One document matched: draft-rescorla-mmusic-ice-trickle-01.xml


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     docName='draft-rescorla-mmusic-ice-trickle-01'>

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  <front>

    <title abbrev='Trickle ICE'>
      Trickle ICE: Incremental Provisioning of Candidates for the
      Interactive Connectivity Establishment (ICE) Protocol
    </title>
    <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='E.' surname='Ivov'
            fullname='Emil Ivov'>
      <organization abbrev='Jitsi'>Jitsi</organization>
      <address>
        <postal>
          <street></street>
          <city>Strasbourg</city>
          <code>67000</code>
          <country>France</country>
        </postal>
        <phone>+33 6 72 81 15 55</phone>
        <email>emcho@jitsi.org</email>
      </address>
    </author>
    <date />
    <abstract>
      <t>
        This document describes an extension to the Interactive
        Connectivity Establishment (ICE) protocol that allows ICE agents
        to send and receive candidates incrementally rather than
        exchanging complete lists. With such incremental provisioning,
        ICE agents can begin connectivity checks while they are still
        gathering candidates and considerably shorten the time necessary
        for ICE processing to complete.
      </t>
      <t>
        The above mechanism is also referred to as "trickle ICE".
      </t>
    </abstract>
  </front>
  <middle>
    <section title='Introduction'>
      <t>
        The Interactive Connectivity Establishment (ICE) protocol
        <xref target="RFC5245"/> describes mechanisms for gathering,
        candidates, prioritizing them, choosing default ones, exchanging
        them with the remote party, pairing them and ordering them into
        check lists. Once all of the above have been completed, and only
        then, the participating agents can begin a phase of connectivity
        checks and eventually select the pair of candidates that will be
        used in the following session.
      </t>
      <t>
        While the above sequence has the advantage of being relatively
        straightforward to implement and debug once deployed, it may
        also prove to be rather lengthy. Gathering candidates or
        candidate harvesting would often involve things like querying
        <xref target="RFC5389">STUN</xref> servers, discovering UPnP
        devices, and allocating relayed candidates at
        <xref target="RFC5766">TURN</xref> servers. All of these can
        be delayed for a noticeable amount of time and while they can be
        run in parallel, they still need to respect the pacing
        requirements from <xref target="RFC5245"/>, which is likely to
        delay them even further. Some or all of the above would also
        have to be completed by the remote agent. Both agents would
        next perform connectivity checks and only then would they be
        ready to begin streaming media.
      </t>
      <t>
        All of the above could lead to relatively lengthy session
        establishment times and degraded user experience.
      </t>
      <t>
        The purpose of this document is to define an alternative mode of
        operation for ICE implementations, also known as "trickle ICE",
        where candidates can be exchanged incrementally. This would
        allow ICE agents to exchange host candidates as soon as a
        session has been initiated. Connectivity checks for a media
        stream would also start as soon as the first candidates for that
        stream have become available.
      </t>
      <t>
        Trickle ICE allows reducing session establishment times in cases
        where connectivity is confirmed for the first exchanged
        candidates (e.g. where the host candidates for one of the agents
        are directly reachable from the second agent). Even when this is
        not the case, running candidate harvesting for both agents and
        connectivity checks all in parallel allows to considerably
        reduce ICE processing times.
      </t>
      <t>
        It is worth pointing out that before being introduced to the
        IETF, trickle ICE had already been included in specifications
        such as <xref target="XEP-0176">XMPP Jingle</xref> and it has
        been in use in various implementations and deployments.
      </t>
      <t>
        In addition to the basics of trickle ICE, this document also
        describes how support for trickle ICE needs to be discovered,
        how regular ICE processing needs to be modified when
        building and updating check lists, and how trickle ICE
        implementations should interoperate with agents that only
        implement <xref target="RFC5245"/> processing.
      </t>
      <t>
        This specification does not define usage of trickle ICE with any
        specific signalling or media description protocol, contrary to
        <xref target="RFC5245"/> which defined a usage for ICE wht SIP
        and SDP. Such usages would have to be specified in separate
        documents.
      </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 by the
        protocol for Interactive Connectivity Establishment in
        <xref target="RFC5245"/>.
      </t>
      <t>
        <list style="hanging">
          <t hangText="Vanilla ICE:">
            The Interactive Connectivity Establishment protocol as
            defined in <xref target="RFC5245"/>.
          </t>
          <t hangText="Candidate Harvester:">
            A module used by an ICE agent to obtain local candidates.
            Candidate harvesters use different mechanisms for
            discovering local candidates. Some of them would typically
            make use of protocols such as STUN or TURN. Others may also
            employ techniques that are not referenced within
            <xref target="RFC5245"/>. UPnP based port allocation and
            XMPP Jingle Relay Nodes <xref target="XEP-0278"/> are among
            the possible examples.
          </t>
          <t hangText="ICE Description:">
            A set of parameters necessary for ICE initiation. Such
            parameters include ice-ufrag, ice-pwd and other attributes
            that agents need to exchange in the beginning of an ICE
            session. ICE descriptions are transmitted by signalling. In
            the case of protocols using Offer/Answer semantics, they are
            always part of the offer and often also part of the answer.
          </t>
        </list>
      </t>
    </section>
    <section title='Incompatibility with Standard ICE'
             anchor='incompat'>
      <t>
        The ICE protocol was designed to be fairly flexible so that it
        would work in and adapt to as many network environments as
        possible. It is hence important to point out at least some of
        the reasons why, despite its flexibility, the specification in
        <xref target="RFC5245"/> would not support trickle-ICE.
      </t>
      <t>
        <xref target="RFC5245"/> 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:
        <list style='empty'>
          <t>
            If there is not a pair in the valid list for each component
            of the media stream, the state of the check list is set to
            Failed.
          </t>
        </list>
        This could be a problem and cause ICE processing to fail
        prematurely in a number of scenarios. Consider the following
        case:
        <list style='symbols'>
          <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
            <xref target="RFC1918"/> block.
          </t>
          <t>
            Alice sends Bob the candidate 10.0.0.10 which also happens
            to correspond to an existing host on Bob's network.
          </t>
          <t>
            Bob creates a check list consisting solely of 10.0.0.10 and
            starts checks.
          </t>
          <t>
            These checks reach the host at 10.0.0.10 in Bob's network,
            which responds with an ICMP "port unreachable" error and per
            <xref target="RFC5245"/> 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 media stream and
        potentially all ICE processing to Fail.
      </t>
      <t>
        A similar race condition would occur if the initial offer from
        Alice only contains candidates that can be determined as
        unreachable (per
        <xref target="I-D.keranen-mmusic-ice-address-selection"/>) from
        any of the candidates that Bob has gathered. 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 sends an offer to a trickle one. Consider the
        following case:
        <list style='symbols'>
          <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>
        After Bob's agent receives Alice's offer it would immediately
        start connectivity checks. It would also start gathering
        candidates, which would take long because of the unreachable
        STUN server. By the time Bob's answer is ready and sent to
        Alice, Bob's connectivity checks may well 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='Detecting Support for Trickle ICE' anchor="disco">
      <t>
        In order to avoid interoperability problems such as those
        described in <xref target="incompat"/>, it is important that
        trickle ICE sessions are only attempted in cases where both
        parties support this specification. This means that usages of
        trickle for specific protocols MUST provide one of the
        following:
      </t>
      <t>
        <list style='symbols'>
          <t>
            A way for agents to verify support of trickle ICE prior to
            initiation a session.
          </t>
          <t>
            A procedure for attempting a trickle ICE session in a way
            that, when received by non-supporting agents, would either
            allow for a smooth fallback to vanilla ICE, or fail in a
            predictable way so that a vanilla ICE session can be retried
            subsequently.
          </t>
        </list>
      </t>
      <t>
        The exact mechanisms that would allow for the verifications
        above are outside the scope of this document and should be
        handled by the signalling protocol that is employing ICE.
      </t>
      <t>
        Examples of how some signalling protocols already handle
        service and capabilities discovery include:
        <list style='symbols'>
          <t>
            <xref target="XEP-0030">Service discovery</xref> and
            <xref target="XEP-0115">Entity capabilities</xref> for XMPP
          </t>
          <t>
            <xref target="RFC3840">Indicating User Agent
            Capabilities</xref> for SIP
          </t>
        </list>
      </t>
      <t>
        Usages of trickle ICE SHOULD make use of these mechanisms where
        they exist and can provide reliable indication.
      </t>
      <t>
        In some cases, agents may choose to just send an offer
        that the remote party would reject as invalid unless it supports
        trickling. One such example would be an offer with no ICE
        candidates and an invalid default address (e.g. 0.0.0.0).
      </t>
      <t>
        Usages of trickle ICE MUST define a way for ICE descriptions
        to indicate support for trickling as well as a clear procedure
        for falling back to vanilla ICE in the absence of such support.
      </t>
    </section>
    <section title='Relationship to the Offer/Answer Model' >
      <t>
        The vanilla ICE specification uses the Offer/Answer model for
        exchanging all ICE parameters. Using just a couple of signalling
        messages is obviously no longer possible with continuous
        candidate provisioning and trying to fit candidate exchanges
        into consecutive offer/answer pairs is clearly not practical.
        This specification therefore loosens the relationship with  the
        Offer/Answer model by splitting trickle ICE signalling into
        two phases: initial ICE Descriptions and subsequent exchange of
        additional candidates.
      </t>
      <t>
        ICE descriptions contain session or media-level parameters that
        are necessary for ICE processing to begin. Those include
        attributes such as ice-ufrag and ice-pwd. Due to their nature
        ICE descriptions are exchanged in the beginning of a session and
        trickle ICE agents MUST NOT send any candidates prior to a
        description. It is however possible for ICE descriptions to be
        accompanied by a first set of candidates.
      </t>
      <t>
        When using trickle ICE with Offer/Answer protocols agents MUST
        include an initial ICE description in their Offers. Answerers in
        this situation MAY send their ICE description at any point after
        receiving that of the offerer but no later than sending their
        answer, which MUST contain an ICE description if the agent did
        not provide one before.
      </t>
      <t>
        After sending an ICE description each agent can continue
        trickling candidates regardless of what the state of the
        Offer/Answer negotiation is.
      </t>
    </section>
    <section title='Sending the Initial Description'
             anchor="initial-desc">
      <t>
        An agent starts 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 session). Contrary to
        vanilla ICE, implementations of trickle ICE do not need to
        gather candidates in a blocking manner, and SHOULD generate and
        transmit their initial ICE description as early as possible.
      </t>
      <t>
        In the case of protocols using the Offer/Answer model, agents
        MUST include the initial ICE description in the corresponding
        offer.
      </t>
      <t>
        Trickle ICE agents MAY include any set of candidates in an ICE
        description. This includes the possibility of generating a
        description with no candidates, or one that contains all the
        candidates that the agent is planning on using in the following
        session.
      </t>
      <t>
        For optimal performance, it is RECOMMENDED that an ICE
        description contains host candidates only. This would allow both
        agents to start gathering server reflexive, relayed and other
        non-host candidates simultaneously, and it would also enable
        them to begin connectivity checks.
      </t>
      <t>
        If the privacy implications of revealing host addresses are a
        concern, agents MAY generate an initial ICE description that
        contains no candidates and then only trickle candidates that do
        not reveal host addresses (e.g. relayed candidates).
      </t>
      <t>
        Prior to actually sending an initial ICE description, agents
        MAY verify if the remote party supports trickle ICE. If absence
        of such support is confirmed agents SHOULD fall back to using
        vanilla ICE or abandon the entire session.
      </t>
      <t>
        All trickle ICE descriptions MUST indicate support of this
        specification. The exact syntax of providing this indication is
        left to the usages that define how signalling protocols employ
        trickle ICE.
      </t>
      <t>
        Calculating priorities and foundations, as well as determining
        redundancy of candidates work the same way they do with vanilla
        ICE.
      </t>
    </section>
    <section title='Receiving the Initial Description' >
      <t>
        When an agent receives an initial ICE description, in the case
        of protocols using Offer/Answer this description will be part
        of the offer, it will check if it indicates support for trickle
        ICE as explained in <xref target="disco"/>. If this is not the
        case, the agent MUST process the description according to the
        <xref target="RFC5245"/> procedures or standard
        <xref target="RFC3264"/> processing in case no ICE support is
        detected at all.
      </t>
      <t>
        If, the description does indicate support for trickle ICE, the
        agent will determine its role, start gathering and prioritizing
        candidates and, while doing so it will also respond by sending
        its own ICE description, so that both agents can start forming
        check lists and begin connectivity checks.
      </t>
      <t>
        Otherwise the agent would simply fallback to vanilla ICE
        processing.
      </t>
      <section title="Responding to an Initial ICE Description">
        <t>
          An agent can respond to an initial ICE description at any
          point while gathering candidates. Just as with initial ICE
          descriptions (<xref target="initial-desc"/>), the agent does
          send the description without any candidates or with all those
          it is planning on using. Again, as with initial descriptions
          it is RECOMMENDED that responses to initial ICE descriptions
          contain host candidates so that the remote party can also
          start forming checklists and performing connectivity checks.
        </t>
        <t>
          The answer MUST indicate support for trickle ICE as described
          by usage specifications.
        </t>
        <t>
          For protocols using Offer/Answer semantics the response to
          the initial ICE description would either be transmitted prior
          to the <xref target="RFC3264"/> answer or as a part of it.
        </t>
      </section>
      <section title="Forming check lists and beginning connectivity
                      checks" anchor="check.lists">
        <t>
          After exchanging descriptions, and as soon as they have
          gathered any candidates, agents will begin forming candidate
          pairs, computing their priorities and creating check lists
          according to the vanilla ICE procedures described in
          <xref target="RFC5245"/>. Obviously in order for candidate
          pairing to be possible, it would be necessary that both
          descriptions contained candidates. If this was not the case
          agents will still create the check lists (so that their
          Active/Frozen state could be monitored and updated) but they
          will only populate them once they have learned any local and
          remote candidates.
        </t>
        <t>
          Initially, all check lists will have their Active/Frozen state
          set to Frozen.
        </t>
        <t>
          Trickle ICE agents will then also attempt to unfreeze the
          check list for the first media stream (i.e. the first media
          stream that was reported to the ICE implementation from the
          using application). If this checklist is still empty however,
          agents will continue examining media streams in the order they
          were reported and will unfreeze the first non-empty checklist.
        </t>
        <t>
          Respecting the order in which lists have been reported to an
          ICE implementation, or in other words, the order in which
          streams had been described by the signalling protocol (e.g.
          SDP), is helpful so that checks for the same media stream
          is more likely to be performed simultaneously by both agents.
        </t>
      </section>
    </section>
    <section title="Receiving a Response to the Initial ICE
                    Description">
      <t>
        When receiving an answer, agents will follow vanilla ICE
        procedures to determine their role and they would then
        form check lists and begin connectivity checks as described in
        <xref target="check.lists"/>.
      </t>
    </section>
    <section title='Performing Connectivity Checks' >
      <t>
        For the most part, trickle ICE agents perform connectivity
        checks following vanilla ICE procedures. Of course, the
        asynchronous nature of candidate harvesting in trickle ICE would
        impose a number of changes:
      </t>
      <section title="Check List and Timer State Updates"
               anchor="state-updates">
        <t>
          The vanilla ICE specification requires that agents update
          check lists and timer states upon completing a connectivity
          check transaction. During such an update vanilla ICE agents
          would set the state of a check list to Failed if the following
          two conditions are satisfied:
          <list style="symbols">
            <t>
              all of the pairs in the check list are either in the
              Failed or Succeeded state;
            </t>
            <t>
              if at least one of the components of the media stream
              has no pairs in its valid list.
            </t>
          </list>
        </t>
        <t>
          With trickle ICE, the above situation would often occur when
          candidate harvesting and trickling are still in progress and
          it is perfectly 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 harvesters have completed and the agent
              is not expecting to learn any new candidates;
            </t>
            <t>
              the remote agent has sent an end-of-candidates message
              for that check list as described in
              <xref target="end-of-candidates"/>.
            </t>
          </list>
        </t>
        <t>
          Vanilla ICE requires that agents then update all other check
          lists, placing one pair in each of them into the Waiting
          state, effectively unfreezing the check list. Given that
          with trickle ICE, other check lists may still be empty at that
          point, a trickle ICE agent SHOULD also maintain an explicit
          Active/Frozen state for every check list, rather than deducing
          it from the state of the pairs it contains. This state should
          be set to Active when unfreezing the first pair in a list
          or when that couldn't happen because a list was empty.
        </t>
      </section>
    </section>
    <section title='Learning and Sending Additional Local Candidates'
             anchor="send-trickling">
      <t>
        After an ICE description has been sent or received, agents will
        most likely continue discovering new local candidates as
        STUN, TURN and other non-host candidate harvesting mechanisms
        begin to yield results. Whenever such a new candidate is
        learned agents will compute its priority, type, foundation and
        component id according to normal vanilla 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 vanilla ICE, trickle ICE agents will
        consider the new candidate redundant regardless of its priority.
        [TODO: is this OK? if not we need to check if the existing
        candidate was already used in conn checks, cancel them, and then
        restart them with the new candidate ... and in this specific
        case there's probably no point to do that].
      </t>
      <t>
        Then, if no remote candidates are currently known for this same
        stream, the new candidate will simply be added to the list of
        local candidates.
      </t>
      <t>
        Otherwise, if the agent has already learned of one or more
        remote candidates for this stream and component, it will begin
        pairing the new local candidates with them and adding the pairs
        to the existing check lists according to their priority. Forming
        candidate pairs will work the way it is described by the vanilla
        ICE specification. Actually adding the new pair to a check list
        however, will happen according to the rules described below.
      </t>
      <t>
        If the new pair's local candidate is server reflexive, the
        server reflexive candidate MUST be replaced by its base before
        adding the pair to the list. Once this is done, the agent
        examines the check list looking for another pair that would be
        redundant with the new one. If such a pair exists and its state
        is:
      </t>
      <t>
        <list style="hanging">
          <t hangText="Succeeded: ">
            the newly formed pair is ignored.
          </t>
          <t hangText="Frozen or Waiting: ">
            the agent chooses the pair with the higher priority local
            candidate, places it in the state that the old pair was in
            (i.e. Frozen or Waiting) and removes the other one as
            redundant.
          </t>
          <t hangText="Failed: ">
            the agent chooses the pair with the higher priority local
            candidate, places it in the Waiting state and removes the
            other one as redundant.
          </t>
          <t hangText="In-Progress: ">
            The agent cancels the in-progress transaction (where
            cancellation happens as explained in Section 7.2.1.4 of
            <xref target="RFC5245"/>), then it chooses the pair with the
            higher priority local candidate, places it in the Waiting
            state and removes the other one as redundant.
          </t>
        </list>
      </t>
      <t>
        For all other pairs, including those with a server reflexive
        local candidate that were not found to be redundant:
        <list style="symbols">
          <t>
            if this check list is Frozen then the new pair will
            also be assigned a Frozen state.
          </t>
          <t>
            else if the check list is Active and it is either empty or
            contains only candidates in the Succeeded and Failed states,
            then the new pair's state is set to Waiting.
          </t>
          <t>
            else if the check list is non-empty and Active, then the new
            pair state will be set to
            <list style="hanging">
              <t hangText="Frozen: ">
                if there is at least one pair in the list whose
                foundation matches the one in the new pair and whose
                state is neither Succeeded nor Failed (eventually the
                new pair will get unfrozen after the the on-going check
                for the existing pair concludes);
              </t>
              <t hangText="Waiting: ">
                if the list contains no pairs with the same foundation
                as the new one, or, in case such pairs exist, they are
                all in either the Succeeded or Failed states.
              </t>
            </list>
          </t>
        </list>
      </t>
      <section title='Announcing End of Candidates'
             anchor="end-of-candidates">
      <t>
        Once all candidate harvesters for a specific media stream
        complete, or expire, the agent MUST generate an
        "end-of-candidates" event for that stream and send it to the
        remote agent via the signalling channel. This would allow the
        remote agent to begin updating check list states and, in case
        valid pairs do not exist for every component in every media
        stream, determine that ICE processing has failed.
      </t>
      <t>
        An agent MAY also choose to generate an "end-of-candidates"
        event before candidate harvesting has actually completed, if the
        agent determines that harvesting has continued for more than an
        acceptable period of time.
      </t>
      <t>
        Once the agent sends the end-of-candidates event, it SHOULD
        update the state of the corresponding check list as explained
        in section <xref target="state-updates"/>
      </t>
      <t>
        [TODO: should we also have an end-of-candidates for the entire
        harvesting process (as opposed to that of a single stream)]
      </t>
      </section>
    </section>
    <section title='Receiving Additional Remote Candidates'
             anchor="recv-trickling">
      <t>
        At any point of ICE processing, a trickle ICE agent may receive
        new candidates from the remote agent. When this happens and no
        local candidates are currently known for this same stream, the
        new remote candidates are simply added to the list of remote
        candidates.
      </t>
      <t>
        Otherwise, the new candidates are used for forming candidate
        pairs with the pool of local candidates.
      </t>
      <t>
        Once the remote agent has completed candidate harvesting, it
        will send an "end-of-candidates" event. Upon receiving such an
        event, the local agent MUST update check list states as per
        <xref target="state-updates"/>. This may lead to some check
        lists being marked as Failed.
      </t>
    </section>
    <section title='Concluding ICE Processing with Trickle ICE'
             anchor="end.ice">
      <t>
        Trickle ICE processing SHOULD be concluded as explained in 
        Section 8 of <xref target="RFC5245"/>.
      </t>
    </section>
    <section title='Interaction with non-Trickle ICE implementations'>
      <t>
        Trickle ICE implementations MUST behave as non-trickle and
        follow <xref target="RFC5245"/> unless they can confirm that
        the remote party supports this specification. [TODO: anything
        else?]
      </t>
    </section>
    <section title='Example Flow'>
      <t>
        A typical successful trickle ICE exchange with an Offer/Answer
        protocol would look this way:
      </t>
      <figure title="Example " anchor="fig-example">
        <artwork>
<![CDATA[
        Alice                                            Bob
          |            Offer+ICE Description              |
          |---------------------------------------------->|
          |            Additional Candidates              |
          |---------------------------------------------->|
          |                                               |
          |            Answer+ICE Description             |
          |<----------------------------------------------|
          |            Additional Candidates              |
          |<----------------------------------------------|
          |                                               |
          | Additional Candidates and Connectivity Checks |
          |<--------------------------------------------->|
          |                                               |
          |<=============== MEDIA FLOWS =================>|

]]>
        </artwork>
      </figure>
    </section>
    <section title='Security Considerations'>
      <t>
        [TODO]
      </t>
    </section>
    <section title='Acknowledgements'>
      <t>
        The authors would like to thank Christer Holmberg and Martin
        Thomson for their reviews and suggestions on improving this
        document.
      </t>
    </section>
  </middle>
  <back>
    <references title='Normative References'>
      <?rfc include="reference.RFC.2119"?>
      <?rfc include="reference.RFC.5245"?>
    </references>
    <references title='Informative References'>
      <?rfc include="reference.RFC.1918"?>
      <?rfc include="reference.RFC.3264"?>
      <?rfc include="reference.RFC.3840"?>
      <?rfc include="reference.RFC.4787"?>
      <?rfc include="reference.RFC.5389"?>
      <?rfc include="reference.RFC.5766"?>
      <?rfc include="reference.I-D.keranen-mmusic-ice-address-selection"?>
      <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'>
            <organization abbrev='Cisco'>Cisco</organization>
          </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'>
            <organization abbrev='Cisco'>Cisco</organization>
          </author>
          <date month="June" year="2008" />
        </front>
        <seriesInfo name="XEP" value="XEP-0030" />
      </reference>
      <reference anchor="XEP-0115">
        <front>
          <title>XEP-0115: Entity Capabilities</title>
          <author initials='J.' surname='Hildebrand'
                  fullname='Joe Hildebrand'>
            <organization abbrev='Cisco'>Cisco</organization>
          </author>
          <author initials='P.' surname='Saint-Andre'
                  fullname='Peter Saint-Andre'>
            <organization abbrev='Cisco'>Cisco</organization>
          </author>
          <author initials='R.' surname='Tronçon'
                  fullname='Remko Tronçon'>
            <organization abbrev='Synopsys'>Synopsys</organization>
          </author>
          <author initials='J.' surname='Konieczny'
                  fullname='Jacek Konieczny'>
          </author>
          <date month="February" year="2008" />
        </front>
        <seriesInfo name="XEP" value="XEP-0115" />
      </reference>
      <reference anchor="XEP-0278">
        <front>
          <title>XEP-0278: Jingle Relay Nodes</title>
          <author initials='T.' surname='Camargo'
                  fullname='T. Camargo'>
          </author>
          <date month="June" year="2011" />
        </front>
        <seriesInfo name="XEP" value="XEP-0278" />
      </reference>
    </references>
    <section title='Open issues'>
      <t>
        At the time of writing of this document the authors have no
        clear view on how and if the following list of issues should
        be address here:
        <list style='numbers'>
          <t>
            Do we need a "stop sending me candidates" message. What
            would be the use case for that.
          </t>
          <t>
            Is there anything specific we need to say about ICE lite?
          </t>
        </list>
      </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 Individual Submission Draft -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>

PAFTECH AB 2003-20262026-04-24 03:13:08