A Distributed Prefix Allocation Scheme for Subordinate MANET

2008 IEEE Asia-Pacific Services Computing Conference

A Distributed Prefix Allocation Scheme for Subordinate MANET Ashutosh Bhatia Advanced Technology Division Samsung India Software Operations [email protected]

Shubhranshu Singh Advanced Technology Division Samsung India Software Operations [email protected]

Abstract

The proposed method is especially useful in the subordinate MANET scenario where there is no dedicated server to allocate prefixes to each of the MANET nodes. The scheme is designed to ensure that it is easy to implement and deploy. The rest of the paper is organized as follows: Section 2 provides architecture details of subordinate MANET deployment scenario as considered in this paper. Section 3 discusses related work. Section 4 proposes a prefix allocation scheme that is especially useful in the scenario presented in section 2. Section 5 compares proposed method performance with the related approaches. Section 6 presents detail simulation results and analysis. The paper concludes with section 7.

Routers are normally manually configured with IPv6 prefixes in the traditional network. However, due to some of the MANET inherent characteristics such as multi-hop, ad hoc and mobility, manual configuration of prefixes is not desirable and practical. Rather than relying on a dedicated server, a distributed way of allocation is more suitable in the MANET environment. In this paper, we propose a distributed scheme to allocate globally routable and topologically correct IPv6 prefix(es) to MANET routers. The allocated prefixes can then be used for address autoconfiguration of MANET router interface(s) and any associated hosts using IETF defined IPv6 Stateless Address Autoconfiguration mechanism. Proposed scheme is implemented using ns-2.31. Simulation results for protocol signal overhead and configuration latency has been analyzed in detail.

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In this paper, we consider subordinate MANET deployment scenario. Subordinate MANET is a MANET which is connected to one or more external network(s) and where such external network(s) imposes addressing hierarchy scheme on the MANET. In this deployment scenario MANET connects to the Internet or other external networks via one or more Border Routers. When these border routers are connected to the Internet, they impose additional requirements on the address autoconfiguration mechanism such as assigned addresses should be topologically correct and globally routable. Nodes are often mobile with wireless interfaces and are free to join or leave the network any time. The proposed method is especially applicable in the subordinate MANET scenario where there is either no dedicated server or a distributed way of allocating prefixes is required in order to allocate prefixes to each of the MANET nodes in the spirit of MANET core design tenets. From the IP link model perspective, this paper assumes MANET as a cloud of individual links. Link is defined as a topological area bounded by routers that decrement IPv4 TTL or IPv6 Hop Limit when forwarding the packet. We have considered MANET as a site containing multiple subnets where each MANET node configures its MANET interface with unique (disjoint) prefix(es). This requirement

INTRODUCTION

A Mobile Ad hoc NETwork (MANET) is a self configuring network of mobile devices. Due to node mobility, ad hoc nature and wireless interface characteristics, the network topology changes rapidly and unpredictably. MANET is deployed either as an autonomous network or subordinate network which is connected to a lager external network. In the recent past, there has been significant increase in the interest of subordinate MANET scenario where MANET is connected with the Internet. The connection of MANET to the Internet is typically established via Border Router (BR). BR is the router with at least two interfaces connecting MANET and the Internet. To communicate with the Internet node, the configuration of only MANET scope addresses are not desirable. The nodes need to be configured with global and topologically correct IP address(es). The main purpose of this paper is to allocate IPv6 prefixes to each of the MANET nodes. A distributed prefix allocation scheme has been proposed which uses Internet Control Message Protocol (ICMPv6) [4] messages. 978-0-7695-3473-2/08 $25.00 © 2008 IEEE DOI 10.1109/APSCC.2008.261

MANET DEPLOYMENT SCENARIO

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is to avoid multi-link subnet issues discussed in [11]

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taining an allocation table and synchronizing them, each node maintains a generation function and a state value to generate a sequence of numbers (addresses). Thus address allocation is totally decentralized and generates zero traffic. The advantages of this approach are zero signal overhead and simple to implement. The paper does not provide any analytic proof if the described function fulfills the necessary conditions mentioned. In addition, the approach is only applicable for large address space, and the utilization of the available address space is not efficient.

RELATED WORK

Previous research in the area of MANET address autoconfiguration primarily focuses on three problems: address generation, Duplicate Address Detection (DAD) and network merges/partition detection. Address assignment in MANET could be classified as stateful or stateless approaches according to the management of the address space. For stateful approaches, the state of each address is kept in such a way that the network has a vision of assigned and non assigned addresses thus address duplication is easily avoided. For stateless approaches, each node randomly chooses its own address and performs the DAD test to ensure that the chosen address is not already configured by another node in the same subnetwork. The Agent Based Addressing proposed in [6], is a stateful autoconfiguration protocol based on a centralized allocation table. In this protocol only one node, the Address Agent (AA) is allowed to assign addresses to requesting nodes, thus mandating that it should be always reachable. The advantage of this protocol is that it guarantees address uniqueness. However, there are some disadvantages of this protocol as well. First, at the initialization phase, the new node is involved in a unicast communication with the AA even though it has no IP address to initiate the communication. How this problem could be solved is not mentioned. Second, it generates a large overhead due to periodic flooding of ’Verify’ messages and their corresponding unicasts from each nodes sent toward the AA. Third, the protocol is too centralized because of the high dependency on the Address Agent (AA). In contrast with Agent Based Addressing where only one node is responsible for assigning addresses and maintaining the allocation table, the idea in MANETconf [7] is based on a common distributed address table, where each node is able to assign IP addresses and maintains an allocation table that contains already allocated addresses and pending allocations. Thus, the synchronization of these distributed tables constitutes the most critical and complex task of this protocol. The advantages of this protocol are that it guarantees address uniqueness and it is totally distributed since each node could possibly assign addresses. The disadvantages of this protocol are its complexity and overhead in term of communication, table maintenance and synchronization. The mechanism requires network-wide flooding and a large number of unicast messages. All nodes should give their permission to the initiator to assign a new address which could result in significant delay. Also, this protocol is very sensitive to network losses because of its dependency on unicasts communications. The main idea proposed in [12] is that rather than main-

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Proposed Prefix Allocation Scheme

4.1

Protocol Operation

In our proposed mechanism, any MANET node can be client as well as server. We refer the MANET router requesting an IP prefix(s) as client node and the MANET node that provides IP prefixes as server node. This is only to facilitate to view the proposed prefix allocation protocol as a handshaking protocol between the server and the client. In the proposed mechanism, we assume that initially there is a pool of IPv6 prefixes allocated to ad hoc network. For example, the service provider initially allocates a fixed range of IPv6 prefixes to the Border Router. From Border router rest of the MANET router gets unique prefixes allocated using the mechanism proposed in this paper. The client may initiate prefix request process in two cases. First, while bootstrapping MANET router has to acquire a prefix or set of prefixes for their MANET and Non-MANET interfaces. Second, when the prefix pool is empty and the client requires new pool of prefixes in order to further allocate them to the arriving nodes. Figures 1 and 2 shows the events flow diagram of an unconfigured node and configured node respectively. Below is step by step detail of the protocol operation. 1. The client periodically sends link local multicast SOLICIT message to ALL-NODE multicast address. If it does not receive any ADVERTISE message within a fixed period it resends the SOLICIT message. After a fixed number of failure attempts, the client assumes that there is no free IP prefix available in the network. 2. On reception of SOLICIT message from the client each node waits for a random amount of time before sending advertise message. While waiting to send ADVERTISE message if a node, say A listens ADVERTISE message from another node, say B then the node A will not send the ADVERTISE message to the client and terminate the process. Otherwise, on expiration of waiting time node A will respond with ADVERTISE message to the client. The random delay in sending

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Figure 1: Flow Diagram of an unconfigured node behavior

Figure 2: Flow Diagram of a configured node behavior

the ADVERTISE message has two advantages. First, it desynchronizes the transmission of ADVERTISE message from different servers, thus reduces the collision. Second, since ADVERTISE message is destined to ALL-NODE multicast address only few nodes respond. This decreases signal overhead and also overall power consumption.

1. If a client node can predict its movement in advance then it informs its departure along with its pool of prefixes by sending LEAVE message to any of neighbor node. 2. When a server receives LEAVE message it extracts the prefix pool from the received message and adds to its own pool of prefixes. After that server sends CONFIRM message back to the the client.

3. On receipt of ADVERTISE message, the client sends unicast PREFIX-REQUEST message to the server. It ignores other ADVERTISE messages, received after PREFIX-REQUEST sent.

3. On receipt of CONFIRM message from the server, client sends ACK to the server and leaves the network.

4. On receipt of PREFIX-REQUEST, the node acting as server divides its prefix pool into two subsets and assign one of them to client node through PREFIXREPLY message.

4.3

We used ICMPv6 protocol message extension by introducing new type value (140) for set of messages related to prefix allocation in MANET. Each individual message from this set is identified by different code value in ICMPv6 message. Figure 3 shows the possible extended ICMPv6 message format with new type value for the proposed prefix allocation protocol.

5. If the client receives PREFIX-REPLY message from the server and if the client is new node then it constructs the global IP address using received prefix similar to IPv6 stateless autoconfiguration. Otherwise, if client is already configured node and requested for additional pool of prefix, it keeps all the prefixes as the available prefix pool to be further delegated.

4.2

Protocol Messages

• Solicit (type-140, code-0): This is a link local multicast message that the client sends periodically to start configuration process or to acquire additional pool of prefixes when its prefix pool is empty. The source of this message is link-local address of client and the destination is ALL-NODE Multicast address.

Departure of nodes

MANET nodes are free to depart from network at any point of time. Wherever possible, when a node departs it should release its pool of prefixes and give to one of the neighbor node to avoid wastage of prefixes. Our proposed mechanism addresses this requirement. Following is the handshake procedure that takes place between client and server nodes. It is a three way handshake mechanism to ensure proper handover of prefixes.

• Advertise (type-140, code-1): This message is link local multicast message that the server sends in response to received solicit message from the client. The source of this message is link local address of server and destination is ALL-NODE multicast address. This message lets the client to identify the server. The

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We compare our proposed protocol with related approaches discussed in this paper based on technical metrics that influence the performance of address autoconfiguration mechanism for Mobile Ad hoc Networks (MANET).

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Performance comparison of proposed protocol with related approaches

• Signal Overhead: this is one of the most important metrics; it also influences the power consumption. The overhead should be described by the required bytes per node; but since some protocols uses packets of variable size and requires two types of communications (periodic floods and per address assignment communications), we have divided the overhead into periodic flood and per address assignment overhead and computed it by number of packets per node.

Figure 3: Extended ICMPv6 message format for MANET prefix allocation server puts link local address of the client performing solicitation, as data in advertise message. This helps the client to identify that the message is destined for it.

• Latency: it is the time spent before configuring a node with a valid IP address. It is important to note that we assume here a reliable medium with zero loss.

• Prefix-Request (type-140, code-2): This is a link local unicast message sent by client performing either address configuration or requesting additional pool of prefixes, to the server. The source is link local address of the client and the destination is link-local address of the server it is interacting with.

• Sensitivity to network loss: network losses are inevitable in mobile ad hoc networks. Autoconfiguration protocols requiring long communications and excessive unicasts are the most sensitive to network losses. Higher sensitivity to network losses involves additional overhead and increased delays.

• Prefix-Reply (type-140, code-3): This is link-local unicast message sent by server to the client in response to prefix-request message received. The source of this message is link-local address of server and the destination is link-local address of the client. In this message the server replies with the size and starting value of prefix block for the allocated pool of prefixes.

• Scalability: This metric is related to the communication overhead and address allocation latency. If the autoconfiguration mechanism requires excessive communications and periodic floods the mechanism would not be scalable, also if the address configuration latency is high the mechanism will not scale well. Table 1 illustrates the comparison between existing approaches based on the overhead, latency and sensitivity to network losses. We assume here zero packet loss. The following notation is adopted:

• Leave (type-140, code-4): This is a link local unicast message sent by client to the server (any neighbor node) when it decides to leave the network. The source is link-local address of the departing node and the destination address is of any neighbor node chosen by client.

• N: total number of nodes • d: the average diameter of the network

• Confirm (type-140, code-5): This is a link local unicast address that is sent by server to the client confirming client that it can leave the network. The source is link local address of server and destination is link local address of client leaving the network.

• l: the average number of neighbors • T: the period of synchronization, flood, or any repetitive procedure if exists • t: the round trip time for one hop communication

• Ack (type-140, code-6): This is the final acknowledgment sent by client in response to confirm message received from the server. The receipt of this message by server confirms the departure of client from network.

For example, if we take the Agent Based Addressing; it requires a request/reply communication with the Address Authority (AA) for each address assignment. If we consider a randomly placed node within the network; it will be

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Agent Based MANET Conf Prophet

Proposed Protocol

Overhead Per Address Assignment d Packets Per address assignment 2l + 2 ∗ N + N ∗ d/2

Periodic Latency Flood Yes T /2 + d ∗ t/2 No (2 + d) ∗ t

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configuration latency, we performed simulation over multiple distribution pattern and took average performance. The transmission range is kept at 100 m. The initial size of prefix pool available at Border Router (BR) is 128. Inter arrival of new nodes is uniformly distributed within the range 0 to the number of nodes for a particular simulation. The underlying routing protocol used is Ad hoc On-Demand Distance Vector (AODV) [9], although our protocol makes no assumption about underlying routing protocol. Simulation is run until all the nodes for a particular simulation scenario gets configured. 1000 "scatter"

Address Allocation Latency (ms)

Table 1: Performance Comparison on average d/2 hops away from the AA. As a consequence the request/reply communication requires 2 packets relayed d/2 times each (d transmission). On the other hand in proposed protocol overhead per address assignment does not depend upon average network diameter, therefore imposes less overhead and easily scalable. Also, Agent Based Addressing requires the AA to flood the network periodically so N packets will be emitted but the proposed protocol does not require periodic flooding. For the latency, each node has to wait for receiving a Verify packet from the AA before initiating its request, as an average it have to wait for T/2 time units given T the flood period then the request/reply communication will take d*t/2 time units because its a communication between d/2 hops away nodes, while proposed protocol requires only local communication (one hop) reduces the address allocation latency. Also it is not dependent on average diameter of network, therefore easily scalable.

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Figure 4: Address Allocation Latency Distribution

6.2

Prefix Allocation Latency

Figure 4 shows a scatter plot of prefix/address allocation latency for one of our simulated network with 100 nodes. More than 60% of nodes got configured in their first attempt. This is because either there was no neighbor node available while performing solicitation or non of the neighbor node has non-empty prefix pool. Figure 5 shows the effect of number of nodes on average allocation latency, keeping prefix pool size fixed at 128. As per the graph maximum average configuration latency is 600 ms for 100 nodes which is a good measure for practical network deployment. Increase in the number of nodes does not effect much on address allocation latency. The graph is initially flat and after a certain point it increases linearly. This is because as the number of nodes increase keeping total prefix pool size fixed, the number of neighbor nodes capable of delegating prefixes decreases. The linear nature of latency graph shows the scalability of proposed autoconfiguration solution using prefix allocation is quite scalable with respect to allocation latency.

Simulation Experiment

To perform simulation experiment and analyze the performance of our proposed solution, we used ns-2 [8] network simulator. The primary focus of the simulation experiment is to gather statistics regarding average prefix/address configuration latency and protocol message overhead.

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Simulation Setup

We simulated MANET over IEEE 802.11 MAC/PHY module. Simulation is performed with random way-point mobility model [1]. While moving from a starting point to a randomly chosen destination the speed is kept constant at 5 m/s. After reaching destination the pause time is 5 s. Then another destination is chosen randomly and same sequence is repeated until simulation ends. Network of size 500m X 500m is simulated with number of nodes varying from 10 to 100. Since the distribution pattern of nodes also effects the

6.3

Signal Overhead

We used only link scoped messages (Multicast and Unicast) in our solution. The link scope messages incur least

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configuration mechanism. Considered deployment scenario is subordinate MANET where there is no dedicated server available and a distributed way of allocating prefixes is required. Proposed mechanism is implemented and analyzed using network simulator ns-2.31.

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[1] J. Broch, D. A. Maltz, D. B. Johnson, Y.-C. Hu, and J. Jetcheva. A performance comparison of muilti-hop wireless ad hoc routing protocols. Proceedings of the Fourth Annual ACM/IEEE International Conference on Mobile Computing and Networking, pages 85–97, October 1998. [2] I. Chakeres, J. Macker, and T. Clausen. Mobile ad hoc network architecture. draft-ietf-autoconf-manetarch-07, IETF, Nov 2007. [3] CMU. http://www.monarch.cs.rice.edu/cmu-ns.html. Proc. IEEE INFOCOM 2003. [4] A. Conta, S. Deering, and M. Gupta. Internet control message protocol (icmpv6). RFC 4443, March 2006. [5] S. Droms, J. Bound, B. Volz, T. Lemon, C. Perkins, and M. Carney. Dynamic host configuration protocol for ipv6 (dhcpv6). RFC 3315, July 2003. [6] M. Gnes and J. Reibel. An ip address configuration algorithm for zeroconf mobile multihop ad hoc networks. Proc. Intl. Wksp. Broadband Wireless Ad Hoc Networks and Services, Sept 2002. [7] S. Nesargi and R. Prakash. Manetconf: Configuration of hosts in a mobile ad hoc network. Proc. IEEE INFOCOM, Mar 2003. [8] NS2. The network simulator ns2. http://www.isi.edu/nsnam/ns. [9] C. E. Perkins, E. M. Belding-Royer, and S. Das. Ad-hoc ondemand distance vector (aodv) routing. RFC 3561, Internet Engineering Task Force, July 2003. [10] S.Thomson, T. Narten, and T. Jinmei. Ipv6 stateless address autoconfiguration. RFC 4862, September 2007. [11] D. Thaler. Multi-link subnet issues. Internet Engineering Task Force, June 2007. [12] M. Zhou, L. M. Ni, and M. W. Mutka. Prophet address allocation for large scale manets. Proc. IEEE INFOCOM 2003, Mar 2003.

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Figure 5: Average Prefix Allocation Latency

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amount of communication overhead and utilization of network resources with respect to site scope messages. Figure 6 shows the effect of number of nodes on average number of protocol message overhead per prefix configuration, keeping prefix pool size fixed at 128. As per simulation results, increase in the number of nodes does not effect much on address allocation overhead until it reaches more than half of total available prefixes. After this point most of the already configured nodes having prefix pool empty initiates the process of acquiring new prefix pool from their neighbor nodes. This increases the signal overhead considerably. The result shows that the proposed solution is quite scalable in terms of signal overhead for the number of nodes less than the half of the total number of available prefixes. In this case we can say that the prefix utilization is 50%. 50 Average Overhead 45

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Figure 6: Average Prefix Allocation Signal Overhead

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Conclusion and Future work

MANET routers require a unique prefix to configure their MANET interface(s) and to advertise them at their non-MANET interface(s). In this paper, we proposed a distributed mechanism for IPv6 prefix(es) allocation to MANET routers. Allocated prefix(es) can then be used for address autoconfiguration of MANET router interface(s) and any attached hosts using IETF defined stateless auto-

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