原版kcptun使用UDP传输层，大流量容易被ISP封停(只能重启拨号或等待一段时间才能恢复).
kcptun(tcp)参考抄袭finalspeed, 用libpcap构造伪TCP包，替代UDP传输.

由于使用pcap发包，需要root权限
windows用户需下载wincap
kcptun(tcp)与原版参数一致，只是多了2个参数：

--tcp                            开启tcp模式，否则和原版一致
--interface value, -i value      指定network interface, 如eth0, 如果没有指定, 启动后手动选择.

注意：客户端只能开一个连接，即--conn 1

安装pcap Developer's Pack
windows: 下载Wincap Developer's Pack
ubuntu: sudo apt-get install libpcap-dev
或者自行编译libpcap: http://www.tcpdump.org/

kcptun(tcp) 启动时会修改防火墙规则, 详见tcp.go

Download precompiled Releases.

KCP Client: ./client_darwin_amd64 -r "KCP_SERVER_IP:4000" -l ":8388" -mode fast2
KCP Server: ./server_linux_amd64 -t "TARGET_IP:8388" -l ":4000" -mode fast2

The above commands will establish port forwarding for 8388/tcp as:

Application -> KCP Client(8388/tcp) -> KCP Server(4000/udp) -> Server(8388/tcp)

$go get -u github.com/xtaci/kcptun/client
$go get -u github.com/xtaci/kcptun/server

All precompiled releases are genereated from build-release.sh script.

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.

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

$ ./client_darwin_amd64 -h
NAME:
   kcptun - client(with SMUX)

USAGE:
   client_darwin_amd64 [global options] command [command options] [arguments...]

VERSION:
   20170120

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, none (default: "aes")
   --mode value                     profiles: fast3, fast2, fast, normal (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)
   --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
   --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
   -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:
   20170120

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, none (default: "aes")
   --mode value                     profiles: fast3, fast2, fast, normal (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
   --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
   -c value                         config from json file, which will override the command from shell
   --help, -h                       show help
   --version, -v                    print the version

In coding theory, the Reed–Solomon code 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.

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.

No matter what encryption you are using for application layer, if you specify -crypt none to kcptun, the header will be PLAINTEXT to everyone; I suggest -crypt aes-128 for encryption at least .

-crypt and -key must be the same on both KCP Client & KCP Server.

NOTICE: -crypt xor is also insecure, do not use this unless you know what you are doing.

Benchmarks for crypto algorithms supported by kcptun:

BenchmarkAES128-4      	  200000	     11182 ns/op
BenchmarkAES192-4      	  200000	     12699 ns/op
BenchmarkAES256-4      	  100000	     13757 ns/op
BenchmarkTEA-4         	   50000	     26441 ns/op
BenchmarkSimpleXOR-4   	 3000000	       441 ns/op
BenchmarkBlowfish-4    	   30000	     48036 ns/op
BenchmarkNone-4        	20000000	       106 ns/op
BenchmarkCast5-4       	   20000	     60222 ns/op
BenchmarkTripleDES-4   	    2000	    878759 ns/op
BenchmarkTwofish-4     	   20000	     68501 ns/op
BenchmarkXTEA-4        	   20000	     77417 ns/op
BenchmarkSalsa20-4     	  300000	      4998 ns/op

Routers, mobile devices are sensitive to memory consumption; by setting GOGC environment(eg: GOGC=20) will lower memory consumption. Reference: https://blog.golang.org/go15gc

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, such as HTTP data.

Compression is enabled by default, you can disable it by setting -nocomp on both KCP Client & KCP Server.

// 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.

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.

1. https://github.com/skywind3000/kcp -- KCP - A Fast and Reliable ARQ Protocol.
2. https://github.com/xtaci/kcp-go/ -- A Production-Grade Reliable-UDP Library for golang
3. https://github.com/klauspost/reedsolomon -- Reed-Solomon Erasure Coding in Go.
4. https://en.wikipedia.org/wiki/Differentiated_services -- DSCP.
5. http://google.github.io/snappy/ -- A fast compressor/decompressor.
6. https://www.backblaze.com/blog/reed-solomon/ -- Reed-Solomon Explained.
7. http://www.qualcomm.cn/products/raptorq -- RaptorQ Forward Error Correction Scheme for Object Delivery.
8. https://en.wikipedia.org/wiki/PBKDF2 -- Key stretching.
9. http://blog.appcanary.com/2016/encrypt-or-compress.html -- Should you encrypt or compress first?
10. https://github.com/hashicorp/yamux -- Connection multiplexing library.
11. https://tools.ietf.org/html/rfc6937 -- Proportional Rate Reduction for TCP.
12. https://tools.ietf.org/html/rfc5827 -- Early Retransmit for TCP and Stream Control Transmission Protocol (SCTP).
13. http://http2.github.io/ -- What is HTTP/2?
14. http://www.lartc.org/ -- Linux Advanced Routing & Traffic Control
15. https://en.wikipedia.org/wiki/Noisy-channel_coding_theorem -- Noisy channel coding theorem