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PROPOSED STANDARD
Errata Exist
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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   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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