I would assume that it's something developed internally at Amazon. Networking inside of AWS isn't standard fare and I doubt something like NetScaler or F5 products would be able to be used. Generally speaking, they aren't using TCP/IP behind the curtain, to move packets between nodes. AWS has even created their own routing hardware/software because no other company could do what they need at the scale that they need. See this video for more information: https://www.youtube.com/watch?v=St3SE4LWhKo
AWS & Amazon uses a LOT of routers from one of these vendors. Although, they would probably try and avoid baking it into a public-facing product like this.
