[riot-devel] Fragmentation support in IP

Thomas C. Schmidt t.schmidt at haw-hamburg.de
Fri Aug 19 10:15:50 CEST 2016


Hi Martine, Adeel,

there had been proposals in 6LowPan + Roll by Pascal Thubert for adding 
reliability to 6LowPan (recovering fragments) ... but if I see things 
correctly, they have never gotten anywhere -> Carsten?

In any case, the design discussion seems a bit out of focus: Whether to 
design a L2 protocol or an application layer (on top of CoAP) should be 
sorted out first. I guess these things depend on the problem scope and 
the objectives.

Cheers,
   Thomas

On 19.08.2016 09:56, Martine Lenders wrote:
> Hi Adeel,
> What do you mean by "lost fragment recovery"? To my knowledge there is
> no such thing in 6LoWPAN fragmentation.
>
> Regards,
> Martine
>
>
> Am 19.08.2016 12:59 vorm. schrieb "Adeel Mohammad Malik"
> <adeel.mohammad.malik at ericsson.com
> <mailto:adeel.mohammad.malik at ericsson.com>>:
>
>     Hi again Thomas,
>
>     Do you mean that 6LoWPAN in RIOT does not support lost fragment
>     recovery? Assuming it does, what I had in mind was that we could
>     segment data in the application to the IP MTU and leverage the lost
>     fragment recovery feature in 6LoWPAN to get a decent performance.
>     This way we may not need to implement lost segment recovery in the
>     application. But if 6LoWPAN in RIOT does not support recovery of
>     lost fragments we might as well run our application direclty on link
>     layer (since we dont need IP routing).
>
>     /Adeel
>     -------- Original message --------
>     From: "Thomas C. Schmidt" <t.schmidt at haw-hamburg.de
>     <mailto:t.schmidt at haw-hamburg.de>>
>     Date: 8/18/2016 23:27 (GMT+01:00)
>     To: RIOT OS kernel developers <devel at riot-os.org
>     <mailto:devel at riot-os.org>>
>     Cc: Börje Ohlman <borje.ohlman at ericsson.com
>     <mailto:borje.ohlman at ericsson.com>>, Joakim Borgh
>     <joakim.borgh at ericsson.com <mailto:joakim.borgh at ericsson.com>>
>     Subject: Re: [riot-devel] Fragmentation support in IP
>
>     Hi Adeel,
>
>     adding to this: you should keep in mind that in LowPANs you may easily
>     trigger multiple layers of fragmentation (loosing one fragment is
>     loosing all!) ... and that receivers may not be ready to handle UDP
>     datagrams of 'arbitrary' sizes.
>
>     So if you really want to stay away from upper layer protocols like
>     CoAP,
>     you should process data segmentation in your application (not sure this
>     is the best idea, though).
>
>     Cheers,
>       Thomas
>
>     On 18.08.2016 22:54, Carsten Bormann wrote:
>     > Hi Adeel,
>     >
>     > IP fragmentation is usually a bad idea*), and more so on a constrained
>     > network.  If you need to transfer payloads beyond a kilobyte or so,
>     > maybe CoAP (RFC 7252) with the block-wise transfer protocol (currently
>     > being published as RFC-to-be 7959) solves your problem.
>     >
>     > Which encryption expands a few bytes of plaintext to kilobytes of
>     > ciphertext?  (You may be thinking about signatures; e.g., hash-based
>     > signatures can be 3-6 KiB or even more.  These might occur in firmware
>     > updates and are covered quite well by CoAP + block-wise.)
>     >
>     > Grüße, Carsten
>     >
>     > *)
>     > https://tools.ietf.org/html/draft-mathis-frag-harmful-00
>     <https://tools.ietf.org/html/draft-mathis-frag-harmful-00>
>     > http://www.hpl.hp.com/techreports/Compaq-DEC/WRL-87-3.pdf
>     <http://www.hpl.hp.com/techreports/Compaq-DEC/WRL-87-3.pdf>
>     >
>     > Adeel Mohammad Malik wrote:
>     >> Hi Thomas,
>     >>
>     >> I agree that IP fragmentation is not an equivalent for data streaming.
>     >> However it still facilitates transporting data that exceeds the MTU. The
>     >> use case we are looking at is encryption of IoT data that may result in
>     >> a few bytes of plaintext being converted to a few kilobytes of
>     >> ciphertext. Had IP supported fragmentation in RIOT it would have been
>     >> possible for us to send such data.
>     >>
>     >> /Adeel
>     >>
>     >>
>     >> -------- Original message --------
>     >> From: "Thomas C. Schmidt" <t.schmidt at haw-hamburg.de <mailto:t.schmidt at haw-hamburg.de>>
>     >> Date: 8/18/2016 18:11 (GMT+01:00)
>     >> To: devel at riot-os.org <mailto:devel at riot-os.org>
>     >> Subject: Re: [riot-devel] Fragmentation support in IP
>     >>
>     >> Hi Adeel,
>     >>
>     >> GNRC in RIOT supports fragmentation, e.g. in the context of 6LowPAN.
>     >>
>     >> However, you seem to be interested in sending UDP datagrams that exceed
>     >> the MTU payload size. I don't think this is common use ... and I don't
>     >> think this is clever, either. IP fragmentation is not an equivalent for
>     >> data streaming.
>     >>
>     >> Cheers,
>     >>   Thomas
>     >>
>     >> On 18.08.2016 18:04, Adeel Mohammad Malik wrote:
>     >>> Hi all,
>     >>>
>     >>>
>     >>>
>     >>> My question is about fragmentation support in IP in RIOT. Does IP in
>     >>> RIOT support fragmentation? The use case I am after is transferring a
>     >>> large blob of data (let’s say 5 kilobytes) on the UDP/IP stack in RIOT.
>     >>> Is that possible?
>     >>>
>     >>>
>     >>>
>     >>> I know that 6LoWPAN supports fragmentation but that is below the IP
>     >>> layer. I am interested in fragmentation at the IP layer so that
>     >>> application running over it in RIOT can send large data.
>     >>>
>     >>>
>     >>>
>     >>> Regards,
>     >>>
>     >>> Adeel
>     >>>
>     >>
>     >> --
>     >>
>     >> Prof. Dr. Thomas C. Schmidt
>     >> ° Hamburg University of Applied Sciences                   Berliner Tor 7 °
>     >> ° Dept. Informatik, Internet Technologies Group    20099 Hamburg, Germany °
>     >> ° http://www.haw-hamburg.de/inet                   Fon:
>     +49-40-42875-8452 <tel:%2B49-40-42875-8452> °
>     >> ° http://www.informatik.haw-hamburg.de/~schmidt
>     <http://www.informatik.haw-hamburg.de/~schmidt>    Fax:
>     +49-40-42875-8409 <tel:%2B49-40-42875-8409> °
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>     --
>
>     Prof. Dr. Thomas C. Schmidt
>     ° Hamburg University of Applied Sciences                   Berliner
>     Tor 7 °
>     ° Dept. Informatik, Internet Technologies Group    20099 Hamburg,
>     Germany °
>     ° http://www.haw-hamburg.de/inet                   Fon:
>     +49-40-42875-8452 <tel:%2B49-40-42875-8452> °
>     ° http://www.informatik.haw-hamburg.de/~schmidt
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-- 

Prof. Dr. Thomas C. Schmidt
° Hamburg University of Applied Sciences                   Berliner Tor 7 °
° Dept. Informatik, Internet Technologies Group    20099 Hamburg, Germany °
° http://www.haw-hamburg.de/inet                   Fon: +49-40-42875-8452 °
° http://www.informatik.haw-hamburg.de/~schmidt    Fax: +49-40-42875-8409 °


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