> From: "David Ready" <[EMAIL PROTECTED]>
> To: "David Schwartz" <[EMAIL PROTECTED]>,
> "Linux PPP Mailing list" <[EMAIL PROTECTED]>
> Subject: Re: Stop Looking!! Re: News Clients When Dialed Up To PPP?
> Date: Mon, 28 Sep 1998 18:16:43 +1000
> You are 100% right. That is the message that the analyser gave me. What
> could be doing it do you think?
This is a known issue with the Microsoft TCP/IP stacks. The
"Dialup Adapter" interface is created, a TCP socket is bound
to it while the MTU on the adapter is still at the default
value (1500). Then the adapter comes up as the call is made
and LCP sets the MRU. Unfortunately, by then TCP has already
decided that its MSS (maximum segment size, i.e. how much data
it is going to send) is MRU - (20 + 20), or 1460 bytes. The
20 + 20 is from the IP and TCP header sizes.
So it happily goes off and sends 1500 byte packets, even though
it has agreed never to send anything bigger than whatever MRU
was negotiated. Only NT gets this "right"... it refuses to
negotiate any MRU other than 1500 bytes. This may or may not
be fixed in '98 and the MSDUN 1.3 package... I haven't tested
it yet.
Dropping ICMP_UNREACH_NEEDFRAG messages is probably a broken
configuration, though you can probe a network for its topology
under the guise of doing path MTU discovery. If they have a
legitimate concern for security, then ok. But if that is not
their reason for filtering ICMPs, then they are unclueful...
Filtering ICMPs is a bad thing to do, except for may ECHOs,
TSTAMPS, MASKREQs, etc. that might be used to gather information
about a network.
One last note: even though you can negotiate another MRU besides
1500, the RFC states that the receiver is required to accept
all sizes up to 1500 (be conservative in what you send, liberal
in what you accept). So the *receiver* is within the spec. But
any *sender* that is exceeding the MRU on data packets that he
sends is broken. The spec should probably spell out the fact that
the 1500 bytes is required for the case where the sender's and
receiver's control protocols get out of sync (LCP, IPCP, whatever),
but there is no reason for data (network) packets to ever be
bigger than the MRU.
-Philip
> David
> -----Original Message-----
> From: David Schwartz <[EMAIL PROTECTED]>
> To: David Ready <[EMAIL PROTECTED]>; Linux PPP Mailing list
> <[EMAIL PROTECTED]>
> Date: Monday, September 28, 1998 3:30 PM
> Subject: RE: Stop Looking!! Re: News Clients When Dialed Up To PPP?
> >
> > Sounds like something's filtering ICMP and you're not getting the
> >'fragmentation needed but DF bit set' message.
> >
> > DS
> >
> >> -----Original Message-----
> >> From: [EMAIL PROTECTED]
> >> [mailto:[EMAIL PROTECTED]]On Behalf Of David Ready
> >> Sent: Sunday, September 27, 1998 10:20 PM
> >> To: Linux PPP Mailing list
> >> Subject: Stop Looking!! Re: News Clients When Dialed Up To PPP?
> >>
> >>
> >> Hi All.
> >>
> >> I found the problem!!! Thanks to Microsoft's NT network analyser.
> >> It was the
> >> MTU 542 size that I had in options. For some reason news servers
> >> can't cope
> >> with a connection setup with an MTU. Just put a "#" in front of it in
> >> options and all is now well.
> >>
> >> regards,
> >>
> >> David
> >>
> >> -----Original Message-----
> >> From: David Ready <[EMAIL PROTECTED]>
> >> To: Linux PPP Mailing list <[EMAIL PROTECTED]>
> >> Date: Monday, September 28, 1998 11:08 AM
> >> Subject: News Clients When Dialed Up To PPP?
> >>
> >>
> >> >Hi,
> >> >
> >> >Is anybody having any problems with PPP dialup client being able to
> >> download
> >> >newsgroups and articles (if not you better check it out otherwise you
> may
> >> >have some unhappy clients)? Winvn and Outlook can connect to the
> >> server but
> >> >they stall on getting even the 1st newsgroups yet while on the
> >> LAN with an
> >> >IP address reserved for PPP callers. There is no problem not
> >> from the VT100
> >> >news client on the RedHat 5.1 host itself. It is all PPP connection
> >> related.
> >> >
> >> >regards,
> >> >
> >> >David
> >> >
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