ch2.pdf

CHAPTER 2 Application Layer Network applications are the raisons d’être of a computer network—if we couldn’t conceive of any useful applications, there wouldn’t be...

CHAPTER 2 Application Layer Network applications are the raisons d’être of a computer network—if we couldn’t conceive of any useful applications, there wouldn’t be any need for networking infra- structure and protocols to support them. Since the Internet’s inception, numerous useful and entertaining applications have indeed been created. These applications have been the driving force behind the Internet’s success, motivating people in homes, schools, govern- ments, and businesses to make the Internet an integral part of their daily activities. Internet applications include the classic text-based applications that became pop- ular in the 1970s and 1980s: text e-mail, remote access to computers, file transfers, and newsgroups. They include the killer application of the mid-1990s, the World Wide Web, encompassing Web surfing, search, and electronic commerce. Since the begin- ning of new millennium, new and highly compelling applications continue to emerge, including voice over IP and video conferencing such as Skype, Facetime, and Google Hangouts; user generated video such as YouTube and movies on demand such as Netflix; and multiplayer online games such as Second Life and World of Warcraft. During this same period, we have seen the emergence of a new generation of social networking applications—such as Facebook, Instagram, and Twitter—which have created human networks on top of the Internet’s network or routers and communi- cation links. And most recently, along with the arrival of the smartphone and the ubiquity of 4G/5G wireless Internet access, there has been a profusion of location based mobile apps, including popular check-in, dating, and road-traffic forecasting apps (such as Yelp, Tinder, and Waz), mobile payment apps (such as WeChat and Apple Pay) and messaging apps (such as WeChat and WhatsApp). Clearly, there has been no slowing down of new and exciting Internet applications. Perhaps some of the readers of this text will create the next generation of killer Internet applications! 111 111 111 M02_KURO5469_08_GE_C02.indd 111 03/05/2021 15:50  112 CHAPTER 2 APPLICATION LAYER In this chapter, we study the conceptual and implementation aspects of network applications. We begin by defining key application-layer concepts, including net- work services required by applications, clients and servers, processes, and trans- port-layer interfaces. We examine several network applications in detail, including the Web, e-mail, DNS, peer-to-peer (P2P) file distribution, and video streaming. We then cover network application development, over both TCP and UDP. In particular, we study the socket interface and walk through some simple client-server applications in Python. We also provide several fun and interesting socket programming assign- ments at the end of the chapter. The application layer is a particularly good place to start our study of protocols. It’s familiar ground. We’re acquainted with many of the applications that rely on the protocols we’ll study. It will give us a good feel for what protocols are all about and will introduce us to many of the same issues that we’ll see again when we study transport, network, and link layer protocols. 2.1 Principles of Network Applications Suppose you have an idea for a new network application. Perhaps this application will be a great service to humanity, or will please your professor, or will bring you great wealth, or will simply be fun to develop. Whatever the motivation may be, let’s now examine how you transform the idea into a real-world network application. At the core of network application development is writing programs that run on different end systems and communicate with each other over the network. For exam- ple, in the Web application there are two distinct programs that communicate with each other: the browser program running in the user’s host (desktop, laptop, tablet, smartphone, and so on); and the Web server program running in the Web server host. As another example, in a Video on Demand application such as Netflix (see Sec- tion!2.6), there is a Netflix-provided program running on the user’s smartphone, tablet, or computer; and a Netflix server program running on the Netflix server host. Servers often (but certainly not always) are housed in a data center, as shown in Figure 2.1. Thus, when developing your new application, you need to write software that will run on multiple end systems. This software could be written, for example, in C, Java, or Python. Importantly, you do not need to write software that runs on net- work-core devices, such as routers or link-layer switches. Even if you wanted to write application software for these network-core devices, you wouldn’t be able to do so. As we learned in Chapter 1, and as shown earlier in Figure 1.24, network-core devices do not function at the application layer but instead function at lower layers— specifically at the network layer and below. This basic design—namely, confining application software to the end systems—as shown in Figure 2.1, has facilitated the rapid development and deployment of a vast array of network applications. M02_KURO5469_08_GE_C02.indd 112 03/05/2021 15:50  2.1 PRINCIPLES OF NETWORK APPLICATIONS 113 Application Transport Network Data Link Physical National or Application Global ISP Transport Network Data Link Mobile Network Physical Datacenter Network Datacenter Network Local or Home Network Regional ISP Content Provider Network Application Transport Network Data Link Physical Enterprise Network Figure 2.1 ♦ Communication for a network application takes place between end systems at the application layer M02_KURO5469_08_GE_C02.indd 113 03/05/2021 15:50  114 CHAPTER 2 APPLICATION LAYER 2.1.1 Network Application Architectures Before diving into software coding, you should have a broad architectural plan for your application. Keep in mind that an application’s architecture is distinctly differ- ent from the network architecture (e.g., the five-layer Internet architecture discussed in Chapter 1). From the application developer’s perspective, the network architec- ture is fixed and provides a specific set of services to applications. The application architecture, on the other hand, is designed by the application developer and dic- tates how the application is structured over the various end systems. In choosing the application architecture, an application developer will likely draw on one of the two predominant architectural paradigms used in modern network applications: the client-server architecture or the peer-to-peer (P2P) architecture. In a client-server architecture, there is an always-on host, called the server, which services requests from many other hosts, called clients. A classic example is the Web application for which an always-on Web server services requests from browsers running on client hosts. When a Web server receives a request for an object from a client host, it responds by sending the requested object to the client host. Note that with the client-server architecture, clients do not directly communicate with each other; for example, in the Web application, two browsers do not directly communi- cate. Another characteristic of the client-server architecture is that the server has a fixed, well-known address, called an IP address (which we’ll discuss soon). Because the server has a fixed, well-known address, and because the server is always on, a cli- ent can always contact the server by sending a packet to the server’s IP address. Some of the better-known applications with a client-server architecture include the Web, FTP, Telnet, and e-mail. The client-server architecture is shown in Figure 2.2(a). Often in a client-server application, a single-server host is incapable of keep- ing up with all the requests from clients. For example, a popular social-networking site can quickly become overwhelmed if it has only one server handling all of its requests. For this reason, a data center, housing a large number of hosts, is often used to create a powerful virtual server. The most popular Internet services—such as search engines (e.g., Google, Bing, Baidu), Internet commerce (e.g., Amazon, eBay, Alibaba), Web-based e-mail (e.g., Gmail and Yahoo Mail), social media (e.g., Facebook, Instagram, Twitter, and WeChat)—run in one or more data centers. As discussed in Section 1.3.3, Google has 19 data centers distributed around the world, which collectively handle search, YouTube, Gmail, and other services. A data center can have hundreds of thousands of servers, which must be powered and maintained. Additionally, the service providers must pay recurring interconnection and band- width costs for sending data from their data centers. In a P2P architecture, there is minimal (or no) reliance on dedicated servers in data centers. Instead the application exploits direct communication between pairs of intermittently connected hosts, called peers. The peers are not owned by the service provider, but are instead desktops and laptops controlled by users, with most of the peers residing in homes, universities, and offices. Because the peers communicate M02_KURO5469_08_GE_C02.indd 114 03/05/2021 15:50  2.1 PRINCIPLES OF NETWORK APPLICATIONS 115 a. Client-server architecture b. Peer-to-peer architecture Figure 2.2 ♦ (a) Client-server architecture; (b) P2P architecture without passing through a dedicated server, the architecture is called peer-to-peer. An example of a popular P2P application is the file-sharing application BitTorrent. One of the most compelling features of P2P architectures is their self- scalability. For example, in a P2P file-sharing application, although each peer generates workload by requesting files, each peer also adds service capacity to the system by distributing files to other peers. P2P architectures are also cost effective, since they normally don’t require significant server infrastructure and server band- width (in contrast with clients-server designs with datacenters). However, P2P appli- cations face challenges of security, performance, and reliability due to their highly decentralized structure. 2.1.2 Processes Communicating Before building your network application, you also need a basic understanding of how the programs, running in multiple end systems, communicate with each other. In the jargon of operating systems, it is not actually programs but processes that M02_KURO5469_08_GE_C02.indd 115 03/05/2021 15:50  116 CHAPTER 2 APPLICATION LAYER communicate. A process can be thought of as a program that is running within an end system. When processes are running on the same end system, they can communicate with each other with interprocess communication, using rules that are governed by the end system’s operating system. But in this book, we are not particularly interested in how processes in the same host communicate, but instead in how processes run- ning on different hosts (with potentially different operating systems) communicate. Processes on two different end systems communicate with each other by exchanging messages across the computer network. A sending process creates and sends messages into the network; a receiving process receives these messages and possibly responds by sending messages back. Figure 2.1 illustrates that processes communicating with each other reside in the application layer of the five-layer pro- tocol stack. Client and Server Processes A network application consists of pairs of processes that send messages to each other over a network. For example, in the Web application a client browser process exchanges messages with a Web server process. In a P2P file-sharing system, a file is transferred from a process in one peer to a process in another peer. For each pair of communicating processes, we typically label one of the two processes as the client and the other process as the server. With the Web, a browser is a client process and a Web server is a server process. With P2P file sharing, the peer that is downloading the file is labeled as the client, and the peer that is uploading the file is labeled as the server. You may have observed that in some applications, such as in P2P file sharing, a process can be both a client and a server. Indeed, a process in a P2P file-sharing system can both upload and download files. Nevertheless, in the context of any given communication session between a pair of processes, we can still label one process as the client and the other process as the server. We define the client and server pro- cesses as follows: In the context of a communication session between a pair of processes, the pro- cess that initiates the communication (that is, initially contacts the other process at the beginning of the session) is labeled as the client. The process that waits to be contacted to begin the session is the server. In the Web, a browser process initializes contact with a Web server process; hence the browser process is the client and the Web server process is the server. In P2P file sharing, when Peer A asks Peer B to send a specific file, Peer A is the cli- ent and Peer B is the server in the context of this specific communication session. When there’s no confusion, we’ll sometimes also use the terminology “client side and server side of an application.” At the end of this chapter, we’ll step through sim- ple code for both the client and server sides of network applications. M02_KURO5469_08_GE_C02.indd 116 03/05/2021 15:50  2.1 PRINCIPLES OF NETWORK APPLICATIONS 117 The Interface Between the Process and the Computer Network As noted above, most applications consist of pairs of communicating processes, with the two processes in each pair sending messages to each other. Any message sent from one process to another must go through the underlying network. A process sends messages into, and receives messages from, the network through a software interface called a socket. Let’s consider an analogy to help us understand processes and sockets. A process is analogous to a house and its socket is analogous to its door. When a process wants to send a message to another process on another host, it shoves the message out its door (socket). This sending process assumes that there is a trans- portation infrastructure on the other side of its door that will transport the message to the door of the destination process. Once the message arrives at the destination host, the message passes through the receiving process’s door (socket), and the receiving process then acts on the message. Figure 2.3 illustrates socket communication between two processes that com- municate over the Internet. (Figure 2.3 assumes that the underlying transport protocol used by the processes is the Internet’s TCP protocol.) As shown in this figure, a socket is the interface between the application layer and the transport layer within a host. It is also referred to as the Application Programming Interface (API) between the application and the network, since the socket is the programming interface with which network applications are built. The application developer has control of everything on the application-layer side of the socket but has little control of the transport-layer side of the socket. The only control that the application developer has on the transport- layer side is (1) the choice of transport protocol and (2) perhaps the ability to fix a few Host or Host or server server Controlled Controlled by application Process Process by application developer developer Socket Socket Controlled TCP with TCP with Controlled by operating buffers, buffers, by operating system variables variables system Internet Figure 2.3 ♦ Application processes, sockets, and underlying transport protocol M02_KURO5469_08_GE_C02.indd 117 03/05/2021 15:50  118 CHAPTER 2 APPLICATION LAYER transport-layer parameters such as maximum buffer and maximum segment sizes (to be covered in Chapter 3). Once the application developer chooses a transport protocol (if a choice is available), the application is built using the transport-layer services provided by that protocol. We’ll explore sockets in some detail in Section 2.7. Addressing Processes In order to send postal mail to a particular destination, the destination needs to have an address. Similarly, in order for a process running on one host to send packets to a process running on another host, the receiving process needs to have an address. To identify the receiving process, two pieces of information need to be specified: (1)!the address of the host and (2) an identifier that specifies the receiving process in the destination host. In the Internet, the host is identified by its IP address. We’ll discuss IP addresses in great detail in Chapter 4. For now, all we need to know is that an IP address is a 32-bit quantity that we can think of as uniquely identifying the host. In addition to know- ing the address of the host to which a message is destined, the sending process must also identify the receiving process (more specifically, the receiving socket) running in the host. This information is needed because in general a host could be running many network applications. A destination port number serves this purpose. Popular applica- tions have been assigned specific port numbers. For example, a Web server is identified by port number 80. A mail server process (using the SMTP protocol) is identified by port number 25. A list of well-known port numbers for all Internet standard protocols can be found at www.iana.org. We’ll examine port numbers in detail in Chapter 3. 2.1.3 Transport Services Available to Applications Recall that a socket is the interface between the application process and the trans- port-layer protocol. The application at the sending side pushes messages through the socket. At the other side of the socket, the transport-layer protocol has the responsi- bility of getting the messages to the socket of the receiving process. Many networks, including the Internet, provide more than one transport-layer protocol. When you develop an application, you must choose one of the available transport-layer protocols. How do you make this choice? Most likely, you would study the services provided by the available transport-layer protocols, and then pick the protocol with the services that best match your application’s needs. The situation is similar to choosing either train or airplane transport for travel between two cities. You have to choose one or the other, and each transportation mode offers different services. (For example, the train offers downtown pickup and drop-off, whereas the plane offers shorter travel time.) What are the services that a transport-layer protocol can offer to applications invoking it? We can broadly classify the possible services along four dimensions: reliable data transfer, throughput, timing, and security. M02_KURO5469_08_GE_C02.indd 118 03/05/2021 15:50  2.1 PRINCIPLES OF NETWORK APPLICATIONS 119 Reliable Data Transfer As discussed in Chapter 1, packets can get lost within a computer network. For exam- ple, a packet can overflow a buffer in a router, or can be discarded by a host or router after having some of its bits corrupted. For many applications—such as electronic mail, file transfer, remote host access, Web document transfers, and financial appli- cations—data loss can have devastating consequences (in the latter case, for either the bank or the customer!). Thus, to support these applications, something has to be done to guarantee that the data sent by one end of the application is delivered cor- rectly and completely to the other end of the application. If a protocol provides such a guaranteed data delivery service, it is said to provide reliable data transfer. One important service that a transport-layer protocol can potentially provide to an applica- tion is process-to-process reliable data transfer. When a transport protocol provides this service, the sending process can just pass its data into the socket and know with complete confidence that the data will arrive without errors at the receiving process. When a transport-layer protocol doesn’t provide reliable data transfer, some of the data sent by the sending process may never arrive at the receiving process. This may be acceptable for loss-tolerant applications, most notably multimedia applica- tions such as conversational audio/video that can tolerate some amount of data loss. In these multimedia applications, lost data might result in a small glitch in the audio/ video—not a crucial impairment. Throughput In Chapter 1, we introduced the concept of available throughput, which, in the context of a communication session between two processes along a network path, is the rate at which the sending process can deliver bits to the receiving process. Because other sessions will be sharing the bandwidth along the network path, and because these other sessions will be coming and going, the available throughput can fluctuate with time. These observations lead to another natural service that a transport-layer protocol could provide, namely, guaranteed available throughput at some specified rate. With such a service, the application could request a guaranteed throughput of r bits/sec, and the transport protocol would then ensure that the avail- able throughput is always at least r bits/sec. Such a guaranteed throughput service would appeal to many applications. For example, if an Internet telephony applica- tion encodes voice at 32 kbps, it needs to send data into the network and have data delivered to the receiving application at this rate. If the transport protocol cannot provide this throughput, the application would need to encode at a lower rate (and receive enough throughput to sustain this lower coding rate) or may have to give up, since receiving, say, half of the needed throughput is of little or no use to this Internet telephony application. Applications that have throughput requirements are said to be bandwidth-sensitive applications. Many current multimedia applications are bandwidth sensitive, although some multimedia applications may use adaptive M02_KURO5469_08_GE_C02.indd 119 03/05/2021 15:50  120 CHAPTER 2 APPLICATION LAYER coding techniques to encode digitized voice or video at a rate that matches the cur- rently available throughput. While bandwidth-sensitive applications have specific throughput requirements, elastic applications can make use of as much, or as little, throughput as happens to be available. Electronic mail, file transfer, and Web transfers are all elastic applica- tions. Of course, the more throughput, the better. There’s an adage that says that one cannot be too rich, too thin, or have too much throughput! Timing A transport-layer protocol can also provide timing guarantees. As with throughput guarantees, timing guarantees can come in many shapes and forms. An example guarantee might be that every bit that the sender pumps into the socket arrives at the receiver’s socket no more than 100 msec later. Such a service would be appealing to interactive real-time applications, such as Internet telephony, virtual environments, teleconferencing, and multiplayer games, all of which require tight timing constraints on data delivery in order to be effective, see [Gauthier 1999; Ramjee 1994]. Long delays in Internet telephony, for example, tend to result in unnatural pauses in the conversation; in a multiplayer game or virtual interactive environment, a long delay between taking an action and seeing the response from the environment (for example, from another player at the end of an end-to-end con- nection) makes the application feel less realistic. For non-real-time applications, lower delay is always preferable to higher delay, but no tight constraint is placed on the end-to-end delays. Security Finally, a transport protocol can provide an application with one or more security services. For example, in the sending host, a transport protocol can encrypt all data transmitted by the sending process, and in the receiving host, the transport-layer pro- tocol can decrypt the data before delivering the data to the receiving process. Such a service would provide confidentiality between the two processes, even if the data is somehow observed between sending and receiving processes. A transport protocol can also provide other security services in addition to confidentiality, including data integrity and end-point authentication, topics that we’ll cover in detail in Chapter 8. 2.1.4 Transport Services Provided by the Internet Up until this point, we have been considering transport services that a computer net- work could provide in general. Let’s now get more specific and examine the type of transport services provided by the Internet. The Internet (and, more generally, TCP/ IP networks) makes two transport protocols available to applications, UDP and TCP. When you (as an application developer) create a new network application for the M02_KURO5469_08_GE_C02.indd 120 03/05/2021 15:50  2.1 PRINCIPLES OF NETWORK APPLICATIONS 121 Application Data Loss Throughput Time-Sensitive File transfer/download No loss Elastic No E-mail No loss Elastic No Web documents No loss Elastic (few kbps) No Internet telephony/ Loss-tolerant Audio: few kbps–1 Mbps Yes: 100s of msec Video conferencing Video: 10 kbps–5 Mbps Streaming stored Loss-tolerant Same as above Yes: few seconds audio/video Interactive games Loss-tolerant Few kbps–10 kbps Yes: 100s of msec Smartphone messaging No loss Elastic Yes and no Figure 2.4 ♦ Requirements of selected network applications Internet, one of the first decisions you have to make is whether to use UDP or TCP. Each of these protocols offers a different set of services to the invoking applications. Figure 2.4 shows the service requirements for some selected applications. TCP Services The TCP service model includes a connection-oriented service and a reliable data transfer service. When an application invokes TCP as its transport protocol, the application receives both of these services from TCP. Connection-oriented service. TCP has the client and server exchange transport- layer control information with each other before the application-level mes- sages begin to flow. This so-called handshaking procedure alerts the client and server, allowing them to prepare for an onslaught of packets. After the handshaking phase, a TCP connection is said to exist between the sockets of the two processes. The connection is a full-duplex connection in that the two processes can send messages to each other over the connection at the same time. When the application finishes sending messages, it must tear down the connec- tion. In Chapter 3, we’ll discuss connection-oriented service in detail and examine how it is implemented. Reliable data transfer service. The communicating processes can rely on TCP to deliver all data sent without error and in the proper order. When one side of the application passes a stream of bytes into a socket, it can count on TCP to deliver the same stream of bytes to the receiving socket, with no missing or duplicate bytes. M02_KURO5469_08_GE_C02.indd 121 03/05/2021 15:50  122 CHAPTER 2 APPLICATION LAYER TCP also includes a congestion-control mechanism, a service for the general welfare of the Internet rather than for the direct benefit of the communicating pro- cesses. The TCP congestion-control mechanism throttles a sending process (client or server) when the network is congested between sender and receiver. As we will see in Chapter 3, TCP congestion control also attempts to limit each TCP connection to its fair share of network bandwidth. UDP Services UDP is a no-frills, lightweight transport protocol, providing minimal services. UDP is connectionless, so there is no handshaking before the two processes start to com- municate. UDP provides an unreliable data transfer service—that is, when a process sends a message into a UDP socket, UDP provides no guarantee that the message will ever reach the receiving process. Furthermore, messages that do arrive at the receiving process may arrive out of order. FOCUS ON SECURITY SECURING T CP Neither TCP nor UDP provides any encryption—the data that the sending process passes into its socket is the same data that travels over the network to the destina- tion process. So, for example, if the sending process sends a password in cleartext (i.e., unencrypted) into its socket, the cleartext password will travel over all the links between sender and receiver, potentially getting sniffed and discovered at any of the intervening links. Because privacy and other security issues have become critical for many applications, the Internet community has developed an enhancement for TCP, called Transport Layer Security (TLS) [RFC 5246]. TCP-enhanced-with-TLS not only does everything that traditional TCP does but also provides critical process-to- process security services, including encryption, data integrity, and end-point authenti- cation. We emphasize that TLS is not a third Internet transport protocol, on the same level as TCP and UDP, but instead is an enhancement of TCP, with the enhancements being implemented in the application layer. In particular, if an application wants to use the services of TLS, it needs to include TLS code (existing, highly optimized librar- ies and classes) in both the client and server sides of the application. TLS has its own socket API that is similar to the traditional TCP socket API. When an application uses TLS, the sending process passes cleartext data to the TLS socket; TLS in the sending host then encrypts the data and passes the encrypted data to the TCP socket. The encrypted data travels over the Internet to the TCP socket in the receiving process. The receiving socket passes the encrypted data to TLS, which decrypts the data. Finally, TLS passes the cleartext data through its TLS socket to the receiving process. We’ll cover TLS in some detail in Chapter 8. M02_KURO5469_08_GE_C02.indd 122 03/05/2021 15:50  2.1 PRINCIPLES OF NETWORK APPLICATIONS 123 UDP does not include a congestion-control mechanism, so the sending side of UDP can pump data into the layer below (the network layer) at any rate it pleases. (Note, however, that the actual end-to-end throughput may be less than this rate due to the limited transmission capacity of intervening links or due to congestion). Services Not Provided by Internet Transport Protocols We have organized transport protocol services along four dimensions: reliable data transfer, throughput, timing, and security. Which of these services are provided by TCP and UDP? We have already noted that TCP provides reliable end-to-end data transfer. And we also know that TCP can be easily enhanced at the application layer with TLS to provide security services. But in our brief description of TCP and UDP, conspicuously missing was any mention of throughput or timing guarantees— services not provided by today’s Internet transport protocols. Does this mean that time- sensitive applications such as Internet telephony cannot run in today’s Internet? The answer is clearly no—the Internet has been hosting time-sensitive applications for many years. These applications often work fairly well because they have been designed to cope, to the greatest extent possible, with this lack of guarantee. Nevertheless, clever design has its limitations when delay is excessive, or the end-to-end throughput is limited. In summary, today’s Internet can often provide satisfactory service to time- sensitive applications, but it cannot provide any timing or throughput guarantees. Figure 2.5 indicates the transport protocols used by some popular Internet appli- cations. We see that e-mail, remote terminal access, the Web, and file transfer all use TCP. These applications have chosen TCP primarily because TCP provides reliable data transfer, guaranteeing that all data will eventually get to its destination. Because Internet telephony applications (such as Skype) can often tolerate some loss but require a minimal rate to be effective, developers of Internet telephony applications Application Application-Layer Protocol Underlying Transport Protocol Electronic mail SMTP [RFC 5321] TCP Remote terminal access Telnet [RFC 854] TCP Web HTTP 1.1 [RFC 7230] TCP File transfer FTP [RFC 959] TCP Streaming multimedia HTTP (e.g., YouTube), DASH TCP Internet telephony SIP [RFC 3261], RTP [RFC 3550], or proprietary UDP or TCP (e.g., Skype) Figure 2.5 ♦ Popular Internet applications, their application-layer protocols, and their underlying transport protocols M02_KURO5469_08_GE_C02.indd 123 03/05/2021 15:50  124 CHAPTER 2 APPLICATION LAYER usually prefer to run their applications over UDP, thereby circumventing TCP’s congestion control mechanism and packet overheads. But because many firewalls are configured to block (most types of) UDP traffic, Internet telephony applications often are designed to use TCP as a backup if UDP communication fails. 2.1.5 Application-Layer Protocols We have just learned that network processes communicate with each other by sending messages into sockets. But how are these messages structured? What are the meanings of the various fields in the messages? When do the processes send the messages? These questions bring us into the realm of application-layer protocols. An application-layer protocol defines how an application’s processes, running on different end systems, pass messages to each other. In particular, an application-layer protocol defines: The types of messages exchanged, for example, request messages and response messages The syntax of the various message types, such as the fields in the message and how the fields are delineated The semantics of the fields, that is, the meaning of the information in the fields Rules for determining when and how a process sends messages and responds to messages Some application-layer protocols are specified in RFCs and are therefore in the public domain. For example, the Web’s application-layer protocol, HTTP (the HyperText Transfer Protocol [RFC 7230]), is available as an RFC. If a browser developer follows the rules of the HTTP RFC, the browser will be able to retrieve Web pages from any Web server that has also followed the rules of the HTTP RFC. Many other application-layer protocols are proprietary and intentionally not avail- able in the public domain. For example, Skype uses proprietary application-layer protocols. It is important to distinguish between network applications and application-layer protocols. An application-layer protocol is only one piece of a network application (albeit, a very important piece of the application from our point of view!). Let’s look at a couple of examples. The Web is a client-server application that allows users to obtain documents from Web servers on demand. The Web application consists of many components, including a standard for document formats (that is, HTML), Web browsers (for example, Chrome and Microsoft Internet Explorer), Web servers (for example, Apache and Microsoft servers), and an application-layer protocol. The Web’s application-layer protocol, HTTP, defines the format and sequence of messages exchanged between browser and Web server. Thus, HTTP is only one piece (albeit, an important piece) of the Web application. As another example, we’ll see in Section 2.6 that Netflix’s video service also has many components, M02_KURO5469_08_GE_C02.indd 124 03/05/2021 15:50  2.2 THE WEB AND HTTP 125 including servers that store and transmit videos, other servers that manage billing and other client functions, clients (e.g., the Netflix app on your smartphone, tablet, or computer), and an application-level DASH protocol defines the format and sequence of messages exchanged between a Netflix server and client. Thus, DASH is only one piece (albeit, an important piece) of the Netflix application. 2.1.6 Network Applications Covered in This Book New applications are being developed every day. Rather than covering a large number of Internet applications in an encyclopedic manner, we have chosen to focus on a small number of applications that are both pervasive and important. In this chapter, we discuss five important applications: the Web, electronic mail, directory service, video streaming, and P2P applications. We first discuss the Web, not only because it is an enormously popular application, but also because its application-layer protocol, HTTP, is straightforward and easy to understand. We then discuss electronic mail, the Internet’s first killer application. E-mail is more complex than the Web in the sense that it makes use of not one but sev- eral application-layer protocols. After e-mail, we cover DNS, which provides a directory service for the Internet. Most users do not interact with DNS directly; instead, users invoke DNS indirectly through other applications (including the Web, file transfer, and electronic mail). DNS illustrates nicely how a piece of core network functionality (network-name to network-address translation) can be implemented at the application layer in the Internet. We then discuss P2P file sharing applications, and complete our application study by discussing video streaming on demand, including distributing stored video over content distribu- tion networks. 2.2 The Web and HTTP Until the early 1990s, the Internet was used primarily by researchers, academics, and university students to log in to remote hosts, to transfer files from local hosts to remote hosts and vice versa, to receive and send news, and to receive and send elec- tronic mail. Although these applications were (and continue to be) extremely useful, the Internet was essentially unknown outside of the academic and research commu- nities. Then, in the early 1990s, a major new application arrived on the scene—the World Wide Web [Berners-Lee 1994]. The Web was the first Internet application that caught the general public’s eye. It dramatically changed how people interact inside and outside their work environments. It elevated the Internet from just one of many data networks to essentially the one and only data network. Perhaps what appeals the most to users is that the Web operates on demand. Users receive what they want, when they want it. This is unlike traditional broadcast M02_KURO5469_08_GE_C02.indd 125 03/05/2021 15:50  126 CHAPTER 2 APPLICATION LAYER radio and television, which force users to tune in when the content provider makes the content available. In addition to being available on demand, the Web has many other wonderful features that people love and cherish. It is enormously easy for any individual to make information available over the Web—everyone can become a publisher at extremely low cost. Hyperlinks and search engines help us navigate through an ocean of information. Photos and videos stimulate our senses. Forms, JavaScript, video, and many other devices enable us to interact with pages and sites. And the Web and its protocols serve as a platform for YouTube, Web-based e-mail (such as Gmail), and most mobile Internet applications, including Instagram and Google Maps. 2.2.1 Overview of HTTP The HyperText Transfer Protocol (HTTP), the Web’s application-layer protocol, is at the heart of the Web. It is defined in [RFC 1945], [RFC 7230] and [RFC 7540]. HTTP is implemented in two programs: a client program and a server program. The client program and server program, executing on different end systems, talk to each other by exchanging HTTP messages. HTTP defines the structure of these messages and how the client and server exchange the messages. Before explaining HTTP in detail, we should review some Web terminology. A Web page (also called a document) consists of objects. An object is simply a file—such as an HTML file, a JPEG image, a Javascrpt file, a CCS style sheet file, or a video clip—that is addressable by a single URL. Most Web pages consist of a base HTML file and several referenced objects. For example, if a Web page contains HTML text and five JPEG images, then the Web page has six objects: the base HTML file plus the five images. The base HTML file refer- ences the other objects in the page with the objects’ URLs. Each URL has two components: the hostname of the server that houses the object and the object’s path name. For example, the URL http://www.someSchool.edu/someDepartment/picture.gif has www.someSchool.edu for a hostname and /someDepartment/picture. gif for a path name. Because Web browsers (such as Internet Explorer and Chrome) implement the client side of HTTP, in the context of the Web, we will use the words browser and client interchangeably. Web servers, which implement the server side of HTTP, house Web objects, each addressable by a URL. Popular Web servers include Apache and Microsoft Internet Information Server. HTTP defines how Web clients request Web pages from Web servers and how servers transfer Web pages to clients. We discuss the interaction between client and server in detail later, but the general idea is illustrated in Figure 2.6. When a user requests a Web page (for example, clicks on a hyperlink), the browser sends M02_KURO5469_08_GE_C02.indd 126 03/05/2021 15:50  2.2 THE WEB AND HTTP 127 Server running Apache Web server HT s t TP ue e re r eq ns HT sp on po TP TP es re se HT P r qu H TT es t PC running Android smartphone Internet Explorer running Google Chrome Figure 2.6 ♦ HTTP request-response behavior HTTP request messages for the objects in the page to the server. The server receives the requests and responds with HTTP response messages that contain the objects. HTTP uses TCP as its underlying transport protocol (rather than running on top of UDP). The HTTP client first initiates a TCP connection with the server. Once the connection is established, the browser and the server processes access TCP through their socket interfaces. As described in Section 2.1, on the client side the socket inter- face is the door between the client process and the TCP connection; on the server side it is the door between the server process and the TCP connection. The client sends HTTP request messages into its socket interface and receives HTTP response mes- sages from its socket interface. Similarly, the HTTP server receives request messages from its socket interface and sends response messages into its socket interface. Once the client sends a message into its socket interface, the message is out of the client’s hands and is “in the hands” of TCP. Recall from Section 2.1 that TCP provides a reliable data transfer service to HTTP. This implies that each HTTP request message sent by a client process eventually arrives intact at the server; similarly, each HTTP response message sent by the server process eventually arrives intact at the client. Here we see one of the great advantages of a layered architecture—HTTP need not worry about lost data or the details of how TCP recovers from loss or reordering of data within the network. That is the job of TCP and the protocols in the lower layers of the protocol stack. It is important to note that the server sends requested files to clients without storing any state information about the client. If a particular client asks for the same object twice in a period of a few seconds, the server does not respond by saying that it just served the object to the client; instead, the server resends the object, as it has completely forgotten what it did earlier. Because an HTTP server maintains M02_KURO5469_08_GE_C02.indd 127 03/05/2021 15:50  128 CHAPTER 2 APPLICATION LAYER no information about the clients, HTTP is said to be a stateless protocol. We also remark that the Web uses the client-server application architecture, as described in Section 2.1. A Web server is always on, with a fixed IP address, and it services requests from potentially millions of different browsers. The original version of HTTP is called HTTP/1.0 and dates back to the early 1990’s [RFC 1945]. As of 2020, the majority of HTTP transactions take place over HTTP/1.1 [RFC 7230]. However, increasingly browsers and Web servers also sup- port a new version of HTTP called HTTP/2 [RFC 7540]. At the end of this section, we’ll provide an introduction to HTTP/2. 2.2.2 Non-Persistent and Persistent Connections In many Internet applications, the client and server communicate for an extended period of time, with the client making a series of requests and the server respond- ing to each of the requests. Depending on the application and on how the application is being used, the series of requests may be made back-to-back, peri- odically at regular intervals, or intermittently. When this client-server interaction is taking place over TCP, the application developer needs to make an important decision—should each request/response pair be sent over a separate TCP connec- tion, or should all of the requests and their corresponding responses be sent over the same TCP connection? In the former approach, the application is said to use non-persistent connections; and in the latter approach, persistent connections. To gain a deep understanding of this design issue, let’s examine the advantages and dis- advantages of persistent connections in the context of a specific application, namely, HTTP, which can use both non-persistent connections and persistent connections. Although HTTP uses persistent connections in its default mode, HTTP clients and servers can be configured to use non-persistent connections instead. HTTP with Non-Persistent Connections Let’s walk through the steps of transferring a Web page from server to client for the case of non-persistent connections. Let’s suppose the page consists of a base HTML file and 10 JPEG images, and that all 11 of these objects reside on the same server. Further suppose the URL for the base HTML file is http://www.someSchool.edu/someDepartment/home.index Here is what happens: 1. The HTTP client process initiates a TCP connection to the server www.someSchool.edu on port number 80, which is the default port number for HTTP. Associated with the TCP connection, there will be a socket at the client and a socket at the server. M02_KURO5469_08_GE_C02.indd 128 03/05/2021 15:50  2.2 THE WEB AND HTTP 129 2. The HTTP client sends an HTTP request message to the server via its socket. The request message includes the path name /someDepartment/home.index. (We will discuss HTTP messages in some detail below.) 3. The HTTP server process receives the request message via its socket, retrieves the object /someDepartment/home.index from its storage (RAM or disk), encapsulates the object in an HTTP response message, and sends the response message to the client via its socket. 4. The HTTP server process tells TCP to close the TCP connection. (But TCP doesn’t actually terminate the connection until it knows for sure that the client has received the response message intact.) 5. The HTTP client receives the response message. The TCP connection termi- nates. The message indicates that the encapsulated object is an HTML file. The client extracts the file from the response message, examines the HTML file, and finds references to the 10 JPEG objects. 6. The first four steps are then repeated for each of the referenced JPEG objects. As the browser receives the Web page, it displays the page to the user. Two different browsers may interpret (that is, display to the user) a Web page in some- what different ways. HTTP has nothing to do with how a Web page is interpreted by a client. The HTTP specifications ([RFC 1945] and [RFC 7540]) define only the communication protocol between the client HTTP program and the server HTTP program. The steps above illustrate the use of non-persistent connections, where each TCP connection is closed after the server sends the object—the connection does not persist for other objects. HTTP/1.0 employes non-persistent TCP connections. Note that each non-persistent TCP connection transports exactly one request message and one response message. Thus, in this example, when a user requests the Web page, 11 TCP connections are generated. In the steps described above, we were intentionally vague about whether the client obtains the 10 JPEGs over 10 serial TCP connections, or whether some of the JPEGs are obtained over parallel TCP connections. Indeed, users can configure some browsers to control the degree of parallelism. Browsers may open multiple TCP con- nections and request different parts of the Web page over the multiple connections. As we’ll see in the next chapter, the use of parallel connections shortens the response time. Before continuing, let’s do a back-of-the-envelope calculation to estimate the amount of time that elapses from when a client requests the base HTML file until the entire file is received by the client. To this end, we define the round-trip time (RTT), which is the time it takes for a small packet to travel from client to server and then back to the client. The RTT includes packet-propagation delays, packet- queuing delays in intermediate routers and switches, and packet-processing delays. (These delays were discussed in Section 1.4.) Now consider what happens when a user clicks on a hyperlink. As shown in Figure 2.7, this causes the browser to initiate a TCP connection between the browser and the Web server; this involves M02_KURO5469_08_GE_C02.indd 129 03/05/2021 15:50  130 CHAPTER 2 APPLICATION LAYER Initiate TCP connection RTT Request file RTT Time to transmit file Entire file received Time Time at client at server Figure 2.7 ♦ Back-of-the-envelope calculation for the time needed to request and receive an HTML file a “three-way handshake”—the client sends a small TCP segment to the server, the server acknowledges and responds with a small TCP segment, and, finally, the cli- ent acknowledges back to the server. The first two parts of the three-way handshake take one RTT. After completing the first two parts of the handshake, the client sends the HTTP request message combined with the third part of the three-way handshake (the acknowledgment) into the TCP connection. Once the request message arrives at the server, the server sends the HTML file into the TCP connection. This HTTP request/response eats up another RTT. Thus, roughly, the total response time is two RTTs plus the transmission time at the server of the HTML file. HTTP with Persistent Connections Non-persistent connections have some shortcomings. First, a brand-new connection must be established and maintained for each requested object. For each of these connections, TCP buffers must be allocated and TCP variables must be kept in both the client and server. This can place a significant burden on the Web server, which may be serving requests from hundreds of different clients simultaneously. Second, M02_KURO5469_08_GE_C02.indd 130 03/05/2021 15:50  2.2 THE WEB AND HTTP 131 as we just described, each object suffers a delivery delay of two RTTs—one RTT to establish the TCP connection and one RTT to request and receive an object. With HTTP/1.1 persistent connections, the server leaves the TCP connection open after sending a response. Subsequent requests and responses between the same client and server can be sent over the same connection. In particular, an entire Web page (in the example above, the base HTML file and the 10 images) can be sent over a single persistent TCP connection. Moreover, multiple Web pages residing on the same server can be sent from the server to the same client over a single persistent TCP connection. These requests for objects can be made back-to-back, without wait- ing for replies to pending requests (pipelining). Typically, the HTTP server closes a connection when it isn’t used for a certain time (a configurable timeout interval). When the server receives the back-to-back requests, it sends the objects back-to- back. The default mode of HTTP uses persistent connections with pipelining. We’ll quantitatively compare the performance of non-persistent and persistent connections in the homework problems of Chapters 2 and 3. You are also encouraged to see [Heidemann 1997; Nielsen 1997; RFC 7540]. 2.2.3 HTTP Message Format The HTTP specifications [RFC 1945; RFC 7230; RFC 7540] include the definitions of the HTTP message formats. There are two types of HTTP messages, request mes- sages and response messages, both of which are discussed below. HTTP Request Message Below we provide a typical HTTP request message: GET /somedir/page.html HTTP/1.1 Host: www.someschool.edu Connection: close User-agent: Mozilla/5.0 Accept-language: fr We can learn a lot by taking a close look at this simple request message. First of all, we see that the message is written in ordinary ASCII text, so that your ordinary computer-literate human being can read it. Second, we see that the message consists of five lines, each followed by a carriage return and a line feed. The last line is fol- lowed by an additional carriage return and line feed. Although this particular request message has five lines, a request message can have many more lines or as few as one line. The first line of an HTTP request message is called the request line; the subsequent lines are called the header lines. The request line has three fields: the method field, the URL field, and the HTTP version field. The method field can take on several different values, including GET, POST, HEAD, PUT, and DELETE. M02_KURO5469_08_GE_C02.indd 131 03/05/2021 15:50  132 CHAPTER 2 APPLICATION LAYER The great majority of HTTP request messages use the GET method. The GET method is used when the browser requests an object, with the requested object identified in the URL field. In this example, the browser is requesting the object /somedir/ page.html. The version is self-explanatory; in this example, the browser imple- ments version HTTP/1.1. Now let’s look at the header lines in the example. The header line Host: www.someschool.edu specifies the host on which the object resides. You might think that this header line is unnecessary, as there is already a TCP connection in place to the host. But, as we’ll see in Section 2.2.5, the information provided by the host header line is required by Web proxy caches. By including the Connection: close header line, the browser is telling the server that it doesn’t want to bother with persistent connections; it wants the server to close the connection after sending the requested object. The User-agent: header line specifies the user agent, that is, the browser type that is making the request to the server. Here the user agent is Mozilla/5.0, a Firefox browser. This header line is useful because the server can actu- ally send different versions of the same object to different types of user agents. (Each of the versions is addressed by the same URL.) Finally, the Accept-language: header indicates that the user prefers to receive a French version of the object, if such an object exists on the server; otherwise, the server should send its default version. The Accept-language: header is just one of many content negotiation headers available in HTTP. Having looked at an example, let’s now look at the general format of a request message, as shown in Figure 2.8. We see that the general format closely follows our earlier example. You may have noticed, however, that after the header lines (and the additional carriage return and line feed) there is an “entity body.” The entity body Request line method sp URL sp Version cr lf header field name: sp value cr lf Header lines header field name: sp value cr lf Blank line cr lf Entity body Figure 2.8 ♦ General format of an HTTP request message M02_KURO5469_08_GE_C02.indd 132 03/05/2021 15:50  2.2 THE WEB AND HTTP 133 is empty with the GET method, but is used with the POST method. An HTTP client often uses the POST method when the user fills out a form—for example, when a user provides search words to a search engine. With a POST message, the user is still requesting a Web page from the server, but the specific contents of the Web page depend on what the user entered into the form fields. If the value of the method field is POST, then the entity body contains what the user entered into the form fields. We would be remiss if we didn’t mention that a request generated with a form does not necessarily have to use the POST method. Instead, HTML forms often use the GET method and include the inputted data (in the form fields) in the requested URL. For example, if a form uses the GET method, has two fields, and the inputs to the two fields are monkeys and bananas, then the URL will have the structure www.somesite.com/animalsearch?monkeys&bananas. In your day-to- day Web surfing, you have probably noticed extended URLs of this sort. The HEAD method is similar to the GET method. When a server receives a request with the HEAD method, it responds with an HTTP message but it leaves out the requested object. Application developers often use the HEAD method for debug- ging. The PUT method is often used in conjunction with Web publishing tools. It allows a user to upload an object to a specific path (directory) on a specific Web server. The PUT method is also used by applications that need to upload objects to Web servers. The DELETE method allows a user, or an application, to delete an object on a Web server. HTTP Response Message Below we provide a typical HTTP response message. This response message could be the response to the example request message just discussed. HTTP/1.1 200 OK Connection: close Date: Tue, 18 Aug 2015 15:44:04 GMT Server: Apache/2.2.3 (CentOS) Last-Modified: Tue, 18 Aug 2015 15:11:03 GMT Content-Length: 6821 Content-Type: text/html! (data data data data data...) Let’s take a careful look at this response message. It has three sections: an initial status line, six header lines, and then the entity body. The entity body is the meat of the message—it contains the requested object itself (represented by data data data data data...). The status line has three fields: the protocol version field, a status code, and a corresponding status message. In this example, the status line indicates that the server is using HTTP/1.1 and that everything is OK (that is, the server has found, and is sending, the requested object). M02_KURO5469_08_GE_C02.indd 133 03/05/2021 15:50  134 CHAPTER 2 APPLICATION LAYER Now let’s look at the header lines. The server uses the Connection: close header line to tell the client that it is going to close the TCP connection after sending the message. The Date: header line indicates the time and date when the HTTP response was created and sent by the server. Note that this is not the time when the object was created or last modified; it is the time when the server retrieves the object from its file system, inserts the object into the response message, and sends the response message. The Server: header line indicates that the message was gener- ated by an Apache Web server; it is analogous to the User-agent: header line in the HTTP request message. The Last-Modified: header line indicates the time and date when the object was created or last modified. The Last-Modified: header, which we will soon cover in more detail, is critical for object caching, both in the local client and in network cache servers (also known as proxy servers). The Content-Length: header line indicates the number of bytes in the object being sent. The Content-Type: header line indicates that the object in the entity body is HTML text. (The object type is officially indicated by the Content-Type: header and not by the file extension.) Having looked at an example, let’s now examine the general format of a response message, which is shown in Figure 2.9. This general format of the response message matches the previous example of a response message. Let’s say a few additional words about status codes and their phrases. The status code and associated phrase indicate the result of the request. Some common status codes and associated phrases include: 200 OK: Request succeeded and the information is returned in the response. 301 Moved Permanently: Requested object has been permanently moved; the new URL is specified in Location: header of the response message. The client software will automatically retrieve the new URL. Status line version sp status code sp phrase cr lf header field name: sp value cr lf Header lines header field name: sp value cr lf Blank line cr lf Entity body Figure 2.9 ♦ General format of an HTTP response message M02_KURO5469_08_GE_C02.indd 134 03/05/2021 15:50  2.2 THE WEB AND HTTP 135 400 Bad Request: This is a generic error code indicating that the request could not be understood by the server. 404 Not Found: The requested document does not exist on this server. 505 HTTP Version Not Supported: The requested HTTP protocol ver- sion is not supported by the server. How would you like to see a real HTTP response message? This is highly rec- ommended and very easy to do! First Telnet into your favorite Web server. Then VideoNote type in a one-line request message for some object that is housed on the server. For Using Wireshark to example, if you have access to a command prompt, type: investigate the HTTP protocol telnet gaia.cs.umass.edu 80! GET /kurose_ross/interactive/index.php HTTP/1.1 Host: gaia.cs.umass.edu (Press the carriage return twice after typing the last line.) This opens a TCP con- nection to port 80 of the host gaia.cs.umass.edu and then sends the HTTP request message. You should see a response message that includes the base HTML file for the interactive homework problems for this textbook. If you’d rather just see the HTTP message lines and not receive the object itself, replace GET with HEAD. In this section, we discussed a number of header lines that can be used within HTTP request and response messages. The HTTP specification defines many, many more header lines that can be inserted by browsers, Web servers, and net- work cache servers. We have covered only a small number of the totality of header lines. We’ll cover a few more below and another small number when we discuss network Web caching in Section 2.2.5. A highly readable and comprehensive dis- cussion of the HTTP protocol, including its headers and status codes, is given in [Krishnamurthy 2001]. How does a browser decide which header lines to include in a request message? How does a Web server decide which header lines to include in a response mes- sage? A browser will generate header lines as a function of the browser type and version, the user configuration of the browser and whether the browser currently has a cached, but possibly out-of-date, version of the object. Web servers behave similarly: There are different products, versions, and configurations, all of which influence which header lines are included in response messages. 2.2.4 User-Server Interaction: Cookies We mentioned above that an HTTP server is stateless. This simplifies server design and has permitted engineers to develop high-performance Web servers that can han- dle thousands of simultaneous TCP connections. However, it is often desirable for a Web site to identify users, either because the server wishes to restrict user access M02_KURO5469_08_GE_C02.indd 135 03/05/2021 15:50  136 CHAPTER 2 APPLICATION LAYER or because it wants to serve content as a function of the user identity. For these pur- poses, HTTP uses cookies. Cookies, defined in [RFC 6265], allow sites to keep track of users. Most major commercial Web sites use cookies today. As shown in Figure 2.10, cookie technology has four components: (1) a cookie header line in the HTTP response message; (2) a cookie header line in the HTTP request message; (3) a cookie file kept on the user’s end system and managed by the user’s browser; and (4) a back-end database at the Web site. Using Figure 2.10, let’s walk through an example of how cookies work. Suppose Susan, who always Client host Server host usua l ht tp requ ebay: 8734 est msg onse Server creates resp a l http 1678 ID 1678 for user us u ie: cook entry in backend Set- database usua l cook http req ie: amazon: 1678 1678 uest msg ebay: 8734 access Cookie-specific msg action onse resp l http usua One week later usua l access cook http req ie: 1678 uest msg amazon: 1678 ebay: 8734 Cookie-specific msg onse action resp l http usua Time Time Key: Cookie file Figure 2.10 ♦ Keeping user state with cookies M02_KURO5469_08_GE_C02.indd 136 03/05/2021 15:50  2.2 THE WEB AND HTTP 137 accesses the Web using Internet Explorer from her home PC, contacts Amazon.com for the first time. Let us suppose that in the past she has already visited the eBay site. When the request comes into the Amazon Web server, the server creates a unique identification number and creates an entry in its back-end database that is indexed by the identification number. The Amazon Web server then responds to Susan’s browser, including in the HTTP response a Set-cookie: header, which contains the identification number. For example, the header line might be: Set-cookie: 1678 When Susan’s browser receives the HTTP response message, it sees the Set-cookie: header. The browser then appends a line to the special cookie file that it manages. This line includes the hostname of the server and the identification number in the Set-cookie: header. Note that the cookie file already has an entry for eBay, since Susan has visited that site in the past. As Susan continues to browse the Amazon site, each time she requests a Web page, her browser consults her cookie file, extracts her identification number for this site, and puts a cookie header line that includes the identification number in the HTTP request. Specifically, each of her HTTP requests to the Amazon server includes the header line: Cookie: 1678 In this manner, the Amazon server is able to track Susan’s activity at the Amazon site. Although the Amazon Web site does not necessarily know Susan’s name, it knows exactly which pages user 1678 visited, in which order, and at what times! Amazon uses cookies to provide its shopping cart service—Amazon can maintain a list of all