RFC 4233
Integrated Services Digital Network (ISDN) Q.921-User Adaptation Layer
4. Procedures
The IUA layer needs to respond to various primitives it receives from other layers as well as messages it receives from the peer IUA layer. This section describes various procedures involved in response to these events.4.1. Procedures to Support Service in Section 1.4.1
These procedures achieve the IUA layer's "Transport of Q.921/Q.931 boundary primitives" service.4.1.1. Q.921 or Q.931 Primitives Procedures
On receiving these primitives from the local layer, the IUA layer will send the corresponding QPTM message (Data, Unit Data, Establish, Release) to its peer. While doing so, the IUA layer needs to fill various fields of the common and specific headers correctly. In addition, the message needs to be sent on the SCTP stream that corresponds to the D channel (Interface Identifier).4.1.2. QPTM Message Procedures
On receiving QPTM messages from a peer IUA layer, the IUA layer on an SG or MGC needs to invoke the corresponding layer primitives (DL-ESTABLISH, DL-DATA, DL-UNIT DATA, DL-RELEASE) to the local Q.921 or Q.931 layer.4.2. Procedures to Support Service in Section 1.4.2
These procedures achieve the IUA layer's "Support for Communication between Layer Managements" service.4.2.1. Layer Management Primitives Procedures
On receiving these primitives from the local Layer Management, the IUA layer will provide the appropriate response primitive across the internal local Layer Management interface. An M-SCTP ESTABLISH request from Layer Management will initiate the establishment of an SCTP association. An M-SCTP ESTABLISH confirm will be sent to Layer Management when the initiated association setup is complete. An M-SCTP ESTABLISH indication is sent to Layer Management upon successful completion of an incoming SCTP association setup from a peer IUA node.
An M-SCTP RELEASE request from Layer Management will initiate the teardown of an SCTP association. An M-SCTP RELEASE confirm will be sent by Layer Management when the association teardown is complete. An M-SCTP RELEASE indication is sent to Layer Management upon successful teardown of an SCTP association initiated by a peer IUA. M-SCTP STATUS request and indication support a Layer Management query of the local status of a particular SCTP association. M-NOTIFY indication and M-ERROR indication indicate to Layer Management the notification or error information contained in a received IUA Notify or Error message, respectively. These indications can also be generated based on local IUA events. M-ASP STATUS request/indication and M-AS-STATUS request/indication support a Layer Management query of the local status of a particular ASP or AS. No IUA peer protocol is invoked. M-ASP-UP request, M-ASP-DOWN request, M-ASP-INACTIVE request, and M-ASP-ACTIVE request allow Layer Management at an ASP to initiate state changes. These requests result in outgoing IUA ASP UP, ASP DOWN, ASP INACTIVE, and ASP ACTIVE messages. M-ASP-UP confirmation, M-ASP-DOWN confirmation, M-ASP-INACTIVE confirmation, and M-ASP-ACTIVE confirmation indicate to Layer Management that the previous request has been confirmed. Upon receipt of an M-TEI Status primitive from Layer Management, the IUA will send the corresponding MGMT message (TEI Status) to its peer. While doing so, the IUA layer needs to fill various fields of the common and specific headers correctly. All MGMT messages are sent on a sequenced stream to ensure ordering. SCTP stream '0' SHOULD be used.4.2.2. Receipt of IUA Peer Management Messages
Upon receipt of IUA Management messages, the IUA layer MUST invoke the corresponding Layer Management primitive indications (e.g., M-AS Status ind., M-ASP Status ind., M-ERROR ind., M-TEI STATUS) to the local layer management. M-NOTIFY indication and M-ERROR indication indicate to Layer Management the notification or error information contained in a received IUA Notify or Error message. These indications can also be generated based on local IUA events.
All MGMT messages are sent on a sequenced stream to ensure ordering. SCTP stream '0' SHOULD be used.4.3. Procedures to Support Service in Section 1.4.3
These procedures achieve the IUA layer's "Support for management of active associations between SG and MGC" service.4.3.1. AS and ASP State Maintenance
The IUA layer on the SG needs to maintain the states of each ASP as well as the state of the AS.4.3.1.1. ASP States
The state of the each ASP, in each AS that it is configured, is maintained in the IUA layer on the SG. The state of an ASP changes due to the following type of events: * Reception of messages from peer IUA layer at that ASP * Reception of some messages from the peer IUA layer at other ASPs in the AS * Reception of indications from SCTP layer * Local Management intervention The ASP state transition diagram is shown in Figure 6. The possible states of an ASP are the following: ASP-DOWN: Application Server Process is unavailable and/or the related SCTP association is down. Initially, all ASPs will be in this state. An ASP in this state SHOULD NOT be sent any IUA 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 Diagram4.3.2. 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
SCTP).
Either IUA peer may optionally send Heartbeat messages periodically,
subject to a provisionable timer T(beat). Upon receiving a Heartbeat
message, the IUA peer MUST respond with a Heartbeat Ack message.
If no Heartbeat Ack message (or any other IUA message) is received
from the IUA peer within 2*T(beat), the remote IUA peer is considered
unavailable. Transmission of Heartbeat messages is stopped and the
signaling process SHOULD attempt to re-establish communication if it
is configured as the client for the disconnected IUA peer.
The BEAT message MAY optionally contain an opaque Heartbeat Data
parameter that MUST be echoed back unchanged in the related Beat Ack
message. The ASP upon examining the contents of the returned BEAT
Ack message MAY choose to consider the remote ASP as unavailable.
The contents/format of the Heartbeat Data parameter is implementation
dependent and only of local interest to the original sender. The
contents MAY be used, for example, to support a Heartbeat sequence
algorithm (to detect missing Heartbeats), and/or a timestamp
mechanism (to evaluate delays).
Note: Heartbeat-related events are not shown in Figure 6, "ASP State
Transition Diagram".
5. Examples
5.1. Establishment of Association and Traffic between SGs and ASPs
5.1.1. Single ASP in an Application Server (1+0 sparing)
This scenario shows the example IUA message flows for the
establishment of traffic between an SG and an ASP, where only one ASP
is configured within an AS (no backup). It is assumed that the SCTP
association is already setup.
SG ASP1
|
|<---------ASP Up----------|
|--------ASP Up Ack------->|
| |
|-----NTFY(AS-INACTIVE)--->|
| |
|<-------ASP Active--------|
|------ASP Active Ack----->|
| |
|------NTFY(AS-ACTIVE)---->|
| |
5.1.2. Two ASPs in Application Server (1+1 sparing)
This scenario shows the example IUA message flows for the
establishment of traffic between an SG and two ASPs in the same
Application Server, where ASP1 is configured to be Active and ASP2 a
standby in the event of communication failure or the withdrawal from
service of ASP1. ASP2 MAY act as a hot, warm, or cold standby
depending on the extent to which ASP1 and ASP2 share call state or
can communicate call state under failure/withdrawal events. The
example message flow is the same whether the ASP Active messages are
Over-ride or Load-share mode although typically this example would
use an Over-ride mode.
SG ASP1 ASP2
| | |
|<--------ASP Up----------| |
|-------ASP Up Ack------->| |
| | |
|----NTFY(AS-INACTIVE)--->| |
| | |
|<-----------------------------ASP Up----------------|
|----------------------------ASP Up Ack------------->|
| | |
| | |
|<-------ASP Active-------| |
|-----ASP Active Ack----->| |
| | |
|-----NTFY(AS-ACTIVE)---->| |
|----------------------NTFY(AS-ACTIVE)-------------->|
5.1.3. Two ASPs in an Application Server (1+1 sparing, load-sharing case)
This scenario shows a similar case to Section 5.1.2 but where the two ASPs are brought to active and load-share the traffic load. In this case, one ASP is sufficient to handle the total traffic load. SG ASP1 ASP2 | | | |<---------ASP Up---------| | |--------ASP Up Ack------>| | | | | |----NTFY(AS-INACTIVE)--->| | | | | |<------------------------------ASP Up---------------| |-----------------------------ASP Up Ack------------>| | | | | | | |<--ASP Active (Ldshr)----| | |----ASP Active Ack------>| | | | | |-----NTFY(AS-ACTIVE)---->| | |----------------------NTFY(AS-ACTIVE)-------------->| | | | |<----------------------------ASP Active (Ldshr)-----| |-----------------------------ASP Active Ack-------->| | | |5.1.4. Three ASPs in an Application Server (n+k sparing, load-sharing case)
This scenario shows the example IUA message flows for the establishment of traffic between an SG and three ASPs in the same Application Server, where two of the ASPs are brought to active and share the load. In this case, a minimum of two ASPs are required to handle the total traffic load (2+1 sparing).
SG ASP1 ASP2 ASP3
| | | |
|<------ASP Up-------| | |
|-----ASP Up Ack---->| | |
| | | |
|-NTFY(AS-INACTIVE)->| | |
| | | |
|<--------------------------ASP Up-------| |
|-----------------------ASP Up Ack------>| |
| | | |
|<---------------------------------------------ASP Up--------|
|--------------------------------------------ASP Up Ack----->|
| | | |
| | | |
|<-ASP Act (Ldshr)---| | |
|----ASP Act Ack---->| | |
| | | |
|<---------------------ASP Act (Ldshr)---| |
|----------------------ASP Act Ack------>| |
| | | |
|--NTFY(AS-ACTIVE)-->| | |
|---------------NTFY(AS-ACTIVE)--------->| |
|------------------------NTFY(AS-ACTIVE)-------------------->|
5.1.5. Interface Identifier Configuration Mismatch Example
This scenario shows the example IUA message flows for the
establishment of traffic between an SG and an ASP in which some of
the Interface Identifiers have been misconfigured on the ASP side.
The SG in this case has Interface Identifiers 1-5 configured for
ASP1.
SG ASP1
| |
| |
|<----ASP Active (IIDs 1-10)-----|
|---ASP Active Ack (IIDs 1-5)--->|
|-------Error (IIDs 6)---------->|
|-------Error (IIDs 7)---------->|
|-------Error (IIDs 8)---------->|
|-------Error (IIDs 9)---------->|
|-------Error (IIDs 10)--------->|
| |
5.2. ASP Traffic Fail-over Examples
5.2.1. (1+1 Sparing, withdrawal of ASP, Backup Over-ride)
The following example shows a case in which an ASP withdraws from service: SG ASP1 ASP2 | | | |<-----ASP Inactive-------| | |----ASP Inactive Ack---->| | | | | |----NTFY(AS-Pending)---->| | |-------------------NTFY(AS-Pending)---------------->| | | | |<------------------------------ ASP Active----------| |-----------------------------ASP Active Ack)------->| | | | |----NTFY(AS-ACTIVE)----->| | |-------------------NTFY(AS-ACTIVE)----------------->| In this case, the SG notifies ASP2 that the AS has moved to the Down state. The SG could have also (optionally) sent a Notify message when the AS moved to the Pending state. Note: If the SG detects loss of the IUA peer (IUA heartbeat loss or detection of SCTP failure), the initial SG-ASP1 ASP Inactive message exchange would not occur.5.2.2. (1+1 Sparing, Backup Over-ride)
The following example shows a case in which ASP2 wishes to override ASP1 and take over the traffic: SG ASP1 ASP2 | | | |<-------------------------------ASP Active----------| |-----------------------------ASP Active Ack-------->| |----NTFY( Alt ASP-Act)-->| | | | In this case, the SG notifies ASP1 that an alternative ASP has overridden it.
5.2.3. (n+k Sparing, Load-sharing case, withdrawal of ASP)
Following on from the example in Section 5.1.4, and ASP1 withdraws from service: SG ASP1 ASP2 ASP3 | | | | |<----ASP Inact------| | | |---ASP Inact Ack--->| | | | | | | |---------------------------------NTFY(Ins. ASPs)----------->| | | | | |<-----------------------------------------ASP Act (Ldshr)---| |-------------------------------------------ASP Act (Ack)--->| | | | | In this case, the SG has knowledge of the minimum ASP resources required (implementation dependent), for example, if the SG knows that n+k = 2+1 for a load-share AS and n currently equals 1. Note: If the SG detects loss of the ASP1 IUA peer (IUA heartbeat loss or detection of SCTP failure), the first SG-ASP1 ASP Inactive message exchange would not occur.5.3. Q.921/Q.931 Primitives Backhaul Examples
When the IUA layer on the ASP has a QPTM message to send to the SG, it will do the following: - Determine the correct SG - Find the SCTP association to the chosen SG - Determine the correct stream in the SCTP association based on the D channel - Fill in the QPTM message, fill in IUA Message Header, fill in Common Header - Send the QPTM message to the remote IUA peer in the SG, over the SCTP association When the IUA layer on the SG has a QPTM message to send to the ASP, it will do the following: - Determine the AS for the Interface Identifier - Determine the Active ASP (SCTP association) within the AS
- Determine the correct stream in the SCTP association based on
the D channel
- Fill in the QPTM message, fill in IUA Message Header, fill in
Common Header
- Send the QPTM message to the remote IUA peer in the ASP, over
the SCTP association
An example of the message flows for establishing a data link on a
signaling channel, passing PDUs and releasing a data link on a
signaling channel is shown below. An active association between MGC
and SG is established (Section 5.1) prior to the following message
flows.
SG ASP
<----------- Establish Request
Establish Confirm ---------->
<----------- Data Request
Data Indication ----------->
<----------- Data Request
Data Indication ----------->
<----------- Data Request
<----------- Data Request
Data Indication ----------->
<----------- Release Request (RELEASE_MGMT)
Release Confirm ---------->
An example of the message flows for a failed attempt to establish a
data link on the signaling channel is shown below. In this case, the
gateway has a problem with its physical connection (e.g., Red Alarm),
so it cannot establish a data link on the signaling channel.
SG ASP
<----------- Establish Request (ESTABLISH_START)
Release Indication ---------->
(RELEASE_PHYS)
5.4. Layer Management Communication Examples
An example of the message flows for communication between Layer
Management modules between SG and ASP is shown below. An active
association between ASP and SG is established (Section 5.1) prior to
the following message flows.
SG ASP
<----------- Data Request
Error Indication ---------->
(INVALID_TEI)
<----------- TEI Status Request
TEI Status Confirm ---------->
(Unassigned)
6. Security
The security considerations discussed in "Security Considerations for
SIGTRAN Protocols", RFC 3788 [3], apply to this document.
7. IANA Considerations
7.1. SCTP Payload Protocol Identifier
The IANA has assigned an IUA value for the Payload Protocol
Identifier in SCTP Payload Data chunk. The following SCTP Payload
Protocol Identifier has been registered:
IUA "1"
The SCTP Payload Protocol Identifier is included in each SCTP Data
chunk, to indicate which protocol the SCTP is carrying. This Payload
Protocol Identifier is not directly used by SCTP but MAY be used by
certain network entities to identify the type of information being
carried in a Data chunk.
The User Adaptation peer MAY use the Payload Protocol Identifier as a
way of determining additional information about the data being
presented to it by SCTP.
7.2. IUA Protocol Extensions
This protocol may also be extended through IANA in three ways:
-- through definition of additional message classes,
-- through definition of additional message types, and
-- through definition of additional message parameters.
The definition and use of new message classes, types, and parameters
are an integral part of SIGTRAN adaptation layers. Thus, these
extensions are assigned by IANA through an IETF Consensus action as
defined in [7].
The proposed extension must in no way adversely affect the general working of the protocol.7.2.1. IETF-Defined Message Classes
The documentation for a new message class MUST include the following information: (a) A long and short name for the message class. (b) A detailed description of the purpose of the message class.7.2.2. IETF-Defined Message Types
Documentation of the message type MUST contain the following information: (a) A long and short name for the new message type. (b) A detailed description of the structure of the message. (c) A detailed definition and description of intended use of each field within the message. (d) A detailed procedural description of the use of the new message type within the operation of the protocol. (e) A detailed description of error conditions when receiving this message type. When an implementation receives a message type that it does not support, it MUST respond with an Error (ERR) message with an Error Code of Unsupported Message Type.7.2.3. IETF-Defined TLV Parameter Extension
Documentation of the message parameter MUST contain the following information: (a) Name of the parameter type. (b) Detailed description of the structure of the parameter field. This structure MUST conform to the general type-length-value format described in Section 3.1.5. (c) Detailed definition of each component of the parameter value. (d) Detailed description of the intended use of this parameter type, and an indication of whether and under what circumstances multiple instances of this parameter type may be found within the same message type.
8. Timer Values
The following are suggestions for default timer values. T(r) 3-5 seconds T(ack) 2-5 seconds T(beat) Heartbeat Timer 30 seconds9. Acknowledgements
The authors would like to thank Alex Audu, Maria Sonia Vazquez Arevalillo, Ming-te Chao, Keith Drage, Norm Glaude, Nikhil Jain, Bernard Kuc, Ming Lin, Stephen Lorusso, John Loughney, Barry Nagelberg, Neil Olson, Lyndon Ong, Heinz Prantner, Jose Luis Jimenez Ramirez, Ian Rytina, Michael Tuexen, and Hank Wang for their valuable comments and suggestions.10. References
10.1. Normative References
[1] ITU-T Recommendation Q.920, 'Digital Subscriber signaling System No. 1 (DSS1) - ISDN User-Network Interface Data Link Layer - General Aspects' [2] Coded Character Set--7-Bit American Standard Code for Information Interchange, ANSI X3.4-1986. [3] Loughney, J., Tuexen, M., and J. Pastor-Balbas, "Security Considerations for Signaling Transport (SIGTRAN) Protocols", RFC 3788, June 2004.10.2. Informative References
[4] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang, L., and V. Paxson, "Stream Control Transmission Protocol", RFC 2960, October 2000. [5] Ong, L., Rytina, I., Garcia, M., Schwarzbauer, H., Coene, L., Lin, H., Juhasz, I., Holdrege, M., and C. Sharp, "Framework Architecture for Signaling Transport", RFC 2719, October 1999. [6] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [7] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[8] Stone, J., Stewart, R., and D. Otis, "Stream Control Transmission Protocol (SCTP) Checksum Change", RFC 3309, September 2002.11. Change Log
Below is a list of the major changes between this document and RFC 3057. 1. The TEI Query message was added. 2. An explanation of the DLCI format (shown in Figure 6) is provided. 3. Aligned the ASP and AS procedures in Section 4 with RFC3331 and RFC3332. 4. Alinged the format of the ASPSM and ASPTM messages with RFC3331 and RFC3332. These changes include removing the Reason field from the ASP Down and ASP Down Ack messages and the Traffic Mode Type field from the ASP Inactive and ASP Inactive Ack messages. 5. Sections 1.3.3 and 1.3.4 were moved to Appendix A. A new section was added in place of Section 1.3.3. 6. The references have been split between Normative and Informative. 7. The new Sigtran security document is referenced and Section 6 has been updated appropriately.
Appendix A
A.1. Signaling Network Architecture
A Signaling Gateway is used to support the transport of Q.921-User signaling traffic to one or more distributed ASPs (e.g., MGCs). Clearly, the IUA protocol is not designed to meet the performance and reliability requirements for such transport by itself. However, the conjunction of distributed architecture and redundant networks does allow for a sufficiently reliable transport of signaling traffic over IP. The IUA protocol is flexible enough to allow its operation and management in a variety of physical configurations, enabling Network Operators to meet their performance and reliability requirements. To meet the ISDN signaling reliability and performance requirements for carrier grade networks, Network Operators SHOULD ensure that there is no single point of failure provisioned in the end-to-end network architecture between an ISDN node and an IP ASP. Depending of course on the reliability of the SG and ASP functional elements, this can typically be met by the provision of redundant Quality of Service (QoS)-bounded IP network paths for SCTP Associations between SCTP End Points, and redundant Hosts, and redundant SGs. The distribution of ASPs within the available Hosts is also important. For a particular Application Server, the related ASPs SHOULD be distributed over at least two Hosts. An example logical network architecture relevant to carrier-grade operation in the IP network domain is shown in Figure 8 below:
Host1 ******** ************** * *_________________________________________* ******** * * * _________* * ASP1 * * * SG1 * SCTP Associations | * ******** * * *_______________________ | * * ******** | | ************** | | ******** | | * *_______________________________| * * | * SG2 * SCTP Associations | * *____________ | * * | | Host2 ******** | | ************** | |_________________* ******** * |____________________________* * ASP1 * * * ******** * * * ************** . . . Figure 8. Logical Model Example For carrier-grade networks, the failure or isolation of a particular ASP SHOULD NOT cause stable calls to be dropped. This implies that ASPs need, in some cases, to share the call state or be able to pass the call state between each other. However, this sharing or communication of call state information is outside the scope of this document.A.2. Application Server Process Redundancy
To avoid a single point of failure, it is recommended that a minimum of two ASPs be in the list, resident in separate hosts and therefore available over different SCTP Associations. For example, in the network shown in Figure 8, all messages from a particular D Channel (Interface Identifier) could be sent to ASP1 in Host1 or ASP1 in Host2. The AS list at SG1 might look like the following: Interface Identifier(s) - Application Server #1 ASP1/Host1 - State=Up, Active ASP1/Host2 - State=Up, Inactive
In this 1+1 redundancy case, ASP1 in Host1 would be sent any incoming
message for the Interface Identifiers registered. ASP1 in Host2
would normally be brought to the active state upon failure of, or
loss of connectivity to, ASP1/Host1. In this example, both ASPs are
Up, meaning that the related SCTP association and far-end IUA peer
are ready.
The AS List at SG1 might also be set up in load-share mode as shown
below:
Interface Identifier(s) - Application Server #1
ASP1/Host1 - State=Up, Active
ASP1/Host2 - State=Up, Active
In this case, both the ASPs would be sent a portion of the traffic.
In the process of fail-over, it is recommended that in the case of
ASPs supporting call processing, stable calls do not get released.
It is possible that calls in transition MAY fail, although measures
of communication between the ASPs involved can be used to mitigate
this problem. For example, the two ASPs MAY share call state via
shared memory, or MAY use an ASP-to-ASP protocol to pass call state
information. The ASP-to-ASP protocol is outside the scope of this
document.
Authors' Addresses
Ken Morneault Cisco Systems Inc. 13615 Dulles Technology Drive Herndon, VA. 20171 USA Phone: +1-703-484-3323 EMail: kmorneau@cisco.com Malleswar Kalla Telcordia Technologies PYA 2J-341 3 Corporate Place Piscataway, NJ 08854 USA Phone: +1-732-699-3728 EMail: mkalla@telcordia.com Selvam Rengasami Tridea Works Phone: +1-732-512-0969 EMail: selvam@trideaworks.com Greg Sidebottom Signatus Technologies Kanata, Ontario, Canada EMail: greg@signatustechnologies.com
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