[asterisk-users] Do I need a sip proxy?

[Andreas Sikkema]

Hi,

> At least
> that is my understanding of NAT. The provider should see me trying to
> register from the same IP with multiple different ports (high number
> ports; not talking about 5060 as this is outbound and not inbound) and
> should be able to differentiate between SIP packets coming from various
> servers. However, it seems to not happen.
> 
> There is some sort of clash and only one of the servers shows registered
> with the provider and other's trunks go down. I have noticed that
> keeping one server works. 

What I have noticed with consumer grade NAT routers is that they seem to
be optimized to only keep track of one single client that is allowed to
connect to a server:port tuple on the outside. So if a SIP client on
local ip_a and port 5060 on the inside of the router is talking to a
server outside of the NAT at ip_s and port 5060 it works fine, but the
minute a second client at local IP ip_b and port 5060 starts to talk to
ip_s at port 5060 on the outside of the same NAT router all traffic from
server_s is sent to ip_b and ip_a will receive nothing.

With NAT entry timeouts and regular traffic from ip_a and ip_b you might
see only one local client being reachable all the time or connectivity
hopping from one to te other at regular intervals.

There are NAT implementations that do not have this problem, but that
might require a more expensive router or you can configure the SIP
clients to all use different local ports. Perhaps this is a result of
some sort of SIP ALG or a stupid basic NAT implementation to reduce code
complexity on the router, but it is a nuisance either way.

-- 
Andreas Sikkema