[ntp:questions] multicast - TTL

Danny Mayer mayer at ntp.isc.org
Fri Feb 23 03:47:34 UTC 2007

Martin wrote:
> wa6zvp wrote:
>> The quick answer is to put this in your config file:
>> ttl 0 1 2 3 4 5 6 7
>> This creates a one-to-one mapping for ttl.
> Roger, thank you for solving the issue.
> I've read the discussion you mentioned. It looks to me that unless overridden
> with the ttl command like the one above, the whole TTL range 0-255 is reduced
> to just 8 values: 0,32,64, ... 224. The ttl option to the broadcast command is
> then not the real TTL value, but an index selecting one from those 8 values.
> It probably causes no harm to use incorrect (larger) TTL, but this feature 
> could be better documented, IMHO.

No, the code should be corrected. In spite of its name, the value is
actually a hop count when you are doing multicasting. Originally when
the IPv4 multicast protocol was being developed it was thought that this
should really be a time-to-live. In practice this didn't happen. The
routers actually take the value and subtract one and send it on if the
new value is greater than 0. With IPv6 multicasting it's actually called
a hop count. Multicasting is not really well developed beyond site-local
and I doubt that any of the Internet backbone routers will pass on
multicast packets outside of their own though I haven't checked that
myself. In fact your border routers should stop them.

In summary, the ttl on the broadcast line should be what you want to set
the outgoing ttl to and not something else. It should always been that
way. I don't understand why you want an extra line to specify what the
value means.


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