Network Working Group                                            E. Ivov
Internet-Draft                                                     Jitsi
Intended status: Standards Track                             E. Rescorla
Expires: April 21, 2016                                       RTFM, Inc.
                                                               J. Uberti
                                                          P. Saint-Andre
                                                        October 19, 2015

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 allows 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 April 21, 2016.

Copyright Notice

   Copyright (c) 2015 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
     4.1.  Encoding the SDP  . . . . . . . . . . . . . . . . . . . .   7
   5.  Receiving the Initial Offer . . . . . . . . . . . . . . . . .   7
     5.1.  Sending the Initial Answer  . . . . . . . . . . . . . . .   8
     5.2.  Forming Check Lists and Beginning Connectivity
           Checks  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.3.  Encoding the SDP  . . . . . . . . . . . . . . . . . . . .   9
   6.  Receiving the Initial Answer  . . . . . . . . . . . . . . . .   9
   7.  Performing Connectivity Checks  . . . . . . . . . . . . . . .   9
     7.1.  Scheduling Checks . . . . . . . . . . . . . . . . . . . .   9
     7.2.  Check List and Timer State Updates  . . . . . . . . . . .   9
   8.  Discovering and Sending Additional Local Candidates . . . . .  10
     8.1.  Pairing Newly Learned Candidates and Updating
           Check Lists . . . . . . . . . . . . . . . . . . . . . . .  12
     8.2.  Encoding the SDP for Additional Candidates  . . . . . . .  13
     8.3.  Announcing End of Candidates  . . . . . . . . . . . . . .  14
   9.  Receiving Additional Remote Candidates  . . . . . . . . . . .  15
   10. Receiving an End-Of-Candidates Notification . . . . . . . . .  16
   11. Trickle ICE and Peer Reflexive Candidates . . . . . . . . . .  16
   12. Concluding ICE Processing . . . . . . . . . . . . . . . . . .  16
   13. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . . .  16
   14. Interaction with ICE Lite . . . . . . . . . . . . . . . . . .  17
   15. Unilateral Use of Trickle ICE (Half Trickle)  . . . . . . . .  18
   16. Example Flow  . . . . . . . . . . . . . . . . . . . . . . . .  19
   17. Security Considerations . . . . . . . . . . . . . . . . . . .  20
   18. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   19. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     19.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     19.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Appendix A.  Open Issues  . . . . . . . . . . . . . . . . . . . .  22
     A.1.  MID/Stream Indices in SDP . . . . . . . . . . . . . . . .  22
     A.2.  Starting Checks . . . . . . . . . . . . . . . . . . . . .  23
     A.3.  Checklist States  . . . . . . . . . . . . . . . . . . . .  23

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     A.4.  Relationship to Continuous Nomination and
           Passive Nomination  . . . . . . . . . . . . . . . . . . .  23
     A.5.  ICE Restarts  . . . . . . . . . . . . . . . . . . . . . .  23
     A.6.  Candidate Redundancy and Priority . . . . . . . . . . . .  23
     A.7.  Make Trickle ICE SDP-Agnostic . . . . . . . . . . . . . .  23
   Appendix B.  Interaction with ICE . . . . . . . . . . . . . . . .  23
   Appendix C.  Changes from Earlier Versions  . . . . . . . . . . .  25
     C.1.  Changes from draft-mmusic-trickle-ice-02  . . . . . . . .  25
     C.2.  Changes from draft-ivov-01 and draft-mmusic-00  . . . . .  25
     C.3.  Changes from draft-ivov-00  . . . . . . . . . . . . . . .  26
     C.4.  Changes from draft-rescorla-01  . . . . . . . . . . . . .  26
     C.5.  Changes from draft-rescorla-00  . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   The Interactive Connectivity Establishment (ICE) protocol [RFC5245]
   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 the above 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 the following session.

   While the above sequence has the advantage of being relatively
   straightforward to implement and debug once deployed, it may also
   prove to be rather lengthy.  Gathering candidates or candidate
   gathering often involves things like querying STUN [RFC5389] servers,
   discovering UPnP devices, and allocating relayed candidates at TURN
   [RFC5766] servers.  All of these can be delayed for a noticeable
   amount of time and while they can be run in parallel, they still need
   to respect the pacing requirements from [RFC5245], which is likely to
   delay them even further.  Some or all of the above would also have to
   be completed by the remote agent.  Both agents would next perform
   connectivity checks and only then would they be ready to begin
   streaming media.

   All of the above can lead to relatively lengthy session establishment
   times and degraded user experience.

   The purpose of this document is to define an alternative mode of
   operation for ICE implementations, also known as "trickle ICE", where
   candidates can be exchanged incrementally.  This would allow ICE
   agents to exchange candidates as soon as a session has been
   initiated.  Connectivity checks for a media stream would also start
   as soon as the first candidates for that stream have become

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   Trickle ICE allows reducing 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).  Even when this is not the case,
   running candidate gathering for both agents and connectivity checks
   all in parallel allows to considerably reduce ICE processing times.

   It is worth pointing out that before being introduced to the IETF,
   trickle ICE had already been included in specifications such as XMPP
   Jingle [XEP-0176] and it has been in use in various implementations
   and deployments.

   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 [RFC5245] processing.

   This specification does not define usage of trickle ICE with any
   specific signalling protocol, different from [RFC5245] which contains
   a usage for ICE with SIP [RFC3261].  Such usages would have to be
   specified in separate documents such as for example
   [I-D.ietf-mmusic-trickle-ice-sip].  However, trickle ICE does however
   reuse and build upon the SDP syntax defined by [RFC5245].

   Although this document mostly describes trickle ICE in terms of the
   offer/answer model [RFC3264], trickle ICE (and ICE itself) can be
   used by application protocols that do not follow the 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 by the
   protocol for Interactive Connectivity Establishment in [RFC5245].

   Vanilla ICE:  The Interactive Connectivity Establishment protocol as
      defined in [RFC5245].  Through the rest of the text, the terms
      vanilla ICE and "RFC5245" are used interchangeably.

   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 [RFC5245].  UPnP based

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      port allocation and XMPP Jingle Relay Nodes [XEP-0278] are among
      the possible examples.

   Trickled Candidates:  Candidates that a trickle ICE agent is sending
      subsequently to but within the context defined by an offer or an
      answer.  Trickled candidates can be sent in parallel with
      candidate gathering and connectivity checks.

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

   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:  Regular mode of operation for trickle ICE agents, used
      in opposition to the half trickle mode of operation.

3.  Determining Support for Trickle ICE

   According to [RFC5245], supported features are to be advertised in
   the ice-options attribute.  Therefore an agent supporting trickle ICE
   MUST include a token of "trickle" in the ice-options attribute every
   time it generates an offer or an answer.  Syntax for this token is
   defined in Section 4.1.

   Agents that receive offers or answers can verify support by examining
   them for the "trickle" ice-options token.  However, agents that are
   about to send an initial offer have no way of doing this.  Thus
   usages of trickle for specific protocols need to either:

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

   o  Make support for trickle ICE mandatory so that support could be
      assumed the agents.

   Alternately, for cases where a protocol provides neither of the
   above, agents may either rely on provisioning/configuration, or use
   the half trickle procedure described in Section 15.

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   Prior to sending an initial offer, agents using signaling protocols
   that support capabilities discovery MAY 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.

   All trickle ICE offers and answers MUST indicate support of this
   specification, as explained in Section 4.1.

   Note that out-of-band discovery semantics and half trickle are only
   necessary prior to session initiation, or in other words, when
   sending the initial offer.  Once a session is established and trickle
   ICE support is confirmed for both parties, either agent can use full
   trickle for subsequent offers.

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 generating one with no candidates,
   or one that contains all the candidates that the agent is planning on
   using in the following session.

   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 are a
   concern, agents MAY 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.

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4.1.  Encoding the SDP

   The process of encoding the SDP [RFC4566] is mostly the same as the
   one used by vanilla ICE.  Still, trickle ICE does require a few
   differences described here.

   Agents MUST indicate support for Trickle ICE by including the
   "trickle" token for the "a=ice-options" attribute:


   As mentioned earlier in this section, offers and answers can contain
   any set of candidates, which means that a trickle ICE session
   description MAY contain no candidates at all.  Doing so enables the
   offerer to receive the answerer's initial candidate list sooner, and
   also enables the answerer to begin candidate gathering more quickly.
   In such cases the agent would still need to place an address in the
   "c=" line(s).  If the use of a host address there is undesirable
   (e.g., for privacy reasons), the agent MAY set the connection address
   to  In this case it MUST also set the port number to 9
   (Discard).  There is no need to include a fictitious candidate for
   the address when doing so.

   It is worth noting that the use of IP6 :: has been selected over IP4, even though [RFC3264] already gives the latter semantics
   appropriate for such use.  The reason for this choice is the historic
   use of as a means of putting a stream on hold [RFC2543] and
   the ambiguity that this may cause with legacy libraries and

   It is also worth mentioning that use of IP6 :: here does not
   constitute any kind of indication as to the actual use of IPv6
   candidates in a session and it can very well appear in a negotiation
   that only involves IPv4 candidates.

5.  Receiving the Initial Offer

   When an agent receives an initial offer, it will first check if it
   indicates support for trickle ICE as explained in Section 3.  If this
   is not the case, the agent MUST process the offer according to the
   [RFC5245] procedures or standard [RFC3264] processing in case no ICE
   support is detected at all.

   It is worth pointing out that in case support for trickle ICE is
   confirmed, an agent will automatically assume support for vanilla ICE
   as well even if the support verification procedure in [RFC5245]

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   indicates otherwise.  Specifically, the rules from RFC 5245 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.  In this case, the IP6 :: address present in the "c="
   line would not "appear in a candidate attribute".  Fallback to
   [RFC3264] is not necessary in this scenario.

   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 them, thus allowing the
   remote party to also start forming checklists and performing
   connectivity checks.

   The answer MUST indicate support for trickle ICE as described by
   Section 3.

5.2.  Forming Check Lists and Beginning Connectivity Checks

   After exchanging offer and answer, and as soon as they have obtained
   local and remote candidates, agents will begin forming candidate
   pairs, computing their priorities and creating check lists according
   to the vanilla ICE procedures described in [RFC5245].  Obviously in
   order for candidate pairing to be possible, candidates would need to
   be provided in both the offer and the answer.  If not, then the
   agents will still create the check lists (so that their Active/Frozen
   state could be monitored and updated) but they will only populate the
   check lists once they actually have the candidate pairs.

   Initially, all check lists will have their Active/Frozen state set to

   Trickle ICE agents will then inspect the first check list and attempt
   to unfreeze all candidates belonging to the first component on the
   first media stream (i.e. the first media stream that was reported to

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   the ICE implementation from the using application).  If this
   checklist is still empty however, agents will hold off further
   processing until this is no longer the case.

   Respecting the order in which lists have been reported to an ICE
   implementation, or in other words, the order in which they appear in
   SDP, is crucial to the frozen candidates algorithm and important when
   making sure that connectivity checks are performed simultaneously by
   both agents.

5.3.  Encoding the SDP

   The process for encoding the SDP at the answerer is identical to the
   process followed by the offerer for both full and lite
   implementations, as described in Section 4.1.

6.  Receiving the Initial Answer

   When receiving an answer, agents will follow vanilla ICE procedures
   to determine their role and they would then 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.  Of course, the asynchronous nature
   of gathering and communicating candidates in trickle ICE would impose
   a number of changes described here.

7.1.  Scheduling Checks

   The ICE specification [RFC5245], 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.

   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 8.3).

7.2.  Check List and Timer State Updates

   The ICE specification [RFC5245], 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:

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   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 8.3.

   Vanilla ICE requires that agents then update all other check lists,
   placing one pair in each of them into the Waiting state, effectively
   unfreezing all remaining check lists.  Given that with trickle ICE,
   other check lists may still be empty at that point, a trickle ICE
   agent SHOULD also maintain an explicit Active/Frozen state for every
   check list, rather than deducing it from the state of the pairs it
   contains.  This state should be set to Active when unfreezing the
   first pair in a list or when that couldn't happen because a list was

8.  Discovering and Sending Additional Local Candidates

   After an offer or an answer have 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
   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 client sends (i.e. trickles) the newly learnt candidate(s)
   to the remote agent.  The actual delivery of the new candidates will

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   be specified by using protocols such as SIP.  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 however, 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 implementation.

   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.

   One important aspect of Vanilla ICE is that connectivity checks for a
   specific foundation and component be 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.

   In order to preserve this feature here, 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 session description contains two
   components (RTP and RTCP), and two foundations (host and the server

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     o=jdoe 2890844526 2890842807 IN IP4
     c=IN IP4
     t=0 0
     m=audio 5000 RTP/AVP 0
     a=rtpmap:0 PCMU/8000
     a=candidate:1 1 UDP 2130706431 5000 typ host
     a=candidate:1 2 UDP 2130706431 5001 typ host
     a=candidate:2 1 UDP 1694498815 5000 typ srflx
         raddr rport 8998
     a=candidate:2 2 UDP 1694498815 5001 typ srflx
         raddr 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 and this is covered by the requirement to always start
   unfreezing candidates starting from the first media stream
   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 this is not the case 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.

8.1.  Pairing Newly Learned Candidates and Updating Check Lists

   Forming candidate pairs will work the way it is described by the ICE
   specification [RFC5245].  Actually adding the new pair to a check
   list however, will happen according to the rules described below.

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   If the check list where the pair is to be added already contains the
   maximum number of candidate pairs (100 by default as per [RFC5245]),
   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, the newly formed pair is ignored.

   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 also be
      assigned a Frozen state.

   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 new pair
      state will be set to

      Frozen:   if there is at least one pair in the 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 the on-going 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.  Encoding the SDP for Additional Candidates

   To facilitate interoperability an ICE agent will encode additional
   candidates using the vanilla ICE SDP syntax.  For example:

       a=candidate:2 1 UDP 1658497328 5000 typ host

   Given that such lines do not provide a relationship between the
   candidate and the m line that it relates to, signalling protocols
   using trickle ICE MUST establish that relation themselves using an
   MID [RFC3388].  Such MIDs use "media stream identification", as
   defined in [RFC3388], to identify a corresponding m-line.  When
   creating candidate lines usages of trickle ICE MUST use the MID if

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   possible, or the m-line index if not.  Obviously, agents MUST NOT
   send individual candidates prior to generating the corresponding SDP
   session description.

   The exact means of transporting additional candidates to a remote
   agent is left to the protocols using trickle ICE.  It is important to
   note, however, that these candidate exchanges are not part of the
   offer/answer model.

8.3.  Announcing End of Candidates

   Once all candidate gatherers for a specific media stream complete, or
   expire, the agents will generate an "end-of-candidates" indication
   for that stream and send it to the remote agent via the signalling
   channel.  Such indications are sent in the form of a media-level
   attribute that has the following form: end-of-candidates.


   The end-of-candidates indications can be sent in the following ways:

   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)

   Controlled trickle ICE agents SHOULD always send end-of-candidates
   indications once gathering for a media stream has completed unless
   ICE processing terminates before they've had a chance to do so.
   Sending the indication is necessary in order to avoid ambiguities and
   speed up ICE conclusion.  Controlling agents on the other hand MAY
   sometimes conclude ICE processing prior to sending end-of-candidates
   notifications for all streams.  This would typically be the case with
   aggressive nomination.  Yet it is RECOMMENDED that controlling agents
   do send such indications whenever possible for the sake of
   consistency and keeping middle boxes and controlled agents up-to-date
   on the state of ICE processing.

   When sending end-of-candidates during trickling, rather than as a
   part of an offer or an answer, it is the responsibility of the using
   protocol to define means that can be used to relate the indication to
   one or more specific m-lines.

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   Receiving an end-of-candidates notification allows 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 allows agents to speed ICE conclusion in cases where
   a candidate pair has been validates but it involves the use of lower-
   preference transports such as TURN.  In such situations some
   implementations may choose to wait in case higher-priority candidates
   are received and end-of-candidates provides an indication that this
   is not going to happen.

   An agent MAY also choose to generate an end-of-candidates event
   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 notification.

   When performing half trickle agents SHOULD send end-of-candidates
   together with their initial offer unless they are planning on
   potentially sending additional candidates in case the remote party
   turns out to actually support trickle ICE.

   When end-of-candidates is sent as part of an offer or an answer it
   can appear as a session-level attribute, which would be equivalent to
   having it appear in all m-lines.

   Once an agent sends the end-of-candidates event, it will update the
   state of the corresponding check list as explained in Section 7.2.
   Past that point agents MUST NOT send any new candidates within this
   ICE session.  Once an agent has received an end-of-candidates
   indication, it MUST also ignore any newly received candidates for
   that media stream, and adding new candidates to the negotiation is
   only possible through an ICE restart.

   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 event.  Upon receiving such an event, 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 notification for a
   specific check list, they will update its state as per Section 7.2.
   In case the list is still in the Active state after the update, the
   agent will persist the the fact that an end-of-candidates
   notification has been received for and take it into account in future
   list updates.

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 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 [RFC5245], 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 [RFC5245] semantics to
   determine whether or not the new offer requires an ICE restart.  If

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   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.  Interaction with ICE Lite

   Behaviour of Trickle ICE capable ICE lite agents does not require any
   particular rules other than those already defined in this
   specification and [RFC5245].  This section is hence added with an
   informational purpose only.

   A Trickle ICE capable ICE Lite agent would generate offers or answers
   as per [RFC5245].  Both will indicate support for trickle ICE
   (Section 4.1) and 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 notification.

   When performing full trickle, a full ICE implementation could send an
   offer or an answer with no candidates and an IP6 :: connection line
   address.  After receiving an answer that identifies the remote agent
   as an ICE lite implementation, the offerer may very well 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:

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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 1: 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.

15.  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 a 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
   declares end of trickling.

   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 it contains a full set of candidates, its
   initial offer can thus be handled by a regular vanilla ICE agent,
   while still allowing a trickle one to use the optimisation 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.  Once both parties have received a session description from

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   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 that the answerer will be able to
   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 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 improve the experience for

16.  Example Flow

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

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

                             Figure 2: Example

17.  Security Considerations

   This specification inherits most of its semantics from [RFC5245] and
   as a result all security considerations described there remain the

18.  Acknowledgements

   The authors would 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 document.

19.  References

19.1.  Normative References

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

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

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

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   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April

19.2.  Informative References

              Ivov, E., Thomas, T., Marocco, E., and C. Holmberg, "A
              Session Initiation Protocol (SIP) usage for Trickle ICE",
              draft-ietf-mmusic-trickle-ice-sip-03 (work in progress),
              October 2015.

              Keraenen, A. and J. Arkko, "Update on Candidate Address
              Selection for Interactive Connectivity Establishment
              (ICE)", draft-keranen-mmusic-ice-address-selection-01
              (work in progress), July 2012.

   [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,

   [RFC2543]  Handley, M., Schulzrinne, H., Schooler, E., and J.
              Rosenberg, "SIP: Session Initiation Protocol", RFC 2543,
              DOI 10.17487/RFC2543, March 1999,

   [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.

   [RFC3388]  Camarillo, G., Eriksson, G., Holler, J., and H.
              Schulzrinne, "Grouping of Media Lines in the Session
              Description Protocol (SDP)", RFC 3388, DOI 10.17487/
              RFC3388, December 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, <>.

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   [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, 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.

Appendix A.  Open Issues

   At the time of writing of this document the authors have no clear
   view on how and if the following list of issues should be addressed.

A.1.  MID/Stream Indices in SDP

   This specification does not currently define syntax for candidate-to-
   stream bindings although it says that they should be implemented with
   MID or a stream index.  Yet, it is reasonable to assume that most
   usages would need to do this within the SDP and it may make sense to
   agree on the format.  Here's one possible way to do this:

         a=candidate:1 1 UDP 1658497328 5000 typ host
         a=candidate:2 1 UDP 1658497328 5000 typ srflx
         a=candidate:2 1 UDP 1658497328 5002 typ srflx

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A.2.  Starting Checks

   Normally vanilla ICE implementations would first activate a check
   list, validate at least one pair in every component and only then
   unfreeze all other checklists.  With trickle ICE this would be
   suboptimal since candidates can arrive randomly and we would be
   wasting time waiting for a checklist to fill (almost as if we were
   doing vanilla ICE).  We need to decide if unfreezing everything
   solely based on foundation is good enough.

A.3.  Checklist States

   It's been proposed that we add a waiting-for-candidates state (e.g.,
   if the checklist is empty and no candidate pairs have been sent or
   received yet).

A.4.  Relationship to Continuous Nomination and Passive Nomination

   Does it make sense to tie trickle ICE more explicitly the continuous
   nomination and passive nomination specs?  In particular, is address
   mobility a goal for the trickle ICE specification?

A.5.  ICE Restarts

   We need to describe how trickle ICE interacts with ICE restarts.
   Specifically, is it sufficient to modify the ufrag and pwd without
   starting a full offer/answer exchange, if the signaling protocol
   being used does not require it or if the restarting entity does not
   include a media description?

A.6.  Candidate Redundancy and Priority

   We need to clarify the relationship between RFC 5245 and trickle ICE
   with respect to candidate redundancy and priority.

A.7.  Make Trickle ICE SDP-Agnostic

   Would it make sense to remove the tie to SDP in the spec?  This is
   similar to what's being done with the ICEbis spec, so consistency
   might be desirable.

Appendix B.  Interaction with ICE

   The ICE protocol was designed to be flexible enough to would work in
   and adapt to as many network environments as possible.  Despite that
   flexibility, ICE as specified in [RFC5245] does not by itself support
   trickle ICE.  This section describes how trickling of candidates
   interacts with ICE.

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   [RFC5245] 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 which also happens to
       correspond to an existing host on Bob's network.

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

   4.  These checks reach the host at in Bob's network, which
       responds with an ICMP "port unreachable" error and per [RFC5245]
       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 (per
   [I-D.keranen-mmusic-ice-address-selection]) from any of the
   candidates that Bob has gathered.  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

   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

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   After Bob's agent receives Alice's offer it would immediately start
   connectivity checks.  It would also start gathering candidates, which
   would take long 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 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-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.2.  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.

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C.3.  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.

   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.4.  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.

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   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

   o  Added two new entries to the open issues section.

C.5.  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
   Strasbourg  67000

   Phone: +33 6 72 81 15 55

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

   Phone: +1 650 678 2350

   Justin Uberti
   747 6th St S
   Kirkland, WA  98033

   Phone: +1 857 288 8888

   Peter Saint-Andre


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