sliplib 0.0.1

Background

The SLIP protocol is described in RFC 1055 (A Nonstandard for Transmission of IP Datagrams over Serial Lines: SLIP, J. Romkey, June 1988). The original purpose of the protocol is to provide a mechanism to indicate the boundaries of IP packets, in particular when the IP packets are sent over a connection that does not provide a framing mechanism, such as serial lines or dial-up connections.

There is, however, nothing specific to IP in the SLIP protocol. SLIP offers a generic framing method that can be used for any type of message that must be transmitted over a (continuous) byte stream. In fact, the main reason for creating this module was the need to communicate with a third-party application that used SLIP over TCP (which is a continuous byte stream) to frame variable length messages.

The SLIP protocol uses four special byte values:

An END byte in the message is encoded as the sequence ESC+ESC_END (b'\xdb\xdc') in the slip packet, and an ESC byte in the message is encoded as the sequence ESC+ESC_ESC (b'\xdb\xdd').

As a consequence, an ESC byte in a slip packet must always be followed by an ESC_END or an ESC_ESC byte; anything else is a protocol error. Although the implementation code proposed by RFC 1055 ignores such errors, the sliplib module raises an exception in such cases.