How do Spark Nodes communicate during a Shuffle?

I've seen from this question that Spark nodes effectively "communicate directly", but I'm less concerned with the theory and more with the implementation. Here it shows, in the "###Encryption" section near the bottom of the page, that you can configure Spark to use a number of SSL protocols for security, which would suggest, to me at least, that it uses some form of HTTP(s) for communication. My question is effectively two parts: what protocol do Spark nodes use to communicate, and how is the data formatted for this transfer?