Subject: [ippm] Re: wire-arrival-time definition (2330 etc)

From: Stephen Donnelly <[email-masked]>
Date: Thu, 28 Jun 2001 10:39:59 +1200

Vern Paxson wrote:
 >> Should the wire arival time erfer to the time of the first link 
layer bit, or the first IP bit?

> Good question.  My inclination would be to consider the wire arrival time
> as reflecting first-link-layer-bit-seen-on-the-wire.  But I don't think
> that's particularly fundamental, given things like preambles.  It would
> be good to nail down, and perhaps worth a thread on the IPPM list.
> 
> 		Vern

Well, my attempt to get a discussion startde on IPPM a month or so back 
came to naught, I tend to agree though.

Although IPPM is an IP metric, it makes most sense to be define OWD as 
being from the first physical layer bit to last physical layer bit. This 
means that the wire-arrival-time at the OWD source network is the same 
as the time at which the host starts to transmit the packet, rather than 
being some link-type dependent delay after that time. That ought to make 
the timeing measurements simpler.

Likewise for the wire exit time at the OWD receiver, you want to define 
it to be the time at which the physical layer packet has ceased 
arriving, not at the earlier time at which the IP part of the packet has 
finished arriving. Since a 'typical' host based packet time-stamping 
system will *at best* generate a time-stamp some tme after the entire 
physical frame has arrived, this time-stamp bes approximates the end of 
the physical packet, not the end of the IP packet. (Even then it's a 
poor time-stamp, but with hardware based systems we can easily 
distinguish between the two, and it's important that it be well defined 
which to use).

Joerg Micheel mentioned that since OWD is specified from first bit sent 
to last bit received, the OWD will also depend on the length of the 
packet, however I assume this is implicitly assumed in the definition of 
the 'Type-P' packet used.

Stephen.
-- 
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     Stephen Donnelly (BCMS)             email: [email-masked]
     WAND Group                      Room GG.15 phone +64 7 838 4086
     Computer Science Department, University of Waikato, New Zealand
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