SIP ALG

[Hoplisenaf]

A new feature that I have noticed on quite a few routers are SIP ALG and FTP ALG. ALG not for Algorithm but for Application Level gateway.

After some research on this new protocol……..I found that it could be quite useful in solving problematic SIP calls. This protocol actually rewrites SIP packets and rectifies the “faulty” parameters in the packets.

Has anyone had some experience where this new Protocol proved usefull…..FTP ALG….not quite sure what this would do….

 

[Roman4604]

Has anyone had some experience where this new Protocol proved usefull

Yes, not really a protocol but rather an extention to the NAT functionality. As you say it can sort out issues sometimes experienced with SIP devices behind NAT (e.g. one-way voice, incoming calls not answering etc.).

A word of warning tho, it can be a double-edged sword as many SIP devices/sw have their own NAT compensation mechanisms e.g. STUN which are sometimes automatically enabled. This causes a clash, so make sure either the router (ALG) or the end device/sw is trying to manage SIP NAT, not both.

 

[Amida]

Is this the thing that inspects the SIP packets and then remove all the local IP addresses and replaces them with the WAN IP?

 

[Roman4604]

Is this the thing that inspects the SIP packets and then remove all the local IP addresses and replaces them with the WAN IP?

Yep, in essence spoofs the SIP proxy into believing the router (WAN IP) is the actual SIP client, thus no NAT exists from the proxy's perspective.

 

[Hoplisenaf]

Thanks, makes more sense now. FTP ALG, is this based on the same concept?