Question

Computer Network

1. Why is the fragmented packet not put back together before reaching the final destination?

a) TCP prevents this from happening
b) This could cause checksum errors
c) The router just wants to dump the packet as fast as possible

2. What is the purpose of the More Fragments flag in the IP header? (Select all that apply)

a) When 0, indicates the packet was not fragmented
b) When 1, indicates no more fragments
c) When 0, indicates the packet is the final fragment
d) When 1, indicates there are more fragments to assemble the packet

Answer 1

1) a--->TCP prevents this from happening

There is no point in assembling the packets before reaching the destination because in future there might be a scenario where we again might want to fragment packets due to network bottleneck so TCP prevent this from happening.

Reassembly of Fragments takes place only at destination and not at routers since packets take independent path(datagram packet switching), so all may not meet at a router and hence a need of fragmentation may arise again. The fragments may arrive out of order also.

2) More fragments (MF = 1 bit) – tells if more fragments ahead of this fragment i.e. if MF = 1, more fragments are ahead of this fragment and if MF = 0, it is the last fragment.

so the answer is

c) When 0, indicates the packet is the final fragment
d) When 1, indicates there are more fragments to assemble the packet

MF with Fragment offset is used to find the packet as shown

MF	FRAGMENT OFFSET
1	0	First packet
1	!=0	Intermediate packet
0	!=0	Last packet
0	0	Invalid

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