The TCP/IP Guide - Miscellaneous TCP/IP Troubleshooting Protocols: Echo, Discard, Character Generator, Quote Of The Day, Active Users, Daytime, Time

The old quip says that the only guarantees in life are death and taxes. When it comes to networking, you can add a few more, including this one: as soon as you set up a network, it will very quickly develop problems that you will need to address. Recognizing that the complexity of TCP/IP internetworks would make diagnosing certain problems difficult, the suite’s architects defined a number of miscellaneous utility protocols that can be helpful in testing and troubleshooting networks. Despite having been around for over 20 years, these protocols are somewhat obscure and get little attention. I too will not devote much time to them (since they are no longer implemented on many systems) but I do feel they are worth a quick look.

These simple protocols are designed to be implemented as services that run on TCP/IP servers. Each listens for requests on a dedicated well-known port number, and then responds with a particular type of information. These protocols can be used with both TCP and UDP, enabling each transport protocol to be tested. In the case of UDP, the server counts each UDP sent to it as a request, and sends a response to it. When used with TCP, a connection is of course first established by the client to the server. In some of the protocols, this connection is then used to send data continuously between the client and server; in others, the establishment of the connection is considered an implied request to the server, which will immediately send a response and then close the connection.

Table 311 provides a brief description of each of these troubleshooting protocols/services under both UDP and TCP. I have shown for each the port number that the service uses, and also the RFC that defines it, if you want additional information.

**Table 311: Miscellaneous TCP/IP Troubleshooting Protocols**
Protocol	Well-Known Port Number	Defining RFC	Description
Echo Protocol	7	862	Echoes received data back to its originator. When used on UDP, the payload of each message is simply packaged into a return UDP datagram and sent back. For TCP, each byte sent by the client is echoed back by the server until the connection is closed.
Discard Protocol	9	863	Throws away all data that is sent to it. I think this should be called the “Black Hole Protocol”. J
Character Generator Protocol	19	864	Generates random characters of data and sends them to a device. When used with UDP, each UDP message sent to the server causes it to send back a UDP message containing a random number (0 to 512 bytes) of data. When used with TCP, the server just starts sending characters as soon as a client establishes a connection, and continues until the connection is terminated by the client.
Quote of the Day Protocol	17	865	Sends a short message (selected by the server’s administrator) to a client device. For UDP, the message is sent for each incoming UDP message; for TCP, the message is sent by the server once when the connection is established, which is then closed.
Active Users	11	866	Sends a list of active users to a device. For UDP, the list is sent for each incoming UDP message; if it is longer than 512 bytes it will be sent in multiple messages. For TCP, the list is sent automatically when the connection is made to the server, and then the connection is terminated.
Daytime Protocol	13	867	Returns the current time on the server in human-readable form, in response to receipt of a UDP message or an incoming TCP connection.
Time Protocol	37	868	Returns the current time in machine-readable form—specifically, the number of seconds since midnight, January 1, 1900 GMT. The time is sent for each UDP message received by the server, or upon establishment of a TCP connection. Note that this protocol cannot be used for time synchronization of servers, because it does not compensate for variability in the time required for the messages to be carried over the internetwork.

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