Information-centric Routing for Opportunistic Wireless Networks
draft-mendes-icnrg-dabber-04

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Network Working Group                                     P. Mendes, Ed.
Internet-Draft                                                    Airbus
Intended status: Experimental                                   R. Sofia
Expires: September 16, 2020                                 fortiss GmbH
                                                          V. Tsaoussidis
                                         Democritus University of Thrace
                                                              C. Borrego
                                      Autonomous University of Barcelona
                                                          March 15, 2020

    Information-centric Routing for Opportunistic Wireless Networks
                      draft-mendes-icnrg-dabber-04

Abstract

   This draft describes the Data reAchaBility BasEd Routing (DABBER)
   protocol, which aims to extend the operation of distributed
   Information Centric Networking frameworks to opportunistic wireless
   networks such as Delay Tolerant Networks.  By "opportunistic wireless
   networks" it is meant multi-hop wireless networks where finding an
   end-to-end path between any pair of nodes at any moment in time may
   be a challenge.  The goal is to assist in better defining
   opportunities for the transmission of Interest and Data packets in a
   store-carry-and-forward manner, based on a combination of proactive
   and reactive approaches.  The document presents an architectural
   overview of DABBER followed by the specification of the proactive
   approach based on the dissemination of name-prefix information, and
   the reactive approach based on the encounters probability.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 16, 2020.

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Copyright Notice

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Applicability . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Assumptions and Requirements  . . . . . . . . . . . . . .   5
     1.3.  Conventions . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  DABBER Architecture . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Routing and Forwarding  . . . . . . . . . . . . . . . . .   6
     2.2.  Contextual Awareness  . . . . . . . . . . . . . . . . . .   8
     2.3.  Device Identifiers  . . . . . . . . . . . . . . . . . . .   9
     2.4.  Faces . . . . . . . . . . . . . . . . . . . . . . . . . .  10
       2.4.1.  OPPFace . . . . . . . . . . . . . . . . . . . . . . .  10
       2.4.2.  DTNFace . . . . . . . . . . . . . . . . . . . . . . .  12
       2.4.3.  CMFace  . . . . . . . . . . . . . . . . . . . . . . .  13
   3.  Routing of Name Prefixes  . . . . . . . . . . . . . . . . . .  13
     3.1.  LSA Dissemination . . . . . . . . . . . . . . . . . . . .  13
     3.2.  Multiple path Computation . . . . . . . . . . . . . . . .  15
       3.2.1.  Name Prefix Cost Computation  . . . . . . . . . . . .  16
       3.2.2.  Update of DABBER internal routing table and LSDB  . .  18
       3.2.3.  Update of RIB on NFD  . . . . . . . . . . . . . . . .  19
     3.3.  Routing Operation Example . . . . . . . . . . . . . . . .  19
   4.  Forwarding of Interest Packets  . . . . . . . . . . . . . . .  21
   5.  Forwarding of Data Packets  . . . . . . . . . . . . . . . . .  22
     5.1.  Time-Evolving Contact Duration  . . . . . . . . . . . . .  24
     5.2.  TECD Importance . . . . . . . . . . . . . . . . . . . . .  25
     5.3.  Forwarding strategy . . . . . . . . . . . . . . . . . . .  26
       5.3.1.  Basic Strategy  . . . . . . . . . . . . . . . . . . .  26
       5.3.2.  Prioritized Strategy  . . . . . . . . . . . . . . . .  27
   6.  Protocol Addictional Functionality  . . . . . . . . . . . . .  27
     6.1.  Adjustment to data source mobility  . . . . . . . . . . .  27
   7.  Interoperability  . . . . . . . . . . . . . . . . . . . . . .  28
     7.1.  Interoperability with ICN routing . . . . . . . . . . . .  28
     7.2.  Interoperability with broadcast based forwarding  . . . .  29

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   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  29
     8.1.  Authenticity  . . . . . . . . . . . . . . . . . . . . . .  30
     8.2.  Confidentiality . . . . . . . . . . . . . . . . . . . . .  30
     8.3.  Privacy . . . . . . . . . . . . . . . . . . . . . . . . .  31
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  32
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  32
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     11.2.  Informative References . . . . . . . . . . . . . . . . .  32
     11.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

1.  Introduction

   In a networking scenario where an increasing number of wireless
   systems, such as end-user nodes and mobile edge nodes, are being
   deployed, there are two networking paradigms that are highly
   correlated to the efficiency of pervasive data sharing: Information-
   Centric Networking (ICN)[RFC7476], and Delay tolerant Networking
   (DTN) [RFC4838].  The latter concerns the capability of exploiting
   any potential wireless communication opportunity to exchange data in
   a multi-hop wireless networks, where it is difficult to find an end-
   to-end path between any pair of nodes at any moment in time.

   Combining ICN and DTN is relevant to efficiently extend the
   applicability of information-centric networking to novel scenarios,
   such as affordable pervasive access; low cost extension of access
   networks; edge computing; vehicular networks.

   This document describes the Data reAchaBility BasEd Routing (DABBER)
   routing protocol aiming to support information-centric delay-tolerant
   networks (ICDTN) [ICN-routing-opp].  These networks are operationally
   located on the Internet fringes.  In such areas, networking
   experiences intermittent connectivity and variable availability of
   nodes due to their movement and/or due to other constrains, e.g.,
   limited battery, storage, and processing.

   It is our understanding that routing in such wireless environments
   needs to be done based on strategies that take into consideration, at
   a network level, the context of wireless nodes (e.g. availability,
   centrality), and not just the history of contacts among wireless
   nodes.  The goal is to assist in better defining opportunities for
   the transmission of Interest and Data packets over time and space:
   DABBER focus on a data plane similar to the one use by CCN/NDN [NFD],
   since these are well established distributed ICN frameworks.

   DABBER brings ICN and DTN together by combining a proactive approach
   to forward Interest packets based on the dissemination of name-prefix

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   information, with a reactive approach to forward Data packets based
   on information collected about custodians and based on encounters
   probability.  The dissemination of name-prefixes and the
   dissemination of Data packets is done based on the context of nodes,
   and not just the history of contacts among wireless nodes.

1.1.  Applicability

   DABBER is being developed to allow the deployment of ICDTN where
   nodes and links can be intermittently available, such as in the case
   of emergency situations [NDN-emergency].  From an end-to-end
   perspective we can consider two scenarios: the NDN wireless network
   is at the fringes of the NDN core; the NDN wireless network can
   interconnect different NDN fixed networks.

   While the latter may support applicability scenarios typical of
   Delay-Tolerant Networks (DTN) for instance tunneling traffic over an
   area lacking network deployment, the former allows the extension of
   the applicability of information-centric networking to novel
   scenarios such as affordable pervasive data access, low cost
   extension of access networks, edge computing, and vehicular networks:

   Affordable pervasive data access: This scenario encompasses the
   implementation of NDN in personal mobile nodes (e.g. smartphones)
   allowing users to share data and messaging services by exploiting
   existing intermittent wireless connections (e.g.  Wi-Fi, Wi-Fi
   direct) in environment without/or limited Internet access.

   Low cost extension of access networks: This scenario refers to the
   usage of wireless nodes (mobile or fix) to extend the reach of an NDN
   networks while reducing CAPEX costs.

   Edge/Fog computing: This scenario is related to the efforts being
   done to bring cloud computing closer to the end-users.  This scenario
   encompasses a large set of heterogeneous (wireless and sometimes
   autonomous) decentralized nodes able of communicating, directly or
   via an infrastructure, in order to perform storage and processing
   tasks without the intervention of third parties.  This scenario deals
   with nodes that might not be continuously connected to a network,
   such as laptops, smartphones, tablets and sensors, as well as nodes
   that may be intermittently available due to scarce resources, such as
   wireless access routers and even Mobile Edge Computing (MEC) servers.

   V2X networks: This scenario deals with the intermittent connectivity
   between vehicles as well as between vehicles and the infrastructure.

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1.2.  Assumptions and Requirements

   DABBER relies on the following assumptions:

   o Mobile nodes are able of exploiting wireless connectivity.

   o Mobile nodes can be a source and destination of data, being able of
   operating as a router: there is not a clear distinction, in terms of
   routing process, between sources, destinations, and routers.

   o Mobile nodes may decide to be the custodians of data transmissions
   based on a set of criteria such as local available resources.

   o In DTNs it is not possible to know the complete network topology.

   o In DTNs it is not efficient to flood the network, as shown by all
   prior solutions based on controlled packet replication forwarding
   ([RFC6693][Dlife][Scorp][Dlife-draft]) instead of broadcast as used
   in Epidemic routing.

   o Selecting the best set of neighbors to replicate packets to, may
   not be efficient if based only on connectivity based information
   (e.g. inter-contact times, contact duration).

   In terms of requirements:

   o Routing informaiton must be exchanged based on Interest / Data
   messages.

   o Routing information should be used to distribute only name prefix
   reachability, since building a network topology based on adjacency
   information is not feasible in an opportunistic network.

   o Routing information must be distributed to multiple next-hops based
   on local information that encodes data reachability.

   o A synchronization mechanism my be used to exchange routing
   information among neighbor node.

   o Forwarding of Interest packets must take into account the
   information stored in the Forwarding Information Base (FIB).

   o Interest packets must carry information about the data consumer ID.

   o Interest packets should carry information about custodians IDs.

   o Forwarding of Interest must take into account the information
   stored in the Forwarding Information Base (FIB).

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   o Forwarding of Data packets must take into account the information
   stored in the Pending Information Table (PIT).

   o The PIT must store information about the data consumer ID.

   o The PIT may store information about custodians IDs.

   o Data sources must set the validity of name prefixes - validity v -
   as an integer that represents the expiration date of the data.

1.3.  Conventions

   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.  In this
   document, these words will appear with that interpretation only when
   in ALL CAPS.  Lower case uses of these words are not to be
   interpreted as carrying significance described in RFC 2119.

2.  DABBER Architecture

   This section presents an overview of the DABBER protocol
   architecture.  DABBER relies on the same message formats, message
   exchange process, and same data structures made available by CCN/NDN:
   Routing Information Base (RIB); Forwarding Information Base (FIB);
   Pending Intent Table (PIT), while adding new elements such as two new
   faces (OPPFace and DTNFace), a contextual manager, and distinct
   forwarding strategies for Interests and Data packets.  On contrary to
   what happens in CCN/NDN, in DTN Data packets may not be able to
   follow the same path followed by Interest packets.

               TBD

   Figure 1: DABBER Architecture.

2.1.  Routing and Forwarding

   DABBER aims to assist in better defining opportunities for the
   transmission of Interest and Data packets in a store-carry-and-
   forward manner, based on a combination of proactive and reactive
   approaches.  DABBER defines a proactive routing approach based on the
   dissemination of name-prefix information, which are use to identifie
   suitable next hops to reach a certain data object.  This location can
   be the source of data or any other custodian.  The proactive routing
   scheme aims to reduce the time needed to reach the requested data
   object.  Without a mechanism able of disseminating routing
   information, devices would need to use try and error approach based

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   on a broadcast forwarding strategy.  Besides the extra delay in
   finding the requested data, such strategy will increase the amount of
   used networked resources.  As shown in figure 2, the proposed
   proactive approach is able of populating the FIB with a list of next
   hops towards each name prefix.  This is done based on the information
   collected from neighbor nodes and stored in the RIB.

       Node A                Node B
             +----------+          +------------+
        N -  |1        2| - N----- |1          2|
             |          |          |            |
             |3        4| - N      |3          4|
             +----------+   |      +------------+
                            |         Node C
                            |       +------------+
                             ------ |1          2|
                                    |            |
                                    |3          4|
                                    +------------+

                RIB                                  FIB
           +----------------------------+      +-----------------------+
           |Prefix Name | Face   | Cost |      | Prefix Name |  Faces  |
           +----------------------------+      +-----------------------+
           |     N      |  2     |  3   |      | N           |  2,1,4  |
           |     N      |  4     |  10  |      |             |         |
           |     N      |  1     |  5   |      +---------------------- +
           +----------------------------+

                  PIT
          +--------------------------------------------+
          | Interest |  Face  | Requester | Custodians |
          +--------------------------------------------+
          |   N      |    1   |    DID1   |  DID2;DID3 |
          |          |    3   |    DID4   |            |
          +--------------------------------------------+

   Figure 2: RIB, FIB and PIT on node A.

   The FIB illustrated in Figure 2 is used by a forwarding strategy
   (c.f. section 4.1) used to transmit Interest packets in the direction
   of one of more copies of the requested data.  This strategy is
   perfectly aligned with the current CCN/NDN architecture.  However the
   same does not happen with the forwarding of Data packets.  On CCN/NDN
   Data packets are transmited in the Faces listed in the PIT for the
   name carried in the Data packet.  Although this breadcumb approach

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   works on a stable/fixed network, the same does not happen in a DTN,
   since faces from which Interest packets were received may be down.
   In this case DABBER forwards Data packets toward the DID of the data
   requester (mandatory), or to any identified custodian (Optional).
   This is done by using new forwarding strategy for Data packets based
   on the encounters probability and contextual awareness, as described
   in section 4.2.

   The inclusion of a forwarding strategy for Data packets is already a
   difference from the CCN/NDN architecture.  To implement such
   forwarding strategy some changes need to be included to handle
   Interest packets (c.f. section 4.1) and to the PIT structure, namely:

   o The Interest packet includes the DID of the requester device, as
   well as of any visited custodian device.  For this the
   ApplicationParameter optional field can be used.

   o The in-record of the PIT entry related to the Interest needs to
   hold the following fields: DID of resquester; list of DID of
   custodians, as illustrated in Figure 2.

   Given the multi-path nature of DABBER, the incoming Face might appear
   among the potential next-hops for a given name prefix.  For this
   reason, DABBER applies the Incoming Face Exclusion principle
   [Loop-free] in order to prevent forwarding packets back though the
   Face them came from, thus removing two-hop loops.

   Furthermore, in order to detect longer forwarding loops (more than
   two hops), DABBER relies on the nonce-based detection scheme
   available in CCN/NDN in order to drop a looping packet as soon as it
   is received the second time.

   In addition, DABBER considers a loop removal mechanism, which takes
   care of disabling the Face responsible for the looping once it is
   detected.

2.2.  Contextual Awareness

   DABBER defines routing and forwarding strategies that take into
   consideration, at a network level, the context of wireless nodes, and
   not just the history of contacts among wireless nodes.  Contextual
   information is obtained in a self-learning approach, by software-
   based agents running in each networked device, and not based on
   network wide orchestration.  Contextual agents are in charge of
   computing node and link related costs concerning availability and
   centrality metrics.  Contextual agents interact with DABBER via a
   well-defined interface: the contextual self-learning process is not
   an integrating part of the DABBER routing framework.

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   The contextual agent (named Contextual Manager [UmobileD45])
   installed in each device can therefore be seen as an end-user
   background service that seamlessly captures wireless data to
   characterize the affinity network (roaming patterns and peers'
   context over time and space) and the usage habits and data interests
   (internal node information) of a node.  Data is captured directly via
   the regular MAC Layer (e.g., Wi-Fi, Bluetooth, LTE) as well as via
   native applications for which the user configures interests or other
   type of personal preferences.  For instance, an application can
   request a one-time configuration of categories of data interests
   (e.g., music, food).

   Based on the defined interface, DABBER is able of querying the local
   Contextual Manager about the characteristics of neighbor nodes, based
   on three types of information: i) Node availability (metric A); ii)
   Node centrality (metric C); iii) Node similarity (metric S):

   o Node Availability (A) gives an estimate of the node availability
   based on the usage of internal resources over time and space, such as
   the time spent per application category (e.g. per day), as well as
   the usage of physical resources (battery status; CPU status, etc).

   o Node Centrality (C) provides awareness about the node's affinity
   network neighborhood context.  This means that a list is kepted with
   the following information about each neigbour: neighbour's node
   degree; Frequency of contacts between the neighbor and other nodes;
   Duration of each contact between the neighbor and other nodes;
   Importance of encountered nodes.

   o Node similarity (S) provides awareness about a node's similarity
   towards neighbor nodes.  This means that a list is kepted with the
   following information about each neigbour: Packet Error Rate of the
   wireless link towards the neighbor; Frequency of contacts with
   neighbor; Duration of each contact with neighbour.

   The Contextual Manager keeps values for the mentioned metrics for
   different periods of time.  Encountered nodes can be of different
   types, such as other mobile devices or wireless access points for
   instance.

2.3.  Device Identifiers

   With DABBER, networked devices (producers, consumers, routers) are
   identified by variable-length identifiers, such as End-points
   Identifiers in DTN and hierarchical names in CCN/NDN.  Using an DID,
   a node is able to determine the source of a Interest packet as well
   as a potential set of custodians that may help the data transmission

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   process.  Each device is required to have at least one DID that
   uniquely identifies it.

   Device ID are expressed syntactically as a Uniform Resource
   Identifier (URI) [RFC3986].  The URI syntax has been designed as a
   way to express names or addresses for a wide range of purposes, and
   is therefore has been used to construct names for DTN endpoints, as
   well as hierarchical names in CCN/NDN.  In URI terminology, each URI
   begins with a scheme name.  The scheme name is an element of the set
   of globally-managed scheme names maintained by IANA.  Lexically
   following the scheme name in a URI is a series of characters
   constrained by the syntax defined by the scheme.  This portion of the
   URI is called the scheme-specific part, and can be quite general.

   Being based on UIRs, device IDs may be kept quite flexible.  They
   might, for example, be constructed based on DNS names, or might look
   like expressions of interest or intentional names.  For instance DIDs
   may be set up to reflect the network operator to which the mobile
   node belongs to and to the home site, in case the mobile operator has
   more than one operational site.  In this case, when a mobile node is
   used outside its home network and some of its requests reach an
   access point of a visited mobile network, the latter may recognize
   may be able of checking if there is a roaming agreement between the
   home network and one of the networks of the visited operator.  If so
   the request may be routed towards an international transit network.

   Based on an URI scheme that may reflect a network operator, the
   information included in the DID may be used to select next hops
   belonging to the same operator network, or nodes that have the same
   home network.  It is assumed that a DTN is build based on wireless
   direct connectivity between nodes that may belong to different
   operators, but that may have roaming patterns that allows them to
   have frequent wireless contacts.

2.4.  Faces

   DABBER leverages the concept of Faces in CCN/NDN to adapt its
   operation to the intermittent property of wireless connections.  This
   is done by the implementation of two new type of faces, called
   Opportunistic Face (OPPFace) and Delay Tolerant Networking Face
   (DTNFace).  Besides these two communication interfaces, DABBER keeps
   a face to the Contextual Manager (CMFace).

2.4.1.  OPPFace

   An OPPFace is based on a system of packet queues to hide intermittent
   connectivity: instead of dispatching packets from the FIB, the
   OPPFace is able of delaying packet transmission until the wireless

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   face is actually connected.  OPPFaces are kept in the Face Table of
   the forwarder and their state reflects the wireless connectivity
   status: they can be in an Up or Down state, depending upon the
   wireless reachability towards neighbor nodes.  Based on this
   information, the OPPFace decides whether to simply queue packets
   (when OPPFace is down) or flush the queue (when OPPFace is up).
   Since packet queuing is concealed inside OPPFaces, existing
   forwarding strategy do not need to be changed.

   OPPFaces can be implemented by using any direct wireless
   communication mode.  The current specification of DABBER considers
   Wi-Fi (Infrastructured, Ad-Hoc, and Direct mode).

   The current version of the NDN port to opportunitic networks based on
   Android (NDN-OPP) makes usage of group communications provided by Wi-
   Fi Direct [NDN-OPP][NDN-opportunisticnets] (NDN-OPP GitHub code [1]).
   In this case there is a one-to-one correspondence between an OPPFace
   and a neighbor node (for each node detected in a Wi-Fi Direct Group,
   a new instance of an OPPFace is created).  In this peer-to-peer
   scenario, OPPFaces can be used in two transmission modes: connection-
   oriented, in which packets are sent to a neighbor node via a reliable
   TCP connection over the group owner; connection-less, in which
   packets are sent directly to a neighbor node during the Wi-Fi direct
   service discovery phase.  In the latter case data transmission is
   limited to the size of the TXT record (900 bytes for Android 5.1 and
   above).  This type of communication is used to exchange small packets
   that require fast transmission (e.g. emergency apps, Chronosync
   status messages).  The connection-less solution allows peers to
   exchange a short number of bytes without the establishment of a TCP
   socket.

   In the peer-to-peer scenario of Wi-Fi direct, DABBER operates as
   follows: routing information is shared among all members of a Wi-Fi
   direct group, while Interest Packets are forwarded to specific
   neighbors.  With Dabber it is the carrier of an Interest packet that
   decides which of the neighbors will get a copy of the Interest
   packet.  Hence, with the current implementation of NDN-OPP, DABBER
   places a copy of the Interest packet in the OPPFaces of selected
   neighbors.  In what concerns the dissemination of routing
   information, it is ensured by: i) node mobility, meaning that nodes
   carry such information between Wi-Fi direct groups; ii) information
   is passed between neighbor groups via nodes that belong to more than
   one group.

   Based on the reception of notifications of Wi-Fi Direct regarding
   changes in the peers detected in the neighborhood, DABBER is able of
   updating its internal peer list (Neighbor Table as illustredted in
   Figure 5).  If it is not currently connected to a Wi-Fi Direct Group,

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   it performs a selection heuristic to determine which node to connect
   to.  The motivation behind this selection process is to attempt to
   minimize the number of Wi-Fi Direct Groups in a certain area given
   that nodes can only transmit packets within the Group they are
   currently connected to.

   By defining OPPFaces implemented based on a broadcast link layer such
   as ad-hoc Wi-Fi, DABBER will need to create only one OPPFace in each
   networked device.  Such OPPFace would be used to exchange packets
   with any neighbor node, making use of the overhearing property of the
   wireless medium.  Since with DABBER, it is the carrier that decides
   which of the neighbors are entitle to get a certain Interest packet,
   DABBER would need to encode in the Interest packet information about
   the ID of the neighbors that should process the overheard Interest
   packet.

2.4.2.  DTNFace

   By defining a DTNFace implemented based on the bundle layer [RFC5050]
   DABBER will make use of the end-to-end protocol, block formats, and
   abstract service description for the exchange of messages (bundles)
   described in the DTN architecture.  A DTNFace provides a robust
   communications platform for the transmission of Data packets towards
   the consumer node, making usage of any available custodian nodes.

   The bundle protocol [RFC5050] introduces the concept of a "bundle
   agent" that manages the interface between applications and the
   "convergence layers" that provide the transport of bundles between
   nodes during communication opportunities.  DABBER defines a DTNFace
   that extends the bundle agent aiming to control the actions of the
   bundle agent during communication opportunities.

   The new DTNFace aims to control the reception and delivery of
   bundles, which are placed in a queue during the forwarding of Data
   packets.  The DTNFace allows the routing process to be aware of the
   bundles placed at the node, and allows it to inform the bundle agent
   about the bundles to be sent to a neighbor node.  Therefore, the
   bundle agent implemented in the DTNFace needs to provide the
   following interface/functionality to the forwarding process:

   Get Bundle List: Returns a list of the stored bundles and their
   attributes to the routing agent.

   Send Bundle: Notifies the bundle agent to send a specified bundle.

   Drop Bundle Advice: Advises the bundle agent that a specified bundle
   may be dropped by the bundle agent if appropriate.

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   Acked Bundle Notification: Bundle agent informs routing agent whether
   a bundle has been delivered to its final destination and time of
   delivery.

2.4.3.  CMFace

   TBD

3.  Routing of Name Prefixes

   Being developed to operate in DTNs, DABBER does not rely on the
   dissemination of Adjacency Link State Advertisements (LSAs) that
   reflect the status of the links towards neighbor nodes; DABBER only
   requires the dissemination of Prefix LSAs, and does not require the
   computation of shortest paths.  DABBER replaces the path cost used by
   protocols used for fixed networks with a data reachability cost
   reducing the impact that topological changes would have on the
   stability of routing information.

   The computation of data reachability costs towards different data
   sources, based on the local dissemination of name prefixes, aims to
   avoid flooding the wireless network with Interest packets that would
   otherwise be broadcast to all potential data sources.

3.1.  LSA Dissemination

   DABBER makes use of Interest/Data packets to have neighbour devices
   exchanging Prefix LSAs.  This means that while IP-based routing
   protocols push updates to other routers, DABBER devices pull updates.
   DABBER can use any underlay communication channels (e.g., TCP/UDP
   tunnels, Link layer TXT records) to exchange LSA information.

   By using Interest/Data packets, DABBER benefits from CCN/NDN built-in
   data authenticity to exchange routing information: since a routing
   update is carried in an Data packet and every Data packet carries a
   signature, a DABBER device can verify the signature of each LSA to
   ensure that it was generated by the claimed origin node and was not
   tampered during dissemination.

   DABBER advertises Prefix LSAs every time a new name prefix is added
   or deleted to the LSA Data Base (LSDB).  Name prefixes are advertised
   with a cost metric related to the validity of the associated data, as
   shown in Figure 3.  Each LSA used by DABBER has the name
   <DID>/DABBER/LSA/Prefix/<version>.  The <DID> is described by a
   scheme based on URIs (c.f. section 2.1); It can be for instance
   <network>/<operator>/<home>/<node>/. The <version> field is increased
   by 1 whenever a device creates a new version of the LSA.

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           Prefix LSA
     +-----------------------------------------------------------------+
     |  LSA  | Number of |Prefix 1|Cost| ... |Prefix N|Cost| Signature |
     |  Name | Prefixes  |        |    |     |        |    |           |
     +-----------------------------------------------------------------+

   Figure 3: Prefix LSA format.

   DABBER disseminates LSAs via a data synchronization mechanism (e.g.
   ChronoSync [ChronoSync], PartialSync [PartialSync]) of the local
   LSDB.  This peer synchronization approach is receiver-driven, meaning
   that a device requests LSAs only when it has CPU cycles.  Thus it is
   less likely a device will be overwhelmed by a flurry of updates.  In
   order to reduce the amount of transfered data, DABBER removes
   obsolete LSAs from the LSDB by periodically refreshing each of its
   own LSAs by generating a newer version.  Every LSA has a lifetime
   associated with it and will be removed from the LSDB when the
   lifetime expires.

   DABBER performs the dissemination of LSAs based on a process able of
   synchronizing the content of LSDBs.  In this sense, all LSAs are kept
   in the LSDB as a name set, and DABBER uses a hash of the LSA name set
   as a compact expression of the set.  Neighbor nodes use the hashes of
   their LSA name sets to detect inconsistencies in their sets.  For
   this reason, neighbor nodes exchange hashes of the LSDB as soon as
   OPPFaces are UP.

   Current version of DABBER makes use of ChronoSync as synchronization
   mechanism.  Chronosync allows DABBER to define a collection of named
   data in a local repo as a slice.  LSA information is synchronized
   among neighbor nodes, since Chronosync keeps the repo slice
   containing the LSA information in sync with identically defined
   slices in neighboring repositories.  If a new LSA name is detected in
   a repo, ChronoSync notifies DABBER to retrieve the corresponding LSA
   in order to update the local LSDB.  DABBER can also request new LSAs
   from Chronosync when resources (e.g.  CPU cycles) are available.

   Figure 4 shows how an LSA is disseminated between two neighbor nodes
   A and B, when the OPPFace is UP.  To synchronize the slice
   representing the LSDB information in the repo, ChronoSync, on each
   node, periodically sends Sync Interests with the hash of its LSA name
   set / slice (step 1).  When Node A has a new Prefix LSA in its LSDB,
   DABBER writes it in the Chronosync slice (step 2).  At this moment,
   the hash value of the LSA slide of node A becomes different from that
   of node B.  As a consequence, the Chronosync in node A replies to the
   Sync Interest of node B with a Sync Reply with the new hash value of
   its local LSA slice (step 3).  The Chronosync in node B identifies
   the LSA that needs to be synchronized and notifies DABBER about the

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   missing LSA, and updates its LSA name set (step 4).  Since DABBER on
   node B has been notified of the missing LSA, DABBER sends an LSA
   Interest message to retrieve the missing LSA (step 5).  DABBER on
   node A sends the missing data in a LSA Data message (step 6).  When
   DABBER on node B receives the LSA data, it inserts the LSA into its
   LSDB.  Chronosync on nodes A and B compute a new hash for updated the
   set and send a new Sync Interest with the new hash (step 7).

          Node A                            Node B
     +----------------------------+     +----------------------------+
     |            +-------------+ |     |+-------------+             |
     |    DABBER  |  Chronosync | |     ||  Chronosync |   DABBER    |
     |            +-------------+ |     |+-------------+             |
     +----------------------------+     +----------------------------+
            |            |  Sync Interest (1) |              |
            |            |------------------->|              |
            |            |<-------------------|              |
            | New LSA (2)|                    |              |
            |----------> |                    |              |
            |            |    Sync Reply (3)  |              |
            |            |------------------->|              |
            |            |                    |  Notify (4)  |
            |            |                    |------------->|
            |            |   LSA Interest (5) |              |
            |<-----------|--------------------|--------------|
            |            |   LSA Data (6)     |              |
            |------------|--------------------|------------->|
            |            |                    |              |
            |            |  Sync Interest (7) |              |
            |            |------------------->|              |
            |            |<-------------------|              |

   Figure 4: LSA exchange process.

   When more than one LSA needs to be synchronized, the issued LSA
   Interest packet will contain information about as many LSAs as
   allowed by the Link maximum transmission unit.  In the same sense one
   LSA Data packet may include also be used to transport information
   about more than one LSA.

3.2.  Multiple path Computation

   By exchanging LSAs each devices becomes aware of potential next-hops
   via which a name prefix N can be reached with a certain cost k.  This
   cost k represents the probability of reaching a data object
   identified by N via a Face F, and is related to the time validity of

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   the name prefix (v).  The rationale for this approach is that the
   selection of faces that have a lower cost k (higher validity v) will
   improve data reachability.  The validity of a name prefix is set by
   the data source as an integer that represents the expiration date of
   the data.

   Since different devices can announce the same name prefix, a certain
   name prefix may be associated with different values of k (as v shall
   differ) over different faces, depending upon the nodes announcing
   such name prefix: this lead to the identification of multiple next
   hops, each one with a different cost.

   The computation of multiple next hops is performed every time DABBER
   has a new Name Prefix LSA (or a new version of an existing Name
   Prefix LSA) in its LSDB.  The sequence of operations, as described in
   the following sub-sections are:

   1) Computes a new value for the validity of a new Name Prefix in the
   LSDB;

   2) Updates DABBER internal routing table;

   3) Updates the LSDB with the data reachability information (validity)
   of the current node towards the new Name Prefix;

   4) Updates the RIB on NDF based on the DABBER internal routing table,
   following a Downwards Path Criterion (FIB is updated by NFD based on
   the RIB content).

   Periodically DABBER updates the validity values of all Name Prefixes
   in its internal routing table, performing the consequent updates of
   the local LSDB and RIB, and needed.

3.2.1.  Name Prefix Cost Computation

   When DABBER is notified that a new Prefix LSA was registered in the
   LSDB or an existing Prefix LSA has a new version, it computes a new
   cost for each name prefix in such Prefix LSA.  The cost of a name
   prefix is given by its validity.

   DABBER starts by computing a new validity v for a prefix N depending
   upon the validity announced by the neighbor, and the relevancy of the
   "relation" between the two neighbor nodes (e.g., node A and node B).
   The cost of the Name Prefix, passed to NFD, will then be computed as
   an inversely proportional value to its validity.

   The relevancy of the "relation" between two neighbor nodes can be,
   e.g., a measure of similarity [UmobileD45], where similarity is seen

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   as a link measure, i.e., it provides a correlation cost between a
   node and its neighbors.  Or such relation can be weighted based on
   metrics derived from average contact duration thus allowing a node to
   adjust the Name Prefix validity based on the probability of meeting
   the respective neighbor again.  The "relation" between two neighbor
   nodes is computed based on the three metrics (A, C, and S) provided
   by the local contextual manager, plus a metric computed by DABBER
   itself: the time reachability.

   The variable considered by DABBER for the computation of the
   validity/cost of a Name prefix passed by a neighbor Na are:

   o Validity (V) - Represents the expiration date of the data
   associated with the Name Prefix.  Currently it is the UNIX Timestamp
   (10 digit integer).

   o Similarity metric (S) towards the neighbor Na, as passed by the
   contextual manager (S >= 0), aiming to select neighbors with whom the
   current node has a good communication link.

   o Availability metric (A) towards the neighbor Na, as passed by the
   contextual manager ( 0 < A < 1), aiming to select neighbors able to
   process Interest packets with high probability.

   o Centrality metric (C) towards the neighbor Na, as passed by the
   contextual manager ( C >= 0), aiming to select neighbors with high
   probability of successfully forwarding Interest packets.

   o Time reachability (T) which corresponds to the RTT between sending
   an Interest packet towards the source of such Name Prefix and
   receiving a data packet. (0 < T < 1).  Currently the value of T is
   computed as (current time when data packet of received - time when
   Interest packet was sent) / Validity of Name Prefix.  Time
   reachability allows DABBER to select next hops that lead to closer
   sources.

             Neighbor table
     +------------------------------------------------------+
     |   Face     | Status | Metric C | Metric A | Metric S |
     +------------------------------------------------------+
     |     1      |    UP  |    6     |   0.3    |    12    |
     |     2      |  DOWN  |    4     |   0.8    |    8     |
     |     3      |    UP  |    1     |   0.8    |    9     |
     +------------------------------------------------------+

   Figure 5: Neighbor table.

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   The values C, A and S provided by the contextual manager are stored
   in a Neighbor Table (c.f.  Figure 5) indexed by the number of faces.
   The higher the values of C, A and S, the most preferential a neighbor
   is.

   T is measured by observing the flow of Interest and Data packets.
   Thus, the lowest the T, the most preferential a Face is.  Although
   different nodes may have a different implementation of a face ranking
   logic, the relevancy of T in comparison to C and A should be higher,
   since T reflects the measured delay to reach a data source, while C
   and A are indicators of the neighbors potential as relays.

   Based on the above mentioned metrics the Validity of a new Name
   Prefix (V) is updated based on two operations:

   o V' = f (V, S') = trunc (V * S'), where:

   S' = (S - Smin) / (Smax - Smin); Smin = 0 and Smax = max (Smax, C)

   o V'' = f (V', C', A, T) = 0.3* trunc (V' * (C'+A)) + 0.7 * trunc (V'
   * T), where:

   C' = (C - Cmin) / (Cmax - Cmin); Where Cmin = 0 and Cmax = max (Cmax,
   C)

3.2.2.  Update of DABBER internal routing table and LSDB

   After the computation of the cost of the Name Prefix taking into
   account the relation of the current node with the neighbor announcing
   it, DABBER updates its internal routing table and its LSDB.  The
   information on the routing table will be used to updated the RIB of
   the local NFD and the information of the LSDB will be announced to
   all neighbors by Chronosync.

   To update the Internal routing table, DABBER adds an entry (Na, V'')
   for the Name Prefix received from Na, where V'' is the computed cost
   of the name prefix (c.f. section 3.2.1).  The routing table is then
   ordered by name prefix in decreased order of validity.

   Since the current node will also disseminate the received Name
   Prefix, DABBER updates the cost of the Name Prefix in the LSA stored
   in its local LSDB in order to consider the computed value V''.  For
   this, DABBER can use two methods:

   o Maximal method: Cost of Name Prefix = max (V'', current cost on
   LSA).

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   o Average method: Cost of Name Prefix = max (average (cost of Name
   Prefix entries on local routing table), current cost on LSA).

3.2.3.  Update of RIB on NFD

   After computing the new value of the cost of a name prefix (c.f.
   section 3.2.2), DABBER updates the RIB entry of that name prefix with
   the face over which the Name Prefix LSA was received and the new
   computed cost.  The cost (k) of the Name Prefix to be stored in the
   RIB is computed based on its validity V'' as k = trunc (100/V'').

   DABBER updates the RIB on NFD with the cost k based on three possible
   logics:

   o Increase diversity - The new Face is included in the RIB entry, if
   the computed cost k helps to increase diversity of the name prefix.
   For instance the new cost k is higher than the average costs already
   stored for that name prefix, affected by a configured diversity
   constant.  This is, this logic include all neighbors with cost =
   trunc (100/V''), such that 1/V'' - Avr (Costs in RIB for N) > X
   (predefined value).

   o Downward Path Criterion - It is a non-equal cost multi-path logic
   that is guaranteed to be loop-free.  Based on the Downward Path
   Criterion, the X faces (the maximum number X of desirable faces can
   be defined by configuration) to be considered for a name prefix
   include the one with the lowest cost k plus X-1 faces that have a
   cost k lower than the cost that the current node has itself to the
   name prefix.  This is, this logic includes X neighbors with cost =
   trunc(100/V''), such that cost is the lowest value of 1/V'' or cost <
   1/ V.

   o Downward Path Criterion extension - Also considers any face over
   which the name prefix can be reached with a cost k equal to the cost
   that the current node has itself to the name prefix.  To avoid packet
   from looping back, there is the need to add a tiebreaker, which
   assures that traffic only crosses one direction of equal-cost links.
   This is, this logic includes X neighbors with cost = trunc (100/V''),
   such that cost is the lowest value of 1/V'' or cost <=1/ V.

3.3.  Routing Operation Example

   In order to illustrate the proactive routing method defined by
   DABBER, let's consider Figure 4, where nodes A, B, and C reside in an
   ICDTN running DABBER, while nodes E and F are wireless edge routers
   running another ICN routing protocol; G is a wireless node running
   DABBER.

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        +--------------------+
        |    +---+           |
        |    | B | .         |
        |    +---+  .2+---+  |   +---+    +---+     +---+
        |+---+        | C |3 ... | E |....| F  |....| G |
        || A |.......1+---+  |   +---+    +---+     +---+
        |+---+               |
        +--------------------+

   Figure 6: End-to-end operational example.

   In our example, Node A starts producing some content derived, for
   instance, from the use of an application (such as a data sharing
   application).  The produced content is stored in its local Content
   Store with the name /NDN/video/Lisbon/nighview.mpg.  Node B stores in
   its Content Store a data object with name /NDN/video/Lisbon/
   river.mpg, which node B received from a neighbor (meaning that B have
   already synchronize its LSDB with such a neighbor).

   Due to the update of the Content Store, the name prefix /NDN/video/
   Lisbon/ is stored in the LSDB of node A and B with a validity of
   864000 and 518400 in the case of node A and B respectively.  In the
   case of node A, the cost k of the name prefix equals the validity v
   of the data object, e.g., v=864000 seconds (10 days) stipulated by
   the application.  In the case of node B the validity is the result of
   the computation process described in section 3.2.1.

   From a routing perspective, storing a name prefix in the local LSDB
   allows the node to share the respective Prefix LSA on all its Faces,
   excepting on the Face over which the LSA was previously received.
   This LSA exchange is done when the OPPFaces are up.  This means that
   Node C, which got a new Prefix LSA from nodes A and B, will:

   o Update its LSDB with the Prefix LSAs received from node A and node
   B.

   o Update its internal routing table with two new entries for the name
   prefix /NDN/video/Lisbon/, associated with the face towards A (face1)
   and with the face towards B (face2), after computing the values of V'
   and V'' for the received validity values:

   o The validity of the name prefix is updated based on the metric
   configured for node C: average inter-contact time.

   o Let's assume that A and C encounter each other frequently, while B
   and C do not meet frequently.  This means that the two entries on the
   routing table of node C for prefix /NDN/video/Lisbon/ will have a
   validity/cost for A higher than the one for B.

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   o Update its LSDB with the validity of the current node towards the
   Name Prefix following the maximal or average methods.

   o Update the RIB with one new entry for the name prefix /NDN/video/
   Lisbon/ with two faces (face 1 and face 2) with a cost inversely
   proportional to the validity of the Name Prefix.

   When node C gets in the range of router E (wireless edge router) it
   will exchange disseminate routing information, based on some
   interoperability issues need to be considered, as described in
   section 4.

4.  Forwarding of Interest Packets

   In order to support the new forward strategy for Data packets,
   devices need to collect information about the DID of the requester
   (mandatory) and of any potential custodian (optional).  Therefore,
   when an Interest packet is received, the following operations need to
   be performed:

   o The DIDs found in the ApplicationParameter field of the Interest
   packet are placed in the PIT entry corresponding to the Face over
   which the Interest packet was received.

   o Before forwarding the Interest packet, DABBER will include the DID
   of the current device if this is a custodian.  In this version the
   role of custodian is pre-configured.  This may be revised to include
   other logics, that may consider the capabilities of the device (e.g.
   available storage; available energy).

   Interest packets are forwarded based on the information that is
   stored in the FIB, which is updated by the NFD based on information
   stored on the RIB.  Independently of the used forwarding strategy, it
   has to respect the ranking of faces done by DABBER on the RIB.  For
   instance an unicast forwarding strategy will use the most important
   face (lower cost), while a multicast forwarding strategy will use all
   the faces indicated for the name prefix.

   After selecting the best set of faces, a copy of the Interest packet
   is sent to the OPPFaces of the selected faces.  The state of an
   OPPFace reflects the fact that the corresponding neighbor device is
   currently reachable or not.  Based on this information, the OPPFace
   decides whether to simply queue the packet or attempt a transmission
   over the associated Opportunistic Channel.

   Based on the feedback provided by the wireless channel (e.g.  Wi-Fi
   direct confirmation), the OPPFace can decide to remove the packet
   from the queue once it has been passed on to its intended peer.  In

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   case the packet was not passed to the intended peers, a new attend to
   forward the packet will be done as soon as the OPPFace is activated:
   the OPPFace integrates a mechanism to automatically flush the queue
   whenever the face is brought up upon detection of the corresponding
   peer being available.

5.  Forwarding of Data Packets

   By following the operation of CCN/NDN, Data packets are forwarded
   based in the information holded in the PIT: the ID of the Faces over
   which a copa of the Data packet must be transmitted.  In a DTN
   network, this setup faces two problems: i) the Face(s) stored in the
   PIT may not be active since neighbour devices are not in range; ii)
   the breadcumb path may not be available, since in a dynamic network
   some of the devices visited by the Interest packet may not be
   reachable.

   To solve these two problems, DABBER makes usage of a new forwarding
   strategy for Data packets by making usage of information stored in
   the PIT (which is different from the standard information used by
   CCN/NDN) and by making usage of an opportunistic forwarding scheme
   aiming to bring the Data packet closer to the requester or to any
   available custodian.

   The new forwarding strategy works as follows:

   o First check if the Face(s) present in the PIT related to that
   Interest are active.  The Data packet is sent to the OPPFace of each
   active Face.  This is a procedure similar to the one used by CCN/NDN.

   o For all Faces that are not active (OPPFace is down), DABBER uses an
   algorithm similar to dlife [Dlife][Dlife-draft] to forward the Data
   packet closer to the requester or any custodian.

   For all OPPFaces that are not active, DABBER starts by collecting the
   DID of the requester of Data, as well as the DID from potential
   Custodian from the in-record PIT entry related to that Interest.
   Based on that information DABBER will forward the Data packet to any
   active neighbour that has high probability to meet any of these DIDs.
   This forwarding is done through a DTNFace, which will create a bundle
   based on the Data packet to be sent.

   To forward Data packets, DABBER applies a social opportunistic
   contact paradigm to decide whether bundle replication is feasible.
   Its decision is based on social weight (w_(x,y)) towards the bundle's
   destination or on the importance (I(x)) of the encountered node
   (i.e., potential next forwarder) in the system.

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   If the encountered node has better relationship with the bundle's
   destination than the carrier in a given daily sample, it receives a
   bundle copy, since there is a much greater chance for the encountered
   node to meet the destination in the future.  If relationship to the
   bundle's destination is unknown, replication happens only if the
   encountered node has higher importance than the bundle's current
   carrier.

   In order to compute the social weight between nodes and their
   importance, DABBER uses parameters that are determined as nodes
   interact in the system.  A brief explanation of these parameters is
   given below:

   o CD_(x,y): Refers to the contact duration between nodes, i.e., time
   nodes spent in the communication range of one another, which would
   allow them to exchange information.  Within a given daily sample,
   different contacts can happen with varied lengths.

   o TCT_(x,y): Refers to the total contact time between nodes within a
   given daily sample.  It is given by the sum of all CD_(x,y) in that
   specific daily sample.

   o AD_(x,y): Refers to the average duration of contacts for the same
   daily sample over different days.  It is a Cumulative Moving Average
   (CMA) of the average duration, considering the TCT_(x,y) of the
   current daily sample and average duration in the same daily sample of
   the previous day, AD_(x,y)_old.

   o w_(x,y): Refers to the social weight between nodes at a given daily
   sample.  It reflects the level of social interaction among such nodes
   throughout their daily routines.

   o I_(x): Refers to the importance of a node in the system.  The
   importance is influenced by how well a node is socially related to
   other important nodes.

   o N_(x): Refers to the neighbor set of a node x, which it encountered
   in the current daily sample.

   o dumping factor (d): Refers the level of randomness considered by
   the forwarding algorithm.

   o daily sample (Ti): Refers to the time period in which the contact
   duration will be measured to determine social weight and node
   importance.

   As nodes interact, their CD_(x,y) is collected and used to determine
   TCT_(x,y), AD_(x,y), w_(x,y), and I_(x) at the end of every daily

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   sample.  If DABBER is configured with a high number of daily samples,
   the social weight and node importance will be more refined.  Thus, it
   is recommended the usage of twenty-four (24) daily samples
   representing each hour of the day: the first daily sample refers
   always to the zero hour of the day when the node is started.

   Being able to identify the current daily sample allows a proper
   computation of social weights and importance.  Hence, in the case of
   node failure (e.g., node crash) or node shutdown (e.g., lack of
   battery), nodes need to know exactly in which daily sample they
   stopped interaction, and more importantly how many daily samples have
   elapsed since then (elapsed_ds).  To guarantee that, the equation
   below is used:

   elapsed_ds = cnds * (ed - 1) + (cds - 1) + (cnds - lds) (1)

   where:

   "cnds" is the configured number of daily samples.

   "ed"refers to the number of elapsed days.

   "cds" refers to the current daily sample (the one in which the node
   came back on).

   "lds" refers to last daily sample (in which the node failed or shut
   down).

   With this, the node knows how many daily samples have elapsed and can
   proceed with the update of social weights and importance to reflect
   the lack of interaction that happen in reality.

5.1.  Time-Evolving Contact Duration

   The TECD utility function considers the duration of contacts
   (representing the intensity of social ties among users) and time-
   evolving interactions (reflecting users' habits over different daily
   samples).

   Regarding the notations used in the equations presented in this sub-
   section: sumk(...) denotes summation for k from 1 to n; sumj(...)
   denotes summation for j from i to i+t-1; sumy denotes summation from
   all y belonging to N(x).

   Two nodes may have a social weight, w_(x,y), that depends on the
   average total contact duration they have had in that same period of
   time over different days.  Within a specific daily sample Ti, node x
   has n contacts with node y, having each contact k a certain contact

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   duration, CD_(x,y).  At the end of each daily sample, the total
   contact time, TCT_(x,y), between nodes x and y is given by the
   equation below where n is the total number of contacts between the
   two nodes.

   TCT_(x,y) = sumk(CD_(x,y)) (2)

   The Total Contact Time between users in the same daily sample over
   consecutive days can be used to estimate the average duration of
   their contacts for that specific daily sample: the average duration
   of contacts between users x and y during a daily sample Ti in a day
   j, denoted by AD_(x,y) is given by a cumulative moving average of
   their TCT in that same daily sample, TCT_(x,y), and the average
   duration of their contacts during the same daily sample Ti on the
   previous day, denoted by AD_(x,y)_old, as shown in the equation
   below.

   AD_(x,y) = (TCT_(x,y)+(j-1)*AD(x,y)_old)/j (3)

   The social strength between users in a specific daily sample Ti may
   also provide some insight about their social strength in consecutive
   k samples in the same day, i+k.  This is what we call Time Transitive
   Property.  This property increases the probability of nodes being
   capable of transmitting large data chunks, since transmission can be
   resumed in the next daily sample with high probability.

   TECD is able to capture the social strength w_(x,y) between any pair
   of users x and y in a daily sample Ti based on the average duration
   AD_(x,y) of contacts between them in such daily sample and in
   consecutive t-1 samples, where t represents the total number of daily
   samples.  When k>t, the corresponding AD_(x,y) value refers to the
   daily sample k-t.  In the equation below the time transitive property
   is given by the weight t/(t+k-i), where the highest weight is
   associated to the average contact duration in the current daily
   sample, being it reduced in consecutive samples.

   TECD = w_(x,y) = sumj(t/(t+k-i)*AD_(x,y)) (4)

5.2.  TECD Importance

   As social interaction may also be modeled to consider the node
   importance, TECDi computes the importance, I_(x), of a node x (cf.
   equation below), considering the weights of the edges between x and
   all the nodes y in its neighbor set, N_(x), at a specific daily
   sample Ti along with their importance.

   TECDi = I_(x) = (1-d)+d*sumy(w_(x,y)*I_(y)/N_(x)) (5)

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   TECDi is based on the PeopleRank function.  However, TECDi considers
   not only node importance, but also the strength of social ties
   between bundle's current carrier and potential next hops.  Another
   difference is that, with TECDi, the neighbor set of a node x only
   includes the nodes which have been in contact with node x within a
   specific daily sample Ti, whereas in PeopleRank the neighbor set of a
   node includes all the nodes that ever had a link to node x.  Note
   that the level of randomness may vary with the application scenario.
   Unless previously experimented, it is suggested that dumping factor
   be set to 0.8.

5.3.  Forwarding strategy

   Independently of the application scenario, each node MUST employ a
   forwarding strategy.  The first rule is that if the encountered node
   is the final destination of a bundle, the carrier SHOULD prioritize
   such bundles by employing the prioritized forwarding strategy,
   described below.

   We use the following notation for the description provided in this
   section.  Nodes A and B are the nodes that encounter each other, and
   the strategies are described as they would be applied by node A.

5.3.1.  Basic Strategy

   Forward the bundle only if w_(B,D) > w_(A,D) or I_(B) > I_(A)

   When two nodes A and B meet in any daily sample Ti, node A gets from
   node B: a) the updated list of all neighbors of B, including the
   social weights that B has towards each of its neighbors, as well as
   the importance of B; b) the list of the bundles that B is carrying
   (bundle identifier, plus Endpoint Identifier (EID) of the
   destination); c) the list of the latest set of bundles acknowledged
   to B (the size of the list of acknowledged bundles returned by B
   depends on the local cache size and policy).  The information about
   the social weight, importance, bundle list, and acknowledged bundles
   received from node B are referenced in node A as w_(B,x)_recv,
   I_(B)_recv, bundleList(IDn, destinationEIDx)_recv, and
   ackedBundleList(IDn, destinationEIDx)_recv, respectively.

   For every bundle that A carries in its buffer, and i) is not carried
   by B, ii) has not been previously acknowledged to B, and iii) B has
   enough buffer space to store it, node A sends a copy to B if B has
   already encountered the bundle's destination D and its weight in
   w_(B,D)_recv is greater than A's weight towards this same destination
   D.  Otherwise, bundles are replicated if I_(B)_recv is greater than
   A's importance in the current daily sample Ti.

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   Finally, node A will update its own ackedBundleList and discard
   bundles that have already been acknowledged to node B.

5.3.2.  Prioritized Strategy

   Similar to the basic forwarding strategy, being the only difference
   the fact that prior to sending bundles, node A will first send those
   bundles that have node B as destination.

6.  Protocol Addictional Functionality

6.1.  Adjustment to data source mobility

   As NDN uses a publish/subscribe communication model, where request
   resolution and data transfer are decoupled, it is especially relevant
   to solve the problem of data source mobility.  Supporting data source
   mobility requires, first of all, finding the new location of the
   source each time data sources move, and, second, updating the name
   resolution system according to the new location, in order to maintain
   the consistency of NDN forwarding.

   This sub-section described a new feature of DABBER which follows a
   new reactive approach to face the challenges of the data source
   mobility and consistent forwarding in Mobile ICNs.  To this end,
   DABBER is using the efficient dissemination method for Opportunistic
   Networks [Optimal-stopping] to efficiently discover data sources by
   replicating Interest messages in an efficient way that avoids network
   flooding.

   With this new feature the prospective forwarders for a given Interest
   message (which are denoted as discoverers) are limited in number and
   carefully selected in terms of three criteria:

   o Centrality: how well connected a node is in the network.  The more
   central a node is, the bigger the chances are to find a data source.

   o Reliability: the likeliness a node does not drop messages.  The
   more reliable a node is, the least probable is that the Interest
   message will be discarded.

   o Similarity: how alike the contacted candidate node is in terms of
   shared acquaintances.  The less similar, the more likely is that it
   will find different nodes in the future.

   A combination of these three criteria defines a reward function
   (discoverer suitability) of an Optimal Stopping (OS) problem.  If a
   node finds a new node with a certain value for the discoverer
   suitability it is difficult to know whether this value is a good one

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   when compared with what a node could find in future contacts.  This
   decision is not trivial: if a node chooses early-contacted discoverer
   candidates, good results are not guaranteed because selected
   discoverers could have a low discoverer suitability metric.  On the
   other end of the spectrum, selecting late-contacted discoverer
   candidates does not guarantee either good discoverer nodes since it
   is likely that good candidates with high discovery suitability values
   would be skipped.

   DABBER is so extended with the ability to perform an OS-based
   strategy that allows nodes to select the most suitable node among all
   of the contacted ones to forward the Interest message.  This strategy
   relies on the existence of an optimal set of stopping values such
   that the nth discoverer node for a certain Interest message is the
   first contacted node which is the best of all the previously explored
   nodes, if the node has contacted the first stopping value.  If this
   node is not found, then it will be the first node which is the second
   best of all the previous nodes, if the node has contacted the second
   stopping value, and so on.  Otherwise, if these nodes are not found,
   then, the nth discoverer node will be the last nth node before
   reaching the last contacted node.  This makes the dissemination of
   the Interest messages in Mobile NDNs efficient, even, and pervasive
   all over the network, increasing the delivery ratio while decreasing
   the network overhead.

7.  Interoperability

   In this section we analyze the interoperability of DABBER with
   routing and forwarding mechanisms used in wired ICN networks, aiming
   to study how DABBER can help in ther interconnection of ICDTNs with
   wired ICN networks.  We analyze the interoperability of DABBER with
   two potential configurations of an ICN access network based on: a
   routing protocol able of disseminating name prefix information; a
   broadcast based forwarding approach.

7.1.  Interoperability with ICN routing

   DABBER LSA dissemination mechanism provides a good interoperability
   with ICN routing protocols based on link state, which normally
   exchange information about adjacency and name prefixes.  In this
   scenario the specification used by DABBER ensures a good level of
   interoperability, since DABBER follows the same message structure and
   sequence used by such protocols, such as the Named Data Link State
   Routing Protocol (NLSR).

   However, when DABBER is executing the LSA dissemination procedure
   over a Wi-Fi face, towards an edge router it will ignore all
   notifications that Chronosync will send related to Adjacency LSAs.

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7.2.  Interoperability with broadcast based forwarding

   Broadcast-based forwarding is a common mechanism in the design of
   some networks, such as switched Ethernet and mobile ad-hoc networks.
   In CCN/NDN networks this means that NFD broadcasts Interest packets
   that do not match an entry in the FIB, inserting then into the FIB
   the forwarding path learned through observation of Data return paths.
   The main challenge in broadcast based forwarding schemes is the
   prefix granularity problem: determine the name prefix of an inserted
   FIB entry from the Data name.  Several solutions exist
   [Self-learning], including the announcements of name prefixes, as
   done by DABBER.

   In any case DABBER interoperability with such CCN/NDN networks relies
   on the following considerations:

   o When in contact with a wireless edge router, DABBER always forward
   Interest packet towards the Wi-Fi Face, even when the Interest packet
   does not match an entry in the FIB.

   o Interest packets received from a wireless edge router will not be
   broadcast.  Interest packets will be forwarded if they match an entry
   in the FIB, or dropped otherwise.

8.  Security Considerations

   DABBER follows the CCN/NDN security framework built on public-key
   cryptography, allows it to secure the exchange of routing messages,
   by being able of verifying the authenticity of routing messages, and
   ensuring the needed levels of confidentiality.  Moreover, DABBER
   ensures the right level of privacy of the involved entities, who
   provide local information to support routing decisions.

   Routing security can be achieved not only by signing routing
   messages, but also by allowing the usage of multiple paths, as done
   by DABBER: when an anomaly is detected routers can retrieve the data
   through alternative paths.

   Besides the presented security and privacy considerations, the issue
   of Denial of Service (DoS) needs to be properly addressed.  An
   example is when a malicious user sends a high rate of broadcast
   messages aiming to exhaust available forwarding resources.

   The remaining of this section provides initial insights about the
   methods used by DABBER to ensure the authenticity, confidentiality of
   the routing message exchange as well as the privacy of the involved
   entities.

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8.1.  Authenticity

   DABBER routing messages are carried in Data packets containing a
   signature.  Hence, a DABBER device can verify the signature of each
   routing message to ensure that it was generated by the claimed origin
   node and was not tampered with during dissemination.  For this
   propose, DABBER makes use of a hierarchical trust model for routing
   to verify the keys used to sign the routing messages.

   Following the name structure described in section 2.3, DABBER can
   model a trust management as a five-level hierarch, although
   reflecting a different administrative structure: <network> represents
   the authority responsible by the international transit network
   allowing roaming services; <operator> represents the operator
   providing the mobile service; <home> represents the network site of
   the mobile operator where the node is registered; <node> represents
   the mobile equipment.  Each node can create a DABBER process that
   produces LSAs.

   With this hierarchical trust model, one can establish a chain of keys
   to authenticate LSAs.  Specifically, a LSA must be signed by a valid
   DABBER process, which runs on the same node where the LSA was
   originated.  To become a valid DABBER process, the process key must
   be signed by the corresponding node key, which in turn should be
   signed by the registered home network of the network operator.  Each
   home network key must be signed by the operator key, which must be
   certified by the network authority using the network key.

   Since keys must be retrieved in order to verify routing updates,
   DABBER allows each node to retrieve keys from its neighbors.  This
   means that a DABBER node will use the Interest/Data exchange process
   to gathers keys from all its direct neighbors.  Upon the reception of
   an Interest of the type /<network>/broadcast/KEYS each neighbor looks
   up the requested keys in their local key storage and return the key
   if it is found.  In case a neighbor does not have the requested key,
   the neighbor can further query its neighbors for such key.  The used
   key retrieval process makes use of a broadcast forwarding strategy,
   stopping at nodes who either own or cache the requested keys.

8.2.  Confidentiality

   Although being deployed under the control of an operator, DABBER
   allows its network to be extended beyond the reach of its
   infrastructure network, into scenarios where wireless communications
   between involved DABBER devices/router may be spoofed.  Hence, DABBER
   requires routing data confidentiality to ensure the setup of a secure
   communication topology.

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   DABBER basic approach relies on the usage of encryption to protect
   the confidentiality of routing messages.  By taking advantage of the
   semantically meaningful names DABBER relies on approaches such as
   Named-based Access Control (NAC) [NAC].  NAC provides content
   confidentiality and access control based on a combination of
   symmetric and asymmetric cryptography algorithms, while using NDN's
   data-centric security and naming convention to automate data access
   control.

   Being implemented in wireless devices that may energy constraint, it
   will be important to verify the efficiency of the cryptographic
   solution, namely since the generation of asymmetric key pairs as well
   as the symmetric and asymmetric encryption/decryption operations may
   be expensive in terms of the usage of resources.  devices.

8.3.  Privacy

   In DABBER, forwarding decisions are taken into account using
   different metrics such as centrality and similarity.  While these
   metrics may be efficient in terms of node selection, they can breach
   privacy of network users carrying networked devices by inferring
   social related information such as position inside groups, as well as
   information about the devices themselves.

   If exchanged as clear text, the information carried in routing
   metrics may potentially compromising the privacy of users.  A way of
   preserving the privacy of the users in DABBER is to use NDN-P2F
   [Privacy], a privacy-preserving forwarding scheme that uses
   homomorphic encryption for information-centric wireless Ad Hoc
   Networks.

   In, NDN-P2F, forwarding decisions are made by performing calculations
   on encrypted forwarding metric values without decrypting them first,
   while maintaining low overhead and delays.  As a result, forwarding
   decisions can be taken preserving the user's privacy.  For these
   purposes, homomorphic encryption is extremely useful.  This
   cryptographic scheme allows computations on ciphertexts and generates
   encrypted results that, when decrypted, match the results of the
   operations as if they had been performed on plaintexts.

   There are many homomorphic cryptosystems.  A good choice for DABBER
   can be the Paillier cryptosystem because it is lightweight and, among
   its properties, it includes the homomorphic addition and
   multiplication of plaintexts and the homomorphic multiplication by a
   scalar.  The Paillier cryptosystem, however, does not provide a way
   of calculating the encrypted subtraction, which is needed for metric
   comparisons.  For these purposes, the mapping scheme proposed in
   [PrivHab] can be used to be able to operate with negative numbers.

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9.  IANA Considerations

   This document has no actions for IANA.

10.  Acknowledgments

   The research leading to these results received funding from the
   European Union (EU) Horizon 2020 research and innovation programmer
   under grant agreement No 645124(Action full title: Universal, mobile-
   centric and opportunistic communications architecture, Action
   Acronym: UMOBILE)[Umobile].

   We thank all contributors, as well as the valuable comments offered
   by Lixia Zhang (UCLA) and Lan Wang (University of Memphis) to improve
   this draft.

11.  References

11.1.  Normative References

   [NDN-OPP]  Tavares, M. and P. Mendes, "NDN-Opp: Named-Data Networking
              in Opportunistic Networks", Technical Report COPE-SITI-TR-
              18-01 , January 2018.

   [NFD]      A. Afanasyev, et al, "NFD Developer's Guide", NDN
              Technical Report NDN-001 , October 2010.

11.2.  Informative References

   [ChronoSync]
              Zhu, Z. and A. Afanasyev, "Lets ChronoSync:Decentralized
              Dataset State Synchronization in Named Data Networking",
              in Proc. IEEE ICNP , October 2013.

   [Dlife]    Moreira, W., Mendes, P., and S. Sargento, "Opportunistic
              Routing based on daily routines", in Proc. of IEEE WoWMoM
              workshop on autonomic and opportunistic communications,
              San Francisco, USA , June 2012.

   [Dlife-draft]
              Moreira, W., Mendes, P., and E. Cerqueira, "Opportunistic
              Routing based on Users Daily Life Routine", IETF Internet
              Draft (draft-moreira-dlife-04) , May 2014.

   [ICN-routing-opp]
              P. Mendes, et al, "Information-centric Routing for
              Opportunistic Wireless Networks", n ACM ICN, Boston, USA ,
              September 2018.

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   [Loop-free]
              Schneider, K. and B. Zhang, "How to Establish Loop-Free
              Multipath Routes in Named Data Networking", NDN Technical
              Report NDN-0044 , April 2017.

   [NAC]      Z. Zhang, et al, "NAC: Automating Access Control via Named
              Data", in IEEE MILCOM , October 2018.

   [NDN-emergency]
              Tavares, M., Aponte, O., and P. Mendes, "Named-data
              Emergency Network Services", in ACM MOBISYS, Munich,
              Germany , June 2018.

   [NDN-opportunisticnets]
              Dynerowicz, S. and P. Mendes, "Named-Data Networking in
              Opportunistic Networks", ACM ICN, Berlin, Germany ,
              September 2017.

   [Optimal-stopping]
              Borrego, C., Borrell, J., and S. Robles, "Efficient
              broadcast in opportunistic networks using optimal stopping
              theory", Ad Hoc Networks , May 2019.

   [PartialSync]
              Zhang, M., Lehman, V., and L. Wang, "PartialSync:Efficient
              Synchronization of a Partial Namespace in NDN", NDN
              Technical Report NDN-0039 , June 2016.

   [Privacy]  C. Borrego, et al, "Privacy-Preserving Forwarding using
              Homomorphic Encryption for Information-Centric Wireless Ad
              Hoc Networks", IEEE Communications Letters , July 2019.

   [PrivHab]  S.Carmona, et al, "PrivHab+: A secure geographic routing
              protocol for DTN", Elsevier Computer Communications , May
              2016.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4838]  Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
              R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
              Networking Architecture", RFC 4838, DOI 10.17487/RFC4838,
              April 2007, <https://www.rfc-editor.org/info/rfc4838>.

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   [RFC5050]  Scott, K. and S. Burleigh, "Bundle Protocol
              Specification", RFC 5050, DOI 10.17487/RFC5050, November
              2007, <https://www.rfc-editor.org/info/rfc5050>.

   [RFC6693]  Lindgren, A., Doria, A., Davies, E., and S. Grasic,
              "Probabilistic Routing Protocol for Intermittently
              Connected Networks", RFC 6693, DOI 10.17487/RFC6693,
              August 2012, <https://www.rfc-editor.org/info/rfc6693>.

   [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>.

   [Scorp]    Moreira, W., Mendes, P., and S. Sargento, "Social-aware
              Opportunistic Routing Protocol based on User's
              Interactions and Interests", in Proc. of AdhocNets,
              Barcelona, Spain , October 2013.

   [Self-learning]
              Shi, J., Newberry, E., and B. Zhang, "On Broadcast-based
              Self-Learning in Named Data Networking", in Proc. Of IFIP
              Networking, Stockholm , June 2017.

   [Umobile]  C. Sarros, et al, "Connecting the Edges: A Universal,
              Mobile centric and Opportunistic Communications
              Architecture", IEEE Communication Magazine , February
              2018.

   [UmobileD45]
              R. Sofia, et al, "UMOBILE D45 - Report on Data Collection
              and Inference Models", Umobile Technical Report ,
              September 2018.

11.3.  URIs

   [1] https://github.com/COPELABS-SITI/ndn-opp

Authors' Addresses

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   Paulo Mendes (editor)
   Airbus
   Willy-Messerschmitt Strasse 1
   Munich  81663
   Germany

   Email: paulo.mendes@airbus.com
   URI:   http://www.paulomilheiromendes.com

   Rute C. Sofia
   fortiss GmbH
   Guerickestrasse 25
   Munich  80805
   Germany

   Email: sofia@fortiss.org
   URI:   http://www.rutesofia.com

   Vassilis Tsaoussidis
   Democritus University of Thrace
   University Campus
   Komotini  69100
   Greece

   Email: vtsaousi@ee.duth.gr

   Carlos Borrego
   Autonomous University of Barcelona
   Department of Information and Communications Engineering
   Bellaterra  08193
   Spain

   Email: carlos.borrego@uab.cat

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