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+4 votes
Which of the following statements is/ are true about the fragmentation which is done at network layer.

(I) The offset of 1st fragment is always offset of parent.

(II) The MF of last fragment is always same MF of parent.

I think I is true and II is false:

Explanation for II :-

Host A connected to R1 and R1 connected to host B.

Link A-R1 MTU = 220 Including IP header of 20 Bytes

Link R1-B MTU = 100 Including IP header of 20 Bytes.

Host A has 400 Bytes of TCP datagram

In link A-R1:-

1st packets <20, 200> MF = 1

2nd packets <20, 200> MF = 0

In link A-R2:-

1st packet in A-R1 is divided into 3 parts

packet 1.1 <20,80> MF = 1

packet 1.2 <20,80> MF = 1

packet 1.3 <20,40> MF = 0

packet 1.3's MF != MF of packet 1 In A-R1.

is it correct.
asked in Computer Networks by Boss (17.8k points) | 449 views
@Bikram Sir, I have solved that question, but no data about MF bit.
MF bit gives the information that are there more fragments after fragment whose is MF is set to 1.

If the MF bit of parent is 1 that means the parent is already fragmented and it is not the last fragment.On further fragmentation of the parent the MF bit of the last fragment is also set to 1 because  there are fragments after its parent.

When MF bit of parent is 0 then MF bit of last fragment is set to 0 because there are no fragments after this fragment or the parent.

So second statement is true.
thanks @Groot,

No of packet having MF bit as 1 os always one. right?
It's the other way around no of fragments having MF bit 0 is always one .This shows that this is the last fragment and receiver will not recieve any other fragments. If the fragments are received by the receiver are out of order offset part of packet helps the receiver rearrange the packets in correct sequence.
Both I and II are true. You may refer page 570  (Fragmentation) in Forouzan edition 5. There is a diagram that demonstrates this.
I think both options are true

(I) The offset of 1st fragment is always offset of parent.


Parent fragment(before fragmentation) will have Offset=0 ,now if this is fragmented into say 3 fragments (F1,F2 & F3) then F1 will have Offset=0 which means no fragment is ahead of it. F2 and F3 will have offset=1 which means fragments are ahead of it

In Fragmentation always the first fragment will have offset=0

(II) The MF of last fragment is always same MF of parent.

Explanation:  MF of Parent fragment(before fragmentation) is 0, and the last fragment will also have fragment =0 which indicates no more fragments are coming after it


MF                                 OFFSET

1                                        0                --------   this is the first packet

1                                      Not 0         ------    Intermediate Fragments

0                                      Not 0    -------      Last Fragment

0                                       0      --------          Single Datagram,no fragmentation

1 Answer

+2 votes

both are correct !

see this table:


1 0 1st fragment
1 not 0 intermediate fragment
0 not 0 last fragment
0 0 not fragmented

MF of last fragment = 0, which is equal to parent because 0 MF in parent shows that original parent data is not fragmented and yet to be fragmented.

Offset of 1st fragment will always be 0 (obvious), offset of parent will also be 0 because its not yet fragmented.

answered by Active (1k points)

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