IP Fragmentation for Dummy like Me!

Recently I encounter this question, and for some reason I answer it wrong 😓


 I answer "d", which if my boss saw me he would just slap me or something, hahahaha.

So what is the right answer? Lets explore IP Fragmentation and seek the truth.

In short. IP Fragmentation means in situation of where before transmission, when the IP packet is too large, larger than the Maximum Transmission Unit (MTU) of an interface, it shall be fragmented (split) or discarded, depends on situation.

There are situation where fragmentation will occur. We will explore these 2 situation.

1 - Fragmentation at Router.

  • During Packet transmission, PC will ask the router what is the MTU.
  • It will later split the packet into several fragments. Those fragments will hold an identifier in the header. It will later reassembled by receiver into an original PDU (Protocol data unit, contains Packet Header and Packet Payload).
  • This has disadvantage over the router where it will make router to work more just to split packets.
  • Router that receive this long packet will create new packet by copying the header. The data section however is divided in potions on a 8 byte boundary. Also the router will include Fragment Offset, or more-fragment flag (MF) in the header noting the packet sequence of the split fragments.
  • The major problem is if multiple hop have smaller and smaller MTU, the packet is keep being fragmented, adding more work to router.

2 - Fragmentation at Sender
  •  Using Path MTU Discovery (PMTUD), Sender already probe using ICMP indicating the MTU size of the next hop. 
  • IP Packet created is later following the MTU so the router did not have to do extra job re-splitting and repackage back the Packet before transmitting again.
  • This is the recommended method used in the Internet. 
From above explanation, you should know what is the correct answer already!
Psst, it is "b" 😛

Also, you should able to answer the next question here:


Source:
  • https://erg.abdn.ac.uk/Users/gorry/course/inet-pages/ip-fragmentatiion.html
  • https://erg.abdn.ac.uk/users/gorry/eg3567/inet-pages/ip-packet.html
  • https://notes.shichao.io/tcpv1/ch10/#ip-fragmentation

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