Representing IPv6 Zone Identifiers in Uniform Resource Identifiers
draft-ietf-6man-uri-zoneid-00

Abstract

This document describes how the Zone Identifier of an IPv6 scoped address can be represented in a Uniform Resource Identifier that includes a literal IPv6 address. It updates RFC 3986 and RFC 4007.

Status of this Memo

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

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This Internet-Draft will expire on August 18, 2012.

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Specification
• 3. Security Considerations
• 4. IANA Considerations
• 5. Acknowledgements
• 6. Change log [RFC Editor: Please remove]
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Authors' Addresses

1. Introduction

[RFC3986] defined how a literal IPv6 address can be represented in the "host" part of a Uniform Resource Identifier (URI). Subsequently, [RFC4007] extended the text representation of limited-scope IPv6 addresses such that a zone identifier may be concatenated to an address, for purposes described in that RFC. Zone identifiers are especially useful in contexts where literal addresses are typically used, for example during fault diagnosis, when it may be essential to specify which interface is used for sending to a link local address. It should be noted that zone identifiers have purely local meaning within the host where they are defined, and they are completely meaningless for any other host.

RFC 4007 does not specify how zone identifiers are to be represented in URIs. Practical experience has shown that this feature is useful, in particular when using a web browser for debugging with link local addresses, but as it is undefined, it is not implemented consistently in URI parsers or in browsers.

This document updates [RFC3986] by adding syntax to allow a zone identifier to be included in a literal IPv6 address. It also clarifies some statements in [RFC4007].

It should be noted that in other contexts than a user interface, a zone identifier is mapped into a numeric zone index or interface number. The MIB textual convention [RFC4001] and the socket interface [RFC3493] define this as a 32 bit unsigned integer. The mapping between the human-readable zone identifier string and the numeric value is a host-specific function that varies between operating systems. The present document is concerned only with the human-readable string.

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. Specification

According to RFC 4007, a zone identifier is attached to the textual representation of an IPv6 address by concatenating "%" followed by <zone_id>, where <zone_id> is a string identifying the zone of the address. However, RFC 4007 gives no precise definition of the character set allowed in <zone_id>. There are no rules or de facto standards for this. For example, the first Ethernet interface in a host might be called %0, %1, %en1, %eth0, or whatever the implementer happened to choose.

In a URI, a literal IPv6 address is always embedded between "[" and "]". This document specifies that <zone_id> may contain any ASCII character classified in RFC 3986 as "unreserved", which conveniently excludes "]" in order to simplify parsing.

There is an additional complication in that "%" is always treated as an escape character in a URI, and according to RFC 3986 it MUST therefore itself be escaped in a URI, in the form "%25". Thus, the scoped address fe80::a%en1 would appear in a produced URI as http://[fe80::a%25en1].

If an operating system uses any other characters in zone or interface identifiers that are not in the "unreserved" character set, they too MUST be escaped with a "%" sign according to RFC 3986.

   IP-literal = "[" ( IPv6address / IPvFuture  ) "]"

In RFC 3986, the IPv6 literal format is formally defined in ABNF [RFC5234] by the following rule:

   IP-literal = "[" ( IPv6addrz / IPvFuture  ) "]"

   ZoneID = 1*( unreserved / pct-encoded )
   
   IPv6addrz = IPv6address [ "%" ZoneID ]

To provide support for a zone identifier, the existing syntax of IPv6address is retained, and a zone identifier may be added optionally to any literal address. This allows flexibility for unknown future uses. The rule quoted above from RFC 3986 is replaced by three rules:

The rules in [RFC5952] SHOULD be applied in producing URIs. The user MUST replace "%" by "%25" when manually constructing such a URI, and similarly for any other characters that are not in the "unreserved" character set of RFC 3986. The option to include such characters SHOULD NOT be used, but is included for the case of operating systems that allow such characters in interface names.

The 6man WG discussed and rejected an alternative in which the existing syntax of IPv6address would be extended by an option to add the ZoneID only for the case of link-local addresses. It was felt that the present solution offers more flexibility for future uses and is more straightforward to implement.

RFC 4007 offers guidance on how the ZoneID affects interface/address selection inside the IPv6 stack. Note that the behaviour of an IPv6 stack if passed a non-zero zone index for an address other than link-local is undefined.

3. Security Considerations

The security considerations of [RFC3986] and [RFC4007] apply. In particular, this URI format creates a specific pathway by which a deceitful zone index might be communicated, as mentioned in the final security consideration of RFC 4007. It is emphasised that the format is intended only for debugging purposes, but of course this intention does not prevent misuse.

To limit this risk, implementations SHOULD NOT allow use of this format except for well-defined usages such as sending to link local addresses under prefix fe80::/10.

An HTTP server or proxy MUST ignore any ZoneID attached to an incoming URI, as it only has local significance at the sending host.

4. IANA Considerations

This document requests no action by IANA.

5. Acknowledgements

The lack of this format was pointed out by Kerry Lynn. Valuable comments and contributions were made by Karl Auer, Brian Haberman, Tatuya Jinmei, Tom Petch, Tomoyuki Sahara, and Juergen Schoenwaelder.

This document was produced using the xml2rfc tool [RFC2629].

6. Change log [RFC Editor: Please remove]

draft-ietf-6man-uri-zoneid-00: adopted by WG, fixed syntax to allow for % encoded characters, 2012-02-17.

draft-carpenter-v6ops-label-balance-01: chose Option 2, removed 15 character limit, added explanation of ID/number mapping and other clarifications, 2012-02-08.

draft-carpenter-v6ops-label-balance-00: original version, 2011-12-07.

7. References

7.1. Normative References

7.2. Informative References

Authors' Addresses