Internet DRAFT - draft-carpenter-6man-rfc6874bis
draft-carpenter-6man-rfc6874bis
6MAN B. Carpenter
Internet-Draft Univ. of Auckland
Obsoletes: 6874 (if approved) S. Cheshire
Updates: 3986, 3987 (if approved) Apple Inc.
Intended status: Standards Track R. Hinden
Expires: 12 August 2022 Check Point Software
8 February 2022
Representing IPv6 Zone Identifiers in Address Literals and Uniform
Resource Identifiers
draft-carpenter-6man-rfc6874bis-03
Abstract
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 and Internationalized Resource
Identifier specifications (RFC 3986, RFC 3987) accordingly, and
obsoletes RFC 6874.
Discussion Venue
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the 6MAN mailing list
(ipv6@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/ipv6/
(https://mailarchive.ietf.org/arch/browse/ipv6/).
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 https://datatracker.ietf.org/drafts/current/.
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 12 August 2022.
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Copyright Notice
Copyright (c) 2022 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 (https://trustee.ietf.org/
license-info) 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 Revised BSD License text as
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Issues with Implementing RFC 6874 . . . . . . . . . . . . . . 4
3. Specification . . . . . . . . . . . . . . . . . . . . . . . . 4
4. URI Parsers . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Options Considered . . . . . . . . . . . . . . . . . 9
Appendix B. Change log . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. 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
future.
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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 or
necessary, in at least three use cases:
1. When using a web browser for simple debugging actions involving
link-local addresses on a host with more than one active link
interface.
2. When using a web browser to configure or reconfigure a device
which only has a link local address and whose only configuration
tool is a web server, again from a host with more than one active
link interface.
3. When using an HTTP-based protocol for establishing link-local
relationships, such as the Apple CUPS printing mechanism [CUPS].
It should be noted that whereas some operating systems and network
APIs support a default zone identifier as described in [RFC4007],
others do not, and for them an appropriate URI syntax is particularly
important.
In the past, some browser versions directly accepted the IPv6 Scoped
Address syntax [RFC4007] for scoped IPv6 addresses embedded in URIs,
i.e., they were 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 is not supported by the URI syntax [RFC3986] or the
Internationalized Resource Identifier (IRI) syntax [RFC3987].
Therefore, this document updates RFC 3986 and RFC 3987 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
human-readable string.
Several alternative solutions were considered while this document was
developed. Appendix A briefly describes the various options and
their advantages and disadvantages.
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This document obsoletes its predecessor [RFC6874] by greatly
simplifying its recommendations and requirements for URI parsers.
Its effect on the formal URI syntax [RFC3986] is different from that
of RFC 6874.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Issues with Implementing RFC 6874
Several issues prevented RFC 6874 being implemented in browsers:
1. There was some disagreement with requiring percent-encoding of
the "%" sign preceding a zone identifier. This requirement is
dropped in the present document.
2. The requirement to delete any zone identifier before emitting a
URI from the host in an HTTP message was considered both too
complex to implement and in violation of normal HTTP practice
[RFC7230]. This requirement has been dropped from the present
document.
3. The suggestion to pragmatically allow a bare "%" sign when this
would be unambiguous was considered both too complex to implement
and confusing for users. This suggestion has been dropped from
the present document since it is now irrelevant.
3. Specification
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. Also, %25 would be valid.
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 the text in Section 2.4 of [RFC3986], "%" must
be percent-encoded as "%25" to be used as data within a URI.
However, in the formal ABNF syntax of RFC 3986, this only applies
where the "pct-encoded" element appears. For this reason, it is
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possible to extend the ABNF such that the scoped address
fe80::abcd%en1 would appear in a URI as http://[fe80::abcd%en1] or
https://[fe80::abcd%en1].
A <zone_id> MUST contain only ASCII characters classified as
"unreserved" for use in URIs [RFC3986]. This excludes characters
such as "]" or even "%" that would complicate parsing. The <zone_id>
"25" cannot be forbidden since it is valid in some operating systems,
so a parser MUST NOT apply percent decoding to a URI such as
http://[fe80::abcd%25].
If an operating system uses any other characters in zone or interface
identifiers that are not in the "unreserved" character set, they
cannot be used in a URI.
We now present the corresponding formal syntax.
The URI syntax specification [RFC3986] formally defines 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 [RFC3986] is
replaced by three rules:
IP-literal = "[" ( IPv6address / IPv6addrz / IPvFuture ) "]"
ZoneID = 1*( unreserved )
IPv6addrz = IPv6address "%" ZoneID
This change also applies to [RFC3987].
This syntax fills the gap that is described at the end of
Section 11.7 of the IPv6 Scoped Address Architecture specification
[RFC4007]. It replaces and obsoletes the syntax in Section 2 of
[RFC6874].
The established rules for textual representation of IPv6 addresses
[RFC5952] SHOULD be applied in producing URIs.
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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
undefined.
4. URI Parsers
This section discusses how URI parsers, such as those embedded in 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.
In practice, although parsers respect the established syntax, they
are coded pragmatically rather than being formally syntax-driven.
Typically, IP address literals are handled by an explicit code path.
Parsers have been inconsistent in providing for ZoneIDs. Most have
no support, but there have been 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 allowed 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 URI parsers to recognise a ZoneID according
to the syntax defined in Section 3.
URIs including a ZoneID have no meaning outside the originating HTTP
client node. However, in some use cases, such as CUPS mentioned
above, the URI will be reflected back to the client.
The various use cases for the ZoneID syntax will cause it to be
entered in a browser's input dialogue box. Thus, URIs including a
ZoneID are unlikely to occur 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.
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5. 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.
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.
6. Acknowledgements
The lack of this format was first pointed out by Margaret Wasserman
and later by Kerry Lynn. A previous draft document by Bill Fenner
and Martin Dürst [LITERAL-ZONE] discussed this topic but was not
finalised. Michael Sweet and Andrew Cady explained some of the
difficulties caused by RFC 6874. The ABNF syntax proposed above was
drafted by Andrew Cady.
Valuable comments and contributions were made by Karl Auer, Carsten
Bormann, Benoit Claise, Martin Dürst, Stephen Farrell, Brian
Haberman, Ted Hardie, Philip Homburg, Tatuya Jinmei, Yves Lafon,
Barry Leiba, Radia Perlman, Tom Petch, Michael Richardson, Tomoyuki
Sahara, Juergen Schoenwaelder, Nico Schottelius, Dave Thaler, Martin
Thomson, Ole Troan, and others.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
January 2005, <https://www.rfc-editor.org/info/rfc3987>.
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[RFC4007] Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
DOI 10.17487/RFC4007, March 2005,
<https://www.rfc-editor.org/info/rfc4007>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952,
DOI 10.17487/RFC5952, August 2010,
<https://www.rfc-editor.org/info/rfc5952>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[CUPS] Apple, "CUPS open source printing system", 2021,
<https://www.cups.org/>.
[LITERAL-ZONE]
Fenner, B. and M. Dürst, "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, DOI 10.17487/RFC3493, February 2003,
<https://www.rfc-editor.org/info/rfc3493>.
[RFC4001] Daniele, M., Haberman, B., Routhier, S., and J.
Schoenwaelder, "Textual Conventions for Internet Network
Addresses", RFC 4001, DOI 10.17487/RFC4001, February 2005,
<https://www.rfc-editor.org/info/rfc4001>.
[RFC6874] Carpenter, B., Cheshire, S., and R. Hinden, "Representing
IPv6 Zone Identifiers in Address Literals and Uniform
Resource Identifiers", RFC 6874, DOI 10.17487/RFC6874,
February 2013, <https://www.rfc-editor.org/info/rfc6874>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
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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:
ping fe80::abcd%en1
Advantage: works today.
Disadvantage: less convenient than using a browser. Leaves some
use cases unsatisfied.
2. Simply use the percent character:
http://[fe80::abcd%en1]
Advantage: allows use of browser; allows cut and paste.
Disadvantage: requires code changes to all URI parsers.
This is the option chosen for standardisation.
3. Simply use an alternative separator:
http://[fe80::abcd-en1]
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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4. Simply use the "IPvFuture" syntax left open in RFC 3986:
http://[v6.fe80::abcd_en1]
Advantage: allows use of browser.
Disadvantage: ugly and redundant; doesn't allow simple cut and
paste.
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]:
http://[fe80::abcd%25en1]
Advantage: allows use of browser; consistent with general URI
syntax.
Disadvantage: somewhat ugly and confusing; doesn't allow simple
cut and paste.
Appendix B. Change log
This section is to be removed before publishing as an RFC.
* draft-carpenter-6man-rfc6874bis-03, 2022-02-08:
- Changed to bare % signs.
- Added IRIs, RFC3987
- Editorial fixes
* draft-carpenter-6man-rfc6874bis-02, 2021-18-12:
- Give details of open issues
- Update authorship
- Editorial fixes
* draft-carpenter-6man-rfc6874bis-01, 2021-07-11:
- Added section on issues with RFC6874
- Removed suggested heuristic for bare % signs
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- Editorial fixes
* draft-carpenter-6man-rfc6874bis-00, 2021-07-05:
- Initial version
Authors' Addresses
Brian Carpenter
School of Computer Science
University of Auckland
PB 92019
Auckland 1142
New Zealand
Email: brian.e.carpenter@gmail.com
Stuart Cheshire
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
United States of America
Email: cheshire@apple.com
Robert M. Hinden
Check Point Software
959 Skyway Road
San Carlos, CA 94070
United States of America
Email: bob.hinden@gmail.com
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