Information Technology Reference
In-Depth Information
servers, such as TELNET and FTP servers. The connection can either be from a fully
specified or an unspecified socket.
•
Active OPEN - TCP actively connects to a foreign host, typically a server (which is
opened with a passive OPEN). Two processes which issue active OPENs to each other,
at the same time, will also be connected.
A connection is established with the transmission of TCP packets with the SYN control flag
set and uses a three-way handshake (see Section 24.6). A connection is cleared by the ex-
change of packets with the FIN control flag set. Data flows in a stream using the SEND call
to send data and RECEIVE to receive data.
The PUSH flag is used to send data in the SEND immediately to the recipient. This is
required as a sending TCP is allowed to collect data from the sending application program
and sends the data in segments when convenient. Thus, the PUSH flag forces it to be sent.
When the receiving TCP sees the PUSH flag, it does not wait for any more data from the
sending TCP before passing the data to the receiving process.
OPEN (active)
OPEN (passive)
Fully specified
the foreign socket
OPEN (active)
OPEN (passive)
Unspecified
the foreign socket
OPEN (active)
OPEN (active)
Figure 24.5
TCP connections
24.5 TCB parameters
Table 24.2 outlines the send and receive packet parameters, as well as the current segment
parameter, which are stored in the TCB. Along with this, the local and remote port number
require to be stored.
24.6 Connection states
Figure 24.6 outlines the states the connection goes into, and the events that cause them. The
events from applications programs are OPEN, SEND, RECEIVE, CLOSE, ABORT and
STATUS, and the events from the incoming TCP packets include the SYN, ACK, RST and
FIN flags. The definition of each of the connection states are:
