B2BUA and Stateful Proxy Operating Modes

The device can operate in one or both of the following SBC modes:

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Back-to-Back User Agent (B2BUA): Maintains independent sessions toward the endpoints, processing an incoming request as a user agent server (UAS) on the inbound leg, and processing the outgoing request as a user agent client (UAC) on the outbound leg. SIP messages are modified regarding headers between the legs and all the device's interworking features may be applied.

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Stateful Proxy Server: SIP messages traverse the device transparently (with minimal interference) between the inbound and outbound legs, for connecting SIP endpoints.

By default, the device's B2BUA mode changes SIP dialog identifiers and topology data in SIP messages traversing through it:

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Call identifiers: Replaces the From-header tag and Call-ID header so that they are different for each leg (inbound and outbound).

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Routing headers:

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Removes all Via headers in incoming requests and sends the outgoing message with its own Via header.

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Doesn't forward any Record-Route headers from the inbound to outbound leg, and vice versa.

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Replaces the address of the Contact header in the incoming message with its own address in the outgoing message.

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Replaces the User-Agent/ Server header value in the outgoing message, and replaces the original value with itself in the incoming message.

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):

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'Remote Representation Mode': Contact and Record-Route headers

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'Keep Incoming Via Headers': Via headers

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'Keep User-Agent Header': User-Agent headers

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'Keep Incoming Routing Headers': Record-Route headers

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'Remote Multiple Early Dialogs': 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:

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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.

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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).

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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.

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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:

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SRDs in the SRDs table (see Configuring SRDs)

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IP Groups in the IP Groups table (see Configuring IP Groups)

If the operation mode is configured in both tables, the operation mode of the IP Group is applied. Once configured, the device uses default settings in the IP Profiles table for handling the SIP headers, as mentioned previously. However, you can change the default settings to enable partial transparency.