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 Reliability and Flow Control Features and Protocol Modifications

TCP Window Management Issues 1 2 3 4 TCP Congestion Handling and Congestion Avoidance Algorithms

Sender SWS Avoidance and Nagle's Algorithm

SWS avoidance by the sender is accomplished generally by imposing “restraint” on the part of the transmitting TCP. Instead of trying to immediately send data as soon as we can, we wait to send until we have a segment of a reasonable size. The specific method for doing this is called Nagle's algorithm, named for its inventor, John Smith. (Just kidding, it was John Nagle. J) Simplified, this algorithm works as follows:

• As long as there is no unacknowledged data outstanding on the connection, as soon as the application wants, data can be immediately sent. For example, in the case of an interactive application like Telnet, a single keystroke can be “pushed” in a segment.
  
  
• While there is unacknowledged data, all subsequent data to be sent is held in the transmit buffer and not transmitted until either all the unacknowledged data is acknowledged, or we have accumulated enough data to send a full-sized (MSS-sized) segment. This applies even if a “push” is requested by the user.

This might seem strange, especially the part about buffering data despite a push request! You might think this would cause applications like Telnet to “break”. In fact, Nagle's algorithm is a very clever method that suits the needs of both low-data-rate interactive applications like Telnet and high-bandwidth file transfer applications.

If you are using something like Telnet where the data is arriving very slowly (humans are very slow compared to computers), the initial data (first keystroke) can be pushed right away. The next keystroke has to wait for an acknowledgment, but this will probably come reasonably soon relative to how long it takes to hit the next key. In contrast, more conventional applications that generate data in large amounts will automatically have the data accumulated into larger segments for efficiency.

Nagle’s algorithm is actually far more complex than this description, but this topic is already getting too long. RFC 896 discusses it in (much) more detail.

TCP Window Management Issues 1 2 3 4 TCP Congestion Handling and Congestion Avoidance Algorithms