AudioCodes Mediant 4000 SBC, User Manual

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CHAPTER 22 SBC Overview
Mediant 4000 SBC | User's Manual
In contrast, when the device operates in Stateful Proxy mode, the device by default forwards SIP
messages transparently (unchanged) between SIP endpoints (from inbound to outbound legs). The
device retains the SIP dialog identifiers and topology headers received in the incoming message
and sends them as is in the outgoing message. The device handles the above mentioned headers
transparently (i.e., they remain unchanged) or according to configuration (enabling partial
transparency), and only adds itself as the top-most Via header and optionally, to the Record-Route
list. To configure the handling of these headers for partial transparency, use the following IP Profile
parameters (see
Configuring IP Profiles ):
■ IpProfile_SBCRemoteRepresentationMode: Contact and Record-Route headers
■ IpProfile_SBCKeepVIAHeaders: Via headers
■ IpProfile_SBCKeepUserAgentHeader: User-Agent headers
■ IpProfile_SBCKeepRoutingHeaders: Record-Route headers
■ IpProfile_SBCRemoteMultipleEarlyDialogs: To-header tags
Thus, the Stateful Proxy mode provides full SIP transparency (no topology hiding) or asymmetric
topology hiding. Below is an example of a SIP dialog-initiating request when operating in Stateful
Proxy mode for full transparency, showing all the incoming SIP headers retained in the outgoing
INVITE message.
Some of the reasons for implementing Stateful Proxy mode include:
■ B2BUA typically hides certain SIP headers for topology hiding. In specific setups, some SIP
servers require the inclusion of these headers to know the history of the SIP request. In such
setups, the requirement may be asymmetric topology hiding, whereby SIP traffic toward the
SIP server must expose these headers whereas SIP traffic toward the users must not expose
these headers.
■ B2BUA changes the call identifiers between the inbound and outbound SBC legs and
therefore, call parties may indicate call identifiers that are not relayed to the other leg. Some
SIP functionalities are achieved by conveying the SIP call identifiers either in SIP specific
headers (e.g., Replaces) or in the message bodies (e.g. Dialog Info in an XML body).
■ In some setups, the SIP client authenticates using a hash that is performed on one or more of
the headers that B2BUA changes (removes). Therefore, implementing B2BUA would cause
authentication to fail.
■ For facilitating debugging procedures, some administrators require that the value in the Call-ID
header remains unchanged between the inbound and outbound SBC legs. As B2BUA changes
the Call-ID header, such debugging requirements would fail.
The operating mode can be configured per the following configuration entities:
■ SRDs in the SRDs table (see Configuring SRDs )
■ IP Groups in the IP Groups table (see Configuring IP Groups )
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