draft-ietf-secevent-http-poll.xml

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<rfc category="std" docName="draft-ietf-secevent-http-poll-12"
	ipr="trust200902">
  <front>
    <title abbrev="draft-ietf-secevent-http-poll">
			Poll-Based Security Event Token (SET) Delivery Using HTTP</title>

    <author fullname="Annabelle Backman" initials="A." surname="Backman" role="editor">
      <organization abbrev="Amazon">Amazon</organization>
      <address>
        <email>richanna@amazon.com</email>
      </address>
    </author>

    <author fullname="Michael B. Jones" initials="M." surname="Jones" role="editor">
        <organization abbrev="Microsoft">Microsoft</organization>
        <address>
            <email>mbj@microsoft.com</email>
            <uri>https://self-issued.info/</uri>
        </address>
    </author>

    <author fullname="Marius Scurtescu" initials="M.S." surname="Scurtescu">
      <organization abbrev="Coinbase">Coinbase</organization>

      <address>
        <email>marius.scurtescu@coinbase.com</email>
      </address>
    </author>

    <author fullname="Morteza Ansari" initials="M." surname="Ansari">
      <organization abbrev="Cisco">Cisco</organization>

      <address>
        <email>morteza.ansari@cisco.com</email>
      </address>
    </author>

    <author fullname="Anthony Nadalin" initials="A." surname="Nadalin">
      <organization abbrev="Microsoft">Microsoft</organization>

      <address>
        <email>tonynad@microsoft.com</email>
      </address>
    </author>

    <date year="2020" month="June" day="24" />

    <area>Security</area>
    <workgroup>Security Events Working Group</workgroup>

    <keyword>Internet-Draft</keyword>

    <abstract>
      <t>
	This specification defines how a series of Security Event Tokens
        (SETs) can 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.
        </t>
    </abstract>
  </front>

  <middle>
    <section anchor="intro" title="Introduction and Overview" toc="default">

      <t>
        This specification defines how a stream of
	Security Event Tokens (SETs) <xref target="RFC8417"/>
        can be transmitted to an intended
        SET Recipient using HTTP <xref target="RFC7231"/>
        over TLS. The specification defines a method to poll for SETs
        using HTTP POST.
	This is an alternative SET delivery method to the one defined in
	<xref target="I-D.ietf-secevent-http-push"/>.
      </t>
      <t>
	Poll-based SET delivery is intended for scenarios where all of
	the following apply:
	<list style="symbols">
	  <t>The recipient of the SET is capable of making outbound HTTP requests.</t>
	  <t>
	    The transmitter is capable of hosting a TLS-enabled HTTP endpoint that is accessible
	    to the recipient.
	  </t>
	  <t>
	    The transmitter and recipient are willing to exchange data with one another.
	  </t>
	</list>
	In some scenarios, either push-based or poll-based delivery could be used,
	and in others, only one of them would be applicable.
      </t>
      <t>
	A mechanism for exchanging configuration metadata such as endpoint URLs,
	cryptographic keys,
	and possible implementation constraints such as buffer size limitations
	between the transmitter and recipient is
        out of scope for this specification.  How SETs are defined and the process
        by which security events are identified for SET Recipients are specified in
	<xref target="RFC8417"/>.
      </t>

      <section anchor="notat" title="Notational Conventions" toc="default">
	<t>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 <xref target="RFC2119"/> <xref target="RFC8174"/>
	when, and only when, they appear in all capitals, as shown here.
	</t>

	<t>
	  Throughout this document, all figures may contain spaces and extra
	  line wrapping for readability and due to space limitations.
	</t>
      </section>

      <section anchor="defs" title="Definitions" toc="default">
	<t>
            This specification utilizes terminology defined in <xref target="RFC8417"/>
            and <xref target="I-D.ietf-secevent-http-push"/>.
	</t>
      </section>
    </section>

    <section anchor="Delivery" title="SET Delivery">
      <t>
	When a SET is available for a SET Recipient, the SET Transmitter
	queues the SET in a buffer so that
	a SET Recipient can poll for SETs using HTTP POST.
      </t>

      <t>
	In poll-based SET delivery using HTTP over TLS, zero or more SETs are
        delivered in a JSON <xref target="RFC8259"/> document
        to a 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 can poll for more.  All requests and
        responses are JSON documents and use a
        <spanx style="verb">Content-Type</spanx> of
        <spanx style="verb">application/json</spanx>, as described in
        <xref target="httpPoll"/>.
      </t>

      <t>After successful (acknowledged) SET delivery, SET
      Transmitters are not required to retain or record SETs for
      retransmission. Once a SET is acknowledged, the SET Recipient SHALL be
      responsible for retention, if needed.
      Transmitters may also discard undelivered SETs under deployment-specific conditions,
      such as if they have not been polled for over too long a period of time
      or if an excessive amount of storage is needed to retain them.
      </t>
      <t>
	Upon receiving a SET, the SET Recipient reads the SET and validates
	it in the manner described in Section 2 of <xref target="I-D.ietf-secevent-http-push"/>.
	The SET Recipient MUST acknowledge receipt to the SET Transmitter,
	and SHOULD do so in a timely fashion, as described in <xref target="pollRequest"/>.
	The SET Recipient SHALL NOT use the event acknowledgement mechanism
        to report event errors other than those relating to the parsing and
        validation of the SET.
      </t>

      <section anchor="httpPoll" title="Polling Delivery using HTTP">
	
	<t>This method allows a SET Recipient to use HTTP POST
	(Section 4.3.3 of <xref target="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,
	per Section 2 of <xref target="RFC6202"/>.
	Note that short polling will result in retrieving zero or more SETs
	whereas long polling will typically result in retrieving one or more SETs
	unless a timeout occurs.
	</t>
	
	<t>The delivery of SETs in this method is facilitated by HTTP
	POST requests initiated by the SET Recipient in which:<list style="symbols">
        <t>The SET Recipient makes a request for available SETs
        using an HTTP POST to a pre-arranged endpoint provided by the SET
        Transmitter or,</t>
        <t>after validating previously received SETs, the SET Recipient
        initiates another poll request using HTTP POST that includes
        acknowledgement of previous SETs and requests the next batch
        of SETs.</t>
	</list></t>
	
	<t>The purpose of the acknowledgement is to inform the
	SET Transmitter that delivery has succeeded and
	redelivery is no longer required.
	Before acknowledgement, SET Recipients validate the received SETs
	and retain them in a manner appropriate to the recipient's
	requirements. The level and method of retention of SETs
	by SET Recipients is out of scope of this specification.</t>
      </section>

      <section anchor="pollReq" title="Polling HTTP Request">

	<t>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 objects.
	</t>
	
	<t>When making a request, the HTTP <spanx style="verb">Content-Type</spanx> header field
	is set to <spanx style="verb">application/json</spanx>.</t>
	
	<t>The following JSON object members are used in a polling request:
	<list style="hanging">
	
	  <t hangText="Request Processing Parameters"><list style="hanging">
	
	    <t hangText="maxEvents"><vspace/>An OPTIONAL integer value
	    indicating the maximum number of unacknowledged SETs to be returned.
	    The SET Transmitter SHOULD NOT send more SETs than the specified maximum.
	    If more than the maximum number of SETs
	    are available, the SET Transmitter determines which to return first;
	    the oldest SETs available MAY returned first,
	    or another selection algorithm MAY be used,
	    such as prioritizing SETs in some manner that makes sense for the use case.
	    first. A value of <spanx style="verb">0</spanx> 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.
	    If this parameter is omitted, no limit is placed on
	    the number of SETs to be returned.
	    </t>
	
	    <t hangText="returnImmediately"><vspace/>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 <spanx style="verb">false</spanx>,
	    which indicates the request is to be treated as an HTTP Long Poll,
	    per Section 2 of <xref target="RFC6202"/>. The timeout for the
	    request is part of the configuration between the participants, which is out of
	    scope of this specification.</t>

	  </list></t>

         <t hangText="SET Acknowledgment Parameters"><list style="hanging">

	   <t hangText="ack">
	     <vspace/>
	     A JSON array of strings whose values
	     are the <spanx style="verb">jti</spanx> <xref target="RFC7519"/> values of
	     successfully received SETs that are being acknowledged.
	     If there are no outstanding SETs to acknowledge, this member is omitted
	     or contains an empty array.
	     Once a SET has been acknowledged, the SET Transmitter is released from
	     any obligation to retain the SET.
	   </t>
	
	   <t hangText="setErrs">
	     <vspace/>
	     A JSON object with one or more members whose keys
	     are the <spanx style="verb">jti</spanx> values of
	     invalid SETs received.
	     The values of these objects are themselves JSON objects that
	     describe the errors detected using the
	     <spanx style="verb">err</spanx> and
	     <spanx style="verb">description</spanx> values
	     specified in <xref target="errorResponse"/>.
	     If there are no outstanding SETs with errors to report, this member is omitted
	     or contains an empty JSON object.
	   </t>

         </list></t>
	
        </list>
        </t>
      </section>

      <section anchor="pollResp" title="Polling HTTP Response">

	<t>In response to a poll request, the SET Transmitter checks for
	available SETs and responds with a JSON document containing
	the following JSON object members:
	<list style="hanging">
	
	  <t hangText="sets"><vspace/>A JSON object containing zero or more SETs being returned.
	  Each member name
	  is the <spanx style="verb">jti</spanx> of a SET to
	  be delivered and its value is a JSON string representing the
	  corresponding SET. If there are no
	  outstanding SETs to be transmitted, the JSON object SHALL be
	  empty.
	  Note that both SETs being transmitted for the first time and
	  SETs that are being re-transmitted after not having been acknowledged
	  are communicated here.
	  </t>
	
	  <t hangText="moreAvailable"><vspace/>A JSON boolean value that
	  indicates if more unacknowledged SETs are available to be returned.
	  This member MAY be omitted, with the meaning being the same as
	  including it with the boolean value <spanx style="verb">false</spanx>.
	  </t>
	</list>
	</t>
	
	<t>When making a response, the HTTP <spanx style="verb">Content-Type</spanx> header field
	is set to <spanx style="verb">application/json</spanx>.</t>
      </section>

      <section anchor="pollRequest" title="Poll Request">

	<t>The SET Recipient performs an HTTP POST (see
	Section 4.3.4 of <xref target="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 <spanx style="verb">ack</spanx> and
	<spanx style="verb">setErrs</spanx> request parameters are omitted.</t>

	<t>
	  After a period of time configured in an out-of-band manner between the SET
	Transmitter and Recipient, a SET Transmitter MAY redeliver 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.
	A SET Transmitter MAY limit the number of times it attempts to
	deliver a SET.
	</t>
	
        <t>If the SET Recipient has received SETs from the
        SET Transmitter, the SET Recipient parses and validates that
        received SETs meet its own requirements and SHOULD acknowledge
        receipt in a timely fashion (e.g., seconds or minutes) so that the SET
        Transmitter can 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.</t>

        <t>Poll requests have three variations:
        <list style="hanging">
          <t hangText="Poll-Only"><vspace/>In which a SET Recipient
          asks for the next set of events where no previous SET deliveries
          are acknowledged (such as in the initial poll request).</t>
          <t hangText="Acknowledge-Only"><vspace/>In which a SET
          Recipient sets the <spanx style="verb">maxEvents</spanx>
          value to <spanx style="verb">0</spanx> along with
          <spanx style="verb">ack</spanx> and
          <spanx style="verb">setErrs</spanx> members indicating the
          SET Recipient is acknowledging previously received SETs and
          does not want to receive any new SETs in response to the
          request. </t>
          <t hangText="Combined Acknowledge and Poll"><vspace/>In
          which a SET Recipient is both acknowledging previously
          received SETs using the <spanx style="verb">ack</spanx> and
          <spanx style="verb">setErrs</spanx> members
          and will wait for the next group of SETs in the SET Transmitters
          response.</t>
        </list></t>

	<section anchor="PollOnlyRequest" title="Poll-Only Request">
	  <t>In the case where no SETs were received in a previous poll (see
	  <xref target="emptyPollResponse"/>), the SET Recipient simply
	  polls without acknowledgement parameters (<spanx style="verb">ack</spanx>
	  and <spanx style="verb">setErrs</spanx>).</t>

	  <figure anchor="pollInitRequest" title="Example Initial Poll Request">
	    <preamble>
	      The following is a non-normative example request made by a SET Recipient
	      that has no outstanding SETs to acknowledge and is polling
	      for available SETs at the endpoint
	      <spanx style="verb">https://notify.idp.example.com/Events</spanx>:
	    </preamble>
	    <artwork><![CDATA[
  POST /Events HTTP/1.1
  Host: notify.idp.example.com
  Content-Type: application/json

  {
   "returnImmediately": true
  }
]]></artwork>
	  </figure>

	  <t>A SET Recipient can poll using default parameter values by passing
	  an empty JSON object.</t>
	
	  <figure anchor="pollDefaultRequest" title="Example Default Poll Request">
	    <preamble>The following is a non-normative example default poll request to the
	    endpoint <spanx style="verb">https://notify.idp.example.com/Events</spanx>:</preamble>
	    <artwork><![CDATA[
  POST /Events HTTP/1.1
  Host: notify.idp.example.com
  Content-Type: application/json

  {}
]]></artwork>
	  </figure>
	</section>
	
	<section anchor="AckOnlyRequest" title="Acknowledge-Only Request">
	  <t>In this variation, the SET Recipient acknowledges previously
	  received SETs and indicates it does not want to receive SETs in
	  response by setting the <spanx style="verb">maxEvents</spanx>
	  value to <spanx style="verb">0</spanx>.
	  This variation might be used, for instance, when a SET Recipient needs to
	  acknowledge received SETs independently (e.g., on separate threads)
	  from the process of receiving SETs.
	  </t>
	  <t>
	    If the poll needs to return immediately, then <spanx style="verb">returnImmediately</spanx>
	    MUST also be present with the value <spanx style="verb">true</spanx>.
	    If it is <spanx style="verb">false</spanx>, then a long poll will still occur
	    until an event is ready to be returned, even though no events will be returned.
	  </t>
	
	  <figure anchor="pollAckOnly" title="Example Acknowledge-Only Request">
	    <preamble>The following is a non-normative example poll request with acknowledgement
	    of SETs received (for example as shown in
	    <xref target="pollResponse"/>):</preamble>
	    <artwork><![CDATA[
  POST /Events HTTP/1.1
  Host: notify.idp.example.com
  Content-Type: application/json

  {
    "ack": [
      "4d3559ec67504aaba65d40b0363faad8",
      "3d0c3cf797584bd193bd0fb1bd4e7d30"
    ],
    "maxEvents": 0,
    "returnImmediately": true
  }
]]></artwork>
	  </figure>
	</section>

	<section anchor="pollAck" title="Poll with Acknowledgement">
	
	  <t>This variation allows a recipient thread to simultaneously
	  acknowledge previously received SETs and wait for the next
	  group of SETs in a single request.</t>
	
	  <figure anchor="pollGoodResponse" title="Example Poll with Acknowledgement and No Errors">
	    <preamble>The following is a non-normative example poll with acknowledgement
	    of the SETs received in <xref target="pollResponse"/>:</preamble>
	    <artwork><![CDATA[
  POST /Events HTTP/1.1
  Host: notify.idp.example.com
  Content-Type: application/json

  {
    "ack": [
      "4d3559ec67504aaba65d40b0363faad8",
      "3d0c3cf797584bd193bd0fb1bd4e7d30"
    ],
    "returnImmediately": false
  }
]]></artwork>
	  </figure>

	  <t>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.</t>
	</section>
	
	<section anchor="pollAckErr" title="Poll with Acknowledgement and Errors">
	  <t>In the case where errors were detected in previously
	  delivered SETs, the SET Recipient MAY use the
	  <spanx style="verb">setErrs</spanx> member to communicate the errors
	  in the following poll request.
	  </t>
	
	  <figure anchor="pollErrorResponse"
		  title="Example Poll Acknowledgement with Error">
	    <preamble>The following is a non-normative example of a response
	    acknowledging one successfully received SET and one SET with an error
	    from the two SETs received in <xref target="pollResponse"/>:</preamble>
	    <artwork><![CDATA[
  POST /Events HTTP/1.1
  Host: notify.idp.example.com
  Content-Language: en-US
  Content-Type: application/json

  {
    "ack": ["3d0c3cf797584bd193bd0fb1bd4e7d30"],
    "setErrs": {
      "4d3559ec67504aaba65d40b0363faad8": {
	"err": "authentication_failed",
	"description": "The SET could not be authenticated"
      }
    },
    "returnImmediately": true
  }
]]></artwork>
	  </figure>

	</section>
      </section>

      <section anchor="pollGetAck"
	       title="Poll Response">

	<t>In response to a valid 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 or the timeout interval has elapsed unless the poll request parameter
	<spanx style="verb">returnImmediately</spanx> is present with the value <spanx style="verb">true</spanx>.
	</t>
	
	<t>As described in <xref target="pollResp"/>, a JSON document
	is returned containing members including
	<spanx style="verb">sets</spanx>, which SHALL contain zero or more
	SETs.</t>
	<figure anchor="pollResponse" title="Example Poll Response">
	  <preamble>The following is a non-normative example response to
	  the request shown in <xref target="pollRequest"/>. This example
	  shows two SETs being returned:</preamble>
	  <artwork><![CDATA[
  HTTP/1.1 200 OK
  Content-Type: application/json

  {
  "sets": {
    "4d3559ec67504aaba65d40b0363faad8":
     "eyJhbGciOiJub25lIn0.
     eyJqdGkiOiI0ZDM1NTllYzY3NTA0YWFiYTY1ZDQwYjAzNjNmYWFkOCIsImlhdCI6MTQ
     1ODQ5NjQwNCwiaXNzIjoiaHR0cHM6Ly9zY2ltLmV4YW1wbGUuY29tIiwiYXVkIjpbIm
     h0dHBzOi8vc2NpbS5leGFtcGxlLmNvbS9GZWVkcy85OGQ1MjQ2MWZhNWJiYzg3OTU5M
     2I3NzU0IiwiaHR0cHM6Ly9zY2ltLmV4YW1wbGUuY29tL0ZlZWRzLzVkNzYwNDUxNmIx
     ZDA4NjQxZDc2NzZlZTciXSwiZXZlbnRzIjp7InVybjppZXRmOnBhcmFtczpzY2ltOmV
     2ZW50OmNyZWF0ZSI6eyJyZWYiOiJodHRwczovL3NjaW0uZXhhbXBsZS5jb20vVXNlcn
     MvNDRmNjE0MmRmOTZiZDZhYjYxZTc1MjFkOSIsImF0dHJpYnV0ZXMiOlsiaWQiLCJuY
     W1lIiwidXNlck5hbWUiLCJwYXNzd29yZCIsImVtYWlscyJdfX19.",
    "3d0c3cf797584bd193bd0fb1bd4e7d30":
     "eyJhbGciOiJub25lIn0.
     eyJqdGkiOiIzZDBjM2NmNzk3NTg0YmQxOTNiZDBmYjFiZDRlN2QzMCIsImlhdCI6MTQ
     1ODQ5NjAyNSwiaXNzIjoiaHR0cHM6Ly9zY2ltLmV4YW1wbGUuY29tIiwiYXVkIjpbIm
     h0dHBzOi8vamh1Yi5leGFtcGxlLmNvbS9GZWVkcy85OGQ1MjQ2MWZhNWJiYzg3OTU5M
     2I3NzU0IiwiaHR0cHM6Ly9qaHViLmV4YW1wbGUuY29tL0ZlZWRzLzVkNzYwNDUxNmIx
     ZDA4NjQxZDc2NzZlZTciXSwic3ViIjoiaHR0cHM6Ly9zY2ltLmV4YW1wbGUuY29tL1V
     zZXJzLzQ0ZjYxNDJkZjk2YmQ2YWI2MWU3NTIxZDkiLCJldmVudHMiOnsidXJuOmlldG
     Y6cGFyYW1zOnNjaW06ZXZlbnQ6cGFzc3dvcmRSZXNldCI6eyJpZCI6IjQ0ZjYxNDJkZ
     jk2YmQ2YWI2MWU3NTIxZDkifSwiaHR0cHM6Ly9leGFtcGxlLmNvbS9zY2ltL2V2ZW50
     L3Bhc3N3b3JkUmVzZXRFeHQiOnsicmVzZXRBdHRlbXB0cyI6NX19fQ."
   }
  }
]]></artwork>
	</figure>
	<t>In the above example, two SETs whose <spanx style="verb">jti</spanx> values
	are <spanx style="verb">4d3559ec67504aaba65d40b0363faad8</spanx>
	and <spanx style="verb">3d0c3cf797584bd193bd0fb1bd4e7d30</spanx>
	are delivered.</t>
	
	<figure anchor="emptyPollResponse" title="Example No SETs Poll Response">
	  <preamble>The following is a non-normative example response to
	  the request shown in <xref target="PollOnlyRequest"/>, which indicates that no new
	  SETs or unacknowledged SETs are available:</preamble>
	  <artwork><![CDATA[
  HTTP/1.1 200 OK
  Content-Type: application/json

  {
   "sets": {}
  }
]]></artwork>
	</figure>
	
	<t>Upon receiving the JSON document (e.g., as shown in
	<xref target="pollResponse"/>), the SET Recipient parses
	and verifies the received SETs and notifies the SET Transmitter
	of successfully received SETs and SETs with errors
	via the next poll request to the SET Transmitter, as described in
	<xref target="pollAck"/> or <xref target="pollAckErr"/>.</t>
	
        <section anchor="PollErrorResponse" title="Poll Error Response">
            <t>In the event of a general HTTP error condition in the context of
                processing a poll request, the service provider responds with
                the applicable HTTP Response Status Code, as defined in Section 6 of
                <xref target="RFC7231" />.</t>

            <t>Service providers MAY respond to any invalid poll request with an HTTP Response
                Status Code of 400 (Bad Request) even when a more specific code might apply, for
                example if the service provider deemed that a more specific code presented an
                information disclosure risk. When no more specific code might apply, the service
                provider SHALL respond to an invalid poll request with an HTTP Status Code of 400.</t>

            <t>
	      The response body for responses to invalid poll requests is left undefined,
	      and its contents SHOULD be ignored.
	    </t>

            <figure title="Example Poll Error Response">
                <preamble>
                    The following is a non-normative example of a response to an invalid poll request:
                </preamble>
                <artwork><![CDATA[
  HTTP/1.1 400 Bad Request
]]></artwork>
            </figure>
        </section>
      </section>

      <section anchor="errorResponse" title="Error Response Handling">
        <t>
	  If a SET is invalid,
	  error codes from the IANA "Security Event Token Delivery Error Codes"
	  registry established by <xref target="I-D.ietf-secevent-http-push"/>
	  are used in error responses.
	  As described in Section 2.3 of <xref target="I-D.ietf-secevent-http-push"/>,
	  an error response is a JSON object providing details about the error
	  that includes the following name/value pairs:
	</t>
	<t>
	  <list style="hanging">
	    <t hangText="err"><vspace />
	      A value from the
	      IANA "Security Event Token Delivery Error Codes" registry
	      that identifies the error.
	    </t>
	    <t hangText="description"><vspace />
	      A human-readable string that provides
	      additional diagnostic information.
	    </t>
	  </list>
	</t>
	<t>
	  When included as part of a batch of SETs, the above JSON is included
	  as part of the <spanx style="verb">setErrs</spanx> member, as
          defined in <xref target="pollReq"/> and <xref target="pollAckErr"/>.
	</t>
        <t>
            When the SET Recipient includes one or more error responses in a request to
            the SET Transmitter, it must also include in the request a 
            <spanx style="verb">Content-Language</spanx> header field whose value indicates the
            language of the error descriptions included in the request.  The method of
            language selection in the case when the SET Recipient can provide error messages
            in multiple languages is out of scope for this specification.
        </t>
      </section>

    </section>

    <section anchor="aa" title="Authentication and Authorization" toc="default">
      <t>The SET delivery method described in this specification is
      based upon HTTP over TLS <xref target="RFC2818"/> and standard
      HTTP authentication and authorization schemes, as per
      <xref target="RFC7235" />.
      The TLS server certificate MUST be validated using DNS-ID <xref target="RFC6125"/>
      and/or DANE <xref target="RFC6698"/>.
      As per Section 4.1 of <xref target="RFC7235"/>, a SET
      delivery endpoint SHALL indicate supported HTTP authentication
      schemes via the <spanx style="verb">WWW-Authenticate</spanx> header field
      when using HTTP authentication.
      </t>

      <t>
	Authorization for the eligibility to provide actionable SETs can be determined by
	using the identity of the SET Issuer,
	validating the identity of the SET Transmitter,
	or via other employed authentication methods.
	Likewise, the SET Transmitter may choose to validate the identity of the SET Recipient,
	perhaps using mutual TLS.
	Because SETs are
      not commands, SET Recipients are free to ignore SETs that
      are not of interest after acknowledging their receipt.</t>

    </section>

    <section anchor="Security" title="Security Considerations" toc="default">

      <section anchor="payloadAuthentication" title="Authentication Using Signed SETs">
	<t>
	  JWS signed SETs can be
	  used (see <xref target="RFC7515"/> and Section 5 of <xref target="RFC8417"/>)
	  to enable the SET Recipient
	  to validate that the SET Issuer is authorized to provide actionable SETs.
	</t>
      </section>

      <section anchor="HTTP" title="HTTP Considerations">
        <t>SET delivery depends on the use of Hypertext Transfer Protocol and is thus
        subject to the security considerations of HTTP Section 9 of <xref
        target="RFC7230"/> and its related specifications.</t>
      </section>

      <section anchor="Confidentiality" title="Confidentiality of SETs">
        <t>
	  SETs may contain sensitive information, including Personally Identifiable Information (PII),
	  or be distributed through third parties.
	  In such cases, SET Transmitters and
	  SET Recipients MUST protect the confidentiality of the SET contents.
	  In some use cases, using TLS to secure the transmitted SETs will be sufficient.
	  In other use cases,
	  encrypting the SET as described in JWE <xref target="RFC7516"/> will also be required.
	  The Event delivery endpoint MUST support at least TLS
	  version 1.2 <xref target="RFC5246"/> and SHOULD support the newest version
	  of TLS that meets its security requirements,
	  which as of the time of this publication is TLS 1.3 <xref target="RFC8446"/>.
	  The client MUST
	  perform a TLS/SSL server certificate check using DNS-ID <xref target="RFC6125"/>
	  and/or DANE <xref target="RFC6698"/>.
          How a SET Recipient determines the expected service identity to match the SET
          Transmitter's server certificate against is out of scope for this document.
	  The implementation security considerations for TLS in
	  "Recommendations for Secure Use of TLS and DTLS" <xref target="RFC7525"/>
	  MUST be followed.
	</t>
      </section>

      <section anchor="AT" title="Access Token Considerations">
        <t>
	  If HTTP Authentication is performed using OAuth access tokens <xref target="RFC6749"/>,
	  implementers MUST take into account the threats
        and countermeasures documented in Section 8 of <xref
        target="RFC7521"/>.</t>

        <section anchor="bearerConsiderations"
		 title="Bearer Token Considerations">
	  <t>
	    Transmitting Bearer tokens <xref target="RFC6750"/> using TLS helps prevent their interception.
	  </t>

	  <t>Bearer tokens SHOULD 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 OAuth 2.0, a client MAY use
	  an OAuth refresh token to obtain a new bearer token after authenticating
	  to an authorization server, per Section 6 of <xref
	  target="RFC6749"/>.</t>

	  <t>Implementations supporting OAuth bearer tokens need to factor in
	  security considerations of this authorization method <xref
	  target="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 <xref
	  target="RFC7617"/>, therefore implementers
	  are encouraged to prefer stronger authentication methods.
	  </t>
	</section>
      </section>


    </section>

    <section anchor="Privacy" title="Privacy Considerations">

      <t>SET Transmitters should attempt to deliver SETs that are
      targeted to the specific business and
      protocol needs of subscribers.</t>

      <t>When sharing personally identifiable information or information
      that is otherwise considered confidential to affected users, SET
      Transmitters and Recipients MUST have the appropriate legal agreements
      and user consent or terms of service in place.
      Furthermore, data that needs confidentiality protection MUST be encrypted,
      at least with TLS
      and sometimes also using JSON Web Encryption (JWE) <xref target="RFC7516"/>.
      </t>

      <t>
	In some cases, subject identifiers themselves may be considered sensitive
	information, such that their inclusion within a SET may be considered a violation
	of privacy.  SET Issuers and SET Transmitters should consider the ramifications of sharing a
	particular subject identifier with a SET Recipient (e.g., whether doing so could
	enable correlation and/or de-anonymization of data) and choose appropriate
	subject identifiers for their use cases.
      </t>

    </section>

    <section anchor="IANA" title="IANA Considerations">

      <t>
	This specification requires no IANA actions.
      </t>

    </section>
  </middle>

  <back>
    <references title="Normative References">
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml' ?>
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2818.xml' ?><!-- HTTP over TLS -->
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5246.xml' ?><!-- TLS 1.2 -->
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6125.xml' ?><!-- TLS Certs -->
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6698.xml' ?><!-- DANE -->
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7231.xml' ?><!-- HTTP Semantics -->

      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7515.xml' ?><!-- JWS -->
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7516.xml' ?><!-- JWE -->
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7519.xml' ?><!-- JWT -->
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7521.xml' ?><!-- Client Auth Assertions -->

      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7525.xml' ?><!-- TLS Recos -->
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml' ?><!-- RFC 2119 bis -->
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8259.xml' ?><!-- JSON -->
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8417.xml' ?><!-- SET -->
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8446.xml' ?><!-- TLS 1.3 -->

      <?rfc include="http://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.draft-ietf-secevent-http-push-12.xml" ?>
    </references>

    <references title="Informative References">
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6202.xml' ?><!-- HTTP Long Polling -->

      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6749.xml' ?><!-- OAuth -->
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6750.xml' ?><!-- OAuth Bearer -->

      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7230.xml' ?><!-- HTTP Msg -->
      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7235.xml' ?><!-- HTTP Auth -->

      <?rfc include='http://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7617.xml' ?><!-- Basic Auth Update -->

    </references>

    <section anchor="Unencrypted" title="Unencrypted Transport Considerations">
      <t>
	Earlier versions of this specification made the use of TLS optional
	and described security and privacy considerations resulting from use
	of unencrypted HTTP as the underlying transport.
	When the working group decided to mandate usage HTTP over TLS,
	it also decided to preserve the description of these considerations
	in a non-normative manner.
      </t>
      <t>
	The considerations for using unencrypted HTTP with this protocol
	are the same as those described in Appendix A of <xref target="I-D.ietf-secevent-http-push"/>,
	and are therefore not repeated here.
      </t>
    </section>

    <section anchor="Others" title="Other Streaming Specifications">
      <t>[[ NOTE TO THE RFC EDITOR: This section to be removed prior to publication ]]</t>
      <t>
	A number of pub/sub, queuing, and streaming systems were reviewed
	as possible solutions or as input to the current draft.
	These are listed in Appendix B of <xref target="I-D.ietf-secevent-http-push"/>,
	and are therefore not repeated here.
      </t>
    </section>

    <section anchor="Acknowledgments" title="Acknowledgments">
      <t>
	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.
	We would like to thank Phil Hunt and the other the authors of draft-ietf-secevent-delivery-02,
	upon which this specification is based.
	We would like to thank the participants in the SecEvents
	working group for their contributions to this specification.
      </t>
      <t>
	Additionally, we would like to thank the following individuals for their reviews of the specification:
	Roman Danyliw,
	Martin Duke,
	Benjamin Kaduk,
	Erik Kline,
	Murray Kucherawy,
	Warren Kumari,
	Barry Leiba,
	Mark Nottingham,
	Alvaro Retana,
	Yaron Sheffer,
	Valery Smyslov,
	Robert Sparks,
	Éric Vyncke,
	and
	Robert Wilton.
      </t>
    </section>

    <section anchor="History" title="Change Log">

      <t>[[ to be removed by the RFC Editor before publication as an RFC ]]</t>

      <t>
	Draft 00 - AB - Based on draft-ietf-secevent-delivery-02 with the
	following additions:
	<list style="symbols">
	  <t>Renamed to "Poll-Based SET Token Delivery Using HTTP"</t>
	  <t>Removed references to the HTTP Push delivery method.</t>
	</list>
      </t>
      <t>
	Draft 01 - mbj:
	<list style="symbols">
	  <t>
	    Addressed problems identified in my 18-Jul-18 review message titled
	    "Issues for both the Push and Poll Specs".
	  </t>
	  <t>
	    Changes to align terminology with RFC 8417, for instance,
	    by using the already defined term SET Recipient rather than SET Receiver.
	  </t>
	  <t>
	    Applied editorial and minor normative corrections.
	  </t>
	  <t>
	    Updated Marius' contact information.
	  </t>
	  <t>
	    Begun eliminating redundancies between this specification and
	    "Push-Based Security Event Token (SET) Delivery Using HTTP"
	    <xref target="I-D.ietf-secevent-http-push"/>,
	    referencing, rather that duplicating common normative text.
	  </t>
	</list>
      </t>
      <t>
	Draft 02 - mbj:
	<list style="symbols">
	  <t>
	    Removed vestigial language remaining from when the push and poll
	    delivery methods were defined in a common specification.
	  </t>
	  <t>
	    Replaced remaining uses of the terms Event Transmitter and Event Recipient
	    with the correct terms SET Transmitter and SET Recipient.
	  </t>
	  <t>
	    Removed uses of the unnecessary term "Event Stream".
	  </t>
	  <t>
	    Removed dependencies between the semantics of
	    <spanx style="verb">maxEvents</spanx> and <spanx style="verb">returnImmediately</spanx>.
	  </t>
	  <t>
	    Said that PII in SETs is to be encrypted with TLS, JWE, or both.
	  </t>
	  <t>
	    Corrected grammar and spelling errors.
	  </t>
	</list>
      </t>
      <t>
	Draft 03 - mbj:
	<list style="symbols">
	  <t>
	    Corrected uses of "attribute" to "member" when describing JSON objects.
	  </t>
	  <t>
	    Further alignment with the push draft.
	  </t>
	</list>
      </t>
      <t>
        Draft 04 - AB + mbj
        <list style="symbols">
          <t>
            Referenced SET Transmitter definition in http-push.
          </t>
          <t>
            Removed incorrect normative text regarding SET construction.
          </t>
          <t>
            Consolidated general out-of-scope items under Introduction.
          </t>
          <t>
            Removed unnecessary HTTP headers in examples and added Content-Type.
          </t>
          <t>
            Added Content-Language requirement for error descriptions, aligning with http-push.
          </t>
	  <t>
	    Stated that bearer tokens SHOULD have a limited lifetime.
	  </t>
          <t>
            Minor editorial fixes.
          </t>
        </list>
      </t>
      <t>
        Draft 05 - AB + mbj
        <list style="symbols">
          <t>
            Added normative text defining how to respond to invalid poll requests.
          </t>
          <t>
            Addressed shepherd comments by Yaron Sheffer.
          </t>
        </list>
      </t>
      <t>
        Draft 06 - mbj
        <list style="symbols">
          <t>
            Addressed nits identified by the idnits tool.
          </t>
        </list>
      </t>
      <t>
        Draft 07 - mbj
        <list style="symbols">
          <t>
            Addressed area director review comments by Benjamin Kaduk.
          </t>
        </list>
      </t>
      <t>
        Draft 08 - mbj + AB
        <list style="symbols">
          <t>
            Corrected editorial nits.
          </t>
        </list>
      </t>
      <t>
        Draft 09 - AB
        <list style="symbols">
          <t>
            Addressed area director review comments by Benjamin Kaduk:
            <list style="symbols">
              <t>Added text clarifying that determining the SET Recipient's service identity is out of scope.</t>
              <t>Removed unelaborated reference to use of authentication to prevent DoS attacks.</t>
            </list>
          </t>
        </list>
      </t>
      <t>
	Draft 10 - mbj
	<list style="symbols">
	  <t>
	    Addressed SecDir review comments by Valery Smyslov
	    on draft-ietf-secevent-http-push-10 that also applied here.
	  </t>
	  <t>
	    Addressed IETF last call comments by Mark Nottingham.
	  </t>
	  <t>
	    Addressed GenArt review comments by Robert Sparks.
	  </t>
	</list>
      </t>
      <t>
	Draft 11 - mbj
	<list style="symbols">
	  <t>
	    Revised to unambiguously require the use of TLS,
	    while preserving descriptions of precautions needed for non-TLS use in an appendix.
	  </t>
	</list>
      </t>
      <t>
	Draft 12 - mbj
	<list style="symbols">
	  <t>
	    Addressed IESG comments.
	  </t>
	</list>
      </t>
    </section>
  </back>
</rfc>