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.
