draft-ietf-dime-load-00.txt   draft-ietf-dime-load-01.txt 
Internet Engineering Task Force B. Campbell Internet Engineering Task Force B. Campbell
Internet-Draft S. Donovan, Ed. Internet-Draft S. Donovan, Ed.
Intended status: Informational Oracle Intended status: Informational Oracle
Expires: January 7, 2016 JJ. Trottin Expires: April 14, 2016 JJ. Trottin
Alcatel-Lucent Alcatel-Lucent
July 6, 2015 October 12, 2015
Diameter Load Information Conveyance Diameter Load Information Conveyance
draft-ietf-dime-load-00 draft-ietf-dime-load-01
Abstract Abstract
This document defines a mechanism for sharing of Diameter load This document defines a mechanism for sharing of Diameter load
information. RFC 7068 describes requirements for Overload Control in information. RFC 7068 describes requirements for Overload Control in
Diameter. This includes a requirement to allow Diameter nodes to Diameter. This includes a requirement to allow Diameter nodes to
send "load" information, even when the node is not overloaded. The send "load" information, even when the node is not overloaded. The
Diameter Overload Information Conveyance (DOIC) solution describes a Diameter Overload Information Conveyance (DOIC) solution describes a
mechanism meeting most of the requirements, but does not currently mechanism meeting most of the requirements, but does not currently
include the ability to send load information. include the ability to send load information.
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This Internet-Draft will expire on January 7, 2016. This Internet-Draft will expire on April 14, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Abbreviations . . . . . . . . . . . . . . . . 3 2. Terminology and Abbreviations . . . . . . . . . . . . . . . . 3
3. Conventions Used in This Document . . . . . . . . . . . . . . 3 3. Conventions Used in This Document . . . . . . . . . . . . . . 4
4. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Differences between Load and Overload information . . . . 4 4.1. Differences between Load and Overload information . . . . 4
4.2. How is Load Information Used? . . . . . . . . . . . . . . 5 4.2. How is Load Information Used? . . . . . . . . . . . . . . 5
5. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 6 5. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Theory of Operation . . . . . . . . . . . . . . . . . . . 7 5.1. Theory of Operation . . . . . . . . . . . . . . . . . . . 7
6. Solution Procedures . . . . . . . . . . . . . . . . . . . . . 8 6. Load Mechanism Procedures . . . . . . . . . . . . . . . . . . 9
6.1. Reporting Node Behavior . . . . . . . . . . . . . . . . . 8 6.1. Reporting Node Behavior . . . . . . . . . . . . . . . . . 9
6.1.1. Endpoint Reporting Node Behavior . . . . . . . . . . 8 6.1.1. End-point Reporting Node Behavior . . . . . . . . . . 10
6.1.2. Agent Reporting Node Behavior . . . . . . . . . . . . 9 6.1.2. Agent Reporting Node Behavior . . . . . . . . . . . . 10
6.2. Receiving Node Behavior . . . . . . . . . . . . . . . . . 9 6.2. Receiving Node Behavior . . . . . . . . . . . . . . . . . 11
6.2.1. Endpoint Receiving Node Behavior . . . . . . . . . . 9 6.3. Extensibility . . . . . . . . . . . . . . . . . . . . . . 12
6.2.2. Agent Receiving Node Behavior . . . . . . . . . . . . 9 7. Attribute Value Pairs . . . . . . . . . . . . . . . . . . . . 12
6.3. Extensibility . . . . . . . . . . . . . . . . . . . . . . 9 7.1. Load AVP . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Attribute Value Pairs . . . . . . . . . . . . . . . . . . . . 9 7.2. Load-Type AVP . . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 7.3. Load-Value AVP . . . . . . . . . . . . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 7.4. SourceID AVP . . . . . . . . . . . . . . . . . . . . . . 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 7.5. Attribute Value Pair flag rules . . . . . . . . . . . . . 13
10.1. Normative References . . . . . . . . . . . . . . . . . . 9 8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
10.2. Informative References . . . . . . . . . . . . . . . . . 10 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
Appendix A. Topology Scenarios . . . . . . . . . . . . . . . . . 10 9.1. AVP Codes . . . . . . . . . . . . . . . . . . . . . . . . 14
A.1. No Agent . . . . . . . . . . . . . . . . . . . . . . . . 10 9.2. New Registries . . . . . . . . . . . . . . . . . . . . . 14
A.2. Single Agent . . . . . . . . . . . . . . . . . . . . . . 10 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
A.3. Multiple Agents . . . . . . . . . . . . . . . . . . . . . 11 10.1. Normative References . . . . . . . . . . . . . . . . . . 14
A.4. Linked Agents . . . . . . . . . . . . . . . . . . . . . . 12 10.2. Informative References . . . . . . . . . . . . . . . . . 14
A.5. Shared Server Pools . . . . . . . . . . . . . . . . . . . 13 Appendix A. Topology Scenarios . . . . . . . . . . . . . . . . . 15
A.6. Agent Chains . . . . . . . . . . . . . . . . . . . . . . 13 A.1. No Agent . . . . . . . . . . . . . . . . . . . . . . . . 15
A.7. Fully Meshed Layers . . . . . . . . . . . . . . . . . . . 14 A.2. Single Agent . . . . . . . . . . . . . . . . . . . . . . 15
A.8. Partitions . . . . . . . . . . . . . . . . . . . . . . . 14 A.3. Multiple Agents . . . . . . . . . . . . . . . . . . . . . 16
A.9. Active-Standby Nodes . . . . . . . . . . . . . . . . . . 14 A.4. Linked Agents . . . . . . . . . . . . . . . . . . . . . . 17
A.10. Addition and removal of Nodes . . . . . . . . . . . . . . 15 A.5. Shared Server Pools . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 A.6. Agent Chains . . . . . . . . . . . . . . . . . . . . . . 18
A.7. Fully Meshed Layers . . . . . . . . . . . . . . . . . . . 19
A.8. Partitions . . . . . . . . . . . . . . . . . . . . . . . 19
A.9. Active-Standby Nodes . . . . . . . . . . . . . . . . . . 19
A.10. Addition and removal of Nodes . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction 1. Introduction
[RFC7068] describes requirements for Overload Control in Diameter [RFC7068] describes requirements for Overload Control in Diameter
[RFC6733]. At the time of this writing, the DIME working group is [RFC6733]. At the time of this writing, the DIME working group is
working on the Diameter Overload Information Conveyance (DOIC) working on the Diameter Overload Information Conveyance (DOIC)
mechanism [I-D.ietf-dime-ovli] . As currently specified, DOIC mechanism [I-D.ietf-dime-ovli] . As currently specified, DOIC
fulfills some, but not all, of the requirements. fulfills some, but not all, of the requirements.
In particular, DOIC does not fulfill Req 24, which requires a In particular, DOIC does not fulfill Req 24, which requires a
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As with DOIC, load information is conveyed by piggy-backing the load As with DOIC, load information is conveyed by piggy-backing the load
AVPs on existing Diameter applications. AVPs on existing Diameter applications.
There are two primary differences. First, there is no capability There are two primary differences. First, there is no capability
negotiation process for load. The sender of the load information is negotiation process for load. The sender of the load information is
sending it with the expectation that any supporting nodes will use it sending it with the expectation that any supporting nodes will use it
when making routing decisions. If there are no nodes that support when making routing decisions. If there are no nodes that support
the load extension then the load information is ignored. the load extension then the load information is ignored.
The second big difference between DOIC and Load is that DOIC The second big difference between DOIC and Load is visibility of the
information is sent end-to-end. The DOIC overload reports much DOIC or Load information within a Diameter network. DOIC information
remain in the message all the way from the reporting node to the node is sent end-to-end resulting in the ability of all nodes in the path
that is the target for the answer message. For the Load mechanism, of the answer message that carries the OC-OLR AVP to act on the
the load information is targeted for the peer of the sender of the information. The DOIC overload reports much remain in the message
Load AVPs. all the way from the reporting node to the node that is the target
for the answer message.
Editor's note: It is still being discussed whether a second type For the Load mechanism there are two types of load reports.
of Load report can be included that is the load of the end-point
and is carried end-to-end.
The Load report applies to an individual node. The Diameter identity The first is the load of the end-point sending the answering the
of the node to which it applies is included in the Load report. message. This load report is carried end-to-end to enable any nodes
that make server selection decisions to use the load status of the
sending end-point as part of the server selection decision.
Note, this is necessary so that supporting nodes can determine if The second type of load report is a peer report. This report is used
the load report came from a peer node. If the identity is for a by Diameter nodes as part of the logic to select the next hop
non peer node than the peer load report can be ignored. Diameter node and, as such, do not have significance beyond the peer
node. These load reports are removed by the first supporting
Diameter node.
In addition to the identity of the node to which the report applies, Because load reports can traverse Diameter nodes that do not support
the load report includes the relative load of the sending node. This the load mechanism, it is necessary to include the identity of the
node to which the load report applies as part of the load report.
This allows for a Diameter node to verify that the load report
applies to its peer or if it should be ignored.
The load report includes the relative load of the sending node. This
relative load is specified in a manner consistent with that defined relative load is specified in a manner consistent with that defined
for DNS SRV [RFC2782]. for DNS SRV [RFC2782].
The method for calculating the load value included in the load report The method for calculating the load value included in the load report
is left as an implementation decision. is left as an implementation decision.
The frequency for sending of load reports is also left as an The frequency for sending of load reports is also left as an
implementation decision. The sending node might choose to send load implementation decision. The sending node might choose to send load
reports in all messages or it might choose to only send load reports reports in all messages or it might choose to only send load reports
when the load value has changed by some implementation specific when the load value has changed by some implementation specific
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Also assume that the request for a Diameter transaction takes the Also assume that the request for a Diameter transaction takes the
following path: following path:
C A1 A4 S[n] C A1 A4 S[n]
| | | | | | | |
|----->|----->|----->| |----->|----->|----->|
xxR xxR xxR xxR xxR xxR
Figure 2: Request Message Path Figure 2: Request Message Path
When sending the answer message, a sending node that supports the When sending the answer message, an end-point node that supports the
Diameter Load mechanism includes it's own load information in the Diameter Load mechanism includes it's own load information in the
answer message: answer message. Because it is a Diameter end-point it includes an
end-point load report.
C A1 A4 S[n] C A1 A4 S[n]
| | | | | | | |
| | |<-----| | | |<-----|
xxA(Load S[n]) | | xxA(Load type:host, source:S[n])
| | | |
Figure 3: Answer Message from S[n] Figure 3: Answer Message from S[n]
If Agent A4 supports the Load mechanism then it will verify that the If Agent A4 supports the Load mechanism then it will verify that the
load information received was from its peer. This is achieved by load information received is valid. For a host load report this is
matching the identity included in the load information with the achieved by matching the identity included in the load information
identity of the peer node from which the answer message was received. with the identity of the host node from which the answer message was
received.
If the load information does not match then the agent will strip the Note: If A4 does not support the load mechanism then it will relay
load AVPs from the message. the answer message without doing any processing on the load
information. In this case the load AVPs will be relayed without
change.
If the identity included in the load information AVPs matches the If the identity included in the load information AVPs matches the
identity of the peer from which the load information is received then identity of the host from which the load information is received then
Agent A4 stores the load information for S[n] in its routing tables. Agent A4 stores the load information for S[n] in its routing tables.
In all cases, A4 strips the load AVPs from the message. Because the load report is an host load report, A4 leaves the load
report in the message it relays.
A4 then calculates its own load information and inserts load A4 then calculates its own load information and inserts load
information AVPs in the message before sending the message to A1: information AVPs in the message before sending the message to A1:
C A1 A4 S[n] C A1 A4 S[n]
| | | | | | | |
| |<-----| | | |<-----| |
xxA(Load A4) | xxA(Load type:peer, source:A4)
| xxA(Load type:host, source:S[n])
| | | |
Figure 4: Answer Message from A4 Figure 4: Answer Message from A4
A1 follows the same procedures as A4, resulting in the following If A1 supports the Load mechanism then it processes each of the Load
message sent to C: reports it receives separately.
For the peer load report, A1 first determines if the source of the
report indicated in the load report matches the DiameterID of the
Diameter node from which the request was received. If the identities
do not match then the peer load report is discarded. If the
identities match then A1 saves the load information in its routing
tables for routing of subsequent request messages. In both cases A1
strips the peer load report from the message.
For the host load report, A1's actions depend on whether A1 is
responsible for doing server selection. If A1 is not doing server
selection then A1 ignores the host load report. If A1 is responsible
for doing server selection then it stores the load information for
S[n] in its routing tables for the handling of subsequent request
messages. In both cases A1 leaves the host report in the message.
A1 then calculates its own load information and inserts load
information AVPs in the message before sending the message to A1:
C A1 A4 S[n] C A1 A4 S[n]
| | | | | | | |
|<-----| | | |<-----| | |
xxA(Load A1) xxA(Load type:peer, source:A1)
xxA(Load type:host, source:S[n])
Figure 5: Answer Message from A1 Figure 5: Answer Message from A1
C follows the same procedure for determining if the Load report was As with A1, C processes each load report separately.
received from the peer from which the report was sent and, when
finding it does, stores the load information for use when making For the peer load report, C follows the same procedure for
future routing decisions. determining if the Load report was received from the peer from which
the report was sent and, when finding it does, stores the load
information for use when making future routing decisions.
For the host load report, C saves the load information only if it is
responsible for doing server selection.
The Load information received by all nodes is then used for routing The Load information received by all nodes is then used for routing
of subsequent request messages. of subsequent request messages.
Editor's note: The above needs to be updated if there is agreement 6. Load Mechanism Procedures
to support end-point Load reports.
6. Solution Procedures This section defines the normative behaviors for the Load mechanism.
6.1. Reporting Node Behavior 6.1. Reporting Node Behavior
6.1.1. Endpoint Reporting Node Behavior This section defines the procedures of Diameter nodes that generate
load reports.
6.1.1. End-point Reporting Node Behavior
A Diameter end-point that supports the Diameter Load mechanism MUST
include a load report of type HOST in sufficient answer messages to
ensure that all consumers of the load information receive timely
updates.
The Diameter end-point MUST include it's own DiameterID in the
Source-ID AVP included in the Load AVP.
The Diameter end-point MUST include a Load-Type AVP of type HOST in
the Load AVP.
The Diameter end-point MUST include its load value in the Value AVP
in the load AVP.
The method for determining the load value included in the load report
is an implementation decision.
The value included MUST be consistant with DNS SRV....
Editor's note: We need to elaborate on this.
The frequency of sending load reports is an implementation decision.
For instance, if the only consumer of the load reports is the end-
points peer then the end-point can choose to only include a load
report when the load of the end-point has changed by a significant
percentage. If there are consumers of the end-point load report
other then the end-points peer (this will be the case if other
nodes are responsible for server selection) then the end-point
might choose to include load reports in all answer messages as a
way of ensuring that all nodes doing server selection get accurate
load information.
6.1.2. Agent Reporting Node Behavior 6.1.2. Agent Reporting Node Behavior
A Diameter agent that supports the Diameter Load mechanism MUST
include a PEER load report in sufficient answer messages to ensure
that all users of the load information receive timely updates.
The Diameter agent MUST include it's own DiameterID in the Source-ID
AVP included in the Load AVP.
The Diameter agent MUST include a Load-Type AVP of type PEER in the
Load AVP.
The Diameter agent MUST include its load value in the Value AVP in
the load AVP.
The method for determining the load value included in the load report
is an implementation decision.
The value included MUST be consistant with DNS SRV....
Editor's note: We need to elaborate on this.
The frequency of sending load reports is an implementation decision.
Note: In the case of peer load reports it is only necessary to
include load reports when the load value has changed by some
significant value. It is also acceptable to include the load
report in every answer message handled by the Diameter agent.
6.2. Receiving Node Behavior 6.2. Receiving Node Behavior
6.2.1. Endpoint Receiving Node Behavior This section defines the behavior of Diameter nodes processing load
reports.
6.2.2. Agent Receiving Node Behavior A Diameter node MUST be prepared to process both load reports of type
HOST and of type PEER, as indicated in the Load-Type AVP included in
the Load AVP.
Note that the node needs to be able to handle messages with no
load reports, messages with just a PEER load report, messages with
just an HOST load report and messages with both types of load
reports.
If the Diameter node is not responsible for doing server selection
then it SHOULD ignore load reports of type HOST.
If the Diameter node is responsible for doing server selection then
it SHOULD save the load value included in the Value AVP included in
the Load AVP of type HOST in its routing tables.
If the Diameter node receives a Load report of type PEER then the
Diameter node MUST determine if the Load report was inserted into the
answer message by the peer from which the message was received. This
is achieved by comparing the DiameterID associated with the
connection from which the message was received with the DiameterID
included in the Source-ID AVP in the Load report.
If the Diameter node determines that the Load report of type PEER was
not received from the peer that sent or relayed the answer message
then the node MUST ignore the Load report.
If the Diameter node determines that the Load report of type PEER was
received from the peer that sent or relayed the answer message then
the node SHOULD save the load information in its routing table.
How a Diameter node uses load information for making routing
decisions is an implememtation decision.
6.3. Extensibility 6.3. Extensibility
The Load mechanism can be extended to include additional information
in the load reports.
Any extension may define new AVPs for use in Load reports. These new
AVPs SHOULD be defined to be extensions to the Load AVPs defined in
this document.
[RFC6733] defined Grouped AVP extension mechanisms apply. This
allows, for example, defining a new feature that is mandatory to be
understood even when piggybacked on an existing application.
As with any Diameter specification, RFC6733 requires all new AVPs to
be registered with IANA. See Section 9 for the required procedures.
7. Attribute Value Pairs 7. Attribute Value Pairs
The section defines the AVPs required for the Load mechanism.
7.1. Load AVP
The Load AVP (AVP code TBD1) is of type Grouped and is used to convey
load information between Diameter nodes.
Load ::= < AVP Header: TBD1 >
[ Load-Type ]
[ Load-Value ]
[ SourceID ]
* [ AVP ]
7.2. Load-Type AVP
The Load-Type AVP (AVP code TBD2) is of type Enumerated. It is used
to convey the type of Diameter node that sent the load information.
The following values are defined:
HOST 0 The load report is for a host.
PEER 1 The load report is for a peer.
7.3. Load-Value AVP
The Load-Value AVP (AVP code TBD3) is of type Unsigned64. It is used
to convey relative load information about the sender of the load
report.
7.4. SourceID AVP
The SourceID AVP is defined in [I-D.ietf-dime-agent-overload]. It is
used to identify the Diameter node that sent the Load report.
7.5. Attribute Value Pair flag rules
+---------+
|AVP flag |
|rules |
+----+----+
AVP Section | |MUST|
Attribute Name Code Defined Value Type |MUST| NOT|
+--------------------------------------------------+----+----+
|Load TBD1 x.1 Grouped | | V |
+--------------------------------------------------+----+----+
|Load-Type TBD2 x.2 Enumerated | | V |
+--------------------------------------------------+----+----+
|Load-Value TBD3 x.3 Unsigned64 | | V |
+--------------------------------------------------+----+----+
|SourceID TBD4 x.4 DiameterID | | V |
+--------------------------------------------------+----+----+
As described in the Diameter base protocol [RFC6733], the M-bit usage
for a given AVP in a given command may be defined by the application.
8. Security Considerations 8. Security Considerations
Load information may be sensitive information in some cases. Load information may be sensitive information in some cases.
Depending on the mechanism, an unauthorized recipient might be able Depending on the mechanism, an unauthorized recipient might be able
to infer the topology of a Diameter network from load information. to infer the topology of a Diameter network from load information.
Load information might be useful in identifying targets for Denial of Load information might be useful in identifying targets for Denial of
Service (DoS) attacks, where a node known to be already heavily Service (DoS) attacks, where a node known to be already heavily
loaded might be a tempting target. Load information might also be loaded might be a tempting target. Load information might also be
useful as feedback about the success of an ongoing DoS attack. useful as feedback about the success of an ongoing DoS attack.
Any load information conveyance mechanism will need to allow Any load information conveyance mechanism will need to allow
operators to avoid sending load information to nodes that are not operators to avoid sending load information to nodes that are not
authorized to receive it. Since Diameter currently only offers authorized to receive it. Since Diameter currently only offers
authentication of nodes at the transport level, any solution that authentication of nodes at the transport level, any solution that
sends load information to non-peer nodes might require a transitive- sends load information to non-peer nodes might require a transitive-
trust model. trust model.
9. IANA Considerations 9. IANA Considerations
This document makes no requests of IANA. 9.1. AVP Codes
New AVPs defined by this specification are listed in
Section Section 7. All AVP codes are allocated from the
'Authentication, Authorization, and Accounting (AAA) Parameters' AVP
Codes registry.
9.2. New Registries
This document makes no new registry requests of IANA.
10. References 10. References
10.1. Normative References 10.1. Normative References
[I-D.ietf-dime-agent-overload]
Donovan, S., "Diameter Agent Overload", draft-ietf-dime-
agent-overload-02 (work in progress), August 2015.
[I-D.ietf-dime-ovli] [I-D.ietf-dime-ovli]
Korhonen, J., Donovan, S., Campbell, B., and L. Morand, Korhonen, J., Donovan, S., Campbell, B., and L. Morand,
"Diameter Overload Indication Conveyance", draft-ietf- "Diameter Overload Indication Conveyance", draft-ietf-
dime-ovli-03 (work in progress), July 2014. dime-ovli-03 (work in progress), July 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC6733] Fajardo, V., Arkko, J., Loughney, J., and G. Zorn, [RFC6733] Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
"Diameter Base Protocol", RFC 6733, October 2012. Ed., "Diameter Base Protocol", RFC 6733,
DOI 10.17487/RFC6733, October 2012,
<http://www.rfc-editor.org/info/rfc6733>.
[RFC7068] McMurry, E. and B. Campbell, "Diameter Overload Control [RFC7068] McMurry, E. and B. Campbell, "Diameter Overload Control
Requirements", RFC 7068, November 2013. Requirements", RFC 7068, DOI 10.17487/RFC7068, November
2013, <http://www.rfc-editor.org/info/rfc7068>.
10.2. Informative References 10.2. Informative References
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782, specifying the location of services (DNS SRV)", RFC 2782,
February 2000. DOI 10.17487/RFC2782, February 2000,
<http://www.rfc-editor.org/info/rfc2782>.
Appendix A. Topology Scenarios Appendix A. Topology Scenarios
This section presents a number of Diameter topology scenarios, and This section presents a number of Diameter topology scenarios, and
discusses how load information might be used in each scenario. discusses how load information might be used in each scenario.
Nothing in this section should be construed to mean that a given Nothing in this section should be construed to mean that a given
scenario is in scope for this effort, or even a good idea. Some scenario is in scope for this effort, or even a good idea. Some
scenarios might be considered as not relevant in practice and scenarios might be considered as not relevant in practice and
subsequently discarded. subsequently discarded.
 End of changes. 37 change blocks. 
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