Multiple Stage Fragmentation

Hi.., we had a discussion on how multiple stage fragmentation works..

First question was:1. where does the re-assembly of fragmented packets occur ?
Consider communication btw Device A and Device B. The IP Datagram is 12000bytes. The first link has a MTU 3300 bytes, the second link has MTU 1300 bytes and the third and final link has MTU 1300 bytes.
When the packet goes thru the first link it is fragmented to 3300 bytes, and the fragmented packets are again fragmented to 1300 bytes while passing through second link. Although the third link has MTU 3300 bytes, it is not assembled back. It is re-assembled only at the receiver Device B.
2. How does the re-assembly occurs at the receiving end ?
Every datagram whether fragmented or not is given an Identification number by the Source of the datagram - in this case Device A. This is a random number. The fragments of the datagram are identified using this number- as it is unique. Even if multiple fragmentation occurs this identification number does not change. So Device B can identify all fragments of a datagram using this.
Also where the fragments fit in is specified using "offset field" and not with an "index number". So the receiving end has no problem in assembling the packets.