ALGs Supported in OmniAccess 5740 USG
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Alcatel-Lucent
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OmniAccess 5740 Unified Services Gateway CLI Configuration Guide
ALGS SUPPORTED IN OMNIACCESS 5740 USG
The ALGs (Application Level Gateway) supported in OmniAccess 5740 USG are
listed below.
SIP
The Session Initiation Protocol (SIP) is an application-layer control protocol that
can establish, modify, and terminate multimedia sessions such as Internet
telephony calls. SIP can also invite participants to already existing sessions.
Media can be added to or removed from an existing session.
A SIP network consists of proxy servers, redirect servers, registration servers and
user agents (UA). For the UA which initiates the session, we call it user agent
client (UAC). The party which accepts the invitation from UAC, we call it user
agent server (UAS). For the purpose of discussion, we call redirect, registration
and proxy servers "SIP servers". SIP is actually a signaling protocol for setting up
sessions between clients over the network. These sessions do not necessarily
have to be Internet telephony sessions.
OmniAccess offers SIP ALG as a service and can screen SIP traffic, allowing and
denying it based on a policy that you configure. SIP ALG is a predefined service in
OmniAccess 5740 USG and uses port 5060 as the destination port.
OmniAccess 5740 USG supports the following:
• Media pinhole creation to support Early and Late media.
• SIP-ALG processing for packets with source-port or destination-port or both other
than 5060.
• Hosted SIP ALG with and without pin-holing.
In scenarios like residential and enterprise where gateway is present, it supports
basic NAT and not VoIP. In case of basic NAT, gateway box translates IP layer
information from private to public address. However, in case of VoIP (SIP), the
payload carries IP address (in SDP), which is used to transmit RTP packets. As
part of basic NAT support, gateway does not translate the IP address in SDP
payload, which carries private IP address. As a result, RTP from public domain
does not reach the private end-point.
The above issue is solved by having an entity called SBC (Session border
controller) on the WAN side (Hosted by service provider). SBC does the SIP-ALG
functionality and translates the private address present in the payload to public
routable address. This mechanism where the gateway (OmniAccess 5740 USG)
does IP layer natting and SBC does ALG is referred as Hosted SIP ALG (on the
gateway side). The gateway must be able to open up pinholes for SIP session and
RTP session. See “To Configure Customized Service” to configure this feature.
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