Session Description Protocol (SDP) Offer/Answer procedures for Interactive Connectivity Establishment (ICE)
draft-ietf-mmusic-ice-sip-sdp-13

MMUSIC                                                 M. Petit-Huguenin
Internet-Draft                                        Impedance Mismatch
Obsoletes: 5245 (if approved)                                 A. Keranen
Intended status: Standards Track                                Ericsson
Expires: December 30, 2017                                 S. Nandakumar
                                                           Cisco Systems
                                                           June 28, 2017

     Session Description Protocol (SDP) Offer/Answer procedures for
              Interactive Connectivity Establishment (ICE)
                    draft-ietf-mmusic-ice-sip-sdp-13

Abstract

   This document describes Session Description Protocol (SDP) Offer/
   Answer procedures for carrying out Interactive Connectivity
   Establishment (ICE) between the agents.

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  ICE Candidate Exchange and Offer/Answer Mapping . . . . . . .   4
   4.  SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . .   4
     4.1.  Initial Offer/Answer Exchange . . . . . . . . . . . . . .   4
       4.1.1.  Sending the Initial Offer . . . . . . . . . . . . . .   4
       4.1.2.  Receiving the Initial Offer . . . . . . . . . . . . .   7
       4.1.3.  Receipt of the Initial Answer . . . . . . . . . . . .   8
       4.1.4.  Performing Connectivity Checks  . . . . . . . . . . .   9
       4.1.5.  Concluding ICE  . . . . . . . . . . . . . . . . . . .   9
     4.2.  Subsequent Offer/Answer Exchanges . . . . . . . . . . . .   9
       4.2.1.  Generating the Offer  . . . . . . . . . . . . . . . .  10
       4.2.2.  Receiving the Offer and Generating an Answer  . . . .  12
       4.2.3.  Receiving the Answer for a Subsequent Offer . . . . .  16
       4.2.4.  Updating the Check and Valid Lists  . . . . . . . . .  17
   5.  Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     5.1.  "candidate" Attribute . . . . . . . . . . . . . . . . . .  18
     5.2.  "remote-candidates" Attribute . . . . . . . . . . . . . .  21
     5.3.  "ice-lite" and "ice-mismatch" Attributes  . . . . . . . .  21
     5.4.  "ice-ufrag" and "ice-pwd" Attributes  . . . . . . . . . .  22
     5.5.  "ice-pacing" Attribute  . . . . . . . . . . . . . . . . .  22
     5.6.  "ice-options" Attribute . . . . . . . . . . . . . . . . .  23
   6.  Keepalives  . . . . . . . . . . . . . . . . . . . . . . . . .  23
   7.  Media Handling  . . . . . . . . . . . . . . . . . . . . . . .  23
     7.1.  Sending Media . . . . . . . . . . . . . . . . . . . . . .  23
       7.1.1.  Procedures for All Implementations  . . . . . . . . .  24
     7.2.  Receiving Media . . . . . . . . . . . . . . . . . . . . .  24
   8.  SIP Considerations  . . . . . . . . . . . . . . . . . . . . .  24
     8.1.  Latency Guidelines  . . . . . . . . . . . . . . . . . . .  24
       8.1.1.  Offer in INVITE . . . . . . . . . . . . . . . . . . .  25
       8.1.2.  Offer in Response . . . . . . . . . . . . . . . . . .  26

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     8.2.  SIP Option Tags and Media Feature Tags  . . . . . . . . .  26
     8.3.  Interactions with Forking . . . . . . . . . . . . . . . .  27
     8.4.  Interactions with Preconditions . . . . . . . . . . . . .  27
     8.5.  Interactions with Third Party Call Control  . . . . . . .  27
   9.  Relationship with ANAT  . . . . . . . . . . . . . . . . . . .  28
   10. Setting Ta and RTO for RTP Media Streams  . . . . . . . . . .  28
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  28
     11.1.  Attacks on the Offer/Answer Exchanges  . . . . . . . . .  28
     11.2.  Insider Attacks  . . . . . . . . . . . . . . . . . . . .  28
       11.2.1.  The Voice Hammer Attack  . . . . . . . . . . . . . .  29
       11.2.2.  Interactions with Application Layer Gateways and SIP  29
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
     12.1.  SDP Attributes . . . . . . . . . . . . . . . . . . . . .  30
       12.1.1.  candidate Attribute  . . . . . . . . . . . . . . . .  31
       12.1.2.  remote-candidates Attribute  . . . . . . . . . . . .  31
       12.1.3.  ice-lite Attribute . . . . . . . . . . . . . . . . .  31
       12.1.4.  ice-mismatch Attribute . . . . . . . . . . . . . . .  32
       12.1.5.  ice-pwd Attribute  . . . . . . . . . . . . . . . . .  32
       12.1.6.  ice-ufrag Attribute  . . . . . . . . . . . . . . . .  33
       12.1.7.  ice-pacing Attribute . . . . . . . . . . . . . . . .  33
       12.1.8.  ice-options Attribute  . . . . . . . . . . . . . . .  33
     12.2.  Interactive Connectivity Establishment (ICE) Options
            Registry . . . . . . . . . . . . . . . . . . . . . . . .  34
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  35
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  35
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  35
     14.2.  Informative References . . . . . . . . . . . . . . . . .  38
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  38
   Appendix B.  The remote-candidates Attribute  . . . . . . . . . .  40
   Appendix C.  Why Is the Conflict Resolution Mechanism Needed? . .  41
   Appendix D.  Why Send an Updated Offer? . . . . . . . . . . . . .  42
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  43

1.  Introduction

   This document describes how Interactive Connectivity Establishment
   (ICE) is used with Session Description Protocol (SDP) offer/answer
   [RFC3264].  The ICE specification [ICE-BIS] describes procedures that
   are common to all usages of ICE and this document gives the
   additional details needed to use ICE with SDP offer/answer.

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 RFC
   2119 [RFC2119].

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   Readers should be familiar with the terminology defined in [RFC3264],
   in [RFC7656], in [ICE-BIS] and the following:

   Default Destination/Candidate:  The default destination for a
      component of a media stream is the transport address that would be
      used by an agent that is not ICE aware.  A default candidate for a
      component is one whose transport address matches the default
      destination for that component.  For the RTP component, the
      default IP address is in the "c=" line of the SDP, and the port is
      in the "m=" line.  For the RTCP component, the address and port
      are indicated using the "a=rtcp" attribute defined in [RFC3605],
      if present; otherwise, the RTCP component address is same as the
      address of the RTP component, and its port is one greater than the
      port of the RTP component.

3.  ICE Candidate Exchange and Offer/Answer Mapping

   [ICE-BIS] defines ICE candidate exchange as the process for ICE
   agents (Initiator and Responder) to exchange their candidate
   information required for ICE processing at the agents.  For the
   purposes of this specification, the candidate exchange process
   corresponds to the [RFC3264] Offer/Answer protocol and the
   terminologies offerer and answerer correspond to the initiator and
   responder terminologies from [ICE-BIS] respectively.

4.  SDP Offer/Answer Procedures

4.1.  Initial Offer/Answer Exchange

4.1.1.  Sending the Initial Offer

   The offerer shall follow the procedures defined in section 4 of
   [ICE-BIS] to gather, prioritize and eliminate the redundant
   candidates.  It then chooses the default candidates and encodes them
   in the SDP to be sent to its peer, the answerer.

4.1.1.1.  Choosing Default Candidates

   A candidate is said to be default if it would be the target of media
   from a non-ICE peer; that target is called the DEFAULT DESTINATION.
   If the default candidates are not selected by the ICE algorithm when
   communicating with an ICE-aware peer, an updated offer/answer will be
   required after ICE processing completes in order to "fix up" the SDP
   so that the default destination for media matches the candidates
   selected by ICE.  If ICE happens to select the default candidates, no
   updated offer/answer is required.

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   An agent MUST choose a set of candidates, one for each component of
   each in-use media stream, to be default.  A media stream is in-use if
   it does not have a port of zero (which is used in RFC 3264 to reject
   a media stream).  Consequently, a media stream is in-use even if it
   is marked as a=inactive [RFC4566] or has a bandwidth value of zero.

   An agent may choose any type of the candidate as the default, if the
   chosen candidates increases the likelihood of success with the peer
   that is being contacted if ICE is not being used.

   It is RECOMMENDED that default candidates be chosen based on the
   likelihood of those candidates to work with the peer that is being
   contacted if ICE is not being used.  Many factors may influence such
   a decision in a given agent.  In scenarios where the agent is fully
   aware of its peer's location and can reach the peer directly,
   choosing the host candidates as the default may well be sufficient.
   If the network configuration under which the agents operates is
   static and known beforehand, either the host or the server reflexives
   candidates can serve as the default candidates (depending on if a
   given agent is behind NAT and their reachability).  If the agent is
   completely unaware of the peer's location or no assumptions can be
   made of network characteristics and the connectivity, the relayed
   candidates might be the only option as the default candidate.  Having
   the decision of choosing the default candidate as a configurable
   option in the implementations might provide agents the flexibility to
   take into account the aforementioned criteria.  Barring such
   configuration flexibility, it is RECOMMENDED that the default
   candidates be the relayed candidates (if relayed candidates are
   available), server reflexive candidates (if server reflexive
   candidates are available), and finally host candidates.

4.1.1.2.  Encoding the SDP

   The process of encoding the SDP is identical between full and lite
   implementations.

   The agent will include an "m=" line for each Source Stream [RFC7656]
   it wishes to use.  The ordering of source streams in the SDP is
   relevant for ICE.  ICE will perform its connectivity checks for the
   first "m=" line first, and consequently media will be able to flow
   for that stream first.  Agents SHOULD place their most important
   source stream, if there is one, first in the SDP.

   There will be a candidate attribute for each candidate for a
   particular source stream.  Section 5 provides detailed rules for
   constructing this attribute.

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   STUN connectivity checks between agents are authenticated using the
   short-term credential mechanism defined for STUN [RFC5389].  This
   mechanism relies on a username and password that are exchanged
   through protocol machinery between the client and server.  The
   username fragment and password are exchanged in the ice-ufrag and
   ice-pwd attributes, respectively.

   If an agent is a lite implementation, it MUST include an "a=ice-lite"
   session-level attribute in its SDP to indicate this.  If an agent is
   a full implementation, it MUST NOT include this attribute.

   Section 9 of [ICE-BIS] defines a new ICE option, 'ice2'.  This option
   is used by ICE Agents to indicate their compliancy with [ICE-BIS]
   specification as compared to the [RFC5245].  If the Offering agent is
   a [ICE-BIS] compliant implementation, a session level ICE option to
   indicate the same (via the "a=ice-options:ice2" SDP line) MUST be
   included.

   The default candidates are added to the SDP as the default
   destination for media.  For source streams based on RTP, this is done
   by placing the IP address and port of the RTP candidate into the "c="
   and "m=" lines, respectively.  If the agent is utilizing RTCP and if
   RTCP candidate is present and is not equal to the same address and
   the next higher port number of the RTP candidate, the agent MUST
   encode the RTCP candidate using the a=rtcp attribute as defined in
   [RFC3605].  If RTCP is not in use, the agent MUST signal that using
   b=RS:0 and b=RR:0 as defined in [RFC3556]

   The transport addresses that will be the default destination for
   media when communicating with non-ICE peers MUST also be present as
   candidates in one or more a=candidate lines.

   ICE provides for extensibility by allowing an offer or answer to
   contain a series of tokens that identify the ICE extensions used by
   that agent.  If an agent supports an ICE extension, it MUST include
   the token defined for that extension in the ice-options attribute.

   The following is an example SDP message that includes ICE attributes
   (lines folded for readability):

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   v=0
   o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
   s=
   c=IN IP4 192.0.2.3
   t=0 0
   a=ice-options:ice2
   a=ice-pwd:asd88fgpdd777uzjYhagZg
   a=ice-ufrag:8hhY
   m=audio 45664 RTP/AVP 0
   b=RS:0
   b=RR:0
   a=rtpmap:0 PCMU/8000
   a=candidate:1 1 UDP 2130706431 10.0.1.1 8998 typ host
   a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr
    10.0.1.1 rport 8998

   Once an agent has sent its offer or its answer, that agent MUST be
   prepared to receive both STUN and media packets on each candidate.
   As discussed in section 11.1 of [ICE-BIS], media packets can be sent
   to a candidate prior to its appearance as the default destination for
   media in an offer or answer.

4.1.2.  Receiving the Initial Offer

   On receiving the offer, the answerer verifies the support for ICE
   (section 4.4 of [ICE-BIS]), determines its role (section 5.1.1 of
   [ICE-BIS]), gathers candidates (section 4 of [ICE-BIS]), encodes the
   candidates in an SDP answer and sends it to its peer, the offerer.
   The answerer shall then follow the steps defined in sections 5.1.3
   and 5.1.4 of [ICE-BIS] to schedule the ICE connectivity checks.

   The below sub-sections provide additional requirements associated
   with the processing of the offerer's SDP pertaining to this
   specification.

4.1.2.1.  ICE Option "ice2" considerations

   If the SDP offer contains a session level ICE option, "ice2" , and if
   the answering ICE Agent is also an [ICE-BIS] compliant
   implementation, then the generated SDP answer MUST include the
   session level "a=ice-options:ice2" SDP line.

4.1.2.2.  Choosing Default Candidates

   The process for selecting default candidates at the answerer is
   identical to the process followed by the offerer, as described in
   Section 4.1.1.1 for full implementations in this specification and
   section 4.2 of [ICE-BIS] for lite implementations.

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4.1.2.3.  Verifying ICE Support

   The agent will proceed with the ICE procedures defined in [ICE-BIS]
   and this specification if, for each media stream in the SDP it
   received, the default destination for each component of that media
   stream appears in a candidate attribute.  For example, in the case of
   RTP, the IP address and port in the "c=" and "m=" lines,
   respectively, appear in a candidate attribute and the value in the
   rtcp attribute appears in a candidate attribute.

   If this condition is not met, the agent MUST process the SDP based on
   normal RFC 3264 procedures, without using any of the ICE mechanisms
   described in the remainder of this specification with the following
   exceptions:

   1.  The agent MUST follow the rules of section 10 of [ICE-BIS], which
       describe keepalive procedures for all agents.

   2.  If the agent is not proceeding with ICE because there were
       a=candidate attributes, but none that matched the default
       destination of the media stream, the agent MUST include an a=ice-
       mismatch attribute in its answer.

   3.  If the default candidates were relayed candidates learned through
       a TURN server, the agent MUST create permissions in the TURN
       server for the IP addresses learned from its peer in the SDP it
       just received.  If this is not done, initial packets in the media
       stream from the peer may be lost.

4.1.2.4.  Determining Role

   In unusual cases, described in Appendix C, it is possible for both
   agents to mistakenly believe they are controlled or controlling.  To
   resolve this, each agent MUST select a random number, called the tie-
   breaker, uniformly distributed between 0 and (2**64) - 1 (that is, a
   64-bit positive integer).  This number is used in connectivity checks
   to detect and repair this case, as described in section 6.1.3 of
   [ICE-BIS].

4.1.3.  Receipt of the Initial Answer

   On receiving the SDP answer, the offerer performs steps similar to
   answerer's processing of the offer.  The offerer verifies the
   answerer's ICE support determines, its role, and processes the
   answerer's candidates to schedule the connectivity checks (section 6
   of [ICE-BIS]).

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   If the offerer had included the "ice2" ICE Option in the offer and
   the SDP answer also includes a similar session level ICE option, then
   the peers are [ICE-BIS] compliant implementations.  On the other
   hand, if the SDP Answer lacks such a ICE option, the offerer defaults
   to the procedures that are backward compatible with the [RFC5245]
   specification.

4.1.3.1.  Verifying ICE Support

   The logic at the offerer is identical to that of the answerer as
   described in section 4.4 of [ICE-BIS], with the exception that an
   offerer would not ever generate a=ice-mismatch attributes in an SDP.

   In some cases, the answer may omit a=candidate attributes for the
   media streams, and instead include an a=ice-mismatch attribute for
   one or more of the media streams in the SDP.  This signals to the
   offerer that the answerer supports ICE, but that ICE processing was
   not used for the session because a signaling intermediary modified
   the default destination for media components without modifying the
   corresponding candidate attributes.  See Section 11.2.2 for a
   discussion of cases where this can happen.  This specification
   provides no guidance on how an agent should proceed in such a failure
   case.

4.1.4.  Performing Connectivity Checks

   The possibility for role conflicts described in section 6.3.1.1 of
   [ICE-BIS] applies to this usage and hence all full agents MUST
   implement the role conflict repairing mechanism.  Also both full and
   lite agents MUST utilize the ICE-CONTROLLED and ICE-CONTROLLING
   attributes as described in section 6.1.3 of [ICE-BIS].

4.1.5.  Concluding ICE

   Once the state of each check list is Completed, If an agent is
   controlling, it examines the highest-priority nominated candidate
   pair for each component of each media stream.  If any of those
   candidate pairs differ from the default candidate pairs in the most
   recent offer/answer exchange, the controlling agent MUST generate an
   updated offer as described in Section 4.2.

4.2.  Subsequent Offer/Answer Exchanges

   Either agent MAY generate a subsequent offer at any time allowed by
   [RFC3264].  The rules in Section 4.1.5 will cause the controlling
   agent to send an updated offer at the conclusion of ICE processing
   when ICE has selected different candidate pairs from the default

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   pairs.  This section defines rules for construction of subsequent
   offers and answers.

   Should a subsequent offer fail, ICE processing continues as if the
   subsequent offer had never been made.

4.2.1.  Generating the Offer

4.2.1.1.  Procedures for All Implementations

4.2.1.1.1.  ICE Restarts

   An agent MAY restart ICE processing for an existing media stream as
   defined in section 8 of [ICE-BIS].

   The rules governing the ICE restart imply that setting the IP address
   in the "c=" line to 0.0.0.0 will cause an ICE restart.  Consequently,
   ICE implementations MUST NOT utilize this mechanism for call hold,
   and instead MUST use a=inactive and a=sendonly as described in
   [RFC3264].

   To restart ICE, an agent MUST change both the ice-pwd and the ice-
   ufrag for the media stream in an offer.  Note that it is permissible
   to use a session-level attribute in one offer, but to provide the
   same ice-pwd or ice-ufrag as a media-level attribute in a subsequent
   offer.  This is not a change in password, just a change in its
   representation, and does not cause an ICE restart.

   An agent sets the rest of the fields in the SDP for this media stream
   as it would in an initial offer of this media stream (see
   Section 4.1.1.2).  Consequently, the set of candidates MAY include
   some, none, or all of the previous candidates for that stream and MAY
   include a totally new set of candidates.

4.2.1.1.2.  Removing a Media Stream

   If an agent removes a media stream by setting its port to zero, it
   MUST NOT include any candidate attributes for that media stream and
   SHOULD NOT include any other ICE-related attributes defined in
   Section 5 for that media stream.

4.2.1.1.3.  Adding a Media Stream

   If an agent wishes to add a new media stream, it sets the fields in
   the SDP for this media stream as if this was an initial offer for
   that media stream (see Section 4.1.1.2).  This will cause ICE
   processing to begin for this media stream.

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4.2.1.2.  Procedures for Full Implementations

   This section describes additional procedures for full
   implementations, covering existing media streams.

4.2.1.2.1.  Existing Media Streams with ICE Running

   If an agent generates an updated offer including a media stream that
   was previously established, and for which ICE checks are in the
   Running state, the agent follows the procedures defined here.

   An agent MUST include candidate attributes for all local candidates
   it had signaled previously for that media stream.  The properties of
   that candidate as signaled in SDP -- the priority, foundation, type,
   and related transport address -- SHOULD remain the same.  The IP
   address, port, and transport protocol, which fundamentally identify
   that candidate, MUST remain the same (if they change, it would be a
   new candidate).  The component ID MUST remain the same.  The agent
   MAY include additional candidates it did not offer previously (see
   section 4.2.4.1.1), but which it has gathered since the last offer/
   answer exchange, including peer reflexive candidates.

   The agent MAY change the default destination for media.  As with
   initial offers, there MUST be a set of candidate attributes in the
   offer matching this default destination.

4.2.1.2.2.  Existing Media Streams with ICE Completed

   If an agent generates an updated offer including a media stream that
   was previously established, and for which ICE checks are in the
   Completed state, the agent follows the procedures defined here.

   The default destination for media (i.e., the values of the IP
   addresses and ports in the "m=" and "c=" lines used for that media
   stream) MUST be the local candidate from the highest-priority
   nominated pair in the valid list for each component.  This "fixes"
   the default destination for media to equal the destination ICE has
   selected for media.

   The agent MUST include candidate attributes for candidates matching
   the default destination for each component of the media stream, and
   MUST NOT include any other candidates.

   In addition, if the agent is controlling, it MUST include the
   a=remote-candidates attribute for each media stream whose check list
   is in the Completed state.  The attribute contains the remote
   candidates from the highest-priority nominated pair in the valid list
   for each component of that media stream.  It is needed to avoid a

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   race condition whereby the controlling agent chooses its pairs, but
   the updated offer beats the connectivity checks to the controlled
   agent, which doesn't even know these pairs are valid, let alone
   selected.  See Appendix B for elaboration on this race condition.

4.2.1.3.  Procedures for Lite Implementations

4.2.1.3.1.  Existing Media Streams with ICE Running

   This section describes procedures for lite implementations for
   existing streams for which ICE is running.

   A lite implementation MUST include all of its candidates for each
   component of each media stream in an a=candidate attribute in any
   subsequent offer.  These candidates are formed identically to the
   procedures for initial offers, as described in section 4.2 of
   [ICE-BIS].

   A lite implementation MUST NOT add additional host candidates in a
   subsequent offer.  If an agent needs to offer additional candidates,
   it MUST restart ICE.

   The username fragments, password, and implementation level MUST
   remain the same as used previously.  If an agent needs to change one
   of these, it MUST restart ICE for that media stream.

4.2.1.3.2.  Existing Media Streams with ICE Completed

   If ICE has completed for a media stream, the default destination for
   that media stream MUST be set to the remote candidate of the
   candidate pair for that component in the valid list.  For a lite
   implementation, there is always just a single candidate pair in the
   valid list for each component of a media stream.  Additionally, the
   agent MUST include a candidate attribute for each default
   destination.

   Additionally, if the agent is controlling (which only happens when
   both agents are lite), the agent MUST include the a=remote-candidates
   attribute for each media stream.  The attribute contains the remote
   candidates from the candidate pairs in the valid list (one pair for
   each component of each media stream).

4.2.2.  Receiving the Offer and Generating an Answer

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4.2.2.1.  Procedures for All Implementations

   When receiving a subsequent offer within an existing session, an
   agent MUST reapply the verification procedures in Section 4.1.2.3
   without regard to the results of verification from any previous
   offer/answer exchanges.  Indeed, it is possible that a previous
   offer/answer exchange resulted in ICE not being used, but it is used
   as a consequence of a subsequent exchange.

4.2.2.1.1.  Detecting ICE Restart

   If the offer contained a change in the a=ice-ufrag or a=ice-pwd
   attributes compared to the previous SDP from the peer, it indicates
   that ICE is restarting for this media stream.  If all media streams
   are restarting, then ICE is restarting overall.

   If ICE is restarting for a media stream:

   o  The agent MUST change the a=ice-ufrag and a=ice-pwd attributes in
      the answer.

   o  The agent MAY change its implementation level in the answer.

   An agent sets the rest of the fields in the SDP for this media stream
   as it would in an initial answer to this media stream (see
   Section 4.1.1.2).  Consequently, the set of candidates MAY include
   some, none, or all of the previous candidates for that stream and MAY
   include a totally new set of candidates.

4.2.2.1.2.  New Media Stream

   If the offer contains a new media stream, the agent sets the fields
   in the answer as if it had received an initial offer containing that
   media stream (see Section 4.1.1.2).  This will cause ICE processing
   to begin for this media stream.

4.2.2.1.3.  Removed Media Stream

   If an offer contains a media stream whose port is zero, the agent
   MUST NOT include any candidate attributes for that media stream in
   its answer and SHOULD NOT include any other ICE-related attributes
   defined in Section 5 for that media stream.

4.2.2.2.  Procedures for Full Implementations

   Unless the agent has detected an ICE restart from the offer, the
   username fragments, password, and implementation level MUST remain
   the same as used previously.  If an agent needs to change one of

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   these it MUST restart ICE for that media stream by generating an
   offer; ICE cannot be restarted in an answer.

   Additional behaviors depend on the state of ICE processing for that
   media stream.

4.2.2.2.1.  Existing Media Streams with ICE Running and no remote-
            candidates

   If ICE is running for a media stream, and the offer for that media
   stream lacked the remote-candidates attribute, the rules for
   construction of the answer are identical to those for the offerer as
   described in Section 4.2.1.2.1.

4.2.2.2.2.  Existing Media Streams with ICE Completed and no remote-
            candidates

   If ICE is Completed for a media stream, and the offer for that media
   stream lacked the remote-candidates attribute, the rules for
   construction of the answer are identical to those for the offerer as
   described in Section 4.2.1.2.2, except that the answerer MUST NOT
   include the a=remote-candidates attribute in the answer.

4.2.2.2.3.  Existing Media Streams and remote-candidates

   A controlled agent will receive an offer with the a=remote-candidates
   attribute for a media stream when its peer has concluded ICE
   processing for that media stream.  This attribute is present in the
   offer to deal with a race condition between the receipt of the offer,
   and the receipt of the Binding Response that tells the answerer the
   candidate that will be selected by ICE.  See Appendix B for an
   explanation of this race condition.  Consequently, processing of an
   offer with this attribute depends on the winner of the race.

   The agent forms a candidate pair for each component of the media
   stream by:

   o  Setting the remote candidate equal to the offerer's default
      destination for that component (e.g., the contents of the "m=" and
      "c=" lines for RTP, and the a=rtcp attribute for RTCP)

   o  Setting the local candidate equal to the transport address for
      that same component in the a=remote-candidates attribute in the
      offer.

   The agent then sees if each of these candidate pairs is present in
   the valid list.  If a particular pair is not in the valid list, the
   check has "lost" the race.  Call such a pair a "losing pair".

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   The agent finds all the pairs in the check list whose remote
   candidates equal the remote candidate in the losing pair:

   o  If none of the pairs are In-Progress, and at least one is Failed,
      it is most likely that a network failure, such as a network
      partition or serious packet loss, has occurred.  The agent SHOULD
      generate an answer for this media stream as if the remote-
      candidates attribute had not been present, and then restart ICE
      for this stream.

   o  If at least one of the pairs is In-Progress, the agent SHOULD wait
      for those checks to complete, and as each completes, redo the
      processing in this section until there are no losing pairs.

   Once there are no losing pairs, the agent can generate the answer.
   It MUST set the default destination for media to the candidates in
   the remote-candidates attribute from the offer (each of which will
   now be the local candidate of a candidate pair in the valid list).
   It MUST include a candidate attribute in the answer for each
   candidate in the remote-candidates attribute in the offer.

4.2.2.3.  Procedures for Lite Implementations

   If the received offer contains the remote-candidates attribute for a
   media stream, the agent forms a candidate pair for each component of
   the media stream by:

   o  Setting the remote candidate equal to the offerer's default
      destination for that component (e.g., the contents of the "m=" and
      "c=" lines for RTP, and the a=rtcp attribute for RTCP).

   o  Setting the local candidate equal to the transport address for
      that same component in the a=remote-candidates attribute in the
      offer.

   It then places those candidates into the Valid list for the media
   stream.  The state of ICE processing for that media stream is set to
   Completed.

   Furthermore, if the agent believed it was controlling, but the offer
   contained the remote-candidates attribute, both agents believe they
   are controlling.  In this case, both would have sent updated offers
   around the same time.  However, the signaling protocol carrying the
   offer/answer exchanges will have resolved this glare condition, so
   that one agent is always the 'winner' by having its offer received
   before its peer has sent an offer.  The winner takes the role of
   controlling, so that the loser (the answerer under consideration in
   this section) MUST change its role to controlled.  Consequently, if

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   the agent was going to send an updated offer since, based on the
   rules in section 7.2 of [ICE-BIS], it was controlling, it no longer
   needs to.

   Besides the potential role change, change in the Valid list, and
   state changes, the construction of the answer is performed
   identically to the construction of an offer as described in
   Section 4.2.1.3.

4.2.3.  Receiving the Answer for a Subsequent Offer

   Some deployments of ICE include e.g.  SDP-Modifying Signaling-only
   Back-to-Back User Agents (B2BUAs) [RFC7092] that modify the SDP body
   during the subsequent offer/answer exchange.  With the B2BUA being
   ICE-unaware, a subsequent answer might be manipulated and might not
   include ICE candidates although the initial answer did.

   An example of a situation where such an "unexpected"blocked", triggering the "any-variant" condition.
   Because in this example "x" has a reflexive variant mapping to itself
   of type "allocatable" the original label "xx" has a reflexive variant
   "xx" that would trigger the "only-variants" condition on the second
   action.

   A label "yy" would have the variants "xy", "yx" and "xx".  Because
   the variant mapping from "y" to "x" is of type "allocatable" and a
   mapping from "y" to "y" is not defined, the labels "xy" and "yx"

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   trigger the "any-variant" condition on the third label.  The variant
   "xx", being generated using the mapping from "y" to "x" of type
   "allocatable", would trigger the "only-variants" condition on the
   section action.  As there is no reflexive variant "yy", the original
   label "yy" cannot trigger any variant type triggers.  However, it
   could still trigger an action defined as matching or not matching a
   rule.

   In each action, one variant type trigger may be present by itself or
   in conjunction with an attribute matching or not-matching a rule.  If
   variant triggers and rule-matching triggers are used together, the
   label MUST "match" or respectively "not-match" the specified rule,
   AND satisfy the conditions on the variant type values given by the
   "any-variant", "all-variants", or "only-variants" attribute.

   A useful convention combines the "any-variant" trigger with reflexive
   variant mappings (Section 5.3.4).  This convention is used, for
   example, when multiple LGRs are defined within the same registry and
   for overlapping repertoire.  In some cases, the delegation of a label
   from one LGR must prohibit the delegation of another label in some
   other LGR.  This can be done using a variant of type "blocked" as in
   this example from an Armenian LGR, where the Armenian, Latin and
   Cyrillic letters all look identical:

       <char cp="0570" comment="Armenian small letter HO">
         <var cp="0068" type="blocked" comment="Latin small letter H" />
         <var cp="04BB" type="blocked"
              comment="Cyrillic small letter SSHA" />
       </char>

   The issue is that the target code points for these two variants are
   both outside the Armenian repertoire.  By using a reflexive variant
   with the following convention:

       <char cp="0068" comment="not part of repertoire">
         <var cp="0068" type="out-of-repertoire-var"
              comment="reflexive mapping" />
         <var cp="04BB" type="blocked"  />
         <var cp="0570" type="blocked"  />
       </char>
         ...

   and associating this with an action of the form:

       <action disp="invalid" any-variant="out-of-repertoire-var" />

   it is possible to list the symmetric and transitive variant mappings
   in the LGR even where they involve out-of-repertoire code points.  By

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   associating the action shown with the special type for these
   reflexive mappings any original labels containing one or more of the
   out-of-repertoire code points are filtered out -- just as if these
   code points had not been listed in the LGR in the first place.
   Nevertheless, they do participate in the permutation of variant
   labels for n-repertoire labels (Armenian in the example), and these
   permuted variants can be used to detect collisions with out-of-
   repertoire labels (see Section 8).

7.2.2.  Example from RFC 3743 Tables

   This section gives an example of using variant type triggers,
   combined with variants with reflexive mappings (Section 5.3.4) to
   achieve LGRs that implement tables like those defined according to
   [RFC3743] where the goal is to allow as variants only labels that
   consist entirely of simplified or traditional variants, in addition
   to the original label.

   Assuming an LGR where all variants have been given suitable "type"
   attributes of "blocked", "simplified", "traditional", or "both",
   similar to the ones discussed in Appendix B.  Given such an LGR, the
   following example actions evaluate the disposition for the variant
   label:

       <action disp="blocked" any-variant="blocked" />
       <action disp="allocatable" only-variants="simplified both" />
       <action disp="allocatable" only-variants="traditional both" />
       <action disp="blocked" all-variants="simplified traditional " />
       <action disp="allocatable" />

   The first action matches any variant label for which at least one of
   the code point variants is of type "blocked".  The second matches any
   variant label for which all of the code point variants are of type
   "simplified" or "both", in other words an all-simplified label.  The
   third matches any label for which all variants are of type
   "traditional" or "both", that is all traditional.  These two actions
   are not triggered by any variant labels containing some original code
   points, unless each of those code points has a variant defined with a
   reflexive mapping (Section 5.3.4).

   The final two actions rely on the fact that actions are evaluated in
   sequence, and that the first action triggered also defines the final
   disposition for a variant label (see Section 7.4).  They further rely
   on the assumption that the only variants with type "both" are also
   reflexive variants.

   Given these assumptions, any remaining simplified or traditional
   variants must then be part of a mixed label, and so are blocked; all

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   labels surviving to the last action are original code points only
   (that is the original label).  The example assumes that an original
   label may be a mixed label; if that is not the case, the disposition
   for the last action would be set to "blocked".

   There are exceptions where the assumption on reflexive mappings made
   above does not hold, so this basic scheme needs some refinements to
   cover all cases.  For a more complete example, see Appendix B.

7.3.  Recommended Disposition Values

   The precise nature of the policy action taken in response to a
   disposition and the name of the corresponding "disp" attributes are
   only partially defined here.  It is strongly RECOMMENDED to use the
   following dispositions only with their conventional sense.

   invalid  The resulting string is not a valid label.  This disposition
        may be assigned implicitly, see Section 7.5.  No variant labels
        should be generated from a variant mapping with this type.

   blocked  The resulting string is a valid label, but should be blocked
        from registration.  This would typically apply for a derived
        variant that is undesirable due to having no practical use or
        being confusingly similar to some other label.

   allocatable  The resulting string should be reserved for use by the
        same operator of the origin string, but not automatically
        allocated for use.

   activated  The resulting string should be activated for use.  (This
        is the same as a preferred variant in [RFC3743].)

   valid  The resultant string is a valid label.  (This is the typical
        default action if no dispositions are defined.)

7.4.  Precedence

   Actions are applied in the order of their appearance in the file.
   This defines their relative precedence.  The first action triggered
   by a label defines the disposition for that label.  To define the
   order of precedence, list the actions in the desired order.  The
   conventional order of precedence for the actions defined in
   Section 7.3 is "invalid", "blocked", "allocatable", "activated" then
   "valid".  This default precedence is used for the default actions
   defined in Section 7.6.

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7.5.  Implied Actions

   The context rules on code points ("not-when" or "when" rules) carry
   an implied action with a disposition of "invalid" (not eligible) if a
   "when" context is not satisfied, or respectively a "not-when" context
   is matched.  These rules are evaluated at the time the code points
   for a label or its variant labels are checked for validity (see
   Section 8).  In other words, they are evaluated before any of the
   whole-label evaluation rules and with higher precedence.  The context
   rules for variant mappings are evaluated when variants are generated
   and/or when variant tables are made symmetric and transitive.  They
   have an implied action with a disposition of "invalid" which means a
   putative variant mapping does not exist whenever the given context
   matches a "not-when" rule or fails to match a "when" rule specified
   for that mapping.  The result of that disposition is that the variant
   mapping is ignored in generating variant labels and the value is
   therefore not accessible to trigger any explicit actions.

   Note that such non-existing variant mapping is different from a
   blocked variant, which is a variant code point mapping that exists
   but results in a label that may not be allocated.

7.6.  Default Actions

   As described in Section 7 any variant mapping may be given a "type"
   attribute.  An action containing an "any-variant", "only-variants",
   or "all-variants" attribute relates these type values to a resulting
   disposition for the entire variant label.

   If no actions are defined for the standard disposition values of
   "invalid", "blocked", "allocatable", "activated", then the following
   implicitly defined default actions are evaluated.  They are shown
   below in their default order of precedence (see Section 7.4).  This
   default order for evaluating dispositions applies only to labels that
   triggered no explicitly defined actions and which are therefore
   handled by implicitly defined default actions.  Default actions have
   a lower order of precedence than explicit actions (see Section 8.3).

   The default actions for variant labels are defined as follows:

       <action disp="invalid" any-variant="invalid"/>
       <action disp="blocked" any-variant="blocked"/>
       <action disp="allocatable" any-variant="allocatable"/>
       <action disp="activated" all-variants="activated"/>

   A final default action sets the disposition to "valid" for any label
   matching the repertoire for which no other action has been triggered.

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   This "catch-all" action also matches all remaining variant labels
   from variants that do not have a type value.

       <action disp="valid" comment="Catch-all if other rules not met"/>

8.  Processing a Label against an LGR

8.1.  Determining Eligibility for a Label

   In order to test a given label for membership in the LGR, a consumer
   of the LGR must iterate through each code point within a given label,
   and test that each instance of a code point is a member of the LGR.
   If any instance of a code point is not a member of the LGR, the label
   shall be deemed as invalid.

   An individual instance of a code point is deemed a member of the LGR
   when it is listed using a "char" element, or is part of a range
   defined with a "range" element, and all necessary conditions in any
   "when" or "not-when" attributes are correctly satisfied for that
   instance.

   Alternatively, an instance of a code point is also deemed a member of
   the LGR when it forms part of a sequence that corresponds to a
   sequence listed using a "char" element for which the "cp" attribute
   defines a sequence, and all necessary conditions in any "when" or
   "not-when" attributes are correctly satisfied for that instance of
   the sequence.

   In determining eligibility, at each position the longest possible
   sequence of code points is evaluated first.  If that sequence matches
   a sequence defined in the LGR and satisfies any required context at
   that position, the instances of its constituent code points are
   deemed members of the LGR and evaluation proceeds with the next code
   point following the sequence.  If the sequence does not match a
   defined sequence or does not satisfy the required context,
   successively shorter sequences are evaluated, until only a single
   code point remains.  The eligibility of that code point is determined
   as described above for an individual code point instance.

   A label must also not trigger any action that results in a
   disposition of "invalid", otherwise it is deemed not eligible.  (This
   step may need to be deferred, until variant code point dispositions
   have been determined).

quot; answer might be
   experienced appears when such a B2BUA introduces a media server
   during call hold using 3rd party call-control procedures.  Omitting
   further details how this is done this could result in an answer being
   received at the holding UA that was constructed by the B2BUA.  With
   the B2BUA being ICE-unaware, that answer would not include ICE
   candidates.

   Receiving an answer without ICE attributes in this situation might be
   unexpected, but would not necessarily impair the user experience.

   In addition to procedures for the expected answer, the following
   section advices on how to recover from the unexpected situation.

4.2.3.1.  Procedures for All Implementations

   When receiving an answer within an existing session for a subsequent
   offer as specified in Section 4.2.1.2.2, an agent MUST verify ICE
   support as specified in Section 4.1.3.1.

   If ICE support is indicated in the SDP answer and the offer was a
   restart, the agent MUST perform ICE restart procedures as specified
   in Section 4.2.4.  If ICE support is no longer indicated in the SDP
   answer, the agent MUST fall-back to [RFC3264] procedures and SHOULD
   NOT drop the dialog just because of missing ICE support.  If the
   agent sends a new offer later on, it SHOULD perform an ICE restart as
   specified in Section 4.2.1.1.1.

   If ICE support is indicated in the SDP answer and ICE is running, the
   agent MUST continue ICE procedures as specified in Section 4.2.4.1.4.
   If ICE support is no longer indicated in the SDP answer, the agent

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   MUST abort the ongoing ICE processing and fall-back to [RFC3264]
   procedures.  The agent SHOULD NOT drop the dialog just because of
   missing ICE support.  If the agent sends a new offer later on, it
   SHOULD perform an ICE restart as specified in Section 4.2.1.1.1.

   If ICE support is indicated in the SDP answer and if ICE is completed
   and the answer conforms to Section 4.2.2.2.3, the agent MUST remain
   in the ICE Completed state.  If ICE support is no longer indicated in
   the SDP answer, the agent MUST fall-back to [RFC3264] procedures and
   SHOULD NOT drop the dialog just because of this unexpected answer.
   Once the agent sends a new offer later on it MUST perform an ICE
   restart.

4.2.4.  Updating the Check and Valid Lists

4.2.4.1.  Procedures for Full Implementations

4.2.4.1.1.  ICE Restarts

   The agent MUST remember the highest-priority nominated pairs in the
   Valid list for each component of the media stream, called the
   previous selected pairs, prior to the restart.  The agent will
   continue to send media using these pairs, as described in
   Section 7.1.  Once these destinations are noted, the agent MUST flush
   the valid and check lists, and then recompute the check list and its
   states as described in section 5.1.2 of [ICE-BIS].

4.2.4.1.2.  New Media Stream

   If the offer/answer exchange added a new media stream, the agent MUST
   create a new check list for it (and an empty Valid list to start of
   course), as described in section 5.1.2 of [ICE-BIS].

4.2.4.1.3.  Removed Media Stream

   If the offer/answer exchange removed a media stream, or an answer
   rejected an offered media stream, an agent MUST flush the Valid list
   for that media stream.  It MUST terminate any STUN transactions in
   progress for that media stream.  An agent MUST remove the check list
   for that media stream and cancel any pending ordinary checks for it.

4.2.4.1.4.  ICE Continuing for Existing Media Stream

   The valid list is not affected by an updated offer/answer exchange
   unless ICE is restarting.

   If an agent is in the Running state for that media stream, the check
   list is updated (the check list is irrelevant if the state is

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   completed).  To do that, the agent recomputes the check list using
   the procedures described in section 5.1.2 of [ICE-BIS].  If a pair on
   the new check list was also on the previous check list, and its state
   was Waiting, In-Progress, Succeeded, or Failed, its state is copied
   over.  Otherwise, its state is set to Frozen.

   If none of the check lists are active (meaning that the pairs in each
   check list are Frozen), the full-mode agent follows steps in
   Section 5.1.2.6 of [ICE-BIS] to place appropriate candidates in the
   Waiting state to further continue ICE processing.

4.2.4.2.  Procedures for Lite Implementations

   If ICE is restarting for a media stream, the agent MUST start a new
   Valid list for that media stream.  It MUST remember the pairs in the
   previous Valid list for each component of the media stream, called
   the previous selected pairs, and continue to send media there as
   described in Section 7.1.  The state of ICE processing for each media
   stream MUST change to Running, and the state of ICE processing MUST
   change to Running.

5.  Grammar

   This specification defines eight new SDP attributes -- the
   "candidate", "remote-candidates", "ice-lite", "ice-mismatch", "ice-
   ufrag", "ice-pwd", "ice-pacing", and "ice-options" attributes.  This
   section also provides non-normative examples of the attributes
   defined.

   The syntax for the attributes follow Augmented BNF as defined in
   [RFC5234].

5.1.  "candidate" Attribute

   The candidate attribute is a media-level attribute only.  It contains
   a transport address for a candidate that can be used for connectivity
   checks.

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   candidate-attribute   = "candidate" ":" foundation SP component-id SP
                           transport SP
                           priority SP
                           connection-address SP     ;from RFC 4566
                           port         ;port from RFC 4566
                           SP cand-type
                           [SP rel-addr]
                           [SP rel-port]
                           *(SP extension-att-name SP
                                extension-att-value)

   foundation            = 1*32ice-char
   component-id          = 1*5DIGIT
   transport             = "UDP" / transport-extension
   transport-extension   = token              ; from RFC 3261
   priority              = 1*10DIGIT
   cand-type             = "typ" SP candidate-types
   candidate-types       = "host" / "srflx" / "prflx" / "relay" / token
   rel-addr              = "raddr" SP connection-address
   rel-port              = "rport" SP port
   extension-att-name    = token
   extension-att-value   = *VCHAR
   ice-char              = ALPHA / DIGIT / "+" / "/"

   This grammar encodes the primary information about a candidate: its
   IP address, port and transport protocol, and its properties: the
   foundation, component ID, priority, type, and related transport
   address:

   <connection-address>:  is taken from RFC 4566 [RFC4566].  It is the
      IP address of the candidate.  When parsing this field, an agent
      can differentiate an IPv4 address and an IPv6 address by presence
      of a colon in its value -- the presence of a colon indicates IPv6.
      An agent MUST ignore candidate lines that include candidates with
      IP address versions that are not supported or recognized.  An IP
      address SHOULD be used, but an FQDN MAY be used in place of an IP
      address.  In that case, when receiving an offer or answer
      containing an FQDN in an a=candidate attribute, the FQDN is looked
      up in the DNS first using an AAAA record (assuming the agent
      supports IPv6), and if no result is found or the agent only
      supports IPv4, using an A record.  The rules from section 6 of
      [RFC6724] is followed by fixing the source address to be one from
      the candidate pair to be matched against destination addresses
      reported by FQDN, in cases where the DNS query returns more than
      one IP address.

   <port>:  is also taken from RFC 4566 [RFC4566].  It is the port of
      the candidate.

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   <transport>:  indicates the transport protocol for the candidate.
      This specification only defines UDP.  However, extensibility is
      provided to allow for future transport protocols to be used with
      ICE, such as the Datagram Congestion Control Protocol (DCCP)
      [RFC4340].

   <foundation>:  is composed of 1 to 32 <ice-char>s.  It is an
      identifier that is equivalent for two candidates that are of the
      same type, share the same base, and come from the same STUN
      server.  The foundation is used to optimize ICE performance in the
      Frozen algorithm as described in section 5.1.2 of [ICE-BIS]

   <component-id>:  is a positive integer between 1 and 256 that
      identifies the specific component of the media stream for which
      this is a candidate.  It MUST start at 1 and MUST increment by 1
      for each component of a particular candidate.  For media streams
      based on RTP, candidates for the actual RTP media MUST have a
      component ID of 1, and candidates for RTCP MUST have a component
      ID of 2.  See section 12 in [ICE-BIS] for additional discussion on
      extending ICE to new media streams.

   <priority>:  is a positive integer between 1 and (2**31 - 1).  The
      procedures for computing candidate's priority is described in
      section 4.1.2 of [ICE-BIS].

   <cand-type>:  encodes the type of candidate.  This specification
      defines the values "host", "srflx", "prflx", and "relay" for host,
      server reflexive, peer reflexive, and relayed candidates,
      respectively.  The set of candidate types is extensible for the
      future.

   <rel-addr> and <rel-port>:  convey transport addresses related to the
      candidate, useful for diagnostics and other purposes.  <rel-addr>
      and <rel-port> MUST be present for server reflexive, peer
      reflexive, and relayed candidates.  If a candidate is server or
      peer reflexive, <rel-addr> and <rel-port> are equal to the base
      for that server or peer reflexive candidate.  If the candidate is
      relayed, <rel-addr> and <rel-port> are equal to the mapped address
      in the Allocate response that provided the client with that
      relayed candidate (see section Appendix B.3 of [ICE-BIS] for a
      discussion of its purpose).  If the candidate is a host candidate,
      <rel-addr> and <rel-port> MUST be omitted.

      In some cases, e.g., for privacy reasons, an agent may not want to
      reveal the related address and port.  In this case the address
      MUST be set to "0.0.0.0" (for IPv4 candidates) or "::" (for IPv6
      candidates) and the port to zero.

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   The candidate attribute can itself be extended.  The grammar allows
   for new name/value pairs to be added at the end of the attribute.  An
   implementation MUST ignore any name/value pairs it doesn't
   understand.

Example: SDP line for UDP server reflexive candidate attribute for the RTP component

a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ
srflx raddr 10.0.1.1 rport 8998

5.2.  "remote-candidates" Attribute

   The syntax of the "remote-candidates" attribute is defined using
   Augmented BNF as defined in [RFC5234].  The remote-candidates
   attribute is a media-level attribute only.

   remote-candidate-att = "remote-candidates:" remote-candidate
                            0*(SP remote-candidate)
   remote-candidate = component-ID SP connection-address SP port

   The attribute contains a connection-address and port for each
   component.  The ordering of components is irrelevant.  However, a
   value MUST be present for each component of a media stream.  This
   attribute MUST be included in an offer by a controlling agent for a
   media stream that is Completed, and MUST NOT be included in any other
   case.

   Example: Remote candidates SDP lines for the RTP and RTCP components:

   a=remote-candidates:1 192.0.2.3 45664
   a=remote-candidates:2 192.0.2.3 45665

5.3.  "ice-lite" and "ice-mismatch" Attributes

   The syntax of the "ice-lite" and "ice-mismatch" attributes, both of
   which are flags, is:

   ice-lite               = "ice-lite"
   ice-mismatch           = "ice-mismatch"

   "ice-lite" is a session-level attribute only, and indicates that an
   agent is a lite implementation.  "ice-mismatch" is a media-level
   attribute only, and when present in an answer, indicates that the
   offer arrived with a default destination for a media component that
   didn't have a corresponding candidate attribute.

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8.1.1.  Determining Eligibility using Reflexive Variant Mappings

   For LGRs that contain reflexive variant mappings (defined in
   Section 5.3.4), the final evaluation of eligibility for the label
   must be deferred until variants are generated.  In essence, LGRs that
   use this feature treat the original label as the (identity) variant
   of itself.  For such LGRs, the ordinary determination of eligibility
   described here is but a first step that generally excludes only a
   subset of invalid labels.

   To further check the validity of a label with reflexive mappings, it
   is not necessary to generate all variant labels.  Only a single
   variant needs to be created, where any reflexive variants are applied
   for each code point, and the label disposition is evaluated (as
   described in Section 8.3).  A disposition of "invalid" results in the
   label being not eligible.  (In the exceptional case where context
   rules are present on reflexive mappings, multiple reflexive variants
   may be defined, but for each original label, at most one of these can
   be valid at each code position.  However, see Section 8.4).

8.2.  Determining Variants for a Label

   For a given eligible label, the set of variant labels is deemed to
   consist of each possible permutation of original code points and
   substituted code points or sequences defined in "var" elements,
   whereby all "when" and "not-when" attributes are correctly satisfied
   for each "char" or "var" element in the given permutation and all
   applicable whole label evaluation rules are satisfied as follows:

   1.  Create each possible permutation of a label, by substituting each
       code point or code point sequence in turn by any defined variant
       mapping (including any reflexive mappings)

   2.  Apply variant mappings with "when" or "not-when" attributes only
       if the conditions are satisfied; otherwise they are not defined

   3.  Record each of the "type" values on the variant mappings used in
       creating a given variant label in a disposition set; for any
       unmapped code point record the "type" value of any reflexive
       variant (see Section 5.3.4)

   4.  Determine the disposition for each variant label per Section 8.3

   5.  If the disposition is "invalid", remove the label from the set

   6.  If final evaluation of the disposition for the unpermuted label
       per Section 8.3 results in a disposition of "invalid", remove all
       associated variant labels from the set.

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   The number of potential permutations can be very large.  In practice,
   implementations would use suitable optimizations to avoid having to
   actually create all permutations.

   In determining the permuted set of variant labels in step (1) above,
   all eligible partitions into sequences must be evaluated.  A label
   "ab" that matches a sequence "ab" defined in the LGR but also matches
   the sequence of individual code points "a" and "b" (both defined in
   the LGR), must be permuted using any defined variant mappings for
   both the sequence "ab" and the code points "a" and "b" individually.

8.3.  Determining a Disposition for a Label or Variant Label

   For a given label (variant or original), its disposition is
   determined by evaluating in order of their appearance all actions for
   which the label or variant label satisfies the conditions.

   1.  For any label that contains code points or sequences not defined
       in the repertoire, or does not satisfy the context rules on all
       of its code points and variants, the disposition is "invalid".

   2.  For all other labels the disposition is given by the value of the
       "disp" attribute for the first action triggered by the label.  An
       action is triggered, if all of the following are true:

       *  the label matches the whole label evaluation rule given in the
          "match" attribute for that action;

       *  the label does not match the whole label evaluation rule given
          in the "not-match" attribute for that action;

       *  any of the recorded variant types for a variant label match
          the types given in the "any-variant" attribute for that
          action;

       *  all of the recorded variant types for a variant label match
          the types given in the "all-variants" or "only-variants"
          attribute given for that action;

       *  in case of an "only-variants" attribute, the label contains
          only code points that are the target of applied variant
          mappings;

       or

       *  the action does not contain any "match", "not-match", "any-
          variant", "all-variants", or "only-variants" attributes:
          catch-all.

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   3.  For any remaining variant label, assign the variant label the
       disposition using the default actions defined in Section 7.6.
       For this step, variant types outside the predefined recommended
       set (see Section 7.3) are ignored.

   4.  For any remaining label, set the disposition to "valid".

8.4.  Duplicate Variant Labels

   For a poorly designed LGR, it is possible to generate duplicate
   variant labels from the same input label, but with different, and
   potentially conflicting dispositions.  Implementations MUST treat any
   duplicate variant labels encountered as an error, irrespective of
   their dispositions.

   This situation can arise in two ways.  One is described in
   Section 5.3.5 and involves defining the same variant mapping with two
   context rules that are formally distinct, but nevertheless overlap so
   that they are not mutually exclusive for the same label.

   The other case involves variants defined for sequences, where one
   sequence is a prefix of another (see Section 5.3.1).  The following
   shows such an example resulting in conflicting reflexive variants:

     <char cp="0061">
       <var cp="0061" type="allocatable"/>
     </char>
     <char cp="0062"/>
     <char cp="0061 0062">
       <var cp="0061 0062" type="blocked"/>
     </char>

   A label "ab" would generate the variant labels "{a}{b}" and "{ab}"
   where the curly braces show the sequence boundaries as they were
   applied during variant mapping.  The result is a duplicate variant
   label "ab", one based on a variant of type "allocatable" plus an
   original code point "b" that has no variant, and another one based on
   a single variant of type "blocked", thus creating two variant labels
   with conflicting dispositions.

   In the general case it is difficult to impossible to prove by
   mechanical inspection of the LGR that duplicate variant labels will
   never occur, so implementations have to be prepared to detect this
   error during variant label generation.  The condition is easily
   avoided by careful design of context rules and special attention to
   the relation among code point sequences with variants.

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8.5.  Checking Labels for Collision

   The obvious method for checking collision between labels is to
   generate the fully permuted set of variants for one of them and see
   whether it contains the other label as a member.  As discussed above,
   this can be prohibitive and is not necessary.

   Because of symmetry and transitivity, all variant mappings form
   disjoint sets.  In each of these sets, the source and target of each
   mapping are also variants of the sources and targets of all the other
   mappings.  As a consequence, if two labels have code points at the
   same position from two different of these variant mapping sets, the
   sets of their variant labels are likewise disjoint.

   Instead of generating all permutations, that is, using each variant
   mapping in each set at a particular code position in the label, it is
   sufficient to substitute an "index" mapping, in effect identifying
   the set of variant code points for that position.  Such an index
   mapping could be, for example, the variant mapping for which the
   target code point (or sequence) comes first in some sorting order.

   To check collision then means generating a single variant label from
   the original by substituting the "index" value as the target for
   mapping from any code point.  This results in an "index label".  Two
   labels collide whenever the index labels for them are the same.

9.  Conversion to and from Other Formats

   Both [RFC3743] and [RFC4290] provide different grammars for IDN
   tables.  The formats in those documents are unable to fully support
   the increased requirements of contemporary IDN variant policies.

   This specification is a superset of functionality provided by the
   older IDN table formats, thus any table expressed in those formats
   can be expressed in this new format.  Automated conversion can be
   conducted between tables conformant with the grammar specified in
   each document.

   For notes on how to translate an RFC 3743-style table, see
   Appendix B.

10.  Media Type

   Well-formed LGRs that comply with this specification SHOULD be
   transmitted with a media type of "application/lgr+xml".  This media
   type will signal to an LGR-aware client that the content is designed
   to be interpreted as an LGR.

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11.  IANA Considerations

   This document requests the following actions from IANA:

11.1.  Media Type Registration

   The media type "application/lgr+xml" should be registered to denote
   transmission of label generation rulesets that are compliant with
   this specification, in accordance with [RFC6838].

   Type name: application

   Subtype name: lgr+xml

   Required parameters: N/A

   Optional parameters: charset (as for application/xml per [RFC7303])

   Security considerations: See the security considerations for
   application/xml in [RFC7303] and the specific security considerations
   for Label Generation Rulesets in RFC XXXX (RFC Editor/IANA, please
   replace XXXX with the final number of this document)

   Interoperability considerations: As for application/xml per [RFC7303]

   Published specification: See RFC XXXX (RFC Editor/IANA, please
   replace XXXX with the final number of this document)

   Applications which use this media type: Software using label
   generation rulesets for international identifiers, such as IDNs,
   including registry applications and client validators.

   Additional information:

      Deprecated alias names for this type: N/A

      Magic number(s): N/A

      File extension(s): .lgr

      Macintosh file type code(s): N/A

   Person & email address for further information:

      Kim Davies <kim.davies@icann.org>

      Asmus Freytag <asmus@unicode.org>

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   Intended Usage: COMMON

   Restrictions on usage: N/A

   Author:

      Kim Davies <kim.davies@icann.org>

      Asmus Freytag <asmus@unicode.org>

   Change Controller: IESG

   Provisional registration? (standards tree only): No

11.2.  URN Registration

   This specification uses a URN to describe the XML namespace, in
   accordance with [RFC3688].

   URI: urn:ietf:params:xml:ns:lgr-1.0

   Registrant Contact: See the Authors of this document.

   XML: None.

11.3.  Disposition Registry

   This document establishes a vocabulary of "Label Generation Ruleset
   Dispositions" which should be reflected as a new IANA registry.  This
   registry should be divided into two sub-registries:

   o  Standard Dispositions - This registry shall list dispositions that
      have been defined in published specifications, i.e. the
      eligibility for such registrations shall be "Specification
      Required" [RFC5226].  The initial set of registrations shall be
      the five dispositions in this document described in Section 7.3.

   o  Private Dispositions - This registry shall list dispositions that
      have been registered "First Come First Served" [RFC5226] by third
      parties with the IANA.  Such dispositions must take the form
      "entity:disposition" where the entity is a prefix that uniquely
      identifies the private user of the namespace.  For example,
      "acme:reserved" could be a private extension used by the
      organization ACME to denote a disposition relating to reserved
      labels.  These extensions are not intended to be interoperable,
      but registration is designed to minimize potential conflicts.  It
      is strongly recommended any new dispositions that require

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      interoperability and have applicability beyond a single
      organization be defined as Standard Dispositions.

   All private dispositions MUST be registered using the prefix-colon
   notation to distinguish them from standard dispositions.

   The IANA registry should provide data on the name of the disposition,
   the intended purposes, and the registrant or defining specification
   for the disposition.

12.  Security Considerations

   A naive implementation attempting to generate all variant labels for
   a given label could lead to the possibility of exhausting the
   resources on the machine running the LGR processor, potentially
   causing denial-of-service consequences.  For many operations, brute
   force generation can be avoided by optimization, and if needed, the
   number of permuted labels can be estimated more cheaply ahead of
   time.

   The implementation of Whole Label Evaluation rules, using certain
   backtracking algorithms, can take exponential time for pathological
   rules or labels and exhaust stack resources.  This can be mitigated
   by proper implementation and enforcing the restrictions on
   permissible label length.

13.  References

13.1.  Normative References

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

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <http://www.rfc-editor.org/info/rfc3339>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <http://www.rfc-editor.org/info/rfc5646>.

   [UAX42]    Unicode Consortium, "Unicode Character Database in XML",
              <http://unicode.org/reports/tr42/>.

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   [Unicode-Stability]
              Unicode Consortium, "Unicode Encoding Stability Policy,
              Property Value Stability",
              <http://www.unicode.org/policies/
              stability_policy.html#Property_Value>.

   [Unicode-Versions]
              Unicode Consortium, "Unicode Version Numbering",
              <http://unicode.org/versions/#Version_Numbering>.

   [XML]      World Wide Web Consortium, "Extensible Markup Language
              (XML) 1.0", <http://www.w3.org/TR/REC-xml/>.

13.2.  Informative References

   [ASIA-TABLE]
              DotAsia Organisation, ".ASIA ZH IDN Language Table".

   [LGR-PROCEDURE]
              Internet Corporation for Assigned Names and Numbers,
              "Procedure to Develop and Maintain the Label Generation
              Rules for the Root Zone in Respect of IDNA Labels",
              <http://www.icann.org/en/resources/idn/
              draft-lgr-procedure-07dec12-en.pdf>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <http://www.rfc-editor.org/info/rfc3688>.

   [RFC3743]  Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
              Engineering Team (JET) Guidelines for Internationalized
              Domain Names (IDN) Registration and Administration for
              Chinese, Japanese, and Korean", RFC 3743, DOI 10.17487/
              RFC3743, April 2004,
              <http://www.rfc-editor.org/info/rfc3743>.

   [RFC4290]  Klensin, J., "Suggested Practices for Registration of
              Internationalized Domain Names (IDN)", RFC 4290, DOI
              10.17487/RFC4290, December 2005,
              <http://www.rfc-editor.org/info/rfc4290>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

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   [RFC5564]  El-Sherbiny, A., Farah, M., Oueichek, I., and A. Al-Zoman,
              "Linguistic Guidelines for the Use of the Arabic Language
              in Internet Domains", RFC 5564, DOI 10.17487/RFC5564,
              February 2010, <http://www.rfc-editor.org/info/rfc5564>.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891, DOI 10.17487/
              RFC5891, August 2010,
              <http://www.rfc-editor.org/info/rfc5891>.

   [RFC5892]  Faltstrom, P., Ed., "The Unicode Code Points and
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5892, DOI 10.17487/RFC5892, August 2010,
              <http://www.rfc-editor.org/info/rfc5892>.

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

   [RFC7303]  Thompson, H. and C. Lilley, "XML Media Types", RFC 7303,
              DOI 10.17487/RFC7303, July 2014,
              <http://www.rfc-editor.org/info/rfc7303>.

   [TDIL-HINDI]
              Technology Development for Indian Languages (TDIL)
              Programme, "Devanagari Script Behaviour for Hindi".

   [UAX44]    Unicode Consortium, "Unicode Character Database",
              <http://unicode.org/reports/tr44/>.

   [WLE-RULES]
              Internet Corporation for Assigned Names and Numbers, "WLE
              Rules", <https://community.icann.org/download/
              attachments/43989034/WLE-Rules.pdf>.

Appendix A.  Example Tables

   The following presents a minimal LGR table defining the lower case
   LDH (letter-digit-hyphen) repertoire and containing no rules or
   metadata elements.  Many simple LGR tables will look quite similar,
   except that they would contain some metadata.

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   <?xml version="1.0" encoding="utf-8"?>
   <lgr xmlns="urn:ietf:params:xml:ns:lgr-1.0">
   <data>
       <char cp="002D" comment="HYPHEN (-)" />
       <range first-cp="0030" last-cp="0039"
         comment="DIGIT ZERO - DIGIT NINE" />
       <range first-cp="0061" last-cp="007A"
         comment="LATIN SMALL LETTER A - LATIN SMALL LETTER Z" />
   </data>
   </lgr>

   In practice, any LGR that includes the hyphen might also contain
   rules invalidating any labels beginning, ending, and containing a
   hyphen in the third and fourth positions as required by [RFC5891].

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   <?xml version="1.0" encoding="utf-8"?>
   <lgr xmlns="urn:ietf:params:xml:ns:lgr-1.0">
   <data>
       <char cp="002D"
             not-when="hyphen-minus-disallowed" />
       <range first-cp="0030" last-cp="0039" />
       <range first-cp="0061" last-cp="007A" />
   </data>
   <rules>
       <rule name="hyphen-minus-disallowed"
             comment="RFC5891 restrictions on U+002D">
         <choice>
           <rule comment="no leading hyphen">
             <look-behind>
               <start />
             </look-behind>
             <anchor />
           </rule>
           <rule comment="no trailing hyphen">
             <anchor />
             <look-ahead>
               <end />
             </look-ahead>
           </rule>
           <rule comment="no consecutive hyphens
                   in third and fourth positions">
             <look-behind>
               <start />
               <any />
               <any />
               <char cp="002D" comment="hyphen-minus" />
             </look-behind>
             <anchor />
           </rule>
         </choice>
       </rule>
   </rules>
   </lgr>

   The following sample LGR shows a more complete collection of the
   elements and attributes defined in this specification in a somewhat
   typical context.

   <?xml version="1.0" encoding="utf-8"?>

   <!-- This example uses a large subset of the features of this
        specification. It does not include every set operator,
        match operator element, or action trigger attribute, their

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        use being largely parallel to the ones demonstrated. -->

   <lgr xmlns="urn:ietf:params:xml:ns:lgr-1.0">
   <!-- meta element with all optional elements -->
     <meta>
       <version comment="initial version">1</version>
       <date>2010-01-01</date>
       <language>sv</language>
       <scope type="domain">example.com</scope>
       <validity-start>2010-01-01</validity-start>
       <validity-end>2013-12-31</validity-end>
       <description type="text/html">
           <![CDATA[
           This language table was developed with the
           <a href="http://swedish.example/">Swedish
           examples institute</a>.
           ]]>
       </description>
       <unicode-version>6.3.0</unicode-version>
       <references>
         <reference id="0" comment="the most recent" >The
               Unicode Standard 6.2</reference>
         <reference id="1" >RFC 5892</reference>
         <reference id="2" >Big-5: Computer Chinese Glyph
            and Character Code Mapping Table, Technical Report
            C-26, 1984</reference>
       </references>
    </meta>
    <!-- the data section describing the repertoire -->
     <data>
       <!-- single code point "char" element -->
       <char cp="002D" ref="1" comment="HYPHEN" />

       <!-- range elements for contiguous code points,  with tags -->
       <range first-cp="0030" last-cp="0039" ref="1" tag="digit" />
       <range first-cp="0061" last-cp="007A" ref ="1" tag="letter" />

       <!-- code point sequence -->
       <char cp="006C 00B7 006C" comment="Catalan middle dot" />

       <!-- alternatively use a when rule -->
       <char cp="00B7" when="catalan-middle-dot" />

        <!-- code point with context rule -->
       <char cp="200D" when="joiner" ref="2" />

       <!-- code points with variants -->
       <char cp="4E16" tag="preferred" ref="0">

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         <var cp="4E17" type="blocked" ref="2" />
         <var cp="534B" type="allocatable" ref="2" />
       </char>
       <char cp="4E17" ref="0">
         <var cp="4E16" type="allocatable" ref="2" />
         <var cp="534B" type="allocatable" ref="2" />
       </char>
       <char cp="534B" ref="0">
         <var cp="4E16" type="allocatable" ref="2" />
         <var cp="4E17" type="blocked" ref="2" />
       </char>
     </data>

     <!-- Context and whole label rules -->
     <rules>
       <!-- Require the given code point to be between two 006C -->
       <rule name="catalan-middle-dot" ref="0">
           <look-behind>
               <char cp="006C" />
           </look-behind>
           <anchor />
           <look-ahead>
               <char cp="006C" />
           </look-ahead>
       </rule>

       <!-- example of a context rule based on property -->
       <class name="virama" property="ccc:9" />
       <rule name="joiner"  ref="1" >
           <look-behind>
               <class by-ref="virama" />
           </look-behind>
           <anchor />
       </rule>

       <!-- example of using set operators -->

       <!-- Subtract vowels from letters to get
            consonant, demonstrating the different
            set notations and the difference operator -->
        <difference name="consonants">
            <class comment="all letters">0061-007A</class>
            <class comment="all vowels">
                    0061 0065 0069 006F 0075
            </class>
        </difference>

        <!-- by using the start and end, rule matches whole label -->

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        <rule name="three-or-more-consonants">
            <start />
            <!-- reference the class defined by the difference
                 and require three or more matches -->
            <class by-ref="consonants" count="3+" />
            <end />
       </rule>

       <!-- rule for negative matching -->
       <rule name="non-preferred"
             comment="matches any non-preferred code point">
           <complement comment="non-preferred" >
               <class from-tag="preferred" />
           </complement>
       </rule>

      <!-- actions triggered by matching rules and/or
           variant types -->
       <action disp="invalid"
               match="three-or-more-consonants" />
       <action disp="blocked" any-variant="blocked" />
       <action disp="allocatable" all-variants="allocatable"
               not-match="non-preferred" />
     </rules>
   </lgr>

Appendix B.  How to Translate RFC 3743 based Tables into the XML Format

   As a background, the [RFC3743] rules work as follows:

   1.  The Original (requested) label is checked to make sure that all
       the code points are a subset of the repertoire.

   2.  If it passes the check, the Original label is allocatable.

   3.  Generate the all-simplified and all-traditional variant labels
       (union of all the labels generated using all the simplified
       variants of the code points) for allocation.

   To illustrate by example, here is one of the more complicated set of
   variants:

       U+4E7E
       U+4E81
       U+5E72
       U+5E79
       U+69A6
       U+6F27

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   The following shows the relevant section of the Chinese language
   table published by the .ASIA registry [ASIA-TABLE].  Its entries
   read:

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5.4.  "ice-ufrag" and "ice-pwd" Attributes

   The "ice-ufrag" and "ice-pwd" attributes convey the username fragment
   and password used by ICE for message integrity.  Their syntax is:

   ice-pwd-att           = "ice-pwd:" password
   ice-ufrag-att         = "ice-ufrag:" ufrag
   password              = 22*256ice-char
   ufrag                 = 4*256ice-char

   The "ice-pwd" and "ice-ufrag" attributes can appear at either the
   session-level or media-level.  When present in both, the value in the
   media-level takes precedence.  Thus, the value at the session-level
   is effectively a default that applies to all media streams, unless
   overridden by a media-level value.  Whether present at the session or
   media-level, there MUST be an ice-pwd and ice-ufrag attribute for
   each media stream.  If two media streams have identical ice-ufrag's,
   they MUST have identical ice-pwd's.

   The ice-ufrag and ice-pwd attributes MUST be chosen randomly at the
   beginning of a session.  The ice-ufrag attribute MUST contain at
   least 24 bits of randomness, and the ice-pwd attribute MUST contain
   at least 128 bits of randomness.  This means that the ice-ufrag
   attribute will be at least 4 characters long, and the ice-pwd at
   least 22 characters long, since the grammar for these attributes
   allows for 6 bits of information per character.  The attributes MAY
   be longer than 4 and 22 characters, respectively, of course, up to
   256 characters.  The upper limit allows for buffer sizing in
   implementations.  Its large upper limit allows for increased amounts
   of randomness to be added over time.  For compatibility with the 512
   character limitation for the STUN username attribute value and for
   bandwidth conservation considerations, the ice-ufrag attribute MUST
   NOT be longer than 32 characters when sending, but an implementation
   MUST accept up to 256 characters when receiving.

   Example shows sample ice-ufrag and ice-pwd SDP lines:

   a=ice-pwd:asd88fgpdd777uzjYhagZg
   a=ice-ufrag:8hhY

5.5.  "ice-pacing" Attribute

   The "ice-pacing" attribute indicates the desired connectivity check
   pacing, in milliseconds, for this agent (see section 13 of
   [ICE-BIS]).  The syntax is:

   ice-pacing-att            = "ice-pacing:" pacing-value
   pacing-value              = 1*10DIGIT

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   Example shows ice-pacing value of 5 ms:

   a=ice-pacing:5

5.6.  "ice-options" Attribute

   The "ice-options" attribute is a session- and media-level attribute.
   It contains a series of tokens that identify the options supported by
   the agent.  Its grammar is:

   ice-options           = "ice-options:" ice-option-tag
                             0*(SP ice-option-tag)
   ice-option-tag        = 1*ice-char

   The existence of an ice-option in an offer indicates that a certain
   extension is supported by the agent and is willing to use it, if the
   peer agent also includes the same extension in the answer.  There
   might be further extension specific negotiations needed between the
   agents that determine how the extensions gets used in a given
   session.  The details of the negotiation procedures, if present, MUST
   be defined by the specification defining the extension.

   Example shows 'rtp+ecn' ice-option SDP line from <<RFC6679>>:

   a=ice-options:rtp+ecn

6.  Keepalives

   All the ICE agents MUST follow the procedures defined in section 10
   of [ICE-BIS] for sending keepalives.  The keepalives MUST be sent
   regardless of whether the media stream is currently inactive,
   sendonly, recvonly, or sendrecv, and regardless of the presence or
   value of the bandwidth attribute.  An agent can determine that its
   peer supports ICE by the presence of a=candidate attributes for each
   media session.

7.  Media Handling

7.1.  Sending Media

   The selected pair for a component of a media stream might not equal
   the default pair for that same component from the most recent offer/
   answer exchange.  When this happens, the selected pair is used for
   media, not the default pair.  When ICE first completes, if the
   selected pairs aren't a match for the default pairs, the controlling
   agent sends an updated offer/answer exchange to remedy this
   disparity.  However, until that updated offer arrives, there will not

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   be a match.  Furthermore, in very unusual cases, the default
   candidates in the updated offer/answer will not be a match.

7.1.1.  Procedures for All Implementations

   Section 11.1.3 of [ICE-BIS] defines procedures for sending media
   common across Full and Lite implementations.

7.2.  Receiving Media

   See section 11.2 of [ICE-BIS] for procedures on receiving media.

8.  SIP Considerations

   Note that ICE is not intended for NAT traversal for SIP, which is
   assumed to be provided via another mechanism [RFC5626].

   When ICE is used with SIP, forking may result in a single offer
   generating a multiplicity of answers.  In that case, ICE proceeds
   completely in parallel and independently for each answer, treating
   the combination of its offer and each answer as an independent offer/
   answer exchange, with its own set of local candidates, pairs, check
   lists, states, and so on.

   Once ICE processing has reached the Completed state for all peers for
   media streams using those candidates, the agent SHOULD wait an
   additional three seconds, and then it MAY cease responding to checks
   or generating triggered checks on that candidate.  It MAY free the
   candidate at that time.  Freeing of server reflexive candidates is
   never explicit; it happens by lack of a keepalive.  The three-second
   delay handles cases when aggressive nomination is used, and the
   selected pairs can quickly change after ICE has completed.

8.1.  Latency Guidelines

   ICE requires a series of STUN-based connectivity checks to take place
   between endpoints.  These checks start from the answerer on
   generation of its answer, and start from the offerer when it receives
   the answer.  These checks can take time to complete, and as such, the
   selection of messages to use with offers and answers can affect
   perceived user latency.  Two latency figures are of particular
   interest.  These are the post-pickup delay and the post-dial delay.
   The post-pickup delay refers to the time between when a user "answers
   the phone" and when any speech they utter can be delivered to the
   caller.  The post-dial delay refers to the time between when a user
   enters the destination address for the user and ringback begins as a
   consequence of having successfully started alerting the called user
   agent.

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   Two cases can be considered -- one where the offer is present in the
   initial INVITE and one where it is in a response.

8.1.1.  Offer in INVITE

   To reduce post-dial delays, it is RECOMMENDED that the caller begin
   gathering candidates prior to actually sending its initial INVITE.
   This can be started upon user interface cues that a call is pending,
   such as activity on a keypad or the phone going off-hook.

   On the receipt of the offer, the answerer SHOULD generate an answer
   in a provisional response once it has compelted candidate gathering.
   ICE requires that a provisional response with an SDP be transmitted
   reliably.  This can be done through the existing Provisional Response
   Acknowledgment (PRACK) mechanism [RFC3262] or through an ICE specific
   optimization, wherein, the agent retransmits the provisional response
   with the exponential backoff timers described in [RFC3262].  Such
   retransmissions MUST cease on receipt of a STUN Binding request for
   one of the media streams signaled in that SDP or on transmission of
   the answer in a 2xx response.  If no Binding request is received
   prior to the last retransmit, the agent does not consider the session
   terminated.  For the ICE lite peers, the agent MUST cease
   retransmitting the 18x after sending it four times (ICE will actually
   work even if the peer never receives the 18x; however, experience has
   shown that sending it is important for middleboxes and firewall
   traversal).

   It should be noted that the ICE specific optimization is very
   specific to provisional response carrying answers that start ICE
   processing and it is not a general technique for 1xx reliability.
   Also such an optimization SHOULD NOT be used if both agents support
   PRACK.

   Despite the fact that the provisional response will be delivered
   reliably, the rules for when an agent can send an updated offer or
   answer do not change from those specified in [RFC3262].
   Specifically, if the INVITE contained an offer, the same answer
   appears in all of the 1xx and in the 2xx response to the INVITE.
   Only after that 2xx has been sent can an updated offer/answer
   exchange occur.

   Alternatively, an agent MAY delay sending an answer until the 200 OK;
   however, this results in a poor user experience and is NOT
   RECOMMENDED.

   Once the answer has been sent, the agent SHOULD begin its
   connectivity checks.  Once candidate pairs for each component of a

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   media stream enter the valid list, the answerer can begin sending
   media on that media stream.

   However, prior to this point, any media that needs to be sent towards
   the caller (such as SIP early media [RFC3960]) MUST NOT be
   transmitted.  For this reason, implementations SHOULD delay alerting
   the called party until candidates for each component of each media
   stream have entered the valid list.  In the case of a PSTN gateway,
   this would mean that the setup message into the PSTN is delayed until
   this point.  Doing this increases the post-dial delay, but has the
   effect of eliminating 'ghost rings'.  Ghost rings are cases where the
   called party hears the phone ring, picks up, but hears nothing and
   cannot be heard.  This technique works without requiring support for,
   or usage of, preconditions [RFC3312].  It also has the benefit of
   guaranteeing that not a single packet of media will get clipped, so
   that post-pickup delay is zero.  If an agent chooses to delay local
   alerting in this way, it SHOULD generate a 180 response once alerting
   begins.

8.1.2.  Offer in Response

   In addition to uses where the offer is in an INVITE, and the answer
   is in the provisional and/or 200 OK response, ICE works with cases
   where the offer appears in the response.  In such cases, which are
   common in third party call control [RFC3725], ICE agents SHOULD
   generate their offers in a reliable provisional response (which MUST
   utilize [RFC3262]), and not alert the user on receipt of the INVITE.
   The answer will arrive in a PRACK.  This allows for ICE processing to
   take place prior to alerting, so that there is no post-pickup delay,
   at the expense of increased call setup delays.  Once ICE completes,
   the callee can alert the user and then generate a 200 OK when they
   answer.  The 200 OK would contain no SDP, since the offer/answer
   exchange has completed.

   Alternatively, agents MAY place the offer in a 2xx instead (in which
   case the answer comes in the ACK).  When this happens, the callee
   will alert the user on receipt of the INVITE, and the ICE exchanges
   will take place only after the user answers.  This has the effect of
   reducing call setup delay, but can cause substantial post-pickup
   delays and media clipping.

8.2.  SIP Option Tags and Media Feature Tags

   [RFC5768] specifies a SIP option tag and media feature tag for usage
   with ICE.  ICE implementations using SIP SHOULD support this
   specification, which uses a feature tag in registrations to
   facilitate interoperability through signaling intermediaries.

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8.3.  Interactions with Forking

   ICE interacts very well with forking.  Indeed, ICE fixes some of the
   problems associated with forking.  Without ICE, when a call forks and
   the caller receives multiple incoming media streams, it cannot
   determine which media stream corresponds to which callee.

   With ICE, this problem is resolved.  The connectivity checks which
   occur prior to transmission of media carry username fragments, which
   in turn are correlated to a specific callee.  Subsequent media
   packets that arrive on the same candidate pair as the connectivity
   check will be associated with that same callee.  Thus, the caller can
   perform this correlation as long as it has received an answer.

8.4.  Interactions with Preconditions

   Quality of Service (QoS) preconditions, which are defined in
   [RFC3312] and [RFC4032], apply only to the transport addresses listed
   as the default targets for media in an offer/answer.  If ICE changes
   the transport address where media is received, this change is
   reflected in an updated offer that changes the default destination
   for media to match ICE's selection.  As such, it appears like any
   other re-INVITE would, and is fully treated in RFCs 3312 and 4032,
   which apply without regard to the fact that the destination for media
   is changing due to ICE negotiations occurring "in the background".

   Indeed, an agent SHOULD NOT indicate that QoS preconditions have been
   met until the checks have completed and selected the candidate pairs
   to be used for media.

   ICE also has (purposeful) interactions with connectivity
   preconditions [RFC5898].  Those interactions are described there.
   Note that the procedures described in Section 8.1 describe their own
   type of "preconditions", albeit with less functionality than those
   provided by the explicit preconditions in [RFC5898].

8.5.  Interactions with Third Party Call Control

   ICE works with Flows I, III, and IV as described in [RFC3725].  Flow
   I works without the controller supporting or being aware of ICE.
   Flow IV will work as long as the controller passes along the ICE
   attributes without alteration.  Flow II is fundamentally incompatible
   with ICE; each agent will believe itself to be the answerer and thus
   never generate a re-INVITE.

   The flows for continued operation, as described in Section 7 of
   [RFC3725], require additional behavior of ICE implementations to
   support.  In particular, if an agent receives a mid-dialog re-INVITE

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   that contains no offer, it MUST restart ICE for each media stream and
   go through the process of gathering new candidates.  Furthermore,
   that list of candidates SHOULD include the ones currently being used
   for media.

9.  Relationship with ANAT

   [RFC4091], the Alternative Network Address Types (ANAT) Semantics for
   the SDP grouping framework, and [RFC4092], its usage with SIP, define
   a mechanism for indicating that an agent can support both IPv4 and
   IPv6 for a media stream, and it does so by including two "m=" lines,
   one for v4 and one for v6.  This is similar to ICE, which allows for
   an agent to indicate multiple transport addresses using the candidate
   attribute.  However, ANAT relies on static selection to pick between
   choices, rather than a dynamic connectivity check used by ICE.

   It is RECOMMENDED that ICE be used in realizing the dual-stack use-
   cases in agents that support ICE.

10.  Setting Ta and RTO for RTP Media Streams

   During the gathering phase of ICE (section 4.1.1 [ICE-BIS]) and while
   ICE is performing connectivity checks (section 6 [ICE-BIS]), an agent
   sends STUN and TURN transactions.  These transactions are paced at a
   rate of one every Ta milliseconds, and utilize a specific RTO.  See
   Section 13 of [ICE-BIS] for details on how the values of Ta and RTO
   are computed with a real-time media stream of known maximum bandwidth
   to rate-control the ICE exchanges.

11.  Security Considerations

11.1.  Attacks on the Offer/Answer Exchanges

   An attacker that can modify or disrupt the offer/answer exchanges
   themselves can readily launch a variety of attacks with ICE.  They
   could direct media to a target of a DoS attack, they could insert
   themselves into the media stream, and so on.  These are similar to
   the general security considerations for offer/answer exchanges, and
   the security considerations in [RFC3264] apply.  These require
   techniques for message integrity and encryption for offers and
   answers, which are satisfied by the TLS mechanism [RFC3261] when SIP
   is used.  As such, the usage of TLS with ICE is RECOMMENDED.

11.2.  Insider Attacks

   In addition to attacks where the attacker is a third party trying to
   insert fake offers, answers, or STUN messages, there are several

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   attacks possible with ICE when the attacker is an authenticated and
   valid participant in the ICE exchange.

11.2.1.  The Voice Hammer Attack

   The voice hammer attack is an amplification attack.  In this attack,
   the attacker initiates sessions to other agents, and maliciously
   includes the IP address and port of a DoS target as the destination
   for media traffic signaled in the SDP.  This causes substantial
   amplification; a single offer/answer exchange can create a continuing
   flood of media packets, possibly at high rates (consider video
   sources).  This attack is not specific to ICE, but ICE can help
   provide remediation.

   Specifically, if ICE is used, the agent receiving the malicious SDP
   will first perform connectivity checks to the target of media before
   sending media there.  If this target is a third-party host, the
   checks will not succeed, and media is never sent.

   Unfortunately, ICE doesn't help if it's not used, in which case an
   attacker could simply send the offer without the ICE parameters.
   However, in environments where the set of clients is known, and is
   limited to ones that support ICE, the server can reject any offers or
   answers that don't indicate ICE support.

   User Agents that are not willing to receive non-ICE answers MUST
   include an "ice" Option Tag in the Require Header Field in their
   offer.  Clients that rejects non-ICE offers SHOULD use a 421 response
   code, together with an Option Tag "ice" in the Require Header Field
   in the response.

11.2.2.  Interactions with Application Layer Gateways and SIP

   Application Layer Gateways (ALGs) are functions present in a Network
   Address Translation (NAT) device that inspect the contents of packets
   and modify them, in order to facilitate NAT traversal for application
   protocols.  Session Border Controllers (SBCs) are close cousins of
   ALGs, but are less transparent since they actually exist as
   application-layer SIP intermediaries.  ICE has interactions with SBCs
   and ALGs.

   If an ALG is SIP aware but not ICE aware, ICE will work through it as
   long as the ALG correctly modifies the SDP.  A correct ALG
   implementation behaves as follows:

   o  The ALG does not modify the "m=" and "c=" lines or the rtcp
      attribute if they contain external addresses.

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   o  If the "m=" and "c=" lines contain internal addresses, the
      modification depends on the state of the ALG:

      *  If the ALG already has a binding established that maps an
         external port to an internal IP address and port matching the
         values in the "m=" and "c=" lines or rtcp attribute, the ALG
         uses that binding instead of creating a new one.

      *  If the ALG does not already have a binding, it creates a new
         one and modifies the SDP, rewriting the "m=" and "c=" lines and
         rtcp attribute.

   Unfortunately, many ALGs are known to work poorly in these corner
   cases.  ICE does not try to work around broken ALGs, as this is
   outside the scope of its functionality.  ICE can help diagnose these
   conditions, which often show up as a mismatch between the set of
   candidates and the "m=" and "c=" lines and rtcp attributes.  The ice-
   mismatch attribute is used for this purpose.

   ICE works best through ALGs when the signaling is run over TLS.  This
   prevents the ALG from manipulating the SDP messages and interfering
   with ICE operation.  Implementations that are expected to be deployed
   behind ALGs SHOULD provide for TLS transport of the SDP.

   If an SBC is SIP aware but not ICE aware, the result depends on the
   behavior of the SBC.  If it is acting as a proper Back-to-Back User
   Agent (B2BUA), the SBC will remove any SDP attributes it doesn't
   understand, including the ICE attributes.  Consequently, the call
   will appear to both endpoints as if the other side doesn't support
   ICE.  This will result in ICE being disabled, and media flowing
   through the SBC, if the SBC has requested it.  If, however, the SBC
   passes the ICE attributes without modification, yet modifies the
   default destination for media (contained in the "m=" and "c=" lines
   and rtcp attribute), this will be detected as an ICE mismatch, and
   ICE processing is aborted for the call.  It is outside of the scope
   of ICE for it to act as a tool for "working around" SBCs.  If one is
   present, ICE will not be used and the SBC techniques take precedence.

12.  IANA Considerations

12.1.  SDP Attributes

   The original ICE specification defined seven new SDP attributes per
   the procedures of Section 8.2.4 of [RFC4566].  The registration
   information is reproduced here.

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12.1.1.  candidate Attribute

   Contact Name:  Jonathan Rosenberg, jdrosen@jdrosen.net.

   Attribute Name:  candidate

   Long Form:  candidate

   Type of Attribute:  media-level

   Charset Considerations:  The attribute is not subject to the charset
      attribute.

   Purpose:  This attribute is used with Interactive Connectivity
      Establishment (ICE), and provides one of many possible candidate
      addresses for communication.  These addresses are validated with
      an end-to-end connectivity check using Session Traversal Utilities
      for NAT (STUN).

   Appropriate Values:  See Section 5 of RFC XXXX.

12.1.2.  remote-candidates Attribute

   Contact Name:  Jonathan Rosenberg, jdrosen@jdrosen.net.

   Attribute Name:  remote-candidates

   Long Form:  remote-candidates

   Type of Attribute:  media-level

   Charset Considerations:  The attribute is not subject to the charset
      attribute.

   Purpose:  This attribute is used with Interactive Connectivity
      Establishment (ICE), and provides the identity of the remote
      candidates that the offerer wishes the answerer to use in its
      answer.

   Appropriate Values:  See Section 5 of RFC XXXX.

12.1.3.  ice-lite Attribute

   Contact Name:  Jonathan Rosenberg, jdrosen@jdrosen.net.

   Attribute Name:  ice-lite

   Long Form:  ice-lite

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   Type of Attribute:  session-level

   Charset Considerations:  The attribute is not subject to the charset
      attribute.

   Purpose:  This attribute is used with Interactive Connectivity
      Establishment (ICE), and indicates that an agent has the minimum
      functionality required to support ICE inter-operation with a peer
      that has a full implementation.

   Appropriate Values:  See Section 5 of RFC XXXX.

12.1.4.  ice-mismatch Attribute

   Contact Name:  Jonathan Rosenberg, jdrosen@jdrosen.net.

   Attribute Name:  ice-mismatch

   Long Form:  ice-mismatch

   Type of Attribute:  session-level

   Charset Considerations:  The attribute is not subject to the charset
      attribute.

   Purpose:  This attribute is used with Interactive Connectivity
      Establishment (ICE), and indicates that an agent is ICE capable,
      but did not proceed with ICE due to a mismatch of candidates with
      the default destination for media signaled in the SDP.

   Appropriate Values:  See Section 5 of RFC XXXX.

12.1.5.  ice-pwd Attribute

   Contact Name:  Jonathan Rosenberg, jdrosen@jdrosen.net.

   Attribute Name:  ice-pwd

   Long Form:  ice-pwd

   Type of Attribute:  session- or media-level

   Charset Considerations:  The attribute is not subject to the charset
      attribute.

   Purpose:  This attribute is used with Interactive Connectivity
      Establishment (ICE), and provides the password used to protect
      STUN connectivity checks.

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   Appropriate Values:  See Section 5 of RFC XXXX.

12.1.6.  ice-ufrag Attribute

   Contact Name:  Jonathan Rosenberg, jdrosen@jdrosen.net.

   Attribute Name:  ice-ufrag

   Long Form:  ice-ufrag

   Type of Attribute:  session- or media-level

   Charset Considerations:  The attribute is not subject to the charset
      attribute.

   Purpose:  This attribute is used with Interactive Connectivity
      Establishment (ICE), and provides the fragments used to construct
      the username in STUN connectivity checks.

   Appropriate Values:  See Section 5 of RFC XXXX.

12.1.7.  ice-pacing Attribute

   Contact Name:  Jonathan Rosenberg, jdrosen@jdrosen.net.

   Attribute Name:  ice-pacing

   Long Form:  ice-pacing

   Type of Attribute:  session-level

   Charset Considerations:  The attribute is not subject to the charset
      attribute.

   Purpose:  This attribute is used with Interactive Connectivity
      Establishment (ICE) to indicate desired connectivity check pacing
      values.

   Appropriate Values:  See Section 5 of RFC XXXX.

12.1.8.  ice-options Attribute

   Contact Name:  Jonathan Rosenberg, jdrosen@jdrosen.net.

   Attribute Name:  ice-options

   Long Form:  ice-options

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   Type of Attribute:  session- or media-level

   Charset Considerations:  The attribute is not subject to the charset
      attribute.

   Purpose:  This attribute is used with Interactive Connectivity
      Establishment (ICE), and indicates the ICE options or extensions
      used by the agent.

   Appropriate Values:  See Section 5 of RFC XXXX.

12.2.  Interactive Connectivity Establishment (ICE) Options Registry

   IANA maintains a registry for ice-options identifiers under the
   Specification Required policy as defined in "Guidelines for Writing
   an IANA Considerations Section in RFCs" [RFC5226].

   ICE options are of unlimited length according to the syntax in
   Section 5.6; however, they are RECOMMENDED to be no longer than 20
   characters.  This is to reduce message sizes and allow for efficient
   parsing.

   In [RFC5245] ICE options could only be defined at the session level.
   ICE options can now also be defined at the media level.  This can be
   used when aggregating between different ICE agents in the same
   endpoint, but future options may require to be defined at the media-
   level.  To ensure compatibility with legacy implementation, the
   media-level ICE options MUST be aggregated into a session-level ICE
   option.  Because aggregation rules depend on the specifics of each
   option, all new ICE options MUST also define in their specification
   how the media-level ICE option values are aggregated to generate the
   value of the session-level ICE option.

   [RFC6679] defines the "rtp+ecn" ICE option.  The aggregation rule for
   this ICE option is that if all aggregated media using ICE contain a
   media-level "rtp+ecn" ICE option then an "rtp+ecn" ICE option MUST be
   inserted at the session-level.  If one of the media does not contain
   the option, then it MUST NOT be inserted at the session-level.

   Section 9 of [ICE-BIS] defines "ice2" ICE option.  Since "ice2" is a
   session level ICE option, no aggregation rules apply.

   A registration request MUST include the following information:

   o  The ICE option identifier to be registered

   o  Name, Email, and Address of a contact person for the registration

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   o  Organization or individuals having the change control

   o  Short description of the ICE extension to which the option relates

   o  Reference(s) to the specification defining the ICE option and the
      related extensions

13.  Acknowledgments

   A large part of the text in this document was taken from [RFC5245],
   authored by Jonathan Rosenberg.

   Some of the text in this document was taken from [RFC6336], authored
   by Magnus Westerlund and Colin Perkins.

   Thanks to Thomas Stach for the text in Section 4.2.3, Roman Shpount
   for suggesting RTCP candidate handling in Section 4.1.1.2 and Simon
   Perreault for advising on IPV6 address selection when candidate-
   address includes FQDN.

   Thanks to following experts for their reviews and constructive
   feedback: Christer Holmberg, Adam Roach and the MMUSIC WG.

14.  References

14.1.  Normative References

   [ICE-BIS]  Keranen, A. and J. Rosenberg, "Interactive Connectivity
              Establishment (ICE): A Protocol for Network Address
              Translator (NAT) Traversal for Offer/Answer Protocols",
              draft-ietf-ice-rfc5245bis-00 (work in progress), March
              2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://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,
              <http://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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc3264>.

   [RFC3312]  Camarillo, G., Ed., Marshall, W., Ed., and J. Rosenberg,
              "Integration of Resource Management and Session Initiation
              Protocol (SIP)", RFC 3312, DOI 10.17487/RFC3312, October
              2002, <http://www.rfc-editor.org/info/rfc3312>.

   [RFC3556]  Casner, S., "Session Description Protocol (SDP) Bandwidth
              Modifiers for RTP Control Protocol (RTCP) Bandwidth",
              RFC 3556, DOI 10.17487/RFC3556, July 2003,
              <http://www.rfc-editor.org/info/rfc3556>.

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

   [RFC4032]  Camarillo, G. and P. Kyzivat, "Update to the Session
              Initiation Protocol (SIP) Preconditions Framework",
              RFC 4032, DOI 10.17487/RFC4032, March 2005,
              <http://www.rfc-editor.org/info/rfc4032>.

   [RFC4091]  Camarillo, G. and J. Rosenberg, "The Alternative Network
              Address Types (ANAT) Semantics for the Session Description
              Protocol (SDP) Grouping Framework", RFC 4091, June 2005,
              <http://www.rfc-editor.org/info/rfc4091>.

   [RFC4092]  Camarillo, G. and J. Rosenberg, "Usage of the Session
              Description Protocol (SDP) Alternative Network Address
              Types (ANAT) Semantics in the Session Initiation Protocol
              (SIP)", RFC 4092, June 2005,
              <http://www.rfc-editor.org/info/rfc4092>.

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

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.

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

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              DOI 10.17487/RFC5245, April 2010,
              <http://www.rfc-editor.org/info/rfc5245>.

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

   [RFC5768]  Rosenberg, J., "Indicating Support for Interactive
              Connectivity Establishment (ICE) in the Session Initiation
              Protocol (SIP)", RFC 5768, DOI 10.17487/RFC5768, April
              2010, <http://www.rfc-editor.org/info/rfc5768>.

   [RFC6336]  Westerlund, M. and C. Perkins, "IANA Registry for
              Interactive Connectivity Establishment (ICE) Options",
              RFC 6336, April 2010,
              <http://www.rfc-editor.org/info/rfc6336>.

   [RFC6679]  Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
              and K. Carlberg, "Explicit Congestion Notification (ECN)
              for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
              2012, <http://www.rfc-editor.org/info/rfc6679>.

   [RFC6724]  Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, September 2012,
              <http://www.rfc-editor.org/info/rfc6724>.

   [RFC7092]  Kaplan, H. and V. Pascual, "A Taxonomy of Session
              Initiation Protocol (SIP) Back-to-Back User Agents",
              RFC 7092, DOI 10.17487/RFC7092, December 2013,
              <http://www.rfc-editor.org/info/rfc7092>.

   [RFC7656]  Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
              B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
              for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
              DOI 10.17487/RFC7656, November 2015,
              <http://www.rfc-editor.org/info/rfc7656>.

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14.2.  Informative References

   [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,
              <http://www.rfc-editor.org/info/rfc3725>.

   [RFC3960]  Camarillo, G. and H. Schulzrinne, "Early Media and Ringing
              Tone Generation in the Session Initiation Protocol (SIP)",
              RFC 3960, DOI 10.17487/RFC3960, December 2004,
              <http://www.rfc-editor.org/info/rfc3960>.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340,
              DOI 10.17487/RFC4340, March 2006,
              <http://www.rfc-editor.org/info/rfc4340>.

   [RFC5626]  Jennings, C., Ed., Mahy, R., Ed., and F. Audet, Ed.,
              "Managing Client-Initiated Connections in the Session
              Initiation Protocol (SIP)", RFC 5626,
              DOI 10.17487/RFC5626, October 2009,
              <http://www.rfc-editor.org/info/rfc5626>.

   [RFC5898]  Andreasen, F., Camarillo, G., Oran, D., and D. Wing,
              "Connectivity Preconditions for Session Description
              Protocol (SDP) Media Streams", RFC 5898,
              DOI 10.17487/RFC5898, July 2010,
              <http://www.rfc-editor.org/info/rfc5898>.

Appendix A.  Examples

   For the example shown in section 12 of [ICE-BIS] the resulting offer
   (message 5) encoded in SDP looks like:

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   v=0
   o=jdoe 2890844526 2890842807 IN IP6 $L-PRIV-1.IP
   s=
   c=IN IP6 $NAT-PUB-1.IP
   t=0 0
   a=ice-pwd:asd88fgpdd777uzjYhagZg
   a=ice-ufrag:8hhY
   m=audio $NAT-PUB-1.PORT RTP/AVP 0
   b=RS:0
   b=RR:0
   a=rtpmap:0 PCMU/8000
   a=candidate:1 1 UDP 2130706431 $L-PRIV-1.IP $L-PRIV-1.PORT typ host
   a=candidate:2 1 UDP 1694498815 $NAT-PUB-1.IP $NAT-PUB-1.PORT typ
    srflx raddr $L-PRIV-1.IP rport $L-PRIV-1.PORT

   The offer, with the variables replaced with their values, will look
   like (lines folded for clarity):

v=0
o=jdoe 2890844526 2890842807 IN IP6 fe80::6676:baff:fe9c:ee4a
s=
c=IN IP6 2001:420:c0e0:1005::61
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 45664 RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 fe80::6676:baff:fe9c:ee4a 8998 typ host
a=candidate:2 1 UDP 1694498815 2001:420:c0e0:1005::61 45664 typ srflx raddr
 fe80::6676:baff:fe9c:ee4a rport 8998

   The resulting answer looks like:

   v=0
   o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP
   s=
   c=IN IP4 $R-PUB-1.IP
   t=0 0
   a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
   a=ice-ufrag:9uB6
   m=audio $R-PUB-1.PORT RTP/AVP 0
   b=RS:0
   b=RR:0
   a=rtpmap:0 PCMU/8000
   a=candidate:1 1 UDP 2130706431 $R-PUB-1.IP $R-PUB-1.PORT typ host

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   With the variables filled in:

   v=0
   o=bob 2808844564 2808844564 IN IP4 192.0.2.1
   s=
   c=IN IP4 192.0.2.1
   t=0 0
   a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
   a=ice-ufrag:9uB6
   m=audio 3478 RTP/AVP 0
   b=RS:0
   b=RR:0
   a=rtpmap:0 PCMU/8000
   a=candidate:1 1 UDP 2130706431 192.0.2.1 3478 typ host

Appendix B.  The remote-candidates Attribute

   The a=remote-candidates attribute exists to eliminate a race
   condition between the updated offer and the response to the STUN
   Binding request that moved a candidate into the Valid list.  This
   race condition is shown in Figure 1.  On receipt of message 4, agent
   L adds a candidate pair to the valid list.  If there was only a
   single media stream with a single component, agent L could now send
   an updated offer.  However, the check from agent R has not yet
   generated a response, and agent R receives the updated offer (message
   7) before getting the response (message 9).  Thus, it does not yet
   know that this particular pair is valid.  To eliminate this
   condition, the actual candidates at R that were selected by the
   offerer (the remote candidates) are included in the offer itself, and
   the answerer delays its answer until those pairs validate.

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   Agent L               Network               Agent R
      |(1) Offer            |                     |
      |------------------------------------------>|
      |(2) Answer           |                     |
      |<------------------------------------------|
      |(3) STUN Req.        |                     |
      |------------------------------------------>|
      |(4) STUN Res.        |                     |
      |<------------------------------------------|
      |(5) STUN Req.        |                     |
      |<------------------------------------------|
      |(6) STUN Res.        |                     |
      |-------------------->|                     |
      |                     |Lost                 |
      |(7) Offer            |                     |
      |------------------------------------------>|
      |(8) STUN Req.        |                     |
      |<------------------------------------------|
      |(9) STUN Res.        |                     |
      |------------------------------------------>|
      |(10) Answer          |                     |
      |<------------------------------------------|

                       Figure 1: Race Condition Flow

Appendix C.  Why Is the Conflict Resolution Mechanism Needed?

   When ICE runs between two peers, one agent acts as controlled, and
   the other as controlling.  Rules are defined as a function of
   implementation type and offerer/answerer to determine who is
   controlling and who is controlled.  However, the specification
   mentions that, in some cases, both sides might believe they are
   controlling, or both sides might believe they are controlled.  How
   can this happen?

   The condition when both agents believe they are controlled shows up
   in third party call control cases.  Consider the following flow:

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   These are the lines corresponding to the set of variants listed above

   U+4E7E;U+4E7E,U+5E72;U+4E7E;U+4E81,U+5E72,U+6F27,U+5E79,U+69A6
   U+4E81;U+5E72;U+4E7E;U+5E72,U+6F27,U+5E79,U+69A6
   U+5E72;U+5E72;U+5E72,U+4E7E,U+5E79;U+4E7E,U+4E81,U+69A6,U+6F27
   U+5E79;U+5E72;U+5E79;U+69A6,U+4E7E,U+4E81,U+6F27
   U+69A6;U+5E72;U+69A6;U+5E79,U+4E7E,U+4E81,U+6F27
   U+6F27;U+4E7E;U+6F27;U+4E81,U+5E72,U+5E79,U+69A6

   The corresponding data section XML format would look like this:

       <data>
       <char cp="4E7E">
       <var cp="4E7E" type="both" comment="identity" />
       <var cp="4E81" type="blocked" />
       <var cp="5E72" type="simp" />
       <var cp="5E79" type="blocked" />
       <var cp="69A6" type="blocked" />
       <var cp="6F27" type="blocked" />
       </char>
       <char cp="4E81">
       <var cp="4E7E" type="trad" />
       <var cp="5E72" type="simp" />
       <var cp="5E79" type="blocked" />
       <var cp="69A6" type="blocked" />
       <var cp="6F27" type="blocked" />
       </char>
       <char cp="5E72">
       <var cp="4E7E" type="trad"/>
       <var cp="4E81" type="blocked"/>
       <var cp="5E72" type="both" comment="identity"/>
       <var cp="5E79" type="trad"/>
       <var cp="69A6" type="blocked"/>
       <var cp="6F27" type="blocked"/>
       </char>
       <char cp="5E79">
       <var cp="4E7E" type="blocked"/>
       <var cp="4E81" type="blocked"/>
       <var cp="5E72" type="simp"/>
       <var cp="5E79" type="trad" comment="identity"/>
       <var cp="69A6" type="blocked"/>
       <var cp="6F27" type="blocked"/>

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       </char>
       <char cp="69A6">
       <var cp="4E7E" type="blocked"/>
       <var cp="4E81" type="blocked"/>
       <var cp="5E72" type="simp"/>
       <var cp="5E79" type="blocked"/>
       <var cp="69A6" type="trad" comment="identity"/>
       <var cp="6F27" type="blocked"/>
       </char>
       <char cp="6F27">
       <var cp="4E7E" type="simp"/>
       <var cp="4E81" type="blocked"/>
       <var cp="5E72" type="blocked"/>
       <var cp="5E79" type="blocked"/>
       <var cp="69A6" type="blocked"/>
       <var cp="6F27" type="trad" comment="identity"/>
       </char>
     </data>

   Here the simplified variants have been given a type of "simp", the
   traditional variants one of "trad" and all other ones are given
   "blocked".

   Because some variant mappings show in more than one column, while the
   XML format allows only a single type value, they have been given the
   type of "both".

   Note that some variant mappings map to themselves (identity), that is
   the mapping is reflexive (see Section 5.3.4).  In creating the
   permutation of all variant labels, these mappings have no effect,
   other than adding a value to the variant type list for the variant
   label containing them.

   In the example so far, all of the entries with type="both" are also
   mappings where source and target are identical.  That is, they are
   reflexive mappings as defined in Section 5.3.4.

   Given a label "U+4E7E U+4E81", the following labels would be ruled
   allocatable under [RFC3743] based on how that standard is commonly
   implemented in domain registries:

       Original label:     U+4E7E U+4E81
       Simplified label 1: U+4E7E U+5E72
       Simplified label 2: U+5E72 U+5E72
       Traditional label:  U+4E7E U+4E7E

   However, if allocatable labels were generated simply by a straight
   permutation of all variants with type other than type="blocked" and

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   without regard to the simplified and traditional variants, we would
   end up with an extra allocatable label of "U+5E72 U+4E7E".  This
   label is comprises both a Simplified Chinese character and a
   Traditional Chinese code point and therefore shouldn't be
   allocatable.

   To more fully resolve the dispositions requires several actions to be
   defined as described in Section 7.2.2 which will override the default
   actions from Section 7.6.  After blocking all labels that contain a
   variant with type "blocked", these actions will set to allocatable
   labels based on the following variant types: "simp", "trad" and
   "both".  Note that these variant types do not directly relate to
   dispositions for the variant label, but that the actions will resolve
   them to the standard dispositions on labels, to with "blocked" and
   "allocatable".

   To resolve label dispositions requires five actions to be defined (in
   the rules section of this document) these actions apply in order and
   the first one triggered, defines the disposition for the label.  The
   actions are:

   1.  block all variant labels containing at least one blocked variant.

   2.  allocate all labels that consist entirely of variants that are
       "simp" or "both"

   3.  also allocate all labels that are entirely "trad" or "both"

   4.  block all surviving labels containing any one of the dispositions
       "simp" or "trad" or "both" because they are now known to be part
       of an undesirable mixed simplified/traditional label

   5.  allocate any remaining label; the original label would be such a
       label.

   The rules declarations would be represented as:

     <rules>
       <!--Action elements - order defines precedence-->
       <action disp="blocked"     any-variant="blocked" />
       <action disp="allocatable"  only-variants="simp both" />
       <action disp="allocatable"  only-variants="trad both" />
       <action disp="blocked"     any-variant="simp trad" />
       <action disp="allocatable"  comment="catch-all" />
     </rules>

   Up to now, variants with type "both" have occurred only associated
   with reflexive variant mappings.  The "action" elements defined above

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   rely on the assumption that this is always the case.  However,
   consider the following set of variants:

       U+62E0;U+636E;U+636E;U+64DA
       U+636E;U+636E;U+64DA;U+62E0
       U+64DA;U+636E;U+64DA;U+62E0

   The corresponding XML would be:

       <char cp="62E0">
       <var cp="636E" type="both" comment="both, but not reflexive" />
       <var cp="64DA" type="blocked

             A         Controller          B
             |(1) INV()     |              |
             |<-------------|              |
             |(2) 200(SDP1) |              |
             |------------->|              |
             |              |(3) INV()     |
             |              |------------->|
             |              |(4) 200(SDP2) |
             |              |<-------------|
             |(5) ACK(SDP2) |              |
             |<-------------|              |
             |              |(6) ACK(SDP1) |
             |              |------------->|

                       Figure 2: Role Conflict Flow

   This flow is a variation on flow III of RFC 3725 [RFC3725].  In fact,
   it works better than flow III since it produces fewer messages.  In
   this flow, the controller sends an offerless INVITE to agent A, which
   responds with its offer, SDP1.  The agent then sends an offerless
   INVITE to agent B, which it responds to with its offer, SDP2.  The
   controller then uses the offer from each agent to generate the
   answers.  When this flow is used, ICE will run between agents A and
   B, but both will believe they are in the controlling role.  With the
   role conflict resolution procedures, this flow will function properly
   when ICE is used.

   At this time, there are no documented flows that can result in the
   case where both agents believe they are controlled.  However, the
   conflict resolution procedures allow for this case, should a flow
   arise that would fit into this category.

Appendix D.  Why Send an Updated Offer?

   Section 11.1 describes rules for sending media.  Both agents can send
   media once ICE checks complete, without waiting for an updated offer.
   Indeed, the only purpose of the updated offer is to "correct" the SDP
   so that the default destination for media matches where media is
   being sent based on ICE procedures (which will be the highest-
   priority nominated candidate pair).

   This begs the question -- why is the updated offer/answer exchange
   needed at all?  Indeed, in a pure offer/answer environment, it would
   not be.  The offerer and answerer will agree on the candidates to use
   through ICE, and then can begin using them.  As far as the agents
   themselves are concerned, the updated offer/answer provides no new
   information.  However, in practice, numerous components along the
   signaling path look at the SDP information.  These include entities

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   performing off-path QoS reservations, NAT traversal components such
   as ALGs and Session Border Controllers (SBCs), and diagnostic tools
   that passively monitor the network.  For these tools to continue to
   function without change, the core property of SDP -- that the
   existing, pre-ICE definitions of the addresses used for media -- the
   "m=" and "c=" lines and the rtcp attribute -- must be retained.  For
   this reason, an updated offer must be sent.

Authors' Addresses

   Marc Petit-Huguenin
   Impedance Mismatch

   Email: marc@petit-huguenin.org

   Ari Keranen
   Ericsson
   Jorvas  02420
   Finland

   Email: ari.keranen@ericsson.com

   Suhas Nandakumar
   Cisco Systems
   707 Tasman Dr
   Milpitas, CA  95035
   USA

   Email: snandaku@cisco.com

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