Getting Started Guide - Glossary of Terms


ICMP Internet Control Message Protocol (defined in RFC 792) provides a mechanism for communicating control message and error reports. Both gateways and hosts use ICMP to transmit problem reports about datagrams back to the datagram originator.. We only deal with the Echo message (ICMP message type 8) and Echo Reply messages (ICMP message type 0) in this document, and subsequently with PingPlotter. You can see all the message types at http://www.iana.org/assignments/icmp-parameters.

Latency The roundtrip time it takes to get a packet of data to a remote network device and back again. It is normally expressed in milliseconds (thousandths of a second).

Ping Ping uses the Internet Control Message Protocol (ICMP) Echo function which is detailed in RFC 792.

A small packet is sent through the network to the target host (this is an ICMP Echo message). The computer that sent the packet then listens for a return packet (an ICMP Echo Reply message back). If the target device (computer, router, etc.) is up, the IP stack (in short the network interface) is up and the host isn’t configured to ignore ICMP Echo requests, it responds back with ICMP Echo Reply messages. Because the originator of the Ping request sets a timer when the Echo message is sent, the roundtrip time/delay can be computed. This roundtrip time is called network latency.

Ping can tell you when a target host is up and operational as well as the roundtrip time/delay to that host.

Traceroute Gives you the ability to “trace the route” to a host so you can see all the hops (more than likely those being routers or smart switches) between you and a host. When a traceroute is initiated, the command sets a timer and then assigns the TTL of a packet to 1 and sends it to the host. The first hop/router in the path decreases the TTL by one to zero and discards it. It then sends an ICMP destination unreachable message back to the originator of the traceroute. What this gives us is the roundtrip time to that hop plus it’s IP address. The traceroute program then sends out another packet to the host with a TTL value of two….the cycle continues until the host is finally reached. PingPlotter improves on this process greatly by doing every hop “real-time” – see the “How PingPlotter Works” section for more information on this.

TTL (Time to Live) Specifies the maximum time that a packet can live. This is important, because in the case of PingPlotter it’s the number of gateways or routers a packet is allowed to go through before being destroyed (hence avoiding loops). When a host answers a ping, it sets the return packet's TTL to 255. Also see Traceroute.