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1200 posts

Uber Geek


# 11713 7-Feb-2007 19:22
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Hello Everyone,

I have a client which is having issues very symptoms very similar to MTU packet size issues (unreliable browsing, Terminal Server sessions failing etc)

This client has a laptop and a 3G card, and sits out on a boat way past great barrier island. They can only get GPRS service, but has been working fine for the last year or so.

I setup my laptop to replicate their settings (Clean install of Windows XP) VMC 7.03.0004 (latest) and a 3G Card locked to GPRS with APN of and internet.

Unusual results. I can ping usually with up to a 1500 MTU from a broadband connection (Clear's DNS servers.)

From the GPRS connection I can only send a ping -f -l 64 and get a reply.

If I send a do not fragment and packet size of 65 and above it does not get a reply - BUT ALSO does not recieve a packet needs to be fragmented.

The maxmum packet I can sent with a fragmentation error is 1476, which seems right from what google says (but most dialup connections are 576 MTU)

My gateway when on Vodafone is I can send packets of up to 1476, but they get eaten after this. Seems to be a GPRS problem, and perhaps even only on this gateway.

I called Vodafone Data support, but the only guy which knew had gone home for the day.

Questions - Whats the MTU meant to be for Vodafone GPRS??? Surely it isn't 64!!!

Tyler - Parnell Geek - iPhone 3G - Lenovo X301 - Kaseya - Great Western Steak House, these are some of my favourite things.

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1200 posts

Uber Geek


  # 60040 7-Feb-2007 19:48
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I dug a little deeper, I did a tracert and then pinged each host, some didn't respond, and the ones that did allowed 1765 sized packets all the way to what looks like the internet gateway then simply dropped them, unless they were 64 bytes or under!

  1   577 ms   695 ms   683 ms [myself]
  2   650 ms   581 ms   580 ms
  3   681 ms   576 ms   581 ms
  4   694 ms   599 ms   599 ms [not pingable]
  5   501 ms   581 ms   737 ms [not pingable]
  6   761 ms   719 ms   839 ms [not pingable]
  7   594 ms   599 ms   737 ms
  8   772 ms   737 ms   719 ms

Tyler - Parnell Geek - iPhone 3G - Lenovo X301 - Kaseya - Great Western Steak House, these are some of my favourite things.

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