Winsock TCP maxsynretransmissions

Question

Hello,

I was inspecting TCP delay and found something not very correct.

In my Windows 10 the default settings for SYN retransmission is "Initial RTO=1000" and "Max SYN retransmissions=4". In this case we should get 1+2+4+8+16=31 seconds time-out after 4 retransmissions. But, the actual timeout is only 21 seconds.

If I set "Max SYN retransmissions=3" and "5", I got correct time delay as 15 and 63 seconds respectively.

Here is the batch file I ran to test it.

@echo Started: %time%

@echo off

telnet 10.10.10.10

@echo on

@echo Completed: %time%

Is this a small bug?

regards,

CHAO GAO

Answer 1

The default settings for SYN retransmission used to be InitialRTO=3000 and MaxSynRetransmissions=2 which results in total timeout being 3+6+12=21 sec.

Ever since Microsoft changed the defaults to be InitialRTO=1000 and MaxSynRetransmissions=4 this specific combination results in 21 sec timeout, instead of being 31 sec.

Both Windows 10 and Windows 11 are affected, although I could not find which builds this was changed in, but seems to be the case for at least 5 years now.

If anyone has additional input on this, it would be much appreciated.