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TCP "Silly Window Syndrome" and Changes To the Sliding Window System For Avoiding Small-Window Problems
(Page 4 of 4)
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
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.
Nagles 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.
Key Concept: Modern TCP implementations incorporate a set of SWS avoidance algorithms. When receiving, devices are programmed not to advertise very small windows, waiting instead until there is enough room in the buffer for one of a reasonable size. Transmitters use Nagles algorithm to ensure that small segments are not generated when there are unacknowledged bytes outstanding.
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Version 3.0 - Version Date: September 20, 2005
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