Subject: [Tsvwg] Draft-floyd-ecn-alternates-00.txt

From: Gorry Fairhurst <[email-masked]>
Date: Sun, 15 May 2005 22:32:42 +0100

Sally,

So I read the above document, and it initially talks about whether
alternative semantics can harm existing conforming sessions, that seems like
the right place to start.

Remarking 11 to another value is not allowed. This seems right.

This leaves some options for remarking to indicate congestion:

1) Remarking to '11'.
That is: 01 -> 11 or 10 -> 11
This is the currently expected norm for ECN.
Remarking 00 to 11 is odd, but seems safe.

2) Alternatively, a different behaviour may arise if a router remarked:
01 -> 10 or 10 -> 01
This seems a potential candidate for another behaviour, possibly to signal
something (less than normal congestion, or at least different to 11) to an
end host. This remarking doesn't ensure a a full congestion response as in
11. If however "ECN-NONCE" were used at the end-hosts, this remarking would
be interpreted as congestion signalling - safe for the network, but perhaps
an expected (more severe) outcome.

You don't say it, but I wonder if it should also be said it that it is
illegal to remark *from* 00?
- Remarking 00  -> 01 (or 00 -> 10) allows a non-ECN capable packet to
pretend to be ECN-capable, and thereby possibly by-pass early discard, by
suggesting to the router that remarking will likely have an effect (be
honoured, even though the end host didn't specify ECN support).

Best wishes,

Gorry

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