It was originally proposed as a way to test and measure an IP network.
A host may connect to a server that supports the Echo Protocol using the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP) on the well-known port number 7.
The server sends back an identical copy of the data it received.
On UNIX-like operating systems an echo server is built into the inetd family of daemons.
It may be enabled by adding the following lines to the file /etc/inetd.conf and telling inetd to reload its configuration:[1] On various routers, this TCP or UDP port 7 for the Echo Protocol for relaying ICMP datagrams (or port 9 for the Discard Protocol) is also configured by default as a proxy to relay Wake-on-LAN (WOL) magic packets from the Internet to hosts on the local network in order to wake them up remotely (these hosts must also have their network adapter configured to accept WOL datagrams and the router must have this proxy setting enabled, and possibly also a configuration of forwarding rules in its embedded firewall to open these ports on the Internet side).