1. Introduction

   The Session Initiation Protocol (SIP) (RFC 3261prop [1]) is a client-
   server protocol used for the initiation and management of
   communications sessions between users.  SIP end systems are called
   user agents, and intermediate elements are known as proxy servers.  A
   typical SIP configuration, referred to as the SIP "trapezoid", is
   shown in Figure 1.  In this diagram, a caller in domain A (UA1)
   wishes to call Joe in domain B (joe@B).  To do so, it communicates
   with proxy 1 in its domain (domain A).  Proxy 1 forwards the request
   to the proxy for the domain of the called party (domain B), which is
   proxy 2.  Proxy 2 forwards the call to the called party, UA 2.

   As part of this call flow, proxy 1 needs to determine a SIP server
   for domain B.  To do this, proxy 1 makes use of DNS procedures, using
   both SRV [2] and NAPTR [3] records.  This document describes the
   specific problems that SIP uses DNS to help solve, and provides a
   solution.