?– 491 Request Pending This is sent by the recipient of a request when that recipient
is waiting for a response from the same requestor.
Remember that these are responses; therefore, they do not come from the client
(originator of the request). Rather, they are used to indicate the session request is being
denied because of a problem in the direction of the client.
5xx Server Failure Responses Server failure responses are sent when a request is denied
because of an error at the server (or destination of a request). Remember that
these can come from a subscriber device or they can come from an application server
providing a service to the subscriber (such as voicemail or video streaming). Following
are the defined 5xx responses:
?– 500 Server Internal Error This is sent, for example when messages are received
out of order.
?– 501 Not Implemented This is sent if the received request contains a method
that is not recognized. For example, if a MESSAGE is received, but the recipient
does not recognize this method, then this response is sent.
68 Chapter 3
?– 502 Bad Gateway This is sent when a downstream server sent an invalid response
while the proxy was acting as a gateway
?– 503 Server Unavailable This is sent by a proxy (such as CSCF) when the proxy is
out-of-order or congested, and not able to process the request. The proxy may indicate
a time when the request can be attempted again using the RETRY-AFTER header.
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