On 9/18/07, Xin Liu <smilerliu@gmail.com> wrote:
Ideally, yes, a protocol should not rely on clock synchronization at all. However, to ensure freshness of messages, we don't have many choices, and clock synchronization seems to be the least painful one.
Xin, Depending on the character of the protocol there are at least two other options for assuring freshness: 1. Sequence numbers. A higher sequence number is fresher and lowered numbered messages should be discarded if received. 2. Lifetime decrement counter (aka TTL). Each router that sees the message decrements the counter. When the counter hits zero the message is stale and gets discarded. Like Robert says, its not even fair to assume that a router has a time of day clock, let alone one that is properly syncronized with everybody else. Even where they do, there's a bootstrapping problem if you put Time of Day in the critical path: the routing has to work before NTP can sync. Regards, Bill Herrin -- William D. Herrin herrin@dirtside.com bill@herrin.us 3005 Crane Dr. Web: <http://bill.herrin.us/> Falls Church, VA 22042-3004