MMUSIC                                                 M. Petit-Huguenin
Internet-Draft                                        Impedance Mismatch
Obsoletes: 5245 (if approved)                                 A. Keranen                              S. Nandakumar
Intended status: Standards Track                                Ericsson                           Cisco Systems
Expires: May September 27, 2018                                      S. Nandakumar
                                                           Cisco Systems
                                                       November 23, 2017                                   A. Keranen
                                                                Ericsson
                                                          March 26, 2018

     Session Description Protocol (SDP) Offer/Answer procedures for
              Interactive Connectivity Establishment (ICE)
                    draft-ietf-mmusic-ice-sip-sdp-16
                    draft-ietf-mmusic-ice-sip-sdp-17

Abstract

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

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on May September 27, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3   4
   3.  ICE Candidate Exchange and  SDP Offer/Answer Mapping Procedures . . . . . . .   4
   4.  SDP Offer/Answer Procedures . . . . . . . . . .   4
     3.1.  Introduction  . . . . . . . .   4
     4.1.  Initial Offer/Answer Exchange . . . . . . . . . . . . . .   4
       4.1.1.  Sending the Initial Offer
     3.2.  Generic Procedures  . . . . . . . . . . . . . .   4
       4.1.2.  Receiving the Initial Offer . . . . .   4
       3.2.1.  Encoding  . . . . . . . .   7
       4.1.3.  Receipt of the Initial Answer . . . . . . . . . . . .   8
       4.1.4.  Performing Connectivity Checks . .   4
       3.2.2.  RTP/RTCP Considerations . . . . . . . . .   9
       4.1.5.  Concluding ICE . . . . . .   6
       3.2.3.  Determining Role  . . . . . . . . . . . . .   9
     4.2.  Subsequent Offer/Answer Exchanges . . . . .   6
       3.2.4.  STUN Considerations . . . . . . .  10
       4.2.1.  Generating the Offer . . . . . . . . . .   6
       3.2.5.  ICE Mismatch  . . . . . .  10
       4.2.2.  Receiving the Offer and Generating an Answer . . . .  13
       4.2.3.  Receiving the Answer for a Subsequent Offer . . . . .  16
       4.2.4.  Updating the Check and Valid Lists . . . . .   6
       3.2.6.  SDP Example . . . .  17
   5.  Grammar . . . . . . . . . . . . . . . . .   7
     3.3.  Initial Offer/Answer Exchange . . . . . . . . . .  18
     5.1.  "candidate" Attribute . . . .   7
       3.3.1.  Sending the Initial Offer . . . . . . . . . . . . . .  18
     5.2.  "remote-candidates" Attribute   7
       3.3.2.  Sending the Initial Answer  . . . . . . . . . . . . .   8
       3.3.3.  Receiving the Initial Answer  .  21
     5.3.  "ice-lite" and "ice-mismatch" Attributes . . . . . . . .  21
     5.4.  "ice-ufrag" and "ice-pwd" Attributes . . .   8
       3.3.4.  Concluding ICE  . . . . . . .  22
     5.5.  "ice-pacing" Attribute . . . . . . . . . . . .   8
     3.4.  Subsequent Offer/Answer Exchanges . . . . .  22
     5.6.  "ice-options" Attribute . . . . . . .   9
       3.4.1.  Sending Subsequent Offer  . . . . . . . . . .  23
   6.  Keepalives . . . .   9
       3.4.2.  Sending Subsequent Answer . . . . . . . . . . . . . .  11
       3.4.3.  Receiving Answer for a Subsequent Offer . . . . . . .  23
   7.  Media Handling  13
   4.  Grammar . . . . . . . . . . . . . . . . . . . . . . .  23
     7.1.  Sending Media . . . .  15
     4.1.  "candidate" Attribute . . . . . . . . . . . . . . . . . .  23
       7.1.1.  Procedures for All Implementations  15
     4.2.  "remote-candidates" Attribute . . . . . . . . .  24
     7.2.  Receiving Media . . . . .  18
     4.3.  "ice-lite" and "ice-mismatch" Attributes  . . . . . . . .  18
     4.4.  "ice-ufrag" and "ice-pwd" Attributes  . . . . . . . .  24
   8.  SIP Considerations . .  19
     4.5.  "ice-pacing" Attribute  . . . . . . . . . . . . . . . . .  20
     4.6.  "ice-options" Attribute . .  24
     8.1.  Latency Guidelines . . . . . . . . . . . . . . .  20
   5.  Keepalives  . . . .  24
       8.1.1.  Offer in INVITE . . . . . . . . . . . . . . . . . . .  25
       8.1.2.  Offer in Response . .  21
   6.  SIP Considerations  . . . . . . . . . . . . . . . .  26

     8.2.  SIP Option Tags and Media Feature Tags . . . . .  21
     6.1.  Latency Guidelines  . . . .  26
     8.3.  Interactions with Forking . . . . . . . . . . . . . . .  21
       6.1.1.  Offer in INVITE .  27
     8.4. . . . . . . . . . . . . . . . . . .  22
       6.1.2.  Offer in Response . . . . . . . . . . . . . . . . . .  23
     6.2.  SIP Option Tags and Media Feature Tags  . . . . . . . . .  23
     6.3.  Interactions with Forking . . . . . . . . . . . . . . . .  24
     6.4.  Interactions with Preconditions . . . . . . . . . . . . .  27
     8.5.  24
     6.5.  Interactions with Third Party Call Control  . . . . . . .  27
   9.  24
   7.  Relationship with ANAT  . . . . . . . . . . . . . . . . . . .  28
   10.  25
   8.  Setting Ta and RTO for RTP Media Streams  . . . . . . . . . .  28
   11.  25
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
     11.1.  25
     9.1.  Attacks on the Offer/Answer Exchanges . . . . . . . . .  28
     11.2. .  25
     9.2.  Insider Attacks . . . . . . . . . . . . . . . . . . . .  28
       11.2.1. .  26
       9.2.1.  The Voice Hammer Attack . . . . . . . . . . . . . .  29
       11.2.2. .  26
       9.2.2.  Interactions with Application Layer Gateways and SIP  29
   12.   26
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
     12.1.  27
     10.1.  SDP Attributes . . . . . . . . . . . . . . . . . . . . .  30
       12.1.1.  27
       10.1.1.  candidate Attribute  . . . . . . . . . . . . . . . .  31
       12.1.2.  28
       10.1.2.  remote-candidates Attribute  . . . . . . . . . . . .  31
       12.1.3.  28
       10.1.3.  ice-lite Attribute . . . . . . . . . . . . . . . . .  31
       12.1.4.  29
       10.1.4.  ice-mismatch Attribute . . . . . . . . . . . . . . .  32
       12.1.5.  29
       10.1.5.  ice-pwd Attribute  . . . . . . . . . . . . . . . . .  32
       12.1.6.  30
       10.1.6.  ice-ufrag Attribute  . . . . . . . . . . . . . . . .  33
       12.1.7.  ice-pacing  30
       10.1.7.  ice-options Attribute  . . . . . . . . . . . . . . . .  33
       12.1.8.  ice-options  31
       10.1.8.  ice-pacing Attribute . . . . . . . . . . . . . . .  33
     12.2. .  31
     10.2.  Interactive Connectivity Establishment (ICE) Options
            Registry . . . . . . . . . . . . . . . . . . . . . . . .  34
   13.  32
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  35
   14.  33
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  35
     14.1.  33
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  35
     14.2.  33
     12.2.  Informative References . . . . . . . . . . . . . . . . .  38  35
     12.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  36
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  38  37
   Appendix B.  The remote-candidates Attribute  . . . . . . . . . .  40  38
   Appendix C.  Why Is the Conflict Resolution Mechanism Needed? . .  41  39
   Appendix D.  Why Send an Updated Offer? . . . . . . . . . . . . .  42  40
   Appendix E.  Contributors . . . . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  43  41

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].

   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 data 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  SDP Offer/Answer Mapping Procedures

3.1.  Introduction

   [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

   Once the procedures defined in section 5 of
   [ICE-BIS] to gather, prioritize initiating agent has gathered, pruned and eliminate the redundant
   candidates.  It then chooses the default prioritized its
   set of candidates and encodes them
   in [ICE-BIS], 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 exchange with the DEFAULT DESTINATION.
   An peer
   agent MUST choose a set of candidates, one begins.

3.2.  Generic Procedures

3.2.1.  Encoding

   Section 4 provides detailed rules 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 constructing various SDP
   attributes defined in RFC 3264 to reject
   a media stream).  Consequently, a media this specification.

3.2.1.1.  Data Streams

   Each data stream [ICE-BIS] is in-use even if it
   is marked as a=inactive [RFC4566] or has represented by an SDP media description
   ("m=" section).

3.2.1.2.  Candidates

   With in a bandwidth value of zero.

   An agent may choose any type of the "m=" section, each candidate as the default, if (including the
   chosen candidates increases the likelihood of success default
   candidate) associated with the peer
   that is being contacted if ICE is not being used.  It data stream is recommended
   that, when multiple candidates are used, UDP based candidates SHOULD
   be included wherever possible and default represented by an SDP
   candidate SHOULD be chosen
   from one of those UDP candidates.  The proto value MUST match attribute.

   Prior to nomination, the
   transport protocol "c=" line associated with an "m=" section
   contains the default candidate.  If UDP
   transport is used for IP address of the default candidate, while the 'proto' value MUST
   include UDP and the 'proto' value MUST be TCP when "m=" line
   contains the port and transport is
   TCP for of the default candidate.

   Since it is RECOMMENDED candidate for that default candidates be chosen based on
   the likelihood of those candidates to work with
   "m=" section.

   After nomination, the peer that is
   being contacted if ICE is not being used.  Many factors may influence
   such a decision in "c=" line for a given agent.  In scenarios where "m=" section contains the agent is
   fully aware
   IP address of its peer's location and can reach the peer directly,
   choosing nominated candidate (the local candidate of the host candidates as
   nominated candidate pair) and the default may well be sufficient.
   If "m=" line contains the network configuration under which port and
   transport corresponding to the agents operates nominated candidate for that "m="
   section.

3.2.1.3.  Username and Password

   The ICE username is
   static represented by an SDP ice-ufrag attribute and known beforehand, either the host
   ICE password is represented by an SDP ice-pwd attribute.

3.2.1.4.  Lite Implementations

   An ICE lite implementation [ICE-BIS] MUST include an SDP ice-lite
   attribute.  A full implementation MUST NOT include that attribute.

3.2.1.5.  ICE Extensions

   An agent uses the SDP ice-options attribute to indicate support of
   ICE extensions.

   An agent compliant to this specification MUST include an SDP ice-
   options attribute with an "ice2" attribute value.  If an agent
   receives an SDP offer or answer that does not include the server reflexives
   candidates can serve as attribute
   value, the default candidates (depending on if a
   given agent assumes that the peer is behind NAT compliant to [RFC5245].

3.2.1.6.  Inactive and their reachability). Disabled Data Streams

   If the agent an "m=" section is
   completely unaware of the peer's location marked as inactive [RFC4566], or no assumptions can be
   made has a
   bandwidth value of network characteristics and the connectivity, the relayed
   candidates might be the only option as zero [RFC4566], the default candidate.  Having agent MUST still include ICE
   related SDP attributes.

   If the decision of choosing port value associated with an "m=" section is set to zero
   (implying a disabled stream), the default agent SHOULD NOT include ICE
   related SDP candidate as a configurable
   option attributes in the implementations might provide agents the flexibility to
   take into account the aforementioned criteria.  Barring such
   configuration flexibility, it is RECOMMENDED that "m=" section, unless an SDP
   extension specifying otherwise is used.

3.2.2.  RTP/RTCP Considerations

   If an agent utilizes both RTP and RTCP, the default
   candidates be agent MUST include SDP
   candidate attributes for both the relayed candidates (if relayed candidates are
   available), server reflexive candidates (if server reflexive
   candidates are available), RTP and finally host candidates.

4.1.1.2.  Encoding the SDP

   The process of encoding RTCP components in the SDP is identical between full "m="
   section.

   If an agent uses separate ports for RTP and lite
   implementations.

   The RTCP, the agent will MUST
   include an SDP rtcp attribute in the "m=" line for each Source Stream [RFC7656]
   it wishes to use.  The ordering of source streams section, as described in
   [RFC3605].  In the SDP is
   relevant for ICE.  ICE will perform its connectivity checks cases where the port number for the
   first "m=" line first, RTCP is one
   higher than the RTP port and consequently media will be able to flow
   for that stream first.  Agents SHOULD place their most important
   source stream, if there RTCP component address is one, first in same as the SDP.

   There will be a candidate
   address of the RTP component, the SDP rtcp attribute for each candidate for a
   particular source stream.  Section 5 provides detailed rules for
   constructing this attribute.

   STUN connectivity checks between agents are authenticated using MUST not be
   included.

   If the
   short-term credential mechanism defined for STUN [RFC5389].  This
   mechanism relies on a username and password agent does not utilize RTCP, it indicates that are exchanged
   through protocol machinery between the client by including
   b=RS:0 and server. b=RR:0 SDP attributes, as described in [RFC3556].

3.2.3.  Determining Role

   The
   username fragment offerer acts as the Initiating agent.  The answerer acts as the
   Responding agent.  The ICE roles (controlling and password controlled) are exchanged in
   determined using the ice-ufrag and
   ice-pwd attributes, respectively.

   If procedures in [ICE-BIS].

3.2.4.  STUN Considerations

   Once an agent is a lite implementation, it MUST include an "a=ice-lite"
   session-level attribute in has provided its SDP local candidates to indicate this.  If its peer, within
   an SDP offer or answer, the agent is
   a full implementation, it MUST NOT include this attribute.

   Section 10 of [ICE-BIS] defines a new ICE option, 'ice2'.  This
   option is used by ICE Agents be prepared to indicate their compliancy receive STUN
   connectivity check Binding requests on those candidates.

3.2.5.  ICE Mismatch

   The agent will proceed 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 procedures defined in [ICE-BIS]
   and this specification if, for each data stream in the SDP as it
   received, the default destination for media. each component of that data
   stream appears in a candidate attribute.  For source streams based on example, in the case of
   RTP, this is done
   by placing the IP address and port of the RTP candidate into in the "c=" and "m=" lines, respectively.  If the agent is utilizing RTCP and if
   RTCP
   respectively, appear in a candidate is present and is not equal to the same address attribute and the next higher port number of the RTP candidate, the agent MUST
   encode the RTCP candidate using value in the a=rtcp
   rtcp attribute as defined appears in
   [RFC3605]. a candidate attribute.

   If RTCP this condition is not in use, met, the agent MUST signal that process the SDP based on
   normal [RFC3264] procedures, without using
   b=RS:0 and b=RR:0 as defined in [RFC3556]

   The transport addresses that will be any of 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 mechanisms
   described in the remainder of tokens that identify this specification with the ICE extensions used by
   that agent.  If an following
   exceptions:

   1.  The agent supports an ICE extension, it MUST include follow the token defined rules of section 11 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 extension matched the default
       destination of the data stream, the agent MUST include an a=ice-
       mismatch attribute in its answer and may omit a=candidate
       attributes for such data streams.  See Section 9.2.2 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.

   3.  If the ice-options attribute. 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 data
       stream from the peer may be lost.

3.2.6.  SDP Example

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

   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 12.1 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

3.3.  Initial Offer/Answer Exchange

3.3.1.  Sending the Initial Offer

   On receiving

   When an offerer generates the initial offer, the answerer verifies the support in each "m=" section it
   MUST include SDP candidate attributes for ICE
   (section 5.4 of [ICE-BIS]), determines its role (section 6.1.1 of
   [ICE-BIS]), gathers candidates (section 5 of [ICE-BIS]), encodes each available candidate
   associated with the
   candidates in "m=" section.  In addition, the offerer MUST
   include an SDP answer ice-ufrag and sends it to its peer, an SDP ice-pwd attribute in the offerer.
   The answerer shall then follow offer.

   Note:  Within the steps defined in sections 6.1.3
   and 6.1.4 of [ICE-BIS] to schedule the ICE connectivity checks.

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

4.1.2.1.  ICE Option "ice2" considerations

   If document, "Initial Offer" refers to
      the first SDP offer contains a session level ICE option, "ice2" , and if
   the answering ICE Agent that is also an [ICE-BIS] compliant
   implementation, then sent in order to negotiate usage of
      ICE.  It might, or might not, be the generated initial SDP answer MUST include offer of the
   session level "a=ice-options:ice2" SDP line.

4.1.2.2.  Choosing Default Candidates
      session.

   Note:  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 procedures in this specification and
   section 5.2 of [ICE-BIS] for lite implementations.

4.1.2.3.  ICE Mismatch

   The agent will proceed document only consider "m=" sections
      associated with the data streams where ICE procedures defined in [ICE-BIS]
   and this specification if, for each media stream in the SDP it
   received, is used.

3.3.2.  Sending the default destination for each component of Initial Answer

   When an answerer receives an initial offer that indicates 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
   offerer supports ICE, and if 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 answerer accepts the rules of section 11 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 7.1.3 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 7
   of [ICE-BIS]).

   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.  ICE Mismatch

   The logic at the offerer is identical to that of the answerer as
   described in section 5.4 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 7.3.1.1 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 7.1.3 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.

   However, If the support for 'ice2' ICE Option is in use, the highest-
   priority nominated candidate is noted and sent in the subsequent
   offer/answer exchange as the default candidate and no updated offer
   is needed to fix the default candidate.

4.2.  Subsequent Offer/Answer Exchanges

   Either agent MAY generate a subsequent offer at any time allowed by
   [RFC3264].  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 9 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 offer and the rest
   usage of the fields ICE, in each "m=" section within the answer, it MUST include
   SDP candidate attributes for this media stream
   as each available candidate associated with
   the "m=" section.  In addition, the answerer MUST include an SDP ice-
   ufrag and an SDP ice-pwd attribute in the answer.

   Once the answerer has sent the answer, it would can start performing
   connectivity checks towards the peer candidates that were provided in an initial
   the offer.

   If the offer does not indicate support of this media stream (see
   Section 4.1.1.2).  Consequently, ICE, the set of candidates MAY include
   some, none, or all of answerer MUST NOT
   accept the previous candidates for that stream and MAY
   include a totally new set usage of candidates.

4.2.1.1.2.  Removing a Media Stream ICE.  If an agent removes a media stream by setting its port to zero, it the answerer still accepts the offer,
   the answerer MUST NOT include any candidate attributes for that media stream and
   SHOULD NOT include any other ICE-related ICE related SDP 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
   answer.  Instead the SDP for this media stream as if this was an initial offer for
   that media stream (see Section 4.1.1.2).  This answerer will cause ICE
   processing generate the answer according to begin for this media stream.

4.2.1.2.  Procedures for Full Implementations

   This section describes additional
   normal offer/answer procedures for full
   implementations, covering existing media streams.

4.2.1.2.1.  Existing Media Streams with ICE Running [RFC3264].

   If an agent generates an updated offer including the answerer detects a media stream that
   was previously established, and for which possibility of the ICE checks are mismatch, procedures
   described in (Section 3.2.5) are followed.

3.3.3.  Receiving the
   Running state, Initial Answer

   When an offerer receives an initial answer that indicates that the agent follows
   answerer supports ICE, it can start performing connectivity checks
   towards the procedures defined here.

   An agent MUST include candidate attributes for all local peer candidates
   it had signaled previously for that media stream.  The properties of that candidate as signaled were provided in SDP -- the priority, foundation, type,
   and related transport address -- SHOULD remain answer.

   If the same.  The IP
   address, port, and transport protocol, which fundamentally identify answer does not indicate that candidate, MUST remain the same (if they change, it would be a
   new candidate).  The component ID MUST remain answerer supports ICE, or if
   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 offerer detects an ICE mismatch in the last offer/
   answer exchange, including peer reflexive candidates.

   The agent MAY change answer, the default destination for media.  As with
   initial offers, there offerer MUST be a set
   terminate the usage of candidate attributes in ICE.  The subsequent actions taken by the
   offer matching
   offerer are implementation dependent and are out of the scope of this default destination.

4.2.1.2.2.  Existing Media Streams with
   specification.

3.3.4.  Concluding ICE Completed

   If an

   Once the state of each check list is Completed, and if the agent generates an updated offer including is
   the controlling, it nominates a media stream that
   was previously established, and for which ICE candidate pair [ICE-BIS], it checks are in the
   Completed state,
   for each data stream whether the agent follows nominated pair matches the procedures defined here.

   The default destination for media (i.e.,
   candidate pair.  If there are one or more data streams with a match,
   the values of controlling agent MUST generate a subsequent offer
   (Section 3.4.1), in which the IP
   addresses address, port and ports transport in the "m=" and
   "c=" and "m=" lines used for that media
   stream) MUST be associated with each data stream match the
   corresponding local candidate from information of the highest-priority nominated pair in the valid list for each component.

   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 that data
   stream.

   However, If the
   a=remote-candidates attribute support for each media stream whose check list 'ice2' ice-option is in use, the Completed state.  The attribute contains the remote
   candidates from the highest-priority
   nominated pair candidate is noted and sent in the valid list
   for each component of that media stream.  It subsequent offer/answer
   exchange as the default candidate and no updated offer is needed to avoid a
   race condition whereby
   fix the default candidate.

   Also as described in [ICE-BIS], once the controlling agent chooses its pairs, but has
   nominated a candidate pair for a data stream, the updated offer beats agent MUST NOT
   nominate another pair for that data stream during the connectivity checks to lifetime of the controlled
   agent, which doesn't even know these pairs are valid, let alone
   selected.  See Appendix B
   ICE session.

3.4.  Subsequent Offer/Answer Exchanges

   Either agent MAY generate a subsequent offer at any time allowed by
   [RFC3264].  This section defines rules for elaboration on this race condition.

4.2.1.3. construction of subsequent
   offers and answers.

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

3.4.1.  Sending Subsequent Offer

3.4.1.1.  Procedures for Lite All Implementations

4.2.1.3.1.  Existing Media Streams with

3.4.1.1.1.  ICE Restarts

   An agent MAY restart ICE processing for an existing data stream
   [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 Running

   This section describes procedures for lite implementations MUST NOT utilize this mechanism for
   existing streams for which ICE is running.

   A lite implementation call hold,
   and instead MUST include all of its candidates 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 each
   component of each media the data stream in an a=candidate offer.  Note that it is permissible
   to use a session-level attribute in any
   subsequent offer.  These candidates are formed identically one offer, but to provide the
   procedures for initial offers,
   same ice-pwd or ice-ufrag as described in section 5.2 of
   [ICE-BIS].

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

   The username fragments,  This is not a change in password, just a change in its
   representation, and implementation level MUST
   remain the same as used previously.  If does not cause 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 restart.

   An agent sets the remote candidate rest of the
   candidate pair for that component ice related fields in the valid list.  For a lite
   implementation, there is always just a single candidate pair SDP for this
   data stream as it would in an initial offer of this data stream (see
   Section 3.2.1).  Consequently, the
   valid list set of candidates MAY include
   some, none, or all of the previous candidates for each component that data stream
   and MAY include a totally new set of candidates.

3.4.1.1.2.  Removing a media stream.  Additionally, the Data Stream

   If an agent removes a data stream by setting its port to zero, it
   MUST NOT include a any candidate attribute attributes for each default
   destination.

   Additionally, if the agent is controlling (which only happens when
   both agents are lite), the agent MUST that data stream and
   SHOULD NOT include the a=remote-candidates
   attribute any other ICE-related attributes defined in
   Section 4 for each media that data stream.  The attribute contains the remote
   candidates from

3.4.1.1.3.  Adding a Data Stream

   If an agent wishes to add a new data stream, it sets the candidate pairs fields in
   the valid list (one pair SDP for
   each component of each media stream).

4.2.2.  Receiving the Offer and Generating this data stream as if this was an Answer

4.2.2.1. initial offer for that
   data stream (see Section 3.2.1).  This will cause ICE processing to
   begin for this data stream.

3.4.1.2.  Procedures for All Full Implementations

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

3.4.1.3.  Before Nomination

   When receiving an offerer sends a subsequent offer, each "m=" section for which
   a candidate pair has not yet been nominated, the offer within an existing session, an
   agent MUST reapply include
   the verification procedures same set of ICE-related information that the offerer included in Section 4.1.2.3
   without regard to
   the results of verification from any previous
   offer/answer exchanges.  Indeed, offer or answer.  The agent MAY include additional
   candidates it is possible that a previous
   offer/answer exchange resulted in ICE did not being used, offer previously, but which it is used
   as a consequence of a subsequent exchange.

4.2.2.1.1.  Detecting ICE Restart

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

   If ICE is restarting default destination.

3.4.1.4.  After Nomination

   Once a candidate pair has been nominated for a media stream:

   o  The agent MUST change data stream, the a=ice-ufrag IP
   address, port and a=ice-pwd attributes transport in each "c=" and "m=" line associated
   with that data stream MUST match the answer.

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

   An agent sets
   nominated pair for that data stream.  In addition, the rest offerer only
   includes SDP candidates representing the local candidates of the fields
   nominated candidate pair.  The offerer MUST NOT include any other SDP
   candidate attributes in the SDP subsequent offer.

   In addition, if the agent is controlling, it MUST include the
   a=remote-candidates attribute for this media each data stream
   as it would whose check list
   is 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 completed state.  The attribute contains the previous remote
   candidates corresponding to the nominated pair in the valid list for that stream and MAY
   include a totally new set
   each component of candidates.

4.2.2.1.2.  New Media Stream

   If the offer contains that data stream.  It is needed to avoid a new media stream, race
   condition whereby the controlling agent sets the fields
   in chooses its pairs, but the answer as if it had received an initial
   updated offer containing that
   media stream (see Section 4.1.1.2).  This will cause ICE processing beats the connectivity checks to begin the controlled agent,
   which doesn't even know these pairs are valid, let alone selected.
   See Appendix B for elaboration on this media stream.

4.2.2.1.3.  Removed Media Stream race condition.

3.4.1.5.  Procedures for Lite Implementations

   If an offer contains a media stream whose port is zero, the agent ICE state is running, a lite implementation MUST NOT include any candidate attributes all
   of its candidates for that media each component of each data stream in
   its answer and SHOULD NOT include any other ICE-related attributes
   defined
   a=candidate attribute 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, any subsequent offer.  The candidates are
   formed identical to the
   username fragments, password, and procedures for initial offers.

   A lite implementation level MUST remain
   the same as used previously.  If an agent needs to NOT add additional host candidates or
   change one of
   these username fragments, password in a subsequent offer.
   Otherwise, it MUST restart ICE.

   If ICE has completed for a data stream and if the agent is
   controlled, the default destination for that media data stream by generating an
   offer; ICE cannot MUST be restarted in an answer.

   Additional behaviors depend on set
   to the state remote candidate of ICE processing the candidate pair for that
   media 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 data
   stream.

4.2.2.2.1.  Existing Media Streams with ICE Running and no remote-
            candidates  Additionally, the agent MUST include a candidate attribute
   for each default destination.

   If ICE state is running for a media stream, completed and if the offer agent is controlling (which only
   happens when both agents are lite), the agent MUST include the
   a=remote-candidates attribute for that media
   stream lacked each data stream.  The attribute
   contains the remote candidates from the remote-candidates attribute, candidate pairs in the rules valid
   list (one pair for
   construction each component 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 each data stream).

3.4.2.  Sending Subsequent Answer

   If ICE is Completed for a media data stream, and the offer for that media data
   stream lacked the remote-candidates a=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, offerer,
   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 data stream when its peer has concluded ICE
   processing for that media data 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 data
   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".

   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 data 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

3.4.2.1.  Detecting ICE Restart

   If the offerer in a subsequent offer requested an ICE restart for a
   data stream, and if the answerer accepts the offer, the answerer
   follows the procedures for generating an initial answer.

   For a given data stream, the answerer MAY include the same candidates
   that were used in the previous ICE session, but it MUST change the
   SDP ice-pwd and ice-ufrag attribute values.

3.4.2.2.  Lite Implementations Implementation specific procedures

   If the received offer contains the remote-candidates attribute for a
   media
   data stream, the agent forms a candidate pair for each component of
   the media data 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 data stream is set to Completed.

   Furthermore, if the agent believed it was controlling, but the offer
   contained the remote-candidates a=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 the agent was going to send an updated offer since,
   based on the rules in section 8.2 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. offer.

3.4.3.  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

3.4.3.1.  Procedures for Full Implementations

   There may be certain situations where the offerer might receive SDP body
   during the subsequent offer/answer exchange.  With the B2BUA being
   ICE-unaware, a subsequent
   answer might be manipulated and might not
   include that lacks ICE candidates although the initial answer did.

   An
   One example of a situation where such an "unexpected" answer might be
   experienced appears happen when such a
   ICE-unaware B2BUA introduces a media server during call hold using
   3rd party call-control procedures.  Omitting further details how this
   is done 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, 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 would not necessarily impair the SDP user experience.

   When the offerer receives an answer and indicating support for ICE, the
   offer performs on of the following actions:

   o  If the offer was a restart, the agent MUST perform ICE restart
      procedures as specified in Section 4.2.4. 3.4.3.1.1

   o  If ICE support is no longer indicated in the SDP
   answer, the offer/answer exchange removed a data stream, or an answer
      rejected an offered data stream, an agent MUST fall-back to [RFC3264] procedures and SHOULD
   NOT drop flush the dialog just because of missing ICE support. Valid
      list for that data stream.  It MUST also terminate any STUN
      transactions in progress for that data stream.

   o  If the offer/answer exchange added a new data stream, the agent sends
      MUST create a new offer later on, check list for it SHOULD perform (and an ICE restart as
   specified in Section 4.2.1.1.1.

   If ICE support is indicated empty Valid list to
      start of course) which in turn triggers the SDP answer and ICE is running, the
   agent MUST continue ICE candidate processing
      procedures as specified in Section 4.2.4.1.4. [ICE-BIS].

   o  If ICE support state is no longer indicated in the SDP answer, running for a given data stream, the agent
   MUST abort
      recomputes the ongoing ICE processing check list.  If a pair on the new check list was
      also on the previous check list, and fall-back its state was Waiting, In-
      Progress, Succeeded, or Failed, its state is copied over.
      Otherwise, its state is set to [RFC3264]
   procedures.  The agent SHOULD NOT drop the dialog just because of
   missing ICE support. Frozen.  If none of the agent sends a new offer later on, it
   SHOULD perform an ICE restart as specified check lists
      are active (meaning that the pairs in Section 4.2.1.1.1.

   If each check list are Frozen),
      appropriate procedures in [ICE-BIS] are performed to move
      candidate(s) to the Waiting state to further continue ICE support is indicated in the SDP answer and if
      processing.

   o  If ICE state is completed and the SDP answer conforms to
      Section 4.2.2.2.3, 3.4.2, the agent MUST remain reman in the ICE Completed completed state.  If

   However, if the 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 the missing ICE support or unexpected answer.
   Once the agent sends a new offer later on 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.

3.4.3.1.1.  ICE Restarts

   The agent MUST remember the highest-priority nominated pairs pair in the Valid list for each
   component of the media data stream, called the previous selected pairs, pair prior
   to the restart.  The agent will continue to send media using these pairs, this
   pair, as described in
   Section 7.1. section 12 of [ICE-BIS].  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 6.1.2 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 6.1.2 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
   completed).  To do that, the agent recomputes the check list using
   the procedures described in section 6.1.2 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 and check lists are active (meaning that
   lists, and then recompute the pairs in each check list are Frozen), and its states, thus
   triggering the full-mode agent follows steps in
   Section 6.1.2.6 of candidate processing procedures [ICE-BIS] to place appropriate candidates in the
   Waiting state to further continue ICE processing.

4.2.4.2.

3.4.3.2.  Procedures for Lite Implementations

   If ICE is restarting for a media data stream, the agent MUST start a new
   Valid list for that media data stream.  It MUST remember the pairs nominated pair
   in the previous Valid list for each component of the media data stream,
   called the previous selected pairs, and continue to send media there
   as described in Section 7.1. section 12 of [ICE-BIS].  The state of ICE processing
   for each media data stream MUST change to Running, and the state of ICE
   processing MUST change to Running.

5. Running

4.  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.

4.1.  "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.

   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.

   <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
      ICE by extending the Datagram Congestion Control Protocol (DCCP)
      [RFC4340]. sub-registry "ICE Transport Protocols" under
      "Interactive Connectivity Establishment (ICE)" registry.

   <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 6.1.2 of [ICE-BIS]

   <component-id>:  is a positive integer between 1 and 256 (inclusive)
      that identifies the specific component of the media dta 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 data 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 14 13 in [ICE-BIS] for additional discussion on
      extending ICE to new media data streams.

   <priority>:  is a positive integer between 1 and (2**31 - 1). 1)
      inclusive.  The procedures for computing candidate's priority is
      described in section 5.1.2 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  Specifications for new candidate types is extensible for MUST define
      how, if at all, various steps in the ICE processing differ from
      the
      future. ones defined by this specification.

   <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.

   The candidate attribute can itself be extended.  The grammar allows
   for new name/value pairs to be added at the end of the attribute.
   Such extensions MUST be made through IETF Review or IESG Approval
   [RFC5226] and the assignments MUST contain the specific extension and
   a reference to the document defining the usage of the extension

   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.

4.2.  "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 data stream.  This
   attribute MUST be included in an offer by a controlling agent for a
   media
   data 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.

4.3.  "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.

5.4.

4.4.  "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 data 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 data stream.  If two media data 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. session (the same applies when ICE is restarting for
   an agent.).

   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.

4.5.  "ice-pacing" Attribute

   The "ice-pacing" is a session level attribute that indicates the
   desired connectivity check pacing, in milliseconds, for this agent
   (see section 15 14 of [ICE-BIS]).  The syntax is:

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

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

   Following the procedures defined in [ICE-BIS], a default value of
   50ms is used for an agent when ice-pacing attribute is omitted in the
   offer or the answer.

   The same rule applies for ice-pacing attribute values lower than
   50ms.  This mandates that, if an agent includes the ice-pacing
   attribute, its value MUST be greater than 50ms or else a value of
   50ms is considered by default for that agent.

   Also the larger of the ice-pacing attribute values between the offer
   and the answer (determined either by the one provided in the ice-
   pacing attribute or by picking the default value) MUST be considered
   for a given ICE session.

   The mux category [I-D.ietf-mmusic-sdp-mux-attributes] for the 'ice-
   pacing' attribute is 'TRANSPORT'.

   Example shows ice-pacing value of 5 ms:

   a=ice-pacing:5

5.6.

4.6.  "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. extension (see
   Section 10.2).

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

   a=ice-options:rtp+ecn

6.

5.  Keepalives

   All the ICE agents MUST follow the procedures defined in section 11
   of [ICE-BIS] for sending keepalives.  The keepalives MUST be sent
   regardless of whether the media data 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
   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 12.1.3 of [ICE-BIS] defines procedures for sending media
   common across Full and Lite implementations.

7.2.  Receiving Media

   See section 12.2 of [ICE-BIS] for procedures on receiving media.

8.

6.  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.

6.1.  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.

   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.

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

6.1.2.  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.

6.2.  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.

8.3.

6.3.  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 data streams, it cannot
   determine which media data 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.

6.4.  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 6.1 describe their own
   type of "preconditions", albeit with less functionality than those
   provided by the explicit preconditions in [RFC5898].

8.5.

6.5.  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
   that contains no offer, it MUST restart ICE for each media data 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.

7.  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 data 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.

8.  Setting Ta and RTO for RTP Media Streams

   During the gathering phase of ICE (section 5.1.1 [ICE-BIS]) and while ICE is performing
   connectivity checks (section 7 [ICE-BIS]), checks, 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 15 14 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.

9.  Security Considerations

11.1.

9.1.  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 data 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.

9.2.  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
   attacks possible with ICE when the attacker is an authenticated and
   valid participant in the ICE exchange.

11.2.1.

9.2.1.  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.

9.2.2.  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.

   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.

10.  IANA Considerations

12.1.

10.1.  SDP Attributes

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

12.1.1. included here with
   modifications to include Mux Category and to align with the recent
   recommendations for populating Contact information.

10.1.1.  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

   Subject to the charset
      attribute. charset:  No

   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 4 of RFC XXXX.

12.1.2.  remote-candidates Attribute

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

   Contact e-mail:  iesg@ietf.org [1]

   Reference:  RFCXXXX

   Mux Category:  TRANSPORT

10.1.2.  remote-candidates Attribute

   Attribute Name:  remote-candidates

   Long Form:  remote-candidates

   Type of Attribute:  media-level

   Charset Considerations:  The attribute is not subject

   Subject to the charset
      attribute. charset:  No

   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 4 of RFC XXXX.

12.1.3.  ice-lite Attribute

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

   Contact e-mail:  iesg@ietf.org [2]

   Reference:  RFCXXXX
   Mux Category:  TRANSPORT

10.1.3.  ice-lite Attribute

   Attribute Name:  ice-lite

   Long Form:  ice-lite

   Type of Attribute:  session-level

   Charset Considerations:  The attribute is not subject

   Subject to the charset
      attribute. charset:  No

   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 4 of RFC XXXX.

12.1.4.  ice-mismatch Attribute

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

   Contact e-mail:  iesg@ietf.org [3]

   Reference:  RFCXXXX

   Mux Category:  NORMAL

10.1.4.  ice-mismatch Attribute

   Attribute Name:  ice-mismatch

   Long Form:  ice-mismatch

   Type of Attribute:  session-level

   Charset Considerations:  The attribute is not subject  ice-mismatch

   Type of Attribute:  media-level

   Subject to the charset
      attribute. charset:  No

   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 4 of RFC XXXX.

12.1.5.  ice-pwd Attribute

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

   Contact e-mail:  iesg@ietf.org [4]

   Reference:  RFCXXXX

   Mux Category:  NORMAL

10.1.5.  ice-pwd Attribute

   Attribute Name:  ice-pwd

   Long Form:  ice-pwd

   Type of Attribute:  session- or media-level

   Charset Considerations:  The attribute is not subject

   Subject to the charset
      attribute. charset:  No

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

   Appropriate Values:  See Section 5 4 of RFC XXXX.

12.1.6.  ice-ufrag Attribute

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

   Contact e-mail:  iesg@ietf.org [5]

   Reference:  RFCXXXX

   Mux Category:  TRANSPORT

10.1.6.  ice-ufrag Attribute

   Attribute Name:  ice-ufrag

   Long Form:  ice-ufrag

   Type of Attribute:  session- or media-level

   Charset Considerations:  The attribute is not subject

   Subject to the charset
      attribute. charset:  No

   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 4 of RFC XXXX.

12.1.7.  ice-pacing Attribute

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

   Contact e-mail:  iesg@ietf.org [6]

   Reference:  RFCXXXX

   Mux Category:  TRANSPORT

10.1.7.  ice-options Attribute

   Attribute Name:  ice-pacing  ice-options

   Long Form:  ice-pacing  ice-options

   Type of Attribute:  session-level

   Charset Considerations:  The attribute is not subject  session- or media-level

   Subject to the charset
      attribute. charset:  No

   Purpose:  This attribute is used with Interactive Connectivity
      Establishment (ICE) to indicate desired connectivity check pacing
      values. (ICE), and indicates the ICE options or extensions
      used by the agent.

   Appropriate Values:  See Section 5 4 of RFC XXXX.

12.1.8.  ice-options Attribute

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

   Contact e-mail:  iesg@ietf.org [7]

   Reference:  RFCXXXX

   Mux Category:  NORMAL

10.1.8.  ice-pacing Attribute

   This specification also defines a new SDP attribute, "ice-pacing"
   according to the following data:

   Attribute Name:  ice-options

   Long Form:  ice-options  ice-pacing

   Type of Attribute:  session- or media-level

   Charset Considerations:  The attribute is not subject  session-level

   Subject to the charset
      attribute. charset:  No

   Purpose:  This attribute is used with Interactive Connectivity
      Establishment (ICE), and indicates the ICE options or extensions
      used by the agent. (ICE) to indicate desired connectivity check pacing
      values.

   Appropriate Values:  See Section 5 4 of RFC XXXX.

12.2.

   Contact Name:  IESG

   Contact e-mail:  iesg@ietf.org [8]

   Reference:  RFCXXXX

   Mux Category:  TRANSPORT

10.2.  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; 4.6; 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 10 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

   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.

11.  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.

   Many thanks to Christer Holmberg for providing text suggestions in
   Section 4 that aligns with [ICE-BIS]

   Thanks to Thomas Stach for the text in Section 4.2.3, help, 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 candidate-address includes FQDN.

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

14.

12.  References

14.1.

12.1.  Normative References

   [I-D.ietf-mmusic-sdp-mux-attributes]
              Nandakumar, S., "A Framework for SDP Attributes when
              Multiplexing", draft-ietf-mmusic-sdp-mux-attributes-17
              (work in progress), February 2018,
              <http://www.rfc-editor.org/info/rfc7656>.

   [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>.

   [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>.

   [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>.

14.2.

12.2.  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>.

12.3.  URIs

   [1] mailto:iesg@ietf.org

   [2] mailto:iesg@ietf.org

   [3] mailto:iesg@ietf.org

   [4] mailto:iesg@ietf.org

   [5] mailto:iesg@ietf.org

   [6] mailto:iesg@ietf.org

   [7] mailto:iesg@ietf.org

   [8] mailto:iesg@ietf.org

   [9] mailto:christer.holmberg@ericsson.com

   [10] mailto:rshpount@turbobridge.com

   [11] mailto:thomass.stach@gmail.com

Appendix A.  Examples

   For the example shown in section 16 of [ICE-BIS] the resulting offer
   (message 5) encoded in SDP looks like:

   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

   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 data 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.

   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:

             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
   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.

Appendix E.  Contributors

   o  Christer Holmberg Ericsson Email: christer.holmberg@ericsson.com
      [9]

   o  Roman Shpount TurboBridge rshpount@turbobridge.com [10]

   o  Thomas Stach thomass.stach@gmail.com [11]

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

   Ari Keranen
   Ericsson
   Jorvas  02420
   Finland

   Email: ari.keranen@ericsson.com