Text 737, 47 rader
Skriven 2006-07-26 19:25:06 av John Beckett (1:379/45)
    Kommentar till text 721 av Geo (1:379/45)
Ärende: Re: My slow Linux Thunderbird on Barkto investigations
==============================================================
From: John Beckett <FirstnameSurname@compuserve.com.omit>

"Geo" <georger@nls.net> wrote in message news:<44c6ea86$4@w3.nls.net>:
> I did have ICMP blocked, but it's enabled now.

Some really old notes I have say this:
-+-
Problem:

Open web page. Some of page may appear. Then nothing happens.

Cause: Your PC is on Ethernet and uses MTU = 1500 but there is a router (e.g.
using PPPoE) that forces lower MTU. The PMTU negotation fails because the web
server admin has blocked all ICMP (including those used for PMTU fragmentation
control).
-+-

Exactly the same would apply to any server providing a TCP service (e.g. a
newsgroup server).

When a TCP connection is established between hosts that are not on the same
segment, each host notifies the other of its MSS (maximum segment size = 1460 =
1500 bytes for standard Ethernet, less 40 for IP+TCP headers).

Each host then uses the smaller of the two values for the amount of data to put
in each datagram sent. The datagram include the DF (Don't Fragment) flag in the
IP header. That tells intermediate routers to not split up the packet if it is
too large. Instead, the router should return an ICMP "need to fragment but DF
is set" error to the sender.

The sender should then lower its packet size and repeat.

Probably, your news server was not lowering its packet size when some weirdness
at Randall's end (PPPoE?) enforced a lower MTU than Randall's computer realised
was required.

Randall could (given infinite time<g>) find the setting to lower his MTU to
what his overall system, including router, is capable of.

Run 'ifconfig' to see the current value (MTU = 1500 for Ethernet, but should be
reduced if you know your router is using PPPoE or some framing like IPSec, that
reduces the amount of data that can be sent in a single Ethernet frame).

John

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