Re: bug

["Jamal Hadi Salim" <hadi@nortelnetworks.com>]

On Sat, 30 Jan 1999, K. K. Ramakrishnan wrote:

> > We also noticed that, in an environmet with all ECN capable
> > connections, marking ALL packets above the high threshold leads to
> > good performance.
> 
> On the one hand, I would say that marking rather than dropping packets
> is desirable. On the other hand, I believe that it is important to keep in mind
> at least a couple of things that we cannot control:
> a) non ECN capable end systems; b) non-cooperative end systems;
> So, with these in mind, if we mark all packets above the high threshold,
> but not drop them, it is possible that we have a case where we're
> marking packets of non-cooperative end systems which don't reduce but
> instead other well-behaved flows to reduce their window.
> Of course, I strongly believe that this is a transient, and will likely
> result in our queue growing up to the max. causing packets to be lost
> anyway. However, it seems to me that if we've set the RED parameters
> and thresholds "appropriately", then going beyond the high threshold
> should be infrequent. Dropping packets when operating above
> the high threshold will likely help in combating non-cooperative users.
> 

I share your opinions.
What Renato is suggesting could easily lead to a DoS.
Imagine an end system lying that it is ECN capable and never having its
packets dropped. Where do you draw the line? Larger queues
(which will happen in this case) imply increased latency.
It would defeat the whole purpose of having RED in the first place (and
even ECN) if the queue grows to a point where the only discard decision is
based on buffer limitations. We are back to Drop Tail.

cheers,
jamal

Computing Technology Labs (CTL), Nortel