[网络协议]websocket简单应用

websocket主要用于浏览器与服务器之间建立长连接，服务器需要主动给浏览器发送数据时。当然，这种情况，websocket不是唯一解决方案。websocket基本流程如下：

这里注意，在①②中，客户端与服务器建立连接请求网页数据和服务器返回网页数据时，还是使用的http协议，从③开始，才是使用websocket。
websocket重点由两部分组成。

握手

websocket握手用于检验连接数据是否合法，主要流程是浏览器发送一个随机base64编码的值，服务端接收后加上一段全局变量，再进行SHA-1和base64编码返回给客户端。客户端检验，正确后建立连接。

int readline(char* allbuf,int level,char* linebuf) 
{    
	int len = strlen(allbuf);    
	for (;level < len; ++level)    //level相当于下标
{        
		if(allbuf[level]=='\r' && allbuf[level+1]=='\n')            
			return level+2;        
		else            
			*(linebuf++) = allbuf[level];    
	}    
	return -1;
}

int handshark(struct sockitem *si, struct reactor *mainloop) 
{
    //si是每个socket独有的参数，包括接收和发送数据的缓冲区。reactor是全局变量，存储epoll相关数据。
	char linebuf[256];
	char sec_accept[32]; 
	int level = 0;
	unsigned char sha1_data[SHA_DIGEST_LENGTH+1] = {0};
	char head[BUFFER_LENGTH] = {0};  //宏定义
	do 
{        
		memset(linebuf, 0, sizeof(linebuf));        
		level = readline(si->recvbuffer, level, linebuf); 
		if (strstr(linebuf,"Sec-WebSocket-Key") != NULL)        
        {   
			strcat(linebuf, GUID);    
			SHA1((unsigned char*)&linebuf+19,strlen(linebuf+19),(unsigned char*)&sha1_data);  
			base64_encode(sha1_data,strlen(sha1_data),sec_accept);           
			sprintf(head, "HTTP/1.1 101 Switching Protocols\r\n" \
				"Upgrade: websocket\r\n" \
				"Connection: Upgrade\r\n" \
				"Sec-WebSocket-Accept: %s\r\n" \
				"\r\n", sec_accept);            
			printf("response\n");            
			printf("%s\n\n\n", head);            
			memset(si->recvbuffer, 0, BUFFER_LENGTH);
			memcpy(si->sendbuffer, head, strlen(head)); // to send 
			si->slength = strlen(head);
			// to set epollout events
			struct epoll_event ev;
			ev.events = EPOLLOUT | EPOLLET;
			//ev.data.fd = clientfd;
			si->sockfd = si->sockfd;
			si->callback = send_cb;
			si->status = WS_DATATRANSFORM;
			ev.data.ptr = si;
			epoll_ctl(mainloop->epfd, EPOLL_CTL_MOD, si->sockfd, &ev);
			break;        
		}    

	} while((si->recvbuffer[level] != '\r' || si->recvbuffer[level+1] != '\n') && level != -1);    
	return 0;
}

通信

握手完成后，开始正常的通信，这里贴出websocket协议封装的代码，协议结构图就不贴了。

char* decode_packet(char *stream, char *mask, int length, int *ret) 
{
	nty_ophdr *hdr =  (nty_ophdr*)stream;  //stream为输入字符串
	unsigned char *data = stream + sizeof(nty_ophdr);
	int size = 0;
	int start = 0;
	//char mask[4] = {0};
	int i = 0;
	//if (hdr->fin == 1) return NULL;
	if ((hdr->mask & 0x7F) == 126) 
{
		nty_websocket_head_126 *hdr126 = (nty_websocket_head_126*)data;
		size = hdr126->payload_length;
		for (i = 0;i < 4;i ++) 
        {
			mask[i] = hdr126->mask_key[i];  //掩码
		}
		start = 8;
	} else if ((hdr->mask & 0x7F) == 127) 
{
		nty_websocket_head_127 *hdr127 = (nty_websocket_head_127*)data;
		size = hdr127->payload_length;
		for (i = 0;i < 4;i ++) 
        {
			mask[i] = hdr127->mask_key[i];  //掩码
		}
		start = 14;
	} else {
		size = hdr->payload_length;

		memcpy(mask, data, 4);
		start = 6;
	}
	*ret = size;
	umask(stream+start, size, mask);
	return stream + start;  //返回解后字符串
}

int encode_packet(char *buffer,char *mask, char *stream, int length) 
{    
    //buffer为输出的封装后字符串，stream为输入的待封装数据
	nty_ophdr head = {0};
	head.fin = 1;
	head.opcode = 1;
	int size = 0;
	if (length < 126) 
{
		head.payload_length = length;
		memcpy(buffer, &head, sizeof(nty_ophdr));
		size = 2;
	} else if (length < 0xffff) 
{
		nty_websocket_head_126 hdr = {0};
		hdr.payload_length = length;
		memcpy(hdr.mask_key, mask, 4);
		memcpy(buffer, &head, sizeof(nty_ophdr));
		memcpy(buffer+sizeof(nty_ophdr), &hdr, sizeof(nty_websocket_head_126));
		size = sizeof(nty_websocket_head_126);
	} else 
{
		nty_websocket_head_127 hdr = {0};
		hdr.payload_length = length;
		memcpy(hdr.mask_key, mask, 4);
		memcpy(buffer, &head, sizeof(nty_ophdr));
		memcpy(buffer+sizeof(nty_ophdr), &hdr, sizeof(nty_websocket_head_127));
		size = sizeof(nty_websocket_head_127);
	}
	memcpy(buffer+2, stream, length);
	return length + 2;
}

再贴出通信的代码

int transform(struct sockitem *si, struct reactor *mainloop) 
{
	int ret = 0;
	char mask[4] = {0};
	char *data = decode_packet(si->recvbuffer, mask, si->rlength, &ret);
	printf("data : %s , length : %d\n", data, ret);
	ret = encode_packet(si->sendbuffer, mask, data, ret);
	si->slength = ret;
	memset(si->recvbuffer, 0, BUFFER_LENGTH);
	struct epoll_event ev;
	ev.events = EPOLLOUT | EPOLLET;
	//ev.data.fd = clientfd;
	si->sockfd = si->sockfd;
	si->callback = send_cb;
	si->status = WS_DATATRANSFORM;
	ev.data.ptr = si;
	epoll_ctl(mainloop->epfd, EPOLL_CTL_MOD, si->sockfd, &ev);
	return 0;
}

这里再提示一下，握手、通信等阶段使用状态机实现。注意使用epoll的话，在每个状态里写epoll_ctl，不要再最后总体结构中写，因为如果数据量大的话，不是一次recv/send就能搞定。
另外，从websocket协议中可以获得启示。今后，在自定义开发基于TCP的协议时，要注意三个方面：操作码、包长度、掩码（非必须）。当然，数据的话，可以使用xml、json等协议。

长连接

最后，关于长连接，再啰嗦一下。TCP中也有保持长连接的keepalive机制，但是不建议使用，不要使用TCP的keepalive去代替应用层的心跳包。因为keepalive超时，TCP会主动回收连接相关资源，并且应用层无法得到任何反馈，可控性不强，只知道超时回收了，没法对超时做出一些判断。所以，在应用层设计心跳包去维持长连接。