Re: Fragment question again ..

HI Shiran

This is not always correct ..

Lets assume that fragment ( last fragment ) with MF bit set to 0
reaches the final (dest) router before the 3.3 fragment in your case .
In such a case ; router has already received a fragment with MF = 0
and hence will not consider any other fragments coming with same
Identifier bytes .

I hope you are getting my ques .

Thnx
Happy !

On Sun, Sep 26, 2010 at 11:00 PM, shiran guez <shiranp3_at_gmail.com> wrote:
> 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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>
>
> --
> Shiran Guez
> MCSE CCNP NCE1 JNCIA-ER CCIE #20572
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Received on Mon Sep 27 2010 - 11:15:21 ART