Network Working Group                                         C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Informational                               T. Watteyne
Expires: September 12, 2019 10, 2020                               Analog Devices
                                                          March 11, 2019 09, 2020

                 Virtual reassembly buffers in 6LoWPAN


   When employing adaptation layer fragmentation in 6LoWPAN, it may be
   beneficial for a forwarder not to have to reassemble each packet in
   its entirety before forwarding it.

   This has been always possible with the original fragmentation design
   of RFC 4944.  Apart from a brief mention of the way to do this in
   Section 2.5.2 of the 6LoWPAN book, this has not been extensively
   described in the literature.  The present document attempts to fill
   that gap.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Reassembly buffers  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Virtual reassembly  . . . . . . . . . . . . . . . . . . . . .   3
   4.  Header compression  . . . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  Security considerations . . . . . . . . . . . . . . . . . . .   4
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   4   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   6LoWPAN [RFC4944] is the seminal standard for the transmission of
   IPv6 packets over IEEE 802.15.4 networks and has served as a
   blueprint for a number of related standards addressing low-power
   radios and other IoT connectivity solutions (the "6Lo suite").

   One of the problems that need to be solved to enable sending IPv6
   packets over low-power radios is that some of these (including IEEE
   802.15.4) do not support frames that are large enough to hold IPv6
   packets of the minimum MTU (Maximum Transmission Unit) defined for
   IPv6, 1280 bytes.  This necessitates providing a fragmentation or
   segmentation scheme in the IP adaptation layer for the radio.

   When employing adaptation layer fragmentation on constrained-node
   networks [RFC7228], it may be beneficial for a forwarder not to have
   to reassemble each packet in its entirety before forwarding it.

   This has been always possible with the original fragmentation design
   of RFC 4944.  Apart from a brief mention of the way to do this in
   Section 2.5.2 of the 6LoWPAN book [BOOK], this has not been
   extensively described in the literature.  The present document
   attempts to fill that gap.


   [I-D.ietf-6lo-minimal-fragment] provides additional context and
   discussion about handling fragment forwarding in the 6Lo standards

2.  Reassembly buffers

   An adaptation layer implementation for 6LoWPAN needs to perform
   reassembly of every fragmented packet received in order to be able to
   forward the packet (re-fragmenting it in the process).

   A reassembly buffer for 6LoWPAN contains:

   o  datagram_size,

   o  datagram_tag and L2 sender and receiver addresses (to which the
      datagram_tag is local),

   o  actual packet data from the fragments received so far, in a form
      that makes it possible to detect when the whole packet has been
      received and can be processed or forwarded,

   o  a timer that allows discarding the partial packet after a timeout.

   This requires a reassembly buffer for each fragmented packet the
   reception of which is in progress.  Since the forwarder may be
   receiving fragments for multiple packets concurrently (e.g., from
   different senders), this means that multiple reassembly buffers are
   needed, easily dominating the memory requirements in a 6LoWPAN
   implementation.  Worse, as this space may still be limited, any lack
   of reassembly buffers may lead to an increased loss rate for
   fragmented packets (which already have to cope with a higher compound
   loss rate).

3.  Virtual reassembly

   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.

   To this end, the reassembly buffer now also stores, as soon as enough
   of the packet is available to make a forwarding decision (i.e., as
   soon as the first fragment has been received):

   o  L2 destination address used for forwarding,

   o  outgoing datagram_tag chosen for this packet.

   A simple implementation may do away with any attempt to keep packet
   data in the virtual reassembly buffer.  It then has to discard all
   non-first fragments for which a reassembly buffer is not already
   available (penalizing reordering, which however may be rare).

   Note that the decision to do local processing of a packet needs to be
   taken with the first fragment - such packets of course do need to be
   fully reassembled (unless transport and application also can cope
   with fragments, which they rarely can in the presence of security).

4.  Header compression

   [RFC6282] defines the header compression format for 6LoWPAN.  One
   important impact of header compression is that the header is no
   longer of a length that stays fixed during forwarding.  In
   particular, changes made by a forwarder may gain or lose the ability
   to use a more highly compressed variant, changing the length of the
   header in the packet.  If the change increases the size, the maximum
   frame size may be exceeded, leading to the need to re-fragment in the
   forwarder.  This is less of a problem with full reassembly, but with
   virtual reassembly can lead to the need for sending an additional
   frame for each packet.

   The well-known approach to minimize the probability of this need is
   for the original sender to put all slack in the frame sizes into the
   _first_ packet, making this the smallest fragment and not the last
   one as would be done in a naive implementation.  (This also has other
   consequences related to delivery probability, which are not discussed
   here.)  This makes sure an additional fragment only needs to be sent
   if the header expansion during forwarding would have created an
   additional fragment with full reassembly as well.

5.  IANA Considerations

   This document makes no requests of IANA.

6.  Security considerations


   There are many security considerations with using fragmentation in
   the first place, even with adaptation layer fragmentation (which is
   not accessible outside the range of that adaptation layer).  Some of
   the more specific ones are documented in
   [I-D.ietf-6lo-minimal-fragment] and will not be duplicated here.

   In general, sending on fragments early from a node will relieve the
   node that is doing the forwarding, but put additional onus on the
   next node.  This may or may not be in favor of an attacker.

7.  References

7.1.  Normative References

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,

7.2.  Informative References

   [BOOK]     Shelby, Z. and C. Bormann, "6LoWPAN", John Wiley & Sons,
              Ltd monograph, DOI 10.1002/9780470686218, November 2009.


              Watteyne, T., Bormann, C., and P. Thubert, "LLN Minimal
              Fragment Forwarding", draft-watteyne-6lo-minimal-
              fragment-02 P., and C. Bormann, "On Forwarding
              6LoWPAN Fragments over a Multihop IPv6 Network", draft-
              ietf-6lo-minimal-fragment-14 (work in progress), July 2018. March

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,


   Many people have mentioned that it would be good to have a
   description of virtual reassembly in 6LoWPAN.  Finally, Thomas
   Watteyne assembled a design team that intends to work on 6Lo
   fragmentation.  Writing up the present document has been motivated by
   that work.

Authors' Addresses

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359

   Phone: +49-421-218-63921
   Thomas Watteyne
   Analog Devices
   32990 Alvarado-Niles Road, Suite 910
   Union City, CA  94587