This is the secdir review, intended for the security AD's. Others should treat this as normal last-call comments. This describes a BGP extension code to increase the size of BGP messages. It nicely explains deployment issues (if a BGP node doesn't understand the extension), and security issues. The extension is disallowed for two operations, although only one has a rationale; it was left to reader to intuit that KEEPALIVE doesn't need the longer size. I suggest making that explicit. That is a nit. Looks good; ship it.