40 lines
2.3 KiB
C
40 lines
2.3 KiB
C
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#ifndef GN_UNWRAP_H
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#define GN_UNWRAP_H
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#include <stdint.h>
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#include <stddef.h>
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#include <stdbool.h>
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// Inverse of geonet_wrap_shb() + dot11p_build_frame(): takes a raw 802.11 frame as delivered by
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// the WiFi driver's promiscuous RX callback and strips 802.11 header -> LLC/SNAP -> GeoNetworking
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// Basic/Common/extended header -> BTP-B header, leaving just the ITS payload (CAM UPER bytes)
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// and the sender's station id (GN_ADDR MID).
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//
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// Deliberately narrow, matching what this project actually transmits: only handles the
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// Single-Hop-Broadcast (TSB, HeaderType=5/Subtype=0) extended header shape, same as
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// geonet_wrap_shb() builds - the same "best-tested decode path" rationale documented there.
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// A real receiver would also need GeoBroadcast (HeaderType=4, used by DENM dissemination in
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// real deployments) and possibly Beacon/GeoUnicast - out of scope for now since nothing this
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// project talks to sends those. Extend header_type handling here if that changes.
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//
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// Only accepts BTP-B destination port 2001 (CAM, per ETSI TS 103 248) - other ports (e.g. 2002
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// DENM) are silently rejected since the phone-side decoder only understands CAM right now.
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//
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// Returns true and fills *out_cam / *out_cam_len (pointing INTO the input frame buffer, not a
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// copy - valid only as long as `frame` is) if this was a well-formed, CAM-carrying SHB frame
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// this project can decode. Returns false otherwise (wrong ethertype, wrong header type, wrong
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// BTP port, truncated, or FCS/promiscuous-capture garbage - all common and expected on an
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// open-air capture, not logged as errors by the caller).
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//
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// No station id is extracted here on purpose: CAM's own ItsPduHeader.stationID (the first real
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// field inside the UPER payload this function hands back, per cam.c) is already the meaningful
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// application-level identifier - the Kotlin-side decoder reads it from there. The GN_ADDR MID
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// this frame also carries is a separate, link-layer-only pseudonym; extracting and forwarding
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// it too would just be a second, easily-confused "station id" for no benefit here.
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//
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// RSSI is NOT extracted here either - it comes from the promiscuous callback's own packet
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// metadata (wifi_pkt_rx_ctrl_t.rssi in main.c), not from anything inside the frame bytes.
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bool gn_unwrap_cam(const uint8_t *frame, int frame_len,
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const uint8_t **out_cam, int *out_cam_len);
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#endif
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