Internet DRAFT - draft-roach-dime-overload-ctrl
draft-roach-dime-overload-ctrl
DIME A. B. Roach
Internet-Draft Mozilla
Intended status: Standards Track E. McMurry
Expires: November 18, 2013 Tekelec
May 17, 2013
A Mechanism for Diameter Overload Control
draft-roach-dime-overload-ctrl-03
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 or stop sending traffic for some period of time. Otherwise,
it must continue to expend resources parsing and responding to
Diameter messages.
This document proposes a concrete, application-independent mechanism
to address the challenge of communicating load and overload state
among Diameter peers, and specifies an algorithm for load abatement
to address such overload conditions as they occur. The load
abatement algorithm is extensible, allowing for future documents to
define additional load abatement approaches.
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
Task Force (IETF). Note that other groups may also distribute
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This Internet-Draft will expire on November 18, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Mechanism Properties . . . . . . . . . . . . . . . . . . . 4
1.2. Overview of Operation . . . . . . . . . . . . . . . . . . 6
1.3. Documentation Conventions . . . . . . . . . . . . . . . . 6
2. Overload Scopes . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Scope Descriptions . . . . . . . . . . . . . . . . . . . . 7
2.2. Combining Scopes . . . . . . . . . . . . . . . . . . . . . 8
3. Diameter Node Behavior . . . . . . . . . . . . . . . . . . . . 9
3.1. Connection Establishment Procedures . . . . . . . . . . . 9
3.2. Diameter Client and Diameter Server Behavior . . . . . . . 11
3.2.1. Sending a Request to a Compliant Peer . . . . . . . . 12
3.2.2. Receiving a Request . . . . . . . . . . . . . . . . . 13
3.2.3. Sending an Answer to a Compliant Peer . . . . . . . . 14
3.2.4. Receiving an Answer from a Compliant Peer . . . . . . 15
3.3. Diameter Agent Behavior . . . . . . . . . . . . . . . . . 16
3.3.1. Proxying a Request . . . . . . . . . . . . . . . . . . 16
3.3.2. Proxying an Answer . . . . . . . . . . . . . . . . . . 16
3.4. Proactive Load and Overload Communication . . . . . . . . 17
3.5. Load Processing . . . . . . . . . . . . . . . . . . . . . 17
3.5.1. Sending Load Information . . . . . . . . . . . . . . . 17
3.5.2. Receiving Load Information . . . . . . . . . . . . . . 18
3.6. Session Establishment for Session Groups . . . . . . . . . 20
3.6.1. Session Group Concepts . . . . . . . . . . . . . . . . 20
3.6.2. Session Group Procedures . . . . . . . . . . . . . . . 22
4. Loss-Based Overload Control Algorithm . . . . . . . . . . . . 22
4.1. Overload-Metric values for the 'Loss' Algorithm . . . . . 23
4.2. Example Implementation . . . . . . . . . . . . . . . . . . 24
5. Diameter AVPs for Overload . . . . . . . . . . . . . . . . . . 28
5.1. Load-Info AVP . . . . . . . . . . . . . . . . . . . . . . 28
5.2. Supported-Scopes AVP . . . . . . . . . . . . . . . . . . . 28
5.3. Overload-Algorithm AVP . . . . . . . . . . . . . . . . . . 29
5.4. Overload-Info-Scope AVP . . . . . . . . . . . . . . . . . 30
5.4.1. Realm Scope . . . . . . . . . . . . . . . . . . . . . 31
5.4.2. Application-ID Scope . . . . . . . . . . . . . . . . . 31
5.4.3. Host Scope . . . . . . . . . . . . . . . . . . . . . . 31
5.4.4. Session Scope . . . . . . . . . . . . . . . . . . . . 31
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5.4.5. Connection Scope . . . . . . . . . . . . . . . . . . . 31
5.4.6. Session Group Scope . . . . . . . . . . . . . . . . . 32
5.5. Overload-Metric AVP . . . . . . . . . . . . . . . . . . . 32
5.6. Period-Of-Validity AVP . . . . . . . . . . . . . . . . . . 32
5.7. Session-Group AVP . . . . . . . . . . . . . . . . . . . . 32
5.8. Load AVP . . . . . . . . . . . . . . . . . . . . . . . . . 32
6. Security Considerations . . . . . . . . . . . . . . . . . . . 32
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
7.1. New Diameter AVPs . . . . . . . . . . . . . . . . . . . . 33
7.2. New Diameter Disconnect-Cause . . . . . . . . . . . . . . 33
7.3. New Diameter Response Code . . . . . . . . . . . . . . . . 34
7.4. New Command Flag . . . . . . . . . . . . . . . . . . . . . 34
7.5. Overload Algorithm Registry . . . . . . . . . . . . . . . 34
7.6. Overload Scope Registry . . . . . . . . . . . . . . . . . 34
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
8.1. Normative References . . . . . . . . . . . . . . . . . . . 35
8.2. Informative References . . . . . . . . . . . . . . . . . . 35
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 35
Appendix B. Requirements Analysis . . . . . . . . . . . . . . . . 36
Appendix C. Extending the Overload Mechanism . . . . . . . . . . 45
C.1. New Algorithms . . . . . . . . . . . . . . . . . . . . . . 45
C.2. New Scopes . . . . . . . . . . . . . . . . . . . . . . . . 46
Appendix D. Design Rationale . . . . . . . . . . . . . . . . . . 46
D.1. Piggybacking . . . . . . . . . . . . . . . . . . . . . . . 47
D.2. Load AVP in All Packets . . . . . . . . . . . . . . . . . 48
D.3. Graceful Failure . . . . . . . . . . . . . . . . . . . . . 48
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49
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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 or stop sending traffic for some period
of time. Otherwise, it must continue to expend resources parsing and
responding to Diameter messages.
This document defines a mechanism for communicating the load and
overload information among Diameter nodes. It also defines a base
algorithm for shedding traffic under overload circumstances. The
design of the mechanism described in this document allows for the
definition of alternate load abatement algorithms as well.
The mechanism proposed in this document is heavily influenced by the
work performed in the IETF Session Initiation Protocol (SIP) Overload
Control Working Group, and draws on the conclusions reached by that
working group after extensive network modeling.
The solution described in this document is intended to satisfy the
requirements described in [I-D.ietf-dime-overload-reqs], with the
exception of REQ 34. As discussed in that document, the intention of
a Diameter overload mechanism is to handle overload of the actual
message processing portions of Diameter servers. This is in contrast
to congestion, which is the inability of the underlying switching and
routing fabric of the network to carry the volume of traffic at the
volume that IP hosts wish to send it. Handling of congestion is
relegated to the underlying transport protocol (TCP or SCTP), and
will not be discussed.
Philosophically, the approach in designing this mechanism is based on
the prospect that building a base-level, fully compliant
implementation should be a very simple and straightforward exercise.
However, the protocol includes many additional features that may be
implemented to allow Diameter nodes to apply increasingly
sophisticated behaviors. This approach gives implementors the
freedom to implement as sophisticated a scheme as they desire, while
freeing them from the burden of unnecessary complexity. By doing so,
the mechanism allows for the rapid development and deployment of the
mechanism followed by a period of steady and gradual improvements as
implementations become more capable.
1.1. Mechanism Properties
The core Diameter overload mechanism described in this document is
fundamentally hop-by-hop. The rationale for using a hop-by-hop
approach is the same as is described in section 5.1 of [RFC6357].
However, due to the fact that Diameter networks frequently have
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traffic that is easily grouped into a few well-defined categories, we
have added some concepts that allow Diameter agents to push back on
subsets of traffic that correspond to certain well-defined and
client-visible constructs (such as Destination-Host, Destination-
Realm, and Application-ID). These constructs are termed "Scopes" in
this document. A more complete discussion of Scopes is found in
Section 2.
The key information transmitted between Diameter peers is the current
server load (to allow for better balancing of traffic, so as to
preempt overload in the first place) as well as an indication of
overload state and severity (overload information). The actual load
and overload information is conveyed as a new compound AVP, added to
any Diameter messages that allow for extensibility. As discussed in
section 3.2 of [RFC6733], all CCFs are encouraged to include AVP-
level extensibility by inclusion of a "* [ AVP ]" construct in their
syntax definition. The document author has conducted an extensive
(although admittedly not exhaustive) audit of existing applications,
and found none lacking this property. The inclusion of load and
overload information in existing messages has the property that the
frequency with which information can be exchanged increases as load
on the system goes up.
For the purpose of grouping the several different parts of load
information together, this mechanism makes use of a Grouped AVP,
called "Load-Info". The Load-Info AVP may appear one or more times
in any extensible command, with the restriction that each instance of
the Load-Info AVP must contain different Scopes.
Load and overload information can be conveyed during times of inter-
node quiescence through the use of DWR/DWA exchanges. These
exchanges can also be used to proactively change the overload or load
level of a server when no other transaction is ready to be sent.
Finally, in the unlikely event that an application is defined that
precludes the inclusion of new AVPs in its commands, DWR/DWA
exchanges can be sent at any rate acceptable to the server in order
to convey load and overload information.
In [RFC3588], the DWR and DWA message syntax did not allow for the
addition of new AVPs in the DWR and DWA messages. This oversight
was fixed in [RFC6733]. To allow for transmission of load
information on quiescent links, implementations of the mechanism
described in this document are expected to correctly handle
extension AVPs in DWR and DWA messages, even if such
implementations have not otherwise been upgraded to support
[RFC6733].
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1.2. Overview of Operation
During the capabilities exchange phase of connection establishment,
peers determine whether the connection will make use of the overload
control mechanism; and, if so, which optional behaviors are to be
employed.
The information sent between adjacent nodes includes two key metrics:
Load (which, roughly speaking, provides a linear metric of how busy
the node is), and Overload-Metric (which is input to the negotiated
load abatement algorithm).
Message originators (whether originating a request or an answer)
include one or more Load-Info AVPs in messages when they form them.
These Load-Info AVPs reflect the originators' own load and overload
state.
Because information is being used on a hop-by-hop basis, it is
exchanged only between adjacent nodes. This means that any Diameter
agent that forwards a message (request or answer) is required to
remove any information received from the previous hop, and act upon
it as necessary. Agents also add their own load and overload
information (which may, at implementors' preference, take previous-
hop information into account) into a new Load-Info AVP before sending
the request or answer along.
Because the mechanism requires affirmative indication of support
in the capabilities exchange phase of connection establishment,
load and overload information will never be sent to intermediaries
that do not support the overload mechanism. Therefore, no special
provisions need to be made for removal of information at such
intermediaries -- it will simply not be sent to them.
Message recipients are responsible for reading and acting upon load
and overload information that they receive in such messages.
1.3. 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].
2. Overload Scopes
In normal operation, a Diameter node may be overloaded for some but
not all possible requests. For example, an agent that supports two
realms (realm A and realm B in this example) may route traffic to one
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set of servers for realm A, and another set of servers for realm B.
If the realm A servers are overloaded but realm B servers are not,
then the agent is effectively overloaded for realm A but not for
realm B.
Despite the fact that Diameter agents can report on scopes that
semantically map to constructs elsewhere in the network, it is
important to keep in mind that overload state is still reported on a
hop-by-hop basis. In other words, the overload state reported for
realm A in the example above represents the aggregate of the agent's
overload state along with the overload state being reported by
applicable upstream servers (those serving realm A).
Even without the use of Diameter agents, similar situations may arise
in servers that need to make use of external resources for certain
applications but not for others. For example, if a single server is
handling two applications, one of which uses an external database
while the other does not, it may become overloaded for the
application that uses the external database when the database
response latency increases.
The indication of scopes for overload information (using the
Overload-Info-Scope AVP; see Section 5.4) allows a node to indicate a
subset of requests to which overload information is to be applied.
This document defines seven scopes; only "Connection" scope is
mandatory to implement. The use of the optional scopes, along with
the use of any additional scopes defined in other documents, is
negotiated at connection establishment time; see Section 3.1.
2.1. Scope Descriptions
Destination-Realm: This scope, which nodes MUST implement, pertains
to all transactions that have a Destination-Realm AVP matching
the indicated value.
Application-ID: This scope, which nodes MUST implement, pertains to
all transactions that contain an Application-ID field matching
the indicated value.
Destination-Host: This scope, which nodes SHOULD implement, pertains
to all transactions that have a Destination-Host AVP matching
the indicated value.
Host: This scope, which nodes SHOULD implement, pertains to all
transactions sent directly to the host matching the indicated
value.
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Connection: This scope, which nodes MUST implement, pertains to all
transactions sent on the same TCP connection or SCTP
association. This scope has no details indicating which
connection or association it applies to; instead, the recipient
of an indication of "Connection" scope is to use the connection
or association on which the message was received as the
indicated connection or association. In other words, any use
of Connection scope applies to "this connection."
Session-Group: This scope, which nodes MAY implement, pertains to
all transactions in a session that has been assigned to the
indicated group. For more information on assigning sessions to
groups, see Section 3.6.
Session: This scope, which nodes MAY implement, pertains to all
transactions in the indicated session.
Some applications do not have long-running sessions containing
multiple transactions. For such applications, the use of "Session-
Group" and "Session" scopes do not make sense. Such applications
will instead make use of the most applicable of the remaining scopes
(plus any negotiated extension scopes) to achieve overload control.
OPEN ISSUE: Is there value to including a stream-level scope for
SCTP? We haven't been able to come up with a use case for doing so
yet, but it wouldn't necessarily be unreasonable.
2.2. Combining Scopes
To allow for the expression of more complicated scopes than the
primitives defined above, multiple Overload-Info-Scope AVPs may be
included in a single Load-Info AVP. Semantically, these scopes are
included in the following way:
o Attributes of the different kinds are logically and-ed together
(e.g., if both "Destination-Realm" and "Application-ID" are
present, the information applies to requests sent that match both
the realm and the application).
o Attributes of the same kind are logically or-ed together (e.g., if
two "Destination-Realm"s are present, the information applies to
requests sent to either realm).
o If a transaction falls within more than one scope, the "most
overloaded" scope is used for traffic shaping.
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To prevent the complexity of implementing arbitrary scope combination
rules, only the following combinations of scopes are allowed (OPEN
ISSUE -- we need to figure out what makes most sense for expressing
these combinations. Formal grammar? Prose? A table of some kind?
For now, they're expressed as a pseudo-ABNF):
o 1*(Destination-Realm) 0*1(Application-ID)
o 1*(Application-ID) 0*1(Destination-Realm)
o 1*(Application-ID) 0*1(Destination-Host)
o 1*(Application-ID) 0*1(Host)
o 1*(Application-ID) 0*1(Connection)
o 1*(Destination-Host)
o 1*1(Host)
o 1*1(Connection)
o 1*(Session-Group) 0*1(Host | Connection)
o 1*(Session) 0*1(Host | Connection)
OPEN ISSUE: Is this the right set of scope combinations? Is there
a need for more? Are any of these unnecessary? Ideally, this
should be the smallest set of combinations that lets nodes report
what they realistically need to report.
Any document that creates additional scopes MUST define how they may
be combined with all scopes registered with IANA at the time of their
publication.
3. Diameter Node Behavior
The following sections outline the behavior expected of Diameter
clients, servers, and agents that implement the overload control
mechanism.
OPEN ISSUE: SIP Overload Control includes a sequence parameter to
ensure that out-of-order messages do not cause the receiver to act on
state that is no longer accurate. Is message reordering a concern in
Diameter? That is, do we need to include sequence numbers in the
messages to ensure that the receiver does not act on stale state
information? Because Diameter uses only reliable, in-order
transports, it seems that this isn't likely to be an issue. Is there
room for a race when multiple connections are in use?
3.1. Connection Establishment Procedures
Negotiation for support of this mechanism is performed during
Diameter capabilities exchange. Optional protocol features and
extensions to this mechanism are also negotiated at this time. No
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provision is provided for renegotiation of mechanism use or
extensions during the course of a connection. If peers wish to make
changes to the mechanism, they must create a new connection to do so.
The connection initiator includes a Load-Info AVP in the CER
(Capabilities-Exchange-Request) message that it sends after
establishing the connection. This Load-Info AVP MUST contain a
Supported-Scopes AVP and an Overload-Algorithm AVP. The Supported-
Scopes AVP includes a comprehensive list of scopes supported that the
connection initiator can receive and understand. See Section 5.2 for
information on the format of the Supported-Scopes AVP.
The Load-Info AVP in a CER message also MAY contain one or more
Overload-Algorithm AVPs. If present, these AVPs indicate every
Overload-Algorithm the connection initiator is willing to support for
the connection that is being established. If the connection
initiator supports only the "Loss" algorithm, it MAY indicate this
fact by omitting the Overload-Algorithm altogether.
The Load-Info AVP in a CER message MAY also contain additional AVPs,
as defined in other documents, for the purpose of negotiation
extensions to the Overload mechanism.
The Diameter node that receives a CER message first examines it for
the presence of a Load-Info AVP. If no such AVP is present, the node
concludes that the overload control mechanism is not supported for
this connection, and no further overload-related negotiation is
performed. If the received CER contains a Load-Info AVP, the
recipient of that message stores that information locally in the
context of the connection being established. It then examines the
Overload-Algorithm AVPs, if present, and selects a single algorithm
from that list. If no Overload-Algorithm is indicated, then the base
"Loss" algorithm is used for the connection. In either case, the
recipient of the CER stores this algorithm in the context of the
connection.
When a node conformant to this specification sends a Capabilities-
Exchange-Answer (CEA) message in answer to a CER that contained a
Load-Info AVP, the CEA MUST contain a Load-Info AVP. This Load-Info
AVP MUST contain a Supported-Scopes AVP that includes a comprehensive
list of scopes supported that the connection initiator can receive
and understand. The CEA also contains zero or one Overload-Algorithm
AVPs. If present, this Overload-Algorithm MUST match one of the
Overload-Algorithm AVPs sent in the CER, and it indicates the
overload control algorithm that will be used for the connection. If
the CEA contains no Overload-Algorithm, the connection will use the
"Loss" algorithm.
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When a node receives a CEA message, it examines it for the presence
of a Load-Info AVP. If no such AVP is present, the node concludes
that the overload mechanism is not supported for this connection. If
the received CEA contains a Load-Info AVP, then the recipient
extracts the Supported-Scopes information, and stores them locally in
the context of the connection being established. It then checks for
the presence of an Overload-Algorithm AVP. If present, this AVP
indicates the overload control algorithm that will be used for the
connection. If absent, then the connection will use the "Loss"
algorithm.
If a node receives a CEA message that indicates support for a scope
that it did not indicate in its CER or which selects an overload
control algorithm that it did not advertise in its CER, then it MUST
terminate the connection by sending a DPR with a Disconnect-Cause of
NEGOTIATION_FAILURE, (128 [actual value TBD]) indicating that the CEA
sender has failed to properly follow the negotiation process
described above.
Note that the Supported-Scopes announcement during capabilities
exchange is a set of mutual advertisements of which scopes the two
nodes are willing to receive information about. It is not a
negotiation. It is perfectly acceptable for a node to send
information for scopes it did not include in the Supported-Scopes AVP
it sent, as long as the recipient indicated support for receiving
such a scope. For example, a Diameter agent, during connection
establishment with a client, may indicate support for receiving only
"Connection" and "Host" scope; however, if the client indicated
support for "Application" scope, then the agent is free to send Load-
Info AVPs that make use of "Application" scope to the client.
3.2. Diameter Client and Diameter Server Behavior
The following sections describe the behavior that Diameter clients
and Diameter servers implement for the overload control mechanism.
Behavior at Diameter Agents is described in Section 3.3.
To implement overload control, Diameter nodes need to keep track of
three important metrics for each of the scopes for which information
has been received: the overload metric for the scope, the period of
validity for that overload metric, and the load within that scope.
Conceptually, these are data records indexed by the scope to which
they apply. In the following sections, we refer to these data
records with the term "scope entry." Further, when it is necessary
to distinguish between those scope entries referring to the load
information received from other nodes and those referring to the load
information sent to other nodes, we use the term "remote scope entry"
to refer to the information received from other nodes, and "local
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scope entry" to refer to that information that is being maintained to
send to other nodes.
In order to allow recipients of overload information to perform
certain performance optimizations, we also define a new command flag,
called 'O'verload. This bit, when set, indicates that the message
contains at least one Load-Info AVP with a non-zero Overload-Metric
-- in other words, the sending node is overloaded for at least one
context. See Section 7.4 for the definition of the 'O'verload bit.
OPEN ISSUE: Is there anything we can do to make this 'O'verload
bit even more useful? Perhaps setting it only when the overload
value has changed, or changed by a certain amount?
3.2.1. Sending a Request to a Compliant Peer
This section applies only to those requests sent to peers who
negotiated use of the overload control mechanism during capabilities
exchange. Requests sent over other connections are handled the same
as they would in the absence of the overload control mechanism.
Before sending a request, a Diameter node must first determine which
scope applies. It does this as follows: first, a next hop host and
connection are determined, according to normal Diameter procedures
(potentially modified as described in Section 3.5.2). The sending
node then searches through its list of remote scope entries (ignoring
any whose Period-of-Validity has expired) to determine which ones
match the combination of the fields in the current request, the next-
hop host, and the selected connection. If none of the matching scope
entries are in overload, then the message is handled normally, and no
additional processing is required.
As an optimization, a sending node MAY choose to track whether any of
its peers are in overload, and to skip the preceding step if it knows
that no scopes are in overload.
If one or more matching scope entries are in overload, then the
sending node determines which scope is most overloaded. The sending
node then sends, drops, or otherwise modifies handling of the request
according to the negotiated overload control algorithm, using the
Overload-Metric from the selected scope entry as input to the
algorithm.
When determining which requests are impacted by the overload control
algorithm, request senders MAY take into account the type of message
being sent and its contents. For example, messages within an
existing session may be prioritized over those that create a new
session. The exact rules for such prioritization will likely vary
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from application to application. The authors expect that
specifications that define or specify the use of specific Diameter
Applications may choose to formally define a set of rules for such
prioritization on a per-Application basis.
The foregoing notwithstanding, senders MUST NOT use the content or
type of request to exempt that request from overload handling. For
example, if a peer requests a 50% decrease in sent traffic using the
"Loss" algorithm (see Section 4), but the traffic that the sending
node wishes to send consists 65% of traffic that the sender considers
critical, then the sender is nonetheless obliged to drop some portion
of that critical traffic (e.g., it may elect to drop all non-critical
traffic and 23% of the critical traffic, resulting in an overall 50%
reduction).
The sending node then inserts one or more Load-Info AVPs (see
Section 5.1) into the request. If the sender inserts more than one
Load-Info AVP, then each Load-Info AVP MUST contain a unique scope,
as specified by the Overload-Scope AVP(s) inside the Load-Info AVP.
Each Load-Info AVP in the request MUST contain an Overload-Metric
(see Section 5.5), indicating whether (and to what degree) the sender
is overloaded for the indicated scope. If this metric is not zero,
then the Load-Info AVP MUST also contain a Period-Of-Validity AVP
(see Section 5.6), indicating the maximum period the recipient should
consider the Overload-Metric to be valid. Any message containing a
non-zero Overload-Metric also MUST set the 'O'verload bit in the
Command Flags field to indicate to the recipient that the message
contains an overload indication. See Section 7.4 for the definition
of the 'O'verload bit.
Each Load-Info AVP MUST also contain a Load AVP, indicating the
server's load level within the context of the indicated scope. See
Section 3.5.1 for details on generating this load metric. Note that
a server's load may frequently be identical for all the scopes for
which it sends information.
3.2.2. Receiving a Request
3.2.2.1. Receiving a Request from a Compliant Peer
A node that receives a request from a peer that has negotiated
support for the overload control mechanism will extract the Load-Info
AVPs from the request and use each of them to update its remote scope
entries. First, the node attempts to locate an existing scope entry
that corresponds to the Overload-Scope indicated in the Load-Info
AVP. If one does not exist, it is created. The scope entry is then
populated with the overload metric, period of validity, and load
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information. The message is then processed as normal.
In some circumstances, request recipients can become sufficiently
overloaded that even those messages received from complaint clients
can overwhelm its processing capabilities. Under such circumstances,
nodes MAY begin treating a subset of such requests as if they were
received from noncompliant peers (as explained in the following
section).
3.2.2.2. Receiving a Request from a Noncompliant Peer
An important aspect of the overload control mechanism is that
Diameter nodes that do not implement the mechanism cannot have an
advantage over those that do. In other words, it is necessary to
prevent the situation that a network in overload will cease servicing
those transactions from overload-compliant nodes in favor of those
sent by those nodes that do not implement the overload control
mechanism. To achieve this goal, message recipients need to track
the overload control metric on behalf of those sending nodes that do
not implement overload, and to reject messages from those nodes that
would have been dropped if the sender had implemented the overload
mechanism.
A node that receives a request from a peer that has not negotiated
support for the overload control mechanism searches through its list
of local scope entries to determine which ones match the combination
of the fields in the received request. (These are the entries that
indicate the Overload-Metric that the node would have sent to the
peer if the peer had supported the overload mechanism). If none of
the matching scope entries are in overload, then the message is sent
normally, and no additional processing is required.
If one or more matching local scope entries are in overload, then the
node determines which scope is most overloaded. The node then
executes the "Loss" overload control algorithm (see Section 4) using
the overload metric in that most overloaded scope. If the result of
running that algorithm determines that a sender who had implemented
the overload control mechanism would have dropped the message, then
the recipient MUST reply to the request with a
DIAMETER_PEER_IN_OVERLOAD response (see Section 7.3).
3.2.3. Sending an Answer to a Compliant Peer
This section applies only to those answers sent to peers who
negotiated use of the overload control mechanism during capabilities
exchange.
When sending an answer, a Diameter node inserts one or more Load-Info
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AVPs (see Section 5.1) into the answer. If the sender inserts more
than one Load-Info AVP, then each Load-Info AVP MUST contain a unique
scope, as specified by the Overload-Scope AVP(s) inside the Load-Info
AVP.
Each Load-Info AVP in the answer MUST contain an Overload-Metric (see
Section 5.5), indicating whether (and to what degree) the server is
overloaded for the indicated scope. If this metric is not zero, then
the Load-Info AVP MUST also contain a Period-Of-Validity AVP (see
Section 5.6), indicating the maximum period the recipient should
consider the Overload-Metric to be valid. Any message containing a
non-zero Overload-Metric also MUST set the 'O'verload bit in the
Command Flags field to indicate to the recipient that the message
contains an overload indication. See Section 7.4 for the definition
of the 'O'verload bit.
It is important to note that using this mechanism creates a closed
feedback loop, with some amount of lag introduced by overload
processing and the network. As such, implementers must be aware of
the potential for such a system to produce oscillations in the
overload level. Without proper control, it is also possible for
these oscillations to diverge, resulting in undesirable behavior.
There are several ways to address this issue, and it is left to
implementors to determine the best way for their particular
situation. However, at a minimum senders of overload control
information SHOULD apply hysteresis to the Overload-Metric, and
signal easing of overload more slowly than signaling increases.
Each Load-Info AVP MUST also contain a Load AVP, indicating the
server's load level within the context of the indicated scope. See
Section 3.5.1 for details on generating this load metric. Note that
a server's load may frequently be identical for all the scopes for
which it sends information.
3.2.4. Receiving an Answer from a Compliant Peer
A node that receives an answer from a peer that has negotiated
support for the overload control mechanism will extract the Load-Info
AVPs from the answer and use each of them to update its remote scope
entries. First, the node attempts to locate an existing scope entry
that corresponds to the Overload-Scope indicated in the Load-Info
AVP. If one does not exist, it is created. The scope entry is then
populated with the overload metric, period of validity, and load
information. The message is then processed as normal.
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3.3. Diameter Agent Behavior
This section discusses the behavior of a Diameter Agent acting as a
Proxy or Relay. Diameter Agents that provide redirect or translation
services behave the same as Diameter Servers for the purpose of
overload control, and follow the procedures defined in Section 3.2.
Whenever sending a request or an answer, Agents MUST include a Load-
Info AVP reflecting the Agent's overload and load information. In
formulating this information, the Agent may choose to use only that
information relating to its own local resources. However, better
network behavior can be achieved if agents incorporate information
received from their peers when generating overload information. The
exact means for incorporating such information is left to local
policy at the agent.
For example: consider an agent that distributes sessions and
transactions among three Diameter servers, each hosting a different
Diameter application. While it would be compliant for the Agent to
only report its own overload state (i.e., at "Host" scope), overall
network behavior would be improved if it chose to also report
overload state for up to three additional scopes (i.e. at
"Application-ID" scope), incorporating the Overload information
received from each server in these scopes.
3.3.1. Proxying a Request
Upon receiving a request, a Diameter Proxy or Relay performs the
steps detailed in Section 3.2.2.
The agent then MUST remove all Load-Info AVPs from the request: Load-
Info is never passed through a Proxy or Relay transparently.
When the Diameter Agent proxies or relays a request, it follows the
process outlined in Section 3.2.1.
3.3.2. Proxying an Answer
Upon receiving an answer, a Diameter Agent follows the process
described in Section 3.2.4 to update its remote scope entries.
The Agent then MUST remove all Load-Info AVPs from the answer: Load-
Info is never passed through a Proxy or Relay transparently.
When the Diameter Agent proxies or relays a response, it follows the
process outlined in Section 3.2.3.
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3.4. Proactive Load and Overload Communication
Because not all Diameter links will have constant traffic, it may be
occasionally necessary to send overload and/or load information over
links that would otherwise be quiescent. To proactively send such
information to peers, the Diameter node with information to convey
may choose to send a Diameter Watchdog Request (DWR) message to its
peers. The procedure described in Section 3.2.1 applies to these
requests, which provides the means to send load and overload
information.
In order to prevent unnecessarily diminished throughput between
peers, a Diameter node SHOULD proactively send a DWR to all its peers
whenever it leaves an overload state. Similarly, in order to provide
peers the proper data for load distribution, nodes SHOULD send DWR
messages to a peer if the load information most recently sent to that
peer has changed by more than 20% and is more than 5 seconds old.
3.5. Load Processing
While the remainder of the mechanism described in this document is
aimed at handling overload situations once they occur, it is far
better for a system if overload can be avoided altogether. In order
to facilitate overload avoidance, the overload mechanism includes the
ability to convey node load information.
Semantically, the Load information sent by a Diameter node indicates
the current utilization of its most constrained resource. It is a
linear scale from 0 (least loaded) to 65535 (most loaded).
It is critical to distinguish between the value conveyed in the Load
AVP and the value conveyed in the Overload-Metric AVP. The Load AVP
is computed and used independent of the Overload-Algorithm selected
for a connection, while the Overload-Metric is meaningful only in the
context of the selected algorithm. Most importantly, the Load
information never has any impact on the behavior specified in the
overload algorithm. If a node reports a Load of 65535, but the
Overload-Metric does not indicate any need to apply the selected
overload control algorithm, then the sender MUST NOT apply the
selected overload control algorithm. Conversely, if a node is
reporting an Overload-Metric that requires the recipient to take
action to reduce traffic, those actions MUST be taken, even if the
node is simultaneously reporting a Load value of 0.
3.5.1. Sending Load Information
Diameter nodes implementing the overload mechanism described in this
document MUST include a Load AVP (inside a Load-Info AVP) in every
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Diameter message (request and answer) they send over a connection
that has been negotiated to use the overload control mechanism. Note
that this requirement does not necessitate calculation of the Load
metric each time a message is sent; the Load value may be calculated
periodically (e.g., every 100 ms), and used for every message sent
until it is recalculated.
The algorithm for generation of the load metric is a matter of local
policy at the Diameter node, and may vary widely based on the
internal software architecture of that node.
For advanced calculations of Load, anticipated inputs to the
computation include CPU utilization, network utilization, processor
interrupts, I/O throughput, and internal message queue depths.
To free implementors from the potential complexity of determining an
optimal calculation for load, we define a very simple, baseline load
calculation that MAY be used for the purpose of populating the Load
AVP. Implementations using this simplified calculation will use a
configured, hard-coded, or Service Level Agreement (SLA)-defined
maximum number of transactions per second (TPS) which a node is known
to be able to support without issue. These implementations simply
report their load as a linear representation of how much of this
known capacity is currently in use:
Load = MIN(Current_TPS * 65535 / Maximum_TPS, 65535)
To prevent rapid fluctuations in the load metric, nodes SHOULD report
a rolling average of the calculated load rather than the actual
instantaneous load at any given moment.
Load information is scoped to the level indicated by the Overload-
Info-Scope AVP present in the Load-Info AVP in which the Load AVP
appears.
3.5.2. Receiving Load Information
While sending load information is mandatory, the actual processing of
load information at a recipient is completely optional. Ideally,
recipients will use the load information as input to a decision
regarding which of multiple equivalent servers to use when initiating
a new connection. Recipients may choose to update load information
on receipt of every message; alternately, they may periodically
"sample" messages from a host to determine the load it is currently
reporting.
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3.5.2.1. Example Load Handling
This section describes a non-normative example of how recipients can
use Load information received from other Diameter nodes. At a high
level, the concept is that received load metrics are used to scale
the distribution algorithm that the node uses for selection of a
server from a group of equivalent servers.
Consider a client that uses DNS to resolve a host name into IP
addresses. In this example, the client is attempting to reach the
server for the realm example.com. It performs a NAPTR query for the
"AAA+D2T" record for that domain, and receives a result pointing to
the SRV record "_diameter._tcp.example.com". Querying for this SRV
record, in turn, results in three entries, with the same priorities:
+------------+----------------------+
| SRV Weight | Server Name |
+------------+----------------------+
| 20 | server-a.example.com |
| 20 | server-b.example.com |
| 60 | server-c.example.com |
+------------+----------------------+
The client then examines the currently reported loads for each of the
three servers. In this example, we are asserting that the reported
load metrics are as follows:
+-------------+----------------------+
| Load | Server Name |
+-------------+----------------------+
| 13107 (20%) | server-a.example.com |
| 26214 (60%) | server-b.example.com |
| 52428 (80%) | server-c.example.com |
+-------------+----------------------+
Based on this load information, the client scales the SRV weights
proportional to each server's reported load; the general formula is:
new_weight = original_weight * (65535 - load) / 65535
The node then calculates a new set of weights for the destination
hosts:
o server-a: new_weight = 20 * (65535 - 13107) / 65535 = 16
o server-b: new_weight = 20 * (65535 - 26214) / 65535 = 12
o server-c: new_weight = 60 * (65535 - 52428) / 65535 = 12
These three new weights (16, 12, and 12) are then used as input to
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the random selection process traditionally used when selecting among
several SRV records.
Note that this example is provided in the context of DNS SRV
processing; however, it works equally well in the case that server
processing weights are provisioned or made available through an
alternate resolution process.
3.6. Session Establishment for Session Groups
The procedure in this section applies to any Diameter operation that
may result in the creation of a new Diameter session. Note that
these operations are performed in addition to any normal message
processing, and in addition to the operations described in the
following sections.
3.6.1. Session Group Concepts
At the time a session is established, the server and/or the client
may choose to assign the newly created session to a Session Group
that they can use to refer to the session (and other sessions in the
same group) in later overload-related messages. This grouping is
intended to be used by servers that have visibility into resources
that may be independently overloaded, but which do not correspond to
an existing Diameter construct (such as Application, Realm, or
Destination Server).
One example of a server having visibility into resources that don't
have a corresponding Diameter construct is a Diameter Agent servicing
a mixed community of users -- say, one authenticated by a "Business"
server, and another authenticated by a "Residential" server. The
client in this network does not know which group any given session
belongs in; the routing of sessions is based on information available
only to the agent.
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+-------------+ +-------------+
| | | |
| Server A | | Server B |
| (Business) | |(Residential)|
| | | |
+-------------+ +-------------+
`. ,'
`. ,'
`. ,'
+-----+---+-----+
| |
| Agent |
| |
+---------------+
^
|
+-------+-------+
| |
| Client |
| |
+---------------+
In this case, the Agent may wish to assign sessions to two client-
visible Session Groups when the session is established. By doing so,
the Agent gains the ability to report Load and Overload metrics to
the Client independently for the two classes of users. This can be
extremely helpful, for example, in allowing the Agent to ask the
Client to throttle traffic for the Residential server when it becomes
overload, without impacting sessions pertaining to the Business
server.
Similar situations can arise even without the presence of Diameter
Agents in the network: a server may have a class of sessions that
require access to an off-board database (which can, itself, become
overloaded), while also servicing a class of sessions that is handled
entirely by a local authentication table. The server can use Session
Groups to assign these two classes of sessions to different groups,
and report overload on the class using the (overloaded) off-board
database without impacting the other sessions.
In some applications, it is possible to have the session established
by one peer (e.g., in the upstream direction), while some subsequent
in-session transactions are initiated by the other peer (e.g., in the
downstream direction). Because of this possibility, the overload
mechanism allows both peers to establish a Session Group at the time
the session is set up. The session identifiers are scoped to the
node that sends them. In other words, if a server assigns a session
to a group called "Residential", this group is not related to a
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client group (if any) by the same name. For clarity, this document
will refer to the session group assigned by the server performing the
processing as a "local session group," and the session group assigned
by the remote node as a "remote session group."
Nodes that send a session-creating request follow normal Diameter
procedures, along with the additional behavior described in
Section 3.2.1 and Section 3.3.1, as appropriate. Such nodes may also
assign the session to a Session Group, as long as the peer to which
they are communicating indicated support for the "Session-Group"
scope during capabilities exchange. Whether to do so and what group
to assign a session to is done according to local policy. To perform
such assignment, the node will include a Session-Group AVP (see
Section 5.7 in the Load-Info AVP for the session creating request.
These nodes also store the assigned name as the session's local
session group.
3.6.2. Session Group Procedures
The procedures in this section only apply on connections for which
support for the "Session-Group" scope has been negotiated during
capabilities exchange. See Section 3.1.
When a node receives a session creating request, it MUST check that
request for the presence for a Session-Group AVP in its Load-Info
AVP. If one is present, it stores that session group name as the
remote session group name for that server. This allows clients to
assign the session to a group, allowing it to indicate overload for
server-initiated transactions in the resulting session.
When a node replies to a session creating request, it can choose to
assign the newly-established session to a session group. Whether it
chooses to do so is independent of whether the remote node assigned
the session to a session group. To perform such an assignment, the
node includes a Session-Group AVP in the Load-Info AVP sent in answer
to the session-creating request. These nodes also store the assigned
name as the session's local session group.
Finally, when a node that has sent a session-creating request
receives a corresponding answer message, it MUST check that answer
for the presence of a Session-Group AVP in its Load-Info AVP. If one
is present, it stores that session group name as the remote session
group name for that server.
4. Loss-Based Overload Control Algorithm
This section describes a baseline, mandatory-to-implement overload
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control algorithm, identified by the indicator "Loss". This
algorithm allows a Diameter peer to ask its peers to reduce the
number of requests they would ordinarily send by a specified
percentage. For example, if a peer requests of another peer that it
reduce the traffic it is sending by 10%, then that peer will
redirect, reject, or treat as failed, 10% of the traffic that would
have otherwise been sent to this Diameter node.
4.1. Overload-Metric values for the 'Loss' Algorithm
A Diameter node entering the overload state for any of the scopes
that it uses with its peers will calculate a value for its Overload
Metric, in the range of 0 to 100 (inclusive). This value indicates
the percentage traffic reduction the Diameter node wishes its peers
to implement. The computation of the exact value for this parameter
is left as an implementation choice at the sending node. It is
acceptable for implementations to request different levels of traffic
reduction to different peers according to local policy at the
Diameter node. These Overload Metrics are then communicated to peers
using the Overload-Metric AVP in requests and answers sent by this
node.
Recipients of Overload-Metric AVPs on connections for which the
"Loss" algorithm has been specified MUST reduce the number of
requests sent in the corresponding scope by that percentage, either
by redirecting them to an alternate destination, or by failing the
request. For a Diameter Agent, these failures are indicated to the
peer who originated the request by sending a
DIAMETER_PEER_IN_OVERLOAD response (see Section 7.3). For diameter
clients, these failures cause the client to behave as if they
received a transient error in response to the request.
It is acceptable, when implementing the "Loss" algorithm, for the
reduction in transactions to make use of a statistical loss function
(e.g., random assignment of transactions into "success" and "failure"
categories based on the indicated percentage). In such a case, the
actual traffic reduction might vary slightly from the percentage
indicated, albeit in an insignificant amount.
The selection of which messages to withhold from sending does not
need to be arbitrary. For example, implementations are allowed to
distinguish between higher-priority and lower-priority messages, and
drop the lower-priority messages in favor of dropping the higher
priority messages, as long as the total reduction in traffic conforms
to the Overload-Metric in effect at the time. The selection of which
messages to prioritize over others will likely vary from application
to application (and may even be subject to standardization as part of
the application definition). One example of such a prioritization
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scheme would be to treat those messages that result in the creation
of a new session as lower priority then those messages sent in the
context of an established session.
4.2. Example Implementation
The exact means a client uses to implement the requirement that it
reduce traffic by a requested percentage is left to the discretion of
the implementor. However, to aid in understanding the nature of such
an implementation, we present an example of a valid implementation in
pseudo-code.
In this example, we consider that the sending node maintains two
classes of request. The first category are considered of lower
priority than the second category. If a reduction in traffic is
required, then these lower priority requests will be dropped before
any of the higher priority requests are dropped.
The sending Diameter node determines the mix of requests falling into
the first category, and those falling into the second category. For
example, 40% of the requests may be in the lower-priority category,
while 60% are in the higher-priority category.
When a node receives an overload indication from one of its peers, it
converts the Overload-Metric value to a value that applies to the
first category of requests. For example, if the Overload-Metric for
the applicable context is "10", and 40% of the requests are in the
lower-priority category, then:
10 / 40 * 100 = 25
Or 25% of the requests in the first category can be dropped, with an
overall reduction in sent traffic of 10%. The sender then drops 25%
of all category 1 requests. This can be done stochastically, by
selecting a random number for each sent packet between 1 to 100
(inclusive), and dropping any packet for which the resulting
percentage is equal to or less than 25. In this set of
circumstances, messages in the second category do not require any
reduction to meet the requirement of 25% traffic reduction.
A reference algorithm is shown below, using pseudo-code.
cat1 := 80.0 // Category 1 --- subject to reduction
cat2 := 100.0 - cat1 // Category 2 --- Under normal operations
// only subject to reduction after category 1 is exhausted.
// Note that the above ratio is simply a reasonable default.
// The actual values will change through periodic sampling
// as the traffic mix changes over time.
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while (true) {
// We're modeling message processing as a single work queue
// that contains both incoming and outgoing messages.
msg := get_next_message_from_work_queue()
update_mix(cat1, cat2) // See Note below
switch (msg.type) {
case outbound request:
destination := get_next_hop(msg)
oc_context := get_oc_scope(destination,msg)
if (we are in overload) {
add_overload_avps(msg)
}
if (oc_context == null) {
send_to_network(msg) // Process it normally by sending the
// request to the next hop since this particular
// destination is not subject to overload
}
else {
// Determine if server wants to enter in overload or is in
// overload
in_oc := extract_in_oc(oc_context)
oc_value := extract_oc(oc_context)
oc_validity := extract_oc_validity(oc_context)
if (in_oc == false or oc_validity is not in effect) {
send_to_network(msg) // Process it normally by sending
// the request to the next hop since this particular
// destination is not subject to overload. Optionally,
// clear the oc context for this server (not shown).
}
else { // Begin perform overload control
r := random()
drop_msg := false
if (cat1 >= cat2) {
category := assign_msg_to_category(msg)
pct_to_reduce_cat2 := 0
pct_to_reduce_cat1 := oc_value / cat1 * 100
if (pct_to_reduce_cat1 > 100) {
// Get remaining messages from category 2
pct_to_reduce_cat2 := 100 - pct_to_reduce_cat1
pct_to_reduce_cat1 := 100
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}
if (category == cat1) {
if (r <= pct_to_reduce_cat1) {
drop_msg := true
}
}
else { // Message from category 2
if (r <= pct_to_reduce_cat2) {
drop_msg := true
}
}
}
else { // More category 2 messages than category 1;
// indicative of an emergency situation. Since
// there are more category 2 messages, don't
// bother distinguishing between category 1 or
// 2 --- treat them equal (for simplicity).
if (r <= oc_value)
drop_msg := true
}
if (drop_msg == false) {
send_to_network(msg) // Process it normally by
// sending the request to the next hop
}
else {
// Do not send request downstream, handle locally by
// generating response (if a proxy) or treating as
// an error (if a user agent).
}
} // End perform overload control
}
end case // outbound request
case outbound answer:
if (we are in overload) {
add_overload_avps(msg)
}
send_to_network(msg)
end case // outbound answer
case inbound answer:
create_or_update_oc_scope() // For the specific server
// that sent the answer, create or update the oc scope;
// i.e., extract the values of the overload AVPs
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// and store them in the proper scopes for later use.
process_msg(msg)
end case // inbound answer
case inbound request:
create_or_update_oc_scope()
if (we are not in overload) {
process_msg(msg)
}
else { // We are in overload
if ( connection supports overload)
process_msg(msg)
}
else { // Sender does not support oc
if (local_policy(msg) says process message) {
process_msg(msg)
}
else {
send_answer(msg, DIAMETER_PEER_IN_OVERLOAD)
}
}
}
end case // inbound request
}
}
A simple way to sample the traffic mix for category 1 and category 2
is to associate a counter with each category of message.
Periodically (every 5-10s), get the value of the counters and
calculate the ratio of category 1 messages to category 2 messages
since the last calculation.
Example: In the last 5 seconds, a total of 500 requests were
scheduled to be sent. Assume that 450 out of 500 were messages
subject to reduction and 50 out of 500 were classified as requests
not subject to reduction. Based on this ratio, cat1 := 90 and cat2
:= 10, or a 90/10 mix will be used in overload calculations.
Of course, this scheme can be generalized to include an arbitrary
number of priorities, depending on how many different classes of
messages make sense for the given application.
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5. Diameter AVPs for Overload
NOTE: THE AVP NUMBERS IN THIS SECTION ARE USED FOR EXAMPLE PURPOSES
ONLY. THE FINAL AVP CODES TO BE USED WILL BE ASSIGNED BY IANA DURING
THE PUBLICATION PROCESS, WHEN AND IF THIS DOCUMENT IS PUBLISHED AS AN
RFC.
+---------------------+-------+-------+-------------+------+--------+
| Attribute Name | AVP | Sec. | Data Type | MUST | MUST |
| | Code | Def. | | | NOT |
+---------------------+-------+-------+-------------+------+--------+
| Load-Info | 1600 | 5.1 | Grouped | | M,V |
| Supported-Scopes | 1601 | 5.2 | Unsigned64 | | M,V |
| Overload-Algorithm | 1602 | 5.3 | Enumerated | | M,V |
| Overload-Info-Scope | 1603 | 5.4 | OctetString | | M,V |
| Overload-Metric | 1604 | 5.5 | Unsigned32 | | M,V |
| Period-Of-Validity | 1605 | 5.6 | Unsigned32 | | M,V |
| Session-Group | 1606 | 5.7 | UTF8String | | M,V |
| Load | 1607 | 5.8 | Unsigned32 | | M,V |
+---------------------+-------+-------+-------------+------+--------+
5.1. Load-Info AVP
The Load-Info AVP (AVP code 1600) is of type Grouped, and is used as
a top-level container to group together all information pertaining to
load and overload information. Every Load-Info AVP MUST contain one
Overload-Information-Scope AVP, and one Overload-Metric AVP.
The Grouped Data field of the Load-Info AVP has the following CCF
grammar:
< Load-Info > ::= < AVP Header: 1600 >
< Overload-Metric >
* { Overload-Info-Scope }
[ Supported-Scopes ]
* [ Overload-Algorithm ]
[ Period-Of-Validity ]
[ Session-Group ]
[ Load ]
* [ AVP ]
5.2. Supported-Scopes AVP
The Supported-Scopes AVP (AVP code 1601) is of type Uint64, and is
used during capabilities exchange to indicate the scopes that a given
node can receive on the connection. Nodes that support the mechanism
defined in this document MUST include a Supported-Scopes AVP in all
CER messages. It also MUST appear in any CEA messages sent in answer
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to a CER message containing a Load-Info AVP. The Supported-Scopes
AVP MUST NOT appear in any other message types. See Section 5.4 for
an initial list of scopes.
The Supported-Scopes AVP contains a bitmap that indicates the scopes
supported by the sender. Within the bitmap, the least significant
bit indicates support for scope 1 (Destination-Realm), while the next
least significant bit indicates support for scope 2 (Application-ID),
and so on. In general, if we consider the bits to be numbered from 0
(LSB) to 63 (MSB), then any bit n corresponds to the scope type
numbered n+1. This scheme allows for up to 64 total scopes to be
supported. More formally, the bitmask used to indicate support for
any specific context is calculated as follows (where the symbol "<<"
indicates a bit shift left):
bitmask = 1 << (n - 1)
For additional clarity, the bitmasks for the scopes defined in this
document are as follows:
+-------+--------------------+-------------------+
| Scope | Bitmask | Scope |
+-------+--------------------+-------------------+
| 1 | 0x0000000000000001 | Destination-Realm |
| 2 | 0x0000000000000002 | Application-ID |
| 3 | 0x0000000000000004 | Destination-Host |
| 4 | 0x0000000000000008 | Host |
| 5 | 0x0000000000000010 | Connection |
| 6 | 0x0000000000000020 | Session-Group |
| 7 | 0x0000000000000040 | Session |
+-------+--------------------+-------------------+
The advertisement process that makes use of the Supported-Scopes AVP
is described in Section 3.1.
5.3. Overload-Algorithm AVP
The Overload-Algorithm AVP (AVP code 1602) is of type Enumerated, and
is used to negotiate the algorithm that will be used for load
abatement. The Overload-Algorithm AVP MAY appear in CER and CEA
messages, and MUST NOT appear in any other message types. If absent,
an Overload Algorithm of type 1 (Loss) is indicated. Additional
values can be registered by other documents; see Appendix C.1.
Initial values for the enumeration are as follows:
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+------------+----------------+------------+
| AVP Values | Attribute Name | Reference |
+------------+----------------+------------+
| 0 | Reserved | - |
| 1 | Loss | [RFC xxxx] |
+------------+----------------+------------+
5.4. Overload-Info-Scope AVP
The Overload-Info-Scope AVP (AVP code 1603) is of type OctetString,
and is used to indicate to which scope the Overload-Metric applies.
See Section 2 for a definition of the different scope types and a
formal description of how they are applied. Other documents may
define additional scopes; see Appendix C.2 for details.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Scope | |
+-+-+-+-+-+-+-+-+ Details |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+-------+-------------------+------------+
| Scope | Attribute Name | Reference |
+-------+-------------------+------------+
| 0 | Reserved | [RFC xxxx] |
| 1 | Destination-Realm | [RFC xxxx] |
| 2 | Application-ID | [RFC xxxx] |
| 3 | Destination-Host | [RFC xxxx] |
| 4 | Host | [RFC xxxx] |
| 5 | Connection | [RFC xxxx] |
| 6 | Session-Group | [RFC xxxx] |
| 7 | Session | [RFC xxxx] |
+-------+-------------------+------------+
Each Overload-Info-Scope has a different encoding, according to the
identifier used to designate the corresponding scope. The formats
for the seven scopes defined in this document are given in the
following section.
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5.4.1. Realm Scope
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | |
+-+-+-+-+-+-+-+-+ Realm (DiameterIdentity) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.4.2. Application-ID Scope
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | Reserved (set to zeros) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Application-ID (Unsigned32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.4.3. Host Scope
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | |
+-+-+-+-+-+-+-+-+ Host (DiameterIdentity) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.4.4. Session Scope
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | |
+-+-+-+-+-+-+-+-+ Session-ID (UTF8String) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.4.5. Connection Scope
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 5 | Reserved (set to zeros) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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5.4.6. Session Group Scope
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 6 | |
+-+-+-+-+-+-+-+-+ Group Name (UTF8String) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.5. Overload-Metric AVP
The Overload-Metric AVP (AVP code 1604) is of type Unsigned32, and is
used as input to the load mitigation algorithm. Its definition and
interpretation is left up to each individual algorithm, with the
exception that an Overload-Metric of "0" always indicates that the
node is not in overload (that is, no load abatement procedures are in
effect) for the indicated scope.
5.6. Period-Of-Validity AVP
The Period-Of-Validity AVP (AVP code 1605) is of type Unsigned32, and
is used to indicate the length of time, in seconds, the Overload-
Metric is to be considered valid (unless overridden by a subsequent
Overload-Metric in the same scope). It MUST NOT be present if the
Overload-Metric is '0', and MUST be present otherwise.
5.7. Session-Group AVP
The Session-Group AVP (AVP code 1606) is of type UTF8String, and is
used to assign a new session to the session group that it names. The
Session-Group AVP MAY appear once in the answer to a session-creating
request, and MUST NOT appear in any other message types.
5.8. Load AVP
The Load AVP (AVP code 1607) is of type Unsigned32, and is used to
indicate the load level of the scope in which it appears. See
Section 3.5 for additional information.
6. Security Considerations
A key concern for recipients of overload metrics and load information
is whether the peer from which the information has been received is
authorized to speak for the indicated scope. For scopes such as
"Host" and "Connection", such authorization is obvious. For other
scopes, such as "Application-ID" and "Realm", the potential for a
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peer to maliciously or accidentally reduce traffic to a third party
is evident. Implementations may choose to ignore indications from
hosts which do not clearly have authority over the indicated scope;
alternately, they may wish to further restrict the scope to apply
only to the host from which the information has been received.
On the other hand, multiple nodes that are under the same
administrative control (or a tightly controlled confederation of
control) may be implicitly trusted to speak for all scopes within
that domain of control. Implementations are encouraged to allow
configuration of inherently trusted servers to which the foregoing
restrictions are not applied.
Open Issue: There are almost certainly other security issues to take
into consideration here. For example, we might need to include
guidance around who gets to see our own load information, and
potentially changing the granularity of information presented based
on trust relationships.
7. IANA Considerations
This document defines new entries in several existing IANA tables.
It also creates two new tables.
7.1. New Diameter AVPs
The following entries are added to the "AVP Codes" table under the
"aaa-parameters" registry.
+----------+---------------------+-----------+
| AVP Code | Attribute Name | Reference |
+----------+---------------------+-----------+
| 1600 | Load-Info | RFC xxxx |
| 1601 | Supported-Scopes | RFC xxxx |
| 1602 | Overload-Algorithm | RFC xxxx |
| 1603 | Overload-Info-Scope | RFC xxxx |
| 1604 | Overload-Metric | RFC xxxx |
| 1605 | Period-Of-Validity | RFC xxxx |
| 1606 | Session-Group | RFC xxxx |
| 1607 | Load | RFC xxxx |
+----------+---------------------+-----------+
7.2. New Diameter Disconnect-Cause
The following entry is added to the "Disconnect-Cause AVP Values
(code 273)" table in the "aaa-parameters" registry:
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+------------------------+---------------------+-----------+
| AVP Values | Attribute Name | Reference |
+------------------------+---------------------+-----------+
| 128 [actual value TBD] | NEGOTIATION_FAILURE | RFC xxxx |
+------------------------+---------------------+-----------+
7.3. New Diameter Response Code
The following entry is added to the "Result-Code AVP Values (code
268) - Transient Failures" table in the "aaa-parameters" registry:
+-------------------------+---------------------------+-----------+
| AVP Values | Attribute Name | Reference |
+-------------------------+---------------------------+-----------+
| 4128 [actual value TBD] | DIAMETER_PEER_IN_OVERLOAD | RFC xxxx |
+-------------------------+---------------------------+-----------+
7.4. New Command Flag
The following entry is added to the "Command Flags" table in the
"aaa-parameters" registry:
+-----+------------+-----------+
| bit | Name | Reference |
+-----+------------+-----------+
| 4 | 'O'verload | RFC xxxx |
+-----+------------+-----------+
7.5. Overload Algorithm Registry
This document defines a new table, to be titled "Overload-Algorithm
Values (code 1602)", in the "aaa-parameters" registry. Its initial
values are to be taken from the table in Section 5.3.
New entries in this table follow the IANA policy of "Specification
Required." (Open Issue: The WG should discuss registration policy to
ensure that we think this is the right balance).
7.6. Overload Scope Registry
This document defines a new table, to be titled "Overload-Info-Scope
Values (code 1603)", in the "aaa-parameters" registry. Its initial
values are to be taken from the table in Section 5.4.
New entries in this table follow the IANA policy of "Specification
Required." (Open Issue: The WG should discuss registration policy to
ensure that we think this is the right balance).
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8. References
8.1. Normative References
[I-D.ietf-dime-overload-reqs]
McMurry, E. and B. Campbell, "Diameter Overload Control
Requirements", draft-ietf-dime-overload-reqs-06 (work in
progress), April 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6733] Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
"Diameter Base Protocol", RFC 6733, October 2012.
8.2. Informative References
[I-D.ietf-soc-overload-control]
Gurbani, V., Hilt, V., and H. Schulzrinne, "Session
Initiation Protocol (SIP) Overload Control",
draft-ietf-soc-overload-control-12 (work in progress),
February 2013.
[I-D.ietf-soc-overload-rate-control]
Noel, E. and P. Williams, "Session Initiation Protocol
(SIP) Rate Control",
draft-ietf-soc-overload-rate-control-04 (work in
progress), April 2013.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[RFC6357] Hilt, V., Noel, E., Shen, C., and A. Abdelal, "Design
Considerations for Session Initiation Protocol (SIP)
Overload Control", RFC 6357, August 2011.
Appendix A. Acknowledgements
This work was inspired by and borrows heavily from the SIP overload
control mechanism described in [I-D.ietf-soc-overload-control]. The
authors of this document are deeply grateful to the editor and
authors of that work, as well as its many contributors.
Thanks to Ben Campbell for significant input to the initial mechanism
design. The author also thanks Martin Dolly, Bob Wallace, John
Gilmore, Matt McCann, Jonathan Palmer, Kedar Karmarkar, Imtiaz
Shaikh, Jouni Korhonen, Uri Baniel, Jianrong Wang, Brian Freeman, and
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Eric Noel for early feedback on the mechanism.
Appendix B. Requirements Analysis
This section analyzes the mechanism described in this document
against the set of requirements detailed in
[I-D.ietf-dime-overload-reqs].
REQ 1: The overload control mechanism MUST provide a communication
method for Diameter nodes to exchange load and overload
information.
Compliant. The mechanism uses new AVPs piggybacked on
existing Diameter messages to exchange load and overload
information.
REQ 2: The mechanism MUST allow Diameter nodes to support overload
control regardless of which Diameter applications they
support. Diameter clients must be able to use the received
load and overload information to support graceful behavior
during an overload condition. Graceful behavior under
overload conditions is best described by REQ 3.
Compliant. Piggybacked AVPs conveying overload control
information is sent on every Diameter message to
compliant peers, without regard to its Application-ID.
The use of the Application-ID scope allows information
relevant to one application to be piggybacked on
messages for other applications.
Information sent to peers includes load and overload
information for use by overload control algorithms,
intended for graceful overload mitigation. The
mechanism is hop-by-hop and has provisions for agents to
forward or aggregate load and overload information
towards clients and servers so that each element can
have appropriate information for graceful overload
control.
REQ 3: The overload control 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.
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Compliant. The mechanism provides information nodes use
to affect the impacts of overload according to agreed
upon algorithms. By controlling or reducing traffic
sent towards overloaded elements, using overload control
information as described in the mechanism, the effects
of overload can be limited. Use of scopes provides a
means to minimize the impact of overload mitigation,
increasing overall useful throughput during overload
conditions.
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.
Compliant. Overload control information can be
piggybacked on any Diameter message. This applies for
requests and answers sent from either side of a
connection.
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.
Compliant. The mechanism makes no assumptions as to how
peers are determined. Discovery of supporting peers is
accomplished as part of the normal capabilities exchange
and does not affect how or where these exchanges occur.
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.
Compliant. The mechanism adds information to the
existing Diameter capabilities exchange mechanism for
determining peer support and overload control
characteristics. Since this is accomplished dynamically
at the start of connections, no provisioning is required
to establish which peers support the mechanism and in
what fashion. Implementations are free to add
configuration for local policy and other control of the
mechanism, but this is not required.
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REQ 7: The overload control mechanism and any associated default
algorithm(s) MUST ensure that the system remains stable. At
some point after an overload condition has ended, the
mechanism MUST enable capacity to stabilize and become equal
to what it would be in the absence of an overload condition.
Note that this also requires that the mechanism MUST allow
nodes to shed load without introducing non converging
oscillations during or after an overload condition.
Compliant. It is possible for an implementation using
this to meet this requirement, and the hop-by-hop nature
limits the impact of overload control actions.
Additional guidance is provided for implementors on
sending of the Overload-Metric and its implications for
the closed loop control system created by this
mechanism.
REQ 8: Supporting nodes MUST be able to distinguish current
overload information from stale information, and SHOULD make
decisions using the most currently available information.
Compliant. The mechanism provides for rapid updates of
overload control information as well as having timeouts
on the validity of overload information that must be
provided by senders.
REQ 9: The mechanism MUST function across fully loaded as well as
quiescent transport connections. This is partially derived
from the requirement for stability in REQ 7.
Compliant. The mechanism uses piggybacked information
transfer, which will generally result in the ability to
transfer information on a similar rate to loading. It
also provides for triggering the use of DWR with
piggybacked information for quiescent connections.
REQ 10: Consumers of overload information MUST be able to determine
when the overload condition improves or ends.
Compliant. The mechanism provides for rapid updates of
overload control information, including abatement
information, as well as mandatory timeouts on the
validity of overload information that must be provided
by senders (it is soft state). Additionally, the
mechanism provides for sending a DWR with piggybacked
information to inform of overload abatement more
quickly.
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REQ 11: The overload control mechanism MUST be able to operate in
networks of different sizes.
Compliant. The hop-by-hop nature of the mechanism
restricts the impacts that large networks might have on
the ability of nodes to deal with overload control
information, as well as restricting the signaling needed
to convey overload information. The use of piggybacked
information transfer limits the additional messaging
imposed by the mechanism for large and small networks
and has the characteristic of scaling with the amount of
Diameter traffic on a network. Additionally, the
dynamic nature of the capabilities exchange reduces the
provisioning burden that can be incurred at large
scales.
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.
Compliant. The mechanism provides for information about
such issues to be conveyed in order for nodes to take
appropriate action to mitigate the situation and prevent
cascades.
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.
Compliant. The mechanism requires sending load and
overload information on all messages exchanged with
compliant peers. It does not, however, require that the
information be recalculated or updated with each
message. The update frequency is up to the
implementation, and each implementation can make
decisions on balancing the update of overload
information along with its other priorities. It is
expected that using a periodically updated grouped AVP
added to all messages sent to compliant peers will not
add substantial additional work. Piggyback base
transport also does not require composition, sending, or
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parsing of new Diameter messages for the purpose of
conveying overload control information.
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 rapid feedback when traffic levels increase.
Compliant. The use of piggybacked information transport
by the mechanism allows for overload control information
to be sent at the same rate as the normal traffic. It
is presumed that the rate of normal traffic will go up
as nodes approach, or enter, overload. Additionally,
DWR messages may be proactively triggered with
piggybacked overload control information to provide
overload control information transfer in an ad hoc
fashion.
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.
Compliant. The mechanism does not require interaction
with any underlying congestion control. It relies
solely on piggybacked transport and does not request or
recommend changes in how the underlying connections are
performed.
REQ 16: The overload control mechanism is likely to be deployed
incrementally. The mechanism MUST support a mixed
environment where some, but not all, nodes implement it.
Compliant. The mechanism specifies behavior for dealing
with non-supporting elements.
REQ 17: In a mixed environment with nodes that support the overload
control mechanism and that do not, the mechanism MUST result
in at least as much useful throughput as would have resulted
if the mechanism were not present. It SHOULD result in less
severe congestion in this environment.
Compliant. When dealing with supporting, and non-
supporting nodes, the mechanism specifies behavior that
attempts to apply relevant information to decisions on
sending to non-compliant hosts. This behavior should
result in reductions in traffic that increase the
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likelihood of successful overload mitigation in mixed
networks.
REQ 18: In a mixed environment of nodes that support the overload
control mechanism and that do not, the mechanism MUST NOT
preclude elements that support overload control from
treating elements that do not support overload control in a
equitable fashion relative to those that do. Users and
operators of nodes that do not support the mechanism MUST
NOT unfairly benefit from the mechanism. The mechanism
specification SHOULD provide guidance to implementors for
dealing with elements not supporting overload control.
Compliant. When dealing with supporting, and non-
supporting nodes, the mechanism specifies behavior that
attempts to apply relevant information to decisions on
sending to non-compliant hosts. This allows nodes to
treat non-supporting elements in a similar, and fair,
fashion relative to non-supporting elements.
REQ 19: It MUST be possible to use the mechanism between nodes in
different realms and in different administrative domains.
Compliant. Scoping of overload information to realms is
explicitly specified by the mechanism. There are no
requirements imposed by the mechanism that would prevent
overload control information from crossing between
adjacent nodes that were in separate administrative
domains.
REQ 20: Any explicit overload indication MUST be clearly
distinguishable from other errors reported via Diameter.
Compliant. A new grouped AVP conveys all overload
control information, and this is transported on existing
messages that are not related to overload control. No
existing Diameter error codes are used by the mechanism.
One new transient error code is defined by the
mechanism.
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 result in at least as much
useful throughput as would have resulted if the overload
control mechanism was not in place.
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Compliant. Procedures are defined cases where
supporting nodes become too overloaded to send overload
information. No retries or sending of additional
messages are required during overload that would reduce
useful throughput in these situations.
REQ 22: The mechanism MUST provide a way for a node to throttle the
amount of traffic it receives from a peer node. This
throttling SHOULD be graded so that it can be applied
gradually as offered load increases. Overload is not a
binary state; there may be degrees of overload.
Compliant. The mechanism provides a 32 bit overload
severity indication. Interpretation of the value is
specific to the algorithm being employed. In the case
of the mandatory to implement loss algorithm, the values
0-100 are used to progressively control the amount of
traffic dropped.
REQ 23: 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.
Compliant. The mechanism provides information so that a
load balancing node can determine that an upstream node
is in overload. Additionally, it provides load
information that can be used as input for balancing
decisions.
REQ 24: 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.
Compliant. The mechanism provides load information in
each message as well as guidelines for implementing the
determination of load to be sent.
REQ 25: The base specification for the overload control mechanism
SHOULD offer general guidance on which message types might
be desirable to send or process over others during times of
overload, based on application-specific considerations. For
example, it may be more beneficial to process messages for
existing sessions ahead of new sessions. Some networks may
have a requirement to give priority to requests associated
with emergency sessions. Any normative or otherwise
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detailed definition of the relative priorities of message
types during an overload condition will be the
responsibility of the application specification.
Compliant. Some guidance is provided for priority
selection and how to deal with different priority
messages is described in an example algorithm
implementation.
REQ 26: 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, not throttled, or processed ahead of others.
Compliant. The mechanism does not place restrictions on
how decisions are made to prioritize messages.
REQ 27: The overload control 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. Note that the Diameter base
specification [RFC6733] lacks end to end security and this
must be considered.
Compliant. The hop-by-hop nature of the mechanism
allows existing Diameter security mechanisms to be used
for securing the connections between peers. ***Detailed
analysis by persons with security expertise would be
beneficial.***
REQ 28: 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.
Compliant. Using a hop-by-hop mechanism limits the
scope of potentially malicious information. Guidance is
provided for trust, in particular relative to scopes.
Additional specification around trust relationships
could be useful to clarify authorization of overload
control information. ***Detailed analysis by persons
with security expertise would be beneficial.***
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REQ 29: It MUST be possible for a supporting node to make
authorization decisions about what information will be sent
to peer nodes based on the identity of those nodes. This
allows a domain administrator who considers the load of
their nodes to be sensitive information to restrict access
to that information. Of course, in such cases, there is no
expectation that the overload control mechanism itself will
help prevent overload from that peer node.
Compliant. The mechanism provides guidance for
authorization decisions and takes no action to restrict
local policy when dealing with authorization.
***Detailed analysis by persons with security expertise
would be beneficial.***
REQ 30: 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.
Compliant. The mechanism allows for local policy
overrides for the bulk of its behavior.
REQ 31: There are multiple situations where a Diameter node may be
overloaded for some purposes but not others. For example,
this can happen to an agent or server that supports multiple
applications, or when a server depends on multiple external
resources, some of which may become overloaded while others
are fully available. The mechanism MUST allow Diameter
nodes to indicate overload with sufficient granularity to
allow clients to take action based on the overloaded
resources without unreasonably forcing available capacity to
go unused. The mechanism MUST support specification of
overload information with granularities of at least
"Diameter node", "realm", and "Diameter application", and
MUST allow extensibility for others to be added in the
future.
Compliant. The mechanism allows for flexible
specification on the scope that overload control
information applies to. It also allows for additional
scopes to be specified as extensions.
REQ 32: The mechanism MUST provide a method for extending the
information communicated and the algorithms used for
overload control.
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Compliant. The mechanism allows for new algorithms to
be specified as extensions. It provides an AVP for
communicating overload information that can be
interpreted differently by different algorithms. It
also provides for extension of information transmitted.
REQ 33: The mechanism MUST provide a default algorithm that is
mandatory to implement.
Compliant. The mechanism specifies the drop algorithm
as mandatory to implement.
REQ 34: 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.
Not Compliant. Additional analysis is needed.
Appendix C. Extending the Overload Mechanism
This specification includes two key extension points to allow for new
behaviors to be smoothly added to the mechanism in the future. The
following sections discuss the means by which future documents are
expected to extend the mechanism.
C.1. New Algorithms
In order to provide the ability for different means of traffic
abatement in the future, this specification allows for descriptions
of new traffic reduction algorithms. In general, documents that
define new algorithms need to describe externally-observable node
behavior in sufficient detail as to allow interoperation.
At a minimum, such description needs to include:
1. The name and IANA-registered number for negotiating the algorithm
(see Section 5.3).
2. A clear description of how the Overload-Metric AVP is to be
interpreted, keeping in mind that "0" is reserved to indicate
that no overload condition exists.
3. An example, proof-of-concept description (preferably in pseudo-
code) of how nodes can implement the algorithm.
New algorithms must be capable of working with all applications, not
just a subset of applications.
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It is generally expected that new algorithms will make use of the
available overload control information as specified in this document.
However, if additional information is needed, the Load-Info AVP
allows for additional optional AVPs to be included. It is
recommended that designers of any new AVPs defined for this purpose
consider reusing existing AVPs first, and also design their AVPS so
that they may be reused by others when possible.
C.2. New Scopes
Because it is impossible to foresee all the potential constructs that
it might be useful to scope operations to for the purposes of
overload, we allow for the registration of new scopes.
At a minimum, such description needs to include:
1. The name and IANA-registered number for negotiating and
indicating the scope (see Section 5.4).
2. A syntax for the "Details" field of the Overload-Info-Scope AVP,
preferably derived from one of the base Diameter data types.
3. An explicit and unambiguous description of how both parties to
the overload control mechanism can determine which transactions
correspond to the indicated scope.
4. A clear and exhaustive list that extends the one in Section 2.2,
indicating exactly which combinations of scopes are allowed with
the new scope. This list must take into account all of the IANA-
registered scopes at the time of its publication.
It is acceptable for new scopes to be specific to constructs within
one or several applications. In other words, it may be desirable to
define scopes that can be applied to one kind of application while
not making sense for another. Extension documents should be very
clear that such is the case, however, if they choose to do so.
Appendix D. Design Rationale
The current design proposed in this document takes into account
several trade-offs and requirements that may not be immediately
obvious. The remainder of this appendix highlights some of the
potentially more controversial and/or non-obvious of these, and
attempts to explain why such decisions were made they way they were.
That said, none of the following text is intended to represent a line
in the sand. All of the decisions can be revisited if necessary,
especially if additional facts are brought into the analysis that
change the balance of the decisions.
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D.1. Piggybacking
The decision to piggyback load information on existing messages
derives primarily from REQ 14 in [I-D.ietf-dime-overload-reqs]: "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."
If we were to introduce new messaging -- say, by defining a new
overload control Application -- then a node in overload would be
required to generate more messages at high load in order to keep
overload information in its peers up-to-date.
If further analysis determines that other factors are ultimately more
important than the provisions of REQ 14, several factors would need
to be considered.
First and foremost would be the prohibition, in the base Diameter
specification ([RFC6733]), against adding new commands to an existing
application. Specifically, section 1.3.4 stipulates: "a new Diameter
application MUST be created when one or more of the following
criteria are met:... A new command is used within the existing
application either because an additional command is added, an
existing command has been modified so that a new Command Code had to
be registered, or a command has been deleted." Because of this
stipulation, the addition of new command codes to existing
applications would require registration of entirely new application
IDs for those applications to support overload control. We consider
this to be too disruptive a change to consider.
By the author's reading, there is no provision that exempts the
"Diameter Common Messages" Application (Application ID 0) from the
above clauses. This effectively prohibits the additional of new
messages to this Application. While it may be theoretically possible
to specify behavior that hijacks the DWR/DWA watchdog messages for
the purpose of overload control messaging, doing so requires a
complete redefinition of their behavior and, fundamentally, their
semantics. This approach seems, at first blush, to be an
unacceptable change to the base Application.
The remaining approach -- defining a new application for overload
control -- has some promise, if we decide not to fulfill REQ 14. It
remains to be seen whether the users of the Diameter protocol,
including other SDOs who define applications for Diameter, are
willing to specify the use of multiple Diameter Applications for use
on a single reference point.
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D.2. Load AVP in All Packets
Some have questioned the currently specified behavior of message
senders including a Load AVP in every message sent. This is being
proposed as a potential performance enhancement, with the idea being
that message recipients can save processing time by examining
arbitrarily selected messages for load information, rather than
looking for a Load AVP in every message that arrives. Of course, to
enable this kind of sampling, the Load AVP must be guaranteed to be
present; otherwise, attempts to find it will occasionally fail.
The reciprocal approach, of sending a Load AVP only when the Load has
changed (or changed by more than a certain amount), requires the
recipient to search through the Load-Info grouped AVP in every
message received in order to determine whether a Load AVP is present.
On a cursory analysis, we determined that appending a Load AVP to
each message is fundamentally a cheaper operation than traversing the
contents of each Load-Info AVP to determine whether a Load AVP is
present.
If a later decision is made to require examination of each message to
determine whether it include a Load AVP, we may be able to obtain
some efficiencies by requiring Load to be the first AVP in the Load-
Info AVP.
D.3. Graceful Failure
Some commenters have raised the question of whether a node can reject
an incoming connection upon recognizing that the remote node does not
support the Diameter overload control mechanism. One suggestion has
been to add a response code to indicate exactly such a situation.
So far, we have opted against doing so. Instead, we anticipate an
incremental deployment of the overload control mechanism, which will
likely consist of a mixture of nodes that support and node that do
not support the mechanism. Were we to allow the rejection of
connections that do not support the mechanism, we would create a
situation that necessitates a "flag day," on which every Diameter
node in a network is required to simultaneously, and in perfect
synchronization, switch from not supporting the overload mechanism,
to supporting it.
Given the operational difficulty of the foregoing, we have decided
that defining a response code, even if optional, that was to be used
to reject connections merely for the lack of overload control
support, would form an attractive nuisance for implementors. The
result could easily be a potential operational nightmare for network
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operators.
Authors' Addresses
Adam Roach
Mozilla
Dallas, TX
US
Email: adam@nostrum.com
Eric McMurry
Tekelec
Dallas, TX
US
Email: emcmurry@computer.org
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