Network Working Group                                         E. McMurry
Internet-Draft                                               B. Campbell
Intended status: Standards Track                                 Tekelec
Expires: July 19, August 26, 2013                                  January 15,                               February 22, 2013

                 Diameter Overload Control Requirements


   When a Diameter server or agent becomes overloaded, it needs to be
   able to gracefully reduce its load, typically by informing clients to
   reduce sending traffic for some period of time.  Otherwise, it must
   continue to expend resources parsing and responding to Diameter
   messages, possibly resulting in congestion collapse.  The existing
   Diameter mechanisms, listed in Section 3 are not sufficient for this
   purpose.  This document describes the limitations of the existing
   mechanisms in Section 4.  Requirements for new overload management
   mechanisms are provided in Section 7.

Status of this Memo

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   This Internet-Draft will expire on July 19, August 26, 2013.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Causes of Overload . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Effects of Overload  . . . . . . . . . . . . . . . . . . .  5
     1.3.  Overload vs. Network Congestion  . . . . . . . . . . . . .  5
     1.4.  Diameter Applications in a Broader Network . . . . . . . .  5
     1.5.  Documentation Conventions  . . . . . . . . . . . . . . . .  6
   2.  Overload Scenarios . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Peer to Peer Scenarios . . . . . . . . . . . . . . . . . .  7
     2.2.  Agent Scenarios  . . . . . . . . . . . . . . . . . . . . .  9
     2.3.  Interconnect Scenario  . . . . . . . . . . . . . . . . . . 12
   3.  Existing Mechanisms  . . . . . . . . . . . . . . . . . . . . . 13
   4.  Issues with the Current Mechanisms . . . . . . . . . . . . . . 14
     4.1.  Problems with Implicit Mechanism . . . . . . . . . . . . . 15
     4.2.  Problems with Explicit Mechanisms  . . . . . . . . . . . . 15
   5.  Diameter Overload Case Studies . . . . . . . . . . . . . . . . 16
     5.1.  Overload in Mobile Data Networks . . . . . . . . . . . . . 16
     5.2.  3GPP Study on Core Network Overload  . . . . . . . . . . . 17
   6.  Extensibility and Application Independence . . . . . . . . . . 18
   7.  Solution Requirements  . . . . . . . . . . . . . . . . . . . . 19
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24 23
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24 23
     9.1.  Access Control . . . . . . . . . . . . . . . . . . . . . . 24
     9.2.  Denial-of-Service Attacks  . . . . . . . . . . . . . . . . 24
     9.3.  Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 25 24
     9.4.  Man-in-the-Middle Attacks  . . . . . . . . . . . . . . . . 25
     9.5.  Compromised Hosts  . . . . . . . . . . . . . . . . . . . . 25
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26 25
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 26 25
     10.2. Informative References . . . . . . . . . . . . . . . . . . 26
   Appendix A.  Contributors  . . . . . . . . . . . . . . . . . . . . 27 26
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 27
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27

1.  Introduction

   When a Diameter [RFC6733] server or agent becomes overloaded, it
   needs to be able to gracefully reduce its load, typically by
   informing clients to reduce sending traffic for some period of time.
   Otherwise, it must continue to expend resources parsing and
   responding to Diameter messages, possibly resulting in congestion
   collapse.  The existing mechanisms provided by Diameter are not
   sufficient for this purpose.  This document describes the limitations
   of the existing mechanisms, and provides requirements for new
   overload management mechanisms.

   This document draws on the work done on SIP overload control
   ([RFC5390], [RFC6357]) as well as on experience gained via overload
   handling in Signaling System No. 7 (SS7) networks and studies done by
   the Third Generation Partnersip Partnership Project (3GPP) (Section 5).

   Diameter is not typically an end-user protocol; rather it is
   generally used as one component in support of some end-user activity.

   For example, a SIP server might use Diameter to authenticate and
   authorize user access.  Overload in the Diameter backend
   infrastructure will likely impact the experience observed by the end
   user in the SIP application.

   The impact of Diameter overload on the client application (a client
   application may use the Diameter protocol and other protocols to do
   its job) is beyond the scope of this document.

   This document presents non-normative descriptions of causes of
   overload along with related scenarios and studies.  Finally, it
   offers a set of normative requirements for an improved overload
   indication mechanism.

1.1.  Causes of Overload

   Overload occurs when an element, such as a Diameter server or agent,
   has insufficient resources to successfully process all of the traffic
   it is receiving.  Resources include all of the capabilities of the
   element used to process a request, including CPU processing, memory,
   I/O, and disk resources.  It can also include external resources such
   as a database or DNS server, in which case the CPU, processing,
   memory, I/O, and disk resources of those elements are effectively
   part of the logical element processing the request.

   Overload can occur for many reasons, including:

   Inadequate capacity:  When designing Diameter networks, that is,
      application layer multi-node Diameter deployments, it can be very
      difficult to predict all scenarios that may cause elevated
      traffic.  It may also be more costly to implement support for some
      scenarios than a network operator may deem worthwhile.  This
      results in the likelihood that a Diameter network will not have
      adequate capacity to handle all situations.

   Dependency failures:  A Diameter node can become overloaded because a
      resource on which it is dependent has failed or become overloaded,
      greatly reducing the logical capacity of the node.  In these
      cases, even minimal traffic might cause the node to go into
      overload.  Examples of such dependency overloads include DNS
      servers, databases, disks, and network interfaces.

   Component failures:  A Diameter node can become overloaded when it is
      a member of a cluster of servers that each share the load of
      traffic, and one or more of the other members in the cluster fail.
      In this case, the remaining nodes take over the work of the failed
      nodes.  Normally, capacity planning takes such failures into
      account, and servers are typically run with enough spare capacity
      to handle failure of another node.  However, unusual failure
      conditions can cause many nodes to fail at once.  This is often
      the case with software failures, where a bad packet or bad
      database entry hits the same bug in a set of nodes in a cluster.

   Network Initiated Traffic Flood:  Issues with the radio access
      network in a mobile network such as radio overlays with frequent
      handovers, and operational changes are examples of network events
      that can precipitate a flood of Diameter signaling traffic, such
      as an avalanche restart.  Failure of a Diameter proxy may also
      result in a large amount of signaling as connections and sessions
      are reestablished.

   Subscriber Initiated Traffic Flood:  Large gatherings of subscribers
      or events that result in many subscribers interacting with the
      network in close time proximity can result in Diameter signaling
      traffic floods.  For example, the finale of a large fireworks show
      could be immediately followed by many subscribers posting
      messages, pictures, and videos concentrated on one portion of a
      network.  Subscriber devices, such as smartphones, may use
      aggressive registration strategies that generate unusually high
      Diameter traffic loads.

   DoS attacks:  An attacker, wishing to disrupt service in the network,
      can cause a large amount of traffic to be launched at a target
      element.  This can be done from a central source of traffic or
      through a distributed DoS attack.  In all cases, the volume of
      traffic well exceeds the capacity of the element, sending the
      system into overload.

1.2.  Effects of Overload

   Modern Diameter networks, comprised of application layer multi-node
   deployments of Diameter elements, may operate at very large
   transaction volumes.  If a Diameter node becomes overloaded, or even
   worse, fails completely, a large number of messages may be lost very
   quickly.  Even with redundant servers, many messages can be lost in
   the time it takes for failover to complete.  While a Diameter client
   or agent should be able to retry such requests, an overloaded peer
   may cause a sudden large increase in the number of transaction
   transactions needing to be retried, rapidly filling local queues or
   otherwise contributing to local overload.  Therefore Diameter devices
   need to be able to shed load before critical failures can occur.

1.3.  Overload vs. Network Congestion

   This document uses the term "overload" to refer to application-layer
   overload at Diameter nodes.  This is distinct from "network
   congestion", that is, congestion that occurs at the lower networking
   layers that may impact the delivery of Diameter messages between
   nodes.  The authors recognize that element overload and network
   congestion are interrelated, and that overload can contribute to
   network congestion and vice versa.

   Network congestion issues are better handled by the transport
   protocols.  Diameter uses TCP and SCTP, both of which include
   congestion management features.  Analysis of whether those features
   are sufficient for transport level congestion between Diameter nodes,
   and any work to further mitigate network congestion is out of scope
   both for this document, and for the work proposed by this document.

1.4.  Diameter Applications in a Broader Network

   Most elements using Diameter applications do not use Diameter
   exclusively.  It is important to realize that overload of an element
   can be caused by a number of factors that may be unrelated to the
   processing of Diameter or Diameter applications.

   A element communicating via protocols other than Diameter that is
   also using a Diameter application needs to be able to signal to
   Diameter peers that it is experiencing overload regardless of the
   cause of the overload, since the overload will affect that element's
   ability to process Diameter transactions.  The element may also need
   to signal this on other protocols depending on its function and the
   architecture of the network and application it is providing services
   for.  Whether that is necessary can only be decided within the
   context of that architecture and application.  A mechanism for
   signaling overload with Diameter, which this specification details
   the requirements for, provides applications the ability to signal
   their Diameter peers of overload, mitigating that part of the issue.
   Applications may need to use this, as well as other mechanisms, to
   solve their broader overload issues.  Indicating overload on
   protocols other than Diameter is out of scope for this document, and
   for the work proposed by this document.

1.5.  Documentation Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   The terms "client", "server", "agent", "node", "peer", "upstream",
   and "downstream" are used as defined in [RFC6733].

2.  Overload Scenarios

   Several Diameter deployment scenarios exist that may impact overload
   management.  The following scenarios help motivate the requirements
   for an overload management mechanism.

   These scenarios are by no means exhaustive, and are in general
   simplified for the sake of clarity.  In particular, the authors
   assume for the sake of clarity that the client sends Diameter
   requests to the server, and the server sends responses to client,
   even though Diameter supports bidirectional applications.  Each
   direction in such an application can be modeled separately.

   In a large scale deployment, many of the nodes represented in these
   scenarios would be deployed as clusters of servers.  The authors
   assume that such a cluster is responsible for managing its own
   internal load balancing and overload management so that it appears as
   a single Diameter node.  That is, other Diameter nodes can treat it
   as single, monolithic node for the purposes of overload management.

   These scenarios do not illustrate the client application.  As
   mentioned in Section 1, Diameter is not typically an end-user
   protocol; rather it is generally used in support of some other client
   application.  These scenarios do not consider the impact of Diameter
   overload on the client application.

2.1.  Peer to Peer Scenarios

   This section describes Diameter peer-to-peer scenarios.  That is,
   scenarios where a Diameter client talks directly with a Diameter
   server, without the use of a Diameter agent.

   Figure 1 illustrates the simplest possible Diameter relationship.
   The client and server share a one-to-one peer-to-peer relationship.
   If the server becomes overloaded, either because the client exceeds
   the server's capacity, or because the server's capacity is reduced
   due to some resource dependency, the client needs to reduce the
   amount of Diameter traffic it sends to the server.  Since the client
   cannot forward requests to another server, it must either queue
   requests until the server recovers, or itself become overloaded in
   the context of the client application and other protocols it may also

                         |                  |
                         |                  |
                         |     Server       |
                         |                  |
                         |                  |
                         |                  |
                         |     Client       |
                         |                  |

                   Figure 1: Basic Peer to Peer Scenario

   Figure 2 shows a similar scenario, except in this case the client has
   multiple servers that can handle work for a specific realm and
   application.  If server 1 becomes overloaded, the client can forward
   traffic to server 2.  Assuming server 2 has sufficient reserve
   capacity to handle the forwarded traffic, the client should be able
   to continue serving client application protocol users.  If server 1
   is approaching overload, but can still handle some number of new
   request, it needs to be able to instruct the client to forward a
   subset of its traffic to server 2.

           +------------------+     +------------------+
           |                  |     |                  |
           |                  |     |                  |
           |     Server 1     |     |     Server 2     |
           |                  |     |                  |
           +--------+-`.------+     +------.'+---------+
                        `.               .'
                          `.           .'
                            `.       .'
                              `.   .'
                        |                  |
                        |                  |
                        |     Client       |
                        |                  |

              Figure 2: Multiple Server Peer to Peer Scenario

   Figure 3 illustrates a peer-to-peer scenario with multiple Diameter
   realm and application combinations.  In this example, server 2 can
   handle work for both applications.  Each application might have
   different resource dependencies.  For example, a server might need to
   access one database for application A, and another for application B.
   This creates a possibility that Server 2 could become overloaded for
   application A but not for application B, in which case the client
   would need to divert some part of its application A requests to
   server 1, but should not divert any application B requests.  This
   requires server 2 to be able to distinguish between applications when
   it indicates an overload condition to the client.

   On the other hand, it's possible that the servers host many
   applications.  If server 2 becomes overloaded for all applications,
   it would be undesirable for it to have to notify the client
   separately for each application.  Therefore it also needs a way to
   indicate that it is overloaded for all possible applications.


   | Application A       +------------------------+----------------------+       +----------------------+----------------------+
   |+------------------+ |  +------------------+  +----------------+  |  +------------------+|
   ||                  | |  |                |  |  |                  ||
   ||                  | |  |                |  |  |                  ||
   ||     Server 1     | |  |    Server 2    |  |  |     Server 3     ||
   ||                  | |  |                |  |  |                  ||
   |+--------+---------+ |  +--------+---------+  +-------+--------+  |  +-+----------------+|
   |         |           |          |           |    |                 |
   +---------+-----------+----------+-----------+    |                 |
             |           |          |                |                 |
             |           |          |                |  Application B  |
             |           +-----------+-----------------+-----------------+           +----------+----------------+-----------------+
             ``-.._                 |                |
                   `-..__           |            _.-''
                        `--._       |        _.-''
                             ``-._  |   _.-''
                            |                |
                            |                |
                            |     Client     |
                            |                |

           Figure 3: Multiple Application Peer to Peer Scenario

2.2.  Agent Scenarios

   This section describes scenarios that include a Diameter agent,
   either in the form of a Diameter relay or Diameter proxy.  These
   scenarios do not consider Diameter redirect agents, since they are
   more readily modeled as end-servers.

   Figure 4 illustrates a simple Diameter agent scenario with a single
   client, agent, and server.  In this case, overload can occur at the
   server, at the agent, or both.  But in most cases, client behavior is
   the same whether overload occurs at the server or at the agent.  From
   the client's perspective, server overload and agent overload is the
   same thing.

                       |                  |
                       |                  |
                       |     Server       |
                       |                  |
                       |                  |
                       |                  |
                       |      Agent       |
                       |                  |
                       |                  |
                       |                  |
                       |     Client       |
                       |                  |

                      Figure 4: Basic Agent Scenario

   Figure 5 shows an agent scenario with multiple servers.  If server 1
   becomes overloaded, but server 2 has sufficient reserve capacity, the
   agent may be able to transparently divert some or all Diameter
   requests originally bound for server 1 to server 2.

   In most cases, the client does not have detailed knowledge of the
   Diameter topology upstream of the agent.  If the agent uses dynamic
   discovery to find eligible servers, the set of eligible servers may
   not be enumerable from the perspective of the client.  Therefore, in
   most cases the agent needs to deal with any upstream overload issues
   in a way that is transparent to the client.  If one server notifies
   the agent that it has become overloaded, the notification should not
   be passed back to the client in a way that the client could
   mistakenly perceive the agent itself as being overloaded.  If the set
   of all possible destinations upstream of the agent no longer has
   sufficient capacity for incoming load, the agent itself becomes
   effectively overloaded.

   On the other hand, there are cases where the client needs to be able
   to select a particular server from behind an agent.  For example, if
   a Diameter request is part of a multiple-round-trip authentication,
   or is otherwise part of a Diameter "session", it may have a
   DestinationHost AVP that requires the request to be served by server
   1.  Therefore the agent may need to inform a client that a particular
   upstream server is overloaded or otherwise unavailable.  Note that
   there can be many ways a server can be specified, which may have
   different implications (e.g. by IP address, by host name, etc).

           +------------------+     +------------------+
           |                  |     |                  |
           |                  |     |                  |
           |     Server 1     |     |     Server 2     |
           |                  |     |                  |
           +--------+-`.------+     +------.'+---------+
                        `.               .'
                         `.           .'
                            `.       .'
                              `.   .'
                        |                  |
                        |                  |
                        |     Agent        |
                        |                  |
                        |                  |
                        |                  |
                        |     Client       |
                        |                  |

                 Figure 5: Multiple Server Agent Scenario

   Figure 6 shows a scenario where an agent routes requests to a set of
   servers for more than one Diameter realm and application.  In this
   scenario, if server 1 becomes overloaded or unavailable, the agent
   may effectively operate at reduced capacity for application A, but at
   full capacity for application B. Therefore, the agent needs to be
   able to report that it is overloaded for one application, but not for


   | Application A       +------------------------+----------------------+       +----------------------+----------------------+
   |+------------------+ |  +------------------+  +----------------+  |  +------------------+|
   ||                  | |  |                |  |  |                  ||
   ||                  | |  |                |  |  |                  ||
   ||     Server 1     | |  |    Server 2    |  |  |     Server 3     ||
   ||                  | |  |                |  |  |                  ||
   |+---------+--------+ |  +--------+---------+  +-------+--------+  |  +--+---------------+|
   |          |          |          |           |     |                |
   +----------+----------+----------+-----------+     |                |
              |          |          |                 |                |
              |          |          |                 | Application B  |
              |          +-----------+------------------+----------------+          +----------+-----------------+----------------+
              |                     |                 |
               ``--.__              |                _.
                      ``-.__        |          __.--''
                            `--.._  |    _..--'
                            |                |
                            |                |
                            |    Agent       |
                            |                |
                            |                |
                            |                |
                            |    Client      |
                            |                |

               Figure 6: Multiple Application Agent Scenario

2.3.  Interconnect Scenario

   Another scenario to consider when looking at Diameter overload is
   that of multiple network operators using Diameter components
   connected through an interconnect service, e.g. using IPX.  IPX (IP
   eXchange) [IR.34] is an Inter-Operator IP Backbone that provides
   roaming interconnection network between mobile operators and service
   providers.  The IPX is also used to transport Diameter signaling
   between operators [IR.88].  Figure 7 shows two network operators with
   an interconnect network in-between.  There could be any number of
   these networks between any two network operator's networks.

               |               Interconnect                |
               |                                           |
               |   +--------------+      +--------------+  |
               |   |   Server 3   |------|   Server 4   |  |
               |   +--------------+      +--------------+  |
               |         .'                      `.        |
                    .'                               `.
                 .-'                                   `.
   ------------.'-----+                             +----`.---------------                             +----`.-------------
         +----------+ |                             | +----------+
         | Server 1 | |                             | | Server 2 |
         +----------+ |                             | +----------+
                      |                             |
   Network Operator 1 |                             | Network Operator 2
   -------------------+                             +---------------------                             +-------------------

                Figure 7: Two Network Interconnect Scenario

   The characteristics of the information that an operator would want to
   share over such a connection are different from the information
   shared between components within a network operator's network.
   Network operators may not want to convey topology or operational
   information, which limits how much overload and loading information
   can be sent.  For the interconnect scenario shown, Server 2 may want
   to signal overload to Server 1, to affect traffic coming from Network
   Operator 1.

   This case is distinct from those internal to a network operator's
   network, where there may be many more elements in a more complicated
   topology.  Also, the elements in the interconnect network may not
   support Diameter overload control, and the network operators may not
   want the interconnect network to use overload or loading information.
   They may only want the information to pass through the interconnect
   network without further processing or action by the interconnect
   network even if the elements in the interconnect network do support
   Diameter overload control.

3.  Existing Mechanisms

   Diameter offers both implicit and explicit mechanisms for a Diameter
   node to learn that a peer is overloaded or unreachable.  The implicit
   mechanism is simply the lack of responses to requests.  If a client
   fails to receive a response in a certain time period, it assumes the
   upstream peer is unavailable, or overloaded to the point of effective
   unavailability.  The watchdog mechanism [RFC3539] ensures that a
   certain rate of transaction responses occur even when there is
   otherwise little or no other Diameter traffic.

   The explicit mechanism can involve specific protocol error responses,
   where an agent or server tells a downstream peer that it is either
   too busy to handle a request (DIAMETER_TOO_BUSY) or unable to route a
   request to an upstream destination (DIAMETER_UNABLE_TO_DELIVER),
   perhaps because that destination itself is overloaded to the point of

   Another explicit mechanism, a DPR (Disconnect-Peer-Request) message,
   can be sent with a Disconnect-Cause of BUSY.  This signals the
   sender's intent to close the transport connection, and requests the
   client not to reconnect.

   Once a Diameter node learns that an upstream peer has become
   overloaded via one of these mechanisms, it can then attempt to take
   action to reduce the load.  This usually means forwarding traffic to
   an alternate destination, if available.  If no alternate destination
   is available, the node must either reduce the number of messages it
   originates (in the case of a client) or inform the client to reduce
   traffic (in the case of an agent.)

   Diameter requires the use of a congestion-managed transport layer,
   currently TCP or SCTP, to mitigate network congestion.  It is
   expected that these transports manage network congestion and that
   issues with transport (e.g. congestion propagation and window
   management) are managed at that level.  But even with a congestion-
   managed transport, a Diameter node can become overloaded at the
   Diameter protocol or application layers due to the causes described
   in Section 1.1 and congestion managed transports do not provide
   facilities (and are at the wrong level) to handle server overload.
   Transport level congestion management is also not sufficient to
   address overload in cases of multi-hop and multi-destination

4.  Issues with the Current Mechanisms

   The currently available Diameter mechanisms for indicating an
   overload condition are not adequate to avoid service outages due to
   overload.  This inadequacy may, in turn, contribute to broader
   congestion collapse due to unresponsive Diameter nodes causing
   application or transport layer retransmissions.  In particular, they
   do not allow a Diameter agent or server to shed load as it approaches
   overload.  At best, a node can only indicate that it needs to
   entirely stop receiving requests, i.e. that it has effectively
   failed.  Even that is problematic due to the inability to indicate
   durational validity on the transient errors available in the base
   Diameter protocol.  Diameter offers no mechanism to allow a node to
   indicate different overload states for different categories of
   messages, for example, if it is overloaded for one Diameter
   application but not another.

4.1.  Problems with Implicit Mechanism

   The implicit mechanism doesn't allow an agent or server to inform the
   client of a problem until it is effectively too late to do anything
   about it.  The client does not know to take action until the upstream
   node has effectively failed.  A Diameter node has no opportunity to
   shed load early to avoid collapse in the first place.

   Additionally, the implicit mechanism cannot distinguish between
   overload of a Diameter node and network congestion.  Diameter treats
   the failure to receive an answer as a transport failure.

4.2.  Problems with Explicit Mechanisms

   The Diameter specification is ambiguous on how a client should handle
   receipt of a DIAMETER_TOO_BUSY response.  The base specification
   [RFC6733] indicates that the sending client should attempt to send
   the request to a different peer.  It makes no suggestion that the
   receipt of a DIAMETER_TOO_BUSY response should affect future Diameter
   messages in any way.

   The Authentication, Authorization, and Accounting (AAA) Transport
   Profile [RFC3539] recommends that a AAA node that receives a "Busy"
   response failover all remaining requests to a different agent or
   server.  But while the Diameter base specification explicitly depends
   on RFC3539 to define transport behavior, it does not refer to RFC3539
   in the description of behavior on receipt of DIAMETER_TOO_BUSY.
   There's a strong likelihood that at least some implementations will
   continue to send Diameter requests to an upstream peer even after
   receiving a DIAMETER_TOO_BUSY error.

   BCP 41 [RFC2914] describes, among other things, how end-to-end
   application behavior can help avoid congestion collapse.  In
   particular, an application should avoid sending messages that will
   never be delivered or processed.  The DIAMETER_TOO_BUSY behavior as
   described in the Diameter base specification fails at this, since if
   an upstream node becomes overloaded, a client attempts each request,
   and does not discover the need to failover the request until the
   initial attempt fails.

   The situation is improved if implementations follow the [RFC3539]
   recommendation and keep state about upstream peer overload.  But even
   then, the Diameter specification offers no guidance on how long a
   client should wait before retrying the overloaded destination.  If an
   agent or server supports multiple realms and/or applications,
   DIAMETER_TOO_BUSY offers no way to indicate that it is overloaded for
   one application but not another.  A DIAMETER_TOO_BUSY error can only
   indicate overload at a "whole server" scope.

   Agent processing of a DIAMETER_TOO_BUSY response is also problematic
   as described in the base specification.  DIAMETER_TOO_BUSY is defined
   as a protocol error.  If an agent receives a protocol error, it may
   either handle it locally or it may forward the response back towards
   the downstream peer.  (The Diameter specification is inconsistent
   about whether a protocol error MAY or SHOULD be handled by an agent,
   rather than forwarded downstream.)  If a downstream peer receives the
   DIAMETER_TOO_BUSY response, it may stop sending all requests to the
   agent for some period of time, even though the agent may still be
   able to deliver requests to other upstream peers.

   DIAMETER_UNABLE_TO_DELIVER, or using DPR with cause code BUSY also
   have no mechanisms for specifying the scope or cause of the failure,
   or the durational validity.

   The issues with error responses in [RFC6733] extend beyond the
   particular issues for overload control and have been addressed in an
   ad hoc fashion by various implementations.  Addressing these in a
   standard way would be a useful exercise, but it us beyond the scope
   of this document.

5.  Diameter Overload Case Studies

5.1.  Overload in Mobile Data Networks

   As the number of Third Generation (3G) and Long Term Evolution (LTE)
   enabled smartphone devices continue to expand in mobility networks,
   there have been situations where high signaling traffic load led to
   overload events at the Diameter-based Home Location Registries (HLR)
   and/or Home Subscriber Servers (HSS) [TR23.843].  The root causes of
   the HLR congestion events were manifold but included hardware failure
   and procedural errors.  The result was high signaling traffic load on
   the HLR and HSS.

   The 3GPP architecture [TS23.002] makes extensive use of Diameter.  It
   is used for mobility management [TS29.272] (and others), (IP
   Multimedia Subsystem) IMS [TS29.228] (and others), policy and
   charging control [TS29.212] (and others) as well as other functions.
   The details of the architecture are out of scope for this document,
   but it is worth noting that there are quite a few Diameter
   applications, some with quite large amounts of Diameter signaling in
   deployed networks.

   The 3GPP specifications do not currently address overload for
   Diameter applications or provide an equivalent load control mechanism
   to those provided in the more traditional SS7 elements in (Global
   System for Mobile Communications) GSM [TS29.002].  The capabilities
   specified in the 3GPP standards do not adequately address the
   abnormal condition where excessively high signaling traffic load
   situations are experienced.

   Smartphones, an increasingly large percentage of mobile devices,
   contribute much more heavily, relative to non-smartphones, to the
   continuation of a registration surge due to their very aggressive
   registration algorithms.  Smartphone behavior contributes to network
   loading and can contribute to overload conditions.  The aggressive
   smartphone logic is designed to:

   a.  always have voice and data registration, and

   b.  constantly try to be on 3G or LTE data (and thus on 3G voice or
       VoLTE) for their added benefits.

   Non-smartphones typically have logic to wait for a time period after
   registering successfully on voice and data.

   The smartphone aggressive registration is problematic in two ways:

   o  first by generating excessive signaling load towards the HLR that
      is ten times that from a non-smartphone,

   o  and second by causing continual registration attempts when a
      network failure affects registrations through the 3G data network.

5.2.  3GPP Study on Core Network Overload

   A study in 3GPP SA2 on core network overload has produced the
   technical report [TR23.843].  This enumerates several causes of
   overload in mobile core networks including portions that are signaled
   using Diameter.  This document is a work in progress and is not
   complete.  However, it is useful for pointing out scenarios and the
   general need for an overload control mechanism for Diameter.

   It is common for mobile networks to employ more than one radio
   technology and to do so in an overlay fashion with multiple
   technologies present in the same location (such as 2nd or 3rd
   generation mobile technologies along with LTE).  This presents
   opportunities for traffic storms when issues occur on one overlay and
   not another as all devices that had been on the overlay with issues
   switch.  This causes a large amount of Diameter traffic as locations
   and policies are updated.

   Another scenario called out by this study is a flood of registration
   and mobility management events caused by some element in the core
   network failing.  This flood of traffic from end nodes falls under
   the network initiated traffic flood category.  There is likely to
   also be traffic resulting directly from the component failure in this
   case.  A similar flood can occur when elements or components recover
   as well.

   Subscriber initiated traffic floods are also indicated in this study
   as an overload mechanism where a large number of mobile devices
   attempting to access services at the same time, such as in response
   to an entertainment event or a catastrophic event.

   While this 3GPP study is concerned with the broader effects of these
   scenarios on wireless networks and their elements, they have
   implications specifically for Diameter signaling.  One of the goals
   of this document is to provide guidance for a core mechanism that can
   be used to mitigate the scenarios called out by this study.

6.  Extensibility and Application Independence

   Given the variety of scenarios Diameter elements can be deployed in,
   and the variety of roles they can fulfill with Diameter and other
   technologies, a single algorithm for handling overload may not be
   sufficient.  This effort cannot anticipate all possible future
   scenarios and roles.  Extensibility, particularly of algorithms used
   to deal with overload, will be important to cover these cases.

   Similarly, the scopes that overload information may apply to may
   include cases that have not yet been considered.  Extensibility in
   this area will also be important.

   The basic mechanism is intended to be application-independent, that
   is, a Diameter node can use it across any existing and future
   Diameter applications and expect reasonable results.  Certain
   Diameter applications might, however, benefit from application-
   specific behavior over and above the mechanism's defaults.  For
   example, an application specification might specify relative
   priorities of messages or selection of a specific overload control

7.  Solution Requirements

   This section proposes requirements for an improved mechanism to
   control Diameter overload, with the goals of improving the issues
   described in Section 4 and supporting the scenarios described in
   Section 2

   REQ 1:   The overload control mechanism MUST provide a communication
            method for Diameter nodes to exchange load and overload

   REQ 2:   The mechanism MUST allow Diameter nodes to support overload
            control regardless of which Diameter applications they

   REQ 3:   The overload control mechanism MUST limit the impact of
            overload on the overall useful throughput of a Diameter
            server, even when the incoming load on the network is far in
            excess of its capacity.  The overall useful throughput under
            load is the ultimate measure of the value of an overload
            control mechanism.

   REQ 4:   Diameter allows requests to be sent from either side of a
            connection and either side of a connection may have need to
            provide its overload status.  The mechanism MUST allow each
            side of a connection to independently inform the other of
            its overload status.

   REQ 5:   Diameter allows nodes to determine their peers via dynamic
            discovery or manual configuration.  The mechanism MUST work
            consistently without regard to how peers are determined.

   REQ 6:   The mechanism designers SHOULD seek to minimize the amount
            of new configuration required in order to work.  For
            example, it is better to allow peers to advertise or
            negotiate support for the mechanism, rather than to require
            this knowledge to be configured at each node.

   REQ 7:   The overload control mechanism and any associated default
            algorithm(s) MUST ensure that the system remains stable.
            When the offered load drops from above  At
            some point after an overload condition has ended, the overall
            mechanism MUST enable capacity
            of the network to below the overall capacity, the throughput
            MUST stabilize and become equal
            to what it would be in the offered load. absence of an overload condition.
            Note that this also requires that the mechanism MUST allow
            nodes to shed load without introducing oscillations. oscillations during
            or after an overload condition.

   REQ 8:   Supporting nodes MUST be able to distinguish current
            overload information from stale information, and SHOULD make
            decisions using the most currently available information.

   REQ 9:   The mechanism MUST function across fully loaded as well as
            quiescent transport connections.  This is partially derived
            from the requirements requirement for stability and hysteresis control
            above. in REQ 7.

   REQ 10:  Consumers of overload state indications information MUST be able to determine
            when the overload condition improves or ends.

   REQ 11:  The overload control mechanism MUST be able to operate in
            networks of different sizes.

   REQ 12:  When a single network node fails, goes into overload, or
            suffers from reduced processing capacity, the mechanism MUST
            make it possible to limit the impact of this on other nodes
            in the network.  This helps to prevent a small-scale failure
            from becoming a widespread outage.

   REQ 13:  The mechanism MUST NOT introduce substantial additional work
            for node in an overloaded state.  For example, a requirement
            for an overloaded node to send overload information every
            time it received a new request would introduce substantial
            work.  Existing messaging is likely to have the
            characteristic of increasing as an overload condition
            approaches, allowing for the possibility of increased
            feedback for information piggybacked on it.

   REQ 14:  Some scenarios that result in overload involve a rapid
            increase of traffic with little time between normal levels
            and overload inducing levels.  The mechanism SHOULD provide
            for rapid feedback when traffic levels increase.

   REQ 15:  The mechanism MUST NOT interfere with the congestion control
            mechanisms of underlying transport protocols.  For example,
            a mechanism that opened additional TCP connections when the
            network is congested would reduce the effectiveness of the
            underlying congestion control mechanisms.

   REQ 16:  The overload control mechanism is likely to be deployed
            incrementally.  The mechanism MUST operate without malfunction in an
            environment with support a mix of nodes that do, and mixed
            environment where some, but not all, nodes that do
            not, support the mechanism. implement it.

   REQ 17:  In a mixed environment with nodes that support the overload
            control mechanism and that do not, the mechanism MUST result
            in at least as much useful throughput as would have resulted
            if the mechanism were not present.  It SHOULD result in less
            severe congestion in this environment.

   REQ 18:  In a mixed environment of nodes that support the overload
            control mechanism and that do not, the mechanism MUST NOT
            preclude elements that support overload control from
            treating elements that do not support overload control in a
            equitable fashion relative to those that do. users and
            operators of nodes that do not support the mechanism MUST
            NOT unfairly benefit from the mechanism.  The mechanism
            specification SHOULD provide guidance to implementors for
            dealing with elements not supporting overload control.

   REQ 19:  It MUST be possible to use the mechanism between nodes in
            different realms and in different administrative domains.

   REQ 20:  Any explicit overload indication MUST distinguish between
            actual overload, as opposed to other, non-overload related

   REQ 21:  In cases where a network node fails, is so overloaded that
            it cannot process messages, or cannot communicate due to a
            network failure, it may not be able to provide explicit
            indications of the nature of the failure or its levels of
            congestion.  The mechanism MUST properly function result in these
            cases. at least as much
            useful throughput as would have resulted if the overload
            control mechanism was not in place.

   REQ 22:  The mechanism MUST provide a way for an node to throttle the
            amount of traffic it receives from an peer node.  This
            throttling SHOULD be graded so that it can be applied
            gradually as offered load increases.  Overload is not a
            binary state; there may be degrees of overload.

   REQ 23:  The mechanism MUST enable a supporting node to minimize the
            chance that retries due to an overloaded or failed node
            result in additional traffic to other overloaded nodes, or
            cause additional nodes to become overloaded.  Moreover, the
            mechanism SHOULD provide unambiguous directions to clients
            on when they should retry a request and when they should not
            considering the various causes of overload such as avalanche
            restart.  REMOVED

   REQ 24:  The mechanism MUST provide sufficient information to enable
            a load balancing node to divert messages that are rejected
            or otherwise throttled by an overloaded upstream node to
            other upstream nodes that are the most likely to have
            sufficient capacity to process them.

   REQ 25:  The mechanism MUST provide a mechanism for indicating load
            levels even when not in an overloaded condition, to assist
            nodes making decisions to prevent overload conditions from

   REQ 26:  The base specification for the overload control mechanism
            SHOULD offer general guidance on which message types might
            be desirable to send or process over others during times of
            overload, based on application-specific considerations.  For
            example, it may be more beneficial to process messages for
            existing sessions ahead of new sessions, or sessions.  Some networks may
            have a requirement to give priority to requests associated
            with emergency sessions or with high
            priority users. sessions.  Any normative or otherwise
            detailed definition of the relative priorities of message
            types during an overload condition will be the
            responsibility of the application specification.

   REQ 27:  The mechanism MUST NOT prevent a node from prioritizing
            requests based on any local policy, so that certain requests
            are given preferential treatment, given additional
            retransmission, not throttled, or processed ahead of others.

   REQ 28:  The overload control mechanism MUST NOT provide new
            vulnerabilities to malicious attack, or increase the
            severity of any existing vulnerabilities.  This includes
            vulnerabilities to DoS and DDoS attacks as well as replay
            and man-in-the middle attacks.  Note that the Diameter base
            specification [RFC6733] lacks end to end security and this
            must be considered.

   REQ 29:  The mechanism MUST provide a means to match an overload
            indication with the node that originated it.  In particular,
            the mechanism MUST allow a node to distinguish between
            overload at a next-hop peer from overload at a node upstream
            of the peer.  For example, in Figure 5, the client must not
            mistake overload at server 1 for overload at the agent,
            whether or not the agent supports the mechanism.( see REQ
            4).  REMOVED

   REQ 30:  The mechanism MUST NOT depend on being deployed in
            environments where all Diameter nodes are completely
            trusted.  It SHOULD operate as effectively as possible in
            environments where other nodes are malicious; this includes
            preventing malicious nodes from obtaining more than a fair
            share of service.  Note that this does not imply any
            responsibility on the mechanism to detect, or take
            countermeasures against, malicious nodes.

   REQ 31:  It MUST be possible for a supporting node to make
            authorization decisions about what information will be sent
            to peer nodes based on the identity of those nodes.  This
            allows a domain administrator who considers the load of
            their nodes to be sensitive information to restrict access
            to that information.  Of course, in such cases, there is no
            expectation that the overload control mechanism itself will
            help prevent overload from that peer node.

   REQ 32:  The mechanism MUST NOT interfere with any Diameter compliant
            method that a node may use to protect itself from overload
            from non-supporting nodes, or from denial of service

   REQ 33:  There are multiple situations where a Diameter node may be
            overloaded for some purposes but not others.  For example,
            this can happen to an agent or server that supports multiple
            applications, or when a server depends on multiple external
            resources, some of which may become overloaded while others
            are fully available.  The mechanism MUST allow Diameter
            nodes to indicate overload with sufficient granularity to
            allow clients to take action based on the overloaded
            resources without unreasonably forcing available capacity to
            go unused.  The mechanism MUST support specification of
            overload information with granularities of at least
            "Diameter node", "realm", "Diameter application", and "Diameter session", application", and
            MUST allow extensibility for others to be added in the

   REQ 34:  The mechanism MUST provide a method for extending the
            information communicated and the algorithms used for
            overload control.

   REQ 35:  The mechanism SHOULD provide a method for exchanging
            overload and load information between elements that are
            connected by intermediaries that do not support the
   REQ 36:  The mechanism MUST provide a default algorithm that is
            mandatory to implement.

8.  IANA Considerations

   This document makes no requests of IANA.

9.  Security Considerations

   A Diameter overload control mechanism is primarily concerned with the
   load and overload related behavior of nodes in a Diameter network,
   and the information used to affect that behavior.  Load and overload
   information is shared between nodes and directly affects the behavior
   and thus is potentially vulnerable to a number of methods of attack.

   Load and overload information may also be sensitive from both
   business and network protection viewpoints.  Operators of Diameter
   equipment want to control visibility to load and overload information
   to keep it from being used for competitive intelligence or for
   targeting attacks.  It is also important that the Diameter overload
   control mechanism not introduce any way in which any other
   information carried by Diameter is sent inappropriately.

   Note that the Diameter base specification [RFC6733] lacks end to end
   security, making verifying the authenticity and ownership of load and
   overload information difficult for non-adjacent nodes.
   Authentication of load and overload information helps to alleviate
   several of the security issues listed in this section.

   This document includes requirements intended to mitigate the effects
   of attacks and to protect the information used by the mechanism.

9.1.  Access Control

   To control the visibility of load and overload information, sending
   should be subject to some form of authentication and authorization of
   the receiver.  It is also important to the receivers that they are
   confident the load and overload information they receive is from a
   legitimate source.  Note that this implies a certain amount of
   configurability on the nodes supporting the Diameter overload control

9.2.  Denial-of-Service Attacks

   An overload control mechanism provides a very attractive target for
   denial-of-service attacks.  A small number of messages may affect a
   large service disruption by falsely reporting overload conditions.
   Alternately, attacking servers nearing, or in, overload may also be
   facilitated by disrupting their overload indications, potentially
   preventing them from mitigating their overload condition.

   A design goal for the Diameter overload control mechanism is to
   minimize or eliminate the possibility of using the mechanism for this
   type of attack.

   As the intent of some denial-of-service attacks is to induce overload
   conditions, an effective overload control mechanism should help to
   mitigate the effects of an such an attack.

9.3.  Replay Attacks

   An attacker that has managed to obtain some messages from the
   overload control mechanism may attempt to affect the behavior of
   nodes supporting the mechanism by sending those messages at
   potentially inopportune times.  In addition to time shifting, replay
   attacks may send messages to other nodes as well (target shifting).

   A design goal for the Diameter overload control mechanism is to
   minimize or eliminate the possibility of causing disruption by using
   a replay attack on the Diameter overload control mechanism.

9.4.  Man-in-the-Middle Attacks

   By inserting themselves in between two nodes supporting the Diameter
   overload control mechanism, an attacker may potentially both access
   and alter the information sent between those nodes.  This can be used
   for information gathering for business intelligence and attack
   targeting, as well as direct attacks.

   A design goal for the Diameter overload control mechanism is to
   minimize or eliminate the possibility of causing disruption man-in-
   the-middle attacks on the Diameter overload control mechanism.  A
   transport using TLS and/or IPSEC may be desirable for this.

9.5.  Compromised Hosts

   A compromised host that supports the Diameter overload control
   mechanism could be used for information gathering as well as for
   sending malicious information to any Diameter node that would
   normally accept information from it.  While is is beyond the scope of
   the Diameter overload control mechanism to mitigate any operational
   interruption to the compromised host, a reasonable design goal is to
   minimize the impact that a compromised host can have on other nodes
   through the use of the Diameter overload control mechanism.  Of
   course, a compromised host could be used to cause damage in a number
   of other ways.  This is out of scope for a Diameter overload control

10.  References

10.1.  Normative References

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

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

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, September 2000.

   [RFC3539]  Aboba, B. and J. Wood, "Authentication, Authorization and
              Accounting (AAA) Transport Profile", RFC 3539, June 2003.

10.2.  Informative References

   [RFC5390]  Rosenberg, J., "Requirements for Management of Overload in
              the Session Initiation Protocol", RFC 5390, December 2008.

   [RFC6357]  Hilt, V., Noel, E., Shen, C., and A. Abdelal, "Design
              Considerations for Session Initiation Protocol (SIP)
              Overload Control", RFC 6357, August 2011.

              3GPP, "Study on Core Network Overload Solutions",
              TR 23.843 0.6.0, October 2012.

   [IR.34]    GSMA, "Inter-Service Provider IP Backbone Guidelines",
              IR 34 7.0, January 2012.

   [IR.88]    GSMA, "LTE Roaming Guidelines", IR 88 7.0, January 2012.

              3GPP, "Network Architecture", TS 23.002 12.0.0,
              September 2012.

              3GPP, "Evolved Packet System (EPS); Mobility Management
              Entity (MME) and Serving GPRS Support Node (SGSN) related
              interfaces based on Diameter protocol", TS 29.272 11.4.0,
              September 2012.

              3GPP, "Policy and Charging Control (PCC) over Gx/Sd
              reference point", TS 29.212 11.6.0, September 2012.

              3GPP, "IP Multimedia (IM) Subsystem Cx and Dx interfaces;
              Signalling flows and message contents", TS 29.228 11.5.0,
              September 2012.

              3GPP, "Mobile Application Part (MAP) specification",
              TS 29.002 11.4.0, September 2012.

Appendix A.  Contributors

   Significant contributions to this document were made by Adam Roach
   and Eric Noel.

Appendix B.  Acknowledgements

   Review of, and contributions to, this specification by Martin Dolly,
   Carolyn Johnson, Jianrong Wang, Imtiaz Shaikh, Jouni Korhonen, Robert
   Sparks, Dieter Jacobsohn, Janet Gunn, Jean-Jacques Trottin, Laurent
   Thiebaut, Andrew Booth, and Lionel Morand were most appreciated.  We
   would like to thank them for their time and expertise.

Authors' Addresses

   Eric McMurry
   17210 Campbell Rd.
   Suite 250
   Dallas, TX  75252


   Ben Campbell
   17210 Campbell Rd.
   Suite 250
   Dallas, TX  75252