Internet Engineering Task Force (IETF) B. Carpenter
Request for Comments: 6874 Univ. of Auckland
Updates: 3986 S. Cheshire
Category: Standards Track Apple Inc.
ISSN: 2070-1721 R. Hinden
February 2013 Representing IPv6 Zone Identifiers in
Address Literals and Uniform Resource Identifiers
This document describes how the zone identifier of an IPv6 scoped
address, defined as <zone_id> in the IPv6 Scoped Address Architecture
(RFC 4007), can be represented in a literal IPv6 address and in a
Uniform Resource Identifier that includes such a literal address. It
updates the URI Generic Syntax specification (RFC 3986) accordingly.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in Section 2 of RFC 5741.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 22. Specification . . . . . . . . . . . . . . . . . . . . . . . . . 33. Web Browsers . . . . . . . . . . . . . . . . . . . . . . . . . 54. Security Considerations . . . . . . . . . . . . . . . . . . . . 65. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 66. References . . . . . . . . . . . . . . . . . . . . . . . . . . 76.1. Normative References . . . . . . . . . . . . . . . . . . . 76.2. Informative References . . . . . . . . . . . . . . . . . . 7Appendix A. Options Considered . . . . . . . . . . . . . . . . . . 81. Introduction
The Uniform Resource Identifier (URI) syntax specification [RFC3986]
defined how a literal IPv6 address can be represented in the "host"
part of a URI. Two months later, the IPv6 Scoped Address
Architecture 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
specification. Zone identifiers are especially useful in contexts in
which 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 in which
they are defined, often being the same as IPv6 interface names. They
are completely meaningless for any other node. Today, they are
meaningful only when attached to addresses with less than global
scope, but it is possible that other uses might be defined in the
The IPv6 Scoped Address Architecture specification [RFC4007] 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 because it is undefined, it is not implemented
consistently in URI parsers or in browsers.
Some versions of some browsers directly accept the IPv6 Scoped
Address syntax [RFC4007] for scoped IPv6 addresses embedded in URIs,
i.e., they have been coded to interpret a "%" sign following the
literal address as introducing a zone identifier [RFC4007], instead
of introducing two hexadecimal characters representing some percent-
encoded octet [RFC3986]. Clearly, interpreting the "%" sign as
introducing a zone identifier is very convenient for users, although
it formally breaches the established URI syntax [RFC3986]. This
document defines an alternative approach that respects and extends
the rules of URI syntax, and IPv6 literals in general, to be
Thus, this document updates the URI syntax specification [RFC3986] by
adding syntax to allow a zone identifier to be included in a literal
IPv6 address within a URI.
It should be noted that in contexts other 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
Several alternative solutions were considered while this document was
developed. Appendix A briefly describes the various options and
their advantages and disadvantages.
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 "Key words for use in
RFCs to Indicate Requirement Levels" [RFC2119].
According to IPv6 Scoped Address syntax [RFC4007], 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, the IPv6 Scoped
Address Architecture specification 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. According to URI syntax [RFC3986], "%" is always treated as
an escape character in a URI, so, according to the established URI
syntax [RFC3986] any occurrences of literal "%" symbols in a URI MUST
be percent-encoded and represented in the form "%25". Thus, the
scoped address fe80::a%en1 would appear in a URI as
A <zone_id> SHOULD contain only ASCII characters classified as
"unreserved" for use in URIs [RFC3986]. This excludes characters
such as "]" or even "%" that would complicate parsing. However, the
syntax described 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 represented using percent encoding [RFC3986].
We now present the necessary formal syntax.
The URI syntax specification [RFC3986] formally defined the IPv6
literal format 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 syntax allows flexibility
for unknown future uses. The rule quoted above from the previous URI
syntax specification [RFC3986] is replaced by three rules:
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 the IPv6 Scoped Address Architecture specification [RFC4007].
The established rules for textual representation of IPv6 addresses
[RFC5952] SHOULD be applied in producing URIs.
The URI syntax specification [RFC3986] 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 at which they originate, since the
ZoneID is of local significance only.
The IPv6 Scoped Address Architecture specification [RFC4007] offers
guidance on how the ZoneID affects interface/address selection inside
the IPv6 stack. Note that the behaviour of an IPv6 stack, if it is
passed a non-null zone index for an address other than link-local, is
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 a bare "%" character,
but this feature was removed for consistency with established syntax
[RFC3986]. As another example, some versions of Internet Explorer
allow use of a ZoneID preceded by a "%" character encoded as "%25",
still beyond the syntax allowed by the established rules [RFC3986].
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 a
percent-encoded "%". 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 manually rewritten to "fe80::a%25ee1" to avoid any risk of
Such bare "%" signs are for user interface convenience, and need to
be turned into properly encoded 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 from the URI syntax specification
[RFC3986] and the IPv6 Scoped Address Architecture specification
[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 the Scoped Address
Architecture specification. 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 has only local significance at the
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 [LITERAL-ZONE] discussed
this topic but was not finalised.
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.
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for
IPv6 Address Text Representation", RFC 5952,
6.2. Informative References
[LITERAL-ZONE] Fenner, B. and M. Duerst, "Formats for IPv6 Scope
Zone Identifiers in Literal Address Formats", Work
in Progress, October 2005.
[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.
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 solution presented in this document offers more
flexibility for future uses and is more straightforward to implement.
The various syntax options considered are now briefly described.
1. Leave the problem unsolved.
This would mean that per-interface diagnostics would still have
to be performed using ping or ping6:
Advantage: works today.
Disadvantage: less convenient than using a browser.
2. Simply use the percent character:
Advantage: allows use of browser; allows cut and paste.
Disadvantage: invalid syntax under RFC 3986; not acceptable to
3. Simply use an 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
4. Simply use the "IPvFuture" syntax left open in RFC 3986:
Advantage: allows use of browser.
Disadvantage: ugly and redundant; doesn't allow simple cut and
5. Retain the percent character already specified for introducing
zone identifiers for IPv6 Scoped Addresses [RFC4007], and then
percent-encode it when it appears in a URI, according to the
already-established URI syntax rules [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 standardisation.
Department of Computer Science
University of Auckland
1 Infinite Loop
Cupertino, CA 95014
Robert M. Hinden
Check Point Software Technologies, Inc.
800 Bridge Parkway
Redwood City, CA 94065