| Network Working Group | E. Ivov |
| Internet-Draft | Jitsi |
| Intended status: Standards Track | E. Rescorla |
| Expires: December 8, 2016 | RTFM, Inc. |
| J. Uberti | |
| P. Saint-Andre | |
| Filament | |
| June 6, 2016 |
Trickle ICE: Incremental Provisioning of Candidates for the Interactive Connectivity Establishment (ICE) Protocol
draft-ietf-ice-trickle-02
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".
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This Internet-Draft will expire on December 8, 2016.
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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 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.
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 signalling 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.
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].
Application protocols that use Trickle ICE should do one of the following:
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 presence 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 both parties, either agent can use full trickle for subsequent offers.
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.
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 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 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.
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.
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.
A Trickle ICE agent MUST initially consider all check lists to be frozen. It then inspects the first check list and attempts 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). However, if this check list is still empty, an agent delays further processing until the check list is non-empty.
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.
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.
For the most part, Trickle ICE agents perform connectivity checks following Vanilla ICE procedures. However, the asynchronous nature of gathering and communicating candidates in Trickle ICE impose a number of changes described as described in the following sections.
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 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).
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:
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:
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).
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 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 are specified by using 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.
One important aspect of Vanilla 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 crucial to unfreezing candidates in 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.
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 example, the following SDP description contains two components (RTP and RTCP) and two foundations (host and server reflexive):
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, 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 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.
Forming candidate pairs works the way it is described by 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, 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:
Once all candidate gathering is completed or expires for a specific media stream, the agents will generate an "end-of-candidates" indication for that stream and send it to the remote agent via the signalling channel. The exact form of the indication depends on the application protocol. The indication can be sent in the following ways:
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.
When sending an end-of-candidate 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 implementations 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 send 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.
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-candidate indications have not been received for all streams.
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 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.
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 active state after the update, the agent will persist the the fact that an end-of-candidates indication has been received and take it into account in future updates to the check list.
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 signalling path.
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.
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:
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).
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.
A typical successful Trickle ICE exchange with an Offer/Answer protocol would look this way:
Alice Bob
| Offer |
|---------------------------------------------->|
| Additional Candidates |
|---------------------------------------------->|
| |
| Answer |
|<----------------------------------------------|
| Additional Candidates |
|<----------------------------------------------|
| |
| Additional Candidates and Connectivity Checks |
|<--------------------------------------------->|
| |
|<=============== MEDIA FLOWS =================>|
Figure 1: Example
This specification requests no actions from IANA.
This specification inherits most of its semantics from [rfc5245bis] and as a result all security considerations described there remain the same.
The authors would 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.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC3264] | Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, DOI 10.17487/RFC3264, June 2002. |
| [RFC4566] | Handley, M., Jacobson, V. and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006. |
| [rfc5245bis] | Keranen, A. and J. Rosenberg, "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal", Internet-Draft draft-ietf-ice-rfc5245bis-00, October 2015. |
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:
This could be a problem and cause ICE processing to fail prematurely in a number of scenarios. Consider the following case:
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:
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.
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.
Such an 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 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 one 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.
Note to the RFC-Editor: please remove this section prior to publication as an RFC.