Ping
Ping is implemented using the required ICMP Echo function, documented in RFC 792 that all hosts should implement.
What Ping can tell you
- Ping places a unique sequence number on each packet it transmits, and reports which sequence numbers it receives back. Thus, you can determine if packets have been dropped, duplicated, or reordered.
- Ping checksums each packet it exchanges. You can detect some forms of damaged packets.
- Ping places a timestamp in each packet, which is echoed back and can easily be used to compute how long each packet exchange took - the Round Trip Time (RTT).
- Ping reports other ICMP messages that might otherwise get buried in the system software. It reports, for example, if a router is declaring the target host unreachable.
What Ping can not tell you
- Some routers may silently discard undeliverable packets. Others may believe a packet has been transmitted successfully when it has not been. (This is especially common over Ethernet, which does not provide link-layer acknowledgments) Therefore, ping may not always provide reasons why packets go unanswered.
- Ping can not tell you why a packet was damaged, delayed, or duplicated. It can not tell you where this happened either, although you may be able to deduce it.
- Ping can not give you a blow-by-blow description of every host that handled the packet and everything that happened at every step of the way. It is an unfortunate fact that no software can reliably provide this information for a TCP/IP network.
