[VoiceOps] What does an ALG actually do?

[Jay Hennigan]

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