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From: dplatt@teknowledge-vaxc.ARPA (Dave Platt)
Newsgroups: comp.unix.wizards,comp.protocols.tcp-ip
Subject: IP fragmentation, and how to avoid it
Message-ID: <12640@teknowledge-vaxc.ARPA>
Date: Mon, 11-May-87 19:50:00 EDT
Article-I.D.: teknowle.12640
Posted: Mon May 11 19:50:00 1987
Date-Received: Fri, 15-May-87 02:01:52 EDT
Organization: Teknowledge, Inc., Palo Alto CA
Lines: 47
Xref: mnetor comp.unix.wizards:2283 comp.protocols.tcp-ip:160
Posting-Front-End: GNU Emacs 18.41.3 of Tue Apr  7 1987 on teknowledge-vaxc (berkeley-unix)


I've run into some problems on a Sun 3/52 workstation running SunOS
3.2 that I've been told may involve IP packet fragmentation.  The
primary symptom is that SMTP mail deliveries "hang up" and abort with
a read timeout.

Background: my Sun is sitting on a 10 Mbit Ethernet with the default
ifconfig for the Ethernet board;  the MTU for the Ethernet interface
is 1500 bytes.  The system is configured so that packets destined for
IP addresses not on our net are sent to our Vax 8650 (Ultrix 1.2),
which ipforwards them to the Internet TIP.  The MTU for the Vax's
"imp0" interface is 1006 bytes.

Problem: if a process on the Sun establishes a TCP connection with a
peer running on a host somewhere on the Internet (e.g. an SMTP
server), and then sends a large burst of data, the Sun will typically
queue up about 4k of data in the TCP buffers at one time.  This
apparently results in the sending of an IP packet that approaches the
Sun's 1500-byte MTU; when the packet passes through the Vax on its way
to the IMP, it is apparently fragmented.  Some system or gateway seems
to drop the fragmented IP packet on the floor.  The Sun's TCP never
receives an acknowledgement for the TCP segment, retries the
transmission periodically, and eventually aborts the connection.

The problem typically occurs in the later stages of an SMTP session.
The Sun's SMTP mailer is able to connect with its peer on another
Internet host, go through the "MAIL FROM" and "RCPT TO" steps, and
receive permission to send the message body.  If the message is short
(< 1k bytes), everything works fine;  if it's too long, then the
timeout occurs.

This problem appears to occur only when the host I'm trying to connect
with lies on a local-area net... and not all LANs are affected.  I've
been told that certain gateways are incapable of reassembling
fragmented IP packets;  other gateways seem to work just fine.

Question for the gurus:  is there any way to reconfigure my Sun's le0
interface so that its MTU doesn't exceed that of the 8650?  If so, how
do I do it?  Or, is there a better solution to the problem?  Or,
finally, have I totally misunderstood the problem?

advTHANKSance,

                Dave Platt

Internet:  dplatt@teknowledge-vaxc.arpa
Usenet: {hplabs|sun|ucbvax|seismo|uw-beaver|decwrl}!teknowledge-vaxc.arpa!dplatt
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