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Internet Engineering Task Force (IETF) E. McMurry
Request for Comments: 7068 B. Campbell
Category: Informational Oracle
ISSN: 2070-1721 November 2013
Diameter Overload Control Requirements
Abstract
When a Diameter server or agent becomes overloaded, it needs to be
able to gracefully reduce its load, typically by advising clients to
reduce traffic for some period of time. Otherwise, it must continue
to expend resources parsing and responding to Diameter messages,
possibly resulting in a progressively severe overload condition. The
existing Diameter mechanisms are not sufficient for managing overload
conditions. This document describes the limitations of the existing
mechanisms. Requirements for new overload management mechanisms are
also provided.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7068.
McMurry & Campbell Informational [Page 1]
RFC 7068 Diameter Overload Control Requirements November 2013
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
McMurry & Campbell Informational [Page 2]
RFC 7068 Diameter Overload Control Requirements November 2013
Table of Contents
1. Introduction ....................................................4
1.1. Documentation Conventions ..................................4
1.2. Causes of Overload .........................................5
1.3. Effects of Overload ........................................6
1.4. Overload vs. Network Congestion ............................6
1.5. Diameter Applications in a Broader Network .................7
2. Overload Control Scenarios ......................................7
2.1. Peer-to-Peer Scenarios .....................................8
2.2. Agent Scenarios ...........................................10
2.3. Interconnect Scenario .....................................14
3. Diameter Overload Case Studies .................................15
3.1. Overload in Mobile Data Networks ..........................15
3.2. 3GPP Study on Core Network Overload .......................16
4. Existing Mechanisms ............................................17
5. Issues with the Current Mechanisms .............................18
5.1. Problems with Implicit Mechanism ..........................18
5.2. Problems with Explicit Mechanisms .........................18
6. Extensibility and Application Independence .....................19
7. Solution Requirements ..........................................20
7.1. General ...................................................20
7.2. Performance ...............................................21
7.3. Heterogeneous Support for Solution ........................22
7.4. Granular Control ..........................................23
7.5. Priority and Policy .......................................23
7.6. Security ..................................................23
7.7. Flexibility and Extensibility .............................24
8. Security Considerations ........................................25
8.1. Access Control ............................................25
8.2. Denial-of-Service Attacks .................................26
8.3. Replay Attacks ............................................26
8.4. Man-in-the-Middle Attacks .................................26
8.5. Compromised Hosts .........................................27
9. References .....................................................27
9.1. Normative References ......................................27
9.2. Informative References ....................................27
Appendix A. Contributors ..........................................29
Appendix B. Acknowledgements ......................................29
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RFC 7068 Diameter Overload Control Requirements November 2013
1. Introduction
A Diameter [RFC6733] node is said to be overloaded when it has
insufficient resources to successfully process all of the Diameter
requests that it receives. When a node becomes overloaded, it needs
to be able to gracefully reduce its load, typically by advising
clients to reduce traffic for some period of time. Otherwise, it
must continue to expend resources parsing and responding to Diameter
messages, possibly resulting in a progressively severe overload
condition. The existing mechanisms provided by Diameter are not
sufficient for managing overload conditions. 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 3).
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. 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 defined in [RFC2119], with the
exception that they are not intended for interoperability of
implementations. Rather, they are used to describe requirements
towards future specifications where the interoperability requirements
will be defined.
The terms "client", "server", "agent", "node", "peer", "upstream",
and "downstream" are used as defined in [RFC6733].
McMurry & Campbell Informational [Page 4]
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1.2. 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.
External resources can include upstream Diameter nodes; for example,
a Diameter agent can become effectively overloaded if one or more
upstream nodes are overloaded.
A Diameter node can become overloaded due to request levels that
exceed its capacity, a reduction of available resources (for example,
a local or upstream hardware failure), or a combination of the two.
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 depends 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 that have failed
or become overloaded.
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.
McMurry & Campbell Informational [Page 5]
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Network-initiated traffic flood: Certain access network events can
precipitate floods of Diameter signaling traffic. For example,
operational changes can trigger avalanche restarts, or frequent
radio overlay handovers can generate excessive authorization
requests. 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.
1.3. Effects of Overload
Modern Diameter networks, composed 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 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.
1.4. 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. This document recognizes that element overload and network
congestion are interrelated, and that overload can contribute to
network congestion and vice versa.
McMurry & Campbell Informational [Page 6]
RFC 7068 Diameter Overload Control Requirements November 2013
Network congestion issues are better handled by the transport
protocols. Diameter uses TCP and the Stream Control Transmission
Protocol (SCTP), both of which include congestion management
features. Analysis of whether those features are sufficient for
transport-level congestion between Diameter nodes and of any work to
further mitigate network congestion is out of scope for both this
document and the work proposed by it.
1.5. 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.
An element that doesn't use Diameter exclusively 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. If the element
communicates with protocols other than Diameter, it may also need to
signal the overload situation on these protocols, depending on its
function and the architecture of the network and application for
which it is providing services. Whether that is necessary can only
be decided within the context of that architecture and use cases.
This specification details the requirements for a mechanism for
signaling overload with Diameter; this mechanism provides Diameter
nodes the ability to inform their Diameter peers of overload,
mitigating that part of the issue. Diameter nodes 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 it.
2. Overload Control 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, this document
assumes for the sake of clarity that the client sends Diameter
requests to the server, and the server sends responses to the 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. This document
assumes that such a cluster is responsible for managing its own
McMurry & Campbell Informational [Page 7]
RFC 7068 Diameter Overload Control Requirements November 2013
internal load-balancing and overload management so that it appears as
a single Diameter node. That is, other Diameter nodes can treat it
as a 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
McMurry & Campbell Informational [Page 8]
RFC 7068 Diameter Overload Control Requirements November 2013
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 that 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
requests, 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 the client should not divert any Application B
requests. This requires that Server 2 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 Informational [Page 9]
RFC 7068 Diameter Overload Control Requirements November 2013
+---------------------------------------------+
| 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, in
the form of either a Diameter relay or Diameter proxy. These
scenarios do not consider Diameter redirect agents, since they are
more readily modeled as end servers. The examples have been kept
simple deliberately, to illustrate basic concepts. Significantly
more complicated topologies are possible with Diameter, including
multiple intermediate agents in a path connected in a variety
of ways.
McMurry & Campbell Informational [Page 10]
RFC 7068 Diameter Overload Control Requirements November 2013
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 are 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
McMurry & Campbell Informational [Page 11]
RFC 7068 Diameter Overload Control Requirements November 2013
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
Destination-Host Attribute-Value Pair (AVP) that requires that the
request 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
McMurry & Campbell Informational [Page 12]
RFC 7068 Diameter Overload Control Requirements November 2013
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 while
Server 2 still has available capacity, 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
McMurry & Campbell Informational [Page 13]
RFC 7068 Diameter Overload Control Requirements November 2013
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 (IP Packet
eXchange). IPX [IR.34] is an Inter-Operator IP Backbone that
provides a roaming interconnection network between mobile operators
and service providers. IPX is also used to transport Diameter
signaling between operators [IR.88]. Figure 7 shows two network
operators with an interconnect network between them. There could be
any number of these networks between any two network operators'
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. For
example, network operators may not want to convey topology or
operational information; this would in turn limit how much overload
and loading information can be sent. For the interconnect scenario
shown in Figure 7, 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
McMurry & Campbell Informational [Page 14]
RFC 7068 Diameter Overload Control Requirements November 2013
network without further processing or action by the interconnect
network, even if the elements in the interconnect network do support
Diameter overload control.
3. Diameter Overload Case Studies
3.1. Overload in Mobile Data Networks
As the number of smartphone devices that are Third Generation (3G)
and Long Term Evolution (LTE) enabled continues to expand in mobile
networks, there have been situations where high signaling traffic
load led to overload events at the Diameter-based Home Location
Registers (HLRs) and/or Home Subscriber Servers (HSS) [TR23.843].
The root causes of the HLR overload 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], the IP Multimedia
Subsystem (IMS) [TS29.228], and policy and charging control
[TS29.212], 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 a load control mechanism equivalent
to those provided in the more traditional SS7 elements in the Global
System for Mobile Communications (GSM); see [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, which comprise an increasingly large percentage of
mobile devices, contribute much more heavily, relative to
non-smartphones, to the continuation of a registration surge, due to
their very aggressive registration algorithms. Smartphone behavior
contributes to network loading and can contribute to overload
conditions. The aggressive smartphone logic is designed to:
a. always have voice and data registration, and
b. constantly try to be on 3G or LTE data (and thus on 3G voice or
Voice over LTE (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.
McMurry & Campbell Informational [Page 15]
RFC 7068 Diameter Overload Control Requirements November 2013
The aggressive smartphone registration is problematic in two ways:
o first, by generating excessive signaling load towards the HSS that
is ten times the load from a non-smartphone, and
o second, by causing continual registration attempts when a network
failure affects registrations through the 3G data network.
3.2. 3GPP Study on Core Network Overload
A study in the 3GPP System Aspects working group 2 (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. [TR23.843] 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 are
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.
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4. 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 that
the upstream peer is unavailable or is overloaded to the point of
effective unavailability. The watchdog mechanism [RFC3539] ensures
that transaction responses occur at a certain rate 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 that
the client not reconnect.
Once a Diameter node learns via one of these mechanisms that an
upstream peer has become overloaded, 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 1.2, 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.
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5. 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
impacts resulting from overload due to unresponsive Diameter nodes
causing application-layer 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.
5.1. 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 that it needs 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.
5.2. 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 a
DIAMETER_TOO_BUSY error. 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.
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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 to 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 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 errors, 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 described 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 is beyond the scope
of this document.
6. Extensibility and Application Independence
Given the variety of scenarios in which Diameter elements can be
deployed and the variety of roles they can fulfill with Diameter and
other technologies, a single algorithm for handling overload may not
be sufficient. For purposes of this discussion, an algorithm is
inclusive of behavior for control of overload but does not encompass
the general mechanism for transporting control information. 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.
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Similarly, the scopes to which overload information may apply 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 addressing the issues
described in Section 5 and supporting the scenarios described in
Section 2. These requirements are stated primarily in terms of
individual node behavior to inform the design of the improved
mechanism; solution designers should keep in mind that the overall
goal is improved overall system behavior across all the nodes
involved, not just improved behavior from specific individual nodes.
7.1. General
REQ 1: The solution MUST provide a communication method for Diameter
nodes to exchange load and overload information.
REQ 2: The solution MUST allow Diameter nodes to support overload
control regardless of which Diameter applications they
support. Diameter clients and agents 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 solution 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 a solution.
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 solution MUST allow each
side of a connection to independently inform the other of its
overload status.
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REQ 5: Diameter allows nodes to determine their peers via dynamic
discovery or manual configuration. The solution MUST work
consistently without regard to how peers are determined.
REQ 6: The solution 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 solution, rather than to require that this knowledge
be configured at each node.
7.2. Performance
REQ 7: The solution and any associated default algorithm(s) MUST
ensure that the system remains stable. At some point after
an overload condition has ended, the solution 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 solution 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.
REQ 9: The solution 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 solution 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 solution MUST
make it possible to limit the impact of the affected node on
other nodes in the network. This helps to prevent a small-
scale failure from becoming a widespread outage.
REQ 13: The solution MUST NOT introduce substantial additional work
for a 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.
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REQ 14: Some scenarios that result in overload involve a rapid
increase of traffic with little time between normal levels
and levels that induce overload. The solution SHOULD provide
for rapid feedback when traffic levels increase.
REQ 15: The solution MUST NOT interfere with the congestion control
mechanisms of underlying transport protocols. For example, a
solution that opened additional TCP connections when the
network is congested would reduce the effectiveness of the
underlying congestion control mechanisms.
7.3. Heterogeneous Support for Solution
REQ 16: The solution is likely to be deployed incrementally. The
solution MUST support a mixed environment where some, but not
all, nodes implement it.
REQ 17: In a mixed environment with nodes that support the solution
and nodes that do not, the solution MUST NOT result in
materially less useful throughput during overload as would
have resulted if the solution were not present. It SHOULD
result in less severe overload in this environment.
REQ 18: In a mixed environment of nodes that support the solution and
nodes that do not, the solution MUST NOT preclude elements
that support overload control from treating elements that do
not support overload control in an equitable fashion relative
to those that do. Users and operators of nodes that do not
support the solution MUST NOT unfairly benefit from the
solution. The solution specification SHOULD provide guidance
to implementors for dealing with elements not supporting
overload control.
REQ 19: It MUST be possible to use the solution 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
overload. The solution MUST result in at least as much
useful throughput as would have resulted if the solution were
not in place.
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7.4. Granular Control
REQ 22: The solution 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 solution 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 solution MUST provide a mechanism for indicating load
levels, even when not in an overload condition, to assist
nodes in making decisions to prevent overload conditions from
occurring.
7.5. Priority and Policy
REQ 25: The base specification for the solution 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 solution 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.
7.6. Security
REQ 27: The solution 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 (see
Security Considerations in this document (Section 8)). Note
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that this requirement was expressed at a high level so as to
not preclude any particular solution. Is is expected that
the solution will address this in more detail.
REQ 28: The solution 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 solution 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 solution itself will help prevent
overload from that peer node.
REQ 30: The solution 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.
7.7. Flexibility and Extensibility
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 solution 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 solution 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 solution MUST provide a method for extending the
information communicated and the algorithms used for overload
control.
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REQ 33: The solution MUST provide a default algorithm that is
mandatory to implement.
REQ 34: The solution SHOULD provide a method for exchanging overload
and load information between elements that are connected by
intermediaries that do not support the solution.
8. Security Considerations
A Diameter overload control mechanism is primarily concerned with the
load-related 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 the information 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 the visibility of 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 it difficult for non-adjacent nodes to verify the
authenticity and ownership of load and overload information.
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.
This section discusses potential security considerations for overload
control solutions. This discussion provides the motivation for
several normative requirements described in Section 7. The
discussion includes specific references to the normative requirements
that apply for each issue.
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. REQ 28 requires that the solution work without
assuming that all Diameter nodes in a network are trusted for the
purposes of exchanging overload and load information. REQ 29
requires that the solution let nodes restrict unauthorized parties
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from seeing overload information. Note that this implies a certain
amount of configurability on the nodes 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 effect 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. More strongly, REQ 27 forbids the solution from
introducing new vulnerabilities to malicious attack. Additionally,
REQ 30 stipulates that the solution not interfere with other
mechanisms used for protection against denial-of-service attacks.
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 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
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 solution is to
minimize or eliminate the possibility of causing disruption by using
a replay attack on the Diameter overload control mechanism.
(Allowing a replay attack using the overload control solution would
violate REQ 27.)
8.4. Man-in-the-Middle Attacks
By inserting themselves 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.
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REQs 27, 28, and 29 imply a need to prevent man-in-the-middle attacks
on the overload control solution. A transport using Transport Layer
Security (TLS) and/or IPsec may be desirable for this purpose.
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 node that would
normally accept information from it. While it is beyond the scope of
the Diameter overload control mechanism to mitigate any operational
interruption to the compromised host, REQs 28 and 29 imply a need 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.
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.
[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.
9.2. 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 (CN) overload solutions",
TR 23.843 1.2.0, Work in Progress, October 2013.
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[IR.34] GSMA, "Inter-Service Provider IP Backbone Guidelines",
IR 34 9.1, May 2013.
[IR.88] GSMA, "LTE Roaming Guidelines", IR 88 9.0, January 2013.
[IR.92] GSMA, "IMS Profile for Voice and SMS", IR 92 7.0,
March 2013.
[TS23.002] 3GPP, "Network Architecture", TS 23.002 12.2.0,
June 2013.
[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 12.2.0,
September 2013.
[TS29.212] 3GPP, "Policy and Charging Control (PCC) over Gx/Sd
reference point", TS 29.212 12.2.0, September 2013.
[TS29.228] 3GPP, "IP Multimedia (IM) Subsystem Cx and Dx interfaces;
Signalling flows and message contents", TS 29.228 12.0.0,
September 2013.
[TS29.002] 3GPP, "Mobile Application Part (MAP) specification",
TS 29.002 12.2.0, September 2013.
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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
Oracle
17210 Campbell Rd.
Suite 250
Dallas, TX 75252
US
EMail: emcmurry@computer.org
Ben Campbell
Oracle
17210 Campbell Rd.
Suite 250
Dallas, TX 75252
US
EMail: ben@nostrum.com
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