ns-2与TCP拥塞模拟 之七 ns-2中自带的应用Ping解读



ns-2与TCP拥塞模拟 之七
ns-2中自带的应用Ping解读

以ns-2中自带的应用Agent/Ping为例，描述一下如何编写一个“应用”。下面引用的是版本2.35中的代码，其他版本可能略有差异。以下是需要注意的地方：

（1）头文件.h中声明的类属性和方法；

（2）Tcl脚本的变量与C++类的变量的绑定；

（3）C++类中的command函数是Agent类与Tcl的接口（Tcl脚本的命令直接作用于该函数）；

（4）C++类中的recv函数，是Agent类功能实现的关键，网络中对于数据包的分类、转发和处理的操作都是通过这个函数来实现的；

（5）TclClass的编写与修改，为Tcl脚本提供接口。

具体的实践过程可以按照以下4个主要步骤进行，举例如下。

（1）添加头文件ns-2.35/apps/ping.h。

/*

* File:Header File for a new 'Ping' Agent Class for the ns

*
network simulator

*Author: Marc Greis (greis@cs.uni-bonn.de), May 1998

*/

#ifndef ns_ping_h

#define ns_ping_h

#include "agent.h"

#include "tclcl.h"

#include "packet.h"

#include "address.h"

#include "ip.h"

struct hdr_ping {



charret;
//从源端出来时值为 0，从目的端回来时值为
1；


doublesend_time;
//源端发送的时间戳，用于往返时延的计算；


doublercv_time;

//接收时刻


intseq;

//序列号


//Header access methods


staticint offset_;
//偏移量


inlinestatic int& offset() { return offset_; }


inlinestatic hdr_ping* access(const Packet* p) {


return(hdr_ping*) p->access(offset_);


}

};

class PingAgent : public Agent {

public:


PingAgent();


intseq;

//发送序号


intoneway;

//标志位


virtualint command(int argc, const char*const* argv);


virtualvoid recv(Packet*, Handler*);

};

#endif // ns_ping_h

（2）添加代码文件ns-2.35/apps/ping.cc。

/*

* 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"

int hdr_ping::offset_;

//以下的两个静态类定义主要完成C++和OTcl的连接


static class PingHeaderClass : publicPacketHeaderClass {

public:


PingHeaderClass(): PacketHeaderClass("PacketHeader/Ping",



sizeof(hdr_ping)) {


bind_offset(&hdr_ping::offset_);


}

} 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),seq(0), oneway(0)
//PingAgent的构造函数

{


bind("packetSize_",&size_);

}

int PingAgent::command(int argc, constchar*const* argv)

{


if(argc == 2) {


if(strcmp(argv[1], "send") == 0) { //如：$p0
send，作为command
函数的输入


//Create a new packet

Packet* pkt = allocpkt();


//Access the Ping header for the new packet:

hdr_ping* hdr = hdr_ping::access(pkt);


//Set the 'ret' field to 0, so the receiving node


//knows that it has to generate an echo packet


hdr->ret = 0;


hdr->seq = seq++;


// Store thecurrent time in the 'send_time' field

hdr->send_time = Scheduler::instance().clock();


//Send the packet

send(pkt, 0);


//return TCL_OK, so the calling function knows that


//the command has been processed

return (TCL_OK);


}


elseif (strcmp(argv[1], "start-WL-brdcast") == 0) {

Packet* pkt = allocpkt();

hdr_ip* iph = HDR_IP(pkt);

hdr_ping* ph = hdr_ping::access(pkt);


iph->daddr() = IP_BROADCAST;

iph->dport() = iph->sport();


ph->ret = 0;


send(pkt, (Handler*) 0);


return (TCL_OK);


}


elseif (strcmp(argv[1], "oneway") == 0) {

oneway=1;

return (TCL_OK);


}


}


// Ifthe command hasn't been processed by PingAgent()::command,


//call the command() function for the base class


return(Agent::command(argc, argv));

}

void PingAgent::recv(Packet* pkt, Handler*)

{


//Access the IP header for the received packet:


hdr_ip*hdrip = hdr_ip::access(pkt);


//Access the Ping header for the received packet:

hdr_ping* hdr = hdr_ping::access(pkt);


//check if in brdcast mode


if((u_int32_t)hdrip->daddr() == IP_BROADCAST) {


if(hdr->ret == 0) {

printf("Recv BRDCAST Ping REQ : at %d.%d from %d.%d\n",here_.addr_, here_.port_,
hdrip->saddr(), hdrip->sport());

Packet::free(pkt);


//create reply

Packet* pktret = allocpkt();

hdr_ping* hdrret = hdr_ping::access(pktret);


hdr_ip* ipret = hdr_ip::access(pktret);

hdrret->ret = 1;


//add brdcast address

ipret->daddr() = IP_BROADCAST;


ipret->dport() = ipret->sport();


send(pktret, 0);


} else {

printf("Recv BRDCAST Ping REPLY : at %d.%d from %d.%d\n",here_.addr_, here_.port_,
hdrip->saddr(), hdrip->sport());

Packet::free(pkt);


}

return;


}


// Isthe '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_time

double stime = hdr->send_time;


intrcv_seq = hdr->seq;


//Discard the packet

Packet::free(pkt);


//Create a new packet

Packet* pktret = allocpkt();


//Access the Ping header for the new packet:

hdr_ping* hdrret = hdr_ping::access(pktret);


//Set the 'ret' field to 1, so the receiver won't send


//another echo

hdrret->ret = 1;


//Set the send_time field to the correct value

hdrret->send_time = stime;


//Added by Andrei Gurtov for one-way delay measurement.

hdrret->rcv_time = Scheduler::instance().clock();

hdrret->seq = rcv_seq;


//Send the packet

send(pktret, 0);


} else{


// Apacket 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.


charout[100];


//Prepare the output to the Tcl interpreter. Calculate the


//round trip time


if(oneway) //AG


sprintf(out,"%s recv %d %d %3.1f %3.1f", name(),



hdrip->src_.addr_ >>Address::instance().NodeShift_[1],


hdr->seq, (hdr->rcv_time -hdr->send_time) * 1000,


(Scheduler::instance().clock()-hdr->rcv_time)* 1000);


elsesprintf(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);


}

}

以上代码的逻辑比较清晰明了，可以看出，比较重要的是command函数中处理“send”命令的部分和recv函数。

①
函数command中处理“send”命令的部分，在逻辑上是发送端，发送第一个报文分组。

②
函数recv中结构“if(hdr->ret
== 0) {…}”部分，逻辑上是接收端处理收到的分组；填充了一个新的报文分组的标志、时间戳、序号等部分后，发送了这个响应包；当然，这个recv函数比较复杂的部分是考虑了广播包和双向反馈。

（3）ns-2中应该做的其他必要修改。

①
修改ns-2.35/common/packet.h文件。

…

// RAP packets

static const packet_t PT_RAP_DATA = 40;

static const packet_t PT_RAP_ACK = 41;

static const packet_t PT_TFRC = 42;

static const packet_t PT_TFRC_ACK = 43;

static const packet_t PT_PING = 44;

static const packet_t PT_PBC = 45;

// Diffusion packets - Chalermek

static const packet_t PT_DIFF = 46;

…

class p_info {

…



staticvoid initName()


{


if(nPkt_>= PT_NTYPE+1)


return;

…


name_[PT_TFRC]="tcpFriend";


name_[PT_TFRC_ACK]="tcpFriendCtl";


name_[PT_PING]="ping";


name_[PT_PBC]= "PBC";

…


}

}

②
修改ns-2.35/tcl/lib/ns-default.tcl文件。

…

##Agent set seqno_ 0 now is gone

##Agent set class_ 0 now is gone

Agent/Ping set packetSize_ 64

Agent/UDP set packetSize_ 1000

Agent/UDP instproc done {} { }

Agent/UDP instproc process_data {from data} {}

…

③
修改ns-2.35/tcl/lib/ns-packet.tcl文件。

set protolist {

…

# Application-Layer Protocols:


Message # a protocol to carry text messages


Ping
# Ping


PBC # PBC

…

}

④
修改ns-2.35/Makefile文件。

OBJ_CC = \


tools/random.otools/rng.o tools/ranvar.o common/misc.o common/timer-handler.o \

…


tcp/tfrc.otcp/tfrc-sink.o mobile/energy-model.o apps/ping.o tcp/tcp-rfc793edu.o \

…

执行make
命令。

（4）运行以下ping.tcl
进行测试。

#ping.tcl
文件

#Create a simulator object

set ns [new Simulator]

#Open a trace file

set tracefd [open out.tr w]

$ns trace-all $tracefd

set nf [open out.nam w]

$ns namtrace-all $nf

#Define a 'finish' procedure

proc finish {} {

global ns nf

$ns flush-trace

close $nf

exec nam out.nam &

exit 0

}

#Create three nodes

set n0 [$ns node]

set n1 [$ns node]

set n2 [$ns node]

#Connect the nodes with two links

$ns duplex-link $n0 $n1 1Mb 10ms DropTail

$ns duplex-link $n1 $n2 1Mb 10ms DropTail

#Create two ping agents and attach them to thenodes n0 and n2

set p0 [new Agent/Ping]

$ns attach-agent $n0 $p0

set p1 [new Agent/Ping]

$ns attach-agent $n2 $p1

#Connect the two agents

$ns connect $p0 $p1

#Schedule events

$ns at 0.2 "$p0send"

$ns at 0.4 "$p1send"

$ns at 0.6 "$p0send"

$ns at 0.6 "$p1send"

$ns at 1.0 "finish"

#Run the simulation

$ns run

整体上来看，与代理模块相比，添加一个新的“应用”，需要在合适的Tcl文件中添加相应的报文类型定义。