<img src="kcp-go.png" alt="kcp-go" height="50px" />
[![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/).
This library intents to provide a **smooth, resilient, ordered, error-checked and anonymous** delivery of streams over **UDP** packets, it has been battle-tested with opensource project [kcptun](https://github.com/xtaci/kcptun). Millions of devices(from low-end MIPS routers to high-end servers) have deployed **kcp-go** powered program in a variety of forms like **online games, live broadcasting, file synchronization and network acceleration**.
[Lastest Release](https://github.com/xtaci/kcp-go/releases)
## Features
1. Designed for **Latency-sensitive** scenarios.
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, which generates completely anonymous packet.
1. Only **A fixed number of goroutines** will be created for the entire server application, costs in **context switch** between goroutines have been taken into consideration.
1. Compatible with [skywind3000's](https://github.com/skywind3000) C version with various improvements.
## 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)
```
## Benchmark
```
Model Name: MacBook Pro
Model Identifier: MacBookPro14,1
Processor Name: Intel Core i5
Processor Speed: 3.1 GHz
Number of Processors: 1
Total Number of Cores: 2
L2 Cache (per Core): 256 KB
L3 Cache: 4 MB
Memory: 8 GB
```
```
$ go test -v -run=^$ -bench .
beginning tests, encryption:salsa20, fec:10/3
goos: darwin
goarch: amd64
pkg: github.com/xtaci/kcp-go
BenchmarkSM4-4 50000 32180 ns/op 93.23 MB/s 0 B/op 0 allocs/op
BenchmarkAES128-4 500000 3285 ns/op 913.21 MB/s 0 B/op 0 allocs/op
BenchmarkAES192-4 300000 3623 ns/op 827.85 MB/s 0 B/op 0 allocs/op
BenchmarkAES256-4 300000 3874 ns/op 774.20 MB/s 0 B/op 0 allocs/op
BenchmarkTEA-4 100000 15384 ns/op 195.00 MB/s 0 B/op 0 allocs/op
BenchmarkXOR-4 20000000 89.9 ns/op 33372.00 MB/s 0 B/op 0 allocs/op
BenchmarkBlowfish-4 50000 26927 ns/op 111.41 MB/s 0 B/op 0 allocs/op
BenchmarkNone-4 30000000 45.7 ns/op 65597.94 MB/s 0 B/op 0 allocs/op
BenchmarkCast5-4 50000 34258 ns/op 87.57 MB/s 0 B/op 0 allocs/op
Benchmark3DES-4 10000 117149 ns/op 25.61 MB/s 0 B/op 0 allocs/op
BenchmarkTwofish-4 50000 33538 ns/op 89.45 MB/s 0 B/op 0 allocs/op
BenchmarkXTEA-4 30000 45666 ns/op 65.69 MB/s 0 B/op 0 allocs/op
BenchmarkSalsa20-4 500000 3308 ns/op 906.76 MB/s 0 B/op 0 allocs/op
BenchmarkCRC32-4 20000000 65.2 ns/op 15712.43 MB/s
BenchmarkCsprngSystem-4 1000000 1150 ns/op 13.91 MB/s
BenchmarkCsprngMD5-4 10000000 145 ns/op 110.26 MB/s
BenchmarkCsprngSHA1-4 10000000 158 ns/op 126.54 MB/s
BenchmarkCsprngNonceMD5-4 10000000 153 ns/op 104.22 MB/s
BenchmarkCsprngNonceAES128-4 100000000 19.1 ns/op 837.81 MB/s
BenchmarkFECDecode-4 1000000 1119 ns/op 1339.61 MB/s 1606 B/op 2 allocs/op
BenchmarkFECEncode-4 2000000 832 ns/op 1801.83 MB/s 17 B/op 0 allocs/op
BenchmarkFlush-4 5000000 272 ns/op 0 B/op 0 allocs/op
BenchmarkEchoSpeed4K-4 5000 259617 ns/op 15.78 MB/s 5451 B/op 149 allocs/op
BenchmarkEchoSpeed64K-4 1000 1706084 ns/op 38.41 MB/s 56002 B/op 1604 allocs/op
BenchmarkEchoSpeed512K-4 100 14345505 ns/op 36.55 MB/s 482597 B/op 13045 allocs/op
BenchmarkEchoSpeed1M-4 30 34859104 ns/op 30.08 MB/s 1143773 B/op 27186 allocs/op
BenchmarkSinkSpeed4K-4 50000 31369 ns/op 130.57 MB/s 1566 B/op 30 allocs/op
BenchmarkSinkSpeed64K-4 5000 329065 ns/op 199.16 MB/s 21529 B/op 453 allocs/op
BenchmarkSinkSpeed256K-4 500 2373354 ns/op 220.91 MB/s 166332 B/op 3554 allocs/op
BenchmarkSinkSpeed1M-4 300 5117927 ns/op 204.88 MB/s 310378 B/op 6988 allocs/op
PASS
ok github.com/xtaci/kcp-go 50.349s
```
## Key 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 leads to 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() lies here:
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 clock time will be updated upon return, and for a single `kcp.flush()` operation, current time will be queried from system once. For most of the time, 5000 connections costs 5000 * 15.6ns = 78us(a fixed cost while no packet needs to be sent), 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**.
## Connection Termination
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.
## FAQ
Q: I'm handling >5K connections on my server, the CPU utilization is so high.
A: A standalone `agent` or `gate` server for running kcp-go is suggested, not only for CPU utilization, but also important to the **precision** of RTT measurements(timing) which indirectly affects retransmission. By increasing update `interval` with `SetNoDelay` like `conn.SetNoDelay(1, 40, 1, 1)` will dramatically reduce system load, but lower the performance.
## 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.
## 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/klauspost/reedsolomon -- Reed-Solomon Erasure Coding in Go