ATOCA                                                     H. Schulzrinne
Internet-Draft                                       Columbia University
Intended status: Informational                                S. Norreys
Expires: July 19, 2011                                          BT Group
                                                                B. Rosen
                                                           NeuStar, Inc.
                                                           H. Tschofenig
                                                  Nokia Siemens Networks
                                                        January 15, 2011

   Requirements, Terminology and Framework for Exigent Communications


   Before, during and after emergency situations various agencies need
   to provide information to a group of persons or to the public within
   a geographical area.  While many aspects of such systems are specific
   to national or local jurisdictions, emergencies span such boundaries
   and notifications need to reach visitors from other jurisdictions.

   This document provides terminology, requirements and an architectural
   description for protocols exchanging alerts between IP-based end

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
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   This Internet-Draft will expire on July 19, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Classical Early Warning Situations . . . . . . . . . . . .  3
     1.2.  Exigent Communications . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Alert Delivery Architecture  . . . . . . . . . . . . . . . . .  5
     3.1.  Responsible Actor Roles  . . . . . . . . . . . . . . . . .  5
       3.1.1.  User Actors  . . . . . . . . . . . . . . . . . . . . .  5
       3.1.2.  Message Handling Service (MHS) Actors  . . . . . . . .  6
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.1.  Requirements for Alert Subscription  . . . . . . . . . . .  9
     4.2.  Requirements for Alert Message Delivery  . . . . . . . . .  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13

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1.  Introduction

1.1.  Classical Early Warning Situations

   During large-scale emergencies, public safety authorities need to
   reliably communicate with citizens in the affected areas, to provide
   warnings, indicate whether citizens should evacuate and how, and to
   dispel misinformation.  Accurate information can reduce the impact of
   such emergencies.

   Traditionally, emergency alerting has used church bells, sirens,
   loudspeakers, radio and television to warn citizens and to provide
   information.  However, techniques, such as sirens and bells, provide
   limited information content; loud speakers cover only very small
   areas and are often hard to understand, even for those not hearing
   impaired or fluent in the local language.  Radio and television offer
   larger information volume, but are hard to target geographically and
   do not work well to address the "walking wounded" or other
   pedestrians.  Both are not suitable for warnings, as many of those
   needing the information will not be listening or watching at any
   given time, particularly during work/school and sleep hours.

   This problem has been illustrated by the London underground bombing
   on July 7, 2006, as described in a government report [July2005].  The
   UK authorities could only use broadcast media and could not, for
   example, easily announce to the "walking wounded" where to assemble.

1.2.  Exigent Communications

   With the usage of the term 'Exigent Communications' this document
   aims to generalize the concept of conveying alerts to IP-based
   systems and at the same time to re-define the actors that participate
   in the messaging communication.  More precisely, exigent
   communications is defined as:

      Communication that requirs immediate action or remedy.
      Information about the reason for action and details about the
      steps that have to be taken are provided in the alert message.

      An alert message (or warning message) is a cautionary advice about
      something imminent (especially imminent danger or other
      unpleasantness).  In the context of exigent communication such an
      alert message refers to a future, ongoing or past event as the
      signaling exchange itself may relate to different stages of the
      lifecycle of the event.  The alert message itself, and not the
      signaling protocol that convey it, provides sufficient context
      about the specific state of the lifecycle the alert message refers

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   Communication typically occurs in two phases:

   Subscription:  In this step Recipients express their interest to
      receive certain types of alerts and happens prior to the actual
      delivery of the alert.  This expression of interest may be in form
      of an explicit communication step by having the Receiver sending a
      subscription message potentially with an indication of the type of
      alerts they are interested in, the duration of the subscription
      and a number of other indicators.  For example, parents may want
      to be alerted of emergencies affecting the school attended by
      their children and adult children may need to know about
      emergencies affecting elderly parents.  The subscription step may,
      however, also happen outside the Internet communication
      infrastructure but rather by the Recipient signing a contract and
      thereby agreeing to receive certain alerts.  Additionally, certain
      subscriptions may happen without the Recipient's explicit consent
      and without the Receiver sending a subscription.  For example, a
      Tsunami flood alert may be delievered to Recipients in case they
      are located in a specific geographical area.

      It is important to note that a protocol interaction initiated by
      the Receiver may need to take place to subscribe to certain types
      of alerts.  In some other cases the subscription does not require
      such interaction from the Receiver.  Orthogonal to the need to
      have a protocol interaction is the question of opt-in vs. opt-out.
      This is a pure policy decision and largely outside the scope of a
      technical specification.

   Alert Delivery  In this step the alert message is distributed to one
      or multiple Receivers.  The Receiver as a software module then
      presents the alert message to the Receipient.  The alert encoding
      is accomplished via the Common Alerting Protocol (CAP) and such an
      alert message contains useful information needed for dealing with
      the imminent danger.

   Note that alert Receivers as software modules may not necessarily
   only be executed on end devices humans typically carry around, such
   as mobile phones, Internet tablets, or laptops.  Instead, alert
   distribution may well directly communicate with displays in subway
   stations, or electronic bill boards.  When a Receiver obtains such an
   alert then it may not necessarily need to interact with a human (as
   the Recipient) but may instead use the alert as input to another
   process to trigger automated behaviors, such as closing vents during
   a chemical spill or activating sirens or other warning systems in
   commercial buildings.

   This document provides terminology, requirements and an architectural
   description.  Note that the requirements focus on the communication

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   protocols for subscription and alert delivery rather than on the
   content of the alert message itself.  With the usage of CAP these
   alert message content requirements are delegated to the authors and
   originators of alerts.

2.  Terminology

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119], with the
   important qualification that, unless otherwise stated, these terms
   apply to the design of a protocol conveying warning messages, not its
   implementation or application.

3.  Alert Delivery Architecture

   This section illustrates the roles useful for alert delivery.

3.1.  Responsible Actor Roles

   The communication system used for the dissemination of alert messages
   builds on top of existing communication infrastructure.  At the time
   of writing this underlying communication infrastructure is the
   Session Initiation Protocol (SIP) and the Extensible Messaging and
   Presence Protocol (XMPP).  These distributed services consist of a
   variety of actors playing different roles.  On a high level we
   differentiate between the User, and the Message Handling Service
   (MHS) actors.  We will describe them in more detail below.

3.1.1.  User Actors

   Users are the sources and sinks of alert messages.  We differentiate
   between two types of users:

   o  Authors
   o  Recipients

   From the user perspective, all alert message transfer activities are
   performed by a monolithic Message Handling Service (MHS), even though
   the actual service can be provided by many independent organizations.  Author

   The Author is a human responsible for creating the alert message, its
   contents, and its intended recipients, even though the exact list of
   recipients may be unknown to the Author at the time of writing the

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   alert message.  The MHS transfers the alert message from the Author
   and delivers it to the Recipients.  Recipient

   The Recipient is a consumer of the delivered alert message.  It is a
   human reading the alert message.

3.1.2.  Message Handling Service (MHS) Actors

   The Message Handling Service (MHS) performs a single end-to-end
   transfer of warning messages on behalf of the Author to reach the
   Recipient.  As a pragmatic heuristic MHS actors actors generate,
   modify or look at only transfer data, rather than the entire message.

   Figure 1 shows the relationships among transfer participants.
   Although it shows the Originator as distinct from the Author and
   Receiver as distinct from Recipient, each pair of roles usually has
   the same actor.  Transfers typically entail one or more Relays.
   However, direct delivery from the Originator to Receiver is possible.
   Delivery of warning messages within a single administrative boundary
   usually only involve a single Relay.

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       ++==========++                        ++===========++
       ||  Author  ||                        || Recipient ||
       ++====++====++                        ++===========++
             ||                                     /\
             ||                                     ||
             \/                                     ||
        +----------+                            +---++----+
        |          |                            |         |
      | |          |     Message Handling       |         |   |
      | |Originator|       System (MHS)         |Receiver |   |
      | |          |                            |         |   |
      | +---++-----+                            +---------+   |
      |     ||                                      /\        |
      |     ||                                      ||        |
      |     \/                                      ||        |
      | +---------+         +---------+        +-+--++---+    |
      | |  Relay  +======-=>|  Relay  +=======>|  Relay  |    |
      | +---------+         +----++---+        +---------+    |
      |                          ||                           |
      |                          ||                           |
      |                          \/                           |
      |                     +---------+                       |
      |                     | Gateway +-->                    |
      |                     +---------+                       |

     Legend: === and || lines indicate primary (possibly
                 indirect) transfers or roles

                 Figure 1: Relationships Among MHS Actors  Originator

   The Originator ensures that a warning message is valid for transfer
   and then submits it to a Relay.  A message is valid if it conforms to
   both communication and warning message encapsulation standards and
   local operational policies.  The Originator can simply review the
   message for conformance and reject it if it finds errors, or it can
   create some or all of the necessary information.

   The Originator serves the Author and can be the same entity in
   absence of a human crafting alert messages.

   The Originator also performs any post-submission, Author-related
   administrative tasks associated with message transfer and delivery.
   Notably, these tasks pertain to sending error and delivery notices,
   enforcing local policies, and dealing with messages from the Author

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   that prove to be problematic for the Internet.  The Originator is
   accountable for the message content, even when it is not responsible
   for it.  The Author creates the message, but the Originator handles
   any transmission issues with it.  Relay

   The Relay performs MHS-level transfer-service routing and store-and-
   forward, by transmitting or retransmitting the message to its
   Recipients.  The Relay may add history information (e.g., as
   available with SIP History Info [RFC4244]) or security related
   protection (e.g., as available with SIP Identity [RFC4474]) but does
   not modify the envelope information or the message content semantics.

   A Message Handling System (MHS) network consists of a set of Relays.
   This MHS network is above any underlying packet-switching network
   that might be used and below any Gateways.  Gateway

   A Gateway is a hybrid of User and Relay that connects heterogeneous
   communication infrastructures.  Its purpose is to emulate a Relay and
   the closer it comes to this, the better.  A Gateway operates as a
   User when it needs the ability to modify message content.

   Differences between the different communication systems can be as
   small as minor syntax variations, but they usually encompass
   significant, semantic distinctions.  Hence, the Relay function in a
   Gateway presents a significant design challenge, if the resulting
   performance is to be seen as nearly seamless.  The challenge is to
   ensure user-to-user functionality between the communication services,
   despite differences in their syntax and semantics.

   The basic test of Gateway design is whether an Author on one side of
   a Gateway can send a useful warning message to a Recipient on the
   other side, without requiring changes to any components in the
   Author's or Receiver's communication service other than adding the
   Gateway.  To each of these otherwise independent services, the
   Gateway appears to be a native participant.  Receiver

   The Receiver performs final delivery or sends the warning message to
   an alternate address.  In case of warning messages it is typically
   responsible for ensuring that the appropriate user interface
   interactions are triggered to interact with the Recipient.

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4.  Requirements

4.1.  Requirements for Alert Subscription

   The requirements listed below refer to the alert subscription phase.


      The protocol solution MUST allow a potential Recipient to indicate
      the language used by alert messages.


      The protocol solution MUST allow a potential Recipient to express
      the geographical area it wants to receive alerts about.


      The protocol solution MUST allow a potential Recipient to indicate
      preferences about the type of alerts it wants to receive.


      The protocol solution MUST allow a potential Recipient to express
      preference for certain media types.  The support for different
      media types depends on the content of the warning message but also
      impacts the communication protocol.  This functionality is, for
      example, useful for hearing and vision impaired persons.

4.2.  Requirements for Alert Message Delivery

   The requirements listed below refer to the delivery of alerts.


      The protocol solution MUST allow delivery of alerts by utilizing
      the lower layer infrastructure ensuring congestion control being
      considered.  Note that congestion does not only focus on over-
      utilization of a network caused by a large number of alerts but
      also in relationship with other traffic not related to exigent
      communication.  Network layer multicast, anycast or broadcast
      mechanisms may be utilized.  The topological network structure may
      be used for efficient alert distribution.

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      The protocol solution MUST allow delivery of messages
      simultaneously to a large audience.


      The protocol solution MUST be independent of the underlying link
      layer technology.


      The protocol solution MUST allow targeting notifications to
      specific individuals and to groups of individuals.


      The protocol solution MUST allow a Recipient to learn the identity
      of the Author of the alert message.

5.  IANA Considerations

   This document does not require actions by IANA.

6.  Security Considerations

   Figure 1 shows the actors for delivering an alert message assuming
   that a prior subscription has taken place already.  The desired
   security properties of an MHS for conveying alerts will depend on the
   number of administrative domains involved.  Each administrative
   domain can have vastly different operating policies and trust-based
   decision-making.  One obvious example is the distinction between
   alert messages that are exchanged within an closed group (such as
   alert messages received by parents affecting the school attended by
   their children) and alert messages that are exchanged between
   independent organizations (e.g., in case of large scale disasters).
   The rules for handling both types of communication architectures tend
   to be quite different.  That difference requires defining the
   boundaries of each.

   Operation of communication systems that are used to convey alert
   messages are typically carried out by different providers (or
   operators).  Since each be in operated in an independent
   administrative domain it is useful to consider administrative domain
   boundaries in the description to facilitate discussion about designs,

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   policies and operations that need to distinguish between internal
   issues and external entities.  Most significant is that the entities
   communicating across administrative boundaries typically have the
   added burden of enforcing organizational policies concerning external
   communications.  For example, routing alerts between administrative
   domains can create requirements, such as needing to route alert
   messages between organizational partners over specially trusted

   The communication interactions are subject to the policies of that
   domain, which cover concerns such as these:

   o  Reliability
   o  Access control
   o  Accountability
   o  Content evaluation, adaptation, and modification

   Many communication system make the distinction of administrative
   domains since they impact the requirements on security solutions.
   However, with the distribution of alert messages a number of
   additional security threats need to be addressed.  Due to the nature
   of alerts it is quite likely that end device implementations will
   offer user interface enhancements to get the Recipients attention
   whenever an alert arrives, which is an attractive property for
   adversaries to exploit.  Below we list the most important threats any
   solution will have to deal with.

   Originator Impersonation:

      An attacker could then conceivably attempt to impersonate the
      Originator of an alert message.  This threat is particularly
      applicable to those deployment environments where authorization
      decisions are based on the identity of the Originator.

   Alert Message Forgery:

      An attacker could forge or alter an alert message in order to
      convey custom messages to Recipients to get their immediate


      An attacker could obtain previously distributed alert messages and
      to replay them at a later time in the hope that Recipients could
      be tricked into believing they are fresh.

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   Unauthorized Distribution:

      When a Receiver receives an alert message it has to determine
      whether the Author distributing the alert messages is genuine to
      avoid accepting messages that are injected by malicious entities
      with the potential desire to at least get the immediate attention
      of the Recipient.

   Amplification Attack:

      An attacker may use the Message Handling System to inject a single
      alert message for distribution that may then be instantly turned
      into potentially millions of alert messages for distribution.

   One important security challenge is related to authorization.  When
   an alert message arrives at the Receiver then certain security checks
   may need to be performed to ensure that the alert message meets
   certain criteria.  The final consumer of the alert message is,
   however, the Recipient - a human.  From a security point of view the
   work split between the Recipient and the Receiver for making the
   authorization decision is important, particularly when an alert
   message is rejected due to a failed security verfication by the
   Receiver.  False positives may be fatal but accepting every alert
   message lowers the trustworthiness in the overall system.

7.  Acknowledgments

   This document re-uses text from [RFC5598].  The authors would like to
   thank Dave Crocker for his work.

   The authors would like to thank Martin Thomson, Carl Reed, Leopold
   Murhammer, and Tony Rutkowski for their comments.

   At IETF#79 the following persons provided feedback leading to changes
   in this document: Keith Drage, Scott Bradner, Ken Carberg, Keeping
   Li, Martin Thomson, Igor Faynberg, Mark Wood, Peter Saint-Andre.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
              July 2009.

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8.2.  Informative References

               ,  ., "Report of the 7 July Review Committee, ISBN 1
              85261 878 7", (PDF document),
              assembly/reports/7july/report.pdf, June 2006.

   [RFC4244]  Barnes, M., "An Extension to the Session Initiation
              Protocol (SIP) for Request History Information", RFC 4244,
              November 2005.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

Authors' Addresses

   Henning Schulzrinne
   Columbia University
   Department of Computer Science
   450 Computer Science Building
   New York, NY  10027

   Phone: +1 212 939 7004

   Steve Norreys
   BT Group
   1 London Road
   Brentwood, Essex  CM14 4QP

   Phone: +44 1277 32 32 20

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   Brian Rosen
   NeuStar, Inc.
   470 Conrad Dr
   Mars, PA  16046


   Hannes Tschhofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600

   Phone: +358 (50) 4871445

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