the four fragments of the first packet will share the same Identification field value.To make this very clear, if three packets are sent from host A to host B and each must be fragmented into four fragments: This does not mean the same ID should be used when fragmenting packets where the source, destination and protocol are the same but that the same ID could be used when they are not. The Identification field (16 bits) is populated with an ID number unique for the combination of source & destination addresses and Protocol field value of the original packet, allowing the destination to distinguish between the fragments of different packets (from the same source).Here’s a reminder of all the fields and their order, with fragmentation headers highlighted:įragmentation’s operation relies upon three IP header fields (32 bits in total), all of which will have very different values in the fragments compared to the original packet: The processes of fragmentation and reassembly involve a number of IP header fields being set in the fragments.
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