Technical data

8 ServerIron ADX Advanced Server Load Balancing Guide

53-1002435-03

SIP SLB and call persistence using ServerIron ADX

FIGURE 3 SIP server farm with DSR mode

Figure 3 demonstrates a typical use case in which the ServerIron ADX application switch provides

Call-ID based server persistence for UDP SIP traffic. The Call-ID attribute that uniquely identifies a

SIP call is used to maintain session persistence. Due to the unique call flow requirements of SIP,

most SIP implementations require you to enable Direct Server Return (DSR) mode on the

ServerIron ADX switch.

Because User1's SIP phone does not know the location of User2's SIP phone, it initiates a new SIP

session by sending an INVITE request to the SIP proxy server. It also generates a unique identifier

(Call-ID) for the call. Because the SIP proxy server used by User1's SIP phone is actually the virtual

IP address hosted on the ServerIron ADX switch, the ServerIron ADX switch receives the INVITE

request and, using a server selection mechanism, identifies the best available SIP server for this

INVITE. The ServerIron ADX uses the Call-ID attribute value to select one of the SIP servers in either

stateless or stateful mode. For all SIP transactions within a dialog that use the same Call-ID, the

ServerIron ADX selects the same SIP server. A new INVITE message with a different Call-ID is again

subjected to server load balancing and may be forwarded to a different SIP server.

The proxy server receives the INVITE request and sends a 100 (Trying) message to User1's SIP

phone. Because the ServerIron ADX switch is configured in DSR mode, the response message that

is sourced from the virtual IP address flows directly to User1's SIP phone, bypassing the ServerIron

ADX. The proxy server then consults the location service and forwards the INVITE request directly to

User2's SIP phone, again bypassing the ServerIron ADX, and is sourced from the proxy server's own

IP address.

INVITE F1

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

RINGING F4

OK F6

From/To SIP Phone To/From Real IP