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I am creating a custom protocol over UDP and I want to use a low number of bits for the sequence number. This means that the number can wrap relatively fast. Is setting the TTL for the outgoing packets to a low value a way to ensure that any packet received was actually sent less than TTL time ago?

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No. The IP packet TTL effectively counts down the L3 or router hops the packet takes. Setting the TTL to a custom value n ensures that the packet cannot take more than n hops on its path. If it does it is discarded by the router on the nth hop.

TTL started out as a combined time/hop counter (hence its name) but quickly evolved to becoming a hop counter only.

To accomplish what you're asking you'd need to use a larger sequence counter or include an actual timestamp. Actually, using a 32-bit sequence counter covers extensive ground while taking up just four bytes in the datagram. Depending on the packet frequency, even 16 bit might suffice.

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