From: Howard C. Berkowitz (hcb@gettcomm.com)
Date: Sun Jun 29 2003 - 18:34:23 GMT-3
At 5:09 PM -0400 6/29/03, Volkov, Dmitry (IDS Canada) wrote:
>How TCP/IP stack reacts on receiving ICMP type 3 code 4 Fragmentation needed
>but DF set ?
>I mean how many bytes will be sent next time after receiving ICMP
>unreachable.
>I lowered IP mtu to 1420 and router sent ICMP and host started send 1420 !!
>I sniffed ICMP packed and I didn't see anything inside ICMP indicating
>allowable MTU.
>How source knows what size frame to retransmit ?
>
More information is needed to answer this. Is the host actively
participating in MTU autodiscovery, or is it just setting DF? There
are valid reasons for the latter. For example, the old IBM RSRB
method of Fast Sequenced Transport sets DF, and then steals the
fragmentation fields in the header for IBM information.
In any case, this is going to be a host implementation matter.
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