Cell Relay Archive

[fahad@cs.pitt.edu (Fahad A Hoymany)]

Vernon Schryver (vjs@calcite.rhyolite.com) wrote:
: >  The MSS value is based on the smaller of the two MSS's advertized by
: >  the two ends.

: I'm sure Rick knows about the MSS option.

: >                When connection is established a smaller MSS may be
: >  used if fragmentation occurs.

: Not quite.  The MSS used is the minimum of the two, regradless of
: any considerations of fragmentation.  Of course, one of the reasons
: for negotiating the MSS is to avoid fragmentation, but you do the
: negotiating and use the negotiated value without noticing whether
: fragmentation would otherwise occur.

  
  Let me see if I understand what you say above.  You are saying the MSS
  value is based on the values reported by the two ends (which is correct)
  but you are also saying that this is the value that's subsequently
  used!  I don't think so.  Like I said above, during establishment the MSS
  is the minimum of the two, but once you send the first data packet, a 
  router that has to fragment your packet reports an ICMP message back to
  you saying it has to fragment (assuming that you set IP's DF bit which 
  is the case in most TCP applications). Then the connection is torn
  down and a new connection is requested with a smaller MSS.


Fahad