Poll-Based Security Event Token (SET) Delivery Using HTTP
draft-ietf-secevent-http-poll-01

Network Working Group                                    A. Backman, Ed.
Internet-Draft                                                    Amazon
Intended status: Standards Track                           M. Jones, Ed.
Expires: April 25, 2019                                        Microsoft
                                                            M. Scurtescu
                                                                Coinbase
                                                               M. Ansari
                                                                   Cisco
                                                              A. Nadalin
                                                               Microsoft
                                                        October 22, 2018

       Poll-Based Security Event Token (SET) Delivery Using HTTP
                    draft-ietf-secevent-http-poll-01

Abstract

   This specification defines how a series of Security Event Tokens
   (SETs) may be delivered to an intended recipient using HTTP POST over
   TLS initiated as a poll by the recipient.  The specification also
   defines how delivery can be assured, subject to the SET Recipient's
   need for assurance.

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

   This Internet-Draft will expire on April 25, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction and Overview . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
     1.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  SET Delivery  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Polling Delivery using HTTP . . . . . . . . . . . . . . .   4
     2.2.  Polling HTTP Request Attributes . . . . . . . . . . . . .   5
     2.3.  Polling HTTP Response Attributes  . . . . . . . . . . . .   6
     2.4.  Poll Request  . . . . . . . . . . . . . . . . . . . . . .   6
       2.4.1.  Poll Only Request . . . . . . . . . . . . . . . . . .   7
       2.4.2.  Acknowledge Only Request  . . . . . . . . . . . . . .   8
       2.4.3.  Poll with Acknowledgement . . . . . . . . . . . . . .   9
       2.4.4.  Poll with Acknowledgement and Errors  . . . . . . . .  10
     2.5.  Poll Response . . . . . . . . . . . . . . . . . . . . . .  10
     2.6.  Error Response Handling . . . . . . . . . . . . . . . . .  12
   3.  Authentication and Authorization  . . . . . . . . . . . . . .  12
     3.1.  Use of Tokens as Authorizations . . . . . . . . . . . . .  13
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     4.1.  Authentication Using Signed SETs  . . . . . . . . . . . .  14
     4.2.  HTTP Considerations . . . . . . . . . . . . . . . . . . .  14
     4.3.  TLS Support Considerations  . . . . . . . . . . . . . . .  14
     4.4.  Access Token Considerations . . . . . . . . . . . . . . .  15
       4.4.1.  Bearer Token Considerations . . . . . . . . . . . . .  15
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  15
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  18
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction and Overview

   This specification defines how a stream of Security Event Tokens
   (SETs) [RFC8417] can be transmitted to an intended SET Recipient
   using HTTP [RFC7231] over TLS.  The specification defines a method to
   poll for SETs using HTTP POST.

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   This specification defines a method of SET delivery in what is known
   as Event Streams.

   This specification does not define the method by which Event Streams
   are defined, provisioned, managed, monitored, and configured and is
   out of scope of this specification.
   [[This work is TBD by the SECEVENTS WG]]

1.1.  Notational Conventions

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

   For purposes of readability, examples are not URL encoded.
   Implementers MUST percent encode URLs as described in Section 2.1 of
   [RFC3986].

   Throughout this document, all figures MAY contain spaces and extra
   line wrapping for readability and due to space limitations.
   Similarly, some URIs contained within examples have been shortened
   for space and readability reasons.

1.2.  Definitions

   This specification utilizes terminology defined in [RFC8417], as well
   as the terms defined below:

   SET Transmitter
      An entity that delivers SETs in its possession to one or more SET
      Recipients.

2.  SET Delivery

   When an event occurs, the SET Transmitter constructs a SET [RFC8417]
   that describes the event.  The SET Transmitter determines the SET
   Recipients that the SET should be distributed to.

   How SETs are defined and the process by which events are identified
   for SET Recipients is out of scope of this specification.

   When a SET is available for an SET Recipient, the SET Transmitter
   attempts to deliver the SET based on the SET Recipient's registered
   delivery mechanism:

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   o  The SET Transmitter queues the SET in a buffer so that an SET
      Recipient MAY poll for SETs using HTTP/1.1 POST.

   o  Or, the SET Transmitter delivers the Event through a different
      method not defined by this specification.

   In Poll-Based SET Delivery Using HTTP, multiple SETs are delivered in
   a JSON document [RFC7159] to an SET Recipient in response to an HTTP
   POST request to the SET Transmitter.  Then in a following request,
   the SET Recipient acknowledges received SETs and MAY poll for more.
   All requests and responses are JSON documents and use a "Content-
   Type" of "application/json" as described in Section 2.1.

   After successful (acknowledged) SET delivery, Event Transmitters
   SHOULD NOT be required to maintain or record SETs for recovery.  Once
   a SET is acknowledged, the SET Recipient SHALL be responsible for
   retention and recovery.

   Transmitted SETs SHOULD be self-validating (e.g., signed) if there is
   a requirement to verify they were issued by the Event Transmitter at
   a later date when de-coupled from the original delivery where
   authenticity could be checked via the HTTP or TLS mutual
   authentication.

   Upon receiving a SET, the SET Recipient reads the SET and validates
   it.  The SET Recipient MUST acknowledge receipt to the SET
   Transmitter, using the defined acknowledgement or error method
   depending on the method of transfer.

   The SET Recipient SHALL NOT use the Event acknowledgement mechanism
   to report Event errors other than relating to the parsing and
   validation of the SET.

2.1.  Polling Delivery using HTTP

   This method allows an SET Recipient to use HTTP POST (Section 4.3.3
   of [RFC7231]) to acknowledge SETs and to check for and receive zero
   or more SETs.  Requests MAY be made at a periodic interval (short
   polling) or requests MAY wait, pending availability of new SETs using
   long polling (see Section 2 of [RFC6202]).

   The delivery of SETs in this method is facilitated by HTTP POST
   requests initiated by the SET Recipient in which:

   o  The SET Recipient makes a request for available SETs using an HTTP
      POST to a pre-arranged endpoint provided by the Event Transmitter.
      Or,

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   o  After validating previously received SETs, the SET Recipient
      initiates another poll request using HTTP POST that includes
      acknowledgement of previous SETs, and waits for the next batch of
      SETs.

   The purpose of the "acknowledgement" is to inform the SET Transmitter
   that has successfully been delivered and attempts to re-deliver are
   no longer required.  Before acknowledgement, Event Recipients SHOULD
   ensure received SETs have been validated and retained in a manner
   appropriate to the recipient's retention requirements.  The level and
   method of retention of SETs by SET Recipients is out of scope of this
   specification.

2.2.  Polling HTTP Request Attributes

   When initiating a poll request, the SET Recipient constructs a JSON
   document that consists of polling request parameters and SET
   acknowledgement parameters in the form of JSON attributes.

   The request payloads are delivered in one of two forms as described
   in Section 2.4 and Section 2.5

   When making a request, the HTTP header "Content-Type" is set to
   "application/json".

   The following JSON Attributes are used in a polling request:

   Request Processing Parameters

      maxEvents
         an OPTIONAL JSON integer value indicating the maximum number of
         unacknowledged SETs that SHOULD be returned.  If more than the
         maximum number of SETs are available, the oldest SETs available
         SHOULD be returned first.  A value of "0" MAY be used by SET
         Recipients that would like to perform an acknowledge only
         request.  This enables the Recipient to use separate HTTP
         requests for acknowledgement and reception of SETs.  When zero
         returned events is requested, the value of the attribute
         "returnImmediately" SHALL be ignored as an immediate response
         is expected.

      returnImmediately
         An OPTIONAL JSON boolean value that indicates the SET
         Transmitter SHOULD return an immediate response even if no
         results are available (short polling).  The default value is
         "false" indicates the request is to be treated as an HTTP Long
         Poll (see Section 2 of [RFC6202]).  The time out for the

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         request is part of the Stream configuration, which is out of
         scope of this specification.

   SET Acknowledgment Parameters

      ack
         Which is an array of Strings that each correspond to the "jti"
         of a successfully received SET.  If there are no outstanding
         SETs to acknowledge, the attribute MAY be omitted.  When
         acknowledging a SET, the SET Transmitter is released from any
         obligation to retain the SET (e.g., for a future retry to
         receive).

      setErrs
         A JSON Object that contains one or more nested JSON attributes
         that correspond to the "jti" of each invalid SET received.  The
         value of each is a JSON object whose contents is an "err"
         attribute and "description" attribute whose value correspond to
         the errors described in Section 2.6.

2.3.  Polling HTTP Response Attributes

   In response to a poll request, the SET Transmitter checks for
   available SETs and responds with a JSON document containing the
   following JSON attributes:

   sets
      A JSON object that contains zero or more nested JSON attributes.
      Each nested attribute corresponds to the "jti" of a SET to be
      delivered and whose value is a JSON String containing the value of
      the encoded corresponding SET.  If there are no outstanding SETs
      to be transmitted, the JSON object SHALL be empty.

   moreAvailable
      A JSON boolean value that indicates if more unacknowledged SETs
      are available to be returned.

   When making a response, the HTTP header "Content-Type" is set to
   "application/json".

2.4.  Poll Request

   The SET Recipient performs an HTTP POST (see Section 4.3.4 of
   [RFC7231]) to a pre-arranged polling endpoint URI to check for SETs
   that are available.  Because the SET Recipient has no prior SETs to
   acknowledge, the "ack" and "errs" request parameters are omitted.

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   If after a period of time, negotiated between the Event Transmitter
   and Recipient, an SET Transmitter MAY reissue SETs it has previously
   delivered.  The SET Recipient SHOULD accept repeat SETs and
   acknowledge the SETs regardless of whether the Recipient believes it
   has already acknowledged the SETs previously.  An SET Transmitter MAY
   limit the number of times it attempts to deliver a SET.

   If the SET Recipient has received SETs from the SET Transmitter, the
   SET Recipient SHOULD parse and validate received SETs to meet its own
   requirements and SHOULD acknowledge receipt in a timely (e.g.,
   minutes) fashion so that the Event Transmitter may mark the SETs as
   received.  SET Recipients SHOULD acknowledge receipt before taking
   any local actions based on the SETs to avoid unnecessary delay in
   acknowledgement, where possible.

   Poll requests have three variations:

   Poll Only
      In which an SET Recipient asks for the next set of Events where no
      previous SET deliveries are acknowledged (such as in the initial
      poll request).

   Acknowledge Only
      In which an Event Recipient sets the "maxEvents" attribute to "0"
      along with "ack" and "err" attributes indicating the SET Recipient
      is acknowledging previously received SETs and does not want to
      receive any new SETs in response to the request.

   Combined Acknowledge and Poll
      In which an SET Recipient is both acknowledging previously
      received SETs using the "ack" and "err" attributes and will wait
      for the next group of SETs in the SET Transmitters response.

2.4.1.  Poll Only Request

   In the case where no SETs were received in a previous poll (see
   Figure 7), the SET Recipient simply polls without acknowledgement
   parameters ("sets" and "setErrs").

   The following is an example request made by an SET Recipient that has
   no outstanding SETs to acknowledge and is polling for available SETs.

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   The following is a non-normative example poll request to the
   endpoint: "https://nofity.exampleidp.com/Events".

   POST /Events  HTTP/1.1

   Host: notify.exampleidp.com
   Authorization: Bearer h480djs93hd8
   Accept: application/json

   {
    "returnImmediately":true
   }

                  Figure 1: Example Initial Poll Request

   An SET Recipient MAY poll with no parameters at all by passing an
   empty JSON object.

   The following is a non-normative example default poll request to the
   endpoint: "https://nofity.exampleidp.com/Events".

   POST /Events  HTTP/1.1

   Host: notify.exampleidp.com
   Authorization: Bearer h480djs93hd8
   Accept: application/json

   {}

                  Figure 2: Example Default Poll Request

2.4.2.  Acknowledge Only Request

   In this variation, the SET Recipient acknowledges previously received
   SETs and indicates it does not want to receive SETs in response by
   setting the "maxEvents" attribute to "0".

   This variation is typically used when an SET Recipient needs to
   acknowledge received SETs independently (e.g., on separate threads)
   from the process of receiving SETs.

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   The following is a non-normative example poll with acknowledgement of
   SETs received (for example as shown in Figure 6).

   POST /Events  HTTP/1.1

   Host: notify.exampleidp.com
   Authorization: Bearer h480djs93hd8
   Content-Type: application/json
   Authorization: Bearer h480djs93hd8

   {
     "ack":[
       "4d3559ec67504aaba65d40b0363faad8",
       "3d0c3cf797584bd193bd0fb1bd4e7d30"
     ],
     "maxEvents":0
   }

                 Figure 3: Example Acknowledge Only equest

2.4.3.  Poll with Acknowledgement

   This variation allows a recipient thread to simultaneously
   acknowledge previously received SETs and wait for the next group of
   SETs in a single request.

   The following is a non-normative example poll with acknowledgement of
   SETs received in Figure 6.

   POST /Events  HTTP/1.1

   Host: notify.exampleidp.com
   Authorization: Bearer h480djs93hd8
   Content-Type: application/json
   Authorization: Bearer h480djs93hd8

   {
     "ack":[
       "4d3559ec67504aaba65d40b0363faad8",
       "3d0c3cf797584bd193bd0fb1bd4e7d30"
     ],
     "returnImmediately":false
   }

         Figure 4: Example Poll With Acknowledgement and No Errors

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   In the above acknowledgement, the SET Recipient has acknowledged
   receipt of two SETs and has indicated it wants to wait until the next
   SET is available.

2.4.4.  Poll with Acknowledgement and Errors

   In the case where errors were detected in previously delivered SETs,
   the SET Recipient MAY use the "setErrs" attribute to indicate errors
   in the following poll request.

   The following is a non-normative example of a response acknowledging
   1 error and 1 receipt of two SETs received in Figure 6.

   POST /Events  HTTP/1.1

   Host: notify.exampleidp.com
   Authorization: Bearer h480djs93hd8
   Content-Type: application/json
   Authorization: Bearer h480djs93hd8

   {
     "ack":["3d0c3cf797584bd193bd0fb1bd4e7d30"],
     "setErrs":{
       "4d3559ec67504aaba65d40b0363faad8":{
         "err":"jwtAud",
         "description":"The audience value was incorrect."
       }
     },
     "returnImmediately":true
   }

             Figure 5: Example Poll Acknowledgement With Error

2.5.  Poll Response

   In response to a poll request, the service provider MAY respond
   immediately if SETs are available to be delivered.  If no SETs are
   available at the time of the request, the SET Transmitter SHALL delay
   responding until a SET is available unless the poll request parameter
   "returnImmediately" is "true".

   As described in Section 2.3 a JSON document is returned containing a
   number of attributes including "sets" which SHALL contain zero or
   more SETs.

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   The following is a non-normative example response to the request
   shown Section 2.4.  This example shows two SETs are returned.

  HTTP/1.1 200 OK
  Content-Type: application/json
  Location: https://notify.exampleidp/Events

  {
  "sets":{
    "4d3559ec67504aaba65d40b0363faad8":
     "eyJhbGciOiJub25lIn0.
     eyJqdGkiOiI0ZDM1NTllYzY3NTA0YWFiYTY1ZDQwYjAzNjNmYWFkOCIsImlhdCI6MTQ
     1ODQ5NjQwNCwiaXNzIjoiaHR0cHM6Ly9zY2ltLmV4YW1wbGUuY29tIiwiYXVkIjpbIm
     h0dHBzOi8vc2NpbS5leGFtcGxlLmNvbS9GZWVkcy85OGQ1MjQ2MWZhNWJiYzg3OTU5M
     2I3NzU0IiwiaHR0cHM6Ly9zY2ltLmV4YW1wbGUuY29tL0ZlZWRzLzVkNzYwNDUxNmIx
     ZDA4NjQxZDc2NzZlZTciXSwiZXZlbnRzIjp7InVybjppZXRmOnBhcmFtczpzY2ltOmV
     2ZW50OmNyZWF0ZSI6eyJyZWYiOiJodHRwczovL3NjaW0uZXhhbXBsZS5jb20vVXNlcn
     MvNDRmNjE0MmRmOTZiZDZhYjYxZTc1MjFkOSIsImF0dHJpYnV0ZXMiOlsiaWQiLCJuY
     W1lIiwidXNlck5hbWUiLCJwYXNzd29yZCIsImVtYWlscyJdfX19.",
    "3d0c3cf797584bd193bd0fb1bd4e7d30":
     "eyJhbGciOiJub25lIn0.
     eyJqdGkiOiIzZDBjM2NmNzk3NTg0YmQxOTNiZDBmYjFiZDRlN2QzMCIsImlhdCI6MTQ
     1ODQ5NjAyNSwiaXNzIjoiaHR0cHM6Ly9zY2ltLmV4YW1wbGUuY29tIiwiYXVkIjpbIm
     h0dHBzOi8vamh1Yi5leGFtcGxlLmNvbS9GZWVkcy85OGQ1MjQ2MWZhNWJiYzg3OTU5M
     2I3NzU0IiwiaHR0cHM6Ly9qaHViLmV4YW1wbGUuY29tL0ZlZWRzLzVkNzYwNDUxNmIx
     ZDA4NjQxZDc2NzZlZTciXSwic3ViIjoiaHR0cHM6Ly9zY2ltLmV4YW1wbGUuY29tL1V
     zZXJzLzQ0ZjYxNDJkZjk2YmQ2YWI2MWU3NTIxZDkiLCJldmVudHMiOnsidXJuOmlldG
     Y6cGFyYW1zOnNjaW06ZXZlbnQ6cGFzc3dvcmRSZXNldCI6eyJpZCI6IjQ0ZjYxNDJkZ
     jk2YmQ2YWI2MWU3NTIxZDkifSwiaHR0cHM6Ly9leGFtcGxlLmNvbS9zY2ltL2V2ZW50
     L3Bhc3N3b3JkUmVzZXRFeHQiOnsicmVzZXRBdHRlbXB0cyI6NX19fQ."
   }
  }

                      Figure 6: Example Poll Response

   In the above example, a two SETs whose "jti" are
   "4d3559ec67504aaba65d40b0363faad8" and
   "3d0c3cf797584bd193bd0fb1bd4e7d30" are delivered.

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   The following is a non-normative example response to the request
   shown Section 2.4 showing no new SETs or unacknowledged SETs are
   available.

   HTTP/1.1 200 OK
   Content-Type: application/json
   Location: https://notify.exampleidp/Events

   {
    "sets":{ }
   }

                  Figure 7: Example No SETs Poll Response

   Upon receiving the JSON document (e.g., as shown in Figure 6), the
   SET Recipient parses and verifies the received SETs and notifies the
   SET Transmitter via the next poll request to the SET Transmitter as
   described in Section 2.4.3 or Section 2.4.4.

2.6.  Error Response Handling

   If a SET is invalid, error codes from the IANA "Security Event Token
   Delivery Error Codes" registry established by
   [I-D.ietf-secevent-http-push] are used in error responses.  An error
   response SHALL include a JSON object which provides details about the
   error.  The JSON object includes the JSON attributes:

   err
      A value from the IANA "Security Event Token Delivery Error Codes"
      registry that identifies the error.

   description
      A human-readable string that provides additional diagnostic
      information.

   When included as part of a batch of SETs, the above JSON is included
   as part of the "setErrs" attribute, as defined in Section 2.3 and
   Section 2.4.4.

3.  Authentication and Authorization

   The SET delivery method described in this specification is based upon
   HTTP and depends on the use of TLS and/or standard HTTP
   authentication and authorization schemes as per [RFC7235].  For
   example, the following methodologies could be used among others:

   TLS Client Authentication

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      Event delivery endpoints MAY request TLS mutual client
      authentication.  See Section 7.3 of [RFC5246].

   Bearer Tokens
      Bearer tokens [RFC6750] MAY be used when combined with TLS and a
      token framework such as OAuth 2.0 [RFC6749].  For security
      considerations regarding the use of bearer tokens in SET delivery
      see Section 4.4.1.

   Basic Authentication
      Usage of basic authentication should be avoided due to its use of
      a single factor that is based upon a relatively static, symmetric
      secret.  Implementers SHOULD combine the use of basic
      authentication with other factors.  The security considerations of
      HTTP BASIC, are well documented in [RFC7617] and SHOULD be
      considered along with using signed SETs (see SET Payload
      Authentication below).

   As per Section 4.1 of [RFC7235], a SET delivery endpoint SHALL
   indicate supported HTTP authentication schemes via the "WWW-
   Authenticate" header.

   Because SET Delivery describes a simple function, authorization for
   the ability to pick-up or deliver SETs can be derived by considering
   the identity of the SET issuer, or via an authentication method
   above.  This specification considers authentication as a feature to
   prevent denial-of-service attacks.  Because SETs are not commands,
   SET Recipients are free to ignore SETs that are not of interest.

   For illustrative purposes only, SET delivery examples show an OAuth2
   bearer token value [RFC6750] in the authorization header.  This is
   not intended to imply that bearer tokens are preferred.  However, the
   use of bearer tokens in the specification does reflect common
   practice.

3.1.  Use of Tokens as Authorizations

   When using bearer tokens or proof-of-possession tokens that represent
   an authorization grant such as issued by OAuth (see [RFC6749]),
   implementers SHOULD consider the type of authorization granted, any
   authorized scopes (see Section 3.3 of [RFC6749]), and the security
   subject(s) that SHOULD be mapped from the authorization when
   considering local access control rules.  Section 6 of the OAuth
   Assertions draft [RFC7521], documents common scenarios for
   authorization including:

   o  Clients using an assertion to authenticate and/or act on behalf of
      itself;

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   o  Clients acting on behalf of a user; and,

   o  A Client acting on behalf of an anonymous user (e.g., see next
      section).

   When using OAuth access tokens, implementers MUST take into account
   the threats and countermeasures documented in the security
   considerations for the use of client authorizations (see Section 8 of
   [RFC7521]).  When using other token formats or frameworks,
   implementers MUST take into account similar threats and
   countermeasures, especially those documented by the relevant
   specifications.

4.  Security Considerations

4.1.  Authentication Using Signed SETs

   In scenarios where HTTP authorization or TLS mutual authentication
   are not used or are considered weak, JWS signed SETs SHOULD be used
   (see [RFC7515] and Security Considerations [RFC8417]).  This enables
   the SET Recipient to validate that the SET issuer is authorized to
   deliver the SET.

4.2.  HTTP Considerations

   SET delivery depends on the use of Hypertext Transfer Protocol and
   thus subject to the security considerations of HTTP Section 9 of
   [RFC7230] and its related specifications.

   As stated in Section 2.7.1 of [RFC7230], an HTTP requestor MUST NOT
   generate the "userinfo" (i.e., username and password) component (and
   its "@" delimiter) when an "http" URI reference is generated with a
   message as they are now disallowed in HTTP.

4.3.  TLS Support Considerations

   SETs may contain sensitive information that is considered PII (e.g.,
   subject claims).  In such cases, SET Transmitters and SET Recipients
   MUST require the use of a transport-layer security mechanism.  Event
   delivery endpoints MUST support TLS 1.2 [RFC5246] and MAY support
   additional transport-layer mechanisms meeting its security
   requirements.  When using TLS, the client MUST perform a TLS/SSL
   server certificate check, per [RFC6125].  Implementation security
   considerations for TLS can be found in "Recommendations for Secure
   Use of TLS and DTLS" [RFC7525].

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4.4.  Access Token Considerations

   When using access tokens such as those issued by OAuth 2.0 [RFC6749],
   implementers MUST take into account threats and countermeasures
   documented in Section 8 of [RFC7521].

4.4.1.  Bearer Token Considerations

   Due to the possibility of interception, Bearer tokens MUST be
   exchanged using TLS.

   Bearer tokens MUST have a limited lifetime that can be determined
   directly or indirectly (e.g., by checking with a validation service)
   by the service provider.  By expiring tokens, clients are forced to
   obtain a new token (which usually involves re-authentication) for
   continued authorized access.  For example, in OAuth2, a client MAY
   use OAuth token refresh to obtain a new bearer token after
   authenticating to an authorization server.  See Section 6 of
   [RFC6749].

   Implementations supporting OAuth bearer tokens need to factor in
   security considerations of this authorization method [RFC7521].
   Since security is only as good as the weakest link, implementers also
   need to consider authentication choices coupled with OAuth bearer
   tokens.  The security considerations of the default authentication
   method for OAuth bearer tokens, HTTP BASIC, are well documented in
   [RFC7617], therefore implementers are encouraged to prefer stronger
   authentication methods.  Designating the specific methods of
   authentication and authorization are out of scope for the delivery of
   SETs, however this information is provided as a resource to
   implementers.

5.  Privacy Considerations

   If a SET needs to be retained for audit purposes, a JWS signature MAY
   be used to provide verification of its authenticity.

   SET Transmitters SHOULD attempt to specialize Event Streams so that
   the content is targeted to the specific business and protocol needs
   of subscribers.

   When sharing personally identifiable information or information that
   is otherwise considered confidential to affected users, Event
   Transmitters and Recipients MUST have the appropriate legal
   agreements and user consent or terms of service in place.

   The propagation of subject identifiers can be perceived as personally
   identifiable information.  Where possible, SET Transmitters and

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   Recipients SHOULD devise approaches that prevent propagation -- for
   example, the passing of a hash value that requires the subscriber to
   already know the subject.

6.  IANA Considerations

   There are no IANA considerations.

7.  References

7.1.  Normative References

   [I-D.ietf-secevent-http-push]
              Backman, A., Jones, M., Scurtescu, M., Ansari, M., and A.
              Nadalin, "Push-Based Security Event Token (SET) Delivery
              Using HTTP", draft-ietf-secevent-http-push-03 (work in
              progress), October 2018.

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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <https://www.rfc-editor.org/info/rfc5988>.

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

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <https://www.rfc-editor.org/info/rfc7159>.

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   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [RFC7517]  Jones, M., "JSON Web Key (JWK)", RFC 7517,
              DOI 10.17487/RFC7517, May 2015,
              <https://www.rfc-editor.org/info/rfc7517>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8417]  Hunt, P., Ed., Jones, M., Denniss, W., and M. Ansari,
              "Security Event Token (SET)", RFC 8417,
              DOI 10.17487/RFC8417, July 2018,
              <https://www.rfc-editor.org/info/rfc8417>.

7.2.  Informative References

   [POSIX.1]  Institute of Electrical and Electronics Engineers, "The
              Open Group Base Specifications Issue 7", IEEE Std 1003.1,
              2013 Edition, 2013.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.

   [RFC6202]  Loreto, S., Saint-Andre, P., Salsano, S., and G. Wilkins,
              "Known Issues and Best Practices for the Use of Long
              Polling and Streaming in Bidirectional HTTP", RFC 6202,
              DOI 10.17487/RFC6202, April 2011,
              <https://www.rfc-editor.org/info/rfc6202>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

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   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,
              <https://www.rfc-editor.org/info/rfc6750>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Authentication", RFC 7235,
              DOI 10.17487/RFC7235, June 2014,
              <https://www.rfc-editor.org/info/rfc7235>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [RFC7516]  Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
              RFC 7516, DOI 10.17487/RFC7516, May 2015,
              <https://www.rfc-editor.org/info/rfc7516>.

   [RFC7521]  Campbell, B., Mortimore, C., Jones, M., and Y. Goland,
              "Assertion Framework for OAuth 2.0 Client Authentication
              and Authorization Grants", RFC 7521, DOI 10.17487/RFC7521,
              May 2015, <https://www.rfc-editor.org/info/rfc7521>.

   [RFC7617]  Reschke, J., "The 'Basic' HTTP Authentication Scheme",
              RFC 7617, DOI 10.17487/RFC7617, September 2015,
              <https://www.rfc-editor.org/info/rfc7617>.

Appendix A.  Acknowledgments

   The editors would like to thank the members of the SCIM working
   group, which began discussions of provisioning events starting with:
   draft-hunt-scim-notify-00 in 2015.

   The editors would like to thank Phil Hunt and the other the authors
   of draft-ietf-secevent-delivery-02, on which this draft is based.

   The editors would like to thank the participants in the the SECEVENTS
   working group for their contributions to this specification.

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Appendix B.  Change Log

   [[ to be removed by the RFC Editor before publication as an RFC ]]

   Draft 00 - AB - Based on draft-ietf-secevent-delivery-02 with the
   following additions:

   o  Renamed to "Poll-Based SET Token Delivery Using HTTP"

   o  Removed references to the HTTP Push delivery method.

   Draft 01 - mbj:

   o  Addressed problems identified in my 18-Jul-18 review message
      titled "Issues for both the Push and Poll Specs".

   o  Changes to align terminology with RFC 8417, for instance, by using
      the already defined term SET Recipient rather than SET Receiver.

   o  Applied editorial and minor normative corrections.

   o  Updated Marius' contact information.

   o  Begun eliminating redundanies between this specification and
      "Push-Based Security Event Token (SET) Delivery Using HTTP"
      [I-D.ietf-secevent-http-push], referencing, rather that
      duplicating common normative text.

Authors' Addresses

   Annabelle Backman (editor)
   Amazon

   Email: richanna@amazon.com

   Michael B. Jones (editor)
   Microsoft

   Email: mbj@microsoft.com
   URI:   http://self-issued.info/

   Marius Scurtescu
   Coinbase

   Email: marius.scurtescu@coinbase.com

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   Morteza Ansari
   Cisco

   Email: morteza.ansari@cisco.com

   Anthony Nadalin
   Microsoft

   Email: tonynad@microsoft.com

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