I'm working on a legacy VB6 app that uses sockets to communicate to various devices.
On a 2012 system, we are noticing the time between calling winSock.Connect() to the connection event being fired is holding at about 9 seconds, across multiple systems on different domains.
On a 2008 R2 or lower system, it's taking 1-3 milliseconds between the call and the event being fired.
Has anyone run into this before, or has any ideas on what could be causing this?
I've done some snooping with Wireshark, and found that the first few TCP transmissions are not connecting and being retransmitted, not sure if that will help.
I ended up finding the answer to this after some extensive digging.
Starting in Windows Server 2012, Microsoft has enabled an extension of TCP called Explicit Congestion Notification (ECN). This allows end-to-end notification of network congestion with the loss of packets. The way this is enabled on a TCP packet is via a flag, which is defined in the definition of ECN (RFC 3168(2001)).
What was happening for me was that the devices my application talks to are older, and don't support the ECN flag. When they received packets with that flag enabled, they wouldn't acknowledge the transmission, leading to a timeout from the server. After two failed transmissions, it looks like Windows shuts off the ECN flag, and the device acknowledged the packets.
I disabled ECN running the following command from an Administrator Command Prompt:
netsh interface tcp set global ecncapability=disabled