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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 IP packet header , and represents the 16-bit result of summation of the header words.
IPv4 is the fourth version in the development of the Internet Protocol, and routes most traffic on the Internet. [1] [non-primary source needed] The IPv4 header includes thirteen mandatory fields and is as small as 20 bytes. A fourteenth optional and infrequently used options field can increase the header size.
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.
The IHL field contains the size of the IPv4 header; it has 4 bits that specify the number of 32-bit words in the header. The minimum value for this field is 5, [33] which indicates a length of 5 × 32 bits = 160 bits = 20 bytes. As a 4-bit field, the maximum value is 15; this means that the maximum size of the IPv4 header is 15 × 32 bits = 480 ...
The type of service (ToS) field is the second byte of the IPv4 header. It has had various purposes over the years, and has been defined in different ways by five RFCs. [1] Prior to the redefinition, the ToS field could specify a datagram's priority and request a route for low-latency, high-throughput, or highly-reliable service.
Field Size (bits) Description Copied: 1: Set to 1 if the options need to be copied into all fragments of a fragmented packet. Option Class: 2: A general options category. 0 is for control options, and 2 is for debugging and measurement. 1 and 3 are reserved.
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 IP header is different from the modern IPv4 header. IEN 44 Latest Header Formats ( June 1978) describes another version of IPv4, also with a header different from the modern IPv4 header. IEN 54 Internetwork Protocol Specification Version 4 ( September 1978) is the first description of IPv4 using the header that would become standardized in ...