IP layer concepts
 

hello,
If a pc wants to send a packet to next hop(say host / router)then
it first does PMTU discovery and if it comes to know that next hop
uses 1500 bytes as its NIC configured and if it got 1450 bytes packet
does it directly forwards packet? and what if reverse condition is
that mean PMTU discovery is must so if pc got 1500 bytes packet and next hop has 1450 MTU then does that packet will be dropped? if yes then what happens at this situation?

regards,
rahul

Yes
the packet is not dropped, it is fragmented in 2 packets ;)
Note that once your packet is fragmented, it is re-put together once the destination is reached and not at the next hop :)

If one host sends a 1500 byte packet and the upstream interface on the next hop has an MTU of 1400 then the upstream host is responsible for fragmenting the IP packet. The destination host is then responsible for re-assembly.

The complication comes when the IP datagram has the do-not-fragment flag set. The upstream router must then discard the packet and send an ICMP "Destination Unreachable - Fragment needed on DF set " message back to the source of the packet. The Source device will then drop the MTU a bit and try again. Eventualy the packet will be reduced enough to get through. This sequence of events is known as Path MTU Discovery or PMTUD.

Using DF was done to reduce unnecessary loading on internet routers, as it pushes the responsibility and thus the processing overhead for getting the MTU correct back onto the host which is more likely to have cpu cycles to spare.