maximum Ethernet throughput


Jeffrey Mogul (mogul@decwrl.dec.com)
1 Mar 1988 1559-PST (Tuesday)


Someone recently asked:
    If anyone has figures on maximum through-put of 10 Mbps Ethernet, I could
    make good use of them.

The snide answer is "10 Mbit/sec". Actually, the theoretical maximum is
about 9.9 Mbit/sec, because of the inter-packet gap, and perhaps closer
to 9.8 or 9.7 Mbit/sec if you count the addresses and CRC as wasted bits.

That's probably not quite what you wanted to know, but the question as
put does not have a single answer. For example, do you want the
aggregate maximum or the maximum for a single pair of hosts?

For the latter, I believe Bill Nowicki of Sun has obtained 5 Mbit/sec
using TCP between Suns, and the Sprite people at Berkeley have
reached somewhere near 5.6 Mbit/sec using their kernel-to-kernel
RPC. These numbers are both from memory; perhaps someone can confirm
or improve them.

The best numbers that I am aware of, for communications between a pair
of hosts, comes from Dave Boggs. Using a pair of 15 MIPs processors
with an interface and software of his own design, and without much
of an operating system or any protocol (aside from the Ethernet data
link headers), he can get about 7.5 Mbits/sec obeying the Ethernet
spec, or at least 8.6 Mbits/sec if he "cheats" by sending 4Kbyte
packets. (He once got >9 Mbits/sec with even larger packets, but
his current code doesn't support that.)

The limiting factor in his measurements seems to be that his interface
can only send one packet at a time; i.e., he must process one interrupt
per transmitted packet, which takes a few hundred microseconds. The
interface can receive up to 16 packets using only one interrupt. With
a more elaborate interface design, the theoretical limit should be
attainable.



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