The IPv4 Fragment Offset field is 13 bits long and records a fragment’s position within the original packet. Together with Identification and Flags, it enables the destination to reassemble fragments, even when they arrive out of order. section 7.1.5
Use the field in the context of IPv4 fragmentation:
- A packet larger than the path’s maximum transmission unit (MTU) may be divided into smaller fragments. section 7.1.5 section 7.4
- The Identification field groups fragments from the same original packet, while the Flags field indicates whether more fragments remain. section 7.1.5
Lower-MTU path
A router with an MTU below the packet’s size fragments the packet; the destination uses the offset and related fields to restore it. The standard MTU is 1,500 bytes. section 7.1.5 section 7.4
