This MIB defines objects describing subscriber sessions, or
more specifically, subscriber sessions terminated by a RAS.  A
subscriber session consists of the traffic between a CPE and a
NAS, as illustrated in the diagram below.
        
                                   Service
                   Access          Provider
                   Network         Network
+--+  +---+  +--+   {   }   +---+   {   }
|PC+--+CPE+--+AN+--{     }--+NAS+--{     }
+--+  +---+  +--+   {   }   +---+   {   }
          |                 |
          |<--------------->|
               subscriber
                session
        
A subscriber session behaves according to the FSM illustrated
below.
        
      +-----------------+
 +--->|  DISCONNECTED   |<-------+
 |    +--------+--------+        |
 |             |                 |
 | failed      | initiated       | disconnect
 |             V                 |
 |    +-----------------+        |
 +----+     PENDING     +--------+
      +--------+--------+        |
               |                 |
               | established     |
               V                 |
+----------------------------+   |
|             UP             |   |
|                            +---+
|  +-----------------+       |
|  | UNAUTHENTICATED |       |
|  +--------+--------+       |
|           |                |
|           | authenticated  |
|           V                |
|  +-----------------+       |
|  |  AUTHENTICATED  |       |
|  +-----------------+       |
|                            |
+----------------------------+
        
A subscriber session in the DISCONNECTED state technically
doesn't exist; that is, the system does not maintain a context
to describe a disconnected subscriber session.
        
Once the system detects the initiation of a subscriber session,
then it creates a context and places the subscriber session in
the PENDING state.  The initiation of a subscriber session can
occur either through provisioning or the reception of a packet.
In the PENDING state, a system does not forward subscriber
traffic.
        
A pending subscriber session can become DISCONNECTED if
it fails to come up (e.g., a timeout) or if the system or the
subscriber explicitly terminates the subscriber session.
        
A pending subscriber session can become UP if the system
successfully configures and applies any relevant policies.
Once in the UP state, a system forwards subscriber traffic.
        
A operationally UP subscriber session can become DISCONNECTED if
either system or the subscriber terminates it.
        
A operationally UP subscriber session can either be
UNAUTHENTICATED or AUTHENTICATED.  When the system is in the
process of checking a the credentials associated with a
subscriber session, it is in the UNAUTHENTICATED state.  When
the system successfully completes this process, it transitions
the subscriber session to the AUTHENTICATED state.  If the
process fails, then the system terminates the subscriber
session.
        
Besides describing individual subscriber sessions, this MIB
module provides an EMS/NMS with the means to perform the
following functions:
        
1)  Enumerate subscriber sessions by ifIndex.
        
2)  Enumerate subscriber sessions by subscriber session type and
    ifIndex.
        
3)  Monitor aggregated statistics relating to subscriber
    sessions:
        
    a.  System-wide
    b.  System-wide by subscriber session type
    c.  Per node
    d.  Per node by subscriber session type
        
4)  Collect 15-minute aggregated performance data history
    relating to subscriber sessions:
        
    a.  System-wide
    b.  System-wide by subscriber session type
    c.  Per node
    d.  Per node by subscriber session type
        
5)  Submit a query for a report containing those subscriber
    sessions that match a specified identity match criteria.
        
6)  Terminate those subscriber session that match a
    specified identify match criteria.
        
GLOSSARY
========
        
Access Concentrator
    See NAS.
        
Access Network
    The network that provides connectivity between a AN and NAS.
    An access network may provide L2 or L3 connectivity. If the
    access network provide L2 connectivity, it may switch
    traffic or tunnel it through a MPLS or IP network.
        
AN (Access Node)
    A device (e.g., a DSLAM) that multiplexes or switches
    traffic between many CPEs and an access network.
        
BRAS (Broadband Remote Access Server)
    See NAS.
        
CPE (Customer Premise Equipment)
    A device (e.g., a DSL modem) that connects a customer's
    network to an AN.
        
DHCP (Dynamic Host Configuration Protocol)
    The protocol that provides a framework for transacting
    configuration information to devices on an IP network, as
    specified by RFC-2131.
        
NAS (Network Access Server)
    A device that switches or routes traffic between subscriber
    sessions and service provider networks.
        
Network Service
    Access to the Internet backbone, voice, video, or other
    content.
        
Node
    A physical entity capable of maintaining a subscriber
    session within a distributed system.  The notion of a node
    is not applicable to a centralized system.
        
PADI (PPPoE Active Discovery Initiation)
    A subscriber broadcasts a PADI packet to start the process
    of discovering access concentrators capable of serving it.
        
PADO (PPPoE Active Discovery Offer)
    The packet sent by an access concentrator to a subscriber
    indicating that it can serve the subscriber.
        
PADR (PPPoE Active Discovery Request)
    The packet sent by a subscriber to an access concentrator
    requesting a PPPoE connection.
        
PADS (PPPoE Active Discovery Session-confirmation)
    The packet sent by an access concentrator to a subscriber
    confirming the request for a PPPoE connection.  Once this
    packet has been sent, then the PPP can proceed as specified
    by RFC-1661.
        
PADT (PPPoE Active Discovery Terminate)
    The packet indicating that a PPPoE connection has been
    terminated.  Either the subscriber or the access
    concentrator can send this packet.
        
PPP (Point-to-Point Protocol)
    The standard method for transporting multi-protocol
    datagrams over point-to-point links, as defined by RFC-1661.
    The PPP specifies the encapsulation for these datagrams and
    the protocols necessary for establishing, configuring, and
    maintaining connectivity.
        
PPPoE (Point-to-Point Protocol over Ethernet)
    The protocol and encapsulation necessary to support a PPP
    connection over an Ethernet connection, as defined by IETF
    RFC-2516.
        
Service Provider Network
    The network that provides connectivity between a NAS and a
    network service.
        
Subscriber
    A customer of a network service.
        
Subscriber Session
    A context maintained by a NAS for the purpose of classifying
    a subscriber's traffic, maintaining a subscriber's identity,
    applying configuration and policies, and maintaining
    statistics.  For more information on the types of subscriber
    sessions, see the CISCO-SUBSCRIBER-SESSION-TC-MIB.