Re: Fragment basic ques

Happy..-:)

In your outputs, the last fragments from all the routers would have the MF
flag unset or 0.(indicating its the last fragment in the series)
The sending router performs the fragmentation for the MTU for the underlying
link.

MF flag would be always set for the first fragment and unset for the last
fragment.

This is an eg between two links

2000 byte packet traversing R1 - R2 (excludes Ip header)
===================================
At R1 it gets fragmented into:-

First fragment
==============
1500 bytes (1480 byte Data + 20 byte Ip header)
The MF flag is set and the offset is 0

Second fragment
===============
540 bytes (520 byte Data and 20 byte Ip header)
The MF flag is unset and offset is 185
The offset is calculated simply by dividing the data by 8 byte blocks.
(1480/8=185)

At the reciever, its calculated like
185*8 + 520 should give you the reassembled data size = 2000 bytes which we
sent initially.

Best Regards,
Tharak Abraham Luke

On Tue, Sep 21, 2010 at 9:12 AM, Happy Singh <happynetworkingsingh_at_gmail.com
> wrote:

> Hi
>
> I have some very basic doubt in fragmentation .
> Lets say we have following scenario :
>
>
> R1 ---------- R2 ------------R3------------R4
>
> R1-R2 MTU 1500
> R2-R3 MTU 800
> R3-R4 MTU 400
>
> Lets say pkt of 2000 bytes arrives at R1 . It splits in 1500 and 500 . Let
> MF means "more fragments"
>
> MF = 1 Fragment offset = 0
> MF =1 Fragment offset = 1501 <<<<<<<<<<<<< MF will be unset since this is
> the last fragment R1 is gonna sent
>
> These are fields written in IP header
>
> Now next link R2 -- R3
>
> MF = 1 Fragment offset = 0 ( 0 to 800)
> MF = 1 Fragment offset = 801 ( 801 to 1500)
> MF = 1 Fragment offset 1501 ( 1501 to 2000)
>
> Next link R3 - R4
>
> MF = 1 Fragment offset =0 ( 0 to 400)
> MF = 1 Fragment offset = 401 ( 401 to 800)
> MF = 1 Fragment offset = 801 ( 801 to 1200 )
> MF = 1 Fragment offset = 1201 ( 1201 to 1500 )
> MF = 1 Fragment offset = 1501 ( 1501 to 1900 )
> MF = 0 Fragment offset = 1901 ( 1901 to 2000 )
>
>
> Please let me know if these calculation is correct ? Please guide where MF
> will be 0 and 1 ..(which router)
>
> Thnx
> Happy
>
>
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Received on Tue Sep 21 2010 - 18:18:39 ART