Internet Engineering Task Force SIP WG
Internet Draft Schulzrinne/Rosenberg
draft-ietf-sip-srv-02.txt Columbia U./dynamicsoft
March 24, 2001
Expires: June 2001
SIP: Session Initiation Protocol -- Locating SIP Servers
STATUS OF THIS MEMO
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all provisions of Section 10 of RFC2026.
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Abstract
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", "MUST NOT", "REQUIRED",
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"SHALL", "SHALLNOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
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
proxies.)
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
information.
Clients MUST re-run the 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-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
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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
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
step.
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
parameter.
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
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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
below.
- 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
capabilities.
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 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
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New York, NY 10027
USA
electronic mail: schulzrinne@cs.columbia.edu
Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Ave
East Hanover, NJ 07936
USA
electronic mail: jdrosen@dynamicsoft.com
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
progress.
[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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