IPv6 was originally designed without tunneling technology, but IPv4 is too popular for applications. IPv6 network transformation is impossible to discard IPv4 networks. This requires IPv4 and IPv6 protocols to coexist in the same network. In use, in some places, the network between IPv6 and IPv6, or between IPv4 and IPv4, is isolated. It is necessary to open IPv6 over IPv4 networks and open IPv4 over IPv6 networks. IPv6 tunneling technology emerges as the times require. Tunneling technology is definitely a standard for network convergence. It is essentially a multi-layer overlay of packet headers, which greatly reduces network forwarding efficiency. Because the maximum length of Ethernet packets is fixed, the length of packet headers is proportional. The larger the transfer efficiency, the lower the efficiency. Regardless of whether it is IPv6-Over-IPv4 or IPv4-Over-IPv6, the content of the message must contain two headers, and the IPv6 header is long. If IPv4 itself is an MPLS network, the content of the header is even more too much.
There are many types of IPv6 tunnels, such as the 6PE technology based on the MPLS network running IPv6. The GRE-based IPv6 tunneling technology can carry IPv6 data packets on the IPv4 GRE tunnel. The IPv6 tunnel technology based on the UNDP solves the problem. Traditional NAT cannot support IPv6-over-IPv4 packet traversal. This technology encapsulates IPv6 data in UDP payloads across NATs, called Teredo tunnels. It also has proxy technology to simplify tunnel configuration and provides automatic configuration means. ISATAP tunnels, manual tunnels, IPv4-compatible IPv6 automatic tunnels, etc. IPv6 tunnels are available in various forms. The reason is that they are mainly used to adapt existing IPv4 networks. Applications such as GRE, MPLS, and NAT in IPv4 networks are common. IPv6 Need to be able to penetrate these networks to truly coexist with IPv4. IPv6 tunneling technology has become an important part of IPv6 technology.
Nowadays, the whole network has started the construction of IPv6 network. Most of them are transformed on the existing IPv4 network, that is, the original IPv4 network application is unchanged, and then the IPv6 network is opened to support IPv6 users. The two network technologies coexist everywhere. Both need tunneling technology. Here we will not specifically talk about these tunnels. The characteristics and implementation of each type of tunnel can be searched on the network, and the control is not complicated. Each network device provides guidance on how to use and configure each tunnel. However, if you really want to use it, you must pay attention to some technical pits, avoid falling in and taking a lot of detours.
MTU problem
The tunnel must first consider the MTU problem, the maximum transmission unit and segmentation of the tunnel: the minimum MTU of IPv6 is 1280 bytes, and the IPv4 header is added after the tunnel to reduce the MTU of the packet from 1500 to 1480 bytes. The packet header with the tunnel encapsulation packet is longer than the normal packet. Before a packet is encapsulated, it does not exceed the maximum Ethernet frame length. The MTU may be exceeded. The packet with the boundary value is required. Handle the fragmentation problem, otherwise it will not pass due to the length of the message. The MTU of the tunnel supports static designation and dynamic negotiation. We can increase the MTU of the tunnel to a larger extent. This avoids the problem that the packet after the tunnel is added exceeds the maximum Ethernet frame and needs to be fragmented. This is handled without problems, but the forwarding efficiency is changed. It is low. If the MTU is added, it is also considered whether other devices on the tunnel link are supported. Otherwise, the MTU size difference will be different, and some packets between the two cannot pass. Sometimes this situation is not easy to check. For example, the BGP neighbors in the tunnel cannot be established. This may be caused by the BGP protocol sending different lengths of packets, and some of them cannot be passed. This problem is analyzed to find the MTU. It's not easy, you can avoid this if you can plan your design ahead of time.
Tunnel neighbor establishment problem
There are many types of IPv6 tunnels, but basically only one method is used in one network, and one network device does not support two or more tunnels at the same time. The traffic between at least two tunnels cannot be interworked. Therefore, in the choice of tunnel type, the first need to be determined according to network requirements. The establishment of a tunnel is very simple. As long as the IP addresses at both ends of the tunnel are reachable, a tunnel can be established. The IPv6 tunnel is not as rich as the VXLAN tunnel. It does not support horizontal splitting. Therefore, it is common to have only one tunnel, which will be opened in two independent networks. As long as it is reachable, the IP addresses at both ends of the tunnel can be interconnected at Layer 2, and can also be interconnected at Layer 3, and various QoS can be performed on the link through which the tunnel passes. Tunnel problems are nothing more than three problems: unreachable, tunnel oscillating, and unreachable. The reasons may be strange and vary from device to device. On the surface, the implementation of the tunnel is complicated. In fact, there are not many problems. When the external tunnel is forwarded, the inner layer can be ignored. When the inner layer is forwarded, the outer layer is not concerned. Compared with other IPv6 technologies, tunnel technology It seems scary on the surface, and the actual control is not complicated.
Tunnel safety
Tunnels are not safer than ordinary network forwarding. This is certain. Why do you say this? It is because the tunnel faces a security threat that overlaps both the inner and outer layers. If the IPv4 address is spoofed, anyone can inject how much traffic into the tunnel, and 6over4 may be subject to address spoofing attacks. The externally forged 6over4 packet may invade the 6over4 domain. Because tunneling technology masks the payload during optimization and the ephemeral ports that are often used by voice calls and FTP clients, this can lead to problems such as the inability to establish effective security policies. It may also cause other types of potential errors in the network connection, according to Mike Morris in the article "Returning to Cisco Subnets": "...suboptimal routing, maximum transmission unit problems, and hardware and software. The risks in terms of scalability are all part of what might happen. In short, the security of tunnel technology will become more prominent. For those data centers that pay special attention to network security, how to establish a secure tunnel is a long-term research topic. There is still no security protection technology specifically for tunnels.
IPv6 tunneling is an essential technology for IPv6 isolated islands in IPv4 networks and IPv4 isolated islands in IPv6 networks. Since the whole network was built in 2018, the applications involving IPv6 tunnels are extremely small, and most of them are to open IPv4/IPv6 dual stacks. As the main way, with the deepening of IPv6 deployment, some local area networks may involve tunnel applications, and the issues mentioned in this article should be carefully considered. Although the theory of IPv6 tunnel is relatively sound, there are few practical application cases, and new problems may be encountered in practice. In the process of IPv6 network transformation, IPv6 tunnels, as an application feature, will play an important role in network transformation.
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