The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP Administration and Troubleshooting Utilities and Protocols

TCP/IP Host Name Utility (hostname) 1 2 3 4 5 TCP/IP Route Tracing Utility (traceroute/tracert/traceroute6)

Operation of the ping Utility

The ping utility is implemented using ICMP Echo (Request) and Echo Reply messages. As explained in the topic discussing these message types, they are designed specifically for these sorts of diagnostic purposes. When Device A sends an ICMP Echo message to device B, device B responds by sending an ICMP Echo Reply message back to device A. The same functionality exists in ICMPv6, the IPv6 version of ICMP; the ICMPv6 Echo and Echo Reply messages only differ from the IPv4 ones slightly in their field structure.

This would seem to indicate that ping would be an extremely simple utility that would send one Echo message and wait to see if an Echo Reply was received back; if so, then this would provide that the two devices were able to communicate, and if not, this would indicate a problem somewhere on the internetwork between the two. However, almost all ping implementations are much more complex than this. They use multiple sets of Echo and Echo Reply messages, along with considerable internal logic, to allow an administrator to determine all of the following, and more:

• Whether or not the two devices can communicate;
  
  
• Whether congestion or other problems exist that might allow communication to succeed sometimes but cause it to fail in others, seen as packet loss—if so, how bad the loss is;
  
  
• How much time it takes to send a simple ICMP message between devices, which gives an indication of the overall latency between the hosts, and also indicates if there are certain types of problems.

TCP/IP Host Name Utility (hostname) 1 2 3 4 5 TCP/IP Route Tracing Utility (traceroute/tracert/traceroute6)