ns3使用入门指南

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"ns3软件使用手册" ns-3是一个开源的网络模拟器，主要用于研究和教育目的，它允许用户在离线环境中模拟网络协议、应用和技术。ns-3 Manual是官方提供的详细使用指南，适用于ns-3.29版本，对于初学者来说是一份非常宝贵的学习资料。该手册首先介绍了软件的组织结构，包括其模块化设计和组件之间的关系。随机变量（Random Variables）部分是模拟中的重要元素，手册详细解释了如何快速理解并创建随机变量，以及如何设定种子以实现可重复的实验。此外，它还涵盖了RandomVariableStream类，这是处理随机变量流的关键，并讨论了不同类型的随机变量以及它们的语义。手册还指导用户如何使用其他伪随机数生成器（PRNG），设置流编号，以及如何发布和记录结果。关于Hash Functions，手册描述了基本用法，如增量哈希和如何使用替代哈希函数。同时，它还阐述了添加新的哈希函数实现的方法以及哈希函数的来源。这对于定制和扩展ns-3的功能非常有用。事件和Simulator章节是ns-3的核心部分。事件是模拟时间推进的基本单位，而Simulator类是管理这些事件的核心。手册详细讨论了事件对象、Simulator接口以及时间处理，包括时间类型和调度器的使用。回调（Callbacks）是ns-3中实现模块间通信的关键机制。手册详细解释了回调的动机、背景知识，以及如何使用回调API，包括绑定回调和追踪回调的使用场景。此外，它还指出了ns-3中回调的具体位置和实现细节。最后，对象模型章节概述了ns-3的面向对象行为，包括基础对象类，这有助于开发者理解如何创建和操作ns-3中的自定义网络组件。这些对象模型的设计使得ns-3能够支持灵活的模拟配置和复用。这份ns3软件使用手册为用户提供了全面的指导，从基本概念到高级特性，一步步引导用户深入理解并熟练使用ns-3进行网络模拟。无论是对网络协议的工作原理感兴趣，还是在进行科研项目，这份手册都是不可或缺的参考资料。

ns-3 Manual, Release ns-3.29

The assignment of streams to a ﬁxed stream number is optional; instances of RandomVariableStream that do not have

a stream value assigned will be assigned the next one from the pool of automatic streams.

To ﬁx a RandomVariableStream to a particular underlying stream, assign its Stream attribute to a non-negative

integer (the default value of -1 means that a value will be automatically allocated).

2.11 Publishing your results

When you publish simulation results, a key piece of conﬁguration information that you should always state is how you

used the random number generator.

• what seeds you used,

• what RNG you used if not the default,

• how were independent runs performed,

• for large simulations, how did you check that you did not cycle.

It is incumbent on the researcher publishing results to include enough information to allow others to reproduce his or

her results. It is also incumbent on the researcher to convince oneself that the random numbers used were statistically

valid, and to state in the paper why such conﬁdence is assumed.

2.12 Summary

Let’s review what things you should do when creating a simulation.

• Decide whether you are running with a ﬁxed seed or random seed; a ﬁxed seed is the default,

• Decide how you are going to manage independent replications, if applicable,

• Convince yourself that you are not drawing more random values than the cycle length, if you are running a very

long simulation, and

• When you publish, follow the guidelines above about documenting your use of the random number generator.

10 Chapter 2. Random Variables

CHAPTER

FOUR

EVENTS AND SIMULATOR

ns-3 is a discrete-event network simulator. Conceptually, the simulator keeps track of a number of events that are

scheduled to execute at a speciﬁed simulation time. The job of the simulator is to execute the events in sequential time

order. Once the completion of an event occurs, the simulator will move to the next event (or will exit if there are no

more events in the event queue). If, for example, an event scheduled for simulation time “100 seconds” is executed,

and the next event is not scheduled until “200 seconds”, the simulator will immediately jump from 100 seconds to 200

seconds (of simulation time) to execute the next event. This is what is meant by “discrete-event” simulator.

To make this all happen, the simulator needs a few things:

1. a simulator object that can access an event queue where events are stored and that can manage the execution of

events

2. a scheduler responsible for inserting and removing events from the queue

3. a way to represent simulation time

4. the events themselves

This chapter of the manual describes these fundamental objects (simulator, scheduler, time, event) and how they are

used.

4.1 Event

To be completed

4.2 Simulator

The Simulator class is the public entry point to access event scheduling facilities. Once a couple of events have

been scheduled to start the simulation, the user can start to execute them by entering the simulator main loop (call

Simulator::Run). Once the main loop starts running, it will sequentially execute all scheduled events in order

from oldest to most recent until there are either no more events left in the event queue or Simulator::Stop has been

called.

To schedule events for execution by the simulator main loop, the Simulator class provides the Simulator::Schedule*

family of functions.

1. Handling event handlers with different signatures

These functions are declared and implemented as C++ templates to handle automatically the wide variety of C++ event

handler signatures used in the wild. For example, to schedule an event to execute 10 seconds in the future, and invoke

a C++ method or function with speciﬁc arguments, you might write this:

ns-3 Manual, Release ns-3.29

void handler (int arg0, int arg1)

{

std::cout << "handler called with argument arg0=" << arg0 << " and

arg1=" << arg1 << std::endl;

}

Simulator::Schedule(Seconds(10), &handler, 10, 5);

Which will output:

handler called with argument arg0=10 and arg1=5

Of course, these C++ templates can also handle transparently member methods on C++ objects:

To be completed: member method example

Notes:

• the ns-3 Schedule methods recognize automatically functions and methods only if they take less than 5 argu-

ments. If you need them to support more arguments, please, ﬁle a bug report.

• Readers familiar with the term ‘fully-bound functors’ will recognize the Simulator::Schedule methods as a way

to automatically construct such objects.

2. Common scheduling operations

The Simulator API was designed to make it really simple to schedule most events. It provides three variants to do so

(ordered from most commonly used to least commonly used):

• Schedule methods which allow you to schedule an event in the future by providing the delay between the current

simulation time and the expiration date of the target event.

• ScheduleNow methods which allow you to schedule an event for the current simulation time: they will execute

_after_ the current event is ﬁnished executing but _before_ the simulation time is changed for the next event.

• ScheduleDestroy methods which allow you to hook in the shutdown process of the Simulator to cleanup simu-

lation resources: every ‘destroy’ event is executed when the user calls the Simulator::Destroy method.

3. Maintaining the simulation context

There are two basic ways to schedule events, with and without context. What does this mean?

Simulator::Schedule (Time const &time, MEM mem_ptr, OBJ obj);

vs.

Simulator::ScheduleWithContext (uint32_t context, Time const &time, MEM mem_ptr, OBJ obj);

Readers who invest time and effort in developing or using a non-trivial simulation model will know the value of the

ns-3 logging framework to debug simple and complex simulations alike. One of the important features that is provided

by this logging framework is the automatic display of the network node id associated with the ‘currently’ running

event.

The node id of the currently executing network node is in fact tracked by the Simulator class. It can be accessed with

the Simulator::GetContext method which returns the ‘context’ (a 32-bit integer) associated and stored in the currently-

executing event. In some rare cases, when an event is not associated with a speciﬁc network node, its ‘context’ is set

to 0xffffffff.

To associate a context to each event, the Schedule, and ScheduleNow methods automatically reuse the context of the

currently-executing event as the context of the event scheduled for execution later.

In some cases, most notably when simulating the transmission of a packet from a node to another, this behavior is

undesirable since the expected context of the reception event is that of the receiving node, not the sending node. To

14 Chapter 4. Events and Simulator

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ns3使用入门指南

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