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10.2.6 Discovering a Registrar
UAs can use three ways to determine the address to which to send
registrations: by configuration, using the address-of-record, and
multicast. A UA can be configured, in ways beyond the scope of this
specification, with a registrar address. If there is no configured
registrar address, the UA SHOULD use the host part of the address-
of-record as the Request-URI and address the request there, using the
normal SIP server location mechanisms [4]. For example, the UA for
the user "sip: This e-mail address is being protected from spambots. You need JavaScript enabled to view it " addresses the REGISTER request to
"sip:chicago.com".
Finally, a UA can be configured to use multicast. Multicast
registrations are addressed to the well-known "all SIP servers"
multicast address "sip.mcast.net" (224.0.1.75 for IPv4). No well-
known IPv6 multicast address has been allocated; such an allocation
will be documented separately when needed. SIP UAs MAY listen to
that address and use it to become aware of the location of other
local users (see [33]); however, they do not respond to the request.
Multicast registration may be inappropriate in some environments,
for example, if multiple businesses share the same local area
network.
10.2.7 Transmitting a Request
Once the REGISTER method has been constructed, and the destination of
the message identified, UACs follow the procedures described in
Section 8.1.2 to hand off the REGISTER to the transaction layer.
If the transaction layer returns a timeout error because the REGISTER
yielded no response, the UAC SHOULD NOT immediately re-attempt a
registration to the same registrar.
An immediate re-attempt is likely to also timeout. Waiting some
reasonable time interval for the conditions causing the timeout to
be corrected reduces unnecessary load on the network. No specific
interval is mandated.
10.2.8 Error Responses
If a UA receives a 423 (Interval Too Brief) response, it MAY retry
the registration after making the expiration interval of all contact
addresses in the REGISTER request equal to or greater than the
expiration interval within the Min-Expires header field of the 423
(Interval Too Brief) response.
