[![GoDoc][1]][2] [![Powered][9]][10] [![MIT licensed][11]][12] [![Build Status][3]][4] [![Go Report Card][5]][6] [![Coverage Statusd][7]][8] [1]: https://godoc.org/github.com/xtaci/kcp-go?status.svg [2]: https://godoc.org/github.com/xtaci/kcp-go [3]: https://travis-ci.org/xtaci/kcp-go.svg?branch=master [4]: https://travis-ci.org/xtaci/kcp-go [5]: https://goreportcard.com/badge/github.com/xtaci/kcp-go [6]: https://goreportcard.com/report/github.com/xtaci/kcp-go [7]: https://codecov.io/gh/xtaci/kcp-go/branch/master/graph/badge.svg [8]: https://codecov.io/gh/xtaci/kcp-go [9]: https://img.shields.io/badge/KCP-Powered-blue.svg [10]: https://github.com/skywind3000/kcp [11]: https://img.shields.io/badge/license-MIT-blue.svg [12]: LICENSE ## Introduction **kcp-go** is a **Production-Grade Reliable-UDP** library for [golang](https://golang.org/). It provides **fast, ordered and error-checked** delivery of streams over **UDP** packets, has been well tested with opensource project [kcptun](https://github.com/xtaci/kcptun). Millions of devices(from low-end MIPS routers to high-end servers) are running with **kcp-go** at present, including applications like **online games, live broadcasting, file synchronization and network acceleration**. [Lastest Release](https://github.com/xtaci/kcp-go/releases) ## Features 1. Optimized for **Realtime Online Games, Audio/Video Streaming and Latency-Sensitive Distributed Consensus**. 1. Compatible with [skywind3000's](https://github.com/skywind3000) C version with language specific optimizations. 1. **Cache friendly** and **Memory optimized** design, offers extremely **High Performance** core. 1. Handles **>5K concurrent connections** on a single commodity server. 1. Compatible with [net.Conn](https://golang.org/pkg/net/#Conn) and [net.Listener](https://golang.org/pkg/net/#Listener), a drop-in replacement for [net.TCPConn](https://golang.org/pkg/net/#TCPConn). 1. [FEC(Forward Error Correction)](https://en.wikipedia.org/wiki/Forward_error_correction) Support with [Reed-Solomon Codes](https://en.wikipedia.org/wiki/Reed%E2%80%93Solomon_error_correction) 1. Packet level encryption support with [AES](https://en.wikipedia.org/wiki/Advanced_Encryption_Standard), [TEA](https://en.wikipedia.org/wiki/Tiny_Encryption_Algorithm), [3DES](https://en.wikipedia.org/wiki/Triple_DES), [Blowfish](https://en.wikipedia.org/wiki/Blowfish_(cipher)), [Cast5](https://en.wikipedia.org/wiki/CAST-128), [Salsa20]( https://en.wikipedia.org/wiki/Salsa20), etc. in [CFB](https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Cipher_Feedback_.28CFB.29) mode. 1. **Fixed number of goroutines** created for the entire server application, minimized goroutine context switch. ## Conventions Control messages like **SYN/FIN/RST** in TCP **are not defined** in KCP, you need some **keepalive/heartbeat mechanism** in the application-level. A real world example is to use some **multiplexing** protocol over session, such as [smux](https://github.com/xtaci/smux)(with embedded keepalive mechanism), see [kcptun](https://github.com/xtaci/kcptun) for example. ## Documentation For complete documentation, see the associated [Godoc](https://godoc.org/github.com/xtaci/kcp-go). ## Specification ``` +-----------------+ | SESSION | +-----------------+ | KCP(ARQ) | +-----------------+ | FEC(OPTIONAL) | +-----------------+ | CRYPTO(OPTIONAL)| +-----------------+ | UDP(PACKET) | +-----------------+ | IP | +-----------------+ | LINK | +-----------------+ | PHY | +-----------------+ (LAYER MODEL OF KCP-GO) ``` ## Usage Client: [full demo](https://github.com/xtaci/kcptun/blob/master/client/main.go) ```go kcpconn, err := kcp.DialWithOptions("192.168.0.1:10000", nil, 10, 3) ``` Server: [full demo](https://github.com/xtaci/kcptun/blob/master/server/main.go) ```go lis, err := kcp.ListenWithOptions(":10000", nil, 10, 3) ``` ## Performance ``` Model Name: MacBook Pro Model Identifier: MacBookPro12,1 Processor Name: Intel Core i5 Processor Speed: 2.7 GHz Number of Processors: 1 Total Number of Cores: 2 L2 Cache (per Core): 256 KB L3 Cache: 3 MB Memory: 8 GB ``` ``` $ go test -v -run=^$ -bench . beginning tests, encryption:salsa20, fec:10/3 BenchmarkAES128-4 200000 8256 ns/op 363.33 MB/s 0 B/op 0 allocs/op BenchmarkAES192-4 200000 9153 ns/op 327.74 MB/s 0 B/op 0 allocs/op BenchmarkAES256-4 200000 10079 ns/op 297.64 MB/s 0 B/op 0 allocs/op BenchmarkTEA-4 100000 18643 ns/op 160.91 MB/s 0 B/op 0 allocs/op BenchmarkXOR-4 5000000 316 ns/op 9486.46 MB/s 0 B/op 0 allocs/op BenchmarkBlowfish-4 50000 35643 ns/op 84.17 MB/s 0 B/op 0 allocs/op BenchmarkNone-4 30000000 56.2 ns/op 53371.83 MB/s 0 B/op 0 allocs/op BenchmarkCast5-4 30000 44744 ns/op 67.05 MB/s 0 B/op 0 allocs/op Benchmark3DES-4 2000 639839 ns/op 4.69 MB/s 2 B/op 0 allocs/op BenchmarkTwofish-4 30000 43368 ns/op 69.17 MB/s 0 B/op 0 allocs/op BenchmarkXTEA-4 30000 57673 ns/op 52.02 MB/s 0 B/op 0 allocs/op BenchmarkSalsa20-4 300000 3917 ns/op 765.80 MB/s 0 B/op 0 allocs/op BenchmarkFlush-4 10000000 226 ns/op 0 B/op 0 allocs/op BenchmarkEchoSpeed4K-4 5000 300030 ns/op 13.65 MB/s 5672 B/op 177 allocs/op BenchmarkEchoSpeed64K-4 500 3202335 ns/op 20.47 MB/s 73295 B/op 2198 allocs/op BenchmarkEchoSpeed512K-4 50 24926924 ns/op 21.03 MB/s 659339 B/op 17602 allocs/op BenchmarkEchoSpeed1M-4 20 64857821 ns/op 16.17 MB/s 1772437 B/op 42869 allocs/op BenchmarkSinkSpeed4K-4 30000 50230 ns/op 81.54 MB/s 2058 B/op 48 allocs/op BenchmarkSinkSpeed64K-4 2000 648718 ns/op 101.02 MB/s 31165 B/op 687 allocs/op BenchmarkSinkSpeed256K-4 300 4635905 ns/op 113.09 MB/s 286229 B/op 5516 allocs/op BenchmarkSinkSpeed1M-4 200 9566933 ns/op 109.60 MB/s 463771 B/op 10701 allocs/op PASS ok _/Users/xtaci/.godeps/src/github.com/xtaci/kcp-go 39.689s ``` ## Design Considerations 1. slice vs. container/list `kcp.flush()` loops through the send queue for retransmission checking for every 20ms(interval). I've wrote a benchmark for comparing sequential loop through *slice* and *container/list* here: https://github.com/xtaci/notes/blob/master/golang/benchmark2/cachemiss_test.go ``` BenchmarkLoopSlice-4 2000000000 0.39 ns/op BenchmarkLoopList-4 100000000 54.6 ns/op ``` List structure introduces **heavy cache misses** compared to slice which owns better **locality**, 5000 connections with 32 window size and 20ms interval will cost 6us/0.03%(cpu) using slice, and 8.7ms/43.5%(cpu) for list for each `kcp.flush()`. 2. Timing accuracy vs. syscall clock_gettime Timing is **critical** to **RTT estimator**, inaccurate timing introduces false retransmissions in KCP, but calling `time.Now()` costs 42 cycles(10.5ns on 4GHz CPU, 15.6ns on my MacBook Pro 2.7GHz), the benchmark for time.Now(): https://github.com/xtaci/notes/blob/master/golang/benchmark2/syscall_test.go ``` BenchmarkNow-4 100000000 15.6 ns/op ``` In kcp-go, after each `kcp.output()` function call, current time will be updated upon return, and each `kcp.flush()` will get current time once. For most of the time, 5000 connections costs 5000 * 15.6ns = 78us(no packet needs to be sent by `kcp.output()`), as for 10MB/s data transfering with 1400 MTU, `kcp.output()` will be called around 7500 times and costs 117us for `time.Now()` in **every second**. ## Tuning Q: I'm handling >5K connections on my server. the CPU utilization is high. A: A standalone `agent` or `gate` server for kcp-go is suggested, not only for CPU utilization, but also important to the **precision** of RTT measurements which indirectly affects retransmission. By increasing update `interval` with `SetNoDelay` like `conn.SetNoDelay(1, 40, 1, 1)` will dramatically reduce system load. ## Who is using this? 1. https://github.com/xtaci/kcptun -- A Secure Tunnel Based On KCP over UDP. 2. https://github.com/getlantern/lantern -- Lantern delivers fast access to the open Internet. 3. https://github.com/smallnest/rpcx -- A RPC service framework based on net/rpc like alibaba Dubbo and weibo Motan. 4. https://github.com/gonet2/agent -- A gateway for games with stream multiplexing. 5. https://github.com/syncthing/syncthing -- Open Source Continuous File Synchronization. 6. https://play.google.com/store/apps/details?id=com.k17game.k3 -- Battle Zone - Earth 2048, a world-wide strategy game. ## Links 1. https://github.com/xtaci/libkcp -- FEC enhanced KCP session library for iOS/Android in C++ 2. https://github.com/skywind3000/kcp -- A Fast and Reliable ARQ Protocol 3. https://github.com/templexxx/reedsolomon -- Reed-Solomon Erasure Coding in Go