tech-invite   World Map     

3GPP     Specs     Glossaries     Architecture     IMS     UICC       IETF     RFCs     Groups     SIP     ABNFs       Search

RFC 2705

 
 
 

Media Gateway Control Protocol (MGCP) Version 1.0

Part 2 of 5, p. 30 to 61
Prev RFC Part       Next RFC Part

 


prevText      Top      Up      ToC       Page 30 
2.3.  Gateway Control Commands

   This section describes the commands of the MGCP. The service consists
   of connection handling and endpoint handling commands. There are nine
   commands in the protocol:

   *  The Call Agent can issue an EndpointConfiguration command to a
      gateway, instructing the gateway about the coding characteristics
      expected by the "line-side" of the endpoint.

   *  The Call Agent can issue a NotificationRequest command to a
      gateway, instructing the gateway to watch for specific events such
      as hook actions or DTMF tones on a specified endpoint .

   *  The gateway will then use the Notify command to inform the Call
      Agent when the requested events occur.

   *  The Call Agent can use the CreateConnection command to create a
      connection that terminates in an "endpoint" inside the gateway.

   *  The Call Agent can use the ModifyConnection command to change the
      parameters associated to a previously established connection.

   *  The Call Agent can use the DeleteConnection command to delete an
      existing connection. The DeleteConnection command may also be used
      by a gateway to indicate that a connection can no longer be
      sustained.

   *  The Call Agent can use the AuditEndpoint and AuditConnection
      commands to audit the status of an "endpoint" and any connections
      associated with it. Network management beyond the capabilities
      provided by these commands are generally desirable, e.g.
      information about the status of the gateway. Such capabilities are
      expected to be supported by the use of the Simple Network
      Management Protocol (SNMP) and definition of a MIB which is
      outside the scope of this specification.

   *  The Gateway can use the RestartInProgress command to notify the
      Call Agent that the gateway, or a group of endpoints managed by
      the gateway, is being taken out of service or is being placed back
      in service.

   These services allow a controller (normally, the Call Agent) to
   instruct a gateway on the creation of connections that terminate in
   an "endpoint" attached to the gateway, and to be informed about
   events occurring at the endpoint. An endpoint may be for example:

Top      Up      ToC       Page 31 
   *  A specific trunk circuit, within a trunk group terminating in a
      gateway,

   *  A specific announcement handled by an announcement server.

   Connections are grouped into "calls". Several connections, that may
   or may not belong to the same call, can terminate in the same
   endpoint .  Each connection is qualified by a "mode" parameter, which
   can be set to "send only" (sendonly), "receive only" (recvonly),
   "send/receive" (sendrecv), "conference" (confrnce), "data",
   "inactive" (inactive), "loopback", "continuity test" (conttest),
   "network loop back" (netwloop) or "network continuity test"
   (netwtest).

   The handling of the audio signals received on these connections is
   determined by the mode parameters:

   *  Audio signals received in data packets through connections in
      "receive", "conference" or "send/receive" mode are mixed and sent
      to the endpoint.

   *  Audio signals originating from the endpoint are transmitted over
      all the connections whose mode is "send", "conference" or
      "send/receive."

   *  In addition to being sent to the endpoint, audio signals received
      in data packets through connections in "conference" mode are
      replicated to all the other connections whose mode is
      "conference."

   The "loopback" and "continuity test" modes are used during
   maintenance and continuity test operations. There are two flavors of
   continuity test, one specified by ITU and one used in the US. In the
   first case, the test is a loopback test. The originating switch will
   send a tone (the go tone) on the bearer circuit and expect the
   terminating switch to loopback the circuit. If the originating switch
   sees the same tone returned (the return tone), the COT has passed. If
   not, the COT has failed. In the second case, the go and return tones
   are different. The originating switch sends a certain go tone. The
   terminating switch detects the go tone, it asserts a different return
   tone in the backwards direction. When the originating switch detects
   the return tone, the COT is passed. If the originating switch never
   detects the return tone, the COT has failed.

   If the mode is set to "loopback", the gateway is expected to return
   the incoming signal from the endpoint back into that same endpoint.
   This procedure will be used, typically, for testing the continuity of
   trunk circuits according to the ITU specifications.

Top      Up      ToC       Page 32 
   If the mode is set to "continuity test", the gateway is informed that
   the other end of the circuit has initiated a continuity test
   procedure according to the GR specification. The gateway will place
   the circuit in the transponder mode required for dual-tone continuity
   tests.

   If the mode is set to "network loopback", the audio signals received
   from the connection will be echoed back on the same connection.

   If the mode is set to "network continuity test", the gateway will
   process the packets received from the connection according to the
   transponder mode required for dual-tone continuity test, and send the
   processed signal back on the connection.

2.3.1.  EndpointConfiguration

   The EndpointConfiguration commands are used to specify the encoding
   of the signals that will be received by the endpoint.  For example,
   in certain international telephony configurations, some calls will
   carry mu-law encoded audio signals, while other will use A-law.  The
   Call Agent will use the EndpointConfiguration command to pass this
   information to the gateway. The configuration may vary on a call by
   call basis, but can also be used in the absence of any connection.

           ReturnCode
           <-- EndpointConfiguration( EndpointId,
                                      BearerInformation)

   EndpointId is the name for the endpoint in the gateway where
   EndpointConfiguration executes, as defined in section 2.1.1.  The
   "any of" wildcard convention shall not be used.  If the "all of"
   wildcard convention is used, the command applies to all the endpoint
   whose name matches the wildcard.

   BearerInformation is a parameter defining the coding of the data
   received from the line side.  These information is encoded as a list
   of sub-parameters.  The only sub-parameter defined in this version of
   the specification is the encoding method, whose values can be set to
   "A-law" and "mu-law".

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

Top      Up      ToC       Page 33 
2.3.2.  NotificationRequest

   The NotificationRequest commands are used to request the gateway to
   send notifications upon the occurrence of specified events in an
   endpoint.  For example, a notification may be requested for when a
   gateway detects that an endpoint is receiving tones associated with
   fax communication.  The entity receiving this notification may decide
   to use a different type of encoding method in the connections bound
   to this endpoint.

       ReturnCode
       <-- NotificationRequest( EndpointId,
                                [NotifiedEntity,]
                                [RequestedEvents,]
                                RequestIdentifier,
                                [DigitMap,]
                                [SignalRequests,]
                                [QuarantineHandling,]
                                [DetectEvents,]
                                [encapsulated EndpointConfiguration])

   EndpointId is the name for the endpoint in the gateway where
   NotificationRequest executes, as defined in section 2.1.1.

   NotifiedEntity is an optional parameter that specifies where the
   notifications should be sent. When this parameter is absent, the
   notifications should be sent to the originator of the
   NotificationRequest.

   RequestIdentifier is used to correlate this request with the
   notifications that it triggers.

   RequestedEvents is a list of events that the gateway is requested to
   detect and report. Such events include, for example, fax tones,
   continuity tones, or on-hook transition.  To each event is associated
   an action, which can be:

   *  Notify the event immediately, together with the accumulated list
      of observed events,

   *  Swap audio,

   *  Accumulate the event in an event buffer, but don't notify yet,

   *  Accumulate according to Digit Map,

   *  Keep Signal(s) active,

Top      Up      ToC       Page 34 
   *  process the Embedded Notification Request,

   *  Ignore the event.

   Some actions can be combined.  In particular:

   *  The "swap audio" action can be combined with "Notify",
      "Accumulate" and "Ignore."

   *  The "keep signal active" action can be combined with "Notify",
      "Accumulate", "Accumulate according to Digit Map", "Ignore" and
      "Embedded Notification Request."

   *  The "Embedded Notification Request" can be combined with
      "Accumulate" and with "Keep signals active." It can also be
      combined with Notify, if the gateway is allowed to issue several
      Notify commands in response to a single Notification request.

   In addition to the requestedEvents parameter specified in the
   command, some profiles of MGCP have introduced the concept of
   "persistent events." According to such profiles, the persistent event
   list is configured in the endpoint, by means outside the scope of
   MGCP. The basic MGCP specification does not specify any persistent
   event.

   If a persistent event is not included in the list of RequestedEvents,
   and the event occurs, the event will be detected anyway, and
   processed like all other events, as if the persistent event had been
   requested with a Notify action. Thus, informally, persistent events
   can be viewed as always being implicitly included in the list of
   RequestedEvents with an action to Notify, although no glare
   detection, etc., will be performed.

   Non-persistent events are those events explicitly included in the
   RequestedEvents list. The (possibly empty) list of requested events
   completely replaces the previous list of requested events. In
   addition to the persistent events, only the events specified in the
   requested events list will be detected by the endpoint. If a
   persistent event is included in the RequestedEvents list, the action
   specified will then replace the default action associated with the
   event for the life of the RequestedEvents list, after which the
   default action is restored. For example, if "Ignore off-hook" was
   specified, and a new request without any off-hook instructions were
   received, the default "Notify off-hook" operation then would be
   restored. A given event MUST NOT appear more than once in a
   RequestedEvents.

Top      Up      ToC       Page 35 
   The gateway will detect the union of the persistent events and the
   requested events. If an event is not specified in either list, it
   will be ignored.

   The Swap Audio action can be used when a gateway handles more than
   one active connection on an endpoint. This will be the case for
   three-way calling, call waiting, and possibly other feature
   scenarios. In order to avoid the round-trip to the Call Agent when
   just changing which connection is attached to the audio functions of
   the endpoint, the NotificationRequest can map an event (usually hook
   flash, but could be some other event) to a local function swap audio,
   which selects the "next" connection in a round robin fashion. If
   there is only one connection, this action is effectively a no-op.

   If signal(s) are desired to start when an event being looked for
   occurs, the "Embedded NotificationRequest" action can be used. The
   embedded NotificationRequest may include a new list of
   RequestedEvents, SignalRequests and a new digit map as well. The
   semantics of the embedded NotificationRequest is as if a new
   NotificationRequest was just received with the same NotifiedEntity,
   and RequestIdentifier. When the "Embedded NotificationRequest" is
   activated, the "current dial string" will be cleared; the list of
   observed events and the quarantine buffer will be unaffected.

   MGCP implementations shall be able to support at least one level of
   embedding.  An embedded NotificationRequest that respects this
   limitation shall not contain another Embedded NotificationRequest.

   DigitMap is an optional parameter that allows the Call Agent to
   provision the gateways with a digit map according to which digits
   will be accumulated. If this optional parameter is absent, the
   previously defined value is retained. This parameter must be defined,
   either explicitly or through a previous command, if the
   RequestedEvent parameters contain an request to "accumulate according
   to the digit map." The collection of these digits will result in a
   digit string. The digit string is initialized to a null string upon
   reception of the NotificationRequest, so that a subsequent
   notification only returns the digits that were collected after this
   request. Digits that were accumulated according to the digit map are
   reported as any other accumulated event, in the order in which they
   occur. It is therefore possible that other events be accumulated may
   be found in between the list of digits.

   SignalRequests is a parameter that contains the set of signals that
   the gateway is asked to apply to the endpoint, such as, for example
   ringing, or continuity tones. Signals are identified by their name,
   which is an event name, and may be qualified by parameters.

Top      Up      ToC       Page 36 
   The action triggered by the SignalRequests is synchronized with the
   collection of events specified in the RequestedEvents parameter. For
   example, if the NotificationRequest mandates "ringing" and the event
   request ask to look for an "off-hook" event, the ringing shall stop
   as soon as the gateway detect an off hook event. The formal
   definition is that the generation of all "Time Out" signals shall
   stop as soon as one of the requested events is detected, unless the
   "Keep signals active" action is associated to the specified event.

   The specific definition of actions that are requested via these
   SignalRequests, such as the duration of and frequency of a DTMF
   digit, is out side the scope of MGCP. This definition may vary from
   location to location and hence from gateway to gateway.

   The RequestedEvents and SignalRequests refer to the same event
   definitions. In one case, the gateway is asked to detect the
   occurrence of the event, and in the other case it is asked to
   generate it. The specific events and signals that a given endpoint
   can detect or perform are determined by the list of event packages
   that are supported by that end point.  Each package specifies a list
   of events and actions that can be detected or performed.  A gateway
   that is requested to detect or perform an event belonging to a
   package that is not supported by the specified endpoint shall return
   an error. When the event name is not qualified by a package name, the
   default package name for the end point is assumed.  If the event name
   is not registered in this default package, the gateway shall return
   an error.

   The Call Agent can send a NotificationRequest whose requested signal
   list is empty. It will do so for example when tone generation should
   stop.

   The optional QuarantineHandling parameter specifies the handling of
   "quarantine" events, i.e. events that have been detected by the
   gateway before the arrival of this NotificationRequest command, but
   have not yet been notified to the Call Agent.  The parameter provides
   a set of handling options:

   *  whether the quarantined events should be processed or discarded
      (the default is to process them.)

   *  whether the gateway is expected to generate at most one
      notification (step by step), or multiple notifications (loop), in
      response to this request (the default is exactly one.)

   When the parameter is absent, the default value is assumed.

Top      Up      ToC       Page 37 
   We should note that the quarantine-handling parameter also governs
   the handling of events that were detected but not yet notified when
   the command is received.

   DetectEvents is an optional parameter that specifies a list of events
   that the gateway is requested to detect during the quarantine period.
   When this parameter is absent, the events that should be detected in
   the quarantine period are those listed in the last received
   DetectEvents list.  In addition, the gateway should also detect the
   events specified in the request list, including those for which the
   "ignore" action is specified.

   Some events and signals, such as the in-line ringback or the quality
   alert, are performed or detected on connections terminating in the
   end point rather than on the endpoint itself.  The structure of the
   event names allow the Call Agent to specify the connection (or
   connections) on which the events should be performed or detected.

   The command may carry an encapsulated EndpointConfiguration command,
   that will apply to the same endpoint.  When this command is present,
   the parameters of the EndpointConfiguration command are inserted
   after the normal parameters of the NotificationRequest, with the
   exception of the EndpointId, which is not replicated.

   The encapsulated EndpointConfiguration command shares the fate of the
   NotificationRequest command.  If the NotificationRequest is rejected,
   the EndpointConfiguration is not executed.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary. .NH 3 Notifications

   Notifications are sent via the Notify command and are sent by the
   gateway when the observed events occur.

               ReturnCode
               <-- Notify( EndpointId,
                           [NotifiedEntity,]
                           RequestIdentifier,
                           ObservedEvents)

   EndpointId is the name for the endpoint in the gateway which is
   issuing the Notify command, as defined in section 2.1.1. The
   identifier should be a fully qualified endpoint identifier, including
   the domain name of the gateway.  The local part of the name shall not
   use the wildcard convention.

Top      Up      ToC       Page 38 
   NotifiedEntity is an optional parameter that identifies the entity to
   which the notifications is sent. This parameter is equal to the last
   received value of the NotifiedEntity parameter.  The parameter is
   absent if there was no such parameter in the triggering request. The
   notification is sent to the "current notified entity" or, if no such
   entity was ever specified, to the address from which the request was
   received.

   RequestIdentifier is parameter that repeats the RequestIdentifier
   parameter of the NotificationRequest that triggered this
   notification.  It is used to correlate this notification with the
   request that triggered it.

   ObservedEvents is a list of events that the gateway detected. A
   single notification may report a list of events that will be reported
   in the order in which they were detected. The list may only contain
   the identification of events that were requested in the
   RequestedEvents parameter of the triggering NotificationRequest. It
   will contain the events that were either accumulated (but not
   notified) or treated according to digit map (but no match yet), and
   the final event that triggered the detection or provided a final
   match in the digit map.

   ReturnCode is a parameter returned by the call agent. It indicates
   the outcome of the command and consists of an integer number
   optionally followed by commentary.

2.3.3.  CreateConnection

   This command is used to create a connection between two endpoints.

            ReturnCode,
            ConnectionId,
            [SpecificEndPointId,]
            [LocalConnectionDescriptor,]
            [SecondEndPointId,]
            [SecondConnectionId]
            <--- CreateConnection(CallId,
                                  EndpointId,
                                  [NotifiedEntity,]
                                  [LocalConnectionOptions,]
                                  Mode,
                                  [{RemoteConnectionDescriptor |
                                    SecondEndpointId}, ]
                                  [Encapsulated NotificationRequest,]
                                  [Encapsulated EndpointConfiguration])

Top      Up      ToC       Page 39 
   A connection is defined by its endpoints. The input parameters in
   CreateConnection provide the data necessary to build a gateway's
   "view" of a connection.

   CallId is a globally unique parameter that identifies the call (or
   session) to which this connection belongs. Connections that belong to
   the same call share the same call-id. The call-id can be used to
   identify calls for reporting and accounting purposes. It does not
   affect the handling of connections by the gateway.

   EndpointId is the identifier for the connection endpoint in the
   gateway where CreateConnection executes. The EndpointId can be
   fully-specified by assigning a value to the parameter EndpointId in
   the function call or it may be under-specified by using the "anyone"
   wildcard convention. If the endpoint is underspecified, the endpoint
   identifier will be assigned by the gateway and its complete value
   returned in the SpecificEndPointId parameter of the response.

   The NotifiedEntity is an optional parameter that specifies where the
   Notify or DeleteConnection commands should be sent. If the parameter
   is absent, the Notify or DeleteConnection commands should be sent to
   the last received Notified Entity, or to originator of the
   CreateConnection command if no Notified Entity was ever received for
   the end point.

   LocalConnectionOptions is a parameter used by the Call Agent to
   direct the handling of the connection by the gateway.  The fields
   contained in LocalConnectionOptions are the following:

   *  Encoding Method,

   *  Packetization period,

   *  Bandwidth,

   *  Type of Service,

   *  Usage of echo cancellation,

   *  Usage of silence suppression or voice activity detection,

   *  Usage of signal level adaptation and noise level reduction, or
      "gain control."

   *  Usage of reservation service,

   *  Usage of RTP security,

Top      Up      ToC       Page 40 
   *  Type of network used to carry the connection.

   This set of field can be completed by vendor specific optional or
   mandatory extensions. The encoding of the first three fields, when
   they are present, will be compatible with the SDP and RTP profiles:

   *  The encoding method shall be specified by using one or several
      valid encoding names, as defined in the RTP AV Profile or
      registered with the IANA.

   *  The packetization period is encoded as either the length of time
      in milliseconds represented by the media in a packet, as specified
      in the "ptime" parameter of SDP, or as a range value, specifying
      both the minimum and maximum acceptable packetization periods.

   *  The bandwidth is encoded as either a single value or a range,
      expressed as an integer number of kilobit per seconds.

   For each of the first three fields, the Call Agent has three options:

   *  It may state exactly one value, which the gateway will then use
      for the connection,

   *  It may provide a loose specification, such as a list of allowed
      encoding methods or a range of packetization periods,

   *  It may simply provide a bandwidth indication, leaving the choice
      of encoding method and packetization period to the gateway.

   The bandwidth specification shall not contradict the specification of
   encoding methods and packetization period. If an encoding method is
   specified, then the gateway is authorized to use it, even if it
   results in the usage of a larger bandwidth than specified.

   The LocalConnectionOptions parameter may be absent in the case of a
   data call.

   The Type of Service specifies the class of service that will be used
   for the connection. When the connection is transmitted over an IP
   network, the parameters encodes the 8-bit type of service value
   parameter of the IP header. When the Type of Service is not
   specified, the gateway shall use a default or configured value.

   The gateways can be instructed to perform a reservation, for example
   using RSVP, on a given connection.  When a reservation is needed, the
   call agent will specify the reservation profile that should be used,
   which is either "controlled load" or "guaranteed service."  The

Top      Up      ToC       Page 41 
   absence of reservation can be indicated by asking for the "best
   effort" service, which is the default value of this parameter. When
   reservation has been asked on a connection, the gateway will:

   *  start emitting RSVP "PATH" messages if the connection is in
      "send-only", "send-receive", "conference", "network loop back" or
      "network continuity test" mode (if a remote connection descriptor
      has been received,)

   *  start emitting RSVP "RESV" messages as soon as it receives "PATH"
      messages if the connection is in "receive-only", "send-receive",
      "conference", "network loop back" or "network continuity test"
      mode.

   The RSVP filters will be deduced from the characteristics of the
   connection. The RSVP resource profiles will be deduced from the
   connection's bandwidth and packetization period.

   By default, the telephony gateways always perform echo cancellation.
   However, it is necessary, for some calls, to turn off these
   operations.  The echo cancellation parameter can have two values,
   "on" (when the echo cancellation is requested) and "off" (when it is
   turned off.)

   The telephony gateways may perform gain control, in order to adapt
   the level of the signal.  However, it is necessary, for example for
   modem calls, to turn off this function.  The gain control parameter
   may either be specified as "automatic", or as an explicit number of
   decibels of gain.  The default is to not perform gain control, which
   is equivalent to specifying a gain of 0 decibels.

   The telephony gateways may perform voice activity detection, and
   avoid sending packets during periods of silence.  However, it is
   necessary, for example for modem calls, to turn off this detection.
   The silence suppression parameter can have two values, "on" (when the
   detection is requested) and "off" (when it is turned off.) The
   default is "off."

   The Call agent can request the gateway to enable encryption of the
   audio Packets.  It does so by providing an key specification, as
   specified in RFC 2327. By default, encryption is not used.

   The Call Agent may instruct the gateway to prepare the connection on
   a specified type of network.  The type of network is encoded as in
   the "connection-field" parameter of the SDP standard.  Possible
   values are IN (Internet), ATM and LOCAL. The parameter is optional;
   if absent, the network is determined by the type of gateway.

Top      Up      ToC       Page 42 
   RemoteConnectionDescriptor is the connection descriptor for the
   remote side of a connection, on the other side of the IP network. It
   includes the same fields as in the LocalConnectionDescriptor, i.e.
   the fields that describe a session according to the SDP standard.
   This parameter may have a null value when the information for the
   remote end is not known yet. This occurs because the entity that
   builds a connection starts by sending a CreateConnection to one of
   the two gateways involved in it. For the first CreateConnection
   issued, there is no information available about the other side of the
   connection. This information may be provided later via a
   ModifyConnection call. In the case of data connections (mode=data),
   this parameter describes the characteristics of the data connection.

   The SecondEndpointId can be used instead of the
   RemoteConnectionDescriptor to establish a connection between two
   endpoints located on the same gateway.  The connection is by
   definition a local connection. The SecondEndpointId can be fully-
   specified by assigning a value to the parameter SecondEndpointId in
   the function call or it may be under-specified by using the "anyone"
   wildcard convention. If the secondendpoint is underspecified, the
   second endpoint identifier will be assigned by the gateway and its
   complete value returned in the SecondEndPointId parameter of the
   response.

   Mode indicates the mode of operation for this side of the connection.
   The mode are "send", "receive", "send/receive", "conference", "data",
   "inactive", "loopback", "continuity test", "network loop back" or
   "network continuity test." The expected handling of these modes is
   specified in the introduction of the "Gateway Handling Function"
   section. Some end points may not be capable of supporting all modes.
   If the command specifies a mode that the endpoint cannot support, and
   error shall be returned.

   The gateway returns a ConnectionId, that uniquely identifies the
   connection within one endpoint, and a LocalConnectionDescriptor,
   which is a session description that contains information about
   addresses and RTP ports, as defined in SDP. The
   LocalConnectionDescriptor is not returned in the case of data
   connections. The SpecificEndPointId is an optional parameter that
   identifies the responding endpoint. It can be used when the
   EndpointId argument referred to a "any of" wildcard name. When a
   SpecificEndPointId is returned, the Call Agent should use it as the
   EndpointId value is successive commands referring to this call.

Top      Up      ToC       Page 43 
   When a SecondEndpointId is specified, the command really creates two
   connections that can be manipulated separately through
   ModifyConnection and DeleteConnection commands.  The response to the
   creation provides a SecondConnectionId parameter that identifies the
   second connection.

   After receiving a "CreateConnection" request that did not include a
   RemoteConnectionDescriptor parameter, a gateway is in an ambiguous
   situation. Because it has exported a LocalConnectionDescriptor
   parameter, it can potentially receive packets. Because it has not yet
   received the RemoteConnectionDescriptor parameter of the other
   gateway, it does not know whether the packets that it receives have
   been authorized by the Call Agent. It must thus navigate between two
   risks, i.e. clipping some important announcements or listening to
   insane data. The behavior of the gateway is determined by the value
   of the Mode parameter:

   *  If the mode was set to ReceiveOnly, the gateway should accept the
      voice signals and transmit them through the endpoint.

   *  If the mode was set to Inactive, Loopback, Continuity Test, the
      gateway should refuse the voice signals.

   *  If the mode was set to Network Loopback or Network Continuity
      Test, the gateway should perform the expected echo or Response.

   Note that the mode values SendReceive, Conference, Data and SendOnly
   don't make sense in this situation. They should be treated as errors,
   and the command should be rejected (Error code 517).

   The command may optionally contain an encapsulated Notification
   Request command, in which case a RequestIdentifier parameter will be
   present, as well as, optionally, the RequestedEvents DigitMap,
   SignalRequests, QuarantineHandling and DetectEvents parameters. The
   encapsulated NotificationRequest is executed simultaneously with the
   creation of the connection. For example, when the Call Agent wants to
   initiate a call to an residential gateway, it should:

   *  ask the residential gateway to prepare a connection, in order to
      be sure that the user can start speaking as soon as the phone goes
      off hook,

   *  ask the residential gateway to start ringing,

   *  ask the residential gateway to notify the Call Agent when the
      phone goes off-hook.

Top      Up      ToC       Page 44 
   This can be accomplished in a single CreateConnection command, by
   also transmitting the RequestedEvent parameters for the off hook
   event, and the SignalRequest parameter for the ringing signal.

   When these parameters are present, the creation and the
   NotificationRequests should be synchronized, which means that
   bothshould be accepted, or both refused. In our example, the
   CreateConnection may be refused if the gateway does not have
   sufficient resources, or cannot get adequate resources from the local
   network access, and the off-hook Notification-Request can be refused
   in the glare condition, if the user is already off-hook. In this
   example, the phone should not ring if the connection cannot be
   established, and the connection should not be established if the user
   is already off hook.

   The NotifiedEntity parameter, if present, applies to both the
   CreateConnection and the NotificationRequest command. It defines the
   new "notified entity" for the endpoint.

   The command may carry an encapsulated EndpointConfiguration command,
   that will apply to the same endpoint.  When this command is present,
   the parameters of the EndpointConfiguration command are inserted
   after the normal parameters of the CreateConnection with the
   exception of the EndpointId, which is not replicated. The
   EndpointConfiguration command may be encapsulated together with an
   encapsulated NotificationRequest command.

   The encapsulated EndpointConfiguration command shares the fate of the
   CreateConnection command.  If the CreateConnection is rejected, the
   EndpointConfiguration is not executed.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

2.3.4.  ModifyConnection

   This command is used to modify the characteristics of a gateway's
   "view" of a connection. This "view" of the call includes both the
   local connection descriptors as well as the remote connection
   descriptor.

Top      Up      ToC       Page 45 
      ReturnCode,
      [LocalConnectionDescriptor]
       <--- ModifyConnection(CallId,
                             EndpointId,
                             ConnectionId,
                             [NotifiedEntity,]
                             [LocalConnectionOptions,]
                             [Mode,]
                             [RemoteConnectionDescriptor,]
                             [Encapsulated NotificationRequest,]
                             [Encapsulated EndpointConfiguration])

   The parameters used are the same as in the CreateConnection command,
   with the addition of a ConnectionId that identifies the connection
   within the endpoint. This parameter is returned by the
   CreateConnection function, as part of the local connection
   descriptor. It uniquely identifies the connection within the context
   of the endpoint.

   The EndpointId should be a fully qualified endpoint identifier.  The
   local name shall not use the wildcard convention.

   The ModifyConnection command can be used to affect parameters of a
   connection in the following ways:

   *  Provide information about the other end of the connection, through
      the RemoteConnectionDescriptor.

   *  Activate or deactivate the connection, by changing the value of
      the Mode parameter. This can occur at any time during the
      connection, with arbitrary parameter values.

   *  Change the sending parameters of the connection, for example by
      switching to a different coding scheme, changing the packetization
      period, or modifying the handling of echo cancellation.

   Connections can only be activated if the RemoteConnectionDescriptor
   has been provided to the gateway. The receive only mode, however, can
   be activated without the provision of this descriptor.

   The command will only return a LocalConnectionDescriptor if the local
   connection parameters, such as RTP ports, were modified. (Usage of
   this feature is actually for further study.)

   The command may optionally contain an encapsulated Notification
   Request command, in which case a RequestIdentifier parameter will be
   present, as well as, optionnally, the RequestedEvents DigitMap,
   SignalRequests, QuarantineHandling and DetectEvents parameters. The

Top      Up      ToC       Page 46 
   encapsulated NotificationRequest is executed simultaneously with the
   modification of the connection. For example, when a connection is
   accepted, the calling gateway should be instructed to place the
   circuit in send-receive mode and to stop providing ringing tones.

   This can be accomplished in a single ModifyConnection command, by
   also transmitting the RequestedEvent parameters, for the on hook
   event, and an empty SignalRequest parameter, to stop the provision of
   ringing tones.

   When these parameters are present, the modification and the
   NotificationRequests should be synchronized, which means that both
   should be accepted, or both refused.  The NotifiedEntity parameter,
   if present, applies to both the ModifyConnection and the
   NotificationRequest command.

   The command may carry an encapsulated EndpointConfiguration command,
   that will apply to the same endpoint.  When this command is present,
   the parameters of the EndpointConfiguration command are inserted
   after the normal parameters of the ModifyConnection with the
   exception of the EndpointId, which is not replicated. The
   EndpointConfiguration command may be encapsulated together with an
   encapsulated NotificationRequest command.

   The encapsulated EndpointConfiguration command shares the fate of the
   ModifyConnection command.  If the ModifyConnection is rejected, the
   EndpointConfiguration is not executed.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

2.3.5.  DeleteConnection (from the Call Agent)

   This command is used to terminate a connection. As a side effect, it
   collects statistics on the execution of the connection.

        ReturnCode,
        Connection-parameters
        <-- DeleteConnection(CallId,
                             EndpointId,
                             ConnectionId,
                             [Encapsulated NotificationRequest,]
                             [Encapsulated EndpointConfiguration])

   The endpoint identifier, in this form of the DeleteConnection
   command, shall be fully qualified.  Wildcard conventions shall not be
   used.

Top      Up      ToC       Page 47 
   In the general case where a connection has two ends, this command has
   to be sent to both gateways involved in the connection. Some
   connections, however, may use IP multicast. In this case, they can be
   deleted individually.

   After the connection has been deleted, any loopback that has been
   requested for the connection should be cancelled. When all
   connections to an endpoint have been deleted, that endpoint should be
   placed in inactive mode.

   In response to the DeleteConnection command, the gateway returns a
   list of parameters that describe the status of the connection. These
   parameters are:

   Number of packets sent:

   The total number of RTP data packets transmitted by the sender since
   starting transmission on this connection. The count is not reset if
   the sender changes its synchronization source identifier (SSRC, as
   defined in RTP), for example as a result of a Modify command. The
   value is zero if the connection was set in "receive only" mode.

   Number of octets sent:

   The total number of payload octets (i.e., not including header or
   padding) transmitted in RTP data packets by the sender since starting
   transmission on this connection. The count is not reset if the sender
   changes its SSRC identifier, for example as a result of a
   ModifyConnection command. The value is zero if the connection was set
   in "receive only" mode.

   Number of packets received:

   The total number of RTP data packets received by the sender since
   starting reception on this connection. The count includes packets
   received from different SSRC, if the sender used several values. The
   value is zero if the connection was set in "send only" mode.

   Number of octets received:

   The total number of payload octets (i.e., not including header or
   padding) transmitted in RTP data packets by the sender since starting
   transmission on this connection. The count includes packets received
   from different SSRC, if the sender used several values. The value is
   zero if the connection was set in "send only" mode.

Top      Up      ToC       Page 48 
   Number of packets lost:

   The total number of RTP data packets that have been lost since the
   beginning of reception. This number is defined to be the number of
   packets expected less the number of packets actually received, where
   the number of packets received includes any which are late or
   duplicates.  The count includes packets received from different SSRC,
   if the sender used several values. Thus packets that arrive late are
   not counted as lost, and the loss may be negative if there are
   duplicates. The count includes packets received from different SSRC,
   if the sender used several values. The number of packets expected is
   defined to be the extended last sequence number received, as defined
   next, less the initial sequence number received. The count includes
   packets received from different SSRC, if the sender used several
   values. The value is zero if the connection was set in "send only"
   mode. This parameter is omitted if the connection was set in "data"
   mode.

   Interarrival jitter:

   An estimate of the statistical variance of the RTP data packet
   interarrival time measured in milliseconds and expressed as an
   unsigned integer. The interarrival jitter J is defined to be the mean
   deviation (smoothed absolute value) of the difference D in packet
   spacing at the receiver compared to the sender for a pair of packets.
   Detailed computation algorithms are found in RFC 1889. The count
   includes packets received from different SSRC, if the sender used
   several values. The value is zero if the connection was set in "send
   only" mode. This parameter is omitted if the connection was set in
   "data" mode.

   Average transmission delay:

   An estimate of the network latency, expressed in milliseconds. This
   is the average value of the difference between the NTP timestamp
   indicated by the senders of the RTCP messages and the NTP timestamp
   of the receivers, measured when this messages are received. The
   average is obtained by summing all the estimates, then dividing by
   the number of RTCP messages that have been received. This parameter
   is omitted if the connection was set in "data" mode.
   When the gateway's clock is not synchronized by NTP, the latency
   value can be computed as one half of the round trip delay, as
   measured through RTCP.
   When the gateway cannot compute the one way delay or the round trip
   delay, the parameter conveys a null value.

   For a detailed definition of these variables, refer to RFC 1889.

Top      Up      ToC       Page 49 
   When the connection was set up over an ATM network, the meaning of
   these parameters may change:

   Number of packets sent:  The total number of ATM cells transmitted
      since starting transmission on this connection.

   Number of octets sent:
      The total number of payload octets transmitted in ATM cells.

   Number of packets received:
      The total number of ATM cells received since starting reception on
      this connection.

   Number of octets received:
      The total number of payload octets received in ATM cells.

   Number of packets lost:
      Should be determined as the number of cell losts, or set to zero
      if the adaptation layer does not enable the gateway to assess
      losses.

   Interarrival jitter:
      Should be understood as the interarrival jitter between ATM cells.

   Average transmission delay:
      The gateway may not be able to assess this parameter over an ATM
      network.  It could simply report a null value.

   When the connection was set up over an LOCAL interconnect, the
   meaning of these parameters is defined as follows:

   Number of packets sent:
     Not significant.

   Number of octets sent:
     The total number of payload octets transmitted over the local
     connection.

   Number of packets received:
     Not significant.

   Number of octets received:
     The total number of payload octets received over the connection.

   Number of packets lost:
     Not significant.  A value of zero is assumed.

Top      Up      ToC       Page 50 
   Interarrival jitter:
     Not significant.  A value of zero is assumed.

   Average transmission delay:
     Not significant.  A value of zero is assumed.

   The standard set of connection parameters can be extended by the
   creation of extension parameters.

   The command may optionally contain an encapsulated Notification
   Request command, in which case a RequestIdentifier parameter will be
   present, as well as, optionnally, the RequestedEvents DigitMap,
   SignalRequests, QuarantineHandling and DetectEvents parameters. The
   encapsulated NotificationRequest is executed simultaneously with the
   deletion of the connection. For example, when a user hang-up is
   notified, the gateway should be instructed to delete the connection
   and to start looking for an off hook event.

   This can be accomplished in a single DeleteConnection command, by
   also transmitting the RequestedEvent parameters, for the off hook
   event, and an empty SignalRequest parameter.

   When these parameters are present, the DeleteConnection and the
   NotificationRequests should be synchronized, which means that both
   should be accepted, or both refused.

   The command may carry an encapsulated EndpointConfiguration command,
   that will apply to the same endpoint.  When this command is present,
   the parameters of the EndpointConfiguration command are inserted
   after the normal parameters of the DeleteConnection with the
   exception of the EndpointId, which is not replicated. The
   EndpointConfiguration command may be encapsulated together with an
   encapsulated NotificationRequest command.

   The encapsulated EndpointConfiguration command shares the fate of the
   DeleteConnection command.  If the DeleteConnection is rejected, the
   EndpointConfiguration is not executed.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

Top      Up      ToC       Page 51 
2.3.6.  DeleteConnection (from the VoIP gateway)

   In some circumstances, a gateway may have to clear a connection, for
   example because it has lost the resource associated with the
   connection, or because it has detected that the endpoint no longer is
   capable or willing to send or receive voice. The gateway terminates
   the connection by using a variant of the DeleteConnection command:

            ReturnCode,
            <-- DeleteConnection( CallId,
                                  EndpointId,
                                  ConnectionId,
                                  Reason-code,
                                  Connection-parameters)

   In addition to the call, endpoint and connection identifiers, the
   gateway will also send the call's parameters that would have been
   returned to the Call Agent in response to a DeleteConnection command.
   The reason code indicates the cause of the disconnection.

   ReturnCode is a parameter returned by the call agent. It indicates
   the outcome of the command and consists of an integer number
   optionally followed by commentary.

2.3.7.  DeleteConnection (multiple connections, from the Call Agent)

   A variation of the DeleteConnection function can be used by the Call
   Agent to delete multiple connections at the same time. The command
   can be used to delete all connections that relate to a Call for an
   endpoint:

            ReturnCode,
            <-- DeleteConnection( CallId,
                                  EndpointId)

   It can also be used to delete all connections that terminate in a
   given endpoint:

            ReturnCode,
            <-- DeleteConnection( EndpointId)

   Finally, Call Agents can take advantage of the hierarchical naming
   structure of endoints to delete all the connections that belong to a
   group of endpoints.  In this case, the "local name" component of the
   EndpointID will be specified using the "all value" wildcarding
   convention. The "any value" convention shall not be used.  For
   example, if endpoints names are structured as the combination of a
   physical interface name and a circuit number, as in "X35V3+A4/13",

Top      Up      ToC       Page 52 
   the Call Agent may replace the circuit number by a wild card
   character "*", as in "X35V3+A4/*".  This "wildcard" command instructs
   the gateway to delete all the connections that where attached to
   circuits connected to the physical interface "X35V3+A4".

   After the connections have been deleted, the endpoint should be
   placed in inactive mode. Any loopback that has been requested for the
   connections should be cancelled.

   This command does not return any individual statistics or call
   parameters.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

2.3.8.  Audit Endpoint

   The AuditEndPoint command can be used by the Call Agent to find out
   the status of a given endpoint.

              ReturnCode,
                EndPointIdList|{
                [RequestedEvents,]
                [DigitMap,]
                [SignalRequests,]
                [RequestIdentifier,]
                [NotifiedEntity,]
                [ConnectionIdentifiers,]
                [DetectEvents,]
                [ObservedEvents,]
                [EventStates,]
                [BearerInformation,]
                [RestartReason,]
                [RestartDelay,]
                [ReasonCode,]
                [Capabilities]}
                        <--- AuditEndPoint(EndpointId,
                                                 [RequestedInfo])

   The EndpointId identifies the endpoint that is being audited. The
   "all of" wildcard convention can be used to start auditing of a group
   of endpoints. If this convention is used, the gateway should return
   the list of endpoint identifiers that match the wildcard in the
   EndPointIdList parameter. It shall not return any parameter specific
   to one of these endpoints.

Top      Up      ToC       Page 53 
   When a non-wildcard EndpointId is specified, the (possibly empty)
   RequestedInfo parameter describes the information that is requested
   for the EndpointId specified. The following endpoint info can be
   audited with this command:

   RequestedEvents, DigitMap, SignalRequests, RequestIdentifier,
   NotifiedEntity, ConnectionIdentifiers, DetectEvents, ObservedEvents,
   EventStates, RestartReason, RestartDelay, ReasonCode, and
   Capabilities.

   The response will in turn include information about each of the items
   for which auditing info was requested:

   *  RequestedEvents: The current value of RequestedEvents the endpoint
      is using including the action associated with each event.
      Persistent events are included in the list.

   *  DigitMap: the digit map the endpoint is currently using.

   *  SignalRequests: A list of the; Time-Out signals that are currently
      active, On/Off signals that are currently "on" for the endpoint
      (with or without parameter), and any pending Brief signals. Time-
      Out signals that have timed-out, and currently playing Brief
      signals are not included.

   *  RequestIdentifier, the RequestIdentifier for the last Notification
      Request received by this endpoint (includes NotificationRequest
      encapsulated in Connection handling primitives). If no
      notification request has been received, the value zero will be
      returned.

   *  QuarantineHandling, the QuarantineHandling for the last
      NotificationRequest received by this endpoint.

   *  DetectEvents, the list of events that are currently detected in
      quarantine mode.

   *  NotifiedEntity, the current notified entity for the endpoint.

   * ConnectionIdentifiers, the list of ConnectionIdentifiers for all
      connections that currently exist for the specified endpoint.

   *  ObservedEvents: the current list of observed events for the
      endpoint.

Top      Up      ToC       Page 54 
   *  EventStates: For events that have auditable states associated with
      them, the event corresponding to the state the endpoint is in,
      e.g., off-hook if the endpoint is off-hook. The definition of the
      individual events will state if the event in question has an
      auditable state associated with it.

   *  BearerInformation: the value of the last received
      BearerInformation parameter for this endpoint.

   *  RestartReason: the value of the restart reason parameter in the
      last RestartInProgress command issued by the endpoint, "restart"
      indicating a fully functional endpoint.

   *  RestartDelay: the value of the  restart delay parameter if a
      RestartInProgress command was issued by the endpoint at the time
      of the response, or zero if the command would not include this
      parameter.

   *  ReasonCode:the value of the Reason-Code parameter in the last
      RestartInProgress or DeleteConnection command issued by the
      gateway for the endpoint, or the special value 000 if the
      endpoint's state is nominal.

   *  The capabilities for the endpoint similar to the
      LocalConnectionOptions parameter and including event packages and
      connection modes.  If there is a need to specify that some
      parameters, such as e.g., silence suppression, are only compatible
      with some

   *  codecs,  then the gateway will return several capability sets:

         Compression Algorithm: a list of supported codecs. The rest of
         the parameters will apply to all codecs specified in this list.

         Packetization Period: A single value or a range may be
         specified.

         Bandwidth: A single value or a range corresponding to the range
         for packetization periods may be specified (assuming no silence
         suppression).

         Echo Cancellation: Whether echo cancellation is supported or
         not.

         Silence Suppression: Whether silence suppression is supported
         or not.

         Type of Service: Whether type of service is supported or not.

Top      Up      ToC       Page 55 
         Event Packages: A list of event packages supported. The first
         event package in the list will be the default package.

         Modes: A list of supported connection modes.

   The Call Agent may then decide to use the AuditConnection command to
   obtain further information about the connections.

   If no info was requested and the EndpointId refers to a valid
   endpoint, the gateway simply returns a positive acknowledgement.

   If no NotifiedEntity has been specified in the last
   NotificationRequest, the notified entity defaults to the source
   address of the last NotificationRequest command received for this
   connection.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

2.3.9.  Audit Connection

   The AuditConnection command can be used by the Call Agent to retrieve
   the parameters attached to a connection:

              ReturnCode,
              [CallId,]
              [NotifiedEntity,]
              [LocalConnectionOptions,]
              [Mode,]
              [RemoteConnectionDescriptor,]
              [LocalConnectionDescriptor,]
              [ConnectionParameters]
                        <--- AuditConnection(EndpointId,
                                         ConnectionId,
                                         RequestedInfo)

   The EndpointId parameter specifies the endpoint that handles the
   connection. The wildcard conventions shall not be used.

   The ConnectionId parameter is the identifier of the audited
   connection, within the context of the specified endpoint.

   The (possibly empty) RequestedInfo describes the information that is
   requested for the ConnectionId within the EndpointId specified. The
   following connection info can be audited with this command:

Top      Up      ToC       Page 56 
      CallId, NotifiedEntity, LocalConnectionOptions, Mode,
      RemoteConnectionDescriptor, LocalConnectionDescriptor,
      ConnectionParameters

   The AuditConnectionResponse will in turn include information about
   each of the items auditing info was requested for:

   *  CallId, the CallId for the call the connection belongs to.

   *  NotifiedEntity, the current notified entity for the Connection.

   *  LocalConnectionOptions, the LocalConnectionOptions that was
      supplied for the connection.

   *  Mode, the current mode of the connection.

   *  RemoteConnectionDescriptor, the RemoteConnectionDescriptor that
      was supplied to the gateway for the connection.

   *  LocalConnectionDescriptor, the LocalConnectionDescriptor the gate-
      way supplied for the connection.

   *  ConnectionParameters, the current value of the connection
      parameters for the connection.

   If no info was requested and the EndpointId is valid, the gateway
   simply checks that the connection exists, and if so returns a
   positive acknowledgement.

   If no NotifiedEntity has been specified for the connection, the
   notified entity defaults to the source address of the last connection
   handling command received for this connection.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

2.3.10.  Restart in progress

   The RestartInProgress command is used by the gateway to signal that
   An endpoint, or a group of endpoint, is taken in or out of service.

          ReturnCode,
          [NotifiedEntity]
                <------- RestartInProgress ( EndPointId,
                                             RestartMethod,
                                             [RestartDelay,]
                                             [Reason-code])

Top      Up      ToC       Page 57 
   The EndPointId identifies the endpoint that are taken in or out of
   service.  The "all of" wildcard convention may be used to apply the
   command to a group of endpoint, such as for example all endpoints
   that are attached to a specified interface, or even all endpoints
   that are attached to a given gateway.  The "any of" wildcard
   convention shall not be used.

   The RestartMethod parameter specified the type of restart.  Three
   values have been defined:

   *  A "graceful" restart method indicates that the specified endpoints
      will Be taken out of service after the specified delay. The
      established connections are not yet affected, but the Call Agent
      should refrain to establish new connections, and should try to
      gracefully tear down the existing connections.

   *  A "forced" restart method indicates that the specified endpoints
      are taken abruptely out of service. The established connections,
      if any, are lost.

   *  A "restart" method indicates that service will be restored on the
      endpoints after the specified "restart delay." There are no
      connections that are currently established on the endpoints.

   *  A "disconnected" method indicates that the endpoint has become
      disconnected and is now trying to establish connectivity. The
      "restart delay" specifies the number of seconds the endpoint has
      been disconnected. Established connections are not affected.

   *  A "cancel-graceful" method indicates that a gateway is canceling a
      previously issued "graceful" restart command.

   The optional "restart delay" parameter is expressed as a number of
   seconds. If the number is absent, the delay value should be
   considered null.  In the case of the "graceful" method, a null delay
   indicates that the call agent should simply wait for the natural
   termination of the existing connections, without establishing new
   connections. The restart delay is always considered null in the case
   of the "forced" method.

   A restart delay of null for the "restart" method indicates that
   service has already been restored. This typically will occur after
   gateway startup/reboot.

   The optional reason code parameter the cause of the restart.

Top      Up      ToC       Page 58 
   Gateways SHOULD send a "graceful" or "forced" RestartInProgress
   message as a courtesy to the Call Agent when they are taken out of
   service, e.g., by being shutdown, or taken out of service by a
   network management system, although the Call Agent cannot rely on
   always receiving such messages. Gateways MUST send a "restart"
   RestartInProgress message with a null delay to their Call Agent when
   they are back in service according to the restart procedure specified
   in Section 4.3.4 - Call Agents can rely on receiving this message.
   Also, gateways MUST send a "disconnected" RestartInProgress message
   to their current "notified entity" according to the "disconnected"
   procedure specified in Section 4.3.5.  The "restart delay" parameter
   MUST NOT be used with the "forced" restart method.

   The RestartInProgress message will be sent to the current notified
   entity for the EndpointId in question. It is expected that a default
   Call Agent, i.e., notified entity, has been provisioned for each
   endpoint so, after a reboot, the default Call Agent will be the
   notified entity for each endpoint. Gateways should take full
   advantage of wild- carding to minimize the number of
   RestartInProgress messages generated when multiple endpoints in a
   gateway restart and the endpoints are managed by the same Call Agent.

   ReturnCode is a parameter returned by the gateway. It indicates the
   outcome of the command and consists of an integer number optionally
   followed by commentary.

   A NotifiedEntity may additionally be returned with the response from
   the Call Agent:

   *  If the response indicated success (return code 200 - transaction
      executed), the restart procedure has  completed, and the
      NotifiedEntity returned is the new "notified entity" for the
      endpoint(s).

   *  If the response from the Call Agent indicated an error, the
      restart procedure is not yet complete, and must therefore be
      initiated again. If a NotifiedEntity parameter was returned, it
      then specifies the new "notified entity" for the endpoint(s),
      which must consequently be used when retrying the restart
      procedure.

2.4.  Return codes and error codes.

   All MGCP commands are acknowledged. The acknowledgment carries a
   return code, which indicates the status of the command. The return
   code is an integer number, for which four ranges of values have been
   defined:

Top      Up      ToC       Page 59 
   *  values between 100 and 199 indicate a provisional response,

   *  values between 200 and 299 indicate a successful completion,

   *  values between 400 and 499 indicate a transient error,

   *  values between 500 and 599 indicate a permanent error.

   The values that have been already defined are listed in the following
   list:

   100  The transaction is currently being executed.  An actual
        completion message will follow on later.

   200  The requested transaction was executed normally.

   250  The connection was deleted.

   400  The transaction could not be executed, due to a transient error.

   401  The phone is already off hook

   402  The phone is already on hook

   403  The transaction could not be executed, because the endpoint does
        not have sufficient resources at this time

   404  Insufficient bandwidth at this time

   500  The transaction could not be executed, because the endpoint is
        unknown.

   01   The transaction could not be executed, because the endpoint is
        not ready.

   502  The transaction could not be executed, because the endpoint does
        not have sufficient resources

   510  The transaction could not be executed, because a protocol error
        was detected.

   11   The transaction could not be executed, because the command
        contained an unrecognized extension.

   512  The transaction could not be executed, because the gateway is
        not equipped to detect one of the requested events.

Top      Up      ToC       Page 60 
   513  The transaction could not be executed, because the gateway is
        not equipped to generate one of the requested signals.

   514  The transaction could not be executed, because the gateway
        cannot send the specified announcement.

   515  The transaction refers to an incorrect connection-id (may have
        been already deleted)

   516  The transaction refers to an unknown call-id.

   517  Unsupported or invalid mode.

   518  Unsupported or unknown package.

   519  Endpoint does not have a digit map.

   520  The transaction could not be executed, because the endpoint is
        "restarting".

   521  Endpoint redirected to another Call Agent.

   522  No such event or signal.

   523  Unknown action or illegal combination of actions

   524  Internal inconsistency in LocalConnectionOptions

   525  Unknown extension in LocalConnectionOptions

   526  Insufficient bandwidth

   527  Missing RemoteConnectionDescriptor

   528  Incompatible protocol version

   529  Internal hardware failure

   530  CAS signaling protocol error.

   531  failure of a grouping of trunks (e.g. facility failure).

Top      Up      ToC       Page 61 
2.5.  Reason Codes

   Reason-codes are used by the gateway when deleting a connection to
   inform the Call Agent about the reason for deleting the connection.
   They may also be used in a RestartInProgress command, to inform the
   gateway of the Restart's reason. The reason code is an integer
   number, and the following values have been defined:

   000  Endpoint state is nominal. (This code is used only in response
        to audit requests.)

   900  Endpoint malfunctioning

   901  Endpoint taken out of service

   902  Loss of lower layer connectivity (e.g., downstream sync)



(page 61 continued on part 3)

Next RFC Part