[ntp:questions] multicast - TTL

David L. Mills mills at udel.edu
Sat Feb 24 00:31:47 UTC 2007


The TTL mechanism in NTP manycast expanding-ring search was designed for 
RFC 2365. Routers of the day applied administrative scoping fences 
specific to multicast packets. I don't know if routers can do that 
today. It actually was quite useful in the MBONE.

Note that the ttl option in the server configuration is the index of an 
entry in the ttl table. The ttl command sets the contents of that table. 
The manycast expanding-rung search uses the entries in the table in 
ascending order.


Danny Mayer wrote:
> 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.
> Danny
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