ICMP

Summary

This chapter shows how to create programs that utilize the Internet Control Message Protocol (ICMP). ICMP allows a network device to quickly send queries and error messages to other network devices. Many types of ICMP packets can be used on the network, the most common of which is the Echo Request packet. When an Echo Request packet is sent to a device, the device responds with an Echo Reply packet. This procedure is the basis for the popular ping program, which tests whether a network device is reachable and how long it takes to send packets to the device.

Programmers can send non-TCP and non-UDP packets with the Socket class, which contains the SocketType.Raw property. This allows raw IP packets to be created and sent on the network. The ProtocolType.Icmp property formats the IP packet use with an ICMP data packet. When working with the raw socket type, you must be careful handling received messages. The received message will include the complete IP packet information, including the IP header fields. You must ensure that your code compensates for the added IP packet information in the returned data by allocating a larger buffer, as well as starting at the right position in the buffer when decoding the ICMP portion of the packet.

To simplify ICMP programs, an ICMP class is established to handle the packet details for ICMP packets. An application program that needs to create or read ICMP packets can create an instance of an ICMP object and place the required data into the data variables of the class. The class also includes a method that allows you to retrieve the raw bytes of the packet so it can be sent using the SendTo() method. When an ICMP packet is received, a separate ICMP class constructor places the raw data from the ICMP packet into the ICMP class variables for easier access.

Several ICMP utility programs are presented in this chapter, including simplified and advanced versions of a ping program. A traceroute program is also created, which exploits how routers handle packets that have an expired IP time to live (TTL) value. When the router receives a packet whose TTL value will expire on the next route, it sends an ICMP Time Exceeded packet back to the packet sender. By starting out with a small TTL value and increasing it, you can track what routers are handling the packet on its way to its final destination. Finally, the ICMP Subnet Request packet type queries network devices to determine the configured subnet mask for a network