linktypes: add DLT for DECT_NR

[stigbjorlykke]

Add a link-layer header type for the DECT New Radio protocol defined in ETSI TS 103 636-4.

[guyharris]

So the current version of that is 1.4.1, although "An update is in preparation."

So is the packet content in a pcap or pcapng file a MAC PDU as specified in section 6.3, beginning with a MAC header type field, and ending with the MIC?

[stigbjorlykke]

The pcap/pcapng packet will start with the "Physical Header Field" in section 6.2 and end with MIC only when MAC security is used.

6.2 Physical Header Field
6.3 MAC PDU
  6.3.2 MAC Header Type
  6.3.3 MAC Common Header
  -- One or more MAC SDUs included in MAC PDU with MAC multiplexing header
    6.3.4 MAC Multiplexing Header
    6.4 MAC Messages and IEs
  5.9.1 Message Integrity Code (MIC)

This is a snapshot from a Wireshark dissector in development:

[guyharris]

The pcap/pcapng packet will start with the "Physical Header Field" in section 6.2

What indicates whether the physical layer header is Type 1 (40 bits/5 bytes) or Type 2 (80 bits/10 bytes)? Is the PHF Size field, as shown, a metadata field that precedes the Physical Header Field?

[stigbjorlykke]

What indicates whether the physical layer header is Type 1 (40 bits/5 bytes) or Type 2 (80 bits/10 bytes)? Is the PHF Size field, as shown, a metadata field that precedes the Physical Header Field?

ETSI TS 103 636-3 chapter 7.5.1 "Overall description" states that

"The receiver shall blind decode both transport block sizes and select the one with a CRC match."

Unfortunately the CRC is not included at this level, and it's currently not possible to determine the size without adding a metadata field. We will ask the spec authors if it's possible to improve this in the format. I will mark this PR as draft until this is sorted out.

[stigbjorlykke]

I have updated the documentation to describe how to handle the two different "Physical Header Field" sizes.