MMUSIC M. Petit-Huguenin
Internet-Draft Impedance Mismatch
Obsoletes: 5245 (if approved) A. Keranen
Intended status: Standards Track Ericsson
Expires: April 5, 2018 S. Nandakumar
Cisco Systems
October 2, 2017
Session Description Protocol (SDP) Offer/Answer procedures for
Interactive Connectivity Establishment (ICE)
draft-ietf-mmusic-ice-sip-sdp-14
Abstract
This document describes Session Description Protocol (SDP) Offer/
Answer procedures for carrying out Interactive Connectivity
Establishment (ICE) between the agents.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. ICE Candidate Exchange and Offer/Answer Mapping . . . . . . . 4
4. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . . 4
4.1. Initial Offer/Answer Exchange . . . . . . . . . . . . . . 4
4.1.1. Sending the Initial Offer . . . . . . . . . . . . . . 4
4.1.2. Receiving the Initial Offer . . . . . . . . . . . . . 7
4.1.3. Receipt of the Initial Answer . . . . . . . . . . . . 8
4.1.4. Performing Connectivity Checks . . . . . . . . . . . 9
4.1.5. Concluding ICE . . . . . . . . . . . . . . . . . . . 9
4.2. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . 10
4.2.1. Generating the Offer . . . . . . . . . . . . . . . . 10
4.2.2. Receiving the Offer and Generating an Answer . . . . 13
4.2.3. Receiving the Answer for a Subsequent Offer . . . . . 16
4.2.4. Updating the Check and Valid Lists . . . . . . . . . 17
5. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.1. "candidate" Attribute . . . . . . . . . . . . . . . . . . 18
5.2. "remote-candidates" Attribute . . . . . . . . . . . . . . 21
5.3. "ice-lite" and "ice-mismatch" Attributes . . . . . . . . 21
5.4. "ice-ufrag" and "ice-pwd" Attributes . . . . . . . . . . 22
5.5. "ice-pacing" Attribute . . . . . . . . . . . . . . . . . 22
5.6. "ice-options" Attribute . . . . . . . . . . . . . . . . . 23
6. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . 23
7. Media Handling . . . . . . . . . . . . . . . . . . . . . . . 23
7.1. Sending Media . . . . . . . . . . . . . . . . . . . . . . 23
7.1.1. Procedures for All Implementations . . . . . . . . . 24
7.2. Receiving Media . . . . . . . . . . . . . . . . . . . . . 24
8. SIP Considerations . . . . . . . . . . . . . . . . . . . . . 24
8.1. Latency Guidelines . . . . . . . . . . . . . . . . . . . 24
8.1.1. Offer in INVITE . . . . . . . . . . . . . . . . . . . 25
8.1.2. Offer in Response . . . . . . . . . . . . . . . . . . 26
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8.2. SIP Option Tags and Media Feature Tags . . . . . . . . . 26
8.3. Interactions with Forking . . . . . . . . . . . . . . . . 27
8.4. Interactions with Preconditions . . . . . . . . . . . . . 27
8.5. Interactions with Third Party Call Control . . . . . . . 27
9. Relationship with ANAT . . . . . . . . . . . . . . . . . . . 28
10. Setting Ta and RTO for RTP Media Streams . . . . . . . . . . 28
11. Security Considerations . . . . . . . . . . . . . . . . . . . 28
11.1. Attacks on the Offer/Answer Exchanges . . . . . . . . . 28
11.2. Insider Attacks . . . . . . . . . . . . . . . . . . . . 28
11.2.1. The Voice Hammer Attack . . . . . . . . . . . . . . 29
11.2.2. Interactions with Application Layer Gateways and SIP 29
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
12.1. SDP Attributes . . . . . . . . . . . . . . . . . . . . . 30
12.1.1. candidate Attribute . . . . . . . . . . . . . . . . 31
12.1.2. remote-candidates Attribute . . . . . . . . . . . . 31
12.1.3. ice-lite Attribute . . . . . . . . . . . . . . . . . 31
12.1.4. ice-mismatch Attribute . . . . . . . . . . . . . . . 32
12.1.5. ice-pwd Attribute . . . . . . . . . . . . . . . . . 32
12.1.6. ice-ufrag Attribute . . . . . . . . . . . . . . . . 33
12.1.7. ice-pacing Attribute . . . . . . . . . . . . . . . . 33
12.1.8. ice-options Attribute . . . . . . . . . . . . . . . 33
12.2. Interactive Connectivity Establishment (ICE) Options
Registry . . . . . . . . . . . . . . . . . . . . . . . . 34
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 35
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
14.1. Normative References . . . . . . . . . . . . . . . . . . 35
14.2. Informative References . . . . . . . . . . . . . . . . . 38
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 38
Appendix B. The remote-candidates Attribute . . . . . . . . . . 40
Appendix C. Why Is the Conflict Resolution Mechanism Needed? . . 41
Appendix D. Why Send an Updated Offer? . . . . . . . . . . . . . 42
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43
1. Introduction
This document describes how Interactive Connectivity Establishment
(ICE) is used with Session Description Protocol (SDP) offer/answer
[RFC3264]. The ICE specification [ICE-BIS] describes procedures that
are common to all usages of ICE and this document gives the
additional details needed to use ICE with SDP offer/answer.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
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Readers should be familiar with the terminology defined in [RFC3264],
in [RFC7656], in [ICE-BIS] and the following:
Default Destination/Candidate: The default destination for a
component of a media stream is the transport address that would be
used by an agent that is not ICE aware. A default candidate for a
component is one whose transport address matches the default
destination for that component. For the RTP component, the
default IP address is in the "c=" line of the SDP, and the port is
in the "m=" line. For the RTCP component, the address and port
are indicated using the "a=rtcp" attribute defined in [RFC3605],
if present; otherwise, the RTCP component address is same as the
address of the RTP component, and its port is one greater than the
port of the RTP component.
3. ICE Candidate Exchange and Offer/Answer Mapping
[ICE-BIS] defines ICE candidate exchange as the process for ICE
agents (Initiator and Responder) to exchange their candidate
information required for ICE processing at the agents. For the
purposes of this specification, the candidate exchange process
corresponds to the [RFC3264] Offer/Answer protocol and the
terminologies offerer and answerer correspond to the initiator and
responder terminologies from [ICE-BIS] respectively.
4. SDP Offer/Answer Procedures
4.1. Initial Offer/Answer Exchange
4.1.1. Sending the Initial Offer
The offerer shall follow the procedures defined in section 4 of
[ICE-BIS] to gather, prioritize and eliminate the redundant
candidates. It then chooses the default candidates and encodes them
in the SDP to be sent to its peer, the answerer.
4.1.1.1. Choosing Default Candidates
A candidate is said to be default if it would be the target of media
from a non-ICE peer; that target is called the DEFAULT DESTINATION.
An agent MUST choose a set of candidates, one for each component of
each in-use media stream, to be default. A media stream is in-use if
it does not have a port of zero (which is used in RFC 3264 to reject
a media stream). Consequently, a media stream is in-use even if it
is marked as a=inactive [RFC4566] or has a bandwidth value of zero.
An agent may choose any type of the candidate as the default, if the
chosen candidates increases the likelihood of success with the peer
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that is being contacted if ICE is not being used. It is recommended
that, when multiple candidates are used, UDP based candidates SHOULD
be included wherever possible and default candidate SHOULD be chosen
from one of those UDP candidates. The proto value MUST match the
transport protocol associated with the default candidate. If UDP
transport is used for the default candidate, the 'proto' value MUST
include UDP and the 'proto' value MUST be TCP when the transport is
TCP for the default candidate.
Since it is RECOMMENDED that default candidates be chosen based on
the likelihood of those candidates to work with the peer that is
being contacted if ICE is not being used. Many factors may influence
such a decision in a given agent. In scenarios where the agent is
fully aware of its peer's location and can reach the peer directly,
choosing the host candidates as the default may well be sufficient.
If the network configuration under which the agents operates is
static and known beforehand, either the host or the server reflexives
candidates can serve as the default candidates (depending on if a
given agent is behind NAT and their reachability). If the agent is
completely unaware of the peer's location or no assumptions can be
made of network characteristics and the connectivity, the relayed
candidates might be the only option as the default candidate. Having
the decision of choosing the default candidate as a configurable
option in the implementations might provide agents the flexibility to
take into account the aforementioned criteria. Barring such
configuration flexibility, it is RECOMMENDED that the default
candidates be the relayed candidates (if relayed candidates are
available), server reflexive candidates (if server reflexive
candidates are available), and finally host candidates.
4.1.1.2. Encoding the SDP
The process of encoding the SDP is identical between full and lite
implementations.
The agent will include an "m=" line for each Source Stream [RFC7656]
it wishes to use. The ordering of source streams in the SDP is
relevant for ICE. ICE will perform its connectivity checks for the
first "m=" line first, and consequently media will be able to flow
for that stream first. Agents SHOULD place their most important
source stream, if there is one, first in the SDP.
There will be a candidate attribute for each candidate for a
particular source stream. Section 5 provides detailed rules for
constructing this attribute.
STUN connectivity checks between agents are authenticated using the
short-term credential mechanism defined for STUN [RFC5389]. This
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mechanism relies on a username and password that are exchanged
through protocol machinery between the client and server. The
username fragment and password are exchanged in the ice-ufrag and
ice-pwd attributes, respectively.
If an agent is a lite implementation, it MUST include an "a=ice-lite"
session-level attribute in its SDP to indicate this. If an agent is
a full implementation, it MUST NOT include this attribute.
Section 9 of [ICE-BIS] defines a new ICE option, 'ice2'. This option
is used by ICE Agents to indicate their compliancy with [ICE-BIS]
specification as compared to the [RFC5245]. If the Offering agent is
a [ICE-BIS] compliant implementation, a session level ICE option to
indicate the same (via the "a=ice-options:ice2" SDP line) MUST be
included.
The default candidates are added to the SDP as the default
destination for media. For source streams based on RTP, this is done
by placing the IP address and port of the RTP candidate into the "c="
and "m=" lines, respectively. If the agent is utilizing RTCP and if
RTCP candidate is present and is not equal to the same address and
the next higher port number of the RTP candidate, the agent MUST
encode the RTCP candidate using the a=rtcp attribute as defined in
[RFC3605]. If RTCP is not in use, the agent MUST signal that using
b=RS:0 and b=RR:0 as defined in [RFC3556]
The transport addresses that will be the default destination for
media when communicating with non-ICE peers MUST also be present as
candidates in one or more a=candidate lines.
ICE provides for extensibility by allowing an offer or answer to
contain a series of tokens that identify the ICE extensions used by
that agent. If an agent supports an ICE extension, it MUST include
the token defined for that extension in the ice-options attribute.
The following is an example SDP message that includes ICE attributes
(lines folded for readability):
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v=0
o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
s=
c=IN IP4 192.0.2.3
t=0 0
a=ice-options:ice2
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 45664 RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 10.0.1.1 8998 typ host
a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr
10.0.1.1 rport 8998
Once an agent has sent its offer or its answer, that agent MUST be
prepared to receive both STUN and media packets on each candidate.
As discussed in section 11.1 of [ICE-BIS], media packets can be sent
to a candidate prior to its appearance as the default destination for
media in an offer or answer.
4.1.2. Receiving the Initial Offer
On receiving the offer, the answerer verifies the support for ICE
(section 4.4 of [ICE-BIS]), determines its role (section 5.1.1 of
[ICE-BIS]), gathers candidates (section 4 of [ICE-BIS]), encodes the
candidates in an SDP answer and sends it to its peer, the offerer.
The answerer shall then follow the steps defined in sections 5.1.3
and 5.1.4 of [ICE-BIS] to schedule the ICE connectivity checks.
The below sub-sections provide additional requirements associated
with the processing of the offerer's SDP pertaining to this
specification.
4.1.2.1. ICE Option "ice2" considerations
If the SDP offer contains a session level ICE option, "ice2" , and if
the answering ICE Agent is also an [ICE-BIS] compliant
implementation, then the generated SDP answer MUST include the
session level "a=ice-options:ice2" SDP line.
4.1.2.2. Choosing Default Candidates
The process for selecting default candidates at the answerer is
identical to the process followed by the offerer, as described in
Section 4.1.1.1 for full implementations in this specification and
section 4.2 of [ICE-BIS] for lite implementations.
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4.1.2.3. Verifying ICE Support
The agent will proceed with the ICE procedures defined in [ICE-BIS]
and this specification if, for each media stream in the SDP it
received, the default destination for each component of that media
stream appears in a candidate attribute. For example, in the case of
RTP, the IP address and port in the "c=" and "m=" lines,
respectively, appear in a candidate attribute and the value in the
rtcp attribute appears in a candidate attribute.
If this condition is not met, the agent MUST process the SDP based on
normal RFC 3264 procedures, without using any of the ICE mechanisms
described in the remainder of this specification with the following
exceptions:
1. The agent MUST follow the rules of section 10 of [ICE-BIS], which
describe keepalive procedures for all agents.
2. If the agent is not proceeding with ICE because there were
a=candidate attributes, but none that matched the default
destination of the media stream, the agent MUST include an a=ice-
mismatch attribute in its answer.
3. If the default candidates were relayed candidates learned through
a TURN server, the agent MUST create permissions in the TURN
server for the IP addresses learned from its peer in the SDP it
just received. If this is not done, initial packets in the media
stream from the peer may be lost.
4.1.2.4. Determining Role
In unusual cases, described in Appendix C, it is possible for both
agents to mistakenly believe they are controlled or controlling. To
resolve this, each agent MUST select a random number, called the tie-
breaker, uniformly distributed between 0 and (2**64) - 1 (that is, a
64-bit positive integer). This number is used in connectivity checks
to detect and repair this case, as described in section 6.1.3 of
[ICE-BIS].
4.1.3. Receipt of the Initial Answer
On receiving the SDP answer, the offerer performs steps similar to
answerer's processing of the offer. The offerer verifies the
answerer's ICE support determines, its role, and processes the
answerer's candidates to schedule the connectivity checks (section 6
of [ICE-BIS]).
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If the offerer had included the "ice2" ICE Option in the offer and
the SDP answer also includes a similar session level ICE option, then
the peers are [ICE-BIS] compliant implementations. On the other
hand, if the SDP Answer lacks such a ICE option, the offerer defaults
to the procedures that are backward compatible with the [RFC5245]
specification.
4.1.3.1. Verifying ICE Support
The logic at the offerer is identical to that of the answerer as
described in section 4.4 of [ICE-BIS], with the exception that an
offerer would not ever generate a=ice-mismatch attributes in an SDP.
In some cases, the answer may omit a=candidate attributes for the
media streams, and instead include an a=ice-mismatch attribute for
one or more of the media streams in the SDP. This signals to the
offerer that the answerer supports ICE, but that ICE processing was
not used for the session because a signaling intermediary modified
the default destination for media components without modifying the
corresponding candidate attributes. See Section 11.2.2 for a
discussion of cases where this can happen. This specification
provides no guidance on how an agent should proceed in such a failure
case.
4.1.4. Performing Connectivity Checks
The possibility for role conflicts described in section 6.3.1.1 of
[ICE-BIS] applies to this usage and hence all full agents MUST
implement the role conflict repairing mechanism. Also both full and
lite agents MUST utilize the ICE-CONTROLLED and ICE-CONTROLLING
attributes as described in section 6.1.3 of [ICE-BIS].
4.1.5. Concluding ICE
Once the state of each check list is Completed, If an agent is
controlling, it examines the highest-priority nominated candidate
pair for each component of each media stream. If any of those
candidate pairs differ from the default candidate pairs in the most
recent offer/answer exchange, the controlling agent MUST generate an
updated offer as described in Section 4.2.
However, If the support for 'ice2' ICE Option is in use, the highest-
priority nominated candidate is noted and sent in the subsequent
offer/answer exchange as the default candidate and no updated offer
is needed to fix the default candidate.
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4.2. Subsequent Offer/Answer Exchanges
Either agent MAY generate a subsequent offer at any time allowed by
[RFC3264]. This section defines rules for construction of subsequent
offers and answers.
Should a subsequent offer fail, ICE processing continues as if the
subsequent offer had never been made.
4.2.1. Generating the Offer
4.2.1.1. Procedures for All Implementations
4.2.1.1.1. ICE Restarts
An agent MAY restart ICE processing for an existing media stream as
defined in section 8 of [ICE-BIS].
The rules governing the ICE restart imply that setting the IP address
in the "c=" line to 0.0.0.0 will cause an ICE restart. Consequently,
ICE implementations MUST NOT utilize this mechanism for call hold,
and instead MUST use a=inactive and a=sendonly as described in
[RFC3264].
To restart ICE, an agent MUST change both the ice-pwd and the ice-
ufrag for the media stream in an offer. Note that it is permissible
to use a session-level attribute in one offer, but to provide the
same ice-pwd or ice-ufrag as a media-level attribute in a subsequent
offer. This is not a change in password, just a change in its
representation, and does not cause an ICE restart.
An agent sets the rest of the fields in the SDP for this media stream
as it would in an initial offer of this media stream (see
Section 4.1.1.2). Consequently, the set of candidates MAY include
some, none, or all of the previous candidates for that stream and MAY
include a totally new set of candidates.
4.2.1.1.2. Removing a Media Stream
If an agent removes a media stream by setting its port to zero, it
MUST NOT include any candidate attributes for that media stream and
SHOULD NOT include any other ICE-related attributes defined in
Section 5 for that media stream.
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4.2.1.1.3. Adding a Media Stream
If an agent wishes to add a new media stream, it sets the fields in
the SDP for this media stream as if this was an initial offer for
that media stream (see Section 4.1.1.2). This will cause ICE
processing to begin for this media stream.
4.2.1.2. Procedures for Full Implementations
This section describes additional procedures for full
implementations, covering existing media streams.
4.2.1.2.1. Existing Media Streams with ICE Running
If an agent generates an updated offer including a media stream that
was previously established, and for which ICE checks are in the
Running state, the agent follows the procedures defined here.
An agent MUST include candidate attributes for all local candidates
it had signaled previously for that media stream. The properties of
that candidate as signaled in SDP -- the priority, foundation, type,
and related transport address -- SHOULD remain the same. The IP
address, port, and transport protocol, which fundamentally identify
that candidate, MUST remain the same (if they change, it would be a
new candidate). The component ID MUST remain the same. The agent
MAY include additional candidates it did not offer previously (see
section 4.2.4.1.1), but which it has gathered since the last offer/
answer exchange, including peer reflexive candidates.
The agent MAY change the default destination for media. As with
initial offers, there MUST be a set of candidate attributes in the
offer matching this default destination.
4.2.1.2.2. Existing Media Streams with ICE Completed
If an agent generates an updated offer including a media stream that
was previously established, and for which ICE checks are in the
Completed state, the agent follows the procedures defined here.
The default destination for media (i.e., the values of the IP
addresses and ports in the "m=" and "c=" lines used for that media
stream) MUST be the local candidate from the highest-priority
nominated pair in the valid list for each component.
The agent MUST include candidate attributes for candidates matching
the default destination for each component of the media stream, and
MUST NOT include any other candidates.
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In addition, if the agent is controlling, it MUST include the
a=remote-candidates attribute for each media stream whose check list
is in the Completed state. The attribute contains the remote
candidates from the highest-priority nominated pair in the valid list
for each component of that media stream. It is needed to avoid a
race condition whereby the controlling agent chooses its pairs, but
the updated offer beats the connectivity checks to the controlled
agent, which doesn't even know these pairs are valid, let alone
selected. See Appendix B for elaboration on this race condition.
4.2.1.3. Procedures for Lite Implementations
4.2.1.3.1. Existing Media Streams with ICE Running
This section describes procedures for lite implementations for
existing streams for which ICE is running.
A lite implementation MUST include all of its candidates for each
component of each media stream in an a=candidate attribute in any
subsequent offer. These candidates are formed identically to the
procedures for initial offers, as described in section 4.2 of
[ICE-BIS].
A lite implementation MUST NOT add additional host candidates in a
subsequent offer. If an agent needs to offer additional candidates,
it MUST restart ICE.
The username fragments, password, and implementation level MUST
remain the same as used previously. If an agent needs to change one
of these, it MUST restart ICE for that media stream.
4.2.1.3.2. Existing Media Streams with ICE Completed
If ICE has completed for a media stream, the default destination for
that media stream MUST be set to the remote candidate of the
candidate pair for that component in the valid list. For a lite
implementation, there is always just a single candidate pair in the
valid list for each component of a media stream. Additionally, the
agent MUST include a candidate attribute for each default
destination.
Additionally, if the agent is controlling (which only happens when
both agents are lite), the agent MUST include the a=remote-candidates
attribute for each media stream. The attribute contains the remote
candidates from the candidate pairs in the valid list (one pair for
each component of each media stream).
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4.2.2. Receiving the Offer and Generating an Answer
4.2.2.1. Procedures for All Implementations
When receiving a subsequent offer within an existing session, an
agent MUST reapply the verification procedures in Section 4.1.2.3
without regard to the results of verification from any previous
offer/answer exchanges. Indeed, it is possible that a previous
offer/answer exchange resulted in ICE not being used, but it is used
as a consequence of a subsequent exchange.
4.2.2.1.1. Detecting ICE Restart
If the offer contained a change in the a=ice-ufrag or a=ice-pwd
attributes compared to the previous SDP from the peer, it indicates
that ICE is restarting for this media stream. If all media streams
are restarting, then ICE is restarting overall.
If ICE is restarting for a media stream:
o The agent MUST change the a=ice-ufrag and a=ice-pwd attributes in
the answer.
o The agent MAY change its implementation level in the answer.
An agent sets the rest of the fields in the SDP for this media stream
as it would in an initial answer to this media stream (see
Section 4.1.1.2). Consequently, the set of candidates MAY include
some, none, or all of the previous candidates for that stream and MAY
include a totally new set of candidates.
4.2.2.1.2. New Media Stream
If the offer contains a new media stream, the agent sets the fields
in the answer as if it had received an initial offer containing that
media stream (see Section 4.1.1.2). This will cause ICE processing
to begin for this media stream.
4.2.2.1.3. Removed Media Stream
If an offer contains a media stream whose port is zero, the agent
MUST NOT include any candidate attributes for that media stream in
its answer and SHOULD NOT include any other ICE-related attributes
defined in Section 5 for that media stream.
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4.2.2.2. Procedures for Full Implementations
Unless the agent has detected an ICE restart from the offer, the
username fragments, password, and implementation level MUST remain
the same as used previously. If an agent needs to change one of
these it MUST restart ICE for that media stream by generating an
offer; ICE cannot be restarted in an answer.
Additional behaviors depend on the state of ICE processing for that
media stream.
4.2.2.2.1. Existing Media Streams with ICE Running and no remote-
candidates
If ICE is running for a media stream, and the offer for that media
stream lacked the remote-candidates attribute, the rules for
construction of the answer are identical to those for the offerer as
described in Section 4.2.1.2.1.
4.2.2.2.2. Existing Media Streams with ICE Completed and no remote-
candidates
If ICE is Completed for a media stream, and the offer for that media
stream lacked the remote-candidates attribute, the rules for
construction of the answer are identical to those for the offerer as
described in Section 4.2.1.2.2, except that the answerer MUST NOT
include the a=remote-candidates attribute in the answer.
4.2.2.2.3. Existing Media Streams and remote-candidates
A controlled agent will receive an offer with the a=remote-candidates
attribute for a media stream when its peer has concluded ICE
processing for that media stream. This attribute is present in the
offer to deal with a race condition between the receipt of the offer,
and the receipt of the Binding Response that tells the answerer the
candidate that will be selected by ICE. See Appendix B for an
explanation of this race condition. Consequently, processing of an
offer with this attribute depends on the winner of the race.
The agent forms a candidate pair for each component of the media
stream by:
o Setting the remote candidate equal to the offerer's default
destination for that component (e.g., the contents of the "m=" and
"c=" lines for RTP, and the a=rtcp attribute for RTCP)
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o Setting the local candidate equal to the transport address for
that same component in the a=remote-candidates attribute in the
offer.
The agent then sees if each of these candidate pairs is present in
the valid list. If a particular pair is not in the valid list, the
check has "lost" the race. Call such a pair a "losing pair".
The agent finds all the pairs in the check list whose remote
candidates equal the remote candidate in the losing pair:
o If none of the pairs are In-Progress, and at least one is Failed,
it is most likely that a network failure, such as a network
partition or serious packet loss, has occurred. The agent SHOULD
generate an answer for this media stream as if the remote-
candidates attribute had not been present, and then restart ICE
for this stream.
o If at least one of the pairs is In-Progress, the agent SHOULD wait
for those checks to complete, and as each completes, redo the
processing in this section until there are no losing pairs.
Once there are no losing pairs, the agent can generate the answer.
It MUST set the default destination for media to the candidates in
the remote-candidates attribute from the offer (each of which will
now be the local candidate of a candidate pair in the valid list).
It MUST include a candidate attribute in the answer for each
candidate in the remote-candidates attribute in the offer.
4.2.2.3. Procedures for Lite Implementations
If the received offer contains the remote-candidates attribute for a
media stream, the agent forms a candidate pair for each component of
the media stream by:
o Setting the remote candidate equal to the offerer's default
destination for that component (e.g., the contents of the "m=" and
"c=" lines for RTP, and the a=rtcp attribute for RTCP).
o Setting the local candidate equal to the transport address for
that same component in the a=remote-candidates attribute in the
offer.
It then places those candidates into the Valid list for the media
stream. The state of ICE processing for that media stream is set to
Completed.
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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 going to send an updated offer since, based on the
rules in section 7.2 of [ICE-BIS], it was controlling, it no longer
needs to.
Besides the potential role change, change in the Valid list, and
state changes, the construction of the answer is performed
identically to the construction of an offer as described in
Section 4.2.1.3.
4.2.3. Receiving the Answer for a Subsequent Offer
Some deployments of ICE include e.g. SDP-Modifying Signaling-only
Back-to-Back User Agents (B2BUAs) [RFC7092] that modify the SDP body
during the subsequent offer/answer exchange. With the B2BUA being
ICE-unaware, a subsequent answer might be manipulated and might not
include ICE candidates although the initial answer did.
An example of a situation where such an "unexpected" answer might be
experienced appears when such a B2BUA introduces a media server
during call hold using 3rd party call-control procedures. Omitting
further details how this is done this could result in an answer being
received at the holding UA that was constructed by the B2BUA. With
the B2BUA being ICE-unaware, that answer would not include ICE
candidates.
Receiving an answer without ICE attributes in this situation might be
unexpected, but would not necessarily impair the user experience.
In addition to procedures for the expected answer, the following
section advices on how to recover from the unexpected situation.
4.2.3.1. Procedures for All Implementations
When receiving an answer within an existing session for a subsequent
offer as specified in Section 4.2.1.2.2, an agent MUST verify ICE
support as specified in Section 4.1.3.1.
If ICE support is indicated in the SDP answer and the offer was a
restart, the agent MUST perform ICE restart procedures as specified
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in Section 4.2.4. If ICE support is no longer indicated in the SDP
answer, the agent MUST fall-back to [RFC3264] procedures and SHOULD
NOT drop the dialog just because of missing ICE support. If the
agent sends a new offer later on, it SHOULD perform an ICE restart as
specified in Section 4.2.1.1.1.
If ICE support is indicated in the SDP answer and ICE is running, the
agent MUST continue ICE procedures as specified in Section 4.2.4.1.4.
If ICE support is no longer indicated in the SDP answer, the agent
MUST abort the ongoing ICE processing and fall-back to [RFC3264]
procedures. The agent SHOULD NOT drop the dialog just because of
missing ICE support. If the agent sends a new offer later on, it
SHOULD perform an ICE restart as specified in Section 4.2.1.1.1.
If ICE support is indicated in the SDP answer and if ICE is completed
and the answer conforms to Section 4.2.2.2.3, the agent MUST remain
in the ICE Completed state. If ICE support is no longer indicated in
the SDP answer, the agent MUST fall-back to [RFC3264] procedures and
SHOULD NOT drop the dialog just because of this unexpected answer.
Once the agent sends a new offer later on it MUST perform an ICE
restart.
4.2.4. Updating the Check and Valid Lists
4.2.4.1. Procedures for Full Implementations
4.2.4.1.1. ICE Restarts
The agent MUST remember the highest-priority nominated pairs in the
Valid list for each component of the media stream, called the
previous selected pairs, prior to the restart. The agent will
continue to send media using these pairs, as described in
Section 7.1. Once these destinations are noted, the agent MUST flush
the valid and check lists, and then recompute the check list and its
states as described in section 5.1.2 of [ICE-BIS].
4.2.4.1.2. New Media Stream
If the offer/answer exchange added a new media stream, the agent MUST
create a new check list for it (and an empty Valid list to start of
course), as described in section 5.1.2 of [ICE-BIS].
4.2.4.1.3. Removed Media Stream
If the offer/answer exchange removed a media stream, or an answer
rejected an offered media stream, an agent MUST flush the Valid list
for that media stream. It MUST terminate any STUN transactions in
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progress for that media stream. An agent MUST remove the check list
for that media stream and cancel any pending ordinary checks for it.
4.2.4.1.4. ICE Continuing for Existing Media Stream
The valid list is not affected by an updated offer/answer exchange
unless ICE is restarting.
If an agent is in the Running state for that media stream, the check
list is updated (the check list is irrelevant if the state is
completed). To do that, the agent recomputes the check list using
the procedures described in section 5.1.2 of [ICE-BIS]. If a pair on
the new check list was also on the previous check list, and its state
was Waiting, In-Progress, Succeeded, or Failed, its state is copied
over. Otherwise, its state is set to Frozen.
If none of the check lists are active (meaning that the pairs in each
check list are Frozen), the full-mode agent follows steps in
Section 5.1.2.6 of [ICE-BIS] to place appropriate candidates in the
Waiting state to further continue ICE processing.
4.2.4.2. Procedures for Lite Implementations
If ICE is restarting for a media stream, the agent MUST start a new
Valid list for that media stream. It MUST remember the pairs in the
previous Valid list for each component of the media stream, called
the previous selected pairs, and continue to send media there as
described in Section 7.1. The state of ICE processing for each media
stream MUST change to Running, and the state of ICE processing MUST
change to Running.
5. Grammar
This specification defines eight new SDP attributes -- the
"candidate", "remote-candidates", "ice-lite", "ice-mismatch", "ice-
ufrag", "ice-pwd", "ice-pacing", and "ice-options" attributes. This
section also provides non-normative examples of the attributes
defined.
The syntax for the attributes follow Augmented BNF as defined in
[RFC5234].
5.1. "candidate" Attribute
The candidate attribute is a media-level attribute only. It contains
a transport address for a candidate that can be used for connectivity
checks.
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candidate-attribute = "candidate" ":" foundation SP component-id SP
transport SP
priority SP
connection-address SP ;from RFC 4566
port ;port from RFC 4566
SP cand-type
[SP rel-addr]
[SP rel-port]
*(SP extension-att-name SP
extension-att-value)
foundation = 1*32ice-char
component-id = 1*5DIGIT
transport = "UDP" / transport-extension
transport-extension = token ; from RFC 3261
priority = 1*10DIGIT
cand-type = "typ" SP candidate-types
candidate-types = "host" / "srflx" / "prflx" / "relay" / token
rel-addr = "raddr" SP connection-address
rel-port = "rport" SP port
extension-att-name = token
extension-att-value = *VCHAR
ice-char = ALPHA / DIGIT / "+" / "/"
This grammar encodes the primary information about a candidate: its
IP address, port and transport protocol, and its properties: the
foundation, component ID, priority, type, and related transport
address:
<connection-address>: is taken from RFC 4566 [RFC4566]. It is the
IP address of the candidate. When parsing this field, an agent
can differentiate an IPv4 address and an IPv6 address by presence
of a colon in its value -- the presence of a colon indicates IPv6.
An agent MUST ignore candidate lines that include candidates with
IP address versions that are not supported or recognized. An IP
address SHOULD be used, but an FQDN MAY be used in place of an IP
address. In that case, when receiving an offer or answer
containing an FQDN in an a=candidate attribute, the FQDN is looked
up in the DNS first using an AAAA record (assuming the agent
supports IPv6), and if no result is found or the agent only
supports IPv4, using an A record. The rules from section 6 of
[RFC6724] is followed by fixing the source address to be one from
the candidate pair to be matched against destination addresses
reported by FQDN, in cases where the DNS query returns more than
one IP address.
<port>: is also taken from RFC 4566 [RFC4566]. It is the port of
the candidate.
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<transport>: indicates the transport protocol for the candidate.
This specification only defines UDP. However, extensibility is
provided to allow for future transport protocols to be used with
ICE, such as the Datagram Congestion Control Protocol (DCCP)
[RFC4340].
<foundation>: is composed of 1 to 32 <ice-char>s. It is an
identifier that is equivalent for two candidates that are of the
same type, share the same base, and come from the same STUN
server. The foundation is used to optimize ICE performance in the
Frozen algorithm as described in section 5.1.2 of [ICE-BIS]
<component-id>: is a positive integer between 1 and 256 that
identifies the specific component of the media stream for which
this is a candidate. It MUST start at 1 and MUST increment by 1
for each component of a particular candidate. For media streams
based on RTP, candidates for the actual RTP media MUST have a
component ID of 1, and candidates for RTCP MUST have a component
ID of 2. See section 12 in [ICE-BIS] for additional discussion on
extending ICE to new media streams.
<priority>: is a positive integer between 1 and (2**31 - 1). The
procedures for computing candidate's priority is described in
section 4.1.2 of [ICE-BIS].
<cand-type>: encodes the type of candidate. This specification
defines the values "host", "srflx", "prflx", and "relay" for host,
server reflexive, peer reflexive, and relayed candidates,
respectively. The set of candidate types is extensible for the
future.
<rel-addr> and <rel-port>: convey transport addresses related to the
candidate, useful for diagnostics and other purposes. <rel-addr>
and <rel-port> MUST be present for server reflexive, peer
reflexive, and relayed candidates. If a candidate is server or
peer reflexive, <rel-addr> and <rel-port> are equal to the base
for that server or peer reflexive candidate. If the candidate is
relayed, <rel-addr> and <rel-port> are equal to the mapped address
in the Allocate response that provided the client with that
relayed candidate (see section Appendix B.3 of [ICE-BIS] for a
discussion of its purpose). If the candidate is a host candidate,
<rel-addr> and <rel-port> MUST be omitted.
In some cases, e.g., for privacy reasons, an agent may not want to
reveal the related address and port. In this case the address
MUST be set to "0.0.0.0" (for IPv4 candidates) or "::" (for IPv6
candidates) and the port to zero.
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The candidate attribute can itself be extended. The grammar allows
for new name/value pairs to be added at the end of the attribute. An
implementation MUST ignore any name/value pairs it doesn't
understand.
Example: SDP line for UDP server reflexive candidate attribute for the RTP component
a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ
srflx raddr 10.0.1.1 rport 8998
5.2. "remote-candidates" Attribute
The syntax of the "remote-candidates" attribute is defined using
Augmented BNF as defined in [RFC5234]. The remote-candidates
attribute is a media-level attribute only.
remote-candidate-att = "remote-candidates:" remote-candidate
0*(SP remote-candidate)
remote-candidate = component-ID SP connection-address SP port
The attribute contains a connection-address and port for each
component. The ordering of components is irrelevant. However, a
value MUST be present for each component of a media stream. This
attribute MUST be included in an offer by a controlling agent for a
media stream that is Completed, and MUST NOT be included in any other
case.
Example: Remote candidates SDP lines for the RTP and RTCP components:
a=remote-candidates:1 192.0.2.3 45664
a=remote-candidates:2 192.0.2.3 45665
5.3. "ice-lite" and "ice-mismatch" Attributes
The syntax of the "ice-lite" and "ice-mismatch" attributes, both of
which are flags, is:
ice-lite = "ice-lite"
ice-mismatch = "ice-mismatch"
"ice-lite" is a session-level attribute only, and indicates that an
agent is a lite implementation. "ice-mismatch" is a media-level
attribute only, and when present in an answer, indicates that the
offer arrived with a default destination for a media component that
didn't have a corresponding candidate attribute.
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5.4. "ice-ufrag" and "ice-pwd" Attributes
The "ice-ufrag" and "ice-pwd" attributes convey the username fragment
and password used by ICE for message integrity. Their syntax is:
ice-pwd-att = "ice-pwd:" password
ice-ufrag-att = "ice-ufrag:" ufrag
password = 22*256ice-char
ufrag = 4*256ice-char
The "ice-pwd" and "ice-ufrag" attributes can appear at either the
session-level or media-level. When present in both, the value in the
media-level takes precedence. Thus, the value at the session-level
is effectively a default that applies to all media streams, unless
overridden by a media-level value. Whether present at the session or
media-level, there MUST be an ice-pwd and ice-ufrag attribute for
each media stream. If two media streams have identical ice-ufrag's,
they MUST have identical ice-pwd's.
The ice-ufrag and ice-pwd attributes MUST be chosen randomly at the
beginning of a session. The ice-ufrag attribute MUST contain at
least 24 bits of randomness, and the ice-pwd attribute MUST contain
at least 128 bits of randomness. This means that the ice-ufrag
attribute will be at least 4 characters long, and the ice-pwd at
least 22 characters long, since the grammar for these attributes
allows for 6 bits of information per character. The attributes MAY
be longer than 4 and 22 characters, respectively, of course, up to
256 characters. The upper limit allows for buffer sizing in
implementations. Its large upper limit allows for increased amounts
of randomness to be added over time. For compatibility with the 512
character limitation for the STUN username attribute value and for
bandwidth conservation considerations, the ice-ufrag attribute MUST
NOT be longer than 32 characters when sending, but an implementation
MUST accept up to 256 characters when receiving.
Example shows sample ice-ufrag and ice-pwd SDP lines:
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
5.5. "ice-pacing" Attribute
The "ice-pacing" attribute indicates the desired connectivity check
pacing, in milliseconds, for this agent (see section 13 of
[ICE-BIS]). The syntax is:
ice-pacing-att = "ice-pacing:" pacing-value
pacing-value = 1*10DIGIT
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Example shows ice-pacing value of 5 ms:
a=ice-pacing:5
5.6. "ice-options" Attribute
The "ice-options" attribute is a session- and media-level attribute.
It contains a series of tokens that identify the options supported by
the agent. Its grammar is:
ice-options = "ice-options:" ice-option-tag
0*(SP ice-option-tag)
ice-option-tag = 1*ice-char
The existence of an ice-option in an offer indicates that a certain
extension is supported by the agent and is willing to use it, if the
peer agent also includes the same extension in the answer. There
might be further extension specific negotiations needed between the
agents that determine how the extensions gets used in a given
session. The details of the negotiation procedures, if present, MUST
be defined by the specification defining the extension.
Example shows 'rtp+ecn' ice-option SDP line from <<RFC6679>>:
a=ice-options:rtp+ecn
6. Keepalives
All the ICE agents MUST follow the procedures defined in section 10
of [ICE-BIS] for sending keepalives. The keepalives MUST be sent
regardless of whether the media stream is currently inactive,
sendonly, recvonly, or sendrecv, and regardless of the presence or
value of the bandwidth attribute. An agent can determine that its
peer supports ICE by the presence of a=candidate attributes for each
media session.
7. Media Handling
7.1. Sending Media
The selected pair for a component of a media stream might not equal
the default pair for that same component from the most recent offer/
answer exchange. When this happens, the selected pair is used for
media, not the default pair. When ICE first completes, if the
selected pairs aren't a match for the default pairs, the controlling
agent sends an updated offer/answer exchange to remedy this
disparity. However, until that updated offer arrives, there will not
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be a match. Furthermore, in very unusual cases, the default
candidates in the updated offer/answer will not be a match.
7.1.1. Procedures for All Implementations
Section 11.1.3 of [ICE-BIS] defines procedures for sending media
common across Full and Lite implementations.
7.2. Receiving Media
See section 11.2 of [ICE-BIS] for procedures on receiving media.
8. SIP Considerations
Note that ICE is not intended for NAT traversal for SIP, which is
assumed to be provided via another mechanism [RFC5626].
When ICE is used with SIP, forking may result in a single offer
generating a multiplicity of answers. In that case, ICE proceeds
completely in parallel and independently for each answer, treating
the combination of its offer and each answer as an independent offer/
answer exchange, with its own set of local candidates, pairs, check
lists, states, and so on.
Once ICE processing has reached the Completed state for all peers for
media streams using those candidates, the agent SHOULD wait an
additional three seconds, and then it MAY cease responding to checks
or generating triggered checks on that candidate. It MAY free the
candidate at that time. Freeing of server reflexive candidates is
never explicit; it happens by lack of a keepalive. The three-second
delay handles cases when aggressive nomination is used, and the
selected pairs can quickly change after ICE has completed.
8.1. Latency Guidelines
ICE requires a series of STUN-based connectivity checks to take place
between endpoints. These checks start from the answerer on
generation of its answer, and start from the offerer when it receives
the answer. These checks can take time to complete, and as such, the
selection of messages to use with offers and answers can affect
perceived user latency. Two latency figures are of particular
interest. These are the post-pickup delay and the post-dial delay.
The post-pickup delay refers to the time between when a user "answers
the phone" and when any speech they utter can be delivered to the
caller. The post-dial delay refers to the time between when a user
enters the destination address for the user and ringback begins as a
consequence of having successfully started alerting the called user
agent.
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Two cases can be considered -- one where the offer is present in the
initial INVITE and one where it is in a response.
8.1.1. Offer in INVITE
To reduce post-dial delays, it is RECOMMENDED that the caller begin
gathering candidates prior to actually sending its initial INVITE.
This can be started upon user interface cues that a call is pending,
such as activity on a keypad or the phone going off-hook.
On the receipt of the offer, the answerer SHOULD generate an answer
in a provisional response once it has compelted candidate gathering.
ICE requires that a provisional response with an SDP be transmitted
reliably. This can be done through the existing Provisional Response
Acknowledgment (PRACK) mechanism [RFC3262] or through an ICE specific
optimization, wherein, the agent retransmits the provisional response
with the exponential backoff timers described in [RFC3262]. Such
retransmissions MUST cease on receipt of a STUN Binding request for
one of the media streams signaled in that SDP or on transmission of
the answer in a 2xx response. If no Binding request is received
prior to the last retransmit, the agent does not consider the session
terminated. For the ICE lite peers, the agent MUST cease
retransmitting the 18x after sending it four times (ICE will actually
work even if the peer never receives the 18x; however, experience has
shown that sending it is important for middleboxes and firewall
traversal).
It should be noted that the ICE specific optimization is very
specific to provisional response carrying answers that start ICE
processing and it is not a general technique for 1xx reliability.
Also such an optimization SHOULD NOT be used if both agents support
PRACK.
Despite the fact that the provisional response will be delivered
reliably, the rules for when an agent can send an updated offer or
answer do not change from those specified in [RFC3262].
Specifically, if the INVITE contained an offer, the same answer
appears in all of the 1xx and in the 2xx response to the INVITE.
Only after that 2xx has been sent can an updated offer/answer
exchange occur.
Alternatively, an agent MAY delay sending an answer until the 200 OK;
however, this results in a poor user experience and is NOT
RECOMMENDED.
Once the answer has been sent, the agent SHOULD begin its
connectivity checks. Once candidate pairs for each component of a
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media stream enter the valid list, the answerer can begin sending
media on that media stream.
However, prior to this point, any media that needs to be sent towards
the caller (such as SIP early media [RFC3960]) MUST NOT be
transmitted. For this reason, implementations SHOULD delay alerting
the called party until candidates for each component of each media
stream have entered the valid list. In the case of a PSTN gateway,
this would mean that the setup message into the PSTN is delayed until
this point. Doing this increases the post-dial delay, but has the
effect of eliminating 'ghost rings'. Ghost rings are cases where the
called party hears the phone ring, picks up, but hears nothing and
cannot be heard. This technique works without requiring support for,
or usage of, preconditions [RFC3312]. It also has the benefit of
guaranteeing that not a single packet of media will get clipped, so
that post-pickup delay is zero. If an agent chooses to delay local
alerting in this way, it SHOULD generate a 180 response once alerting
begins.
8.1.2. Offer in Response
In addition to uses where the offer is in an INVITE, and the answer
is in the provisional and/or 200 OK response, ICE works with cases
where the offer appears in the response. In such cases, which are
common in third party call control [RFC3725], ICE agents SHOULD
generate their offers in a reliable provisional response (which MUST
utilize [RFC3262]), and not alert the user on receipt of the INVITE.
The answer will arrive in a PRACK. This allows for ICE processing to
take place prior to alerting, so that there is no post-pickup delay,
at the expense of increased call setup delays. Once ICE completes,
the callee can alert the user and then generate a 200 OK when they
answer. The 200 OK would contain no SDP, since the offer/answer
exchange has completed.
Alternatively, agents MAY place the offer in a 2xx instead (in which
case the answer comes in the ACK). When this happens, the callee
will alert the user on receipt of the INVITE, and the ICE exchanges
will take place only after the user answers. This has the effect of
reducing call setup delay, but can cause substantial post-pickup
delays and media clipping.
8.2. SIP Option Tags and Media Feature Tags
[RFC5768] specifies a SIP option tag and media feature tag for usage
with ICE. ICE implementations using SIP SHOULD support this
specification, which uses a feature tag in registrations to
facilitate interoperability through signaling intermediaries.
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8.3. Interactions with Forking
ICE interacts very well with forking. Indeed, ICE fixes some of the
problems associated with forking. Without ICE, when a call forks and
the caller receives multiple incoming media streams, it cannot
determine which media stream corresponds to which callee.
With ICE, this problem is resolved. The connectivity checks which
occur prior to transmission of media carry username fragments, which
in turn are correlated to a specific callee. Subsequent media
packets that arrive on the same candidate pair as the connectivity
check will be associated with that same callee. Thus, the caller can
perform this correlation as long as it has received an answer.
8.4. Interactions with Preconditions
Quality of Service (QoS) preconditions, which are defined in
[RFC3312] and [RFC4032], apply only to the transport addresses listed
as the default targets for media in an offer/answer. If ICE changes
the transport address where media is received, this change is
reflected in an updated offer that changes the default destination
for media to match ICE's selection. As such, it appears like any
other re-INVITE would, and is fully treated in RFCs 3312 and 4032,
which apply without regard to the fact that the destination for media
is changing due to ICE negotiations occurring "in the background".
Indeed, an agent SHOULD NOT indicate that QoS preconditions have been
met until the checks have completed and selected the candidate pairs
to be used for media.
ICE also has (purposeful) interactions with connectivity
preconditions [RFC5898]. Those interactions are described there.
Note that the procedures described in Section 8.1 describe their own
type of "preconditions", albeit with less functionality than those
provided by the explicit preconditions in [RFC5898].
8.5. Interactions with Third Party Call Control
ICE works with Flows I, III, and IV as described in [RFC3725]. Flow
I works without the controller supporting or being aware of ICE.
Flow IV will work as long as the controller passes along the ICE
attributes without alteration. Flow II is fundamentally incompatible
with ICE; each agent will believe itself to be the answerer and thus
never generate a re-INVITE.
The flows for continued operation, as described in Section 7 of
[RFC3725], require additional behavior of ICE implementations to
support. In particular, if an agent receives a mid-dialog re-INVITE
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that contains no offer, it MUST restart ICE for each media stream and
go through the process of gathering new candidates. Furthermore,
that list of candidates SHOULD include the ones currently being used
for media.
9. Relationship with ANAT
[RFC4091], the Alternative Network Address Types (ANAT) Semantics for
the SDP grouping framework, and [RFC4092], its usage with SIP, define
a mechanism for indicating that an agent can support both IPv4 and
IPv6 for a media stream, and it does so by including two "m=" lines,
one for v4 and one for v6. This is similar to ICE, which allows for
an agent to indicate multiple transport addresses using the candidate
attribute. However, ANAT relies on static selection to pick between
choices, rather than a dynamic connectivity check used by ICE.
It is RECOMMENDED that ICE be used in realizing the dual-stack use-
cases in agents that support ICE.
10. Setting Ta and RTO for RTP Media Streams
During the gathering phase of ICE (section 4.1.1 [ICE-BIS]) and while
ICE is performing connectivity checks (section 6 [ICE-BIS]), an agent
sends STUN and TURN transactions. These transactions are paced at a
rate of one every Ta milliseconds, and utilize a specific RTO. See
Section 13 of [ICE-BIS] for details on how the values of Ta and RTO
are computed with a real-time media stream of known maximum bandwidth
to rate-control the ICE exchanges.
11. Security Considerations
11.1. Attacks on the Offer/Answer Exchanges
An attacker that can modify or disrupt the offer/answer exchanges
themselves can readily launch a variety of attacks with ICE. They
could direct media to a target of a DoS attack, they could insert
themselves into the media stream, and so on. These are similar to
the general security considerations for offer/answer exchanges, and
the security considerations in [RFC3264] apply. These require
techniques for message integrity and encryption for offers and
answers, which are satisfied by the TLS mechanism [RFC3261] when SIP
is used. As such, the usage of TLS with ICE is RECOMMENDED.
11.2. Insider Attacks
In addition to attacks where the attacker is a third party trying to
insert fake offers, answers, or STUN messages, there are several
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attacks possible with ICE when the attacker is an authenticated and
valid participant in the ICE exchange.
11.2.1. The Voice Hammer Attack
The voice hammer attack is an amplification attack. In this attack,
the attacker initiates sessions to other agents, and maliciously
includes the IP address and port of a DoS target as the destination
for media traffic signaled in the SDP. This causes substantial
amplification; a single offer/answer exchange can create a continuing
flood of media packets, possibly at high rates (consider video
sources). This attack is not specific to ICE, but ICE can help
provide remediation.
Specifically, if ICE is used, the agent receiving the malicious SDP
will first perform connectivity checks to the target of media before
sending media there. If this target is a third-party host, the
checks will not succeed, and media is never sent.
Unfortunately, ICE doesn't help if it's not used, in which case an
attacker could simply send the offer without the ICE parameters.
However, in environments where the set of clients is known, and is
limited to ones that support ICE, the server can reject any offers or
answers that don't indicate ICE support.
User Agents that are not willing to receive non-ICE answers MUST
include an "ice" Option Tag in the Require Header Field in their
offer. Clients that rejects non-ICE offers SHOULD use a 421 response
code, together with an Option Tag "ice" in the Require Header Field
in the response.
11.2.2. Interactions with Application Layer Gateways and SIP
Application Layer Gateways (ALGs) are functions present in a Network
Address Translation (NAT) device that inspect the contents of packets
and modify them, in order to facilitate NAT traversal for application
protocols. Session Border Controllers (SBCs) are close cousins of
ALGs, but are less transparent since they actually exist as
application-layer SIP intermediaries. ICE has interactions with SBCs
and ALGs.
If an ALG is SIP aware but not ICE aware, ICE will work through it as
long as the ALG correctly modifies the SDP. A correct ALG
implementation behaves as follows:
o The ALG does not modify the "m=" and "c=" lines or the rtcp
attribute if they contain external addresses.
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o If the "m=" and "c=" lines contain internal addresses, the
modification depends on the state of the ALG:
* If the ALG already has a binding established that maps an
external port to an internal IP address and port matching the
values in the "m=" and "c=" lines or rtcp attribute, the ALG
uses that binding instead of creating a new one.
* If the ALG does not already have a binding, it creates a new
one and modifies the SDP, rewriting the "m=" and "c=" lines and
rtcp attribute.
Unfortunately, many ALGs are known to work poorly in these corner
cases. ICE does not try to work around broken ALGs, as this is
outside the scope of its functionality. ICE can help diagnose these
conditions, which often show up as a mismatch between the set of
candidates and the "m=" and "c=" lines and rtcp attributes. The ice-
mismatch attribute is used for this purpose.
ICE works best through ALGs when the signaling is run over TLS. This
prevents the ALG from manipulating the SDP messages and interfering
with ICE operation. Implementations that are expected to be deployed
behind ALGs SHOULD provide for TLS transport of the SDP.
If an SBC is SIP aware but not ICE aware, the result depends on the
behavior of the SBC. If it is acting as a proper Back-to-Back User
Agent (B2BUA), the SBC will remove any SDP attributes it doesn't
understand, including the ICE attributes. Consequently, the call
will appear to both endpoints as if the other side doesn't support
ICE. This will result in ICE being disabled, and media flowing
through the SBC, if the SBC has requested it. If, however, the SBC
passes the ICE attributes without modification, yet modifies the
default destination for media (contained in the "m=" and "c=" lines
and rtcp attribute), this will be detected as an ICE mismatch, and
ICE processing is aborted for the call. It is outside of the scope
of ICE for it to act as a tool for "working around" SBCs. If one is
present, ICE will not be used and the SBC techniques take precedence.
12. IANA Considerations
12.1. SDP Attributes
The original ICE specification defined seven new SDP attributes per
the procedures of Section 8.2.4 of [RFC4566]. The registration
information is reproduced here.
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12.1.1. candidate Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: candidate
Long Form: candidate
Type of Attribute: media-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides one of many possible candidate
addresses for communication. These addresses are validated with
an end-to-end connectivity check using Session Traversal Utilities
for NAT (STUN).
Appropriate Values: See Section 5 of RFC XXXX.
12.1.2. remote-candidates Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: remote-candidates
Long Form: remote-candidates
Type of Attribute: media-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the identity of the remote
candidates that the offerer wishes the answerer to use in its
answer.
Appropriate Values: See Section 5 of RFC XXXX.
12.1.3. ice-lite Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-lite
Long Form: ice-lite
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Type of Attribute: session-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates that an agent has the minimum
functionality required to support ICE inter-operation with a peer
that has a full implementation.
Appropriate Values: See Section 5 of RFC XXXX.
12.1.4. ice-mismatch Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-mismatch
Long Form: ice-mismatch
Type of Attribute: session-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates that an agent is ICE capable,
but did not proceed with ICE due to a mismatch of candidates with
the default destination for media signaled in the SDP.
Appropriate Values: See Section 5 of RFC XXXX.
12.1.5. ice-pwd Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-pwd
Long Form: ice-pwd
Type of Attribute: session- or media-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the password used to protect
STUN connectivity checks.
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Appropriate Values: See Section 5 of RFC XXXX.
12.1.6. ice-ufrag Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-ufrag
Long Form: ice-ufrag
Type of Attribute: session- or media-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the fragments used to construct
the username in STUN connectivity checks.
Appropriate Values: See Section 5 of RFC XXXX.
12.1.7. ice-pacing Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-pacing
Long Form: ice-pacing
Type of Attribute: session-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE) to indicate desired connectivity check pacing
values.
Appropriate Values: See Section 5 of RFC XXXX.
12.1.8. ice-options Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-options
Long Form: ice-options
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Type of Attribute: session- or media-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates the ICE options or extensions
used by the agent.
Appropriate Values: See Section 5 of RFC XXXX.
12.2. Interactive Connectivity Establishment (ICE) Options Registry
IANA maintains a registry for ice-options identifiers under the
Specification Required policy as defined in "Guidelines for Writing
an IANA Considerations Section in RFCs" [RFC5226].
ICE options are of unlimited length according to the syntax in
Section 5.6; however, they are RECOMMENDED to be no longer than 20
characters. This is to reduce message sizes and allow for efficient
parsing.
In [RFC5245] ICE options could only be defined at the session level.
ICE options can now also be defined at the media level. This can be
used when aggregating between different ICE agents in the same
endpoint, but future options may require to be defined at the media-
level. To ensure compatibility with legacy implementation, the
media-level ICE options MUST be aggregated into a session-level ICE
option. Because aggregation rules depend on the specifics of each
option, all new ICE options MUST also define in their specification
how the media-level ICE option values are aggregated to generate the
value of the session-level ICE option.
[RFC6679] defines the "rtp+ecn" ICE option. The aggregation rule for
this ICE option is that if all aggregated media using ICE contain a
media-level "rtp+ecn" ICE option then an "rtp+ecn" ICE option MUST be
inserted at the session-level. If one of the media does not contain
the option, then it MUST NOT be inserted at the session-level.
Section 9 of [ICE-BIS] defines "ice2" ICE option. Since "ice2" is a
session level ICE option, no aggregation rules apply.
A registration request MUST include the following information:
o The ICE option identifier to be registered
o Name, Email, and Address of a contact person for the registration
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o Organization or individuals having the change control
o Short description of the ICE extension to which the option relates
o Reference(s) to the specification defining the ICE option and the
related extensions
13. Acknowledgments
A large part of the text in this document was taken from [RFC5245],
authored by Jonathan Rosenberg.
Some of the text in this document was taken from [RFC6336], authored
by Magnus Westerlund and Colin Perkins.
Thanks to Thomas Stach for the text in Section 4.2.3, Roman Shpount
for suggesting RTCP candidate handling in Section 4.1.1.2 and Simon
Perreault for advising on IPV6 address selection when candidate-
address includes FQDN.
Thanks to following experts for their reviews and constructive
feedback: Christer Holmberg, Adam Roach and the MMUSIC WG.
14. References
14.1. Normative References
[ICE-BIS] Keranen, A. and J. Rosenberg, "Interactive Connectivity
Establishment (ICE): A Protocol for Network Address
Translator (NAT) Traversal for Offer/Answer Protocols",
draft-ietf-ice-rfc5245bis-00 (work in progress), March
2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, DOI 10.17487/RFC3262, June 2002,
<http://www.rfc-editor.org/info/rfc3262>.
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[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<http://www.rfc-editor.org/info/rfc3264>.
[RFC3312] Camarillo, G., Ed., Marshall, W., Ed., and J. Rosenberg,
"Integration of Resource Management and Session Initiation
Protocol (SIP)", RFC 3312, DOI 10.17487/RFC3312, October
2002, <http://www.rfc-editor.org/info/rfc3312>.
[RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth",
RFC 3556, DOI 10.17487/RFC3556, July 2003,
<http://www.rfc-editor.org/info/rfc3556>.
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605,
DOI 10.17487/RFC3605, October 2003,
<http://www.rfc-editor.org/info/rfc3605>.
[RFC4032] Camarillo, G. and P. Kyzivat, "Update to the Session
Initiation Protocol (SIP) Preconditions Framework",
RFC 4032, DOI 10.17487/RFC4032, March 2005,
<http://www.rfc-editor.org/info/rfc4032>.
[RFC4091] Camarillo, G. and J. Rosenberg, "The Alternative Network
Address Types (ANAT) Semantics for the Session Description
Protocol (SDP) Grouping Framework", RFC 4091, June 2005,
<http://www.rfc-editor.org/info/rfc4091>.
[RFC4092] Camarillo, G. and J. Rosenberg, "Usage of the Session
Description Protocol (SDP) Alternative Network Address
Types (ANAT) Semantics in the Session Initiation Protocol
(SIP)", RFC 4092, June 2005,
<http://www.rfc-editor.org/info/rfc4092>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
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[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
DOI 10.17487/RFC5245, April 2010,
<http://www.rfc-editor.org/info/rfc5245>.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<http://www.rfc-editor.org/info/rfc5389>.
[RFC5768] Rosenberg, J., "Indicating Support for Interactive
Connectivity Establishment (ICE) in the Session Initiation
Protocol (SIP)", RFC 5768, DOI 10.17487/RFC5768, April
2010, <http://www.rfc-editor.org/info/rfc5768>.
[RFC6336] Westerlund, M. and C. Perkins, "IANA Registry for
Interactive Connectivity Establishment (ICE) Options",
RFC 6336, April 2010,
<http://www.rfc-editor.org/info/rfc6336>.
[RFC6679] Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
and K. Carlberg, "Explicit Congestion Notification (ECN)
for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
2012, <http://www.rfc-editor.org/info/rfc6679>.
[RFC6724] Thaler, D., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, September 2012,
<http://www.rfc-editor.org/info/rfc6724>.
[RFC7092] Kaplan, H. and V. Pascual, "A Taxonomy of Session
Initiation Protocol (SIP) Back-to-Back User Agents",
RFC 7092, DOI 10.17487/RFC7092, December 2013,
<http://www.rfc-editor.org/info/rfc7092>.
[RFC7656] Lennox, J., Gross, K., Nandakumar, S., Salgueiro, G., and
B. Burman, Ed., "A Taxonomy of Semantics and Mechanisms
for Real-Time Transport Protocol (RTP) Sources", RFC 7656,
DOI 10.17487/RFC7656, November 2015,
<http://www.rfc-editor.org/info/rfc7656>.
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14.2. Informative References
[RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
Camarillo, "Best Current Practices for Third Party Call
Control (3pcc) in the Session Initiation Protocol (SIP)",
BCP 85, RFC 3725, DOI 10.17487/RFC3725, April 2004,
<http://www.rfc-editor.org/info/rfc3725>.
[RFC3960] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing
Tone Generation in the Session Initiation Protocol (SIP)",
RFC 3960, DOI 10.17487/RFC3960, December 2004,
<http://www.rfc-editor.org/info/rfc3960>.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340,
DOI 10.17487/RFC4340, March 2006,
<http://www.rfc-editor.org/info/rfc4340>.
[RFC5626] Jennings, C., Ed., Mahy, R., Ed., and F. Audet, Ed.,
"Managing Client-Initiated Connections in the Session
Initiation Protocol (SIP)", RFC 5626,
DOI 10.17487/RFC5626, October 2009,
<http://www.rfc-editor.org/info/rfc5626>.
[RFC5898] Andreasen, F., Camarillo, G., Oran, D., and D. Wing,
"Connectivity Preconditions for Session Description
Protocol (SDP) Media Streams", RFC 5898,
DOI 10.17487/RFC5898, July 2010,
<http://www.rfc-editor.org/info/rfc5898>.
Appendix A. Examples
For the example shown in section 12 of [ICE-BIS] the resulting offer
(message 5) encoded in SDP looks like:
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v=0
o=jdoe 2890844526 2890842807 IN IP6 $L-PRIV-1.IP
s=
c=IN IP6 $NAT-PUB-1.IP
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio $NAT-PUB-1.PORT RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 $L-PRIV-1.IP $L-PRIV-1.PORT typ host
a=candidate:2 1 UDP 1694498815 $NAT-PUB-1.IP $NAT-PUB-1.PORT typ
srflx raddr $L-PRIV-1.IP rport $L-PRIV-1.PORT
The offer, with the variables replaced with their values, will look
like (lines folded for clarity):
v=0
o=jdoe 2890844526 2890842807 IN IP6 fe80::6676:baff:fe9c:ee4a
s=
c=IN IP6 2001:420:c0e0:1005::61
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 45664 RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 fe80::6676:baff:fe9c:ee4a 8998 typ host
a=candidate:2 1 UDP 1694498815 2001:420:c0e0:1005::61 45664 typ srflx raddr
fe80::6676:baff:fe9c:ee4a rport 8998
The resulting answer looks like:
v=0
o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP
s=
c=IN IP4 $R-PUB-1.IP
t=0 0
a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
a=ice-ufrag:9uB6
m=audio $R-PUB-1.PORT RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 $R-PUB-1.IP $R-PUB-1.PORT typ host
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With the variables filled in:
v=0
o=bob 2808844564 2808844564 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
a=ice-ufrag:9uB6
m=audio 3478 RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 192.0.2.1 3478 typ host
Appendix B. The remote-candidates Attribute
The a=remote-candidates attribute exists to eliminate a race
condition between the updated offer and the response to the STUN
Binding request that moved a candidate into the Valid list. This
race condition is shown in Figure 1. On receipt of message 4, agent
L adds a candidate pair to the valid list. If there was only a
single media stream with a single component, agent L could now send
an updated offer. However, the check from agent R has not yet
generated a response, and agent R receives the updated offer (message
7) before getting the response (message 9). Thus, it does not yet
know that this particular pair is valid. To eliminate this
condition, the actual candidates at R that were selected by the
offerer (the remote candidates) are included in the offer itself, and
the answerer delays its answer until those pairs validate.
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Agent L Network Agent R
|(1) Offer | |
|------------------------------------------>|
|(2) Answer | |
|<------------------------------------------|
|(3) STUN Req. | |
|------------------------------------------>|
|(4) STUN Res. | |
|<------------------------------------------|
|(5) STUN Req. | |
|<------------------------------------------|
|(6) STUN Res. | |
|-------------------->| |
| |Lost |
|(7) Offer | |
|------------------------------------------>|
|(8) STUN Req. | |
|<------------------------------------------|
|(9) STUN Res. | |
|------------------------------------------>|
|(10) Answer | |
|<------------------------------------------|
Figure 1: Race Condition Flow
Appendix C. Why Is the Conflict Resolution Mechanism Needed?
When ICE runs between two peers, one agent acts as controlled, and
the other as controlling. Rules are defined as a function of
implementation type and offerer/answerer to determine who is
controlling and who is controlled. However, the specification
mentions that, in some cases, both sides might believe they are
controlling, or both sides might believe they are controlled. How
can this happen?
The condition when both agents believe they are controlled shows up
in third party call control cases. Consider the following flow:
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A Controller B
|(1) INV() | |
|<-------------| |
|(2) 200(SDP1) | |
|------------->| |
| |(3) INV() |
| |------------->|
| |(4) 200(SDP2) |
| |<-------------|
|(5) ACK(SDP2) | |
|<-------------| |
| |(6) ACK(SDP1) |
| |------------->|
Figure 2: Role Conflict Flow
This flow is a variation on flow III of RFC 3725 [RFC3725]. In fact,
it works better than flow III since it produces fewer messages. In
this flow, the controller sends an offerless INVITE to agent A, which
responds with its offer, SDP1. The agent then sends an offerless
INVITE to agent B, which it responds to with its offer, SDP2. The
controller then uses the offer from each agent to generate the
answers. When this flow is used, ICE will run between agents A and
B, but both will believe they are in the controlling role. With the
role conflict resolution procedures, this flow will function properly
when ICE is used.
At this time, there are no documented flows that can result in the
case where both agents believe they are controlled. However, the
conflict resolution procedures allow for this case, should a flow
arise that would fit into this category.
Appendix D. Why Send an Updated Offer?
Section 11.1 describes rules for sending media. Both agents can send
media once ICE checks complete, without waiting for an updated offer.
Indeed, the only purpose of the updated offer is to "correct" the SDP
so that the default destination for media matches where media is
being sent based on ICE procedures (which will be the highest-
priority nominated candidate pair).
This begs the question -- why is the updated offer/answer exchange
needed at all? Indeed, in a pure offer/answer environment, it would
not be. The offerer and answerer will agree on the candidates to use
through ICE, and then can begin using them. As far as the agents
themselves are concerned, the updated offer/answer provides no new
information. However, in practice, numerous components along the
signaling path look at the SDP information. These include entities
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performing off-path QoS reservations, NAT traversal components such
as ALGs and Session Border Controllers (SBCs), and diagnostic tools
that passively monitor the network. For these tools to continue to
function without change, the core property of SDP -- that the
existing, pre-ICE definitions of the addresses used for media -- the
"m=" and "c=" lines and the rtcp attribute -- must be retained. For
this reason, an updated offer must be sent.
Authors' Addresses
Marc Petit-Huguenin
Impedance Mismatch
Email: marc@petit-huguenin.org
Ari Keranen
Ericsson
Jorvas 02420
Finland
Email: ari.keranen@ericsson.com
Suhas Nandakumar
Cisco Systems
707 Tasman Dr
Milpitas, CA 95035
USA
Email: snandaku@cisco.com
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