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In: MissingRef
Network Working Group                                            E. Ivov
Internet-Draft                                                     Jitsi
Intended status: Standards Track                                T. Stach
Expires: June 25, 2018                                      Unaffiliated
                                                              E. Marocco
                                                          Telecom Italia
                                                             C. Holmberg
                                                                Ericsson
                                                       December 22, 2017


       A Session Initiation Protocol (SIP) usage for Trickle ICE
                  draft-ietf-mmusic-trickle-ice-sip-12

Abstract

   The Interactive Connectivity Establishment (ICE) protocol describes a
   Network Address Translator (NAT) traversal mechanism for UDP-based
   multimedia sessions established with the Offer/Answer model.  The ICE
   extension for Incremental Provisioning of Candidates (Trickle ICE)
   defines a mechanism that allows ICE Agents to shorten session
   establishment delays by making the candidate gathering and
   connectivity checking phases of ICE non-blocking and by executing
   them in parallel.

   This document defines usage semantics for Trickle ICE with the
   Session Initiation Protocol (SIP) and defines a new SIP Info Package.

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 https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on June 25, 2018.







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Copyright Notice

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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Discovery issues  . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Relationship with the Offer/Answer Model  . . . . . . . .   6
   4.  Incremental Signaling of ICE candidates . . . . . . . . . . .   7
     4.1.  Initial Offer/Answer exchange . . . . . . . . . . . . . .   8
       4.1.1.  Sending the initial Offer . . . . . . . . . . . . . .   8
       4.1.2.  Receiving the initial Offer . . . . . . . . . . . . .   9
       4.1.3.  Sending the initial Answer  . . . . . . . . . . . . .   9
       4.1.4.  Receiving the initial Answer  . . . . . . . . . . . .  10
     4.2.  Subsequent Offer/Answer exchanges . . . . . . . . . . . .  10
     4.3.  Establishing the dialog . . . . . . . . . . . . . . . . .  10
       4.3.1.  Establishing dialog state through reliable
               Offer/Answer delivery . . . . . . . . . . . . . . . .  11
       4.3.2.  Establishing dialog state through unreliable
               Offer/Answer delivery . . . . . . . . . . . . . . . .  12
       4.3.3.  Initiating Trickle ICE without an SDP Answer  . . . .  14
       4.3.4.  Considerations for Third Party Call Control . . . . .  15
     4.4.  Delivering candidates in INFO messages  . . . . . . . . .  17
   5.  Initial discovery of Trickle ICE support  . . . . . . . . . .  21
     5.1.  Provisioning support for Trickle ICE  . . . . . . . . . .  22
     5.2.  Trickle ICE discovery with Globally Routable User Agent
           URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  22
     5.3.  Fall-back to Half Trickle . . . . . . . . . . . . . . . .  23
   6.  Considerations for RTP and RTCP multiplexing  . . . . . . . .  24
   7.  Considerations for Media Multiplexing . . . . . . . . . . . .  26
   8.  SDP 'end-of-candidate' Attribute  . . . . . . . . . . . . . .  29
     8.1.  Definition  . . . . . . . . . . . . . . . . . . . . . . .  29
     8.2.  Offer/Answer procedures . . . . . . . . . . . . . . . . .  30
   9.  Content Type 'application/trickle-ice-sdpfrag'  . . . . . . .  30



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     9.1.  Overall Description . . . . . . . . . . . . . . . . . . .  30
     9.2.  Grammar . . . . . . . . . . . . . . . . . . . . . . . . .  30
   10. Info Package  . . . . . . . . . . . . . . . . . . . . . . . .  32
     10.1.  Rationale - Why INFO?  . . . . . . . . . . . . . . . . .  32
     10.2.  Overall Description  . . . . . . . . . . . . . . . . . .  33
     10.3.  Applicability  . . . . . . . . . . . . . . . . . . . . .  33
     10.4.  Info Package Name  . . . . . . . . . . . . . . . . . . .  33
     10.5.  Info Package Parameters  . . . . . . . . . . . . . . . .  33
     10.6.  SIP Option Tags  . . . . . . . . . . . . . . . . . . . .  33
     10.7.  Info Request Body Parts  . . . . . . . . . . . . . . . .  34
     10.8.  Info Package Usage Restrictions  . . . . . . . . . . . .  34
     10.9.  Rate of INFO Requests  . . . . . . . . . . . . . . . . .  34
     10.10. Info Package Security Considerations . . . . . . . . . .  34
   11. Deployment Considerations . . . . . . . . . . . . . . . . . .  34
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34
     12.1.  SDP 'end-of-candidate' Attribute . . . . . . . . . . . .  35
     12.2.  application/trickle-ice-sdpfrag Media Type . . . . . . .  35
     12.3.  SIP Info Package 'trickle-ice' . . . . . . . . . . . . .  37
     12.4.  SIP Option Tag 'trickle-ice' . . . . . . . . . . . . . .  37
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  38
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  38
   15. Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .  38
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  41
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  41
     16.2.  Informative References . . . . . . . . . . . . . . . . .  44
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  44

1.  Introduction

   The Interactive Connectivity Establishment (ICE) protocol
   [I-D.ietf-ice-rfc5245bis] describes a mechanism for Network Address
   Translator (NAT) traversal that consists of three main phases.

   During the first phase an agent gathers a set of candidate transport
   addresses (source IP address, port and transport protocol).  This is
   followed by a second phase where these candidates are sent to a
   remote agent.  There, the gathering procedure is repeated and
   candidates are sent to the first agent.  Finally, a third phase
   starts where connectivity between all candidates in both sets is
   checked (connectivity checks).  Once these phases have been
   completed, and only then, both agents can begin communication.

   According to [I-D.ietf-ice-rfc5245bis] the three phases above happen
   consecutively, in a blocking way, which can introduce undesirable
   setup delay during session establishment.  The Trickle ICE extension
   [I-D.ietf-ice-trickle] defines generic semantics required for these
   ICE phases to happen in a parallel, non-blocking way and hence speed
   up session establishment.



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   This specification defines a usage of Trickle ICE with the Session
   Initiation Protocol (SIP)[RFC3261].  It describes how ICE candidates
   are to be exchanged incrementally with SIP INFO requests [RFC6086]
   and how the Half Trickle and Full Trickle modes defined in
   [I-D.ietf-ice-trickle] are to be used by SIP User Agents (UAs)
   depending on their expectations for support of Trickle ICE by a
   remote agent.

   This document defines a new Info Package as specified in [RFC6086]
   for use with Trickle ICE.

2.  Terminology

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

   This specification makes use of terminology defined by the protocol
   for Interactive Connectivity Establishment in
   [I-D.ietf-ice-rfc5245bis] and its Trickle ICE extension
   [I-D.ietf-ice-trickle].  It is assumed that the reader will be
   familiar with the terminology from both documents.

3.  Protocol Overview

   When using ICE for SIP according to [I-D.ietf-mmusic-ice-sip-sdp] the
   ICE candidates are exchanged solely via SDP Offer/Answer as per
   [RFC3264].  This specification defines an additional mechanism where
   candidates can be exchanged using SIP INFO messages and a newly
   defined Info Package [RFC6086].  This allows ICE candidates also to
   be sent in parallel to an ongoing Offer/Answer negotiation and/or
   after the completion of the Offer/Answer negotiation.

   Typically, in cases where Trickle ICE is fully supported, the Offerer
   would send an INVITE request containing a subset of candidates.  Once
   an early dialog is established the Offerer can continue sending
   candidates in INFO requests within that dialog.

   Similarly, an Answerer can send ICE candidates using INFO requests
   within the dialog established by its 18x provisional response.
   Figure 1 shows such a sample exchange:








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      STUN/Turn                                                STUN/TURN
       Servers          Alice                      Bob          Servers
          |               |                         |                |
          |  STUN Bi.Req. |     INVITE (Offer)      |                |
          |<--------------|------------------------>|                |
          |               |      183 (Answer)       | TURN Alloc Req |
          | STUN Bi.Resp. |<------------------------|--------------->|
          |-------------->|  INFO/OK (SRFLX Cand.)  |                |
          |               |------------------------>| TURN Alloc Resp|
          |               |  INFO/OK (Relay Cand.)  |<---------------|
          |               |<------------------------|                |
          |               |                         |                |
          |               |  More Cands & ConnChecks|                |
          |               |<=======================>|                |
          |               |                         |                |
          |               |          200 OK         |                |
          |               |<------------------------|                |
          |               |            ACK          |                |
          |               |------------------------>|                |
          |               |                         |                |
          |               |<===== MEDIA FLOWS =====>|                |
          |               |                         |                |

          Note: SRFLX denotes server-reflexive candidates

              Figure 1: Sample Trickle ICE scenario with SIP

3.1.  Discovery issues

   In order to benefit from Trickle ICE's full potential and reduce
   session establishment latency to a minimum, Trickle ICE agents need
   to generate SDP Offers and Answers that contain incomplete,
   potentially empty sets of candidates.  Such Offers and Answers can
   only be handled meaningfully by agents that actually support
   incremental candidate provisioning, which implies the need to confirm
   such support before actually using it.

   Contrary to other protocols, where "in advance" capability discovery
   is widely implemented, the mechanisms that allow this for SIP (i.e.,
   a combination of UA Capabilities [RFC3840] and GRUU [RFC5627]) have
   only seen low levels of adoption.  This presents an issue for Trickle
   ICE implementations as SIP UAs do not have an obvious means of
   verifying that their peer will support incremental candidate
   provisioning.

   The Half Trickle mode of operation defined in the Trickle ICE
   specification [I-D.ietf-ice-trickle] provides one way around this, by
   requiring the first Offer to contain a complete set of local ICE



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   candidates and only using incremental provisioning of remote
   candidates for the rest of the session.

   While using Half Trickle does provide a working solution it also
   comes at the price of increased latency.  Section 5 therefore makes
   several alternative suggestions that enable SIP UAs to engage in Full
   Trickle right from their first Offer: Section 5.1 discusses the use
   of on-line provisioning as a means of allowing use of Trickle ICE for
   all endpoints in controlled environments.  Section 5.2 describes
   anticipatory discovery for implementations that actually do support
   GRUU and UA Capabilities and Section 5.3 discusses the implementation
   and use of Half Trickle by SIP UAs where none of the above are an
   option.

3.2.  Relationship with the Offer/Answer Model

   From the perspective of SIP middle boxes and proxies the Offer/Answer
   exchange looks partly similar for Trickle ICE as it would for regular
   ICE for SIP [I-D.ietf-mmusic-ice-sip-sdp].  However, in order to have
   the full picture of the candidate exchange, the newly introduced INFO
   messages need to be considered as well.






























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   +-------------------------------+  +-------------------------------+
   |   Alice      +--------------+ |  | +--------------+       Bob    |
   |              | Offer/Answer | |  | | Offer/Answer |              |
   | +--------+   |    Module    | |  | |    Module    |   +--------+ |
   | |  ICE   |   +--------------+ |  | +--------------+   |  ICE   | |
   | | Module |         |          |  |        |           | Module | |
   | +--------+         |          |  |        |           +--------+ |
   +-------------------------------+  +-------------------------------+
         |              |                      |                |
         |              |    INVITE (Offer)    |                |
         |              |--------------------->|                |
         |              |     183 (Answer)     |                |
         |              |<---------------------|                |
         |              |                      |                |
         |                                                      |
         |             SIP INFO (more candidates)               |
         |----------------------------------------------------->|
         |             SIP INFO (more candidates)               |
         |<-----------------------------------------------------|
         |                                                      |
         |          STUN Binding Requests/Responses             |
         |----------------------------------------------------->|
         |          STUN Binding Requests/Responses             |
         |<-----------------------------------------------------|
         |                                                      |



       Figure 2: Distinguishing between Trickle ICE and traditional
                                signaling.

   From an architectural viewpoint, as displayed in Figure 2, exchanging
   candidates through SIP INFO requests could be represented as
   signaling between ICE modules and not between Offer/Answer modules of
   SIP User Agents.  Then, such INFO requests do not impact the state of
   the Offer/Answer transaction other than providing additional
   candidates.  Consequently, INFO requests are not considered Offers or
   Answers.  Nevertheless, candidates that have been exchanged using
   INFO requests SHALL be included in subsequent Offers or Answers.  The
   version number in the "o=" line of that subsequent Offer would need
   to be incremented by 1 per the rules in [RFC3264].

4.  Incremental Signaling of ICE candidates

   Trickle ICE Agents will exchange ICE descriptions compliant to
   [I-D.ietf-ice-trickle] via Offer/Answer procedures and/or INFO
   request bodies.  This requires the following SIP-specific extensions:




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   1.  Trickle ICE Agents MUST indicate support for Trickle ICE by
       including the SIP option-tag 'trickle-ice' in a SIP Supported:
       header field within all SIP INVITE requests and responses.

   2.  Trickle ICE Agents MUST indicate support for Trickle ICE by
       including the ice-option 'trickle' within all SDP Offers and
       Answers in accordance to [I-D.ietf-ice-trickle].

   3.  Trickle ICE Agents MAY include any number of ICE candidates, i.e.
       from zero to the complete set of candidates, in their initial
       Offer or Answer.  If the complete candidate set is included
       already in the initial Offer, this is called Half-Trickle.

   4.  Trickle ICE Agents MAY exchange additional ICE candidates using
       INFO requests within an existing INVITE dialog usage (including
       an early dialog) as specified in [RFC6086].  The INFO requests
       carry an Info-Package: trickle-ice.  Trickle ICE Agents MUST be
       prepared to receive INFO requests within that same dialog usage,
       containing additional candidates and/or an indication that
       trickling of such candidates has ended.

   5.  Trickle ICE Agents MAY exchange additional ICE candidates before
       the Answerer has sent the Answer provided that an invite dialog
       usage is established at both Trickle ICE Agents.  Note that in
       case of forking multiple early dialogs may exist.

   The following sections provide further details on how Trickle ICE
   Agents perform the initial Offers/Answers exchange (Section 4.1),
   perform subsequent Offers/Answers exchanges (Section 4.2) and
   establish the INVITE dialog usage (Section 4.3) such that they can
   incrementally trickle candidates (Section 4.4).

4.1.  Initial Offer/Answer exchange

4.1.1.  Sending the initial Offer

   If the Offerer includes candidates in its initial Offer, it MUST
   encode these candidates as specified in
   [I-D.ietf-mmusic-ice-sip-sdp].

   If the Offerer wants to send its initial Offer before knowing any
   candidate for one or more media descriptions, it MUST set the port to
   the default value '9' for these media descriptions.  If the Offerer
   does not want to include the host IP address in the corresponding
   c-line, e.g. due to privacy reasons, it SHOULD include a default
   address in the c-line, which is set to the IPv4 address 0.0.0.0 or to
   the IPv6 equivalent ::.




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   In this case, the Offerer obviously cannot know the RTCP transport
   address and, thus, MUST NOT include the "a=rtcp" attribute [RFC6086].
   This avoids potential ICE mismatch (see
   [I-D.ietf-mmusic-ice-sip-sdp]) for the RTCP transport address.

   If the Offerer wants to use RTCP multiplexing [RFC5761] and/or
   exclusive RTCP multiplexing [I-D.ietf-mmusic-mux-exclusive], it still
   will include the "a=rtcp-mux" and/or "a=rctp-mux-only" attribute in
   the initial Offer.

   In any case, the Offerer MUST include the attribute "a=ice-
   options:trickle" in accordance to [I-D.ietf-ice-trickle] and MUST
   include in each "m="-line a "a=mid:" attribute in accordance to
   [RFC5888].

4.1.2.  Receiving the initial Offer

   If the initial Offer included candidates, the Answerer uses these
   candidates to start ICE processing as specified in
   [I-D.ietf-ice-trickle].

   If the initial Offer included the attribute a=ice-options:trickle,
   the Answerer MUST be prepared for receiving trickled candidates later
   on.

   In case of a "m/c=" line with default values none of the eventually
   trickled candidates will match the default destination.  This
   situation MUST NOT cause an ICE mismatch (see
   [I-D.ietf-mmusic-ice-sip-sdp]).

4.1.3.  Sending the initial Answer

   If the Answerer includes candidates in its initial Answerer, it MUST
   encode these candidates as specified in
   [I-D.ietf-mmusic-ice-sip-sdp].

   If the Answerer wants to send its initial Answer before knowing any
   candidate for one or more media descriptions, it MUST set the port to
   the default value '9' for these media descriptions.  If the Answerer
   does not want to include the host IP address in the corresponding
   c-line, e.g. due to privacy reasons, it SHOULD include a default
   address in the c-line, which is set to the IPv4 address 0.0.0.0 or to
   the IPv6 equivalent ::.

   In this case, the Answerer obviously cannot know the RTCP transport
   address and, thus, MUST NOT include the "a=rtcp" attribute [RFC6086].
   This avoids potential ICE mismatch (see
   [I-D.ietf-mmusic-ice-sip-sdp]) for the RTCP transport address.



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   If the Answerer accepts to use RTCP multiplexing [RFC5761] and/or
   exclusive RTCP multiplexing [I-D.ietf-mmusic-mux-exclusive], it will
   include the "a=rtcp-mux" attribute in the initial Answer.

   In any case, the Answerer MUST include the attribute "a=ice-
   options:trickle" in accordance to [I-D.ietf-ice-trickle] and MUST
   include in each "m="-line a "a=mid:" attribute in accordance to
   [RFC5888].

4.1.4.  Receiving the initial Answer

   If the initial Answer included candidates, the Offerer uses these
   candidates to start ICE processing as specified in
   [I-D.ietf-ice-trickle].

   If the initial Answer included the attribute a=ice-options:trickle,
   the Offerer MUST be prepared for receiving trickled candidates later
   on.

   In case of a "m/c=" line with default values none of the eventually
   trickled candidates will match the default destination.  This
   situation MUST NOT cause an ICE mismatch (see
   [I-D.ietf-mmusic-ice-sip-sdp]).

4.2.  Subsequent Offer/Answer exchanges

   Subsequent Offer/Answer exchanges are handled as for regular ICE (see
   section 4.2 of [I-D.ietf-mmusic-ice-sip-sdp]).

   If an Offer or Answer needs to be sent while the ICE agents are in
   the middle of trickling section 4.2.1.2.1 of
   [I-D.ietf-mmusic-ice-sip-sdp]) applies.  This means that an ICE agent
   includes candidate attributes for all local candidates it had
   trickled previously for a specific media stream.

   [RFC EDITOR NOTE: The section 4.2.1.2.1 in above sentence is correct
   for version 16 of said I-D.  Authors need to cross-check during
   Auth48 since it could have have changed in the meantime.]

4.3.  Establishing the dialog

   In order to be able to start trickling, the following two conditions
   need to be satisfied at the SIP UAs:

   o  Trickle ICE support at the peer agent MUST be confirmed.

   o  The dialog at both peers MUST be in early or confirmed state.




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   Section 5 discusses in detail the various options for satisfying the
   first of the above conditions.  Regardless of those mechanisms,
   however, agents are certain to have a clear understanding of whether
   their peers support trickle ICE once an Offer and an Answer have been
   exchanged, which also allows for ICE processing to commence (see
   Figure 3).

4.3.1.  Establishing dialog state through reliable Offer/Answer delivery

                   Alice                      Bob
                     |                         |
                     |     INVITE (Offer)      |
                     |------------------------>|
                     |      183 (Answer)       |
                     |<------------------------|
                     |        PRACK/OK         |
                     |------------------------>|
                     |                         |
             +----------------------------------------+
             |Alice and Bob know that both can trickle|
             |and know that the dialog is in the early|
             |state. Send INFO!                       |
             +----------------------------------------+
                     |                         |
                     |  INFO/OK (+SRFLX Cand.) |
                     |------------------------>|
                     |  INFO/OK (+SRFLX Cand.) |
                     |<------------------------|
                     |                         |

           Note: SRFLX denotes server-reflexive candidates


    Figure 3: SIP Offerer can freely trickle as soon as it receives an
                                  Answer.

   As shown in Figure 3 satisfying both conditions is relatively trivial
   for ICE Agents that have sent an Offer in an INVITE and that have
   received an Answer in a reliable provisional response.  It is
   guaranteed to have confirmed support for Trickle ICE at the Answerer
   (or lack thereof) and to have fully initialized the SIP dialog at
   both ends.  Offerers and Answerers (after receipt of the PRACK
   request) in the above situation can therefore freely commence
   trickling within the newly established dialog.







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4.3.2.  Establishing dialog state through unreliable Offer/Answer
        delivery

   The situation is a bit more delicate for agents that have received an
   Offer in an INVITE request and have sent an Answer in an unreliable
   provisional response because, once the response has been sent, the
   Answerer does not know when or if it has been received (Figure 4).

                   Alice                      Bob
                     |                         |
                     |     INVITE (Offer)      |
                     |------------------------>|
                     |      183 (Answer)       |
                     |<------------------------|
                     |                         |
                     |               +----------------------+
                     |               |Bob:  I don't know if |
                     |               |Alice got my 183 or if|
                     |               |her dialog is already |
                     |               |in the early state.   |
                     |               |  Can I send INFO???  |
                     |               +----------------------+
                     |                         |

    Figure 4: A SIP UA that sent an Answer in an unreliable provisional
    response does not know if it was received and if the dialog at the
              side of the Offerer has entered the early state

   In order to clear this ambiguity as soon as possible, the Answerer
   needs to retransmit the provisional response with the exponential
   back-off timers described in [RFC3262].  These retransmissions MUST
   cease on receipt of an INFO request or on transmission of the Answer
   in a 2xx response.  This is similar to the procedure described in
   section 8.1.1 of [I-D.ietf-mmusic-ice-sip-sdp] except that the STUN
   binding Request is replaced by the INFO request.

   [RFC EDITOR NOTE: The section 8.1.1 in above sentence is correct for
   version 16 of said I-D.  Authors need to cross-check during Auth48
   since it could have have changed in the meantime.]

   The Offerer MUST send a Trickle ICE INFO request as soon as it
   receives an SDP Answer in an unreliable provisional response.  This
   INFO request MUST repeat the candidates that were already provided in
   the Offer (as would be the case when Half Trickle is performed or
   when new candidates have not been learned since then).

   If available, the Offerer SHOULD also deliver newly learned
   candidates in this INFO request, unless it wants to hold back some



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   candidates in reserve, e.g. in case that these candidates are
   expensive to use and would only be trickled if all other candidates
   failed.

   The Offerer SHOULD include an end-of-candidates attribute in case
   candidate discovery has ended in the mean time and no further
   candidates are to be trickled.

   As soon as an Answerer has received such an INFO request, the
   Answerer has an indication that a dialog is established at both ends
   and can begin trickling (Figure 5).

   Note: The +SRFLX in Figure 5 indicates that additionally newly
   learned server-reflexive candidates are included.

                   Alice                      Bob
                     |                         |
                     |     INVITE (Offer)      |
                     |------------------------>|
                     |      183 (Answer)       |
                     |<------------------------|
                     |  INFO/OK (+SRFLX Cand.) |
                     |------------------------>|
                     |                         |
                     |               +----------------------+
                     |               |Bob:  Now I know Alice|
                     |               | is ready. Send INFO! |
                     |               +----------------------+
                     |  INFO/OK (+SRFLX Cand.) |
                     |<------------------------|
                     |                         |
                     |    200/ACK (Answer)     |
                     |<------------------------|

             Note: SRFLX denotes server-reflexive candidates


     Figure 5: A SIP UA that received an INFO request after sending an
   unreliable provisional response knows that the dialog at the side of
                 the receiver has entered the early state

   When sending the Answer in the 200 OK response to the INVITE request,
   the Answerer needs to repeat exactly the same Answer that was
   previously sent in the unreliable provisional response in order to
   fulfill the corresponding requirements in [RFC3264].  Thus, the
   Offerer needs to be prepared for receiving a different number of
   candidates in that repeated Answer than previously exchanged via




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   trickling and MUST ignore the candidate information in that 200 OK
   response.

4.3.3.  Initiating Trickle ICE without an SDP Answer

   The ability to convey arbitrary candidates in INFO message bodies
   allows ICE Agents to initiate trickling without actually sending an
   Answer.  Trickle ICE Agents can therefore respond to an INVITE
   request with provisional responses without an SDP Answer [RFC3261].
   Such provisional responses serve for establishing an early dialog.

   Agents that choose to establish the dialog in this way, MUST
   retransmit these responses with the exponential back-off timers
   described in [RFC3262].  These retransmissions MUST cease on receipt
   of an INFO request or on transmission of the Answer in a 2xx
   response.  This is again similar to the procedure described in
   section 8.1.1 of [I-D.ietf-mmusic-ice-sip-sdp] except that an Answer
   is not yet provided.

   [RFC EDITOR NOTE: The section 8.1.1 in above sentence is correct for
   version 16 of said I-D.  Authors need to cross-check during Auth48
   since it could have have changed in the meantime.]

   Note: The +SRFLX in Figure 6 indicates that additionally newly
   learned server-reflexive candidates are included.


























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                   Alice                      Bob
                     |                         |
                     |     INVITE (Offer)      |
                     |------------------------>|
                     |      183 (-)            |
                     |<------------------------|
                     |  INFO/OK (SRFLX Cand.)  |
                     |------------------------>|
                     |                         |
                     |               +----------------------+
                     |               |Bob:  Now I know again|
                     |               | that Alice is ready. |
                     |               | Send INFO!           |
                     |               +----------------------+
                     |  INFO/OK (SRFLX Cand.)  |
                     |<------------------------|
                     |    183 (Answer) opt.    |
                     |<------------------------|
                     |  INFO/OK (SRFLX Cand.)  |
                     |<------------------------|
                     |    200/ACK (Answer)     |
                     |<------------------------|

          Note: SRFLX denotes server-reflexive candidates


    Figure 6: A SIP UA sends an unreliable provisional response without
                an Answer for establishing an early dialog

   When sending the Answer, the agent MUST repeat all currently known
   and used candidates, if any, and MAY include all newly gathered
   candidates since the last INFO request was sent.  However, if that
   Answer was already sent in a unreliable provisional response, the
   Answerers MUST repeat exactly the same Answer in the 200 OK response
   to the INVITE request in order to fulfill the corresponding
   requirements in [RFC3264].  In case that trickling continued, an
   Offerer needs to be prepared for receiving fewer candidates in that
   repeated Answer than previously exchanged via trickling and MUST
   ignore the candidate information in that 200 OK response.

4.3.4.  Considerations for Third Party Call Control

   Third Party Call Control (3PCC) for SIP can be performed using
   several signaling variants as described in [RFC3725].  We give
   specific consideration for 3PCC that starts with an offerless INVITE
   request [RFC3261].  Then, a User Agent Client (UAC) has the option to
   send its Offer in a reliable provisional response [RFC3262] or in the
   200 OK response to the INVITE request.



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   Agents that had sent an Offer in a reliable provisional response and
   that received an Answer in a PRACK request [RFC3262] are also in a
   situation where support for Trickle ICE is confirmed and the SIP
   dialog is guaranteed to be in a state that would allow in-dialog INFO
   requests (see Figure 7).

                   Alice                      Bob
                     |                         |
                     |        INVITE           |
                     |------------------------>|
                     |      183 (Offer)        |
                     |<------------------------|
                     |     PRACK (Answer)      |
                     |------------------------>|
                     |                         |
                     |               +----------------------+
                     |               |Bob:  I know Alice can|
                     |               |trickle and I know her|
                     |               |dialog is in the early|
                     |               |state. Send INFO!     |
                     |               +----------------------+
                     |                         |
                     |  INFO/OK (SRFLX Cand.)  |
                     |<------------------------|
                     |                         |
                     |  INFO/OK (SRFLX Cand.)  |
                     |------------------------>|
                     |     200 OK/ACK          |
                     |<------------------------|

          Note: SRFLX denotes server-reflexive candidates


     Figure 7: A SIP Offerer in a 3PCC scenario can also freely start
                trickling as soon as it receives an Answer.

   Trickle ICE Agents that send an Offer in a 200 OK response and
   receive an Answer in an ACK message can still create a dialog and
   confirm support for Trickle ICE by sending an unreliable provisional
   response similar to Section 4.3.3.  According to [RFC3261], this
   unreliable response cannot contain an Offer.

   The Trickle ICE Agent, i.e. the user Agent server (UAS), retransmits
   the provisional response with the exponential back-off timers
   described in [RFC3262].  Retransmits MUST cease on receipt of an INFO
   request or on transmission of the Answer in a 2xx response.  The peer
   Trickle ICE Agent (the UAC) MUST send a Trickle ICE INFO request as




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   soon as they receive an unreliable provisional response (see
   Figure 8).

                   Alice                      Bob
                     |                         |
                     |        INVITE           |
                     |------------------------>|
                     |      183 (-)            |
                     |<------------------------|
                     |  INFO/OK (SRFLX Cand.)  |
                     |------------------------>|
                     |                         |
                     |               +-----------------------+
                     |               |Bob:  I know Alice can |
                     |               |trickle and I know her |
                     |               |dialog is in the early |
                     |               |state.                 |
                     |               |INFO can be sent.      |
                     |               +-----------------------+
                     |                         |
                     |  INFO/OK (SRFLX Cand.)  |
                     |<------------------------|
                     |                         |
                     |      200 (Offer)        |
                     |<------------------------|
                     |       ACK (Answer)      |
                     |------------------------>|
                     |                         |

          Note: SRFLX denotes server-reflexive candidates

       Figure 8: A SIP UAC in a 3PCC scenario can also freely start
   trickling as soon as it receives an unreliable provisional response.

4.4.  Delivering candidates in INFO messages

   Whenever new ICE candidates become available for sending, agents
   would encode them in "a=candidate" attributes as described by
   [I-D.ietf-mmusic-ice-sip-sdp].  For example:


      a=candidate:2 1 UDP 1694498815 2001:db8:a0b:12f0::3 5000 typ srflx
          raddr 2001:db8:a0b:12f0::1 rport 8998


   The use of SIP INFO requests happens within the context of the Info
   Package as defined Section 10.  The Media Type [RFC6838] for their




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   payload MUST be set to 'application/trickle-ice-sdpfrag' as defined
   in Section 9.

   Since neither the "a=candidate" nor the "a=end-of-candidates"
   attributes contain information that would allow correlating them to a
   specific "m=" line, this is handled through the use of pseudo "m="
   lines and identification tags in "a=mid:" attributes as defined in
   [RFC5888].  Pseudo "m=" lines follow the SDP syntax for "m=" lines as
   defined in [RFC4566], but provide no semantics other than indicating
   to which "m=" line a candidate belongs.  Consequently, the receiving
   agent MUST ignore any remaining content of the pseudo "m=" line,
   which is not defined in this document.  This guarantees that the
   'application/trickle-ice-sdpfrag' bodies do not interfere with the
   Offer/Answer procedures as specified in [RFC3264].

   When sending the INFO request, the agent MAY, if already known to the
   agent, include the same content into the pseudo "m=" line as for the
   "m=" line in the corresponding Offer or Answer.  However, since
   Trickle-ICE might be decoupled from the Offer/Answer negotiation this
   content might be unknown to the agent.  In this case, the agent MUST
   include the following default values.

   o  The media field is set to 'audio'.

   o  The port value is set to '9'.

   o  The proto value is set to 'RTP/AVP'.

   o  The fmt field MUST appear only once and is set to '0'

   Agents MUST include a pseudo "m=" line and an identification tag in a
   "a=mid:" attribute for every "m=" line whose candidate list they
   intend to update.  Such "a=mid:" attributes MUST immediately precede
   the list of candidates for that specific "m=" line.  All
   "a=candidate" or "a=end-of-candidates" attributes following an
   "a=mid:" attribute, up until (and excluding) the next occurrence of a
   pseudo "m=" line, pertain to the "m=" line identified by that
   identification tag.  An "a=end-of-candidates" attribute, preceding
   any pseudo "m=" line, indicates the end of all trickling from that
   agent, as opposed to end of trickling for a specific "m=" line, which
   would be indicated by a media level "a=end-of-candidates" attribute.

   Refer to Figure 9 for an example of the INFO request content.

   The use of pseudo "m=" lines allows for a structure similar to the
   one in SDP Offers and Answers where separate media-level and session-
   level sections can be distinguished.  In the current case, lines
   preceding any pseudo "m=" line are considered to be session-level.



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   Lines appearing in between or after pseudo "m=" lines will be
   interpreted as media-level.

      Note that while this specification uses the "a=mid:" attribute
      from [RFC5888], it does not define any grouping semantics.
      Consequently, the "a=group:" attribute from that same
      specification is neither needed nor used in Trickle ICE for SIP.

   All INFO requests MUST carry the "a=ice-pwd:" and "a=ice-ufrag:"
   attributes that would allow mapping them to a specific ICE
   generation.  An agent MUST discard any received INFO requests
   containing "a=ice-pwd:" and "a=ice-ufrag:" attributes that do not
   match those of the current ICE Negotiation Session.

   The "a=ice-pwd:" and "a=ice-ufrag:" attributes MUST appear at the
   same level as the ones in the Offer/Answer exchange.  In other words,
   if they were present as session-level attributes, they will also
   appear at the beginning of all INFO request payloads, i.e. preceding
   all pseudo "m=" lines.  If they were originally exchanged as media
   level attributes, potentially overriding session-level values, then
   they will also be included in INFO request payloads following the
   corresponding pseudo "m=" lines.

   Note that [I-D.ietf-ice-trickle] requires that when candidates are
   trickled, each candidate must be delivered to the receiving Trickle
   ICE implementation not more than once and in the same order as it was
   conveyed.  If the signaling protocol provides any candidate
   retransmissions, they need to be hidden from the ICE implementation.
   This requirement is fulfilled as follows.

   Since the agent is not fully aware of the state of the ICE
   Negotiation Session at its peer it MUST include all currently known
   and used local candidates in every INFO request.  I.e. the agent MUST
   repeat in the INFO request body all candidates that were previously
   sent under the same combination of "a=ice-pwd:" and "a=ice-ufrag:" in
   the same order as they were gathered.  In other words, the sequence
   of a previously sent list of candidates MUST NOT change in subsequent
   INFO requests and newly gathered candidates MUST be added at the end
   of that list.  Although repeating all candidates creates some
   overhead, it also allows easier handling of problems that could arise
   from unreliable transports, like e.g. loss of messages and
   reordering, which can be detected through the CSeq: header field in
   the INFO request.

   When receiving INFO requests carrying any candidates, agents will
   therefore first identify and discard the attribute lines containing
   candidates they have already received in previous INFO requests or in
   the Offer/Answer exchange preceding them.  Two candidates are



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   considered to be equal if their IP address port, transport and
   component ID are the same.  After identifying and discarding known
   candidates, the agents MUST forward the actually new candidates to
   the ICE Agents in the same order as they were received in the INFO
   request body.  The ICE Agents will then process the new candidates
   according to the rules described in [I-D.ietf-ice-trickle].

   Receiving an "a=end-of-candidates" attribute in an INFO request body
   - with the "a=ice-ufrag" and "a=ice-pwd" attributes matching the
   current ICE generation - is an indication from the peer agent that it
   will not send any further candidates.  When included at session
   level, i.e. before any pseudo "m=" line, this indication applies to
   the whole session; when included at media level the indication
   applies only to the corresponding "m=" line.  Handling of such end-
   of-candidate indications is defined in [I-D.ietf-ice-trickle].

   Note: At the time of writing this specification there were ongoing
   discussions if a functionality for removing already exchanged
   candidates would be useful.  Such a functionality is out of the scope
   of this specification and most likely needs to be signaled by means
   of a yet to be defined ICE extension, although it could in principle
   be achieved quite easily, e.g. without anticipating any solution by
   simply omitting a previously sent candidate from a subsequent INFO
   request.  However, if an implementation according to this
   specification receives such an INFO request with a missing candidate
   it would have to treat that as an exceptional case.  Implementing
   appropriate recovery procedures at the receiving side is advisable
   for this situation.  Ignoring that a candidate was missing might be a
   sensible strategy.

   The example in Figure 9 shows the content of a candidate delivering
   INFO request.  In the example the "a=end-of-candidates" attributes
   indicate that the candidate gathering is finished and that no further
   INFO requests follow.

















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      INFO sip:alice@example.com SIP/2.0
      ...
      Info-Package: trickle-ice
      Content-type: application/trickle-ice-sdpfrag
      Content-Disposition: Info-Package
      Content-length: 862

      a=ice-pwd:asd88fgpdd777uzjYhagZg
      a=ice-ufrag:8hhY
      m=audio 9 RTP/AVP 0
      a=mid:1
      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
      a=end-of-candidates
      m=audio 9 RTP/AVP 0
      a=mid:2
      a=candidate:1 1 UDP 2130706431 2001:db8:a0b:12f0::1 6000 typ host
      a=candidate:1 2 UDP 2130706431 2001:db8:a0b:12f0::1 6001 typ host
      a=candidate:2 1 UDP 1694498815 2001:db8:a0b:12f0::3 6000 typ srflx
         raddr 2001:db8:a0b:12f0::1 rport 9998
      a=candidate:2 2 UDP 1694498815 2001:db8:a0b:12f0::3 6001 typ srflx
         raddr 2001:db8:a0b:12f0::1 rport 9998
      a=end-of-candidates


      Note: In a real INFO request there would be no line breaks
            in the a=candidate: attributes


          Figure 9: An Example for the Content of an INFO Request

5.  Initial discovery of Trickle ICE support

   SIP User Agents (UAs) that support and intend to use trickle ICE are
   required by [I-D.ietf-ice-trickle] to indicate that in their Offers
   and Answers using the following attribute: "a=ice-options:trickle".
   This makes discovery fairly straightforward for Answerers or for
   cases where Offers need to be generated within existing dialogs
   (i.e., when sending UPDATE or re-INVITE requests).  In both scenarios
   prior SDP would have provided the necessary information.

   Obviously, prior SDP is not available at the time a first Offer is
   being constructed and it is therefore impossible for ICE Agents to




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   determine support for incremental provisioning that way.  The
   following options are suggested as ways of addressing this issue.

5.1.  Provisioning support for Trickle ICE

   In certain situations it may be possible for integrators deploying
   Trickle ICE to know in advance that some or all endpoints reachable
   from within the deployment will support Trickle ICE.  This is the
   case, for example, if Session Border Controllers (SBC) with support
   for this specification are used to connect to UAs that do not support
   Trickle ICE.

   While the exact mechanism for allowing such provisioning is out of
   scope here, this specification encourages trickle ICE implementations
   to allow the option in the way they find most appropriate.

5.2.  Trickle ICE discovery with Globally Routable User Agent URIs

   [RFC3840] provides a way for SIP User Agents to query for support of
   specific capabilities using, among others, OPTIONS requests.  Support
   for Globally Routable User Agent URIs (GRUU) according to [RFC5627]
   on the other hand allows SIP requests to be addressed to specific UAs
   (as opposed to arbitrary instances of an address of record).
   Combining the two and using the "trickle-ice" option tag defined in
   Section 10.6 provides SIP UAs with a way of learning the capabilities
   of specific SIP UA instances and then addressing them directly with
   INVITE requests that require Trickle ICE support.

   Such learning of capabilities may happen in different ways.  One
   option for a SIP UA would be to learn the GRUU instance ID of a peer
   through presence and then to query its capabilities with an OPTIONS
   request.  Alternatively, it can also just send an OPTIONS request to
   the AOR it intends to contact and then inspect the returned
   response(s) for support of both GRUU and Trickle ICE (Figure 10).  It
   is noted that using the GRUU means that the INVITE request can go
   only to that particular device.  This circumvents to use of forking
   for that request.














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            Alice                                                Bob
              |                                                   |
              |        OPTIONS sip:b1@example.com SIP/2.0         |
              |-------------------------------------------------->|
              |                                                   |
              |                      200 OK                       |
              |    Contact: sip:b1@example.com;gr=hha9s8d-999a    |
              |            ;audio;video|;trickle-ice;...          |
              |<--------------------------------------------------|
              |                                                   |
              | INVITE sip:b1@example.com;gr=hha9s8d-999a SIP/2.0 |
              |-------------------------------------------------->|
              |                                                   |
              |                  183 (Answer)                     |
              |<--------------------------------------------------|
              |                INFO/OK (Trickling)                |
              |<------------------------------------------------->|
              |                                                   |
              |                      ...                          |
              |                                                   |


      Figure 10: Trickle ICE support discovery with OPTIONS and GRUU

   Confirming support for Trickle ICE through [RFC3840] gives SIP UAs
   the options to engage in Full Trickle negotiation (as opposed to the
   more lengthy Half Trickle) from the very first Offer they send.

5.3.  Fall-back to Half Trickle

   In cases where none of the other mechanisms in this section are
   acceptable, SIP UAs should use the Half Trickle mode defined in
   [I-D.ietf-ice-trickle].  With Half Trickle, agents initiate sessions
   the same way they would when using ICE for SIP
   [I-D.ietf-mmusic-ice-sip-sdp].  This means that, prior to actually
   sending an Offer, agents would first gather ICE candidates in a
   blocking way and then send them all in that Offer.  The blocking
   nature of the process would likely imply that some amount of latency
   will be accumulated and it is advised that agents try to anticipate
   it where possible, like for example, when user actions indicate a
   high likelihood for an imminent call (e.g., activity on a keypad or a
   phone going off-hook).

   Using Half Trickle would result in Offers that are compatible with
   both ICE SIP endpoints and legacy [RFC3264] endpoints.






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   STUN/Turn                                                STUN/TURN
   Servers          Alice                      Bob          Servers
      |               |                             |               |
      |<--------------|                             |               |
      |               |                             |               |
      |               |                             |               |
      |   Candidate   |                             |               |
      |               |                             |               |
      |               |                             |               |
      |   Discovery   |                             |               |
      |               |                             |               |
      |               |                             |               |
      |-------------->|       INVITE (Offer)        |               |
      |               |---------------------------->|               |
      |               |        183 (Answer)         |-------------->|
      |               |<----------------------------|               |
      |               |  INFO (repeated candidates) |               |
      |               |---------------------------->|               |
      |               |                             |               |
      |               |    INFO (more candidates)   |   Candidate   |
      |               |<----------------------------|               |
      |               |    Connectivity Checks      |               |
      |               |<===========================>|   Discovery   |
      |               |   INFO (more candidates)    |               |
      |               |<----------------------------|               |
      |               |    Connectivity Checks      |<--------------|
      |               |<===========================>|               |
      |               |                             |               |
      |               |          200 OK             |               |
      |               |<----------------------------|               |
      |               |                             |               |
      |               |<======= MEDIA FLOWS =======>|               |
      |               |                             |               |


    Figure 11: Example - A typical (Half) Trickle ICE exchange with SIP

   It is worth reminding that once a single Offer or Answer had been
   exchanged within a specific dialog, support for Trickle ICE will have
   been determined.  No further use of Half Trickle will therefore be
   necessary within that same dialog and all subsequent exchanges can
   use the Full Trickle mode of operation.

6.  Considerations for RTP and RTCP multiplexing

   The following consideration describe options for Trickle-ICE in order
   to give some guidance to implementors on how trickling can be
   optimized with respect to providing RTCP candidates.



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   Handling of the "a=rtcp" attribute [RFC3605] and the "a=rtcp-mux"
   attribute for RTP/RTCP multiplexing [RFC5761] is already considered
   in section 5.1.1.1.  of [I-D.ietf-ice-rfc5245bis] and as well in
   [RFC5761] itself.  These considerations are still valid for Trickle
   ICE, however, trickling provides more flexibility for the sequence of
   candidate exchange in case of RTCP multiplexing.

   [RFC EDITOR NOTE: The section 5.1.1.1 in above sentence is correct
   for version 15 of said I-D.  Authors need to cross-check during
   Auth48 since it could have have changed in the meantime.]

   If the Offerer supports RTP/RTCP multiplexing exclusively as
   specified in [I-D.ietf-mmusic-mux-exclusive], the procedures in that
   document apply for the handling of the "a=rtcp-mux-only", "a=rtcp"
   and the "a=rtcp-mux" attributes.

   While a Half Trickle Offerer would have to send an Offer compliant to
   [I-D.ietf-mmusic-ice-sip-sdp] and [RFC5761] including candidates for
   all components, this flexibility allows a Full Trickle Offerer to
   send only RTP candidates (component 1) in the initial Offer if it
   assumes that RTCP multiplexing is supported by the Answerer.  A Full
   Trickle Offerer would need to start gathering and trickling RTCP
   candidates (component 2) only after having received an indication in
   the Answer that the Answerer unexpectedly does not support RTCP
   multiplexing.

   A Trickle Answerer MAY include an "a=rtcp-mux" attribute [RFC5761] in
   the application/trickle-ice-sdpfrag body if it supports and uses RTP
   and RTCP multiplexing.  The Trickle Answerer needs to follow the
   guidance on the usage of the "a=rtcp" attribute as given in
   [I-D.ietf-mmusic-ice-sip-sdp] and [RFC3605].  Receipt of this
   attribute at the Offerer in an INFO request prior to the Answer
   indicates that the Answerer supports and uses RTP and RTCP
   multiplexing.  The Offerer can use this information e.g. for stopping
   gathering of RTCP candidates and/or for freeing corresponding
   resources.

   This behavior is illustrated by the following example Offer that
   indicates support for RTP and RTCP multiplexing.












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       v=0
       o=alice 2890844526 2890844526 IN IP6 atlanta.example.com
       s=
       c=IN IP6 2001:db8:a0b:12f0::3
       t=0 0
       a=ice-pwd:777uzjYhagZgasd88fgpdd
       a=ice-ufrag:Yhh8
       m=audio 5000 RTP/AVP 0
       a=mid:1
       a=rtcp-mux
       a=candidate:1 1 UDP 1658497328 2001:db8:a0b:12f0::3 5000 typ host


   Once the dialog is established as described in section Section 4.3
   the Answerer sends the following INFO request.


       INFO sip:alice@example.com SIP/2.0
       ...
       Info-Package: trickle-ice
       Content-type: application/sdp
       Content-Disposition: Info-Package
       Content-length: 161

       a=ice-pwd:asd88fgpdd777uzjYhagZg
       a=ice-ufrag:8hhY
       m=audio 9 RTP/AVP 0
       a=mid:1
       a=rtcp-mux
       a=candidate:1 1 UDP 1658497382 2001:db8:a0b:12f0::4 6000 typ host


   This INFO request indicates that the Answerer supports and uses RTP
   and RTCP multiplexing as well.  It allows the Offerer to omit
   gathering of RTCP candidates or releasing already gathered RTCP
   candidates.  If the INFO request did not contain the a=rtcp-mux
   attribute, the Offerer would have to gather RTCP candidates unless it
   wants to wait until receipt of an Answer that eventually confirms
   support or non-support for RTP and RTCP multiplexing.

7.  Considerations for Media Multiplexing

   The following considerations describe options for Trickle-ICE in
   order to give some guidance to implementors on how trickling can be
   optimized with respect to providing candidates in case of Media
   Multiplexing [I-D.ietf-mmusic-sdp-bundle-negotiation].  It is assumed
   that the reader is familiar with
   [I-D.ietf-mmusic-sdp-bundle-negotiation].



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   ICE candidate exchange is already considered in section 11 of
   [I-D.ietf-mmusic-sdp-bundle-negotiation].  These considerations are
   still valid for Trickle ICE, however, trickling provides more
   flexibility for the sequence of candidate exchange, especially in
   Full Trickle mode.

   Except for bundle-only "m=" lines, a Half Trickle Offerer would have
   to send an Offer with candidates for all bundled "m=" lines.  The
   additional flexibility, however, allows a Full Trickle Offerer to
   initially send only candidates for the "m=" line with the suggested
   Offerer BUNDLE address.

   On receipt of the Answer, the Offerer will detect if BUNDLE is
   supported by the Answerer and if the suggested Offerer BUNDLE address
   was selected.  In this case, the Offerer does not need to trickle
   further candidates for the remaining "m=" lines in a bundle.
   However, if BUNDLE is not supported, the Full Trickle Offerer needs
   to gather and trickle candidates for the remaining "m=" lines as
   necessary.  If the Answerer selects an Offerer BUNDLE address
   different from the suggested Offerer BUNDLE address, the Full Trickle
   Offerer needs to gather and trickle candidates for the "m=" line that
   carries the selected Offerer BUNDLE address.

   A Trickle Answerer SHOULD include an "a=group: BUNDLE" attribute
   [I-D.ietf-mmusic-sdp-bundle-negotiation] in the application/trickle-
   ice-sdpfrag body if it supports and uses bundling.  When doing so,
   the Answerer MUST include all identification-tags in the same order
   that is used or will be used in the Answer.

   Receipt of this attribute at the Offerer in an INFO request prior to
   the Answer indicates that the Answerer supports and uses bundling.
   The Offerer can use this information e.g. for stopping the gathering
   of candidates for the remaining "m=" lines in a bundle and/or for
   freeing corresponding resources.

   This behaviour is illustrated by the following example Offer that
   indicates support for Media Multiplexing.














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      v=0
      o=alice 2890844526 2890844526 IN IP6 atlanta.example.com
      s=
      c=IN IP6 2001:db8:a0b:12f0::3
      t=0 0
      a=group:BUNDLE foo bar
      a=ice-pwd:777uzjYhagZgasd88fgpdd
      a=ice-ufrag:Yhh8
      m=audio 10000 RTP/AVP 0
      a=mid:foo
      a=rtcp-mux
      a=rtpmap:0 PCMU/8000
      a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid
      a=candidate:1 1 UDP 1658497328 2001:db8:a0b:12f0::3 10000 typ host
      m=video 10002 RTP/AVP 31
      a=mid:bar
      a=rtcp-mux
      a=rtpmap:31 H261/90000
      a=extmap 1 urn:ietf:params:rtp-hdrext:sdes:mid


   The example Offer indicates support for RTP and RTCP multiplexing and
   contains a "a=candidate" attribute only for the m-line with the
   suggested Offerer bundle address.  Once the dialog is established as
   described in Section 4.3 the Answerer sends the following INFO
   request.


       INFO sip:alice@example.com SIP/2.0
       ...
       Info-Package: trickle-ice
       Content-type: application/sdp
       Content-Disposition: Info-Package
       Content-length: 219

       a=group:BUNDLE foo bar
       a=ice-pwd:asd88fgpdd777uzjYhagZg
       a=ice-ufrag:8hhY
       m=audio 9 RTP/AVP 0
       a=mid:foo
       a=rtcp-mux
       a=candidate:1 1 UDP 1658497328 2001:db8:a0b:12f0::3 5000 typ host
       m=audio 9 RTP/AVP 0
       a=mid:bar







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   This INFO request indicates that the Answerer supports and uses Media
   Multiplexing as well.  Note, that the second "m=" line shows the
   default values as specified in section Section 4.4, e.g. media set
   'audio' although 'video' was offered.  The receiving ICE Agents MUST
   ignore these default values in the pseudo "m=" lines.

   The INFO request also indicates that the Answerer accepted the
   suggested Offerer Bundle Address.  This allows the Offerer to omit
   gathering of RTP and RTCP candidates for the other "m=" lines or
   releasing already gathered candidates.  If the INFO request did not
   contain the a=group:BUNDLE attribute, the Offerer would have to
   gather RTP and RTCP candidates for the other "m=" lines unless it
   wants to wait until receipt of an Answer that eventually confirms
   support or non-support for Media Multiplexing.

   Independent of using Full Trickle or Half Trickle mode, the rules
   from [I-D.ietf-mmusic-sdp-mux-attributes] apply to both, Offerer and
   Answerer, when putting attributes as specified in Section 9.2 in the
   application/trickle-ice-sdpfrag body.

8.  SDP 'end-of-candidate' Attribute

8.1.  Definition

   This section defines a new SDP media-level and session-level
   attribute [RFC4566] 'end-of-candidate'. 'end-of-candidate' is a
   property attribute [RFC4566], and hence has no value.  By including
   this attribute in an Offer or Answer the sending agent indicates that
   it will not trickle further candidates.  When included at session
   level this indication applies to the whole session, when included at
   media level the indication applies only to the corresponding media
   description.

      Name: end-of-candidate

      Value: N/A

      Usage Level: media and session-level

      Charset Dependent: no

      Mux Category: IDENTICAL

      Example: a=end-of-candidate







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8.2.  Offer/Answer procedures

   The Offerer or Answerer MAY include an "a=end-of-candidates"
   attribute in case candidate discovery has ended and no further
   candidates are to be trickled.  The Offerer or Answerer MUST provide
   the "a=end-of-candidates" attribute together with the "a=ice-ufrag"
   and "a=ice-pwd" attributes of the current ICE generation as required
   by [I-D.ietf-ice-trickle].  When included at session level this
   indication applies to the whole session; when included at media level
   the indication applies only to the corresponding media description.

   Receipt of an "a=end-of-candidates" attribute at an Offerer or
   Answerer - with the "a=ice-ufrag" and "a=ice-pwd" attributes matching
   the current ICE generation - indicates that gathering of candidates
   has ended at the peer, either for the session or only for the
   corresponding media description as specified above.  The receiving
   agent forwards an end-of-candidates indication to the ICE Agent,
   which in turn acts as specified in [I-D.ietf-ice-trickle].

9.  Content Type 'application/trickle-ice-sdpfrag'

9.1.  Overall Description

   A application/trickle-ice-sdpfrag body is used exclusively by the
   'trickle-ice' Info Package.  Other SDP related applications need to
   define their own media type.  The INFO request body uses a subset of
   the possible SDP lines as defined by the grammar defined in
   [RFC4566].  A valid body uses only pseudo "m=" lines and certain
   attributes that are needed and/or useful for trickling candidates.
   The content adheres to the following grammar.

9.2.  Grammar

   The grammar of an 'application/trickle-ice-sdpfrag' body is based on
   the following ABNF [RFC5234].  It specifies the subset of existing
   SDP attributes, that is needed or useful for trickling candidates.
   The grammar uses the indicator for case-sensitivity %s as defined in
   [RFC7405], but also imports grammars for other SDP attributes that
   precede the production of [RFC7405].  A sender SHOULD stick to lower-
   case for such grammars, but a receiver MUST treat them case-
   insensitive.










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         ;  Syntax
         trickle-ice-sdpfrag =   session-level-fields
                           pseudo-media-descriptions
         session-level-fields = [bundle-group-attribute CRLF]
                           [ice-lite-attribute CRLF]
                            ice-pwd-attribute CRLF
                            ice-ufrag-attribute CRLF
                           [ice-options-attribute CRLF]
                           [ice-pacing-attribute CRLF]
                           [end-of-candidates-attribute CRLF]
                           extension-attribute-fields
                                 ; for future extensions

         ice-lite-attribute     = %s"a" "=" ice-lite
         ice-pwd-attribute      = %s"a" "=" ice-pwd-att
         ice-ufrag-attribute    = %s"a" "=" ice-ufrag-att
         ice-pacing-attribute   = %s"a" "=" ice-pacing-att
         ice-options-attribute  = %s"a" "=" ice-options
         bundle-group-attribute = %s"a" "=" %s"group:" bundle-semantics
                                    *(SP identification-tag)
         bundle-semantics = "BUNDLE"
         end-of-candidates-attribute  = %s"a" "=" end-of-candidates
         end-of-candidates            = %s"end-of-candidates"
         extension-attribute-fields   = attribute-fields

         pseudo-media-descriptions    =  *( media-field
                                    trickle-ice-attribute-fields
                                   [extension-attribute-fields] )
                                       ; for future extensions
         trickle-ice-attribute-fields = %s"a" "="  mid-attribute CRLF
                                 [%s"a" "=" %s"rtcp" CRLF]
                                 [%s"a" "=" %s"rtcp-mux" CRLF]
                                 [%s"a" "=" %s"rtcp-mux-only" CRLF]
                                *(candidate-attributes CRLF)
                                 [ice-pwd-attribute CRLF]
                                 [ice-ufrag-attribute CRLF]
                                 [remote-candidate-attribute CRLF]
                                 [end-of-candidates-attribute CRLF]
         remote-candidate-attribute   = %s"a" "=" remote-candidate-att
         candidate-attributes         = %s"a" "=" candidate-attribute

   with ice-lite, ice-pwd-att, remote-candidate-att, ice-ufrag-att, ice-
   pacing-att, ice-options, candidate-attribute remote-candidate-att
   from [I-D.ietf-mmusic-ice-sip-sdp], identification-tag, mid-attribute
   ; from [RFC5888], media-field, attribute-fields from [RFC4566].  The
   "a=rtcp" attribute is defined in [RFC3605], the "a=rtcp-mux"
   attribute in [RFC5761] and the "a=rtcp-mux-only" attribute in




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   [I-D.ietf-mmusic-mux-exclusive].  The latter attributes lack a formal
   grammar in their corresponding RFC and are reproduced here.

   An Agent MUST ignore any received unknown extension-attribute-fields.

10.  Info Package

10.1.  Rationale - Why INFO?

   The decision to use SIP INFO requests as a candidate transport method
   is based primarily on their lightweight nature.  Once a dialog has
   been established, INFO requests can be exchanged both ways with no
   restrictions on timing and frequency and no risk of collision.

   On the other hand, using Offer/Answer and UPDATE requests [RFC3311]
   would introduce the following complications:

   Blocking of messages:   [RFC3264] defines Offer/Answer as a strictly
      sequential mechanism.  There can only be a maximum of one active
      exchange at any point of time.  Both sides cannot simultaneously
      send Offers nor can they generate multiple Offers prior to
      receiving an Answer.  Using UPDATE requests for candidate
      transport would therefore imply the implementation of a candidate
      pool at every agent where candidates can be stored until it is
      once again that agent's "turn" to emit an Answer or a new Offer.
      Such an approach would introduce non-negligible complexity for no
      additional value.

   Elevated risk of glare:   The sequential nature of Offer/Answer also
      makes it impossible for both sides to send Offers simultaneously.
      What's worse is that there are no mechanisms in SIP to actually
      prevent that.  [RFC3261], where the situation of Offers crossing
      on the wire is described as "glare", only defines a procedure for
      addressing the issue after it has occurred.  According to that
      procedure both Offers are invalidated and both sides need to retry
      the negotiation after a period between 0 and 4 seconds.  The high
      likelihood for glare to occur and the average two second back-off
      intervals would imply Trickle ICE processing duration would not
      only fail to improve but actually exceed those of regular ICE.

   INFO messages decouple the exchange of candidates from the Offer/
   Answer negotiation and are subject to none of the glare issues
   described above, which makes them a very convenient and lightweight
   mechanism for asynchronous delivery of candidates.

   Using in-dialog INFO messages also provides a way of guaranteeing
   that candidates are delivered end-to-end, between the same entities
   that are actually in the process of initiating a session.  Out-of-



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   dialog alternatives would have implied requiring support for Globally
   Routable UA URI (GRUU) [RFC5627] which, given GRUUs relatively low
   adoption levels, would have constituted too strong of a constraint to
   the adoption of Trickle ICE.

10.2.  Overall Description

   This specification defines an Info Package for use by SIP User Agents
   implementing Trickle ICE.  INFO requests carry ICE candidates
   discovered after the peer user agents have confirmed mutual support
   for Trickle ICE.

10.3.  Applicability

   The purpose of the ICE protocol is to establish a media path in the
   presence of NAT and firewalls.  The candidates are transported in
   INFO requests and are part of this establishment.

   Candidates sent by a Trickle ICE Agent after the Offer, follow the
   same signaling path and reach the same entity as the Offer itself.
   While it is true that GRUUs can be used to achieve this, one of the
   goals of this specification is to allow operation of Trickle ICE in
   as many environments as possible including those without GRUU
   support.  Using out-of-dialog SUBSCRIBE/NOTIFY requests would not
   satisfy this goal.

10.4.  Info Package Name

   This document defines a SIP Info Package as per [RFC6086].  The Info
   Package token name for this package is "trickle-ice"

10.5.  Info Package Parameters

   This document does not define any Info Package parameters.

10.6.  SIP Option Tags

   [RFC6086] allows Info Package specifications to define SIP option-
   tags.  This specification extends the option-tag construct of the SIP
   grammar as follows:

    option-tag /= "trickle-ice"

   SIP entities that support this specification MUST place the 'trickle-
   ice' option-tag in a SIP Supported: header field within all SIP
   INVITE requests and responses.





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   When responding to, or generating a SIP OPTIONS request a SIP entity
   MUST also include the 'trickle-ice' option-tag in a SIP Supported:
   header field.

10.7.  Info Request Body Parts

   Entities implementing this specification MUST include a payload of
   type 'application/trickle-ice-sdpfrag' as defined in Section 9.2 in
   SIP INFO requests.  The payload is used to convey SDP-encoded ICE
   candidates.

10.8.  Info Package Usage Restrictions

   This document does not define any Info Package Usage Restrictions.

10.9.  Rate of INFO Requests

   A Trickle ICE Agent with many network interfaces might create a high
   rate of INFO requests if every newly detected candidate is trickled
   individually without aggregation.  Implementors that are concerned
   about loss of packets in such a case might consider aggregating ICE
   candidates and sending INFOs only at some configurable intervals.

10.10.  Info Package Security Considerations

   See Section 13

11.  Deployment Considerations

   Trickle ICE uses two mechanism for exchange of candidate information.
   This imposes new requirements to certain middleboxes that are used in
   some networks, e.g. for monitoring purposes.  While the first
   mechanism, SDP Offers and Answers, is already used by regular ICE and
   is assumed to be supported, the second mechanism, INFO request
   bodies, needs to be considered by such middleboxes as well when
   trickle ICE is used.  Such middleboxes need to make sure that they
   remain in the signaling path of the INFO requests and need to
   understand the INFO request body.

12.  IANA Considerations

   [RFC EDITOR NOTE: Please replace RFCXXXX with the RFC number of this
   document. ]








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12.1.  SDP 'end-of-candidate' Attribute

   This section defines a new SDP media-level and session-level
   attribute [RFC4566] , 'end-of-candidate'. 'end-of-candidate' is a
   property attribute [RFC4566] , and hence has no value.


     Name: end-of-candidate

      Value: N/A

      Usage Level: media and session

      Charset Dependent: no

      Purpose: The sender indicates that it will not trickle
               further ICE candidates.

      O/A Procedures: RFCXXX defines the detailed
                      SDP Offer/Answer procedures for
                      the 'end-of-candidate' attribute.

      Mux Category: IDENTICAL

      Reference: RFCXXXX

      Example:

      a=end-of-candidate


12.2.  application/trickle-ice-sdpfrag Media Type

   This section defines a new Media Type 'application/trickle-ice-
   sdpfrag' in accordance with [RFC6838].

      Type name: application

      Subtype name: trickle-ice-sdpfrag

      Required parameters: None.

      Optional parameters: None.

      Encoding considerations:






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         SDP files are primarily UTF-8 format text.  Although the
         initially defined content of a trickle-ice-sdpfrag body does
         only include ASCII characters, UTF-8 encoded content might be
         introduced via extension attributes.  The "a=charset:"
         attribute may be used to signal the presence of other character
         sets in certain parts of a trickle-ice-sdpfrag body (see
         [RFC4566]).  Arbitrary binary content cannot be directly
         represented in SDP or a trickle-ice-sdpfrag body.

      Security considerations:



         See [RFC4566] and RFCXXXX

      Interoperability considerations:



         See RFCXXXX

      Published specification:



         See RFCXXXX

      Applications which use this Media Type:



         Trickle-ICE

      Fragment identifier considerations: N/A

      Additional information:



         Deprecated alias names for this type: N/A

         Magic number(s): N/A

         File extension(s): N/A

         Macintosh File Type Code(s): N/A

      Person and email address to contact for further information:



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         The IESG (iesg@ietf.org)

      Intended usage:



         Trickle-ICE for SIP as specified in RFCXXXX.

      Restrictions on usage: N/A

      Author/Change controller:



         The IESG (iesg@ietf.org)

      Provisional registration? (standards tree only): N/A

12.3.  SIP Info Package 'trickle-ice'

   This document defines a new SIP Info Package named 'trickle-ice' and
   updates the Info Packages Registry with the following entry.

       +-------------+-----------+
       | Name        | Reference |
       +-------------+-----------+
       | trickle-ice | [RFCXXXX] |
       |             |           |
       +-------------+-----------+

12.4.  SIP Option Tag 'trickle-ice'

   This specification registers a new SIP option tag 'trickle-ice' as
   per the guidelines in Section 27.1 of [RFC3261] and updates the
   "Option Tags" section of the SIP Parameter Registry with the
   following entry:

       +-------------+-------------------------------------+-----------+
       | Name        | Description                         | Reference |
       +-------------+-------------------------------------+-----------+
       | trickle-ice | This option tag is used to indicate | [RFCXXXX] |
       |             | that a UA supports and understands  |           |
       |             | Trickle-ICE.                        |           |
       +-------------+-------------------------------------+-----------+







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13.  Security Considerations

   The Security Considerations of [I-D.ietf-mmusic-ice-sip-sdp],
   [RFC6086] and [I-D.ietf-ice-trickle] apply.  This document clarifies
   how the above specifications are used together for trickling
   candidates and does not create additional security risks.

   The new Info Package 'trickle-ice' and the new Media Type
   'application/trickle-ice-sdpfrag' do not introduce additional
   security considerations when used in the context of Trickle ICE.
   Both are not intended to be used for other applications, so any
   security considerations for its use in other contexts is out of the
   scope of this document

14.  Acknowledgements

   The authors would like to thank Flemming Andreasen, Ayush Jain, Paul
   Kyzivat, Jonathan Lennox, Simon Perreault, Roman Shpount and Martin
   Thomson for reviewing and/or making various suggestions for
   improvements and optimizations.

   The authors would also like to thank Flemming Andreasen for
   shepherding this document and Ben Campbell for his AD review and
   suggestions.

15.  Change Log

   [RFC EDITOR NOTE: Please remove this section when publishing].

   Changes from draft-ietf-mmusic-trickle-ice-sip-01

   o  Editorial Clean up

   o  IANA Consideration added

   o  Security Consideration added

   o  RTCP and BUNDLE Consideration added with rules for including
      "a=rtcp-mux" and "a=group: BUNDLLE" attributes

   o  3PCC Consideration added

   o  Clarified that 18x w/o answer is sufficient to create a dialog
      that allows for trickling to start

   o  Added remaining Info Package definition sections as outlined in
      section 10 of [RFC6086]




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   o  Added definition of application/sdpfrag making draft-ivov-mmusic-
      sdpfrag obsolete

   o  Added pseudo m-lines as additional separator in sdpfrag bodies for
      Trickle ICE

   o  Added ABNF for sdp-frag bodies and Trickle-ICE package

   Changes from draft-ietf-mmusic-trickle-ice-sip-02

   o  Removed definition of application/sdpfrag

   o  Replaced with new type application/trickle-ice-sdpfrag

   o  RTCP and BUNDLE Consideration enhanced with some examples

   o  draft-ietf-mmusic-sdp-bundle-negotiation and RFC5761 changed to
      normative reference

   o  Removed reference to 4566bis

   o  Addressed review comment from Simon Perreault

   Changes from draft-ietf-mmusic-trickle-ice-sip-03

   o  replaced reference to RFC5245 with draft-ietf-mmusic-rfc5245bis
      and draft-ietf-mmusic-ice-sip-sdp

   o  Corrected Figure 10, credits to Ayush Jain for finding the bug

   o  Referencing a=rtcp and a=rtcp-mux handling from draft-ietf-mmusic-
      ice-sip-sdp

   o  Referencing a=rtcp-mux-exclusive handling from draft-ietf-mmusic-
      mux-exclusive, enhanced ABNF to support a=rtcp-mux-exclusive

   o  Clarifying that draft-ietf-mmusic-sdp-mux-attributes applies for
      the application/trickle-ice-sdpfrag body

   Changes from draft-ietf-mmusic-trickle-ice-sip-04

   o  considered comments from Christer Holmberg

   o  corrected grammar for INFO package, such that ice-ufrag/pwd are
      also allowed on media-level as specified in
      [I-D.ietf-mmusic-ice-sip-sdp]

   o  Added new ice-pacing-attribute fom [I-D.ietf-mmusic-ice-sip-sdp]



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   o  Added formal definition for the end-of-candidates attribute

   Changes from draft-ietf-mmusic-trickle-ice-sip-05

   o  considered further comments from Christer Holmberg

   o  editorial comments on section 3 addressed

   o  moved section 3.1 to section 10.1 and applied some edits

   o  replaced the term "previously sent candidates" with "currently
      known and used candidates".

   Changes from draft-ietf-mmusic-trickle-ice-sip-06

   o  editorial fixes

   o  additional text on the content of the INFO messages.

   o  recommendation on what to do if a previously sent candidate is
      unexpectedly missing in a subsequent INFO

   o  terminology alignment with draft-ietf-ice-trickle-07

   Changes from draft-ietf-mmusic-trickle-ice-sip-07

   o  editorial fixes

   o  clarification on ordering of candidates for alignment with draft-
      ietf-ice-trickle-12

   o  O/A procedures for end-of-candidates attribute described here
      after corresponding procedures have been removed from draft-ietf-
      ice-trickle-11

   o  using IPv6 addresses in examples

   Changes from draft-ietf-mmusic-trickle-ice-sip-08

   o  editorial fixes/clarification based on Flemmings review

   o  Description of Trickle specifics in O/A procedures for initial O/A
      exchange and specification of ICE mismatch exception

   Changes from draft-ietf-mmusic-trickle-ice-sip-09

   o  editorial fixes/correction of references




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   o  adding missing Ref to RFC3605 in section 6, 5th para

   o  replaced remaining IPv4 adresses with IPv6

   o  Added text for handling a=rtcp in case of default RTP address
      0.0.0.0:9 based on comment from Roman Shpount.

   Changes from draft-ietf-mmusic-trickle-ice-sip-10

   o  editorial fixes due to idnits output

   Changes from draft-ietf-mmusic-trickle-ice-sip-11

   o  addressing comments from Ben Campell's AD review and Christer's
      review

   o  Numerous editorial improvements/corrections

   o  Added [RFC8174] boiler plate and adapted usage of normative
      language

   o  Clarified terminology ICE modules .vs.  ICE agent

   o  Added more detailed OA procedures

   o  Corrected default values in m-line and usage of "a=mid:" attribute
      explicitly mentioned for offer/answer

   o  Removed explicit mentioning of XMPP

   o  Added Deployment Considerations section

   o  Fixed ref for rfc5245bis

16.  References

16.1.  Normative References

   [I-D.ietf-ice-rfc5245bis]
              Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", draft-ietf-ice-
              rfc5245bis-15 (work in progress), November 2017.








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   [I-D.ietf-ice-trickle]
              Ivov, E., Rescorla, E., Uberti, J., and P. Saint-Andre,
              "Trickle ICE: Incremental Provisioning of Candidates for
              the Interactive Connectivity Establishment (ICE)
              Protocol", draft-ietf-ice-trickle-15 (work in progress),
              November 2017.

   [I-D.ietf-mmusic-ice-sip-sdp]
              Petit-Huguenin, M., Keranen, A., and S. Nandakumar,
              "Session Description Protocol (SDP) Offer/Answer
              procedures for Interactive Connectivity Establishment
              (ICE)", draft-ietf-mmusic-ice-sip-sdp-16 (work in
              progress), November 2017.

   [I-D.ietf-mmusic-mux-exclusive]
              Holmberg, C., "Indicating Exclusive Support of RTP/RTCP
              Multiplexing using SDP", draft-ietf-mmusic-mux-
              exclusive-12 (work in progress), May 2017.

   [I-D.ietf-mmusic-sdp-bundle-negotiation]
              Holmberg, C., Alvestrand, H., and C. Jennings,
              "Negotiating Media Multiplexing Using the Session
              Description Protocol (SDP)", draft-ietf-mmusic-sdp-bundle-
              negotiation-43 (work in progress), December 2017.

   [I-D.ietf-mmusic-sdp-mux-attributes]
              Nandakumar, S., "A Framework for SDP Attributes when
              Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-16
              (work in progress), December 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC3262]  Rosenberg, J. and H. Schulzrinne, "Reliability of
              Provisional Responses in Session Initiation Protocol
              (SIP)", RFC 3262, DOI 10.17487/RFC3262, June 2002,
              <https://www.rfc-editor.org/info/rfc3262>.






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   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,
              <https://www.rfc-editor.org/info/rfc3264>.

   [RFC3605]  Huitema, C., "Real Time Control Protocol (RTCP) attribute
              in Session Description Protocol (SDP)", RFC 3605,
              DOI 10.17487/RFC3605, October 2003,
              <https://www.rfc-editor.org/info/rfc3605>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <https://www.rfc-editor.org/info/rfc4566>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761,
              DOI 10.17487/RFC5761, April 2010,
              <https://www.rfc-editor.org/info/rfc5761>.

   [RFC5888]  Camarillo, G. and H. Schulzrinne, "The Session Description
              Protocol (SDP) Grouping Framework", RFC 5888,
              DOI 10.17487/RFC5888, June 2010,
              <https://www.rfc-editor.org/info/rfc5888>.

   [RFC6086]  Holmberg, C., Burger, E., and H. Kaplan, "Session
              Initiation Protocol (SIP) INFO Method and Package
              Framework", RFC 6086, DOI 10.17487/RFC6086, January 2011,
              <https://www.rfc-editor.org/info/rfc6086>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC7405]  Kyzivat, P., "Case-Sensitive String Support in ABNF",
              RFC 7405, DOI 10.17487/RFC7405, December 2014,
              <https://www.rfc-editor.org/info/rfc7405>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.





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16.2.  Informative References

   [RFC3311]  Rosenberg, J., "The Session Initiation Protocol (SIP)
              UPDATE Method", RFC 3311, DOI 10.17487/RFC3311, October
              2002, <https://www.rfc-editor.org/info/rfc3311>.

   [RFC3725]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
              Camarillo, "Best Current Practices for Third Party Call
              Control (3pcc) in the Session Initiation Protocol (SIP)",
              BCP 85, RFC 3725, DOI 10.17487/RFC3725, April 2004,
              <https://www.rfc-editor.org/info/rfc3725>.

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840,
              DOI 10.17487/RFC3840, August 2004,
              <https://www.rfc-editor.org/info/rfc3840>.

   [RFC5627]  Rosenberg, J., "Obtaining and Using Globally Routable User
              Agent URIs (GRUUs) in the Session Initiation Protocol
              (SIP)", RFC 5627, DOI 10.17487/RFC5627, October 2009,
              <https://www.rfc-editor.org/info/rfc5627>.

Authors' Addresses

   Emil Ivov
   Jitsi
   Strasbourg  67000
   France

   Phone: +33 6 72 81 15 55
   Email: emcho@jitsi.org


   Thomas Stach
   Unaffiliated
   Vienna  1130
   Austria

   Email: thomass.stach@gmail.com











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   Enrico Marocco
   Telecom Italia
   Via G. Reiss Romoli, 274
   Turin  10148
   Italy

   Email: enrico.marocco@telecomitalia.it


   Christer Holmberg
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: christer.holmberg@ericsson.com



































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