[e2e] Queue size of routers

[Alexandre L. Grojsgold]

On Thu, 16 Jan 2003, Vadim Antonov wrote:

> Date: Thu, 16 Jan 2003 22:56:38 -0800 (PST)
> From: Vadim Antonov <avg at kotovnik.com>
> To: Greg Minshall <minshall at acm.org>
> Cc: end2end-interest at postel.org
> Subject: Re: [e2e] Queue size of routers
>
>
> On Thu, 16 Jan 2003, Greg Minshall wrote:
>
> > i think routers in the core don't have a delay*bandwidth of buffer memory
> > (though i could be wrong).
>
> Routers in real backbones have the delay*bw of buffer space.


My understanding is that the delay*bandwidth product applies to protocol
engines that implement flow/error control, and need to receive acks before
thay can go over.

It holds true for TCP sessions (the bandwith and delay being the
throughput and delay seen across the network), and for connection based
packet switches (like X.25 switches).

I can't see a good reason for IP routers having longer queues when the
bandwith increases or when the delay increases. The same routers are used
at the ends of a terrestrial or satellite link, unnmodified. I've read
papers proposing mods to TCP when used over satellite paths, but I've
never seen a word saying that routers should have their queues enlarged.


On a STM-1 link, with 20ms one way delay, the bandwith*delay product leads
to aprox. 1500 packets (if we consider 256 octets as the mean packet
length). This is clearly a queue length that will not be found in actual
routers.


-Alexandre.