MMUSIC M. Petit-Huguenin
Internet-Draft Impedance Mismatch
Obsoletes: 5245 (if approved) S. Nandakumar
Intended status: Standards Track Cisco Systems
Expires: May 13, 2019 A. Keranen
Ericsson
November 9, 2018
Session Description Protocol (SDP) Offer/Answer procedures for
Interactive Connectivity Establishment (ICE)
draft-ietf-mmusic-ice-sip-sdp-24
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. SDP Offer/Answer Procedures . . . . . . . . . . . . . . . . . 4
3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Generic Procedures . . . . . . . . . . . . . . . . . . . 4
3.2.1. Encoding . . . . . . . . . . . . . . . . . . . . . . 4
3.2.2. RTP/RTCP Considerations . . . . . . . . . . . . . . . 6
3.2.3. Determining Role . . . . . . . . . . . . . . . . . . 6
3.2.4. STUN Considerations . . . . . . . . . . . . . . . . . 6
3.2.5. Verifying ICE Support Procedures . . . . . . . . . . 6
3.2.6. SDP Example . . . . . . . . . . . . . . . . . . . . . 7
3.3. Initial Offer/Answer Exchange . . . . . . . . . . . . . . 7
3.3.1. Sending the Initial Offer . . . . . . . . . . . . . . 7
3.3.2. Sending the Initial Answer . . . . . . . . . . . . . 8
3.3.3. Receiving the Initial Answer . . . . . . . . . . . . 8
3.3.4. Concluding ICE . . . . . . . . . . . . . . . . . . . 8
3.4. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . 9
3.4.1. Sending Subsequent Offer . . . . . . . . . . . . . . 9
3.4.2. Sending Subsequent Answer . . . . . . . . . . . . . . 11
3.4.3. Receiving Answer for a Subsequent Offer . . . . . . . 13
4. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. "candidate" Attribute . . . . . . . . . . . . . . . . . . 15
4.2. "remote-candidates" Attribute . . . . . . . . . . . . . . 18
4.3. "ice-lite" and "ice-mismatch" Attributes . . . . . . . . 18
4.4. "ice-ufrag" and "ice-pwd" Attributes . . . . . . . . . . 18
4.5. "ice-pacing" Attribute . . . . . . . . . . . . . . . . . 19
4.6. "ice-options" Attribute . . . . . . . . . . . . . . . . . 20
5. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . 20
6. SIP Considerations . . . . . . . . . . . . . . . . . . . . . 21
6.1. Latency Guidelines . . . . . . . . . . . . . . . . . . . 21
6.1.1. Offer in INVITE . . . . . . . . . . . . . . . . . . . 21
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6.1.2. Offer in Response . . . . . . . . . . . . . . . . . . 23
6.2. SIP Option Tags and Media Feature Tags . . . . . . . . . 23
6.3. Interactions with Forking . . . . . . . . . . . . . . . . 23
6.4. Interactions with Preconditions . . . . . . . . . . . . . 23
6.5. Interactions with Third Party Call Control . . . . . . . 24
7. Relationship with ANAT . . . . . . . . . . . . . . . . . . . 24
8. Security Considerations . . . . . . . . . . . . . . . . . . . 25
8.1. Attacks on the Offer/Answer Exchanges . . . . . . . . . . 25
8.2. Insider Attacks . . . . . . . . . . . . . . . . . . . . . 25
8.2.1. The Voice Hammer Attack . . . . . . . . . . . . . . . 25
8.2.2. Interactions with Application Layer Gateways and SIP 26
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
9.1. SDP Attributes . . . . . . . . . . . . . . . . . . . . . 27
9.1.1. candidate Attribute . . . . . . . . . . . . . . . . . 27
9.1.2. remote-candidates Attribute . . . . . . . . . . . . . 27
9.1.3. ice-lite Attribute . . . . . . . . . . . . . . . . . 28
9.1.4. ice-mismatch Attribute . . . . . . . . . . . . . . . 28
9.1.5. ice-pwd Attribute . . . . . . . . . . . . . . . . . . 29
9.1.6. ice-ufrag Attribute . . . . . . . . . . . . . . . . . 29
9.1.7. ice-options Attribute . . . . . . . . . . . . . . . . 30
9.1.8. ice-pacing Attribute . . . . . . . . . . . . . . . . 30
9.2. Interactive Connectivity Establishment (ICE) Options
Registry . . . . . . . . . . . . . . . . . . . . . . . . 31
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 31
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 32
11.1. Normative References . . . . . . . . . . . . . . . . . . 32
11.2. Informative References . . . . . . . . . . . . . . . . . 33
11.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 35
Appendix B. The remote-candidates Attribute . . . . . . . . . . 36
Appendix C. Why Is the Conflict Resolution Mechanism Needed? . . 37
Appendix D. Why Send an Updated Offer? . . . . . . . . . . . . . 38
Appendix E. Contributors . . . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
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.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
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"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
Readers should be familiar with the terminology defined in [RFC3264],
in [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 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. SDP Offer/Answer Procedures
3.1. Introduction
[RFC8445] defines ICE candidate exchange as the process for ICE
agents (Initiator and Responder) to exchange their candidate
information required for ICE processing at the agents. For the
purposes of this specification, the candidate exchange process
corresponds to the [RFC3264] Offer/Answer protocol and the
terminologies offerer and answerer correspond to the initiator and
responder terminologies 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.
3.2. Generic Procedures
3.2.1. Encoding
Section 4 provides detailed rules for constructing various SDP
attributes defined in this specification.
3.2.1.1. Data Streams
Each data stream [RFC8445] is represented by an SDP media description
("m=" section).
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3.2.1.2. Candidates
With in a "m=" section, each candidate (including the default
candidate) associated with the data stream is represented by an SDP
candidate attribute.
Prior to nomination, the "c=" line associated with an "m=" section
contains the IP address of the default candidate, while the "m=" line
contains the port and transport of the default candidate for that
"m=" section.
After nomination, the "c=" line for a given "m=" section contains the
IP address of the nominated candidate (the local candidate of the
nominated candidate pair) and the "m=" line contains the port and
transport corresponding to the nominated candidate for that "m="
section.
3.2.1.3. Username and Password
The ICE username is represented by an SDP ice-ufrag attribute and the
ICE password is represented by an SDP ice-pwd attribute.
3.2.1.4. Lite Implementations
An ICE lite implementation [RFC8445] MUST include an SDP ice-lite
attribute. A full implementation MUST NOT include that attribute.
3.2.1.5. ICE Extensions
An agent uses the SDP ice-options attribute to indicate support of
ICE extensions.
An agent compliant to this specification MUST include an SDP ice-
options attribute with an "ice2" attribute value. If an agent
receives an SDP offer or answer with ICE attributes but without the
"ice2" ice-options attribute value, the agent assumes that the peer
is compliant to [RFC5245].
3.2.1.6. Inactive and Disabled Data Streams
If an "m=" section is marked as inactive [RFC4566], or has a
bandwidth value of zero [RFC4566], the agent MUST still include ICE
related SDP attributes.
If the port value associated with an "m=" section is set to zero
(implying a disabled stream) as defined in section 8.2 of [RFC3264],
the agent SHOULD NOT include ICE related SDP candidate attributes in
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that "m=" section, unless an SDP extension specifying otherwise is
used.
3.2.2. RTP/RTCP Considerations
If an agent utilizes both RTP and RTCP, the agent MUST include SDP
candidate attributes for both the RTP and RTCP components in the "m="
section.
If an agent uses separate ports for RTP and RTCP, the agent MUST
include an SDP rtcp attribute in the "m=" section, as described in
[RFC3605]. In the cases where the port number for the RTCP is one
higher than the RTP port and RTCP component address is same as the
address of the RTP component, the SDP rtcp attribute MAY be omitted.
If the agent does not utilize RTCP, it indicates that by including
b=RS:0 and b=RR:0 SDP attributes, as described in [RFC3556].
3.2.3. 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].
3.2.4. STUN Considerations
Once an agent has provided its local candidates to its peer in an SDP
offer or answer, the agent MUST be prepared to receive STUN
connectivity check Binding requests on those candidates.
3.2.5. Verifying ICE Support Procedures
The agents will proceed with the ICE procedures defined in [RFC8445]
and this specification if, for each data stream in the SDP it
received, the default destination for each component of that data
stream appears in a candidate attribute. For example, in the case of
RTP, the 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 agents MUST process the SDP based
on normal [RFC3264] procedures, without using any of the ICE
mechanisms described in the remainder of this specification with the
few exceptions noted below:
1. The presence of certain application layer gateways MAY modify the
transport address information as described in Section 8.2.2. The
behavior of the responding agent in such a situation is
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implementation defined. Informally, the responding agent MAY
consider the mismatched transport address information as a
plausible new candidate learnt from the peer and continue its ICE
processing with that transport address included. Alternatively,
the responding agent MAY include an "a=ice-mismatch" attribute in
its answer and MAY also omit a=candidate attributes for such data
streams.
2. The transport address from the peer for the default destination
correspond to IP address values "0.0.0.0"/"::" and port value of
"9". This MUST not be considered as a ICE failure by the peer
agent and the ICE processing MUST continue as usual.
Also to note, this specification provides no guidance on how an
controlling/initiator agent should proceed in scenarios where the the
SDP answer includes "a=ice-mismatch" from the peer.
3.2.6. SDP Example
The following is an example SDP message that includes ICE attributes
(lines folded for readability):
v=0
o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
s=
c=IN IP4 192.0.2.3
t=0 0
a=ice-options:ice2
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 45664 RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 10.0.1.1 8998 typ host
a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr
10.0.1.1 rport 8998
3.3. Initial Offer/Answer Exchange
3.3.1. Sending the Initial Offer
When an offerer generates the initial offer, in each "m=" section it
MUST include SDP candidate attributes for each available candidate
associated with the "m=" section. In addition, the offerer MUST
include an SDP ice-ufrag and an SDP ice-pwd attribute in the offer.
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Note: Within the scope of this document, "Initial Offer" refers to
the first SDP offer that is sent in order to negotiate usage of
ICE. It might, or might not, be the initial SDP offer of the SDP
session.
Note: The procedures in this document only consider "m=" sections
associated with data streams where ICE is used.
3.3.2. Sending the Initial Answer
When an answerer receives an initial offer that indicates that the
offerer supports ICE, and if the answerer accepts the offer and the
usage of ICE, in each "m=" section within the answer, it MUST include
SDP candidate attributes for each available candidate associated with
the "m=" section. In addition, the answerer MUST include an SDP ice-
ufrag and an SDP ice-pwd attribute in the answer.
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, 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 the ICE mismatch, procedures
described in (Section 3.2.5) are followed.
3.3.3. 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 offerer detects an ICE mismatch in the answer, the offerer MUST
terminate the usage of ICE. The subsequent actions taken by the
offerer are implementation dependent and are out of the scope of this
specification.
3.3.4. Concluding ICE
Once the state of each check list is Completed, and if the agent is
the controlling agent, it nominates a candidate pair [RFC8445] and
checks for each data stream whether the nominated pair matches the
default candidate pair. If there are one or more data streams with a
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match, and the peer did not indicate support for the 'ice2' ice-
option, the controlling agent MUST generate a subsequent offer
(Section 3.4.1), in which the IP address, port and transport in the
"c=" and "m=" lines associated with each data stream match the
corresponding local information of the nominated pair for that data
stream.
However, If the support for 'ice2' ice-option is in use, the
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.
Also as described in [RFC8445], once the controlling agent has
nominated a candidate pair for a data stream, the agent MUST NOT
nominate another pair for that data stream during the lifetime of the
ICE session (i.e. until ICE is restarted).
3.4. 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.
3.4.1. Sending Subsequent Offer
3.4.1.1. Procedures for All Implementations
3.4.1.1.1. ICE Restarts
An agent MAY restart ICE processing for an existing data stream
[RFC8445].
The rules governing the ICE restart imply that setting the IP address
in the "c=" line to 0.0.0.0 (for IPv4)/ :: (for IPv6) will cause an
ICE restart. Consequently, ICE implementations MUST NOT utilize this
mechanism for call hold, and instead MUST use a=inactive and
a=sendonly as described in [RFC3264].
To restart ICE, an agent MUST change both the ice-pwd and the ice-
ufrag for the data stream in an offer. However, it is permissible to
use a session-level attribute in one offer, but to provide the same
ice-pwd or ice-ufrag as a media-level attribute in a subsequent
offer. This MUST NOT be considered as ICE restart.
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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 (see
Section 3.2.1). 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.
3.4.1.1.2. Removing a Data Stream
If an agent removes a data stream by setting its port to zero, it
MUST NOT include any candidate attributes for that data stream and
SHOULD NOT include any other ICE-related attributes defined in
Section 4 for that data stream.
3.4.1.1.3. 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 was an initial offer for that
data stream (see Section 3.2.1). This will cause ICE processing to
begin for this data stream.
3.4.1.2. Procedures for Full Implementations
This section describes additional procedures for full
implementations, covering existing data streams.
3.4.1.2.1. Before Nomination
When an offerer sends a subsequent offer; in each "m=" section for
which a candidate pair has not yet been nominated, the offer MUST
include the same set of ICE-related information that the offerer
included in the previous offer or answer. The agent MAY include
additional candidates it did not offer previously, but which it has
gathered since the last offer/ answer exchange, including peer
reflexive candidates.
The agent MAY change the default destination for media. As with
initial offers, there MUST be a set of candidate attributes in the
offer matching this default destination.
3.4.1.2.2. After Nomination
Once a candidate pair has been nominated for a data stream, the IP
address, port and transport 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 representing the local candidates of the
nominated candidate pair. The offerer MUST NOT include any other SDP
candidate attributes in the subsequent offer.
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In addition, if the agent is controlling, it MUST include the
a=remote-candidates attribute for each data stream whose check list
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.
3.4.1.3. 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 attribute in any subsequent offer. The candidates are
formed identical to the procedures for initial offers.
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. Similarly, the username fragments or passwords
MUST remain the same as used previously. If an agent needs to change
one of these, it MUST restart ICE for that media stream.
If ICE has completed for a data stream and if the agent is
controlled, the default destination for that data stream MUST be set
to the remote candidate of the candidate pair for that component in
the valid list. For a lite implementation, there is always just a
single candidate pair in the valid list for each component of a data
stream. Additionally, the agent MUST include a candidate attribute
for each default destination.
If ICE state is completed and if the agent is controlling (which only
happens when both agents are lite), the agent MUST include the
a=remote-candidates attribute for each data stream. The attribute
contains the remote candidates from the candidate pairs in the valid
list (one pair for each component of each data stream).
3.4.2. Sending Subsequent Answer
If ICE is Completed for a data stream, and the offer for that data
stream lacked the a=remote-candidates attribute, the rules for
construction of the answer are identical to those for the offerer,
except that the answerer MUST NOT include the a=remote-candidates
attribute in the answer.
A controlled agent will receive an offer with the a=remote-candidates
attribute for a data stream when its peer has concluded ICE
processing for that data stream. This attribute is present in the
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offer to deal with a race condition between the receipt of the offer,
and the receipt of the Binding Response that tells the answerer the
candidate that will be selected by ICE. See Appendix B for an
explanation of this race condition. Consequently, processing of an
offer with this attribute depends on the winner of the race.
The agent forms a candidate pair for each component of the data
stream by:
o Setting the remote candidate equal to the offerer's default
destination for that component (i.e. the contents of the "m=" and
"c=" lines for RTP, and the a=rtcp attribute for RTCP)
o Setting the local candidate equal to the transport address for
that same component in the a=remote-candidates attribute in the
offer.
The agent then sees if each of these candidate pairs is present in
the valid list. If a particular pair is not in the valid list, the
check has "lost" the race. Call such a pair a "losing pair".
The agent finds all the pairs in the 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 data stream as if the remote-
candidates attribute had not been present, and then restart ICE
for this stream.
o If at least one of the pairs is In-Progress, the agent SHOULD wait
for those checks to complete, and as each completes, redo the
processing in this section until there are no losing pairs.
Once there are no losing pairs, the agent can generate the answer.
It MUST set the default destination for media to the candidates in
the remote-candidates attribute from the offer (each of which will
now be the local candidate of a candidate pair in the valid list).
It MUST include a candidate attribute in the answer for each
candidate in the remote-candidates attribute in the offer.
3.4.2.1. ICE Restart
If the offerer in a subsequent offer requested an ICE restart for a
data stream, and if the answerer accepts the offer, the answerer
follows the procedures for generating an initial answer.
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For a given data stream, the answerer MAY include the same candidates
that were used in the previous ICE session, but it MUST change the
SDP ice-pwd and ice-ufrag attribute values.
3.4.2.2. Lite Implementation specific procedures
If the received offer contains the remote-candidates attribute for a
data stream, the agent forms a candidate pair for each component of
the data stream by:
o Setting the remote candidate equal to the offerer's default
destination for that component (i.e., the contents of the "m=" and
"c=" lines for RTP, and the a=rtcp attribute for RTCP).
o Setting the local candidate equal to the transport address for
that same component in the a=remote-candidates attribute in the
offer.
The state of ICE processing for that data stream is set to Completed.
Furthermore, if the agent believed it was controlling, but the offer
contained the a=remote-candidates attribute, both agents believe they
are controlling. In this case, both would have sent updated offers
around the same time.
However, the signaling protocol carrying the offer/answer exchanges
will have resolved this glare condition, so that one agent is always
the 'winner' by having its offer received before its peer has sent an
offer. The winner takes the role of controlling, so that the loser
(the answerer under consideration in this section) MUST change its
role to controlled.
Consequently, if the agent was going to send an updated offer since,
based on the rules in section 8.2 of [RFC8445], it was controlling,
it no longer needs to.
Besides the potential role change, change in the Valid list, and
state changes, the construction of the answer is performed
identically to the construction of an offer.
3.4.3. Receiving Answer for a Subsequent Offer
3.4.3.1. Procedures for Full Implementations
There may be certain situations where the offerer receives an SDP
answer that lacks ICE candidates although the initial answer did.
One example of such an "unexpected" answer might be happen when an
ICE-unaware B2BUA introduces a media server during call hold using
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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.
When the offerer receives an answer indicating support for ICE, the
offer performs on of the following actions:
o If the offer was a restart, the agent MUST perform ICE restart
procedures as specified in Section 3.4.3.1.1
o If the offer/answer exchange removed a data stream, or an answer
rejected an offered data stream, an agent MUST flush the Valid
list for that data stream. It MUST also terminate any STUN
transactions in progress for that data stream.
o If the offer/answer exchange added a new data stream, the agent
MUST create a new check list for it (and an empty Valid list to
start of course) which in turn triggers the candidate processing
procedures [RFC8445].
o If ICE state is running for a given data stream, the agent
recomputes the check list. If a pair on the new check list was
also on the previous check list, and its state is not Frozen, 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), appropriate procedures in [RFC8445] are
performed to move candidate(s) to the Waiting state to further
continue ICE processing.
o If ICE state is completed and the SDP answer conforms to
Section 3.4.2, the agent MUST reman in the ICE completed state.
However, if the ICE support is no longer indicated in the SDP answer,
the agent MUST fall-back to [RFC3264] procedures and SHOULD NOT drop
the dialog because of the missing ICE support or unexpected answer.
Once the agent sends a new offer later on, it MUST perform an ICE
restart.
3.4.3.1.1. ICE Restarts
The agent MUST remember the nominated pair in the Valid list for each
component of the data stream, called the previous selected pair prior
to the restart. The agent will continue to send media using this
pair, as described in section 12 of [RFC8445]. Once these
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destinations are noted, the agent MUST flush the valid and check
lists, and then recompute the check list and its states, thus
triggering the candidate processing procedures [RFC8445]
3.4.3.2. Procedures for Lite Implementations
If ICE is restarting for a data stream, the agent MUST start a new
Valid list for that data stream. It MUST remember the nominated pair
in the previous Valid list for each component of the data stream,
called the previous selected pairs, and continue to send media there
as described in section 12 of [RFC8445]. The state of ICE processing
for each data stream MUST change to Running, and the state of ICE
processing MUST change to Running
4. Grammar
This specification defines eight new SDP attributes -- the
"candidate", "remote-candidates", "ice-lite", "ice-mismatch", "ice-
ufrag", "ice-pwd", "ice-pacing", and "ice-options" attributes.
This section also provides non-normative examples of the attributes
defined.
The syntax for the attributes follow Augmented BNF as defined in
[RFC5234].
4.1. "candidate" Attribute
The candidate attribute is a media-level attribute only. It contains
a transport address for a candidate that can be used for connectivity
checks.
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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.
<port>: is also taken from RFC 4566 [RFC4566]. It is the port of
the candidate.
<transport>: indicates the transport protocol for the candidate.
This specification only defines UDP. However, extensibility is
provided to allow for future transport protocols to be used with
ICE by extending the sub-registry "ICE Transport Protocols" under
"Interactive Connectivity Establishment (ICE)" registry.
<foundation>: is composed of 1 to 32 <ice-char>s. It is an
identifier that is equivalent for two candidates that are of the
same type, share the same base, and come from the same STUN
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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 dta stream for which
this is a candidate. It MUST start at 1 and MUST increment by 1
for each component of a particular candidate. For data streams
based on RTP, candidates for the actual RTP media MUST have a
component ID of 1, and candidates for RTCP MUST have a component
ID of 2. See section 13 in [RFC8445] for additional discussion on
extending ICE to new data streams.
<priority>: is a positive integer between 1 and (2**31 - 1)
inclusive. The procedures for computing candidate's priority is
described in section 5.1.2 of [RFC8445].
<cand-type>: encodes the type of candidate. This specification
defines the values "host", "srflx", "prflx", and "relay" for host,
server reflexive, peer reflexive, and relayed candidates,
respectively. Specifications for new candidate types MUST define
how, if at all, various steps in the ICE processing differ from
the ones defined by this specification.
<rel-addr> and <rel-port>: convey transport addresses related to the
candidate, useful for diagnostics and other purposes. <rel-addr>
and <rel-port> MUST be present for server reflexive, peer
reflexive, and relayed candidates. If a candidate is server or
peer reflexive, <rel-addr> and <rel-port> are equal to the base
for that server or peer reflexive candidate. If the candidate is
relayed, <rel-addr> and <rel-port> are equal to the mapped address
in the Allocate response that provided the client with that
relayed candidate (see Appendix B.3 of [RFC8445] for a discussion
of its purpose). If the candidate is a host candidate, <rel-addr>
and <rel-port> MUST be omitted.
In some cases, e.g., for privacy reasons, an agent may not want to
reveal the related address and port. In this case the address
MUST be set to "0.0.0.0" (for IPv4 candidates) or "::" (for IPv6
candidates) and the port to zero.
The candidate attribute can itself be extended. The grammar allows
for new name/value pairs to be added at the end of the attribute.
Such extensions MUST be made through IETF Review or IESG Approval
[RFC5226] and the assignments MUST contain the specific extension and
a reference to the document defining the usage of the extension
An implementation MUST ignore any name/value pairs it doesn't
understand.
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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
4.2. "remote-candidates" Attribute
The syntax of the "remote-candidates" attribute is defined using
Augmented BNF as defined in [RFC5234]. The remote-candidates
attribute is a media-level attribute only.
remote-candidate-att = "remote-candidates:" remote-candidate
0*(SP remote-candidate)
remote-candidate = component-ID SP connection-address SP port
The attribute contains a connection-address and port for each
component. The ordering of components is irrelevant. However, a
value MUST be present for each component of a data stream. This
attribute MUST be included in an offer by a controlling agent for a
data stream that is Completed, and MUST NOT be included in any other
case.
Example: Remote candidates SDP lines for the RTP and RTCP components:
a=remote-candidates:1 192.0.2.3 45664
a=remote-candidates:2 192.0.2.3 45665
4.3. "ice-lite" and "ice-mismatch" Attributes
The syntax of the "ice-lite" and "ice-mismatch" attributes, both of
which are flags, is:
ice-lite = "ice-lite"
ice-mismatch = "ice-mismatch"
"ice-lite" is a session-level attribute only, and indicates that an
agent is a lite implementation. "ice-mismatch" is a media-level
attribute only, and when present in an answer, indicates that the
offer arrived with a default destination for a media component that
didn't have a corresponding candidate attribute.
4.4. "ice-ufrag" and "ice-pwd" Attributes
The "ice-ufrag" and "ice-pwd" attributes convey the username fragment
and password used by ICE for message integrity. Their syntax is:
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ice-pwd-att = "ice-pwd:" password
ice-ufrag-att = "ice-ufrag:" ufrag
password = 22*256ice-char
ufrag = 4*256ice-char
The "ice-pwd" and "ice-ufrag" attributes can appear at either the
session-level or media-level. When present in both, the value in the
media-level takes precedence. Thus, the value at the session-level
is effectively a default that applies to all data streams, unless
overridden by a media-level value. Whether present at the session or
media-level, there MUST be an ice-pwd and ice-ufrag attribute for
each data stream. If two data streams have identical ice-ufrag's,
they MUST have identical ice-pwd's.
The ice-ufrag and ice-pwd attributes MUST be chosen randomly at the
beginning of a session (the same applies when ICE is restarting for
an agent).
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
4.5. "ice-pacing" Attribute
The "ice-pacing" is a session level attribute that indicates the
desired connectivity check pacing, in milliseconds, for this agent
(see section 14 of [RFC8445]). The syntax is:
ice-pacing-att = "ice-pacing:" pacing-value
pacing-value = 1*10DIGIT
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Following the procedures defined in [RFC8445], a default value of
50ms is used for an agent when ice-pacing attribute is omitted in the
offer or the answer.
The same rule applies for ice-pacing attribute values lower than
50ms. This mandates that, if an agent includes the ice-pacing
attribute, its value MUST be greater than 50ms or else a value of
50ms is considered by default for that agent.
Also the larger of the ice-pacing attribute values between the offer
and the answer (determined either by the one provided in the ice-
pacing attribute or by picking the default value) MUST be considered
for a given ICE session.
Example shows ice-pacing value of 5 ms:
a=ice-pacing:5
4.6. "ice-options" Attribute
The "ice-options" attribute is a session- and media-level attribute.
It contains a series of tokens that identify the options supported by
the agent. Its grammar is:
ice-options = "ice-options:" ice-option-tag
0*(SP ice-option-tag)
ice-option-tag = 1*ice-char
The existence of an ice-option in an offer indicates that a certain
extension is supported by the agent and is willing to use it, if the
peer agent also includes the same extension in the answer. There
might be further extension specific negotiation 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 (see
Section 9.2).
Example shows 'rtp+ecn' ice-option SDP line from <<RFC6679>>:
a=ice-options:rtp+ecn
5. Keepalives
All the ICE agents MUST follow the procedures defined in section 11
of [RFC8445] for sending keepalives. The keepalives MUST be sent
regardless of whether the data stream is currently inactive,
sendonly, recvonly, or sendrecv, and regardless of the presence or
value of the bandwidth attribute. An agent can determine that its
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peer supports ICE by the presence of a=candidate attributes for each
media session.
6. SIP Considerations
Note that ICE is not intended for NAT traversal for SIP, which is
assumed to be provided via another mechanism [RFC5626].
When ICE is used with SIP, forking may result in a single offer
generating a multiplicity of answers. In that case, ICE proceeds
completely in parallel and independently for each answer, treating
the combination of its offer and each answer as an independent offer/
answer exchange, with its own set of local candidates, pairs, check
lists, states, and so on.
Once ICE processing has reached the Completed state for all peers for
media streams using those candidates, the agent SHOULD wait an
additional three seconds, and then it MAY cease responding to checks
or generating triggered checks on that candidate. It MAY free the
candidate at that time. Freeing of server reflexive candidates is
never explicit; it happens by lack of a keepalive. The three-second
delay handles cases when aggressive nomination is used, and the
selected pairs can quickly change after ICE has completed.
6.1. Latency Guidelines
ICE requires a series of STUN-based connectivity checks to take place
between endpoints. These checks start from the answerer on
generation of its answer, and start from the offerer when it receives
the answer. These checks can take time to complete, and as such, the
selection of messages to use with offers and answers can affect
perceived user latency. Two latency figures are of particular
interest. These are the post-pickup delay and the post-dial delay.
The post-pickup delay refers to the time between when a user "answers
the phone" and when any speech they utter can be delivered to the
caller. The post-dial delay refers to the time between when a user
enters the destination address for the user and ringback begins as a
consequence of having successfully started alerting the called user
agent.
Two cases can be considered -- one where the offer is present in the
initial INVITE and one where it is in a response.
6.1.1. Offer in INVITE
To reduce post-dial delays, it is RECOMMENDED that the caller begin
gathering candidates prior to actually sending its initial INVITE, so
that the candidates can be provided in the INVITE. This can be
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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 transport address matching candidate
address for one of the data streams signaled in that SDP or on
transmission of the answer in a 2xx response. If no Binding request
is received prior to the last retransmit, the agent does not consider
the session terminated. For the ICE lite peers , the agent MUST
cease retransmitting the 18x after sending it four times since there
will be no Binding request sent and the number four is arbitrarily
chosen to limit the number of 18x retransmits ('poor man's version of
[RFC3262]' basically). (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).
Once the answer has been sent, the agent SHOULD begin its
connectivity checks. Once candidate pairs for each component of a
data stream enter the valid list, the answerer can begin sending
media on that data stream.
However, prior to this point, any media that needs to be sent towards
the caller (such as SIP early media [RFC3960]) MUST NOT be
transmitted. For this reason, implementations SHOULD delay alerting
the called party until candidates for each component of each data
stream have entered the valid list. In the case of a PSTN gateway,
this would mean that the setup message into the PSTN is delayed until
this point. Doing this increases the post-dial delay, but has the
effect of eliminating 'ghost rings'. Ghost rings are cases where the
called party hears the phone ring, picks up, but hears nothing and
cannot be heard. This technique works without requiring support for,
or usage of, preconditions [RFC3312]. It also has the benefit of
guaranteeing that not a single packet of media will get clipped, so
that post-pickup delay is zero. If an agent chooses to delay local
alerting in this way, it SHOULD generate a 180 response once alerting
begins.
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6.1.2. Offer in Response
In addition to uses where the offer is in an INVITE, and the answer
is in the provisional and/or 200 OK response, ICE works with cases
where the offer appears in the response. In such cases, which are
common in third party call control [RFC3725], ICE agents SHOULD
generate their offers in a reliable provisional response (which MUST
utilize [RFC3262]), and not alert the user on receipt of the INVITE.
The answer will arrive in a PRACK. This allows for ICE processing to
take place prior to alerting, so that there is no post-pickup delay,
at the expense of increased call setup delays. Once ICE completes,
the callee can alert the user and then generate a 200 OK when they
answer. The 200 OK would contain no SDP, since the offer/answer
exchange has completed.
Alternatively, agents MAY place the offer in a 2xx instead (in which
case the answer comes in the ACK). When this happens, the callee
will alert the user on receipt of the INVITE, and the ICE exchanges
will take place only after the user answers. This has the effect of
reducing call setup delay, but can cause substantial post-pickup
delays and media clipping.
6.2. SIP Option Tags and Media Feature Tags
[RFC5768] specifies a SIP option tag and media feature tag for usage
with ICE. ICE implementations using SIP SHOULD support this
specification, which uses a feature tag in registrations to
facilitate interoperability through signaling intermediaries.
6.3. Interactions with Forking
ICE interacts very well with forking. Indeed, ICE fixes some of the
problems associated with forking. Without ICE, when a call forks and
the caller receives multiple incoming 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.
6.4. Interactions with Preconditions
Quality of Service (QoS) preconditions, which are defined in
[RFC3312] and [RFC4032], apply only to the transport addresses listed
as the default targets for media in an offer/answer. If ICE changes
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the transport address where media is received, this change is
reflected in an updated offer that changes the default destination
for media to match ICE's selection. As such, it appears like any
other re-INVITE would, and is fully treated in RFCs 3312 and 4032,
which apply without regard to the fact that the destination for media
is changing due to ICE negotiations occurring "in the background".
Indeed, an agent SHOULD NOT indicate that QoS preconditions have been
met until the checks have completed and selected the candidate pairs
to be used for media.
ICE also has (purposeful) interactions with connectivity
preconditions [RFC5898]. Those interactions are described there.
Note that the procedures described in Section 6.1 describe their own
type of "preconditions", albeit with less functionality than those
provided by the explicit preconditions in [RFC5898].
6.5. Interactions with Third Party Call Control
ICE works with Flows I, III, and IV as described in [RFC3725]. Flow
I works without the controller supporting or being aware of ICE.
Flow IV will work as long as the controller passes along the ICE
attributes without alteration. Flow II is fundamentally incompatible
with ICE; each agent will believe itself to be the answerer and thus
never generate a re-INVITE.
The flows for continued operation, as described in Section 7 of
[RFC3725], require additional behavior of ICE implementations to
support. In particular, if an agent receives a mid-dialog re-INVITE
that contains no offer, it MUST restart ICE for each 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.
7. Relationship with ANAT
[RFC4091], the Alternative Network Address Types (ANAT) Semantics for
the SDP grouping framework, and [RFC4092], its usage with SIP, define
a mechanism for indicating that an agent can support both IPv4 and
IPv6 for a data stream, and it does so by including two "m=" lines,
one for v4 and one for v6. This is similar to ICE, which allows for
an agent to indicate multiple transport addresses using the candidate
attribute. However, ANAT relies on static selection to pick between
choices, rather than a dynamic connectivity check used by ICE.
It is RECOMMENDED that ICE be used in realizing the dual-stack use-
cases in agents that support ICE.
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8. Security Considerations
8.1. 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 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.
8.2. Insider Attacks
In addition to attacks where the attacker is a third party trying to
insert fake offers, answers, or STUN messages, there are several
attacks possible with ICE when the attacker is an authenticated and
valid participant in the ICE exchange.
8.2.1. 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.
SIP User Agents (UA) [RFC3261] that are not willing to receive non-
ICE answers MUST include an "ice" Option Tag in the SIP Require
Header Field in their offer. UAs 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.
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8.2.2. Interactions with Application Layer Gateways and SIP
Application Layer Gateways (ALGs) are functions present in a Network
Address Translation (NAT) device that inspect the contents of packets
and modify them, in order to facilitate NAT traversal for application
protocols. Session Border Controllers (SBCs) are close cousins of
ALGs, but are less transparent since they actually exist as
application-layer SIP intermediaries. ICE has interactions with SBCs
and ALGs.
If an ALG is SIP aware but not ICE aware, ICE will work through it as
long as the ALG correctly modifies the SDP. A correct ALG
implementation behaves as follows:
o The ALG does not modify the "m=" and "c=" lines or the rtcp
attribute if they contain external addresses.
o If the "m=" and "c=" lines contain internal addresses, the
modification depends on the state of the ALG:
* If the ALG already has a binding established that maps an
external port to an internal IP address and port matching the
values in the "m=" and "c=" lines or rtcp attribute, the ALG
uses that binding instead of creating a new one.
* If the ALG does not already have a binding, it creates a new
one and modifies the SDP, rewriting the "m=" and "c=" lines and
rtcp attribute.
Unfortunately, many ALGs are known to work poorly in these corner
cases. ICE does not try to work around broken ALGs, as this is
outside the scope of its functionality. ICE can help diagnose these
conditions, which often show up as a mismatch between the set of
candidates and the "m=" and "c=" lines and rtcp attributes. The ice-
mismatch attribute is used for this purpose.
ICE works best through ALGs when the signaling is run over TLS. This
prevents the ALG from manipulating the SDP messages and interfering
with ICE operation. Implementations that are expected to be deployed
behind ALGs SHOULD provide for TLS transport of the SDP.
If an SBC is SIP aware but not ICE aware, the result depends on the
behavior of the SBC. If it is acting as a proper Back-to-Back User
Agent (B2BUA), the SBC will remove any SDP attributes it doesn't
understand, including the ICE attributes. Consequently, the call
will appear to both endpoints as if the other side doesn't support
ICE. This will result in ICE being disabled, and media flowing
through the SBC, if the SBC has requested it. If, however, the SBC
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passes the ICE attributes without modification, yet modifies the
default destination for media (contained in the "m=" and "c=" lines
and rtcp attribute), this will be detected as an ICE mismatch, and
ICE processing is aborted for the call. It is outside of the scope
of ICE for it to act as a tool for "working around" SBCs. If one is
present, ICE will not be used and the SBC techniques take precedence.
9. IANA Considerations
9.1. SDP Attributes
The original ICE specification defined seven new SDP attributes per
the procedures of Section 8.2.4 of [RFC4566]. The registration
information from the original specification is included here with
modifications to include Mux Category and also defines a new SDP
attribute 'ice-pacing'.
9.1.1. candidate Attribute
Attribute Name: candidate
Type of Attribute: media-level
Subject to charset: No
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides one of many possible candidate
addresses for communication. These addresses are validated with
an end-to-end connectivity check using Session Traversal Utilities
for NAT (STUN).
Appropriate Values: See Section 4 of RFC XXXX.
Contact Name: IESG
Contact e-mail: iesg@ietf.org [1]
Reference: RFCXXXX
Mux Category: TRANSPORT
9.1.2. remote-candidates Attribute
Attribute Name: remote-candidates
Type of Attribute: media-level
Subject to charset: No
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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 4 of RFC XXXX.
Contact Name: IESG
Contact e-mail: iesg@ietf.org [2]
Reference: RFCXXXX
Mux Category: TRANSPORT
9.1.3. ice-lite Attribute
Attribute Name: ice-lite
Type of Attribute: session-level
Subject to charset: No
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates that an agent has the minimum
functionality required to support ICE inter-operation with a peer
that has a full implementation.
Appropriate Values: See Section 4 of RFC XXXX.
Contact Name: IESG
Contact e-mail: iesg@ietf.org [3]
Reference: RFCXXXX
Mux Category: NORMAL
9.1.4. ice-mismatch Attribute
Attribute Name: ice-mismatch
Type of Attribute: media-level
Subject to charset: No
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates that an agent is ICE capable,
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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 4 of RFC XXXX.
Contact Name: IESG
Contact e-mail: iesg@ietf.org [4]
Reference: RFCXXXX
Mux Category: NORMAL
9.1.5. ice-pwd Attribute
Attribute Name: ice-pwd
Type of Attribute: session- or media-level
Subject to charset: No
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the password used to protect
STUN connectivity checks.
Appropriate Values: See Section 4 of RFC XXXX.
Contact Name: IESG
Contact e-mail: iesg@ietf.org [5]
Reference: RFCXXXX
Mux Category: TRANSPORT
9.1.6. ice-ufrag Attribute
Attribute Name: ice-ufrag
Type of Attribute: session- or media-level
Subject to charset: No
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the fragments used to construct
the username in STUN connectivity checks.
Appropriate Values: See Section 4 of RFC XXXX.
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Contact Name: IESG
Contact e-mail: iesg@ietf.org [6]
Reference: RFCXXXX
Mux Category: TRANSPORT
9.1.7. ice-options Attribute
Attribute Name: ice-options
Long Form: ice-options
Type of Attribute: session-level
Subject to charset: No
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates the ICE options or extensions
used by the agent.
Appropriate Values: See Section 4 of RFC XXXX.
Contact Name: IESG
Contact e-mail: iesg@ietf.org [7]
Reference: RFCXXXX
Mux Category: NORMAL
9.1.8. ice-pacing Attribute
This specification also defines a new SDP attribute, "ice-pacing"
according to the following data:
Attribute Name: ice-pacing
Type of Attribute: session-level
Subject to charset: No
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE) to indicate desired connectivity check pacing
values.
Appropriate Values: See Section 4 of RFC XXXX.
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Contact Name: IESG
Contact e-mail: iesg@ietf.org [8]
Reference: RFCXXXX
Mux Category: NORMAL
9.2. 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 4.6; 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 leve..
A registration request MUST include the following information:
o The ICE option identifier to be registered
o Name, Email, and Address of a contact person for the registration
o Organization or individuals having the change control
o Short description of the ICE extension to which the option relates
o Reference(s) to the specification defining the ICE option and the
related extensions
10. Acknowledgments
A large part of the text in this document was taken from [RFC5245],
authored by Jonathan Rosenberg.
Some of the text in this document was taken from [RFC6336], authored
by Magnus Westerlund and Colin Perkins.
Many thanks to Christer Holmberg for providing text suggestions in
Section 3 that aligns with [RFC8445]
Thanks to Thomas Stach for text help, Roman Shpount for suggesting
RTCP candidate handling and Simon Perreault for advising on IPV6
address selection when candidate-address includes FQDN.
Many thanks to Flemming Andreasen for shepherd review feedback.
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Thanks to following experts for their reviews and constructive
feedback: Christer Holmberg, Adam Roach, Peter Saint-Andre and the
MMUSIC WG.
11. References
11.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, DOI 10.17487/RFC3262, June 2002,
<http://www.rfc-editor.org/info/rfc3262>.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
DOI 10.17487/RFC3264, June 2002,
<http://www.rfc-editor.org/info/rfc3264>.
[RFC3312] Camarillo, G., Ed., Marshall, W., Ed., and J. Rosenberg,
"Integration of Resource Management and Session Initiation
Protocol (SIP)", RFC 3312, DOI 10.17487/RFC3312, October
2002, <http://www.rfc-editor.org/info/rfc3312>.
[RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth",
RFC 3556, DOI 10.17487/RFC3556, July 2003,
<http://www.rfc-editor.org/info/rfc3556>.
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605,
DOI 10.17487/RFC3605, October 2003,
<http://www.rfc-editor.org/info/rfc3605>.
[RFC4032] Camarillo, G. and P. Kyzivat, "Update to the Session
Initiation Protocol (SIP) Preconditions Framework",
RFC 4032, DOI 10.17487/RFC4032, March 2005,
<http://www.rfc-editor.org/info/rfc4032>.
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[RFC4091] Camarillo, G. and J. Rosenberg, "The Alternative Network
Address Types (ANAT) Semantics for the Session Description
Protocol (SDP) Grouping Framework", RFC 4091, June 2005,
<http://www.rfc-editor.org/info/rfc4091>.
[RFC4092] Camarillo, G. and J. Rosenberg, "Usage of the Session
Description Protocol (SDP) Alternative Network Address
Types (ANAT) Semantics in the Session Initiation Protocol
(SIP)", RFC 4092, June 2005,
<http://www.rfc-editor.org/info/rfc4092>.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
July 2006, <http://www.rfc-editor.org/info/rfc4566>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
[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>.
[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,
<http://www.rfc-editor.org/info/rfc8445>.
11.2. 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>.
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[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>.
[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>.
[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>.
11.3. URIs
[1] mailto:iesg@ietf.org
[2] mailto:iesg@ietf.org
[3] mailto:iesg@ietf.org
[4] mailto:iesg@ietf.org
[5] mailto:iesg@ietf.org
[6] mailto:iesg@ietf.org
[7] mailto:iesg@ietf.org
[8] mailto:iesg@ietf.org
[9] mailto:christer.holmberg@ericsson.com
[10] mailto:rshpount@turbobridge.com
[11] mailto:thomass.stach@gmail.com
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Appendix A. Examples
For the example shown in section 15 of [RFC8445] the resulting offer
(message 5) encoded in SDP looks like:
v=0
o=jdoe 2890844526 2890842807 IN IP6 $L-PRIV-1.IP
s=
c=IN IP6 $NAT-PUB-1.IP
t=0 0
a=ice-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:
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v=0
o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP
s=
c=IN IP4 $R-PUB-1.IP
t=0 0
a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
a=ice-ufrag:9uB6
m=audio $R-PUB-1.PORT RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 $R-PUB-1.IP $R-PUB-1.PORT typ host
With the variables filled in:
v=0
o=bob 2808844564 2808844564 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
a=ice-ufrag:9uB6
m=audio 3478 RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 192.0.2.1 3478 typ host
Appendix B. The remote-candidates Attribute
The a=remote-candidates attribute exists to eliminate a race
condition between the updated offer and the response to the STUN
Binding request that moved a candidate into the Valid list. This
race condition is shown in Figure 1. On receipt of message 4, agent
L adds a candidate pair to the valid list. If there was only a
single data stream with a single component, agent L could now send an
updated offer. However, the check from agent R has not yet generated
a response, and agent R receives the updated offer (message 7) before
getting the response (message 9). Thus, it does not yet know that
this particular pair is valid. To eliminate this condition, the
actual candidates at R that were selected by the offerer (the remote
candidates) are included in the offer itself, and the answerer delays
its answer until those pairs validate.
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Agent L Network Agent R
|(1) Offer | |
|------------------------------------------>|
|(2) Answer | |
|<------------------------------------------|
|(3) STUN Req. | |
|------------------------------------------>|
|(4) STUN Res. | |
|<------------------------------------------|
|(5) STUN Req. | |
|<------------------------------------------|
|(6) STUN Res. | |
|-------------------->| |
| |Lost |
|(7) Offer | |
|------------------------------------------>|
|(8) STUN Req. | |
|<------------------------------------------|
|(9) STUN Res. | |
|------------------------------------------>|
|(10) Answer | |
|<------------------------------------------|
Figure 1: Race Condition Flow
Appendix C. Why Is the Conflict Resolution Mechanism Needed?
When ICE runs between two peers, one agent acts as controlled, and
the other as controlling. Rules are defined as a function of
implementation type and offerer/answerer to determine who is
controlling and who is controlled. However, the specification
mentions that, in some cases, both sides might believe they are
controlling, or both sides might believe they are controlled. How
can this happen?
The condition when both agents believe they are controlled shows up
in third party call control cases. Consider the following flow:
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A Controller B
|(1) INV() | |
|<-------------| |
|(2) 200(SDP1) | |
|------------->| |
| |(3) INV() |
| |------------->|
| |(4) 200(SDP2) |
| |<-------------|
|(5) ACK(SDP2) | |
|<-------------| |
| |(6) ACK(SDP1) |
| |------------->|
Figure 2: Role Conflict Flow
This flow is a variation on flow III of RFC 3725 [RFC3725]. In fact,
it works better than flow III since it produces fewer messages. In
this flow, the controller sends an offerless INVITE to agent A, which
responds with its offer, SDP1. The agent then sends an offerless
INVITE to agent B, which it responds to with its offer, SDP2. The
controller then uses the offer from each agent to generate the
answers. When this flow is used, ICE will run between agents A and
B, but both will believe they are in the controlling role. With the
role conflict resolution procedures, this flow will function properly
when ICE is used.
At this time, there are no documented flows that can result in the
case where both agents believe they are controlled. However, the
conflict resolution procedures allow for this case, should a flow
arise that would fit into this category.
Appendix D. Why Send an Updated Offer?
Section 11.1 describes rules for sending media. Both agents can send
media once ICE checks complete, without waiting for an updated offer.
Indeed, the only purpose of the updated offer is to "correct" the SDP
so that the default destination for media matches where media is
being sent based on ICE procedures (which will be the highest-
priority nominated candidate pair).
This 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.
Appendix E. Contributors
Following experts have contributed textual and structural
improvements for this work
1. Christer Holmberg
* Ericsson
* Email: christer.holmberg@ericsson.com [9]
2. Roman Shpount
* TurboBridge
* rshpount@turbobridge.com [10]
3. Thomas Stach
* thomass.stach@gmail.com [11]
Authors' Addresses
Marc Petit-Huguenin
Impedance Mismatch
Email: marc@petit-huguenin.org
Suhas Nandakumar
Cisco Systems
707 Tasman Dr
Milpitas, CA 95035
USA
Email: snandaku@cisco.com
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Ari Keranen
Ericsson
Jorvas 02420
Finland
Email: ari.keranen@ericsson.com
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