Interactive Connectivity Establishment Patiently Awaiting Connectivity (ICE PAC)

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Last updated 2020-01-25 (latest revision 2019-10-13)
Replaces draft-holmberg-ice-pac
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ICE Working Group                                            C. Holmberg
Internet-Draft                                                  Ericsson
Updates: 8445 (if approved)                                    J. Uberti
Intended status: Standards Track                                  Google
Expires: April 15, 2020                                 October 13, 2019

 Interactive Connectivity Establishment Patiently Awaiting Connectivity
                               (ICE PAC)


   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 proposes
   a way to avoid premature failure.  This document updates RFC 8445 and

   [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.]

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   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 April 15, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
   publication of this document.  Please review these documents
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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.  Update to RFC XXXX  . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  IANA considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

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

   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.

   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.

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   This specification updates [RFC8445], 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.

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 check
   transactions, typically tens of seconds, will prevent this problem
   from occurring.  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 agent 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 peer agent 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 peer agent will
   be empty.

3.2.  All Candidates Discarded

   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

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   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.

   In these scenarios, when the candidates are discarded, the checklist
   at the peer agent will once again be empty, leading to immediate ICE

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 transaction timeout.

   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.

   In this situation, the checklist at the peer agent 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 a full ICE implementation begins its
   ICE processing, as described in [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.

   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 [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.

   The RECOMMENDED duration for the timer is equal to the agent's
   connectivity check transaction timeout, including all
   retransmissions.  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

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   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.

   While the timer is running, the ICE agent MUST NOT set the state of a
   checklist to Failed, even if the checklist has no pairs left 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

   When the timer eventually elapses, the ICE agent MUST resume typical
   ICE processing, including setting any checklists containing only
   Failed pairs to the Failed state, as usual, and handling any
   consequences as indicated in [RFC8445], Section 8.1.2.  Naturally, if
   there are no such checklists, no action is necessary.

   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.

   Note also that the PAC timer is potentially relevant to the ICE
   nomination procedure described in [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 timer elapsing.

5.  Update to RFC XXXX

   [RFC EDITOR NOTE: Please replace RFC XXXX with the RFC number of
   draft-ietf-ice-trickle once it has been published.]

   Trickle ICE [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 [I-D.ietf-ice-trickle], Section 8,
   is to wait until an end-of-candidates indication has been received
   before determining ICE failure.

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   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.

   Note also that sending an end-of-candidates indication is only a
   SHOULD-strength requirement, which means that ICE agents will need to
   implement an 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.

6.  Security Considerations

   The security considerations for ICE are defined in [RFC8445].  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.

7.  IANA considerations

   This specification makes no requests to IANA.

8.  Acknowledgements

9.  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-

              Ivov, E., Rescorla, E., Uberti, J., and P. Saint-Andre,
              "Trickle ICE: Incremental Provisioning of Candidates for
              the Interactive Connectivity Establishment (ICE)
              Protocol", draft-ietf-ice-trickle-21 (work in progress),
              April 2018.

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Authors' Addresses

   Christer Holmberg
   Hirsalantie 11
   Jorvas  02420


   Justin Uberti
   747 6th St W
   Kirkland  98033


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