Packet

All sizes in bits.

packet_header:
    [ 1] flag_compression
    [ 1] flag_request_resend
    [ 1] flag_connless
    [ 1] flag_control
    [ 2] padding
    [10] ack
    [ 8] num_chunks

    FFFF ppAA  AAAA AAAA  nnnn nnnn

NOTE: padding must be zeroed, it's incorrectly used as part of the ack field while unpacking in the reference implementation.

chunk_header_vital:
    [ 1] flag_resend
    [ 1] flag_vital
    [ 6] <----------
    [ 4] padding   |-- size
    [ 4] <----------

    FFss ssss  PPPP ssss


chunk_header_nonvital:
    [ 1] flag_resend
    [ 1] flag_vital
    [ 6] <----------
    [ 4] sequence  |-- size
    [ 4] <----------
    [ 8] sequence part 2

    FFss ssss  SSSS ssss  SSSS SSSS

In the packed form, the first four bits of sequence correspond to bits 10 to 6, and the second part corresponds to the bits 8 to 1.

NOTE: The reference implementation just uses bitwise-or to resolve contradictions in the overlapping bits.