Spa9000 Media Proxy - Linksys SPA9000 Administration Manual

UserB
UserA
Internet (WAN)
SPA9000
SIP Proxy with
media proxy enabled
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 A-2 also 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. Even after the SIP session is established, the
SPA9000 continues to direct RTP packets between UserA and the ITSP.

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, go to the Voice > SIP page, PBX Parameters section, and set the 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.
Local traffic is not affected by this configuration. When UserA initiates a call to UserB, RTP traffic
still flows directly between the two UAs. The media proxy only affects RTP traffic to a UA
connected through the ITSP.
Linksys Voice System Administration Guide
Figure A-2 SPA9000 as a SIP Proxy
IP Router (firewall)
Hub/switch
Broadband modem
Interface
Technology Background
UserC
Internet
ISP
ITSP
132