Subject: probablility of successful TIME_WAIT reuse

From: Rick Jones <[email-masked]>
Date: Wed, 04 Nov 1998 14:13:32 -0800

If I have:

*) a passive acceptor of connections on a single port and IP address
*) an active connector to that tuple cycling through 60K port id's
*) roughly 15KB of data transfered on average on each connection
*) the acceptor being the initiator of graceful close

what are the chances of the active connector's SYN segments having ISN's
meeting the requirements for successful transition from TIME_WAIT to
ESTABLISHED (SYN_RECVD?) such that the active connector could generate
say 2000 connections per second over a fifteen minute interval?

rick jones

-- 
these opinions are mine, all mine; HP might not want them anyway... :)
feel free to email, or post, but please do not do both...
my email address is raj in the cup.hp.com domain...

---

From: David Borman <[email-masked]>
Date: Wed, 4 Nov 1998 17:19:43 -0600 (CST)

> Date: Wed, 04 Nov 1998 14:13:32 -0800
> From: Rick Jones <[email-masked]>
> Subject: probablility of successful TIME_WAIT reuse
>
> If I have:
>

In BSD/OS ISS increments 250KB/second, plus 1/4 of that per connection.
With 60000 ports and 2000 connections per second, it'll take 30 seconds
to reuse a connection.  That means the ISS will go up by:
   60000 * 250K/4 + 30 * 250K, => 250K*(15000+30) => 3847680000.
The upper half of the 32 bit sequence space is 2147483648-4294967296,
so you fall right into it, (the 15Kb of transfered data is in the
noise) and the checks for allowing the transition
from TIME_WAIT -> SYN_RCVD will fail.  So, probability 0%.

Of course, if you are putting timestamps into the packets, then
with PAWS the sequence wrap around isn't an issue, and all new
connections should be accepted (and as I say this, (1) I look at the
4.4 BSD code and note that it doesn't do a PAWS check for the
TIME_WAIT -> SYN_RCVD transition, and (2) RFC 1323 doesn't address
this issue.)

Is that what you were looking for?

			-David Borman, [email-masked]

PS: at 15KB/connection, and 2000 connections/second, you will be
transfering over 245 Mbits/second.