The other day I had a customer experiencing a problem where TCP sessions couldn’t be established. We checked the firewall and core and it all looked fine. We could see the source host sending a TCP SYN and the destination was immediately responding with a Reset. Based on the IP TTL the Reset was coming from the destination host so I asked for a packet capture taken on the destination. In this case they didn’t have netmon or wireshark installed on the destination so suggested using the built in capability in 08R2 and newer – netsh trace scenario…
Once I got the capture ( an etl file) and loaded it into netmon with the latest parsers I did a filter on description where it contains the IP address. I found the SYN and Reset but the next two messages were the surprising bonus.
Notice the message at the end of the message:
Wscore_MicrosoftWindowsWinsockAFD:datagram dropped: 2 (0x2): Process 0xFFFFFA8007E3E060, Endpoint 0xFFFFFA8008149620, Buffer 0xFFFFFA80060E06D0, Length 40 (0x28), Address <IP Address>:0, Seq 10001 (0x2711), Reason Insufficient local buffer space
Given the low buffer space message the decision was made to reboot and that cleared the problem. Of course there is now the task to determine what caused the memory issue but that’s another story.