[Search] [txt|xml|pdfized|bibtex] [Tracker] [Email] [Diff1] [Diff2] [Nits]
Versions: 00 01 02                                                      
ICN Research Group                                           C. Gundogan
Internet-Draft                                                T. Schmidt
Intended status: Experimental                                HAW Hamburg
Expires: January 4, 2018                                    M. Waehlisch
                                                    link-lab & FU Berlin
                                                            July 3, 2017


Publish-Subscribe Deployment Option for NDN in the Constrained Internet
                               of Things
                    draft-gundogan-icnrg-pub-iot-01

Abstract

   Constrained IoT devices often operate more efficiently in a loosely
   coupled environment without maintaining end-to-end connectivity
   between nodes.  Information Centric Networking naturally supports
   this demand by replicated data distribution and hop wise forwarding.
   This document outlines a deployment option for NDN in low-power and
   lossy networks (LLNs) that follows a publish-subscribe pattern.  The
   proposed protocol scheme simplifies name-based routing significantly
   and facilitates even large off-duty cycles for constrained nodes.

Status of This Memo

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

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

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

   This Internet-Draft will expire on January 4, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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



Gundogan, et al.         Expires January 4, 2018                [Page 1]


Internet-Draft           NDN Pub-Sub in the IoT                July 2017


   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Baseline Scenarios  . . . . . . . . . . . . . . . . . . .   3
     1.2.  Benefits of Loose Coupling in the IoT . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Publish-Subscribe in IoT Edge Networks  . . . . . . . . . . .   5
     3.1.  Topology Maintenance and Routing  . . . . . . . . . . . .   5
     3.2.  Mapping Publish to NDN  . . . . . . . . . . . . . . . . .   7
     3.3.  Mapping Subscribe to NDN  . . . . . . . . . . . . . . . .   8
     3.4.  Content Replication in partitioned networks . . . . . . .   9
     3.5.  Content Replication between Proxy Instances . . . . . . .   9
     3.6.  Alerting and group communication  . . . . . . . . . . . .   9
   4.  Control Plane Messaging . . . . . . . . . . . . . . . . . . .  10
     4.1.  Prefix Advertisement Message (PAM)  . . . . . . . . . . .  10
     4.2.  Name Advertisement Message (NAM)  . . . . . . . . . . . .  10
       4.2.1.  Name Option . . . . . . . . . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   In the emerging Internet of Things (IoT), it is expected that large
   quantities of very constrained sensors and actuators collect,
   communicate, and process massive amounts of machine data.  Early
   experiments with constrained nodes show promising results for
   different deployments of ICN communication [NDN-EXP].

   Characteristics of constrained nodes:

   o  Battery-powered with sleep cycles

   o  Failing nodes

   o  Low power lossy networks

   Challenges of NDN deployment [RFC7927]

   o  Complexity of name-based routing




Gundogan, et al.         Expires January 4, 2018                [Page 2]


Internet-Draft           NDN Pub-Sub in the IoT                July 2017


   o  State management at nodes

   o  Clear separation between control and data plane

   o  Adaptation to constrained wireless transmission

   o  Mobility management

1.1.  Baseline Scenarios

   Multiple scenarios have been discussed in [RFC7476] and [IWMT] that
   evaluate the applicability of ICN in IoT.

   We consider two characteristic constrained IoT scenarios with the
   enumerated challenges:

   Stationary IoT nodes within reach of fixed uplinks  for home,
      building, and factory automation, stationary monitoring, ...

      *  Reliability, resilience of operation

      *  Radio coordination, coverage

      *  Energy constraints, device lifetime

      *  Interference with rivaling appliances

   Mobile IoT nodes with sparse coverage or intermittent connectivity
      for urban or rural mobility and sensing, industrial Internet in
      widespread environments, disaster recovery and rescue ...

      *  Exploit connectivity when available

      *  Large off-duty cycles of nodes

      *  Partitioned networks

      *  Limited dependability

      *  Environmental impact and disturbance

   IoT scenarios usually impose routing requirements to support mobile
   nodes, handle failing links and to be resilient against attacks.  A
   secure and autonomous bootstrapping is essential, especially for
   large-scale IoT deployments.






Gundogan, et al.         Expires January 4, 2018                [Page 3]


Internet-Draft           NDN Pub-Sub in the IoT                July 2017


1.2.  Benefits of Loose Coupling in the IoT

   ICN decouples content consumers from data producers (decoupling in
   space).  A more sophisticated decoupling can be provided with the
   publish-subscribe messaging pattern that further adds a decoupling in
   time and synchronization.  Constrained devices in LLNs can leverage
   this loose coupling to increase sleep cycles and delegate the
   authority over as much information as possible to more powerful
   devices that act as content proxies.  In Figure 1, once content is
   published to the content proxy (CP) by a producer (P), consumers (C)
   can retrieve this content from (CP) without interacting with the
   producer.  This indirection when retrieving information allows (P) to
   align sleep cycles accordingly to the period of generating new sensor
   readings, instead of handling content requests from any consumers
   (C).

                                 (CP)
                                / | \
                               /  |  `-----.
                              /   |   |  |  \
                            (P)  (C) ...... (C)

        Figure 1: Content Proxy (CP) - Producer (P) - Consumer (C)

   TODO: The problem of PUSH

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].
   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:

   Converge Cast  Many-to-One communication pattern, where multiple
      devices send sensor readings to one gateway.

   Content Proxy  Stable node for replicating content.

   Cloud Gateway  A Gateway that enables content transfer to and from a
      remote cloud storage, possibly by performing some kind of protocol
      translation.



Gundogan, et al.         Expires January 4, 2018                [Page 4]


Internet-Draft           NDN Pub-Sub in the IoT                July 2017


   PAM  Prefix Advertisement Message.

   NAM  Name Advertisement Message.

3.  Publish-Subscribe in IoT Edge Networks

   The publish-subscribe system is centered around prefix-specific
   content proxies (CPs) that are deployed in IoT edge networks.  Such
   proxy function can be hosted on the Cloud- or Internet Gateway, or
   may reside on a stable, less constrained node within the IoT
   infrastructure.  It is assumed that a CP is present for each prefix
   covering publishable content.

   Implementing a pub-sub NDN involves several steps that are bound to
   the tight requirements of resource-constrained devices.  These steps
   include:

   1.  Building the prefix-specific routing topology tailored to
       constrained networks

   2.  Mapping _Publish_ to NDN semantics

   3.  Mapping _Subscribe_ to NDN semantics

3.1.  Topology Maintenance and Routing

   A (sensor) node that wants to publish a data item needs to rely on
   path information towards the Content Proxy.  Following the approach
   of PANINI [PANINI], default routes will be established as follows.

   Each CP in the local IoT sub-network advertises the prefix(es) it
   represents to the routing system.  It does so by broadcasting Prefix
   Advertisement Messages (PAMs) on the link layer (see Section 4 for
   the corresponding protocol details).  Nodes that newly receive PAM
   advertisements will add or refresh a prefix-specific default route in
   their FIB.  Intermediate nodes in a multi-hop environment also re-
   broadcast PAMs, so that the entire sub-network is flooded and default
   route entries build a shortest path tree (SPT) towards the CP as
   shown in Table 1 (alternatively, a DODAG w.r.t.  a gateway for
   redundant CPs).











Gundogan, et al.         Expires January 4, 2018                [Page 5]


Internet-Draft           NDN Pub-Sub in the IoT                July 2017


                                 (CP)
                             PAM /  \
                                /    \ PAM
                              (A)    (B)
                              /|\    /|\
                             : : :  : : : PAM

      Figure 2: SPT building by Prefix Advertisement Messages (PAMs)

   Information flowing from constrained sensor nodes towards a gateway
   is the prevalent communication pattern in the IoT (converge cast).
   The publish-subscribe system hence establishes a default routing (see
   sample FIB in Table 1) and uses the tree topology with default routes
   towards the CP as a first step of content aggregation.  Content
   replication towards other CPs, an Internet gateway, or into a cloud
   can follow subsequently.

                       +--------+------+----------+
                       | Prefix | Face | Lifetime |
                       +--------+------+----------+
                       | /P/    |  Fx  |    Ft    |
                       | ...    | ...  |   ...    |
                       +--------+------+----------+

             Table 1: FIB with a prefix-specific default route

   It is noteworthy that the role of the new PAM message remains
   orthogonal to the existing Interest or Data semantics.  A PAM never
   carries data nor requests, but persists on the control plane of name-
   based routing.  User applications stay unaffected, and continue to
   rely on the NDN-specific request-response paradigm.

   Each node in the tree calculates a rank based on the rank of its
   parent and a routing metric.  Hence, the rank is an indication for
   the position within the tree and is strictly monotonically increasing
   in the downwards direction from root to leaf.  For simplicity, this
   document uses the hop-count routing metric to calculate the rank.
   Future work can focus on more elaborate routing metrics, e.g., to
   reduce packet retransmissions or improve load balancing for publish
   operations.

   The Routing Information Base (RIB) contains the following
   information:

   SPT

      Prefix:




Gundogan, et al.         Expires January 4, 2018                [Page 6]


Internet-Draft           NDN Pub-Sub in the IoT                July 2017


         Variable length prefix to configure a prefix-specific default
         route

      Rank:
         16-bit unsigned integer indicating a node's rank

      Flags:
         8-bit unsigned integer to store SPT related flags

      Parent
         The appropriate face towards the parent node is stored.

   NAM Cache (NC)
      Entries in the NAM Cache are _<name,downstream_face>_ tuples.  The
      NC is consolidated when propagating name advertisements.

3.2.  Mapping Publish to NDN

   In classical publish-subscribe systems, a _Publish_ is typically
   implemented as a push mechanism on the data plane.  However, this
   contradicts the request-response paradigm employed by NDN.  To adapt
   the _Publish_ operation to NDN semantics, it is split into two phases
   and the required push mechanism is performed on the control plane
   with a link-local scope.  The two phases consist of:

   1.  Announcing names of Named Data Objects (NDO) on the control plane

   2.  Requesting NDOs on the data plane

   In the first phase, the name of the NDO to publish is advertised to
   the prefix-specific upstream neighbor by encoding it with TLV format
   in a unicast link-local Name Advertisement Message (NAM) adopted from
   PANINI [PANINI].  Once an upstream neighbor receives an unsolicited
   NAM, the name is extracted and along with the incoming face stored in
   the NAM Cache (NC) as a _<name,downstream_face>_ tuple.  This link-
   local content signaling does not establish any data paths in the PIT,
   nor does it modify the FIB or the Content Store (CS).

   In the second phase and as a result of an unsolicited NAM, the
   content is requested from the downstream neighbor accordingly to the
   standard NDN scheme with an Interest message for the name recorded in
   the NC on the recorded face.  When a downstream neighbor replies with
   the content (Data message), this neighbor removes the corresponding
   NC entry and the NDO to publish is successfully replicated one hop
   closer towards the content proxy.  NC entries are thus short-lived
   for hop-wise replication only.





Gundogan, et al.         Expires January 4, 2018                [Page 7]


Internet-Draft           NDN Pub-Sub in the IoT                July 2017


   Both phases are iteratively repeated hop-wise until the content proxy
   is reached.  Being the root of this prefix-specific spanning tree,
   the content proxy has no further prefix-specific upstream neighbor
   and the replication terminates.  It is noteworthy, that both phases
   can be interleaved, so that NAMs are signaled to the upstream
   neighbor, while the content is requested from the downstream
   neighbor.

   Figure 3 (a) depicts the hop-wise replication of a published content
   on the gradient towards the (CP).  In this example, the name _/HAW/
   temp_t_ is advertised by the producer (P) to its parent node (A).
   (A) extracts the name from the NAM and stores it along with the
   incoming face in its NC.  (A) then propagates the NAM further to its
   parent (CP) and simultaneously requests the content from (P) as
   depicted in Figure 3 (b).  Likewise in Figure 3 (c), (CP) reacts to
   the incoming NAM by requesting the content from (A).  NC states from
   (A) and (P) are removed as soon as they respond to the request.

   +-------------+   +------------------------+   +-------------------+
   |             |   |                        |   | Interest          |
   |    (CP)     |   |            ,->(CP)     |   |    ,----(CP)<-,   |
   |    /        |   |        NAM |  /        |   |    |    /    /    |
   |   /         |   |            | /         |   |    |   /    /     |
   | (A)<-,      |   |          ,-(A)<-,      |   |    `>(A)---' Data |
   |   \  | NAM  |   |          |   \  | Data |   |        \          |
   |    \ |      |   |          |    \ |      |   |         \         |
   |    (P)      |   | Interest `--->(P)      |   |         (P)       |
   | /HAW/temp_t |   |            /HAW/temp_t |   |      /HAW/temp_t  |
   +-------------+   +------------------------+   +-------------------+
    (a) Adv. Name      (b) Replicate Content       (c) Finish publish

                             Figure 3: Publish

3.3.  Mapping Subscribe to NDN

   In the proposed publish-subscribe system, the _Subscribe_ operation
   is equivalent to an Interest-based request of previously learned
   content names.  A device can learn about new content in various ways:

   a.  Name Advertisements of the CP via dedicated prefix path (TODO) or
       broadcast.

   b.  By polling a dedicated data structure at the CP that records
       publishings and updates.  Such a data structure may contain
       indicators about the periodicity of sensor readings to align
       periodic polling schemes to sensor reading intervals.





Gundogan, et al.         Expires January 4, 2018                [Page 8]


Internet-Draft           NDN Pub-Sub in the IoT                July 2017


   c.  By encoding topics into a hierarchical naming scheme of the form
       _routing prefix / topic / unique_part_. Inerest timeouts for
       these names serve as subscription periods and must be refreshed
       or retriggered for every publishing to adhere to the flow control
       approach of NDN / CCNx.

   TODO

3.4.  Content Replication in partitioned networks

   The hop-wise replication described in Section 3.2 transparently
   supports publish operations in partitioned networks.  When a publish
   operation fails to replicate content and no backup parent is in the
   vicinity (Figure 4 (b)), the node marks its sub-DAG as _floating_ by
   propagating PAMs with the floating bit set and the NC entry is
   preserved.  Once a sub-DAG becomes connected to another parent, the
   publishing is resumed (Figure 4 (c)).

        +-------------+       +-------------+       +-------------+
        |             |       |             |       |             |
        |    (CP)     |       |    (CP)     |       |    (CP)<-,  |
        |             |       |             |       |    /    /   |
        |             |       |        X    |       |   /    /    |
        | (A)<-,      |       | (A)---'     |       | (A)---'     |
        |   \  | PUB  |       |   \   PUB   |       |   \   PUB   |
        |    \ |      |       |    \        |       |    \        |
        |    (P)      |       |    (P)      |       |    (P)      |
        | /HAW/temp_t |       | /HAW/temp_t |       | /HAW/temp_t |
        +-------------+       +-------------+       +-------------+
      (a) Publish content    (b) Publish fails    (c) Finish Publish

                 Figure 4: Publish in partitioned network

   A node receiving a PAM from its parent with the floating bit set may
   rejoin the tree using another parent in the neighborhood that is
   connected to the content proxy.  Rejoining the tree may result in a
   worse rank.

3.5.  Content Replication between Proxy Instances

   TODO

3.6.  Alerting and group communication

   TODO






Gundogan, et al.         Expires January 4, 2018                [Page 9]


Internet-Draft           NDN Pub-Sub in the IoT                July 2017


4.  Control Plane Messaging

   TODO: add ABNF

   TODO: add mappings of PAM and NAM to the NDN and CCNx dialects

4.1.  Prefix Advertisement Message (PAM)

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type = PAM   |F|  Reserved   |             Rank              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Prefix Length         |            Prefix ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 5: Prefix Advertisement Message (PAM)

   Message type:
               8-bit unsigned integer.  Indicates a PAM.

   Floating (F):
               1-bit floating flag.  Indicates that a sub-tree is not
               connected to the content proxy, when set.

   Reserved:
               7-bit unused field.  It MUST be initialized to zero by
               the sender and MUST be ignored by the receiver.

   Rank:
               16-bit unsigned integer.  Indicates a node's position in
               the SPT.

   Prefix Length:
               16-bit unsigned integer.  Indicates the length of the
               following prefix in bytes.

   Prefix:
               Variable length prefix used to configure a default route
               within the SPT.

4.2.  Name Advertisement Message (NAM)









Gundogan, et al.         Expires January 4, 2018               [Page 10]


Internet-Draft           NDN Pub-Sub in the IoT                July 2017


      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type = NAM   |   Reserved    |        Options Length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                            Options                            :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 6: Name Advertisement Message (NAM)

   Type:
               8-bit unsigned integer.  Indicates a NAM.

   Reserved:
               8-bit unused field.  It MUST be initialized to zero by
               the sender and MUST be ignored by the receiver.

   Length:
               16-bit unsigned integer.  Indicates the accumulated
               length of all options.

4.2.1.  Name Option

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Type = 0x00  |          Name Length          |   Name ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 7: Name option format

   Type:
               8-bit unsigned integer.  Indicates the name option
               (0x00).

   Name Length:
               16-bit unsigned integer.  Indicates the length of the
               name, excluding the type and length field.

   Name:
               Variable length name.

5.  Security Considerations

   TODO






Gundogan, et al.         Expires January 4, 2018               [Page 11]


Internet-Draft           NDN Pub-Sub in the IoT                July 2017


6.  References

6.1.  Normative References

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

6.2.  Informative References

   [IWMT]     Kutscher, D. and S. Farrell, "Towards an Information-
              Centric Internet with more Things", Position Paper,
              Interconnecting Smart Objects with the Internet
              Workshop IAB, 2011.

   [NDN-EXP]  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.

   [PANINI]   Schmidt, TC., Woelke, S., Berg, N., and M. Waehlisch,
              "Let's Collect Names: How PANINI Limits FIB Tables in Name
              Based Routing", Proc. of 15th IFIP Networking
              Conference IEEE Press, pp. 458-466, Mai 2016.

   [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,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7945>.







Gundogan, et al.         Expires January 4, 2018               [Page 12]


Internet-Draft           NDN Pub-Sub in the IoT                July 2017


Acknowledgments

   This work was stimulated by fruitful discussions ... We would like to
   thank all active members for constructive thoughts and feedback.  In
   particular, the authors would like to thank (in alphabetical order)
   Emmanuel Baccelli, Michael Frey, Oliver Hahm, Peter Kietzmann, Dirk
   Kutscher, Martine Lenders, Joerg Ott, Hauke Petersen, and Felix Shzu-
   Juraschek.  This work was partly funded by the German Federal
   Ministry of Education and Research, project I3.

Authors' Addresses

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

   Phone: +4940428758067
   EMail: Cenk.Guendogan@haw-hamburg.de


   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











Gundogan, et al.         Expires January 4, 2018               [Page 13]