Re: timestamp in Packet Data

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

There are many factors that make the timestamps an approximation.  In addition to the ones already mentioned, the 
timestamps is typically taken several kernel layers higher than the driver.  

You can avoid these approximations on Linux by using tcpdump -j / -J with a NIC that is capable of hardware 
timestamping (plug: such as the Intel 82580 NIC).  The -j / -J options are only available in the git repository.

Scott  

-----Original Message-----
From: tcpdump-workers-owner () lists tcpdump org [mailto:tcpdump-workers-owner () lists tcpdump org] On Behalf Of 
Sanjay Sundaresan
Sent: Saturday, July 09, 2011 7:59 PM
To: tcpdump-workers () lists tcpdump org
Subject: Re: [tcpdump-workers] timestamp in Packet Data

Is the approximation because of the fact that NIC card generarates interrupt
only after some number of packets arrive ?. Does device polling affect time
stamp ? At what stage of capture time stamping is done ?

On Sat, Jul 9, 2011 at 6:59 PM, Alokat <mailing () alokat org> wrote:

> On 07/09/11 21:56, Guy Harris wrote:
> > On Jul 9, 2011, at 4:41 PM, Alokat wrote:
> >
> > > I'm wondering what is in the pcap_data (pcap file format) and what is
> not?
> > > Especially the timestamp ... is it just in the packet_header or in the
> > > packet_data too?
> > A pcap file starts with a header.  Following the header are zero or more
> packet records.  A packet record has a header, which includes the packet
> time stamp, followed by packet data, which is just the raw data as supplied
> to libpcap/WinPcap by whatever mechanism it uses.  That mechanism supplies
> the packet time stamp for inclusion in the header, so there is no reason to
> expect that it will also be in the packet data, especially given that no
> link layers would include that time stamp (it's not in an Ethernet header,
> for example), so the time stamp is just in the packet header, not the packet
> data.
> > The time stamp is an approximation of the time when the packet was
> received by the machine that captured it.-
> > This is the tcpdump-workers list.
> > Visit https://cod.sandelman.ca/ to unsubscribe.
> Okay,
>
> Thanks for your answer ...
>
> Regards,
> alokat
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