Re: [6lo] draft-ietf-6lo-fragment-recovery maximum fragment size too small

["Pascal Thubert (pthubert)" <[e-mail]>]

Hello Michel

Your proposal is feasible; the datagram space is probably overly large anyway. But it seems to be tailored around your exact problem as opposed to generic.
There are alternates like use a different unit, e.g., 4 bytes words as the existing text suggests. Maybe we could make the unit dependent on the MTU, like 1 byte under 128, 2 bytes till 256, and 4 bytes above that?
It seems that you are not concerned with the overall IP packet size, which is constrained to 2K by the fragment_offset. Maybe we should also look at that?

All the best

Pascal


From: 6lo <[e-mail]> On Behalf Of Michel Veillette
Sent: mardi 8 janvier 2019 18:38
To: [e-mail]
Subject: [6lo] draft-ietf-6lo-fragment-recovery maximum fragment size too small

Hi authors of "draft-ietf-6lo-fragment-recovery"

I'm currently working on a fragment forwarding implementation based on "draft-ietf-6lo-fragment-recovery".
In some cases, relatively large fragments need to be supported (i.e. minimum MTU transferred using 3 fragments).
However, the current format of the RFRAG Dispatch type limits fragment size to only 128 bytes.

Is it possible to modify the RFRAG Dispatch type (https://tools.ietf.org/html/draft-ietf-6lo-fragment-recovery-00#section-5.1)
and corresponding RFRAG Acknowledgment Dispatch type (https://tools.ietf.org/html/draft-ietf-6lo-fragment-recovery-00#section-5.2)
to allow larger fragments?

For example:

                           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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |1 1 1 0 1 0 0|  fragment_size  |X|E|     datagram_tag          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |sequence |  fragment_offset    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           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
                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      |1 1 1 0 1 0 1 Y| R |     datagram_tag          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          RFRAG Acknowledgment Bitmap (32 bits)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



R = reserved


Regards,
Michel Veillette