For data packets whose length exceeds the MTU of the interface, fragmentation processing is required. The fields related to fragmentation in the IP header are as follows:

Identification - Used to confirm whether different fragments belong to the same IP packet.
Flags - where IP_MF indicates that there are also fragments, which are intermediate fragments.
Fragment Offset - Represents the offset address of this fragment in the entire message.

I have a question, how to understand the Flags is IP_MF, what will happen? is the IP fragmentation which flags = IP_MF will be divide to other fragmentations(I call it sub-fragmentation)? If is, how can the sub-fragmentation relate to the IP fragmentation which flags = IP_MF?

1 Answer 1


The More Fragments flag is set for all fragments except the last fragment. It is possible that a packet is fragmented into more than two fragments, and each fragment except the last must have the flag set to tell the destination host that more fragments are expected.

For example, a router could receive a full-size packet on a serial interface with an MTU of 4500 and need to send it to an ethernet interface with an MTU of 1500. That would result in multiple fragments, all of which, except the last fragment, would have the MF flag set.

The full IPv4 fragmentation process (IPv6 does not allow intermediate devices fragment packets) is explained in RFC 791, Internet Protocol. This is the main explanation, but the entire RFC has details on fragmentation (fragmentation is one of the main features of IPv4):


Fragmentation of an internet datagram is necessary when it originates in a local net that allows a large packet size and must traverse a local net that limits packets to a smaller size to reach its destination.

An internet datagram can be marked "don't fragment." Any internet datagram so marked is not to be internet fragmented under any circumstances. If internet datagram marked don't fragment cannot be delivered to its destination without fragmenting it, it is to be discarded instead.

Fragmentation, transmission and reassembly across a local network which is invisible to the internet protocol module is called intranet fragmentation and may be used [6].

The internet fragmentation and reassembly procedure needs to be able to break a datagram into an almost arbitrary number of pieces that can be later reassembled. The receiver of the fragments uses the identification field to ensure that fragments of different datagrams are not mixed. The fragment offset field tells the receiver the position of a fragment in the original datagram. The fragment offset and length determine the portion of the original datagram covered by this fragment. The more-fragments flag indicates (by being reset) the last fragment. These fields provide sufficient information to reassemble datagrams.

The identification field is used to distinguish the fragments of one datagram from those of another. The originating protocol module of an internet datagram sets the identification field to a value that must be unique for that source-destination pair and protocol for the time the datagram will be active in the internet system. The originating protocol module of a complete datagram sets the more-fragments flag to zero and the fragment offset to zero.

To fragment a long internet datagram, an internet protocol module (for example, in a gateway), creates two new internet datagrams and copies the contents of the internet header fields from the long datagram into both new internet headers. The data of the long datagram is divided into two portions on a 8 octet (64 bit) boundary (the second portion might not be an integral multiple of 8 octets, but the first must be). Call the number of 8 octet blocks in the first portion NFB (for Number of Fragment Blocks). The first portion of the data is placed in the first new internet datagram, and the total length field is set to the length of the first datagram. The more-fragments flag is set to one. The second portion of the data is placed in the second new internet datagram, and the total length field is set to the length of the second datagram. The more-fragments flag carries the same value as the long datagram. The fragment offset field of the second new internet datagram is set to the value of that field in the long datagram plus NFB.

This procedure can be generalized for an n-way split, rather than the two-way split described.

To assemble the fragments of an internet datagram, an internet protocol module (for example at a destination host) combines internet datagrams that all have the same value for the four fields: identification, source, destination, and protocol. The combination is done by placing the data portion of each fragment in the relative position indicated by the fragment offset in that fragment's internet header. The first fragment will have the fragment offset zero, and the last fragment will have the more-fragments flag reset to zero.

It is also possible that fragments will need to be further fragmented later in the path. The router performing the further fragmentation will fragment the fragments in the same way.

There are questions with answers here that explain this. For example: this question and this question.

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