[Tcp](https://en.wikipedia.org/wiki/Transmission_Control_Protocol#Congestion_control) is a protocol built on top of IP. It is by far the most popular protocol on the internet. Everything you are seeing in this page was sent to your browser via TCP. It is designed to be simple to use, scalable and for transmiting large amounts of data.
A server opens a port and waits for connections. Clients send an initial message to establish the connection (handshake) and after that a connection is considered established. Data flows both ways as a stream, one byte after another, always in the right order, without missing anything. Some of the key features include:
* Reliable: if you try to send data, it will either make it to the other end, or you will get an error, nothing is ever silently dropped.
* Fragmented: Your network card cannot just send 1 MB of data, it can only send small packets of 1.5Kb or less. If you try to send a lot of data, TCP will split your data into small packets and reassemble the data on the receiving end.
* Sequenced: If you send data a,b,c in that order, TCP guarantees that it will arrive in the same order or an error will be generated.
* Connection oriented: you don't just send data, TCP will send a message first to let the server know that they will be talking together. Both the client and server can disconnect and react to the disconnection.
* Congestion control: If a server is being overwhelmed, TCP will throttle the data to avoid congestion collapse.
Suppose an object is moving from point a to b to c. Our server sends 3 messages: a, b, c. Suppose b gets lost (wify drops a lot of packages for example) and c arrives fine. We could skip b and move towards c instead, but we can't because the operating system won't give us c until b is retransmited.
[UDP](https://en.wikipedia.org/wiki/User_Datagram_Protocol) is also a protocol based on IP. It is used for real time applications such as fast paced action games or voice over ip, where low latency is more important than reliability.
A server opens a port and waits for messages. Clients send messages to such port, and the server may send messages back. Data flows in both ways as individual messages.
There is no concept of connection, so there is no built in way to determine if a client disconnects. Messages are delivered as soon as possible, there is no guarantee that the order will be preserved or that they will be delivered at all. Messages must be small, typically 1.5Kb or less.
Mirror does need reliability, fragmentation, sequenced, connections for many things, so we would not use raw UDP. We would use a library that implements those features on top of UDP such as [ENet](http://enet.bespin.org/), [LiteNetLib](https://github.com/RevenantX/LiteNetLib) or LLAPI, typically refered to as RUDP (Reliable UDP)
The obvious question is: do RUDP libraries just reinventing TCP? yes, to some degree they do. But the point is that those features would be optional and we would be able to send messages without the extra features for low latency data such as movement or voice.
Back in 2015 when we started uMMORPG and Cubica, we originally used Unity's built in Networking system aka UNET. UNET used LLAPI, an RUDP library that avoided garbage collection at all costs.
What sounds good in theory, was terrible in practice. We spent about half our work hours from 2015 to 2018 dealing with UNET bugs. There was packet loss, highly complex code due to GC avoidance, synchronization issues, memory leaks and random errors. Most importantly, no decent way to debug any of it.
After 3 years in UDP/LLAPI hell, we realized if we ever wanted to finish our games, we would need a networking layer that just works. We could have tried other RUDP transports, but we would end up debugging them instead.
We acknowledge not everyone will agree with our reasoning. Rather than push our views on users, we made Mirror transport independent. A few months later, Unity did the same thing. You can easily swap out the transport for one of the several RUDP implementations simply by dragging it into your NetworkManager gameobject. Pick whatever works best for you. We recommend you profile your game and collect real world numbers before you make a final decision.
After we made Mirror transport independent, the community stepped up and several RUDP transports have been adapted to work with Mirror as we hoped. While the default is Telepathy (simple "just works" TCP transport), you can choose any of [these](../Transports) transports or even write your own.
