Representing IPv6 Zone Identifiers in Address Literals and Uniform Resource Identifiers
draft-ietf-6man-rfc6874bis-06

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: 8 October 2023                             Check Point Software
                                                            6 April 2023

   Representing IPv6 Zone Identifiers in Address Literals and Uniform
                          Resource Identifiers
                     draft-ietf-6man-rfc6874bis-06

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 8 October 2023.

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Copyright Notice

   Copyright (c) 2023 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
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   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 . . . . . . . . . . . . . .   5
   3.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Scope and Deployment  . . . . . . . . . . . . . . . . . . . .   8
   5.  URI Parsers . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Options Considered . . . . . . . . . . . . . . . . .  14
   Appendix B.  Change log . . . . . . . . . . . . . . . . . . . . .  16
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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.  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, usually being the same as IPv6 interface names.  They
   are completely meaningless for any other node.  Today, they are
   meaningful only when attached to link-local addresses, but it is
   possible that other uses might be defined in the future.

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   The IPv6 Scoped Address Architecture specification does not specify
   how zone identifiers are to be represented in URIs.  Practical
   experience has shown that this feature is necessary in various use
   cases, including the following:

   1.  A web browser may be used for simple debugging actions involving
       link-local addresses on a host with more than one active link
       interface.  For example, the existence of a device may today be
       checked via "ping fe80::1234%eth0" but not via
       "https://[fe80::1234%eth0]".

   2.  A web browser must sometimes be used to configure or reconfigure
       a device which only has a link-local address and whose only
       configuration tool is a web server, again in a host with more
       than one active link interface.  For example, a typical home
       router may today be configured via "http://192.168.178.1" but not
       via "http://[fe80::1%eth0]".

   3.  The Apple and open-source CUPS printing mechanism [CUPS]
       [OP-CUPS] uses an HTTP-based protocol [RFC3510][RFC7472] to
       establish link-local relationships, so requires the specification
       of the relevant interface.

   4.  The Microsoft Web Services for Devices (WSD) virtual printer port
       mechanism can generate an IPv6 link-local URL such as
       "http://[fe80::823b:f9ff:fe7b:d9dc%10]:80/WebServices/Device" in
       which the zone identifier is present, but is not recognized by
       any current browser.

   5.  The National Marine Electronics Association (NMEA) has recently
       defined its "OneNet Marine IPv6 Ethernet Networking Standard"
       [ONE-NET], which includes a specific requirement for device
       configuration via a browser using link-local addresses.  Such
       requirements have already spawned a hack to work around the
       current limitation [LL-HACK].

   For these use cases, it is highly desirable that a complete IPv6
   link-local address can be cut and pasted from one context (such as
   the output from a system command) to another (such as a browser
   dialogue box).  Since such addresses may include quite long
   hexadecimal strings, any solution except cut-and-paste is highly
   error prone.

   The use cases listed above apply to relatively simple actions on end
   systems, not to network infrastructure devices.  The latter may have
   large numbers of interfaces, which are commonly named for network
   management purposes in styles such as "Ethernet1/0/1" or "ge-
   0/0/0.0", reflecting the hardware structure and depending on the

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   manufacturer.  Such names commonly involve upper case and non-
   alphanumeric characters, both of which are intrinsically problematic
   in the host part of a URI.  Generally speaking they are handled by
   various network management mechanisms and specialized commands, and
   this specification does not show how to support them in URIs.

   As IPv6 deployment becomes widespread, the lack of a solution for
   handling complete link-local addresses in web browsers is becoming an
   acute problem for increasing numbers of operational and support
   personnel.  It will become critical as IPv6-only networks, with no
   native IPv4 support, appear.  For example, the NMEA use case
   mentioned above is an immediate requirement.  This is the principal
   reason for documenting this requirement and its solution now.

   It should be noted that whereas some operating systems and network
   APIs support a default zone identifier as described in the IPv6
   scoped address architecture [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 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, instead of introducing two hexadecimal
   characters representing some percent-encoded octet as explained in
   Section 2.1 of [RFC3986].  Clearly, interpreting the "%" sign as
   introducing a zone identifier is very convenient for users, although
   it is not supported by the URI syntax in RFC 3986 or the
   Internationalized Resource Identifier (IRI) syntax in [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.
   (However, note that interface numbers are limited to positive signed
   32-bit integers (see InterfaceIndex defined in [RFC2863] and if-index
   defined in [RFC8343]) while the zone index allows for unsigned 32-bit
   integers.)

   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.  However, in some operating systems it is
   possible to use the underlying interface number, represented as a
   decimal integer, as an alternative to the human-readable string.  For
   example, on Linux, a user can determine interface numbers simply by

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   issuing the command "ip link show" and then, for example, use
   "fe80::1%5" instead of "fe80::1%Ethernet1/0/1", if the interface
   number happens to be 5.

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

   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
       [RFC9110].  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, %25, %en1, %eth0, or whatever the

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   implementer happened to choose.

   In a URI, a literal IPv6 address is always embedded between "[" and
   "]".  This document specifies how a zone identifier can be appended
   to the address.  The URI syntax defined by [RFC3986] does not allow
   the presence of a percent ("%") character within an IPv6 address
   literal.  For this reason, it is backwards compatible to allow the
   use of "%" within an IPv6 address literal as a delimiter only, such
   that the scoped address "fe80::abcd%en1" would appear in a URI as
   "http://[fe80::abcd%en1]" or "https://[fe80::abcd%en1]".

   This use of "%" as a delimiter applies only within an IPv6 address
   literal, and is irrelevant to and exempt from the percent-encoding
   mechanism of RFC 3986.

   A zone identifier MUST contain only ASCII characters classified as
   "unreserved" for use in URIs by RFC 3986.  This excludes characters
   such as "/", "]" or even "%" that would complicate parsing.  For the
   avoidance of doubt, note that a zone identifier consisting of "25" or
   starting with "25" is valid and is used in some operating systems.  A
   parser MUST NOT apply percent decoding to the IPv6 address literal in
   a URI, including cases such as "http://[fe80::abcd%25]" and
   "http://[fe80::abcd%25xy]".

   If an operating system uses any characters in zone or interface
   identifiers that are not in the "unreserved" character set,
   identifiers including them cannot be used in a URI.

   If an operating system supports case-sensitive zone or interface
   identifiers, identifiers including upper case letters cannot be used
   in a URI.

   We now present the corresponding formal syntax.

   The URI syntax specification in RFC 3986 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 RFC 3986 is
   replaced by three rules:

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      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.

   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 zone identifier are an example of this, since the zone
   identifier is of local significance only.  Such a zone identifier
   cannot be correctly interpreted outside the host to which it applies,
   so it must be treated as an opaque string.

   RFC 3986 also states that the host subcomponent of a URI is case-
   insensitive and is normalized to lower case.  Section 6.2.2.1 of RFC
   3986 states unambiguously that "the scheme and host are case-
   insensitive and therefore should be normalized to lowercase".  The
   mechanism described here will therefore fail for zone identifiers
   that contain upper case letters, since RFC 4007 implies case-
   sensitivity.  This specification therefore RECOMMENDS the use of
   lower case letters only when assigning zone identifiers to
   interfaces.

   RFC 4007 offers guidance on how the zone identifier 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.

   In cases where the RFC 6874 encoding is currently used between
   specific software components rather than between a browser and a web
   server, such usage MAY continue indefinitely.

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4.  Scope and Deployment

   A URI (or IRI) using this format has no meaning outside the scope of
   the individual host that originates it and of the specific layer 2
   link concerned.  It may in fact be delivered in an HTTP message to a
   server that does not support this format and which will reject the
   message as invalid.  For the diagnostic use cases concerned, this is
   of no importance: an HTTP error response will serve the diagnostic
   purpose of establishing that the link and remote host are
   operational.  The other use cases shown above are only meaningful if
   the remote host also accepts this format; otherwise they will fail
   with an HTTP error response.  As a result, this format can be
   deployed progressively as required, with no wider consequences.

   It is worth noting that there is nothing new about a URI that refers
   to a local resource.  URIs referring to local domains under ".local"
   are normal.  Any URI such as "https://169.254.0.1" (link-local IPv4,
   [RFC3927]), "https://10.1.1.1" (private IPv4, [RFC1918]), or
   "https://[fd63:45eb:cd14:0:80b2:5c79:62ae:d341]" (IPv6 unique local
   address, [RFC4193]) refers to a local resource and has no meaning off
   the link or outside the local domain.  In operating systems with
   support for a default zone identifier, URLs such as
   "https://[fe80::2e3a:12cd:fea4:dde7]" already work as expected.
   Deployment of support for link-local IPv6 addresses with zone
   identifiers introduces no new principle compared to these four
   currently operational cases.

   There has been considerable concern about potential security concerns
   caused by locally scoped URIs.  A recent W3C Community Group draft
   report [LNA-REP] provides background on the issue of cross-origin
   resource sharing (CORS), a mechanism which "allows a server to
   indicate any origins (domain, scheme, or port) other than its own
   from which a browser should permit loading resources."  This
   mechanism was originally devised for the case of private IPv4
   addresses, but has been expanded to cover other cases, explicitly
   including link-local IPv6 addresses.  Addresses are sorted into three
   scopes: loopback, local and public.  It could be argued that link-
   local addresses which include a zone ID should be treated on the same
   basis as a loopback address, since they are meaningless outside the
   originating host.  In any case, they can clearly be handled by the
   CORS mechanism.

5.  URI Parsers

   This section discusses how URI (or IRI) parsers, such as those
   embedded in web browsers, might handle this syntax extension.

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   In practice, although parsers respect the established syntax, many
   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 zone identifiers.
   Most have no support, but there have been examples of ad hoc support.
   For example, some versions of Firefox allowed the use of a zone
   identifier preceded by a bare "%" character, but this feature was
   removed for consistency with the established syntax of RFC 3986.  As
   another example, some versions of Internet Explorer allowed use of a
   zone identifier preceded by a "%" character encoded as "%25", still
   beyond the syntax allowed by the established rules.  This syntax
   extension is in fact used internally in the Windows operating system
   and some of its APIs.

   URI parsers SHOULD accept a zone identifier according to the syntax
   defined in Section 3, rather than treating the URI as invalid as they
   do today.  An IPv6 address literal never contains percent-encodings.
   In terms of Section 2.4 of [RFC3986], the "%" character preceding a
   zone identifier is acting as a delimiter, not as data.  Any code
   handling percent-encoding or percent-decoding must be aware of this.

   While the ABNF syntax defined above is consistent, there are many
   existing URI parsers that apply percent decoding liberally (including
   within IPv6 literals) regardless of the ABNF, so the probability of
   practical and operational problems is claimed to be very high,
   especially during the period when some parsers have been updated and
   others have not.  For example, the URI "http://[fe80::cd%21]" might
   be incorrectly decoded as "http://[fe80::cd!]", which will fail.
   However, as discussed in the first paragraph of Section 4, errors of
   this type will not prevent progressive deployment of the new syntax
   on devices that need it.

   As noted above, a zone identifier included in a URI has no meaning
   outside the originating HTTP client node.  However, in some use
   cases, such as CUPS, the host address embedded in the URI will be
   reflected back to the client, using exactly the representation of the
   zone identifier that the client sent.

   The various use cases for the zone identifier syntax will usually
   require it to be entered in a browser's input dialogue box.  However,
   URIs including a zone identifier might occur in HTML documents.  For
   example, a diagnostic script in an HTML page might be tailored for a
   particular host.  Because of such usage, it is appropriate for
   browsers to treat such URIs in the same way whether they are entered
   in the dialogue box or encountered in an HTML document.

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6.  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.

   However, this format is only meaningful for link-local addresses
   under prefix fe80::/10.  It is not necessary for web browsers to
   verify this, or to validate the zone identifier, because the
   operating system will do so when the address is passed to the socket
   API, and return an error code if the zone identifier is invalid.
   This is in addition to the protection offered by CORS as discussed in
   Section 4.

   A zone identifier in a URI will be revealed to the recipient of an
   HTTP message containing it (typically in the "Host" field [RFC9110]).
   A server that receives a zone identifier in an HTTP message or
   otherwise SHOULD NOT make use of it, for validation of authority or
   any other purpose, since it has no meaning outside the originating
   host.  Existing practice for controlling cross-origin resource
   sharing applies, as discussed above Section 4.

   Visibility of the zone identifier to a server is anyway a minor
   security concern, since the information revealed is of local
   significance only and will be exploitable only if both the client
   host and the server have both already been compromised.

   Unfortunately there is no formal limit on the length of the zone
   identifier string in RFC 4007.  An implementation SHOULD apply a
   reasonable length limit when generating a URI, in order to minimize
   the risk of a buffer overrun.  For example, a limit to 16 ASCII
   characters would correspond to the existing limit on Linux interface
   names.

   An implementation SHOULD NOT include ASCII NULL characters in a zone
   identifier string as this could cause inconsistencies in subsequent
   string processing.

   It is conceivable that this format could be misused to remotely probe
   a local network configuration or to fingerprint a host.  In
   particular, a script included in an HTML web page could originate
   HTTP messages intended to determine if a particular link-local
   address is valid, for example to discover and misuse the address of
   the first-hop router.  However, such attacks are already possible, by
   probing IPv4 addresses, routeable IPv6 addresses or link-local

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   addresses without a zone identifier.  Indeed, with a zone identifier
   present, the attacker's job is harder because they must also guess
   the zone identifier itself; the zone identifier increases the search
   space compared to guessing only the interface identifier.  Zone
   identifiers vary widely between operating systems; in some cases they
   are easily guessed small integers or conventional names such as
   "eth0" but in other cases they contain arbitrary characters derived
   from MAC addresses.  In any case, an attacker must discover them
   before probing any link-local addresses.  This argues against the
   recommendation of [RFC4007] to support a default zone identifier.
   Nevertheless, the principal defence against scanning attacks remains
   the 64 bit size of the IPv6 interface identifier [RFC7707].

   In the case that a zone identifier contains the hexadecimal MAC
   address of a network interface, it will be revealed to the HTTP
   recipient and to any observer on the link.  Since the MAC address
   will also be visible in the underlying layer 2 frame, this is not a
   new exposure.  Nevertheless, this method of naming interfaces might
   be considered to be a privacy issue.

   It should be noted that if a node uses an interface identifier in the
   outdated Modified EUI format [RFC4291] for its link-local address,
   the search space for an attacker is very significantly reduced, as
   discussed in Section 4.1.1.1 of [RFC7707].  The resultant
   recommendations of [RFC8064] apply to all nodes, including routers,
   since they ensure that the search space for an attacker is of size
   2**64, which is impracticably large.

   Nevertheless, even a Modified EUI link-local address is significantly
   harder to guess than typical IPv4 addresses for devices such as home
   routers, which are often included in published documentation.

7.  IANA Considerations

   This document makes no request of IANA.

8.  References

8.1.  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>.

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   [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>.

   [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>.

   [RFC8064]  Gont, F., Cooper, A., Thaler, D., and W. Liu,
              "Recommendation on Stable IPv6 Interface Identifiers",
              RFC 8064, DOI 10.17487/RFC8064, February 2017,
              <https://www.rfc-editor.org/info/rfc8064>.

   [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>.

8.2.  Informative References

   [CUPS]     Apple, "Apple CUPS", 2022, <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.

   [LL-HACK]  Jin, P., "Snippets: IPv6 link local connect hack", 2021,
              <https://website.peterjin.org/wiki/
              Snippets:IPv6_link_local_connect_hack>.

   [LNA-REP]  Rigoudy, T., Ed., "Local Network Access", 2023,
              <https://wicg.github.io/local-network-access/>.

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   [ONE-NET]  NMEA, "The OneNet Standard for IP Networking of Marine
              Electronic Devices", 2023,
              <https://www.nmea.org/nmea-onenet.html>.

   [OP-CUPS]  Sweet, M., "OpenPrinting CUPS", 2022,
              <https://openprinting.github.io/cups/>.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              J., and E. Lear, "Address Allocation for Private
              Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
              February 1996, <https://www.rfc-editor.org/info/rfc1918>.

   [RFC2863]  McCloghrie, K. and F. Kastenholz, "The Interfaces Group
              MIB", RFC 2863, DOI 10.17487/RFC2863, June 2000,
              <https://www.rfc-editor.org/info/rfc2863>.

   [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>.

   [RFC3510]  Herriot, R. and I. McDonald, "Internet Printing
              Protocol/1.1: IPP URL Scheme", RFC 3510,
              DOI 10.17487/RFC3510, April 2003,
              <https://www.rfc-editor.org/info/rfc3510>.

   [RFC3927]  Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
              Configuration of IPv4 Link-Local Addresses", RFC 3927,
              DOI 10.17487/RFC3927, May 2005,
              <https://www.rfc-editor.org/info/rfc3927>.

   [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>.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
              <https://www.rfc-editor.org/info/rfc4193>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [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>.

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   [RFC7472]  McDonald, I. and M. Sweet, "Internet Printing Protocol
              (IPP) over HTTPS Transport Binding and the 'ipps' URI
              Scheme", RFC 7472, DOI 10.17487/RFC7472, March 2015,
              <https://www.rfc-editor.org/info/rfc7472>.

   [RFC7707]  Gont, F. and T. Chown, "Network Reconnaissance in IPv6
              Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016,
              <https://www.rfc-editor.org/info/rfc7707>.

   [RFC8343]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 8343, DOI 10.17487/RFC8343, March 2018,
              <https://www.rfc-editor.org/info/rfc8343>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

Appendix A.  Options Considered

   The syntax defined above allows a zone identifier 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 zone identifier 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 use
       cases unsatisfied.

   2.  Simply use the percent character:

       http://[fe80::abcd%en1]

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

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       Disadvantage: requires code changes to all URI parsers, some of
       which differ in their interpretation of the percent-encoding
       rules.

       This is the option chosen for standardisation.

   3.  Use an alternative separator:

       http://[fe80::abcd-en1]

       Advantage: allows use of browser; simple syntax.

       Disadvantages: requires code changes to all URI parsers; requires
       manual editing during 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 may
       become invisible or unclear when a user interface automatically
       underlines URLs.

   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 [RFC3986]:

       http://[fe80::abcd%25en1]

       Advantage: allows use of browser; consistent with general URI
       syntax.

       Disadvantages: somewhat ugly and confusing; requires manual
       editing during cut and paste; requires code changes to all URI
       parsers, some of which differ in their interpretation of the
       percent-encoding rules.

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Appendix B.  Change log

   This section is to be removed before publishing as an RFC.

   *  draft-ietf-6man-rfc6874bis-06, 2023-04-07:

      -  Noted potential exposure of MAC addresses in zone IDs.

      -  Expanded detail on lower-case normalization.

      -  Added specific use case examples.

      -  Added NMEA use case.

      -  Clearly explained cut-and-paste requirement.

      -  Indicated that network infrastructure devices are out of scope.

      -  Noted the work-around using interface numbers.

      -  Mentioned .local as another case of locally significant URIs.

      -  Added discussion of CORS.

      -  Update descriptions of rejected alternatives

      -  Noted parsing fragility re % sign.

      -  Other IESG review nits.

   *  draft-ietf-6man-rfc6874bis-05, 2022-11-07:

      -  Noted lower case issue.

   *  draft-ietf-6man-rfc6874bis-04, 2022-10-19:

      -  should accept -> SHOULD.

      -  Suggested maximum length of zone ID.

   *  draft-ietf-6man-rfc6874bis-03, 2022-09-30:

      -  Strengthened motivation for publishing this requirement now.

      -  Removed unnecessary sentence about browsers.

      -  Noted that zone ID will be revealed to HTTP server.

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      -  Noted that servers should make no use of received zone IDs.

      -  Noted that zone IDs have no length limit.

      -  Added section on scope and deployment, specifically noting that
         URIs with local scope are nothing new.

      -  Other Last Call clarifications and nits.

   *  draft-ietf-6man-rfc6874bis-02, 2022-07-05:

      -  Improve discussion of URLs in HTML documents

      -  Discuss scripting attack and Modified EUI IIDs

      -  Several editorial clarifications

      -  Some nits fixed

   *  draft-ietf-6man-rfc6874bis-01, 2022-04-07:

      -  Extended use cases

      -  Clarified relationship with RFC3986 language

      -  Allow for legacy use of RFC6874 format

      -  Augmented security considerations

      -  Editorial and reference improvements

   *  draft-ietf-6man-rfc6874bis-00, 2022-03-19:

      -  WG adoption

      -  Clarified security considerations

   *  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

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      -  Update authorship

      -  Editorial fixes

   *  draft-carpenter-6man-rfc6874bis-01, 2021-07-11:

      -  Added section on issues with RFC6874

      -  Removed suggested heuristic for bare % signs

      -  Editorial fixes

   *  draft-carpenter-6man-rfc6874bis-00, 2021-07-05:

      -  Initial version

Appendix C.  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, Carlos
   Bernardos, Carsten Bormann, Benoit Claise, Martin Dürst, David
   Farmer, Stephen Farrell, Brian Haberman, Ted Hardie, Philip Homburg,
   Tatuya Jinmei, Leif Johansson, Nate Karstens, Yves Lafon, Barry
   Leiba, Ted Lemon, Ben Maddison, Radia Perlman, Tom Petch, Michael
   Richardson, Tomoyuki Sahara, Jürgen Schönwälder, Nico Schottelius,
   Dave Thaler, Martin Thomson, Philipp S.  Tiesel, Ole Troan, Éric
   Vyncke, Shang Ye, several IESG members, and others.

Authors' Addresses

   Brian Carpenter
   School of Computer Science
   University of Auckland
   PB 92019
   Auckland 1142
   New Zealand
   Email: brian.e.carpenter@gmail.com

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   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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