http2/http2-spec — pull request #91 by jasnell (2013-05-10 16:30:40+00:00)
Title: Design: Limit Ping to 8-bytes of frame data

Currently, the PING frame allows for up to (2^16)-1 bytes of arbitrary data.. that's just silly insane for a PING mechanism and opens the door for significant abuse (i.e. ping flood). Let's limit it to 8-bytes max.. that gives us enough room for a timestamp or a correlation ID without introducing too much risk.