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RE: MTU/MRU Settings . .

Hi Rilindo,

An MTU (maximum transmission unit) of 1,500 bytes is the PPP default. It's
the amount of data PPP sends in each packet, including header (note: some
PPP implementations define MTU as data only, without the header).

By and large, it's a good thing to use the default MTU if the peer you're
connecting to also supports it (and ideally, the router the peer connects
to). Smaller MTUs/MRUs (maximum receive unit) means the peer will have to
fragment packets before sending them to you, which causes performance
degradation due extra overhead (each packet is fragmented, has headers
tacked on, the headers are processed, the packets are routed and reassembled
etc) and increased latencies.

However, this is not an absolute rule -- some old routers/access servers are
set to MTUs of 576 bytes, so Ken would have to check to see what kind of
equipment he's connecting to and experiment. Windows has an easy way to
determine the MTU with the help of ping (you send different packet sizes and
disallow packet fragmenting -- when ping complains that it can't send the
packet because it needs to be fragmented, you've found the MTU) but as far
as I can tell, the Linux ping can't do the same. Instead, try using
Linuxconf and enter the MTU/MRU values for PPP (or do it manually in your
connect script).

I doubt it's an MTU problem Ken is seeing. V.90 is very sensitive to line
impediments such as other devices (phones, faxes) on the same phone line as
the modem, plus other stuff beyond your control like the actual phone line
and the gear at the telco central office/exchange. Rockwell-based modems
appear to be more difficult than USR ones, but a modem firmware upgrade
often cures the problem in my experience. Having the right init string is
also crucial. You need error correction and hardware data compression with
V.90 for best performance.

Another thing that I've found beneficial is turning off all software
compression for PPP -- edit /etc/ppp/options and add:


to disable software compression altogether. Your modem does a much better
job of it.

There's the RWIN (receive window) thing too, which determines how much data
can be received before pppd stops and sends back an ACK, but that should be
OK on Linux (32,768 bytes max).

With these settings and a clean modem line (no other devices, new
installation) I get 50 to 52Kbps connects each time to my ISP, with download
speeds ranging from 5.7KBps to 6.2KBps for ftp (compressed files) and up to
12KBps for HTTP and text files. Not bad for a modem.

Hope this helps!

-- Juha

> -----Original Message-----
> From: Rilindo Foster [mailto:Rilindo foster Teletech com]
> Sent: Wednesday, 25 August 1999 04:12
> To: 'Redhat Mailing List'
> Subject: MTU/MRU Settings . .
> I got this post and I am trying to see how I can help him out :
> -------FWD--------------
> Anyone having problems with v90 modem throughput?
> Have linux set at 1500 MTU but got some packet loss and overall
> slow performance over the weekend.  Truthfully, I'm not too sure what
> this even is... but the default is set at 1500 ... does that sound right?
> Are there any other issues with this that may hamper customers getting
> connected - and could someone explain what exactly the MRU/MTU
> does... exactly...
> Ken (the greenie with linux)
> -----end-FWD------------
> - Rilindo Foster
> Teletech Telecommunications
> 818-295-6245
> "You will cut down the biggest tree in the forest - WITH A HERRING!!!" -
> King Arthur and the Quest for the Holy Grail

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