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DTN Research Group                                           A. Lindgren
Internet-Draft                                                      SICS
Intended status: Experimental                                   A. Doria
Expires: April 26, 2011                   Lulea University of Technology
                                                               E. Davies
                                                        Folly Consulting
                                                               S. Grasic
                                          Lulea University of Technology
                                                        October 23, 2010


  Probabilistic Routing Protocol for Intermittently Connected Networks
                      draft-irtf-dtnrg-prophet-08

Abstract

   This document is a product of the Delay Tolerant Networking Research
   Group and has been reviewed by that group.  No objections to its
   publication as an RFC were raised.

   This document defines PRoPHET, a Probabilistic Routing Protocol using
   History of Encounters and Transitivity.  PRoPHET is a variant of the
   epidemic routing protocol for intermittently connected networks that
   operates by pruning the epidemic distribution tree to minimize
   resource usage while still attempting to achieve the best case
   routing capabilities of epidemic routing.  It is intended for use in
   sparse mesh networks where there is no guarantee that a fully
   connected path between source and destination exists at any time,
   rendering traditional routing protocols unable to deliver messages
   between hosts.  These networks are examples of networks where there
   is a disparity between the latency requirements of applications and
   the capabilities of the underlying network (networks often referred
   to as Delay and Disruption Tolerant).  The document presents an
   architectural overview followed by the protocol specification.

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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   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



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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 26, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Relation to the Delay Tolerant Networking architecture . .  7
     1.2.  Applicability of the protocol  . . . . . . . . . . . . . .  8
     1.3.  PRoPHET as Compared to Regular Routing Protocols . . . . .  9
     1.4.  Requirements notation  . . . . . . . . . . . . . . . . . . 10
   2.  Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 11
     2.1.  PRoPHET  . . . . . . . . . . . . . . . . . . . . . . . . . 11
       2.1.1.  Delivery Predictability Calculation  . . . . . . . . . 11
       2.1.2.  Optional Delivery Predictability Optimizations . . . . 14
       2.1.3.  Forwarding Strategies and Queueing Policies  . . . . . 15
     2.2.  Bundle Agent to Routing Agent Interface  . . . . . . . . . 16
     2.3.  PRoPHET Zone Gateways  . . . . . . . . . . . . . . . . . . 17
     2.4.  Lower Layer Requirements and Interface . . . . . . . . . . 18
   3.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . . 19
     3.1.  Neighbor Awareness . . . . . . . . . . . . . . . . . . . . 19
     3.2.  Information Exchange Phase . . . . . . . . . . . . . . . . 19
       3.2.1.  Routing Information Base Dictionary  . . . . . . . . . 20
     3.3.  Routing Algorithm  . . . . . . . . . . . . . . . . . . . . 20
     3.4.  Bundle Passing . . . . . . . . . . . . . . . . . . . . . . 23
       3.4.1.  Custody  . . . . . . . . . . . . . . . . . . . . . . . 23
     3.5.  When a Bundle Reaches its Destination  . . . . . . . . . . 23
     3.6.  Forwarding Strategies  . . . . . . . . . . . . . . . . . . 24
     3.7.  Queueing Policies  . . . . . . . . . . . . . . . . . . . . 26
   4.  Message Formats  . . . . . . . . . . . . . . . . . . . . . . . 29
     4.1.  Header . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     4.2.  TLV Structure  . . . . . . . . . . . . . . . . . . . . . . 34
     4.3.  TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
       4.3.1.  Hello TLV  . . . . . . . . . . . . . . . . . . . . . . 34
       4.3.2.  Error TLV  . . . . . . . . . . . . . . . . . . . . . . 36
       4.3.3.  Routing Information Base Dictionary TLV  . . . . . . . 37
       4.3.4.  Routing Information Base TLV . . . . . . . . . . . . . 38
       4.3.5.  Bundle Offer and Response TLV  . . . . . . . . . . . . 39
   5.  Detailed Operation . . . . . . . . . . . . . . . . . . . . . . 42
     5.1.  High Level State Tables  . . . . . . . . . . . . . . . . . 42
     5.2.  Hello Procedure  . . . . . . . . . . . . . . . . . . . . . 44
       5.2.1.  State Tables . . . . . . . . . . . . . . . . . . . . . 46
       5.2.2.  Interaction with Nodes Using Version 1 of PRoPHET  . . 48
     5.3.  Information Exchange and Bundle Passing Phase  . . . . . . 49
       5.3.1.  State Tables . . . . . . . . . . . . . . . . . . . . . 49
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 54
     6.1.  Attacks on the Operation of the Protocol . . . . . . . . . 54
       6.1.1.  Black Hole Attack  . . . . . . . . . . . . . . . . . . 54
       6.1.2.  Limited Black Hole Attack/Identity Spoofing  . . . . . 55
       6.1.3.  Fake PRoPHET ACKs  . . . . . . . . . . . . . . . . . . 56
       6.1.4.  Bundle Store Overflow  . . . . . . . . . . . . . . . . 56
       6.1.5.  Bundle Store Overflow with Delivery Predictability



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               Manipulation . . . . . . . . . . . . . . . . . . . . . 56
     6.2.  Interactions with External Routing Domains . . . . . . . . 57
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 58
     7.1.  DTN Routing Protocol Number  . . . . . . . . . . . . . . . 59
     7.2.  PRoPHET Version  . . . . . . . . . . . . . . . . . . . . . 59
     7.3.  Header Flags . . . . . . . . . . . . . . . . . . . . . . . 60
     7.4.  Result . . . . . . . . . . . . . . . . . . . . . . . . . . 60
     7.5.  Code . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
     7.6.  Error and Log Messages . . . . . . . . . . . . . . . . . . 61
     7.7.  TLV Type . . . . . . . . . . . . . . . . . . . . . . . . . 62
     7.8.  Hello TLV Flags  . . . . . . . . . . . . . . . . . . . . . 62
     7.9.  Error TLV Flags  . . . . . . . . . . . . . . . . . . . . . 63
     7.10. RIB Base Dictionary TLV Flags  . . . . . . . . . . . . . . 63
     7.11. RIB TLV Flags  . . . . . . . . . . . . . . . . . . . . . . 63
     7.12. RIB Flags  . . . . . . . . . . . . . . . . . . . . . . . . 64
     7.13. Bundle Flags . . . . . . . . . . . . . . . . . . . . . . . 64
   8.  Implementation Experience  . . . . . . . . . . . . . . . . . . 65
   9.  Deployment Experience  . . . . . . . . . . . . . . . . . . . . 66
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 67
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 68
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 68
     11.2. Informative References . . . . . . . . . . . . . . . . . . 68
   Appendix A.  PRoPHET Example . . . . . . . . . . . . . . . . . . . 70
   Appendix B.  Neighbor Discovery Example  . . . . . . . . . . . . . 72
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 73


























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1.  Introduction

   The Probabilistic Routing Protocol using History of Encounters and
   Transitivity (PRoPHET) algorithm enables communication between
   participating nodes wishing to communicate in an intermittently
   connected network where at least some of the nodes are mobile.  One
   of the most basic requirements for 'traditional' (IP) networking is
   that there must exist a fully connected path between communication
   endpoints for the duration of a communication session in order for
   communication to be possible.  There are, however, a number of
   scenarios where connectivity is intermittent so that this is not the
   case (thus rendering the end-to-end use of traditional networking
   protocols impossible), but where it still is desirable to allow
   communication between nodes.

   Consider a network of mobile nodes using wireless communication with
   a limited range which is less than the typical excursion distances
   over which the nodes travel.  Communication between a pair of nodes
   at a particular instant is only possible when the distance between
   the nodes is less than the range of the wireless communication.  This
   means that, even if messages are forwarded through other nodes acting
   as intermediate routes, there is no guarantee of finding a viable
   continuous path when it is needed to transmit a message.

   One way to enable communication in such scenarios, is by allowing
   messages to be buffered at intermediate nodes for a longer time than
   normally occurs in the queues of conventional routers (c.f.  Delay
   and Disruption Tolerant Networking [RFC4838]).  It would then be
   possible to exploit the mobility of a subset of the nodes to bring
   messages closer to their destination by transferring them to other
   nodes as they meet.  Figure 1 shows how the mobility of nodes in such
   a scenario can be used to eventually deliver a message to its
   destination.  In this figure, the four sub-figures (a) - (d)
   represent the physical positions of four nodes (A, B, C, and D) at
   four time instants, increasing from (a) to (d) and associated radio
   ranges.  At the start time node A has a message (indicated by a *
   next to that node) to be delivered to node D, but there does not
   exist a path between nodes A and D because of the limited range of
   available wireless connections.  As shown in sub-figures (a) - (d),
   the mobility of the nodes allows the message to first be transferred
   to node B, then to node C, and when finally node C moves within range
   of node D, it can deliver the message to its final destination.  This
   technique is known as 'transitive networking'.

   Mobility and contact patterns in real application scenarios are
   likely to be non-random, but rather be predictable, based on the
   underlying activities of the higher level application (this could for
   example stem from human mobility having regular traffic patterns



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   based on repeating behavioral patterns (e.g., going to work or the
   market and returning home) and social interactions, or from any
   number of other node mobility situations where a proportion of nodes
   are mobile and move in ways that are not completely random over time
   but have a degree of predictability over time).  This means that if a
   node has visited a location or been in contact with a certain node
   several times before, it is likely that it will visit that location
   or meet that node again.

   PRoPHET can also be used in some networks where such mobility as
   described above does not take place.  Predictable patterns in node
   contacts can also occur among static nodes where varying radio
   conditions or power-saving sleeping schedules cause connection
   between nodes to be intermittent.

   In previously discussed mechanisms to enable communication in
   intermittently connected networks, such as Epidemic Routing
   [vahdat_00], very general approaches have been taken to the problem
   at hand.  In an environment where buffer space and bandwidth are
   infinite, Epidemic Routing will give an optimal solution to the
   problem of routing in an intermittently connected network with regard
   to message delivery ratio and latency.  However, in most cases
   neither bandwidth nor buffer space is infinite, but instead they are
   rather scarce resources, especially in the case of sensor networks.

   PRoPHET is fundamentally an epidemic protocol with strict pruning.
   An epidemic protocol works by transferring its data to each and every
   node it meets.  As data is passed from node to node, it is eventually
   passed to all nodes, including the target node.  One of the
   advantages of an epidemic protocol is that by trying every path, it
   is guaranteed to try the best path.  One of the disadvantages of an
   epidemic protocol is the extensive use of resources with every node
   needing to carry every packet and the associated transmission costs.
   PRoPHET's goal is to gain the advantages of an epidemic protocol
   without paying the price in storage and communication resources
   incurred by the basic epidemic protocol.  That is, PRoPHET offers an
   alternative to basic Epidemic Routing, with lower demands on buffer
   space and bandwidth, and with equal or better performance in cases
   where those resources are limited, and without loss of generality in
   scenarios where it is suitable to use PRoPHET.











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     +----------------------------+   +----------------------------+
     |                      ___   |   |                      ___   |
     |      ___            /   \  |   |                     /   \  |
     |     /   \          (  D  ) |   |                    (  D  ) |
     |    (  B  )          \___/  |   |     ___             \___/  |
     |     \___/    ___           |   |    /___\    ___            |
     |___          /   \          |   |   (/ B*\)  /   \           |
     |   \        (  C  )         |   |   (\_A_/) (  C  )          |
     | A* )        \___/          |   |    \___/   \___/           |
     |___/                        |   |                            |
     +----------------------------+   +----------------------------+
              (a) Time t                     (b) Time (t + dt)
     +----------------------------+   +----------------------------+
     |        _____         ___   |   |        ___           ___   |
     |       / / \ \       /   \  |   |       /   \         /___\  |
     |      ( (B C* )     (  D  ) |   |      (  B  )       (/ D*\) |
     |       \_\_/_/       \___/  |   |       \___/        (\_C_/) |
     |     ___                    |   |     ___             \___/  |
     |    /   \                   |   |    /   \                   |
     |   (  A  )                  |   |   (  A  )                  |
     |    \___/                   |   |    \___/                   |
     |                            |   |                            |
     +----------------------------+   +----------------------------+
          (c) Time (t + 2*dt)               (d) Time (t + 3*dt)


               Figure 1: Example of transitive communication

   This document presents a framework for probabilistic routing in
   intermittently connected networks, using an assumption of non-random
   mobility of nodes to improve the delivery rate of messages while
   keeping buffer usage and communication overhead at a low level.
   First, a probabilistic metric called delivery predictability is
   defined.  The document then goes on to define a probabilistic routing
   protocol using this metric.

1.1.  Relation to the Delay Tolerant Networking architecture

   The Delay Tolerant Networking (DTN) architecture[RFC4838] defines an
   architecture for communication in environments where traditional
   communication protocols can not be used due to excessive delays, link
   outages and other extreme conditions.  The intermittently connected
   networks considered here are a subset of those covered by the DTN
   architecture.  The DTN architecture defines routes to be computed
   based on a collection of 'contacts' indicating the start time,
   duration, endpoints, forwarding capacity and latency of a link in the
   topology graph.  These contacts may be deterministic, or may be
   derived from estimates.  The architecture defines some different



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   types of intermittent contacts.  The ones called opportunistic and
   predicted are the ones addressed by this protocol.

   Opportunistic contacts are those that are not scheduled, but rather
   present themselves unexpectedly and frequently arise due to node
   mobility.  Predicted contacts are like opportunistic contacts, but
   based on some information, it might be possible to draw some
   statistical conclusion as to whether or not a contact will be present
   soon.

   The DTN architecture also introduces the bundle protocol [RFC5050],
   which provides a way for applications to 'bundle' an entire session,
   including both data and meta-data, into a single message, or bundle,
   that can be sent as a unit.  The bundle protocol also provides end-
   to-end addressing and acknowledgments.  PRoPHET is specifically
   intended to provide routing services in a network environment that
   uses bundles as its data transfer mechanism, but could be also be
   used in other intermittent environments.

1.2.  Applicability of the protocol

   The PRoPHET routing protocol is mainly targeted at situations where
   at least some of the nodes are mobile with mobility that creates
   connectivity patterns that are not completely random over time but
   have a degree of predictability.  Such connectivity patterns can also
   occur in networks where nodes switch off radios to preserve power.
   Human mobility patterns (often containing daily or weekly periodic
   activities) provide one such example where PRoPHET is expected to be
   applicable, but the applicability is not limited to scenarios
   including humans.

   In order for PRoPHET to benefit from such predictability in the
   contact patterns between nodes, it is expected the network exist
   under similar circumstances over a longer time-scale (in terms of
   node encounters) so that the predictability can be accurately
   estimated.

   The PRoPHET protocol expects nodes to be able to establish a local
   TCP link in order to exchange the information needed by the PRoPHET
   protocol.  Protocol signaling is done out-of-band over this TCP link,
   without involving the Bundle Protocol agent [RFC5050].  The PRoPHET
   protocol is however expected to interact with the Bundle Protocol
   agent to retrieve information about available bundles as well as
   requesting that a bundle is sent to another node (it is expected that
   the associated bundle agents are then able to establish a link
   (probably over the TCP convergence layer) to perform this transfer).

   While PRoPHET is currently defined to run over TCP, in future



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   versions the information exchange may take place over other transport
   protocols as well and these may not provide message segmentation.
   Hence the capability is provided to segment protocol messages
   directly in the PRoPHET layer.

   In a large Delay and Disruption Tolerant Network (DTN), network
   conditions may vary widely, and in different parts of the network,
   different routing protocols may be appropriate.  In this
   specification, we consider routing within a single 'PRoPHET zone',
   which is a set of nodes among which messages are routed using
   PRoPHET.  In many cases, a PRoPHET zone will not span the entire DTN,
   but there will be other parts of the network with other
   characteristics that run other routing protocols.  To handle this,
   there may be nodes within the zone that act as gateways to other
   nodes that are the destinations for bundles generated within the zone
   or that insert bundles into the zone.  Thus, PRoPHET is not
   necessarily used end-to-end, but only within regions of the network
   where its use is appropriate.

1.3.  PRoPHET as Compared to Regular Routing Protocols

   While PRoPHET uses a mechanism for pruning the epidemic forwarding
   tree that is similar to the mechanism used in Metric-based Vector
   Routing protocols (where the metric might be distance or cost), it
   should not be confused with a metric vector protocol.

   In a traditional metric-based vector routing protocol, the
   information passed from node to node is used to create a single non-
   looping path from source to destination that is optimal given the
   metric used.  The path consists of a set of directed edges selected
   from the complete graph of communications links between the network
   nodes.

   In PRoPHET, that information is used to prune the epidemic tree of
   paths by removing paths that look less likely to provide an effective
   route for delivery of data to its intended destination.  One of the
   effects of this difference is that the regular notions of split
   horizon do not apply to PRoPHET.  The purpose of split horizon is to
   prevent a distance vector protocol from ever passing a packet back to
   the node that sent it the packet because it is well known that the
   source does not lie in that direction as determined when the directed
   path was computed.

   In an epidemic protocol, where that previous system already has the
   data, the notion of passing the data back to the node is redundant:
   the protocol can readily determine that such a transfer is not
   required.  Further, given the mobility and constant churn of
   encounters possible in a DTN that is dominated by opportunistic



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   encounters, it is quite possible that on a future encounter, that
   node might have become a better option for reaching the destination.
   Such a later encounter may require a re-transfer of the data if
   resource constraints have resulted in the data being deleted from the
   original carrier between the encounters.

   The logic of metric routing protocols does not map directly onto the
   family of epidemic protocols.  In particular it is inappropriate to
   try to assess such protocols against the criteria used to assess
   conventional routing protocols such as the metric vector protocols;
   this is not to say that the family of epidemic protocols do not have
   weaknesses but they have to be considered independently of
   traditional protocols.

1.4.  Requirements notation

   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.
































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2.  Architecture

2.1.  PRoPHET

   This section presents an overview of the main architecture of
   PRoPHET, a Probabilistic Routing Protocol using History of Encounters
   and Transitivity.  The protocol leverages the observations made on
   the non-randomness of mobility patterns present in many application
   scenarios to improve routing performance.  Instead of doing blind
   epidemic replication of bundles through the network as previous
   protocols have done, it applies 'probabilistic routing'.

   To accomplish this, a metric called 'delivery predictability',
   0 <= P_(A,B) <= 1, is established at every node A for each known
   destination B. This metric is calculated so that a node with a higher
   value for a certain destination is estimated to be a better candidate
   for delivering a bundle to that destination (i.e., if
   P_(A,B)>P_(C,B), bundles for destination B are preferable to forward
   to A rather than C).  It is later used when making forwarding
   decisions.  As routes in a DTN are likely to be asymmetric, the
   calculation of the delivery predictability reflects this, and P_(A,B)
   may be different from P_(B,A).

   The delivery predictability values in each node evolve over time both
   as a result of decay of the metrics between encounters between nodes
   and due to changes resulting from encounters when metric information
   for the encountered node is updated to reflect the encounter and
   metric information about other nodes is exchanged.

   When two PRoPHET nodes have a communication opportunity, they first
   exchange the delivery predictabilities for all destinations known by
   the nodes.  This information is used by the nodes to update the
   internal delivery predictability vector as described below.  After
   that, the nodes exchange information (including destination and size)
   about the bundles each node carries and the information is used in
   conjunction with the updated delivery predictabilities to decide
   which bundles to request to be forwarded from the other node based on
   the forwarding strategy used (as discussed in Section 2.1.3).

2.1.1.  Delivery Predictability Calculation

   As stated above, PRoPHET relies on calculating a metric based on the
   probability of encountering a certain node, and using that to support
   the decision of whether or not to forward a bundle to a certain node.
   This section describes the operations performed on the metrics stored
   in a node when it encounters another node and a communications
   opportunity arises.  In the operations described by the equations
   that follow, the updates are being performed by node A, P_(A,B) is



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   the delivery predictability value that node A will have stored for
   the destination B after the encounter and P_(A,B)_old is the
   corresponding value that was stored before the encounter.  If no
   delivery predictability value is stored for a particular destination
   B, P_(A,B) is considered to be zero.

   As a special case, the metric value for a node itself is always
   defined to be 1 (i.e., P_(A,A)=1).

   The equations use a number of parameters that can be selected to
   match the characteristics of the mobility pattern in the PRoPHET zone
   where the node is located.  Recommended settings for the various
   parameters are given in Section 3.3.  The impact on the evolution of
   delivery predictabilities if encountering nodes have different
   parameter setting is discussed in Section 2.1.1.1.

   The calculation of the updates to the delivery predictabilities
   during an encounter has three parts.

   When two nodes meet, the first thing they do is to update the
   delivery predictability for each other, so that nodes that are often
   encountered have a high delivery predictability.  If node B has not
   met node A for a long time or has never met node B, such that
   P_(A,B) < P_first_threshold, then P_(A,B) should be set to
   P_encounter_first.  Because PRoPHET generally has no prior knowledge
   about whether this is an encounter that will be repeated relatively
   frequently or one that will be a rare event, P_encounter_first SHOULD
   be set to 0.5 unless the node has extra information obtained other
   than through the PRoPHET protocol about the likelihood of future
   encounters.  Otherwise, P_(A,B) should be calculated as shown in
   Equation 1, where 0 <= P_encounter <= 1 is a scaling constant setting
   the rate at which the predictability increases on encounters after
   the first and delta is a small positive number that effectively sets
   an upper bound for P_(A,B).  The limit is set so that
   predictabilities between different nodes stay strictly less than 1.
   The value of delta should normally be very small (e.g., 0.01) so as
   not to significantly restrict the range of available
   predictabilities, but can be chosen to make calculations efficient
   where this is important.

   P_(A,B) = P_(A,B)_old + ( 1 - delta - P_(A,B)_old ) * P_encounter (1)

   If a pair of nodes do not encounter each other during an interval,
   they are less likely to be good forwarders of bundles to each other,
   thus the delivery predictability values must age, being reduced in
   the process.  The second part of the updates of the metric values is
   application of the aging equation shown in Equation 2, where
   0 <= gamma <= 1 is the aging constant, and K is the number of time



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   units that have elapsed since the last time the metric was aged.  The
   time unit used can differ, and should be defined based on the
   application and the expected delays in the targeted network.

   P_(A,B) = P_(A,B)_old * gamma^K (2)

   The delivery predictabilities are aged according to Equation 2 before
   being passed to an encountered node so that they reflect the time
   that has passed since the node had its last encounter with any other
   node.

   The delivery predictability also has a transitive property, that is
   based on the observation that if node A frequently encounters node B,
   and node B frequently encounters node C, then node C probably is a
   good node to forward bundles destined for node A to.  Equation 3
   shows how this transitivity affects the delivery predictability,
   where 0 <= beta <= 1 is a scaling constant that controls how large an
   impact the transitivity should have on the delivery predictability.

   P_(A,C) = MAX( P_(A,C)_old, P_(A,B) * P_(B,C)_recv * beta )    (3)

   Node A uses Equation 3 and the metric values received from the
   encountered node B (e.g., P_(B,C)_recv) in the third part of updating
   the metric values stored in node A.

2.1.1.1.  Impact of Encounters Between Nodes with Different Parameter
          Settings

   The various parameters used in the three equations described in
   Section 2.1.1 are set independently in each node and it is therefore
   possible that encounters may take place between nodes that have been
   configured with different values of the parameters.  This section
   considers whether this could be problematic for the operation of
   PRoPHET in that zone.

   It is desirable that all the nodes operating in a PRoPHET zone should
   use closely matched values of the parameters and that the parameters
   should be set to values that are appropriate for the operating zone.
   More details of how to select appropriate values are given in
   Section 3.3.  Using closely matched values means that delivery
   predictabilities will evolve in the same way in each node leading to
   consistent decision making about the bundles that should be exchanged
   during encounters.

   Before going on to consider the impact of reasonable but different
   settings, it should be noted that malicious nodes can use
   inappropriate settings of the parameters to disrupt delivery of
   bundles in a PRoPHET zone as described in Section 6.



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   Firstly and importantly, use of different, but legitimate, settings
   in encountering nodes will not cause problems in the protocol itself.
   Apart from P_encounter_first, the other parameters control the rate
   of change of the the metric values or limit the range of valid values
   that will be stored in a node.  None of the calculations in a node
   will be invalidated or result in illegal values if the metric values
   received from another node had been calculated using different
   parameters.

   Simulation work indicates that the update calculations are quite
   stable in the face of changes to the rate parameters, so that minor
   discrepancies will not have a major impact on the performance of the
   protocol.  The protocol is explicitly designed to deal with
   situations where there are random factors in the opportunistic nature
   of node encounters and this randomness dominates over the
   discrepancies in the parameters.

   More major discrepancies may lead to sub-optimal behavior of the
   protocol as certain paths might be more preferred or more deprecated
   inappropriately.  However, since the protocol overall is epidemic in
   nature, this would not generally lead to non-delivery of bundles as
   they would also be passed to other nodes and would still be delivered
   though possibly not on the optimal path.

2.1.2.  Optional Delivery Predictability Optimizations

2.1.2.1.  Smoothing

   To give the delivery predictability a smoother rate of change, a node
   MAY apply one of the following methods to smooth the metric:

   1.  Keep a list of NUM_P (the recommended value is 4, which has been
       shown in simulations to give a good trade off between smoothness
       and rate of response to changes) values for each destination
       instead of only a single value.  The list is held in order of
       acquisition.  When a delivery predictability is updated, the
       value at the 'newest' position in the list is used as input to
       the equations in Section 2.1.1.  The oldest value in the list is
       then discarded and the new value is written in the 'newest'
       position of the list.  When a delivery predictability value is
       needed (either for sending to a peering PRoPHET node, or for
       making a forwarding decision), the average of the values in the
       list is calculated, and that value is then used.  If less than
       NUM_P values have been entered into the list, only the positions
       that have been filled should be used for the averaging.

   2.  In addition to keeping the delivery predictability as described
       in Section 2.1.1, a node MAY also keep an exponential weighted



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       moving average (EWMA) of the delivery predictability.  The EWMA
       is then used for making forwarding decisions and to report to
       peering nodes, but the value calculated according to
       Section 2.1.1 is still used as input to the calculations of new
       delivery predictabilities.  The EWMA is calculated according to
       Equation 4, where 0 <= alpha <= 1 is the weight of the most
       current value.


   P_ewma = P_ewma_old * (1 - alpha) + P * alpha (4)

   The appropriate choice of alpha may vary depending on application
   scenario circumstances.  Unless prior knowledge of the scenario is
   available, it is suggested that alpha is set to 0.5.

2.1.2.2.  Removal of Low Delivery Predictabilities

   To reduce the data to be transferred between two nodes, a node MAY
   treat delivery predictabilities smaller than epsilon, where epsilon
   is a small number, as if they were zero, and thus they do not need to
   be included in the list sent during the information exchange phase.
   If this optimization is used, care must be taken to select epsilon to
   be smaller than delivery predictability values normally present in
   the network for destinations for which this node is a forwarder.  It
   is possible that epsilon could be calculated based on delivery
   predictability ranges and the amount they change historically, but
   this has not been investigated yet.

2.1.3.  Forwarding Strategies and Queueing Policies

   In traditional routing protocols, choosing where to forward a message
   is usually a simple task; the message is sent to the neighbor that
   has the path to the destination with the lowest cost (often the
   shortest path).  Normally the message is also only sent to a single
   node since the reliability of paths is relatively high.  However, in
   the settings we envision here, things are radically different.  The
   first possibility that must be considered when a bundle arrives at a
   node is that there might not be a path to the destination available,
   so the node has to buffer the bundle and upon each encounter with
   another node, the decision must be made whether or not to transfer a
   particular bundle.  Furthermore, having duplicates of messages (on
   different nodes, as the bundle offer/request mechanism described in
   Section 4.3.5 ensures that a node does not receive a bundle it
   already carries) may also be sensible, as forwarding a bundle to
   multiple nodes can increase the delivery probability of that bundle.

   Unfortunately, these decisions are not trivial to make.  In some
   cases it might be sensible to select a fixed threshold and only give



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   a bundle to nodes that have a delivery predictability over that
   threshold for the destination of the bundle.  On the other hand, when
   encountering a node with a low delivery predictability, it is not
   certain that a node with a higher metric will be encountered within
   reasonable time.  Thus, there can also be situations where we might
   want to be less strict in deciding who to give bundles to.
   Furthermore, there is the problem of deciding how many nodes to give
   a certain bundle to.  Distributing a bundle to a large number of
   nodes will of course increase the probability of delivering that
   particular bundle to its destination, but this comes at the cost of
   consuming more system resources for bundle storage and possibly
   reducing the probability of other bundles being delivered.  On the
   other hand, giving a bundle to only a few nodes (maybe even just a
   single node) will use less system resources, but the probability of
   delivering a bundle is lower, and the delay incurred high.

   When resources are constrained, nodes may suffer from storage
   shortage, and may have to drop bundles before they have been
   delivered to their destinations.  Similarly to when deciding whether
   or not to forward a bundle, deciding which bundle to drop to still
   maintain good performance might require different policies in
   different scenarios.

   Nodes MAY define their own forwarding strategies and queueing
   policies that take into account the special conditions applicable to
   the nodes, and local resource constraints.  Some default strategies
   and policies that should be suitable for most normal operation are
   defined in Section 3.6 and Section 3.7.

2.2.  Bundle Agent to Routing Agent Interface

   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.  This specification extends
   the bundle agent with a routing agent that controls the actions of
   the bundle agent during an (opportunistic) communications
   opportunity.

   This specification defines the details of the PRoPHET routing agent,
   but the interface defines a more general interface that is also
   applicable to alternative routing protocols.

   To enable the PRoPHET routing agent to operate properly, it must be
   aware of the bundles stored at the node, and it must also be able to
   tell the bundle agent of that node to send a bundle to a peering
   node.  Therefore, the bundle agent needs to provide the following
   interface/functionality to the routing agent:



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

   Send Bundle
        Makes the bundle agent send a specified bundle.

   Accept Bundle
        Gives the bundle agent a new bundle to store.

   Bundle Delivered
        Tells the bundle agent that a bundle was delivered to its
        destination.

   Drop Bundle Advice
        Advises the bundle agent that a specified bundle should not be
        offered for forwarding in future and may be dropped by the
        bundle agent if appropriate.

   Route Import
        Can be used by a gateway node in a PRoPHET zone to import
        reachability information about EIDs that are external to the
        PRoPHET zone.  Translation functions dependent on the external
        routing protocol will be used to set the appropriate delivery
        predictabilities for imported destinations as described in
        Section 2.3.

   Route Export
        Can be used by a gateway node in a PRoPHET zone to export
        reachability information (destination EIDs and corresponding
        delivery predictabilities) for use by routing protocols in other
        parts of the DTN.

2.3.  PRoPHET Zone Gateways

   PRoPHET is designed to handle routing primarily within a "PRoPHET
   zone," i.e. a set of nodes that all implement the PRoPHET routing
   scheme.  However, since we recognise that a PRoPHET routing zone is
   unlikely to encompass an entire DTN, there may be nodes within the
   zone that act as gateways to other nodes that are the destinations
   for bundles generated within the zone or that insert bundles into the
   zone.

   PRoPHET MAY elect to export and import routes across a bundle agent
   interface.  The delivery predictability to use for routes that are
   imported depends on the routing protocol used to manage those routes.
   If a translation function between the external routing protocol and
   PRoPHET exists, it SHOULD be used to set the delivery predictability.



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   If no such translation function exists, the delivery predictability
   SHOULD be set to 1.  For those routes that are exported, the current
   delivery predictability will be exported with the route.

2.4.  Lower Layer Requirements and Interface

   PRoPHET can be run on a large number of underlying networking
   technologies.  To accommodate its operation on all kinds of lower
   layers, it requires the lower layers to provide the following
   functionality and interfaces.

   Neighbor discovery and maintenance
        A PRoPHET node needs to know the identity of its neighbors and
        when new neighbors appear and old neighbors disappear.  Some
        wireless networking technologies might already contain
        mechanisms for detecting neighbors and maintaining this state.
        To avoid redundancies and inefficiencies, neighbor discovery is
        thus not included as a part of PRoPHET, but PRoPHET relies on
        such mechanism in lower layers.  The lower layers MUST provide
        the two functions listed below.  If the underlying networking
        technology does not support such services, a simple neighbor
        discovery scheme using local broadcasts of beacon messages could
        be run in-between PRoPHET and the underlying layer.  An example
        of a simple neighbor discovery mechanism that could be used is
        shown in Appendix B.

        New Neighbor
             Signals to the PRoPHET agent that a new node has become a
             neighbor.  A neighbor is here defined as another node that
             is currently within communication range of the wireless
             networking technology in use.  The PRoPHET agent should now
             start the Hello procedure as described in Section 5.2.

        Neighbor Gone
             Signals to the PRoPHET agent that one of its neighbors have
             left.

   Local Address
        An address used by the underlying communication layer (e.g. an
        IP or MAC address) that identifies the sender address of the
        current message.  This address must be unique among the nodes
        that can currently communicate, and is only used in conjunction
        with the Instance numbers to identify a communicating pair of
        nodes as described in Section 4.1.  This address and its format
        is dependent on the convergence layer that is being used by the
        bundle layer.





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3.  Protocol Overview

3.1.  Neighbor Awareness

   Since the operation of the protocol is dependent on the encounters of
   nodes running PRoPHET, the nodes must be able to detect when a new
   neighbor is present.  The protocol may be run on several different
   networking technologies, and as some of them might already have
   methods available for detecting neighbors, PRoPHET does not include a
   mechanism for neighbor discovery.  Instead, it requires the
   underlying layer to provide a mechanism to notify the protocol of
   when neighbors appear and disappear as described in Section 2.4.

   When a new neighbor has been detected, the protocol starts to set up
   a link with that node through the Hello message exchange as described
   in Section 5.2.  Once the link has been set up the protocol may
   continue to the Information Exchange Phase (see Section 3.2).  Once
   this has been completed the nodes will normally recalculate the
   delivery predictabilities using the equations and mechanisms
   described in Section 2.1.1 and Section 2.1.2.

   If the nodes have already done this exchange within a time interval
   shorter than, say, 5% of the characteristic intercontact time
   expected, the nodes MAY omit this recalculation phase depending on
   the characteristics of the communications mechanism and expected node
   behavior in the PRoPHET zone.  For example mobile nodes communicating
   with each other using Wi-Fi ad hoc mode may produce apparent multiple
   encounters with a short interval between them but these are
   frequently due to artifacts of the underlying physical network when
   using wireless connections, where transmission problems or small
   changes in location may result in repeated reconnections.  Treating
   such multiple reconnections as new encounters can give an
   inappropriate view of the real probability that the two nodes will
   encounter each other in the future and updating the predictabilities
   repeatedly will mirror this inappropriate view in the predictability
   values, which should be avoided.  On the other hand there may be some
   networks where it is desirable to update predictabilities after every
   encounter.  Also if one or other of the nodes has encountered a third
   node in the meantime it MAY be appropriate to carry out the exchange
   even though there has only been a short interval between encounters.
   Nodes can agree to suppress the recalculation phase but it MUST be
   carried out if one or other node does not request suppression.

3.2.  Information Exchange Phase

   The first step in the Information Exchange Phase is for the protocol
   to send a Routing Information Base Dictionary TLV to the node it is
   peering with.  This is a dictionary of the Endpoint Identifiers



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   (EIDs) of the nodes that will be listed in the Routing Information
   Base.  After this, a Routing Information Base TLV is sent.  This TLV
   contains a list of the EIDs that the node has knowledge of, and the
   corresponding delivery predictabilities for those nodes, and flags
   describing the capabilities of the sending node.  Upon reception of
   this TLV, the node updates its delivery predictability table
   according to the equations in Section 2.1.1, and using its forwarding
   strategy (see Section 2.1.3) determines which of its stored bundles
   it wishes to offer the peering node.  After making this decision, a
   Bundle Offer TLV is prepared, listing the bundle identifiers and
   their destinations for all bundles it wishes to offer the other node.
   If the Bundle Offer TLV lists a bundle for which the destination was
   not included in the first Routing Information Base Dictionary TLV
   sent, a new such TLV is sent first with an incremental update of the
   dictionary.  When the peering node has a dictionary with all
   necessary EIDs, the Bundle Offer TLV is sent to it.  The Bundle Offer
   TLV also contains a list of PRoPHET ACKs (see Section 3.5).  This
   phase of the protocol is described in more detail in Section 5.3.

   When a new bundle arrives at a node, the node MAY inspect its list of
   available neighbors, and if one of them is a candidate to forward the
   bundle, a new Bundle Offer TLV MAY be sent to that node.  If two
   nodes remain connected over a longer period of time, the Information
   Exchange Phase will be periodically re-initiated when the WAIT_INFO
   timer expires to allow new delivery predictability information to be
   spread through the network and new bundle exchanges to take place.

3.2.1.  Routing Information Base Dictionary

   To reduce the overhead of the protocol, the Routing Information Base
   and Bundle Offer/Request TLVs utilize an EID dictionary.  This
   dictionary maps long variable length EIDs as defined in [RFC4838] to
   shorter SDNV (see Section 4.1. of RFC 5050 [RFC5050]) identifiers
   that are used in place of the EIDs in subsequent TLVs.  The
   dictionary established only persist through a single encounter with a
   node (while the same link set up by the Hello procedure, with the
   same instance numbers, remains).

3.3.  Routing Algorithm

   The basic routing algorithm of the protocol is described in
   Section 2.1.  The algorithm uses some parameter values in the
   calculation of the delivery predictability metric.  These parameters
   are configurable depending on the usage scenario, but Figure 2
   provides some recommended default values.  A brief explanation of the
   parameters is given below.





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   P_encounter
        P_encounter is used to increase the delivery predictability for
        a destination when the destination node is encountered.  A
        larger value of P_encounter will increase the delivery
        predictability faster and fewer encounters will be required for
        the delivery predictability to reach a certain level.  Given
        that relative rather than absolute delivery predictability
        values are what is interesting for the forwarding mechanisms
        defined, the protocol is very robust to different values of
        P_encounter as long as the same value is chosen for all nodes.
        In order to reduce the effect of spurious encounters, a lower
        value, P_encounter_first, is used when a node is encountered for
        the first time, or when the last encounter was sufficiently long
        ago so that the delivery predictability for that node has
        decayed to a value below the threshold P_first_threshold.  We
        have found the values given in the table below to be suitable.

   alpha
        The alpha parameter is used in the optional smoothing of the
        delivery predictabilities described in Section 2.1.2.1.  It is
        used to determine the weight of the most current P-value in the
        calculation of an EWMA.

   beta
        The beta parameter adjusts the weight of the transitive property
        of PRoPHET, that is, how much consideration should be given to
        information about destinations that is received from encountered
        nodes.  If beta is set to zero, the transitive property of
        PRoPHET will not be active and only direct encounters will be
        used in the calculation of the delivery predictability.  The
        higher the value of beta the more rapidly encounters will
        increase predictabilities through the transitive rule.

   gamma
        The gamma parameter determines how quickly delivery
        predictabilities age.  A lower value of gamma will cause the
        delivery predictability to age faster.  The value of gamma
        should be chosen according to the scenario and environment in
        which the protocol will be used.  If encounters are expected to
        be very frequent, a lower value should be chosen for gamma than
        if encounters are expected to be rare.

   To set an appropriate gamma value, one should consider the 'average
   expected delivery' time T_aed in the PRoPHET zone where the protocol
   is to be used, and the time unit used (the resolution with which the
   delivery predictability is being updated).  The T_aed time interval
   can be estimated according to the average number of hops that bundles
   have to pass and average encounter frequency.  Clearly if bundles



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   have a Time To Live (TTL) that is less than T_aed they are unlikely
   to survive in the network to be delivered to a node in this PRoPHET
   zone, but the TTL for bundles created in nodes in this zone should
   not be chosen solely on this basis because they may pass through
   other networks.

   After estimating T_aed and selecting how much we want the delivery
   predictability to age in one T_aed time period (call this A), we can
   calculate the number of time unit in one T_aed as K=T_aed/timeunit.
   This can then be used to calculate gamma as gamma=Kth-root(A).  These
   instructions on how to set gamma are only given as a possible method
   for selecting an appropriate value, but network operators are free to
   set gamma as they choose.

   Recommended starting parameter values when specific network
   measurements have not been done are below.  Note: there are no "one
   size fits all" default values and the ideal values vary based on
   network characteristics.  It is not inherently necessary for the
   parameter values to be identical at all nodes, but it is recommended
   that similar values are used at all nodes within a PRoPHET zone as
   discussed in Section 2.1.1.1.


     +========================================+
     |      Parameter     | Recommended value |
     +========================================+
     |     P_encounter    |       0.5         |
     +----------------------------------------+
     |  P_encounter_first |       0.5         |
     +----------------------------------------+
     |  P_first_threshold |       0.1         |
     +----------------------------------------+
     |        alpha       |       0.5         |
     +----------------------------------------+
     |        beta        |       0.9         |
     +----------------------------------------+
     |        gamma       |       0.999       |
     +----------------------------------------+
     |        delta       |       0.01        |
     +========================================+


                   Figure 2: Default parameter settings








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3.4.  Bundle Passing

   Upon reception of the Bundle Offer TLV, the node inspects the list of
   bundles and decides which bundles it is willing to store for future
   forwarding, or that it is able to deliver to their destination.  This
   decision has to be made using local policies and considering
   parameters such as available buffer space.  For each such acceptable
   bundle, the node sends a Bundle Request TLV to its peering node,
   which in response to that sends the requested bundle.  If a node has
   some bundles it would prefer to receive ahead of others offered (e.g.
   bundles that it can deliver to their final destination), it MAY
   request the bundles in that priority order.  This is often desirable
   as there is no guarantee that the nodes will remain in contact with
   each other for long enough to transfer all the acceptable bundles.
   Otherwise, the node SHOULD assume that the bundles are listed in a
   priority order determined by the peering node's forwarding strategy,
   and request bundles in that order.

3.4.1.  Custody

   To free up local resources, a node may give custody of a bundle to
   another node that offers custody.  This is done to move the
   retransmission requirement further toward the destination.  The
   concept of custody transfer, and more details on the motivation for
   its use can be found in [RFC4838].  PRoPHET takes no responsibilities
   for making custody decisions.  Such decisions should be made by a
   higher layer.

3.5.  When a Bundle Reaches its Destination

   When a bundle reaches its destination within the PRoPHET zone (i.e.,
   within the part of the network where PRoPHET is used for routing; not
   necessarily the final destination of the bundle), a PRoPHET ACK for
   that bundle is issued.  A PRoPHET ACK is a confirmation that a bundle
   has been delivered to its destination in the PRoPHET zone (bundles
   might traverse several different types of networks using different
   routing protocols; thus, this might not be the final destination of
   the bundle).  When nodes exchange Bundle Offer TLVs, bundles that
   have been ACKed are also listed, having the "PRoPHET ACK" flag set.
   The node that receives this list updates its own list of ACKed
   bundles to be the union of its previous list and the received list.
   To prevent the list of ACKed bundles growing indefinitely, each
   PRoPHET ACK should have a timeout that MUST NOT be longer than the
   timeout of the bundle to which the ACK corresponds.

   When a node receives a PRoPHET ACK for a bundle it is carrying, it
   MAY delete that bundle from its storage, unless the node holds
   custody of that bundle.  The PRoPHET ACK only indicates that a bundle



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   has been delivered to its destination within the PRoPHET zone, so the
   reception of a PRoPHET ACK is not a guarantee that the bundle has
   been delivered to its final destination.

   Nodes MAY keep track of which nodes they have sent PRoPHET ACKs for
   certain bundles to, and MAY in that case refrain from sending
   multiple PRoPHET ACKs for the same bundle to the same node.

   If necessary in order to preserve system resources, nodes MAY drop
   PRoPHET ACKs prematurely, but SHOULD refrain from doing so if
   possible.

   It is important to keep in mind that PRoPHET ACKs and bundle
   ACKs[RFC5050] are different things.  PRoPHET ACKs are only valid
   within the PRoPHET part of the network, while bundle ACKs are end-to-
   end acknowledgments that may go outside of the PRoPHET network.

3.6.  Forwarding Strategies

   During the information exchange phase, nodes need to decide on which
   bundles they wish to exchange with the peering node.  Because of the
   large number of scenarios and environments that PRoPHET can be used
   in, and because of the wide range of devices that may be used, it is
   not certain that this decision will be based on the same strategy in
   every case.  Therefore, each node uses a _forwarding strategy_ to
   make this decision.  Nodes may define their own strategies, but this
   section defines a few basic forwarding strategies that nodes can use.
   Note: If the node being encountered is the destination of any of the
   bundles being carried, those bundles SHOULD be offered to the
   destination, even if that would violate the forwarding strategy.
   Some of the forwarding strategies listed here have been evaluated
   (together with a number of queueing policies) through simulations,
   and more information about that and recommendations on which
   strategies to use in different situations can be found in
   [lindgren_06].  If not chosen differently due to the characteristics
   of the deployment scenario, nodes SHOULD choose GRTR as the default
   forwarding strategy.

   The short names applied to the Forwarding Strategies should be read
   as mnemonic handles rather as specific acronyms for any set of words
   in the specification.

   We use the following notation in our descriptions below.  A and B are
   the nodes that encounter each other, and the strategies are described
   as they would be applied by node A. The destination node is D.
   P_(X,Y) denotes the delivery predictability stored at node X for
   destination Y, and NF is the number of times A has given the bundle
   to some other node.



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   GRTR
        Forward the bundle only if P_(B,D) > P_(A,D).

        When two nodes meet, a bundle is sent to the other node if the
        delivery predictability of the destination of the bundle is
        higher at the other node.  The first node does not delete the
        bundle after sending it as long as there is sufficient buffer
        space available (since it might encounter a better node, or even
        the final destination of the bundle in the future).

   GTMX
        Forward the bundle only if P_(B,D) > P_(A,D) && NF < NF_max.

        This strategy is like the previous one, but each bundle is given
        to at most NF_max other nodes apart from the destination.

   GTHR
        Forward the bundle only if
        P_(B,D) > P_(A,D) OR P_(B,D) > FORW_thres,
        where FORW_thres is a threshold value, above which a bundle
        should always be given to the node unless it is already present
        at the other node.

        This strategy is similar to GRTR, but among nodes with very high
        delivery predictability, bundles for that particular destination
        are spread epidemically.

   GRTR+
        Forward the bundle only if Equation 5 holds, where P_max is the
        largest delivery predictability reported by a node to which the
        bundle has been sent so far.


             P_(B,D) > P_(A,D) && P_(B,D) > P_max (5)

        This strategy is like GRTR, but nodes keep track of the largest
        delivery predictability of any node it has forwarded this bundle
        to, and only forward the bundle again if the currently
        encountered node has a greater delivery predictability than the
        maximum previously encountered.

   GTMX+
        Forward the bundle only if Equation 6 holds.



             P_(B,D) > P_(A,D) && P_(B,D) > P_max && NF < NF_max (6)




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        This strategy is like GTMX, but nodes keep track of P_max as in
        GRTR+.

   GRTRSort
        Select bundles in descending order of the value of
        P_(B,D) - P_(A,D).
        Forward the bundle only if P_(B,D) > P_(A,D).

        This strategy is like GRTR, but instead of just going through
        the bundle queue linearly, this strategy looks at the difference
        in delivery predictabilities for each bundle between the two
        nodes, and forwards the bundles with the largest difference
        first.  As bandwidth limitations or disrupted connections may
        result in not all bundles that would be desirable being
        exchanged, it could be desirable to first send bundles that get
        a large improvement in delivery predictability.

   GRTRMax
        Select bundles in descending order of P_(B,D).
        Forward the bundle only if P_(B,D) > P_(A,D).

        This strategy begins by considering the bundles for which the
        encountered node has the highest delivery predictability.  The
        motivation for doing this is the same as in GRTRSort, but based
        on the idea that it is better to give bundles to nodes with high
        absolute delivery predictabilities, instead of trying to
        maximize the improvement.

3.7.  Queueing Policies

   Because of limited buffer resources, nodes may need to drop some
   bundles.  As is the case with the forwarding strategies, which bundle
   to drop is also dependent on the scenario.  Therefore, each node also
   has a queueing policy that determines how its bundle queue is
   handled.  This section defines a few basic queueing policies, but
   nodes MAY use other policies if desired.  Some of the queueing
   policies listed here have been evaluated (together with a number of
   forwarding strategies) through simulations.  More information about
   that and recommendations on which policies to use in different
   situations can be found in [lindgren_06].  If not chosen differently
   due to the characteristics of the deployment scenario, nodes SHOULD
   choose FIFO as the default queueing policy.

   The short names applied to the Queueing Policies should be read as
   mnemonic handles rather as specific acronyms for any set of words in
   the specification.





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   FIFO
        Handle the queue in a First In First Out (FIFO) order.

        The bundle that was first entered into the queue is the first
        bundle to be dropped.

   MOFO - Evict most forwarded first
        In an attempt to maximize the delivery rate of bundles, this
        policy requires that the routing agent keeps track of the number
        of times each bundle has been forwarded to some other node.  The
        bundle that has been forwarded the largest number of times is
        the first to be dropped.

   MOPR - Evict most favorably forwarded first
        Keep a variable FAV for each bundle in the queue, initialized to
        zero.  Each time the bundle is forwarded, update FAV according
        to Equation 7, where P is the predictability metric the node the
        bundle is forwarded to has for its destination.


             FAV_new = FAV_old + ( 1 - FAV_old ) * P (7)

        The bundle with the highest FAV value is the first to be
        dropped.

   Linear MOPR - Evict most favorably forwarded first; linear increase
        Keep a variable FAV for each bundle in the queue, initialized to
        zero.  Each time the bundle is forwarded, update FAV according
        to Equation 8, where P is the predictability metric the node the
        bundle is forwarded to has for its destination.


             FAV_new = FAV_old + P (8)

        The bundle with the highest FAV value is the first to be
        dropped.

   SHLI - Evict shortest life time first
        As described in [RFC5050], each bundle has a timeout value
        specifying when it no longer is meaningful to its application
        and should be deleted.  Since bundles with short remaining time
        to life will soon be dropped anyway, this policy decides to drop
        the bundle with the shortest remaining life time first.  To
        successfully use a policy like this, there needs to be some form
        of time synchronization between nodes so that it is possible to
        know the exact lifetimes of bundles.  This is however not
        specific to this routing protocol, but a more general DTN
        problem.



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   LEPR - Evict least probable first
        Since the node is least likely to deliver a bundle for which it
        has a low delivery predictability, drop the bundle for which the
        node has the lowest delivery predictability, and that has been
        forwarded at least MF times, which is a minimum number of
        forwards that a bundle must have been forwarded before being
        dropped (if such a bundle exists).

   More than one queueing policy MAY be combined in an ordered set,
   where the first policy is used primarily, the second only being used
   if there is a need to tie-break between bundles given the same
   eviction priority by the primary policy, and so on.  As an example,
   one could select the queueing policy to be {MOFO; SHLI; FIFO}, which
   would start by dropping the bundle that has been forwarded the
   largest number of times.  If more than one bundle has been forwarded
   the same number of times, the one with the shortest remaining life
   time will be dropped, and if that also is the same, the FIFO policy
   will be used to drop the bundle first received.

   It is worth noting that obviously nodes MUST NOT drop bundles for
   which it has custody unless the lifetime expires.






























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4.  Message Formats

   This section defines the message formats of the PRoPHET routing
   protocol.  In order to allow for variable length fields, many numeric
   fields are encoded as Self-Delimiting Numeric Values (SDNVs).  The
   format of SDNVs is defined in [RFC5050].


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                            Header                             ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                             TLV 1                             ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                .                              |
      ~                                .                              ~
      |                                .                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                             TLV n                             ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                      Figure 3: Basic message format





















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4.1.  Header


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Protocol Number|Version| Flags |     Result    |     Code      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Receiver Instance        |      Sender Instance          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Transaction Identifier                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |S|      SubMessage Number      |         Length (SDNV)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                          Message Body                         ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                             Figure 4: Header

   Protocol Number
        The DTN Routing Protocol Number encoded as 8 bit unsigned
        integer in network bit order.  The value of this field is 0.
        The PRoPHET header is organized in this way so that in principle
        PRoPHET messages could be sent as the Protocol Data Unit of an
        IP packet if an IP protocol number was allocated for PRoPHET.
        At present PRoPHET is only specified to use a TCP transport for
        carriage of PRoPHET packets so that the protocol number serves
        only to identify the PRoPHET protocol within DTN.  Transmitting
        PRoPHET packets directly as an IP protocol on a public IP
        network such as the Internet would generally not work well
        because middle boxes such as firewalls and NAT boxes would be
        unlikely to allow the protocol to pass through and the protocol
        does not provide any congestion control.  However it could be so
        used on private networks for experimentation.  Also in future
        other protocols that require transmission of metadata between
        DTN nodes could potentially use the same format and protocol
        state machinery but with a different Protocol Number.

   Version
        The Version of the PRoPHET Protocol.  Encoded as a four bit
        unsigned integer in network bit order.  This document defines
        version 2.







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   Flags
        Reserved

   Result
        Field that is used to indicate whether a response is required to
        the request message if the outcome is successful.  A value of
        "NoSuccessAck" indicates that the request message does not
        expect a response if the outcome is successful, and a value of
        "AckAll" indicates that a response is expected if the outcome is
        successful.  In both cases a failure response MUST be generated
        if the request fails.

        In a response message, the result field can have two values:
        "Success," and "Failure".  The "Success" results indicates a
        success response.  All messages that belong to the same success
        response will have the same Transaction Identifier.  The
        "Success" result indicates a success response that may be
        contained in a single message or the final message of a success
        response spanning multiple messages.

        ReturnReceipt is a result field used to indicate that an
        acknowledgement is required for the message.  The default for
        Messages is that the controller will not acknowledge responses.
        In the case where an acknowledgement is required, it will set
        the Result Field to ReturnReceipt in the header of the Message.



        The result field is encoded as an 8 bit unsigned integer in
        network bit order.  The following values are currently defined:

           NoSuccessAck:       Result = 1
           AckAll:             Result = 2
           Success:            Result = 3
           Failure:            Result = 4
           ReturnReceipt       Result = 5

   Code
        Field gives further information concerning the result in a
        response message.  It is mostly used to pass an error code in a
        failure response but can also be used to give further
        information in a success response message or an event message.
        In a request message, the code field is not used and is set to
        zero.
        If the Code field indicates that the Error TLV is included in
        the message, further information on the error will be found in
        the Error TLV, which MUST be the the first TLV after the header.




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        The Code field is encoded as an 8 bit unsigned integer in
        network bit order.  The following value ranges are defined:

                  PRoPHET Error Messages       0x00 - 0x99
                  Reserved                     0xA0 - 0xFE
                  Error TLV in message             0xFF

   Sender Instance
        For messages during the Hello phase with the Hello SYN, Hello
        SYNACK, and Hello ACK functions (which are explained in
        Section 5.2), it is the sender's instance number for the link.
        It is used to detect when the link comes back up after going
        down or when the identity of the entity at the other end of the
        link changes.  The instance number is a 16-bit number that is
        guaranteed to be unique within the recent past and to change
        when the link or node comes back up after going down.  Zero is
        not a valid instance number.  For the RSTACK function (also
        explained in detail in Section 5.2), the Sender Instance field
        is set to the value of the Receiver Instance field from the
        incoming message that caused the RSTACK function to be
        generated.  Messages sent after the Hello phase is completed
        should use the sender's instance number for the link.  The
        Sender Instance is encoded as an unsigned integer in network bit
        order.

   Receiver Instance
        For messages during the Hello phase with the Hello SYN, Hello
        SYNACK, and Hello ACK functions, is what the sender believes is
        the current instance number for the link, allocated by the
        entity at the far end of the link.  If the sender of the message
        does not know the current instance number at the far end of the
        link, this field SHOULD be set to zero.  For the RSTACK message,
        the Receiver Instance field is set to the value of the Sender
        Instance field from the incoming message that caused the RSTACK
        message to be generated.  Messages sent after the Hello phase is
        completed should use what the sender believes is the current
        instance number for the link, allocated by the entity at the far
        end of the link.  The Sender Instance is encoded as a 16-bit
        unsigned integer in network bit order.

   Transaction Identifier
        Used to associate a message with its response message.  This
        should be set in request messages to a value that is unique for
        the sending host within the recent past.  Reply messages contain
        the Transaction Identifier of the request they are responding
        to.  The Transaction Identifier is a 32-bit bit pattern.





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   S-flag
        If S is set (value 1) then the SubMessage Number field indicates
        the total number of SubMessage segments that compose the entire
        message.  If it is not set (value 0) then the SubMessage Number
        field indicates the sequence number of this SubMessage segment
        within the whole message. the S field will only be set in the
        first sub-message of a sequence.

   SubMessage Number
        When a message is segmented because it exceeds the MTU of the
        link layer, each segment will include a SubMessage Number to
        indicate its position.  Alternatively, if it is the first sub-
        message in a sequence of sub-messages, the S flag will be set
        and this field will contain the total count of SubMessage
        segments.  The SubMessage Number is encoded as a 15-bit unsigned
        integer in network byte order

   Length
        Length in octets of this message including headers and message
        body.  If the message is fragmented, this field contains the
        length of this SubMessage.  The Length is encoded as an SDNV.

   The protocol also requires extra information about the link that the
   underlying communication layer MUST provide.  This information is
   used in the Hello procedure described in more detail in Section 5.2.
   Since this information is available from the underlying layer, there
   is no need to carry it in PRoPHET messages.  The following values are
   defined to be provided by the underlying layer:

   Sender Local Address
        An address used by the underlying communication layer as
        described in Section 2.4 that identifies the sender address of
        the current message.  This address must be unique among the
        nodes that can currently communicate, and is only used in
        conjunction with the Receiver Local Address and the Receiver
        Instance and Sender Instance to identify a communicating pair of
        nodes.

   Receiver Local Address
        An address used by the underlying communication layer as
        described in Section 2.4 that identifies the receiver address of
        the current message.  This address must be unique among the
        nodes that can currently communicate, and is only used in
        conjunction with the Sender Local Address and the Receiver
        Instance and Sender Instance to identify a communicating pair of
        nodes.

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   nodes are used as Sender and Receiver Local Addresses.

4.2.  TLV Structure

   All TLVs have the following format, and can be nested.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    TLV Type   |  TLV Flags    |      TLV Length (SDNV)        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                           TLV Data                            ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                           Figure 5: TLV Format

   TLV Type
        Specific TLVs are defined in Section 4.3.  The TLV Type is
        encoded as an 8 bit unsigned integer in network bit order.  Each
        TLV will have fields defined that are specific to the function
        of that TLV.

   TLV Flags
        These are defined per TLV type.  Flag n corresponds to bit 15-n
        in the TLV.

   TLV Length
        Length of the TLV in octets, including the TLV header and any
        nested TLVs.  Encoded as an SDNV.

4.3.  TLVs

4.3.1.  Hello TLV

   The Hello TLV is used to set up and maintain a link between two
   PRoPHET nodes.  Hello messages with the SYN function are transmitted
   periodically as beacons.  The Hello TLV is the first TLV exchanged
   between two PRoPHET nodes when they encounter each other.  No other
   TLVs can be exchanged until the first Hello sequence is completed.

   Once a communication link is established between two PRoPHET nodes,
   the Hello TLV will be sent once for each interval as defined in the
   interval timer.  If a node experiences the lapse of HELLO_DEAD Hello
   intervals without receiving a Hello TLV on an ESTAB connection (as
   defined in the state machine in Section 5.2), the connection SHOULD



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   be assumed broken.


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | TLV Type=0x01 | Hello Function|          TLV Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Timer (SDNV)  |EID Length,SDNV|  Sender EID (variable length) |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                        Figure 6: Hello TLV Format

   Hello Function
        Specifies the function of the Hello TLV.  Four main functions
        are specified for the Hello TLV, as well as one flag to indicate
        that the information exchange phase can be suppressed (to be
        used together with either the SYNACK or ACK flag).  The
        functions are encoded as an 8 bit unsigned integer.
        The encoding of the Hello Function is:


                SYN:     Hello Function = 1
                SYNACK:
                   Requesting execution of recalculation phase:
                         Hello Function = 2
                   Requesting suppression of recalculation phase:
                         Hello Function = 130

                ACK:
                   Requesting execution of recalculation phase:
                         Hello Function = 3
                   Requesting suppression of recalculation phase:
                         Hello Function = 131
                RSTACK:  Hello Function = 4


        If the SYNACK message requests suppression of recalculation
        (function 130) and the ACK message requests suppression of
        recalculation (function 131) then recalculation MUST be
        suppressed.  In all other cases recalculation must be done.

   TLV Data






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   Timer
        The Timer field is used to inform the receiver of the timer
        value used in the Hello processing of the sender.  The timer
        specifies the nominal time between periodic Hello messages.  It
        is a constant for the duration of a session.  The timer field is
        specified in units of 100ms and is encoded as an SDNV.

   EID Length
        The EID Length field is used to specify the length of the Sender
        EID field in octets.  If the Endpoint Identifier (EID) has
        already been sent at least once in a message with the current
        Sender Instance, a node MAY choose to set this field to zero,
        omitting the Sender EID from the Hello TLV.  The EID Length is
        encoded as an SDNV and the field is thus of variable length.

   Sender EID
        The Sender EID field specifies the DTN endpoint identifier (EID)
        of the sender that is to be used in updating routing information
        and making forwarding decisions.  If a node has multiple EIDs,
        one should be chosen for PRoPHET routing.  This field is of
        variable length.

4.3.2.  Error TLV


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | TLV type=0x02 |     Flags     |          TLV Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                               Data                            ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                        Figure 7: Error TLV Format

   TLV Flags
        Reserved

   TLV Data
        Reserved








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4.3.3.  Routing Information Base Dictionary TLV

   The Routing Information Base Dictionary includes the list of endpoint
   identifiers used in making routing decisions.  The referents remain
   constant for the duration of a session over a link where the instance
   numbers remain the same and can be used by both the Routing
   Information Base messages and the bundle offer messages.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | TLV type=0xA0 |   Flags       |           Length              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     RIBD  Entry Count (SDNV)  |          Reserved             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                                                               ~
      ~           Variable Length Routing Address Strings             ~
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Routing Address String

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        String ID 1 (SDNV)     | Length (SDNV) |    Resv       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                   Endpoint Identifier 1 (variable)            ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                .                              |
      ~                                .                              ~
      |                                .                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        String ID n (SDNV)     | Length (SDNV) |    Resv       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                   Endpoint Identifier n (variable)            ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


               Figure 8: Routing Information Base Dictionary

   TLV Flags
        Reserved







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   RIBD Entry Count
        Number of entries in the database.  Encoded as SDNV.

   String ID
        SDNV identifier that is constant for the duration of a session.
        String ID zero is predefined as the node initiating the session
        through sending the Hello SYN message, and String ID one is
        predefined as the node responding with the Hello SYNACK message.

   Length
        Length of Address String.  Encoded as SDNV.

4.3.4.  Routing Information Base TLV

   The Routing Information Base lists the destinations (endpoints) a
   node knows of, and the delivery predictabilities it has associated
   with them.  This information is needed by the PRoPHET algorithm to
   make decisions on routing and forwarding.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | TLV Type=0xA1 |   Flags       |           Length              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     RIB String Count (SDNV)   |           Reserved            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     RIBD String ID 1 (SDNV)   |            P-Value            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  RIB Flags 1  |               .                               ~
      +-+-+-+-+-+-+-+-+               .                               ~
      ~                               .                               ~
      ~                               .                               ~
      ~                               .                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     RIBD String ID n (SDNV)   |            P-Value            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  RIB Flags n  |
      +-+-+-+-+-+-+-+-+

                 Figure 9: Routing Information Base Header

   Flags
        The encoding of the Header flag field relates to the
        capabilities of the Source node sending the RIB:







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             Flag 0: More RIB TLVs       0b1
             Flag 1: Reserved            0b1
             Flag 2: Reserved            0b1
             Flag 3: Reserved            0b1
             Flag 4: Reserved            0b1
             Flag 5: Reserved            0b1
             Flag 6: Reserved            0b1
             Flag 7: Reserved            0b1

        The "More RIB TLVs" flag is set to 1 if the RIB requires more
        TLVs to be fully transferred.  This flag is set to 0 if this is
        the final TLV of this RIB.

   RIB String Count
        Number of routing entries in the TLV.  Encoded as SDNV.

   RIBD String ID
        ID string as predefined in the dictionary TLV.  Encoded as SDNV.

   P-value
        Delivery predictability for the destination of this entry as
        calculated according to the equations in Section 2.1.1, encoded
        as a 16-bit unsigned integer.  The encoding of this field is a
        linear mapping from [0,1] to [0, 0xFFFF] (e.g., for a P-value of
        0.75, the mapping would be 0.75*65535=49151=0xBFFF; thus the
        P-value would be encoded as 0xBFFF).

   RIB Flag
        The encoding of the RIB flag field is:


             Flag 0: Reserved            0b1
             Flag 1: Reserved            0b1
             Flag 2: Reserved            0b1
             Flag 3: Reserved            0b1
             Flag 4: Reserved            0b1
             Flag 5: Reserved            0b1
             Flag 6: Reserved            0b1
             Flag 7: Reserved            0b1

4.3.5.  Bundle Offer and Response TLV

   After the routing information has been passed, the node will ask the
   other node to review available bundles and determine which bundles it
   will accept for relay.  The source relay will determine which bundles
   to offer based on relative delivery predictabilities as explained in
   Section 3.6.  The Bundle Offer TLV also lists the bundles that a
   PRoPHET acknowledgement has been issued for.  Those bundles have the



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   PRoPHET ACK flag set in their entry in the list.  When a node
   receives a PRoPHET ACK for a bundle, it SHOULD, if possible, signal
   to the bundle agent that this bundle is no longer required for
   transmission by PRoPHET.  Despite no longer transmitting the bundle,
   it SHOULD keep an entry of the acknowledged bundle to be able to
   further propagate the PRoPHET ACK.

   The Response message is identical to the request message with the
   exception of the TLV Type field.


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    TLV Type   |   Flags       |           Length              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Bundle Offer Count       |       Reserved                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Bundle Dest String Id 1 (SDNV)|    B_flags    |    resv       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 Bundle 1 Creation Timestamp time              |
      |                     (variable length SDNV)                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Bundle 1 Creation Timestamp sequence number         |
      |                     (variable length SDNV)                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                               .                               ~
      ~                               .                               ~
      ~                               .                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Bundle Dest String Id n (SDNV)|    B_flags    |    resv       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                 Bundle n Creation Timestamp time              |
      |                     (variable length SDNV)                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Bundle n Creation Timestamp sequence number         |
      |                     (variable length SDNV)                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                   Figure 10: Bundle Offer and Response

   TLV Type
        The TLV Type for a Bundle Offer is 0xA2.  The TLV Type for a
        Bundle Response is 0xA3.






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   TLV Flags
        Reserved

   Bundle Offer Count
        Number of bundle offer/response entries.

   Bundle Dest String Id
        ID string of the destination of the bundle as predefined in the
        dictionary TLV.  Encoded as SDNV.

   B-Flags
        The encoding of the B_Flags are:


             Flag 0: Bundle Accepted     0b1
             Flag 1: Reserved            0b1
             Flag 2: Reserved            0b1
             Flag 3: Reserved            0b1
             Flag 4: Reserved            0b1
             Flag 5: Reserved            0b1
             Flag 6: Reserved            0b1
             Flag 7: PRoPHET ACK         0b1





























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5.  Detailed Operation

   In this section, some more details on the operation of PRoPHET is
   given along with state tables to help in implementing the protocol.

5.1.  High Level State Tables

   This section gives high level state tables for the operation of
   PRoPHET.  The following sections will describe each part of the
   operation in more detail (including state tables for the internal
   states of those procedures).

   The following states are used in the state tables:

   WAIT_NB  This is the state all nodes start in.  Nodes remain in this
         state until they are notified that a new neighbor is available.
         At that point, the Hello procedure should be started with the
         new neighbor, and the node move into the HELLO state.  It does
         also needs to remain in the WAIT_NB state to ensure that it can
         detect new neighbors.  This can be handled by creating a new
         thread or process that enters the HELLO state and takes care of
         the communication with the new neighbor while the parent
         remains in WAIT_NB.

   HELLO Nodes are in the HELLO state from when a new neighbor is
         detected until the Hello procedure is completed and a link is
         established (which happens when the Hello procedure enters the
         ESTAB state as described in Section 5.2 - during this
         procedure, the states ESTAB, SYNSENT, and SYNRCVD will be used,
         but those are internal to the Hello procedure and are not
         listed here).  If the node is notified that the neighbor is no
         longer in range before a link has been established, it returns
         to the WAIT_NB state.

   INFO_EXCH  After a link has been set up by the Hello procedure, a
         node enters the INFO_EXCH state where the information exchange
         and bundle passing is done.  The node remains in this state as
         long as Information Exchange Phase TLVs (Routing RIB, Routing
         RIB Dictionary) and bundle passing TLVs (Bundle Offer, Bundle
         Request) are being received.  When an empty Bundle Request TLV
         (i.e., no more bundles to send) is received, the node starts a
         timer and enters the WAIT_INFO state.  If the node is notified
         that the neighbor is no longer in range before all information
         and bundles have been exchanged, it returns to the WAIT_NB
         state.






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   WAIT_INFO  Nodes enter the WAIT_INFO state after a completed
         Information Exchange Phase and bundle passing phase.  Nodes
         remain in this state until a timer expires that means that the
         Information Exchange Phase should be re-initiated.  If the node
         is notified that the neighbor is no longer in range before the
         timer has expired, it returns to the WAIT_NB state.




    State: WAIT_NB

    +==================================================================+
    |     Condition    |               Action              | New State |
    +==================+===================================+===========+
    |   New Neighbor   | Start Hello procedure for neighbor|   HELLO   |
    |                  |  Keep waiting for more neighbors  |  WAIT_NB  |
    +==================================================================+





    State: HELLO

    +==================================================================+
    |    Condition     |               Action              | New State |
    +==================+===================================+===========+
    |  Hello TLV rcvd  |                                   |   HELLO   |
    +------------------+-----------------------------------+-----------+
    | Enter ESTAB state|  Start Information Exchange Phase | INFO_EXCH |
    +------------------+-----------------------------------+-----------+
    |  Neighbor Gone   |                                   |  WAIT_NB  |
    +==================================================================+





    State: INFO_EXCH

    +==================================================================+
    |    Condition     |               Action              | New State |
    +==================+===================================+===========+
    |Info Exch TLV rcvd|                                   | INFO_EXCH |
    +------------------+-----------------------------------+-----------+
    | No more bundles  |       Start WAIT_INFO timer       | WAIT_INFO |
    +------------------+-----------------------------------+-----------+



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    |  Neighbor Gone   |                                   |  WAIT_NB  |
    +==================================================================+




    State: WAIT_INFO

    +==================================================================+
    |    Condition     |               Action              | New State |
    +==================+===================================+===========+
    |  Timer expires   | Restart Information Exchange Phase| INFO_EXCH |
    +------------------+-----------------------------------+-----------+
    |  Neighbor Gone   |                                   |  WAIT_NB  |
    +==================================================================+



5.2.  Hello Procedure

   The Hello TLV procedure is described by the following rules and state
   tables.

   The rules and state tables use the following operations:

   o  The "Update Peer Verifier" operation is defined as storing the
      values of the Sender Instance and Sender Local Address fields from
      a Hello SYN or Hello SYNACK function received from the entity at
      the far end of the link.

   o  The procedure "Reset the link" is defined as:

      1.  Generate a new instance number for the link.

      2.  Delete the peer verifier (set to zero the values of Sender
          Instance and Sender Local Address previously stored by the
          Update Peer Verifier operation).

      3.  Send a SYN message.

      4.  Enter the SYNSENT state.

   o  The state tables use the following Boolean terms and operators:

      A     The Sender Instance in the incoming message matches the
            value stored from a previous message by the "Update Peer
            Verifier" operation.




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      B     The Sender Instance and Sender Local Address fields in the
            incoming message match the values stored from a previous
            message by the "Update Peer Verifier" operation.

      C     The Receiver Instance and Receiver Local Address fields in
            the incoming message match the values of the Sender Instance
            and Sender Local Address used in outgoing Hello SYN, Hello
            SYNACK, and Hello ACK messages.

      SYN   A Hello SYN TLV has been received.

      SYNACK  A Hello SYNACK TLV has been received.

      ACK   A Hello ACK TLV has been received.

      "&&"  Represents the logical AND operation

      "||"  Represents the logical OR operation

      "!"   Represents the logical negation (NOT) operation.

   o  A timer is required for the periodic generation of Hello SYN,
      Hello SYNACK, and Hello ACK messages.  The value of the timer is
      announced in the Timer field.  To avoid synchronization effects,
      uniformly distributed random jitter of +/-5% of the Timer field
      SHOULD be added to the actual interval used for the timer.

      There are two independent events: the timer expires, and a packet
      arrives.  The processing rules for these events are:


             Timer Expires:  Reset Timer
                             If state = SYNSENT Send SYN
                             If state = SYNRCVD Send SYNACK
                             If state = ESTAB   Send ACK
















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             Packet Arrives:
                 If incoming message is an RSTACK:
                     If (A && C && !SYNSENT) Reset the link
                     Else discard the message.
                 If incoming message is a SYN, SYNACK, or ACK:
                     Response defined by the following State Tables.
                 If incoming message is any other PRoPHET TLV and
                     state != ESTAB:
                     Discard incoming message.
                     If state = SYNSENT Send SYN (Note 1)
                     If state = SYNRCVD Send SYNACK (Note 1)

            Note 1: No more than two SYN or SYNACK messages should be
            sent within any time period of length defined by the timer.

   o  A connection across a link is considered to be achieved when the
      protocol reaches the ESTAB state.  All TLVs, other than Hello
      TLVs, that are received before synchronisation is achieved, will
      be discarded.

5.2.1.  State Tables



    State: SYNSENT

    +==================================================================+
    |     Condition    |               Action              | New State |
    +==================+===================================+===========+
    |   SYNACK && C    | Update Peer Verifier; Send ACK    |   ESTAB   |
    +------------------+-----------------------------------+-----------+
    |   SYNACK && !C   |           Send RSTACK             |  SYNSENT  |
    +------------------+-----------------------------------+-----------+
    |       SYN        | Update Peer Verifier; Send SYNACK |  SYNRCVD  |
    +------------------+-----------------------------------+-----------+
    |       ACK        |           Send RSTACK             |  SYNSENT  |
    +==================================================================+



   Note: When sending SYNACK determine if recalculation of
   predictabilities should be executed or suppressed and send
   appropriate flag accompanying SYNACK.  See Figure 6.

   Note: When sending ACK, if received SYNACK requested suppression of
   predictability recalculation, determine if recalculation of
   predictabilities should be executed or suppressed and send
   appropriate flag accompanying ACK; otherwise send ACK indicating



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   recalculation should be executed.


    State: SYNRCVD

    +==================================================================+
    |    Condition     |               Action              | New State |
    +==================+===================================+===========+
    |   SYNACK && C    | Update Peer Verifier; Send ACK    |   ESTAB   |
    +------------------+-----------------------------------+-----------+
    |   SYNACK && !C   |           Send RSTACK             |  SYNRCVD  |
    +------------------+-----------------------------------+-----------+
    |       SYN        | Update Peer Verifier; Send SYNACK |  SYNRCVD  |
    +------------------+-----------------------------------+-----------+
    |  ACK && B && C   |             Send ACK              |   ESTAB   |
    +------------------+-----------------------------------+-----------+
    | ACK && !(B && C) |           Send RSTACK             |  SYNRCVD  |
    +==================================================================+



   Note: When sending SYNACK determine if recalculation of
   predictabilities should be executed or suppressed and send
   appropriate flag accompanying SYNACK.  See Figure 6.

   Note: When sending ACK, if received SYNACK requested suppression of
   predictability recalculation, determine if recalculation of
   predictabilities should be executed or be suppressed and send
   appropriate flag accompanying ACK; otherwise send ACK indicating
   recalculation should be executed.


    State: ESTAB

    +==================================================================+
    |    Condition     |               Action              | New State |
    +==================+===================================+===========+
    |  SYN || SYNACK   |    Send ACK (notes 2, 4, and 5)   |   ESTAB   |
    +------------------+-----------------------------------+-----------+
    |  ACK && B && C   |    Send ACK (notes 3, 4, and 5)   |   ESTAB   |
    +------------------+-----------------------------------+-----------+
    | ACK && !(B && C) |             Send RSTACK           |   ESTAB   |
    +==================================================================+








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      Note 2: No more than two ACKs should be sent within any time
      period of length defined by the timer.  Thus, one ACK MUST be sent
      every time the timer expires.  In addition, one further ACK may be
      sent between timer expirations if the incoming message is a SYN or
      SYNACK.  This additional ACK allows the Hello functions to reach
      synchronisation more quickly.


      Note 3: No more than one ACK should be sent within any time period
      of length defined by the timer.


      Note 4: No more than one ACK should be sent within any time period
      of length defined by the timer.


      Note 5: When sending ACK, if received SYNACK requested suppression
      of predictability recalculation, determine if recalculation of
      predictabilities should be executed or be suppressed and send
      appropriate flag accompanying ACK; otherwise send ACK indicating
      recalculation should be executed.

5.2.2.  Interaction with Nodes Using Version 1 of PRoPHET

   There are existing implementations of PRoPHET based on drafts of this
   specification prior to version 06 that use version 1 of the protocol.
   There are two significant areas of difference between version 1 and
   version 2 as described in this document:

   o  the delivery predictability update equations were significantly
      different, and in the case of the transitivity equation (Equation
      3) could lead to incorrect behavior in some circumstances in
      version 1, and

   o  the versions of the SYNACK and ACK messages requesting suppression
      of the recalculation phase were not present in version 1.

   A node implementing version 2 of the PRoPHET protocol as defined in
   this document MAY choose either to ignore a communication opportunity
   with a node that sends a HELLO message indicating that it uses
   version 1 or it may partially downgrade and respond to messages as if
   it were a version 1 node.  This means that the version field in all
   message headers MUST contain 1, and the SYNACK and ACK messages
   indicating use of recalculation phase suppression MUST not be used
   (i.e., recalculation will always be done).  It is RECOMMENDED that
   the version 2 node use the metric update equations defined in this
   document even when communicating with a version 1 node as this will



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   partially inhibit the problems with the transitivity equation in
   version 1, and that the version 2 node should modify any received
   metrics that are greater than (1 - delta) to be (1 - delta) to avoid
   becoming a 'sink' for bundles that are not destined for this node.

   Generally, nodes using version 1 should be upgraded if at all
   possible because of problems that have been identified.

5.3.  Information Exchange and Bundle Passing Phase

   After the Hello messages have been exchanged, and the nodes are in
   the ESTAB state, the information exchange and bundle passing phase is
   initiated.  This section describes the procedure and shows the state
   transitions necessary in this phase, and the following sections
   describe the various TLVs passed in this phase in detail.

5.3.1.  State Tables

   This section shows the state transitions that nodes goes through
   during the information exchange and bundle passing phase.  State
   tables are given for a "Listener" and for a "Initiator".  Both nodes
   should assume both roles during this phase, and this can be done
   either concurrently or sequentially, depending on the implementation.


    Listener:
    ---------

    State: WAIT_DICT

    +==================================================================+
    |     Condition    |               Action              | New State |
    +==================+===================================+===========+
    | Dictionary rcvd  |  Update local dictionary (note 1) | WAIT_RIB  |
    +------------------+-----------------------------------+-----------+
    |   ACK received   |                                   | WAIT_DICT |
    +------------------+-----------------------------------+-----------+
    |  Timeout(peer)   |        Send ACK (note 2)          | WAIT_DICT |
    +==================================================================+












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     State: WAIT_RIB

    +==================================================================+
    |     Condition    |               Action              | New State |
    +==================+===================================+===========+
    | RIB TLV received |       Save RIB information.       | WAIT_RIB  |
    | More RIB TLVs=1  |      Wait for more RIB TLVs.      |           |
    +------------------+-----------------------------------+-----------+
    | RIB TLV received |   Update P ; Send offer (note 3)  |   OFFER   |
    | More RIB TLVs=0  |                                   |           |
    +------------------+-----------------------------------+-----------+
    |   ACK received   |                                   | WAIT_DICT |
    +------------------+-----------------------------------+-----------+
    | Dictionary rcvd  |      Update local dictionary      | WAIT_RIB  |
    +------------------+-----------------------------------+-----------+
    | Bundle req rcd   |            Send ACK               | WAIT_DICT |
    +------------------+-----------------------------------+-----------+
    |  Timeout(peer)   |            Send ACK               | WAIT_DICT |
    +==================================================================+





     State: OFFER

    +==================================================================+
    |     Condition    |               Action              | New State |
    +==================+===================================+===========+
    | Bundle req rcvd  |      Send requested bundle(s)     |   OFFER   |
    | #req bundles!=0  |                                   |           |
    +------------------+-----------------------------------+-----------+
    | Bundle req rcvd  |            (note 4)               | WAIT_DICT |
    | #req bundles==0  |                                   |           |
    +------------------+-----------------------------------+-----------+
    |   ACK received   |                                   | WAIT_DICT |
    +------------------+-----------------------------------+-----------+
    |  Timeout(info)   |    Resend bundle offer (note 5)   |   OFFER   |
    +------------------+-----------------------------------+-----------+
    | Dictionary or ACK|         Resend bundle offer       |   OFFER   |
    |    received      |                                   |           |
    +==================================================================+









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      Note 1: Both the dictionary and the RIB TLVs may come in the same
      PRoPHET message.  In that case, the state will change to WAIT_RIB
      and the RIB will then immediately be processed.


      Note 2: Send an ACK if the timer for the peering node expires.
      Either the link has been broken, and then the link setup will
      restart, or it will trigger the information exchange phase to
      restart.


      Note 3: When the RIB is received it is possible for the PRoPHET
      agent to update its delivery predictabilities according to
      Section 2.1.1.  If the SYNACK message requests suppression of
      recalculation (function 130) and the ACK message requests
      suppression of recalculation (function 131) then recalculation
      MUST be suppressed.  In all other cases recalculation must be
      done.  The delivery predictabilities and the RIB is then used
      together with the forwarding strategy in use to create a bundle
      offer TLV.  This is sent to the peering node.


      Note 4: No more bundles are requested by the other node, transfer
      is complete.


      Note 5: No response to the bundle offer has been received before
      the timer expired, so we resend the bundle offer.



    Initiator:
    ----------

    State: CREATE_DR

    +==================================================================+
    |     Condition    |               Action              | New State |
    +==================+===================================+===========+
    |      Always      | Create & send dict & RIB (note 1) |  SEND_DR  |
    +==================================================================+









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    State: SEND_DR

    +==================================================================+
    |     Condition    |               Action              | New State |
    +==================+===================================+===========+
    |  Timeout(info)   |  Resend dictionary & RIB (note 2) |  SEND_DR  |
    +------------------+-----------------------------------+-----------+
    | Bundle offer rcvd|         Send bundle request       |  REQUEST  |
    +==================================================================+





    State: REQUEST

    +==================================================================+
    |     Condition    |               Action              | New State |
    +==================+===================================+===========+
    |  Timeout(info)   |     Send bundle request for       |  REQUEST  |
    |                  |     missing bundles (note 3)      |           |
    +------------------+-----------------------------------+-----------+
    | Bundle rcvd &&   |       Wait for more bundles       |  REQUEST  |
    | REQ not fulfilled|            (note 4)               |           |
    +------------------+-----------------------------------+-----------+
    | Bundle rcvd &&   |     Send empty bundle request     |  REQUEST  |
    |  REQ fulfilled   |            (note 4)               |           |
    +------------------+-----------------------------------+-----------+
    |   ACK received   |                                   | CREATE_DR |
    +==================================================================+




      Note 1: The Initiator always starts by creating dictionary and RIB
      TLVs, and send them to its peering node.


      Note 2: No response to the RIB has been received before the timer
      expired, so we resend the dictionary and RIB TLVs.


      Note 3: If the timer expires, and not all requested bundles have
      been received, send a new bundle request for the missing bundles.







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      Note 4: While bundles are received, but there still are requested
      bundles that have not been received, continue waiting for more
      bundles.  If all desired bundles have been received, send an empty
      bundles request message to the peering node to signal that no more
      bundles should be passed.













































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6.  Security Considerations

   Currently, PRoPHET does not specify any special security measures.
   As a routing protocol for intermittently connected networks, PRoPHET
   is a target for various attacks.  The various known possible
   vulnerabilities are discussed in this section.

   The attacks described here are not problematic if all nodes in the
   network can be trusted and are working towards a common goal.  If
   there exist such a set of nodes, but there also exist malicious
   nodes, these security problems can be solved by introducing an
   authentication mechanism when two nodes meet, for example using a
   public key system.  Thus, only nodes that are known to be members of
   the trusted group of nodes are allowed to participate in the routing.
   This of course introduces the additional problem of key distribution,
   but that is not addressed here.

   Where suitable, the mechanisms (such as key management and bundle
   authentication or integrity checks) and terminology specified by the
   Bundle Security Protocol[symington_09] is to be used.

6.1.  Attacks on the Operation of the Protocol

   There are a number of kinds of attacks on the operation of the
   protocol that it would be possible to stage on a PRoPHET network.
   The attacks and possible remedies are listed here.

6.1.1.  Black Hole Attack

   A malicious node sets its delivery predictabilities for all
   destinations to a value close to or exactly equal to 1 and/or
   requests all bundles from nodes it meets, and does not forward any
   bundles.  This has two effects, both causing messages to be drawn
   towards the black hole, instead of to its correct destination.

   1.  A node encountering a malicious node will try to forward all its
       bundles to the malicious node, creating the belief that the
       bundle has been very favorably forwarded.  Depending on the
       forwarding strategy and queueing policy in use, this might hamper
       future forwarding of the bundle and/or lead to premature dropping
       of the bundle.

   2.  Due to the transitivity, the delivery predictabilities reported
       by the malicious node will affect the delivery predictabilities
       of other nodes.  This will create a gradient for all destinations
       with the black hole as the "center of gravity" towards which all
       bundles traverse.  This should be particularly severe in
       connected parts of the network.



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6.1.1.1.  Attack detection

   A node receiving a set of delivery predictabilities that are all at
   or close to 1 should be suspicious.  Similarly, a node which accepts
   all bundles and offers none might be considered suspicious.  However
   these conditions are not impossible in normal operation.

6.1.1.2.  Attack prevention/solution

   To prevent this attack, authentication between nodes that meet needs
   to be present.  Nodes can also inspect the received metrics and
   bundle acceptances/offers for suspicious patterns and terminate
   communications with nodes that appear suspicious.  The natural
   evolution of delivery predictabilities should mean that a genuine
   node would not be permanently ostracised even if the values lead to
   termination of a communication opportunity on one occasion.  The
   epidemic nature of PRoPHET would mean that such a termination would
   not generally lead to non-delivery of bundles.

6.1.2.  Limited Black Hole Attack/Identity Spoofing

   A malicious node misrepresents itself by claiming to be someone else.
   The effects of this attack are:

   1.  The effects of the black hole attack listed above hold for this
       attack as well, with the exception that only the delivery
       predictabilities and bundles for one particular destination are
       affected.  This could be used to "steal" the data that should be
       going to a particular node.

   2.  In addition to the above problems, PRoPHET ACKs will be issued
       for the bundles that are delivered to the malicious node.  This
       will cause these bundles to be removed from the network, reducing
       the chance that they will reach their real destination.

6.1.2.1.  Attack Detection

   It is possible for the destination to detect that this kind of attack
   has occurred (but it will not be able to prevent it) if it receives a
   PRoPHET ACK for a bundle destined to itself but for which it did not
   receive the corresponding bundle.

6.1.2.2.  Attack Prevention/Solution

   To prevent this attack, authentication between nodes that meet needs
   to be present.





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6.1.3.  Fake PRoPHET ACKs

   A malicious node may issue fake PRoPHET ACKs for all bundles (or only
   bundles for a certain destination if the attack is targeted at a
   single node) carried by nodes it meet.  The affected bundles will be
   deleted from the network, greatly reducing their probability of being
   delivered to the destination.

6.1.3.1.  Attack Prevention/Solution

   If a public key cryptography system is in place, this attack can be
   prevented by mandating that all PRoPHET ACKs be signed by the
   destination.  Similarly to other solutions using public key
   cryptography, this introduces the problem of key distribution.

6.1.4.  Bundle Store Overflow

   After encountering and receiving the delivery predictability
   information from the victim, a malicious node may generate a large
   number of fake bundles for the destination for which the victim has
   the highest delivery predictability.  This will cause the victim to
   most likely accept these bundles, filling up its bundle storage,
   possibly at the expense of other, legitimate, bundles.  This problem
   is transient as the messages will be removed when the victim meets
   the destination and delivers the messages.

6.1.4.1.  Attack Detection

   If it is possible for the destination to figure out that the bundles
   it is receiving are fake, it could report that malicious actions are
   underway.

6.1.4.2.  Attack Prevention/Solution

   This attack could be prevented by requiring sending nodes to sign all
   bundles they send.  By doing this, intermediate nodes could verify
   the integrity of the messages before accepting them for forwarding.

6.1.5.  Bundle Store Overflow with Delivery Predictability Manipulation

   A more sophisticated version of the attack in the previous section
   can be attempted.  The effect of the previous attack was lessened
   since the destination node of the fake bundles existed.  This caused
   fake bundles to be purged from the network when the destination was
   encountered.  The malicious node may now use the transitive property
   of the protocol to boost the victim's delivery predictabilities for a
   non-existent destination.  After this, it creates a large number of
   fake bundles for this non-existent destination and offers them to the



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   victim.  As before, these bundles will fill up the bundle storage of
   the victim.  The impact of this attack will be greater as there is no
   probability of the destination being encountered and the bundles
   being acknowledged.  Thus, they will remain in the bundle storage
   until they time out (the malicious node may set the timeout to a
   large value) or until they are evicted by the queueing policy.

   The delivery predictability for the fake destination may spread in
   the network due to the transitivity, but this is not a problem, as it
   will eventually age and fade away.

   The impact of this attack could be increased if multiple malicious
   nodes collude, as network resources can be consumed at a greater
   speed and at many different places in the network simultaneously.

6.2.  Interactions with External Routing Domains

   Users may opt to connect two regions of sparsely connected nodes
   through a connected network such as the Internet where another
   routing protocol is running.  To this network, PRoPHET traffic would
   look like any other application layer data.  Extra care must be taken
   in setting up these gateway nodes and their interconnections to make
   sure that malicious nodes cannot use them to launch attacks on the
   infrastructure of the connected network.  In particular, the traffic
   generated should not be significantly more than what a single regular
   user end host could create on the network.

























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7.  IANA Considerations

   Following the policies outlined in "Guidelines for Writing an IANA
   Considerations Section in RFCs" (RFC 5226 [RFC5226]), the following
   name spaces are defined in PRoPHET:

   o  DTN Routing Protocol Number Section 4.1

   o  PRoPHET Version Section 4.1

   o  Header FlagsSection 4.1

   o  Result Section 4.1

   o  Code Section 4.1

   o  Error and Log Messages

   o  TLV Type Section 4.2

   o  Hello TLV Flags

   o  Error TLV Flags

   o  Routing Information Base Dictionary TLV Flags Section 4.3.3

   o  Routing Information Base TLV Flags Section 4.3.3

   o  RIB entry Flag Section 4.3.4

   o  Bundle Offer/Response TLV FlagsSection 4.3.5

   The following subsections lists the registries that are requested to
   be created.  Initial values for the registries are given below;
   future assignments are to be made through the Specification Required
   policy.  Where specific values are defined in the IANA registries to
   be setup according to the specifications in the sub-sections below,
   the registry should refer to this document as defining the
   allocation.












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7.1.  DTN Routing Protocol Number

               The encoding of the Protocol Number field is:

         +------------------+-----------+------------------------+
         |     Protocol     |   Value   |   Allocation Control   |
         +------------------+-----------+------------------------+
         | PRoPHET Protocol |    0x00   |                        |
         |                  |           |                        |
         |     Reserved     | 0x01-0xEF | Specification required |
         |                  |           |                        |
         |      Private     | 0xF0-0xFE |      Experimental      |
         +------------------+-----------+------------------------+

7.2.  PRoPHET Version

               The encoding of the PRoPHET Version field is:

   +------------------------------+-----------+------------------------+
   |            Version           |   Value   |   Allocation Control   |
   +------------------------------+-----------+------------------------+
   |           Reserved           |    0x00   |                        |
   |                              |           |                        |
   |        Earlier Drafts        |    0x01   |                        |
   |                              |           |                        |
   |         This protocol        |    0x02   |                        |
   |                              |           |                        |
   |           Reserved           | 0x03-0xEF | Specification required |
   |                              |           |                        |
   |            Private           | 0xF0-0xFE |      Experimental      |
   |                              |           |                        |
   |      Reserved for future     |    0xFF   | Specification required |
   |           expansion          |           |                        |
   +------------------------------+-----------+------------------------+

















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7.3.  Header Flags

                       The flags for the Header are:

           +--------------+----------+------------------------+
           | Bit Position |  Meaning |       Explanation      |
           +--------------+----------+------------------------+
           |     Bit 0    | Reserved | Specification required |
           |              |          |                        |
           |     Bit 1    | Reserved | Specification required |
           |              |          |                        |
           |     Bit 2    | Reserved | Specification required |
           |              |          |                        |
           |     Bit 3    | Reserved | Specification required |
           +--------------+----------+------------------------+

7.4.  Result

                   The encoding of the result field is:

         +---------------+-------------+------------------------+
         |  Result Value |    Value    |   Allocation Control   |
         +---------------+-------------+------------------------+
         |  NoSuccessAck |     0x01    |                        |
         |               |             |                        |
         |     AckAll    |     0x02    |                        |
         |               |             |                        |
         |    Success    |     0x03    |                        |
         |               |             |                        |
         |    Failure    |     0x04    |                        |
         |               |             |                        |
         | ReturnReceipt |     0x05    |                        |
         |               |             |                        |
         |    Reserved   | 0x06 - 0x7F | Specification required |
         |               |             |                        |
         |    Private    | 0x80 - 0xFF |      Experimental      |
         +---------------+-------------+------------------------+














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7.5.  Code

                         The encoding for Code is:

      +----------------------+-------------+------------------------+
      |     Message Type     |    Range    |   Allocation Control   |
      +----------------------+-------------+------------------------+
      |    Error Responses   | 0x00 - 0x33 | Specification required |
      |                      |             |                        |
      |   Success Responses  | 0x34 - 0x66 | Specification required |
      |                      |             |                        |
      |      Event Codes     | 0x67 - 0x99 | Specification required |
      |                      |             |                        |
      |        Private       | 0xA0 - 0xFE |      Experimental      |
      |                      |             |                        |
      | Error TLV in message |     0xFF    |                        |
      +----------------------+-------------+------------------------+

7.6.  Error and Log Messages

   Messages defined in range 0x00 - 0x33 of Code defined in Section 7.5

             +-----------------+-------+--------------------+
             |  Error Message  | Value | Allocation Control |
             +-----------------+-------+--------------------+
             | Undefined Error |   1   |                    |
             +-----------------+-------+--------------------+
























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7.7.  TLV Type

                 The list of TLVs Defined for PRoPHET are:

       +--------------------+-------------+------------------------+
       |        Type        |    Value    |   Allocation Control   |
       +--------------------+-------------+------------------------+
       |      Hello TLV     |     0x01    |                        |
       |                    |             |                        |
       |      Error TLV     |     0x02    |                        |
       |                    |             |                        |
       |      Reserved      | 0x03 - 0x9F | Specification required |
       |                    |             |                        |
       | RIB dictionary TLV |     0xA0    |                        |
       |                    |             |                        |
       |       RIB TLV      |     0xA1    |                        |
       |                    |             |                        |
       |    Bundle Offer    |     0xA2    |                        |
       |                    |             |                        |
       |   Bundle Response  |     0xA3    |                        |
       |                    |             |                        |
       |      Reserved      | 0xA4 - 0xCF | Specification required |
       |                    |             |                        |
       |       Private      | 0xD0 - 0xFF |      Experimental      |
       +--------------------+-------------+------------------------+

7.8.  Hello TLV Flags

            The following flags are defined for the Hello TLV:

            +----------+-------------+------------------------+
            |   Type   |    Value    |   Allocation Control   |
            +----------+-------------+------------------------+
            |    SYN   |     0x01    |                        |
            |          |             |                        |
            |  SYNACK  |     0x02    |                        |
            |          |             |                        |
            |    ACK   |     0x03    |                        |
            |          |             |                        |
            |  RSTACK  |     0x04    |                        |
            |          |             |                        |
            | Reserved | 0x05 - 0x0F | Specification required |
            |          |             |                        |
            |  Private | 0x10 - 0xFF |      Experimental      |
            +----------+-------------+------------------------+






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7.9.  Error TLV Flags

            The following flags are defined for the Error TLV:

            +----------+-------------+------------------------+
            |   Type   |    Value    |   Allocation Control   |
            +----------+-------------+------------------------+
            | Reserved | 0x00 - 0x7F | Specification required |
            |          |             |                        |
            |  Private | 0x80 - 0xFF |      Experimental      |
            +----------+-------------+------------------------+

7.10.  RIB Base Dictionary TLV Flags

     The following flags are defined for the RIB Base Dictionary TLV:

            +----------+-------------+------------------------+
            |   Type   |    Value    |   Allocation Control   |
            +----------+-------------+------------------------+
            | Reserved | 0x00 - 0x7F | Specification required |
            |          |             |                        |
            |  Private | 0x80 - 0xFF |      Experimental      |
            +----------+-------------+------------------------+

7.11.  RIB TLV Flags

            The following flags are defined for the Error TLV:

            +----------+-------------+------------------------+
            |   Type   |    Value    |   Allocation Control   |
            +----------+-------------+------------------------+
            | Reserved | 0x00 - 0x7F | Specification required |
            |          |             |                        |
            |  Private | 0x80 - 0xFF |      Experimental      |
            +----------+-------------+------------------------+
















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7.12.  RIB Flags

            The following flags are defined for the Error TLV:

            +----------+-------------+------------------------+
            |   Type   |    Value    |   Allocation Control   |
            +----------+-------------+------------------------+
            | Reserved | 0x00 - 0x7F | Specification required |
            |          |             |                        |
            |  Private | 0x80 - 0xFF |      Experimental      |
            +----------+-------------+------------------------+

7.13.  Bundle Flags

           The flags for the Bundle Offer and Response TLV are:

        +--------------+-----------------+------------------------+
        | Bit Position |     Meaning     |   Allocation Control   |
        +--------------+-----------------+------------------------+
        |     Bit 0    | Bundle Accepted |                        |
        |              |                 |                        |
        |     Bit 1    |     Reserved    | Specification required |
        |              |                 |                        |
        |     Bit 2    |     Reserved    | Specification required |
        |              |                 |                        |
        |     Bit 3    |     Reserved    | Specification required |
        |              |                 |                        |
        |     Bit 4    |     Reserved    | Specification required |
        |              |                 |                        |
        |     Bit 5    |     Reserved    | Specification required |
        |              |                 |                        |
        |     Bit 6    |     Reserved    | Specification required |
        |              |                 |                        |
        |     Bit 7    |   PRoPHET ACK   |                        |
        +--------------+-----------------+------------------------+
















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8.  Implementation Experience

   Multiple independent implementations of the PRoPHET protocol exist.

   The first implementation is written in Java, and has been optimized
   to run on the Lego MindStorms platform that has very limited
   resources.  Due to the resource constraints, some parts of the
   protocol have been simplified or omitted, but the implementation
   contains all the important mechanisms to ensure proper protocol
   operation.  The implementation is also highly modular and can be run
   on another system with only minor modifications (it has currently
   been shown to run on the Lego MindStorms platform and on regular
   laptops).

   Another implementation is written in C++ and runs in the OmNet++
   simulator to enable testing and evaluation of the protocol and new
   features.  Experience and feedback from the implementors on early
   versions of the protocol have been incorporated into the current
   version.

   An implementation compliant to version 2 of the predecessor draft
   (draft-lindgren-prophet-02.txt) has been written at Baylor
   University.  This implementation has been integrated into the DTN2
   reference implementation.

   An implementation of the protocol in C++ was developed by one of the
   authors (Samo Grasic) at Lulea University of Technology (LTU) as part
   of the Saami Networking Connectivity project (see Section 9) and
   continues to track the development of the protocol.  This work is now
   part of the Networking for Communications Challenged Communities
   (N4C) project and is used in N4C testbeds.




















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9.  Deployment Experience

   During a week in August 2006, a proof-of-concept deployment of a DTN
   system, using the LTU PRoPHET implementation for routing was made in
   the Swedish mountains - the target area for the Saami Network
   Connectivity project [ccnc07][doria_02].  Four fixed camps with
   application gateways, one Internet gateway, and seven mobile relays
   were deployed.  The deployment showed PRoPHET to be able to route
   bundles generated by different applications such as e-mail and web
   caching.

   Within the realms of the SNC and N4C projects, multiple other
   deployments, both during summer and winter conditions have been done
   in various scale during 2007-2009. [winsdr08]





































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10.  Acknowledgements

   The authors would like to thank Olov Schelen and Kaustubh S. Phanse
   for contributing with valuable feedback regarding various aspects of
   the protocol.  We would also like to thank all other reviewers and
   the DTNRG chairs for the feedback in the process of developing the
   protocol.  The Hello TLV mechanism is loosely based on Adjacency
   message developed for RFC3292.  Luka Birsa and Jeff Wilson have
   provided us with feedback from doing implementations of the protocol
   based on various preliminary versions of the draft.  Their feedback
   has helped us make the draft easier to read for an implementor and
   has improved the protocol.







































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11.  References

11.1.  Normative References

   [RFC5050]  Scott, K. and S. Burleigh, "Bundle Protocol
              Specification", RFC 5050, November 2007.

11.2.  Informative References

   [RFC4838]  Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
              R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
              Network Architecture", RFC 4838, April 2007.

   [RFC5226]  Narten, T. and H. Tveit Alvestrand, "Guidelines for
              Writing an IANA Considerations Section in RFCs", RFC 5226,
              May 2008.

   [ccnc07]   Lindgren, A. and A. Doria, "Experiences from Deploying a
              Real-life DTN System", Proceedings of the 4th Annual IEEE
              CONSUMER COMMUNICATIONS and NETWORKING CONFERENCE (CCNC
              2007), Las Vegas, Nevada, USA , January 2007.

   [doria_02]
              Doria, A., Uden, M., and D. Pandey, "Providing
              connectivity to the Saami nomadic community", Proceedings
              of the 2nd International Conference on Open Collaborative
              Design for Sustainable Innovation (dyd 02), Bangalore,
              India , December 2002.

   [lindgren_06]
              Lindgren, A. and K. Phanse, "Evaluation of Queueing
              Policies and Forwarding Strategies for Routing in
              Intermittently Connected Networks", Proceedings of
              COMSWARE 2006 , January 2006.

   [symington_09]
              Symington, S., Farrell, S., Weiss, H., and P. Lovell,
              "Bundle Security Protocol Specification", Internet Draft
              draft-irtf-dtnrg-bundle-security-11.txt , November 2009.

   [vahdat_00]
              Vahdat, A. and D. Becker, "Epidemic Routing for Partially
              Connected Ad Hoc Networks", Duke University Technical
              Report CS-200006, April 2000.

   [winsdr08]
              Lindgren, A., Doria, A., Lindblom, J., and M. Ek,
              "Networking in the Land of Northern Lights - Two Years of



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              Experiences from DTN System Deployments", Proceedings of
              the ACM Wireless Networks and Systems for Developing
              Regions Workshop(WiNS-DR), San Francisco, California,
              USA , September 2008.















































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Appendix A.  PRoPHET Example

   To help grasp the concepts of PRoPHET, an example is provided to give
   a understanding of the transitive property of the delivery
   predictability, and the basic operation of PRoPHET.  In Figure 11, we
   revisit the scenario where node A has a message it wants to send to
   node D. In the bottom right corner of subfigures a)-c), the delivery
   predictability tables for the nodes are shown.  Assume that nodes C
   and D encounter each other frequently (Figure 11a) ), making the
   delivery predictability values they have for each other high.  Now
   assume that node C also frequently encounters node B (Figure 11b) ).
   B and C will get high delivery predictability values for each other,
   and the transitive property will also increase the value B has for D
   to a medium level.  Finally, node B meets node A (Figure 11c) ) that
   has a message for node D. Figure 11d) shows the message exchange
   between node A and node B. Summary vectors and delivery
   predictability information is exchanged, delivery predictabilities
   are updated, and node A then realized that P_(b,d) > P_(a,d), and
   thus forwards the message for D to node B.
































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   +----------------------------+   +----------------------------+
   |                            |   |                            |
   |                  C         |   |                       D    |
   |                   D        |   |                            |
   |       B                    |   |       B C                  |
   |                            |   |                            |
   |                            |   |                            |
   |                            |   |                            |
   |                            |   |                            |
   | A*                         |   | A*                         |
   +-------------+--------------+   +-------------+--------------+
   |   A  |   B  |   C   |  D   |   |   A  |   B  |   C   |  D   |
   |B:low |A:low |A:low  |A:low |   |B:low |A:low |A:low  |A:low |
   |C:low |C:low |B:low  |B:low |   |C:low |C:high|B:high |B:low |
   |D:low |D:low |D:high |C:high|   |D:low |D:med |D:high |C:high|
   +-------------+--------------+   +-------------+--------------+
                 a)                               b)
   +----------------------------+   A                            B
   |                            |   |                            |
   |                       D    |   |Summary vector&delivery pred|
   |                            |   |--------------------------->|
   |         C                  |   |Summary vector&delivery pred|
   |                            |   |<---------------------------|
   |                            |   |                            |
   |   B*                       |  Update delivery predictabilities
   |  A                         |   |                            |
   |                            |  Packet for D not in SV        |
   +-------------+--------------+  P(b,d)>P(a,d)                 |
   |   A  |   B  |   C   |  D   |  Thus, send                    |
   |B:low |A:low |A:low  |A:low |   |                            |
   |C:med |C:high|B:high |B:low |   |      Packet for D          |
   |D:low+|D:med |D:high |C:high|   |--------------------------->|
   +-------------+--------------+   |                            |
                 c)                               d)

                        Figure 11: PRoPHET example















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Appendix B.  Neighbor Discovery Example

   This section outlines an example of a simple neighbor discovery
   protocol that can be run in-between PRoPHET and underlying layer in
   case lower layers do not provide methods for neighbor discovery.  It
   assumes that the underlying layer supports broadcast messages as
   would be the case if a wireless infrastructure was involved.

   Each node needs to maintain a list of its active neighbors.  The
   operation of the protocol is as follows:

   1.  Every BEACON_INTERVAL milliseconds, the node does a local
       broadcast of a beacon that contains its identity and address, as
       well as the BEACON_INTERVAL value used by the node.

   2.  Upon reception of a beacon, the following can happen:

       1.  The sending node is already in the list of active neighbors.
           Update its entry in the list with the current time, and the
           node's BEACON_INTERVAL if it has changed.

       2.  The sending node is not in the list of active neighbors.  Add
           the node to the list of active neighbors and record the
           current time and the node's BEACON_INTERVAL.  Notify the
           PRoPHET agent that a new neighbor is available ("New
           Neighbor", as described in Section 2.4).

   3.  If a beacon has not been received from a node in the list of
       active neighbors within a time period of NUM_ACCEPTED_LOSSES *
       BEACON_INTERVAL (for the BEACON_INTERVAL used by that node), it
       should be assumed that this node is no longer a neighbor.  The
       entry for this node should be removed from the list of active
       neighbors, and the PRoPHET agent should be notified that a
       neighbor has left ("Neighbor Gone", as described in Section 2.4).

















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Authors' Addresses

   Anders F. Lindgren
   Swedish Institute of Computer Science
   Box 1263
   Kista  SE-164 29
   SE

   Phone: +46707177269
   Email: andersl@sics.se
   URI:   http://www.sics.se/~andersl


   Avri Doria
   Lulea University of Technology
   Lulea  SE-971 87
   SE

   Phone:
   Email: avri@acm.org
   URI:   http://psg.com/~avri


   Elwyn Davies
   Folly Consulting
   Soham
   UK

   Phone:
   Email: elwynd@folly.org.uk
   URI:


   Samo Grasic
   Lulea University of Technology
   Lulea  SE-971 87
   SE

   Phone:
   Email: samo.grasic@ltu.se
   URI:










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