The ACK method is used to acknowledge final responses to INVITE requests. Final responses to all other requests are never acknowledged.An ACK may contain an application/sdp message body. This is permitted if the initial INVITE did not contain a SDP message body. If the INVITE CONTAINED a message body, the ACK may not contain a message body. For 2xx responses, the ACK is end-to-end, but for all other final responses it is done on a hop-by-hop basis when stateful proxies are involved. A hop-by-hop ACK reuses the same BRANCH ID as the INVITE SINCE it is considered part of the same transaction. An end-to-end ACK USES a different branch ID as it is considered a new transaction.

The ACK method is used to acknowledge final responses to INVITE requests. Final responses to all other requests are never acknowledged.An ACK may contain an application/sdp message body. This is permitted if the initial INVITE did not contain a SDP message body. If the INVITE contained a message body, the ACK may not contain a message body. For 2xx responses, the ACK is end-to-end, but for all other final responses it is done on a hop-by-hop basis when stateful proxies are involved. A hop-by-hop ACK reuses the same branch ID as the INVITE since it is considered part of the same transaction. An end-to-end ACK uses a different branch ID as it is considered a new transaction.