Design & roadmap#
pw_hdlc: Simple, robust, and efficient serial communication
pw_hdlc implements a subset of the High-Level Data Link Control (HDLC) protocol.
HDLC is a data link layer protocol intended for serial communication between
devices and is standardized as ISO/IEC 13239:2002.
Overview#
The pw_hdlc module provides a simple, robust frame-oriented transport that
uses a subset of the HDLC protocol. pw_hdlc supports sending between
embedded devices or the host. It can be used with pw_rpc to enable
remote procedure calls (RPCs) on embedded devices.
pw_hdlc has two primary functions:
Encoding data by constructing a frame with the escaped payload bytes and frame check sequence.
Decoding data by unescaping the received bytes, verifying the frame check sequence, and returning successfully decoded frames.
Design considerations#
pw_hdlconly supports unnumbered information frames.It uses special escaped bytes to mark the beginning and end of a frame.
Frame data is stored in a buffer until the end-of-frame delimiter byte is read.
Protocol Description#
Frames#
The HDLC implementation in pw_hdlc supports only HDLC unnumbered
information frames. These frames are encoded as follows:
_________________________________________
| | | | | | |...
| | | | | | |... [More frames]
|_|_|_|__________________________|____|_|...
F A C Payload FCS F
F = flag byte (0x7e, the ~ character)
A = address field
C = control field
FCS = frame check sequence (CRC-32)
Encoding and sending data#
This module first writes an initial frame delimiter byte (0x7E) to indicate
the beginning of the frame. Before sending any of the payload data through
serial, the special bytes are escaped:
Unescaped Special Bytes |
Escaped Special Bytes |
|---|---|
|
|
|
|
The bytes of the payload are escaped and written in a single pass. The
frame check sequence is calculated, escaped, and written after. After this, a
final frame delimiter byte (0x7E) is written to mark the end of the frame.
Decoding received bytes#
Frames may be received in multiple parts, so pw_hdlc needs to store the
received data in a buffer until the ending frame delimiter (0x7E) is read.
When the pw_hdlc decoder receives data, it unescapes it and adds it to a
buffer. When the frame is complete, it calculates and verifies the frame check
sequence and does the following:
If correctly verified, the decoder returns the decoded frame.
If the checksum verification fails, the frame is discarded and an error is reported.
Roadmap#
Expanded protocol support -
pw_hdlccurrently only supports unnumbered information frames. Support for different frame types and extended control fields may be added in the future.