tcp window size, urgent pointer

THE WINDOW FLAG

The Window size is considered to be one of the most important flags within the TCP header. This field is used by the receiver to indicate to the sender the amount of data that it is able to accept.

The Window size field uses 'bytes' as a metric. So in our example above, the number 64,240 is equal to 64,240 bytes, or 62.7 kb (64,240/1024).

The 62.7 kbytes reflects the amount of data the receiver is able to accept, before transmitting to the sender (the server) a new Window value. When the amount of data transmitted is equal to the current Window value, the sender will expect a new Window value from the receiver, along with an acknowledgement for the Window just received.

In this example, Host A is connected to a Web server via a 10 Mbit link. According to our formula, to calculate the best Window value we need the following information: Bandwidth and Delay. We are aware of the link's bandwidth: 10,000,000 bits (10 Mbits), and we can easily find out the delay by issuing a 'ping' from Host A to the Web server which gives us an average Round Trip Time response (RTT) of 10 milliseconds or 0.01 seconds.

We are then able to use this information to calculate the most efficient Window size (WS):

WS = 10,000,000 x 0.01 => WS = 100,000 bits or (100,000/8)/1024 = 12,5 kbytes

For 10 Mbps bandwidth and a round-trip delay of 0.01 sec, this gives a window size of about 12 kb or nine 1460-byte segments:

When a client connects to a web server, the server is required to reserve a small amount of memory (RAM) aside for the client's session. The amount of required memory is the same amount as the window size and, as we have seen, this value depends on the bandwidth and delay between the client and server.

If you had a web server that served 10,000 clients on a local area network (LAN) running at 100 Mbits with a 0.1 second round trip delay and wanted maximum performance/efficiency for your file transfers, according to our formula, you would need to allocate a window of 1.25 MB for each client, or 12 Gigs of memory for all your clients!.

THE URGENT POINTER

The urgent pointer flag in the TCP Flag allows us to mark a segment of data as 'urgent', while this urgent pointer field specifies where exactly the urgent data ends.