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New Member

ip tcp adjust-mss unidirection or bidirectional?

If i configure this command on my cisco CPE with a value of 1440, why do i still have packets who has a mss of 1460, while i clearly see the TCP three-way handshake? I'm no wireshark expert, but maybe you guys can tell me what i am doing wrong? I have made a capture between two hosts who are communicating with each other. 

 

Here is the direct link for a more clearer picture http://s16.postimg.org/4vyeqpg91/syn_bit.png

 

wireshark output

 

 

1 ACCEPTED SOLUTION

Accepted Solutions

Hi there,Correct me if i m

Hi there,

Correct me if i m wrong, is the capture taken from a PC connected to Cisco? 

 

The default MSS is 1460 which MTU 1500 - 40 Header = 1460 which is announced by the PC in syn and as you can see from the second packet which is syn ack received on the PC through the router the MSS is set to 1440, which means the MSS was modified / adjusted by the router.

Please refer below link for more information and testing MSS.

 

http://www.cisco.com/c/en/us/td/docs/ios/12_2sb/12_2sba/feature/guide/sb_admss.pdf

 

HTH

Hitesh

2 REPLIES

Hi there,Correct me if i m

Hi there,

Correct me if i m wrong, is the capture taken from a PC connected to Cisco? 

 

The default MSS is 1460 which MTU 1500 - 40 Header = 1460 which is announced by the PC in syn and as you can see from the second packet which is syn ack received on the PC through the router the MSS is set to 1440, which means the MSS was modified / adjusted by the router.

Please refer below link for more information and testing MSS.

 

http://www.cisco.com/c/en/us/td/docs/ios/12_2sb/12_2sba/feature/guide/sb_admss.pdf

 

HTH

Hitesh

New Member

I agree with you completely!

I agree with you completely! Although the funny thing is, that the host is still sending packets out with a MTU of 1500 bytes. (DF BIT not set) i could say that this particular node is not RFC compliant.

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