 side or the other will "win" the race condition and have the re-INVITE processed. An example is shown in Figure 5.5. 5.4.34 493 Request Undecipherable The 493 Request Undecipherable response is used when an S/MIME message body can not be decrypted because the public key is unavailable. If the UAS does not support S/MIME, no message body will be present in the response. If the UAS does support S/MIME, the response will contain a message SIP Response Messages Media session INVITE INVITE 491 Request Pending 491 Request Pending INVITE 200 OK New media session Figure 5.5 Simultaneous re-INVITE resolution example. body containing a public key suitable for the UAC to use for S/MIME encryp- tion. See Section 3.7 for more details on S/MIME encryption. 5.5 Server Error This class of responses is used to indicate that the request cannot be processed because of an error with the server. The response may contain a Retry- - After header field if the server anticipates being available within a specific time period. The request can be tried at other locations because there are no errors indicated in the request. 5.5.1 500 Server Internal Error This server error class response indicates that the server has experienced some kind of error that is preventing it from processing the request. The reason phrase SIP: Understanding the Session Initiation Protocol can be used to identify the type of failure. The client can retry the request again at this server after several seconds. 5.5.2 501 Not Implemented This response indicates that the server is unable to process the request because it is not supported. This response can be used to decline a request containing an unknown method. A proxy, however, will forward a request containing an unknown request method. Thus, a proxy will forward an unknown SELF-DESTRUCT request, assuming that the UAS will generate this response if the method is not known. 5.5.3 502 Bad Gateway This response is sent by a proxy that is acting as a gateway to another network, and indicates that some problem in the 