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List:       isp-bgp
Subject:    [isp-bgp] Re: Please help me..
From:       "Edward B. Dreger" <eddy+public+spam () noc ! everquick ! net>
Date:       2004-06-24 17:05:35
Message-ID: Pine.LNX.4.44.0406241645040.27744-100000 () a ! mx ! ict1 ! everquick ! net
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DM> Date: Wed, 23 Jun 2004 21:38:03 +0700
DM> From: Dwi Mahindra

DM> EDDY>What happens if you telnet to port 80 of the sites that don't
DM> EDDY>work?  Do you establish the connection?
DM>
DM> I can telnet port 80 and session has established
DM> This following the capture result from host/PC :

Okay.  It's not a BGP problem.  If the problem were BGP-related,
you'd be unable to connect.  I've found many people learning
about BGP also need to learn about this, so I'll keep the mail
on-list a bit longer.  Someone flame me if I go too far off-
topic. :)


DM> C:\>telnet 202.130.175.172 80

Now try connecting via telnet and typing

	GET / HTTP/1.0

followed by pressing the <ENTER> key twice.  Does the page ever
come up?  If not, it's probably an MTU problem.


DM> What the meaning of path MTU and how I check it ?

	http://www.google.com/search?q=%22path+mtu%22

Depending on your version of Windows, use the "ping" command with
"do not fragment" set.


DM> So what I must be do to solve it ?

Make sure ICMP type 3 is not blocked anywhere along the path.

Unfortunately, I think the problem is with the shangintranet.com
site.  Here's why:

* As a test, I did a "dig +norec @shangintranet.com foo." to send
  a UDP packet to shangintranet.com.  Normally, a server that
  doesn't allow that will send an ICMP error message.  I received
  a DNS timeout, which indicates 1) the DNS request was dropped
  silently, or 2) someone is blocking ICMP unreachables.

* I decreased my MTU to 1496 and tried pulling up the Web page.
  Nothing happened.  I set my MTU back to 1500, and the site
  loaded instantly.

I suspect you have something limiting your MTU, such as PPPoE,
GRE, or MPLS.  That's not standard, but that's not the problem --
sites MUST allow ICMP unreachables for MTU discovery.  When
someone foolishly blocks ICMP type 3 (like shangintranet.com is
doing), it breaks things for people with smaller MTUs.


Eddy
--
EverQuick Internet - http://www.everquick.net/
A division of Brotsman & Dreger, Inc. - http://www.brotsman.com/
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