udp用0就行了,不是必须的
Checksum is the 16-bit one's complement of the one's complement sum of a
pseudo header of information from the IP header, the UDP header, and the
data, padded with zero octets at the end (if necessary) to make a
multiple of two octets.
The pseudo header conceptually prefixed to the UDP header contains the
source address, the destination address, the protocol, and the UDP
length. This information gives protection against misrouted datagrams.
This checksum procedure is the same as is used in TCP.
0 7 8 15 16 23 24 31
+--------+--------+--------+--------+
| source address |
+--------+--------+--------+--------+
| destination address |
+--------+--------+--------+--------+
| zero |protocol| UDP length |
+--------+--------+--------+--------+
If the computed checksum is zero, it is transmitted as all ones (the
equivalent in one's complement arithmetic). An all zero transmitted
checksum value means that the transmitter generated no checksum (for
debugging or for higher level protocols that don't care).
tcp的规则是这么描述的
Checksum: 16 bits
The checksum field is the 16 bit one's complement of the one's
complement sum of all 16 bit words in the header and text. If a
segment contains an odd number of header and text octets to be
checksummed, the last octet is padded on the right with zeros to
form a 16 bit word for checksum purposes. The pad is not
transmitted as part of the segment. While computing the checksum,
the checksum field itself is replaced with zeros.
The checksum also covers a 96 bit pseudo header conceptually