在上节中，我们把教程要求的3.1-3.3过了一遍，这一次回到正途上来。看看到底是怎么完成一个新的协议的。
本节中的代码实现了一些简单的“ping”协议（灵感来自“ns注释和文档”（现在更名为ns手册）的第9.6章中的“ping请求者”，但相当不同）。 一个节点将能够发送一个数据包到另一个节点，它将立即返回，以便可以计算往返时间。
1、头文件
在新的头文件'ping.h'中，首先必须声明将携带相关数据的新的Ping包头的数据结构。
struct hdr_ping {char ret;double send_time;
};以下代码段将类“PingAgent”声明为类“Agent”的子类。
class PingAgent : public Agent {public:PingAgent();int command(int argc, const char*const* argv);void recv(Packet*, Handler*);protected:int off_ping_;
}; int off_ping_将用于访问数据包的ping头。 请注意，对于具有本地对象范围的变量通常使用尾随“_”。
2、c++代码
首先必须定义C ++代码和Tcl代码之间的联系。
首先，定义了协议头类以及应用类（C++中）
static class PingHeaderClass : public PacketHeaderClass {
public:PingHeaderClass() : PacketHeaderClass("PacketHeader/Ping", sizeof(hdr_ping)) {}
} class_pinghdr;static class PingClass : public TclClass {
public:PingClass() : TclClass("Agent/Ping") {}TclObject* create(int, const char*const*) {return (new PingAgent());}
} class_ping;下一段代码是类“PingAgent”的构造函数。 它绑定了必须在Tcl和C ++中访问的变量。TCL中的（packetSize）——>C++（size）
PingAgent::PingAgent() : Agent(PT_PING)
{bind("packetSize_", &size_);bind("off_ping_", &off_ping_);
}当执行类“PingAgent”的Tcl命令时，将调用函数'command（）'。 在本案例中，将是'$ pa send'（假设'pa'是Agent / Ping类的一个实例），因为我们希望将ping数据包从代理发送到另一个ping代理。 我们基本上必须在'command（）'函数中解析命令，如果没有匹配，则必须将其参数传递给基类的command（）函数（在这种情况下为“代理” ：：命令（）'（详见代码倒数第二行的return））
int PingAgent::command(int argc, const char*const* argv)
{if (argc == 2) {if (strcmp(argv[1], "send") == 0) {// Create a new packetPacket* pkt = allocpkt();// Access the Ping header for the new packet:hdr_ping* hdr = (hdr_ping*)pkt->access(off_ping_);// Set the 'ret' field to 0, so the receiving node knows// that it has to generate an echo packethdr->ret = 0;// Store the current time in the 'send_time' fieldhdr->send_time = Scheduler::instance().clock();// Send the packetsend(pkt, 0);// return TCL_OK, so the calling function knows that the// command has been processedreturn (TCL_OK);}}// If the command hasn't been processed by PingAgent()::command,// call the command() function for the base classreturn (Agent::command(argc, argv));
}函数recv（）定义了接收到数据包时要执行的操作。 如果'ret'字段为0，则必须返回“send_time”字段的值相同但“ret”字段设置为1的数据包。 如果'ret'为1，则会调用Tcl函数（必须由Tcl中的用户定义）函数并处理该事件。
void PingAgent::recv(Packet* pkt, Handler*)
{// Access the IP header for the received packet:hdr_ip* hdrip = (hdr_ip*)pkt->access(off_ip_);// Access the Ping header for the received packet:hdr_ping* hdr = (hdr_ping*)pkt->access(off_ping_);// Is the 'ret' field = 0 (i.e. the receiving node is being pinged)?if (hdr->ret == 0) {// Send an 'echo'. First save the old packet's send_timedouble stime = hdr->send_time;// Discard the packetPacket::free(pkt);// Create a new packetPacket* pktret = allocpkt(); // Access the Ping header for the new packet:hdr_ping* hdrret = (hdr_ping*)pktret->access(off_ping_);// Set the 'ret' field to 1, so the receiver won't send another echohdrret->ret = 1; // Set the send_time field to the correct valuehdrret->send_time = stime; // Send the packet send(pktret, 0); } else { // A packet was received. Use tcl.eval to call the Tcl// interpreter with the ping results.// Note: In the Tcl code, a procedure 'Agent/Ping recv {from rtt}'// has to be defined which allows the user to react to the ping// result. char out[100]; // Prepare the output to the Tcl interpreter. Calculate the round// trip time sprintf(out, "%s recv %d %3.1f", name(), hdrip->src_.addr_ >> Address::instance().NodeShift_[1],(Scheduler::instance().clock()-hdr->send_time) * 1000);Tcl& tcl = Tcl::instance(); tcl.eval(out); // Discard the packet Packet::free(pkt); }
} 完整代码：
/** File: Code for a new 'Ping' Agent Class for the ns* network simulator* Author: Marc Greis (greis@cs.uni-bonn.de), May 1998**/#include "ping.h"static class PingHeaderClass : public PacketHeaderClass {
public:PingHeaderClass() : PacketHeaderClass("PacketHeader/Ping", sizeof(hdr_ping)) {}
} class_pinghdr;static class PingClass : public TclClass {
public:PingClass() : TclClass("Agent/Ping") {}TclObject* create(int, const char*const*) {return (new PingAgent());}
} class_ping;PingAgent::PingAgent() : Agent(PT_PING)
{bind("packetSize_", &size_);bind("off_ping_", &off_ping_);
}int PingAgent::command(int argc, const char*const* argv)
{if (argc == 2) {if (strcmp(argv[1], "send") == 0) {// Create a new packetPacket* pkt = allocpkt();// Access the Ping header for the new packet:hdr_ping* hdr = (hdr_ping*)pkt->access(off_ping_);// Set the 'ret' field to 0, so the receiving node knows// that it has to generate an echo packethdr->ret = 0;// Store the current time in the 'send_time' fieldhdr->send_time = Scheduler::instance().clock();// Send the packetsend(pkt, 0);// return TCL_OK, so the calling function knows that the// command has been processedreturn (TCL_OK);}}// If the command hasn't been processed by PingAgent()::command,// call the command() function for the base classreturn (Agent::command(argc, argv));
}void PingAgent::recv(Packet* pkt, Handler*)
{// Access the IP header for the received packet:hdr_ip* hdrip = (hdr_ip*)pkt->access(off_ip_);// Access the Ping header for the received packet:hdr_ping* hdr = (hdr_ping*)pkt->access(off_ping_);// Is the 'ret' field = 0 (i.e. the receiving node is being pinged)?if (hdr->ret == 0) {// Send an 'echo'. First save the old packet's send_timedouble stime = hdr->send_time;// Discard the packetPacket::free(pkt);// Create a new packetPacket* pktret = allocpkt();// Access the Ping header for the new packet:hdr_ping* hdrret = (hdr_ping*)pktret->access(off_ping_);// Set the 'ret' field to 1, so the receiver won't send another echohdrret->ret = 1;// Set the send_time field to the correct valuehdrret->send_time = stime;// Send the packetsend(pktret, 0);} else {// A packet was received. Use tcl.eval to call the Tcl// interpreter with the ping results.// Note: In the Tcl code, a procedure 'Agent/Ping recv {from rtt}'// has to be defined which allows the user to react to the ping// result.char out[100];// Prepare the output to the Tcl interpreter. Calculate the round// trip timesprintf(out, "%s recv %d %3.1f", name(), hdrip->src_ >> Address::instance().NodeShift_[1], (Scheduler::instance().clock()-hdr->send_time) * 1000);Tcl& tcl = Tcl::instance();tcl.eval(out);// Discard the packetPacket::free(pkt);}
}
)
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