





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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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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   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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      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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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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   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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   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>.






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   [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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   [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>.




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   [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


















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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

























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