Some users may find that the wireless connection between their computer and mobile phone to WiFi is slow and sometimes the web page cannot be opened after encountering a light cat or router that has enabled IPv6. Here are the solutions.
Recently, it has been frequently mentioned that the issue of slow IPv6 connection speed has been activated. At present, there is indeed a problem in China where there are not enough servers and CDN nodes supporting IPv6, and the international bandwidth of IPv6 is smaller than that of IPv6. However, it is not likely that domestic websites will be blocked. Usually, encountering this issue indicates that there is a PMTU black hole on the link to the target server.
4G Industrial Router About PMTU black holes
MTU (Maximum transmission unit) is the maximum size (including IP packet header) of three-layer data packets that can pass through a link. The default MTU on Ethernet is 1500 bytes, but there may be links smaller than MTU 1500 on the path between you and the target server. The minimum MTU value on this path is the PMTU value of the entire path. When the router forwards packets, packets larger than the MTU size will be fragmented (Transmission), meaning that a large packet will be divided into multiple small packets that do not exceed the MTU size for transportation, resulting in a decrease in transmission efficiency.
5G Industrial Router 
When the terminal device is contracted, it can also set the DF (Don't Fragment) flag to tell the router not to fragment. At this point, the intermediate router will drop packets exceeding the MTU and reply with an ICMP Fragmentation Needed message. After receiving this packet, the sender will send a smaller packet next time, which is called PMTU Discovery. In reality, it can be seen that HTTPS traffic is mostly marked with DF.
Vehicle Router However, there are a large number of intermediate devices on the internet that do not respond to ICMP for the sake of so-called "security" or incorrect configuration
Fragmentation Needed packet, which causes a packet with a size exceeding PMTU to be silently discarded when accessing certain websites, until the TCP protocol detects a timeout loss and retransmits it, which is very slow. In this situation, we can say that there is a PMTU black hole on the path between you and the target server.
In addition, IPV6 does not support sharding, in other words, it can be understood that all packets under IPv6 are marked with DF. When the intermediate router encounters a situation where the packet size is greater than the MTU, it should respond to the ICMPv6 Packet Too Big message. Similarly, due to various reasons, some intermediate devices may directly lose packets without responding to ICMPV6 Packet Too Big messages until the TCP protocol detects a timeout loss and retransmits it.
Why doesn't IPv4 have this issue?
In fact, IPv4 also has this issue. I have seen netizens say more than once that the soft route I set up is very slow to access certain websites, but switching back to a hard route is normal. This is because most home routers default to turning on MSS (maximum segment size) Clamping for TCP under IPv4 (friends who use OpenWPT soft routing can find the MSS Clamping switch in firewall settings). MSS Clamping is a Workround for PMTU black holes. Simply put, during TCP handshake, there is an MSS field that determines the maximum size of a single TCP packet. Routers can sniff TCP handshake packets and reduce the MSS value, so that the final size of the three-layer IP packet (MSS+TCP header size+IP header size) does not exceed a specific value.
Summary:
Nowadays, domestic ISPs generally establish WAN connections through PPPoE virtual dialing. The default MTU for Ethernet is 1500, but the PPPoE tunnel has an overhead of 8 bytes, so the MTU for PPPoE virtual connections is 1500-8=1492, minus IPv4
The packet header (20 bytes) and TCP packet header (20 bytes) indicate that MSS needs to be set below 1452 under IPv4.
The packet header of IPv6 is 40 bytes, so it is necessary to set MSS below 1432 under IPv6.
At this point, the problem arises. Currently, many optical cats and home routers have poor optimization of IPv6 and do not support MSS Clamping of TCP packets under IPv6. This leads to slow opening when accessing IPv6 websites with PMTU black holes in the path.
