ICE Working Group                                            C. Holmberg
Internet-Draft                                                  Ericsson
Updates: 8445 (if approved)                                    J. Uberti
Intended status: Standards Track                                  Google
Expires: September 27, 2019                               March 26, 2019

 Interactive Connectivity Establishment Patiently Awaiting Connectivity
                               (ICE PAC)


   During the process of creating a peer-to-peer connection, ICE
   implementations 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 proposes a way to avoid premature failure.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 27, 2019.

Copyright Notice

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Relevant Scenarios  . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  No Candidates From Peer . . . . . . . . . . . . . . . . .   3
     3.2.  All Candidates Discarded  . . . . . . . . . . . . . . . .   3
     3.3.  Immediate Candidate Pair Failure  . . . . . . . . . . . .   4
   4.  Update to RFC 8445  . . . . . . . . . . . . . . . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   [RFC8445] describes a protocol, Interactive Connectivity
   Establishment (ICE), for Network Address Translator (NAT) traversal
   for UDP-based communication.

   Typically, when using ICE, endpoints will exchange candidate
   addresses, 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 is considered to have

   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 race
   condition 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 race
   condition can occur in many common situations.

   This specification updates [RFC8445], by simply requiring that an
   endpoint 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

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   of peer-reflexive candidates, which may eventually lead to ICE
   processing completing successfully.

2.  Conventions

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

3.  Relevant Scenarios

   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.

   In most ICE scenarios, the lengthy timeouts for connectivity checks,
   typically tens of seconds, will prevent this problem for occuring.
   However, there are certain specific cases where this problem will
   frequently occur.

3.1.  No Candidates From Peer

   It is entirely legal for an ICE gent to 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 remote endpoint can discover the
   appropriate local candidate via connectivity checks.

   However, following the procedures from [RFC8445] strictly will result
   in immediate ICE failure, since the checklist at the remote endpoint
   will be empty.

3.2.  All Candidates Discarded

   Even if the ICE agent provides candidates, they may be discarded by
   the remote endpoint if it does not know what to do with them.  For
   example, candidates may use an address family that the remote
   endpoint 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 (e.g., a candidate that contains a FQDN that fails to

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   In these scenarios, when the candidates are discarded, the checklist
   at the remote endpoint will once again be empty, leading to immediate
   ICE failure.

3.3.  Immediate Candidate Pair Failure

   Section of [RFC8445] describes several situations in which a
   candidate pair will be considered to have failed, well before the
   connectivity check timeout.

   As a result, even if the ICE agent provides usable candidates, the
   pairs created by the remote endpoint may fail immediately when
   checked, e.g., a check to a nonroutable address that receives an
   immediate ICMP error.

   In this situation, the checklist at the remote endpoint may contain
   only failed pairs, resulting in immediate ICE failure.

4.  Update to RFC 8445

   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 full ICE implementations begin their
   ICE processing, as described in [RFC8445], Section 6.1, the ICE agent
   MUST set a timer, and its duration SHOULD be equal to the ICE agent's
   connectivity check timeout.

   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 ICE agent waits some reasonable amount of time.

   While the timer is running, if a checklist has no pairs left to
   check, i.e., there are no pairs that are not in the failed state, the
   ICE agent MUST not conclude that ICE processing has failed, and MUST
   wait for the timer to elapse before doing so.

   One consequence of this behavior is that in cases where ICE should
   fail, e.g., where both sides provide candidates with unresolvable
   FQDNs ICE will no longer fail immediately, and only fail when the
   aforementioned 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.

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5.  Security Considerations

   The security considerations for ICE are defined in [RFC8445].  This
   specification only recommends ICE endpoints to wait for a certain
   time of period before they declare ICE failure, and does not
   introduce new security considerations.

6.  IANA considerations

   This specification makes no requests to IANA.

7.  Acknowledgements

8.  Normative References

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018, <https://www.rfc-

Authors' Addresses

   Christer Holmberg
   Hirsalantie 11
   Jorvas  02420


   Justin Uberti
   747 6th St W
   Kirkland  98033


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