Network Working Group P. Saint-Andre

Network Working Group P. Saint-Andre
Request for Comments: 5122 XSF
Obsoletes: 4622 February 2008
Category: Standards Track

Internationalized Resource Identifiers (IRIs) and
Uniform Resource Identifiers (URIs) for
the Extensible Messaging and Presence Protocol (XMPP)

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Abstract

This document defines the use of Internationalized Resource
Identifiers (IRIs) and Uniform Resource Identifiers (URIs) in
identifying or interacting with entities that can communicate via the
Extensible Messaging and Presence Protocol (XMPP).

This document obsoletes RFC 4622.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Use of XMPP IRIs and URIs . . . . . . . . . . . . . . . . . . 4
2.1. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Form . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Authority Component . . . . . . . . . . . . . . . . . . . 6
2.4. Path Component . . . . . . . . . . . . . . . . . . . . . . 7
2.5. Query Component . . . . . . . . . . . . . . . . . . . . . 8
2.6. Fragment Identifier Component . . . . . . . . . . . . . . 9
2.7. Generation of XMPP IRIs/URIs . . . . . . . . . . . . . . . 10
2.8. Processing of XMPP IRIs/URIs . . . . . . . . . . . . . . . 13
2.9. Internationalization . . . . . . . . . . . . . . . . . . . 16
3. IANA Registration of xmpp URI Scheme . . . . . . . . . . . . . 16
3.1. URI Scheme Name . . . . . . . . . . . . . . . . . . . . . 16
3.2. Status . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.3. URI Scheme Syntax . . . . . . . . . . . . . . . . . . . . 17
3.4. URI Scheme Semantics . . . . . . . . . . . . . . . . . . . 17
3.5. Encoding Considerations . . . . . . . . . . . . . . . . . 17
3.6. Applications/Protocols That Use This URI Scheme Name . . . 18
3.7. Interoperability Considerations . . . . . . . . . . . . . 18
3.8. Security Considerations . . . . . . . . . . . . . . . . . 18
3.9. Contact . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.10. Author/Change Controller . . . . . . . . . . . . . . . . . 18
3.11. References . . . . . . . . . . . . . . . . . . . . . . . . 18
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
5. Security Considerations . . . . . . . . . . . . . . . . . . . 19
5.1. Reliability and Consistency . . . . . . . . . . . . . . . 19
5.2. Malicious Construction . . . . . . . . . . . . . . . . . . 20
5.3. Back-End Transcoding . . . . . . . . . . . . . . . . . . . 20
5.4. Sensitive Information . . . . . . . . . . . . . . . . . . 20
5.5. Semantic Attacks . . . . . . . . . . . . . . . . . . . . . 21
5.6. Spoofing . . . . . . . . . . . . . . . . . . . . . . . . . 21
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.1. Normative References . . . . . . . . . . . . . . . . . . . 21
7.2. Informative References . . . . . . . . . . . . . . . . . . 22
Appendix A. Differences from RFC 4622 . . . . . . . . . . . . . . 25
Appendix B. Copying Conditions . . . . . . . . . . . . . . . . . 25

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1. Introduction

The Extensible Messaging and Presence Protocol (XMPP) is a streaming
XML technology that enables any two entities on a network to exchange
well-defined but extensible XML elements (called "XML stanzas") at a
rate close to real time.

As specified in [XMPP-CORE], entity addresses as used in
communications over an XMPP network must not be prepended with a
Uniform Resource Identifier (URI) scheme (as specified in [URI]).
However, applications external to an XMPP network may need to
identify XMPP entities either as URIs or, in a more modern fashion,
as Internationalized Resource Identifiers (IRIs; see [IRI]).
Examples of such external applications include databases that need to
store XMPP addresses and non-native user agents such as web browsers
and calendaring applications that provide interfaces to XMPP
services.

The format for an XMPP address is defined in [XMPP-CORE]. Such an
address may contain nearly any Unicode character [UNICODE] and must
adhere to various profiles of stringprep [STRINGPREP]. The result is
that an XMPP address is fully internationalizable and is very close
to being an IRI without a scheme. However, given that there is no
freestanding registry of IRI schemes, it is necessary to define XMPP
identifiers primarily as URIs rather than as IRIs, and to register an
XMPP URI scheme instead of an IRI scheme. Therefore, this document
does the following:

o Specifies how to identify XMPP entities as IRIs or URIs.

o Specifies how to interact with XMPP entities as IRIs or URIs.

o Formally defines the syntax for XMPP IRIs and URIs.

o Specifies how to transform XMPP IRIs into URIs and vice versa.

o Registers the xmpp URI scheme.

1.1. Terminology

This document inherits terminology from [IRI], [URI], and
[XMPP-CORE].

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 RFC 2119 [TERMS].

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2. Use of XMPP IRIs and URIs

2.1. Rationale

As described in [XMPP-IM], instant messaging and presence
applications of XMPP must handle im: and pres: URIs (as specified by
[CPIM] and [CPP]). However, there are many other applications of
XMPP (including network management, workflow systems, generic
publish-subscribe, remote procedure calls, content syndication,
gaming, and middleware), and these applications do not implement
instant messaging and presence semantics. Furthermore, a generic
XMPP entity does not implement the semantics of any existing URI
scheme, such as the http:, ftp:, or mailto: scheme. Therefore, it is
appropriate to define a new URI scheme that makes it possible to
identify or interact with any XMPP entity (not just instant messaging
and presence entities) as an IRI or URI.

XMPP IRIs and URIs are defined for use by non-native interfaces and
applications. In order to ensure interoperability on XMPP networks,
when data is routed to an XMPP entity (e.g., when an XMPP address is
contained in the 'to' or 'from' attribute of an XML stanza) or an
XMPP entity is otherwise identified in standard XMPP protocol
elements, the entity MUST be addressed as <[node@]domain[/resource]>
(i.e., without a prepended scheme), where the "node identifier",
"domain identifier", and "resource identifier" portions of an XMPP
address conform to the definitions provided in Section 3 of
[XMPP-CORE].

Note: For historical reasons, the term "resource identifier" is used
in XMPP to refer to the optional portion of an XMPP address that
follows the domain identifier and the "/" separator character (for
details, refer to Section 3.4 of [XMPP-CORE]); this use of the term
"resource identifier" is not to be confused with the meanings of
"resource" and "identifier" provided in Section 1.1 of [URI].

XMPP IRIs and URIs are defined primarily for the purpose of
identification rather than of interaction (regarding this
distinction, see Section 1.2.2 of [URI]). The "Internet resource"
identified by an XMPP IRI or URI is an entity that can communicate
via XMPP over a network. An XMPP IRI or URI can contain additional
information above and beyond the identified resource; in particular,
as described under Section 2.5 a query component can be included to
specify suggested semantics for an interaction with the identified
resource. It is envisioned that when an XMPP application resolves an
XMPP IRI or URI containing suggested interaction semantics, the
application will generate an XMPP stanza and send it to the
identified resource, where the generated stanza may include user or

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application inputs that are consistent with the suggested interaction
semantics (for details, see Section 2.8.1).

2.2. Form

As described in [XMPP-CORE], an XMPP address used natively on an XMPP
network is a string of Unicode characters that (1) conforms to a
certain set of stringprep [STRINGPREP] profiles and IDNA restrictions
[IDNA], (2) follows a certain set of syntax rules, and (3) is encoded
as UTF-8 [UTF-8]. The form of such an address can be represented
using Augmented Backus-Naur Form [ABNF] as:

[ node "@" ] domain [ "/" resource ]

In this context, the "node" and "resource" rules rely on distinct
profiles of stringprep [STRINGPREP], and the "domain" rule relies on
the concept of an internationalized domain name as described in
[IDNA]. (Note: There is no need to refer to punycode in the IRI
syntax itself, since any punycode representation would occur only
inside an XMPP application in order to represent internationalized
domain names. However, it is the responsibility of the processing
application to convert IRI syntax [IRI] into IDNA syntax [IDNA]
before addressing XML stanzas to the specified entity on an XMPP
network.)

Certain characters are allowed in XMPP node identifiers and XMPP
resource identifiers but not in the relevant portion of an IRI or
URI. The characters are as follows:

In node identifiers: [ \ ] ^ ` { | }

In resource identifiers: " < > [ \ ] ^ ` { | }

The node identifier characters are not allowed in userinfo by the
sub-delims rule and the resource identifier characters are not
allowed in segment by the pchar rule. These characters MUST be
percent-encoded when transforming an XMPP address into an XMPP IRI or
URI.

Naturally, in order to be converted into an IRI or URI, an XMPP
address must be prepended with a scheme (specifically, the xmpp
scheme) and may also need to undergo transformations that adhere to
the rules defined in [IRI] and [URI]. Furthermore, in order to
enable more advanced interaction with an XMPP entity rather than
simple identification, it is desirable to take advantage of
additional aspects of URI syntax and semantics, such as authority
components, query components, and fragment identifier components.

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Therefore, the ABNF syntax for an XMPP IRI is defined as shown below
using Augmented Backus-Naur Form specified by [ABNF], where the
"ifragment", "ihost", and "iunreserved" rules are defined in [IRI]
and the "pct-encoded" rule is defined in [URI]:

xmppiri = "xmpp" ":" ihierxmpp
[ "?" iquerycomp ]
[ "#" ifragment ]
ihierxmpp = iauthpath / ipathxmpp
iauthpath = "//" iauthxmpp [ "/" ipathxmpp ]
iauthxmpp = inodeid "@" ihost
ipathxmpp = [ inodeid "@" ] ihost [ "/" iresid ]
inodeid = *( iunreserved / pct-encoded / nodeallow )
nodeallow = "!" / "$" / "(" / ")" / "*" / "+" / "," / ";" / "="
iresid = *( iunreserved / pct-encoded / resallow )
resallow = "!" / "$" / "&" / "'" / "(" / ")" /
"*" / "+" / "," / ":" / ";" / "="
iquerycomp = iquerytype [ *ipair ]
iquerytype = *iunreserved
ipair = ";" ikey "=" ivalue
ikey = *iunreserved
ivalue = *( iunreserved / pct-encoded )

However, the foregoing syntax is not appropriate for inclusion in the
registration of the xmpp URI scheme, since the IANA recognizes only
URI schemes and not IRI schemes. Therefore, the ABNF syntax for an
XMPP URI rather than for IRI is defined as shown in Section 3.3 of
this document. If it is necessary to convert the IRI syntax into URI
syntax, an application MUST adhere to the mapping procedure specified
in Section 3.1 of [IRI].

The following is an example of a basic XMPP IRI/URI used for purposes
of identifying a node associated with an XMPP server:

xmpp:[email protected]

Descriptions of the various components of an XMPP IRI/URI are
provided in the following sections.

2.3. Authority Component

As explained in Section 2.8 of this document, in the absence of an
authority component, the processing application would authenticate as
a configured user at a configured XMPP server. That is, the
authority component section is unnecessary and should be ignored if
the processing application has been configured with a set of default
credentials.

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In accordance with Section 3.2 of RFC 3986 [URI], the authority
component is preceded by a double slash ("//") and is terminated by
the next slash ("/"), question mark ("?"), or number sign ("#")
character, or by the end of the IRI/URI. As explained more fully in
Section 2.8.1 of this document, the presence of an authority
component signals the processing application to authenticate as the
node@domain specified in the authority component rather than as a
configured node@domain (see the Security Considerations section of
this document regarding authentication). (While it is unlikely that
the authority component will be included in most XMPP IRIs or URIs,
the scheme allows for its inclusion, if appropriate.) Thus, the
following XMPP IRI/URI indicates to authenticate as
"[email protected]":

xmpp://[email protected]

Note well that this is quite different from the following XMPP IRI/
URI, which identifies a node "[email protected]" but does not signal
the processing application to authenticate as that node:

xmpp:[email protected]

Similarly, using a possible query component of "?message" to trigger
an interface for sending a message, the following XMPP IRI/URI
signals the processing application to authenticate as
"[email protected]" and to send a message to "[email protected]":

xmpp://[email protected]/[email protected]?message

By contrast, the following XMPP IRI/URI signals the processing
application to authenticate as its configured default account and to
send a message to "[email protected]":

xmpp:[email protected]?message

2.4. Path Component

The path component of an XMPP IRI/URI identifies an XMPP address or
specifies the XMPP address to which an XML stanza shall be directed
at the end of IRI/URI processing.

For example, the following XMPP IRI/URI identifies a node associated
with an XMPP server:

xmpp:[email protected]

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The following XMPP IRI/URI identifies a node associated with an XMPP
server along with a particular XMPP resource identifier associated
with that node:

xmpp:[email protected]/some-resource

Inclusion of a node is optional in XMPP addresses, so the following
XMPP IRI/URI simply identifies an XMPP server:

xmpp:example.com

2.5. Query Component

There are many potential use cases for encapsulating information in
the query component of an XMPP IRI/URI for the purpose of specifying
suggested interaction semantics (see Section 2.1); examples include,
but are not limited to:

o sending an XMPP message stanza (see [XMPP-IM]),

o adding a roster item (see [XMPP-IM]),

o sending a presence subscription (see [XMPP-IM]),

o probing for current presence information (see [XMPP-IM]),

o triggering a remote procedure call (see [XEP-0009]),

o discovering the identity or capabilities of another entity (see
[XEP-0030]),

o joining an XMPP-based text chat room (see [XEP-0045]),

o interacting with publish-subscribe channels (see [XEP-0060]),

o providing a SOAP interface (see [XEP-0072]), and

o registering with another entity (see [XEP-0077]).

Many of these potential use cases are application specific, and the
full range of such applications cannot be foreseen in advance given
the continued expansion in XMPP development. However, there is
agreement within the Jabber/XMPP developer community that all the
uses envisioned to date can be encapsulated via a "query type",
optionally supplemented by one or more "key-value" pairs (this is
similar to the "application/x-www-form-urlencoded" MIME type
described in [HTML]).

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As an example, an XMPP IRI/URI intended to launch an interface for
sending a message to the XMPP entity "[email protected]" might
be represented as follows:

xmpp:[email protected]?message

Similarly, an XMPP IRI/URI intended to launch an interface for
sending a message to the XMPP entity "[email protected]" with
a particular subject might be represented as follows:

xmpp:[email protected]?message;subject=Hello%20World

If the processing application does not understand query components or
the specified query type, it MUST ignore the query component and
treat the IRI/URI as consisting of, for example,
<xmpp:[email protected]> rather than
<xmpp:[email protected]?query>. If the processing application
does not understand a particular key within the query component, it
MUST ignore that key and its associated value.

As noted, there exist many kinds of XMPP applications (both actual
and potential), and such applications may define query types and keys
for use in the query component portion of XMPP URIs. The XMPP
Registrar function (see [XEP-0053]) of the XMPP Standards Foundation
maintains a registry of such query types and keys at
<http://www.xmpp.org/registrar/querytypes.html>. To help ensure
interoperability, any application using the formats defined in this
document SHOULD submit any associated query types and keys to that
registry in accordance with the procedures specified in [XEP-0147].

Note: The delimiter between key-value pairs is the ";" character
instead of the "&" character used in many other URI schemes. This
delimiter was chosen in order to avoid problems with escaping of the
& character in HTML and XML applications.

2.6. Fragment Identifier Component

As stated in Section 3.5 of [URI], "The fragment identifier component
of a URI allows indirect identification of a secondary resource by
reference to a primary resource and additional identifying
information." Because the resource identified by an XMPP IRI/URI
does not make available any media type (see [MIME]) and therefore (in
the terminology of [URI]) no representation exists at an XMPP
resource, the semantics of the fragment identifier component in XMPP
IRIs/URIs are to be "considered unknown and, effectively,
unconstrained" (ibid.). Particular XMPP applications MAY make use of
the fragment identifier component for their own purposes. However,
if a processing application does not understand fragment identifier

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components or the syntax of a particular fragment identifier
component included in an XMPP IRI/URI, it MUST ignore the fragment
identifier component.

2.7. Generation of XMPP IRIs/URIs

2.7.1. Generation Method

In order to form an XMPP IRI from an XMPP node identifier, domain
identifier, and resource identifier, the generating application MUST
first ensure that the XMPP address conforms to the rules specified in
[XMPP-CORE], including encoding as a UTF-8 [UTF-8] string and
application of the relevant stringprep profiles [STRINGPREP].
Because IRI syntax [IRI] specifies that the characters in an IRI are
the original Unicode characters themselves [UNICODE], when generating
an XMPP IRI the generating application MUST then decode the UTF-8
[UTF-8] characters of a native XMPP address to their original Unicode
form. The generating application then MUST concatenate the
following:

1. The "xmpp" scheme and the ":" character.

2. Optionally (if an authority component is to be included before
the node identifier), the characters "//", an authority component
of the form node@domain, and the character "/".

3. Optionally (if the XMPP address contained an XMPP "node
identifier"), a string of Unicode characters that conforms to the
"inodeid" rule, followed by the "@" character.

4. A string of Unicode characters that conforms to the "ihost" rule.

5. Optionally (if the XMPP address contained an XMPP "resource
identifier"), the character "/" and a string of Unicode
characters that conforms to the "iresid" rule.

6. Optionally (if a query component is to be included), the "?"
character and query component.

7. Optionally (if a fragment identifier component is to be
included), the "#" character and fragment identifier component.

In order to form an XMPP URI from the resulting IRI, an application
MUST adhere to the mapping procedure specified in Section 3.1 of
[IRI].

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2.7.2. Generation Notes

Certain characters are allowed in the node identifier, domain
identifier, and resource identifier portions of a native XMPP address
but prohibited by the "inodeid", "ihost", and "iresid" rules of an
XMPP IRI. Specifically, the "#" and "?" characters are allowed in
node identifiers, and the "/", "?", "#", and "@" characters are
allowed in resource identifiers, but these characters are used as
delimiters in XMPP IRIs. In addition, the " " ([US-ASCII] space)
character is allowed in resource identifiers but prohibited in IRIs.
Therefore, all the foregoing characters MUST be percent-encoded when
transforming an XMPP address into an XMPP IRI.

Consider the following nasty node in an XMPP address:

nasty!#$%()*+,-.;=?[\]^_`{|}[email protected]

That address would be transformed into the following XMPP IRI (split
into two lines for layout purposes):

xmpp:nasty!%23$%25()*+,-.;=%3F%5B%5C%5D%5E_%60%7B%7C%7D~node
@example.com

Consider the following repulsive resource in an XMPP address (split
into two lines for layout purposes):

[email protected]
/repulsive !#"$%&'()*+,-./:;<=>?@[\]^_`{|}~resource

That address would be transformed into the following XMPP IRI (split
into three lines for layout purposes):

xmpp:[email protected]
/repulsive%20!%23%22$%25&'()*+,-.%2F:;%3C=
%3E%3F%40%5B%5C%5D%5E_%60%7B%7C%7D~resource

Furthermore, virtually any character outside the US-ASCII range
[US-ASCII] is allowed in an XMPP address and therefore also in an
XMPP IRI, but URI syntax forbids such characters directly and
specifies that such characters MUST be percent-encoded. In order to
determine the URI associated with an XMPP IRI, an application MUST
adhere to the mapping procedure specified in Section 3.1 of [IRI].

The following table may assist implementors in understanding the
respective encodings and "carrier units" of the identifiers discussed
in this document, namely: (1) native XMPP addresses, (2) IRIs, and
(3) URIs. For details, refer to Section 3.5 of this document as well

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as Section 3 of [XMPP-CORE], Section 6.4 of [IRI], and Section 2 of
[URI].

+--------------+-----------+-----------+
| Identifier | Encoding | Units |
+--------------+-----------+-----------+
| XMPP address | UTF-8 | Octets |
+--------------+-----------+-----------+
| IRI | Unicode | 16/32-bit |
| | | values |
+--------------+-----------+-----------+
| URI | Percent- | US-ASCII |
| | encoded | |
| | UTF-8 | |
+--------------+-----------+-----------+

2.7.3. Generation Example

Consider the following XMPP address:

<jiři@čechy.example/v Praze>

Note: The string "ř" stands for the Unicode character LATIN
SMALL LETTER R WITH CARON, and the string "č" stands for the
Unicode character LATIN SMALL LETTER C WITH CARON. The "&#x..." form
is used in this document as a notational device to represent Unicode
characters, following the "XML Notation" used in [IRI] to represent
characters that cannot be rendered in ASCII-only documents. An XMPP
IRI MUST contain the Unicode characters themselves, not the
representation in XML Notation (in particular, note that the "#"
character is forbidden in IRI syntax). An XMPP URI MUST properly
escape such characters, as described below. The '<' and '>'
characters are not part of the address itself but are provided to set
off the address for legibility. (For those who do not understand the
Czech language, this example could be Anglicized as
"[email protected]/In Prague".)

In accordance with the process specified above, the generating
application would do the following to generate a valid XMPP IRI from
this address:

1. Ensure that the XMPP address conforms to the rules specified in
[XMPP-CORE], including application of the relevant stringprep
profiles [STRINGPREP] and encoding as a UTF-8 string [UTF-8].

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2. Concatenate the following:

1. The "xmpp" scheme and the ":" character.

2. An "authority component" if included (not shown in this
example).

3. A string of Unicode characters that represents the XMPP
address, transformed in accordance with the "inodeid",
"ihost", and "iresid" rules.

4. The "?" character followed by a "query component" if
appropriate to the application (not shown in this example).

5. The "#" character followed by a "fragment identifier
component" if appropriate to the application (not shown in
this example).

The result is the following XMPP IRI (note again that, in accordance
with the "XML Notation" used in [IRI], the string "ř" stands
for the Unicode character LATIN SMALL LETTER R WITH CARON and the
string "č" stands for the Unicode character LATIN SMALL LETTER
C WITH CARON; an XMPP IRI would contain the Unicode characters
themselves).

<xmpp:jiři@čechy.example/v%20Praze>

In order to generate a valid XMPP URI from the foregoing IRI, the
application MUST adhere to the procedure specified in Section 3.1 of
[IRI], resulting in the following URI:

<xmpp:ji%C5%99i@%C4%8Dechy.example/v%20Praze>

2.8. Processing of XMPP IRIs/URIs

2.8.1. Processing Method

If a processing application is presented with an XMPP URI and not
with an XMPP IRI, it MUST first convert the URI into an IRI by
following the procedure specified in Section 3.2 of [IRI].

In order to decompose an XMPP IRI for interaction with the entity it
identifies, a processing application MUST separate:

1. The "xmpp" scheme and the ":" character.

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2. The authority component, if included (the string of Unicode
characters between the "//" characters and the next "/"
character, the "?" character, the "#" character, or the end of
the IRI).

3. A string of Unicode characters that represents an XMPP address as
transformed in accordance with the "inodeid", "ihost", and
"iresid" rules.

4. Optionally the query component, if included, using the "?"
character as a separator.

5. Optionally the fragment identifier component, if included, using
the "#" character as a separator.

At this point, the processing application MUST ensure that the
resulting XMPP address conforms to the rules specified in
[XMPP-CORE], including application of the relevant stringprep
profiles [STRINGPREP]. The processing application then would either
(1) complete further XMPP handling itself or (2) invoke a helper
application to complete XMPP handling; such XMPP handling would most
likely consist of the following steps:

1. If not already connected to an XMPP server, connect either as the
user specified in the authority component or as the configured
user at the configured XMPP server, normally by adhering to the
XMPP connection procedures defined in [XMPP-CORE]. (Note: The
processing application SHOULD ignore the authority component if
it has been configured with a set of default credentials.)

2. Optionally, determine the nature of the intended recipient (e.g.,
via [XEP-0030]).

3. Optionally, present an appropriate interface to a user based on
the nature of the intended recipient and/or the contents of the
query component.

4. Generate an XMPP stanza that translates any user or application
inputs into their corresponding XMPP equivalents.

5. Send the XMPP stanza via the authenticated server connection for
delivery to the intended recipient.

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2.8.2. Processing Notes

It may help implementors to note that the first two steps of "further
XMPP handling", as described at the end of Section 2.8.1, are similar
to HTTP authentication [HTTP-AUTH], while the next three steps are
similar to the handling of mailto: URIs [MAILTO].

As noted in Section 2.7.2 of this document, certain characters are
allowed in the node identifier, domain identifier, and resource
identifier portions of a native XMPP address but prohibited by the
"inodeid", "ihost", and "iresid" rules of an XMPP IRI. The percent-
encoded octets corresponding to these characters in XMPP IRIs MUST be
transformed into the characters allowed in XMPP addresses when
processing an XMPP IRI for interaction with the represented XMPP
entity.

Consider the following nasty node in an XMPP IRI (split into two
lines for layout purposes):

xmpp:nasty!%23$%25()*+,-.;=%3F%5B%5C%5D%5E_%60%7B%7C%7D~node
@example.com

That IRI would be transformed into the following XMPP address:

nasty!#$%()*+,-.;=?[\]^_`{|}[email protected]

Consider the following repulsive resource in an XMPP IRI (split into
three lines for layout purposes):

xmpp:[email protected]
/repulsive%20!%23%22$%25&'()*+,-.%2F:;%3C
=%3E%3F%40%5B%5C%5D%5E_%60%7B%7C%7D~resource

That IRI would be transformed into the following XMPP address (split
into two lines for layout purposes):

[email protected]
/repulsive !#"$%&'()*+,-./:;<=>?@[\]^_`{|}~resource

2.8.3. Processing Example

Consider the XMPP URI that resulted from the previous example (see
Section 2.7.3):

<xmpp:ji%C5%99i@%C4%8Dechy.example/v%20Praze>

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RFC 5122 XMPP IRIs/URIs February 2008

In order to generate a valid XMPP IRI from that URI, the application
MUST adhere to the procedure specified in Section 3.2 of [IRI],
resulting in the following IRI:

<xmpp:jiři@čechy.example/v%20Praze>

In accordance with the process specified above, the processing
application would remove the "xmpp" scheme and ":" character to
extract the XMPP address from this XMPP IRI, converting any percent-
encoded octets from the "inodeid", "ihost", and "iresid" rules into
their character equivalents (e.g., "%20" into the space character).

The result is this XMPP address:

<jiři@čechy.example/v Praze>

2.9. Internationalization

Because XMPP addresses are UTF-8 strings [UTF-8] and because octets
outside the US-ASCII range [US-ASCII] within XMPP addresses can be
easily converted to percent-encoded octets, XMPP addresses are
designed to work well with Internationalized Resource Identifiers
[IRI]. In particular, with the exceptions of stringprep
verification, the conversion of syntax-relevant US-ASCII characters
(e.g., "?"), and the conversion of percent-encoded octets from the
"inodeid", "ihost", and "iresid" rules into their character
equivalents (e.g., "%20" into the US-ASCII space character), an XMPP
IRI can be constructed directly by prepending the "xmpp" scheme and
":" character to an XMPP address. Furthermore, an XMPP IRI can be
converted into URI syntax by adhering to the procedure specified in
Section 3.1 of [IRI], and an XMPP URI can be converted into IRI
syntax by adhering to the procedure specified in Section 3.2 of
[IRI], thus ensuring interoperability with applications that are able
to process URIs but unable to process IRIs.

3. IANA Registration of xmpp URI Scheme

In accordance with [URI-SCHEMES], this section provides the
information required to register the xmpp URI scheme.

3.1. URI Scheme Name

xmpp

3.2. Status

permanent

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RFC 5122 XMPP IRIs/URIs February 2008

3.3. URI Scheme Syntax

The syntax for an xmpp URI is defined below using Augmented Backus-
Naur Form as specified by [ABNF], where the "fragment", "host", "pct-
encoded", and "unreserved" rules are defined in [URI]:

xmppuri = "xmpp" ":" hierxmpp [ "?" querycomp ] [ "#" fragment ]
hierxmpp = authpath / pathxmpp
authpath = "//" authxmpp [ "/" pathxmpp ]
authxmpp = nodeid "@" host
pathxmpp = [ nodeid "@" ] host [ "/" resid ]
nodeid = *( unreserved / pct-encoded / nodeallow )
nodeallow = "!" / "$" / "(" / ")" / "*" / "+" / "," / ";" / "="
resid = *( unreserved / pct-encoded / resallow )
resallow = "!" / "$" / "&" / "'" / "(" / ")" /
"*" / "+" / "," / ":" / ";" / "="
querycomp = querytype [ *pair ]
querytype = *( unreserved / pct-encoded )
pair = ";" key "=" value
key = *( unreserved / pct-encoded )
value = *( unreserved / pct-encoded )

3.4. URI Scheme Semantics

The xmpp URI scheme identifies entities that natively communicate
using the Extensible Messaging and Presence Protocol (XMPP), and is
mainly used for identification rather than for resource location.
However, if an application that processes an xmpp URI enables
interaction with the XMPP address identified by the URI, it MUST
follow the methodology defined in Section 2 of this document, Use of
XMPP IRIs and URIs, to reconstruct the encapsulated XMPP address,
connect to an appropriate XMPP server, and send an appropriate XMPP
"stanza" (XML fragment) to the XMPP address. (Note: There is no MIME
type associated with the xmpp URI scheme.)

3.5. Encoding Considerations

In addition to XMPP URIs, there will also be XMPP Internationalized
Resource Identifiers (IRIs). Prior to converting an Extensible
Messaging and Presence Protocol (XMPP) address into an IRI (and in
accordance with [XMPP-CORE]), the XMPP address must be represented as
a string of UTF-8 characters [UTF-8] by the generating application
(e.g., by transforming an application's internal representation of
the address as a UTF-16 string into a UTF-8 string). Because IRI
syntax [IRI] specifies that the characters in an IRI are the original
Unicode characters themselves [UNICODE], when generating an XMPP IRI
the generating application MUST decode the UTF-8 characters of a
native XMPP address to their original Unicode form. Because URI

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RFC 5122 XMPP IRIs/URIs February 2008

syntax [URI] specifices that the characters in a URI are US-ASCII
characters [US-ASCII] only, when generating an XMPP URI the
generating application MUST escape the Unicode characters of an XMPP
IRI to US-ASCII characters by adhering to the procedure specified in
RFC 3987.

3.6. Applications/Protocols That Use This URI Scheme Name

The xmpp URI scheme is intended to be used by interfaces to an XMPP
network from non-native user agents, such as web browsers, as well as
by non-native applications that need to identify XMPP entities as
full URIs or IRIs.

3.7. Interoperability Considerations

There are no known interoperability concerns related to use of the
xmpp URI scheme. In order to help ensure interoperability, the XMPP
Registrar function of the XMPP Standards Foundation maintains a
registry of query types and keys that can be used in the query
components of XMPP URIs and IRIs, located at
<http://www.xmpp.org/registrar/querytypes.html>.

3.8. Security Considerations

See Section 5 of this document, Security Considerations.

3.9. Contact

Peter Saint-Andre [mailto:[email protected],
xmpp:[email protected]]

3.10. Author/Change Controller

This scheme is registered under the IETF tree. As such, the IETF
maintains change control.

3.11. References

[XMPP-CORE]

4. IANA Considerations

This document obsoletes the URI scheme registration created by RFC
4622. The registration template can be found in Section 3 of this
document. In order to help ensure interoperability, the XMPP
Registrar function of the XMPP Standards Foundation maintains a
registry of query types and keys that can be used in the query

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RFC 5122 XMPP IRIs/URIs February 2008

components of XMPP URIs and IRIs, located at
<http://www.xmpp.org/registrar/querytypes.html>.

5. Security Considerations

Providing an interface to XMPP services from non-native applications
introduces new security concerns. The security considerations
discussed in [IRI], [URI], and [XMPP-CORE] apply to XMPP IRIs, and
the security considerations discussed in [URI] and [XMPP-CORE] apply
to XMPP URIs. In accordance with Section 2.7 of [URI-SCHEMES] and
Section 7 of [URI], particular security considerations are specified
in the following sections.

5.1. Reliability and Consistency

Given that XMPP addresses of the form [email protected] are typically
created via registration at an XMPP server or provisioned by an
administrator of such a server, it is possible that such addresses
may also be unregistered or deprovisioned. Therefore, the XMPP IRI/
URI that identifies such an XMPP address may not reliably and
consistently be associated with the same principal, account owner,
application, or device.

XMPP addresses of the form [email protected]/resource are typically
even more ephemeral (since a given XMPP resource identifier is
typically associated with a particular, temporary session of an XMPP
client at an XMPP server). Therefore, the XMPP IRI/URI that
identifies such an XMPP address probably will not reliably and
consistently be associated with the same session. However, the
procedures specified in Section 10 of [XMPP-CORE] effectively
eliminate any potential confusion that might be introduced by the
lack of reliability and consistency for the XMPP IRI/URI that
identifies such an XMPP address.

XMPP addresses of the form domain.tld are typically long-lived XMPP
servers or associated services. Although naturally it is possible
for server or service administrators to decommission the server or
service at any time, typically the IRIs/URIs that identify such
servers or services are the most reliable and consistent of XMPP
IRIs/URIs.

XMPP addresses of the form domain.tld/resource are not yet common on
XMPP networks; however, the reliability and consistency of XMPP IRIs/
URIs that identify such XMPP addresses would likely fall somewhere
between those that identify XMPP addresses of the form domain.tld and
those that identify XMPP addresses of the form [email protected].

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5.2. Malicious Construction

Malicious construction of XMPP IRIs/URIs is made less likely by the
prohibition on port numbers in XMPP IRIs/URIs (since port numbers are
to be discovered using DNS SRV records [DNS-SRV], as specified in
[XMPP-CORE]).

5.3. Back-End Transcoding

Because the base XMPP protocol is designed to implement the exchange
of messages and presence information and not the retrieval of files
or invocation of similar system functions, it is deemed unlikely that
the use of XMPP IRIs/URIs would result in harmful dereferencing.
However, if an XMPP protocol extension defines methods for
information retrieval, it MUST define appropriate controls over
access to that information. In addition, XMPP servers SHOULD NOT
natively parse XMPP IRIs/URIs but instead SHOULD accept only the XML
wire protocol specified in [XMPP-CORE] and any desired extensions
thereto.

5.4. Sensitive Information

The ability to interact with XMPP entities via a web browser or other
non-native application may expose sensitive information (such as
support for particular XMPP application protocol extensions) and
thereby make it possible to launch attacks that are not possible or
that are unlikely on a native XMPP network. Due care must be taken
in deciding what information is appropriate for representation in
XMPP IRIs or URIs.

In particular, advertising XMPP IRIs/URIs in publicly accessible
locations (e.g., on websites) may make it easier for malicious users
to harvest XMPP addresses from the authority and path components of
XMPP IRIs/URIs and therefore to send unsolicited bulk communications
to the users or applications represented by those addresses. Due
care should be taken in balancing the benefits of open information
exchange against the potential costs of unwanted communications.

To help prevent leaking of sensitive information, passwords and other
user credentials are forbidden in the authority component of XMPP
IRIs/URIs; in fact they are not needed, since the fact that
authentication in XMPP occurs via the Simple Authentication and
Security Layer [SASL] makes it possible to use the SASL ANONYMOUS
mechanism, if desired.

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RFC 5122 XMPP IRIs/URIs February 2008

5.5. Semantic Attacks

Despite the existence of non-hierarchical URI schemes such as
[MAILTO], by association human users may expect all URIs to include
the "//" characters after the scheme name and ":" character.
However, in XMPP IRIs/URIs, the "//" characters precede the authority
component rather than the path component. Thus,
xmpp://[email protected] indicates to authenticate as
"[email protected]", whereas xmpp:[email protected] identifies the
node "[email protected]". Processing applications MUST clearly
differentiate between these forms, and user agents SHOULD discourage
human users from including the "//" characters in XMPP IRIs/URIs
since use of the authority component is envisioned to be helpful only
in specialized scenarios, not more generally.

5.6. Spoofing

The ability to include effectively the full range of Unicode
characters in an XMPP IRI may make it easier to execute certain forms
of address mimicking (also called "spoofing"). However, XMPP IRIs
are no different from other IRIs in this regard, and applications
that will present XMPP IRIs to human users must adhere to best
practices regarding address mimicking in order to help prevent
attacks that result from spoofed addresses (e.g., the phenomenon
known as "phishing"). For details, refer to the Security
Considerations of [IRI].

6. Acknowledgements

Thanks to Martin Duerst, Lisa Dusseault, Frank Ellerman, Roy
Fielding, Joe Hildebrand, and Ralph Meijer for their comments.

7. References

7.1. Normative References

[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2007.

[IRI] Duerst, M. and M. Suignard, "Internationalized
Resource Identifiers (IRIs)", RFC 3987, January 2005.

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

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

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RFC 5122 XMPP IRIs/URIs February 2008

[XMPP-CORE] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 3920, October 2004.

7.2. Informative References

[CPIM] Peterson, J., "Common Profile for Instant Messaging
(CPIM)", RFC 3860, August 2004.

[CPP] Peterson, J., "Common Profile for Presence (CPP)",
RFC 3859, August 2004.

[DNS-SRV] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR
for specifying the location of services (DNS SRV)",
RFC 2782, February 2000.

[HTML] Raggett, D., "HTML 4.0 Specification", W3C REC REC-
html40-19980424, April 1998.

[HTTP-AUTH] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence,
S., Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access
Authentication", RFC 2617, June 1999.

[IDNA] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications
(IDNA)", RFC 3490, March 2003.

[MAILTO] Hoffman, P., Masinter, L., and J. Zawinski, "The
mailto URL scheme", RFC 2368, July 1998.

[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part Two: Media Types",
RFC 2046, November 1996.

[SASL] Melnikov, A. and K. Zeilenga, "Simple Authentication
and Security Layer (SASL)", RFC 4422, June 2006.

[STRINGPREP] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("STRINGPREP")", RFC 3454,
December 2002.

Saint-Andre Standards Track [Page 22]

RFC 5122 XMPP IRIs/URIs February 2008

[UNICODE] The Unicode Consortium, "The Unicode Standard, Version
3.2.0", 2000.

The Unicode Standard, Version 3.2.0 is defined by The
Unicode Standard, Version 3.0 (Reading, MA, Addison-
Wesley, 2000. ISBN 0-201-61633-5), as amended by the
Unicode Standard Annex #27: Unicode 3.1
(http://www.unicode.org/reports/tr27/) and by the
Unicode Standard Annex #28: Unicode 3.2
(http://www.unicode.org/reports/tr28/).

[URI-SCHEMES] Hansen, T., Hardie, T., and L. Masinter, "Guidelines
and Registration Procedures for New URI Schemes",
RFC 4395, February 2006.

[US-ASCII] American National Standards Institute, "Coded
Character Set - 7-bit American Standard Code for
Information Interchange", ANSI X3.4, 1986.

[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.

[XEP-0009] Adams, D., "Jabber-RPC", XSF XEP 0009, February 2006.

[XEP-0030] Hildebrand, J., Millard, P., Eatmon, R., and P. Saint-
Andre, "Service Discovery", XSF XEP 0030,
February 2007.

[XEP-0045] Saint-Andre, P., "Multi-User Chat", XSF XEP 0045,
April 2007.

[XEP-0053] Saint-Andre, P., "XMPP Registrar Function", XSF
XEP 0053, December 2006.

[XEP-0060] Millard, P., Saint-Andre, P., and R. Meijer, "Publish-
Subscribe", XSF XEP 0060, September 2006.

[XEP-0072] Forno, F. and P. Saint-Andre, "SOAP Over XMPP", XSF
XEP 0072, December 2005.

[XEP-0077] Saint-Andre, P., "In-Band Registration", XSF XEP 0077,
January 2006.

[XEP-0147] Saint-Andre, P., "XMPP URI Scheme Query Components",
XSF XEP 0147, September 2006.

Saint-Andre Standards Track [Page 23]

RFC 5122 XMPP IRIs/URIs February 2008

[XMPP-IM] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Instant Messaging and Presence",
RFC 3921, October 2004.

Saint-Andre Standards Track [Page 24]

RFC 5122 XMPP IRIs/URIs February 2008

Appendix A. Differences from RFC 4622

Several errors were found in RFC 4622. This document corrects those
errors. The resulting differences from RFC 4622 are as follows:

o Specified that the characters "[", "\", "]", "^", "`", "{", "|",
and "}" are allowed in XMPP node identifiers but not allowed in
IRIs or URIs according to the sub-delims rule.

o Specified that the characters '"', "<", ">", "[", "\", "]", "^",
"`", "{", "|", and "}" are allowed in XMPP resource identifiers
but not allowed in IRIs or URIs according to the pchar rule.

o Specified that the foregoing characters must be percent-encoded
when constructing an XMPP URI.

o Corrected the ABNF accordingly.

o Updated the examples accordingly.

Appendix B. Copying Conditions

Regarding this entire document or any portion of it, the author makes
no guarantees and is not responsible for any damage resulting from
its use. The author grants irrevocable permission to anyone to use,
modify, and distribute it in any way that does not diminish the
rights of anyone else to use, modify, and distribute it, provided
that redistributed derivative works do not contain misleading author
or version information. Derivative works need not be licensed under
similar terms.

Author's Address

Peter Saint-Andre
XMPP Standards Foundation

EMail: [email protected]
URI: xmpp:[email protected]

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RFC 5122 XMPP IRIs/URIs February 2008

Full Copyright Statement

Copyright (C) The IETF Trust (2008).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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Saint-Andre Standards Track [Page 26]