[e2e] Header length field in IP header

[Joe Touch]

Mark Allman wrote:

>>The header length HL of IP header has 4 bits.  Normally, the header
>>length HL would be a number from 5 (no options) to
>>15. Since the IP header is at least 5, I wonder if the field HL can take
>>the values 0, 1, 2, 3, or 4 to mean something else. If it takes such values
>>(0-4), what is the meaning?
> 
> 
> BTW, Eddie Kohler has suggested doing just this for TCP to allow for
> more option space.  Basically, a TCP offset of 0=48 bytes of TCP
> options, 1=64 bytes, 2=128 bytes, 3=256 bytes and 4=there is no payload.

IP specificially prohibits values under 5. TCP has no such prohibition, 
though it doesn't address it either way. That which is not prohibited 
MUST be allowed, IMO. Your experiments below validate that approach, 
happily ;-)

However, I still believe (and will post to TCPM separately) that Eddy's 
approach which uses an option to override vanilla TCP data length field 
is a better way to go; it corresponds to window scaling, and how IP 
jumbograms are handled. I don't believe there is a measurable difference 
to how uncompliant middleboxes would handle either version - both could 
fail where not supported.

Joe

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