| This interface represents the Reason header, as defined by
RFC3326, this header is
not part of RFC3261.
The ReasonHeader provides information on why a SIP request was issued, often
useful when creating services and also used to encapsulate a final status code in
a provisional response, which is needed to resolve the "Heterogeneous Error
Response Forking Problem".
The Reason header field appears to be most useful for BYE and CANCEL requests,
but it can appear in any request within a dialog, in any CANCEL request and
in 155 (Update Requested) responses. When used in requests, clients and
servers are free to ignore this header field. It has no impact on protocol
processing.
Examples of the ReasonHeader usage are:
- A SIP CANCEL request can be issued if the call has completed on another
branch or was abandoned before answer. While the protocol and system behavior
is the same in both cases, namely, alerting will cease, the user interface
may well differ. In the second case, the call may be logged as a missed call,
while this would not be appropriate if the call was picked up elsewhere.
- Third party call controllers sometimes generate a SIP request upon
reception of a SIP response from another dialog. Gateways generate SIP
requests after receiving messages from a different protocol than SIP. In
both cases the client may be interested in knowing what triggered the SIP
request.
- An INVITE can sometimes be rejected not because the session initiation
was declined, but because some aspect of the request was not acceptable. If
the INVITE forked and resulted in a rejection, the error response may never
be forwarded to the client unless all the other branches also reject the
request. This problem is known as the "Heterogeneous Error Response Forking
Problem". The header field defined in this draft allows encapsulating the
final error response in a 155 (Update Requested) provisional response.
A server must ignore Headers that it does not understand. A proxy must not
remove or modify Headers that it does not understand.
author:
BEA Systems, NIST
version:
1.2 |