Mirror is a high level Networking Library that can use several different low level transports. To use a transport, simply add it as component to the NetworkManager and drag it into the NetworkManager's Transport field.
KCP - KCP Transport based on kcp.c v1.7, nearly translated 1:1.
TCP - Libuv2k based on Native C networking backend used by Node.js.
TCP - Telepathy Simple, message based, MMO Scale TCP networking in C#. And no magic.
TCP - Apathy Apathy is a fast, lightweight, allocation-free low level TCP library for Unity developed by vis2k. Apathy was developed in native C for maximum MMO Scale networking performance.
TCP - Booster The Mirror Booster uncorks your multiplayer game by moving the Networking load out of Unity!
WebSockets - SimpleWebTransport WebSockets transport layer for Mirror that target WebGL clients.
Multiplexer Multiplexer is a bridging transport to allow a server to handle clients on different transports concurrently, for example desktop clients using Telepathy together with WebGL clients using Websockets.
Fallback Fallback is a compatibility transport for transports that don't run on all platforms and need fallback options to cover all other platforms.
UDP - Ignorance Ignorance implements a reliable and unreliable sequenced UDP transport based on ENet.
Steam - FizzySteamworks Transport utilising Steam P2P network, building on Steamworks.NET.
Steam - FizzyFacepunch Transport utilising Steam P2P network, building on Facepunch.Steamworks.