Internet Engineering Task Force                                   SIP WG
Internet Draft                                     Schulzrinne/Rosenberg
draft-ietf-sip-srv-00.txt                        Columbia U./dynamicsoft
October 6, 2000
Expires: January 2000

        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 supercedes 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", "REQUIRED",

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   "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].

   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

   At each step, unless stated otherwise, the client SHOULD try to
   contact a server at the port number listed in the Request-URI.  If no
   port number is present in the Request-URI, the client uses port 5060.
   If the Request-URI specifies a protocol, the client contacts the
   server using that protocol. If no protocol is specified, the client
   tries UDP (if UDP is supported). If the attempt fails with an ICMP
   error of "destination unreachable", code "port unreachable" or
   "protocol unreachable" or a time out, or if the client doesn't
   support UDP but supports other protocols, it tries those protocols in
   some unspecified order.

   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 example, in
   socket-based programs, connect() for TCP 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

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   SHOULD behave as if it had received a 400-class error response to
   that request.

   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

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

        1.   If the maddr SIP URI parameter exists, it becomes the
             destination address used below; if not, the host element in
             the Request-URI is the destination address.

        2.   If the destination address is an IP address, the client
             contacts the server at the given address and the port
             number specified in the Request-URI or, if none is
             specified, the default port and ignores the remaining

        3.   The Request-URI is examined. If it contains no port number
             or port 5060, the transport parameter is inspected:

             1.   There are three cases: the Request-URI does not
                  specify a transport protocol, it specifies a client-
                  supported transport protocol, or it specifies a
                  protocol that is not supported by the client. We
                  discuss these cases below in turn.

                  If the Request-URI 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 below for stateless
                  proxies. 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 below.

                  If the Request-URI specifies a transport protocol and

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                  the transport 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 Request-URI.

                  If the Request-URI specifies a transport protocol that
                  is not supported by the client, the client gives up.

             If the Request-URI contains 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
             capabilities. If the DNS server returns no address records,
             the client gives up.

   Within a transaction, a stateless proxy MUST always select the same
   destination within the set of hosts with the same priority. This can
   be accomplished, for example, by using the modulo N of a hash of the
   Call-ID value or some other combination of 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 MAY cache the list of DNS query results if one of the
   addresses was contacted successfully. Request for the same
   transaction SHOULD be sent to the same network address. Other
   requests from the same client select a server from the list of
   addresses cached, using the SRV load-balancing mechanism if
   applicable. The client must invalidate this list and retry the DNS
   query according to the rules in RFC1035 [5].

   A client MAY omit attempting to reach a server which it had failed to
   reach for a previous request.

   The results of the DNS lookup operation do not, in general, lead to a
   modification of the Request-URI.

        A proxy is free to modify the Request-URI to any value
        desired, but the DNS lookups are usually based on the
        Request-URI obtained from a location server.

        If the DNS time-to-live value exceeds a few minutes,
        servers generating a large number of requests are probably
        well advised to retry failed servers every few minutes.

3 Security Considerations

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   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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