1. Introduction

   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.