2.1.3 Transport Services Available to Applications

• 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

 

• The situation is similar to choosing either train or airplane transport for travel between two cities.

• We can broadly classify the possible services along four dimensions: reliable data transfer, throughput, timing, and security.

 

Reliable Data Transfer

• As discussed in Chapter 1, packets can get lost within a computer network.

• One important service that a transport-layer protocol can potentially provide to an application is process-to-process reliable data transfer.

 

 

Throughput

• With such a service, the application could request a guaranteed throughput of r bits/sec, and the transport protocol would then ensure that the available throughput is always at least r bits/sec.

 

Timing 

• A transport-layer protocol can also provide timing guarantees.

• 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

 

Security

• 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 protocol can decrypt the data before delivering the data to the receiving process

 

 

2.1.4 Transport Services Provided by the Internet

 

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 messages begin to flow. This so-called

  handshaking procedure alerts the client and server, allowing them to prepare for an onslaught of packets.

 

• Reliable data transfer service.

• 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.

 

 

UDP Services

• UDP is connectionless, so there is no handshaking before the two processes start to communicate.

• 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

 

Services Not Provided by Internet Transport Protocols

 

 

• In summary, today’s Internet can often provide satisfactory service to time-sensitive applications, but it cannot provide any timing or throughput guarantees

• 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 usually prefer to run their applications over UDP,

 

 

2.1.5 Application-Layer Protocols

• 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

 

• It is important to distinguish between network applications and application-layer protocols. An application-layer protocol is only one piece of a network application

• 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.