UDP Telemetry Receiver
A sender and a receiver that talk to each other over UDP using a binary protocol I designed, carrying simulated GPS and IMU readings. The point was to build the opposite of a framework project. Every byte on the wire is there because I put it there, and anything that would normally be a library call is written out.
The wire format
Twenty-eight bytes, fixed:
| Field | Bytes | Purpose |
|---|---|---|
magic | 2 | Says this is our protocol. Anything else gets dropped |
version | 1 | Format version |
sensor_type | 1 | GPS or IMU |
sequence_number | 4 | Counts up, so you can tell order and spot gaps |
timestamp | 4 | Milliseconds since the sender started |
values[3] | 12 | Three float readings |
checksum | 4 | CRC-32 over the 24 bytes before it |
The bits worth doing by hand
CRC-32 is generated, not linked. A 256 entry lookup table built from the
reflected 0xEDB88320 polynomial the first time it’s needed, then a byte at a time
over the payload. It’s about thirty lines, and the reason to write them yourself is
that when a packet fails validation you can actually reason about why.
Byte order is spelled out everywhere. Every multi-byte field goes through htons or htonl on the way out and ntohs or ntohl on the way back, so the
format is big-endian on the wire no matter what machine is on either end. Floats are
the fiddly case. They get copied into a uint32_t with memcpy, byte-swapped, and
copied back. Never cast through a pointer and never punned through a union, which
are the two usual ways people quietly introduce undefined behaviour into a parser
that looks like it works.
Parsing returns std::optional<Packet>. A packet only gets out of that optional
by clearing three checks: the datagram is exactly 28 bytes, the magic number
matches, and the CRC I recompute agrees with the one in the packet. Everything else
gets logged and dropped. If you’re sitting on an open socket you should assume every
datagram is junk until it proves otherwise.
The build is CMake, with the parsing and checksum code in a static library that both the sender and the receiver link against, so there’s only one copy of the protocol and the two ends can’t drift apart.
Where it’s at
The format carries sequence_number and the receiver reads and prints it, but gap
and reorder detection isn’t built yet. That’s next, along with proper tests for the
codec.