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5 years ago What Is the Internet?
In this book, we’ll use the public Internet, a specific computer network, as our principal vehicle for discussing computer networks and their protocols. But what is the Internet?
基于硬件和软件的描述
In Internet jargon, all of these devices are called hosts or end systems. End systems are connected together by a network of communication links and packet switches.
When one end system has data to send to another end system, the sending end system segments the data and adds header bytes to each segment. The resulting packages of information, known as packets in the jargon of computer networks, are then sent through the network to the destination end system, where they are reassembled into the original data.
Packet switches come in many shapes and flavors, but the two most prominent types in today’s Internet are routers and link-layer switches.
Link-layer switches are typically used in access networks, while routers are typically used in the network core.
The sequence of communication links and packet switches traversed by a packet from the sending end system to the receiving end system is known as a route or path through the network.
Packet-switched networks (which transport packets) are in many ways similar to transportation networks of highways, roads, and intersections (which transport vehicles).
In many ways, packets are analogous to trucks, communication links are analogous to highways and roads, packet switches are analogous to intersections, and end systems are analogous to buildings. Just as a truck takes a path through the transportation network, a packet takes a path through a computer network.
End systems access the Internet through Internet Service Providers (ISPs). Each ISP is in itself a network of packet switches and communication links. ISPs provide a variety of types of network access to the end systems. The Internet is all about connecting end systems to each other, so the ISPs that provide access to end systems must also be interconnected.
End systems, packet switches, and other pieces of the Internet run protocols that control the sending and receiving of information within the Internet. The Transmission Control Protocol (TCP) and the Internet Protocol (IP) are two of the most important protocols in the Internet. The IP protocol specifies the format of the packets that are sent and received among routers and end systems. The Internet’s principal protocols are collectively known as TCP/IP.
The IETF standards documents are called requests for comments (RFCs).
基于服务的描述
We can also describe the Internet from an entirely different angle—namely, as an infrastructure that provides services to applications. These applications include electronic mail, Web surfing, social networks, instant messaging, Voice-over-IP (VoIP), video streaming, distributed games, peer-to-peer (P2P) file sharing, television over the Internet, remote login, and much, much more.
The applications are said to be distributed applications, since they involve multiple end systems that exchange data with each other. Importantly, Internet applications run on end systems—they do not run in the packet switches in the network core. Although packet switches facilitate the exchange of data among end systems, they are not concerned with the application that is the source or sink of data.
Because applications run on end systems, you are going to need to write programs that run on the end systems. How does one program running on one end system instruct the Internet to deliver data to another program running on another end system? End systems attached to the Internet provide an Application Programming Interface (API) that specifies how a program running on one end system asks the Internet infrastructure to deliver data to a specific destination program running on another end system.
This Internet API is a set of rules that the sending program must follow so that the Internet can deliver the data to the destination program.
The Internet provides multiple services to its applications. When you develop an Internet application, you too must choose one of the Internet’s services for your application.
因特网的核心：什么是协议？
We have just given two descriptions of the Internet; one in terms of its hardware and software components, the other in terms of an infrastructure for providing services to distributed applications.
What are packet switching and TCP/IP? What are routers? What kinds of communication links are present in the Internet? What is a distributed application? How can a toaster or a weather sensor be attached to the Internet? If you feel a bit overwhelmed by all of this now, don’t worry—the purpose of this book is to introduce you to both the nuts and bolts of the Internet and the principles that govern how and why it works.
Now that we’ve got a bit of a feel for what the Internet is, let’s consider another important buzzword in computer networking: protocol. What is a protocol? What does a protocol do?
It is probably easiest to understand the notion of a computer network protocol by first considering some human analogies, since we humans execute protocols all of the time.
A network protocol is similar to a human protocol, except that the entities exchanging messages and taking actions are hardware or software components of some device. Protocols are running everywhere in the Internet, and consequently much of this book is about computer network protocols.
A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.
Mastering the field of computer networking is equivalent to understanding the what, why, and how of networking protocols.
A residence typically obtains DSL Internet access from the same local telephone company (telco) that provides its wired local phone access. Thus, when DSL is used, a customer’s telco is also its ISP. The residential telephone line carries both data and traditional telephone signals simultaneously, which are encoded at different frequencies.
While DSL makes use of the telco’s existing local telephone infrastructure, cable Internet access makes use of the cable television company’s existing cable television infrastructure.
One important characteristic of cable Internet access is that it is a shared broadcast medium.
Wide-Area Wireless Access: 3G and LTE, for smartphones.
At time L/R seconds, since the router has just received the entire packet, it can begin to transmit the packet onto the outbound link towards the destination; at time 2L/R, the router has transmitted the entire packet, and the entire packet has been received by the destination. Thus, the total delay is 2L/R.
as we will discuss in Section 1.4, routers need to receive, store, and process the entire packet before forwarding.
If an arriving packet needs to be transmitted onto a link but finds the link busy with the transmission of another packet, the arriving packet must wait in the output buffer. Thus, in addition to the store-and-forward delays, packets suffer output buffer queuing delays. Since the amount of buffer space is finite, an arriving packet may find that the buffer is completely full with other packets waiting for transmission. In this case, packet loss will occur—either the arriving packet or one of the already-queued packets will be dropped.
But how does the router determine which link it should forward the packet onto? Packet forwarding is actually done in different ways in different types of computer networks. Here, we briefly describe how it is done in the Internet.
Proponents of packet switching have always argued that circuit switching is wasteful because the dedicated circuits are idle during silent periods. 
Each layer provides its service by (1) performing certain actions within that layer (for example, at the gate layer, loading and unloading people from an airplane) and by (2) using the services of the layer directly below it (for example, in the gate layer, using the runway-to- runway passenger transfer service of the takeoff/landing layer).
俯瞰计算机网络与因特网
This first chapter presents a broad overview of computer networking and the Internet.
After introducing some basic terminology and concepts, we’ll first examine the basic hardware and software components that make up a network. We’ll begin at the network’s edge and look at the end systems and network applications running in the network. We’ll then explore the core of a computer network, examining the links and the switches that transport data, as well as the access networks and physical media that connect end systems to the network core. We’ll learn that the Internet is a network of networks, and we’ll learn how these networks connect with each other.
After having completed this overview of the edge and core of a computer network, we’ll take the broader and more abstract view in the second half of this chapter. We’ll examine delay, loss, and throughput of data in a computer network and provide simple quantitative models for end-to-end throughput and delay: models that take into account transmission, propagation, and queuing delays. We’ll then introduce some of the key architectural principles in computer networking, namely, protocol layering and service models. We’ll also learn that computer networks are vulnerable to many different types of attacks; we’ll survey some of these attacks and consider how computer networks can be made more secure. Finally, we’ll close this chapter with a brief history of computer networking.