📄 rfc3057.txt
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Host1
******** **************
* *_________________________________________* ******** *
* * _________* * ASP1 * *
* SG1 * SCTP Associations | * ******** *
* *_______________________ | * *
******** | | **************
| |
******** | |
* *_______________________________|
* * |
* SG2 * SCTP Associations |
* *____________ |
* * | | Host2
******** | | **************
| |_________________* ******** *
|____________________________* * ASP1 * *
* ******** *
* *
**************
.
.
.
Figure 2 - 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.
1.3.4 ASP Fail-over Model and Terminology
The IUA layer supports ASP fail-over functions in order to support a
high availability of call processing capability. All Q.921-User
messages incoming to an SG are assigned to a unique Application
Server, based on the Interface Identifier of the message.
The Application Server is, in practical terms, a list of all ASPs
configured to process Q.921-User messages from certain Interface
Identifiers. One or more ASPs in the list are normally active (i.e.,
handling traffic) while any others MAY be unavailable or inactive, to
be possibly used in the event of failure or unavailability of the
active ASP(s).
Morneault, et al. Standards Track [Page 7]
RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
The fail-over model supports an n+k redundancy model, where n ASP(s)
are the minimum number of redundant ASPs required to handle traffic
and k ASPs are available to take over for a failed or unavailable
ASP. Note that 1+1 active/standby redundancy is a subset of this
model. A simplex 1+0 model is also supported as a subset, with no
ASP 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 2, 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 is
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.
1.3.5 Client/Server Model
It is recommended that the SG and ASP be able to support both client
and server operation. The peer endpoints using IUA SHOULD be
configured so that one always takes on the role of client and the
Morneault, et al. Standards Track [Page 8]
RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
other the role of server for initiating SCTP associations. The
default orientation would be for the SG to take on the role of server
while the ASP is the client. In this case, ASPs SHOULD initiate the
SCTP association to the SG.
The SCTP (and UDP/TCP) Registered User Port Number Assignment for IUA
is 9900.
1.4 Services Provided by the IUA Layer
1.4.1 Support for transport of Q.921/Q.931 boundary primitives
In the backhaul scenario, the Q.921/Q.931 boundary primitives are
exposed. IUA layer needs to support all of the primitives of this
boundary to successfully backhaul Q.931.
This includes the following primitives [1]:
DL-ESTABLISH
The DL-ESTABLISH primitives are used to request, indicate and confirm
the outcome of the procedures for establishing multiple frame
operation.
DL-RELEASE
DL-RELEASE primitives are used to request, indicate, and confirm the
outcome of the procedures for terminating a previously established
multiple frame operation, or for reporting an unsuccessful
establishment attempt.
DL-DATA
The DL-DATA primitives are used to request and indicate layer 3
(Q.931) messages which are to be transmitted, or have been received,
by the Q.921 layer using the acknowledged information transfer
service.
DL-UNIT DATA
The DL-UNIT DATA primitives are used to request and indicate layer 3
(Q.931) messages which are to be transmitted, by the Q.921 layer
using the unacknowledged information transfer service.
Morneault, et al. Standards Track [Page 9]
RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
1.4.2 Support for communication between Layer Management modules on SG
and MGC
It is envisioned that the IUA layer needs to provide some services
that will facilitate communication between Layer Management modules
on the SG and MGC. These primitives are pointed out in [2], which
are shown below:
M-TEI STATUS
The M-TEI STATUS primitives are used to request, confirm and indicate
the status (assigned/unassigned) of a TEI.
M-ERROR
The M-ERROR primitive is used to indicate an error with a received
IUA message (e.g., interface identifier value is not known to the
SG).
1.4.3 Support for management of active associations between SG and MGC
A set of primitives between the IUA layer and the Layer Management
are defined below to help the Layer Management manage the SCTP
association(s) between the SG and MGC. The IUA layer can be
instructed by the Layer Management to establish an SCTP association
to a peer IUA node. This procedure can be achieved using the M-SCTP
ESTABLISH primitive.
M-SCTP ESTABLISH
The M-SCTP ESTABLISH primitives are used to request, indicate, and
confirm the establishment of an SCTP association to a peer IUA node.
M-SCTP RELEASE
The M-SCTP RELEASE primitives are used to request, indicate, and
confirm the release of an SCTP association to a peer IUA node.
The IUA layer MAY also need to inform the status of the SCTP
associations to the Layer Management. This can be achieved using the
M-SCTP STATUS primitive.
M-SCTP STATUS
The M-SCTP STATUS primitives are used to request and indicate the
status of the underlying SCTP association(s).
Morneault, et al. Standards Track [Page 10]
RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
The Layer Management MAY need to inform the IUA layer of an AS/ASP
status (i.e., failure, active, etc.), so that messages can be
exchanged between IUA layer peers to stop traffic to the local IUA
user. This can be achieved using the M-ASP STATUS primitive.
M-ASP STATUS
The ASP status is stored inside IUA layer on both the SG and MGC
sides. The M-ASP STATUS primitive can be used by Layer Management to
request the status of the Application Server Process from the IUA
layer. This primitive can also be used to indicate the status of the
Application Server Process.
M-ASP-UP
The M-ASP-UP primitive can be used by Layer Management to send a ASP
Up message for the Application Server Process. It can also be used
to generate an ASP Up Acknowledgement.
M-ASP-DOWN
The M-ASP-DOWN primitive can be used by Layer Management to send a
ASP Down message for the Application Server Process. It can also be
used to generate an ASP Down Acknowledgement.
M-ASP-ACTIVE
The M-ASP-UP primitive can be used by Layer Management to send a ASP
Active message for the Application Server Process. It can also be
used to generate an ASP Active Acknowledgement.
M-ASP-INACTIVE
The M-ASP-UP primitive can be used by Layer Management to send a ASP
Inactive message for the Application Server Process. It can also be
used to generate an ASP Inactive Acknowledgement.
M-AS STATUS
The M-AS STATUS primitive can be used by Layer Management to request
the status of the Application Server. This primitive can also be
used to indicate the status of the Application Server.
Morneault, et al. Standards Track [Page 11]
RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
1.5 Functions Implemented by the IUA Layer
1.5.1 Mapping
The IUA layer MUST maintain a map of the Interface Identifier to a
physical interface on the Signaling Gateway. A physical interface
would be a T1 line, E1 line, etc., and could include the TDM
timeslot. In addition, for a given interface the SG MUST be able to
identify the associated signaling channel. IUA layers on both SG and
MGC MAY maintain the status of TEIs and SAPIs.
The SG maps an Interface Identifier to an SCTP association/stream
only when an ASP sends an ASP Active message for a particular
Interface Identifier. It MUST be noted, however, that this mapping
is dynamic and could change at any time due to a change of ASP state.
This mapping could even temporarily be invalid, for example during
failover of one ASP to another. Therefore, the SG MUST maintain the
states of AS/ASP and reference them during the routing of an messages
to an AS/ASP.
One example of the logical view of relationship between D channel,
Interface Identifier, AS and ASP in the SG is shown below:
/---------------------------------------------------+
/ /------------------------------------------------|--+
/ / v |
/ / +----+ act+-----+ +-------+ -+--+-|+--+-
D chan1-------->|IID |-+ +-->| ASP |--->| Assoc | v
/ +----+ | +----+ | +-----+ +-------+ -+--+--+--+-
/ +->| AS |--+ Streams
/ +----+ | +----+ stb+-----+
D chan2-------->|IID |-+ | ASP |
+----+ +-----+
where IID = Interface Identifier
Note that an ASP can be in more than one AS.
1.5.2 Status of ASPs
The IUA layer on the SG MUST maintain the state of the ASPs it is
supporting. The state of an ASP changes because of reception of
peer-to-peer messages (ASPM messages as described in Section 3.3.2)
or reception of indications from the local SCTP association. ASP
state transition procedures are described in Section 4.3.1.
Morneault, et al. Standards Track [Page 12]
RFC 3057 ISDN Q.921-User Adaptation Layer February 2001
At a SG, an Application Server list MAY contain active and inactive
ASPs to support ASP load-sharing and fail-over procedures. When, for
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