You’ve seen this wait type before, haven’t you? Well, it is certainly a wait type that has caused lots of confusion. The immediate focus is generally given to the word “NETWORK” but more times than not it has nothing to do with the network!
The ASYNC_NETWORK_IO wait type will typically yield two types of symptoms, the first being session workload is waiting for the corresponding client application to acknowledge/process a given set of data and let SQL Server know it is ready to process/acknowledge more data. The second symptom is that there may be a performance issue in the network between the application and database instance.Behind the scenes, SQL Server keeps the data held in an output buffer until an acknowledgement is received from the client determining whether the consumption of data is complete. ASYNC_NETWORK_IO is a tell-tale sign that the application lacks efficiency in reading data it requires from its backend database. The underlying network may also have issues which can produce lengthier waits while data is processed, and signals are being sent back from the client to the server.