Diameter Load Information Conveyance
draft-ietf-dime-load-01
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| Authors | Ben Campbell , Steve Donovan , Jean-Jacques Trottin | ||
| Last updated | 2015-10-14 | ||
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draft-ietf-dime-load-01
Internet Engineering Task Force B. Campbell
Internet-Draft S. Donovan, Ed.
Intended status: Informational Oracle
Expires: April 14, 2016 JJ. Trottin
Alcatel-Lucent
October 12, 2015
Diameter Load Information Conveyance
draft-ietf-dime-load-01
Abstract
This document defines a mechanism for sharing of Diameter load
information. RFC 7068 describes requirements for Overload Control in
Diameter. This includes a requirement to allow Diameter nodes to
send "load" information, even when the node is not overloaded. The
Diameter Overload Information Conveyance (DOIC) solution describes a
mechanism meeting most of the requirements, but does not currently
include the ability to send load information.
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 April 14, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Abbreviations . . . . . . . . . . . . . . . . 3
3. Conventions Used in This Document . . . . . . . . . . . . . . 4
4. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Differences between Load and Overload information . . . . 4
4.2. How is Load Information Used? . . . . . . . . . . . . . . 5
5. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Theory of Operation . . . . . . . . . . . . . . . . . . . 7
6. Load Mechanism Procedures . . . . . . . . . . . . . . . . . . 9
6.1. Reporting Node Behavior . . . . . . . . . . . . . . . . . 9
6.1.1. End-point Reporting Node Behavior . . . . . . . . . . 10
6.1.2. Agent Reporting Node Behavior . . . . . . . . . . . . 10
6.2. Receiving Node Behavior . . . . . . . . . . . . . . . . . 11
6.3. Extensibility . . . . . . . . . . . . . . . . . . . . . . 12
7. Attribute Value Pairs . . . . . . . . . . . . . . . . . . . . 12
7.1. Load AVP . . . . . . . . . . . . . . . . . . . . . . . . 12
7.2. Load-Type AVP . . . . . . . . . . . . . . . . . . . . . . 12
7.3. Load-Value AVP . . . . . . . . . . . . . . . . . . . . . 13
7.4. SourceID AVP . . . . . . . . . . . . . . . . . . . . . . 13
7.5. Attribute Value Pair flag rules . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9.1. AVP Codes . . . . . . . . . . . . . . . . . . . . . . . . 14
9.2. New Registries . . . . . . . . . . . . . . . . . . . . . 14
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
10.1. Normative References . . . . . . . . . . . . . . . . . . 14
10.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. Topology Scenarios . . . . . . . . . . . . . . . . . 15
A.1. No Agent . . . . . . . . . . . . . . . . . . . . . . . . 15
A.2. Single Agent . . . . . . . . . . . . . . . . . . . . . . 15
A.3. Multiple Agents . . . . . . . . . . . . . . . . . . . . . 16
A.4. Linked Agents . . . . . . . . . . . . . . . . . . . . . . 17
A.5. Shared Server Pools . . . . . . . . . . . . . . . . . . . 18
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
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1. Introduction
[RFC7068] describes requirements for Overload Control in Diameter
[RFC6733]. At the time of this writing, the DIME working group is
working on the Diameter Overload Information Conveyance (DOIC)
mechanism [I-D.ietf-dime-ovli] . As currently specified, DOIC
fulfills some, but not all, of the requirements.
In particular, DOIC does not fulfill Req 24, which requires a
mechanism where Diameter nodes can indicate their current load, even
if they are not currently overloaded. DOIC also does not fulfill Req
23, which requires that nodes that divert traffic away from
overloaded nodes be provided with sufficient information to select
targets that are most likely to have sufficient capacity.
There are several other requirements in RFC 7068 that mention both
overload and load information that are only partially fulfilled by
DOIC.
The DIME working group explicitly chose not to fulfill these
requirements in DOIC due to several reasons. A principal reason was
that the working group did not agree on a general approach for
conveying load information. It chose to progress the rest of DOIC,
and defer load information conveyance to a DOIC extension or a
separate mechanism.
This document defines a mechanism that addresses the load-related
requirements from RFC 7068.
2. Terminology and Abbreviations
DOIC
Diameter Overload Information Conveyance
Load
The relative capacity of a Diameter node. A low value indicates
that the Diameter node is under utilized. A high value indicated
that the node is closer to being fully utilized.
Offered Load
The actual traffic sent to the reporting node after overload
abatement and routing decisions are made.
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3. Conventions Used in This Document
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 RFC 2119 [RFC2119].
RFC 2119 [RFC2119] interpretation does not apply for the above listed
words when they are not used in all-caps format.
4. Background
4.1. Differences between Load and Overload information
Previous discussions of how to solve the load-related requirements in
[RFC7068] have shown that people have not had an agreed-upon concept
of how "load" information differs from "overload" information. While
the two concepts are highly interrelated, in the opinion of the
authors, there are two primary differences. First, a Diameter node
always has a load. At any given time that load maybe effectively
zero, effectively fully loaded, or somewhere in between. In
contrast, overload is an exceptional condition. A node only has
overload information when it is in an overloaded state. Furthermore,
the relationship between a node's load level and overload state at
any given time may be vague. For example, a node may normally
operate at a "fully loaded" level, but still not be considered
overloaded. Another node may declare itself to be "overloaded" even
though it might not be fully "loaded".
Second, Overload information, in the form of a DOIC Overload Report
(OLR) [I-D.ietf-dime-ovli] indicates an explicit request for action
on the part of the reacting node. That is, the OLR requests that the
reacting node reduce the offered load -- the actual traffic sent to
the reporting node after overload abatement and routing decisions are
made -- by an indicated amount or to an indicated level.
Effectively, DOIC provides a contract between the reporting node and
the reacting node.
In contrast, load is informational. That is, load information can be
considered a hint to the recipient node. That node may use the load
information for load balancing purposes, as an input to certain
overload abatement techniques, to make inferences about the
likelihood that the sending node becomes overloaded in the immediate
future, or for other purposes.
None of this prevents a Diameter node from deciding to reduce the
offered load based on load information. The fundamental difference
is that an overload report requires that reduction. It is also
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reasonable for a Diameter node to decide to increase the offered load
based on load information.
4.2. How is Load Information Used?
[RFC7068] contemplates two primary uses for load information. Req 23
discusses how load information might be used when performing
diversion as an overload abatement technique, as described in
[I-D.ietf-dime-ovli]. When a reacting node diverts traffic away from
an overloaded node, it needs load information for the other
candidates for that traffic in order to effectively load balance the
diverted load between potential candidates. Otherwise, diversion has
a greater potential to drive other nodes into overload.
Req 24 discusses how Diameter load information might be used when no
overload condition currently exists. Diameter nodes can use the load
information to make decisions to try to avoid overload conditions in
the first place. Normal load-balancing falls into this category. A
node might also take other proactive steps to reduce offered load
based on load information, so that the loaded node never goes into
overload in the first place.
If the loaded nodes are Diameter servers (or clients in the case of
server-to-client transactions), both of these uses are most
effectively accomplished by a Diameter node that performs server
selection. Typically, server selection is performed by a node (a
client or an agent) that is an immediate peer of the server.
However, there are scenarios (see Appendix A) where a client or proxy
that is not the immediate peer to the selected servers performs
server selection. In this case, the client or proxy enforces the
server selection by inserting a Destination-Host AVP.
For example, a Diameter node (e.g. client) can use a redirect
agent to get candidate destination host addresses. The redirect
agent might return several destination host addresses, from which
the Diameter node selects one. The Diameter node can use load
information received from these hosts to make the selection.
Just as load information can be used as part of server selection, it
can also be used as input to the selection of the next-hop peer to
which a request is to be routed.
Editor's Note: One area that requires thought is how load
information is used, if at all, in the presence of an overload
report from the same Diameter node. It might be that the load
information from that Diameter node is ignored for the duration of
the time that the overload report is in effect. It might also be
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possible that the load information can aid in the diverting of
non-abated requests targeted for the overloaded Diameter node.
5. Solution Overview
The mechanism defined here for the conveyance of load information is
similar in some says to the mechanism defined for DOIC and is
different in other ways.
As with DOIC, load information is conveyed by piggy-backing the load
AVPs on existing Diameter applications.
There are two primary differences. First, there is no capability
negotiation process for load. The sender of the load information is
sending it with the expectation that any supporting nodes will use it
when making routing decisions. If there are no nodes that support
the load extension then the load information is ignored.
The second big difference between DOIC and Load is visibility of the
DOIC or Load information within a Diameter network. DOIC information
is sent end-to-end resulting in the ability of all nodes in the path
of the answer message that carries the OC-OLR AVP to act on the
information. The DOIC overload reports much remain in the message
all the way from the reporting node to the node that is the target
for the answer message.
For the Load mechanism there are two types of load reports.
The first is the load of the end-point sending the answering the
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.
The second type of load report is a peer report. This report is used
by Diameter nodes as part of the logic to select the next hop
Diameter node and, as such, do not have significance beyond the peer
node. These load reports are removed by the first supporting
Diameter node.
Because load reports can traverse Diameter nodes that do not support
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
for DNS SRV [RFC2782].
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The method for calculating the load value included in the load report
is left as an implementation decision.
The frequency for sending of load reports is also left as an
implementation decision. The sending node might choose to send load
reports in all messages or it might choose to only send load reports
when the load value has changed by some implementation specific
value. The important consideration is that all nodes needing the
load information have a sufficiently accurate view of the nodes load.
5.1. Theory of Operation
This section outlines how the Diameter load mechanism is expected to
work.
For this discussion, assume the following Diameter network
configuration:
---A1---A3----S[1], S[2]...S[p]
/ | \ /
C | x
\ | / \
---A2---A4----S[p+1], S[p+2] ...S[n]
Figure 1: Example Diameter Network
Also assume that the request for a Diameter transaction takes the
following path:
C A1 A4 S[n]
| | | |
|----->|----->|----->|
xxR xxR xxR
Figure 2: Request Message Path
When sending the answer message, an end-point node that supports the
Diameter Load mechanism includes it's own load information in the
answer message. Because it is a Diameter end-point it includes an
end-point load report.
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C A1 A4 S[n]
| | | |
| | |<-----|
| | xxA(Load type:host, source:S[n])
| | | |
Figure 3: Answer Message from S[n]
If Agent A4 supports the Load mechanism then it will verify that the
load information received is valid. For a host load report this is
achieved by matching the identity included in the load information
with the identity of the host node from which the answer message was
received.
Note: If A4 does not support the load mechanism then it will relay
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
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.
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
information AVPs in the message before sending the message to A1:
C A1 A4 S[n]
| | | |
| |<-----| |
| xxA(Load type:peer, source:A4)
| xxA(Load type:host, source:S[n])
| | | |
Figure 4: Answer Message from A4
If A1 supports the Load mechanism then it processes each of the Load
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
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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]
| | | |
|<-----| | |
xxA(Load type:peer, source:A1)
xxA(Load type:host, source:S[n])
Figure 5: Answer Message from A1
As with A1, C processes each load report separately.
For the peer load report, C follows the same procedure for
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
of subsequent request messages.
6. Load Mechanism Procedures
This section defines the normative behaviors for the Load mechanism.
6.1. Reporting Node Behavior
This section defines the procedures of Diameter nodes that generate
load reports.
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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
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.
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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
This section defines the behavior of Diameter nodes processing load
reports.
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.
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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
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
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.
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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
Load information may be sensitive information in some cases.
Depending on the mechanism, an unauthorized recipient might be able
to infer the topology of a Diameter network from load information.
Load information might be useful in identifying targets for Denial of
Service (DoS) attacks, where a node known to be already heavily
loaded might be a tempting target. Load information might also be
useful as feedback about the success of an ongoing DoS attack.
Any load information conveyance mechanism will need to allow
operators to avoid sending load information to nodes that are not
authorized to receive it. Since Diameter currently only offers
authentication of nodes at the transport level, any solution that
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sends load information to non-peer nodes might require a transitive-
trust model.
9. IANA Considerations
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
10.1. 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]
Korhonen, J., Donovan, S., Campbell, B., and L. Morand,
"Diameter Overload Indication Conveyance", draft-ietf-
dime-ovli-03 (work in progress), July 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC6733] Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
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
Requirements", RFC 7068, DOI 10.17487/RFC7068, November
2013, <http://www.rfc-editor.org/info/rfc7068>.
10.2. Informative References
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[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<http://www.rfc-editor.org/info/rfc2782>.
Appendix A. Topology Scenarios
This section presents a number of Diameter topology scenarios, and
discusses how load information might be used in each scenario.
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
scenarios might be considered as not relevant in practice and
subsequently discarded.
A.1. No Agent
Figure 6 shows a simple client-server scenario, where a client picks
from a set of candidate servers available for a particular realm and
application. The client selects the server for a given transaction
using the load information received from each server.
------S1
/
C
\
------S2
Figure 6: Basic Client Server Scenario
Open Issue: Will a Diameter node include potential peers that it
is not currently connected to as part of the candidate set? It is
unlikely the client would have load information from peers that it
is not currently connected to.
Note: The use of dynamic connections needs to be considered.
A.2. Single Agent
Figure 7 shows a client that sends requests to an agent. The agent
selects the request destination from a set of candidate servers,
using load information received from each server. The client does
not need to receive load information, since it does not select
between multiple agents.
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------S1
/
C----A
\
------S2
Figure 7: Simple Agent Scenario
A.3. Multiple Agents
Figure 8 shows a client selecting between multiple agents, and each
agent selecting from multiple servers. The client selects an agent
based on the load information received from each agent. Each agent
selects a server based on the load information received from its
servers.
This scenario adds a complication that one set of servers may be more
loaded than the other set. If, for example, S4 was the least loaded
server, C would need to know to select agent A2 to reach S4. This
might require C to receive load information from the servers as well
as the agents. Alternatively, each agent might use the load of its
servers as an input into calculating its own load, in effect
aggregating upstream load.
Similarly, if C sends a host-routed request [I-D.ietf-dime-ovli], it
needs to know which agent can deliver requests to the selected
server. Without some special, potentially proprietary, knowledge of
the topology upstream of A1 and A2, C would select the agent based on
the normal peer selection procedures for the realm and application,
and perhaps consider the load information from A1 and A2. If C sends
a request to A1 that contains a Destination-Host AVP with a value of
S4, A1 will not be able to deliver the request.
-----S3
/
---A1------S1
/
C
\
---A2------S2
\
---- S4
Figure 8: Multiple Agents and Servers
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A.4. Linked Agents
Figure 9 shows a scenario similar to that of Figure 8, except that
the agents are linked, so that A1 can forward a request to A2, and
vice-versa. Each agent could receive load information from the
linked agent, as well as its connected servers.
This somewhat simplifies the complication from Figure 8, due to the
fact that C does not necessarily need to choose a particular agent to
reach a particular server. But it creates a similar question of how,
for example, A1 might know that S4 was less loaded than S1 or S3.
Additionally, it creates the opportunity for sub-optimal request
paths. For example [C,A1,A2,S4] vs. [C,A2,S4].
A likely application for linked agents is when each agent prefers to
route only to directly connected servers and only forwards requests
to another agent under exceptional circumstances. For example, A1
might not forward requests to A2 unless both S1 and S3 are
overloaded. In this case, A1 might use the load information from S1
and S3 to select between those, and only consider the load
information from A2 (and other connected agents) if it needs to
divert requests to different agents.
-----S3
/
---A1------S1
/ |
C |
\ |
---A2------S2
\
---- S4
Figure 9: Linked Agents
Figure 10 is a variant of Figure 9. In this case, C1 sends all
traffic through A1 and C2 sends all traffic through A2. By default,
A1 will load balance traffic between S1 and S3 and A2 will load
balance traffic between S2 and S4.
Now, if S1 S3 are significantly more loaded than S2 S4, A1 may route
some C1 traffic to A2. This is non optimal path but allows a better
load balancing between the servers. To achieve this, A1 needs to
receive some load info from A2 about S2/S4 load.
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-----S3
/
C1----A1------S1
|
|
|
C2----A2------S2
\
---- S4
Figure 10: Linked Agents
A.5. Shared Server Pools
Figure 11 is similar to Figure 9, except that instead of a link
between agents, each agent is linked to all servers. (The links to
each set of servers should be interpreted as a link to each server.
The links are not shown separately due to the limitations of ASCII
art.)
In this scenario, each agent can select among all of the servers,
based on the load information from the servers. The client need only
be concerned with the load information of the agents.
---A1---S[1], S[2]...S[p]
/ \ /
C x
\ / \
---A2---S[p+1], S[p+2] ...S[n]
Figure 11: Shared Server Pools
A.6. Agent Chains
The scenario in Figure 12 is similar to that of Figure 8, except
that, instead of the client possibly needing to select an agent that
can route requests to the least loaded server, in this case A1 and A2
need to make similar decisions when selecting between A3 or A4. As
the former scenario, this could be mitigated if A3 and A4 aggregate
upstream loads into the load information they report downstream.
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---A1---A3----S[1], S[2]...S[p]
/ | \ /
C | x
\ | / \
---A2---A4----S[p+1], S[p+2] ...S[n]
Figure 12: Agent Chains
A.7. Fully Meshed Layers
Figure 13 extends the scenario in Figure 11 by adding an extra layer
of agents. But since each layer of nodes can reach any node in the
next layer, each node only needs to consider the load of its next-hop
peer.
---A1---A3---S[1], S[2]...S[p]
/ | \ / |\ /
C | x | x
\ | / \ |/ \
---A2---A4---S[p+1], S[p+2] ...S[n]
Figure 13: Full Mesh
A.8. Partitions
A Diameter network with multiple is said to be "partitioned" when
only a subset of available servers can server a particular realm-
routed request. For example, one group of servers may handle users
whose names start with "A" through "M", and another group may handle
"N" through "Z".
In such a partitioned network, nodes cannot load-balance requests
across partitions, since not all servers can handle the request. A
client, or an intermediate agent, may still be able to load-balance
between servers inside a partition.
A.9. Active-Standby Nodes
The previous scenarios assume that traffic can be load balanced among
all peers that are eligible to handle a request. That is, the peers
operate in an "active-active" configuration. In an "active-standby"
configuration, traffic would be load-balanced among active peers.
Requests would only be sent to peers in a "standby" state if the
active peers became unavailable. For example, requests might be
diverted to a stand-by peer if one or more active peers becomes
overloaded.
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A.10. Addition and removal of Nodes
When a Diameter node is added, the new node will start by advertising
its load. Downstream nodes will need to factor the new load
information into load balancing decisions. The downstream nodes
should attempt to ensure a smooth increase of the traffic to the new
node, avoiding an immediate spike of traffic to the new node. It
should be determined if this use case is in the scope of the load
control mechanism.
When removing a node in a controlled way (e.g. for maintenance
purpose, so outside a failure case), it might be appropriate to
progressively reduce the traffic to this node by routing traffic to
other nodes. Simple load information (load percentage) would be not
sufficient. It should be determined if this use case is in the scope
of the load control mechanism.
Authors' Addresses
Ben Campbell
Oracle
7460 Warren Parkway # 300
Frisco, Texas 75034
USA
Email: ben@nostrum.com
Steve Donovan (editor)
Oracle
7460 Warren Parkway # 300
Frisco, Texas 75034
United States
Email: srdonovan@usdonovans.com
Jean-Jacques Trottin
Alcatel-Lucent
Route de Villejust
91620 Nozay
France
Email: jean-jacques.trottin@alcatel-lucent.com
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