SIP ALG "Standard Mode" vs. "Compatibility Mode"; what changes?


#1

Hi, under “Service Passthrough” the SIP ALG mode is selectable between “Standard Mode” and “Compatibility Mode”. The 5.2.0 user manual (section 18.4) doesn’t say much more. Operationally what exactly changes (i.e. what makes “standard” less compatible) between the two modes?

From the manual:
<blockquote>

Session Initiation Protocol, aka SIP, is a voice-over-IP protocol. Peplink Balance can act as a SIP Application Layer Gateway (ALG) which binds connections for the same SIP session to the same WAN connection and translate IP address in the SIP packets correctly in NAT mode. Such passthrough support is always enabled and there are two modes for selection: Standard Mode and Compatibility Mode
</blockquote>

Thanks,

Josh


#2

Hi Joshua,

For the “Standard Mode”, it will turn on the SIP Application Layer Gateway (ALG) while the “Compatibility Mode” will turn off the SIP Application Layer Gateway (ALG).

Thanks


#3

So SIP ALG is not always enabled? Now curious what special SIP of passthrough support is the manual referring to as being “always enabled”?


#4

Hi Joshua,

In addition to my previous response, SIP Passthrough contains two parts.

First, it is used to make sure the SIP and related RTP will go along the same WAN link.

Second is SIP ALG translates the IP address inside the SIP packet during NAT.

In both Standard Mode and Compatibility Mode, the first feature that I mentioned above is always enable.

For second feature - SIP ALG, Standard Mode = on, Compatibility Mode = Off

Thanks


#5

Ahh, thank you, that makes more sense. One more question (I’m setting up a Peplink Balance in a small VOIP environment and want to be certain I know how it will behave); if the WAN link that SIP/RTP are communicating over goes down, I assume those protocols will then migrate to a working WAN link along with other traffic. Then when the link comes back up will protocols using SIP Passthrough migrate back, or stay on that backup link?

Thanks,

Josh


#6

Hi Joshua,

There is an option called “Terminate Sessions on Link Recovery” in Outbound Policy.

If this option is enabled(checked) in a priority rule, all the traffic will fallback to the higher priority WAN once the active link is up.

If the feature is not enabled, after higher priority WAN link resumed, existing traffic will still be routed through the backup link while all the new traffic will be routed to the active link.

Thanks