Re: [Roll] capability handling

[Rahul Jadhav <[e-mail]>]

Please find comments inline.

________________________________
From: Roll <[e-mail]> on behalf of Michael Richardson <[e-mail]>
Sent: 09 May 2020 02:05 AM
To: Routing Over Low power and Lossy networks <[e-mail]>
Subject: Re: [Roll] capability handling


Rahul Jadhav <[e-mail]> wrote:
    > There was ​another point we discussed during the interim in context to
    > Capability Indicators: How to arrange the Indicator bits?  a. Using
    > left to right b. Or using right to left (currently in the draft [1])

    > If we use right to left then we can give every bit a number, for e.g.,
    > currently the 'T' bit is 0x1. The length can be set accordingly i.e. if
    > the indicators are less than 8 bits then the length will be one.  This
    > is simpler to handle in implementation as well. Thoughts?

(1) If we number from right to left, can we expand the field to be as many octets
    are we like?  So len=3 is just an example?  Don't transmit zero bytes.

[RJ] Yeah len=3 is just an example.

(2) If we number from left to right, then we could number them "bit 0" through
    "bit 7" of byte 0 through 255.

The IANA considerations for this might be a bit weird either way.
I guess there is a maximum number of octets that we can get into the field
anyway, so we declare it to be a field with that many bits and ask IANA to
allocate contiguously.

The effect is the same: we don't transmit bytes which are zeros, it's just
whether we consider those zeros on the left or right.

I think that (2) might easier to code because the offset of each flag will
not move as the structure gets longer, and it doesn't confuse people into
thinking they need 64-bit ints, which then fail on capability #65. (actually #64)

[RJ] This point makes sense. If the indicators go more than (or eq to) 64 then the operations are not so straightforward to handle. With left to right, the complexity is constant regardless of the number of bits.