Research on all-IP communication between wireless sensor networks and IPv6 networks

Computer Standards & Interfaces 35 (2013) 403–414

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Computer Standards & Interfaces journal homepage: www.elsevier.com/locate/csi

Research on all-IP communication between wireless sensor networks and IPv6 networks Wang Xiaonan a,⁎, Qian Huanyan b a b

Changshu Institute of Technology, Jiangsu, Changshu 215500, China Nanjing University of Science & Technology, Jiangsu, Nanjing 210094, China

a r t i c l e

i n f o

Article history: Received 2 June 2012 Received in revised form 24 October 2012 Accepted 28 December 2012 Available online 31 January 2013 Keywords: Wireless sensor network IPv6 network Address configuration Routing Mobility

a b s t r a c t This paper proposes a scheme for achieving all-IP communication between wireless sensor networks and IPv6 networks. The network architecture based on trees is proposed. Based on the architecture, the hierarchical IPv6 address structure is created and the address configuration algorithm is proposed. During the address configuration process, the trees are also established simultaneously. Based on the hierarchical address structure, the routing algorithm is proposed. In the algorithm, the routing can be performed automatically through the trees in the link layer. The paper analyzes the scheme's performance parameters and the data results show that the performance of the scheme is better. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Currently, the scalable and interoperable communication mechanism is urgently required in order to support the rapid development of WSN (wireless sensor network) applications. IPv6 is the protocol of the nextgeneration Internet and has many advantages, such as abundant address resources and mobility support, so all-IP communication between WSN and IPv6 networks has become an inevitable trend in the future. All-IP WSN can be applied to many fields. For example, in the healthcare field, a doctor can communicate with each sensor node attached to a patient in the end-to-end way through the Internet to acquire the patient's condition parameters in order to take timely treatment. At present, the following key technologies on achieving all-IP communication between WSN and IPv6 networks need further researches [1]. 1.1. Address configuration The typical IPv6 address configuration protocols, including the stateful address configuration [2] and stateless address configuration [3], are not suitable for all-IP WSN. For example, in the stateful address configuration, the server/client communication mode is used to achieve

⁎ Corresponding author. E-mail address: [email protected] (X. Wang). 0920-5489/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.csi.2012.12.001

the address configuration. The stateless address configuration is based on the neighbor discovery protocol [4], and the uniqueness of each assigned address is ensured by DAD (duplicate address detection) [5] which leads to a lot of control messages. Therefore, it is necessary to establish a new IPv6 address configuration mechanism for all-IP WSN. 1.2. Routing mechanism The current WSN routing protocols are based on the data-centric working mechanism, so they are not suitable for all-IP WSN with the address-centric working mechanism. Moreover, the WSN architecture is different from the IPv6 network architecture. For example, a sensor node works as both an ordinary node and a router while an IPv6 node works only as an ordinary node. One IPv6 hop usually contains all nodes sharing the same IPv6 subnet prefix while one WSN hop includes all nodes that are reachable by a sender's symmetric radio range. Therefore, it is difficult to apply the existing IPv6 routing protocols to all-IP WSN, and it is necessary to establish an IP-based routing mechanism to achieve all-IP communication between WSN and IPv6 networks. 1.3. Mobility The tunnel-based mobility protocols, such as HMIPv6 [6], are not suitable for WSN because a node has to send/receive a lot of control information to ensure the communication continuity. Similarly, the routing-based mobility schemes, such as HAWAII [7], cannot work

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in WSN because a node needs to periodically send control messages to achieve the mobility handover. In addition, the existing mobility protocols are performed in the network layer [8] and each control message must include one IPv6 header, so the transmission cost is increased. Therefore, it is necessary to propose a low-overhead mobility support scheme for all-IP WSN. In this situation, this paper proposes a scheme for achieving all-IP communication between WSN and IPv6 networks, and it has the following contributions. 1) The network architecture based on IPv6 ingress gateway trees is proposed. Based on the architecture, the hierarchical IPv6 address structure is created and the address configuration algorithm is proposed. In the algorithm, a child node obtains a unique IPv6 address from its parent node without duplicate address detection, and the address configuration in the different branches of a tree can be performed simultaneously. During the address configuration process, the IPv6 ingress gateway trees are also established simultaneously, so the routing cost and delay are saved. 2) Based on the hierarchical address structure, the routing algorithm is proposed. In the routing algorithm, the routing can be performed automatically through the IPv6 ingress gateway trees in the link layer without either route discovery or route establishment, and the intermediate nodes only deal with the frame header and the mesh addressing header without processing the headers in the above layers, such as the fragment header or IPv6 header. The fragments of an IPv6 packet except the first fragment do not need to include the IPv6 header, so both the fragment utilization and the routing efficiency are improved. 3) A mobile node only works as a leaf node and its position change has no influence on the routing paths, so the routing robustness is enhanced. This scheme is based on our previous works [9–11], and the differences between the proposed scheme and the previous works are as follows. 1) In the work found in [9], the all-IP communication scheme is based on sensor nodes' location information, so the network architecture, the address configuration algorithm, the routing algorithm and the mobility algorithm are all different from the ones in the proposed scheme, as shown as follows. • The network architecture in the previous work is based on the logical grids and the one in the proposed scheme is based on an IPv6 ingress gateway tree. • In the previous work the location information on sensor nodes is utilized to achieve the address configuration. In the proposed scheme, the hierarchical IPv6 address structure based on the network architecture is presented, and a sensor node obtains a unique address through joining an IPv6 ingress gateway tree. • In the previous work, the location information on sensor nodes is utilized to achieve the routing discovery and establishment. In the proposed scheme, the routing is performed automatically through IPv6 ingress gateway trees without either routing discovery or establishment. • In the previous work, a mobile sensor node is always identified by its home address during the mobility process. In the proposed scheme, a mobile node works as a leaf node of an IPv6 ingress gateway tree, and it needs a temporary address to achieve the mobility handover process. 2) The works [10,11] only focus on the address configuration and they do not address the routing and mobility issues. The differences between the works [10,11] and the proposed scheme are as follows. • The network architectures are different. In the work found in [10], the network architecture is flat and in the work found in [11] the network architecture is based on clusters. In the

proposed scheme, the network architecture is hierarchical and it adopts the tree topology where a mobile node only works as a leaf node. • The address configuration algorithms are different. In the work found in [10], it is a gateway that manages all the address resources and is responsible for the reclaim and reassignment of the address resources. In the work found in [11], it is a cluster head that manages all the address resources and assigns the addresses for its cluster members. In the proposed scheme, a node obtains a unique address through joining an IPv6 ingress gateway tree. The remainder of this paper is organized as follows. In Section 2, the related work on all-IP WSN is discussed. We discuss the scheme on achieving all-IP communication between WSN and IPv6 networks in Section 3, and analyze the performance of the scheme in Section 4. We conclude this paper with a summary in Section 5. 2. Related work Reference [12] proposed an address configuration scheme based on location information, but the scheme was built on IPv4 and was not suitable for all-IP WSN based on IPv6. Reference [13] proposed a proxy-based IPv6 address configuration scheme, and the experimental results showed that the scheme effectively shortened the address configuration delay and reduced the power consumption. The scheme adopted the random ID and the time stamp to identify a new node. If multiple nodes with the same ID and the same time stamp sent address request messages to the same proxy node simultaneously, then the proxy node was unable to distinguish between these nodes. In addition, NS (Neighbor Solicitation) and NA (Neighbor Advertisement) were adopted to communicate between a new node and a proxy node while REQ and REP were used to communicate between a proxy node and a gateway node. As a result, a proxy node had to store the mapping between these two kinds of messages, so the address configuration delay was increased. In the scheme [14], the transmission scope of control messages was controlled within twohop scope, so the address configuration cost was reduced and the delay was shortened. However, the scheme was unable to reuse the address resources occupied by failed proxy nodes. If a proxy node's address resources ran out, it broadcasted an address recovery message in the whole network. After a proxy node received the message, it also broadcasted an address recovery message in the whole network. If a node's address was within the recovery address space, then the node returned a response message to show its normal work state. During one address recovery process, all the proxy nodes in the network almost needed to broadcast a message, so a lot of network resources were consumed. In the scheme [15], a node acquired its location information by GPS, and DAD was performed through the distributed DAD servers. Since the resources of sensor nodes were limited, it was difficult for a sensor node to work as a DAD server. Reference [16] adopted the mapping between 16-bit short addresses and 64-bit EUI (Extended Unique Identifier) addresses to achieve the address configuration. However, when a node moved from one subnet to another subnet, the address conflict might be incurred. Reference [17] proposed the address configuration scheme based on multi-way trees. The scheme required one parameter (MC, the maximum number of child nodes) to achieve the address configuration, so it limited the WSN scalability. Reference [18] pointed out that 6LoWPAN routing and mobility remained an open issue. References [19,20] defined the reduced IPv6 stack where the adaptation layer was introduced to achieve fragment and reassembly of an IPv6 packet. Reference [21] proposed a scheme for achieving all-IP WSN, but it did not discuss the address configuration algorithm and the routing algorithm. The scheme [22] set up a number of

X. Wang, H. Qian / Computer Standards & Interfaces 35 (2013) 403–414

IPv6 ingress gateway

which was supposed to store the information on neighbor PAN coordinators. According to the information, a node's partner node performed the pre-configuration process in order to shorten the delay. If a node moved relatively fast or its mobile angle suddenly changed, then the pre-configuration process might fail. To sum up, the above schemes have the following shortcomings.

IPv6 ingress gateway tree

Fixed sensor node

Mobile sensor node Fig. 1. IPv6 ingress gateway tree.

mobile routers in WSN which performed the mobility management. The scheme added the type of IPv6 load (Mobility header) by modifying 6LoWPAN dispatch to achieve the mobility handover, which was not in line with the layered design principle of IPv6. SPMIPv6 (Sensor Proxy Mobile IPv6) [23] presented the network architecture and the message format, and also evaluated the performance including signaling cost, mobility cost and energy consumption. The analytical results showed that SPMIPv6 reduced the energy consumption significantly. Reference [24] proposed to adopt the link attributes of IEEE 802.15.4 [25] to enhance the performance of all-IP WSN. Border routers stored routing information on all sensor nodes in all-IP WSN and the extension header, Routing Header, was included in each packet to achieve the routing. Therefore, the data transmission power consumption was increased and the fragment efficiency was reduced. Reference [26] proposed a scheme for routing discovery and maintenance in low-power and lossy networks. The scheme discussed how to establish routing paths reaching the destination subnet, but it did not analyze the relationship between the hierarchical structure of IPv6 addresses and the routing process. In the scheme [27], the source node established multiple disjoint routing paths reaching the destination node through one routing discovery process and then ranked the disjoint routing paths according to the accumulated link costs of each path. The routing path with the minimum link costs was selected as the primary path. If the primary path failed, then the second best routing path was chosen as the primary path to continue routing the data. The scheme effectively reduced the power consumed by reestablishing failed routing paths, but maintaining multiple disjoint routing paths increased the power consumption and reduced the routing performance. Reference [28] proposed the 6LoWPAN mobile support scheme which reduced the mobility handoff cost and the tunnel establishment cost. The scheme employed Dispatch type to determine source/destination of one packet, and it also added one header structure between the adaptation layer and the IP layer. Therefore, the transmission power was increased. In the scheme [29], each mobile node was equipped with a partner node

1) The address configuration does not help achieve either the routing discovery or the routing establishment. In the above schemes, the address configuration process is independent of the routing process, so the performance of WSN is influenced by the address configuration cost and routing cost. 2) The above routing schemes do not utilize the hierarchy of an IPv6 address structure to improve the routing performance. 3) In the above mobility support scheme, a routing path is made up of multiple mobile nodes. As a result, if a mobile node in a routing path changes its position, then the routing discovery has to be performed to establish a new path. 3. All-IP communication 3.1. Network architecture IEEE 802.15.4 defines two nodes, FFD (full-function device) and RFD (reduced-function device). In the scheme, the MAC protocol adopts IEEE 802.15.4, and one WSN includes three kinds of sensor nodes. • IPv6 ingress gateway: it connects WSN to the IPv6 networks. • Fixed sensor node: it has routing/forwarding function. • Mobile sensor node: it has no routing/forwarding function. Among them, a gateway/fixed node is a FFD, and a mobile node is a RFD. Multiple gateways in one WSN communicate with each another in the multicast way through the IPv6 networks. One gateway and multiple sensor nodes form a tree structure which is called an IPv6 ingress gateway tree. The root node is the gateway, the intermediate nodes are the fixed nodes and the leaf nodes are the fixed/mobile nodes, as shown in Fig. 1. In the scheme, the total number of gateways and fixed nodes in a WSN is determined by the network size, and all the gateways and fixed nodes should cover the WSN area. For example, a hospital is covered by fixed nodes, and each patient is equipped with multiple sensor nodes used to collect medical parameters such as blood pressure and heartbeat frequency. The fixed node closest to an access router in the IPv6 Internet can be selected as a gateway and connect the WSN to the IPv6 Internet. In this way, a doctor can use the Internet to acquire real-time parameters of patients for monitoring. Compared with the existing tree architectures [30,31], the proposed IPv6 ingress gateway tree has the following strengths. 1) The proposed architecture is compatible with the IPv6 network architecture. For example, a fixed node corresponds to an IPv6 router, as shown in Fig. 2.

IPv6 router IPv6 subnet 1

405

IPv6 router IPv6 subnet 2

Fixed sensor node

IPv6 subnet 3 Mobile sensor node

Fig. 2. IPv6 network architecture and IPv6 ingress gateway tree.

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X. Wang, H. Qian / Computer Standards & Interfaces 35 (2013) 403–414

Table 1 IPv6 address structure. 64 bits

n bits

i bits

(64-n-i)bits

Global routing prefix

Parent ID Sensor node ID

Node ID

Padding(0)

density of sensor nodes and the scale of WSN. Parent ID is divided into multiple levels, and i bits represent one level. The level of an IPv6 ingress gateway is 1, and the level of its child nodes is 2, and so on. A node can calculate its level l according to formula (1). l ¼ n=i þ 1

2) In the proposed architecture, the hierarchical IPv6 address structure can be employed to achieve the IPv6 address configuration. 3) The hierarchical IPv6 address structure can be used to improve the routing performance. 4) The position change of a mobile node has no influence on the routing paths. In addition, in the proposed architecture, a mobile sensor node only communicates with its parent node, so flooding is avoided. When the number of fixed sensor nodes which constitute the backbone routing network is relatively small, the routing cost is also reduced. 3.2. IPv6 address configuration 3.2.1. IPv6 address structure According to the hierarchical structure of an IPv6 address and the characteristics of WSN, the scheme adopts the following IPv6 address structure for WSN, as shown in Table 1. An IPv6 address includes two parts. The first part is global routing prefix which is 64-bit long. The global routing prefixes of all sensor nodes in one all-IP WSN are identical, and the value is equal to the global routing prefix of the gateway in the same WSN. The second part is sensor node ID which uniquely identifies a sensor node in one all-IP WSN. Sensor node ID is made up of three parts: parent ID which is n-bit long, node ID which is i-bit long and padding whose value is 0. The writing format of sensor node ID is the same as that one of an IPv6 address. Routing is based on the IPv6 address structure. If gateways in WSN can dynamically change the value of i, then both the address configuration algorithm and the routing protocol have to be updated. As a result, a lot of costs can be incurred and the routing error can also be caused. Therefore, the value of i is predetermined according to the

3FE8:1:1:3::/64

ð1Þ

When i is equal to 4, one all-IP WSN can include up to 15 (except 0) trees whose depth is up to 16, and a node in a tree can have up to 15 (except 0) child nodes. For example, if the sensor node ID of a node is 1000:0000:0000:0000 (abbreviated as 1000::), then the sensor node IDs of its child nodes are 1x00::, where x = 1, 2… F, as shown in Fig. 2. In general, a sensor node in WSN is used to collect the physical parameters. In the scheme, in the situation that fixed sensor nodes are uniformly distributed, if the address resources in one area are exhausted, it means that the density of sensor nodes which have been configured with an IPv6 address in the area is large enough and that these sensor nodes are able to cooperate to accomplish the data collection task. For example, in the case that i is equal to 4, if a node's address space is exhausted, then there are 15 nodes within the node's one-hop scope. Moreover, when the node density is very high, i can be set to a relative large value to avoid the address exhaustion. In the scheme, when a node cannot obtain an IPv6 address due to the address space exhaustion, it returns to a dormant state. After some addresses are released by failed nodes, a node can acquire an IPv6 address and continue the data collection task. In this way, WSN life span can be extended. In the scheme, the sensor node ID, which is acquired by a mobile node when joining one all-IP WSN, is called the node's permanent ID, and the tree where a mobile node acquires its permanent ID is called the node's home tree. The sensor node ID, which a mobile sensor node acquires when it leaves its home tree and moves to another tree, is called the node's temporary ID, and the tree where a node acquires its temporary ID is called the node's temporary tree. 3.2.2. Acquisition of a sensor node ID A node acquires its sensor node ID by joining an IPv6 ingress gateway tree. A node X adopts the following address configuration algorithm to obtain its sensor node ID: 1) X broadcasts a request beacon frame whose payload is empty.

IPv6 Internet

3FE8:1:1:1::/64 3FE8:1:1:1:1000::/68 2100::

SensornodeID:1100::

3FE8:1:1:1:2000::/68

Y 1200:: 2300:: 1110:: 2200:: 2210:: 2220::

1210:: 1211::

X acquires a sensor node ID: 1220::

Fig. 3. A sensor node acquires a sensor node ID.

X. Wang, H. Qian / Computer Standards & Interfaces 35 (2013) 403–414

2) Within the coverage area of the request frame, a fixed node Y which has the sensor node ID receives the frame. If Y has the address resources, then it returns a response beacon frame whose payload is the assigned sensor node ID, marks the state of the assigned sensor node ID as Occupied and records X's type (fixed or mobile). 3) Within the specified time, X receives multiple response beacon frames. X calculates the distance from the fixed node returning a response frame to the root node of the corresponding tree, then selects the node F with the shortest distance as its parent node, and returns an ACK frame to F. If X is a fixed node, then it sets its sensor node ID to the one assigned by F. If X is a mobile node and its permanent ID is 0 (that means that X is a new mobile node), then X sets its permanent ID to the sensor node ID assigned by F and does the permanent ID registration operation (as shown in Section 3.2.3.1). If X is a mobile node and its permanent ID is neither 0 nor the assigned sensor node ID, then it sets its temporary ID to the assigned sensor node ID and does the temporary ID registration operation (as shown in Section 3.2.3.2). If X is a mobile node and its permanent ID is equal to the assigned sensor node ID, then X sets the temporary ID to 0 and does the temporary ID registration operation. 4) After F receives the ACK frame, it confirms that X becomes its child node. 5) In this way, X successfully acquires its sensor node ID. In the scheme, only a fixed node which has the sensor node ID can return a response beacon frame and the address configuration begins from the nodes in the second level of a tree. After the nodes in the second level are configured, the nodes in the third level can receive a response beacon frame from neighbor nodes in the second level. As a result, the specified time is a function of the maximum number d of the levels in the tree, and is set to α ⋅ d ⋅ t. Among them, α is an adjustment coefficient and it is determined by the characteristics of WSN and the physical features of a sensor node, d is set to 64/i, and t is the delay taken by transmitting a frame between two neighbor nodes. In the above process, when a fixed sensor node returns a response beacon frame to X, it marks the state of the assigned sensor node ID as Occupied. In this way, even in the situation that the ACK frame is lost, the duplicate assignment of the same address can be avoided. If a fixed sensor node does not receive a beacon frame from X, then it can determine that X selects another fixed node as the parent node, and recover the assigned sensor node ID. In the scheme, the process of a node acquiring a sensor node ID is also the one of it joining a tree, as shown in Fig. 3. In Fig. 3, after a node X broadcasts a request beacon frame, the fixed nodes with the sensor node IDs 1210::, 1211::, 2220:: and 1200:: respectively return a response beacon frame. X selects the node Y with the sensor node ID 1200:: as its parent node and acquires its sensor node ID 1220::. In the scheme, a node selects its parent node from its neighbor fixed nodes due to the following reasons: 1) Reduce the address configuration cost and delay. 2) Acquire the routing path reaching a gateway.

407

If a gateway selects the parent node for a sensor node in order to achieve the load balancing, then the following problems should be taken into account: 1) Before a node acquires a unique IPv6 address, it is nearly impossible to communicate with a remote gateway. 2) The number of a fixed node's child nodes dynamically changes. Each time a gateway selects the parent node for a sensor node, it has to communicate with every fixed node to acquire the number of the child nodes. As a result, the address configuration cost and delay are increased greatly. In the scheme, in order to achieve load balancing, the fixed nodes should be distributed evenly as far as possible around the working area. For example, in a hospital, the number of fixed nodes in the same area should tend to be equal. 3.2.3. Registration In the scheme, the root node of a tree has an address table which records the permanent ID and temporary ID of a mobile node which acquires its permanent ID in the tree. 3.2.3.1. Permanent ID registration operation. If a mobile node X acquires its permanent ID, then it sends a PReg command frame to the root node R of its home tree. After R receives the PReg frame, it adds into the address table one entry whose permanent ID is X's permanent ID and whose temporary ID is 0. In general, a WSN includes up to ten thousand sensor nodes. In the scheme, only a mobile node performs the registration operation and the registration tasks of all mobile nodes are distributed around 2i IPv6 ingress gateways. The value of i can be adjusted according to the density of sensor nodes and the scale of WSN. When the number of mobile nodes is relatively large, i is also correspondingly enlarged. In this way, the registration cost of each gateway is reduced substantially. For example, if there are 216 mobile nodes and i is set to 4, then a gateway needs up to 4 K entries to record the registration information. 3.2.3.2. Temporary ID registration operation. If a mobile node X acquires a temporary ID, then it sends to the root node R of its home tree a CReg command frame whose payload is its permanent ID. After R receives the CReg frame, it locates X's entry according to X's permanent ID and then updates the temporary ID in X's entry with the new temporary ID. The scheme implements the PReg/CReg command frame through expanding the type of the IEEE 802.15.4 command frame, as shown in Table 2 where the MAC payload includes 1-byte command frame identifier. 3.2.4. Failure/mobility of sensor nodes In the scheme, if a sensor node's power is less than a threshold, it is considered as a failed node. When a sensor node X detects that it is imminent to fail, it does the following operations: 1) If X is a fixed node, it sends a FLeave command frame to both its parent node and its child nodes. Otherwise, X sends a MLeave command frame to the parent node F where X acquires its permanent ID, the root node R of its home tree, and its current parent node F′. 2) After a mobile child node receives the FLeave frame, it sets both its permanent ID and temporary ID to 0 and rejoins a tree to obtain a permanent ID. Otherwise, after a fixed child node receives the

Table 2 Frame format. 2 bytes

1 byte

8 bytes

8 bytes

n bytes

2 bytes

Frame control

Sequence number

Destination address

Source address

MAC payload

Frame check sequence

Table 3 MAC payload. Mesh addressing header

Fragment header

IPv6 header

IPv6 payload

408

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Table 4 Mesh addressing header. 2 bits

1 bit

1 bit

4 bits

8 bytes

8 bytes

10

O

F

Hop limit

Source address

Destination address

FLeave frame, it rejoins a tree to obtain a sensor node ID and forwards the FLeave frame to its child nodes which repeat the above operations. After X's parent node receives the FLeave frame, it sets to Free the address space occupied by X's sensor node ID. 3) After F receives the MLeave frame, it sets to Free the address space occupied by X's permanent ID. After R receives the MLeave frame, it deletes X's entry from its address table. After F′ receives the MLeave frame, it sets to Free the address space occupied by X's temporary ID. 4) The process of dealing with X's failure ends. In the scheme, after a sensor node acquires its sensor node ID, it periodically broadcasts a beacon frame within one-hop scope. After a mobile node obtains its sensor node ID, it periodically sends a beacon frame to both its parent node and the root node of its home tree. If a fixed/mobile node X does not receive a beacon frame from its parent node within the specified time, then it considers that its parent node fails without alerting. In this situation, X sets both its permanent ID and temporary ID to 0, and rejoins a tree to obtain a permanent ID. If a fixed node does not receive a beacon frame from its child node within the specified time, then it considers that its child node fails without alerting and recovers the address space occupied by the child node. If the root node of a mobile node's home tree does not receive the beacon frame from the mobile node within the specified time, then it considers that the mobile node fails without alerting and deletes the mobile node's entry from its address table. The specified time is set to α⋅d⋅ 1f where f is the frequency of broadcasting/sending a beacon frame. For a beacon frame from a mobile node, d is set to 64/i, and for a beacon frame from a fixed node, d is set to 1. From the above process, it can be seen that the scheme can recover the address resources occupied by the failed nodes. In the scheme, the

onse /Resp

est

Requ

t

packe

parent ID is divided into multiple levels, and i bits represent one level, as shown in Table 1. When i is set to 4, a node in a tree can assign an address for up to 15 nodes. Therefore, the address recovery is important because it can improve the success rate of the address configuration. In the scheme, if a fixed node fails, then its child nodes rejoin a tree to obtain a permanent ID. Since all the fixed nodes with an address space can assign an address for other nodes and a node always acquires an address from a neighbor fixed node, the distributed address configuration for those child nodes is achieved. As a result, the rejoining storm problem is avoided. In IEEE 802.15.4, the maximum beacon interval is 252 s at 250 kbps [25] and reference [32] shows that the transmission of beacon frames has little influence on the performance of WSN. In Table 2, the maximum length of the MAC payload is 106 bytes. In Table 3, the mesh addressing header is 17-byte long and the fragment header is 5-byte long [20]. If a node sends a 1280-byte IPv6 data packet to the IPv6 network every hour, then the size of the first 15 data frames is 127 bytes and the size of the 16th frame is 63 bytes. In the scheme, the beacon frame format is shown in Table 2 where the MAC payload is 1-byte command frame identifier. When the beacon interval is 252 s, the power consumed by transmitting beacon frames is nearly 3.2% of that one by transmitting data frames. In the scheme, a mobile node adopts the method in reference [33] to determine whether it is imminent to leave the communication scope of its parent node. It is assumed that at the time t1 the distance between the mobile node and its parent node is d1, and at the more recent time t2 the distance between the mobile node and its parent node is d2. Thus, at any given time tany, the distance dest between the mobile node and its parent node can be estimated according to formula (2) [33].

dest ¼ d2 þ

 d2 −d1
⋅ t any −t 2 ; t 1 bt 2 bt any t 2 −t 1

In this way, if the current time is t3, then the distance d0.1 between the mobile node and its parent node at the time (t3 + 0.1) can be estimated according to formula (2). If the mobile node detects that

IPv6 Internet

3FE8:1:1:3::/64 3FE8:1:1:1::/64

N T': 3FE8:1:1:1:1000::/68 Y: 1100::

Request packet with tunnel header T: 3FE8:1:1:1:2000::/68

1200:: 2200:: 2300:: 1220::

1110:: 1210::

2210:: 2220::

1211::

ð2Þ

X: Permanent ID: 2100:: Temporary ID: 1212:: Fig. 4. Routing process.

d0.1 is greater than the transmission range, then it determines that it is imminent to leave the communication scope of its parent node. If a mobile sensor node X detects that it is imminent to leave the communication scope of its parent node, then it does the following operations: 1) If X's temporary ID is not 0, then X sends a MLeave command frame to its current parent node F′. 2) After F′ receives the MLeave frame, it sets to Free the address space occupied by X's temporary ID. 3) The process of dealing with X's mobility ends.

Address configuration cost

X. Wang, H. Qian / Computer Standards & Interfaces 35 (2013) 403–414

40 35 30 25

v=5m/s v=15m/s

20 15 10 5 0

100

1) The intermediate nodes only need to deal with the frame header and the mesh addressing header without processing the above headers, such as the fragment header or IPv6 header, so the power consumption is reduced and the routing efficiency is increased. 2) Only the first fragment of one IPv6 packet includes the IPv6 header, so the transmission cost is reduced and the fragment utilization is improved. In the scheme, a sensor node's link address is its sensor node ID. 3.3.1. Data frame format The data frame is shown in Table 2 where the destination address is the sensor node ID of the next hop, the source address is the sensor node ID of the node forwarding the frame, and the content of MAC payload is shown in Table 3. Since the sensor node ID is 64-bit long, the address type adopts the EUI-64 address. The format of the mesh addressing header is shown in Table 4. If a sensor node communicates with the Internet, then only the packets from the Internet include the mesh addressing header. When a mobile sensor node sends a packet to the Internet, the destination node is the root node of the corresponding IPv6 ingress gateway tree by default. Therefore, the packets from the sensor nodes do not include the mesh addressing header. In Table 4, the hop limit is the distance from the source node to the destination node, namely, the depth of the destination node in its current tree, and it decreases by 1 with one hop. The source address is the sensor node ID of the root node of the tree where the destination node is located, and the destination address is the sensor node ID of the destination node. According to reference [34], the mesh addressing header belongs to the layer-two forwarding. 3.3.2. Routing process An IPv6 node N uses the permanent IPv6 address of a mobile sensor node X to acquire the collected data, and the communication process between N and X is as follows: 1) N sends a request packet to X in order to request the collected data, and the packet's destination address is X's permanent IPv6 address.

Table 5 Address configuration cost and delay. Performance parameter Cost Delay

Broadcasting 2O(N1) 2O(1)

Unicasting O(1) O(1)

Total 2O(N1) + O(1) 3O(1)

200

250

300

350

400

450

500

Fig. 5. Simulation of address configuration cost.

2) In the IPv6 network, the packet is routed to the root node T of X's home tree. 3) T searches its address table for X's entry, and then checks if the temporary ID in the entry is 0. If not, then T encapsulates the packet with a tunnel header and sends the encapsulated packet to the root node T′ of X's temporary tree through the IPv6 networks. 4) T/T′ performs the packet fragmentation, and encapsulates each fragment with the mesh addressing header where the source address is the sensor node ID of T/T′ and the destination address is X's current sensor node ID, and the frame header where the source address is the sensor node ID of T/T′ and the destination address is the sensor node ID of its child node in the branch where X is located. Then T/T′ sends the frames. 5) After the child node receives the frame, it first checks if it is X. If it is, then it goes to step 6). Otherwise, according to the destination address of the mesh addressing header, the child node updates the destination address in the frame header with the sensor node ID of its child node in the branch where X is located and the source address with its own sensor node ID, forwards the frames and goes to step 5). 6) After X receives all the frames, it reassembles the frames into a packet and deals with the packet. Then X performs the fragmentation of the response packet, encapsulates each fragment with the frame header where source address is its current sensor node ID and the destination address is the sensor node ID of X's current parent node, and then sends the frames. 7) After the parent node receives the frames, it first checks if it is T/T′. If it is, then it goes to step 8). Otherwise, the parent node updates the destination address in the frame header with the sensor node ID of its parent node and the source address with its own sensor node ID, forwards the frames and goes to step 7). 8) After T/T′ receives all the frames, it reassembles the frames into one response packet and sends the packet to the IPv6 network where the packet is routed to N, as shown in Fig. 4.

Address configuration delay(ms)

In the scheme, the routing strengths are as follows:

150

Number of nodes

The scheme designs and implements the MLeave/FLeave command frame through expanding the type of the IEEE 802.15.4 command frame, as shown in Table 2 where the MAC payload only includes 1-byte command frame identifier. 3.3. Routing process

409

60 50 40 30

v=5m/s v=15m/s

20 10 0

100

150

200

250

300

350

400

Number of nodes Fig. 6. Simulation of address configuration delay.

450

500

X. Wang, H. Qian / Computer Standards & Interfaces 35 (2013) 403–414

In Fig. 4, one request packet destined for the mobile sensor node X with the permanent ID 2100:: is first routed to the root node T of X's home tree. T forwards the packet to the root node T′ of X's temporary tree. Then, T′ performs the fragmentation of the packet, encapsulates each fragment into one frame, and forwards the frames to its child node with sensor node ID 1200::. Through the corresponding branch where X is located, the frames finally reach X, and the routing path is shown in the dotted line. In the scheme, a sensor node periodically broadcasts a beacon frame within one-hop scope, so a node can acquire the IPv6 addresses of its neighbor nodes. If a sensor node X wants to achieve the horizontal communication with another sensor node Y, then it first checks if Y is its neighbor node. If it is, then X directly communicates with Y. Otherwise, X achieves the communication with Y through the following process: 1) X sends frames to its parent node. 2) According to Y's address, the parent node can determine whether Y is its descendant. If it is, then the parent node sends the frames to its child node in the branch where Y is located. In this way, the frames can reach Y. Otherwise, if the parent node is a gateway, then it goes to step 3), or it forwards the frames to its parent node and goes to step 2). 3) As the payloads of IPv6 packets, the frames are routed to the gateway of the tree where Y is located according to Y's address. 4) The gateway extracts the frames from the packets and sends the frames to its child node in the branch where Y is located. In this way, the frames can reach Y. 5) The horizontal communication between sensor nodes is achieved, as shown in Fig. 4. In Fig. 4, the sensor node X with the permanent ID 2100:: wants to communicate with the destination node Y with the sensor node ID 1100::. Through the IPv6 ingress tree, the frames finally reach the gateway T′. Since Y is the child node of T′, T′ forwards the frames to Y. In this way, the horizontal communication between X and Y is achieved. 4. Performance evaluation Hereinafter, the proposed scheme is referred to as the scheme and the proposal of the literature is referred to as the existing scheme. 4.1. Address configuration 4.1.1. Analysis of address configuration When a node acquires an address, the address configuration cost and delay are analyzed and proved by using the proof methods in reference [35].

Redundant delay time(ms)

410

Redundant delay time

4 3 2 1 0

1

2

3

4

5

6

7

8

9

10

Fig. 8. Analysis of redundant delay time.

In the scheme, a node broadcasts a request frame to request an address. After the node receives the response frames broadcasted by neighbor fixed nodes, it returns an ACK frame to the parent node in order to confirm that it acquires a unique address. Therefore, the upper bounds of the cost and delay for a node acquiring an address are 2O(N1) + O(1) and 3O(1) respectively. 4.1.1.1. Proof. In the scheme, the upper bounds of the cost and delay of a node broadcasting a request frame are O(N1) and O(1) respectively, and N1 is the number of a node's neighbor fixed nodes with an address. Also, the upper bounds of the cost and delay of a neighbor fixed node returning a response frame are also O(N1) and O(1) respectively. The upper bounds of the cost and delay of a node returning an ACK frame are O(1) and O(1) respectively. Therefore, the upper bounds of the cost and delay of a node acquiring an address are 2O(N1) + O(1) and 3O(1) respectively, as shown Table 5. 4.1.2. Simulation of address configuration In ns-2, the simulation area is 200×200 m2, and the region includes 2 IPv6 ingress gateways and the number of sensor nodes ranges from 100 to 500. The MAC protocol is IEEE 802.15.4, the bandwidth is 250 kbps, a mobile sensor node moves in the random walk mobility model [36], the communication scope ranges from 10 m to 100 m and the maximum speed is 20 m/s. The size of a packet is 1280 bytes. In the initial state, fixed nodes are distributed uniformly and mobile nodes are distributed randomly around the simulation area. The confidence interval is 95% and the simulation time is 200 s. Depending on the choice of seeds, random variables created with these seeds have correlation [37]. Therefore, we adopt the new method to seed the RNG [38] in order to avoid the correlation. The address configuration cost and delay based on speed are simulated, as shown in Figs. 5 and 6 where v is a node's speed. When the number of nodes increases, the number of a node's neighbor nodes also grows. Therefore, the address configuration cost tends to increase in proportion to the number of nodes. The address configuration

2000

120

Redundant power

Power consumption(uJ)

Redundant power(uJ)

5

d

140

100 80 60 40 20 0

6

1

2

3

4

5

6

7

d Fig. 7. Analysis of redundant power.

8

9

10

Proposed scheme Existing scheme

1500 1000 500 0

1

2

3

4

5

6

7

8

d Fig. 9. Simulation of routing power consumption.

9

10

X. Wang, H. Qian / Computer Standards & Interfaces 35 (2013) 403–414

Delay time(ms)

200 Proposed scheme Existing scheme

150

e ¼ ET ðk; rÞ þ ER ðkÞ

ð4Þ


 2 ET ðk; r Þ ¼ k Etx þ εr

ð5Þ

ER ðkÞ ¼ kErx

ð6Þ

100 50 0

1

2

3

4

5

6

7

8

9

10

d Fig. 10. Simulation of routing delay time.

delay has no relation to the node density. However, when the number of nodes increases, the traffic also grows. As a result, the packet loss rate grows. Therefore, the address configuration cost and delay have a small amount of increment. In addition, when the speed increases, the packet loss rate also grows [9]. As a result, the address configuration cost and delay have also a small amount of increment. 4.2. Routing 4.2.1. Analysis of routing At present, the existing routing in all-IP WSN [21,22] is performed in the network layer while the proposed routing is performed in the link layer. In the network-layer routing, the IPv6 header in each frame includes the 128-bit source/destination IPv6 address while in the proposed link-layer routing each frame only contains the 64-bit source/destination link-layer address. Therefore, when an IPv6 packet is fragmented, 128 bits are saved from the second fragment on. It is assumed that the power consumed by transmitting one 127-byte frame, including sending consumption and receiving consumption between two neighbor nodes, is e, and the distance from the source node to the destination node is d. In practical applications, the power consumed by data transmission is more than the one consumed by data processing by several orders of magnitude, so the power consumed by data processing can be ignored [39]. Therefore, only the power consumed by data transmission is taken into account. In all-IP WSN, both the network-layer routing and the link-layer routing needs the fragment header, so in the network-layer routing the redundant power E consumed by transmitting one frame from the source to the destination can be calculated from formula (3) where e can be calculated from formulas (4), (5) and (6) [40]. E¼

411

64⋅2⋅e⋅d ¼ 0:126⋅e⋅d 127  8

Fig. 11. Analysis of mobility cost.

ð3Þ

In formulas (4), (5) and (6), ET(k,r) is the total power consumed by sending k bits, Etx is the power consumed by sending 1 bit, ε is the magnification of the signal amplifier, r is the distance between two sensor nodes, ER(k) is the total power consumed by receiving k bits, and Erx is the power consumed by receiving 1 bit. According to reference [40], the parameters are set to the following values: Etx = Erx = 50 nJ/bit, ε = 10 pJ/b/m2, k = (127× 8) bits. When sensor nodes are distributed evenly and r is set to 20 m, the analysis of the redundant power consumed by routing one frame is shown in Fig. 7. It is assumed that the delay taken by data processing in the existing routing is the same as that one in the proposed routing, and the delay taken by transmitting 1 bit between two neighbor nodes is t′. Since a frame is a transmission unit and the size of the redundant data in one frame in the network-layer routing is 64 bits, the redundant delay time T taken by transmitting one frame from the source to the destination can be calculated from formula (7). 0

T ¼ 64⋅2⋅∑ t ¼ 128⋅∑ t d

0

d

ð7Þ

When t′ is set to 4 μs, the analysis of the redundant delay time taken by routing one frame in the existing scheme is shown in Fig. 8. 4.2.2. Simulation of routing In the simulation environment in Section 4.1.2, the number of nodes is 500 and the routing performance is analyzed. Reference [22] has improved the work in reference [21]. Moreover, reference [22] has proposed the routing and mobility support scheme for all-IP WSN while other existing schemes have only addressed one issue of all-IP WSN, either routing or mobility support. Therefore, the existing scheme in reference [22] is selected to compare with the proposed scheme. The routing power consumption and delay time are simulated, as shown in Figs. 9 and 10. From Figs. 9 and 10, it can be seen that the routing performance in the proposed scheme is better. The reasons are analyzed as follows: 1) In the proposed scheme, the routing process is performed in the link layer, so the routing power is reduced and the delay is shortened, as shown Figs. 7 and 8. 2) In the proposed scheme, the routing can be achieved automatically through IPv6 ingress gateway trees without either routing discovery or routing establishment.

Fig. 12. Analysis of mobility delay time.

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140

5

r=20m

4

r=40m

3 2 1 0

2

4

6

120

Mobility delay (ms)

Packet loss rate(%)

6

8

10

12

14

16

18

100 80 60 Proposed scheme Existing scheme

40 20 0

20

2

4

6

8

Speed(m/s) Fig. 13. Analysis of packet loss rate.

p 1−p 1−p p

 ð8Þ

It is assumed that π0 and π1 are respectively the long-term steadystate probability of a sensor node being in each sub-PAN and the long-term steady-state probability of a sensor node moving to a new sub-PAN, then formula (9) can be obtained. 8 <

π0 þ π1 ¼ 1 π0 ¼ pπ0 þ ð1−pÞπ1 : π1 ¼ ð1−pÞπ0 þ pπ1

ð9Þ

For any value of p, π0 and π1 can be calculated from formula (9). A sensor node's mobility cost CT can be calculated from formula (10) [22] where CIntra − subPAN refers to the cost taken by a sensor node moving within its current sub-PAN, CInter − subPAN refers to the

CT ¼

20

4 3 2 1 4

6

8

10

ð10Þ

In the scheme, when a mobile sensor node moves within its current sub-PAN, CIntra−subPAN is made up of two parts: the routing discovery Routing Register cost CIntra−subPAN and the registration cost CIntra−subPAN . When a mobile sensor node moves between two sub-PAN, CInter−subPAN is also made up Routing of two parts: the routing discovery cost CInter−subPAN and the registration Register cost CInter−subPAN , as shown in formulas (11) and (12). Routing

Register

ð11Þ

Routing

Register

ð12Þ

C Intra−subPAN ¼ C Intra−subPAN þ C Intra−subPAN C Inter−subPAN ¼ C Inter−subPAN þ C Inter−subPAN

Since the routing is automatically performed through IPv6 ingress Routing Routing gateway trees, both CIntra − subPAN and CInter − subPAN are 0. In this way, CIntra − subPAN and CInter − subPAN are shown in formulas (13) and (14). Register

C Intra−subPAN ¼ C Intra−subPAN ¼ ðDSN−HGW −1Þ⋅κ þ C HGW

ð13Þ

Register

C Inter−subPAN ¼ C Inter−subPAN ¼ ðDSN−TGW −1Þ⋅κ þ ðDTGW−HGW −1Þ⋅τ þ C HGW ð14Þ In formulas (13) and (14), DSN − HGW is the distance from a mobile node to the root node of the node's home tree, DTGW − HGW is the distance from the root node of a mobile node's temporary tree to the root node of the node's home tree, CHGWis the cost of the root node of a mobile node's home tree processing the registration information, κ/τ is the cost of transmitting a unit of data in WSN/IPv6 networks. According to references [22,41,42], the parameters are set to the

Packet loss rate(%)

Mobility cost (Kb)

18

3.5

Proposed scheme Existing scheme

5

2

16

π0⋅C Intra−subPAN þ π1⋅C Inter−subPAN T

6

0

14

cost taken by a sensor node moving between two sub-PANs, and T refers to a sensor node's average residence time.

4.3.1. Analysis of mobility We compare the proposed scheme with the existing scheme [22] from two perspectives: mobility cost and mobility delay time. The methodology in reference [22] is used to evaluate the mobility performance of the proposed scheme. A sensor node moves in the random walk mobility model [36], the speed of a sensor node ranges from 2 m/s to 20 m/s and the movement direction ranges from 0 to 2π. During one movement process, both the speed and the movement direction keep invariable [36]. One all-IP WSN includes 2 IPv6 ingress gateways. It is assumed that the probability of a sensor node remaining in its current sub-PAN (namely, the IPv6 ingress gateway tree) is p and the probability of a sensor node moving to a new sub-PAN is 1-p, then Markov chain model can be applied to predict the position of the sensor node, as shown in formula (8). 

12

Fig. 15. Simulation of mobility delay time.

4.3. Mobility

P¼

10

Speed(m/s)

12

14

Speed(m/s) Fig. 14. Simulation of mobility cost.

16

18

20

3

Proposed scheme Existing scheme

2.5 2 1.5 1 0.5 0

2

4

6

8

10

12

14

Speed(m/s) Fig. 16. Simulation of packet loss rate.

16

18

20

X. Wang, H. Qian / Computer Standards & Interfaces 35 (2013) 403–414

following values: τ = 1, κ = 2,DSN − HGW = 6,CHGW = 24. Then, the analysis of the mobility cost is shown in Fig. 11. Mobility delay time DT includes the delay TDetection of the mobility detection of a mobile node, the delay time TA_Configure of a mobile node's temporary ID configuration, and the delay time TRegister of the registration process, as shown in formulas (15), (16), (17) and (18) [22]. DT ¼ T Detection þ T AXConfigure þ T Register T Detection ¼ t s þ

ð15Þ

P Beacon þ t r þ Lwireless BW wireless

ð16Þ

P Request þ P Response þ P ACK þ 3t r þ 3Lwireless BW wireless

ð17Þ

T AConfigure ¼ 2t s þ



P Creg þ t r þ Lwireless BW DSN−TGW wireless   P Creg þ t r þ Lwired þ ∑ DTGW−HGW BW wired



This paper proposes a scheme for achieving all-IP communication between WSN and IPv6 networks. The paper analyzes the scheme's performance parameters, including the address configuration cost/delay, the routing cost/delay and the mobility cost/delay. The data results show that the proposed scheme has better performance. IEEE 802.15.4 is the low-rate protocol and WSN is a typical lowrate network, so our current works mainly focus on all-IP WSN based on IEEE 802.15.4. In the future works, we plan to do some works on all-IP WSN with heterogeneous features.

This work is supported by the National Natural Science Foundation of China (61202440). References

ð18Þ

In formulas (15), (16), (17) and (18), ts is the delay of a node processing data and it is set to 1 ms [43,44]. tr is the delay of an intermediate node processing routing information. In the proposed scheme, tr is set to 0 because the routing is automatically performed through IPv6 ingress gateway trees. In the existing scheme, tr is set to 0.001 ms [22].Lwireless/Lwired includes the queuing delay and the data forwarding delay, and is set to 2 ms/0.5 ms [22]. PBeacon/PRequest/PResponse/PACK/PCreg is the length of a beacon frame/request frame/response frame/ACK frame/CReg frame. BWwireless/BWwired is the bandwidth of WSN/IPv6 networks and is set to 250 kbps/100 Mbps. When DTGW-HGW is equal to 5, the analysis of the mobility delay is shown in Fig. 12. According to formula (19) [9] where v is a node's speed and R is a node's communication range, the packet loss rate P can be acquired. v :D R T

5. Conclusion

Acknowledgments

T Register ¼ t s þ ∑

P¼

413

ð19Þ

When DSN − HGW is equal to 6, the analysis of the packet loss is shown in Fig. 13. 4.3.2. Simulation of mobility In the simulation environment in Section 4.1.2, the mobility performance is simulated, as shown in Figs. 14–16. When the speed increases, the probability of a node moving between subPANs also grows. Since the inter-subPAN mobility cost and delay are more than the intra-subPAN ones, the average mobility cost and delay increase with the speed. In addition, when the speed increases, the packet loss rate also grows. As a result, the mobility cost and delay also have a small amount of increment. From Figs. 14–16, it can be seen that the mobility performance in the proposed scheme is better. The reasons are analyzed as follows: 1. In the proposed scheme, the routing performance is better, as shown Figs. 9 and 10. 2. In the proposed scheme, a mobile node does not perform routing/ forwarding function, so the change of a mobile node's position has no influence on the routing paths. As a result, the packet loss caused by the routing path disruption due to a node's mobility is avoided. 3. In the proposed scheme, the mobility delay is shorter and the mobility cost is smaller, as shown in Figs. 14 and 15, so the packet loss rate is lower.

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Wang Xiaonan received her PHD degree in computer science and engineering from Nanjing University of Science and Technology. She is currently working at Changshu Institute of Technology. Her research interests are the next-generation network architecture and protocol, and the all-IP communication between IPv6 networks and wireless sensor networks.

Qian Huanyan is a professor in Nanjing University of Science and Technology. His research interests are the next-generation network architecture and protocol.