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Representing IPv6 Zone Identifiers in Address Literals and Uniform Resource Identifiers

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6874.
Authors Brian E. Carpenter , Stuart Cheshire , Bob Hinden
Last updated 2020-07-29 (Latest revision 2012-12-07)
Replaces draft-carpenter-6man-uri-zoneid
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Ole Trøan
Shepherd write-up Show Last changed 2012-10-10
IESG IESG state Became RFC 6874 (Proposed Standard)
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Consensus boilerplate Unknown
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Responsible AD Brian Haberman
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6MAN                                                        B. Carpenter
Internet-Draft                                         Univ. of Auckland
Updates: 3986 (if approved)                                  S. Cheshire
Intended status: Standards Track                              Apple Inc.
Expires: June 10, 2013                                         R. Hinden
                                                             Check Point
                                                        December 7, 2012

   Representing IPv6 Zone Identifiers in Address Literals and Uniform
                          Resource Identifiers


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

Status of this Memo

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

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

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

   This Internet-Draft will expire on June 10, 2013.

Copyright Notice

   Copyright (c) 2012 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

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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Specification  . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Web Browsers . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  6
   7.  Change log [RFC Editor: Please remove] . . . . . . . . . . . .  7
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  7
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  8
   Appendix A.  Options Considered  . . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   The Uniform Resource Identifier (URI) syntax [RFC3986] defined how a
   literal IPv6 address can be represented in the "host" part of a URI.
   A subsequent specification [RFC4007] extended the text representation
   of limited-scope IPv6 addresses such that a zone identifier may be
   concatenated to a literal 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 node where they
   are defined, often being the same as IPv6 interface names.  They are
   completely meaningless for any other node.  Today, they are only
   meaningful when attached to addresses with less than global scope,
   but it is possible that other uses might be defined in the future.

   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.

   Some versions of some browsers accept the RFC 4007 syntax for scoped
   IPv6 addresses embedded in URIs, i.e., they have been coded to
   interpret the "%" sign according to RFC 4007 instead of RFC 3986.
   Clearly this approach is very convenient for users, although it
   formally breaches the syntax rules of RFC 3986.  The present document
   defines an alternative approach that respects and extends the rules
   of URI syntax, and IPv6 literals in general, to be consistent.

   Thus, this document updates [RFC3986] by adding syntax to allow a
   zone identifier to be included in a literal IPv6 address within a

   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 InetZoneIndex [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.

   Several alternative solutions were considered while this document was
   developed.  The Appendix briefly describes the various options and
   their advantages and disadvantages.

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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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 how a <zone_id> can be appended to the
   address.  Unfortunately "%" is always treated as an escape character
   in a URI, and according to RFC 3986 it MUST therefore itself be
   percent-encoded in a URI, in the form "%25".  Thus, the scoped
   address fe80::a%en1 would appear in a URI as http://[fe80::a%25en1].

   A <zone_id> SHOULD contain only ASCII characters classified in RFC
   3986 as "unreserved".  This excludes characters such as "]" or even
   "%" which would complicate parsing.  However, the syntax below does
   allow such characters to be percent-encoded, for compatibility with
   existing devices that use them.

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

   We now present the necessary formal syntax.

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

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

   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:

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      IP-literal = "[" ( IPv6address / IPv6addrz / IPvFuture  ) "]"

      ZoneID = 1*( unreserved / pct-encoded )

      IPv6addrz = IPv6address "%25" ZoneID

   This syntax fills the gap that is described at the end of Section
   11.7 of RFC 4007.

   The rules in [RFC5952] SHOULD be applied in producing URIs.

   RFC 3986 states that URIs have a global scope, but that in some cases
   their interpretation depends on the end-user's context.  URIs
   including a ZoneID are to be interpreted only in the context of the
   host where they originate, since the ZoneID is of local significance

   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-null zone index for an address other than link-
   local is undefined.

3.  Web Browsers

   This section discusses how web browsers might handle this syntax
   extension.  Unfortunately there is no formal distinction between the
   syntax allowed in a browser's input dialogue box and the syntax
   allowed in URIs.  For this reason, no normative statements are made
   in this section.

   Due to the lack of defined syntax, web browsers have been
   inconsistent in providing for ZoneIDs.  Many have no support, but
   there are examples of ad hoc support.  For example, some versions of
   Firefox allowed the use of a ZoneID preceded by an unescaped "%"
   character, but this was removed for consistency with RFC 3986.  As
   another example, some versions of Internet Explorer allow use of a
   ZoneID preceded by a "%" character escaped as "%25", still beyond the
   syntax allowed by RFC 3986.  This syntax extension is in fact used
   internally in the Windows operating system and some of its APIs.

   It is desirable for all browsers to recognise a ZoneID preceded by an
   escaped "%".  In the spirit of "be liberal with what you accept", we
   also suggest that URI parsers accept bare "%" signs when possible
   (i.e., a "%" not followed by two valid and meaningful hexadecimal
   characters).  This would make it possible for a user to copy and
   paste a string such as "fe80::a%en1" from the output of a "ping"
   command and have it work.  On the other hand, "%ee1" would need to be

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   manually escaped as "fe80::a%25ee1" to avoid any risk of

   Such bare "%" signs are for user interface convenience, and need to
   be turned into properly escaped characters (where "%25" encodes "%")
   before the URI is used in any protocol or HTML document.  However,
   URIs including a ZoneID have no meaning outside the originating node.
   It would therefore be highly desirable for a browser to remove the
   ZoneID from a URI before including that URI in an HTTP request.

   The normal diagnostic usage for the ZoneID syntax will cause it to be
   entered in the browser's input dialogue box.  Thus, URIs including a
   ZoneID are unlikely to be encountered in HTML documents.  However, if
   they do (for example, in a diagnostic script coded in HTML) it would
   be appropriate to treat them exactly as above.

4.  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 MUST NOT allow use of this format
   except for well-defined usages such as sending to link local
   addresses under prefix fe80::/10.  At the time of writing, this is
   the only well-defined usage known.

   An HTTP client, proxy or other intermediary MUST remove any ZoneID
   attached to an outgoing URI, as it only has local significance at the
   sending host.

5.  IANA Considerations

   This document requests no action by IANA.

6.  Acknowledgements

   The lack of this format was first pointed out by Margaret Wasserman
   some years ago, and more recently by Kerry Lynn.  A previous draft
   document by Martin Duerst and Bill Fenner [I-D.fenner-literal-zone]
   discussed this topic but was not finalised.

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   Valuable comments and contributions were made by Karl Auer, Carsten
   Bormann, Benoit Claise, Stephen Farrell, Brian Haberman, Ted Hardie,
   Tatuya Jinmei, Yves Lafon, Barry Leiba, Radia Perlman, Tom Petch,
   Tomoyuki Sahara, Juergen Schoenwaelder, Dave Thaler, Martin Thomson,
   and Ole Troan.

   Brian Carpenter was a visitor at the Computer Laboratory, Cambridge
   University during part of this work.

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

7.  Change log [RFC Editor: Please remove]

   draft-ietf-6man-uri-zoneid-06: responding to IETF Last Call and IESG
   comments, 2012-12-07.

   draft-ietf-6man-uri-zoneid-05: tuned ABNF, clarified RFC 4007 text,

   draft-ietf-6man-uri-zoneid-04: additional author, 2012-09-21.

   draft-ietf-6man-uri-zoneid-03: reverted to percent-encoded model
   following WGLC, 2012-09-10.

   draft-ietf-6man-uri-zoneid-02: additional WG comments, 2012-07-11.

   draft-ietf-6man-uri-zoneid-01: use "-" instead of %25, listed
   alternatives in Appendix, according to WG debate, added suggestion
   for browser developers, 2012-05-29.

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

   draft-carpenter-6man-uri-zoneid-01: chose Option 2, removed 15
   character limit, added explanation of ID/number mapping and other
   clarifications, 2012-02-08.

   draft-carpenter-6man-uri-zoneid-00: original version, 2011-12-07.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4007]  Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
              B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
              March 2005.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952, August 2010.

8.2.  Informative References

              Fenner, B. and M. Duerst, "Formats for IPv6 Scope Zone
              Identifiers in Literal Address Formats",
              draft-fenner-literal-zone-02 (work in progress),
              October 2005.

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, February 2003.

   [RFC4001]  Daniele, M., Haberman, B., Routhier, S., and J.
              Schoenwaelder, "Textual Conventions for Internet Network
              Addresses", RFC 4001, February 2005.

   [chrome]   Google, "Use the address bar (omnibox)", 2012, <http://

Appendix A.  Options Considered

   The syntax defined above allows a ZoneID to be added to any IPv6
   address.  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.

   The various syntax options considered are now briefly described.

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   1.  Leave the problem unsolved.

       This would mean that per-interface diagnostics would still have
       to be performed using ping or ping6:

       ping fe80::a%en1

       Advantage: works today.

       Disadvantage: less convenient than using a browser.

   2.  Simply using the percent character.


       Advantage: allows use of browser, allows cut and paste.

       Disadvantage: invalid syntax under RFC 3986; not acceptable to
       URI community.

   3.  Escaping the escape character as allowed by RFC 3986:


       Advantage: allows use of browser, consistent with general URI

       Disadvantage: somewhat ugly and confusing, doesn't allow simple
       cut and paste.

       This is the option chosen for standardization.

   4.  Alternative separator


       Advantage: allows use of browser, simple syntax

       Disadvantage: Requires all IPv6 address literal parsers and
       generators to be updated in order to allow simple cut and paste;
       inconsistent with existing tools and practice.

       Note: the initial proposal for this choice was to use an
       underscore as the separator, but it was noted that this becomes
       effectively invisible when a user interface automatically
       underlines URLs.

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   5.  With the "IPvFuture" syntax left open in RFC 3986:


       Advantage: allows use of browser.

       Disadvantage: ugly and redundant, doesn't allow simple cut and

Authors' Addresses

   Brian Carpenter
   Department of Computer Science
   University of Auckland
   PB 92019
   Auckland,   1142
   New Zealand


   Stuart Cheshire
   Apple Inc.
   1 Infinite Loop
   Cupertino, CA  95014


   Robert M. Hinden
   Check Point Software Technologies, Inc.
   800 Bridge Parkway
   Redwood City, CA  94065


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