[tcpdump-workers] Link-layer header type request for libopenvizsla

Guy Harris gharris at sonic.net
Wed Aug 22 23:11:32 EDT 2018


On Aug 22, 2018, at 1:57 AM, Matwey V. Kornilov <matwey.kornilov at gmail.com> wrote:

> Current OpenVizsla data encapsulation format is described here:
> https://github.com/matwey/libopenvizsla/wiki/OpenVizsla-protocol-description

Why are:

	the magic header;

	the size field;

needed?

The magic number is always the same, so it contains zero bits of information (in the Claude Shannon sense).

If the size field is just the number of bytes in the Data field, it's just the total packet size (from the "length" field in pcap files, the "Original Packet Length" field in pcapng Packet Blocks, Enhanced Packet Blocks, and Simple Packet Blocks) minus the number of header bytes prepended (which would just be 5, for the flags and timestamp fields, if none of the fields mentioned above are needed).

Presumably the timestamp field is useful because either

	it doesn't have to be converted to seconds-and-microseconds or seconds-and-nanoseconds units for pcap or 10^-N/2^-N ticks for pcapng

or

	it's a relative timestamp and can't be conveniently converted to an absolute time stamp

so that it's not redundant with the timestamp field of the pcap record or pcapng block.


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