path: root/doc/rfc/rfc8839.txt

Internet Engineering Task Force (IETF)                 M. Petit-Huguenin
Request for Comments: 8839                            Impedance Mismatch
Obsoletes: 5245, 6336                                      S. Nandakumar
Category: Standards Track                                  Cisco Systems
ISSN: 2070-1721                                              C. Holmberg
                                                              A. Keränen
                                                                Ericsson
                                                              R. Shpount
                                                             TurboBridge
                                                            January 2021


     Session Description Protocol (SDP) Offer/Answer Procedures for
              Interactive Connectivity Establishment (ICE)

Abstract

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

   This document obsoletes RFCs 5245 and 6336.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8839.

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   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   than English.

Table of Contents

   1.  Introduction
   2.  Conventions
   3.  Terminology
   4.  SDP Offer/Answer Procedures
     4.1.  Introduction
     4.2.  Generic Procedures
       4.2.1.  Encoding
       4.2.2.  RTP/RTCP Considerations
       4.2.3.  Determining Role
       4.2.4.  STUN Considerations
       4.2.5.  Verifying ICE Support Procedures
       4.2.6.  SDP Example
     4.3.  Initial Offer/Answer Exchange
       4.3.1.  Sending the Initial Offer
       4.3.2.  Sending the Initial Answer
       4.3.3.  Receiving the Initial Answer
       4.3.4.  Concluding ICE
     4.4.  Subsequent Offer/Answer Exchanges
       4.4.1.  Sending Subsequent Offer
       4.4.2.  Sending Subsequent Answer
       4.4.3.  Receiving Answer for a Subsequent Offer
   5.  Grammar
     5.1.  "candidate" Attribute
     5.2.  "remote-candidates" Attribute
     5.3.  "ice-lite" and "ice-mismatch" Attributes
     5.4.  "ice-ufrag" and "ice-pwd" Attributes
     5.5.  "ice-pacing" Attribute
     5.6.  "ice-options" Attribute
   6.  Keepalives
   7.  SIP Considerations
     7.1.  Latency Guidelines
       7.1.1.  Offer in INVITE
       7.1.2.  Offer in Response
     7.2.  SIP Option Tags and Media Feature Tags
     7.3.  Interactions with Forking
     7.4.  Interactions with Preconditions
     7.5.  Interactions with Third Party Call Control
   8.  Interactions with Application Layer Gateways and SIP
   9.  Security Considerations
     9.1.  IP Address Privacy
     9.2.  Attacks on the Offer/Answer Exchanges
     9.3.  The Voice Hammer Attack
   10. IANA Considerations
     10.1.  SDP Attributes
       10.1.1.  "candidate" Attribute
       10.1.2.  "remote-candidates" Attribute
       10.1.3.  "ice-lite" Attribute
       10.1.4.  "ice-mismatch" Attribute
       10.1.5.  "ice-pwd" Attribute
       10.1.6.  "ice-ufrag" Attribute
       10.1.7.  "ice-options" Attribute
       10.1.8.  "ice-pacing" Attribute
     10.2.  Interactive Connectivity Establishment (ICE) Options
            Registry
     10.3.  Candidate Attribute Extension Subregistry Establishment
   11. Changes from RFC 5245
   12. References
     12.1.  Normative References
     12.2.  Informative References
   Appendix A.  Examples
   Appendix B.  The "remote-candidates" Attribute
   Appendix C.  Why Is the Conflict Resolution Mechanism Needed?
   Appendix D.  Why Send an Updated Offer?
   Acknowledgements
   Contributors
   Authors' Addresses

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.

   This document obsoletes RFCs 5245 and 6336.

   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 11 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 RTP
      Control Protocol (RTCP) component, the address and port are
      indicated using the "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 Offer/Answer protocol [RFC3264], 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.

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 with 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"
   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
   "RS:0" and "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 (Session
   Traversal Utilities for NAT, [RFC5389]) connectivity check Binding
   requests on those candidates.

4.2.5.  Verifying ICE Support Procedures

   An ICE agent indicates 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 learned from the peer and continue its
       ICE processing with that transport address included.
       Alternatively, the responding agent MAY include an "ice-mismatch"
       attribute in its answer for such data streams.  If an agent
       chooses to include an "ice-mismatch" attribute in its answer for
       a data stream, then it MUST also omit "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 an ICE failure by the
       peer agent, and the ICE processing MUST continue as usual.

   3.  In some cases, the controlling/initiator agent may receive an SDP
       answer that may omit "candidate" attributes for the data stream,
       and instead include a media-level "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
       the resolution result, this MUST NOT be considered as an 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
   include 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 have the default destination set to the IP address
   values "0.0.0.0"/"::" and the port value to "9".  This implies that
   the offering agent is only going to use peer-reflexive candidates or
   will provide additional candidates in subsequent signaling messages.

   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 indicating that the
   offerer supports ICE, and if the answerer accepts the offer and the
   usage of ICE, the answerer MUST include in each "m=" section within
   the answer the 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
   answer (if not included, the default value will apply).  A lite ICE
   implementation MUST NOT include the "ice-pacing" attribute in the
   answer (as it will not perform connectivity checks).

   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 uses 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 have the default destination set to the IP
   address values "0.0.0.0"/"::" and the port value to "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].

   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 "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 "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 "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 send a subsequent offer,
   it MUST do the alignment as described above.

   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).

   [RFC8863] 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.

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
   "inactive" and "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 "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.

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 "candidate" attributes (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
   "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
   "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 to
   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
   "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).

4.4.2.  Sending Subsequent Answer

   If ICE is Completed for a data stream, and the offer for that data
   stream lacked the "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 "remote-candidates"
   attribute in the answer.

   A controlled agent will receive an offer with the "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:

   *  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 "rtcp" attribute for RTCP)

   *  Setting the local candidate equal to the transport address for
      that same component in the "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:

   *  If none of the pairs is 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.

   *  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.

4.4.2.1.  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:

   *  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 "rtcp" attribute for RTCP).

   *  Setting the local candidate equal to the transport address for
      that same component in the "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 "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 controlling based on the rules in
   Section 8.2 of [RFC8445] and was going to send an updated offer, 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 happen when
   an ICE-unaware Back-to-Back User Agent (B2BUA) introduces a media
   server during call hold using third party call control procedures
   [RFC3725].  Omitting further details on how this is done, this could
   result in an answer that was constructed by the B2BUA being received
   at the holding UA.  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:

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

   *  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.

   *  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].

   *  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.

   *  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.
   When the agent sends a new offer, 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.

   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 FQDNs
      or IP address versions that are not supported or recognized.  The
      procedures for generation and handling of FQDN candidates, as well
      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 subregistry "ICE Transport Protocols" under
      the "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 of [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 a candidate's priority
      are 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-
      reflexive or peer-reflexive, <rel-addr> and <rel-port> are equal
      to the base for that server-reflexive 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 6.3 of [RFC5766]).
      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.

   The following is an example SDP line for a 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.

   The following is an example of "remote-candidates" SDP lines for the
   RTP and RTCP components:

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

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

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

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

   "ice-lite" is a session-level attribute only, and indicates that an
   agent is a lite implementation. "ice-mismatch" is a media-level
   attribute 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
   "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 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.

   The following 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].

   As defined in [RFC8445], regardless of the Ta value chosen for each
   agent, the combination of all transactions from all agents (if a
   given implementation runs several concurrent agents) will not be sent
   more often than once every 5 ms.

   As defined in [RFC8445], once both agents have indicated the pacing
   value they want to use, both agents will use the larger of the
   indicated values.

   The following 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-level 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-options" 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).

   The following 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.  As defined in [RFC8445], the
   keepalives will 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 "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.

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 response 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.

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
   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 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:

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

   *  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.

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.  The ICE extension
   defined in [RFC7675] can be used to further protect against voice
   hammer attacks.

   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.

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 [RFC8859] 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 8839.

   Contact Name:  IESG

   Contact Email:  iesg@ietf.org

   Reference:  RFC 8839

   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 8839.

   Contact Name:  IESG

   Contact Email:  iesg@ietf.org

   Reference:  RFC 8839

   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 8839.

   Contact Name:  IESG

   Contact Email:  iesg@ietf.org

   Reference:  RFC 8839

   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 8839.

   Contact Name:  IESG

   Contact e-mail:  iesg@ietf.org

   Reference:  RFC 8839

   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 8839.

   Contact Name:  IESG

   Contact e-mail:  iesg@ietf.org

   Reference:  RFC 8839

   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 8839.

   Contact Name:  IESG

   Contact e-mail:  iesg@ietf.org

   Reference:  RFC 8839

   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 8839.

   Contact Name:  IESG

   Contact e-mail:  iesg@ietf.org

   Reference:  RFC 8839

   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 8839.

   Contact Name:  IESG

   Contact e-mail:  iesg@ietf.org

   Reference:  RFC 8839

   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:

   *  The ICE option identifier to be registered

   *  Name and email address of organization or individuals having
      change control

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

   *  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 subregistry, "Candidate Attribute
   Extensions", under the SDP Parameters registry:
   http://www.iana.org/assignments/sdp-parameters.

   The purpose of the subregistry is to register SDP "candidate"
   attribute extensions.

   When a "candidate" extension is registered in the subregistry, 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:

   *  The name of the attribute extension.

   *  Name and email address of organization or individuals having
      change control

   *  A short description of the attribute extension.

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

11.  Changes from RFC 5245

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

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

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

   *  Definition and usage of SDP "ice-pacing" attribute.

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

   *  Relax requirement to include SDP "rtcp" attribute.

   *  Generic clarifications of SDP offer/answer procedures.

   *  ICE mismatch is now optional, and an agent has an option to not
      trigger mismatch and instead treat the default candidate as an
      additional candidate.

   *  FQDNs and "0.0.0.0"/"::" IP addresses with port "9" default
      candidates do not trigger ICE mismatch.

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <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>.

   [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>.

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

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766,
              DOI 10.17487/RFC5766, April 2010,
              <https://www.rfc-editor.org/info/rfc5766>.

   [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>.

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,
              <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>.

   [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>.

   [RFC7675]  Perumal, M., Wing, D., Ravindranath, R., Reddy, T., and M.
              Thomson, "Session Traversal Utilities for NAT (STUN) Usage
              for Consent Freshness", RFC 7675, DOI 10.17487/RFC7675,
              October 2015, <https://www.rfc-editor.org/info/rfc7675>.

   [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>.

   [RFC8859]  Nandakumar, S., "A Framework for Session Description
              Protocol (SDP) Attributes When Multiplexing", RFC 8859,
              DOI 10.17487/RFC8859, January 2021,
              <https://www.rfc-editor.org/info/rfc8859>.

   [RFC8863]  Holmberg, C. and J. Uberti, "Interactive Connectivity
              Establishment Patiently Awaiting Connectivity (ICE PAC)",
              RFC 8863, DOI 10.17487/RFC8863, January 2021,
              <https://www.rfc-editor.org/info/rfc8863>.

Appendix A.  Examples

   For the example shown in Section 15 of [RFC8445], the resulting offer
   (message 5) encoded in SDP looks like (lines folded for clarity):

   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):

   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:

   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 "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 received
   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 12.1 of [RFC8445] 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
   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.

Acknowledgements

   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 Kühlewind, Alexey Melnikov, and
   Éric Vyncke for their detailed reviews.

Contributors

   The following experts have contributed textual and structural
   improvements for this work:

   Thomas Stach

   Email: 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
   United States of America

   Email: snandaku@cisco.com


   Christer Holmberg
   Ericsson
   Hirsalantie 11
   FI-02420 Jorvas
   Finland

   Email: christer.holmberg@ericsson.com


   Ari Keränen
   Ericsson
   FI-02420 Jorvas
   Finland

   Email: ari.keranen@ericsson.com


   Roman Shpount
   TurboBridge
   4905 Del Ray Avenue, Suite 300
   Bethesda, MD 20814
   United States of America

   Email: rshpount@turbobridge.com