LPRng-HOWTO: 15 Oct 2001 (For LPRng-3.7.9)
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# 11.4. AppSocket TCP/IP Protocol

The AppSocket interface is supported by Tektronix and some other printer vendors. It is similar to the Socket API, with a couple of significant differences.

1. The printer has two ports for network connections: a TCP port 9100 for TCP/IP stream connections and a UDP port for UDP packet connections.

2. When a 0 length UDP packet or a UDP packet containing only `CR/LF` is sent to UDP port 9101, the printer will return a packet to the sender containing print status information. This information indicates the printers current status (busy, idle, printing) and any error conditions.

3. The format, reliability, and repeatability of the UDP format and information is totally undocumented, and the UPD port facility should be regarded as, at best, an advisory function of the printer.

4. To send a job to the printer, a connection to TCP port is made. This connection will be refused while the printer is busy or has a connection to another host.

5. When the TCP connection is established, the information to be printed can be sent over the TCP connection. Bytes sent on this stream will be placed in the input buffer of the Print Engine and processed.

6. An end of job (EOJ) sequence indication in the data stream will cause the printer to terminate the connection. This is different than the Socket API, where the printer will keep the connection open. This means that if the PostScript CTRL-D (end of job) character is sent in a job, then the connection will be terminated.

7. Some models of printers modify this behavior slightly and will not terminate the connection, but will simply ignore any data following the EOJ indication.

8. Some printers support bidirectional AppSocket communication, and while the connection is open will return error indications or status information.

9. Once all the data has been received and the job has finished printing, the connection will be terminated by the printer.

The ifhp filter, one of the helper programs for LPRng, is used with LPRng to provide AppSocket support. For details, please see the [IFHP-HOWTO] in the ifhp Distribution and Tektronix P450 and Family for details. The following is a typical printcap entry for the AppSocket protocol. The actual network connection to the printer is made by the ifhp filter:

        lp:
          # LPRng opens a dummy connection for consistency
          :lp=/dev/null
          # we pass the ifhp filter options indicating that the
          # Tektronics printer will need the appsocket protocol
          # and to use port 35 at 10.0.0.1 to make the connection
          # The ifhp filter may open and close the connection several
          # times during the file transfer in order to ensure that
          # the printer handles the job correctly.
          :ifhp=model=tek,appsocket,dev=10.0.0.1%35
          :filter=/usr/local/libexec/filters/ifhp

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