I have been wondering this for a few days. I basically understand how networks and packets work. But what happends when an unexpected packet arrives? Like, when I didn't send a request for a website's index, but it sends the index to my machine anyways ? Does my browser pop up? Does it ignore it? Anyway to actually catch it?

closed as not a real question by EL Yusubov, Dynamic, gnat, Martijn Pieters, Walter Jan 26 '13 at 22:55

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    Consider what is meant by "unexpected" -- there is no program registered to handle that packet. Thus, an unexpected packet is ignored. In your example, if there were a program registered to listen for spontaneous HTTP responses, then the packets wouldn't really be "unexpected", would they? There would be some service waiting for them. – apsillers Jan 26 '13 at 19:57

Simple : It gets thrown away.

If either TCP, or network layer can't accept the packet, then it is thrown away. This might happen if port is not open or packet was re-send, but original packet arrived, so it is not necessary anymore.

TCP and most network system contains rigorous checks on what packets to accept and what packet are bad or corrupted in some way. If such packet arives, it is ignored and client requests a re-send.

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    Actually, if it's a TCP packet, a response is sent back to the originating machine refusing to accept the packet. – Ross Patterson Jan 26 '13 at 22:41
  • @RossPatterson: Sometimes. A lot of systems are configured to not respond whatsoever to unexpected packets. – whatsisname Jan 27 '13 at 5:59

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