[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04

Media Gateway Control (Megaco)                  Julian Mitchell
Internet Draft                                       Tom Taylor
Document: draft-ietf-megaco-naspkg-04.txt       Alan V. Whitton
Category: Standards Track                       Nortel Networks
                                               Ravi Subramaniam
                                                  Cisco Systems

                                                     April 2002


                       Megaco/H.248 NAS Packages


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-
   Drafts. Internet-Drafts are draft documents valid for a maximum of
   six months and may be updated, replaced, or obsoleted by other
   documents at any time. It is inappropriate to use Internet- Drafts
   as reference material or to cite them other than as "work in
   progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt
   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.


Abstract

   This document is intended to satisfy the requirements in section
   11.2.5 of the Megaco/H.248 requirements document.  It defines five
   packages:
    - the base NAS package contains properties and events supported by
      all NAS terminations;
    - the NAS Incoming package contains properties and events
      supported by NAS terminations involved in calls initiated by the
      circuit network;
    - the NAS Outgoing package contains properties supported by NAS
      terminations involved in calls outgoing to the circuit network;
    - the NAS Control package contains an event supported by a NAS
      Control termination, which allows the MG to indicate a request to
      initiate a data connection to a terminal served by the switched
      circuit network;
    - the NAS ROOT package contains properties supported by an MG which
      is also capable of supporting at least the NAS and NAS Incoming
      packages.


Taylor et al     Standards Track - Expires January 2002              1
 
                       Megaco/H.248 NAS Package             April 2002


1. Introduction

   One of the applications for which the Megaco/H.248 protocol has been
   designed is the control of a Network Access Server (NAS).  Such a
   server typically connects calls incoming on circuits to modems, and
   directs the modem output through a data network to its destination.
   The NAS may also provide dialout services.  The architecture of the
   NAS arrangement is described more fully in section 4 of this
   document.

   To realize the NAS control functions, this document specifies five
   packages.  The base NAS package, the NAS Incoming package, and the
   NAS ROOT package are supported by all media gateways performing the
   NAS function.  The NAS Outgoing package is required for callback and
   other types of dialout service.  The NAS Control package is required
   to support dialout where the call originates in the data network.

   The present version differs from the previous one as follows:
         .  An introduction is added to bring the document into line
            with RFC formatting requirements.
         .  Where previously missing, text is added to indicate either
            the default values for parameters and properties or that
            (in the case of parameters) that the parameters MUST be
            present.
         .  The error codes defined in section 5 are documented using
            the template provided by H.248 Annex L.  Their numeric
            values are changes to be in the 6xx range.
         .  A specific reference is added for the documentation of SDP
            encodings of bearer capability properties.
         .  The type description of the nampat and ctlnam properties in
            the nasroot package is changed to conform to the VALUE
            construct.


2. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC-2119 [1].


3. ABBREVIATIONS AND DEFINITIONS

   AAA: Authentication, authorization, and accounting

   NAS: Network Access Server

   VPN: Virtual Private Network

   NAS termination: an ephemeral termination which represents the data
   network side of a modem connection.  It shares a context with a

Taylor et al     Standards Track - Expires October 2002              2
 
                       Megaco/H.248 NAS Package             April 2002


   termination which carries the modem tones, but the NAS termination
   supports no explicit media flows.

   NAS Control termination: a persistent termination representing all
   data network entities capable of originating outgoing data calls.
   It resides in the NULL context.


4. ARCHITECTURE

   RFC 2805 [5] section 11.2.5 shows the gross arrangement of network
   elements assumed in the present document.  As is usual for
   Megaco/H.248 systems, call signalling is processed at the MGC.  The
   MG performs the NAS function, which mediates between dial-up modem
   connections and one or more data networks.  The MG and MGC are
   supported by one or more AAA servers, the interface to which is out
   of scope of this document (see e.g. RADIUS [2]).  Typically the MG
   will have the task of communicating with the AAA server, and this is
   a basic assumption of the present document.

   To set up an incoming NAS call, the MGC sends to the MG:
    - a session identifier, to allow correlation of billing records
      between the circuit and packet network sides of the call
    - the calling number, if available
    - calling number status, which may indicate that it is private
    - the called number
    - information about the incoming circuit, possibly including
      continuity test requirements.
   All of this information except the session identifier is derived
   from call signalling.

   An incoming NAS call is modelled in the MG as a circuit or circuit-
   emulating termination sharing a context with a NAS termination.  The
   session identifier and calling and called number attributes are
   properties of the NAS termination.

   Outgoing NAS calls can be originated in several ways:
    (a) as callback to a dialled-in user
    (b) as outgoing service for a dialled-in user
    (c) as outgoing service for an user connected to the NAS via
        the data network.

   In all of these cases, the MG must provide the MGC with the number
   to be dialled for the outgoing call.  However, the detailed
   procedures and modelling differ between them.

   Case (a) (callback) begins with the context with the original
   circuit and NAS terminations.  The original circuit is subtracted
   when the incoming call is released, and a new circuit is added when
   the outgoing call is made.


Taylor et al     Standards Track - Expires October 2002              3
 
                       Megaco/H.248 NAS Package             April 2002


   Case (b) begins like case (a), with a circuit and a NAS termination
   sharing a context.  These terminations remain in the context and
   another circuit termination is added to it.  Physically, this
   represents demodulation and remodulation of the modem tones as they
   are forwarded from one circuit to another.

   Case (c) does not begin with an established context, so an event on
   a persistent termination established for control purposes only (a
   NAS Control termination) is used to report the information required
   by the MGC to set up the call.  As well as the number to be dialled,
   this includes an MG-generated handle to the user data connection.
   Case (c) eventually results in a context with a NAS termination and
   a circuit termination.

   Deletion of a NAS termination from a context terminates NAS service
   if it is active.  If more than one circuit termination is present in
   the context after the NAS termination is deleted, the context
   assumes the role of an ordinary circuit-to-circuit connection.


5. BASE NAS PACKAGE

   PackageID: nas(serial number TBD)
   Version: 1
   Extends: None

   Description:

   This package is supported by and distinguishes all NAS terminations.
   It contains the session identifier property, the connection type
   property, the NAS Failure event and the NAS Release Event.

5.1   Properties

5.1.1 Session Identifier

   PropertyId: sessid (0x0001)

   Description: provides a value which may be used to correlate data
   and circuit network billing records.

   Type: octet string suitable for use as the value of the RADIUS
   attribute Acct-Session-Id as defined in RFC2866 [4].  In the text
   encoding of Megaco/H.248 this is represented as a hex string.

   Possible Values: as configured in the MGC.  Default value is empty.

   Defined in: TerminationState for a NAS termination.

   Characteristics: read/write.

5.1.2 Connection Type

Taylor et al     Standards Track - Expires October 2002              4
 
                       Megaco/H.248 NAS Package             April 2002



   PropertyId: conntyp (0x0002)

   Description: indicates the nature of the current connection.

   Type: integer, 0 to 99.

   Possible values:
     1: incoming dial-up connection to data network (default)
     2: callback connection
     3: incoming dial-up connection to outgoing circuit
     4: data connection to outgoing circuit

   Defined in: TerminationState for a NAS termination.

   Characteristics: read/write.

5.2 Events

5.2.1 NAS Failure

   EventID: nasfail (0x0001)

   Description: indicates a failure in the processing of the NAS
   connection attempt.  Prior to successful initiation of service, it
   may occur as a result of any failure cause.  Once service has begun,
   it indicates specifically that the NAS was unable to communicate
   with the AAA server.

   EventDescriptor Parameters: none added by this package

   ObservedEventDescriptor Parameters:

     Error code (required parameter)
       ParameterID: ec (0x0001)
       Description: describes the failure reason
       Type: integer, 0 to 99
       Possible values:
         1: NAS internal error
         2: modems failed to train
         3: no response from AAA server

5.2.2 NAS Release

   EventID: nasrel (0x0002)

   Description: The NAS Release event indicates that the NAS session
   has terminated at a point in time subsequent to registration of the
   first accounting record for the service.

   EventDescriptor Parameters: none added by this package


Taylor et al     Standards Track - Expires October 2002              5
 
                       Megaco/H.248 NAS Package             April 2002


   ObservedEventDescriptor Parameters:

     reason
       ParameterID: reason (0x0001)
       Description: reason for termination of service.
       Type: integer, 0 to 99

       Possible values: as defined for the RADIUS Acct-Terminate-Cause
       event (see RFC 2866 [4]).  The list of values is reproduced here
       for information.  See RFC 2866 for detailed definitions.
         1       User Request
         2       Lost Carrier
         3       Lost Service
         4       Idle Timeout
         5       Session Timeout
         6       Admin Reset
         7       Admin Reboot
         8       Port Error
         9       NAS Error
         10      NAS Request
         11      NAS Reboot
         12      Port Unneeded
         13      Port Preempted
         14      Port Suspended
         15      Service Unavailable
         16      Callback
         17      User Error
         18      Host Request (default)

5.3 Signals

   None.

5.4 Statistics

   None.

5.5 Error Codes

   The following additional Megaco/H.248 error codes are documented
   here:

   Error code #: 613
   Name: ISP port limit overrun
   Definit
   ion:
   A NAS cannot connect an incoming or outgoing call because allocation
   of a modem would cause the configured port limit for the ISP to be
   exceeded.
   Package: NAS base package (nas).
   Reference: RFC xxxx (draft-ietf-megaco-naspkg-04.txt) section 5.5.
   Error text in the Error Descriptor: none.

Taylor et al     Standards Track - Expires October 2002              6
 
                       Megaco/H.248 NAS Package             April 2002


   Comment: attempt to create a NAS termination failed because an
   administrative limit has been reached.

   Error code #: 614
   Name: No modems available.
   Definition:
   A NAS cannot connect an incoming
   or outgoing call because it has no more modems available to use for
   this call.
   Package: NAS base package (nas).
   Reference: RFC xxxx (draft-ietf-megaco-naspkg-04.txt) section 5.5.
   Error text in the Error Descriptor: none.
   Comment: attempt to create a NAS termination failed because a
   resource limit has been reached.

   Error code #: 615
   Name: Calling n
   umber unacceptable
   Definition:
   The calling number is being blocked or is not allowed to call this
   service.
   Package: NAS base package (nas).
   Reference: RFC xxxx (draft-ietf-megaco-naspkg-04.txt) section 5.5.
   Error text in the Error Descriptor: none.
   Comment: attempt to create a NAS termination failed because of
   administrative restrictions.

   Error code #: 616
   Name: Called number unacceptable
   Definition:
   The called number supplied is unknown or blocked by this gateway.
   Package: NAS base package (nas).
   Reference: RFC xxxx (draft-ietf-megaco-naspkg-04.txt) section 5.5.
   Error text in the Error Descriptor: none.
   Comment: attempt to create a NAS termination failed because of
   administrative restrictions.

5.6 Procedures

   If the MGC specifies a non-empty Session Identifier value for an
   incoming dial-up session and callback or outgoing service is
   authorized, the MGC MUST set a new Session Identifier value at the
   beginning of the outgoing phase, at the same time as it updates the
   Connection Type.  If the Session Identifier is non-empty, the MG
   MUST use the supplied value as the value of the Acct-Session-Id
   attribute for the corresponding session in communications with the
   AAA server.  If the Session Identifier is empty (unspecified), the
   MG provides its own value for the Acct-Session-Id.

   The MGC initially sets the Connection Type property either to 1
   (incoming dial-up call) or 4 (outgoing from data connection) as the
   case may be.  Where the call starts as an incoming dial-up call and

Taylor et al     Standards Track - Expires October 2002              7
 
                       Megaco/H.248 NAS Package             April 2002


   either moves to callback or to an outgoing call, the MGC resets the
   Connection Type property accordingly to ensure state synchronization
   with the MG.

   The MGC will normally enable the NAS Failure event for the life of
   the NAS termination.  The MGC will typically enable the NAS Release
   event if it is prepared to initiate call clearing to the circuit
   network upon receipt of a notification of that event, but it must
   then also be prepared to handle race conditions due to simultaneous
   call release by the subscriber.


6. NAS INCOMING PACKAGE

   PackageID: nasin (serial number TBD)
   Version: 1
   Extends: nas (0xTBD)

   Description:

   This package provides the Authorization Exception event and
   additional properties which the MGC must supply to the MG for an
   incoming dial-up NAS call.  These properties are assigned to the NAS
   termination when it is created and given a Connection Type value of
   1 (incoming dial-up connection).

6.1 Properties

6.1.1 Calling Number Status

   PropertyId: clgstat (0x0003)

   Description: indicates whether the calling number has not been set
   because it was unavailable at the MGC, has not been set by the MGC
   because it was witheld, has been set but is private (may not be used
   for presentation purposes on the AAA server), or has been set and
   may be passed to the AAA server.

   Type: integer, 0 to 99

   Possible values:
     1: calling number has not been set because it is unavailable
     (default)
     2: calling number has not been set because it is witheld
     3: calling number has been set but is presentation restricted
     4: calling number has been set and may be presented.

   Defined in: TerminationState for a NAS termination

   Characteristics: read/write


Taylor et al     Standards Track - Expires October 2002              8
 
                       Megaco/H.248 NAS Package             April 2002


6.1.2 Calling Number

   PropertyId: clgnum (0x0004)

   Description: value of the calling number.  This property MUST NOT be
   specified if clgstat is set to 1 (calling number unavailable) or 2
   (calling number withheld); it MUST be specified otherwise.

   Type: string of hexadecimal digits, with no preceding "0x".  Each
   hex digit represents one dialled digit.

   Possible values: as determined by the numbering plan. Default value
   if unspecified is "absent".

   Defined in: TerminationState for a NAS termination

   Characteristics: read/write

6.1.3 Called Number

   PropertyId: cldnum (0x0005)

   Description: value of the called number as provided by call
   signalling.

   Type: string of hexadecimal digits, with no preceding "0x".  Each
   hex digit represents one dialled digit.

   Possible values: as determined by the numbering plan. No default
   value.

   Defined in: TerminationState for a NAS termination

   Characteristics: read/write.

6.2 Events

6.2.1 Authorization Exception

   EventID: authex (0x0003)

   Description: this event reports an unexpected outcome of a request
   for authorization of the network access request.  The expected
   outcome is that the request is accepted and involves straightforward
   incoming service.

   EventDescriptor Parameters: none added by this package

   ObservedEventDescriptor Parameters:

     result (required parameter)
       ParameterID: res (0x0001)

Taylor et al     Standards Track - Expires October 2002              9
 
                       Megaco/H.248 NAS Package             April 2002


       Description: outcome of the service request to the AAA server
       (see for example RFC 2865 (RADIUS) [2]).
       Type: integer, 0 to 99
       Possible values:
         1: service denied
         2: callback service authorized
         3: outgoing service authorized

     number to dial
       ParameterID: dialnum (0x0002)
       Description: number to dial for call-back or outgoing service,
       as conveyed, for example, by the Callback number in RADIUS [2].
       Type: string of hexadecimal digits, without preceding "0x".
       Each hex digit represents one digit to be dialled/signalled.
       Possible values: as determined by the dialling plan.  This
       parameter MUST be absent if result was "service denied", and
       MUST be present if result was "callback service authorized" or
       "outgoing service authorized".

6.3 Signals

   None.

6.4 Statistics

   None.

6.5 Procedures

   It is assumed that a NAS incoming connection is identified by the
   MGC on the basis of incoming call signalling.  The MGC maps from
   signalled circuit identifier to the incoming terminationId in the
   usual way.  It may perform continuity testing on the incoming
   termination before beginning the procedures described here.  It will
   typically add the bearer capability properties (described in [7] for
   SDP or [6] Annex C for binary encoding) to Local and Remote for the
   circuit termination based on call signalling.  To enable modem tones
   to be sent and received, the MGC MUST set streamMode in the
   LocalDescriptor on the circuit termination to SendReceive.

   The MGC adds a NAS termination to the context in which the incoming
   termination has also been placed, setting the properties defined by
   the NAS and NAS Incoming packages and enabling the NAS Failure and
   Authorization Exception events of the NAS package.  The MGC may also
   enable the NAS Release event.

   The MG may be unable to carry out the Add command for the NAS
   termination, in which case it will return the appropriate error
   code, typically one of the codes documented in section 5.5 above.

   Once the incoming termination and the NAS termination have been
   associated in the same context, modem training is carried out and

Taylor et al     Standards Track - Expires October 2002             10
 
                       Megaco/H.248 NAS Package             April 2002


   the MG issues a request for authorization to the AAA server.  If
   either of these tasks is unsuccessful, the NAS termination generates
   a NAS Failure event with the appropriate reason code:
     1: NAS internal error, if an internal condition such as hardware
     or software failure is recognized
     2: modems failed to train, if loss of carrier is detected or the
     NAS times out waiting for the connection to be established.
     3: no response from AAA server, if no response to the
     authorization request was received.

   Assuming that the AAA server response is received, the NAS
   termination may generate an Authorization Exception event.
   Depending on the result, the MGC should take the following actions:

   Result = 1 (denied):

     Subtract all terminations from the context.  Either initiate call
     clearing or wait for the subscriber to clear the call, depending
     on local policy.

   Result = 2 (callback):

     Initiate call clearing for the current circuit connection.
     Remaining actions as for the next case.

     Note that if the NAS Release event is enabled an instance will be
     generated with reason code 16 (callback) following the
     Authorization Exception event.

   Result = 3 (outgoing):

     Update the Session Identifier and Connection Type of the NAS
     termination and add Dialout Number (see the NAS Outgoing package).
     Select and add a new circuit termination (possibly wildcarded) for
     the outgoing call.  Initiate an outgoing call to the number
     provided by the Authorization Exception event.  Add bearer
     capability parameters to the new circuit termination when
     available.  Perform continuity testing on the new circuit
     termination if required, before setting streamMode to SendReceive.

   If no Authorization Exception event has been received, or in the two
   success cases once other actions have been taken, maintain the
   connection until cleared by call signalling or alerted by a NAS
   Release or NAS Failure event.

   In all cases of failure, or if the NAS Release event is enabled and
   reported, the MGC has the option of waiting for call clearing from
   the circuit network or initiating call clearing to the circuit
   network.  In the latter case, as already mentioned, it must be
   prepared to handle signalling race conditions due to simultaneous
   release at the subscriber end.


Taylor et al     Standards Track - Expires October 2002             11
 
                       Megaco/H.248 NAS Package             April 2002



7. NAS OUTGOING PACKAGE

   PackageID: nasout (serial number TBD)
   Version: 1
   Extends: nas (0xTBD)

   Description:

   This package provides two properties used for outgoing NAS service:
   the Dialout Number and the Data User Handle.

7.1 Properties

7.1.1 Dialout Number

   PropertyID: dialnum (0x0003)

   Description: the number to dial for an outgoing connection, copied
   from the dialnum parameter of an Authorization Result event or an
   Outgoing Call event (see NAS Control package).

   Type: string of hexadecimal digits, not preceded by "0x".  Each hex
   digit represents one dialled digit.

   Possible values: as defined by the dialling plan.  Default if
   unspecified is "absent".

   Defined in: TerminationState for a NAS termination

   Characteristics: read/write

7.1.2 Data User Handle

   PropertyID: handle (0x0004)

   Description: handle to an existing user data network connection
   which is to be provided outgoing service.  This value is generated
   by and is of local significance to the MG.  It is copied from the
   handle parameter of an Outgoing Call event (see NAS Control
   package).

   Type: a binary value represented as a hexadecimal string (text
   encoding) or OCTET STRING (binary encoding).

   Possible values: as set by the MG.  Default if unspecified is
   "absent".

   Defined in: TerminationState for a NAS termination

   Characteristics: read/write


Taylor et al     Standards Track - Expires October 2002             12
 
                       Megaco/H.248 NAS Package             April 2002


7.2 Events

   None.

7.3 Signals

   None

7.4 Statistics

   None

7.5 Procedures

   The Dialout Number property MUST be specified if and only if the
   Connection Type is 2 (callback), 3 (dial-in to circuit), or 4
   (outgoing from data network).

   The Data User Handle property MUST be specified if and only if the
   connection type is 4 (outgoing from data network).


8. NAS CONTROL PACKAGE

   PackageId: nasctl (serial number TBD)
   Version: 1
   Extends: none

   Description:

   This package is supported by and distinguishes all NAS Control
   terminations.  It contains one event, used by the MG to indicate
   that a data network user has requested outgoing service.

8.1 Properties

   None.

8.2 Events

8.2.1 Outgoing Call

   EventID: callreq (0x0001)

   Description: this event indicates that a data network user wishes to
   make an outgoing call to the circuit network.

   EventDescriptor Parameters: none added by this package

   ObservedEventDescriptor Parameters:

     number to dial (required parameter)

Taylor et al     Standards Track - Expires October 2002             13
 
                       Megaco/H.248 NAS Package             April 2002


       ParameterID: dialnum (0x0001)
       Description: number to dial for outgoing service, as conveyed,
       for example, by the Callback number in RADIUS [2].
       Type: string of hexadecimal digits, not preceded by "0x".  Each
       hex digit represents one digit to be dialled/signalled.
       Possible values: as determined by the dialling plan.

     data user handle (required parameter)
       ParameterID: handle (0x0002)
       Description: handle to an existing user data network connection
       which is to be provided outgoing service.  This value is
       generated by and is of local significance to the MG.
       Type: a binary value represented as a hexadecimal string (text
       encoding) or OCTET STRING (binary encoding).

8.3 Signals

   None.

8.4 Statistics

   None.

8.5 Procedures

   The NAS Control termination is a persistent termination logically
   present in the NULL context of an MG which supports outgoing service
   initiated from data network users.  The MGC must enable the Outgoing
   Call event on this termination in order to receive requests for such
   service.

   When a data network user requests outgoing service (by means outside
   the scope of this document) and the MG has obtained authorization
   for such service from the AAA server, it generates an Outgoing Call
   event.  The value of the dialnum property is set to the value
   provided directly by the data network user or (possibly) provided by
   the AAA server.  (Note that no attribute is specifically defined for
   this purpose in RADIUS.)  The handle property is set to a value
   which the MG can use to retrieve the state of the call, including
   the data connection and information passed to it by the AAA server.

   Upon receipt of an Outgoing Call event, the MGC initiates a call to
   the number provided in dialnum.  It passes a request back to the MG
   to add a terminationId (possibly wildcarded) of a circuit
   termination to a context.  During call setup it may perform
   continuity testing on the circuit, and will typically add Bearer
   Capability parameter values to it in the Local/Remote descriptors.

   At an appropriate point it will request the MG to add a NAS
   termination to the context.  Session Identifier for this termination
   MAY be set.  Connection Type MUST be set to 4 (outgoing call from
   data network).  Both the nasout/dialnum and nasout/handle properties

Taylor et al     Standards Track - Expires October 2002             14
 
                       Megaco/H.248 NAS Package             April 2002


   MUST be set as described in the NAS Outgoing package.  The MGC
   SHOULD enable the NAS Failure event and MAY enable the NAS Release
   event.

   Processing from this point onwards is similar to that for an
   incoming call, except that no Authorization Result event is
   expected.


9. NAS ROOT PACKAGE

   PackageId: nasroot (serial number TBD)
   Version: 1
   Extends: none

   Description: this package provides properties applicable to an MG
   which provides NAS services.  These include the naming pattern for
   NAS terminations, the name of the NAS Control termination (if any),
   the maximum number of NAS terminations supported by the MG, and the
   expected execution time for adding NAS terminations to a context.

9.1 Properties

9.1.1 NAS Termination Naming Pattern

   PropertyId: nampat (0x0001)

   Description: pattern of termination identifiers specifically used to
   identify NAS terminations.

   Type: for binary encoding, an OCTET STRING encapsulating a value of
   ASN.1 type TerminationIDList.  For text encoding, a string delimited
   by double quotes, consisting of one or more terms separated by
   commas.  Each term conforms to the syntax of the pathName production
   in [6] Annex B.  In either encoding, the CHOOSE wildcard MUST NOT be
   used in any of the terms.  Typically individual terms will contain
   ALL wildcards at one or more positions.

   Possible values: as configured in the MG.

   Defined in: TerminationState for ROOT.

   Characteristics: read only.

9.1.2 NAS Control Termination Name

   PropertyId: ctlnam (0x0002)

   Description: name of the NAS Control Termination, if any, supported
   by the MG.


Taylor et al     Standards Track - Expires October 2002             15
 
                       Megaco/H.248 NAS Package             April 2002


   Type: for binary encoding, an OCTET STRING encapsulating a value of
   ASN.1 type TerminationID.  For text encoding, a string conforming to
   the syntax of the pathName production in [6] Annex B.  In either
   encoding, wildcards MUST NOT be present.

   Possible values: as provisioned in the MG.  If the MG does not
   support outgoing calls originated from the data network, the MG MUST
   return an empty value for this property to any audit.

   Defined in: TerminationState for ROOT.

   Characteristics: read only.

9.1.3 Available Modems

   PropertyId: avalmodems (0x0003)

   Description: number of idle in-service modems currently available on
   the MG for assignment.

   Type: integer.

   Possible values: 1 to 9999999.

   Defined in: TerminationState for ROOT.

   Characteristics: read only.

9.1.4 Maximum Time To Add A NAS Termination

   PropertyId: nasaddtime (0x0004)

   Type: integer.

   Possible values: as configured in the MG.  Maximum expected time
   required to add a NAS termination to a context, milliseconds.  See
   procedural description below.

   Defined in: TerminationState for ROOT.

   Characteristics: read only.

9.2 Events

   None.

9.3 Signals

   None.


Taylor et al     Standards Track - Expires October 2002             16
 
                       Megaco/H.248 NAS Package             April 2002


9.4 Statistics

   None.

9.5 Procedures

   The MGC reads the values of the NAS Root Package properties by
   performing an AuditValue of ROOT.  If these properties have been
   set, they indicate that the MG supports the NAS function.

   The value of nasroot/nasaddtime is advisory.  The MGC SHOULD take
   this value into account if it sets the value of the base root
   property root/normalMGexecutiontime.

   The MGC may manage load balancing between alternative MGs by
   periodically auditing the value of nasroot/avalmodems and assigning
   calls according to a policy based on this value.

10. Security Implications

   Security for Megaco/H.248 signalling is considered in the base
   protocol specification [6].  Special consideration must be given to
   protection of caller privacy, as this is the subject of regulatory
   requirements in many jurisdictions.  The basic principle is that if
   the caller has requested privacy (typically through subscription),
   then:
    - the requirement for protection of caller identity MUST be
      propagated between successive entities in the call chain;
    - caller identity itself MUST NOT be sent to an untrusted entity.

   In the NAS case, the trust boundary may lie between the MGC and MG,
   between the MG and the AAA server, or beyond the AAA server.  The
   MGC MUST be provisioned to be able to distinguish between a trusted
   and untrusted MG.  Where the MG is trusted, the MG MUST be
   provisioned with the knowledge of whether the AAA server is trusted
   or untrusted.

11. IANA Considerations

   This document adds the Megaco/H.248 error codes defined in section
   5.5.

12. Acknowledgements

   This document was improved by comments from Matt Holdrege and
   Christian Groves.

13. References

   [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
   Levels", BCP 14, RFC 2119, March 1997.


Taylor et al     Standards Track - Expires October 2002             17
 
                       Megaco/H.248 NAS Package             April 2002


   [2] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
   Authentication Dial In Service (RADIUS)", RFC 2865, June 2000.

   [3] Townsley, W., et al, "Layer Two Tunneling Protocol(L2TP)", RFC
   2661, August 1999

   [4] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

   [5] N. Greene, M. Ramalho, B. Rosen, "Media Gateway control protocol
   architecture and requirements", RFC 2805, April 2000.

   [6] F. Cuervo, N. Greene, C. Huitema, A. Rayhan, B. Rosen, J.
   Segers, "Media Gateway Control Protocol", RFC 3015, November 2000.

   [7] T. Taylor, "Conventions for the use of the Session Description
   Protocol (SDP) for Digital Circuit Connections", draft-taylor-
   mmusic-sdp-tdm-xx.txt, work in progress.


14. AUTHORS' ADDRESSES

   Tom Taylor
   Nortel Networks
   Phone: +1 613 736 0961
   Email: taylor@nortelnetworks.com

   Alan V. Whitton
   Nortel Networks
   Phone: +1 613 763 8871
   Email: awhitton@nortelnetworks.com

   Julian Mitchell
   Nortel Networks
   Phone: +44-(0)1628-434695
   Email: julianm@nortelnetworks.com

   Ravi Subramaniam
   Cisco Systems
   Email: ravis@cisco.com



Taylor et al     Standards Track - Expires October 2002             18


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/