IPv6 Hop-by-Hop Header Handling
draft-baker-6man-hbh-header-handling-03

Abstract

This note updates the IPv6 Specification (RFC 2460), specifically commenting on the Hop-by-Hop Options Header (section 4.3) and option format and handling (section 4.2).

It also updates RFC 7045, which noted that RFC 2460 is widely violated in this respect, but merely legitimized this situation with a SHOULD. The present document tries to address the issue more fundamentally.

Status of This Memo

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Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 2. Handling of options in extension headers
• 2.1. Hop-by_hop Options
• 2.2. Changing options in transit
• 2.3. Adding headers or options in transit
• 2.4. Interactions with the Security Extension Header
• 3. Interoperation with RFC 2460
• 4. IANA Considerations
• 5. Security Considerations
• 6. Privacy Considerations
• 7. Acknowledgements
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Appendix A. Change Log
• Author's Address

1. Introduction

The IPv6 Specification [RFC2460] specifies a number of extension headers. These, and the ordering considerations given, were defined based on experience with IPv4 options. They were, however, prescient with respect to their actual use - the IETF community did not know how they would be used. In at least one case, the Hop-by-Hop option, most if not all implementations implement it by punting to a software path. In the words of [RFC7045],

• The IPv6 Hop-by-Hop Options header SHOULD be processed by intermediate forwarding nodes as described in [RFC2460]. However, it is to be expected that high-performance routers will either ignore it or assign packets containing it to a slow processing path. Designers planning to use a Hop-by-Hop option need to be aware of this likely behaviour.

Fernando Gont, in his Observations on IPv6 EH Filtering in the Real World [I-D.ietf-v6ops-ipv6-ehs-in-real-world], and the operational community in IPv6 Operations, consider any punt to a software path to be an attack vector. Hence, IPv6 packets containing the Hop-by-Hop Extension Header (and in some cases, any extension header) get dropped in transit.

The subject of this document is implementation approaches to obviate or mitigate the attack vector, and updating the Hop-by-Hop option with respect to current issues.

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

2. Handling of options in extension headers

Packets containing the Hop-by-Hop Extension Header SHOULD be processed at substantially the same rate as packets that do not.

If a hop-by-hop header option is not implemented, or is not in use, in a given system (such as, for example, an interface that is not configured for RSVP receiving an RSVP Alert Option), the option MUST be skipped.

If a hop-by-hop header is present in a packet's extension header chain and it is not the first extension header, the packet MUST be discarded by the first system that observes the fact (Section 2.2 of [RFC7045]). This will normally be in the system using the IPv6 address in the Destination Address, as [RFC2460] precludes other routers from parsing the header chain. The only obvious exception to that is a router or firewall configured to parse the IPv6 header chain.

2.1. Hop-by_hop Options

At this writing, there are several defined Hop-by-Hop options:

PAD Options:

The PAD1 and PADn options [RFC2460] define empty space.

Router Alert Option:

The IPv6 Router Alert Option [RFC2711] [RFC6398] is intended to force the punting of a datagram to software, in cases in which RSVP or other protocols need that to happen.

Jumbo Payload:

Carries a length field for a packet whose length exceeds 0xFFFF octets. [RFC2675]

RPL Option:

The RPL option carries routing information used in a RPL network[RFC6553]

Quickstart Option

Identifies TCP quick-start configuration, and allows an intermediate router to reduce the configuration parameters as appropriate. [RFC4782]

Common Architecture Label IPv6 Security Option:

Encodes security labels on packets [RFC5570]

SMF Option:

Simplified Multicast Forwarding Option[RFC6621]

MPL Option:

Supports multicast in an RPL network [I-D.ietf-roll-trickle-mcast]

DFF Option:

Depth-First Forwarding [RFC6971]

There are also options that have been defined for the Destination Options header. These are not listed here.

While this is not true of older implementations, modern equipment is capable of parsing the Extension Header chain, and can be extended to perform at least a cursory examination of the Hop-by-Hop options. For example, such implementations SHOULD be able to identify and skip the PAD1 and PADn options, and perform more complicated processing only if configured by software to do so. More to the point: it isn't clear what the purpose of the JumboFrame option is if not to be understood by anyone that looks at it.

• Question asked by a reviewer: "Is this configurable? How will router know that HbH needs to be skipped on one interface and not on others."
• Answer: the system knows whether RSVP has been configured on an interface. When such configuration is present, it can configure the hardware with what it wants done with the Router Alert. In the absence of such configuration, hardware should be configured to skip the option if found.

2.2. Changing options in transit

Section 4.2 of [RFC2460] explicitly allows for options that may be updated in transit. It is likely that the original authors intended that to be very simple, such as having the originating end system provide the container, and having intermediate systems update it - perhaps performing some calculation, and in any event storing the resulting value. Examples of such a use might be in [XCP] or [RCP].

As a side comment, the Routing Header, which is an extension header rather than a list of options, is treated similarly; when a system is the destination of a packet and not the last one in the Routing Header's list, it swaps the destination address with the indicated address in the list, and updates the hop count and the list depth accordingly.

Such options must be marked appropriately (their option type is of the form XX1XXXXX), and are excluded from checksum calculations in AH and ESP.

2.3. Adding headers or options in transit

Use cases under current consideration take this a step further: a router or middleware process MAY add an extension header, MAY add an option to the header, which may extend the length of the Hop-by-Hop Extension Header, or MAY process such an option in a manner that extends both the length of the option and the Extension Header containing it. The obvious implication is that other equipment in the network may not understand or implement the new option type. As such, the Option Type value of such an option MUST indicate that it is to be skipped by a system that does not understand it. Since, by definition, it is being updated in transit and not included in any AH or ESP integrity check if present, the Option Type MUST also indicate that it may be updated in transit, and so is excluded from AH and ESP processing. By implication, such an Option Type MUST be of the form 001XXXXX.

2.4. Interactions with the Security Extension Header

The interactions with the IP Authentication Header [RFC4302] and IP Encapsulating Security Payload (ESP) [RFC4303], as in the case of existing option uses, is minimally defined. AH and ESP call for the exclusion of mutable data in their calculations by zeroing it out prior to performing the integrity check calculation. However, in the case that network operation has changed the length of the option or the extension header, that may still cause the integrity check to fail. Specifications that define such options SHOULD consider the implications of this for AH and ESP. An option whose insertion would affect the integrity check MUST be removed prior to the integrity check, and as a result the packet restored to its state as originally sent.

3. Interoperation with RFC 2460

There are four possible modes of interaction with routers that don't implement the Hop-By-Hop Option in the fast path:

1. Presume that they cannot handle the Hop-By-Hop option at close to wire speed, and that's OK.
2. Presume that they will drop traffic containing Hop-By-Hop options.
3. Presume that they can handle the Hop-By-Hop option at or close to wire speed, and are configured to do so.
4. Presume that they don't exist, perhaps because older routers are configured to ignore all Hop-by-Hop options.

If the first model actually works in a given network, it may be acceptable in that domain. It is not a model that will work in the general Internet, however.

The second model (which is most probable at this writing) is a description of the general Internet in 2015.

The third and fourth models, if applicable in a given context, are what one might hope for. Vendors are in a position to either have an option to ignore the Hop-By-Hop header in older equipment, or add such an option in upgraded software (fourth model). New equipment is expected to follow the third model by implementing the recommendations in Section 2.

4. IANA Considerations

This memo asks the IANA for no new parameters.

5. Security Considerations

In general, modification of a datagram in transit is considered very closely from the viewpoint of the End-to-End Principle, which in this context may be summarized as "the network should do nothing that is of concern to the communicating applications or introduces operational issues." The concept of changing the length of an Extension Header or an option contained within it (Section 2.3) is of concern in that context. The obvious concern is around the interaction with AH or ESP, and a less obvious concern relates to Path MTU, which might change if the size of an underlying header changes. Section 2.4 is intended to mitigate that issue. However, some ramifications, such as with Path MTU, may not be completely solvable in the general Internet, but require use cases to be confined to a network or set of consenting networks.

6. Privacy Considerations

Data formats in this memo reveal no personally identifying information.

7. Acknowledgements

This note grew out of a discussion among the author, Ole Troan, Mark Townsley, Frank Brockners, and Shwetha Bhandari, and benefited from comments by Dennis Ferguson, Brian Carpenter, Panos Kampanakis, JINMEI Tatuya, and Joe Touch.

8. References

8.1. Normative References

8.2. Informative References

Appendix A. Change Log

Initial Version:

June 2015

01 Version:

June 2015, responding to list discussion

02 Version:

July 2015, discussed at IETF 93

03 Version:

October 2015

Author's Address