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<font size="-1"><font face="Helvetica, Arial, sans-serif">Three forms
of "best efforts":<br>
<br>
1. In current QoS terminology, "Best Efforts" is the default QoS level,
between expedited delivery and background. See the 802.1d VLAN tag's
QoS field, the 802.11e EDCF's QoS specs, or the DiffServ specs. 802.1d
has 8 levels of QoS, 802.11e has 4, DiffServ has 63, the original IP
header had 6 or 8.<br>
<br>
2. CSMA/CD Ethernet's MAC provided a "Best Efforts" frame delivery
service, where BE meant high reliability but no end-to-end error
checking. The specs for the CSMA/CD PHY called for an rate of
undetected errors on the order of 10^-8, and collisions were detectable
by the transmitter (as a voltage drop on the cable.) Transmitters
retried 16 times after collisions. Therefore, the system was
essentially as reliable as an end-to-end ACK/NAK protocol. Given that,
"best efforts" in CSMA/CD is very, very close to "absolute reliability."<br>
<br>
3. The third definition is the loosey-goosey, pseudo-philosophical grad
student musing about the role of dependent functions in a modular
system consisting of lots of parts with varying degrees of reliability.<br>
<br>
RB</font></font><br>
<br>
On 6/3/2010 8:48 AM, Matthias Bärwolff wrote:
<blockquote cite="mid:4C07CEE5.5080201@cs.tu-berlin.de" type="cite">
<pre wrap="">
On 06/03/2010 05:10 PM, Dave Crocker wrote:
</pre>
<blockquote type="cite">
<pre wrap="">
</pre>
<blockquote type="cite">
<blockquote type="cite">
<pre wrap="">The complexity of the Arpanet design and layering might permit a bit of
debate about whether it qualified as being based on best effort.
Alohanet's simplicity does not (permit debate.)
</pre>
</blockquote>
<pre wrap="">
Why not? Alohanet (pure Aloha) capacity was at 1/(2e), and downlink data
was not even acknowledged (for performance reasons). Ethernet capacity
has been at some 98 percent right away, and packets hardly ever got lost
(safe in screwed up installations). Best effort certainly doesn's mean
no effort (as in Alohanet), but has probably always meant "reasonable",
"sane" effort. (But that's just my two cents, Richard was gonna
enlighten us us to the three meanings of best effort.)
</pre>
</blockquote>
<pre wrap="">
Alohanet was not no effort. It had retransmission.
But it merely kept things extremely simple. That its version of 'best'
was relatively poor and that the Ethernet's version was a lot better
does not make either less an example of 'best'.
</pre>
</blockquote>
<pre wrap="">
Just an aside correction: Alohanet only had acknowledgments and
retransmission on the uplink. The broadcast channel (from the central
hub to all stations) had no retransmission as it was considered
reasonably reliable (there could be no collisions, after all). Another
fun fact about the Alohanet: a station would have retransmitted only 2
times, after which it would give up and leave retransmissions to the
user -- the rationale being that this way the third retransmission
interval would be larger and more random so as to avoid yet another
collision. It rarely happened given the fairly low volume use of the
network, but it was in the specs (probably written after the fact, but
anyway).
Matthias
</pre>
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<pre wrap="">d/
</pre>
</blockquote>
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</pre>
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<br>
<pre class="moz-signature" cols="72">--
Richard Bennett
Research Fellow
Information Technology and Innovation Foundation
Washington, DC</pre>
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