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Re: MTU of the Internet?

  • From: Steve Carter
  • Date: Thu Feb 05 19:13:12 1998

My common sense tells me that your concepts are kind of half flawed,
but, of course, they may not be >;).  Let me explain why I think this. 
Basically, there are two types of connectivity/traffic; interactive and
non-interactive.  As examples, I would put telnet in the former and FTP
& HTTP/WWW in the latter.

Theory tells me that for both types of traffic it is probably better,
for response times sake, to have an asymetrical MTU (send = smaller,
receive = bigger from the clients perspective).  Servers set big MTU's,
clients set their's smaller.

Irrespective of your MTU size, the file or web page, etc. size is always
going to be the same, therefore, if you set a smaller MTU at the server
or within the network, fragmentation occurs, meaning greater overhead
for a file of a given size and due to this the end station will have to
reconstitute the data stream out of smaller packets, meaning more CPU


Steve Carter                       [email protected]
GTE Internetworking              Phone: (602) 308 2386    
     finger [email protected] for public key

> I have my MTU set to 552 and it helps quite a bit.  It's not an issue of
> the Win95 stack being broken.  I'm running Linux.
> The reason I chose to use a low MTU, and sometimes I knock it down even
> further, is to be able to improve my telnet interactive response over a
> 33.6k link that I also run as many as 4 concurrent downloads on.
> Here's what I suspect is happening:
> With web surfing, a page loads with many images, each of which is often
> larger than a sliding window worth of packets.  The browser will nearly
> concurrently connect and request for every image.  Thus for N images you
> now get N sliding windows worth of packets slammed at you.  This takes up
> a _lot_ of buffer space in the dialup routers for all these concurrent
> TCP connections all sending data at the same time over a high speed net
> to a low speed final link.
> With this happening, buffer space is exhausted and packets are discarded.
> If you set the MTU smaller, then the size of all those packets is smaller
> and the chance of being discarded due to memory exhaustion is reduced,
> even if you're the only one on that server with small packets.