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 RFCand 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 RFCdoes 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 RFCfor 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 [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 RFCemploying 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 RFCwhere 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 [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 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 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 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.
ANNEX A - Binary encoding of the protocol
This annex specifies the syntax of messages using the notation
defined in Recommendation X.680; Information technology - Abstract
Syntax Notation One (ASN.1): Specification of basic notation.
Messages shall be encoded for transmission by applying the basic
encoding rules specified in Recommendation X.690, Information
Technology - ASN.1 Encoding Rules: Specification of Basic Encoding
Rules (BER), Canonical Encoding Rules (CER) and Distinguished
Encoding Rules.
A.1 Coding of wildcards
The use of wildcards ALL and CHOOSE is allowed in the protocol. This
allows a MGC to partially specify Termination IDs and to let the MG
choose from the values that conform to the partial specification.
Termination IDs may encode a hierarchy of names. This hierarchy is
provisioned. For instance, a TerminationID may consist of a trunk
group, a trunk within the group and a circuit. Wildcarding must be
possible at all levels. The following paragraphs explain how this is
achieved.
The ASN.1 description uses octet strings of up to 8 octets in length
for Termination IDs. This means that Termination IDs consist of at
most 64 bits. A fully specified Termination ID may be preceded by a
sequence of wildcarding fields. A wildcarding field is one octet in
length. Bit 7 (the most significant bit) of this octet specifies
what type of wildcarding is invoked: if the bit value equals 1, then
the ALL wildcard is used; if the bit value if 0, then the CHOOSE
wildcard is used. Bit 6 of the wildcarding field specifies whether
the wildcarding pertains to one level in the hierarchical naming
scheme (bit value 0) or to the level of the hierarchy specified in
the wildcarding field plus all lower levels (bit value 1). Bits 0
through 5 of the wildcarding field specify the bit position in the
Termination ID at which the wildcarding starts.
We illustrate this scheme with some examples. In these examples, the
most significant bit in a string of bits appears on the left hand
side.
Assume that Termination IDs are three octets long and that each octet
represents a level in a hierarchical naming scheme. A valid
Termination ID is:
00000001 00011110 01010101.
Addressing ALL names with prefix 00000001 00011110 is done as
follows:
wildcarding field: 10000111
Termination ID: 00000001 00011110 xxxxxxxx.
The values of the bits labeled "x" is irrelevant and shall be ignored
by the receiver.
Indicating to the receiver that it must choose a name with 00011110
as the second octet is done as follows:
wildcarding fields: 00010111 followed by 00000111
Termination ID: xxxxxxxx 00011110 xxxxxxxx.
The first wildcard field indicates a CHOOSE wildcard for the level in
the naming hierarchy starting at bit 23, the highest level in our
assumed naming scheme. The second wildcard field indicates a CHOOSE
wildcard for the level in the naming hierarchy starting at bit 7, the
lowest level in our assumed naming scheme.
Finally, a CHOOSE-wildcarded name with the highest level of the name
equal to 00000001 is specified as follows:
wildcard field: 01001111
Termination ID: 0000001 xxxxxxxx xxxxxxxx .
Bit value 1 at bit position 6 of the first octet of the wildcard
field indicates that the wildcarding pertains to the specified level
in the naming hierarchy and all lower levels.
Context IDs may also be wildcarded. In the case of Context IDs,
however, specifying partial names is not allowed. Context ID 0x0
SHALL be used to indicate the NULL Context, Context ID 0xFFFFFFFE
SHALL be used to indicate a CHOOSE wildcard, and Context ID
0xFFFFFFFF SHALL be used to indicate an ALL wildcard.
TerminationID 0xFFFFFFFFFFFFFFFF SHALL be used to indicate the ROOT
Termination.
A.2 ASN.1 syntax specification
This subclause contains the ASN.1 specification of the H.248.1
protocol syntax.
NOTE 1 - In case a transport mechanism is used that employs
application level framing, the definition of Transaction below
changes. Refer to the annex or to the Recommendation of the H.248
sub-series defining the transport mechanism for the definition that
applies in that case.
NOTE 2 - The ASN.1 specification below contains a clause defining
TerminationIDList as a sequence of TerminationIDs. The length of
this sequence SHALL be one, except possibly when used in
contextAuditResult.
NOTE 3 - This syntax specification does not enforce all
restrictions on element inclusions and values. Some additional
restrictions are stated in comments and other restrictions appear
in the text of this RFC. These additional restrictions
are part of the protocol even though not enforced by this
specification.
NOTE 4 - The ASN.1 module in this Annex uses octet string types to
encode values for property parameter, signal parameter and event
parameter values and statistics. The actual types of these values
vary and are specified in Annex C or the relevant package
definition.
A value is first BER-encoded based on its type using the table below.
The result of this BER-encoding is then encoded as an ASN.1 octet
string, "double wrapping" the value. The format specified in Annex C
or the package relates to BER encoding according to the following
table:
Type Specified in Package ASN.1 BER Type
String IA5String or UTF8String (Note 4)
Integer (4 Octet) INTEGER
Double (8 octet signed int) INTEGER (Note 3)
Character (UTF-8, Note 1) IA5String
Enumeration ENUMERATED
Boolean BOOLEAN
Unsigned Integer (Note 2) INTEGER (Note 3)
Octet (String) OCTET STRING
Note 1: Can be more than one byte
Note 2: Unsigned integer is referenced in Annex C
Note 3: The BER encoding of INTEGER does not imply the use of 4
bytes.
Note 4: String should be encoded as IA5String when the contents
are all ASCII characters, but as UTF8String if it contains any
Non-ASCII characters.
See ITU-T Rec. X.690, 8.7, for the definition of the encoding of an
octet string value.
MEDIA-GATEWAY-CONTROL DEFINITIONS AUTOMATIC TAGS::=
BEGIN
MegacoMessage ::= SEQUENCE
{
authHeader AuthenticationHeader OPTIONAL,
mess Message
}
AuthenticationHeader ::= SEQUENCE
{
secParmIndex SecurityParmIndex,
seqNum SequenceNum,
ad AuthData
}
SecurityParmIndex ::= OCTET STRING(SIZE(4))
SequenceNum ::= OCTET STRING(SIZE(4))
AuthData ::= OCTET STRING (SIZE (12..32))
Message ::= SEQUENCE
{
version INTEGER(0..99),
-- The version of the protocol defined here is equal to 1.
mId MId, -- Name/address of message originator
messageBody CHOICE
{
messageError ErrorDescriptor,
transactions SEQUENCE OF Transaction
},
...
}
MId ::= CHOICE
{
ip4Address IP4Address,
ip6Address IP6Address,
domainName DomainName,
deviceName PathName,
mtpAddress OCTET STRING(SIZE(2..4)),
-- Addressing structure of mtpAddress:
-- 25 - 15 0
-- | PC | NI |
-- 24 - 14 bits 2 bits
-- Note: 14 bits are defined for international use.
-- Two national options exist where the point code is 16 or 24
-- bits.
-- To octet align the mtpAddress, the MSBs shall be encoded as 0s.
...
}
DomainName ::= SEQUENCE
{
name IA5String,
-- The name starts with an alphanumeric digit followed by a
-- sequence of alphanumeric digits, hyphens and dots. No two
-- dots shall occur consecutively.
portNumber INTEGER(0..65535) OPTIONAL
}
IP4Address ::= SEQUENCE
{
address OCTET STRING (SIZE(4)),
portNumber INTEGER(0..65535) OPTIONAL
}
IP6Address ::= SEQUENCE
{
address OCTET STRING (SIZE(16)),
portNumber INTEGER(0..65535) OPTIONAL
}
PathName ::= IA5String(SIZE (1..64))
-- See A.3
Transaction ::= CHOICE
{
transactionRequest TransactionRequest,
transactionPending TransactionPending,
transactionReply TransactionReply,
transactionResponseAck TransactionResponseAck,
-- use of response acks is dependent on underlying transport
...
}
TransactionId ::= INTEGER(0..4294967295) -- 32-bit unsigned integer
TransactionRequest ::= SEQUENCE
{
transactionId TransactionId,
actions SEQUENCE OF ActionRequest,
...
}
TransactionPending ::= SEQUENCE
{
transactionId TransactionId,
...
}
TransactionReply ::= SEQUENCE
{
transactionId TransactionId,
immAckRequired NULL OPTIONAL,
transactionResult CHOICE
{
transactionError ErrorDescriptor,
actionReplies SEQUENCE OF ActionReply
},
...
}
TransactionResponseAck ::= SEQUENCE OF TransactionAck
TransactionAck ::= SEQUENCE
{
firstAck TransactionId,
lastAck TransactionId OPTIONAL
}
ErrorDescriptor ::= SEQUENCE
{
errorCode ErrorCode,
errorText ErrorText OPTIONAL
}
ErrorCode ::= INTEGER(0..65535)
-- See clause 13 for IANA Considerations with respect to error codes
ErrorText ::= IA5String
ContextID ::= INTEGER(0..4294967295)
-- Context NULL Value: 0
-- Context CHOOSE Value: 4294967294 (0xFFFFFFFE)
-- Context ALL Value: 4294967295 (0xFFFFFFFF)
ActionRequest ::= SEQUENCE
{
contextId ContextID,
contextRequest ContextRequest OPTIONAL,
contextAttrAuditReq ContextAttrAuditRequest OPTIONAL,
commandRequests SEQUENCE OF CommandRequest
}
ActionReply ::= SEQUENCE
{
contextId ContextID,
errorDescriptor ErrorDescriptor OPTIONAL,
contextReply ContextRequest OPTIONAL,
commandReply SEQUENCE OF CommandReply
}
ContextRequest ::= SEQUENCE
{
priority INTEGER(0..15) OPTIONAL,
emergency BOOLEAN OPTIONAL,
topologyReq SEQUENCE OF TopologyRequest OPTIONAL,
...
}
ContextAttrAuditRequest ::= SEQUENCE
{
topology NULL OPTIONAL,
emergency NULL OPTIONAL,
priority NULL OPTIONAL,
...
}
CommandRequest ::= SEQUENCE
{
command Command,
optional NULL OPTIONAL,
wildcardReturn NULL OPTIONAL,
...
}
Command ::= CHOICE
{
addReq AmmRequest,
moveReq AmmRequest,
modReq AmmRequest,
-- Add, Move, Modify requests have the same parameters
subtractReq SubtractRequest,
auditCapRequest AuditRequest,