On Aug 18, 2009, at 15:37, Ralph Droms wrote:
does anyone have empirical evidence about the impact of fragment
loss in 802.15.4 networks that would motivate the need for reliable
fragment delivery?
(no WG chair hat:)
I can't answer that question.
But it is important to describe the applicability of http://tools.ietf.org/html/draft-thubert-6lowpan-simple-fragment-recovery-06
some more.
As it stands, the draft addresses the mesh-under case. There is
little gain for the hop-by-hop fragmentation/reassembly implied by
route-over; it is also not clear how to apply the draft to the route-
over fragmentation optimization I described in the other sub-thread
(who would send the FRACK?).
We do have "fragment recovery" in 4944 in the sense that it is (lower-
case) recommended to use 802.15.4's link-layer ACK to increase the
reliability of each (L2) hop (4944, Section 2, first paragraph).
What we don't have is recovery on the whole L2 path from the
fragmenting L3/L2 boundary (mesh-under ingress, MUI) to the
reassembling L2/L3 boundary (mesh-under egress, MUE). The argument I
have understood for the draft is that there may be (MU) route changes
(possibly including MU multipath routing) between MUI and MUE during
transmission of the fragments of a single (larger) packet, causing
some parts to be lost even with good use of per-hop acknowledgements.
A 1280 byte packet will take some 40 ms of pure airtime on 2.4 GHz or
some 500 ms on .868 GHz. Multiply this by a realistic media sharing
multiplier, and there does appear some opportunity for radio
characteristics to change during that time. But that is just a hunch,
and some numbers from a real-life network would indeed help.
For now, my (WG member) answer to the WG chair's question whether we
should adopt that draft is a luke-warm "I wouldn't mind".
Gruesse, Carsten
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