[tcpdump-workers] Request for link-layer header type (XRA)

Bruno Verstuyft bruno.verstuyft at gmail.com
Fri Jan 19 08:03:51 EST 2018


2018-01-19 10:10 GMT+01:00 Guy Harris <guy at alum.mit.edu>:

> On Jan 4, 2018, at 5:47 AM, Bruno Verstuyft <bruno.verstuyft at gmail.com>
> wrote:
>
> > are any more clarifications needed for the XRA header spec?
>
> How is the Symbol ID used for timing calculations?
>

DOCSIS 3.1 PLC (PHY Link Channel) frames in the downstream contain an
absolute timestamp. For every packet in the downstream, the difference in
symbols between the previous PLC frame and the packet is calculated. With
the help of parameters from the OCD (discrete fourier transform size and
cyclic prefix), this difference in symbols can be converted in a difference
in nanoseconds. This time difference is then added to the absolute
timestamp of the previous PLC frame, which yields the absolute timestamp of
the packet. This timestamp is stored in the timestamp of the pcap packet
header. For debugging reasons, the symbol id TLV can also be added to the
packet.


>
> Is the Burst ID just a sequence of octets?
>

 For the moment, the Burst ID  is a uint64_t. Should this not be not enough
in future implementations, it can be increased to e.g. uint128_t


> What does the Burst ID Reference field contain?  Another burst ID?


The Burst ID reference is the same as the Burst ID. Burst IDs are used in
databursts, while Burst ID references are used in Mac Frames. For the
moment, these are both uint64_t.


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