Any new request may receive 3xx responses themselves containing
the original URI as a contact. Two locations can be configured to
redirect to each other. Placing any given URI in the target set
only once prevents infinite redirection loops.
As the target set grows, the client MAY generate new requests to the
URIs in any order. A common mechanism is to order the set by the "q"
parameter value from the Contact header field value. Requests to the
URIs MAY be generated serially or in parallel. One approach is to
process groups of decreasing q-values serially and process the URIs
in each q-value group in parallel. Another is to perform only serial
processing in decreasing q-value order, arbitrarily choosing between
contacts of equal q-value.
If contacting an address in the list results in a failure, as defined
in the next paragraph, the element moves to the next address in the
list, until the list is exhausted. If the list is exhausted, then
the request has failed.
Note that in some instances, header fields that have been
communicated in the contact address may instead append to existing
request header fields in the original redirected request. As a
general rule, if the header field can accept a comma-separated list
of values, then the new header field value MAY be appended to any
existing values in the original redirected request. If the header
field does not accept multiple values, the value in the original
redirected request MAY be overwritten by the header field value
communicated in the contact address. For example, if a contact
address is returned with the following value:
sip:user@host?Subject=foo&Call-Info=<http://www.foo.com>
Then any Subject header field in the original redirected request is
overwritten, but the HTTP URL is merely appended to any existing
Call-Info header field values.
It is RECOMMENDED that the UAC reuse the same To, From, and Call-ID
used in the original redirected request, but the UAC MAY also choose
to update the Call-ID header field value for new requests, for
example.
Finally, once the new request has been constructed, it is sent using
a new client transaction, and therefore MUST have a new branch ID in
the top Via field as discussed in Section 8.1.1.7.
