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RFC 791 describes the procedure for IP fragmentation, and transmission and reassembly of IP packets. [1] RFC 815 describes a simplified reassembly algorithm. [2] The Identification field along with the foreign and local internet address and the protocol ID, and Fragment offset field along with Don't Fragment and More Fragments flags in the IP header are used for fragmentation and reassembly of ...
An IP header is at least 20 bytes long, so the maximum value for "Fragment Offset" is restricted to 8189, which leaves room for 3 bytes in the last fragment. Because an IP internet can be connectionless, fragments from one packet may be interleaved with those from another at the destination.
This field specifies the offset of a particular fragment relative to the beginning of the original unfragmented IP datagram. Fragments are specified in units of 8 bytes, which is why fragment lengths are always a multiple of 8; except the last, which may be smaller. [39] The fragmentation offset value for the first fragment is always 0.
When fragmentation is performed, each IP fragment needs to carry information about which part of the original IP packet it contains. This information is kept in the Fragment Offset field, in the IP header. The field is 13 bits long, and contains the offset of the data in the current IP fragment, in the original IP packet.
Each subsequent packet consists of three parts: the per-fragment headers, followed by the Fragment extension header, and by a part of the original payload as identified by a Fragment Offset. The per-fragment headers are determined based on whether the original contains Routing or Hop-by-Hop extension header. If neither exists, the per-fragment ...
This field is the length of the encapsulated IP packet (including Outer IP Header, Inner IP Header, IP Payload). Identification: 16 bits This field is used to identify the fragments of a datagram which will be helpful while reassembling the datagram as the encapsulator might fragment the datagram. For the Outer IP Header, a new number is generated.
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.
In IPv4, the AH protects the IP payload and all header fields of an IP datagram except for mutable fields (i.e. those that might be altered in transit), and also IP options such as the IP Security Option. [20] Mutable (and therefore unauthenticated) IPv4 header fields are DSCP/ToS, ECN, Flags, Fragment Offset, TTL and Header Checksum. [12]