Int. J. Electron. Commun. (AEÜ) 60 (2006) 25–29

www.elsevier.de/aeue

Structured peer-to-peer systems for telecommunications and mobileenvironments

Wolfgang Kellerera,∗, Gerald Kunzmannb, Rüdiger Schollmeierb, Stefan Zölsb

aDoCoMo Communications Laboratories Europe GmbH, Landsberger Str. 312, 80687 Munich, GermanybTechnische Universität München, Institute of Communication Networks, Arcistr. 21, 80333 Munich, Germany

Dedicated to Professor Jörg Eberspächer on the occasion of his 60th birthday

Abstract

The peer-to-peer (P2P) paradigm not only allows end users to share own resources, but is also regarded as a new networkingparadigm due to its robust, self-organizing character, abandoning infrastructure and relying on end-users equipment. In contrastto common fixed-line Internet file sharing P2P systems, in this paper, we address critical issues regarding scalable personalcommunications and heterogeneous mobile systems. We discuss challenges and present our solutions based on structured P2P.� 2005 Elsevier GmbH. All rights reserved.

Keywords: Peer-to-peer; DHT; Self-organizing networks

1. Introduction

Peer-to-peer (P2P) networking started with Napster in1999, shortly followed by Gnutella, causing a highly in-creased traffic in the Internet. In this new networkingparadigm participants establish overlay networks in a self-organizing way by contributing their own resources, e.g.,processing power and storage capacity. P2P’s primary goalis to share content directly between users. Every peer in aP2P network operates as a router (to route requests to a peerproviding the desired content), as a server (to serve match-ing content requests), and as a client (to initiate requests forcontent) in the overlay network at the same time.

Within the last few years, P2P networking has been usedfor an increasing number of applications, primarily for con-tent distribution but also for applications like media stream-ing, distributed collaboration, personal communication suchas Voice over IP (VoIP) [1], or Grid Computing [2]. As a

∗ Corresponding author. Tel.: +49 89 56824 222; fax: +49 89 56824 300.E-mail address: kellerer@docomolab-euro.com (W. Kellerer).

1434-8411/$ - see front matter � 2005 Elsevier GmbH. All rights reserved.doi:10.1016/j.aeue.2005.10.005

result, today P2P traffic in the Internet exceeds even WWW’sHTTP-traffic which was the dominating source for a longtime.

P2P concepts are not only interesting to individual userssharing files, but also for operators and service providers.Operators are interested in leveraging their infrastructure us-ing self-organizing concepts to save on infrastructure costand enable new user-driven innovative applications. How-ever, in carrier-grade, i.e., large-scale highly available over-lay networks like VoIP, or resource-constraint environmentssuch as mobile networks, the deployed P2P concepts do notscale in terms of signaling overhead as shown in the follow-ing sections.

In this paper, we address the issue of P2P’s applicabilityfor carrier grade networks, along with the most challengingareas of personal communication (VoIP) and heterogeneouscommunication infrastructure. We present our advancedsolutions based on the Chord [3] concept belonging to theclass of structured P2P systems, to be able to develop moreefficient P2P systems in these areas. Section 2 describesthe requirements and challenges of P2P in personal and

26 W. Kellerer et al. / Int. J. Electron. Commun. (AEÜ) 60 (2006) 25–29

mobile communications. In Section 3, we give an overviewover Chord [3]. In Sections 4 and 5 we describe our exten-sions to provide scalable VoIP services and P2P in mobileenvironments, respectively. Section 6 concludes.

2. Requirements and challenges

The basic requirement for P2P networks is to allow peersto find a shared resource hosted by one of the participat-ing peers in the established overlay network. P2P overlaynetworks are only based on the equipment of participatingusers without additional infrastructure. Thus, they have tobe robust against peer failures and users leaving and joiningthe network.

Currently deployed P2P solutions, such as Gnutella, useflooding of requests among all participating peers. To over-come this inefficient overhead caused by flooding, conceptsrecently emerged that establish knowledge about the struc-ture of distributed resources before execution of search.Moreover, these structured P2P protocols, such as Chord canguarantee the resolution of a query, whether the requestedresource is available or not.

Expanding the application area of P2P from searchfor content to applications where the main focus is self-organization in order to achieve infrastructure cost saving,new issues must be managed. P2P concepts also have to dealwith finding unique resources that are usually not replicatedbetween users, e.g., ‘telephone numbers’ in VoIP systems.Efficient operation to comply with the user expectations forsimilar services, i.e., traditional telephony, requires suchlookups to be performed in extremely short time among alarge customer base.

Scalability of P2P in mobile networks poses additionalchallenges. Mobile networks, using wireless connections,are usually resource constrained systems limited by low datarates, low processing power and storage capacity of mobileterminals. Furthermore, they are characterized by frequentjoining and leaving of peers. High churn rates result frompeer failures, e.g., when moving out of coverage, dischargedbatteries, or from short session times based on the user be-havior, e.g., due to high online costs. When expanding theP2P paradigm to mobile networks we encounter highly het-erogeneous environments rather than desktop computers infixed Internet P2P networks. This heterogeneity is not yettaken into account by existing structured P2P concepts. Inthe following, we focus on this heterogeneity in terms ofdevice and network capabilities and user behavior. Hence,our target environment includes powerful and limited de-vices and connections. Pure ad hoc wireless networks arenot in the focus of this work. Here, unstructured P2P con-cepts might provide good solutions as shown in [4].

From our point of view, structured P2P such as inChord [3] is a promising approach to overcome the de-scribed challenges of P2P in an operator-grade environ-ment. In fact, we have selected Chord representing a simple

one-dimensional identifier space-based P2P concept as thebasis for our work. In the following sections, we introduceextensions to the conventional Chord protocol, in order toadapt this structured P2P protocol to the special require-ments mentioned above. In principle, these advancementscan also be applied to other state of the art one-dimensionalstructured P2P systems such as Pastry, Tapestry or Kademlia.

3. Structured P2P systems

Structured P2P networks use hash functions to map peers,as well as references to shared content, onto a commonID space. This is why they are often also referred to asdistributed hash tables (DHT). In contrast to unstructuredP2P networks, where a routing path is generated duringthe lookup, structured P2P networks use preexisting infor-mation stored in routing tables. Therefore, the completerouting path is determined by the protocol and the routingentries. Hence, users are able to find even infrequent itemsstored in the network (= scope) within a scaling numberof hops (= efficiency). DHTs also scale well with an in-creasing number of peers; however, current DHT-basedsystems hardly scale with increasing churn rates. As peersfrequently join and leave the overlay network, signalingtraffic necessary to keep the overlay structure up-to-dateincreases. Therefore, we propose using hybrid approachesin highly dynamic (e.g., mobile) scenarios (see Section 5).

An example of a DHT-based system is the Chord proto-col [3] that uses a one-dimensional m-bit hash function andarranges the participating peers in a ring structure. Each peeris responsible for a portion of the ID space between its ownID and the ID of its predecessor on the ring, or in otherwords, shared content is assigned to that peer with the IDclosest following the content ID in clockwise direction. Tobe able to route queries each peer has to know at least itssuccessor on the ring. Lookups can then be routed from suc-cessor to successor until they arrive at the peer that is respon-sible for the queried ID. As such a simple routing schemedoes not scale, additional routing entries called fingers arestored at each peer. A peer n’s fingers can be determinedusing the following equation:

ith finger = successor(n.id + 2i ) with i ∈ [0..m − 1]The logarithmical structure of the finger table guarantees

that with each hop the distance to the queried ID can atleast be halved. Therefore, lookups can be resolved withinO(log2 N) hops, with N as the total number of peers in theoverlay network.

Each peer p periodically asks its successor s about thepredecessor of s. The usual case would be that the answeris p, but whenever a new peer n has joined the networkbetween the two peers, peer s will return this new peer n.Peer p then changes its successor entry to peer n and allrouting entries are correct again. If a peer wants to leavethe network it informs its predecessor and successor, so that

W. Kellerer et al. / Int. J. Electron. Commun. (AEÜ) 60 (2006) 25–29 27

they can update their routing entries. Moreover, peer n mustassume that its successor s has failed if peer s does not replyto stabilization requests any more. For this reason, each peermaintains a list of multiple successors. If a peer’s directsuccessor fails, it can try to contact the next successor in thelist, avoiding that the ring structure breaks apart. Again, thestabilization algorithm updates all routing entries afterwards.

4. Chord for voice over IP

Most current VoIP solutions (e.g., based on SIP [5]) stillrely on network-based servers to provide basic services likeestablishing calls or billing and accounting. P2P conceptsallow for infrastructure cost saving and eliminate servers assingle-point of failure resulting in Voice over P2P (VoP2P)applications such as Skype [1].

From our point of view, structured P2P protocols couldincrease the efficiency of such a system. In this section, weevaluate the applicability of the Chord protocol for signal-ing purposes in VoP2P systems. Establishing a call can bedivided into three steps:

1. Looking up the person in the white pages (optional).2. Finding out the person’s current IP address.3. Calling the person.

The connection itself (step three) is established directly be-tween the two peers, whereas step one and two requirelookups in centralized or decentralized databases. To providea certain Quality of Service (QoS) these lookups must be an-swered fast and reliably. To be able to perform fast lookupsin structured P2P networks it is essential that the networkstructure is stable and that there occur as little timeouts aspossible during queries. This is why we present a symmetri-cal variant of the Chord protocol that performs faster lookupsand increases the stability of Chord’s ring structure. Eachpeer in our symmetrical variant stores a list of n neighborsin both directions on the ring. In addition to the Chord fin-gers, all peers store a list of other peers that point to it. Wecall these fingers ‘freebie fingers’ [6], as there is no addi-tional traffic necessary to learn about them because they docontact the peer anyway. Both freebie fingers and standardchord fingers are arranged in a way that most fingers pointto close nodes and only few fingers point to nodes fartheraway. Thus lookups can be routed in both directions on thering. The next hop is always given by the node that is closestto the queried ID. Simulations prove that the average lookuppath length is reduced by 25% by using freebie fingers.

As we use symmetrical structures we also recommenddefining a symmetric replication group. Due to the stabiliza-tion scheme, peers have a more accurate knowledge aboutthe status of their direct neighbors than about neighbors far-ther away. If we define replication groups that consist ofclose neighbors instead of only neighbors in clockwise di-rection we can improve operations like replicating content atthese peers. Content in a VoP2P application is information

0%

20%

40%

60%

80%

0 20 40 60 80 100 120 140

Average Online Time [min]

Per

cent

age

of S

ucce

ssor

Err

ors

Standard Chord

Token Chord

Fig. 1. Simulations prove that the number of errors in the succes-sor lists is noticeably smaller when our token-based stabilizationmechanism is used.

about users, e.g., current IP address, nickname, full name,postal address, or services a user has registered with. Forindexing and discovering content we use techniques that aresimilar to [7].

As mentioned above, the stability of the DHT structure isvery important as sequentially contacting failed peers resultsin timeouts and therefore lookups are delayed significantly.For this reason we improve the basic Chord stabilizationscheme. We propose a token-like stabilization procedure,where two or more tokens circulate counter-directional onthe ring [8]. Each token contains information about thelast h peers it has passed before. Receiving a token fromtheir direct neighbor, peers acknowledge the token and usethe information included in the token to update their listsof neighbors. Routing these tokens requires symmetricalneighbor lists, because if a neighbor has failed and a tokenis not acknowledged the peer can try forwarding the tokento the next entry in its list of neighbors. To provide an evenfaster recognition of failures, keep-alive messages are ex-changed between direct neighbors. If a peer detects a failedneighbor it deletes this peer from its list of neighbors andsends a ‘Notify Token’ in both directions. Notify Tokensequal normal tokens but are discarded after h hops.

The advantages of using token stabilize compared to stan-dard Chord stabilization are shown in Fig. 1. Especially forhigh churn rates the Chord stabilization scheme is no longerable to correct its successor lists fast enough. Both tokenand Chord stabilization period are set to 30 s in these simu-lations. Chord stabilization generates a constant data rate, asstabilize is called periodically. Tokens are sent periodicallyand when changes in the neighbor list are detected. There-fore, its data rate increases with higher churn rates, but it isless than twice the stabilize data rate of basic Chord for allsimulated values.

5. Chord for heterogeneous environments

In heterogeneous environments, where also resource-constraint devices such as PDAs or mobile phones take

28 W. Kellerer et al. / Int. J. Electron. Commun. (AEÜ) 60 (2006) 25–29

part in the overlay, the underlying P2P system has to dealwith several challenges. The overlay protocol must beable to handle the limited resources of mobile devices aswell as high churn rates and increased failure probabilitiesof peers. From our point of view, structured DHT-basedP2P networks are a promising approach to meet these re-quirements. In contrast to unstructured P2P networks, theycan completely avoid the flooding of messages throughthe network, as queries can be routed directly to the re-sponsible peer. Therefore, signaling traffic can be reducedsignificantly.

However, when a new peer joins the structured overlaynetwork it has to receive all object references it is respon-sible for (according to the new peer’s ID) from its succes-sor. Analogous when a peer leaves, its object referenceshave to be shifted to its successor, in order to maintainthe hash key mapping rules. In environments with highchurn rates this leads to a high amount of maintenancetraffic for the involved peers, as object references are fre-quently shifted through the overlay network. Moreover, ifa peer fails, all object references stored on this peer arelost. In overlay networks with high failure probabilitiesthis may lead to the (temporary) unavailability of sharedcontent.

In order to overcome these disadvantages of structuredDHT-based P2P networks in scenarios with mobile partici-pants, we suggest adapting the different tasks in a structuredDHT-based P2P overlay network to the different capacitiesof the participating peers. As a result, resource-constraintmobile peers are intended to perform only simple, non-critical tasks, while more important tasks are performed bymore reliable peers, e.g., UMTS/WLAN equipped laptopsor hard-wired office computers.

A possible realization of such an adaptation is the HybridChord Protocol (HCP) [9]. HCP subdivides the participat-ing peers into two groups and assigns different tasks to bothgroups. On the one hand, we define so-called static peers.These peers are highly available and have a high data rateconnection as well as large CPU and storage capacities. Analgorithm periodically checks whether a peer fulfills the cri-teria of becoming a static peer. On the other hand, we clas-sify temporary peers. These are resource-constraint peersthat join the overlay network usually only for a short periodof time. Typical examples for temporary peers are PDAs andmobile phones.

In an HCP overlay network, the routing tasks are per-formed by all peers in the network, while the more criticaltask of storing and providing references to shared objects isprocessed only by the static peers. Thus, we can achieve anincreased availability of shared content, as object referencesare stored only on highly available peers. Temporary peersin the HCP network store only a pointer to their closest staticsuccessor. Whenever a temporary peer receives an INSERTor QUERY message (due to its responsibility for an ID) itsimply forwards the message to its closest static successor,which in turn stores the according object reference.

Fig. 2. Simulations show that the number of transferred objectreferences in HCP is reduced by 1/a in comparison to conventionalChord.

The main advantage of HCP – besides increased avail-ability of shared content – is the significantly reduced main-tenance traffic in the overlay network. As shown in [9], thetraffic that is generated by the shifting of object referencescan be decreased by a factor of 1/a, assuming that staticHCP peers have an a-times longer average session lengththan conventional Chord peers (see Fig. 2). This traffic re-duction yields directly from the fact that object references areshifted only when static peers (which usually have long ses-sion times) join or leave the network. As a result, resource-constraint mobile devices are prevented from storing andshifting object references and therefore have a significantlydecreased traffic load.

6. Conclusion

Structured P2P networks cause high signaling overhead inhighly dynamic networks, i.e., in networks with high churnrates, even if the participating peers do not initiate searchrequests. This is especially harmful in heterogeneous envi-ronments, where high churn rates have to be expected. Onthe other hand, structured P2P networks are able to returna distinctive search result. This is especially important forVoIP applications where participating users expect a highavailability, although the replication rate is low.

In this work, we presented enhancements of existing struc-tured P2P approaches to increase stability and thus alsoavailability in structured P2P networks. Hence, we are ableto use the advantages of structured P2P networks even inhighly dynamic environments and with high availability de-mands at acceptable signaling overhead costs.

With our approaches we can support the general trendtowards applications established upon self-organizing net-works in fixed network scenarios as well as in the mobilearea, which we could observe since the end of the 1990s.This also results in more and more intelligence distributedover a whole network and pushed to the place where it isdemanded, i.e., at the edges of the networks.

W. Kellerer et al. / Int. J. Electron. Commun. (AEÜ) 60 (2006) 25–29 29

References

[1] Skype. Skype Homepage, 2005.[2] Foster I, Iamnitchi A. On death, taxes, and the convergence of

peer-to-peer and grid computing. Berkeley, CA, USA: IPTPS;2003. p. 118–28.

[3] Stoica I, Morris R, Karger D, Kaashoek M, BalakrishnanH. Chord: a scalable peer-to-peer lookup service for Internetapplications. San Diego, CA, USA: ACM SIG-COMM; 2001.

[4] Gruber I, Schollmeier R, Kellerer W. Peer-to-peercommunication in mobile ad hoc networks. Ad Hoc andSensor Wireless Networks (OCP Science Journals) 2005.

[5] Rosenberg J, Schulzrinne H, Camarillo G, Johnston A,Peterson J, Sparks R, Handley M, Schooler E. SIP: the sessioninitiation protocol. RFC 2002; 3261.

[6] Kunzmann G, Schollmeier R. Exploiting the overhead ina DHT to improve lookup latency. Colmenarejo, Spain:EUNICE; 2005.

[7] Garces-Erice L, Felber PA, Biersack EW, Urvoy-Keller G,Ross KW. Data indexing in peer-to-peer DHT networks.Tokyo, Japan: ICDCS; 2004.

[8] Kunzmann G, Nagel R, Eberspächer J. Increasing thereliability of structured P2P networks. Island of Ischia, Italy:ICON; 2005.

[9] Zöls S, Schollmeier R, Kellerer W, Tarlano A. The hybridchord protocol: a peer-to-peer lookup service for context-aware mobile applications. Reunion Island: ICN; 2005.

Wolfgang Kellerer is a Manager of theUbiquitous Services Platform group ofNTT DoCoMo’s European ResearchLaboratories in Munich, Germany. Hereceived his Dipl.-Ing. (M.Sc.) and Dr.-Ing. degree from Technische UniversitätMünchen (TUM), Munich, Germany,in 1995 and 2002, respectively.

Gerald Kunzmann has studied Electri-cal Engineering at TUM. He receivedhis B.Sc. and Dipl.-Ing. in 2002 and2004, respectively. Since 2004, he isworking as a Ph.D. student at the Insti-tute of Communication Networks at thesame university. His research focuseson current P2P systems and the applica-bility of P2P-based telecommunicationscenarios.

Rüdiger Schollmeier received hisDipl.-Ing. in 2001 from TUM. From2001 until 2005 he worked on differentresearch projects focusing on differentaspects of P2P networks. In 2005, hereceived his Dr.-Ing. degree from TUM.He is now working in the productiondivision of the BMW AG.

Stefan Zöls received his Dipl.-Ing. de-gree in Electrical Engineering and Infor-mation Technology in 2003 from TUM,where he is currently working at theInstitute of Communication Networks.His research interests include the anal-ysis and development of P2P conceptswith a special focus on mobile P2P net-working.