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@@ -1472,3 +1472,27 @@ HTTP is an application layer protocol relying on lower-level protocols such as *
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* [HTTP](https://www.nginx.com/resources/glossary/http/)
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* [README](https://www.quora.com/What-is-the-difference-between-HTTP-protocol-and-TCP-protocol)
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+
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+### Transmission control protocol (TCP)
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+
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+<p align="center">
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+ <img src="http://i.imgur.com/JdAsdvG.jpg">
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+ <br/>
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+ <i><a href=http://www.wildbunny.co.uk/blog/2012/10/09/how-to-make-a-multi-player-game-part-1/>Source: How to make a multiplayer game</a></i>
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+</p>
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+
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+TCP is a connection-oriented protocol over an [IP network](https://en.wikipedia.org/wiki/Internet_Protocol). Connection is established and terminated using a [handshake](https://en.wikipedia.org/wiki/Handshaking). All packets sent are guaranteed to reach the destination in the original order and without corruption through:
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+
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+* Sequence numbers and [checksum fields](https://en.wikipedia.org/wiki/Transmission_Control_Protocol#Checksum_computation) for each packet
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+* [Acknowledgement](https://en.wikipedia.org/wiki/Acknowledgement_(data_networks)) packets and automatic retransmission
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+
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+If the sender does not receive a correct response, it will resend the packets. If there are multiple timeouts, the connection is dropped. TCP also implements [flow control](https://en.wikipedia.org/wiki/Flow_control_(data)) and [congestion control](https://en.wikipedia.org/wiki/Network_congestion#Congestion_control). These guarantees cause delays and generally results in less efficient transmission than UDP.
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+
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+To ensure high throughput, web servers can keep a large number of TCP connections open, resulting in high memory usage. It can be expensive to have a large number of open connections between web server threads and say, a [memcached](#memcached) server. [Connection pooling](https://en.wikipedia.org/wiki/Connection_pool) can help in addition to switching to UDP where applicable.
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+
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+TCP is useful for applications that require high reliability but are less time critical. Some examples include web servers, database info, SMTP, FTP, and SSH.
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+
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+Use TCP over UDP when:
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+
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+* You need all of the data to arrive in tact
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+* You want to automatically make a best estimate use of the network throughput
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Donne Martin