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Versions: (draft-gundogan-icnrg-ccnlowpan) 00 01 02 03 04

ICN Research Group                                           C. Gundogan
Internet-Draft                                               TC. Schmidt
Intended status: Experimental                                HAW Hamburg
Expires: January 9, 2020                                    M. Waehlisch
                                                    link-lab & FU Berlin
                                                               C. Scherb
                                                               C. Marxer
                                                             C. Tschudin
                                                     University of Basel
                                                            July 8, 2019


             ICN Adaptation to LowPAN Networks (ICN LoWPAN)
                     draft-irtf-icnrg-icnlowpan-03

Abstract

   In this document, a convergence layer for CCNx and NDN over IEEE
   802.15.4 LoWPAN networks is defined.  A new frame format is specified
   to adapt CCNx and NDN packets to the small MTU size of IEEE 802.15.4.
   For that, syntactic and semantic changes to the TLV-based header
   formats are described.  To support compatibility with other LoWPAN
   technologies that may coexist on a wireless medium, the dispatching
   scheme provided by 6LoWPAN is extended to include new dispatch types
   for CCNx and NDN.  Additionally, the link fragmentation component of
   the 6LoWPAN dispatching framework is applied to ICN chunks.  In its
   second part, the document defines stateless and stateful compression
   schemes to improve efficiency on constrained links.  Stateless
   compression reduces TLV expressions to static header fields for
   common use cases.  Stateful compression schemes elide state local to
   the LoWPAN and replace names in data packets by short local
   identifiers.

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."




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   This Internet-Draft will expire on January 9, 2020.

Copyright Notice

   Copyright (c) 2019 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
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview of ICN LoWPAN  . . . . . . . . . . . . . . . . . . .   5
     3.1.  Link-Layer Convergence  . . . . . . . . . . . . . . . . .   5
     3.2.  Stateless Header Compression  . . . . . . . . . . . . . .   6
     3.3.  Stateful Header Compression . . . . . . . . . . . . . . .   7
   4.  IEEE 802.15.4 Adaptation  . . . . . . . . . . . . . . . . . .   7
     4.1.  LoWPAN Encapsulation  . . . . . . . . . . . . . . . . . .   7
     4.2.  Link Fragmentation  . . . . . . . . . . . . . . . . . . .   8
   5.  Space-efficient Message Encoding for NDN  . . . . . . . . . .   9
     5.1.  TLV Encoding  . . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Name TLV Compression  . . . . . . . . . . . . . . . . . .  10
     5.3.  Interest Messages . . . . . . . . . . . . . . . . . . . .  11
     5.4.  Data Messages . . . . . . . . . . . . . . . . . . . . . .  14
   6.  Space-efficient Message Encoding for CCNx . . . . . . . . . .  16
     6.1.  TLV Encoding  . . . . . . . . . . . . . . . . . . . . . .  16
     6.2.  Name TLV Compression  . . . . . . . . . . . . . . . . . .  16
     6.3.  Interest Messages . . . . . . . . . . . . . . . . . . . .  17
     6.4.  Content Objects . . . . . . . . . . . . . . . . . . . . .  23
   7.  Compressed Time Encoding  . . . . . . . . . . . . . . . . . .  26
   8.  Stateful Header Compression . . . . . . . . . . . . . . . . .  27
     8.1.  LoWPAN-local State  . . . . . . . . . . . . . . . . . . .  27
     8.2.  En-route State  . . . . . . . . . . . . . . . . . . . . .  28
     8.3.  Integrating Stateful Header Compression . . . . . . . . .  30
   9.  ICNLoWPAN Constants and Variables . . . . . . . . . . . . . .  30
   10. Implementation Report and Guidance  . . . . . . . . . . . . .  30
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  31
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31
     12.1.  Page Switch Dispatch Type  . . . . . . . . . . . . . . .  31
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  31
     13.2.  Informative References . . . . . . . . . . . . . . . . .  32



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   Appendix A.  Estimated Size Reduction . . . . . . . . . . . . . .  35
     A.1.  NDN . . . . . . . . . . . . . . . . . . . . . . . . . . .  35
       A.1.1.  Interest  . . . . . . . . . . . . . . . . . . . . . .  35
       A.1.2.  Data  . . . . . . . . . . . . . . . . . . . . . . . .  36
     A.2.  CCNx  . . . . . . . . . . . . . . . . . . . . . . . . . .  38
       A.2.1.  Interest  . . . . . . . . . . . . . . . . . . . . . .  38
       A.2.2.  Content Object  . . . . . . . . . . . . . . . . . . .  39
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  40
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  40

1.  Introduction

   The Internet of Things (IoT) has been identified as a promising
   deployment area for Information Centric Networks (ICN), as
   infrastructureless access to content, resilient forwarding, and in-
   network data replication demonstrated notable advantages over the
   traditional host-to-host approach on the Internet [NDN-EXP1],
   [NDN-EXP2].  Recent studies [NDN-MAC] have shown that an appropriate
   mapping to link layer technologies has a large impact on the
   practical performance of an ICN.  This will be even more relevant in
   the context of IoT communication where nodes often exchange messages
   via low-power wireless links under lossy conditions.  In this memo,
   we address the base adaptation of data chunks to such link layers for
   the ICN flavors NDN [NDN] and CCNx.

   The IEEE 802.15.4 [ieee802.15.4] link layer is used in low-power and
   lossy networks (see "LLN" in [RFC7228]), in which devices are
   typically battery-operated and constrained in resources.
   Characteristics of LLNs include an unreliable environment, low
   bandwidth transmissions, and increased latencies.  IEEE 802.15.4
   admits a maximum physical layer packet size of 127 octets.  The
   maximum frame header size is 25 octets, which leaves 102 octets for
   the payload.  IEEE 802.15.4 security features further reduce this
   payload length by up to 21 octets, yielding a net of 81 octets for
   CCNx or NDN packet headers, signatures and content.

   6LoWPAN [RFC4944], [RFC6282] is a convergence layer that provides
   frame formats, header compression and link fragmentation for IPv6
   packets in IEEE 802.15.4 networks.  The 6LoWPAN adaptation introduces
   a dispatching framework that prepends further information to 6LoWPAN
   packets, including a protocol identifier for IEEE 802.15.4 payload
   and meta information about link fragmentation.

   Prevalent Type-Length-Value (TLV) based packet formats such as in
   CCNx and NDN are designed to be generic and extensible.  This leads
   to header verbosity which is inappropriate in constrained
   environments of IEEE 802.15.4 links.  This document presents ICN
   LoWPAN, a convergence layer for IEEE 802.15.4 motivated by 6LoWPAN



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   that compresses packet headers of CCNx as well as NDN and allows for
   an increased payload size per packet.  Additionally, reusing the
   dispatching framework defined by 6LoWPAN enables compatibility
   between coexisting wireless networks of competing technologies.  This
   also allows to reuse the link fragmentation scheme specified by
   6LoWPAN for ICN LoWPAN.

   ICN LoWPAN defines a more space efficient representation of CCNx and
   NDN packet formats.  This syntactic change is described for CCNx and
   NDN separately, as the header formats and TLV encodings differ
   largely.  For further reductions, default header values suitable for
   constrained IoT networks are selected in order to elide corresponding
   TLVs.  Experimental evaluations of the ICN LoWPAN header compression
   schemes in [ICNLOWPAN] illustrate a reduced message overhead, a
   shortened message airtime, and an overall decline in power
   consumption for typical Class 2 devices compared to uncompressed ICN
   messages.

   In a typical IoT scenario (see Figure 1), embedded devices are
   interconnected via a quasi-stationary infrastructure using a border
   router (BR) that uplinks the constrained LoWPAN network by some
   Gateway with the public Internet.  In ICN based IoT networks, non-
   local Interest and Data messages transparently travel through the BR
   up and down between a Gateway and the embedded devices situated in
   the constrained LoWPAN.

                               |Gateway Services|
                               -------------------------
                                     |
                                 ,--------,
                                 |        |
                                 |   BR   |
                                 |        |
                                 '--------'
                                              LoWPAN
                               O            O
                                      O
                             O                O   embedded
                               O      O     O     devices
                                O         O

                        Figure 1: IoT Stub Network

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 RFC 2119 [RFC2119].



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   The use of the term, "silently ignore" is not defined in RFC 2119.
   However, the term is used in this document and can be similarly
   construed.

   This document uses the terminology of [RFC7476], [RFC7927], and
   [RFC7945] for ICN entities.

   The following terms are used in the document and defined as follows:

   ICN LoWPAN:   Information-Centric Networking over Low-power Wireless
                 Personal Area Network

   LLN           Low-Power and Lossy Network

   CCNx:         Content-Centric Networking Architecture

   NDN:          Named Data Networking Architecture

   time-value:   a time value measured in seconds

   time-code:    an 8-bit encoded time-value

3.  Overview of ICN LoWPAN

3.1.  Link-Layer Convergence

   ICN LoWPAN provides a convergence layer that maps ICN packets onto
   constrained link-layer technologies.  This includes features such as
   link-layer fragmentation, protocol separation on the link-layer
   level, and link-layer address mappings.  The stack traversal is
   visualized in Figure 2.

         Device 1                                         Device 2
   ,------------------,           Router            ,------------------,
   |  Application   . |     __________________      | ,-> Application  |
   |----------------|-|    |    NDN / CCNx    |     |-|----------------|
   |  NDN / CCNx    | |    | ,--------------, |     | |    NDN / CCNx  |
   |----------------|-|    |-|--------------|-|     |-|----------------|
   |  ICN LoWPAN    | |    | |  ICN LoWPAN  | |     | |    ICN LoWPAN  |
   |----------------|-|    |-|--------------|-|     |-|----------------|
   |  Link-Layer    | |    | |  Link-Layer  | |     | |    Link-Layer  |
   '----------------|-'    '-|--------------|-'     '-|----------------'
                    '--------'              '---------'

         Figure 2: ICN LoWPAN convergence layer for IEEE 802.15.4

   Section 4 of this document defines the convergence layer for IEEE
   802.15.4.



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3.2.  Stateless Header Compression

   ICN LoWPAN also defines a stateless header compression scheme with
   the main purpose of reducing header overhead of ICN packets.  This is
   of particular importance for link-layers with small MTUs.  The
   stateless compression does not require pre-configuration of global
   state.

   The CCNx and NDN header formats are composed of Type-Length-Value
   (TLV) fields to encode header data.  The advantage of TLVs is its
   native support of variable-sized data.  The main disadvantage of TLVs
   is the verbosity that results from storing the type and length of the
   encoded data.

   The stateless header compression scheme makes use of compact bit
   fields to indicate the presence of mandatory and optional TLVs in the
   uncompressed packet.  The order of set bits in the bit fields
   corresponds to the order of each TLV in the packet.  Further
   compression is achieved by specifying default values and reducing the
   codomain of certain header fields.

   Figure 3 demonstrates the stateless header compression idea.  In this
   example, the first type of the first TLV is removed and the
   corresponding bit in the bit field is set.  The second TLV represents
   a fixed-length TLV (e.g., the Nonce TLV in NDN), so that the type and
   the length fields are removed.  The third TLV represents a boolean
   TLV (e.g., the MustBeFresh selector in NDN) and is missing the type,
   length and the value field.

                +---+---+---+---+---+---+---+---+
                | 1 | 0 | 1 | 0 | 0 | 0 | 0 | 1 |  Bit field
                +---+---+---+---+---+---+---+---+
                  |       |                   |
               ,--'       '-----------,       '- boolean
               |                      |
              +-------+--------------+-------------+
              |  LEN  |     VALUE    |    VALUE    |
              +-------+--------------+-------------+

     Figure 3: Compression using a compact bit field to encode context
                               information.

   Stateless TLV compression for NDN is defined in Section 5.  Section 6
   defines the stateless TLV compression for CCNx.







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3.3.  Stateful Header Compression

   ICN LoWPAN further employs two orthogonal stateful compression
   schemes for packet size reductions which are defined in Section 8.
   These mechanisms rely on shared contexts that are either distributed
   and maintained in the entire LoWPAN, or are generated on-demand hop-
   wise on a particular Interest-data path.

   The shared context identification is defined in Section 8.1.  The
   hop-wise name compression "en-route" is specified in Section 8.2.

4.  IEEE 802.15.4 Adaptation

4.1.  LoWPAN Encapsulation

   The IEEE 802.15.4 frame header does not provide a protocol identifier
   for its payload.  This causes problems of misinterpreting frames when
   several network layers coexist on the same link.  To mitigate errors,
   6LoWPAN defines dispatches as encapsulation headers for IEEE 802.15.4
   frames (see Section 5 of [RFC4944]).  Multiple LoWPAN encapsulation
   headers can prepend the actual payload and each encapsulation header
   is identified by a dispatch type.

   [RFC8025] further specifies dispatch pages to switch between
   different contexts.  When a LoWPAN parser encounters a "Page switch"
   LoWPAN encapsulation header, then all following encapsulation headers
   are interpreted by using a dispatch table as specified by the "Page
   switch" header.  Page 0 and page 1 are reserved for 6LoWPAN.  This
   document uses page 2 ("1111 0010 (0xF2)") for NDN and page 3 ("1111
   0011 (0xF3)") for CCNx.

   The base dispatch format (Figure 4) is used and extended by CCNx and
   NDN in Section 5 and Section 6.

                               0   1   2  ...
                             +---+---+--------
                             | C | M |
                             +---+---+--------

               Figure 4: Base dispatch format for ICN LoWPAN

   C: Compression

       0:          The message is uncompressed.

       1:          The message is compressed.

   M: Message Type



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       0:          The payload contains an Interest message.

       1:          The payload contains a Data message.

   ICN LoWPAN frames with compressed CCNx and NDN messages (C=1) use the
   extended dispatch format in Figure 5.

                               0   1   2  ...
                             +---+---+--------
                             | 1 | M |CID|
                             +---+---+--------

       Figure 5: Extended dispatch format for compressed ICN LoWPAN

   CID: Context Identifier

       0:          No context identifiers are present.

       1:          1..n context identifiers are present.

   The encapsulation format for ICN LoWPAN is displayed in Figure 6.

    +------...------+------...-----+--------+-------...-------+-----...
    | IEEE 802.15.4 | RFC4944 Disp.|  Page  | ICN LoWPAN Disp.| Payl. /
    +------...------+------...-----+--------+-------...-------+-----...

              Figure 6: LoWPAN Encapsulation with ICN-LoWPAN

   IEEE 802.15.4:  The IEEE 802.15.4 header.

   RFC4944 Disp.:  Optional additional dispatches defined in Section 5.1
                   of [RFC4944]

   Page:           Page Switch. 2 for NDN and 3 for CCNx.

   ICN LoWPAN:     Dispatches defined in Section 5 and Section 6.

   Payload:        The actual (un-)compressed CCNx or NDN message.

4.2.  Link Fragmentation

   Small payload sizes in the LoWPAN require fragmentation for various
   network layers.  Therefore, Section 5.3 of [RFC4944] defines a
   protocol-independent fragmentation dispatch type, a fragmentation
   header for the first fragment, and a separate fragmentation header
   for subsequent fragments.  ICN LoWPAN adopts this fragmentation
   handling of [RFC4944].




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   The Fragmentation LoWPAN header can encapsulate other dispatch
   headers.  The order of dispatch types is defined in Section 5 of
   [RFC4944].  Figure 7 shows the fragmentation scheme.  The reassembled
   ICN LoWPAN frame does not contain any fragmentation headers and is
   depicted in Figure 8.

    +------...------+----...----+--------+------...-------+--------...
    | IEEE 802.15.4 | Frag. 1st |  Page  |   ICN LoWPAN   | Payload  /
    +------...------+----...----+--------+------...-------+--------...

    +------...------+----...----+--------...
    | IEEE 802.15.4 | Frag. 2nd | Payload  /
    +------...------+----...----+--------...

                    .
                    .
                    .

    +------...------+----...----+--------...
    | IEEE 802.15.4 | Frag. Nth | Payload  /
    +------...------+----...----+--------...

                      Figure 7: Fragmentation scheme

          +------...------+--------+------...-------+--------...
          | IEEE 802.15.4 |  Page  |   ICN LoWPAN   | Payload  /
          +------...------+--------+------...-------+--------...

                  Figure 8: Reassembled ICN LoWPAN frame

5.  Space-efficient Message Encoding for NDN

5.1.  TLV Encoding

   The NDN packet format consists of TLV fields using the TLV encoding
   that is described in [NDN-PACKET-SPEC].  Type and length fields are
   of variable size, where numbers greater than 252 are encoded using
   multiple octets.

   If the type or length number is less than "253", then that number is
   encoded into the actual type or length field.  If the number is
   greater or equals "253" and fits into 2 octets, then the type or
   lengh field is set to "253" and the number is encoded in the next
   following 2 octets in network byte order, i.e., from the most
   significant byte (MSB) to the least significant byte (LSB).  If the
   number is greater than 2 octets and fits into 4 octets, then the type
   or length field is set to "254" and the number is encoded in the
   subsequent 4 octets in network byte order.  For larger numbers, the



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   type or length field is set to "255" and the number is encoded in the
   subsequent 8 octets in network byte order.

   In this specification, compressed NDN TLVs make use of a different
   TLV encoding scheme that reduces size.  Instead of using the first
   octet as a marker for the number of following octets, the compressed
   NDN TLV scheme uses a method to chain a variable number of octets
   together.  If an octet equals "255 (0xFF)", then the following octet
   will also be interpreted.  The actual value of a chain equals the sum
   of all links.

   If the type or length number is less than "255", then that number is
   encoded into the actual type or length field (Figure 9 a).  If the
   type or length number (X) fits into 2 octets, then the first octet is
   set to "255" and the subsequent octet equals "X mod 255" (Figure 9
   b).  Following this scheme, a variable-sized number (X) is encoded
   using multiple octets of "255" with a trailing octet containing "X
   mod 255" (Figure 9 c).

       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
   a) |   < 255 (X)   | = X
      +-+-+-+-+-+-+-+-+

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   b) |      255      |   < 255 (X)   | = 255 + X
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       0
       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+-+-+-.....-+-+-+-+-+-+-+-+-+-+-+
   c) |      255      |      255      |   < 255 (X)   | = (N * 255) + X
      +-+-+-+-+-+-+-+-+-+-+-.....-+-+-+-+-+-+-+-+-+-+-+
                           (N - 1)

               Figure 9: Compressed NDN TLV encoding scheme

5.2.  Name TLV Compression

   This Name TLV compression encodes length fields of two consecutive
   NameComponent TLVs into one octet, using 4 bits each.  This process
   limits the length of a NameComponent TLV to 15 octets.  A length of 0
   marks the end of the compressed Name TLV.






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                     Name: /HAW/Room/481/Humid/99

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 1 1|0 1 0 0|       H       |       A       |       W       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       R       |       o       |       o       |       m       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |0 0 1 1|0 1 0 1|       4       |       8       |       1       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       H       |       u       |       m       |       i       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       d       |0 0 1 0|0 0 0 0|       9       |       9       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 10: Name TLV compression for /HAW/Room/481/Humid/99

5.3.  Interest Messages

5.3.1.  Uncompressed Interest Messages

   An uncompressed Interest message uses the base dispatch format (see
   Figure 4) and sets the C as well as the M flag to "0" (Figure 11).
   "resv" MUST be set to 0.  The Interest message is handed to the NDN
   network stack without modifications.

                       0   1            ...        7
                     +---+---+-----------------------+
                     | 0 | 0 |         resv          |
                     +---+---+-----------------------+

     Figure 11: Dispatch format for uncompressed NDN Interest messages

5.3.2.  Compressed Interest Messages

   The compressed Interest message uses the extended dispatch format
   (Figure 5) and sets the C flag to "1" and the M flag to "0".  This
   specification assumes the presence of a HopLimit TLV, which will be
   set to a default value of DEFAULT_NDN_HOPLIMIT prior to the
   compression if absent in the original message.  In the default use
   case, the Interest message is compressed with the following minimal
   rule set:

   1.  The "Type" field of the outermost MessageType TLV is removed.

   2.  The Name TLV is compressed according to Section 5.2.  For this,
       all NameComponents are expected to be of type



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       GenericNameComponent.  Otherwise, the message MUST be sent
       uncompressed.

   3.  InterestLifetime TLV is encoded as described in Section 7.  If a
       lifetime is not a valid time-value, then the lifetime is rounded
       up to the nearest valid time-value (see Section 7).

   4.  The Nonce TLV, HopLimit TLV and InterestLifetime TLV MUST be
       moved to the end of the compressed Interest, keeping the order 1)
       Nonce TLV, 2) HopLimit TLV and 3) InterestLifetime TLV.

   5.  The Type and Length fields of Nonce TLV, HopLimit TLV and
       InterestLifetime TLV are elided.  The Nonce value has a length of
       4 octets and the HopLimit value has a length of 1 octet.  The
       compressed InterestLifetime (Section 7) has a length of 1 octet.
       The presence of an InterestLifetime TLV is deduced from the
       remaining length to parse.

   The compressed NDN LoWPAN Interest message is visualized in
   Figure 12.

        T = Type, L = Length, V = Value

        +--------+--------+                    +--------+
        | Msg T  | Msg L  |                    | Msg L  |
        +--------+--------+--------+           +--------+
        | Name T | Name L | Name V |           | Name V |
        +--------+--------+--------+           +--------+--------+
        | CBPfx T| CBPfx L|                    | FWDH L | FWDH V |
        +--------+--------+                    +--------+--------+
        | MBFr T | MBFr L |                    | PRM L  | PRM V  |
        +--------+--------+--------+    ==>    +--------+--------+
        | FWDH T | FWDH L | FWDH V |           | NONC V |
        +--------+--------+--------+           +--------+
        | NONC T | NONC L | NONC V |           | HPL V  |
        +--------+--------+--------+           +--------+
        | ILT T  | ILT L  | ILT V  |           | ILT V  |
        +--------+--------+--------+           +--------+
        | HPL T  | HPL L  | HPL V  |
        +--------+--------+--------+
        | PRM T  | PRM L  | PRM V  |
        +--------+--------+--------+

           Figure 12: Compression of NDN LoWPAN Interest Message

   Further TLV compression is indicated by the ICN LoWPAN dispatch in
   Figure 13.




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                       0   1   2   3   4   5   6   7
                     +---+---+---+---+---+---+---+---+
                     | 1 | 0 |CID|DIG|PFX|FRE|FWD|PRM|
                     +---+---+---+---+---+---+---+---+

      Figure 13: Dispatch format for compressed NDN Interest messages

   CID: Context Identifier  See Figure 5.

   DIG: ImplicitSha256DigestComponent TLV

       0:          The name does not include an
                   ImplicitSha256DigestComponent as the last TLV.

       1:          The name does include an
                   ImplicitSha256DigestComponent as the last TLV.  The
                   Type and Length fields are omitted.

   PFX: CanBePrefix TLV

       0:          The uncompressed message does not include a
                   CanBePrefix TLV.

       1:          The uncompressed message does include a CanBePrefix
                   TLV and is removed from the compressed message.

   FRE: MustBeFresh TLV

       0:          The uncompressed message does not include a
                   MustBeFresh TLV.

       1:          The uncompressed message does include a MustBeFresh
                   TLV and is removed from the compressed message.

   FWD: ForwardingHint TLV

       0:          The uncompressed message does not include a
                   ForwardingHint TLV.

       1:          The uncompressed message does include a
                   ForwardingHint TLV.  The Type field is removed from
                   the compressed message.

   PRM: Parameters TLV

       0:          The uncompressed message does not include a
                   Parameters TLV.




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       1:          The uncompressed message does include a Parameters
                   TLV.  The Type field is removed from the compressed
                   message.

5.4.  Data Messages

5.4.1.  Uncompressed Data Messages

   An uncompressed Data message uses the base dispatch format and sets
   the C flag to "0" and the M flag to "1" (Figure 14). "resv" MUST be
   set to 0.  The Data message is handed to the NDN network stack
   without modifications.

                       0   1            ...        7
                     +---+---+-----------------------+
                     | 0 | 1 |         resv          |
                     +---+---+-----------------------+

       Figure 14: Dispatch format for uncompressed NDN Data messages

5.4.2.  Compressed Data Messages

   The compressed Data message uses the extended dispatch format
   (Figure 5) and sets the C flag as well as the M flag to "1".  By
   default, the Data message is compressed with the following base rule
   set:

   1.  The "Type" field of the outermost MessageType TLV is removed.

   2.  The Name TLV is compressed according to Section 5.2.  For this,
       all NameComponents are expected to be of type
       GenericNameComponent.  Otherwise, the message MUST be sent
       uncompressed.

   3.  The MetaInfo TLV as well as Content TLV Type and Length fields
       are elided from the compressed Data message.

   4.  If present, the FinalBlockId TLV is encoded according to
       Section 5.2.

   5.  The FreshnessPeriod TLV MUST be moved to the end of the
       compressed Data message and the length is set to 1.  Type and
       Length fields are elided and the value is encoded as described in
       Section 7.  If the freshness period is not a valid time-value,
       then the message MUST be sent uncompressed in order to preserve
       the security envelope of the Data message.  The presence of a
       FreshnessPeriod TLV is deduced from the remaining length to
       parse.



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   The compressed NDN LoWPAN Data message is visualized in Figure 15.

        T = Type, L = Length, V = Value

        +--------+--------+                    +--------+
        | Msg T  | Msg L  |                    | Msg L  |
        +--------+--------+--------+           +--------+
        | Name T | Name L | Name V |           | Name V |
        +--------+--------+--------+           +--------+
        | Meta T | Meta L |                    | CTyp V |
        +--------+--------+--------+           +--------+
        | CTyp T | CTyp L | CTyp V |           | FBID V |
        +--------+--------+--------+    ==>    +--------+--------+
        | FrPr T | FrPr L | FrPr V |           | CONT L | CONT V |
        +--------+--------+--------+           +--------+--------+
        | FBID T | FBID L | FBID V |           | Sig L  |
        +--------+--------+--------+           +--------+--------+
        | CONT T | CONT L | CONT V |           | SInf L | SInf V |
        +--------+--------+--------+           +--------+--------+
        | Sig T  | Sig L  |                    | SVal L | SVal V |
        +--------+--------+--------+           +--------+--------+
        | SInf T | SInf L | SInf V |           | FrPr V |
        +--------+--------+--------+           +--------+
        | SVal T | SVal L | SVal V |
        +--------+--------+--------+

             Figure 15: Compression of NDN LoWPAN Data Message

   Further TLV compression is indicated by the ICN LoWPAN dispatch in
   Figure 16.

                       0   1   2   3   4   5   6   7
                     +---+---+---+---+---+---+---+---+
                     | 1 | 1 |CID|DIG|FBI|CON|  SIG  |
                     +---+---+---+---+---+---+---+---+

        Figure 16: Dispatch format for compressed NDN Data messages

   CID: Context Identifier  See Figure 5.

   DIG: ImplicitSha256DigestComponent TLV

       0:          The name does not include an
                   ImplicitSha256DigestComponent as the last TLV.

       1:          The name does include an
                   ImplicitSha256DigestComponent as the last TLV.  The
                   Type and Length fields are omitted.



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   FBI: FinalBlockId TLV

       0:          The uncompressed message does not include a
                   FinalBlockId TLV.

       1:          The uncompressed message does include a FinalBlockId.

   CON: ContentType TLV

       0:          The uncompressed message does not include a
                   ContentType TLV.

       1:          The uncompressed message does include a ContentType
                   TLV.  The Type field is removed from the compressed
                   message.

   SIG: Signature TLV

       00:         The Type fields of the SignatureInfo TLV,
                   SignatureType TLV and SignatureValue TLV are removed.

       01:         Reserved.

       10:         Reserved.

       11:         Reserved.

6.  Space-efficient Message Encoding for CCNx

6.1.  TLV Encoding

   The generic CCNx TLV encoding is described in
   [I-D.irtf-icnrg-ccnxmessages].  Type and Length fields attain the
   common fixed length of 2 octets.

   The TLV encoding for CCNx LoWPAN is changed to the more space
   efficient encoding described in Section 5.1.  Hence NDN and CCNx use
   the same compressed format for writing TLVs.

6.2.  Name TLV Compression

   Name TLVs are compressed using the scheme already defined in
   Section 5.2 for NDN.  If a Name TLV contains T_IPID, T_APP, or
   organizational TLVs, then the name remains uncompressed.







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6.3.  Interest Messages

6.3.1.  Uncompressed Interest Messages

   An uncompressed Interest message uses the base dispatch format (see
   Figure 4) and sets the C as well as the M flag to "0" (Figure 17).
   "resv" MUST be set to 0.  The Interest message is handed to the CCNx
   network stack without modifications.

                       0   1            ...        7
                     +---+---+-----------------------+
                     | 0 | 0 |         resv          |
                     +---+---+-----------------------+

    Figure 17: Dispatch format for uncompressed CCNx Interest messages

6.3.2.  Compressed Interest Messages

   The compressed Interest message uses the extended dispatch format
   (Figure 5) and sets the C flag to "1" and the M flag to "0".  In the
   default use case, the Interest message is compressed with the
   following minimal rule set:

   1.  The Type and Length fields of the CCNx Message TLV are elided and
       are obtained from the Fixed Header on decompression.

   The compressed CCNx LoWPAN Interest message is visualized in
   Figure 18.























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    T = Type, L = Length, V = Value

    +--------------------------+           +--------------------------+
    |  Uncompr. Fixed Header   |           |   Compr. Fixed Header    |
    +--------------------------+           +--------------------------+
    +--------+--------+--------+           +--------+
    | ILT T  | ILT L  | ILT V  |           | ILT V  |
    +--------+--------+--------+           +--------+
    | MSGH T | MSGH L | MSGH V |           | MSGH V |
    +--------+--------+--------+           +--------+
    +--------+--------+                    +--------+
    | MSGT T | MSGT L |                    | Name V |
    +--------+--------+--------+           +--------+
    | Name T | Name L | Name V |    ==>    | KIDR V |
    +--------+--------+--------+           +--------+
    | KIDR T | KIDR L | KIDR V |           | OBHR V |
    +--------+--------+--------+           +--------+--------+
    | OBHR T | OBHR L | OBHR V |           | PAYL L | PAYL V |
    +--------+--------+--------+           +--------+--------+
    | PAYL T | PAYL L | PAYL V |           | VALG L | VALG V |
    +--------+--------+--------+           +--------+--------+
    | VALG T | VALG L | VALG V |           | VPAY L | VPAY V |
    +--------+--------+--------+           +--------+--------+
    | VPAY T | VPAY L | VPAY V |
    +--------+--------+--------+

          Figure 18: Compression of CCNx LoWPAN Interest Message

   Further TLV compression is indicated by the ICN LoWPAN dispatch in
   Figure 19.

       0                                       1
       0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
     | 1 | 0 |CID|VER|FLG|PTY|HPL|FRS|PAY|ILT|MGH|KIR|CHR|VAL|EXT|RSV|
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

     Figure 19: Dispatch format for compressed CCNx Interest messages

   CID: Context Identifier  See Figure 5.

   VER: CCNx protocol version in the fixed header

       0:      The Version field equals 1 and is removed from the fixed
               header.

       1:      The Version field is carried in-line.




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   FLG: Flags field in the fixed header

       0:      The Flags field equals 0 and is removed from the Interest
               message.

       1:      The Flags field is carried in-line.

   PTY: PacketType field in the fixed header

       0:      The PacketType field is elided and assumed to be
               "PT_INTEREST"

       1:      The PacketType field is elided and assumed to be
               "PT_RETURN"

   HPL: HopLimit field in the fixed header

       0:      The HopLimit field is carried in-line

       1:      The HopLimit field is elided and assumed to be "1"

   FRS: Reserved field in the fixed header

       0:      The Reserved field is carried in-line

       1:      The Reserved field is elided and assumed to be "0"

   PAY: Optional Payload TLV

       0:      The Payload TLV is absent.

       1:      The Payload TLV is present and the type field is elided.

   ILT: Optional Hop-By-Hop InterestLifetime TLV

               See Section 6.3.2.1 for further details on the ordering
               of hop-by-hop TLVs.

       0:      No InterestLifetime TLV is present in the Interest
               message.

       1:      An InterestLifetime TLV is present with a fixed length of
               1 octet and is encoded as described in Section 7.  The
               type and length fields are elided.  If a lifetime is not
               a valid time-value, then the lifetime is rounded up to
               the nearest valid time-value (see Section 7).

   MGH: Optional Hop-By-Hop MessageHash TLV



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               See Section 6.3.2.1 for further details on the ordering
               of hop-by-hop TLVs.

               This TLV is expected to contain a T_SHA-256 TLV.  If
               another hash is contained, then the Interest MUST be sent
               uncompressed.

       0:      The MessageHash TLV is absent.

       1:      A T_SHA-256 TLV is present and the type as well as the
               length fields are removed.  The length field is assumed
               to represent 32 octets.  The outer Message Hash TLV is
               omitted.

   KIR: Optional KeyIdRestriction TLV

               This TLV is expected to contain a T_SHA-256 TLV.  If
               another hash is contained, then the Interest MUST be sent
               uncompressed.

       0:      The KeyIDRestriction TLV is absent.

       1:      A T_SHA-256 TLV is present and the type as well as the
               length fields are removed.  The length field is assumed
               to represent 32 octets.  The outer KeyIdRestriction TLV
               is omitted.

   CHR: Optional ContentObjectHashRestriction TLV

               This TLV is expected to contain a T_SHA-256 TLV.  If
               another hash is contained, then the Interest MUST be sent
               uncompressed.

       0:      The ContentObjectHashRestriction TLV is absent.

       1:      A T_SHA-256 TLV is present and the type as well as the
               length fields are removed.  The length field is assumed
               to represent 32 octets.  The outer
               ContentObjectHashRestriction TLV is omitted.

   VAL: Optional ValidationAlgorithm and ValidationPayload TLVs

       0:      No validation related TLVs are present in the Interest
               message.

       1:      Validation related TLVs are present in the Interest
               message.  An additional octet follows immediately that




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               handles validation related TLV compressions and is
               described in Section 6.3.2.2.

   EXT: Extension

       0:      No extension octet follows.

       1:      An extension octet follows immediately.  Extension octets
               are used to extend the compression scheme, but are out of
               scope of this document.

   RSV: Reserved  Must be set to 0.

6.3.2.1.  Hop-By-Hop Header TLVs Compression

   Hop-By-Hop Header TLVs are unordered.  For an Interest message, two
   optional Hop-By-Hop Header TLVs are defined in
   [I-D.irtf-icnrg-ccnxmessages], but several more can be defined in
   higher level specifications.  For a compressed representation, this
   document defines the following ordering of Hop-By-Hop TLVs:

   1.  Interest Lifetime TLV

   2.  Message Hash TLV

   Note: If the original Interest message includes Hop-By-Hop Header
   TLVs that follow a different ordering, then the message MUST be sent
   uncompressed.

6.3.2.2.  Validation

     0       1       2       3       4       5       6       7       8
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |         ValidationAlg         |     KeyID     |   Reserved    |
     +-------+-------+-------+-------+-------+-------+-------+-------+

               Figure 20: Dispatch for Interset Validations

   ValidationALg: Optional ValidationAlgorithm TLV

       0000:   An uncompressed ValidationAlgorithm TLV is included.

       0001:   A T_CRC32C ValidationAlgorithm TLV is assumed, but no
               ValidationAlgorithm TLV is included.

       0010:   A T_CRC32C ValidationAlgorithm TLV is assumed, but no
               ValidationAlgorithm TLV is included.  Additionally, a
               Sigtime TLV is inlined without a type and a length field.



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       0011:   A T_HMAC-SHA256 ValidationAlgorithm TLV is assumed, but
               no ValidationAlgorithm TLV is included.

       0100:   A T_HMAC-SHA256 ValidationAlgorithm TLV is assumed, but
               no ValidationAlgorithm TLV is inclued.  Additionally, a
               Sigtime TLV is inlined without a type and a length field.

       0101:   Reserved.

       0110:   Reserved.

       0111:   Reserved.

       1000:   Reserved.

       1001:   Reserved.

       1010:   Reserved.

       1011:   Reserved.

       1100:   Reserved.

       1101:   Reserved.

       1110:   Reserved.

       1111:   Reserved.

   KeyID: Optional KeyID TLV within the ValidationAlgorithm TLV

       00:     The KeyId TLV is absent.

       01:     The KeyId TLV is present and uncompressed.

       10:     A T_SHA-256 TLV is present and the type field as well as
               the length fields are removed.  The length field is
               assumed to represent 32 octets.  The outer KeyId TLV is
               omitted.

       11:     A T_SHA-512 TLV is present and the type field as well as
               the length fields are removed.  The length field is
               assumed to represent 64 octets.  The outer KeyId TLV is
               omitted.

   The ValidationPayload TLV is present if the ValidationAlgorithm TLV
   is present.  The type field is omitted.




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6.4.  Content Objects

6.4.1.  Uncompressed Content Objects

   An uncompressed Content object uses the base dispatch format (see
   Figure 4) and sets the C flag to "0" and the M flag to "1"
   (Figure 21). "resv" MUST be set to 0.  The Content object is handed
   to the CCNx network stack without modifications.

                       0   1            ...        7
                     +---+---+-----------------------+
                     | 0 | 1 |         resv          |
                     +---+---+-----------------------+

     Figure 21: Dispatch format for uncompressed CCNx Content objects

6.4.2.  Compressed Content Objects

   The compressed Content object uses the extended dispatch format
   (Figure 5) and sets the C flag as well as the M flag to "1".  By
   default, the Content object is compressed with the following base
   rule set:

   1.  The PacketType field is elided from the Fixed Header.

   2.  The Type and Length fields of the CCNx Message TLV are elided and
       are obtained from the Fixed Header on decompression.

   The compressed CCNx LoWPAN Data message is visualized in Figure 22.






















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    T = Type, L = Length, V = Value

    +--------------------------+           +--------------------------+
    |  Uncompr. Fixed Header   |           |   Compr. Fixed Header    |
    +--------------------------+           +--------------------------+
    +--------+--------+--------+           +--------+
    | RCT T  | RCT L  | RCT V  |           | RCT V  |
    +--------+--------+--------+           +--------+--------+
    | MSGH T | MSGH L | MSGH V |           | MSGH L | MSGH V |
    +--------+--------+--------+           +--------+--------+
    +--------+--------+                    +--------+
    | MSGT T | MSGT L |                    | Name V |
    +--------+--------+--------+           +--------+
    | Name T | Name L | Name V |    ==>    | EXPT V |
    +--------+--------+--------+           +--------+--------+
    | PTYP T | PTYP L | PTYP V |           | PAYL L | PAYL V |
    +--------+--------+--------+           +--------+--------+
    | EXPT T | EXPT L | EXPT V |           | VALG L | VALG V |
    +--------+--------+--------+           +--------+--------+
    | PAYL T | PAYL L | PAYL V |           | VPAY L | VPAY V |
    +--------+--------+--------+           +--------+--------+
    | VALG T | VALG L | VALG V |
    +--------+--------+--------+
    | VPAY T | VPAY L | VPAY V |
    +--------+--------+--------+

            Figure 22: Compression of CCNx LoWPAN Data Message

   Further TLV compression is indicated by the ICN LoWPAN dispatch in
   Figure 23.

       0                                       1
       0   1   2   3   4   5   6   7   8   9   0   1   2   3   4   5
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
     | 1 | 1 |CID|VER|FLG|FRS|PAY|RCT|MGH| PLTYP |EXP|VAL|EXT|  RSV  |
     +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

      Figure 23: Dispatch format for compressed CCNx Content objects

   CID: Context Identifier  See Figure 5.

   VER: CCNx protocol version in the fixed header

       0:      The Version field equals 1 and is removed from the fixed
               header.

       1:      The Version field is carried in-line.




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   FLG: Flags field in the fixed header  See Section 6.3.2.

   FRS: Reserved field in the fixed header  See Section 6.3.2.

   PAY: Optional Payload TLV  See Section 6.3.2.

   RCT: Optional Hop-By-Hop RecommendedCacheTime TLV

       0:      The Recommended Cache Time TLV is absent.

       1:      The Recommended Cache Time TLV is present and the type as
               well as the length fields are elided.

   MGH: Optional Hop-By-Hop MessageHash TLV

               See Section 6.4.2.1 for further details on the ordering
               of hop-by-hop TLVs.

               This TLV is expected to contain a T_SHA-256 TLV.  If
               another hash is contained, then the Content Object MUST
               be sent uncompressed.

       0:      The MessageHash TLV is absent.

       1:      A T_SHA-256 TLV is present and the type as well as the
               length fields are removed.  The length field is assumed
               to represent 32 octets.  The outer Message Hash TLV is
               omitted.



       PLTYP: Optional PayloadType TLV

           00:     The PayloadType TLV is absent.

           01:     The PayloadType TLV is absent and T_PAYLOADTYPE_DATA
                   is assumed.

           10:     The PayloadType TLV is absent and T_PAYLOADTYPE_KEY
                   is assumed.

           11:     The PayloadType TLV is present and uncompressed.

   EXP: Optional ExpiryTime TLV

       0:      The ExpiryTime TLV is absent.





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       1:      The ExpiryTime TLV is present and the type as well as the
               length fields are elided.

   RSV: Reserved  Must be set to 0.

   VAL: Optional ValidationAlgorithm and ValidationPayload TLVs  See Sec
       tion 6.3.2.

   EXT: Extension  See Section 6.3.2.

6.4.2.1.  Hop-By-Hop Header TLVs Compression

   Hop-By-Hop Header TLVs are unordered.  For a Content Object message,
   two optional Hop-By-Hop Header TLVs are defined in
   [I-D.irtf-icnrg-ccnxmessages], but several more can be defined in
   higher level specifications.  For better compression, an ordering of
   Hop-By-Hop TLVs is required as follows:

   1.  Recommended Cache Time TLV

   2.  Message Hash TLV

   With this ordering in place, Type fields are elided from the
   Recommended Cache Time TLV and Message Hash TLV.

   Note: If the original Content Object message includes Hop-By-Hop
   Header TLVs with a different ordering, then they remain uncompressed.

7.  Compressed Time Encoding

   This document defines a compressed TLV encoding format for time-
   values that is inspired from [RFC5497]. 8-bit time-codes are used to
   represent time-values ranging from milliseconds to days.

   Time-codes are constructed using the formula:

   time-code := 8 * b + a

   where a is the mantissa and b the exponent of a time-value that
   follows the form:

   time-value := (1 + a/8) * 2^b * C

   The least significant 3 bits of a time-code represents the mantissa
   (a) and the most significant 5 bits represent the exponent (b).  C is
   set to 1/1024 seconds in order to achieve a millisecond resolution.





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   A time-code of all-bits zero MUST be decoded as a time-value of all-
   bits zero.  The smallest representable time-value is thus 0 (a=0,
   b=0), the second smallest is ~1 ms (a=1, b=0), and the largest time-
   value is ~45 days (a=7, b=31).

   An invalid time-value (t, in seconds) MUST be rounded up to the
   nearest valid time-value using this algorithm:

   o  set b := floor(log2(t/C))

   o  set a := 8 * (t / (C * 2^b) - 1)

8.  Stateful Header Compression

   Stateful header compression in ICN LoWPAN enables packet size
   reductions in two ways.  First, common information that is shared
   throughout the local LoWPAN may be memorized in context state at all
   nodes and ommitted from communication.  Second, redundancy in a
   single Interest-data exchange may be removed from ICN stateful
   forwarding on a hop-by-hop bases and memorized in en-route state
   tables.

8.1.  LoWPAN-local State

   A context identifier (CID) is an octet that refers to a particular
   conceptual context between network devices and MAY be used to replace
   frequently appearing information, like name prefixes, suffixes, or
   meta information, such as Interest lifetime.

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

                      Figure 24: Context Identifier.

   The ContextID refers to a locally-scoped unique identifyer that
   represents contextual state shared between sender and receiver of the
   corresponding frame (see Figure 24).

   The initial distribution and maintenance of shared context is out of
   scope of this document.  Frames containing unknown or invalid CIDs
   MUST be silently discarded.








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8.2.  En-route State

   In CCNx and NDN, Name TLVs are included in Interest messages, and
   they return in data messages.  Returning Name TLVs either equal the
   original Name TLV, or they contain the original Name TLV as a prefix.
   ICN LoWPAN reduces this redundancy in responses by replacing Name
   TLVs with single octets that represent link-local HopIDs.  HopIDs are
   carried as Context Identifiers of link-local scope as shown in
   Figure 25.

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

                  Figure 25: Context Identifier as HopID.

   A HopID is valid, if not all ID bits are set to zero and invalid
   otherwise.  This yields 127 distinct HopIDs.  If this range (1...128)
   is exhausted, the messages MUST be sent without en-route state
   compression until new HopIDs are available.  An ICN LoWPAN node that
   forwards without replacing the name by a HopID (without en-route
   compression) MUST invalidate the HopID by setting all ID-bits to
   zero.

   While an Interest is traversing, a forwarder generates an ephemeral
   HopID that is tied to a PIT entry.  Each HopID MUST be unique within
   the local PIT and only exists during the lifetime of a PIT entry.  To
   maintain HopIDs, the local PIT is extended by two new columns: HIDi
   (inbound HopIDs) and HIDo (outbound HopIDs).

   HopIDs are included in Interests and stored on the next hop with the
   resulting PIT entry in the HIDi column.  The HopID is replaced with a
   newly generated local HopID before the Interest is forwarded.  This
   new HopID is stored in the HIDo column of the local PIT (see
   Figure 26).















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       PIT of B      PIT Extension          PIT of C      PIT Extension
   +--------+------++------+------+     +--------+------++------+------+
   | Prefix | Face || HIDi | HIDo |     | Prefix | Face || HIDi | HIDo |
   +========+======++======+======+     +========+======++======+======+
   |  /p0   | F_A  || h_A  | h_B  |     |  /p0   | F_A  || h_A  |      |
   +--------+------++------+------+     +--------+------++------+------+
                       ^       |                            ^
                 store |       '----------------------, ,---' store
                       |                 send         v |
   ,---,         /p0, h_A          ,---,         /p0, h_B          ,---,
   | A | ------------------------> | B | ------------------------> | C |
   '---'                           '---'                           '---'

         Figure 26: Setting compression state en-route (Interest).

   Responses include HopIDs that were obtained from Interests.  If the
   returning Name TLV equals the original Name TLV, then the name is
   entirely elided.  Otherwise, the distinct suffix is included along
   with the HopID.  When a response is forwarded, the contained HopID is
   extracted and used to match against the correct PIT entry by
   performing a lookup on the HIDo column.  The HopID is then replaced
   with the corresponding HopID from the HIDi column prior to forwarding
   the reponse (Figure 27).

       PIT of B      PIT Extension          PIT of C      PIT Extension
   +--------+------++------+------+     +--------+------++------+------+
   | Prefix | Face || HIDi | HIDo |     | Prefix | Face || HIDi | HIDo |
   +========+======++======+======+     +========+======++======+======+
   |  /p0   | F_A  || h_A  | h_B  |     |  /p0   | F_A  || h_A  |      |
   +--------+------++------+------+     +--------+------++------+------+
                       |       ^                            |
                  send |       '----------------------, ,---' send
                       v                 match        | v
   ,---,              h_A          ,---,              h_B          ,---,
   | A | <------------------------ | B | <------------------------ | C |
   '---'                           '---'                           '---'

         Figure 27: Eliding Name TLVs using en-route state (data).

   It should be noted that each forwarder of an Interest in an ICN
   LoWPAN network can individuall decide whether to paricipate in en-
   route compression or not.  However, an ICN LoWPAN node SHOULD use en-
   route compression whenever the stateful compression mechanism is
   activated.

   Note also that the extensions of the PIT data structure are required
   only at ICN LoWPAN nodes, while regular NDN/CCNx forwarders outside
   of an ICN LoWPAN domain do not need to implement these extensions.



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8.3.  Integrating Stateful Header Compression

   A CID appears whenever the CID flag is set (see Figure 5).  The CID
   is appended to the last ICN LoWPAN dispatch octet as shown in
   Figure 28.

          ...-------+--------+-------...-------+--...-+-------...
          /  ...    |  Page  | ICN LoWPAN Disp.| CIDs | Payload /
          ...-------+--------+-------...-------+--...-+-------...

         Figure 28: LoWPAN Encapsulation with ICN LoWPAN and CIDs

   Multiple CIDs are chained together, with the most significant bit
   indicating the presence of a subsequent CID (Figure 29).

       +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+
       |1|     CID     | --> |1|     CID     | --> |0|     CID     |
       +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+     +-+-+-+-+-+-+-+-+

                Figure 29: Chaining of context identifiers.

   The HopID is always included as the very first CID.

9.  ICNLoWPAN Constants and Variables

   This is a summary of all ICNLoWPAN constants and variables.

   DEFAULT_NDN_HOPLIMIT:  255

10.  Implementation Report and Guidance

   The ICN LoWPAN scheme defined in this document has been implemented
   as an extension of the NDN/CCNx software stack [CCN-LITE] in its IoT
   version on RIOT [RIOT].  An experimental evaluation with varying
   configurations is performed in [ICNLOWPAN].

   The header compression performance depends on certain aspects and
   configurations.  It works best for the following cases:

   o  Each name component is of GenericNameComponent type and is limited
      to a length of 15 bytes.

   o  Time-values for content freshness TLVs represent valid time-values
      as per Section 7.  Interest lifetimes will round up to the nearest
      valid encoded time-value.

   o  Contextual state is distributed, such that long names are elided
      from Interest and data messages.



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11.  Security Considerations

   Main memory is typically a scarce resource of constrained networked
   devices.  Fragmentation as described in this memo preserves fragments
   and purges them only after a packet is reassembled, which requires a
   buffering of all fragments.  This scheme is able to handle fragments
   for distinctive packets simultaneously, which can lead to overflowing
   packet buffers which cannot hold all necessary fragments for packet
   reassembly.  Implementers are thus urged to make use of appropriate
   buffer replacement strategies for fragments.

   The stateful header compression generates ephemeral HopIDs for
   incoming and outgoing Interests and consumes them on returning Data
   packets.  Forged Interests can deplete the number of available
   HopIDs, thus leading to a denial of compression service for
   subsequent content requests.

   To further alleviate the problems caused by forged fragments or
   Interest initiations, proper protective mechanisms for accessing the
   link-layer should be deployed.

12.  IANA Considerations

12.1.  Page Switch Dispatch Type

   This document makes use of "Page 2" from the existing paging
   dispatches in [RFC8025].

13.  References

13.1.  Normative References

   [ieee802.15.4]
              "IEEE Std. 802.15.4-2015", April 2016,
              <https://standards.ieee.org/findstds/
              standard/802.15.4-2015.html>.

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

   [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>.





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   [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>.

13.2.  Informative References

   [CCN-LITE]
              "CCN-lite: A lightweight CCNx and NDN implementation",
              <http://ccn-lite.net/>.

   [I-D.irtf-icnrg-ccnxmessages]
              Mosko, M., Solis, I., and C. Wood, "CCNx Messages in TLV
              Format", draft-irtf-icnrg-ccnxmessages-09 (work in
              progress), January 2019.

   [I-D.irtf-icnrg-ccnxsemantics]
              Mosko, M., Solis, I., and C. Wood, "CCNx Semantics",
              draft-irtf-icnrg-ccnxsemantics-10 (work in progress),
              January 2019.

   [ICNLOWPAN]
              Gundogan, C., Kietzmann, P., Schmidt, TC., and M.
              Waehlisch, "ICNLoWPAN -- Named-Data Networking in Low
              Power IoT Networks", Proc. of 18th IFIP Networking
              Conference , May 2019.

   [NDN]      Jacobson, V., Smetters, D., Thornton, J., and M. Plass,
              "Networking Named Content", 5th Int. Conf. on emerging
              Networking Experiments and Technologies (ACM CoNEXT),
              2009, <https://doi.org/10.1145/1658939.1658941>.

   [NDN-EXP1]
              Baccelli, E., Mehlis, C., Hahm, O., Schmidt, TC., and M.
              Waehlisch, "Information Centric Networking in the IoT:
              Experiments with NDN in the Wild", Proc. of 1st ACM Conf.
              on Information-Centric Networking (ICN-2014) ACM DL, pp.
              77-86, September 2014,
              <http://dx.doi.org/10.1145/2660129.2660144>.

   [NDN-EXP2]
              Gundogan, C., Kietzmann, P., Lenders, M., Petersen, H.,
              Schmidt, TC., and M. Waehlisch, "NDN, CoAP, and MQTT: A
              Comparative Measurement Study in the IoT", Proc. of 5th
              ACM Conf. on Information-Centric Networking (ICN-2018) ACM
              DL, pp. 159-171, September 2018,
              <https://doi.org/10.1145/3267955.3267967>.




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   [NDN-MAC]  Kietzmann, P., Gundogan, C., Schmidt, TC., Hahm, O., and
              M. Waehlisch, "The Need for a Name to MAC Address Mapping
              in NDN: Towards Quantifying the Resource Gain", Proc. of
              4th ACM Conf. on Information-Centric Networking (ICN-
              2017) ACM DL, pp. 36-42, September 2017,
              <https://doi.org/10.1145/3125719.3125737>.

   [NDN-PACKET-SPEC]
              "NDN Packet Format Specification",
              <http://named-data.net/doc/NDN-packet-spec/0.3/>.

   [RFC5497]  Clausen, T. and C. Dearlove, "Representing Multi-Value
              Time in Mobile Ad Hoc Networks (MANETs)", RFC 5497,
              DOI 10.17487/RFC5497, March 2009,
              <https://www.rfc-editor.org/info/rfc5497>.

   [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>.

   [RFC7476]  Pentikousis, K., Ed., Ohlman, B., Corujo, D., Boggia, G.,
              Tyson, G., Davies, E., Molinaro, A., and S. Eum,
              "Information-Centric Networking: Baseline Scenarios",
              RFC 7476, DOI 10.17487/RFC7476, March 2015,
              <https://www.rfc-editor.org/info/rfc7476>.

   [RFC7927]  Kutscher, D., Ed., Eum, S., Pentikousis, K., Psaras, I.,
              Corujo, D., Saucez, D., Schmidt, T., and M. Waehlisch,
              "Information-Centric Networking (ICN) Research
              Challenges", RFC 7927, DOI 10.17487/RFC7927, July 2016,
              <https://www.rfc-editor.org/info/rfc7927>.

   [RFC7945]  Pentikousis, K., Ed., Ohlman, B., Davies, E., Spirou, S.,
              and G. Boggia, "Information-Centric Networking: Evaluation
              and Security Considerations", RFC 7945,
              DOI 10.17487/RFC7945, September 2016,
              <https://www.rfc-editor.org/info/rfc7945>.

   [RFC8025]  Thubert, P., Ed. and R. Cragie, "IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Paging Dispatch",
              RFC 8025, DOI 10.17487/RFC8025, November 2016,
              <https://www.rfc-editor.org/info/rfc8025>.

   [RIOT]     Baccelli, E., Guenes, M., Hahm, O., Schmidt, TC., and M.
              Waehlisch, "RIOT OS: Towards an OS for the Internet of
              Things", Proc. of the 32nd IEEE INFOCOM IEEE Press, pp.
              79-80, April 2013, <http://riot-os.org/>.



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   [TLV-ENC-802.15.4]
              "CCN and NDN TLV encodings in 802.15.4 packets",
              <https://datatracker.ietf.org/meeting/interim-2015-icnrg-
              01/materials/slides-interim-2015-icnrg-1-2>.

   [WIRE-FORMAT-CONSID]
              "CCN/NDN Protocol Wire Format and Functionality
              Considerations", <https://datatracker.ietf.org/meeting/
              interim-2015-icnrg-01/materials/
              slides-interim-2015-icnrg-1-8>.









































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Appendix A.  Estimated Size Reduction

   In the following a theoretical evaluation is given to estimate the
   gains of ICN LoWPAN compared to uncompressed CCNx and NDN messages.

   We assume that "n" is the number of name components, "comps_n"
   denotes the sum of n name component lengths.  We also assume that the
   length of each name component is lower than 16 bytes.  The length of
   the content is given by "clen".  The lengths of TLV components is
   specific to the CCNx or NDN encoding and outlined below.

A.1.  NDN

   The NDN TLV encoding has variable-sized TLV fields.  For simplicity,
   the 1 octet form of each TLV component is assumed.  A typical TLV
   component therefore is of size 2 (type field + length field) + the
   actual value.

A.1.1.  Interest

   Figure 30 depicts the size requirements for a basic, uncompressed NDN
   Interest containing a CanBePrefix TLV, a MustBeFresh TLV, a
   InterestLifetime TLV set to 4 seconds and a HopLimit TLV set to 6.
   Numbers below represent the amount of octets.

         ------------------------------------,
         Interest TLV            = 2         |
           ---------------------,            |
           Name                 |  2 +       |
             NameComponents      = 2n +      |
                                |  comps_n   |
           ---------------------'             = 21 + 2n + comps_n
           CanBePrefix           = 2         |
           MustBeFresh           = 2         |
           Nonce                 = 6         |
           InterestLifetime      = 4         |
           HopLimit              = 3         |
         ------------------------------------'

         Figure 30: Estimated size of an uncompressed NDN Interest

   Figure 31 depicts the size requirements after compression.









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         ------------------------------------,
         Dispatch Page Switch    = 1         |
         NDN Interset Dispatch   = 1         |
         Interest TLV            = 1         |
         -----------------------,            |
         Name                   |             = 9 + n/2 + comps_n
           NameComponents        = n/2 +     |
                                |  comps_n   |
         -----------------------'            |
         Nonce                   = 4         |
         HopLimit                = 1         |
         InterestLifetime        = 1         |
         ------------------------------------'

          Figure 31: Estimated size of a compressed NDN Interest

   The size difference is:
   12 + 1.5n octets.

   For the name "/DE/HH/HAW/BT7", the total size gain is 18 octets,
   which is 46% of the uncompressed packet.

A.1.2.  Data

   Figure 32 depicts the size requirements for a basic, uncompressed NDN
   Data containing a FreshnessPeriod as MetaInfo.  A FreshnessPeriod of
   1 minute is assumed and the value is encoded using 1 octet.  An
   HMACWithSha256 is assumed as signature.  The key locator is assumed
   to contain a Name TLV of length klen.






















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        ------------------------------------,
        Data TLV                = 2         |
          ---------------------,            |
          Name                 |  2 +       |
            NameComponents      = 2n +      |
                               |  comps_n   |
          ---------------------'            |
          ---------------------,            |
          MetaInfo             |            |
            FreshnessPeriod     = 6          = 53 + 2n + comps_n +
                               |            |  clen + klen
          ---------------------'            |
          Content               = 2 + clen  |
          ---------------------,            |
          SignatureInfo        |            |
            SignatureType      |            |
              KeyLocator        = 41 + klen |
          SignatureValue       |            |
            DigestSha256       |            |
          ---------------------'            |
        ------------------------------------'

           Figure 32: Estimated size of an uncompressed NDN Data

   Figure 33 depicts the size requirements for the compressed version of
   the above Data packet.

        ------------------------------------,
        Dispatch Page Switch    = 1         |
        NDN Data Dispatch       = 1         |
        -----------------------,            |
        Name                   |             = 37 + n/2 + comps_n +
          NameComponents        = n/2 +     |  clen + klen
                               |  comps_n   |
        -----------------------'            |
        Content                 = 1 + clen  |
        KeyLocator              = 1 + klen  |
        DigestSha256            = 32        |
        FreshnessPeriod         = 1         |
        ------------------------------------'

            Figure 33: Estimated size of a compressed NDN Data

   The size difference is:
   16 + 1.5n octets.

   For the name "/DE/HH/HAW/BT7", the total size gain is 22 octets.




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A.2.  CCNx

   The CCNx TLV encoding defines a 2-octet encoding for type and length
   fields, summing up to 4 octets in total without a value.

A.2.1.  Interest

   Figure 34 depicts the size requirements for a basic, uncompressed
   CCNx Interest.  No Hop-By-Hop TLVs are included, the protocol version
   is assumed to be 1 and the reserved field is assumed to be 0.  A
   KeyIdRestriction TLV with T_SHA-256 is included to limit the
   responses to Content Objects containing the specific key.

         ------------------------------------,
         Fixed Header            = 8         |
         Message                 = 4         |
           ---------------------,            |
           Name                 |  4 +        = 56 + 4n + comps_n
             NameSegments        = 4n +      |
                                |  comps_n   |
           ---------------------'            |
           KeyIdRestriction      = 40        |
         ------------------------------------'

        Figure 34: Estimated size of an uncompressed CCNx Interest

   Figure 35 depicts the size requirements after compression.

         ------------------------------------,
         Dispatch Page Switch    = 1         |
         CCNx Interest Dispatch  = 2         |
         Fixed Header            = 3         |
         -----------------------,            |
         Name                   |             = 38 + n/2 + comps_n
           NameSegments          = n/2 +     |
                                |  comps_n   |
         -----------------------'            |
         T_SHA-256               = 32        |
         ------------------------------------'

          Figure 35: Estimated size of a compressed CCNx Interest

   The size difference is:
   18 + 3.5n octets.

   For the name "/DE/HH/HAW/BT7", the size is reduced by 53 octets,
   which is 53% of the uncompressed packet.




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A.2.2.  Content Object

   Figure 36 depicts the size requirements for a basic, uncompressed
   CCNx Content Object containing an ExpiryTime Message TLV, an
   HMAC_SHA-256 signature, the signature time and a hash of the shared
   secret key.  In the fixed header, the protocol version is assumed to
   be 1 and the reserved field is assumed to be 0

     ------------------------------------,
     Fixed Header            = 8         |
     Message                 = 4         |
       ---------------------,            |
       Name                 |  4 +       |
         NameSegments        = 4n +      |
                            |  comps_n   |
       ---------------------'            |
       ExpiryTime            = 12         = 124 + 4n + comps_n + clen
       Payload               = 4 + clen  |
       ---------------------,            |
       ValidationAlgorithm  |            |
         T_HMAC-256          = 56        |
           KeyId            |            |
         SignatureTime      |            |
       ---------------------'            |
       ValidationPayload     = 36        |
     ------------------------------------'

     Figure 36: Estimated size of an uncompressed CCNx Content Object

   Figure 37 depicts the size requirements for a basic, compressed CCNx
   Data.




















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     ------------------------------------,
     Dispatch Page Switch    = 1         |
     CCNx Content Dispatch   = 3         |
     Fixed Header            = 2         |
     -----------------------,            |
     Name                   |            |
       NameSegments          = n/2 +     |
                            |  comps_n    = 89 + n/2 + comps_n + clen
     -----------------------'            |
     ExpiryTime              = 8         |
     Payload                 = 1 + clen  |
     T_HMAC-SHA256           = 32        |
     SignatureTime           = 8         |
     ValidationPayload       = 34        |
     ------------------------------------'

        Figure 37: Estimated size of a compressed CCNx Data Object

   The size difference is:
   35 + 3.5n octets.

   For the name "/DE/HH/HAW/BT7", the size is reduced by 70 octets,
   which is 40% of the uncompressed packet containing a 4-octet payload.

Acknowledgments

   This work was stimulated by fruitful discussions in the ICNRG
   research group and the communities of RIOT and CCNlite.  We would
   like to thank all active members for constructive thoughts and
   feedback.  In particular, the authors would like to thank (in
   alphabetical order) Peter Kietzmann, Dirk Kutscher, Martine Lenders.
   The hop-wise stateful name compression was brought up in a discussion
   by Dave Oran, which is gratefully acknowledged.  Larger parts of this
   work are inspired by [RFC4944] and [RFC6282].  Special mentioning
   goes to Mark Mosko as well as G.Q.  Wang and Ravi Ravindran as their
   previous work in [TLV-ENC-802.15.4] and [WIRE-FORMAT-CONSID] provided
   a good base for our discussions on stateless header compression
   mechanisms.  This work was supported in part by the German Federal
   Ministry of Research and Education within the projects I3 and
   RAPstore.

Authors' Addresses









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   Cenk Gundogan
   HAW Hamburg
   Berliner Tor 7
   Hamburg  D-20099
   Germany

   Phone: +4940428758067
   EMail: cenk.guendogan@haw-hamburg.de
   URI:   http://inet.haw-hamburg.de/members/cenk-gundogan


   Thomas C. Schmidt
   HAW Hamburg
   Berliner Tor 7
   Hamburg  D-20099
   Germany

   EMail: t.schmidt@haw-hamburg.de
   URI:   http://inet.haw-hamburg.de/members/schmidt


   Matthias Waehlisch
   link-lab & FU Berlin
   Hoenower Str. 35
   Berlin  D-10318
   Germany

   EMail: mw@link-lab.net
   URI:   http://www.inf.fu-berlin.de/~waehl


   Christopher Scherb
   University of Basel
   Spiegelgasse 1
   Basel  CH-4051
   Switzerland

   EMail: christopher.scherb@unibas.ch


   Claudio Marxer
   University of Basel
   Spiegelgasse 1
   Basel  CH-4051
   Switzerland

   EMail: claudio.marxer@unibas.ch




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   Christian Tschudin
   University of Basel
   Spiegelgasse 1
   Basel  CH-4051
   Switzerland

   EMail: christian.tschudin@unibas.ch












































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