IP fragmentation – Questions and answers

John Howard December 16, 2021December 16, 2021 0 Comments

Networking Questions and answers to prepare for exams, tests, and certifications. These questions are taken from a real written exam and some parts are taken from an interview. So you will find questions on IP fragmentation. This Questions and answers will easily prepare anyone to pass their exam.

1. What is IP fragmentation?

The fragmentation of an IP datagram into two or more smaller IP datagrams is called an IP fragmentation.

2. When does IP fragmentation occur?

Fragmentation occurs when an IP datagram crosses a network whose maximum transmission unit (MTU) is less than the datagram size. For example, let’s take a standard Ethernet datagram of 1500 bytes. If a larger datagram were to cross an Ethernet network, fragmentation would be required to prevent it from being dropped somewhere on the network. The fragments continue to their destination, where the receiving host reassembles them into the original datagram.

3. Why is an IP datagram fragmented?

Each medium limits the maximum size of a frame (MTU) that it can transmit. Since IP datagrams are encapsulated in frames, the size of the IP datagram is also limited. If the size of an IP datagram exceeds this limit, it must be fragmented.

4. Which RFCs deal with IP fragmentation?

RFC 791 and RFC 815 deal with IP datagrams, fragmentation and reassembly.

5. Is it possible to select an IP datagram size to always avoid fragmentation?

It is not possible to select a particular size of an IP datagram to always avoid fragmentation, depending on the MTU. However, it is possible, for a given path, to select a size that does not lead to fragmentation. This operation is called Path MTU Discovery (PMTUd) and is described in RFC 1191 which allows to determine the MTU size on the path between two IP hosts, in order to avoid packet fragmentation. The TCP transport protocol tries to avoid fragmentation by using the MSS (Maximum Segment Size) option.

6. Where an IP datagram can be fragmented?

An IP datagram can be fragmented on the sending host or on one of the intermediate routers.

7. Where does the IP datagram fragment get reassembled?

IP fragments are reassembled only on the destination host.

8. How to prevent an IP datagram from fragmentation?

Fragmentation of an IP datagram can be prevented by setting the DF (Don’t Fragment) flag in the IP header.

9. What happens when a datagram must be fragmented to cross a network, but the DF (Don’t Fragment) flag in the datagram is on?

The datagram with the DF (Don’t Fragment) flag set, the datagram is dropped if it needs to be fragmented to cross a network. In addition, an ICMP error message is returned to the sender of the datagram.

10. Will all fragments of a datagram reach the destination via the same path?

Different fragments of the same IP datagram can take the same or different paths to the destination.

11. Will all fragments of a datagram reach the destination system in the correct order?

The different fragments of the same IP datagram can reach the destination system in no particular order.

12. What happens to the original datagram when one or more fragments are lost?

When one or more fragments of an IP datagram are lost, the entire IP datagram is deleted after a timeout.

13. What is the minimum size of an IP fragment?

The minimum size of an IP fragment is the minimum size of an IP header plus eight bytes of data. Most firewall devices remove the initial IP fragment (offset 0) that does not contain enough data to hold the transport headers. This means that the IP fragment normally requires 20 bytes of data in addition to the IP header in order to pass through a firewall if the offset is 0.

14. What are the limitations on the fragment size?

The size of an IP datagram fragment is limited by :

The amount of data remaining in the original datagram
MTU of the network
Must be a multiple of 8, except the last fragment.

15. How is an IP datagram fragment distinguished from an unfragmented IP datagram?

A complete IP datagram is distinguished from an IP fragment by the offset field and the MF (More Fragments) indicator. For a non-fragmented IP datagram, the fragment offset will be zero and the MF (More Fragments) indicator will be set to zero.

16. How are fragments of a single IP datagram identified?

The “Identifier” field in the IP header is used to identify fragments of a single IP datagram. The value of this field is defined by the sending computer. It is unique for the source and destination as long as the datagram is active.

17. How is the last fragment of an IP datagram identified?

The last fragment of an IP datagram is identified by the “More Fragments” (MF) flag. The “More Fragment” (MF) flag is set to zero for the last fragment.

18. How is the total length of the IP datagram calculated from the received IP fragments?

Using the fragment offset field and the length of the last fragment, the total length of the IP datagram is calculated.

19. How an IP datagram is fragmented?

In the following example, an IP datagram is fragmented into two fragments. This same algorithm can be used to fragment the datagram into “n” fragments.

The IP layer creates two new IP datagrams, whose length meets the requirements of the network in which the original datagram is going to be sent.
The IP header of the original IP datagram is copied into the two new datagrams.
The data from the original IP datagram is split into two 8-byte blocks. The number of 8-byte blocks in the first part is called the Number of Fragment Blocks (NFB) or Number of fragment blocks.
The first part of the data is placed in the first new IP datagram.
The length field of the first new IP datagram is set to the length of the first datagram.
The fragment offset field in the first IP datagram is set to the value of this field in the original datagram.
The “more fragments” (MF) field of the first IP datagram is set to one.
The second part of the data is placed in the second new IP datagram.
The length field of the second new IP datagram is set to the length of the second datagram.
The “More Fragments” (MF) field of the second IP datagram is set to the same value as the original IP datagram.
The fragment offset field in the second IP datagram is set to the value of this field in the original datagram plus NFB.