Internet Engineering Task Force (IETF)                      M. Boucadair
Request for Comments: 9284                                        Orange
Category: Informational                                       T. Reddy.K
ISSN: 2070-1721                                                    Nokia
                                                                  W. Pan
                                                     Huawei Technologies
                                                             August 2022


  Multihoming Deployment Considerations for DDoS Open Threat Signaling
                                 (DOTS)

Abstract

   This document discusses multihoming considerations for DDoS Open
   Threat Signaling (DOTS).  The goal is to provide some guidance for
   DOTS clients and client-domain DOTS gateways when multihomed.

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/rfc9284.

Copyright Notice

   Copyright (c) 2022 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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Requirements Language
   3.  Terminology
   4.  Multihoming Scenarios
     4.1.  Multihomed Residential: Single CPE
     4.2.  Multihomed Enterprise: Single CPE, Multiple Upstream ISPs
     4.3.  Multihomed Enterprise: Multiple CPEs, Multiple Upstream
           ISPs
     4.4.  Multihomed Enterprise with the Same ISP
   5.  DOTS Multihoming Deployment Considerations
     5.1.  Residential CPE
     5.2.  Multihomed Enterprise: Single CPE, Multiple Upstream ISPs
     5.3.  Multihomed Enterprise: Multiple CPEs, Multiple Upstream
           ISPs
     5.4.  Multihomed Enterprise: Single ISP
   6.  Security Considerations
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   In many deployments, it may not be possible for a network to
   determine the cause of a DDoS attack [RFC4732].  Rather, the network
   may just realize that some resources appear to be under attack.  To
   help with such situations, the IETF has specified the DDoS Open
   Threat Signaling (DOTS) architecture [RFC8811], where a DOTS client
   can inform an upstream DOTS server that its network is under a
   potential attack and that appropriate mitigation actions are
   required.  The DOTS protocols can be used to coordinate real-time
   mitigation efforts that can evolve as the attacks mutate, thereby
   reducing the impact of an attack and leading to more-efficient
   responsive actions.  [RFC8903] identifies a set of scenarios for
   DOTS; most of these scenarios involve a Customer Premises Equipment
   (CPE).

   The high-level base DOTS architecture is illustrated in Figure 1
   (repeated from Section 2 of [RFC8811]):

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

                     Figure 1: Basic DOTS Architecture

   [RFC8811] specifies that the DOTS client may be provided with a list
   of DOTS servers; each of these servers is 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 DOTS sessions by
   connecting to the provided addresses for the DOTS server or servers
   [RFC8973].

   DOTS may be deployed within networks that are connected to one single
   upstream provider.  DOTS can also be enabled within networks that are
   multihomed.  The reader may refer to [RFC3582] for an overview of
   multihoming goals and motivations.  This document discusses DOTS
   multihoming considerations.  Specifically, the document aims to:

   1.  Complete the base DOTS architecture with multihoming specifics.
       Those specifics need to be taken into account because:

       *  Sending a DOTS mitigation request to an arbitrary DOTS server
          will not necessarily help in mitigating a DDoS attack.

       *  Randomly replicating all DOTS mitigation requests among all
          available DOTS servers is suboptimal.

       *  Sequentially contacting DOTS servers may increase the delay
          before a mitigation plan is enforced.

   2.  Identify DOTS deployment schemes in a multihoming context, where
       DOTS services can be offered by all or a subset of upstream
       providers.

   3.  Provide guidelines and recommendations for placing DOTS requests
       in multihomed networks, for example:

       *  Select the appropriate DOTS server(s).

       *  Identify cases where anycast is not recommended for DOTS.

   This document adopts the following methodology:

   *  Identify and extract viable deployment candidates from [RFC8903].

   *  Augment the description with multihoming technicalities, for
      example:

      -  One vs. multiple upstream network providers

      -  One vs. multiple interconnect routers

      -  Provider-Independent (PI) vs. Provider-Aggregatable (PA) IP
         addresses

   *  Describe the recommended behavior of DOTS clients and client-
      domain DOTS gateways for each case.

   Multihomed DOTS agents are assumed to make use of the protocols
   defined in [RFC9132] and [RFC8783].  This document does not require
   any specific extension to the base DOTS protocols for deploying DOTS
   in a multihomed context.

2.  Requirements Language

   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.

3.  Terminology

   This document makes use of the terms defined in [RFC8811], [RFC8612],
   and [RFC4116].  In particular:

   Provider-Aggregatable (PA) addresses:  globally unique addresses
      assigned by a transit provider to a customer.  The addresses are
      considered "aggregatable" because the set of routes corresponding
      to the PA addresses are usually covered by an aggregate route set
      corresponding to the address space operated by the transit
      provider, from which the assignment was made (Section 2 of
      [RFC4116]).

   Provider-Independent (PI) addresses:  globally unique addresses that
      are not assigned by a transit provider, but are provided by some
      other organization, usually a Regional Internet Registry (RIR)
      (Section 2 of [RFC4116]).

   IP indifferently refers to IPv4 or IPv6.

4.  Multihoming Scenarios

   This section describes some multihoming scenarios that are relevant
   to DOTS.  In the following subsections, only the connections of
   border routers are shown; internal network topologies are not
   elaborated.

   A multihomed network may enable DOTS for all or a subset of its
   upstream interconnection links.  In such a case, DOTS servers can be
   explicitly configured or dynamically discovered by a DOTS client
   using means such as those discussed in [RFC8973].  These DOTS servers
   can be owned by the upstream provider, managed by a third-party
   (e.g., mitigation service provider), or a combination thereof.

   If a DOTS server is explicitly configured, it is assumed that an
   interface is also provided to bind the DOTS service to an
   interconnection link.  If no interface is provided, the DOTS server
   can be reached via any active interface.

   This section distinguishes between residential CPEs and enterprise
   CPEs because PI addresses may be used for enterprises, which is not
   the current practice for residential CPEs.

   In the following subsections, all or a subset of interconnection
   links are associated with DOTS servers.

4.1.  Multihomed Residential: Single CPE

   The scenario shown in Figure 2 is characterized as follows:

   *  The home network is connected to the Internet using one single
      CPE.

   *  The CPE is connected to multiple provisioning domains (i.e., both
      fixed and mobile networks).  Provisioning Domain (PvD) is
      explained in [RFC7556].

      In a typical deployment scenario, these provisioning domains are
      owned by the same provider (Section 1 of [RFC8803]).  Such a
      deployment is meant to seamlessly use both fixed and cellular
      networks for bonding, faster handovers, or better resiliency
      purposes.

   *  Each of these provisioning domains assigns IP addresses or
      prefixes to the CPE and provides additional configuration
      information such as a list of DNS servers, DNS suffixes associated
      with the network, the default gateway address, and the DOTS
      server's name [RFC8973].  These addresses or prefixes are assumed
      to be Provider-Aggregatable (PA).

   *  Because of ingress filtering, packets forwarded by the CPE towards
      a given provisioning domain must be sent with a source IP address
      that was assigned by that domain [RFC8043].

                  +-------+            +-------+
                  |Fixed  |            |Mobile |
                  |Network|            |Network|
                  +---+---+            +---+---+
                      |                    |     Service Providers
          ............|....................|.......................
                      +---------++---------+     Home Network
                                ||
                             +--++-+
                             | CPE |
                             +-----+
                                   ... (Internal Network)

                Figure 2: Typical Multihomed Residential CPE

4.2.  Multihomed Enterprise: Single CPE, Multiple Upstream ISPs

   The scenario shown in Figure 3 is characterized as follows:

   *  The enterprise network is connected to the Internet using a single
      router.

   *  That router is connected to multiple provisioning domains managed
      by distinct administrative entities.

   Unlike the previous scenario, two sub-cases can be considered for an
   enterprise network with regard to assigned addresses:

   1.  PI addresses or prefixes: The enterprise is the owner of the IP
       addresses or prefixes; the same address or prefix is then used
       when establishing communications over any of the provisioning
       domains.

   2.  PA addresses or prefixes: Each of the provisioning domains
       assigns IP addresses or prefixes to the enterprise network.
       These addresses or prefixes are used when communicating over the
       provisioning domain that assigned them.

                  +------+              +------+
                  | ISP1 |              | ISP2 |
                  +---+--+              +--+---+
                      |                    |     Service Providers
          ............|....................|.......................
                      +---------++---------+     Enterprise Network
                                ||
                             +--++-+
                             | CPE |
                             +-----+
                                   ... (Internal Network)

      Figure 3: Multihomed Enterprise Network (Single CPE Connected to
                             Multiple Networks)

4.3.  Multihomed Enterprise: Multiple CPEs, Multiple Upstream ISPs

   This scenario is similar to the one described in Section 4.2; the
   main difference is that dedicated routers (CPE1 and CPE2) are used to
   connect to each provisioning domain.

                            +------+    +------+
                            | ISP1 |    | ISP2 |
                            +---+--+    +--+---+
                                |          |     Service Providers
          ......................|..........|.......................
                                |          |     Enterprise Network
                            +---+--+    +--+---+
                            | CPE1 |    | CPE2 |
                            +------+    +------+

                                  ... (Internal Network)

      Figure 4: Multihomed Enterprise Network (Multiple CPEs, Multiple
                                   ISPs)

4.4.  Multihomed Enterprise with the Same ISP

   This scenario is a variant of Sections 4.2 and 4.3 in which
   multihoming is supported by the same ISP (i.e., same provisioning
   domain).

5.  DOTS Multihoming Deployment Considerations

   Table 1 provides some sample, non-exhaustive deployment schemes to
   illustrate how DOTS agents may be deployed for each of the scenarios
   introduced in Section 4.

    +=========================+=======================+===============+
    |         Scenario        |      DOTS Client      | Client-Domain |
    |                         |                       |  DOTS Gateway |
    +=========================+=======================+===============+
    |     Residential CPE     |          CPE          |      N/A      |
    +-------------------------+-----------------------+---------------+
    |   Single CPE, multiple  | Internal hosts or CPE |      CPE      |
    |   provisioning domains  |                       |               |
    +-------------------------+-----------------------+---------------+
    | Multiple CPEs, multiple | Internal hosts or all |   CPEs (CPE1  |
    |   provisioning domains  |  CPEs (CPE1 and CPE2) |   and CPE2)   |
    +-------------------------+-----------------------+---------------+
    |  Multihomed enterprise, | Internal hosts or all |   CPEs (CPE1  |
    |   single provisioning   |  CPEs (CPE1 and CPE2) |   and CPE2)   |
    |          domain         |                       |               |
    +-------------------------+-----------------------+---------------+

                      Table 1: Sample Deployment Cases

   These deployment schemes are further discussed in the following
   subsections.

5.1.  Residential CPE

   Figure 5 depicts DOTS sessions that need to be established between a
   DOTS client (C) and two DOTS servers (S1, S2) within the context of
   the scenario described in Section 4.1.  As listed in Table 1, the
   DOTS client is hosted by the residential CPE.

                                            +--+
                                  ----------|S1|
                                /           +--+
                               /    DOTS Server Domain #1
                              /
                        +---+/
                        | C |
                        +---+\
                         CPE  \
                               \
                                \           +--+
                                  ----------|S2|
                                            +--+
                                    DOTS Server Domain #2

        Figure 5: DOTS Associations for a Multihomed Residential CPE

   The DOTS client MUST resolve the DOTS server's name provided by each
   provisioning domain using the DNS servers either learned from the
   respective provisioning domain or associated with the interface(s)
   for which a DOTS server was explicitly configured (Section 4).
   IPv6-capable DOTS clients MUST use the source address selection
   algorithm defined in [RFC6724] to select the candidate source
   addresses to contact each of these DOTS servers.  DOTS sessions MUST
   be established and MUST be maintained with each of the DOTS servers
   because the mitigation scope of each of these servers is restricted.
   The DOTS client MUST use the security credentials (a certificate,
   typically) provided by a provisioning domain to authenticate itself
   to the DOTS server(s) provided by the same provisioning domain.  How
   such security credentials are provided to the DOTS client is out of
   the scope of this document.  The reader may refer to Section 7.1 of
   [RFC9132] for more details about DOTS authentication methods.

   When conveying a mitigation request to protect the attack target(s),
   the DOTS client MUST select an available DOTS server whose network
   has assigned the IP prefixes from which target addresses or prefixes
   are derived.  This implies that if no appropriate DOTS server is
   found, the DOTS client MUST NOT send the mitigation request to any
   other available DOTS server.

   For example, a mitigation request to protect target resources bound
   to a PA IP address or prefix cannot be satisfied by a provisioning
   domain other than the one that owns those addresses or prefixes.
   Consequently, if a CPE detects a DDoS attack that spreads over all
   its network attachments, it MUST contact all DOTS servers for
   mitigation purposes.

   The DOTS client MUST be able to associate a DOTS server with each
   provisioning domain it serves.  For example, if the DOTS client is
   provisioned with S1 using DHCP when attaching to a first network and
   with S2 using Protocol Configuration Option (PCO) [TS.24008] when
   attaching to a second network, the DOTS client must record the
   interface from which a DOTS server was provisioned.  A DOTS signaling
   session to a given DOTS server must be established using the
   interface from which the DOTS server was provisioned.  If a DOTS
   server is explicitly configured, DOTS signaling with that server must
   be established via the interfaces that are indicated in the explicit
   configuration or via any active interface if no interface is
   configured.

5.2.  Multihomed Enterprise: Single CPE, Multiple Upstream ISPs

   Figure 6 illustrates the DOTS sessions that can be established with a
   client-domain DOTS gateway (hosted within the CPE as per Table 1)
   that is enabled within the context of the scenario described in
   Section 4.2.  This deployment is characterized as follows:

   *  One or more DOTS clients are enabled in hosts located in the
      internal network.

   *  A client-domain DOTS gateway is enabled to aggregate and then
      relay the requests towards upstream DOTS servers.

                                               +--+
              ....................   ----------|S1|
              .    +---+         . /           +--+
              .    | C1|----+    ./     DOTS Server Domain #1
              .    +---+    |    .
              .             |   /.
              .+---+      +-+-+/ .
              .| C3|------| G |  .
              .+---+      +-+-+\ .
              .            CPE  \.
              .     +---+    |   .
              .     | C2|----+   .\
              .     +---+        . \          +--+
              '..................'  ----------|S2|
                                              +--+
               DOTS Client Domain     DOTS Server Domain #2

       Figure 6: Multiple DOTS Clients, Single DOTS Gateway, Multiple
                                DOTS Servers

   When PA addresses or prefixes are in use, the same considerations
   discussed in Section 5.1 need to be followed by the client-domain
   DOTS gateway to contact its DOTS server(s).  The client-domain DOTS
   gateways can be reachable from DOTS clients by using a unicast
   address or an anycast address (Section 3.2.4 of [RFC8811]).

   Nevertheless, when PI addresses or prefixes are assigned, and absent
   any policy, the client-domain DOTS gateway SHOULD send mitigation
   requests to all its DOTS servers.  Otherwise, the attack traffic may
   still be delivered via the ISP that hasn't received the mitigation
   request.

   An alternate deployment model is depicted in Figure 7.  This
   deployment assumes that:

   *  One or more DOTS clients are enabled in hosts located in the
      internal network.  These DOTS clients may use [RFC8973] to
      discover their DOTS server(s).

   *  These DOTS clients communicate directly with upstream DOTS
      servers.

                                ..........
                                .  +--+  .
                          +--------|C1|--------+
                          |     .  +--+  .     |
                          |     .        .     |
                         +--+   .  +--+  .   +--+
                         |S2|------|C3|------|S1|
                         +--+   .  +--+  .   +--+
                          |     .        .     |
                          |     .  +--+  .     |
                          +--------|C2|--------+
                                .  +--+  .
                                '........'
                               DOTS Client
                                 Domain

           Figure 7: Multiple DOTS Clients, Multiple DOTS Servers

   If PI addresses or prefixes are in use, the DOTS client MUST send a
   mitigation request to all the DOTS servers.  The use of the same
   anycast addresses to reach these DOTS servers is NOT RECOMMENDED.  If
   a well-known anycast address is used to reach multiple DOTS servers,
   the CPE may not be able to select the appropriate provisioning domain
   to which the mitigation request should be forwarded.  As a
   consequence, the request may not be forwarded to the appropriate DOTS
   server.

   If PA addresses or prefixes are used, the same considerations
   discussed in Section 5.1 need to be followed by the DOTS clients.
   Because DOTS clients are not embedded in the CPE and multiple
   addresses or prefixes may not be assigned to the DOTS client
   (typically in an IPv4 context), some issues may arise in how to steer
   traffic towards the appropriate DOTS server by using the appropriate
   source IP address.  These complications discussed in [RFC4116] are
   not specific to DOTS.

   Another deployment approach is to enable many DOTS clients; each of
   them is responsible for handling communications with a specific DOTS
   server (see Figure 8).

                                ..........
                                .  +--+  .
                          +--------|C1|  .
                          |     .  +--+  .
                         +--+   .  +--+  .   +--+
                         |S2|   .  |C2|------|S1|
                         +--+   .  +--+  .   +--+
                                '........'
                               DOTS Client
                                 Domain

                    Figure 8: Single-Homed DOTS Clients

   For both deployments depicted in Figures 7 and 8, each DOTS client
   SHOULD be provided with policies (e.g., a prefix filter that is used
   to filter DDoS detection alarms) that will trigger DOTS
   communications with the DOTS servers.  Such policies will help the
   DOTS client to select the appropriate destination DOTS server.  The
   CPE MUST select the appropriate source IP address when forwarding
   DOTS messages received from an internal DOTS client.

5.3.  Multihomed Enterprise: Multiple CPEs, Multiple Upstream ISPs

   The deployments depicted in Figures 7 and 8 also apply to the
   scenario described in Section 4.3.  One specific problem for this
   scenario is to select the appropriate exit router when contacting a
   given DOTS server.

   An alternative deployment scheme is shown in Figure 9:

   *  DOTS clients are enabled in hosts located in the internal network.

   *  A client-domain DOTS gateway is enabled in each CPE (CPE1 and CPE2
      per Table 1).

   *  Each of these client-domain DOTS gateways communicates with the
      DOTS server of the provisioning domain.

                     .................................
                     .                 +---+         .
                     .    +------------| C1|----+    .
                     .    |            +---+    |    .
                     .    |                     |    .
              +--+   .  +-+-+      +---+      +-+-+  .   +--+
              |S2|------|G2 |------| C3|------|G1 |------|S1|
              +--+   .  +-+-+      +---+      +-+-+  .   +--+
                     .  CPE2                   CPE1  .
                     .    |            +---+    |    .
                     .    +------------| C2|----+    .
                     .                 +---+         .
                     '...............................'
                            DOTS Client Domain

     Figure 9: Multiple DOTS Clients, Multiple DOTS Gateways, Multiple
                                DOTS Servers

   When PI addresses or prefixes are used, DOTS clients MUST contact all
   the client-domain DOTS gateways to send a DOTS message.  Client-
   domain DOTS gateways will then relay the request to the DOTS servers
   as a function of local policy.  Note that (same) anycast addresses
   cannot be used to establish DOTS sessions between DOTS clients and
   client-domain DOTS gateways because only one DOTS gateway will
   receive the mitigation request.

   When PA addresses/prefixes are used, but no filter rules are provided
   to DOTS clients, the DOTS clients MUST contact all client-domain DOTS
   gateways simultaneously to send a DOTS message.  Client-domain DOTS
   gateways MUST check whether a received request is to be forwarded
   upstream (if the target IP prefix is managed by the upstream server)
   or rejected.

   When PA addresses or prefixes are used, but specific filter rules are
   provided to DOTS clients using some means that are out of scope of
   this document, the clients MUST select the appropriate client-domain
   DOTS gateway to reach.  The use of the same anycast addresses is NOT
   RECOMMENDED to reach client-domain DOTS gateways.

5.4.  Multihomed Enterprise: Single ISP

   The key difference between the scenario described in Section 4.4 and
   the other scenarios is that multihoming is provided by the same ISP.
   Concretely, that ISP can decide to provision the enterprise network
   with:

   *  The same DOTS server for all network attachments.

   *  Distinct DOTS servers for each network attachment.  These DOTS
      servers need to coordinate when a mitigation action is received
      from the enterprise network.

   In both cases, DOTS agents enabled within the enterprise network MAY
   decide to select one or all network attachments to send DOTS
   mitigation requests.

6.  Security Considerations

   A set of security threats related to multihoming is discussed in
   [RFC4218].

   DOTS-related security considerations are discussed in Section 5 of
   [RFC8811].

   DOTS clients should control the information that they share with peer
   DOTS servers.  In particular, if a DOTS client maintains DOTS
   sessions with specific DOTS servers per interconnection link, the
   DOTS client SHOULD NOT leak information specific to a given link to
   DOTS servers on different interconnection links that are not
   authorized to mitigate attacks for that given link.  Whether this
   constraint is relaxed is deployment specific and must be subject to
   explicit consent from the DOTS client domain administrator.  How to
   seek such consent is implementation and deployment specific.

7.  IANA Considerations

   This document has no IANA actions.

8.  References

8.1.  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>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <https://www.rfc-editor.org/info/rfc6724>.

   [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>.

   [RFC8811]  Mortensen, A., Ed., Reddy.K, T., Ed., Andreasen, F.,
              Teague, N., and R. Compton, "DDoS Open Threat Signaling
              (DOTS) Architecture", RFC 8811, DOI 10.17487/RFC8811,
              August 2020, <https://www.rfc-editor.org/info/rfc8811>.

8.2.  Informative References

   [RFC3582]  Abley, J., Black, B., and V. Gill, "Goals for IPv6 Site-
              Multihoming Architectures", RFC 3582,
              DOI 10.17487/RFC3582, August 2003,
              <https://www.rfc-editor.org/info/rfc3582>.

   [RFC4116]  Abley, J., Lindqvist, K., Davies, E., Black, B., and V.
              Gill, "IPv4 Multihoming Practices and Limitations",
              RFC 4116, DOI 10.17487/RFC4116, July 2005,
              <https://www.rfc-editor.org/info/rfc4116>.

   [RFC4218]  Nordmark, E. and T. Li, "Threats Relating to IPv6
              Multihoming Solutions", RFC 4218, DOI 10.17487/RFC4218,
              October 2005, <https://www.rfc-editor.org/info/rfc4218>.

   [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>.

   [RFC7556]  Anipko, D., Ed., "Multiple Provisioning Domain
              Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,
              <https://www.rfc-editor.org/info/rfc7556>.

   [RFC8043]  Sarikaya, B. and M. Boucadair, "Source-Address-Dependent
              Routing and Source Address Selection for IPv6 Hosts:
              Overview of the Problem Space", RFC 8043,
              DOI 10.17487/RFC8043, January 2017,
              <https://www.rfc-editor.org/info/rfc8043>.

   [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>.

   [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>.

   [RFC8803]  Bonaventure, O., Ed., Boucadair, M., Ed., Gundavelli, S.,
              Seo, S., and B. Hesmans, "0-RTT TCP Convert Protocol",
              RFC 8803, DOI 10.17487/RFC8803, July 2020,
              <https://www.rfc-editor.org/info/rfc8803>.

   [RFC8903]  Dobbins, R., Migault, D., Moskowitz, R., Teague, N., Xia,
              L., and K. Nishizuka, "Use Cases for DDoS Open Threat
              Signaling", RFC 8903, DOI 10.17487/RFC8903, May 2021,
              <https://www.rfc-editor.org/info/rfc8903>.

   [RFC8973]  Boucadair, M. and T. Reddy.K, "DDoS Open Threat Signaling
              (DOTS) Agent Discovery", RFC 8973, DOI 10.17487/RFC8973,
              January 2021, <https://www.rfc-editor.org/info/rfc8973>.

   [RFC9132]  Boucadair, M., Ed., Shallow, J., and T. Reddy.K,
              "Distributed Denial-of-Service Open Threat Signaling
              (DOTS) Signal Channel Specification", RFC 9132,
              DOI 10.17487/RFC9132, September 2021,
              <https://www.rfc-editor.org/info/rfc9132>.

   [TS.24008] 3GPP, "Mobile radio interface Layer 3 specification; Core
              network protocols; Stage 3", 3GPP TS 24.008 16.3.0,
              December 2019,
              <https://www.3gpp.org/DynaReport/24008.htm>.

Acknowledgements

   Thanks to Roland Dobbins, Nik Teague, Jon Shallow, Dan Wing, and
   Christian Jacquenet for sharing their comments on the mailing list.

   Thanks to Kirill Kasavchenko for the comments.

   Thanks to Kathleen Moriarty for the secdir review, Joel Jaeggli for
   the opsdir review, Mirja Kühlewind for the tsvart review, and Dave
   Thaler for the intdir review.

   Many thanks to Roman Danyliw for the careful AD review.

   Thanks to Lars Eggert, Robert Wilton, Paul Wouters, Erik Kline, and
   Éric Vyncke for the IESG review.

Authors' Addresses

   Mohamed Boucadair
   Orange
   35000 Rennes
   France
   Email: mohamed.boucadair@orange.com


   Tirumaleswar Reddy.K
   Nokia
   Email: kondtir@gmail.com


   Wei Pan
   Huawei Technologies
   Email: william.panwei@huawei.com