Re: Linktype allocation for ATSC link layer protocol

[Nick Kelsey <nickk () silicondust com>]

> > 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

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