Network Working Group E. Ivov
Internet-Draft Atlassian
Intended status: Standards Track E. Rescorla
Expires: March 12, 2017 RTFM, Inc.
J. Uberti
Google
P. Saint-Andre
Filament
September 8, 2016
Trickle ICE: Incremental Provisioning of Candidates for the Interactive
Connectivity Establishment (ICE) Protocol
draft-ietf-ice-trickle-04
Abstract
This document describes 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. This mechanism is called "Trickle ICE".
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-
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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 March 12, 2017.
Copyright Notice
Copyright (c) 2016 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Determining Support for Trickle ICE . . . . . . . . . . . . . 5
4. Sending the Initial Offer . . . . . . . . . . . . . . . . . . 6
5. Receiving the Initial Offer . . . . . . . . . . . . . . . . . 6
5.1. Sending the Initial Answer . . . . . . . . . . . . . . . 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 . . . . . 9
8.1. Pairing Newly Learned Candidates and Updating
Check Lists . . . . . . . . . . . . . . . . . . . . . . . 12
8.2. Announcing End of Candidates . . . . . . . . . . . . . . 13
9. Receiving Additional Remote Candidates . . . . . . . . . . . 15
10. Receiving an End-Of-Candidates Notification . . . . . . . . . 15
11. Trickle ICE and Peer Reflexive Candidates . . . . . . . . . . 15
12. Concluding ICE Processing . . . . . . . . . . . . . . . . . . 16
13. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . . . 16
14. Unilateral Use of Trickle ICE (Half Trickle) . . . . . . . . 16
15. Example Flow . . . . . . . . . . . . . . . . . . . . . . . . 17
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
17. Security Considerations . . . . . . . . . . . . . . . . . . . 18
18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
19.1. Normative References . . . . . . . . . . . . . . . . . . 19
19.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Interaction with ICE . . . . . . . . . . . . . . . . 20
Appendix B. Interaction with ICE Lite . . . . . . . . . . . . . 21
Appendix C. Changes from Earlier Versions . . . . . . . . . . . 22
C.1. Changes from draft-ietf-ice-trickle-02 . . . . . . . . . 22
C.2. Changes from draft-ietf-ice-trickle-01 . . . . . . . . . 23
C.3. Changes from draft-ietf-ice-trickle-00 . . . . . . . . . 23
C.4. Changes from draft-mmusic-trickle-ice-02 . . . . . . . . 23
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C.5. Changes from draft-ivov-01 and draft-mmusic-00 . . . . . 23
C.6. Changes from draft-ivov-00 . . . . . . . . . . . . . . . 24
C.7. Changes from draft-rescorla-01 . . . . . . . . . . . . . 25
C.8. Changes from draft-rescorla-00 . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
The Interactive Connectivity Establishment (ICE) protocol
[rfc5245bis] describes mechanisms for gathering candidates,
prioritizing them, choosing default ones, exchanging them with the
remote party, pairing them, and ordering them into check lists. Once
all of these actions have been completed (and only then), the
participating agents can begin a phase of connectivity checks and
eventually select the pair of candidates that will be used in a media
session.
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, discovering UPnP
devices, 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 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 a session has been initiated. Connectivity
checks for a media stream can also start as soon as the first
candidates for that stream become available.
Trickle ICE can reduce session establishment times in cases where
connectivity is confirmed for the first exchanged candidates (e.g.,
where the host candidates for one of the agents are directly
reachable from the second agent, such as host candidates at a media
relay). Even when this is not the case, running candidate gathering
for both agents and connectivity checks in parallel can considerably
shorten ICE processing times.
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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 so-called "Vanilla 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]).
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.
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].
Vanilla ICE: The Interactive Connectivity Establishment protocol as
defined in [rfc5245bis].
Candidate Gatherer: A module used by an ICE agent to obtain local
candidates. Candidate gatherers use different mechanisms for
discovering local candidates. Some of them would typically make
use of protocols such as STUN or TURN. Others may also employ
techniques that are not referenced within [rfc5245bis] (e.g., UPnP
based port allocation or XMPP Jingle Relay Nodes [XEP-0278]).
Trickled Candidates: Candidates that a Trickle ICE agent sends after
an offer or answer but within the same context. Trickled
candidates can be sent in parallel with candidate gathering and
connectivity checks.
Trickling/Trickle (v.): The act of sending trickled candidates.
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Half Trickle: A Trickle ICE mode of operation where the offerer
gathers its first generation of candidates strictly before
creating and sending the offer. Once sent, that offer can be
processed by Vanilla ICE agents and does not require support for
this specification. It also allows Trickle ICE capable answerers
to still gather candidates and perform connectivity checks in a
non-blocking way, thus roughly offering "half" the advantages of
Trickle ICE. The mechanism is mostly meant for use in cases where
support for Trickle ICE cannot be confirmed prior to sending an
initial offer.
Full Trickle: The regular mode of operation for Trickle ICE agents,
in which an initial offer can include any number of candidates
(even zero candidates) and does not need to include the entire
first generation of candidates as in half trickle.
3. Determining Support for Trickle ICE
Application protocols that use Trickle ICE should do one of the
following:
o Provide a way for agents to verify support of Trickle ICE prior to
initiating a session (XMPP's Service Discovery [XEP-0030] is one
such mechanism).
o Make support for Trickle ICE mandatory so that user agents can
assume support.
Alternately, for cases where a protocol provides neither of the
foregoing methods, agents may rely on provisioning/configuration or
use the half trickle procedure described in Section 14.
Prior to sending an initial offer, 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 Vanilla ICE or abandon the entire session.
In application protocols that use SDP, a user agent supporting
Trickle ICE MUST include a token of "trickle" in the ice-options
attribute every time it generates an offer or an answer. This
enables an agent that receives offers or answers to verify support by
checking for inclusion of the token.
Dedicated discovery semantics and half trickle are needed only prior
to session initiation (e.g., when sending the initial offer). After
a session is established and Trickle ICE support is confirmed for
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both parties, either agent can use full trickle for subsequent
offers.
4. Sending the Initial Offer
An agent starts 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). Contrary to Vanilla ICE,
implementations of Trickle ICE do not need to gather candidates in a
blocking manner. Therefore, unless half trickle is being used,
agents SHOULD generate and transmit their initial offer as early as
possible, in order to allow the remote party to start gathering and
trickling candidates.
Trickle ICE agents MAY include any set of candidates in an offer.
This includes the possibility of sending an offer that contains all
the candidates that the agent plans to use (as in half trickle mode),
sending an offer that contains only a publically-reachable IP address
(e.g., a host candidate at a media relay that is known to not be
behind a firewall), or sending an offer with no candidates at all (in
which case the offerer can receive the answerer's initial candidate
list sooner and the answerer can begin candidate gathering more
quickly).
For optimal performance, it is RECOMMENDED that the candidates in an
initial offer (if any) be host candidates only. This would allow
both agents to start gathering server reflexive, relayed, and other
non-host candidates simultaneously, and it would also enable them to
begin connectivity checks.
If the privacy implications of revealing host addresses on an
endpoint device are a concern, agents can generate an offer that
contains no candidates and then only trickle candidates that do not
reveal host addresses (e.g., relayed candidates).
Methods for calculating priorities and foundations, as well as
determining redundancy of candidates, work just as with Vanilla ICE.
5. Receiving the Initial Offer
When an agent receives an initial offer, it will first check if the
offer or offerer indicates support for Trickle ICE as explained in
Section 3. If this is not the case, the agent MUST process the offer
according to Vanilla ICE procedures [rfc5245bis] or offer/answer
processing rules [RFC3264] if no ICE support is detected at all.
If support for Trickle ICE is confirmed, an agent will automatically
assume support for Vanilla ICE as well even if the support
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verification procedure in [rfc5245bis] indicates otherwise.
Specifically, the rules from [rfc5245bis] would imply that ICE itself
is not supported if the initial offer includes no candidates in the
offer; however, such a conclusion is not warranted if the answerer
can confirm that the offerer supports Trickle ICE and thus fallback
to [RFC3264] is not necessary.
If the offer does indicate support for Trickle ICE, the agent will
determine its role, start gathering and prioritizing candidates;
while doing so, it will also respond by sending its own answer, so
that both agents can start forming check lists and begin connectivity
checks.
5.1. Sending the Initial Answer
An agent can respond to an initial offer at any point while gathering
candidates. The answer can again contain any set of candidates,
including all candidates or no candidates. (The benefit of including
no candidates is to send the answer as quickly as possible, so that
both parties can consider the overall session to be under active
negotiation as soon as possible.) Unless the answering agent is
protecting host addresses for privacy reasons, it would typically
construct this initial answer including only host addresses, thus
enabling the remote party to also start forming check lists and
performing connectivity checks.
In application protocols that use SDP, the answer MUST indicate
support for Trickle ICE as described in Section 3.
5.2. Forming Check Lists and Beginning Connectivity Checks
After exchanging the offer and answer, and as soon as they have
obtained local and remote candidates, agents begin forming candidate
pairs, computing candidate pair priorities and ordering candidate
pairs, pruning duplicate pairs, and creating check lists according to
the Vanilla ICE procedures described in [rfc5245bis].
According to those procedures, in order for candidate pairing to be
possible and for duplicate candidates to be pruned, the candidates
would need to be provided in both the offer and the answer. 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: the agents still
create the check lists, but they only populate the check lists after
they actually have the candidate pairs.
Note: According to [rfc5245bis], "A check list with at least one
pair that is Waiting is called an active check list, and a check
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list with all pairs Frozen is called a frozen check list."
Formally speaking an active check list does not have a state of
Active and a frozen check list does not have a state of Frozen,
because the only check list states are Running, Completed, and
Failed.
A Trickle ICE agent initially considers all check lists to be frozen.
It then inspects the first check list and attempts to unfreeze all
candidates 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.
With regard to pruning of duplicate candidate pairs, a Trickle ICE
agent SHOULD follow a policy of "first one wins" and not re-apply the
pruning procedure if a higher-priority candidate pair is received
from the remote agent.
Respecting the order in which check lists have been reported to an
ICE implementation is crucial to the frozen candidates algorithm, so
that connectivity checks are performed simultaneously by both agents.
6. Receiving the Initial Answer
When receiving an answer, agents follow Vanilla ICE procedures to
determine their role, after which they form check lists (as described
in Section 5.2) and begin connectivity checks.
7. Performing Connectivity Checks
For the most part, Trickle ICE agents perform connectivity checks
following Vanilla 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 5.8, 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.
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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).
7.2. Check List and Timer State Updates
The ICE specification [rfc5245bis], Section 7.1.3.3, requires that
agents update check lists and timer states upon completing a
connectivity check transaction. During such an update, Vanilla 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 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.
Vanilla ICE requires that agents then update all other check lists,
placing one pair from each of them into the Waiting state,
effectively unfreezing all remaining check lists. However, under
Trickle ICE other check lists might still be empty at that point.
Therefore a Trickle ICE agent SHOULD monitor whether a check list is
active or frozen independently of the state of the candidate pairs
that the check list contains. A Trickle ICE agent SHOULD consider a
check list to be active either when unfreezing the first candidate
pair in the check list or when there is no candidate pair in the
check list (i.e., when the check list is empty).
8. Discovering and Sending Additional Local Candidates
After an offer or an answer has 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
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priority, type, foundation and component ID according to normal
Vanilla 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
Vanilla 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 signaling protocols such as SIP or XMPP.
Trickle ICE imposes no restrictions on the way this is done or
whether it is done at all. For example, some applications may choose
not to send trickle updates for server reflexive candidates and rely
on the discovery of peer reflexive ones instead.
When trickle updates are sent, each candidate MUST be delivered to
the receiving Trickle ICE implementation not more than once and in
the same order that they were sent. In other words, if there are any
candidate retransmissions, they must be hidden from the ICE
implementation.
Also, candidate trickling needs to be correlated to a specific ICE
negotiation 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 SDP that
represents candidates 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.
Note: The signaling protocol needs to provide a mechanism for both
parties to indicate and agree on the ICE negotiation session (ufrag)
in force 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).
One important aspect of Vanilla ICE is that connectivity checks for a
specific foundation and component are attempted simultaneously by
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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 crucial to
unfreezing candidates at the right time.
In order to preserve this feature in Trickle ICE, when trickling
candidates agents MUST respect the order of the components as they
appear (implicitly or explicitly) in the offer/answer 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):
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.
Note: The order restriction only applies among candidates that belong
to the same foundation.
It is also equally important to preserve this order across media
streams, which is covered by the requirement to always start
unfreezing candidates starting from the first media stream as
described under Section 5.2.
Once the candidate has been sent to the remote party, the agent
checks if any remote candidates are currently known for this same
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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 its type 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 pre-existing pair
is peer reflexive, the agent MUST replace it with the new candidate
it received (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.
Note: Replacing pre-existing pairs with seemingly equivalent
higher-priority ones helps guarantee that both agents will have
the same view of candidate priorities. This is particularly
important during aggressive nomination, when priority is sometimes
the only way a controlled agent can determine the selected pair.
It is for that same reason that peer-reflexive candidates need to
always be updated if equivalent alternatives are received through
signaling.
For all other pairs, including those with a server reflexive local
candidate that were not found to be redundant:
o if this check list is frozen then the new pair will be assigned a
state of Frozen.
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o else if the check list is active and it is either empty or
contains only candidates in the Succeeded and Failed states, then
the new pair's state is set to Waiting.
o else if the check list is non-empty and active, then the state of
the new pair will be set to
Frozen: if there is at least one pair in the check list whose
foundation matches the one in the new pair and whose state is
neither Succeeded nor Failed (eventually the new pair will get
unfrozen after the ongoing check for the existing pair
concludes);
Waiting: if the list contains no pairs with the same foundation
as the new one, or in case such pairs exist but they are all in
either the Succeeded or Failed states.
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 offer (which would typically be the case with half
trickle initial offers)
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)
A controlled Trickle ICE agent SHOULD send end-of-candidates
indications after gathering for a media stream has completed, unless
ICE processing terminates before the agent has had a chance to do so.
Sending the indication is necessary in order to avoid ambiguities and
speed up the conclusion of ICE processing. On the other hand, a
controlling agent MAY conclude ICE processing prior to sending end-
of-candidates indications for all streams. This would typically be
the case with aggressive nomination. However, it is RECOMMENDED that
controlling agents do send such indications whenever possible for the
sake of consistency and to keep middle boxes and controlled agents
up-to-date on the state of ICE processing.
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When sending an end-of-candidates indication during trickling (rather
than as a part of an offer or an answer), it is the responsibility of
the using protocol to define methods for relating the indication to
one or more specific media streams.
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 notificaiton 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 offer unless it is
planning to potentially send additional candidates (e.g., in case the
remote party turns out to support Trickle ICE).
When an end-of-candidates indication is sent as part of an offer or
an answer, it can be considered to apply to the session as a whole,
which is equivalent to having it apply to all media streams.
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 Vanilla 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 will still conclude even if end-of-
candidates indications have not been received for all streams.
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9. Receiving Additional Remote Candidates
At any time during ICE processing, a Trickle ICE agent may 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 simply 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.
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 may 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
check list, it will update the state of the 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, their processing could be
impacted in implementations. 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.
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12. Concluding ICE Processing
This specification does not directly modify the procedures ending ICE
processing described in Section 8 of [rfc5245bis], and Trickle ICE
implementations will follow the same rules.
13. Subsequent Offer/Answer Exchanges
Either agent MAY generate a subsequent offer at any time allowed by
[RFC3264]. When this happens agents will use [rfc5245bis] semantics
to determine whether or not the new offer requires an ICE restart.
If this is the case then agents would perform Trickle ICE as they
would in an initial offer/answer exchange.
The only differences between an ICE restart and a brand new media
session are that:
o during the restart, media can continue to be sent to the
previously validated pair.
o both agents are already aware whether or not their peer supports
Trickle ICE, and there is no longer need for performing half
trickle or confirming support with other mechanisms.
14. Unilateral Use of Trickle ICE (Half Trickle)
In half trickle mode, the offerer sends a regular, Vanilla ICE offer,
with a complete set of candidates. This ensures that the offer can
be processed by a Vanilla ICE answerer and is mostly meant for use in
cases where support for Trickle ICE cannot be confirmed prior to
sending an initial offer. The initial offer indicates support for
Trickle ICE, so that the answerer can respond with an incomplete set
of candidates and continue trickling the rest. Half trickle offers
typically contain an end-of-candidates indication, although this is
not mandatory because if trickle support is confirmed then the
offerer 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 offer contains a full set of
candidates, it can thus be handled by a regular Vanilla 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).
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Use of half trickle is only necessary during an initial offer/answer
exchange. 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 offer/answer 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 offer, 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 offer. Because the answerer will be able to
trickle candidates, both agents will be able to start connectivity
checks and complete ICE processing earlier than with Vanilla 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 Vanilla ICE because it would result in a better experience for
answerers.
15. Example Flow
A typical successful Trickle ICE exchange with an Offer/Answer
protocol would look this way:
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Alice Bob
| Offer |
|---------------------------------------------->|
| Additional Candidates |
|---------------------------------------------->|
| |
| Answer |
|<----------------------------------------------|
| Additional Candidates |
|<----------------------------------------------|
| |
| Additional Candidates and Connectivity Checks |
|<--------------------------------------------->|
| |
|<=============== MEDIA FLOWS =================>|
Figure 1: Example
16. IANA Considerations
This specification requests no actions from IANA.
17. Security Considerations
This specification inherits most of its semantics from [rfc5245bis]
and as a result all security considerations described there apply to
Trickle ICE.
18. Acknowledgements
The authors would like to thank Taylor Brandstetter for identifying
the need to replace pre-existing peer-reflexive candidates with
higher-priority ones received from trickling and the fact that not
doing so could break aggressive nomination.
The authors would also like to thank Bernard Aboba, Flemming
Andreasen, Rajmohan Banavi, Christer Holmberg, Jonathan Lennox,
Enrico Marocco, Pal Martinsen, Martin Thomson, Dale R. Worley, and
Brandon Williams for their reviews and suggestions on improving this
document.
19. References
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19.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[rfc5245bis]
Keranen, A. and J. Rosenberg, "Interactive Connectivity
Establishment (ICE): A Protocol for Network Address
Translator (NAT) Traversal", draft-ietf-ice-rfc5245bis-00
(work in progress), October 2015.
19.2. 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-05 (work in progress),
August 2016.
[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.
[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.
[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.
[XEP-0278]
Camargo, T., "XEP-0278: Jingle Relay Nodes", XEP XEP-0278,
June 2011.
Appendix A. Interaction with 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 [RFC1918] block.
2. Alice sends Bob the candidate 2001:db8:a0b:12f0::10 which also
happens to correspond to an existing host on Bob's network.
3. Bob creates a check list consisting solely of
2001:db8:a0b:12f0::10 and starts checks.
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4. These checks reach the host at 2001:db8:a0b:12f0::10 in Bob's
network, which responds with an ICMP "port unreachable" error and
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 offer 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 sends an offer to a trickle one. 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 offer 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 offers or answers as per
[rfc5245bis]. Both its offers and answers will indicate support for
Trickle ICE. Given that they will contain a complete set of
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candidates (the agent's host candidates), these offers and answers
would also be accompanied with an end-of-candidates indication.
When performing full trickle, a full ICE implementation could send an
offer or an answer with no candidates. After receiving an answer
that identifies the remote agent as an ICE lite implementation, the
offerer may choose to not send any additional candidates. The same
is also true in the case when the ICE lite agent is making the offer
and the full ICE agent is answering. 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 2: Example
In addition to reducing signaling traffic this approach also removes
the need to discover STUN bindings, or to make TURN or UPnP
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-02
o Adjusted unfreezing behavior when there are disparate foundations.
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C.2. Changes from draft-ietf-ice-trickle-01
o Changed examples to use IPv6.
C.3. 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.
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.4. 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.5. 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.
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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.6. 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.
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.
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C.7. 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.
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.8. 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
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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
Justin Uberti
Google
747 6th St S
Kirkland, WA 98033
USA
Phone: +1 857 288 8888
Email: justin@uberti.name
Peter Saint-Andre
Filament
Email: peter@filament.com
URI: https://filament.com/
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