Technical data

8 ServerIron ADX Advanced Server Load Balancing Guide

53-1002435-01

SIP SLB and Call Persistence using ServerIron ADX

FIGURE 3 SIP server farm with DSR mode

Figure 3 demonstrates a typical use case - the ServerIron 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 switch.

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

session by sending INVITE request to 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 switch, the ServerIron switch receives the INVITE request and,

using a server selection mechanism, identifies the best available SIP server for this INVITE. The

ServerIron uses the call-ID attribute value to select one of SIP servers in either stateless or stateful

mode. For all SIP transactions within a dialog that use same call-ID, the ServerIron 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 100 (Trying) message to User1's SIP

phone. Since the ServerIron 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.

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

User2's SIP phone, again bypassing the ServerIron 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