Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!MATHOM.CISCO.COM!BILLW
From: BILLW@MATHOM.CISCO.COM (William Westfield)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Number of TCP retries
Message-ID: <12533869350.17.BILLW@MATHOM.CISCO.COM>
Date: 13 Oct 89 22:00:41 GMT
References: <242@spectra.COM>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
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    The environment is a common Ethernet lan.  No gateways, routers,
    bridges, brouters, long-haul networks, etc,  are involved.

No. No.  Please do NOT make any assumptions about the environment
a TCP will be located in.  I can't tell you how many problems
assumptions like this have caused (they always end up used in an
environment other than the one they were designed for originally.


    1) If a packet is not acknowledged, how many times should TCP
       retry sending the packet before giving up?

Well, one philosophy says "forever", which is to say that this should
be controlled by the application, rather than by TCP. If the application
is telnet, you let the user decide when to abort.  If it is something
like mail, the application might abort after several minutes.


    2) What is a reasonable time for a SYN packet to time out?

0.5 seconds, with exponential backoff and retransmission, with a total
timeout of 30 seconds or so.  The exponential backoff is real important
here.  The short initial retransmission will let ARP drop packets.


    3) What is a reasonable fixed TCP/IP timeout for an Ethernet lan when
       you don't want to do fancy retry timing ala Karn's work?

Never use a fixed timeout.  An exponential timeout is easy to implement.
Even Karn/Jacobson algorithms, although "fancy" turn out to be
wonderfully uncomplicated to implement.


    4) When programming an Ethernet controller (e.g. Lance chip), how many
       times should the controller retransmit the packet when it detects
       collisions and other errors before giving up and reporting an error
       to the driver?

This is fixed by the ethernet spec, and usually by the ethernet
controller.  It is 16 times total.  The LANCE does not permit you
to change this number, although some other controllers might.

Bill Westfield
cisco Systems.
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