TCP Flow Control:

TCP Congestion Control

  • RTT: time difference between the TCP segment and its ACK. RTT measurement is done constantly during the TCP connection.
  • MSS(Maximum-Segment Size):Maximum payload size a receiver accept in single packet.
  • CWND(Congestion Window): Variable that limits the amount of data the TCP can send into the network before receiving an ACK.
  • ssthresh: Slow start threshold. At first, its size is equal to window size.
  • Slow Start Phase: Initially, cwnd =1MSS. Since increase of speed depends on RTT value, in general, cwnd increments exponentially in that phase till the ssthresh limit.
  • Congestion Avoidance Phase:After reaching the sshtresh, cwnd increments by 1MSS. So, cwnd=cwnd+1.
  • Congestion Detection Phase: This phase is the one at which senders detects the packet loss. How senders understand there is packet loss. There is two ways to understand it. First, sender waits RTO timer(retransmission timeout) until it gets an ACK for the packet sent. If it does not get an ACK, it thinks there is a packet drop and retransmit the packet. The other way of understanding packet loss is that if sender gets 3 DUP-ACK message which is named as Fast Retransmission.


TCP Reno Bandwidth Usage with %1 Packet Loss
TCP BBR Bandwidth Usage with %1 Packet Loss




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