2008/10/13 Jiri Kuthan jiri@iptel.org:
The thing here is that actually a load-balancer vendor is free to build stuff his way -- he is not compelled to build a proxy or B2BUA and go to some certification authority, he is supposed to build something that load-balances well. I'm intimately aware of some load-balancers that are close to being a kind of "transparent proxy", which is just fine: it doesn't put itself in signaling and it handles routing by state table.
Well, but what I mean is that the vendor needs, not just a custom LB which doesn't add "Via" header, but also devices behind the LB (other proxies or gateways) being not SIP compliant in points 18.2.1 & 18.2.2 of RFC3261.
This is, it's not enough if the vendor customizes the LB, it also needs to modify the SIP devices after the LB. If not, the proxies/gateways behind the LB will always reply to the source IP (the IP of LB) regardless of the content/existance of a Via added (or not) by LB.
An example: if a vendor just controls the LB and use gateways of different carriers, the LB will always receive the responses to the initial INVITE transaction (except if the gateways are not SIP compliant when sending a response).
I think this is very clear in points 18.2.1 and 18.2.2, am I wrong?
Thanks a lot for your comments.