Session Description Protocol (SDP) Offer/Answer procedures for Interactive Connectivity Establishment (ICE)
draft-ietf-mmusic-ice-sip-sdp-39

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MMUSIC                                                 M. Petit-Huguenin
Internet-Draft                                        Impedance Mismatch
Obsoletes: 5245 (if approved)                              S. Nandakumar
Intended status: Standards Track                           Cisco Systems
Expires: February 14, 2020                                   C. Holmberg
                                                              A. Keranen
                                                                Ericsson
                                                              R. Shpount
                                                             TurboBridge
                                                         August 13, 2019

     Session Description Protocol (SDP) Offer/Answer procedures for
              Interactive Connectivity Establishment (ICE)
                    draft-ietf-mmusic-ice-sip-sdp-39

Abstract

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

   This document obsoletes RFC 5245.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on February 14, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

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   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . .   4
     4.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Generic Procedures  . . . . . . . . . . . . . . . . . . .   5
       4.2.1.  Encoding  . . . . . . . . . . . . . . . . . . . . . .   5
       4.2.2.  RTP/RTCP Considerations . . . . . . . . . . . . . . .   6
       4.2.3.  Determining Role  . . . . . . . . . . . . . . . . . .   6
       4.2.4.  STUN Considerations . . . . . . . . . . . . . . . . .   6
       4.2.5.  Verifying ICE Support Procedures  . . . . . . . . . .   7
       4.2.6.  SDP Example . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  Initial Offer/Answer Exchange . . . . . . . . . . . . . .   8
       4.3.1.  Sending the Initial Offer . . . . . . . . . . . . . .   8
       4.3.2.  Sending the Initial Answer  . . . . . . . . . . . . .   9
       4.3.3.  Receiving the Initial Answer  . . . . . . . . . . . .  10
       4.3.4.  Concluding ICE  . . . . . . . . . . . . . . . . . . .  10
     4.4.  Subsequent Offer/Answer Exchanges . . . . . . . . . . . .  11
       4.4.1.  Sending Subsequent Offer  . . . . . . . . . . . . . .  11
       4.4.2.  Sending Subsequent Answer . . . . . . . . . . . . . .  14
       4.4.3.  Receiving Answer for a Subsequent Offer . . . . . . .  16
   5.  Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.1.  "candidate" Attribute . . . . . . . . . . . . . . . . . .  18
     5.2.  "remote-candidates" Attribute . . . . . . . . . . . . . .  20
     5.3.  "ice-lite" and "ice-mismatch" Attributes  . . . . . . . .  21
     5.4.  "ice-ufrag" and "ice-pwd" Attributes  . . . . . . . . . .  21

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     5.5.  "ice-pacing" Attribute  . . . . . . . . . . . . . . . . .  22
     5.6.  "ice-options" Attribute . . . . . . . . . . . . . . . . .  22
   6.  Keepalives  . . . . . . . . . . . . . . . . . . . . . . . . .  23
   7.  SIP Considerations  . . . . . . . . . . . . . . . . . . . . .  23
     7.1.  Latency Guidelines  . . . . . . . . . . . . . . . . . . .  23
       7.1.1.  Offer in INVITE . . . . . . . . . . . . . . . . . . .  24
       7.1.2.  Offer in Response . . . . . . . . . . . . . . . . . .  25
     7.2.  SIP Option Tags and Media Feature Tags  . . . . . . . . .  25
     7.3.  Interactions with Forking . . . . . . . . . . . . . . . .  25
     7.4.  Interactions with Preconditions . . . . . . . . . . . . .  25
     7.5.  Interactions with Third Party Call Control  . . . . . . .  26
   8.  Interactions with Application Layer Gateways and SIP  . . . .  26
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  27
     9.1.  IP Address Privacy  . . . . . . . . . . . . . . . . . . .  28
     9.2.  Attacks on the Offer/Answer Exchanges . . . . . . . . . .  28
     9.3.  The Voice Hammer Attack . . . . . . . . . . . . . . . . .  28
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
     10.1.  SDP Attributes . . . . . . . . . . . . . . . . . . . . .  29
       10.1.1.  candidate Attribute  . . . . . . . . . . . . . . . .  29
       10.1.2.  remote-candidates Attribute  . . . . . . . . . . . .  29
       10.1.3.  ice-lite Attribute . . . . . . . . . . . . . . . . .  30
       10.1.4.  ice-mismatch Attribute . . . . . . . . . . . . . . .  30
       10.1.5.  ice-pwd Attribute  . . . . . . . . . . . . . . . . .  31
       10.1.6.  ice-ufrag Attribute  . . . . . . . . . . . . . . . .  31
       10.1.7.  ice-options Attribute  . . . . . . . . . . . . . . .  32
       10.1.8.  ice-pacing Attribute . . . . . . . . . . . . . . . .  32
     10.2.  Interactive Connectivity Establishment (ICE) Options
            Registry . . . . . . . . . . . . . . . . . . . . . . . .  33
     10.3.  Candidate Attribute Extension Subregistry Establishment   33
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  34
   12. Changes from RFC 5245 . . . . . . . . . . . . . . . . . . . .  34
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  34
     13.2.  Informative References . . . . . . . . . . . . . . . . .  36
   Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . .  37
   Appendix B.  The remote-candidates Attribute  . . . . . . . . . .  39
   Appendix C.  Why Is the Conflict Resolution Mechanism Needed? . .  40
   Appendix D.  Why Send an Updated Offer? . . . . . . . . . . . . .  41
   Appendix E.  Contributors . . . . . . . . . . . . . . . . . . . .  42
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  42

1.  Introduction

   This document describes how Interactive Connectivity Establishment
   (ICE) is used with Session Description Protocol (SDP) offer/answer
   [RFC3264].  The ICE specification [RFC8445] 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.

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   This document obsoletes RFC 5245.

   NOTE: Previously both the common ICE procedures, and the SDP offer/
   answer specific details, were described in[RFC5245].  [RFC8445]
   obsoleted [RFC5245], and the SDP offer/answer specific details were
   removed from the document.  Section 12 describes the changes to the
   SDP offer/answer specific details specified in this document.

2.  Conventions

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

3.  Terminology

   Readers should be familiar with the terminology defined in [RFC3264],
   in [RFC8445] and the following:

   Default Destination/Candidate:  The default destination for a
      component of a 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 connection address is in the "c=" line of the SDP, and the
      port and transport protocol are 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 the same as the address of the RTP component,
      and its port is one greater than the port of the RTP component.

4.  SDP Offer/Answer Procedures

4.1.  Introduction

   [RFC8445] 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 terms
   "offerer" and "answerer" correspond to the initiator and responder
   roles from [RFC8445] respectively.

   Once the initiating agent has gathered, pruned, and prioritized its
   set of candidates [RFC8445], the candidate exchange with the peer
   agent begins.

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4.2.  Generic Procedures

4.2.1.  Encoding

   Section 5 provides detailed rules for constructing various SDP
   attributes defined in this specification.

4.2.1.1.  Data Streams

   Each data stream [RFC8445] is represented by an SDP media description
   ("m=" section).

4.2.1.2.  Candidates

   Within an "m=" section, each candidate (including the default
   candidate) associated with the data stream is represented by an SDP
   candidate attribute.

   Prior to nomination, the "c=" line associated with an "m=" section
   contains the connection address of the default candidate, while the
   "m=" line contains the port and transport protocol of the default
   candidate for that "m=" section.

   After nomination, the "c=" line for a given "m=" section contains the
   connection address of the nominated candidate (the local candidate of
   the nominated candidate pair) and the "m=" line contains the port and
   transport protocol corresponding to the nominated candidate for that
   "m=" section.

4.2.1.3.  Username and Password

   The ICE username is represented by an SDP ice-ufrag attribute and the
   ICE password is represented by an SDP ice-pwd attribute.

4.2.1.4.  Lite Implementations

   An ICE lite implementation [RFC8445] MUST include an SDP ice-lite
   attribute.  A full implementation MUST NOT include that attribute.

4.2.1.5.  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 [RFC8445].  If an
   agent receives an SDP offer or answer that indicates ICE support, but
   that does not contain an SDP ice-options attribute with an "ice2"

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   attribute value, the agent can assume that the peer is compliant to
   [RFC5245].

4.2.1.6.  Inactive and Disabled Data Streams

   If an "m=" section is marked as inactive [RFC4566], or has a
   bandwidth value of zero [RFC4566], the agent MUST still include ICE-
   related SDP attributes.

   If the port value associated with an "m=" section is set to zero
   (implying a disabled stream) as defined in section 8.2 of [RFC3264],
   the agent SHOULD NOT include ICE-related SDP candidate attributes in
   that "m=" section, unless an SDP extension specifying otherwise is
   used.

4.2.2.  RTP/RTCP Considerations

   If an agent utilizes both RTP and RTCP, and separate ports are used
   for RTP and RTCP, the agent MUST include SDP candidate attributes for
   both the RTP and RTCP components.

   The agent includes an SDP rtcp attribute following the procedures in
   [RFC3605].  Hence, in the cases where the RTCP port value is one
   higher than the RTP port value and the RTCP component address the
   same as the address of the RTP component, the SDP rtcp attribute
   might be omitted.

   NOTE: [RFC5245] required that an agent always includes the SDP rtcp
   attribute, even if the RTCP port value was one higher than the RTP
   port value.  This specification aligns the rtcp attribute procedures
   with [RFC3605].

   If the agent does not utilize RTCP, it indicates that by including
   b=RS:0 and b=RR:0 SDP attributes, as described in [RFC3556].

4.2.3.  Determining Role

   The offerer acts as the Initiating agent.  The answerer acts as the
   Responding agent.  The ICE roles (controlling and controlled) are
   determined using the procedures in [RFC8445].

4.2.4.  STUN Considerations

   Once an agent has provided its local candidates to its peer in an SDP
   offer or answer, the agent MUST be prepared to receive STUN
   connectivity check Binding requests on those candidates.

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4.2.5.  Verifying ICE Support Procedures

   An ICE agent is considered to indicate support of ICE by including at
   least the SDP ice-pwd and ice-ufrag attributes in an offer or answer.
   An ICE agent compliant with this specification MUST also include an
   SDP ice-options attribute with an "ice2" attribute value.

   The agents will proceed with the ICE procedures defined in [RFC8445]
   and this specification if, for each data stream in the SDP it
   received, the default destination for each component of that data
   stream appears in a candidate attribute.  For example, in the case of
   RTP, the connection address, port, and transport protocol in the "c="
   and "m=" lines, respectively, appear in a candidate attribute and the
   value in the rtcp attribute appears in a candidate attribute.

   This specification provides no guidance on how an agent should
   proceed in the cases where the above condition is not met with the
   few exceptions noted below:

   1.  The presence of certain application layer gateways might modify
       the transport address information as described in Section 8.  The
       behavior of the responding agent in such a situation is
       implementation dependent.  Informally, the responding agent might
       consider the mismatched transport address information as a
       plausible new candidate learnt from the peer and continue its ICE
       processing with that transport address included.  Alternatively,
       the responding agent MAY include an "a=ice-mismatch" attribute in
       its answer for such data streams.  If an agent chooses to include
       an "a=ice-mismatch" attribute in its answer for a data stream,
       then it MUST also omit "a=candidate" attributes, MUST terminate
       the usage of ICE procedures and [RFC3264] procedures MUST be used
       instead for this data stream.

   2.  The transport address from the peer for the default destination
       is set to IPv4/IPv6 address values "0.0.0.0"/"::" and port value
       of "9".  This MUST NOT be considered as a ICE failure by the peer
       agent and the ICE processing MUST continue as usual.

   3.  In some cases, the controlling/initiator agent may receive the
       SDP answer that may omit "a=candidate" attributes for the data
       stream, and instead include a media level "a=ice-mismatch"
       attribute.  This signals to the offerer that the answerer
       supports ICE, but that ICE processing was not used for this data
       stream.  In this case, ICE processing MUST be terminated for this
       data stream and [RFC3264] procedures MUST be followed instead.

   4.  The transport address from the peer for the default destination
       is an FQDN.  Regardless of the procedures used to resolve FQDN or

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       the resolution result, this MUST NOT be considered as a ICE
       failure by the peer agent and the ICE processing MUST continue as
       usual.

4.2.6.  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 203.0.113.141
   s=
   c=IN IP4 192.0.2.3
   t=0 0
   a=ice-options:ice2
   a=ice-pacing:50
   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 203.0.113.141 8998 typ host
   a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr
    203.0.113.141 rport 8998

4.3.  Initial Offer/Answer Exchange

4.3.1.  Sending the Initial Offer

   When an offerer generates the initial offer, in each "m=" section it
   MUST include SDP candidate attributes for each available candidate
   associated with the "m=" section.  In addition, the offerer MUST
   include an SDP ice-ufrag attribute, an SDP ice-pwd attribute and an
   SDP ice-options attribute with an "ice2" attribute value in the
   offer.  If the offerer is a full ICE implementation, it SHOULD
   include an ice-pacing attribute in the offer (if not included, the
   default value will apply).  A lite ICE implementation MUST NOT
   included the ice-pacing attribute in the offer (as it will not
   perform connectivity checks).

   It is valid for an offer "m=" line to include no SDP candidate
   attributes and with default destination set to the IP address values
   "0.0.0.0"/"::" and port value of "9".  This implies that the offering
   agent is only going to use peer reflexive candidates or that
   additional candidates would be provided in subsequent signaling
   messages.

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   Note:  Within the scope of this document, "Initial Offer" refers to
      the first SDP offer that is sent in order to negotiate usage of
      ICE.  It might, or might not, be the initial SDP offer of the SDP
      session.

   Note:  The procedures in this document only consider "m=" sections
      associated with data streams where ICE is used.

4.3.2.  Sending the Initial Answer

   When an answerer receives an initial offer that indicates that the
   offerer supports ICE, and if the answerer accepts the offer and the
   usage of ICE, in each "m=" section within the answer, it MUST include
   SDP candidate attributes for each available candidate associated with
   the "m=" section.  In addition, the answerer MUST include an SDP ice-
   ufrag attribute, an SDP ice-pwd attribute and an SDP ice-options
   attribute with an "ice2" attribute value in the answer.  If the
   answerer is a full ICE implementation, it SHOULD include an ice-
   pacing attribute in the answerer (if not included, the default value
   will apply).  A lite ICE implementation MUST NOT included the ice-
   pacing attribute in the answer (as it will not perform connectivity
   chekcks).

   In each "m=" line, the answerer MUST use the same transport protocol
   as was used in the offer "m=" line.  If none of the candidates in the
   "m=" line in the answer use the same transport protocol as indicated
   in the offer "m=" line, then, in order to avoid ICE mismatch, the
   default destination MUST be set to IP address values "0.0.0.0"/"::"
   and port value of "9".

   It is also valid for an answer "m=" line to include no SDP candidate
   attributes and with default destination set to the IP address values
   "0.0.0.0"/"::" and port value of "9".  This implies that the
   answering agent is only going to use peer reflexive candidates or
   that additional candidates would be provided in subsequent signaling
   messages.

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

   If the offer does not indicate support of ICE Section 4.2.5, the
   answerer MUST NOT accept the usage of ICE.  If the answerer still
   accepts the offer, the answerer MUST NOT include any ICE-related SDP
   attributes in the answer.  Instead the answerer will generate the
   answer according to normal offer/answer procedures [RFC3264].

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   If the answerer detects a possibility of an ICE mismatch, procedures
   described in Section 4.2.5 are followed.

4.3.3.  Receiving the Initial Answer

   When an offerer receives an initial answer that indicates that the
   answerer supports ICE, it can start performing connectivity checks
   towards the peer candidates that were provided in the answer.

   If the answer does not indicate that the answerer supports ICE, or if
   the answerer included "a=ice-mismatch" attributes for all the active
   data streams in the answer, the offerer MUST terminate the usage of
   ICE for the entire session and [RFC3264] procedures MUST be followed
   instead.

   On the other hand, if the answer indicates support for ICE but
   includes "a=ice-mismatch" in certain active data streams, then the
   offerer MUST terminate the usage of ICE procedures and [RFC3264]
   procedures MUST be used instead for only these data streams.  Also,
   ICE procedures MUST be used for data streams where an "a=ice-
   mismatch" attribute was not included.

   If the offerer detects an ICE mismatch for one or more data streams
   in the answer, as described in Section 4.2.5, the offerer MUST
   terminate the usage of ICE for the entire session.  The subsequent
   actions taken by the offerer are implementation dependent and are out
   of the scope of this specification.

4.3.4.  Concluding ICE

   Once the agent has successfully nominated a pair [RFC8445], the state
   of the checklist associated with the pair is set to Completed.  Once
   the state of each checklist is set to either Completed or Failed, for
   each Completed checklist the agent checks whether the nominated pair
   matches the default candidate pair.  If there are one or more pairs
   that do not match, and the peer did not indicate support for the
   'ice2' ice-option, the controlling agent MUST generate a subsequent
   offer, in which the connection address, port and transport protocol
   in the "c=" and "m=" lines associated with each data stream match the
   corresponding local information of the nominated pair for that data
   stream (Section 4.4.1.2.2).  If the peer did indicate support for the
   'ice2' ice-option, the controlling agent does not immediately need to
   generate an updated offer in order to align a connection address,
   port and protocol with a nominated pair.  However, later in the
   session, whenever the controlling agent does sent a subsequent offer,
   it MUST do the alignment as described above.

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   If there are one or more checklists with the state set to Failed, the
   controlling agent MUST generate a subsequent offer in order to remove
   the associated data streams by setting the port value of the data
   streams to zero (Section 4.4.1.1.2), even if the peer did indicate
   support for the 'ice2' ice-option.  If needed, such offer is used to
   align the connection address, port and transport protocol, as
   described above.

   As described in [RFC8445], once the controlling agent has nominated a
   candidate pair for a checklist, the agent MUST NOT nominate another
   pair for that checklist during the lifetime of the ICE session (i.e.
   until ICE is restarted).

   [draft-ietf-ice-pac] provides a mechanism for allowing the ICE
   process to run long enough in order to find working candidate pairs,
   by waiting for potential peer-reflexive candidates, even though no
   candidate pairs were received from the peer or all current candidate
   pairs associated with a checklist have either failed or been
   discarded.  It is OPTIONAL for an ICE agent to support the mechanism.

4.4.  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.

4.4.1.  Sending Subsequent Offer

4.4.1.1.  Procedures for All Implementations

4.4.1.1.1.  ICE Restart

   An agent MAY restart ICE processing for an existing data stream
   [RFC8445].

   The rules governing the ICE restart imply that setting the connection
   address in the "c=" line to "0.0.0.0" (for IPv4)/ "::" (for IPv6)
   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 data stream in an offer.  However, it is permissible to
   use a session-level attribute in one offer, but to provide the same

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   ice-pwd or ice-ufrag as a media-level attribute in a subsequent
   offer.  This MUST NOT be considered as ICE restart.

   An agent sets the rest of the ICE-related fields in the SDP for this
   data stream as it would in an initial offer of this data stream
   (Section 4.2.1).  Consequently, the set of candidates MAY include
   some, none, or all of the previous candidates for that data stream
   and MAY include a totally new set of candidates.  The agent MAY
   modify the attribute values of the SDP ice-options and SDP ice-pacing
   attributes, and it MAY change its role using the SDP ice-lite
   attribute.  The agent MUST NOT modify the SDP ice-options, ice-pacing
   and ice-lite attributes in a subsequent offer unless the offer is
   sent in order to request an ICE restart.

4.4.1.1.2.  Removing a Data Stream

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

4.4.1.1.3.  Adding a Data Stream

   If an agent wishes to add a new data stream, it sets the fields in
   the SDP for this data stream as if this were an initial offer for
   that data stream (Section 4.2.1).  This will cause ICE processing to
   begin for this data stream.

4.4.1.2.  Procedures for Full Implementations

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

4.4.1.2.1.  Before Nomination

   When an offerer sends a subsequent offer; in each "m=" section for
   which a candidate pair has not yet been nominated, the offer MUST
   include the same set of ICE-related information that the offerer
   included in the previous offer or answer.  The agent MAY include
   additional candidates it did not offer previously, but which it has
   gathered since the last offer/answer exchange, including peer
   reflexive candidates.

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

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4.4.1.2.2.  After Nomination

   Once a candidate pair has been nominated for a data stream, the
   connection address, port and transport protocol in each "c=" and "m="
   line associated with that data stream MUST match the data associated
   with the nominated pair for that data stream.  In addition, the
   offerer only includes SDP candidates (one per component) representing
   the local candidates of the nominated candidate pair.  The offerer
   MUST NOT include any other SDP candidate attributes in the subsequent
   offer.

   In addition, if the agent is controlling, it MUST include the
   "a=remote-candidates" attribute for each data stream whose checklist
   is in the Completed state.  The attribute contains the remote
   candidates corresponding to the nominated pair in the valid list for
   each component of that data stream.  It is needed to avoid a race
   condition whereby the controlling agent chooses its pairs, but the
   updated offer beats the connectivity checks to the controlled agent,
   which doesn't even know these pairs are valid, let alone selected.
   See Appendix B for elaboration on this race condition.

4.4.1.3.  Procedures for Lite Implementations

   If the ICE state is Running, a lite implementation MUST include all
   of its candidates for each component of each data stream in
   "a=candidate" attributes in any subsequent offer.  The candidates are
   formed identically to the procedures for initial offers.

   A lite implementation MUST NOT add additional host candidates in a
   subsequent offer, and MUST NOT modify the username fragments and
   passwords.  If an agent needs to offer additional candidates, or
   modify the username fragments and passwords, it MUST request an ICE
   restart (Section 4.4.1.1.1) for that data stream.

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

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

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4.4.2.  Sending Subsequent Answer

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

   A controlled agent will receive an offer with the "a=remote-
   candidates" attribute for a data stream when its peer has concluded
   ICE processing for that 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 data
   stream by:

   o  Setting the remote candidate equal to the offerer's default
      destination for that component (i.e. 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 checklist 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 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

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   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.

4.4.2.1.  ICE Restart

   If the offerer in a subsequent offer requested an ICE restart
   (Section 4.4.1.1.1) 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.

   The answerer MAY modify the attribute values of the SDP ice-options
   and SDP ice-pacing attributes, and it MAY change its role using the
   SDP ice-lite attribute.  The answerer MUST NOT modify the SDP ice-
   options, ice-pacing and ice-lite attributes in a subsequent answer
   unless the answer is sent for an offer that was used to request an
   ICE restart (Section 4.4.1.1.1).  If any of the SDP attributes have
   been modified in a subsequent offer that is not used to request an
   ICE restart, the answerer MUST reject the offer.

4.4.2.2.  Lite Implementation specific procedures

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

   o  Setting the remote candidate equal to the offerer's default
      destination for that component (i.e., 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 state of the checklist associated with that data stream is set to
   Completed.

   Furthermore, if the agent believed it was controlling, but the offer
   contained the "a=remote-candidates" attribute, both agents believe
   they are controlling.  In this case, both would have sent updated
   offers around the same time.

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   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 [RFC8445], 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.

4.4.3.  Receiving Answer for a Subsequent Offer

4.4.3.1.  Procedures for Full Implementations

   There may be certain situations where the offerer receives an SDP
   answer that lacks ICE candidates although the initial answer included
   them.  One example of such an "unexpected" answer might be happen
   when an ICE-unaware Back-to-Back User Agent (B2BUA) introduces a
   media server during call hold using 3rd party call-control procedures
   [RFC3725].  Omitting further details how this is done, this could
   result in an answer being received at the holding UA that was
   constructed by the B2BUA.  With the B2BUA being ICE-unaware, that
   answer would not include ICE candidates.

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

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

   o  If the offer was a restart, the agent MUST perform ICE restart
      procedures as specified in Section 4.4.3.1.1

   o  If the offer/answer exchange removed a data stream, or an answer
      rejected an offered data stream, an agent MUST flush the 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
      MUST create a new checklist for it (and an empty Valid list to
      start of course) which in turn triggers the candidate processing
      procedures [RFC8445].

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   o  If the checklist state associated with a data stream is Running,
      the agent recomputes the checklist.  If a pair on the new
      checklist was also on the previous checklist, its candidate pair
      state is copied over.  Otherwise, its candidate pair state is set
      to Frozen.  If none of the checklists are active (meaning that the
      candidate pair states in each checklist are Frozen), appropriate
      procedures in [RFC8445] are performed to move candidate pair(s) to
      the Waiting state to further continue ICE processing.

   o  If the ICE state is Completed and the SDP answer conforms to
      Section 4.4.2, the agent MUST remain in the Completed ICE state.

   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 because of the missing ICE support or unexpected answer.
   Once the agent sends a new offer later on, it MUST perform an ICE
   restart.

4.4.3.1.1.  ICE Restarts

   The agent MUST remember the nominated pair in the Valid list for each
   component of the data stream, called the "previous selected pair",
   prior to the restart.  The agent will continue to send media using
   this pair, as described in section 12 of [RFC8445].  Once these
   destinations are noted, the agent MUST flush the Valid lists and
   checklists, and then recompute the checklist and its states, thus
   triggering the candidate processing procedures [RFC8445]

4.4.3.2.  Procedures for Lite Implementations

   If ICE is restarting for a data stream, the agent MUST create a new
   Valid list for that data stream.  It MUST remember the nominated pair
   in the previous Valid list for each component of the data stream,
   called the "previous selected pairs", and continue to send media
   there as described in section 12 of [RFC8445].  The state of each
   checklist for each data stream MUST change to Running, and the ICE
   state MUST be set to Running.

5.  Grammar

   This specification defines eight new SDP attributes -- the
   "candidate", "remote-candidates", "ice-lite", "ice-mismatch", "ice-
   ufrag", "ice-pwd", "ice-pacing", and "ice-options" attributes.

   This section also provides non-normative examples of the attributes
   defined.

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

5.1.  "candidate" Attribute

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

   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 cand-extension)

   foundation            = 1*32ice-char
   component-id          = 1*3DIGIT
   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
   cand-extension        = extension-att-name SP extension-att-value
   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, allowing for IPv4 addresses, IPv6
      addresses, and fully qualified domain names (FQDNs).  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 generating local candidates MUST
      NOT use FQDN addresses.  An agent processing remote candidates
      MUST ignore candidate lines that include candidates with FQDN or
      IP address versions that are not supported or recognized.  The
      procedures for generation and handling of FQDN candidates, as well

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      as, how agents indicate support for such procedures, need to be
      specified in an extension specification.

   <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 by extending the 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 [RFC8445]

   <component-id>:  is a positive integer between 1 and 256 (inclusive)
      that identifies the specific component of the data 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 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 13 in [RFC8445] for additional
      discussion on extending ICE to new data streams.

   <priority>:  is a positive integer between 1 and (2**31 - 1)
      inclusive.  The procedures for computing candidate's priority is
      described in section 5.1.2 of [RFC8445].

   <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.  Specifications for new candidate types MUST define
      how, if at all, various steps in the ICE processing differ from
      the 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 Appendix B.3 of [RFC8445] for a discussion

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      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 '9'.

   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
   [RFC8126] 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
   203.0.113.141 rport 8998

5.2.  "remote-candidates" Attribute

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

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

   The attribute contains a connection-address and port for each
   component.  The ordering of components is irrelevant.  However, a
   value MUST be present for each component of a data stream.  This
   attribute MUST be included in an offer by a controlling agent for a
   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

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

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

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

   "ice-lite" is a session-level attribute only, and indicates that an
   agent is a lite implementation. "ice-mismatch" is a media-level
   attribute and only reported in the answer.  It indicates that the
   offer arrived with a default destination for a media component that
   didn't have a corresponding candidate attribute.  Inclusion of
   "a=ice-mismatch" attribute for a given data stream implies that even
   though both agents support ICE, ICE procedures MUST NOT be used for
   this data stream and [RFC3264] procedures MUST be used instead.

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

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

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

   The "ice-pwd" and "ice-ufrag" attributes can appear at either the
   session-level or media-level.  When present in both, the value in the
   media-level takes precedence.  Thus, the value at the session-level
   is effectively a default that applies to all 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 data stream.  If two 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 (the same applies when ICE is restarting for
   an agent).

   [RFC8445] requires the ice-ufrag attribute to contain at least 24
   bits of randomness, and the ice-pwd attribute to 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

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   upper limit allows for increased amounts of randomness to be added
   over time.  For compatibility with the 512 character limitation for
   the STUN username attribute value and for bandwidth conservation
   considerations, the ice-ufrag attribute MUST NOT be longer than 32
   characters when sending, but an implementation MUST accept up to 256
   characters when receiving.

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

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

5.5.  "ice-pacing" Attribute

   The "ice-pacing" is a session level attribute that indicates the
   desired connectivity check pacing (Ta interval), in milliseconds,
   that the sender wishes to use.  See section 14.2 of [RFC8445] for
   more information regarding selecting a pacing value.  The syntax is:

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

   If absent in an offer or answer the default value of the attribute is
   50 ms, which is the recommended value specified in [RFC8445].

   Once both agents have indicated the pacing value they with to use,
   both agents MUST use the larger of the indicated values.

   Example shows an ice-pacing SDP line with value '50':
   a=ice-pacing:50

5.6.  "ice-options" Attribute

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

   ice-options           = "ice-options:" ice-option-tag
                             *(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 it is willing to use it, if
   the peer agent also includes the same extension in the answer.  There
   might be further extension specific negotiation needed between the
   agents that determine how the extension gets used in a given session.
   The details of the negotiation procedures, if present, MUST be
   defined by the specification defining the extension (Section 10.2).

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Example shows an ice-options SDP line with 'ice2' and 'rtp+ecn' [RFC6679] values:

a=ice-options:ice2 rtp+ecn

6.  Keepalives

   All the ICE agents MUST follow the procedures defined in section 11
   of [RFC8445] for sending keepalives.  The keepalives MUST be sent
   regardless of whether the 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.  SIP Considerations

   Note that ICE is not intended for NAT traversal for SIP signaling,
   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,
   checklists, states, and so on.

7.1.  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.

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7.1.1.  Offer in INVITE

   To reduce post-dial delays, it is RECOMMENDED that the caller begin
   gathering candidates prior to actually sending its initial INVITE, so
   that the candidates can be provided in the 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 as soon as it has completed gathering the
   candidates.  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 with the transport address matching the
   candidate address for one of the 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 since
   there will be no Binding request sent and the number four is
   arbitrarily chosen to limit the number of 18x retransmits.

   Once the answer has been sent, the agent SHOULD begin its
   connectivity checks.  Once candidate pairs for each component of a
   data stream enter the valid list, the answerer can begin sending
   media on that 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 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.

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7.1.2.  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.

7.2.  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.

7.3.  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 data streams, it cannot
   determine which 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.

7.4.  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

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

7.5.  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 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.

8.  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:

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   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 connection 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.

9.  Security Considerations

   The generic ICE security considerations are defined in [RFC8445], and
   the generic SDP offer/answer security considerations are defined in
   [RFC3264].  These security considerations also apply to
   implementations of this document.

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9.1.  IP Address Privacy

   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 '9'.

9.2.  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 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.

9.3.  The Voice Hammer Attack

   The voice hammer attack is an amplification attack, and can be
   triggered even if the attacker is an authenticated and valid
   participant in a session.  In this attack, the attacker initiates
   sessions to other agents, and maliciously includes the connection
   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).  The use of ICE can
   help to prevent against this attack.

   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.

   SIP User Agents (UA) [RFC3261] that are not willing to receive non-
   ICE answers MUST include an "ice" Option Tag [RFC5768] in the SIP
   Require Header Field in their offer.  UAs that reject non-ICE offers
   will generally use a 421 response code, together with an Option Tag
   "ice" in the Require Header Field in the response.

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10.  IANA Considerations

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 included here with
   modifications to include Mux Category and also defines a new SDP
   attribute 'ice-pacing'.

10.1.1.  candidate Attribute

   Attribute Name:  candidate

   Type of Attribute:  media-level

   Subject to 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 of RFC XXXX.

   Contact Name:  IESG

   Contact Email:  iesg@ietf.org

   Reference:  RFCXXXX

   Mux Category:  TRANSPORT

10.1.2.  remote-candidates Attribute

   Attribute Name:  remote-candidates

   Type of Attribute:  media-level

   Subject to 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 of RFC XXXX.

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   Contact Name:  IESG

   Contact Email:  iesg@ietf.org

   Reference:  RFCXXXX

   Mux Category:  TRANSPORT

10.1.3.  ice-lite Attribute

   Attribute Name:  ice-lite

   Type of Attribute:  session-level

   Subject to 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 of RFC XXXX.

   Contact Name:  IESG

   Contact Email:  iesg@ietf.org

   Reference:  RFCXXXX

   Mux Category:  NORMAL

10.1.4.  ice-mismatch Attribute

   Attribute Name:  ice-mismatch

   Type of Attribute:  media-level

   Subject to 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 of RFC XXXX.

   Contact Name:  IESG

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   Contact e-mail:  iesg@ietf.org

   Reference:  RFCXXXX

   Mux Category:  NORMAL

10.1.5.  ice-pwd Attribute

   Attribute Name:  ice-pwd

   Type of Attribute:  session- or media-level

   Subject to 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 of RFC XXXX.

   Contact Name:  IESG

   Contact e-mail:  iesg@ietf.org

   Reference:  RFCXXXX

   Mux Category:  TRANSPORT

10.1.6.  ice-ufrag Attribute

   Attribute Name:  ice-ufrag

   Type of Attribute:  session- or media-level

   Subject to 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 of RFC XXXX.

   Contact Name:  IESG

   Contact e-mail:  iesg@ietf.org

   Reference:  RFCXXXX

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   Mux Category:  TRANSPORT

10.1.7.  ice-options Attribute

   Attribute Name:  ice-options

   Long Form:  ice-options

   Type of Attribute:  session-level

   Subject to charset:  No

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

   Appropriate Values:  See Section 5 of RFC XXXX.

   Contact Name:  IESG

   Contact e-mail:  iesg@ietf.org

   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-pacing

   Type of Attribute:  session-level

   Subject to charset:  No

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

   Appropriate Values:  See Section 5 of RFC XXXX.

   Contact Name:  IESG

   Contact e-mail:  iesg@ietf.org

   Reference:  RFCXXXX

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   Mux Category:  NORMAL

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" [RFC8126].

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

   A registration request MUST include the following information:

   o  The ICE option identifier to be registered

   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

10.3.  Candidate Attribute Extension Subregistry Establishment

   This section creates a new sub-registry, "Candidate Attribute
   Extensions", under the sdp-parameters registry:
   http://www.iana.org/assignments/sdp-parameters.

   The purpose of the sub-registry is to register SDP candidate
   attribute extensions.

   When a candidate extension is registered in the sub-registry, it
   needs to meet the "Specification Required" policies defined in
   [RFC8126].

   Candidate attribute extensions MUST follow the 'cand-extension'
   syntax.  The attribute extension name MUST follow the 'extension-att-
   name' syntax, and the attribute extension value MUST follow the
   'extension-att-value' syntax.

   A registration request MUST include the following information:

   o  The name of the attribute extension.

   o  A short description of the attribute extension.

   o  A reference to a specification that describes the semantics, usage
      and possible values of the attribute extension.

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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 Flemming Andreasen for shepherd review feedback.

   Thanks to following experts for their reviews and constructive
   feedback: Thomas Stach, Adam Roach, Peter Saint-Andre, Roman Danyliw,
   Alissa Cooper, Benjamin Kaduk, Mirja Kuhlewind, Alexey Melnikov, Eric
   Vyncke for their detailed reviews.

12.  Changes from RFC 5245

   [RFC8445] describes the changes that were done to the common SIP
   procedures, including removal of aggressive nomination, modifying the
   procedures for calculating candidate pair states and scheduling
   connectivity checks and the calculation of timer values.

   This document defines the following SDP offer/answer specific
   changes:

   o  SDP offer/answer realization and usage of of 'ice2' option.

   o  Definition and usage of SDP 'ice-pacing' attribute.

   o  Explicit text that an ICE agent must not generate candidates with
      FQDNs, and must discard such candidates if received from the peer
      agent.

   o  Relax requirement to include SDP 'rtcp' attribute.

   o  Generic clarifications of SDP offer/answer procedures.

13.  References

13.1.  Normative References

   [draft-ietf-ice-pac]
              Holmberg, C. and J. Uberti, "Interactive Connectivity
              Establishment Patiently Awaiting Connectivity (ICE PAC)",
              draft-ietf-ice-pac-02 (work in progress), July 2019,
              <http://www.ietf.org/internet-drafts/
              draft-ietf-ice-pac-02.txt>.

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

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

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

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

   [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, <https://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,
              <https://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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc4032>.

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

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

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

   [RFC6336]  Westerlund, M. and C. Perkins, "IANA Registry for
              Interactive Connectivity Establishment (ICE) Options",
              RFC 6336, DOI 10.17487/RFC6336, July 2011,
              <https://www.rfc-editor.org/info/rfc6336>.

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

   [RFC8445]  Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,
              <https://www.rfc-editor.org/info/rfc8445>.

13.2.  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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc3960>.

   [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,
              <https://www.rfc-editor.org/info/rfc5245>.

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   [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,
              <https://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,
              <https://www.rfc-editor.org/info/rfc5898>.

   [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, <https://www.rfc-editor.org/info/rfc6679>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

Appendix A.  Examples

   For the example shown in section 15 of [RFC8445] 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-options:ice2
   a=ice-pacing:50
   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):

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  v=0
  o=jdoe 2890844526 2890842807 IN IP6 fe80::6676:baff:fe9c:ee4a
  s=
  c=IN IP6 2001:db8:8101:3a55:4858:a2a9:22ff:99b9
  t=0 0
  a=ice-options:ice2
  a=ice-pacing:50
  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:db8:8101:3a55:4858:a2a9:22ff:99b9
  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-options:ice2
   a=ice-pacing:50
   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:

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   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-options:ice2
   a=ice-pacing:50
   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 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.

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

                       Figure 1: Race Condition Flow

Appendix C.  Why Is the Conflict Resolution Mechanism Needed?

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

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

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

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

Appendix E.  Contributors

   Following experts have contributed textual and structural
   improvements for this work

   1.  Thomas Stach

       *  thomass.stach@gmail.com

Authors' Addresses

   Marc Petit-Huguenin
   Impedance Mismatch

   Email: marc@petit-huguenin.org

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

   Email: snandaku@cisco.com

   Christer Holmberg
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: christer.holmberg@ericsson.com

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   Ari Keranen
   Ericsson
   Jorvas  02420
   Finland

   Email: ari.keranen@ericsson.com

   Roman Shpount
   TurboBridge
   4905 Del Ray Avenue, Suite 300
   Bethesda, MD  20814
   USA

   Phone: +1 (240) 292-6632
   Email: rshpount@turbobridge.com

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