Re: Linktype allocation for ATSC link layer protocol

[Nick Kelsey via tcpdump-workers <tcpdump-workers () lists tcpdump org>]

> --- Begin Message ---
>
>
> From: Nick Kelsey <nickk () silicondust com>
>
> Date: Fri, 20 Mar 2020 10:28:27 -0700
>
> After going to interop/test events and discussions with other engineers 
> in the field it looks like a lot of the junk in the ATSC ALP protocol 
> will never be used and can be ignored.
>
> The protocol is defined in the ATSC A/330 standard.
>
> Proposed implementation:
>
> 1) Check the packet_type (first 3 bits of the packet). If the packet 
> type is 7 (MPEG2 transport stream) then report "MPEG2 transport stream" 
> and stop parsing. It is likely this packet type will never be used in 
> real-world use. Note that the fields that follow don't apply when the 
> packet type is 7 - all we can do is report the packet type and stop.
>
> 2) If the payload_configuration bit is 1 check the 
> segmentation_concatenation bit.
>
> If the segmentation_concatenation bit is 0 report that it is a 
> "segmented packet" and stop parsing. It is likely this mode will never 
> be used in real world use (the layer below this protocol does 
> segmentation/reassembly).
>
> If the segmentation_concatenation bit is 1 report that it is a 
> "concatenation packet" and stop parsing. It is likely this mode will 
> never be used in real world use.
>
> 3) If the payload_configuration bit is 0 then the packet is useful!
>
> If header_mode is 0 then report the length and the packet_type. The 
> packet type will typically be IPv4 packet (packet_type = 0) where the 
> payload is a normal IPv4 packet.
>
> The other possible packet type is a link layer signalling packet 
> (packet_type = 4). In this case there is an "Addition Header for 
> Signaling" header followed by a table. Note that the payload_length 
> tells you the table length - it does not include the "Addition Header 
> for Signaling" header length.
>
> If header_mode is 1 then parse the single_packet_hdr().
>
> If SIF flag is set then parse/skip sub_stream_identification() header.
>
> If HEF flag is set the parse/skip header_extension() header.
>
> At this point you look at the packet type, following the same logic as 
> header_mode 0 above.
>
> Nick
>
>
>
> --- End Message ---
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