Question

I can’t find the answer to this question anywhere on the Internet. I was wondering what happens when you receive a keep alive message on a bittorrent protocol? are we suppose to send a keep alive message back?

Answer 1

From the Unofficial BitTorrent Specification:

keep-alive: <len=0000>

The keep-alive message is a message with zero bytes, specified with the length prefix set to zero. There
is no message ID and no payload. Peers may close a connection if they
receive no messages (keep-alive or any other message) for a
certain period of time, so a keep-alive message must be sent to
maintain the connection alive if no command have been sent for a given
amount of time. This amount of time is generally two minutes.

I’m no BitTorrent expert, but based on this and what knowledge of network protocols I do have, it sounds like the answer to your question

are we suppose to send a keep alive message back?

is no. As for

what happens when you receive a keep alive message?

well, nothing — what matters is what happens when you don’t receive a keep-alive (or any other) message after the mentioned "given amount of time." In a BitTorrent implementation, receiving a keep-alive message would probably reset a timer, or update a "last message received" timestamp.