RFC 2885
Megaco Protocol version 0.8
7.3 Command Error Codes
Errors consist of an IANA registered error code and an explanatory string. Sending the explanatory string is optional. Implementations are encouraged to append diagnostic information to the end of the string. When a MG reports an error to a MGC, it does so in an error descriptor. An error descriptor consists of an error code and optionally the associated explanatory string. The identified error codes are: 400 - Bad Request 401 - Protocol Error 402 - Unauthorized 403 - Syntax Error in Transaction 404 - Syntax Error in TransactionReply 405 - Syntax Error in TransactionPending 406 - Version Not Supported 410 - Incorrect identifier 411 - The transaction refers to an unknown ContextId 412 - No ContextIDs available 421 - Unknown action or illegal combination of actions 422 - Syntax Error in Action 430 - Unknown TerminationID 431 - No TerminationID matched a wildcard 432 - Out of TerminationIDs or No TerminationID available 433 - TerminationID is already in a Context 440 - Unsupported or unknown Package 441 - Missing RemoteDescriptor 442 - Syntax Error in Command 443 - Unsupported or Unknown Command 444 - Unsupported or Unknown Descriptor 445 - Unsupported or Unknown Property 446 - Unsupported or Unknown Parameter 447 - Descriptor not legal in this command 448 - Descriptor appears twice in a command 450 - No such property in this package 451 - No such event in this package 452 - No such signal in this package 453 - No such statistic in this package 454 - No such parameter value in this package 455 - Parameter illegal in this Descriptor 456 - Parameter or Property appears twice in this Descriptor 461 - TransactionIDs in Reply do not match Request
462 - Commands in Transaction Reply do not match commands in
request
463 - TerminationID of Transaction Reply does not match
request
464 - Missing reply in Transaction Reply
465 - TransactionID in Transaction Pending does not match any
open request
466 - Illegal Duplicate Transaction Request
467 - Illegal Duplicate Transaction Reply
471 - Implied Add for Multiplex failure
500 - Internal Gateway Error
501 - Not Implemented
502 - Not ready.
503 - Service Unavailable
504 - Command Received from unauthorized entity
505 - Command Received before Restart Response
510 - Insufficient resources
512 - Media Gateway unequipped to detect requested Event
513 - Media Gateway unequipped to generate requested Signals
514 - Media Gateway cannot send the specified announcement
515 - Unsupported Media Type
517 - Unsupported or invalid mode
518 - Event buffer full
519 - Out of space to store digit map
520 - Media Gateway does not have a digit map
521 - Termination is "ServiceChangeing"
526 - Insufficient bandwidth
529 - Internal hardware failure
530 - Temporary Network failure
531 - Permanent Network failure
581 - Does Not Exist
8. TRANSACTIONS
Commands between the Media Gateway Controller and the Media Gateway
are grouped into Transactions, each of which is identified by a
TransactionID. Transactions consist of one or more Actions. An
Action consists of a series of Commands that are limited to operating
within a single Context. Consequently each Action typically
specifies a ContextID. However, there are two circumstances where a
specific ContextID is not provided with an Action. One is the case
of modification of a Termination outside of a Context. The other is
where the controller requests the gateway to create a new Context.
Following is a graphic representation of the Transaction, Action and
Command relationships.
+----------------------------------------------------------+
| Transaction x |
| +----------------------------------------------------+ |
| | Action 1 | |
| | +---------+ +---------+ +---------+ +---------+ | |
| | | Command | | Command | | Command | | Command | | |
| | | 1 | | 2 | | 3 | | 4 | | |
| | +---------+ +---------+ +---------+ +---------+ | |
| +----------------------------------------------------+ |
| |
| +----------------------------------------------------+ |
| | Action 2 | |
| | +---------+ | |
| | | Command | | |
| | | 1 | | |
| | +---------+ | |
| +----------------------------------------------------+ |
| |
| +----------------------------------------------------+ |
| | Action 3 | |
| | +---------+ +---------+ +---------+ | |
| | | Command | | Command | | Command | | |
| | | 1 | | 2 | | 3 | | |
| | +---------+ +---------+ +---------+ | |
| +----------------------------------------------------+ |
+----------------------------------------------------------+
Figure 5 Transactions, Actions and Commands
Transactions are presented as TransactionRequests. Corresponding
responses to a TransactionRequest are received in a single reply,
possibly preceded by a number of TransactionPending messages (see
section 8.2.3).
Transactions guarantee ordered Command processing. That is, Commands
within a Transaction are executed sequentially. Ordering of
Transactions is NOT guaranteed - transactions may be executed in any
order, or simultaneously.
At the first failing Command in a Transaction, processing of the
remaining Commands in that Transaction stops. If a command contains
a wildcarded TerminationID, the command is attempted with each of the
actual TerminationIDs matching the wildcard. A response within the
TransactionReply is included for each matching TerminationID, even if
one or more instances generated an error. If any TerminationID
matching a wildcard results in an error when executed, any commands
following the wildcarded command are not attempted. Commands may be
marked as "Optional" which can override this behaviour - if a
command marked as Optional results in an error, subsequent commands in the Transaction will be executed. A TransactionReply includes the results for all of the Commands in the corresponding TransactionRequest. The TransactionReply includes the return values for the Commands that were executed successfully, and the Command and error descriptor for any Command that failed. TransactionPending is used to periodically notify the receiver that a Transaction has not completed yet, but is actively being processed. Applications SHOULD implement an application level timer per transaction. Expiration of the timer should cause a retransmission of the request. Receipt of a Reply should cancel the timer. Receipt of Pending should restart the timer.8.1 Common Parameters
8.1.1 Transaction Identifiers
Transactions are identified by a TransactionID, which is assigned by sender and is unique within the scope of the sender.8.1.2 Context Identifiers
Contexts are identified by a ContextID, which is assigned by the Media Gateway and is unique within the scope of the Media Gateway. The Media Gateway Controller shall use the ContextID supplied by the Media Gateway in all subsequent Transactions relating to that Context. The protocol makes reference to a distinguished value that may be used by the Media Gateway Controller when referring to a Termination that is currently not associated with a Context, namely the null ContextID. The CHOOSE wildcard is used to request that the Media Gateway create a new Context. The MGC shall not use partially specified ContextIDs containing the CHOOSE wildcard. The MGC may use the ALL wildcard to address all Contexts on the MG.8.2 Transaction Application Programming Interface
Following is an Application Programming Interface (API) describing the Transactions of the protocol. This API is shown to illustrate the Transactions and their parameters and is not intended to specify implementation (e.g. via use of blocking function calls). It will describe the input parameters and return values expected to be used by the various Transactions of the protocol from a very high level. Transaction syntax and encodings are specified in later subsections.
8.2.1 TransactionRequest
The TransactionRequest is invoked by the sender. There is one Transaction per request invocation. A request contains one or more Actions, each of which specifies its target Context and one or more Commands per Context. TransactionRequest(TransactionId { ContextID {Command _ Command}, . . . ContextID {Command _ Command } }) The TransactionID parameter must specify a value for later correlation with the TransactionReply or TransactionPending response from the receiver. The ContextID parameter must specify a value to pertain to all Commands that follow up to either the next specification of a ContextID parameter or the end of the TransactionRequest, whichever comes first. The Command parameter represents one of the Commands mentioned in the "Command Details" subsection titled "Application Programming Interface".8.2.2 TransactionReply
The TransactionReply is invoked by the receiver. There is one reply invocation per transaction. A reply contains one or more Actions, each of which must specify its target Context and one or more Responses per Context. TransactionReply(TransactionID { ContextID { Response _ Response }, . . . ContextID { Response _ Response } }) The TransactionID parameter must be the same as that of the corresponding TransactionRequest. The ContextID parameter must specify a value to pertain to all Responses for the action. The ContextID may be specific or null. Each of the Response parameters represents a return value as mentioned in section 7.2, or an error descriptor if the command execution encountered an error. Commands after the point of failure are not processed and, therefore, Responses are not issued for them.
An exception to this occurs if a command has been marked as optional in the Transaction request. If the optional command generates an error, the transaction still continues to execute, so the Reply would, in this case, have Responses after an Error. If the receiver encounters an error in processing a ContextID, the requested Action response will consist of the context ID and a single error descriptor, 422 Syntax Error in Action. If the receiver encounters an error such that it cannot determine a legal Action, it will return a TransactionReply consisting of the TransactionID and a single error descriptor, 422 Syntax Error in Action. If the end of an action cannot be reliably determined but one or more Actions can be parsed, it will process them and then send 422 Syntax Error in Action as the last action for the transaction. If the receiver encounters an error such that is cannot determine a legal Transaction, it will return a TransactionReply with a null TransactionID and a single error descriptor (403 Syntax Error in Transaction). If the end of a transaction can not be reliably determined and one or more Actions can be parsed, it will process them and then return 403 Syntax Error in Transaction as the last action reply for the transaction. If no Actions can be parsed, it will return 403 Syntax Error in Transaction as the only reply If the terminationID cannot be reliably determined it will send 442 Syntax Error in Command as the action reply. If the end of a command cannot be reliably determined it will return 442 Syntax Error in Transaction as the reply to the last action it can parse.8.2.3 TransactionPending
The receiver invokes the TransactionPending. A TransactionPending indicates that the Transaction is actively being processed, but has not been completed. It is used to prevent the sender from assuming the TransactionRequest was lost where the Transaction will take some time to complete. TransactionPending(TransactionID { } ) The TransactionID parameter must be the same as that of the corresponding TransactionRequest. A property of root (normalMGExecutionTime) is settable by the MGC to indicate the interval within which the MGC expects a response to any transaction from the MG. Another property (normalMGCExecutionTime) is settable
by the MGC to indicate the interval within which the MG should expects a response to any transaction from the MGC. Senders may receive more than one TransactionPending for a command. If a duplicate request is received when pending, the responder may send a duplicate pending immediately, or continue waiting for its timer to trigger another Transaction Pending.8.3 Messages
Multiple Transactions can be concatenated into a Message. Messages have a header, which includes the identity of the sender. The Message Identifier (MID) of a message is set to a provisioned name (e.g. domain address/domain name/device name) of the entity transmitting the message. Domain name is a suggested default. Every Message contains a Version Number identifying the version of the protocol the message conforms to. Versions consist of one or two digits, beginning with version 1 for the present version of the protocol. The transactions in a message are treated independently. There is no order implied, there is no application or protocol acknowledgement of a message.9. TRANSPORT
The transport mechanism for the protocol should allow the reliable transport of transactions between an MGC and MG. The transport shall remain independent of what particular commands are being sent and shall be applicable to all application states. There are several transports defined for the protocol, which are defined in normative Annexes to this document. Additional Transports may be defined as additional annexes in subsequent editions of this document, or in separate documents. For transport of the protocol over IP, MGCs shall implement both TCP and UDP/ALF, an MG shall implement TCP or UDP/ALF or both. The MG is provisioned with a name or address (such as DNS name or IP address) of a primary and zero or more secondary MGCs (see section 7.2.8) that is the address the MG uses to send messages to the MGC. If TCP or UDP is used as the protocol transport and the port to which the initial ServiceChange request is to be sent is not otherwise known, that request should be sent to the default port number for the protocol. This port number is 2944 for text-encoded operation or 2945 for binary-encoded operation, for either UDP or TCP. The MGC receives the message containing the ServiceChange request from the MG and can determine the MG's address from it. As described in section 7.2.8, either the MG or the MGC may supply an address in the
ServiceChangeAddress parameter to which subsequent transaction requests must be addressed, but responses (including the response to the initial ServiceChange request) must always be sent back to the address which was the source of the corresponding request.9.1 Ordering of Commands
This document does not mandate that the underlying transport protocol guarantees the sequencing of transactions sent to an entity. This property tends to maximize the timeliness of actions, but it has a few drawbacks. For example: . Notify commands may be delayed and arrive at the MGC after the transmission of a new command changing the EventsDescriptor . If a new command is transmitted before a previous one is acknowledged, there is no guarantee that prior command will be executed before the new one. Media Gateway Controllers that want to guarantee consistent operation of the Media Gateway may use the following rules. These rules are with respect to commands that are in different transactions. Commands that are in the same transaction are executed in order (see section 8). 1. When a Media Gateway handles several Terminations, commands pertaining to the different Terminations may be sent in parallel, for example following a model where each Termination (or group of Terminations) is controlled by its own process or its own thread. 2. On a Termination, there should normally be at most one outstanding command (Add or Modify or Move), unless the outstanding commands are in the same transaction. However, a Subtract command may be issued at any time. In consequence, a Media Gateway may sometimes receive a Modify command that applies to a previously subtracted Termination. Such commands should be ignored, and an error code should be returned. 3. On a given Termination, there should normally be at most one outstanding Notify command at any time. 4. In some cases, an implicitly or explicitly wildcarded Subtract command that applies to a group of Terminations may step in front of a pending Add command. The Media Gateway Controller should individually delete all Terminations for which an Add command was pending at the time of the global Subtract command. Also, new Add
commands for Terminations named by the wild-carding (or implied in
a Multiplex descriptor) should not be sent until the wild-carded
Subtract command is acknowledged.
5. AuditValue and AuditCapability are not subject to any sequencing.
6. ServiceChange shall always be the first command sent by a MG as
defined by the restart procedure. Any other command or response
must be delivered after this ServiceChange command.
These rules do not affect the command responder, which should always
respond to commands.
9.2 Protection against Restart Avalanche
In the event that a large number of Media Gateways are powered on
simultaneously and they were to all initiate a ServiceChange
transaction, the Media Gateway Controller would very likely be
swamped, leading to message losses and network congestion during the
critical period of service restoration. In order to prevent such
avalanches, the following behavior is suggested:
1. When a Media Gateway is powered on, it should initiate a restart
timer to a random value, uniformly distributed between 0 and a
maximum waiting delay (MWD). Care should be taken to avoid
synchronicity of the random number generation between multiple
Media Gateways that would use the same algorithm.
2. The Media Gateway should then wait for either the end of this
timer or the detection of a local user activity, such as for
example an off-hook transition on a residential Media Gateway.
3. When the timer elapses, or when an activity is detected, the Media
Gateway should initiate the restart procedure.
The restart procedure simply requires the MG to guarantee that the
first message that the Media Gateway Controller sees from this MG is
a ServiceChange message informing the Media Gateway Controller about
the restart.
Note - The value of MWD is a configuration parameter that depends on
the type of the Media Gateway. The following reasoning may be used to
determine the value of this delay on residential gateways.
Media Gateway Controllers are typically dimensioned to handle the
peak hour traffic load, during which, in average, 10% of the lines
will be busy, placing calls whose average duration is typically 3
minutes. The processing of a call typically involves 5 to 6 Media
Gateway Controller transactions between each Media Gateway and the Media Gateway Controller. This simple calculation shows that the Media Gateway Controller is expected to handle 5 to 6 transactions for each Termination, every 30 minutes on average, or, to put it otherwise, about one transaction per Termination every 5 to 6 minutes on average. This suggests that a reasonable value of MWD for a residential gateway would be 10 to 12 minutes. In the absence of explicit configuration, residential gateways should adopt a value of 600 seconds for MWD. The same reasoning suggests that the value of MWD should be much shorter for trunking gateways or for business gateways, because they handle a large number of Terminations, and also because the usage rate of these Terminations is much higher than 10% during the peak busy hour, a typical value being 60%. These Terminations, during the peak hour, are this expected to contribute about one transaction per minute to the Media Gateway Controller load. A reasonable algorithm is to make the value of MWD per "trunk" Termination six times shorter than the MWD per residential gateway, and also inversely proportional to the number of Terminations that are being restarted. For example MWD should be set to 2.5 seconds for a gateway that handles a T1 line, or to 60 milliseconds for a gateway that handles a T3 line.10. SECURITY CONSIDERATIONS
This section covers security when using the protocol in an IP environment.10.1 Protection of Protocol Connections
A security mechanism is clearly needed to prevent unauthorized entities from using the protocol defined in this document for setting up unauthorized calls or interfering with authorized calls. The security mechanism for the protocol when transported over IP networks is IPsec [RFC2401 to RFC2411]. The AH header [RFC2402] affords data origin authentication, connectionless integrity and optional anti-replay protection of messages passed between the MG and the MGC. The ESP header [RFC2406] provides confidentiality of messages, if desired. For instance, the ESP encryption service should be requested if the session descriptions are used to carry session keys, as defined in SDP. Implementations of the protocol defined in this document employing the ESP header SHALL comply with section 5 of [RFC2406], which defines a minimum set of algorithms for integrity checking and
encryption. Similarly, implementations employing the AH header SHALL comply with section 5 of [RFC2402], which defines a minimum set of algorithms for integrity checking using manual keys. Implementations SHOULD use IKE [RFC2409] to permit more robust keying options. Implementations employing IKE SHOULD support authentication with RSA signatures and RSA public key encryption.10.2 Interim AH scheme
Implementation of IPsec requires that the AH or ESP header be inserted immediately after the IP header. This cannot be easily done at the application level. Therefore, this presents a deployment problem for the protocol defined in this document where the underlying network implementation does not support IPsec. As an interim solution, an optional AH header is defined within the H.248 protocol header. The header fields are exactly those of the SPI, SEQUENCE NUMBER and DATA fields as defined in [RFC2402]. The semantics of the header fields are the same as the "transport mode" of [RFC2402], except for the calculation of the Integrity Check value (ICV). In IPsec, the ICV is calculated over the entire IP packet including the IP header. This prevents spoofing of the IP addresses. To retain the same functionality, the ICV calculation should be performed across the entire transaction prepended by a synthesized IP header consisting of a 32 bit source IP address, a 32 bit destination address and an 16 bit UDP encoded as 10 hex digits. When the interim AH mechanism is employed when TCP is the transport Layer, the UDP Port above becomes the TCP port, and all other operations are the same. Implementations of the H.248 protocol SHALL implement IPsec where the underlying operating system and the transport network supports IPsec. Implementations of the protocol using IPv4 SHALL implement the interim AH scheme. However, this interim scheme SHALL NOT be used when the underlying network layer supports IPsec. IPv6 implementations are assumed to support IPsec and SHALL NOT use the interim AH scheme. All implementations of the interim AH mechanism SHALL comply with section 5 of [RFC2402] which defines a minimum set of algorithms for integrity checking using manual keys. The interim AH interim scheme does not provide protection against eavesdropping; thus forbidding third parties from monitoring the connections set up by a given termination. Also, it does not provide protection against replay attacks. These procedures do not necessarily protect against denial of service attacks by misbehaving
MGs or misbehaving MGCs. However, they will provide an identification of these misbehaving entities, which should then be deprived of their authorization through maintenance procedures.10.3 Protection of Media Connections
The protocol allows the MGC to provide MGs with "session keys" that can be used to encrypt the audio messages, protecting against eavesdropping. A specific problem of packet networks is "uncontrolled barge-in". This attack can be performed by directing media packets to the IP address and UDP port used by a connection. If no protection is implemented, the packets must be decompressed and the signals must be played on the "line side". A basic protection against this attack is to only accept packets from known sources, checking for example that the IP source address and UDP source port match the values announced in the Remote Descriptor. This has two inconveniences: it slows down connection establishment and it can be fooled by source spoofing: . To enable the address-based protection, the MGC must obtain the remote session description of the egress MG and pass it to the ingress MG. This requires at least one network roundtrip, and leaves us with a dilemma: either allow the call to proceed without waiting for the round trip to complete, and risk for example, "clipping" a remote announcement, or wait for the full roundtrip and settle for slower call-set-up procedures. . Source spoofing is only effective if the attacker can obtain valid pairs of source destination addresses and ports, for example by listening to a fraction of the traffic. To fight source spoofing, one could try to control all access points to the network. But this is in practice very hard to achieve. An alternative to checking the source address is to encrypt and authenticate the packets, using a secret key that is conveyed during the call set-up procedure. This will not slow down the call set-up, and provides strong protection against address spoofing.11. MG-MGC CONTROL INTERFACE
The control association between MG and MGC is initiated at MG cold start, and announced by a ServiceChange message, but can be changed by subsequent events, such as failures or manual service events. While the protocol does not have an explicit mechanism to support
multiple MGCs controlling a physical MG, it has been designed to support the multiple logical MG (within a single physical MG) that can be associated with different MGCs.11.1 Multiple Virtual MGs
A physical Media Gateway may be partitioned into one or more Virtual MGs. A virtual MG consists of a set of statically partitioned physical Terminations and/or sets of ephemeral Terminations. A physical Termination is controlled by one MGC. The model does not require that other resources be statically allocated, just Terminations. The mechanism for allocating Terminations to virtual MGs is a management method outside the scope of the protocol. Each of the virtual MGs appears to the MGC as a complete MG client. A physical MG may have only one network interface, which must be shared across virtual MGs. In such a case, the packet/cell side Termination is shared. It should be noted however, that in use, such interfaces require an ephemeral instance of the Termination to be created per flow, and thus sharing the Termination is straightforward. This mechanism does lead to a complication, namely that the MG must always know which of its controlling MGCs should be notified if an event occurs on the interface. In normal operation, the Virtual MG will be instructed by the MGC to create network flows (if it is the originating side), or to expect flow requests (if it is the terminating side), and no confusion will arise. However, if an unexpected event occurs, the Virtual MG must know what to do with respect to the physical resources it is controlling. If recovering from the event requires manipulation of a physical interface's state, only one MGC should do so. These issues are resolved by allowing any of the MGCs to create EventsDescriptors to be notified of such events, but only one MGC can have read/write access to the physical interface properties; all other MGCs have read-only access. The management mechanism is used to designate which MGC has read/write capability, and is designated the Master MGC. Each virtual MG has its own Root Termination. In most cases the values for the properties of the Root Termination are independently settable by each MGC. Where there can only be one value, the parameter is read-only to all but the Master MGC. ServiceChange may only be applied to a Termination or set of Terminations partitioned to the Virtual MG or created (in the case of ephemeral Terminations) by that Virtual MG.
11.2 Cold Start
A MG is pre-provisioned by a management mechanism outside the scope of this protocol with a Primary and (optionally) an ordered list of Secondary MGCs. Upon a cold start of the MG, it will issue a ServiceChange command with a "Restart" method, on the Root Termination to its primary MGC. If the MGC accepts the MG, it will send a Transaction Accept, with the ServiceChangeMgcId set to itself. If the MG receives an ServiceChangeMgcId not equal to the MGC it contacted, it sends a ServiceChange to the MGC specified in the ServiceChangeMgcId. It continues this process until it gets a controlling MGC to accept its registration, or it fails to get a reply. Upon failure to obtain a reply, either from the Primary MGC, or a designated successor, the MG tries its pre-provisioned Secondary MGCs, in order. If the MG is unable to comply and it has established a transport connection to the MGC, it should close that connection. In any event, it should reject all subsequent requests from the MGC with Error 406 Version Not Supported. It is possible that the reply to a ServiceChange with Restart will be lost, and a command will be received by the MG prior to the receipt of the ServiceChange response. The MG shall issue error 505 - Command Received before Restart Response.11.3 Negotiation of Protocol Version
The first ServiceChange command from an MG shall contain the version number of the protocol supported by the MG in the ServiceChangeVersion parameter. Upon receiving such a message, if the MGC supports only a lower version, then the MGC shall send a ServiceChangeReply with the lower version and thereafter all the messages between MG and MGC shall conform to the lower version of the protocol. If the MG is unable to comply and it has established a transport connection to the MGC, it should close that connection. In any event, it should reject all subsequent requests from the MGC with Error 406 Version Not supported. If the MGC supports a higher version than the MG but is able to support the lower version proposed by the MG, it shall send a ServiceChangeReply with the lower version and thereafter all the messages between MG and MGC shall conform to the lower version of the protocol. If the MGC is unable to comply, it shall reject the association, with Error 406 Version Not Supported. Protocol version negotiation may also occur at "handoff" and "failover" ServiceChanges.
When extending the protocol with new versions, the following rules
should be followed.
1. Existing protocol elements, i.e., procedures, parameters,
descriptor, property, values, should not be changed unless a
protocol error needs to be corrected or it becomes necessary to
change the operation of the service that is being supported by the
protocol.
2. The semantics of a command, a parameter, descriptor, property,
value should not be changed.
3. Established rules for formatting and encoding messages and
parameters should not be modified.
4. When information elements are found to be obsolete they can be
marked as not used. However, the identifier for that information
element will be marked as reserved. In that way it can not be used
in future versions.
11.4 Failure of an MG
If a MG fails, but is capable of sending a message to the MGC, it
sends a ServiceChange with an appropriate method (graceful or forced)
and specifies the Root TerminationID. When it returns to service, it
sends a ServiceChange with a "Restart" method.
Allowing the MGC to send duplicate messages to both MGs accommodates
pairs of MGs that are capable of redundant failover of one of the
MGs. Only the Working MG shall accept or reject transactions. Upon
failover, the Primary MG sends a ServiceChange command with a
"Failover" method and a "MG Impending Failure" reason. The MGC then
uses the primary MG as the active MG. When the error condition is
repaired, the Working MG can send a "ServiceChange" with a "Restart"
method.
11.5 Failure of an MGC
If the MG detects a failure of its controlling MGC, it attempts to
contact the next MGC on its pre-provisioned list. It starts its
attempts at the beginning (Primary MGC), unless that was the MGC that
failed, in which case it starts at its first Secondary MGC. It sends
a ServiceChange message with a "Failover" method and a " MGC
Impending Failure" reason.
In partial failure, or manual maintenance reasons, an MGC may wish to
direct its controlled MGs to use a different MGC. To do so, it sends
a ServiceChange method to the MG with a "HandOff" method, and its
designated replacement in ServiceChangeMgcId. The MG should send a ServiceChange message with a "Handoff" method and a "MGC directed change" reason to the designated MGC. If it fails to get a reply, or fails to see an Audit command subsequently, it should behave as if its MGC failed, and start contacting secondary MGCs. If the MG is unable to establish a control relationship with any MGC, it shall wait a random amount of time as described in section 9.2 and then start contacting its primary, and if necessary, its secondary MGCs again. No recommendation is made on how the MGCs involved in the Handoff maintain state information; this is considered to be out of scope of this recommendation. The MGC and MG may take the following steps when Handoff occurs. When the MGC initiates a HandOff, the handover should be transparent to Operations on the Media Gateway. Transactions can be executed in any order, and could be in progress when the ServiceChange is executed. Accordingly, commands in progress continue, transaction replies are sent to the new MGC (after a new control association is established), and the MG should expect outstanding transaction replies from the new MGC. No new messages shall be sent to the new MGC until the control association is established. Repeated transaction requests shall be directed to the new MGC. The MG shall maintain state on all terminations and contexts. It is possible that the MGC could be implemented in such a way that a failed MGC is replaced by a working MGC where the identity of the new MGC is the same as the failed one. In such a case, ServiceChangeMgcId would be specified with the previous value and the MG shall behave as if the value was changed, and send a ServiceChange message, as above. Pairs of MGCs that are capable of redundant failover can notify the controlled MGs of the failover by the above mechanism.12. PACKAGE DEFINITION
The primary mechanism for extension is by means of Packages. Packages define additional Properties, Events, Signals and Statistics that may occur on Terminations. Packages defined by IETF will appear in separate RFCs. Packages defined by ITU-T may appear in the relevant recommendations (e.g. as annexes).
1. A public document or a standard forum document, which can be
referenced as the document that describes the package following
the guideline above, should be specified.
2. The document shall specify the version of the Package that it
describes.
3. The document should be available on a public web server and should
have a stable URL. The site should provide a mechanism to provide
comments and appropriate responses should be returned.
12.1 Guidelines for defining packages
Packages define Properties, Events, Signals, and Statistics.
Packages may also define new error codes according to the guidelines
given in section 13.2. This is a matter of documentary convenience:
the package documentation is submitted to IANA in support of the
error code registration. If a package is modified, it is unnecessary
to provide IANA with a new document reference in support of the error
code unless the description of the error code itself is modified.
Names of all such defined constructs shall consist of the PackageID
(which uniquely identifies the package) and the ID of the item (which
uniquely identifies the item in that package). In the text encoding
the two shall be separated by a forward slash ("/") character.
Example: togen/playtone is the text encoding to refer to the play
tone signal in the tone generation package.
A Package will contain the following sections:
12.1.1 Package
Overall description of the package, specifying:
. Package Name: only descriptive,
. PackageID: Is an identifier
. Description:
. Version: A new version of a package can only add additional
Properties, Events, Signals, Statistics and new possible values
for an existing parameter described in the original package. No
deletions or modifications shall be allowed. A version is an
integer in the range from 1 to 99.
. Extends (Optional): A package may extend an existing package. The
version of the original package must be specified. When a package
extends another package it shall only add additional Properties,
Events, Signals, Statistics and new possible values for an
existing parameter described in the original package. An extended
package shall not redefine or overload a name defined in the
original package. Hence, if package B version 1 extends package A
version 1, version 2 of B will not be able to extend the A version
2 if A version 2 defines a name already in B version 1.
12.1.2 Properties
Properties defined by the package, specifying:
. Property Name: only descriptive.
. PropertyID: Is an identifier
. Description:
. Type: One of:
String: UTF-8 string
Integer: 4 byte signed integer
Double: 8 byte signed integer
Character: Unicode UTF-8 encoding of a single letter.
Could be more than one octet.
Enumeration: One of a list of possible unique values (See 12.3)
Sub-list: A list of several values from a list
Boolean
. Possible Values:
. Defined in: Which H.248 descriptor the property is defined in.
LocalControl is for stream dependent properties. TerminationState
is for stream independent properties.
. Characteristics: Read / Write or both, and (optionally), global:
Indicates whether a property is read-only, or read-write, and if
it is global. If Global is omitted, the property is not global.
If a property is declared as global, the value of the property is
shared by all terminations realizing the package.
12.1.3 Events
Events defined by the package, specifying:
. Event name: only descriptive.
. EventID: Is an identifier
. Description:
. EventsDescriptor Parameters: Parameters used by the MGC to
configure the event, and found in the EventsDescriptor. See
section 12.2.
. ObservedEventsDescriptor Parameters: Parameters returned to the
MGC in Notify requests and in replies to command requests from
the MGC that audit ObservedEventsDescriptor, and found in the
ObservedEventsDescriptor. See section 12.2.
12.1.4 Signals
. Signals defined by the package, specifying:
. Signal Name: only descriptive.
. SignalID: Is an identifier. SignalID is used in a
SignalsDescriptor
. Description
. SignalType: One of:
- OO (On/Off)
- TO (TimeOut)
- BR (Brief)
Note - SignalType may be defined such that it is dependent on the
value of one or more parameters. Signals that would be played with
SignalType BR should have a default duration. The package has to
define the default duration and signalType.
. Duration: in hundredths of seconds
. Additional Parameters: See section 12.2
12.1.5 Statistics
Statistics defined by the package, specifying:
. Statistic name: only descriptive.
. StatisticID: Is an identifier. StatisticID is used in a
StatisticsDescriptor.
. Description
. Units: unit of measure, e.g. milliseconds, packets.
12.1.6 Procedures
Additional guidance on the use of the package.
12.2 Guidelines to defining Properties, Statistics and Parameters to
Events and Signals.
. Parameter Name: only descriptive
. ParameterID: Is an identifier
. Type: One of:
String: UTF-8 octet string
Integer: 4 octet signed integer
Double: 8 octet signed integer
Character: Unicode UTF-8 encoding of a single letter. Could be
more than one octet.
Enumeration: One of a list of possible unique values (See 12.3)
Sub-list: A list of several values from a list
Boolean
. Possible values:
. Description:
12.3 Lists
Possible values for parameters include enumerations. Enumerations
may be defined in a list. It is recommended that the list be IANA
registered so that packages that extend the list can be defined
without concern for conflicting names.
12.4 Identifiers
Identifiers in text encoding shall be strings of up to 64 characters,
containing no spaces, starting with an alphanumeric character and
consisting of alphanumeric characters and / or digits, and possibly
including the special character underscore ("_").
Identifiers in binary encoding are 2 octets long.
Both text and binary values shall be specified for each identifier,
including identifiers used as values in enumerated types.
12.5 Package Registration
A package can be registered with IANA for interoperability reasons.
See section 13 for IANA considerations.
13. IANA CONSIDERATIONS
13.1 Packages
The following considerations SHALL be met to register a package with
IANA:
1. A unique string name, unique serial number and version number is
registered for each package. The string name is used with text
encoding. The serial number shall be used with binary encoding.
Serial Numbers 60000-64565 are reserved for private use. Serial
number 0 is reserved.
2. A contact name, email and postal addresses for that contact shall
be specified. The contact information shall be updated by the
defining organization as necessary.
3. A reference to a document that describes the package, which should
be public:
The document shall specify the version of the Package that it
describes.
If the document is public, it should be located on a public web
server and should have a stable URL. The site should provide a
mechanism to provide comments and appropriate responses should be
returned.
4. Packages registered by other than recognized standards bodies
shall have a minimum package name length of 8 characters.
5. All other package names are first come-first served if all other
conditions are met
13.2 Error Codes
The following considerations SHALL be met to register an error code
with IANA:
1. An error number and a one line (80 character maximum) string is
registered for each error.
2. A complete description of the conditions under which the error is
detected shall be included in a publicly available document. The
description shall be sufficiently clear to differentiate the error
from all other existing error codes.
3. The document should be available on a public web server and should
have a stable URL.
4. Error numbers registered by recognized standards bodies shall have
3 or 4 character error numbers.
5. Error numbers registered by all other organizations or individuals
shall have 4 character error numbers.
6. An error number shall not be redefined, nor modified except by the
organization or individual that originally defined it, or their
successors or assigns.
13.3 ServiceChange Reasons
The following considerations SHALL be met to register service change reason with IANA: 1. A one phrase, 80-character maximum, unique reason code is registered for each reason. 2. A complete description of the conditions under which the reason is used is detected shall be included in a publicly available document. The description shall be sufficiently clear to differentiate the reason from all other existing reasons. 3. The document should be available on a public web server and should have a stable URL.
(next page on part 4)
