Trickle ICE: Incremental Provisioning of Candidates for the Interactive Connectivity Establishment (ICE) Protocol
draft-ietf-ice-trickle-08
The information below is for an old version of the document.
| Document | Type | Active Internet-Draft (ice WG) | |
|---|---|---|---|
| Authors | Emil Ivov , Eric Rescorla , Justin Uberti , Peter Saint-Andre | ||
| Last updated | 2017-03-27 | ||
| Replaces | draft-ietf-mmusic-trickle-ice | ||
| Stream | Internet Engineering Task Force (IETF) | ||
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draft-ietf-ice-trickle-08
Network Working Group E. Ivov
Internet-Draft Atlassian
Intended status: Standards Track E. Rescorla
Expires: September 28, 2017 RTFM, Inc.
J. Uberti
Google
P. Saint-Andre
Filament
March 27, 2017
Trickle ICE: Incremental Provisioning of Candidates for the Interactive
Connectivity Establishment (ICE) Protocol
draft-ietf-ice-trickle-08
Abstract
This document describes "Trickle ICE", an extension to the
Interactive Connectivity Establishment (ICE) protocol that enables
ICE agents to send and receive candidates incrementally rather than
exchanging complete lists. With such incremental provisioning, ICE
agents can begin connectivity checks while they are still gathering
candidates and considerably shorten the time necessary for ICE
processing to complete.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 28, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) 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
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Determining Support for Trickle ICE . . . . . . . . . . . . . 5
4. Sending the Initial ICE Description . . . . . . . . . . . . . 6
5. Receiving the Initial ICE Description . . . . . . . . . . . . 7
5.1. Sending the Initial Response . . . . . . . . . . . . . . 7
5.2. Forming Check Lists and Beginning Connectivity
Checks . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Receiving the Initial Answer . . . . . . . . . . . . . . . . 8
7. Performing Connectivity Checks . . . . . . . . . . . . . . . 8
7.1. Scheduling Checks . . . . . . . . . . . . . . . . . . . . 8
7.2. Check List and Timer State Updates . . . . . . . . . . . 9
8. Discovering and Sending Additional Local Candidates . . . . . 10
8.1. Pairing Newly Learned Candidates and Updating
Check Lists . . . . . . . . . . . . . . . . . . . . . . . 11
8.1.1. Inserting a New Pair in a Check List . . . . . . . . 12
8.2. Announcing End of Candidates . . . . . . . . . . . . . . 13
9. Receiving Additional Remote Candidates . . . . . . . . . . . 14
10. Receiving an End-Of-Candidates Notification . . . . . . . . . 15
11. Trickle ICE and Peer Reflexive Candidates . . . . . . . . . . 15
12. Concluding ICE Processing . . . . . . . . . . . . . . . . . . 15
13. Subsequent Exchanges . . . . . . . . . . . . . . . . . . . . 15
14. Unilateral Use of Trickle ICE (Half Trickle) . . . . . . . . 16
15. Requirements for Signaling Protocols . . . . . . . . . . . . 17
16. Preserving Candidate Order while Trickling . . . . . . . . . 17
17. Example Flow . . . . . . . . . . . . . . . . . . . . . . . . 18
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
19. Security Considerations . . . . . . . . . . . . . . . . . . . 19
20. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
21. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
21.1. Normative References . . . . . . . . . . . . . . . . . . 20
21.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Interaction with Regular ICE . . . . . . . . . . . . 21
Appendix B. Interaction with ICE Lite . . . . . . . . . . . . . 22
Appendix C. Changes from Earlier Versions . . . . . . . . . . . 23
C.1. Changes from draft-ietf-ice-trickle-07 . . . . . . . . . 23
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C.2. Changes from draft-ietf-ice-trickle-06 . . . . . . . . . 24
C.3. Changes from draft-ietf-ice-trickle-05 . . . . . . . . . 24
C.4. Changes from draft-ietf-ice-trickle-04 . . . . . . . . . 24
C.5. Changes from draft-ietf-ice-trickle-03 . . . . . . . . . 24
C.6. Changes from draft-ietf-ice-trickle-03 . . . . . . . . . 24
C.7. Changes from draft-ietf-ice-trickle-02 . . . . . . . . . 24
C.8. Changes from draft-ietf-ice-trickle-01 . . . . . . . . . 24
C.9. Changes from draft-ietf-ice-trickle-00 . . . . . . . . . 24
C.10. Changes from draft-mmusic-trickle-ice-02 . . . . . . . . 25
C.11. Changes from draft-ivov-01 and draft-mmusic-00 . . . . . 25
C.12. Changes from draft-ivov-00 . . . . . . . . . . . . . . . 25
C.13. Changes from draft-rescorla-01 . . . . . . . . . . . . . 26
C.14. Changes from draft-rescorla-00 . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Introduction
The Interactive Connectivity Establishment (ICE) protocol
[rfc5245bis] describes mechanisms for gathering candidates,
prioritizing them, choosing default ones, exchanging them with a
remote party, pairing them, and ordering them into check lists. Once
all of these actions have been completed (and only then), the parties
can begin a phase of connectivity checks and eventually select the
pair of candidates that will be used in a media session or for a
given media stream.
Although the sequence described above has the advantage of being
relatively straightforward to implement and debug once deployed, it
can also be rather lengthy. Candidate gathering often involves
things like querying STUN [RFC5389] servers and allocating relayed
candidates at TURN [RFC5766] servers. All of these actions can be
delayed for a noticeable amount of time; although they can be run in
parallel, they still need to respect the pacing requirements from
[rfc5245bis], which is likely to delay them even further. Some or
all of these actions also need be completed by the remote agent.
Both agents would next perform connectivity checks and only then
would they be ready to begin streaming media.
These factors can lead to relatively lengthy session establishment
times and thus to a degraded user experience.
This document defines an alternative or supplementary mode of
operation for ICE implementations, known as "Trickle ICE", in which
candidates can be exchanged incrementally. This enables ICE agents
to exchange candidates as soon as an ICE session has been initiated.
Connectivity checks for a media stream can also start as soon as the
first candidates for that stream become available.
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Trickle ICE can reduce session establishment times in cases where
connectivity is confirmed for the first exchanged candidates (e.g.,
where candidates for one of the agents are directly reachable from
the second agent, such as candidates at a media relay). Even when
this is not the case, performing candidate gathering for both agents
and connectivity checks in parallel can considerably shorten ICE
processing times.
It is worth noting that there is quite a bit of operational
experience with the Trickle ICE technique, going back as far as 2005
(when the XMPP Jingle extension defined a "dribble mode" as specified
in [XEP-0176]); this document incorporates feedback from those who
have implemented and deployed the technique.
In addition to the basics of Trickle ICE, this document also
describes how to discover support for Trickle ICE, how regular ICE
processing needs to be modified when building and updating check
lists, and how Trickle ICE implementations interoperate with agents
that only implement regular ICE processing as defined in
[rfc5245bis].
This specification does not define the usage of Trickle ICE with any
specific signaling protocol (however, see
[I-D.ietf-mmusic-trickle-ice-sip] for usage with SIP [RFC3261] and
[XEP-0176] for usage with XMPP [RFC6120]). Similarly, it does not
define Trickle ICE in terms of the Session Description Protocol (SDP)
[RFC4566] or the offer/answer model [RFC3264] because the technique
can be and already is used in application protocols that are not tied
to SDP or to offer/answer semantics. However, because SDP and the
offer/answer model are familiar to most readers of this
specification, some examples in this document use those particulars
in order to explain the underlying concepts.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification makes use of all terminology defined for
Interactive Connectivity Establishment in [rfc5245bis]. In addition,
it defines the following terms:
Candidate Gatherer: A module used by an ICE agent to obtain local
candidates. Candidate gatherers use different mechanisms for
discovering local candidates, such as STUN and TURN.
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Generation: All of the candidates sent within an ICE session; these
are the candidates that are associated with a local/remote ufrag
pair (which will change on ICE restart, if any).
ICE Description: Any session-related (as opposed to candidate-
related) attributes required to configure an ICE agent. These
include but are not limited to "ice-ufrag", "ice-pwd", and "ice-
options".
Trickled Candidates: Candidates that a Trickle ICE agent sends after
sending an initial ICE description or responding to an initial ICE
description, but within the same ICE session. Trickled candidates
can be sent in parallel with candidate gathering and connectivity
checks.
Trickling: The act of sending trickled candidates.
Half Trickle: A Trickle ICE mode of operation where the initiator
gathers a full generation of candidates strictly before creating
and sending the initial ICE description. Once sent, that ICE
description can be processed by regular ICE agents and does not
require support for this specification. It also allows Trickle
ICE capable responders to still gather candidates and perform
connectivity checks in a non-blocking way, thus roughly providing
"half" the advantages of Trickle ICE. The mechanism is mostly
meant for use in cases where the remote agent's support for
Trickle ICE cannot be confirmed prior to sending an initial ICE
description.
Full Trickle: The typical mode of operation for Trickle ICE agents,
in which an 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.
3. Determining Support for Trickle ICE
To fully support Trickle ICE, applications SHOULD incorporate one of
the following mechanisms to enable implementations to determine
whether Trickle ICE is supported:
1. Provide a capabilities discovery method so that agents can verify
support of Trickle ICE prior to initiating a session (XMPP's
Service Discovery [XEP-0030] is one such mechanism).
2. Make support for Trickle ICE mandatory so that user agents can
assume support.
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If an application 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 Section 14.
Prior to sending an initial ICE description, agents using signaling
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 signaling protocol does not include a capabilities
discovery method, a user agent can provide an indication within the
ICE description that it supports Trickle ICE (e.g., in SDP this would
be a token of "trickle" in the ice-options attribute).
Dedicated discovery semantics and half trickle are needed only prior
to session initiation. After a session is established and Trickle
ICE support is confirmed for both parties, either agent can use full
trickle for subsequent exchanges.
4. Sending the Initial ICE Description
An 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 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,
agents SHOULD generate and transmit their initial ICE description as
early as possible, so that the remote party can start gathering and
trickling candidates.
Trickle ICE agents MAY include any mix of candidates in an ICE
description. This includes the possibility of sending an ICE
description that contains all the candidates that the agent plans to
use (as in half trickle mode), sending an ICE description that
contains only a publicly-reachable IP address (e.g., a candidate at a
media relay that is known to not be behind a firewall), or sending an
ICE description with 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).
Methods for calculating priorities and foundations, as well as
determining redundancy of candidates, work just as with regular ICE
(with the exception of pruning of duplicate peer reflexive candidates
as described under Section 5.2).
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5. Receiving the Initial ICE Description
When a responder receives an initial ICE description, it will first
check if the ICE description or initiator indicates support for
Trickle ICE as explained in Section 3. If this is not the case, the
agent MUST process the ICE description according to regular ICE
procedures [rfc5245bis] (or, if no ICE support is detected at all,
according to relevant processing rules for the underlying signaling
protocol, such as offer/answer processing rules [RFC3264]).
If support for Trickle ICE is confirmed, an agent will automatically
assume support for regular ICE as well even if the support
verification procedure in [rfc5245bis] indicates otherwise.
Specifically, the rules from [rfc5245bis] would imply that ICE itself
is not supported if the initial ICE description includes no
candidates; however, such a conclusion is not warranted if the
responder can confirm that the initiator supports Trickle ICE; in
this case, fallback to [RFC3264] is not necessary.
If the initial ICE description does indicate support for Trickle ICE,
the agent will determine its role and start gathering and
prioritizing candidates; while doing so, it will also respond by
sending its own ICE description, so that both agents can start
forming check lists and begin connectivity checks.
5.1. Sending the Initial Response
An agent can respond to an initial ICE description at any point while
gathering candidates. Here again the ICE description MAY contain any
set of candidates, including all candidates or no candidates. (The
benefit of including no candidates is to send the ICE description as
quickly as possible, so that both parties can consider the overall
session to be under active negotiation as soon as possible.)
As noted in Section 3, in application protocols that use SDP the
responder's ICE description can indicate support for Trickle ICE by
including a token of "trickle" in the ice-options attribute.
5.2. Forming Check Lists and Beginning Connectivity Checks
After the initiator and responder exchange ICE descriptions, and as
soon as they have obtained local and remote candidates, agents begin
forming candidate pairs, computing candidate pair priorities,
ordering candidate pairs, pruning duplicate pairs, and creating check
lists according to regular ICE procedures [rfc5245bis].
According to those procedures, in order for candidate pairing to be
possible and for duplicate candidates to be pruned, the candidates
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would need to be provided in the relevant ICE descriptions. Under
Trickle ICE, check lists can be empty until candidate pairs are sent
or received. Therefore Trickle ICE agents handle check lists and
candidate pairing in a slightly different way than regular ICE
agents: the agents still create the check lists, but they populate
the check lists only after they actually have the candidate pairs.
A Trickle ICE agent initially considers all check lists to be frozen.
It then inspects the first check list and attempts to unfreeze all
candidate pairs it has received so far that belong to the first
component on the first media stream (i.e., the first media stream
that was reported to the ICE implementation from the using
application). If that first component of the first media stream does
not contain candidates for one or more of the currently known pair
foundations, and if candidate pairs already exist for that foundation
in one of the following components or media streams, then the agent
unfreezes the first of those candidate pairs.
With regard to pruning of duplicate candidate pairs, a Trickle ICE
agent SHOULD follow a policy of pruning the lower priority candidate
unless it is peer reflexive.
6. Receiving the Initial Answer
When processing an ICE description from a responder, the initiator
follows regular ICE procedures to determine its role, after which it
forms check lists (as described in Section 5.2) and begins
connectivity checks.
7. Performing Connectivity Checks
For the most part, Trickle ICE agents perform connectivity checks
following regular ICE procedures. However, the fact that gathering
and communicating candidates is asynchronous in Trickle ICE imposes a
number of changes as described in the following sections.
7.1. Scheduling Checks
The ICE specification [rfc5245bis], Section 5.1.5, requires that
agents terminate the timer for a triggered check in relation to an
active check list once the agent has exhausted all frozen pairs in
the check list. This will not work with Trickle ICE, because more
pairs will be added to the check list incrementally.
Therefore, a Trickle ICE agent SHOULD NOT terminate the timer until
the state of the check list is Completed or Failed as specified
herein (see Section 8.2).
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7.2. Check List and Timer State Updates
The ICE specification [rfc5245bis], Section 6.1.2.4.3, requires that
agents update check lists and timer states upon completing a
connectivity check transaction. During such an update, regular ICE
agents would set the state of a check list to Failed if both of the
following two conditions are satisfied:
o all of the pairs in the check list are either in the Failed state
or Succeeded state; and
o there is not a pair in the valid list for each component of the
media 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:
o all candidate gatherers have completed and the agent is not
expecting to discover any new local candidates;
o the remote agent has sent an end-of-candidates indication for that
check list as described in Section 8.2.
When a check list is set to Failed as described above, regular ICE
requires the agent to update all other check lists, placing one pair
from each check list into the Waiting state - effectively unfreezing
all remaining check lists. However, under Trickle ICE other check
lists might still be empty at this point (because candidates have not
yet been received), and following only the rules from regular ICE
would prevent the agent from unfreezing those check lists (because
the state of a check list depends on the state of the candidate pairs
in that check list, but there are none yet). Therefore a Trickle ICE
agent MUST monitor whether a check list is active or frozen
independently of the state of the candidate pairs in the check list
(since there might not be any pairs yet), and MUST consider a check
list to be active whenever it adds the first candidate pair with a
state of Waiting to the check list (see Section 8.1.1).
With regard to empty check lists, by default a Trickle ICE agent MAY
consider an empty check list to be either active or frozen. When a
Trickle ICE agent considers an empty check list to be frozen, during
the candidate checking process it SHOULD change the check list to
active if checking of another check list is completely finished
(i.e., if every pair in the other check list is either Successful or
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Failed), or if another check list has a valid candidate pair for all
components.
8. Discovering and Sending Additional Local Candidates
After ICE descriptions have been sent, agents will most likely
continue discovering 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 sends (i.e., trickles) the newly discovered
candidate(s) to the remote agent. The actual delivery of the new
candidates is handled by a signaling protocol such as SIP or XMPP.
Trickle ICE imposes no restrictions on the way this is done (e.g.,
some applications may choose not to send trickle updates for server
reflexive candidates and instead rely on the discovery of peer
reflexive ones).
When trickle updates are sent, each candidate MUST be delivered to
the receiving Trickle ICE implementation not more than once. If
there are 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, applications that choose to signal
candidates via SDP may include a ufrag 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 may include a ufrag in
the JavaScript objects that represent candidates.
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Note: The signaling protocol needs to provide a mechanism for both
parties to indicate and agree on the ICE session in force (as
identified by the ufrag) 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 Section 13).
Once the candidate has been sent to the remote party, the agent
checks if any remote candidates are currently known for this same
stream. If not, the new candidate will simply be added to the list
of local candidates.
Otherwise, if the agent has already learned of one or more remote
candidates for this stream and component, it will begin pairing the
new local candidates with them and adding the pairs to the existing
check lists according to their priority.
8.1. Pairing Newly Learned Candidates and Updating Check Lists
Forming candidate pairs works as described in the ICE specification
[rfc5245bis]. However, actually adding the new pair to a check list
happens according to the rules described below.
If the check list where the pair is to be added already contains the
maximum number of candidate pairs (100 by default as per
[rfc5245bis]), the new pair is discarded.
If the new pair's local candidate is server reflexive, the server
reflexive candidate MUST be replaced by its base before adding the
pair to the list.
Once this is done, the agent examines the check list looking for
another pair that would be redundant with the new one. If such a
pair exists and the type of its remote candidate is not peer
reflexive, the pair with the higher priority is kept and the one with
the lower priority is discarded. If, on the other hand, the type of
the remote candidate in the pre-existing pair is peer reflexive, the
agent MUST replace it with the newly formed pair (regardless of their
respective priorities); this is done by setting the priority of the
new candidate to the priority of the pre-existing candidate and then
re-sorting the check list.
For all other pairs, including those with a server reflexive local
candidate that were not found to be redundant, the rules specified in
the following section apply.
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8.1.1. Inserting a New Pair in a Check List
Consider the following tabular representation of all checklists in an
agent:
+------------+------+------+------+------+------+
| | f1 | f2 | f3 | f4 | f5 |
+------------+------+------+------+------+------+
| Audio.RTP | cp | cp | cp | | |
+------------+------+------+------+------+------+
| Audio.RTCP | cp | cp | cp | cp | |
+------------+------+------+------+------+------+
| Video.RTP | cp | | | | cp |
+------------+------+------+------+------+------+
| Video.RTCP | cp | | | | cp |
+------------+------+------+------+------+------+
Figure 1: Trickle State Updates
Each row in the table represents a component for a given media
stream. Each column represents one foundation. Each cell represents
one candidate pair.
When an agent commences ICE processing as per [rfc5245bis], it will
unfreeze (i.e., place in the Waiting state) the topmost candidate
pair in every column. Then, as the checks proceed, for each pair
that enters the Succeeded state the agent will unfreeze the pair that
is immediately underneath the pair that succeeded (e.g., if the pair
in column 1, row 1 succeeds then the agent will unfreeze the pair in
column 1, row 2). ICE also specifies that, if all the pairs in a
media stream for one foundation are unfrozen (e.g., column 1, rows 1
and 2 representing both components for the audio stream), then all of
the candidate pairs in the entire column are unfrozen (e.g., column
1, rows 3 and 4).
Trickle ICE preserves all of these rules. This implies that if, for
some reason, a Trickle 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 candidates
can arrive after connectivity checks have started. When this
happens, an agent sets the state of the newly formed pair as follows:
Waiting: if the newly formed pair is the topmost pair in this
column;
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Waiting: if the pair immediately above the newly formed pair in
this column is in the Succeeded state;
Waiting: if there is at least one pair in this column below the row
of the newly formed pair whose state is either Succeeded or
Failed.
Frozen: in all other cases.
8.2. Announcing End of Candidates
Once all candidate gathering is completed or expires for a specific
media stream, the agent will generate an "end-of-candidates"
indication for that stream and send it to the remote agent via the
signaling channel. The exact form of the indication depends on the
application protocol. The indication can be sent in the following
ways:
o As part of an initiation request (which would typically be the
case with an initial ICE description for half trickle)
o Along with the last candidate an agent can send for a stream
o As a standalone notification (e.g., after STUN Binding requests or
TURN Allocate requests to a server time out and the agent has no
other active gatherers)
Sending 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:
o A controlled Trickle ICE agent SHOULD send an end-of-candidates
indication after it has completed gathering for a media stream,
unless ICE processing terminates before the agent has had a chance
to complete gathering.
o A controlling agent MAY conclude ICE processing prior to sending
end-of-candidates indications for all streams. However, it is
RECOMMENDED for a controlling agent to send 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 sending an end-of-candidates indication during trickling (rather
than as a part of an initial ICE description or response), it is the
responsibility of the using protocol to define methods for relating
the indication to one or more specific media streams.
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Receiving an end-of-candidates indication enables an agent to update
check list states and, in case valid pairs do not exist for every
component in every media stream, determine that ICE processing has
failed. It also enables agents to speed up the conclusion of ICE
processing when a candidate pair has been validated but it involves
the use of lower-preference transports 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.
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 send any more candidates
after it has sent an end-of-candidates indication.
When performing half trickle, an agent SHOULD send an end-of-
candidates indication together with its initial ICE description
unless it is planning to potentially send additional candidates
(e.g., in case the remote party turns out to support Trickle ICE).
After an agent sends the end-of-candidates indication, it will update
the state of the corresponding check list as explained in
Section 7.2. Past that point, an agent MUST NOT send any new
candidates within this ICE session. After an agent has received an
end-of-candidates indication, it MUST also ignore any newly received
candidates for that media stream or media session. Therefore, adding
new candidates to the negotiation is possible only through an ICE
restart (see Section 13).
This specification does not override regular ICE semantics for
concluding ICE processing. Therefore, even if end-of-candidates
indications are sent, agents will still have to go through pair
nomination. Also, if pairs have been nominated for components and
media streams, ICE processing MAY still conclude even if end-of-
candidates indications have not been received for all streams.
9. Receiving Additional Remote Candidates
At any time during ICE processing, a Trickle ICE agent might receive
new candidates from the remote agent. When this happens and no local
candidates are currently known for this same stream, the new remote
candidates are added to the list of remote candidates.
Otherwise, the new candidates are used for forming candidate pairs
with the pool of local candidates and they are added to the local
check lists as described in Section 8.1.
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Once the remote agent has completed candidate gathering, it will send
an end-of-candidates indication. Upon receiving such an indication,
the local agent MUST update check list states as per Section 7.2.
This might lead to some check lists being marked as Failed.
10. Receiving an End-Of-Candidates Notification
When an agent receives an end-of-candidates indication for a specific
media stream, it will update the state of the relevant check list as
per Section 7.2. If the check list is still in the Active state
after the update, the agent will persist the fact that an end-of-
candidates indication has been received and take it into account in
future updates to the check list.
11. Trickle ICE and Peer Reflexive Candidates
Even though Trickle ICE does not explicitly modify the procedures for
handling peer-reflexive candidates, use of Trickle ICE can have an
impact on how they are processed. With Trickle ICE, it is possible
that server reflexive candidates can be discovered as peer reflexive
in cases where incoming connectivity checks are received from these
candidates before the trickle updates that carry them.
While this would certainly increase the number of cases where ICE
processing nominates and selects candidates discovered as peer-
reflexive, it does not require any change in processing.
It is also likely that some applications would 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.
12. Concluding ICE Processing
This specification does not directly modify the procedures for ending
ICE processing described in Section 6.2 of [rfc5245bis], and Trickle
ICE implementations follow the same rules.
13. Subsequent Exchanges
Either agent MAY generate a subsequent ICE description at any time
allowed by the signaling protocol in use. When this happens agents
will use [rfc5245bis] semantics to determine whether or not the new
ICE description requires an ICE restart. If an ICE restart occurs,
the user agents can assume that Trickle ICE is still supported if
support was determined previously, and thus can engage in Trickle ICE
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behavior as they would in an initial exchange of ICE descriptions
where support was determined through a capabilities discovery method.
14. Unilateral Use of Trickle ICE (Half Trickle)
In half trickle mode, the initiator sends a regular ICE description
with a 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 sending an initial ICE description. The initial
ICE description indicates support for Trickle ICE, which means the
responder can respond with something less than a full generation of
candidates and then trickle the rest. A half trickle ICE description
would typically 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
sends 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 (hence the name
of the mechanism).
Use of half trickle is only necessary during an initial exchange of
ICE descriptions. After both parties have received a session
description from their peer, they can each reliably determine Trickle
ICE support and use it for all subsequent exchanges.
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 send the ICE description. 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
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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.
15. Requirements for Signaling Protocols
In order to fully enable the use of Trickle ICE, this specification
defines the following requirements for signaling protocols.
o A signaling protocol SHOULD provide a way for parties to advertise
and discover support for Trickle ICE before an ICE session begins
(see Section 3).
o A signaling protocol MUST provide methods for incrementally
sending (i.e., "trickling") additional candidates after sending
the initial ICE description (see Section 8).
o A signaling protocol MUST provide a mechanism for both parties to
indicate and agree on the ICE session in force (see Section 8).
o A signaling protocol MUST provide a way for parties to communicate
the end-of-candidates indication (see Section 8.2).
16. 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 MUST respect the
order in which the components and streams as they have been
negotiated appear (implicitly or explicitly) in the relevant ICE
descriptions. Therefore a candidate for a specific component MUST
NOT be sent prior to candidates for other components within the same
foundation.
For example, the following SDP description contains two components
(RTP and RTCP) and two foundations (host and server reflexive):
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v=0
o=jdoe 2890844526 2890842807 IN IP6 2001:db8:a0b:12f0::1
s=
c=IN IP4 2001:db8:a0b:12f0::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 2001:db8:a0b:12f0::1 5000 typ host
a=candidate:1 2 UDP 2130706431 2001:db8:a0b:12f0::1 5001 typ host
a=candidate:2 1 UDP 1694498815 2001:db8:a0b:12f0::3 5000 typ srflx
raddr 2001:db8:a0b:12f0::1 rport 8998
a=candidate:2 2 UDP 1694498815 2001:db8:a0b:12f0::3 5001 typ srflx
raddr 2001:db8:a0b:12f0::1 rport 8998
For this description the RTCP host candidate MUST NOT be sent prior
to the RTP host candidate. Similarly the RTP server reflexive
candidate MUST be sent together with or prior to the RTCP server
reflexive candidate.
Similar considerations apply at the level of media streams in
addition to foundations; this is covered by the requirement to always
start unfreezing candidates starting from the first media stream as
described under Section 5.2.
17. Example Flow
As an example, a typical successful Trickle ICE exchange with a
signaling protocol that follows the offer/answer model would look
this way:
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Alice Bob
| Offer |
|---------------------------------------------->|
| Additional Candidates |
|---------------------------------------------->|
| |
| Answer |
|<----------------------------------------------|
| Additional Candidates |
|<----------------------------------------------|
| |
| Additional Candidates and Connectivity Checks |
|<--------------------------------------------->|
| |
|<=============== MEDIA FLOWS =================>|
Figure 2: Example
18. IANA Considerations
This specification requests no actions from IANA.
19. Security Considerations
This specification inherits most of its semantics from [rfc5245bis]
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, agents can generate an ICE description
that contains no candidates and then only trickle candidates that do
not reveal host addresses (e.g., relayed candidates).
20. Acknowledgements
The authors would like to thank Bernard Aboba, Flemming Andreasen,
Rajmohan Banavi, Taylor Brandstetter, Christer Holmberg, Ari Keranen,
Paul Kyzivat, Jonathan Lennox, Enrico Marocco, Pal Martinsen, Thomas
Stach, Peter Thatcher, Martin Thomson, Dale R. Worley, and Brandon
Williams for their reviews and suggestions on improving this
document. Thanks also to Ari Keranen and Peter Thatcher for chairing
the ICE Working Group.
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21. References
21.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[rfc5245bis]
Keranen, A., Keranen, A., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", draft-ietf-ice-
rfc5245bis-08 (work in progress), December 2016.
21.2. Informative References
[I-D.ietf-mmusic-trickle-ice-sip]
Ivov, E., Thomas, T., Marocco, E., and C. Holmberg, "A
Session Initiation Protocol (SIP) usage for Trickle ICE",
draft-ietf-mmusic-trickle-ice-sip-07 (work in progress),
March 2017.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
<http://www.rfc-editor.org/info/rfc1918>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4787] Audet, F., Ed. and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
2007, <http://www.rfc-editor.org/info/rfc4787>.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<http://www.rfc-editor.org/info/rfc5389>.
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[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
[XEP-0030]
Hildebrand, J., Millard, P., Eatmon, R., and P. Saint-
Andre, "XEP-0030: Service Discovery", XEP XEP-0030, June
2008.
[XEP-0176]
Beda, J., Ludwig, S., Saint-Andre, P., Hildebrand, J.,
Egan, S., and R. McQueen, "XEP-0176: Jingle ICE-UDP
Transport Method", XEP XEP-0176, June 2009.
Appendix A. Interaction 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 [rfc5245bis] does not by itself
support trickle ICE. This section describes how trickling of
candidates interacts with ICE.
[rfc5245bis] describes the conditions required to update check lists
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 pair in the valid list for each component of the
media stream, the state of the check list 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:
1. Alice and Bob are both located in different networks with Network
Address Translation (NAT). Alice and Bob themselves have
different address but both networks use the same private internet
block (e.g., the "20-bit block" 172.16/12 specified in
[RFC1918]).
2. Alice sends Bob the candidate 172.16.0.1 which also happens to
correspond to an existing host on Bob's network.
3. Bob creates a check list consisting solely of 172.16.0.1 and
starts checks.
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4. These checks reach the host at 172.16.0.1 in Bob's network, which
responds with an ICMP "port unreachable" error; per [rfc5245bis]
Bob marks the transaction as Failed.
At this point the check list only contains Failed candidates and the
valid list is empty. This causes the media stream and potentially
all ICE processing to fail.
A similar race condition would occur if the initial ICE description
from Alice only contains 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:
1. Alice's client has a non-Trickle ICE implementation.
2. Bob's client has support for Trickle ICE.
3. Alice and Bob are behind NATs with address-dependent filtering
[RFC4787].
4. 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 sent to Alice,
Bob's connectivity checks may well 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.
Appendix B. 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 [rfc5245bis]. This section is hence provided
only for informational purposes.
An ICE lite agent would generate an ICE description as per
[rfc5245bis] and would indicate support for Trickle ICE. Given that
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the ICE description 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 send an
initial ICE description or response with no candidates. After
receiving a response that identifies the remote agent as an ICE lite
implementation, the initiator can choose to not send 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:
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| |
| |
|<=============== MEDIA FLOWS =================>|
Figure 3: Example
In addition to reducing signaling traffic this approach also removes
the need to discover STUN bindings or make TURN allocations, which
may considerably lighten ICE processing.
Appendix C. Changes from Earlier Versions
Note to the RFC-Editor: please remove this section prior to
publication as an RFC.
C.1. Changes from draft-ietf-ice-trickle-07
o Addressed editorial feedback from chairs review.
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C.2. Changes from draft-ietf-ice-trickle-06
o Clarified terminology regarding generations.
C.3. Changes from draft-ietf-ice-trickle-05
o Rewrote the text on inserting a new pair into a check list.
C.4. Changes from draft-ietf-ice-trickle-04
o Removed dependency on SDP and offer/answer model.
o Removed mentions of aggressive nomination, since it is deprecated
in 5245bis.
o Added section on requirements for signaling protocols.
o Clarified terminology.
o Addressed various WG feedback.
C.5. Changes from draft-ietf-ice-trickle-03
o Copy edit.
C.6. Changes from draft-ietf-ice-trickle-03
o Provided more detailed description of unfreezing behavior,
specifically how to replace pre-existing peer-reflexive candidates
with higher-priority ones received via trickling.
C.7. Changes from draft-ietf-ice-trickle-02
o Adjusted unfreezing behavior when there are disparate foundations.
C.8. Changes from draft-ietf-ice-trickle-01
o Changed examples to use IPv6.
C.9. Changes from draft-ietf-ice-trickle-00
o Removed dependency on SDP (which is to be provided in a separate
specification).
o Clarified text about the fact that a check list can be empty if no
candidates have been sent or received yet.
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o Clarified wording about check list states so as not to define new
states for "Active" and "Frozen" because those states are not
defined for check lists (only for candidate pairs) in ICE core.
o Removed open issues list because it was out of date.
o Completed a thorough copy edit.
C.10. Changes from draft-mmusic-trickle-ice-02
o Addressed feedback from Rajmohan Banavi and Brandon Williams.
o Clarified text about determining support and about how to proceed
if it can be determined that the answering agent does not support
Trickle ICE.
o Clarified text about check list and timer updates.
o Clarified when it is appropriate to use half trickle or to send no
candidates in an offer or answer.
o Updated the list of open issues.
C.11. Changes from draft-ivov-01 and draft-mmusic-00
o Added a requirement to trickle candidates by order of components
to avoid deadlocks in the unfreezing algorithm.
o Added an informative note on peer-reflexive candidates explaining
that nothing changes for them semantically but they do become a
more likely occurrence for Trickle ICE.
o Limit the number of pairs to 100 to comply with 5245.
o Added clarifications on the non-importance of how newly discovered
candidates are trickled/sent to the remote party or if this is
done at all.
o Added transport expectations for trickled candidates as per Dale
Worley's recommendation.
C.12. Changes from draft-ivov-00
o Specified that end-of-candidates is a media level attribute which
can of course appear as session level, which is equivalent to
having it appear in all m-lines. Also made end-of-candidates
optional for cases such as aggressive nomination for controlled
agents.
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o Added an example for ICE lite and Trickle ICE to illustrate how,
when talking to an ICE lite agent doesn't need to send or even
discover any candidates.
o Added an example for ICE lite and Trickle ICE to illustrate how,
when talking to an ICE lite agent doesn't need to send or even
discover any candidates.
o Added wording that explicitly states ICE lite agents have to be
prepared to receive no candidates over signaling and that they
should not freak out if this happens. (Closed the corresponding
open issue).
o It is now mandatory to use MID when trickling candidates and using
m-line indexes is no longer allowed.
o Replaced use of 0.0.0.0 to IP6 :: in order to avoid potential
issues with RFC2543 SDP libraries that interpret 0.0.0.0 as an on-
hold operation. Also changed the port number here from 1 to 9
since it already has a more appropriate meaning. (Port change
suggested by Jonathan Lennox).
o Closed the Open Issue about use about what to do with cands
received after end-of-cands. Solution: ignore, do an ICE restart
if you want to add something.
o Added more terminology, including trickling, trickled candidates,
half trickle, full trickle,
o Added a reference to the SIP usage for Trickle ICE as requested at
the Boston interim.
C.13. Changes from draft-rescorla-01
o Brought back explicit use of Offer/Answer. There are no more
attempts to try to do this in an O/A independent way. Also
removed the use of ICE Descriptions.
o Added SDP specification for trickled candidates, the trickle
option and 0.0.0.0 addresses in m-lines, and end-of-candidates.
o Support and Discovery. Changed that section to be less abstract.
As discussed in IETF85, the draft now says implementations and
usages need to either determine support in advance and directly
use trickle, or do half trickle. Removed suggestion about use of
discovery in SIP or about letting implementing protocols do what
they want.
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o Defined Half Trickle. Added a section that says how it works.
Mentioned that it only needs to happen in the first o/a (not
necessary in updates), and added Jonathan's comment about how it
could, in some cases, offer more than half the improvement if you
can pre-gather part or all of your candidates before the user
actually presses the call button.
o Added a short section about subsequent offer/answer exchanges.
o Added a short section about interactions with ICE Lite
implementations.
o Added two new entries to the open issues section.
C.14. Changes from draft-rescorla-00
o Relaxed requirements about verifying support following a
discussion on MMUSIC.
o Introduced ICE descriptions in order to remove ambiguous use of
3264 language and inappropriate references to offers and answers.
o Removed inappropriate assumption of adoption by RTCWEB pointed out
by Martin Thomson.
Authors' Addresses
Emil Ivov
Atlassian
303 Colorado Street, #1600
Austin 78701
USA
Phone: +1-512-640-3000
Email: eivov@atlassian.com
Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
USA
Phone: +1 650 678 2350
Email: ekr@rtfm.com
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Justin Uberti
Google
747 6th St S
Kirkland, WA 98033
USA
Phone: +1 857 288 8888
Email: justin@uberti.name
Peter Saint-Andre
Filament
P.O. Box 787
Parker, CO 80134
USA
Phone: +1 720 256 6756
Email: peter@filament.com
URI: https://filament.com/
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