wdiff rfc9098v1.txt rfc9098.txt

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
July
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.

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. 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 the 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 the 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
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 for to
nodes processing IPv4 packets. The IPv4 header length is specified
in the Internet "Internet Header Length Length" (IHL) header field of the mandatory IPv4
header and must be in the range of 20 octets (the minimum IPv4 header
size) to 60 octets (accommodating octets, accommodating at most 40 octets of
options). 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
extension 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 how deep into the
IPv6 packet" packet the upper layer 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 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 that specify and have specified updates to RFC 2460 [RFC2460], an [RFC2460]
(an earlier version of the IPv6 standard, many standard). Many of which 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 and formally forbids such fragmentation). 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]
[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
further,
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.

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
form
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-forwarding 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- software-based
forwarding plane that is distinct both from the hardware-forwarding 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 and 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 TLV 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 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 lookup capability 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, could impact the data-plane throughput performance on the router.
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
packet
protocol header information. This n-tuple will typically use the src/
dst
src/dst Media Access Control (MAC) address, addresses, src/dst IP address, 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 "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 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, Flow Label,
and hash-based ECMP/load-sharing devices also did not employ the flow
label 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 Flow
Label for ECMP and load balancing [Cunha-2020]. [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
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/seconds 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 because more potential as a result of the increased attack vectors
open.
surface.

7.3. DDoS Management and Customer Requests for Filtering

The case of customer DDoS 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 flowspec BGP Flowspec
[RFC8955] [RFC8956].

For example, a website that normally only handled 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 traditional [RFC5635] source-based, real-time 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
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,
NIDSs, but they can also prevent intrusions by reacting to detected
attack
attempts, e.g., attempts by e.g., triggering packet-filtering 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 NIDS/IPS 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 NIDS/IPS 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
architecture;
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 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 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 Internet Header Length (IHL) IHL field of the IPv4 header field, 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. 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 other devices such as
| firewalls, NIDS, NIDSs, 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-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 Headers are Header is employed to allow 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 leakage, 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>.

[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]
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,
opsec-ipv6-eh-filtering-08, 3 June 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-opsec-
ipv6-eh-filtering>.
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, Éric 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: http://www.apnic.net https://www.apnic.net

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

Email: liushucheng@huawei.com