Network Working Group E. M. McMurry Internet-Draft B. C. Campbell Intended status: Standards Track Tekelec Expires: November 18, 2012 May 17, 2012 Diameter Overload Control Requirements draft-mcmurry-dime-overload-reqs-00 Abstract 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 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. Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on November 18, 2012. Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must McMurry & Campbell Expires November 18, 2012 [Page 1] Internet-Draft Diameter Overload Control Requirements May 2012 include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Causes of Overload . . . . . . . . . . . . . . . . . . . . 3 1.2. Effects of Overload . . . . . . . . . . . . . . . . . . . 4 1.3. Documentation Conventions . . . . . . . . . . . . . . . . 5 2. Overload Scenarios . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Peer to Peer Scenarios . . . . . . . . . . . . . . . . . . 6 2.2. Agent Scenarios . . . . . . . . . . . . . . . . . . . . . 8 3. Existing Mechanisms . . . . . . . . . . . . . . . . . . . . . 11 4. Issues with the Current Mechanisms . . . . . . . . . . . . . . 12 4.1. Problems with Implicit Mechanism . . . . . . . . . . . . . 12 4.2. Problems with Explicit Mechanisms . . . . . . . . . . . . 12 5. 3GPP Study on Core Network Overload . . . . . . . . . . . . . 13 6. Solution Requirements . . . . . . . . . . . . . . . . . . . . 14 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 8. Security Considerations . . . . . . . . . . . . . . . . . . . 19 8.1. Access Control . . . . . . . . . . . . . . . . . . . . . . 19 8.2. Denial-of-Service Attacks . . . . . . . . . . . . . . . . 20 8.3. Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 20 8.4. Man-in-the-Middle Attacks . . . . . . . . . . . . . . . . 20 8.5. Compromised Hosts . . . . . . . . . . . . . . . . . . . . 21 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21 9.1. Normative References . . . . . . . . . . . . . . . . . . . 21 9.2. Informative References . . . . . . . . . . . . . . . . . . 21 Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 21 Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22 McMurry & Campbell Expires November 18, 2012 [Page 2] Internet-Draft Diameter Overload Control Requirements May 2012 1. Introduction When a Diameter [I-D.ietf-dime-rfc3588bis] 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 [RFC5390] and the work done on SIP overload control as well as on overload practices in SS7 networks and studies done by 3GPP. 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 WiFi access point might use Diameter to authenticate and authorize user access via 802.11. Overload in the Diameter network will likely spill over into the end-user application network. 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 servers are effectively part of the logical element processing the request. Overload can occur for many reasons, including: Inadequate capacity: When designing Diameter networks, 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 McMurry & Campbell Expires November 18, 2012 [Page 3] Internet-Draft Diameter Overload Control Requirements May 2012 adequate capacity to handle all situations. Dependency failures: A Diameter element can become overloaded because a resource on which it is dependent has failed or become overloaded, greatly reducing the logical capacity of the element. In these cases, even minimal traffic might cause the server to go into overload. Examples of such dependency overloads include DNS servers, databases, disks, and network interfaces. Component failures: A Diameter element 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 elements take over the work of the failed elements. Normally, capacity planning takes such failures into account, and servers are typically run with enough spare capacity to handle failure of another element. However, unusual failure conditions can cause many elements 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 elements 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 signaling traffic on a Diameter network, 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 signaling traffic floods on Diameter networks. 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. DoS attacks: An attacker, wishing to disrupt service in the network, can cause a large amount of traffic to be launched at a target server. 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 server, sending the system into overload. 1.2. Effects of Overload Modern Diameter networks may operate at very large transaction volumes. If a Diameter node becomes overloaded, or even worse, fails McMurry & Campbell Expires November 18, 2012 [Page 4] Internet-Draft Diameter Overload Control Requirements May 2012 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. Diameter depends heavily on The "Authentication, Authorization, and Accounting (AAA) Transport Profile" [RFC3539], which states assumptions about the scale of AAA services which may be incorrect for current uses of Diameter. In particular, the document suggests that AAA services will typically be low volume and that traffic will typically be application-driven. Section 2.1 of that document uses an example of a 48 port NAS. However, Diameter is commonly used in large-scale mobile data environments, where a typical client could be a packet gateway that serves millions of users, and generates Diameter messages at network-driven rates. 1.3. 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 [I-D.ietf-dime-rfc3588bis]. 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 McMurry & Campbell Expires November 18, 2012 [Page 5] Internet-Draft Diameter Overload Control Requirements May 2012 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 McMurry & Campbell Expires November 18, 2012 [Page 6] Internet-Draft Diameter Overload Control Requirements May 2012 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. McMurry & Campbell Expires November 18, 2012 [Page 7] Internet-Draft Diameter Overload Control Requirements May 2012 +----------------------------------------------+ | 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. McMurry & Campbell Expires November 18, 2012 [Page 8] Internet-Draft Diameter Overload Control Requirements May 2012 +------------------+ | | | | | 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 where 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 McMurry & Campbell Expires November 18, 2012 [Page 9] Internet-Draft Diameter Overload Control Requirements May 2012 1. Therefore the agent may need to inform a client that a particular upstream server is overloaded or otherwise unavailable. +------------------+ +------------------+ | | | | | | | | | 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. McMurry & Campbell Expires November 18, 2012 [Page 10] Internet-Draft Diameter Overload Control Requirements May 2012 +----------------------------------------------+ | Application A +------------------------+----------------------+ |+------------------+ | +------------------+ | +------------------+| || | | | | | | || || | | | | | | || || Server 1 | | | Server 2 | | | Server 3 || || | | | | | | || |+---------+--------+ | +--------+---------+ | +--+---------------+| | | | | | | | +----------+----------+-----------+------------+ | | | | | | | | | | | Application B | | +-----------+------------------+----------------+ | | | ``--.__ | _. ``-.__ | __.--'' `--.._ | _..--' +-----``-+.-''-----+ | | | | | Agent | | | +--------+---------+ | | +--------+---------+ | | | | | Client | | | +------------------+ Figure 6: Multiple Application Agent Scenario 3. Existing Mechanisms Diameter requires the use of a congestion-managed transport layer, currently TCP or SCTP, to mitigate network congestion. But even with a congestion-managed transport, a Diameter node can become overloaded at the protocol layer due to the causes described in Section 1.1. 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 McMurry & Campbell Expires November 18, 2012 [Page 11] Internet-Draft Diameter Overload Control Requirements May 2012 certain rate of transaction responses occur even when there is otherwise little or no other Diameter traffic. The explicit mechanism involves specific protocol error responses, where an agent or server can tell 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. 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.) 4. Issues with the Current Mechanisms The currently available Diameter mechanisms for indicating an overload condition are not adequate to avoid congestion collapse. 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 needs to entirely stop receiving requests, i.e. that it has effectively failed. 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 [I-D.ietf-dime-rfc3588bis] indicates that the sending client should McMurry & Campbell Expires November 18, 2012 [Page 12] Internet-Draft Diameter Overload Control Requirements May 2012 attempt to send the request to a different peer. It makes no suggestion that a 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 only 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. 5. 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 McMurry & Campbell Expires November 18, 2012 [Page 13] Internet-Draft Diameter Overload Control Requirements May 2012 overload in mobile core networks including portions that are signaled using 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 GSM or CDMA 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 elements falls under the network initiated traffic flood category. There is likely to also be traffic resulting directly from the component failure in this case. 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 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. 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 mechanism MUST provide a communication method for Diameter nodes to exchange overload information. REQ 2: The overload mechanism MUST be useable with any existing or future Diameter application. It MUST NOT require specification changes for existing Diameter applications. This may be achieved using a mechanism in the Diameter base protocol that all applications could make use of. McMurry & Campbell Expires November 18, 2012 [Page 14] Internet-Draft Diameter Overload Control Requirements May 2012 REQ 3: The overload 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 elements 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 mechanism MUST ensure that the system remains stable. When the offered load drops from above the overall capacity of the network to below the overall capacity, the throughput MUST stabilize and become equal to the offered load. REQ 8: The mechanism MUST allow nodes to shed load without introducing oscillations. Note that this requirement implies a need for supporting nodes to be able to distinguish current overload information from stale information, and to 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 for stability and hysteresis control above. REQ 10: Consumers of overload state indications MUST be able to determine when the overload condition improves or ends. McMurry & Campbell Expires November 18, 2012 [Page 15] Internet-Draft Diameter Overload Control Requirements May 2012 REQ 11: The overload mechanism MUST be scalable. That is, it MUST be able to operate in different sized networks. REQ 12: When a single network element fails, goes into overload, or suffers from reduced processing capacity, the mechanism MUST make it possible to limit the impact of this on other elements 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 increased feedback when traffic levels increase. The mechanism MUST NOT do this in such a way that it increases the number of messages while at high loads. REQ 15: The mechanism MUST NOT interfere with the congestion control mechanisms of underlying transport protocols. REQ 16: The mechanism MUST operate without malfunction in an environment with a mix of elements that do, and elements that do not, support the mechanism. REQ 17: In a mixed environment with elements that support the overload control mechanism and that do not, the mechanism MUST NOT result in less useful throughput than would have resulted if it were not present. It SHOULD result in less severe congestion in this environment. REQ 18: In a mixed environment of elements that support the overload control mechanism and that do not, users and operators of elements that do not support the mechanism MUST NOT benefit from the mechanism more than users and operators of elements that support the mechanism. McMurry & Campbell Expires November 18, 2012 [Page 16] Internet-Draft Diameter Overload Control Requirements May 2012 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 failures. REQ 21: In cases where a network element 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 in these cases. REQ 22: The mechanism MUST provide a way for an element to throttle the amount of traffic it receives from an peer element. 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 element result in additional traffic to other overloaded elements, or cause additional elements 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. 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 element to other upstream elements 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 elements making decisions to prevent overload conditions from occurring. REQ 26: The specification for the overload mechanism SHOULD offer guidance on which message types might be desirable to process over others during times of overload, based on Diameter-specific considerations. For example, it may be more beneficial to process messages for existing sessions ahead of new sessions. McMurry & Campbell Expires November 18, 2012 [Page 17] Internet-Draft Diameter Overload Control Requirements May 2012 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, or processed ahead of others. REQ 28: The overload 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. 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). 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 elements are malicious; this includes preventing malicious elements 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 elements. REQ 31: It MUST be possible for a supporting node to make authorization decisions about what information will be sent to peer elements based on the identity of those elements. This allows a domain administrator who considers the load of their elements to be sensitive information to restrict access to that information. Of course, in such cases, there is no expectation that the overload mechanism itself will help prevent overload from that peer element. 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 attacks. 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 McMurry & Campbell Expires November 18, 2012 [Page 18] Internet-Draft Diameter Overload Control Requirements May 2012 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 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", and SHOULD allow extensibility for others to be added in the future. REQ 34: The mechanism MUST provide a method for extending the information communicated and the algorithms used for overload control. 7. IANA Considerations This document makes no requests of IANA. 8. Security Considerations A Diameter overload control mechanism is primarily concerned with the load and overload related behavior of elements in a Diameter network, and the information used to affect that behavior. Load and overload information is shared between elements 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. This document includes requirements intended to mitigate the effects of attacks and to protect the information used by the mechanism. 8.1. 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 McMurry & Campbell Expires November 18, 2012 [Page 19] Internet-Draft Diameter Overload Control Requirements May 2012 configurability on the elements supporting the Diameter overload control mechanism. 8.2. 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. 8.3. Replay Attacks An attacker that has managed to obtain some messages from the overload control mechanism may attempt to affect the behavior of elements 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. 8.4. Man-in-the-Middle Attacks By inserting themselves in between two elements supporting the Diameter overload control mechanism, an attacker may potentially both access and alter the information sent between those elements. 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. McMurry & Campbell Expires November 18, 2012 [Page 20] Internet-Draft Diameter Overload Control Requirements May 2012 8.5. 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 element 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 elements 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. 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [I-D.ietf-dime-rfc3588bis] Fajardo, V., Arkko, J., Loughney, J., and G. Zorn, "Diameter Base Protocol", draft-ietf-dime-rfc3588bis-33 (work in progress), May 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. 9.2. Informative References [RFC5390] Rosenberg, J., "Requirements for Management of Overload in the Session Initiation Protocol", RFC 5390, December 2008. [TR23.843] 3GPP, "Study on Core Network Overload Solutions", TR 23.843 0.4.0, April 2011. Appendix A. Contributors Significant contributions to this document were made by Adam Roach and Eric Noel. McMurry & Campbell Expires November 18, 2012 [Page 21] Internet-Draft Diameter Overload Control Requirements May 2012 Appendix B. Acknowledgements Review of, and contributions to, this specification by Martin Dolly, Carolyn Johnson, Jianrong Wang, Imtiaz Shaikh, and Robert Sparks 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: emcmurry@estacado.net Ben Campbell Tekelec 17210 Campbell Rd. Suite 250 Dallas, TX 75252 US Email: ben@nostrum.com McMurry & Campbell Expires November 18, 2012 [Page 22]