3.4 keep-alive Details

This section describes the negotiation and the hop-by-hop keep-alive mechanism. The keep-alive mechanism serves two purposes. First, it serves to keep the connection between the protocol client and the outbound proxy alive by keeping network components operating below the TCP layer between the protocol client and its first hop SIP proxy from timing out and disconnecting the TCP connection because of inactivity. In addition, if the first hop SIP proxy is also the SIP registrar (registrar) for the protocol client, the registrar uses the lack of periodic keepalive message the protocol client endpoint for inactivity. The hop-by-hop keep-alive mechanism does not forward the keepalive message across multiple SIP entities. The outbound proxy MUST NOT behave as if every connection originating from the protocol client endpoints has keep-alive enabled. The keep-alive mechanism is enabled or disabled for each individual connection.