NAT mapping

[David30]

Would someone please explain how NAT-mapping is implemented in the SPA112?

In my case STUN is off and its associated parameters are not set, however NAT keep-alive is enabled.  I can't find any reference to the Port Control Protocol (PCP) in either the SPA112 documentation or the router's User Guide.

Regards,

David

[Dan Lukes]

Port Control Protocol (PCP) allows hosts to control how the incoming packets are translated and forwarded by an upstream  router. No PCP is implemented in SPA112. You need to configure the router in question by self, manually.

[David30]

Hi Dan,

would you please explain how "NAT mapping" does work in the SPA112?

Regards, David

[Dan Lukes]

SPA112 supports no NAT-PMP. (NAT Port Mapping Protocol).No way to explain how it works - because unsupported.

Just to avoid confusion - "NAT Port Mapping Protocol (NAT-PMP) is a protocol for establishing NAT & port forwarding automatically without user effort. WIth no NAT-PMP supported you just need to configure settings manually. Of course - on device doing NAT, not on the SPA112 as it do no NAT.

[David30]

I apologise for the confusion here Dan but we seem to be at cross-purposes, though the information you've provided so far is useful.

The SPA112 has a parameter "NAT Mapping Enable" in the LINE configuration, and I'm asking what it does; would you please explain that?

[Dan Lukes]

It's described in Admin guide - search for "NAT support parameters".

Also, Linksys ATA Administrator Users Guide, section Network Address Translation may help you to understand.

[David30]

The Admin Guide was the first place I looked, and this Cisco site somewhat confusingly restates it:

This feature specifies whether IP addresses has been mapped externally or not.
• Yes — Use externally mapped IP and SIP/RTP ports in SIP messages (i.e. do not use ATA mapping?)
• No — Do not use mapping. This is the default value

To put the question another way, what does the ATA do differently (in a little technical detail please) depending on whether this parameter is set 'yes' or 'no'?

General opinion - e.g. here - seems to be that this parameter should be set to 'yes' and NAT keep-alive messages should be on when the ATA is connected to an internet-connected router.

[David30]

Not understanding what the NAT-mapping parameter actually does makes it hard to configure sensibly!!

An SPA112 ATA will be separated from some NAT device between it and the public network, and it doesn't support PMP or PCP, so it's not obvious how it can do anything to determine its public IP address and port number. 

To take a specific case, if the NAT device (router) has a SIP & RTSP application level gateway, should NAT-mapping be enabled or not?

David

[Dan Lukes]

This feature specifies whether IP addresses has been mapped externally or not.

Either there's a NAT on the path, so it should to be turned on, or there's no NAT, so it shall be turned off. It's very simple condition and decision algorithm. I can't rephrase it to be simpler.

May be you should explain particular issue you have with configuration instead.

An SPA112 ATA will be separated from some NAT device between it and the public network, and it doesn't support PMP or PCP, so it's not obvious how it can do anything to determine its public IP address and port number.

SPA112 can use STUN protocol to autodetect parameters of NAT on the path (for example public IP) or parameters (for example public IP) needs to be configured manually (so they are known to SPA112).

[David30]

I'm looking for an explanation of what it does, not when to click on the button!

STUN is enabled by a separate parameter, and thus seems to be independent of "NAT Mapping Enable".  I wonder whether NAT-mapping tries to work out the external addressing by other means?

If a NAT-enabled router has either a well implemented SIP/RTSP ALG enabled or port-forwarding correctly set up, I assume a connected SPA will see the world as though there were no NAT in place and NAT-mapping can be disabled.  Is this correct?

[Dan Lukes · Accepted solution]

It do "nothing" itself. It just turn on NAT mapping engine. Exact behavior depends on the configuration of other NAT-related options and/or data discovered during STUN (if enabled). Do you wish to see exact differences between content of INVITE packets with and without the feature turned on ? Just capture them and compare.

If a NAT-enabled router has either a well implemented SIP/RTSP ALG enabled or port-forwarding correctly set up

True - fully transparent NAT is transparent - so end devices need not be configured differently.

False - just port forwarding is not enough to pass SIP/SDP thru NAT. SPA112 needs to modify the content of packets accordingly and this is what the NAT mappings is here for.

[David30]

Thank you!

I've set up three SPA112 users, two with different, but recent, models of DrayTek router and one with a 3-year old NetComm router.  All have a SIP/RTSP ALG enabled, and all work fine with NAT-mapping off.  The only difference is that NAT keep-alive needs to be on with the DrayTek routers, but the NetComm ALG keeps a session up itself.

[Dan Lukes]

Glad to hear you are satisfied.