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When and Why do we decrease MSS instead of MTU.

Here is the question.

When and Why do we decrease MSS instead of MTU.

In the router that we can configure  both of them separately.

Suppose I have kept MSS as 1260.I have not made any changes to the MTU. Now if the TCP and the IP header do not have any options then they will be using 40 bytes( 20 TCP + 20 IP).

So does this mean that the MTU in this case is 1300.

Now the second scenario.

I decreased the MTU to 1300. I did not make any change to the MSS. Again there are no options in the TCP and IP options.

So does this mean that the MSS in this case is 1260.

If both the scenarios have same outcome then please help me understand the following.

  1. In which case should we decrease the MTU and leave MSS untouched ?
  2. In which case should we decrease the MSS and leave MTU untouched ?

Thank You

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Perhaps the following will help you understand a typical reason for reducing MSS but not MTU:

http://www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsulation-gre/25885-pmtud-ipfrag.html

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