Figure 4.1 depicts the IP header and its fields. The first field in the IP header is the version field. This is used to identify which version of IP was used to create the header. This is important in internets because not every network is running the same version of a protocol. If the IP header was created in a network using the latest version of IP, it may contain information not recognizable by an older version of IP.

When this occurs, the receiving network (running the older version of IP) knows to ignore unrecognizable fields because the version field indicates a version newer than its own. This is valuable information for an internet and can be found in a number of the TCP/IP protocols.

The version field is followed by the header length field, which provides the length of the header itself. The data portion is not indicated here. One would think that the IP header would always be the same length, but there are some variable options which can be included in the header. The length is measured in 32-bit units (or words). A value of 1 indicates a header length of 32 bits, while a value of 2 indicates a header length of 64 bits.

Following the header length field is the type of service field (see Fig. 4.2). In other protocols, this is often referred to as Quality of Service (QoS). It stipulates the level of service the data requires. This field contains the four values discussed below.

Precedence is used to assign a level of priority to a data unit. It can be used for congestion management (lower-priority data units could be discarded while higher-priority data units are allowed to pass) and flow control. Not all networks implement this parameter. The precedence field consists of a 3-bit code indicating the type of precedence assigned. The values are shown in Table 4.1

NEXT PAGE: IP Header (cont.)

Copied with permission,McGraw-Hill Telecommunications from the book Telecommunications Protocols, by Travis Russell, 1997 McGraw-Hill Telecommunications, pages 149-155.