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8. Concluding ICE Processing

This section describes how an agent completes ICE.

8.1. Procedures for Full Implementations​

Concluding ICE involves nominating pairs by the controlling agent and updating of state machinery.

8.1.1. Nominating Pairs​

The controlling agent nominates pairs to be selected by ICE by using one of two techniques: regular nomination or aggressive nomination. If its peer has a lite implementation, an agent MUST use a regular nomination algorithm. If its peer is using ICE options (present in an ice-options attribute from the peer) that the agent does not understand, the agent MUST use a regular nomination algorithm. If its peer is a full implementation and isn't using any ICE options or is using ICE options understood by the agent, the agent MAY use either the aggressive or the regular nomination algorithm. However, the regular algorithm is RECOMMENDED since it provides greater stability.

8.1.1.1. Regular Nomination​

With regular nomination, the agent lets some number of checks complete, each of which omit the USE-CANDIDATE attribute. Once one or more checks complete successfully for a component of a media stream, valid pairs are generated and added to the valid list. The agent lets the checks continue until some stopping criterion is met, and then picks amongst the valid pairs based on an evaluation criterion. The criteria for stopping the checks and for evaluating the valid pairs is entirely a matter of local optimization.

When the controlling agent selects the valid pair, it repeats the check that produced this valid pair (by enqueuing the pair that generated the check into the triggered check queue), this time with the USE-CANDIDATE attribute. This check should succeed (since the previous did), causing the nominated flag of that and only that pair to be set. Consequently, there will be only a single nominated pair in the valid list for each component, and when the state of the check list moves to completed, that exact pair is selected by ICE for sending and receiving media for that component.

Regular nomination provides the most flexibility, since the agent has control over the stopping and selection criteria for checks. The only requirement is that the agent MUST eventually pick one and only one candidate pair and generate a check for that pair with the USE-CANDIDATE attribute present. Regular nomination also improves ICE's resilience to variations in implementation (see Section 14). Regular nomination is also more stable, allowing both agents to converge on a single pair for media without any transient selections, which can happen with the aggressive algorithm. The drawback of regular nomination is that it is guaranteed to increase latencies because it requires an additional check to be done.

8.1.1.2. Aggressive Nomination​

With aggressive nomination, the controlling agent includes the USE-CANDIDATE attribute in every check it sends. Once the first check for a component succeeds, it will be added to the valid list and have its nominated flag set. When all components have a nominated pair in the valid list, media can begin to flow using the highest priority nominated pair. However, because the agent included the USE-CANDIDATE attribute in all of its checks, another check may yet complete, causing another valid pair to have its nominated flag set. ICE always selects the highest-priority nominated candidate pair from the valid list as the one used for media. Consequently, the selected pair may actually change briefly as ICE checks complete, resulting in a set of transient selections until it stabilizes.

8.1.2. Updating States​

For both controlling and controlled agents, the state of ICE processing depends on the presence of nominated candidate pairs in the valid list and on the state of the check list. Note that, at any time, more than one of the following cases can apply:

  • If there are no nominated pairs in the valid list for a media stream and the state of the check list is Running, ICE processing continues.

  • If there is at least one nominated pair in the valid list for a media stream and the state of the check list is Running:

    • The agent MUST remove all Waiting and Frozen pairs in the check list and triggered check queue for the same component as the nominated pairs for that media stream.

    • If an In-Progress pair in the check list is for the same component as a nominated pair, the agent SHOULD cease retransmissions for its check if its pair priority is lower than the lowest-priority nominated pair for that component.

  • Once there is at least one nominated pair in the valid list for every component of at least one media stream and the state of the check list is Running:

    • The agent MUST change the state of processing for its check list for that media stream to Completed.

    • The agent MUST continue to respond to any checks it may still receive for that media stream, and MUST perform triggered checks if required by the processing of Section 7.2.

    • The agent MUST continue retransmitting any In-Progress checks for that check list.

    • The agent MAY begin transmitting media for this media stream as described in Section 11.1.

  • Once the state of each check list is Completed:

    • The agent sets the state of ICE processing overall to Completed.

    • If an agent is controlling, it examines the highest-priority nominated candidate pair for each component of each media stream. If any of those candidate pairs differ from the default candidate pairs in the most recent offer/answer exchange, the controlling agent MUST generate an updated offer as described in Section 9. If the controlling agent is using an aggressive nomination algorithm, this may result in several updated offers as the pairs selected for media change. An agent MAY delay sending the offer for a brief interval (one second is RECOMMENDED) in order to allow the selected pairs to stabilize.

  • If the state of the check list is Failed, ICE has not been able to complete for this media stream. The correct behavior depends on the state of the check lists for other media streams:

    • If all check lists are Failed, ICE processing overall is considered to be in the Failed state, and the agent SHOULD consider the session a failure, SHOULD NOT restart ICE, and the controlling agent SHOULD terminate the entire session.

    • If at least one of the check lists for other media streams is Completed, the controlling agent SHOULD remove the failed media stream from the session in its updated offer.

    • If none of the check lists for other media streams are Completed, but at least one is Running, the agent SHOULD let ICE continue.

8.2. Procedures for Lite Implementations​

Concluding ICE for a lite implementation is relatively straightforward. There are two cases to consider:

The implementation is lite, and its peer is full.

The implementation is lite, and its peer is lite.

The effect of ICE concluding is that the agent can free any allocated host candidates that were not utilized by ICE, as described in Section 8.3.

8.2.1. Peer Is Full​

In this case, the agent will receive connectivity checks from its peer. When an agent has received a connectivity check that includes the USE-CANDIDATE attribute for each component of a media stream, the state of ICE processing for that media stream moves from Running to Completed. When the state of ICE processing for all media streams is Completed, the state of ICE processing overall is Completed.

The lite implementation will never itself determine that ICE processing has failed for a media stream; rather, the full peer will make that determination and then remove or restart the failed media stream in a subsequent offer.

8.2.2. Peer Is Lite​

Once the offer/answer exchange has completed, both agents examine their candidates and those of its peer. For each media stream, each agent pairs up its own candidates with the candidates of its peer for that media stream. Two candidates are paired up when they are for the same component, utilize the same transport protocol (UDP in this specification), and are from the same IP address family (IPv4 or IPv6).

  • If there is a single pair per component, that pair is added to the Valid list. If all of the components for a media stream had one pair, the state of ICE processing for that media stream is set to Completed. If all media streams are Completed, the state of ICE processing is set to Completed overall. This will always be the case for implementations that are IPv4 only.

  • If there is more than one pair per component:

    • The agent MUST select a pair based on local policy. Since this case only arises for IPv6, it is RECOMMENDED that an agent follow the procedures of RFC 3484 [RFC3484] to select a single pair.

    • The agent adds the selected pair for each component to the valid list. As described in Section 11.1, this will permit media to begin flowing. However, it is possible (and in fact likely) that both agents have chosen different pairs.

    • To reconcile this, the controlling agent MUST send an updated offer as described in Section 9.1.3, which will include the remote-candidates attribute.

    • The agent MUST NOT update the state of ICE processing when the offer is sent. If this subsequent offer completes, the controlling agent MUST change the state of ICE processing to Completed for all media streams, and the state of ICE processing overall to Completed. The states for the controlled agent are set based on the logic in Section 9.2.3.

8.3. Freeing Candidates​

8.3.1. Full Implementation Procedures​

The procedures in Section 8 require that an agent continue to listen for STUN requests and continue to generate triggered checks for a media stream, even once processing for that stream completes. The rules in this section describe when it is safe for an agent to cease sending or receiving checks on a candidate that was not selected by ICE, and then free the candidate.

When ICE is used with SIP, and an offer is forked to multiple recipients, ICE proceeds in parallel and independently with each answerer, all using the same local candidates. Once ICE processing has reached the Completed state for all peers for media streams using those candidates, the agent SHOULD wait an additional three seconds, and then it MAY cease responding to checks or generating triggered checks on that candidate. It MAY free the candidate at that time. Freeing of server reflexive candidates is never explicit; it happens by lack of a keepalive. The three-second delay handles cases when aggressive nomination is used, and the selected pairs can quickly change after ICE has completed.

8.3.2. Lite Implementation Procedures​

A lite implementation MAY free candidates not selected by ICE as soon as ICE processing has reached the Completed state for all peers for all media streams using those candidates.

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