An Operating Systems Vade Mecum

310 Co-operating Processes Chapter 9These particular divisions are not necessarily standard. The CCITT/ISO model, forexample, distinguishes seven levels of protocol. Each network tends to build its own protocols,and the levels at which services are defined tend to fit only roughly into any standarddescription.Each protocol level builds an abstraction, according to the Beautification Principle,both to hide details of the lower levels and to introduce new structures. In the languageintroduced by the ISO model, each level provides ‘‘service data units’’ to the next levelup. In order to perform its function, it uses ‘‘protocol data units’’ built on the servicedata units of next lower level. This structure is shown in Figure 9.9, which shows thenames of some of the ISO levels. The protocol data units and service data units for theISO model change at each layer. At the atomic level, electrons are discrete. (We will notconcern ourselves with quantum-mechanical details.) At the aggregate level, physicsbecomes continuous, with measures such as voltage and current. The physical level (thelowest ISO level) is discrete, providing bits. The data-link level groups bits into frames,which it transports reliably. This level uses acknowledgements and sequence numbers aspart of its protocol data units. The network layer converts frames into packets, whichprovide routing, congestion control, and accounting services. The transport layer providesarbitrarily long data segments, transmitting them as numbered packets andreassembling them at the other end. The division into packets is hidden from view, butsegment boundaries are still made visible. (The U. S. Department of Defense TCP protocol,in contrast, also hides the sender’s segment boundaries from the receiver.) Higherlevels also provide new services built on lower ones, such as checkpointing and convertingbetween different floating-point formats.transporttransportnetworknetworkdata linkdata linkservice data unitsphysicalprotocol data unitsphysicalComputer AComputer BFigure 9.9 Protocol levels