Re: My goal is a _really slow_ connection

From: Tauno Voipio (tauno.voipio_at_iki.fi.NOSPAM.invalid)
Date: 12/03/04 

Date: Fri, 03 Dec 2004 20:48:09 GMT

Helmut Giese wrote:
> Hello out there,
> I have an extremely slow connection between 2 machines (a research
> project involving powerline modems over 10.000 Volt lines). The
> connection itself is reliable - data which is sent out eventually
> arrives at the other machine. Both boxes are running Suse Linux 9.0.
> We try to transfer a small file (187 byte) from server 172.16.3.1 via
> tftp 172.16.3.1:minihttp.tcl
> This doesn't work, due to timing problems: The server starts to
> repeat, and the IP level on the client gives up too early (at least
> that's what I suppose).
>
> I have 2 main questions:
> 1) How do I keep the server (/usr/sbin/in.tftpd) from repeating too
> early (or repeating at all)? I tried the -T option with values like
> 6000000 (6 minutes) - but then it didn't even start. Am I missing
> something here?
> 1a) tftpd has an option -v, saying it will log information. But where?
> I didn't see anything in /var/log.
> 2) Is there an option / a command to make the IP level on the client
> wait sufficiently long?
>
> Below is tcpdump's result on the client (172.17.1.2). I numbered the
> lines since my news reader will surely break them up. If someone is
> familiar with tcpdump's output: Please explain, what's going on.
> - Line 1 evidently is the initial request 'send file minihttp.tcl'
> - To me it seems, that in steps 2 thru 4 the requested transfer takes
> place. (What does 'udp 4 (DF)' mean ?)
> - In line 5 the server starts anew (as I would interprete it) - ok,
> could be the server hit it's timeout (see above).
> - Now what do lines 7 & 8 signify?
> - The whole story repeats 5 times and that's it. BTW, this was a
> succesful transfer - in the end, the file existed on the client.
>
> 1) 21:54:42.061902 172.17.1.2.filenet-tms > 172.16.3.1.tftp: 21 RRQ
> "minihttp.tcl" (DF)
> 2) 21:54:52.882742 172.16.3.1 > 172.17.1.2: udp (frag 105:71@128)
> 3) 21:55:04.469749 172.16.3.1.filenet-tms > 172.17.1.2.filenet-tms:
> udp 191 (frag 105:128@0+)
> 4) 21:55:04.470402 172.17.1.2.filenet-tms > 172.16.3.1.filenet-tms:
> udp 4 (DF)
>
> 5) 21:55:16.910742 172.16.3.1 > 172.17.1.2: udp (frag 106:71@128)
> 6) 21:55:28.014755 172.16.3.1.filenet-tms > 172.17.1.2.filenet-tms:
> udp 191 (frag 106:128@0+)
> 7) 21:55:28.015311 172.17.1.2 > 172.16.3.1: icmp (frag 4167:2@128)
> [tos 0xc0]
> 8) 21:55:28.015319 172.17.1.2 > 172.16.3.1: icmp: 172.17.1.2 udp port
> filenet-tms unreachable (frag 4167:128@0+) [tos 0xc0]
>
> [lines 5 - 8 repeated several times]
>
> 17) 21:56:34.261744 172.16.3.1 > 172.17.1.2: udp (frag 109:71@128)
> 18) 21:56:49.339758 172.16.3.1.filenet-tms > 172.17.1.2.filenet-tms:
> udp 191 (frag 109:128@0+)
> 19) 21:56:49.340314 172.17.1.2 > 172.16.3.1: icmp (frag 4170:2@128)
> [tos 0xc0]
> 20) 21:56:49.340322 172.17.1.2 > 172.16.3.1: icmp: 172.17.1.2 udp port
> filenet-tms unreachable (frag 4170:128@0+) [tos 0xc0]
>
> Thanks for any help and best regards
> Helmut Giese

The timeout mechanism in TFTP (running on top of UDP) is pretty dumb.

Can you use a TCP -based protocol (like FTP) instead? TCP has much
more intelligent timeout mechanism. It should be able to use anything
up to the pigeon link tested by Trondheim University of Technology.

In Linux, ATFTP is a somewhat more intelligent client then the plain
tftp.

HTH 

-- 
Tauno Voipio
tauno voipio (at) iki fi

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