The following is the fourth installment of a multi-part series on the fundamentals of routing. Each tip is excerpted
from Routing First-Step by William Parkhurst, published by Cisco Press. Check back frequently for the next installment, or go to the main series page for all the installments.
IP header format
Unlike the post office, a router or computer cannot determine the size of a package without additional information. A person can look at a letter or box and determine how big it is, but a router cannot. Therefore, additional information is required at the IP layer, in addition to the source and destination IP addresses. Figure 3-12 is a logical representation of the information that is used at the IP layer to enable the delivery of electronic data. This information is called a header, and is analogous to the addressing information on an envelope. A header contains the information required to route data on the Internet, and has the same format regardless of the type of data being sent. This is the same for an envelope where the address format is the same regardless of the type of letter being sent.
Figure 3-12 - IP Header Format
The fields in the IP header and their descriptions are
The IP Precedence field can be used to prioritize IP traffic. (See Table 3-9.) This is the same as the postal system having different classes of mail such as priority, overnight, and 2-day delivery. Routers can choose to use this field to give preferential treatment to certain types of IP traffic.
The ToS bits were originally designed to influence the delivery of data based on delay, throughput, reliability and cost. (See Table 3-10.) They are usually not used and are therefore set to zero.
The IP Precedence field can have 8 or 23 possible values. Routers use two of these values, 6 and 7, for routing protocol traffic. That leaves six values that can be used to prioritize user traffic. Because the ToS bits are typically not used, the IP Precedence field can be extended from 3 to 6 bits by using 3 bits from the ToS field. (See Figure 3-13.)
This new field is called the Differentiated Services Code Point (DSCP). That gives you 64 or 26 possible values that can be used to prioritize traffic. Although there are 64 possible DSCP values, only 14 are used typically. (See Table 3-11 and the explanation that follows.)
Notice that the first 3 bits of the DSCP value are the 3 bits from the IP precedence. An IP precedence of 000 maps into a DSCP value of 000 000, and both represent best effort delivery. An IP precedence of 101 (Critical) maps into a DSCP value of 101 110 (High Priority or Expedited Forwarding). The remaining 4 IP precedence values are each mapped into 3 DSCP values. The additional 3-bit portion is used to identify a drop probability within one of the four assured forwarding (AF) classes.
This discussion of the contents of the IP header is meant as an overview. If you are interested in learning more details regarding the IP header, refer to the references at the end of this chapter. The important concept to take away from this discussion is that the IP header contains the source and destination IP addresses. Routers use the destination IP address to determine a route; therefore, the IP layer in the layered model is the routing layer.
At this point, we could stop our discussion of the layered protocol model. This book is about routing, and routing is the second or third layer depending on which model is used. A router does not care what application sent the data, or how the application is going to receive the data. The job of the router is to get the packet to the proper destination. It is then the responsibility of the destination host to deliver the data to the application. The incomplete layered model in Figure 3-8 is sufficient for the remainder of this book. But, to be complete, let's go ahead and finish the model.
All parts reproduced from the book Routing First-Step, ISBN 1587201224, Copyright 2005, Cisco Systems, Inc. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240. Written permission from Pearson Education, Inc. is required for all other uses. Visit www.ciscopress.com for a detailed description and to learn how to purchase this title.