Re: [RFC PATCH 0/2]: hw timestamp support

["Mcmillan, Scott A" <scott.a.mcmillan () intel com>]

Guy,

Both the 'raw' and 'nic' timestamps are in the form of seconds since the Unix epoch, plus fractions of a second.  
Please see my response to Darren for more info on the difference between these two timestamp sources.

The only reason I can think of to use 'raw' is for debugging.  

Regarding whether one would always want to use hw timestamps if they are available... I think in general, one would 
want to use hw timestamps.  The reason I didn't make it the default is that limitations of current hw can lead to 
potentially confusing results.  For example, the Intel 82580 nic only hw timestamps RX general purpose packets, meaning 
TX general purpose packets are still timestamped in sw.  Hence, timestamps may not be monotonic in a capture stream 
(although RX timestamps will be monotonic and TX timestamps will be monotonic).  

If the consensus is to use hw timestamps by default if available, I have no issues modifying the patch accordingly.  
Similarly, if the 'raw' timesource is not seen as potentially useful, I can change that too.

Scott

-----Original Message-----
From: tcpdump-workers-owner () lists tcpdump org [mailto:tcpdump-workers-owner () lists tcpdump org] On Behalf Of Guy 
Harris
Sent: Tuesday, May 25, 2010 8:37 PM
To: tcpdump-workers () lists tcpdump org
Subject: Re: [tcpdump-workers] [RFC PATCH 0/2]: hw timestamp support


On May 24, 2010, at 7:26 AM, Mcmillan, Scott A wrote:

> [My apologies if this double posts.  The mail server didn't care for the first submission.]
>
> This patch adds the capability to select the packet timestamp source.

Is there ever any reason *NOT* to use the hardware timestamp if it's available?

> A new command line option was added to tcpdump, -j, to specify the source of the packet timestamp.  Valid options are 
> 'raw' to use the raw NIC HW timestamp and 'nic' to use the NIC HW timestamp transformed into the system clock basis.

Is the raw timestamp in the form of seconds since January 1, 1970, 00:00:00 UTC, plus fractions of a second?

I assume "NIC HW timestamp transformed into the system clock basis" means that the time stamp in question *is* in the 
form of seconds since January 1, 1970, 00:00:00 UTC, plus fractions of a second?

If "raw" is not in that form, and "nic" is in that form, is there any reason to use "raw"?-
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