Introduction

Trace utility traces the path of an IP packet. To understand the process, its important to understand ICMP protocol we are going to use for this purpose.

ICMP Protocol

The Internet Protocol (IP) is used for host-to-host datagram service in a system of interconnected networks. Occasionally a destination host will communicate with a source host, for example, to report an error in datagram processing. For such purposes the Internet Control Message Protocol (ICMP), is used. ICMP, uses the basic support of IP as if it were a higher level protocol, however, ICMP is actually an integral part of IP, and must be implemented by every IP module.

ICMP messages are sent in several situations: for example, when a datagram cannot reach its destination, when the gateway does not have the buffering capacity to forward a datagram, and when the gateway can direct the host to send traffic on a shorter route.

The Internet Protocol is not designed to be absolutely reliable. The purpose of these control messages is to provide feedback about problems in the communication environment, not to make IP reliable. There are still no guarantees that a datagram will be delivered or a control message will be returned. Some datagrams may still be undelivered without any report of their loss. The higher level protocols that use IP must implement their own reliability procedures if reliable communication is required.

The ICMP messages typically report errors in the processing of datagrams. To avoid the infinite regress of messages about messages etc., no ICMP messages are sent about ICMP messages. Also ICMP messages are only sent about errors in handling fragment zero of fragemented datagrams. (Fragment zero has the fragment offeset equal zero).

Basics of Trace utility

Apart from other fields each ICMP header consists of a field called Time to Live (TTL). TTL field is decremented at each machine in which the datagram is processed. Thus if my packet routes through Machine A-> Machine B-> Machine C, and if I set initial TTL to 3 then TTL at B would be 2 and at C would be 1. If the gateway processing a datagram finds TTL field is zero it discards the datagram. The gateway also notifies the source host via the time exceeded message.

Thus to get our utility working we send a packet containing echo request to the destination machine with increasing number of TTL starting from 1. Each time TTL goes to zero the machine that was currently processing datagram returns the packet with time exceeded message. We remeber the ip of this machine and send the packet back with incremented TTL. We repeat this till we successfully receive an echo reply.

You can compile the .cs file on command line by command

csc Trace.cs

and run it