On 2/27/13 1:33 PM, John Levine wrote:
I realize that an ALG is a hack in a router that is supposed to allow SIP packets to go through a NAT router. I also realize that for modern SIP equipment, ALG usually causes more problems than it solves, and that it's described in RFCs 2663, 3424, and others.
What I can't find anywhere is what a SIP ALG actually does to the packets. Is that written down anywhere, or is it just network folklore?
A lot of this depends on what the ALG vendor is selling, but it typically functions like a stateful packet inspection firewall for SIP. To make it more interesting, different vendors use their own proprietary terms to describe similar or identical functions making apples-to-apples comparisons challenging. Some ALG functions (not every ALG does all of these): * NAT including fixup of source IP address embedded in payload. * SIP proxy, B2BUA or some combination. * Registration pacing * Other header manipulation (which can break things that aren't broken as well as fix things that are). * Various flavors of QoS. * Various flavors of survivability including PSTN backup. -- Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net Impulse Internet Service - http://www.impulse.net/ Your local telephone and internet company - 805 884-6323 - WB6RDV