Diameter Routing Message Priority
RFC 7944

Internet Engineering Task Force (IETF)                        S. Donovan
Request for Comments: 7944                                        Oracle
Category: Standards Track                                    August 2016
ISSN: 2070-1721

                   Diameter Routing Message Priority

Abstract

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

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 7841.

   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/rfc7944.

Copyright Notice

   Copyright (c) 2016 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Applicability . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology and Abbreviations . . . . . . . . . . . . . . . .   4
   3.  Conventions Used in This Document . . . . . . . . . . . . . .   4
   4.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  First-Responder-Related Signaling . . . . . . . . . . . .   6
     5.2.  Emergency-Call-Related Signaling  . . . . . . . . . . . .   6
     5.3.  Differentiated Services . . . . . . . . . . . . . . . . .   7
     5.4.  Application-Specific Priorities . . . . . . . . . . . . .   7
   6.  Theory of Operation . . . . . . . . . . . . . . . . . . . . .   8
   7.  Extensibility . . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Normative Behavior  . . . . . . . . . . . . . . . . . . . . .  10
   9.  Attribute Value Pairs . . . . . . . . . . . . . . . . . . . .  12
     9.1.  DRMP AVP  . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.2.  Attribute Value Pair Flag Rules . . . . . . . . . . . . .  13
   10. Considerations When Defining Application Priorities . . . . .  14
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     11.1.  AVP Codes  . . . . . . . . . . . . . . . . . . . . . . .  15
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  15
     12.1.  Potential Threat Modes . . . . . . . . . . . . . . . . .  15
     12.2.  Denial-of-Service Attacks  . . . . . . . . . . . . . . .  16
     12.3.  End-to-End Security Issues . . . . . . . . . . . . . . .  16
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     13.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The Diameter Overload Indication Conveyance (DOIC) solution [RFC7683]
   for Diameter overload control introduces scenarios where Diameter
   routing decisions made by Diameter nodes can be influenced by the
   overload state of other Diameter nodes.  This includes the scenarios
   where Diameter endpoints and Diameter Agents can throttle requests as
   a result of the target for the request being overloaded.

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

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

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

1.1.  Applicability

   There are two primary considerations that must be addressed for the
   mechanism described in this document to work effectively.  The first
   takes into consideration the fact that the Diameter base protocol
   defined in [RFC6733]  is designed to transport multiple Diameter
   applications and that Diameter nodes can be implemented that support
   multiple applications.  In order for the Diameter Routing Message
   Priority (DRMP) mechanism to work, the priorities defined for all
   messages across all applications used in a Diameter administrative
   domain must be defined in a consistent and coordinated fashion,
   taking the default priority into account.  See Section 10 for a
   discussion of some of the considerations that need to be factored
   into the setting of DRMPs used by Diameter applications.

      Note that this consideration does not apply to Diameter networks
      where all Diameter nodes only support a single application.

   Without this cross application priority design taken into
   consideration, it is possible for messages for one application to
   gain unwarranted preferential treatment over messages for other
   applications.

   This mechanism also depends on all of the messages that carry the
   DRMP Attribute Value Pair (AVP) that are inserted into Diameter
   messages by trusted nodes within the Diameter administrative domain.
   As discussed in Section 12, misbehaving nodes have the ability to use
   the DRMP mechanism to gain unwarranted preferential treatment.

   When messages cross Diameter administrative boundaries, care should
   be taken to either strip or modify the DRMP values in these messages.
   If the priority definitions vary between the two Diameter
   administrative domains, then it is possible for messages from a
   foreign domain to gain unwarranted preferential treatment.

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2.  Terminology and Abbreviations

   Diversion

      As defined in [RFC7683].  An overload abatement treatment where
      the reacting node selects alternate destinations or paths for
      requests.

   DOIC

      Diameter Overload Indication Conveyance.

   DRMP

      Diameter Routing Message Priority.

   Overload Abatement

      As defined in [RFC7683].  Reaction to receipt of an overload
      report resulting in a reduction in traffic sent to the reporting
      node.  Abatement actions include diversion and throttling.

   Priority

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

   Throttling

      As defined in [RFC7683].  An abatement treatment that limits the
      number of requests sent by the DOIC 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 [RFC2119].

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

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4.  Problem Statement

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

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

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

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

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

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

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

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

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5.  Use Cases

   This section discusses various scenarios where Diameter transactions
   can benefit from the use of priority information.

   It is important to note that for priority information to be reliably
   usable, the Diameter nodes sending and consuming DRMP AVPs must have
   pre-established trust relationships of the sort described in
   Section 12.

5.1.  First-Responder-Related Signaling

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

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

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

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

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

5.2.  Emergency-Call-Related Signaling

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

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5.3.  Differentiated Services

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

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

5.4.  Application-Specific Priorities

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

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

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

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

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

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      One example of this is in the 3GPP application [S6a] where an
      Update Location Request (ULR) resulting from a Mobility Management
      Entity (MME) restoration procedure might be given a higher
      priority than a ULR resulting from an initial attach.

6.  Theory of Operation

   This section outlines the envisioned usage of DRMP.

   The expected behavior depends on the role (request sender, agent, or
   request handler) of the Diameter node handling the request.

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

   1.  Request sender -- The sender of a request, be it a Diameter
       Client or a Diameter Server, determines the relative priority of
       the request and includes that priority information in the
       request.  The method for determining the relative priority is
       application specific and is outside the scope of this
       specification.  The request sender also saves the priority
       information with the transaction state.  This will be used when
       handling the answer messages.

   2.  Agents handling the request -- Agents use the priority
       information when making routing decisions.  This can include
       determining which requests to route first, which requests to
       throttle, and where the request is routed.  For instance,
       requests with higher priority might have a lower probability of
       being throttled.  The mechanism for how the agent determines
       which requests are candidates to be throttled is implementation
       dependent and is outside the scope of this document.  Before
       forwarding request messages, agents generally do not modify the
       priority information present in the received request message nor
       include the priority information when absent in the received
       request message.  However, in some scenarios, agents can modify
       the priority information, for example, edge agents modifying the
       priority values set by an adjacent operator.  There might be
       other scenarios where a Diameter endpoint does not support the
       DRMP mechanism, and agents insert the priority information in the
       request messages for that non-supporting endpoint.  When
       forwarding the request messages, the agent also saves the
       transaction priority in the transaction state either as locally
       managed state or using the Proxy-Info mechanism defined in
       [RFC6733].  This will be used when handling the associated answer
       message for the transaction.

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   3.  Request handler -- The handler of the request, be it a Diameter
       Server or a Diameter Client, can use the priority information to
       determine how to handle the request.  This could include
       determining the order in which requests are handled and resources
       that are applied to the handling of the request.

   4.  Answer sender -- The handler of the request is also the sender of
       the answer.  The answer sender uses the priority information
       received in the request message when sending the answer.  This
       implies that answers for higher-priority transactions are given
       preferential treatment over lower-priority transactions.  The
       answer sender also has the option of including priority
       information in the answer message.  This is done when the answer
       message needs to have a different priority than the priority
       carried in the request message.  The priority included by the
       answer sender is application specific.

   5.  Agent handling the answer -- By default, agents handling answer
       messages use the priority information stored with the transaction
       state to determine the priority of relaying the answer message.
       However, priority information included in the answer message,
       when present, is used in place of the stored priority
       information.  The use of priority information implies that
       answers for higher-priority transactions are given preferential
       treatment over lower-priority transactions.  When forwarding the
       answer messages, agents generally do not modify the priority
       information present in the received answer messages nor include
       the priority information when absent in the received answer
       messages.  However, in some scenarios, agents can modify the
       priority information, for example, edge agents modifying the
       priority values set by an adjacent operator.  There might be
       other scenarios where a Diameter endpoint does not support the
       DRMP mechanism, and agents insert the priority information for
       that non-supporting endpoint.

   6.  Answer handler -- The answer handler uses the same method as the
       agent to determine the priority of the answer message.  By
       default, the handler of the answer message uses the priority
       saved in the transaction's state.  Priority information in the
       answer message is used when present.  The priority is used when
       allocating resources for processing that occurs after the receipt
       of the answer message.

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7.  Extensibility

   This document does not define extensibility mechanisms that are
   specific to the DRMP mechanism.  As a result, any extension that
   requires new AVPs will be required to use existing Diameter
   extensibility mechanisms defined in [RFC6733].

8.  Normative Behavior

   This section contains the normative behavior associated with DRMP.

   When routing priority information is available, Diameter nodes SHOULD
   include Diameter routing message priority in the DRMP AVP in all
   Diameter request messages.

      Note: The method of determining the priority value included in the
      request is application specific and is not in the scope of this
      specification.

   The priority marking scheme does not require the Diameter Agents to
   understand application-specific AVPs.

   When available, Diameter nodes SHOULD use routing priority
   information included in the DRMP AVP when making Diameter overload
   throttling decisions.

   Diameter Agents MAY use routing priority information included in the
   DRMP AVP when relaying request and answer messages.  This includes
   the selection of routes and the ordering of messages relayed.

      Note: The priority information included in the DRMP AVP in request
      messages applies to both the request message and, by default, the
      answer message associated with the transaction.

   While done only in exceptional circumstances, Diameter Agents MAY
   modify priority information when relaying request and answer
   messages.

      Note: There might be scenarios where a Diameter Agent does modify
      priority information.  For instance, an edge agent might need to
      modify the priority values set by an adjacent operator.

   While done only in exceptional circumstances, Diameter Agents MAY add
   priority information when relaying request and answer messages.

      Note: There might be scenarios where a Diameter endpoint does not
      support the DRMP mechanism, and agents insert priority information
      for that non-supporting endpoint.

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   Diameter endpoints MAY use routing priority information included in
   the DRMP AVP when making resource allocation decisions for the
   transaction associated with the request message that contains the
   DRMP information.

   Diameter endpoints MAY use routing priority information included in
   the DRMP AVP when making resource allocation decisions for the
   transaction associated with the answer messages using the DRMP
   information associated with the transaction.

   Diameter endpoints MAY include the DRMP AVP in answer messages.  This
   is done when the priority for the answer message needs to have a
   different priority than the priority carried in the request message.

   When determining the priority to apply to answer messages, Diameter
   nodes SHOULD use the priority indicated in the DRMP AVP carried in
   the answer message, if it exists.  If there is not DRMP AVP in the
   answer message, then the Diameter node SHOULD use the priority
   indicated in the DRMP AVP of the associated request message.

      Note: One method to determine what priority to apply to an answer
      when there is no DRMP AVP in the answer message is to save the
      priority included in the request message in the state associated
      with the Diameter transaction.  Another is to use the Proxy-Info
      mechanism defined in [RFC6733].

   Diameter nodes MUST have a default priority to apply to transactions
   that do not have an explicit priority set in the DRMP AVP.

   In order to guarantee consistent handling of messages from non-
   upgraded Diameter Clients, Diameter nodes SHOULD use the PRIORITY_10
   priority as this default priority value.

      PRIORITY_10 is a midrange priority that corresponds to "normal"
      traffic and thus would be a suitable default for most deployments,
      while still allowing different Diameter applications to designate
      other priorities for lower- and higher-priority traffic.

      Note: This does not imply that a DRMP AVP is added to the message.
      Rather, the message is treated the same as a message that has a
      DRMP AVP with a priority value of PRIORITY_10.

   Diameter nodes MUST support the ability for the default priority to
   be modified through local configuration interfaces.

      Note: There are scenarios where operators might want to specify a
      different default value for transactions that do not have an
      explicit priority.  In this case, the operator-defined local

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      policy would override the use of PRIORITY_10 as the default
      priority.

   When using DRMP information, Diameter nodes MUST use the default
   priority for transactions that do not have priority specified in a
   DRMP AVP.

      Note: This guidance on the handling of messages without a priority
      does not result in a Diameter Agent inserting a DRMP AVP into the
      message.  Rather, it gives guidance on how that specific
      transaction should be treated when its priority is compared with
      other requests.  When a Diameter Agent relays the request, it will
      not insert a DRMP AVP with a priority value of 10.

   When setting and using priorities, for all integers x,y in [0,15],
   treat PRIORITY_<x> as lower priority than PRIORITY_<y> when y<x.

      Note: As a result, PRIORITY_0 is the highest priority.

9.  Attribute Value Pairs

   This section describes the encoding and semantics of the Diameter
   Routing Message Priority AVP defined in this document.

9.1.  DRMP AVP

   The DRMP (AVP code 301) is of type Enumerated.  The value of the AVP
   indicates the routing message priority for the transaction.  The
   following values are defined:

   PRIORITY_15 15  PRIORITY_15 is the lowest priority.

   PRIORITY_14 14  PRIORITY_14 is a higher priority than PRIORITY_15 and
      a lower priority than PRIORITY_13.

   PRIORITY_13 13  PRIORITY_13 is a higher priority than PRIORITY_14 and
      a lower priority than PRIORITY_12.

   PRIORITY_12 12  PRIORITY_12 is a higher priority than PRIORITY_13 and
      a lower priority than PRIORITY_11.

   PRIORITY_11 11  PRIORITY_11 is a higher priority than PRIORITY_12 and
      a lower priority than PRIORITY_10.

   PRIORITY_10 10  PRIORITY_10 is a higher priority than PRIORITY_11 and
      a lower priority than PRIORITY_9.

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   PRIORITY_9 9  PRIORITY_9 is a higher priority than PRIORITY_10 and a
      lower priority than PRIORITY_8.

   PRIORITY_8 8  PRIORITY_8 is a higher priority than PRIORITY_9 and a
      lower priority than PRIORITY_7.

   PRIORITY_7 7  PRIORITY_7 is a higher priority than PRIORITY_8 and a
      lower priority than PRIORITY_6.

   PRIORITY_6 6  PRIORITY_6 is a higher priority than PRIORITY_7 and a
      lower priority than PRIORITY_5.

   PRIORITY_5 5  PRIORITY_5 is a higher priority than PRIORITY_6 and a
      lower priority than PRIORITY_4.

   PRIORITY_4 4  PRIORITY_4 is a higher priority than PRIORITY_5 and a
      lower priority than PRIORITY_3.

   PRIORITY_3 3  PRIORITY_3 is a higher priority than PRIORITY_4 and a
      lower priority than PRIORITY_2.

   PRIORITY_2 2  PRIORITY_2 is a higher priority than PRIORITY_3 and a
      lower priority than PRIORITY_1.

   PRIORITY_1 1  PRIORITY_1 is a higher priority than PRIORITY_2 and a
      lower priority than PRIORITY_0.

   PRIORITY_0 0  Priority 0 is the highest priority.

9.2.  Attribute Value Pair Flag Rules

                                                         +---------+
                                                         |AVP Flag |
                                                         |Rules    |
                                                         +----+----+
                              AVP   Section              |    |MUST|
       Attribute Name         Code  Defined  Value Type  |MUST| NOT|
      +--------------------------------------------------+----+----+
      |DRMP                    301  9.1      Enumerated  |    | V  |
      +--------------------------------------------------+----+----+

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10.  Considerations When Defining Application Priorities

   As discussed in Section 1.1, it is important that the definition of
   priority values used by all applications within a single Diameter
   administrative domain be done in a consistent and coordinated manner.

   The following are some things to be considered when defining the
   DRMPs to be used in Diameter networks that support Diameter nodes
   handling multiple applications.

   1.  As with any prioritization scheme, it is possible for higher-
       priority messages to block lower-priority messages from ever
       being handled.  In a Diameter network, this will often result in
       those Diameter transactions being retried.  This can result in
       more traffic than the network would have handled without use of
       the DRMP mechanism.

       One potential guideline to prevent unwanted starving of lower-
       priority messages is to have higher-priority messages represent a
       relatively small portion of messages handled by the Diameter
       network under normal scenarios.

          Note that there are scenarios, such as first responder
          messages, where the blocking of lower-priority messages is a
          requirement.

   2.  When setting priorities for any of the use cases outlined in
       Section 5, it is important to use the same priority values across
       applications.  For instance, when defining priority for the first
       responder use case discussed in Section 5.1 and the emergency
       call use case discussed in Section 5.2, one high-priority value
       might be used for all first responder messages, say PRIORITY_2,
       and a slightly lower-priority value, say PRIORITY_3, might be
       used for emergency-call-related messages.  These values should be
       specified for these use cases across all applications used within
       the Diameter administrative domain.

          Note that the values mentioned here are strictly for
          illustrative purposes.  The actual values used for these use
          cases are likely to be different.

   3.  Messages without the DRMP AVP will be given default priority
       value treatment.  This will include messages from Diameter
       Clients that have not been updated to support the DRMP mechanism.
       It might also include messages from foreign administrative
       domains if the DRMP AVPs are stripped from messages crossing the
       Diameter administrative domains.

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   4.  The process used to introduce the DRMP mechanism into a Diameter
       network should also be taken into consideration.  Messages of the
       same type within the same application might get different
       treatment depending on whether those messages are sent from nodes
       that are upgraded to support the DRMP mechanism versus nodes that
       have not yet been upgraded to support the DRMP mechanism.

11.  IANA Considerations

11.1.  AVP Codes

   The new AVP defined by this specification is listed in Section 9.
   All AVP codes are allocated from the "AVP Codes" subregistry of the
   "Authentication, Authorization, and Accounting (AAA) Parameters"
   registry.

12.  Security Considerations

   DRMP gives Diameter nodes the ability to influence which requests are
   throttled during overload scenarios.  In addition, DRMP can be used
   in determining the routing decisions for request messages.  Improper
   use of the DRMP mechanism could result in the malicious Diameter node
   gaining preferential treatment, by reducing the probability of its
   requests being throttled, over other Diameter nodes.  This would be
   achieved by the malicious node inserting priority values that are
   artificially high.

   Diameter does not include features to provide end-to-end
   authentication, integrity protection, or confidentiality.  This opens
   the possibility that malicious or compromised agents in the path of a
   request could modify the DRMP AVP to reflect a priority different
   than that asserted by the sender of the request.

12.1.  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., the Transmission Control Protocol (TCP) or Stream Control
   Transmission Protocol (SCTP)) connection, or the messages may
   traverse one or more intermediaries, known as Diameter Agents.
   Diameter nodes use Transport Layer Security (TLS), Datagram Transport
   Layer Security (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 occurs at the
   transport layer, agents can read, and perhaps modify, any part of a
   Diameter message, including the DRMP AVP.

   There are several ways an attacker might attempt to exploit the DRMP
   mechanism.  A malicious or compromised Diameter node might insert
   invalid priority values resulting in either preferential treatment,
   resulting from higher values, or degraded treatment resulting from
   lower values, for that node.

   A similar attack involves a malicious or compromised Diameter Agent
   changing the priority value resulting in the sending Diameter node
   getting either preferential or degraded service.

   The DRMP mechanism can be used to aid in overload throttling
   decisions.  When this is the case, then the above attacks are limited
   in scope to when one or more Diameter nodes are in an overloaded
   state.

   The DRMP mechanism can also be used to influence the order in which
   Diameter messages are handled by Diameter nodes.  The above attacks
   have a potentially greater impact in this scenario as the priority
   indication impacts the handling of all requests at all times,
   independent of the overload status of Diameter nodes in the Diameter
   network.

12.2.  Denial-of-Service Attacks

   The DRMP mechanism does not open direct denial-of-service attack
   vectors.  Rather, it introduces a mechanism where a node can gain
   unwarranted preferential treatment.  It also introduces a mechanism
   where a node can get degraded service in the scenario where a rogue
   agent changes the priority value included in messages.

12.3.  End-to-End Security Issues

   The lack of end-to-end integrity features in Diameter [RFC6733] makes
   it difficult to establish trust in DRMP AVPs received from non-
   adjacent nodes.  Any agents in the message path may insert or modify
   DRMP AVPs.  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

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   DRMP MUST give operators the ability to select which peers are
   trusted to deliver DRMP AVPs, and whether they are trusted to forward
   the DRMP AVPs from non-adjacent nodes.  Diameter nodes MUST strip
   DRMP AVPs from messages received from peers that are not trusted for
   DRMP purposes.

   It is expected that work on end-to-end Diameter security might make
   it easier to establish trust in non-adjacent nodes for DRMP purposes.
   Readers should be reminded, however, that the DRMP mechanism allows
   Diameter Agents to modify AVPs in 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 DRMP will require careful consideration and are beyond
   the scope of this document.

13.  References

13.1.  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>.

   [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>.

13.2.  Informative References

   [RFC7683]  Korhonen, J., Ed., Donovan, S., Ed., Campbell, B., and L.
              Morand, "Diameter Overload Indication Conveyance",
              RFC 7683, DOI 10.17487/RFC7683, October 2015,
              <http://www.rfc-editor.org/info/rfc7683>.

   [S6a]      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, 14.0.0, June 2016,
              <http://www.3gpp.org/ftp/Specs/html-info/29272.htm>.

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Contributors

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

   o  Janet P. Gunn

Author's Address

   Steve Donovan
   Oracle
   7460 Warren Parkway
   Frisco, Texas  75034
   United States of America

   Email: srdonovan@usdonovans.com

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