Network Working Group B. Campbell
Internet-Draft Tekelec
Intended status: Standards Track H. Tschofenig
Expires: August 22, 2013 Nokia Siemens Networks
J. Korhonen
Renesas Mobile
A. Roach
Mozilla
February 18, 2013
Diameter Overload Data Analysis
draft-campbell-dime-overload-data-analysis-00
Abstract
When a Diameter server or agent becomes overloaded, it needs to be
able to gracefully reduce its load, typically by informing clients to
reduce sending traffic for some period of time. Multiple mechanisms
have been proposed for transporting overload and load information.
While these proposals differ in many ways, they share similar data
requirements. This document analyzes the data requirements of each
proposal with a view towards proposing a common set of of Diameter
Attribute-Value Pairs (AVPs).
Status of this Memo
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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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Documentation Conventions . . . . . . . . . . . . . . . . . . 3
3. Overload Control Data Usage . . . . . . . . . . . . . . . . . 3
4. Mechanism Differences that Affect Data Structures . . . . . . 4
4.1. Non-Adjacent Nodes . . . . . . . . . . . . . . . . . . . . 4
4.2. Stateless Negotiation . . . . . . . . . . . . . . . . . . 4
4.3. Overload Scopes . . . . . . . . . . . . . . . . . . . . . 5
4.4. Hard or Soft Overload State . . . . . . . . . . . . . . . 5
5. Naming Conventions . . . . . . . . . . . . . . . . . . . . . . 5
6. Data Element Comparison . . . . . . . . . . . . . . . . . . . 6
6.1. Data Elements for Connection Establishment and
Negotiation . . . . . . . . . . . . . . . . . . . . . . . 6
6.1.1. Supported Scope Selection . . . . . . . . . . . . . . 6
6.1.2. Algorithm Selection . . . . . . . . . . . . . . . . . 6
6.1.3. Application Selection . . . . . . . . . . . . . . . . 7
6.1.4. Frequency of Reports . . . . . . . . . . . . . . . . . 7
6.1.5. Grouping . . . . . . . . . . . . . . . . . . . . . . . 7
6.2. Data Elements for Overload and Load reporting . . . . . . 7
6.2.1. Scope of Report . . . . . . . . . . . . . . . . . . . 7
6.2.2. Overload Severity . . . . . . . . . . . . . . . . . . 8
6.2.3. Report Algorithm . . . . . . . . . . . . . . . . . . . 8
6.2.4. Report Expiration . . . . . . . . . . . . . . . . . . 8
6.2.5. Current Load . . . . . . . . . . . . . . . . . . . . . 9
6.2.6. Applications covered by a Report . . . . . . . . . . . 9
6.2.7. Report Action . . . . . . . . . . . . . . . . . . . . 9
6.2.8. Priority . . . . . . . . . . . . . . . . . . . . . . . 10
6.2.9. Session Groups . . . . . . . . . . . . . . . . . . . . 10
6.3. Result Codes . . . . . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Contributors . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
When a Diameter [RFC6733] server or agent becomes overloaded, it
needs to be able to gracefully reduce its load, typically by
informing clients to reduce sending traffic for some period of time.
The Diameter Overload Control Requirements
[I-D.ietf-dime-overload-reqs] describe requirements for overflow
control mechanisms.
At the time of this writing, there have been two proposals for
Diameter overload control mechanisms. "A Mechanism for Diameter
Overload Control" (MDOC) [I-D.roach-dime-overload-ctrl] defines a
mechanism that piggybacks overload and load state information over
existing Diameter messages. "The Diameter Overload Control
Application" (DOCA) [I-D.korhonen-dime-ovl] defines a mechanism that
uses a new and distinct Diameter application to communicate similar
information. While there are significant differences between the two
proposals, they carry similar information. Each proposal includes
its own set of Diameter AVPs.
This document is intended as a framework for discussing the data
requirements of the two proposals. It includes an analysis of the
differences and similarities of their respective data elements, with
a view towards rationalizing the AVPs from the two proposals.
The authors expect that a follow-on effort will eventually specify
a common data model for reporting Diameter overload information.
This document assumes that Diameter nodes exchange overload control
information via Diameter, rather than via some out-of-band channel.
This document does not address the specific difference of either
mechanism proposal, except where they impact the AVP definitions.
2. Documentation Conventions
This document uses terms defined in [RFC6733] and
[I-D.ietf-dime-overload-reqs].
3. Overload Control Data Usage
A Diameter overload control mechanism based on the overload control
requirements [I-D.ietf-dime-overload-reqs] involves the exchange of
information between two or more Diameter nodes. The exchanged
information serves three distinct purposes:
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Negotiation: Diameter nodes need to negotiate support for the
overload control mechanism in general. Nodes that support
overload control need to advertise the overload control scopes
they can support. Finally, they need to select an overload
control algorithm.
Communication of Overload State: Nodes need to report that an
overload condition is in effect, to what degree they are
overloaded, and the scope of the overload condition. We refer to
such a communication as an "Overload Report".
Communication of Load: Nodes need to communicate their current load
status, even when not in an overloaded state.
Overload Control information may be communicated between adjacent
Diameter nodes, or it may cross one or more intervening nodes.
Overload Control information can be communicated in either direction;
that is, a downstream node can indicate overload to an upstream node,
or vice-versa.
Open Issue: There is an ongoing discussion about whether the
overload control mechanism should be strictly hop-by-hop, or
whether it should support communication between non-adjacent
nodes. The results of this discussion may have implications for
overload control data elements.
4. Mechanism Differences that Affect Data Structures
While a thorough comparison of the two proposed mechanisms is out of
scope for this document, there are a few differences that directly
impact the choice of data elements.
4.1. Non-Adjacent Nodes
MDOC only supports hop-by-hop communication of overload information.
DOCA allows for the possibility of communication between non-adjacent
nodes. For hop-by-hop communication, the originator of an overload
report is always the directly connected node. If non-adjacent
communication is to be allowed, the data model needs a way to express
the identity of the originating node.
4.2. Stateless Negotiation
Both MDOC and DOCA allow overload control parameters to be negotiated
at the beginning of a connection, and persist for the duration of the
connection. DOCA also allows a "stateless" mode, where the
parameters do not persist between overload reports. This requires
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the sender of an overload report to restate any relevant parameters
for each report. Thus, the DOCA overload report format includes the
ability to express all such parameters at any time, not just during
negotiation.
Note that stateless negotiation does not mean that no state may ever
be saved. Nodes may use implementation-specific methods of
remembering certain parameters, or out-of-band configuration methods
to do the same.
4.3. Overload Scopes
As described in [I-D.ietf-dime-overload-reqs], it's possible for a
Diameter node to experience overload that impacts some subset of
potential traffic. For example, a Diameter agent might route traffic
to different servers based on realm. If the server for one realm
experienced an outage or overload condition, the agent report that it
is overloaded for that realm, but can process traffic for other
realms normally. We use the term "overload scope", or simply
"scope", to refer to the set of potential messages affected by an
overload report.
MDOC includes a richer (and therefore more complex) concept of
overload scopes. A node may include multiple scopes in an overload
report. Each scope entry indicates both the type of scope, and the
value of the scope, where the value is interpreted according to the
type.
DOCA also allows a node to include multiple scopes in a report. But
DOCA's current set of scope types only affect the interpretation of
the originating node identity. Therefore the DOCA scope entries do
not include a value.
4.4. Hard or Soft Overload State
MDOC assumes that overload information is soft state. That is, it
expires if not refreshed within a stated interval. DOCA also treats
most overload information as soft state, but there are situations
where it may be treated as hard-state. For example, if the OC-Level
is set to "Hold", the expiration time is not honored.
5. Naming Conventions
MDOC and DOCA use somewhat different naming conventions for their
respective AVPs. DOCA prefixes each AVP name with "OC". (for
example, "OC-Scope"). MDOC prefixes AVPs that can appear in the root
of messages with "Overload", and leaves those that occur inside an
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overload related grouped AVP to be identified by context. (For
example, "Overload Info" and "Supported Scopes"). The working group
should consider picking one approach or the other.
6. Data Element Comparison
6.1. Data Elements for Connection Establishment and Negotiation
The following sections describe data elements used for initial
negotiation.
6.1.1. Supported Scope Selection
o DOCA: OC-Scope : Bitmap of scopes supported by the sender.
Currently defined values are "Host scope", "Realm Scope", "Only
origin realm", "Application Information", "Node Utilization
Information", and "Application Priorities".
o MDOC: Supported-Scopes : Bitmap of scopes supported by the sender.
Currently defined values are "Destination-Realm",
"Application-ID", "Destination-Host", "Host", "Connection",
"Session-Group", and "Session".
DOCA uses OC-Scope both to declare supported scopes, and to list the
scopes associated with a particular overload report. MDOC uses
separate dedicated AVPs for the two purposes. DOCA overloads OC-
Scope to include indicators that load information and priority
information may be included.
6.1.2. Algorithm Selection
o DOCA: OC-Algorithm : Bitmap of supported algorithms. Currently
defined values are "Drop", "Throttle", and "Prioritize". Multiple
values allowed.
o MDOC: Overload-Algorithm: Enumeration of supported algorithms.
Multiple instances allowed in negotiation. Currently, there is
one algorithm described, namely "loss".
Both mechanisms support algorithm extensibility. MDOC only allows
Overload-Algorithm to occur in a CER or CEA message, and negotiates a
single algorithm for the duration of the connection. DOCA allows the
algorithm to be selected at report time. (Open Issue: what does it
mean to indicate multiple algorithms in a congestion report?)
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6.1.3. Application Selection
o DOCA: OC-Applications: Indications of the applications that are of
interest.
o MDOC: MDOC assumes that overload reports can apply to any and all
applications, and does not negotiate the list upfront. The
"application" scope is used to select one or more applications on
a per-report basis.
Open Issue: Are there use cases for the up front negotiation of
applications of interest?
6.1.4. Frequency of Reports
o DOCA: OC-Tocl: Indicates how frequent reports shall be sent.
o MDOC: N/A
Since MDOC piggybacks overload reports in existing messages, the rate
of overload reports is the same as the overall message rate. This
may have advantage of giving more rapid and precise feedback as load
increases.
Open Issue: We need further discussion about the appropriate rate(s)
for overload reporting, regardless of which mechanism may be
selected.
6.1.5. Grouping
o DOCA: n/a - negotiation AVPs included at message root.
o MDOC: Load-Info: Grouped AVP acting as a container for the other
AVPs used for negotiation.
6.2. Data Elements for Overload and Load reporting
6.2.1. Scope of Report
o DOCA: OC-Scope (See Section 6.1.1)
o MDOC: Load-Info-Scope: Octet-String giving the scope of the
overload report. The string contains a type indicator and a
value. One or more instances required.
MDOC has a richer and more complex concept of scopes. Multiple
scopes can be combined for a given overload report. Allowable scope
combinations are described in [I-D.roach-dime-overload-ctrl].
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6.2.2. Overload Severity
o DOCA: OC-Level: OctetString(1): Values 1-6 define discreet
overload levels of increasing severity, with 1 meaning no overload
condition, and 6 meaning clients should switch to a different
server.
o DOCA: OC-Sending-Rate: Float32: Used when the "throttle" algorithm
is in effect to indicate the maximum desired Diameter message
rate.
o MDOC: Overload-Metric (Unsigned32): A numeric representation of
load. The meaning is up to the interpretation of the selected
algorithm, with the exception that a value of zero always means
that no overload abatement is in effect. For the "Loss"
algorithm, Overload Metric is a numeric value in the range of zero
through 100, indicating the percentage of traffic reduction
requested.
The Overload-Metric AVP used by MDOC is more general than OC-Level,
in that it's interpretation is left to the algorithm. The meaning of
the OC-Level values appear to be fixed regardless of algorithm
choice. the OC-Level meanings could be used in MDOC by defining a new
algorithm that interpreted Overload-Metric values 1-6 in the same way
as defined for OC-Level.
Since MDOC does not define an algorithm similar to "throttle", it has
no built in analog to OC-Sending-Rate. However, since MDOC allows
algorithm extensibility, one could define a similar algorithm, and if
necessary, add an extension AVP to state sending-rate.
6.2.3. Report Algorithm
o DOCA: OC-Algorithm (See Section 6.1.2)
o MDOC: The overload control algorithm is set during negotiation,
and doesn't change for the duration of the connection.
Open Issue: DOCA's reuse of the OC-Algorithm AVP seems to allow more
than one algorithm to be assigned to a single overload report. It's
not clear what that would mean.
6.2.4. Report Expiration
o DOCA: OC-Best-Before: (Time) Time of report expiration.
o MDOC: Period-Of-Validity (Unsigned32)- Number of seconds until
expiration.
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DOCA defines expiration to be a point in time. MDOC uses a duration,
i.e. number of seconds until expiration. The DOCA approach seems to
require clock synchronization.
DOCA contains an open issue about whether to allow reports to expire
vs. requiring explicit signaling.
6.2.5. Current Load
o DOCA: OC-Utilization: Indicates the overall load situation as a
value between 0 and 100.
o MDOC: Load: The load situation in terms of 0 - 65535.
Current load indicates the existing load on an otherwise non-
overloaded node. MDOC's range of 0-65535 was selected to harmonize
with the DNS service location (SRV) [RFC2782] record's "Weight"
field.
6.2.6. Applications covered by a Report
o DOCA: OC-Applications: Indications what applications are of
interest for load reporting.
o MDOC does not use a separate AVP for this purpose. Rather, one or
more applications can be indicated using the application scope
type.
6.2.7. Report Action
o DOCA: OC-Action: Indicates the start, interim, and end of an
overload period.
o MDOC: MDOC does not have a separate AVP to indicate the start and
stop of an overload condition. Rather, a report with a non-zero
Overload-Metric value starts the condition, and a report with a
zero value, or the expiration of the Period-of-Validity value,
indicate an end. Subsequent reports with non-zero Overload-Metric
values serve the same purpose as a DOCA report with an OC-Action
value of "interim".
Open Issue: Is OC-Action redundant? DOCA also has the ability to
express a non-overload condition in OC-Level, so an approach similar
to that of MDOC should be workable.
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6.2.8. Priority
o DOCA: OC-Priority: Unsigned32: When used in an OC-Information AVP,
sets the relative priority of applications listed in OC-
Applications. As specified, may also be used to set the priority
of a given Diameter message. [Open Issue: Is OC-Priority only in
effect when the "Prioritize" algorithm is in effect?]
o MDOC: N/A
MDOC does not have an explicit priority data element. Relative
priority between applications can be managed using the "Application"
scope. This is not exactly the same as stating inter-application
priority explicitly, but it may be possible to accomplish similar
behavior.
6.2.9. Session Groups
o DOCA: N/A
o MDOC: Session-Group: UTF8String: Session-Group allows a node to
assign a session to a named group. Overload Reports can refer to
all sessions in a group using the Session-Group AVP.
A common application for Session-Group is when a Diameter agent load
balances Diameter sessions across a set of servers. If the agent
assigns all of the sessions assigned to a particular server to a
group, and that server later becomes overloaded, the agent can send
one overload report that applies to all sessions in the group, but
does not apply to sessions assigned to other, non-overloaded,
servers.
DOCA may be able to do something similar using by using the OC-Origin
AVP to identify the overloaded server. However, the server-group
approach can work even if the Diameter agent performs topology
hiding.
6.3. Result Codes
DOCA defines the following Diameter result codes:
o DIAMETER_NO_COMMON_SCOPE (Permanent Failure): The Diameter peers
are unable to negotiate one or more scopes in common.
o DIAMETER_NO_COMMON_ALGORITHM (Permanent Failure): The Diameter
peers are unable to negotiate one or more algorithms in common.
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o DIAMETER_TOCL_TOO_SMALL (Permanent Failure): The peer included an
OC-TOCL AVP with an unacceptably low value.
o DIAMETER_TOCL_TOO_BIG (Permanent Failure): The peer included an
OC-TOCL AVP with an unacceptably high value.
o DIAMETER_RATE_TOO_BIG (Permanent Failure): The peer included an
OC-SENDING-RATE AVP with an unacceptably high value.
A failure to negotiate Overload Control support does not cause a
connection failure in MDOC. Instead, overload control is just not
invoked on the connection.
MDOC defines the following result codes:
o DIAMETER_PEER_IN_OVERLOAD (Transient Failure): When a Diameter
node drops a request due to overload, it responds with this result
code. This is primarily used when the peer does not support
overload control, and therefore fails to reduce load as it would
be expected to do so if it supported overload control.
DIAMETER_PEER_IN_OVERLOAD may be of value to both mechanisms. The
Overload Control Requirements [I-D.ietf-dime-overload-reqs] argues
that the result codes in the Diameter base protocol are insufficient
for reporting failures due to congestion.
7. IANA Considerations
This draft makes no requests of IANA. The authors expect that a
follow-on effort will specify a common set of Overload Control
AVPs.This may introduce additional IANA considerations.
8. Security Considerations
This document compares the data elements used by "DOCA
[I-D.korhonen-dime-ovl] and MDOC [I-D.roach-dime-overload-ctrl]. It
introduces no security considerations beyond those in the respective
documents.
The authors expect that a follow-on effort will specify a common set
of Overload Control AVPs. This may introduce additional security
considerations.
The authors made no attempt to analyze the security considerations in
the DOCA and MDOC specifications for completeness.
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9. References
9.1. Normative References
[RFC6733] Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
"Diameter Base Protocol", RFC 6733, October 2012.
[I-D.ietf-dime-overload-reqs]
McMurry, E. and B. Campbell, "Diameter Overload Control
Requirements", draft-ietf-dime-overload-reqs-03 (work in
progress), January 2013.
[I-D.roach-dime-overload-ctrl]
Roach, A., "A Mechanism for Diameter Overload Control",
draft-roach-dime-overload-ctrl-01 (work in progress),
October 2012.
[I-D.korhonen-dime-ovl]
Korhonen, J., "Diameter Overload Control Application",
draft-korhonen-dime-ovl-00 (work in progress),
October 2012.
9.2. Informative References
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
Appendix A. Contributors
Eric McMurry made significant contributions to the analysis in this
draft.
Authors' Addresses
Ben Campbell
Tekelec
17210 Campbell Rd.
Suite 250
Dallas, TX 75252
US
Email: ben@nostrum.com
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Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland
Email: Hannes.Tschofenig@nsn.com
Jouni Korhonen
Renesas Mobile
Porkkalankatu 24
Helsinki FIN-00180
Finland
Email: jouni.nospam@gmail.com
Adam Roach
Mozilla
Dallas, TX
Email: adam@nostrum.com
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