Fragment question again ..

R1 -------------- R2 -------------- R3
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| |
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R4---R5----R6------R8-----------

Hi All

Last week I discussed a few points abt fragmntation in IP networks .Thnx for
all help . Here is a interesting scenario which i wanted to clearify

Let 5000 byte pkt is rx on R1 . R1 makes 4 fragments : 1500 , 1500 , 1500 ,
500

Let is sends Frag 1 ( offset 0 ) , Frag 3 ( offset 3001 ) , fragment 4 (
offset 4501 ) to link1
Let it sends Frag 2 ( offset 1501) to link 2

Last fragment on link 1 will have MF = 0 ; others will have MF = 1 ( MF =
more fragments )

Now the pkt that goes via R4--R5--R6--R8 is required to be further
fragmented ( due to even smaller MTU )
Let it is broken as frag 1 ( offset 1501) ; frag 2 ( offset 2001) ; frag 3 (
offset 2501 ) : 4 sub-fragments of 500 bytes each .
Now these will have MF = 1 and fragment with offset 2501 will have MF = 0

Question is : AT assembling router (R3) ; we are getting 2 fragments with
MF = 0 . This will cause problem because if fragment 4 ( offset 4501 ) with
MF = 0 is recieved before frag # ( offset 2501 ) then it might assume that
since MF = 0 ; I am not expecting any other fragments .

Please tell me how is this taken care of in routers ?
I hope I have made my question clear ,

Thnx
Happy !

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Received on Sun Sep 26 2010 - 14:59:26 ART