    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

    Abstract

       This 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.

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    RFC 8612                    DOTS Requirements                   May 2019

    Table of Contents

       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

    1.  Introduction

    1.1.  Context and Motivation

       Distributed Denial-of-Service (DDoS) attacks afflict networks
       connected to the Internet, plaguing network operators at service
       providers and enterprises around the world.  High-volume attacks
       saturating inbound links are now common as attack scale and frequency
       continue to increase.

       The prevalence and impact of these DDoS attacks has led to an
       increased focus on coordinated attack response.  However, many
       enterprises lack the resources or expertise to operate on-premise
       attack mitigation solutions themselves, or are constrained by local
       bandwidth limitations.  To address such gaps, service providers have
       begun to offer on-demand traffic scrubbing services, which are
       designed to separate the DDoS attack traffic from legitimate traffic
       and forward only the latter.

       Today, these services offer proprietary interfaces for subscribers to
       request attack mitigation.  Such proprietary interfaces tie a
       subscriber to a service and limit the abilities of network elements
       that would otherwise be capable of participating in attack
       mitigation.  As a result of signaling interface incompatibility,

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       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].

    1.2.  Terminology

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
       "OPTIONAL" in this document are to be interpreted as described in
       BCP 14 [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.

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       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.

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       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.

    2.  Requirements

       The expected layout and interactions amongst DOTS entities is
       described in the DOTS Architecture [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

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       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.

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    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
          proprietary DDoS defenses.  Future extensions MUST be backward
          compatible.  Implementations of older protocol versions MUST
          ignore optional information added to DOTS messages as part of
          newer protocol versions.  Implementations of older protocol
          versions MUST reject DOTS messages carrying mandatory information
          as part of newer protocol versions.

       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 caused by attack
          traffic.  Additional means to enhance the resilience of DOTS
          protocols, including when multiple DOTS servers are provisioned to
          the DOTS clients, SHOULD be considered.  The protocol MUST be
          resilient, that is, continue operating despite message loss and
          out-of-order or redundant message delivery.  In support of
          signaling protocol robustness, DOTS signals SHOULD be conveyed
          over transport and application protocols not susceptible to head-
          of-line blocking.  These requirements are at SHOULD strength to
          handle middle-boxes and firewall traversal.

       GEN-003  Bulk Data Exchange: Infrequent bulk data exchange between
          DOTS agents can also significantly augment attack response
          coordination, permitting such tasks as population of drop- or
          accept-listed source addresses, address or prefix group aliasing,
          exchange of incident reports, and other hinting or configuration
          supplementing attack responses.

          As the resilience requirements for the DOTS signal channel mandate
          a small signal message size, a separate, secure data channel
          utilizing a reliable transport protocol MUST be used for bulk data
          exchange.  However, reliable bulk data exchange may not be
          possible during attacks causing network congestion.

       GEN-004  Mitigation Hinting: DOTS clients may have access to attack
          details that can be used to inform mitigation techniques.  Example
          attack details might include locally collected fingerprints for an
          on-going attack, or anticipated or active attack focal points
          based on other threat intelligence.  DOTS clients MAY send
          mitigation hints derived from attack details to DOTS servers, with
          the full understanding that the DOTS server MAY ignore mitigation
          hints.  Mitigation hints MUST be transmitted across the signal
          channel, as the data channel may not be functional during an
          attack.  DOTS-server handling of mitigation hints is
          implementation-specific.

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       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.

    2.2.  Signal Channel Requirements

       SIG-001  Use of Common Transport Protocols: DOTS MUST operate over
          common, widely deployed and standardized transport protocols.
          While connectionless transport such as the User Datagram Protocol
          (UDP) [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.

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       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.

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       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.

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          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

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          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.

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       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].

    2.3.  Data Channel Requirements

       The data channel is intended to be used for bulk data exchanges
       between DOTS agents.  Unlike the signal channel, 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 provides a protocol for DOTS configuration and
       management.  For example, a DOTS client may submit to a 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.

       DATA-001  Reliable transport: Messages sent over the data channel
          MUST be delivered reliably in the order sent.

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    RFC 8612                    DOTS Requirements                   May 2019

       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.

    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 signal or data channel is considered
          valid.  The method of authentication is not specified in this
          document but should follow current IETF best practices [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

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    RFC 8612                    DOTS Requirements                   May 2019

          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.

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    RFC 8612                    DOTS Requirements                   May 2019

    2.5.  Data Model Requirements

       A well-structured DOTS data model is critical to the development of
       successful DOTS protocols.

       DM-001  Structure: The data-model structure for the DOTS protocol MAY
          be described by a single module or be divided into related
          collections of hierarchical modules and submodules.  If the data
          model structure is split across modules, those distinct modules
          MUST allow references to describe the overall data model's
          structural dependencies.

       DM-002  Versioning: To ensure interoperability between DOTS protocol
          implementations, data models MUST be versioned.  How the protocols
          represent data-model versions is not defined in this document.

       DM-003  Mitigation Status Representation: The data model MUST provide
          the ability to represent a request for mitigation and the
          withdrawal of such a request.  The data model MUST also support a
          representation of currently-requested mitigation status, including
          failures and their causes.

       DM-004  Mitigation Scope Representation: The data model MUST support
          representation of a requested mitigation's scope.  As mitigation
          scope may be represented in several different ways, per SIG-008,
          the data model MUST include extensible representation of
          mitigation scope.

       DM-005  Mitigation Lifetime Representation: The data model MUST
          support representation of a mitigation request's lifetime,
          including mitigations with no specified end time.

       DM-006  Mitigation Efficacy Representation: The data model MUST
          support representation of a DOTS client's understanding of the
          efficacy of a mitigation enabled through a mitigation request.

       DM-007  Acceptable Signal Loss Representation: The data model MUST be
          able to represent the DOTS agent's preference for acceptable
          signal loss when establishing a signal channel.  Measurements of
          loss might include, but are not restricted to, number of
          consecutive missed heartbeat messages, retransmission count, or
          request timeouts.

       DM-008  Heartbeat Interval Representation: The data model MUST be
          able to represent the DOTS agent's preferred heartbeat interval,
          which the client may include when establishing the signal channel,
          as described in SIG-003.

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    RFC 8612                    DOTS Requirements                   May 2019

       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.

    3.  Congestion Control Considerations

    3.1.  Signal Channel

       As part of a protocol expected to operate over links affected by DDoS
       attack traffic, the DOTS signal channel MUST NOT contribute
       significantly to link congestion.  To meet the signal channel
       requirements above, DOTS signal channel implementations SHOULD
       support connectionless transports.  However, some connectionless
       transports, when deployed naively, can be a source of network
       congestion, as discussed in [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.

    3.2.  Data Channel

       As specified in DATA-001, the data channel requires reliable, in-
       order message delivery.  Data channel implementations using an IETF
       standard congestion-controlled transport protocol may rely on the
       transport implementation's built-in congestion control mechanisms.

    4.  Security Considerations

       This document informs future protocols under development and so does
       not have security considerations of its own.  However, operators
       should be aware of potential risks involved in deploying DOTS.  DOTS
       agent impersonation and signal blocking are discussed here.
       Additional DOTS security considerations may be found in [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.

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    RFC 8612                    DOTS Requirements                   May 2019

       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.

    5.  IANA Considerations

       This document has no IANA actions.

    6.  References

    6.1.  Normative References

       [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>.

    Mortensen, et al.             Informational                    [Page 18]

    RFC 8612                    DOTS Requirements                   May 2019

       [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>.

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    RFC 8612                    DOTS Requirements                   May 2019

       [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>.

    6.2.  Informative References

       [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>.

    Mortensen, et al.             Informational                    [Page 20]

    RFC 8612                    DOTS Requirements                   May 2019

       [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>.

    Acknowledgments

       Thanks to Roman Danyliw, Matt Richardson, Joe Touch, Scott Bradner,
       Robert Sparks, Brian Weis, Benjamin Kaduk, Eric Rescorla, Alvaro
       Retana, Suresh Krishnan, Ben Campbell, Mirja Kuehlewind, and Jon
       Shallow for their careful reading and feedback.

    Contributors

       Mohamed Boucadair
          Orange

          mohamed.boucadair@orange.com

       Flemming Andreasen
          Cisco Systems, Inc.

          fandreas@cisco.com

       Dave Dolson
          Sandvine

          ddolson@sandvine.com

    Mortensen, et al.             Informational                    [Page 21]

    RFC 8612                    DOTS Requirements                   May 2019

    Authors' Addresses

       Andrew Mortensen
       Arbor Networks
       2727 S. State St.
       Ann Arbor, MI  48104
       United States of America

       Email: andrewmortensen@gmail.com

       Tirumaleswar Reddy
       McAfee
       Embassy Golf Link Business Park
       Bangalore, Karnataka  560071
       India

       Email: TirumaleswarReddy_Konda@McAfee.com

       Robert Moskowitz
       Huawei
       Oak Park, MI  42837
       United States of America

       Email: rgm@htt-consult.com

    Mortensen, et al.             Informational                    [Page 22]