Some hosting weirdness...

Eric F Crist ecrist at
Wed Jul 11 18:15:04 UTC 2007

On Jul 11, 2007, at 7:40 AMJul 11, 2007, Heiko Wundram (Beenic) wrote:

> 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  
> 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  
> 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.

Well, I performed a tcpdump as you suggested, and my mss is exactly  
1460, not the 1452 you suggest.  What does this mean?

Eric F Crist
Secure Computing Networks

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