Types of NAT
The availability of the TCP-hole-punching technique depends on the type of computer port allocation used by the NAT. When two peers, A and B, initiate TCP connections by binding to local ports Pa and Pb, respectively, they need to know the remote endpoint NAT port in order to make the connection.
A NAT port allocation can be one of the two:
- predictable: the gateway uses a simple algorithm to map the local port to the NAT port. Most of the time a NAT will use port preservation, which is to say that the local port is mapped to the same port on the NAT.
- non predictable: the gateways uses an algorithm that is either random or too impractical to predict.
Connection matrix representing the different cases:
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A predictable A non-predictable B predictable YES YES B non-predictable YES NO
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- YES: the connection will work all the time
- NO: the connection will almost never work
Read more about this topic: TCP Hole Punching
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