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draft-ietf-mmusic-trickle-ice-01.txt
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Network Working Group E. Ivov
Internet-Draft Jitsi
Intended status: Standards Track E. Rescorla
Expires: August 11, 2014 RTFM, Inc.
J. Uberti
Google
February 7, 2014
Trickle ICE: Incremental Provisioning of Candidates for the Interactive
Connectivity Establishment (ICE) Protocol
draft-ietf-mmusic-trickle-ice-01
Abstract
This document describes an extension to the Interactive Connectivity
Establishment (ICE) protocol that allows 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.
The above mechanism is also referred to as "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-
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 August 11, 2014.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
Ivov, et al. Expires August 11, 2014 [Page 1]
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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. Incompatibility with Standard ICE . . . . . . . . . . . . . . 5
4. Determining Support for Trickle ICE . . . . . . . . . . . . . 6
4.1. Unilateral Use of Trickle ICE (Half Trickle) . . . . . . 7
5. Sending the Initial Offer . . . . . . . . . . . . . . . . . . 8
5.1. Encoding the SDP . . . . . . . . . . . . . . . . . . . . 9
6. Receiving the Initial Offer . . . . . . . . . . . . . . . . . 9
6.1. Sending the Initial Answer . . . . . . . . . . . . . . . 10
6.2. Forming check lists and beginning connectivity
checks . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.3. Encoding the SDP . . . . . . . . . . . . . . . . . . . . 11
7. Receiving the Initial Answer . . . . . . . . . . . . . . . . 11
8. Performing Connectivity Checks . . . . . . . . . . . . . . . 11
8.1. Check List and Timer State Updates . . . . . . . . . . . 11
9. Discovering and Sending Additional Local Candidates . . . . . 12
9.1. Pairing newly learned candidates and updating
check lists . . . . . . . . . . . . . . . . . . . . . . . 14
9.2. Encoding the SDP for Additional Candidates . . . . . . . 15
9.3. Announcing End of Candidates . . . . . . . . . . . . . . 15
10. Receiving Additional Remote Candidates . . . . . . . . . . . 17
11. Receiving an End Of Candidates Notification . . . . . . . . . 17
12. Trickle ICE and Peer Reflexive Candidates . . . . . . . . . . 17
13. Concluding ICE Processing . . . . . . . . . . . . . . . . . . 18
14. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . . . 18
15. Interaction with ICE Lite . . . . . . . . . . . . . . . . . . 18
16. Example Flow . . . . . . . . . . . . . . . . . . . . . . . . 19
17. Security Considerations . . . . . . . . . . . . . . . . . . . 20
18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
19. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
19.1. Normative References . . . . . . . . . . . . . . . . . . 20
19.2. Informative References . . . . . . . . . . . . . . . . . 21
Appendix A. Open issues . . . . . . . . . . . . . . . . . . . . 22
A.1. MID/Stream Indices in SDP . . . . . . . . . . . . . . . . 22
A.2. Starting checks . . . . . . . . . . . . . . . . . . . . . 23
Appendix B. Changes From Earlier Versions . . . . . . . . . . . 23
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B.1. Changes From draft-ivov-01 and draft-mmusic-00 . . . . . 23
B.2. Changes From draft-ivov-00 . . . . . . . . . . . . . . . 23
B.3. Changes From draft-rescorla-01 . . . . . . . . . . . . . 24
B.4. Changes From draft-rescorla-00 . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
The Interactive Connectivity Establishment (ICE) protocol [RFC5245]
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 the above 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 the following session.
While the above sequence has the advantage of being relatively
straightforward to implement and debug once deployed, it may also
prove to be rather lengthy. Gathering candidates or candidate
harvesting would often involve things like querying STUN [RFC5389]
servers, discovering UPnP devices, and allocating relayed candidates
at TURN [RFC5766] servers. All of these can be delayed for a
noticeable amount of time and while they can be run in parallel, they
still need to respect the pacing requirements from [RFC5245], which
is likely to delay them even further. Some or all of the above would
also have to be completed by the remote agent. Both agents would
next perform connectivity checks and only then would they be ready to
begin streaming media.
All of the above could lead to relatively lengthy session
establishment times and degraded user experience.
The purpose of this document is to define an alternative mode of
operation for ICE implementations, also known as "trickle ICE", where
candidates can be exchanged incrementally. This would allow ICE
agents to exchange host candidates as soon as a session has been
initiated. Connectivity checks for a media stream would also start
as soon as the first candidates for that stream have become
available.
Trickle ICE allows reducing 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). Even when this is not the case,
running candidate harvesting for both agents and connectivity checks
all in parallel allows to considerably reduce ICE processing times.
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It is worth pointing out that before being introduced to the IETF,
trickle ICE had already been included in specifications such as XMPP
Jingle [XEP-0176] and it has been in use in various implementations
and deployments.
In addition to the basics of trickle ICE, this document also
describes how support for trickle ICE needs to be discovered, how
regular ICE processing needs to be modified when building and
updating check lists, and how trickle ICE implementations should
interoperate with agents that only implement [RFC5245] processing.
This specification does not define usage of trickle ICE with any
specific signalling protocol, contrary to [RFC5245] which contains a
usage for ICE with SIP. Such usages would have to be specified in
separate documents such as for example
[I-D.ivov-mmusic-trickle-ice-sip].
Trickle ICE does however reuse and build upon the SDP syntax defined
by vanilla ICE.
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 by the
protocol for Interactive Connectivity Establishment in [RFC5245].
Vanilla ICE: The Interactive Connectivity Establishment protocol as
defined in [RFC5245].
Candidate Harvester: A module used by an ICE agent to obtain local
candidates. Candidate harvesters 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 [RFC5245]. UPnP based
port allocation and XMPP Jingle Relay Nodes [XEP-0278] are among
the possible examples.
Trickled Candidates: Candidates that a trickle ICE agent is sending
subsequently to but within the context defined by an offer or an
answer. Trickled candidates can be sent in parallel with
candidate harvesting 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 a
first offer.
Full Trickle: Regular mode of operation for trickle ICE agents, used
in opposition to the half trickle mode of operation.
3. Incompatibility with Standard ICE
The ICE protocol was designed to be fairly flexible so that it would
work in and adapt to as many network environments as possible. It is
hence important to point out at least some of the reasons why,
despite its flexibility, the specification in [RFC5245] would not
support trickle ICE.
[RFC5245] 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:
o 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.
o Alice sends Bob the candidate 10.0.0.10 which also happens to
correspond to an existing host on Bob's network.
o Bob creates a check list consisting solely of 10.0.0.10 and starts
checks.
o These checks reach the host at 10.0.0.10 in Bob's network, which
responds with an ICMP "port unreachable" error and per [RFC5245]
Bob marks the transaction as Failed.
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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 (per
[I-D.keranen-mmusic-ice-address-selection]) from any of the
candidates that Bob has gathered. 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:
o Alice's client has a non-trickle ICE implementation
o Bob's client has support for trickle ICE.
o Alice and Bob are behind NATs with address-dependent filtering
[RFC4787].
o 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 long 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.
4. Determining Support for Trickle ICE
According to [RFC5245] every time an agent supporting trickle ICE
generates an offer or an answer, it MUST include the "trickle" token
in the ice-options attribute. Syntax for this token is defined in
Section 5.1.
Additionally, in order to avoid interoperability problems such as
those described in Section 3, it is important that trickle ICE
negotiation is only attempted in cases where the remote party
actually supports this specification. Agents that receive offers or
answers can verify support by examining them for the "trickle" ice-
options token. However, agents that are about to send a first offer,
have no immediate way of doing this. This means that usages of
trickle for specific protocols would need to either:
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o Provide a way for agents to verify support of trickle ICE prior to
initiating a session. XMPP's Service discovery [XEP-0030] is an
example for one such mechanism;
o Make support for trickle ICE mandatory so that support could be
assumed the agents.
Alternately, for cases where a protocol provides neither of the
above, agents may either rely on provisioning/configuration, or use
the half trickle procedure described in Section 4.1.
Note that out-of-band discovery semantics and half trickle are only
necessary prior to session initiation, or in other words, when
sending the initial offer. Once a session is established and trickle
ICE support is confirmed for both parties, either agent can use full
trickle for subsequent offers.
4.1. Unilateral Use of Trickle ICE (Half Trickle)
The idea of using half trickle is about having the caller send a
regular, vanilla ICE offer, with a complete set of candidates. This
offer still indicates support for trickle ice, so the answerer is
able to respond with an incomplete set of candidates and continue
trickling the rest. Half trickle offers will typically contain an
end-of-candidates indication.
The 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 it contains a full set of candidates, its first offer can
thus be handled by a regular vanilla ICE agent, while still allowing
a trickle one to use the optimisation 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 offer/answer
exchange. Once 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.
It is worth pointing out that using half trickle may actually bring
more than just half the improvement in terms of user experience.
This can happen in cases where an agent starts gathering candidates
upon user interface cues that a call is pending, such as activity on
a keypad or the phone going off hook. This would mean a part or all
candidate harvesting could have completed before the agent actually
needs to send the offer. Given that the answerer will be able to
trickle candidates, both agents will be able to start connectivity
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checks and complete ICE processing earlier than with vanilla ICE and
potentially even as early as with full trickle.
However, such anticipation is not 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.
Still, even in these cases, using half trickle would be an
improvement over vanilla ICE as it would optimize performance for
answerers.
5. 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 generating one with no candidates,
or one that contains all the candidates that the agent is planning on
using in the following session.
For optimal performance, it is RECOMMENDED that an initial offer
contains 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 are a
concern, agents MAY generate an offer that contains no candidates and
then only trickle candidates that do not reveal host addresses (e.g.
relayed candidates).
Prior to actually sending an initial offer, agents MAY verify if the
remote party supports trickle ICE, where such mechanisms actually
exist. If absence of such support is confirmed agents MUST fall back
to using vanilla ICE or abandon the entire session.
All trickle ICE offers and answers MUST indicate support of this
specification, as explained in Section 5.1.
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Calculating priorities and foundations, as well as determining
redundancy of candidates work the same way they do with vanilla ICE.
5.1. Encoding the SDP
The process of encoding the SDP [RFC4566] is mostly the same as the
one used by vanilla ICE. Still, trickle ICE does require a few
differences described here.
Agents MUST indicate support for Trickle ICE by including the
"trickle" token for the "a=ice-options" attribute:
a=ice-options:trickle
As mentioned earlier in this section, Offers and Answers can contain
any set of candidates, which means that a trickle ICE session
description MAY contain no candidates at all. In such cases the
agent would still need to place an address in the "c=" line(s). If
the use of a host address there is undesirable (e.g. for privacy
reasons), the agent MAY set the connection address to IP6 ::. In this
case it MUST also set the port number to 9 (Discard). There is no
need to include a fictitious candidate for the IP6 :: address when
doing so.
It is worth noting that the use of IP6 :: has been selected over IP4
0.0.0.0, even though [RFC3264] already gives the latter semantics
appropriate for such use. The reason for this choice is the historic
use of 0.0.0.0 as a means of putting a stream on hold [RFC2543] and
the ambiguity that this may cause with legacy libraries and
applications.
It is also worth mentioning that use of IP6 :: here does not
constitute any kind of indication as to the actual use of IPv6
candidates in a session and it can very well appear in a negotiation
that only involves IPv4 candidates.
6. Receiving the Initial Offer
When an agent receives an initial offer, it will first check if it
indicates support for trickle ICE as explained in Section 4. If this
is not the case, the agent MUST process the offer according to the
[RFC5245] procedures or standard [RFC3264] processing in case no ICE
support is detected at all.
It is worth pointing out that in case support for trickle ICE is
confirmed, an agent will automatically assume support for vanilla ICE
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as well even if the support verification procedure in [RFC5245]
indicates otherwise. Specifically, such verification would indicate
lack of support when the offer contains no candidates. The IP6 ::
address present in the c= line in that case would not "appear in a
candidate attribute". Obviously, a fallback to [RFC3264] is not
required when this happens.
If, the offer does indicate support for trickle ICE, the agent will
determine its role, start gathering and prioritizing candidates and,
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.
6.1. 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 none or all of them. Unless it is protecting host
addresses for privacy reasons, the agent would typically construct
this initial answer including only them, thus allowing the remote
party to also start forming checklists and performing connectivity
checks.
The answer MUST indicate support for trickle ICE as described by
Section 4.
6.2. Forming check lists and beginning connectivity checks
After exchanging offer and answer, and as soon as they have obtained
local and remote candidates, agents will begin forming candidate
pairs, computing their priorities and creating check lists according
to the vanilla ICE procedures described in [RFC5245]. Obviously in
order for candidate pairing to be possible, it would be necessary
that both the offer and the answer contained candidates. If this was
not the case agents will still create the check lists (so that their
Active/Frozen state could be monitored and updated) but they will
only populate them once they actually have the candidate pairs.
Initially, all check lists will have their Active/Frozen state set to
Frozen.
Trickle ICE agents will then inspect the first check list and attempt
to unfreeze all candidates belonging 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 this
checklist is still empty however, agents will hold off further
processing until this is no longer the case.
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Respecting the order in which lists have been reported to an ICE
implementation, or in other words, the order in which they appear in
SDP, is crucial to the frozen candidates algorithm and important when
making sure that connectivity checks are performed simultaneously by
both agents.
6.3. Encoding the SDP
The process for encoding the SDP at the answerer is identical to the
process followed by the offerer for both full and lite
implementations, as described in Section 5.1.
7. Receiving the Initial Answer
When receiving an answer, agents will follow vanilla ICE procedures
to determine their role and they would then form check lists (as
described in Section 6.2) and begin connectivity checks .
8. Performing Connectivity Checks
For the most part, trickle ICE agents perform connectivity checks
following vanilla ICE procedures. Of course, the asynchronous nature
of candidate harvesting in trickle ICE would impose a number of
changes described here.
8.1. Check List and Timer State Updates
The vanilla ICE specification 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 the following two conditions are satisfied:
o all of the pairs in the check list are either in the Failed or
Succeeded state;
o if at least one of the components of the media stream has no pairs
in its valid list.
With trickle ICE, the above situation would often occur when
candidate harvesting and trickling are still in progress and it is
perfectly possible that future checks will succeed. For this reason
trickle ICE agents add the following conditions to the above list:
o all candidate harvesters 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 9.3.
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Vanilla ICE requires that agents then update all other check lists,
placing one pair in each of them into the Waiting state, effectively
unfreezing all remaining check lists. Given that with trickle ICE,
other check lists may still be empty at that point, a trickle ICE
agent SHOULD also maintain an explicit Active/Frozen state for every
check list, rather than deducing it from the state of the pairs it
contains. This state should be set to Active when unfreezing the
first pair in a list or when that couldn't happen because a list was
empty.
9. Discovering and Sending Additional Local Candidates
After an offer or an answer have been sent, agents will most likely
continue discovering new local candidates as STUN, TURN and other
non-host candidate harvesting 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 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 client sends (i.e. trickles) the newly learnt candidate(s)
to the remote agent. The actual delivery of the new candidates will
be specified by using protocols such as SIP. 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 however, 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.
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One important aspect of Vanilla ICE is that connectivity checks for a
specific foundation and component be 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 crucial to unfreezing candidates
in the right time.
In order to preserve this feature here, 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 session description contains two
components (RTP and RTCP), and two foundations (host and the server
reflexive):
v=0
o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
s=
c=IN IP4 10.0.1.1
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 5000 RTP/AVP 0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 10.0.1.1 5000 typ host
a=candidate:1 2 UDP 2130706431 10.0.1.1 5001 typ host
a=candidate:2 1 UDP 1694498815 192.0.2.3 5000 typ srflx
raddr 10.0.1.1 rport 8998
a=candidate:2 2 UDP 1694498815 192.0.2.3 5001 typ srflx
raddr 10.0.1.1 rport 8998
For this 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 that 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 and this is covered by the requirement to always start
unfreezing candidates starting from the first media stream
Section 6.2.
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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 this is not the case 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.
9.1. Pairing newly learned candidates and updating check lists
Forming candidate pairs will work the way it is described by the
vanilla ICE specification. Actually adding the new pair to a check
list however, will happen 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 [RFC5245]),
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, the newly formed pair is ignored.
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 also be
assigned a Frozen state.
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 new pair
state will be set to
Frozen: if there is at least one pair in the 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 the on-going 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.
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9.2. Encoding the SDP for Additional Candidates
To facilitate interoperability an ICE agent will encode additional
candidates using the vanilla ICE SDP syntax. For example:
a=candidate:2 1 UDP 1658497328 198.51.100.33 5000 typ host
Given that such lines do not provide a relationship between the
candidate and the m line that it relates to, signalling protocols
using trickle ICE MUST establish that relation themselves using an
MID [RFC3388]. Such MIDs use "media stream identification", as
defined in [RFC3388], to identify a corresponding m-line. When
creating candidate lines usages of trickle ICE MUST use the MID if
possible, or the m-line index if not. Obviously, agents MUST NOT
send individual candidates prior to generating the corresponding SDP
session description.
The exact means of transporting additional candidates to a remote
agent is left to the protocols using trickle ICE. It is important to
note, however, that these candidate exchanges are not part of the
offer/answer model.
9.3. Announcing End of Candidates
Once all candidate harvesters for a specific media stream complete,
or expire, the agents will generate an "end-of-candidates" indication
for that stream and send it to the remote agent via the signalling
channel. Such indications are sent in the form of a media-level
attribute that has the following form: end-of-candidates.
a=end-of-candidates
The end-of-candidates indications can be sent as part of an offer,
which would typically be the case with half trickle initial offers,
they can accompany the last candidate an agent can send for a stream,
and they can also be sent alone (e.g. after STUN Binding requests or
TURN Allocate requests to a server timeout and the agent has no other
active harvesters).
Controlled trickle ICE agents SHOULD always send end-of-candidates
indications once harvesting for a media stream has completed unless
ICE processing terminates before they've had a chance to do so.
Sending the indication is necessary in order to avoid ambiguities and
speed up ICE conclusion. This is necessary in order to avoid
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ambiguities and speed up ICE conclusion. Controlling agents on the
other hand MAY sometimes conclude ICE processing prior to sending
end-of-candidates notifications for all streams. This would
typically be the case with aggressive nomination. Yet it is
RECOMMENDED that controlling agents do send such indications whenever
possible for the sake of consistency and keeping middle boxes and
controlled agents up-to-date on the state of ICE processing.
When sending end-of-candidates during trickling, rather than as a
part of an offer or an answer, it is the responsibility of the using
protocol to define means that can be used to relate the indication to
one or more specific m-lines.
Receiving an end-of-candidates notification allows 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 allows agents to speed ICE conclusion in cases where
a candidate pair has been validates but it involves the use of lower-
preference transports such as TURN. In such situations some
implementations may choose to wait in case higher-priority candidates
are received and end-of-candidates provides an indication that this
is not going to happen.
An agent MAY also choose to generate an end-of-candidates event
before candidate harvesting has actually completed, if the agent
determines that harvesting has continued for more than an acceptable
period of time. However, an agent MUST NOT send any more candidates
after it has send an end-of-candidates notification.
When performing half trickle agents SHOULD send end-of-candidates
together with their initial offer unless they are planning on
potentially sending additional candidates in case the remote party
turns out to actually support trickle ICE.
When end-of-candidates is sent as part of an offer or an answer it
can appear as a session-level attribute, which would be equivalent to
having it appear in all m-lines.
Once an agent sends the end-of-candidates event, it will update the
state of the corresponding check list as explained in section
Section 8.1. Past that point agents MUST NOT send any new
candidates. Once an agent has received an end-of-candidates
indication, it MUST also ignore any newly received candidates for
that media stream. Adding new candidates to the negotiation is hence
only possible through an ICE restart.
It is important to note that This specification does not override
vanilla ICE semantics for concluding ICE processing. This means that
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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-candidate indications have not been received for all
streams.
10. Receiving Additional Remote Candidates
At any point of 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 9.1.
Once the remote agent has completed candidate harvesting, it will
send an end-of-candidates event. Upon receiving such an event, the
local agent MUST update check list states as per Section 8.1. This
may lead to some check lists being marked as Failed.
11. Receiving an End Of Candidates Notification
When an agent receives an end-of-candidates notification for a
specific check list, they will update its state as per Section 8.1.
In case the list is still in the Active state after the update, the
agent will persist the the fact that an end-of-candidates
notification has been received for and take it into account in future
list updates.
12. 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 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
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to reach the destination faster than the trickle update that travels
through the signalling path.
13. Concluding ICE Processing
This specification does not directly modify the procedures ending ICE
processing described in Section 8 of [RFC5245], and trickle ICE
implementations will follow the same rules.
14. Subsequent Offer/Answer Exchanges
Either agent MAY generate a subsequent offer at any time allowed by
[RFC3264]. When this happens agents will use [RFC5245] 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.
15. Interaction with ICE Lite
Behaviour of Trickle ICE capable ICE lite agents does not require any
particular rules other than those already defined in this
specification and [RFC5245]. This section is hence added with an
informational purpose only.
A Trickle ICE capable ICE Lite agent would generate offers or answers
as per [RFC5245]. Both will indicate support for trickle ICE
(Section 5.1) and given that they will contain a complete set of
candidates (the agent's host candidates) these offers and answers
would also be accompanied with an end-of-candidates notification.
When performing full trickle, a full ICE implementation could send an
offer or an answer with no candidates and an IP6 :: connection line
address. After receiving an answer that identifies the remote agent
as an ICE lite implementation, the offerer may very well choose to