Explain the various steps in TCP congestion control.

TCP Congestion Control Algorithms One big difference between TCP and UDP is the congestion control algorithm. The TCP congestion algorithm prevents a sender from overrunning the capacity of the network (for example, slower WAN links). TCP can adapt the sender's rate to network capacity and attempt to avoid potential congestion situations. In order to understand the difference between TCP and UDP, understanding basic TCP congestion control algorithms is very helpful. Several congestion control enhancements have been added and suggested to TCP over the years. This is still an active and ongoing research area, but modern implementations of TCP contain four intertwined algorithms as basic Internet standards:

·        Slow

·        Congestion

·        Fast

·        Fast recovery

Slow Start: Old implementations of TCP start a connection with the sender injecting multiple segments into the network, up to the window size advertised by the receiver. Although this is OK when the two hosts are on the same LAN, if there are routers and slower links between the sender and the receiver, problems can arise. Some intermediate routers cannot handle it, packets get dropped, and retransmission results and performance is degraded.

Congestion Avoidance: The assumption of the algorithm is that packet loss caused by damage is very small (much less than 1%). Therefore, the loss of a packet signals congestion somewhere in the network between the source and destination. There are two indications of packet loss:
A timeout occurs.
Duplicate ACKs are received.
Congestion avoidance and slow start are independent algorithms with different objectives. But when congestion occurs, TCP must slow down itstransmission rate of packets into the network and invoke slow start to get things going again. In practice, they are implemented together. Congestion avoidance and slow start require that two variables be maintained for each connection:
A congestion window, cwnd
A slow start threshold size, ssthresh

Fast Retransmit: Fast retransmit avoids having TCP wait for a timeout to resend lost segments. Modifications to the congestion avoidance algorithm were proposed in 1990. Before describing the change, realize that TCP can generate an immediate acknowledgment (a duplicate ACK) when an out-of-order segment is received. This duplicate ACK should not be delayed. The purpose of this duplicate ACK is to let the other end know that a segment was received out of order and to tell it what sequence number is expected.

Fast recovery: After fast retransmit sends what appears to be the missing segment, congestion avoidance, but not slow start, is performed. This is the fast recovery algorithm. It is an improvement that allows high throughput under moderate congestion, especially for large windows. The reason for not performing slow start in this case is that the receipt of the duplicate ACKs tells TCP more than just a packet has been lost. Because the receiver can only generate the duplicate ACK when another segment is received, that segment has left the network and is in the receiver's buffer. That is, there is still data flowing between the two ends, and TCP does not want to reduce the flow abruptly by going into slow start.