- Open Access
DRO: domain-based route optimization scheme for nested mobile networks
© Chuang and Lee; licensee Springer. 2011
Received: 1 December 2010
Accepted: 19 August 2011
Published: 19 August 2011
The network mobility (NEMO) basic support protocol is designed to support NEMO management, and to ensure communication continuity between nodes in mobile networks. However, in nested mobile networks, NEMO suffers from the pinball routing problem, which results in long packet transmission delays. To solve the problem, we propose a domain-based route optimization (DRO) scheme that incorporates a domain-based network architecture and ad hoc routing protocols for route optimization. DRO also improves the intra-domain handoff performance, reduces the convergence time during route optimization, and avoids the out-of-sequence packet problem. A detailed performance analysis and simulations were conducted to evaluate the scheme. The results demonstrate that DRO outperforms existing mechanisms in terms of packet transmission delay (i.e., better route-optimization), intra-domain handoff latency, convergence time, and packet tunneling overhead.
Recently, vehicular networks have received a significant amount of attention in the field of wireless mobile networking. On public methods of transportation, such as taxies, trains, buses, and airplanes, many mobile network nodes (MNNs) move together as a large-scale vehicular network. In such environments, people can use mobile devices for accessing services, such as VoIP, video conferencing, web-browsing, and music downloading, anytime-anywhere. With the emergence of vehicular networks, users require seamless and efficient communications on the move. Therefore, developing a route optimization scheme has become an important research issue.
The NEMO routing protocol can be divided into (1) inter-domain routing, which means the MNN and the CN are in different nested mobile networks; and (2) intra-domain routing, where the MNN and the CN are in the same nested mobile network. Most approaches focus on the inter-domain routing problem and use a hierarchical architecture to achieve route optimization. However, hierarchy-based schemes may suffer from the non-optimal route problem when the CN and the MNN are located in the same nested mobile network (i.e., intra-domain routing). Moreover, such schemes do not cope with the handoff procedure well, resulting in long convergence time in route optimization or communication disruption. Actually, the handoff procedure has a substantial impact on the performance of route optimization because it is implemented before route optimization. If the handoff latency (HL) is long, then it disrupts communications or causes long convergence time in route optimization. Therefore, we also consider the handoff problem to reduce the latency in route optimization. Similar to the NEMO routing protocol, inter-domain handoff means that the MR hands off to a different nested mobile network; while intra-domain handoff means the MR hands off within the same nested mobile network. Hence, the proposed mechanism considers route optimization for inter-domain and intra-domain routing, and reduces the HL in both scenarios.
In this article, we propose a domain-based route optimization (DRO) scheme. The domain-based network architecture incorporates the operations of ad hoc routing protocols for performing route optimization and reduce HL. Moreover, we use a double buffer mechanism in DRO to prevent the packet out-of-sequence problem during the route optimization procedure. We compare DRO's performance with that of existing route optimization schemes via analysis and simulations. The results demonstrate that DRO outperforms the compared schemes in terms of packet transmission delay, HL, convergence time, and packet tunneling overhead.
The remainder of this article is organized as follows. Section 2 contains a review of related work. In Section 3, we describe the proposed DRO scheme. In Section 4, we evaluate the scheme's performance in terms of packet delay (PD), HL, packet overhead during tunneling, and total cost (TC). Section 5 contains some concluding remarks.
2. Related work
In this section, we discuss existing schemes for solving the pinball routing problem, out-of-sequence problem, and route optimization using the concepts of mobile ad hoc networks (MANETs).
The reverse routing header  uses new extension headers to inform the HAs of an MR in the nested structure. However, this header modification needs to be performed by each MR that an outgoing packet passes through. Moreover, the modification and re-computation overhead of the packet checksum or CRC increases with the level of the nested mobile network. The recursive binding update (RBU)  allows the HAs to maintain the binding information for the care-of-address (CoA) of the root mobile router (RMR). Consequently, RBU can use the BU messages to find the optimal route. However, RBU needs long convergence time to find the optimal route when there are many handoff events because the HAs need to repeat the RBU procedure for each event. Calderon et al.  propose the Mobile IPv6 route optimization scheme for NEMO (MIRON) based on the protocol for carrying authentication for network access (PANA)  and the dynamic host configuration protocol (DHCPv6) . However, MIRON needs to modify all MRs and visiting mobile nodes (VMNs). Moreover, MIRON will not work well if the VMNs do not have PANA client software, or the MR does not have PANA client and server software. SIP-NEMO  extends SIP to support NEMO so that the packets can be transmitted directly between the MNN and the CN, but the scheme only applies to applications that use SIP. The route optimization using tree information option (ROTIO) scheme  has a fast convergence time during route optimization. However, if an inter-domain handoff event occurs, the communication may be disconnected since ROTIO does not handle inter-domain handoff well. Kuo and Ji  proposed an enhanced hierarchical NEMO protocol called HRO+, which reduces the PD in inter-domain and intra-domain routing. In inter-domain routing, the CN sends the packets to the RMR directly without passing through any HA because the MR binds the NEMO prefix of RMR to the CN. In intra-domain routing, each MR records the routing information of sub-MRs. Therefore, the MR can find an optimal path when the sender and receiver belong to its sub-MR. However, HRO+ does not consider inter-domain handoff and it also suffers from the suboptimal routing problem in intra-domain routing (i.e., the sender and the receiver do not have the same parent MR). N-PMIPv6  uses Proxy Mobile IPv6 (PMIPv6) protocol  to reduce HL in a NEMO environment, but it does not address the route optimization issue.
During the route optimization procedure, the MNN may receive out-of-sequence packets, as shown in Figure 2. In this situation, receivers will transmit duplicate ACKs so that the performance of TCP will be degraded. Zheng et al.  and Tandjaoui et al.  anticipate the arrival time of packets from the old link to adjust the transmission time of packets from the new link. The drawback of these schemes is that, since they are based on prediction methods, they suffer from packet loss or inaccurate time estimation when the network environment varies.
MANEMO integrates MANET and NEMO technologies to provide IP connectivity across nested mobile networks. Clausen et al.  used the optimized link state routing (OLSR) protocol to support route optimization, but the scheme does not consider the handoff situation of the MR. McCarthy et al. [18, 19] introduced the MANEMO concept and identified two key solution areas in the MANEMO problem domain, namely, NEMO-Centric MANEMO (NCM) and MANET-Centric MANEMO (MCM). McCarthy et al. [20, 21] and Tsukada and Ernst  built testbeds for implementing and experimenting with the MANEMO protocols. Although their results show that MANEMO outperforms the traditional NEMO protocol, they only considered inter-domain route optimization and measured the packet transmission delay between the CN and the MNN. They did not describe the route optimization mechanism in detail or solve the mobility problem in NEMO.
A MANET comprises a collection of mobile nodes that form a temporary network without any infrastructure. Each mobile node in a MANET can act as a sender and cooperate with other nodes and act as a relay in multi-hop transmissions. Moreover, mobile nodes can self-organize and maintain the routing information through routing protocols. In general, the routing protocols for MANETs can be classified as proactive routing protocols  and on-demand routing protocols [24, 25] based on whether each node maintains the routing tables or finds the route to destination before transmitting data. These routing protocols find the optimal path from the source to the destination based on certain routing metrics. They also have mechanisms to deal with dynamic topology changes because of node mobility or link failures.
We discuss more related work in this journal version.
We describe the proposed scheme in detail such as the intra-domain routing and the inter-domain handoff procedures. Moreover, we propose the double buffer mechanism to avoid the packet out-of-sequence problem. We also correct some flaws of the conference version.
In the preliminary version, we only use the numerical analysis to evaluate the HL and the PD of intra-domain and inter-domain handoff procedures. However, in this version, we add detailed analytical models for 'Convergence Time of Route Optimization during Inter-Domain Handoff', 'Packet Overhead Ratio (POR)', 'TC', and 'Discussion of Double Buffer Mechanism'. Moreover, we use NS-2 simulations to evaluate the performance of DRO compared with existing mechanisms and verify the analytical models.
3. The DRO scheme
Route optimization involves minimizing the packet transmission delay between the sender and the receiver. Although many hierarchy-based route optimization schemes [11, 12] support route optimization for inter-domain routing, a non-optimal route is formed when the CN and the MNN are located in the same nested mobile network (i.e., intra-domain routing). Moreover, these schemes do not cope with the handoff procedure well, resulting in a long convergence time during route optimization or communication disruption. To resolve these problems, we propose a novel NEMO support protocol with a DRO scheme. The domain-based network architecture incorporates the routing techniques of MANETs for route optimization. We also use the architecture to reduce intra-domain HL and provide a fast handoff scheme to achieve low inter-domain HL. In addition, we use a double buffer mechanism to avoid the packet out-of-sequence problem during the route optimization procedure.
3.1 MANET routing protocols
Our DRO scheme is based on MANET routing protocols since these routing protocols find the optimal path from the source to the destination. Moreover, they also have mechanisms to deal with dynamic topology changes because of node mobility or link failures. Therefore, we use the protocols to find the shortest/optimal path among MRs in nested mobile networks in order to achieve route optimization. Most hierarchy-based schemes do not adopt these routing protocols because they use tree-based network architectures for mobility management. In contrast, our domain-based network architecture functions like a mesh network; hence, it is compatible with all MANET routing protocols.
3.2 Domain construction
In our domain-based network architecture, when an MR moves in the mobile network, it works as the RMR in the domain if it receives an router advertisement (RA) message from access router (AR). Moreover, the new RMR configures its CoA according to the prefix of the AR, binds its new CoA to the HA, inserts its prefix in RA message, and then broadcasts the RA message. However, if the MR receives an RA message from other intermediate MRs (IMRs), it acts as an IMR, joins this domain, generates its CoA based on the prefix of the RMR, and rebroadcasts the RA message. Then, it finds the shortest path to the RMR based on the routing protocol adopted by the mobile network and binds the CoA of the RMR to its HA. In DRO, each MR sends two kinds of BU messages: a local BU and a global BU. The former is sent to the RMR and other MRs in the domain, and the latter is for the HA and CN of the MR. Finally, every MR follows the routing information recorded in the network's routing protocol so that the network nodes can communicate via the optimal routes.
We use the following example to describe the advantage of our domain-based network architecture. In hierarchy-based schemes, the CoA of each sub-MR is based on the prefix of its parent-MR, and every parent-MR is responsible for recording the routing information of its sub-MRs. Therefore, hierarchy-based schemes provide shorter routes and reduce the packet transmission delay than NEMO. However, they still suffer from the suboptimal routing problem if the source and destination MRs are in the same nested mobile network (i.e., intra-domain routing), but they have different parent-MRs. Figure 3a illustrates the inefficiency of intra-domain routing in hierarchy-based schemes. The parent-MRs in such schemes are only responsible for managing the routing information of their sub-MRs. Hence, in the figure, MR3 forwards the packets for MR5 to its parent-MR (i.e., MR2), since it only handles the routing to MNN1 and has no routing information about MR5. The packets are forwarded up the tree until the parent-MR has the routing information for the destination MNN. Therefore, if MNN1 wants to communicate with MNN2, the routing path is: MNN1 → MR3 → MR2 → MR1 → MR4 → MR5 → MNN2. However, there are many shorter routing paths, e.g., MNN1 → MR3 → MR7 → MR5 → MNN2 as shown in Figure 3b.
In addition, hierarchy-based schemes still do not cope with intra-domain handoff well in a nested mobile network. If an MR performs intra-domain handoff, then it suffers from long HL since it needs to perform the local duplicate address detection (DAD) procedure and generate a new CoA. Furthermore, the convergence time is directly proportional to the HL. Therefore, hierarchy-based schemes cannot handle the handoff procedure efficiently, so there is a long convergence time during route optimization. In our domain-based scheme, a network domain consists of an RMR and a set of its descendant MRs. The descendant MRs (i.e., MR2-MR7 in Figure 3b; A:A:A::/56-A:A:F::/56) create their CoAs from the MNP of the RMR (i.e., MR1 in Figure 3b; A:A::/48), rather than the prefix of their parent-MR as in hierarchy-based schemes. The RMR acts as the domain root and manages all descendant MRs in the network domain and every descendant MR records a default routing path to the RMR. It is noted that the RMR will notify the sub-MR to generate a new sub-prefix if the sub-prefix of the sub-MR is not unique in the domain. When an MR moves within the same nested mobile network (i.e., intra-domain handoff), it only updates its RMR with the routing information and it does not need to change its address. Our domain-based scheme reduces the HL substantially because the MR does not need to perform the DAD procedure. Consequently, the nested mobile network in DRO functions like a MANET, and each MR in the network uses existing ad hoc routing protocols to find the optimal paths to communicate with other MRs. At present, if the MR3 has a routing entry to MR5 via MR7, the MR3 can find better routing path to achieve the intra-domain route optimization.
3.3 Inter-domain routing
3.4 Intra-domain routing
DRO works in the same way as hierarchy-based routing schemes before the route optimization procedure is performed. Then, the RMR checks its binding cache. If an entry's network prefix field is equal to the destination's prefix, then the destination MR is located in its nested mobile network and intra-domain route optimization is performed. The RMR sends a notification message to the source MR (i.e., MR5) when the source MR and destination MR (i.e., MR3) are located in the same nested mobile network. Then, MR5 implements the return routability procedure and executes the route optimization procedure based on the ad hoc routing protocols to find the optimal route. For example, in the route optimization procedure, MR5 can send a route request (RREQ) message to find MR3. Then, MR3 replies by sending a route reply (RREP) message to MR5. Since the domain-based network architecture is compatible with all kinds of ad hoc routing protocols, after the route optimization procedure, DRO can find an optimal path from the source to the destination. Moreover, intra-domain route optimization under DRO is not based on tunneling, and the packets for transmission do not require encapsulation from the source to the destination. As a result, DRO reduces the packet transmission delay and the header overhead for encapsulation.
3.5 Inter-domain handoff
Many studies have focused on route optimization for solving the pinball routing problem, but the schemes do not handle inter-domain handoff well. This is a critical problem because the route optimization procedure is performed after the handoff procedure. The convergence time of the route optimization process will be long if the handoff procedure is inefficient. Although fast Mobile IPv6 (FMIPv6)  provides seamless handoff, it may suffer from handoff failure since it only uses a simple link layer trigger to assist the handoff procedure . Moreover, FMIPv6 is not suitable for network environments with multiple ARs because it cannot select the best AR to connect. In contrast, DRO provides reliable and seamless inter-domain handoff by integrating the pre-handoff procedure with the handoff procedure.
The differences between our scheme and FMIPv6 are the number of link layer triggers and the binding update procedure. To overcome the disadvantage of FMIPv6, DRO uses three types of link layer triggers, namely, a link weakness trigger (LWT), a link down trigger (LDT), and a link up trigger (LUT) to ensure successful handoff. In the pre-handoff procedure, the AR broadcasts an RA message, which includes the neighbor advertisement (NB_ADV) periodically. The NB_ADV contains the new CoA of the AR/RMR and the prefix of new AR (NAR)/RMR. When the LWT is triggered, the MR sends a fast binding update (FBU) message to the candidate ARs and performs the DAD procedure using the information of NB_ADV in the RA message before the handoff occurs. The MR confirms that the pre-handoff procedure is finished when it receives the FBACK message. Then, the MR selects the best AR to connect and binds the CoA of NAR to its CN/HA, when the LDT is triggered. At the same time, the packets are forwarded to the NAR from the previous AR (PAR) and the NAR buffers the packets. After the MR connects to the new nested mobile network (i.e., the LUT is triggered), it sends a fast neighbor advertisement (FNA) message to the NAR, and then downloads its packets.
3.6 Intra-domain handoff
When the MR attaches to a different parent-MR in the same nested mobile network, it performs intra-domain handoff. In NEMO, when an MR moves from one subnet to another one, it needs to configure a new CoA and register with its HA, resulting in high HL. Although the hierarchical architecture helps mitigate the problem, each MR still has to configure the new local CoA and register with the RMR. In contrast, when an MR in DRO performs intra-domain handoff, it simply updates the RMR with its routing information and creates a new routing entry between the RMR and itself. The MR does not need to generate a new CoA or send a binding update to its HA because the CoA of each MR is configured according to the prefix of the RMR. Moreover, our scheme reduces the HL from the RMR to the HA of the MR and therefore saves the local DAD time.
3.7 Double buffer mechanism
The route optimization mechanism may affect the performance of TCP because of the out-of-sequence problem illustrated in Figure 2. Since the anticipation schemes in [15, 16] do not fit a dynamic network environment, we use a double buffer mechanism in DRO to avoid the packet out-of-sequence problem. There are two kinds of buffers: a forwarding packet buffer (FPB) and a new packet buffer (NPB). FPB stores the packets from the old link before the optimal route is built, while NPB stores the packets from the new link after the optimal route has been built. The steps of the double buffer mechanism are as follows:
Step 1: The FPB of the MR of the MNN starts to buffer packets when the binding update message is sent by the MR of the MNN.
Step 2: The MR of the CN records a new route entry from the MR of the CN to the MR of the MNN when the MR of the CN receives the binding update message. Then, the MR of the CN replies with a binding update acknowledge (BACK) message to the MR of the MNN. The BACK message includes the sequence number of the last packet that passed through the old link. Then, the packet will be transmitted via the new link.
Step 3: The MR of the MNN receives the packets, checks their sequence numbers, and put them in the corresponding buffer.
Step 4: After the route optimization procedure, the packets in the FPB will be transmitted prior to those in the NPB. Consequently, the MNN receives the packets in sequence.
4. Performance analysis
PD: The PD is defined as the time interval from the time that the CN transmits the packet to the MNN until the MNN receives the packet.
HL: The HL is the disrupt time that an MR changes its association. The total HL is the sum of the movement detection (MD) delay, the DAD delay, the registration delay, and the processing time of the network entities.
POR: The POR means how many packet overheads (i.e., the original packet header plus the tunneling packet header) are occupying in a packet.
TC: The TC is composed of the signaling cost (SC) (e.g., BU, LBU, etc.) and the packet delivery cost.
Parameter values for numerical analysis
The processing delay of MR i
The propagation delay between MR i and MR i+1
The propagation delay between Router i and Router i+1
The processing delay of HA i
The propagation delay between a CN and a router
The propagation delay between an HA and a router
The propagation delay between an MR and an MNN
The propagation delay between an AR and a router
The propagation delay between an AR and an RMR
D MD_MinIn t
The minimum route advertisement interval
The maximum route advertisement interval
The DAD time
The parameter values used in the simulations
1,600 m*1,600 m
Number of MRs
Number of MNN in each MR
Wireless link bandwidth
Moving speed (v)
Route advertisement interval
Radius of wireless cell
IEEE 802.11 DCF
Hello message interval
Packet arrival rate
4.1 PD in inter-domain routing
where n (n ≥ 1) is the number of nesting level of MNN, LD i-j is the propagation delay between entities i and j, and D i is the processing delay of entity i.
4.2. PD in intra-domain routing
In intra-domain routing, we consider two scenarios in hierarchy-based schemes: (1) there is no common parent-MR for the MNN and the CN (e.g., Figure 8b); and (2) the MNN and the CN have k common parent-MRs (e.g., Figure 8c).
where k (k ≥ 1) is the number of common parent-MRs.
4.3 Convergence time of route optimization during inter-domain handoff
4.4 Intra-domain HL
4.5 Packet overhead ratio (POR)
4.6 Total Cost (TC)
In this section, we discuss the TC of route optimization under NEMO, ROTIO, HRO+, and DRO schemes. The TC is composed of the SC and the packet delivery cost. The SC is the sum of the signaling messages for handoff and route optimization procedures, and the packet delivery cost is the sum of data packets sent from the network entities. Moreover, the packet delivery cost is proportion to the hops between the CN and the MNN.
Compared with HRO+ and ROTIO, although our proposed DRO has more TC, their difference is not high. Therefore, we believe it is worth incurring a little extra SC to achieve a better performance (i.e., low PD, low HL, and low POR) for DRO.
4.7 Discussion of double buffer mechanism
5. Concluding remarks
We have proposed a DRO scheme for nested mobile networks. The scheme utilizes a domain-based network architecture and incorporates ad hoc routing techniques to solve the pinball routing problem, reduce HL, and achieve route optimization for NEMO. Moreover, the scheme uses a double buffer mechanism to prevent the out-of-sequence packet problem during the route optimization procedure. We compare the DRO scheme with existing route optimization schemes via numerical analysis and simulations. The results demonstrate that it outperforms the compared schemes in terms of packet transmission delay, inter-domain and intra-domain HL, the convergence time required for route optimization, and the POR.
In our future study, we will investigate two issues. (1) Adjustment of the domain size: we will investigate the optimum domain size to reduce the SC and improve the scheme's performance. (2) Route optimization for new mobility management model: we will consider the route optimization mechanism for network-based localized mobility management (e.g., PMIPv6) in a nested mobile network environment.
aThe lengths of the tunneling header and the original IP header in IPv6 are both 40 bytes. bThe length of address in IPv6 is 16 bytes. cAccording to the CoA of the MR of CN, the MR of the MNN can determine if the MR and the CN are located in the same domain.
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