I have an application that is communicating to a SQL server. Every once in a while, the queries the application calls are taking much longer. I've captured this with Wireshark. When everything is going fine, the log looks like this. These 5 entries take about 1ms. Client -> SQL Server : Remote Procedure Call SQL Server -> Client: Response Client -> SQL Server : SQL Batch SQL Server -> Client : [ACK] Seq=3218061 Ack=4449533 Win=131328 Len = 0 SQL Server -> Client: Response When things start to get slow, the SQL Server/Client communication looks like this. The entire transaction takes about 500ms Client -> SQL Server : Remote Procedure Call SQL Server -> Client: Response Client -> SQL Server : SQL Batch SQL Server -> Client : [ACK] Seq=3222735 Ack=4458045 Win=131328 Len = 0 (Packet length = 60) Client -> SQL Server: [ACK] Seq=596422 Ack=560033 Win=65280 Len = 0 (Packet length = 54) SQL Server -> Client: Response Notice the extra ACK from the client to the server. There is also a mixture of ARP requests in there, SSDP Notify's, STP, NBNS. I'm not sure why there are a bunch of other calls in there suddenly when there weren't before, but from everything I've found it should be pretty harmless. What I don't understand is why would the client be sending an ACK back to the SQL server? Any thoughts? Thanks in advance. asked 22 Oct '15, 07:23  kw2107 |
One Answer:
ACK is a TCP feature (Layer 4) to assure that the segments are transmitted correctly. For example to show the sender that all transmitted segments have reached its destination. A response is an application things and is different from application to application. The TCP behaviour is equal to all applications the at use the TCP. answered 22 Oct '15, 09:41  Christian_R edited 22 Oct '15, 09:42 |
Sure, I understand that. I guess my question is why does the client sometimes send back an ACK and sometimes it doesn't? And when it does send back the ACK, there is a 500ms delay in the SQL server response?
Depends on the moment. Without a trace I can't say much more. But in fact every packet is an ACK, too.