Name: Ling Huang SID: 15053127 E-mail: hlion@newton.berkeley.edu

Review for <<Congestion Avoidance and Control>>

With the explosive growth of computer network, as well as the problem of transport protocol implementation, internet comes severe congestion problems. This paper briefly describes several new algorithms dealing with the congestion problem and their implementation in the 4BSD TCP. The main principle behind those algorithms is the conservation of packets. Those algorithms include:

1) Main idea is "conservation of packet" : At the balance point, a new packet can’t put into the network until an old packet leaves.

2) Slow-start algorithm increases the congestion window exponentially, thus getting to equilibrium quickly at starting or restarting after a packet lost. This can help connection get a rough estimation of optimal cwnd size. The algorithm goes as follows:

> A congestion window, cwnd, is added to the per-connection state.

> Cwnd is set to on when starting or restarting after a loss.

> On each ack for new data, increase cwnd by on.

> Send the minimum of the receiver’s advertised window and cwnd when sending.

3) A new round-trip-time variance estimation algorithm to eliminates spurious retransmissions: estimating ß and using exponential back off.

4) A congestion avoidance algorithm:

> A packet loss is due to congestion in the network.

> Employ additive increase/multiplicative decrease policy to adjust the window size: on congestion w(i) = d*w(i-1) and d < 1; on no congestion, w(i) = w(i-1) + u.

> On any timeout, set cwnd = 0.5 * (current windows size); on each ack for new data, increase cwnd by 1/cwnd.

Finally the paper presents future work of gateway side of congestion control. The main idea is that gateway have enough information to control sharing and fair allocation, so it can detect congestion early and small adjustment to the senders’ window can cure it.