SI6 Networks

Segurola y Habana 4310, 7mo Piso Villa Devoto Ciudad Autonoma de Buenos Aires Argentina fgont@si6networks.com https://www.si6networks.com

INEX

4027 Kingswood Road Dublin 24 Ireland nick@inex.ie

SpaceNet AG

Joseph-Dollinger-Bogen 14 Muenchen D-80807 Germany gert@space.net

Google

1600 Amphitheatre Parkway Mountain View CA 94043 US warren@kumari.net

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

Huawei Technologies

Bantian, Longgang District Shenzhen 518129 China liushucheng@huawei.com

Operations and Management IPv6 Operations Working Group (v6ops) 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 .

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 () 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

• .  Introduction
• .  Terminology
• .  Disclaimer
• .  Background Information
• .  Previous Work on IPv6 Extension Headers
• .  Packet-Forwarding Engine Constraints
  • .  Recirculation
• .  Requirement to Process Layer 3 / Layer 4 Information in Intermediate Systems
  • .  ECMP and Hash-Based Load Sharing
  • .  Enforcing Infrastructure ACLs
  • .  DDoS Management and Customer Requests for Filtering
  • .  Network Intrusion Detection and Prevention
  • .  Firewalling
• .  Operational and Security Implications
  • .  Inability to Find Layer 4 Information
  • .  Route-Processor Protection
  • .  Inability to Perform Fine-Grained Filtering
  • .  Security Concerns Associated with IPv6 Extension Headers
• .  IANA Considerations
• . Security Considerations
• . References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Authors' Addresses

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 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.

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

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.

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.

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.

IPv4 Packet Structure Protocol, IHL +--------+ | | | v +------//-----+------------------------+ | | | | IPv4 | Upper-Layer | | Header | Protocol | | | | +-----//------+------------------------+ variable length <------------->

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.

IPv6 Packet Structure 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 <------------> <-------------------------------->

This packet structure has the following implications:

• 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.

Previous Work on IPv6 Extension Headers Some of the operational and security implications of IPv6 extension headers have been discussed in the IETF:

• discusses a rationale for which operators drop IPv6 fragments.
• discusses possible issues arising from "long" IPv6 header chains.
• 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.
• analyzes the security implications of IPv6 EHs, as well as the operational implications of dropping packets that employ IPv6 EHs and associated options.
• discusses how some popular Router Advertisement Guard (RA-Guard) implementations are subject to evasion by means of IPv6 extension headers.
• 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 (an earlier version of the IPv6 standard). Many of these updates have now been incorporated into the current IPv6 core standard or the IPv6 node requirements . Namely,

• discusses the security implications of Routing Header Type 0 (RHT0) and deprecates it.
• analyzes the security implications of overlapping fragments and provides recommendations in this area.
• clarifies how intermediate nodes should deal with IPv6 extension headers.
• 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.
• discusses a flawed (but common) processing of the so-called IPv6 "atomic fragments" and specifies improved processing of such packets.
• deprecates the generation of IPv6 atomic fragments.
• clarifies processing rules for packets with extension headers and also allows hosts to enforce limits on the number of options included in IPv6 EHs.
• discusses the security implications of predictable fragment Identification values and provides recommendations for the generation of these values.
• analyzes the security implications of employing IPv6 fragmentation with Neighbor Discovery for IPv6 and formally recommends against such usage.

Additionally, has relaxed the requirement that "all nodes must examine and process the Hop-by-Hop Options header" from , 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:

• and present some preliminary measurements regarding the extent to which packets containing IPv6 EHs are dropped in the public Internet.
• presents more comprehensive results and documents the methodology used to obtain these results.
• and measure packet drops resulting from IPv6 fragmentation when communicating with DNS servers.

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 and 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. 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. 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.

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.

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.

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" . 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. 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 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 . A contributing factor could be the issues that have been found in host implementations and middleboxes . Clearly, widespread support of would relieve intermediate systems from having to process the entire IPv6 header chain, making Flow Label-based ECMP and load sharing 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.

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.

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 . 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 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.

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., ).

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.

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 (), 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 .
• 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., ).

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 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.

Operational and Security Implications

Inability to Find Layer 4 Information As discussed in , 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., ), while processing packets outside of the normal forwarding path will usually open the door to Denial-of-Service (DoS) attacks (see, e.g., ). Thus, in these scenarios, devices often simply resort to dropping such packets unconditionally.

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 . provides advice regarding protection of a router's control plane.

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").

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. 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 . 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 and for further details). 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., , , and ). 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 , , and .

IANA Considerations This document has no IANA actions.

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

References Normative References The functionality provided by IPv6's Type 0 Routing Header can be exploited in order to achieve traffic amplification over a remote path for the purposes of generating denial-of-service traffic. This document updates the IPv6 specification to deprecate the use of IPv6 Type 0 Routing Headers, in light of this security concern. [STANDARDS-TRACK] The fragmentation and reassembly algorithm specified in the base IPv6 specification allows fragments to overlap. This document demonstrates the security issues associated with allowing overlapping fragments and updates the IPv6 specification to explicitly forbid overlapping fragments. [STANDARDS-TRACK] The IPv6 specification allows packets to contain a Fragment Header without the packet being actually fragmented into multiple pieces (we refer to these packets as "atomic fragments"). Such packets are typically sent by hosts that have received an ICMPv6 "Packet Too Big" error message that advertises a Next-Hop MTU smaller than 1280 bytes, and are currently processed by some implementations as normal "fragmented traffic" (i.e., they are "reassembled" with any other queued fragments that supposedly correspond to the same original packet). Thus, an attacker can cause hosts to employ atomic fragments by forging ICMPv6 "Packet Too Big" error messages, and then launch any fragmentation-based attacks against such traffic. This document discusses the generation of the aforementioned atomic fragments and the corresponding security implications. Additionally, this document formally updates RFC 2460 and RFC 5722, such that IPv6 atomic fragments are processed independently of any other fragments, thus completely eliminating the aforementioned attack vector. This document analyzes the security implications of employing IPv6 fragmentation with Neighbor Discovery (ND) messages. It updates RFC 4861 such that use of the IPv6 Fragmentation Header is forbidden in all Neighbor Discovery messages, thus allowing for simple and effective countermeasures for Neighbor Discovery attacks. Finally, it discusses the security implications of using IPv6 fragmentation with SEcure Neighbor Discovery (SEND) and formally updates RFC 3971 to provide advice regarding how the aforementioned security implications can be mitigated. The IPv6 specification allows IPv6 Header Chains of an arbitrary size. The specification also allows options that can, in turn, extend each of the headers. In those scenarios in which the IPv6 Header Chain or options are unusually long and packets are fragmented, or scenarios in which the fragment size is very small, the First Fragment of a packet may fail to include the entire IPv6 Header Chain. This document discusses the interoperability and security problems of such traffic, and updates RFC 2460 such that the First Fragment of a packet is required to contain the entire IPv6 Header Chain. This document discusses the security implications of the generation of IPv6 atomic fragments and a number of interoperability issues associated with IPv6 atomic fragments. It concludes that the aforementioned functionality is undesirable and thus documents the motivation for removing this functionality from an upcoming revision of the core IPv6 protocol specification (RFC 2460). This document specifies version 6 of the Internet Protocol (IPv6). It obsoletes RFC 2460. This document defines requirements for IPv6 nodes. It is expected that IPv6 will be deployed in a wide range of devices and situations. Specifying the requirements for IPv6 nodes allows IPv6 to function well and interoperate in a large number of situations and deployments. This document obsoletes RFC 6434, and in turn RFC 4294. Informative References IEEE INFOCOM 2020 Juniper Networks APNIC Blog Black Hat Europe 2012 Black Hat Europe 2014 NANOG 58 Cisco Cisco The FreeBSD Project This memo specifies requirements for IPv6 forwarders as they process packets with long header chains. It also provides guidance for application developers whose applications might rely on long headers chains. As background, this memo explains how many ASIC-based IPv6 forwarders process packets and why processing of packets with long header chains might be problematic. Work in Progress APNIC Blog NPS/CAIDA 2020 Virtual IPv6 Workshop Juniper Networks Juniper Networks IEPG 94 Work in Progress APNIC Blog IEPG 90 Microsoft CVE-2021-24094 Chapter 15. Nmap Reference Guide Work in Progress Work in Progress University of Amsterdam, MSc. Systems & Network Engineering This document specifies version 6 of the Internet Protocol (IPv6), also sometimes referred to as IP Next Generation or IPng. [STANDARDS-TRACK] Remote Triggered Black Hole (RTBH) filtering is a popular and effective technique for the mitigation of denial-of-service attacks. This document expands upon destination-based RTBH filtering by outlining a method to enable filtering by source address as well. This memo provides information for the Internet community. This memo provides a method for protecting a router's control plane from undesired or malicious traffic. In this approach, all legitimate router control plane traffic is identified. Once legitimate traffic has been identified, a filter is deployed in the router's forwarding plane. That filter prevents traffic not specifically identified as legitimate from reaching the router's control plane, or rate-limits such traffic to an acceptable level. Note that the filters described in this memo are applied only to traffic that is destined for the router, and not to all traffic that is passing through the router. This document is not an Internet Standards Track specification; it is published for informational purposes. This document specifies the IPv6 Flow Label field and the minimum requirements for IPv6 nodes labeling flows, IPv6 nodes forwarding labeled packets, and flow state establishment methods. Even when mentioned as examples of possible uses of the flow labeling, more detailed requirements for specific use cases are out of the scope for this document. The usage of the Flow Label field enables efficient IPv6 flow classification based only on IPv6 main header fields in fixed positions. [STANDARDS-TRACK] The IPv6 flow label has certain restrictions on its use. This document describes how those restrictions apply when using the flow label for load balancing by equal cost multipath routing and for link aggregation, particularly for IP-in-IPv6 tunneled traffic. [STANDARDS-TRACK] Various IPv6 extension headers have been standardised since the IPv6 standard was first published. This document updates RFC 2460 to clarify how intermediate nodes should deal with such extension headers and with any that are defined in the future. It also specifies how extension headers should be registered by IANA, with a corresponding minor update to RFC 2780. This document describes how the currently specified IPv6 flow label can be used to enhance layer 3/4 (L3/4) load distribution and balancing for large server farms. The IPv6 Router Advertisement Guard (RA-Guard) mechanism is commonly employed to mitigate attack vectors based on forged ICMPv6 Router Advertisement messages. Many existing IPv6 deployments rely on RA-Guard as the first line of defense against the aforementioned attack vectors. However, some implementations of RA-Guard have been found to be prone to circumvention by employing IPv6 Extension Headers. This document describes the evasion techniques that affect the aforementioned implementations and formally updates RFC 6105, such that the aforementioned RA-Guard evasion vectors are eliminated. IPv6 specifies the Fragment Header, which is employed for the fragmentation and reassembly mechanisms. The Fragment Header contains an "Identification" field that, together with the IPv6 Source Address and the IPv6 Destination Address of a packet, identifies fragments that correspond to the same original datagram, such that they can be reassembled together by the receiving host. The only requirement for setting the Identification field is that the corresponding value must be different than that employed for any other fragmented datagram sent recently with the same Source Address and Destination Address. Some implementations use a simple global counter for setting the Identification field, thus leading to predictable Identification values. This document analyzes the security implications of predictable Identification values, and provides implementation guidance for setting the Identification field of the Fragment Header, such that the aforementioned security implications are mitigated. This document presents real-world data regarding the extent to which packets with IPv6 Extension Headers (EHs) are dropped in the Internet (as originally measured in August 2014 and later in June 2015, with similar results) and where in the network such dropping occurs. The aforementioned results serve as a problem statement that is expected to trigger operational advice on the filtering of IPv6 packets carrying IPv6 EHs so that the situation improves over time. This document also explains how the results were obtained, such that the corresponding measurements can be reproduced by other members of the community and repeated over time to observe changes in the handling of packets with IPv6 EHs. This document describes IP fragmentation and explains how it introduces fragility to Internet communication. This document also proposes alternatives to IP fragmentation and provides recommendations for developers and network operators. This document defines a Border Gateway Protocol Network Layer Reachability Information (BGP NLRI) encoding format that can be used to distribute (intra-domain and inter-domain) traffic Flow Specifications for IPv4 unicast and IPv4 BGP/MPLS VPN services. This allows the routing system to propagate information regarding more specific components of the traffic aggregate defined by an IP destination prefix. It also specifies BGP Extended Community encoding formats, which can be used to propagate Traffic Filtering Actions along with the Flow Specification NLRI. Those Traffic Filtering Actions encode actions a routing system can take if the packet matches the Flow Specification. This document obsoletes both RFC 5575 and RFC 7674. "Dissemination of Flow Specification Rules" (RFC 8955) provides a Border Gateway Protocol (BGP) extension for the propagation of traffic flow information for the purpose of rate limiting or filtering IPv4 protocol data packets. This document extends RFC 8955 with IPv6 functionality. It also updates RFC 8955 by changing the IANA Flow Spec Component Types registry. IPv6 Hackers Meeting #1

Acknowledgements The authors would like to thank (in alphabetical order) , , , , , , , , , , , , , , , , and for providing valuable comments on earlier draft versions of this document. would like to thank / Go6 Lab , , and for providing access to systems and networks that were employed to perform experiments and measurements involving packets with IPv6 extension headers.

Authors' Addresses SI6 Networks

Segurola y Habana 4310, 7mo Piso Villa Devoto Ciudad Autonoma de Buenos Aires Argentina fgont@si6networks.com https://www.si6networks.com

INEX

4027 Kingswood Road Dublin 24 Ireland nick@inex.ie

SpaceNet AG

Joseph-Dollinger-Bogen 14 Muenchen D-80807 Germany gert@space.net

Google

1600 Amphitheatre Parkway Mountain View CA 94043 US warren@kumari.net

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

Huawei Technologies

Bantian, Longgang District Shenzhen 518129 China liushucheng@huawei.com