Internet DRAFT - draft-mcmurry-dime-overload-reqs
draft-mcmurry-dime-overload-reqs
Network Working Group E. McMurry
Internet-Draft B. Campbell
Intended status: Standards Track Tekelec
Expires: March 1, 2013 August 28, 2012
Diameter Overload Control Requirements
draft-mcmurry-dime-overload-reqs-02
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
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This Internet-Draft will expire on March 1, 2013.
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Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Causes of Overload . . . . . . . . . . . . . . . . . . . . 3
1.2. Effects of Overload . . . . . . . . . . . . . . . . . . . 5
1.3. Overload vs. Network Congestion . . . . . . . . . . . . . 5
1.4. Diameter Applications in a Broader Network . . . . . . . . 5
1.5. Documentation Conventions . . . . . . . . . . . . . . . . 6
2. Overload Scenarios . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Peer to Peer Scenarios . . . . . . . . . . . . . . . . . . 7
2.2. Agent Scenarios . . . . . . . . . . . . . . . . . . . . . 9
2.3. Interconnect Scenario . . . . . . . . . . . . . . . . . . 12
3. Existing Mechanisms . . . . . . . . . . . . . . . . . . . . . 13
4. Issues with the Current Mechanisms . . . . . . . . . . . . . . 14
4.1. Problems with Implicit Mechanism . . . . . . . . . . . . . 15
4.2. Problems with Explicit Mechanisms . . . . . . . . . . . . 15
5. Diameter Overload Case Studies . . . . . . . . . . . . . . . . 16
5.1. Overload in Mobile Data Networks . . . . . . . . . . . . . 16
5.2. 3GPP Study on Core Network Overload . . . . . . . . . . . 17
6. Solution Requirements . . . . . . . . . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
8. Security Considerations . . . . . . . . . . . . . . . . . . . 22
8.1. Access Control . . . . . . . . . . . . . . . . . . . . . . 23
8.2. Denial-of-Service Attacks . . . . . . . . . . . . . . . . 23
8.3. Replay Attacks . . . . . . . . . . . . . . . . . . . . . . 23
8.4. Man-in-the-Middle Attacks . . . . . . . . . . . . . . . . 24
8.5. Compromised Hosts . . . . . . . . . . . . . . . . . . . . 24
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
9.1. Normative References . . . . . . . . . . . . . . . . . . . 24
9.2. Informative References . . . . . . . . . . . . . . . . . . 25
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 25
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
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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 a network that
uses Diameter applications 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 elements are effectively
part of the logical element processing the request.
Overload can occur for many reasons, including:
Inadequate capacity: When designing Diameter networks, that is,
application layer multi-node Diameter deployments, it can be very
difficult to predict all scenarios that may cause elevated
traffic. It may also be more costly to implement support for some
scenarios than a network operator may deem worthwhile. This
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results in the likelihood that a Diameter network will not have
adequate capacity to handle all situations.
Dependency failures: A Diameter node can become overloaded because a
resource on which it is dependent has failed or become overloaded,
greatly reducing the logical capacity of the node. In these
cases, even minimal traffic might cause the node to go into
overload. Examples of such dependency overloads include DNS
servers, databases, disks, and network interfaces.
Component failures: A Diameter node can become overloaded when it is
a member of a cluster of servers that each share the load of
traffic, and one or more of the other members in the cluster fail.
In this case, the remaining nodes take over the work of the failed
nodes. Normally, capacity planning takes such failures into
account, and servers are typically run with enough spare capacity
to handle failure of another node. However, unusual failure
conditions can cause many nodes to fail at once. This is often
the case with software failures, where a bad packet or bad
database entry hits the same bug in a set of nodes in a cluster.
Network Initiated Traffic Flood: Issues with the radio access
network in a mobile network such as radio overlays with frequent
handovers, and operational changes are examples of network events
that can precipitate a flood of Diameter signaling traffic, such
as an avalanche restart. Failure of a Diameter proxy may also
result in a large amount of signaling as connections and sessions
are reestablished.
Subscriber Initiated Traffic Flood: Large gatherings of subscribers
or events that result in many subscribers interacting with the
network in close time proximity can result in Diameter signaling
traffic floods. For example, the finale of a large fireworks show
could be immediately followed by many subscribers posting
messages, pictures, and videos concentrated on one portion of a
network. Subscriber devices, such as smartphones, may use
aggressive registration strategies that generate unusually high
Diameter traffic loads.
DoS attacks: An attacker, wishing to disrupt service in the network,
can cause a large amount of traffic to be launched at a target
element. This can be done from a central source of traffic or
through a distributed DoS attack. In all cases, the volume of
traffic well exceeds the capacity of the element, sending the
system into overload.
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1.2. Effects of Overload
Modern Diameter networks, comprised of application layer multi-node
deployments of Diameter elements, may operate at very large
transaction volumes. If a Diameter node becomes overloaded, or even
worse, fails completely, a large number of messages may be lost very
quickly. Even with redundant servers, many messages can be lost in
the time it takes for failover to complete. While a Diameter client
or agent should be able to retry such requests, an overloaded peer
may cause a sudden large increase in the number of transaction
transactions needing to be retried, rapidly filling local queues or
otherwise contributing to local overload. Therefore Diameter devices
need to be able to shed load before critical failures can occur.
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. Overload vs. Network Congestion
This document uses the term "overload" to refer to application-layer
overload at Diameter nodes. This is distinct from "network
congestion", that is, congestion that occurs at the lower networking
layers that may impact the delivery of Diameter messages between
nodes. The authors recognize that element overload and network
congestion are interrelated, and that overload can contribute to
network congestion and vice versa.
Network congestion issues are better handled by the transport
protocols. Diameter uses TCP and SCTP, both of which include
congestion management features. Analysis of whether those features
are sufficient for transport level congestion between Diameter nodes,
and any work to further mitigate network congestion is out of scope
both for this document, and for the work proposed by this document.
1.4. Diameter Applications in a Broader Network
Most elements using Diameter applications do not use Diameter
exclusively. It is important to realize that overload of an element
can be caused by a number of factors that may be unrelated to the
processing of Diameter or Diameter applications.
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A element communicating via protocols other than Diameter that is
also using a Diameter application needs to be able to signal to
Diameter peers that it is experiencing overload regardless of the
cause of the overload, since the overload will affect that element's
ability to process Diameter transactions. The element may also need
to signal this on other protocols depending on its function and the
architecture of the network and application it is providing services
for. Whether that is necessary can only be decided within the
context of that architecture and application. A mechanism for
signaling overload with Diameter, which this specification details
the requirements for, provides applications the ability to signal
their Diameter peers of overload, mitigating that part of the issue.
Applications may need to use this, as well as other mechanisms, to
solve their broader overload issues. Indicating overload on
protocols other than Diameter is out of scope for this document, and
for the work proposed by this document.
1.5. Documentation Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The terms "client", "server", "agent", "node", "peer", "upstream",
and "downstream" are used as defined in [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
as single, monolithic node for the purposes of overload management.
These scenarios do not illustrate the client application. As
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mentioned in Section 1, Diameter is not typically an end-user
protocol; rather it is generally used in support of some other client
application. These scenarios do not consider the impact of Diameter
overload on the client application.
2.1. Peer to Peer Scenarios
This section describes Diameter peer-to-peer scenarios. That is,
scenarios where a Diameter client talks directly with a Diameter
server, without the use of a Diameter agent.
Figure 1 illustrates the simplest possible Diameter relationship.
The client and server share a one-to-one peer-to-peer relationship.
If the server becomes overloaded, either because the client exceeds
the server's capacity, or because the server's capacity is reduced
due to some resource dependency, the client needs to reduce the
amount of Diameter traffic it sends to the server. Since the client
cannot forward requests to another server, it must either queue
requests until the server recovers, or itself become overloaded in
the context of the client application and other protocols it may also
use.
+------------------+
| |
| |
| Server |
| |
+--------+---------+
|
|
+--------+---------+
| |
| |
| Client |
| |
+------------------+
Figure 1: Basic Peer to Peer Scenario
Figure 2 shows a similar scenario, except in this case the client has
multiple servers that can handle work for a specific realm and
application. If server 1 becomes overloaded, the client can forward
traffic to server 2. Assuming server 2 has sufficient reserve
capacity to handle the forwarded traffic, the client should be able
to continue serving client application protocol users. If server 1
is approaching overload, but can still handle some number of new
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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.
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+----------------------------------------------+
| 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.
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+------------------+
| |
| |
| 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
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DestinationHost AVP that requires the request to be served by server
1. Therefore the agent may need to inform a client that a particular
upstream server is overloaded or otherwise unavailable. Note that
there can be many ways a server can be specified, which may have
different implications (e.g. by IP address, by host name, etc).
+------------------+ +------------------+
| | | |
| | | |
| Server 1 | | Server 2 |
| | | |
+--------+-`.------+ +------.'+---------+
`. .'
`. .'
`. .'
`. .'
+-------`.'--------+
| |
| |
| Agent |
| |
+--------+---------+
|
|
|
+--------+---------+
| |
| |
| Client |
| |
+------------------+
Figure 5: Multiple Server Agent Scenario
Figure 6 shows a scenario where an agent routes requests to a set of
servers for more than one Diameter realm and application. In this
scenario, if server 1 becomes overloaded or unavailable, the agent
may effectively operate at reduced capacity for application A, but at
full capacity for application B. Therefore, the agent needs to be
able to report that it is overloaded for one application, but not for
another.
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+----------------------------------------------+
| Application A +------------------------+----------------------+
|+------------------+ | +------------------+ | +------------------+|
|| | | | | | | ||
|| | | | | | | ||
|| Server 1 | | | Server 2 | | | Server 3 ||
|| | | | | | | ||
|+---------+--------+ | +--------+---------+ | +--+---------------+|
| | | | | | |
+----------+----------+-----------+------------+ | |
| | | | |
| | | | Application B |
| +-----------+------------------+----------------+
| | |
``--.__ | _.
``-.__ | __.--''
`--.._ | _..--'
+-----``-+.-''-----+
| |
| |
| Agent |
| |
+--------+---------+
|
|
+--------+---------+
| |
| |
| Client |
| |
+------------------+
Figure 6: Multiple Application Agent Scenario
2.3. Interconnect Scenario
Another scenario to consider when looking at Diameter overload is
that of multiple network operators using Diameter components
connected through an interconnect service, e.g. using IPX. Figure 7
shows two network operators with an interconnect network in-between.
There could be any number of these networks between any two network
operator's networks.
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+-------------------------------------------+
| Interconnect |
| |
| +--------------+ +--------------+ |
| | Edge Agent 1 |------| Edge Agent 2 | |
| +--------------+ +--------------+ |
| .' `. |
+------.-'--------------------------`.------+
.' `.
.-' `.
------------.'-----+ +----`.---------------
+----------+ | | +----------+
| 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 than the information
shared between components within a network operator's network.
Network operators may not want to convey topology or operational
information, which limits how overload and loading information can be
sent. For the interconnect scenario shown, Server 2 may want to
signal overload to Server 1, to affect traffic coming from Network
Operator 1.
This is different than internal to an 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 to use overload or loading information intended to pass
through the interconnect even if the elements in the interconnect
network do support diameter overload control.
3. Existing Mechanisms
Diameter offers both implicit and explicit mechanisms for a Diameter
node to learn that a peer is overloaded or unreachable. The implicit
mechanism is simply the lack of responses to requests. If a client
fails to receive a response in a certain time period, it assumes the
upstream peer is unavailable, or overloaded to the point of effective
unavailability. The watchdog mechanism [RFC3539] ensures that a
certain rate of transaction responses occur even when there is
otherwise little or no other Diameter traffic.
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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.)
Diameter requires the use of a congestion-managed transport layer,
currently TCP or SCTP, to mitigate network congestion. It is
expected that these transports manage network congestion and that
issues with transport (e.g. congestion propagation and window
management) are managed at that level. But even with a congestion-
managed transport, a Diameter node can become overloaded at the
Diameter protocol or application layers due to the causes described
in Section 1.1 and congestion managed transports do not provide
facilities (and are at the wrong level) to handle server overload.
Transport level congestion management is also not sufficient to
address overload in cases of multi-hop and multi-destination
signaling.
4. Issues with the Current Mechanisms
The currently available Diameter mechanisms for indicating an
overload condition are not adequate to avoid service outages due to
overload. This may, in turn, contribute to broader congestion
collapse due to unresponsive Diameter nodes causing application or
transport layer retransmissions. In particular, they do not allow a
Diameter agent or server to shed load as it approaches overload. At
best, a node can only indicate that it needs to entirely stop
receiving requests, i.e. that it has effectively failed. Even that
is problematic due to the inability to indicate durational validity
on the transient errors available in the base Diameter protocol.
Diameter offers no mechanism to allow a node to indicate different
overload states for different categories of messages, for example, if
it is overloaded for one Diameter application but not another.
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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
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.
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Agent processing of a DIAMETER_TOO_BUSY response is also problematic
as described in the base specification. DIAMETER_TOO_BUSY is defined
as a protocol error. If an agent receives a protocol error, it may
either handle it locally or it may forward the response back towards
the downstream peer. (The Diameter specification is inconsistent
about whether a protocol error MAY or SHOULD be handled by an agent,
rather than forwarded downstream.) If a downstream peer receives the
DIAMETER_TOO_BUSY response, it may stop sending all requests to the
agent for some period of time, even though the agent may still be
able to deliver requests to other upstream peers.
DIAMETER_UNABLE_TO_DELIVER also has no mechanisms for specifying the
scope or cause of the failure, or the durational validity.
5. Diameter Overload Case Studies
5.1. Overload in Mobile Data Networks
As the number of Third Generation (3G) and Long Term Evolution (LTE)
enabled smartphone devices continue to expand in mobility networks,
there have been situations where high signaling traffic load led to
overload events at the Diameter-based Home Location Registries (HLR)
and/or Home Subscriber Servers (HSS). The root causes of the HLR
congestion events were manifold but included hardware failure and
procedural errors. The result was high signaling traffic load on the
HLR and HSS.
The 3GPP standards specification[need citation] for the end-to-end
signaling call flows in 3G and LTE, from the end user device
traversing through the radio and the core networks to the HLR/HSS,
did not have an equivalent load control mechanism which is provided
in the more traditional SS7 elements in GSM [need citation]. The
capabilities specified in the 3GPP standards do not adequately
address the abnormal condition where excessively high signaling
traffic load situations are experienced.
Smartphones contribute much more heavily to the continuation of a
registration surge due to their very aggressive registration
algorithms. The aggressive smartphone logic is designed to:
a. always have voice and data registration, and
b. constantly try to be on 3G data (and thus on 3G voice) for their
added benefits.
Non-smartphones typically have logic to wait for a time period after
registering successfully on voice and data.
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The smartphone aggressive registration is problematic in two ways:
o first by generating excessive signaling load towards the HLR that
is ten times that from a non-smartphone,
o and second by causing continual registration attempts when a
network failure affects registrations through the 3G data network.
5.2. 3GPP Study on Core Network Overload
A study in 3GPP SA2 on core network overload has produced the
technical report [TR23.843]. This enumerates several causes of
overload in mobile core networks including portions that are signaled
using Diameter. This document is a work in progress and is not
complete. However, it is useful for pointing out scenarios and the
general need for an overload control mechanism for Diameter.
It is common for mobile networks to employ more than one radio
technology and to do so in an overlay fashion with multiple
technologies present in the same location (such as GSM, UMTS 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 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.
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
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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.
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 nodes to determine their peers via dynamic
discovery or manual configuration. The mechanism MUST work
consistently without regard to how peers are determined.
REQ 6: The mechanism designers SHOULD seek to minimize the amount
of new configuration required in order to work. For
example, it is better to allow peers to advertise or
negotiate support for the mechanism, rather than to require
this knowledge to be configured at each node.
REQ 7: The overload 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.
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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.
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 node fails, goes into overload, or
suffers from reduced processing capacity, the mechanism MUST
make it possible to limit the impact of this on other nodes
in the network. This helps to prevent a small-scale failure
from becoming a widespread outage.
REQ 13: The mechanism MUST NOT introduce substantial additional work
for node in an overloaded state. For example, a requirement
for an overloaded node to send overload information every
time it received a new request would introduce substantial
work. Existing messaging is likely to have the
characteristic of increasing as an overload condition
approaches, allowing for the possibility of increased
feedback for information piggybacked on it.
REQ 14: Some scenarios that result in overload involve a rapid
increase of traffic with little time between normal levels
and overload inducing levels. The mechanism SHOULD provide
for 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. For example,
a mechanism that opened additional TCP connections when the
network is congested would reduce the effectiveness of the
underlying congestion control mechanisms.
REQ 16: The mechanism MUST operate without malfunction in an
environment with a mix of nodes that do, and nodes that do
not, support the mechanism.
REQ 17: In a mixed environment with nodes 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.
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REQ 18: In a mixed environment of nodes that support the overload
control mechanism and that do not, users and operators of
nodes that do not support the mechanism MUST NOT benefit
from the mechanism more than users and operators of nodes
that support the mechanism.
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 node fails, is so overloaded that
it cannot process messages, or cannot communicate due to a
network failure, it may not be able to provide explicit
indications of the nature of the failure or its levels of
congestion. The mechanism MUST properly function in these
cases.
REQ 22: The mechanism MUST provide a way for an node to throttle the
amount of traffic it receives from an peer node. This
throttling SHOULD be graded so that it can be applied
gradually as offered load increases. Overload is not a
binary state; there may be degrees of overload.
REQ 23: The mechanism MUST enable a supporting node to minimize the
chance that retries due to an overloaded or failed node
result in additional traffic to other overloaded nodes, or
cause additional nodes to become overloaded. Moreover, the
mechanism SHOULD provide unambiguous directions to clients
on when they should retry a request and when they should not
considering the various causes of overload such as avalanche
restart.
REQ 24: The mechanism MUST provide sufficient information to enable
a load balancing node to divert messages that are rejected
or otherwise throttled by an overloaded upstream node to
other upstream nodes that are the most likely to have
sufficient capacity to process them.
REQ 25: The mechanism MUST provide a mechanism for indicating load
levels even when not in an overloaded condition, to assist
nodes making decisions to prevent overload conditions from
occurring.
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REQ 26: The specification for the overload mechanism SHOULD offer
guidance on which message types might be desirable to send
or 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.
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 nodes are malicious; this includes
preventing malicious nodes from obtaining more than a fair
share of service. Note that this does not imply any
responsibility on the mechanism to detect, or take
countermeasures against, malicious nodes.
REQ 31: It MUST be possible for a supporting node to make
authorization decisions about what information will be sent
to peer nodes based on the identity of those nodes. This
allows a domain administrator who considers the load of
their nodes to be sensitive information to restrict access
to that information. Of course, in such cases, there is no
expectation that the overload mechanism itself will help
prevent overload from that peer node.
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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
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.
REQ 35: The mechanism SHOULD provide a method for exchanging
overload and load information between elements that are
connected by intermediaries that do not support the
mechanism. A separate mechanism or extension of the
mechanism to support this may be warranted for this.
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 nodes in a Diameter network,
and the information used to affect that behavior. Load and overload
information is shared between nodes and directly affects the behavior
and thus is potentially vulnerable to a number of methods of attack.
Load and overload information may also be sensitive from both
business and network protection viewpoints. Operators of Diameter
equipment want to control visibility to load and overload information
to keep it from being used for competitive intelligence or for
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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
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 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
nodes supporting the mechanism by sending those messages at
potentially inopportune times. In addition to time shifting, replay
attacks may send messages to other nodes as well (target shifting).
A design goal for the Diameter overload control mechanism is to
minimize or eliminate the possibility of causing disruption by using
a replay attack on the Diameter overload control mechanism.
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8.4. Man-in-the-Middle Attacks
By inserting themselves in between two nodes supporting the Diameter
overload control mechanism, an attacker may potentially both access
and alter the information sent between those nodes. This can be used
for information gathering for business intelligence and attack
targeting, as well as direct attacks.
A design goal for the Diameter overload control mechanism is to
minimize or eliminate the possibility of causing disruption man-in-
the-middle attacks on the Diameter overload control mechanism. A
transport using TLS and/or IPSEC may be desirable for this.
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 is is beyond the scope of
the Diameter overload control mechanism to mitigate any operational
interruption to the compromised host, a reasonable design goal is to
minimize the impact that a compromised host can have on other nodes
through the use of the Diameter overload control mechanism. Of
course, a compromised host could be used to cause damage in a number
of other ways. This is out of scope for a Diameter overload control
mechanism.
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-34
(work in progress), June 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.
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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.
Appendix B. Acknowledgements
Review of, and contributions to, this specification by Martin Dolly,
Carolyn Johnson, Jianrong Wang, Imtiaz Shaikh, Jouni Korhonen, 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
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