3.1 Transport Layer Services and Principles

Residing between the application and network layers, the transport layer is in the core of the layered network architecture. It has the critical role of providing communication services directly to the application processes running on different hosts. In this chapter we'll examine the possible services provided by a transport layer protocol and the principles underlying various approaches towards providing these services. We'll also look at how these services are implemented and instantiated in existing protocols; as usual, particular emphasis will be given to the Internet protocols, namely, TCP and UDP transport layer protocols.

In the previous two chapters we have touched on the role of the transport layer and the services that it provides. Let's quickly review what we have already learned about the transport layer:

A transport layer protocol provides for logical communication between application processes running on different hosts. By "logical" communication, we mean that although the communicating application processes are not physically connected to each other (indeed, they may be on different sides of the planet, connected via numerous routers and a wide range of link types), from the applications' viewpoint, it is as if they were physically connected. Application processes use the logical communication provided by the transport layer to send messages to each other, free for the worry of the details of the physical infrastructure used to carry these messages. Figure 3.1-1 illustrates the notion of logical communication.

As shown in Figure 3.1-1, transport layer protocols are implemented in the end systems but not in network routers. Network routers only act on the network-layer fields of the layer-3 PDUs; they do not act on the transport-layer fields.

At the sending side, the transport layer converts the messages it receives from a sending application process into 4-PDUs (that is, transport-layer protocol data units). This is done by (possibly) breaking the application messages into smaller chunks and adding a transport-layer header to each chunk to create 4-PDUs. The transport layer then passes the 4-PDUs to the network layer, where each 4-PDU is encapsulated into a 3-PDU. At the receiving side, the transport layer receives the 4-PDUs from the network layer, removes the transport header from the 4-PDUs, reassembles the messages and passes them to a receiving application process.

A computer network can make more than one transport layer protocol available to network applications. For example, the Internet has two protocols -- TCP and UDP. Each of these protocols provides a different set of transport layer services to the invoking application.

All transport layer protocols provide an application multiplexing/demultiplexing service. This service will be described in detail in the next section. As discussed in Section 2.1, in addition to multiplexing/demultiplexing service, a transport protocol can possibly provide other services to invoking applications, including reliable data transfer, bandwidth guarantees, and delay guarantees.

Tranpsort layer: logical versus physical communication

Figure 3.1-1: The transport layer provides logical rather than physical communication between applications.

3.1.1 Relationship between Transport and Network Layers

From the perspective of network applications, the transport layer is the underlying communication infrastructure. Of course, there is more to the communication infrastructure than just the transport layer. For example, the network layer lies just below the transport layer in the protocol stack. Whereas a transport layer protocol provides logical communication between processes running on different hosts, a network layer protocol provides logical communication between hosts. This distinction is subtle but important. Let's examine this distinction with the aid of a household analogy.

Consider two houses, one on the East Coast and the other on the West Coast, with each house being home to a dozen kids. The kids in the East Coast household are cousins with the kids in the West Coast households. The kids in the two households love to write each other -- each kid writes each cousin every week, with each letter delivered by the traditional postal service in a separate envelope. Thus, each household sends 144 letters to the other household every week. (These kids would save a lot of money if they had e-mail!). In each of the households there is one kid -- Alice in the West Coast house and Bob in the East Coast house -- responsible for mail collection and mail distribution. Each week Alice visits all her brothers and sisters, collects the mail, and gives the mail to a postal-service mail person who makes daily visits to the house. When letters arrive to the West Coast house, Alice also has the job of distributing the mail to her brothers and sisters. Bob has a similar job on the East coast.

In this example, the postal service provides logical communication between the two houses -- the postal service moves mail from house to house, not from person to person. On the other hand, Alice and Bob provide logical communication between the cousins -- Alice and Bob pick up mail from and deliver mail to, their brothers and sisters. Note that, from the cousins' perspective, Alice and Bob are the mail service, even though Alice and Bob are only a part (the end system part) of the end-to-end delivery process. This household example serves as a nice analogy for explaining how the transport layer relates to the network layer:

hosts (also called end systems) = houses
processes = cousins
application messages = letters in envelope
network layer protocol = postal service (including mail persons)
transport layer protocol = Alice and Bob

Continuing with this analogy, observe that Alice and Bob do all their work within their respective homes; they are not involved, for example, in sorting mail in any intermediate mail center or in moving mail from one mail center to another. Similarly, transport layer protocols live in the end systems. Within an end system, a transport protocol moves messages from application processes to the network edge (i.e., the network layer) and vice versa; but it doesn't have any say about how the messages are moved within the network core. In fact, as illustrated in Figure 3.1-1, intermediate routers neither act on, nor recognize, any information that the transport layer may have appended to the application messages.

Continuing with our family saga, suppose now that when Alice and Bob go on vacation, another cousin pair -- say, Susan and Harvey -- substitute for them and provide the household-internal collection and delivery of mail. Unfortunately for the two families, Susan and Harvey do not do the collection and delivery in exactly the same way as Alice and Bob. Being younger kids, Susan and Harvey pick up and drop off the mail less frequently and occasionally lose letters (which are sometimes chewed up by the family dog). Thus, the cousin-pair Susan and Harvey do not provide the same set of services (i.e., the same service model) as Alice and Bob. In an analogous manner, a computer network may make available multiple transport protocols, with each protocol offering a different service model to applications.

The possible services that Alice and Bob can provide are clearly constrained by the possible services that the postal service provides. For example, if the postal service doesn't provide a maximum bound on how long it can take to deliver mail between the two houses (e.g., three days), then there is no way that Alice and Bob can guarantee a maximum delay for mail delivery between any of the cousin pairs. In a similar manner, the services that a transport protocol can provide are often constrained by the service model of the underlying network-layer protocol. If the network layer protocol cannot provide delay or bandwidth guarantees for 4-PDUs sent between hosts, then the transport layer protocol can not provide delay or bandwidth guarantees for the messages sent between processes.

Nevertheless, certain services can be offered by a transport protocol even when the underlying network protocol doesn't offer the corresponding service at the network layer. For example, as we'll see in this chapter, a transport protocol can offer reliable data transfer service to an application even when the underlying network protocol is unreliable, that is, even when the network protocol loses, garbles and duplicates packets. As another example (which we'll explore in Chapter 7 when we discuss network security), a transport protocol can use encryption to guarantee that application messages are not read by intruders, even when the network layer cannot guarantee the secrecy of 4-PDUs.

3.1.2 Overview of the Transport Layer in the Internet

The Internet, and more generally a TCP/IP network, makes available two distinct transport-layer protocols to the application layer. One of these protocols is UDP (User Datagram Protocol), which provides an unreliable, connectionless service to the invoking application. The second of the these protocols is TCP (Transmission Control Protocol), which provides a reliable, connection-oriented service to the invoking application. When designing a network application, the application developer must specify one of these two transport protocols. As we saw in Sections 2.6 and 2.7, the application developer selects between UDP and TCP when creating sockets.

To simplify terminology, when in an Internet context, we refer to the 4-PDU as a segment. We mention, however, that the Internet literature (e.g., the RFCs) also refers to the PDU for TCP as a segment but often refers to the PDU for UDP as a datagram. But this same Internet literature also uses the terminology datagram for the network-layer PDU! For an introductory book on computer networking such as this one, we believe that it is less confusing to refer to both TCP and UDP PDUs as segments, and reserve the terminology datagram for the network-layer PDU.

Before preceding with our brief introduction of UDP and TCP, it is useful to say a few words about the Internet's network layer. (The network layer is examined in detail in Chapter 4.) The Internet's network-layer protocol has a name -- IP, which abbreviates "Internet Protocol". IP provides logical communication between hosts. The IP service model is a best-effort delivery service. This means that IP makes its "best effort" to deliver segments between communicating hosts, but it makes no guarantees. In particular, it does not guarantee segment delivery, it does not guarantee orderly delivery of segments, and it does it guarantee the integrity of the data in the segments. For these reasons, IP is said to be an unreliable service. We also mention here that every host has an IP address. We will examine IP addressing in detail in Chapter 4; for this chapter we need only keep in mind that each host has a unique IP address.

Having taken a glimpse at the IP service model, let's now summarize the service model of UDP and TCP. The most fundamental responsibility of UDP and TCP is to extend IP's delivery service between two end systems to a delivery service between two processes running on the end systems. Extending host-to-host delivery to process-to-process delivery is called application multiplexing and demultiplexing. We'll discuss application multiplexing and demultiplexing in the next section. UDP and TCP also provide integrity checking by including error detection fields in its header. These two minimal transport-layer services -- host-to-host data delivery and error checking -- are the only two services that UDP provides! In particular, like IP, UDP is an unreliable service -- it does not guarantee data sent by one process will arrive in tact to the destination process. UDP is discussed in detail in Section 3.3.

TCP, on the other hand, offers several additional services to applications.. First and foremost, it provides reliable data transfer. Using flow control, sequence numbers, acknowledgments and timers (techniques we'll explore in detail in this Chapter), TCP's guarantee of reliable data transfer ensures that data is delivered from sending process to receiving process, correctly and in order. TCP thus converts IP's unreliable service between end systems into a reliable data transport service between processes. TCP also uses congestion control. Congestion control is not so much a service provided to the invoking application as it is a service for the Internet as a whole -- a service for the general good. In loose terms, TCP congestion control prevents any one TCP connection from swamping the links and switches between communicating hosts with an excessive amount of traffic. In principle, TCP permits TCP connections traversing a congested network link to equally share that link's bandwidth. This is done by regulating the rate at which an the sending-side TCPs can send traffic into the network. UDP traffic, on the other hand, is unregulated. A an application using UDP transport can send traffic at any rate it pleases, for as long as it pleases.

A protocol that provides reliable data transfer and congestion control is necessarily complex. We will need several sections to cover the principles of reliable data transfer and congestion control, and additional sections to cover the TCP protocol itself. These topics are investigated in Sections 3.4 through 3.8. The approach taken in this chapter is to alternative between the basic principles and the TCP protocol. For example, we first discuss reliable data transfer in a general setting and then discuss how TCP specifically provides reliable data transfer. Similarly, we first discuss congestion control in a general setting and then discuss how TCP uses congestion control. But before getting into all this good stuff, let's first look at application multiplexing and demultiplexing in the next section.

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