Cisco Systems Understanding the SPA9000 as a SIP Proxy and Media Proxy

Chapter 1 Using the Linksys Voice System

Technology Background

In Figure 1-3, UserA and UserB are client stations (UAs) that register over the local area network to which the SPA9000 PBX is connected. When UserA calls UserB, the SPA9000 acts as a SIP proxy and establishes a session between the two UAs. After the session is established, RTP traffic flows directly between the two client stations.

Figure 1-3 SPA9000 as a SIP Proxy

UserC

UserA

UserB

Hub/switch	IP Router (firewall)
	Broadband modem

ISP

Internet (WAN)

Interface

Internet

SPA9000

SIP Proxy with

media proxy enabled

ITSP

When a user picks up the handset in an LVS system, the SPA9000 collects DTMF digits from a touchtone analog telephone or the locally connected SPA900 Series IP phones. Unless the call is for a local client station, the SPA9000 system sends the full number in a SIP INVITE message to another SIP proxy server for further call processing.

To minimize dialing delay, a dial plan is maintained that is matched against the cumulative number entered by the user. Invalid phone numbers that are not compatible with the dial plan are detected and the user is alerted using a configurable tone (reorder) or announcement.

Figure 1-3also illustrates connectivity between the SPA9000 and the ITSP over the Internet. When UserA calls UserC, the SPA9000 directs the request to the SIP proxy at the ITSP, which is then responsible for routing the request to UserC. Again, once the session is established, RTP packets are exchanged directly between UserA and UserC. However, this requires that the firewall on the Internet routers allow UserA access to the Internet. Because the SIP UAs are generally assigned IP addresses dynamically through DHCP, this makes implementing a secure firewall policy more difficult.

SPA9000 Media Proxy

To address this possible security issue, the SPA9000 can also function as a media (RTP) proxy. This option forces RTP traffic destined for the Internet (or IP WAN) to be directed to the SPA9000, which then directs it to the remote UA. This configuration may simplify firewall configuration because the client stations do not require direct access to the Internet through the firewall.

To enable the media proxy, set the PBX Parameters:<Force Media Proxy> parameter to True. With the media proxy enabled, when UserA calls User C, the SPA9000 still acts as the SIP proxy and forwards the request to the SIP server on the ITSP. However, even after the SIP session is established, the SPA9000 continues to direct RTP packets between UserA and the ITSP.

	Linksys SPA9000 Administrator Guide
Document Version 3.01		1-7