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Re: probablility of successful TIME_WAIT reuse

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  • David Borman
    ... 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
    Message 1 of 2 , Nov 4, 1998
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      > Date: Wed, 04 Nov 1998 14:13:32 -0800
      > From: Rick Jones <raj@...>
      > Subject: probablility of successful TIME_WAIT reuse
      >
      > 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?

      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, dab@...

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