IP Fragmentation and Reassembly
I am currently going through my networking slides and was wondering if someone could help me with the concept of fragmentation and reassembly.
I understand how it works, namely how datagrams are split into smaller chunks because network links have a MTU. However the example in the picture is confusing me.
So the first two sections show a length of 1500, because this is the MSU, but shouldn't this mean that the last one should have 1000 (for a total of 4000 bytes) and not 1040? Where did these extra 40 bytes come from? My guess is that because the previous two fragments both had a header of 20 bytes, this extra 40 bytes of data needed to go somewhere, so it will arrive in the last fragment?
Fragflag essentially means that there is another fragment, so all of them will have a Fragflag of 1 except the last fragment which will be at zero. However I don't understand what offset is or how it is calculated. Why is the first offset at zero? Why did we divide the bytes in the datafield (1480) by 8 to get the second offset? Where did this 8 come from? Aside from that, I am assuming that each fragments offset will just increase by this value?
For example, the first fragment will have a offset of 0, the second 185, the third 370 and the fourth 555? (370+185)
Thanks for any help!
Answer
There is a 20 byte header in each packet. So the original packet contains 3,980 bytes of data. The fragments contain 1480, 1480, and 1020 bytes of data. 1480 + 1480 + 1020 = 3980
Every bit in the header is precious. Dividing the offset by 8 allows it to fit in 13 bits instead of 16. This means every packet but the last must contain a number of data bytes that is a multiple of 8, which isn't a problem.