Reassembly of IP Fragments

CDF.

When a large IP datagram is sent over a network that cannot handle its size, the sending host must fragment it into smaller pieces, which are then sent individually and reassembled by the receiving host. To reassemble IP fragments into a complete IP datagram, the receiver references the following three IP header fields:

1. Identification Field: This field identifies the IP datagram to which the fragment belongs. All fragments belonging to the same IP datagram have the same identification value.

2. Fragment Offset Field: This field indicates the position of the fragment relative to the beginning of the original IP datagram. The first fragment has a fragment offset of zero, and the remaining fragments have a fragment offset that is a multiple of 8.

3. More Fragment (MF) Flag: This flag indicates whether there are more fragments to follow in the sequence. If the flag is set to 1, then there are more fragments to follow. If the flag is set to 0, then this is the last fragment in the sequence.

The other three IP header fields mentioned in the question are not used in the reassembly process.

The Don't Fragment (DF) flag is used by the sender to indicate that the IP datagram should not be fragmented. If a router encounters a DF flag while forwarding an IP datagram that is too large for the next hop, it drops the datagram and sends an ICMP "Fragmentation Needed" message back to the sender.

The Packet is Fragmented flag is not a valid IP header field.

The Number of Fragments field is not an IP header field. The number of fragments is calculated based on the total length of the IP datagram and the Maximum Transmission Unit (MTU) of the network over which the IP datagram is being sent.

In summary, the three IP header fields referenced by the receiver when reassembling IP fragments into a complete IP datagram are: Identification Field, Fragment Offset Field, and More Fragment (MF) Flag.