[e2e] ECN vs. source quench

[Black_David at emc.com]

> -> Sending the congestion signal directly back to the source 
> -> might seem like a 
> -> possible option for the IP layer to implement (source quench 
> -> was the name), 
> -> but in fact it is quite incompatible with many high-level 
> -> protocols.   
> 
>   I got exactly the opposite supportive argument when I read 
>   RFC 2884 (http://www.ietf.org/rfc/rfc2884.txt). This RFC 
>   clearly states that the explicit congestion signal is efficient 
>   is most of the situations.

This is confused.  The ECN congestion signal does not go directly
back to the source; it goes to the destination and is reflected back
to the source, piggybacking on TCP traffic.  In contrast, source
quench goes directly to the source.  This difference is important
for several reasons, including:
	- ECN takes advantage of the destination being able to
		reach the source (source quench may not, as ICMP
		is not end-to-end).
	- ECN takes advantage of the TCP destination being able to
		retain state to retransmit the ECN signal (asking a
		router to retain state for source quench retransmission
		is problematic for a number of reasons).
	- ECN introduces no new traffic (a conservative "do no harm"
		property, as adding traffic to deal with too much traffic
		has the potential to make things worse).

>   Coming back to compatibility and portability of network layer 
>   congestion notification to higher level protocols, ECN for example, 
>   is in fact implemented for TCP, UDP applications. Recent
>   transport protocols, like DCCP and SCTP also adapted the ECN 
>   usage.

ECN for UDP -  really??  How is it reflected back to the source
and what does the source do with it?

Thanks,
--David
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David L. Black, Senior Technologist
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