Network Working Group                                            E. Ivov
Internet-Draft                                                 Atlassian
Intended status: Standards Track                             E. Rescorla
Expires: January 21, 2017                                     RTFM, Inc.
                                                               J. Uberti
                                                          P. Saint-Andre
                                                           July 20, 2016

Trickle ICE: Incremental Provisioning of Candidates for the Interactive
               Connectivity Establishment (ICE) Protocol


   This document describes 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.  This mechanism is called "trickle ICE".

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

   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 January 21, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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.  Sending the Initial Offer . . . . . . . . . . . . . . . . . .   6
   5.  Receiving the Initial Offer . . . . . . . . . . . . . . . . .   6
     5.1.  Sending the Initial Answer  . . . . . . . . . . . . . . .   7
     5.2.  Forming Check Lists and Beginning Connectivity
           Checks  . . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Receiving the Initial Answer  . . . . . . . . . . . . . . . .   8
   7.  Performing Connectivity Checks  . . . . . . . . . . . . . . .   8
     7.1.  Scheduling Checks . . . . . . . . . . . . . . . . . . . .   8
     7.2.  Check List and Timer State Updates  . . . . . . . . . . .   9
   8.  Discovering and Sending Additional Local Candidates . . . . .   9
     8.1.  Pairing Newly Learned Candidates and Updating
           Check Lists . . . . . . . . . . . . . . . . . . . . . . .  12
     8.2.  Announcing End of Candidates  . . . . . . . . . . . . . .  13
   9.  Receiving Additional Remote Candidates  . . . . . . . . . . .  14
   10. Receiving an End-Of-Candidates Notification . . . . . . . . .  15
   11. Trickle ICE and Peer Reflexive Candidates . . . . . . . . . .  15
   12. Concluding ICE Processing . . . . . . . . . . . . . . . . . .  15
   13. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . . .  15
   14. Unilateral Use of Trickle ICE (Half Trickle)  . . . . . . . .  16
   15. Example Flow  . . . . . . . . . . . . . . . . . . . . . . . .  17
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   17. Security Considerations . . . . . . . . . . . . . . . . . . .  18
   18. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   19. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     19.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     19.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Interaction with ICE . . . . . . . . . . . . . . . .  20
   Appendix B.  Interaction with ICE Lite  . . . . . . . . . . . . .  21
   Appendix C.  Changes from Earlier Versions  . . . . . . . . . . .  22
     C.1.  Changes from draft-ietf-ice-trickle-02  . . . . . . . . .  22
     C.2.  Changes from draft-ietf-ice-trickle-01  . . . . . . . . .  22
     C.3.  Changes from draft-ietf-ice-trickle-00  . . . . . . . . .  22
     C.4.  Changes from draft-mmusic-trickle-ice-02  . . . . . . . .  23

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     C.5.  Changes from draft-ivov-01 and draft-mmusic-00  . . . . .  23
     C.6.  Changes from draft-ivov-00  . . . . . . . . . . . . . . .  23
     C.7.  Changes from draft-rescorla-01  . . . . . . . . . . . . .  24
     C.8.  Changes from draft-rescorla-00  . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

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

   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, discovering UPnP
   devices, 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 degraded user experience.

   This document defines an alternative 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 a session has been initiated.  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 the host candidates for one of the agents are directly
   reachable from the second agent, such as host candidates at a media
   relay).  Even when this is not the case, running candidate gathering
   for both agents and connectivity checks in parallel can considerably
   shorten ICE processing times.

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   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 so-called "Vanilla ICE" processing as defined in

   This specification does not define the usage of Trickle ICE with any
   specific signalling protocol (however, see
   [I-D.ietf-mmusic-trickle-ice-sip] for usage with SIP [RFC3261]).
   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

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   This specification makes use of all terminology defined for
   Interactive Connectivity Establishment in [rfc5245bis].

   Vanilla ICE:  The Interactive Connectivity Establishment protocol as
      defined in [rfc5245bis].

   Candidate Harvester:  A module used by an ICE agent to obtain local
      candidates.  Candidate gatherers use different mechanisms for
      discovering local candidates.  Some of them would typically make
      use of protocols such as STUN or TURN.  Others may also employ
      techniques that are not referenced within [rfc5245bis] (e.g., UPnP
      based port allocation or XMPP Jingle Relay Nodes [XEP-0278]).

   Trickled Candidates:  Candidates that a Trickle ICE agent sends after
      an offer or answer but within the same context.  Trickled
      candidates can be sent in parallel with candidate gathering and
      connectivity checks.

   Trickling/Trickle (v.):  The act of sending trickled candidates.

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   Half Trickle:  A Trickle ICE mode of operation where the offerer
      gathers its first generation of candidates strictly before
      creating and sending the offer.  Once sent, that offer can be
      processed by Vanilla ICE agents and does not require support for
      this specification.  It also allows Trickle ICE capable answerers
      to still gather candidates and perform connectivity checks in a
      non-blocking way, thus roughly offering "half" the advantages of
      Trickle ICE.  The mechanism is mostly meant for use in cases where
      support for trickle ICE cannot be confirmed prior to sending a
      initial offer.

   Full Trickle:  The regular mode of operation for Trickle ICE agents,
      in which an initial offer can include any number of candidates
      (even zero candidates) and does not need to include the entire
      first generation of candidates as in half trickle.

3.  Determining Support for Trickle ICE

   Application protocols that use Trickle ICE should do one of the

   o  Provide a way for agents to verify support of Trickle ICE prior to
      initiating a session (XMPP's Service Discovery [XEP-0030] is one
      such mechanism).

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

   Alternately, for cases where a protocol provides neither of the
   foregoing methods, agents may rely on provisioning/configuration or
   use the half trickle procedure described in Section 14.

   Prior to sending an initial offer, 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 Vanilla ICE or abandon the entire session.

   In application protocols that use SDP, a user agent supporting
   Trickle ICE MUST include a token of "trickle" in the ice-options
   attribute every time it generates an offer or an answer.  This
   enables an agent that receives offers or answers to verify support by
   checking for presence of the token.

   Dedicated discovery semantics and half trickle are needed only prior
   to session initiation (e.g., when sending the initial offer).  After
   a session is established and Trickle ICE support is confirmed for

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   both parties, either agent can use full trickle for subsequent

4.  Sending the Initial Offer

   An agent starts gathering candidates as soon as it has an indication
   that communication is imminent (e.g., a user interface cue or an
   explicit request to initiate a session).  Contrary to Vanilla ICE,
   implementations of Trickle ICE do not need to gather candidates in a
   blocking manner.  Therefore, unless half trickle is being used,
   agents SHOULD generate and transmit their initial offer as early as
   possible, in order to allow the remote party to start gathering and
   trickling candidates.

   Trickle ICE agents MAY include any set of candidates in an offer.
   This includes the possibility of sending an offer that contains all
   the candidates that the agent plans to use (as in half trickle mode),
   sending an offer that contains only a publically-reachable IP address
   (e.g., a host candidate at a media relay that is known to not be
   behind a firewall), or sending an offer with no candidates at all (in
   which case the offerer can receive the answerer's initial candidate
   list sooner and the answerer can begin candidate gathering more

   For optimal performance, it is RECOMMENDED that the candidates in an
   initial offer (if any) be host candidates only.  This would allow
   both agents to start gathering server reflexive, relayed, and other
   non-host candidates simultaneously, and it would also enable them to
   begin connectivity checks.

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

   Methods for calculating priorities and foundations, as well as
   determining redundancy of candidates, work just as with vanilla ICE.

5.  Receiving the Initial Offer

   When an agent receives an initial offer, it will first check if the
   offer or offerer indicates support for Trickle ICE as explained in
   Section 3.  If this is not the case, the agent MUST process the offer
   according to Vanilla ICE procedures [rfc5245bis] or offer/answer
   processing rules [RFC3264] if no ICE support is detected at all.

   If support for Trickle ICE is confirmed, an agent will automatically
   assume support for Vanilla ICE as well even if the support

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   verification procedure in [rfc5245bis] indicates otherwise.
   Specifically, the rules from [rfc5245bis] would imply that ICE itself
   is not supported if the initial offer includes no candidates in the
   offer; however, such a conclusion is not warranted if the answerer
   can confirm that the offerer supports Trickle ICE and thus fallback
   to [RFC3264] is not necessary.

   If the offer does indicate support for Trickle ICE, the agent will
   determine its role, start gathering and prioritizing candidates and
   while doing so it will also respond by sending its own answer, so
   that both agents can start forming check lists and begin connectivity

5.1.  Sending the Initial Answer

   An agent can respond to an initial offer at any point while gathering
   candidates.  The answer can again contain any set of candidates,
   including all candidates or no candidates.  (The benefit of including
   no candidates is to send the answer as quickly as possible, so that
   both parties can consider the overall session to be under active
   negotiation as soon as possible.)  Unless the answering agent is
   protecting host addresses for privacy reasons, it would typically
   construct this initial answer including only host addresses, thus
   enabling the remote party to also start forming check lists and
   performing connectivity checks.

   In application protocols that use SDP, the answer MUST indicate
   support for Trickle ICE as described in Section 3.

5.2.  Forming Check Lists and Beginning Connectivity Checks

   After exchanging the offer and answer, and as soon as they have
   obtained local and remote candidates, agents begin forming candidate
   pairs, computing candidate pair priorities and ordering candidate
   pairs, pruning duplicate pairs, and creating check lists according to
   the Vanilla ICE procedures described in [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 both the offer and the answer.  Under
   Trickle ICE, check lists can be empty until candidate pairs are sent
   or received.  Therefore Trickle ICE agents handle check lists and
   candidate pairing in a slightly different way: the agents still
   create the check lists, but they only populate the check lists after
   they actually have the candidate pairs.

      Note: According to [rfc5245bis], "A check list with at least one
      pair that is Waiting is called an active check list, and a check

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      list with all pairs Frozen is called a frozen check list."
      Formally speaking an active check list does not have a state of
      Active and a frozen check list does not have a state of Frozen,
      because the only check list states are Running, Completed, and

   A Trickle ICE agent initially considers all check lists to be frozen.
   It then inspects the first check list and attempts to unfreeze all
   candidates 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.

   With regard to pruning of duplicate candidate pairs, a Trickle ICE
   agent SHOULD follow a policy of "first one wins" and not re-apply the
   pruning procedure if a higher-priority candidate pair is received
   from the remote agent.

   Respecting the order in which check lists have been reported to an
   ICE implementation is crucial to the frozen candidates algorithm, so
   that connectivity checks are performed simultaneously by both agents.

6.  Receiving the Initial Answer

   When receiving an answer, agents follow Vanilla ICE procedures to
   determine their role, after which they form check lists (as described
   in Section 5.2) and begin connectivity checks.

7.  Performing Connectivity Checks

   For the most part, Trickle ICE agents perform connectivity checks
   following Vanilla ICE procedures.  However, the asynchronous nature
   of gathering and communicating candidates in Trickle ICE impose a
   number of changes described as described in the following sections.

7.1.  Scheduling Checks

   The ICE specification [rfc5245bis], Section 5.8, 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 check
   list.  This will not work with Trickle ICE, because more pairs will
   be added to the check list incrementally.

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   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, requires that
   agents update check lists and timer states upon completing a
   connectivity check transaction.  During such an update, Vanilla ICE
   agents would set the state of a check list to Failed if both of the
   following two conditions are satisfied:

   o  all of the pairs in the check list are either in the Failed 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

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

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

   Vanilla ICE requires that agents then update all other check lists,
   placing one pair from each of them into the Waiting state,
   effectively unfreezing all remaining check lists.  However, under
   Trickle ICE other check lists might still be empty at that point.
   Therefore a Trickle ICE agent SHOULD monitor whether a check list is
   active or frozen independently of the state of the candidate pairs
   that the check list contains.  A Trickle ICE agent SHOULD consider a
   check list to be active either when unfreezing the first candidate
   pair in the check list or when there is no candidate pair in the
   check list (i.e., when the check list is empty).

8.  Discovering and Sending Additional Local Candidates

   After an offer or an answer has been sent, 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

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   priority, type, foundation and component ID according to normal
   Vanilla 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
   Vanilla ICE, Trickle ICE agents will consider the new candidate
   redundant regardless of its priority.

   Next the agent sends (i.e., trickles) the newly discovered
   candidate(s) to the remote agent.  The actual delivery of the new
   candidates are specified by using protocols such as SIP or XMPP.
   Trickle ICE imposes no restrictions on the way this is done or
   whether it is done at all.  For example, some applications may choose
   not to send trickle updates for server reflexive candidates and rely
   on the discovery of peer reflexive ones instead.

   When trickle updates are sent, each candidate MUST be delivered to
   the receiving Trickle ICE implementation not more than once and in
   the same order that they were sent.  In other words, if there are any
   candidate retransmissions, they must be hidden from the ICE

   Also, candidate trickling needs to be correlated to a specific ICE
   negotiation 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 candidates via SDP may include a ufrag value in the SDP that
   represents candidates such as:

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

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

   One important aspect of Vanilla 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 crucial to
   unfreezing candidates in the right time.

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   In order to preserve this feature in Trickle ICE, when trickling
   candidates agents MUST respect the order of the components as they
   appear (implicitly or explicitly) in the offer/answer descriptions.
   Therefore a candidate for a specific component MUST NOT be sent prior
   to candidates for other components within the same foundation.

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

     o=jdoe 2890844526 2890842807 IN IP6 2001:db8:a0b:12f0::1
     c=IN IP4 2001:db8:a0b:12f0::1
     t=0 0
     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 description the RTCP host candidate MUST NOT be sent prior
   to the RTP host candidate.  Similarly the RTP server reflexive
   candidate MUST be sent together with or prior to the RTCP server
   reflexive candidate.

   Note that the order restriction only applies among candidates that
   belong to the same foundation.

   It is also equally important to preserve this order across media
   streams, which is covered by the requirement to always start
   unfreezing candidates starting from the first media stream as
   described under Section 5.2.

   Once the candidate has been sent to the remote party, the agent
   checks if any remote candidates are currently known for this same
   stream.  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

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   new local candidates with them and adding the pairs to the existing
   check lists according to their priority.

8.1.  Pairing Newly Learned Candidates and Updating Check Lists

   Forming candidate pairs works the way it is described by 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 its type 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 the pre-existing pair
   is peer reflexive, the agent MUST replace it with the new candidate
   it received, regardless of their priorities.

      Note: Replacing pre-existing pairs with seemingly equivalent
      higher-priority ones helps guarantee that both agents will have
      the same view of candidate priorities.  This is particularly
      important during aggressive nomination, when priority is sometimes
      the only way a controlled agent can determine the selected pair.
      It is for that same reason that peer-reflexive candidates need to
      always be updated if equivalent alternatives are received through

   For all other pairs, including those with a server reflexive local
   candidate that were not found to be redundant:

   o  if this check list is frozen then the new pair will be assigned a
      state of Frozen.

   o  else if the check list is active and it is either empty or
      contains only candidates in the Succeeded and Failed states, then
      the new pair's state is set to Waiting.

   o  else if the check list is non-empty and active, then the state of
      the new pair will be set to

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      Frozen:   if there is at least one pair in the check list whose
         foundation matches the one in the new pair and whose state is
         neither Succeeded nor Failed (eventually the new pair will get
         unfrozen after the ongoing check for the existing pair

      Waiting:   if the list contains no pairs with the same foundation
         as the new one, or, in case such pairs exist but they are all
         in either the Succeeded or Failed states.

8.2.  Announcing End of Candidates

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

   o  As part of an offer (which would typically be the case with half
      trickle initial offers)

   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 timeout and the agent has no
      other active gatherers)

   A controlled Trickle ICE agent SHOULD send end-of-candidates
   indications after gathering for a media stream has completed, unless
   ICE processing terminates before the agent has had a chance to do so.
   Sending the indication is necessary in order to avoid ambiguities and
   speed up the conclusion of ICE processing.  On the other hand, a
   controlling agent MAY conclude ICE processing prior to sending end-
   of-candidates indications for all streams.  This would typically be
   the case with aggressive nomination.  However, it is RECOMMENDED that
   controlling agents do send such indications whenever possible for the
   sake of consistency and to keep middle boxes and controlled agents
   up-to-date on the state of ICE processing.

   When sending an end-of-candidate indication during trickling (rather
   than as a part of an offer or an answer), 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

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   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 implementations may choose to wait and see if higher-
   priority candidates are received; in this case the end-of-candidates
   indication provides a notificaiton that such candidates are not

   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 send any more candidates
   after it has send an end-of-candidates indication.

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

   When an end-of-candidates indication is sent as part of an offer or
   an answer, it can be considered to apply to the session as a whole,
   which is equivalent to having it apply to all media streams.

   After an agent sends 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 send 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

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

9.  Receiving Additional Remote Candidates

   At any point of ICE processing, a Trickle ICE agent may receive new
   candidates from the remote agent.  When this happens and no local
   candidates are currently known for this same stream, the new remote
   candidates are simply added to the list of remote candidates.

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   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 send
   an end-of-candidates indication.  Upon receiving such an indication,
   the local agent MUST update check list states as per Section 7.2.
   This may lead to some check lists being marked as Failed.

10.  Receiving an End-Of-Candidates Notification

   When an agent receives an end-of-candidates indication for a specific
   check list, it will update the state of the check list as per
   Section 7.2.  If the check list is still active state after the
   update, the agent will persist the 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, their processing could be
   impacted in implementations.  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 signalling path.

12.  Concluding ICE Processing

   This specification does not directly modify the procedures ending ICE
   processing described in Section 8 of [rfc5245bis], and Trickle ICE
   implementations will follow the same rules.

13.  Subsequent Offer/Answer Exchanges

   Either agent MAY generate a subsequent offer at any time allowed by
   [RFC3264].  When this happens agents will use [rfc5245bis] semantics
   to determine whether or not the new offer requires an ICE restart.

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   If this is the case then agents would perform Trickle ICE as they
   would in an initial offer/answer exchange.

   The only differences between an ICE restart and a brand new media
   session are that:

   o  during the restart, media can continue to be sent to the
      previously validated pair.

   o  both agents are already aware whether or not their peer supports
      Trickle ICE, and there is no longer need for performing half
      trickle or confirming support with other mechanisms.

14.  Unilateral Use of Trickle ICE (Half Trickle)

   In half trickle mode, the offerer sends a regular, Vanilla ICE offer,
   with a complete set of candidates.  This ensures that the offer can
   be processed by a Vanilla ICE answerer and is mostly meant for use in
   cases where support for Trickle ICE cannot be confirmed prior to
   sending an initial offer.  The initial offer indicates support for
   Trickle ICE, so that the answerer can respond with an incomplete set
   of candidates and continue trickling the rest.  Half trickle offers
   typically contain an end-of-candidates indication, although this is
   not mandatory because if trickle support is confirmed then the
   offerer can choose to trickle additional candidates before it sends
   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 offer contains a full set of
   candidates, it can thus be handled by a regular Vanilla 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 offer/answer
   exchange.  After both parties have received a session description
   from their peer, they can each reliably determine Trickle ICE support
   and use it for all subsequent offer/answer 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 offer, 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 send the offer.  Because the answerer will be able to

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   trickle candidates, both agents will be able to start connectivity
   checks and complete ICE processing earlier than with Vanilla 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
   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 vanilla ICE because it would result in a better experience for

15.  Example Flow

   A typical successful Trickle ICE exchange with an Offer/Answer
   protocol would look this way:

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

                             Figure 1: Example

16.  IANA Considerations

   This specification requests no actions from IANA.

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17.  Security Considerations

   This specification inherits most of its semantics from [rfc5245bis]
   and as a result all security considerations described there remain
   the same.

18.  Acknowledgements

   The authors would like to thank Taylor Brandstetter for identifying
   the need to replace pre-existing peer-reflexive candidates with
   higher-priority ones received from trickling and the fact that not
   doing so could break aggressive nomination.

   The authors would also like to thank Bernard Aboba, Flemming
   Andreasen, Rajmohan Banavi, Christer Holmberg, Jonathan Lennox,
   Enrico Marocco, Pal Martinsen, Martin Thomson, Dale R.  Worley, and
   Brandon Williams for their reviews and suggestions on improving this

19.  References

19.1.  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,

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <>.

              Keranen, A. and J. Rosenberg, "Interactive Connectivity
              Establishment (ICE): A Protocol for Network Address
              Translator (NAT) Traversal", draft-ietf-ice-rfc5245bis-00
              (work in progress), October 2015.

19.2.  Informative References

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              Ivov, E., Thomas, T., Marocco, E., and C. Holmberg, "A
              Session Initiation Protocol (SIP) usage for Trickle ICE",
              draft-ietf-mmusic-trickle-ice-sip-04 (work in progress),
              May 2016.

   [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,

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,

   [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, <>.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008,

   [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,
              DOI 10.17487/RFC5766, April 2010,

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

              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.

              Camargo, T., "XEP-0278: Jingle Relay Nodes", XEP XEP-0278,
              June 2011.

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Appendix A.  Interaction with 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.

   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 [RFC1918] block.

   2.  Alice sends Bob the candidate 2001:db8:a0b:12f0::10 which also
       happens to correspond to an existing host on Bob's network.

   3.  Bob creates a check list consisting solely of
       2001:db8:a0b:12f0::10 and starts checks.

   4.  These checks reach the host at 2001:db8:a0b:12f0::10 in Bob's
       network, which responds with an ICMP "port unreachable" error and
       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 offer from Alice
   only contains 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 sends an offer to a trickle one.  Consider the
   following case:

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

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

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

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

   After Bob's agent receives Alice's offer 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 sent to Alice, Bob's connectivity
   checks may well have failed: until Alice gets Bob's answer, she won't
   be able to start connectivity checks and punch holes in her NAT.  The
   NAT would hence be filtering Bob's checks as originating from an
   unknown endpoint.

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.

   Such an agent would generate offers or answers as per [rfc5245bis].
   Both its offers and answers will indicate support for Trickle ICE.
   Given that they will contain a complete set of candidates (the
   agent's host candidates), these offers and answers would also be
   accompanied with an end-of-candidates indication.

   When performing full trickle, a full ICE implementation could send an
   offer or an answer with no candidates.  After receiving an answer
   that identifies the remote agent as an ICE lite implementation, the
   offerer may choose to not send any additional candidates.  The same
   is also true in the case when the ICE lite agent is making the offer
   and the full ICE one is answering.  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:

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           ICE Lite                                          Bob
              |   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 2: Example

   In addition to reducing signaling traffic this approach also removes
   the need to discover STUN bindings, or to make TURN or UPnP
   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-02

   o  Adjusted unfreezing behavior when there are disparate foundations.

C.2.  Changes from draft-ietf-ice-trickle-01

   o  Changed examples to use IPv6.

C.3.  Changes from draft-ietf-ice-trickle-00

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

   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.

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   o  Completed a thorough copy edit.

C.4.  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.

   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.5.  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.6.  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

   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.

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   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 signalling 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.

   o  Replaced use of to IP6 :: in order to avoid potential
      issues with RFC2543 SDP libraries that interpret 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.7.  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 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

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      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

   o  Added two new entries to the open issues section.

C.8.  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
   303 Colorado St#1600
   Austin  78701

   Phone: +1-512-640-3000

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

   Phone: +1 650 678 2350

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   Justin Uberti
   747 6th St S
   Kirkland, WA  98033

   Phone: +1 857 288 8888

   Peter Saint-Andre


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