I have few doubts regarding telnet and echo which I haven't got so far lucky with google (also to say I have done my homework)
I am not sure if there is telnet / protocol specific child stackoverflow site? If yes, can somebody point me there?
The IANA Service Name and Transport Protocol Port Number Registry contains all port number registrations and for Telnet it cites RFC 854 as specification which defines
Port Assignment
When used for remote user access to service hosts (i.e., remote
terminal access) this protocol is assigned server port 23
(27 octal). That is L=23.
For the Echo Protocol the RFC number RFC 862 which similarily defines
TCP Based Echo Service
One echo service is defined as a connection based application on TCP.
A server listens for TCP connections on TCP port 7. Once a
connection is established any data received is sent back. This
continues until the calling user terminates the connection.
The port number 10001 is registered for "SCP Configuration".