ip tcp adjust-mss not working

Answered Question
Mar 2nd, 2010
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Hello,


I'm configuring ip tcp adjust-mss to 1200 on a router interface and when doing a packet trace I still see the MSS value showing up as 1460 in the intial SYN packet.


The setup is as follows; I have a pc connected to a router on Fa0/0.  I have placed the command 'ip tcp adjust-mss 1200' in interface Fa0/0 configuration.  This router then connects to another router.  I issue a telnet from the host pc to the remote router, so the SYN is going through the router I have lowered the MSS value on and it just isn't changing.  I'm using wireshark to verify.  Am I missing something or is this command supposed to tell the router to intercept SYN packets and change the MSS value?


Any help is much appreciated.


Sincerely,


David

Correct Answer by Lei Tian about 7 years 3 months ago

Hi Dave,


The SYN ACK with MSS as 536 is expected. Most platforms use 536 as the default MSS. During TCP 3 way handshakes, whichever side has smaller size MSS will be used.


HTH,

Lei Tian

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Lei Tian Tue, 03/02/2010 - 21:35
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Hi David,


What did you see in the SYN ACK? I think you should check the intial SYN packet on the remote router see if the MSS value got changed.


HTH,

Lei Tian

d.serra Wed, 03/03/2010 - 05:18
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Interestingly enough it is 536 in the SYN ACK.  I've run a few more experiments and when using a couple of switches as the source host and destination host with an 800 series router in between without changing the MSS value the SYN MSS is 536 and SYN ACK MSS is 516.  Very strange.  I've not had any luck with the original scenario which I've tried with both 3700 and 7200 series router.


When using the switches as source and destination for telnet with an 800 series router in between having the 'ip tcp adjust-mss 500' set on both ingress and egress interfaces, I did see the MSS value change to 500.  So I think I'm making some progress but it is very strange.


Since I now can see that this command does work (on some platforms) I'm going to open a TAC case and see what they say.


Thanks again!


Dave

Correct Answer
Lei Tian Wed, 03/03/2010 - 06:05
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  • Cisco Employee,

Hi Dave,


The SYN ACK with MSS as 536 is expected. Most platforms use 536 as the default MSS. During TCP 3 way handshakes, whichever side has smaller size MSS will be used.


HTH,

Lei Tian

d.serra Wed, 03/03/2010 - 07:30
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Ah...looks like everyone was expecting it except me


Thanks for your help Lei.  It is much appreciated.


sincerely,


David

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