How do PGP keyservers synchronize the keys?

I want to know technical details about how public PGP keyservers synchronize the keys.

If I send my key to one keyserver, how exactly does it "travel" to all the other ones? Who sends it to who and how? What would I need to know it I wanted to write my own public key server software from scratch?

I am trying to look for either some protocol description or even the actual code that takes care of this, but I cannot find it.

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One of the examples I had in mind were keyservers like http://pgp.mit.edu and http://ift.tt/1vZ2z2Y. As far as I know, if I upload a key to the first one, it gets somehow "magically" transfered to the other one, and then dozens of other keyservers publicly found on the internet.

I am asking for concrete standards how those keyservers exchange the submitted keys, and if there is not one concrete strategy, then what strategies are usually used.