The demand for open networks has generated a need for published standards by which manufacturers can supply equipment and software that function properly with products from other vendors. One standard that has resulted is the Open System Interconnection (OSI) reference model, produced by the Interna- tional Organization for Standardization. This standard is based on a seven-level hierarchy as opposed to the four-level hierarchy we have just described. It is an often-quoted model because it carries the authority of an international organiza- tion, but it has been slow to replace the four-level point of view, mainly because it was established after the four-level hierarchy had already become the de facto standard for the Internet.
The TCP/IP protocol suite is a collection of protocol standards used by the Internet to implement the four-level communication hierarchy implemented in the Internet. Actually, the Transmission Control Protocol (TCP) and the Internet Protocol (IP)are the names of only two of the protocols in this vast collection—so the fact that the entire collection is referred to as the TCP/IP pro- tocol suite is rather misleading. More precisely, TCP defines a version of the transport layer. We say a versionbecause the TCP/IP protocol suite provides for more than one way of implementing the transport layer; one of the other options is defined by the User Datagram Protocol (UDP).This diversity is analogous to the fact that when shipping a package, you have a choice of different shipping companies, each of which offers the same basic service but with its own unique
characteristics. Thus, depending on the particular quality of service required, a unit within the application layer might choose to send data via a TCP or UDP version of the transport layer (Figure 4.15).
There are several differences between TCP and UDP. One is that before send- ing a message as requested by the application layer, a transport layer based on TCP sends its own message to the transport layer at the destination telling it that a message is about to be sent. It then waits for this message to be acknowledged before starting to send the application layer’s message. In this manner, a TCP transport layer is said to establish a connection before sending a message. A transport layer based on UDP does not establish such a connection prior to sending a message. It merely sends the message to the address it was given and forgets about it. For all it knows, the destination computer might not even be opera- tional. For this reason, UDP is called a connectionless protocol.
Another difference between TCP and UDP is that TCP transport layers at the origin and destination work together by means of acknowledgments and packet retransmissions to assure that all segments of a message are successfully transferred to the destination. For this reason TCP is called a reliable protocol, whereas UDP, which does not offer such retransmission services, is said to be an unreliable protocol.
Still another distinction between TCP and UDP is that TCP provides for both flow control, meaning that a TCP transport layer at a message’s origin can reduce the rate at which it transmits segments to keep from overwhelming its counterpart at the destination, as well as congestion control,meaning that a TCP transport layer at a message’s origin can adjust its transmission rate to alle- viate congestion between it and the message’s destination.
All this does not mean that UDP is a poor choice. After all, a transport layer based on UDP is more streamlined than a layer based on TCP, and thus if an application is prepared to handle the potential consequences of UDP, that option might be the better choice. For example, the efficiency of UDP makes it the protocol of choice for DNS lookups and VoIP. However, because email is less time sensitive, mail servers use TCP to transfer email.
Figure 4.15 Choosing between TCP and UDP
Application layer
?
Transport layer
More “reliable” but less efficient
TCP
More efficient but less “reliable”
IP is the Internet’s standard for implementing the tasks assigned to the net- work layer. We have already observed that this task consists of forwarding,which involves relaying packets through the Internet, and routing, which involves updating the layer’s forwarding table to reflect changing conditions. For instance, a router may malfunction, meaning that traffic should no longer be forwarded in its direction, or a section of the Internet may become congested, meaning that traffic should be routed around the blockage. Much of the IP standard associated with routing deals with the protocols used for communication among neighboring network layers as they interchange routing information.
An interesting feature associated with forwarding is that each time an IP net- work layer at a message’s origin prepares a packet, it appends a value called a hop count,or time to live, to that packet. This value is a limit to the number of times the packet should be forwarded as it tries to find its way through the Internet. Each time an IP network layer forwards a packet, it decrements that packet’s hop count by one. With this information, the network layer can protect the Internet from packets circling endlessly within the system. Although the Internet continues to grow on a daily basis, an initial hop count of 64 remains more than sufficient to allow a packet to find its way through the maze of routers within today’s ISPs.
For years a version of IP known as IPv4 (IP version four) has been used for implementing the network layer within the Internet. However, the Internet is rapidly outgrowing the 32-bit internet addressing system dictated by IPv4. To solve this problem as well as to implement other improvements such as multicast, a new version of IP known as IPv6, which uses internet addresses consisting of 128 bits, has been established. The process of converting from IPv4 to IPv6 is currently underway—this is the conversion that was alluded to in our introduc- tion of Internet addresses in Section 4.2—and it is expected that the use of 32-bit addresses within the Internet will be extinct by 2025.
4.5
Security
When a computer is connected to a network, it becomes subject to unauthorized access and vandalism. In this section we address topics associated with these problems.