Kernel bug handling TCP_RTO_MAX?

From: Andreani Stefano (
Date: Thu Dec 12 2002 - 14:15:42 EST

Problem: I need to change the max value of the TCP retransmission timeout.

Background: According to Karn's exponential backoff algorithm, when the receiver doesn't acknowledge packets for a while, the sender should retransmit the latest not acknowledged packet several times increasing the delay (RTO) since this delay reaches the Max Retransmission Timeout Value.

Testing environment: Red Hat Linux release 7.2 (Enigma), Kernel 2.4.7-10 on an i686, Kernel 2.4.7-10.

Test details: I supposed this timeout in Linux was TCP_RTO_MAX, so I changed in /include/net/tcp.h the following line:

#define TCP_RTO_MAX ((unsigned)(6*HZ)) //It was: ((unsigned)(120*HZ))

Then I recompiled the kernel, rebooted the machine and tested the solution. The result I obtained was the same I had before this modification.

I'm confident there isn't an error in the testing procedure because I already tested with a Solaris server the same procedure (changing the tcp_rexmit_interval_max variable) and it works. I'm just trying to reproduce the modification of that parameter in Linux.

Could it be a bug on the RTO calculation algorithm, or there is something I mistook?

This is the first time I get into the linux kernel, so please be patient!



        Stefano Andreani
    Freelance ICT Consultant
      H3G IOT Team - Italy
      tel. +39 347 8215965

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