Preconditions Interworking
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The provides a preconditions support feature to reserve network resources as a preventative measure to minimize chances of session failure due to lack of resources before alerting the called party. This prevents a "ghost ring" where the called party is alerted before the call is set up.The supports preconditions interworking functionality only when one leg of a call supports preconditions and other does not. The SBC supports preconditions functionality on either of the legs (egress or ingress). With this feature, preconditions interworking is decided based on the precondition profile configuration, present in the Common IP Attributes of the IP Signaling Profile (IPSP).This feature supports ISUP-SIP and SIP-SIP preconditions inter-working, and provides transparency by dropping/adding of preconditions attributes between supporting or non-supporting peers. The preconditions are configured from the at the SIP Trunk Group level using the parameters “preconditions” (for incoming requests) and “transmitPreconditions” (for outgoing requests). |
The CLI syntax is shown below:
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% set addressContext <addressContext_name> zone <zone_name> sipTrunkGroup <sipTrunkGroup_name> services preconditions [none | required | supported]
% set addressContext <addressContext_name> zone <zone_name> sipTrunkGroup <sipTrunkGroup_name> services transmitPreconditions [none | required | supported] |
Preconditions Interworking Using 183 Response
The SBC supports preconditions interworking for generic call flows based on the ingress IPTG or on the message content of the same IPTG. A new SIP Message Manipulation (SMM) variable is used to determine the applicable IPTG configurables for an initial INVITE, SUBSCRIBE, REGISTER, or an Out-Of-Dialog request.
If support of an IPTG prefix-based Call Admission Control (CAC) together with SMM-based ingress IPTG selection is desired, use IPTGs which are selected based on the SMM. The SMM rules in the same profile are used for the selection, and each profile is associated with an IP prefix (by virtue of being attached to an IPTG which is selected based on IP prefix). A Parent IPTG is defined covering all such SMM-based IPTGs, and the parent IPTG CAC corresponds to the IP prefix-based IPTG.
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Note The presence, absence, or value of a SIP information element such as a proprietary Header or a proprietary Parameter are examples of the type of message contents that trigger the enhanced preconditions. |
The message content policy selection is supported through Flexible Policy.
The SBC Flexible Policy supports the selection of the IPTG for the ingress leg though SIP SMM rules. The IPTG that is selected using SMM configuration is applied to all the incoming initial INVITE, initial SUBSCRIBE, initial REGISTER, and/or Out-Of-Dialog request. The CAC is applied to both the IP-prefix based IPTG and the SMM based IPTG.
Support of a IPTG prefix based CAC, along with SMM based ingress IPTG selection, it is accomplished as follows:
- IPTGs selected based on the SMM, uses the SMM rules in the same profile for selection and each profile is associated with an IP prefix (since it is attached to an IPTG, which is selected based on an IP prefix).
- A parent IPTG is defined as covering all the SMM based IPTGs, and the parent IPTG CAC corresponds to the IP prefix based IPTG.
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Note - If an IPTG configured using SMM, and configured in the system, the call is rejected with a code 488 "Request not acceptable here" message.
- The IPTG is not validated while configuring via SMM; it is validated at the time of execution since IPTGs can be added/deleted in between SMM.
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The
also provides preconditions support for 183 responses using following CLI syntax, (This flag is for specific call flows and is not intended for generic use)
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% set addressContext <addressContext_name> zone <zone_name> sipTrunkGroup <sipTrunkGroup_name> services preconditionIntwkUsing183 <disable|enable> |
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Do not enable preConditionIntwkUsing183 when End-to-End Prack is configured. |
Preconditions Interworking Properties
The preconditions interworking is provided based on a new Preconditions Profile (PP), and the interpretation of what IPTG configuration to apply is driven by the ingress/egress PP configuration. This is required to provide flexibility for environments where mixed combinations of functionality is required as shown in the following table:
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| 0 | Table |
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| 1 | Preconditions Interworking Properties |
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| 3 | Preconditions Interworking Properties |
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| Ingress | Egress | Preconditions Interworking |
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| IPTG-1 | IPTG-2 | Requires preconditions interworking in one direction, but not the other direction | | IPTG-1 | IPTG-3 | Does not require preconditions interworking | | IPTG-1 | IPTG-4 | Requires preconditions interworking for all calls |
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The PP is present in the "Common IP Attributes" of the IP Signaling Profile (IPSP) for the PSX/ERE.Interworking functionality is triggered, and ingress/egress PP is evaluated only under the following two circumstances:
- Ingress session offer has precondition attributes and egress TG PP
SupportIfEgressIPTG is disabled. - Ingress session offer has no precondition attributes and egress TG PP
SupportIfEgressIPTG is enabled.
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Note Under any other circumstances, precondition interworking is not present and PP does not have any effect. |
Precondition Profile Flags Interaction and Outcome
Refer to the table below for flags interactions between the ingress and egress trunk groups. For a description of the flags, see SIP Trunk Group - Services - CLI section below.
Case 1:
Ingress session offer has precondition attributes and egress TG PP SupportIfEgressIPTG is disabled.
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| 0 | Table |
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| 1 | Precondition Profile Flags Interaction and Outcome |
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| Scenarios | Ingress Flags Assumptions | Egress Flags Assumptions | SBC Behavior |
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| 1 | IPTG Flags:
Preconditions: Transparent 100rel Support: Enabled IPSP PP flags: Preconditions Support If Egress IPTG: Disabled/Enabled Preconditions Strength Mandatory: Policy: Disabled Preconditions Strength Mandatory: Priority: 2 | IPTG Flags: Preconditions: Transparent 100rel Support: Enabled IPSP PP flags: Preconditions Support If Egress IPTG: Disabled Preconditions Strength Mandatory: Policy: Enabled Preconditions Strength Mandatory: Priority: 1 | As Egress Preconditions Strength Mandatory: Priority has highest value, its policy is considered Interworking is desired
| | 2 | IPTG Flags: Preconditions: Transparent 100rel Support: Enabled IPSP PP flags: Preconditions Support If Egress IPTG: Disabled/Enabled Preconditions Strength Mandatory: Policy: Disabled Preconditions Strength Mandatory: Priority: 1 | IPTG Flags: Preconditions: Transparent 100rel Support: Enabled IPSP PP flags: Preconditions Support If Egress IPTG: Disabled Preconditions Strength Mandatory: Policy: Enabled Preconditions Strength Mandatory: Priority: 2 | As Ingress Preconditions Strength Mandatory: Priority has highest value, its policy is considered Interworking is not desired. As Egress IPTG has preconditions: Transparent is configured, Preconditions Parameters is transparently sent to Egress | | 3 | IPTG Flags: Preconditions: Transparent 100rel Support: Enabled IPSP PP flags: Preconditions Support If Egress IPTG: Disabled/Enabled Preconditions Strength Mandatory: Policy: Disabled Preconditions Strength Mandatory: Priority: 1 | IPTG Flags: Preconditions: Transparent 100rel Support: Enabled IPSP PP flags: Preconditions Support If Egress IPTG: Disabled Preconditions Strength Mandatory: Policy: Enabled Preconditions Strength Mandatory: Priority: 1 | As priority is same Ingress Preconditions Strength is considered Interworking is not desired As Egress IPTG has preconditions: Transparent is configured, Preconditions Parameters is transparently sent to Egress |
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Priority 1 (lowest value) has the highest priority. |
Case 2:
Ingress session offer has no precondition attributes and egress TG PP “Preconditions Support If Egress IPTG” = Enabled.
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| 0 | Table |
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| 1 | Precondition Profile Flags Interaction and Outcome |
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| Scenarios | Ingress Flags Assumptions | Egress Flags Assumptions | SBC Behavior |
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| 1 | IPTG Flags: Preconditions: Transparent 100rel Support: Enabled IPSP PP flags: Preconditions Support If Egress IPTG: Disabled/Enabled Preconditions Strength Mandatory: Policy: Enabled/Disabled Preconditions Strength Mandatory: Priority: 2 Use UPDATE when Signaling Preconditions Locally Met: Policy:Enabled Use UPDATE when Signaling Preconditions Locally Met: Priority:2 | IPTG Flags: Preconditions: Transparent 100rel Support: Enabled IPSP PP flags: Preconditions Support If Egress IPTG: Enabled Preconditions Strength Mandatory: Policy: Enabled/Disabled Preconditions Strength Mandatory: Priority: 1 Use UPDATE when Signaling Preconditions Locally Met: Policy: Disabled Use UPDATE when Signaling Preconditions Locally Met: Priority: 1 | As Egress Preconditions Support If Egress IPTG: Enabled Irrespective of Preconditions Strength Mandatory: Policy/Priority Interworking is desired and Outgoing Offer will have preconditions with preconditions strength as "mandatory"
As Egress Use UPDATE when Signaling Preconditions Locally Met: Policy will be considered as its priority is more
As value is disabled in initial outgoing Invite local: met is sent | | 2 | IPTG Flags: Preconditions: Transparent 100rel Support: Enabled IPSP PP flags: Preconditions Support If Egress IPTG: Disabled/Enabled Preconditions Strength Mandatory: Policy: Enabled/Disabled Preconditions Strength Mandatory: Priority: 2 Use UPDATE when Signaling Preconditions Locally Met: Policy: Enabled Use UPDATE when Signaling Preconditions Locally Met: Priority:1 | IPTG Flags: Preconditions: Transparent 100rel Support: Enabled IPSP PP flags: Preconditions Support If Egress IPTG: Enabled Preconditions Strength Mandatory: Policy: Enabled/Disabled Preconditions Strength Mandatory: Priority: 1 Use UPDATE when Signaling Preconditions Locally Met: Policy: Disabled Use UPDATE when Signaling Preconditions Locally Met: Priority: 2 | As Egress Preconditions Support If Egress IPTG: Enabled Irrespective of Preconditions Strength Mandatory: Policy/Priority Interworking is desired and Outgoing Offer will have preconditions with preconditions strength as "mandatory"
As Ingress Use UPDATE when Signaling Preconditions Locally Met: Policy is considered as its priority is more
As value is Enabled in initial outgoing Invite local: not met is sent |
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- If both PP and the flag
preconditionIntwkUsing183 is configured, PP has a higher precedence. Settings of “preconditions=transparent”/”100rel support=yes” on both legs is the only supported configuration for proper precondition interworking functionality. - Preconditions interworking configured as “Use UPDATE”, user should configure Allow UPDATE flag as well
- Preconditions interworking configured as “Use UPDATE”, But 183 from peer does not have UPDATE in Allow header, SBC will not send UPDATE and does not take any other action either, so that call timeout timer fires to terminate the call.
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