Unix Programming - Application Protocol Metaformats

The Classical Internet Application Metaprotocol

Marshall Rose's RFC 3117, On the Design of Application Protocols,^[55] provides an excellent overview of the design issues in Internet application protocols. It makes explicit several of the tropes in classical Internet application protocols that we observed in our examination of SMTP, POP, and IMAP, and provides an instructive taxonomy of such protocols. It is recommended reading.

The classical Internet metaprotocol is textual. It uses single-line requests and responses, except for payloads which may be multiline. Payloads are shipped either with a preceding length in octets or with a terminator that is the line ".\r\n". In the latter case the payload is byte-stuffed; all lines that start with a period get another period prepended, and the receiver side is responsible for both recognizing the termination and stripping away the stuffing. Response lines consist of a status code followed by a human-readable message.

One final advantage of this classical style is that it is readily extensible. The parsing and state-machine framework doesn't need to change much to accommodate new requests, and it is easy to code implementations so that they can parse unknown requests and return an error or simply ignore them. SMTP, POP3, and IMAP have all been extended in minor ways fairly often during their lifetimes, with minimal interoperability problems. Navely designed binary protocols are, by contrast, notoriouslybrittle.