[e2e] TCP Local Area Normal behaviour? any references?

[Jonathan Stone]

In message <20050121193447.910DA21A at aland.bbn.com>,
Craig Partridge writes:

>In message <Pine.LNX.4.58.0501211335290.11281 at tesla.psc.edu>, Matt Mathis write
>s:
>
>>But the more pragmatic solution (adopted here at PSC and may other places) is
>>to declare half duplex Ethernet to be broken, and eradicate it wherever
>>possible.  Where not possible, tell people that the maximum theoretical
>>utilization is 1/e (35%), and they should be pleased if they get any better
>>than that, because they are operating beyond the designed operating point for
>>the media.
>
>That 1/e is not consistent with Boggs & Mogul's work from SIGCOMM 1988.
>Van Jacobson also reported results inconsistent with 1/e.  Indeed, I'd
>thought 1/e had generally been discredited as a mistaken result from
>inaccurate models.

If (very) dim memory serves, 1/e is valid for slotted Aloha.

But Ethernet -- even half-duplex Ethernet -- is not Aloha.  Indeed, I
beleive different Ethernet chips in fast enough workstations (33 MHz
r3000a or thereabouts) would repeatably give different saturation
throughputs for ttcp on a two-host half-duplex Ethernet, due to small
differences in collision-detect and BEB hardware implementation
yielding slightly more idle time on the wire, or something like that.

I think Lance versus SEEQ is the pair I once noted in my lab book.  I
can ask some more determined practitioners of the time, if you care
for more details.



>Boggs & Mogul used multiple TCP's.  Van used a single one.
>
>So what did they do such that the capture effect didn't happen?  

I seem to recall they used comparatively slow CPUs (DECWRL Titan) and
an Ethernet chip that required a software intervention after each
packet send. According to the tech report cited below, the driver
interaction took about 100 usec, or about 2 10Mbit contention-slot
times. (I have not checked the arithmetic.)


>And does capture really yield 1/e or something different?

See ``A New Binary Logarithmic Arbitration Method for Ethernet'', Mart
L. Molle, Tech report CSRI-298, Computer Science Research Insitute,
University of Toronto, 1994.  Molle examined the results of Boggs and
Mogul, and proposed a better, fairer backoff algorithm, BLAM that (in
Molle's words) gave a logarithmic rather than linear estimator of the
offered workload, yielding a less non-work-preserving discipline than
binary exponential backoff.  BLAM never went anywhere, since
half-duplex shared Ethernet became obsolescent about that time.