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Versions: 00 01

Network Working Group                                        A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Informational                                L. Toutain
Expires: December 23, 2016     Institut MINES TELECOM ; TELECOM Bretagne
                                                           June 21, 2016


    6LPWA Static Context Header Compression (SCHC) for IPV6 and UDP
             draft-toutain-6lpwa-ipv6-static-context-hc-01

Abstract

   This document describes a header compression scheme for IPv6, IPv6/
   UDP based on static contexts.  This technique is especially tailored
   for LPWA networks and could be extended to other protocol stacks.

   During the IETF history several compression mechanisms have been
   proposed.  First mechanisms, such as RoHC, are using a context to
   store header field values and send smaller incremental differences on
   the link.  Values in the context evolve dynamically with information
   contained in the compressed header.  The challenge is to maintain
   sender's and receiver's contexts synchronized even with packet
   losses.  Based on the fact that IPv6 contains only static fields,
   6LoWPAN developed an efficient context-free compression mechanisms,
   allowing better flexibility and performance.

   The Static Context Header Compression (SCHC) combines the advantages
   of RoHC context which offers a great level of flexibility in the
   processing of fields, and 6LoWPAN behavior to elide fields that are
   known from the other side.  Static context means that values in the
   context field do not change during the transmission, avoiding complex
   resynchronization mechanisms, incompatible with LPWA characteristics.
   In most of the cases, IPv6/UDP headers are reduced to a small
   identifier.

   This document focuses on IPv6/UDP headers compression, but the
   mechanism can be applied to other protocols such as CoAP.  It will be
   described in a separate document.

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/.



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   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 December 23, 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
   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.

1.  Introduction

   Headers compression is mandatory to bring the internet protocols to
   the node within a LPWA network [I-D.minaburo-lp-wan-gap-analysis].

   Nevertheless, LPWA networks offer good properties for an efficient
   header compression:

   o  Topology is star oriented, therefore all the packets follows the
      same path.  For the needs of this draft, the architecture can be
      summarized to End-Systems (ES) exchanging information with a
      single LPWA Compressor (LC).  In most of the cases, End Systems
      and LC form a star topology.  ESs and LC maintain a context for
      compression.

   o  Traffic flows are mostly deterministic, since End-Systems embed
      built-in applications.  Contrary to computers or smartphones, new
      applications cannot be easily installed.

   First mechanisms such as RoHC use a context to store header field
   values and send smaller incremental differences on the link.  The
   first version of RoHC targeted IP/UDP/RTP stack.  RoHCv2 extends the
   principle to any protocol and introduces a formal notation [RFC4997]
   describing the header and associating compression functions to each
   field.  To be efficient the sender and the receiver must check that
   the context remains synchronized (i.e. contains the same values).



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   Context synchronization imposes to periodically send a full header or
   at least dynamic fields.  If fully compressed, the header can be
   compatible with LPWA constraints.  However, the first exchanges or
   context resynchronisations impose to send uncompressed headers, which
   may be bigger than the original one.  This will force the use of
   inefficient fragmentation mechanisms.  For some LPWA technologies,
   duty cycle limits can also delay the resynchronization.  Figure 1
   illustrates this behavior.

                       sync
             ^         +-+         sync     sync             ^
             | IPv6    | |         +-+       +-+             | IPv6
             v         | |         | |       | |             v
      +------------+   | +-+-+     | |       | |    +------------+
      |       +--+ |   | | | |     | |       | |    | +--+       |
      |       | c| |   | | | +-+-+-+ +-+-+-+-+ |    | | c|       |
      |       | t| |   | | | | | | | | | | | | |    | | t|       |
      |       | x| |   +-+-+-+-+-+-+-+-+-+-+-+-+    | | x|       |
      |       | t| | <----------------------------> | | t|       |
      |       +--+ |                                | +--+       |
      +------------+                                +------------+



             Figure 1: RoHC Compressed Header size evolution.

   On the other hand, 6LoWPAN [RFC4944] is context-free based on the
   fact that IPv6, its extensions or UDP headers do not contain
   incremental fields.  The compression mechanism described in [RFC6282]
   is based on sending a 2-byte bitmap, which describes how the header
   should be decompressed, either using some standard values or sending
   information after this bitmap.  [RFC6282] also allows for UDP
   compression.

   In the best case, when Hop limit is a standard value, flow label,
   DiffServ fields are set to 0 and Link Local addresses are used over a
   single hop network, the 6LoWPAN compressed header is reduced to 4
   bytes.  This compression ratio is possible because the IID are
   derived from the MAC addresses and the link local prefix is known
   from both sides.  In that case, the IPv6 compression is 4 bytes and
   UDP compression is 2 bytes, which fills half of the payload of a
   SIGFOX frame, or more than 10% of a LoRaWAN payload (with spreading
   factor 12).

   The Static Context Header Compression (SCHC) combines the advantages
   of RoHC context, which offers a great level of flexibility in the
   processing of fields, and 6LoWPAN behavior to elide fields that are
   known from the other side.  Static context means that values in the



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   context field do not change during the transmission, avoiding complex
   resynchronization mechanisms, incompatible with LPWA characteristics.
   In most of the cases, IPv6/UDP headers are reduced to a small context
   identifier.

2.  Static Context Header Compression

   Static Context Header Compression (SCHC) avoids context
   synchronization, which is the most bandwidth-consuming operation in
   RoHC.  Based on the fact that the nature of data flows is highly
   predictable in LPWA networks, a static context may be stored on the
   End-System (ES).  The other end, the LPWA Compressor (LC) can learn
   the context through a provisionning protocol during the
   identification phase (for instance, as it learns the encryption key).

   The context contains an ordered list of rules.  Each rule is a vector
   of entries.  Each entry is composed of a field descriptor, a
   prescribed matching value, a matching rule for the compression side,
   a matching rule for the decompression side and a compression/
   decompression action.  Contexts in the compressor and decompressor
   are the same.  A rule is identified by a rule identifier.  If the
   layer 2 allows it, the rule id can be carried in the layer 2 header.
   Otherwise the rule id is located in the first byte of the L2 payload.

   Being at the boundary between Layer 2 and Layer 3, the rule id will
   also be called a shim id.  Different ES will use the same shim id to
   identify their own context.  An LC may also use the ES device id to
   identify the appropriate rule.























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            +---------------------------------------------------------------------+
            |                      Rule N                                         |
       +---------------------------------------------------------------------+    |
       |                    Rule i                                           |    |
+---------------------------------------------------------------------+      |    |
|                    Rule 1                                           |      |    |
|   +---------+-------+------------+--------------+-----------------+ |      |    |
|   | Field 1 | Value |match. comp.| match decomp | Action function | |      |    |
|   +---------+-------+------------+--------------+-----------------+ |      |    |
|   | Field 2 | Value |match. comp.| match decomp | Action function | |      |    |
|   +---------+-------+------------+--------------+-----------------+ |      |    |
|   | ...     | ...   |...         | ...          | ...             | |      |    |
|   +---------+-------+------------+--------------+-----------------+ |      |----+
|   | Field N | Value |match. comp.| match decomp | Action function | |      |
|   +---------+-------+------------+--------------+-----------------+ |------+
|                                                                     |
+---------------------------------------------------------------------+


                          Figure 2: Context in LC

   The compression/decompression process follows several steps:

   o  compression rule selection: the goal is to identify which rule
      will be used to compress the headers.  To each field is associated
      a matching rule for compression.  Each header field's value is
      compared to the corresponding value stored in the rule for that
      field using the matching operator.  If all the fields match, the
      packet is processed using this rule action functions and the rule
      list exploration is aborted.  Otherwise the next rule is tested.
      If no rule is found, then the packet is dropped.

   o  compression: the action function indicates is the field is send on
      the link or not.  A field can also be partially sent regarding the
      matching operator.  The resulting compressed header must be
      aligned on byte boundaries.

   o  decompression rule selection, as for compression, a rule has to be
      selected to uncompress incoming packets.  A matching operator is
      defined on the compress header and works as for compression.

   o  decompression: the same action function indicates how the field
      value can be rebuilt, either from bits received on the link, a
      value stored in the rule or by using a specific algorithm.







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3.  Matching operators

   Matching a field with a value and header compression are related
   operations; If a field matches a rule containing the value, it is not
   necessary to send it on the link.  Since context are synchronized,
   reading the rule's value is enough to reconstruct the field's value
   at the other end.

   On some other cases, the value need to be sent on the link to inform
   the other end.  The field value may vary from one packet to another,
   therefore the field cannot be used to select the rule id.

   It may exist some intermediary cases, where part of the value may be
   used to select a field and a variable part has to be sent on the
   link.  This is true for Least Significant Bits (LSB) where the most
   significant bit can be used to select a rule id and the least
   significant bits has to be sent on the link.

   Several matching operators are defined:

   o  = : a field value in a packet matches with a field value in a rule
      if they are equal.

   o  no : no check is done between a field value in a packet matches
      with a field value in the rule

   o  lbs(L) : a field value of length T in a packet matches with a
      field value in a rule if the most significant T-L bits are equal.

4.  Action functions

   The action functions describe the action taken by the compression and
   inversely the action taken by the decompressor to restore the
   original value.

















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   /--------------------+-------------+--------------------------\
   | Function           | Compression | Decompression            |
   |                    |             |                          |
   +--------------------+-------------+--------------------------+
   |elided              |not sent     |use value stored in ctxt  |
   |send-value          |send         |build field from value    |
   |compute-IPv6-length |elided       |compute IPv6 length       |
   |compute-UDP-length  |elided       |compute UDP length        |
   |compute-UDP-checksum|elided       |compute UDP checksum      |
   |ESiid-DID           |elided       |build IID from L2 ES addr |
   |LCiid-DID           |elided       |build IID from L2 LA addr |
   \--------------------+-------------+--------------------------/


              Figure 3: Simplified Protocol Stack for LP-WAN

   Figure 3 lists all the functions defined to compress and decompress a
   field.  The first column gives the function's name.  The second and
   third columns outlines the compression/decompression process.

   As with 6LoWPAN, the compression process may produce some data, where
   fields that were not compressed (or were partially compressed) will
   be sent in the order of the original packet.  Information added by
   the compression phase must be aligned on byte boundaries, but each
   individual compression function may generate any size.


























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/-----------------+---------------------+----------------------------------------\
| Field           |Function             | Behavior                               |
+-----------------+---------------------+----------------------------------------+
|IPv6 version     |elided               |The value is not sent, but each end     |
|IPv6 DiffServ    |                     |agrees on a value, which can be         |
|IPv6 Flow Label  |                     |different from 0.                       |
|IPv6 Next Header |send-value           |Depending on the matching operator, the |
|                 |                     |entire field value is sent or an        |
|                 |                     |adjustment to the context value         |
+-----------------+---------------------+----------------------------------------+
|IPv6 Length      |compute-IPv6-length  |Dedicated function to reconstruct value |
+-----------------+---------------------+----------------------------------------+
|IPv6 Hop Limit   |elided+no matching   |The receiver will put a value stored in |
|                 |                     |the context. It may be different from   |
|                 |                     |one originally sent, but in a star      |
|                 |                     |topology, there is not risk of loops    |
|                 |elided+matching      |Receiver and sender agree on the value. |
|                 |                     |If the value is not correct the packet  |
|                 |                     |the rule is not selected                |
|                 |send-value           |Explicitly sent                         |
+-----------------+---------------------+----------------------------------------+
|IPv6 ESPrefix    |elided               |The 64 bit prefix is stored on the ctxt |
|IPv6 LCPrefix    |send-value           |Explicitly send 64 bits on the link     |
+-----------------+---------------------+----------------------------------------+
|IPv6 ESiid       |elided               |IID is not sent, but stored in the ctxt |
|IPv6 LCiid       |ESiid-DID | LCiid-DID|IID is built from the ES Device ID      |
|                 |send-value           |IID is explicitly sent on the link. The |
|                 |                     |size depends of the L2 technology       |
+-----------------+---------------------+----------------------------------------+
|UDP ESport       |elided               |In the context                          |
|UDP LCport       |send-value           |Send the 2 bytes of the port number     |
|                 |                     |or less if lsb matching is specified in |
|                 |                     |the matching operator.                  |
+-----------------+---------------------+----------------------------------------+
|UDP length       |compute-UDP-length   |Dedicated function to reconstruct value |
+-----------------+---------------------+----------------------------------------+
|UDP Checksum     |compute-UDP-checksum |Dedicated function to reconstruct value |
+-----------------+---------------------+----------------------------------------+

       Figure 4: SCHC functions' example assignment for IPv6 and UDP

   Figure 4 gives an example of function assignment to IPv6/UDP fields.

4.1.  Action functions







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4.1.1.  Elided

   The compressor do not sent the field value on the link.  The
   decompressor restore the field value with the one stored in the
   matched rule.

4.1.2.  Send-value

   The compressor send the field value on the link, if the matching
   operator is "=".  Otherwise the matching operator indicates the
   information that will be sent on the link.  For a LSB operator only
   the Least Significant Bits are sent.

4.1.3.  ESiid-DID, LCiid-DID

   These functions are used to process respectively the End System and
   the LC Device Identifier (DID).  The IID value is computed from
   device ID present in the Layer 2 header.  The computation depends of
   the technology and the device ID size.

5.  Examples

   This section gives some scenarios of the compression mechanism for
   IPv6/UDP.  The goal is to illustrate the SCHC behaviour.

5.1.  IPv6/UDP compression in a star topology

   The most common case will be a LPWA end-system embeds some
   applications running over CoAP.  In this example, the first flow is
   for instance for the device management based on CoAP using Link Local
   addresses and UDP ports 123 and 124.  The second flow will be a CoAP
   server for measurements done by the end-system (using ports 5683) and
   Global Addresses alpha::IID/64 to beta::1/64.  The last flow is for
   legacy applications using different ports numbers, the destination is
   gamma::1/64.

   Figure 5 presents the protocol stack for this end-system.  IPv6 and
   UDP are represented with dotted lines since these protocols are
   compressed on the radio link.  The rule ID is represented by a shim
   id (respectively 0, 1 and 2).











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    Managment    Data
   +----------+---------+---------+
   |   CoAP   |  CoAP   | legacy  |
   +----||----+---||----+---||----+
   .   UDP    .  UDP    |   UDP   |
   ................................
   .   IPv6   .  IPv6   .  IPv6   .
   +--SHIM0------SHIM1-----SHIM2--+
   |      6LPWA L2 technologies   |
   +------------------------------+
         End System or LPWA GW


              Figure 5: Simplified Protocol Stack for LP-WAN

   Note that in some LPWA technologies, only End Systems have a device
   ID . Therefore it is necessary to define statically an IID for the
   Link Local address for the LPWA Compressor.

   +----------------+---------+--------+--------+-------------++------+
   | Field          | Value   | Match  | Match  | Function    || Sent |
   +----------------+---------+-----------------+-------------++------+
   |LPWA SHIM       |0        | No     | =      | send-value  || 0    |
   |ESDevice-ID     |dev-id   | No     | =      | elided      ||      |
   +================+=========+========+========+=============++======+
   |IPv6 version    |6        | =      | No     | elided      ||      |
   |IPv6 DiffServ   |0        | =      | No     | elided      ||      |
   |IPv6 Flow Label |0        | =      | No     | elided      ||      |
   |IPv6 Length     |XXXXXXXXX| No     | No     | comp-IPv6-l ||      |
   |IPv6 Next Header|17       | =      | No     | elided      ||      |
   |IPv6 Hop Limit  |255      | No     | No     | elided      ||      |
   |IPv6 ESprefix   |FE80::/64| =      | No     | elided      ||      |
   |IPv6 ESiid      |         | No     | No     | ESiid-DID   ||      |
   |IPv6 LCprefix   |FE80::/64| =      | No     | elided      ||      |
   |IPv6 LCiid      |::1      | =      | No     | elided      ||      |
   +================+=========+========+========+=============++======+
   |UDP ESport      |123      | =      | No     | elided      ||      |
   |UDP LCport      |124      | =      | No     | elided      ||      |
   |UDP Length      |XXXXXXXXX| No     | No     | comp-UDP-l  ||      |
   |UDP checksum    |XXXXXXXXX| No     | No     | comp-UDP-c  ||      |
   +================+=========+========+========+=============++======+

   +----------------+---------+--------+--------+-------------++------+
   | Field          | Value   | Match  | Match  | Function    || Sent |
   +----------------+---------+-----------------+-------------++------+
   |LPWA SHIM       |1        | No     | =      | send-value  || 1    |
   |ESDevice-ID     |dev-id   | No     | =      | elided      ||      |
   +================+=========+========+========+=============++======+



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   |IPv6 version    |6        | =      | No     | elided      ||      |
   |IPv6 DiffServ   |0        | =      | No     | elided      ||      |
   |IPv6 Flow Label |0        | =      | No     | elided      ||      |
   |IPv6 Length     |XXXXXXXXX| No     | No     | comp-IPv6-l ||      |
   |IPv6 Next Header|17       | =      | No     | elided      ||      |
   |IPv6 Hop Limit  |255      | No     | No     | elided      ||      |
   |IPv6 ESprefix   |alpha/64 | =      | No     | elided      ||      |
   |IPv6 ESiid      |         | No     | No     | ESiid-DID   ||      |
   |IPv6 LCprefix   |beta/64  | =      | No     | elided      ||      |
   |IPv6 LCiid      |::1000   | =      | No     | elided      ||      |
   +================+=========+========+========+=============++======+
   |UDP ESport      |5683     | =      | No     | elided      ||      |
   |UDP LCport      |5683     | =      | No     | elided      ||      |
   |UDP Length      |XXXXXXXXX| No     | No     | comp-UDP-l  ||      |
   |UDP checksum    |XXXXXXXXX| No     | No     | comp-UDP-c  ||      |
   +================+=========+========+========+=============++======+

    +----------------+---------+--------+--------+-------------++------+
   | Field          | Value   | Match  | Match  | Function    || Sent |
   +----------------+---------+-----------------+-------------++------+
   |LPWA SHIM       |2        | No     | =      | send-value  || 2    |
   |ESDevice-ID     |dev-id   | No     | =      | elided      ||      |
   +================+=========+========+========+=============++======+
   |IPv6 version    |6        | =      | No     | elided      ||      |
   |IPv6 DiffServ   |0        | =      | No     | elided      ||      |
   |IPv6 Flow Label |0        | =      | No     | elided      ||      |
   |IPv6 Length     |XXXXXXXXX| No     | No     | comp-IPv6-l ||      |
   |IPv6 Next Header|17       | =      | No     | elided      ||      |
   |IPv6 Hop Limit  |255      | No     | No     | elided      ||      |
   |IPv6 ESprefix   |alpha/64 | =      | No     | elided      ||      |
   |IPv6 ESiid      |         | No     | No     | ESiid-DID   ||      |
   |IPv6 LCprefix   |gamma/64 | =      | No     | elided      ||      |
   |IPv6 LCiid      |::1000   | =      | No     | elided      ||      |
   +================+=========+========+========+=============++======+
   |UDP ESport      |8720     | lsb(4) | No     | elided      || lsb  |
   |UDP LCport      |8720     | lsb(4) | No     | elided      || lsb  |
   |UDP Length      |XXXXXXXXX| No     | No     | comp-UDP-l  ||      |
   |UDP checksum    |XXXXXXXXX| No     | No     | comp-UDP-c  ||      |
   +================+=========+========+========+=============++======+



              Figure 6: Simplified Protocol Stack for LP-WAN

   All the fields described in the three rules Figure 6 are present in
   the IPv6 and UDP headers.  Two fields have been added at the begin,
   they are used to identify the rule id for decompression when the
   other end receives the compressed header.  The shim id is read either



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   from the L2 header or from the first bit in the payload depending on
   the technology.  The ESDevice-ID value is found in the L2 header.

   The second and third rules use global addresses.  The way the ES
   learn the prefix is not in the scope of the document.  One possible
   way is to use a management protocol to set up in both end rules the
   prefix used on the LPWA network.

   The third rule compresses port numbers on 4 bits.  This value is
   selected to maintain alignment on byte boundaries for the compressed
   header.

6.  Acknowledgements

   Thanks to Dominique Barthel, Alexander Pelov, Juan Carlos Zuniga for
   useful design consideration.

7.  Normative 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.

   [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>.

   [RFC4997]  Finking, R. and G. Pelletier, "Formal Notation for RObust
              Header Compression (ROHC-FN)", RFC 4997,
              DOI 10.17487/RFC4997, July 2007,
              <http://www.rfc-editor.org/info/rfc4997>.

   [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>.

Authors' Addresses

   Ana Minaburo
   Acklio
   2bis rue de la Chataigneraie
   35510 Cesson-Sevigne Cedex
   France

   Email: ana@ackl.io



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   Laurent Toutain
   Institut MINES TELECOM ; TELECOM Bretagne
   2 rue de la Chataigneraie
   CS 17607
   35576 Cesson-Sevigne Cedex
   France

   Email: Laurent.Toutain@telecom-bretagne.eu











































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