Subject: [ippm] rfc2330 'wire-time' discussion

From: Stephen Donnelly <[email-masked]>
Date: Thu, 24 May 2001 14:22:09 +1200

Hi,

I am doing some detailed investigation of active and passive measurement
as part of my thesis work, including OWD. I have been working with both
RIPE and Advanced in performing passive measurements of their rfc2679
Type-P-One-way-Delay measurement systems.

In the discussion of these terms, it became clear that the IPPM
definition of the probe packet transmission and reception times needs
clarification. It was suggested that a discussion thread on the IPPM
mailing list may be useful.

rfc2330 states in section 10.2:

   In order to provide a general way of talking about these effects, we
   introduce two notions of "wire time".  These notions are only defined
   in terms of an Internet host H observing an Internet link L at a
   particular location:

 +    For a given packet P, the 'wire arrival time' of P at H on L is
      the first time T at which any bit of P has appeared at H's
      observational position on L.

 +    For a given packet P, the 'wire exit time' of P at H on L is the
      first time T at which all the bits of P have appeared at H's
      observational position on L.

And rfc2679 states in section 3.4:

   For a real number dT, >>the *Type-P-One-way-Delay* from Src to Dst at
   T is dT<< means that Src sent the first bit of a Type-P packet to Dst
   at wire-time* T and that Dst received the last bit of that packet at
   wire-time T+dT.

It is not clear however if the packet P is taken to be the IP packet
itself, a layer 2 packet such as an Ethernet MAC frame, or a layer 1
packet including synchronisation signals such as Ethernet preamble.

Being an IP metric, I assume that P refers to the IP packet, do you
agree? Is this the correct definition for the wire-exit-time and
wire-arrival-time?

This means that when timestamping packets for measurement purposes, care
must be taken to correct for the offset within the physical packet
between the start of the IP packet and the point at which the timestamp
is generated. In order to do this, the data link rate of the interface
must also be known.

Although these corrections may potentially be small compared to other
delay components, there are now a number of passive measurement systems
capable of resolving packet timestamps to byte or bit time resolution,
and these distinctions can be made.

Stephen.
-- 
-----------------------------------------------------------------------
    Stephen Donnelly (BCMS)             email: [email-masked]
    WAND Group                      Room GG.15 phone +64 7 838 4086
    Computer Science Department, University of Waikato, New Zealand
-----------------------------------------------------------------------
_______________________________________________
ippm mailing list
[email-masked]
http://mailhost.advanced.org/mailman/listinfo/ippm