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The Internet checksum, [1] [2] also called the IPv4 header checksum is a checksum used in version 4 of the Internet Protocol (IPv4) to detect corruption in the header of IPv4 packets. It is carried in the IPv4 packet header, and represents the 16-bit result of the summation of the header words. [3] The IPv6 protocol does not use header checksums.
Packets that hold Internet Protocol data carry a 4-bit IP version number as the first field of its header. [1] [2] Currently, only IPv4 and IPv6 packets are seen on the Internet, having IP version numbers 4 and 6, respectively.
This is a list of the IP protocol numbers found in the field Protocol of the IPv4 header and the Next Header field of the IPv6 header. It is an identifier for the encapsulated protocol and determines the layout of the data that immediately follows the header. Both fields are eight bits wide.
This field indicates the transport layer protocol of the datagram following this header. The value is set to 4 for IP in IP. Not to be mistaken with value 4 in the Version field, which indicates IPv4. Header Checksum: 16 bits This field is the IP checksum of outer header. Source IP Address: 32 bits This field is the IP address of the encapsulator.
The field is 13 bits wide, so the offset value ranges from 0 to 8191 (from (2 0 – 1) to (2 13 – 1)). Therefore, it allows a maximum fragment offset of (2 13 – 1) × 8 = 65,528 bytes, with the header length included (65,528 + 20 = 65,548 bytes), supporting fragmentation of packets exceeding the maximum IP length of 65,535 bytes.
When UDP runs over IPv4, the checksum is computed using a pseudo header that contains some of the same information from the real IPv4 header. [7]: 2 The pseudo header is not the real IPv4 header used to send an IP packet, it is used only for the checksum calculation. UDP checksum computation is optional for IPv4.
The content of such spam may often vary in its details, which would render normal checksumming ineffective. By contrast, a "fuzzy checksum" reduces the body text to its characteristic minimum, then generates a checksum in the usual manner. This greatly increases the chances of slightly different spam emails producing the same checksum.
Checksum schemes include parity bits, check digits, and longitudinal redundancy checks. Some checksum schemes, such as the Damm algorithm, the Luhn algorithm, and the Verhoeff algorithm, are specifically designed to detect errors commonly introduced by humans in writing down or remembering identification numbers.