RFC 3525
Gateway Control Protocol Version 1
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. EventBufferDescriptor returns the same information as EventsDescriptor. 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.
The following illustrates other information that can be obtained with
the AuditCapabilties 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 MG 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 Same as for AuditValue
All Specific Same as for AuditValue
7.2.7 Notify
The Notify Command allows the Media Gateway to notify the Media
Gateway Controller of events occurring within the Media Gateway.
TerminationID
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 optionally 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). The ErrorDescriptor may be sent in the Notify Command as a result of Error 518 - Event buffer full.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 on the
Termination(s) affected by the serviceChange command. 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 may be lost. For non-Root terminations, 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". For the root
termination, the MGC can assume that all connections are lost on
the MG and thus can consider that all the terminations have been
subtracted.
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 to the same MGC (possibly after trying other
MGCs on a pre-provisioned list). 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. This serviceChange
method is also sent from the MG to the MGC when the MG detects
that MGC has failed.
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, optionally, 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. Note that the use of
ServiceChangeAddress is not encouraged. MGCs and MGs must be able to
cope with the ServiceChangeAddress being either a full address or
just a port number in the case of TCP transports.
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
11.3).
The optional TimeStamp parameter specifies the actual time as kept by
the sender. As such, it is not necessarily absolute time according
to, for example, a local time zone - it merely establishes an
arbitrary starting time against which all future timestamps
transmitted by a sender during this association shall be compared.
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.
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 may also announce a registration
command by specifying the "Root" for the TerminationID and
ServiceChangeMethod equal to Failover when the MG detects MGC
failures. 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, except when the MGC has returned
an alternative ServiceChangeMgcId as described in the following
paragraph. 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 clause 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 a 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 MGC specified in a
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 a
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 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 subclauses
apply to Terminations. This subclause specifies how Contexts are
manipulated and audited.
Commands are grouped into actions (see clause 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 RFC 2234 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 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. H.248.8 contains the error codes supported by Recommendations in the H.248 sub-series.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 non-empty series of Commands, Context property modifications, or Context property audits 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. Figure 8 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 8: 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 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. If a command fails, the MG shall as far as possible restore the state that existed prior to the attempted execution of the command before continuing with command processing. 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. A response containing an error descriptor to indicate that the TransactionID is missing in a request shall use TransactionID 0 in the corresponding TransactionReply.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 may use the ALL wildcard to address all Contexts on the MG. The null Context is not included when the ALL wildcard is used.
The MGC shall not use partially specified ContextIDs containing the CHOOSE or ALL wildcards.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 subclauses.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 7.2 (Command 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. The TransactionReply is invoked by the responder when it has processed the TransactionRequest.
A TransactionRequest has been processed:
- when all actions in that TransactionRequest have been processed;
or
- when an error is encountered in processing that
TransactionRequest, except when the error is in an optional
command.
A command has been processed when all descriptors in that command
have been processed.
A SignalsDescriptor is considered to have been processed when it has
been established that the descriptor is syntactically valid, the
requested signals are supported and they have been queued to be
applied.
An EventsDescriptor or EventBufferDescriptor is considered to have
been processed when it has been established that the descriptor is
syntactically valid, the requested events can be observed, any
embedded signals can be generated, any embedded events can be
detected, and the MG has been brought into a state in which the
events will be detected.
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, all or
null.
Each of the Response parameters represents a return value as
mentioned in 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.
Section 7.1.19 Error Descriptor specifies the generation of error
descriptors. The text below discusses several individual cases.
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 commands 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 TransactionRequest). If the end of a transaction cannot 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 TransactionRequest 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 Command 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 expect 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 TransactionPending.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. An H.248.1 entity (MG/MGC) must consistently use the same MID in all messages it originates for the duration of control association with the peer (MGC/MG). 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. A message is essentially a transport mechanism. For example, message X containing transaction requests A, B, and C may be responded to with message Y containing replies to A and C and message Z containing the reply to B. Likewise, message L containing request D and message M containing request E may be responded to with message N containing replies to both D and E.9 Transport
The transport mechanism for the protocol should allow the reliable transport of transactions between a 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 Annexes to this RFC and other Recommendations of the H.248 sub-series. Additional Transports may be defined as additional Recommendations of the H.248 sub-series. For transport of the protocol over IP, MGCs shall implement both TCP and UDP/ALF, a 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 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 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. For example, in IP networks, this is the source address in the IP header and the source port number in the TCP/UDP/SCTP header.9.1 Ordering of Commands
This RFC 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 clause 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) For transports that do not guarantee in-sequence delivery of
messages (i.e., UDP), there should normally be on a given
Termination 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 wildcarding (or implied in
a Multiplex descriptor) should not be sent until the wildcarded
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 behaviour 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 clause 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 RFC for setting up
unauthorized calls or interfering with authorized calls. The
security mechanism for the protocol when transported over IP networks
is IPsec [RFC 2401 to RFC 2411].
The AH header [RFC 2402] affords data origin authentication,
connectionless integrity and optional anti-replay protection of
messages passed between the MG and the MGC. The ESP header [RFC
2406] 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 RFC employing the ESP header SHALL comply with section 5 of [RFC 2406], which defines a minimum set of algorithms for integrity checking and encryption. Similarly, implementations employing the AH header SHALL comply with section 5 of [RFC 2402], which defines a minimum set of algorithms for integrity checking using manual keys. Implementations SHOULD use IKE [RFC 2409] 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 RFC where the underlying network implementation does not support IPsec. As an interim solution, an optional AH header is defined within the H.248.1 protocol header. The header fields are exactly those of the SPI, SEQUENCE NUMBER and DATA fields as defined in [RFC 2402]. The semantics of the header fields are the same as the "transport mode" of [RFC 2402], 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 all the transactions (concatenated) in the message prepended by a synthesized IP header consisting of a 32-bit source IP address, a 32-bit destination address and a 16-bit UDP destination port encoded as 20 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.248.1 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 RFC 2402 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 round trip, 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 round trip 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 sends a Transaction Reply not including a ServiceChangeMgcId parameter. If the MGC does not accept the MG's registration, it sends a Transaction Reply, providing the address of an alternate MGC to be contacted by including a ServiceChangeMgcId parameter. If the MG receives a Transaction Reply that includes a ServiceChangeMgcId parameter, 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 establish a control relationship with any MGC, it shall wait a random amount of time as described in 9.2 and then start contacting its primary, and if necessary, its secondary MGCs again. 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 a ServiceChange Reply has been received.11.3 Negotiation of protocol version
The first ServiceChange command from a 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, a descriptor, a property,
or a 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 a 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 secondary MG as the active MG. When the error condition is
repaired, the Working MG can send a "ServiceChange" with a "Restart"
method.
Note: Redundant failover MGs require a reliable transport, because
the protocol provides no means for a secondary MG running ALF to
acknowledge messages sent from the MGC.
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. 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 again contacting its primary,
and (if necessary) its secondary MGCs. When contacting its
previously controlling MGC, the MG sends the ServiceChange message
with "Disconnected" method.
In partial failure, or for 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. If "HandOff"
is supported, the MG shall 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 from the designated MGC, the MG
shall behave as if its MGC failed, and start contacting secondary
MGCs as specified in the previous paragraph. If the MG is unable to
establish a control relationship with any MGC, it shall wait a random
amount of time as described in 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 RFC. 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 and replies to all commands from the original MGC must be
sent to the transport address from which they were sent. If the
service relationship with the sending MGC has ended, the replies
should be discarded. The MG may receive 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 Recommendations of the H.248 sub-series). 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 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.
Designed to be extended only (Optional):
This indicates that the package has been expressly designed to
be extended by others, not to be directly referenced. For
example, the package may not have any function on its own or be
nonsensical on its own. The MG SHOULD NOT publish this
PackageID when reporting packages.
Extends (Optional): existing package Descriptor
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 an identifier
defined in the original package and packages it may have
extended (multiple levels of extension). 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: Boolean String: UTF-8 string Octet String: A number of octets. See Annex A and Annex B.3 for encoding 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. The type of sub-list SHALL also be specified. The type shall be chosen from the types specified in this section (with the exception of sub-list). For example, Type: sub-list of enumeration. The encoding of sub-lists is specified in Annexes A and B.3. Possible values: A package MUST specify either a specific set of values or a description of how values are determined. A package MUST also specify a default value or the default behaviour when the value is omitted from its descriptor. For example, a package may specify that procedures related to the property are suspended when its value is omitted. A default value (but not procedures) may be specified as provisionable. Defined in: Which H.248.1 descriptor the property is defined in.
LocalControl is for stream dependent properties.
TerminationState is for stream independent properties. These
are expected to be the most common cases, but it is possible
for properties to be defined in other descriptors.
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 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 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. The package MUST specify a
default signal type. If the default type is TO, the package MUST
specify a default duration which may be provisioned. A default
duration is meaningless for BR.
Duration: in hundredths of seconds
Additional Parameters: see 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 Parameters to Events and Signals
Parameter Name: only descriptive
ParameterID: is an identifier. The textual ParameterID of parameters
to Events and Signals shall not start with "EPA" and "SPA",
respectively. The textual ParameterID shall also not be "ST",
"Stream", "SY", "SignalType", "DR", "Duration", "NC",
"NotifyCompletion", "KA", "Keepactive", "EB", "Embed", "DM" or
"DigitMap".
Type: One of:
Boolean
String: UTF-8 octet string
Octet String: A number of octets. See Annex A and Annex B.3 for
encoding
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 (not supported for
statistics). The type of sub-list SHALL also be specified. The
type shall be chosen from the types specified in this section
(with the exception of sub-list). For example, Type: sub-list of
enumeration. The encoding of sub-lists is specified in Annexes A
and B.3.
Possible values:
A package MUST specify either a specific set of values or a
description of how values are determined. A package MUST also
specify a default value or the default behavior when the value is
omitted from its descriptor. For example, a package may specify
that procedures related to the parameter are suspended when it
value is omitted. A default value (but not procedures) may be
specified as provisionable.
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 alphabetic 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 clause 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 0x8000 to 0xFFFF 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.