The BSG uses policies to control system behavior and to determine
how VoIP signaling is handled on a services PIC or MS-DPC. All incoming
VoIP requests are matched against policies, and the actions defined
in the matching policies determine how the request is handled; for
example, which INVITE requests to accept and which to reject. Each
BSG feature, such as QoS, call admission control, and routing of SIP
requests, is controlled by policies and has its own actions.
There are two types of policies:
New transaction policies—Define how the BSG handles
signaling for new dialogs and for out-of-dialog transactions. A new
transaction event is raised when a new SIP request, such as an INVITE,
either opens a new dialog or is not related to any dialog. The actions
that you can take on requests that match conditions in new transaction
policies include accepting, rejecting, or tracing traffic, routing
of SIP requests, or applying CAC (call admission control).
New call policies—Define how the BSG handles media
sessions (voice and video). New call policies classify media and provision
the actions to take on media streams, such as mark, drop, applying
QoS, or applying media anchoring.
BSG Policy Model
BSG policies are made up of terms that contain conditions and
actions that cause the BSG to handle incoming requests in a certain
way.
Condition—The from statement in the policy.
Defines values or fields that a request must contain before an action
is triggered; for example, a source address, contents of the contact
or request URI fields, a SIP method, or a media type, such as audio
or video.
If you have multiple matching fields defined in a from clause within the same term, an AND function is between
the conditions. For example, if you have both a contact defined and
a SIP method defined, the term must match the values defined for both
the contact and the SIP method.
If you have multiple definitions for the same field in
a from clause, an OR function is between the values in the
condition. For example, if you have multiple values defined for a
contact, the term must match one the values defined.
Action—The then statement in the policy.
Specifies the action that is performed on incoming traffic that matches
the condition; for example, accept or reject, mark with a DSCP code
point, apply QoS, or route to a next-hop or egress point.
If you have a policy that includes multiple terms, the software
applies the actions in the first term that matches the policy.
Policy Sets
You can configure policy sets, which are a list of policies
that you can then apply to a service point. All policies in a set
are evaluated. The order in which you add policies to the set determines
the order in which the BSG processes the policies. In each policy,
the action in the first term that matches is the action that is applied.
Service Points
Service points identify a service interface and transport parameters
for incoming requests. You attach policies to the service point, and
all requests that arrive at the service point are handled by these
policies. You can also configure a service point to be used as an
egress service point to which SIP requests are routed. Each BSG can
have five service points.
A VPN can be configured on the service point so when you set
the egress service point, the packet can be sent on a VPN.
