    Internet Engineering Task Force (IETF)                  J. Korhonen, Ed.
    Request for Comments: 7683                          Broadcom Corporation
    Category: Standards Track                                S. Donovan, Ed.
    ISSN: 2070-1721                                              B. Campbell
                                                                      Oracle
                                                                   L. Morand
                                                                 Orange Labs
                                                                October 2015

                    Diameter Overload Indication Conveyance

    Abstract

       This specification defines a base solution for Diameter overload
       control, referred to as Diameter Overload Indication Conveyance
       (DOIC).

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 5741.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc7683.

    Copyright Notice

       Copyright (c) 2015 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
       (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
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

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    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
       2.  Terminology and Abbreviations . . . . . . . . . . . . . . . .   3
       3.  Conventions Used in This Document . . . . . . . . . . . . . .   5
       4.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   5
         4.1.  Piggybacking  . . . . . . . . . . . . . . . . . . . . . .   6
         4.2.  DOIC Capability Announcement  . . . . . . . . . . . . . .   7
         4.3.  DOIC Overload Condition Reporting . . . . . . . . . . . .   9
         4.4.  DOIC Extensibility  . . . . . . . . . . . . . . . . . . .  11
         4.5.  Simplified Example Architecture . . . . . . . . . . . . .  12
       5.  Solution Procedures . . . . . . . . . . . . . . . . . . . . .  12
         5.1.  Capability Announcement . . . . . . . . . . . . . . . . .  12
           5.1.1.  Reacting Node Behavior  . . . . . . . . . . . . . . .  13
           5.1.2.  Reporting Node Behavior . . . . . . . . . . . . . . .  13
           5.1.3.  Agent Behavior  . . . . . . . . . . . . . . . . . . .  14
         5.2.  Overload Report Processing  . . . . . . . . . . . . . . .  15
           5.2.1.  Overload Control State  . . . . . . . . . . . . . . .  15
           5.2.2.  Reacting Node Behavior  . . . . . . . . . . . . . . .  19
           5.2.3.  Reporting Node Behavior . . . . . . . . . . . . . . .  20
         5.3.  Protocol Extensibility  . . . . . . . . . . . . . . . . .  22
       6.  Loss Algorithm  . . . . . . . . . . . . . . . . . . . . . . .  23
         6.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  23
         6.2.  Reporting Node Behavior . . . . . . . . . . . . . . . . .  24
         6.3.  Reacting Node Behavior  . . . . . . . . . . . . . . . . .  24
       7.  Attribute Value Pairs . . . . . . . . . . . . . . . . . . . .  25
         7.1.  OC-Supported-Features AVP . . . . . . . . . . . . . . . .  25
         7.2.  OC-Feature-Vector AVP . . . . . . . . . . . . . . . . . .  25
         7.3.  OC-OLR AVP  . . . . . . . . . . . . . . . . . . . . . . .  26
         7.4.  OC-Sequence-Number AVP  . . . . . . . . . . . . . . . . .  26
         7.5.  OC-Validity-Duration AVP  . . . . . . . . . . . . . . . .  26
         7.6.  OC-Report-Type AVP  . . . . . . . . . . . . . . . . . . .  27
         7.7.  OC-Reduction-Percentage AVP . . . . . . . . . . . . . . .  27
         7.8.  AVP Flag Rules  . . . . . . . . . . . . . . . . . . . . .  28
       8.  Error Response Codes  . . . . . . . . . . . . . . . . . . . .  28
       9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
         9.1.  AVP Codes . . . . . . . . . . . . . . . . . . . . . . . .  29
         9.2.  New Registries  . . . . . . . . . . . . . . . . . . . . .  29
       10. Security Considerations . . . . . . . . . . . . . . . . . . .  30
         10.1.  Potential Threat Modes . . . . . . . . . . . . . . . . .  30
         10.2.  Denial-of-Service Attacks  . . . . . . . . . . . . . . .  31
         10.3.  Noncompliant Nodes . . . . . . . . . . . . . . . . . . .  32
         10.4.  End-to-End Security Issues . . . . . . . . . . . . . . .  32
       11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
         11.1.  Normative References . . . . . . . . . . . . . . . . . .  34
         11.2.  Informative References . . . . . . . . . . . . . . . . .  34

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    RFC 7683                          DOIC                      October 2015

       Appendix A.  Issues Left for Future Specifications  . . . . . . .  35
         A.1.  Additional Traffic Abatement Algorithms . . . . . . . . .  35
         A.2.  Agent Overload  . . . . . . . . . . . . . . . . . . . . .  35
         A.3.  New Error Diagnostic AVP  . . . . . . . . . . . . . . . .  35
       Appendix B.  Deployment Considerations  . . . . . . . . . . . . .  35
       Appendix C.  Considerations for Applications Integrating the DOIC
                    Solution . . . . . . . . . . . . . . . . . . . . . .  36
         C.1.  Application Classification  . . . . . . . . . . . . . . .  36
         C.2.  Implications of Application Type Overload . . . . . . . .  37
         C.3.  Request Transaction Classification  . . . . . . . . . . .  38
         C.4.  Request Type Overload Implications  . . . . . . . . . . .  39
       Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  41
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  42

    1.  Introduction

       This specification defines a base solution for Diameter overload
       control, referred to as Diameter Overload Indication Conveyance
       (DOIC), based on the requirements identified in [RFC7068].

       This specification addresses Diameter overload control between
       Diameter nodes that support the DOIC solution.  The solution, which
       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.

       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.  It is the responsibility of the Diameter application
       designers to define how overload control mechanisms work on that
       application.

       Note that the overload control solution defined in this specification
       does not address all the requirements listed in [RFC7068].  A number
       of features related to overload control are left for future
       specifications.  See Appendix A for a list of extensions that are
       currently being considered.

    2.  Terminology and Abbreviations

       Abatement

          Reaction to receipt of an overload report resulting in a reduction
          in traffic sent to the reporting node.  Abatement actions include
          diversion and throttling.

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       Abatement Algorithm

          An extensible method requested by reporting nodes and used by
          reacting nodes to reduce the amount of traffic sent during an
          occurrence of overload control.

       Diversion

          An overload abatement treatment where the reacting node selects
          alternate destinations or paths for requests.

       Host-Routed Requests

          Requests that a reacting node knows will be served by a particular
          host, either due to the presence of a Destination-Host Attribute
          Value Pair (AVP) or by some other local knowledge on the part of
          the reacting node.

       Overload Control State (OCS)

          Internal state maintained by a reporting or reacting node
          describing occurrences of overload control.

       Overload Report (OLR)

          Overload control information for a particular overload occurrence
          sent by a reporting node.

       Reacting Node

          A Diameter node that acts upon an overload report.

       Realm-Routed Requests

          Requests sent by a reacting node where the reacting node does not
          know to which host the request will be routed.

       Reporting Node

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

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       Throttling

          An abatement treatment that limits the number of requests sent by
          the reacting node.  Throttling can include a Diameter Client
          choosing to not send requests, or a Diameter Agent or Server
          rejecting requests with appropriate error responses.  In both
          cases, the result of the throttling is a permanent rejection of
          the transaction.

    3.  Conventions Used in This Document

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

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

    4.  Solution Overview

       The Diameter Overload Information Conveyance (DOIC) solution allows
       Diameter nodes to request that other Diameter nodes perform overload
       abatement actions, that is, actions to reduce the load offered to the
       overloaded node or realm.

       A Diameter node that supports DOIC is known as a "DOIC node".  Any
       Diameter node can act as a DOIC node, including Diameter Clients,
       Diameter Servers, and Diameter Agents.  DOIC nodes are further
       divided into "Reporting Nodes" and "Reacting Nodes."  A reporting
       node requests overload abatement by sending Overload Reports (OLRs).

       A reacting node acts upon OLRs and performs whatever actions are
       needed to fulfill the abatement requests included in the OLRs.  A
       reporting node may report overload on its own behalf or on behalf of
       other nodes.  Likewise, a reacting node may perform overload
       abatement on its own behalf or on behalf of other nodes.

       A Diameter node's role as a DOIC node is independent of its Diameter
       role.  For example, Diameter Agents may act as DOIC nodes, even
       though they are not endpoints in the Diameter sense.  Since Diameter
       enables bidirectional applications, where Diameter Servers can send
       requests towards Diameter Clients, a given Diameter node can
       simultaneously act as both a reporting node and a reacting node.

       Likewise, a Diameter Agent may act as a reacting node from the
       perspective of upstream nodes, and a reporting node from the
       perspective of downstream nodes.

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       DOIC nodes do not generate new messages to carry DOIC-related
       information.  Rather, they "piggyback" DOIC information over existing
       Diameter messages by inserting new AVPs into existing Diameter
       requests and responses.  Nodes indicate support for DOIC, and any
       needed DOIC parameters, by inserting an OC-Supported-Features AVP
       (Section 7.1) into existing requests and responses.  Reporting nodes
       send OLRs by inserting OC-OLR AVPs (Section 7.3).

       A given OLR applies to the Diameter realm and application of the
       Diameter message that carries it.  If a reporting node supports more
       than one realm and/or application, it reports independently for each
       combination of realm and application.  Similarly, the OC-Supported-
       Features AVP applies to the realm and application of the enclosing
       message.  This implies that a node may support DOIC for one
       application and/or realm, but not another, and may indicate different
       DOIC parameters for each application and realm for which it supports
       DOIC.

       Reacting nodes perform overload abatement according to an agreed-upon
       abatement algorithm.  An abatement algorithm defines the meaning of
       some of the parameters of an OLR and the procedures required for
       overload abatement.  An overload abatement algorithm separates
       Diameter requests into two sets.  The first set contains the requests
       that are to undergo overload abatement treatment of either throttling
       or diversion.  The second set contains the requests that are to be
       given normal routing treatment.  This document specifies a single
       "must-support" algorithm, namely, the "loss" algorithm (Section 6).
       Future specifications may introduce new algorithms.

       Overload conditions may vary in scope.  For example, a single
       Diameter node may be overloaded, in which case, reacting nodes may
       attempt to send requests to other destinations.  On the other hand,
       an entire Diameter realm may be overloaded, in which case, such
       attempts would do harm.  DOIC OLRs have a concept of "report type"
       (Section 7.6), where the type defines such behaviors.  Report types
       are extensible.  This document defines report types for overload of a
       specific host and for overload of an entire realm.

       DOIC works through non-supporting Diameter Agents that properly pass
       unknown AVPs unchanged.

    4.1.  Piggybacking

       There is no new Diameter application defined to carry overload-
       related AVPs.  The overload control AVPs defined in this
       specification have been designed to be piggybacked on top of existing

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       application messages.  This is made possible by adding the optional
       overload control AVPs OC-OLR and OC-Supported-Features into existing
       commands.

       Reacting nodes indicate support for DOIC by including the
       OC-Supported-Features AVP in all request messages originated or
       relayed by the reacting node.

       Reporting nodes indicate support for DOIC by including the
       OC-Supported-Features AVP in all answer messages that are originated
       or relayed by the reporting node and that are in response to a
       request that contained the OC-Supported-Features AVP.  Reporting
       nodes may include overload reports using the OC-OLR AVP in answer
       messages.

       Note that the overload control solution does not have fixed server
       and client roles.  The DOIC node 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., sent by a "reporting node").
       Therefore, in a typical client-server deployment, the Diameter Client
       may report its overload condition to the Diameter Server for any
       Diameter-Server-initiated message exchange.  An example of such is
       the Diameter Server requesting a re-authentication from a Diameter
       Client.

    4.2.  DOIC Capability Announcement

       The DOIC solution supports the ability for Diameter nodes to
       determine if other nodes in the path of a request support the
       solution.  This capability is referred to as DOIC Capability
       Announcement (DCA) and is separate from the Diameter Capability
       Exchange.

       The DCA mechanism uses the OC-Supported-Features AVPs to indicate the
       Diameter overload features supported.

       The first node in the path of a Diameter request that supports the
       DOIC solution inserts the OC-Supported-Features AVP in the request
       message.

       The individual features supported by the DOIC nodes are indicated in
       the OC-Feature-Vector AVP.  Any semantics associated with the
       features will be defined in extension specifications that introduce
       the features.

          Note: As discussed elsewhere in the document, agents in the path
          of the request can modify the OC-Supported-Features AVP.

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          Note: The DOIC solution must support deployments where Diameter
          Clients and/or Diameter Servers do not support the DOIC solution.
          In this scenario, Diameter Agents that support the DOIC solution
          may handle overload abatement for the non-supporting Diameter
          nodes.  In this case, the DOIC agent will insert the OC-Supported-
          Features AVP in requests that do not already contain one, telling
          the reporting node that there is a DOIC node that will handle
          overload abatement.  For transactions where there was an
          OC-Supporting-Features AVP in the request, the agent will insert
          the OC-Supported-Features AVP in answers, telling the reacting
          node that there is a reporting node.

       The OC-Feature-Vector AVP will always contain an indication of
       support for the loss overload abatement algorithm defined in this
       specification (see Section 6).  This ensures that a reporting node
       always supports at least one of the advertised abatement algorithms
       received in a request messages.

       The reporting node inserts the OC-Supported-Features AVP in all
       answer messages to requests that contained the OC-Supported-Features
       AVP.  The contents of the reporting node's OC-Supported-Features AVP
       indicate the set of Diameter overload features supported by the
       reporting node.  This specification defines one exception -- the
       reporting node only includes an indication of support for one
       overload abatement algorithm, independent of the number of overload
       abatement algorithms actually supported by the reacting node.  The
       overload abatement algorithm indicated is the algorithm that the
       reporting node intends to use should it enter an overload condition.
       Reacting nodes can use the indicated overload abatement algorithm to
       prepare for possible overload reports and must use the indicated
       overload abatement algorithm if traffic reduction is actually
       requested.

          Note that the loss algorithm defined in this document is a
          stateless abatement algorithm.  As a result, it does not require
          any actions by reacting nodes prior to the receipt of an overload
          report.  Stateful abatement algorithms that base the abatement
          logic on a history of request messages sent might require reacting
          nodes to maintain state in advance of receiving an overload report
          to ensure that the overload reports can be properly handled.

       While it should only be done in exceptional circumstances and not
       during an active occurrence of overload, a reacting node that wishes
       to transition to a different abatement algorithm can stop advertising
       support for the algorithm indicated by the reporting node, as long as
       support for the loss algorithm is always advertised.

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       The DCA mechanism must also allow the scenario where the set of
       features supported by the sender of a request and by agents in the
       path of a request differ.  In this case, the agent can update the
       OC-Supported-Features AVP to reflect the mixture of the two sets of
       supported features.

          Note: The logic to determine if the content of the OC-Supported-
          Features AVP should be changed is out of scope for this document,
          as is the logic to determine the content of a modified
          OC-Supported-Features AVP.  These are left to implementation
          decisions.  Care must be taken not to introduce interoperability
          issues for downstream or upstream DOIC nodes.  As such, the agent
          must act as a fully compliant reporting node to the downstream
          reacting node and as a fully compliant reacting node to the
          upstream reporting node.

    4.3.  DOIC Overload Condition Reporting

       As with DOIC capability announcement, overload condition reporting
       uses new AVPs (Section 7.3) to indicate an overload condition.

       The OC-OLR AVP is referred to as an overload report.  The OC-OLR AVP
       includes the type of report, a sequence number, the length of time
       that the report is valid, and AVPs specific to the abatement
       algorithm.

       Two types of overload reports are defined in this document: host
       reports and realm reports.

       A report of type "HOST_REPORT" is sent to indicate the overload of a
       specific host, identified by the Origin-Host AVP of the message
       containing the OLR, for the Application-ID indicated in the
       transaction.  When receiving an OLR of type "HOST_REPORT", a reacting
       node applies overload abatement treatment to the host-routed requests
       identified by the overload abatement algorithm (as defined in
       Section 2) sent for this application to the overloaded host.

       A report of type "REALM_REPORT" is sent to indicate the overload of a
       realm for the Application-ID indicated in the transaction.  The
       overloaded realm is identified by the Destination-Realm AVP of the
       message containing the OLR.  When receiving an OLR of type
       "REALM_REPORT", a reacting node applies overload abatement treatment
       to realm-routed requests identified by the overload abatement
       algorithm (as defined in Section 2) sent for this application to the
       overloaded realm.

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       This document assumes that there is a single source for realm reports
       for a given realm, or that if multiple nodes can send realm reports,
       that each such node has full knowledge of the overload state of the
       entire realm.  A reacting node cannot distinguish between receiving
       realm reports from a single node or from multiple nodes.

          Note: Known issues exist if there are multiple sources for
          overload reports that apply to the same Diameter entity.  Reacting
          nodes have no way of determining the source and, as such, will
          treat them as coming from a single source.  Variance in sequence
          numbers between the two sources can then cause incorrect overload
          abatement treatment to be applied for indeterminate periods of
          time.

       Reporting nodes are responsible for determining the need for a
       reduction of traffic.  The method for making this determination is
       implementation specific and depends on the type of overload report
       being generated.  A host report might be generated by tracking use of
       resources required by the host to handle transactions for the
       Diameter application.  A realm report generally impacts the traffic
       sent to multiple hosts and, as such, requires tracking the capacity
       of all servers able to handle realm-routed requests for the
       application and realm.

       Once a reporting node determines the need for a reduction in traffic,
       it uses the DOIC-defined AVPs to report on the condition.  These AVPs
       are included in answer messages sent or relayed by the reporting
       node.  The reporting node indicates the overload abatement algorithm
       that is to be used to handle the traffic reduction in the
       OC-Supported-Features AVP.  The OC-OLR AVP is used to communicate
       information about the requested reduction.

       Reacting nodes, upon receipt of an overload report, apply the
       overload abatement algorithm to traffic impacted by the overload
       report.  The method used to determine the requests that are to
       receive overload abatement treatment is dependent on the abatement
       algorithm.  The loss abatement algorithm is defined in this document
       (Section 6).  Other abatement algorithms can be defined in extensions
       to the DOIC solution.

       Two types of overload abatement treatment are defined, diversion and
       throttling.  Reacting nodes are responsible for determining which
       treatment is appropriate for individual requests.

       As the conditions that lead to the generation of the overload report
       change, the reporting node can send new overload reports requesting
       greater reduction if the condition gets worse or less reduction if
       the condition improves.  The reporting node sends an overload report

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       with a duration of zero to indicate that the overload condition has
       ended and abatement is no longer needed.

       The reacting node also determines when the overload report expires
       based on the OC-Validity-Duration AVP in the overload report and
       stops applying the abatement algorithm when the report expires.

       Note that erroneous overload reports can be used for DoS attacks.
       This includes the ability to indicate that a significant reduction in
       traffic, up to and including a request for no traffic, should be sent
       to a reporting node.  As such, care should be taken to verify the
       sender of overload reports.

    4.4.  DOIC Extensibility

       The DOIC solution is designed to be extensible.  This extensibility
       is based on existing Diameter-based extensibility mechanisms, along
       with the DOIC capability announcement mechanism.

       There are multiple categories of extensions that are expected.  This
       includes the definition of new overload abatement algorithms, the
       definition of new report types, and the definition of new scopes of
       messages impacted by an overload report.

       A DOIC node communicates supported features by including them in the
       OC-Feature-Vector AVP, as a sub-AVP of OC-Supported-Features.  Any
       non-backwards-compatible DOIC extensions define new values for the
       OC-Feature-Vector AVP.  DOIC extensions also have the ability to add
       new AVPs to the OC-Supported-Features AVP, if additional information
       about the new feature is required.

       Overload reports can also be extended by adding new sub-AVPs to the
       OC-OLR AVP, allowing reporting nodes to communicate additional
       information about handling an overload condition.

       If necessary, new extensions can also define new AVPs that are not
       part of the OC-Supported-Features and OC-OLR group AVPs.  It is,
       however, recommended that DOIC extensions use the OC-Supported-
       Features AVP and OC-OLR AVP to carry all DOIC-related AVPs.

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    4.5.  Simplified Example Architecture

       Figure 1 illustrates the simplified architecture for Diameter
       overload information conveyance.

        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)  <----------------------> <---------------------->
                     Diameter Application Y   Diameter Application Y

         Figure 1: Simplified Architecture Choices for Overload Indication
                                     Delivery

       In Figure 1, the Diameter overload indication can be conveyed (1)
       end-to-end between servers and clients or (2) between servers and the
       Diameter Agent inside the realm and then between the Diameter Agent
       and the clients.

    5.  Solution Procedures

       This section outlines the normative behavior for the DOIC solution.

    5.1.  Capability Announcement

       This section defines DOIC Capability Announcement (DCA) behavior.

          Note: This specification assumes that changes in DOIC node
          capabilities are relatively rare events that occur as a result of
          administrative action.  Reacting nodes ought to minimize changes
          that force the reporting node to change the features being used,
          especially during active overload conditions.  But even if

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          reacting nodes avoid such changes, reporting nodes still have to
          be prepared for them to occur.  For example, differing
          capabilities between multiple reacting nodes may still force a
          reporting node to select different features on a per-transaction
          basis.

    5.1.1.  Reacting Node Behavior

       A reacting node MUST include the OC-Supported-Features AVP in all
       requests.  It MAY include the OC-Feature-Vector AVP, as a sub-AVP of
       OC-Supported-Features.  If it does so, it MUST indicate support for
       the "loss" algorithm.  If the reacting node is configured to support
       features (including other algorithms) in addition to the loss
       algorithm, it MUST indicate such support in an OC-Feature-Vector AVP.

       An OC-Supported-Features AVP in answer messages indicates there is a
       reporting node for the transaction.  The reacting node MAY take
       action, for example, creating state for some stateful abatement
       algorithm, based on the features indicated in the OC-Feature-Vector
       AVP.

          Note: The loss abatement algorithm does not require stateful
          behavior when there is no active overload report.

       Reacting nodes need to be prepared for the reporting node to change
       selected algorithms.  This can happen at any time, including when the
       reporting node has sent an active overload report.  The reacting node
       can minimize the potential for changes by modifying the advertised
       abatement algorithms sent to an overloaded reporting node to the
       currently selected algorithm and loss (or just loss if it is the
       currently selected algorithm).  This has the effect of limiting the
       potential change in abatement algorithm from the currently selected
       algorithm to loss, avoiding changes to more complex abatement
       algorithms that require state to operate properly.

    5.1.2.  Reporting Node Behavior

       Upon receipt of a request message, a reporting node determines if
       there is a reacting node for the transaction based on the presence of
       the OC-Supported-Features AVP in the request message.

       If the request message contains an OC-Supported-Features AVP, then a
       reporting node MUST include the OC-Supported-Features AVP in the
       answer message for that transaction.

          Note: Capability announcement is done on a per-transaction basis.
          The reporting node cannot assume that the capabilities announced
          by a reacting node will be the same between transactions.

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       A reporting node MUST NOT include the OC-Supported-Features AVP,
       OC-OLR AVP, or any other overload control AVPs defined in extension
       documents in response messages for transactions where the request
       message does not include the OC-Supported-Features AVP.  Lack of the
       OC-Supported-Features AVP in the request message indicates that there
       is no reacting node for the transaction.

       A reporting node knows what overload control functionality is
       supported by the reacting node based on the content or absence of the
       OC-Feature-Vector AVP within the OC-Supported-Features AVP in the
       request message.

       A reporting node MUST select a single abatement algorithm in the
       OC-Feature-Vector AVP.  The abatement algorithm selected MUST
       indicate the abatement algorithm the reporting node wants the
       reacting node to use when the reporting node enters an overload
       condition.

       The abatement algorithm selected MUST be from the set of abatement
       algorithms contained in the request message's OC-Feature-Vector AVP.

       A reporting node that selects the loss algorithm may do so by
       including the OC-Feature-Vector AVP with an explicit indication of
       the loss algorithm, or it MAY omit the OC-Feature-Vector AVP.  If it
       selects a different algorithm, it MUST include the OC-Feature-Vector
       AVP with an explicit indication of the selected algorithm.

       The reporting node SHOULD indicate support for other DOIC features
       defined in extension documents that it supports and that apply to the
       transaction.  It does so using the OC-Feature-Vector AVP.

          Note: Not all DOIC features will apply to all Diameter
          applications or deployment scenarios.  The features included in
          the OC-Feature-Vector AVP are based on local policy of the
          reporting node.

    5.1.3.  Agent Behavior

       Diameter Agents that support DOIC can ensure that all messages
       relayed by the agent contain the OC-Supported-Features AVP.

       A Diameter Agent MAY take on reacting node behavior for Diameter
       endpoints that do not support the DOIC solution.  A Diameter Agent
       detects that a Diameter endpoint does not support DOIC reacting node
       behavior when there is no OC-Supported-Features AVP in a request
       message.

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       For a Diameter Agent to be a reacting node for a non-supporting
       Diameter endpoint, the Diameter Agent MUST include the OC-Supported-
       Features AVP in request messages it relays that do not contain the
       OC-Supported-Features AVP.

       A Diameter Agent MAY take on reporting node behavior for Diameter
       endpoints that do not support the DOIC solution.  The Diameter Agent
       MUST have visibility to all traffic destined for the non-supporting
       host in order to become the reporting node for the Diameter endpoint.
       A Diameter Agent detects that a Diameter endpoint does not support
       DOIC reporting node behavior when there is no OC-Supported-Features
       AVP in an answer message for a transaction that contained the
       OC-Supported-Features AVP in the request message.

       If a request already has the OC-Supported-Features AVP, a Diameter
       Agent MAY modify it to reflect the features appropriate for the
       transaction.  Otherwise, the agent relays the OC-Supported-Features
       AVP without change.

          Example: If the agent supports a superset of the features reported
          by the reacting node, then the agent might choose, based on local
          policy, to advertise that superset of features to the reporting
          node.

       If the Diameter Agent changes the OC-Supported-Features AVP in a
       request message, then it is likely it will also need to modify the
       OC-Supported-Features AVP in the answer message for the transaction.
       A Diameter Agent MAY modify the OC-Supported-Features AVP carried in
       answer messages.

       When making changes to the OC-Supported-Features or OC-OLR AVPs, the
       Diameter Agent needs to ensure consistency in its behavior with both
       upstream and downstream DOIC nodes.

    5.2.  Overload Report Processing

    5.2.1.  Overload Control State

       Both reacting and reporting nodes maintain Overload Control State
       (OCS) for active overload conditions.  The following sections define
       behavior associated with that OCS.

       The contents of the OCS in the reporting node and in the reacting
       node represent logical constructs.  The actual internal physical
       structure of the state included in the OCS is an implementation
       decision.

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    5.2.1.1.  Overload Control State for Reacting Nodes

       A reacting node maintains the following OCS per supported Diameter
       application:

       o  a host-type OCS entry for each Destination-Host to which it sends
          host-type requests and

       o  a realm-type OCS entry for each Destination-Realm to which it
          sends realm-type requests.

       A host-type OCS entry is identified by the pair of Application-ID and
       the node's DiameterIdentity.

       A realm-type OCS entry is identified by the pair of Application-ID
       and realm.

       The host-type and realm-type OCS entries include the following
       information (the actual information stored is an implementation
       decision):

       o  Sequence number (as received in OC-OLR; see Section 7.3)

       o  Time of expiry (derived from OC-Validity-Duration AVP received in
          the OC-OLR AVP and time of reception of the message carrying
          OC-OLR AVP)

       o  Selected abatement algorithm (as received in the OC-Supported-
          Features AVP)

       o  Input data that is abatement algorithm specific (as received in
          the OC-OLR AVP -- for example, OC-Reduction-Percentage for the
          loss abatement algorithm)

    5.2.1.2.  Overload Control State for Reporting Nodes

       A reporting node maintains OCS entries per supported Diameter
       application, per supported (and eventually selected) abatement
       algorithm, and per report type.

       An OCS entry is identified by the tuple of Application-ID, report
       type, and abatement algorithm, and it includes the following
       information (the actual information stored is an implementation
       decision):

       o  Sequence number

       o  Validity duration

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       o  Expiration time

       o  Input data that is algorithm specific (for example, the reduction
          percentage for the loss abatement algorithm)

    5.2.1.3.  Reacting Node's Maintenance of Overload Control State

       When a reacting node receives an OC-OLR AVP, it MUST determine if it
       is for an existing or new overload condition.

          Note: For the remainder of this section, the term "OLR" refers to
          the combination of the contents of the received OC-OLR AVP and the
          abatement algorithm indicated in the received OC-Supported-
          Features AVP.

       When receiving an answer message with multiple OLRs of different
       supported report types, a reacting node MUST process each received
       OLR.

       The OLR is for an existing overload condition if a reacting node has
       an OCS that matches the received OLR.

       For a host report, this means it matches the Application-ID and the
       host's DiameterIdentity in an existing host OCS entry.

       For a realm report, this means it matches the Application-ID and the
       realm in an existing realm OCS entry.

       If the OLR is for an existing overload condition, then a reacting
       node MUST determine if the OLR is a retransmission or an update to
       the existing OLR.

       If the sequence number for the received OLR is greater than the
       sequence number stored in the matching OCS entry, then a reacting
       node MUST update the matching OCS entry.

       If the sequence number for the received OLR is less than or equal to
       the sequence number in the matching OCS entry, then a reacting node
       MUST silently ignore the received OLR.  The matching OCS MUST NOT be
       updated in this case.

       If the reacting node determines that the sequence number has rolled
       over, then the reacting node MUST update the matching OCS entry.
       This can be determined by recognizing that the number has changed
       from a value within 1% of the maximum value in the OC-Sequence-Number
       AVP to a value within 1% of the minimum value in the OC-Sequence-
       Number AVP.

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       If the received OLR is for a new overload condition, then a reacting
       node MUST generate a new OCS entry for the overload condition.

       For a host report, this means a reacting node creates an OCS entry
       with the Application-ID in the received message and DiameterIdentity
       of the Origin-Host in the received message.

          Note: This solution assumes that the Origin-Host AVP in the answer
          message included by the reporting node is not changed along the
          path to the reacting node.

       For a realm report, this means a reacting node creates an OCS entry
       with the Application-ID in the received message and realm of the
       Origin-Realm in the received message.

       If the received OLR contains a validity duration of zero ("0"), then
       a reacting node MUST update the OCS entry as being expired.

          Note: It is not necessarily appropriate to delete the OCS entry,
          as the recommended behavior is that the reacting node slowly
          returns to full traffic when ending an overload abatement period.

       The reacting node does not delete an OCS when receiving an answer
       message that does not contain an OC-OLR AVP (i.e., absence of OLR
       means "no change").

    5.2.1.4.  Reporting Node's Maintenance of Overload Control State

       A reporting node SHOULD create a new OCS entry when entering an
       overload condition.

          Note: If a reporting node knows through absence of the
          OC-Supported-Features AVP in received messages that there are no
          reacting nodes supporting DOIC, then the reporting node can choose
          to not create OCS entries.

       When generating a new OCS entry, the sequence number SHOULD be set to
       zero ("0").

       When generating sequence numbers for new overload conditions, the new
       sequence number MUST be greater than any sequence number in an active
       (unexpired) overload report for the same application and report type
       previously sent by the reporting node.  This property MUST hold over
       a reboot of the reporting node.

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          Note: One way of addressing this over a reboot of a reporting node
          is to use a timestamp for the first overload condition that occurs
          after the report and to start using sequences beginning with zero
          for subsequent overload conditions.

       A reporting node MUST update an OCS entry when it needs to adjust the
       validity duration of the overload condition at reacting nodes.

          Example: If a reporting node wishes to instruct reacting nodes to
          continue overload abatement for a longer period of time than
          originally communicated.  This also applies if the reporting node
          wishes to shorten the period of time that overload abatement is to
          continue.

       A reporting node MUST update an OCS entry when it wishes to adjust
       any parameters specific to the abatement algorithm, including, for
       example, the reduction percentage used for the loss abatement
       algorithm.

          Example: If a reporting node wishes to change the reduction
          percentage either higher (if the overload condition has worsened)
          or lower (if the overload condition has improved), then the
          reporting node would update the appropriate OCS entry.

       A reporting node MUST increment the sequence number associated with
       the OCS entry anytime the contents of the OCS entry are changed.
       This will result in a new sequence number being sent to reacting
       nodes, instructing them to process the OC-OLR AVP.

       A reporting node SHOULD update an OCS entry with a validity duration
       of zero ("0") when the overload condition ends.

          Note: If a reporting node knows that the OCS entries in the
          reacting nodes are near expiration, then the reporting node might
          decide not to send an OLR with a validity duration of zero.

       A reporting node MUST keep an OCS entry with a validity duration of
       zero ("0") for a period of time long enough to ensure that any
       unexpired reacting node's OCS entry created as a result of the
       overload condition in the reporting node is deleted.

    5.2.2.  Reacting Node Behavior

       When a reacting node sends a request, it MUST determine if that
       request matches an active OCS.

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       If the request matches an active OCS, then the reacting node MUST use
       the overload abatement algorithm indicated in the OCS to determine if
       the request is to receive overload abatement treatment.

       For the loss abatement algorithm defined in this specification, see
       Section 6 for the overload abatement algorithm logic applied.

       If the overload abatement algorithm selects the request for overload
       abatement treatment, then the reacting node MUST apply overload
       abatement treatment on the request.  The abatement treatment applied
       depends on the context of the request.

       If diversion abatement treatment is possible (i.e., a different path
       for the request can be selected where the overloaded node is not part
       of the different path), then the reacting node SHOULD apply diversion
       abatement treatment to the request.  The reacting node MUST apply
       throttling abatement treatment to requests identified for abatement
       treatment when diversion treatment is not possible or was not
       applied.

          Note: This only addresses the case where there are two defined
          abatement treatments, diversion and throttling.  Any extension
          that defines a new abatement treatment must also define its
          interaction with existing treatments.

       If the overload abatement treatment results in throttling of the
       request and if the reacting node is an agent, then the agent MUST
       send an appropriate error as defined in Section 8.

       Diameter endpoints that throttle requests need to do so according to
       the rules of the client application.  Those rules will vary by
       application and are beyond the scope of this document.

       In the case that the OCS entry indicated no traffic was to be sent to
       the overloaded entity and the validity duration expires, then
       overload abatement associated with the overload report MUST be ended
       in a controlled fashion.

    5.2.3.  Reporting Node Behavior

       If there is an active OCS entry, then a reporting node SHOULD include
       the OC-OLR AVP in all answers to requests that contain the
       OC-Supported-Features AVP and that match the active OCS entry.

          Note: A request matches 1) if the Application-ID in the request
          matches the Application-ID in any active OCS entry and 2) if the
          report type in the OCS entry matches a report type supported by
          the reporting node as indicated in the OC-Supported-Features AVP.

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       The contents of the OC-OLR AVP depend on the selected algorithm.

       A reporting node MAY choose to not resend an overload report to a
       reacting node if it can guarantee that this overload report is
       already active in the reacting node.

          Note: In some cases (e.g., when there are one or more agents in
          the path between reporting and reacting nodes, or when overload
          reports are discarded by reacting nodes), a reporting node may not
          be able to guarantee that the reacting node has received the
          report.

       A reporting node MUST NOT send overload reports of a type that has
       not been advertised as supported by the reacting node.

          Note: A reacting node implicitly advertises support for the host
          and realm report types by including the OC-Supported-Features AVP
          in the request.  Support for other report types will be explicitly
          indicated by new feature bits in the OC-Feature-Vector AVP.

       A reporting node SHOULD explicitly indicate the end of an overload
       occurrence by sending a new OLR with OC-Validity-Duration set to a
       value of zero ("0").  The reporting node SHOULD ensure that all
       reacting nodes receive the updated overload report.

       A reporting node MAY rely on the OC-Validity-Duration AVP values for
       the implicit cleanup of overload control state on the reacting node.

          Note: All OLRs sent have an expiration time calculated by adding
          the validity duration contained in the OLR to the time the message
          was sent.  Transit time for the OLR can be safely ignored.  The
          reporting node can ensure that all reacting nodes have received
          the OLR by continuing to send it in answer messages until the
          expiration time for all OLRs sent for that overload condition have
          expired.

       When a reporting node sends an OLR, it effectively delegates any
       necessary throttling to downstream nodes.  If the reporting node also
       locally throttles the same set of messages, the overall number of
       throttled requests may be higher than intended.  Therefore, before
       applying local message throttling, a reporting node needs to check if
       these messages match existing OCS entries, indicating that these
       messages have survived throttling applied by downstream nodes that
       have received the related OLR.

       However, even if the set of messages match existing OCS entries, the
       reporting node can still apply other abatement methods such as
       diversion.  The reporting node might also need to throttle requests

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       for reasons other than overload.  For example, an agent or server
       might have a configured rate limit for each client and might throttle
       requests that exceed that limit, even if such requests had already
       been candidates for throttling by downstream nodes.  The reporting
       node also has the option to send new OLRs requesting greater
       reductions in traffic, reducing the need for local throttling.

       A reporting node SHOULD decrease requested overload abatement
       treatment in a controlled fashion to avoid oscillations in traffic.

          Example: A reporting node might wait some period of time after
          overload ends before terminating the OLR, or it might send a
          series of OLRs indicating progressively less overload severity.

    5.3.  Protocol Extensibility

       The DOIC solution can be extended.  Types of potential extensions
       include new traffic abatement algorithms, new report types, or other
       new functionality.

       When defining a new extension that requires new normative behavior,
       the specification must define a new feature for the OC-Feature-Vector
       AVP.  This feature bit is used to communicate support for the new
       feature.

       The extension may define new AVPs for use in the DOIC Capability
       Announcement and for use in DOIC overload reporting.  These new AVPs
       SHOULD be defined to be extensions to the OC-Supported-Features or
       OC-OLR AVPs defined in this document.

       The Grouped AVP extension mechanisms defined in [RFC6733] apply.
       This allows, for example, defining a new feature that is mandatory to
       be understood even when piggybacked on an existing application.

       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 OC-Supported-Feature and OC-OLR AVPs can be expanded with
       optional sub-AVPs only if a legacy DOIC implementation can safely
       ignore them without breaking backward compatibility for the given
       OC-Report-Type AVP value.  Any new sub-AVPs must not require that the
       M-bit be set.

       Documents that introduce new report types must describe any
       limitations on their use across non-supporting agents.

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       As with any Diameter specification, RFC 6733 requires all new AVPs to
       be registered with IANA.  See Section 9 for the required procedures.
       New features (feature bits in the OC-Feature-Vector AVP) and report
       types (in the OC-Report-Type AVP) MUST be registered with IANA.

    6.  Loss Algorithm

       This section documents the Diameter overload loss abatement
       algorithm.

    6.1.  Overview

       The DOIC specification supports the ability for multiple overload
       abatement algorithms to be specified.  The abatement algorithm used
       for any instance of overload is determined by the DOIC Capability
       Announcement process documented in Section 5.1.

       The loss algorithm described in this section is the default algorithm
       that must be supported by all Diameter nodes that support DOIC.

       The loss algorithm is designed to be a straightforward and stateless
       overload abatement algorithm.  It is used by reporting nodes to
       request a percentage reduction in the amount of traffic sent.  The
       traffic impacted by the requested reduction depends on the type of
       overload report.

       Reporting nodes request the stateless reduction of the number of
       requests by an indicated percentage.  This percentage reduction is in
       comparison to the number of messages the node otherwise would send,
       regardless of how many requests the node might have sent in the past.

       From a conceptual level, the logic at the reacting node could be
       outlined as follows.

       1.  An overload report is received, and the associated OCS is either
           saved or updated (if required) by the reacting node.

       2.  A new Diameter request is generated by the application running on
           the reacting node.

       3.  The reacting node determines that an active overload report
           applies to the request, as indicated by the corresponding OCS
           entry.

       4.  The reacting node determines if overload abatement treatment
           should be applied to the request.  One approach that could be
           taken for each request is to select a uniformly selected random
           number between 1 and 100.  If the random number is less than or

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           equal to the indicated reduction percentage, then the request is
           given abatement treatment; otherwise, the request is given normal
           routing treatment.

    6.2.  Reporting Node Behavior

       The method a reporting node uses to determine the amount of traffic
       reduction required to address an overload condition is an
       implementation decision.

       When a reporting node that has selected the loss abatement algorithm
       determines the need to request a reduction in traffic, it includes an
       OC-OLR AVP in answer messages as described in Section 5.2.3.

       When sending the OC-OLR AVP, the reporting node MUST indicate a
       percentage reduction in the OC-Reduction-Percentage AVP.

       The reporting node MAY change the reduction percentage in subsequent
       overload reports.  When doing so, the reporting node must conform to
       overload report handling specified in Section 5.2.3.

    6.3.  Reacting Node Behavior

       The method a reacting node uses to determine which request messages
       are given abatement treatment is an implementation decision.

       When receiving an OC-OLR in an answer message where the algorithm
       indicated in the OC-Supported-Features AVP is the loss algorithm, the
       reacting node MUST apply abatement treatment to the requested
       percentage of request messages sent.

          Note: The loss algorithm is a stateless algorithm.  As a result,
          the reacting node does not guarantee that there will be an
          absolute reduction in traffic sent.  Rather, it guarantees that
          the requested percentage of new requests will be given abatement
          treatment.

       If the reacting node comes out of the 100% traffic reduction
       (meaning, it has received an OLR indicating that no traffic should be
       sent, as a result of the overload report timing out), 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%
       reduction.

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          Note: The goal of this behavior is to reduce the probability of
          overload condition thrashing where an immediate transition from
          100% reduction to 0% reduction results in the reporting node
          moving quickly back into an overload condition.

    7.  Attribute Value Pairs

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

       Refer to Section 4 of [RFC6733] for more information on AVPs and AVP
       data types.

    7.1.  OC-Supported-Features AVP

       The OC-Supported-Features AVP (AVP Code 621) is of type Grouped and
       serves two purposes.  First, it announces a node's support for the
       DOIC solution in general.  Second, it contains the description of the
       supported DOIC features of the sending node.  The OC-Supported-
       Features AVP MUST be included in every Diameter request message a
       DOIC supporting node sends.

          OC-Supported-Features ::= < AVP Header: 621 >
                                    [ OC-Feature-Vector ]
                                  * [ AVP ]

    7.2.  OC-Feature-Vector AVP

       The OC-Feature-Vector AVP (AVP Code 622) is of type Unsigned64 and
       contains a 64-bit flags field of announced capabilities of a DOIC
       node.  The value of zero (0) is reserved.

       The OC-Feature-Vector sub-AVP is used to announce the DOIC features
       supported by the DOIC node, in the form of a flag-bits field in which
       each bit announces one feature or capability supported by the node.
       The absence of the OC-Feature-Vector AVP in request messages
       indicates that only the default traffic abatement algorithm described
       in this specification is supported.  The absence of the OC-Feature-
       Vector AVP in answer messages indicates that the default traffic
       abatement algorithm described in this specification is selected
       (while other traffic abatement algorithms may be supported), and no
       features other than abatement algorithms are supported.

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       The following capability is defined in this document:

       OLR_DEFAULT_ALGO (0x0000000000000001)

          When this flag is set by the a DOIC reacting node, it means that
          the default traffic abatement (loss) algorithm is supported.  When
          this flag is set by a DOIC reporting node, it means that the loss
          algorithm will be used for requested overload abatement.

    7.3.  OC-OLR AVP

       The OC-OLR AVP (AVP Code 623) is of type Grouped and contains the
       information necessary to convey an overload report on an overload
       condition at the reporting node.  The application the OC-OLR AVP
       applies to is identified by the Application-ID found in the Diameter
       message header.  The host or realm the OC-OLR AVP concerns is
       determined from the Origin-Host AVP and/or Origin-Realm AVP found in
       the encapsulating Diameter command.  The OC-OLR AVP is intended to be
       sent only by a reporting node.

          OC-OLR ::= < AVP Header: 623 >
                     < OC-Sequence-Number >
                     < OC-Report-Type >
                     [ OC-Reduction-Percentage ]
                     [ OC-Validity-Duration ]
                   * [ AVP ]

    7.4.  OC-Sequence-Number AVP

       The OC-Sequence-Number AVP (AVP Code 624) is of type Unsigned64.  Its
       usage in the context of overload control is described in Section 5.2.

       From the functionality point of view, the OC-Sequence-Number AVP is
       used as a nonvolatile increasing counter for a sequence of overload
       reports between two DOIC nodes for the same overload occurrence.
       Sequence numbers are treated in a unidirectional manner, i.e., two
       sequence numbers in each direction between two DOIC nodes are not
       related or correlated.

    7.5.  OC-Validity-Duration AVP

       The OC-Validity-Duration AVP (AVP Code 625) is of type Unsigned32 and
       indicates in seconds the validity time of the overload report.  The
       number of seconds is measured after reception of the first OC-OLR AVP
       with a given value of OC-Sequence-Number AVP.  The default value for
       the OC-Validity-Duration AVP is 30 seconds.  When the OC-Validity-
       Duration AVP is not present in the OC-OLR AVP, the default value
       applies.  The maximum value for the OC-Validity-Duration AVP is

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       86,400 seconds (24 hours).  If the value received in the OC-Validity-
       Duration is greater than the maximum value, then the default value
       applies.

    7.6.  OC-Report-Type AVP

       The OC-Report-Type AVP (AVP Code 626) is of type Enumerated.  The
       value of the AVP describes what the overload report concerns.  The
       following values are initially defined:

       HOST_REPORT 0
          The overload report is for a host.  Overload abatement treatment
          applies to host-routed requests.

       REALM_REPORT 1
          The overload report is for a realm.  Overload abatement treatment
          applies to realm-routed requests.

       The values 2-4294967295 are unassigned.

    7.7.  OC-Reduction-Percentage AVP

       The OC-Reduction-Percentage AVP (AVP Code 627) is of type Unsigned32
       and describes the percentage of the traffic that the sender is
       requested to reduce, compared to what it otherwise would send.  The
       OC-Reduction-Percentage AVP applies to the default (loss) algorithm
       specified in this specification.  However, the AVP can be reused for
       future abatement algorithms, if its semantics fit into the new
       algorithm.

       The value of the Reduction-Percentage AVP is between zero (0) and one
       hundred (100).  Values greater than 100 are ignored.  The value of
       100 means that all traffic is to be throttled, 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 need for the reacting node to apply any traffic abatement.

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    7.8.  AVP Flag Rules

                                                             +---------+
                                                             |AVP flag |
                                                             |rules    |
                                                             +----+----+
                                  AVP   Section              |    |MUST|
           Attribute Name         Code  Defined  Value Type  |MUST| NOT|
          +--------------------------------------------------+----+----+
          |OC-Supported-Features  621   7.1      Grouped     |    | V  |
          +--------------------------------------------------+----+----+
          |OC-Feature-Vector      622   7.2      Unsigned64  |    | V  |
          +--------------------------------------------------+----+----+
          |OC-OLR                 623   7.3      Grouped     |    | V  |
          +--------------------------------------------------+----+----+
          |OC-Sequence-Number     624   7.4      Unsigned64  |    | V  |
          +--------------------------------------------------+----+----+
          |OC-Validity-Duration   625   7.5      Unsigned32  |    | V  |
          +--------------------------------------------------+----+----+
          |OC-Report-Type         626   7.6      Enumerated  |    | V  |
          +--------------------------------------------------+----+----+
          |OC-Reduction                                      |    |    |
          |  -Percentage          627   7.7      Unsigned32  |    | V  |
          +--------------------------------------------------+----+----+

       As described in the Diameter base protocol [RFC6733], the M-bit usage
       for a given AVP in a given command may be defined by the application.

    8.  Error Response Codes

       When a DOIC node rejects a Diameter request due to overload, the DOIC
       node MUST select an appropriate error response code.  This
       determination is made based on the probability of the request
       succeeding if retried on a different path.

          Note: This only applies for DOIC nodes that are not the originator
          of the request.

       A reporting node rejecting a Diameter request due to an overload
       condition SHOULD send a DIAMETER_TOO_BUSY error response, if it can
       assume that the same request may succeed on a different path.

       If a reporting node knows or assumes that the same request will not
       succeed on a different path, the DIAMETER_UNABLE_TO_COMPLY error
       response SHOULD be used.  Retrying would consume valuable resources
       during an occurrence of overload.

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          For instance, if the request arrived at the reporting node without
          a Destination-Host AVP, then the reporting node might determine
          that there is an alternative Diameter node that could successfully
          process the request and that retrying the transaction would not
          negatively impact the reporting node.  DIAMETER_TOO_BUSY would be
          sent in this case.

          If the request arrived at the reporting node with a Destination-
          Host AVP populated with its own Diameter identity, then the
          reporting node can assume that retrying the request would result
          in it coming to the same reporting node.
          DIAMETER_UNABLE_TO_COMPLY would be sent in this case.

          A second example is when an agent that supports the DOIC solution
          is performing the role of a reacting node for a non-supporting
          client.  Requests that are rejected as a result of DOIC throttling
          by the agent in this scenario would generally be rejected with a
          DIAMETER_UNABLE_TO_COMPLY response code.

    9.  IANA Considerations

    9.1.  AVP Codes

       New AVPs defined by this specification are listed in Section 7.  All
       AVP codes are allocated from the "AVP Codes" sub-registry under the
       "Authentication, Authorization, and Accounting (AAA) Parameters"
       registry.

    9.2.  New Registries

       Two new registries have been created in the "AVP Specific Values"
       sub-registry under the "Authentication, Authorization, and Accounting
       (AAA) Parameters" registry.

       A new "OC-Feature-Vector AVP Values (code 622)" registry has been
       created.  This registry contains the following:

          Feature Vector Value Name

          Feature Vector Value

          Specification defining the new value

       See Section 7.2 for the initial Feature Vector Value in the registry.
       This specification defines the value.  New values can be added to the
       registry using the Specification Required policy [RFC5226].

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       A new "OC-Report-Type AVP Values (code 626)" registry has been
       created.  This registry contains the following:

          Report Type Value Name

          Report Type Value

          Specification defining the new value

       See Section 7.6 for the initial assignment in the registry.  New
       types can be added using the Specification Required policy [RFC5226].

    10.  Security Considerations

       DOIC gives Diameter nodes the ability to request that downstream
       nodes send fewer Diameter requests.  Nodes do this by exchanging
       overload reports that directly effect 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.  For
       instance, a series of injected realm OLRs with a requested reduction
       percentage of 100% could be used to completely eliminate any traffic
       from being sent to that realm.

       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 their
       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.

    10.1.  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., 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.

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       When agents are involved, this presents an effectively transitive
       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 occur 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 an answer.  Such an answer would also need to
       arrive at a Diameter node via a protected transport connection.
       Therefore, implementations MUST validate that an answer containing an
       overload report is a properly constructed response to a pending
       request prior to acting on the overload report, and that the answer
       was received via an appropriate transport connection.

       A similar attack involves a compromised but otherwise authorized 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 a peer that applies to a realm
       not handled by that peer is suspect.  This may require out-of-band,
       non-Diameter agreements and/or mechanisms.

          This attack is partially mitigated by the fact that the
          application, as well as host and realm, for a given OLR is
          determined implicitly by respective AVPs in the enclosing answer.
          If a reporting node modifies any of those AVPs, the enclosing
          transaction will also be affected.

    10.2.  Denial-of-Service Attacks

       Diameter overload reports, especially realm 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 attacks.
       Furthermore, since Diameter is almost always used in support of other

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       protocols, a DoS attack on Diameter is likely to impact those
       protocols as well.  In the worst case, where the Diameter application
       is being used for access control into an IP network, a coordinated
       DoS attack could result in the blockage of all traffic into that
       network.  Therefore, Diameter nodes MUST NOT honor or forward OLRs
       received from peers that are not trusted to send them.

       An attacker might use the information in an OLR to assist in DoS
       attacks.  For example, an attacker could use information about
       current overload conditions to time an attack for maximum effect, or
       use subsequent overload reports as a feedback mechanism to learn the
       results of a previous or ongoing attack.  Operators need the ability
       to ensure that OLRs are not leaked to untrusted parties.

    10.3.  Noncompliant Nodes

       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 throttle 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.

       When a Diameter node sends an overload report, it cannot assume that
       all nodes will comply, even if they indicate support for DOIC.  A
       noncompliant node might continue to send requests with no reduction
       in load.  Such noncompliance could be done accidentally or
       maliciously to gain an unfair advantage over compliant nodes.
       Requirement 28 in [RFC7068] indicates that the overload control
       solution cannot assume that all Diameter nodes in a network are
       trusted.  It also requires that malicious nodes not be allowed to
       take advantage of the overload control mechanism to get more than
       their fair share of service.

    10.4.  End-to-End Security Issues

       The lack of end-to-end integrity features makes it difficult to
       establish trust in overload reports received 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

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       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.  DOIC
       nodes MUST strip DOIC AVPs from messages received from peers that are
       not trusted for DOIC purposes.

       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.

          A DOIC node cannot always automatically detect that a peer also
          supports DOIC.  For example, a node might have a peer that is a
          non-supporting agent.  If nodes on the other side of that agent
          send OC-Supported-Features AVPs, the agent is likely to forward
          them as unknown AVPs.  Messages received across the non-supporting
          agent may be indistinguishable from messages received across a
          DOIC supporting agent, giving the false impression that the non-
          supporting agent actually supports DOIC.  This complicates the
          transitive-trust nature of DOIC.  Operators need to be careful to
          avoid situations where a non-supporting agent is mistakenly
          trusted to enforce DOIC-related authorization policies.

       It is expected that work on end-to-end Diameter security 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 allows 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.

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    11.  References

    11.1.  Normative References

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <http://www.rfc-editor.org/info/rfc2119>.

       [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
                  IANA Considerations Section in RFCs", BCP 26, RFC 5226,
                  DOI 10.17487/RFC5226, May 2008,
                  <http://www.rfc-editor.org/info/rfc5226>.

       [RFC6733]  Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
                  Ed., "Diameter Base Protocol", RFC 6733,
                  DOI 10.17487/RFC6733, October 2012,
                  <http://www.rfc-editor.org/info/rfc6733>.

    11.2.  Informative References

       [Cx]       3GPP, "Cx and Dx interfaces based on the Diameter
                  protocol; Protocol details", 3GPP TS 29.229 12.7.0,
                  September 2015.

       [PCC]      3GPP, "Policy and charging control architecture", 3GPP
                  TS 23.203 12.10.0, September 2015.

       [RFC4006]  Hakala, H., Mattila, L., Koskinen, J-P., Stura, M., and J.
                  Loughney, "Diameter Credit-Control Application", RFC 4006,
                  DOI 10.17487/RFC4006, August 2005,
                  <http://www.rfc-editor.org/info/rfc4006>.

       [RFC7068]  McMurry, E. and B. Campbell, "Diameter Overload Control
                  Requirements", RFC 7068, DOI 10.17487/RFC7068, November
                  2013, <http://www.rfc-editor.org/info/rfc7068>.

       [S13]      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
                  12.8.0, September 2015.

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    Appendix A.  Issues Left for Future Specifications

       The base solution for overload control does not cover all possible
       use cases.  A number of solution aspects were intentionally left for
       future specification and protocol work.  The following subsections
       define some of the potential extensions to the DOIC solution.

    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 7.2 and 9 for the required IANA
       steps.

    A.2.  Agent Overload

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

    A.3.  New Error Diagnostic AVP

       This specification indicates the use of existing error messages when
       nodes reject requests due to overload.  There is an expectation that
       additional error codes or AVPs will be defined in a separate
       specification to indicate that overload was the reason for the
       rejection of the message.

    Appendix B.  Deployment Considerations

       Non-supporting Agents

          Due to the way that realm-routed requests are handled in Diameter
          networks with the server selection for the request done by an
          agent, network operators should enable DOIC at agents that perform
          server selection first.

       Topology-Hiding Interactions

          There exist proxies that implement what is referred to as Topology
          Hiding.  This can include cases where the agent modifies the
          Origin-Host in answer messages.  The behavior of the DOIC solution
          is not well understood when this happens.  As such, the DOIC
          solution does not address this scenario.

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       Inter-Realm/Administrative Domain Considerations

          There are likely to be special considerations for handling DOIC
          signaling across administrative boundaries.  This includes
          considerations for whether or not information included in the DOIC
          signaling should be sent across those boundaries.  In addition,
          consideration should be taken as to whether or not a reacting node
          in one realm can be trusted to implement the requested overload
          abatement handling for overload reports received from a separately
          administered realm.

    Appendix C.  Considerations for Applications Integrating the DOIC
                 Solution

       This section outlines considerations to be taken into account when
       integrating the DOIC solution into Diameter applications.

    C.1.  Application Classification

       The following is a classification of Diameter applications and
       request types.  This discussion is meant to document factors that
       play into decisions made by the Diameter entity responsible for
       handling 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.

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

       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.

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       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 [S13], 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 [Cx] is an
          example of a pseudo-session application.

       The handling of overload reports must take the type of application
       into consideration, as discussed in Appendix C.2.

    C.2.  Implications of Application Type Overload

       This section discusses considerations for mitigating overload
       reported by a Diameter entity.  This discussion focuses on the type
       of application.  Appendix C.3 discusses considerations for handling
       various request types when the target server is known to be in an
       overloaded state.

       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 it 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.

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       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
          repeated.

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

    C.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.

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       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 Policy and Charging Control (PCC)
          architecture [PCC], where multiple apparently independent Diameter
          application sessions are actually correlated and must be handled
          by the same Diameter server.

       Intra-session Request:

          An intra-session request is a request that uses the same Session-
          Id as 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 request.

       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 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
          [Cx] application are examples of pseudo-session requests.

    C.4.  Request Type Overload Implications

       The request classes identified in Appendix C.3 have implications on
       decisions about which requests should be throttled first.  The
       following list of request treatments regarding throttling is provided
       as guidelines for application designers when implementing the
       Diameter overload control mechanism described in this document.  The
       exact behavior regarding throttling is a matter of local policy,
       unless specifically defined for the application.

       Independent Requests:

          Independent requests can generally be given equal treatment when
          making throttling decisions, unless otherwise indicated by
          application requirements or local policy.

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       Session-Initiating Requests:

          Session-initiating requests often represent more work than
          independent or intra-session requests.  Moreover, session-
          initiating requests are typically followed by other session-
          related requests.  Since the main objective of 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.  In the absence of
          local policies or application-specific requirements to the
          contrary, individual session-initiating requests can be given
          equal treatment when making throttling decisions.

       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, it 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 types of intra-sessions requests, requests that
          terminate a session and the remainder of intra-session requests.
          Implementers and operators may choose to throttle session-
          terminating requests less aggressively in order to gracefully
          terminate sessions, allow cleanup of the related resources (e.g.,
          session state), and avoid the need for additional intra-session
          requests.  Favoring session termination requests may reduce 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.

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    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

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    Authors' Addresses

       Jouni Korhonen (editor)
       Broadcom Corporation
       3151 Zanker Road
       San Jose, CA  95134
       United States

       Email: jouni.nospam@gmail.com

       Steve Donovan (editor)
       Oracle
       7460 Warren Parkway
       Frisco, Texas  75034
       United States

       Email: srdonovan@usdonovans.com

       Ben Campbell
       Oracle
       7460 Warren Parkway
       Frisco, Texas  75034
       United States

       Email: ben@nostrum.com

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

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

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