



Internet Engineering Task Force (IETF)                      M. Boucadair
Request for Comments: 8973                                        Orange
Category: Standards Track                                     T. Reddy.K
ISSN: 2070-1721                                                   McAfee
                                                            January 2021


           DDoS Open Threat Signaling (DOTS) Agent Discovery

Abstract

   This document specifies mechanisms to configure DDoS Open Threat
   Signaling (DOTS) clients with their DOTS servers.  The discovery
   procedure also covers the DOTS signal channel Call Home.  It can be
   useful to know the appropriate DOTS server for a given location in
   order to engage mitigation actions.  This is true even in cases where
   the DOTS client cannot localize the attack: cases where it only knows
   that some resources are under attack and that help is needed.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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/rfc8973.

Copyright Notice

   Copyright (c) 2021 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.  Introduction
   2.  Terminology
   3.  Why Multiple Discovery Mechanisms?
   4.  DOTS Discovery Procedure
   5.  DHCP Options for DOTS Agent Discovery
     5.1.  DHCPv6 DOTS Options
       5.1.1.  Format of DOTS Reference Identifier Option
       5.1.2.  Format of DOTS Address Option
       5.1.3.  DHCPv6 Client Behavior
     5.2.  DHCPv4 DOTS Options
       5.2.1.  Format of DOTS Reference Identifier Option
       5.2.2.  Format of DOTS Address Option
       5.2.3.  DHCPv4 Client Behavior
   6.  Discovery Using Service Resolution
   7.  DNS Service Discovery
   8.  Security Considerations
     8.1.  DHCP
     8.2.  Service Resolution
     8.3.  DNS Service Discovery
   9.  IANA Considerations
     9.1.  Service Name and Transport Protocol Port Number Registry
     9.2.  DHCPv6 Options
     9.3.  DHCPv4 Options
     9.4.  Application Service & Application Protocol Tags
       9.4.1.  DOTS Application Service Tag Registration
       9.4.2.  DOTS Call Home Application Service Tag Registration
       9.4.3.  signal.udp Application Protocol Tag Registration
       9.4.4.  signal.tcp Application Protocol Tag Registration
       9.4.5.  data.tcp Application Protocol Tag Registration
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   DDoS Open Threat Signaling (DOTS) [RFC8811] specifies an architecture
   in which a DOTS client can inform a DOTS server that the network is
   under a potential attack and that appropriate mitigation actions are
   required.  Indeed, because the lack of a common method to coordinate
   a real-time response among involved actors and network domains
   inhibits the effectiveness of DDoS attack mitigation, the DOTS signal
   channel protocol [RFC8782] is meant to carry requests for DDoS attack
   mitigation.  With this approach, DOTS can reduce the impact of an
   attack and lead to more efficient defensive actions in various
   deployment scenarios, such as those discussed in [DOTS-USE-CASES].
   Moreover, DOTS clients can instruct a DOTS server to install named
   filtering rules by means of the DOTS data channel protocol [RFC8783].

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

                 +-----------+            +-------------+
                 | 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 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 DOTS server addresses.

   This document specifies methods for DOTS clients to discover their
   DOTS server(s).  The rationale for specifying multiple discovery
   mechanisms is discussed in Section 3.

   The discovery methods can also be used by a DOTS server to locate a
   DOTS client in the context of DOTS signal channel Call Home
   [DOTS-SIG-CALL-HOME].  The basic high-level DOTS Call Home
   architecture is illustrated in Figure 2.

                 +---------------+        +-------------+
                 | Alert         | ~~~~~~ |  Call Home  |
                 |               |        | DOTS Client |
                 +---------------+        +------+------+
                                                 |
                                                 |
                                                 |
                 +---------------+        +------+------+
                 |    Attack     | ~~~~~~ |  Call Home  |
                 |   Source(s)   |        | DOTS Server |
                 +---------------+        +-------------+

   Figure 2: Basic DOTS Signal Channel Call Home Functional Architecture

   A DOTS agent may be used to establish base DOTS channels, DOTS Call
   Home, or both.  This specification accommodates all these deployment
   cases.

   Considerations for the selection of DOTS server(s) by multihomed DOTS
   clients are out of this document's scope; readers should refer to
   [DOTS-MULTIHOMING] for more details.

   This document assumes that security credentials to authenticate DOTS
   server(s) are pre-provisioned to a DOTS client using a mechanism such
   as (but not limited to) those discussed in [RFC8572] or
   [BTSRP-KEYINFR].  DOTS clients use those credentials for
   authentication purposes following the rules documented in [RFC8782].

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The reader should be familiar with the terms defined in [RFC3958].

   This document makes use of the following terms:

   DHCP:  refers to both DHCPv4 [RFC2131] and DHCPv6 [RFC8415].

   DOTS client:  refers to a DOTS-aware software module responsible for
      requesting attack response coordination with other DOTS-aware
      elements.

   DOTS server:  is a DOTS-aware software module handling and responding
      to messages from DOTS clients.  The DOTS server enables mitigation
      on behalf of the DOTS client, if requested, by communicating the
      DOTS client's request to the mitigator and returning selected
      mitigator feedback to the requesting DOTS client.

   Call Home DOTS client or server:  refers to a DOTS client or server
      deployed in a Call Home scenario (Figure 2).

   DOTS agent:  is any DOTS-aware software module capable of
      participating in a DOTS channel.

   Peer DOTS agent:  refers to the peer DOTS server (base DOTS
      operation) or to a peer Call Home DOTS client (for DOTS signal
      channel Call Home).

3.  Why Multiple Discovery Mechanisms?

   Analysis of the various use cases sketched in [DOTS-USE-CASES]
   reveals that it is unlikely that one single discovery method can be
   suitable for all the sample deployments.  Concretely:

   *  Many of the use cases discussed in [DOTS-USE-CASES] do involve
      Customer Premises Equipment (CPE).  Multiple CPEs connected to
      distinct network providers may even be considered.  It is
      intuitive to leverage existing mechanisms, such as discovery using
      service resolution or DHCP, to provision the CPE acting as a DOTS
      client with the DOTS server(s).

   *  Resolving a DOTS server domain name offered by an upstream transit
      provider provisioned to a DOTS client into IP address(es) requires
      the use of the appropriate DNS resolvers; otherwise, resolving
      those names will fail.  The use of protocols, such as DHCP, does
      allow associating provisioned DOTS server domain names with a list
      of DNS servers to be used for name resolution.  Furthermore, DHCP
      allows for directly providing IP addresses, therefore, avoiding
      the need for extra lookup delays.

   *  Some of the use cases may allow DOTS clients to have direct
      communications with upstream DOTS servers, that is, no DOTS
      gateway is involved.  Leveraging existing protocol behaviors that
      do not require specific features on the node embedding the DOTS
      client may ease DOTS deployment.  Typically, the use of
      Straightforward-Naming Authority Pointer (S-NAPTR) lookups
      [RFC3958] allows the DOTS server administrators to provision the
      preferred DOTS transport protocol between the DOTS client and the
      DOTS server and allows the DOTS client to discover this
      preference.

   *  The upstream network provider is not the DDoS mitigation provider
      for some of these use cases.  It is safe to assume that, for such
      deployments, the DOTS server(s) domain name is provided during the
      service subscription (i.e., manual/local configuration).

   *  Multiple DOTS clients may be enabled within a network (e.g., an
      enterprise network).  Dynamic discovery needs to be deterministic
      from an operational standpoint.

   *  Some of the use cases may involve a DOTS gateway that is
      responsible for selecting the appropriate DOTS server(s) to relay
      requests received from DOTS clients.

   Consequently, this document describes a unified discovery logic
   (Section 4) that involves the following mechanisms:

   *  dynamic discovery using DHCP (Section 5)

   *  a resolution mechanism based on S-NAPTR resource records in the
      DNS (Section 6)

   *  DNS Service Discovery (Section 7)

4.  DOTS Discovery Procedure

   Operators will need a consistent set of ways in which DOTS clients
   can discover this information and a consistent priority among these
   options.  If some devices prefer manual configuration over dynamic
   discovery while others prefer dynamic discovery over manual
   configuration, the result will be a process where the operator must
   find devices that are using the wrong DOTS server(s), determine how
   to ensure the devices are configured properly, and then reconfigure
   the device through the preferred method.

   All DOTS clients MUST support at least one of the three mechanisms
   below to determine a DOTS server list.  All DOTS clients SHOULD
   implement all three, or as many as are practical for any specific
   device, of the following ways to discover DOTS servers in order to
   facilitate the deployment of DOTS in large-scale environments.  For
   example, a CPE will support the first two mechanisms, a host within a
   LAN will support the last two mechanisms, or an application server
   will support a local configuration.  More examples are discussed in
   Section 3.  DOTS clients will prefer information received from the
   discovery methods in the order listed below.

   1.  Explicit Configuration:

       Local/Manual Configuration:  A DOTS client will learn the DOTS
          server(s) by means of local or manual DOTS configuration
          (i.e., DOTS servers configured at the system level).
          Configuration discovered from a DOTS client application is
          considered a local configuration.

          An implementation may give the user an opportunity (e.g., by
          means of configuration file options or menu items) to specify
          DOTS server(s) for each address family.  These may be
          specified either as a list of IP addresses or the DNS name of
          a DOTS server.  When only DOTS server IP addresses are
          configured, a reference identifier must also be configured for
          authentication purposes.

       Automatic Configuration (e.g., DHCP):  The DOTS client attempts
          to discover DOTS server(s) names and/or addresses from DHCP,
          as described in Section 5.

   2.  Service Resolution: The DOTS client attempts to discover DOTS
       server name(s) using service resolution, as specified in
       Section 6.

   3.  DNS-SD: DNS-based Service Discovery.  The DOTS client attempts to
       discover DOTS server name(s) using DNS service discovery, as
       specified in Section 7.

   Some of these mechanisms imply the use of DNS to resolve the IP
   address(es) of the DOTS server, while others imply an IP address of
   the relevant DOTS server is obtained directly.  Implementation
   options may vary on a per-device basis, as some devices may not have
   DNS capabilities and/or suitable DNS configuration.

   On hosts with more than one interface or address family (IPv4/IPv6),
   the DOTS server discovery procedure has to be performed for each
   interface-/address-family combination.  A DOTS client may choose to
   perform the discovery procedure only for a desired interface/address
   combination if the client does not wish to discover a DOTS server for
   all interface-/address-family combinations.

   This procedure is also followed by a Call Home DOTS server to
   discover its Call Home DOTS client in the context of
   [DOTS-SIG-CALL-HOME].

   The discovery method is performed upon the bootstrapping of a DOTS
   agent and is reiterated by the DOTS agent upon the following events:

   *  expiry of a validity timer (e.g., DHCP lease, DHCP information
      refresh time, or DNS TTL) associated with a discovered DOTS agent

   *  expiry of the certificate of a peer DOTS agent currently in use

   *  attachment to a new network

5.  DHCP Options for DOTS Agent Discovery

   As reported in Section 1.7.2 of [RFC6125]:

   |  Some certification authorities issue server certificates based on
   |  IP addresses, but preliminary evidence indicates that such
   |  certificates are a very small percentage (less than 1%) of issued
   |  certificates.

   In order to allow for PKIX-based authentication between a DOTS client
   and server while accommodating the current best practices for issuing
   certificates, this document allows DOTS agents to retrieve the names
   of their peer DOTS agents.  These names can be used for two purposes:
   (1) to retrieve the list of IP addresses of a peer DOTS agent or (2)
   to be presented as a reference identifier for authentication
   purposes.

   Defining the option to include a list of IP addresses would avoid
   depending on an underlying name resolution, but that design requires
   also supplying a name for PKIX-based authentication purposes.

   Given that DOTS gateways can be involved in a DOTS session, a peer
   DOTS agent can be reachable using a link-local address.  Such
   addresses can also be discovered using the options defined in
   Section 5.1.

   The list of the IP addresses returned by DHCP servers is typically
   used to feed the DOTS server selection procedure, including when DOTS
   agents are provided with primary and backup IP addresses of their
   peer DOTS agents.  An example of the DOTS server selection procedure
   is specified in Section 4.3 of [RFC8782].

   The design assumes that the same peer DOTS agent is used for
   establishing both signal and data channels.  For more customized
   configurations (e.g., transport-specific configuration and distinct
   DOTS servers for the signal and data channels), an operator can
   supply only a DOTS reference identifier that will be then passed to
   the procedure described in Section 6.

   The design allows terminating the base DOTS channels and DOTS Call
   Home on the same or distinct peer DOTS agents.  If distinct peer DOTS
   agents are deployed, the DHCP option can return, for example, a list
   of IP addresses to a requesting DOTS agent.  This list includes the
   IP address to be used for the base DOTS channels and the IP address
   for the DOTS Call Home.  The DOTS client (or Call Home DOTS server)
   will then use the address selection procedure specified in
   Section 4.3 of [RFC8782] to identify the IP address of the peer DOTS
   server (or Call Home DOTS client).

   For example, let's consider that the DOTS server is reachable at
   2001:db8:122:300::1, while the Call Home DOTS client is reachable at
   2001:db8:122:300::2.  The DHCP server will then return one DOTS
   reference identifier and a list that includes both
   2001:db8:122:300::1 and 2001:db8:122:300::2 to a requesting DHCP
   client.  That list is passed to the DOTS client (or Call Home DOTS
   server), which will try to establish connections to the addresses of
   that list and destination port number 4646 (or the Call Home port
   number).  As a result, the DOTS client (or Call Home DOTS server)
   will select 2001:db8:122:300::1 (or 2001:db8:122:300::2) as a DOTS
   server (or Call Home DOTS client).

5.1.  DHCPv6 DOTS Options

5.1.1.  Format of DOTS Reference Identifier Option

   The DHCPv6 DOTS Reference Identifier option is used to configure the
   name of the DOTS server (or the name of the Call Home DOTS client).
   The format of this option is shown in Figure 3.

    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     OPTION_V6_DOTS_RI         |         Option-length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                      dots-agent-name (FQDN)                   |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 3: DHCPv6 DOTS Reference Identifier Option

   The fields of the option shown in Figure 3 are as follows:

   Option-code:  OPTION_V6_DOTS_RI (141, see Section 9.2).
   Option-length:  Length of the dots-agent-name field in octets.
   dots-agent-name:  A fully qualified domain name of the peer DOTS
      agent.  This field is formatted as specified in Section 10 of
      [RFC8415].

   An example of the dots-agent-name encoding is shown in Figure 4.
   This example conveys the FQDN "dots.example.com", and the resulting
   Option-length field is 18.

   +------+------+------+------+------+------+------+------+------+
   | 0x04 |   d  |   o  |   t  |  s   | 0x07 |   e  |   x  |   a  |
   +------+------+------+------+------+------+------+------+------+
   |   m  |   p  |   l  |   e  | 0x03 |   c  |   o  |   m  | 0x00 |
   +------+------+------+------+------+------+------+------+------+

            Figure 4: An Example of the dots-agent-name Encoding


5.1.2.  Format of DOTS Address Option

   The DHCPv6 DOTS Address option can be used to configure a list of
   IPv6 addresses of a DOTS server (or a Call Home DOTS client).  The
   format of this option is shown in Figure 5.

    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  OPTION_V6_DOTS_ADDRESS       |         Option-length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                    DOTS ipv6-address                          |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                    DOTS ipv6-address                          |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              ...                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 5: DHCPv6 DOTS Address Option

   The fields of the option shown in Figure 5 are as follows:

   Option-code:  OPTION_V6_DOTS_ADDRESS (142, see Section 9.2).
   Option-length:  Length of the DOTS ipv6-address fields in octets.
      This MUST be a multiple of 16.
   DOTS ipv6-address:  Includes one or more IPv6 addresses [RFC4291] of
      the peer DOTS agent to be used by a DOTS agent for establishing a
      DOTS session.  The addresses are listed in the order of preference
      for use by the DOTS agent.

   Note that IPv4-mapped IPv6 addresses (Section 2.5.5.2 of [RFC4291])
   may be included in this option when there is no DHCPv4 server able to
   advertise the DHCPv4 DOTS options (Section 5.2) and when only IPv4
   connectivity is possible to the peer DOTS agent.


5.1.3.  DHCPv6 Client Behavior

   DHCP clients MAY request options OPTION_V6_DOTS_RI and
   OPTION_V6_DOTS_ADDRESS, as defined in Sections 18.2.1, 18.2.2,
   18.2.4, 18.2.5, 18.2.6, and 21.7 of [RFC8415].  As a convenience to
   the reader, it is mentioned here that the DHCP client includes the
   requested option codes in the Option Request option.

   If the DHCP client receives more than one instance of option
   OPTION_V6_DOTS_RI (or OPTION_V6_DOTS_ADDRESS), it MUST use only the
   first instance of that option.

   The DHCP client MUST silently discard multicast and host loopback
   addresses [RFC6890] conveyed in OPTION_V6_DOTS_ADDRESS.

   If the DHCP client receives and validates both OPTION_V6_DOTS_RI and
   OPTION_V6_DOTS_ADDRESS, the content of OPTION_V6_DOTS_RI is used as
   the reference identifier for authentication purposes (e.g., PKIX
   [RFC6125]), while the valid addresses included in
   OPTION_V6_DOTS_ADDRESS are used to reach the peer DOTS agent.  In
   other words, the name conveyed in OPTION_V6_DOTS_RI MUST NOT be
   passed to an underlying resolution library in the presence of a valid
   OPTION_V6_DOTS_ADDRESS in a response.

   If the DHCP client receives OPTION_V6_DOTS_RI only, but
   OPTION_V6_DOTS_RI contains more than one name, the DHCP client MUST
   use only the first name.  Once the name is validated (Section 10 of
   [RFC8415]), the name is passed to a name resolution library.
   Moreover, that name is also used as a reference identifier for
   authentication purposes.

   If the DHCP client receives OPTION_V6_DOTS_ADDRESS only, the
   address(es) included in OPTION_V6_DOTS_ADDRESS are used to reach the
   peer DOTS agent.  In addition, these addresses can be used as
   identifiers for authentication.

5.2.  DHCPv4 DOTS Options

5.2.1.  Format of DOTS Reference Identifier Option

   The DHCPv4 [RFC2132] DOTS Reference Identifier option is used to
   configure a name of the peer DOTS agent.  The format of this option
   is illustrated in Figure 6.

    Code  Length   Peer DOTS agent name
   +-----+-----+-----+-----+-----+-----+-----+--
   | 147 |  n  |  s1 |  s2 |  s3 |  s4 | s5  |  ...
   +-----+-----+-----+-----+-----+-----+-----+--

             Figure 6: DHCPv4 DOTS Reference Identifier Option

   The values s1, s2, s3, etc. represent the domain name labels in the
   domain name encoding.

   The fields of the option shown in Figure 6 are as follows:

   Code:  OPTION_V4_DOTS_RI (147, see Section 9.3).
   Length:  Includes the length of the "Peer DOTS agent name" field in
      octets.
   Peer DOTS agent name:  The domain name of the peer DOTS agent.  This
      field is formatted as specified in Section 10 of [RFC8415].

5.2.2.  Format of DOTS Address Option

   The DHCPv4 DOTS Address option can be used to configure a list of
   IPv4 addresses of a peer DOTS agent.  The format of this option is
   illustrated in Figure 7.

    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Code=148     |     Length    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               |
   |       DOTS IPv4 Address       |
   |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  ---
   |                               |   |
   |       DOTS IPv4 Address       |   |
   |                               | optional
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |
   .             ...               .   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  ---

                    Figure 7: DHCPv4 DOTS Address Option

   The fields of the option shown in Figure 7 are as follows:

   Code:  OPTION_V4_DOTS_ADDRESS (148, see Section 9.3).
   Length:  Set to 4*N, where N is the number of IPv4 addresses included
      in the option.
   DOTS IPv4 Address(es):  Contains one or more IPv4 addresses of the
      peer DOTS agent to be used by a DOTS agent.  The addresses are
      listed in the order of preference for use by the DOTS agent.

   OPTION_V4_DOTS_ADDRESS is a concatenation-requiring option.  As such,
   the mechanism specified in [RFC3396] MUST be used if
   OPTION_V4_DOTS_ADDRESS exceeds the maximum DHCPv4 option size of 255
   octets.

5.2.3.  DHCPv4 Client Behavior

   To discover a peer DOTS agent, the DHCPv4 client MUST include both
   OPTION_V4_DOTS_RI and OPTION_V4_DOTS_ADDRESS in a Parameter Request
   List option [RFC2132].

   If the DHCP client receives more than one instance of
   OPTION_V4_DOTS_RI option, it MUST use only the first instance of that
   option.

   The DHCP client MUST silently discard multicast and host loopback
   addresses [RFC6890] conveyed in OPTION_V4_DOTS_ADDRESS.

   If the DHCP client receives and validates both OPTION_V4_DOTS_RI and
   OPTION_V4_DOTS_ADDRESS, the content of OPTION_V4_DOTS_RI is used as
   the reference identifier for authentication purposes (e.g., PKIX
   [RFC6125]), while the valid addresses included in
   OPTION_V4_DOTS_ADDRESS are used to reach the peer DOTS agent.  In
   other words, the name conveyed in OPTION_V4_DOTS_RI MUST NOT be
   passed to an underlying resolution library in the presence of valid
   OPTION_V4_DOTS_ADDRESS in a response.

   If the DHCP client receives OPTION_V4_DOTS_RI only, but
   OPTION_V4_DOTS_RI option contains more than one name, as
   distinguished by the presence of multiple root labels, the DHCP
   client MUST use only the first name.  Once the name is validated
   (Section 10 of [RFC8415]), the name is passed to a name resolution
   library.  Moreover, that name is also used as a reference identifier
   for authentication purposes.

   If the DHCP client receives OPTION_V4_DOTS_ADDRESS only, the
   address(es) included in OPTION_V4_DOTS_ADDRESS are used to reach the
   peer DOTS server.  In addition, these addresses can be used as
   identifiers for authentication.

6.  Discovery Using Service Resolution

   This mechanism is performed in two steps:

   1.  A DNS domain name is retrieved for each combination of interface
       and address family.  A DOTS agent has to determine the domain in
       which it is located relying on dynamic means, such as DHCP
       (Section 5).  Implementations may allow the user to specify a
       default name that is used if no specific name has been
       configured.

   2.  Retrieved DNS domain names are then used for S-NAPTR lookups
       [RFC3958].  Further DNS lookups may be necessary to determine the
       peer DOTS agent IP address(es).

   Once the DOTS agent has retrieved its DNS domain or discovered the
   peer DOTS agent name that needs to be resolved, an S-NAPTR lookup
   with the appropriate application service and the desired protocol tag
   is made to obtain information necessary to connect to the
   authoritative peer DOTS agent within the given domain.

   This specification defines "DOTS" and "DOTS-CALL-HOME" as application
   service tags (Sections 9.4.1 and 9.4.2).  It also defines
   "signal.udp" (Section 9.4.3), "signal.tcp" (Section 9.4.4), and
   "data.tcp" (Section 9.4.5) as application protocol tags.  An example
   is provided in Figure 8.

   In the example below, for domain "example.net", the resolution
   algorithm will result in IP address, port, tag, and protocol tuples
   listed in Table 1.

   example.net.
   IN NAPTR 100 10 "" DOTS:signal.udp "" signal.example.net.
   IN NAPTR 200 10 "" DOTS:signal.tcp "" signal.example.net.
   IN NAPTR 300 10 "" DOTS:data.tcp "" data.example.net.

   signal.example.net.
   IN NAPTR 100 10 "s" DOTS:signal.udp "" _dots-signal._udp.example.net.
   IN NAPTR 200 10 "s" DOTS:signal.tcp "" _dots-signal._tcp.example.net.

   data.example.net.
   IN NAPTR 100 10 "s" DOTS:data.tcp "" _dots-data._tcp.example.net.
   IN NAPTR 200 10 "a" DOTS:data.tcp "" b.example.net.

   _dots-signal._udp.example.net.
   IN SRV   0 0 5000 a.example.net.

   _dots-signal._tcp.example.net.
   IN SRV   0 0 5001 a.example.net.

   _dots-data._tcp.example.net.
   IN SRV   0 0 5002 a.example.net.

   a.example.net.
   IN AAAA  2001:db8::1

   b.example.net.
   IN AAAA  2001:db8::2

        Figure 8: Example of Discovery of DOTS Servers Using Service
                                 Resolution

            +=======+==========+=============+======+========+
            | Order | Protocol | IP address  | Port | Tag    |
            +=======+==========+=============+======+========+
            | 1     | UDP      | 2001:db8::1 | 5000 | Signal |
            +-------+----------+-------------+------+--------+
            | 2     | TCP      | 2001:db8::1 | 5001 | Signal |
            +-------+----------+-------------+------+--------+
            | 3     | TCP      | 2001:db8::1 | 5002 | Data   |
            +-------+----------+-------------+------+--------+
            | 4     | TCP      | 2001:db8::2 | 443  | Data   |
            +-------+----------+-------------+------+--------+

                       Table 1: Resolution Results

   An example is provided in Figure 9 for the Call Home case.  In this
   example, the resolution algorithm will result in IP address, port,
   and protocol tuples listed in Table 2 for domain "example.net".

   example.net.
   IN NAPTR 100 10 "" DOTS-CALL-HOME:signal.udp "" signal.example.net.
   IN NAPTR 200 10 "" DOTS-CALL-HOME:signal.tcp "" signal.example.net.

   signal.example.net.
   IN NAPTR 100 10 "s" DOTS-CALL-HOME:signal.udp ""
               _dots-call-home._udp.example.net.
   IN NAPTR 200 10 "s" DOTS-CALL-HOME:signal.tcp ""
               _dots-call-home._tcp.example.net.

   _dots-call-home._udp.example.net.
   IN SRV   0 0 6000 b.example.net.

   _dots-call-home._tcp.example.net.
   IN SRV   0 0 6001 b.example.net.

   b.example.net.
   IN AAAA  2001:db8::2

   Figure 9: Example of Discovery of DOTS Call Home Client Using Service
                                 Resolution

                 +=======+==========+=============+======+
                 | Order | Protocol | IP address  | Port |
                 +=======+==========+=============+======+
                 | 1     | UDP      | 2001:db8::2 | 6000 |
                 +-------+----------+-------------+------+
                 | 2     | TCP      | 2001:db8::2 | 6001 |
                 +-------+----------+-------------+------+

                  Table 2: Resolution Results (Call Home)

   Note that customized port numbers are used for the DOTS signal
   channel, DOTS data channel, and DOTS signal channel Call Home in the
   examples shown in Figures 8 and 9 for illustration purposes.  If
   default port numbers are used in a deployment, the discovery
   procedure will return 4646 (DOTS signal channel) and 443 (DOTS data
   channel) as DOTS service port numbers.

   If no DOTS-specific S-NAPTR records can be retrieved, the discovery
   procedure fails for this domain name (and the corresponding interface
   and IP protocol version).  If more domain names are known, the
   discovery procedure MAY perform the corresponding S-NAPTR lookups
   immediately.  However, before retrying a lookup that has failed, a
   DOTS client MUST wait a time period that is appropriate for the
   encountered error (e.g., NXDOMAIN, timeout, etc.).

7.  DNS Service Discovery

   DNS-based Service Discovery (DNS-SD) [RFC6763] provides generic
   solutions for discovering services.  DNS-SD defines a set of naming
   rules for certain DNS record types that they use for advertising and
   discovering services.

   Section 4.1 of [RFC6763] specifies that a service instance name in
   DNS-SD has the following structure:

   <Instance> . <Service> . <Domain>

   The <Domain> portion specifies the DNS subdomain where the service
   instance is registered.  It is a conventional domain name, such as
   "example.com".

   The <Service> portion of the DOTS service instance name MUST be
   "_dots-signal._udp", "_dots-signal._tcp", "_dots-data._tcp", "_dots-
   call-home._udp", or "_dots-call-home._tcp".

   This document does not define any keys; the TXT record of a DNS-SD
   service is thus empty (Section 6 of [RFC6763]).

   Figure 10 depicts an excerpt of the DNS zone configuration file
   listing record examples to discover two DOTS signal channel servers.
   In this example, only UDP is supported as transport for the
   establishment of the DOTS signal channel.

   _dots-signal._udp.example.net. PTR  a._dots-signal._udp.example.net.
   _dots-signal._udp.example.net. PTR  b._dots-signal._udp.example.net.
   a._dots-signal._udp.example.net. SRV 0 0 4646 a.example.net.
   b._dots-signal._udp.example.net. SRV 0 0 4646 b.example.net.
   a._dots-signal._udp.example.net. TXT ""
   b._dots-signal._udp.example.net. TXT ""

      Figure 10: An Example of DNS-SD Records for the UDP DOTS Signal
            Channel Involving Two Servers with the Same Priority

8.  Security Considerations

   DOTS-related security considerations are discussed in Section 5 of
   [RFC8811].  As a reminder, DOTS agents must authenticate each other
   using (D)TLS before a DOTS session is considered valid according to
   the [RFC8782].

   An attacker may block some protocol messages (e.g., DHCP) to force
   the client to use a discovery mechanism with a lower priority.  The
   security implications of such attack are those inherent to the
   fallback discovery mechanism discussed in the following subsections.

   The results of the discovery procedure are a function of the
   interface/address family.  Contacting a discovered DOTS server via an
   interface to which it is not bound may exacerbate the delay required
   to establish a DOTS channel.  Moreover, such behavior may reveal that
   a DOTS service is enabled by a DOTS client domain and exposes the
   identity of the DOTS service provider (which can be inferred from the
   name and the destination IP address) to external networks.

   Security considerations related to how security credentials to
   authenticate DOTS server(s) are provisioned to a DOTS client are
   those inherent to the mechanism used for that purpose (for example,
   see [RFC8572]).

8.1.  DHCP

   The security considerations in [RFC2131] and [RFC8415] are to be
   considered.  In particular, issues related to rogue DHCP servers and
   means to mitigate many of these attacks are discussed in Section 22
   of [RFC8415].

   An attacker can get a domain name, get a domain-validated public
   certificate from a certification authority (CA), and host a DOTS
   agent.  An active attacker can then spoof DHCP responses to include
   the attacker's DOTS agent.  Such an attacker can also launch other
   attacks, as discussed in Section 22 of [RFC8415].  In addition to the
   mitigations listed in Section 22 of [RFC8415], a DOTS agent may be
   preconfigured with a list of trusted DOTS domain names.  If such a
   list is preconfigured, a DOTS agent will accept a DHCP-discovered
   name if it matches a name in that list.  Also, the DOTS agent has to
   check that the "DNS-ID" identifier type within subjectAltName in the
   server certificate matches a preconfigured name.  If the DOTS agent
   is instructed to trust subdomains of the names in that list as well
   (e.g., "*.example.com"), a DOTS agent will accept a DHCP-discovered
   name that matches a name in the preconfigured list (e.g., "dots-
   1.example.com" or "dots-2.example.com").

   Relying on an underlying resolution library to resolve a supplied
   reference identifier has similar security issues as those discussed
   in Section 8.2 (e.g., an active attacker may modify DNS messages used
   to resolve the supplied reference identifier and point the client to
   an attacker server).

   Supplying both an IP address and the reference identifier makes it
   easier to use a mis-issued certificate.

8.2.  Service Resolution

   The primary attack against the methods described in Section 6 is one
   that would lead to impersonation of a peer DOTS agent.  An attacker
   could attempt to compromise the S-NAPTR resolution.

   The DOTS client (or a Call Home DOTS server) constructs one reference
   identifier for the DOTS server (or a Call Home DOTS client) based on
   the domain name that is used for S-NAPTR lookup: DNS-ID.  If the
   reference identifier is found (as described in Section 6 of
   [RFC6125]) in the PKIX certificate's subjectAltName extension, the
   DOTS client should accept the certificate for the server.

   DNS Security Extensions (DNSSEC) [RFC4033] uses cryptographic keys
   and digital signatures to provide authentication of DNS data.  The
   information that is retrieved from the S-NAPTR lookup and that is
   validated using DNSSEC is thereby proved to be the authoritative
   data.

8.3.  DNS Service Discovery

   Since DNS-SD is a specification for how to name and use records in
   the existing DNS system, it has no specific additional security
   requirements over and above those that already apply to DNS queries
   and DNS updates.  For DNS queries, DNSSEC SHOULD be used where the
   authenticity of information is important.  For DNS updates, secure
   updates [RFC2136] [RFC3007] SHOULD generally be used to control which
   clients have permission to update DNS records.

   Note that means such as DNS over TLS (DoT) [RFC7858] or DNS over
   HTTPS (DoH) [RFC8484] can be used to prevent eavesdroppers from
   accessing DNS messages.

9.  IANA Considerations


9.1.  Service Name and Transport Protocol Port Number Registry

   IANA has allocated the following service names from the registry
   available at: <https://www.iana.org/assignments/service-names-port-
   numbers/>.

   Service Name:            dots-data
   Port Number:             N/A
   Transport Protocol(s):   TCP
   Description:             DOTS Data Channel Protocol.  The service
                            name is used to construct the SRV service
                            name "_dots-data._tcp" for discovering DOTS
                            servers used to establish DOTS data channel.
   Assignee:                IESG: iesg@ietf.org
   Contact:                 IETF Chair: chair@ietf.org
   Reference:               [RFC8973]

   Service Name:            dots-call-home
   Transport Protocol(s):   TCP/UDP
   Description:             DOTS Signal Channel Call Home Protocol.  The
                            service name is used to construct the SRV
                            service names "_dots-call-home._udp" and
                            "_dots-call-home._tcp" for discovering Call
                            Home DOTS clients used to establish DOTS
                            signal channel Call Home.
   Assignee:                IESG: iesg@ietf.org
   Contact:                 IETF Chair: chair@ietf.org
   Reference:               [RFC8973]

   IANA has updated the following entry from the registry available at:
   <https://www.iana.org/assignments/service-names-port-numbers/>.

   Port Number:             4646
   Transport Protocol(s):   TCP/UDP
   Description:             DOTS Signal Channel Protocol.  The service
                            name is used to construct the SRV service
                            names "_dots-signal._udp" and "_dots-
                            signal._tcp" for discovering DOTS servers
                            used to establish DOTS signal channel.
   Assignee:                IESG: iesg@ietf.org
   Contact:                 IETF Chair: chair@ietf.org
   Reference:               [RFC8782][RFC8973]

9.2.  DHCPv6 Options

   IANA has assigned the following new DHCPv6 Option Codes in the
   registry maintained in <https://www.iana.org/assignments/
   dhcpv6-parameters/>.

    +=======+========================+============+==================+
    | Value | Description            | Client ORO | Singleton Option |
    +=======+========================+============+==================+
    | 141   | OPTION_V6_DOTS_RI      | Yes        | Yes              |
    +-------+------------------------+------------+------------------+
    | 142   | OPTION_V6_DOTS_ADDRESS | Yes        | Yes              |
    +-------+------------------------+------------+------------------+

                         Table 3: DHCPv6 Options

9.3.  DHCPv4 Options

   IANA has assigned the following new DHCPv4 Option Codes in the
   registry maintained in <https://www.iana.org/assignments/bootp-dhcp-
   parameters/>.

   +========================+=====+=========+==============+===========+
   |                   Name | Tag | Data    | Meaning      | Reference |
   |                        |     | Length  |              |           |
   +========================+=====+=========+==============+===========+
   |      OPTION_V4_DOTS_RI | 147 | N       | The name     | [RFC8973] |
   |                        |     |         | of the       |           |
   |                        |     |         | peer DOTS    |           |
   |                        |     |         | agent.       |           |
   +------------------------+-----+---------+--------------+-----------+
   | OPTION_V4_DOTS_ADDRESS | 148 | N (the  | N/4 IPv4     | [RFC8973] |
   |                        |     | minimal | addresses    |           |
   |                        |     | length  | of peer      |           |
   |                        |     | is 4)   | DOTS         |           |
   |                        |     |         | agent(s).    |           |
   +------------------------+-----+---------+--------------+-----------+

                          Table 4: DHCPv4 Options


9.4.  Application Service & Application Protocol Tags

   IANA has made the following allocations from the registries available
   at <https://www.iana.org/assignments/s-naptr-parameters/> for
   application service tags and application protocol tags.

9.4.1.  DOTS Application Service Tag Registration

   Application Service Tag:           DOTS
   Intended Usage:                    See Section 6
   Security Considerations:           See Section 8
   Interoperability Considerations:   None
   Relevant Publications:             RFC 8973

9.4.2.  DOTS Call Home Application Service Tag Registration

   Application Service Tag:           DOTS-CALL-HOME
   Intended Usage:                    See Section 6
   Security Considerations:           See Section 8
   Interoperability Considerations:   None
   Relevant Publications:             RFC 8973

9.4.3.  signal.udp Application Protocol Tag Registration

   Application Protocol Tag:          signal.udp
   Intended Usage:                    See Section 6
   Security Considerations:           See Section 8
   Interoperability Considerations:   None
   Relevant Publications:             RFC 8973

9.4.4.  signal.tcp Application Protocol Tag Registration

   Application Protocol Tag:          signal.tcp
   Intended Usage:                    See Section 6
   Security Considerations:           See Section 8
   Interoperability Considerations:   None
   Relevant Publications:             RFC 8973

9.4.5.  data.tcp Application Protocol Tag Registration

   Application Protocol Tag:          data.tcp
   Intended Usage:                    See Section 6
   Security Considerations:           See Section 8
   Interoperability Considerations:   None
   Relevant Publications:             RFC 8973

10.  References

10.1.  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>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
              <https://www.rfc-editor.org/info/rfc2132>.

   [RFC3396]  Lemon, T. and S. Cheshire, "Encoding Long Options in the
              Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
              DOI 10.17487/RFC3396, November 2002,
              <https://www.rfc-editor.org/info/rfc3396>.

   [RFC3958]  Daigle, L. and A. Newton, "Domain-Based Application
              Service Location Using SRV RRs and the Dynamic Delegation
              Discovery Service (DDDS)", RFC 3958, DOI 10.17487/RFC3958,
              January 2005, <https://www.rfc-editor.org/info/rfc3958>.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <https://www.rfc-editor.org/info/rfc4291>.

   [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>.

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153,
              RFC 6890, DOI 10.17487/RFC6890, April 2013,
              <https://www.rfc-editor.org/info/rfc6890>.

   [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>.

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

10.2.  Informative References

   [BTSRP-KEYINFR]
              Pritikin, M., Richardson, M. C., Eckert, T., Behringer, M.
              H., and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", Work in Progress, Internet-
              Draft, draft-ietf-anima-bootstrapping-keyinfra-45, 11
              November 2020, <https://tools.ietf.org/html/draft-ietf-
              anima-bootstrapping-keyinfra-45>.

   [DOTS-MULTIHOMING]
              Boucadair, M., Reddy, T., and W. Pan, "Multi-homing
              Deployment Considerations for Distributed-Denial-of-
              Service Open Threat Signaling (DOTS)", Work in Progress,
              Internet-Draft, draft-ietf-dots-multihoming-05, 23
              November 2020, <https://tools.ietf.org/html/draft-ietf-
              dots-multihoming-05>.

   [DOTS-SIG-CALL-HOME]
              Reddy, T., Boucadair, M., and J. Shallow, "Distributed
              Denial-of-Service Open Threat Signaling (DOTS) Signal
              Channel Call Home", Work in Progress, Internet-Draft,
              draft-ietf-dots-signal-call-home-13, 11 January 2021,
              <https://tools.ietf.org/html/draft-ietf-dots-signal-call-
              home-13>.

   [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>.

   [RFC2136]  Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, DOI 10.17487/RFC2136, April 1997,
              <https://www.rfc-editor.org/info/rfc2136>.

   [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
              Update", RFC 3007, DOI 10.17487/RFC3007, November 2000,
              <https://www.rfc-editor.org/info/rfc3007>.

   [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>.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

   [RFC8572]  Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero
              Touch Provisioning (SZTP)", RFC 8572,
              DOI 10.17487/RFC8572, April 2019,
              <https://www.rfc-editor.org/info/rfc8572>.

   [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>.

   [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>.

Acknowledgements

   Thanks to Brian Carpenter for the review of the Bootstrapping Remote
   Secure Key Infrastructure (BRSKI) text used in previous draft
   versions of the specification.

   Many thanks to Russ White for the review, comments, and text
   contribution.

   Thanks to Dan Wing, Pei Wei, Valery Smyslov, and Jon Shallow for the
   review and comments.

   Thanks to Bernie Volz for the review of the DHCP section.

   Many thanks to Benjamin Kaduk for the detailed AD review.

   Thanks to Zhen Cao, Kyle Rose, Nagendra Nainar, and Peter Yee for the
   directorate reviews.

   Thanks to Barry Leiba, Martin Duke, Roman Danyliw, Éric Vyncke, and
   Magnus Westerlund for the IESG review.

Contributors

   Prashanth Patil
   Cisco Systems, Inc.

   Email: praspati@cisco.com


Authors' Addresses

   Mohamed Boucadair
   Orange
   35000 Rennes
   France

   Email: mohamed.boucadair@orange.com


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

   Email: TirumaleswarReddy_Konda@McAfee.com
