theoritic question about routers

I would like to ask a theoritic question about
routers.
When a router is receiving a packet what happens?

a)It always waits until the whole packet is received and then it forwards the packet to its   destination.

OR

b)It's possible to start forwarding the packet
at the same time(as far as the header is received and the destination is known)  
ellostronAsked:
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sunray_2003Commented:
Check this

http://computer.howstuffworks.com/router.htm

should answer your question

Sunray
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ShineOnCommented:
The type and model of router will determine that, as well as the configuration.  Not all routers have to do store-and-forward and I can't say (since I haven't seen every router ever made) that all routers can.  Same goes for the pass-through type routing you described in b.  

You will get less lag with store-and-forward disabled but you will have fewer lost packets and retransmits with it enabled, IIRC
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Steve JenningsIT ManagerCommented:
ellostron,

Your (a) and (b) describe store and forward vs. cuthrough switching. But there's a lot more that goes on in a router that has to do with answering your question. For example the protocol being routed may require that the check_sum value be valid before forwarding. Different rules have to be applied to different conditions such as fragmentation, reassembly, ACK status in TCP . . . and on and on.

If you really want an answer to this question read Richard Stevens book Understanding IP, the Protocols.

Good luck.
Steve
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ShineOnCommented:
Good point, SteveJ.
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