[6lo] Question on draft-ietf-6lo-fragment-recovery and VRB

Martine Lenders <m.lenders@fu-berlin.de> Fri, 30 August 2019 09:41 UTC

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From: Martine Lenders <m.lenders@fu-berlin.de>
Date: Fri, 30 Aug 2019 11:40:48 +0200
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Subject: [6lo] Question on draft-ietf-6lo-fragment-recovery and VRB
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I have an implementation question for the virtual reassembly buffer with
regards to [draft-ietf-6lo-fragment-recovery]. Section 6.1 of this draft
states that a tuple (source address, tag) is used to identify a VRB entry
and cites [draft-ietf-6lo-minimal-fragment] for that. However, as far as I
understand [draft-ietf-6lo-minimal-fragment] and
[draft-ietf-lwig-6lowpan-virtual-reassembly] this is not the case.
[draft-ietf-6lo-minimal-fragment] doesn't mention VRB entry identification
and only refers to [draft-ietf-lwig-6lowpan-virtual-reassembly]. That in
turn only recounts the classic (source address, destination address, size,
tag) tuple defined in RFC4944 and states that the only difference between
the classic and VRB is the lack of the reassembled packet and addition of
the next hop's address and the newly assigned tag:

To reduce the memory requirement for reassembly buffers, the implementation
> may opt to not keep the actual packet data in the reassembly buffer.
> Instead, it may attempt to send out the data for a fragment in the form of
> a forwarded fragment, as soon as all necessary information for that is
> available. Obviously, all fragments need to be sent with the same outgoing
> address (otherwise a full reassembly implementation would discard the
> fragments) and the same datagram_tag.

So my question is: Is the tuple definition in
[draft-ietf-6lo-fragment-recovery] really correct? For exclusion datagram
size I agree that it could be somewhat redundant. However, a node could
have multiple destination addresses (either via multiple interfaces or
IEEE-802.15.4-style short and long address). So, as the tag is
link-specific (defined by a (source, destination) tuple), there could be
distinct datagrams that have the same tuple (source, tag), or am I missing

I already implemented the VRB with draft-ietf-6lo-minimal-fragment in mind
and thus used the classic 4-tuple for indexing. But now I'm wondering: Can
I reduce its index or would that be not advised?

Kind regards,