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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.
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.
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.
Internet Protocol version 4 (IPv4) is the first version of the Internet Protocol (IP) ... the router must calculate a new header checksum before sending it out again.
When TCP runs over IPv4, the method used to compute the checksum is defined as follows: [16] The checksum field is the 16-bit ones' complement of the ones' complement sum of all 16-bit words in the header and text. The checksum computation needs to ensure the 16-bit alignment of the data being summed.
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A checksum of a message is a modular arithmetic sum of message code words of a fixed word length (e.g., byte values). The sum may be negated by means of a ones'-complement operation prior to transmission to detect unintentional all-zero messages. Checksum schemes include parity bits, check digits, and longitudinal redundancy checks.
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