1. 下表显示了 NS2 和 TCP/IP、OSI七层网络结构的大致对应关系（这个表很有好处哦）
TCP NS2 OSI
应用层
应用层 应用层
表示层
会话层
传输层（TCP/UDP）
网络层
代理（Agent） 传输层
网络层
物理层 节点和连接
（NODE & Link）
数据链路层
物理层
2. 下面我们将演示 在NS2中实现自己编写的Agent的全过程，对于传输层和网络层模拟的研究有着基本的指导意义。
实现的参考： Marc Greis‘ Tutorial http://www.isi.edu/nsnam/ns/tutorial/
VII. A new protocol for NS
我用的是NS-2.34版本的， 其中已经添加了 Agent/Ping，此处仅验证 Marc Greis' Tutotial的可行性；
下面是整个实现的过程：（过程是一致的，但是目录、实现的细节有些区别，注意哦，哈哈！）
（相信在看过前面的实例，并具体运行测试过后，阅读并实现以下的Task，应该是不会有问题的！）
一般在NS2下实现一个协议，主要是编写.h 和 .cc 两个文件，但是当问题比较复杂后，可能需要编写很多个文件，但一般都是每个 .h 文件就对应着相应的 .cc 文件。 .h 文件，一般用于定义 数据包格式和类， 而 .cc文件，需要完成以下工作：
(1). .h中定义的类方法的实现，
(2). TclClass的编写，为TCL脚本提供接口， 此函数的代码基本不变，每次只做简单的替换即可；
(3). TCL脚本的变量和 C++ 中类的变量的绑定函数，也只直接对应着填写即可！
(4). 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！
(5). 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！ 参看具体的应用再编写吧！
以下演示Marc Greis‘ Tutorial 中Agent/Ping的实现过程，最后给出测试结果！
1. ping.h 文件：
#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 {
char ret; //从源端出来时值为 0， 从目的端回来时值为 1；
double send_time; //源端发送的时间锉，用于往返时延的计算；
};
class PingAgent : public Agent {
public:
PingAgent();
int command(int argc, const char*const* argv);
void recv(Packet*, Handler*);
protected:
int off_ping_; //it will be used to access a packet's ping header
};
#endif
2. ping.cc 文件：
#include "ping.h"
//以下的两个函数主要完成C++和OTCL的连接，每次可套用，做相应的修改即可！
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) //PingAgent的构造函数
{
bind("packetSize_", &size_);
bind("off_ping_", &off_ping_);
}
/× The function 'command()' is called when a Tcl command for the class 'PingAgent' is executed. In our case that would be '$pa send' (assuming 'pa' is an instance of the Agent/Ping class), because we want to send ping packets from the Agent to another ping agent. You basically have to parse the command in the 'command()' function, and if no match is found, you have to pass the command with its arguments to the 'command()' function of the base class (in this case 'Agent::command()'). ×/
// $pa send 命令作为command 函数的输入
int PingAgent::command(int argc, const char*const* argv)
{
if (argc == 2) {
if (strcmp(argv[1], "send") == 0) {
// Create a new packet
Packet* 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 packet
hdr->ret = 0;
// Store the current 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);
}
}
// If the command hasn't been processed by PingAgent()::command,
// call the command() function for the base class
return (Agent::command(argc, argv));
}
/× The function 'recv()' defines the actions to be taken when a packet is received. If the 'ret' field is 0, a packet with the same value for the 'send_time' field, but with the 'ret' field set to 1 has to be returned. If 'ret' is 1, a Tcl function (which has to be defined by the user in Tcl) is called and processed the event. */
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_time
double stime = hdr->send_time;
// 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*)pktret->access(off_ping_);
// 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;
// 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_ >> Address::instance().NodeShift_[1],
(Scheduler::instance().clock()-hdr->send_time) * 1000);
Tcl& tcl = Tcl::instance();
tcl.eval_r(out);
// Discard the packet
Packet::free(pkt);
}
}
简单的Protocol， 这两个文件可以直接放在 ~/ns-2.xx/ 目录下。 复杂的情况，将在后面的实例中做更详尽的描述！
3. NS2中应该做的必要修改
(1). "packet.h" 文件
enum packet_t {
PT_TCP,
PT_UDP,
......
// insert new packet types here
PT_TFRC,
PT_TFRC_ACK,
PT_PING, // packet protocol ID for our ping-agent
PT_NTYPE // This MUST be the LAST one
};
packet.h ： home/username/ns-allinone-2.34/ns-2.34/common/
我的机子上的 packet.h 对 PT_PING 的定义没有采用枚举，而是直接定义成为了static const packet_t 类型的:
.......
static const packet_t PT_TFRC = 42;
static const packet_t PT_TFRC_ACK = 43;
static const packet_t PT_PING = 44;
.......
修改 class p_info {} 结构：
class p_info {
public:
p_info() {
name_[PT_TCP]= "tcp";
name_[PT_UDP]= "udp";
...........
name_[PT_TFRC]= "tcpFriend";
name_[PT_TFRC_ACK]= "tcpFriendCtl";
name_[PT_PING]="Ping"; //添加的
name_[PT_NTYPE]= "undefined";
}
.....
}
我机子上该类的实现为：
class p_info {
public:
p_info() {
initName(); //因此我应该到initName()中做修改！！
}
............
static void initName()
{
if(nPkt_ >= PT_NTYPE+1)
return;
char **nameNew = new char*[PT_NTYPE+1];
...............
name_[PT_TFRC]= "tcpFriend";
name_[PT_TFRC_ACK]= "tcpFriendCtl";
name_[PT_PING]="ping"; //应该添加！
................
................
(2). tcl/lib/ns-default.tcl 文件 （这个好找到的哦！）
##Agent set seqno_ 0 now is gone
##Agent set class_ 0 now is gone
Agent/Ping set packetSize_ 64
(3). tcl/lib/ns-packet.tcl 文件
{ SRMEXT off_srm_ext_}
{ Ping off_ping_ }} {
set cl PacketHeader/[lindex $pair 0]
我机子上：
# { UMP off_ump_ }
# { TFRC off_tfrm_ }
# { Ping off_ping_ } //注意，已经注释掉了
# { rtProtoLS off_LS_ }
# { MPLS off_mpls_ }
# { GAF off_gaf_ }
# { LDP off_ldp_ }
# } {
# create-packet-header [lindex $pair 0] [lindex $pair 1]
# }
而是利用 foreach prot {} {} 结构实现的
foreach prot {
# Common:
Common
Flags
IP # IP
# Routing Protocols:
........
# Application-Layer Protocols:
Message # a protocol to carry text messages
Ping # Ping
PBC # PBC
........
# Other:
Encap # common/encap.cc
IPinIP # IP encapsulation
HDLC # High Level Data Link Control
} {
add-packet-header $prot
}
具体实现机制还不甚了解，但是这不影响的， 只要一葫芦画瓢即可！ 哈哈！
(4). Makefile 文件 （这个文件肯定知道在哪吧？ 哈哈，要不知道，就很可惜，你不大可能能测试通过，Good Luck！）
sessionhelper.o delaymodel.o srm-ssm.o \
srm-topo.o \
ping.o \
$(LIB_DIR)int.Vec.o $(LIB_DIR)int.RVec.o \
$(LIB_DIR)dmalloc_support.o \
在我机子上，如下：
diffusion/hash_table.o diffusion/routing_table.o diffusion/iflist.o \
tcp/tfrc.o tcp/tfrc-sink.o mobile/energy-model.o apps/ping.o tcp/tcp-rfc793edu.o \
queue/rio.o queue/semantic-rio.o tcp/tcp-sack-rh.o tcp/scoreboard-rh.o \
从中，可以看到，我机子上ping.h 和 ping.cc 文件都存储在ns-2.34/apps/ 文件夹中， （make命令之后理应在该文件夹下生成ping.o的，但是没找到，而ping.tcl代码却执行如常，哈哈，以后再来思考这点吧！）
(5). 执行make 命令
或
make clean
make depend
make
(6). 运行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
#Define a 'recv' function for the class 'Agent/Ping'
Agent/Ping instproc recv {from rtt} {
$self instvar node_
puts "node [$node_ id] received ping answer from \
$from with round-trip-time $rtt ms."
}
#Create two ping agents and attach them to the nodes 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
$ns at 0.2 "$p0 send"
$ns at 0.4 "$p1 send"
$ns at 0.6 "$p0 send"
$ns at 0.6 "$p1 send"
$ns at 1.0 "finish"
#Run the simulation
$ns run