Ping is a program that sends a series of packets over a network or the Internet to a specific computer in order to generate a response from that computer. The other computer responds with an acknowledgment that it received the packets. Ping was created to verify whether a specific computer on a network or the Internet exists and is connected.
Ping uses ICMP (Internet Control Message Protocol) packets. The packet from the origin computer is called an "ICMP_echo_request", and the response from the target is called an "ICMP_echo_reply". Each packet contains by default either 32 or 64 bytes of data and 8 bytes of protocol reader information, but ping can be configured at the command line to use different sized packets. You can access a list of switches and additional functions by invoking the help file for ping:
In an IP network, `ping' sends a short data burst - a single packet - and listens for a single packet in reply. Since this tests the most basic function of an IP network (delivery of single packet), it's easy to see how you can learn a lot from some `pings'.
Ping is implemented using the required ICMP Echo function, documented in RFC 792 that all hosts should implement. Of course, administrators can disable ping messages (this is rarely a good idea, unless security considerations dictate that the host should be unreachable anyway), and some implementations have (gasp) even been known not to implement all required functions. However, ping is usually a better bet than almost any other network software.
Many versions of ping are available. For the remainder of this discussion, I assume use of BSD UNIX's ping, a freely available, full-featured ping available for many UNIX systems. Most PC-based pings do not have the advanced features I describe. As always, read the manual for whatever version you use.
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.
Traceroute is the program that shows you the route over the network between two systems, listing all the intermediate routers a connection must pass through to get to its destination. It can help you determine why your connections to a given server might be poor, and can often help you figure out where exactly the problem is. It also shows you how systems are connected to each other, letting you see how your ISP connects to the Internet as well as how the target system is connected.
Traceroute utilities work by sending packets with low time-to-live (TTL) fields. The TTL value specifies how many hops the packet is allowed before it is returned. When a packet can't reach its destination because the TTL value is too low, the last host returns the packet and identifies itself. By sending a series of packets and incrementing the TTL value with each successive packet, traceroute finds out who all the intermediary hosts are.
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