Trickle ICE: Incremental Provisioning of Candidates for the Interactive Connectivity Establishment (ICE) Protocol8x8, Inc. / Jitsi675 Creekside WayCampbellCA95008United States of America+1 512 420 6968emcho@jitsi.orgGoogle747 6th Street SKirklandWA98033United States of America+1 857 288 8888justin@uberti.nameMozillaP.O. Box 787ParkerCO80134United States of America+1 720 256 6756stpeter@mozilla.comhttps://www.mozilla.com/
This document describes "Trickle ICE", an extension to the Interactive
Connectivity Establishment (ICE) protocol that enables ICE agents
to begin connectivity checks while they are still gathering
candidates, by incrementally exchanging candidates over time instead
of all at once. This method can considerably accelerate the process
of establishing a communication session.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by
the Internet Engineering Steering Group (IESG). Further
information on Internet Standards is available in Section 2 of
RFC 7841.
Information about the current status of this document, any
errata, and how to provide feedback on it may be obtained at
.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
() in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
respect to this document. Code Components extracted from this
document must include Simplified BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Simplified BSD License.
Table of Contents
. Introduction
. Terminology
. Determining Support for Trickle ICE
. Generating the Initial ICE Description
. Handling the Initial ICE Description and Generating the Initial ICE Response
. Handling the Initial ICE Response
. Forming Checklists
. Performing Connectivity Checks
. Gathering and Conveying Newly Gathered Local Candidates
. Pairing Newly Gathered Local Candidates
. Receiving Trickled Candidates
. Inserting Trickled Candidate Pairs into a Checklist
. Generating an End-of-Candidates Indication
. Receiving an End-of-Candidates Indication
. Subsequent Exchanges and ICE Restarts
. Half Trickle
. Preserving Candidate Order While Trickling
. Requirements for Using Protocols
. IANA Considerations
. Security Considerations
. References
. Normative References
. Informative References
. Interaction with Regular ICE
. Interaction with ICE-Lite
Acknowledgements
Authors' Addresses
Introduction
The Interactive Connectivity Establishment (ICE) protocol
describes how an ICE agent
gathers candidates, exchanges candidates with a peer ICE
agent, and creates candidate pairs. Once the pairs have been
gathered, the ICE agent will perform connectivity checks and
eventually nominate and select pairs that will be used for
sending and receiving data within a communication session.
Following the procedures in
can lead to somewhat lengthy establishment times for communication
sessions, because candidate gathering often involves querying Session
Traversal Utilities for NAT (STUN) servers and allocating relayed candidates on Traversal
Using Relay NAT (TURN) servers . Although many ICE procedures can be completed in
parallel, the pacing requirements from still need to be followed.
This document defines "Trickle ICE", a supplementary mode of ICE
operation in which candidates can be exchanged
incrementally as soon as they become available (and simultaneously
with the gathering of other candidates). Connectivity checks can
also start as soon as candidate pairs have been created. Because
Trickle ICE enables candidate gathering and connectivity checks
to be done in parallel, the method can considerably accelerate
the process of establishing a communication session.
This document also defines how to discover support for
Trickle ICE, how the procedures in are
modified or supplemented when using Trickle ICE, and how a Trickle
ICE agent can interoperate with an ICE agent compliant to
.
This document does not define any protocol-specific usage of Trickle
ICE. Instead, protocol-specific details for Trickle ICE are defined
in separate usage documents.
Examples of such documents are
(which defines usage
with the Session Initiation Protocol (SIP)
and the Session Description Protocol (SDP) ) and
(which defines usage with the Extensible Messaging and Presence Protocol (XMPP)
). However, some of the examples in the
document use SDP and the Offer/Answer model
to explain the underlying concepts.
The following diagram illustrates a successful Trickle ICE exchange with a
using protocol that follows the Offer/Answer model:
Flow
Alice Bob
| Offer |
|---------------------------------------------->|
| Additional Candidates |
|---------------------------------------------->|
| Answer |
|<----------------------------------------------|
| Additional Candidates |
|<----------------------------------------------|
| Additional Candidates and Connectivity Checks |
|<--------------------------------------------->|
|<========== CONNECTION ESTABLISHED ===========>|
The main body of this document is structured to describe the behavior
of Trickle ICE agents in roughly the order of operations and interactions
during an ICE session:
Determining support for Trickle ICE
Generating the initial ICE description
Handling the initial ICE description and generating the initial ICE response
Handling the initial ICE response
Forming checklists, pruning candidates, performing connectivity checks, etc.
Gathering and conveying candidates after the initial ICE description and response
Handling inbound trickled candidates
Generating and handling the end-of-candidates indication
Handling ICE restarts
There is quite a bit of operational experience with the technique behind
Trickle ICE, going back as far as 2005 (when the XMPP Jingle extension
defined a "dribble mode" as specified in ); this
document incorporates feedback from those who have implemented and
deployed the technique over the years.
Terminology
The key words "MUST", "MUST NOT",
"REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT",
"RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be
interpreted as described in BCP 14 when, and only when, they appear in all capitals, as
shown here.
This specification makes use of all terminology defined
for Interactive Connectivity Establishment in
. In addition, it defines the following terms:
Empty Checklist:
A checklist that initially does not contain any candidate pairs
because they will be incrementally added as they are trickled.
(This scenario does not arise with a regular ICE agent, because all
candidate pairs are known when the agent creates the checklist set.)
Full Trickle:
The typical mode of operation for Trickle ICE agents, in which
the initial ICE description can include any number of candidates (even
zero candidates) and does not need to include a full generation
of candidates as in half trickle.
Generation:
All of the candidates conveyed within an ICE session (correlated
with a particular Username Fragment and Password combination).
Half Trickle:
A Trickle ICE mode of operation in which the initiator gathers
a full generation of candidates strictly before creating
and conveying the initial ICE description. Once conveyed,
this candidate information can be
processed by regular ICE agents, which do not require support
for Trickle ICE. It also allows Trickle-ICE-capable
responders to still gather candidates and perform
connectivity checks in a non-blocking way, thus providing roughly
"half" the advantages of Trickle ICE. The half-trickle mechanism
is mostly meant for use when the responder's support for Trickle
ICE cannot be confirmed prior to conveying the initial ICE description.
ICE Description:
Any attributes related to the ICE session (other than candidates)
required to configure an ICE agent. These include but are not
limited to the Username Fragment, the Password, and other attributes.
Trickled Candidates:
Candidates that a Trickle ICE agent conveys after conveying or responding to the initial
ICE description, but within
the same ICE session. Trickled candidates can be conveyed in
parallel with candidate gathering and connectivity checks.
Trickling:
The act of incrementally conveying trickled candidates.
Determining Support for Trickle ICE
To fully support Trickle ICE, using protocols
SHOULD incorporate one of the following mechanisms so that implementations
can determine whether Trickle ICE is supported:
Provide a capabilities discovery method so that agents can verify
support of Trickle ICE prior to initiating a session (XMPP's
Service Discovery is
one such mechanism).
Make support for Trickle ICE mandatory so that user agents
can assume support.
If a using protocol does not provide a method of determining
ahead of time whether Trickle ICE is supported, agents can make use of
the half-trickle procedure described in .
Prior to conveying the initial ICE description, agents that implement using protocols
that support capabilities discovery can attempt to verify whether or
not the remote party supports Trickle ICE. If an agent determines
that the remote party does not support Trickle ICE, it MUST fall back
to using regular ICE or abandon the entire session.
Even if a using protocol does not include a capabilities discovery
method, a user agent can provide an indication within the ICE description
that it supports Trickle ICE by communicating an ICE option of 'trickle'.
This token MUST be provided either at the session level or, if at the data
stream level, for every data stream (an agent MUST NOT specify Trickle ICE
support for some data streams but not others).
Note: The encoding of the 'trickle' ICE option, and the message(s) used to
carry it to the peer, are protocol specific; for instance, the encoding for
SDP is defined in
.
Dedicated discovery semantics and half trickle are needed only prior
to initiation of an ICE session. After an ICE session is established
and Trickle ICE support is confirmed for both parties, either
agent can use full trickle for subsequent exchanges (see also
).
Generating the Initial ICE Description
An ICE agent can start gathering candidates as soon as it has an
indication that communication is imminent (e.g., a user-interface
cue or an explicit request to initiate a communication session). Unlike in
regular ICE, in Trickle ICE implementations do not need to
gather candidates in a blocking manner. Therefore, unless half
trickle is being used, the user experience is improved if the
initiating agent generates and transmits its initial ICE description
as early as possible (thus enabling the remote party to start
gathering and trickling candidates).
An initiator MAY include any mix of candidates when conveying
the initial ICE description. This includes the possibility of conveying
all the candidates the initiator plans to use
(as in half trickle), conveying only a
publicly reachable IP address (e.g., a candidate at a data
relay that is known to not be behind a firewall), or conveying
no candidates at all (in which case the initiator can obtain the
responder's initial candidate list sooner, and the responder can begin
candidate gathering more quickly).
For candidates included in the initial ICE description, the methods
for calculating priorities and foundations, determining redundancy
of candidates, and the like work just as in regular ICE
.
Handling the Initial ICE Description and Generating the Initial ICE Response
When a responder receives the initial ICE description, it will first check if
the ICE description or initiator indicates support for Trickle ICE as explained
in . If not, the responder MUST
process the initial ICE description according to regular ICE procedures
(or, if no ICE support is detected at all,
according to relevant processing rules for the using
protocol, such as Offer/Answer processing rules ).
However, if support for Trickle ICE is confirmed, a responder will
automatically assume support for regular ICE as well.
If the initial ICE description indicates support for Trickle ICE, the
responder will determine its role and start gathering and prioritizing
candidates; while doing so, it will also respond by conveying an
initial ICE response, so that both the initiator
and the responder can form checklists and begin connectivity checks.
A responder can respond to the initial ICE description at any point while
gathering candidates. The initial ICE response MAY contain any set of
candidates, including all candidates or no candidates. (The benefit of
including no candidates is to convey the initial ICE response as
quickly as possible, so that both parties can consider the
ICE session to be under active negotiation as soon as
possible.)
As noted in , in using protocols that use
SDP, the initial ICE response can indicate support for Trickle ICE
by including a token of 'trickle' in the ice-options attribute.
Handling the Initial ICE Response
When processing the initial ICE response, the initiator follows regular ICE
procedures to determine its role, after which it
forms checklists ()
and performs connectivity checks ().
Forming Checklists
According to regular ICE procedures ,
in order for candidate pairing
to be possible and for redundant candidates to be pruned, the
candidates would need to be provided in the initial ICE description
and initial ICE response.
By contrast, under Trickle ICE, checklists can be empty until
candidates are conveyed or received. Therefore, a Trickle ICE agent
handles checklist formation and candidate pairing in a slightly different
way than a regular ICE agent: the agent still forms the checklists, but
it populates a given checklist only after it actually has candidate
pairs for that checklist. Every checklist is initially placed in the
Running state, even if the checklist is empty (this is consistent
with ).
Performing Connectivity Checks
As specified in , whenever timer
Ta fires, only checklists in the Running state will be picked
when scheduling connectivity checks for candidate pairs.
Therefore, a Trickle ICE agent MUST keep each checklist in
the Running state as long as it expects candidate pairs to be
incrementally added to the checklist. After that, the checklist
state is set according to the procedures in
.
Whenever timer Ta fires and an empty checklist is picked, no action
is performed for the list. Without waiting for timer Ta to expire
again, the agent selects the next checklist in the Running state,
in accordance with .
requires that agents update checklists and timer states upon
completing a connectivity check transaction. During such an
update, regular ICE agents would set the state of a checklist
to Failed if both of the following two conditions are satisfied:
all of the pairs in the checklist are in either the
Failed state or the Succeeded state; and
there is not a pair in the valid list for each component
of the data stream.
With Trickle ICE, the above situation would often occur when
candidate gathering and trickling are still in progress, even
though it is quite possible that future checks will succeed. For
this reason, Trickle ICE agents add the following conditions to
the above list:
all candidate gathering has completed, and the agent
is not expecting to discover any new local candidates; and
the remote agent has conveyed an end-of-candidates indication
for that checklist as described in
.
Gathering and Conveying Newly Gathered Local Candidates
After Trickle ICE agents have conveyed initial ICE descriptions
and initial ICE responses, they will most
likely continue gathering new local candidates as STUN, TURN,
and other non-host candidate gathering mechanisms begin to
yield results. Whenever an agent discovers such a new candidate,
it will compute its priority, type, foundation, and component ID
according to regular ICE procedures.
The new candidate is then checked for redundancy against the
existing list of local candidates. If its transport address and
base match those of an existing candidate, it will be considered
redundant and will be ignored. This would often happen for
server-reflexive candidates that match the host addresses they
were obtained from (e.g., when the latter are public IPv4
addresses). Contrary to regular ICE, Trickle ICE agents will
consider the new candidate redundant regardless of its priority.
Next, the agent "trickles" the newly discovered
candidate(s) to the remote agent. The actual delivery of the new
candidates is handled by a using protocol such as SIP or XMPP.
Trickle ICE imposes no restrictions on the way this is done
(e.g., some using protocols might
choose not to trickle updates for server-reflexive
candidates and instead rely on the discovery of peer-reflexive ones).
When candidates are trickled, the using protocol MUST deliver each
candidate (and any end-of-candidates indication as described in
) to the receiving Trickle ICE implementation
exactly once
and in the same order it was conveyed. If the using protocol
provides any candidate retransmissions, they need to be hidden
from the ICE implementation.
Also, candidate trickling needs to be correlated to a specific
ICE session, so that if there is an ICE restart, any
delayed updates for a previous session can be recognized as such
and ignored by the receiving party. For example, using protocols
that signal candidates via SDP might include a Username
Fragment value in the corresponding a=candidate line, such as:
a=candidate:1 1 UDP 2130706431 2001:db8::1 5000 typ host ufrag 8hhY
Or, as another example, WebRTC implementations might include a Username
Fragment in the JavaScript objects that represent candidates.
Note: The using protocol needs to provide a mechanism for both
parties to indicate and agree on the ICE session in force
(as identified by the Username Fragment and Password combination),
so that they have a consistent view of which candidates are
to be paired. This is especially important in the case of ICE
restarts (see ).
Note: A using protocol might prefer not to
trickle server-reflexive candidates to entities that are known
to be publicly accessible and where sending a direct STUN
binding request is likely to reach the destination faster than
the trickle update that travels through the signaling path.
Pairing Newly Gathered Local Candidates
As a Trickle ICE agent gathers local candidates, it needs
to form candidate pairs; this works as described in
the ICE specification , with the
following provisos:
A Trickle ICE agent MUST NOT pair a local candidate until it
has been trickled to the remote party.
Once the agent has conveyed the local candidate to the remote
party, the agent checks if any remote candidates are currently
known for this same stream and component. If not, the agent
merely adds the new candidate to the list of local candidates
(without pairing it).
Otherwise, if the agent has already learned of one or more
remote candidates for this stream and component, it attempts
to pair the new local candidate as described in the ICE
specification .
If a newly formed pair has a local candidate whose type is server-reflexive,
the agent MUST replace the local candidate with its
base before completing the relevant redundancy tests.
The agent prunes redundant pairs by following the rules
in but checks
existing pairs only if they have a state of Waiting or Frozen;
this avoids removal of pairs for which connectivity checks are
in flight (a state of In‑Progress) or for which connectivity
checks have already yielded a definitive result (a state of
Succeeded or Failed).
If, after completing the relevant redundancy tests, the checklist where the
pair is to be added already contains the maximum number of candidate
pairs (100 by default as per ), the agent
SHOULD discard any pairs in the Failed state to make room for the
new pair. If there are no such pairs, the agent SHOULD discard a
pair with a lower priority than the new pair in order to make room
for the new pair, until the number of pairs is equal to the maximum
number of pairs. This processing is consistent with
.
Receiving Trickled Candidates
At any time during an ICE session, a Trickle ICE agent might receive
new candidates from the remote agent, from which it will attempt to
form a candidate pair; this works as described in the ICE specification
, with the following provisos:
The agent checks if any local candidates are currently known for
this same stream and component. If not, the agent merely adds the
new candidate to the list of remote candidates (without pairing it).
Otherwise, if the agent has already gathered one or more
local candidates for this stream and component, it attempts
to pair the new remote candidate as described in the ICE
specification .
If a newly formed pair has a local candidate whose type is server-reflexive, the agent MUST replace the local candidate with its
base before completing the redundancy check in the next step.
The agent prunes redundant pairs as described below but checks
existing pairs only if they have a state of Waiting or Frozen;
this avoids removal of pairs for which connectivity checks are
in flight (a state of In-Progress) or for which connectivity
checks have already yielded a definitive result (a state of
Succeeded or Failed).
If the agent finds a redundancy between two pairs and one of
those pairs contains a newly received remote candidate whose
type is peer-reflexive, the agent SHOULD discard the
pair containing that candidate, set the priority of the
existing pair to the priority of the discarded pair, and
re-sort the checklist.
(This policy helps to eliminate
problems with remote peer-reflexive candidates for which
a STUN Binding request is received before signaling of the
candidate is trickled to the receiving agent, such as a
different view of pair priorities between the local agent
and the remote agent, because the same candidate could be
perceived as peer-reflexive by one agent and as server-reflexive
by the other agent.)
The agent then applies the rules defined in
.
If, after completing the relevant redundancy tests, the checklist where the
pair is to be added already contains the maximum number of candidate
pairs (100 by default as per ), the agent
SHOULD discard any pairs in the Failed state to make room for the
new pair. If there are no such pairs, the agent SHOULD discard a
pair with a lower priority than the new pair in order to make room
for the new pair, until the number of pairs is equal to the maximum
number of pairs. This processing is consistent with
.
Inserting Trickled Candidate Pairs into a Checklist
After a local agent has trickled a candidate and formed a candidate
pair from that local candidate (), or after
a remote agent has received a trickled candidate and formed a candidate
pair from that remote candidate (), a Trickle
ICE agent adds the new candidate pair to a checklist as defined in
this section.
As an aid to understanding the procedures defined in this section,
consider the following tabular representation of all checklists in
an agent (note that initially for one of the foundations, i.e., f5,
there are no candidate pairs):
Example of Checklist State
f1
f2
f3
f4
f5
s1 (Audio.RTP)
F
F
F
s2 (Audio.RTCP)
F
F
F
F
s3 (Video.RTP)
F
s4 (Video.RTCP)
F
Each row in the table represents a component for a given data
stream (e.g., s1 and s2 might be the RTP and RTP Control Protocol (RTCP) components
for audio) and thus a single checklist in the checklist set.
Each column represents one foundation. Each cell represents one
candidate pair. In the tables shown in this section, "F" stands
for "frozen", "W" stands for "waiting", and "S" stands for
"succeeded"; in addition, "^^" is used to notate newly added
candidate pairs.
When an agent commences ICE processing, in accordance with
, for each
foundation it will unfreeze the pair with the lowest component
ID and, if the component IDs are equal, with the highest priority
(this is the topmost candidate pair in every column).
This initial state is shown in the following table.
Initial Checklist State
f1
f2
f3
f4
f5
s1 (Audio.RTP)
W
W
W
s2 (Audio.RTCP)
F
F
F
W
s3 (Video.RTP)
F
s4 (Video.RTCP)
F
Then, as the checks proceed (see
), for each pair
that enters the Succeeded state (denoted here by "S"),
the agent will unfreeze all pairs for all data streams with the same
foundation (e.g., if the pair in column 1, row 1 succeeds then
the agent will unfreeze the pairs in column 1, rows 2, 3, and 4).
Checklist State with Succeeded Candidate Pair
f1
f2
f3
f4
f5
s1 (Audio.RTP)
S
W
W
s2 (Audio.RTCP)
W
F
F
W
s3 (Video.RTP)
W
s4 (Video.RTCP)
W
Trickle ICE preserves all of these rules as they apply to
"static" checklist sets. This implies that if
a Trickle ICE agent were to begin connectivity checks with all
of its pairs already present, the way that pair states change
is indistinguishable from that of a regular ICE agent.
Of course, the major difference with Trickle ICE is that checklist
sets can be dynamically updated because candidates can
arrive after connectivity checks have started. When this happens, an
agent sets the state of the newly formed pair as described below.
Rule 1: If the newly formed pair has the lowest component ID and,
if the component IDs are equal, the highest priority of any candidate
pair for this foundation (i.e., if it is the topmost pair in the
column), set the state to Waiting. For example, this would be the
case if the newly formed pair were placed in column 5, row 1. This
rule is consistent with .
Checklist State with Newly Formed Pair, Rule 1
f1
f2
f3
f4
f5
s1 (Audio.RTP)
S
W
W
^W^
s2 (Audio.RTCP)
W
F
F
W
s3 (Video.RTP)
W
s4 (Video.RTCP)
W
Rule 2: If there is at least one pair in the Succeeded state for
this foundation, set the state to Waiting. For example, this would be
the case if the pair in column 5, row 1 succeeded and the newly formed
pair were placed in column 5, row 2. This rule is consistent with
.
Checklist State with Newly Formed Pair, Rule 2
f1
f2
f3
f4
f5
s1 (Audio.RTP)
S
W
W
S
s2 (Audio.RTCP)
W
F
F
W
^W^
s3 (Video.RTP)
W
s4 (Video.RTCP)
W
Rule 3: In all other cases, set the state to Frozen. For example,
this would be the case if the newly formed pair were placed in
column 3, row 3.
Checklist State with Newly Formed Pair, Rule 3
f1
f2
f3
f4
f5
s1 (Audio.RTP)
S
W
W
S
s2 (Audio.RTCP)
W
F
F
W
W
s3 (Video.RTP)
W
^F^
s4 (Video.RTCP)
W
Generating an End-of-Candidates Indication
Once all candidate gathering is completed or expires for an
ICE session associated with a specific data stream, the agent will generate an
"end-of-candidates" indication for that session and convey it to
the remote agent via the signaling channel. Although the exact form of
the indication depends on the using protocol, the indication
MUST specify the generation (Username Fragment and Password combination), so that an agent
can correlate the end-of-candidates indication with a particular ICE
session. The indication can be conveyed in the following ways:
As part of an initiation request (which would typically be the case with
the initial ICE description for half trickle)
Along with the last candidate an agent can send for a stream
As a standalone notification (e.g., after STUN Binding requests
or TURN Allocate requests to a server time out and the agent
is no longer actively gathering candidates)
Conveying an end-of-candidates indication in a timely manner is important
in order to avoid ambiguities and speed up the conclusion of ICE processing.
In particular:
A controlled Trickle ICE agent SHOULD convey an end-of-candidates
indication after it has completed gathering for a data stream,
unless ICE processing terminates before the agent has had a chance
to complete gathering.
A controlling agent MAY conclude ICE processing prior to conveying
end-of-candidates indications for all streams. However, it is
RECOMMENDED for a controlling agent to convey end-of-candidates
indications whenever possible for the sake of consistency and to
keep middleboxes and controlled agents up-to-date on the state of
ICE processing.
When conveying an end-of-candidates indication during trickling
(rather than as a part of the initial ICE description or a response thereto),
it is the responsibility of the
using protocol to define methods for associating the
indication with one or more specific data streams.
An agent MAY also choose to generate an end-of-candidates
indication before candidate gathering has actually completed, if the
agent determines that gathering has continued for more than an
acceptable period of time. However, an agent MUST NOT convey any
more candidates after it has conveyed an end-of-candidates
indication.
When performing half trickle, an agent SHOULD convey an
end-of-candidates indication together with its initial ICE description unless
it is planning to potentially trickle additional candidates (e.g., in
case the remote party turns out to support Trickle ICE).
After an agent conveys the end-of-candidates indication, it will
update the state of the corresponding checklist as explained
in . Past that point, an
agent MUST NOT trickle any new candidates within this ICE session.
Therefore, adding new candidates to the
negotiation is possible only through an ICE restart (see
).
This specification does not
override regular ICE semantics for concluding ICE processing.
Therefore, even if end-of-candidates indications are conveyed,
an agent will still need to go through pair nomination. Also, if
pairs have been nominated for components and data streams, ICE
processing MAY still conclude even if end-of-candidates
indications have not been received for all streams. In all cases,
an agent MUST NOT trickle any new candidates within an ICE session
after nomination of a candidate pair as described in
.
Receiving an End-of-Candidates Indication
Receiving an end-of-candidates indication enables an agent to
update checklist states and, in case valid pairs do not exist
for every component in every data stream, determine that ICE
processing has failed. It also enables an agent to speed up the
conclusion of ICE processing when a candidate pair has been validated
but uses a lower-preference transport such as
TURN. In such situations, an implementation MAY choose to wait
and see if higher-priority candidates are received; in this case,
the end-of-candidates indication provides a notification that such
candidates are not forthcoming.
When an agent receives an end-of-candidates indication for a
specific data stream, it will update the state of the relevant
checklist as per (which might lead to
some checklists being marked as Failed).
If the checklist is
still in the Running state after the update, the agent will note that an end-of-candidates indication has been
received and take it into account in future updates
to the checklist.
After an agent has received an end-of-candidates indication, it
MUST ignore any newly received candidates for that data
stream or data session.
Subsequent Exchanges and ICE Restarts
Before conveying an end-of-candidates indication,
either agent MAY convey subsequent candidate information at any time allowed
by the using protocol. When this happens, agents will use semantics from
(e.g., checking of the
Username Fragment and Password combination) to determine whether or not
the new candidate information requires an ICE restart.
If an ICE restart
occurs, the agents can assume that Trickle ICE is still supported
if support was determined previously; thus, they can engage in Trickle ICE
behavior as they would in an initial exchange of ICE descriptions where
support was determined through a capabilities discovery method.
Half Trickle
In half trickle, the initiator conveys the initial ICE description
with a usable but not necessarily full generation of candidates. This
ensures that the ICE description can be processed by a regular ICE
responder and is mostly meant for use in cases where support for
Trickle ICE cannot be confirmed prior to conveying the initial ICE
description. The initial ICE description indicates support for
Trickle ICE, so that the responder can respond with something less
than a full generation of candidates and then trickle the rest.
The initial ICE description for half trickle can contain
an end-of-candidates indication, although this is not mandatory
because if trickle support is confirmed, then the initiator can
choose to trickle additional candidates before it conveys an
end-of-candidates indication.
The half-trickle mechanism can be used in cases where there is
no way for an agent to verify in advance whether a remote
party supports Trickle ICE. Because the initial ICE description contains
a full generation of candidates, it can thus be handled by a regular
ICE agent, while still allowing a Trickle ICE agent to use
the optimization defined in this specification. This prevents
negotiation from failing in the former case while still giving
roughly half the Trickle ICE benefits in the latter.
Use of half trickle is only necessary during an initial
exchange of ICE descriptions. After both parties have received
an ICE description from their peer, they can each reliably
determine Trickle ICE support and use it for all subsequent
exchanges (see ).
In some instances, using half trickle might bring more than
just half the improvement in terms of user experience.
This
can happen when an agent starts gathering candidates upon user-interface
cues that the user will soon be initiating an interaction,
such as activity on a keypad or the phone going off hook. This
would mean that some or all of the candidate
gathering could be completed before the agent actually
needs to convey the candidate information. Because the responder will be able
to trickle candidates, both agents will be able to start
connectivity checks and complete ICE processing earlier than
with regular ICE and potentially even as early as with full
trickle.
However, such anticipation is not always possible. For
example, a multipurpose user agent or a WebRTC web page where
communication is a non-central feature (e.g., calling a support
line in case of a problem with the main features) would not
necessarily have a way of distinguishing between call
intentions and other user activity. In such cases, using full
trickle is most likely to result in an ideal user experience.
Even so, using half trickle would be an improvement over regular
ICE because it would result in a better experience for responders.
Preserving Candidate Order While Trickling
One important aspect of regular ICE is that connectivity checks
for a specific foundation and component are attempted
simultaneously by both agents, so that any firewalls or NATs
fronting the agents would whitelist both endpoints and allow
all except for the first ("suicide") packets to go through. This
is also important to unfreezing candidates at the right time. While
not crucial, preserving this behavior in Trickle ICE is likely to
improve ICE performance.
To achieve this, when trickling candidates, agents SHOULD respect the
order of components as reflected by their component IDs; that is,
candidates for a given component
SHOULD NOT be conveyed prior to candidates for a component with a
lower ID number within the same foundation. In addition, candidates
SHOULD be paired, following the procedures in ,
in the same order they are conveyed.
For example, the following SDP description contains two
components (RTP and RTCP) and two foundations (host and
server-reflexive):
v=0
o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
s=
c=IN IP4 10.0.1.1
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 5000 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 10.0.1.1 5000 typ host
a=candidate:1 2 UDP 2130706431 10.0.1.1 5001 typ host
a=candidate:2 1 UDP 1694498815 192.0.2.3 5000 typ srflx
raddr 10.0.1.1 rport 8998
a=candidate:2 2 UDP 1694498815 192.0.2.3 5001 typ srflx
raddr 10.0.1.1 rport 8998
For this candidate information, the RTCP host candidate would not be conveyed
prior to the RTP host candidate. Similarly, the RTP server-reflexive
candidate would be conveyed together with or prior to the
RTCP server-reflexive candidate.
Requirements for Using Protocols
In order to fully enable the use of Trickle ICE, this specification
defines the following requirements for using protocols.
A using protocol SHOULD provide a way for parties to advertise
and discover support for Trickle ICE before an ICE
session begins (see ).
A using protocol MUST provide methods for incrementally
conveying (i.e., "trickling") additional candidates after
conveying the initial ICE description (see
).
A using protocol MUST deliver each trickled candidate
or end-of-candidates indication exactly once
and in the same order it was conveyed (see
).
A using protocol MUST provide a mechanism for both parties
to indicate and agree on the ICE session in force
(see ).
A using protocol MUST provide a way for parties to communicate the
end-of-candidates indication, which MUST specify the particular
ICE session to which the indication applies (see ).
IANA Considerations
IANA has registered the following ICE option in the "ICE
Options" subregistry of the "Interactive Connectivity Establishment
(ICE) registry", following the procedures defined in
.
ICE Option:
trickle
Contact:
IESG <iesg@ietf.org>
Change controller:
IESG
Description:
An ICE option of 'trickle' indicates support for incremental
communication of ICE candidates.
Reference:
RFC 8838
Security Considerations
This specification inherits most of its semantics from
, and as a result, all security
considerations described there apply to Trickle ICE.
If the privacy implications of revealing host addresses on an
endpoint device are a concern (see, for example, the discussion
in and in
), agents can generate ICE descriptions that contain no
candidates and then only trickle candidates that do not reveal
host addresses (e.g., relayed candidates).
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.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 ReferencesAddress Allocation for Private InternetsThis document describes address allocation for private internets. 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]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]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]Network Address Translation (NAT) Behavioral Requirements for Unicast UDPThis document defines basic terminology for describing different types of Network Address Translation (NAT) behavior when handling Unicast UDP and also defines a set of requirements that would allow many applications, such as multimedia communications or online gaming, to work consistently. Developing NATs that meet this set of requirements will greatly increase the likelihood that these applications will function properly. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.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]Extensible Messaging and Presence Protocol (XMPP): CoreThe Extensible Messaging and Presence Protocol (XMPP) is an application profile of the Extensible Markup Language (XML) that enables the near-real-time exchange of structured yet extensible data between any two or more network entities. This document defines XMPP's core protocol methods: setup and teardown of XML streams, channel encryption, authentication, error handling, and communication primitives for messaging, network availability ("presence"), and request-response interactions. This document obsoletes RFC 3920. [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]WebRTC IP Address Handling RequirementsA Session Initiation Protocol (SIP) Usage for Incremental Provisioning of Candidates for the Interactive Connectivity Establishment (Trickle ICE)XEP-0030: Service DiscoveryCiscoXEP-0176: Jingle ICE-UDP Transport MethodGoogleGoogleCiscoGoogleCollaboraInteraction with Regular ICE
The ICE protocol was designed to be flexible enough to
work in and adapt to as many network environments as
possible. Despite that flexibility, ICE as specified in
does not by itself support Trickle
ICE. This section describes how trickling of candidates
interacts with ICE.
describes the conditions required to
update checklists and timer states while an ICE agent is in the
Running state. These conditions are verified upon transaction
completion, and one of them stipulates that:
if there is not a
valid pair in the valid list for each component of the data stream
associated with the checklist, the state of the checklist is set to
Failed.
This could be a problem and cause ICE processing to fail
prematurely in a number of scenarios. Consider the following
case:
Alice and Bob are both located in different networks with
Network Address Translation (NAT). Alice and Bob themselves
have different addresses, but both networks use the same
private internet block (e.g., the "20-bit block"
172.16/12 specified in ).
Alice conveys to Bob the candidate 172.16.0.1, which also happens
to correspond to an existing host on Bob's network.
Bob creates a candidate pair from his host candidate and
172.16.0.1, puts this one pair into a checklist, and starts
checks.
These checks reach the host at 172.16.0.1 in Bob's network,
which responds with an ICMP "port unreachable" error; per
, Bob marks the transaction as
Failed.
At this point, the checklist only contains a Failed pair, and
the valid list is empty.
This causes the data stream and
potentially all ICE processing to fail, even though Trickle ICE agents
can subsequently convey candidates that could succeed.
A similar race condition would occur if the initial ICE description from
Alice contains only candidates that can be determined as
unreachable from
any of the candidates that Bob has gathered (e.g., this would be the
case if Bob's candidates only contain IPv4 addresses and the
first candidate that he receives from Alice is an IPv6 one).
Another potential problem could arise when a non-Trickle
ICE implementation initiates an interaction with a Trickle ICE
implementation. Consider the following case:
Alice's client has a non-Trickle ICE implementation.
Bob's client has support for Trickle ICE.
Alice and Bob are behind NATs with address-dependent
filtering .
Bob has two STUN servers, but one of them is currently
unreachable.
After Bob's agent receives Alice's initial ICE description, it would
immediately start connectivity checks. It would also start gathering
candidates, which would take a long time because of the unreachable
STUN server. By the time Bob's answer is ready and conveyed to
Alice, Bob's connectivity checks might have failed: until
Alice gets Bob's answer, she won't be able to start connectivity
checks and punch holes in her NAT. The NAT would hence be
filtering Bob's checks as originating from an unknown endpoint.
Interaction with ICE-Lite
The behavior of ICE-lite agents that are capable of Trickle ICE does not
require any particular rules other than those already defined
in this specification and . This section
is hence provided only for informational purposes.
An ICE-lite agent would generate candidate information
as per and
would indicate support for Trickle ICE. Given
that the candidate information will contain a full generation of candidates,
it would also be accompanied by an end-of-candidates indication.
When performing full trickle, a full ICE implementation could
convey the initial ICE description or response thereto with no candidates. After receiving
a response that
identifies the remote agent as an ICE-lite implementation, the
initiator can choose to not trickle any additional
candidates. The same is also true in the case when the ICE-lite
agent initiates the interaction and the full ICE agent is the responder. In
these cases, the connectivity checks would be enough for the ICE-lite
implementation to discover all potentially useful
candidates as peer-reflexive. The following example illustrates
one such ICE session using SDP syntax:
Example
ICE-Lite Bob
Agent
| Offer (a=ice-lite a=ice-options:trickle) |
|---------------------------------------------->|
| |no cand
| Answer (a=ice-options:trickle) |trickling
|<----------------------------------------------|
| Connectivity Checks |
|<--------------------------------------------->|
peer rflx| |
cand disco| |
|<========== CONNECTION ESTABLISHED ===========>|
In addition to reducing signaling traffic, this approach also
removes the need to discover STUN Bindings or make TURN
allocations, which can considerably lighten ICE processing.
Acknowledgements
The authors would like to thank
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
, and
for their reviews and
suggestions on improving this document. , , and completed OPSDIR, GenART, and security
reviews, respectively. Thanks also to and
in their role as chairs and in his role as responsible
Area Director.
Authors' Addresses8x8, Inc. / Jitsi675 Creekside WayCampbellCA95008United States of America+1 512 420 6968emcho@jitsi.orgGoogle747 6th Street SKirklandWA98033United States of America+1 857 288 8888justin@uberti.nameMozillaP.O. Box 787ParkerCO80134United States of America+1 720 256 6756stpeter@mozilla.comhttps://www.mozilla.com/