Re: Linktype allocation for ATSC link layer protocol

[Nick Kelsey <nickk () silicondust com>]

> But they all have the Base Header, and that header has:
>         3 bits of packet type;
>         1 bit of payload configuration;
>         1 bit of:
>                 header mode, if the payload configuration bit is 0;
>                 segmentation/concatenation, if the payload configuration bit is 1;
>         11 bits of length;
> for a total of 16 bits, right?

Yes, unless the packet type is 111b in which case there the header is only 8 bits - you have the packet type, 4 bits 
indicating the count of TS frames (where 0 means a count of 16) and 1 bit indicating if header deletion mode is used.

> If so, in a capture file:
>         1) the Base Header has 2 octets - in what order in the file are the two octets?
>         2) in what order are the bit fields?

Order - byte at a time, most significant bit first.
The first 3 bits are the packet type - ie the most significant 3 bits of the first byte.

> For example, does the first octet of a packet contain:
>         3 bits of packet type in its upper 3 bits;
>         1 bit of payload configuration in the next lower bit;
>         1 bit of header mode or of segmentation/concatenation in the next lower bit;
>         the uppermost 3 bits of length in the lowest 3 bits;
> and does the second octet contain the lowermost 8 bits of length?

Correct, noting that the length field can be larger than 11 bits as follows:

payload_configuration = 0, header_mode = 0: the length field is 3+8=11 bits long.
payload_configuration = 0, header_mode = 1: the third byte contains another 5 bits of length (most significant) making 
the length field 16 bits long.
payload_configuration = 1, segmentation_concatenation = 0: the length field is 3+8=11 bits long.
payload_configuration = 1, segmentation_concatenation = 1: the third byte contains another 4 bits of length (most 
significant) making the length field 15 bits long.

If the packet type is 4 (link layer signaling packet) then there is an additional 5 bytes of link layer signaling 
header after the optional headers, before you start counting the length.

BTW - packets seem to often contain junk at the end.

Nick

_______________________________________________
tcpdump-workers mailing list
tcpdump-workers () lists tcpdump org
https://lists.sandelman.ca/mailman/listinfo/tcpdump-workers