tcp_fast_open的概念 作用以及实现（转）

引言

三次握手的过程中，当用户首次访问server时，发送syn包，server根据用户IP生成cookie，并与syn+ack一同发回client；client再次访问server时，在syn包携带TCP cookie；如果server校验合法，则在用户回复ack前就可以直接发送数据；否则按照正常三次握手进行。

TFO提高性能的关键是省去了热请求的三次握手，这在充斥着小对象的移动应用场景中能够极大提升性能。

Google研究发现TCP 二次握手是页面延迟时间的重要部分，所以提出TFO

TFO的fast open标志体现在TCP报文的头部的OPTION字段

TCP Fast Open的标准文档是rfc7413

TFO与2.6.34内核合并到主线，lwn通告地址

TFO的使用目前还是有些复杂的，从linux的network文档来看：

TFO的配置说明：

tcp_fastopen - INTEGER
	Enable TCP Fast Open feature (draft-ietf-tcpm-fastopen) to send data
	in the opening SYN packet. To use this feature, the client application must use sendmsg() or sendto() with MSG_FASTOPEN flag rather than connect() to perform a TCP handshake automatically. The values (bitmap) are 1: Enables sending data in the opening SYN on the client w/ MSG_FASTOPEN. 2: Enables TCP Fast Open on the server side, i.e., allowing data in a SYN packet to be accepted and passed to the application before 3-way hand shake finishes. 4: Send data in the opening SYN regardless of cookie availability and without a cookie option. 0x100: Accept SYN data w/o validating the cookie. 0x200: Accept data-in-SYN w/o any cookie option present. 0x400/0x800: Enable Fast Open on all listeners regardless of the TCP_FASTOPEN socket option. The two different flags designate two different ways of setting max_qlen without the TCP_FASTOPEN socket option. Default: 1 Note that the client & server side Fast Open flags (1 and 2 respectively) must be also enabled before the rest of flags can take effect. See include/net/tcp.h and the code for more details. 

为了启用 tcp fast open功能

- client需要使用sendmsg或者sento系统调用，加上MSG_FASTOPEN flag，来连接server端，代替connect系统调用。 - 对server端不做要求。 

linux系统（高版本内核）默认tcp_fastopen为1：

$ sysctl -a | grep fastopen net.ipv4.tcp_fastopen = 1 

测试代码： server.c

// reference: http://blog.csdn.net/hanhuili/article/details/8540227 #include <string.h> #include <sys/types.h> /* See NOTES */ #include <sys/socket.h> #include <netinet/in.h> int main(){ int portno = 6666; socklen_t clilen; char buffer[256]; struct sockaddr_in serv_addr, cli_addr; int cfd; int sfd = socket(AF_INET, SOCK_STREAM, 0); // Create socket bzero((char *) &serv_addr, sizeof(serv_addr)); serv_addr.sin_family = AF_INET; serv_addr.sin_addr.s_addr = INADDR_ANY; serv_addr.sin_port = htons(portno); bind(sfd, &serv_addr,sizeof(serv_addr)); // Bind to well known address int qlen = 5; // Value to be chosen by application int err = setsockopt(sfd, IPPROTO_TCP/*SOL_TCP*/, 23/*TCP_FASTOPEN*/, &qlen, sizeof(qlen)); listen(sfd,1); // Mark socket to receive connections while(1){ cfd = accept(sfd, NULL, 0); // Accept connection on new socket while(1){ int len = read(cfd,buffer,256); if(len) printf("tcp fast open: %s\n",buffer); else break; // read and write data on connected socket cfd } memset(buffer, 0, 256); close(cfd); } } 

测试代码：client.c

#include <sys/socket.h> #include <netinet/in.h> #include <netdb.h> #include <errno.h> int main(){ struct sockaddr_in serv_addr; struct hostent *server; char *data = "Hello, tcp fast open"; int data_len = strlen(data); int sfd = socket(AF_INET, SOCK_STREAM, 0); server = gethostbyname("localhost"); bzero((char *) &serv_addr, sizeof(serv_addr)); serv_addr.sin_family = AF_INET; bcopy((char *)server->h_addr, (char *)&serv_addr.sin_addr.s_addr, server->h_length); serv_addr.sin_port = htons(6666); // /usr/src/linux-headers-4.4.0-22/include/linux/socket.h:#define MSG_FASTOPEN 0x20000000 /* Send data in TCP SYN */ // int len = sendto(sfd, data, data_len, 0x20000000/*MSG_FASTOPEN*/, int len = sendto(sfd, data, data_len, MSG_FASTOPEN/*MSG_FASTOPEN*/, (struct sockaddr *) &serv_addr, sizeof(serv_addr)); if(errno != 0){ printf("error: %s\n", strerror(errno)); } close(sfd); } 

通信过程：tcpdump

$ sudo tcpdump -i any port 6666 -X # 第一次 ./client.o 00:29:34.820187 IP localhost.51388 > localhost.6666: Flags [S], seq 755101042, win 43690, options [mss 65495,sackOK,TS val 17053 ecr 0,nop,wscale 7,unknown-34,nop,nop], length 0 0x0000: 4500 0040 afef 4000 4006 8cc6 7f00 0001 E..@..@.@....... 0x0010: 7f00 0001 c8bc 1a0a 2d01 ed72 0000 0000 ........-..r.... 0x0020: b002 aaaa fe34 0000 0204 ffd7 0402 080a .....4.......... 0x0030: 0000 429d 0000 0000 0103 0307 2202 0101 ..B........."... 00:29:34.820284 IP localhost.6666 > localhost.51388: Flags [S.], seq 3725111481, ack 755101043, win 43690, options [mss 65495,sackOK,TS val 17053 ecr 17053,nop,wscale 7], length 0 0x0000: 4500 003c 0000 4000 4006 3cba 7f00 0001 E..<..@.@.<..... 0x0010: 7f00 0001 1a0a c8bc de08 b0b9 2d01 ed73 ............-..s 0x0020: a012 aaaa fe30 0000 0204 ffd7 0402 080a .....0.......... 0x0030: 0000 429d 0000 429d 0103 0307 ..B...B..... 00:29:34.820372 IP localhost.51388 > localhost.6666: Flags [P.], seq 1:21, ack 1, win 342, options [nop,nop,TS val 17053 ecr 17053], length 20 0x0000: 4500 0048 aff0 4000 4006 8cbd 7f00 0001 E..H..@.@....... 0x0010: 7f00 0001 c8bc 1a0a 2d01 ed73 de08 b0ba ........-..s.... 0x0020: 8018 0156 fe3c 0000 0101 080a 0000 429d ...V.<........B. 0x0030: 0000 429d 4865 6c6c 6f2c 2074 6370 2066 ..B.Hello,.tcp.f 0x0040: 6173 7420 6f70 656e ast.open 00:29:34.820433 IP localhost.6666 > localhost.51388: Flags [.], ack 21, win 342, options [nop,nop,TS val 17053 ecr 17053], length 0 0x0000: 4500 0034 f227 4000 4006 4a9a 7f00 0001 E..4.'@.@.J..... 0x0010: 7f00 0001 1a0a c8bc de08 b0ba 2d01 ed87 ............-... 0x0020: 8010 0156 fe28 0000 0101 080a 0000 429d ...V.(........B. 0x0030: 0000 429d ..B. 00:29:34.859246 IP localhost.6666 > localhost.51388: Flags [.], ack 22, win 342, options [nop,nop,TS val 17063 ecr 17053], length 0 0x0000: 4500 0034 f228 4000 4006 4a99 7f00 0001 E..4.(@.@.J..... 0x0010: 7f00 0001 1a0a c8bc de08 b0ba 2d01 ed88 ............-... 0x0020: 8010 0156 fe28 0000 0101 080a 0000 42a7 ...V.(........B. 0x0030: 0000 429d ..B. # 第二次 ./client.o 00:29:39.271936 IP localhost.51398 > localhost.6666: Flags [S], seq 2362540136, win 43690, options [mss 65495,sackOK,TS val 18166 ecr 0,nop,wscale 7,exp-tfo cookiereq], length 0 0x0000: 4500 0040 c69e 4000 4006 7617 7f00 0001 E..@..@.@.v..... 0x0010: 7f00 0001 c8c6 1a0a 8cd1 8068 0000 0000 ...........h.... 0x0020: b002 aaaa fe34 0000 0204 ffd7 0402 080a .....4.......... 0x0030: 0000 46f6 0000 0000 0103 0307 fe04 f989 ..F............. 00:29:39.271986 IP localhost.6666 > localhost.51398: Flags [S.], seq 3703577773, ack 2362540137, win 43690, options [mss 65495,sackOK,TS val 18166 ecr 18166,nop,wscale 7], length 0 0x0000: 4500 003c 0000 4000 4006 3cba 7f00 0001 E..<..@.@.<..... 0x0010: 7f00 0001 1a0a c8c6 dcc0 1cad 8cd1 8069 ...............i 0x0020: a012 aaaa fe30 0000 0204 ffd7 0402 080a .....0.......... 0x0030: 0000 46f6 0000 46f6 0103 0307 ..F...F..... 00:29:39.272038 IP localhost.51398 > localhost.6666: Flags [P.], seq 1:21, ack 1, win 342, options [nop,nop,TS val 18166 ecr 18166], length 20 0x0000: 4500 0048 c69f 4000 4006 760e 7f00 0001 E..H..@.@.v..... 0x0010: 7f00 0001 c8c6 1a0a 8cd1 8069 dcc0 1cae ...........i.... 0x0020: 8018 0156 fe3c 0000 0101 080a 0000 46f6 ...V.<........F. 0x0030: 0000 46f6 4865 6c6c 6f2c 2074 6370 2066 ..F.Hello,.tcp.f 0x0040: 6173 7420 6f70 656e ast.open 00:29:39.272072 IP localhost.6666 > localhost.51398: Flags [.], ack 21, win 342, options [nop,nop,TS val 18166 ecr 18166], length 0 0x0000: 4500 0034 5a58 4000 4006 e269 7f00 0001 E..4ZX@.@..i.... 0x0010: 7f00 0001 1a0a c8c6 dcc0 1cae 8cd1 807d ...............} 0x0020: 8010 0156 fe28 0000 0101 080a 0000 46f6 ...V.(........F. 0x0030: 0000 46f6 ..F. 00:29:39.311280 IP localhost.6666 > localhost.51398: Flags [.], ack 22, win 342, options [nop,nop,TS val 18176 ecr 18166], length 0 0x0000: 4500 0034 5a59 4000 4006 e268 7f00 0001 E..4ZY@.@..h.... 0x0010: 7f00 0001 1a0a c8c6 dcc0 1cae 8cd1 807e ...............~ 0x0020: 8010 0156 fe28 0000 0101 080a 0000 4700 ...V.(........G. 0x0030: 0000 46f6 ..F. 

奇怪的是，在代码中启用tcp_fastopen的结果和不启用，并没有区别。那这是什么原因呢？

通过搜索，发现在介绍tcp fast open优化shadowsocks时，设置net.ipv4.tcp_fastopen为3，虽然奇怪，但是可以试试：

$ sysctl -a | grep fastopen net.ipv4.tcp_fastopen = 3 

# 第一次，server返回cookie unknown-34 0x38af51c10bf41ca4 00:36:36.667932 IP localhost.52220 > localhost.6666: Flags [S], seq 3662514892, win 43690, options [mss 65495,sackOK,TS val 122515 ecr 0,nop,wscale 7,unknown-34,nop,nop], length 0 0x0000: 4500 0040 545f 4000 4006 e856 7f00 0001 E..@T_@.@..V.... 0x0010: 7f00 0001 cbfc 1a0a da4d 8acc 0000 0000 .........M...... 0x0020: b002 aaaa fe34 0000 0204 ffd7 0402 080a .....4.......... 0x0030: 0001 de93 0000 0000 0103 0307 2202 0101 ............"... 00:36:36.667990 IP localhost.6666 > localhost.52220: Flags [S.], seq 3186866007, ack 3662514893, win 43690, options [mss 65495,sackOK,TS val 122515 ecr 122515,nop,wscale 7,unknown-34 0x38af51c10bf41ca4,nop,nop], length 0 0x0000: 4500 0048 0000 4000 4006 3cae 7f00 0001 E..H..@.@.<..... 0x0010: 7f00 0001 1a0a cbfc bdf3 b757 da4d 8acd ...........W.M.. 0x0020: d012 aaaa fe3c 0000 0204 ffd7 0402 080a .....<.......... 0x0030: 0001 de93 0001 de93 0103 0307 220a 38af ............".8. 0x0040: 51c1 0bf4 1ca4 0101 Q....... 00:36:36.668050 IP localhost.52220 > localhost.6666: Flags [P.], seq 1:21, ack 1, win 342, options [nop,nop,TS val 122515 ecr 122515], length 20 0x0000: 4500 0048 5460 4000 4006 e84d 7f00 0001 E..HT`@.@..M.... 0x0010: 7f00 0001 cbfc 1a0a da4d 8acd bdf3 b758 .........M.....X 0x0020: 8018 0156 fe3c 0000 0101 080a 0001 de93 ...V.<.......... 0x0030: 0001 de93 4865 6c6c 6f2c 2074 6370 2066 ....Hello,.tcp.f 0x0040: 6173 7420 6f70 656e ast.open 00:36:36.668109 IP localhost.6666 > localhost.52220: Flags [.], ack 21, win 342, options [nop,nop,TS val 122515 ecr 122515], length 0 0x0000: 4500 0034 69cb 4000 4006 d2f6 7f00 0001 E..4i.@.@....... 0x0010: 7f00 0001 1a0a cbfc bdf3 b758 da4d 8ae1 ...........X.M.. 0x0020: 8010 0156 fe28 0000 0101 080a 0001 de93 ...V.(.......... 0x0030: 0001 de93 .... 00:36:36.707264 IP localhost.6666 > localhost.52220: Flags [.], ack 22, win 342, options [nop,nop,TS val 122525 ecr 122515], length 0 0x0000: 4500 0034 69cc 4000 4006 d2f5 7f00 0001 E..4i.@.@....... 0x0010: 7f00 0001 1a0a cbfc bdf3 b758 da4d 8ae2 ...........X.M.. 0x0020: 8010 0156 fe28 0000 0101 080a 0001 de9d ...V.(.......... 0x0030: 0001 de93 .... # 第二次，client发送请求时，将cookie写在syn包中，同时带上发送的数据；server端校验后(kernel和tcp/ip协议栈做校验)后返回成功，如此在3次握手中节省了一次rtt时间 00:36:38.744954 IP localhost.52226 > localhost.6666: Flags [S], seq 1820632025:1820632045, win 43690, options [mss 65495,sackOK,TS val 123034 ecr 0,nop,wscale 7,unknown-34 0x38af51c10bf41ca4,nop,nop], length 20 0x0000: 4500 005c 4343 4000 4006 f956 7f00 0001 E..\CC@.@..V.... 0x0010: 7f00 0001 cc02 1a0a 6c84 a3d9 0000 0000 ........l....... 0x0020: d002 aaaa fe50 0000 0204 ffd7 0402 080a .....P.......... 0x0030: 0001 e09a 0000 0000 0103 0307 220a 38af ............".8. 0x0040: 51c1 0bf4 1ca4 0101 4865 6c6c 6f2c 2074 Q.......Hello,.t 0x0050: 6370 2066 6173 7420 6f70 656e cp.fast.open 00:36:38.745022 IP localhost.6666 > localhost.52226: Flags [S.], seq 3848342665, ack 1820632046, win 43690, options [mss 65495,sackOK,TS val 123034 ecr 123034,nop,wscale 7], length 0 0x0000: 4500 003c 0000 4000 4006 3cba 7f00 0001 E..<..@.@.<..... 0x0010: 7f00 0001 1a0a cc02 e561 0c89 6c84 a3ee .........a..l... 0x0020: a012 aaaa fe30 0000 0204 ffd7 0402 080a .....0.......... 0x0030: 0001 e09a 0001 e09a 0103 0307 ............ 00:36:38.745072 IP localhost.52226 > localhost.6666: Flags [.], ack 1, win 342, options [nop,nop,TS val 123034 ecr 123034], length 0 0x0000: 4500 0034 4344 4000 4006 f97d 7f00 0001 E..4CD@.@..}.... 0x0010: 7f00 0001 cc02 1a0a 6c84 a3ee e561 0c8a ........l....a.. 0x0020: 8010 0156 fe28 0000 0101 080a 0001 e09a ...V.(.......... 0x0030: 0001 e09a .... 00:36:38.745127 IP localhost.52226 > localhost.6666: Flags [F.], seq 1, ack 1, win 342, options [nop,nop,TS val 123034 ecr 123034], length 0 0x0000: 4500 0034 4345 4000 4006 f97c 7f00 0001 E..4CE@.@..|.... 0x0010: 7f00 0001 cc02 1a0a 6c84 a3ee e561 0c8a ........l....a.. 0x0020: 8011 0156 fe28 0000 0101 080a 0001 e09a ...V.(.......... 0x0030: 0001 e09a .... 00:36:38.747232 IP localhost.6666 > localhost.52226: Flags [.], ack 2, win 342, options [nop,nop,TS val 123035 ecr 123034], length 0 0x0000: 4500 0034 ec10 4000 4006 50b1 7f00 0001 E..4..@.@.P..... 0x0010: 7f00 0001 1a0a cc02 e561 0c8a 6c84 a3ef .........a..l... 0x0020: 8010 0156 fe28 0000 0101 080a 0001 e09b ...V.(.......... 0x0030: 0001 e09a .... 

• 上述通信过程中
  • 第一次，server返回cookie unknown-34 0x38af51c10bf41ca4
  • 第二次，client发送请求时，将cookie写在syn包中，同时带上发送的数据；server端校验后(kernel和tcp/ip协议栈做校验)后返回成功，如此在3次握手中节省了一次rtt时间
• 也就是说，在net.ipv4.tcp_fastopen设置为3时，tcp fastopen特性使能

关于如何使能TFO，在前文中的TFO的配置说明中，我们可以看到，

	The values (bitmap) are
	1: Enables sending data in the opening SYN on the client w/ MSG_FASTOPEN. 使能client端的TFO特性 2: Enables TCP Fast Open on the server side, i.e., allowing data in a SYN packet to be accepted and passed to the application before 3-way hand shake finishes. 使能server端的TFO特性 4: Send data in the opening SYN regardless of cookie availability and without a cookie option. 

并且这个标志是位操作，如果我在本机做实验，将本机作为sever端和client端的话，需要两个位都使能，所以应该将该值设置为3.

同时我们可以看到，tcp fast open是非常向后兼容的，升级成本不高，需要高于3.7+版本内核，但总体来说值得采用。

nginx 1.5.18（2013年）开始支持tcp fast open

TODO LIST

• TFO在移动端场景中的性能体现：android+nginx
• tcp fast open 在内核中的实现

参考链接

• TFO—google tcp fast open protocol
• wikipedia
• TFO简介
• tfo的golang实现(github)
• 上一行项目的作者bradley falzon
• google关于tfo的论文