[ntar-workers] Timestamp resolution: if_tsaccur option unclear!

[Stephen Donnelly]

On Tue, 2005-08-23 at 15:04 +0200, Ulf Lamping wrote:
> Hi List!
> 
> I'm currently looking for a way to bring nanosecond timestamp resolution to Ethereal. Unfortunately this is urgent, so I cannot wait until the NTAR lib is integrated into Ethereal. To achieve this goal I'm planning to use a new libpcap DLT_ value and simply put nanosecond resolution into the microsecond timestamp field.
> 
> As I don't want to reinvent the wheel, I had a look at how the timestamp resolution is specificated in the new PCAP file format and simply use as much as possible of this spec.

> P.S: With the current timestamp definition, the finest resolution would be 232,8 picoseconds (when I've done a correct calculation) until the lower 32 bit part of the timestamp will wrap around. Will this be enough even for 10GBit Ethernet?

Depends what you want to measure?

My opinion is that the minimum timestamp accuracy required for packet
measurement is that minimum sized packets with minimum inter-packet gap
should always receive unique tmiestamps, e.g. timestamps on sequential
packets should always be strictly increasing.

For Gigabit Ethernet with minimum Ethernet frame size of 64 Bytes (plus
IFG etc) this is approximately 576ns. This implies that the microsecond
resolution of libpcap is already insufficient.

For 10GE this would be approximately 58ns, implying that nanosecond
resolution would be more than sufficient.

I think the highest resolution that is likely to be useful is the
byte-time of the link layer, since most link layers are essentially byte
oriented (bitwise HDLC is an exception). In this case the resolution
required for Gigabit Ethernet would be 8 nanoseconds, and for 10GE 1
nanosecond.

Stephen.
-- 
-----------------------------------------------------------------------
    Stephen Donnelly BCMS PhD           email: sfd at endace.com
    Endace Technology Ltd               phone: +64 7 839 0540
    Hamilton, New Zealand               cell:  +64 21 1104378
-----------------------------------------------------------------------