The IPv6 over IEEE 802.15.4 has specified way how to carry IPv6 packets over IEEE 802.15.4 standardized devices and similar networks with the introduction of an adaptation header which comes before the IP header and just after MAC layer, a link in such a 6LoWPAN is characterized as lossy, with minimum power consumption, minimum data rate, small range, with many nodes saving energy with hibernating or deep sleep schedules. IEEE standards are based on low memory devices which has a limited MTU size keeping this under consideration we have to see how we can fit IPv6 address in that limited space we have and even UDP packets also which are used in our transmission. IPv6 already posses a big memory but in use of 6LoWPAN all the headers are compressed with special compression algorithm so that they can be fit in the limited space of IEEE standardized MTU. Compressing algorithm are bit different according to the use or according to the network topology, as 6LoWPAN support both ad hoc and star technically used as Mesh Under and Route Over.
| Application Layer |
| Transport Layer (TCP/UDP) |
| Network +———+ |
| Layer | Routing | |
| (IPv6) +———+ |
| 6LoWPAN Adaptation Layer |
| IEEE 802.15.4 (MAC) |
| IEEE 802.15.4 (PHY) |
Figure X: New adaption layer introduces
IEEE 802.15.4 has some several types of frames like beacon frames, MAC command frames, acknowledge frames and off course data frames also, IEEE standardize addressing mode also which are further described in this documents these are 2 basic i.e. 16-bit short addresses or a unique identifier and 64-bit unique addresses.
A particular LoWPAN header will be like this after compressed field containing all the field like IEEE header Mesh header or if Route over is used depending on the network topology used then fragment header and in the end IPv6 compressed header.
Figure X: Typical LoWPAN Header Stack
Several compression methods are used or developed to keep the header compressed and fitted in limited amount of space and should be working according to the desired need, compression algorithm like gzip, deflate, ROHC but normally and best fit for this scenarios are the HC1 or HC2 compression header formats. They are predefined formats having their own formatting to represent data and data types .
This can best describe us how the header look like after all there field filled and necessary information saved for transmitting the packet from node to other may be in the LoWPAN or may be on other LoWPAN, this is only the structure defined and use by the LoWPAN nodes. Now we can consider a node which wanted to join a network and what necessary things will be needed and what processes it has to go through to final get his IPv6 address, as IPv6 has some standard predefined for normal Ethernet use like multicast as used in classic IPv6 Neighbor Discovery is not useable and desirable in such a wireless low-power, lossy network. Network discovery is just a simple procedure as defined in classical IPv6 to get the fresh node bind with the network.
6LoWPAN Neighbor Discovery (6LoWPAN-ND) is specially designed for LoWPAN networks which provides some basic operations of bootstrapping and other simple network operation, even they some advanced features includes claim and defends address generation and Extended LoWPAN over backbone links, while avoiding the flooding from multicast; this solution supports the use of both link-layer and LoWPAN-level Mesh Under and Route Over solutions.
6LoWPAN-ND design introduces a registration method over the radio edge of the network and proxy operation over the federating backhaul, this registration mechanism provides a examination similar to the Multicast Address Resolution Server (MARS) for a restricted purpose, and in a much simpler and generic way for those link scope multicasts is unavoidable, such as for Router Advertisements, optimizations may be used to optimize the distribution of the information in the LoWPAN. The new concept behind LoWPAN Whiteboard located at Edge Routers (ERs) is introduced which works as the cache to maintain he entries from the registration of nodes, which allows for Duplicate Address Detection for the entire LoWPAN. A new registration/confirmation message sequence is specified so that nodes can register themselves with their IPv6 addresses with an Edge Router. These white boards are the cache of edge routers contain all the information regarding the nodes which are registered with the LoWPAN with their unique IPv6 address and network prefixes.
The Whiteboard uses soft bindings of nodes meaning the entries will be expires until particular time if not getting updates from nodes before the life time of registration of the nodes expires, thus nodes can send periodic registration messages in order to update their bindings with the edge routers and to keep the cache record updated which node was last updated and registered to which LoWPAN router range, changes in network topology and mobility between ERs and LoWPAN are supported and sometimes unreachability of routers can be detected in this way. This research also put an eye on the combination of an Extended LoWPAN with more than one edge router working in same LoWPAN sharing same backbone link (e.g. Ethernet) getting advantage of having a single IPv6 subnet. This allows nodes to keep the same IPv6 address throughout a large network if they move from the domain of one edge router to other and registers them with a new edge router which allows for easy communications and even routing among the same LoWPAN is easy with nodes over the backbone link and with other IPv6 hosts.
Edge Routers backed by Ethernet link working on an edge of LoWPAN networks in this particular scenario one edge router will be maintaining or working as DHCPv6 server and other routers in his domain will be working as relaying router. A LoWPAN network typically uses two types of Layer 2 addresses aa‚¬” for example 16-bit short addresses or a unique identifier and 64-bit unique addresses. Additionally, the available Layer 2 payload size with respect to payload we might need to use header compression as explained earlier and use a minimum payload on the order of less than 100 bytes. These network is lossy and battery-powered, and it does not support multicast capability at the link-layer, thus pretending multicast actions by both using broadcast or sending a number of unicast messages which are costly for the battery powered network and the low-processing capable nodes. Often these battery-powered nodes save energy by using sleep schedules and keeping themselves in sleep mode to save battery power; waking them up just to receive IPv6 signaling messages so that they can only be updated in our oneaa‚¬a„?s domain only is not useful on this case what nodes usually programmed as they will get themselves updated and the registration has a life time and if the node did not update himself before that the entry in the white board will be discarded, these nodes do schedule there sleep time but keeping the timer of registration a little longer then there sleeping time so usually they wake up before the cache entry expires, also they are not competent of processing RA for their neighbors efficiently. Sometimes due to the strong radio signal from a neighbor router or its own strength, a node may often move from one network to other network from one router to another without physically moving. Considering the above characteristics in a LoWPAN, and the IPv6 Neighbor Discovery basic requirement of his protocol, it was finalized that classic ND protocol of IPv6 is not good to be used in LoWPAN so a little different protocol for LoWPAN has been designed as 6LoWPAN-specific ND.
4.1 6LoWPAN Neighbor Discovery Protocol Overview
6LoWPAN Neighbor Discovery optimizes with a mechanism which is on its own least yet very important for LoWPAN IPv6 operation. 6LoWPAN-ND defines a registration process which optimizing the node-router interface, this mechanism do not work on flooding which reduces link-local multicast frequency. 6LoWPAN-ND can work with non-transitive links also, the use of mesh-under and route-over techniques and makes no hypothesis about node synchronization.
6LoWPAN-ND is compatible so that they will be used with the link-layer mesh techniques, which improve the if not non-transitive characteristic nature of wireless links if they are used throughout the LoWPAN, this normally called Mesh Under topology which make the whole link IP layer as having a link-local scope making all the 6LoWPAN interfaces in the LoWPAN. This kind of LoWPAN is consisting of hosts and Edge Routers, this link still lossy, slow data rate, irregular behavior along with sleep mode. The non-transitive nature of the link can be overcome simply by using routing based on IP with in single domain of a LoWPAN, also called a Route over topology. Routing between all nodes in the LoWPAN is done by LoWPAN Routers in the LoWPAN. Mesh Under and Route Over techniques are not limited.
A Host performs independent address auto configuration of its link-local unique and single t address for each LoWPAN interface from its EUI-64. The process of joining a LoWPAN is done either by listening of RA or either by sending a broadcast message of Router Solicitation (RS) and in return it will receive RA an acknowledgement message responses from local routers, there may be more than one router it a particular range. It mainly depending if a true prefix is advertised in the RA, the host will also form a positive global unique address with auto configuration, this is the point the node can choose more than onr router if its available to get himself registered its only if it have signals from more than one LoWPAN.
After selecting the desired router the node will make some effort to first perform node registration and get itself bind with a router, registration is performed with a link-local Edge Router or LoWPAN Router by sending a unicast Node Registration (NR) message to it from host to router. It is more good to get yourself registered directly with an Edge Router, although all other routers with in a LoWPAN has capability to work as a relaying agent on the LoWPAN have the capability to relay NR/NC messages on behalf of a node. The NR contains the addresses of the node who wants to register. A node may also request a short address of 16 bit to be generated on its behalf when the Edge Router get a request of NR its replies either directly with a Node Confirmation (NC) message to the host or through the help of relaying router which is in between node and edge router, relaying routers only exist in Route Over configurations or network topology and in pure Mesh Under network topology and scenario, configurations nodes are within link-local scope or range of an Edge Router. This NC message includes the set of unique addresses which will be updated and also made an entry in the Whiteboard and address will be bound to the ER the Host is now registered and can use the LoWPAN.
4.1.2 Basic operation
The node is now ready to use and may send packets to any IPv6 address inside or outside the LoWPAN or over the internet every request for Next-hop will be sent to the predefined default router but only local address which will redirected to as they are present in local domain. The white board entries and binding table of LoWPAN Router must be renewed on every pre defined intervals as per advertised interval and the lifetime of the binding, the white board entry will be valid in till it expires and then the current entry will deleted from the routers cache. This is achieved by occasionally sending a new NR message when ever host changes his location from original LoWPAN to another LoWPAN or may be some change in network topology occurs or may be some certain router is no longer in reach then host have to get them registered again with the nearest available route. Host then starts the registration process with another router if the LoWPAN does not change for host its IPv6 addresses remain the same. Addresses bound or the entry in the Whiteboard must be remembered by the host and updated in order to keep the address. If the host moves to a different LoWPAN, the bootstrapping process is initiated again the host may be moved while he is sleeping mode. LoWPAN Routers at times send RAs to their neighbors in order to keep themselves and their neighbors updated. The Edge Router triggered the first RAs, and information from these RAs is included in the RAs of each further router where this RA travel, causing the information to be spread throughout the LoWPAN and to every router on it.
4.2 Address collision detection and resolution
The address collision process can be carried out within the Edge Router if the Edge Router already has an entry in his cache for registration of a given address in his white board or over the Backbone Link using Duplicate Address Detection mechanism designed to clear the mess if more than one router are keeping the same entry of a node in there white board. For this case, a new ND option is introduced in the NS/NA messages from routers to carry additional information to resolve the conflicts in all over domain range if any duplicate identity is present; the Owner Interface Identifier Option is used in NS/NA messages to carry the added information necessary for the resolution of conflicts: Transaction ID, Owner Interface Identifier, and Owner Nonce. In any case generally, the Edge Router of LoWPAN is in charge of the resolution which is the same Edge Router that handles all the registration processes of the LoWPAN nodes. A conflict only occurs when a Node already registered and router again receives registration request making it a duplicated address over the same network for that node and an IPv6 address that is already registered with a different OII at the same or another Edge Router. The address collision is only occurred while one node may be in sleeping node or normally moves from one LoWPAN to another, when it receives new LoWPAN it will try to get himself registered to that new edge router or may be contact the relaying router to get himself registered. At that very moment it will be checked which router has the registration entry of this node or its new node for this LoWPAN.
Mobility of the nodes with in a same subnet or within a single domain of a LoWPAN router subnet is supported it has least effects on entries on white board, as the node is with on domain of a router no extra registration is necessary making no duplication of addresses in an Extended LoWPAN, a LoWPAN Node may move around from one Edge router to a new Edge Router visibly and at any time. The protocol for this mobility detection is designed to differentiate the mobility and clear out the registration states or any duplication among routers white board. The node registration triggered by an Edge Router that processes usually takes over an existing registration present among his domain or maintained by a defendant Edge Router this decision to transfer the registration from an Edge Router to another edge router is made by the Edge Router that is processing a Node Registration message depending on its states for that particular registration and ND exchanges over the Backbone Link if we talk about extend LoWPAN. If the node is already registered on a backbone link or to any other router with in a LoWPAN the chances of getting conflicts occurs when getting the registration of the same node start by, the resolution of such conflict is overcome by using classic Duplicate Address Detection mechanism; itaa‚¬a„?s quite simple who claim and have proof of the nodes ownership will succeed.
Once a node registration message received to an edge router, the Edge Router checks for the existing registration for that IPv6 address in its LoWPAN whiteboard. If the entry does not exist then the Edge Router concerns the address over the Backbone Link using duplicate address detection mechanism. The edge router who is defending the address when gets the DAD message will reply back to this message with an acknowledgment and with owner identifier otherwise if the message is timed out then the edge router who advertised the DAD message will accept the registration of this node. It will create an entry on its white board and update the node with a positive confirmation message.
If some router claims the ownership of the replies with his owner identifier and when the edge router get this message it will check the owner identifier interface in the message and matches it with the entries in his white board. If its present then the address is duplicate and already present on the network, taking this on account the request for registration is discarded and duplicate address detection is successful completed his task. If there are by any chance two routers claims to be the owner then checking the owner identifiers and matching them with the OII present in NA message will be checked and who has the Transaction ID will be claimed as owner and other request will be discarded. The router who a loss the registration has to keep the entry in his white board till the time period of this registration is expires.