Internet printing protocol (IPP) encoding and transport

Abstract

■ This article discusses the encoding of IPP and overall transport using HTTP and TCP/IP. It describes the IPP protocol elements, mapping of IPP to the wire, pros and cons of using HTTP to transfer IPP and various implementation considerations. PP is operations encoding and format coupled with an application-level protocol developed to address modern, distributed printing using existing Internet technologies. The IPP protocol consists of request and response messages. IPP clients send request messages to IPP printers and get response messages back in return. The request message consists of an IPP operation code, attributes and (optionally) print data. IPP response messages consist of a status code and attributes. IPP request/ response message encoding is referred to as the “operation layer” and forms a new Internet MIME media type called “application/ipp.” The IPP operation layer is transport-independent and, as such, requires some application-layer means of transfer. The first widespread, standardized embodiment of IPP is based on the use of HTTP1.1 as the transfer protocol. In this context, printing with IPP consists of a series of HTTP posts, generated by the IPP client, and appropriate responses from the IPP printer. The ability for HTTP to encapsulate the print data stream and represent printer and print job objects as URIs meets the requirements of the IPP operation layer. Furthermore, the ability for HTTP/1.1 to manage print data as a sequence of chunks, each with a known length, makes it ideal for a wide range of print applications where the total length of the data is not known prior to submission. To distinguish the use of HTTP with IPP vs. generic browsing, the Internet Assigned Numbers Authority (IANA) has assigned the well-known port 631, which is considered the default port for IPP over HTTP. Implementations may support additional ports (including port 80). An actual IPP URL scheme (ipp://) is under consideration for future versions of the IPP standard to further distinguish the role of HTTP in printing.