overridden by provisioning,
. otherwise, the provisioned value.
7.2.4 Move
The Move Command moves a Termination to another Context from its
current Context in one atomic operation. The Move command is the
only command that refers to a Termination in a Context different from
that to which the command is applied. The Move command shall not be
used to move Terminations to or from the null Context.
TerminationID
[,MediaDescriptor]
[,ModemDescriptor]
[,MuxDescriptor]
[,EventsDescriptor]
[,SignalsDescriptor]
[,DigitMapDescriptor]
[,ObservedEventsDescriptor]
[,EventBufferDescriptor]
[,StatisticsDescriptor]
[,PackagesDescriptor]
Move( TerminationID
[, MediaDescriptor]
[, ModemDescriptor]
[, MuxDescriptor]
[, EventsDescriptor]
[, SignalsDescriptor]
[, DigitMapDescriptor]
[, AuditDescriptor]
)
The TerminationID specifies the Termination to be moved. It may be
wildcarded. If the wildcard matches more than one TerminationID, all
possible matches are attempted, with results reported for each one.
The order of attempts when multiple TerminationIDs match is not
specified. By convention, the Termination is subtracted from its
previous Context. The Context to which the Termination is moved is
indicated by the target ContextId in the Action. If the last
remaining Termination is moved out of a Context, the Context is
deleted.
The remaining descriptors are processed as in the Modify Command.
The AuditDescriptor with the Statistics option, for example, would
return statistics on the Termination just prior to the Move.
Possible descriptors returned from Move are the same as for Add.
Move SHALL NOT be used on a Termination with a serviceState of
"OutofService".
7.2.5 AuditValue
The AuditValue Command returns the current values of properties,
events, signals and statistics associated with Terminations.
TerminationID
[,MediaDescriptor]
[,ModemDescriptor]
[,MuxDescriptor]
[,EventsDescriptor]
[,SignalsDescriptor]
[,DigitMapDescriptor]
[,ObservedEventsDescriptor]
[,EventBufferDescriptor]
[,StatisticsDescriptor]
[,PackagesDescriptor]
AuditValue(TerminationID,
AuditDescriptor
)
TerminationID may be specific or wildcarded. If the wildcard matches
more than one TerminationID, all possible matches are attempted, with
results reported for each one. The order of attempts when multiple
TerminationIDs match is not specified. If a wildcarded response is
requested, only one command return is generated, with the contents
containing the union of the values of all Terminations matching the
wildcard. This convention may reduce the volume of data required to
audit a group of Terminations. Use of CHOOSE is an error.
The appropriate descriptors, with the current values for the
Termination, are returned from AuditValue. Values appearing in
multiple instances of a descriptor are defined to be alternate values
supported, with each parameter in a descriptor considered
independent.
ObservedEvents returns a list of events in the EventBuffer,
PackagesDescriptor returns a list of packages realized by the
Termination. DigitMapDescriptor returns the name or value of the
current DigitMap for the Termination. DigitMap requested in an
AuditValue command with TerminationID ALL returns all DigitMaps in
the gateway. Statistics returns the current values of all statistics
being kept on the Termination. Specifying an empty Audit Descriptor
results in only the TerminationID being returned. This may be useful
to get a list of TerminationIDs when used with wildcard.
AuditValue results depend on the Context, viz. specific, null, or
wildcarded. The TerminationID may be specific, or wildcarded. The
following illustrates other information that can be obtained with the
Audit Command:
ContextID TerminationID Information Obtained
Specific wildcard Audit of matching
Terminations in a Context
Specific specific Audit of a single
Termination in a Context
Null Root Audit of Media Gateway state
and events
Null wildcard Audit of all matching
Terminations in the Null
Context
Null specific Audit of a single
Termination outside of any
Context
All wildcard Audit of all matching
Terminations and the Context
to which they are associated
All Root List of all ContextIds
7.2.6 AuditCapabilities
The AuditCapabilities Command returns the possible values of
properties, events, signals and statistics associated with
Terminations.
TerminationID
[,MediaDescriptor]
[,ModemDescriptor]
[,MuxDescriptor]
[,EventsDescriptor]
[,SignalsDescriptor]
[,ObservedEventsDescriptor]
[,EventBufferDescriptor]
[,StatisticsDescriptor]
AuditCapabilities(TerminationID,
AuditDescriptor
)
The appropriate descriptors, with the possible values for the
Termination are returned from AuditCapabilities. Descriptors may be
repeated where there are multiple possible values. If a wildcarded
response is requested, only one command return is generated, with the
contents containing the union of the values of all Terminations
matching the wildcard. This convention may reduce the volume of data
required to audit a group of Terminations.
Interpretation of what capabilities are requested for various values
of ContextID and TerminationID is the same as in AuditValue.
The EventsDescriptor returns the list of possible events on the
Termination together with the list of all possible values for the
EventsDescriptor Parameters. The SignalsDescriptor returns the list
of possible signals that could be applied to the Termination together
with the list of all possible values for the Signals Parameters.
StatisticsDescriptor returns the names of the statistics being kept
on the termination. ObservedEventsDescriptor returns the names of
active events on the termination. DigitMap and Packages are not
legal in AuditCapability.
7.2.7 Notify
The Notify Command allows the Media Gateway to notify the Media
Gateway Controller of events occurring within the Media Gateway.
Notify(TerminationID,
ObservedEventsDescriptor,
[ErrorDescriptor]
)
The TerminationID parameter specifies the Termination issuing the
Notify Command. The TerminationID shall be a fully qualified name.
The ObservedEventsDescriptor contains the RequestID and a list of
events that the Media Gateway detected in the order that they were
detected. Each event in the list is accompanied by parameters
associated with the event and an indication of the time that the
event was detected. Procedures for sending Notify commands with
RequestID equal to 0 are for further study.
Notify Commands with RequestID not equal to 0 shall occur only as the
result of detection of an event specified by an Events Descriptor
which is active on the termination concerned.
The RequestID returns the RequestID parameter of the EventsDescriptor
that triggered the Notify Command. It is used to correlate the
notification with the request that triggered it. The events in the
list must have been requested via the triggering EventsDescriptor or
embedded events descriptor unless the RequestID is 0 (which is for
further study).
7.2.8 ServiceChange
The ServiceChange Command allows the Media Gateway to notify the
Media Gateway Controller that a Termination or group of Terminations
is about to be taken out of service or has just been returned to
service. The Media Gateway Controller may indicate that
Termination(s) shall be taken out of or returned to service. The
Media Gateway may notify the MGC that the capability of a Termination
has changed. It also allows a MGC to hand over control of a MG to
another MGC.
TerminationID,
[ServiceChangeDescriptor]
ServiceChange(TerminationID,
ServiceChangeDescriptor
)
The TerminationID parameter specifies the Termination(s) that are
taken out of or returned to service. Wildcarding of Termination
names is permitted, with the exception that the CHOOSE mechanism
shall not be used. Use of the "Root" TerminationID indicates a
ServiceChange affecting the entire Media Gateway.
The ServiceChangeDescriptor contains the following parameters as
required:
. ServiceChangeMethod
. ServiceChangeReason
. ServiceChangeDelay
. ServiceChangeAddress
. ServiceChangeProfile
. ServiceChangeVersion
. ServiceChangeMgcId
. TimeStamp
The ServiceChangeMethod parameter specifies the type of ServiceChange
that will or has occurred:
1) Graceful - indicates that the specified Terminations will be taken
out of service after the specified ServiceChangeDelay; established
connections are not yet affected, but the Media Gateway Controller
should refrain from establishing new connections and should
attempt to gracefully tear down existing connections. The MG
should set termination serviceState at the expiry of
ServiceChangeDelay or the removal of the termination from an
active context (whichever is first), to "out of service".
2) Forced - indicates that the specified Terminations were taken
abruptly out of service and any established connections associated
with them were lost. The MGC is responsible for cleaning up the
context (if any) with which the failed termination is associated.
At a minimum the termination shall be subtracted from the context.
The termination serviceState should be "out of service".
3) Restart - indicates that service will be restored on the specified
Terminations after expiration of the ServiceChangeDelay. The
serviceState should be set to "inService" upon expiry of
ServiceChangeDelay.
4) Disconnected - always applied with the Root TerminationID,
indicates that the MG lost communication with the MGC, but it was
subsequently restored. Since MG state may have changed, the MGC
may wish to use the Audit command to resynchronize its state with
the MG's.
5) Handoff - sent from the MGC to the MG, this reason indicates that
the MGC is going out of service and a new MGC association must be
established. Sent from the MG to the MGC, this indicates that the
MG is attempting to establish a new association in accordance with
a Handoff received from the MGC with which it was previously
associated.
6) Failover - sent from MG to MGC to indicate the primary MG is out
of service and a secondary MG is taking over.
7) Another value whose meaning is mutually understood between the MG
and the MGC.
The ServiceChangeReason parameter specifies the reason why the
ServiceChange has or will occur. It consists of an alphanumeric
token (IANA registered) and an explanatory string.
The optional ServiceChangeAddress parameter specifies the address
(e.g., IP port number for IP networks) to be used for subsequent
communications. It can be specified in the input parameter
descriptor or the returned result descriptor. ServiceChangeAddress
and ServiceChangeMgcId parameters must not both be present in the
ServiceChangeDescriptor or the ServiceChangeResultDescriptor. The
serviceChangeAddress provides an address to be used within the
context of the association currently being negotiated, while the
ServiceChangeMgcId provides an alternate address where the MG should
seek to establish another association.
The optional ServiceChangeDelay parameter is expressed in seconds.
If the delay is absent or set to zero, the delay value should be
considered to be null. In the case of a "graceful"
ServiceChangeMethod, a null delay indicates that the Media Gateway
Controller should wait for the natural removal of existing
connections and should not establish new connections. . For
"graceful" only, a null delay means the MG must not set serviceState
"out of service" until the termination is in the null context.
The optional ServiceChangeProfile parameter specifies the Profile (if
any) of the protocol supported. The ServiceChangeProfile includes
the version of the profile supported.
The optional ServiceChangeVersion parameter contains the protocol
version and is used if protocol version negotiation occurs (see
section 11.3).
The optional TimeStamp parameter specifies the actual time as kept by
the sender. It can be used by the responder to determine how its
notion of time differs from that of its correspondent. TimeStamp is
sent with a precision of hundredths of a second, and is expressed in
UTC.
The optional Extension parameter may contain any value whose meaning
is mutually understood by the MG and MGC.
A ServiceChange Command specifying the "Root" for the TerminationID
and ServiceChangeMethod equal to Restart is a registration command by
which a Media Gateway announces its existence to the Media Gateway
Controller. The Media Gateway is expected to be provisioned with the
name of one primary and optionally some number of alternate Media
Gateway Controllers. Acknowledgement of the ServiceChange Command
completes the registration process. The MG may specify the transport
ServiceChangeAddress to be used by the MGC for sending messages in
the ServiceChangeAddress parameter in the input
ServiceChangeDescriptor. The MG may specify an address in the
ServiceChangeAddress parameter of the ServiceChange request, and the
MGC may also do so in the ServiceChange reply. In either case, the
recipient must use the supplied address as the destination for all
subsequent transaction requests within the association. At the same
time, as indicated in section 9, transaction replies and pending
indications must be sent to the address from which the corresponding
requests originated. This must be done even if it implies extra
messaging because commands and responses cannot be packed together.
The TimeStamp parameter shall be sent with a registration command and
its response.
The Media Gateway Controller may return an ServiceChangeMgcId
parameter that describes the Media Gateway Controller that should
preferably be contacted for further service by the Media Gateway. In
this case the Media Gateway shall reissue the ServiceChange command
to the new Media Gateway Controller. The Gateway specified in an
ServiceChangeMgcId, if provided, shall be contacted before any
further alternate MGCs. On a HandOff message from MGC to MG, the
ServiceChangeMgcId is the new MGC that will take over from the
current MGC.
The return from ServiceChange is empty except when the Root
terminationID is used. In that case it includes the following
parameters as required:
. ServiceChangeAddress, if the responding MGC wishes to specify an
new destination for messages from the MG for the remainder of the
association;
. ServiceChangeMgcId, if the responding MGC does not wish to
sustain an association with the MG;
. ServiceChangeProfile, if the responder wishes to negotiate the
profile to be used for the association;
. ServiceChangeVersion, if the responder wishes to negotiate the
version of the protocol to be used for the association.
The following ServiceChangeReasons are defined. This list may be
extended by an IANA registration as outlined in section 13.3
900 Service Restored
901 Cold Boot
902 Warm Boot
903 MGC Directed Change
904 Termination malfunctioning
905 Termination taken out of service
906 Loss of lower layer connectivity (e.g. downstream sync)
907 Transmission Failure
908 MG Impending Failure
909 MGC Impending Failure
910 Media Capability Failure
911 Modem Capability Failure
912 Mux Capability Failure
913 Signal Capability Failure
914 Event Capability Failure
915 State Loss
7.2.9 Manipulating and Auditing Context Attributes
The commands of the protocol as discussed in the preceding sections
apply to terminations. This section specifies how contexts are
manipulated and audited.
Commands are grouped into actions (see section 8). An action applies
to one context. In addition to commands, an action may contain
context manipulation and auditing instructions.
An action request sent to a MG may include a request to audit
attributes of a context. An action may also include a request to
change the attributes of a context.
The context properties that may be included in an action reply are
used to return information to a MGC. This can be information
requested by an audit of context attributes or details of the effect
of manipulation of a context.
If a MG receives an action which contains both a request to audit
context attributes and a request to manipulate those attributes, the
response SHALL include the values of the attributes after processing
the manipulation request.
7.2.10 Generic Command Syntax
The protocol can be encoded in a binary format or in a text format.
MGCs should support both encoding formats. MGs may support both
formats.
The protocol syntax for the binary format of the protocol is defined
in Annex A. Annex C specifies the encoding of the Local and Remote
descriptors for use with the binary format.
A complete ABNF of the text encoding of the protocol per RFC2234 is
given in Annex B. SDP is used as the encoding of the Local and
Remote Descriptors for use with the text encoding as modified in
section 7.1.8.
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.