The TCP Accelerated Open (TAO) [3] is a mechanism introduced by T/TCP that is designed to cut down the number of packets needed to establish connection with a host.
There are a number of new options that T/TCP introduces. These options allow the establishment of a connection with a host using the TAO. T/TCP uses a 32-bit incarnation number, called a connection count (CC). This option is carried in the options part of a T/TCP segment, figure 3.1. A distinct CC value is assigned to each direction of an open connection. Incremental CC values are assigned to each connection that a host establishes, either actively or passively.
The 3-Way Handshake is bypassed using the CC value. Each server host caches in memory, or a file, the last valid CC value it received from each different client host. This CC value is sent with the initial SYN segment to the server. If the initial CC value for a particular client host is larger than the corresponding cached value, the property of the CC options (the increasing numbers) ensures that the SYN segment is new and can be accepted immediately.
The TAO test fails if the CC options that arrives in the SYN segment is smaller than the last CC value received that the host has cached, or if a CCnew option is sent. The server then initiates a 3-Way handshake in the normal TCP/IP fashion. Thus it can be seen that the TAO test is an enhancement to TCP, with the normal 3-Way Handshake to fall back on for reliability and backwards compatibility.