[Bloat] enabling outgoing tcp-ecn by default for ipv6 links

[Hannes Frederic Sowa]

Hello!

I am thinking about advancing the ipv4/tcp_ecn knob in linux to
distinguish between ipv4 and ipv6 transport and enable ecn signaling
on outgoing ipv6 connections by default (for me, at least). I am
currently doing so with the help of iptables (echo 1 > tcp_ecn and -j
ECN --ecn-tcp-remove).

Currently linux does only respond to ecn signaling but does not actively
try to signal ecn capabilities by default because of the high(?) fail rate
of connections in ipv4 land. I wonder if this situation has improved in
ipv6 world because I assume most providers have upgraded their gear in
recent times to support this shiny new protocol.

Does someone know about recent data if such a change would be reasonable?

Btw, has someone looked at RFC5562 - Adding Explicit Congestion
Notification (ECN) Capability to TCP's SYN/ACK Packets;
experimental? Adam Langley has provided a patch for linux enabling ecn on
syn/ack packets some while ago[1].  This seems to be a nice addition but
I fear for new problems arising because of buggy routers. Perhaps after the
retransmit of the initial syn without the ecn-capable codepoint one could
mark this connection as non-ecn compatible in the destination cache to
reduce further latencies on follow-up syns (kind of path ecn discovery).

Greetings,

  Hannes

[1] https://www.ietf.org/mail-archive/web/tcpm/current/msg03988.html