Session Description Protocol (SDP) Offer/Answer Procedures for Interactive Connectivity Establishment (ICE)Impedance Mismatchmarc@petit-huguenin.orgCisco Systems707 Tasman DrMilpitasCA95035United States of Americasnandaku@cisco.comEricssonHirsalantie 11Jorvas02420Finlandchrister.holmberg@ericsson.comEricssonJorvas02420Finlandari.keranen@ericsson.comTurboBridge4905 Del Ray Avenue, Suite 300BethesdaMD20814United States of Americarshpount@turbobridge.com
ART
MMUSICSIPSession Initial Protocol
This document describes Session Description Protocol (SDP) Offer/Answer
procedures for carrying out Interactive Connectivity Establishment (ICE)
between the agents.
This document obsoletes RFCs 5245 and 6336.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by
the Internet Engineering Steering Group (IESG). Further
information on Internet Standards is available in Section 2 of
RFC 7841.
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errata, and how to provide feedback on it may be obtained at
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Table of Contents
. Introduction
. Conventions
. Terminology
. SDP Offer/Answer Procedures
. Introduction
. Generic Procedures
. Encoding
. RTP/RTCP Considerations
. Determining Role
. STUN Considerations
. Verifying ICE Support Procedures
. SDP Example
. Initial Offer/Answer Exchange
. Sending the Initial Offer
. Sending the Initial Answer
. Receiving the Initial Answer
. Concluding ICE
. Subsequent Offer/Answer Exchanges
. Sending Subsequent Offer
. Sending Subsequent Answer
. Receiving Answer for a Subsequent Offer
. Grammar
. "candidate" Attribute
. "remote-candidates" Attribute
. "ice-lite" and "ice-mismatch" Attributes
. "ice-ufrag" and "ice-pwd" Attributes
. "ice-pacing" Attribute
. "ice-options" Attribute
. Keepalives
. SIP Considerations
. Latency Guidelines
. Offer in INVITE
. Offer in Response
. SIP Option Tags and Media Feature Tags
. Interactions with Forking
. Interactions with Preconditions
. Interactions with Third Party Call Control
. Interactions with Application Layer Gateways and SIP
. Why Is the Conflict Resolution Mechanism Needed?
. Why Send an Updated Offer?
Acknowledgements
Contributors
Authors' Addresses
Introduction
This document describes how Interactive Connectivity Establishment (ICE) is
used with Session Description Protocol (SDP) offer/answer . The ICE specification describes procedures that are common to
all usages of ICE, and this document gives the additional details needed to
use ICE with SDP offer/answer.
This document obsoletes RFCs 5245 and 6336.
NOTE: Previously both the common ICE procedures, and the SDP offer/answer
specific details, were described in . obsoleted , and the SDP offer/answer-specific details
were removed from the document.
describes the changes to the SDP offer/answer-specific details specified in
this document.
Conventions
The key words "MUST", "MUST NOT",
"REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are
to be interpreted as described in BCP 14 when, and only when, they appear in all capitals,
as shown here.
Terminology
Readers should be familiar with the terminology defined in , in , and the following:
Default Destination/Candidate:
The default destination for a component of a data stream is the transport
address that would be used by an agent that is not ICE aware. A default
candidate for a component is one whose transport address matches the default
destination for that component. For the RTP component, the default connection
address is in the "c=" line of the SDP, and the port and transport protocol
are in the "m=" line. For the RTP Control Protocol (RTCP) component, the
address and port are indicated using the "rtcp" attribute defined in , if present; otherwise, the RTCP component
address is the same as the address of the RTP component, and its port is one
greater than the port of the RTP component.
SDP Offer/Answer ProceduresIntroduction defines ICE candidate
exchange as the process for ICE agents (initiator and responder) to
exchange their candidate information required for ICE processing at
the agents. For the purposes of this specification, the candidate
exchange process corresponds to the Offer/Answer protocol , and the terms "offerer" and
"answerer" correspond to the initiator and responder roles from respectively.
Once the initiating agent has gathered, pruned, and prioritized its set of
candidates , the candidate exchange
with the peer agent begins.
Generic ProceduresEncoding provides detailed
rules for constructing various SDP attributes defined in this
specification.
Data Streams
Each data stream is represented by
an SDP media description ("m=" section).
Candidates
Within an "m=" section, each candidate (including the default candidate) associated
with the data stream is represented by an SDP "candidate" attribute.
Prior to nomination, the "c=" line associated with an "m=" section contains
the connection address of the default candidate, while the "m=" line contains the port
and transport protocol of the default candidate for that "m=" section.
After nomination, the "c=" line for a given "m=" section contains the
connection address of the nominated candidate (the local candidate of the
nominated candidate pair), and the "m=" line contains the port and
transport protocol corresponding to the nominated candidate for that
"m=" section.
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.
Lite Implementations
An ICE-lite implementation MUST include an SDP "ice-lite" attribute.
A full implementation MUST NOT include that attribute.
ICE Extensions
An agent uses the SDP "ice-options" attribute to indicate support of ICE
extensions.
An agent compliant with this specification MUST include an SDP "ice-options"
attribute with an "ice2" attribute value . If an agent receives an SDP offer
or answer that indicates ICE support, but that does not contain an SDP "ice-options" attribute with an "ice2" attribute value,
the agent can assume that the peer is compliant to .
Inactive and Disabled Data Streams
If an "m=" section is marked as inactive , or has a bandwidth
value of zero , 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 , the agent SHOULD NOT
include ICE-related SDP "candidate" attributes in that "m=" section, unless an
SDP extension specifying otherwise is used.
RTP/RTCP Considerations
If an agent utilizes both RTP and RTCP, and separate ports
are used for RTP and RTCP, the agent MUST include SDP "candidate"
attributes for both the RTP and RTCP components.
The agent includes an SDP "rtcp" attribute following the procedures
in . Hence, in the cases where the RTCP
port value is one higher than the RTP port value and the RTCP component
address the same as the address of the RTP component, the SDP "rtcp" attribute
might be omitted.
NOTE: required that an agent always includes the
SDP "rtcp" attribute, even if the RTCP port value was one higher than the
RTP port value. This specification aligns the "rtcp" attribute procedures
with .
If the agent does not utilize RTCP, it indicates that by including "RS:0"
and "RR:0" SDP attributes as described in .
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 .
STUN Considerations
Once an agent has provided its local candidates to its peer in an SDP
offer or answer, the agent MUST be prepared to receive STUN
(Session Traversal Utilities for NAT, ) connectivity
check Binding requests on those candidates.
Verifying ICE Support Procedures
An ICE agent indicates support of ICE by including
at least the SDP "ice-pwd" and "ice-ufrag" attributes in an offer or answer.
An ICE agent compliant with this specification MUST also include an
SDP "ice-options" attribute with an "ice2" attribute value.
The agents will proceed with the ICE procedures defined in and
this specification if, for each data stream in the SDP it received, the
default destination for each component of that data stream appears in
a "candidate" attribute. For example, in the case of RTP, the
connection address, port, and transport protocol in the
"c=" and "m=" lines, respectively, appear in a "candidate"
attribute, and the value in the "rtcp" attribute appears in a "candidate"
attribute.
This specification provides no guidance on how an agent should proceed
in the cases where the above condition is not met with the few
exceptions noted below:
The presence of certain Application Layer Gateways might modify
the transport address information as described in .
The behavior of the responding agent in such a situation is
implementation dependent. Informally, the responding agent might
consider the mismatched transport address information as a
plausible new candidate learned from the peer and continue its
ICE processing with that transport address included.
Alternatively, the responding agent MAY include an "ice-mismatch"
attribute in its answer for such data streams. If an agent chooses to
include an "ice-mismatch" attribute in its answer for a data stream,
then it MUST also omit "candidate" attributes, MUST terminate
the usage of ICE procedures, and
procedures MUST be used instead for this data stream.
The transport address from the peer for the default destination
is set to IPv4/IPv6 address values "0.0.0.0"/"::" and port value of "9".
This MUST NOT be considered as an ICE failure by the peer agent, and
the ICE processing MUST continue as usual.
In some cases, the controlling/initiator agent may receive an SDP answer
that may omit "candidate" attributes for the data stream, and instead
include a media-level "ice-mismatch" attribute. This signals to the
offerer that the answerer supports ICE, but that ICE processing was not
used for this data stream. In this case, ICE processing MUST be terminated
for this data stream, and procedures MUST be followed instead.
The transport address from the peer for the default destination is
an FQDN. Regardless of the procedures used to resolve FQDN or the
resolution result, this MUST NOT be considered as an ICE failure by
the peer agent, and the ICE processing MUST continue as usual.
SDP Example
The following is an example SDP message that includes ICE attributes
(lines folded for readability):
v=0
o=jdoe 2890844526 2890842807 IN IP4 203.0.113.141
s=
c=IN IP4 192.0.2.3
t=0 0
a=ice-options:ice2
a=ice-pacing:50
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 45664 RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 203.0.113.141 8998 typ host
a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr
203.0.113.141 rport 8998
Initial Offer/Answer ExchangeSending the Initial Offer
When an offerer generates the initial offer, in each "m=" section it MUST
include SDP "candidate" attributes for each available candidate associated
with the "m=" section. In addition, the offerer MUST include an SDP "ice-ufrag"
attribute, an SDP "ice-pwd" attribute, and an SDP "ice-options" attribute with
an "ice2" attribute value in the offer. If the offerer is a full ICE implementation,
it SHOULD include an "ice-pacing" attribute in the offer (if not included, the
default value will apply). A lite ICE implementation MUST NOT include the "ice-pacing"
attribute in the offer (as it will not perform connectivity checks).
It is valid for an offer "m=" line to include no SDP "candidate" attributes
and have the default destination set to the IP address values
"0.0.0.0"/"::" and the port value to "9".
This implies that the offering agent is only going to use peer-reflexive
candidates or will provide additional candidates in subsequent signaling
messages.
Note:
Within the scope of this document, "initial offer" refers to the first
SDP offer that is sent in order to negotiate usage of ICE. It might, or
might not, be the initial SDP offer of the SDP session.
Note:
The procedures in this document only consider "m=" sections associated
with data streams where ICE is used.
Sending the Initial Answer
When an answerer receives an initial offer indicating
that the offerer supports ICE, and if the answerer accepts
the offer and the usage of ICE, the answerer MUST include
in each "m=" section within the answer the SDP "candidate"
attributes for each available candidate associated with
the "m=" section.
In addition, the answerer MUST include an SDP "ice-ufrag"
attribute, an SDP "ice-pwd" attribute, and an SDP "ice-options"
attribute with an "ice2" attribute value in the answer. If the
answerer is a full ICE implementation, it SHOULD include an
"ice-pacing" attribute in the answer (if not included, the
default value will apply). A lite ICE implementation MUST NOT
include the "ice-pacing" attribute in the answer (as it will
not perform connectivity checks).
In each "m=" line, the answerer MUST use the same transport
protocol as was used in the offer "m=" line. If none of
the candidates in the "m=" line in the answer uses the same
transport protocol as indicated in the offer "m=" line,
then, in order to avoid ICE mismatch, the default destination
MUST be set to IP address values "0.0.0.0"/"::" and
port value of "9".
It is also valid for an answer "m=" line to include no SDP
"candidate" attributes and have the default destination set
to the IP address values "0.0.0.0"/"::" and the port value to "9".
This implies that the answering agent is only going to use
peer-reflexive candidates or that additional candidates would be
provided in subsequent signaling messages.
Once the answerer has sent the answer, it can start performing
connectivity checks towards the peer candidates that were provided
in the offer.
If the offer does not indicate support of ICE (), 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 .
If the answerer detects a possibility of an ICE mismatch,
procedures described in are followed.
Receiving the Initial Answer
When an offerer receives an initial answer that indicates
that the answerer supports ICE, it can start performing
connectivity checks towards the peer candidates that were
provided in the answer.
If the answer does not indicate that the answerer supports ICE, or if the
answerer included "ice-mismatch" attributes for all the active data streams
in the answer, the offerer MUST terminate the usage of ICE for
the entire session, and procedures
MUST be followed instead.
On the other hand, if the answer indicates support for
ICE but includes "ice-mismatch" in certain active data
streams, then the offerer MUST terminate the usage of ICE
procedures, and procedures
MUST be used instead for only these data streams. Also, ICE
procedures MUST be used for data streams where an "ice-mismatch"
attribute was not included.
If the offerer detects an ICE mismatch for one or more data streams in the
answer, as described in ,
the offerer MUST terminate the usage of ICE for the entire
session. The subsequent actions taken by the offerer are implementation
dependent and are out of the scope of this specification.
Concluding ICE
Once the agent has successfully nominated a pair , the state of the checklist associated with the pair is set
to Completed. Once the state of each checklist is set to either Completed or
Failed, for each Completed checklist, the agent checks whether the nominated
pair matches the default candidate pair. If there are one or more pairs that
do not match, and the peer did not indicate support for the 'ice2' ice-option,
the controlling agent MUST generate a subsequent offer in which
the connection address, port, and transport protocol in the "c=" and "m="
lines associated with each data stream match the corresponding local
information of the nominated pair for that data stream (). If the peer
did indicate support for the 'ice2' ice-option, the controlling agent does not
immediately need to generate an updated offer in order to align a connection
address, port, and protocol with a nominated pair. However, later in the
session, whenever the controlling agent does send a subsequent offer, it
MUST do the alignment as described above.
If there are one or more checklists with the state set to Failed, the
controlling agent MUST generate a subsequent offer in order to
remove the associated data streams by setting the port value of the data
streams to zero (), even if the peer did indicate support for the 'ice2'
ice-option. If needed, such offer is used to align the connection address,
port, and transport protocol, as described above.
As described in , once the controlling agent has nominated
a candidate pair for a checklist, the agent MUST NOT nominate another pair
for that checklist during the lifetime of the ICE session (i.e., until
ICE is restarted).
provides a mechanism for
allowing the ICE process to run long enough in order to find working candidate pairs,
by waiting for potential peer-reflexive candidates, even though no candidate pairs were
received from the peer or all current candidate pairs associated with a checklist have
either failed or been discarded.
Subsequent Offer/Answer Exchanges
Either agent MAY generate a subsequent offer at any time allowed by
. 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.
Sending Subsequent OfferProcedures for All ImplementationsICE Restart
An agent MAY restart ICE processing for an existing data stream .
The rules governing the ICE restart imply that setting the connection address
in the "c=" line to "0.0.0.0" (for IPv4)/ "::" (for IPv6) will cause an ICE restart.
Consequently, ICE implementations MUST NOT utilize this mechanism for call hold,
and instead MUST use "inactive" and "sendonly" as described in
.
To restart ICE, an agent MUST change both the "ice-pwd" and the "ice-ufrag" for
the data stream in an offer. However, it is permissible to use a session-level
attribute in one offer, but to provide the same "ice-pwd" or "ice-ufrag" as a
media-level attribute in a subsequent offer. This MUST NOT be considered
as ICE restart.
An agent sets the rest of the ICE-related fields in the SDP for this data stream as it
would in an initial offer of this data stream ().
Consequently, the set of candidates MAY include some, none, or all of the
previous candidates for that data stream and MAY include a totally new set of
candidates. The agent MAY modify the attribute values of the SDP "ice-options" and
SDP "ice-pacing" attributes, and it MAY change its role using the SDP "ice-lite" attribute.
The agent MUST NOT modify the SDP "ice-options", "ice-pacing", and "ice-lite" attributes in a
subsequent offer unless the offer is sent in order to request an ICE restart.
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 for that data stream.
Adding a Data Stream
If an agent wishes to add a new data stream, it sets the fields in the SDP for
this data stream as if this were an initial offer for that data stream
(). This will cause ICE processing to begin for this data stream.
Procedures for Full Implementations
This section describes additional procedures for full implementations,
covering existing data streams.
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.
After Nomination
Once a candidate pair has been nominated for a data stream, the connection address,
port, and transport protocol in each "c=" and "m=" line associated with that data
stream MUST match the data associated with the nominated pair for that
data stream. In addition, the offerer only includes SDP "candidate" attributes
(one per component) representing the local candidates of the nominated candidate pair.
The offerer MUST NOT include any other SDP "candidate" attributes in the
subsequent offer.
In addition, if the agent is controlling, it MUST include the
"remote-candidates" attribute for each data stream whose checklist
is in the Completed state. The attribute contains the remote candidates
corresponding to the nominated pair in the valid list for each
component of that data stream. It is needed to avoid a race condition
whereby the controlling agent chooses its pairs, but the updated offer
beats the connectivity checks to the controlled agent, which doesn't
even know these pairs are valid, let alone selected. See
for elaboration on this race condition.
Procedures for Lite Implementations
If the ICE state is Running, a lite implementation MUST include all of
its candidates for each component of each data stream in "candidate"
attributes in any subsequent offer. The candidates are formed identically
to the procedures for initial offers.
A lite implementation MUST NOT add additional host candidates in a
subsequent offer, and MUST NOT modify the username fragments and
passwords. If an agent needs to offer additional candidates, or
to modify the username fragments and passwords, it MUST request an
ICE restart () for that data stream.
If ICE has completed for a data stream, and if the agent is controlled,
the default destination for that data stream MUST be set to the
remote candidate of the candidate pair for that component in the valid list.
For a lite implementation, there is always just a single candidate pair in
the valid list for each component of a data stream. Additionally, the agent
MUST include a "candidate" attribute for each default destination.
If the ICE state is Completed, and if the agent is controlling (which only
happens when both agents are lite), the agent MUST include the
"remote-candidates" attribute for each data stream. The attribute
contains the remote candidates from the candidate pairs in the
valid list (one pair for each component of each data stream).
Sending Subsequent Answer
If ICE is Completed for a data stream, and the offer for that data
stream lacked the "remote-candidates" attribute, the rules for
construction of the answer are identical to those for the offerer,
except that the answerer MUST NOT include the "remote-candidates"
attribute in the answer.
A controlled agent will receive an offer with the "remote-candidates"
attribute for a data stream when its peer has concluded ICE processing
for that data stream. This attribute is present in the
offer to deal with a race condition between the receipt of the offer,
and the receipt of the Binding response that tells the answerer the
candidate that will be selected by ICE. See for an
explanation of this race condition. Consequently, processing of an
offer with this attribute depends on the winner of the race.
The agent forms a candidate pair for each component of the data stream by:
Setting the remote candidate equal to the offerer's default
destination for that component (i.e., the contents of the "m=" and
"c=" lines for RTP, and the "rtcp" attribute for RTCP)
Setting the local candidate equal to the transport address for
that same component in the "remote-candidates" attribute in the
offer.
The agent then sees if each of these candidate pairs is present
in the valid list. If a particular pair is not in the valid list,
the check has "lost" the race. Call such a pair a "losing pair".
The agent finds all the pairs in the checklist whose remote
candidates equal the remote candidate in the losing pair:
If none of the pairs is In-Progress, and at least one is Failed,
it is most likely that a network failure, such as a network
partition or serious packet loss, has occurred. The agent SHOULD
generate an answer for this data stream as if the "remote-
candidates" attribute had not been present, and then restart ICE
for this stream.
If at least one of the pairs is In-Progress, the agent SHOULD wait
for those checks to complete, and as each completes, redo the
processing in this section until there are no losing pairs.
Once there are no losing pairs, the agent can generate the answer.
It MUST set the default destination for media to the candidates in
the "remote-candidates" attribute from the offer (each of which will
now be the local candidate of a candidate pair in the valid list).
It MUST include a "candidate" attribute in the answer for each
candidate in the "remote-candidates" attribute in the offer.
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.
For a given data stream, the answerer MAY include the same
candidates that were used in the previous ICE session, but
it MUST change the SDP "ice-pwd" and "ice-ufrag" attribute
values.
The answerer MAY modify the attribute values of the SDP "ice-options" and
SDP "ice-pacing" attributes, and it MAY change its role using the SDP "ice-lite" attribute.
The answerer MUST NOT modify the SDP "ice-options", "ice-pacing", and "ice-lite" attributes in a
subsequent answer unless the answer is sent for an offer that was used to request an ICE restart
(). If any of the SDP attributes have been modified in
a subsequent offer that is not used to request an ICE restart, the answerer MUST reject the
offer.
Lite Implementation Specific Procedures
If the received offer contains the "remote-candidates" attribute for a
data stream, the agent forms a candidate pair for each component of the
data stream by:
Setting the remote candidate equal to the offerer's default destination
for that component (i.e., the contents of the "m=" and "c=" lines for RTP,
and the "rtcp" attribute for RTCP).
Setting the local candidate equal to the transport address for that same
component in the "remote-candidates" attribute in the offer.
The state of the checklist associated with that data stream is set to Completed.
Furthermore, if the agent believed it was controlling, but the offer contained
the "remote-candidates" attribute, both agents believe they are controlling.
In this case, both would have sent updated offers around the same time.
However, the signaling protocol carrying the offer/answer exchanges
will have resolved this glare condition, so that one agent is always
the 'winner' by having its offer received before its peer has sent
an offer. The winner takes the role of controlling, so that the
loser (the answerer under consideration in this section) MUST
change its role to controlled.
Consequently, if the agent was controlling based on the rules
in
and was going to send an updated offer, it no longer needs to.
Besides the potential role change, change in the valid list, and
state changes, the construction of the answer is performed identically
to the construction of an offer.
Receiving Answer for a Subsequent OfferProcedures for Full Implementations
There may be certain situations where the offerer receives
an SDP answer that lacks ICE candidates although the initial answer
included them. One example of such an "unexpected" answer might
happen when an ICE-unaware Back-to-Back User Agent (B2BUA)
introduces a media server during call hold using third party
call control procedures .
Omitting further details on how this is done, this could
result in an answer that was constructed by the B2BUA
being received at the holding UA. With the B2BUA being
ICE-unaware, that answer would not include ICE candidates.
Receiving an answer without ICE attributes in this situation might be
unexpected, but would not necessarily impair the user experience.
When the offerer receives an answer indicating support for ICE, the
offer performs one of the following actions:
If the offer was a restart, the agent MUST perform ICE restart
procedures as specified in .
If the offer/answer exchange removed a data stream, or an
answer rejected an offered data stream, an agent MUST flush the
valid list for that data stream. It MUST also terminate any
STUN transactions in progress for that data stream.
If the offer/answer exchange added a new data stream, the agent
MUST create a new checklist for it (and an empty valid list to
start of course), which in turn triggers the candidate
processing procedures .
If the checklist state associated with a data stream is Running, the agent
recomputes the checklist. If a pair on the new checklist was
also on the previous checklist, its candidate pair state is copied over.
Otherwise, its candidate pair state is set to Frozen. If none of the checklists
are active (meaning that the candidate pair states in each checklist
are Frozen), appropriate procedures in
are performed to move candidate pair(s) to the Waiting state to
further continue ICE processing.
If the ICE state is Completed, and the SDP answer conforms to
, the agent MUST remain in the
Completed ICE state.
However, if the ICE support is no longer indicated in the SDP answer,
the agent MUST fall back to
procedures and SHOULD NOT drop the dialog because of the
missing ICE support or unexpected answer.
When the agent sends a new offer, it MUST perform an ICE restart.
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 . Once these destinations are
noted, the agent MUST flush the valid lists and checklists, and then recompute
the checklist and its states, thus triggering the candidate processing
procedures .Procedures for Lite Implementations
If ICE is restarting for a data stream, the agent MUST create a new
valid list for that data stream. It MUST remember the nominated pair in the
previous valid list for each component of the data stream, called
the "previous selected pairs", and continue to send media there as
described in . The state of each
checklist for each data stream MUST change to Running, and the ICE state
MUST be set to Running.
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 .
"candidate" Attribute
The "candidate" attribute is a media-level attribute only.
It contains a transport address for a candidate that can be used for connectivity checks.
candidate-attribute = "candidate" ":" foundation SP component-id SP
transport SP
priority SP
connection-address SP ;from RFC 4566
port ;port from RFC 4566
SP cand-type
[SP rel-addr]
[SP rel-port]
*(SP cand-extension)
foundation = 1*32ice-char
component-id = 1*3DIGIT
transport = "UDP" / transport-extension
transport-extension = token ; from RFC 3261
priority = 1*10DIGIT
cand-type = "typ" SP candidate-types
candidate-types = "host" / "srflx" / "prflx" / "relay" / token
rel-addr = "raddr" SP connection-address
rel-port = "rport" SP port
cand-extension = extension-att-name SP extension-att-value
extension-att-name = token
extension-att-value = *VCHAR
ice-char = ALPHA / DIGIT / "+" / "/"
This grammar encodes the primary information about a candidate: its IP address,
port and transport protocol, and its properties: the foundation, component ID, priority,
type, and related transport address:
<connection-address>:
is taken from RFC 4566 .
It is the IP address of the candidate, allowing for
IPv4 addresses, IPv6 addresses, and fully qualified domain names (FQDNs).
When parsing this field, an agent can differentiate an IPv4 address
and an IPv6 address by presence of a colon in its value -
the presence of a colon indicates IPv6. An agent generating
local candidates MUST NOT use FQDN addresses. An agent processing remote
candidates MUST ignore "candidate" lines that include candidates with
FQDNs or IP address versions that are not supported or recognized.
The procedures for generation and handling of FQDN candidates, as well as,
how agents indicate support for such procedures, need to be specified in an
extension specification.
<port>:
is also taken from RFC 4566 .
It is the port of the candidate.
<transport>:
indicates the transport protocol for the candidate.
This specification only defines UDP. However, extensibility is provided to allow for
future transport protocols to be used with ICE by extending the subregistry
"ICE Transport Protocols" under the "Interactive Connectivity Establishment (ICE)" registry.
<foundation>:
is composed of 1 to 32 <ice-char>s.
It is an identifier that is equivalent for two candidates that are of the same type,
share the same base, and come from the same STUN server.
The foundation is used to optimize ICE performance in the Frozen algorithm as
described in .
<component-id>:
is a positive integer between 1 and 256 (inclusive) that
identifies the specific component of the data stream for which this is a candidate.
It MUST start at 1 and MUST increment by 1 for each component of a particular candidate.
For data streams based on RTP, candidates for the actual RTP media MUST have a component
ID of 1, and candidates for RTCP MUST have a component ID of 2.
See for additional discussion on extending ICE to new data streams.
<priority>:
is a positive integer between 1 and (2**31 - 1) inclusive. The procedures
for computing a candidate's priority are described in .
<cand-type>:
encodes the type of candidate.
This specification defines the values "host", "srflx", "prflx", and "relay" for host,
server-reflexive, peer-reflexive, and relayed candidates, respectively.
Specifications for new candidate types MUST define how, if at all, various steps in the ICE
processing differ from the ones defined by this specification.
<rel-addr> and <rel-port>:
convey transport addresses related to the candidate, useful for diagnostics
and other purposes. <rel-addr> and <rel-port> MUST be present for
server-reflexive, peer-reflexive, and relayed candidates.
If a candidate is server-reflexive or peer-reflexive, <rel-addr> and <rel-port>
are equal to the base for that server-reflexive or peer-reflexive candidate.
If the candidate is relayed, <rel-addr> and <rel-port> are equal to
the mapped address in the Allocate response that provided the client with
that relayed candidate (see ).
If the candidate is a host candidate, <rel-addr> and <rel-port> MUST be omitted.
In some cases, e.g., for privacy reasons, an agent may not want to reveal the related
address and port. In this case the address MUST be set to "0.0.0.0" (for IPv4 candidates)
or "::" (for IPv6 candidates) and the port to "9".
The "candidate" attribute can itself be extended. The grammar allows for new name/value pairs
to be added at the end of the attribute. Such extensions MUST be made through IETF Review or
IESG Approval , and the assignments MUST contain the specific extension and a
reference to the document defining the usage of the extension.
An implementation MUST ignore any name/value pairs it doesn't understand.
The following is an example SDP line for a UDP server-reflexive "candidate" attribute for
the RTP component:
a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr
203.0.113.141 rport 8998
"remote-candidates" Attribute
The syntax of the "remote-candidates" attribute is defined using Augmented BNF
as defined in .
The "remote-candidates" attribute is a media-level attribute only.
remote-candidate-att = "remote-candidates:" remote-candidate
0*(SP remote-candidate)
remote-candidate = component-id SP connection-address SP port
The attribute contains a connection-address and port for each component. The ordering
of components is irrelevant. However, a value MUST be present for each component of a
data stream. This attribute MUST be included in an offer by a controlling agent for
a data stream that is Completed, and MUST NOT be included in any other case.
The following is an example of "remote-candidates" SDP lines for the RTP and RTCP components:
a=remote-candidates:1 192.0.2.3 45664
a=remote-candidates:2 192.0.2.3 45665
"ice-lite" and "ice-mismatch" Attributes
The syntax of the "ice-lite" and "ice-mismatch" attributes, both of which are flags, is:
ice-lite = "ice-lite"
ice-mismatch = "ice-mismatch"
"ice-lite" is a session-level attribute only, and indicates that an agent is a
lite implementation. "ice-mismatch" is a media-level attribute and only
reported in the answer. It indicates that the offer arrived with a default
destination for a media component that didn't have a corresponding "candidate"
attribute. Inclusion of "ice-mismatch" attribute for a given data stream implies
that even though both agents support ICE, ICE procedures MUST NOT be used for this data
stream, and procedures MUST be used instead.
"ice-ufrag" and "ice-pwd" Attributes
The "ice-ufrag" and "ice-pwd" attributes convey the username fragment and password used by ICE for message integrity.
Their syntax is:
ice-pwd-att = "ice-pwd:" password
ice-ufrag-att = "ice-ufrag:" ufrag
password = 22*256ice-char
ufrag = 4*256ice-char
The "ice-pwd" and "ice-ufrag" attributes can appear at either the session-level
or media-level. When present in both, the value in the media-level takes precedence.
Thus, the value at the session-level is effectively a default that applies to all
data streams, unless overridden by a media-level value. Whether present at the session
or media-level, there MUST be an "ice-pwd" and "ice-ufrag" attribute for each data stream.
If two data streams have identical "ice-ufrag"s, they MUST have identical "ice-pwd"s.
The "ice-ufrag" and "ice-pwd" attributes MUST be chosen randomly at the beginning of
a session (the same applies when ICE is restarting for an agent).
requires the "ice-ufrag" attribute to contain at least 24 bits of
randomness, and the "ice-pwd" attribute to contain at least 128 bits of
randomness. This means that the "ice-ufrag"
attribute will be at least 4 characters long, and the "ice-pwd" at least 22 characters long,
since the grammar for these attributes allows for 6 bits of information per character.
The attributes MAY be longer than 4 and 22 characters, respectively, of course, up to
256 characters. The upper limit allows for buffer sizing in implementations.
Its large upper limit allows for increased amounts of randomness to be added over time.
For compatibility with the 512-character limitation for the STUN username attribute value
and for bandwidth conservation considerations, the "ice-ufrag" attribute MUST NOT be longer
than 32 characters when sending, but an implementation MUST accept up to 256 characters
when receiving.
The following example shows sample "ice-ufrag" and "ice-pwd" SDP lines:
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
"ice-pacing" Attribute
The "ice-pacing" is a session-level attribute that indicates the desired connectivity-check pacing (Ta interval), in milliseconds, that the sender wishes to use. See
for more information regarding selecting a pacing value.
The syntax is:
ice-pacing-att = "ice-pacing:" pacing-value
pacing-value = 1*10DIGIT
If absent in an offer or answer, the default value of the attribute is 50 ms,
which is the recommended value specified in .
As defined in , regardless of the Ta value
chosen for each agent, the combination of all transactions from all agents (if a given
implementation runs several concurrent agents) will not be sent more often than
once every 5 ms.
As defined in , once both agents have
indicated the pacing value they want to use, both agents will use the larger of the
indicated values.
The following example shows an "ice-pacing" SDP line with value '50':
a=ice-pacing:50
"ice-options" Attribute
The "ice-options" attribute is a session-level and media-level attribute.
It contains a series of tokens that identify the options supported by the agent.
Its grammar is:
ice-options = "ice-options:" ice-option-tag
*(SP ice-option-tag)
ice-option-tag = 1*ice-char
The existence of an "ice-options" in an offer indicates that a certain extension
is supported by the agent, and it is willing to use it if the peer agent also includes
the same extension in the answer. There might be further extension-specific
negotiation needed between the agents that determine how the extension gets used
in a given session. The details of the negotiation procedures, if present, MUST be
defined by the specification defining the extension ().
The following example shows an "ice-options" SDP line with 'ice2' and
'rtp+ecn' values.
a=ice-options:ice2 rtp+ecn
Keepalives
All the ICE agents MUST follow the procedures defined in
for sending keepalives. As defined in ,
the keepalives will be sent regardless of whether the data stream is currently
inactive, sendonly, recvonly, or sendrecv, and regardless
of the presence or value of the bandwidth attribute. An agent can determine that its
peer supports ICE by the presence of "candidate" attributes for each media session.
SIP Considerations
Note that ICE is not intended for NAT traversal for SIP signaling, which is assumed to be
provided via another mechanism .
When ICE is used with SIP, forking may result in a single offer generating a
multiplicity of answers. In that case, ICE proceeds completely in parallel and
independently for each answer, treating the combination of its offer and
each answer as an independent offer/answer exchange, with its own set of local
candidates, pairs, checklists, states, and so on.
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.
Offer in INVITE
To reduce post-dial delays, it is RECOMMENDED that the caller begin gathering
candidates prior to actually sending its initial INVITE, so that the candidates
can be provided in the INVITE. This can be started upon
user interface cues that a call is pending, such as activity on a keypad or
the phone going off-hook.
On the receipt of the offer, the answerer SHOULD generate an answer in a
provisional response as soon as it has completed gathering
the candidates. ICE requires that a provisional response with an SDP be
transmitted reliably. This can be done through the existing
Provisional Response Acknowledgment (PRACK)
mechanism or through an ICE-specific optimization, wherein,
the agent retransmits the provisional response with the exponential backoff
timers described in . Such retransmissions MUST cease on receipt
of a STUN Binding request with the transport address matching the candidate address
for one of the data streams signaled in that SDP or on transmission of the answer
in a 2xx response. If no Binding request is received prior to the last retransmit,
the agent does not consider the session terminated. For the ICE-lite peers, the
agent MUST cease retransmitting the 18x response after
sending it four times since there will be no Binding request sent, and
the number four is arbitrarily chosen to limit the number of 18x retransmits.
Once the answer has been sent, the agent SHOULD begin its connectivity checks.
Once candidate pairs for each component of a data stream enter the valid list,
the answerer can begin sending media on that data stream.
However, prior to this point, any media that needs to be sent towards the
caller (such as SIP early media ) 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 . 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.
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 , ICE
agents SHOULD generate their offers in a reliable provisional response
(which MUST utilize ), 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.
SIP Option Tags and Media Feature Tags 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.
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.
Interactions with Preconditions
Quality of Service (QoS) preconditions, which are defined in
and , apply only to the transport addresses listed as the default
targets for media in an offer/answer.
If ICE changes the transport address where media is received, this change
is reflected in an updated offer that changes the default destination for
media to match ICE's selection. As such, it appears like any other re-INVITE would,
and is fully treated in RFCs 3312 and 4032, which apply without regard to the fact
that the destination for media is changing due to ICE negotiations occurring
"in the background".
Indeed, an agent SHOULD NOT indicate that QoS preconditions have been met
until the checks have completed and selected the candidate pairs to be used for media.
ICE also has interactions with connectivity preconditions .
Those interactions are described there.
Note that the procedures described in describe their own type of "preconditions", albeit with less functionality than those provided by the explicit preconditions in .
Interactions with Third Party Call Control
ICE works with Flows I, III, and IV as described in .
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 ,
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.
Interactions with Application Layer Gateways and SIP
Application Layer Gateways (ALGs) are functions present in a Network Address Translation (NAT)
device that inspect the contents of packets and modify them, in order to facilitate NAT traversal
for application protocols. Session Border Controllers (SBCs) are close cousins of ALGs, but are
less transparent since they actually exist as application-layer SIP intermediaries. ICE has
interactions with SBCs and ALGs.
If an ALG is SIP aware but not ICE aware, ICE will work through it as long as the ALG correctly
modifies the SDP. A correct ALG implementation behaves as follows:
The ALG does not modify the "m=" and "c=" lines or the "rtcp" attribute if they contain
external addresses.
If the "m=" and "c=" lines contain internal addresses, the modification depends on the state of the ALG:
If the ALG already has a binding established that maps an external port to an internal connection
address and port matching the values in the "m=" and "c=" lines or "rtcp" attribute, the ALG uses that
binding instead of creating a new one.
If the ALG does not already have a binding, it creates a new one and modifies the SDP, rewriting
the "m=" and "c=" lines and "rtcp" attribute.
Unfortunately, many ALGs are known to work poorly in these corner cases.
ICE does not try to work around broken ALGs, as this is outside the scope of its functionality.
ICE can help diagnose these conditions, which often show up as a mismatch between the set of candidates and
the "m=" and "c=" lines and "rtcp" attributes. The "ice-mismatch" attribute is used for this purpose.
ICE works best through ALGs when the signaling is run over TLS.
This prevents the ALG from manipulating the SDP messages and interfering with ICE operation.
Implementations that are expected to be deployed behind ALGs SHOULD provide for TLS transport of the SDP.
If an SBC is SIP aware but not ICE aware, the result depends on the behavior of the SBC.
If it is acting as a proper Back-to-Back User Agent (B2BUA), the SBC will remove any SDP attributes
it doesn't understand, including the ICE attributes. Consequently, the call will appear to both
endpoints as if the other side doesn't support ICE. This will result in ICE being disabled, and
media flowing through the SBC, if the SBC has requested it. If, however, the SBC passes the ICE attributes
without modification, yet modifies the default destination for media (contained in the "m=" and "c=" lines
and "rtcp" attribute), this will be detected as an ICE mismatch, and ICE processing is aborted for the call.
It is outside of the scope of ICE for it to act as a tool for "working around" SBCs.
If one is present, ICE will not be used and the SBC techniques take precedence.
Security Considerations
The generic ICE security considerations are defined in , and the
generic SDP offer/answer security considerations are defined in . These
security considerations also apply to implementations of this document.
IP Address Privacy
In some cases, e.g., for privacy reasons, an agent may not want to reveal the related
address and port. In this case the address MUST be set to "0.0.0.0" (for IPv4 candidates)
or "::" (for IPv6 candidates) and the port to '9'.
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
apply. These require techniques for message integrity and encryption
for offers and answers, which are satisfied by the TLS mechanism when
SIP is used. As such, the usage of TLS with ICE is RECOMMENDED.
The Voice Hammer Attack
The voice hammer attack is an amplification attack, and can be triggered even if the attacker
is an authenticated and valid participant in a session.
In this attack, the attacker initiates sessions to other agents, and maliciously includes
the connection address and port of a DoS target as the destination for media traffic
signaled in the SDP. This causes substantial amplification; a single offer/answer exchange
can create a continuing flood of media packets, possibly at high rates (consider video sources).
The use of ICE can help to prevent against this attack.
Specifically, if ICE is used, the agent receiving the malicious SDP will first perform connectivity
checks to the target of media before sending media there. If this target is a third-party host, the
checks will not succeed, and media is never sent. The ICE extension defined in
can be used to further protect against voice hammer attacks.
Unfortunately, ICE doesn't help if it's not used, in which case an attacker could simply
send the offer without the ICE parameters. However, in environments where the set of clients is known,
and is limited to ones that support ICE, the server can reject any offers or answers that don't
indicate ICE support.
SIP user agents (UA) 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
reject non-ICE offers will generally use a 421 response code, together with an option tag "ice" in the
Require header field in the response.
IANA ConsiderationsSDP Attributes
The original ICE specification defined seven new SDP attributes per the procedures of
.
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".
"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 of RFC 8839.
Contact Name:
IESG
Contact Email:
iesg@ietf.org
Reference:
RFC 8839
Mux Category:
TRANSPORT
"remote-candidates" Attribute
Attribute Name:
remote-candidates
Type of Attribute:
media-level
Subject to charset:
No
Purpose:
This attribute is used with Interactive Connectivity Establishment (ICE),
and provides the identity of the remote candidates that the offerer wishes the answerer
to use in its answer.
Appropriate Values:
See of RFC 8839.
Contact Name:
IESG
Contact Email:
iesg@ietf.org
Reference:
RFC 8839
Mux Category:
TRANSPORT
"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 of RFC 8839.
Contact Name:
IESG
Contact Email:
iesg@ietf.org
Reference:
RFC 8839
Mux Category:
NORMAL
"ice-mismatch" Attribute
Attribute Name:
ice-mismatch
Type of Attribute:
media-level
Subject to charset:
No
Purpose:
This attribute is used with Interactive Connectivity Establishment (ICE), and indicates that an agent is ICE capable, but did not proceed with ICE due to a mismatch of candidates with the default destination for media signaled in the SDP.
Appropriate Values:
See of RFC 8839.
Contact Name:
IESG
Contact e-mail:
iesg@ietf.org
Reference:
RFC 8839
Mux Category:
NORMAL
"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 of RFC 8839.
Contact Name:
IESG
Contact e-mail:
iesg@ietf.org
Reference:
RFC 8839
Mux Category:
TRANSPORT
"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 of RFC 8839.
Contact Name:
IESG
Contact e-mail:
iesg@ietf.org
Reference:
RFC 8839
Mux Category:
TRANSPORT
"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 of RFC 8839.
Contact Name:
IESG
Contact e-mail:
iesg@ietf.org
Reference:
RFC 8839
Mux Category:
NORMAL
"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 of RFC 8839.
Contact Name:
IESG
Contact e-mail:
iesg@ietf.org
Reference:
RFC 8839
Mux Category:
NORMAL
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" .
ICE options are of unlimited length according to the syntax in
; however, they are RECOMMENDED to be no longer
than 20 characters. This is to reduce message sizes and allow for
efficient parsing. ICE options are defined at the session level.
A registration request MUST include the following information:
The ICE option identifier to be registered
Name and email address of organization or individuals having change control
Short description of the ICE extension to which the option relates
Reference(s) to the specification defining the ICE option and the related extensions
Candidate Attribute Extension Subregistry Establishment
This section creates a new subregistry, "Candidate Attribute
Extensions", under the SDP Parameters
registry: .
The purpose of the subregistry is to register SDP "candidate" attribute extensions.
When a "candidate" extension is registered in the subregistry, it needs to meet the "Specification Required"
policies defined in .
"candidate" attribute extensions MUST follow the 'cand-extension' syntax. The attribute extension
name MUST follow the 'extension-att-name' syntax, and the attribute extension value MUST follow the
'extension-att-value' syntax.
A registration request MUST include the following information:
The name of the attribute extension.
Name and email address of organization or individuals having change control
A short description of the attribute extension.
A reference to a specification that describes the semantics, usage and possible
values of the attribute extension.
Changes from RFC 5245 describes the changes made to the
common SIP procedures, including removal of aggressive nomination,
modifying the procedures for calculating candidate pair states,
scheduling connectivity checks, and the calculation of timer values.
This document defines the following SDP offer/answer specific changes:
SDP offer/answer realization and usage of 'ice2' option.
Definition and usage of SDP "ice-pacing" attribute.
Explicit text that an ICE agent must not generate candidates with FQDNs, and
must discard such candidates if received from the peer agent.
Relax requirement to include SDP "rtcp" attribute.
Generic clarifications of SDP offer/answer procedures.
ICE mismatch is now optional, and an agent has an option to not
trigger mismatch and instead treat the default candidate as an
additional candidate.
FQDNs and "0.0.0.0"/"::" IP addresses with port "9" default
candidates do not trigger ICE mismatch.
ReferencesNormative ReferencesKey words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.SIP: Session Initiation ProtocolThis document describes Session Initiation Protocol (SIP), an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions include Internet telephone calls, multimedia distribution, and multimedia conferences. [STANDARDS-TRACK]Reliability of Provisional Responses in Session Initiation Protocol (SIP)This document specifies an extension to the Session Initiation Protocol (SIP) providing reliable provisional response messages. This extension uses the option tag 100rel and defines the Provisional Response ACKnowledgement (PRACK) method. [STANDARDS-TRACK]An Offer/Answer Model with Session Description Protocol (SDP)This document defines a mechanism by which two entities can make use of the Session Description Protocol (SDP) to arrive at a common view of a multimedia session between them. In the model, one participant offers the other a description of the desired session from their perspective, and the other participant answers with the desired session from their perspective. This offer/answer model is most useful in unicast sessions where information from both participants is needed for the complete view of the session. The offer/answer model is used by protocols like the Session Initiation Protocol (SIP). [STANDARDS-TRACK]Integration of Resource Management and Session Initiation Protocol (SIP)Session Description Protocol (SDP) Bandwidth Modifiers for RTP Control Protocol (RTCP) BandwidthThis document defines an extension to the Session Description Protocol (SDP) to specify two additional modifiers for the bandwidth attribute. These modifiers may be used to specify the bandwidth allowed for RTP Control Protocol (RTCP) packets in a Real-time Transport Protocol (RTP) session. [STANDARDS-TRACK]Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)The Session Description Protocol (SDP) is used to describe the parameters of media streams used in multimedia sessions. When a session requires multiple ports, SDP assumes that these ports have consecutive numbers. However, when the session crosses a network address translation device that also uses port mapping, the ordering of ports can be destroyed by the translation. To handle this, we propose an extension attribute to SDP.Update to the Session Initiation Protocol (SIP) Preconditions FrameworkThis document updates RFC 3312, which defines the framework for preconditions in SIP. We provide guidelines for authors of new precondition types and describe how to use SIP preconditions in situations that involve session mobility. [STANDARDS-TRACK]SDP: Session Description ProtocolThis memo defines the Session Description Protocol (SDP). SDP is intended for describing multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation. [STANDARDS-TRACK]Augmented BNF for Syntax Specifications: ABNFInternet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK]Session Traversal Utilities for NAT (STUN)Session Traversal Utilities for NAT (STUN) is a protocol that serves as a tool for other protocols in dealing with Network Address Translator (NAT) traversal. It can be used by an endpoint to determine the IP address and port allocated to it by a NAT. It can also be used to check connectivity between two endpoints, and as a keep-alive protocol to maintain NAT bindings. STUN works with many existing NATs, and does not require any special behavior from them.STUN is not a NAT traversal solution by itself. Rather, it is a tool to be used in the context of a NAT traversal solution. This is an important change from the previous version of this specification (RFC 3489), which presented STUN as a complete solution.This document obsoletes RFC 3489. [STANDARDS-TRACK]Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN)If a host is located behind a NAT, then in certain situations it can be impossible for that host to communicate directly with other hosts (peers). In these situations, it is necessary for the host to use the services of an intermediate node that acts as a communication relay. This specification defines a protocol, called TURN (Traversal Using Relays around NAT), that allows the host to control the operation of the relay and to exchange packets with its peers using the relay. TURN differs from some other relay control protocols in that it allows a client to communicate with multiple peers using a single relay address. [STANDARDS-TRACK]Indicating Support for Interactive Connectivity Establishment (ICE) in the Session Initiation Protocol (SIP)This specification defines a media feature tag and an option tag for use with the Session Initiation Protocol (SIP). The media feature tag allows a User Agent (UA) to communicate to its registrar that it supports ICE. The option tag allows a UA to require support for ICE in order for a call to proceed. [STANDARDS-TRACK]IANA Registry for Interactive Connectivity Establishment (ICE) OptionsIt has been identified that "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols" (RFC 5245) is missing a registry for ICE options. This document defines this missing IANA registry and updates RFC 5245. [STANDARDS-TRACK]Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) TraversalThis document describes a protocol for Network Address Translator (NAT) traversal for UDP-based communication. This protocol is called Interactive Connectivity Establishment (ICE). ICE makes use of the Session Traversal Utilities for NAT (STUN) protocol and its extension, Traversal Using Relay NAT (TURN).This document obsoletes RFC 5245.Informative ReferencesBest Current Practices for Third Party Call Control (3pcc) in the Session Initiation Protocol (SIP)Third party call control refers to the ability of one entity to create a call in which communication is actually between other parties. Third party call control is possible using the mechanisms specified within the Session Initiation Protocol (SIP). However, there are several possible approaches, each with different benefits and drawbacks. This document discusses best current practices for the usage of SIP for third party call control. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Early Media and Ringing Tone Generation in the Session Initiation Protocol (SIP)This document describes how to manage early media in the Session Initiation Protocol (SIP) using two models: the gateway model and the application server model. It also describes the inputs one needs to consider in defining local policies for ringing tone generation. This memo provides information for the Internet community.Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer ProtocolsThis document describes a protocol for Network Address Translator (NAT) traversal for UDP-based multimedia sessions established with the offer/answer model. This protocol is called Interactive Connectivity Establishment (ICE). ICE makes use of the Session Traversal Utilities for NAT (STUN) protocol and its extension, Traversal Using Relay NAT (TURN). ICE can be used by any protocol utilizing the offer/answer model, such as the Session Initiation Protocol (SIP). [STANDARDS-TRACK]Managing Client-Initiated Connections in the Session Initiation Protocol (SIP)The Session Initiation Protocol (SIP) allows proxy servers to initiate TCP connections or to send asynchronous UDP datagrams to User Agents in order to deliver requests. However, in a large number of real deployments, many practical considerations, such as the existence of firewalls and Network Address Translators (NATs) or the use of TLS with server-provided certificates, prevent servers from connecting to User Agents in this way. This specification defines behaviors for User Agents, registrars, and proxy servers that allow requests to be delivered on existing connections established by the User Agent. It also defines keep-alive behaviors needed to keep NAT bindings open and specifies the usage of multiple connections from the User Agent to its registrar. [STANDARDS-TRACK]Connectivity Preconditions for Session Description Protocol (SDP) Media StreamsThis document defines a new connectivity precondition for the Session Description Protocol (SDP) precondition framework. A connectivity precondition can be used to delay session establishment or modification until media stream connectivity has been successfully verified. The method of verification may vary depending on the type of transport used for the media. For unreliable datagram transports such as UDP, verification involves probing the stream with data or control packets. For reliable connection-oriented transports such as TCP, verification can be achieved simply by successful connection establishment or by probing the connection with data or control packets, depending on the situation. [STANDARDS-TRACK]Explicit Congestion Notification (ECN) for RTP over UDPThis memo specifies how Explicit Congestion Notification (ECN) can be used with the Real-time Transport Protocol (RTP) running over UDP, using the RTP Control Protocol (RTCP) as a feedback mechanism. It defines a new RTCP Extended Report (XR) block for periodic ECN feedback, a new RTCP transport feedback message for timely reporting of congestion events, and a Session Traversal Utilities for NAT (STUN) extension used in the optional initialisation method using Interactive Connectivity Establishment (ICE). Signalling and procedures for negotiation of capabilities and initialisation methods are also defined. [STANDARDS-TRACK]Session Traversal Utilities for NAT (STUN) Usage for Consent FreshnessTo prevent WebRTC applications, such as browsers, from launching attacks by sending traffic to unwilling victims, periodic consent to send needs to be obtained from remote endpoints.This document describes a consent mechanism using a new Session Traversal Utilities for NAT (STUN) usage.Guidelines for Writing an IANA Considerations Section in RFCsMany protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA).To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry.This is the third edition of this document; it obsoletes RFC 5226.A Framework for Session Description Protocol (SDP) Attributes When MultiplexingInteractive Connectivity Establishment Patiently Awaiting Connectivity (ICE PAC)Examples
For the example shown in ,
the resulting offer (message 5) encoded in SDP looks like (lines folded for clarity):
v=0
o=jdoe 2890844526 2890842807 IN IP6 $L-PRIV-1.IP
s=
c=IN IP6 $NAT-PUB-1.IP
t=0 0
a=ice-options:ice2
a=ice-pacing:50
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio $NAT-PUB-1.PORT RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 $L-PRIV-1.IP $L-PRIV-1.PORT typ host
a=candidate:2 1 UDP 1694498815 $NAT-PUB-1.IP $NAT-PUB-1.PORT typ
srflx raddr $L-PRIV-1.IP rport $L-PRIV-1.PORT
The offer, with the variables replaced with their values, will look like (lines folded for clarity):
v=0
o=jdoe 2890844526 2890842807 IN IP6 fe80::6676:baff:fe9c:ee4a
s=
c=IN IP6 2001:db8:8101:3a55:4858:a2a9:22ff:99b9
t=0 0
a=ice-options:ice2
a=ice-pacing:50
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 45664 RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 fe80::6676:baff:fe9c:ee4a 8998
typ host
a=candidate:2 1 UDP 1694498815 2001:db8:8101:3a55:4858:a2a9:22ff:99b9
45664 typ srflx raddr fe80::6676:baff:fe9c:ee4a rport 8998
The resulting answer looks like:
v=0
o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP
s=
c=IN IP4 $R-PUB-1.IP
t=0 0
a=ice-options:ice2
a=ice-pacing:50
a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
a=ice-ufrag:9uB6
m=audio $R-PUB-1.PORT RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 $R-PUB-1.IP $R-PUB-1.PORT typ host
With the variables filled in:
v=0
o=bob 2808844564 2808844564 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=ice-options:ice2
a=ice-pacing:50
a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
a=ice-ufrag:9uB6
m=audio 3478 RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 192.0.2.1 3478 typ host
The "remote-candidates" Attribute
The "remote-candidates" attribute exists to eliminate a race condition between the updated offer and the response to the STUN Binding request that moved a candidate into the valid list.
This race condition is shown in .
On receipt of message 4, agent L adds a candidate pair to the valid list.
If there was only a single data stream with a single component, agent L could now send an updated offer.
However, the check from agent R has not yet
received a response, and agent R receives the updated offer
(message 7) before getting the response (message 9).
Thus, it does not yet know that this particular pair is valid.
To eliminate this condition, the actual candidates at R that were selected by the offerer (the remote candidates) are included in the offer itself, and the answerer delays its answer until those pairs validate.
Race Condition Flow
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 | |
|<------------------------------------------|
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:
Role Conflict Flow
A Controller B
|(1) INV() | |
|<-------------| |
|(2) 200(SDP1) | |
|------------->| |
| |(3) INV() |
| |------------->|
| |(4) 200(SDP2) |
| |<-------------|
|(5) ACK(SDP2) | |
|<-------------| |
| |(6) ACK(SDP1) |
| |------------->|
This flow is a variation on flow III of RFC 3725 .
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.
Why Send an Updated Offer?
describes rules for sending media.
Both agents can send media once ICE checks complete, without waiting for an updated offer.
Indeed, the only purpose of the updated offer is to "correct" the SDP so that the default destination for media matches where media is being sent based on ICE procedures (which will be the highest-priority nominated candidate pair).
This raises the question -- why is the updated offer/answer exchange needed at all?
Indeed, in a pure offer/answer environment, it would not be.
The offerer and answerer will agree on the candidates to use through ICE, and then can begin using them.
As far as the agents themselves are concerned, the updated offer/answer provides no new information.
However, in practice, numerous components along the signaling path look at the SDP information.
These include entities performing off-path QoS reservations, NAT traversal components such as ALGs and Session Border Controllers (SBCs), and diagnostic tools that passively monitor the network.
For these tools to continue to function without change, the core property of SDP -- that the existing, pre-ICE definitions of the addresses used for media -- the "m=" and "c=" lines and the "rtcp" attribute -- must be retained.
For this reason, an updated offer must be sent.
Acknowledgements
A large part of the text in this document was taken from , authored by .
Some of the text in this document was taken from ,
authored by and .
Many thanks to for shepherd review feedback.
Thanks to following experts for their reviews and constructive feedback:
, ,
, ,
, ,
, , and
for their detailed reviews.
Contributors
The following experts have contributed textual and structural improvements for this work:
thomass.stach@gmail.comAuthors' AddressesImpedance Mismatchmarc@petit-huguenin.orgCisco Systems707 Tasman DrMilpitasCA95035United States of Americasnandaku@cisco.comEricssonHirsalantie 11Jorvas02420Finlandchrister.holmberg@ericsson.comEricssonJorvas02420Finlandari.keranen@ericsson.comTurboBridge4905 Del Ray Avenue, Suite 300BethesdaMD20814United States of Americarshpount@turbobridge.com