654 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 15, NO. 2, SECOND QUARTER 2013 Routing in Delay/Disruption Tolerant Networks: A Taxonomy, Survey and Challenges Yue Cao and Zhili Sun, Member, IEEE Applications Abstract —The introduction of intelligent devices with short of range wireless communication techniques has motivated the DTNs development of Mobile Ad hoc NETworks (MANETs) during the last few years. However, traditional end-to-end based routing algorithms designed for MANETs are not much robust in the challenged networks suffering from frequent disruption, sparse network density and limited device capability. Such challenged Space Terrestrial networks, also known as Intermittently Connected Networks Application Application (ICNs) adopt the Store-Carry-Forward (SCF) behavior arising from the mobility of mobile nodes for message relaying. In this article, we consider the term ICNs as Delay/Disruption Tolerant Networks (DTNs) for the purpose of generalization, IPNs since DTNs have been envisioned for different applications with a large number of proposed routing algorithms. Motivated by the great interest from the research community, we fi rstly review the existing unicasting issue of DTNs because of its extensive research stage. Then, we also address multicasting and anycasting issues in DTNs considering their perspectives. A detail survey based on our taxonomy over the period from 2006 to 2010 is not only Suburb UWNs PSNs VANETs ANs Networks provided but also a comparison is given. We further identify the remaining challenges and open issues followed by an evaluation framework proposed for routing in DTNs. Finally, we summarize Fig. 1. Applications of DTNs our contribution with three future research topics highlighted. Index Terms —Delay/Disruption Tolerant Networks, Intermit- tently Connected Networks, Routing, Store-Carry-Forward. temporaneous end-to-end connectivity is unavailable. Further- more, the global knowledge about network is not essential for the mobile nodes in ICNs. Given the lack of contemporaneous I. I NTRODUCTION end-to-end connectivity that prevents the conventional routing D UE to the characteristic of challenged environment suf- algorithms designed for MANETs from working effectively fering from frequent disruption, sparse network density in ICNs, the Bundle Protocol [3] borrowing from the concept and limited device capability, routing algorithms designed of Email protocol is proposed by the Internet Research Task for Mobile Ad hoc NETworks (MANETs) can not perform Force (IRTF) Delay Tolerant Networking Research Group effectively under these constraints, since the availability of (DTNRG) [4], to behave as a convergence layer protocol on contemporaneous end-to-end connectivity is essential for con- top of the Transmission Control Protocol (TCP) layer for ventional routing algorithms such as Ad hoc On-Demand enhancing the transmission reliability. Distance Vector (AODV) [1] or Dynamic Source Routing Thanks to the most recent tutorial [5], providing a rigor- (DSR) [2]. However, this does not prevent bridging com- ous de fi nition about the difference between Delay/Disruption munication between the disconnected areas, as the concept Networking (DTN) [6] and ICNs. Also, taking into account of Intermittently Connected Networks (ICNs) is proposed the understanding from the authors in [7] 1 , we replace the to overcome these dif fi culties using the Store-Carry-Forward term ICNs with Delay/Disruption Tolerant Networks (DTNs) (SCF) routing behavior. in this article for the purpose of generalization, since we focus on routing issue for this type of networks without investigating A. Concept and Applications of DTNs the DTN architecture. As illustrated in Fig.1, the space application of DTNs In Intermittently Connected Networks (ICNs), mobile nodes is for InterPlanetary Networks (IPNs) [8] with a low net- are capable of communicating with each other even if the con- work dynamic. In mobile wireless networks, the terrestrial Manuscript received 2 April 2011; revised 11 October 2011 and 20 applications of DTNs have been envisioned for UnderWater February 2012. The funding leading to this work is from the EU FP7 MONET Networks (UWNs) [9], Pocket Switched Networks (PSNs) project and EPSRC UK-CHINA Science Bridge UC4G project. [10], Vehicular Ad hoc NETworks (VANETs) [11], Airborne Y. Cao is with the Center for Communication Systems Research, University of Surrey, Guildford, UK (e-mail: Y.Cao@surrey.ac.uk). 1 In [7], the authors provide the concept of Opportunistic Networks (ONs) Z. Sun is with the Center for Communication Systems Research, University of Surrey, Guildford, UK (e-mail: Z.Sun@surrey.ac.uk). and interpret it is as a more fl exible environment than Delay/Disruption Digital Object Identi fi er 10.1109/SURV.2012.042512.00053 Tolerant Networks (DTNs). 1553-877X/13/$31.00 c � 2013 IEEE
CAO and SUN: ROUTING IN DELAY/DISRUPTION TOLERANT NETWORKS: A TAXONOMY, SURVEY AND CHALLENGES 655 Time 1 C A M Initially, A carries B message M Time 1 Time 2 B C B C M M Time 2 A A C Afterwards, A relays M to B when B is in A proximity M B Time 3 M B C Time 3 C M Finally, M is delivered A by C when B encounters C A B Message M is relayed via A-B- C symmetrically from Time 1 to Time 3 (a) Routing in MANETs (b) Routing in DTNs Fig. 2. Illustration of Routing in MANETs and DTNs Networks (ANs) [12] and suburb networks for developing Security : In DTNs, it would be hard for a certi fi cate author- region [13]. ity to exchange cryptographic message with a particular node. Apart from key management, DoS attacks, access control, privacy and anonymity are also being investigated. B. Existing Research Activities of DTNs Up to now, the research activities in DTNs are being C. Organization of This Article investigated for application layer design [14], convergence As our focus, routing is an important research area in DTNs layer design [15], routing [16], congestion control [17], fl ow not only because of its unique characteristic, but also due to control [18] and security [19], which are brie fl y introduced as the extensive attention from the research community. follows: In section II, we provide the relevant background of routing Application Layer Design : The design of application layer in DTNs together with our taxonomy illustrated in III. We protocol is the most challenging issue since the network further provide the overview of unicasting, multicasting and architecture needs to deal with system component, which is anycasting issues based on our taxonomy in section IV, V, fi xed and known. However, the application has to deal with VI respectively. Given the comparison and discussion for user interest, which is more dynamic. the reviewed algorithms in section VII, we further identify Convergence Layer Design : This research issue is sepa- the remaining challenges and open issues in section VIII, rated into the proposal for space DTNs (or referred to IPNs) followed by a proposed evaluation framework in section IX. and terrestrial DTNs. More speci fi cally, the long delay is more Finally, section X summarizes our contribution with three concerned for space DTNs even when the connectivity exists. topics highlighted for future investigations. In contrast, the communication in terrestrial DTNs somehow is with frequent disruption. As such, these properties have to II. B ACKGROUND OF R OUTING IN DTN S be considered for these two types of applications. Routing : In contrast to routing in MANETs, routing in Given the examples illustrated in Fig.2(a) and Fig.2(b) DTNs is more dif fi cult due to the lack of the most recent where message M is relayed from node A to node C via network topology information. node B , the difference between routing in MANETs and Congestion Control : Congestion control in DTNs is af- DTNs is that the former relies more on symmetric relaying fected by the acknowledgement strategy since once the mes- the message with a multi-hop routing behavior, thanks to sage is acknowledged, the cached message can be discarded the contemporaneous end-to-end connectivity. Whereas the to alleviate the buffer space exhaustion. latter relies more on the mobility of mobile nodes to create Flow Control : Instead of the traditional end-to-end based encounter opportunity for an asymmetric routing behavior, approach, fl ow control in DTNs requires a hop-by-hop behav- under the assumption of intermittent connectivity. ior to provide the information on traf fi c and local resource As illustrated in TABLE I, routing in DTNs suffers more availability that can also be used from upper layer. from long delivery delay than that of in MANETs due to the
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