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PMTUD question

I wonder when and who sends path MTU discovery?? Is sends by PC when communication is esthablised, is it resend periodically??? What about situation when ICMP blocked and PMTUD cannot be send

Thx

2 REPLIES
Hall of Fame Super Silver

Re: PMTUD question

Milan

PMTUD is done between end stations on a connection. So if one of the end stations is a PC then yes it is sent from the PC.

PMTUD is performed when the connection is established. And it is performed by both end stations. Whether it is performed later in the connection depends on how the IP stack in the end station is written.

The essence of PMTUD is sending a packet with the DF bit set in the header. If the end station receives an ICMP error indicating that fragmentation was required but DF set, then the end station knows that it needs to use a smaller size frame and makes adjustment. If ICMP is blocked and the error response is not received then the end station never knows what the optimum size is and may send frames that are too large, they will be discarded, and it will appear that the application is not working. I have encountered this many times. We get a complaint from users that something in the network is not working. And when we troubleshoot it, we find that someone was blocking ICMP, and that PMTUD did not discover the frame size was too large, and that is why the application is not working. Blocking all ICMP may sound like a good idea to some peoople who are concentrating on making their network more secure. But this discussion shows one reason why blocking all ICMP is counter-productive.

HTH

Rick

Silver

Re: PMTUD question

The response from Rick pretty much covers everything so I am just going to provide you with a link in case you want to get into more specifics of PMTUD and real world scenarios:

http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml

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