Dual Stack

Dual-stack node using the technology to run IPv4 and IPv6 protocol stack sets. This is the IPv6 nodes to maintain compatibility with the pure IPv4 node most direct way to communicate targeted at end nodes (including hosts, routers). IPv4 and IPv6 in this way provides a fully compatible, but for the IP address depletion problem is not any help. Due to the need dual-route infrastructure, this approach actually increased the complexity of the network.
 First, the dual-stack work Dual Stack refers to a single node to support both IPv4 and IPv6 protocol stacks of two. IPv6 and IPv4 is the function of the network layer protocol similar, both are based on the same physical platform and its load on the transport layer protocol TCP and UDP are basically no difference, therefore, support both dual-stack node and IPv4 node communication protocol support, but also with the support of IPv6 protocol nodes. It is believed that the network all the major service providers in the network before the upgrade to IPv6, will support dual-stack operation. 1. Receives the packet Dual-stack node and other types of multi-stack nodes work the same way. Link layer data segment is received, open and check the header. IPv4/IPv6 header if the first field, the IP packet version number is 4, the packet to the IPv4 stack to deal with; If the version number is 6, by the IPv6 stack processing; If the establishment of an automatic tunneling mechanism, the use of appropriate technology will be re-integrated into IPv6 packets of data packets handled by the IPv6 stack. 2. Send data packets As dual-stack IPv4 and IPv6 host support both two kinds of agreements, so when its time to communicate in the network needs to determine the circumstances in which a protocol stack used for communication, which need to develop dual-stack work. In the network communication process, the destination address as the main parameters of the routing, so depending on the application procedures used by the destination address of the protocol type of the dual-stack work to make the following conventions: (1) If the application uses the destination address for the IPv4 address, IPv4 protocol is used; Suppose node A and node B communications, A for the dual stack node, node B supports IPv4 protocol (the destination address for the IPv4 address). A dual stack node will work: Node B using the IPv4 communication protocol. Completeness: Node A has two options, the use of IPv4 or IPv6 protocol agreement with the host B communicate. Under the given conditions, A, B nodes are supported IPv4 protocol, so I chose to IPv4 protocol can guarantee the normal communication, it is agreed upon is sufficient. If you use the IPv6 protocol for communication, because we can not determine whether the dual stack node B (for a given condition is just the destination address for the IPv4 address), so when the node B does not support the IPv6 protocol can not guarantee the normal communication, it is agreed is necessary. At this point that the convention (1) is complete. (2) If the destination address for the IPv6 address, and for the local online network, use the IPv6 protocol. (3) If the application uses the destination address for the IPv4-compatible IPv6 addresses, and non-local online network, use the IPv4 protocol, IPv6 will be at this time In encapsulated in IPv4. IPv4-compatible IPv6 address, IPv6 protocol specification is provided in a special address. Such addresses are 0, high 96, low 32 bits contain IPv4 addresses. IPv4-compatible address is the node for the router to tunnel through the transmission of IPv4 IPv6 packets, the nodes to understand both IPv4 and understanding of IPv6. Can automatically tunnel IPv6 packets in IPv4 network mode IPv4/IPv6 dual stack node transmission will use these addresses. IPv6 address specifications under the agreement, and (3) the assumed conditions, can determine the destination node B is also a dual stack node, and IPv4/IPv6 transition period with the basic situation of the network, node A and node B will be across the IPv4 network communication, so using the IPv4 protocol is feasible. Of course, you can also communicate using the IPv6 protocol, but in the early transition years, (3) the agreement will be superior to using the IPv6 protocol. (4) If the application uses the destination address is non-compatible IPv6 address IPv4, non-local online network, use the IPv6 protocol. Similar to the convention (2), using the IPv6 protocol can guarantee the normal communication, and if it is pure IPv4 network across the communication mechanisms such as the use of the tunnel for communication; and if the local network, the tunnel mechanism is not required to complete the communication. (5) If the application uses the domain name as the destination address, domain name server to start the corresponding IPv4/IPv6 address, then address the appropriate handling of the situation. Above (1) to (5) is the dual-stack work, with the constant expansion of IPv6 network, these methods will do the appropriate changes and additions, which will depend on the transition process and IPv6 networks continue to evolve . Second, based on dual-stack applications 1. Based on dual-stack domain name service domain name system (referred to as: DNS) The main function of the domain name and IP address through the interaction between the corresponding relationship to precisely locate network resources that, according to domain name queries IP addresses, and vice versa. DNS is the Internet's infrastructure, many network services are built based on the DNS system. Therefore, DNS importance of self-evident. IPv4 address of a positive resolution of the resource record is "A" record. There are two kinds of IPv6 address resolution resource records, that is, "AAAA" and "A6" records. "A6" record to support a number of "AAAA" do not have new features, such as address aggregation, change of address (Renumber) and so on. Exist in the DNS server "A" record and "AAAA" (or "A6") records. Since the node can handle both IPv4 protocol, IPv6 protocol can handle, so no DNS ALG and other switching equipment. DNS server to answer either "A" record or "AAAA" records, can communicate. 2. Based BBS services, dual-stack BBS is a communication platform on the Internet, the Web services play an important role, along with the smooth transition of IPv4 to IPv6, created based on dual-stack BBS service system becomes necessary. According to the "basic support for IPv6, socket (socket) interface expansion", modify the relevant parts of the code, you can achieve IPv6 and IPv4-compatible BBS system.
 (1) IPv6 socket address structure Defined in <sys/socket.h> IPv6 --- AF_INET6 address family and protocol family --- PF_INET6, or the new sockaddr_in6 data structure sockaddr_in data structure with the original distinction, which will create the socket when use. (2) IPv6 address several specific structures to support IPv6, BBS systems, use the following two special IPv6 addresses. ① IPv4 addresses mapped IPv6 address. ② IPv6 wildcard address.
 (3) How to make the program compatible with IPv4/IPv6 most of the existing IPv4 network application is written, the principle can be seen from the above, except for some closely related with the characteristics of IPv4 procedures (such as the use of multicasting, IP options or the original set of interface programs), most of the application to convert IPv4 IPv4/IPv6 application compatibility is not very difficult. Based on this idea, according to the IPv4 and IPv6 Socket API made compatible with the support of the existing BBS system (IPv4-based application system) a series of changes to support IPv6, to construct a BBS system. Take the following major steps: ① to construct a dual stack (Dual Stack) BBS server, IPv4 and IPv6 addresses were given, making its services running on the BBS handle both IPv4 clients use, capable of handling the use of IPv6 customers.
 ② IPv4 socket address of the original part of the modification, creation based on IPv6 sockets.
 ③ IPv6 socket will be bound to the IPv6 wildcard address and listening port 23.
 ④ The transfer function compatible with IPv4/IPv6 addresses instead of IPv4-only support address translation function. ⑤ to amend the IP address displayed in the format of the relevant code, with sufficient length Show support for IPv6 addresses. 3. IPv6 Campus Transition Plan During the transition process, IPv6 IPv4 access network segment as the islands, and between network segments to achieve IPv6 IPv4, IPv6 interoperability between network segments, a variety of transitional technologies must be integrated to optimize network structure, network security and reliable operation, and a gradual transition, saving investment under the premise of the transition program design. Based on the above basic principles, and network operating conditions, for the early stages of the transition program of Campus Network Planning are as follows: (1) cross-communication between IPv4 network using IPv6 tunneling technology; (2) server based on the escalation of IPv4 to dual stack node server; (3) IPv4/IPv6 interoperability client can use NAT-PT technology; (4) The export of local IPv6 router access to higher segment associated IPv6 network. In carrying out the early stages of IPv6 applications should try to ensure the existing network system safe and stable operation, so for the new IPv6 network segments, should be independent of access to higher link IPv6 network. Select dual-stack IPv6 router segment as the access node, to ensure that the new IPv6 network segment can also use the local network services.