【正文】 μIP is operating system dependable.In this paper, we propose a new design of dualstack for embedded systems, which supports IPv4 and IPv6 simultaneously. Our contributions can be summarized as follows:? We first develop a new IPv6 stack, called EIPv6, which shares the same TCP interface and API with LwIP。 ? The IPv4/IPv6 dual stack is implemented on embedded evaluation board LM3s8962.Experimental results show that our dual stack takes up only about 33KB of ROM and 6KB of RAM during execution, while the data transmission speed reaches 40KB/s. It proves that our protocol stack can effectively satisfy most embedded applications with limited resource. The rest of this paper is organized as follows: Section II gives a short description of our design of EIPv6. The architecture of our dualstack is presented in Section III Section IV gives the implementation of the integration. Experimental results are shown in Section V followed by conclusion in Section VI. II. EMBEDDED IPV6 STACKEIPV6EIPv6 is a lightweight IPv6 stack we designed for embedded system, which is highly portable to various platforms and network devices. The code is carefully designed to support a minimal set of TCP/IPv6 protocols, plying with the resource constraint of embedded systems. Just as LwIP, it is independent of any operating systems. Generally, the TCP/IP model can be divided into four layers from bottom to top: link layer, network layer, transport layer and application layer. Accordingly, our stack uses the corresponding layered architecture. To be independent of specific application and device, we focus on implementing the protocol for the transport layer and the network layer, and simply create a framework for the link layer and API for the application layer. The transport layer includes only Transport Control Protocol (TCP) while User Datagram Protocol (UDP) is dropped because it is seldom used and can be replaced by TCP equivalence. The network layer is further divided into four main modules: Basic IPv6 module, Neighbor Discovery (ND) module, Stateless ADDRess CONfiguration (ADDRCONF) module and Internet Control Message Protocol for IPv6 (ICMPv6) module. The layered architecture is illustrated in Fig. 1, where the modules in network layer are grouped into three sub layers. It means a module may depend on another. For example, the ND protocol reuses ICMPv6 message format for neighbor solicitation and advertisement, etc., and the ICMPv6 itself in turn is based on IPv6 PDUs (Protocol Data Units).Figure 1. Output stream of LwIP and EIPv6An embedded system cannot generally afford a fullfeatured IPv6 stack。 ? Simplified neighbor discovery。 and ? Simplified ICMPv6 processing.III. ARCHITECTURE OF DUAL STACKWe design the dual stack by integrating LwIP and EIPv6. Since the two stacks share the same architecture of standard TCP/IP model, the integrated dual stack follows the layered architecture. However, straightforward merger will result in redundant parts in some layers. In order to find out the mon parts and determine the detailed architecture, the output and input streams of both stacks are analyzed as follows.These functions are called by output functions in network layer to perform packets sending. In LwIP, the packet arriving at link layer will be put into a mailbox through function netifinput. The main thread of stack reads the packet from the mailbox and calls different input functions according to the type of packet. EIPv6 adopts the same mechanism but only ip6_input is called, because there is only one type of valid Ethernet packet in IPv6 protocol.After the analysis of input and output streams, we discuss the feasibility of integration from top layer to bottom layer. EIPv6 provides the same API as LwIP despite of different formats of address. Similarly, IPv4 and IPv6 share the same TCP protocol according to related RFCs by omitting the difference between addresses. If the formats of addresses in these two layers can be unified, the integrations are practicable. The solution is the dualstack address, which will be discussed in detail in Section IV. In the network layer, IPv6 protocol adopts totally different mechanism from IPv4, such as a new address resolution process. In addition, Internet Control Message Protocol for IPv6 (ICMPv6) changed largely pared to ICMPv4. Therefore it is much suitable to reserve their respective network modules in the dual stack. Although different interface functions are called by network output functions, the final extra function of both protocols is the same link output function. So the dual stack can provide a unified network interface by merging all the different interface functions of both stacks and reserving one copy of the same functions. The network layer contains two distinct parts of modules, which can be grouped into IPv4 and IPv6 family. Other three layers are shared by both IPv4 and IPv6 protocols in dual stack. The integrations of these layers make the dual stack be more reduced to fit in embedded systems. Furthermore, the dual stack provides the API and network interface similar to LwIP, so application and port code for LwIP can be applied to it with few modifications.IV. IMPLEMENTATION DETAILSSection III discusses the overall design of dual stack。 3: union{ 4: struct ip_addr addr4。 7: u32_t zone_id。 9: }uniAddr。This structure contains an identifier named family, which indicates that a certain address is an IPv4 address of an IPv6 address. The other member is a union posed of an IPv4 address and a structure strAddr6, which contains an IPv6 address as well as a 32bits zone ID. The RFC requires a unique value, but does not define how to assign the zone ID. In our implementation, it is assigned with first 32 bits of pointer to the sending interface which the linklocal address belongs to. This practice can bring two