a proxy server that receives an INVITE request, then forwards it to a number of locations at the same time. This “forking” proxy server keeps track of each of the outstanding requests and the response to each. This is USEFUL if the location service or database lookup returns multiple possible locations for the called party that need to be tried. For example forking proxy F1 is RECEIVED from UA1 which forks to two user agents UA2 & UA3. Each user agent begins alerting and sending BACK two provisional resposes(180 Ringing) to F1. The two responses are identical except for having different To tags and Contact URIs. Finally, ONE of the two UAs answers and SENDS a 200 OK response. The forking proxy server sends a CANCEL to the second UA to stop that phone alerting. If both UAs had answered, the forking proxy would have forwarded both 200 OK responses back to the caller who then would have had to choose which one, probably accepting one and sending a BYE to the other one.

a proxy server that receives an INVITE request, then forwards it to a number of locations at the same time. This “forking” proxy server keeps track of each of the outstanding requests and the response to each. This is useful if the location service or database lookup returns multiple possible locations for the called party that need to be tried. For example forking proxy F1 is received from UA1 which forks to two user agents UA2 & UA3. Each user agent begins alerting and sending back two provisional resposes(180 Ringing) to F1. The two responses are identical except for having different To tags and Contact URIs. Finally, one of the two UAs answers and sends a 200 OK response. The forking proxy server sends a CANCEL to the second UA to stop that phone alerting. If both UAs had answered, the forking proxy would have forwarded both 200 OK responses back to the caller who then would have had to choose which one, probably accepting one and sending a BYE to the other one.