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Versions: (draft-bormann-6lo-rpl-mesh) 00 01 02

6lo                                                      P. Thubert, Ed.
Internet-Draft                                                     Cisco
Updates: 6282 (if approved)                                   C. Bormann
Intended status: Standards Track                                     TZI
Expires: April 27, 2015                                 October 24, 2014


               A compression mechanism for the RPL option
                      draft-thubert-6lo-rpl-nhc-02

Abstract

   This specification defines the RPL Packet Information (RPI) NHC
   compression, a method to compress RPL Option (RFC6553) information
   within 6LoWPAN-style ("6lo") adaptation layers.  This extends 6LoWPAN
   Header Compression (RFC6282), saving up to 48 bits in each frame
   compared to the uncompressed form in RFC 6553.

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
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   This Internet-Draft will expire on April 27, 2015.

Copyright Notice

   Copyright (c) 2014 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
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   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Updating RFC 6282 . . . . . . . . . . . . . . . . . . . . . .   4
   4.  The RPL Packet Information NHC  . . . . . . . . . . . . . . .   4
     4.1.  Compressing the RPLInstanceId . . . . . . . . . . . . . .   5
     4.2.  Compressing the SenderRank  . . . . . . . . . . . . . . .   5
     4.3.  The RPI_NHC encoding  . . . . . . . . . . . . . . . . . .   5
       4.3.1.  The Greedy Approach . . . . . . . . . . . . . . . . .   7
       4.3.2.  The Conservative Approach . . . . . . . . . . . . . .   7
       4.3.3.  The Efficient Approach  . . . . . . . . . . . . . . .   8
         4.3.3.1.  The NHC escape mechanism  . . . . . . . . . . . .   8
         4.3.3.2.  RPI_NHC Encoding  . . . . . . . . . . . . . . . .   9
         4.3.3.3.  Operation . . . . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   The design of Low Power and Lossy Networks (LLNs) is generally
   focused on saving energy, which is the most constrained resource of
   all.  The other constraints, such as the memory capacity and the duty
   cycling of the LLN devices, derive from that primary concern.  Energy
   is typically available from batteries that are expected to last for
   years, or scavenged from the environment in very limited quantities.
   Any protocol that is intended for use in LLNs must be designed with
   the primary concern of saving energy as a strict requirement.

   Controlling the amount of data transmission is one possible venue to
   save energy.  In a number of LLN standards, the frame size is limited
   to much smaller values than the IPv6 maximum transmission unit (MTU)
   of 1280 bytes.  In particular, an LLN that relies on the classical
   Physical Layer (PHY) of IEEE 802.14.5 [IEEE802154] is limited to 127
   bytes per frame.  The need to compress IPv6 packets over IEEE
   802.14.5 led to the 6LoWPAN Header Compression [RFC6282] work
   (6LoWPAN-HC).

   The Routing Protocol for Low Power and Lossy Networks [RFC6550] (RPL)
   is designed to optimize the routing operations in constrained LLNs.



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   As part of this optimization, RPL requires the addition of RPL Packet
   Information (RPI) in every packet, as defined in Section 11.2 of
   [RFC6550].

           ------+---------                            ^
                 |          Internet                   |
                 |                                     | Native IPv6
              +-----+                                  |
              |     | Border Router (RPL Root)    ^    |    ^
              |     |                             |    |    |
              +-----+                             |    |    | IPv6 in
                 |                                |    |    | IPv6 + RPI
           o    o   o    o                        |    |    |
       o o   o  o   o  o  o o   o                 |    |    |
      o  o o  o o    o   o   o  o  o              |    |    |
      o   o    o  o     o  o    o  o  o           |    |    |
     o  o   o  o   o         o   o o              v    v    v
     o          o             o     o
                       LLN

              Figure 1: IP-in-IP Encapsulation within the LLN

   The RPI is used to tag the packet with the RPL Instance ID and other
   information that RPL requires for its operation within the RPL
   domain.  In particular, the SenderRank, which is the scalar metric
   computed by an specialized Objective Function such as [RFC6552],
   indicates the Rank of the sender and is modified at each hop.  The
   SenderRank allows to validate that the packet progresses in the
   expected direction, either upwards or downwards, along the DODAG.

   RPL defines the RPL Option for Carrying RPL Information in Data-Plane
   Datagrams [RFC6553] to transport the RPI, which is carried in an IPv6
   Hop-by-Hop Options Header [RFC2460], typically consuming eight bytes
   per packet.

   6TiSCH [I-D.ietf-6tisch-architecture] specifies the operation of IPv6
   over the TimeSlotted Channel Hopping [I-D.ietf-6tisch-tsch] (TSCH)
   mode of operation of IEEE 802.14.5.  The architecture requires the
   use of both 6LoWPAN HC and RPL over IEEE 802.14.5.  Because it
   inherits the constraints on the frame size from the MAC layer, 6TiSCH
   cannot afford to spend 8 bytes per packet on the RPI.  Hence the
   requirement for a 6LoWPAN header compression of the RPI.

   This specification extends the 6lo adaptation layer framework
   ([RFC4944], [RFC6282]) to carry the same information in a 6LoWPAN RPL
   Packet Information (RPI) NHC Next-header compression (NHC) header,
   usually eliminating the Hop-by-Hop Options Header saving up to six
   bytes per packet.



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2.  Terminology

   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].

   The Terminology used in this document is consistent with and
   incorporates that described in `Terminology in Low power And Lossy
   Networks' [RFC7102] and [RFC6550].

   The term "byte" is used in its now customary sense as a synonym for
   "octet".

3.  Updating RFC 6282

   This specification proposes a new 6LoWPAN Next Header Compression
   (NHC) for the RPL option [RFC6553], called RPI_NHC, to be placed in a
   packet that is compressed per [RFC6282].

   It updates [RFC6282] in that the "necessary property of encoding
   headers using LOWPAN_NHC" becomes that "the immediately preceding
   header must be encoded using either LOWPAN_IPHC, RPI_NHC or
   LOWPAN_NHC".

   Additionally, the necessary property of encoding headers using
   RPI_NHC is that the immediately preceding header must be encoded
   using either LOWPAN_IPHC or LOWPAN_NHC.

   (Discuss: Is this really an update of RFC 6282 or a straightforward
   addition to it?)

4.  The RPL Packet Information NHC

   [RFC6550], Section 11.2, specifies the RPL Packet Information (RPI)
   as a set of fields that are to be added to the IP packets for the
   purpose of Instance Identification, as well as Loop Avoidance and
   Detection.

   [RFC6553] defines an encoding for the RPI as a RPL option located in
   the IPv6 Hop-by-hop Option Header.  The present NHC compression
   mechanism compresses IPv6 Hop-by-hop Headers that contain only that
   RPL option.

   The fields in the RPI include an 'O', an 'R', and an 'F' bit, an
   8-bit RPLInstanceID (with some internal structure), and a 16-bit
   SenderRank.





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   This section defines the format of the RPL Packet Information NHC
   (RPI_NHC) that is used to compress the RPI in 6LoWPAN networks.

4.1.  Compressing the RPLInstanceId

   RPL Instances are discussed in [RFC6550], Section 5.  A number of
   simple use cases will not require more than one instance, and in such
   a case, the instance is expected to be the global Instance 0.  A
   global RPLInstanceID is encoded in a RPLInstanceID field as follows:

       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |0|     ID      |  Global RPLInstanceID in 0..127
      +-+-+-+-+-+-+-+-+

         Figure 2: RPLInstanceID Field Format for Global Instances

   For the particular case of the global Instance 0, the RPLInstanceID
   field is all zeroes.  This specification allows to elide a
   RPLInstanceID field that is all zeroes, and defines a 'I' flag that,
   when set, signals that the field is elided.

4.2.  Compressing the SenderRank

   The SenderRank is the result of the DAGRank operation on the rank of
   the sender; here the DAGRank operation is defined in [RFC6550],
   Section 3.5.1, as:

      DAGRank(rank) = floor(rank/MinHopRankIncrease)

   If MinHopRankIncrease is set to a multiple of 256, the least
   significant 8 bits of the SenderRank will be all zeroes; by eliding
   those, the SenderRank can be compressed into a single byte.  This
   idea is used in [RFC6550] by defining DEFAULT_MIN_HOP_RANK_INCREASE
   as 256 and in [RFC6552] that defaults MinHopRankIncrease to
   DEFAULT_MIN_HOP_RANK_INCREASE.

   This specification allows to encode the SenderRank as either one or
   two bytes, and defines a 'K' flag that, when set, signals that a
   single byte is used.

4.3.  The RPI_NHC encoding

   [RFC6553] defines an encoding for the RPL information as a RPL Option
   located in an IPv6 Hop-by-Hop Option Header.  The RPI_NHC provides a
   compressed form for that information and is constructed as follows:





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   The RPI_NHC is immediately followed by the RPLInstanceID, unless that
   is elided (I=1), and then the SenderRank, which is either compressed
   into one byte (K=1) or fully inlined as the whole 2 bytes (K=0).
   Bits in the RPI_NHC indicate whether the RPLInstanceID is elided and/
   or the SenderRank is compressed:

   O, R, and F bits:  The O, R, and F bits as defined in [RFC6550],
         Section 11.2.

   NH:   1-bit flag.  The Next Header (NH) bit is defined in [RFC6282],
         Section 4.2, and it is set to indicate that the next header is
         encoded using LOWPAN_NHC

   I:    1-bit flag.  If it is set, the Instance ID is elided and the
         RPLInstanceID is the Global RPLInstanceID 0.  If it is not set,
         the byte immediately following the RPI_NHC contains the
         RPLInstanceID as specified in [RFC6550], Section 5.1.

   K:    1-bit flag.  If it is set, the SenderRank is compressed into
         one byte, and the least significant byte is elided.  If it is
         not set, the SenderRank is fully inlined as 2 bytes.

   In Figure 3, the RPLInstanceID is the Global RPLInstanceID 0, and the
   MinHopRankIncrease is a multiple of 256 so the least significant byte
   is all zeroes and can be elided:

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | NHC: I=1, K=1 |  SenderRank   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 3: The most compressed RPI_NHC

   In Figure 4, the RPLInstanceID is the Global RPLInstanceID 0, but
   both bytes of the SenderRank are significant so it can not be
   compressed:

       0                   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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | NHC: I=1, K=0 |      SenderRank               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 4: Eliding the RPLInstanceID

   In Figure 5, the RPLInstanceID is not the Global RPLInstanceID 0, and
   the MinHopRankIncrease is a multiple of 256:



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       0                   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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | NHC: I=0, K=1 | RPLInstanceID |  SenderRank   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 5: Compressing SenderRank

   In Figure 6, the RPLInstanceID is not the Global RPLInstanceID 0, and
   both bytes of the SenderRank are significant:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | NHC: I=0, K=0 | RPLInstanceID |      SenderRank               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 6: Least compressed form of RPI_NHC

   The next sections provide alternatives for format of the RPI_NHC.

4.3.1.  The Greedy Approach

   The RPI_NHC is constructed as follows:

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      | 1 | 0 | I | K | O | R | F |NH |
      +---+---+---+---+---+---+---+---+

                   Figure 7: The RPI_NHC, Greedy Version

   Depending on the RPLInstanceID and the MinHopRankIncrease, the
   proposed format thus squeezes the RPL information into 16 to 32 bits,
   which compares to 64 bits when using a Hop-by-hop option with the RPL
   option as specified in [RFC6553].

   (This is called the "greedy" approach as it consumes 1/4 of the NHC
   space just for the RPI compression.)

4.3.2.  The Conservative Approach

   In this approach, the encoding of the RPL Packet Information takes
   two bytes: one byte to indicate the NH type, and then one byte to
   signal the compressed information.






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   The NH type is indicated in an extension to existing LOWPAN_NHC
   encodings.  Section 4.2 of [RFC6553] defines LOWPAN_NHC encodings for
   IPv6 Extension Headers as in Figure 8:

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      | 1 | 1 | 1 | 0 |    EID    |NH |
      +---+---+---+---+---+---+---+---+

                 Figure 8: IPv6 Extension Header Encoding

   Values 5 and 6 of the IPv6 Extension Header ID (EID) are still
   reserved.  This specification uses EID of 5 to indicate that the next
   byte is a RPI_NHC.  The RPI_NHC is constructed as shown in Figure 9:

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      | I | K | O | R | F | reserved  |
      +---+---+---+---+---+---+---+---+

                Figure 9: The RPI_NHC, Conservative Version

   The bits 5 to 7 of the RPI_NHC are reserved for future use and MUST
   be sent as zero.

   (This is called the "conservative" approach as it consumes only 1/256
   of the NHC space.)

4.3.3.  The Efficient Approach

4.3.3.1.  The NHC escape mechanism

   The NHC space of [RFC6282] is limited to 256 code points.  For the
   case some infrequent bit combinations do not fit into the 256 code
   points, this specification assigns four code points:

           0   1   2   3   4   5   6   7
         +---+---+---+---+---+---+---+---+
         | 0 | 1 | 0 | 0 | 0 | 1 | X | Y |
         +---+---+---+---+---+---+---+---+

                        Figure 10: NHC Escape Codes

   Each NHC escape code is followed by a further NHC code point.  The
   latter MUST be a code point for which special semantics for a
   preceding escape code are defined, i.e., an escape code MUST NOT be
   used in front of an NHC code point that does not define special
   semantics for this escape code.



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   An escape code followed by another escape code supplies additional
   semantics; again, a sequence of such escape codes MUST NOT be used
   unless the final NHC code following this sequence defines the
   semantics for the specific sequence.

4.3.3.2.  RPI_NHC Encoding

   The RPI_NHC provides a compressed form for the RPI and is constructed
   as follows:

           0   1   2   3   4   5   6   7
         +---+---+---+---+---+---+---+---+
         | 1 | 0 | 0 | 0 | O | I | K |NH |
         +---+---+---+---+---+---+---+---+

                   Figure 11: RPI NHC, efficient version

   The R and F bits, as defined in [RFC6550], Section 11.2, are
   represented as follows:

   If R=0 and F=0, the NHC code is used as defined above.  If either is
   non-zero, a single escape code with X=R and Y=F is prepended in front
   of the NHC code.  (An escape code with X=0 and Y=0 MUST NOT be used
   with RPI_NHC.  A sequence of two or more escape codes MUST NOT be
   used with RPI_NHC.)

   Depending on the RPLInstanceID and the MinHopRankIncrease, the
   proposed format thus squeezes the RPI into 16 to 40 bits, which
   compares to 64 bits when using a Hop-by-hop option with the RPL
   option as specified in [RFC6553].

   (This is called the "efficient" approach as it consumes only 1/16 of
   the NHC space, but, depending on the frequency of set R or set F
   flags, is almost as efficient as the greedy approach.)

4.3.3.3.  Operation

   A 6lo compressor that is about to create either an RFC 6282 IPHC
   header [RFC6282] or a Frag1 header [RFC4944] and finds a Hop-by-Hop
   Options header [RFC2460] with an RPL Option [RFC6553] in it, performs
   the following checks:

   1.  Does the compression scheme apply?  I.e.:

       A.  is no sub-tlv present in the RPL Option?

       B.  is the RPL Option the only option in the Hop-by-Hop Options
           header?



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   2.  Does the additional compression for I=1 apply?  I.e.: is
       RPLInstanceID == 0?

   3.  Does the additional compression for K=1 apply?  I.e.: is
       SenderRank < 256?

   4.  Is both R=0 and F=0, or do we need an escape code?

   If check 1 succeeds, the compressor removes the Hop-by-Hop Options
   header (replacing the zero-valued next header field in the IPv6
   header with the value of the next header field of the Hop-by-Hop
   Options header), and, depending on the outcome of check 2 and 3,
   generates an RPI_NHC Header with I and K set from the payload
   information in the RPL Option.  If one or both of R and F are non-
   zero (check 4), it precedes the first byte in the RPI_NHC header with
   an escape code with X=R and Y=F.  It then continues generating the
   RFC 6282 IPHC or RFC 4944 Frag1 header, filling in the continuation
   of the RPL Information header as defined in Section 4.3.3.2.

   A 6lo decompressor that encounters a RPL Information header reverses
   this process, creating a Hop-by-Hop Options header with a single RPL
   Option carrying the information in the RPL Information header.

5.  Security Considerations

   The security considerations of [RFC4944], [RFC6282], and [RFC6553]
   apply.

   Using a compressed format as opposed to the full inline RPL option is
   logically equivalent and does not create an opening for a new threat
   when compared to [RFC6553].

6.  IANA Considerations

   (greedy variant:)

   This document updates IANA registry for the LOWPAN_NHC defined in
   [RFC6282] and assigns the previously unassigned:

      10IKORFN: RPL Information [RFCthis]

   Capital letters in bit positions represent class-specific bit
   assignments.  IKORF represents variables specific to RPL Information
   compression defined in Section 4.  N indicates whether or not
   additional LOWPAN_NHC encodings follow, as defined in Section 4.2 of
   [RFC6553].

   (efficient variant:)



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   This draft requests IANA to assign the following LOWPAN_NHC types in
   the "IPv6 Low Power Personal Area Network Parameters" registry:

      010001XY: Escape X=0/Y=0 to X=1/Y=1 [RFCthis]

      1000IOKN: RPL Information [RFCthis]

7.  Acknowledgements

   The author wishes to thank Laurent Toutain for suggesting this work
   and and Martin Turon for his constructive contributions.  Ralph Droms
   supplied a number of helpful comments on the -00 draft of
   [I-D.bormann-6lo-rpl-mesh], which was superseded by the present
   document, turning into the "efficient approach".  The discussion in
   the 6man and roll working groups also was helpful.

8.  References

8.1.  Normative References

   [IEEE802154]
              IEEE standard for Information Technology, "IEEE std.
              802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
              and Physical Layer (PHY) Specifications for Low-Rate
              Wireless Personal Area Networks", June 2011.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

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

   [RFC6550]  Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
              Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
              Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
              Lossy Networks", RFC 6550, March 2012.

   [RFC6552]  Thubert, P., "Objective Function Zero for the Routing
              Protocol for Low-Power and Lossy Networks (RPL)", RFC
              6552, March 2012.







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   [RFC6553]  Hui, J. and JP. Vasseur, "The Routing Protocol for Low-
              Power and Lossy Networks (RPL) Option for Carrying RPL
              Information in Data-Plane Datagrams", RFC 6553, March
              2012.

8.2.  Informative References

   [I-D.bormann-6lo-rpl-mesh]
              Bormann, C., "NHC compression for RPL Packet Information",
              draft-bormann-6lo-rpl-mesh-02 (work in progress), October
              2014.

   [I-D.ietf-6tisch-architecture]
              Thubert, P., Watteyne, T., and R. Assimiti, "An
              Architecture for IPv6 over the TSCH mode of IEEE
              802.15.4e", draft-ietf-6tisch-architecture-03 (work in
              progress), July 2014.

   [I-D.ietf-6tisch-tsch]
              Watteyne, T., Palattella, M., and L. Grieco, "Using
              IEEE802.15.4e TSCH in an IoT context: Overview, Problem
              Statement and Goals", draft-ietf-6tisch-tsch-02 (work in
              progress), October 2014.

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

   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, January 2014.

Authors' Addresses

   Pascal Thubert (editor)
   Cisco Systems
   Village d'Entreprises Green Side
   400, Avenue de Roumanille
   Batiment T3
   Biot - Sophia Antipolis  06410
   FRANCE

   Phone: +33 4 97 23 26 34
   Email: pthubert@cisco.com








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   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

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











































Thubert & Bormann        Expires April 27, 2015                [Page 13]


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