Distributed Denial of Service (DDoS) Open Threat Signaling Requirements
draft-ietf-dots-requirements-01

Abstract

This document defines the requirements for the Distributed Denial of Service (DDoS) Open Threat Signaling (DOTS) protocols coordinating attack response against DDoS attacks.

Status of This Memo

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Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Context and Motivation
• 1.2. Terminology
• 2. Requirements
• 2.1. General Requirements
• 2.2. Operational Requirements
• 2.3. Data Channel Requirements
• 2.4. Security requirements
• 3. Congestion Control Considerations
• 4. Security Considerations
• 5. Contributors
• 6. Acknowledgments
• 7. Change Log
• 7.1. 01 revision
• 7.2. 00 revision
• 7.3. Initial revision
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Authors' Addresses

1. Introduction

1.1. Context and Motivation

Distributed Denial of Service (DDoS) attacks continue to plague networks around the globe, from Tier-1 service providers on down to enterprises and small businesses. Attack scale and frequency similarly have continued to increase, in part as a result of software vulnerabilities leading to reflection and amplification attacks. Once staggering attack traffic volume is now the norm, and the impact of larger-scale attacks attract the attention of international press agencies.

The greater impact of contemporary DDoS attacks has led to increased focus on coordinated attack response. Many institutions and enterprises lack the resources or expertise to operate on-premise attack prevention solutions themselves, or simply find themselves constrained by local bandwidth limitations. To address such gaps, security service providers have begun to offer on-demand traffic scrubbing services. Each service offers its own interface for subscribers to request attack mitigation, tying subscribers to proprietary implementations while also limiting the subset of network elements capable of participating in the attack response. As a result of incompatibility across services, attack responses may be fragmentary or otherwise incomplete, leaving key players in the attack path unable to assist in the defense.

The lack of a common method to coordinate a real-time response among involved actors and network domains inhibits the speed and effectiveness of DDoS attack mitigation. This document describes the required characteristics of DOTS protocols that would mitigate contemporary DDoS attack impact and lead to more efficient defensive strategies.

DOTS is less concerned with the form of defensive action than with communicating the need for that action. DOTS supplements calls for help with pertinent details about the detected attack, allowing entities participating in DOTS to form ad hoc, adaptive alliances against DDoS attacks as described in the DOTS use cases [I-D.ietf-dots-use-cases]. The requirements in this document are derived from those use cases.

1.2. Terminology

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].

This document adopts the following terms:

DDoS:

A distributed denial-of-service attack, in which of traffic originating from multiple sources are directed at a target on a network. DDoS attacks are intended to cause a negative impact on the availability of servers, services, applications, and/or other functionality of an attack target. Denial-of-service considerations are discussed in detail in [RFC4732].

DDoS attack target:

A networked server, network service or application that is the focus of a DDoS attack.

DDoS attack telemetry:

Collected network traffic characteristics defining the nature of a DDoS attack. This document makes no assumptions regarding telemetry collection methodology.

Countermeasure:

An action or set of actions taken to recognize and filter out DDoS attack traffic while passing legitimate traffic to the attack target.

Mitigation:

A set of countermeasures enforced against traffic destined for the target or targets of a detected or reported DDoS attack, where countermeasure enforcement is managed by an entity in the network path between attack sources and the attack target. Mitigation methodology is out of scope for this document.

Mitigator:

An entity, typically a network element, capable of performing mitigation of a detected or reported DDoS attack. For the purposes of this document, this entity is a black box capable of mitigation, making no assumptions about availability or design of countermeasures, nor about the programmable interface between this entity and other network elements. The mitigator and DOTS server are assumed to belong to the same administrative entity.

DOTS client:

A DOTS-aware software module responsible for requesting attack response coordination with other DOTS-aware elements.

DOTS server:

A DOTS-aware software module handling and responding to messages from DOTS clients. The DOTS server MAY enable mitigation on behalf of the DOTS client, if requested, by communicating the DOTS client’s request to the mitigator and relaying any mitigator feedback to the requesting DOTS client. A DOTS server may also be a mitigator.

DOTS relay:

A DOTS-aware software module positioned between a DOTS server and a DOTS client in the signaling path. A DOTS relay receives messages from a DOTS client and relays them to a DOTS server, and similarly passes messages from the DOTS server to the DOTS client. A DOTS relay acts as a proxy or bridge between stateful and stateless transport signaling, and may also aggregate signaling from multiple downstream DOTS clients into a single session with an upstream DOTS server or DOTS relay.

DOTS agents:

Any DOTS functional element, including DOTS clients, DOTS servers and DOTS relays.

Signal channel:

A bidirectional, mutually authenticated communication layer between DOTS agents characterized by resilience even in conditions leading to severe packet loss, such as a volumetric DDoS attack causing network congestion.

DOTS signal:

A concise authenticated status/control message transmitted between DOTS agents, used to indicate client’s need for mitigation, as well as to convey the status of any requested mitigation.

Heartbeat:

A message transmitted between DOTS agents over the signal channel, used as a keep-alive and to measure peer health.

Client signal:

A message sent from a DOTS client to a DOTS server or DOTS relay over the signal channel, indicating the DOTS client’s need for mitigation, as well as the scope of any requested mitigation, optionally including additional attack details to supplement server-initiated mitigation.

Server signal:

A message sent from a DOTS server to a DOTS client or DOTS relay over the signal channel. Note that a server signal is not a response to client signal, but a DOTS server-initiated status message sent to DOTS clients with which the server has established signaling sessions, containing information about the status of DOTS client-requested mitigation and its efficacy.

Data channel:

A secure communication layer between DOTS clients and DOTS servers used for infrequent bulk exchange of data not easily or appropriately communicated through the signal channel under attack conditions.

Blacklist:

A list of addresses, prefixes and/or other identifiers indicating sources from which traffic should be blocked, regardless of traffic content.

Whitelist:

A list of addresses, prefixes and/or other identifiersfrom indicating sources from which traffic should always be allowed, regardless of contradictory data gleaned in a detected attack.

Multi-homed DOTS client:

A DOTS client exchanging messages with multiple DOTS servers, each in a separate administrative domain.

2. Requirements

This section describes the required features and characteristics of the DOTS protocols.

DOTS is an advisory protocol. An active DDoS attack against the entity controlling the DOTS client need not be present before establishing DOTS communication between DOTS agents. Indeed, establishing a relationship with peer DOTS agents during nominal network conditions provides the foundation for more rapid attack response against future attacks, as all interactions setting up DOTS, including any business or service level agreements, are already complete.

DOTS must at a minimum make it possible for a DOTS client to request a DOTS server’s aid in mounting a coordinated defense against a detected attack, signaling inter- or intra-domain as requested by local operators. DOTS clients should similarly be able to withdraw aid requests. DOTS requires no justification from DOTS clients for requests for help, nor must DOTS clients justify withdrawing help requests: the decision is local to the entity owning the DOTS clients. Regular feedback between DOTS clients and DOTS server supplement the defensive alliance by maintaining a common understanding of DOTS peer health and activity. Bidirectional communication between DOTS clients and DOTS servers is therefore critical.

Yet DOTS must also work with a set of competing operational goals. On the one hand, the protocol must be resilient under extremely hostile network conditions, providing continued contact between DOTS agents even as attack traffic saturates the link. Such resiliency may be developed several ways, but characteristics such as small message size, asynchronous, redundant message delivery and minimal connection overhead (when possible given local network policy) will tend to contribute to the robustness demanded by a viable DOTS protocol. Operators of peer DOTS-enabled domains may enable quality- or class-of-service traffic tagging to increase the probability of successful DOTS signal delivery, but DOTS requires no such policies be in place. The DOTS solution indeed must be viable especially in their absence.

On the other hand, DOTS must include protections ensuring message confidentiality, integrity and authenticity to keep the protocol from becoming another vector for the very attacks it’s meant to help fight off. DOTS clients must be able to authenticate DOTS servers, and vice versa, for DOTS to operate safely, meaning the DOTS agents must have a way to negotiate and agree upon the terms of protocol security. Attacks against the transport protocol should not offer a means of attack against the message confidentiality, integrity and authenticity.

The DOTS server and client must also have some common method of defining the scope of any mitigation performed by the mitigator, as well as making adjustments to other commonly configurable features, such as listen ports, exchanging black- and white-lists, and so on.

Finally, DOTS should provide sufficient extensibility to meet local, vendor or future needs in coordinated attack defense, although this consideration is necessarily superseded by the other operational requirements.

2.1. General Requirements

GEN-001

Extensibility: Protocols and data models developed as part of DOTS MUST be extensible in order to keep DOTS adaptable to operational and proprietary DDoS defenses. Future extensions MUST be backward compatible.

GEN-002

Resilience and Robustness: The signaling protocol MUST be designed to maximize the probability of signal delivery even under the severely constrained network conditions imposed by particular attack traffic. The protocol MUST be resilient, that is, continue operating despite message loss and out-of-order or redundant signal delivery.

GEN-003

Bidirectionality: To support peer health detection, to maintain an open signal channel, and to increase the probability of signal delivery during attack, the signal channel MUST be bidirectional, with client and server transmitting signals to each other at regular intervals, regardless of any client request for mitigation. Unidirectional messages MUST be supported within the bidirectional signal channel to allow for unsolicited message delivery, enabling asynchronous notifications between agents.

GEN-004

Sub-MTU Message Size: To avoid message fragmentation and the consequently decreased probability of message delivery, signaling protocol message size MUST be kept under signaling path Maximum Transmission Unit (MTU), including the byte overhead of any encapsulation, transport headers, and transport- or message-level security.

GEN-005

Bulk Data Exchange: Infrequent bulk data exchange between DOTS agents can also significantly augment attack response coordination, permitting such tasks as population of black- or white-listed source addresses; address or prefix group aliasing; exchange of incident reports; and other hinting or configuration supplementing attack response.

As the resilience requirements for the DOTS signal channel mandate small signal message size, a separate, secure data channel utilizing an established reliable transport protocol MUST be used for bulk data exchange.

2.2. Operational Requirements

OP-001

Use of Common Transport Protocols: DOTS MUST operate over common widely deployed and standardized transport protocols. While the User Datagram Protocol (UDP) [RFC0768] SHOULD be used for the signal channel, the Transmission Control Protocol (TCP) [RFC0793] MAY be used if necessary due to network policy or middlebox capabilities or configurations. The data channel MUST use TCP; see Section 2.3 below.

OP-002

Session Health Monitoring: Peer DOTS agents MUST regularly send heartbeats to each other after mutual authentication in order to keep the DOTS session open. A session MUST be considered active until a DOTS agent explicitly ends the session, or either DOTS agent fails to receive heartbeats from the other after a mutually negotiated timeout period has elapsed.

OP-003

Session Redirection: In order to increase DOTS operational flexibility and scalability, DOTS servers SHOULD be able to redirect DOTS clients to another DOTS server or relay at any time. Due to the decreased probability of DOTS server signal delivery due to link congestion, it is RECOMMENDED DOTS servers avoid redirecting while mitigation is enabled during an active attack against a target in the DOTS client’s domain. Either the DOTS servers have to fate-share the security state, the client MUST have separate security state with each potential redirectable server, or be able to negotiate new state as part of redirection.

OP-004

Mitigation Status: DOTS MUST provide a means to report the status of an action requested by a DOTS client. In particular, DOTS clients MUST be able to request or withdraw a request for mitigation from the DOTS server. The DOTS server MUST acknowledge a DOTS client’s request to withdraw from coordinated attack response in subsequent signals, and MUST cease mitigation activity as quickly as possible. However, a DOTS client rapidly toggling active mitigation may result in undesirable side-effects for the network path, such as route or DNS [RFC1034] flapping. A DOTS server therefore MAY continue mitigating for a mutually negotiated period after receiving the DOTS client’s request to stop.

A server MAY refuse to engage in coordinated attack response with a client. To make the status of a client’s request clear, the server MUST indicate in server signals whether client-initiated mitigation is active. When a client-initiated mitigation is active, and threat handling details such as mitigation scope and statistics are available to the server, the server SHOULD include those details in server signals sent to the client. DOTS clients SHOULD take mitigation statistics into account when deciding whether to request the DOTS server cease mitigation.

OP-005

Mitigation Lifetime: A DOTS client SHOULD indicate the desired lifetime of any mitigation requested from the DOTS server. As DDoS attack duration is unpredictable, the DOTS client SHOULD be able to extend mitigation lifetime with periodic renewed requests for help. When the mitigation lifetime comes to an end, the DOTS server SHOULD delay session termination for a protocol-defined grace period to allow for delivery of delayed mitigation renewals over the signal channel. After the grace period elapses, the DOTS server MAY terminate the session at any time.

If a DOTS client does not include a mitigation lifetime in requests for help sent to the DOTS server, the DOTS server will use a reasonable default as defined by the protocol. As above, the DOTS client MAY extend a current mitigation request’s lifetime trivially with renewed requests for help.

A DOTS client MAY also request an indefinite mitigation lifetime, enabling architectures in which the mitigator is always in the traffic path to the resources for which the DOTS client is requesting protection. DOTS servers MAY refuse such requests for any reason. The reasons themselves are not in scope.

OP-006

Mitigation Scope: DOTS clients MUST indicate the desired address or prefix space coverage of any mitigation, for example by using Classless Internet Domain Routing (CIDR) [RFC1518],[RFC1519] prefixes, [RFC2373] for IPv6 [RFC2460] prefixes, the length/prefix convention established in the Border Gateway Protocol (BGP) [RFC4271], SIP URIs [RFC3261], E.164 numbers, DNS names, or by a prefix group alias agreed upon with the server through the data channel.

If there is additional information available narrowing the scope of any requested attack response, such as targeted port range, protocol, or service, DOTS clients SHOULD include that information in client signals. DOTS clients MAY also include additional attack details. Such supplemental information is OPTIONAL, and DOTS servers MAY ignore it when enabling countermeasures on the mitigator.

As an active attack evolves, clients MUST be able to adjust as necessary the scope of requested mitigation by refining the address space requiring intervention.

OP-007

Mitigation Efficacy: When a mitigation request by a DOTS client is active, DOTS clients SHOULD transmit a metric of perceived mitigation efficacy to the DOTS server, per “Automatic or Operator-Assisted CPE or PE Mitigators Request Upstream DDoS Mitigation Services” in [I-D.ietf-dots-use-cases]. DOTS servers MAY use the efficacy metric to adjust countermeasures activated on a mitigator on behalf of a DOTS client.

OP-008

Conflict Detection and Notification: Multiple DOTS clients controlled by a single administrative entity may send conflicting mitigation requests for pool of protected resources, as a result of misconfiguration, operator error, or compromised DOTS clients. DOTS servers attempting to honor conflicting requests may flap network route or DNS information, degrading the networks attempting to participate in attack response with the DOTS clients. DOTS servers SHALL detect such conflicting requests, and SHALL notify the DOTS clients in conflict. The notification SHOULD indicate the nature and scope of the conflict, for example, the overlapping prefix range in a conflicting mitigation request.

OP-009:

Lookup Caching: DOTS agents SHOULD cache resolved names, PKI validation chains, and similarly queried data as necessary. Network-based lookups and validation may be inhibited or unavailable during an active attack due to link congestion. For example, DOTS agents SHOULD cache resolved names and addresses of peer DOTS agents, and SHOULD refer to those agents by IPv4 [RFC0791] or IPv6 address for all communications following initial name resolution.

OP-010:

Network Address Translator Traversal: The DOTS protocol MUST operate over networks in which Network Address Translation (NAT) is deployed. As UDP is the recommended transport for DOTS, all considerations in “Middlebox Traversal Guidelines” in [RFC5405] apply to DOTS. Regardless of transport, DOTS protocols MUST follow established best common practices (BCPs) for NAT traversal.

2.3. Data Channel Requirements

The data channel is intended to be used for bulk data exchanges between DOTS agents. Unlike the signal channel, which must operate nominally even when confronted with despite signal degradation due to packet loss, the data channel is not expected to be constructed to deal with attack conditions. As the primary function of the data channel is data exchange, a reliable transport is required in order for DOTS agents to detect data delivery success or failure.

The data channel must be extensible. We anticipate the data channel will be used for such purposes as configuration or resource discovery. For example, a DOTS client may submit to the DOTS server a collection of prefixes it wants to refer to by alias when requesting mitigation, to which the server would respond with a success status and the new prefix group alias, or an error status and message in the event the DOTS client’s data channel request failed. The transactional nature of such data exchanges suggests a separate set of requirements for the data channel, while the potentially sensitive content sent between DOTS agents requires extra precautions to ensure data privacy and authenticity.

DATA-001

Reliable transport: Transmissions over the data channel MUST be transactional, requiring reliable, in-order packet delivery.

DATA-002

Data privacy and integrity: Transmissions over the data channel is likely to contain operationally or privacy-sensitive information or instructions from the remote DOTS agent. Theft or modification of data channel transmissions could lead to information leaks or malicious transactions on behalf of the sending agent (see Section 4 below). Consequently data sent over the data channel MUST be encrypted and authenticated using current industry best practices. DOTS servers and relays MUST enable means to prevent leaking operationally or privacy-sensitive data. Although administrative entities participating in DOTS may detail what data may be revealed to third-party DOTS agents, such considerations are not in scope for this document.

DATA-003

Black- and whitelist management: DOTS servers SHOULD provide methods for DOTS clients to manage black- and white-lists of source addresses of traffic destined for addresses belonging to a client.

For example, a DOTS client should be able to create a black- or whitelist entry; retrieve a list of current entries from either list; update the content of either list; and delete entries as necessary.

How the DOTS server determines client ownership of address space is not in scope.

2.4. Security requirements

DOTS must operate within a particularly strict security context, as an insufficiently protected signal or data channel may be subject to abuse, enabling or supplementing the very attacks DOTS purports to mitigate.

SEC-001

Peer Mutual Authentication: DOTS agents MUST authenticate each other before a DOTS session is considered valid. The method of authentication is not specified, but should follow current industry best practices with respect to any cryptographic mechanisms to authenticate the remote peer.

SEC-002

Message Confidentiality, Integrity and Authenticity: DOTS protocols MUST take steps to protect the confidentiality, integrity and authenticity of messages sent between client and server. While specific transport- and message-level security options are not specified, the protocols MUST follow current industry best practices for encryption and message authentication.

In order for DOTS protocols to remain secure despite advancements in cryptanalysis and traffic analysis, DOTS agents MUST be able to negotiate the terms and mechanisms of protocol security, subject to the interoperability and signal message size requirements above.

SEC-003

Message Replay Protection: In order to prevent a passive attacker from capturing and replaying old messages, DOTS protocols MUST provide a method for replay detection.

3. Congestion Control Considerations

The DOTS signal channel will not contribute measurably to link congestion, as the protocol’s transmission rate will be negligible regardless of network conditions. Bulk data transfers are performed over the data channel, which should use a reliable transport with built-in congestion control mechanisms, such as TCP.

4. Security Considerations

DOTS is at risk from three primary attacks:

• DOTS agent impersonation
• Traffic injection
• Signaling blocking

The DOTS protocol MUST be designed for minimal data transfer to address the blocking risk. Impersonation and traffic injection mitigation can be managed through current secure communications best practices. See Section 2.4 above for a detailed discussion.

5. Contributors

Med Boucadair

6. Acknowledgments

Thanks to Roman Danyliw and Matt Richardson for careful reading and feedback.

7. Change Log

7.1. 01 revision

2016-03-21

• Reconciled terminology with -00 revision of [I-D.ietf-dots-use-cases].
• Terminology clarification based on working group feedback.
• Moved security-related requirements to separate section.
• Made resilience/robustness primary general requirement to align with charter.
• Clarified support for unidirectional communication within the bidirection signal channel.
• Added proposed operational requirement to support session redirection.
• Added proposed operational requirement to support conflict notification.
• Added proposed operational requirement to support mitigation lifetime in mitigation requests.
• Added proposed operational requirement to support mitigation efficacy reporting from DOTS clients.
• Added proposed operational requirement to cache lookups of all kinds.
• Added proposed operational requirement regarding NAT traversal.
• Removed redundant mutual authentication requirement from data channel requirements.

7.2. 00 revision

2015-10-15

7.3. Initial revision

2015-09-24 Andrew Mortensen

8. References

8.1. Normative References

8.2. Informative References

Authors' Addresses