[VoiceOps] What does an ALG actually do?

[Moises Silva]

On Wed, Feb 27, 2013 at 4:33 PM, John Levine <johnl at taugh.com> 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?
>
>
I am unaware if there is any RFC stating exactly what a SIP ALG should do.
Most likely, just like with SBCs it is a term that can mean different
things to different people and there is not a "spec" to follow (although
for SBCs there is RFC 5853 which has very general guide lines), but rather
some general functionality that is expected from it.

>From what I remember the Linux kernel for example has some SIP ALG
functionality (which probably breaks more things than it fixes) which will
modify the SDP to "fix" the IP addresses for the media streams, patching
the IP when you're behind NAT (and using the public IP in the SDP), you can
expect other routers (some incidentally based on Linux) to perform similar
tricks.

I've never met someone who has good things to say about SIP ALGs :)

*Moises Silva
**Manager, Software Engineering***

msilva at sangoma.com

Sangoma Technologies

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