Re: packet fragmentation

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Date: Thu, 29 Sep 2005 21:47:00 GMT

roboftheblues <robwood894@hotmail.com> wrote:
> we have a unix box that receives ICMP fragmentation requests because
> the MTU size is too large, but it continues to retransmit the packet
> without changing the MTU size. I don't know Unix, how can I check
> if there is a setting on this box that is preventing the packet from
> fragmenting

picking nits perhaps, but your host is receiving ICMP Destination
Unreachable messages with a subcode of "fragmentation needed but Don't
Fragment set" (something like that) because the ip _datagram_ size is
larger than the MTU of the next hop link.

upon receipt of this message, your host is not altering the size of
the IP datagrams it is sending. it should never change the link-local
MTU in response to the ICMP message.

in other words, MTU is a function of a link, not a packet.

You either have to have the host stop setting the DF bit on the IP
datagrams it sends - that will be related to the aforementioned path
MTU discovery stuff. Under the AIX variant of Unix I _believe_ that
settings like that are controlled via the "no" (network options?)
command.

rick jones

-- 
Process shall set you free from the need for rational thought. 
these opinions are mine, all mine; HP might not want them anyway... :)
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

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