IMPP WG                                                       D. Crocker
Internet-Draft                                               Brandenburg
Expires: August 29, 2003                                     J. Peterson
                                                       February 28, 2003

         Address Resolution for Instant Messaging and Presence

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   Presence and instant messaging are defined in RFC2778 [5].  The
   Common Profiles for Presence [2] and Instant Messaging [1] define two
   URI schemes: 'im' for INSTANT INBOXes and 'pres' for PRESENTITIES.
   This document provides guidance for locating the resources associated
   with URIs that employ these schemes.

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Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2. Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3. Address Resolution . . . . . . . . . . . . . . . . . . . . . . . 4
   4. Domain Name Lookup . . . . . . . . . . . . . . . . . . . . . . . 4
   5. Processing SRV RRs . . . . . . . . . . . . . . . . . . . . . . . 4
   6. Processing Multiple Addresses  . . . . . . . . . . . . . . . . . 5
   7. Security Considerations  . . . . . . . . . . . . . . . . . . . . 5
   8. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . . 6
   9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 6
      Normative References . . . . . . . . . . . . . . . . . . . . . . 6
      Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
      Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 8

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1. Introduction

   Presence and instant messaging are defined in RFC2778 [5].  The
   Common Profiles for Presence (CPP [2]) and Instant Messaging (CPIM
   [1]) define two 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 [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 in order to provide greater compatibility with
   clients that support only one instant messaging protocol.  A client
   would look up the record corresponding 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 MX records.

   The system described in this document relies on the use of DNS SRV
   [7] records and A records.

2. Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as

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   described in RFC2119 [3] and indicate requirement levels for
   compliant implementations.

   This memos makes use of the vocabulary defined in RFC2778 [5].  Terms
   the same meaning as defined therein.

3. Address Resolution

   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 IM URI (i.e., domain names that can
   be resolved to SRV [7] or A RRs).

4. Domain Name Lookup

   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 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.

5. Processing SRV RRs

   Taking the IM URI for a concrete example, a lookup is performed for
   SRVs for the target domain and a desired IM transfer protocol.

   For example, if the destination INSTANT INBOX is
   "", and the sender wishes to use an IM transfer
   protocol called "SIP", then a SRV lookup is performed for:

   The returned RRs, if any, specify the next-hop server.

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   The choice of IM transfer protocol is a local configuration option
   for each system.

   Receiving systems that are registed for this DNS-based SRV resolution
   service list the transfer protocols by which they can be reached,
   either directly or through a translating gateway.  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.

   The same logic is used for PRES URIs.

6. Processing Multiple Addresses

   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, multihoming, or both.  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.  Two types of information are used
   to rank the domain addresses: multiple SRV records, and multihomed

   Multiple SRV records contain a preference indication that MUST be
   used in sorting.  Lower numbers are preferable to higher ones.  If
   there are multiple destinations with the same preference, and there
   is no clear reason to favor one (e.g., by recognition of an easily-
   reached address), then the sender MUST randomize them to spread the
   load across multiple servers for a specific destination.

   The destination domain (perhaps taken from the preferred SRV record)
   may be multihomed, in which case the resolver will return a list of
   alternative IP addresses.  It is the responsibility of the resolver
   to have ordered this list by decreasing preference if necessary, and
   the sender MUST try them in the order presented.

7. Security Considerations

   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]).

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8. IANA Considerations

   This document introduces no new considerations for IANA.

9. Contributors

   The following individuals made substantial textual contributions to
   this document:

      Athanassios Diacakis (

      Florencio Mazzoldi (

      Christian Huitema (

      Graham Klyne (

      Jonathan Rosenberg (

      Robert Sparks (

      Hiroyasu Sugano (

Normative References

   [1]  Crocker, D. and J. Peterson, "Common Profile: Instant
        Messaging", draft-ietf-impp-im-00 (work in progress), October

   [2]  Crocker, D. and J. Peterson, "Common Profile: Presence", draft-
        ietf-impp-pres-00 (work in progress), October 2002.

   [3]  Bradner, S., "Key words for use in RFCs to indicate requirement
        levels", RFC 2119, March 1997.

   [4]  Mockapetris, P., "Domain Names - Concepts and Facilities", RFC
        1034, STD 13, 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

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Authors' Addresses

   Dave Crocker
   Brandenburg InternetWorking
   675 Spruce Drive
   Sunnyvale, CA  94086

   Phone: +1 408/246-8253

   Jon Peterson
   NeuStar, Inc.
   1800 Sutter St
   Suite 570
   Concord, CA  94520

   Phone: +1 925/363-8720

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