[113383] in North American Network Operators' Group
Re: Forward Erasure Correction (FEC) and network performance
daemon@ATHENA.MIT.EDU (Matthew Kaufman)
Fri Apr 10 13:46:23 2009
Date: Fri, 10 Apr 2009 10:43:26 -0700
From: Matthew Kaufman <matthew@eeph.com>
To: Marshall Eubanks <tme@multicasttech.com>
In-Reply-To: <B84DE4F1-B8AA-4D1A-A10E-78B3CB810FC2@multicasttech.com>
Cc: "nanog@nanog.org list" <nanog@nanog.org>
Reply-To: matthew@eeph.com
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
Marshall Eubanks wrote:
> If there is some consensus around this, it would effectively set an
> upper bound for the need for FEC in network transit.
The bit error rate of copper is better than 1 error in 10^9 bits. The
bit error rate of fiber is better than 1 error in 10^12 bits. So the
packet loss rate of the transport media is approximately zero.*
Thus any packet loss you see is congestion. If you see packet loss, you
should SLOW DOWN, not just keep sending and using more and more FEC to
get recoverable video out the far end. (And by "SLOW DOWN" I mean in a
way that is TCP-friendly, as anything less will starve out TCP flows
unfairly and anything more will itself find that it is starved out by TCP)
Matthew Kaufman
* The bit error rate of RF-based connections like Wi-Fi is higher *but*
because they need to transport TCP, and TCP interprets loss as
congestion, they implement link-level ARQ in order to emulate the
bit-error-rate performance of wire as best they can.
