TCP-Vegas

TCP Vegas senses the congestion in the network before any packet loss occurs, and instantly it decreases the window size. So, TCP Vegas handles the congestion without any packet loss occurring.

• Uses diff = expected rate - actual rate to regulate the sending rate.

• Expected rate = window size divided by baseRTT.

• Actual rate = window size divided by currentRTT.

• baseRTT = It is the minimum RTT measured so far.

• The expected rate is always larger than the Actual rate.

Rate-based congestion control.

• diff = expected rate – actual rate.

• If diff < a, Vegas increases cwnd linearly.

• If diff > b, Vegas decreases cwnd linearly.

• If a< diff < b, Vegas keeps cwnd unchanged.

Modified slow-start.

• Allows cwnd to grow exponentially only in every other RTT.

• If diff > c, Vegas switches from slow-start to congestion avoidance.

New retransmission.

• Reads and records transmission time.

• When DUPACK arrives, check if it is expired.

• Retransmits without waiting for a third DUPACK.