[Zope] IE 5.5+ and login problems

danielle.d-avout danielle.d-avout@wanadoo.fr
Tue, 18 Mar 2003 01:14:23 +0100

----- Original Message -----
From: "AM" <list_subscriber@neurobs.com>
To: <lsh@wo.cz>
Cc: "Zope" <zope@zope.org>
Sent: Tuesday, March 18, 2003 12:26 AM
Subject: Re: [Zope] IE 5.5+ and login problems

> Well unfortunately I saw the same behavior on a new install of XP which
> came with IE 6

I had this kind of problems  with 6.0.2 (and a lot more)
before I changed my MTU
according to the following explanations  (

Joe User has a DSL line.  Like many DSL lines, Joe's line actually comes
from a large DSL wholesaler and uses PPPoE (PPP over Ethernet) to create a
logical channel between Joe and his favorite ISP.  The DSL line physically
connects to a small PPPoE router with a built-in hub, which Joe uses as the
basis for his home network.
The MTU on Joe's network is Ethernet's usual 1500 bytes.  The MTU on his DSL
line is also 1500 bytes, but PPPoE uses 8 of these bytes for an 8-byte PPPoE
header, so the MTU of his PPPoE channel to his ISP is only 1492 bytes.
A computer on Joe's Ethernet doesn't know about the PPPoE channel.  The
largest packet it can receive is a 1500-byte Ethernet packet, so when Joe
uses his computer to connect to a remote server, his computer tells the
server it's ok to send packets up to 1500 bytes.[*]
Establishing connections uses only very small packets, so Joe can connect to
the server without any MTU issues, but as soon as Joe asks for enough data
to fill a 1500-byte packet, the server sends a 1500-byte packet.  When that
packet gets to Joe's ISP, it doesn't fit down the PPPoE channel.  The packet
has the DF bit set, telling Joe's ISP to drop the packet and send the server
an ICMP saying what packet size will fit.  The ISP sends an ICMP saying the
largest size is 1492 bytes.
If the server gets that ICMP, it re-sends the first 1492 bytes as a new
packet.  This new packets fits down the PPPoE channel, so Joe's computer
gets it and sends back an acknowledgement.  The server continues sending
1492-byte packets until Joe has what he asked for.  In most cases the server
will remember the reduced path MTU, typically for ten minutes, and use it
for future connections from Joe's computer.
However, if the server does not get that ICMP, things go downhill fast.  The
server is expecting an acknowledgement from Joe's computer, but Joe's
computer didn't get the packet, so the acknowledgment never comes.  After
awhile, the server gives up waiting and sends the same 1500-byte packet
again.  Joe's ISP sends back another ICMP.  The server doesn't get it again
and tries sending the 1500-byte packet a third time, then a fourth, a fifth,
and so on.
Meanwhile Joe's computer can't tell this is happening and is waiting for a
response from the server.  Eventually, it gives up and sends the server a
connection reset.  It reports a network failure to Joe, who is left
wondering what happened.  He may soon discover he can access nearly all
other sites, just not the one he wanted.

If the server he's trying to access is a web server, he may get part of the
website, such as getting the ads but not the main page content.  This
happens because the pages normally displayed for many websites are assembled
from data obtained from multiple servers on a variety of networks.  The
networks dispensing the ads all support path MTU discovery.
If Joe complains to the server operator, they may tell him the problem has
to be his ISP because other users can view their server.  However, Joe's ISP
is not doing anything wrong, but the server operator is.  PPPoE and other
connections with reduced MTUs are perfectly legitimate and increasingly
common. Having a server ask for ICMP noptification that it refuses to accept
is what's broken behavior.  Some later day, when the server site has gotten
enough complaints from people like Joe to believe something is wrong, it may
either stop asking for the ICMPs or start accepting the useful ones.  Then
Joe will be able to access it

> Is there ANY way to circumvent the Evil Empires nefarious attempts to
> break everything holy and standardized?
> AM
> Jaroslav Lukesh wrote:
> >: Odesílatel: AM <list_subscriber@neurobs.com>
> >: The problem we are facing seems to manifest itself only in the IE 5.5+
> >: browsers. We have been unable to reproduce it on Netscape or Mozilla or
> >: Opera.
> >
> >IE5.5 have authentication problems with Zope. Please use fresh install of
> >IE4, IE5, IE6. Does not make upgrade of IE5.5. But you could try it.
> >
> >Regards, JL.
> >
> >
>  >> Chris Withers wrote:
>  >>
>  >> Sounds like something, somewhere is caching the incorrect
> information for one of you Zope threads...
>  >>
>  >> cheers,
> >
> >
> --
> ==================================================================
>  Aseem Mohanty
>  Neurobehavioral Systems Inc, 828 San Pablo Ave, Albany, CA 94706
>  (R) 510 7696011 (M) 510 3014871 (O) 510 5279231
> ==================================================================
>  "I saw `cout' being shifted "Hello world" times to the left and
>   stopped right there!!"                        -- Steve Gonedes
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