Re: Comparing two pcap files for latency

[jobhunts02 () aol com]

The reason I want this information is that I am streaming video over a communications link and am getting poor quality 
video.  I used the program TPCAT and found that no packets are being lost.  I assume that the poor video must be due to 
latency.  I'd like to look at the total latency from source to destination and see if it fluctuates.  If it does, I 
will then see if there is a backlog in Linux on either end or in the link.  TPCAT is supposed to be able to measure 
latency but it stops responding when there are large numbers of packets like in a video stream.  The author told me 
this is a known issue.  I was hoping there might be an alternative tool to measure latency similar to TPCAT that 
compares pcap files for latency.


On Jan 31, 2011, at 4:44 PM, Guy Harris <guy () alum mit edu> wrote:

> On Jan 31, 2011, at 3:20 PM, Martin Visser wrote:
>
> > As Guy has said, you can't use the absolute times unless you can
> > ensure the machines at each end are millisecond synchronized.
>
> ...and that the time stamps given to transmitted and received packets correspond to the times you're interested in.  
> If you care *only* about transit time on the network - e.g., the time from the point at which the first bit of the 
> packet is put onto the first network hop and the time at which the last bit of the packet is received from the last 
> network hop - then those time stamps almost certainly do *not* correspond to the times you're interested in.  If you 
> care about latency *including network stack latency*, it might be better, although the time stamp applied to outgoing 
> packets is probably later than the time at which the outgoing packet was initially handed to the network stack, and 
> the time stamp applied to incoming packets is probably earlier than the time at which the incoming packet was handed 
> up from the network stack.
>
> =
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