[tcpm] Question on draft-gomez-tcpm-ack-rate-request

["Scharf, Michael" <[e-mail]>]

Hi Carles,

I wonder if it is possible to save one byte of space in the option proposed in draft-gomez-tcpm-ack-rate-request-02.

As far as I understand the proposal, the "N" field is only used if the "R" field is 0. So, no matter whether R is set to 0 or >0, almost one byte of the TCP option just consists of zero bits. If that was true, it might be inefficient to transfer a large number of zero bits in the TCP header. TCP option space is rather scarce and should not be wasted.

Wouldn't be one byte less be sufficient?

Example:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Kind      |     Length    |              ExID             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |I|X|Value|
     +-+-+-+-+-+-+

The bit "X"=0 would be equivalent to "R"=0, i.e., immediately request ACKs. In that case, the 6 bit field "Value" could encode the value of "N" described in draft-gomez-tcpm-ack-rate-request-02.

The bit "X"=1 would enable delayed ACKs ("R">0) and the 6 bit field "Value" would encode the number that "R" defined in draft-gomez-tcpm-ack-rate-request-02.

The "I" bit would be defined as in draft-gomez-tcpm-ack-rate-request-02. (It may make sense to change the order and or naming of the fields, or other details of the encoding - this is just an example.)

As far as I can see, there *is* a difference between this encoding and the one in draft-gomez-tcpm-ack-rate-request-02: There is one bit less to encode the value of "R", and two bits less to encode the value of "N". But would that matter? Do we really expect values larger than 63 (or 63) for "R" and "N"?

(And, if significantly large values were needed for "R" and "N", one could use another encoding instead of a raw binary number, such as encoding an exponent - then 6 bit would be a lot).

Or do I just miss something basic and need more coffee?

Thanks

Michael