draft-ietf-ice-pac.xml

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<!ENTITY RFC0822 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml">
<!ENTITY RFC0822 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.8174.xml">
<!ENTITY RFC8445 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.8445.xml">
]>
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<?rfc compact="yes" ?>
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<?rfc sortrefs="no" ?>
<?rfc strict="yes" ?>
<rfc ipr="trust200902" category="std" docName="draft-ietf-ice-pac-latest" obsoletes="" updates="8445" submissionType="IETF" xml:lang="en">
  <front>
    <title abbrev="ICE PAC">
      Interactive Connectivity Establishment Patiently Awaiting Connectivity (ICE PAC)
    </title>
    <author initials="C.H." surname="Holmberg" fullname="Christer Holmberg">
      <organization>Ericsson</organization>
      <address>
        <postal>
          <street>Hirsalantie 11</street>
          <code>02420</code>
          <city>Jorvas</city>
          <country>Finland</country>
        </postal>
        <email>christer.holmberg@ericsson.com</email>
      </address>
    </author>
    <author initials="J." surname="Uberti" fullname="Justin Uberti">
      <organization>Google</organization>
      <address>
        <postal>
          <street>747 6th St W</street>
          <code>98033</code>
          <city>Kirkland</city>
          <country>USA</country>
        </postal>
        <email>justin@uberti.name</email>
      </address>
    </author>

    <date year="2020"/>
    <area>Transport</area>
    <workgroup>ICE Working Group</workgroup>
    <keyword>ICE</keyword>
    <keyword>PAC</keyword>
    <keyword>Candidate</keyword>
    <abstract>
      <t>
        During the process of establishing peer-to-peer connectivity,
        ICE agents can encounter situations where they have
        no candidate pairs to check, and, as a result, conclude that
        ICE processing has failed. However, because additional
        candidate pairs can be discovered during ICE processing,
        declaring failure at this point may be premature. This
        document discusses when these situations can occur and
        updates RFC8445 and RFC XXXX by requiring that an ICE agent
        wait a minimum amount of time before declaring ICE failure, 
        even if there are no candidate pairs left to check.
      </t>
      <t>
        [RFC EDITOR NOTE: Please replace RFC XXXX with the RFC number of
        draft-ietf-ice-trickle once it has been published. Please also
        indicate that this specification updates RFC XXXX.]
      </t>
    </abstract>
  </front>

  <middle>
    <section title="Introduction" toc="default">
      <t>
        <xref target="RFC8445"></xref> describes a protocol, Interactive Connectivity Establishment (ICE),
        for Network Address Translator (NAT) traversal for UDP-based communication.
      </t>
      <t>
        When using ICE, endpoints will typically exchange ICE candidates,
        form a list of candidate pairs, and then test each candidate pair to see
        if connectivity can be established. If the test for a given pair fails,
        it is marked accordingly, and if all pairs have failed, the overall
        ICE process typically is considered to have failed.
      </t>
      <t>
        During the process of connectivity checks, additional candidates may
        be created as a result of successful inbound checks from the remote
        peer. Such candidates are referred to as peer-reflexive candidates,
        and once discovered, will be used to form new candidate pairs which will
        be tested like any other. However, there is an inherent problem
        here; if, before learning about any peer-reflexive candidates, an
        endpoint runs out of candidate pairs to check, either because it has
        none, or it considers them all to have failed, it will prematurely
        declare failure and terminate ICE processing. This problem can
        occur in many common situations.
      </t>
      <t>
        This specification updates <xref target="RFC8445"></xref>, by simply
        requiring that an ICE agent wait a minimum amount of time before
        declaring ICE failure, even if there are no candidate pairs to check,
        or if all candidate pairs have failed. This delay provides enough time
        for the discovery of peer-reflexive candidates, which may eventually
        lead to ICE processing completing successfully.
      </t>
    </section>

    <section title="Conventions">
      <t>
        The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
        NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
        "MAY", and "OPTIONAL" in this document are to be interpreted as
        described in BCP 14 <xref target="RFC2119"></xref> <xref target="RFC8174"></xref>
        when, and only when, they appear in all capitals, as shown here.
      </t>
    </section>

    <section title="Relevant Scenarios">
        <t>
          As noted above, the core problem this specification attempts to
          address is the situation where even after local gathering and remote
          candidate signaling has completed, the ICE agent immediately ends up
          with no valid pairs and no candidate pairs left to check, resulting in
          a premature ICE failure. This failure is premature because not
          enough time has elapsed to allow for discovery of peer-reflexive
          candidates from inbound connectivity checks; if discovered, these
          candidates are very likely to result in a valid pair.
        </t>
        <t>
          In most ICE scenarios, the lengthy timeouts for connectivity check transactions,
          typically tens of seconds, will prevent this problem from occurring. However, there
          are certain specific cases where this problem will frequently occur.
        </t>
        <section title="No Candidates From Peer">
          <t>
            Per RFC XXXX, an ICE agent can provide zero candidates of
            its own. If the agent somehow knows that the remote endpoint is
            directly reachable, gathering local candidates is unnecessary and
            will only cause delays; the peer agent can discover the
            appropriate local candidate via connectivity checks.
          </t>
          <t>
            However, following the procedures from
            <xref target="RFC8445"></xref> strictly will result in immediate
            ICE failure, since the checklist at the peer agent will be
            empty.
          </t>
        </section>
        <section title="All Candidates Discarded">
          <t>
            Even if the ICE agent provides candidates, they may be discarded
            by the peer agent if it does not know what to do with them.
            For example, candidates may use an address family that the peer
            agent does not support, (e.g., a host candidate with an IPv6
            address in a NAT64 scenario), or may not be usable for some other
            reason.
          </t>
          <t>
            In these scenarios, when the candidates are discarded, the
            checklist at the peer agent will once again be empty, leading
            to immediate ICE failure.
          </t>
        </section>
        <section title="Immediate Candidate Pair Failure">
          <t>
            Section 7.2.5.2 of <xref target="RFC8445"></xref> describes several
            situations in which a candidate pair will be considered to have
            failed, well before the connectivity check transaction timeout.
          </t>
          <t>
            As a result, even if the ICE agent provides usable candidates,
            the pairs created by the peer agent may fail immediately when
            checked, e.g., a check to a non-routable address that receives an
            immediate ICMP error.
          </t>
          <t>
            In this situation, the checklist at the peer agent may contain
            only failed pairs, resulting in immediate ICE failure.
          </t>
        </section>
    </section>

    <section title="Update to RFC 8445">
      <t>
        In order to avoid the problem raised by this document, the ICE agent
        needs to wait enough time to allow peer-reflexive candidates to be
        discovered. Accordingly, when a full ICE implementation begins its
        ICE processing, as described in <xref target="RFC8445" />, Section
        6.1, it MUST set a timer, henceforth known as the PAC timer, to 
        ensure ICE will run for a minimum amount of time before determining
        failure.
      </t>
      <t>
        Specifically, the ICE agent will start its timer once it believes
        ICE connectivity checks are starting. This occurs when the agent has
        sent the values needed to perform connectivity checks
        (e.g., the Username Fragment and Password denoted in
        <xref target="RFC8445" />, Section 5.3)
        and has received some indication that the remote side is
        ready to start connectivity checks, typically via receipt of the values
        mentioned above. Note that the agent will start the timer even if it
        has not sent or received any ICE candidates.
      </t>
      <t>
        The RECOMMENDED duration for the PAC timer is equal to the agent's
        connectivity check transaction timeout, including all retransmissions.
        When using default values for RTO and Rc, this amounts to 39.5 seconds,
        as explained in <xref target="RFC5389" />, Section 7.2.1.
        This timeout value is chosen to roughly coincide with the maximum
        possible duration of ICE connectivity checks from the remote peer,
        which, if successful, could create peer-reflexive candidates. Because
        the ICE agent doesn't know the exact number of candidate pairs and pacing
        interval in use by the remote side, this timeout value is simply a
        guess, albeit an educated one. Regardless, for this particular problem,
        the desired benefits will be realized as long as the agent waits
        some reasonable amount of time, and, as usual, the application is in
        the best position to determine what is reasonable for its scenario.
      </t>
      <t>
        While the timer is still running, the ICE agent MUST NOT update a checklist state
        from Running to Failed, even if there are no pairs left in the checklist to check.
        As a result, the ICE agent will not remove any data streams or set the state of the
        ICE session to Failed as long as the timer is running.
      </t>
      <t>
        When the timer eventually elapses, the ICE agent MUST resume typical
        ICE processing, including setting the state of any checklists to Failed if they
        have no pairs left to check, and handling any consequences as indicated
        in <xref target="RFC8445" />, Section 8.1.2.  Naturally, if there are no
        such checklists, no action is necessary.
      </t>
      <t>
        One consequence of this behavior is that in cases where ICE should fail,
        e.g., where both sides provide candidates with unsupported address families,
        ICE will no longer fail immediately, and only fail when the
        PAC timer expires. However, because most ICE scenarios
        require an extended period of time to determine failure, the
        fact that some specific scenarios no longer fail fast should have
        minimal application impact, if any.
      </t>
      <t>
        Note also that the PAC timer is potentially relevant to the ICE nomination
        procedure described in <xref target="RFC8445" />, Section 8.1.1. That
        specification does not define a minimum duration for ICE processing
        prior to nomination of a candidate pair, but in order to select the
        best candidate pair, ICE needs to run for enough time in order to allow
        peer-reflexive candidates to be discovered and checked, as noted above.
        Accordingly, the controlling ICE agent SHOULD wait a sufficient amount
        of time before nominating candidate pairs, and it MAY use the PAC timer
        to do so. As always, the controlling ICE agent retains
        full discretion, and MAY decide, based on its own criteria, to nominate
        pairs prior to the PAC timer elapsing.
      </t>
    </section>

    <section title="Update to RFC XXXX">
      <t>
        [RFC EDITOR NOTE: Please replace RFC XXXX with the RFC number of
         draft-ietf-ice-trickle once it has been published.]
      </t>
      <t>
        Trickle ICE <xref target="I-D.ietf-ice-trickle" />
        considers a similar problem, namely whether an ICE agent should allow
        a checklist to enter the Failed state if more candidates might
        still be provided by the remote peer. The solution, specified in
        <xref target="I-D.ietf-ice-trickle" />, Section 8, is to
        wait until an end-of-candidates indication has been received
        before determining ICE failure.
      </t>
      <t>
        However, for the same reasons described above,
        the ICE agent may discover peer-reflexive candidates after it has
        received the end-of-candidates indication, and so the solution
        proposed by this document MUST still be used even when
        the ICE agent is using Trickle ICE.
      </t>
      <t>
        Note also that sending an end-of-candidates indication is only a
        SHOULD-strength requirement, which means that ICE agents will need
        to implement a backup mechanism to decide when all candidates
        have been received, typically a timer. Accordingly, ICE agents
        MAY use the PAC timer to also serve as an end-of-candidates fallback.
      </t>
    </section>

    <section anchor="section.sec" title="Security Considerations">
      <t>
        The security considerations for ICE are defined in <xref target="RFC8445" pageno="false" format="default"/>.
        This specification only recommends that ICE agents wait for a certain time of period before they declare
        ICE failure, and does not introduce new security considerations.
      </t>
    </section>

    <section anchor="section.iana" title="IANA considerations">
      <t>
        This specification makes no requests to IANA.
      </t>
    </section>

    <section anchor="sec-acks" title="Acknowledgements" toc="default">
      <t>
        Roman Shpount, Nils Ohlmeier and Peter Thatcher provided lots of useful input and
        comments.
      </t>
    </section>
  </middle>
  <back>
    <references title="Normative References">
      <?rfc include="reference.RFC.2119"?>
      <?rfc include="reference.RFC.5389"?>
      <?rfc include="reference.RFC.8174"?>
      <?rfc include="reference.RFC.8445"?>
      <?rfc include='reference.I-D.ietf-ice-trickle'?>
    </references>
  </back>
</rfc>