Re: Fragment question again ..

Happy,

I was thinking why you didnt ask this question in the beginning..-:)
Well, i hope to answer your query and if not there are better and smarter
ppl out here for that.

The situation that you explained is less likely to happen (still wondering
if it can...)

When the fragments arrive, a small data buffer is created and if at all a
fragment with MF 1 arrives a descriptor list is created and it waits to fill
the list until all the 4 factors of ID, length, offset and MF are satisfied.

Now adding to Carlos, Gaurav and Shiran's thread

The last device will eventually get only one fragment with MF=0, i.e the
last fragment of the initial jumbo packet/frame.
Adding to Gaurav's eg: the fragment 3 which is a 1500 byte one when get
fragmented again would make 3.1, 3.2 and 3.3 of 500 bytes packet with all MF
= 1 and the offset would be calculated in an increasing order.
Carlos is correct as far as i understood him.

The last fragment 4.3 in your eg would have MF=0 and the offset value would
be so that it can recompute the initial packet.
i.e the offset value of the last fragment * 8 + the data size it carries
gives the original 5000 byte packet which Happy send..-:)

You may test it with 3 routers, but may not be able to see the MF unless you
have sniffers deployed.
There is a capture option in Dynagen which saves the capture per interface
to .cap files which can be opened in Wireshark. (if someone has time to try
it will help all of us)

Best Regards,
Tharak Abraham Luke

On Sun, Sep 26, 2010 at 2:59 PM, 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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Received on Mon Sep 27 2010 - 23:30:42 ART