NetScaler TCP Test

These measures include connections in the Opening, Established, and Closing states.

A connection is said to be OPENING if the handshakes between the server and the client are not completed.

An established connection indicates that the data transfer can occur between the ADC device and the client/server.

A connection is said to be in the CLOSING state when the connection termination process has been initiated but not completed.

The ADC device can be used to limit the number of simultaneous requests that are passed on to a server. When a request is completed, additional requests are forwarded to the server. If a request arrives and the server is handling the maximum configured number of requests, the ADC device places the new request in a surge queue, where the request waits for its turn to be sent to the server for processing. The surge queue allows a server to run at peak capacity without the risk of having it spiral out of control because of a surge of incoming requests. The surge queue length indicates whether a server is able to keep up with its incoming workload or not. If the surge queue is consistently greater than 0, this indicates that the server is not able to keep up with the workload and additional server capacity is required. On the other hand, a periodic surge is not a cause for concern.

When a surge in client requests overloads a server, server response becomes slow, and the server is unable to respond to new requests. The Surge Protection feature ensures that connections to the server occur at a rate that the server can handle. The response rate depends on how surge protection is configured. The ADC appliance also tracks the number of connections to the server, and uses that information to adjust the rate at which it opens new server connections.

The ADC does connection multiplexing between clients and physical servers. When it receives a client request to access a service on a server, the ADC looks for an already established connection to the server that is free. If it finds a free/spare connection, it uses that connection to establish a virtual link between the client and the server.

To save time and resources in establishing another connection for a new client, the connection on the server is not closed after completing the request from the first client and is available for serving future requests.

The client connection would be flushed when the client has remained idle for a specified time.

The server connections would be flushed when there are no client requests in the queue for a specified time.

A half-opened connection often refers to the TCP connection that is in the process of being established. These connections are flushed when the three-way handshake (SYN, SYN/ACK and ACK) process is not completed.

The TCP protocol has a three-way handshake process for opening a connection. First, the originating endpoint (A) sends a SYN packet to the destination (B). A is now in an embryonic state (specifically, SYN_SENT), and awaiting a response. B now updates its kernel information to indicate the incoming connection from A, and sends out a request to open a channel back (the SYN/ACK packet).

At this point, B is also in an embryonic state (specifically, SYN_RCVD). Note that B was put into this state by another machine, outside of B's control.

Under normal circumstances (see denial-of-service attack for deliberate failure cases), A will receive the SYN/ACK from B, update its tables (which now have enough information for A to both send and receive), and send a final ACK back to B.

Once B receives this final ACK, it also has sufficient information for two-way communication, and the connection is fully open. Both endpoints are now in an established state.

A half-closed connection refers to the connections closed by the client/server and there is no activity taking place on the connection. A half-closed connection may also be referred to as the connection through which the client/server would have stopped sending data but still data is received through the same.

A passive half-closed connection refers to the connections closed by the ADC and there is no activity taking place on the connection.

Every time a connection is flushed, it is marked for cleanup. The zombie cleanup function clears all these connections at predefined intervals.

These are good indicators of the load on the ADC appliance.

The ADC sends a SYN packet to the server to check its availability and expects a SYN_ACK packet from the server before a specified response timeout.

Packets cannot be transferred on a connection in this state.

The SYN packets would be held when the ADC device is waiting for the server connection.

The SYN packets would be flushed when there is no response from the server for 3 or more seconds.