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 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 12, 2008. Copyright Notice Copyright (C) The IETF Trust (2008). 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 Lindgren & Doria Expires August 12, 2008 [Page 1] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 2] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 3] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 4] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 5] Internet-Draft PRoPHET February 2008 +----------------------------+ +----------------------------+ | ___ | | ___ | | ___ / \ | | / \ | | / \ ( 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. Lindgren & Doria Expires August 12, 2008 [Page 6] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 7] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 8] Internet-Draft PRoPHET February 2008 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, Lindgren & Doria Expires August 12, 2008 [Page 9] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 10] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 11] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 12] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 13] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 14] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 15] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 16] Internet-Draft PRoPHET February 2008 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) Lindgren & Doria Expires August 12, 2008 [Page 17] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 18] Internet-Draft PRoPHET February 2008 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, Lindgren & Doria Expires August 12, 2008 [Page 19] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 20] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 21] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 22] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 23] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 24] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 25] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 26] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 27] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 28] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 29] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 30] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 31] Internet-Draft PRoPHET February 2008 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 | +------------------+-----------------------------------+-----------+ Lindgren & Doria Expires August 12, 2008 [Page 32] Internet-Draft PRoPHET February 2008 | 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. Lindgren & Doria Expires August 12, 2008 [Page 33] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 34] Internet-Draft PRoPHET February 2008 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 | +==================================================================+ Lindgren & Doria Expires August 12, 2008 [Page 35] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 36] Internet-Draft PRoPHET February 2008 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 | +==================================================================+ Lindgren & Doria Expires August 12, 2008 [Page 37] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 38] Internet-Draft PRoPHET February 2008 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 | +==================================================================+ Lindgren & Doria Expires August 12, 2008 [Page 39] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 40] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 41] Internet-Draft PRoPHET February 2008 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 Lindgren & Doria Expires August 12, 2008 [Page 42] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 43] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 44] Internet-Draft PRoPHET February 2008 [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 Lindgren & Doria Expires August 12, 2008 [Page 45] Internet-Draft PRoPHET February 2008 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. Lindgren & Doria Expires August 12, 2008 [Page 46] Internet-Draft PRoPHET February 2008 +----------------------------+ +----------------------------+ | | | | | 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: Lindgren & Doria Expires August 12, 2008 [Page 47] Internet-Draft PRoPHET February 2008 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). Lindgren & Doria Expires August 12, 2008 [Page 48] Internet-Draft PRoPHET February 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). 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The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Lindgren & Doria Expires August 12, 2008 [Page 49]