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It has always been a puzzle for me how a certain type of information can be discarded from a system when it is unused. Say, for example, the Internet Protocol (IP) specification states that packets whose version is neither 4 nor 6 are 'discarded'. Since the packets' representation is a signal of some sort, how is this information 'discarded'? Where does it go? Is it dissipated as heat or something else?

(I don't know how to tag this, or whether it's in the right section so excuse me if I'm wrong here)

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Every classical signal is dissipated as heat, no matter if the information is discarded or not! Only in a quantum computer there's a difference here. –  leftaroundabout May 1 at 19:23
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Possibly related physics.stackexchange.com/questions/110739/… –  DigitalTrauma May 1 at 19:30
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Discarded means the same as "ignored" in this context (after examination to determine you don't care about it any further). There isn't a bin someplace with a pile of discarded bits. –  Olin Lathrop May 1 at 22:01
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The circuit includes a node with the netname /dev/nul, and there's a demultiplexor... –  Ben Voigt May 1 at 22:12

2 Answers 2

up vote 14 down vote accepted

In this case, the packet is received into a memory buffer of some sort, so that its protocol version field can be examined. If it has a correct value, it is then forwarded to the next stage of processing, typically by copying the data to a different area of memory.

If not, then the memory buffer is simply over-written by the next packet received, and the data from the first packet is lost.

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I think part of the mental challenge here is remembering that a packet is not something physical. The packet information must be sent through a physical medium, though, such as copper wire, fiber-optic cable, radio waves, etc. which is why it seems confusing.

A packet can be thought of like a sentence. If we rephrase your question in the context of a verbal conversation, it would go something like this:

"When your brother tells a story that you have already heard, what happens to the sentences? How are they discarded? As heat?"

The processing component in the IP stack treats the 'discarded' packet the same way. It just ignores it and doesn't waste any energy processing it further.

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