Dear friends;
We were making some FTP transfers between a Sun and a NT machine through a
satellite link simulator, with a round trip delay of about 600 msec through
a 256 Kbps link. This application runs in our network (really through
satellite) and it is too slow. That's why we tried to simulate it to better
diagnose the problem.
We were monitoring the transfer with a LAN Analyzer, and we saw a message
reporting excessive retransmissions. We looked at the traces, and found that
the Sun was periodically retransmitting some packets. There was no packet
discard in the routers, no CRC errors, and no TCP segment asking for
retransmission (with an already acknowledged ACK). So, we concluded that the
retransmission timeout was expiring at the Sun. We were surprised, because
the RTO for TCP should be dinamically adjusted to reflect the round trip
delay of the link.
Searching in the Internet, we found an App Note :"Internet Performance on a
Satellite Link", from Intelsat, that said that at the Sun there was a
parameter for minimum retransmit time, and that the protocol periodically
resets the RTO to this minimum, causing some retransmits. We found that
there is a parameter called tcp_rxmit_interval_min that was defaulted to 200
msec. We changed this parameter to 600 msec and the retransmissions
disappeared.
We have some questions :
- Is it normal for the RTO to periodically reset to its minimum value ?
- Is there any available specification of the algoritm that calculates the
RTO, that is used by the implementors ?
Then we tried to do an ftp, transfering a file from the NT to the Sun, and
the problem still happened in that direction. We didn't find any parameter
equivalent to the parameter in the Sun to configure in the NT or 95 .
- Do you know if the problem in the NT is the same as in the Sun?
- Do you know how can we change it in the NT ? We looked at the parameters
in the registry and didn't find anyone.
Any help is welcome.
Best Regards,
Jaques Rechtand
VICOM S.A.
System Engineer
Tel : 55-21-5366136
Fax : 55-21-5366149
e-mail : [email protected]
This archive was generated by hypermail 2b29 : Mon Feb 14 2000 - 16:14:55 EST