D-Link NetDefend DFL-210 User Manual page 217

6.2.7. The SIP ALG
SIP Components
The following components are the logical building blocks for SIP communication:
User Agents
Proxy Servers
Registrars
SIP Media-related Protocols
A SIP session makes use of a number of protocols. These are:
SDP Session Description Protocol (RFC4566) is used for media session initialization.
RTP Real-time Transport Protocol (RFC3550) is used as the underlying packet format for
delivering audio and video streaming via IP using the UDP protocol.
RTCP Real-time Control Protocol (RFC3550) is used in conjunction with RTP to provide
out-of-band control flow management.
NetDefendOS SIP Setup
When configuring NetDefendOS to handle SIP sessions the following steps are needed:
• Define a single Service object for SIP communication.
• Define a SIP ALG object which is associated with the Service object.
• Define the appropriate IP rules for SIP communications which use the defined Service object.
SIP ALG Options
The following options can be configured for a SIP ALG object:
Maximum Sessions per ID
Maximum Registration Time
These are the end points or clients that are involved in the client-to-client
communication. These would typically be the workstation or device used in an
IP telephony conversation. The term client will be used throughout this
section to describe a user agent.
These act as routers in the SIP protocol, performing both as client and server
when receiving client requests. They forward requests to a client's current
location as well as authenticating and authorizing access to services. They also
implement provider call-routing policies.
The proxy is often located on the external, unprotected side of the D-Link
Firewall but can have other locations. All of these scenarios are supported by
NetDefendOS.
A server that handles SIP REGISTER requests is given the special name of
Registrar. The Registrar server has the task of locating the host where the
other client is reachable.
The Registrar and Proxy Server are logical entities and may, in fact, reside on
the same physical server.
The number of simultaneous sessions that a single client can
be involved with is restricted by this value. The default
number is 5.
The maximum time for registration with a SIP Registrar. The
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Chapter 6. Security Mechanisms