cvs commit: src/sys/net bpf.c

[Sam Leffler]

Jung-uk Kim wrote:
> On Monday 24 July 2006 01:38 pm, David Malone wrote:
>>> Thanks for taking care of this! It is not very desirable for the
>>> same packet to have different timestamps associated with it
>>> across different bpf peers. It certainly could cause a problem if
>>> people are using timestamps to correlate events from different
>>> programs on the same system.
>> Indeed - calling microtime more than necessary and generating
>> inconsistent timestamps just didn't seem good.
>>
>> I think jkim@ has interesting work related to this that might allow
>> us to exploit hardware that does hardware timestamping of packets.
>> It is some way down the road though.
> 
> Darn, I guess I have to finish the work now since you said it. ;-)  
> The idea is simply to pass timeval to bpf_tap() from network drivers 
> when it is available and bpf is attached.  (Something like 
> bpf_*tap_ts()?)   Additionally we may add an ioctl for bpf to select 
> timestamping method, e.g., microtime(9), getmicrotime(9), 
> timestamping from lower layer, or no timestamping at all because we 
> don't need (accurate) timestamps in many places.  For the 
> hardware-based accurate timestamping, the big challenge is actually 
> timecounter to timeval conversion, not the bpf API itself because we 
> can only select one hardware timecounter at a given time if the 
> controller does not give us timeval, bintime, or something similar.  
> We have to come up with some API to register additional timecounter, 
> which is not primary timecounter but synchronized with system uptime.  
> On top of that, the timecounter in the given descriptor is not 
> current timecounter.  So we need 'what time was it then', not 'what 
> time is it now.'

Why not leverage something like radiotap?  It already has a mechanism
for associating a 64-bit microsecond timestamp w/ each packet.  It's
presently somewhat wireless-oriented but could be used w/ wired devices
too and tools like ethereal and tcpdump already grok it.

	Sam