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Re: Terminology harmonization: a proposal
thanks for opening up the discussion on terminology.
Maurizio Molina wrote:
> 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).
I think it will be too limiting to say that type (a) sampling must
be blind to packet content. Implementations may want to use content
from the packet stream as a cheap way to insert or seed randomness into
sampling decisions. (In a different arena, this approach has been
proposed for importance sampling of flow statistics).
> 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.
I agree that at a high level one can regard hash-based selection as a
of filtering, since it relies on a complex but deterministic function of
the packet contents. Perhaps it is worth noting that any good hash-based
selection function would be infeasible to express as a composition of
match/mask filters; I've been asked this.
But I'm concerned that the (a) /(b) division may close the door to
selectors that people may invent, or implementations (e.g. of sampling)
that are really a hybrid of the two approaches.
And why stop there? At an abstract level one could regard all the
selectors as (i) calculating some quantity depending on the packet
content and/or other variables; then (ii) selecting packet if the
quantity falls in a given range. A danger with trying to couch the
framework this way is of losing focus of the basic functionality
that we want (e.g. sample 1 in N packets, somehow) and dwelling on
implementations (e.g. different ways of getting 1 in N sampling in this
framework, e.g. decrement a counter, or calculate a hash, or use a
well-known random number generator, or seed a counter with the packet
> 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."
Yes, I have been wondering whether this should be reworked after Peram
brought up this point a little while ago.
The reason for excluding router state from the primitive operations
was arhictectural: we didn't want to assume that the routing state would
be available to a filter that could be required to operate at line rate.
(Any comments on this from implementors?)
But we do assume that routing state, if present in the measuring network
element, will be available to form the packet reports, so it should be
feasible to do filtering based on routing state when reports
are formed i.e. after all the other selection primitives have operated.
> Maurizio Molina
> Research Staff member
> Network Laboratories Heidelberg
> NEC Europe Ltd.
> Adenauerplatz 6, D-69115 Heidelberg, Germany.
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