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Network Working Group
Request for Comments: 3661
Updates: 3435
Category: Informational
B. Foster
C. Sivachelvan
Cisco Systems
December 2003

Media Gateway Control Protocol (MGCP) Return Code Usage

Status of this Memo

This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2003). All Rights Reserved.

Abstract

This document provides implementation guidelines for the use of return codes in RFC 3435, Media Gateway Control Protocol (MGCP) Version 1.0. Return codes in RFC 3435 do not cover all possible specific situations that may ever occur in a gateway. That is not possible and not necessary. What is important is to ensure that the Call Agent that receives a return code behaves appropriately and consistently for the given situation. The purpose of this document is to provide implementation guidelines to ensure that consistency.

Table of Contents

   1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   2
       1.2.  Document Organization . . . . . . . . . . . . . . . . .   2
   2.  Return Code Usage . . . . . . . . . . . . . . . . . . . . . .   2
       2.1.  Return Code Categories. . . . . . . . . . . . . . . . .   2
       2.2.  Return Code Situations and Categories . . . . . . . . .   3
       2.3.  Summary of Return Code Categories . . . . . . . . . . .  19
   3.  Additional Guidelines . . . . . . . . . . . . . . . . . . . .  21
       3.1.  Gateway Recommendations . . . . . . . . . . . . . . . .  21
       3.2.  Call Agent Recommendations. . . . . . . . . . . . . . .  21
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   5.  Acknowledgements. . . . . . . . . . . . . . . . . . . . . . .  22
   6.  Normative References. . . . . . . . . . . . . . . . . . . . .  22
   7.  Authors' Addresses. . . . . . . . . . . . . . . . . . . . . .  23
   8.  Full Copyright Statement. . . . . . . . . . . . . . . . . . .  24

1. Introduction

This document provides implementation guidelines for the use of return codes in the Media Gateway Control Protocol MGCP 1.0 [1]. Return codes in [1] do not cover all possible specific situations that may ever occur in the gateway. That is not possible and not necessary. What is important is to ensure that the Call Agent that receives that return code behaves appropriately and consistently for the situation that occurred. The solution described in this document is to categorize return codes that gateways return based on the expected behavior for the Call Agents that receive them.

Categorizing errors helps both Call Agent and gateway developers: it helps gateway developers choose an appropriate return code when a specific one for the situation is not available; it also helps Call Agent developers ensure that there is consistent behavior for the return code that is received.

1.2. Document Organization

In addition to categorizing return codes (section 2.1), section 2.2 provides a consolidated list of return codes in terms of "situations" that may have triggered and the "categories" that they fall under. This provides some additional implementation guidelines for the use of these return codes. Section 2.3 includes a summary of the return codes and their categories. Section 3 provides some additional implementation guidelines for Call Agent and gateway developers.

2. Return Code Usage

2.1. Return Code Categories

The following categorizes return codes from gateways based on expected Call Agent behavior.
   Category normal: These return codes are used in normal operation and
      do not represent error conditions.
   Category none (specific errors requiring specific action): A return
      code associated with a specific situation in the gateway that will
      invoke a corresponding specific Call Agent behavior.  As such,
      these return codes are not categorized into a common behavioral
      category.
   Category "Service Failure": A category in which the endpoint is
      either out-of-service or the treatment by the Call Agent is
      expected to be the same as for an out-of-service endpoint.
   Category "Provisioning Mismatch": A situation where the gateway has
      indicated that it does not support what the Call Agent has asked
      it to do.  This may be caused by a lack of synchronization between
      the provisioning of the Call Agent and the gateway.  Note that
      attempts should be made to weed out these types of error
      situations during integration testing.
   Category "Temporary Failure": The transient nature of this error is
      such that this particular call is likely to be permanently
      affected but later calls on the same endpoint may proceed
      successfully.  Typically the situation that caused this error is
      not going to disappear unless there is some change in state in the
      gateway or network (e.g., more bandwidth becomes available, more
      CPU resources become available etc.).  This situation is not
      likely to change in a few 10's of milliseconds but could change
      within some number of seconds or minutes later (as resources
      become free), i.e., within the time period that you might expect a
      different call to be tried on that endpoint.
   Category "State Mismatch":  A case where there is a state mismatch
      between the Call Agent and the gateway that can be resolved by the
      Call Agent making a request that is more appropriate to the
      gateway state.  Although categorized with a common category
      indicator the behavior of the Call Agent will depend on the
      situation (the type of state mismatch that has occurred as well as
      other state information, e.g., call state).
   Category "Remote Connection Descriptor Error": This indicates some
      mismatch between the two gateways involved in the call.  Note that
      per RFC 2327, all gateways should ignore SDP attributes that they
      do not recognize (i.e., lack of recognition of an SDP attribute
      should not be the cause of an error indication).

The exact behavior of the Call Agent for the above categories may depend on the type of endpoint (analog, ISUP trunk, CAS trunk, etc.), whether this is the originating or terminating endpoint in the call and possibly other information related to call state. This document does not attempt to outline the Call Agent behavior based on call state. Instead, it just recommends that the Call Agent behavior be consistent based on a combination of call state and the specific category of error received.

2.2. Return Code Situations and Categories

This section describes return codes in MGCP 1.0 [1] in terms of "situations" that may have triggered that return code and "categories" to which the return code belongs. The purpose is to provide developers additional guidelines for return code use.
Note that any indication that a response is valid for a NotificationRequest (RQNT) is also an indication that it is valid for a connection handling request, i.e., CreateConnection (CRCX), ModifyConnection (MDCX), or DeleteConnection (DLCX) with an encapsulated RQNT. The same holds for the EndpointConfiguration (EPCF) command.
   000 - Response acknowledgement
        Response valid for:    Confirmation of a final response after a
        provisional response (3-way handshake).
        Situation:   If the final response that follows a provisional
        response contains an empty response acknowledgement parameter, a
        Response Acknowledgement is used to acknowledge the final
        response (section 3.5.6 of [1]).
        Category:    normal.
   100 - Transaction in progress
        Response valid for:    Any command that may result in a long
        transaction execution time, e.g., more than 200 ms.
        Situation:   When a transaction is expected to take a processing
        time that is beyond the normal retry timer, the gateway will
        return a provisional response.  A final response will be
        provided later, after the transaction has completed.  Refer to
        section 3.5.6 of [1].  An example of this might be a
        CreateConnection command using RSVP, where the time to create
        the connection may be longer than usual because of the need to
        perform the network resource reservation.
        Category:    normal.
   101 - Transaction has been queued for execution
        Response valid for:    Any command.
        Situation:   As described in [1], Section 4.4.8, we assume that
        Call Agents and gateways conceptually maintain a queue of
        incoming transactions to be executed.  Associated with this
        transaction queue is a high-water and a low-water mark.  Once
        the queue length reaches the high-water mark, the entity should
        start issuing 101 provisional responses (transaction queued)
        until the queue length drops to the low-water mark.  This
        applies to new transactions as well as to retransmissions.  A
        final response will be provided later, after the transaction has
        completed.  In this case, the Call Agent should throttle back
        its request rate for this gateway.
        Category:    normal.
   200 - Transaction executed normally
        Response valid for:    Any command (including DeleteConnection).
        Situation:   Normal response as a result of successful
        execution.  The 250 response code can be used to acknowledge a
        successful completion of a DeleteConnection command.  However, a
        200 response code is also appropriate.
        Category:    normal
   250 - The connection was deleted
        Response valid for:    DeleteConnection.
        Situation:   Response to a successful DeleteConnection command.
        Category:   normal
   400 - Unspecified transient error
        Response valid for:    Any command.
        Situation:   Unspecified transient error.  A more specific error
        code should be used if one is available since this error code
        provides very little information.  If used, some specific
        commentary should be included to aid in debug.
        Category:    "Temporary Failure".
   401 - The phone is already off-hook
        Response valid for:    NotificationRequest.
        Situation:   This is returned in response to a request for an
        off-hook transition requested event when the phone is already
        off-hook.  It is also returned when a request is made to
        generate a signal that has an explicit on-hook precondition in
        the signal definition, such as the ringing signal ("rg") in the
        Line package [2].  It is also returned when requesting an
        incoming off-hook/seizure indication for a Channel Associated
        Signaling (CAS) trunk when the incoming hook-state for that
        trunk is already off-hook.
        Category: "State Mismatch".  If the Call Agent makes the request
        with a requested event indicating a different hook-state, the
        request should not result in this return code again.
   402 - The phone is already on-hook
        Response valid for:    NotificationRequest.
        Situation: This is returned in response to a request for an on-
        hook or hook-flash requested event when the phone is already
        on-hook.  It is also returned when a request is made to generate
        a signal that has an explicit off-hook precondition in the
        signal definition, such as the dial tone ("dl") in the Line
        package [2].  It is also returned when requesting an incoming
        on-hook indication for a CAS trunk when the incoming hook-state
        for that trunk is already on-hook.
        Category:    "State Mismatch".  If the Call Agent makes the
        request with a requested event indicating a different hook-
        state, the request should not result in this error again.
   403 - Insufficient resources available at this time
        Response valid for:    Any command.
        Situation:   This is returned if the request cannot be processed
        due to a temporary lack of gateway resources, such as CPU
        utilization, DSP resources, memory etc; however, the command may
        succeed at a later time when resources free up.  Note that lack
        of network resources should not result in this code (i.e.,
        return code 404 would be more appropriate).
        Category:    "Temporary Failure".

404 - Insufficient bandwidth at this time.

        Response valid for:    CreateConnection, ModifyConnection.
        Situation:   This is an indication that there is not enough
        bandwidth available to sustain the call.  It is as a result of
        some failed bandwidth check (could be RSVP or some other
        mechanism).  It is possible that the Call Agent could request a
        codec requiring lower bandwidth codec and have a successful
        result.  Alternatively, it could treat this as a "Temporary
        Failure" for this codec.
Category: "Temporary Failure". Although categorized under this general category, note that the Call Agent could apply some specific behavior (try a lower bandwidth codec) depending on policy.
   405 - Endpoint is restarting
        Response valid for:    Any command.
        Situation:   It may be returned to requests made when the
        endpoint is in-service and has initiated the restart procedures
        (see [1], Section 4.4.6) but the procedure has not yet
        completed.  If the request is made at a later time, it may be
        "successful" but may not be appropriate (because of possible
        state mismatch).  The Call Agent should proceed after it
        believes the restart procedure has completed.
        Category:    "Temporary Failure"
   406 - Transaction Timeout
        Response valid for:    Any command.
        Situation:   The transaction took longer than expected and has
        been aborted.  An example might be a transaction where a
        provisional response (100 response code) was returned.
        Following that, the gateway determined that the actual
        transaction was taking longer than should reasonably be expected
        and as a result it aborted the transaction and returned 406 as
        the final response.
        Category:    "Temporary Failure".  If this error code is
        returned repeatedly, it could indicate a more serious problem.

407 - Transaction aborted by some external action.

        Response valid for:    Any command.
        Situation:   This is returned to indicate cancellation of a
        pending request (see [1] Section 4.4.4).  For example,
        DeleteConnection is received while processing a CreateConnection
        or ModifyConnection.  Also, if either a ModifyConnection,
        NotificationRequest, or EndpointConfiguration command is in
        progress, and the same command is received with a different
        transaction Id, 407 will be returned.
        Category: none (specific situation and behavior).
   409 - Internal overload
        Response valid for:    Any command.
        Situation: Gateway is overloaded (e.g., too many requests per
        second from the Call Agent) and is unable to process any more
        transactions at this time. In this case, the Call Agent SHOULD
        throttle back its request rate for this gateway as described in
        [1], Section 4.4.8.
        Category:    "Temporary Failure". Note that although the Call
        Agent behavior with respect to the call being set up corresponds
        to this general category, there is some specific Call Agent
        behavior implied as well (i.e., the Call Agent throttling back).
   410 - No endpoint available
        Response valid for: CreateConnection using an "any of" wildcard.
        Situation:   A CreateConnection request was made with an "any
        of" ("$") wildcard and no endpoint was available to execute the
        request. As described in [1], Section 2.3.5, when the "any of"
        wildcard is used with the CreateConnection command, the endpoint
        assigned MUST be in-service and MUST NOT already have any
        connections on it.
        Category: none (specific situation and behavior).
   500 - Endpoint unknown
        Response valid for:    Any command.
        Situation:   There is no endpoint matching the EndpointId
        provided with the command. This could be the result of a
        provisioning mismatch between the Call Agent and the gateway or
        it could be because a card was removed from the gateway so that
        the endpoint is no longer available (in which case a
        RestartInProgress should be received, although the Call Agent
        cannot depend on this). Note that the endpoint is not just out-
        of-service (in which case 501 would be used); it is completely
        unknown/unavailable to the MGCP.
        Category:    "Provisioning Mismatch".
   501 - Endpoint is not ready or is out of service
        Response valid for:    Any command.
        Situation:   This is returned if the endpoint is in a permanent
        "not ready" state. This includes maintenance states such as
        out-of-service. Note that an endpoint that has initiated the
        restart procedure is in-service, and hence should not use this
        return code, even if the restart procedure has not yet completed
        (see [1], Section 4.4.5).
        Category:    "Service Failure".

502 - Insufficient resources (permanent).

        Response valid for:    Any command.
        Situation:   This is returned when the endpoint does not have
        sufficient resources and future requests on this endpoint are
        expected to fail, meaning some resources dedicated to the
        endpoint are broken (e.g., return code 529 - "hardware failure"
        might be a more specific indication). For situations where
        resources may become available in the future (i.e., resources
        are pooled and not available at the present time), return code
        403 should be used instead.
        Category:    "Service Failure".

503 - "All of" wildcard too complicated.

        Response valid for:    Any command.
        Situation:   This is returned when the wildcard convention used
        in the request is understood, but the requested command cannot
        be processed with the specified wildcarding. An example of this
        would be a NotificationRequest with a request such that a
        failure would make it too difficult to roll back the state of
        all the endpoints to what they were prior to the request.
        Category: Normally treated as a "Provisioning Mismatch". Note
        however, that the Call Agent could treat it differently by
        recovering with some specific behavior (e.g., generate a number
        of individual requests without wildcards instead of a single one
        with the wildcard).
504 - Unknown or unsupported command.
        Response valid for:    Any unknown command.
        Situation:   A command was requested other than those specified
        in the MGCP specification [1], and the command is not supported.
        Category:    "Provisioning Mismatch".

505 - Unsupported remote connection descriptor.

        Response valid for: CreateConnection, ModifyConnection.
        Situation:   One or more mandatory parameters or values in the
        RemoteConnectionDescriptor are not supported by the gateway.
        Note that, per [3], unsupported attribute lines must be ignored
        and hence should not result in any errors.
        Category:    "Remote Connection Descriptor Error".
   506 - Inability to satisfy both local connection options and remote
        connection descriptor simultaneously.
        Response valid for: CreateConnection, ModifyConnection.
        Situation:   The LocalConnectionOptions and
        RemoteConnectionDescriptor contain one or more mandatory
        parameters or values that conflict with each other and/or cannot
        be supported at the same time (except for codec negotiation
        failure - see error code 534).
        Category:    "Remote Connection Descriptor Error".
   507 - Unsupported Functionality.  Note that this error code SHOULD
        only be used if there is no other more specific error code for
        the unsupported functionality.
        Response valid for:    Any command.
        Situation:   Any situation where a request from the Call Agent
        is not supported by the gateway - beyond the situations already
        covered by other more specific return codes.
        Category:    "Provisioning Mismatch".

508 - Unknown or unsupported quarantine handling.

        Response valid for:    NotificationRequest.
        Situation:   The endpoint does not support or does not recognize
        the requested quarantine handling.
        Category:    "Provisioning Mismatch".
   509 - Error in RemoteConnectionDescriptor
        Response valid for: CreateConnection, ModifyConnection.
        Situation: There is a syntax or semantic error in the Remote
        Connection Descriptor.  For example, there is no IP address for
        an RTP media stream.
        Category: "Remote Connection Descriptor Error".
   510 - Protocol error
        Response valid for:    Any command.
        Situation:   Some unspecified protocol error was detected.
        Gateways should use this error as a last resort since it
        provides very little information.  If used, some specific
        commentary should be included to aid in debug.
        Category:    "Provisioning Mismatch".

511 - Unrecognized parameter extension.

        Response valid for:    Any command.
        Situation:   It is returned if the requested command contains an
        unrecognized mandatory parameter extension ("X+").  In MGCP 1.0,
        this specifically refers to unrecognized parameters, since other
        error codes are available for unrecognized connection modes
        (517), unrecognized packages (518), unrecognized local
        connection options (541), etc.
        Category: "Provisioning Mismatch".

512 - Gateway not equipped to detect one of the requested events.

        Response valid for:    NotificationRequest.
        Situation:   A valid event was requested however the gateway is
        not equipped to detect this event (i.e., the package is only
        implemented partially).  Of course, such an implementation would
        not conform to [1].  Note that if an invalid event was
        requested, i.e., an event not defined in the relevant package,
        then error code 522 should be used.  Also note, that if the
        package is unknown or unsupported, then error code 518 should be
        used.
        Category:    "Provisioning Mismatch".
   513 - gateway is not equipped to generate one of the requested
        signals.
        Response valid for:    NotificationRequest.
        Situation: A valid signal was requested, however the gateway is
        not equipped to generate this signal (i.e., the package is only
        implemented partially).  Of course, such an implementation would
        not conform to [1].  Note that if an invalid signal was
        requested, i.e., a signal not defined in the relevant package,
        then error code 522 should be used.  Also note, that if the
        package is unknown or unsupported, then error code 518 should be
        used.
        Category:    "Provisioning Mismatch".

514 - The gateway cannot send the specified announcement.

        Response valid for:    NotificationRequest with a request for an
        announcement to be played.
        Situation:   This is a specific situation with respect to
        playing announcements on an endpoint or connection associated
        with the endpoint.  Error code 538 could be used instead.
        Category:    "Provisioning Mismatch".

515 - Incorrect connection-id.

        Response valid for:    CreateConnection, ModifyConnection,
        DeleteConnection, NotificationRequest, AuditConnection.
        Situation:   An unknown connection-id has been specified.  It is
        possible that the connection has already been deleted.  It
        should be noted that a connection-id can also supplied with
        events and signals (e.g., "S: L/rt@connId").  Note that a
        mismatch between connection-id and call-id should use error code
        516.
        Category:    "State Mismatch".
516 - Unknown or incorrect call-id.
        Response valid for: ModifyConnection, DeleteConnection.
        Situation:   Unknown call-id, or the call-id supplied is
        incorrect (e.g., connection-id not associated with this call-
        id).
        Category:    "State Mismatch".

517 - Invalid or unsupported mode.

        Response valid for:    CreateConnection, ModifyConnection.
        Situation:   This is returned if the command specifies a
        connection mode that the endpoint does not support (note that
        not all endpoints will support all modes).  Note that if the
        unsupported mode is an extension connection mode, error code 518
        (unsupported package) should be used instead.
        Category:    "Provisioning Mismatch".

518 - Unsupported or unknown package.

        Response valid for:    Any command
        Situation:   A package name included in a request is not
        supported (or unknown).  Note that the package name may be a
        prefix to an event or other things (e.g., a parameter) as
        defined in [1].  Note that it is recommended to include a
        PackageList parameter with a list of supported packages in the
        response.
        Category:    "Provisioning Mismatch".

519 - Endpoint does not have a digit map.

        Response valid for: NotificationRequest.
        Situation:   Request was made to detect digits based on a digit
        map and the gateway does not have a digit map.
        Category:    "State Mismatch".  The Call Agent needs to send
        down a digit map in order to continue with the call.
520 - Endpoint is restarting.
        Situation: This is normally a transient error in which error
        code 405 SHOULD be used.  Gateways SHOULD not use this error
        code unless there is some relevant situation that warrants the
        category of "Service Failure".  Note that this was included in
        [1] only to maintain backwards compatibility with previous
        releases of the MGCP specification.
        Category:    If it is returned, this return code will be treated
        as category "Service Failure", i.e., as if this endpoint is
        out-of-service.

521 - Endpoint re-directed to another Call Agent.

        Response valid for: RestartInProgress.
        Situation:   A RestartInProgress command was sent to the Call
        Agent and the Call Agent returns this return code along with a
        NotifiedEntity parameter pointing to another Call Agent.  The
        gateway then sends a new RestartInProgress command to the Call
        Agent specified in the Notified Entity.
        Category:    none (specific situation and behavior).

522 - No such event or signal.

        Response valid for:    NotificationRequest.
        Situation:   This is returned if the requested event/signal name
        is not registered with this package.  If on the other hand the
        signal or event is part of the package but is not supported by
        the gateway, then return code 512 or 513 SHOULD be provided
        instead.  If the package is not supported, return code 518
        SHOULD be returned.
        Category:    "Provisioning Mismatch".

523 - Unknown action or illegal combination of actions.

        Response valid for: NotificationRequest with one or more
        requested events.
        Situation:   Request was made with a requested event(s) that
        included an action or actions defined in [1] that are either
        unknown, unsupported or an illegal combination as indicated in
        section 2.3.3 of [1].  Note that unsupported extension actions
        should generate error code 518 (unsupported package).
        Category:    "Provisioning Mismatch".

524 - Internal inconsistency in Local Connection Options

        Response valid for:    CreateConnection, ModifyConnection.
        Situation:   This is returned if one or more of the
        LocalConnectionOptions (LCO) parameters are coded with values
        that are not consistent with each other (e.g., other LCO
        parameters inconsistent with the network type).
        Category:    "Provisioning Mismatch".

525 - Unknown extension in Local Connection Options.

        Response valid for: CreateConnection, ModifyConnection.
        Situation: This is returned if the request contains a Local
        Connection Option with one or more unrecognized mandatory ("x+")
        extensions.  Note that unsupported package extensions should use
        error code 518 (unsupported package) instead.
        Category: "Provisioning Mismatch".
   526 - Insufficient bandwidth
        Response valid for:    CreateConnection, ModifyConnection.
        Situation:   In most cases where there is insufficient
        bandwidth, a 404 return code should be used.  526 would be used
        in cases where future requests are destined to fail.  An example
        might be a very restricted bandwidth case, where there is not
        enough bandwidth available for the codec requested even for a
        single endpoint.  Making a request with the same codec in the
        future will fail.  However, making a request for some other
        codec (with a higher degree of compression) may pass.  For
        cases, where the bandwidth is pooled over multiple endpoints and
        could free up at some future time (because an endpoint becomes
        inactive), then 404 is more appropriate.
        Category:    If it is returned, this return code will be treated
        as category "Provisioning Mismatch", e.g., the codec was
        incorrectly provisioned for the bandwidth available.

527 - Missing RemoteConnectionDescriptor.

        Response valid for: CreateConnection, ModifyConnection,
        NotificationRequest.
        Situation: This is returned if the connection has not yet
        received a RemoteConnectionDescriptor when one is required to
        support the request.  This can for example happen if a
        connection is attempted to be placed in "send/receive mode", or
        if a signal is applied on a connection.
        Category:    "Remote Connection Descriptor Error" in the case
        where the other end did not provide a connection descriptor.
        Alternatively, if this is an initial request made by a Call
        Agent (such there is no remote connection descriptor), then this
        is a "State Mismatch" problem.
   528 -Incompatible protocol version
        Response valid for:    Any command.
        Situation:   A command was received with a protocol version that
        was not supported.
        Category:    "Provisioning Mismatch".  This could also be
        treated as a "State Mismatch" problem if the there is a recovery
        mechanism (e.g., Call Agent recognizes the protocol version
        mismatch and switches to the correct protocol version)

529 - Internal Hardware Error.

        Response valid for:    Any command.
        Situation: A hardware fault occurred during the execution of a
        command such that repeating this command will result in a
        failure indication once again.  This is a slightly more specific
        error code than error 502, although more commentary should be
        provided (for debug purposes) if possible.
        Category: "Service Failure".

530 - CAS Signaling Protocol Error.

        Response valid for:    NotificationRequest.
        Situation:   This is specific to Channel Associated Signaling
        (CAS) interfaces.  A typical example might be an attempt to
        outpulse digits failed for some reason.
        Category:    none (specific situation and behavior).
531 - Failure of a grouping of trunks (e.g., facility failure)
        Response valid for:    CreateConnection, ModifyConnection,
        NotificationRequest.
        Situation:   Request made to an endpoint that has a failed trunk
        connection (e.g., T1 or E1 failed).  Note that an RSIP should
        have been sent as a result of the facility failure.  This is a
        more specific response than return code 501.
        Category:    "Service Failure".

532 - Unsupported value(s) in Local Connection Options.

        Response valid for: CreateConnection, ModifyConnection.
        Situation:   This is returned if one or more of the
        LocalConnectionOptions parameters are coded with a value that
        the gateway does not support.
        Category:    "Provisioning Mismatch".
   533 - Response too large
        Response valid for:    Any command.
        Situation: This would only be likely to occur in the case of an
        audit where the maximum response packet size would end up being
        too large.
        Category:    none (specific situation and behavior).
   534 - Codec negotiation failure
        Response valid for: CreateConnection, ModifyConnection.
        Situation:   The intersection between the list of codecs that
        the gateway supports, the codecs allowed by the local connection
        options and the codecs supplied in the Remote Connection
        Descriptor (if provided) is empty.
        Category:    "Provisioning Mismatch" if the error resulted from
        an empty approved list of codes as described in [1], Section
        2.6).  "Remote Connection Descriptor Error" if the error
        resulted from an empty negotiated list of codecs, as described
        in [1], Section 2.6.
   535 - Packetization period not supported
        Response valid for:    CreateConnection, ModifyConnection.
        Situation:   Normally this error should not be generated since
        if the gateway is unable to support the packetization period
        specified in the local connection options for the codec
        indicated, it should follow the behavior specified in [1] (which
        is to pick an appropriate value rather than failing the
        request).
        Category:    "none".
   536 - Unknown or unsupported Restart Method
        Response valid for: RestartInProgress.
        Situation:   This error is generated by the Call Agent if it
        receives a RestartInProgress command with an unsupported restart
        method.  Note that if the restart method is an extension restart
        method, error code 518 (unsupported package) should be used
        instead.
        Category:    "Provisioning Mismatch".
   537 - Unknown or unsupported digit map extension
        Response valid for: NotificationRequest.
        Situation:   Digit map letter in the digit map unknown or
        unsupported.  Note that this code does apply to extension digit
        map letters as well.
        Category:    "Provisioning Mismatch".
   538 - Event/signal parameter error
        Response valid for:    NotificationRequest.
        Situation:   It is returned if the event/signal parameter is in
        error or not supported.  If the event/signal or a package is not
        supported, then one of 512, 513, 518, or 522 should be used
        instead.
        Category:    "Provisioning Mismatch".
   539 - Invalid or unsupported command parameter
        Response valid for:    Any command.
        Situation: This is returned if the command contains an invalid
        or unsupported parameter, which is neither a package (which
        would use return code 518) nor vendor specific extension (which
        would use return code 511).  For example, if an endpoint does
        not support the BearerInformation parameter of the
        EndpointConfiguration command, this return code could be used.
        Of course, such an implementation would not conform to [1].
        Category:    "Provisioning Mismatch".
   540 - Per endpoint connection limit exceeded
        Response valid for: CreateConnection.
        Situation:   A CreateConnection command was made, but the
        gateway cannot support any additional connections on that
        endpoint.
        Category:    "State Mismatch".
   541 - Invalid or unsupported Local Connection Options
        Response valid for:    CreateConnection, ModifyConnection.
        Situation:   This is returned if the command contains an invalid
        or unsupported LocalConnectionOption, which is neither a package
        (which would use return code 518) nor vendor specific extension
        (which would use return code 511).
        Category: "Provisioning Mismatch".

2.3. Summary of Return Code Categories

A summary of the categories of the various error codes is included in the following table. This information is also repeated in the detailed error descriptions in the next section.
    ------------------------------------------------------------------
   |  Category   | Return Codes                                       |
   |-------------|----------------------------------------------------|
   |   normal    | 000, 100, 101, 200, 250                            |
   |-------------|----------------------------------------------------|
   |    none     | 405, 407, 410, 510, 521, 530, 533, 535             |
   |-------------|----------------------------------------------------|
   | "Service    | 501, 502, 520, 529, 531                            |
   |  Failure"   |                                                    |
   |-------------|----------------------------------------------------|
   |"Provisioning| 500, 503*, 504, 507, 508, 510, 511, 512, 513, 514, |
   | Mismatch"   | 517, 518, 522, 523, 524, 525, 526, 528*, 532, 534*,|
   |             | 536, 537, 538, 539, 541                            |
   |-------------|----------------------------------------------------|
   | "Temporary  | 400, 403, 404*, 405, 406, 409                      |
   |  Failure"   |                                                    |
   |-------------|----------------------------------------------------|
   | "State      | 401, 402, 515, 516, 519, 540                       |
   |  Mismatch"  |                                                    |
   |-------------|----------------------------------------------------|
   | "Remote     | 505, 506, 509, 527*                                |
   |  Connection |                                                    |
   |  Descriptor |                                                    |
   |  Error"     |                                                    |
    ------------------------------------------------------------------

Notes:

   *  404: may be treated as a "Temporary Failure", but specific
      behavior is possible (e.g., trying an alternate codec with lower
      bandwidth requirement rather than failing this call).
   *  503: rather than treating this as a "Provisioning Mismatch", it is
      possible for the Call Agent to recover from this error.
   *  527: See the detailed description for this error code in section
      2.2. This could be treated as a "State Mismatch" depending on the
      circumstances.
   *  528: See the detailed description for this error code in section
      2.2. This could be treated as a "State Mismatch" depending on the
      circumstances.
* 534: See the note on error code 534 in the detailed description section (2.2) of this document (may be treated as a "Remote Connection Descriptor Error" if no local connection options were supplied).

3. Additional Guidelines

This section provides additional guidelines to Gateway and Call Agent developers.

3.1. Gateway Recommendations

The following guidelines are recommended for gateway implementations:
   *  For uncategorized return codes (category "none") that involve
      specific situations, gateways should make sure they do an accurate
      mapping between the situation and the return code.
   *  Also for category "State Mismatch", it is equally important that
      the situation (and state) is accurately mapped to the specific
      error code.
   *  For situations similar to those involving return codes in "Service
      Failure", Provisioning Mismatch", "Temporary Failure" and "Remote
      Connection Descriptor Error" categories, the gateway should make
      sure that it uses a return code in the correct category.
   *  MGCP allows additional commentary to be included with the return
      code. It is important that the gateway includes more specific
      information concerning the situation for debug purposes.
   *  It is recommended that return codes 502, 520 and 526 not be used
      unless there is something that makes these permanent situations.
      As indicated in the detailed description of these return codes,
      403, 405 and 404 respectively are more appropriate in almost all
      situations. If a gateway presently uses 502, 520 and 526 for
      temporary situations and expects to upgrade to 403, 405 and 404,
      the gateway should refrain from using 502, 520 and 526 for some
      other use immediately after the upgrade. This is to avoid problems
      where a Call Agent is expected to treat the same error code in two
      different ways, e.g., 403 is a category "Temporary Failure" which
      requires a different Call Agent behavior from 502 which is in
      category "Service Failure".

3.2. Call Agent Recommendations

The following guidelines are recommended for gateway implementations:
   *  Call Agents should handle return codes they do not recognize (or
      do not expect) based on the first digit in the return code as
      outlined in [1].
   *  For categories "Service Failure", "Provisioning Mismatch",
      "Temporary Failure", and "Remote Connection Descriptor Error",
      Call Agents are expected to treat return codes that are within the
      same category in the same way (i.e., make the same decision, based
      on the return code and other state information available to them).
   *  Because there was little guidance given for return codes 502, 520
      and 526 in RFC 2705 [4], Call Agents may have to treat these as
      403, 405 and 404 respectively for gateways that have not been
      updated according to [1] and these recommendations. The gateway
      implementer should be consulted for information on the gateway
      behavior for (now and in the future) for these return codes (i.e.,
      it may be that return codes 502, 520 and 526 are presently used
      incorrectly but will be replaced with 403, 405 and 404 in the
      future).

4. Security Considerations

This document merely provides a convenient way to categorize MGCP return codes in order to facilitate decisions related to failure conditions; it does not impact MGCP security in any way.

5. Acknowledgements

Thanks also to Kevin Miller, Joe Stone, Flemming Andreasen, Bob Biskner for input contributions used in this document.

6. Normative References

   [1]  Andreasen, F. and B. Foster, "Media Gateway Control Protocol
        (MGCP) Version 1.0", RFC 3435, January 2003.
   [2]  Foster, B. and F. Andreasen, "Basic Media Gateway Control
        Protocol (MGCP) Packages", RFC 3660, December 2003.
   [3]  Handley, M. and V. Jacobson, "SDP: Session Description
        Protocol", RFC 2327, April 1998.
   [4]  Arango, M., Dugan, A., Elliott, I., Huitema, C. and S. Pickett,
        "Media Gateway Control Protocol (MGCP) Version 1.0", RFC 2705,
        October 1999.

7. Authors' Addresses

   C. Sivachelvan
   Cisco Systems
   2200 East President George Bush Turnpike
   Richardson, TX, 75082
   EMail: chelliah@cisco.com

B. Foster Cisco Systems

   EMail: bfoster@cisco.com

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