ATSC 3.0 includes a new link layer protocol known as ALP (ATSC Link-layer protocol).
So the first two octets of the packet data contain the Base Header? That appears to be a 16-bit header; in a capture, in what order are the bytes of the Base Header, and in what order are the bit fields? The same applies to other fields.
There are a few different packet formats... If the most significant 3 bits of the first byte are 111b then the ALP packet contains MPEG-TS frames and the remaining bits in the first byte convey the count and if header deletion mode is used. The MPEG-TS frames are missing the leading 0x47 and optionally more of the header requiring minor reconstruction. The most common packet type is a single packet without additional header - this format is a simple 2 byte link layer header. Most commonly used to carry an IPv4 packet. Single packet with additional header - 3 bytes plus optional bytes. Segmentation packet where the payload has been split into multiple ALP packets - 3 bytes plus optional bytes, requires multi-packet reassembly to recover the payload. Concatenation packet where the ALP packet contains multiple payload frames - 3 bytes plus 12 bits per payload frame plus optional bytes. This one seems a little silly because it requires 12 bits (1.5 bytes) per concatenated payload where the overhead of putting each payload in a single packet is 2 bytes if you don't need additional headers. In single packet (with or without additional header), segmentation, and concatenation modes the payload type may be a type that has an additional header which must be accounted for before doing reassembly or splitting. In real-world captures I have seen single packets without additional header and single packets with additional header, carrying link-layer-signaling payloads and IPv4 payloads. I haven't seen MPEG-TS payloads, segmentation, or concatenation. Nick _______________________________________________ tcpdump-workers mailing list tcpdump-workers@lists.tcpdump.org https://lists.sandelman.ca/mailman/listinfo/tcpdump-workers
