Question on Loosely Synchronized Router Clocks

William Herrin herrin-nanog at
Tue Sep 18 18:15:52 UTC 2007

On 9/18/07, Xin Liu <smilerliu at> 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.


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.

Bill Herrin

William D. Herrin                  herrin at  bill at
3005 Crane Dr.                        Web: <>
Falls Church, VA 22042-3004

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