Lecture 11: Reliable Transport -- TCP

TCP Connections

An application process requests TCP to establish, or open, a (reliable) connection to a server process running on a specified edge-system, and awaiting connections at a known port number. After allocating an unused client-side port number^[5], TCP initiates an exchange of connection establishment "control segments":

This exchange of segments is called a 3-way handshake (for obvious reasons), and is necessary because any one of the three segments can be lost, etc. The ACK and SYN segment names refer to "control bits" in the TCP header: for example, if the ACK bit is set, then this is an ACK segment.
Each TCP chooses an random initial sequence number (the x and y in this example). This is crucial to the protocol's operation if there's a small chance that "old" segments (from a closed connection) might be interpreted as valid within the current connection.
A connection is closed by another 3-way handshake of control segments. It's possible for a connection to be half open if one end requests close, and the other doesn't respond with an appropriate segment.

^[5] Port numbers less than 1024 are reserved for "well-known" services, so the client port number is always numerically greater than this. NB: this is the "source" port in segments sent from the client process to the server process, and the dest port in segments travelling in the opposite direction.

Lecture 11: Reliable Transport -- TCP

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