[e2e] on local ethernet throughput?

[Vernon Schryver]

> From: Dan HE <he at enseirb.fr>

> ...
> Is the >90% throughput maitained for every nodes communicating at the
> same time or not?
> Vernon's email hints that if there are n machines sharing one common
> segment,
> the bandwidth of each node can be fairly allocated up to 1/n of the
> total bandwidth.
> because of "colliding". So does colliding result in triggering CSMA/CD
> restransmit?
> I remember that one email said "NO". It concludes that packet loss is
> possibly caused
> by ethernet collision, right?

Ethernet collisions are no more than the mechanism by which CSMA/CD
distributes the available bandwidth of the shared wire among requesting
stations.  The best you can hope for a wire shared among N machines
is that each machine will get 1/N of it.  (Priority schemes such as
in FDDI have all failed, perhaps because unlike WAN QoS, it is cheaper,
more reliable, and easier to use separate wires for separate classes
of service.)  For decades people have heard the word "collision," not
bothered to understand the protocol definition, and assumed that a
"collision" is something bad.  It is not bad but merely a "bus
arbitration event."  At least one of those who chose the word "collision,"
Rich Seifert, has often agreed in public that the choice of the word
"collision" was a mistake because of what lazy people assume.

As I recall, IEEE-802.3 says that stations "retransmit" after a
"collision," but that is another unfortunate word choice that allows
people to falsely assume something bad.  In fact after a "collision"
only the first "slot time" worth of the packet is sent again, because
only that much has been transmitted when the "collision" stopped things.
A CSMA/CD collision rate of 100% (one collision for every successful
packet) for a typical TCP/IP/Ethernet stream (67% 1460 byte data segments
and 33% ACKs) spends only about 6.5% of the available bandwidth on
"collisions" and "retransmissions."

I don't think this mailing list is about Ethernet or local area networks.
Anyone who really cares about CSMA/CD should read the IEEE standard or
the preceding "blue book."  Rich Seifert's book "Gigabit Ethernet,"
ISBN 0-201-18553-9, might be more accessible, and since the author is
one the inventors of CSMA/CD, almost as authoritative as the IEEE
standard.  The Boggs et al paper that Mr. Cannara mentioned is readable.
relevant, contains useful references, and can be found via Google at
ftp://gatekeeper.dec.com/pub/DEC/WRL/research-reports/WRL-TR-88.4.pdf


Vernon Schryver    vjs at rhyolite.com