Internet Engineering Task Force                                   SIP WG
Internet Draft                                     Schulzrinne/Rosenberg
draft-ietf-sip-srv-02.txt                        Columbia U./dynamicsoft
January 15,
March 24, 2001
Expires: June 2001

        SIP: Session Initiation Protocol -- Locating SIP Servers


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

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   This document describes how a SIP client locates a SIP server based
   on the Request-URI or a preconfigured outbound proxy server. This
   document updates the process described in RFC 2543.

1 Introduction

   This document updates Sections 1.3 and 1.4.2 and supersedes Appendix
   D of RFC 2543 [1]. Inter alia, it defines the term outbound proxy and
   replaces references to the obsoleted RFC 2052 with current references
   to RFC 2782.

1.1 Terminology

   In this document, the key words "MUST", "MUSTNOT", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [2] and
   indicate requirement levels for compliant SIP implementations.

1.2 Definitions

        Outbound proxy: A proxy that is located near the originator of
             requests. It receives all outgoing requests from a
             particular UAC, including those requests whose Request-URLs
             identify a host other than the outbound proxy. The outbound
             proxy sends these requests, after any local processing, to
             the address indicated in the Request-URI. (All other proxy
             servers are simply referred as proxies, not inbound

2 Locating a SIP Server

   When a client wishes to send a request, the client either sends it to
   a locally configured SIP proxy server, the so-called outbound proxy ,
   independent of the Request-URI, or sends it to the IP address and
   port corresponding to the Request-URI. The outbound proxy can be
   configured by any mechanism, including DHCP [3] and can be specified
   either as a set of parameters such as network address or host name,
   protocol port and transport protocol, or as a SIP URI.

   If the Request-URI is used, the client needs to determine the
   protocol, port and IP address of a server to which to send the
   request. A client SHOULD follow the steps below to obtain this

   Clients MUST determine re-run the destination address above selection algorithm, re-drawing any
   random numbers involved, once per transaction rather than for each
   request, i.e., requests within the same transaction MUST be sent to
   the same network address. Thus, the same address is used for the
   request, any retransmissions, any associated CANCEL requests and ACK
   requests for non-2xx responses. However, ACKs for 2xx responses use
   another iteration of the selection algorithm. (Indeed, in many cases,
   they may have different request URIs.)

   A stateless proxy can accomplish this, for example, by using the
   modulo N of a hash of the Call-ID value or some other combination of transaction-
   transaction-identifying headers as the uniform random number
   described in the weighting algorithm of RFC 2782. Here, N is the sum
   of weights within the priority class.

   A client SHOULD be able to interpret explicit network notifications
   (such as ICMP messages) which indicate that a server is not
   reachable, rather than relying solely on timeouts. (For socket-based
   programs:  For TCP, connect() returns ECONNREFUSED if the client
   could not connect to a server at that address. For UDP, the socket
   needs to be bound to the destination address using connect() rather
   than sendto() or similar so that a second write() or send() fails
   with ECONNREFUSED if there is no server listening) If the client
   finds the server is not reachable at a particular address, it SHOULD
   behave as if it had received a 400-class error response to that

   The client tries to find one or more addresses for the SIP server by
   querying DNS. If a step elicits no addresses, the client continues to
   the next step. However if a step elicits one or more addresses, but
   no SIP server at any of those addresses responds, then the client
   concludes the server is down and does not continue on to the next

   If the client is configured with the address of an outbound proxy,
   the parameters of the outbound proxy, including transport protocol
   and port, become the destination used below.

   If there is no outbound proxy, the destination is the Request-URI.
   The destination address is the maddr parameter if it exists and the
   host element if not. The transport protocol is the transport

   The service identifier for DNS SRV records [4] is "_sip".

        1.   If the destination address is a numeric IP address, the
             client contacts the server at the given address and the
             port number specified in the SIP-URI or, if not specified,
             the default port (5060).

             If the destination specifies a protocol, the client
             contacts the server using that protocol. If no protocol is
             specified, the client first tries UDP. If attempt fails, or
             if the client does not support UDP but supports other
             protocols, it tries those protocols in some
             implementation-defined order.

             The client then skips the remaining steps.

        2.   If the destination specifies no port number or port number
             5060, the transport protocol determines the use of one of
             the following three rules:

             - If the destination does not specify a transport protocol,
               DNS SRV records are retrieved according to RFC 2782 [4].
               The results of the query or queries are merged and
               ordered based on priority, keeping only records with
               transport protocols that the client supports.  Then, the
               searching technique outlined in RFC 2782 [4] is used to
               select servers in order. Server selection across requests
               is independent of previous choices, except as noted above
               for stateless proxies. Message length or other request
               properties do not influence the server selection. The
               client attempts to contact each server in the order
               listed, at the port number specified in the SRV record.
               If none of the servers can be contacted, the client gives
               up. If there are no SRV records (with any transport
               protocol), DNS address records are used, as described

             - If a transport protocol is specified and this protocol is
               supported by the client, the procedure in the paragraph
               above is used, limited to DNS resource records with the
               transport protocol specified in the SIP-URI.

             - If the transport protocol specified is not supported by
               the client, the client gives up.

             If there are no SRV records, the next step applies.

        3.   If the destination specifies a port number other than 5060
             or if there are no SRV records, the client queries the DNS
             server for address records for the destination address.
             Address records include A RR's, AAAA RR's, or other similar
             records, chosen according to the client's network protocol

             If the DNS server returns no address records, the client
             gives up. If there are address records, the same rules as
             in step 2 apply.

   Clients MUSTNOT MUST NOT cache query results except according to the rules in
   RFC 1035 [5].

3 Security Considerations

   The security considerations in RFC 2543 [1] apply.

4 Authors' Addresses

   Henning Schulzrinne
   Dept. of Computer Science
   Columbia University
   1214 Amsterdam Avenue
   New York, NY 10027
   electronic mail:

   Jonathan Rosenberg
   72 Eagle Rock Ave
   East Hanover, NJ 07936
   electronic mail:

5 Bibliography

   [1] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
   session initiation protocol," Request for Comments 2543, Internet
   Engineering Task Force, Mar. 1999.

   [2] S. Bradner, "Key words for use in RFCs to indicate requirement
   levels," Request for Comments 2119, Internet Engineering Task Force,
   Mar. 1997.

   [3] G. Nair and H. Schulzrinne, "DHCP option for SIP servers,"
   Internet Draft, Internet Engineering Task Force, Apr. 2000.  Work in

   [4] A. Gulbrandsen, P. Vixie, and L. Esibov, "A DNS RR for specifying
   the location of services (DNS SRV)," Request for Comments 2782,
   Internet Engineering Task Force, Feb. 2000.

   [5] P. V. Mockapetris, "Domain names - implementation and
   specification," Request for Comments 1035, Internet Engineering Task
   Force, Nov. 1987.

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