a.  Per-subscriber limits MUST be configurable by the CGN
       administrator.

   b.  Per-subscriber limits MAY be configurable independently per
       transport protocol.

   c.  Additionally, it is RECOMMENDED that the CGN include
       administrator-adjustable thresholds to prevent a single
       subscriber from consuming excessive CPU resources from the CGN
       (e.g., rate-limit the subscriber's creation of new mappings).

Justification:  A CGN can be considered a network resource that is
   shared by competing subscribers.  Limiting the number of external
   ports assigned to each subscriber mitigates the denial-of-service
   (DoS) attack that a subscriber could launch against other
   subscribers through the CGN in order to get a larger share of the
   resource.  It ensures fairness among subscribers.  Limiting the
   rate of allocation mitigates a similar attack where the CPU is the
   resource being targeted instead of port numbers.  However, this
   requirement is not a MUST because it is very hard to explicitly
   call out all CPU-consuming events.