nanog mailing list archives
Re: NAP/ISP Saturation WAS: Re: Exchanges that matter...
From: Dennis Ferguson <dennis () jnx com>
Date: Tue, 17 Dec 1996 11:07:20 -0800
* Head of queue blocking in the Gigaswitch Even though the Gigaswitch has input and output queues, your output queue will block until the other providers input queue is free. My (admittedly second hand) understanding is that the Gigaswitch/FDDI actually has minimal amounts of buffering. During a congestion event, it simply withholds the token, resulting in buffering in the routers. Queues there eventually overflow, and ...
This matches my understanding, though I think it understates the problem. Gigaswitches are essentially input-queued. When their teeny tiny buffers fill they flow-control everyone to slow them down. What this means is that a single congested output port will cause all inputs, including packets to the other `n' uncongested output ports, to be pushed back, so the overall throughput drops. And there appear to be additional problems caused when two gigaswitches are connected together, on the link between them, I assume because flow control sucks even worse when neither of the guys on the ends of the link has buffers. In any case, the head-of-line blocking means these switches only work really well when they're unloaded, something I've always suspected. Providing suitable buffering in switches is both necessary and sufficient. The evil that flow control attempts to hide is still evil, you just have to work harder to see it. Dennis Ferguson - - - - - - - - - - - - - - - - -
Current thread:
- Re: Exchanges that matter..., (continued)
- Re: Exchanges that matter... Alec H. Peterson (Dec 08)
- Re: Exchanges that matter... Nathan Stratton (Dec 08)
- Re: Exchanges that matter... Jeremy Porter (Dec 08)
- Re: Exchanges that matter... Alec H. Peterson (Dec 08)
- NAP/ISP Saturation WAS: Re: Exchanges that matter... Forrest W. Christian (Dec 15)
- Re: NAP/ISP Saturation WAS: Re: Exchanges that matter... Mike Leber (Dec 16)
- Re: NAP/ISP Saturation WAS: Re: Exchanges that matter... dave o'leary (Dec 16)
- Re: NAP/ISP Saturation WAS: Re: Exchanges that matter... Dima Volodin (Dec 16)
- Re: NAP/ISP Saturation WAS: Re: Exchanges that matter... Craig Nordin (Dec 16)
- Re: NAP/ISP Saturation WAS: Re: Exchanges that matter... Tony Li (Dec 17)
- Re: NAP/ISP Saturation WAS: Re: Exchanges that matter... Dennis Ferguson (Dec 17)
- Re: NAP/ISP Saturation WAS: Re: Exchanges that matter... David Schwartz (Dec 17)
- Re: NAP/ISP Saturation WAS: Re: Exchanges that matter... Hank Nussbacher (Dec 17)
- Re: Exchanges that matter... Alex.Bligh (Dec 05)
- RE: Exchanges that matter... Deepak Jain (Dec 05)
- Building exchanges that matter .. Joe Rhett (Dec 07)
- Re: Exchanges that matter... Wayne Bouchard (Dec 05)