Re: Fragment question again ..

The router that will frag the fragment will take the 1500 byte and will set
the offset for the new frag

so you will have

1 pkt 1500 with offset 0 from 5000
2 pkt 1500 with offset 1501 from 5000
3 pkt 1500 with offset 3001 from 5000--> now lets say that packet 3 is going
through another frag off 500 byte
           3.1 pkt 500 with offset 0 from 1500
           3.2 pkt 500 with offset 501 from 1500
           3.3 pkt 500 with offset 1001 from 1500
4 pkt 500 with offset 4501 from 5000

so the router will receive fragment 1 2 and now 3 he will see another
fragment and only after 3 is assemble he will be able to assemble 4

On Sun, Sep 26, 2010 at 11:29 AM, Happy Singh <
happynetworkingsingh_at_gmail.com> wrote:

> R1 -------------- R2 -------------- R3
> | |
> | |
> | |
> 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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