Name: Daniel Meredith
Course: CS 125 Computer Networks
Comentary Due: 03-22-00
Submit Date: 03-22-00
Journal Reference: Internet Draft - draft-fpeng-wecn-01
The mechanisms built into the TCP protocol for congestion control were all
based around the assumtion of a wired network and that the only cause of
packet loss was congestion on the network. The designers did realize that there
could be hardware corruptions that cause some packet corruption and therefore
packet loss, but they were assumed to be minial and not worth addeding to the
complexity of the protocol to deal with them independently.
These assumptions have been true for the passed 30 years, but with the movement
of the population to a more mobile form of connectivity the assumptions of a
wired network are no longer vaild. The wireless networks of today have at best
spotty constant connections and very poor signal quality. These factors lead
to a very high rate of packet loss due to hardware and connectivity issues
rather than solely due to congestion on the network.
TCP is design to combat packet loss in any form as packet loss due to
congestion. This is very inefficient if the cause of packet loss is not due to
congestion. TCP handles congestion by shrinking the window size to reduce the
possiblity of packet loss. There is nothing wrong with this approach to
handling packet loss from congestion, but if the cause is not congestion it
only make the problem worse.
The authors of this draft propose adding a Wireless Explicit Congestion Network
flag to the headers of all packets going out on an obviously congested network.
This signals to the reciever that the packet loss is due to congestion and to
take meassures against it. The ability to differentiate between hardware
related failures and congestion related failures allows the reciever to take
an appropriate course of action. This is much more efficient. By informing the
sender with Wireless-ECN packets whenever the congestion loss occurs, packet
losses due to transmission errors could not reduce the TCP flow control window,
which results in a signifigant performance improvement.
In order to insure the reception of the WECN packet the reciever must set a
WECN-echo bit on its reply packets and then when the sender send a packet
marked with the CWR ( congetsion window reduced ) flag the echos are stopped.
In an area such as wireless networks where the rate of transfer is low every
bit counts. The proposed protocol sets the 5th bit in the ECN packet of the
IPv4 TOS octet. The normal bits to be set for ECN messages are the 6th and 7th.
This means that the protocol has some issues with backward compatiblity with
routers not using the WECN mechanism.