Fragmentation on Lan

Hi, I have a small Lan with 2 systems. System one is a Linux server,
and system 2 is a windows xp pro client.
While doing a tcpdump of a ssh session between the two systems I
noticed that the 'ip id field' was not zero, but the offset was. If I
understand this properly, the datagrams are getting fragmented, but I
do not understand why this would happen on a lan with no overhead such
as tunnels or encryption?

If the id field was not zero would'nt the offset be set also?

I also tried setting the 'PREROUTING -o ethx' --set-mss to 1400, and
did not see any changes.

any suggestions insight appreciated

jason

Hi Jason,

The ID feild does not mean that the packet is fragmented, The
originating protocol module of an internet datagram sets the
identification field to a value that must be unique for that
source-destination pair and protocol for the time the datagram will be
active in the internet system.

It can be used to identify the fragments but non-zero id feild does not
always mean that the packet is framneted.

The originating protocol module of a complete datagram clears the MF
bit to zero and the Fragment Offset field to zero.

So i think in your case there is no fragmentation going on.

Please let me know if u have any more questions.

Thanks,
Sunil
http://geocities.com/sunil3112000

jasonsig wrote:
> Hi, I have a small Lan with 2 systems. System one is a Linux server,
> and system 2 is a windows xp pro client.
> While doing a tcpdump of a ssh session between the two systems I
> noticed that the 'ip id field' was not zero, but the offset was. If I
> understand this properly, the datagrams are getting fragmented, but I
> do not understand why this would happen on a lan with no overhead such
> as tunnels or encryption?
>
> If the id field was not zero would'nt the offset be set also?
>
> I also tried setting the 'PREROUTING -o ethx' --set-mss to 1400, and
> did not see any changes.
>
> any suggestions insight appreciated
>
> jason