[e2e] Fwd: Re: Why do we need congestion control?
detlef.bosau at web.de
Tue Apr 2 16:11:52 PDT 2013
Dave Reed sent me the following comments via PM, I think it may be
helpful for the discussion, particularly his remark on the asymptotic
behaviour, so I forward this to the list (with explicit permission of DPR).
-------- Original-Nachricht --------
Betreff: Re: [e2e] Why do we need congestion control?
Datum: Tue, 2 Apr 2013 18:59:46 -0400 (EDT)
Von: dpreed at reed.com
An: Detlef Bosau <detlef.bosau at web.de>
Kopie (CC): end2end-interest at postel.org
"Erasure codes" is not a proper term for this. Which creates lots of
confusion on the list, and that will get worse. Erasure codes correct
erasures, but that is not the key attribute of fountain codes. You need
The correct technical term is "rateless erasure codes". That is, codes
that correct erasures but are not dependent on a particular "error
rate". A "rateless erasure code" has the property that the goodput over
the long term on *any* erasure channel is asymptotically equal to the
total number of symbols actually delivered (or symbols sent minus
symbols erased). An erasure channel is an information theoretic
"channel" where the loss process causes erasure of zero or more symbols
in the stream, but all symbols actually delivered to the destination are
Fountain codes are one example of a rateless erasure code. If you look
up "rateless erasure code" on Google, you can learn all about them.
Feel free to forward this clarification to e2e, since I cannot post there.
PS: rateless erasure codes do not obviate the need for congestion
signalling within a network.
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