第10章廣播與多播編程廣播一個IP地址由網絡號和主機號組成。所有主機號部分為全1的IP地址是廣播地址。發送應用進程UDPIPv4數據鏈路UDPIPv4數據鏈路數據鏈路IPv4UDP接收應用進程以太網頭部IPv4頭部UDP頭部UDP數據子網128.7.6目的以太網=ff:ff:ff:ff:ff:ff幀類型=0800目的IP=128.7.6.255協議=UDP目的端口=520Sendto目的IP=128.7.6.255目的端口=520丟棄協議=UDP幀類型0800128.7.6.99=單播128.7.6.255=廣播128.7.6.5=單播128.7.6.255=廣播協議=UDP端口=520廣播的實現應用程序只能通過UDP方式發送廣播。一般情況下，如果調用sendto，只能向非廣播地址發送數據報。如果要發送廣播數據報，必須告訴內核，可以通過設置SO_BROADCAST套接口選項來做到這一點。inton=1;setsockopt(sockfd,SOL_SOCKET,SO_BROADCAST,&on,sizeof(int));廣播實例服務端為tserv.c，運行時需指定廣播地址和端口號，運行后每3秒發送一次廣播，將本機時間通知本子網內所有主機。服務器端程序tserv.c#include<sys/socket.h>#include<arpa/inet.h>#include<sys/ioctl.h>#include<stdio.h>#defineBUFLEN255voidgetcurtime(char*curtime){time_ttm;time(&tm);snprintf(curtime,BUFLEN,"%s\n",ctime(&tm));}intmain(intargc,char**argv){structsockaddr_inpeeraddr;intsockfd,on=1;intnum,i;charmsg[BUFLEN+1];if(argc!=3){printf("usage:%s<ipaddress><port>\n",argv[0]);exit(0);}sockfd=socket(AF_INET,SOCK_DGRAM,0);if(sockfd<0){fprintf(stderr,"socketcreatingerrorintserv.c\n");exit(1);}setsockopt(sockfd,SOL_SOCKET,SO_BROADCAST,&on,sizeof(int));memset(&peeraddr,0,sizeof(structsockaddr_in));peeraddr.sin_family=AF_INET;if(inet_pton(AF_INET,argv[1],&peeraddr.sin_addr)<=0){printf("WrongdestIPaddress\n");exit(0);}peeraddr.sin_port=htons(atoi(argv[2]));for(;;){getcurtime(msg);inta;a=sendto(sockfd,msg,strlen(msg),0,(structsockaddr*)&peeraddr,sizeof(structsockaddr_in));printf("%d",a);fflush(stdout);sleep(3);}客戶端程序tcli.c(只需偵聽某一固定端口的數據報,接收一個數據報就返回。)#include<sys/types.h>#include<sys/socket.h>#include<arpa/inet.h>#include<stdio.h>#defineBUFLEN255intmain(intargc,char**argv){structsockaddr_inlocaladdr;intsockfd,n;charmsg[BUFLEN+1];if(argc!=2){printf("usage:%s<port>\n",argv[0]);exit(0);}sockfd=socket(AF_INET,SOCK_DGRAM,0);if(sockfd<0){fprintf(stderr,"socketcreatingerrorintcli.c\n");exit(1);}memset(&localaddr,0,sizeof(structsockaddr_in));localaddr.sin_port=htons(atoi(argv[1]));localaddr.sin_addr.s_addr=htonl(INADDR_ANY);intopt=SO_REUSEADDR;setsockopt(sockfd,SOL_SOCKET,SO_REUSEADDR,&opt,sizeof(opt));if(bind(sockfd,(structsockaddr*)&localaddr,sizeof(structsockaddr_in))<0){fprintf(stderr,"binderrorintcli.c\n");exit(2);}n=read(sockfd,msg,BUFLEN);if(n==-1){fprintf(stderr,"readerrorintcli.c\n");exit(3);}else{msg[n]=0;printf("%s",msg);}}程序運行結果如果在服務器端輸入./serv222.18.113.1711234則兩個客戶不會同時收到數據報。多播多播是通過D類地址進行的，D類地址的前4位為1110，后面28位為群播的組標識。地址范圍224.0.0.0到239.255.255.255特殊的IPv4多播地址：224.0.0.0保留224.0.0.1本子網上所有主機224.0.0.2本子網上所有網關224.0.1.1NTP(網絡時間協議)組多播的原理當一個多播分組到達一個以太網時，形成幀后它的MAC地址為01:00:5e:xx:xx:xx，其后23位由多播組標識的后23位映射而成。例如目的地址為224.0.1.1的多播分組，在以太網上幀的MAC地址就為01:00:5e:00:01:01，如下圖示。11100000000000000000000100000001000000010000000001011110000000000000000100000001IP地址MAC地址由于多播IP地址中組標識有28位，而映射到MAC地址的只有23位，還差5位，所以有32個組將映射成相同的MAC地址，例如224.0.0.1，225.0.1.1,239.128.1.1都映射到MAC地址01:00:5e:00:01:01。因此要由IP層來檢驗到達的多播分組是否是自已所加入的多播組。如果不是，則拋棄該分組。應用程序UDPIP層132.0.0.100數據鏈路層52:37:4a:6d:7f:5e應用程序UDPIP層132.0.0.129應用程序UDPIP層132.0.0.168數據鏈路層52:23:4e:6f:2c:35數據鏈路層00:2e:2c:5f:ae:3f發送數據報IP:224.0.1.1端口1234通過ARP解析224.0.1.1對應01:00:5e:00:01:01接收數據報端口1234應用程序指定132.0.0.129加入多播組224.0.1.1IP層指示接收MAC地址為01:00:5e:00:01:01的幀加入225.0.1.1丟棄多播數據報在子網中的發送接收多播的實現

應用程序只需向多播組地址發送數據報，接收應用程序加入這個多播組。在指定端口上進行偵聽。加入多播組的方法是設置套接口選項IP_ADD_MEMBERSHIP.它要用要如下結構：Structip_mreq{structin_addrimr_multiaddr;/*IPv4的D類多播地址*/Structin_addrimr_interface;/*本地接口IPv4地址*/};例：setsockopt(sockfd,IPPROTO_IP,IP_ADD_MEMBERSHIP,&mcaddr,sizeof(structip_mreq))多播的實現服務器端程序沒有什么改變，只要將設置socket選項SO_BROADCAST的那行去掉就可以了。客戶程序源碼如下頁：源程序mtcli.c#include<sys/types.h>#include<sys/socket.h>#include<arpa/inet.h>#include<stdio.h>#defineBUFLEN255intmain(intargc,char**argv){structsockaddr_inlocaladdr;intsockfd,n;structip_mreqmcaddr;charmsg[BUFLEN+1];if(argc<3){printf("usage:%s<multicast><port>\n",argv[0]);exit(0);}sockfd=socket(AF_INET,SOCK_DGRAM,0);if(sockfd<0){fprintf(stderr,"socketcreatingerrorintcli.c\n");exit(1);}memset(&localaddr,0,sizeof(structsockaddr_in));localaddr.sin_port=htons(atoi(argv[2]));localaddr.sin_addr.s_addr=htonl(INADDR_ANY);if(inet_pton(AF_INET,"222.18.113.171",&mcaddr.imr_interface)<=0){perror("failure");exit(0);}if(inet_pton(AF_INET,argv[1],&mcaddr.imr_multiaddr)<=0){printf("WrongmulticastIPaddress\n");exit(0);}printf("ip:%s",inet_ntoa(mcaddr.imr_multiaddr));fflush(stdout);if(setsockopt(sockfd,IPPROTO_IP,IP_ADD_MEMBERSHIP,&mcaddr,sizeof(structip_mreq))<0){perror("setsockopterrorinmtcli.c,joinmulticastfailed\n");exit(4);}if(bind(sockfd,(structsockaddr*)&localaddr,sizeof(structsockaddr_in))<0){