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Re: sysctl should disable ECN by default

> RFC793 says 
> Reserved:  6 bits
>     Reserved for future use.  Must be zero.
> The last statement is the cause of all confusions. s/Must/Should/ would
> have been better.

No; to be forward compatible, a TCP must set the bits
to zero.  2481 describes the operation of those bits and
augments 793, much like SACK augments it.  Some TCP's
have a bug where they set the bits in the SYN/ACK if
they received reserved bits in the SYN, which is why ECN
negotiation is asymmetric.

If you want to blame routers for not implementing
standards, try 791.

  The internet protocol is specifically limited in scope
  to provide the functions necessary to deliver a package
  of bits (an internet datagram) from a source to a
  destination over an interconnected system of networks.


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