[e2e] ECN, RED, dropping packets, etc

[RJ Atkinson]

On Apr 27, 2004, at 10:05, David P. Reed wrote:
> At 11:49 AM 4/26/2004, Guy T Almes wrote:
>> And, in the meantime, the designed-in behavior of growing cwnd until 
>> something breaks does have some negative environmental impact.
>
> You won't get any argument with me there!   If the router guys would 
> just stop trying to "help" by squirrelling packets away in buffers 
> until the burden becomes intolerable, and instead drop early, set ECN 
> where possible, etc. they would probably find that from their worm's 
> eye view of the world, it just gets better and better.
>
> To the extent that endpoint implementers are lazy, any stacks that 
> still don't support ECN should be motivated by smoother and higher 
> performance.

David,

Generally speaking, the router implementers did their job a few years
back. Most routers (both the largish ones and the relatively lower
cost "layer-3 switch" variety) support Random Early Drop, and other
similar techniques -- and have for a few years now.  Certainly we do.
However, our experience is that most operators do not enable those
features (unclear to me why, possible just conservative in what they
deploy, others here might be better informed as to why).

And, by the way, most "core WAN backbone routers" (e.g. the kit that
Juniper make) do tend to have (trans-Pacific RTT * interface speed)
milliseconds of buffer on each interface.  That buffer is *designed*
to operate empty -- so that it can handle transients without dropping
any packets inside the WAN core.  I have not myself seen any data
indicating that such packet buffer is operating in any other way in
practice.  Measured data from some WAN operator's core routers would
likely help focus the discussion on that tangent.

[A quick note on terminology might be in order here.  Most "enterprise
core routers" only have {Gig, 10 Gig} Ethernet interfaces are typically
built out of what marketing folks call a "layer-3 switch".  These latter
are very different beasts than a "core WAN backbone router".  These
"layer-3" switches are generally thin on per-interface packet buffering
(for cost reasons; enterprise end users won't pay the extra cost to get
deep buffering on each interface) and many of them (though not all, 
sigh)
are designed to have non-blocking switch fabrics and I/O 
configurations.]

I'm a bit out of touch with the latest facts on most widely deployed
host TCP stack, so I won't comment on the state of end systems' TCP
implementation in this note.

Ran
rja at extremenetworks.com