On 30 mar 2010, at 12.08, Avasarala Ranjit-A20990 wrote:

> No, it's just 2 networks which can go up and down under different
> conditions.
> There is nothing special about them.
> (They could be both wireless but not necessarily) There are just
> effectively 2 gateways, and each gateway goes to a different network.
> Each network could be a completely different service provider.
> 
> SIP UA---------------|-------192.168.0.1 GW1 ROUTER--external IP1-->
> ISP1
> 192.168.0.254        |-------192.168.0.2 GW2 ROUTER--external IP2-->
> ISP2
> 
> So for making a call, for example:
> 1. only GW1 has a connection to ISP
> 2. route call from SIP UA via GW1
> 3. call is still up and GW2 now has a link to ISP2 4. call is still up
> but GW1 loses link 5. (what I want) SIP and RTP continue seamlessly to
> use GW2 for
>    the call
> 
> The SIP UA user should have no idea that call is flowing through
> different router.



First I thought that this is just a matter of IP routing, but it's more 
complicated than that... Since you are using NAT, the SIP/RTP session actually 
changes if you switch from ISP1 to ISP2, since the NAT is the "endpoint" and 
not the SIP UA. As far as I understand it, the SIP session must be modified 
with an SIP Update or Re-Invite in order to modify the session with ISP2 public 
address. 

If you'd been using public Internet addresses, this would be a matter of 
multi-homed IP routing and the SIP layer would never be involved. 

I might be missing something though... =)


Best regards,
/Staffan


--
Staffan Kerker
mail/sip/xmpp: [email protected]

"There is absolutely no money above the 5th fret..." /Donald "Duck" Dunn

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