[MUD-Dev] Transport layer (UDP vs TCP)

Jon Leonard jleonard at divcom.umop-ap.com
Tue Mar 17 11:18:40 New Zealand Daylight Time 1998


On Tue, Mar 17, 1998 at 10:58:58AM +0000, Niklas Elmqvist wrote:
> On Tue, 17 Mar 1998, Ben Greear wrote:
> > 
> > I'm pondering a multi level transport stream.  The important stuff
> > would go over tcp/ip, and the client-side-cache updates would run
> > over UDP.
> > 
> > The question is, what is the packet size (max) that UDP can handle?
> 
> A good place to find information about anything that even remotely
> pertains to networks and the Internet are the RFC docs. A bit technical
> (but that's how we like them) but usually of good standard. You can find a
> directory containing ASCII-text versions of all the RFCs at
> <URL:http://ftp.sunet.se/pub/Internet-documents/rfc/> (the index is called
> rfc-index.txt).
> 
> RFC 1180 is a TCP/IP tutorial, and a good one at that, but only brushes
> through UDP. RFC 768 concerns itself solely with UDP, the answer to your
> question might be found there.

It's likely more complicated than that.  The maximum size of a UDP packet
is about the maximum size of an IP datagram (quite large).  Realisticly,
though, you want largest packet that won't get broken up into fragments.

The largest non-fragmenting packet (MTU = Maximum Transfer Unit) depends
on the networks involved.  For Ethernet, it's about 1500 bytes.  Most high
speed networks have larger maxima, but you could see almost anything.

A search for "MTU discovery" should probably find something on how some
TCP implementations figure out what the optimal packet size is.  Anyone
seriously considering a UDP tranport protocol should write something similar.
The basic idea is set the "don't fragment" bit, and play with packet size
until you find out what works.

Jon Leonard



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