17 views I am facing serious troubles while calculating number of fragments. I read a formula #fragments = ceil (payload/ mtu-header)  but this isn’t giving correct answer in all the cases.

Before I show my approach, I would like to ask what does PDU imply? Does it mean header is already added? Because in questions like “TL PDU is X bytes”, I assume that according to NL, X is payload, to which it adds IP header. So NL PDU is X bytes or (X+ ip header) bytes?

For question 26:

Assuming NL PDU means it includes header(20B) , then useful data (payload) = 17056 bytes which is to be fragmented. MTU is 200B out of which 20B is for header. so it can carry 180 of useful data. So for the formula I read till now(calling it formula 1), #frags= ceil (17056/180)=95

But the fragments can’t carry 180B of data, it has to be multiple of 8. So it carries 176B. (calling this formula 2). So #frags=ceil(17056/176)=97

Now assuming NL PDU means only useful data, ie. payload is 17076B which is to be fragmented. Using the formula 1 #frags= ceil(17076/180)=95

Using formula 2, #frags= ceil(17076/176)=98

No match either. Where am I going wrong?

| 17 views