Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 35
IP Options Field

This field is found on IPv4 packet headers. It contains information on source routing (nothing to do with Token Ring), tracing a route, timestamping the packet as it traverses routers, and security entries. These fields may or may not be in the header (which allows for the variable length header). It was found that most of these features were not used or were better implemented in other protocols, so IPv6 does not implement them as a function of the IP header.

Source routing is the ability of the originating station to place route information into the datagram to be interpreted by routers. Router will forward the datagram based on information in the source route fields, and in some cases, it will be blind. The originator indicates the path it wishes to take, and the routers must obey, even if there is a better route. There are two types: loose source route (LSR) and strict source route (SSR).

The difference between the two is relatively simple. Routes (IP addresses) are placed in a field of the IP header. The IP addresses indicate the route the datagram would like to take to the destination. Loose source route allows a router to forward the datagram to any router it feels is correct to service the next route indicated in the source route field. A complete list of IP addresses from the source to the destination is probably not in the IP header, but some points in the Internet should be used to forward the datagram. For example, IP multicast uses LSR for tunneling its IP multicast datagrams over the nonmulticast–enabled IPv4 Internet. Strict source routing forces a router to forward a datagram to its destination completely based on the routes indicated by the source route field.

IP Options Field

The Traceroute is a very useful utility. It allows the echoing of the forwarding path of a datagram. With this option set, the points to which the datagram is routed are echoed back to the sender. This allows you to follow a datagram along a path. It is very often used in troubleshooting IP networks. If you have Windows 95, you have this utility. Type in (DOS prompt) “tracert <IP address>” and watch the echo points on your screen.

IPv6 eliminated this field and those functions that were not used or were better implemented by other protocols.