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Versions: 00 01 02 draft-ietf-mmusic-trickle-ice-sip

Network Working Group                                            E. Ivov
Internet-Draft                                                     Jitsi
Intended status: Standards Track                              E. Marocco
Expires: December 19, 2014                                Telecom Italia
                                                             C. Holmberg
                                                                Ericsson
                                                           June 17, 2014


       A Session Initiation Protocol (SIP) usage for Trickle ICE
                  draft-ivov-mmusic-trickle-ice-sip-02

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).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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

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

   This Internet-Draft will expire on December 19, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Rationale. Why INFO . . . . . . . . . . . . . . . . . . .   4
     3.2.  Discovery issues  . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Relationship with the Offer/Answer Model  . . . . . . . .   6
   4.  Incremental signalling of ICE candidates  . . . . . . . . . .   8
     4.1.  Asserting Offer/Answer delivery and dialog state  . . . .   8
     4.2.  Delivering candidates in INFO messages  . . . . . . . . .  12
   5.  Initial discovery of trickle ICE support  . . . . . . . . . .  14
     5.1.  Provisioning support for trickle ICE  . . . . . . . . . .  14
     5.2.  Trickle ICE discovery with GRUU . . . . . . . . . . . . .  15
     5.3.  Trickle ICE discovery through other protocols . . . . . .  16
     5.4.  Fallback to half trickle  . . . . . . . . . . . . . . . .  16
   6.  Info Package  . . . . . . . . . . . . . . . . . . . . . . . .  18
     6.1.  Overall Description . . . . . . . . . . . . . . . . . . .  18
     6.2.  Applicability . . . . . . . . . . . . . . . . . . . . . .  18
     6.3.  Info Package Name . . . . . . . . . . . . . . . . . . . .  18
     6.4.  Info Package Parameters . . . . . . . . . . . . . . . . .  18
     6.5.  SIP Option-Tags . . . . . . . . . . . . . . . . . . . . .  18
     6.6.  Info Message Body Parts . . . . . . . . . . . . . . . . .  19
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Appendix A.  Open issues  . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

1.  Introduction

   The vanilla specification of the Interactive Connectivity
   Establishment (vanilla ICE) protocol [RFC5245] describes a mechanism
   for NAT traversal that consists of three main phases: a phase where
   an agent gathers a set of candidate transport addresses (source IP
   address, port and transport protocol), a second phase where these



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   candidates are sent to a remote agent and this gathering procedure is
   repeated and, finally, a third phase where connectivity between all
   candidates in both sets is checked (connectivity checks).  Once these
   phases have been completed, and only then, can both agents begin
   communication.  According to the Vanilla ICE specification the three
   phases above happen consecutively, in a blocking way, which may lead
   to undesirable latency during session establishment.

   The trickle ICE extension defined in [I-D.ietf-mmusic-trickle-ice]
   defines generic semantics required for these ICE phases to happen
   simultaneously, in a non-blocking way and hence speed up session
   establishment.

   The present specification defines a usage of trickle ICE with the
   Session Initiation Protocol (SIP).  It describes how ICE candidates
   are to be incrementally exchanged with SIP INFO requests and how the
   half and full-trickle modes defined in [I-D.ietf-mmusic-trickle-ice]
   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 [RFC6086] for use with
   Trickle ICE.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   This specification makes use of all terminology defined by the
   protocol for Interactive Connectivity Establishment in [RFC5245] and
   its Trickle ICE extension [I-D.ietf-mmusic-trickle-ice].  It is
   assumed that the reader will be familiar with the terminology from
   both of them.

3.  Protocol Overview

   The semantics that vanilla ICE [RFC5245] defines for exchanging ICE
   candidates are exclusively based on use of Offers and Answers as per
   [RFC3264] over the Session Description Protocol (SDP) [RFC4566].
   This specification extends these mechanism by allowing ICE candidates
   to also be sent after the completion of Offer/Answer negotiation,
   through the use of SIP INFO messages and a newly defined Info Package
   [RFC6086].

   Specifically, in cases where trickle ICE is fully supported, a
   typical exchange would happen along the following lines: The offerer
   would send an INVITE containing a subset of candidates and then wait



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   for an early dialog to be established.  Once that happens, it will be
   able to continue sending candidates through in INFO requests and
   within the same dialog.

   Similarly, once it has sent an answer (though not earlier) an
   answerer can continue "trickling" ICE candidates using INFO messages
   within the dialog established by its 18x provisional response.
   Figure 1 shows such a sample exchange:

      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          |                |
          |               |------------------------>|                |
          |               |                         |                |
          |               |    5245 SIP re-INVITE   |                |
          |               |------------------------>|                |
          |               |          200 OK         |                |
          |               |<------------------------|                |
          |               |            ACK          |                |
          |               |------------------------>|                |
          |               |                         |                |
          |               |<===== MEDIA FLOWS =====>|                |
          |               |                         |                |

              Figure 1: Sample trickle ICE scenario with SIP

3.1.  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 messages can be exchanged both ways with no
   restrictions on timing and frequency and no risk of collision.




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   On the other hand, using offer/answer and UPDATE requests, which from
   an [RFC5245] perspective is the traditional way of transporting ICE
   candidates, introduces the following complications:

   Need for a turn-based mechanism:   [RFC3264] defines Offer/Answer as
      a strictly sequential mechanism.  There can only be a maximum of
      one exchange at any point of time.  Both sides cannot
      simultaneously send offers nor can they generate multiple offers
      prior to receiving an answer.  Using UPDATEs 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 backoff
      intervals would imply trickle ICE processing duration would not
      only fail to improve but actually exceed those of vanilla ICE.

   INFO messages have no impact on 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.  The
   alternative 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 constraint to
   the adoption of trickle ICE.

3.2.  Discovery issues

   In order for 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




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   incremental candidate provisioning, which implies the need to confirm
   such support before actually using it.

   Contrary to other protocols, like XMPP [RFC6120], 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-mmusic-trickle-ice] provides one way around
   this, by requiring first offers to contain a complete set of ICE
   candidates and only using incremental provisioning for the rest of
   the sessions.

   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 5.4 discusses the implementation
   and use of "half trickle" by SIP UAs where none of the above are an
   option.

3.3.  Relationship with the Offer/Answer Model

   It is important to note that this specification does not require,
   define, or even assume any mechanisms that would have an impact on
   the Offer/Answer model beyond the way it is already used by vanilla
   ICE [RFC5245].  From the perspective of all SIP middle boxes and
   proxies, and with the exception of the actual INFO messages,
   signalling in general and Offer/Answer exchanges in particular would
   look the same way for trickle as they would for vanilla ICE.














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   +-------------------------------+  +-------------------------------+
   |   Alice      +--------------+ |  | +--------------+       Bob    |
   |              | Offer/Answer | |  | | Offer/Answer |              |
   | +-------+    |    Module    | |  | |    Module    |    +-------+ |
   | |  ICE  |    +--------------+ |  | +--------------+    |  ICE  | |
   | | Agent |          |          |  |        |            | Agent | |
   | +-------+          |          |  |        |            +-------+ |
   +-------------------------------+  +-------------------------------+
         |              |                      |                |
         |              |    INVITE (Offer)    |                |
         |              |--------------------->|                |
         |              |     183 (Answer)     |                |
         |              |<---------------------|                |
         |              |                      |                |
         |                                                      |
         |             SIP INFO (more candidates)               |
         |----------------------------------------------------->|
         |             SIP INFO (more candidates)               |
         |<-----------------------------------------------------|
         |                                                      |
         |          STUN Binding Requests/Responses             |
         |----------------------------------------------------->|
         |          STUN Binding Requests/Responses             |
         |<-----------------------------------------------------|
         |                                                      |
         |              |                      |                |
         |              |  5245 SIP re-INVITE  |                |
         |              |--------------------->|                |
         |              |        200 OK        |                |
         |              |<---------------------|                |



       Figure 2: Distinguishing between trickle ICE and traditional
                                signalling.

   It is important to note that, as displayed on Figure 2, exchanging
   candidates through SIP INFO messages are best represented as
   signalling between ICE agents and not between the traditional SIP and
   Offer/Answer modules of SIP User Agents.  Such INFO requests do not
   impact the state of Offer/Answer, nor do they have an impact on the
   version number in the "o=" line.  In that regard they are actually
   comparable to Peer Reflexive candidates that ICE agents can discover
   during ICE processing.







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4.  Incremental signalling of ICE candidates

   Trickle ICE agents will construct offers and answers the same way a
   [RFC5245] compatible agent would, with the following two main
   differences:

   o  First, all offers and answers sent by the trickle ICE capable
      agents MUST indicate support for trickle ICE through the "trickle"
      ice-options tag defined in [I-D.ietf-mmusic-trickle-ice]:


         a=ice-options:trickle


   o  Second, offers and answers may contain all, some, or no ICE
      candidates at all.

   Once an agent has sent an offer or an answer indicating support for
   trickle ICE, it MUST be prepared to receive SIP INFO requests within
   that same dialog, containing additional candidates or an indication
   for the end of such candidates.

   Similarly, as soon as a SIP UA has established a dialog (including an
   early state one) it MAY begin sending INFO requests containing
   additional candidates or end-of-candidates indications.  Such INFO
   requests MUST be sent within that same dialog.  This is necessary so
   that the candidates from these INFO messages could be delivered to
   the same entities that initiated the session.

4.1.  Asserting Offer/Answer delivery and dialog state

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

   o  ICE support in the peer agent MUST be confirmed.

   o  The SIP dialog at both sides MUST be at least in the early state.

   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 and an answer have
   been exchanged, which needs to happen anyway for ICE processing to
   commence (see Figure 3).







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

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

   Satisfying both conditions is also relatively trivial for ICE agents
   that have sent an offer in an INVITE and that have received an
   answer.  Regardless of how that answer was delivered, it is
   guaranteed to have confirmed support for trickle ICE within the
   answerer (or lack thereof) and to have fully initialized the SIP
   dialog at both ends.  Offerers in the above situation can therefore
   freely commence trickling within the newly established dialog (see
   Figure 4).






















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                 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.)  |
                   |<------------------------|
                   |                         |

     Figure 4: A SIP Offerer in a 3PCC scenario can also freely start
                trickling as soon as it receives an answer.

   Agents that have sent an offer in a reliable provisional response or
   in a 200 OK and that receive an answer in a PRACK or in an ACK are
   also in a similar situation because, by the time the offer and the
   answer are exchanged, support for trickle ICE will be confirmed and
   the SIP dialog is guaranteed to be in a state that would allow in-
   dialog INFO requests.

   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, there
   is no way for the answerer to know when or if it has been received
   (Figure 5).
















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                 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 5: A SIP UA that has answer-ed in an unreliable provisional
   response cannot know exactly when it is received and when the dialog
          at the side of the receiver has entered the early state

   In order to clear this ambiguity as soon as possible, trickle ICE SIP
   UAs MUST send a trickle ICE INFO request as soon as they receive an
   SDP Answer in an unreliable provisional response.  This INFO message
   can only contain the candidates that were already provided in the
   offer (as would be the case when half trickle is performed or when no
   new candidates have been learned since then) or they can also deliver
   new information, such as new candidates (if available) or an end-of-
   candidates indication in case candidate discovery has ended in the
   mean time.

   As soon as answerers have received such INFO requests, they would
   have an indication that a dialog is well established at both ends and
   they MAY begin trickling (Figure 6).


















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

    Figure 6: A SIP UA that has answer-ed in an unreliable provisional
   response cannot know exactly when it is received and when the dialog
          at the side of the receiver has entered the early state

   Obviously, if PRACK [RFC3262] requests are supported and used, there
   is no need for the above as the PRACKs themselves would provide
   sufficient indication for the state of the dialog.

4.2.  Delivering candidates in INFO messages

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


           a=candidate:2 1 UDP 1694498815 192.0.2.3 5000 typ srflx
                     raddr 10.0.1.1 rport 8998


   The use of SIP INFO requests happens within the context of an Info
   Package specifically defined for the purpose (Section 6).

   Such INFO requests MUST be sent within the existing SIP dialog.  The
   MIME type for their payload MUST be set to 'application/sdpfrag' as
   defined in [I-D.ivov-dispatch-sdpfrag].

   Since neither the "a=candidate" nor the "a=end-of-candidates" lines
   contain information that would allow correlating them to a specific
   "m=" line, this is handled through the use of MID [RFC3388].  Agents
   MUST include the corresponding "a=mid" line for every "m=" line whose
   candidate list they intend to update.  Such "a=mid" lines MUST



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   immediately precede the list of candidates for that specific "m="
   line.  All "a=candidate" or "a=end-of-candidates" lines following an
   "a=mid" line, up until (and excluding) the next occurrence of an
   "a=mid" line, pertain the the "m=" line identified by that MID.
   "a=end-of-candidates" lines preceding any "a=mid" lines indicate end
   of all trickling from that agent (as opposed to end of trickling for
   a specific "m=" line.

   The use of "a=mid" lines allows for a structure similar to the one in
   SDP offers and answers where one can distinguish separate media-level
   and session-level sections.  In the current case lines preceding any
   "a=mid" lines are considered to be session-level.  Lines appearing in
   between or after "a=mid" lines will be interpreted as media-level.

   All INFO requests MUST carry the "ice-pwd" and "ice-ufrag" attributes
   that would allow mapping them to a specific ICE generation.  INFO
   requests containing ice-ufrag and ice-pwd values that do not match
   those of the current ICE processing session MUST be discarded.  The
   "ice-pwd" and "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 sesssion-level attributes there, they will also appear at
   the beginning of all INFO message payloads, preceding all "a=mid"
   lines.  If they were originally exchanged as media level attributes,
   potentially overriding session-level values, then they will also be
   included in INFO message payloads, following the corresponding
   "a=mid' line.

   In every INFO request agents MUST include all local candidates they
   have previously signalled.  This is necessary in order to more easily
   avoid problems that would arise from misordering and unreliability.

   When receiving INFO requests carrying any candidates, agents will
   therefore first identify and discard the SDP lines containing
   candidates they have already received in previous INFO requests or in
   the Offer/Answer exchange preceding them.  Two candidates are
   considered to be equal if their IP address port, transport and
   component ID are the same.  After identifying and discarding known
   candidates, agents will then process them remaining ones (the actual
   new candidates) according to the rules described in
   [I-D.ietf-mmusic-trickle-ice].

   The following example shows the content of one sample candidate
   delivering INFO request:








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

         a=ice-pwd:asd88fgpdd777uzjYhagZg
         a=ice-ufrag:8hhY
         a=mid:1
         a=candidate:1 1 UDP 1658497328 192.168.100.33 5000 typ host
         a=candidate:2 1 UDP 1658497328 96.1.2.3 5000 typ srflx
                 raddr 10.0.1.1 rport 8998
         a=mid:2
         a=candidate:2 1 UDP 1658497328 96.1.2.3 5002 typ srflx
                 raddr 10.0.1.1 rport 9000
         a=end-of-candidates


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-mmusic-trickle-ice] 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 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
   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 likely
   to be the case, for example, for WebRTC clients that will always be
   communicating with other WebRTC clients or known Session Border
   Controllers (SBC) with support for this specification.

   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.





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5.2.  Trickle ICE discovery with GRUU

   [RFC3840] provides a way for SIP user agents to query for support of
   specific capabilities using, among others, OPTIONS requests.  GRUU
   support 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 6.5 provides SIP UAs with a way of learning the
   capabilities of specific US instances and then addressing them
   directly with INVITE requests that require SIP support.

   Such targeted trickling may happen in different ways.  One option
   would be for a SIP UA to learn the GRUU instance ID of a peer through
   presence and to then query its capabilities direction with an OPTIONS
   request.  Alternately, 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 7).

            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 7: 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.






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5.3.  Trickle ICE discovery through other protocols

   Protocols like XMPP [RFC6120] define advanced discovery mechanisms
   that allow specific features to be queried priory to actually
   attempting to use them.  Solutions like [RFC7081] define ways of
   using SIP and XMPP together which also provides a way for dual stack
   SIP+XMPP endpoints to make use of such features and verify trickle
   ICE support for a specific SIP endpoint through XMPP.  [TODO expand
   on a specific way to do this]

5.4.  Fallback 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-mmusic-trickle-ice].  With half trickle, agents initiate
   sessions the same way they would when using vanilla ICE [RFC5245].
   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 likelyhood for an imminent call
   (e.g., activity on a keypad or a phone going offhook).

   Using half trickle would result in offers that are compatible with
   both vanilla ICE and legacy [RFC3264] endpoints both.

   A typical (half) trickle ICE exchange with SIP would look this way:























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      STUN/Turn                                                STUN/TURN
       Servers          Alice                      Bob          Servers
          |               |                         |               |
          |<--------------|                         |               |
          |               |                         |               |
          |               |                         |               |
          |   Candidate   |                         |               |
          |               |                         |               |
          |               |                         |               |
          |   Discovery   |                         |               |
          |               |                         |               |
          |               |                         |               |
          |-------------->|     INVITE (Offer)      |               |
          |               |------------------------>|               |
          |               |      183 (Answer)       |-------------->|
          |               |<------------------------|               |
          |               |                         |               |
          |               |  INFO (more candidates) |   Candidate   |
          |               |<------------------------|               |
          |               |  Connectivity Checks    |               |
          |               |<=======================>|   Discovery   |
          |               | INFO (more candidates)  |               |
          |               |<------------------------|               |
          |               |  Connectivity Checks    |<--------------|
          |               |<=======================>|               |
          |               |                         |               |
          |               |          200 OK         |               |
          |               |<------------------------|               |
          |               |                         |               |
          |               |    5245 SIP re-INVITE   |               |
          |               |------------------------>|               |
          |               |          200 OK         |               |
          |               |<------------------------|               |
          |               |                         |               |
          |               |                         |               |
          |               |<===== MEDIA FLOWS =====>|               |
          |               |                         |               |


                             Figure 8: Example

   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.





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6.  Info Package

6.1.  Overall Description

   This specification defines an Info Package meant for use by SIP user
   agents implementing Trickle ICE.  Typically INFO requests would carry
   ICE candidates discovered after the user agent has sent or received a
   trickle-ice offer.

6.2.  Applicability

   The purpose of the ICE protocol is to establish a media path.  The
   candidates that this specification transports in INFO requests are
   part of this establishment.  There is hence no way for them to be
   transported through the not yet existing media path.

   Candidates sent by a trickle ICE agent after the offer, are meant to
   follow the same signalling 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 with
   no GRUU support.  Using out-of-dialog SUBSCRIBE/NOTIFY requests would
   not satisfy this goal.

6.3.  Info Package Name

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

6.4.  Info Package Parameters

   This document does not define any Info package parameters.

6.5.  SIP Option-Tags

   [RFC6086] allows Info Package specifications to define SIP option-
   tags.  This document therefore stipulates that SIP entities that
   support trickle ICE and this specification MUST place the 'trickle-
   ice' option-tag in a SIP Supported header field.

   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.








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6.6.  Info Message Body Parts

   Entities implementing this specification MUST include SDP encoded ICE
   candidates in all SIP INFO requests.  The MIME type for the payload
   MUST be of type 'application/sdp' as defined in Section 4.2 and
   [I-D.ietf-mmusic-trickle-ice].

7.  Security Considerations

   [TODO]

8.  Acknowledgements

   The authors would like to thank Thomas Stack for suggesting the INFO
   acknowledgements used in the specification as a way of avoiding
   making PRACKs mandatory, Paul Kyzivat and Jonathan Lennox for making
   various suggestions for improvements and optimisations.

9.  References

9.1.  Normative References

   [I-D.ietf-mmusic-trickle-ice]
              Ivov, E., Rescorla, E., and J. Uberti, "Trickle ICE:
              Incremental Provisioning of Candidates for the Interactive
              Connectivity Establishment (ICE) Protocol", draft-ietf-
              mmusic-trickle-ice-01 (work in progress), February 2014.

   [I-D.ivov-dispatch-sdpfrag]
              Ivov, E. and A. Roach, "Internet Media Type application/
              sdpfrag", draft-ivov-dispatch-sdpfrag-03 (work in
              progress), October 2013.

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

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

   [RFC3262]  Rosenberg, J. and H. Schulzrinne, "Reliability of
              Provisional Responses in Session Initiation Protocol
              (SIP)", RFC 3262, June 2002.

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



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

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April
              2010.

   [RFC6086]  Holmberg, C., Burger, E., and H. Kaplan, "Session
              Initiation Protocol (SIP) INFO Method and Package
              Framework", RFC 6086, January 2011.

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

9.2.  Informative References

   [RFC3388]  Camarillo, G., Eriksson, G., Holler, J., and H.
              Schulzrinne, "Grouping of Media Lines in the Session
              Description Protocol (SDP)", RFC 3388, December 2002.

   [RFC3840]  Rosenberg, J., Schulzrinne, H., and P. Kyzivat,
              "Indicating User Agent Capabilities in the Session
              Initiation Protocol (SIP)", RFC 3840, August 2004.

   [RFC5627]  Rosenberg, J., "Obtaining and Using Globally Routable User
              Agent URIs (GRUUs) in the Session Initiation Protocol
              (SIP)", RFC 5627, October 2009.

   [RFC7081]  Ivov, E., Saint-Andre, P., and E. Marocco, "CUSAX:
              Combined Use of the Session Initiation Protocol (SIP) and
              the Extensible Messaging and Presence Protocol (XMPP)",
              RFC 7081, November 2013.

Appendix A.  Open issues

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

   1.  Should we allow for full trickle if support can be verified
       automatically and confirmed for a gruu with [RFC3840].

   2.  Can we pick between MID and stream indices for stream
       identification.






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Authors' Addresses

   Emil Ivov
   Jitsi
   Strasbourg  67000
   France

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


   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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