TCP/IP 协议栈 -- 编写UDP客户端注意细节

上节我们说到了TCP 客户端编写的主要细节，本节我们来看一下UDP client的几种情况，测试代码如下：
server：

#include <stdio.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <string.h>
#define PORT 6666
int main()
{int serverfd = socket(AF_INET, SOCK_DGRAM, 0);if(serverfd < 0)return -1;struct sockaddr_in serveraddr;serveraddr.sin_port = htons(PORT);serveraddr.sin_family = AF_INET;serveraddr.sin_addr.s_addr = htonl(INADDR_ANY);bind(serverfd, (const struct sockaddr *)&serveraddr, sizeof(serveraddr));char buff[256] = {0};struct sockaddr_in clientaddr;socklen_t len = sizeof(clientaddr);while(1){recvfrom(serverfd, buff, 256, 0, (struct sockaddr *)&clientaddr, &len);printf("RECV:  %s\n", inet_ntoa( clientaddr.sin_addr));printf("TEXT: %s\n", buff);memset(buff, 0, 256);sendto(serverfd, "recvd hello", 20, 0, (const struct sockaddr*)&clientaddr, sizeof(clientaddr));}return 0;
}

client：

#include <stdio.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <string.h>
#include <unistd.h>
#define PORT 6666
int main(int argc, char *argv[])
{if(argc != 2){fprintf(stderr, "+++ IPaddress\n");return -1;}int clientfd = socket(AF_INET, SOCK_DGRAM, 0);if(clientfd < 0)return -1;struct sockaddr_in serveraddr;serveraddr.sin_port = htons(PORT);serveraddr.sin_family = AF_INET;char *buff = "hello world";size_t n = strlen(buff);char rebuff[256] = {0};if(inet_pton(AF_INET, argv[1], (void *)&serveraddr.sin_addr) == 0){printf("IP address error\n");return -1;}socklen_t len = sizeof(serveraddr);while(1){sleep(2);sendto(clientfd, buff, n, 0, (const struct sockaddr *)&serveraddr, len);recvfrom(clientfd, rebuff, 256, 0, NULL, NULL);printf("SERVER :%s\n", rebuff);}return 0;
}

UDP 无连接不可靠的协议它并不会向TCP一样会建立连接后再发送数据。
1.server 进程没有打开

Desktop# tcpdump -i lo  -A -e -nn 
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on lo, link-type EN10MB (Ethernet), capture size 65535 bytes
12:43:03.656237 00:00:00:00:00:00 > 00:00:00:00:00:00, ethertype IPv4 (0x0800), length 53: 127.0.0.1.46066 > 127.0.0.1.6666: UDP, length 11
E..'..@.@.#$...........
...&hello world
12:43:03.656275 00:00:00:00:00:00 > 00:00:00:00:00:00, ethertype IPv4 (0x0800), length 81: 127.0.0.1 > 127.0.0.1: ICMP 127.0.0.1 udp port 6666 unreachable, length 47
E..C%p..@.V.................E..'..@.@.#$...........
...&hello world

这里以127地址做实验这里可以看到若主机server进程没有打开，则client会阻塞在recvfrom处这里可以在sendto和 recvfrom 之间插入printf进行测试。这就是UDP的特性， ICMP其实是sendto造成的它是一个异步错误但是这个ICMP并没有返回到应用程序， client 并不知道。这里的解决方案就是用connect 这里的connect与TCP里的connect建立三次握手差别很大，对于UDP它就是为了收取这个内核接到的这个ICMP错误。内核只是检查是否存在立即可知的错误，记录对端的IP地址和端口号，然后立即返回到调用进程。那么这里就要注意到connect里面传入server的ip和port，那么这里的sendto和recvfrom就不要在传入iP和port，那么就可以用read和write来代替。这里还有一点就是因为UDP是无连接的， connect并不会像tcp一样会发送SYN， TCP发送SYN时，若server进程没有开启那么会立即受到RST，但对于UDP来说当write 后才会收到主机的ICMP错误。
这里写图片描述

2.server 主机不存在。

Desktop# tcpdump -i eth0  -A -e -nn host 15.15.12.13
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
13:11:25.331059 00:0c:29:61:1a:9b > 00:1e:2a:67:f3:2e, ethertype IPv4 (0x0800), length 53: 192.168.1.2.44646 > 15.15.12.13.6666: UDP, length 11
.f.'".@.@.;........
....hello world
13:11:25.331233 00:0c:29:61:1a:9b > 00:1e:2a:67:f3:2e, ethertype IPv4 (0x0800), length 60: 192.168.1.2.44646 > 15.15.12.13.6666: UDP, length 11
.f.'".@.@.;........
....hello world.......

这里可以看到一直阻塞在recvfrom这里这里的解决方案就是在recvfrom添加超时中断，alarm 或者 select检测fd是否可读设立超时。

3.client 正在发送数据， server 进程被kill掉

13:17:29.059557 00:00:00:00:00:00 > 00:00:00:00:00:00, ethertype IPv4 (0x0800), length 62: 127.0.0.1.6666 > 127.0.0.1.57718: UDP, length 20
E..0r.@.@............
.v.../recvd hello.......;4
13:17:31.059842 00:00:00:00:00:00 > 00:00:00:00:00:00, ethertype IPv4 (0x0800), length 53: 127.0.0.1.57718 > 127.0.0.1.6666: UDP, length 11
E..'s.@.@............v.
...&hello world
13:17:31.059922 00:00:00:00:00:00 > 00:00:00:00:00:00, ethertype IPv4 (0x0800), length 81: 127.0.0.1 > 127.0.0.1: ICMP 127.0.0.1 udp port 6666 unreachable, length 47
E..C....@.............qs....E..'s.@.@............v.
...&hello world

这里可以看到client也收到了ICMP 解决方案同1.

4.client 正在发生数据， server 断电。

13:17:29.059557 00:00:00:00:00:00 > 00:00:00:00:00:00, ethertype IPv4 (0x0800), length 62: 127.0.0.1.6666 > 127.0.0.1.57718: UDP, length 20
E..0r.@.@............
.v.../recvd hello.......;4
13:17:31.059842 00:00:00:00:00:00 > 00:00:00:00:00:00, ethertype IPv4 (0x0800), length 53: 127.0.0.1.57718 > 127.0.0.1.6666: UDP, length 11
E..'s.@.@............v.
...&hello world