What is Congestion Control?

 Congestion Control

• When large amount of data is transferred to system (which is not capable of handling it) congestion occurs.
• Congestion control refers to the techniques used to control or prevent congestion.
• TCP may use three algorithms for congestion control:

TCP may use three algorithms for congestion control:

1. Additive increase, Multiplicative Decrease
2. Slow Start
3. Timeout React

Timer Management

• TCP uses different types of timer to control and management various tasks:

Keep-alive timer

• This timer is used to check the integrity and validity of a connection.
• When keep-alive time expires, the host sends a probe to check if the connection still exists.

Re-transmission timer

• This timer maintains statefull session of data sent.
• If the acknowledgement of sent data does not receive within the Re-transmission time, the data segment is sent again.

Persist timer

• TCP session can be paused by either host by sending Window Size 0.
• To resume the session a host needs to send Window Size with some larger value.
• If this segment never reaches the other end, both ends may wait for each other for infinite time.
• When the Persist timer expires, the host re-sends its window size to let the other end know.
• Persist Timer helps avoid deadlocks in communication.

Timed-Wait

• After releasing a connection, either of the hosts waits for a Timed-Wait time to terminate the connection completely.
• This is in order to make sure that the other end has received the acknowledgement of its connection termination request.
• Timed-out can be a maximum of 240 seconds (4 minutes).

Crash Recovery

• TCP is very reliable protocol. It provides sequence number to each of byte sent in segment. 
• It provides the feedback mechanism i.e. when a host receives a packet, it is bound to ACK that packet having the next sequence number expected (if it is not the last segment).
• When a TCP Server crashes mid-way communication and re-starts its process it sends TPDU broadcast to all its hosts. The hosts can then send the last data segment which was never unacknowledged and carry onwards.