Subject: [6lowpan] post-IETF-last-call fix: Extending the Datagram-Tag

From: "David Culler" <[email-masked]>
Date: Mon, 19 Mar 2007 10:00:38 -0700

Carsten,
   I think that you have captured the discussion very well and this is a big
improvement.  My only concern is that we don't confuse the upper two bits.
They should be reserved and undefined at this stage, rather than counted on
to be zero.  If one is masking the upper bits to produce a valid 16 bit
size, all upper 5 should be masked.

NEW:
                           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 0 R R|    datagram_size    |         datagram_tag          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 10: First Fragment

NEW:
                           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 R R|    datagram_size    |         datagram_tag          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |datagram_offset|
      +-+-+-+-+-+-+-+-+

                      Figure 11: Subsequent Fragments
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From: Carsten Bormann <[email-masked]>
Date: Mon, 19 Mar 2007 18:37:44 +0100

On Mar 19 2007, at 18:00, David Culler wrote:

> They should be reserved and undefined at this stage

But this is exactly what the proposed text says -- it only defines  
the 11000 and 11100 prefixes, and all other bit combinations are not  
defined.

I would not want to pre-suppose 11xNN needs to be a Fragmentation  
header -- it might be something completely different.

(Maybe I don't understand your concern.)

BTW, this also touches on the IESG comment below.

Gruesse, Carsten

#################################
* Editorial 1

 From Gen-ART review by Joel Halpern:

         I don't know that I have ever seen a document before that  
says "thou
shalt not extend this."  (Section 5, last sentence before 5.1, "All  
headers used
in LOWPAN adaptation layer SHALL be defined in this format document.")

===> INTERPRET COMMENT
My view of this is that this is indeed the intention. Of course,
evolving this spec itself should be possible.  Note that this would
require some version management, which will need to be addressed by
bootstrapping.