What determines the MTU size and what is the minimum value? During TCP communication, if a packet of data is larger than 1MTU, will it be unpacked?

What MTU value to set depends on your Internet access method?

Fiber optic: 1500?

PPPoE/ADSL: 1492?

Dial Up/Modem:576?

I don’t understand why you focus on the smallest value. It’s like focusing on how small a child can swallow rice.

If you want to discuss the smallest value Topic:

Take Ethernet encapsulation as an example. It requires min posts=46. If it is less than 46, it will automatically supplement it; then 46+6(src mac)+6(dst mac)+2(protocol) =60, if you add the 4-digit CRC at the end, it will be 64;

For IP packets, there seems to be no minimum requirement, right? At least I haven't noticed this problem at ordinary times. I have tried sending packets with only 20 bytes of IP header;

For TCP packets, packets without data, such as SYN packets, etc.

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IP fragmentation is often discussed in many materials, but I think it is not important whether it is fragmented or not. What is important is another thing. A

data packet passes through a large network, and it passes through multiple networks, and the MTU value of each network is different. Just like the barrel effect, we should have an effective way to discover this value. Perhaps the stupidest way is to first use traceroute to view all nodes, and then ping one by one...

In the IP header, three flags are used to describe a fragmented packet:

1. Fragmentation flag: If a packet is fragmented, it is set to 1, except for the last fragment. ;——In this way, for the receiving end, this flag can be used as one of the important basis for reorganization;

2. Fragment offset flag: only A flag bit indicates "whether it is a fragmented packet" is not enough. The offset flag bit indicates where the fragment is located in the original datagram.

Obviously, these two flags are Combined, it is easy to reassemble fragmented packages.

3. Fragmentation not allowed flag: If the data packet forcibly sets this flag, then the data packet will be discarded when it should be fragmented, but an ICMP unreachable error will be received, telling You,need to shard.

In the Internet protocol, the "path maximum transmission unit" of an Internet transmission path is defined as the minimum value of the maximum transmission unit of all IP hops on the "path" from the source address to the destination address. Or from another perspective, it is the maximum number of transmission units that can traverse this "path" without further fragmentation.

RFC 1191 describes the "Path Maximum Transmission Unit Discovery Method", which is a technique for determining the maximum transmission unit of a path between two IP hosts, with the purpose of avoiding IP fragmentation. In this technology, the source address sets the DF (Don't Fragment) bit of the datagram, and then gradually increases the size of the datagram sent - any device on the path that needs to fragment the packet will This datagram is discarded and a "datagram too large" ICMP response is returned to the source address - in this way, the source host "learns" the largest maximum transmission unit that can pass this path without fragmentation.

Unfortunately, more and more networks are blocking ICMP transmission (for example, to prevent DDOS attacks) - this makes the path maximum transmission unit discovery method not work properly, and its common manifestation is a connection Works fine with low data traffic, but hangs immediately once a lot of data is sent at the same time (for example, when using IRC, customers will find that after sending a ping with IP spoofing disabled, they will not get any responded because the connection was clogged with a large number of welcome messages). Moreover, in a network using Internet Protocol, the "path" from source address to destination address is often dynamically modified in response to various events (load balancing, congestion, power outages, etc.) - this This may cause the path's maximum transmission unit to change during transmission—sometimes even repeatedly. The result is that more packets are lost while the host searches for a new maximum transmission unit that can safely operate.