Shulou(Shulou.com)06/01 Report--

1. IP Datagram format

2 、 MTU

MTU (maximum transmission unit): the maximum length of a data field in a frame format.

3. Slice

1) conditions for slicing

When the length of the Datagram exceeds the maximum transmission unit MTU allowed by the network, the overlong Datagram must be sliced before it can be transmitted on the network.

2) how to slice

For each IP Datagram sent by the sender, the identity field contains a unique value. This value is copied to each slice when the Datagram is sliced. The flag field uses one of the bits to represent "more slices". Except for the last piece, every other piece that makes up the Datagram has to set the bit to 1. The slice offset field refers to the position where the slice offsets the beginning of the original Datagram. In addition, when the Datagram is sliced, the total length of each slice is changed to the length of the slice.

Finally, there is a bit in the flag field called the "unfragmented" bit. The Datagram will not be sliced if this bit is set to 1Q IP.

When the IP Datagram is fragmented, each piece becomes a packet, has its own IP header, and is independent of other packets when routing. In this way, these slices of the Datagram may be out of order when they reach the destination, but there is enough information in the IP header to enable the receiver to assemble these Datagram chips correctly.

4. Assembly

In order to reassemble these Datagram fragments, the receiving host allocates a storage buffer when the first shard arrives. The host will also start a timer. When subsequent fragments of the Datagram arrive, the data is copied to the location indicated by the slice offset field in the buffer memory. When all the fragments arrive, the complete unfragmented original packet is restored. Processing continues as if it were an unfragmented Datagram.

5. Three-digit mark

The highest bit in the flag field currently has no meaning

The lowest bit in the flag field is marked as MF (More Fragment), and MF=1 represents the Datagram that is followed by "fragmentation". MF=0 says this is the last of several datagrams.

The bit in the middle of the flag field is marked as DF (Don't Fragment), which means "cannot be sliced". Slicing is allowed only when DF=0.

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