Technical data

ServerIron ADX Advanced Server Load Balancing Guide 9

53-1002435-01

SIP SLB and Call Persistence using ServerIron ADX

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 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 and SIP server using VIP as source address

In this implementation, the SIP proxy server must use 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 same call-ID.

Design #2: SIP Server Load Balancing with ServerIron non-DSR mode

Figure shows a SIP server farm with proxy servers connected inline (non-DSR mode) with the

ServerIron 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