The DARPA Packet Radio Network Protocols JOHN JUBIN AND JANET D. TORNOW, ASSOCIATE, IEEE Invited Paper In this paper we describe the current state of the DARPA packet scribe the algorithms used to route a packet through the radio network. Fully automated algorithms and protocols to orga- packet radio communications subnet.In Section V, we ex- nize, control, maintain, and move traffic through the packet radio network have been designed, implemented, and tested. By means amine the protocols for transmittingpackets. In Section VI, of protocols, networks of about 50 packet radios with some degree wedescribesomeofthehardwarecapabilitiesofthepacket of nodal mobility can be organized and maintained under a fully radio that strongly influence the design and characteristics distributed mode of control. We have described the algorithms and of the PRNET protocols. We conclude by looking brieflyat illustrated how the PRNET provides highly reliable network trans- some applications of packet radio networks and by sum- port and datagram service, by dynamically determining optimal routes, effectively controlling congestion, and fairly allocating the marizing the state of the current technology. channel in the face of changing link conditions, mobility, and vary- ing traffic loads. II. DESCRIPTIONOF THE PACKET RADIO SYSTEM I. INTRODUCrlON A. Broadcast Radio In 1973, the Defense AdvancedResearch Projects Agency (DARPA) initiated research on the feasibilityof usingpacket- The PRNET provides, via a common radio channel, the switched, store-and-forward radio communicationsto pro- exchange of data between computers that are geographi- vide reliable computer communications [I].This devel- cally separated. As a communications medium, broadcast opment was motivated by the need to provide computer radio (as opposed to wires and antennadirected radio) pro- network access to mobilehosts and terminals, andto pro- vides important advantages to theuser of the network. One vide computer communications in a mobile environment. of the benefits is mobility; a packet radio(PR) can operate Packet radio networking offers a highly efficient way of while in motion. Second, the network can be installed or using a multiple-access channel, particularly with bursty deployed quickly; there are no wires to set up. A thirdad- traffic [2]. The DARPA Packet Radio Network (PRNET) has vantage is the ease of reconfiguration and redeployment. evolved through the years to be a robust, reliable, opera- The PRNET protocols take advantage of broadcasting and tional experimental network[3]. The development process common-channel properties to allow the PRNET to be ex- has been of an incremental, evolutionary nature [4]; as al- panded or contracted automatically and dynamically. A gorithms were designed and implemented, new versions group of packet radios leavingthe original area simply de- ofthe PRNET with increasedcapabilities were demon- parts. Having done so, it can function as an autonomous strated. The PRNET has been in daily operation for exper- group and may later rejoin the original network or joinan- imental purposes for nearly tenyears. In this paper wede- other group. scribe the currentstate of the DARPA PRNET. The broadcasting and common channel propertiesof ra- We begin by providing asynopsis of the PRNET system dio have disadvantages too. These properties, for all prac- concepts, attributes, and physical components in Section tical purposes, prohibit the building ofa radio that is able 11. In Section Ill, we illustrate the mechanisms by which a to transmit and receive at the same time. Therefore, the packet radio automaticallykeeps track of a potentially con- PRNET protocols must attemptto schedule each transmis- tinuouslychanging network topology. InSection IV, we de- sion when the intendedPR is not itself transmitting. Also, transmissions often reach unintended PRs and interfere Manuscript received February l, 1986; revised July 30,1986.The with intended receptions. Therefore, the protocols must work of j. jubin was supported by the Defense Advanced Research attempt to schedule each transmission when the intended Projects Agency of the Department of Defense under Contract PR is not receiving another PR’s transmission. MDA903-85-C4205. Thework of J. D. Tornow was supported by the Defense Advanced Research Projects Agency of the Department of Defense under Contract MDA90MSC4254. B. Automated Network Management J.Jubin is with Collins Defense Communications, Rockwell In- ternational, Richardson, TX 75081, USA. The PRNET features fully automated network manage- j. D. Tornow is with SRI International, Menlo Park, CA 94025, ment. It is self-configuring upon network initialization,re- USA. configures upon gain or loss of packet radios, and has dy- 0018-9219/87/01ooM)21501.000 1987 IEEE PROCEEDINGS OF THE IEEE, VOL. 75, NO. 1, JANUARY 1987 21 namic routing. The network operator simplyhas to ensure tion. The digital subsystem controls the routing and flow that the packet radios are deployed so that every packet of packets between PRs while the RF subsystem transmits radio is situated so as to have at leastone otherpacket radio and receives packets over the radio channel. The PR re- within line-of-sight, and then turn theradios on; the packet ceives packetsof data either fromits wire interface or from radio is intended to operate unattended. Once installed, the radio channel. Each PR is responsible for receiving a the system discovers the radio connectivity betweenpacket packet and relaying it on toa PR that is one hopcloser to radios and organizesrouting strategies dynamically on the the final destination. The packets can be routed eitherto basis of this connectivity. After initialization, most com- another PR over the radio channel oran to attached device munication networks maintaina static topology. A unique (i.e., host computer or terminal) viathe wireinterface. The feature of the PRNET is the ease with which network to- protocols that runin the PRs encompass the Physical, Data pology can be altered without affecting user‘sthe ability to Link, and Network layers, asdefined in ISO’s Open Systems communicate. Although RF connectivity is difficult to pre- Interconnection Reference Model 161. dictandmayabruptlychangeinunexpectedwaysasmobile It is the local broadcast nature of the packet radio that packet radios move about, the automated network man- givesthePRNETitsuniquenetworkingcharacteristic:aPR’s agement procedures usedin the PRNET are capableof sens- transmission is received by allPRs within line-of-sight; e.g., ingtheexistingconnectivityin realtimeandthenexploiting in Fig. 2, a connecting line indicates which PRs are within this connectivity in order to continuously transport data line-of-sight of each other. Since PRs L, N, and Q are all and control packets, all in a way that is totally transparent within line-of-sight of M, a transmission by M can be re- to the users. ceived by allof these PRs; they aresaid to be one hopaway from M. Note, however, that a transmission by, say, PR P C. Network Components can be received only by PRs L and N. In general, a PRNET consists of many PRs that are not all within line-of-sight of The PRNET system comprises: each other, and packets must traverse multiple hops to The PRNET subnet, which consists of the packet radios. reach their destination. The PRNET subnet provides the means of interconnecting a community of users. The collection of devices (host computers and termi- nals), each attached to a packet radio via awire high-level data link control (HDLC) interface, that wish to exchange data in real time. The primary component of the packet radio communi- cation network system is the packet radio. The generation Fig. 2. Small packet radio network. of PR that is supporting the currentresearch is designed to supportthe development and evaluation of advanced packet networking concepts and techniques. This packet In order fora user to send data across a PRNET, a device radio equipmenthas been designatedthe LowcostPacket (such as a small host computer) must be connected to a Radio (LPR) [5], Fig. 1. The LPR consists of both digital and packet radio viaan HDLC wire interface. Since the DARPA RF subsystems. It is capable of omnidirectional, spread- PRNET is a part of theDARPA Experimental Internet System spectrum, half-duplex transmissionlreception at 40@ and [7l, the devices are responsible for running theDoD-stan- IWkbit/s data rates. It supports pseudo-noise code mod- dard internetwork-, transport-, and application-level pro- ulation of the information bits and forward error correc- tocols (IP, TCP, and TELNET). These protocols ensure that the end-to-end communication between hosts is reliable and robust, and allowhosts on thePRNET to communicate with computers on various other packet-switched satellite, terrestrial, radio, and local area networks that also partic- ipate in the DARPA Internet. A host computer may be di- rectly interfacedto a PR. If a user wishes to send data across the PRNET from a terminal or host that does not run the required protocols,a Network Interface Unit (NIU)[8], Fig. 3, may be used between the terminal or host and the PR. Fig. 1. Low-cost Packet Radio. Fig. 3. NetworkInterface Unit. 22 PROCEEDINGS OF THE IEEE, VOL. 75, NO. 1, JANUARY 1987 The NIU performs thenecessary host-to-host and terminal- congested neighborhoods. However, performanceis likely specific protocols. The PRNET can also be accessed from to suffer when “actual” neighborhoodsare large. Enhance- other networks via an Internet gateway. In