User Level: Advanced
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, it is used 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.
- 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 and so forth, 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 the Trace Utility
Apart from other fields, each ICMP header consists of a field called Time to Live (TTL). The 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 the initial TTL to 3, TTL at B it would be 2 and at C it would be 1. If the gateway processing a datagram finds the 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 an echo request to the destination machine with an increasing TTL number, starting from 1. Each time the TTL goes to zero, the machine that was currently processing the datagram returns the packet with a time-exceeded message. We remeber the IP of this machine and send the packet back with an incremented TTL. We repeat this until we successfully receive an echo reply.
You can compile the .cs file on command line by using the command
and running it.
Questions? Send them to me at firstname.lastname@example.org.
About the Author
Sanjay Ahuja is a Bachelor of Engineering and has done his CDAC from Pune, India. He is currently working as a consultant for Verizon.