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(a): Implement a managed interface for each external telecommunication service;
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(b): Establish a traffic flow policy for each managed interface;
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(c): Protect the confidentiality and integrity of the information being transmitted across each interface;
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(d): Document each exception to the traffic flow policy with a supporting mission or business need and duration of that need;
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(e): Review exceptions to the traffic flow policy [the frequency at which to review exceptions to traffic flow policy is defined;] and remove exceptions that are no longer supported by an explicit mission or business need;
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(f): Prevent unauthorized exchange of control plane traffic with external networks;
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(g): Publish information to enable remote networks to detect unauthorized control plane traffic from internal networks; and
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(h): Filter unauthorized control plane traffic from external networks.
Supplemental
External telecommunications services can provide data and/or voice communications services. Examples of control plane traffic include Border Gateway Protocol (BGP) routing, Domain Name System (DNS), and management protocols. See [SP 800-189](#f5edfe51-d1f2-422e-9b27-5d0e90b49c72) for additional information on the use of the resource public key infrastructure (RPKI) to protect BGP routes and detect unauthorized BGP announcements.