Some hosting weirdness...

Heiko Wundram (Beenic) wundram at
Wed Jul 11 13:08:57 UTC 2007

On Wednesday 11 July 2007 14:19:09 Eric F Crist wrote:
> <snip>
> What should I look for?  Is there possibly some weird caching issues
> at their ISPs?  How can I fix this?

Do a tcpdump when someone connects from their network and check for TCP-MSS 
issues, which would be my first guess when small files/items load fine over 
HTTP but items larger than a single TCP-packet won't (which basically fits 
the symptoms you describe).

As some ISPs will do IP fragmentation when a packet too large to fit over the 
downlink to a customer arrives, you'll not see this problem with these. Those 
ISPs that don't do IP fragmentation on the downlink (quite a few) generally 
should send out an ICMP-message with a "Fragmentation needed" error (which 
appears in the tcpdump), but some don't do that either.

Generally, the MSS in their SYN-packet when connecting to your webserver 
should be below 1460; most probably at 1452 (which is DSL and cable AFAIK), 
or more generally speaking (their) MTU-40, and the _IP_ packet size your host 
sends back should always be equal to or below the minimum of your MSS (which 
is sent in the SYN/ACK packet) and their MSS, plus 40. If this is not the 
case, you have an issue.

Heiko Wundram
Product & Application Development
Beenic Networks GmbH
Mailänder Straße 2
30539 Hannover
Fon        +49 511 / 590 935 - 15
Fax        +49 511 / 590 935 - 29
Mail       wundram at

Beenic Networks GmbH
Sitz der Gesellschaft: Hannover
Geschäftsführer: Jorge Delgado
Registernummer: HRB 61869
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