From proposal236: ``` From dir-spec.txt: "Guard" -- A router is a possible 'Guard' if its Weighted Fractional Uptime is at least the median for "familiar" active routers, and if its bandwidth is at least median or at least 250KB/s. When this proposal becomes effective, authorities should change the bandwidth threshold for being a guard node to 2000KB/s instead of 250KB/s. ```