]> Broadcom Communications

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Oracle

17210 Campbell Road Dallas Texas 75254 United States srdonovan@usdonovans.com

Oracle

17210 Campbell Road Dallas Texas 75254 United States ben@nostrum.com

Operations and Management Diameter Maintenance and Extensions (DIME) Internet-Draft Diameter Overload This specification documents a Diameter Overload Control (DOC) base solution and the dissemination of the overload report information. 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.

This specification defines a base solution for the Diameter Overload Control (DOC). The requirements for the solution are described and discussed in the corresponding design requirements document . Note that the overload control solution defined in this specification does not address all the requirements listed in . A number of overload control related features are left for the future specifications. See for more detailed discussion on those. The solution defined in this specification addresses the Diameter overload control between two endpoints (see ). Furthermore, the solution is designed to apply to existing and future Diameter applications, requires no changes to the Diameter base protocol and is deployable in environments where some Diameter nodes do not implement the Diameter overload control solution defined in this specification.

A set of Diameter servers that can handle any request for a given set of Diameter applications. While these servers support the same set of applications, they do not necessarily all have the same capacity. An individual server farm might also support a subset of the users for a Diameter Realm. [OpenIssue: Is a server farm assumed to support a single realm? That is, does it support a set of applications in a single realm?] A Server Front End (SFE) is a role that can be performed by a Diameter agent -- either a relay or a proxy -- that sits between Diameter clients and a Server Farm. An SFE can perform various functions for the server farm it sits in front of. This includes some or all of the following functions: Diameter Routing Diameter layer load balancing Load Management Overload Management Topology Hiding Server Farm Identity Management [OpenIssue: We used the concept of a server farm and SFE for internal discussions. Do we still need those concepts to explain the mechanism? It doesn't seem like we use them much.] Diameter Routing determines the destination of Diameter messages addressed to either a Diameter Realm and Application in general, or to a specific server using Destination-Host. This function is defined in . Application level routing specifications that expand on also exist. Diameter layer load balancing allows Diameter requests to be distributed across the set of servers. Definition of this function is outside the scope of this document. This functionality ensures that the consolidated load state for the server farm is collected, and processed. The exact algorithm for computing the load at the SFE is implementation specific but enough semantic of the conveyed load information needs to be specified so that deterministic behavior can be ensured. The SFE is the entity that understands the consolidated overload state for the server farm. Just as it is outside the scope of this document to specify how a Diameter server calculates its overload state, it is also outside the scope of this document to specify how an SFE calculates the overload state for the set of servers. This document describes how the SFE communicates Overload information to Diameter Clients. Topology Hiding is loosely defined as ensuring that no Diameter topology information about the server farm can be discovered from Diameter messages sent outside a predefined boundary (typically an administrative domain). This includes obfuscating identifiers and address information of Diameter entities in the server farm. It can also include hiding the number of various Diameter entities in the server farm. Identifying information can occur in many Diameter Attribute-Value Pairs (AVPs), including Origin-Host, Destination-Host, Route-Record, Proxy-Info, Session-ID and other AVPs. Server Farm Identity Management (SFIM) is a mechanism that can be used by the SFE to present a single Diameter identity that can be used by clients to send Diameter requests to the server farm. This requires that the SFE modifies Origin-Host information in answers coming from servers in the server farm. An agent that performs SFIM appears as a server from the client's perspective. Throttling is the reduction of the number of requests sent to an entity. Throttling can include a client dropping requests, or an agent rejecting requests with appropriate error responses. Clients and agents can also choose to redirect throttled requests to some other entity or entities capable of handling them. A Diameter node that generates an overload report. (This may or may not be the actually overloaded node.) A Diameter node that consumes and acts upon a report. Note that "act upon" does not necessarily mean the reacting node applies an abatement algorithm; it might decide to delegate that downstream, in which case it also becomes a "reporting node". Oveload Report.

This section describes the high-level architectural and semantic assumptions that underly the Diameter Overload Control Mechanism.

The following is a classification of Diameter applications and requests. This discussion is meant to document factors that play into decisions made by the Diameter identity responsible for handling overload reports. Section 8.1 of defines two state machines that imply two types of applications, session-less and session-based. The primary differentiator between these types of applications is the lifetime of Session-IDs. For session-based applications, the session-id is used to tie multiple requests into a single session. In session-less applications, the lifetime of the session-id is a single Diameter transaction. The 3GPP-defined S6a application is an example of a session-less application. The following, copied from section 7.1.4 of 29.272, explicitly states that sessions are implicitly terminated and that the server does not maintain session state: "Between the MME and the HSS and between the SGSN and the HSS and between the MME and the EIR, Diameter sessions shall be implicitly terminated. An implicitly terminated session is one for which the server does not maintain state information. The client shall not send any re-authorization or session termination requests to the server. The Diameter base protocol includes the Auth-Session-State AVP as the mechanism for the implementation of implicitly terminated sessions. The client (server) shall include in its requests (responses) the Auth-Session-State AVP set to the value NO_STATE_MAINTAINED (1), as described in . As a consequence, the server shall not maintain any state information about this session and the client shall not send any session termination request. Neither the Authorization-Lifetime AVP nor the Session-Timeout AVP shall be present in requests or responses." For the purposes of this discussion, session-less applications are further divided into two types of applications: Requests within a stateless application have no relationship to each other. The 3GPP defined S13 application is an example of a stateless application. While this class of application does not use the Diameter Session-ID AVP to correlate requests, there is an implied ordering of transactions defined by the application. The 3GPP defined Cx application [reference] is an example of a pseudo-session application. [OpenIssue: Do we assume that all requests in a pseudo-session typically need to go to the same server?] The accounting application defined in and the Credit-Control application defined in are examples of Diameter session-based applications. The handling of overload reports must take the type of application into consideration, as discussed in .

This section discusses considerations for mitigating overload reported by a Diameter entity. This discussion focuses on the type of application. discusses considerations for handling various request types when the target server is known to be in an overloaded state. discusses considerations for handling overload conditions based on the network deployment scenario. These discussions assume that the strategy for mitigating the reported overload is to reduce the overall workload sent to the overloaded entity. The concept of applying overload treatment to requests targeted for an overloaded Diameter entity is inherent to this discussion. The method used to reduce offered load is not specified here but could include routing requests to another Diameter entity known to be able to handle them, or it could mean rejecting certain requests. For a Diameter agent, rejecting requests will usually mean generating appropriate Diameter error responses. For a Diameter client, rejecting requests will depend upon the application. For example, it could mean giving an indication to the entity requesting the Diameter service that the network is busy and to try again later. By definition there is no relationship between individual requests in a stateless application. As a result, when a request is sent or relayed to an overloaded Diameter entity - either a Diameter Server or a Diameter Agent - the sending or relaying entity can choose to apply the overload treatment to any request targeted for the overloaded entity. Pseudo stateful applications are also stateless applications in that there is no session Diameter state maintained between transactions. There is, however, an implied ordering of requests. As a result, decisions about which transactions to reject as a result of an overloaded entity could take the command-code of the request into consideration. This generally means that transactions later in the sequence of transactions should be given more favorable treatment than messages earlier in the sequence. This is because more work has already been done by the Diameter network for those transactions that occur later in the sequence. Rejecting them could result in increasing the load on the network as the transactions earlier in the sequence might also need to be repeated. Overload handling for stateful applications must take into consideration the work associated with setting up an maintaining a session. As such, the entity handling overload of a Diameter entity for a stateful application might tend to reject new session requests before rejecting intra-session requests. In addition, session ending requests might be given a lower priority of being rejected as rejecting session ending requests could result in session status being out of sync between the Diameter clients and servers. Nodes that reject mid-session requests will need to consider whether the rejection invalidates the session, and any session clean-up that may be required.

An independent request is not a part of a Diameter session and, as such, the lifetime of the session-id is constrained to an individual transaction. A session-initiating request is the initial message that establishes a Diameter session. The ACR message defined in is an example of a session-initiating request. There are cases, most notably in the 3GPP PCC architecture, where multiple Diameter sessions are correlated and must be handled by the same Diameter server. This is a special case of a Session-Initiating Request. Gx CCR-I requests and Rx AAR messages are examples of correlated session-initiating requests. [OpenIssue: The previous paragraph needs references.] An intra session request is a request that uses a session-id for an already established request. An intra session request generally needs to be delivered to the server that handled the session creating request for the session. The STR message defined in is an example of an intra-session requests. CCR-U and CCR-T requests defined in are further examples of intra-session requests. Pseudo session requests are independent requests and, as such, the request transactions are not tied together using the Diameter session-id. There exist Diameter applications that define an expected ordering of transactions. This sequencing of independent transactions results in a pseudo session. The AIR, MAR and SAR requests in the 3GPP defined Cx application are examples of pseudo-session requests.

The request classes identified in have implications on decisions about which requests should be throttled first. Independent requests can be given equal treatment when making throttling decisions. Session-creating requests represent more work than independent or intra-session requests. As such, throttling decisions might favor intra-session requests over session-creating requests. Individual session-creating requests can be given equal treatment when making throttling decisions. A Request that results in a new binding, where the binding is used for routing of subsequent session-creating requests, represents more work than other requests. As such, these requests might be throttled more frequently than other request types. Throttling decisions for pseudo-session requests can take where individual requests fit into the overall sequence of requests within the pseudo session. Requests that are earlier in the sequence might be throttled more aggressively than requests that occur later in the sequence. There are two classes of intra-sessions requests. The first is a request that ends a session. The second is a request that is used to convey session related state between the Diameter client and server. Session ending request should be throttled less aggressively in order to keep session state consistent between the client and server, and possibly reduce the sessions impact on the overloaded entity. The default handling of other intra-session requests might be to treat them equally when making throttling decisions. There might also be application level considerations whether some request types are favored over others.

This section discusses various Diameter network deployment scenarios and the implications of those deployment models on handling of overload reports. The scenarios vary based on the following: The presence or absence of Diameter agents Which Diameter entities support the DOC extension The amount of the network topology understood by Diameter clients The complexity of the Diameter server deployment for a Diameter application Number of Diameter applications supported by Diameter clients and Diameter servers Without consideration for which elements support the DOC extension, the following is a representative list of deployment scenarios: Client --- Server Client --- Multiple equivalent servers Client --- Agent --- Multiple equivalent servers Client --- Agent [ --- Agent ] --- Partitioned server Client --- Edge Agent [ --- Edge Agent] --- { Multiple Equivalent Servers | Partitioned Servers } Client --- Session Correlating Agent --- Multiple Equivalent Servers [OpenIssue: Do the "multiple equivalent servers" cases change for session-stateful applications? Do we need to distinguish equivalence for session-initiation requests vs intra-session requests?] The following is a list of representative DOC deployment scenarios: Direct connection between a DOC client and a DOC server DOC client --- non-DOC agent --- DOC server DOC client --- DOC agent --- DOC server Non-DOC client --- DOC agent --- DOC server Non-DOC client --- DOC agent --- Mix of DOC and non-DOC servers DOC client --- agent --- Partitioned/Segmented DOC server DOC client --- agent --- agent --- Partitioned/Segmented DOC server DOC client --- edge agent --- edge agent --- DOC server [OpenIssue: In the last 3 list entries, are the agents DOC or non-DOC?]

In the context of the Diameter Overload Indication Conveyance (DOIC) and reacting to the overload information, the functional behaviour of Diameter agents in front of servers, especially Diameter proxies, needs to be common. This is important because agents may actively participate in the handling of an overload conditions. For example, they may make intelligent next hop selection decisions based on overload conditions, or aggregate overload information to be disseminated downstream. Diameter agents may have other deployment related tasks that are not defined in the Diameter base protocol . These include, among other tasks, topology hiding, and acting as a server front end for a server farm of real Diameter servers. Since the solution defined in this specification must not break the Diameter base protocol at any time, great care has to be taken not to assume functionality from the Diameter agents that would break base protocol behavior, or to assume agent functionality beyond the Diameter base protocol. Effectively this means the following from a Diameter agent: If a Diameter agent presents itself as the "end node", perhaps acting as an topology hiding SFE, the DOC mechanism MUST NOT leak information of the Diameter nodes behind it. From the Diameter client point of view the final destination to its requests and the original source for the answers MUST be the Diameter agent. This requirement means that such a Diameter agent acts as a back-to-back-agent for DOC purposes. How the agent in this case appears to the Diameter nodes it is representing (i.e. the real Diameter servers), is an implementation and a deployment specific within the realm the Diameter agent is deployed. This requirement also implies that if the Diameter agent does not impersonate the servers behind it, the Diameter dialogue is established between clients and servers and any overload information received by a client would be from a given server identified by the Origin-Host identity. [OpenIssue: We've discussed multiple situations where an agent might insert an OLR. I don't think we mean to force them to always perform topology hiding or SFIM in order to do so. We cannot assume that an OLR is always "from" or "about" the Origin-Host. Also, the section seems to assume that topology hiding agents act as b2b overload agents, but non-topology hiding agents never do. It don't think that's the right abstraction. It's possible that topology-hiding agents must do this, but I don't think we can preclude non-topology hiding agents from also doing it, at least some of the time.]

illustrates the simplified architecture for Diameter overload control. See for more discussion and details how different Diameter nodes fit into the architecture from the DOIC point of view.

<----------------------> +--^-----+ : (optional) : |Diameter| : : |Server A|--+ .--. : +---^----+ : .--. +--------+ | _( `. : |Diameter| : _( `. +---^----+ +--( )--:-| Agent |-:--( )--|Diameter| +--------+ | ( ` . ) ) : +-----^--+ : ( ` . ) ) | Client | |Diameter|--+ `--(___.-' : : `--(___.-' +-----^--+ |Server B| : : +---^----+ : : Overload Indication A Overload Indication A' 1) <----------------------> <----------------------> standard base protocol standard base protocol End-to-end Overload Indication 2) <-----------------------------------------------> standard base protocol ]]>

The following features describe new Diameter AVPs used for sending overload reports, and for declaring support for certain DOC features.

Since the Diameter overload control mechanism is also designed to work over existing application (i.e., the piggybacking principle), a proper negotiation is hard to accomplish. The "capability negotiation" is based on the existense of specific non-mandatory APV, such as the OC-Feature-Vector AVP (see . Although the OC-Feature-Vector AVP can be used to advertise a certain set of new or existing Diameter overload control capabilities, it is not a versioning solution per se, however, it can be used to achieve the same result.

The Diameter overload control APVs SHOULD always be sent as an optional AVPs. This requirement stems from the fact that piggybacking overload control information on top of existing application cannot really use AVPs with the M-bit set. However, there are certain exceptions as explained in . From the Diameter overload control functionality point of view, the "Reacting node" is always the requester of the overload report information and the "Reporting node" is the provider of the overload report. The overload report or the capability information in the request message is always interpreted as an announcement of a "capability". The overload report and the capability information in the answer is always interpreted as a report of supported commond functionality and as a status report of an overload condition (of a node).

Diameter nodes can request a reduction in offered load by indicating an overload condition in the form of an overload report. The overload report contains information about how much load should be reduced, and may contain other information about the overload condition. This information is encoded in Diameter Attribute Value Pairs (AVPs). Certain new AVPs may also be used to declare certain DOIC capabilities and extensions.

This section describes the encoding and semantics of Overload Indication Attribute Value Pairs (AVPs).

The OC-Feature-Vector AVP (AVP code TBD1) is type of Unsigned64 and contains a 64 bit flags field of announced capabilities of an overload control endpoint. Sending or receiving the OC-Feature-Vector AVP with the value 0 indicates that the endpoint only support the capabilities defined in this specification. An overload control endpoint (a reacting node) includes this AVP to indicate its capabilities to the other overload control endpoint (the reporting node). For example, the endpoint (reacting node) may indicate which (future defined) traffic abatement algorithms it supports in addition to the default. During the message exchange the overload control endpoints express their common set of supported capabilities. The endpoint sending a request (the reacting node) includes the OC-Feature-Vector AVP with those flags set that correspond what it supports. The endpoint that sends the answer (the reporting node) also includes the OC-Feature-Vector AVP with flags set to describe the capabilities it both supports and agrees with the request sender (e.g., based on the local policy and/or configuration). The answer sending endpoint (the reporting node) does not need to advertise those capabilities it is not going to use with the request sending endpoint (the reacting node). This specification does not define any additional capability flag. The implicity capability (all flags set to zero) indicates the support for this specification only.

The OC-OLR AVP (AVP code TBD2) is type of Grouped and contains the necessary information to convey an overload report. OC-OLR may also be used to convey additional information about an extension that is declared in the OC-Feature-Vector AVP. The OC-OLR AVP does not contain explicit information to which application it applies to and who inserted the AVP or whom the specific OC-OLR AVP concerns to. Both these information is implicitly learned from the encapsulating Diameter message/command. The application the OC-OLR AVP applies to is the same as the Application-Id found in the Diameter message header. The identity the OC-OLR AVP concerns is determined from the Origin-Host AVP found from the encapsulating Diameter command.

< TimeStamp > [ Reduction-Percentage ] [ ValidityDuration ] [ ReportType ] * [ AVP ] ]]>

The TimeStamp AVP indicates when the original OC-OLR AVP with the current content was created. It is possible to replay the same OC-OLR AVP multiple times between the overload endpoints, however, when the OC-OLR AVP content changes or the other information sending endpoint wants the receiving endpoint to update its overload control information, then the TimeStamp AVP MUST contain a new value. [OpenIssue: Is this necessarily a timestamp, or is it just a sequence number that can be implemented as a timestamp? Is this timestamp used to calculate expiration time? (propose no.). We should also consider whether either a timestamp or sequence number is needed for protection against replay attacks.]

The TimeStamp AVP (AVP code TBD3) is type of Time. Its usage in the context of the overload control is described in . From the functionality point of view, the TimeStamp AVP is merely used as a non-volatile increasing counter between two overload control endpoints.

The ValidityDuration AVP (AVP code TBD4) is type of Unsigned32 and describes the number of seconds the OC-OLR AVP and its content is valid since the creation of the OC-OLR AVP (as indicated by the TimeStamp AVP). A timeout of the overload report has specific concerns that need to be taken into account by the endpoint acting on the earlier received overload report(s). discusses the impacts of timeout in the scope of the traffic abatement algorithms. As a general guidance for implementations it is RECOMMENDED never to let any overload report to timeout. Rather, an overload endpoint should explicitly signal, e.g. the end of overload condition. This leaves no need for the other overload endpoint to reason or guess the condition the other endpoint is at.

The ReportType AVP (AVP code TBD5) is type of Enumerated. The value of the AVP describes what the overload report concerns. The following values are initially defined: Reserved. Destination-Host report. The overload treatment should apply to requests that the sender knows will reach the overloaded server. For example, requests with a Destination-Host AVP indicating the server. Realm (aggregated) report. The overload treatment should apply to all requests bound for the overloaded realm. The ReportType AVP is envisioned to be useful for situations where a reacting node needs to apply different overload treatments for different "types" of overload. For example, the reacting node(s) might need to throttle requests that are targeted to a specific server by the presence of a Destination-Host AVP than for requests that can be handled by any server in a realm. The example in illustrates this usage. [OpenIssue: There is an ongoing discussion about whether the ReportType AVP is the right way to solve that issue, and whether it's needed at all.]

The Reduction-Percentage AVP (AVP code TBD8) is type of Unsigned32 and describes the percentage of the traffic that the sender is requested to reduce, compared to what it otherwise would have sent. The value of the Reduction-Percentage AVP is between zero (0) and one hundred (100). Values greater than 100 are interpreted as 100. The value of 100 means that no traffic is expected, i.e. the sender of the information is under a severe load and ceases to process any new messages. The value of 0 means that the sender of the information is in a stable state and has no requests to the other endpoint to apply any traffic abatement. [Open Issue: We should consider an algorithm independent way to end an overload condition. Perhaps setting the validitytime to zero? Counter comment; since the abatement is based on a specific algorithm, it is natural to indicate that from the abatement algorithm point of view status quo has been reached.] If an overload control endpoint comes out of the 100 percent traffic reduction as a result of the overload report timing out, the following concerns are RECOMMENDED to be applied. The endpoint sending the traffic should be conservative and, for example, first send few "probe" messages to learn the overload condition of the other endpoint before converging to any traffic amount/rate decided by the sender. Similar concerns actually apply in all cases when the overload report times out unless the previous overload report stated 0 percent reduction. [Open Issue: It is still open whether we need an AVP to indicate the exact used traffic abatement algorithm. Currently it assumed that the reacting node is able to figure out what to do based on the Reducttion-Percentage AVP and possible other embedded information inside the OC-OLR AVP.]

The overload control solution can be considered as an overlay on top of an arbitrary Diameter network. The overload control information is exchanged over on a "DOIC association" between two communicatin endpoints. The endpoints, namely the "reacting node" and the "reporting node" do not need to be adjacent Diameter peer nodes, nor they need to be the end-to-end Diameter nodes in a typical "client-server" deployment with multiple intermediate Diameter agent nodes in between. The overload control endpoint are the two Diameter nodes that decide to exchange overload control information between each other. How the endpoints are determined is specific to a deployment, a Diameter node role in that deployment and local configuration. The following diagrams illustrate the concept of Diameter Overload End-Points and how they differ from the standard defined client, server and agent Diameter nodes. The following is the key to the elements in the diagrams: Diameter client as defined in . Diameter server as defined in . Diameter agent, in either a relay or proxy mode, as defined in . Diameter Overload End-Point as defined in this document. In the following figures a DEP may terminate two different DOIC associations being a reporter and reactor at the same time. A Diameter session as defined in . A DOIC association exists between two Diameter Overload End-Points. One of the end-points is the overload reporter and the other is the overload reactor. illustrates the most basic configuration where a client is connected directly to a server. In this case, the session and association are both between the client and server.

In there is an agent that is not participating directly in the exchange of overload reports. As a result, the DOIC association is still between the client and the server.

illustrates the case where the client does not support Diameter overload. In this case, the DOIC association is between the agent and the server. The agent handles the role of the reactor for overload reports generated by the server.

In there is a DOIC association between the client and the agent and a second DOIC association between the agent and the server. One use case requiring this configuration is when the agent is serving as a SFE/SFIM for a set of servers.

illustrates a deployment where some clients support Diameter overload control and some do not. In this case the agent must support Diameter overload control for the non supporting client. It might also need to have a DOIC association with the server, as shown here, to handle overload for a server farm and/or for managing Realm overload.

illustrates a deployment where some agents support Diameter overload control and others do not.

The overload control solution defined AVPs are essentially piggybacked on top of existing application message exchanges. This is made possible by adding overload control top level AVPs, the OC-OLR AVP and the OC-Feature-Vector AVP into existing commands (this has an assumption that the application CCF allows adding new AVPs into the Diameter messages. In a case of newly defined Diameter applications, it is RECOMMENDED to add and defined how overload control mechanisms works on that application. using OC-Feature-Vector and OC-OLR AVPs in a non-mandatory manner is intended only existing applications. Note that the overload control solution does not have fixed server and client roles. The endpoint role is determined based on the sent message type: whether the message is a request (i.e. sent by a "reacting node") or an answer (i.e. send by a "reporting node"). Therefore, in a typical "client-server" deployment, the "client" MAY report its overload condition to the "server" for any server initiated message exchange. An example of such is the server requesting a re-authentication from a client.

Since the overload control solution relies on the piggybacking principle for the overload reporting and the overload control endpoint are likely not adjacent peers, finding out whether the other endpoint supports the overload control or what is the common traffic abatement algorithm to apply for the traffic. The approach defined in this specification for the end-to-end capability capability announcement relies on the exchange of the OC-Feature-Vector between the endpoints. The feature announcement solution also works when carried out on existing applications. For the newly defined application the negotiation can be more exact based on the application specification. The announced set of capabilities MUST NOT change during the life time of the Diameter session (or transaction in a case of non-session maintaining applications).

The basic principle is that the request message initiating endpoint (i.e. the "reacting node") announces its support for the overload control mechanism by including in the request message the OC-Feature-Vector AVP with those capability flag bits set that it supports and is willing to use for this Diameter session (or transaction in a case of a non-session state maintaining applications). In a case of session maintaining applications the request message initiating endpoint does not need to do the capability announcement more than once for the lifetime of the Diameter session. In a case of non-session maintaining applications, it is RECOMMENDED that the request message initiating endpoint includes the capability announcement into every request regardless it has had prior message exchanges with the give remote endpoint. [OpenIssue: We need to think about the lifetime of a capabilities declaration. It's probably not the same as for a session. We have had proposals that the feature vector needs to go into every request sent by an OC node. For peer to peer cases, this can be associated with connection lifetime, but it's more complex for non-adjacent OC support.] Once the endpoint that initiated the request message receives an answer message from the remote endpoint, it can detect from the received answer message whether the remote endpoint supports the overload control solution and in a case it does, what features are supported. The support for the overload control solution is based on the presence of the OC-Feature-Vector AVP in the Diameter answer for existing application. For the newly defined applications the support for the overload control MAY already be part of the application specification. Based on capability knowledge the request message initiating endpoint can select the preferred common traffic abatement algorithm and act accordingly for the subsequent message exchanges.

When a remote endpoint (i.e. a "reporting node") receives a request message in can detect whether the request message initiating endpoint has support for the overload control solution based on the presence of the OC-Feature-Vector AVP. For the newly defined applications the overload control solution support can be part of the application specification. Based on the content of the OC-Feature-Vector AVP the request message receiving endpoint knows what overload control functionality the other endpoint supports and then act accordingly for the subsequent answer messages it initiates. It is RECOMMENDED that the answer message initiating endpoint selects one common traffic abatement algorithm even if it would support multiple. The answer message initiating endpoint MUST NOT include any overload control solution defined AVPs into its answer messages if the request message initiating endpoint has not indicated support at the beginning of the the created session (or transaction in a case of non-session state maintaining applications).

The overload control solution can be extended, e.g. with new traffic abatement algorithms or new functionality. The new features and algorithms MUST be registered with the IANA and for the ppossible use with the OC-Feature-Vector for announcing the support for the new features (see for the required procedures). It should be noted that defined Grouped AVP extension mechanisms also apply. This allows, for example, defining a new feature that is mandatory to understand even when piggybacked on an existing applications. More specifically, the sub-AVPs inside the OC-OLR AVP MAY have the M-bit set. However, when overload control AVPs are piggybacked on top of an existing applications, setting M-bit in sub-AVPs is NOT RECOMMENDED.

[OpenIssue: did we now agree that e.g. a server can refrain sending OLR in answers based on some magical algorithm? (Note: We seem to have consensus that a server MAY repeat OLRs in subsequent messages, but is not required to do so, based on local policy.)]

In order to reduce overload control introduced additional AVP and message processing it might be desirable/beneficial to signal whether the Diameter command carries overload control information that should be of interest of an overload aware Diameter node. Should such indication be include is not part of this specification. It has not either been concluded at what layer such possible indication should be. Obvious candidates include transport layer protocols (e.g., SCTP PPID or TCP flags) or Diameter command header flags.

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

Three new registries are needed under the 'Authentication, Authorization, and Accounting (AAA) Parameters' registry. defines a new "Overload Control Feature Vector" registry including the initial assignments. New values can be added into the registry using the Specification Required policy . defines a new "Overload Report Type" registry with its initial assignments. New types can be added using the Specification Required policy .

This mechanism gives Diameter nodes the ability to request that downstream nodes send fewer Diameter requests. Nodes do this by exchanging overload reports that directly affect this reduction. This exchange is potentially subject to multiple methods of attack, and has the potential to be used as a Denial-of-Service (DoS) attack vector. Overload reports may contain information about the topology and current status of a Diameter network. This information is potentially sensitive. Network operators may wish to control disclosure of overload reports to unauthorized parties to avoid its use for competitive intelligence or to target attacks. Diameter does not include features to provide end-to-end authentication, integrity protection, or confidentiality. This may cause complications when sending overload reports between non-adjacent nodes.

The Diameter protocol involves transactions in the form of requests and answers exchanged between clients and servers. These clients and servers may be peers, that is,they may share a direct transport (e.g. TCP or SCTP) connection, or the messages may traverse one or more intermediaries, known as Diameter Agents. Diameter nodes use TLS, DTLS, or IPSec to authenticate peers, and to provide confidentiality and integrity protection of traffic between peers. Nodes can make authorization decisions based on the peer identities authenticated at the transport layer. When agents are involved, this presents an effectively hop-by-hop trust model. That is, a Diameter client or server can authorize an agent for certain actions, but it must trust that agent to make appropriate authorization decisions about its peers, and so on. Since confidentiality and integrity protection occurs at the transport layer. Agents can read, and perhaps modify, any part of a Diameter message, including an overload report. There are several ways an attacker might attempt to exploit the overload control mechanism. An unauthorized third party might inject an overload report into the network. If this third party is upstream of an agent, and that agent fails to apply proper authorization policies, downstream nodes may mistakenly trust the report. This attack is at least partially mitigated by the assumption that nodes include overload reports in Diameter answers but not in requests. This requires an attacker to have knowledge of the original request in order to construct a response. Therefore, implementations SHOULD validate that an answer containing an overload report is a properly constructed response to a pending request prior to acting on the overload report. A similar attack involves an otherwise authorized Diameter node that sends an inappropriate overload report. For example, a server for the realm "example.com" might send an overload report indicating that a competitor's realm "example.net" is overloaded. If other nodes act on the report, they may falsely believe that "example.net" is overloaded, effectively reducing that realm's capacity. Therefore, it's critical that nodes validate that an overload report received from a peer actually falls within that peer's responsibility before acting on the report or forwarding the report to other peers. For example, an overload report from an peer that applies to a realm not handled by that peer is suspect. An attacker might use the information in an overload report to assist in certain attacks. For example, an attacker could use information about current overload conditions to time a DoS attack for maximum effect, or use subsequent overload reports as a feedback mechanism to learn the results of a previous or ongoing attack.

Diameter overload reports can cause a node to cease sending some or all Diameter requests for an extended period. This makes them a tempting vector for DoS tacks. Furthermore, since Diameter is almost always used in support of other protocols, a DoS attack on Diameter is likely to impact those protocols as well. Therefore, Diameter nodes MUST NOT honor or forward overload reports from unauthorized or otherwise untrusted sources.

When a Diameter node sends an overload report, it cannot assume that all nodes will comply. A non-compliant node might continue to send requests with no reduction in load. Requirement 28 indicates that the overload control solution cannot assume that all Diameter nodes in a network are necessarily trusted, and that malicious nodes not be allowed to take advantage of the overload control mechanism to get more than their fair share of service. In the absence of an overload control mechanism, Diameter nodes need to implement strategies to protect themselves from floods of requests, and to make sure that a disproportionate load from one source does not prevent other sources from receiving service. For example, a Diameter server might reject a certain percentage of requests from sources that exceed certain limits. Overload control can be thought of as an optimization for such strategies, where downstream nodes never send the excess requests in the first place. However, the presence of an overload control mechanism does not remove the need for these other protection strategies.

The lack of end-to-end security features makes it far more difficult to establish trust in overload reports that originate from non-adjacent nodes. Any agents in the message path may insert or modify overload reports. Nodes must trust that their adjacent peers perform proper checks on overload reports from their peers, and so on, creating a transitive-trust requirement extending for potentially long chains of nodes. Network operators must determine if this transitive trust requirement is acceptable for their deployments. Nodes supporting Diameter overload control MUST give operators the ability to select which peers are trusted to deliver overload reports, and whether they are trusted to forward overload reports from non-adjacent nodes. [OpenIssue: This requires that a responding node be able to tell a peer-generated OLR from one generated by a non-adjacent node. One way of doing this would be to include the identity of the node that generated the report as part of the OLR] [OpenIssue: Do we need further language about what rules an agent should apply before forwarding an OLR?] The lack of end-to-end protection creates a tension between two requirements from the overload control requirements document. Requirement 34 requires the ability to send overload reports across intermediaries (i.e. agents) that do not support overload control mechanism. Requirement 27 forbids the mechanism from adding new vulnerabilities or increasing the severity of existing ones. A non-supporting agent will most likely forward overload reports without inspecting them or applying any sort of validation or authorization. This makes the transitive trust issue considerably more of a problem. Without the ability to authenticate and integrity protect overload reports across a non-supporting agent, the mechanism cannot comply with both requirements. [OpenIssue: What do we want to do about this? Req27 is a normative MUST, while Req34 is "merely" a SHOULD. This would seem to imply that 27 has to take precedent. Can we say that overload reports MUST NOT be sent to and/or accepted from non-supporting agents until such time we can use end-to-end security?] The lack of end-to-end confidentiality protection means that any Diameter agent in the path of an overload report can view the contents of that report. In addition to the requirement to select which peers are trusted to send overload reports, operators MUST be able to select which peers are authorized to receive reports. A node MUST not send an overload report to a peer not authorized to receive it. Furthermore, an agent MUST remove any overload reports that might have been inserted by other nodes before forwarding a Diameter message to a peer that is not authorized to receive overload reports. At the time of this writing, the DIME working group is studying requirements for adding end-to-end security features to Diameter. These features, when they become available, might make it easier to establish trust in non-adjacent nodes for overload control purposes. Readers should be reminded, however, that the overload control mechanism encourages Diameter agents to modify AVPs in, or insert additional AVPs into, existing messages that are originated by other nodes. If end-to-end security is enabled, there is a risk that such modification could violate integrity protection. The details of using any future Diameter end-to-end security mechanism with overload control will require careful consideration, and are beyond the scope of this document.

The following people contributed substantial ideas, feedback, and discussion to this document: Eric McMurry Hannes Tschofenig Ulrich Wiehe Jean-Jacques Trottin Lionel Morand Maria Cruz Bartolome Martin Dolly Nirav Salot Susan Shishufeng

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&RFC2119; &RFC6733; &RFC5226; &I-D.ietf-dime-overload-reqs; &RFC4006; &I-D.ietf-dime-e2e-sec-req;

The base solution for the overload control does not cover all possible use cases. A number of solution aspects were intentionally left for future specification and protocol work.

This specification describes only means for a simple loss based algorithm. Future algorithms can be added using the designed solution extension mechanism. The new algorithms need to be registered with IANA. See Sections and for the required IANA steps.

This specification focuses on Diameter end-point (server or client) overload. A separate extension will be required to outline the handling the case of agent overload.

The current behaviour in a case of DIAMETER_TOO_BUSY is somewhat underspecified. For example, there is no information how long the specific Diameter node is willing to be unavailable. A specification updating should clarify the handling of DIAMETER_TOO_BUSY from the error answer initiating Diameter node point of view and from the original request initiating Diameter node point of view. Further, the inclusion of possible additional information providing APVs should be discussed and possible be recommended to be used.

The following section analyses, which Diameter Overload Control requirements are met by this specification. Key: S - Supported P - Partial N - Not supported Rqmt # S/ P/ N Notes REQ 1 P The DOIC solution only addresses overload information. Load information is left as future work. In addition, the DOIC solution does not address agent overload scenarios. - REQ 2 P The DOIC solution supports overload reports that implicitly indicate the application impacted by the report. The DOIC solution does not support reporting load information. The DOIC solution is thought to support graceful behavior. Allowing an application specific capabilities negotiation mechanism violates application-independence. Suggested different wording: The DOIC solution supports overload reports that are applicable to any Diameter application. The DOIC solution does not support reporting load information. The DOIC solution allows to support graceful behavior; this will be enhanced when the Load information will be defined. Comment: Can we removed the words "is thought"? - REQ 3 S The DOIC solution is thought to address this requirement. Comment: Can we removed the words "is thought"? - REQ 4 P The DOIC solution does allow for both both a Diameter server and a Diameter client to send overload reports. The DOIC solution only addresses Diameter end-point (server and client) overload. Agent overload is being addressed in a separate draft. - REQ 5 S The DOIC solution does not depend on how the end-points are discovered. Comment: it might be worth working through at least one use case showing DNS based dynamic peer discovery to make sure we haven’t missed anything. - REQ 6 ? Need to update text as some configuation is required. Need to determin if the current discussion on overload application id increases the amount of configuration which would change this to a N. - REQ 7 S The DOIC solution supports the loss algorithm, which is expected to address this requirement. There is concern about the ability to address oscillations. Wording is included for how a reacting node starts to increase traffic after an overload report expires to address this concern. Suggested different wording: The DOIC solution supports a baseline mechanism relying on traffic reduction percentage that is a loss algorithm, which allows to address this requirement. Oscillations are avoided or quite minimized by sending successive OLR reports with the values to converge to the optimal traffic or to smoothly come back to normal traffic conditions when overload decreases and ends. - REQ 8 ? The DOIC solution supports a timestamp which is meant to serve as a report version indication to address this requirement. Comment: The use of the timestamp is under discussion. - REQ 9 ? The DOIC solution uses a piggybacking strategy for carrying overload reports, which scales lineraly with the amount of traffic. As such, the first part of the requirement is addressed. The DOIC solution does not support a mechanism for sending overload reports over a quiescent transport connections or, more generally, to Diameter nodes that are not producing traffic. Suggested different wording: The DOIC solution uses a piggybacking strategy for carrying overload reports. As such, the first part of the requirement is addressed. For a connection that has become quiescent due to OLRs with a 100% traffic reduction, the validity timer allows to handle this case. Other cases of quiescent connections are outside the scope of Diameter overload (e.g. their handling may be done through the watch dog of the Diameter base protocol). - REQ 10 S The DOIC solution supports two methods for managing the length of an overload condition. First, all overload reports must contain a duration indication, after which the node reacting to the report can consider the overload condition as ended. Secondly, the solution supports the method for the node originating the overload report to explicitly communicate that the condition has ended. This latter mechanism depends on traffic to be sent from the reacting node and, as such, can not be depended upon in all circumstances. - REQ 11 ? The DOIC solution works well for small network configurations and for network configurations with a single Diameter agent hop. More analysis is required to determine how well the DOIC solution handles very large Diameter network with partitioned or segmented server farms requiring multiple hops through Diameter agents. - REQ 12 P The DOIC solution focuses on Diameter end-point overload and meets this requirement for those Diameter nodes. The DOIC solution does not address Diameter Agent overload and does not meet this requirement for those Diameter nodes. - REQ 13 ? The DOIC solution requires including of the overload report in all answer messages in some situations. It is not agreed, however, that this constitutes substantial work. This can also be mitigated by the sender of the overload report keeping state to record who has received overload reports. It is left to implementation decisions as to which approach is taken -- send in all messages or send once with a record of who has received the report. Another way is to let the request sender (reacting node) insert information in the request to say whether a throttling is actually performed. The reporting node then can base its decision on information received in the request; no need for keeping state to record who has received overload reports. The DOIC solution also requires capabilities negotiation in every request and response message, which increases the baseline work required for any node supporting the DOIC solution. Suggested additional text: It does not, however, require that the information be recalculated or updated with each message. The update frequency is up to the implementation, and each implementation can make decisions on balancing the update of overload information along with its other priorities. It is expected that using a periodically updated OLR report added to all messages sent to overload control endpoints will not add substantial additional work. Piggyback base transport also does not require composition, sending, or parsing of new Diameter messages for the purpose of conveying overload control information. There is still discussion on the substantial additional work due to have OLR in each answer message. - REQ 14 S The DOIC solution uses the piggybacking method to deliver overload report, which scales lineraly with the amount of traffic. This allows for immediate feedback to any node generating traffic toward another overloaded node. - REQ 15 S The DOIC solution does not interfere with transport protocols. - REQ 16 ? The DOIC solution allows for a mixed network of supporting and non supporting Diameter end-points. It isn't clear how realm overload is handled in a network with agents that do not support the DOIC solution. Suggested additional wording: Evaluation of Realm overload may require a DA supporting DOIC, if the realm overload is not evaluated by the client. Realm overload handling is still under discussion. - REQ 17 ? Suggested wording: The DOIC solution addresses this requirement through the loss algorithm (DOIC baseline mechanism) with the following possibilities. A DA supporting DOIC can act on behalf of clients not supporting DOIC. A reporting node is also aware of the nodes not supporting the DOIC as there is no advertisement of the DOIC support. It may then apply a particular throttling of the requests coming from these non supporting DOIC clients. - REQ 18 ? It isn't clear yet that if this requirement is addressed. There has been a proposal to mark messages that survived overload throttling as one method for an overloaded node to address fairness but this proposal is not yet part of the solution. It is also possible that the overloaded node could use state gathered as part of the capability advertisement mechanism to know if the sending node supports the DOIC solution and if not, to apply a particular throttling of the requests coming from these non supporting DOIC clients. - REQ 19 S The DOIC solution supports the ability for the overloaded node and the reacting node to be in different administrative domains. - REQ 20 ? This mechanism is still under discussion. Comment 1: I think this is a “S”. OLRs are clearly distinguishable from any error code. The fact that an agent would need to send errors if it throttles is not an overload indication per se. It needs to do that even without DoC. OTOH, if we apply some DOC related fix to TOO_BUSY, we probably need a new code. Comment 2: New AVPs conveys overload control information, and this is transported on existing answer messages, so distinguishable from Diameter errors. - REQ 21 S The inability for a node to send overload reports will result in equivalent through put to a network that does not support the DOIC solution. - REQ 22 S The DOIC solution gives this node generating the overload report the ability to control the amount of throttling done by the reacting node using the reduction percentage parameter in the overload report. - REQ 23 ? Initial text: The DOIC mechanism supports two abatement strategies by reacting nodes, routing to an alternative node or dropping traffic. The routing to an alternative node will be enhanced when the Load extension is defined. Comment: This is a N. There’s no good way to determine which nodes are likely to have sufficient capacity without some sort of load metric for non-overloaded nodes. - REQ 24 N The DOIC solution does not address delivering load information. - REQ 25 S The DOIC solution contains some guideance. - REQ 26 S The DOIC solution does not constrain a nodes ability to determine which requests are trottled. - REQ 27 ? Initial text: The DOIC solution does add a new line of attack in the ability for a malicious entity to insert overload reports that would reduce or eliminate traffic. This, however, is no worse than an attacker that would assert erroneous error responses such as a TOO BUSY response. It is recognized that the end-to-end security solution currently being worked on by the DIME working group is needed to close these types of vulurabilities. Comment: Sending a malicious OLR with a type of “realm” will have considerably more impact than a TOO_BUSY. Personally, I don’t think we can achieve this requirement without either being hop-by-hop or requiring e2e security. We probably need further analysis of the security implications of the capabilities negotiation as well. Suggested additional verbage: An OLR only relates to the traffic between a reporting node and a reacting node and can effectively block the traffic from a client which would be an important impact. Nevertheless OLRs are regularly sent in all answers, so a malicious OLR will have a short transient effect, as quickly overridden by a new OLR. To have a significant impact would require a continuous flow of answers with malicious OLRs. There is the exception of the OLR with a value of 100% reduction traffic which has a higher vulnerability and the use of which should be avoided when possible. In addition such malicious OLRs must be in answers, which means the capability to insert the malicious OLR in an existing answer rather than to create an answer which is much less easy than to create a request. To have a network wide applicability would request to generate malicious OLRs messages towards all reacting nodes. It can be considered that the baseline mechanism offer a relevant level of security. Further analysis with a security expertise would be beneficial. - REQ 28 ? See REQ 18 and REQ 27. Suggested additional verbage: Guidance may be provided for detection of non compliant/abnormal use of OLRs, not only by endpoints but also by intermediate DA that can be aware of OLRs, an example being edge DAs with external networks. Further analysis with a security expertise would be beneficial. - REQ 29 ? This requirement is not explicitly addressed by the DOIC solution. There is nothing in the DOIC solution that would prevent the goals of this requirement from being achieved. Non-adjacent DOIC without e2e security could be an issue here. - REQ 30 ? It isn't clear how a solution would interfere. Suggested wording: A node can have methods on how to protect from overload from nodes non supporting DOIC. The DOIC mechanism used with DOIC supporting nodes will not interfere with the appliance of these methods. There is the remark that the use of these methods may impact the global overload of the node and the evaluation of the traffic reduction that the reporting node will send in OLRs. If a node has methods to protect against denial of service attacks, the use of DOIC will not interfere with them. A denial of service attack concerning the DOIC itself is addressed in REQ 27. - REQ 31 ? Initial text with an S: The DOIC solution addresses node and realm directly. The application to which a report applies is implicitly determined based on the application level message carrying the report. Note that there is no way with DOIC for an overloaded node to communicate multiple nodes, realms or applications in a single overload report. So the inverse of this requirement is not supported. Comment: The inverse is also not _required_ :-) But I think we are "P" here, in that we don’t support “node” per se. we do support “server.” “Node” includes agents. (I also interpreted this to mean that each granularity needed to be supported independently—that is, a potential to say “all traffic to a realm” or “all traffic to a host” independently of application.) - REQ 32 ? Initial text with an S: The DOIC solution supports extensibility of both the information communicated and in the definition of new overload abatement algorithms. Comment 1: Recent discussions have made this a ?. It can be changed to S/N/P once these discussions come to a conclusion and new text is added to the draft. Comment 2: Suggested wording - The DOIC solution supports extensibility of both the information communicated and in the definition of new overload abatement algorithms or strategies. It should be noted that, according to the applications or to reacting node implementations, many algorithms may be applied on top of the DOIC baseline solution (without contradicting it), e.g. regarding which type of request to throttle, prioritized messages handling, mapping of the reduction % to an internal algorithm (eg 1 message out of ten etc..) but such algorithms are out of scope of DOIC. - REQ 33 ? Initial text with P: The DOIC solution currently defines the loss algorithm as the default algorithm. It does not specify it as mandatory to implement. Comment 1: Then I think that’s a “n”. The MTI part is the crux of the requirement. Comment 2: Suggested wording: In the DOIC baseline solution, the reacting node has to apply the received Reduction-Percentage, and for achieving this, the reacting node can do requests rerouting (when it is possible) or drop/reject requests. This DOIC baseline solution is a loss algorithm and DOIC should not require further specification. The answer to REQ32 indicates the possibility to add other algorithms on top of the DOIC baseline solution. The DOIC solution currently defines this loss algorithm as the default algorithm. It is still under discussion to make it as mandatory to implement. - REQ 34 P The ability to communicate overload reports between supporting Diameter nodes does not require agents to support the DOIC solution. Load information exchange is not currently defined.

[TBD: Would cover S6a MME-HSS communication with several topology choices (such as with or without DRA, and with "generic" agents).]

[TBD: Would cover Gx/Rx and maybe S9..]

[TBD: Add example showing the use of Destination-Host type OLRs and Realm type OLRs.]