 Welcome to the session. In this session, we will see fragmentation. Learning outcomes at the end of this session, student will be able to discuss fragmentation and assembly of datagram and how the original datagram can be recovered out of fragmented ones. Now, let us see the need of fragmentation first. A datagram can be travelled through different networks. Router, de-capsulate the IP datagram, process it and encapsulate it into a new frame. The format and size of the sent and received frame is depends on the protocol used by physical network. When the frame is transmitted from one physical network to another, the size of the IP datagram get changed. Some physical network cannot accept the large IP datagram, hence there is a need of fragmentation. For example, if a router connects a LAN to a WAN, it receives the frame in the LAN format and send the frame in the WAN format. Fragmentation in fragmentation, the data field of a large IP packet is fragmented. The fragments are sent into a series of smaller IP packets fitting to a network MTU. Here, MTU stands for maximum transfer unit. Fragmentation is done by router or source host. Fragmentation may be done multiple times along the route. If IP packet is longer than the MTU, the router breaks the packet into smaller packets called as IP datagram, sorry, IP fragments. Fragments are still known as IP packets. Now, let us see, in this diagram there is a sender and receiver. Router sends IP packet to a router, router receives it and decides whether to fragment or not. Now, this IP packet is fragmented into three IP packets. Then these three IP packets are travelled through an internet and received by router too. Router too forward it to the receiver. Now, here router does the fragmentation function. These router or the host, source host, they can able to fragment the IP packets. But on the another side, that means on the receiver side, only destination host can be able to reassemble the packets. So, this is a fragmentation. Now, let us see a very important topic that is MTU, which is occur in a frame. MTU is a maximum transfer unit. Each data link layer protocol has its own frame format in most protocols. One of the field defined in the format is the maximum size of the data field. The datagram is encapsulated in a frame. The total size of the datagram must be less than the maximum size. Now, here this IP datagram is encapsulated into this frame and this frame contains header and trailer. Now, the maximum length of the data that can be encapsulated in a frame is known as MTU. Value of the MTU is differ from one physical network protocol to another. Example, the value of Ethernet LAN is 1500 bytes. Similarly, for the FDDI LAN it is 4352 bytes and for point to point protocol it is 296 bytes. So, the total data in the datagram is fragmented whereas the other fields like addresses, source address and destination address and also a checksum part these things are not or these fields are not fragmented. Now, let us see the fields related with the fragmentation. Fragmentation has total three fields identification, flags and fragmentation offset. Let us see the fragmentation first, sorry identification first. This 16 bit field identifies a datagram originating from the source host. The combination of the identification and source IP address must uniquely define a datagram as it leaves the source host. To guarantee uniqueness, the IPv4 protocol uses a counter to label the datagram, counter is initialized to a positive number, counter is kept in a main memory and uniqueness is guaranteed. All fragments have the same identification number same as the original datagram. Now, the second field related to the fragmentation is flag. Flag is a 3 bit field, the first bit is the reserved and the second bit is known as do not fragment bit which is denoted by D. Now, if its value is 1, the machine must not fragment the datagram. If it cannot pass the datagram through any available physical network, it discard the datagram and send an ICMP error message to the source host. ICMP stands for Internet Control Message Protocol. When this d bit is set to 0, the datagram cannot be fragmented if necessary. Now, the third bit is called the more fragment bit. If its value is 1, it means the datagram is not last fragment. There are more fragments after this one. So, this diagram shows D and M bit. So, D if it is set to 0, datagram can be fragmented. If it is set to 1, do not fragment. If M bit is set to 0, then the last fragment or only fragment and if it is set to 1, that is not the last fragment. Now, let us see the third important field that is fragmentation offset. The 13 bit field shows the relative position of this fragment with respect to the whole datagram. It is the offset of the data in the original datagram measured in the unit of 8 bytes. Now, pause the video and write down the question and write down the answer of the question. A packet has arrived with the D bit value of 0 and M bit value of 1. What is the position of fragment? If the D bit is 0, it means that received datagram is fragmented and if M bit is 1, it means that there is at least one more fragment. The fragment can be the first one or middle one, but not the last. Now, let us continue with the fragmentation offset. Figure shows the datagram of 4000 bytes. This IP datagram is divided into three segments. This is the first fragment, another fragment and the third fragment. First fragment having a size 1400 bytes. Second fragment again having the size 1400 bytes and the third fragment is 1200 bytes. Here the fragmentation offset is calculated that is 0 divided by 8 equal to 0. So the fragmentation offset of the original datagram is 0. Then the fragmentation offset of the first fragment is 0 and the fragmentation offset of the second fragment is 175. Similarly, the fragmentation offset of the third fragment is 350. Now, this is the explanation of previous diagram. The value of the offset is measured in unit of 8 bytes because the length of the offset field is 13 bit long and cannot represent the sequence of bytes greater than 8191. Hence host or routers fragment the datagram to choose the size of each fragment to that the first byte number is divisible by 8. Now, let us see the same example in detail. Now this is the original IP datagram where it shows the total length of the IP datagram. Now we have IP datagram of 4000 bytes, this 20 is nothing but the header bytes. So therefore, it shows the total length of the IP datagram is 4020. This is the identification number which is 14567. This is the m flag set to 0 that means it is the only fragment there is no more fragments and this 0 0 0 is the fragmentation offset. So, this original datagram is divided into 3 fragments fragment 1, fragment 2 and fragment 3. Now look at this diagram here all 3 fragments have the same identification number that is 14567, its fragmentation offset is 0, now its second fragment have the fragmentation offset 175 and the third fragment have the fragmentation offset 350. Now the second fragment is divided into 2 more fragments that is fragments 2.1 and the fragment 2.2. Now the fragmentation offset of the fragment 2.1 is 175 and the 2.2 is 275. So this is the fragmentation. Now in this session we have studied what actually a fragmentation is, why there is a need of fragmentation and how the fragments are reassembled. This is the reference, thank you.