Paper review: Congestion Avoidance and Control
- How does TCP effectively
avoid network issues associated with congestion?
- The paper contains algorithms
rooted in the idea of achieving network stability by forcing the transport
connection to obey a packet conservation principle. It also contains case studies of network
congestion problems and what effect the new procedures would have on the
congestion in terms of network performance. This problem will always continue to be
a problem and will increase in importance as the internet grows with time.
- The most important ideas are:
- Dynamic window sizing. A host will only transmit a certain
window size before waiting to receive an ack. This prevents the host from sending too
much information that will congest the network even more, have to be
dropped, and must be sent again.
This allows the absence of acks to
indicate a congested network, and be used for flow control.
- Slow Start. Forces the host to first send one
segment and wait for an ack and with each
success work its way to using the full bandwidth. This prevents a routers queue from being
filled by a host that began with a full windows worth of information
because it did not see the congestion coming. They were able to show through studies
that begin with a smaller number of segments and then increasingly taking
more bandwidth is a small price to pay in comparison to a congested
network without Slow Start.
- Fast Retransmit. If an end host receives a packet out of
order it will immediately send an ack that the
packet has not been received and that it must be sent again. The end host is unable to use any of the
packets until the lost one has been received. This means that the host will not have
to wait for the retransmission timer to expire to send the lost
packet. If this time is saved then
all the out of order packets that would overflow the buffer and the end
host do not have to be sent until they will be received in order.
- Critique the main
System researchers and
builders should recognize that to more effectively deal with congestion
the network must be able to give an indication of how many packets are
getting through. This is solved by
the ack packets.
They must also realize that the problem of sharing the bandwidth is
very dynamic with hosts leaving and entering very quickly. The sender must be always adjusting his
window size according to how many of his packets are being dropped or are
out of order. One of the key points
to this is to also always error on the side of caution when deciding how
much bandwidth to allow a host.
- Significance- 5 The study of
congestion control was new at this point and was increasingly becoming a
problem on the internet. It was
very significant because it led to more efficient use of the bandwidth
without having to change any other standards.
- Convincing- 5 The
algorithms are backed by case studies that show how the small performance
gain that is lost in non congested networks is gained many times over in
a congested network. Plotted
graphs are even better at showing just haw dramatic the change is.