cyphal_udp_header.dsdl

# All Cyphal/UDP traffic is sent to port 9382.
# The subject multicast group address is composed as 239.0.0.0 (=0xEF000000) + subject_id (23 bits).
# All frames of a transfer must share the same field values unless otherwise noted.
# Frames may arrive out-of-order, possibly interleaved with neighboring transfers; implementations must cope.

uint5 version                   #=2 in this version.
uint3 priority                  # 0=highest, 7=lowest.

uint2 KIND_MSG_BEST_EFFORT = 0  # No ack must be sent.
uint2 KIND_MSG_RELIABLE    = 1  # Remote must acknowledge reception by sending an ACK frame back.
uint2 KIND_ACK             = 2  # Sent P2P; the transfer_id is of the acknowledged frame. Payload empty/ignored.
uint2 kind
uint6 reserved_incompat         # Discard frame if any incompatibility flags are set that are not understood.

void16                          # Reserved for compatibility flags and fields (transmit zero, ignore on reception).

# Payload reassembly information.
# We provide both the frame index and the frame payload offset to allow various reassembly strategies depending on the
# preferences of the implementation. The provided information is sufficient for zero-copy out-of-order reassembly.
# Offset 4 bytes.

uint24 frame_index              # Zero-based index of the payload fragment carried by this frame.
void8
uint32 frame_payload_offset     # The offset of the frame payload relative to the start of the transfer payload.
uint32 transfer_payload_size    # Total for all frames.

# Transfer identification information.
# The transfer-ID is a single field that segregates transfers by topic hash and epoch (publisher sequence restarts).
# Offset 16 bytes.

uint64 transfer_id              # For multi-frame reassembly and dedup. ACK specifies the acked tfer here.
uint64 sender_uid               # Origin identifier ensures invariance to the source IP address for reassembly.

# Integrity checking information.
# Offset 32 bytes.

uint32 prefix_crc32c            # crc32c(payload[0:(frame_payload_offset+payload_size)])
uint32 header_crc32c            # Covers all fields above. Same as the transfer payload CRC.

# End of header at 40 bytes. Payload follows.