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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Transport Layer Protocols            9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)                 9  TCP/IP Transmission Control Protocol (TCP)                      9  TCP Message Formatting and Data Transfer

TCP Maximum Segment Size (MSS) and Relationship to IP Datagram Size 1 2 3 4 5 6 TCP Immediate Data Transfer: "Push" Function

Real-World Complications of the Sliding Window Mechanism

I'm sure this seems rather complicated, but in fact, the example is highly simplified, to show you how the basic data transfer mechanism works without too much going on. Scary, isn’t it? J A real world connection would include several complications:

• Overlapping Transmissions: I intentionally showed only one request from the client and the response from the server. In reality, the client and server could be pumping many requests and responses at each other in rapid-fire succession; the client would be acknowledging segments received from the server with segments that themselves contained new requests, and so on.
  
  
• Acknowledgment of Multiple Segments: I also didn't show a case where two segments are received by a device and acknowledged with a single acknowledgment, though this can certainly happen. Suppose that in the example above, the two parts of the 280-byte file were sent at once and received by the client at the same time. The client would acknowledge both by sending a single segment with an Acknowledgment Number of 601. Remember, this field is a cumulative acknowledgment of all segments containing data through the number preceding it, so this would acknowledge all data up to byte 600.
  
  
• Fluctuating Window Sizes For Flow Control: The window sizes in the example above remained constant, but in a real connection this will not always be the case. A very busy server may not be able to process and remove data from its buffer as fast as it acknowledges it. It may need to reduce its receive window to reduce the amount of data the client sends it, and then increase the window when more space becomes available. This is how TCP implements flow control.
  
  
• Lost Transmissions: In a real connection, some transmitted segments will be lost and need to be retransmitted. This is handled by TCP's retransmission scheme.
  
  
• Avoiding Small Window Problems: I hinted in the table above that we don't necessarily always want to send data as fast as we can, when it means we have to send a very small segment. The reason is that this can lead to performance degradation, including a phenomenon called silly window syndrome. This too will be explored in the section that follows, where we will see how handling it requires that we change the simple sliding windows scheme we have seen until the point.
  
  
• Congestion Handling and Avoidance: The basic sliding window mechanism has been changed over the years to avoid having TCP connections cause internetwork congestion and to have them handle congestion when it is detected. Congestion issues are discussed, you guessed it, in the next section.

TCP Maximum Segment Size (MSS) and Relationship to IP Datagram Size 1 2 3 4 5 6 TCP Immediate Data Transfer: "Push" Function