Subject: comments on draft-pilc-link-arq-issues-00

From: "Gary Kenward" <[email-masked]>
Date: Wed, 29 Nov 2000 13:30:28 -0500

I have two comments on the draft draft-pilc-link-arq-issues-00:

 1. Under Section 2, ARQ Persistence, the second paragraph attempts
     to define "retransmission persistence".  There are many dimensions to
     persistence at the link layer, and I feel the current description is
     not clear enough. From an ARQ perspective, there is not only the
     duration of repeated attempts, but also the frequency and priority 
     of the attempts.

     The reason I was initially confused by the text is that I am more
     familiar with the definition of persistence as used with stochastic
     contention resolution protocols such as CSMA; e.g. p-persistent
     CSMA refers to the probability that a retransmission will be 
     attempted within a certain time period.

     Perhaps this is too much detail for a draft of this scope, however,
     given that the concept of persistence is used throughout the draft,
     it should be defined more crisply, and, the definition should 
     be complete.

  2. In the subsections 2.n, you characterize different types of persistence
      as a trade off between delay and reliability. Again, frequency
      and priority of repeats must be accounted for in your taxonomy.

      However, I also wanted to point out that the reason many protocols,
      including Ethernet, use a finite number of repeated transfer attempts
      (or retransmissions in Ethernet) is to ensure *stability* of the
channel.
      An ARQ protocol with an infinite number of repeated attempts is
      generally unstable.

      A finite number of retransmissions does not necessarily ensure
stability.
      I do not have the references at hand (I could probably find them if
you
      are interested), but for a finite population size, stability bounds on
the 
      number of repeats have been derived for particular protocols (e.g.
CSMA,
      ALOHA, S-CSMA, S-ALOHA).

I hope these comments help. If I have missed something, I would happy
to know it.

Gary Kenward
     
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Gary W. Kenward
[email-masked]                      

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As far as the laws of mathematics refer to reality, they are not certain;
and as far as they are certain, they do not refer to reality.
-- Albert Einstein