[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-bormann-lwig-6lowpan-virtual-reassembly) 00

Network Working Group                                         C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Informational                               T. Watteyne
Expires: January 3, 2019                                  Analog Devices
                                                           July 02, 2018


                 Virtual reassembly buffers in 6LoWPAN
             draft-ietf-lwig-6lowpan-virtual-reassembly-00

Abstract

   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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 3, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



Bormann & Watteyne       Expires January 3, 2019                [Page 1]


Internet-Draft    Virtual reassembly buffers in 6LoWPAN        July 2018


   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     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.watteyne-6lo-minimal-fragment] provides additional context and
   discussion about handling fragment forwarding in the 6Lo standards
   suite.




Bormann & Watteyne       Expires January 3, 2019                [Page 2]


Internet-Draft    Virtual reassembly buffers in 6LoWPAN        July 2018


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.




Bormann & Watteyne       Expires January 3, 2019                [Page 3]


Internet-Draft    Virtual reassembly buffers in 6LoWPAN        July 2018


   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 fixed length.  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

   TBD

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,
              <https://www.rfc-editor.org/info/rfc4944>.




Bormann & Watteyne       Expires January 3, 2019                [Page 4]


Internet-Draft    Virtual reassembly buffers in 6LoWPAN        July 2018


7.2.  Informative References

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

   [I-D.watteyne-6lo-minimal-fragment]
              Watteyne, T., Bormann, C., and P. Thubert, "LLN Minimal
              Fragment Forwarding", draft-watteyne-6lo-minimal-
              fragment-01 (work in progress), March 2018.

   [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,
              <https://www.rfc-editor.org/info/rfc6282>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

Acknowledgements

   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
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org


   Thomas Watteyne
   Analog Devices
   32990 Alvarado-Niles Road, Suite 910
   Union City, CA  94587
   USA

   Email: thomas.watteyne@analog.com




Bormann & Watteyne       Expires January 3, 2019                [Page 5]


Html markup produced by rfcmarkup 1.128, available from https://tools.ietf.org/tools/rfcmarkup/