Re: [dpdk-dev] [PATCH 1/2] net/pcap: support software Tx nanosecond timestamp

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 3 Jun 2020 18:50:51 +0100
Ferruh Yigit <ferruh.yigit@intel.com> wrote:

> On 5/23/2020 6:21 PM, Vivien Didelot wrote:
> > When capturing packets into a PCAP file, DPDK currently uses
> > microseconds for the timestamp. But libpcap supports interpreting
> > tv_usec as nanoseconds depending on the file timestamp precision.
> > 
> > To support this, use PCAP_TSTAMP_PRECISION_NANO when creating the
> > empty PCAP file as specified by PCAP_OPEN_DEAD(3PCAP) and implement
> > nanosecond timeval addition. This also ensures that the precision
> > reported by capinfos is nanoseconds (9).  
> 
> Overall good idea and patch looks good.
> 
> Only concern is change in the libpcap dependency. Do you know which libpcap
> version supports 'PCAP_TSTAMP_PRECISION_NANO'?
> If a user of pcap PMD updates to latest DPDK and the environment doesn't have
> new version of the libpcap, this change will require an environment update and
> this may or may not be easy to do. That is why not sure if the updates require
> dependency change should wait for the LTS or not.
> 
> Another things is the backward compatibility, as far as I understand the pcap
> file recorded with nanosecond precision can be read and parsed without problem
> by old application that doesn't know the nanosecond precision, but can you
> please confirm this?

We should do pcapng instead of doing the pcap with nano timestamp.
My impression is that libpcap is a dormant project at this point, and the
finer grain timestamp is a hack that is only in some verisions.
That was one of the reasons pcapng started.