messages.
ASP-INACTIVE: The remote IUA peer at the ASP is available (and the
related SCTP association is up) but application traffic is stopped.
In this state, the ASP can be sent any non-QPTM IUA messages (except
for TEI Status messages).
ASP-ACTIVE: The remote IUA peer at the ASP is available and
application traffic is active.
+--------------+
+----------------------| |
| Alternate +-------| ASP-ACTIVE |
| ASP | +--------------+
| Takeover | ^ |
| | ASP | | ASP Inactive /
| | Active | | ASP Up
| | | v
| | +--------------+
| | | |
| +------>| ASP-INACTIVE |
| +--------------+
| ^ |
ASP Down/ | ASP | | ASP Down /
SCTP CDI/ | Up | | SCTP CDI /
SCTP RI | | v SCTP RI
| +--------------+
+--------------------->| |
| ASP-DOWN |
+--------------+
Figure 6. ASP State Transition Diagram
SCTP CDI: The local SCTP layer’s Communication Down Indication to
the Upper Layer Protocol (IUA) on an SG. The local SCTP will send
this indication when it detects the loss of connectivity to the ASP’s
peer SCTP layer. SCTP CDI is understood as either a SHUTDOWN
COMPLETE notification and COMMUNICATION LOST notification from the
SCTP.
SCTP RI: The local SCTP layer’s Restart indication to the upper layer
protocol (IUA) on an SG. The local SCTP will send this indication
when it detects a restart from the ASP’s peer SCTP layer.
4.3.1.2. AS States
The state of the AS is maintained in the IUA layer on the SG.
The state of an AS changes due to events. These events include the
following:
* ASP state transitions
* Recovery timer triggers
The possible states of an AS are the following:
AS-DOWN: The Application Server is unavailable. This state implies
that all related ASPs are in the ASP-DOWN state for this AS.
Initially, the AS will be in this state.
AS-INACTIVE: The Application Server is available but no application
traffic is active (i.e., one or more related ASPs are in the
ASP-INACTIVE state, but none in the ASP-ACTIVE state). The recovery
timer T(r) is not running or has expired.
AS-ACTIVE: The Application Server is available and application
traffic is active. This state implies that at least one ASP is in
the ASP-ACTIVE state.
AS-PENDING: An active ASP has transitioned from active to inactive or
down and it was the last remaining active ASP in the AS. A recovery
timer T(r) will be started and all incoming SCN messages will be
queued by the SG. If an ASP becomes active before T(r) expires, the
AS will move to AS-ACTIVE state and all the queued messages will be
sent to the active ASP.
If T(r) expires before an ASP becomes active, the SG stops queuing
messages and discards all previously queued messages. The AS will
move to AS-INACTIVE if at least one ASP is in ASP-INACTIVE state,
otherwise it will move to AS-DOWN state.
+----------+ one ASP trans to ASP-ACTIVE +-------------+
| AS- |---------------------------->| AS- |
| INACTIVE | | ACTIVE |
| |<--- | |
+----------+ \ +-------------+
^ | \ Tr Expiry, ^ |
| | \ at least one | |
| | \ ASP in ASP-INACTIVE | |
| | \ | |
| | \ | |
| | \ | |
one ASP | | all ASP \ one ASP | | Last ACTIVE
trans | | trans to \ trans to | | ASP trans to
to | | ASP-DOWN -------\ ASP- | | ASP-INACTIVE
ASP- | | \ ACTIVE | | or ASP-DOWN
INACTIVE| | \ | | (start Tr)
| | \ | |
| | \ | |
| v \ | v
+----------+ \ +-------------+
| | --| |
| AS-DOWN | | AS-PENDING |
| | | (queueing) |
| |<----------------------------| |
+----------+ Tr Expiry and no ASP +-------------+
in ASP-INACTIVE state
Tr = Recovery Timer
Figure 7: AS State Transition Diagram
4.3.2. ASPM Procedures for Primitives
Before the establishment of an SCTP association, the ASP state at
both the SG and ASP is assumed to be in the state ASP-DOWN.
As the ASP is responsible for initiating the setup of an SCTP
association to an SG, the IUA layer at an ASP receives an M-SCTP
ESTABLISH request primitive from the Layer Management, the IUA layer
will try to establish an SCTP association with the remote IUA peer at
an SG. Upon reception of an eventual SCTP-Communication Up confirm
primitive from the SCTP, the IUA layer will invoke the primitive
M-SCTP ESTABLISH confirm to the Layer Management.
At the SG, the IUA layer will receive an SCTP Communication Up
indication primitive from the SCTP. The IUA layer will then invoke
the primitive M-SCTP ESTABLISH indication to the Layer Management.
Once the SCTP association is established and assuming that the local
IUA-User is ready, the local ASP IUA Application Server Process
Maintenance (ASPM) function will initiate the ASPM procedures, using
the ASP Up/-Down/-Active/-Inactive messages to convey the ASP state
to the SG (see Section 4.3.3).
The Layer Management and the IUA layer on SG can communicate the
status of the application server using the M-AS_STATUS primitives.
The Layer Management and the IUA layer on both the SG and ASP can
communicate the status of an SCTP association using the M-SCTP_STATUS
primitives.
If the Layer Management on SG or ASP wants to bring down an SCTP
association for management reasons, it would send M-SCTP RELEASE
request primitive to the local IUA layer. The IUA layer would
release the SCTP association and upon receiving the SCTP-
COMMUNICATION_DOWN indication from the underlying SCTP layer, it
would inform the local Layer Management using M-SCTP_RELEASE confirm
primitive.
If the IUA layer receives an SCTP-COMMUNICATION_DOWN indication from
the underlying SCTP layer, it will inform the Layer Management by
invoking the M-SCTP RELEASE indication primitive. The state of the
ASP will be moved to "Down" at both the SG and ASP.
At an ASP, the Layer Management MAY try to reestablish the SCTP
association using M-SCTP_ESTABLISH request primitive.
In the case of an SCTP-RESTART indication at an ASP, the ASP is now
considered by its IUA peer to be in the ASP-DOWN state. The ASP, if
it is to recover, must begin any recovery with the ASP Up procedure.
4.3.3. ASPM Procedures for Peer-to-Peer Messages
All ASPM messages are sent on a sequenced stream to ensure ordering.
SCTP stream ’0’ SHOULD be used.
4.3.3.1. ASP Up Procedures
After an ASP has successfully established an SCTP association to an
SG, the SG waits for the ASP to send an ASP Up message, indicating
that the ASP IUA peer is available. The ASP is always the initiator
of the ASP Up message. This action MAY be initiated at the ASP by an
M-ASP_UP request primitive from Layer Management or MAY be initiated
automatically by an IUA management function.
When an ASP Up message is received at an SG and internally the remote
ASP is in the ASP-DOWN state and not considered locked out for local
management reasons, the SG marks the remote ASP in the state
ASP-INACTIVE and informs Layer Management with an M-ASP_Up indication
primitive. If the SG is aware, via current configuration data, which
Application Servers the ASP is configured to operate in, the SG
updates the ASP state to ASP-INACTIVE in each AS that it is a member.
Alternatively, the SG may move the ASP into a pool of Inactive ASPs
available for future configuration within Application Server(s),
determined in a subsequent ASP Active procedure. If the ASP Up
message contains an ASP Identifier, the SG should save the ASP
Identifier for that ASP. The SG MUST send an ASP Up Ack message in
response to a received ASP Up message even if the ASP is already
marked as ASP-INACTIVE at the SG.
If for any local reason (e.g., management lockout) the SG cannot
respond with an ASP Up Ack message, the SG responds to an ASP Up
message with an Error message with reason "Refused - Management
Blocking".
At the ASP, the ASP Up Ack message received is not acknowledged.
Layer Management is informed with an M-ASP_UP confirm primitive.
When the ASP sends an ASP Up message, it starts timer T(ack). If the
ASP does not receive a response to an ASP Up message within T(ack),
the ASP MAY restart T(ack) and resend ASP Up messages until it
receives an ASP Up Ack message. T(ack) is provisionable, with a
default of 2 seconds. Alternatively, retransmission of ASP Up
messages MAY be put under control of Layer Management. In this
method, expiry of T(ack) results in an M-ASP_UP confirm primitive
carrying a negative indication.
The ASP must wait for the ASP Up Ack message before sending any other
IUA messages (e.g., ASP Active). If the SG receives any other IUA
messages before an ASP Up message is received (other than ASP Down;
see Section 4.3.3.2), the SG MAY discard them.
If an ASP Up message is received and internally the remote ASP is in
the ASP-ACTIVE state, an ASP Up Ack message is returned, as well as
an Error message ("Unexpected Message"), and the remote ASP state is
changed to ASP-INACTIVE in all relevant Application Servers.
If an ASP Up message is received and internally the remote ASP is
already in the ASP-INACTIVE state, an ASP Up Ack message is returned
and no further action is taken.
4.3.3.2. ASP Down Procedures
The ASP will send an ASP Down message to an SG when the ASP wishes to
be removed from the list of ASPs in all Application Servers that it
is a member and no longer receive any IUA QPTM or ASPTM messages.
This action MAY be initiated at the ASP by an M-ASP_DOWN request
primitive from Layer Management or MAY be initiated automatically by
an IUA management function.
Whether the ASP is permanently removed from an AS is a function of
configuration management.
The SG marks the ASP as ASP-DOWN, informs Layer Management with an
M-ASP_Down indication primitive, and returns an ASP Down Ack message
to the ASP.
The SG MUST send an ASP Down Ack message in response to a received
ASP Down message from the ASP even if the ASP is already marked as
ASP-DOWN at the SG.
At the ASP, the ASP Down Ack message received is not acknowledged.
Layer Management is informed with an M-ASP_DOWN confirm primitive.
If the ASP receives an ASP Down Ack without having sent an ASP Down
message, the ASP should now consider itself as in the ASP-DOWN state.
If the ASP was previously in the ASP-ACTIVE or ASP-INACTIVE state,
the ASP should then initiate procedures to return itself to its
previous state.
When the ASP sends an ASP Down message, it starts timer T(ack). If
the ASP does not receive a response to an ASP Down message within
T(ack), the ASP MAY restart T(ack) and resend ASP Down messages until
it receives an ASP Down Ack message. T(ack) is provisionable, with a
default of 2 seconds. Alternatively, retransmission of ASP Down
messages MAY be put under control of Layer Management. In this
method, expiry of T(ack) results in an M-ASP_DOWN confirm primitive
carrying a negative indication.
4.3.3.3. IUA Version Control
If a ASP Up message with an unsupported version is received, the
receiving end responds with an Error message, indicating the version
the receiving node supports and notifies Layer Management.
This is useful when protocol version upgrades are being performed in
a network. A node upgraded to a newer version SHOULD support the
older versions used on other nodes it is communicating with. Because
ASPs initiate the ASP Up procedure it is assumed that the Error
message would normally come from the SG.
4.3.3.4. ASP Active Procedures
Any time after the ASP has received an ASP Up Ack from the SG, the
ASP sends an ASP Active message to the SG indicating that the ASP is
ready to start processing traffic. This action MAY be initiated at
the ASP by an M-ASP_ACTIVE request primitive from Layer Management or
MAY be initiated automatically by an IUA management function. In the
case where an ASP is configured/registered to process the traffic for
more than one Application Server across an SCTP association, the
ASPAC contains one or more Interface Identifiers to indicate for
which Application Servers the ASPAC applies.
If the Application Server can be successfully activated, the SG
responds to the ASP with an ASPAC Ack message acknowledging that the
ASPAC message was received and starts sending traffic for the
Application Server to that ASP.
In the case where an "out-of-the-blue" ASP Active message is received
(i.e., the ASP has not registered with the SG or the SG has no static
configuration data for the ASP), the message MAY be silently
discarded.
The SG MUST send an ASP Active Ack message in response to a received
ASP Active message from the ASP, if the ASP is already marked in the
ASP-ACTIVE state at the SG.
At the ASP, the ASP Active Ack message received is not acknowledged.
Layer Management is informed with an M-ASP_ACTIVE confirm primitive.
It is possible for the ASP to receive Data message(s) before the ASP
Active Ack message as the ASP Active Ack and Data messages from an SG
may be sent on different SCTP streams. Message loss is possible as
the ASP does not consider itself in the ASP-ACTIVE state until
reception of the ASP Active Ack message.
When the ASP sends an ASP Active message, it starts timer T(ack). If
the ASP does not receive a response to an ASP Active message within
T(ack), the ASP MAY restart T(ack) and resend ASP Active messages
until it receives an ASP Active Ack message. T(ack) is
provisionable, with a default of 2 seconds. Alternatively,
retransmission of ASP Active messages MAY be put under control of
Layer Management. In this method, expiry of T(ack) results in an M-
ASP_ACTIVE confirm primitive carrying a negative indication.
The ASP MUST wait for the ASP Active Ack message from the SG before
sending any Data messages or it will risk message loss. If the SG
receives QPTM messages before an ASP Active is received, the SG
SHOULD discard these messages.
There are two modes of Application Server traffic handling in the SG
IUA: Over-ride and Load-sharing. The Type parameter in the ASPAC
message indicates the mode used in a particular Application Server.
If the SG determines that the mode indicates in an ASPAC is
incompatible with the traffic handling mode currently used in the AS,
the SG responds with an Error message indicating Unsupported Traffic
Handling Mode.
In the case of an Over-ride mode AS, reception of an ASPAC message at
an SG causes the redirection of all traffic for the AS to the ASP
that sent the ASPAC. The SG responds to the ASPAC with an ASP Active
Ack message to the ASP. Any previously active ASP in the AS is now
considered Inactive and will no longer receive traffic from the SG
within the AS. The SG sends a Notify (Alternate ASP Active) to the
previously active ASP in the AS, after stopping all traffic to that
ASP.
In the case of a load-share mode AS, reception of an ASPAC message at
an SG causes the direction of traffic to the ASP sending the ASPAC,
in addition to all the other ASPs that are currently active in the
AS. The algorithm at the SG for load-sharing traffic within an AS to
all the active ASPs is implementation dependent. The algorithm
could, for example, be round-robin or based on information in the
Data message, such as Interface Identifier, depending on the
requirements of the application and the call state handling
assumptions of the collection of ASPs in the AS. The SG responds to
the ASPAC with an ASP Active Ack message to the ASP.
4.3.3.5. ASP Inactive Procedures
When an ASP wishes to withdraw from receiving traffic within an AS,
the ASP sends an ASP Inactive message to the SG. This action MAY be
initiated at the ASP by an M-ASP_INACTIVE request primitive from
Layer Management or MAY be initiated automatically by an IUA
management function. In the case where an ASP is configured/
registered to process the traffic for more than one Application
Server across an SCTP association, the ASPIA contains one or more
Interface Identifiers to indicate for which Application Servers the
ASP Inactive message applies.
There are two modes of Application Server traffic handling in the SG
IUA when withdrawing an ASP from service: Over-ride and Load-sharing.
In the case of an Over-ride mode AS, where normally another ASP has
already taken over the traffic within the AS with an Over-ride ASPAC
message, the ASP that sends the ASPIA message is already considered
by the SG to be ASP-INACTIVE. An ASPIA Ack message is sent to the
ASP, after ensuring that all traffic is stopped to the ASP.
In the case of a Load-share mode AS, the SG moves the ASP to the
ASP-INACTIVE state and the AS traffic is re-allocated across the
remaining ASP-ACTIVE ASPs per the load-sharing algorithm currently
used within the AS. An ASPIA Ack message is sent to the ASP after
all traffic is halted to the ASP. A Notify (Insufficient ASPs)
message MAY be sent to all inactive ASPs, if required.
When the ASP sends an ASP Inactive message it starts timer T(ack).
If the ASP does not receive a response to an ASP Inactive message
within T(ack), the ASP MAY restart T(ack) and resend ASP Inactive
messages until it receives an ASP Inactive Ack message. T(ack) is
provisionable, with a default of 2 seconds. Alternatively,
retransmission of ASP Inactive messages MAY be put under control of
Layer Management. In this method, expiry of T(ack) results in a M-
ASP_Inactive confirm primitive carrying a negative indication.
If no other ASPs in the Application Server are in the state
ASP-ACTIVE, the SG MUST send a Notify ("AS-Pending") message to all
of the ASPs in the AS that are in the state ASP-INACTIVE. The SG
SHOULD start buffering the incoming messages for T(r) seconds, after
which messages MAY be discarded. T(r) is configurable by the network
operator. If the SG receives an ASP Active message from an ASP in
the AS before expiry of T(r), the buffered traffic is directed to
that ASP and the timer is cancelled. If T(r) expires, the AS is
moved to the AS-INACTIVE state.
At the ASP, the ASP Inactive Ack message received is not
acknowledged. Layer Management is informed with an M-ASP_INACTIVE
confirm primitive. If the ASP receives an ASP Inactive Ack without
having sent an ASP Inactive message, the ASP should now consider
itself as in the ASP-INACTIVE state. If the ASP was previously in
the ASP-ACTIVE state, the ASP should then initiate procedures to
return itself to its previous state.
4.3.3.6. Notify Procedures
A Notify message reflecting a change in the AS state MUST be sent to
all ASPs in the AS, except those in the ASP-DOWN state, with
appropriate Status Information and any ASP Identifier of the failed
ASP. At the ASP, Layer Management is informed with an M-NOTIFY
indication primitive. The Notify message must be sent whether the AS
state change was a result of an ASP failure or reception of an ASP
State Management (ASPSM) / ASP Traffic Management (ASPTM) message.
In the second case, the Notify message MUST be sent after any related
acknowledgement messages (e.g., ASP Up Ack, ASP Down Ack, ASP Active
Ack, or ASP Inactive Ack).
In the case where a Notify ("AS-Pending") message is sent by an SG
that now has no ASPs active to service the traffic, or a NTFY
("Insufficient ASPs") is sent in the Load-share mode, the Notify does
not explicitly compel the ASP(s) receiving the message to become
active. The ASPs remain in control of what (and when) action is
taken.
4.3.3.7. Heartbeat
The optional Heartbeat procedures MAY be used when operating over
transport layers that do not have their own heartbeat mechanism for
detecting loss of the transport association (i.e., other than the