Diameter Maintenance and Extensions                     J. Korhonen, Ed.
(DIME)                                                          Broadcom
Internet-Draft                                                S. Donovan
Intended status: Standards Track                             B. Campbell
Expires: June 20, 2014                                            Oracle
                                                               L. Morand
                                                             Orange Labs
                                                       December 17, 2013

                Diameter Overload Indication Conveyance


   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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 20, 2014.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology and Abbreviations  . . . . . . . . . . . . . . . .  4
   3.  Solution Overview  . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Architectural Assumptions  . . . . . . . . . . . . . . . .  5
       3.1.1.  Application Classification . . . . . . . . . . . . . .  5
       3.1.2.  Application Type Overload Implications . . . . . . . .  6
       3.1.3.  Request Transaction Classification . . . . . . . . . .  8
       3.1.4.  Request Type Overload Implications . . . . . . . . . .  9
       3.1.5.  Diameter Agent Behaviour . . . . . . . . . . . . . . . 10
       3.1.6.  Simplified Example Architecture  . . . . . . . . . . . 11
     3.2.  Conveyance of the Overload Indication  . . . . . . . . . . 11
       3.2.1.  DOIC Capability Discovery  . . . . . . . . . . . . . . 12
     3.3.  Overload Condition Indication  . . . . . . . . . . . . . . 12
   4.  Attribute Value Pairs  . . . . . . . . . . . . . . . . . . . . 12
     4.1.  OC-Supported-Features AVP  . . . . . . . . . . . . . . . . 13
     4.2.  OC-Feature-Vector AVP  . . . . . . . . . . . . . . . . . . 14
     4.3.  OC-OLR AVP . . . . . . . . . . . . . . . . . . . . . . . . 14
     4.4.  OC-Sequence-Number AVP . . . . . . . . . . . . . . . . . . 15
     4.5.  OC-Validity-Duration AVP . . . . . . . . . . . . . . . . . 15
     4.6.  OC-Report-Type AVP . . . . . . . . . . . . . . . . . . . . 16
     4.7.  OC-Reduction-Percentage AVP  . . . . . . . . . . . . . . . 16
     4.8.  Attribute Value Pair flag rules  . . . . . . . . . . . . . 17
   5.  Overload Control Operation . . . . . . . . . . . . . . . . . . 18
     5.1.  Overload Control Endpoints . . . . . . . . . . . . . . . . 18
     5.2.  Piggybacking Principle . . . . . . . . . . . . . . . . . . 21
     5.3.  Capability Announcement  . . . . . . . . . . . . . . . . . 22
       5.3.1.  Reacting Node Endpoint Considerations  . . . . . . . . 22
       5.3.2.  Reporting Node Endpoint Considerations . . . . . . . . 23
     5.4.  Protocol Extensibility . . . . . . . . . . . . . . . . . . 23
     5.5.  Overload Report Processing . . . . . . . . . . . . . . . . 24
       5.5.1.  Overload Control State . . . . . . . . . . . . . . . . 24
       5.5.2.  Reacting Node Considerations . . . . . . . . . . . . . 24
       5.5.3.  Reporting Node Considerations  . . . . . . . . . . . . 27
   6.  Transport Considerations . . . . . . . . . . . . . . . . . . . 27
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 28
     7.1.  AVP codes  . . . . . . . . . . . . . . . . . . . . . . . . 28
     7.2.  New registries . . . . . . . . . . . . . . . . . . . . . . 28

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   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
     8.1.  Potential Threat Modes . . . . . . . . . . . . . . . . . . 28
     8.2.  Denial of Service Attacks  . . . . . . . . . . . . . . . . 30
     8.3.  Non-Compliant Nodes  . . . . . . . . . . . . . . . . . . . 30
     8.4.  End-to End-Security Issues . . . . . . . . . . . . . . . . 30
   9.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 31
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 32
     10.2. Informative References . . . . . . . . . . . . . . . . . . 32
   Appendix A.  Issues left for future specifications . . . . . . . . 33
     A.1.  Additional traffic abatement algorithms  . . . . . . . . . 33
     A.2.  Agent Overload . . . . . . . . . . . . . . . . . . . . . . 33
     A.3.  DIAMETER_TOO_BUSY clarifications . . . . . . . . . . . . . 33
   Appendix B.  Examples  . . . . . . . . . . . . . . . . . . . . . . 33
     B.1.  Mix of Destination-Realm routed requests and
           Destination-Host routed requests . . . . . . . . . . . . . 33
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37

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1.  Introduction

   This specification defines a base solution for Diameter Overload
   Control (DOC).  The requirements for the solution are described and
   discussed in the corresponding design requirements document
   [RFC7068].  Note that the overload control solution defined in this
   specification does not address all the requirements listed in
   [RFC7068].  A number of overload control related features are left
   for the future specifications.

   The solution defined in this specification addresses the Diameter
   overload control between two endpoints (see Section 5.1).
   Furthermore, the solution is designed to apply to existing and future
   Diameter applications, requires no changes to the Diameter base
   protocol [RFC6733] and is deployable in environments where some
   Diameter nodes do not implement the Diameter overload control
   solution defined in this specification.

2.  Terminology and Abbreviations

   Server Farm

      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.  A server farm may host
      a single or multiple realms.

   Diameter Routing:

      Diameter Routing between non-adjacent nodes relies on the
      Destination-Realm AVP to determine the Diameter realm in which the
      request needs to be processed.  A Destination-Host AVP may also be
      present in the request to address a specific server inside the
      Diameter realm.  This function is defined in [RFC6733].  However,
      it is possible to enhance the routing decisions with application
      level knowledge as it done in 3GPP PCC [3GPP.23.203] and NAI-based
      source routing [RFC5729].

   Diameter layer Load Balancing:

      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.

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   Topology Hiding:

      Topology Hiding is loosely defined as ensuring that no Diameter
      topology information about a Diameter network 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
      Diameter network.  It can also include hiding the number of
      various Diameter entities in the Diameter network.  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.


      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.

   Reporting Node

      A Diameter node that generates an overload report.  (This may or
      may not be the actually overloaded node.)

   Reacting 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".

   OLR  Overload Report.

3.  Solution Overview

3.1.  Architectural Assumptions

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

3.1.1.  Application Classification

   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

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   overload reports.

   Section 8.1 of [RFC6733] defines two state machines that imply two
   types of applications, session-less and session-based applications.
   The primary difference 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, i.e. the session is implicitly
   terminated after a single Diameter transaction and a new Session-Id
   is generated for each Diameter request.

   For the purposes of this discussion, session-less applications are
   further divided into two types of applications:

   Stateless applications:

      Requests within a stateless application have no relationship to
      each other.  The 3GPP defined S13 application is an example of a
      stateless application [3GPP.29.272], where only a Diameter command
      is defined between a client and a server and no state is
      maintained between two consecutive transactions.

   Pseudo-session applications:

      Applications that do not rely on the Session-Id AVP for
      correlation of application messages related to the same session
      but use other session-related information in the Diameter requests
      for this purpose.  The 3GPP defined Cx application [3GPP.29.229]
      is an example of a pseudo-session application.

   The Credit-Control application defined in [RFC4006] is an example of
   a Diameter session-based application.

   The handling of overload reports must take the type of application
   into consideration, as discussed in Section 3.1.2.

3.1.2.  Application Type Overload Implications

   This section discusses considerations for mitigating overload
   reported by a Diameter entity.  This discussion focuses on the type
   of application.  Section 3.1.3 discusses considerations for handling
   various request types when the target server is known to be in an
   overloaded state.

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   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.

   Stateless applications:

      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-session applications:

      For pseudo-session applications, there is an implied ordering of
      requests.  As a result, decisions about which requests towards an
      overloaded entity to reject 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

   Session-based applications:

      Overload handling for session-based applications must take into
      consideration the work load associated with setting up and
      maintaining a session.  As such, the entity sending requests
      towards an overloaded Diameter entity for a session-based
      application might tend to reject new session requests prior to
      rejecting intra-session requests.  In addition, session ending
      requests might be given a lower probability of being rejected as
      rejecting session ending requests could result in session status
      being out of sync between the Diameter clients and servers.

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      Application designers that would decide to reject mid-session
      requests will need to consider whether the rejection invalidates
      the session and any resulting session clean-up procedures.

3.1.3.  Request Transaction Classification

   Independent Request:

      An independent request is not correlated to any other requests
      and, as such, the lifetime of the session-id is constrained to an
      individual transaction.

   Session-Initiating Request:

      A session-initiating request is the initial message that
      establishes a Diameter session.  The ACR message defined in
      [RFC6733] is an example of a session-initiating request.

   Correlated Session-Initiating Request:

      There are cases when multiple session-initiated requests must be
      correlated and managed by the same Diameter server.  It is notably
      the case in the 3GPP PCC architecture [3GPP.23.203], where
      multiple apparently independent Diameter application sessions are
      actually correlated and must be handled by the same Diameter

   Intra-Session Request:

      An intra session request is a request that uses the same
      Session-Id than the one used in a previous 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 [RFC6733] is an example of an intra-session

   Pseudo-Session Requests:

      Pseudo-session requests are independent requests and do not use
      the same Session-Id but are correlated by other session-related
      information contained in the request.  There exists 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.

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3.1.4.  Request Type Overload Implications

   The request classes identified in Section 3.1.3 have implications on
   decisions about which requests should be throttled first.  The
   following list of request treatment regarding throttling is provided
   as guidelines for application designers when implementing the
   Diameter overload control mechanism described in this document.
   Exact behavior regarding throttling must be defined per application.

   Independent requests:

      Independent requests can be given equal treatment when making
      throttling decisions.

   Session-initiating requests:

      Session-initiating requests represent more work than independent
      or intra-session requests.  Moreover, session-initiating requests
      are typically followed by other related session-related requests.
      As such, as the main objective of the overload control is to
      reduce the total number of requests sent to the overloaded entity,
      throttling decisions might favor allowing intra-session requests
      over session-initiating requests.  Individual session-initiating
      requests can be given equal treatment when making throttling

   Correlated session-initiating requests:

      A Request that results in a new binding, where the binding is used
      for routing of subsequent session-initiating requests to the same
      server, represents more work load than other requests.  As such,
      these requests might be throttled more frequently than other
      request types.

   Pseudo-session requests:

      Throttling decisions for pseudo-session requests can take into
      consideration 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.

   Intra-session requests

      There are two classes of intra-sessions requests.  The first class
      consists of requests that terminate a session.  The second one
      contains the set of requests that are used by the Diameter client
      and server to maintain the ongoing session state.  Session

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      terminating requests should be throttled less aggressively in
      order to gracefully terminate sessions, allow clean-up of the
      related resources (e.g. session state) and get rid of the need for
      other intra-session requests, reducing the session management
      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.

3.1.5.  Diameter Agent Behaviour

   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
   [RFC6733].  These include, among other tasks, topology hiding, or
   agent acting as a Server Front End (SFE) for a farm of Diameter

   Since the solution defined in this specification must not break the
   Diameter base protocol [RFC6733] 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:

   o  If a Diameter agent presents itself as the "end node", as an agent
      acting as an topology hiding SFE, the agent is the final
      destination of requests initiated by Diameter clients, the
      original source for the corresponding answers and server-initiated
      requests.  As a consequence, the DOIC mechanism MUST NOT leak
      information of the Diameter nodes behind it.  This requirement
      means that such a Diameter agent acts as a back-to-back-agent for
      DOIC purposes.  How the Diameter agent in this case appears to the
      Diameter servers in the farm, is specific to the implementation
      and deployment within the realm the Diameter agent is deployed.

   o  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
      the server identified by the Origin-Host identity contained in the
      Diameter message.

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3.1.6.  Simplified Example Architecture

   Figure 1 illustrates the simplified architecture for Diameter
   overload information conveyance.  See Section 5.1 for more discussion
   and details how different Diameter nodes fit into the architecture
   from the DOIC point of view.

    Realm X                                  Same or other Realms
   <--------------------------------------> <---------------------->

   +--^-----+                 : (optional) :
   |Diameter|                 :            :
   |Server A|--+     .--.     : +---^----+ :     .--.
   +--------+  |   _(    `.   : |Diameter| :   _(    `.   +---^----+
               +--(        )--:-|  Agent |-:--(        )--|Diameter|
   +--------+  | ( `  .  )  ) : +-----^--+ : ( `  .  )  ) | Client |
   |Diameter|--+  `--(___.-'  :            :  `--(___.-'  +-----^--+
   |Server B|                 :            :
   +---^----+                 :            :

                       End-to-end Overload Indication
          1)  <----------------------------------------------->
                          Diameter Application Y

               Overload Indication A    Overload Indication A'
          2)  <----------------------> <---------------------->
              standard base protocol   standard base protocol

     Figure 1: Simplified architecture choices for overload indication

   In Figure 1, the Diameter overload indication can be conveyed (1)
   end-to-end between servers and clients or (2) between servers and
   Diameter agent inside the realm and then between the Diameter agent
   and the clients when the Diameter agent acting as back-to-back-agent
   for DOIC purposes.

3.2.  Conveyance of the Overload Indication

   The following sections describe new Diameter AVPs used for sending
   overload reports, and for declaring support for certain DOIC

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3.2.1.  DOIC Capability Discovery

   Support of DOIC may be specified as part of the functionality
   supported by a new Diameter application.  In this way, support of the
   considered Diameter application (discovered during capabilities
   exchange phase as defined in Diameter base protocol [RFC6733])
   indicates implicit support of the DOIC mechanism.

   When the DOIC mechanism is introduced in existing Diameter
   applications, a specific capability discovery mechanism is required.
   The "DOIC capability discovery mechanism" is based on the presence of
   specific optional AVPs in the Diameter messages, such as the OC-
   Supported-Features AVP (see Section 4.1).  Although the OC-Supported-
   Features 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.

   From the Diameter overload control functionality point of view, the
   "Reacting node" is the requester of the overload report information
   and the "Reporting node" is the provider of the overload report.  The
   OC-Supported-Features AVP in the request message is always
   interpreted as an announcement of "DOIC supported capabilities".  The
   OC-Supported-Features AVP in the answer is also interpreted as a
   report of "DOIC supported capabilities" and at least one of supported
   capabilities MUST be common with the "Reacting node" (see
   Section 4.1).

3.3.  Overload Condition Indication

   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 conveyed in Diameter Attribute Value
   Pairs (AVPs).

   Certain new AVPs may also be used to declare certain DOIC
   capabilities and extensions.

4.  Attribute Value Pairs

   This section describes the encoding and semantics of the Diameter
   Overload Indication Attribute Value Pairs (AVPs) defined in this

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4.1.  OC-Supported-Features AVP

   The OC-Supported-Features AVP (AVP code TBD1) is type of Grouped and
   serves for two purposes.  First, it announces node's support for the
   DOIC in general.  Second, it contains the description of the
   supported DOIC features of the sending node.  The OC-Supported-
   Features AVP SHOULD be included into every Diameter message a DOIC
   supporting node sends (and intends to use for DOIC purposes).

   OC-Supported-Features ::= < AVP Header: TBD1 >
                             < OC-Sequence-Number >
                             [ OC-Feature-Vector ]
                           * [ AVP ]

   The OC-Sequence-Number AVP is used to indicate whether the contents
   of the OC-Supported-Features AVP has changed since last time the node
   included the OC-Supported-Features AVP (see Section 4.4).  Although
   sending the OC-Sequence-Number AVP is mandatory in the OC-Supported-
   Features AVP, the receiving node MAY always choose to ignore the
   sequence number if it can determine the feature support changes

   The OC-Feature-Vector sub-AVP is used to announced the DOIC features
   supported by the endpoint, in the form of a flag bits field in which
   each bit announces one feature or capability supported by the node
   (see Section 4.2).  The absence of the OC-Feature-Vector AVP
   indicates that only the default traffic abatement algorithm described
   in this specification is supported.

   A reacting node includes this AVP to indicate its capabilities to a
   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 reacting node
   includes the OC-Supported-Features AVP that announces what it
   supports.  The reporting node that sends the answer also includes the
   OC-Supported-Features AVP that describes the capabilities it
   supports.  The set of capabilities advertised by the reporting node
   depends on local policies.  At least one of the announced
   capabilities MUST match mutually.  If there is no single matching
   capability the reacting node MUST act as if it does not implement
   DOIC and cease inserting any DOIC related AVPs into any Diameter
   messages with this specific reacting node.

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4.2.  OC-Feature-Vector AVP

   The OC-Feature-Vector AVP (AVP code TBD6) is type of Unsigned64 and
   contains a 64 bit flags field of announced capabilities of an
   overload control endpoint.  The value of zero (0) is reserved.

   The following capabilities are defined in this document:

   OLR_DEFAULT_ALGO (0x0000000000000001)

      When this flag is set by the overload control endpoint it means
      that the default traffic abatement (loss) algorithm is supported.

4.3.  OC-OLR AVP

   The OC-OLR AVP (AVP code TBD2) is type of Grouped and contains the
   necessary information to convey an overload report.  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 (and Origin-Realm AVP as well)
   found from the encapsulating Diameter command.  The OC-OLR AVP is
   intended to be sent only by a reporting node.

   OC-OLR ::= < AVP Header: TBD2 >
              < OC-Sequence-Number >
              [ OC-Report-Type ]
              [ OC-Reduction-Percentage ]
              [ OC-Validity-Duration ]
            * [ AVP ]

   The Sequence-Number AVP indicates the "freshness" of the OC-OLR AVP.
   It is possible to replay the same OC-OLR AVP multiple times between
   the overload control endpoints, however, when the OC-OLR AVP content
   changes or sending endpoint otherwise wants the receiving endpoint to
   update its overload control information, then the OC-Sequence-Number
   AVP MUST contain a new greater value than the previously received.
   The receiver SHOULD discard an OC-OLR AVP with a sequence number that
   is less than previously received one.

   Note that if a Diameter command were to contain multiple OC-OLR AVPs
   they all MUST have different OC-Report-Type AVP value.  OC-OLR AVPs
   with unknown values SHOULD be silently discarded and the event SHOULD
   be logged.

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   The OC-OLR AVP can be expanded with optional sub-AVPs only if a
   legacy implementation can safely ignore them without breaking
   backward compatibility for the given OC-Report-Type AVP value implied
   report handling semantics.  If the new sub-AVPs imply new semantics
   for the report handling, then a new OC-Report-Type AVP value MUST be

4.4.  OC-Sequence-Number AVP

   The OC-Sequence-Number AVP (AVP code TBD3) is type of Time.  Its
   usage in the context of the overload control is described in Sections
   4.1 and 4.3.

   From the functionality point of view, the OC-Sequence-Number AVP MUST
   be used as a non-volatile increasing counter between two overload
   control endpoints (neglecting the fact that the contents of the AVP
   is a 64-bit NTP timestamp [RFC5905]).  The sequence number is only
   required to be unique between two overload control endpoints.
   Sequence numbers are treated in uni-directional manner, i.e. two
   sequence numbers on each direction between two endpoints are not
   related or correlated.

   When generating sequence numbers, the new sequence number MUST be
   greater than any sequence number previously seen between two
   endpoints within a time window that tolerates the wraparound of the
   NTP timestamp (i.e. approximately 68 years).

4.5.  OC-Validity-Duration AVP

   The OC-Validity-Duration AVP (AVP code TBD4) is type of Unsigned32
   and describes the number of seconds the "new and fresh" OC-OLR AVP
   and its content is valid since the reception of the new OC-OLR AVP.
   The default value for the OC-Validity-Duration AVP value is 5 (i.e.,
   5 seconds).  When the OC-Validity-Duration AVP is not present in the
   OC-OLR AVP, the default value applies.  Validity duration values 0
   (i.e., 0 seconds) and above 86400 (i.e., 24 hours) MUST NOT be used.
   Invalid validity duration values are treated as if the OC-Validity-
   Duration AVP were not present.

   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).  Section 4.7 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.  Following to this rule, an
   overload endpoint should explicitly signal the end of overload
   condition and not rely on the expiration of the validity time of the

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   overload report in the reacting node.  This leaves no need for the
   reacting node to reason or guess the overload condition of the
   reporting node.

4.6.  OC-Report-Type AVP

   The OC-Report-Type 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:

   0  A host report.  The overload treatment should apply to requests
      the reacting node knows that will reach the overloaded node.  For
      example, requests with a Destination-Host AVP indicating the
      endpoint.  The reacting node learns the "host" implicitly from the
      Origin-Host AVP of the received message that contained the OC-OLR

   1  A realm report.  The overload treatment should apply to all
      requests bound for the overloaded realm.  The reacting node learns
      the "realm" implicitly from the Origin-Realm AVP of the received
      message that contained the OC-OLR AVP.

   The default value of the OC-Report-Type AVP is 0 (i.e. the host

   The OC-Report-Type 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 differently requests sent to a specific server
   (identified by the Destination-Host AVP in the request) and requests
   that can be handled by any server in a realm.  The example in
   Appendix B.1 illustrates this usage.

   When defining new report type values, the corresponding specification
   MUST define the semantics of the new report types and how they affect
   the OC-OLR AVP handling.  The specification MUST also reserve a
   corresponding new feature, see the OC-Supported-Features and OC-
   Feature-Vector AVPs.

4.7.  OC-Reduction-Percentage AVP

   The OC-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 OC-Reduction-Percentage AVP applies to the default (loss like)
   algorithm specified in this specification.  However, the AVP can be
   reused for future abatement algorithms, if its semantics fit into the
   new algorithm.

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   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 reporting
   node is under a severe load and ceases to process any new messages.
   The value of 0 means that the reporting node is in a stable state and
   has no requests to the other endpoint to apply any traffic abatement.
   The default value of the OC-Reduction-Percentage AVP is 0.  When the
   OC-Reduction-Percentage AVP is not present in the overload report,
   the default value applies.

   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 reacting node
   sending the traffic should be conservative and, for example, first
   send "probe" messages to learn the overload condition of the
   overloaded node before converging to any traffic amount/rate decided
   by the sender.  Similar concerns apply in all cases when the overload
   report times out unless the previous overload report stated 0 percent

4.8.  Attribute Value Pair flag rules

                                                      |AVP flag |
                                                      |rules    |
                           AVP   Section              |    |MUST|
    Attribute Name         Code  Defined  Value Type  |MUST| NOT|
   |OC-Supported-Features  TBD1  x.x      Grouped     |    | V  |
   |OC-OLR                 TBD2  x.x      Grouped     |    | V  |
   |OC-Sequence-Number     TBD3  x.x      Time        |    | V  |
   |OC-Validity-Duration   TBD4  x.x      Unsigned32  |    | V  |
   |OC-Report-Type         TBD5  x.x      Enumerated  |    | V  |
   |OC-Reduction                                      |    |    |
   |  -Percentage          TBD8  x.x      Unsigned32  |    | V  |
   |OC-Feature-Vector      TBD6  x.x      Unsigned64  |    | V  |

   As described in the Diameter base protocol [RFC6733], the M-bit
   setting for a given AVP is relevant to an application and each

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   command within that application that includes the AVP.

   The Diameter overload control AVPs SHOULD always be sent with the
   M-bit cleared when used within existing Diameter applications to
   avoid backward compatibility issues.  Otherwise, when reused in newly
   defined Diameter applications, the DOC related AVPs SHOULD have the
   M-bit set.

5.  Overload Control Operation

5.1.  Overload Control Endpoints

   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" established between two
   communication 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 endpoints 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

   The following diagrams illustrate the concept of Diameter Overload
   End-Points and how they differ from the standard [RFC6733] defined
   client, server and agent Diameter nodes.  The following is the key to
   the elements in the diagrams:

   C  Diameter client as defined in [RFC6733].

   S  Diameter server as defined in [RFC6733].

   A  Diameter agent, in either a relay or proxy mode, as defined in

   DEP  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.

   Diameter Session  A Diameter session as defined in [RFC6733].

   DOIC Association  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.

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   Figure 2 illustrates the most basic configuration where a client is
   connected directly to a server.  In this case, the Diameter session
   and the DOIC association are both between the client and server.

   +-----+            +-----+
   |  C  |            |  S  |
   +-----+            +-----+
   | DEP |            | DEP |
   +--+--+            +--+--+
      |                  |
      |                  |
      |{Diameter Session}|
      |                  |
      |{DOIC Association}|
      |                  |

                      Figure 2: Basic DOIC deployment

   In Figure 3 there is an agent that is not participating directly in
   the exchange of overload reports.  As a result, the Diameter session
   and the DOIC association are still established between the client and
   the server.

   +-----+            +-----+            +-----+
   |  C  |            |  A  |            |  S  |
   +-----+            +--+--+            +-----+
   | DEP |               |               | DEP |
   +--+--+               |               +--+--+
      |                  |                  |
      |                  |                  |
      |----------{Diameter Session}---------|
      |                  |                  |
      |----------{DOIC Association}---------|
      |                  |                  |

          Figure 3: DOIC deployment with non participating agent

   Figure 4 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.

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   +-----+            +-----+            +-----+
   |  C  |            |  A  |            |  S  |
   +--+--+            +-----+            +-----+
      |               | DEP |            | DEP |
      |               +--+--+            +--+--+
      |                  |                  |
      |                  |                  |
      |----------{Diameter Session}---------|
      |                  |                  |
      |                  |{DOIC Association}|
      |                  |                  |

   Figure 4: DOIC deployment with non-DOIC client and DOIC enabled agent

   In Figure 5 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 for a set of servers.

   +-----+            +-----+            +-----+
   |  C  |            |  A  |            |  S  |
   +-----+            +-----+            +-----+
   | DEP |            | DEP |            | DEP |
   +--+--+            +--+--+            +--+--+
      |                  |                  |
      |                  |                  |
      |----------{Diameter Session}---------|
      |                  |                  |
      |{DOIC Association}|{DOIC Association}|
      |                  |                and/or
      |----------{DOIC Association}---------|
      |                  |                  |

            Figure 5: A deployment where all nodes support DOIC

   Figure 6 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

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   +-----+            +-----+            +-----+            +-----+
   | C1  |            | C2  |            |  A  |            |  S  |
   +-----+            +--+--+            +-----+            +-----+
   | DEP |               |               | DEP |            | DEP |
   +--+--+               |               +--+--+            +--+--+
      |                  |                  |                  |
      |                  |                  |                  |
      |-------------------{Diameter Session}-------------------|
      |                  |                  |                  |
      |                  |--------{Diameter Session}-----------|
      |                  |                  |                  |
      |---------{DOIC Association}----------|{DOIC Association}|
      |                  |                  |                and/or
      |-------------------{DOIC Association}-------------------|
      |                  |                  |                  |

     Figure 6: A deployment with DOIC and non-DOIC supporting clients

   Figure 7 illustrates a deployment where some agents support Diameter
   overload control and others do not.

   +-----+            +-----+            +-----+            +-----+
   |  C  |            |  A  |            |  A  |            |  S  |
   +-----+            +--+--+            +-----+            +-----+
   | DEP |               |               | DEP |            | DEP |
   +--+--+               |               +--+--+            +--+--+
      |                  |                  |                  |
      |                  |                  |                  |
      |-------------------{Diameter Session}-------------------|
      |                  |                  |                  |
      |                  |                  |                  |
      |---------{DOIC Association}----------|{DOIC Association}|
      |                  |                  |                and/or
      |-------------------{DOIC Association}-------------------|
      |                  |                  |                  |

      Figure 7: A deployment with DOIC and non-DOIC supporting agents

5.2.  Piggybacking Principle

   The overload control AVPs defined in this specification have been
   designed to be 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-Supported-Features AVP as
   optional AVPs into existing commands when the corresponding Command
   Code Format (CCF) specification allows adding new optional AVPs (see

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   Section 1.3.4 of [RFC6733]).

   When added to existing commands, both OC-Feature-Vector and OC-OLR
   AVPs SHOULD have the M-bit flag cleared to avoid backward
   compatibility issues.

   A new application specification can incorporate the overload control
   mechanism specified in this document by making it mandatory to
   implement for the application and referencing this specification
   normatively.  In such a case, the OC-Feature-Vector and OC-OLR AVPs
   reused in newly defined Diameter applications SHOULD have the M-bit
   flag set.  However, it is the responsibility of the Diameter
   application designers to define how overload control mechanisms works
   on that application.

   Note that the overload control solution does not have fixed server
   and client roles.  The endpoint role is determined based on the
   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.

5.3.  Capability Announcement

   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 announcement
   relies on the exchange of the OC-Supported-Features 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 case of non-session
   maintaining applications).

5.3.1.  Reacting Node Endpoint Considerations

   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-
   Supported-Features AVP with those capabilities it supports and is
   willing to use for this Diameter session (or transaction in a case of
   a non-session state maintaining applications, see Section 3.1.2 for

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   more details on Diameter sessions).  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.  In a case of a Diameter
   session maintaining application, sending the OC-Supported-Features
   AVP in every message is not really necessary after the initial
   capability announcement or until there is a change in supported

   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-Supported-Features AVP in the Diameter answer
   for existing application.

5.3.2.  Reporting Node Endpoint Considerations

   When a remote endpoint (i.e. a "reporting node") receives a request
   message, it can detect whether the request message initiating
   endpoint supports the overload control solution based on the presence
   of the OC-Supported-Features 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-Supported-Features 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.  The answer message
   initiating endpoint MAY announce as many supported capabilities as it
   has (the announced set is a subject to local policy and
   configuration).  However, at least one of the announced capabilities
   MUST be the same as received in the request message.

   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 created session (or transaction in a case of non-
   session state maintaining applications).  The same also applies if
   none of the announced capabilities match between the two endpoints.

5.4.  Protocol Extensibility

   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 possible use
   with the OC-Supported-Features for announcing the support for the new
   features (see Section 7 for the required procedures).

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   It should be noted that [RFC6733] 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.

5.5.  Overload Report Processing

5.5.1.  Overload Control State

   Both reacting and reporting nodes maintain an overload condition
   state for each endpoint (a host or a realm) they communicate with and
   both endpoints have announced support for DOIC.  See Sections 4.1 and
   5.3 for discussion about how the support for DOIC is determined.  The
   overload condition state SHOULD be able to make a difference between
   a realm and a specific host in that realm.

   The overload condition state could include the following information
   (per host or realm):

   o  The endpoint information (Diameter identity of the realm and/or
      host, application identifier, etc)

   o  Reduction percentage

   o  Validity period timer

   o  Sequence number

   o  Supported/selected traffic abatement algorithm

   The overload control state information SHOULD be maintained as long
   as the other endpoint is known to support DOIC (based on the presence
   of the DOIC AVPs or by a future application specification).

5.5.2.  Reacting Node Considerations

   Once a reacting node receives an OC-OLR AVP from a reporting node, it
   applies the traffic abatement based on the commonly supported
   algorithm with the reporting node and the current overload condition.
   The reacting node learns the reporting node supported abatement
   algorithms directly from the received answer message containing the
   OC-Supported-Features AVP or indirectly remembering the previously
   used traffic abatement algorithm with the given reporting node.

   The received OC-Supported-Features AVP does not change the existing

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   overload condition and/or traffic abatement algorithm settings if the
   OC-Sequence-Number AVP contains a value that is equal to the
   previously received/recorded one.  If the OC-Supported-Features AVP
   is received for the first time for the reporting node or the OC-
   Sequence-Number AVP value is less than the previously received/
   recorded one (and is outside the valid overflow window), then either
   the sequence number is stale (e.g. an intentional or unintentional
   replay) and SHOULD be silently discarded.

   The OC-OLR AVP contains the necessary information of the overload
   condition on the reporting node.  Similarly to the OC-Supported-
   Features's sequence numbering, the OC-OLR AVP also has the OC-
   Sequence-Number AVP and its handling is similar to the one in the OC-
   Supported-Features AVP.  The reacting node MUST update its overload
   condition state whenever receiving the OC-OLR AVP for the first time
   or the OC-Sequence-Number sub-AVP indicates a change in the OC-OLR

   As described in Section 4.3, the OC-OLR AVP contains the necessary
   information of the overload condition on the reporting node.

   From the OC-Report-Type AVP contained in the OC-OLR AVP, the reacting
   node learns whether the overload condition report concerns a specific
   host (as identified by the Origin-Host AVP of the answer message
   containing the OC-OLR AVP) or the entire realm (as identified by the
   Origin-Realm AVP of the answer message containing the OC-OLR AVP).
   The reacting node learns the Diameter application to which the
   overload report applies from the Application-ID of the answer message
   containing the OC-OLR AVP.  The reacting node MUST use this
   information as an input for its traffic abatement algorithm.  The
   idea is that the reacting node applies different handling of the
   traffic abatement, whether sent request messages are targeted to a
   specific host (identified by the Diameter-Host AVP in the request) or
   to any host in a realm (when only the Destination-Realm AVP is
   present in the request).  Note that future specifications MAY define
   new OC-Report-Type AVP values that imply different handling of the
   OC-OLR AVP.  For example, in a form of new additional AVPs inside the
   Grouped OC-OLR AVP that would define report target in a finer
   granularity than just a host.

   In the context of this specification and the default traffic
   abatement algorithm, the OC-Reduction-Percentage AVP value MUST be
   interpreted in the following way:

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   value == 0

      Indicates explicitly the end of overload condition and the
      reacting node SHOULD NOT apply the traffic abatement algorithm
      procedures anymore for the given reporting node (or realm).

   value == 100

      Indicates that the reporting node (or realm) does not want to
      receive any traffic from the reacting node for the application the
      report concerns.  The reacting node MUST do all measure not to
      send traffic to the reporting node (or realm) as long as the
      overload condition changes or expires.

   0 < value < 100

      Indicates that the reporting node urges the reacting node to
      reduce its traffic by a given percentage.  For example if the
      reacting node has been sending 100 packets per second to the
      reporting node, then a reception of OC-Reduction-Percentage value
      of 10 would mean that from now on the reacting node MUST only send
      90 packets per second.  How the reacting node achieves the "true
      reduction" transactions leading to the sent request messages is up
      to the implementation.  The reacting node MAY simply drop every
      10th packet from its output queue and let the generic application
      logic try to recover from it.

   If the OC-OLR AVP is received for the first time, the reacting node
   MUST create an overload condition state associated with the related
   realm or a specific host in the realm identified in the message
   carrying the OC-OLR AVP, as described in Section 5.5.1.

   If the value of the OC-Sequence-Number AVP contained in the received
   OC-OLR AVP is equal to or less than the value stored in an existing
   overload condition state, the received OC-OLR AVP SHOULD be silently
   discarded.  If the value of the OC-Sequence-Number AVP contained in
   the received OC-OLR AVP is greater than the value stored in an
   existing overload condition state or there is no previously recorded
   sequence number, the reacting node MUST update the overload condition
   state associated with the realm or the specific node is the realm.

   When an overload condition state is created or updated, the reacting
   node MUST apply the traffic abatement requested in the OC-OLR AVP
   using the algorithm announced in the OC-Supported-Features AVP
   contained in the received answer message along with the OC-OLR AVP.

   The validity duration of the overload information contained in the
   OC-OLR AVP is either explicitly indicated in the OC-Validity-Duration

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   AVP or is implicitly equals to the default value (5 seconds) if the
   OC-Validity-Duration AVP is absent of the OC-OLR AVP.  The reacting
   node MUST maintain the validity duration in the overload condition
   state.  Once the validity duration times out, the reacting node MUST
   assume the overload condition reported in a previous OC-OLR AVP has

5.5.3.  Reporting Node Considerations

   A reporting node is a Diameter node inserting an OC-OLR AVP in a
   Diameter message in order to inform a reacting node about an overload
   condition and request Diameter traffic abatement.

   The operation on the reporting node is rather straight forward.  The
   reporting node learns the capabilities of the reacting node when it
   receives the OC-Supported-Features AVP as part of any Diameter
   request message.  If the reporting node shares at least one common
   feature with the reacting node, then the DOIC can be enabled between
   these two endpoints.  See Section 5.3 for further discussion on the
   capability and feature announcement between two endpoints.

   When a traffic reduction is required due to an overload condition and
   the overload control solution is supported by the sender of the
   Diameter request, the reporting node MUST include an OC-Supported-
   Features AVP and an OC-OLR AVP in the corresponding Diameter answer.
   The OC-OLR AVP contains the required traffic reduction and the OC-
   Supported-Features AVP indicates the traffic abatement algorithm to
   apply.  This algorithm MUST be one of the algorithms advertised by
   the request sender.

   A reporting node MAY rely on the OC-Validity-Duration AVP values for
   the implicit overload condition state cleanup on the reacting node.
   However, it is RECOMMENDED that the reporting node always explicitly
   indicates the end of a overload condition.

6.  Transport Considerations

   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

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7.  IANA Considerations

7.1.  AVP codes

   New AVPs defined by this specification are listed in Section 4.  All
   AVP codes allocated from the 'Authentication, Authorization, and
   Accounting (AAA) Parameters' AVP Codes registry.

7.2.  New registries

   Three new registries are needed under the 'Authentication,
   Authorization, and Accounting (AAA) Parameters' registry.

   Section 4.2 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 [RFC5226].  See
   Section 4.2 for the initial assignment in the registry.

   Section 4.6 defines a new "Overload Report Type" registry with its
   initial assignments.  New types can be added using the Specification
   Required policy [RFC5226].

8.  Security Considerations

   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

   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.

8.1.  Potential Threat Modes

   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.

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   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.

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8.2.  Denial of Service Attacks

   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.

8.3.  Non-Compliant Nodes

   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 [RFC7068]
   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.

8.4.  End-to End-Security Issues

   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.

   The lack of end-to-end confidentiality protection means that any
   Diameter agent in the path of an overload report can view the

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   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
   [I-D.ietf-dime-e2e-sec-req] 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

9.  Contributors

   The following people contributed substantial ideas, feedback, and
   discussion to this document:

   o  Eric McMurry

   o  Hannes Tschofenig

   o  Ulrich Wiehe

   o  Jean-Jacques Trottin

   o  Maria Cruz Bartolome

   o  Martin Dolly

   o  Nirav Salot

   o  Susan Shishufeng

10.  References

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10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5905]  Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
              Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, June 2010.

   [RFC6733]  Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
              "Diameter Base Protocol", RFC 6733, October 2012.

10.2.  Informative References

              3GPP, "Policy and charging control architecture", 3GPP
              TS 23.203 10.9.0, September 2013.

              3GPP, "Cx and Dx interfaces based on the Diameter
              protocol; Protocol details", 3GPP TS 29.229 10.5.0,
              March 2013.

              3GPP, "Evolved Packet System (EPS); Mobility Management
              Entity (MME) and Serving GPRS Support Node (SGSN) related
              interfaces based on Diameter protocol", 3GPP TS 29.272
              10.8.0, June 2013.

              Tschofenig, H., Korhonen, J., Zorn, G., and K. Pillay,
              "Diameter AVP Level Security: Scenarios and Requirements",
              draft-ietf-dime-e2e-sec-req-00 (work in progress),
              September 2013.

   [RFC4006]  Hakala, H., Mattila, L., Koskinen, J-P., Stura, M., and J.
              Loughney, "Diameter Credit-Control Application", RFC 4006,
              August 2005.

   [RFC5729]  Korhonen, J., Jones, M., Morand, L., and T. Tsou,
              "Clarifications on the Routing of Diameter Requests Based
              on the Username and the Realm", RFC 5729, December 2009.

   [RFC7068]  McMurry, E. and B. Campbell, "Diameter Overload Control

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              Requirements", RFC 7068, November 2013.

Appendix A.  Issues left for future specifications

   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.

A.1.  Additional traffic abatement algorithms

   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 4.1 and 7 for the required IANA

A.2.  Agent Overload

   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.

A.3.  DIAMETER_TOO_BUSY clarifications

   The current [RFC6733] behaviour in a case of DIAMETER_TOO_BUSY is
   somewhat under specified.  For example, there is no information how
   long the specific Diameter node is willing to be unavailable.  A
   specification updating [RFC6733] 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 AVPs should be discussed and possible be
   recommended to be used.

Appendix B.  Examples

B.1.  Mix of Destination-Realm routed requests and Destination-Host
      routed requests

   Diameter allows a client to optionally select the destination server
   of a request, even if there are agents between the client and the
   server.  The client does this using the Destination-Host AVP.  In
   cases where the client does not care if a specific server receives
   the request, it can omit Destination-Host and route the request using
   the Destination-Realm and Application Id, effectively letting an
   agent select the server.

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   Clients commonly send mixtures of Destination-Host and Destination-
   Realm routed requests.  For example, in an application that uses user
   sessions, a client typically won't care which server handles a
   session-initiating requests.  But once the session is initiated, the
   client will send all subsequent requests in that session to the same
   server.  Therefore it would send the initial request with no
   Destination-Host AVP.  If it receives a successful answer, the client
   would copy the Origin-Host value from the answer message into a
   Destination-Host AVP in each subsequent request in the session.

   An agent has very limited options in applying overload abatement to
   requests that contain Destination-Host AVPs.  It typically cannot
   route the request to a different server than the one identified in
   Destination-Host.  It's only remaining options are to throttle such
   requests locally, or to send an overload report back towards the
   client so the client can throttle the requests.  The second choice is
   usually more efficient, since it prevents any throttled requests from
   being sent in the first place, and removes the agent's need to send
   errors back to the client for each dropped request.

   On the other hand, an agent has much more leeway to apply overload
   abatement for requests that do not contain Destination-Host AVPs.  If
   the agent has multiple servers in its peer table for the given realm
   and application, it can route such requests to other, less overloaded

   If the overload severity increases, the agent may reach a point where
   there is not sufficient capacity across all servers to handle even
   realm-routed requests.  In this case, the realm itself can be
   considered overloaded.  The agent may need the client to throttle
   realm-routed requests in addition to Destination-Host routed
   requests.  The overload severity may be different for each server,
   and the severity for the realm at is likely to be different than for
   any specific server.  Therefore, an agent may need to forward, or
   originate, multiple overload reports with differing ReportType and
   Reduction-Percentage values.

   Figure 8 illustrates such a mixed-routing scenario.  In this example,
   the servers S1, S2, and S3 handle requests for the realm "realm".
   Any of the three can handle requests that are not part of a user
   session (i.e. routed by Destination-Realm).  But once a session is
   established, all requests in that session must go to the same server.

     Client     Agent      S1        S2        S3
        |         |         |         |         |
        |(1) Request (DR:realm)       |         |
        |-------->|         |         |         |
        |         |         |         |         |

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        |         |         |         |         |
        |         |Agent selects S1   |         |
        |         |         |         |         |
        |         |         |         |         |
        |         |         |         |         |
        |         |(2) Request (DR:realm)       |
        |         |-------->|         |         |
        |         |         |         |         |
        |         |         |         |         |
        |         |         |S1 overloaded, returns OLR
        |         |         |         |         |
        |         |         |         |         |
        |         |         |         |         |
        |         |(3) Answer (OR:realm,OH:S1,OLR:RT=DH)
        |         |<--------|         |         |
        |         |         |         |         |
        |         |         |         |         |
        |         |sees OLR,routes DR traffic to S2&S3
        |         |         |         |         |
        |         |         |         |         |
        |         |         |         |         |
        |(4) Answer (OR:realm,OH:S1, OLR:RT=DH) |
        |<--------|         |         |         |
        |         |         |         |         |
        |         |         |         |         |
        |Client throttles requests with DH:S1   |
        |         |         |         |         |
        |         |         |         |         |
        |         |         |         |         |
        |(5) Request (DR:realm)       |         |
        |-------->|         |         |         |
        |         |         |         |         |
        |         |         |         |         |
        |         |Agent selects S2   |         |
        |         |         |         |         |
        |         |         |         |         |
        |         |         |         |         |
        |         |(6) Request (DR:realm)       |
        |         |------------------>|         |
        |         |         |         |         |
        |         |         |         |         |
        |         |         |         |S2 is overloaded...
        |         |         |         |         |
        |         |         |         |         |
        |         |         |         |         |
        |         |(7) Answer (OH:S2, OLR:RT=DH)|
        |         |<------------------|         |
        |         |         |         |         |

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        |         |         |         |         |
        |         |Agent sees OLR, realm now overloaded
        |         |         |         |         |
        |         |         |         |         |
        |         |         |         |         |
        |(8) Answer (OR:realm,OH:S2, OLR:RT=DH, OLR: RT=R)
        |<--------|         |         |         |
        |         |         |         |         |
        |         |         |         |         |
        |Client throttles DH:S1, DH:S2, and DR:realm
        |         |         |         |         |
        |         |         |         |         |
        |         |         |         |         |
        |         |         |         |         |
        |         |         |         |         |

      Figure 8: Mix of Destination-Host and Destination-Realm Routed

   1.  The client sends a request with no Destination-Host AVP (that is,
       a Destination-Realm routed request.)

   2.  The agent follows local policy to select a server from its peer
       table.  In this case, the agent selects S2 and forwards the

   3.  S1 is overloaded.  It sends a answer indicating success, but also
       includes an overload report.  Since the overload report only
       applies to S1, the ReportType is "Destination-Host".

   4.  The agent sees the overload report, and records that S1 is
       overloaded by the value in the Reduction-Percentage AVP.  It
       begins diverting the indicated percentage of realm-routed traffic
       from S1 to S2 and S3.  Since it can't divert Destination-Host
       routed traffic, it forwards the overload report to the client.
       This effectively delegates the throttling of traffic with
       Destination-Host:S1 to the client.

   5.  The client sends another Destination-Realm routed request.

   6.  The agent selects S2, and forwards the request.

   7.  It turns out that S2 is also overloaded, perhaps due to all that
       traffic it took over for S1.  S2 returns an successful answer
       containing an overload report.  Since this report only applies to
       S2, the ReportType is "Destination-Host".

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   8.  The agent sees that S2 is also overloaded by the value in
       Reduction-Percentage.  This value is probably different than the
       value from S1's report.  The agent diverts the remaining traffic
       to S3 as best as it can, but it calculates that the remaining
       capacity across all three servers is no longer sufficient to
       handle all of the realm-routed traffic.  This means the realm
       itself is overloaded.  The realm's overload percentage is most
       likely different than that for either S1 or S2.  The agent
       forward's S2's report back to the client in the Diameter answer.
       Additionally, the agent generates a new report for the realm of
       "realm", and inserts that report into the answer.  The client
       throttles requests with Destination-Host:S1 at one rate, requests
       with Destination-Host:S2 at another rate, and requests with no
       Destination-Host AVP at yet a third rate.  (Since S3 has not
       indicated overload, the client does not throttle requests with

Authors' Addresses

   Jouni Korhonen (editor)
   Porkkalankatu 24
   Helsinki  FIN-00180

   Email: jouni.nospam@gmail.com

   Steve Donovan
   17210 Campbell Road
   Dallas, Texas  75254
   United States

   Email: srdonovan@usdonovans.com

   Ben Campbell
   17210 Campbell Road
   Dallas, Texas  75254
   United States

   Email: ben@nostrum.com

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   Lionel Morand
   Orange Labs
   38/40 rue du General Leclerc
   Issy-Les-Moulineaux Cedex 9  92794

   Phone: +33145296257
   Email: lionel.morand@orange.com

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