[Serusers] About a SIP load balancing document involving SER

[Iñaki Baz Castillo]

Hi, I've found a document about SIP load balancing in iptel.org website and I 
would like to comment an error I've found in the document.

The document is this:
"Towards Effective SIP load Balancing"
http://www.iptel.org/voipsecurity/doc/14%20-%20Kambourakis%20-%20Towards%20Effective%20SIP%20load%20Balancing.pdf


Page 7 says:
--------------------
SIP Proxies insert a VIA header in all the incoming SIP requests
   So…
     In case the LB (Load Balancer) is implemented as another proxy
     all SIP responses will pass through that proxy
Request that belong to a specific dialog should not pass through LB
--------------------


This is completely true, but next page (8) says:
--------------------
Transparency for responses
  Prevent Load Balancer from inserting a VIA header
     E.g. in SER utilizing the SEND core command
  Modify the SIP's Proxy core to ignore the VIA-header
  added by the Load Balancer
-------------------


There is an important error that unfortunatelly I've realized it's very 
common. Section 18.2.2 of RFC 3261 says clearly that the responses are 
*always* sent through the same nodes the request came from. So the response 
should always traverse the load balancer.

1) Load balancer --- (SIP UDP) ---> UAS
In this case the UAS would always reply to the *real* source IP (if this is 
different of the Via "sent-by" then UAS adds "received" parameter and replies 
there).

2) Load balancer --- (SIP TCP/SCTP) ---> UAS
By definition a UAS must reply using the incoming TCP connection.


So it's extrange for me that a document about SIP load balancing tries to 
offer solutions that are not SIP compliant and also unfeasible (UAS will 
always reply to the real source IP regardless of the Via content).

Since the document proposes SER based solutions (using "SEND" command) I'd 
just like to confirm if I'm completely right, or maybe it's common those not 
SIP compliant methods by some vendors in order to provide a load balancing 
solution.


Thanks a lot for any comment and best regards.



-- 
Iñaki Baz Castillo