> For the complete documentation index, see [llms.txt](https://hanfak.gitbook.io/workspace/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hanfak.gitbook.io/workspace/networks/tcp-ip/transmission-layer-protocol.md).

# TCP - Transmission Control Protocol

* TCP is a utility built on top of IP
* TCP was created to solve a problem with IP
  * Data over IP is typically sent in multiple packets because each packet is fairly small (2^16 bytes).
  * Multiple packets can result in
    * lost or dropped packets and
    * disordered packets
    * thus corrupting the transmitted data
  * TCP solves both of these by guaranteeing transmission of packets in an ordered way.
* the packet has a header called the TCP header in addition to the IP header.
  * TCP header contains information about the ordering of packets, and the number of packets and so on.
* TCP needs to establish a connection between source and destination before it transmits the packets, and it does this via a "handshake".
  * This connection itself is established using packets where the source informs the destination that it wants to open a connection, and the destination says OK, and then a connection is opened.
  * is what happens when a server "listens" at a port - just before it starts to listen there is a handshake, and then the connection is opened (listening starts).
  * one sends the other a message that it is about to close the connection, and that ends the connection.

## links

* <https://www.youtube.com/watch?v=xdQ9sgpkrX8>
* <https://www.youtube.com/watch?v=NdvWI6RH1eo>
* How TCP Works - The Handshake: <https://www.youtube.com/watch?v=HCHFX5O1IaQ>
* How TCP really works // Three-way handshake // TCP/IP Deep Dive <https://youtu.be/rmFX1V49K8U>