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

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 6/3/2020 9:11 PM, Stephen Hemminger wrote:
> 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.
> 

When there is a way to get better precision timestamp from pcap, why not benefit
from it. We can discuss switching to pcapng separately I think.