[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 and scalable.
Servers open a TCP port and wait for connections. Clients send an initial message (handshake) to establish the connection then send data. Data flows one byte after another, always in the right order, without missing anything. Some of the key features include:
* Reliable: if a packet gets lost, TCP will resend it. All data is either transmitted successfully or you get an error and the connection is closed. Applications don't have to worry about missing packets.
* Fragmented: network cards cannot just send 1 MB of data. They can only send small packets of 1.5Kb or less. If a lot of data is sent by the application, TCP will split it into small packets and reassemble the data on the receiving end.
* Sequenced: If you send data "a" and "b" you will not receive "b" and "a". TCP guarantees that every byte will arrive in the same order it was sent.
* Connection oriented: TCP has the concept of a connection. A client sends an initial handshake message. A connection is considered established until either the client and server decides to disconnect. Both the client and server get notified when the connection ends and can react accordingly, for example saving and destroying player object.
Suppose an object is moving from point a to b to c. The server sends 3 messages: move to a, b, c. Suppose b gets lost (wifi drops a lot of packets 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 retransmitted.
[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 the 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 referred 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 are optional and we can 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 integrated several RUDP transports with Mirror. While the default is Telepathy (simple "just works" TCP transport) you can choose among several [transports](../Transports/index.md) or even write your own.
