Skip to main content

Trickle ICE: Incremental Provisioning of Candidates for the Interactive Connectivity Establishment (ICE) Protocol
draft-ietf-ice-trickle-09

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8838.
Authors Emil Ivov , Eric Rescorla , Justin Uberti , Peter Saint-Andre
Last updated 2017-04-23
Replaces draft-ietf-mmusic-trickle-ice
RFC stream Internet Engineering Task Force (IETF)
Formats
Reviews
Additional resources Mailing list discussion
Stream WG state WG Document
Document shepherd (None)
IESG IESG state Became RFC 8838 (Proposed Standard)
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD (None)
Send notices to (None)
draft-ietf-ice-trickle-09
Network Working Group                                            E. Ivov
Internet-Draft                                                 Atlassian
Intended status: Standards Track                             E. Rescorla
Expires: October 25, 2017                                     RTFM, Inc.
                                                               J. Uberti
                                                                  Google
                                                          P. Saint-Andre
                                                                Filament
                                                          April 23, 2017

Trickle ICE: Incremental Provisioning of Candidates for the Interactive
               Connectivity Establishment (ICE) Protocol
                       draft-ietf-ice-trickle-09

Abstract

   This document describes "Trickle ICE", an extension to the
   Interactive Connectivity Establishment (ICE) protocol that enables
   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.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 25, 2017.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

Ivov, et al.            Expires October 25, 2017                [Page 1]
Internet-Draft                 Trickle ICE                    April 2017

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Determining Support for Trickle ICE . . . . . . . . . . . . .   5
   4.  Conveying the Initial ICE Parameters  . . . . . . . . . . . .   6
   5.  Receiving the Initial ICE Parameters  . . . . . . . . . . . .   7
     5.1.  Conveying the Initial Response  . . . . . . . . . . . . .   7
     5.2.  Forming Check Lists and Beginning Connectivity
           Checks  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Receiving the Initial Answer  . . . . . . . . . . . . . . . .   8
   7.  Performing Connectivity Checks  . . . . . . . . . . . . . . .   8
     7.1.  Scheduling Checks . . . . . . . . . . . . . . . . . . . .   9
     7.2.  Check List and Timer State Updates  . . . . . . . . . . .   9
   8.  Discovering and Conveying Additional Local Candidates . . . .  10
     8.1.  Pairing Newly Learned Candidates and Updating
           Check Lists . . . . . . . . . . . . . . . . . . . . . . .  11
       8.1.1.  Inserting a New Pair in a Check List  . . . . . . . .  12
     8.2.  Announcing End of Candidates  . . . . . . . . . . . . . .  16
   9.  Receiving Additional Remote Candidates  . . . . . . . . . . .  17
   10. Receiving an End-Of-Candidates Notification . . . . . . . . .  18
   11. Trickle ICE and Peer Reflexive Candidates . . . . . . . . . .  18
   12. Concluding ICE Processing . . . . . . . . . . . . . . . . . .  18
   13. Subsequent Exchanges  . . . . . . . . . . . . . . . . . . . .  18
   14. Unilateral Use of Trickle ICE (Half Trickle)  . . . . . . . .  19
   15. Requirements for Signaling Protocols  . . . . . . . . . . . .  20
   16. Preserving Candidate Order while Trickling  . . . . . . . . .  20
   17. Example Flow  . . . . . . . . . . . . . . . . . . . . . . . .  21
   18. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   19. Security Considerations . . . . . . . . . . . . . . . . . . .  22
   20. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  23
   21. References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     21.1.  Normative References . . . . . . . . . . . . . . . . . .  23
     21.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Appendix A.  Interaction with Regular ICE . . . . . . . . . . . .  24
   Appendix B.  Interaction with ICE Lite  . . . . . . . . . . . . .  26
   Appendix C.  Changes from Earlier Versions  . . . . . . . . . . .  27
     C.1.  Changes from draft-ietf-ice-trickle-09  . . . . . . . . .  27

Ivov, et al.            Expires October 25, 2017                [Page 2]
Internet-Draft                 Trickle ICE                    April 2017

     C.2.  Changes from draft-ietf-ice-trickle-08  . . . . . . . . .  27
     C.3.  Changes from draft-ietf-ice-trickle-07  . . . . . . . . .  27
     C.4.  Changes from draft-ietf-ice-trickle-06  . . . . . . . . .  27
     C.5.  Changes from draft-ietf-ice-trickle-05  . . . . . . . . .  27
     C.6.  Changes from draft-ietf-ice-trickle-04  . . . . . . . . .  27
     C.7.  Changes from draft-ietf-ice-trickle-03  . . . . . . . . .  28
     C.8.  Changes from draft-ietf-ice-trickle-03  . . . . . . . . .  28
     C.9.  Changes from draft-ietf-ice-trickle-02  . . . . . . . . .  28
     C.10. Changes from draft-ietf-ice-trickle-01  . . . . . . . . .  28
     C.11. Changes from draft-ietf-ice-trickle-00  . . . . . . . . .  28
     C.12. Changes from draft-mmusic-trickle-ice-02  . . . . . . . .  28
     C.13. Changes from draft-ivov-01 and draft-mmusic-00  . . . . .  29
     C.14. Changes from draft-ivov-00  . . . . . . . . . . . . . . .  29
     C.15. Changes from draft-rescorla-01  . . . . . . . . . . . . .  30
     C.16. Changes from draft-rescorla-00  . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   The Interactive Connectivity Establishment (ICE) protocol
   [rfc5245bis] describes mechanisms for gathering candidates,
   prioritizing them, choosing default ones, exchanging them with a
   remote party, pairing them, and ordering them into check lists.  Once
   all of these actions have been completed (and only then), the parties
   can begin a phase of connectivity checks and eventually select the
   pair of candidates that will be used in a media session or for a
   given media stream.

   Although the sequence described above has the advantage of being
   relatively straightforward to implement and debug once deployed, it
   can also be rather lengthy.  Candidate gathering often involves
   things like querying STUN [RFC5389] servers and allocating relayed
   candidates at TURN [RFC5766] servers.  All of these actions can be
   delayed for a noticeable amount of time; although they can be run in
   parallel, they still need to respect the pacing requirements from
   [rfc5245bis], which is likely to delay them even further.  Some or
   all of these actions also need be completed by the remote agent.
   Both agents would next perform connectivity checks and only then
   would they be ready to begin streaming media.

   These factors can lead to relatively lengthy session establishment
   times and thus to a degraded user experience.

   This document defines an alternative or supplementary mode of
   operation for ICE implementations, known as "Trickle ICE", in which
   candidates can be exchanged incrementally.  This enables ICE agents
   to exchange candidates as soon as an ICE session has been initiated.

Ivov, et al.            Expires October 25, 2017                [Page 3]
Internet-Draft                 Trickle ICE                    April 2017

   Connectivity checks for a media stream can also start as soon as the
   first candidates for that stream become available.

   Trickle ICE can reduce session establishment times in cases where
   connectivity is confirmed for the first exchanged candidates (e.g.,
   where candidates for one of the agents are directly reachable from
   the second agent, such as candidates at a media relay).  Even when
   this is not the case, performing candidate gathering for both agents
   and connectivity checks in parallel can considerably shorten ICE
   processing times.

   It is worth noting that there is quite a bit of operational
   experience with the Trickle ICE technique, going back as far as 2005
   (when the XMPP Jingle extension defined a "dribble mode" as specified
   in [XEP-0176]); this document incorporates feedback from those who
   have implemented and deployed the technique.

   In addition to the basics of Trickle ICE, this document also
   describes how to discover support for Trickle ICE, how regular ICE
   processing needs to be modified when building and updating check
   lists, and how Trickle ICE implementations interoperate with agents
   that only implement regular ICE processing as defined in
   [rfc5245bis].

   This specification does not define the usage of Trickle ICE with any
   specific signaling protocol (however, see
   [I-D.ietf-mmusic-trickle-ice-sip] for usage with SIP [RFC3261] and
   [XEP-0176] for usage with XMPP [RFC6120]).  Similarly, it does not
   define Trickle ICE in terms of the Session Description Protocol (SDP)
   [RFC4566] or the offer/answer model [RFC3264] because the technique
   can be and already is used in application protocols that are not tied
   to SDP or to offer/answer semantics.  However, because SDP and the
   offer/answer model are familiar to most readers of this
   specification, some examples in this document use those particulars
   in order to explain the underlying concepts.

2.  Terminology

   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 [RFC2119].

   This specification makes use of all terminology defined for
   Interactive Connectivity Establishment in [rfc5245bis].  In addition,
   it defines the following terms:

Ivov, et al.            Expires October 25, 2017                [Page 4]
Internet-Draft                 Trickle ICE                    April 2017

   Candidate Gatherer:  A module used by an ICE agent to obtain local
      candidates.  Candidate gatherers use different mechanisms for
      discovering local candidates, such as STUN and TURN.

   Generation:  All of the candidates conveyed within an ICE session;
      these are the candidates that are associated with a specific local
      /remote ufrag pair (which will change on ICE restart, if any
      occurs).

   ICE Parameters:  Any session-related (as opposed to candidate-
      related) attributes required to configure an ICE agent.  These
      include but are not limited to the username fragment, password,
      and other options.

   Trickled Candidates:  Candidates that a Trickle ICE agent conveys
      after conveying initial candidate information or responding to
      initial candidate information, but within the same ICE session.
      Trickled candidates can be conveyed in parallel with candidate
      gathering and connectivity checks.

   Trickling:  The act of conveying trickled candidates.

   Half Trickle:  A Trickle ICE mode of operation where the initiator
      gathers a full generation of candidates strictly before creating
      and conveying the initial candidate information.  Once conveyed,
      this candidate information can be processed by regular ICE agents,
      which do not require support for this specification.  It also
      allows Trickle ICE capable responders to still gather candidates
      and perform connectivity checks in a non-blocking way, thus
      roughly providing "half" the advantages of Trickle ICE.  The
      mechanism is mostly meant for use in cases where the remote
      agent's support for Trickle ICE cannot be confirmed prior to
      conveying the initial candidate information.

   Full Trickle:  The typical mode of operation for Trickle ICE agents,
      in which the initial candidate information can include any number
      of candidates (even zero candidates) and does not need to include
      a full generation of candidates as in half trickle.

3.  Determining Support for Trickle ICE

   To fully support Trickle ICE, applications SHOULD incorporate one of
   the following mechanisms to enable implementations to determine
   whether Trickle ICE is supported:

   1.  Provide a capabilities discovery method so that agents can verify
       support of Trickle ICE prior to initiating a session (XMPP's
       Service Discovery [XEP-0030] is one such mechanism).

Ivov, et al.            Expires October 25, 2017                [Page 5]
Internet-Draft                 Trickle ICE                    April 2017

   2.  Make support for Trickle ICE mandatory so that user agents can
       assume support.

   If an application 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 Section 14.

   Prior to conveying the initial candidate information, agents using
   signaling 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.

   Even if a signaling protocol does not include a capabilities
   discovery method, a user agent can provide an indication within the
   candidate information that it supports Trickle ICE, either at the
   session level or for every media stream at the media stream level
   (e.g., in SDP this would be a token of "trickle" in the ice-options
   attribute).

   Dedicated discovery semantics and half trickle are needed only prior
   to session initiation.  After a session is established and Trickle
   ICE support is confirmed for both parties, either agent can use full
   trickle for subsequent exchanges.

4.  Conveying the Initial ICE Parameters

   An 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 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,
   agents SHOULD generate and transmit their initial candidate
   information as early as possible, so that the remote party can start
   gathering and trickling candidates.

   Trickle ICE agents MAY include any mix of candidates when conveying
   candidate information.  This includes the possibility of conveying
   ICE paramters that contain all the candidates the agent plans to use
   (as in half trickle mode), conveying candidate information that
   contain only a publicly-reachable IP address (e.g., a candidate at a
   media relay that is known to not be behind a firewall), or conveying
   candidate information with 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).

Ivov, et al.            Expires October 25, 2017                [Page 6]
Internet-Draft                 Trickle ICE                    April 2017

   Methods for calculating priorities and foundations, as well as
   determining redundancy of candidates, work just as with regular ICE
   (with the exception of pruning of duplicate peer reflexive candidates
   as described under Section 5.2).

5.  Receiving the Initial ICE Parameters

   When a responder receives initial candidate information, it will
   first check if the candidate information or initiator indicates
   support for Trickle ICE as explained in Section 3.  If this is not
   the case, the agent MUST process the candidate information according
   to regular ICE procedures [rfc5245bis] (or, if no ICE support is
   detected at all, according to relevant processing rules for the
   underlying signaling protocol, such as offer/answer processing rules
   [RFC3264]).

   If support for Trickle ICE is confirmed, an agent will automatically
   assume support for regular ICE as well even if the support
   verification procedure in [rfc5245bis] indicates otherwise.
   Specifically, the rules from [rfc5245bis] would imply that ICE itself
   is not supported if the initial candidate information include no
   candidates; however, such a conclusion is not warranted if the
   responder can confirm that the initiator supports Trickle ICE; in
   this case, fallback to [RFC3264] is not necessary.

   If the initial candidate information do indicate support for Trickle
   ICE, the agent will determine its role and start gathering and
   prioritizing candidates; while doing so, it will also respond by
   conveying its own candidate information, so that both agents can
   start forming check lists and begin connectivity checks.

5.1.  Conveying the Initial Response

   An agent can respond to initial candidate information at any point
   while gathering candidates.  Here again the candidate information MAY
   contain any set of candidates, including all candidates or no
   candidates.  (The benefit of including no candidates is to convey the
   candidate information as quickly as possible, so that both parties
   can consider the overall session to be under active negotiation as
   soon as possible.)

   As noted in Section 3, in application protocols that use SDP the
   responder's candidate information can indicate support for Trickle
   ICE by including a token of "trickle" in the ice-options attribute.

Ivov, et al.            Expires October 25, 2017                [Page 7]
Internet-Draft                 Trickle ICE                    April 2017

5.2.  Forming Check Lists and Beginning Connectivity Checks

   After the initiator and responder exchange candidate information, and
   as soon as they have obtained local and remote candidates, agents
   begin forming candidate pairs, computing candidate pair priorities,
   ordering candidate pairs, pruning duplicate pairs, and creating check
   lists according to regular ICE procedures [rfc5245bis].

   According to those procedures, in order for candidate pairing to be
   possible and for duplicate candidates to be pruned, the candidates
   would need to be provided in the relevant candidate information.
   Under Trickle ICE, check lists can be empty until candidate pairs are
   conveyed or received.  Therefore Trickle ICE agents handle check
   lists and candidate pairing in a slightly different way than regular
   ICE agents: the agents still create the check lists, but they
   populate the check lists only after they actually have the candidate
   pairs.

   A Trickle ICE agent initially considers all check lists to be frozen.
   It then inspects the first check list and attempts to unfreeze all
   candidate pairs it has received so far that belong to the first
   component on the first media stream (i.e., the first media stream
   that was reported to the ICE implementation from the using
   application).  If that first component of the first media stream does
   not contain candidates for one or more of the currently known pair
   foundations, and if candidate pairs already exist for that foundation
   in one of the following components or media streams, then the agent
   unfreezes the first of those candidate pairs.

   With regard to pruning of duplicate candidate pairs, a Trickle ICE
   agent SHOULD follow a policy of keeping the higher priority candidate
   unless it is peer reflexive.

6.  Receiving the Initial Answer

   When processing candidate information from a responder, the initiator
   follows regular ICE procedures to determine its role, after which it
   forms check lists (as described in Section 5.2) and begins
   connectivity checks.

7.  Performing Connectivity Checks

   For the most part, Trickle ICE agents perform connectivity checks
   following regular ICE procedures.  However, the fact that gathering
   and communicating candidates is asynchronous in Trickle ICE imposes a
   number of changes as described in the following sections.

Ivov, et al.            Expires October 25, 2017                [Page 8]
Internet-Draft                 Trickle ICE                    April 2017

7.1.  Scheduling Checks

   The ICE specification [rfc5245bis], Section 5.1.5, requires that
   agents terminate the timer for a triggered check in relation to an
   active check list once the agent has exhausted all frozen pairs in
   the check list.  This will not work with Trickle ICE, because more
   pairs will be added to the check list incrementally.

   Therefore, a Trickle ICE agent SHOULD NOT terminate the timer until
   the state of the check list is Completed or Failed as specified
   herein (see Section 8.2).

7.2.  Check List and Timer State Updates

   The ICE specification [rfc5245bis], Section 6.1.2.4.3, 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:

   o  all of the pairs in the check list are either in the Failed state
      or Succeeded state; and

   o  there is not a pair in the valid list for each component of the
      media stream.

   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:

   o  all candidate gatherers have completed and the agent is not
      expecting to discover any new local candidates;

   o  the remote agent has conveyed an end-of-candidates indication for
      that check list as described in Section 8.2.

   When a check list is set to Failed as described above, regular ICE
   requires the agent to update all other check lists, placing one pair
   from each check list into the Waiting state - effectively unfreezing
   all remaining check lists.  However, under Trickle ICE other check
   lists might still be empty at this point (because candidates have not
   yet been received), and following only the rules from regular ICE
   would prevent the agent from unfreezing those check lists (because
   the state of a check list depends on the state of the candidate pairs
   in that check list, but there are none yet).  Therefore a Trickle ICE
   agent needs to monitor whether a check list is active or frozen

Ivov, et al.            Expires October 25, 2017                [Page 9]
Internet-Draft                 Trickle ICE                    April 2017

   independently of the state of the candidate pairs in the check list
   (since there might not be any pairs yet).  With regard to empty check
   lists, by default a Trickle ICE agent MAY consider an empty check
   list to be either active or frozen.  When a Trickle ICE agent
   considers an empty check list to be frozen, during the candidate
   checking process it SHOULD change the check list to active if
   checking of another check list is completely finished (i.e., if every
   pair in the other check list is either Successful or Failed), if
   another check list has a valid candidate pair for all components, or
   if it adds a candidate pair to the check list (because, in accordance
   with Section 8.1.1, when inserting a new candidate pair into an empty
   check list, the agent sets the pair to a state of Waiting).

8.  Discovering and Conveying Additional Local Candidates

   After candidate information have been conveyed, agents will most
   likely continue discovering 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.

   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.

   Next the agent "trickles" the newly discovered candidate(s) to the
   remote agent.  The actual delivery of the new candidates is handled
   by a signaling protocol such as SIP or XMPP.  Trickle ICE imposes no
   restrictions on the way this is done (e.g., some applications may
   choose not to trickle updates for server reflexive candidates and
   instead rely on the discovery of peer reflexive ones).

   When candidates are trickled, each candidate MUST be delivered to the
   receiving Trickle ICE implementation not more than once and in the
   same order it was conveyed.  If the signaling protocol provides any
   candidate retransmissions, they need to be hidden from the ICE
   implementation.

   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, applications that choose to signal

Ivov, et al.            Expires October 25, 2017               [Page 10]
Internet-Draft                 Trickle ICE                    April 2017

   candidates via SDP may include a ufrag value in the corresponding
   a=candidate line such as:

     a=candidate:1 1 UDP 2130706431 2001:db8::1 5000 typ host ufrag 8hhY

   Or as another example, WebRTC implementations may include a ufrag in
   the JavaScript objects that represent candidates.

   Note: The signaling protocol needs to provide a mechanism for both
   parties to indicate and agree on the ICE session in force (as
   identified by the ufrag) 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 Section 13).

   Once the candidate has been conveyed to the remote party, the agent
   checks if any remote candidates are currently known for this same
   stream and component.  If not, the new candidate will simply be added
   to the list of local candidates.

   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.

   Note: A Trickle ICE agent MUST NOT pair a local candidate until it
   has been trickled to the remote agent.

8.1.  Pairing Newly Learned Candidates and Updating Check Lists

   Forming candidate pairs works as described in the ICE specification
   [rfc5245bis].  However, actually adding the new pair to a check list
   happens according to the rules described below.

   If the check list where the pair is to be added already contains the
   maximum number of candidate pairs (100 by default as per
   [rfc5245bis]), the new pair is discarded.

   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 the type of its remote candidate is not peer
   reflexive, the pair with the higher priority is kept and the one with
   the lower priority is discarded.  If, on the other hand, the type of

Ivov, et al.            Expires October 25, 2017               [Page 11]
Internet-Draft                 Trickle ICE                    April 2017

   the remote candidate in the pre-existing pair is peer reflexive, the
   agent MUST replace it with the newly formed pair (regardless of their
   respective priorities); this is done by setting the priority of the
   new candidate to the priority of the pre-existing candidate and then
   re-sorting the check list.

   For all other pairs, including those with a server reflexive local
   candidate that were not found to be redundant, the rules specified in
   the following section apply.

8.1.1.  Inserting a New Pair in a Check List

   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):

   +-----------------+------+------+------+------+------+
   |                 |  f1  |  f2  |  f3  |  f4  |  f5  |
   +-----------------+------+------+------+------+------+
   | m1 (Audio.RTP)  |  F   |  F   |  F   |      |      |
   +-----------------+------+------+------+------+------+
   | m2 (Audio.RTCP) |  F   |  F   |  F   |  F   |      |
   +-----------------+------+------+------+------+------+
   | m3 (Video.RTP)  |  F   |      |      |      |      |
   +-----------------+------+------+------+------+------+
   | m4 (Video.RTCP) |  F   |      |      |      |      |
   +-----------------+------+------+------+------+------+

                   Figure 1: Example of Check List State

   Each row in the table represents a component for a given media stream
   (e.g., m1 and m2 might be the RTP and RTCP components for audio).
   Each column represents one foundation.  Each cell represents one
   candidate pair.  In the foregoing table, "F" stands for "frozen"; in
   the tables below, "W" stands for "waiting" and "S" stands for
   "succeeded".

   When an agent commences ICE processing, in accordance with
   Section 5.1.3.6 of [rfc5245bis] it will unfreeze (i.e., place in the
   Waiting state) the topmost candidate pair in every column (i.e., the
   pair with the lowest component ID).  This state is shown in the
   following table, with candidate pairs in the Waiting state marked by
   "W".

Ivov, et al.            Expires October 25, 2017               [Page 12]
Internet-Draft                 Trickle ICE                    April 2017

   +-----------------+------+------+------+------+------+
   |                 |  f1  |  f2  |  f3  |  f4  |  f5  |
   +-----------------+------+------+------+------+------+
   | m1 (Audio.RTP)  |  W   |  W   |  W   |      |      |
   +-----------------+------+------+------+------+------+
   | m2 (Audio.RTCP) |  F   |  F   |  F   |  W   |      |
   +-----------------+------+------+------+------+------+
   | m3 (Video.RTP)  |  F   |      |      |      |      |
   +-----------------+------+------+------+------+------+
   | m4 (Video.RTCP) |  F   |      |      |      |      |
   +-----------------+------+------+------+------+------+

                    Figure 2: Initial Check List State

   Then, as the checks proceed (see Section 6.1.2.4.2.3 of
   [rfc5245bis]), for each pair that enters the Succeeded state (denoted
   here by "S"), the agent will unfreeze all pairs for the same media
   stream and foundation (e.g., if the pair in column 1, row 1 succeeds
   then the agent will unfreeze the pair in column 1, row 2).

   +-----------------+------+------+------+------+------+
   |                 |  f1  |  f2  |  f3  |  f4  |  f5  |
   +-----------------+------+------+------+------+------+
   | m1 (Audio.RTP)  |  S   |  W   |  W   |      |      |
   +-----------------+------+------+------+------+------+
   | m2 (Audio.RTCP) |  W   |  F   |  F   |  W   |      |
   +-----------------+------+------+------+------+------+
   | m3 (Video.RTP)  |  F   |      |      |      |  W   |
   +-----------------+------+------+------+------+------+
   | m4 (Video.RTCP) |  F   |      |      |      |  F   |
   +-----------------+------+------+------+------+------+

             Figure 3: Check List State after Succeeded Check

   ICE also specifies that, if all the pairs in a media stream for one
   foundation are unfrozen (e.g., column 1, rows 1 and 2 representing
   both components for the audio stream), then all of the candidate
   pairs in the entire column are unfrozen (e.g., column 1, rows 3 and
   4).

Ivov, et al.            Expires October 25, 2017               [Page 13]
Internet-Draft                 Trickle ICE                    April 2017

   +-----------------+------+------+------+------+------+
   |                 |  f1  |  f2  |  f3  |  f4  |  f5  |
   +-----------------+------+------+------+------+------+
   | m1 (Audio.RTP)  |  S   |  W   |  W   |      |      |
   +-----------------+------+------+------+------+------+
   | m2 (Audio.RTCP) |  W   |  F   |  F   |  W   |      |
   +-----------------+------+------+------+------+------+
   | m3 (Video.RTP)  |  W   |      |      |      |  W   |
   +-----------------+------+------+------+------+------+
   | m4 (Video.RTCP) |  W   |      |      |      |  F   |
   +-----------------+------+------+------+------+------+

           Figure 4: Check List State with Unfrozen Media Stream

   Trickle ICE preserves all of these rules as they apply to what we
   might call "static" check list sets.  This implies that if, for some
   reason, a Trickle 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.

   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.

   Case 1: If the newly formed pair is the topmost pair in this column
   (i.e.  the topmost pair among all the check lists for this
   foundation), set the state to Waiting (e.g., this would be the case
   if the newly formed pair were placed in column 5, row 1).

   +-----------------+------+------+------+------+------+
   |                 |  f1  |  f2  |  f3  |  f4  |  f5  |
   +-----------------+------+------+------+------+------+
   | m1 (Audio.RTP)  |  S   |  W   |  W   |      |  W   |
   +-----------------+------+------+------+------+------+
   | m2 (Audio.RTCP) |  W   |  F   |  F   |  W   |      |
   +-----------------+------+------+------+------+------+
   | m3 (Video.RTP)  |  W   |      |      |      |      |
   +-----------------+------+------+------+------+------+
   | m4 (Video.RTCP) |  W   |      |      |      |      |
   +-----------------+------+------+------+------+------+

         Figure 5: Check List State with Newly Formed Pair, Case 1

Ivov, et al.            Expires October 25, 2017               [Page 14]
Internet-Draft                 Trickle ICE                    April 2017

   Case 2: If the pair immediately above the newly formed pair in this
   column is in the Succeeded state, set the state to Waiting (e.g.,
   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);

   +-----------------+------+------+------+------+------+
   |                 |  f1  |  f2  |  f3  |  f4  |  f5  |
   +-----------------+------+------+------+------+------+
   | m1 (Audio.RTP)  |  S   |  W   |  W   |      |  S   |
   +-----------------+------+------+------+------+------+
   | m2 (Audio.RTCP) |  W   |  F   |  F   |  W   |  W   |
   +-----------------+------+------+------+------+------+
   | m3 (Video.RTP)  |  W   |      |      |      |      |
   +-----------------+------+------+------+------+------+
   | m4 (Video.RTCP) |  W   |      |      |      |      |
   +-----------------+------+------+------+------+------+

         Figure 6: Check List State with Newly Formed Pair, Case 2

   Case 3: If there is at least one pair in this column above the row of
   the newly formed pair whose state is either Succeeded or Failed, set
   the state to Waiting (e.g., this would be the case if the pair in
   column 5, row 1 succeeded and two newly formed pairs were placed in
   column 5, rows 3 and 4).

   +-----------------+------+------+------+------+------+
   |                 |  f1  |  f2  |  f3  |  f4  |  f5  |
   +-----------------+------+------+------+------+------+
   | m1 (Audio.RTP)  |  S   |  W   |  W   |      |  S   |
   +-----------------+------+------+------+------+------+
   | m2 (Audio.RTCP) |  W   |  F   |  F   |  W   |  W   |
   +-----------------+------+------+------+------+------+
   | m3 (Video.RTP)  |  W   |      |      |      |  W   |
   +-----------------+------+------+------+------+------+
   | m4 (Video.RTCP) |  W   |      |      |      |  W   |
   +-----------------+------+------+------+------+------+

         Figure 7: Check List State with Newly Formed Pair, Case 3

   Case 4: In all other cases, set the state to Frozen.

Ivov, et al.            Expires October 25, 2017               [Page 15]
Internet-Draft                 Trickle ICE                    April 2017

8.2.  Announcing End of Candidates

   Once all candidate gathering is completed or expires for a specific
   media stream, the agent will generate an "end-of-candidates"
   indication for that stream and convey it to the remote agent via the
   signaling channel.  The exact form of the indication depends on the
   application protocol.  The indication can be conveyed in the
   following ways:

   o  As part of an initiation request (which would typically be the
      case with initial candidate information for half trickle)

   o  Along with the last candidate an agent can send for a stream

   o  As a standalone notification (e.g., after STUN Binding requests or
      TURN Allocate requests to a server time out and the agent has no
      other active gatherers)

   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:

   o  A controlled Trickle ICE agent SHOULD convey an end-of-candidates
      indication after it has completed gathering for a media stream,
      unless ICE processing terminates before the agent has had a chance
      to complete gathering.

   o  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.

   When conveying an end-of-candidates indication during trickling
   (rather than as a part of initial candidate information or a response
   thereto), it is the responsibility of the using protocol to define
   methods for relating the indication to one or more specific media
   streams.

   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 media stream, determine that ICE processing has
   failed.  It also enables agents 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

Ivov, et al.            Expires October 25, 2017               [Page 16]
Internet-Draft                 Trickle ICE                    April 2017

   indication provides a notification that such candidates are not
   forthcoming.

   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.

   When performing half trickle, an agent SHOULD convey an end-of-
   candidates indication together with its initial candidate information
   unless it is planning to potentially trickle additional candidates
   (e.g., in case the remote party turns out to support Trickle ICE).

   After an agent conveys the end-of-candidates indication, it will
   update the state of the corresponding check list as explained in
   Section 7.2.  Past that point, an agent MUST NOT trickle any new
   candidates within this ICE session.  After an agent has received an
   end-of-candidates indication, it MUST also ignore any newly received
   candidates for that media stream or media session.  Therefore, adding
   new candidates to the negotiation is possible only through an ICE
   restart (see Section 13).

   This specification does not override regular ICE semantics for
   concluding ICE processing.  Therefore, even if end-of-candidates
   indications are conveyed, agents will still have to go through pair
   nomination.  Also, if pairs have been nominated for components and
   media streams, ICE processing MAY still conclude even if end-of-
   candidates indications have not been received for all streams.

9.  Receiving Additional Remote Candidates

   At any time during ICE processing, a Trickle ICE agent might 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 added to the list of remote candidates.

   Otherwise, the new candidates are used for forming candidate pairs
   with the pool of local candidates and they are added to the local
   check lists as described in Section 8.1.

   Once the remote agent has completed candidate gathering, it will
   convey an end-of-candidates indication.  Upon receiving such an
   indication, the local agent MUST update check list states as per
   Section 7.2.  This might lead to some check lists being marked as
   Failed.

Ivov, et al.            Expires October 25, 2017               [Page 17]
Internet-Draft                 Trickle ICE                    April 2017

10.  Receiving an End-Of-Candidates Notification

   When an agent receives an end-of-candidates indication for a specific
   media stream, it will update the state of the relevant check list as
   per Section 7.2.  If the check list is still in the Active 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.

11.  Trickle ICE and Peer Reflexive Candidates

   Even though Trickle ICE does not explicitly modify the procedures for
   handling peer-reflexive candidates, use of Trickle ICE can have an
   impact on how they are processed.  With Trickle ICE, it is possible
   that server reflexive candidates can be discovered as peer reflexive
   in cases where incoming connectivity checks are received from these
   candidates before the trickle updates that carry them.

   While this would certainly increase the number of cases where ICE
   processing nominates and selects candidates discovered as peer-
   reflexive, it does not require any change in processing.

   It is also likely that some applications would 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.

12.  Concluding ICE Processing

   This specification does not directly modify the procedures for ending
   ICE processing described in Section 6.2 of [rfc5245bis], and Trickle
   ICE implementations follow the same rules.

13.  Subsequent Exchanges

   Either agent MAY convey subsequent candidate information at any time
   allowed by the signaling protocol in use.  When this happens agents
   will use [rfc5245bis] semantics to determine whether or not the new
   candidate information require an ICE restart.  If an ICE restart
   occurs, the user 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
   candidate information where support was determined through a
   capabilities discovery method.

Ivov, et al.            Expires October 25, 2017               [Page 18]
Internet-Draft                 Trickle ICE                    April 2017

14.  Unilateral Use of Trickle ICE (Half Trickle)

   In half trickle mode, the initiator conveys regular candidate
   information with a full generation of candidates.  This ensures that
   the candidate information 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 initial candidate information.
   The initial candidate information indicate support for Trickle ICE,
   which means the responder can respond with something less than a full
   generation of candidates and then trickle the rest.  candidate
   information for half trickle would typically 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.

   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 candidate information 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 (hence the name
   of the mechanism).

   Use of half trickle is only necessary during an initial exchange of
   candidate information.  After both parties have received a candidate
   information from their peer, they can each reliably determine Trickle
   ICE support and use it for all subsequent exchanges.

   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.

   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

Ivov, et al.            Expires October 25, 2017               [Page 19]
Internet-Draft                 Trickle ICE                    April 2017

   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.

15.  Requirements for Signaling Protocols

   In order to fully enable the use of Trickle ICE, this specification
   defines the following requirements for signaling protocols.

   o  A signaling protocol SHOULD provide a way for parties to advertise
      and discover support for Trickle ICE before an ICE session begins
      (see Section 3).

   o  A signaling protocol MUST provide methods for incrementally
      conveying (i.e., "trickling") additional candidates after
      conveying the initial candidate information (see Section 8).

   o  A signaling protocol MUST deliver each trickled candidate not more
      than once and in the same order it was conveyed (see Section 8).

   o  A signaling protocol MUST provide a mechanism for both parties to
      indicate and agree on the ICE session in force (see Section 8).

   o  A signaling protocol MUST provide a way for parties to communicate
      the end-of-candidates indication (see Section 8.2).

16.  Preserving Candidate Order while Trickling

   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.

   To achieve this, when trickling candidates, agents MUST respect the
   order in which the components and streams appear (implicitly or
   explicitly) as they have been negotiated by means of the relevant
   candidate information.  Therefore a candidate for a specific
   component MUST NOT be conveyed prior to candidates for other
   components within the same foundation.  In addition, candidates MUST
   be paired, following the procedures in Section 8.1.1, in the same
   order they are conveyed.

Ivov, et al.            Expires October 25, 2017               [Page 20]
Internet-Draft                 Trickle ICE                    April 2017

   For example, the following SDP description contains two components
   (RTP and RTCP) and two foundations (host and server reflexive):

     v=0
     o=jdoe 2890844526 2890842807 IN IP6 2001:db8:a0b:12f0::1
     s=
     c=IN IP4 2001:db8:a0b:12f0::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 2001:db8:a0b:12f0::1 5000 typ host
     a=candidate:1 2 UDP 2130706431 2001:db8:a0b:12f0::1 5001 typ host
     a=candidate:2 1 UDP 1694498815 2001:db8:a0b:12f0::3 5000 typ srflx
         raddr 2001:db8:a0b:12f0::1 rport 8998
     a=candidate:2 2 UDP 1694498815 2001:db8:a0b:12f0::3 5001 typ srflx
         raddr 2001:db8:a0b:12f0::1 rport 8998

   For this candidate information the RTCP host candidate MUST NOT be
   conveyed prior to the RTP host candidate.  Similarly the RTP server
   reflexive candidate MUST be conveyed together with or prior to the
   RTCP server reflexive candidate.

   Similar considerations apply at the level of media streams in
   addition to foundations; this is covered by the requirement to always
   start unfreezing candidates starting from the first media stream as
   described under Section 5.2.

17.  Example Flow

   As an example, a typical successful Trickle ICE exchange with a
   signaling protocol that follows the offer/answer model would look
   this way:

Ivov, et al.            Expires October 25, 2017               [Page 21]
Internet-Draft                 Trickle ICE                    April 2017

           Alice                                            Bob
             |                     Offer                     |
             |---------------------------------------------->|
             |            Additional Candidates              |
             |---------------------------------------------->|
             |                                               |
             |                     Answer                    |
             |<----------------------------------------------|
             |            Additional Candidates              |
             |<----------------------------------------------|
             |                                               |
             | Additional Candidates and Connectivity Checks |
             |<--------------------------------------------->|
             |                                               |
             |<=============== MEDIA FLOWS =================>|

                             Figure 8: Example

18.  IANA Considerations

   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 [RFC6336].

   ICE Option:  trickle

   Contact:  Emil Ivov, eivov@atlassian.com

   Change control:  IESG

   Description:  An ICE option of "trickle" indicates support for
      incremental communication of ICE candidates.

   Reference:  RFC XXXX

19.  Security Considerations

   This specification inherits most of its semantics from [rfc5245bis]
   and as a result all security considerations described there apply to
   Trickle ICE.

   If the privacy implications of revealing host addresses on an
   endpoint device are a concern, agents can generate candidate
   information that contain no candidates and then only trickle
   candidates that do not reveal host addresses (e.g., relayed
   candidates).

Ivov, et al.            Expires October 25, 2017               [Page 22]
Internet-Draft                 Trickle ICE                    April 2017

20.  Acknowledgements

   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, Thomas Stach, Peter Thatcher, Martin Thomson, Dale R.
   Worley, and Brandon Williams for their reviews and suggestions on
   improving this document.  Thanks also to Ari Keranen and Peter
   Thatcher for chairing the ICE Working Group.

21.  References

21.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [rfc5245bis]
              Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", draft-ietf-ice-
              rfc5245bis-09 (work in progress), April 2017.

21.2.  Informative References

   [I-D.ietf-mmusic-trickle-ice-sip]
              Ivov, E., Thomas, T., Marocco, E., and C. Holmberg, "A
              Session Initiation Protocol (SIP) usage for Trickle ICE",
              draft-ietf-mmusic-trickle-ice-sip-07 (work in progress),
              March 2017.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
              <http://www.rfc-editor.org/info/rfc1918>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264, June
              2002.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

Ivov, et al.            Expires October 25, 2017               [Page 23]
Internet-Draft                 Trickle ICE                    April 2017

   [RFC4787]  Audet, F., Ed. and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
              2007, <http://www.rfc-editor.org/info/rfc4787>.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,
              <http://www.rfc-editor.org/info/rfc5389>.

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.

   [RFC6336]  Westerlund, M. and C. Perkins, "IANA Registry for
              Interactive Connectivity Establishment (ICE) Options", RFC
              6336, DOI 10.17487/RFC6336, July 2011,
              <http://www.rfc-editor.org/info/rfc6336>.

   [XEP-0030]
              Hildebrand, J., Millard, P., Eatmon, R., and P. Saint-
              Andre, "XEP-0030: Service Discovery", XEP XEP-0030, June
              2008.

   [XEP-0176]
              Beda, J., Ludwig, S., Saint-Andre, P., Hildebrand, J.,
              Egan, S., and R. McQueen, "XEP-0176: Jingle ICE-UDP
              Transport Method", XEP XEP-0176, June 2009.

Appendix A.  Interaction with Regular ICE

   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 [rfc5245bis] does not by itself
   support trickle ICE.  This section describes how trickling of
   candidates interacts with ICE.

   [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:

      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.

Ivov, et al.            Expires October 25, 2017               [Page 24]
Internet-Draft                 Trickle ICE                    April 2017

   This could be a problem and cause ICE processing to fail prematurely
   in a number of scenarios.  Consider the following case:

   1.  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
       [RFC1918]).

   2.  Alice conveys Bob the candidate 172.16.0.1 which also happens to
       correspond to an existing host on Bob's network.

   3.  Bob creates a check list consisting solely of 172.16.0.1 and
       starts checks.

   4.  These checks reach the host at 172.16.0.1 in Bob's network, which
       responds with an ICMP "port unreachable" error; per [rfc5245bis]
       Bob marks the transaction as Failed.

   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.

   A similar race condition would occur if the initial candidate
   information 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).

   Another potential problem could arise when a non-trickle ICE
   implementation initiates an interaction with a Trickle ICE
   implementation.  Consider the following case:

   1.  Alice's client has a non-Trickle ICE implementation.

   2.  Bob's client has support for Trickle ICE.

   3.  Alice and Bob are behind NATs with address-dependent filtering
       [RFC4787].

   4.  Bob has two STUN servers but one of them is currently
       unreachable.

   After Bob's agent receives Alice's initial candidate information 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

Ivov, et al.            Expires October 25, 2017               [Page 25]
Internet-Draft                 Trickle ICE                    April 2017

   conveyed 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.

Appendix B.  Interaction with ICE Lite

   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 [rfc5245bis].  This section is hence provided
   only for informational purposes.

   An ICE lite agent would generate candidate information as per
   [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.

   When performing full trickle, a full ICE implementation could
   conveying initial candidate information 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:

           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|                                               |
              |                                               |
              |<=============== MEDIA FLOWS =================>|

                             Figure 9: Example

Ivov, et al.            Expires October 25, 2017               [Page 26]
Internet-Draft                 Trickle ICE                    April 2017

   In addition to reducing signaling traffic this approach also removes
   the need to discover STUN bindings or make TURN allocations, which
   may considerably lighten ICE processing.

Appendix C.  Changes from Earlier Versions

   Note to the RFC-Editor: please remove this section prior to
   publication as an RFC.

C.1.  Changes from draft-ietf-ice-trickle-09

   o  Reinstated text about in-order processing of messages as a
      requirement for signaling protocols.

   o  Added IANA registration template for ICE option.

   o  Corrected Case 3 rule in Section 8.1.1 to ensure consistency with
      regular ICE rules.

   o  Added tabular representations to Section 8.1.1 in order to
      illustrate the new pair rules.

C.2.  Changes from draft-ietf-ice-trickle-08

   o  Changed "ICE description" to "candidate information" for
      consistency with 5245bis.

C.3.  Changes from draft-ietf-ice-trickle-07

   o  Addressed editorial feedback from chairs review.

C.4.  Changes from draft-ietf-ice-trickle-06

   o  Clarified terminology regarding generations.

C.5.  Changes from draft-ietf-ice-trickle-05

   o  Rewrote the text on inserting a new pair into a check list.

C.6.  Changes from draft-ietf-ice-trickle-04

   o  Removed dependency on SDP and offer/answer model.

   o  Removed mentions of aggressive nomination, since it is deprecated
      in 5245bis.

   o  Added section on requirements for signaling protocols.

Ivov, et al.            Expires October 25, 2017               [Page 27]
Internet-Draft                 Trickle ICE                    April 2017

   o  Clarified terminology.

   o  Addressed various WG feedback.

C.7.  Changes from draft-ietf-ice-trickle-03

   o  Copy edit.

C.8.  Changes from draft-ietf-ice-trickle-03

   o  Provided more detailed description of unfreezing behavior,
      specifically how to replace pre-existing peer-reflexive candidates
      with higher-priority ones received via trickling.

C.9.  Changes from draft-ietf-ice-trickle-02

   o  Adjusted unfreezing behavior when there are disparate foundations.

C.10.  Changes from draft-ietf-ice-trickle-01

   o  Changed examples to use IPv6.

C.11.  Changes from draft-ietf-ice-trickle-00

   o  Removed dependency on SDP (which is to be provided in a separate
      specification).

   o  Clarified text about the fact that a check list can be empty if no
      candidates have been sent or received yet.

   o  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.

   o  Removed open issues list because it was out of date.

   o  Completed a thorough copy edit.

C.12.  Changes from draft-mmusic-trickle-ice-02

   o  Addressed feedback from Rajmohan Banavi and Brandon Williams.

   o  Clarified text about determining support and about how to proceed
      if it can be determined that the answering agent does not support
      Trickle ICE.

   o  Clarified text about check list and timer updates.

Ivov, et al.            Expires October 25, 2017               [Page 28]
Internet-Draft                 Trickle ICE                    April 2017

   o  Clarified when it is appropriate to use half trickle or to send no
      candidates in an offer or answer.

   o  Updated the list of open issues.

C.13.  Changes from draft-ivov-01 and draft-mmusic-00

   o  Added a requirement to trickle candidates by order of components
      to avoid deadlocks in the unfreezing algorithm.

   o  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.

   o  Limit the number of pairs to 100 to comply with 5245.

   o  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.

   o  Added transport expectations for trickled candidates as per Dale
      Worley's recommendation.

C.14.  Changes from draft-ivov-00

   o  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.

   o  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.

   o  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.

   o  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).

   o  It is now mandatory to use MID when trickling candidates and using
      m-line indexes is no longer allowed.

Ivov, et al.            Expires October 25, 2017               [Page 29]
Internet-Draft                 Trickle ICE                    April 2017

   o  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).

   o  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.

   o  Added more terminology, including trickling, trickled candidates,
      half trickle, full trickle,

   o  Added a reference to the SIP usage for Trickle ICE as requested at
      the Boston interim.

C.15.  Changes from draft-rescorla-01

   o  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.

   o  Added SDP specification for trickled candidates, the trickle
      option and 0.0.0.0 addresses in m-lines, and end-of-candidates.

   o  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.

   o  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.

   o  Added a short section about subsequent offer/answer exchanges.

   o  Added a short section about interactions with ICE Lite
      implementations.

   o  Added two new entries to the open issues section.

Ivov, et al.            Expires October 25, 2017               [Page 30]
Internet-Draft                 Trickle ICE                    April 2017

C.16.  Changes from draft-rescorla-00

   o  Relaxed requirements about verifying support following a
      discussion on MMUSIC.

   o  Introduced ICE descriptions in order to remove ambiguous use of
      3264 language and inappropriate references to offers and answers.

   o  Removed inappropriate assumption of adoption by RTCWEB pointed out
      by Martin Thomson.

Authors' Addresses

   Emil Ivov
   Atlassian
   303 Colorado Street, #1600
   Austin, TX  78701
   USA

   Phone: +1-512-640-3000
   Email: eivov@atlassian.com

   Eric Rescorla
   RTFM, Inc.
   2064 Edgewood Drive
   Palo Alto, CA  94303
   USA

   Phone: +1 650 678 2350
   Email: ekr@rtfm.com

   Justin Uberti
   Google
   747 6th St S
   Kirkland, WA  98033
   USA

   Phone: +1 857 288 8888
   Email: justin@uberti.name

Ivov, et al.            Expires October 25, 2017               [Page 31]
Internet-Draft                 Trickle ICE                    April 2017

   Peter Saint-Andre
   Filament
   P.O. Box 787
   Parker, CO  80134
   USA

   Phone: +1 720 256 6756
   Email: peter@filament.com
   URI:   https://filament.com/

Ivov, et al.            Expires October 25, 2017               [Page 32]