ATM ABR offers explicit closed loop feedback about congestion experienced, and
will drop cells/frames if that feedback is not followed. In certain types of
VBR
the CLP bit can be used to indicate that a cell was out of profile. Such cells
do
not have to be dropped if the network has room for them.
Fred Baker wrote:
At 05:57 PM 3/9/00 +0800, Jianbo Huang wrote:
Dear Sirs and Madams,
A friend of mine are working on the paper on "critically compare the
congestion control on TCP/IP and ATM", and she ask me for help. But I do
not get much idea on the "congestion control on ATM". So, is there anyone
can give me any idea on this topic, while my friend and I processing on this?
that's easy; there isn't any. There is ingress port policing, which is
something different, and there may be PNNI call routing. But there is not
anything that corresponds to what TCP expects from its underlying layers.
There have been some papers written and a fair bit of experience with a
technique for mitigating this, called Early packet Discard. In essence, if
a link is becoming congested, rather than dropping a single cell, if it has
to drop an AAL5 cell it drops the entire packet containing the cell. This
may sound odd, but it is actually quite sensible - if the other cells were
not also dropped, then they would uselessly occupy bandwidth on subsequent
links, and at the final delivery point would consume memory unnecessarily
until the SAR was able to determine that the cell had been dropped.