Robust MANET Routing using Adaptive Path Redundancy and Coding

Robust MANET Routing using Adaptive Path Redundancy and Coding Soon Y. Oh and Mario Gerla Computer Science Department, University of California, Los Angeles Los Angeles, CA 90095-1596 USA Email: soonoh,[email protected]

Abstract—Providing efficient networking services in MANETs is very challenging due to mobility and unpredictable radio channel: a significant number of packets can be corrupted and/or lost. To increase reliability, various measures have been proposed. A popular approach is to use multiple paths and transmit an identical copy of the packet on each path (i.e., path redundancy). A more efficient way is to use Network Coding on top of path redundancy and send different, encoded packets on each path. Network coding can improve throughput efficiency. However, it increases delays. If channel disruption is intermittent, it behooves us to “turn on” path redundancy and/or Network Coding only when packet loss is severe. In this paper we compare via simulation the performance of multipath routing with and without Network Coding (and with/without dynamic adaptation) for various motion and packet loss scenarios in terms of reliability, efficiency, robustness, and scalability.

I. I NTRODUCTION In mobile environments without infrastructure, nodes must be self-organize and create a wireless ad hoc network in order to communicate or exchange messages with each other. Providing reliable and efficient networking services in a rapidly moving environment such as vehicular networks and tactical scenarios, however, is very challenging due to high mobility and external interference (e.g., jamming by the adversary). In such a “disruptive” environment, conventional links, networks, and transport protocols fail to operate properly; thus, a significant fraction of the packets can be received in error and/or can be lost. Various error control methods have been proposed to handle channel errors/losses. Automatic Repeat-reQuest (ARQ) uses acknowledgments and timeouts to achieve reliable data transmission. For example, the sender, if it does not receive an acknowledgment before the timeout, will re-transmit the packet until it receives an ACK. ARQ can increase reliability, but it is not proper to inelastic flow such as video and audio streaming because of high delay and delay variance. Unlike ARQ, in Forward Error Correction (FEC), the sender adds redundant data to its messages, in the form of an error correcting code. This allows the destination to detect and correct errors (within some limits) without the need to ask the sender for retransmission. FEC is traditionally implemented at the physical layer so it corrects channel errors caused by interference or bit distortion. FEC error correction, however, becomes inefficient if the channel is extremely bad. A novel form of coding that has many features in common with FEC is

Network Coding [1]. In Network Coding, intermediate nodes store, modify, and forward packets. Network Coding increases network robustness and reliability by combining error correction and multipath packet redundancy at intermediate nodes. In fact, it is difficult to achieve the full benefits of Network Coding in disruptive single path connections since packet redundancy is limited. The question then arises, why not use multipath redundancy alone? Indeed, multipath packet replication is another solution for making communications more robust. Namely, each forwarding node exploits multiple path spatial redundancy by transmitting the same copy of the packet over all the available paths. As a difference from the ARQ scheme, feedback is not used. In this study we evaluate multipath routing with/without Network Coding. We only consider inelastic unicast flows though we expect the same concepts and tradeoffs to apply to multicast. When a source has packets to send, a braided style multipath is established between a source and a destination. Duplicated packets arrive at the destination traveling over multiple paths so that packet delivery increases in the presence of channel loss. The main drawback of multipath routing (with replicated copies) is channel overhead. We seek to reduce channel overhead (due to brute force replication) and improve efficiency by employing Network Coding on top of multipath routing. The source and the intermediate nodes modify/encode received packets using random linear coding before forwarding. Strictly speaking, no duplicated packets are injected, thus each packet potentially carries innovative information (as opposed to the brute force replication scheme), though redundancy is built in “adaptively” along the path as the channel becomes increasingly disruptive. The destination reconstructs the original “generation” upon collecting enough linearly independent packets. The careful reader will note that similar benefits can be achieved using end to end encoding, e.g. rateless coding. We compared network coding to rateless coding (for multicast) discovering that in extreme disruption (mobility + jamming) and dense networks, Network Coding outperforms end to end erasure coding [5]. We expect this to hold true for unicast as well. So, in this study, we limit ourselves to the evaluation of Network Coding versus brute force, replicated multipath. Fig 1 is a simple example illustrating multipath routing with/without Network Coding. In Fig 1 (a), F1 and F2 forward two packets, P1 and P2, so a total of 4 packets are forwarded

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by intermediate nodes. If we use Network Coding, however, a destination can recover original packets when F1 and F2 forward only one re-encoded packet. Since intermediate nodes generate a new packet using re-encoding, packets are not duplicated and thus Network Coding yields network resource savings. Consequently, Network Coding can decrease overhead and increase throughput in disruptive conditions. Another contribution of this paper is the design of an adaptive multipath routing protocol which dynamically changes the routing mode based on channel loss condition. The protocol has three routing modes, single path and multipath with/without Network Coding mode. The protocol starts packet transmission by a single path routing mode and switches to multipath routing with or without Network Coding mode if packet error rate exceeds the maximum threshold. Single path routing mode is reinstated if channel quality is above the threshold. The rest of paper is organized as following. Section II introduces related work in terms of multipath routing and routing using Network Coding. Section III describes the multipath strategy with/without Network Coding and section IV presents dynamic routing that adapts to varying channel condition. We show simulation results in section V and conclude in section VI. II. BACKGROUND AND R ELATED W ORK In this section we review previous MANET multipath routing schemes. We also review the use of Network Coding in MANETs.

Fig. 3. SMR routing example. SMR establishes 3 disjoint paths between the source and the destination.

A. Multipath Routing Multipath routing allows packets with the same source and the destination to travel more than one possible path. Multipath routing allows packets to travel over multiple source/destination paths with benefits ranging from load balancing to higher aggregate bandwidth and fault-tolerance. Load balancing can alleviate congestion and bottlenecks by spreading the traffic along multiple paths. If multiple paths deliver different packets simultaneously, the aggregated bandwidth increases as function of the number of disjoint paths. However, to get the throughput benefit in MANETs is a challenge: there represent bandwidth bottlenecks between a source and a destination; moreover, topology changes dynamically due to motion, requiring the recomputation of node disjoint paths. Due to this limitation, multipath routing is often employed for fault-tolerance. Since a packet travels along multiple paths, as long as at least one of the paths does not fail, the destination receives the packet. Fig 1 is a simple example of multiple paths providing fault-tolerance. As long as at least one path lives, packet transmission succeeds. AODV-BR (Backup Routing) [6] provides a braided path redundant structure in AODV without extra control messages. In AODV-BR packets travel on the primary route. Alternate paths (in the braid) deliver packets when the primary path breaks. Fig 2 shows AODV-BR primary and alternate paths. Lim et.al. [10] uses multipath as a backup route in TCP traffic. Split Multipath Routing (SMR) [7] is an extension of AODV like AODV-BR. As a difference from the latter, that builds maximally disjoint paths. Fig 3 is a SMR routing example. Three disjoint paths exist between the source and the destination and they deliver duplicated packets independently.

To establish disjoint paths, intermediate nodes do not discard duplicate route request packets and only a destination node sends back a reply packet after choosing paths. The destination first sends a reply over the shortest path and it chooses the path that is maximally disjoint from the shortest path. Data traffic is split into multiple routes to avoid congestion and to use network resources efficiently. AODVM [17] and AOMDV [12] are extension of AODV [14] and they employ disjoint multiple paths. Like SMR, Multipath TCP [3] establishes disjoint paths and duplicated TCP packets are transmitted over multiple paths. CHAMP (Caching and Multipath Routing) [16] uses multiple paths by round-robin traffic allocation. After establishing equal length multiple paths, a source sends a packet along the least used route. CHAMP accepts non-disjoint paths and each node stores recently forwarded packets. If packet forwarding to the next hop fails, the route is removed from the route table and the node forwards packets using alternate paths if they exist. If there is no alternate path, the node broadcasts an error message. Upon receiving this error message, each neighbor searches an alternative path from its own route table. If there is an alternative path, the route is rebuilt. Otherwise, the node re-broadcasts an error message. Manfredi et. al. [11] recently proposed local “braided” routing to avoid network-wide route recalculations. The shortest path between a source and a destination is established by existing single path routing algorithms, e.g., AODV [14], and an intermediate node on the shortest path decides the next hop based on link quality. If shortest link quality is bad, it looks for node in the braided path. If link quality to a node on the braided path is good, the intermediate node transmits a packet to it setting a braided flag on the packet. This algorithm does not use multiple paths simultaneously nor backup routes. It locally selects a high reliable link and it performs minimum throughput gain compare to AODV. B. Network Coding in MANETs CodeCast [13] and CodeTorrent [9] are two well-known MANET protocols using Network Coding. CodeCast is a multicast protocol; it uses random linear coding. The source divides stream of data from application layer into blocks of packets and then encodes a block before transmission. Forwarders re-encode and relay received packets. Destinations re-construct the original data upon receiving enough encoded packets. CodeCast broadcasts encoded packets without establishing a sub-graph route and it prunes unnecessary routes using passive acknowledges while transmitting. CodeTorrent is a P2P file swarming protocol that transmits blocks of a file after encoding. It uses epidemic dissemination style contents distribution. A sender broadcasts a control packet to one-hop neighbors and it transmits encoded packets when it discovers a node that wants the data. A recently proposed protocol that uses Network Coding is MORE [2] - an opportunistic routing protocol for a wireless mesh networks. MORE randomly mixes packets using Network Coding before forwarding them. This randomness

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ensures that routers hearing the same transmission do not forward the same packets. Consequently, MORE is MAC independent and performance gains of up to 45% over non-coded opportunistic routing protocols. MORE is a protocol developed for stationary, wireless urban meshes, e.g., community wireless networks. It has not been tested for highly mobile, tactical scenarios. III. M ULTIPATH C ODED ROUTING In this section, we introduce our proposed multipath routing protocol with and without Network Coding. A. Multipath Routing without Network Coding We propose a multipath braid generated by an on-demand style scheme such as AODV. When a sender has packets to send, it broadcast a Join Query control packet into the whole network. All nodes rebroadcast the Join Query after recording the upstream node address (i.e., reverse path learning). Upon receiving the Join Query packet, the destination sends back a Join Reply to the source through the shortest path. If a node receives the Join Reply packet, it sets itself as a forwarder; it then relays the Joint Reply to the source using the reverse path information in its table. To generate the multipath braid, neighbors overhearing the Join Reply packet also take on the role of forwarders; they do not relay the Joint Reply, however. Fig 4 (a) shows multipath establishment. The Join Reply travels the shortest path and nodes overheard the Join Reply as well as nodes on the shortest path become forwarders. This process is similar to AODV-BR. However, the latter distinguishes between primary and alternative paths. The alternative paths are used only if the primary route is broken.

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In the proposed braided scheme we use all paths in parallel to deliver packets. In Fig 4 (b), data packets reach the destination passing through multiple paths. On reception of non-duplicated packets, forwarders rebroadcast; thus, packets can reach the destination through any path in the braid. In essence, the forwarding on the braid is the same as the forwarding on an ODMRP mesh [8]. Since each forwarder receives several replicas of the packet, delivery is robust to channel errors. The most difficult challenge in designing a MANET protocol is dynamic topology change. Route is frequently broken due to node mobility. To cope with mobility, in our scheme the source periodically refreshes routes through Join Query and Join Reply exchange. If the destination has become disconnected because some of the intermediate forwarders have failed or have walked away, the route refresh re-connects the destination. This periodic route refresh is differs from the event driven route repatch strategy used by AODV and AODV inspired multipath protocols. We recall that in AODV, when a path fails, neighbors of the failed node are searched for local reconnect; in high mobility this strategy may backfire. The periodic route refresh (as opposed to event driven route repatch) makes the protocol more robust, even though it may increases control traffic, Braided multipath routing employs a soft-state approach; no explicit control message is required to terminate a session. When a source node has no more packets to send, it simply stops sending Join Queries. If a destination does not want to receive packets any more, it does not respond to Join Queries from the source. A forwarding node becomes a non-forwarder if it reaches the forwarder timeout (a multiple of the refresh interval) without receiving the Join Reply. B. Braided vs. Disjoint Multipath Forwarding: End-to-End Packet Drop Probability We use a “braided” multipath with interleaved forwarding; other multipath routing protocols use “disjoint” multipath forwarding. The main difference is that in the braided multipath



Fig. 6. Packet error probability of single, disjoint, and braided path as a function of channel/link error rate. h = 4, m = 3, and r = 2.

scheme, an intermediate node can forward packets received from any upstream neighbors, while in disjoint multipath forwarding, a node can receive only from a single upstream neighbor. . Fig 5 is a simple topology where h is the number of hop between the source and the destination and r is number of redundant paths. Fig 5 (a) has 3 disjoint paths and Fig 5 (b) is a braided multipath in which each hop has 3 nodes. Each routing has its own benefit; the disjoin paths can achieve traffic balancing and bandwidth increase with intelligent path selection; the braided multipath can gain reliability and robustness. Note we can verify the robustness of the braided multipath based on the simple routing model, Fig 5. First we know that the probability of packet error, Pr , between the source and the destination in the single path routing is: Pr = 1 − (1 − e)h

(1)

where e is error rate and h is the number of hop. Now we can find that the probability of packet error in the disjoint paths is: P r = {1 − (1 − e)h }m (2) where m is the number of disjoint paths. The braided multipath is not disjointing rather it works similar to mesh. Says, each node can receive multiple duplicated packets from multiple upstream nodes. If there is n nodes at each hop and nodes recive r duplicated packets from upstream nodes, then the packet error probability is: P r = 1 − (1 − erm )h

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Fig 6 shows of the plots of formula 1, 2, and 3 in which hop,h, is 4, the number of path, m is 3, and the duplicated packets, r is 2. In both single and disjoint multipath, the end to end packet drop rate rapidly increases as a function of channel/error rate. In fact, drop rate exceed 80% with channel error rate more than 50%. In contrast, the packet drop rate in the braided multipath increases much slower with channel rate, so that it shows negligible packet error rate as long as channel error rate is less than 50%. Therefore, the braided multipath is the better scheme in terms of robustness. In all fairness, we must also compare the throughput achieved by the two

schemes in absence of errors. The disjoint multipath scheme delivers one packet per time slot (assuming 3D pacing). Namely, the rate is 1/3 on each path, times the number of independent paths =3. In contrast, the braid achieves only 1/3 packet/slot since the three paths are interfering with each other. The above analysis is approximate, since it is difficult in practice to avoid interference among paths in Fig 5 (a), unless we use directional or MIMO antennas. Nevertheless, the “qualitative” tradeoffs between throughput and reliability are quite evident even in this simple example. C. Network Coding with Multipath In wireless networks, nodes can leverage the broadcast nature of wireless medium that allows them to overhear packets from different sources in a “promiscuous” way. Nodes can then combine the overheard packets to generate and forward “mixed” (i.e., coded) packets. Figuratively speaking, the encoded packet contains more information than the original packet. Although some packets are lost due to interference, jamming and motion, the destination can recover the original data by efficiently exploiting the redundant coded stream. For example, a node may not be available because of mobility (or topology changes), yet other nodes around it have sufficient information to recover from the loss. For this reason, Network Coding works well only with multipath routing. To implement Network Coding, we use the “Random linear Network Coding” scheme [4]. Given n packets, a node generates Pa coded packet by linearly combining all the packets, i.e., ek pk where ek is a random number drawn from a finite field GF (2n ), pk is the k th packet, and k ranges from 1 to n. n is called generation size and n = 8 in our simulation. Note that each packet has an encoding vector e used for the decoding process and it is carried recoding in the header of the packet indicating how it constructed. A source splits the original stream into n packets. It groups packets in generations. It mixes packets randomly within the same generation and transmits the coded packets. Upon overhearing packets, intermediate nodes store them into their local buffer if they are linearly independent of previously received packets in the same generation. A packet linearly independent of the so far processed set is called “innovative”. Given that an intermediate node has collected n coded packets in the same generation or certain period has passed since the first packet in that generation arrived, all of received coded packets in the generation are used to generate another coded packet. Once a destination collects n innovative packets, it can recover the original packets via Gaussian elimination. If a destination collects less than n packets in the generation, it cannot decode that generation. Thus, it requests retransmission from one hop neighbors. To this end, the destination starts timeout, i.e., 1 second in our simulation, when it receives the first coded packet of the generation. When reaching timeout, the destination checks completion of the generation and if necessary sends request to one hop neighbors. Upon receiving this request, a neighbor who can help retransmits coded

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packets. This request and retransmission increase the delivery ratio and throughput; yet they also increase overhead. Fig 7 is an example of Network Coding with multiple paths. An intermediate node re-encodes received packets and the destination reconstructs original data using matrix inversion. Since an intermediate node modifies/generates a new coded packet, a destination receives different packets from different paths. In contrast, in the multipath version without Network Coding replicated packets arrive at the destination via different paths. Therefore, Network Coding uses network resources more efficiently than simple multipath routing. IV. DYNAMIC ROUTING The most important feature of the multipath with/without Network Coding routing protocol is the “mode” change to adapt to channel conditions. When channel conditions degrade below a given threshold, the network switches routing strategy to a more robust mode, e.g., from single path to multipath with/without Network Coding. If channel conditions return to normal, single path routing is resumed to reduce overhead and save resources. A source first starts packet transmissions using single path routing. As explain earlier the source broadcasts a Join Query packet when it has packets to send. Intermediate nodes rebroadcast a non-duplicated Join Query. The destination replies using a Join Reply packet. This process is the same as to establish a multipath braid. The difference is that only nodes on the shortest path become forwarders. The source node periodically refreshes the single path to cope with mobility. During the data transmission phase, the source counts the number of sent data in the period and records it in the Join Query. The destination also counts the number of received packets and upon receiving the Join Query, calculates the packet delivery ratio. If packet loss rate exceeds a specified threshold, the destination sets “routing upgrade flag” in its Join Reply and sends it back. Nodes that overheard the Join Reply set themselves as forwarders based on the destination’s request. Namely, the intermediate nodes organize themselves in a braid. If channel error is extremely bad (e.g., error rate >

50%), the destination can speed up the routing mode change by initiates an unsolicited Join Reply along the current shortest path before receiving the Join Query. If channel conditions improve, (i.e., packet error rate is 0% for a specific period), the routing mode switches back from multipath to single path routing. In our simulation, routing starts with a single path and routing mode switches to the multipath without Network Coding if packet error in the period exceeds 10%. If packet error still exceeds 10%, Network Coding starts. Dynamic route switching does not increase robustness rather it uses resource more efficiently. In fact, multipath routing uses more bandwidth than single path. Network Coding introduces extra coding overhead (mainly, processing and latency). Thus, multipath routing increases the load in the network and may lead to congestion when competing with other flows. In contrast, single path routing is lean and resource conservative. Ideally, dynamic routing allows us to adapt to network conditions and maintain the best compromise between efficient use of resources and robustness. V. S IMULATION R ESULTS In this section, we validate single path, multipath with/without Network Coding routing using Qualnet v3.9.5 [15], a packet level network simulator. We implement three routing protocols in Qualnet and compare the performance of them. We also report dynamic routing (with mode switching) simulation experiments and compare performance with fixed mode routing. A. Simulation Setup We use 802.11 with 376m effective reception range and 2Mbps channel capacity. The packet size is 512 bytes and traffic is 10 packets per second unless otherwise specified. This very low loading factor allows us to focus on robustness independent of throughput. 30 nodes are randomly distributed in 1000 by 1000 meter area and Random Way Point mobility model is applied. We use Qualnet default values for MAC and Physical layer configuration parameters. The Join Query refresh interval is 3 seconds and forwarder life time for multipath is 3 times the refresh interval. Forwarder life time is the same as the refresh interval in single path routing to facilitate clean start in multipath establishment. In single and multipath routing, forwarders relay every nonduplicated packet. Thus, many of the received packets are dropped as duplicates. In Network Coding duplicates are rare since Network Coding modifies the received packets before forwarding. To prevent overload, Network Coding forwards less packets than it received. For example, if a forwarder receives n packets or reaches the timeout, it re-encodes and sends out k < n coded packets. In our simulation, n = 8 which is generation size, and k = n/2. In general the parameter k can be used to adjust the degree of redundancy in Network Coding. For performance evaluation, we use four metrics: Packet Delivery Ratio is the fraction of received data; Normalized Packet Overhead is the total number of packet transmissions

by the network divided by the total number of data packets actually received by the destination; Average End-to-End Delay is the average time taken for a packet to be transmitted across the network from a source to a destination; Throughput is the total received byte of data packet divided by the total simulation time. All numbers are averaged over 100 simulation runs. B. Varying Node Speed Figure 8, 9, and 10 illustrate each protocol’s performance with varying node speed. The maximum node speed varies from 1m/s to 50m/s and minimum node speed is fixed, 1m/s. Node pause time is 0 second. As we expected, the packet delivery ratios of multiple path with and without Network Coding outperform single path routing. Redundant multiple paths increase the probability of packet delivery. In Fig 8, as node maximum speed increases, multipath and Network Coding packet delivery ratios remain high while delivery ratio of the single path routing significantly decreases. It is difficult to explain Network Coding superiority over mere redundant multipath (without Network Coding) from theses results. However, we must note that if Network Coding redundancy is high enough and “generation size” is large enough, Network Coding will recover from the simultaneous loss of multiple packets across the multipath shown in Fig 4. The multipath solution, on the other hand, maintains multiple identical copies across the multiple paths; it will suffer a loss if all multiple copies are lost. As mentioned in III-B, the multiple packet loss probability is statistically too small to be detected in the simulation, but, it is quite possible in a hostile jamming scenario. Another potential advantage of Network Coding not evidenced in this experiment is the increase in throughput, over redundant multipath without Network Coding, when the network allows the full exploitation of the “degrees of freedom” of the multiple paths. In practice, this is not easy. Going back to Fig 4, we note that if all node transmission speeds are the same, the “bottleneck” occurs at the source (and destination) node. The multiple paths yield no throughput advantages. However, if transmission speed is adaptively adjusted based on interference like in 802.11 radios, it is very likely that interference will be much higher in the middle of the network than at end points (where hostile interferers may be detected and eliminated). Thus, transmission rate will be higher at end points. With higher source and destination transmission speeds, one finds that Network Coding will also exhibit a throughput advantage. We plan to further explore these Network Coding advantages over multipath in future experiments. Fig 9 shows the normalized packet overhead with various speeds. The packet overhead of the multipath routing is extremely high compared to single path routing since many nodes near the shortest path participate in packet forwarding. In Fig 9, the overhead is near 20 in the multipath routing while the single path routing overhead is less than 5. Network Coding with multipath overhead is in the middle. As we mentioned in the previous section III-C, forwarders in Network

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