Diameter Routing Message Priority
draft-donovan-dime-drmp-00.txt

Abstract

When making routing and resource allocation decisions, Diameter nodes currently have no generic mechanism to determine the relative priority of Diameter requests. This document defines a mechanism to allow Diameter endpoints to indicate the relative priority of Diameter requests/transactions/messages. With this information Diameter nodes can factor the relative priority of requests/transactions/messages into routing, resource allocation and overload abatement decisions.

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Table of Contents

• 1. Introduction
• 2. Terminology and Abbreviations
• 3. Conventions Used in This Document
• 4. Problem Statement
• 5. Use Cases
• 5.1. First Responder Related Signaling
• 5.2. Emergency Call Related Signaling
• 5.3. Differentiated Services
• 5.4. Application Specific Priorities
• 6. Design Considerations and Questions
• 6.1. Relationship with SIP Resource Priority
• 6.2. Priority Encoding Method
• 6.3. Base Protocol versus Application Extension
• 6.4. Scope of Priority Setting
• 7. Theory of Operation
• 8. Normative Behavior
• 9. Attribute Value Pairs
• 10. IANA Considerations
• 10.1. AVP codes
• 10.2. New registries
• 11. Security Considerations
• 12. References
• 12.1. Normative References
• 12.2. Informative References
• Author's Address

1. Introduction

The DOIC solution [I-D.ietf-dime-ovli] for Diameter overload control introduces scenarios where Diameter routing decisions made by Diameter nodes can be influenced by the overload state of other Diameter nodes. This includes the scenarios where Diameter endpoints and Diameter agents can throttle requests as a result of the target for the request being overloaded.

With currently available mechanisms these Diameter nodes do not have a clean mechanism to differentiate request message priorities when making these throttling decisions. As such, all requests are treated the same meaning that all requests have the same probability of being throttled.

There are scenarios where treating all requests the same can cause issues. For instance it might be considered important to reduce the probability of transactions involving first responders during a period of heavy signaling resulting from a natural disaster being throttled during overload scenarios.

This document defines a mechanism that allows Diameter nodes to indicate the relative priority of Diameter requests/transactions/messages. With this information other Diameter nodes can factor the relative priority of requests into routing and throttling decisions.

• Editor's Note: It is an open decision as to whether priority applies to a request, transaction or message. Once this decision is made the above construct of "requests/transactions/messages" can be change to reflect the decision.

2. Terminology and Abbreviations

Abatement

As defined in [I-D.ietf-dime-ovli]. Reaction to receipt of an overload report resulting in a reduction in traffic sent to the reporting node. Abatement actions include diversion and throttling.

Diversion

As defined in [I-D.ietf-dime-ovli]. An overload abatement treatment where the reacting node selects alternate destinations or paths for requests.

DOIC

Diameter Overload Indication Conveyance.

DRMP

Diameter Routing Message Priority.

Priority Name Space

An identifier for a group of priority values.

Priority

The relative importance of a Diameter message. A higher priority value implies a higher relative importance of the message.

Throttling

As defined in [I-D.ietf-dime-ovli]. An abatement treatment that limits the number of requests sent by the DIOC reacting node. Throttling can include a Diameter Client choosing to not send requests, or a Diameter Agent or Server rejecting requests with appropriate error responses. In both cases the result of the throttling is a permanent rejection of the transaction.

3. Conventions Used in This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

RFC 2119 [RFC2119] interpretation does not apply for the above listed words when they are not used in all-caps format.

4. Problem Statement

With the introduction of overload control mechanisms, Diameter nodes will be required to make decisions regarding which Diameter request messages should be throttled as a result of overloaded Diameter nodes.

There is currently no generic mechanism to indicate which request messages should be given preferential treatment when these throttling decisions are made.

As a result, all messages are by definition treated equally and, as such, have an equal probability of being throttled.

There are a number of scenarios where it is appropriate for an application to mark a request as being of a higher priority than other application requests. These are discussed in the next section.

This document defines a mechanism for applications to indicate priority for individual requests/messages/transactions, reducing the probability of those requests/messages/transactions being throttled if there are other lower priority transactions that are eligible for being throttled.

While the primary usage of DRMP defined priorities is for input to Diameter overload control related throttling decisions, it is also expected that the priority information could also be used for other routing related functionality. This might include giving higher priority requests/messages/transactions preferential treatment when selecting routes.

It is also envisioned that DRMP priority information could be used by Diameter endpoints to make resource allocation decisions. For instance, a Diameter Server might choose to use the priority information to treat higher priority requests ahead of lower priority requests.

• Note: There are a number of application specific definitions indicating various views of application level priority for different requests. Using these application specific priority AVPs as input to throttling and other Diameter routing decisions would require Diameter agents to understand all applications and do application specific parsing of all messages in order to determine the priority of individual messages. This is considered an unacceptable level of complexity to put on elements whose primary responsibility is to route Diameter messages.

5. Use Cases

This section discussed various scenarios where Diameter requests/messages/transactions can benefit from the use of priority information.

5.1. First Responder Related Signaling

Natural disasters can be a considerable increase in usage of network resources. This can be made worse if the disaster results in a loss of network capacity.

The combination of added load and reduced capacity can lead to Diameter nodes becoming overloaded and, as a result, the use of DOIC mechanisms to request a reduction in traffic. This in turn results in requests being throttled in an attempt to control the overload scenario and prevent the overloaded node from failing.

There is the need for first responders and other individuals responsible for handling the after effects of the disaster to be assured that they can gain access to the network resources in order to communicate both between themselves and with other network resources.

Signaling associated with first responders needs to be given a higher priority to help ensure they can most effectively do their job.

The United States Wireless Priority Services (WPS) and Government Emergency Telecommunications Service (GETS) are examples of systems designed to address these first responder needs.

5.2. Emergency Call Related Signaling

Similar to the first responder scenario, there is also signaling associated with emergency calls. Given the critical nature of these emergency calls, this signaling should also be given preferential treatment when possible.

5.3. Differentiated Services

Operators may desire to differentiate network-based services by providing a service level agreement that includes preferential Diameter routing behavior. This might be modeled as Platinum, Gold and Silver levels of service.

In this scenario an operator might offer a Platinum SLA the includes ensuring that all signaling for a customer who purchases the Platinum service being marked as having a higher priority than signaling associated with Gold and Silver customers.

5.4. Application Specific Priorities

There are scenarios within Diameter applications where it might be appropriate to give a subset of the transactions for the application a higher priority than other transactions for that application.

For instance, when there is a series of transactions required for a user to gain access to network services, it might be appropriate to mark transactions that occur later in the series at a higher priority than those that occur early in the series. This would recognize that there was potentially significant work done by the network already that would be lost if those later transactions were throttled.

There are also scenarios where an agent cannot easily differentiate a request that starts a session from requests that update or end sessions. In these scenarios it might be appropriate to mark the requests that establish new sessions with a lower priority than updates and session ending requests. This also recognizes that more work has already taken place for established sessions and, as a result, it might be more harmful if the session update and session ending requests were to be throttled.

There are also scenarios where the priority of requests for individual command codes within an application depends on the context that exists when the request is sent. There isn't always information in the message from which this context can be determined by Diameter nodes other than the node that originates the request.

This is similar to the scenario where a series of requests are needed to access a network service. It is different in that the series of requests involve different application command-codes. In this scenario it is requests with the same command-code that have different implied priorities.

• One example of this is in the 3GPP S6a application [S6a] where a ULR request resulting from an MME restoration procedure might be given a higher priority than a ULR resulting from an initial attach.

6. Design Considerations and Questions

This section contains a list of questions that will influence the design of the DRMP mechanism. It is expected that this section will be removed once the DRMP mechanism is defined.

6.1. Relationship with SIP Resource Priority

Question 1: Is there value with aligning the Diameter Routing Message Priority design with the SIP Resource Priority [RFC4412]work?

Current thoughts: It is worth investigating if there is synergy between the work that has already occurred for the SIP protocol to see if it can be leveraged for DRMP.

Question 2: If so, is there value in reusing the existing SIP Resource Priority name spaces and request handling strategies?

Current thoughts: This depends on the answer to question 1.

6.2. Priority Encoding Method

Question 3: Is there a preference for handling DRMP by introducing AVPs or by using existing bits in the Diameter Command Flags field?

Current thoughts: The advantage of using bits in the Command Flags field is that it would reduce parsing overhead for elements that need access to the routing priority information. The question is whether this optimization in parsing overhead is worth the expense of using the reserved bits.

There are four bits remaining in the Command Flags header. If this approach is taken then the expectation would be that three of the bits would be used, allowing for eight priority levels.

This approach has questionable utility if multiple namespaces are to be used as the namespace identity would still require an AVP. Once the requirement for parsing the namespace AVP is introduced the incremental savings from utilizing the Command Flags would be minimal.

6.3. Base Protocol versus Application Extension

Question 4: Should DRMP be base protocol behavior or should Diameter applications be required to explicitly incorporate DRMP behavior?

Current Thoughts: If Command Flags are used then it is by definition a base Diameter protocol capability. If AVPs are used then it can be base protocol behavior by defining the DRMP AVPs as routing AVPs, in the same fashion as Destination-Host, Origin-Host, Destination-Realm and Origin-Realm.

It is the current thought of the author that this should be part of the base Diameter protocol and not application specific behavior.

6.4. Scope of Priority Setting

Question 5: Which of the following does the DRMP priority apply to:

• Messages - meaning that a separate priority can be set for request messages and answer messages?
• Transactions - meaning that the priority set in the request message also applies to the answer messages?
• Request messages - meaning that answer message priority always has an implied higher priority than all request messages?

Current thoughts: The most straightforward approach is to have the explicitly set priority apply only to request messages with answer messages having an implied higher priority than request messages.

The question is whether this meets all use cases.

7. Theory of Operation

This section outlines the envisioned usage of DRMP.

The expected behavior depends on the role of the Diameter node handling the request.

The following behavior is expected during the flow of a Diameter transaction.

• Note: It is currently TBD if the priority applies only to the request, applies to the transaction or applies to individual messages.
• This decision impacts answer handling the most.
• If the priority applies to the transaction then routing of answer messages can be impacted by the priority but there is no need for answer messages to carry priority information as it can be saved in transaction state by all nodes handling the transaction.
• If the priority applies to individual messages then a request message and an answer message in the same transaction can have different priorities. In this case the answer message will need to include priority information.
• If the priority applies only to request messages then the assumption is that answer messages are a higher priority than all request messages and there would be no need to include priority information in the answer message.
• The current wording in this section assumes that priority applies to request messages.

1. Request sender - The sender of a request, be it a Diameter Client or a Diameter Server, determines the relative priority of the request and includes that priority in the request. The method for determining the relative priority is application specific and is outside the scope of this specification.
2. Agents handing the request - Agents use the priority information when making routing decisions. This can include determining which requests to route first, which requests to throttle and where the request is routed. For instance, requests with higher priority might have a lower probability of being throttled. The mechanism for how the agent determines which requests are throttled is implementation dependent and is outside the scope of this document.
3. Request handler - The handler of the request, be it a Diameter Server or a Diameter Client, can use the priority information to determine how to handle the request. This could include determining the order in which requests are handled and resources that are applied to handling of the request.
4. Answer sender - The handler of the request is also the sender of the answer. The answer sender uses normal procedures to send the answer.
   
   Note: The assumption with this flow is that the DRMP message priority only applies to request messages and that answer messages have an implied higher priority.
5. Agent handling the answer - Agents handling answer messages give answer messages an implied higher priority than all request messages.
6. Answer handler - The handler of the answer message does not change behavior based on the DRMP priority setting.

8. Normative Behavior

TBD

9. Attribute Value Pairs

TBD.

10. IANA Considerations

10.1. AVP codes

New AVPs defined by this specification are listed in Section 9. All AVP codes are allocated from the 'Authentication, Authorization, and Accounting (AAA) Parameters' AVP Codes registry.

10.2. New registries

TBD

11. Security Considerations

TBD

12. References

12.1. Normative References

12.2. Informative References

Author's Address