How protocols work

Most computer networks require that information transferred between two nodes be divided into blocks, called packets. These packets make the information more manageable for sending and receiving nodes, as well as any intermediate nodes (bridges or routers). In addition to the information, or data, that is being transferred, each packet contains control information used for error-checking, addressing, and other purposes.

The protocols used on the network define the content of this control information. In most cases, multiple protocols exist within a packet; each protocol defines a different portion of the control information for the packet, and the control information for each protocol serves a different purpose.

When multiple protocols are used, the control information for the highest level protocol is placed around the data first, then the control information for each subsequent protocol in the protocol stack is added to the beginning and/or end of the packet. This process is called ``enveloping'' and is illustrated in Table 4-2.

Table 4-2 Example of multiple protocols in a packet

 ---------------------------------------------
| Header Control Information for Protocol #3 |
|--------------------------------------------|
| Header Control Information for Protocol #2 |
|--------------------------------------------|
| Header Control Information for Protocol #l |
|--------------------------------------------|
|                    Data                    |
|--------------------------------------------|
| Trailer Control Information for Protocol #l|
|--------------------------------------------|
| Trailer Control Information for Protocol #2|
|--------------------------------------------|
| Trailer Control Information for Protocol #3|
|--------------------------------------------|

The enveloping pattern illustrated in the previous table is common in the computer communications industry. However, the tasks assigned to each protocol in the packet differ for different vendors' implementations. In an effort to standardize the definition of protocols -- and therefore make the networking implementations of different vendors interoperable -- several standards organizations have been formed by governments and corporations. One of these, the International Standards Organization (ISO), has developed a model, called the Open Systems Interconnection (OSI) model, that specifies how protocols should be defined in the future.

The OSI model separates the functions required for effective computer communications (such as error-checking and addressing) into the following seven categories, or layers, presented below from the highest to the lowest:

Application layer

the content of the application layer is up to the individual user

Presentation layer

performs functions that are requested sufficiently often to warrant finding a general solution for them

Session layer

the user's interface into the network

Transport layer

accepts data from the session layer, splits it into smaller units if necessary, passes these to the network layer, and ensures that the pieces all arrive correctly at the other end

Network layer

controls the operation of the subnet

Data link layer

takes a raw transmission facility and transforms it into a line that appears free of transmission errors to the network layer

Physical layer

concerned with transmitting raw bits over a communication channel

Having been defined prior to the finalization of the OSI model, the protocols used by NetWare do not all correspond exactly to the OSI model's definitions. NetWare uses a variety of protocols. Some of these protocols were developed specifically for NetWare; some are used throughout the networking industry. The protocols required for communications between NetWare workstations and file servers are:

Medium-access protocols

Define the addressing that distinguishes each node an a NetWare network.

Internetwork Packet Exchange (IPX)

Defines the addressing schemes used on a NetWare network.

Sequenced Packet Exchange (SPX)

Provides security and reliability to the IPX protocol.

Routing Information Protocol (RIP)

Facilitates the exchange of routing information on a NetWare network.

Service Advertising Protocol (SAP)

Allows service-providing nodes to advertise their services and addresses.

NetWare Core Protocol (NCP)

Defines the connection control and service request encoding that make possible the interaction between clients and servers.

These protocols are described in greater detail in the following sections.

Figure 4-1 provides a relative mapping of the NetWare protocols -- also called the NetWare protocol stack -- to the OSI model; in actuality, a direct correlation to the layer boundaries of the two architectures does not exist. The NetWare protocols follow the enveloping pattern previously shown in Table 4-2. More specifically, the upper level protocols (NCP, SAP, and RIP) are enveloped by IPX, and IPX is subsequently enveloped by a medium-access protocol header and trailer.

Figure 4-1 Mapping of NetWare protocols to OSI model

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