North American Network Operators Group

Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical

Re: backbone routers' priority settings for ICMP & UDP

  • From: Dave Siegel
  • Date: Wed Feb 04 16:23:05 1998

> > Which side of the implementation do you mean?  as a client, or as a gateway?
> > I suppose it doesn't really matter.  Since source quenches are not supposed
> > to be used on routers anymore, the expectation of receiving a source
> > quench on a large network (like the Internet) is a bad one, so the TCP
> > implementations have to implement congestion controls through other means
> > anyhow.
> As a client, of course, since end-to-end source quench is the only
> alternative available.  And consider the near future scenario where a user
> with a cable-modem connected via Ethernet to their nice new NC (with
> cheapest bus design possible to contain costs) has requested a URL from a
> <insert whomping fast server here> connected via OC-3 to the Internet.  It
> seems likely a source quench will come in handy to provide flow control.

Yes, but it seems more likely that the <whomping fast server> would be 
receiving such source quenches, if they were provided by <whomping fast
caching agent> in the cable network.

> > TCP/IP Illus. Vol. I by W. Richard Stevens has a pretty good explanation
> > of what source quenches are.
> Don't have Mr. Stevens handy, but from RFC777 (1981!), when both types of
> source quench were defined:
> ...A destination host may also send a source quench message if datagrams
> arrive too fast to be processed.  The source quench message is a request
> to the host to cut back the rate at which it is sending traffic to the
> internet destination.
> This is what I was getting at.  Flow control versus congestion control.

When it comes to IP, it's sometimes hard to distinguish between the
two.  Since you've already lost packets, is it really flow control, or
congestion control?  Flow control usually assumes a "hold that thought"

For example, if you are using TCP, the initialization period when packets
are lost is definitely considered congestion control.  Once that period
of time is over, the negotiated TCP rate, and associated buffers, are
considered flow control.

When talking about UDP, though, most implementations ignore SQ for udp
altogether, so UDP does not really implement congestion or flow control.
If congestion/flow control are to be done with UDP, it has to be done
at a higher layer (application).


Dave Siegel				[email protected]
Network Engineer			[email protected] (alpha pager)
					(520)579-0450 (home office)