Solaris retransmission bug eats Internet links

Hank Nussbacher hank at
Tue May 21 08:41:06 UTC 1996

If you can think of a better list to post this, please forward it...

The following TCP bug has been measured to consume as much as
15% of Internet links.  ISPs who run heavily congested links
and/or satellite links will lose 15% of their bandwidth as we
have due to this error by Sun.

Please distribute this announcement to those running Solaris 2 as often as

Hank Nussbacher
IBM Israel

During the last few weeks we were conducting a survey of our internet
communication performance. As a result of the survey we have discovered
that your host is involved in a large portion of TCP/IP retransmissions.
This phenomena significantly decreases the communication performance.

If your host is a Sun running Solaris 2, than we think we know what is the
source of the problem. It appears that Solaris 2 has a bug in the TCP/IP
implementation, causing excessive retransmissions to occur. The following
is an excerpt from the May 3rd Solaris 2 FAQ:

+5.49) Solaris 2.5 and Solaris 2.4 patch 101945-34+ have poor TCP performance
over slow links.

    Solaris 2.5 and Solaris 2.4 kernel patch 101945-34 and later have
    a bug in their TCP retransmission algorithm that cause excessive
    retransmissions over slow links, Sun's bug ID is #1233827.

    A work around for this bug is running the following commands
    at system boot, e.g., by adding them to /etc/init.d/inetinit
    (values are in milliseconds):

        /usr/sbin/ndd -set /dev/tcp tcp_rexmit_interval_min 3000
        /usr/sbin/ndd -set /dev/tcp tcp_rexmit_interval_initial 3000

Please note that the above mentioned patch (101945-34) included a "fix"
to the retransmission problem, but apparently, the fix is not good enough
(see ""
section: "How can I improve my Web server's http performance?").

To summarize, it seems that all Solaris 2 versions suffer from this problem!

We recommend that you apply the above workaround until the problem is
resolved in a future Sun release.  Please note that the problem affects
mainly your host and users accessing it over large latency links, such as
satellite links (as we do), or over slow links (e.g. dialup lines) or a
congested network .

We will appreciate if you let us know how do you intend to proceed with this.

Thanks for your cooperation,

Oded Comay (comay at
Operating Systems group
Computing Center
Tel Aviv University

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