TCP/IP Analysis and Troubleshooting Toolkit

CHAPTER 4

Internet Control Message Protocol

Chapter 3 introduced the Internet Protocol (IP), its addressing, and how IP traffic gets from a source to a destination. But what happens if there is a problem along that path? What if a router doesn't have a route to the destination? What if the IP datagrams are too large to be transmitted onto an outgoing link? What if the destination host doesn't respond?

IP is what is called an unreliable connectionless protocol. Although it is responsible for getting our data from one place to another, it does not guarantee that it will make it. When transmitting traffic across a Layer 2 link, such as Ethernet, the data link layer is responsible for guaranteeing the integrity of the data. For example:

• If a user on a shared Ethernet segment transmits at the same instant as another station, a collision results. Both stations “hear” the collision and attempt to transmit again.
• If a cable connecting a next-hop router is faulty and corrupts a packet that is being transmitted over the link, a CRC error results. The destination station (or router) checks the CRC on the frame; when it sees that it is incorrect, it drops the packet. Eventually the sending station will notice that it hasn't received a response, and it will transmit the data again.

Both of these examples illustrate how the data link layer handles events that occur in the physical layer. Collisions and CRC errors are manifestations of problems that occur in the physical layer of the network. ...