Notes: NAT Addressing and Port Mapping and Filter Behavior - RFC4787
NAT Addressing and Port Mapping
Endpoint-Independent Mapping
The NAT reuses the port mapping for subsequent packets sent from the same internal IP address and port (X:x) to any external IP address and port. Specifically, X1':x1' equals X2':x2' for all values of Y2:y2.
Address-Dependent Mapping
The NAT reuses the port mapping for subsequent packets sent from the same internal IP address and port (X:x) to the same external IP address, regardless of the external port. Specifically, X1':x1' equals X2':x2' if and only if, Y2 equals Y1.
Address and Port-Dependent Mapping
The NAT reuses the port mapping for subsequent packets sent from the same internal IP address and port (X:x) to the same external IP address and port while the mapping is still active. Specially, X1':x1' equals X2:x2' if and only if Y2:y2 equals Y1:y1.
REQ-1: A NAT MUST have an "Endpoint-Independent Mapping" behavior.
NAT Filtering Behavior
The key behavior to describe is what criteria are used by the NAT to filter
packets originating from specific external endpoints.
Endpoint-Independent Filtering
The NAT filters out only packets not destined to the internal address and port X:x, regardless of the external IP address and port source (Z:z). The NAT forwards any packets destined to X:x. In other words, sending packets from the internal side of the NAT to any external IP address is sufficient to allow any packets back to the internal endpoint.
Address-Dependent Filtering
The NAT filters out packets not destined to the internal address X:x. Additionally, the NAT will filter out packets from Y:y destined for the internal endpoint X:x if X:x has not sent packets to Y previously(independently of the port used by Y). In other words, for receiving packets from a specific external endpoint, it is necessary for the internal endpoint to send packets first to that specific external endpoint's IP address.
Address and Port-Dependent Filtering
This is similar to the previours behavior, except that the external port is also relevant. The NAT filters out packets not destined for the internal address X:x. Additionally, the NAT will filter out packets from Y:y destined for the internal endpoint X:x if X:x has not sent packets to Y:y previously. In other words, for receiving packets from a specific external endpoint, it is necessary for the internal endpoint to send packets first to that external endpoint's IP address and port.
REQ-8: If application transparency is most important, it is RECOMMENDED that a NAT have an "Endpoint-Independent Filtering" behavior. If a more stringent filtering behavior is most import, it is RECOMMENDED that a NAT have an "Address-Dependent Filtering" behavior.
a) The filtering behavior MAY be an option configurable by the administrator of the NAT.
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