Classify an 802.11 frame as mgmt/ctrl/data

Challenge

Classify an 802.11 frame by its type field — the first decision a Wi-Fi forensics tool makes before it knows the rest of the layout. Pure bit work on a fixed header byte; no NIC or capture.

Task

Implement int classify_frame(const uint8_t *hdr) that extracts the frame TYPE from the Frame Control field's first byte and maps it to a category.

The first byte is laid out as:

bits 7..4 : subtype
bits 3..2 : TYPE        <- extract this: (hdr[0] >> 2) & 0x3
bits 1..0 : protocol version

Map the 2-bit type:

• 0 -> return 0 (management)
• 1 -> return 1 (control)
• 2 -> return 2 (data)
• 3 -> return -1 (reserved)

Input

• hdr: a pointer to the header bytes (only hdr[0] is read), or NULL. The grader passes fixed bytes.

Output

Returns int: 0, 1, or 2 for valid types; -1 for the reserved type or a NULL pointer.

Example

hdr[0] with type bits = 0   ->   0   (management)
hdr[0] with type bits = 1   ->   1   (control)
hdr[0] with type bits = 2   ->   2   (data)
hdr[0] with type bits = 3   ->   -1  (reserved)
NULL                        ->   -1

Edge cases

• NULL hdr returns -1.
• The subtype bits (4..7) and protocol-version bits (0..1) do not affect the answer — mask them out.

Rules

• One shift and one mask; no loops or string work.

Why this matters

Every Wi-Fi forensics tool starts by classifying the frame type before it knows the layout of the rest of the bytes.

Input format

A pointer hdr to at least one byte (only hdr[0] is read), or NULL.

Output format

An int: 0/1/2 for the type, or -1 for reserved/NULL.

Constraints

Extract (hdr[0] >> 2) & 0x3; mask out subtype and protocol-version bits.

Starter code

#include <stdint.h>
int classify_frame(const uint8_t *hdr) {
    /* TODO */
    (void)hdr;
    return -1;
}

Common mistakes

Reading the wrong bit position. Forgetting the NULL check. Allowing the reserved type 3 through.

Edge cases to handle

All-zeros byte (type 0). Top bits set (subtype) should not affect classification.

Complexity

O(1).

Background lessons

• Classify an 802.11 frame from its control byte

Up next

• Extract the BLE Complete Local Name from an advertisement