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From: field@elvis.cs.pitt.edu (Gus)
Newsgroups: comp.protocols.tcp-ip
Subject: UDP/IP over ether weirdness
Message-ID: <8945@pitt.UUCP>
Date: 23 Oct 90 15:33:02 GMT
Sender: news@pitt.UUCP
Reply-To: field@elvis.cs.pitt.edu (Gus)
Organization: Computer Science Dept., Univ. of Pittsburgh
Lines: 38


I've got an application that would like to send the largest UDP packet
it can.  (sun3 - sun3 running 4.0.3 over 10MB ethernet).  The only
reference to max UDP packet size is in the RPC section where it says
the max size the UDP transport mechanism can handle is 8K bytes.
(sendto () does not return EMSGSIZE until I try to send >9000
bytes per datagram).  Anyway, when I send 8K byte packets, sendto ()
doesn't complain, but the packets never arrive at the receiver.  In
fact, if I attempt to send a datagram larger than 2048 bytes, it never
arrives at the receiver.  Running etherfind on a third machine, and
watching the traffic between the UDP src and dst's I see:

-----
2048 byte packet:

 1514  udp pebbles	 wilma		       1788       4343
* 610  udp

This datagram is delivered correctly to the destination application.
   
-----
2049 byte packet:

 1066  udp pebbles	 wilma		       1790       4343
* 611  udp

Now, something is definitely wrong here.  

-----


So, what is the limit on UDP size messages (besides the 16 bit length
field limit)?  Is 2048 bytes per UDP datagram the limit?

Thanks
Brian
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field@cs.pitt.edu