[e2e] How many TCP flows fit in the Internet?

Fred Baker (fred) fred at cisco.com
Sun Mar 31 17:24:41 PDT 2013

On Mar 30, 2013, at 7:29 AM, Detlef Bosau <detlef.bosau at web.de> wrote:

> Actually, the Internet has limited, if huge, capacity and the issue is to share this in a reasonable manner.

Numbers in the integer space may be very large, but are generally finite. Yes, one can have only so many simultaneously-active active TCP sessions on any given link that actually accomplish something.

The next question is how to figure out what "so many" means. I suspect that the most useful number is related somehow to a measure of bandwidth. If we assume that a mouse might move one MSS each way, or an elephant will want to move one MSS per RTT for several RTTs, the question is the duration of an MSS on the wire and the duration of a typical RTT; you're probably not going to have more active sessions simultaneously than some small multiple of the ratio of RTT to MSS. 

That question, of course, has similarities to the question of how many angels can dance on the head of a pin. That will perhaps have as a limiting function the ratio of the surface area of a head of a pin to the sole of an angel's foot. But when dancing, don't angels sometimes have their feet in the air? TCP will have the same problem.

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