Distributed Denial of Service (DDoS) Open Threat Signaling Requirements
draft-ietf-dots-requirements-01

DOTS                                                        A. Mortensen
Internet-Draft                                      Arbor Networks, Inc.
Intended status: Informational                              R. Moskowitz
Expires: September 19, 2016                               HTT Consulting
                                                                T. Reddy
                                                     Cisco Systems, Inc.
                                                          March 18, 2016

Distributed Denial of Service (DDoS) Open Threat Signaling Requirements
                    draft-ietf-dots-requirements-01

Abstract

   This document defines the requirements for the Distributed Denial of
   Service (DDoS) Open Threat Signaling (DOTS) protocols coordinating
   attack response against DDoS attacks.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 19, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Context and Motivation  . . . . . . . . . . . . . . . . .   2
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  General Requirements  . . . . . . . . . . . . . . . . . .   6
     2.2.  Operational Requirements  . . . . . . . . . . . . . . . .   7
     2.3.  Data Channel Requirements . . . . . . . . . . . . . . . .  10
     2.4.  Security requirements . . . . . . . . . . . . . . . . . .  11
   3.  Congestion Control Considerations . . . . . . . . . . . . . .  12
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   5.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  01 revision . . . . . . . . . . . . . . . . . . . . . . .  13
     7.2.  00 revision . . . . . . . . . . . . . . . . . . . . . . .  13
     7.3.  Initial revision  . . . . . . . . . . . . . . . . . . . .  13
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

1.1.  Context and Motivation

   Distributed Denial of Service (DDoS) attacks continue to plague
   networks around the globe, from Tier-1 service providers on down to
   enterprises and small businesses.  Attack scale and frequency
   similarly have continued to increase, in part as a result of software
   vulnerabilities leading to reflection and amplification attacks.
   Once staggering attack traffic volume is now the norm, and the impact
   of larger-scale attacks attract the attention of international press
   agencies.

   The greater impact of contemporary DDoS attacks has led to increased
   focus on coordinated attack response.  Many institutions and
   enterprises lack the resources or expertise to operate on-premise
   attack prevention solutions themselves, or simply find themselves
   constrained by local bandwidth limitations.  To address such gaps,
   security service providers have begun to offer on-demand traffic
   scrubbing services.  Each service offers its own interface for
   subscribers to request attack mitigation, tying subscribers to
   proprietary implementations while also limiting the subset of network

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   elements capable of participating in the attack response.  As a
   result of incompatibility across services, attack responses may be
   fragmentary or otherwise incomplete, leaving key players in the
   attack path unable to assist in the defense.

   The lack of a common method to coordinate a real-time response among
   involved actors and network domains inhibits the speed and
   effectiveness of DDoS attack mitigation.  This document describes the
   required characteristics of DOTS protocols that would mitigate
   contemporary DDoS attack impact and lead to more efficient defensive
   strategies.

   DOTS is less concerned with the form of defensive action than with
   communicating the need for that action.  DOTS supplements calls for
   help with pertinent details about the detected attack, allowing
   entities participating in DOTS to form ad hoc, adaptive alliances
   against DDoS attacks as described in the DOTS use cases
   [I-D.ietf-dots-use-cases].  The requirements in this document are
   derived from those use cases.

1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   This document adopts the following terms:

   DDoS:  A distributed denial-of-service attack, in which of traffic
      originating from multiple sources are directed at a target on a
      network.  DDoS attacks are intended to cause a negative impact on
      the availability of servers, services, applications, and/or other
      functionality of an attack target.  Denial-of-service
      considerations are discussed in detail in [RFC4732].

   DDoS attack target:  A networked server, network service or
      application that is the focus of a DDoS attack.

   DDoS attack telemetry:  Collected network traffic characteristics
      defining the nature of a DDoS attack.  This document makes no
      assumptions regarding telemetry collection methodology.

   Countermeasure:  An action or set of actions taken to recognize and
      filter out DDoS attack traffic while passing legitimate traffic to
      the attack target.

   Mitigation:  A set of countermeasures enforced against traffic
      destined for the target or targets of a detected or reported DDoS

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      attack, where countermeasure enforcement is managed by an entity
      in the network path between attack sources and the attack target.
      Mitigation methodology is out of scope for this document.

   Mitigator:  An entity, typically a network element, capable of
      performing mitigation of a detected or reported DDoS attack.  For
      the purposes of this document, this entity is a black box capable
      of mitigation, making no assumptions about availability or design
      of countermeasures, nor about the programmable interface between
      this entity and other network elements.  The mitigator and DOTS
      server are assumed to belong to the same administrative entity.

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

   DOTS server:  A DOTS-aware software module handling and responding to
      messages from DOTS clients.  The DOTS server MAY enable mitigation
      on behalf of the DOTS client, if requested, by communicating the
      DOTS client's request to the mitigator and relaying any mitigator
      feedback to the requesting DOTS client.  A DOTS server may also be
      a mitigator.

   DOTS relay:  A DOTS-aware software module positioned between a DOTS
      server and a DOTS client in the signaling path.  A DOTS relay
      receives messages from a DOTS client and relays them to a DOTS
      server, and similarly passes messages from the DOTS server to the
      DOTS client.  A DOTS relay acts as a proxy or bridge between
      stateful and stateless transport signaling, and may also aggregate
      signaling from multiple downstream DOTS clients into a single
      session with an upstream DOTS server or DOTS relay.

   DOTS agents:  Any DOTS functional element, including DOTS clients,
      DOTS servers and DOTS relays.

   Signal channel:  A bidirectional, mutually authenticated
      communication layer between DOTS agents characterized by
      resilience even in conditions leading to severe packet loss, such
      as a volumetric DDoS attack causing network congestion.

   DOTS signal:  A concise authenticated status/control message
      transmitted between DOTS agents, used to indicate client's need
      for mitigation, as well as to convey the status of any requested
      mitigation.

   Heartbeat:  A message transmitted between DOTS agents over the signal
      channel, used as a keep-alive and to measure peer health.

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   Client signal:  A message sent from a DOTS client to a DOTS server or
      DOTS relay over the signal channel, indicating the DOTS client's
      need for mitigation, as well as the scope of any requested
      mitigation, optionally including additional attack details to
      supplement server-initiated mitigation.

   Server signal:  A message sent from a DOTS server to a DOTS client or
      DOTS relay over the signal channel.  Note that a server signal is
      not a response to client signal, but a DOTS server-initiated
      status message sent to DOTS clients with which the server has
      established signaling sessions, containing information about the
      status of DOTS client-requested mitigation and its efficacy.

   Data channel:  A secure communication layer between DOTS clients and
      DOTS servers used for infrequent bulk exchange of data not easily
      or appropriately communicated through the signal channel under
      attack conditions.

   Blacklist:  A list of addresses, prefixes and/or other identifiers
      indicating sources from which traffic should be blocked,
      regardless of traffic content.

   Whitelist:  A list of addresses, prefixes and/or other
      identifiersfrom indicating sources from which traffic should
      always be allowed, regardless of contradictory data gleaned in a
      detected attack.

   Multi-homed DOTS client:  A DOTS client exchanging messages with
      multiple DOTS servers, each in a separate administrative domain.

2.  Requirements

   This section describes the required features and characteristics of
   the DOTS protocols.

   DOTS is an advisory protocol.  An active DDoS attack against the
   entity controlling the DOTS client need not be present before
   establishing DOTS communication between DOTS agents.  Indeed,
   establishing a relationship with peer DOTS agents during nominal
   network conditions provides the foundation for more rapid attack
   response against future attacks, as all interactions setting up DOTS,
   including any business or service level agreements, are already
   complete.

   DOTS must at a minimum make it possible for a DOTS client to request
   a DOTS server's aid in mounting a coordinated defense against a
   detected attack, signaling inter- or intra-domain as requested by
   local operators.  DOTS clients should similarly be able to withdraw

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   aid requests.  DOTS requires no justification from DOTS clients for
   requests for help, nor must DOTS clients justify withdrawing help
   requests: the decision is local to the entity owning the DOTS
   clients.  Regular feedback between DOTS clients and DOTS server
   supplement the defensive alliance by maintaining a common
   understanding of DOTS peer health and activity.  Bidirectional
   communication between DOTS clients and DOTS servers is therefore
   critical.

   Yet DOTS must also work with a set of competing operational goals.
   On the one hand, the protocol must be resilient under extremely
   hostile network conditions, providing continued contact between DOTS
   agents even as attack traffic saturates the link.  Such resiliency
   may be developed several ways, but characteristics such as small
   message size, asynchronous, redundant message delivery and minimal
   connection overhead (when possible given local network policy) will
   tend to contribute to the robustness demanded by a viable DOTS
   protocol.  Operators of peer DOTS-enabled domains may enable quality-
   or class-of-service traffic tagging to increase the probability of
   successful DOTS signal delivery, but DOTS requires no such policies
   be in place.  The DOTS solution indeed must be viable especially in
   their absence.

   On the other hand, DOTS must include protections ensuring message
   confidentiality, integrity and authenticity to keep the protocol from
   becoming another vector for the very attacks it's meant to help fight
   off.  DOTS clients must be able to authenticate DOTS servers, and
   vice versa, for DOTS to operate safely, meaning the DOTS agents must
   have a way to negotiate and agree upon the terms of protocol
   security.  Attacks against the transport protocol should not offer a
   means of attack against the message confidentiality, integrity and
   authenticity.

   The DOTS server and client must also have some common method of
   defining the scope of any mitigation performed by the mitigator, as
   well as making adjustments to other commonly configurable features,
   such as listen ports, exchanging black- and white-lists, and so on.

   Finally, DOTS should provide sufficient extensibility to meet local,
   vendor or future needs in coordinated attack defense, although this
   consideration is necessarily superseded by the other operational
   requirements.

2.1.  General Requirements

   GEN-001  Extensibility: Protocols and data models developed as part
      of DOTS MUST be extensible in order to keep DOTS adaptable to

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      operational and proprietary DDoS defenses.  Future extensions MUST
      be backward compatible.

   GEN-002  Resilience and Robustness: The signaling protocol MUST be
      designed to maximize the probability of signal delivery even under
      the severely constrained network conditions imposed by particular
      attack traffic.  The protocol MUST be resilient, that is, continue
      operating despite message loss and out-of-order or redundant
      signal delivery.

   GEN-003  Bidirectionality: To support peer health detection, to
      maintain an open signal channel, and to increase the probability
      of signal delivery during attack, the signal channel MUST be
      bidirectional, with client and server transmitting signals to each
      other at regular intervals, regardless of any client request for
      mitigation.  Unidirectional messages MUST be supported within the
      bidirectional signal channel to allow for unsolicited message
      delivery, enabling asynchronous notifications between agents.

   GEN-004  Sub-MTU Message Size: To avoid message fragmentation and the
      consequently decreased probability of message delivery, signaling
      protocol message size MUST be kept under signaling path Maximum
      Transmission Unit (MTU), including the byte overhead of any
      encapsulation, transport headers, and transport- or message-level
      security.

   GEN-005  Bulk Data Exchange: Infrequent bulk data exchange between
      DOTS agents can also significantly augment attack response
      coordination, permitting such tasks as population of black- or
      white-listed source addresses; address or prefix group aliasing;
      exchange of incident reports; and other hinting or configuration
      supplementing attack response.

      As the resilience requirements for the DOTS signal channel mandate
      small signal message size, a separate, secure data channel
      utilizing an established reliable transport protocol MUST be used
      for bulk data exchange.

2.2.  Operational Requirements

   OP-001  Use of Common Transport Protocols: DOTS MUST operate over
      common widely deployed and standardized transport protocols.
      While the User Datagram Protocol (UDP) [RFC0768] SHOULD be used
      for the signal channel, the Transmission Control Protocol (TCP)
      [RFC0793] MAY be used if necessary due to network policy or
      middlebox capabilities or configurations.  The data channel MUST
      use TCP; see Section 2.3 below.

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   OP-002  Session Health Monitoring: Peer DOTS agents MUST regularly
      send heartbeats to each other after mutual authentication in order
      to keep the DOTS session open.  A session MUST be considered
      active until a DOTS agent explicitly ends the session, or either
      DOTS agent fails to receive heartbeats from the other after a
      mutually negotiated timeout period has elapsed.

   OP-003  Session Redirection: In order to increase DOTS operational
      flexibility and scalability, DOTS servers SHOULD be able to
      redirect DOTS clients to another DOTS server or relay at any time.
      Due to the decreased probability of DOTS server signal delivery
      due to link congestion, it is RECOMMENDED DOTS servers avoid
      redirecting while mitigation is enabled during an active attack
      against a target in the DOTS client's domain.  Either the DOTS
      servers have to fate-share the security state, the client MUST
      have separate security state with each potential redirectable
      server, or be able to negotiate new state as part of redirection.

   OP-004  Mitigation Status: DOTS MUST provide a means to report the
      status of an action requested by a DOTS client.  In particular,
      DOTS clients MUST be able to request or withdraw a request for
      mitigation from the DOTS server.  The DOTS server MUST acknowledge
      a DOTS client's request to withdraw from coordinated attack
      response in subsequent signals, and MUST cease mitigation activity
      as quickly as possible.  However, a DOTS client rapidly toggling
      active mitigation may result in undesirable side-effects for the
      network path, such as route or DNS [RFC1034] flapping.  A DOTS
      server therefore MAY continue mitigating for a mutually negotiated
      period after receiving the DOTS client's request to stop.

      A server MAY refuse to engage in coordinated attack response with
      a client.  To make the status of a client's request clear, the
      server MUST indicate in server signals whether client-initiated
      mitigation is active.  When a client-initiated mitigation is
      active, and threat handling details such as mitigation scope and
      statistics are available to the server, the server SHOULD include
      those details in server signals sent to the client.  DOTS clients
      SHOULD take mitigation statistics into account when deciding
      whether to request the DOTS server cease mitigation.

   OP-005  Mitigation Lifetime: A DOTS client SHOULD indicate the
      desired lifetime of any mitigation requested from the DOTS server.
      As DDoS attack duration is unpredictable, the DOTS client SHOULD
      be able to extend mitigation lifetime with periodic renewed
      requests for help.  When the mitigation lifetime comes to an end,
      the DOTS server SHOULD delay session termination for a protocol-
      defined grace period to allow for delivery of delayed mitigation

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      renewals over the signal channel.  After the grace period elapses,
      the DOTS server MAY terminate the session at any time.

      If a DOTS client does not include a mitigation lifetime in
      requests for help sent to the DOTS server, the DOTS server will
      use a reasonable default as defined by the protocol.  As above,
      the DOTS client MAY extend a current mitigation request's lifetime
      trivially with renewed requests for help.

      A DOTS client MAY also request an indefinite mitigation lifetime,
      enabling architectures in which the mitigator is always in the
      traffic path to the resources for which the DOTS client is
      requesting protection.  DOTS servers MAY refuse such requests for
      any reason.  The reasons themselves are not in scope.

   OP-006  Mitigation Scope: DOTS clients MUST indicate the desired
      address or prefix space coverage of any mitigation, for example by
      using Classless Internet Domain Routing (CIDR) [RFC1518],[RFC1519]
      prefixes, [RFC2373] for IPv6 [RFC2460] prefixes, the length/prefix
      convention established in the Border Gateway Protocol (BGP)
      [RFC4271], SIP URIs [RFC3261], E.164 numbers, DNS names, or by a
      prefix group alias agreed upon with the server through the data
      channel.

      If there is additional information available narrowing the scope
      of any requested attack response, such as targeted port range,
      protocol, or service, DOTS clients SHOULD include that information
      in client signals.  DOTS clients MAY also include additional
      attack details.  Such supplemental information is OPTIONAL, and
      DOTS servers MAY ignore it when enabling countermeasures on the
      mitigator.

      As an active attack evolves, clients MUST be able to adjust as
      necessary the scope of requested mitigation by refining the
      address space requiring intervention.

   OP-007  Mitigation Efficacy: When a mitigation request by a DOTS
      client is active, DOTS clients SHOULD transmit a metric of
      perceived mitigation efficacy to the DOTS server, per "Automatic
      or Operator-Assisted CPE or PE Mitigators Request Upstream DDoS
      Mitigation Services" in [I-D.ietf-dots-use-cases].  DOTS servers
      MAY use the efficacy metric to adjust countermeasures activated on
      a mitigator on behalf of a DOTS client.

   OP-008  Conflict Detection and Notification: Multiple DOTS clients
      controlled by a single administrative entity may send conflicting
      mitigation requests for pool of protected resources, as a result
      of misconfiguration, operator error, or compromised DOTS clients.

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      DOTS servers attempting to honor conflicting requests may flap
      network route or DNS information, degrading the networks
      attempting to participate in attack response with the DOTS
      clients.  DOTS servers SHALL detect such conflicting requests, and
      SHALL notify the DOTS clients in conflict.  The notification
      SHOULD indicate the nature and scope of the conflict, for example,
      the overlapping prefix range in a conflicting mitigation request.

   OP-009:  Lookup Caching: DOTS agents SHOULD cache resolved names, PKI
      validation chains, and similarly queried data as necessary.
      Network-based lookups and validation may be inhibited or
      unavailable during an active attack due to link congestion.  For
      example, DOTS agents SHOULD cache resolved names and addresses of
      peer DOTS agents, and SHOULD refer to those agents by IPv4
      [RFC0791] or IPv6 address for all communications following initial
      name resolution.

   OP-010:  Network Address Translator Traversal: The DOTS protocol MUST
      operate over networks in which Network Address Translation (NAT)
      is deployed.  As UDP is the recommended transport for DOTS, all
      considerations in "Middlebox Traversal Guidelines" in [RFC5405]
      apply to DOTS.  Regardless of transport, DOTS protocols MUST
      follow established best common practices (BCPs) for NAT traversal.

2.3.  Data Channel Requirements

   The data channel is intended to be used for bulk data exchanges
   between DOTS agents.  Unlike the signal channel, which must operate
   nominally even when confronted with despite signal degradation due to
   packet loss, the data channel is not expected to be constructed to
   deal with attack conditions.  As the primary function of the data
   channel is data exchange, a reliable transport is required in order
   for DOTS agents to detect data delivery success or failure.

   The data channel must be extensible.  We anticipate the data channel
   will be used for such purposes as configuration or resource
   discovery.  For example, a DOTS client may submit to the DOTS server
   a collection of prefixes it wants to refer to by alias when
   requesting mitigation, to which the server would respond with a
   success status and the new prefix group alias, or an error status and
   message in the event the DOTS client's data channel request failed.
   The transactional nature of such data exchanges suggests a separate
   set of requirements for the data channel, while the potentially
   sensitive content sent between DOTS agents requires extra precautions
   to ensure data privacy and authenticity.

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   DATA-001  Reliable transport: Transmissions over the data channel
      MUST be transactional, requiring reliable, in-order packet
      delivery.

   DATA-002  Data privacy and integrity: Transmissions over the data
      channel is likely to contain operationally or privacy-sensitive
      information or instructions from the remote DOTS agent.  Theft or
      modification of data channel transmissions could lead to
      information leaks or malicious transactions on behalf of the
      sending agent (see Section 4 below).  Consequently data sent over
      the data channel MUST be encrypted and authenticated using current
      industry best practices.  DOTS servers and relays MUST enable
      means to prevent leaking operationally or privacy-sensitive data.
      Although administrative entities participating in DOTS may detail
      what data may be revealed to third-party DOTS agents, such
      considerations are not in scope for this document.

   DATA-003  Black- and whitelist management: DOTS servers SHOULD
      provide methods for DOTS clients to manage black- and white-lists
      of source addresses of traffic destined for addresses belonging to
      a client.

      For example, a DOTS client should be able to create a black- or
      whitelist entry; retrieve a list of current entries from either
      list; update the content of either list; and delete entries as
      necessary.

      How the DOTS server determines client ownership of address space
      is not in scope.

2.4.  Security requirements

   DOTS must operate within a particularly strict security context, as
   an insufficiently protected signal or data channel may be subject to
   abuse, enabling or supplementing the very attacks DOTS purports to
   mitigate.

   SEC-001  Peer Mutual Authentication: DOTS agents MUST authenticate
      each other before a DOTS session is considered valid.  The method
      of authentication is not specified, but should follow current
      industry best practices with respect to any cryptographic
      mechanisms to authenticate the remote peer.

   SEC-002  Message Confidentiality, Integrity and Authenticity: DOTS
      protocols MUST take steps to protect the confidentiality,
      integrity and authenticity of messages sent between client and
      server.  While specific transport- and message-level security

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      options are not specified, the protocols MUST follow current
      industry best practices for encryption and message authentication.

      In order for DOTS protocols to remain secure despite advancements
      in cryptanalysis and traffic analysis, DOTS agents MUST be able to
      negotiate the terms and mechanisms of protocol security, subject
      to the interoperability and signal message size requirements
      above.

   SEC-003  Message Replay Protection: In order to prevent a passive
      attacker from capturing and replaying old messages, DOTS protocols
      MUST provide a method for replay detection.

3.  Congestion Control Considerations

   The DOTS signal channel will not contribute measurably to link
   congestion, as the protocol's transmission rate will be negligible
   regardless of network conditions.  Bulk data transfers are performed
   over the data channel, which should use a reliable transport with
   built-in congestion control mechanisms, such as TCP.

4.  Security Considerations

   DOTS is at risk from three primary attacks:

   o  DOTS agent impersonation

   o  Traffic injection

   o  Signaling blocking

   The DOTS protocol MUST be designed for minimal data transfer to
   address the blocking risk.  Impersonation and traffic injection
   mitigation can be managed through current secure communications best
   practices.  See Section 2.4 above for a detailed discussion.

5.  Contributors

   Med Boucadair

6.  Acknowledgments

   Thanks to Roman Danyliw and Matt Richardson for careful reading and
   feedback.

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7.  Change Log

7.1.  01 revision

   2016-03-21

   o  Reconciled terminology with -00 revision of
      [I-D.ietf-dots-use-cases].

   o  Terminology clarification based on working group feedback.

   o  Moved security-related requirements to separate section.

   o  Made resilience/robustness primary general requirement to align
      with charter.

   o  Clarified support for unidirectional communication within the
      bidirection signal channel.

   o  Added proposed operational requirement to support session
      redirection.

   o  Added proposed operational requirement to support conflict
      notification.

   o  Added proposed operational requirement to support mitigation
      lifetime in mitigation requests.

   o  Added proposed operational requirement to support mitigation
      efficacy reporting from DOTS clients.

   o  Added proposed operational requirement to cache lookups of all
      kinds.

   o  Added proposed operational requirement regarding NAT traversal.

   o  Removed redundant mutual authentication requirement from data
      channel requirements.

7.2.  00 revision

   2015-10-15

7.3.  Initial revision

   2015-09-24 Andrew Mortensen

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8.  References

8.1.  Normative References

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

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405, DOI
              10.17487/RFC5405, November 2008,
              <http://www.rfc-editor.org/info/rfc5405>.

8.2.  Informative References

   [I-D.ietf-dots-use-cases]
              Dobbins, R., Fouant, S., Migault, D., Moskowitz, R.,
              Teague, N., and L. Xia, "Use cases for DDoS Open Threat
              Signaling", draft-ietf-dots-use-cases-00 (work in
              progress), October 2015.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, DOI
              10.17487/RFC0791, September 1981,
              <http://www.rfc-editor.org/info/rfc791>.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <http://www.rfc-editor.org/info/rfc1034>.

   [RFC1518]  Rekhter, Y. and T. Li, "An Architecture for IP Address
              Allocation with CIDR", RFC 1518, DOI 10.17487/RFC1518,
              September 1993, <http://www.rfc-editor.org/info/rfc1518>.

   [RFC1519]  Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
              Inter-Domain Routing (CIDR): an Address Assignment and
              Aggregation Strategy", RFC 1519, DOI 10.17487/RFC1519,
              September 1993, <http://www.rfc-editor.org/info/rfc1519>.

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   [RFC2373]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 2373, DOI 10.17487/RFC2373, July 1998,
              <http://www.rfc-editor.org/info/rfc2373>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [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,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc4732>.

Authors' Addresses

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

   Email: amortensen@arbor.net

   Robert Moskowitz
   HTT Consulting
   Oak Park, MI  42837
   United States

   Email: rgm@htt-consult.com

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   Tirumaleswar Reddy
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103
   India

   Email: tireddy@cisco.com

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