# 
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[14]: https://github.com/xtaci/kcptun/releases/latest
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[16]: https://github.com/xtaci/kcptun/releases
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[18]: https://github.com/skywind3000/kcp
> *Disclaimer: kcptun maintains a single website — [github.com/xtaci/kcptun](https://github.com/xtaci/kcptun). Any websites other than [github.com/xtaci/kcptun](https://github.com/xtaci/kcptun) are not endorsed by xtaci.*
### QuickStart
Increase the number of open files on your server, as:
`ulimit -n 65535`, or write it in `~/.bashrc`.
Suggested `sysctl.conf` parameters for better handling of UDP packets:
```
net.core.rmem_max=26214400 // BDP - bandwidth delay product
net.core.rmem_default=26214400
net.core.wmem_max=26214400
net.core.wmem_default=26214400
net.core.netdev_max_backlog=2048 // proportional to -rcvwnd
```
You can also increase the per-socket buffer by adding parameter(default 4MB):
```
-sockbuf 16777217
```
for **slow processors**, increasing this buffer is **CRITICAL** to receive packets properly.
Download a corresponding one from precompiled [Releases](https://github.com/xtaci/kcptun/releases).
```
KCP Client: ./client_darwin_amd64 -r "KCP_SERVER_IP:4000" -l ":8388" -mode fast3 -nocomp -autoexpire 900 -sockbuf 16777217 -dscp 46
KCP Server: ./server_linux_amd64 -t "TARGET_IP:8388" -l ":4000" -mode fast3 -nocomp -sockbuf 16777217 -dscp 46
```
The above commands will establish port forwarding channel for 8388/tcp as:
> Application -> **KCP Client(8388/tcp) -> KCP Server(4000/udp)** -> Target Server(8388/tcp)
which tunnels the original connection:
> Application -> Target Server(8388/tcp)
### Install from source
```
$go get -u github.com/xtaci/kcptun/...
```
All precompiled releases are genereated from `build-release.sh` script.
### Performance
### Basic Tuning Guide
#### Improving Thoughput
> **Q: I have a high speed network link, how to reach the maximum bandwidth?**
> **A:** Increase `-rcvwnd` on KCP Client and `-sndwnd` on KCP Server **simultaneously & gradually**, the mininum one decides the maximum transfer rate of the link, as `wnd * mtu / rtt`; Then try downloading something and to see if it meets your requirements.
(mtu is adjustable by `-mtu`)
#### Improving Latency
> **Q: I'm using kcptun for game, I don't want any lag happening.**
> **A:** Lag means packet loss for most of the time, lags can be improved by changing `-mode`.
> eg: `-mode fast3`
> Aggresiveness/Responsiveness on retransmission for embeded modes are:
> *fast3 > fast2 > fast > normal > default*
### Expert Tuning Guide
#### Overview
#### Usage
```
$ ./client_darwin_amd64 -h
NAME:
kcptun - client(with SMUX)
USAGE:
client_darwin_amd64 [global options] command [command options] [arguments...]
VERSION:
20180922
COMMANDS:
help, h Shows a list of commands or help for one command
GLOBAL OPTIONS:
--localaddr value, -l value local listen address (default: ":12948")
--remoteaddr value, -r value kcp server address (default: "vps:29900")
--key value pre-shared secret between client and server (default: "it's a secrect") [$KCPTUN_KEY]
--crypt value aes, aes-128, aes-192, salsa20, blowfish, twofish, cast5, 3des, tea, xtea, xor, sm4, none (default: "aes")
--mode value profiles: fast3, fast2, fast, normal, manual (default: "fast")
--conn value set num of UDP connections to server (default: 1)
--autoexpire value set auto expiration time(in seconds) for a single UDP connection, 0 to disable (default: 0)
--scavengettl value set how long an expired connection can live(in sec), -1 to disable (default: 600)
--mtu value set maximum transmission unit for UDP packets (default: 1350)
--sndwnd value set send window size(num of packets) (default: 128)
--rcvwnd value set receive window size(num of packets) (default: 512)
--datashard value, --ds value set reed-solomon erasure coding - datashard (default: 10)
--parityshard value, --ps value set reed-solomon erasure coding - parityshard (default: 3)
--dscp value set DSCP(6bit) (default: 0)
--nocomp disable compression
--sockbuf value (default: 4194304)
--keepalive value (default: 10)
--snmplog value collect snmp to file, aware of timeformat in golang, like: ./snmp-20060102.log
--snmpperiod value snmp collect period, in seconds (default: 60)
--log value specify a log file to output, default goes to stderr
--quiet to suppress the 'stream open/close' messages
-c value config from json file, which will override the command from shell
--help, -h show help
--version, -v print the version
$ ./server_darwin_amd64 -h
NAME:
kcptun - server(with SMUX)
USAGE:
server_darwin_amd64 [global options] command [command options] [arguments...]
VERSION:
20180922
COMMANDS:
help, h Shows a list of commands or help for one command
GLOBAL OPTIONS:
--listen value, -l value kcp server listen address (default: ":29900")
--target value, -t value target server address (default: "127.0.0.1:12948")
--key value pre-shared secret between client and server (default: "it's a secrect") [$KCPTUN_KEY]
--crypt value aes, aes-128, aes-192, salsa20, blowfish, twofish, cast5, 3des, tea, xtea, xor, sm4, none (default: "aes")
--mode value profiles: fast3, fast2, fast, normal, manual (default: "fast")
--mtu value set maximum transmission unit for UDP packets (default: 1350)
--sndwnd value set send window size(num of packets) (default: 1024)
--rcvwnd value set receive window size(num of packets) (default: 1024)
--datashard value, --ds value set reed-solomon erasure coding - datashard (default: 10)
--parityshard value, --ps value set reed-solomon erasure coding - parityshard (default: 3)
--dscp value set DSCP(6bit) (default: 0)
--nocomp disable compression
--sockbuf value (default: 4194304)
--keepalive value (default: 10)
--snmplog value collect snmp to file, aware of timeformat in golang, like: ./snmp-20060102.log
--snmpperiod value snmp collect period, in seconds (default: 60)
--pprof start profiling server on :6060
--log value specify a log file to output, default goes to stderr
--quiet to suppress the 'stream open/close' messages
-c value config from json file, which will override the command from shell
--help, -h show help
--version, -v print the version
```
#### Forward Error Correction
In coding theory, the [Reed–Solomon code](https://en.wikipedia.org/wiki/Reed%E2%80%93Solomon_error_correction) belongs to the class of non-binary cyclic error-correcting codes. The Reed–Solomon code is based on univariate polynomials over finite fields.
It is able to detect and correct multiple symbol errors. By adding t check symbols to the data, a Reed–Solomon code can detect any combination of up to t erroneous symbols, or correct up to ⌊t/2⌋ symbols. As an erasure code, it can correct up to t known erasures, or it can detect and correct combinations of errors and erasures. Furthermore, Reed–Solomon codes are suitable as multiple-burst bit-error correcting codes, since a sequence of b + 1 consecutive bit errors can affect at most two symbols of size b. The choice of t is up to the designer of the code, and may be selected within wide limits.
Setting parameters of RS-Code with ```-datashard m -parityshard n``` on **BOTH** KCP Client & KCP Server **MUST** be **IDENTICAL**.
#### DSCP
Differentiated services or DiffServ is a computer networking architecture that specifies a simple, scalable and coarse-grained mechanism for classifying and managing network traffic and providing quality of service (QoS) on modern IP networks. DiffServ can, for example, be used to provide low-latency to critical network traffic such as voice or streaming media while providing simple best-effort service to non-critical services such as web traffic or file transfers.
DiffServ uses a 6-bit differentiated services code point (DSCP) in the 8-bit differentiated services field (DS field) in the IP header for packet classification purposes. The DS field and ECN field replace the outdated IPv4 TOS field.
setting each side with ```-dscp value```, Here are some [Commonly used DSCP values](https://en.wikipedia.org/wiki/Differentiated_services#Commonly_used_DSCP_values).
#### Cryptanalysis
kcptun is shipped with builtin packet encryption powered by various block encryption algorithms and works in [Cipher Feedback Mode](https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Cipher_Feedback_(CFB)), for each packet to be sent, the encryption process will start from encrypting a [nonce](https://en.wikipedia.org/wiki/Cryptographic_nonce) from the [system entropy](https://en.wikipedia.org/wiki//dev/random), so encryption to same plaintexts never leads to a same ciphertexts thereafter.
The contents of the packets are completely anonymous with encryption, including the headers(FEC,KCP), checksums and contents. Note that, no matter which encryption method you choose on you upper layer, if you disable encryption by specifying `-crypt none` to kcptun, the transmit will be insecure somehow, since the header is ***PLAINTEXT*** to everyone it would be susceptible to header tampering, such as jamming the *sliding window size*, *round-trip time*, *FEC property* and *checksums*. ```aes-128``` is suggested for minimal encryption since modern CPUs are shipped with [AES-NI](https://en.wikipedia.org/wiki/AES_instruction_set) instructions and performs even better than `salsa20`(check the table below).
Other possible attacks to kcptun includes: a) [traffic analysis](https://en.wikipedia.org/wiki/Traffic_analysis), dataflow on specific websites may have pattern while interchanging data, but this type of eavesdropping has been mitigated by adapting [smux](https://github.com/xtaci/smux) to mix data streams so as to introduce noises, perfect solution to this has not appeared yet, theroretically by shuffling/mixing messages on larger scale network may mitigate this problem. b) [replay attack](https://en.wikipedia.org/wiki/Replay_attack), since the asymmetrical encryption has not been introduced into kcptun for some reason, capturing the packets and replay them on a different machine is possible, (notice: hijacking the session and decrypting the contents is still *impossible*), so upper layers should contain a asymmetrical encryption system to guarantee the authenticity of each message(to process message exactly once), such as HTTPS/OpenSSL/LibreSSL, only by signing the requests with private keys can eliminate this type of attack.
Important:
1. `-crypt` and `-key` must be the same on both KCP Client & KCP Server.
2. `-crypt xor` is also insecure and vulnerable to [known-plaintext attack](https://en.wikipedia.org/wiki/Known-plaintext_attack), do not use this unless you know what you are doing. (*cryptanalysis note: any type of [counter mode](https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Counter_(CTR)) is insecure in packet encryption due to the shorten of counter period and leads to iv/nonce collision*)
Benchmarks for crypto algorithms supported by kcptun:
```
BenchmarkSM4-4 50000 32087 ns/op 93.49 MB/s 0 B/op 0 allocs/op
BenchmarkAES128-4 500000 3274 ns/op 916.15 MB/s 0 B/op 0 allocs/op
BenchmarkAES192-4 500000 3587 ns/op 836.34 MB/s 0 B/op 0 allocs/op
BenchmarkAES256-4 300000 3828 ns/op 783.60 MB/s 0 B/op 0 allocs/op
BenchmarkTEA-4 100000 15359 ns/op 195.32 MB/s 0 B/op 0 allocs/op
BenchmarkXOR-4 20000000 90.2 ns/op 33249.02 MB/s 0 B/op 0 allocs/op
BenchmarkBlowfish-4 50000 26885 ns/op 111.58 MB/s 0 B/op 0 allocs/op
BenchmarkNone-4 30000000 45.8 ns/op 65557.11 MB/s 0 B/op 0 allocs/op
BenchmarkCast5-4 50000 34370 ns/op 87.29 MB/s 0 B/op 0 allocs/op
Benchmark3DES-4 10000 117893 ns/op 25.45 MB/s 0 B/op 0 allocs/op
BenchmarkTwofish-4 50000 33477 ns/op 89.61 MB/s 0 B/op 0 allocs/op
BenchmarkXTEA-4 30000 45825 ns/op 65.47 MB/s 0 B/op 0 allocs/op
BenchmarkSalsa20-4 500000 3282 ns/op 913.90 MB/s 0 B/op 0 allocs/op
```
Benchmark result from openssl
```
$ openssl speed -evp aes-128-cfb
Doing aes-128-cfb for 3s on 16 size blocks: 157794127 aes-128-cfb's in 2.98s
Doing aes-128-cfb for 3s on 64 size blocks: 39614018 aes-128-cfb's in 2.98s
Doing aes-128-cfb for 3s on 256 size blocks: 9971090 aes-128-cfb's in 2.99s
Doing aes-128-cfb for 3s on 1024 size blocks: 2510877 aes-128-cfb's in 2.99s
Doing aes-128-cfb for 3s on 8192 size blocks: 310865 aes-128-cfb's in 2.98s
OpenSSL 1.0.2p 14 Aug 2018
built on: reproducible build, date unspecified
options:bn(64,64) rc4(ptr,int) des(idx,cisc,16,int) aes(partial) idea(int) blowfish(idx)
compiler: clang -I. -I.. -I../include -fPIC -fno-common -DOPENSSL_PIC -DOPENSSL_THREADS -D_REENTRANT -DDSO_DLFCN -DHAVE_DLFCN_H -arch x86_64 -O3 -DL_ENDIAN -Wall -DOPENSSL_IA32_SSE2 -DOPENSSL_BN_ASM_MONT -DOPENSSL_BN_ASM_MONT5 -DOPENSSL_BN_ASM_GF2m -DSHA1_ASM -DSHA256_ASM -DSHA512_ASM -DMD5_ASM -DAES_ASM -DVPAES_ASM -DBSAES_ASM -DWHIRLPOOL_ASM -DGHASH_ASM -DECP_NISTZ256_ASM
The 'numbers' are in 1000s of bytes per second processed.
type 16 bytes 64 bytes 256 bytes 1024 bytes 8192 bytes
aes-128-cfb 847216.79k 850770.86k 853712.05k 859912.39k 854565.80k
```
The encrytion performance in kcptun is as fast as in openssl library(if not faster).
#### Memory Usage Control
Routers, mobile devices are susceptible to memory consumption; by setting GOGC environment(eg: GOGC=20) will make the garbage collector to recycle faster.
Reference: https://blog.golang.org/go15gc
#### Compression
kcptun has builtin snappy algorithms for compressing streams:
> Snappy is a compression/decompression library. It does not aim for maximum
> compression, or compatibility with any other compression library; instead,
> it aims for very high speeds and reasonable compression. For instance,
> compared to the fastest mode of zlib, Snappy is an order of magnitude faster
> for most inputs, but the resulting compressed files are anywhere from 20% to
> 100% bigger.
> Reference: http://google.github.io/snappy/
Compression may save bandwidth for **PLAINTEXT** data, it's quite useful for specific scenarios as cross-datacenter replications, by compressing the redologs in dbms or kafka-like message queues and then transfer the data streams across the continent can be much faster.
Compression is enabled by default, you can disable it by setting ```-nocomp``` on **BOTH** KCP Client & KCP Server **MUST** be **IDENTICAL**.
#### SNMP
```go
// Snmp defines network statistics indicator
type Snmp struct {
BytesSent uint64 // raw bytes sent
BytesReceived uint64
MaxConn uint64
ActiveOpens uint64
PassiveOpens uint64
CurrEstab uint64 // count of connections for now
InErrs uint64 // udp read errors
InCsumErrors uint64 // checksum errors from CRC32
KCPInErrors uint64 // packet iput errors from kcp
InSegs uint64
OutSegs uint64
InBytes uint64 // udp bytes received
OutBytes uint64 // udp bytes sent
RetransSegs uint64
FastRetransSegs uint64
EarlyRetransSegs uint64
LostSegs uint64 // number of segs infered as lost
RepeatSegs uint64 // number of segs duplicated
FECRecovered uint64 // correct packets recovered from FEC
FECErrs uint64 // incorrect packets recovered from FEC
FECSegs uint64 // FEC segments received
FECShortShards uint64 // number of data shards that's not enough for recovery
}
```
Sending a `SIGUSR1` signal to KCP Client or KCP Server will dump SNMP information to console, just like `/proc/net/snmp`. You can use this information to do fine-grained tuning.
### Manual Control
https://github.com/skywind3000/kcp/blob/master/README.en.md#protocol-configuration
`-mode manual -nodelay 1 -interval 20 -resend 2 -nc 1`
Low-level KCP configuration can be altered by using manual mode like above, make sure you really **UNDERSTAND** what these means before doing **ANY** manual settings.
### Identical Parmeters
The parameters below **MUST** be **IDENTICAL** on **BOTH** side:
1. -key
1. -crypt
1. -nocomp
1. -datashard
1. -parityshard
### References
1. https://github.com/skywind3000/kcp -- KCP - A Fast and Reliable ARQ Protocol.
1. https://github.com/xtaci/kcp-go/ -- A Production-Grade Reliable-UDP Library for golang
1. https://github.com/klauspost/reedsolomon -- Reed-Solomon Erasure Coding in Go.
1. https://en.wikipedia.org/wiki/Differentiated_services -- DSCP.
1. http://google.github.io/snappy/ -- A fast compressor/decompressor.
1. https://www.backblaze.com/blog/reed-solomon/ -- Reed-Solomon Explained.
1. http://www.qualcomm.cn/products/raptorq -- RaptorQ Forward Error Correction Scheme for Object Delivery.
1. https://en.wikipedia.org/wiki/PBKDF2 -- Key stretching.
1. http://blog.appcanary.com/2016/encrypt-or-compress.html -- Should you encrypt or compress first?
1. https://github.com/hashicorp/yamux -- Connection multiplexing library.
1. https://tools.ietf.org/html/rfc6937 -- Proportional Rate Reduction for TCP.
1. https://tools.ietf.org/html/rfc5827 -- Early Retransmit for TCP and Stream Control Transmission Protocol (SCTP).
1. http://http2.github.io/ -- What is HTTP/2?
1. http://www.lartc.org/ -- Linux Advanced Routing & Traffic Control
1. https://en.wikipedia.org/wiki/Noisy-channel_coding_theorem -- Noisy channel coding theorem
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