Noname manuscript No.(will be inserted by the editor)

Illustrating a Publish-Subscribe Internet Architecture

Nikos Fotiou · Dirk Trossen · George C. Polyzos

Received: date / Accepted: date

Abstract The Publish-Subscribe Internet RoutingParadigm (PSIRP) project aims at developing and eval-uating an information-centric architecture for the fu-ture Internet. The ambition is to provide a new form ofinternetworking which will offer the desired functional-ity, flexibility, and performance, but will also supportavailability, security, and mobility, as well as innovativeapplications and new market opportunities. This pa-per illustrates the high level architecture developed inthe PSIRP project, revealing its principles, core com-ponents, and basic operations through example usagescenarios. While the focus of this paper is specifically onthe operations within the architecture, the revelation ofthe workings through our use cases can also be consid-ered relevant more generally for publish-subscribe ar-chitectures.

Keywords Future Internet · Clean Slate · Networkingusage scenarios

1 Introduction

The current Internet architecture has remained rela-tively unchanged since its inception. The Internet was

Nikos Fotiou, George C. Polyzos

Mobile Multimedia LaboratoryAthens University of Economics and Business

Athens, 113 62 GreeceTel.: +30 210 8203 693E-mail: {fotiou,polyzos}@aueb.gr

Dirk Trossen

Computer Laboratory

University of CambridgeWilliam Gates Building

15 JJ Thomson Avenue, Rm FE08, UK

Tel.: +44 7918 711 695E-mail: dirk.trossen@cl.cam.ac.uk

initially designed in a way that somewhat resembles atelephone network, where uniquely addressed endpointstrust each other and exchange data through an internet-working infrastructure. However, nowadays this designdoes not cope with current networking trends, neitherwith applications needs. Viruses and worms have led toa state where two arbitrary network nodes do not trusteach other anymore. End-to-end communication doesnot seem to be the prevailing paradigm as data requestsare likely to be served by an intermediate element–suchas a content delivery network, or a proxy server–andpopular applications (e.g., p2p file sharing applications)focus on the information itself rather than on the end-points sotring the information or the location.

Furthermore, an imbalance of powers exists in thecurrent Internet, where the network is designed to for-ward data from senders to receivers, whether the re-ceivers want to receive the data or not, leading to prob-lems such as denial of service attacks and spam email.Various mechanisms such as network address transla-tion (NAT) and spam filters have been deployed in or-der to restore these issues; however they do not solve theproblem completely. Moreover challenges related to se-curity, mobility, scalability, quality of service, and eco-nomics have increased the need for a clean slate ap-proach to a new Internet architecture [8].

We underline the critical importance of informationwith the following realistic scenario of future Internetusage. We focus on the challenges that arise, manifest-ing this way a shift towards an information orientedfuture Internet.

During the summer Olympic games, various me-dia providers offer live streaming video from the sta-dium. Wireless cameras deployed in the field, camerason blimps and dozens of cameramen are used to offer abreathtaking experience to the viewers worldwide. In-

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ternet users are able to enjoy the streams in a varietyof devices ranging from high definition TV to 3G mo-bile phones. Moreover users are able to choose amongvarious coverage angles, as well as select the languageof narration etc. Media providers also offer exclusivevideos from the preparation room as well as interviewswith the athletes and their coaches. All these videos areprovided upon subscription. Meanwhile in the stadium,thousands of spectators shoot pictures, which they in-stantly upload to their photo blogs, record videos thatthey share in popular video sharing Web sites and makevideo calls in order to show to their friends famous ath-letes.

Various challenges can be identified in this scenario.Information needs to be accessed seamlessly by a vari-ety of devices which can be fixed or mobile. Tussles ariseas spectators and media providers compete for the net-work resources inside the stadium. Tussle may also arisebetween athlete sponsors, that may want to control themedia regarding their athletes, and media providers,that want to reveal as much information as possible.The dissemination of information–especially concern-ing the exclusive videos and the video calls–needs to belimited to only the eligible users, assuring the respectof digital rights as well as the privacy of the users. Fi-nally it needs to assure that end-users will receive theinformation which they are really interested in, exclud-ing from the information delivery path malicious users,such as spammers.

We believe that an information-centric communi-cation paradigm would address these challenges in amore successful way than the current Internet. Such aparadigm is the publish/subscribe (pub/sub). Pub/subis an information-centric paradigm that shifts the poweraway from the data sender, i.e., data consumers expresstheir interest in specific pieces of information explicitly,which are forwarded to them by the network when theybecome available. As a result, information is propagatedonly to those nodes which really want it.

The Publish/Subscribe Internet Routing Paradigm(PSIRP) project [18] is a multi-organization FP7 EUfunded research effort aiming at creating a clean-slate architecture for the future Internet based on thepub/sub communication paradigm, taking nothing forgranted. The contribution of this paper is to present themain architectural concepts that are underpinning thePSIRP approach. In particular, we outline the archi-tectural concept equivalent to layering in today’s net-working, the so-called bubble concept. This concept isdirectly based on (a) the network functions being iden-tified as central in PSIRP and (b) the information struc-tures being defined within PSIRP. We expect this con-cept to have a direct impact on the way network nodes

will be implemented as well as on how they will func-tion in a networked environment. This paper presentsthis concept as well as the general architectural thrustof PSIRP at conceptual as well as qualitative level.

We have implemented various parts of the presentedarchitecture and installed early testbeds for upcom-ing performance results of meaningful scale. Further-more, early performance results have been obtained forparts of the architecture with simulation and emulation.However, we left out the presentation of these resultsdue to the architectural thrust of the paper, but wedo recognize the need to extend the performance eval-uation of various aspects of the architecture in orderto fully appreciate and understand its benefits and inparticular of the proposed bubbles concept.

The remainder of this paper is organized as follows.Section 2 presents related work in the area. Section 3gives an overview of the PSIRP architecture, present-ing its core elements and its basic operations as wellas how information is organized and provided. In Sec-tion 4 PSIRP usage scenarios are given, illustrating atconsiderable level of detail the supported functionalityand operations. Finally, Section 5 presents applicationsconsiderations for the PSIRP architecture and Section6 presents our conclusions and plans for future work.

2 Related work

Publish/subscribe overlay systems have been widelystudied, especially in cases of event notification archi-tectures. Siena [6] and Hermes [17] are two notableexamples of large scale pub/sub systems with intra-domain rendezvous functionality. Siena was particularlysuccessful in demonstrating the applicability and effec-tiveness of the pub/sub paradigm in multi-domain en-vironments, whereas Hermes provides middleware thataccelerates the development of applications that op-erate in these environments. PSIRP extends the ser-vice model of these systems by applying the pub/subparadigm at all levels of its architecture, targeting atthe same time the provision of the necessary tools andAPIs that will allow applications to harvest the full po-tential of this paradigm in a seamless way.

Various research efforts–such as the Internet Indi-rection Infrastructure (i3) [19] and the Host IdentityProtocol (HIP) [2]–advocate indirection as the solu-tion to the problems that point-to-point communicationposes to mobility, multicast and multihoming. i3 imple-ments an IP overlay network that replaces the point-to-point communication model with a rendezvous-basedparadigm where senders send packets to a specificrendezvous-point while receivers issue triggers on spe-cific packet identifiers. HIP introduces a new layer in the

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internetwork stack between the IP layer and the trans-port layer. This new layer decouples host identity fromlocation identity. PSIRP uses similar concepts throughthe rendezvous and topology formation processes.

The problem of routing based on flat informationidentifiers rather than on hierarchical location-basedidentifiers has also been studied in the Data-OrientedNetwork Architecture (DONA) [15] and in the Routingon Flat Labels (ROFL) [4] projects. DONA proposes anew identification scheme based on flat, self-certifyingidentifications as a replacement to the DNS namingresolution scheme that enables ‘finding’ and ‘fetching’content. ROFL investigates the possibility of having aninternetworking architecture solely based on flat identi-fiers, using DHTs and hierarchical DHTs. The evalua-tion results of ROFL show that this approach is feasibleand it can incorporate all the internetworking struc-tures that exist in the current Internet. PSIRP bor-rows the information identification concept of DONA,but chooses a separate inter-domain architecture withslow and fast paths. Moreover PSIRP extends ROLFtowards flat identifiers within hierarchical scopes, whichare expected to offer faster information scoping and dis-semination.

PSIRP is not the only current research project aim-ing at redesigning the Internet with an information-centric or content-centric perspective. CCNx [7] is aresearch effort that proposes routing based on hierar-chical naming. In CCNx consumers ask for content bybroadcasting ‘Interest’ packets that contain the name ofthe content in request. Any ‘Data’ packet whose contentname is a suffix of the name in the ‘Interest’ packet isconspired that it satisfies this interest. PSIRP, on theother hand, introduces flat label identifiers organizedinto scopes, allowing for a variety of naming approachesto be layered on top of the internetworking architecture.Moreover although overlaying PSIRP over the currentInternet is possible, it is the declared goal of the PSIRPproject to investigate a native solution that will replacecurrent internetworking technology. This leads to a fo-cus on inter-domain functions, which is not found inCCNx. 4WARD [1], another FP7 EU funded ongoingresearch project, also advocates an information-centricInternet which will enable network diversity, allowingvarious types of networks to co-exist and cooperate ina smooth and cost-efficient manner. It envisions an In-ternet where networks will be self-manageable and net-work paths will be an active networking componentthat it will be able to affect transport services. 4WARDborrows concepts from DONA in terms of labeling andintends to shed light on business aspects, similarly tothe socio-economic work in PSIRP.

Other ongoing research investigates PSIRP perfor-mance and effectiveness regarding forwarding, caching,and mobility. PSIRP’s forwarding is based on the for-mation of a bloom filter based data structure–calledzFilter–that includes the identifiers of the links thata packet needs to traverse in order to reach its des-tinations. Jokela et al. implemented zFilter-based for-warding in NetFPGAs using temporary link identifiersachieving secure forwarding at line speed [9]. Kat-saros et al. investigated content delivery in a PSIRP-like environment, where multicast is the primary de-livery method and pub/sub based caches are used inedge routers; they showed that the approach has the po-tential to achieve substantial reduction in interdomaintraffic and download time [13]. Finally it was demon-strated that even an overlay environment (which suffersfrom the inefficiency of stretch) abiding to PSIRP prin-ciples, i.e, pub/sub and multicast can be more effectivethan mobile IPv6 [11], [12].

3 A Publish-Subscribe Inter-DomainArchitecture

The PSIRP architecture [18] is based on the premise ofinterconnecting information rather than endpoints, i.e.,endpoint topologies are dynamically created based onthe expressed availability (publication) and need (sub-scription) of information. With this, we envision a ro-bust and scalable architecture where mobility will bethe norm and data morphing will allow users to ac-cess information anywhere through any medium. Per-formance and efficiency will be achieved with the useof innovative multicasting and caching techniques, andsecurity will be a native component of the architecture.Specific consideration is given to the ability to placefunctions in trustworthy points within our architecture,taking into account the Trust-to-Trust principle [3].

3.1 Information Concepts

Information is the core element in the PSIRP archi-tecture; everything is information and information iseverything [20]. Information is organized in a hierar-chical way, so small ‘meaningless’ pieces of data, whichcan be arbitrary chunks of data, are combined into largecomplex information items–such as files, documents pic-tures and videos. An information item may be used as areference to other items, providing information such asdata size, information owner, permissions, compositionelements. These items are referred to as the metadataand they can be used to group information based onsome specific semantics. Scoping mechanisms are used

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to limit the reachability of the information to the par-ties having access to that particular scope [10]. Withina scope the architecture is neutral with regard to thesemantics and structure of the data, although gover-nance rules regarding the available information may bedefined. Scopes can be regarded as the equivalent of IPtopologies, i.e., as IP topology mechanisms allow thecreation of a topological inter-network, scoping mecha-nisms allow for building information networks. Scopeshave a hierarchical structure where parent-children andsibling relationships exist. In PSIRP, there can existphysical scopes, e.g., a University of X network, andlogical scopes, e.g., a social network (hierarchy). Everypiece of information is attached at least to one specificscope, which is represented by the scope identifier thatpublishers set when they publish information. Severalmechanisms are used to control the scope of a piece ofinformation. These mechanisms include access control,DRM, user authentication, and many others. Informa-tion items may be part of multiple scopes. For example,an information item (such as an image) may belong toa University and at the same time it may belong to aspecific family scope.

As for identification, every piece of information isidentified with a (statistically) unique label. This labelis used in order for subscriber interests to be matchedwith published information. The function of match-ing subscribers interests with published information isknown as the rendezvous function and for this rea-son this label is referred to as the rendezvous identi-fier (RId). A subclass of rendezvous identifiers is thescope identifier (SId). SId denotes the specific scopewithin which the information is reachable. Rids andSids are independent from the endpoints producing andconsuming the associated information items. Flat andendpoint independent labels seem to be a natural choicefor information oriented architectures as they clearlyseparate location from identity allowing for properlyincorporating mobility, multicasting, and multihominginto the architecture. as well as a more comprehensivenotion of identity [4]. The PSIRP architecture includespublishers, subscribers and rendezvous points. Publish-ers are information providers that feed information el-ements into the pub/sub network by virtue of publica-tions. Subscribers are consumers that explicitly expresstheir interest in a specific publication by issuing sub-scription messages. These messages contain the criteriathat a publication should fulfill in order to be forwardedto a subscriber. Publications may have different ver-sions, and whenever a new version of a publication iscreated, all subscribers are being informed.

3.2 Bubbles: A Layer Concept for anInformation-Centric World

As outlined above, information in the PSIRP architec-ture is organized using scopes with an identificationstructure that allows for hierarchically organizing infor-mation. In the following, we outline the so-called bubbleconcept which is akin to the layering model in our cur-rent Internet model, i.e., it defines the concepts andmethods which are used to provide information itemswithin a set of scopes within particular usage scenarios.In other words, bubbles complement the scope conceptwith a notion of provisioning information within a par-ticular scope.

Each bubble implements scope-specific Rendezvous,Topology and Forwarding (RTF) functions to enablethe provisioning of the information within the scope.The Rendezvous function is responsible for match-ing subscribers’ interests with publications. The nodewhere the matching of a publisher’s content with a sub-scriber’s interest takes place is referred to as the ren-dezvous point (RP). These elements initiate routing,forwarding, and distribution decisions, eventually lead-ing to the delivery of the content from publishers tosubscribers. Hence, RPs ensure a balance of power be-tween sender and receiver of information, i.e., no infor-mation is delivered without explicit signaling of avail-ability (publish) and interest (subscribe). Publicationand subscription operations are decoupled in time andspace as they do not have to be synchronized; they donot block each other and publishers do not have to beaware of the subscribers–and vice versa.

Whenever publishers wish to issue a new publicationthey have to use two identifiers: RId and SId. A pub-lication’s RId can be derived by an application specificfunction. A publication’s SId should denote to whichextent the publisher wishes the publication to be avail-able. Prior to publishing an information element, pub-lishers have to locate the nodes that are responsible formanaging the desired scope. One of these nodes will bethe RP for the publication. The nodes that are respon-sible for managing a scope and are eligible for becominga RP are referred to as the rendezvous nodes. What isactually published to the RP is the publication’s meta-data, which contain information specific to the actualpublication; this can be for instance the author of thepublication, its size and perhaps a small description ofit. In order for a subscriber to access a publication shemust be aware of its RId and SId. She expresses herinterest about a specific publication by issuing a sub-scription message towards the publication’s RP, iden-tified by the SId. The RP is responsible for matchingpublications with subscriptions and for initializing the

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forwarding of a publication from the publisher towardsthe subscriber.

The topology function monitors the network topol-ogy and detects changes using various techniques, de-pending on the bubble it is implemented in. Moreover,the topology function is responsible for creating infor-mation delivery paths at different levels of the inter-domain system.

Finally, the forwarding function implements infor-mation forwarding throughout the delivery paths us-ing MPLS-like label-based forwarding mechanisms. Theforwarding is implemented through assigning a stack ofpath specific identifiers to the publication, named for-warding identifiers (FId). These FIds are used by inter-mediary forwarding nodes to forward the publicationto the desired destination. Publications may be cachedalong the path. In case that more than one subscribersubscribe to a specific RId, a multicast tree is createdin order to deliver the publication.

The particular implementation fro RTF functionsdepends on the specific context in which the bubblehas been created. For instance, methods applying lo-cality could be utilized for the Access Point (AP) bub-ble, having simple forms of rendezvous, largely drivenby the local attachment and by virtue of the local linkdiscovery (i.e., literally the L2 discovery of the chan-nel). The topology function running on the OperatingSystem (OS) bubble is responsible for maintaining con-nectivity and for predefining (forwarding) labels thatwill be used by the forwarding function in order to for-ward information items through the various interfaces.Larger bubbles, such as the global one, need to solvemore complex ‘matching,’ but also topology problems,which leads to more complex solutions in these areas.

As for the organization of bubbles, they can be in-cluded in each other or can just touch each other (im-plementing a sequence of information traversals). Infor-mation within each bubble traverses through the bub-ble from points on its membrane–the traversal imple-mented through the proper RTF functions. The pointson the membrane constitute publishers and/or sub-scribers of information within the enclosed but also theenclosing or touching bubble.

The bubble concept bears similarities with the Re-cursive Network Architecture (RNA) [21], as well aswith Netlets [22]. RNA uses a single, tunable protocolfor different layers of the protocol stack, reusing ba-sic protocol operations across different protocol layerswhilst Netlets, encapsulate protocol stacks, with theprinciple of hiding protocol details but yet providinga number of properties via its interfaces. Our modelof bubbles differs from the model of recursive layeringbecause a particular layering is not assumed. Bubbles

Fig. 1 An instance of the bubble model

describe the conveyance of information within a partic-ular region in which this bubble makes sense. Informa-tion at the edge of the bubble can be conveyed further(within another bubble) at the same ‘layer’ but usingdifferent methods for RTF. In other words, bubbles caninclude other bubbles (akin to recursive layering), butalso touch each other, i.e, similar layers with differentfunctionality for RTF. Moreover, in contrast to Netlets,PSIRP bubbles predefine the functionality that all bub-bles shall offer, i.e, the RTF functions.

Figure 1 shows a particular instance of the bubblemodel, demonstrating the inclusive and touching natureof bubbles. An information item from the shonw OS inLAN1 can traverse the following bubbles in order tobe delivered/accessed by a user in the Provider 3 bub-ble: OS-AP-Router-(LAN1)-LAN2-LAN3-(Provider1)-Provider2-Provider3. Additional steps may be requiredwithin Provider2–and at different points–but they areabstracted out here. In particular, points where inter-nal bubbles touch external bubbles at the same pointas that of a bubble containing them, the traversal tothe exterior bubble need not to be explicit. This case isshown with the parenthesis for LAN1 and Provider1 inthis example.

3.3 Mobility

User mobility is regarded as a two-dimensional problem.The first dimension of the problem concerns the scale of

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Fig. 2 Static/Local mobility handled through isolation

the mobility which can be local or global. The second di-mension of the problem reflects how the mobility is han-dled by the architecture. Mobility can be handled eitherin a static or in a dynamic way. When static mobility isinvolved, bubbles simply ‘fly around’ within the perime-ter of their containing bubble detaching and attachingto other bubbles. In the case of dynamic mobility, tem-porary bubbles are created that allow for informationtransition between different environments–which can bedifferent wireless technologies or even different adminis-trative domains. Table 1 gives an overview of mobilitycategorization in PSIRP. A user moving around withhis laptop inside a campus covered by a (multi-AP)WLAN is an example of static-local mobility, whereasa Vehicular Network which involves sensing from vari-ous sensors deployed along the road directly or receiv-ing the same information indirectly through commu-nication with other cars is a typical example of dy-namic/local mobility. On the other hand handover ofa roaming, user to another provider, i.e., a new admin-istrative domain, similar to today’s roaming is a static-global case of mobility, while vertical handover withoutroaming is a case of dynamic-global mobility.1

Figure 2 demonstrates a static/local mobility sce-nario. A mobile terminal (MT) is moving around withinthe premises of LAN1. Its initial AP is AP1, and itmoves to AP3 through AP2. This MT is receiving an in-formation flow from Provider 3 through Provider 2 and

1 In this case the user is a client of both providers separately

and no roaming is initiated

Local Global

Static Handover in managed

WLAN environment

Nets between cars

Dynamic Handover with roam-ing

Handover withoutroaming

Table 1 Mobility categorization in PSIRP through examples;static refers to no change in bubble; dynamic refers to the case

where a new bubble is created

LANs 3 and 2 of Provider 1. When the MT changesAP the information delivery tree is simply re-built bythe Router bubble of LAN1, which forwards informationto the appropriate AP. In order to achieve informationdelivery tree reconstruction, the MT needs to issue anew subscription message from the new location. Thissubscription message will ultimately trigger the Topol-ogy and Forwarding functions of the Router bubble aswell as that of the new AP bubble, leading to informa-tion flow redirection. Outside LAN1’s bubble the mo-bility of the MT is transparent as it is ‘isolated’ insideLAN1’s bubble, causing no change to any other bubblein the network. A dynamic-global mobility scenario isdepicted in Figure 3. This scenario involves a MT thathas a 3G and a WLAN interface. Initially the MT re-ceives an information flow from Provider 3 through its3G interface. It then detects an available WLAN and itdecides to perform a vertical handover. Although thisscenario involves little–or no–physical MT movement, itmay cause global information-flow shift, as the 3G op-erator and WLAN provider can be different. The newlocation of the MT needs to be informed about the up-coming arrival and state needs to be transfered fromthe CELL2 bubble of Provider V to the LAN1 bubbleof Provider 1. In order for this state transfer to occurrethe MT needs to inform CELL2 about its movement.CELL2 in return creates a dynamic bubble between itsbubble and the AP1 bubble. This ‘dynamically createdbubble’ enables state transfer from the 3G bubble to theAP to which the MT is going to be attached. The bub-ble of the AP in return is going to perform all the neces-sary actions in order to redirect the MT’s information-flow to the new location and when it is ready, it willinform the CELL2, using the dynamic bubble. The in-formation flow redirection requires the activation of theRTF functions of all bubbles between the AP1 bubbleand the Provider1 bubble as new subscription messagesneed to be sent from the new location. The dynami-cally created bubble not only allows two different me-dia to communicate, but it enables information transferbetween two differently managed environments, whichinvolves trust relationships and business agreements.PSIRP security mechanisms will handle the security is-sues that will be possibly raised by the bubble creation.

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Fig. 3 Dynamic/Global mobility, involving vertical handover, handled through temporary bubble creation

3.4 Security

The Trust-to-Trust principle [3] is one of the core princi-ples of PSIRP. Rendezvous, topology management andforwarding functions take place in trustworthy points.The pub/sub nature of PSIRP, its decentralized natureof address space management, as well as its rendezvous-driven operation offers some significant security advan-tages. The network balance is shifted towards the in-formation receiver, relieving the information providerfrom the burden of constant information requests. Thisbalance shift is expected to attenuate DDoS attacks,that plague the current Internet as well as to constrainunwanted traffic such as spam since no information2

is delivered without explicit signaling of availability(publish) and interest (subscribe). Moreover some levelanonymity is inherited from the pub/sub paradigm asinteracting parties do not need to know each other oreven come to direct contact. The publication and sub-scription operations are also decoupled in location andtime, since they do not have to be synchronized, and,thus receiver and sender unlinkability is also achieved.Furthermore, by using multicast as the preferred de-livery method, it is expected to have better resourceutilization and information availability.

PSIRP considers security even at the packet level.Packet Level Authentication (PLA) [5] is a novel tech-nique, for protecting the networking architecture. PLAis based on the assumption that per packet public keycryptographic operations are possible at wire speedin high speed networks due to new cryptographic al-gorithms and advances in semiconductor technology.Moreover PSIRP’s forwarding mechanism is based ondynamically generated forwarding identifiers [9] mak-ing it almost impossible for an attacker to forward ma-

2 except to RPs, but those are few, key points of the architec-

ture that can be adequately protected

licious data packets. When it comes to the networklevel, attachment procedure in PSIRP [14] assures theproper user authentication, protecting both users fromimproper configuration, as well as network from DoSattacks. Finally at the application level, it has beenfound that, although networking protocols will be re-disgned, current cryptographic protocol analysis can beapplied to a certain extent, with only minor modifica-tions mostly in notation [16].

4 PSIRP usage scenario

After we outlined the general architecture in the previ-ous section, we now illustrate the workings of the archi-tecture in some typical usage scenarios. With this, weattempt to further clarify the relations and operationswithin the architecture, but also shed some light ontothe development of typical applications.

4.1 Scenario setup

A user, USERA, works in a university UNIA, in the de-partment DEPTA. USERA has prepared a presentationand he wants to make it available to his colleagues inhis department. USERA has three colleagues; USERB,USERC and USERD. USERB and USERC want to ac-cess USERA’s presentation through UNIA’s local net-work and USERD wants to access it from his homenetwork, NETA. Figure 4 depicts this scenario setup.

4.2 Publication

The UNIA network consists of four rendezvous nodesRN001, RN002, RN003 and RN004. DEPTA has itsown scope with SId 00A1. Scope 00A1 is managed by

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Fig. 4 A PSIRP usage scenario setup

RN003 and RN004, and every potential publicationto scope 00A1 will be forwarded to either RN003 orRN004. Scope 00A1 implements the following accesspolicy: Only members of DEPTA are allowed to pub-lish information and to subscribe for publications inthat specific scope. USERA wants his presentation tobe accessed only by members of DEPTA, so he decidesto publish it in scope 00A1. A publication is created andan application specific function generates an RId iden-tification number for this publication, which is AA12.USERA’s application running on SERVER01–which islocated inside the UNIA network–issues a publish mes-sage with SId 00A1 and RId AA12, and this messageis forwarded to RN003. The publish message contains,along with the identifiers, USERA’s presentation meta-data. The publication operation ends with RN003 up-dating its internal RIds database by adding AA12 andbecoming the rendezvous point for this RId. The publi-cation operation is depicted in Figure 5. The successfulcompletion of the publication operation presumes thatUSERA has properly authenticated himself to the scope00A1.

4.3 Subscription from the Internal Network

USERB learns about USERA’s publication throughsome form of discovery, e.g., an internal listing of pre-sentation or some (new form) of search engine. Inorder for USERB to subscribe to USERA’s publica-tion, he has to authenticate himself to scope 00A1.He achieves that by logging in to SERVER02 withcredentials that allow him to subscribe for publica-tions in scope 00A1. USERB’s application running onSERVER02 issues a subscribe message towards scope00A1, denoting that he is interested in the publica-tion with RId AA12. RN003 receives USERB’s sub-

Fig. 5 PSIRP’s publication operation

scription message and it initiates the creation of a for-warding path between publication’s AA12 location, i.e.,SERVER01, and SERVER02.

In scope 00A1, a publication with Rid F20E exists,that contains the RIds of every presentation availablein this scope. This publication is provided by a pre-sentation announcement service. USERC is interestedin every presentation in scope 00A1 so she has sub-scribed for publication F20E. USERC uses SERVER03with credentials that allow her to subscribe for publica-tions in scope 00A1. When USERA publishes his pre-

Fig. 6 PSIRP’s subscription operation. The numbers inside

brackets denote originator and the sequence of each message,e.g.,[1] is the first message that USERD sends.

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Fig. 7 PSIRP’s subscription operation from an external network. The numbers inside brackets denote the sequence of message.

sentation, the presentation announcement service cre-ates a new version of F20E, which contains USERA’spublication RId. USERC receives the new version ofF20E and she becomes aware of USERA’s publicationso, she subscribes to it. At this point a procedure sim-ilar to the one described for USERB’s subscription isfollowed in order for the publication to be forwardedto SERVER03. Figure 6. depicts PSIRP’s subscriptionoperation.

4.4 Subscription from an External Network

USERD wants to access USERA’s presentation from hishome network NETA. He creates a subscription mes-sage with destination SId being 00A1 and he encryptshis authentication data in the message payload. Thesubscription message is forwarded to NETA’s defaultRP which is 000A. 000A forwards the subscription mes-sage to RN001. RN001 determines that the destinationscope is 00A1 and it forwards the subscription messagetowards this scope. RN003, which is the RP for thisspecific RId, finally receives the subscription message.However, instead of creating a forwarding path towardsUSERD it initially checks for the validity of USERD’scredentials, sent encrypted along with the subscriptionmessage. If USERD credentials are valid, a forwardingpath from SERVER03 to USERD is created and thedesired publication is forwarded using this path. Theoverall operation is depicted in Figure 7.

4.5 Forwarding

In PSIRP forwarding solely depends on informationidentifiers, i.e. RIds and SIds, thus MPLS-like label

switching protocols are used. In order for an informa-tion item to be forwarded a stack of forwarding iden-tifiers is determined for the links along the forward-ing path. Each forwarding node along the path main-tains a forwarding table which contains the incomingforwarding identifier and its corresponding outgoing in-terface and identifier. If we consider again the subscrip-tion from the internal network case, upon the successfulcompletion of the subscription operation, a forwardingpath is created from SERVER01 towards SERVER02and SERVER03. The forwarding path can be seen inFigure 8. The box next to the dashed lines represents adata packet with its FId. The table below each forward-ing node shows its forwarding table. The forwardingtable contains the incoming Fid, the outgoing FId andthe outgoing interface. SERVER01 sends a packet withFId 12. The first forwarding node checks its forward-ing table and finds that it has to forward this packetto interface 2 with FId 14. In a similar way the secondforwarding node duplicates and forwards the packet toits interface 2 with FId 19 and to its interface 3 withFId 20. This way the publication will reach USERB.

4.6 RTF function execution within bubbles

During the whole operation, sequences of RTF func-tions are executed in a recursive way. In order to demon-strate this, we slightly modify the publication use case.We consider that the publishing application of USERAis running in bubble LAN1 of Figure 1 and the pub-lication’s RP is located in bubble Provider 3. The ap-plication needs to get to the global bubble Rendezvous(R) function and in order to do so it has to go throughsome of the relevant bubbles in-between. Initially theapplication makes the publication available to the OS

10

Fig. 8 PSIRP forwarding

R function. The OS R function determines that theOS bubble is not the destination bubble and asks theOS Topology (T) function to create a forwarding pathtowards the Provider 3 bubble. The OS T functiondetermines, e.g., using static routing tables, that thepublication message should be forwarded through thewireless interface and it informs the R function aboutthe appropriate forwarding label that should be used.The R function instructs the application to forward thepublication using the forwarding label of the wirelessinterfaces. The Forwarding (F) function makes all thenecessary steps in order for the publication to reachthe AP Bubble. Using a similar sequence of RTF exe-cutions the publication message will reach the Routerbubble. The Router bubble is responsible for advanc-ing the publication message to a higher layer bubble,thus, it has to make use of the more advanced RTFfunctions of the LAN 1 bubble. The R function of theLAN 1 bubble, identifies that the RP of the publica-tion message is located in the Provider 3 bubble andit asks the T function for a path towards that bubble.The T function, that implements an intra-domain rout-ing protocol, recognizes that the publication messageneeds to be forwarded through the LAN 3 to anothernetwork and it creates a forwarding path from LAN 1to LAN 3 through LAN 2. Edge routers in these LANsupdate their routing tables with the new path and theF functions running in these routers make all the nec-essary forwarding decisions in order for the publicationmessage to reach the LAN 3 edge router that intercon-nects the Provider 1 bubble with the Provider 2 bubble.At this point the publication message will be advancedto an even higher level bubble, so more complex RTFfunctions need to be executed and these are the RTFfunctions of the Provider’s 1 bubble. The R function

identifies the exact location of the Provider 3 RP andasks the T function, which implements an inter-domainrouting protocol, to create a forwarding path towardsthis location. The T function creates a path from theProvider 1 bubble towards Provider 3 bubble, throughthe Provider 3 bubble.

5 An application developer’s view

PSIRP’s use cases are meant to also shed some light onapplication perspectives towards this architecture. Theidentifiers used in PSIRP are not application orientedand, their structure although very useful for the overallsystem effectiveness, it may pose a burden on appli-cation developers, as e.g., they should assure that the(publication) identifiers their applications generate areunique within a scope. However, even at these earlysteps of PSIRP, an in-kernel module that generatespublication identifiers is provided. Scopes in PSIRP areused for describing the extent of dissemination of in-formation, and not the structure underneath, thereforethey are a more general notion than what could be con-sidered their first order approximation in the current In-ternet, i.e., networks. Scopes are managed by lower levelmechanisms and their operation is completely trans-parent to applications. Even more transparent will bethe operation of the forwarding function; forwardingidentifiers will be completely hidden from applications.Applications will be able to smoothly operate withoutbeing aware of user mobility, information multicastingand multihoming, or any other specific Internet accessmechanism. On the other hand, applications will be of-fered the possibility to define policies and requirementsthat will affect routing.

Another fundamental concept that may trouble de-velopers is the use of trust with every transaction. Pre-serving trust is a core principle of the PSIRP archi-tecture and all functions should take place at trustedpoints in the internetwork. PSIRP will provide to ap-plications all the means for determining if another nodein the network behaves in a trustful way, however ap-plications are also expected to implement their own,application specific, trust evaluation algorithms, sinceat the application layer there are various trust expec-tations (and high-level information), which cannot bepredicted by the lower layers of the architecture.

6 Conclusions, Ongoing and Future Work

PSIRP set out with an ambitious goal to define anew and information-centric inter-domain architecture.

11

Progress toward this goal has been made with first re-sults available and work still progressing at a good pace.

Many lessons have been learned from this effort.Some of these lessons relate to the design of such ar-chitecture. Not only did PSIRP apply a method ofcombining top-down (requirements-driven) as well asbottom-up (learning from code) approaches, but it alsoincluded socio-economic viewpoints very early in thedesign phase of the architecture. This has led to a deepunderstanding of what the core proposition of the ar-chitecture ought to be, namely its foundations in anidentifier configuration that provides structures akin tocomplex application-layer information structures whilebeing simple enough to scale to the desired size of theFuture Internet.

The PSIRP architecture effectively handles informa-tion provisioning using the concept of bubbles. Bubblesnot only enable layering, but they also provide a mecha-nism for seamlessly integrating new functionality withinthe PSIRP architecture. By defining what should be im-plemented within a bubble–and not how, in conjunctionwith a common inter-bubble communication interface,the PSIRP architecture is expected to be extensible andflexible. The bubbles concept will allow the easy intro-duction of new technologies and protocols as well as forlarge scale architectural modifications, leveraging thisway the architecture’s composability.

Apart from the obvious technological advantages itoffers, the concept of bubbles is also expected to havea significant socio-economic impact. PSIRP envisions a‘bubble-market’, where providers will be able to sell agreat variety of services ranging from access technolo-gies to elaborate information retrieval and manipula-tion applications. Moreover the bubble concept is envi-sioned to play a key role for the easy integration andconnection to the network of even more and diverse de-vices (as information appliances).

Our socio-economic work furthermore sheds light onthe various tussles that are likely to occur in such archi-tecture, explaining some of the design choices aroundtopology formation and rendezvous from the angle offuture markets in which such architecture would livein. And last but not least, we consider a solid think-ing on potential migration crucial for the success ofPSIRP. Work on overlay implementations of the archi-tecture have been part of the efforts from the start,realizing that a ‘native’ deployment of the architecturewould be nothing short of unrealistic. That gives us therequired options to gradually deploy the architecturepresented here, while still remaining ‘clean-slate’ in itsdesign through questioning the necessary fundamentalsof today’s Internet.

However, the most important lesson learned through-out the efforts of PSIRP is certainly to better formulatethe necessary and crucial questions we need to continueasking when envisioning an information-centric Inter-net.

PSIRP is an ongoing research effort. During thisphase of development, part of PSIRP’s functionality hasbeen implemented (on top of the FreeBSD operatingsystem) and it is being tested (at this point mostly inthe local area but a wide-area testbed is in existence andbeing exploited for experimentation). The implementa-tion is available under open source license terms [18]with the potential to create a growing developer com-munity for this work.

Future work includes defining an inter-domaintopology formation protocol, exploring trust related is-sues in PSIRP networks, implementing a fast forward-ing mechanism, porting PSIRP functionality to otheroperating systems and incorporating existing applica-tions to the PSIRP environment.

Acknowledgements The work reported in this paper was sup-

ported in part by the FP7 ICT project PSIRP, under contract

ICT-2007- 216173.

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fotiou.jpg Nikos Fotiou, is a Phd Candidate in Com-

puter Science at AUEB, participates in thePSIRP project and in the Euro-NF Net-

work of Excellence. He has received his Dipl.in Information and Communication Systems

Eng. from the University of the Aegean in

Samos, Greece (2005) and his M.Sc. in Inter-networking from the Royal Institute of Tech-

nology (KTH) in Stockholm, Sweden (2007).

His current research interests include pub-lish/subscribe architectures and information

oriented security mechanisms.

trossen.jpg Dirk Trossen, Senior Researcher in the Com-puter Laboratory at Cambridge University,

has more than ten years of experience in

network architectures and wireless technologywith main contributions in the area of inter-

domain networking, seamless handovers andphysical network overlays. He designed a plat-

form for participatory wireless sensing, avail-

able under open source license. Dirk is one ofthe main drivers of the EIFFEL think tank. He

is currently technical lead for the EU project

PSIRP, working on a novel internetworking ar-chitecture based on information. He also tech-

nical leads a UK-funded project on lifestyle

management. Dirk held prior positions as aChief Researcher with BT Research and as a

Principal Scientist at Nokia Research. He holds

a Ph.D. degree in Computer Science fromTechnical University of Aachen, Germany. He

published more than 50 peer-reviewed papers

in international conferences and journals andhas currently 23 international patents

polyzos.jpg George C. Polyzos, Professor of ComputerScience at AUEB since 1999, is leading the Mo-

bile Multimedia Laboratory and is currently

the Chair of the Division of Computer andCommunications Systems. Previously, he was

Professor of Computer Science and Engineer-

ing at the University of California, San Diego,where he was co-director of the Computer

Systems Laboratory, member of the Steering

Committee of the UCSD Center for WirelessCommunications, and Senior Fellow of the San

Diego Supercomputer Center. He has received

his Dipl. in EE from the National TechnicalUniversity in Athens, Greece (1982) and his

M.A.Sc. in EE (1985) and Ph.D. in ComputerScience (1989) from the University of Toronto.

His current research interests include Internet

architecture and protocols, mobile multimediacommunications, wireless networks, ubiquitous

computing, network security, and performanceanalysis of computer and communications sys-tems. He is an organizer of the EIFFEL Think

Tank, on the Steering Board of the Euro-NF

Network of Excellence and participates in thePSIRP project. Prof. Polyzos is on the edito-

rial board of the journal Wireless Communi-cations and Mobile Computing and has beena guest editor for IEEE Personal Communica-

tions, ACM/Kluwer Mobile Networks and Ap-plications, IEEE Journal on Selected Areas in

Communications, and Computer Networks. He

has been on the Program Committees of manyconferences and workshops and a reviewer for

many scientific journals and research funding

agencies.