We then localized where (with respect to our measurement system) in the network topology the GFW operates, as follows. For a given measurement packet, we can control how far into the network it transits from our measurement system to its destination by controlling the packet’s TTL value. The TTL value determines after how many intermediate hops a packet will be discarded by the Internet’s internal routers. We sent the “Falun” queries from our test system to the Baidu server with TTL values increasing from 1 on up. We observed that the GFW’s TCP Reset injection only occurred when we sent packets with TTL values >= 18, suggesting that the GFW acts on traffic flowing between the 17th and 18th hop along the path from our test system to the Baidu server (which was itself 24 hops away from our test system). This packet capture shows our localization results.11
接 下来我们定位了GFW在网络拓扑上的位置（向我们的测量系统表示尊敬），接下来开始说明：对于一个给出的用于测量的包，我们可以通过控制包的 TTL(Time To Live，存活时间或跳步数)值来控制包能深入网络的最远距离。TTL值决定了经过多少中间跳板后一个包会被互联网内部的路由器丢弃。我们从测试系统发送 了“Falun”查询请求，目标是百度服务器，TTL值则从1开始逐渐增加。我们观察到GFW的TCP Reset包注入只发生在包的TTL值>=18时，这表明对于我们的测试系统和百度服务器（和测试系统之间有24跳的距离）之间的通信，GFW在第 17跳和第18跳之间对通信流进行了处理。这个抓包结果显示了我们的定位结果。[11]
The GC operates as a separate, in-path system
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