| Diameter Maintenance and Extensions (DIME) | J. Korhonen, Ed. |
| Internet-Draft | Broadcom |
| Intended status: Standards Track | S. Donovan |
| Expires: May 26, 2014 | B. Campbell |
| Oracle | |
| November 22, 2013 |
Diameter Overload Indication Conveyance
draft-ietf-dime-ovli-00.txt
This specification documents a Diameter Overload Control (DOC) base solution and the dissemination of the overload report information.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
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 working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
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This Internet-Draft will expire on May 26, 2014.
Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.
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This specification defines a base solution for the Diameter Overload Control (DOC). The requirements for the solution are described and discussed in the corresponding design requirements document [I-D.ietf-dime-overload-reqs]. Note that the overload control solution defined in this specification does not address all the requirements listed in [I-D.ietf-dime-overload-reqs]. A number of overload control related features are left for the future specifications. See Appendix A for more detailed discussion on those.
The solution defined in this specification addresses the Diameter overload control between two endpoints (see Section 5.1). Furthermore, the solution is designed to apply to existing and future Diameter applications, requires no changes to the Diameter base protocol [RFC6733] and is deployable in environments where some Diameter nodes do not implement the Diameter overload control solution defined in this specification.
This section describes the high-level architectural and semantic assumptions that underly the Diameter Overload Control Mechanism.
The following is a classification of Diameter applications and requests. This discussion is meant to document factors that play into decisions made by the Diameter identity responsible for handling overload reports.
Section 8.1 of [RFC6733] defines two state machines that imply two types of applications, session-less and session-based. The primary differentiator between these types of applications is the lifetime of Session-IDs.
For session-based applications, the session-id is used to tie multiple requests into a single session.
In session-less applications, the lifetime of the session-id is a single Diameter transaction.
The 3GPP-defined S6a application is an example of a session-less application. The following, copied from section 7.1.4 of 29.272, explicitly states that sessions are implicitly terminated and that the server does not maintain session state:
For the purposes of this discussion, session-less applications are further divided into two types of applications:
[OpenIssue: Do we assume that all requests in a pseudo-session typically need to go to the same server?]
The accounting application defined in [RFC6733] and the Credit-Control application defined in [RFC4006] are examples of Diameter session-based applications.
The handling of overload reports must take the type of application into consideration, as discussed in Section 3.1.2.
This section discusses considerations for mitigating overload reported by a Diameter entity. This discussion focuses on the type of application. Section 3.1.3 discusses considerations for handling various request types when the target server is known to be in an overloaded state. Section 3.1.5 discusses considerations for handling overload conditions based on the network deployment scenario.
These discussions assume that the strategy for mitigating the reported overload is to reduce the overall workload sent to the overloaded entity. The concept of applying overload treatment to requests targeted for an overloaded Diameter entity is inherent to this discussion. The method used to reduce offered load is not specified here but could include routing requests to another Diameter entity known to be able to handle them, or it could mean rejecting certain requests. For a Diameter agent, rejecting requests will usually mean generating appropriate Diameter error responses. For a Diameter client, rejecting requests will depend upon the application. For example, it could mean giving an indication to the entity requesting the Diameter service that the network is busy and to try again later.
The request classes identified in Section 3.1.3 have implications on decisions about which requests should be throttled first.
This section discusses various Diameter network deployment scenarios and the implications of those deployment models on handling of overload reports.
The scenarios vary based on the following:
Without consideration for which elements support the DOC extension, the following is a representative list of deployment scenarios:
[OpenIssue: Do the "multiple equivalent servers" cases change for session-stateful applications? Do we need to distinguish equivalence for session-initiation requests vs intra-session requests?]
The following is a list of representative DOC deployment scenarios:
[OpenIssue: In the last 3 list entries, are the agents DOC or non-DOC?]
In the context of the Diameter Overload Indication Conveyance (DOIC) and reacting to the overload information, the functional behaviour of Diameter agents in front of servers, especially Diameter proxies, needs to be common. This is important because agents may actively participate in the handling of an overload conditions. For example, they may make intelligent next hop selection decisions based on overload conditions, or aggregate overload information to be disseminated downstream. Diameter agents may have other deployment related tasks that are not defined in the Diameter base protocol [RFC6733]. These include, among other tasks, topology hiding, and acting as a server front end for a server farm of real Diameter servers.
Since the solution defined in this specification must not break the Diameter base protocol [RFC6733] at any time, great care has to be taken not to assume functionality from the Diameter agents that would break base protocol behavior, or to assume agent functionality beyond the Diameter base protocol. Effectively this means the following from a Diameter agent:
[OpenIssue: We've discussed multiple situations where an agent might insert an OLR. I don't think we mean to force them to always perform topology hiding or SFIM in order to do so. We cannot assume that an OLR is always "from" or "about" the Origin-Host. Also, the section seems to assume that topology hiding agents act as b2b overload agents, but non-topology hiding agents never do. It don't think that's the right abstraction. It's possible that topology-hiding agents must do this, but I don't think we can preclude non-topology hiding agents from also doing it, at least some of the time.]
Figure 1 illustrates the simplified architecture for Diameter overload control. See Section 5.1 for more discussion and details how different Diameter nodes fit into the architecture from the DOIC point of view.
Realm X Other Realms
<--------------------------------------> <---------------------->
+--^-----+ : (optional) :
|Diameter| : :
|Server A|--+ .--. : +---^----+ : .--.
+--------+ | _( `. : |Diameter| : _( `. +---^----+
+--( )--:-| Agent |-:--( )--|Diameter|
+--------+ | ( ` . ) ) : +-----^--+ : ( ` . ) ) | Client |
|Diameter|--+ `--(___.-' : : `--(___.-' +-----^--+
|Server B| : :
+---^----+ : :
Overload Indication A Overload Indication A'
1) <----------------------> <---------------------->
standard base protocol standard base protocol
End-to-end Overload Indication
2) <----------------------------------------------->
standard base protocol
Figure 1: Simplified architecture choices for overload indication delivery
The following features describe new Diameter AVPs used for sending overload reports, and for declaring support for certain DOC features.
Since the Diameter overload control mechanism is also designed to work over existing application (i.e., the piggybacking principle), a proper negotiation is hard to accomplish. The "capability negotiation" is based on the existense of specific non-mandatory APV, such as the OC-Feature-Vector AVP (see Section 4.1. Although the OC-Feature-Vector AVP can be used to advertise a certain set of new or existing Diameter overload control capabilities, it is not a versioning solution per se, however, it can be used to achieve the same result.
The Diameter overload control APVs SHOULD always be sent as an optional AVPs. This requirement stems from the fact that piggybacking overload control information on top of existing application cannot really use AVPs with the M-bit set. However, there are certain exceptions as explained in Section 5.4.
From the Diameter overload control functionality point of view, the "Reacting node" is always the requester of the overload report information and the "Reporting node" is the provider of the overload report. The overload report or the capability information in the request message is always interpreted as an announcement of a "capability". The overload report and the capability information in the answer is always interpreted as a report of supported commond functionality and as a status report of an overload condition (of a node).
Diameter nodes can request a reduction in offered load by indicating an overload condition in the form of an overload report. The overload report contains information about how much load should be reduced, and may contain other information about the overload condition. This information is encoded in Diameter Attribute Value Pairs (AVPs).
Certain new AVPs may also be used to declare certain DOIC capabilities and extensions.
This section describes the encoding and semantics of Overload Indication Attribute Value Pairs (AVPs).
The OC-Feature-Vector AVP (AVP code TBD1) is type of Unsigned64 and contains a 64 bit flags field of announced capabilities of an overload control endpoint. Sending or receiving the OC-Feature-Vector AVP with the value 0 indicates that the endpoint only support the capabilities defined in this specification.
An overload control endpoint (a reacting node) includes this AVP to indicate its capabilities to the other overload control endpoint (the reporting node). For example, the endpoint (reacting node) may indicate which (future defined) traffic abatement algorithms it supports in addition to the default.
During the message exchange the overload control endpoints express their common set of supported capabilities. The endpoint sending a request (the reacting node) includes the OC-Feature-Vector AVP with those flags set that correspond what it supports. The endpoint that sends the answer (the reporting node) also includes the OC-Feature-Vector AVP with flags set to describe the capabilities it both supports and agrees with the request sender (e.g., based on the local policy and/or configuration). The answer sending endpoint (the reporting node) does not need to advertise those capabilities it is not going to use with the request sending endpoint (the reacting node).
This specification does not define any additional capability flag. The implicity capability (all flags set to zero) indicates the support for this specification only.
The OC-OLR AVP (AVP code TBD2) is type of Grouped and contains the necessary information to convey an overload report. OC-OLR may also be used to convey additional information about an extension that is declared in the OC-Feature-Vector AVP.
The OC-OLR AVP does not contain explicit information to which application it applies to and who inserted the AVP or whom the specific OC-OLR AVP concerns to. Both these information is implicitly learned from the encapsulating Diameter message/command. The application the OC-OLR AVP applies to is the same as the Application-Id found in the Diameter message header. The identity the OC-OLR AVP concerns is determined from the Origin-Host AVP found from the encapsulating Diameter command.
OC-OLR ::= < AVP Header: TBD2 >
< TimeStamp >
[ Reduction-Percentage ]
[ ValidityDuration ]
[ ReportType ]
* [ AVP ]
The TimeStamp AVP indicates when the original OC-OLR AVP with the current content was created. It is possible to replay the same OC-OLR AVP multiple times between the overload endpoints, however, when the OC-OLR AVP content changes or the other information sending endpoint wants the receiving endpoint to update its overload control information, then the TimeStamp AVP MUST contain a new value.
[OpenIssue: Is this necessarily a timestamp, or is it just a sequence number that can be implemented as a timestamp? Is this timestamp used to calculate expiration time? (propose no.). We should also consider whether either a timestamp or sequence number is needed for protection against replay attacks.]
The TimeStamp AVP (AVP code TBD3) is type of Time. Its usage in the context of the overload control is described in Section 4.2. From the functionality point of view, the TimeStamp AVP is merely used as a non-volatile increasing counter between two overload control endpoints.
The ValidityDuration AVP (AVP code TBD4) is type of Unsigned32 and describes the number of seconds the OC-OLR AVP and its content is valid since the creation of the OC-OLR AVP (as indicated by the TimeStamp AVP).
A timeout of the overload report has specific concerns that need to be taken into account by the endpoint acting on the earlier received overload report(s). Section 4.6 discusses the impacts of timeout in the scope of the traffic abatement algorithms.
As a general guidance for implementations it is RECOMMENDED never to let any overload report to timeout. Rather, an overload endpoint should explicitly signal, e.g. the end of overload condition. This leaves no need for the other overload endpoint to reason or guess the condition the other endpoint is at.
The ReportType AVP (AVP code TBD5) is type of Enumerated. The value of the AVP describes what the overload report concerns. The following values are initially defined:
The ReportType AVP is envisioned to be useful for situations where a reacting node needs to apply different overload treatments for different "types" of overload. For example, the reacting node(s) might need to throttle requests that are targeted to a specific server by the presence of a Destination-Host AVP than for requests that can be handled by any server in a realm. The example in Appendix C.3 illustrates this usage.
[OpenIssue: There is an ongoing discussion about whether the ReportType AVP is the right way to solve that issue, and whether it's needed at all.]
The Reduction-Percentage AVP (AVP code TBD8) is type of Unsigned32 and describes the percentage of the traffic that the sender is requested to reduce, compared to what it otherwise would have sent.
The value of the Reduction-Percentage AVP is between zero (0) and one hundred (100). Values greater than 100 are interpreted as 100. The value of 100 means that no traffic is expected, i.e. the sender of the information is under a severe load and ceases to process any new messages. The value of 0 means that the sender of the information is in a stable state and has no requests to the other endpoint to apply any traffic abatement.
[Open Issue: We should consider an algorithm independent way to end an overload condition. Perhaps setting the validitytime to zero? Counter comment; since the abatement is based on a specific algorithm, it is natural to indicate that from the abatement algorithm point of view status quo has been reached.]
If an overload control endpoint comes out of the 100 percent traffic reduction as a result of the overload report timing out, the following concerns are RECOMMENDED to be applied. The endpoint sending the traffic should be conservative and, for example, first send few "probe" messages to learn the overload condition of the other endpoint before converging to any traffic amount/rate decided by the sender. Similar concerns actually apply in all cases when the overload report times out unless the previous overload report stated 0 percent reduction.
[Open Issue: It is still open whether we need an AVP to indicate the exact used traffic abatement algorithm. Currently it assumed that the reacting node is able to figure out what to do based on the Reducttion-Percentage AVP and possible other embedded information inside the OC-OLR AVP.]
+---------+
|AVP flag |
|rules |
+----+----+
AVP Section | |MUST|
Attribute Name Code Defined Value Type |MUST| NOT|
+--------------------------------------------------+----+----+
|OC-Feature-Vector TBD1 x.x Unsigned64 | | V |
+--------------------------------------------------+----+----+
|OC-OLR TBD2 x.x Grouped | | V |
+--------------------------------------------------+----+----+
|TimeStamp TBD3 x.x Time | | V |
+--------------------------------------------------+----+----+
|ValidityPeriod TBD4 x.x Unsigned32 | | V |
+--------------------------------------------------+----+----+
|ReportType TBD5 x.x Enumerated | | V |
+--------------------------------------------------+----+----+
|Reduction | | |
| -Percentage TBD8 x.x Unsigned32 | | V |
+--------------------------------------------------+----+----+
The overload control solution can be considered as an overlay on top of an arbitrary Diameter network. The overload control information is exchanged over on a "DOIC association" between two communicatin endpoints. The endpoints, namely the "reacting node" and the "reporting node" do not need to be adjacent Diameter peer nodes, nor they need to be the end-to-end Diameter nodes in a typical "client-server" deployment with multiple intermediate Diameter agent nodes in between. The overload control endpoint are the two Diameter nodes that decide to exchange overload control information between each other. How the endpoints are determined is specific to a deployment, a Diameter node role in that deployment and local configuration.
The following diagrams illustrate the concept of Diameter Overload End-Points and how they differ from the standard [RFC6733] defined client, server and agent Diameter nodes. The following is the key to the elements in the diagrams:
Figure 2 illustrates the most basic configuration where a client is connected directly to a server. In this case, the session and association are both between the client and server.
+-----+ +-----+
| C | | S |
+-----+ +-----+
| DEP | | DEP |
+--+--+ +--+--+
| |
| |
|{Diameter Session}|
| |
|{DOIC Association}|
| |
Figure 2: Basic DOIC deployment
In Figure 3 there is an agent that is not participating directly in the exchange of overload reports. As a result, the DOIC association is still between the client and the server.
+-----+ +-----+ +-----+
| C | | A | | S |
+-----+ +--+--+ +-----+
| DEP | | | DEP |
+--+--+ | +--+--+
| | |
| | |
|----------{Diameter Session}---------|
| | |
|----------{DOIC Association}---------|
| | |
Figure 3: DOIC deployment with non participating agent
Figure 4 illustrates the case where the client does not support Diameter overload. In this case, the DOIC association is between the agent and the server. The agent handles the role of the reactor for overload reports generated by the server.
+-----+ +-----+ +-----+
| C | | A | | S |
+--+--+ +-----+ +-----+
| | DEP | | DEP |
| +--+--+ +--+--+
| | |
| | |
|----------{Diameter Session}---------|
| | |
| |{DOIC Association}|
| | |
Figure 4: DOIC deployment with non-DOIC client and DOIC enabled agent
In Figure 5 there is a DOIC association between the client and the agent and a second DOIC association between the agent and the server. One use case requiring this configuration is when the agent is serving as a SFE/SFIM for a set of servers.
+-----+ +-----+ +-----+
| C | | A | | S |
+-----+ +-----+ +-----+
| DEP | | DEP | | DEP |
+--+--+ +--+--+ +--+--+
| | |
| | |
|----------{Diameter Session}---------|
| | |
|{DOIC Association}|{DOIC Association}|
| | |
Figure 5: A deployment where all nodes support DOIC
Figure 6 illustrates a deployment where some clients support Diameter overload control and some do not. In this case the agent must support Diameter overload control for the non supporting client. It might also need to have a DOIC association with the server, as shown here, to handle overload for a server farm and/or for managing Realm overload.
+-----+ +-----+ +-----+ +-----+
| C1 | | C2 | | A | | S |
+-----+ +--+--+ +-----+ +-----+
| DEP | | | DEP | | DEP |
+--+--+ | +--+--+ +--+--+
| | | |
| | | |
|-------------------{Diameter Session}-------------------|
| | | |
| |--------{Diameter Session}-----------|
| | | |
|---------{DOIC Association}----------|{DOIC Association}|
| | | |
Figure 6: A deployment with DOIC and non-DOIC supporting clients
Figure 7 illustrates a deployment where some agents support Diameter overload control and others do not.
+-----+ +-----+ +-----+ +-----+
| C | | A | | A | | S |
+-----+ +--+--+ +-----+ +-----+
| DEP | | | DEP | | DEP |
+--+--+ | +--+--+ +--+--+
| | | |
| | | |
|-------------------{Diameter Session}-------------------|
| | | |
| | | |
|---------{DOIC Association}----------|{DOIC Association}|
| | | |
Figure 7: A deployment with DOIC and non-DOIC supporting agents
The overload control solution defined AVPs are essentially piggybacked on top of existing application message exchanges. This is made possible by adding overload control top level AVPs, the OC-OLR AVP and the OC-Feature-Vector AVP into existing commands (this has an assumption that the application CCF allows adding new AVPs into the Diameter messages.
In a case of newly defined Diameter applications, it is RECOMMENDED to add and defined how overload control mechanisms works on that application. using OC-Feature-Vector and OC-OLR AVPs in a non-mandatory manner is intended only existing applications.
Note that the overload control solution does not have fixed server and client roles. The endpoint role is determined based on the sent message type: whether the message is a request (i.e. sent by a "reacting node") or an answer (i.e. send by a "reporting node"). Therefore, in a typical "client-server" deployment, the "client" MAY report its overload condition to the "server" for any server initiated message exchange. An example of such is the server requesting a re-authentication from a client.
Since the overload control solution relies on the piggybacking principle for the overload reporting and the overload control endpoint are likely not adjacent peers, finding out whether the other endpoint supports the overload control or what is the common traffic abatement algorithm to apply for the traffic. The approach defined in this specification for the end-to-end capability capability announcement relies on the exchange of the OC-Feature-Vector between the endpoints. The feature announcement solution also works when carried out on existing applications. For the newly defined application the negotiation can be more exact based on the application specification. The announced set of capabilities MUST NOT change during the life time of the Diameter session (or transaction in a case of non-session maintaining applications).
The basic principle is that the request message initiating endpoint (i.e. the "reacting node") announces its support for the overload control mechanism by including in the request message the OC-Feature-Vector AVP with those capability flag bits set that it supports and is willing to use for this Diameter session (or transaction in a case of a non-session state maintaining applications). In a case of session maintaining applications the request message initiating endpoint does not need to do the capability announcement more than once for the lifetime of the Diameter session. In a case of non-session maintaining applications, it is RECOMMENDED that the request message initiating endpoint includes the capability announcement into every request regardless it has had prior message exchanges with the give remote endpoint.
[OpenIssue: We need to think about the lifetime of a capabilities declaration. It's probably not the same as for a session. We have had proposals that the feature vector needs to go into every request sent by an OC node. For peer to peer cases, this can be associated with connection lifetime, but it's more complex for non-adjacent OC support.]
Once the endpoint that initiated the request message receives an answer message from the remote endpoint, it can detect from the received answer message whether the remote endpoint supports the overload control solution and in a case it does, what features are supported. The support for the overload control solution is based on the presence of the OC-Feature-Vector AVP in the Diameter answer for existing application. For the newly defined applications the support for the overload control MAY already be part of the application specification. Based on capability knowledge the request message initiating endpoint can select the preferred common traffic abatement algorithm and act accordingly for the subsequent message exchanges.
When a remote endpoint (i.e. a "reporting node") receives a request message in can detect whether the request message initiating endpoint has support for the overload control solution based on the presence of the OC-Feature-Vector AVP. For the newly defined applications the overload control solution support can be part of the application specification. Based on the content of the OC-Feature-Vector AVP the request message receiving endpoint knows what overload control functionality the other endpoint supports and then act accordingly for the subsequent answer messages it initiates. It is RECOMMENDED that the answer message initiating endpoint selects one common traffic abatement algorithm even if it would support multiple. The answer message initiating endpoint MUST NOT include any overload control solution defined AVPs into its answer messages if the request message initiating endpoint has not indicated support at the beginning of the the created session (or transaction in a case of non-session state maintaining applications).
The overload control solution can be extended, e.g. with new traffic abatement algorithms or new functionality. The new features and algorithms MUST be registered with the IANA and for the ppossible use with the OC-Feature-Vector for announcing the support for the new features (see Section 7 for the required procedures).
It should be noted that [RFC6733] defined Grouped AVP extension mechanisms also apply. This allows, for example, defining a new feature that is mandatory to understand even when piggybacked on an existing applications. More specifically, the sub-AVPs inside the OC-OLR AVP MAY have the M-bit set. However, when overload control AVPs are piggybacked on top of an existing applications, setting M-bit in sub-AVPs is NOT RECOMMENDED.
[OpenIssue: did we now agree that e.g. a server can refrain sending OLR in answers based on some magical algorithm? (Note: We seem to have consensus that a server MAY repeat OLRs in subsequent messages, but is not required to do so, based on local policy.)]
In order to reduce overload control introduced additional AVP and message processing it might be desirable/beneficial to signal whether the Diameter command carries overload control information that should be of interest of an overload aware Diameter node.
Should such indication be include is not part of this specification. It has not either been concluded at what layer such possible indication should be. Obvious candidates include transport layer protocols (e.g., SCTP PPID or TCP flags) or Diameter command header flags.
New AVPs defined by this specification are listed in Section 4. All AVP codes allocated from the 'Authentication, Authorization, and Accounting (AAA) Parameters' AVP Codes registry.
Three new registries are needed under the 'Authentication, Authorization, and Accounting (AAA) Parameters' registry.
Section 4.1 defines a new "Overload Control Feature Vector" registry including the initial assignments. New values can be added into the registry using the Specification Required policy [RFC5226].
Section 4.5 defines a new "Overload Report Type" registry with its initial assignments. New types can be added using the Specification Required policy [RFC5226].
This mechanism gives Diameter nodes the ability to request that downstream nodes send fewer Diameter requests. Nodes do this by exchanging overload reports that directly affect this reduction. This exchange is potentially subject to multiple methods of attack, and has the potential to be used as a Denial-of-Service (DoS) attack vector.
Overload reports may contain information about the topology and current status of a Diameter network. This information is potentially sensitive. Network operators may wish to control disclosure of overload reports to unauthorized parties to avoid its use for competitive intelligence or to target attacks.
Diameter does not include features to provide end-to-end authentication, integrity protection, or confidentiality. This may cause complications when sending overload reports between non-adjacent nodes.
The Diameter protocol involves transactions in the form of requests and answers exchanged between clients and servers. These clients and servers may be peers, that is,they may share a direct transport (e.g. TCP or SCTP) connection, or the messages may traverse one or more intermediaries, known as Diameter Agents. Diameter nodes use TLS, DTLS, or IPSec to authenticate peers, and to provide confidentiality and integrity protection of traffic between peers. Nodes can make authorization decisions based on the peer identities authenticated at the transport layer.
When agents are involved, this presents an effectively hop-by-hop trust model. That is, a Diameter client or server can authorize an agent for certain actions, but it must trust that agent to make appropriate authorization decisions about its peers, and so on.
Since confidentiality and integrity protection occurs at the transport layer. Agents can read, and perhaps modify, any part of a Diameter message, including an overload report.
There are several ways an attacker might attempt to exploit the overload control mechanism. An unauthorized third party might inject an overload report into the network. If this third party is upstream of an agent, and that agent fails to apply proper authorization policies, downstream nodes may mistakenly trust the report. This attack is at least partially mitigated by the assumption that nodes include overload reports in Diameter answers but not in requests. This requires an attacker to have knowledge of the original request in order to construct a response. Therefore, implementations SHOULD validate that an answer containing an overload report is a properly constructed response to a pending request prior to acting on the overload report.
A similar attack involves an otherwise authorized Diameter node that sends an inappropriate overload report. For example, a server for the realm "example.com" might send an overload report indicating that a competitor's realm "example.net" is overloaded. If other nodes act on the report, they may falsely believe that "example.net" is overloaded, effectively reducing that realm's capacity. Therefore, it's critical that nodes validate that an overload report received from a peer actually falls within that peer's responsibility before acting on the report or forwarding the report to other peers. For example, an overload report from an peer that applies to a realm not handled by that peer is suspect.
An attacker might use the information in an overload report to assist in certain attacks. For example, an attacker could use information about current overload conditions to time a DoS attack for maximum effect, or use subsequent overload reports as a feedback mechanism to learn the results of a previous or ongoing attack.
Diameter overload reports can cause a node to cease sending some or all Diameter requests for an extended period. This makes them a tempting vector for DoS tacks. Furthermore, since Diameter is almost always used in support of other protocols, a DoS attack on Diameter is likely to impact those protocols as well. Therefore, Diameter nodes MUST NOT honor or forward overload reports from unauthorized or otherwise untrusted sources.
When a Diameter node sends an overload report, it cannot assume that all nodes will comply. A non-compliant node might continue to send requests with no reduction in load. Requirement 28 [I-D.ietf-dime-overload-reqs] indicates that the overload control solution cannot assume that all Diameter nodes in a network are necessarily trusted, and that malicious nodes not be allowed to take advantage of the overload control mechanism to get more than their fair share of service.
In the absence of an overload control mechanism, Diameter nodes need to implement strategies to protect themselves from floods of requests, and to make sure that a disproportionate load from one source does not prevent other sources from receiving service. For example, a Diameter server might reject a certain percentage of requests from sources that exceed certain limits. Overload control can be thought of as an optimization for such strategies, where downstream nodes never send the excess requests in the first place. However, the presence of an overload control mechanism does not remove the need for these other protection strategies.
The lack of end-to-end security features makes it far more difficult to establish trust in overload reports that originate from non-adjacent nodes. Any agents in the message path may insert or modify overload reports. Nodes must trust that their adjacent peers perform proper checks on overload reports from their peers, and so on, creating a transitive-trust requirement extending for potentially long chains of nodes. Network operators must determine if this transitive trust requirement is acceptable for their deployments. Nodes supporting Diameter overload control MUST give operators the ability to select which peers are trusted to deliver overload reports, and whether they are trusted to forward overload reports from non-adjacent nodes.
[OpenIssue: This requires that a responding node be able to tell a peer-generated OLR from one generated by a non-adjacent node. One way of doing this would be to include the identity of the node that generated the report as part of the OLR]
[OpenIssue: Do we need further language about what rules an agent should apply before forwarding an OLR?]
The lack of end-to-end confidentiality protection means that any Diameter agent in the path of an overload report can view the contents of that report. In addition to the requirement to select which peers are trusted to send overload reports, operators MUST be able to select which peers are authorized to receive reports. A node MUST not send an overload report to a peer not authorized to receive it. Furthermore, an agent MUST remove any overload reports that might have been inserted by other nodes before forwarding a Diameter message to a peer that is not authorized to receive overload reports.
The following people contributed substantial ideas, feedback, and discussion to this document:
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| [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. |
| [RFC5226] | Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. |
| [I-D.ietf-dime-overload-reqs] | McMurry, E. and B. Campbell, "Diameter Overload Control Requirements", Internet-Draft draft-ietf-dime-overload-reqs-13, September 2013. |
| [RFC4006] | Hakala, H., Mattila, L., Koskinen, J-P., Stura, M. and J. Loughney, "Diameter Credit-Control Application", RFC 4006, August 2005. |
| [I-D.ietf-dime-e2e-sec-req] | Tschofenig, H., Korhonen, J., Zorn, G. and K. Pillay, "Diameter AVP Level Security: Scenarios and Requirements", Internet-Draft draft-ietf-dime-e2e-sec-req-00, September 2013. |
The base solution for the overload control does not cover all possible use cases. A number of solution aspects were intentionally left for future specification and protocol work.
This specification describes only means for a simple loss based algorithm. Future algorithms can be added using the designed solution extension mechanism. The new algorithms need to be registered with IANA. See Sections 4.1 and 7 for the required IANA steps.
This specification focuses on Diameter end-point (server or client) overload. A separate extension will be required to outline the handling the case of agent overload.
The current [RFC6733] behaviour in a case of DIAMETER_TOO_BUSY is somewhat underspecified. For example, there is no information how long the specific Diameter node is willing to be unavailable. A specification updating [RFC6733] should clarify the handling of DIAMETER_TOO_BUSY from the error answer initiating Diameter node point of view and from the original request initiating Diameter node point of view. Further, the inclusion of possible additional information providing APVs should be discussed and possible be recommended to be used.
The following section analyses, which Diameter Overload Control requirements [I-D.ietf-dime-overload-reqs] are met by this specification.
Key:
| Rqmt # | S/ P/ N | Notes |
|---|---|---|
| REQ 1 | P | The DOIC solution only addresses overload information. Load information is left as future work. In addition, the DOIC solution does not address agent overload scenarios. |
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| REQ 2 | P | The DOIC solution supports overload reports that implicitly indicate the application impacted by the report. The DOIC solution does not support reporting load information. The DOIC solution is thought to support graceful behavior. Allowing an application specific capabilities negotiation mechanism violates application-independence. Suggested different wording: The DOIC solution supports overload reports that are applicable to any Diameter application. The DOIC solution does not support reporting load information. The DOIC solution allows to support graceful behavior; this will be enhanced when the Load information will be defined. Comment: Can we removed the words "is thought"? |
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| REQ 3 | S | The DOIC solution is thought to address this requirement. Comment: Can we removed the words "is thought"? |
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| REQ 4 | P | The DOIC solution does allow for both both a Diameter server and a Diameter client to send overload reports. The DOIC solution only addresses Diameter end-point (server and client) overload. Agent overload is being addressed in a separate draft. |
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| REQ 5 | S | The DOIC solution does not depend on how the end-points are discovered. Comment: it might be worth working through at least one use case showing DNS based dynamic peer discovery to make sure we haven’t missed anything. |
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| REQ 6 | ? | Need to update text as some configuation is required. Need to determin if the current discussion on overload application id increases the amount of configuration which would change this to a N. |
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| REQ 7 | S | The DOIC solution supports the loss algorithm, which is expected to address this requirement. There is concern about the ability to address oscillations. Wording is included for how a reacting node starts to increase traffic after an overload report expires to address this concern. Suggested different wording: The DOIC solution supports a baseline mechanism relying on traffic reduction percentage that is a loss algorithm, which allows to address this requirement. Oscillations are avoided or quite minimized by sending successive OLR reports with the values to converge to the optimal traffic or to smoothly come back to normal traffic conditions when overload decreases and ends. |
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| REQ 8 | ? | The DOIC solution supports a timestamp which is meant to serve as a report version indication to address this requirement. Comment: The use of the timestamp is under discussion. |
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| REQ 9 | ? | The DOIC solution uses a piggybacking strategy for carrying overload reports, which scales lineraly with the amount of traffic. As such, the first part of the requirement is addressed. The DOIC solution does not support a mechanism for sending overload reports over a quiescent transport connections or, more generally, to Diameter nodes that are not producing traffic. Suggested different wording: The DOIC solution uses a piggybacking strategy for carrying overload reports. As such, the first part of the requirement is addressed. For a connection that has become quiescent due to OLRs with a 100% traffic reduction, the validity timer allows to handle this case. Other cases of quiescent connections are outside the scope of Diameter overload (e.g. their handling may be done through the watch dog of the Diameter base protocol). |
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| REQ 10 | S | The DOIC solution supports two methods for managing the length of an overload condition. First, all overload reports must contain a duration indication, after which the node reacting to the report can consider the overload condition as ended. Secondly, the solution supports the method for the node originating the overload report to explicitly communicate that the condition has ended. This latter mechanism depends on traffic to be sent from the reacting node and, as such, can not be depended upon in all circumstances. |
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| REQ 11 | ? | The DOIC solution works well for small network configurations and for network configurations with a single Diameter agent hop. More analysis is required to determine how well the DOIC solution handles very large Diameter network with partitioned or segmented server farms requiring multiple hops through Diameter agents. |
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| REQ 12 | P | The DOIC solution focuses on Diameter end-point overload and meets this requirement for those Diameter nodes. The DOIC solution does not address Diameter Agent overload and does not meet this requirement for those Diameter nodes. |
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| REQ 13 | ? | The DOIC solution requires including of the overload report in all answer messages in some situations. It is not agreed, however, that this constitutes substantial work. This can also be mitigated by the sender of the overload report keeping state to record who has received overload reports. It is left to implementation decisions as to which approach is taken -- send in all messages or send once with a record of who has received the report. Another way is to let the request sender (reacting node) insert information in the request to say whether a throttling is actually performed. The reporting node then can base its decision on information received in the request; no need for keeping state to record who has received overload reports. The DOIC solution also requires capabilities negotiation in every request and response message, which increases the baseline work required for any node supporting the DOIC solution. Suggested additional text: 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 OLR report added to all messages sent to overload control endpoints will not add substantial additional work. Piggyback base transport also does not require composition, sending, or parsing of new Diameter messages for the purpose of conveying overload control information. There is still discussion on the substantial additional work due to have OLR in each answer message. |
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| REQ 14 | S | The DOIC solution uses the piggybacking method to deliver overload report, which scales lineraly with the amount of traffic. This allows for immediate feedback to any node generating traffic toward another overloaded node. |
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| REQ 15 | S | The DOIC solution does not interfere with transport protocols. |
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| REQ 16 | ? | The DOIC solution allows for a mixed network of supporting and non supporting Diameter end-points. It isn't clear how realm overload is handled in a network with agents that do not support the DOIC solution. Suggested additional wording: Evaluation of Realm overload may require a DA supporting DOIC, if the realm overload is not evaluated by the client. Realm overload handling is still under discussion. |
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| REQ 17 | ? | Suggested wording: The DOIC solution addresses this requirement through the loss algorithm (DOIC baseline mechanism) with the following possibilities. A DA supporting DOIC can act on behalf of clients not supporting DOIC. A reporting node is also aware of the nodes not supporting the DOIC as there is no advertisement of the DOIC support. It may then apply a particular throttling of the requests coming from these non supporting DOIC clients. |
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| REQ 18 | ? | It isn't clear yet that if this requirement is addressed. There has been a proposal to mark messages that survived overload throttling as one method for an overloaded node to address fairness but this proposal is not yet part of the solution. It is also possible that the overloaded node could use state gathered as part of the capability advertisement mechanism to know if the sending node supports the DOIC solution and if not, to apply a particular throttling of the requests coming from these non supporting DOIC clients. |
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| REQ 19 | S | The DOIC solution supports the ability for the overloaded node and the reacting node to be in different administrative domains. |
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| REQ 20 | ? | This mechanism is still under discussion. Comment 1: I think this is a “S”. OLRs are clearly distinguishable from any error code. The fact that an agent would need to send errors if it throttles is not an overload indication per se. It needs to do that even without DoC. OTOH, if we apply some DOC related fix to TOO_BUSY, we probably need a new code. Comment 2: New AVPs conveys overload control information, and this is transported on existing answer messages, so distinguishable from Diameter errors. |
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| REQ 21 | S | The inability for a node to send overload reports will result in equivalent through put to a network that does not support the DOIC solution. |
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| REQ 22 | S | The DOIC solution gives this node generating the overload report the ability to control the amount of throttling done by the reacting node using the reduction percentage parameter in the overload report. |
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| REQ 23 | ? | Initial text: The DOIC mechanism supports two abatement strategies by reacting nodes, routing to an alternative node or dropping traffic. The routing to an alternative node will be enhanced when the Load extension is defined. Comment: This is a N. There’s no good way to determine which nodes are likely to have sufficient capacity without some sort of load metric for non-overloaded nodes. |
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| REQ 24 | N | The DOIC solution does not address delivering load information. |
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| REQ 25 | S | The DOIC solution contains some guideance. |
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| REQ 26 | S | The DOIC solution does not constrain a nodes ability to determine which requests are trottled. |
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| REQ 27 | ? | Initial text: The DOIC solution does add a new line of attack in the ability for a malicious entity to insert overload reports that would reduce or eliminate traffic. This, however, is no worse than an attacker that would assert erroneous error responses such as a TOO BUSY response. It is recognized that the end-to-end security solution currently being worked on by the DIME working group is needed to close these types of vulurabilities. Comment: Sending a malicious OLR with a type of “realm” will have considerably more impact than a TOO_BUSY. Personally, I don’t think we can achieve this requirement without either being hop-by-hop or requiring e2e security. We probably need further analysis of the security implications of the capabilities negotiation as well. Suggested additional verbage: An OLR only relates to the traffic between a reporting node and a reacting node and can effectively block the traffic from a client which would be an important impact. Nevertheless OLRs are regularly sent in all answers, so a malicious OLR will have a short transient effect, as quickly overridden by a new OLR. To have a significant impact would require a continuous flow of answers with malicious OLRs. There is the exception of the OLR with a value of 100% reduction traffic which has a higher vulnerability and the use of which should be avoided when possible. In addition such malicious OLRs must be in answers, which means the capability to insert the malicious OLR in an existing answer rather than to create an answer which is much less easy than to create a request. To have a network wide applicability would request to generate malicious OLRs messages towards all reacting nodes. It can be considered that the baseline mechanism offer a relevant level of security. Further analysis with a security expertise would be beneficial. |
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| REQ 28 | ? | See REQ 18 and REQ 27. Suggested additional verbage: Guidance may be provided for detection of non compliant/abnormal use of OLRs, not only by endpoints but also by intermediate DA that can be aware of OLRs, an example being edge DAs with external networks. Further analysis with a security expertise would be beneficial. |
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| REQ 29 | ? | This requirement is not explicitly addressed by the DOIC solution. There is nothing in the DOIC solution that would prevent the goals of this requirement from being achieved. Non-adjacent DOIC without e2e security could be an issue here. |
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| REQ 30 | ? | It isn't clear how a solution would interfere. Suggested wording: A node can have methods on how to protect from overload from nodes non supporting DOIC. The DOIC mechanism used with DOIC supporting nodes will not interfere with the appliance of these methods. There is the remark that the use of these methods may impact the global overload of the node and the evaluation of the traffic reduction that the reporting node will send in OLRs. If a node has methods to protect against denial of service attacks, the use of DOIC will not interfere with them. A denial of service attack concerning the DOIC itself is addressed in REQ 27. |
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| REQ 31 | ? | Initial text with an S: The DOIC solution addresses node and realm directly. The application to which a report applies is implicitly determined based on the application level message carrying the report. Note that there is no way with DOIC for an overloaded node to communicate multiple nodes, realms or applications in a single overload report. So the inverse of this requirement is not supported. Comment: The inverse is also not _required_ :-) But I think we are "P" here, in that we don’t support “node” per se. we do support “server.” “Node” includes agents. (I also interpreted this to mean that each granularity needed to be supported independently—that is, a potential to say “all traffic to a realm” or “all traffic to a host” independently of application.) |
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| REQ 32 | ? | Initial text with an S: The DOIC solution supports extensibility of both the information communicated and in the definition of new overload abatement algorithms. Comment 1: Recent discussions have made this a ?. It can be changed to S/N/P once these discussions come to a conclusion and new text is added to the draft. Comment 2: Suggested wording - The DOIC solution supports extensibility of both the information communicated and in the definition of new overload abatement algorithms or strategies. It should be noted that, according to the applications or to reacting node implementations, many algorithms may be applied on top of the DOIC baseline solution (without contradicting it), e.g. regarding which type of request to throttle, prioritized messages handling, mapping of the reduction % to an internal algorithm (eg 1 message out of ten etc..) but such algorithms are out of scope of DOIC. |
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| REQ 33 | ? | Initial text with P: The DOIC solution currently defines the loss algorithm as the default algorithm. It does not specify it as mandatory to implement. Comment 1: Then I think that’s a “n”. The MTI part is the crux of the requirement. Comment 2: Suggested wording: In the DOIC baseline solution, the reacting node has to apply the received Reduction-Percentage, and for achieving this, the reacting node can do requests rerouting (when it is possible) or drop/reject requests. This DOIC baseline solution is a loss algorithm and DOIC should not require further specification. The answer to REQ32 indicates the possibility to add other algorithms on top of the DOIC baseline solution. The DOIC solution currently defines this loss algorithm as the default algorithm. It is still under discussion to make it as mandatory to implement. |
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| REQ 34 | P | The ability to communicate overload reports between supporting Diameter nodes does not require agents to support the DOIC solution. Load information exchange is not currently defined. |
[TBD: Would cover S6a MME-HSS communication with several topology choices (such as with or without DRA, and with "generic" agents).]
[TBD: Would cover Gx/Rx and maybe S9..]
[TBD: Add example showing the use of Destination-Host type OLRs and Realm type OLRs.]