wdiff rfc8839v1.txt rfc8839.txt

Internet Engineering Task Force (IETF) M. Petit-Huguenin
Request for Comments: 0000 8839 Impedance Mismatch
Obsoletes: 5245, 6336 S. Nandakumar
Category: Standards Track Cisco Systems
ISSN: 2070-1721 C. Holmberg
A. Keränen
Ericsson
R. Shpount
TurboBridge
May 2020
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/rfc0000.
https://www.rfc-editor.org/info/rfc8839.

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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 "candidate" Attribute
10.1.2. remote-candidates "remote-candidates" Attribute
10.1.3. ice-lite "ice-lite" Attribute
10.1.4. ice-mismatch "ice-mismatch" Attribute
10.1.5. ice-pwd "ice-pwd" Attribute
10.1.6. ice-ufrag "ice-ufrag" Attribute
10.1.7. ice-options "ice-options" Attribute
10.1.8. ice-pacing "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 "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 "a=rtcp" "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
"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 "ice-ufrag" attribute, and
the ICE password is represented by an SDP ice-pwd "ice-pwd" attribute.

4.2.1.4. Lite Implementations

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

4.2.1.5. ICE Extensions

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

An agent compliant with this specification MUST include an SDP ice-
options "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 "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 "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 "candidate" attributes
for both the RTP and RTCP components.

The agent includes an SDP rtcp "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 "rtcp" attribute
might be omitted.

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

If the agent does not utilize RTCP, it indicates that by including
"b=RS:0"
"RS:0" and "b=RR:0" "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
"ice-pwd" and ice-ufrag "ice-ufrag" attributes in an offer or answer. An ICE
agent compliant with this specification MUST also include an SDP ice-
options
"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 "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 "candidate" attribute,
and the value in the rtcp "rtcp" attribute appears in a candidate "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 learned from the peer and continue its
ICE processing with that transport address included.
Alternatively, the responding agent MAY include an "a=ice-mismatch" "ice-mismatch"
attribute in its answer for such data streams. If an agent
chooses to include an "a=ice-mismatch" "ice-mismatch" attribute in its answer for
a data stream, then it MUST also omit "a=candidate" "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 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 "a=candidate" "candidate" attributes for the data stream,
and instead include a media-level "a=ice-mismatch" "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 a 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 "candidate" attributes for each available candidate
associated with the "m=" section. In addition, the offerer MUST
include an SDP ice-ufrag "ice-ufrag" attribute, an SDP ice-pwd "ice-pwd" attribute, and
an SDP ice-options "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 "ice-pacing" attribute in the offer (if not included, the
default value will apply). A lite ICE implementation MUST NOT
include the ice-pacing "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 "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 peer-reflexive candidates or
that
will provide additional candidates would be provided 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 that indicates 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, it MUST include answer the SDP candidate "candidate" attributes for each available
candidate associated with the "m=" section. In addition, the
answerer MUST include an SDP ice-
ufrag "ice-ufrag" attribute, an SDP ice-pwd "ice-pwd"
attribute, and an SDP ice-options "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 "ice-pacing" attribute in the
answer (if not included, the default value will apply). A lite ICE
implementation MUST NOT include the ice-
pacing "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
"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 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 "a=ice-mismatch" "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" "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" "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).

[ICE-PAC]

[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
"a=inactive"
"inactive" and "a=sendonly" "sendonly" as described in [RFC3264].

To restart ICE, an agent MUST change both the ice-pwd "ice-pwd" and the ice-
ufrag "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 "ice-pwd" or ice-ufrag "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 "ice-options" and SDP ice-pacing "ice-
pacing" attributes, and it MAY change its role using the SDP ice-lite "ice-
lite" attribute. The agent MUST NOT modify the SDP ice-options, ice-
pacing, "ice-options",
"ice-pacing", and ice-lite "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 "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 peer-
reflexive candidates.

The agent MAY change the default destination for media. As with
initial offers, there MUST be a set of candidate "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 candidates "candidate" attributes (one
per component) representing the local candidates of the nominated
candidate pair. The offerer MUST NOT include any other SDP candidate
"candidate" attributes in the subsequent offer.

In addition, if the agent is controlling, it MUST include the
"a=remote-candidates"
"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"
"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 "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"
"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 "a=remote-candidates" "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" "remote-candidates"
attribute in the answer.

A controlled agent will receive an offer with the "a=remote-
candidates" "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 "a=rtcp" "rtcp" attribute for RTCP)

* Setting the local candidate equal to the transport address for
that same component in the "a=remote-candidates" "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 "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 "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 "candidate" attribute in the answer for each
candidate in the remote-candidates "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 "ice-pwd" and ice-ufrag "ice-ufrag" attribute values.

The answerer MAY modify the attribute values of the SDP ice-options "ice-options"
and SDP ice-pacing "ice-pacing" attributes, and it MAY change its role using the
SDP ice-lite "ice-lite" attribute. The answerer MUST NOT modify the SDP ice-
options, ice-pacing, "ice-
options", "ice-pacing", and ice-lite "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 "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 "a=rtcp" "rtcp" attribute for RTCP).

* Setting the local candidate equal to the transport address for
that same component in the "a=remote-candidates" "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" "remote-candidates" attribute, both agents believe they
are controlling. In this case, both would have sent updated offers
around the same time.

However, the signaling protocol carrying the offer/answer exchanges
will have resolved this glare condition, so that one agent is always
the 'winner' by having its offer received before its peer has sent an
offer. The winner takes the role of controlling, so that the loser
(the answerer under consideration in this section) MUST change its
role to controlled.

Consequently, if the agent was going to send an updated offer since, controlling based on the rules in
Section 8.2 of [RFC8445], it [RFC8445] and was controlling, going to send an updated offer, it
no longer needs to.

Besides the potential role change, change in the Valid 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 being received at the holding UA that was constructed by the B2BUA. 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 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 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.
Once
When the agent sends a new offer later on, offer, it MUST perform an ICE restart.

4.4.3.1.1. ICE Restarts

The agent MUST remember the nominated pair in the Valid 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 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
valid list for that data stream. It MUST remember the nominated pair
in the previous Valid 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 "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 "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,
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 server-reflexive, peer-
reflexive, and relayed candidates. If a candidate is server server-
reflexive or
peer reflexive, peer-reflexive, <rel-addr> and <rel-port> are equal
to the base for that server server-reflexive or peer reflexive 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 Section 6.3 of its purpose). [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 "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 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 "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 "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 "candidate" attribute. Inclusion of
"a=ice-mismatch"
"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 "ice-pwd" and ice-ufrag "ice-ufrag" attribute for
each data stream. If two data streams have identical ice-ufrag's, "ice-ufrag"s,
they MUST have identical ice-pwd's. "ice-pwd"s.

The ice-ufrag "ice-ufrag" and ice-pwd "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 "ice-ufrag" attribute to contain at least 24
bits of randomness, and the ice-pwd "ice-pwd" attribute to contain at least
128 bits of randomness. This means that the ice-ufrag "ice-ufrag" attribute
will be at least 4 characters long, and the ice-pwd "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 "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 "ice-ufrag" and ice-pwd "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 "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-option "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 "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 "a=candidate" "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 (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:

* The ALG does not modify the "m=" and "c=" lines or the rtcp "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 "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
"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 "rtcp" attributes. The ice-
mismatch
"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 "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'. "ice-pacing".

10.1.1. candidate "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 "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 "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 "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 "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 "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 "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 "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 "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 "candidate"
attribute extensions.

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

Candidate

"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' "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' "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

[ICE-PAC] Holmberg, C. and J. Uberti, "Interactive Connectivity
Establishment Patiently Awaiting Connectivity (ICE PAC)",
Work in Progress, Internet-Draft, draft-ietf-ice-pac-06,
29 April 2020,
<https://tools.ietf.org/html/draft-ietf-ice-pac-06>.

[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: 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 "remote-candidates" Attribute

The "a=remote-candidates" "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 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 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 11.1 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 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 "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