Presence and instant messaging are defined in RFC 2778 [5]. The
Common Profiles for Presence (CPP) [2] and Instant Messaging (CPIM)
[1] define two Universal Resource Identifier (URI) schemes: 'im' for
INSTANT INBOXes and 'pres' for PRESENTITIES. This document provides
rules for locating the resources associated with URIs that employ
these schemes via the Domain Name Service (DNS) [4]. These rules
could no doubt be applied to the resolution of other URI schemes that
are unrelated to instant messaging and presence.
CPIM and CPP both specify operations that have 'source' and
'destination' attributes. While only the semantics, not the syntax,
of these attributes are defined by CPIM and CPP, many instant
messaging and presence protocols today support the use of URIs to
reflect the source and destination of their operations. The 'im' and
'pres' URI schemes allow such protocols to express the identities of
the principals associated with a protocol exchange. When these
operations pass through a CPIM or CPP gateway, these URIs could be
relayed without modification, which has a number of desirable
properties for the purposes of interoperability.
These URI schemes are also useful in cases where no CPIM/CPP
gatewaying will occur. If a particular principal's endpoint supports
multiple instant messaging applications, for example, then a domain
that identifies that host might use the sort of DNS records described
in this document to provide greater compatibility with clients that
support only one instant messaging protocol. A client would look up
the corresponding record to the supported protocol, and learn how to
contact the endpoint for that protocol. The principal in this
instance would use an IM URI as their canonical address.
In some architectures, these URIs might also be used to locate a CPIM
or CPP gateway that serves a particular domain. If a particular IM
service provider wishes to operate CPIM/CPP gateways in its own
domain that map standard Internet protocols to an internal
proprietary protocol, that gateway could be identified by an IM URI.
In that case, the DNS records used to dereference the IM URI would
serve a purpose similar to that of Mail Exchange (MX) records.
The system described in this document relies on the use of DNS
service (SRV) [7] records and address (A) records.
Peterson Standards Track [Page 2]
RFC 3861 IM&P SRV August 2004
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
described in BCP 14, RFC 2119 [3] and indicate requirement levels for
compliant implementations.
This memo makes use of the vocabulary defined in RFC 2778 [5]. Terms
such as CLOSED, INSTANT INBOX, INSTANT MESSAGE, and OPEN are used in
the same meaning as defined therein.
A client determines the address of an appropriate system running a
server, on behalf of the system referenced by the domain, by
resolving the destination domain name that is part of the identifier
to either an intermediate relay system or a final target system.
Only resolvable, fully-qualified, domain names (FQDNs) are permitted
when domain names are used in an Instant Messaging (IM) URI (i.e.,
domain names that can be resolved to SRV [7] or A Resource Record
(RR)).
The symbolic name used in the Service field of the SRV record is
"_im" for instant messaging and "_pres" for presence (matching their
respective URI schemes). However, the advertisement of these
services in the DNS is incomplete if it does not include the protocol
that will be used to instantiate the instant messaging or presence
operations. Thus, the Protocol field of the SRV record contains an
IANA-registered label corresponding to the underlying instant
messaging or presence protocol being advertised (see Section 8 for
more information on valid Protocol fields).
Taking the IM URI as a concrete example, a lookup is performed for
SRVs for the target domain, a desired service (using the "_im"
Service label) and a desired IM transfer protocol. If the
destination INSTANT INBOX is "im:fred@example.com", and the sender
wishes to use an IM transfer protocol called "BIP" (and supposing
"_bip" were registered with IANA as a valid Protocol label for the IM
Service), then a SRV lookup is performed for:
_im._bip.example.com.
The same procedure is used for PRES URIs, with the "_pres" Service
label.
Peterson Standards Track [Page 3]
RFC 3861 IM&P SRV August 2004
Some clients may support multiple instant messaging or presence
protocols; in these cases they may make several such SRV queries, in
an application-specific order, until they find one supported in
common with the target domain.
Once a client lexically identifies a domain to which instant
messaging or presence operations will be delivered for processing, a
DNS lookup MUST be performed to resolve the domain. The names MUST
be fully-qualified domain names (FQDNs) -- mechanisms for inferring
FQDNs from partial names or local aliases are a local matter.
The lookup first attempts to locate SRV RRs associated with the
domain. If a canonical name (CNAME) RR is found instead, the
resulting domain is processed as if it were the initial domain.
If one or more SRV RRs are found for a given domain, a sender MUST
NOT utilize any A RRs associated with that domain unless they are
located using the SRV RRs. If no SRV RRs are found, but an A RR is
found, then the A RR is treated as if it was associated with an
implicit SRV RR, with a preference of 0, pointing to that domain.
The returned DNS RRs, if any, specify the next-hop server, which may
be a protocol gateway or an endpoint.
Receiving systems that are registered for this DNS-based SRV
resolution service list the transfer protocols by which they can be
reached, either directly or through a translating gateway (using
combinations of Service and Protocol labels as described above). The
transfer-time choice of the IM transfer protocol to be used (and,
therefore, to be resolved) is a local configuration option for each
sending system.
Using this mechanism, seamless routing of IM traffic is possible,
regardless of whether a gateway is necessary for interoperation. To
achieve this transparency, a separate RR for a gateway must be
present for each transfer protocol and domain pair that it serves.
Peterson Standards Track [Page 4]
RFC 3861 IM&P SRV August 2004
When the lookup succeeds, the mapping can result in a list of
alternative delivery addresses rather than a single address, because
of multiple SRV records. For reliable operations, the client MUST be
able to try each of the relevant addresses in this list in order,
until a delivery attempt succeeds. However, there MAY also be a
configurable limit on the number of alternate addresses that can be
tried. In any case, the client SHOULD try at least two addresses.
Resolvers must follow the standard procedures in RFC 2782 [7] for
handling the priority and weight fields of SRV records.
The usage of IM and PRES URIs, and the DNS procedures in this
document, introduce no security considerations beyond those described
in the requirements for instant messaging and presence ([6]) and the
SRV specification ([7]).
Subsequent registrations of Protocol labels for use with the "_im" or
"_pres" Service labels MUST, however, explain any security
considerations that arise from the use of the protocol in question
with SRV.
This document reserves the use of "_im" and "_pres" Service labels.
Since these relate to a service which may pass messages over a number
of different message transports, they must be associated with a
specific instant messaging or presence service.
In order to ensure that the association between "_im" and "_pres" and
their respective underlying services are deterministic, the IANA has
created two independent registries: the Instant Messaging SRV
Protocol Label registry and the Presence SRV Protocol Label registry.
For each registry, an entry shall consist of a label name and a
pointer to a specification describing how the protocol named in the
label uses SRV. Specifications should conform to the requirements
listed in RFC 2434 [8] for "specification required".
Protocol labels compliant with this specification MUST begin with the
underscore character "_" and follow all other rules for SRV Protocol
labels described in [7].
Peterson Standards Track [Page 5]
RFC 3861 IM&P SRV August 2004
Dave Crocker edited earlier versions of this document.
The following individuals made substantial textual contributions to
this document:
Athanassios Diacakis (thanos.diacakis@openwave.com)
Florencio Mazzoldi (flo@networkprojects.com)
Christian Huitema (huitema@microsoft.com)
Graham Klyne (gk@ninebynine.org)
Jonathan Rosenberg (jdrosen@dynamicsoft.com)
Robert Sparks (rsparks@dynamicsoft.com)
Hiroyasu Sugano (suga@flab.fujitsu.co.jp)
[1] Peterson, J., "Common Profile for Instant Messaging (CPIM)", RFC
3860, August 2004.
[2] Peterson, J., "Common Profile for Presence (CPP)", RFC 3859,
August 2004.
[3] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", BCP 14, RFC 2119, March 1997.
[4] Mockapetris, P., "Domain Names - Concepts and Facilities", STD
13, RFC 1034, November 1987.
[5] Day, M., Rosenberg, J., and H. Sugano, "A Model for Presence and
Instant Messaging", RFC 2778, February 2000.
[6] Day, M., Aggarwal, S., and J. Vincent, "Instant Messaging /
Presence Protocol Requirements", RFC 2779, February 2000.
[7] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
Specifying the Location of Services (SRV)", RFC 2782, February
2000.
[8] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC 2434, BCP 26, October 1998.
Peterson Standards Track [Page 6]
RFC 3861 IM&P SRV August 2004
Jon Peterson
NeuStar, Inc.
1800 Sutter St
Suite 570
Concord, CA 94520
US
Phone: +1 925/363-8720
EMail: jon.peterson@neustar.biz
Peterson Standards Track [Page 7]
RFC 3861 IM&P SRV August 2004
Copyright (C) The Internet Society (2004). 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
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND 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
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Peterson Standards Track [Page 8]