<img src="kcp-go.png" alt="kcp-go" height="50px" />
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[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
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[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
<img src="frame.png" alt="Frame Format" height="109px" />
```
+-----------------+
| 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