1.
     6.1.  Recirculation . . . . . . . . . . . . . . . . . . . . . .   8
   6.
   7.  Requirement to Process Layer-3/layer-4 information in
       Intermediate Systems  . . . . . . . . . . . . . . . . . . . .   8
     6.1.
     7.1.  ECMP and Hash-based Load-Sharing  . . . . . . . . . . . .   8
     6.2.
     7.2.  Enforcing infrastructure ACLs . . . . . . . . . . . . . .   9
     6.3.
     7.3.  DDoS Management and Customer Requests for Filtering . . .   9
     6.4.  10
     7.4.  Network Intrusion Detection and Prevention  . . . . . . .  10
     6.5.
     7.5.  Firewalling . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  11
   8.  Operational and Security Implications . . . . . . . . . . . . . . . . . .  11
     7.1.  12
     8.1.  Inability to Find Layer-4 Information . . . . . . . . . .  11
     7.2.  12
     8.2.  Route-Processor Protection  . . . . . . . . . . . . . . .  11
     7.3.  12
     8.3.  Inability to Perform Fine-grained Filtering . . . . . . .  12
     7.4.
     8.4.  Security Concerns Associated with IPv6 Extension Headers   12
   8.
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   9.  14
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  13
   10.  14
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   11.
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     11.1.
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     11.2.
     12.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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
   middle-boxes, and evidence exists that IPv6 packets with EHs are
   intentionally dropped in the public Internet in some network
   deployments. circumstances.

   This document has the following goals:

   o  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.

   o  Highlight areas where current IPv6 support by networking devices
      maybe sub-optimal, such that the aforementioned support is
      improved.

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

   Section 3 4 provides background information about the IPv6 packet
   structure and associated implications.  Section 4 5 of this document
   summarizes the previous work that has been carried out in the area of
   IPv6 extension headers.  Section 5 6 discusses packet forwarding engine
   constraints in contemporary routers.  Section 6 7 discusses why
   contemporary routers and middle-boxes
   intermediate systems may need to access Layer-4 information to make a
   forwarding decision.  Finally, Section 7 8 discusses the operational
   implications of IPv6 EHs.

2.  Terminology

   This document uses the term "intermediate system" to describe both
   routers and middle-boxes, 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.

3.

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 by
   nodes processing IPv4 packets.  The IPv4 header length is specified
   in the IHL header field of the mandatory IPv4 header, and must be in
   the range from 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
   linked-list-like packet structure, 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" type that follows in the IPv6 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:

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

   o  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 may be found.

   o  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.

4.

5.  Previous Work on IPv6 Extension Headers

   Some of the operational and security implications of IPv6 Extension
   Headers have been discussed at the IETF:

   o  [I-D.taylor-v6ops-fragdrop] discusses a rationale for which
      operators drop IPv6 fragments.

   o  [I-D.wkumari-long-headers] discusses possible issues arising from
      "long" IPv6 header chains.

   o  [I-D.kampanakis-6man-ipv6-eh-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.

   o  [I-D.ietf-opsec-ipv6-eh-filtering] analyzes the security
      implications of IPv6 EHs, and the operational implications of
      dropping packets that employ IPv6 EHs and associated options.

   o  [RFC7113] discusses how some popular RA-Guard implementations are
      subject to evasion by means of IPv6 extension headers.

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

   A number of recent RFCs have discussed issues related to IPv6
   extension headers, specifying updates to a previous revision of the
   IPv6 standard [RFC2460], many of which have now been incorporated
   into the current IPv6 core standard [RFC8200] or the IPv6 Node
   Requirements [RFC8504].  Namely,
   o  [RFC5095] discusses the security implications of Routing Header
      Type 0 (RTH0), and deprecates it.

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

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

   o  [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).

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

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

   o  [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.

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

   o  [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
   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:

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

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

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

5.

6.  Packet Forwarding Engine Constraints

   Most contemporary carrier-grade routers use dedicated hardware (e.g.,
   ASICs hardware, e.g.
   application-specific integrated circuits (ASICs) or NPUs) network
   processing units (NPUs), to determine how to forward packets across
   their internal fabrics (see [IEPG94-Scudder] and [APNIC-Scudder] for
   details).  One of the common methods of handling next-hop lookup is
   to send a small portion of the ingress packet to a lookup engine with
   specialised hardware (e.g., hardware, e.g. ternary CAM content-addressable memory (TCAM)
   or Reduced Latency DRAM) 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 look-up engine, then the router will normally drop
   the packet.  Section 6 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
   form 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 forwarding capacity of the
   router and processing capacity of the control plane and the size of
   the management link which connects the control plane to the
   forwarding plane.  Other platforms may have a separate software
   forwarding plane that is distinct both from the hardware 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 that it exceeds the
   packet look-up 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.

5.1.

6.1.  Recirculation

   Although TLV chains are amenable to iterative processing on
   architectures that have packet look-up engines with deep inspection
   capabilities, some packet forwarding engines manage IPv6 Extension
   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 look-up capability, because it allows arbitrarily long header
   chains to be processed without the complexity and cost associated
   with packet forwarding engines which have deep look-up 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, this could impact data-plane throughput
   performance on the router.

6.

7.  Requirement to Process Layer-3/layer-4 information in Intermediate
    Systems

   The following subsections discuss some of the reasons for which
   contemporary routers and middle-boxes
   intermediate systems may need to process Layer-3/
   layer-4 Layer-3/layer-4 information
   to make a forwarding decision.

6.1.

7.1.  ECMP and Hash-based Load-Sharing

   In the case of ECMP (equal equal cost multi path) multi-path (ECMP) load sharing, the router
   on the sending side of the link
   intermediate system needs to make a decision regarding which of the links its
   interfaces to use to forward a given packet.  Since round-
   robin 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
   packet header information.  This n-tuple will typically use the src/dst src/
   dst MAC address, src/dst IP address, 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 [Cunha-2020].  A contributing
   factor could be the issues that have been found in host
   implementations and middle-boxes [Jaeggli-2018].

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

6.2.

   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 ACLs (iACLs) drop unwanted packets destined to a
   network's infrastructure IP addresses. 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:

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

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

6.3.

   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 due to having more potential attack
   vectors open.

7.3.  DDoS Management and Customer Requests for Filtering

   The case of customer 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 Flowspec [RFC5575].

   For example, a web site that normally only handled traffic on TCP
   ports 80 and 443 could be subject to a volumetric DDoS attack using
   NTP and DNS packets with randomised source IP address, thereby
   rendering traditional [RFC5635] source-based real-time black hole
   mechanisms useless.  In this situation, DDoS protection ACLs 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.

6.4.

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 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
   NIDS's, 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:

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

   o  Use of unknown extension headers can prevent an NIDS/IPS from
      processing layer-4 information.

   o  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.

6.5.

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 NIDS/IPS (Section 6.4), 7.4), use of IPv6 extension headers can
   represent a challenge to network firewalls, since:

   o  Extension headers increase the complexity of resulting traffic,
      and the associated work and system requirements to process it (see
      e.g., [Zack-FW-Benchmark]). it, as
      outlined in [Zack-FW-Benchmark].

   o  Use of unknown extension headers can prevent firewalls from
      processing layer-4 information.

   o  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.

7.

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

8.  Operational and Security Implications

7.1.

8.1.  Inability to Find Layer-4 Information

   As discussed in Section 6, routers and middle-boxes 7, intermediate systems that need to find the
   layer-4 header must process the entire IPv6 extension 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 6.5), 7.5), while processing packets outside of the normal
   forwarding path will usually open the door to DoS attacks (see e.g.,
   Section 5). 6).  Thus, in these scenarios, devices often simply resort to
   dropping such packets unconditionally.

7.2.

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 currently drop IPv6
   packets containing this extension header. header [RFC7872].  [RFC6192]
   provides advice regarding protection of a router's control plane.

7.3.

8.3.  Inability to Perform Fine-grained Filtering

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

7.4.

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:

   o  Evasion of security controls
   o  DoS due to processing requirements

   o  DoS due to implementation errors

   o  Extension Header-specific issues

   Unlike IPv4 packets where the upper-layer protocol can be trivially
   found by means of the "IHL" ("Internet Header Length") IPv4 header
   field, 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.  [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
   [I-D.kampanakis-6man-ipv6-eh-parsing] [Atlasis2014] [BH-EU-2014].

   Sometimes packets with IPv6 Extension Headers can impact throughput
   performance on routers and middleboxes. intermediate systems.  Unless appropriate mitigations
   are put in place (e.g., packet dropping and/or rate-
   limiting), rate-limiting), an
   attacker could simply send a large amount of IPv6 traffic employing
   IPv6 Extension Headers with the purpose of performing a Denial of
   Service (DoS) attack (see Section 7 6.1 and Section 8 for further
   details).

   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 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 other devices such as
      firewalls, Network Intrusion Detection Systems (NIDS), etc.
      [Zack-FW-Benchmark].  The extent to which performance is affected
      on these devices is implementation-dependent.

   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-Frag1],
   [Cisco-Frag2], [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.

   IPv6 Fragment Headers are employed to allow fragmentation 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 denial of service attacks to information leakage, as
   discussed in [RFC7739], [Bonica-NANOG58] and [Atlasis2012]).

8.

9.  IANA Considerations

   This document has no IANA actions.

9.

10.  Security Considerations

   The security implications of IPv6 extension headers are discussed in
   Section 7.4. 8.4.  This document does not introduce any new security
   issues.

10.

11.  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, Eric Vyncke,
   Rob Wilton, Jingrong Xie, and Andrew Yourtchenko, for providing
   valuable comments on earlier 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.

11.

12.  References

11.1.

12.1.  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.

12.2.  Informative References

   [APNIC-Scudder]
              Scudder, J., "Modern router architecture and IPv6",  APNIC
              Blog, June 4, 2020, <https://blog.apnic.net/2020/06/04/
              modern-router-architecture-and-ipv6/>.

   [Atlasis2012]
              Atlasis, A., "Attacking IPv6 Implementation Using
              Fragmentation",  BlackHat Europe 2012. Amsterdam,
              Netherlands. March 14-16, 2012,
              <https://media.blackhat.com/bh-eu-12/Atlasis/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",  BlackHat 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. New Orleans, Louisiana, USA. June
              3-5, 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-Frag1]
              Cisco, "Cisco IOS Software IPv6 Virtual Fragmentation
              Reassembly Denial of Service Vulnerability", September
              2013, <http://tools.cisco.com/security/center/content/
              CiscoSecurityAdvisory/cisco-sa-20130925-ipv6vfr>.

   [Cisco-Frag2]

   [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>.

   [Cunha-2020]
              Cunha, I., "IPv4 vs IPv6 load balancing in Internet
              routes",  NPS/CAIDA 2020 Virtual IPv6 Workshop, 2020,
              <https://www.cmand.org/workshops/202006-v6/slides/
              cunha.pdf>.

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

   [Huston-2017]
              Huston, G., "Dealing with IPv6 fragmentation in the
              DNS",  APNIC Blog, 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, 2020,
              <https://www.cmand.org/workshops/202006-v6/
              slides/2020-06-16-xtn-hdrs.pdf>.

   [I-D.ietf-opsec-ipv6-eh-filtering]
              Gont, F. and W. LIU, Liu, "Recommendations on the Filtering of
              IPv6 Packets Containing IPv6 Extension Headers at Transit
              Routers", draft-ietf-opsec-ipv6-eh-filtering-07 (work in
              progress), January 2021.

   [I-D.kampanakis-6man-ipv6-eh-parsing]
              Kampanakis, P., "Implementation Guidelines for parsing
              IPv6 Extension Headers", draft-kampanakis-6man-ipv6-eh-
              parsing-01 (work in progress), August 2014.

   [I-D.taylor-v6ops-fragdrop]
              Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
              M., and T. Taylor, "Why Operators Filter Fragments and
              What It Implies", draft-taylor-v6ops-fragdrop-02 (work in
              progress), December 2013.

   [I-D.wkumari-long-headers]
              Kumari, W., Jaeggli, J., Bonica, R., R. P., and J. Linkova,
              "Operational Issues Associated With Long IPv6 Header
              Chains", draft-wkumari-long-headers-03 (work in progress),
              June 2015.

   [IEPG94-Scudder]
              Petersen, B. and J. Scudder, "Modern Router Architecture
              for Protocol Designers",  IEPG 94. Yokohama, Japan.
              November 1, 2015, <http://www.iepg.org/2015-11-01-ietf94/
              IEPG-RouterArchitecture-jgs.pdf>.

   [Jaeggli-2018]
              Jaeggli, J., "IPv6 flow label: misuse in hashing",  APNIC
              Blog, 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. Toronto, ON, Canada. July 20,
              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]     Fyodor, "Dealing with IPv6 fragmentation in the
              DNS",  Firewall/IDS Evasion and Spoofing,
              <https://nmap.org/book/man-bypass-firewalls-ids.html>.

   [PMTUD-Blackholes]
              De Boer, M. and J. Bosma, "Discovering Path MTU black
              holes on the Internet using RIPE Atlas", 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>.

   [RFC5575]  Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
              and D. McPherson, "Dissemination of Flow Specification
              Rules", RFC 5575, DOI 10.17487/RFC5575, August 2009,
              <https://www.rfc-editor.org/info/rfc5575>.

   [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>.

   [Zack-FW-Benchmark]
              Zack, E., "Firewall Security Assessment and Benchmarking
              IPv6 Firewall Load Tests",  IPv6 Hackers Meeting #1,
              Berlin, Germany. June 30, 2013,
              <https://www.ipv6hackers.org/files/meetings/ipv6-hackers-
              1/zack-ipv6hackers1-firewall-security-assessment-and-
              benchmarking.pdf>.

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
   IE

   Email: nick@inex.ie

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

   Email: gert@space.net

   Warren Kumari
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   Email: warren@kumari.net

   Geoff Huston

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

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

   Email: liushucheng@huawei.com