Technical data

ServerIron ADX Advanced Server Load Balancing Guide 9
53-1002435-03
SIP SLB and call persistence using ServerIron ADX
1
NOTE
The proxy server's IP address must be reachable from all SIP clients.
User2's SIP phone receives the INVITE and alerts User2 of an incoming call. User2 replies with a
Ringing message to the proxy server. If User2 answers the call, a 200 OK message is sent to the
proxy server. The proxy server forwards this message to User1's SIP phone. Upon receiving the 200
OK message, User1's SIP phone sends an acknowledgement (ACK) message directly to User2's SIP
phone, bypassing the proxy server. User1 and User2 SIP phones now begin media exchange and,
upon completion, a BYE message closes the call.
Some SIP servers may be configured to use a virtual IP address (VIP) as the source address for all
communications. Figure 4 shows SIP packet flows in this type of configuration.
FIGURE 4 SIP server farm with DSR mode and SIP server using VIP as source address
In this implementation, the SIP proxy server must use the same Call-ID for both legs of
communication (the same Call-ID for message exchange with both SIP clients within a given SIP
dialog). Session persistence and transaction integrity can only be achieved if the proxy server uses
the same Call-ID.
SIP server load balancing with non-DSR mode
Figure 5 shows a SIP server farm with proxy servers connected inline (non-DSR mode) with the
ServerIron ADX switch.
INVITE F1
RINGING F4
OK F6
INVITE F1
RINGING F4
OK F6
ACK F8
MEDIA RTP
BYE F9 OK F10
TRYING F3
RINGING F5
OK F7
INVITE F2
ServerIron
SIP Proxy Server
user2
user1
From/To SIP Phone To/From VIP
L2/3
Infrastructure
SI