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RFCs & Standards Voice over IP

NAT solutions used in VoIP

NAT is still common in Voice networks, and while we’re all awaiting the full scale adoption of IPv6, it’s still going to be a thing for some time.

I thought I’d dive into some of the NAT “solutions” that are currently in use.

Old RFC 3489 Definitions

These were the first NAT implementations used, and are still often used today.

Full cone NAT

A request from a private address is mapped to a public address and a publicly available port.

Traffic can be sent from any external device to this public address / port combination, and will be sent the internal device.

This is often statically setup, where you’d log into your router and put a NAT rule saying “Traffic on Port 5060 I want forwarded to my desk phone on 192.168.1.2” for example, and is sometimes just called a “Port forward”.

This can work fine if you’ve just got one unchanging internal address, but starts to have issues with multiple devices or dynamically assigned IPs.

Restricted Cone NAT

A request from a private address is mapped to a public address.

Traffic sent to this public address from an allowed IP will be routed to the internal device, regardless of port used.

Port Restricted Cone

Like restricted cone but only a single port may be used, traffic sent to any other port will not be routed to the internal device.

Symmetric NAT

Each request to an external destination gets a unique Public IP / Port combination to be used only by that destination, and each new request with a different source port on the internal side, or different destination on the external side, sets up a new NAT path.

RFC 5389 NAT Definitions

Endpoint Independent Mapping

Each request to an external destination gets the same public IP address / Port combination used for the outbound traffic.

Return traffic from the external destination is routed based on the source address, to the internal IP of the originating user.

It’s possible to have multiple internal devices communicating with multiple external destinations, using the same public IP address / port combination for each of them.

The source IP address of the traffic back from the external destination is used to map the path back to the internal IP.

This is efficient (doesn’t need to keep using outbound ports on the public IP) but means that it’ll only work to the requested external destination’s IP.

If you register to a SIP server on one IP, and media comes in on another, an Endpoint Independent Mapping NAT will see you with one-way audio.

Address Dependant Mapping

Each request to an external destination gets a unique public IP address / port combination used for outbound traffic.

It is reused for packets sent to the same destination, regardless of which destination port is used.

Address and Port Dependant Mapping

Same as Address Dependant Mapping but a new mapping is created for each destination and port.

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