Internet Engineering Task Force (IETF) A. Mortensen Request for Comments: 8612 Arbor Networks Category: Informational T. Reddy ISSN: 2070-1721 McAfee R. Moskowitz Huawei May 2019 DDoS Open Threat Signaling (DOTS) Requirements
AbstractThis document defines the requirements for the Distributed Denial-of- Service (DDoS) Open Threat Signaling (DOTS) protocols enabling coordinated response to DDoS attacks. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are candidates for any level of Internet Standard; see Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8612. Copyright Notice Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Context and Motivation . . . . . . . . . . . . . . . . . 2 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. General Requirements . . . . . . . . . . . . . . . . . . 7 2.2. Signal Channel Requirements . . . . . . . . . . . . . . . 8 2.3. Data Channel Requirements . . . . . . . . . . . . . . . . 13 2.4. Security Requirements . . . . . . . . . . . . . . . . . . 14 2.5. Data Model Requirements . . . . . . . . . . . . . . . . . 16 3. Congestion Control Considerations . . . . . . . . . . . . . . 17 3.1. Signal Channel . . . . . . . . . . . . . . . . . . . . . 17 3.2. Data Channel . . . . . . . . . . . . . . . . . . . . . . 17 4. Security Considerations . . . . . . . . . . . . . . . . . . . 17 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 6.1. Normative References . . . . . . . . . . . . . . . . . . 18 6.2. Informative References . . . . . . . . . . . . . . . . . 20 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 21 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
attack responses may be fragmented or otherwise incomplete, leaving operators in the attack path unable to assist in the defense. A standardized method to coordinate a real-time response among involved operators will increase the speed and effectiveness of DDoS attack mitigation and reduce the impact of these attacks. This document describes the required characteristics of protocols that enable attack response coordination and mitigation of DDoS attacks. DDoS Open Threat Signaling (DOTS) communicates the need for defensive action in anticipation of or in response to an attack, but it does not dictate the implementation of these actions. The DOTS use cases are discussed in [DOTS-USE], and the DOTS architecture is discussed in [DOTS-ARCH]. RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. These capitalized words are used to signify the requirements for the DOTS protocols design. This document adopts the following terms: DDoS: A distributed denial-of-service attack in which traffic originating from multiple sources is directed at a target on a network. DDoS attacks are intended to cause a negative impact on the availability and/or functionality of an attack target. Denial-of-service considerations are discussed in detail in [RFC4732]. DDoS attack target: A network-connected entity that is the target of a DDoS attack. Potential targets include (but are not limited to) network elements, network links, servers, and services. DDoS attack telemetry: Collected measurements and behavioral characteristics defining the nature of a DDoS attack. Countermeasure: An action or set of actions focused on recognizing and filtering out specific types of DDoS attack traffic while passing legitimate traffic to the attack target. Distinct countermeasures can be layered to defend against attacks combining multiple DDoS attack types.
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. The means by which this entity performs these mitigations and how they are requested of it are out of scope for this document. The mitigator and DOTS server receiving a mitigation request 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 enables mitigation on behalf of the DOTS client, if requested, by communicating the DOTS client's request to the mitigator and returning selected mitigator feedback to the requesting DOTS client. DOTS agent: Any DOTS-aware software module capable of participating in a DOTS signal or data channel. It can be a DOTS client, DOTS server, or, as a logical agent, a DOTS gateway. DOTS gateway: A DOTS-aware software module resulting from the logical concatenation of the functionality of a DOTS server and a DOTS client into a single DOTS agent. This functionality is analogous to a Session Initiation Protocol (SIP) [RFC3261] Back- to-Back User Agent (B2BUA) [RFC7092]. A DOTS gateway has a client-facing side, which behaves as a DOTS server for downstream clients, and a server-facing side, which performs the role of a DOTS client for upstream DOTS servers. Client-domain DOTS gateways are DOTS gateways that are in the DOTS client's domain, while server-domain DOTS gateways denote DOTS gateways that are in the DOTS server's domain. A DOTS gateway may terminate multiple discrete DOTS client connections and may aggregate these into one or more connections. DOTS gateways are described further in [DOTS-ARCH]. Signal channel: A bidirectional, mutually authenticated communication channel between DOTS agents that is resilient even in conditions leading to severe packet loss such as a volumetric DDoS attack causing network congestion.
DOTS signal: A status/control message transmitted over the authenticated signal channel between DOTS agents, used to indicate the client's need for mitigation or 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. Data channel: A bidirectional, mutually authenticated communication channel between two DOTS agents used for infrequent but reliable bulk exchange of data not easily or appropriately communicated through the signal channel. Reliable bulk data exchange may not function well or at all during attacks causing network congestion. The data channel is not expected to operate in such conditions. Filter: A specification of a matching network traffic flow or set of flows. The filter will typically have a policy associated with it, e.g., rate-limiting or discarding matching traffic [RFC4949]. Drop-list: A list of filters indicating sources from which traffic should be blocked regardless of traffic content. Accept-list: A list of filters indicating sources from which traffic should always be allowed regardless of contradictory data gleaned in a detected attack. Multihomed DOTS client: A DOTS client exchanging messages with multiple DOTS servers, each in a separate administrative domain. DOTS-ARCH]. The goal of the DOTS requirements specification is to specify the requirements for DOTS signal channel and data channel protocols that have different application and transport-layer requirements. This section describes the required features and characteristics of the DOTS protocols. The goal of DOTS protocols is to enable and manage mitigation on behalf of a network domain or resource that is or may become the focus of a DDoS attack. An active DDoS attack against the entity controlling the DOTS client need not be present before establishing a communication channel between DOTS agents. Indeed, establishing a relationship with peer DOTS agents during normal network conditions provides the foundation for a more rapid attack response against future attacks, as all interactions setting up DOTS, including any
business or service-level agreements, are already complete. Reachability information of peer DOTS agents is provisioned to a DOTS client using a variety of manual or dynamic methods. Once a relationship between DOTS agents is established, regular communication between DOTS clients and servers enables a common understanding of the DOTS agents' health and activity. The DOTS protocol must, at a minimum, make it possible for a DOTS client to request aid mounting a defense against a suspected attack. This defense could be coordinated by a DOTS server and include signaling within or between domains 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 do DOTS clients need to justify withdrawing help requests; the decision is local to the DOTS clients' domain. Multihomed DOTS clients must be able to select the appropriate DOTS server(s) to which a mitigation request is to be sent. The method for selecting the appropriate DOTS server in a multihomed environment is out of scope for this document. DOTS protocol implementations face competing operational goals when maintaining this bidirectional communication stream. On the one hand, DOTS must include measures to ensure message confidentiality, integrity, authenticity, and replay protection to keep the protocols from becoming additional vectors for the very attacks it is meant to help fight off. On the other 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 notifications, 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 either quality-of- service or class-of-service traffic tagging to increase the probability of successful DOTS signal delivery, but DOTS does not require such policies be in place and should be viable in their absence. The DOTS server and client must also have some standardized method of defining the scope of any mitigation, as well as managing other mitigation-related configurations. Finally, DOTS should be sufficiently extensible to meet future needs in coordinated attack defense, although this consideration is necessarily superseded by the other operational requirements.
GEN-005 Loop Handling: In certain scenarios, typically involving misconfiguration of DNS or routing policy, it may be possible for communication between DOTS agents to loop. Signal and data channel implementations should be prepared to detect and terminate such loops to prevent service disruption. RFC768] SHOULD be used for the signal channel, the Transmission Control Protocol (TCP) [RFC793] MAY be used if necessary due to network policy or middlebox capabilities or configurations. SIG-002 Sub-MTU Message Size: To avoid message fragmentation and the consequently decreased probability of message delivery over a congested link, signaling protocol message size MUST be kept under the signaling Path Maximum Transmission Unit (PMTU), including the byte overhead of any encapsulation, transport headers, and transport- or message-level security. If the total message size exceeds the PMTU, the DOTS agent MUST split the message into separate messages; for example, the list of mitigation scope types could be split into multiple lists and each list conveyed in a new message. DOTS agents can attempt to learn PMTU using the procedures discussed in [IP-FRAG-FRAGILE]. If the PMTU cannot be discovered, DOTS agents MUST assume a PMTU of 1280 bytes, as IPv6 requires that every link in the Internet have an MTU of 1280 octets or greater as specified in [RFC8200]. If IPv4 support on legacy or otherwise unusual networks is a consideration and the PMTU is unknown, DOTS implementations MAY assume a PMTU of 576 bytes for IPv4 datagrams, as every IPv4 host must be capable of receiving a packet whose length is equal to 576 bytes as discussed in [RFC791] and [RFC1122]. SIG-003 Bidirectionality: To support peer health detection, to maintain an active signal channel, and to increase the probability of signal delivery during an 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. The bidirectional signal channel MUST support unidirectional messaging to enable notifications between DOTS agents.
SIG-004 Channel Health Monitoring: DOTS agents MUST support exchange of heartbeat messages over the signal channel to monitor channel health. These keep-alives serve to maintain any on-path NAT or Firewall bindings to avoid cryptographic handshake for new mitigation requests. The heartbeat interval during active mitigation could be negotiable based on NAT/Firewall characteristics. Absent information about the NAT/Firewall characteristics, DOTS agents need to ensure its on-path NAT or Firewall bindings do not expire, by using the keep-alive frequency discussed in Section 3.5 of [RFC8085]. To support scenarios in which loss of heartbeat is used to trigger mitigation, and to keep the channel active, DOTS servers MUST solicit heartbeat exchanges after successful mutual authentication. When DOTS agents are exchanging heartbeats and no mitigation request is active, either agent MAY request changes to the heartbeat rate. For example, a DOTS server might want to reduce heartbeat frequency or cease heartbeat exchanges when an active DOTS client has not requested mitigation, in order to control load. Following mutual authentication, a signal channel MUST be considered active until a DOTS agent explicitly ends the session. When no attack traffic is present, the signal channel MUST be considered active until either DOTS agent fails to receive heartbeats from the other peer after a mutually agreed upon retransmission procedure has been exhausted. Peer DOTS agents MUST regularly send heartbeats to each other while a mitigation request is active. Because heartbeat loss is much more likely during volumetric attack, DOTS agents SHOULD avoid signal channel termination when mitigation is active and heartbeats are not received by either DOTS agent for an extended period. The exception circumstances to terminating the signal channel session during active mitigation are discussed below: * To handle a possible DOTS server restart or crash, the DOTS clients MAY attempt to establish a new signal channel session but MUST continue to send heartbeats on the current session so that the DOTS server knows the session is still alive. If the new session is successfully established, the DOTS client can terminate the current session. * DOTS servers are assumed to have the ability to monitor the attack, using feedback from the mitigator and other available sources, and MAY use the absence of attack traffic and lack of client heartbeats as an indication the signal channel is defunct.
SIG-005 Channel Redirection: In order to increase DOTS operational flexibility and scalability, DOTS servers SHOULD be able to redirect DOTS clients to another DOTS server at any time. DOTS clients MUST NOT assume the redirection target DOTS server shares security state with the redirecting DOTS server. DOTS clients are free to attempt abbreviated security negotiation methods supported by the protocol, such as DTLS session resumption, but MUST be prepared to negotiate new security state with the redirection target DOTS server. The redirection DOTS server and redirecting DOTS server MUST belong to the same administrative domain. Due to the increased likelihood of packet loss caused by link congestion during an attack, DOTS servers SHOULD NOT redirect while mitigation is enabled during an active attack against a target in the DOTS client's domain. SIG-006 Mitigation Requests and Status: Authorized DOTS clients MUST be able to request scoped mitigation from DOTS servers. DOTS servers MUST send status to the DOTS clients about mitigation requests. If a DOTS server rejects an authorized request for mitigation, the DOTS server MUST include a reason for the rejection in the status message sent to the client. DOTS servers MUST regularly send mitigation status updates to authorized DOTS clients that have requested and been granted mitigation. If unreliable transport is used for the signal channel protocol, due to the higher likelihood of packet loss during a DDoS attack, DOTS servers need to send the mitigation status multiple times at regular intervals following the data transmission guidelines discussed in Section 3.1.3 of [RFC8085]. When DOTS client-requested mitigation is active, DOTS server status messages MUST include the following mitigation metrics: * Total number of packets blocked by the mitigation * Current number of packets per second blocked * Total number of bytes blocked * Current number of bytes per second blocked DOTS clients MAY take these metrics into account when determining whether to ask the DOTS server to cease mitigation.
A DOTS client MAY withdraw a mitigation request at any time regardless of whether mitigation is currently active. The DOTS server MUST immediately acknowledge a DOTS client's request to stop mitigation. To protect against route or DNS flapping caused by a client rapidly toggling mitigation, and to dampen the effect of oscillating attacks, DOTS servers MAY allow mitigation to continue for a limited period after acknowledging a DOTS client's withdrawal of a mitigation request. During this period, DOTS server status messages SHOULD indicate that mitigation is active but terminating. DOTS clients MAY reverse the mitigation termination during this active-but-terminating period with a new mitigation request for the same scope. The DOTS server MUST treat this request as a mitigation lifetime extension (see SIG-007). The initial active-but-terminating period is both implementation- and deployment-specific, but SHOULD be sufficiently long enough to absorb latency incurred by route propagation. If a DOTS client refreshes the mitigation before the active-but-terminating period elapses, the DOTS server MAY increase the active-but-terminating period up to a maximum of 300 seconds (5 minutes). After the active-but-terminating period elapses, the DOTS server MUST treat the mitigation as terminated, as the DOTS client is no longer responsible for the mitigation. SIG-007 Mitigation Lifetime: DOTS servers MUST support mitigations for a negotiated time interval and MUST terminate a mitigation when the lifetime elapses. DOTS servers also MUST support renewal of mitigation lifetimes in mitigation requests from DOTS clients, allowing clients to extend mitigation as necessary for the duration of an attack. DOTS servers MUST treat a mitigation terminated due to lifetime expiration exactly as if the DOTS client originating the mitigation had asked to end the mitigation, including the active- but-terminating period, as described above in SIG-005. DOTS clients MUST include a mitigation lifetime in all mitigation requests. DOTS servers SHOULD support indefinite mitigation lifetimes, enabling architectures in which the mitigator is always in the traffic path to the resources for which the DOTS client is requesting protection. DOTS clients MUST be prepared to not be granted mitigations with indefinite lifetimes. DOTS servers MAY refuse mitigations with indefinite lifetimes for policy reasons. The reasons themselves are out of scope for this document. If the
DOTS server does not grant a mitigation request with an indefinite mitigation lifetime, it MUST set the lifetime to a value that is configured locally. That value MUST be returned in a reply to the requesting DOTS client. SIG-008 Mitigation Scope: DOTS clients MUST indicate desired mitigation scope. The scope type will vary depending on the resources requiring mitigation. All DOTS agent implementations MUST support the following required scope types: * IPv4 prefixes [RFC4632] * IPv6 prefixes [RFC4291] [RFC5952] * Domain names [RFC1035] The following mitigation scope type is OPTIONAL: * Uniform Resource Identifiers [RFC3986] DOTS servers MUST be able to resolve domain names and (when supported) URIs. How name resolution is managed on the DOTS server is implementation-specific. DOTS agents MUST support mitigation scope aliases, allowing DOTS clients and servers to refer to collections of protected resources by an opaque identifier created through the data channel, direct configuration, or other means. Domain name and URI mitigation scopes may be thought of as a form of scope alias in which the addresses to which the domain name or URI resolve represent the full scope of the mitigation. 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 mitigation requests. DOTS clients MAY also include additional attack details. DOTS servers MAY ignore such supplemental information when enabling countermeasures on the mitigator. As an active attack evolves, DOTS clients MUST be able to adjust as necessary the scope of requested mitigation by refining the scope of resources requiring mitigation. A DOTS client may obtain the mitigation scope through direct provisioning or through implementation-specific methods of discovery. DOTS clients MUST support at least one mechanism to obtain mitigation scope.
SIG-009 Mitigation Efficacy: When a mitigation request is active, DOTS clients MUST be able to transmit a metric of perceived mitigation efficacy to the DOTS server. DOTS servers MAY use the efficacy metric to adjust countermeasures activated on a mitigator on behalf of a DOTS client. SIG-010 Conflict Detection and Notification: Multiple DOTS clients controlled by a single administrative entity may send conflicting mitigation requests as a result of misconfiguration, operator error, or compromised DOTS clients. DOTS servers in the same administrative domain 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 in a single administrative domain SHALL detect such conflicting requests and SHALL notify the DOTS clients in conflict. The notification MUST indicate the nature and scope of the conflict, for example, the overlapping prefix range in a conflicting mitigation request. SIG-011 Network Address Translator Traversal: DOTS clients may be deployed behind a Network Address Translator (NAT) and need to communicate with DOTS servers through the NAT. DOTS protocols MUST therefore be capable of traversing NATs. If UDP is used as the transport for the DOTS signal channel, all considerations in "Middlebox Traversal Guidelines" in [RFC8085] apply to DOTS. Regardless of transport, DOTS protocols MUST follow established best common practices established in BCP 127 for NAT traversal [RFC4787] [RFC6888] [RFC7857].
DATA-003 Resource Configuration: To help meet the general and signal channel requirements in Sections 2.1 and 2.2, DOTS server implementations MUST provide an interface to configure resource identifiers, as described in SIG-008. DOTS server implementations MAY expose additional configurability. Additional configurability is implementation-specific. DATA-004 Policy Management: DOTS servers MUST provide methods for DOTS clients to manage drop- and accept-lists of traffic destined for resources belonging to a client. For example, a DOTS client should be able to create a drop- or accept-list entry, retrieve a list of current entries from either list, update the content of either list, and delete entries as necessary. How a DOTS server authorizes DOTS client management of drop- and accept-list entries is implementation-specific. RFC7525] 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 IETF best practices [RFC7525] for encryption and message authentication. Client-domain DOTS gateways are more trusted than DOTS clients, while server-domain DOTS gateways and DOTS servers share the same level of trust. A security mechanism at the transport layer (TLS or DTLS) is thus adequate to provide security between peer DOTS agents. In order for DOTS protocols to remain secure despite advancements in cryptanalysis and traffic analysis, DOTS agents MUST support secure negotiation of the terms and mechanisms of protocol
security, subject to the interoperability and signal message size requirements in Section 2.2. While the interfaces between downstream DOTS server and upstream DOTS client within a DOTS gateway are implementation-specific, those interfaces nevertheless MUST provide security equivalent to that of the signal channels bridged by gateways in the signaling path. For example, when a DOTS gateway consisting of a DOTS server and DOTS client is running on the same logical device, the two DOTS agents could be implemented within the same process security boundary. SEC-003 Data Privacy and Integrity: Transmissions over the DOTS protocols are likely to contain operationally or privacy-sensitive information or instructions from the remote DOTS agent. Theft, modification, or replay of message transmissions could lead to information leaks or malicious transactions on behalf of the sending agent (see Section 4). Consequently, data sent over the DOTS protocols MUST be encrypted using secure transports (TLS or DTLS). DOTS servers 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. SEC-004 Message Replay Protection: To prevent a passive attacker from capturing and replaying old messages, and thereby potentially disrupting or influencing the network policy of the receiving DOTS agent's domain, DOTS protocols MUST provide a method for replay detection and prevention. Within the signal channel, messages MUST be uniquely identified such that replayed or duplicated messages can be detected and discarded. Unique mitigation requests MUST be processed at most once. SEC-005 Authorization: DOTS servers MUST authorize all messages from DOTS clients that pertain to mitigation, configuration, filtering, or status. DOTS servers MUST reject mitigation requests with scopes that the DOTS client is not authorized to manage. Likewise, DOTS servers MUST refuse to allow creation, modification, or deletion of scope aliases and drop- or accept- lists when the DOTS client is unauthorized. The modes of authorization are implementation-specific.
DM-009 Relationship to Transport: The DOTS data model MUST NOT make any assumptions about specific characteristics of any given transport into the data model, but instead represent the fields in the model explicitly. RFC8085]. Signal channel implementations using such connectionless transports, such as UDP, therefore MUST include a congestion control mechanism. Signal channel implementations using an IETF standard congestion- controlled transport protocol (like TCP) may rely on built-in transport congestion control support. DOTS-ARCH] and DOTS protocol documents. Impersonation of either a DOTS server or a DOTS client could have catastrophic impact on operations in either domain. If an attacker has the ability to impersonate a DOTS client, that attacker can affect policy on the network path to the DOTS client's domain up to and including instantiation of drop-lists blocking all inbound traffic to networks for which the DOTS client is authorized to request mitigation.
Similarly, an impersonated DOTS server may be able to act as a sort of malicious DOTS gateway, intercepting requests from the downstream DOTS client and modifying them before transmission to the DOTS server to inflict the desired impact on traffic to or from the DOTS client's domain. Among other things, this malicious DOTS gateway might receive and discard mitigation requests from the DOTS client, ensuring no requested mitigation is ever applied. To detect misuse, as detailed in Section 2.4, DOTS implementations require mutual authentication of DOTS agents in order to make agent impersonation more difficult. However, impersonation may still be possible as a result of credential theft, implementation flaws, or DOTS agents being compromised. To detect compromised DOTS agents, DOTS operators should carefully monitor and audit DOTS agents to detect misbehavior and deter misuse while employing best current practices to secure network communications to reduce attack surface. Blocking communication between DOTS agents has the potential to disrupt the core function of DOTS, which is to request mitigation of active or expected DDoS attacks. The DOTS signal channel is expected to operate over congested inbound links, and, as described in Section 2.2, the signal channel protocol must be designed for minimal data transfer to reduce the incidence of signal loss. [RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, August 1980, <https://www.rfc-editor.org/info/rfc768>. [RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, DOI 10.17487/RFC0791, September 1981, <https://www.rfc-editor.org/info/rfc791>. [RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981, <https://www.rfc-editor.org/info/rfc793>.
[RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, <https://www.rfc-editor.org/info/rfc1035>. [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, DOI 10.17487/RFC1122, October 1989, <https://www.rfc-editor.org/info/rfc1122>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, <https://www.rfc-editor.org/info/rfc3986>. [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2006, <https://www.rfc-editor.org/info/rfc4291>. [RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan", BCP 122, RFC 4632, DOI 10.17487/RFC4632, August 2006, <https://www.rfc-editor.org/info/rfc4632>. [RFC4787] Audet, F., Ed. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January 2007, <https://www.rfc-editor.org/info/rfc4787>. [RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6 Address Text Representation", RFC 5952, DOI 10.17487/RFC5952, August 2010, <https://www.rfc-editor.org/info/rfc5952>. [RFC6888] Perreault, S., Ed., Yamagata, I., Miyakawa, S., Nakagawa, A., and H. Ashida, "Common Requirements for Carrier-Grade NATs (CGNs)", BCP 127, RFC 6888, DOI 10.17487/RFC6888, April 2013, <https://www.rfc-editor.org/info/rfc6888>. [RFC7857] Penno, R., Perreault, S., Boucadair, M., Ed., Sivakumar, S., and K. Naito, "Updates to Network Address Translation (NAT) Behavioral Requirements", BCP 127, RFC 7857, DOI 10.17487/RFC7857, April 2016, <https://www.rfc-editor.org/info/rfc7857>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085, March 2017, <https://www.rfc-editor.org/info/rfc8085>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, July 2017, <https://www.rfc-editor.org/info/rfc8200>. [DOTS-ARCH] Mortensen, A., Ed., Reddy, T., Ed., Andreasen, F., Teague, N., and R. Compton, "Distributed-Denial-of-Service Open Threat Signaling (DOTS) Architecture", Work in Progress, draft-ietf-dots-architecture-13, April 2019. [DOTS-USE] Dobbins, R., Migault, D., Fouant, S., Moskowitz, R., Teague, N., Xia, L., and K. Nishizuka, "Use cases for DDoS Open Threat Signaling", Work in Progress, draft-ietf-dots- use-cases-17, January 2019. [IP-FRAG-FRAGILE] Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O., and F. Gont, "IP Fragmentation Considered Fragile", Work in Progress, draft-ietf-intarea-frag-fragile-10, May 2019. [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002, <https://www.rfc-editor.org/info/rfc3261>. [RFC7092] Kaplan, H. and V. Pascual, "A Taxonomy of Session Initiation Protocol (SIP) Back-to-Back User Agents", RFC 7092, DOI 10.17487/RFC7092, December 2013, <https://www.rfc-editor.org/info/rfc7092>. [RFC4732] Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet Denial-of-Service Considerations", RFC 4732, DOI 10.17487/RFC4732, December 2006, <https://www.rfc-editor.org/info/rfc4732>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007, <https://www.rfc-editor.org/info/rfc4949>. [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015, <https://www.rfc-editor.org/info/rfc7525>.