DTN Research Group A. Lindgren
Internet-Draft University College London
Expires: August 12, 2008 A. Doria
Lulea University of Technology
February 11, 2008
Probabilistic Routing Protocol for Intermittently Connected Networks
draft-irtf-dtnrg-prophet-00
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Abstract
This document defines PRoPHET, a Probabilistic Routing Protocol using
History of Encounters and Transitivity. PRoPHET is a routing
protocol for intermittently connected 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 the Delay-Tolerant Network architecture[1] is
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applicable. The document presents an architectural overview followed
by the protocol specification.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2 Relation to the Delay-Tolerant Networking architecture . . 8
2. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1 PRoPHET . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1.1 Delivery predictability calculation . . . . . . . . . 9
2.1.2 Forwarding Strategies and Queueing Policies . . . . . 11
2.2 Bundle Agent to Routing Agent Interface . . . . . . . . . 12
2.3 Lower Layer Requirements and Interface . . . . . . . . . . 12
3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 13
3.1 Neighbor Awareness . . . . . . . . . . . . . . . . . . . . 13
3.2 Information Exchange Phase . . . . . . . . . . . . . . . . 13
3.2.1 Routing Information Base Dictionary . . . . . . . . . 14
3.3 Routing Algorithm . . . . . . . . . . . . . . . . . . . . 14
3.4 Bundle Passing . . . . . . . . . . . . . . . . . . . . . . 15
3.4.1 Custody . . . . . . . . . . . . . . . . . . . . . . . 16
3.5 When a Bundle Reaches its Destination . . . . . . . . . . 16
3.6 Forwarding Strategies . . . . . . . . . . . . . . . . . . 17
3.7 Queueing Policies . . . . . . . . . . . . . . . . . . . . 18
4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 20
4.1 Messages . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.2 Header . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.3 TLV Structure . . . . . . . . . . . . . . . . . . . . . . 24
4.4 TLVs . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.4.1 Hello TLV . . . . . . . . . . . . . . . . . . . . . . 24
4.4.2 Error TLV . . . . . . . . . . . . . . . . . . . . . . 26
4.4.3 Routing Information Base Dictionary TLV . . . . . . . 27
4.4.4 Routing Information Base TLV . . . . . . . . . . . . . 28
4.4.5 Bundle Offer and Response TLV . . . . . . . . . . . . 29
5. Detailed Operation . . . . . . . . . . . . . . . . . . . . . . 31
5.1 High Level State Tables . . . . . . . . . . . . . . . . . 31
5.2 Hello Procedure . . . . . . . . . . . . . . . . . . . . . 33
5.2.1 State Tables . . . . . . . . . . . . . . . . . . . . . 35
5.3 Information exchange and bundle passing phase . . . . . . 36
5.3.1 State Tables . . . . . . . . . . . . . . . . . . . . . 37
6. Security Considerations . . . . . . . . . . . . . . . . . . . 40
6.1 Attacks on the operation of the protocol . . . . . . . . . 40
6.1.1 Black hole attack . . . . . . . . . . . . . . . . . . 40
6.1.2 Limited black hole attack/identity spoofing . . . . . 41
6.1.3 Fake PRoPHET ACKs . . . . . . . . . . . . . . . . . . 41
6.1.4 Bundle store overflow . . . . . . . . . . . . . . . . 42
6.1.5 Bundle store overflow with delivery predictability
manipulation . . . . . . . . . . . . . . . . . . . . . 42
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6.2 Interactions with External Routing Domains . . . . . . . . 43
7. Implementation Experience . . . . . . . . . . . . . . . . . . 43
8. Deployment Experience . . . . . . . . . . . . . . . . . . . . 44
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 44
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 45
A. PRoPHET Example . . . . . . . . . . . . . . . . . . . . . . . 45
B. Neighbor Discovery Example . . . . . . . . . . . . . . . . . . 47
Intellectual Property and Copyright Statements . . . . . . . . 49
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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 (see Section 1.1 for a survey of such
scenarios).
To introduce the work, 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
Tolerant Networking [1]). It would then be possible to exploit the
mobility of a subset of the nodes to bring messages closer to their
destination by transferring messages 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'.
Real users are not likely to move around randomly, but rather move in
a predictable fashion based on human traffic patterns (e.g., roads or
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foot paths), and on repeating behavioral patterns (e.g., going to
work or the market and returning home). 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.
In previously discussed mechanisms to enable communication in
intermittently connected networks, such as Epidemic Routing[3], very
general approaches have been taken to the problem at hand. There
have, however, not been any attempts to make use of assumed knowledge
of different (mobility) properties of the nodes in the network in a
truly distributed way.
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. We define an
alternative to 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 applicable.
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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 Background
The kinds of communication networks addressed in this document are
only viable for applications that can tolerate long delays and are
able to deal with extended periods of being disconnected. In
practice there are many different scenarios and situations where
communication is inherently intermittent, and in which it is of high
value to develop methods of communication despite the limitations on
applications. This section presents a selection of situations where
it appears that these kinds of communication offer valuable solutions
to realistic problems.
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The semi-nomadic Saami population of reindeer herders in the north of
Sweden follow the movement of the reindeer and when in their summer
camps, no fixed infrastructure is available. The herders would find
it useful to be able to communicate with the rest of the world
through, for example, mobile relays attached to snowmobiles and all-
terrain vehicles (ATVs), or carried as small devices in a backpack,
to (for example) obtain weather forecasts, conduct herd business,
receive RSS feeds, send and receive personal email, and maintain the
supply of educational material for children in the parties[6].
Similar problems exist between rural villages in India and in other
regions where the Internet infrastructure is less well developed or
is only available at prices which are beyond the means of the local
population. The DakNet project[7] has deployed store-and-forward
ring networks connecting a number of villages through relays on buses
and motorcycles in India and Cambodia.
While satellite networks often rely on very well defined trajectories
and predictable encounters, there are cases when connectivity between
them can be intermittent and opportunistic. In military war-time
scenarios and disaster recovery situations, soldiers, human rights
observers, or rescue personnel are often in hostile environments
where no infrastructure can be assumed to be present, or if present,
cannot be relied upon. Furthermore, the units may be sparsely
distributed so that connectivity between them is intermittent and
infrequent.
In sensor networks, a large number of sensors are usually deployed in
the area in which measurements are to be performed. To ensure
connectivity among the sensors and to get measurements from the
entire area, it is common to deploy a very large number of sensors.
If sensors can be mobile and transitive communication techniques can
be used between them, the number of sensors required can be reduced,
and new areas where regular sensor networks have been too expensive
or difficult to deploy, can be monitored.
Experiments have been done with attaching sensors to seals, vastly
increasing the number of oceanic temperature readings compared to
using a number of fixed sensors, and in a similar project sensors are
attached to whales[5]. To allow scientists to analyze the collected
data, it must somehow be transferred to a data sink, even though
connectivity among the seals and whales is very sparse and
intermittent, so the mobility of the animals (and their occasional
encounters with each other and networked buoys at feeding grounds)
must be relied upon for successful data delivery.
Resembling the vast areas of the oceans are the plains of Africa in
that there are many remote areas with almost no fixed infrastructure
and where satellite connectivity is prohibitively expensive. In the
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ZebraNet project, an attempt is made to gain a better understanding
of the life and movements of the wildlife in a certain part of Africa
by equipping zebras with tracking collars communicating in fashions
similar to the ones described above. Yet another example concerns
weather monitoring over large areas such as a national park, where a
number of electronic display boards showing weather reports from
other parts of the park have been installed. By equipping hikers
with small networked devices, their mobility through the park can be
used to spread the weather information throughout the entire park.
1.2 Relation to the Delay-Tolerant Networking architecture
The Delay-Tolerant Networking (DTN) architecture[1] 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
types of intermittent contacts. The ones called opportunistic and
predicted are the ones addressed by this protocoll.
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 on if a contact will be present soon.
The DTN architecture also defines the bundle protocol [2], 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 bundling protocol also provides end-
to-end addressing and reliability. We build on the bundling
protocol, using bundles as the basic data transfer unit.
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 human mobility to improve routing performance.
Instead of doing blind epidemic replication of messages through the
network as previous protocols have done, it applies 'probabilistic
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routing'.
To accomplish this, a probabilistic metric called 'delivery
predictability', 0 <= P_(A,B) <= 1, is established at every node A
for each known destination B. This indicates how good node A is as a
forwarder for a message to destination B.
When two PRoPHET nodes have a communication opportunity, they
exchange information about the bundles each node carries, and also
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
information is used to decide which messages to request from the
other node based on the forwarding strategy used (as discussed in
Section 2.1.2).
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 message to a certain
node. In the equations that follow, the updates are being performed
by node A, and P_(A,B) is the delivery predictability value that node
A has stored for the destination B. If no delivery predictability
values is stored for a particular destination B, P_(A,B) is
considered to be zero. Recommended settings for the various
parameters are given in Section 3.3. The calculation of the delivery
predictabilities 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. This calculation is shown in Equation 1, where 0 <=
P_encounter <= 1 is an initialization constant.
P_(A,B) = P_(A,B)_old + ( 1 - 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 messages to each other,
thus the delivery predictability values must age, being reduced in
the process. The aging equation is shown in Equation 2, where 0 <=
gamma <= 1 is the aging constant, and K is the number of time 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 predictability also has a transitive property, that is
based on the observation that if node A frequently encounters node B,
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and node B frequently encounters node C, then node C probably is a
good node to forward messages 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) = P_(A,C)_old + ( 1 - P_(A,C)_old ) * P_(A,B) *
P_(B,C) * beta (3)
2.1.1.1 Optional delivery predictability optimizations
2.1.1.1.1 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 tradeoff 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
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 smoothing algorithm in various
circumstances is the subject of ongoing research and a future version
of this protocol specification may contain additional advice.
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2.1.1.1.2 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 investigate yet.
2.1.2 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 message arrives at a
node is that there might not be a path to the destination available,
so the node has to buffer the message and upon each encounter with
another node, the decision must be made on whether or not to transfer
a particular message. Furthermore, it may also be sensible to
forward a message to multiple nodes to increase the probability that
a message is really delivered to its destination.
Unfortunately, these decisions are not trivial to make. In some
cases it might be sensible to select a fixed threshold and only give
a message to nodes that have a delivery predictability over that
threshold for the destination of the message. 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 messages to.
Furthermore, there is the problem of deciding how many nodes to give
a certain message to. Distributing a message to a large number of
nodes will of course increase the probability of delivering that
particular message to its destination, but this comes at the cost of
consuming more system resources for message storage and possibly
reducing the probability of other messages being delivered. On the
other hand, giving a message to only a few nodes (maybe even just a
single node) will use less system resources, but the probability of
delivering a message is lower, and the delay incurred high.
When resources are constrained, nodes may suffer from storage
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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 message, deciding which message 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
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:
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
Makes the bundle agent drop a specified bundle.
2.3 Lower Layer Requirements and Interface
PRoPHET can be run on a large number of underlying networking
technologies. To accomodate 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 wireless
networking technology does not support such services, a simple
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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 4.2. This address and its format
is dependent on the convergence layer that is being used by the
bundle layer.
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 2.3.
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 continues
to the Information Exchange Phase (see Section 3.2).
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
(EIDs) of the nodes that will be listed in the Routing Information
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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 2.1.2) 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 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 [1] to
shorter 16 bit 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.
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.
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.
Recommended parameter values
+==================================+
| Parameter | Recommended value |
+==================================+
| P_encounter | 0.75 |
+----------------------------------+
| beta | 0.25 |
+----------------------------------+
| gamma | 0.99 |
+==================================+
Figure 2
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
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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 desireable
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 [1]. 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
SHOULD delete that bundle from its storage, unless the node holds
custody of that bundle.
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.
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It is important to keep in mind that PRoPHET ACKs and bundle ACKs[2]
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 [4].
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.
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.
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)
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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)
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 predictabilites 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 queuing 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 [4].
FIFO
Handle the queue in a FIFO order. The bundle that was first
entered into the queue is the first bundle to be dropped.
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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 [2], 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 need 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.
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,
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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.
4. Message Formats
4.1 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Header ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ TLV 1 ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
~ . ~
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ TLV n ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Basic message format
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4.2 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | Flags | Result | Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Instance | Sender Instance |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transaction Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| SubMessage Number | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Message Body ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Header
Version
This version of the PRoPHET Protocol = 1.
Flags
TBD
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.
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The encoding of the result field is:
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.
The encoding is:
PRoPHET Error messages 0x000 - 0x099
Reserved 0x0A0 - 0x0FE
Error TLV in message 0x0FF
Sender Instance
For messages during the Hello phase with the Hello SYN, Hello
SYNACK, and Hello ACK functions, 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 18-
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, 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.
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
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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.
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 Indentifier of the request they are responding
to.
S-flag
If S is set then the SubMessage Number field indicates the total
number of SubMessage segments that compose the entire message.
If it is not set 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
submessage in a sequence of submessages, the S flag will be set
and this field will contain the total count of submessage
segments.
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 protocol also uses a pseudo header with information that MUST be
provided by the underlying communication layer. The following pseudo
header fields are defined:
Sender Local Address
An address used by the underlying communication layer as
described in Section 2.3 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 2.3 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
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nodes.
4.3 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ TLV Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: TLV Format
TLV Type
Specific TLVs are defined in Section 4.4. Each TLV will have
fields defined that are specific to the function of that TLV.
TLV Flags
These are defined per TLV type.
TLV Length
Length of the TLV in octets, including the TLV header and any
nested TLVs.
4.4 TLVs
4.4.1 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 sequenece 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
be assumed broken.
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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 | resv | HF | TLV Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Timer | Name Length | Sender Name (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Hello TLV Format
HF
Specifies the function of the Hello TLV. Four functions are
specified for the Hello TLV:
SYN: HF = 1
SYNACK: HF = 2
ACK: HF = 3
RSTACK: HF = 4.
TLV Data
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.
Name Length
The Name Length field is used to specify the length of the
Sender Name field in octets. If the name 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
Name from the Hello TLV.
Sender Name
The Sender Name field specifies the routable DTN name of the
sender that is to be used in updating routing information and
making forwarding decisions.
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4.4.2 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 10: Error TLV Format
TLV Flags
TBD
TLV Data
TBD
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4.4.3 Routing Information Base Dictionary TLV
The Routing Information Base Dictionary includes the list of
addresses 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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ Variable Length Routing Address Strings ~
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Routing Address String
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| String ID 1 | Length | Resv |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Routing Address String 1(variable) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
~ . ~
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| String ID n | Length | Resv |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Routing Address String n ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Routing Information Base Dictionary
TLV Flags
TBD
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RIBD Entry Count
Number of entries in the database
String ID 16 bit 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.
4.4.4 Routing Information Base TLV
The Routing Information Base lists the destinations 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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RIB String ID 1 | P-Value | RIB Flag 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ . ~
~ . ~
~ . ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RIBD String ID n | P-Value | RIB Flags n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Routing Information Base Header
Flags
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The encoding of the Header flag field relates to the
capabilities of the Source node sending the RIB:
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
RIB String Count
Number of routing entries in the TLV
RIB String ID
ID string as predefined in the dictionary TLV.
P-value
Delivery predictability for the destination of this entry as
calculated according to the equations in Section 2.1.1. The
encoding of this field is a linear mapping from [0,1] to [0,
0xFF].
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
4.4.5 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
PRoPHET ACK flag set in their entry in the list. When a node
receives a PRoPHET ACK for a bundle, it MUST remove any copies of
that bundle from its buffers, but SHOULD keep an entry of the
acknowledged bundle to be able to further propagate the PRoPHET ACK.
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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 | 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 | B_flags | resv |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bundle n Creation Timestamp time |
| (variable length SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bundle n Creation Timestamp sequence number |
| (variable length SDNV) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15: Bundle Offer and Response
TLV Type
The TLV Type for a Bundle Request is 0xA2. The TLV Type for a
Bundle Response is 0xA3.
TLV Flags
TBD
Bundle Offer Count
Number of bundle offer entries.
Bundle Dest String Id
ID string of the destination of the bundle as predefined in the
dictionary TLV.
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The encoding of the B_Flags in the request are:
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: PRoPHET ACK 0b1
The encoding of the B_flag values in the response 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: Reserved 0b1
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.
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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). 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.
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 reinitiated. 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 |
+------------------+-----------------------------------+-----------+
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| Neighbor Gone | | WAIT_NB |
+==================================================================+
State: INFO_EXCH
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
|Info Exch TLV rcvd| | INFO_EXCH |
+------------------+-----------------------------------+-----------+
| No more bundles | Start timer | WAIT_INFO |
+------------------+-----------------------------------+-----------+
| 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.
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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.
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 |
+==================================================================+
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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 |
+==================================================================+
State: ESTAB
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| SYN || SYNACK | Send ACK (note 2) | ESTAB |
+------------------+-----------------------------------+-----------+
| ACK && B && C | Send ACK (note 3) | ESTAB |
+------------------+-----------------------------------+-----------+
| ACK && !(B && C) | Send RSTACK | ESTAB |
+==================================================================+
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.
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
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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 |
+==================================================================+
State: WAIT_RIB
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| RIB received | Update P ; Send offer (note 3) | OFFER |
+------------------+-----------------------------------+-----------+
| ACK received | | WAIT_DICT |
+------------------+-----------------------------------+-----------+
| Dictionary rcvd | Update local dictionary | WAIT_RIB |
+------------------+-----------------------------------+-----------+
| Bundle req rcd | Send ACK | WAIT_DICT |
+------------------+-----------------------------------+-----------+
| Timeout(peer) | Send ACK | WAIT_DICT |
+==================================================================+
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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 | | |
+==================================================================+
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. This 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.
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Initiator:
----------
State: CREATE_DR
+==================================================================+
| Condition | Action | New State |
+==================+===================================+===========+
| Always | Create & send dict & RIB (note 1) | SEND_DR |
+==================================================================+
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 |
+==================================================================+
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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.
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.
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.
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 nodes to
1, 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 send all its
bundles to the malicious node, creating the belief that the
bundle has been very favorably forwarded. Depending on the
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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.
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 message destined to itself but for which it did not
receive the corresponding bundle.
6.1.2.2 Attack prevention/solution
To prevent this attack, some form of authentication between nodes
that meet is needed. One way to achieve this is to use public key
cryptography, but then the problem of key distribution needs to be
solved.
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 be
delivered to the destination.
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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
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 acked. 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
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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.
7. 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.
A prototype implementation of the protocol in C++ has been written at
Lulea University of Technology (LTU).
An implementation compliant to version 2 of the draft has been
written at Baylor University. This implementation has been
integrated into the DTN2 reference implementation.
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8. 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. 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.
During the winter of 2008, another deployment was made in the same
region to try out new hardware that is more suitable for the harsh
environment. Once again, PRoPHET was used for the routing in the
system.
9. Acknowledgements
The authors would like to thank Elwyn Davies for contributing with
valuable feedback, both on the technical details of the the protocol
as well as the structure of the draft. We would also like to thank
Olov Schelen, and Kaustubh S. Phanse for valuable discussions
regarding various aspects of the protocol. The Hello TLV mechanism
is loosely based on Adjacency message developed for RFC3292. Luka
Birsa, Samo Grasic, 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.
10. References
[1] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst, R.,
Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant Network
Architecture", Internet Draft draft-irtf-dtnrg-arch-04.txt,
December 2005.
[2] Scott, K. and S. Burleigh, "Bundle Protocol Specification",
Internet Draft draft-irtf-dtnrg-bundle-spec-04.txt,
November 2005.
[3] Vahdat, A. and D. Becker, "Epidemic Routing for Partially
Connected Ad Hoc Networks", Duke University Technical Report CS-
200006, April 2000.
[4] Lindgren, A. and K. Phanse, "Evaluation of Queueing Policies and
Forwarding Strategies for Routing in Intermittently Connected
Networks", Proceedings of COMSWARE 2006 , January 2006.
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[5] Small, T. and Z. Haas, "The Shared Wireless Infostation Model -
A New Ad Hoc Networking Paradigm (or Where there is a Whale,
there is a Way)", Proceedings of The Fourth ACM International
Symposium on Mobile Ad Hoc Networking and Computing (MobiHoc
2003) pp 233-244, June 2003.
[6] 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.
[7] Pentland, A., Fletcher, R., and A. Hasson, "A Road to Universal
Broadband Connectivity", Proceedings of the 2nd International
Conference on Open Collaborative Design for Sustainable
Innovation (dyd 02), Bangalore, India , December 2002.
Authors' Addresses
Anders F. Lindgren
University College London
Gower Street
London WC1E 6BT
United Kingdom
Phone: +447942205289
Email: dugdale@gmail.com
URI: http://www.sm.luth.se/~dugdale
Avri Doria
Lulea University of Technology
Lulea SE-971 87
Sweden
Phone:
Email: avri@acm.org
URI: http://psg.com/~avri
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 20, 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
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and D encounter each other frequently (Figure 20a) ), making the
delivery predictability values they have for each other high. Now
assume that node C also frequently encounters node B (Figure 20b) ).
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 20c) ) that
has a message for node D. Figure 20d) 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 20: PRoPHET example
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:
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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 2.3).
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 2.3).
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