H.323 call setup

Depending on whether a gatekeeper is being used or not, establishing an H.323v1 call can take about a dozen [exact number?] of packets and about 6 to 7 round-trip times, depending on how connections and packets are overlapped, including setting up the Q.931 and H.245 TCP connections. For a modem connection, where transmission delays are substantial, setting up an H.323 call can take several seconds. in many cases the sender does not have to wait for a response before sending the next message. We count a RTT at each point where the sender has to wait for a response before it can proceed. Also, the termcap set, master/slave, and open logical channel messages from the caller and callee happen in parallel. Opening multiple channels (audio and video) can happen completely in parallel, so it doesn't add to the number of round-trip times. Since TCP (Solaris implementation) waits 3.5 and 6.4 seconds to retransmit SYN packets after one or two packet losses, call set up delays may be much higher for some calls if the network suffers from appreciable packet loss.

H.323v2 with "Fast Connect" may use a single exchange and reduces the delay to 2.5 RTTs. The addition of Annex E (UDP-based signaling) [May 1999] may reduce this to 1.5 RTTs.

Last updated by Henning Schulzrinne