LINK MANAGEMENT IN THE AIR FORCE AIRBORNE NETWORK

Rafols RamirezThe MITRE Corporation

Colorado Springs, CO 80910

ABSTRACT

The envisionedA ir Force A irborne Network (AFAN) circa2020 will most likely consist of a backbone network andmany edge networks. The backbone nodes are expected toexperience semi-permanent connectivity whereas the edgenetwork nodes may experience rapidly changing connec-tivity in an ad-hoc manner. In addition, nodes may joinand leave any edge network at fairly rapid rates. In thisenvironment there is a need to manarge link connectivity,node membership and the overall network topology insuch a way as to minimize the impact ofrapidly-changingconnectivity and node network membership on the per-formance of user information transfers. This rapid detec-tionlresponse management of links, nodes and the overalltopology is defined as link management (LM). This paperdescribes the LM approach being taken for the AF ANcirca 2020. The following topics are addressed: (a) highlevel LM functions from a network perspective, (b) LMarchitecture alternatives based on the extent ofcentraliza-tionldecentralization employed, and (c) a proposed LMfunctional allocation between platform and terminalequipment. Finally, this paper identifies sample issuespertaining to the successful development ofan AiNLMsys-tem, along with possible methods to resolve these issues.

1. INTRODUCTION

The US Air Force (AF) Airborne Network (AN) will be aninfrastructure that enables DOD Global Information Grid(GIG) information exchanges via airborne platforms. It isthe official airborne piece of ConstellationNet, which inturn is the AF portion of the GIG. The AN will be capableof interfacing with GIG surface networks (terrestrial andmaritime, fixed and mobile) as well as the future GIGspace network, i.e., Transformational Satellite Communi-cation System (TSAT). The AN will handle informationexchanges for all GIG users, including those aboard air-borne platforms. These exchanges support the followingtraffic: IP traffic source to/from IP traffic destination, leg-acy (non-IP) traffic source to/from legacy traffic destina-tion; IP traffic source to/from legacy traffic destination, viaa gateway; and legacy traffic source to/from IP traffic des-tination, also via a gateway. Figure 1 shows the AN in thecontext of the GIG. The envisioned AN will consist of abackbone network and many edge networks (e.g., tacticalsubnets, ISR subnets, legacy (non-IP based subnets)). Thebackbone nodes are expected to experience relatively sta-

ble semi-permanent connectivity whereas the edge net-work nodes may experience rapidly changing connectivityin an ad-hoc manner.

Figure 1. The Airborne Network within the GIG

2. AN CHARACTERISTICS and MAJORFUNCTIONS

Some of the key characteristics of the AN are as follows.First, airborne platforms (network nodes) will "carry" theAN routing infrastructure. This means that every platformis a potential routing node, although routing may be dis-abled on certain platforms when so desired. Second, theAN will be capable of operating disconnected from the restof the GIG. This leads to platforms having to also containmost if not all, of the functionality for network manage-ment, information assurance, and network services (e.g.,network timing, name resolution, dynamic address con-figuration). Third, the AN will operate in a very challeng-ing environment. This includes airborne radio communi-cations channels characterized by high error rates, fading,and susceptibility to jamming and unintentional interfer-ence, intermittent link performance due to platform blockage of radiated signals unidirectional links and limitedelectromagnetic spectrum in a region. Fourth, AN func-tionality within a platform will be defined statically as wellas dynamically based upon the individual networking ca-pabilities of each platform It is expected that early ANfunctionality will be mostly fixed in advance whereas fu-ture functionality will be dynamically "composable.Fifth, the network structure will be self-forming, self-organizing and self-healing. This means that nodes joinleave, and reorganize quickly.

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The AN will provide the following major functions: (a)Inter-node Connectivity, which includes routing/switching,QoS/CoS, backbone connectivity, subnet connectivity, andnetwork access connectivity; (b) Network Services, whichincludes name resolution service, dynamic network con-figuration management and network time; (c) InformationAssurance, which includes policy-based security manage-ment, data protection, authentication/integrity/access con-trol, key management, and protocol security; (d) NetworkManagement, which includes fault/configuration/account-ing/performance/security (FCAPS) management, networksituation awareness, network resource planning, and pol-icy-based network management; and (e) Link Manage-ment, which includes node advertising and discovery, es-tablishing and restoring, link/node monitoring and evalua-tion, and topology management.

3. AN LINK MANAGEMENT OVERVIEW

AN Link Management (LM) provides the means to man-age nodes, links and the overall network topology in sucha way as to minimize the impact of the rapidly-changingAN environment characteristics (e.g., data rates, errorrates, fading, jamming, platform blockage of radiated sig-nals) and the rapidly-changing AN node membership (dueto nodes joining/leaving the AN as a whole as well asnodes moving within the AN) on the performance of userinformation transfers. LM enables autonomous fast detec-tion of and response to these rapidly-changing conditions.The high level functions performed by LM are as follows.

Node Advertising and Discovery - Each node will becapable of advertising its identity and location so it can bediscovered by other AN nodes. The LM will identify, au-thenticate identity and locate nodes requesting admittanceto the AN. It will also determine the role/functionality ofauthenticated nodes, the service requested by such nodes,and the availability of resources needed to provide suchservice. The LM will admit nodes for which proper ser-vice can be provided without jeopardizing the health andmission-operations of the AN as a whole.

Establishing and Restoring - The LM will establish linksamong admitted AN nodes to satisfy mission operationalrequirements and topology configurations in accordancewith current policies. It will also pre-configure/re-configure links, and provision (i.e., authorize the use of)configured links. The LM will be able to adjust link pa-rameters as well as establish, replace and/or remove linksbetween admitted nodes to compensate for degraded per-formance or for changes in node membership. Examplesof parameters that can be adjusted are channel data rateand channel frequency. The LM will also control the de-establishment and establishment of links during handoffs(e.g., cases where a node moves from one AN subnetwork

to another AN subnetwork). In addition, the LM will beable to pre-configure/re-configure the role and/or function-ality of nodes based on mission operational needs and cur-rent policies.

Link/Node Monitoring and Evaluation - The LM willmonitor the operational state of all links and admitted ANnodes. It will also monitor and determine the performanceof links and of network components (e.g., routers) withinadmitted nodes, and it will predict link degradations due toexpected blockages or range change. The LM will deter-mine the performance ofAN regions as well as the overallAN. In addition, the LM will keep track of changes innode membership (joining, leaving nodes) within the AN.Finally, the LM decides whether or not the current topol-ogy needs alteration to satisfy current mission operationalrequirements or safeguard the AN as a whole.

Topology Management - The AN LM will determinehow nodes will be interconnected in order to establish andmaintain an AN topology that satisfies current missionoperational requirements and preserves the health of theAN as a whole. Once the LM determines necessary topol-ogy changes, it achieves them by performing functionssuch as those described earlier in this section under nodeadvertising/discovery and establishing/restoring.

4. LM FUNCTIONAL ARCHITECTUREALTERNATIVES

The LM system needs to support autonomous fast detec-tion of and response to the rapidly-changing environmentcharacteristics and the rapidly-changing node membershiptypical of the AN. This section describes two alternativesfor a modular LM functional architecture that minimizesthe impact of the above conditions on user informationtransfers. The first alternative employs a hybrid central-ized/decentralized approach whereas the second one em-ploys a partially decentralized approach. Both of thesearchitectures are currently under consideration. Two otherarchitecture variants, one employing a fully centralizedapproach and the other a fully decentralized approach, arenot under consideration. A major problem with the fullycentralized approach in the AN environment is that it re-quires a large volume of information exchanges betweenthe central control node and each of the other nodes in theAN. This, in turn, uses a lot of bandwidth which other-wise would be available to AN user data transfers. A ma-jor problem with the fully decentralized approach in theAN environment is that each node performs many func-tions, including local control functions. This, in turn, leadsto implementations requiring a significant amount ofphysical space. The small AN platforms do not have suf-ficient physical space to support this approach.

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4.1 Alternative 1: Hybrid Centralized/DecentralizedArchitecture

Overview - Figure 2 shows the first architecture alterna-tive which is composed of three types of functional mod-ules: a high level LM Executive, LM Autonomous Manag-ers, and LM Agents. There are two classes ofLM Agents:terminal and node. However, this distinction is not shownin the figure due to physical space limitations for the draw-ing. There is one Terminal LM Agent in each terminalassociated with an airborne platform, and one Node LMAgent in each airborne platform. There is one LMAutonomous Manager per each region of the overall AN.Each of these managers controls the LM Agents in its re-gion. The number of LM Agents under the control of anLM Autonomous Manager is initially tailored to each re-gion but can be adjusted based on changes in traffic, envi-ronment and subnetwork membership. There is one mainLM Executive and one backup for the entire AN, each in aseparate AN backbone node. The backup module will bekept in synchronism with the main module for reliabilitypurposes. The main LM Executive controls all the LMAutonomous Managers within the AN. In this approachthe LM Agents perform many local functions autono-mously, and provide most of their data only upon request,to minimize overhead traffic between themselves and theirLM Autonomous Manager. However, these agents willautonomously report certain local exceptional conditionsto their LM Autonomous Manager according to pre-defined criteria. Similarly, the LM Autonomous Managerswill perform many regional functions autonomously, andprovide most of their data only upon request, to minimizeoverhead traffic between themselves and the LM Execu-tive. However, these Managers will autonomously reportcertain regional exceptional conditions to the LM Execu-tive according to pre-defined criteria. Peer LM Autono-mous Managers cooperate to accomplish certain functionssuch as inter-region handoffs.

Terminal LM Agent - This LM Agent (LMA) autono-mously performs the following functions pertaining tonodes: (a) advertises the identity and location of its hostnode, (b) discovers other advertising nodes, (c) authenti-cates identity of discovered nodes, and (d) requests admis-sion of authenticated nodes from its LM AutonomousManager. A terminal LMA autonomously performs thefollowing functions pertaining to links within its control:(a) pre-configures and re-configures links (set link parame-ters) to meet requirements, (b) provisions (authorize theuse of) configured links, (c) sets and modifies link opera-tional status, (d) monitors and evaluates performance oflinks, and (e) adjusts link parameters, and (f) establishes,replaces and/or removes links to compensate for degradedlink performance. In addition, each terminal LMA proc-esses and responds to messages sent by its LM Autono-mous Manager. For example, messages are sent to (a) no-

tify node admittance disposition, (b) request operationalstatus, performance and/or configuration information onlinks, (c) modify link parameters (e.g., data rate, transmit-ting/receiving frequency), (d) modify link operationalstatus, and (e) modify connectivity by the establishment,replacement and/or removal of links. Finally, this LMAautonomously reports local exceptional conditions to itsLM Autonomous Manager according to pre-defined crite-ria. An example of a reported exceptional condition is thefailure to establish a link that is critical to the accomplish-ment of a mission.

LM NOTE:

AN Platform EtikfiW LM Autonomous Manager &LM Agents

are present but not shown

+0--Opo- May traverse intermediate AN platforms

Figure 2. LM Architecture Alternative 1

Node LM Agent - This LMA autonomously performs thefollowing functions pertaining to its host AN node: (a)pre-configures and reconfigures node network devices(e.g., routers) as well as the role of the node to meet re-quirements, (b) sets and modifies the node operationalstatus, (c) monitors and evaluates performance of networkdevices, and (d) adjusts network device parameters. Inaddition, each node LMA processes and responds to mes-sages sent by its LM Autonomous Manager. For example,messages are sent to (a) request node operational status,node role, network device performance, and/or networkdevice configuration, (b) modify node operational status(including the disabling of the node), node role, and/ornetwork device parameters. Finally, this LMA autono-mously reports local exceptional conditions to its LMAutonomous Manager according to pre-defined criteria.An example of a reported exceptional condition is the fail-ure to change the role of its host node, where this change iscritical to the accomplishment of a mission.

LM Autonomous Manager - An LMAM autonomouslyperforms the following functions pertaining to nodeswithin its region: (a) determines the role of nodes authen-ticated by terminals as well as the service requested bysuch nodes, (b) determines the resources needed to provideproper service to each requesting node, (c) admits nodesfor which proper service can be provided, and (d) reportsnode admittance disposition to the Terminal LM Agent

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that discovered the node. In addition, an LMAM (a) sendsrequests to node LM Agents for node operational status,node role, network device performance, and/or networkdevice configuration, and (b) sends commands to modifynode operational status (including the disabling of thenode), node role, and/or network device parameters. Eachautonomous manager receives reports from node LMAgents on exceptional conditions pertaining to the nodewhere the reporting agent resides. An LMAM autono-mously performs the following functions pertaining tolinks within its region: (a) predicts link degradations dueto range or blockages, and (b) sends requests to TerminalLM Agents for the operational status, performance and/orconfiguration of links. In addition, it sends commands toTerminal LM Agents to (a) modify link parameters (e.g.,data rate, transmitting/receiving frequency), (b) modifylink operational status, and (c) modify connectivity by es-tablishing, replacing and/or removing links (e.g., to com-pensate for degraded network performance and to supportnode handoffs). Each autonomous manager receives re-ports from terminal LM Agents on exceptional conditionspertaining to links under the control of the reporting agent.Based on information obtained via requests to and reportsfrom LMAs, an LMAM keeps track of the connectivitybetween nodes, nodes joining/leaving the AN, nodeschanging subnetwork membership, failed nodes, and net-work performance within its region. In addition, anLMAM autonomously determines changes needed to thetopology within its region and directs the execution ofsuch changes via commands to LMAs. An LMAMautonomously reports exceptional regional conditions tothe LM Executive. An example of a reported exceptionalcondition is the failure to properly reconfigure node(s) keyto the accomplishment of a mission. Each autonomousmanager exchanges information with peer autonomousmanagers to accomplish operations involving more thanone topology region (e.g., inter-region handoff). Finally,an LMAM processes messages sent by the LM Executive.For example, messages are sent by the LME to alter thetopology under control of the recipient LMAM.

LM Executive - The LM Executive (LME) autonomously(a) sends requests to LMAMs for regional topology infor-mation, (b) builds and maintain a view of the overall ANtopology, (c) determines when the AN topology needs tobe altered as well as the corresponding actions to be taken,and (d) directs the execution of these actions by command-ing the appropriate LMAMs. Actions to alter the AN to-pology will be undertaken by the LME only when abso-lutely necessary, to ensure that mission requirements areproperly supported or to safeguard the overall health of theAN. The LME receives reports from LM AutonomousManagers on exceptional conditions pertaining to the to-pology region under control of the reporting AutonomousManager.

4.2 Alternative 2: Partially Decentralized Architecture

Overview - Figure 3 shows the second architecture alter-native which is composed of two types of functional mod-ules: LM Autonomous Managers and LM Agents. As inalternative 1, there are two classes ofLM Agents: terminaland node. However, this distinction is not shown in thefigure due to physical space limitations for the drawing.There is one Terminal LM Agent in each terminal associ-ated with an airborne platform, and one Node LM Agent ineach airborne platform. There is one LM AutonomousManager per each region of the overall AN. Each of thesemanagers controls the LM Agents in its region. However,this alternative does not include an LM Executive. TheLM Agents perform exactly the same functions as theircounterparts in alternative 1. The LM Autonomous Man-agers will perform most of the functions performed bytheir counterparts in alternative 1. The differences aredocumented in the rest of this section.

I ..+ May traverse intermediate AN platforms|

Figure 3. LM Architecture Alternative 2

LM Autonomous Manager - The collective LMAMsneed to perform the following functions: (a) build andmaintain a fairly accurate view of the overall AN topology,(b) determine when the overall AN topology needs to bealtered as well as the corresponding actions to be taken,and (c) cause the execution of these actions by collabora-tion among peer LMAMs. Actions to alter the AN topol-ogy will be undertaken only when absolutely necessary, toensure that mission requirements are properly supported orto safeguard the overall health of the AN. An LMAMsends requests to peer LMAMs for regional topology in-formation, as needed, and autonomously reports excep-tional regional conditions (e.g., unexpected loss of nodeand failure to reconfigure another node, which will hinderthe accomplishment of a mission) to peer LMAMs. Fi-nally, an LMAM processes commands sent by peerLMAMs. For example, commands are sent to alter thetopology under control of the recipient LMAM.

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Of the two alternatives described, alternative 2 providesthe most robust approach for operations involving theoverall AN topology because it does not depend on a cen-tral node. However, due to the lack of a central node, al-ternative 2 requires more complexity than alternative 1 todetermine the overall AN topology. In addition, alterna-tive 2 requires more information exchanges than alterna-tive 1, because information from each LMAM must besent to all other LMAMs instead of a central point. Thisall-to-all communication requirement also causes alterna-tive 2 to be less scalable than alternative 1. The lack ofrobustness in alternative 1 can be mitigated by employinga backup LME. However, this requires that the main andbackup LMEs be kept in synchronism to achieve seamlessswitching between these two units. Studies need to beconducted under expected AN mission scenarios and envi-ronment conditions to determine the best alternative.

5. ISSUES and RESOLUTION METHODS

This section identifies sample issues and possible resolu-tion methods pertaining to the successful development ofan AN Link Management system.

5.1 Examples of Issues

Definition/Determination of:- Performance factors to be measured, measurement fre-quency, and reporting thresholds.

- Device and link parameters that can be dynamically ad-justed by LMAs, and by LMAMs.

- Criteria to determine the specific environmental changesthat are worthy of compensation.

- Effective compensation steps for these worthy changes.- Criteria to determine exceptional conditions worthy ofbeing reported immediately by LMAs, and by LMAMs.

- Network resources required to satisfy the services re-quested by a node.

- Criteria to determine the conditions that warrant the al-tering of the overall AN topology.

- Specific actions to be taken for each condition warrant-ing the alteration of the AN topology.

- The most effective division of labor between LMAs andLMAMs, and between LMAMs and the LME.

- The extent of regional topology information to be sharedby peer LMAMs.

- Criteria for node admittance.- Conditions that indicate the need for a handoff.- Criteria for node authentication.- Criteria to establish that performance has been suffi-

ciently degraded to warrant corrective actions.- Specific corrective actions to be undertaken upon detec-

tion of degraded performance.

- Data exchanges with other AN functional areas such asQoS, Routing, Information Assurance and NetworkManagement.

- Data exchanges with systems external to the AN.- Cases for which pre-planned reaction scripts are suitable.- Relative benefits and tradeoffs of a cross-layer approach

(i.e., physical, MAC, network and application layer tech-niques in combination with one another) to LM.

- Tradeoffs among factors such as computation overhead,optimal node power, desired connectivity, maximumdata rate for links, vulnerability to jamming, networksurvivability, and graceful performance degradation un-der dynamic changes.

Efficient Algorithms to:- Advertise node identity and location.- Discover advertising nodes.- Predict link degradations due to range or blockages.- Accomplish seamless intra-region and inter-region hand-

offs.- Elect the node that should perform as the LMAM within

region.- Determine the optimum number of LMAs to be under

the control of a specific LMAM.- Build and maintain a fairly accurate view of the overallAN topology.

- Maintain synchronization between the main and backupLMEs.

Efficient Measurement Techniques to:- Determine the operational status of nodes and of links.- Determine the performance of links, routers, and the net-work as a whole.

Efficient Protocols for Information Exchanges:- Between LMAs and LMAMs, between LMAMs and theLME, and between peer LMAMs.

- Between LM and other AN functional areas such asQoS, Routing, Information Assurance and NetworkManagement.

- Between LM and systems external to the AN.

5.2 High Priority Issues

All the issues listed above are important; however, thoselisted in this section are the most difficult to resolve, and,therefore, have high priority.

Efficient Algorithms to:- Predict link degradations due to range or blockages.- Accomplish seamless intra-region and inter-region hand-

offs.

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- Build and maintain a fairly accurate view of the overallAN topology.

Efficient Measurement Techniques to:- Determine the performance of the network as a whole.

Efficient Protocols for Information Exchanges:- Between LMAs and LMAMs, between LMAMs and theLME, and between peer LMAMs.

Definition/Determination of:- Performance factors to be measured, measurement fre-quency, and reporting thresholds.

- Effective compensation steps for environmental changes.- Network resources required to satisfy the services re-quested by a node.

- Specific actions to be taken for each condition warrant-ing the alteration of the AN topology.

- The most effective division of labor between LMAs andLMAMs, and between LMAMs and the LME.

- Specific corrective actions to be undertaken upon detec-tion of degraded performance.

- Relative benefits and tradeoffs of a cross-layer approach(i.e., physical, MAC, network and application layer tech-niques in combination with one another) to LM.

- Tradeoffs among factors such as computation overhead,optimal node power, desired connectivity, maximumdata rate for links, vulnerability to jamming, networksurvivability, and graceful performance degradation un-der dynamic changes.

5.3 Possible Resolution Methods

There are several methods that can be employed to resolvethe above issues. Examples are studies as well as researchand development performed by DOD/USAF contractors,industry or academia.

6. SUMMARY and RECOMMENDATIONS

6.1 Summary

This paper describes the USAF AN which will be an infra-structure that enables DOD GIG information exchangesvia airborne platforms. The AN will operate in a verychallenging environment. This includes airborne radiocommunications channels characterized by high error

rates, fading, and susceptibility to jamming and uninten-tional interference; intermittent link performance due toplatform blockage of radiated signals; unidirectional links;and limited electromagnetic spectrum in a region. In addi-tion, the network structure will be self-forming, self-organizing, and self-healing. This means that nodes join,leave, and reorganize quickly. AN Link Management(LM) provides the means to manage nodes, links and theoverall network topology in such a way as to minimize theimpact of the rapidly-changing AN environment character-istics and the rapidly-changing AN node membership onthe performance of user information transfers. LM enablesautonomous fast detection of and response to these rap-idly-changing conditions. It performs the following highlevel functions: node advertising and discovery, establish-ing and restoring, link/node monitoring and evaluation,and topology management. Two alternatives were pre-sented for a modular LM functional architecture thatminimizes the impact of the above conditions on user in-formation transfers. The first architecture is hybrid cen-tralized/decentralized whereas the second one is partiallydecentralized. In addition, a proposed LM functional allo-cation between platform and terminal equipment was pre-sented. The paper concluded by identifying high priorityissues and possible resolution methods pertaining to thesuccessful development of an AN Link Management sys-tem.

6.2 Recommendations

Recommend that DOD/USAF coordinates the resolutionefforts for the high-priority issues identified in this paper.In this role, it would convene joint forums involving DODcontractors, industry and academia in an attempt to opti-mize the use of resources toward the resolution of theseissues. In addition, recommend that coordination effortsbe undertaken by DOD/USAF with platform and terminalprograms in order to better define the allocation of LMfunctionality, and to sponsor studies to determine the op-timum LM functional architecture.

7. REFERENCE

Airborne Network Architecture System CommunicationsDescription and Technical Architecture Profile, Version1. 1, Prepared by HQ ESC/GIGSG/NA for the USAF Air-borne Network Special Interest Group, dated 7 October2004.

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