Wireshark Grab bag found 1506 bytes Package

Ethernet has a maximum MTU of 1506 bytes, but when I grabbed the packet I found a large number of 1506 bytes of packet, which made me baffled

Where does the MTU of 1,tcp come from?

The MTU of TCP generally uses default values, and of course, programmers can set their own. We are talking about the Ethernet MTU value, and the Ethernet MTU value is 1500.

2, the MTU of Ethernet is 1500, why does the packet length reach 1506?

The maximum transmission unit of Ethernet is 1500, but this length does not include the link layer, this value is the link layer for the network layer design, we can look at the link layer frame structure

Here we discuss the most common Ethernet frame packaging structure, more than 99% of the network is the Ethernet frame package structure

By the Ethernet frame structure, it is easy to see that the data link layer is restricted to the network layer data, while the length of the Wireshark is the whole frame.

i.e. 14+20+20+1452 = 1506

1452 this value is easily obtained from the TCP layer that catches the packet.

Thus, we conclude that the value of MSS exchange at the time of the TCP layer three handshake depends on the network layer and is limited by the Data Link layer protocol.

In the Ethernet data frame, the beginning part is the Variable length field (IP header and TCP header, the data part is fixed, but does not affect), moving them to the tail (CRC), so that the data copied to the kernel, you can

The data part of the frame is mapped to a hardware page, which saves the memory-to-memory copy process. That is, the CRC is not necessary after the data has been in the memory repeatedly copied again to use. The length of the TCP datagram is 512 bytes

Integer multiples, which can be handled by the page table in the kernel.

We notice a detail that the CRC is not counted to the link layer data length

CRC is in the data sent during the calculation, is the entire frame of the school, that is, when a frame seal outflow, CRC calibration is completed immediately, therefore, CRC is not counted into the link layer. We assume that if you put the CRC in the frame head, or frame, then you need to do the whole frame 2 times

Operation can be completed, because after the completion of the frame calibration, if not attached to the tail, it is necessary to do the data insertion operation.

We also know that an Ethernet data stream can reach up to 1500 + 18 or 1518 bytes.

end!

Wireshark Grab bag found 1506 bytes Package