Internet Engineering Task Force                                K. Larose
Internet-Draft                                                  Sandvine
Intended status: Informational                                 D. Dolson
Expires: September 18, 2018                               March 17, 2018

                          CAPPORT Architecture


   This document aims to document consensus on the CAPPORT architecture.
   DHCP or Router Advertisements, ICMP, and an HTTP API are used to
   provide the solution.  The role of Provisioning Domains (PvDs) is

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   This Internet-Draft will expire on September 18, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Components  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  User Equipment  . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Provisioning Service  . . . . . . . . . . . . . . . . . .   6
       2.2.1.  DHCP or Router Advertisements . . . . . . . . . . . .   6
       2.2.2.  Provisioning Domains  . . . . . . . . . . . . . . . .   6
     2.3.  Captive Portal API Server . . . . . . . . . . . . . . . .   7
     2.4.  Captive Portal Enforcement  . . . . . . . . . . . . . . .   7
     2.5.  ICMP/ICMP6  . . . . . . . . . . . . . . . . . . . . . . .   8
     2.6.  Component Diagram . . . . . . . . . . . . . . . . . . . .   9
   3.  User Equipment Identity . . . . . . . . . . . . . . . . . . .  10
     3.1.  Identifiers . . . . . . . . . . . . . . . . . . . . . . .  10
     3.2.  Recommended Properties  . . . . . . . . . . . . . . . . .  10
       3.2.1.  Uniquely Identify User Equipment  . . . . . . . . . .  11
       3.2.2.  Hard to Spoof . . . . . . . . . . . . . . . . . . . .  11
       3.2.3.  Visible to the API  . . . . . . . . . . . . . . . . .  11
       3.2.4.  Visible to the Enforcement Device . . . . . . . . . .  11
     3.3.  Evaluating an Identifier  . . . . . . . . . . . . . . . .  12
     3.4.  Examples of an Identifier . . . . . . . . . . . . . . . .  12
       3.4.1.  Physical Interface  . . . . . . . . . . . . . . . . .  12
       3.4.2.  IP Address  . . . . . . . . . . . . . . . . . . . . .  13
   4.  Solution Workflow . . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  Initial Connection  . . . . . . . . . . . . . . . . . . .  14
     4.2.  Conditions Expire . . . . . . . . . . . . . . . . . . . .  14
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
     7.1.  Trusting the Network  . . . . . . . . . . . . . . . . . .  15
     7.2.  Authenticated APIs  . . . . . . . . . . . . . . . . . . .  16
     7.3.  Secure APIs . . . . . . . . . . . . . . . . . . . . . . .  16
     7.4.  Risk of Nuisance Captive Portal . . . . . . . . . . . . .  16
     7.5.  User Options  . . . . . . . . . . . . . . . . . . . . . .  17
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   In this document, "Captive Portal" is used to describe a network to
   which a device may be voluntarily attached, such that network access
   is limited until some requirements have been fulfilled.  Typically a
   user is required to use a web browser to fulfil requirements imposed

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   by the network operator, such as reading advertisements, accepting an
   acceptable-use policy, or providing some form of credentials.

   Implementations generally require a web server, some method to allow/
   block traffic, and some method to alert the user.  Common methods of
   alerting the user involve modifying HTTP or DNS traffic.

   Problems with captive portal implementations have been described in
   [I-D.nottingham-capport-problem].  [If that document cannot be
   published, consider putting its best parts into an appendix of this

   This document standardizes an architecture for implementing captive
   portals that provides tools for addressing most of those problems.
   We are guided by these principles:

   o  Solutions SHOULD NOT require the forging of responses from DNS or
      HTTP servers, or any other protocol.  In particular, solutions
      SHOULD NOT require man-in-the-middle proxy of TLS traffic.

   o  Solutions MUST operate at the layer of Internet Protocol (IP) or
      above, not being specific to any particular access technology such
      as Cable, WiFi or 3GPP.

   o  Solutions SHOULD allow a device to be alerted that it is in a
      captive network when attempting to use any application on the
      network.  (Versus requiring a user to visit a clear-text HTTP site
      in order to receive a notification.)

   o  Solutions SHOULD allow a device to learn that it is in a captive
      network before any application attempts to use the network.

   o  The state of captivity SHOULD be explicitly available to devices
      (in contrast to modification of DNS or HTTP, which is only
      indirectly machine-detectable by the client--by comparing
      responses to well-known queries with expected responses).

   o  The architecture MUST provide a path of incremental migration,
      acknowledging a huge variety of portals and end-user device
      implementations and software versions.

   o  The architecture SHOULD improve security by providing mechanisms
      for trust, allowing alerts from trusted network operators to be
      distinguished from attacks from untrusted agents.

   A side-benefit of the architecture described in this document is that
   devices without user interfaces are able to identify parameters of

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   captivity.  However, this document does not yet describe a mechanism
   for such devices to escape captivity.

   The architecture uses the following mechanisms:

   o  Network provisioning protocols provide end-user devices with a URI
      for the API that end-user devices query for information about what
      is required to escape captivity.  DHCP, DHCPv6, and Router-
      Advertisement options for this purpose are available in [RFC7710].
      Other protocols (such as RADIUS), Provisioning Domains
      [I-D.ietf-intarea-provisioning-domains], or static configuration
      may also be used.  A device MAY query this API at any time to
      determine whether the network is holding the device in a captive

   o  End-user devices are notified of captivity with ICMP/ICMP6
      messages in response to traffic.  This notification can work with
      any Internet protocol, not just clear-text HTTP.  This
      notification does not carry the portal URI; rather it provides a
      notification to the User Equipment that it is in a captive state.

   o  Receipt of the ICMP/ICMP6 messages informs an end-user device that
      it is captive.  In response, the device SHOULD query the
      provisioned API to obtain information about the network state.
      The device MAY take immediate action to satisfy the portal
      (according to its configuration/policy).

   The architecture attempts to provide privacy, authentication, and
   safety mechanisms to the extent possible.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2.  Terminology

   Captive Network: A network which limits communication of attached
   devices to restricted hosts until the user has satisfied Captive
   Portal Conditions, after which access is permitted to a wider set of
   hosts (typically the internet).

   Captive Portal Conditions: site-specific requirements that a user or
   device must satisfy in order to gain access to the wider network.

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   Captive Portal Enforcement: The network equipment which enforces the
   traffic restriction and notifies the User Equipment it is in a
   captive state.

   Captive Portal User Equipment: Also known as User Equipment.  A
   device which has voluntarily joined a network for purposes of
   communicating beyond the constraints of the captive network.

   Captive Portal Server: The web server providing a user interface for
   assisting the user in satisfying the conditions to escape captivity.

2.  Components

2.1.  User Equipment

   The User Equipment is the device that a user desires to be attached
   to a network with full access to all hosts on the network (e.g., to
   have Internet access).  The User Equipment communication is typically
   restricted by the Captive Portal Enforcement, described in
   Section 2.4, until site-specific requirements have been met.

   At this time we consider only devices with web browsers, with web
   applications being the means of satisfying Captive Portal Conditions.

   o  An example interactive User Equipment is a smart phone.

   o  SHOULD support provisioning of the URI for the Captive Portal API
      (e.g., by DHCP)

   o  SHOULD distinguish Captive Portal API access per network
      interface, in the manner of Provisioning Domain Architecture

   o  SHOULD have a mechanism for notifying the user of the Captive

   o  SHOULD have a web browser so that the user may navigate the
      Captive Portal user interface.

   o  SHOULD be able to receive and validate the Captive Portal ICMP
      message types, and to access the Captive Portal API in response.

   o  MAY restrict application access to networks not granting full
      network access.  E.g., a device connected to a mobile network may
      be connecting to a WiFi network; the operating system MAY avoid
      updating the default route until network access restrictions have
      been lifted (excepting access to the Captive Portal server).  This
      has been termed "make before break".

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   None of the above requirements are mandatory because (a) we do not
   wish to say users or devices must seek access beyond the captive
   network, (b) the requirements may be fulfilled by manually visiting
   the captive portal web application, and (c) legacy devices must
   continue to be supported.

2.2.  Provisioning Service

   Here we discuss candidate mechanisms for provisioning the User
   Equipment with the URI of the API to query captive portal state and
   navigate the portal.

2.2.1.  DHCP or Router Advertisements

   A standard for providing a portal URI using DHCP or Router
   Advertisements is described in [RFC7710].  The CAPPORT architecture
   expects this URI to indicate the API described in Section 2.3.

   Although it is not clear from RFC7710 what protocol should be
   executed at the specified URI, some readers might have assumed it to
   be an HTML page, and hence there might be User Equipment assuming a
   browser should open this URI.  For backwards compatibility, it is
   RECOMMENDED that the server check the "Accept" field when serving the
   URI, and serve HTML pages for "text/html" and serve the API for
   "application/json".  [REVISIT: are these details appropriate?]

2.2.2.  Provisioning Domains

   Although still a work in progress,
   [I-D.ietf-intarea-provisioning-domains] proposes a mechanism for User
   Equipment to be provided with PvD Bootstrap Information containing
   the URI for a JSON file containing key-value pairs to be downloaded
   over HTTPS.  This JSON file would fill the role of the Captive Portal
   API described in Section 2.3.

   The PvD security model provides secure binding between the
   information provided by the trusted Router Advertisement and the
   HTTPS server.

   One key-value pair can be used to indicate the network has restricted
   access, requiring captive portal navigation by a user.  E.g.,
   key="captivePortal" and value=<URI of portal>.  The key-value pair
   should provide a different result when access is not restricted.
   E.g., key="captivePortal" and value="".

   This JSON file is extensible, allowing new key-value pairs to
   indicate such things as network access expiry time, URI for API
   access by IOT devices, etc.

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   The PvD server MUST support multiple (repeated) queries from each
   User Equipment, always returning the current captive portal
   information.  The User Equipment is expected to make this query upon
   receiving (and validating) an ICMP Captive Portal message (see
   Section 2.5).

2.3.  Captive Portal API Server

   The purpose of a Captive Portal API is to permit a query of Captive
   Portal state without interrupting the user.  This API thereby removes
   the need for a device to perform clear-text "canary" HTTP queries to
   check for response tampering.

   The URI of this API will have been provisioned to the User Equipment.
   (Refer to Section 2.2).

   This architecture expects the User Equipment to query the API when
   the User Equipment attaches to the network and multiple times
   thereafter.  Therefore the API MUST support multiple repeated queries
   from the same User Equipment, returning current state of captivity
   for the equipment.

   At minimum the API MUST provide: (1) the state of captivity and (2) a
   URI for a browser to present the portal application to the user.  The
   API SHOULD provide evidence to the caller that it supports the
   present architecture.

   When user equipment receives (and validates) ICMP Captive Portal
   alerts, the user equipment SHOULD query the API to check the state.
   The User Equipment SHOULD rate-limit these API queries in the event
   of ICMP flooding by an attacker.  (See Section 7.)

   The API MUST be extensible to support future use-cases by allowing
   extensible information elements.  Suggestions include quota
   information, expiry time, method of providing credentials, security
   token for validating ICMP messages.

   The API MUST use TLS for privacy and server authentication.  The
   implementation of the API MUST ensure both privacy and integrity of
   any information provided by or required by it.

   This document does not specify the details of the API.

2.4.  Captive Portal Enforcement

   The Captive Portal Enforcement component restricts network access to
   User Equipment according to site-specific policy.  Typically User

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   Equipment is permitted access to a small number of services and is
   denied general network access until it has performed some action.

   The Captive Portal Enforcement component:

   o  Allows traffic through for allowed User Equipment.

   o  Blocks (discards) traffic and sends ICMP notifications for
      disallowed User Equipment.

   o  Permits disallowed User Equipment to access necessary APIs and web
      pages to fulfill requirements of exiting captivity.

   o  Updates allow/block rules per User Equipment in response to
      operations from the Captive Portal back-end.

   As an upgrade path, captive portals MAY continue to support methods
   that work today, such as modification of port-80 HTTP responses to
   redirect users to the portal.  Various user-equipment vendors probe
   canary URLs to identify the state captivity [reference Mariko
   Kobayashi's survey].  While doing so, ICMP messages SHOULD also be
   sent, to activate work-flows in supporting devices.  [TODO: give some
   thought to precise recommendations for backwards compatibility.]

2.5.  ICMP/ICMP6

   ICMP messages have been selected for indicating to a sender that
   packets could not be delivered for reason of a network policy (in
   particular, captive portal).  ICMP is already used to indicate
   reasons that packets could not be delivered (network unreachable,
   port unreachable, packet too large, etc.).

   A mechanism to trigger captive portal work-flows in the User
   Equipment is proposed in [I-D.wkumari-capport-icmp-unreach].

   The Captive Portal Enforcement function is REQUIRED to send such ICMP
   messages when disallowed User Equipment attempts to send to the
   network.  These ICMP messages MUST be rate-limited to a configurable

   The ICMP messages MUST NOT be sent to the Internet devices.  The
   indications are only sent to the User Equipment.

   The User Equipment operating system is NOT REQUIRED to deliver the
   impact of the ICMP message to the application that triggered it.  The
   User Equipment might be able to satisfy the Captive Portal
   requirements quickly enough that existing transport connections are
   not impacted.

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2.6.  Component Diagram

   The following diagram shows the communication between each component.

   o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o
   . CAPTIVE NETWORK                                               .
   .                                            +--------------+   .
   . +------------+   Provision API URI         | Provisioning |   .
   . |            |<---------------------------+|  Service     |   .
   . |   User     |                             +--------------+   .
   . | Equipment  |   Query CAPPORT status;     +-------------+    .
   . |            |+--------------------------->| CAPPORT API |    .
   . |            |                             |  Server     |    .
   . |            |                             +------+------+    .
   . |            |                                    |Status     .
   . |            |   Portal user interface     +------+------+    .
   . |            |+--------------------------> | CAPPORT     |    .
   . +------------+                             | web portal  |    .
   .       ^  |  Connection Attempt             +-------------+    .
   .       |  |  to prohibited service.                   |        .
   .       |  +------------------> +---------------+  Allow/Deny   .
   .       |                       |               |    Rules      .
   .       |   ICMP Unreachable    | Captive Portal|     |         .
   .       +---------------------+ | Enforcement   | <---+         .
   .                               +---------------+               .
   .                                       |                       .
   .                            To/from external network           .
   .                                       |                       .
   .                                       |                       .
   o . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . o
                                     EXTERNAL NETWORK

           Figure 1: Captive Portal Architecture Component Diagram

   In the diagram:

   o  During provisioning (e.g., DHCP), the User Equipment acquires the
      URI for the CAPPORT API.

   o  The User Equipment queries the API to learn of its state of
      captivity.  If captive, the User Equipment presents the portal
      user interface to the user.

   o  The User Equipment attempts to communicate to the external network
      through the Captive Portal enforcement device.

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   o  The Captive Portal Enforcement device either allows the User
      Equipment's packets to the external network, or responds with an
      ICMP message.

   o  The CAPPORT web portal server directs the Captive Portal
      Enforcement device to either allow or deny external network access
      for the User Equipment.

   Although the provisioning, API, and web portal functions are shown as
   discrete blocks, they could of course be combined into a single

3.  User Equipment Identity

   Multiple components in the architecture interact with both the User
   Equipment and each other.  Since the User Equipment is the focus of
   these interactions, the components must be able to both identify the
   user equipment from their interactions with it, and be able to agree
   on the identify of the user equipment when interacting with each

   The methods by which the components interact restrict the type of
   information that may be used as an identifying characteristics.  This
   section discusses the identifying characteristics.

3.1.  Identifiers

   An Identifier is a chatacteristic of the User Equipment used by the
   components of a Captive Portal to uniquely determine which specific
   User Equipment is interacting with them.  An Identifier MAY be a
   field contained in packets sent by the User Equipment to the External
   Network.  Or, an Identifier MAY be an ephemeral property not
   contained in packets destined for the External Network, but instead
   correlated with such information through knowledge available to the
   different components.

3.2.  Recommended Properties

   The set of possible identifiers is quite large.  However, in order to
   be considered a good identifier, an identifier SHOULD meet the
   following criteria.  Note that the optimal identifier will likely
   change depending on the position of the components in the network as
   well as the information available to them.  An identifier SHOULD:

   o  Uniquely Identify the User Equipment

   o  Be Hard to Spoof

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   o  Be Visible to the API

   o  Be Visible to the Enforcement Device

3.2.1.  Uniquely Identify User Equipment

   In order to uniquely identify the User Equipment, at most one user
   equipment interacting with the other components of the Captive Portal
   MUST have a given value of the identifier.

   Over time, the user equipment identified by the value MAY change.
   Allowing the identified device to change over time ensures that the
   space of possible identifying vallues need not be overly large.

   Independent Captive Portals MAY use the same identifying value to
   identify different User Equipment.  Allowing indendent captive
   portals to reuse identifying values allows the identifier to be a
   property of the local network, expanding the space of possible

3.2.2.  Hard to Spoof

   A good identifier does not lend itself to being easily spoofed.  At
   no time should it be simple or straightforward for one User Equipment
   to pretend to be another User Equipment, regardless of whether both
   are active at the same time.  This property is particularly important
   when the user equipment is extended externally to devices such as
   billing systems, or where the identity of the User Equipment could
   imply liability.

3.2.3.  Visible to the API

   Since the API will need to perform operations which rely on the
   identify of the user equipment, such as query whether it is captive,
   the API needs to be able to relate requests to the User Equipment
   making the request.

3.2.4.  Visible to the Enforcement Device

   The Enforcement Device will decide on a per packet basis whether it
   should be permitted to communicate with the external network.  Since
   this decision depends on which User Equipment sent the packet, the
   Enforcement Device requires that it be able to map the packet to its
   concept of the User Equipment.

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3.3.  Evaluating an Identifier

   To evaluate whether an identifer is appropriate, one should consider
   every recommended property from the perspective of interactions among
   the components in the architecture.  When comparing identifiers,
   choose the one which best satifies all of the recommended properties.
   The architecture does not provide an exact measure of how well an
   identifier satisfies a given property; care should be taken in
   performing the evaluation.

3.4.  Examples of an Identifier

   This section provides some examples of identifiers, along with some
   evaluation of whether they are good identifiers.  The list of
   identifiers is not exhaustive.  Other identifiers may be used.  An
   important point to note is that whether the identifiers are good
   depends heavily on the capabilities of the components and where in
   the network the components exist.

3.4.1.  Physical Interface

   The physical interface by which the User Equipment is attached to the
   network can be used to identify the User Equipment.  This identifier
   has the property of being extremely difficult to spoof: the User
   Equipment is unaware of the property; one User Equipment cannot
   manipulate its interactions to appear as though it is another.

   Further, if only a single User Equipment is attatched to a given
   physical interface, then the identifier will be unique.  If multiple
   User Equipment is attached to the network on the same physical
   interface, then this property is not appropriate.

   Another consideration related to uniqueness of the User Equipment is
   that if the attached User Equipment changes, both the API server and
   the Enforcement Device must invalidate their state related to the
   User Equipment.

   The Enforcement Device needs to be aware of the physical interface,
   which constrains the environment: it must either be part of the
   device providing physical access (e.g., implemented in firmware), or
   packets traversing the network must be extended to include
   information about the source physical interface (e.g.  a tunnel).

   The API server faces a similar problem, implying that it should co-
   exist with the Enforcement Device, or that the enforcement device
   should extend requests to it with the identifying information.

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3.4.2.  IP Address

   A natural identifier to consider is the IP address of the User
   Equipment.  At any given time, no device on the network can have the
   same IP address without causing the network to malfunction, so it is
   appropriate from the perspective of uniqueness.

   However, it may be possible to spoof the IP address, particularly for
   malicious reasons where proper functioning of the network is not
   necessary for the malicious actor.  Consequently, any solution using
   the IP address should proactively try to prevent spoofing of the IP
   address.  Similarily, if the mapping of IP address to User Equipment
   is changed, the components of the architecture must remove or update
   their mapping to prevent spoofing.  Demonstrations of return
   routeability, such as that required for TCP connection establishment,
   might be sufficient defense against spoofing, though this might not
   be sufficient in networks that use broadcast media (such as some
   wireless networks).

   Since the IP address may traverse multiple segments of the network,
   more flexibility is afforded to the Enforcement Device and the API
   server: they simply must exist on a segment of the network where the
   IP address is still unique.  However, consider that a NAT may be
   deployed between the User Equipment and the Enforcement Device.  In
   such cases, it is possible for the components to still uniquely
   identify the device if they are aware of the port mapping.

   In some situtations, the User Equipment may have multiple IP
   addresses, while still satisfying all of the recommended properties.
   This raises some challenges to the components of the network.  For
   example, if the user equipment tries to access the network with
   multiple IP addresses, should the enforcement device and API server
   treat each IP address as a unique User Equipment, or should it tie
   the multiple addresses together into one view of the subscriber?  An
   implementation MAY do either.  Attention should be paid to IPv6 and
   the fact that it is expected for a device to have multiple IPv6
   addresses on a single link.  In such cases, idenfication could be
   performed by subnet, such as the /64 to which the IP belongs.

4.  Solution Workflow

   This section aims to improve understanding by describing a possible
   workflow of solutions adhering to the architecture.

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4.1.  Initial Connection

   This section describes a possible work-flow when User Equipment
   initially joins a Captive Network.

   1.  The User Equipment joins the Captive Network by acquiring a DHCP
       lease, RA, or similar, acquiring provisioning information.

   2.  The User Equipment learns the URI for the Captive Portal API from
       the provisioning information (e.g., [RFC7710]).

   3.  The User Equipment accesses the CAPPORT API to receive parameters
       of the Captive Network, including web-portal URI.  (This step
       replaces the clear-text query to a canary URL.)

   4.  If necessary, the User navigates the web portal to gain access to
       the external network.

   5.  The Captive Portal API server indicates to the Captive Portal
       Enforcement device that the User Equipment is allowed to access
       the external network.

   6.  The User Equipment attempts a connection outside the captive

   7.  If the requirements have been satisfied, the access is permitted;
       otherwise the "Expired" behavior occurs.

   8.  The User Equipment accesses the network until conditions Expire.

4.2.  Conditions Expire

   This section describes a possible work-flow when conditions expire
   and the user visits the portal again (e.g., low quota, or time

   1.  Pre-condition: the Captive Portal Enforcement has been configured
       to detect an expiry condition, which has now occurred.

   2.  The User Equipment sends a packet to the outside network.

   3.  The Captive Portal Enforcement detects that the packet is from an
       expired User Equipment.

   4.  The Captive Portal Enforcement sends an ICMP message to the User
       Equipment indicating that it needs to refresh its access.

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   5.  The User Equipment verifies the ICMP message is plausible.

   6.  The User Equipment queries the Captive Portal API to refresh
       parameters and status of the Captive Network.

   7.  If necessary, the User once again navigates the web portal to
       gain access to the external network.

   8.  The Captive Portal API Server gives more quota (time, bytes,
       etc.) to the User Equipment by indicating to the Captive Portal
       Enforcement the new, extended quota.

   9.  The User Equipment accesses the external network.

5.  Acknowledgements

   The authors thank various individuals for their feedback on the
   mailing list and during the IETF98 hackathon: David Bird, Eric Kline,
   Alexis La Goulette, Alex Roscoe, Darshak Thakore, and Vincent van

6.  IANA Considerations

   This memo includes no request to IANA.

7.  Security Considerations

7.1.  Trusting the Network

   When joining a network, some trust is placed in the network operator.
   This is usually considered to be a decision by a user on the basis of
   the reputation of an organization.  However, once a user makes such a
   decision, protocols can support authenticating a network is operated
   by who claims to be operating it.  The Provisioning Domain
   Architecture [RFC7556] provides some discussion on authenticating an

   Given that a user chooses to visit a Captive Portal URI, the URI
   location SHOULD be securely provided to the user's device.  E.g., the
   DHCPv6 AUTH option can sign this information.

   If a user decides to incorrectly trust an attacking network, they
   might be convinced to visit an attacking web page and unwittingly
   provide credentials to an attacker.  Browsers can authenticate
   servers but cannot detect cleverly misspelled domains, for example.

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7.2.  Authenticated APIs

   The solution described here assumes that when the User Equipment
   needs to trust the API server, server authentication will be
   performed using TLS mechanisms.

7.3.  Secure APIs

   The solution described here requires that the API be secured using
   TLS.  This is required to allow the user equipment and API server to
   exchange secrets which can be used to validate future interactions.
   The API must ensure the integrity of this information, as well as its

7.4.  Risk of Nuisance Captive Portal

   It is possible for any user on the Internet to send ICMP packets in
   an attempt to cause the receiving equipment to go to the captive
   portal.  This has been considered and addressed in the following

      The ICMP packet does not carry the URL, making this method safer
      than HTTP 3xx-redirect methods currently in use.  The User
      Equipment does not have to use clear-text HTTP to solicit the URL
      of the portal.

      Because the ICMP messages will carry embedded packets sent by the
      sender, the receiver of the ICMP message can validate that the
      transport header is plausibly one it sent (i.e., the transport-
      layer 5-tuple matches an open connection; there is no need to save
      every packet it sent).  This validation requires an off-path
      attacker to guess the 5-tuple in order to affect a flow.  It is
      trivial for an on-path attacker to send a plausible ICMP packet.
      (This is not a new ICMP attack.)  The impact of getting a valid
      ICMP packet to the User Equipment is that it will visit the
      CAPPORT API to check the status.  For this reason we recommend the
      User Equipment rate-limit these requests to the API.

      We considered that the ICMP packet could carry a short secret
      token that would be known to the User Equipment and Captive Portal
      Enforcement device but would not be available to an attacker, even
      to an on-path attacker.  Although possible to guess by brute
      force, the impact would be at worst a nuisance visit to the API.
      We suggest that a 32-bit token would be sufficient to deter
      nuisance attacks.

      Even when redirected, the User Equipment securely authenticates
      with API servers.

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7.5.  User Options

   The ICMP messaging informs the User Equipment that it is being held
   captive.  There is no requirement that the User Equipment do
   something about this.  Devices MAY permit users to disable automatic
   reaction to captive-portal indications for privacy reasons.  However,
   there is the trade-off that the user doesn't get notified when
   network access is restricted.  Hence, end-user devices MAY allow
   users to manually control captive portal interactions, possibly on
   the granularity of Provisioning Domains.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC7556]  Anipko, D., Ed., "Multiple Provisioning Domain
              Architecture", RFC 7556, DOI 10.17487/RFC7556, June 2015,

   [RFC7710]  Kumari, W., Gudmundsson, O., Ebersman, P., and S. Sheng,
              "Captive-Portal Identification Using DHCP or Router
              Advertisements (RAs)", RFC 7710, DOI 10.17487/RFC7710,
              December 2015, <>.

8.2.  Informative References

              Pfister, P., Vyncke, E., Pauly, T., and D. Schinazi,
              "Discovering Provisioning Domain Names and Data", draft-
              ietf-intarea-provisioning-domains-01 (work in progress),
              February 2018.

              Nottingham, M., "Captive Portals Problem Statement",
              draft-nottingham-capport-problem-01 (work in progress),
              April 2016.

              Bird, D. and W. Kumari, "Captive Portal ICMP Messages",
              draft-wkumari-capport-icmp-unreach-02 (work in progress),
              April 2017.

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Authors' Addresses

   Kyle Larose
   408 Albert Street
   Waterloo, ON  N2L 3V3

   Phone: +1 519 880 2400

   David Dolson


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