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

Versions: 00 01 02 03

Network Working Group                                           C. Gomez
Internet-Draft                                              J. Paradells
Intended status: Standards Track                               UPC/i2CAT
Expires: September 21, 2016                                 J. Crowcroft
                                                 University of Cambridge
                                                          March 20, 2016


            Optimized 6LoWPAN Fragmentation Header for LPWAN
               draft-gomez-lpwan-fragmentation-header-01

Abstract

   LPWAN technologies are characterized by a very limited data unit and/
   or payload size, often one order of magnitude below the one in IEEE
   802.15.4.  However, many such technologies do not support layer 2
   fragmentation.  The 6LoWPAN fragmentation header defined in RFC 4944
   represents very high overhead for LPWAN technologies, and it even
   does not support transporting IPv6 datagrams that require
   fragmentation over layer 2 technologies of a payload size below 13
   bytes.  This specification defines an optimized 6LoWPAN fragmentation
   header for LPWAN.

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 http://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 September 21, 2016.

Copyright Notice

   Copyright (c) 2016 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
   (http://trustee.ietf.org/license-info) in effect on the date of



Gomez, et al.          Expires September 21, 2016               [Page 1]


Internet-Draft         LPWAN fragmentation header             March 2016


   publication of this document.  Please review these documents
   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
     1.1.  Conventions used in this document . . . . . . . . . . . .   3
   2.  6LoFHL rules and format . . . . . . . . . . . . . . . . . . .   3
   3.  Changes from RFC 4944 fragmentation header and rationale  . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Annex A. Quantitative comparison of RFC 4944 fragmentation
       header with 6LoFHL  . . . . . . . . . . . . . . . . . . . . .   6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   Low Power Wide Area Network (LPWAN) technologies are characterized,
   among others, by a very reduced data unit and/or payload size
   [I-D.minaburo-lp-wan-gap-analysis].  However, many such technologies
   do not support layer two fragmentation, therefore the only option for
   these to support IPv6 (and, in particular, its MTU requirement of
   1280 bytes [RFC2460]) is the use of a fragmentation header at the
   adaptation layer below IPv6, such as the 6LoWPAN fragmentation header
   defined in [RFC4944].

   While the aforementioned 6LoWPAN fragmentation header is appropriate
   for IEEE 802.15.4-2003 (which has a frame payload size of 81 to 102
   bytes), it is not suitable for several LPWAN technologies, many of
   which have a maximum payload size that is one order of magnitude
   below that of IEEE 802.15.4-2003.  The overhead of the 6LoWPAN
   fragmentation header is high, considering the reduced payload size of
   LPWAN technologies and the limited energy availability of the devices
   using such technologies.  Furthermore, its datagram offset field is
   expressed in increments of eight octets.  In some LPWAN technologies,
   the 6LoWPAN fragmentation header plus eight octets from the original
   datagram exceeds the available space in the layer 2 (L2) payload.






Gomez, et al.          Expires September 21, 2016               [Page 2]


Internet-Draft         LPWAN fragmentation header             March 2016


   This specification defines an optimized 6LoWPAN Fragmentation Header
   for LPWAN (6LoFHL).  Nevertheless, other L2 technologies beyond the
   LPWAN category may benefit from using 6LoFHL.

   It is expected that this specification will be used jointly with
   other 6Lo(WPAN) mechanisms such as [RFC6282] based header
   compression.

   The benefits of using 6LoFHL are the following:

   -- While the 6LoWPAN fragmentation header defined in RFC 4944 has a
   size of 4 bytes (first fragment) or 5 bytes (subsequent fragments),
   6LoFHL has a size of 3 bytes (any fragment).  This reduces
   significantly both the L2 overhead and the adaptation layer overhead
   for transporting an IPv6 packet that requires fragmentation (see
   Annex A).

   -- Because the datagram offset can be expressed in increments of a
   single octet, 6LoFHL enables the transport of IPv6 packets over L2
   data units with a maximum payload size as small as only 4 bytes in
   the most extreme case.

1.1.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119]

2.  6LoFHL rules and format

   If an entire payload (e.g., IPv6) datagram fits within a single L2
   data unit, it is unfragmented and a fragmentation header is not
   needed.  If the datagram does not fit within a single L2 data unit,
   it SHALL be broken into fragments.  The first fragment SHALL contain
   the first fragment header as defined in Figure 1.

                              1                   2
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |1 1 0 0 1|    datagram_size    |  datagram_tag |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                         Figure 1: First Fragment

   The second and subsequent fragments (up to and including the last)
   SHALL contain a fragmentation header that conforms to the format
   shown in Figure 2.



Gomez, et al.          Expires September 21, 2016               [Page 3]


Internet-Draft         LPWAN fragmentation header             March 2016


                              1                   2
          0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |1 1 0 1 0|   datagram_offset   |  datagram_tag |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                      Figure 2: Subsequent Fragments

   datagram_size: This 11-bit field encodes the size of the entire IP
   packet before link-layer fragmentation (but after IP layer
   fragmentation).  For IPv6, the datagram size SHALL be 40 octets (the
   size of the uncompressed IPv6 header) more than the value of Payload
   Length in the IPv6 header [RFC4944] of the packet.  Note that this
   packet may already be fragmented by hosts involved in the
   communication, i.e., this field needs to encode a maximum length of
   1280 octets (the required by IPv6).

   datagram_tag: The value of datagram_tag (datagram tag) SHALL be the
   same for all fragments of a payload (e.g., IPv6) datagram.  The
   sender SHALL increment datagram_tag for successive, fragmented
   datagrams.  The incremented value of datagram_tag SHALL wrap from 255
   back to zero.  This field is 8 bits long, and its initial value is
   not defined.

   datagram_offset: This field is present only in the second and
   subsequent fragments and SHALL specify the offset, in increments of 1
   octet, of the fragment from the beginning of the payload datagram.
   The first octet of the datagram (e.g., the start of the IPv6 header)
   has an offset of zero; the implicit value of datagram_offset in the
   first fragment is zero.  This field is 11 bits long.

   The recipient of link fragments SHALL use (1) the sender's L2 source
   address, (2) the destination's L2 address, (3) datagram_size, and (4)
   datagram_tag to identify all the fragments that belong to a given
   datagram.

   Upon receipt of a link fragment, the recipient starts constructing
   the original unfragmented packet whose size is datagram_size.  It
   uses the datagram_offset field to determine the location of the
   individual fragments within the original unfragmented packet.  For
   example, it may place the data payload (except the encapsulation
   header) within a payload datagram reassembly buffer at the location
   specified by datagram_offset.  The size of the reassembly buffer
   SHALL be determined from datagram_size.

   If a fragment recipient disassociates from its L2 network, the
   recipient MUST discard all link fragments of all partially



Gomez, et al.          Expires September 21, 2016               [Page 4]


Internet-Draft         LPWAN fragmentation header             March 2016


   reassembled payload datagrams, and fragment senders MUST discard all
   not yet transmitted link fragments of all partially transmitted
   payload (e.g., IPv6) datagrams.  Similarly, when a node first
   receives a fragment with a given datagram_tag, it starts a reassembly
   timer.  When this time expires, if the entire packet has not been
   reassembled, the existing fragments MUST be discarded and the
   reassembly state MUST be flushed.  The reassembly timeout MUST be set
   to a maximum of TBD seconds).

3.  Changes from RFC 4944 fragmentation header and rationale

   The main changes introduced in this specification to the
   fragmentation header format defined in RFC 4944 are listed below,
   together with their rationale:

   -- The datagram size field is only included in the first fragment.
   Rationale: In the RFC 4944 fragmentation header, the datagram size
   was included in all fragments to ease the task of reassembly at the
   receiver, since in an IEEE 802.15.4 mesh network, the fragment that
   arrives earliest to a destination is not necessarily the first
   fragment transmitted by the source.  However, in LPWAN, such
   reordering effects are not expected.  LPWAN technologies typically
   define star topology networks, peripheral to peripheral
   communications are not expected, and the central device is not
   expected to perform priority queuing operations.  Nevertheless, the
   fragmentation format defined in this document supports limited
   reordering.

   -- The datagram tag size is reduced from 2 bytes to 1 byte.
   Rationale: Given the low bit rate, as well as the low message rate of
   LPWAN technologies, ambiguities due to datagram tag wrapping events
   are not expected to occur despite the reduced tag space.

   -- The datagram offset size is increased from 8 bits to 11 bits.
   Rationale: This allows to express the datagram offset in single-octet
   increments.

4.  IANA Considerations

   This document allocates the following sixteen RFC 4944 Dispatch type
   values:

   11001 000

   through

   11001 111




Gomez, et al.          Expires September 21, 2016               [Page 5]


Internet-Draft         LPWAN fragmentation header             March 2016


   and

   11010 000

   through

   11010 111

5.  Security Considerations

   TBD

6.  Acknowledgments

   In section 2, the authors have reused extensive parts of text
   available in section 5.3 of RFC 4944, and would like to thank the
   authors of RFC 4944.

   Carles Gomez has been funded in part by the Spanish Government
   (Ministerio de Educacion, Cultura y Deporte) through the Jose
   Castillejo grant CAS15/00336.  His contribution to this work has been
   carried out during his stay as a visiting scholar at the Computer
   Laboratory of the University of Cambridge.

7.  Annex A.  Quantitative comparison of RFC 4944 fragmentation header
    with 6LoFHL

                   +-------------------------------------------------------+
                   |                IPv6 datagram size (bytes)             |
                   +-------------+-------------+-------------+-------------+
                   |     11      |    40       |     100     |     1280    |
+------------------+-------------+-------------+-------------+-------------+
|L2 payload (bytes)| 4944 |6LoFHL| 4944 |6LoFHL| 4944 |6LoFHL| 4944 |6LoFHL|
+------------------+-------------+-------------+-------------+-------------+
|       10         | ---- |   2  | ---- |   6  | ---- |  15  | ---- |  183 |
+------------------+-------------+------+------+------+------+-------------+
|       15         |   1  |   1  |   5  |   4  |  13  |  9   |  160 |  107 |
+------------------+-------------+------+------+------+------+-------------+
|       20         |   1  |   1  |   4  |   3  |  12  |  6   |  159 |  76  |
+------------------+-------------+------+------+------+------+-------------+
|       25         |   1  |   1  |   3  |   2  |   7  |  5   |   80 |  59  |
+------------------+-------------+------+------+------+------+-------------+
|       30         |   1  |   1  |   2  |   2  |   5  |  4   |   54 |  48  |
+------------------+-------------+------+------+------+------+-------------+

       Figure 3: L2 overhead (in terms of L2 data units) required to
                        transport an IPv6 datagram




Gomez, et al.          Expires September 21, 2016               [Page 6]


Internet-Draft         LPWAN fragmentation header             March 2016


                   +-------------------------------------------------------+
                   |                IPv6 datagram size (bytes)             |
                   +-------------+-------------+-------------+-------------+
                   |     11      |     40      |     100     |     1280    |
+------------------+-------------+-------------+-------------+-------------+
|L2 payload (bytes)| 4944 |6LoFHL| 4944 |6LoFHL| 4944 |6LoFHL| 4944 |6LoFHL|
+------------------+-------------+-------------+-------------+-------------+
|       10         | ---- |   6  | ---- |  18  | ---- |  45  | ---- |  768 |
+------------------+-------------+------+------+------+------+-------------+
|       15         |   0  |   0  |  24  |  12  |  64  |  27  |  799 |  321 |
+------------------+-------------+------+------+------+------+-------------+
|       20         |   0  |   0  |  19  |   9  |  59  |  18  |  794 |  228 |
+------------------+-------------+------+------+------+------+-------------+
|       25         |   0  |   0  |  14  |   6  |  34  |  15  |  399 |  177 |
+------------------+-------------+------+------+------+------+-------------+
|       30         |   0  |   0  |   9  |   6  |  24  |  12  |  269 |  144 |
+------------------+-------------+------+------+------+------+-------------+

   Figure 4: Adaptation layer fragmentation overhead (in bytes) required
                       to transport an IPv6 datagram

   Note 1: with the RFC 4944 fragmentation header it is not possible to
   transport IPv6 datagrams of the considered sizes over a 10-byte
   payload L2 technology.

   Note 2: 11 bytes is the size of an IPv6 datagram with a 3-byte RFC
   6282 compressed header (the shortest possible IPv6 header that uses
   global addresses), a 4-byte RFC 6282 UDP compressed header, and a
   CoAP message without header options and without payload.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

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




Gomez, et al.          Expires September 21, 2016               [Page 7]


Internet-Draft         LPWAN fragmentation header             March 2016


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

8.2.  Informative References

   [I-D.minaburo-lp-wan-gap-analysis]
              Minaburo, A., Pelov, A., and L. Toutain, "LP-WAN GAP
              Analysis", draft-minaburo-lp-wan-gap-analysis-01 (work in
              progress), February 2016.

Authors' Addresses

   Carles Gomez
   UPC/i2CAT
   C/Esteve Terradas, 7
   Castelldefels  08860
   Spain

   Email: carlesgo@entel.upc.edu


   Josep Paradells
   UPC/i2CAT
   C/Jordi Girona, 1-3
   Barcelona  08034
   Spain

   Email: josep.paradells@entel.upc.edu


   Jon Crowcroft
   University of Cambridge
   JJ Thomson Avenue
   Cambridge, CB3 0FD
   United Kingdom

   Email: jon.crowcroft@cl.cam.ac.uk












Gomez, et al.          Expires September 21, 2016               [Page 8]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/