[Q] TCP segment sent is larger than peers advertised MSS

[Thomas Portmann]

On 06/26/2015 03:21 PM, Flavien Lebarbe wrote:
> Thomas Portmann ecrivait :
>> I am in the process of troubleshooting another problem and noticed a
>> rather strange behavior with the openssh client. A client (OpenSSH
>> 5.3) and a server (6.7p1) do the TCP handshake, the client announces
>> a MSS of 1460 (its MTU is 1500), while the server announces a MSS of
>> 1260 (its MTU is set to 1300). What troubles me is, that the client
>> is sending the server a frame of 2034 bytes (TCP segment length of
>> 1968, during the "Client: Key Exchange Init"). Why is it that the
>> client seems to ignore the servers MSS?
>  From what you say, the client not only disregards the server MSS, but
> also the client MSS itself : 2034 is larger than 1500. That's weird...
> Do you have a pcap capture of it ? Where do you see the 2034 ? In a
> strace by chance ?

I have a tcpdump I made on the client. Since it is rather small (~7KB), 
I will just attach it to the mail. The 2034 is taken from the capture.


     tom