Subject: [Fwd: comments on draft-pilc-link-arq-issues-00 : MAC issues]

From: Dr G Fairhurst <[email-masked]>
Date: Tue, 12 Dec 2000 19:31:25 +0000

I enclose a mail thread originating from Gary Kenward 
which comments on the link-ARQ draft
(Gary didn’t originally copy the list in on this, 
but has subsequently agreed to me sending it).

The emails discus issues relating to the way in which persistency is
used in
the MAC world. At first glance, this may appear an orthogonal issue to
link 
ARQ, but there is some interesting overlap of issues and jargon...

==================================================================

Gary Kenward wrote:

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 

==================================================================

Gorry Fairhurst wrote:

It is interesting that you raise the issue of persistence within 
MAC protocols as well as within link ARQ protocols.  I guess this 
is a much more common usage of the word - but I'm not quite sure the 
two uses are exactly the same.  In ARQ it describes the giving up of 
reliability (as a trade off with maximum delay) - at least that 
is how I believe we meant to use it in the document. 

In MAC protocols, the giving up may be for the same reason - 
because the end-point address simply doesn't exist (or may not 
be reached) - but also with back-off to control congestion 
within the MAC domain, as you describe. 

If you think there are common issues, I'd be interested in 
further email - or if you'd like to suggest a "tuned" definition 
which covers both (or a note to clarify things?) 

Gorry 
     
==================================================================

I understand you perspective better now. I do not want to suggest that
MAC layer 
design issues be included into the document. It was my intent only to
identify a need 
to clarify the language, for the reasons I outlined in my email. 

The first issue was whether, at the Link layer, the definition of
persistency is sufficient. 
In re-reading the second paragraph of section 2, I realise that you have
equated the 
retransmission timeout to be the expected round trip delay (packet
transmission to 
ack/nack receipt delay). If this is the case, then the "persistence" as
you describe it 
is given by either the maximum number of attempts allowed, or the total
duration of 
all attempts. 

However, in many packet radio systems the delay between attempts is
greater then 
the round trip delay, and frequently increasing in duration. This is
done for reasons 
similar to having increasing backoffs in CSMA - to decrease the offered
channel load 
in the presence of impairments to transmission that may persist for an
extended 
duration. In CSMA the channel impairment is congestion that results in a
high probability 
of collisions. 

In a wireless channel, for example, a mobile may be experiencing poor
coverage due to 
fading, shadowing or interference: in this situation, it is wasteful to
consume bandwidth 
by re-transmitting to the wireless terminal. One simple solution is to
delay retransmissions 
in the hope that the channel conditions will improve. 

Thus, the "persistence" of the ARQ is characterized by the delay between
attempts, 
as well as the number of attempts. 

If you agree to this generalization of the definition, then I will be
happy to write up 
some text. 

Also, in paragraph 3, section 2, the reference to Ethernet introduces
some confusion (for me, 
at least, this is where the confusion with persistent media access
protocols was introduced). 
Clearly, Ethernet is both a MAC and Link protocol, but it is my
understanding that the 
retransmission algorithm of Ethernet was defined to compensate for
collisions on a 
CSMA channel. In the theoretical definition of CSMA, busy periods and
channel collisions 
are treated as identical events, and both cause a retransmission to be
scheduled; both are 
non-persistent. 

Regards, 
Gary 

==================================================================

Gary  Kenward  wrote:                      

I *think* the issue can be kept general. We need an English language, IP
centric 
description of an algorithm that makes repeated attempts to transfer a
copy of a 
given IP packet while keep the actual link layer mechanisms abstract. 

The number of attempts and the delay between attempts I believe can be
covered by 
expanding the description of "persistence" in your i-d. However, this
expanded definition 
would have to be propagated into the subsequent sections where you
describe the 
various trade offs. 

The use of p-persistence, adaptive FEC (e.g. hybrid ARQ), power control,
etc. etc. at the 
MAC layer might be abstracted as the "tenacity" of each transfer attempt
(or maybe 
I go to far with the anthropomorphism). To be more specific then this
would probably 
go to far into link layer technology, and it would be a huge effort to
attempt to describe 
the trade-offs. So perhaps an abstract definition with a few examples
will have to suffice. 

Are there any more attributes of ARQ protocols that might be described?
Nothing 
comes to mind at the moment. 

Gary 
======== 
Gary W. Kenward                                  
[email-masked] 
Advisor, Wireless Architecture               (613) 765-1437 
Advanced Technology Labs                   
Wireless Solutions                                  FAX: (613) 763-2686 
Nortel Networks