draft-ietf-ice-trickle.xml

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     docName='draft-ietf-ice-trickle-21'>

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  <front>

    <title abbrev='Trickle ICE'>
        Trickle ICE: Incremental Provisioning of Candidates for the Interactive 
        Connectivity Establishment (ICE) Protocol
    </title>
    <author initials='E.' surname='Ivov'
            fullname='Emil Ivov'>
      <organization abbrev='Atlassian'>Atlassian</organization>
      <address>
        <postal>
          <street>303 Colorado Street, #1600</street>
          <city>Austin</city>
          <region>TX</region>
          <code>78701</code>
          <country>USA</country>
        </postal>
        <phone>+1-512-640-3000</phone>
        <email>eivov@atlassian.com</email>
      </address>
    </author>
    <author fullname="Eric Rescorla" initials="E.K." surname="Rescorla">
      <organization>RTFM, Inc.</organization>
      <address>
        <postal>
          <street>2064 Edgewood Drive</street>
          <city>Palo Alto</city>
          <region>CA</region>
          <code>94303</code>
          <country>USA</country>
        </postal>
        <phone>+1 650 678 2350</phone>
        <email>ekr@rtfm.com</email>
      </address>
    </author>
    <author fullname="Justin Uberti" initials="J." surname="Uberti">
      <organization>Google</organization>
      <address>
        <postal>
          <street>747 6th St S</street>
          <city>Kirkland</city>
          <region>WA</region>
          <code>98033</code>
          <country>USA</country>
        </postal>
        <phone>+1 857 288 8888</phone>
        <email>justin@uberti.name</email>
      </address>
    </author>
    <author initials="P." surname="Saint-Andre" fullname="Peter Saint-Andre">
      <organization>Mozilla</organization>
      <address>
        <postal>
          <street>P.O. Box 787</street>
          <city>Parker</city>
          <region>CO</region>
          <code>80134</code>
          <country>USA</country>
        </postal>
        <phone>+1 720 256 6756</phone>
        <email>stpeter@mozilla.com</email>
        <uri>https://www.mozilla.com/</uri>
      </address>
    </author>
    <date />
    <abstract>
      <t>
        This document describes "Trickle ICE", an extension to the Interactive
        Connectivity Establishment (ICE) protocol that enables ICE agents
        to begin connectivity checks while they are still gathering 
        candidates, by incrementally exchanging candidates over time instead
        of all at once. This method can considerably accelerate the process 
        of establishing a communication session.
      </t>
    </abstract>
  </front>
  <middle>
    <section title='Introduction'>
      <t>
        The Interactive Connectivity Establishment (ICE) protocol 
        <xref target="rfc5245bis"/> describes how an ICE agent 
        gathers candidates, exchanges candidates with a peer ICE 
        agent, and creates candidate pairs. Once the pairs have been 
        gathered, the ICE agent will perform connectivity checks, and 
        eventually nominate and select pairs that will be used for 
        sending and receiving data within a communication session.
      </t>
      <t>
        Following the procedures in <xref target="rfc5245bis"/> can 
        lead to somewhat lengthy establishment times for communication sessions, 
        because candidate gathering often involves querying STUN servers 
        <xref target="RFC5389"/> and allocating relayed candidates using 
        TURN servers <xref target="RFC5766"/>. Although many ICE procedures 
        can be completed in parallel, the pacing requirements from 
        <xref target="rfc5245bis"/> still need to be followed.
      </t>
      <t>
        This document defines "Trickle ICE", a supplementary mode of ICE 
        operation in which candidates can be exchanged 
        incrementally as soon as they become available (and simultaneously 
        with the gathering of other candidates). Connectivity checks can 
        also start as soon as candidate pairs have been created. Because 
        Trickle ICE enables candidate gathering and connectivity checks
        to be done in parallel, the method can considerably accelerate
        the process of establishing a communication session.
      </t>
      <t>
        This document also defines how to discover support for
        Trickle ICE, how the procedures in <xref target="rfc5245bis"/> are 
        modified or supplemented when using Trickle ICE, and how a Trickle 
        ICE agent can interoperate with an ICE agent compliant to 
        <xref target="rfc5245bis"/>.
      </t>
      <t>
        This document does not define any protocol-specific usage of Trickle 
        ICE. Instead, protocol-specific details for Trickle ICE are defined 
        in separate usage documents. Examples of such documents are 
        <xref target="I-D.ietf-mmusic-trickle-ice-sip"/> (which defines usage 
        with the Session Initiation Protocol (SIP) <xref target='RFC3261'/> 
        and the Session Description Protocol <xref target='RFC3261'/>) and 
        <xref target='XEP-0176'/> (which defines usage with XMPP 
        <xref target='RFC6120'/>). However, some of the examples in the 
        document use SDP and the offer/answer model <xref target='RFC3264'/> 
        to explain the underlying concepts.
      </t>
      <t>
        The following diagram illustrates a successful Trickle ICE exchange with a
        using protocol that follows the offer/answer model:
      </t>
      <figure title="Flow" anchor="fig-flow">
        <artwork>
<![CDATA[
        Alice                                            Bob
          |                     Offer                     |
          |---------------------------------------------->|
          |            Additional Candidates              |
          |---------------------------------------------->|
          |                     Answer                    |
          |<----------------------------------------------|
          |            Additional Candidates              |
          |<----------------------------------------------|
          | Additional Candidates and Connectivity Checks |
          |<--------------------------------------------->|
          |<========== CONNECTION ESTABLISHED ===========>|

]]>
        </artwork>
      </figure>
      <t>
        The main body of this document is structured to describe the behavior 
        of Trickle ICE agents in roughly the order of operations and interactions 
        during an ICE session:
        <list style='numbers'>
          <t>Determining support for trickle ICE</t>
          <t>Generating the initial ICE description</t>
          <t>Handling the initial ICE description and generating the initial ICE response</t>
          <t>Handling the initial ICE response</t>
          <t>Forming check lists, pruning candidates, performing connectivity checks, etc.</t>
          <t>Gathering and conveying candidates after the initial ICE description and response</t>
          <t>Handling inbound trickled candidates</t>
          <t>Generating and handling the end-of-candidates indication</t>
          <t>Handling ICE restarts</t>
        </list>
      </t>
      <t>
        There is quite a bit of operational experience with the technique behind 
        Trickle ICE, going back as far as 2005 (when the XMPP Jingle extension 
        defined a "dribble mode" as specified in <xref target='XEP-0176'/>); this 
        document incorporates feedback from those who have implemented and 
        deployed the technique over the years.
      </t>
    </section>
    <section title="Terminology">
      <t>
        The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
        NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
        "OPTIONAL" in this document are to be interpreted as described
        in <xref target="RFC2119"/>.
      </t>
      <t>
        This specification makes use of all terminology defined 
        for Interactive Connectivity Establishment in
        <xref target="rfc5245bis"/>. In addition, it defines the following terms:
      </t>
      <t>
        <list style="hanging">
          <t hangText="Full Trickle:">
            The typical mode of operation for Trickle ICE agents, in which
            the initial ICE description can include any number of candidates (even
            zero candidates) and does not need to include a full generation
            of candidates as in half trickle.
          </t>
          <t hangText="Generation:">
            All of the candidates conveyed within an ICE session (correlated
            with a particular Username Fragment and Password combination).
          </t>
          <t hangText="Half Trickle:">
            A Trickle ICE mode of operation in which the initiator gathers
            a full generation of candidates strictly before creating
            and conveying the initial ICE description. Once conveyed, 
            this candidate information can be
            processed by regular ICE agents, which do not require support
            for Trickle ICE. It also allows Trickle ICE capable
            responders to still gather candidates and perform
            connectivity checks in a non-blocking way, thus providing roughly 
            "half" the advantages of Trickle ICE. The half trickle mechanism
            is mostly meant for use when the responder's support for Trickle
            ICE cannot be confirmed prior to conveying the initial ICE description.
          </t>
          <t hangText="ICE Description:">
            Any attributes related to the ICE session (not candidates) 
            required to configure an ICE agent. These include but are not 
            limited to the username fragment, password, and other attributes.
          </t>
          <t hangText="Trickled Candidates:">
            Candidates that a Trickle ICE agent conveys after conveying the initial 
            ICE description or responding to the initial ICE description, but within 
            the same ICE session.  Trickled candidates can be conveyed in 
            parallel with candidate gathering and connectivity checks.
          </t>
          <t hangText="Trickling:">
            The act of incrementally conveying trickled candidates.
          </t>
          <t hangText="Empty Check List:">
            A check list that initially does not contain any candidate pairs 
            because they will be incrementally added as they are trickled.
           (This scenario does not arise with a regular ICE agent, because all 
           candidate pairs are known when the agent creates the check list set).
          </t>
        </list>
      </t>
    </section>
    <section title='Determining Support for Trickle ICE' anchor="support">
      <t>
        To fully support Trickle ICE, using protocols
        SHOULD incorporate one of the following mechanisms so that implementations
        can determine whether Trickle ICE is supported:
      </t>
      <t>
        <list style='numbers'>
          <t>
            Provide a capabilities discovery method so that agents can verify 
            support of Trickle ICE prior to initiating a session (XMPP's
            <xref target="XEP-0030">Service Discovery</xref> is 
            one such mechanism).
          </t>
          <t>
            Make support for Trickle ICE mandatory so that user agents
            can assume support.
          </t>
        </list>
      </t>
      <t>
        If a using protocol does not provide a method of determining
        ahead of time whether Trickle ICE is supported, agents can make use of 
        the half trickle procedure described in <xref target="half-trickle"/>.
      </t>
      <t>
        Prior to conveying the initial ICE description, agents that implement using protocols
        that support capabilities discovery can attempt to verify whether or 
        not the remote party supports Trickle ICE. If an agent determines 
        that the remote party does not support Trickle ICE, it MUST fall back 
        to using regular ICE or abandon the entire session.
      </t>
      <t>
        Even if a using protocol does not include a capabilities discovery 
        method, a user agent can provide an indication within the ICE description 
        that it supports Trickle ICE by communicating an ICE option of 'trickle'.  
        This token MUST be provided either at the session level or, if at the data 
        stream level, for every data stream (an agent MUST NOT specify Trickle ICE 
        support for some data streams but not others).

        Note: The encoding of the 'trickle' ICE option, and the message(s) used to 
        carry it to the peer, are protocol specific; for instance, the encoding for 
        the Session Description Protocol (SDP) <xref target='RFC4566'/> is defined in
        <xref target='I-D.ietf-mmusic-trickle-ice-sip'/>.
      </t>
      <t>
        Dedicated discovery semantics and half trickle are needed only prior 
        to initiation of an ICE session. After an ICE session is established
        and Trickle ICE support is confirmed for both parties, either
        agent can use full trickle for subsequent exchanges (see also
        <xref target='subsequent'/>).
      </t>
    </section>
    <section title='Generating the Initial ICE Description' anchor="initial">
      <t>
        An ICE agent can start gathering candidates as soon as it has an
        indication that communication is imminent (e.g., a user interface
        cue or an explicit request to initiate a communication session). Unlike in
        regular ICE, in Trickle ICE implementations do not need to
        gather candidates in a blocking manner. Therefore, unless half
        trickle is being used, the user experience is improved if the 
        initiating agent generates and transmits its initial ICE description 
        as early as possible (thus enabling the remote party to start 
        gathering and trickling candidates).
      </t>
      <t>
        An initiator MAY include any mix of candidates when conveying
        the initial ICE description. This includes the possibility of conveying 
        all the candidates the initiator plans to use
        (as in half trickle), conveying only a 
        publicly-reachable IP address (e.g., a candidate at a data 
        relay that is known to not be behind a firewall), or conveying 
        no candidates at all (in which case the initiator can obtain the 
        responder's initial candidate list sooner and the responder can begin 
        candidate gathering more quickly).
      </t>
      <t>
        For candidates included in the initial ICE description, the methods 
        for calculating priorities and foundations, determining redundancy 
        of candidates, and the like work just as in regular ICE 
        <xref target="rfc5245bis"/>.
      </t>
    </section>
    <section title='Handling the Initial ICE Description and Generating the Initial ICE Response' >
      <t>
        When a responder receives the initial ICE description, it will first check if
        the ICE description or initiator indicates support for Trickle ICE as explained 
        in <xref target="support"/>. If not, the responder MUST
        process the initial ICE description according to regular ICE procedures 
        <xref target="rfc5245bis"/> (or, if no ICE support is detected at all,
        according to relevant processing rules for the using
        protocol, such as offer/answer processing rules <xref target="RFC3264"/>).
        However, if support for Trickle ICE is confirmed, a responder will 
        automatically assume support for regular ICE as well. 
      </t>
      <t>
        If the initial ICE description indicates support for Trickle ICE, the 
        responder will determine its role and start gathering and prioritizing
        candidates; while doing so, it will also respond by conveying an 
        initial ICE response, so that both the initiator 
        and the responder can form check lists and begin connectivity checks.
      </t>
      <t>
        A responder can respond to the initial ICE description at any point while
        gathering candidates. The initial ICE response MAY contain any set of
        candidates, including all candidates or no candidates. (The benefit of 
        including no candidates is to convey the initial ICE response as 
        quickly as possible, so that both parties can consider the 
        ICE session to be under active negotiation as soon as 
        possible.) 
      </t>
      <t>
        As noted in <xref target="support"/>, in using protocols that use 
        SDP the initial ICE response can indicate support for Trickle ICE 
        by including a token of "trickle" in the ice-options attribute.
      </t>
    </section>
    <section title="Handling the Initial ICE Response">
      <t>
        When processing the initial ICE response, the initiator follows regular ICE
        procedures to determine its role, after which it
        forms check lists (<xref target="checklists"/>)
        and performs connectivity checks (<xref target='checks'/>).
      </t>
    </section>
    <section title='Forming Check Lists' anchor='checklists'>
      <t>
        According to regular ICE procedures <xref target="rfc5245bis"/>, 
        in order for candidate pairing 
        to be possible and for redundant candidates to be pruned, the 
        candidates would need to be provided in the initial ICE description
        and initial ICE response.
        By contrast, under Trickle ICE check lists can be empty until 
        candidates are conveyed or received. Therefore a Trickle ICE agent
        handles check list formation and candidate pairing in a slightly different
        way than a regular ICE agent: the agent still forms the check lists, but 
        it populates a given check list only after it actually has candidate 
        pairs for that check list. Every check list is initially placed in the 
        Running state, even if the check list is empty (this is consistent
        with Section 6.1.2.1 of <xref target='rfc5245bis'/>).
      </t>
    </section>
    <section title='Performing Connectivity Checks' anchor='checks'>
      <t>
        As specified in <xref target='rfc5245bis'/>, whenever timer 
        Ta fires, only check lists in the Running state will be picked 
        when scheduling connectivity checks for candidate pairs.
        Therefore, a Trickle ICE agent MUST keep each check list in 
        the Running state as long as it expects candidate pairs to be 
        incrementally added to the check list. After that, the check 
        list state is set according to the procedures in
        <xref target='rfc5245bis'/>.
      </t>
      <t>
        Whenever timer Ta fires and an empty check list is picked, no action 
        is performed for the list. Without waiting for timer Ta to expire 
        again, the agent selects the next check list in the Running state, 
        in accordance with Section 6.1.4.2 of <xref target='rfc5245bis'/>.
      </t>
      <t>
        Section 7.2.5.3.3 of <xref target='rfc5245bis'/>
        requires that agents update check lists and timer states upon 
        completing a connectivity check transaction. During such an 
        update, regular ICE agents would set the state of a check list 
        to Failed if both of the following two conditions are satisfied:
      </t>
      <t>
        <list style="symbols">
          <t>
            all of the pairs in the check list are either in the
            Failed state or Succeeded state; and
          </t>
          <t>
            there is not a pair in the valid list for each component 
            of the data stream.
          </t>
        </list>
      </t>
      <t>
        With Trickle ICE, the above situation would often occur when
        candidate gathering and trickling are still in progress, even
        though it is quite possible that future checks will succeed. For
        this reason, Trickle ICE agents add the following conditions to
        the above list:
      </t>
      <t>
        <list style="symbols">
          <t>
            all candidate gathering has completed and the agent
            is not expecting to discover any new local candidates; and
          </t>
          <t>
            the remote agent has conveyed an end-of-candidates indication
            for that check list as described in
            <xref target="end-of-candidates.send"/>.
          </t>
        </list>
      </t>
    </section>
    <section title='Gathering and Conveying Newly Gathered Local Candidates'
             anchor="trickle-send">
      <t>
        After Trickle ICE agents have conveyed initial ICE descriptions
        and initial ICE responses, they will most
        likely continue gathering new local candidates as STUN, TURN,
        and other non-host candidate gathering mechanisms begin to
        yield results. Whenever an agent discovers such a new candidate
        it will compute its priority, type, foundation, and component ID
        according to regular ICE procedures.
      </t>
      <t>
        The new candidate is then checked for redundancy against the
        existing list of local candidates. If its transport address and
        base match those of an existing candidate, it will be considered
        redundant and will be ignored. This would often happen for
        server reflexive candidates that match the host addresses they
        were obtained from (e.g., when the latter are public IPv4
        addresses). Contrary to regular ICE, Trickle ICE agents will
        consider the new candidate redundant regardless of its priority.
      </t>
      <t>
        Next the agent "trickles" the newly discovered
        candidate(s) to the remote agent. The actual delivery of the new
        candidates is handled by a using protocol such as SIP or XMPP.
        Trickle ICE imposes no restrictions on the way this is done 
        (e.g., some using protocols might
        choose not to trickle updates for server reflexive
        candidates and instead rely on the discovery of peer reflexive ones).
      </t>
      <t>
        When candidates are trickled, the using protocol MUST deliver each 
        candidate (and any end-of-candidates indication as described in
        <xref target='end-of-candidates.send'/>) to the receiving Trickle ICE implementation 
        exactly once
        and in the same order it was conveyed. If the using protocol 
        provides any candidate retransmissions, they need to be hidden 
        from the ICE implementation.
      </t>
      <t>
        Also, candidate trickling needs to be correlated to a specific
        ICE session, so that if there is an ICE restart, any
        delayed updates for a previous session can be recognized as such
        and ignored by the receiving party.  For example, using protocols
        that signal candidates via SDP might include a Username
        Fragment value in the corresponding a=candidate line, such as:
        <figure>
          <artwork>
<![CDATA[
  a=candidate:1 1 UDP 2130706431 2001:db8::1 5000 typ host ufrag 8hhY
]]>
          </artwork>
        </figure>
        Or, as another example, WebRTC implementations might include a Username
        Fragment in the JavaScript objects that represent candidates.
      </t>
      <t>
        Note: The using protocol needs to provide a mechanism for both
        parties to indicate and agree on the ICE session in force
        (as identified by the Username Fragment and Password combination)
        so that they have a consistent view of which candidates are
        to be paired.  This is especially important in the case of ICE
        restarts (see <xref target='subsequent'/>).
      </t>
      <t>
        Note: A using protocol might prefer not to
        trickle server reflexive candidates to entities that are known
        to be publicly accessible and where sending a direct STUN
        binding request is likely to reach the destination faster than
        the trickle update that travels through the signaling path.
      </t>
    </section>
    <section title='Pairing Newly Gathered Local Candidates' anchor="local-pairing">
      <t>
        As a Trickle ICE agent gathers local candidates, it needs
        to form candidate pairs; this works as described in
        the ICE specification <xref target='rfc5245bis'/>, with the 
        following provisos:
        <list style='numbers'>
          <t>
            A Trickle ICE agent MUST NOT pair a local candidate until it
            has been trickled to the remote party.
          </t>
          <t>
            Once the agent has conveyed the local candidate to the remote 
            party, the agent checks if any remote candidates are currently 
            known for this same stream and component. If not, the agent
            merely adds the new candidate to the list of local candidates
            (without pairing it).
          </t>
          <t>
            Otherwise, if the agent has already learned of one or more
            remote candidates for this stream and component, it attempts
            to pair the new local candidate as described in the ICE
            specification <xref target='rfc5245bis'/>.
          </t>
          <t>
            If a newly formed pair has a local candidate whose type is server 
            reflexive, the agent MUST replace the local candidate with its 
            base before completing the relevant redundancy tests.
          </t>
          <t>
            The agent prunes redundant pairs by following the rules
            in Section 6.1.2.4 of <xref target='rfc5245bis'/>, but checks
            existing pairs only if they have a state of Waiting or Frozen;
            this avoids removal of pairs for which connectivity checks are 
            in flight (a state of In-Progress) or for which connectivity 
            checks have already yielded a definitive result (a state of
            Succeeded or Failed).
          </t>
          <t>
            If after the relevant redundancy tests the check list where the 
            pair is to be added already contains the maximum number of candidate 
            pairs (100 by default as per <xref target="rfc5245bis"/>), the agent
            SHOULD discard any pairs in the Failed state to make room for the 
            new pair. If there are no such pairs, the agent SHOULD discard a
            pair with a lower priority than the new pair in order to make room
            for the new pair, until the number of pairs is equal to the maximum
            number of pairs. This processing is consistent with Section 6.1.2.5
            of <xref target='rfc5245bis'/>.
          </t>
        </list>
      </t>
    </section>
    <section title='Receiving Trickled Candidates' anchor="trickle-recv">
      <t>
        At any time during an ICE session, a Trickle ICE agent might receive
        new candidates from the remote agent, from which it will attempt to
        form a candidate pair; this works as described in the ICE specification 
        <xref target='rfc5245bis'/>, with the following provisos:
        <list style='numbers'>
          <t>
            The agent checks if any local candidates are currently known for 
            this same stream and component. If not, the agent merely adds the 
            new candidate to the list of remote candidates (without pairing it).
          </t>
          <t>
            Otherwise, if the agent has already gathered one or more
            local candidates for this stream and component, it attempts
            to pair the new remote candidate as described in the ICE
            specification <xref target='rfc5245bis'/>.
          </t>
          <t>
            If a newly formed pair has a local candidate whose type is server 
            reflexive, the agent MUST replace the local candidate with its 
            base before completing the redundancy check in the next step.
          </t>
          <t>
            The agent prunes redundant pairs as described below, but checks
            existing pairs only if they have a state of Waiting or Frozen;
            this avoids removal of pairs for which connectivity checks are 
            in flight (a state of In-Progress) or for which connectivity 
            checks have already yielded a definitive result (a state of
            Succeeded or Failed).
            <list style='letters'>
              <t>
                If the agent finds a redundancy between two pairs and one of 
                those pairs contains a newly received remote candidate whose 
                type is peer reflexive, the agent SHOULD discard the
                pair containing that candidate, set the priority of the
                existing pair to the priority of the discarded pair, and
                re-sort the check list. (This policy helps to eliminate
                problems with remote peer reflexive candidates for which
                a STUN binding request is received before signaling of the
                candidate is trickled to the receiving agent, such as a
                different view of pair priorities between the local agent
                and the remote agent, because the same candidate could be
                perceived as peer reflexive by one agent and as server
                reflexive by the other agent.)
              </t>
              <t>
                The agent then applies the rules defined in 
                Section 6.1.2.4 of <xref target='rfc5245bis'/>.
              </t>
            </list>
          </t>
          <t>
            If after the relevant redundancy tests the check list where the 
            pair is to be added already contains the maximum number of candidate 
            pairs (100 by default as per <xref target="rfc5245bis"/>), the agent
            SHOULD discard any pairs in the Failed state to make room for the 
            new pair. If there are no such pairs, the agent SHOULD discard a
            pair with a lower priority than the new pair in order to make room
            for the new pair, until the number of pairs is equal to the maximum
            number of pairs. This processing is consistent with Section 6.1.2.5
            of <xref target='rfc5245bis'/>.
          </t>
        </list>
      </t>
    </section>
    <section title='Inserting Trickled Candidate Pairs into a Check List' 
             anchor="trickle-insert">
      <t>
        After a local agent has trickled a candidate and formed a candidate 
        pair from that local candidate (<xref target='trickle-send'/>), or after 
        a remote agent has received a trickled candidate and formed a candidate 
        pair from that remote candidate (<xref target='trickle-recv'/>), a Trickle 
        ICE agent adds the new candidate pair to a check list as defined in 
        this section.
      </t>
      <t>
        As an aid to understanding the procedures defined in this section, 
        consider the following tabular representation of all check lists in 
        an agent (note that initially for one of the foundations, i.e., f5, 
        there are no candidate pairs): 
      </t>
      <t>
        <figure title="Example of Check List State" anchor="fig-checklist-0">
          <artwork>
<![CDATA[
+-----------------+------+------+------+------+------+
|                 |  f1  |  f2  |  f3  |  f4  |  f5  |
+-----------------+------+------+------+------+------+
| s1 (Audio.RTP)  |  F   |  F   |  F   |      |      |
+-----------------+------+------+------+------+------+
| s2 (Audio.RTCP) |  F   |  F   |  F   |  F   |      |
+-----------------+------+------+------+------+------+
| s3 (Video.RTP)  |  F   |      |      |      |      |
+-----------------+------+------+------+------+------+
| s4 (Video.RTCP) |  F   |      |      |      |      |
+-----------------+------+------+------+------+------+
]]>
          </artwork>
        </figure>
      </t>
      <t>
        Each row in the table represents a component for a given data
        stream (e.g., s1 and s2 might be the RTP and RTCP components
        for audio) and thus a single check list in the check list set. 
        Each column represents one foundation. Each cell represents one 
        candidate pair. In the tables shown in this section, "F" stands 
        for "frozen", "W" stands for "waiting", and "S" stands for 
        "succeeded"; in addition, "^^" is used to notate newly-added 
        candidate pairs.
      </t>
      <t>
        When an agent commences ICE processing, in accordance with 
        Section 6.1.2.6 of <xref target="rfc5245bis"/>, for each 
        foundation it will unfreeze the pair with the lowest component 
        ID and, if the component IDs are equal, with the highest priority 
        (this is the topmost candidate pair in every column).
        This initial state is shown in the following table.
      </t>
      <t>
        <figure title="Initial Check List State" anchor="fig-checklist-initial">
          <artwork>
<![CDATA[
+-----------------+------+------+------+------+------+
|                 |  f1  |  f2  |  f3  |  f4  |  f5  |
+-----------------+------+------+------+------+------+
| s1 (Audio.RTP)  |  W   |  W   |  W   |      |      |
+-----------------+------+------+------+------+------+
| s2 (Audio.RTCP) |  F   |  F   |  F   |  W   |      |
+-----------------+------+------+------+------+------+
| s3 (Video.RTP)  |  F   |      |      |      |      |
+-----------------+------+------+------+------+------+
| s4 (Video.RTCP) |  F   |      |      |      |      |
+-----------------+------+------+------+------+------+
]]>
          </artwork>
        </figure>
      </t>
      <t>
        Then, as the checks proceed (see Section 7.2.5.4 of
        <xref target="rfc5245bis"/>), for each pair 
        that enters the Succeeded state (denoted here by "S"),
        the agent will unfreeze all pairs for all data streams with the same
        foundation (e.g., if the pair in column 1, row 1 succeeds then
        the agent will unfreeze the pair in column 1, rows 2, 3, and 4). 
      </t>
      <t>
        <figure title="Check List State with Succeeded Candidate Pair" anchor="fig-checklist-succeeded">
          <artwork>
<![CDATA[
+-----------------+------+------+------+------+------+
|                 |  f1  |  f2  |  f3  |  f4  |  f5  |
+-----------------+------+------+------+------+------+
| s1 (Audio.RTP)  |  S   |  W   |  W   |      |      |
+-----------------+------+------+------+------+------+
| s2 (Audio.RTCP) |  W   |  F   |  F   |  W   |      |
+-----------------+------+------+------+------+------+
| s3 (Video.RTP)  |  W   |      |      |      |      |
+-----------------+------+------+------+------+------+
| s4 (Video.RTCP) |  W   |      |      |      |      |
+-----------------+------+------+------+------+------+
]]>
          </artwork>
        </figure>
      </t>
      <t>
        Trickle ICE preserves all of these rules as they apply to 
        "static" check list sets. This implies that if 
        a Trickle ICE agent were to begin connectivity checks with all 
        of its pairs already present, the way that pair states change 
        is indistinguishable from that of a regular ICE agent. 
      </t>
      <t>
        Of course, the major difference with Trickle ICE is that check list 
        sets can be dynamically updated because candidates can 
        arrive after connectivity checks have started. When this happens, an 
        agent sets the state of the newly formed pair as described below.
      </t>
      <t>
        Rule 1: If the newly formed pair has the lowest component ID and, 
        if the component IDs are equal, the highest priority of any candidate 
        pair for this foundation (i.e., if it is the topmost pair in the 
        column), set the state to Waiting. For example, this would be the 
        case if the newly formed pair were placed in column 5, row 1. This 
        rule is consistent with Section 6.1.2.6 of <xref target="rfc5245bis"/>.
      </t>
      <t>
        <figure title="Check List State with Newly Formed Pair, Rule 1" anchor="fig-checklist-rule1">
          <artwork>
<![CDATA[
+-----------------+------+------+------+------+------+
|                 |  f1  |  f2  |  f3  |  f4  |  f5  |
+-----------------+------+------+------+------+------+
| s1 (Audio.RTP)  |  S   |  W   |  W   |      | ^W^  |
+-----------------+------+------+------+------+------+
| s2 (Audio.RTCP) |  W   |  F   |  F   |  W   |      |
+-----------------+------+------+------+------+------+
| s3 (Video.RTP)  |  W   |      |      |      |      |
+-----------------+------+------+------+------+------+
| s4 (Video.RTCP) |  W   |      |      |      |      |
+-----------------+------+------+------+------+------+
]]>
          </artwork>
        </figure>
      </t>
      <t> 
        Rule 2: If there is at least one pair in the Succeeded state for 
        this foundation, set the state to Waiting. For example, this would be 
        the case if the pair in column 5, row 1 succeeded and the newly formed 
        pair were placed in column 5, row 2. This rule is consistent with 
        Section 7.2.5.3.3 of <xref target="rfc5245bis"/>.
      </t>
      <t>
        <figure title="Check List State with Newly Formed Pair, Rule 2" anchor="fig-checklist-rule2">
          <artwork>
<![CDATA[
+-----------------+------+------+------+------+------+
|                 |  f1  |  f2  |  f3  |  f4  |  f5  |
+-----------------+------+------+------+------+------+
| s1 (Audio.RTP)  |  S   |  W   |  W   |      |  S   |
+-----------------+------+------+------+------+------+
| s2 (Audio.RTCP) |  W   |  F   |  F   |  W   | ^W^  |
+-----------------+------+------+------+------+------+
| s3 (Video.RTP)  |  W   |      |      |      |      |
+-----------------+------+------+------+------+------+
| s4 (Video.RTCP) |  W   |      |      |      |      |
+-----------------+------+------+------+------+------+
]]>
          </artwork>
        </figure>
      </t>
      <t>
        Rule 3: In all other cases, set the state to Frozen. For example, 
        this would be the case if the newly formed pair were placed in
        column 3, row 3.
      </t>
      <t>
        <figure title="Check List State with Newly Formed Pair, Rule 3" anchor="fig-checklist-rule3">
          <artwork>
<![CDATA[
+-----------------+------+------+------+------+------+
|                 |  f1  |  f2  |  f3  |  f4  |  f5  |
+-----------------+------+------+------+------+------+
| s1 (Audio.RTP)  |  S   |  W   |  W   |      |  S   |
+-----------------+------+------+------+------+------+
| s2 (Audio.RTCP) |  W   |  F   |  F   |  W   |  W   |
+-----------------+------+------+------+------+------+
| s3 (Video.RTP)  |  W   |      | ^F^  |      |      |
+-----------------+------+------+------+------+------+
| s4 (Video.RTCP) |  W   |      |      |      |      |
+-----------------+------+------+------+------+------+
]]>
          </artwork>
        </figure>
      </t>
    </section>
    <section title='Generating an End-of-Candidates Indication'
             anchor="end-of-candidates.send">
      <t>
        Once all candidate gathering is completed or expires for an
        ICE session associated with a specific data stream, the agent will generate an
        "end-of-candidates" indication for that session and convey it to
        the remote agent via the signaling channel. Although the exact form of
        the indication depends on the using protocol, the indication
        MUST specify the generation (Username Fragment and Password combination) so that an agent
        can correlate the end-of-candidates indication with a particular ICE
        session. The indication can be conveyed in the following ways:
        <list style='symbols'>
          <t>As part of an initiation request (which would typically be the case with 
             the initial ICE description for half trickle)</t>
          <t>Along with the last candidate an agent can send for a stream</t>
          <t>As a standalone notification (e.g., after STUN Binding requests 
             or TURN Allocate requests to a server time out and the agent 
             is no longer actively gathering candidates)</t>
        </list>
      </t>
      <t>
        Conveying an end-of-candidates indication in a timely manner is important 
        in order to avoid ambiguities and speed up the conclusion of ICE processing. 
        In particular: 
        <list style='symbols'>
          <t>
            A controlled Trickle ICE agent SHOULD convey an end-of-candidates 
            indication after it has completed gathering for a data stream, 
            unless ICE processing terminates before the agent has had a chance 
            to complete gathering. 
          </t>
          <t>
            A controlling agent MAY conclude ICE processing prior to conveying 
            end-of-candidates indications for all streams.  However, it is 
            RECOMMENDED for a controlling agent to convey end-of-candidates 
            indications whenever possible for the sake of consistency and to 
            keep middleboxes and controlled agents up-to-date on the state of 
            ICE processing.
          </t>
        </list>
      </t>
      <t>
        When conveying an end-of-candidates indication during trickling
        (rather than as a part of the initial ICE description or a response thereto), 
        it is the responsibility of the
        using protocol to define methods for associating the
        indication with one or more specific data streams.
      </t>
      <t>
        An agent MAY also choose to generate an end-of-candidates
        indication before candidate gathering has actually completed, if the
        agent determines that gathering has continued for more than an
        acceptable period of time. However, an agent MUST NOT convey any
        more candidates after it has conveyed an end-of-candidates
        indication.
      </t>
      <t>
        When performing half trickle, an agent SHOULD convey an
        end-of-candidates indication together with its initial ICE description unless
        it is planning to potentially trickle additional candidates (e.g., in
        case the remote party turns out to support Trickle ICE).
      </t>
      <t>
        After an agent conveys the end-of-candidates indication, it will
        update the state of the corresponding check list as explained
        in <xref target="checks"/>. Past that point, an
        agent MUST NOT trickle any new candidates within this ICE session. 
        Therefore, adding new candidates to the 
        negotiation is possible only through an ICE restart (see
        <xref target='subsequent'/>).
      </t>
      <t>
        This specification does not
        override regular ICE semantics for concluding ICE processing.
        Therefore, even if end-of-candidates indications are conveyed,
        an agent will still need to go through pair nomination. Also, if
        pairs have been nominated for components and data streams, ICE
        processing MAY still conclude even if end-of-candidates
        indications have not been received for all streams. In all cases,
        an agent MUST NOT trickle any new candidates within an ICE session
        after nomination of a candidate pair as described in Section 8.1.1
        of <xref target='rfc5245bis'/>. 
      </t>
    </section>
    <section title='Receiving an End-of-Candidates Indication'
             anchor="end-of-candidates.recv">
      <t>
        Receiving an end-of-candidates indication enables an agent to
        update check list states and, in case valid pairs do not exist
        for every component in every data stream, determine that ICE
        processing has failed. It also enables an agent to speed up the
        conclusion of ICE processing when a candidate pair has been validated
        but it involves the use of lower-preference transports such as
        TURN. In such situations, an implementation MAY choose to wait
        and see if higher-priority candidates are received; in this case
        the end-of-candidates indication provides a notification that such
        candidates are not forthcoming.
      </t>
      <t>
        When an agent receives an end-of-candidates indication for a 
        specific data stream, it will update the state of the relevant 
        check list as per <xref target="checks"/> (which might lead to 
        some check lists being marked as Failed). If the check list is 
        still in the Running state after the update, the agent will persist 
        the fact that an end-of-candidates indication has been
        received and take it into account in future updates
        to the check list.
      </t>
      <t>
        After an agent has received an end-of-candidates indication, it 
        MUST ignore any newly received candidates for that data 
        stream or data session. 
      </t>
    </section>
    <section title='Subsequent Exchanges and ICE Restarts'
             anchor="subsequent">
      <t>
        Before conveying an end-of-candidates indication, 
        either agent MAY convey subsequent candidate information at any time allowed
        by the using protocol. When this happens, agents will use
        <xref target="rfc5245bis"/> semantics (e.g., checking of the 
        Username Fragment and Password combination) to determine whether or not
        the new candidate information requires an ICE restart.  
      </t>
      <t>
        If an ICE restart
        occurs, the agents can assume that Trickle ICE is still supported
        if support was determined previously, and thus can engage in Trickle ICE 
        behavior as they would in an initial exchange of ICE descriptions where
        support was determined through a capabilities discovery method.
      </t>
    </section>
    <section title='Half Trickle' anchor="half-trickle">
      <t>
        In half trickle, the initiator conveys the initial ICE description
        with a usable but not necessarily full generation of candidates. This 
        ensures that the ICE description can be processed by a regular ICE 
        responder and is mostly meant for use in cases where support for 
        Trickle ICE cannot be confirmed prior to conveying the initial ICE 
        description.  The initial ICE description indicates support for 
        Trickle ICE, so that the responder can respond with something less 
        than a full generation of candidates and then trickle the rest.  
        The initial ICE description for half trickle can contain 
        an end-of-candidates indication, although this is not mandatory
        because if trickle support is confirmed then the initiator can
        choose to trickle additional candidates before it conveys an
        end-of-candidates indication.
      </t>
      <t>
        The half trickle mechanism can be used in cases where there is
        no way for an agent to verify in advance whether a remote
        party supports Trickle ICE. Because the initial ICE description contain
        a full generation of candidates, it can thus be handled by a regular
        ICE agent, while still allowing a Trickle ICE agent to use
        the optimization defined in this specification. This prevents
        negotiation from failing in the former case while still giving
        roughly half the Trickle ICE benefits in the latter.
      </t>
      <t>
        Use of half trickle is only necessary during an initial
        exchange of ICE descriptions. After both parties have received 
        an ICE description from their peer, they can each reliably
        determine Trickle ICE support and use it for all subsequent
        exchanges (see <xref target='subsequent'/>).
      </t>
      <t>
        In some instances, using half trickle might bring more than 
        just half the improvement in terms of user experience. This 
        can happen when an agent starts gathering candidates upon user 
        interface cues that the user will soon be initiating an interaction, 
        such as activity on a keypad or the phone going off hook. This 
        would mean that some or all of the candidate
        gathering could be completed before the agent actually
        needs to convey the candidate information. Because the responder will be able
        to trickle candidates, both agents will be able to start
        connectivity checks and complete ICE processing earlier than
        with regular ICE and potentially even as early as with full
        trickle.
      </t>
      <t>
        However, such anticipation is not always possible. For
        example, a multipurpose user agent or a WebRTC web page where
        communication is a non-central feature (e.g., calling a support
        line in case of a problem with the main features) would not
        necessarily have a way of distinguishing between call
        intentions and other user activity. In such cases, using full
        trickle is most likely to result in an ideal user experience.
        Even so, using half trickle would be an improvement over regular
        ICE because it would result in a better experience for responders.
      </t>
    </section>
    <section title='Preserving Candidate Order while Trickling'>
      <t>
        One important aspect of regular ICE is that connectivity checks
        for a specific foundation and component are attempted
        simultaneously by both agents, so that any firewalls or NATs
        fronting the agents would whitelist both endpoints and allow
        all except for the first ("suicide") packets to go through. This
        is also important to unfreezing candidates at the right time. While 
        not crucial, preserving this behavior in Trickle ICE is likely to 
        improve ICE performance.
      </t>
      <t>
        To achieve this, when trickling candidates, agents SHOULD respect the 
        order of components as reflected by their component IDs; that is,
        candidates for a given component 
        SHOULD NOT be conveyed prior to candidates for a component with a 
        lower ID number within the same foundation. In addition, candidates 
        SHOULD be paired, following the procedures in <xref target='trickle-insert'/>, 
        in the same order they are conveyed.
      </t>
      <t>
        For example, the following SDP description contains two
        components (RTP and RTCP) and two foundations (host and 
        server reflexive):
        <figure>
          <artwork>
<![CDATA[
  v=0
  o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
  s=
  c=IN IP4 10.0.1.1
  t=0 0
  a=ice-pwd:asd88fgpdd777uzjYhagZg
  a=ice-ufrag:8hhY
  m=audio 5000 RTP/AVP 0
  a=rtpmap:0 PCMU/8000
  a=candidate:1 1 UDP 2130706431 10.0.1.1 5000 typ host
  a=candidate:1 2 UDP 2130706431 10.0.1.1 5001 typ host
  a=candidate:2 1 UDP 1694498815 192.0.2.3 5000 typ srflx
      raddr 10.0.1.1 rport 8998
  a=candidate:2 2 UDP 1694498815 192.0.2.3 5001 typ srflx
      raddr 10.0.1.1 rport 8998
]]>
          </artwork>
        </figure>
        For this candidate information the RTCP host candidate would not be conveyed
        prior to the RTP host candidate. Similarly the RTP server
        reflexive candidate would be conveyed together with or prior to the
        RTCP server reflexive candidate.
      </t>
    </section>
    <section title='Requirements for Using Protocols' anchor="reqs">
      <t>
        In order to fully enable the use of Trickle ICE, this specification
        defines the following requirements for using protocols.
        <list style='symbols'>
          <t>
            A using protocol SHOULD provide a way for parties to advertise 
            and discover support for Trickle ICE before an ICE 
            session begins (see <xref target='support'/>).
          </t>
          <t>
            A using protocol MUST provide methods for incrementally
            conveying (i.e., "trickling") additional candidates after 
            conveying the initial ICE description (see 
            <xref target='trickle-send'/>).
          </t>
          <t>
            A using protocol MUST deliver each trickled candidate 
            or end-of-candidates indication exactly once
            and in the same order it was conveyed (see 
            <xref target='trickle-send'/>).
          </t>
          <t>
            A using protocol MUST provide a mechanism for both parties 
            to indicate and agree on the ICE session in force
            (see <xref target='trickle-send'/>).
          </t>
          <t>
            A using protocol MUST provide a way for parties to communicate the 
            end-of-candidates indication, which MUST specify the particular ICE 
            session to which the indication applies (see <xref target='end-of-candidates.send'/>).
          </t>
        </list>
      </t>
    </section>
    <section title='IANA Considerations'>
      <t>
        IANA is requested to register the following ICE option in the "ICE
        Options" sub-registry of the "Interactive Connectivity Establishment
        (ICE) registry", following the procedures defined in
        <xref target='RFC6336'/>.
      </t>
      <t>
        <list style='hanging'>
          <t hangText="ICE Option:">trickle</t>
          <t hangText="Contact:">IESG, iesg@ietf.org</t>
          <t hangText="Change control:">IESG</t>
          <t hangText="Description:">
            An ICE option of "trickle" indicates support for incremental
            communication of ICE candidates.
          </t>
          <t hangText="Reference:">RFC XXXX</t>
        </list>
      </t>
    </section>
    <section title='Security Considerations'>
      <t>
        This specification inherits most of its semantics from
        <xref target="rfc5245bis"/> and as a result all security
        considerations described there apply to Trickle ICE.
      </t>
      <t>
        If the privacy implications of revealing host addresses on an
        endpoint device are a concern (see for example the discussion
        in <xref target='I-D.ietf-rtcweb-ip-handling'/> and in Section 19
        of <xref target="rfc5245bis"/>), agents can generate ICE descriptions that contain no
        candidates and then only trickle candidates that do not reveal
        host addresses (e.g., relayed candidates).
      </t>
    </section>
    <section title='Acknowledgements'>
     <t>   
        The authors would like to thank Bernard Aboba, Flemming Andreasen, 
        Rajmohan Banavi, Taylor Brandstetter, Philipp Hancke, Christer Holmberg, 
        Ari Keranen, Paul Kyzivat, Jonathan Lennox, Enrico Marocco, Pal Martinsen, 
        Nils Ohlmeier, Thomas Stach, Peter Thatcher, Martin Thomson, Brandon Williams,
        and Dale Worley for their reviews and suggestions on improving this document. 
        Sarah Banks, Roni Even, and David Mandelberg completed opsdir, genart, and
        security reviews, respectively. Thanks also to Ari Keranen and Peter Thatcher 
        in their role as chairs, and Ben Campbell in his role as responsible Area 
        Director.
      </t>
    </section>
  </middle>
  <back>
    <references title='Normative References'>
      <?rfc include="reference.RFC.2119"?>

<reference anchor='rfc5245bis'>
<front>
<title>Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal</title>
<author initials='A' surname='Keranen' fullname='Ari Keranen'>
    <organization />
</author>
<author initials='C' surname='Holmberg' fullname='Christer Holmberg'>
    <organization />
</author>
<author initials='J' surname='Rosenberg' fullname='Jonathan Rosenberg'>
    <organization />
</author>
<date month='March' day='8' year='2018' />
<abstract><t>This document describes a protocol for Network Address Translator (NAT) traversal for UDP-based multimedia.  This protocol is called Interactive Connectivity Establishment (ICE).  ICE makes use of the Session Traversal Utilities for NAT (STUN) protocol and its extension, Traversal Using Relay NAT (TURN).  This document obsoletes RFC 5245.</t></abstract>
</front>
<seriesInfo name='Internet-Draft' value='draft-ietf-ice-rfc5245bis-20' />
<format type='TXT'
        target='http://www.ietf.org/internet-drafts/draft-ietf-ice-rfc5245bis-20.txt' />
</reference>

    </references>
    <references title='Informative References'>
      <?rfc include="reference.RFC.1918"?>
      <?rfc include="reference.RFC.3261"?>
      <?rfc include="reference.RFC.3264"?>
      <?rfc include="reference.RFC.4566"?>
      <?rfc include="reference.RFC.4787"?>
      <?rfc include="reference.RFC.5389"?>
      <?rfc include="reference.RFC.5766"?>
      <?rfc include="reference.RFC.6120"?>
      <?rfc include="reference.RFC.6336"?>
<reference anchor='I-D.ietf-mmusic-trickle-ice-sip'>
<front>
<title>A Session Initiation Protocol (SIP) usage for Trickle ICE</title>
<author initials='E' surname='Ivov' fullname='Emil Ivov'>
    <organization />
</author>
<author initials='T' surname='Stach' fullname='Thomas Stach'>
    <organization />
</author>
<author initials='E' surname='Marocco' fullname='Enrico Marocco'>
    <organization />
</author>
<author initials='C' surname='Holmberg' fullname='Christer Holmberg'>
    <organization />
</author>
<date month='February' day='24' year='2018' />
<abstract><t>The Interactive Connectivity Establishment (ICE) protocol describes a Network Address Translator (NAT) traversal mechanism for UDP-based multimedia sessions established with the Offer/Answer model.  The ICE extension for Incremental Provisioning of Candidates (Trickle ICE) defines a mechanism that allows ICE agents to shorten session establishment delays by making the candidate gathering and connectivity checking phases of ICE non-blocking and by executing them in parallel.  This document defines usage semantics for Trickle ICE with the Session Initiation Protocol (SIP).</t></abstract>
</front>
<seriesInfo name='Internet-Draft' value='draft-ietf-mmusic-trickle-ice-sip-14' />
<format type='TXT'
        target='http://www.ietf.org/internet-drafts/draft-ietf-mmusic-trickle-ice-sip-14.txt' />
</reference>
<reference anchor='I-D.ietf-rtcweb-ip-handling'>
<front>
<title>WebRTC IP Address Handling Requirements</title>
<author initials='J' surname='Uberti' fullname='Justin Uberti'>
    <organization />
</author>
<author initials='G' surname='Shieh' fullname='Guo-wei Shieh'>
    <organization />
</author>
<date month='March' day='1' year='2018' />
<abstract><t>This document provides information and requirements for how IP addresses should be handled by WebRTC implementations.</t></abstract>
</front>
<seriesInfo name='Internet-Draft' value='draft-ietf-rtcweb-ip-handling-06' />
<format type='TXT'
        target='http://www.ietf.org/internet-drafts/draft-ietf-rtcweb-ip-handling-06.txt' />
</reference>
      <reference anchor="XEP-0176">
        <front>
          <title>XEP-0176: Jingle ICE-UDP Transport Method</title>
          <author initials='J.' surname='Beda' fullname='Joe Beda'>
                  <organization abbrev='Google'>Google</organization>
          </author>
          <author initials='S.' surname='Ludwig'
                  fullname='Scott Ludwig'>
            <organization abbrev='Google'>Google</organization>
          </author>
          <author initials='P.' surname='Saint-Andre'
                  fullname='Peter Saint-Andre'>
          </author>
          <author initials='J.' surname='Hildebrand'
                  fullname='Joe Hildebrand'>
            <organization abbrev='Cisco'>Cisco</organization>
          </author>
          <author initials='S.' surname='Egan' fullname='Sean Egan'>
            <organization abbrev='Google'>Google </organization>
          </author>
          <author initials='R.' surname='McQueen'
                      fullname='Robert McQueen'>
            <organization abbrev='Collabora'>Collabora</organization>
          </author>
          <date month="June" year="2009" />
        </front>
        <seriesInfo name="XEP" value="XEP-0176" />
      </reference>
      <reference anchor="XEP-0030">
        <front>
          <title>XEP-0030: Service Discovery</title>
          <author initials='J.' surname='Hildebrand'
                  fullname='Joe Hildebrand'>
            <organization abbrev='Cisco'>Cisco</organization>
          </author>
          <author initials='P.' surname='Millard'
                  fullname='Peter Millard'>
          </author>
          <author initials='R.' surname='Eatmon'
                  fullname='Ryan Eatmon'>
          </author>
          <author initials='P.' surname='Saint-Andre'
                  fullname='Peter Saint-Andre'>
          </author>
          <date month="June" year="2008" />
        </front>
        <seriesInfo name="XEP" value="XEP-0030" />
      </reference>
    </references>
    <section title='Interaction with Regular ICE'
             anchor='interaction'>
      <t>
        The ICE protocol was designed to be flexible enough to
        work in and adapt to as many network environments as
        possible. Despite that flexibility, ICE as specified in
        <xref target="rfc5245bis"/> does not by itself support trickle 
        ICE. This section describes how trickling of candidates 
        interacts with ICE.
      </t>
      <t>
        <xref target="rfc5245bis"/> describes the conditions required to
        update check lists and timer states while an ICE agent is in the
        Running state. These conditions are verified upon transaction
        completion and one of them stipulates that:
      </t>
      <t>
        <list style='empty'>
          <t>
            If there is not a pair in the valid list for each component
            of the data stream, the state of the check list is set to
            Failed.
          </t>
        </list>
      </t>
      <t>
        This could be a problem and cause ICE processing to fail
        prematurely in a number of scenarios. Consider the following
        case:
      </t>
      <t>
        <list style='numbers'>
          <t>
            Alice and Bob are both located in different networks with
            Network Address Translation (NAT). Alice and Bob themselves
            have different address but both networks use the same
            private internet block (e.g., the "20-bit block"
            172.16/12 specified in <xref target="RFC1918"/>).
          </t>
          <t>
            Alice conveys to Bob the candidate 172.16.0.1 which also happens
            to correspond to an existing host on Bob's network.
          </t>
          <t>
            Bob creates a check list consisting solely of 172.16.0.1 and
            starts checks.
          </t>
          <t>
            These checks reach the host at 172.16.0.1 in Bob's network,
            which responds with an ICMP "port unreachable" error; per
            <xref target="rfc5245bis"/> Bob marks the transaction as
            Failed.
          </t>
        </list>
        At this point the check list only contains Failed candidates and
        the valid list is empty. This causes the data stream and
        potentially all ICE processing to fail, even though if Trickle ICE agents
        could subsequently convey candidates that
        would cause previously empty check lists to become non-empty.
      </t>
      <t>
        A similar race condition would occur if the initial ICE description from
        Alice contain only candidates that can be determined as
        unreachable from
        any of the candidates that Bob has gathered (e.g., this would be the
        case if Bob's candidates only contain IPv4 addresses and the
        first candidate that he receives from Alice is an IPv6 one).
      </t>
      <t>
        Another potential problem could arise when a non-trickle
        ICE implementation initiates an interaction with a Trickle ICE 
        implementation. Consider the following case:
        <list style='numbers'>
          <t>
            Alice's client has a non-Trickle ICE implementation.
          </t>
          <t>
            Bob's client has support for Trickle ICE.
          </t>
          <t>
            Alice and Bob are behind NATs with address-dependent
            filtering <xref target="RFC4787"/>.
          </t>
          <t>
            Bob has two STUN servers but one of them is currently
            unreachable.
          </t>
        </list>
      </t>
      <t>
        After Bob's agent receives Alice's initial ICE description it would 
        immediately start connectivity checks. It would also start gathering
        candidates, which would take a long time because of the unreachable
        STUN server. By the time Bob's answer is ready and conveyed to
        Alice, Bob's connectivity checks might have failed: until
        Alice gets Bob's answer, she won't be able to start connectivity
        checks and punch holes in her NAT. The NAT would hence be
        filtering Bob's checks as originating from an unknown endpoint.
      </t>
    </section>
    <section title='Interaction with ICE Lite'>
      <t>
        The behavior of ICE lite agents that are capable of Trickle ICE does not
        require any particular rules other than those already defined
        in this specification and <xref target="rfc5245bis"/>. This section
        is hence provided only for informational purposes.
      </t>
      <t>
        An ICE lite agent would generate candidate information 
        as per <xref target="rfc5245bis"/> and 
        would indicate support for Trickle ICE. Given
        that the candidate information will contain a full generation of candidates,
        it would also be accompanied by an end-of-candidates indication.
      </t>
      <t>
        When performing full trickle, a full ICE implementation could
        convey the initial ICE description or response thereto with no candidates. After receiving 
        a response that
        identifies the remote agent as an ICE lite implementation, the
        initiator can choose to not trickle any additional
        candidates. The same is also true in the case when the ICE lite
        agent initiates the interaction and the full ICE agent is the responder. In
        these cases the connectivity checks would be enough for the ICE
        lite implementation to discover all potentially useful
        candidates as peer reflexive. The following example illustrates
        one such ICE session using SDP syntax:
      </t>
      <figure title="Example " anchor="fig-ice-lite">
        <artwork>
<![CDATA[
        ICE Lite                                          Bob
         Agent
           |   Offer (a=ice-lite a=ice-options:trickle)    |
           |---------------------------------------------->|
           |                                               |no cand
           |         Answer (a=ice-options:trickle)        |trickling
           |<----------------------------------------------|
           |              Connectivity Checks              |
           |<--------------------------------------------->|
  peer rflx|                                               |
 cand disco|                                               |
           |<========== CONNECTION ESTABLISHED ===========>|

]]>
        </artwork>
      </figure>
      <t>
        In addition to reducing signaling traffic this approach also
        removes the need to discover STUN bindings or make TURN 
        allocations, which can considerably lighten ICE processing.
      </t>
    </section>
    <section title='Changes from Earlier Versions'>
      <t>
        Note to the RFC Editor: please remove this section prior to
        publication as an RFC.
      </t>
      <section title='Changes from draft-ietf-ice-trickle-20'>
        <t>
          <list style='symbols'>
            <t>
              Slight corrections to hanlding of peer reflexive candidates. 
            </t>
            <t>
              Wordsmithing in a few sections.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-19'>
        <t>
          <list style='symbols'>
            <t>
              Further clarified handling of remote peer reflexive 
              candidates.
            </t>
            <t>
              To improve readibility, renamed and restructured some 
              sections and subsections, and modified some wording.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-18'>
        <t>
          <list style='symbols'>
            <t>
              Cleaned up pairing and redundancy checking rules for 
              newly discovered candidates per IESG feedback and WG
              discussion.
            </t>
            <t>
              Improved wording in half trickle section.
            </t>
            <t>
              Changed "not more than once" to "exactly once".
            </t>
            <t>
              Changed NAT examples back to IPv4.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-17'>
        <t>
          <list style='symbols'>
            <t>
              Simplified the rules for inserting a new pair in a check list.
            </t>
            <t>
              Clarified it is not allowed to nominate a candidate 
              pair after a pair has already been nominated (a.k.a. 
              renomination or continuous nomination).
            </t>
            <t>
              Removed some text that referenced older versions of
              rfc5245bis.
            </t>
            <t>
              Removed some text that duplicated concepts and procedures 
              specified in rfc5245bis.
            </t>
            <t>
              Removed the ill-defined concept of stream order.
            </t>
            <t>
              Shortened the introduction.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-16'>
        <t>
          <list style='symbols'>
            <t>
              Made "ufrag" terminology consistent with 5245bis.
            </t>
            <t>
              Applied in-order delivery rule to end-of-candidates indication.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-15'>
        <t>
          <list style='symbols'>
            <t>
              Adjustments to address AD review feedback.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-14'>
        <t>
          <list style='symbols'>
            <t>
              Minor modifications to track changes to ICE core.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-13'>
        <t>
          <list style='symbols'>
            <t>
              Removed independent monitoring of check list "states" of
              frozen or active, since this is handled by placing a check
              list in the Running state defined in ICE core.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-12'>
        <t>
          <list style='symbols'>
            <t>
              Specified that the end-of-candidates indication must
              include the generation (ufrag/pwd) to enable association
              with a particular ICE session.
            </t>
            <t>
              Further editorial fixes to address WGLC feedback.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-11'>
        <t>
          <list style='symbols'>
            <t>
              Editorial and terminological fixes to address WGLC feedback.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-10'>
        <t>
          <list style='symbols'>
            <t>
              Minor editorial fixes.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-09'>
        <t>
          <list style='symbols'>
            <t>
              Removed immediate unfreeze upon Fail.
            </t>
            <t>
              Specified MUST NOT regarding ice-options.
            </t>
            <t>
              Changed terminology regarding initial ICE parameters
              to avoid implementer confusion.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-08'>
        <t>
          <list style='symbols'>
            <t>
              Reinstated text about in-order processing of messages
              as a requirement for signaling protocols.
            </t>
            <t>
              Added IANA registration template for ICE option.
            </t>
            <t>
              Corrected Case 3 rule in Section 8.1.1 to ensure
              consistency with regular ICE rules.
            </t>
            <t>
              Added tabular representations to Section 8.1.1 in order
              to illustrate the new pair rules.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-07'>
        <t>
          <list style='symbols'>
            <t>
              Changed "ICE description" to "candidate information" for 
              consistency with 5245bis.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-06'>
        <t>
          <list style='symbols'>
            <t>
              Addressed editorial feedback from chairs' review.
            </t>
            <t>
              Clarified terminology regarding generations.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-05'>
        <t>
          <list style='symbols'>
            <t>
              Rewrote the text on inserting a new pair into a 
              check list.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-04'>
        <t>
          <list style='symbols'>
            <t>
              Removed dependency on SDP and offer/answer model.
            </t>
            <t>
              Removed mentions of aggressive nomination, since it is
              deprecated in 5245bis.
            </t>
            <t>
              Added section on requirements for signaling protocols.
            </t>
            <t>
              Clarified terminology.
            </t>
            <t>
              Addressed various WG feedback.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-03'>
        <t>
          <list style='symbols'>
            <t>
              Provided more detailed description of unfreezing behavior, specifically
              how to replace pre-existing peer-reflexive candidates with higher-priority 
              ones received via trickling.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-02'>
        <t>
          <list style='symbols'>
            <t>
              Adjusted unfreezing behavior when there are disparate foundations.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-01'>
        <t>
          <list style='symbols'>
            <t>
              Changed examples to use IPv6.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ietf-ice-trickle-00'>
        <t>
          <list style='symbols'>
            <t>
              Removed dependency on SDP (which is to be provided 
              in a separate specification).
            </t>
            <t>
              Clarified text about the fact that a check list
              can be empty if no candidates have been sent or
              received yet.
            </t>
            <t>
              Clarified wording about check list states so as not
              to define new states for "Active" and "Frozen" because
              those states are not defined for check lists (only for
              candidate pairs) in ICE core.
            </t>
            <t>
              Removed open issues list because it was out of date.
            </t>
            <t>
              Completed a thorough copy edit.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-mmusic-trickle-ice-02'>
        <t>
          <list style='symbols'>
            <t>
              Addressed feedback from Rajmohan Banavi and Brandon Williams.
            </t>
            <t>
              Clarified text about determining support and about how to 
              proceed if it can be determined that the answering agent
              does not support Trickle ICE.
            </t>
            <t>
              Clarified text about check list and timer updates.
            </t>
            <t>
              Clarified when it is appropriate to use half trickle or
              to send no candidates in an offer or answer.
            </t>
            <t>
              Updated the list of open issues.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ivov-01 and draft-mmusic-00'>
        <t>
          <list style='symbols'>
            <t>
              Added a requirement to trickle candidates by order of
              components to avoid deadlocks in the unfreezing algorithm.
            </t>
            <t>
              Added an informative note on peer-reflexive candidates
              explaining that nothing changes for them semantically but
              they do become a more likely occurrence for Trickle ICE.
            </t>
            <t>
              Limit the number of pairs to 100 to comply with 5245.
            </t>
            <t>
              Added clarifications on the non-importance of how newly
              discovered candidates are trickled/sent to the remote
              party or if this is done at all.
            </t>
            <t>
              Added transport expectations for trickled candidates
              as per Dale Worley's recommendation.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-ivov-00'>
        <t>
          <list style='symbols'>
            <t>
              Specified that end-of-candidates is a media level
              attribute which can of course appear as session level,
              which is equivalent to having it appear in all m-lines.
              Also made end-of-candidates optional for cases such as
              aggressive nomination for controlled agents.
            </t>
            <t>
              Added an example for ICE lite and Trickle ICE to
              illustrate how, when talking to an ICE lite agent doesn't
              need to send or even discover any candidates.
            </t>
            <t>
              Added an example for ICE lite and Trickle ICE to
              illustrate how, when talking to an ICE lite agent doesn't
              need to send or even discover any candidates.
            </t>
            <t>
              Added wording that explicitly states ICE lite agents
              have to be prepared to receive no candidates over
              signaling and that they should not freak out if this
              happens. (Closed the corresponding open issue).
            </t>
            <t>
              It is now mandatory to use MID when trickling candidates
              and using m-line indexes is no longer allowed.
            </t>
            <t>
              Replaced use of 0.0.0.0 to IP6 :: in order to avoid
              potential issues with RFC2543 SDP libraries that interpret
              0.0.0.0 as an on-hold operation. Also changed the port
              number here from 1 to 9 since it already has a more
              appropriate meaning. (Port change suggested by Jonathan
              Lennox).
            </t>
            <t>
              Closed the Open Issue about use about what to do with
              cands received after end-of-cands. Solution: ignore, do
              an ICE restart if you want to add something.
            </t>
            <t>
              Added more terminology, including trickling, trickled
              candidates, half trickle, full trickle,
            </t>
            <t>
              Added a reference to the SIP usage for Trickle ICE as
              requested at the Boston interim.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-rescorla-01'>
        <t>
          <list style='symbols'>
            <t>
              Brought back explicit use of Offer/Answer. There are no
              more attempts to try to do this in an O/A independent way.
              Also removed the use of ICE Descriptions.
            </t>
            <t>
              Added SDP specification for trickled candidates, the
              trickle option and 0.0.0.0 addresses in m-lines, and
              end-of-candidates.
            </t>
            <t>
              Support and Discovery. Changed that section to be less
              abstract. As discussed in IETF85, the draft now says
              implementations and usages need to either determine
              support in advance and directly use trickle, or do
              half trickle. Removed suggestion about use of discovery in
              SIP or about letting implementing protocols do what they
              want.
            </t>
            <t>
              Defined Half Trickle. Added a section that says how it
              works. Mentioned that it only needs to happen in the first
              o/a (not necessary in updates), and added Jonathan's
              comment about how it could, in some cases, offer more than
              half the improvement if you can pre-gather part or all of
              your candidates before the user actually presses the call
              button.
            </t>
            <t>
              Added a short section about subsequent offer/answer
              exchanges.
            </t>
            <t>
              Added a short section about interactions with ICE Lite
              implementations.
            </t>
            <t>
              Added two new entries to the open issues section.
            </t>
          </list>
        </t>
      </section>
      <section title='Changes from draft-rescorla-00'>
        <t>
          <list style='symbols'>
            <t>
              Relaxed requirements about verifying support following
              a discussion on MMUSIC.
            </t>
            <t>
              Introduced ICE descriptions in order to remove ambiguous
              use of 3264 language and inappropriate references to
              offers and answers.
            </t>
            <t>
              Removed inappropriate assumption of adoption by RTCWEB
              pointed out by Martin Thomson.
            </t>
          </list>
        </t>
      </section>
    </section>
  </back>
</rfc>