Minimum Number of TCP Sessions

C.

Fast start allows for H323 media connections to be started at the beginning of a call.

This is helpful for ringback scenarios, and also reduces the amount of time calls take to establish media.

H245 is still negotiated later, but the actual media can be done earlier through H225 messages.

The H.323 protocol is a standard for transmitting audio and video traffic over IP networks. When two H.323 gateways establish a call, they need to create TCP sessions to exchange control messages and media traffic.

In this context, slow start refers to the process by which a TCP connection gradually increases its transmission rate until it reaches the maximum allowed by the network. Slow start is used to avoid congestion and packet loss.

To answer the question, we need to understand how the H.323 protocol works and how many TCP sessions are required for different phases of a call.

During the call setup phase, the H.323 gateways exchange information about the capabilities and addresses of the endpoints. This information is transmitted using the H.225 protocol, which relies on TCP for reliable delivery.

Typically, the call setup process involves the following steps:

1. The originating gateway sends an H.225 Setup message to the destination gateway.
2. The destination gateway responds with an H.225 Call Proceeding message to indicate that it has received the Setup message.
3. The destination gateway sends an H.225 Alerting message to indicate that the destination endpoint is being alerted.
4. The destination endpoint sends an H.245 Capability Exchange message to negotiate the audio and video codecs to be used.
5. The originating endpoint sends an H.245 Capability Exchange message to confirm the codec selection and exchange additional capabilities.
6. The H.245 channel is opened using TCP to exchange control messages related to media negotiation.

During the call setup phase, at least one TCP session is needed for the H.225 protocol, and an additional TCP session is needed for the H.245 protocol. Therefore, the minimum number of TCP sessions needed to complete a H.323 call between two gateways is two.

Option D (2) is the correct answer to the question.