Re: libpcap timestamp for sending a packet ?

[Daniel Lawson <daniel () meta net nz>]

> Hello
> I am setting up a framework for measuring one way delay between two Internet
> end points.
> For higher accuracy I am using libpcap time stamps (from the packet header)
> at the reciever.
> Does any one know if there is any method for improving accuracy at the
> sender side.
> Is it possible for sender's NIC to log the time stamp the packet was sent?
> or any other idea ?
>   

I assume you've read the various proposals on one-way delay measurement
protocols, such as IPMP or  OWAMP / OWDP ? IPMP is an IP level protocol
similar to ICMP in some respects. It records the local timestamp and id
of every IPMP-enabled hop it passes through.
(http://watt.nlanr.net/AMP/IPMP/)

 OWDP is a client/server/control framework over UDP for one-way delay
measurement, pref. using local GPS sources for accurate timestamping.

My understanding of libpcap is that timestamping is not guaranteed to be
accurate. If the machines are heavily loaded the timestamps might drift
a bit. I'm also not aware of any standard NIC that will do what you ask,
but I'm not claiming this is definitely the case. One option for
accurate timestamping are the DAG cards (www.endace.com) but these are
relatively expensive, and requiring custom hardware at the end points
limits the flexibility of your framework. libpcap supports DAG cards
however, so your app could be written in a way that transparently makes
use of DAG cards if they are available.


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