gigabit router (was Re: Getting a "portable" /19 or /20)

Craig Partridge craig at aland.bbn.com
Wed Apr 11 01:02:19 UTC 2001



In message <Pine.BSF.4.21.0104101937230.98098-100000 at overlord.e-gerbil.net>, "R
ichard A. Steenbergen" writes:

>On Tue, 10 Apr 2001, Craig Partridge wrote:

>> OK, so your bus has 4.2 Gb/s of bandwidth.  But, alas, you're in a PC
>> so you have to copy each packet from the line card, into main memory,
>> examine it, and push it back out to a line card.  ...
>
>Bad assumptions. Like I said, the Alteon Tigon 2 firmware is opensource,
>and you don't HAVE to DMA the entire packet into main memory. You can
>easily coalasce and preprocess packets on the card, transfer only packet
>headers or smaller across the PCI bus, and then DMA between cards for the
>rest of the payload. The limitation is the switch fabric, and poor
>assumptions.

Played that game: packet headers (plus Ether headers and the periodic
need to peek into transport headers if you do QoS) says you transfer about
500 bits per packet, about half the average packet size.  So OK, we've
cut our bandwidth needs in half.  So we get 4 Ethernet cards, assuming
you're really driving the PCI fully.  (Last time I played with this problem,
getting more than about 60% out of the PCI was a challenge, and having
multiple bus masters made it even more fun...)


>Try ~ 10ns ram which you can buy 256MB of for ~ $60-80
>(www.pricewatch.com). At any rate, A raw 3 or 4 level mtrie FIB fully
>populated with the real 100k+ routes on the internet consumes less then
>900kb, and all the interesting parts fit in the L2 cache of a Celeron A
>where you can do about 22,000 lookups per MHz. Hardly excessive memory
>bandwidth. The packet ram on the gige cards is also very fast, and could
>easily accomidate a dCEF approach.

I did a quick look and all I saw was SDRAM at 10ns, which creates its
own set of challenges because of the different access rules.  Yes
you can fit routing tables into cache, but I thought you wanted to run
an OS so much of the cache may get pushed out to main memory?

Stepping back for a moment -- the answer, clearly, is yes sometime in the
next few years you'll be able to route gigabits through a PC platform.  Having
been down this path a few times, I think you underestimate the difficulty
and are a couple of years too early, but what the heck, if you want to,
go ahead and build one.  At minimum, we'll all learn something from your
experience and hey, you might succeed.  Then be prepared to fight to
maintain your turf against $200 ASIC based 4-port gigabit ethernet
routers... :-)

Craig




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