Subject: [PSAMP] PSAMP Protocol: Selection Sequence Statistics

From: Gerhard Muenz <[email-masked]>
Date: Wed, 20 Aug 2008 13:00:48 +0200

Dear all,

The PSAMP protocol draft specifies:

6.5.3 Selection Sequence Statistics Report Interpretation

   A Selector MAY be used in multiple Selection Sequences.  However,
   each use of a Selector must be independent, so each separate logical
   instance of a Selector MUST maintain its own individual Selection
   State and statistics.

The motivation is that the number of observed and selected packets is to
be counted for each Selector (or selection step) along a Selection
Sequence, right?

If I interprete the above paragraph correctly, it forbids the deployment
of the same Selector instance in two different Selection Sequences.
However, this would be over-restrictive.

Consider the following setup of two Selection Sequences consisting of
three Selector instances Sampler 1, Filter 1 and Filter 2:

              +-> Filter 1
  Sampler 1 --+
              +-> Filter 2

  Selection Sequence 1: Sampler 1, Filter 1
  Selection Sequence 2: Sampler 1, Filter 2

Although this setup is not allowed according to the PSAMP protocol
specification, it still allows gathering the desired statistics. Yet,
PSAMP forces me to deploy one more Selector instance:

  Sampler 1 --> Filter 1

  Sampler 2 --> Filter 2

This is a waste of resources. Also, the result is not necessarily the
same if the packet selection in Sampler 1 and Sampler 2 is independent.

In order to gather the desired statistics for each Selector instance
along a Selection Sequence, it is sufficient to fulfill the following:

   A single instance of a Selector MAY be used in multiple Selection
   Sequences if it sees the same packets in all Selection Sequences and
   if the packets are either observed by a single Observation Point or
   selected by a single Selector instance. In any other case, separate
   instances of the Selector MUST be used in order to maintain separate
   individual Selection State and statistics.

Note that this specification still disallows setups like:

  Sampler 1 --+
              +-> Filter 1
  Sampler 2 --+

In this case, the statistics gathered in Filter 1 are based on
aggregated packet streams of two Selection Sequences, so the following
sentence from the PSAMP protocol is not fulfilled:

   Within a Selection Sequence composed of several Primitive Selectors,
   the number of packets selected for one Selector is equal to the
   number of packets seen by the next Selector.

Regards,
Gerhard

-- 
Dipl.-Ing. Gerhard Münz
Computer Networks and Internet
Wilhelm Schickard Institute for Computer Science
University of Tuebingen
Sand 13 (Room B309), D-72076 Tuebingen, Germany
Phone:  +49 7071 29-70534       Fax: +49 7071 29-5220
E-mail: [email-masked]
WWW:    http://net.informatik.uni-tuebingen.de/~muenz

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