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Terminology harmonization: a proposal

    After browsing through Nick's draft, the PSAMP mailing list, and
Tanja's draft proposal, I see there's still the need to harmonize some
terminology (and concepts) regarding packet selection,  sampling and

1) From Nick's draft:
3.2 Packet selection
..........Packet selection is performed through combination a number
of   measurement primitives described below.......
- Hashing
- Filtering
- Sampling

2) From a Nick's mail:
......With the words currently at my disposal, my usage is:
1. sampling = 1 in N (periodic or statistical) or hash-based
2. filtering = filtering
3. (primitive) selectors = either 1 or 2, and further methods TBD
4. (composite) selectors = composites of methods from 3

3) Tanja's draft: it focuses on samping, but it mentions some methods
(e.g. stratified sampling, or sampling dependent on the packet content)
that im my view are already a combination of filtering + sampling. This
point is clarified later.

My proposal for harmonizing terminology/concepts would be the following:

a) "primitives" for packet selection are only sampling and filtering.
Composite packet selection methodologies can then be built by a
combination of the two.
b) sampling is always "blind" to packet content. A packet is sampled out
of a stream only dependening on the packet position (which can be
spatial or temporal) and/or on the result of a sampling algorithm (which
can be deterministic or probabilistic).
c) filtering is on the contrary "blind" to the packet position in the
stream, but it is based on the packet properties. A packet property may
be simply its content, or the content of a set of subfields, or the
result of a function taking as an input (part of) the packet content.

Some notes:
n1) filtering is always deterministic.
n2) hashing is a sub-case of filtering.
n3) how "complex" a composite selector can be still needs to be
discussed, but by sure the methodologies Tanja mentioned (stratified
sampling, or sampling dependent on the packet content) can be
implemented by a cascaded filter->sampler.
n4) Another example of a filtering function could be taking the source
and/or destination address, lookup the source/destination AS and filter
on the basis of the result. While such a complex filtering function
doesn't  make sense at the line rate, it may make sense if a sampler is
placed in front of a filter to reduce the rate of packets to be
processed. In this respect, the text appearing in Nick's draft at the
bottom of 3.2 reported below (unavailability of router state to
measurement primitives) should be reconsidered.

   "In order to be able to function at line rates, each measurement
   primitive take as its input only a packet itself, or quantities
   that have been calculated from the packet previously by other
   measurement primitives. Router state is not assumed to be available
   to the measurement primitives."

Maurizio Molina
Research Staff member
Network Laboratories Heidelberg
NEC Europe Ltd.
Adenauerplatz 6, D-69115 Heidelberg, Germany.
Tel. (49)6221 90511-18 Fax: (49)6221 90511-55
e-mail: molina@ccrle.nec.de
Web: www.ccrle.nec.de

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