



Internet Engineering Task Force (IETF)                 A. Mortensen, Ed.
Request for Comments: 8811                                    Forcepoint
Category: Informational                                  T. Reddy.K, Ed.
ISSN: 2070-1721                                             McAfee, Inc.
                                                            F. Andreasen
                                                                   Cisco
                                                               N. Teague
                                                           Iron Mountain
                                                              R. Compton
                                                                 Charter
                                                             August 2020


             DDoS Open Threat Signaling (DOTS) Architecture

Abstract

   This document describes an architecture for establishing and
   maintaining Distributed Denial-of-Service (DDoS) Open Threat
   Signaling (DOTS) within and between domains.  The document does not
   specify protocols or protocol extensions, instead focusing on
   defining architectural relationships, components, and concepts used
   in a DOTS deployment.

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/rfc8811.

Copyright Notice

   Copyright (c) 2020 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.

Table of Contents

   1.  Context and Motivation
     1.1.  Terminology
       1.1.1.  Key Words
       1.1.2.  Definition of Terms
     1.2.  Scope
     1.3.  Assumptions
   2.  DOTS Architecture
     2.1.  DOTS Operations
     2.2.  Components
       2.2.1.  DOTS Client
       2.2.2.  DOTS Server
       2.2.3.  DOTS Gateway
     2.3.  DOTS Agent Relationships
       2.3.1.  Gatewayed Signaling
   3.  Concepts
     3.1.  DOTS Sessions
       3.1.1.  Preconditions
       3.1.2.  Establishing the DOTS Session
       3.1.3.  Maintaining the DOTS Session
     3.2.  Modes of Signaling
       3.2.1.  Direct Signaling
       3.2.2.  Redirected Signaling
       3.2.3.  Recursive Signaling
       3.2.4.  Anycast Signaling
       3.2.5.  Signaling Considerations for Network Address
               Translation
     3.3.  Triggering Requests for Mitigation
       3.3.1.  Manual Mitigation Request
       3.3.2.  Automated Conditional Mitigation Request
       3.3.3.  Automated Mitigation on Loss of Signal
   4.  IANA Considerations
   5.  Security Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Acknowledgments
   Contributors
   Authors' Addresses

1.  Context and Motivation

   Signaling the need for help to defend against an active distributed
   denial-of-service (DDoS) attack requires a common understanding of
   mechanisms and roles among the parties coordinating a defensive
   response.  The signaling layer and supplementary messaging are the
   focus of DDoS Open Threat Signaling (DOTS).  DOTS defines a method of
   coordinating defensive measures among willing peers to mitigate
   attacks quickly and efficiently, enabling hybrid attack responses
   coordinated locally at or near the target of an active attack, or
   anywhere in path between attack sources and target.  Sample DOTS use
   cases are elaborated in [DOTS-USE-CASES].

   This document describes an architecture used in establishing,
   maintaining, or terminating a DOTS relationship within a domain or
   between domains.

1.1.  Terminology

1.1.1.  Key Words

   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.

1.1.2.  Definition of Terms

   This document uses the terms defined in [RFC8612].

1.2.  Scope

   In this architecture, DOTS clients and servers communicate using DOTS
   signal channel [RFC8782] and data channel [RFC8783] protocols.

   The DOTS architecture presented here is applicable across network
   administrative domains, for example, between an enterprise domain and
   the domain of a third-party attack mitigation service, as well as to
   a single administrative domain.  DOTS is generally assumed to be most
   effective when aiding coordination of attack response between two or
   more participating networks, but single domain scenarios are valuable
   in their own right, as when aggregating intra-domain DOTS client
   signals for an inter-domain coordinated attack response.

   This document does not address any administrative or business
   agreements that may be established between involved DOTS parties.
   Those considerations are out of scope.  Regardless, this document
   assumes necessary authentication and authorization mechanisms are put
   in place so that only authorized clients can invoke the DOTS service.

   A detailed set of DOTS requirements are discussed in [RFC8612], and
   the DOTS architecture is designed to follow those requirements.  Only
   new behavioral requirements are described in this document.

1.3.  Assumptions

   This document makes the following assumptions:

   *  All domains in which DOTS is deployed are assumed to offer the
      required connectivity between DOTS agents and any intermediary
      network elements, but the architecture imposes no additional
      limitations on the form of connectivity.

   *  Congestion and resource exhaustion are intended outcomes of a DDoS
      attack [RFC4732].  Some operators may utilize non-impacted paths
      or networks for DOTS.  However, in general, conditions should be
      assumed to be hostile, and DOTS must be able to function in all
      circumstances, including when the signaling path is significantly
      impaired.  Congestion control requirements are discussed in
      Section 3 of [RFC8612].  The DOTS signal channel defined in
      [RFC8782] is designed to be extremely resilient under extremely
      hostile network conditions, and it provides continued contact
      between DOTS agents even as DDoS attack traffic saturates the
      link.

   *  There is no universal DDoS attack scale threshold triggering a
      coordinated response across administrative domains.  A network
      domain administrator or service or application owner may
      arbitrarily set attack scale threshold triggers, or manually send
      requests for mitigation.

   *  Mitigation requests may be sent to one or more upstream DOTS
      servers based on criteria determined by DOTS client administrators
      and the underlying network configuration.  The number of DOTS
      servers with which a given DOTS client has established
      communications is determined by local policy and is deployment
      specific.  For example, a DOTS client of a multihomed network may
      support built-in policies to establish DOTS relationships with
      DOTS servers located upstream of each interconnection link.

   *  The mitigation capacity and/or capability of domains receiving
      requests for coordinated attack response is opaque to the domains
      sending the request.  The domain receiving the DOTS client signal
      may or may not have sufficient capacity or capability to filter
      any or all DDoS attack traffic directed at a target.  In either
      case, the upstream DOTS server may redirect a request to another
      DOTS server.  Redirection may be local to the redirecting DOTS
      server's domain or may involve a third-party domain.

   *  DOTS client and server signals, as well as messages sent through
      the data channel, are sent across any transit networks with the
      same probability of delivery as any other traffic between the DOTS
      client domain and the DOTS server domain.  Any encapsulation
      required for successful delivery is left untouched by transit
      network elements.  DOTS servers and DOTS clients cannot assume any
      preferential treatment of DOTS signals.  Such preferential
      treatment may be available in some deployments (e.g., intra-domain
      scenarios), and the DOTS architecture does not preclude its use
      when available.  However, DOTS itself does not address how that
      may be done.

   *  The architecture allows for, but does not assume, the presence of
      Quality-of-Service (QoS) policy agreements between DOTS-enabled
      peer networks or local QoS prioritization aimed at ensuring
      delivery of DOTS messages between DOTS agents.  QoS is an
      operational consideration only, not a functional part of the DOTS
      architecture.

   *  The signal and data channels are loosely coupled and might not
      terminate on the same DOTS server.  How the DOTS servers
      synchronize the DOTS configuration is out of scope of this
      specification.

2.  DOTS Architecture

   The basic high-level DOTS architecture is illustrated in Figure 1:

       +-----------+            +-------------+
       | Mitigator | ~~~~~~~~~~ | DOTS Server |
       +-----------+            +-------------+
                                       |
                                       |
                                       |
       +---------------+        +-------------+
       | Attack Target | ~~~~~~ | DOTS Client |
       +---------------+        +-------------+

                     Figure 1: Basic DOTS Architecture

   A simple example instantiation of the DOTS architecture could be an
   enterprise as the attack target for a volumetric DDoS attack and an
   upstream DDoS mitigation service as the mitigator.  The service
   provided by the mitigator is called "DDoS mitigation service".  The
   enterprise (attack target) is connected to the Internet via a link
   that is getting saturated, and the enterprise suspects it is under
   DDoS attack.  The enterprise has a DOTS client, which obtains
   information about the DDoS attack and signals the DOTS server for
   help in mitigating the attack.  In turn, the DOTS server invokes one
   or more mitigators, which are tasked with mitigating the actual DDoS
   attack and, hence, aim to suppress the attack traffic while allowing
   valid traffic to reach the attack target.

   The scope of the DOTS specifications is the interfaces between the
   DOTS client and DOTS server.  The interfaces to the attack target and
   the mitigator are out of scope of DOTS.  Similarly, the operation of
   both the attack target and the mitigator is out of scope of DOTS.
   Thus, DOTS specifies neither how an attack target decides it is under
   DDoS attack nor does DOTS specify how a mitigator may actually
   mitigate such an attack.  A DOTS client's request for mitigation is
   advisory in nature and might not lead to any mitigation at all,
   depending on the DOTS server domain's capacity and willingness to
   mitigate on behalf of the DOTS client domain.

   The DOTS client may be provided with a list of DOTS servers, each
   associated with one or more IP addresses.  These addresses may or may
   not be of the same address family.  The DOTS client establishes one
   or more sessions by connecting to the provided DOTS server addresses.

   As illustrated in Figure 2, there are two interfaces between a DOTS
   server and a DOTS client: a signal channel and (optionally) a data
   channel.

     +---------------+                                 +---------------+
     |               | <------- Signal Channel ------> |               |
     |  DOTS Client  |                                 |  DOTS Server  |
     |               | <=======  Data Channel  ======> |               |
     +---------------+                                 +---------------+

                         Figure 2: DOTS Interfaces

   The primary purpose of the signal channel is for a DOTS client to ask
   a DOTS server for help in mitigating an attack and for the DOTS
   server to inform the DOTS client about the status of such mitigation.
   The DOTS client does this by sending a client signal that contains
   information about the attack target(s).  The client signal may also
   include telemetry information about the attack, if the DOTS client
   has such information available.  In turn, the DOTS server sends a
   server signal to inform the DOTS client of whether it will honor the
   mitigation request.  Assuming it will, the DOTS server initiates
   attack mitigation and periodically informs the DOTS client about the
   status of the mitigation.  Similarly, the DOTS client periodically
   informs the DOTS server about the client's status, which, at a
   minimum, provides client (attack target) health information; it
   should also include efficacy information about the attack mitigation
   as it is now seen by the client.  At some point, the DOTS client may
   decide to terminate the server-side attack mitigation, which it
   indicates to the DOTS server over the signal channel.  A mitigation
   may also be terminated if a DOTS client-specified mitigation lifetime
   is exceeded.  Note that the signal channel may need to operate over a
   link that is experiencing a DDoS attack and, hence, is subject to
   severe packet loss and high latency.

   While DOTS is able to request mitigation with just the signal
   channel, the addition of the DOTS data channel provides for
   additional, more efficient capabilities.  The primary purpose of the
   data channel is to support DOTS-related configuration and policy
   information exchange between the DOTS client and the DOTS server.
   Examples of such information include, but are not limited to:

   *  Creating identifiers, such as names or aliases, for resources for
      which mitigation may be requested.  Such identifiers may then be
      used in subsequent signal channel exchanges to refer more
      efficiently to the resources under attack.

   *  Drop-list management, which enables a DOTS client to inform the
      DOTS server about sources to suppress.

   *  Accept-list management, which enables a DOTS client to inform the
      DOTS server about sources from which traffic is always accepted.

   *  Filter management, which enables a DOTS client to install or
      remove traffic filters dropping or rate-limiting unwanted traffic.

   *  DOTS client provisioning.

   Note that, while it is possible to exchange the above information
   before, during, or after a DDoS attack, DOTS requires reliable
   delivery of this information and does not provide any special means
   for ensuring timely delivery of it during an attack.  In practice,
   this means that DOTS deployments should rely on such information
   being exchanged only under normal traffic conditions.

2.1.  DOTS Operations

   DOTS does not prescribe any specific deployment models; however, DOTS
   is designed with some specific requirements around the different DOTS
   agents and their relationships.

   First of all, a DOTS agent belongs to a domain that has an identity
   that can be authenticated and authorized.  DOTS agents communicate
   with each other over a mutually authenticated signal channel and
   (optionally) data channel.  However, before they can do so, a service
   relationship needs to be established between them.  The details and
   means by which this is done is outside the scope of DOTS; however, an
   example would be for an enterprise A (DOTS client) to sign up for
   DDoS service from provider B (DOTS server).  This would establish a
   (service) relationship between the two that enables enterprise A's
   DOTS client to establish a signal channel with provider B's DOTS
   server.  A and B will authenticate each other, and B can verify that
   A is authorized for its service.

   From an operational and design point of view, DOTS assumes that the
   above relationship is established prior to a request for DDoS attack
   mitigation.  In particular, it is assumed that bidirectional
   communication is possible at this time between the DOTS client and
   DOTS server.  Furthermore, it is assumed that additional service
   provisioning, configuration, and information exchange can be
   performed by use of the data channel if operationally required.  It
   is not until this point that the mitigation service is available for
   use.

   Once the mutually authenticated signal channel has been established,
   it will remain active.  This is done to increase the likelihood that
   the DOTS client can signal the DOTS server for help when the attack
   target is being flooded, and similarly raise the probability that
   DOTS server signals reach the client regardless of inbound link
   congestion.  This does not necessarily imply that the attack target
   and the DOTS client have to be co-located in the same administrative
   domain, but it is expected to be a common scenario.

   DDoS mitigation with the help of an upstream mitigator may involve
   some form of traffic redirection whereby traffic destined for the
   attack target is steered towards the mitigator.  Common mechanisms to
   achieve this redirection depend on BGP [RFC4271] and DNS [RFC1035].
   In turn, the mitigator inspects and scrubs the traffic and forwards
   the resulting (hopefully non-attack) traffic to the attack target.
   Thus, when a DOTS server receives an attack mitigation request from a
   DOTS client, it can be viewed as a way of causing traffic redirection
   for the attack target indicated.

   DOTS relies on mutual authentication and the pre-established service
   relationship between the DOTS client domain and the DOTS server
   domain to provide authorization.  The DOTS server should enforce
   authorization mechanisms to restrict the mitigation scope a DOTS
   client can request, but such authorization mechanisms are deployment
   specific.

   Although co-location of DOTS server and mitigator within the same
   domain is expected to be a common deployment model, it is assumed
   that operators may require alternative models.  Nothing in this
   document precludes such alternatives.

2.2.  Components

2.2.1.  DOTS Client

   A DOTS client is a DOTS agent from which requests for help
   coordinating an attack response originate.  The requests may be in
   response to an active, ongoing attack against a target in the DOTS
   client domain, but no active attack is required for a DOTS client to
   request help.  Operators may wish to have upstream mitigators in the
   network path for an indefinite period and are restricted only by
   business relationships when it comes to duration and scope of
   requested mitigation.

   The DOTS client requests attack response coordination from a DOTS
   server over the signal channel, including in the request the DOTS
   client's desired mitigation scoping, as described in [RFC8612] (SIG-
   008).  The actual mitigation scope and countermeasures used in
   response to the attack are up to the DOTS server and mitigator
   operators, as the DOTS client may have a narrow perspective on the
   ongoing attack.  As such, the DOTS client's request for mitigation
   should be considered advisory: guarantees of DOTS server availability
   or mitigation capacity constitute Service Level Agreements (SLAs) and
   are out of scope for this document.

   The DOTS client adjusts mitigation scope and provides available
   mitigation feedback (e.g., mitigation efficacy) at the direction of
   its local administrator.  Such direction may involve manual or
   automated adjustments in response to updates from the DOTS server.

   To provide a metric of signal health and distinguish an idle signal
   channel from a disconnected or defunct session, the DOTS client sends
   a heartbeat over the signal channel to maintain its half of the
   channel.  The DOTS client similarly expects a heartbeat from the DOTS
   server and may consider a session terminated in the extended absence
   of a DOTS server heartbeat.

2.2.2.  DOTS Server

   A DOTS server is a DOTS agent capable of receiving, processing, and
   possibly acting on requests for help coordinating attack responses
   from DOTS clients.  The DOTS server authenticates and authorizes DOTS
   clients as described in Section 3.1 and maintains session state,
   tracks requests for mitigation, reports on the status of active
   mitigations, and terminates sessions in the extended absence of a
   client heartbeat or when a session times out.

   Assuming the preconditions discussed below exist, a DOTS client
   maintaining an active session with a DOTS server may reasonably
   expect some level of mitigation in response to a request for
   coordinated attack response.

   For a given DOTS client (administrative) domain, the DOTS server
   needs to be able to determine whether a given resource is in that
   domain.  For example, this could take the form of associating a set
   of IP addresses and/or prefixes per DOTS client domain.  The DOTS
   server enforces authorization of signals for mitigation, filtering
   rules, and aliases for resources from DOTS clients.  The mechanism of
   enforcement is not in scope for this document but is expected to
   restrict mitigation requests, filtering rules, aliases for addresses
   and prefixes, and/or services owned by the DOTS client domain, such
   that a DOTS client from one domain is not able to influence the
   network path to another domain.  A DOTS server MUST reject mitigation
   requests, filtering rules, and aliases for resources not owned by the
   requesting DOTS client's administrative domain.  The exact mechanism
   for the DOTS servers to validate that the resources are within the
   scope of the DOTS client domain is deployment specific.  For example,
   if the DOTS client domain uses Provider-Aggregatable prefixes for its
   resources and leverages the DDoS mitigation service of the Internet
   Transit Provider (ITP); the ITP knows the prefixes assigned to the
   DOTS client domain because they are assigned by the ITP itself.
   However, if the DDoS Mitigation is offered by a third-party DDoS
   mitigation service provider; it does not know the resources owned by
   the DOTS client domain.  The DDoS mitigation service provider and the
   DOTS client domain can opt to use the identifier validation
   challenges discussed in [RFC8555] and [RFC8738] to identify whether
   or not the DOTS client domain actually controls the resources.  The
   challenges for validating control of resources must be performed when
   no attack traffic is present and works only for "dns" and "ip"
   identifier types.  Further, if the DOTS client lies about the
   resources owned by the DOTS client domain, the DDoS mitigation
   service provider can impose penalties for violating the SLA.  A DOTS
   server MAY also refuse a DOTS client's mitigation request for
   arbitrary reasons, within any limits imposed by business or SLAs
   between client and server domains.  If a DOTS server refuses a DOTS
   client's request for mitigation, the DOTS server MUST include the
   refusal reason in the server signal sent to the client.

   A DOTS server is in regular contact with one or more mitigators.  If
   a DOTS server accepts a DOTS client's request for help, the DOTS
   server forwards a translated form of that request to the mitigator(s)
   responsible for scrubbing attack traffic.  Note that the form of the
   translated request passed from the DOTS server to the mitigator is
   not in scope; it may be as simple as an alert to mitigator operators,
   or highly automated using vendor or open application programming
   interfaces supported by the mitigator.  The DOTS server MUST report
   the actual scope of any mitigation enabled on behalf of a client.

   The DOTS server SHOULD retrieve available metrics for any mitigations
   activated on behalf of a DOTS client and SHOULD include them in
   server signals sent to the DOTS client originating the request for
   mitigation.

   To provide a metric of signal health and distinguish an idle signal
   channel from a disconnected or defunct channel, the DOTS server MUST
   send a heartbeat over the signal channel to maintain its half of the
   channel.  The DOTS server similarly expects a heartbeat from the DOTS
   client and MAY consider a session terminated in the extended absence
   of a DOTS client heartbeat.

2.2.3.  DOTS Gateway

   Traditional client/server relationships may be expanded by chaining
   DOTS sessions.  This chaining is enabled through "logical
   concatenation" of a DOTS server and a DOTS client, resulting in an
   application analogous to the Session Initiation Protocol (SIP)
   [RFC3261] logical entity of a Back-to-Back User Agent (B2BUA)
   [RFC7092].  The term "DOTS gateway" is used here in the descriptions
   of selected scenarios involving this application.

   A DOTS gateway may be deployed client side, server side, or both.
   The gateway may terminate multiple discrete client connections and
   may aggregate these into a single or multiple DOTS session(s).

   The DOTS gateway will appear as a server to its downstream agents and
   as a client to its upstream agents, a functional concatenation of the
   DOTS client and server roles, as depicted in Figure 3:

                         +-------------+
                         |    | D |    |
         +----+          |    | O |    |         +----+
         | c1 |----------| s1 | T | c2 |---------| s2 |
         +----+          |    | S |    |         +----+
                         |    | G |    |
                         +-------------+

                           Figure 3: DOTS Gateway

   The DOTS gateway MUST perform full stack DOTS session termination and
   reorigination between its client and server side.  The details of how
   this is achieved are implementation specific.

2.3.  DOTS Agent Relationships

   So far, we have only considered a relatively simple scenario of a
   single DOTS client associated with a single DOTS server; however,
   DOTS supports more advanced relationships.

   A DOTS server may be associated with one or more DOTS clients, and
   those DOTS clients may belong to different domains.  An example
   scenario is a mitigation provider serving multiple attack targets
   (Figure 4).

      DOTS clients       DOTS server
      +---+
      | c |-----------
      +---+           \
      c1.example.org   \
                        \
      +---+              \ +---+
      | c |----------------| S |
      +---+              / +---+
      c1.example.com    /  dots1.example.net
                       /
      +---+           /
      | c |-----------
      +---+
      c2.example.com

                Figure 4: DOTS Server with Multiple Clients

   A DOTS client may be associated with one or more DOTS servers, and
   those DOTS servers may belong to different domains.  This may be to
   ensure high availability or coordinate mitigation with more than one
   directly connected ISP.  An example scenario is for an enterprise to
   have DDoS mitigation service from multiple providers, as shown in
   Figure 5.

      DOTS client        DOTS servers
                          +---+
               -----------| S |
              /           +---+
             /            dots1.example.net
            /
      +---+/              +---+
      | c |---------------| S |
      +---+\              +---+
            \             dots.example.org
             \
              \           +---+
               -----------| S |
                          +---+
      c.example.com       dots2.example.net

                      Figure 5: Multihomed DOTS Client

   Deploying a multihomed client requires extra care and planning, as
   the DOTS servers with which the multihomed client communicates might
   not be affiliated.  Should the multihomed client simultaneously
   request for mitigation from all servers with which it has established
   signal channels, the client may unintentionally inflict additional
   network disruption on the resources it intends to protect.  In one of
   the worst cases, a multihomed DOTS client could cause a permanent
   routing loop of traffic destined for the client's protected services,
   as the uncoordinated DOTS servers' mitigators all try to divert that
   traffic to their own scrubbing centers.

   The DOTS protocol itself provides no fool-proof method to prevent
   such self-inflicted harms as a result of deploying multihomed DOTS
   clients.  If DOTS client implementations nevertheless include support
   for multihoming, they are expected to be aware of the risks, and
   consequently to include measures aimed at reducing the likelihood of
   negative outcomes.  Simple measures might include:

   *  Requesting mitigation serially, ensuring only one mitigation
      request for a given address space is active at any given time;

   *  Dividing the protected resources among the DOTS servers, such that
      no two mitigators will be attempting to divert and scrub the same
      traffic;

   *  Restricting multihoming to deployments in which all DOTS servers
      are coordinating management of a shared pool of mitigation
      resources.

2.3.1.  Gatewayed Signaling

   As discussed in Section 2.2.3, a DOTS gateway is a logical function
   chaining DOTS sessions through concatenation of a DOTS server and
   DOTS client.

   An example scenario, as shown in Figure 6 and Figure 7, is for an
   enterprise to have deployed multiple DOTS-capable devices that are
   able to signal intra-domain using TCP [RFC0793] on uncongested links
   to a DOTS gateway that may then transform these to a UDP [RFC0768]
   transport inter-domain where connection-oriented transports may
   degrade; this applies to the signal channel only, as the data channel
   requires a connection-oriented transport.  The relationship between
   the gateway and its upstream agents is opaque to the initial clients.

         +---+
         | c |\
         +---+ \              +---+
                \-----TCP-----| D |               +---+
         +---+                | O |               |   |
         | c |--------TCP-----| T |------UDP------| S |
         +---+                | S |               |   |
                /-----TCP-----| G |               +---+
         +---+ /              +---+
         | c |/
         +---+
         example.com       example.com           example.net
         DOTS clients      DOTS gateway (DOTSG)  DOTS server

               Figure 6: Client-Side Gateway with Aggregation

         +---+
         | c |\
         +---+ \              +---+
                \-----TCP-----| D |------UDP------+---+
         +---+                | O |               |   |
         | c |--------TCP-----| T |------UDP------| S |
         +---+                | S |               |   |
                /-----TCP-----| G |------UDP------+---+
         +---+ /              +---+
         | c |/
         +---+
         example.com       example.com           example.net
         DOTS clients      DOTS gateway (DOTSG)  DOTS server

             Figure 7: Client-Side Gateway without Aggregation

   This may similarly be deployed in the inverse scenario where the
   gateway resides in the server-side domain and may be used to
   terminate and/or aggregate multiple clients to a single transport as
   shown in Figure 8 and Figure 9.

         +---+
         | c |\
         +---+ \              +---+
                \-----UDP-----| D |               +---+
         +---+                | O |               |   |
         | c |--------TCP-----| T |------TCP------| S |
         +---+                | S |               |   |
                /-----TCP-----| G |               +---+
         +---+ /              +---+
         | c |/
         +---+
         example.com       example.net           example.net
         DOTS clients      DOTS gateway (DOTSG)  DOTS server

               Figure 8: Server-Side Gateway with Aggregation

         +---+
         | c |\
         +---+ \              +---+
                \-----UDP-----| D |------TCP------+---+
         +---+                | O |               |   |
         | c |--------TCP-----| T |------TCP------| S |
         +---+                | S |               |   |
                /-----UDP-----| G |------TCP------+---+
         +---+ /              +---+
         | c |/
         +---+
         example.com       example.net           example.net
         DOTS clients      DOTS gateway (DOTSG)  DOTS server

             Figure 9: Server-Side Gateway without Aggregation

   This document anticipates scenarios involving multiple DOTS gateways.
   An example is a DOTS gateway at the network client's side and another
   one at the server side.  The first gateway can be located at Customer
   Premises Equipment (CPE) to aggregate requests from multiple DOTS
   clients enabled in an enterprise network.  The second DOTS gateway is
   deployed on the provider side.  This scenario can be seen as a
   combination of the client-side and server-side scenarios.

3.  Concepts

3.1.  DOTS Sessions

   In order for DOTS to be effective as a vehicle for DDoS mitigation
   requests, one or more DOTS clients must establish ongoing
   communication with one or more DOTS servers.  While the preconditions
   for enabling DOTS in or among network domains may also involve
   business relationships, SLAs, or other formal or informal
   understandings between network operators, such considerations are out
   of scope for this document.

   A DOTS session is established to support bilateral exchange of data
   between an associated DOTS client and a DOTS server.  In the DOTS
   architecture, data is exchanged between DOTS agents over signal and
   data channels.  As such, a DOTS session can be a DOTS signal channel
   session, a DOTS data channel session, or both.  The DOTS server
   couples the DOTS signal and data channel sessions using the DOTS
   client identity.  The DOTS session is further elaborated in the DOTS
   signal channel protocol defined in [RFC8782] and the DOTS data
   channel protocol defined in [RFC8783].

   A DOTS agent can maintain one or more DOTS sessions.

   A DOTS signal channel session is associated with a single transport
   connection (TCP or UDP session) and a security association (a TLS or
   DTLS session).  Similarly, a DOTS data channel session is associated
   with a single TCP connection and a TLS security association.

   Mitigation requests created using the DOTS signal channel are not
   bound to the DOTS signal channel session.  Instead, mitigation
   requests are associated with a DOTS client and can be managed using
   different DOTS signal channel sessions.

3.1.1.  Preconditions

   Prior to establishing a DOTS session between agents, the owners of
   the networks, domains, services or applications involved are assumed
   to have agreed upon the terms of the relationship involved.  Such
   agreements are out of scope for this document but must be in place
   for a functional DOTS architecture.

   It is assumed that, as part of any DOTS service agreement, the DOTS
   client is provided with all data and metadata required to establish
   communication with the DOTS server.  Such data and metadata would
   include any cryptographic information necessary to meet the message
   confidentiality, integrity, and authenticity requirement (SEC-002) in
   [RFC8612] and might also include the pool of DOTS server addresses
   and ports the DOTS client should use for signal and data channel
   messaging.

3.1.2.  Establishing the DOTS Session

   With the required business agreements in place, the DOTS client
   initiates a DOTS session by contacting its DOTS server(s) over the
   signal channel and (possibly) the data channel.  To allow for DOTS
   service flexibility, neither the order of contact nor the time
   interval between channel creations is specified.  A DOTS client MAY
   establish the signal channel first, and then the data channel, or
   vice versa.

   The methods by which a DOTS client receives the address and
   associated service details of the DOTS server are not prescribed by
   this document.  For example, a DOTS client may be directly configured
   to use a specific DOTS server IP address and port, and be directly
   provided with any data necessary to satisfy the Peer Mutual
   Authentication requirement (SEC-001) in [RFC8612], such as symmetric
   or asymmetric keys, usernames, passwords, etc.  All configuration and
   authentication information in this scenario is provided out of band
   by the domain operating the DOTS server.

   At the other extreme, the architecture in this document allows for a
   form of DOTS client auto-provisioning.  For example, the domain
   operating the DOTS server or servers might provide the client domain
   only with symmetric or asymmetric keys to authenticate the
   provisioned DOTS clients.  Only the keys would then be directly
   configured on DOTS clients, but the remaining configuration required
   to provision the DOTS clients could be learned through mechanisms
   similar to DNS SRV [RFC2782] or DNS Service Discovery [RFC6763].

   The DOTS client SHOULD successfully authenticate and exchange
   messages with the DOTS server over both the signal and (if used) data
   channel as soon as possible to confirm that both channels are
   operational.

   As described in [RFC8612] (DM-008), the DOTS client can configure
   preferred values for acceptable signal loss, mitigation lifetime, and
   heartbeat intervals when establishing the DOTS signal channel
   session.  A DOTS signal channel session is not active until DOTS
   agents have agreed on the values for these DOTS session parameters, a
   process defined by the protocol.

   Once the DOTS client begins receiving DOTS server signals, the DOTS
   session is active.  At any time during the DOTS session, the DOTS
   client may use the data channel to manage aliases, manage drop- and
   accept-listed prefixes or addresses, leverage vendor-specific
   extensions, and so on.  Note that unlike the signal channel, there is
   no requirement that the data channel remains operational in attack
   conditions.  (See "Data Channel Requirements" Section 2.3 of
   [RFC8612]).

3.1.3.  Maintaining the DOTS Session

   DOTS clients and servers periodically send heartbeats to each other
   over the signal channel, discussed in [RFC8612] (SIG-004).  DOTS
   agent operators SHOULD configure the heartbeat interval such that the
   frequency does not lead to accidental denials of service due to the
   overwhelming number of heartbeats a DOTS agent must field.

   Either DOTS agent may consider a DOTS signal channel session
   terminated in the extended absence of a heartbeat from its peer
   agent.  The period of that absence will be established in the
   protocol definition.

3.2.  Modes of Signaling

   This section examines the modes of signaling between agents in a DOTS
   architecture.

3.2.1.  Direct Signaling

   A DOTS session may take the form of direct signaling between the DOTS
   clients and servers, as shown in Figure 10.

           +-------------+                            +-------------+
           | DOTS client |<------signal session------>| DOTS server |
           +-------------+                            +-------------+

                        Figure 10: Direct Signaling

   In a direct DOTS session, the DOTS client and server are
   communicating directly.  Direct signaling may exist inter- or intra-
   domain.  The DOTS session is abstracted from the underlying networks
   or network elements the signals traverse; in direct signaling, the
   DOTS client and server are logically adjacent.

3.2.2.  Redirected Signaling

   In certain circumstances, a DOTS server may want to redirect a DOTS
   client to an alternative DOTS server for a DOTS signal channel
   session.  Such circumstances include but are not limited to:

   *  Maximum number of DOTS signal channel sessions with clients has
      been reached;

   *  Mitigation capacity exhaustion in the mitigator with which the
      specific DOTS server is communicating;

   *  Mitigator outage or other downtime such as scheduled maintenance;

   *  Scheduled DOTS server maintenance;

   *  Scheduled modifications to the network path between DOTS server
      and DOTS client.

   A basic redirected DOTS signal channel session resembles the
   following, as shown in Figure 11.

           +-------------+                            +---------------+
           |             |<-(1)--- DOTS signal ------>|               |
           |             |      channel session 1     |               |
           |             |<=(2)== redirect to B ======|               |
           | DOTS client |                            | DOTS server A |
           |             |X-(4)--- DOTS signal ------X|               |
           |             |      channel session 1     |               |
           |             |                            |               |
           +-------------+                            +---------------+
                  ^
                  |
                 (3) DOTS signal channel
                  |      session 2
                  v
           +---------------+
           | DOTS server B |
           +---------------+

                      Figure 11: Redirected Signaling

   1.  Previously established DOTS signal channel session 1 exists
       between a DOTS client and DOTS server A.

   2.  DOTS server A sends a server signal redirecting the client to
       DOTS server B.

   3.  If the DOTS client does not already have a separate DOTS signal
       channel session with the redirection target, the DOTS client
       initiates and establishes DOTS signal channel session 2 with DOTS
       server B.

   4.  Having redirected the DOTS client, DOTS server A ceases sending
       server signals.  The DOTS client likewise stops sending client
       signals to DOTS server A.  DOTS signal channel session 1 is
       terminated.

3.2.3.  Recursive Signaling

   DOTS is centered around improving the speed and efficiency of a
   coordinated response to DDoS attacks.  One scenario not yet discussed
   involves coordination among federated domains operating DOTS servers
   and mitigators.

   In the course of normal DOTS operations, a DOTS client communicates
   the need for mitigation to a DOTS server, and that server initiates
   mitigation on a mitigator with which the server has an established
   service relationship.  The operator of the mitigator may in turn
   monitor mitigation performance and capacity, as the attack being
   mitigated may grow in severity beyond the mitigating domain's
   capabilities.

   The operator of the mitigator has limited options in the event a DOTS
   client-requested mitigation is being overwhelmed by the severity of
   the attack.  Out-of-scope business or SLAs may permit the mitigating
   domain to drop the mitigation and let attack traffic flow unchecked
   to the target, but this only encourages attack escalation.  In the
   case where the mitigating domain is the upstream service provider for
   the attack target, this may mean the mitigating domain and its other
   services and users continue to suffer the incidental effects of the
   attack.

   A recursive signaling model as shown in Figure 12 offers an
   alternative.  In a variation of the use case "Upstream DDoS
   Mitigation by an Upstream Internet Transit Provider" described in
   [DOTS-USE-CASES], a domain operating a DOTS server and mitigator also
   operates a DOTS client.  This DOTS client has an established DOTS
   session with a DOTS server belonging to a separate administrative
   domain.

   With these preconditions in place, the operator of the mitigator
   being overwhelmed or otherwise performing inadequately may request
   mitigation for the attack target from this separate DOTS-aware
   domain.  Such a request recurses the originating mitigation request
   to the secondary DOTS server in the hope of building a cumulative
   mitigation against the attack.

                        example.net domain
                        . . . . . . . . . . . . . . . . .
                        .    Gn                         .
          +----+    1   .  +----+       +-----------+   .
          | Cc |<--------->| Sn |~~~~~~~| Mitigator |   .
          +----+        .  +====+       |     Mn    |   .
                        .  | Cn |       +-----------+   .
        example.com     .  +----+                       .
           client       .    ^                          .
                        . . .|. . . . . . . . . . . . . .
                             |
                           2 |
                             |
                        . . .|. . . . . . . . . . . . . .
                        .    v                          .
                        .  +----+       +-----------+   .
                        .  | So |~~~~~~~| Mitigator |   .
                        .  +----+       |     Mo    |   .
                        .               +-----------+   .
                        .                               .
                        . . . . . . . . . . . . . . . . .
                        example.org domain

                       Figure 12: Recursive Signaling

   In Figure 12, client Cc signals a request for mitigation across
   inter-domain DOTS session 1 to the DOTS server Sn belonging to the
   example.net domain.  DOTS server Sn enables mitigation on mitigator
   Mn.  DOTS server Sn is half of DOTS gateway Gn, being deployed
   logically back to back with DOTS client Cn, which has preexisting
   inter-domain DOTS session 2 with the DOTS server So belonging to the
   example.org domain.  At any point, DOTS server Sn MAY recurse an
   ongoing mitigation request through DOTS client Cn to DOTS server So,
   in the expectation that mitigator Mo will be activated to aid in the
   defense of the attack target.

   Recursive signaling is opaque to the DOTS client.  To maximize
   mitigation visibility to the DOTS client, however, the recursing
   domain SHOULD provide recursed mitigation feedback in signals
   reporting on mitigation status to the DOTS client.  For example, the
   recursing domain's DOTS server should incorporate available metrics
   such as dropped packet or byte counts from the recursed domain's DOTS
   server into mitigation status messages.

   DOTS clients involved in recursive signaling must be able to withdraw
   requests for mitigation without warning or justification per SIG-006
   in [RFC8612].

   Operators recursing mitigation requests MAY maintain the recursed
   mitigation for a brief protocol-defined period in the event the DOTS
   client originating the mitigation withdraws its request for help, as
   per the discussion of managing mitigation toggling in SIG-006 of
   [RFC8612].

   Deployment of recursive signaling may result in traffic redirection,
   examination, and mitigation extending beyond the initial bilateral
   relationship between DOTS client and DOTS server.  As such, client
   control over the network path of mitigated traffic may be reduced.
   DOTS client operators should be aware of any privacy concerns and
   work with DOTS server operators employing recursive signaling to
   ensure shared sensitive material is suitably protected.  Typically,
   there is a contractual SLA negotiated among the DOTS client domain,
   the recursed domain, and the recursing domain to meet the privacy
   requirements of the DOTS client domain and authorization for the
   recursing domain to request mitigation for the resources controlled
   by the DOTS client domain.

3.2.4.  Anycast Signaling

   The DOTS architecture does not assume the availability of anycast
   within a DOTS deployment, but neither does the architecture exclude
   it.  Domains operating DOTS servers MAY deploy DOTS servers with an
   anycast Service Address as described in BCP 126 [RFC4786].  In such a
   deployment, DOTS clients connecting to the DOTS Service Address may
   be communicating with distinct DOTS servers, depending on the network
   configuration at the time the DOTS clients connect.  Among other
   benefits, anycast signaling potentially offers the following:

   *  Simplified DOTS client configuration, including service discovery
      through the methods described in [RFC7094].  In this scenario, the
      "instance discovery" message would be a DOTS client initiating a
      DOTS session to the DOTS server anycast Service Address, to which
      the DOTS server would reply with a redirection to the DOTS server
      unicast address the client should use for DOTS.

   *  Region- or customer-specific deployments, in which the DOTS
      Service Addresses route to distinct DOTS servers depending on the
      client region or the customer network in which a DOTS client
      resides.

   *  Operational resiliency, spreading DOTS signaling traffic across
      the DOTS server domain's networks, and thereby also reducing the
      potential attack surface, as described in BCP 126 [RFC4786].

3.2.4.1.  Anycast Signaling Considerations

   As long as network configuration remains stable, anycast DOTS
   signaling is to the individual DOTS client indistinct from direct
   signaling.  However, the operational challenges inherent in anycast
   signaling are anything but negligible, and DOTS server operators must
   carefully weigh the risks against the benefits before deploying.

   While the DOTS signal channel primarily operates over UDP per SIG-001
   in [RFC8612], the signal channel also requires mutual authentication
   between DOTS agents, with associated security state on both ends.

   Network instability is of particular concern with anycast signaling,
   as DOTS signal channels are expected to be long lived and potentially
   operating under congested network conditions caused by a volumetric
   DDoS attack.

   For example, a network configuration altering the route to the DOTS
   server during active anycast signaling may cause the DOTS client to
   send messages to a DOTS server other than the one with which it
   initially established a signaling session.  That second DOTS server
   might not have the security state of the existing session, forcing
   the DOTS client to initialize a new DOTS session.  This challenge
   might in part be mitigated by use of resumption via a pre-shared key
   (PSK) in TLS 1.3 [RFC8446] and DTLS 1.3 [DTLS-PROTOCOL] (session
   resumption in TLS 1.2 [RFC5246] and DTLS 1.2 [RFC6347]), but keying
   material must then be available to all DOTS servers sharing the
   anycast Service Address, which has operational challenges of its own.

   While the DOTS client will try to establish a new DOTS session with
   the DOTS server now acting as the anycast DOTS Service Address, the
   link between DOTS client and server may be congested with attack
   traffic, making signal session establishment difficult.  In such a
   scenario, anycast Service Address instability becomes a sort of
   signal session flapping, with obvious negative consequences for the
   DOTS deployment.

   Anycast signaling deployments similarly must also take into account
   active mitigations.  Active mitigations initiated through a DOTS
   session may involve diverting traffic to a scrubbing center.  If the
   DOTS session flaps due to anycast changes as described above,
   mitigation may also flap as the DOTS servers sharing the anycast DOTS
   service address toggles mitigation on detecting DOTS session loss,
   depending on whether or not the client has configured mitigation on
   loss of signal (Section 3.3.3).

3.2.5.  Signaling Considerations for Network Address Translation

   Network address translators (NATs) are expected to be a common
   feature of DOTS deployments.  The middlebox traversal guidelines in
   [RFC8085] include general NAT considerations that are applicable to
   DOTS deployments when the signal channel is established over UDP.

   Additional DOTS-specific considerations arise when NATs are part of
   the DOTS architecture.  For example, DDoS attack detection behind a
   NAT will detect attacks against internal addresses.  A DOTS client
   subsequently asked to request mitigation for the attacked scope of
   addresses cannot reasonably perform the task, due to the lack of
   externally routable addresses in the mitigation scope.

   The following considerations do not cover all possible scenarios but
   are meant rather to highlight anticipated common issues when
   signaling through NATs.

3.2.5.1.  Direct Provisioning of Internal-to-External Address Mappings

   Operators may circumvent the problem of translating internal
   addresses or prefixes to externally routable mitigation scopes by
   directly provisioning the mappings of external addresses to internal
   protected resources on the DOTS client.  When the operator requests
   mitigation scoped for internal addresses, directly or through
   automated means, the DOTS client looks up the matching external
   addresses or prefixes and issues a mitigation request scoped to that
   externally routable information.

   When directly provisioning the address mappings, operators must
   ensure the mappings remain up to date or they risk losing the ability
   to request accurate mitigation scopes.  To that aim, the DOTS client
   can rely on mechanisms such as [RFC8512] or [RFC7658] to retrieve
   static explicit mappings.  This document does not prescribe the
   method by which mappings are maintained once they are provisioned on
   the DOTS client.

3.2.5.2.  Resolving Public Mitigation Scope with Port Control Protocol
          (PCP)

   Port Control Protocol (PCP) [RFC6887] may be used to retrieve the
   external addresses/prefixes and/or port numbers if the NAT function
   embeds a PCP server.

   A DOTS client can use the information retrieved by means of PCP to
   feed the DOTS protocol(s) messages that will be sent to a DOTS
   server.  These messages will convey the external addresses/prefixes
   as set by the NAT.

   PCP also enables discovery and configuration of the lifetime of port
   mappings instantiated in intermediate NAT devices.  Discovery of port
   mapping lifetimes can reduce the dependency on heartbeat messages to
   maintain mappings and, therefore, reduce the load on DOTS servers and
   the network.

3.2.5.3.  Resolving Public Mitigation Scope with Session Traversal
          Utilities (STUN)

   An internal resource, e.g., a web server, can discover its reflexive
   transport address through a STUN Binding request/response
   transaction, as described in [RFC8489].  After learning its reflexive
   transport address from the STUN server, the internal resource can
   export its reflexive transport address and internal transport address
   to the DOTS client, thereby enabling the DOTS client to request
   mitigation with the correct external scope, as depicted in Figure 13.
   The mechanism for providing the DOTS client with the reflexive
   transport address and internal transport address is unspecified in
   this document.

   In order to prevent an attacker from modifying the STUN messages in
   transit, the STUN client and server must use the message-integrity
   mechanism discussed in Section 9 of [RFC8489] or use STUN over DTLS
   [RFC7350] or STUN over TLS.  If the STUN client is behind a NAT that
   performs Endpoint-Dependent Mapping [RFC5128], the internal service
   cannot provide the DOTS client with the reflexive transport address
   discovered using STUN.  The behavior of a NAT between the STUN client
   and the STUN server could be discovered using the experimental
   techniques discussed in [RFC5780], but note that there is currently
   no standardized way for a STUN client to reliably determine if it is
   behind a NAT that performs Endpoint-Dependent Mapping.

               Binding         Binding
   +--------+  request  +---+  request  +--------+
   |  STUN  |<----------| N |<----------|  STUN  |
   | server |           | A |           | client |
   |        |---------->| T |---------->|        |
   +--------+  Binding  +---+ Binding   +--------+
               response       response    |
                                          | reflexive transport address
                                          | & internal transport address
                                          v
                                        +--------+
                                        |  DOTS  |
                                        | client |
                                        +--------+

              Figure 13: Resolving Mitigation Scope with STUN

3.2.5.4.  Resolving Requested Mitigation Scope with DNS

   DOTS supports mitigation scoped to DNS names.  As discussed in
   [RFC3235], using DNS names instead of IP addresses potentially avoids
   the address translation problem, as long as the same domain name is
   internally and externally resolvable.  For example, a detected
   attack's internal target address can be mapped to a DNS name through
   a reverse lookup.  The DNS name returned by the reverse lookup can
   then be provided to the DOTS client as the external scope for
   mitigation.  For the reverse DNS lookup, DNS Security Extensions
   (DNSSEC) [RFC4033] must be used where the authenticity of response is
   critical.

3.3.  Triggering Requests for Mitigation

   [RFC8612] places no limitation on the circumstances in which a DOTS
   client operator may request mitigation, nor does it demand
   justification for any mitigation request, thereby reserving
   operational control over DDoS defense for the domain requesting
   mitigation.  This architecture likewise does not prescribe the
   network conditions and mechanisms triggering a mitigation request
   from a DOTS client.

   However, considering selected possible mitigation triggers from an
   architectural perspective offers a model for alternative or
   unanticipated triggers for DOTS deployments.  In all cases, what
   network conditions merit a mitigation request are at the discretion
   of the DOTS client operator.

   The mitigation request itself is defined by DOTS; however, the
   interfaces required to trigger the mitigation request in the
   following scenarios are implementation specific.

3.3.1.  Manual Mitigation Request

   A DOTS client operator may manually prepare a request for mitigation,
   including scope and duration, and manually instruct the DOTS client
   to send the mitigation request to the DOTS server.  In context, a
   manual request is a request directly issued by the operator without
   automated decision making performed by a device interacting with the
   DOTS client.  Modes of manual mitigation requests include an operator
   entering a command into a text interface, or directly interacting
   with a graphical interface to send the request.

   An operator might do this, for example, in response to notice of an
   attack delivered by attack detection equipment or software, and the
   alerting detector lacks interfaces or is not configured to use
   available interfaces to translate the alert to a mitigation request
   automatically.

   In a variation of the above scenario, the operator may have
   preconfigured on the DOTS client mitigation requests for various
   resources in the operator's domain.  When notified of an attack, the
   DOTS client operator manually instructs the DOTS client to send the
   relevant preconfigured mitigation request for the resources under
   attack.

   A further variant involves recursive signaling, as described in
   Section 3.2.3.  The DOTS client in this case is the second half of a
   DOTS gateway (back-to-back DOTS server and client).  As in the
   previous scenario, the scope and duration of the mitigation request
   are preexisting but, in this case, are derived from the mitigation
   request received from a downstream DOTS client by the DOTS server.
   Assuming the preconditions required by Section 3.2.3 are in place,
   the DOTS gateway operator may at any time manually request mitigation
   from an upstream DOTS server, sending a mitigation request derived
   from the downstream DOTS client's request.

   The motivations for a DOTS client operator to request mitigation
   manually are not prescribed by this architecture but are expected to
   include some of the following:

   *  Notice of an attack delivered via email or alternative messaging

   *  Notice of an attack delivered via phone call

   *  Notice of an attack delivered through the interface(s) of
      networking monitoring software deployed in the operator's domain

   *  Manual monitoring of network behavior through network monitoring
      software

3.3.2.  Automated Conditional Mitigation Request

   Unlike manual mitigation requests, which depend entirely on the DOTS
   client operator's capacity to react with speed and accuracy to every
   detected or detectable attack, mitigation requests triggered by
   detected attack conditions reduce the operational burden on the DOTS
   client operator and minimize the latency between attack detection and
   the start of mitigation.

   Mitigation requests are triggered in this scenario by operator-
   specified network conditions.  Attack detection is deployment
   specific and not constrained by this architecture.  Similarly, the
   specifics of a condition are left to the discretion of the operator,
   though common conditions meriting mitigation include the following:

   *  Detected attack exceeding a rate in packets per second (pps).

   *  Detected attack exceeding a rate in bytes per second (bps).

   *  Detected resource exhaustion in an attack target.

   *  Detected resource exhaustion in the local domain's mitigator.

   *  Number of open connections to an attack target.

   *  Number of attack sources in a given attack.

   *  Number of active attacks against targets in the operator's domain.

   *  Conditional detection developed through arbitrary statistical
      analysis or deep learning techniques.

   *  Any combination of the above.

   When automated conditional mitigation requests are enabled,
   violations of any of the above conditions, or any additional
   operator-defined conditions, will trigger a mitigation request from
   the DOTS client to the DOTS server.  The interfaces between the
   application detecting the condition violation and the DOTS client are
   implementation specific.

3.3.3.  Automated Mitigation on Loss of Signal

   To maintain a DOTS signal channel session, the DOTS client and the
   DOTS server exchange regular but infrequent messages across the
   signal channel.  In the absence of an attack, the probability of
   message loss in the signaling channel should be extremely low.  Under
   attack conditions, however, some signal loss may be anticipated as
   attack traffic congests the link, depending on the attack type.

   While [RFC8612] specifies the DOTS protocol be robust when signaling
   under attack conditions, there are nevertheless scenarios in which
   the DOTS signal is lost in spite of protocol best efforts.  To handle
   such scenarios, a DOTS operator may request one or more mitigations,
   which are triggered only when the DOTS server ceases receiving DOTS
   client heartbeats beyond the miss count or interval permitted by the
   protocol.

   The impact of mitigating due to loss of signal in either direction
   must be considered carefully before enabling it.  Attack traffic
   congesting links is not the only reason why signal could be lost, and
   as such, mitigation requests triggered by signal channel degradation
   in either direction may incur unnecessary costs due to scrubbing
   traffic, adversely impact network performance and operational expense
   alike.

4.  IANA Considerations

   This document has no IANA actions.

5.  Security Considerations

   This section describes identified security considerations for the
   DOTS architecture.

   Security considerations and security requirements discussed in
   [RFC8612] need to be taken into account.

   DOTS is at risk from three primary attack vectors: agent
   impersonation, traffic injection, and signal blocking.  These vectors
   may be exploited individually or in concert by an attacker to
   confuse, disable, take information from, or otherwise inhibit DOTS
   agents.

   Any attacker with the ability to impersonate a legitimate DOTS client
   or server or, indeed, inject false messages into the stream may
   potentially trigger/withdraw traffic redirection, trigger/cancel
   mitigation activities or subvert drop-/accept-lists.  From an
   architectural standpoint, operators MUST ensure conformance to the
   security requirements defined in Section 2.4 of [RFC8612] to secure
   data in transit.  Similarly, as the received data may contain network
   topology, telemetry, and threat and mitigation information that could
   be considered sensitive in certain environments, it SHOULD be
   protected at rest per required local policy.

   DOTS agents MUST perform mutual authentication to ensure authenticity
   of each other, and DOTS servers MUST verify that the requesting DOTS
   client is authorized to request mitigation for specific target
   resources (see Section 2.2.2).

   A man-in-the-middle (MITM) attacker can intercept and drop packets,
   preventing the DOTS peers from receiving some or all of the DOTS
   messages; automated mitigation on loss of signal can be used as a
   countermeasure but with risks discussed in Section 3.3.3.

   An attacker with control of a DOTS client may negatively influence
   network traffic by requesting and withdrawing requests for mitigation
   for particular prefixes, leading to route or DNS flapping.  DOTS
   operators should carefully monitor and audit DOTS clients to detect
   misbehavior and deter misuse.

   Any attack targeting the availability of DOTS servers may disrupt the
   ability of the system to receive and process DOTS signals resulting
   in failure to fulfill a mitigation request.  DOTS servers MUST be
   given adequate protections in accordance with best current practices
   for network and host security.

6.  References

6.1.  Normative References

   [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>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/info/rfc4033>.

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
              December 2006, <https://www.rfc-editor.org/info/rfc4786>.

   [RFC6887]  Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
              P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
              DOI 10.17487/RFC6887, April 2013,
              <https://www.rfc-editor.org/info/rfc6887>.

   [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>.

   [RFC8612]  Mortensen, A., Reddy, T., and R. Moskowitz, "DDoS Open
              Threat Signaling (DOTS) Requirements", RFC 8612,
              DOI 10.17487/RFC8612, May 2019,
              <https://www.rfc-editor.org/info/rfc8612>.

6.2.  Informative References

   [DOTS-USE-CASES]
              Dobbins, R., Migault, D., Moskowitz, R., Teague, N., Xia,
              L., and K. Nishizuka, "Use cases for DDoS Open Threat
              Signaling", Work in Progress, Internet-Draft, draft-ietf-
              dots-use-cases-25, 5 July 2020,
              <https://tools.ietf.org/html/draft-ietf-dots-use-cases-
              25>.

   [DTLS-PROTOCOL]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-38, 29 May 2020,
              <https://tools.ietf.org/html/draft-ietf-tls-dtls13-38>.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <https://www.rfc-editor.org/info/rfc2782>.

   [RFC3235]  Senie, D., "Network Address Translator (NAT)-Friendly
              Application Design Guidelines", RFC 3235,
              DOI 10.17487/RFC3235, January 2002,
              <https://www.rfc-editor.org/info/rfc3235>.

   [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>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [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>.

   [RFC5128]  Srisuresh, P., Ford, B., and D. Kegel, "State of Peer-to-
              Peer (P2P) Communication across Network Address
              Translators (NATs)", RFC 5128, DOI 10.17487/RFC5128, March
              2008, <https://www.rfc-editor.org/info/rfc5128>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC5780]  MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
              Using Session Traversal Utilities for NAT (STUN)",
              RFC 5780, DOI 10.17487/RFC5780, May 2010,
              <https://www.rfc-editor.org/info/rfc5780>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [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>.

   [RFC7094]  McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
              "Architectural Considerations of IP Anycast", RFC 7094,
              DOI 10.17487/RFC7094, January 2014,
              <https://www.rfc-editor.org/info/rfc7094>.

   [RFC7350]  Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport
              Layer Security (DTLS) as Transport for Session Traversal
              Utilities for NAT (STUN)", RFC 7350, DOI 10.17487/RFC7350,
              August 2014, <https://www.rfc-editor.org/info/rfc7350>.

   [RFC7658]  Perreault, S., Tsou, T., Sivakumar, S., and T. Taylor,
              "Deprecation of MIB Module NAT-MIB: Managed Objects for
              Network Address Translators (NATs)", RFC 7658,
              DOI 10.17487/RFC7658, October 2015,
              <https://www.rfc-editor.org/info/rfc7658>.

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8489]  Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
              D., Mahy, R., and P. Matthews, "Session Traversal
              Utilities for NAT (STUN)", RFC 8489, DOI 10.17487/RFC8489,
              February 2020, <https://www.rfc-editor.org/info/rfc8489>.

   [RFC8512]  Boucadair, M., Ed., Sivakumar, S., Jacquenet, C.,
              Vinapamula, S., and Q. Wu, "A YANG Module for Network
              Address Translation (NAT) and Network Prefix Translation
              (NPT)", RFC 8512, DOI 10.17487/RFC8512, January 2019,
              <https://www.rfc-editor.org/info/rfc8512>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.

   [RFC8738]  Shoemaker, R.B., "Automated Certificate Management
              Environment (ACME) IP Identifier Validation Extension",
              RFC 8738, DOI 10.17487/RFC8738, February 2020,
              <https://www.rfc-editor.org/info/rfc8738>.

   [RFC8782]  Reddy.K, T., Ed., Boucadair, M., Ed., Patil, P.,
              Mortensen, A., and N. Teague, "Distributed Denial-of-
              Service Open Threat Signaling (DOTS) Signal Channel
              Specification", RFC 8782, DOI 10.17487/RFC8782, May 2020,
              <https://www.rfc-editor.org/info/rfc8782>.

   [RFC8783]  Boucadair, M., Ed. and T. Reddy.K, Ed., "Distributed
              Denial-of-Service Open Threat Signaling (DOTS) Data
              Channel Specification", RFC 8783, DOI 10.17487/RFC8783,
              May 2020, <https://www.rfc-editor.org/info/rfc8783>.

Acknowledgments

   Thanks to Matt Richardson, Roman Danyliw, Frank Xialiang, Roland
   Dobbins, Wei Pan, Kaname Nishizuka, Jon Shallow, Paul Kyzivat, Warren
   Kumari, Benjamin Kaduk, and Mohamed Boucadair for their comments and
   suggestions.

   Special thanks to Roman Danyliw for the AD review.

Contributors

      Mohamed Boucadair
         Orange
         mohamed.boucadair@orange.com

      Cristopher Gray
         Christopher_Gray3@cable.comcast.com

Authors' Addresses

   Andrew Mortensen (editor)
   Forcepoint
   United States of America

   Email: andrewmortensen@gmail.com


   Tirumaleswar Reddy.K (editor)
   McAfee, Inc.
   Embassy Golf Link Business Park
   Bangalore 560071
   Karnataka
   India

   Email: kondtir@gmail.com


   Flemming Andreasen
   Cisco
   United States of America

   Email: fandreas@cisco.com


   Nik Teague
   Iron Mountain
   United States of America

   Email: nteague@ironmountain.co.uk


   Rich Compton
   Charter

   Email: Rich.Compton@charter.com
