TWC (AS11351) blocking all NTP?

Dobbins, Roland rdobbins at
Mon Feb 3 06:16:23 UTC 2014

On Feb 3, 2014, at 1:02 PM, Dobbins, Roland <rdobbins at> wrote:

> b) enforce their AUPs (most broadband operators prohibit operating servers) by blocking *inbound* UDP/123 traffic towards their customers at the customer aggregation edge

Actually, this can cause problems for ntpds operating in symmetric mode, where both the source and destination ports are UDP/123.  Allowing inbound UDP/123 - UDP/123 and then rate-limiting it would be one approach; another would be to block outbound UDP/123 emanating from customers based upon packet size, if one's hardware allows matching on size in ACLs.

Roland Dobbins <rdobbins at> // <>

	  Luck is the residue of opportunity and design.

		       -- John Milton

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