Andrew Tanenbaum - Distributed operating systems

Fig. 2-2.A typical message as it appears on the network.

In the OSI model, there are not two layers, but seven, as we saw in Fig. 2-1. The collection of protocols used in a particular system is called a protocol suiteor protocol stack.In the following sections, we will briefly examine each of the layers in turn, starting at the bottom. Where appropriate, we will also point out some of the protocols used in each layer.

The physical layer is concerned with transmitting the 0s and 1s. How many volts to use for 0 and 1, how many bits per second can be sent, and whether transmission can take place in both directions simultaneously are key issues in the physical layer. In addition, the size and shape of the network connector (plug), as well as the number of pins and meaning of each are of concern here.

The physical layer protocol deals with standardizing the electrical, mechanical, and signaling interfaces so that when one machine sends a 0 bit it is actually received as a 0 bit and not a 1 bit. Many physical layer standards have been developed (for different media), for example, the RS-232-C standard for serial communication lines.

The physical layer just sends bits. As long as no errors occur, all is well. However, real communication networks are subject to errors, so some mechanism is needed to detect and correct them. This mechanism is the main task of the data link layer. What it does is to group the bits into units, sometimes called frames,and see that each frame is correctly received.

The data link layer does its work by putting a special bit pattern on the start and end of each frame, to mark them, as well as computing a checksumby adding up all the bytes in the frame in a certain way. The data link layer appends the checksum to the frame. When the frame arrives, the receiver recomputes the checksum from the data and compares the result to the checksum following the frame. If they agree, the frame is considered correct and is accepted. It they disagree, the receiver asks the sender to retransmit it. Frames are assigned sequence numbers (in the header), so everyone can tell which is which.

In Fig. 2-3 we see a (slightly pathological) example of A trying to send two messages, 0 and 1, to B. At time 0, data message 0 is sent, but when it arrives, at time 1, noise on the transmission line has caused it to be damaged, so the checksum is wrong. B notices this, and at time 2 asks for a retransmission using a control message. Unfortunately, at the same time, A is sending data message 1. When A gets the request for retransmission, it resends 0. However, when B gets message 1, instead of the requested message 0, it sends control message 1 to A complaining that it wants 0, not 1. When A sees this, it shrugs its shoulders and sends message 0 for the third time.

Fig. 2-3.Discussion between a receiver and a sender in the data link layer.

The point here is not so much whether the protocol of Fig. 2-3 is a great one (it is not), but rather to illustrate that in each layer there is a need for discussion between the sender and the receiver. Typical messages are "Please retransmit message n," "I already retransmitted it," "No you did not," "Yes I did," "All right, have it your way, but send it again," and so forth. This discussion takes place in the header field, where various requests and responses are defined, and parameters (such as frame numbers) can be supplied.

On a LAN, there is usually no need for the sender to locate the receiver. It just puts the message out on the network and the receiver takes it off. A wide-area network, however, consists of a large number of machines, each with some number of lines to other machines, rather like a large-scale map showing major cities and roads connecting them. For a message to get from the sender to the receiver it may have to make a number of hops, at each one choosing an outgoing line to use. The question of how to choose the best path is called routing,and is the primary task of the network layer.

The problem is complicated by the fact that the shortest route is not always the best route. What really matters is the amount of delay on a given route, which, in turn, is related to the amount of traffic and the number of messages queued up for transmission over the various lines. The delay can thus change over the course of time. Some routing algorithms try to adapt to changing loads, whereas others are content to make decisions based on long-term averages.

Two network-layer protocols are in widespread use, one connection-oriented and one connectionless. The connection-oriented one is called X.25, and is favored by the operators of public networks, such as telephone companies and the European PTTs. The X.25 user first sends a Call Request to the destination, which can either accept or reject the proposed connection. If the connection is accepted, the caller is given a connection identifier to use in subsequent requests. In many cases, the network chooses a route from the sender to the receiver during this setup, and uses it for subsequent traffic.

The connectionless one is called IP (Internet Protocol) and is part of the DoD (U.S. Department of Defense) protocol suite. An IP packet(the technical term for a message in the network layer) can be sent without any setup. Each IP packet is routed to its destination independent of all others. No internal path is selected and remembered as is often the case with X.25.

Packets can be lost on the way from the sender to the receiver. Although some applications can handle their own error recovery, others prefer a reliable connection. The job of the transport layer is to provide this service. The idea is that the session layer should be able to deliver a message to the transport layer with the expectation that it will be delivered without loss.

Upon receiving a message from the session layer, the transport layer breaks it into pieces small enough for each to fit in a single packet, assigns each one a sequence number, and then sends them all. The discussion in the transport layer header concerns which packets have been sent, which have been received, how many more the receiver has room to accept, and similar topics.

Reliable transport connections (which by definition are connection-oriented) can be built on top of either X.25 or IP. In the former case all the packets will arrive in the correct sequence (if they arrive at all), but in the latter case it is possible for one packet to take a different route and arrive earlier than the packet sent before it. It is up to the transport layer software to put everything back in order to maintain the illusion that a transport connection is like a big tube — you put messages into it and they come out undamaged and in the same order in which they went in.

The official ISO transport protocol has five variants, known as TP0through TP4.The differences relate to error handling and the ability to send several transport connections over a single X.25 connection. The choice of which one to use depends on the properties of the underlying network layer.

The DoD transport protocol is called TCP (Transmission Control Protocol)and is described in detail in (Comer, 1991). It is similar to TP4. The combination TCP/IP is widely used at universities and on most UNIX systems. The DoD protocol suite also supports a connectionless transport protocol called UDP(universal Datagram Protocol), which is essentially just IP with some minor additions. User programs that do not need a connection-oriented protocol normally use UDP.