Subject: Re: (IPng) mechanisms for managing the flow ID field in IPv6

From: [email-masked] (Noel Chiappa)
Date: Thu, 1 Dec 94 01:56:04 -0500

From: Craig Partridge <[email-masked]>

    Take Van J's fast IP forwarding code and write it up in assembler for your
    favorite current processor.

Do any major router vendors actually use this approach (plain old RISC CPU's)
for forwarding in their high-end boxes? If not, aren't we solving a non-
problem?

    Flows force one to split the first level cache into two caches -- one of
    flow IDs and one of routing entries

See, if you made flows the basic forwarding element, like I'm suggesting, this
problem would go away! :-)

    What's more, the flow space is bigger, so rather than using the, say, 100
    or 200 most recent routes (ala Feldmeier's and Mogul's studies), we may
    have to cache the most recent 200-500 flow IDs.

Craig, is this a guess, or is there some study we can look at?  Feldmeier's
TM-352, "Estimated Performance of a Gateway Routing Cache" is old (it was
done in '88), but it contains some interesting data when studied closely.

For instance, it shows, for number of active "trains" (defined as ones whose
interpacket arrival times do not exceed 10 seconds) in the most active
internal router at MIT, a nice bell curve centered on about 65, with a
standard deviation of about 13. There is more of a tail on the high side,
true, but it peters out by about 120.

I'd expect these numbers to be higher today, of course, but is traffic growth
faster than growth in cache size? (Remember that cache size will grow as the
*square* of the feature size, whereas speed goes only as the feature size.)

	Noel

---

From: Curtis Villamizar <[email-masked]>
Date: Thu, 01 Dec 1994 12:16:46 -0500

> for forwarding in their high-end boxes? If not, aren't we solving a non-

Ours use an I960 on each interface card, which do the forwarding.  The
RS/6K just runs the AIX system, daemons, routing protocols, snmp, etc.

A few spercomputer centers run DEC Alpha or SGIs as FDDI routers
because they are faster than commercial routers.

> problem would go away! :-)

The problem is then the amount of state you need.  Remember that
commercial routers are currently having trouble with 20,000 CIDR
prefixes.  We have seen close to 8,000 active destination host
addresses in random 90 second data samples (samples taken about 6
months ago).  If we start routing on flow IDs and flow IDs are unique
per TCP connection, I think we are going to have a space requirement
explosion.  That is also why I think connection oriented will
eventually fail to scale.

> done in '88), but it contains some interesting data when studied closely.

Try closer to 10,000 most recent routes on Internet today.  Right now
there are already commercial routers failing (theashing their cache
and pitching packets into the bit bucket at high rates) because there
cache size is too small.

> *square* of the feature size, whereas speed goes only as the feature size.)

Where is hwb when you need him?  :-)  You've seen his "flows.ps" paper?

Curtis