Network Working Group E. McMurry
Internet-Draft B. Campbell
Intended status: Informational Tekelec
Expires: December 08, 2013 June 06, 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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Table of Contents

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 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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 use.

                      |                  |          
                      |                  |         
                      |     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 another.

| 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 unavailability.

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 signaling.

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. 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 mobile 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:

  1. always have voice and data registration, and

  2. constantly try to be on 3G or LTE data (and thus on 3G voice or VoLTE [IR.92]) 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:

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

  • 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 algorithm.

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 information.

REQ 2:
The mechanism MUST allow Diameter nodes to support overload control regardless of which Diameter applications they support. Diameter clients must be able to use the received load and overload information to support graceful behavior during an overload condition. Graceful behavior under overload conditions is best described by REQ 3.

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. At some point after an overload condition has ended, the mechanism MUST enable capacity to stabilize and become equal to what it would be in the absence of an overload condition. Note that this also requires that the mechanism MUST allow nodes to shed load without introducing non converging 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 requirement for stability in REQ 7.

REQ 10:
Consumers of overload 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 support a mixed environment where some, but not all, nodes 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 be clearly distinguishable from other errors reported via Diameter.

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 result in 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 a node to throttle the amount of traffic it receives from a 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 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 24:
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 occurring.

REQ 25:
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. Some networks may have a requirement to give priority to requests associated with emergency 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 26:
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 27:
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 28:
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 29:
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 30:
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 attacks.

REQ 31:
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", and "Diameter application", and MUST allow extensibility for others to be added in the future.

REQ 32:
The mechanism MUST provide a method for extending the information communicated and the algorithms used for overload control.

REQ 33:
The mechanism MUST provide a default algorithm that is mandatory to implement.

REQ 34:
The mechanism SHOULD provide a method for exchanging overload and load information between elements that are connected by intermediaries that do not support the mechanism.

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 mechanism.

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 mechanism.

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.
[TR23.843] 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.
[IR.92] GSMA , "IMS Profile for Voice and SMS", IR 92 7.0, March 2013.
[TS23.002] 3GPP , "Network Architecture", TS 23.002 12.0.0, September 2012.
[TS29.272] 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.
[TS29.212] 3GPP , "Policy and Charging Control (PCC) over Gx/Sd reference point", TS 29.212 11.6.0, September 2012.
[TS29.228] 3GPP , "IP Multimedia (IM) Subsystem Cx and Dx interfaces; Signalling flows and message contents", TS 29.228 11.5.0, September 2012.
[TS29.002] 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 Tekelec 17210 Campbell Rd. Suite 250 Dallas, TX 75252 US EMail:
Ben Campbell Tekelec 17210 Campbell Rd. Suite 250 Dallas, TX 75252 US EMail: