Robust Packet framing

AX.25 values at the library boundary exclude FCS bytes. A bare radio packet carries the AX.25 FCS; a container instead carries one CRC over its header, length words and contents. No FCS is synthesized for delivered container frames. Bare framing is measured on four public frames. [P: 3 stations] Container framing is covered by generated vectors. [T: 1 implementation, synthetic frames, 2026-09-30]

Addresses compress from seven bytes to five using a base-40 callsign integer and the original SSID/flags byte. The bare packet prepends a byte carrying padding and speed suggestion. PN9 scrambling excludes the final zero tail byte; a K=9 rate-1/2 convolutional code and a length-dependent interleaver follow. [P: 3 stations, level 0]

Classes 0–14 hold 15, 21, 45, 75, 105, 126, 147, 168, 189, 210, 231, 252, 273, 294 and 312 bytes including the tail. Class 15 adds a second header and holds 345 + 30e bytes for extensions e = 0–16. The smallest fitting class is selected, including exact-capacity extended packets. Bare padding must still fit its five-bit field. [T: 1 implementation, synthetic frames, 2026-09-30] Public frames cover class 3 at three stations [P: 3 stations, 3 frames] and class 4 at one station. [P: 1 station, 1 frame]

brubru.frame.pack_frames returns packets in input order. A frame that cannot share the container's address representation goes bare. A frame that fits no permitted bare class is logged and skipped; other frames retain their order. Later container frames store only one C bit, so both destination/source combinations 00 and 11 must stay bare. One-frame containers are accepted on receive and are never emitted. [T: 1 implementation, synthetic frames, 2026-09-30]

Both boundaries validate all addresses, control/PID presence, declared lengths and the applicable CRC before returning frames. Receive results contain only validated, unchanged AX.25 bytes. Invalid headers, unsupported extensions, truncation, malformed containers and failed checks produce no delivered frame. [T: 1 implementation, malformed-input tests, 2026-09-30]

Padding outside the CRC is discarded; its byte values do not change the delivered frame. [T: 1 implementation, 2 padding cases, 2026-09-30]

The Python literal vectors pin the preamble, convolutional encoder pair, interleaver checks, all 64 header codewords, compressed callsign examples and CRC check value. Public corpus tests additionally pin each recording's digest, payload and measured regeneration extent. The second radioscanner frame is 227 symbols and 1808 differentials; its acceptance currently requires a valid FCS and exact packet reconstruction, without a regeneration-count claim.

Container header bits 3–4 and length-word bits 9–14 are ignored on receive; length uses bits 0–8. The first word's C bit is ignored: its addresses carry the original flags. Later words supply the destination C bit, with its complement used for the source. The container CRC still covers every header and length bit. The public frames do not establish container behaviour. [T: 1 implementation, 10 bit/flag cases, 2026-09-30]