4. Sending the Initial Offer
In order to send the initial offer in an offer/answer exchange, an
agent must (1) gather candidates, (2) prioritize them, (3) eliminate
redundant candidates, (4) choose default candidates, and then (5)
formulate and send the SDP offer. All but the last of these five
steps differ for full and lite implementations.
4.1. Full Implementation Requirements
4.1.1. Gathering Candidates
An agent gathers candidates when it believes that communication is
imminent. An offerer can do this based on a user interface cue, or
based on an explicit request to initiate a session. Every candidate
is a transport address. It also has a type and a base. Four types
are defined and gathered by this specification -- host candidates,
server reflexive candidates, peer reflexive candidates, and relayed
candidates. The server reflexive candidates are gathered using STUN
or TURN, and relayed candidates are obtained through TURN. Peer
reflexive candidates are obtained in later phases of ICE, as a
consequence of connectivity checks. The base of a candidate is the
candidate that an agent must send from when using that candidate.
18.104.22.168. Host Candidates
The first step is to gather host candidates. Host candidates are
obtained by binding to ports (typically ephemeral) on a IP address
attached to an interface (physical or virtual, including VPN
interfaces) on the host.
For each UDP media stream the agent wishes to use, the agent SHOULD
obtain a candidate for each component of the media stream on each IP
address that the host has. It obtains each candidate by binding to a
UDP port on the specific IP address. A host candidate (and indeed
every candidate) is always associated with a specific component for
which it is a candidate. Each component has an ID assigned to it,
called the component ID. For RTP-based media streams, the RTP itself
has a component ID of 1, and RTCP a component ID of 2. If an agent
is using RTCP, it MUST obtain a candidate for it. If an agent is
using both RTP and RTCP, it would end up with 2*K host candidates if
an agent has K IP addresses.
The base for each host candidate is set to the candidate itself.
22.214.171.124. Server Reflexive and Relayed Candidates
Agents SHOULD obtain relayed candidates and SHOULD obtain server
reflexive candidates. These requirements are at SHOULD strength to
allow for provider variation. Use of STUN and TURN servers may be
unnecessary in closed networks where agents are never connected to
the public Internet or to endpoints outside of the closed network.
In such cases, a full implementation would be used for agents that
are dual stack or multihomed, to select a host candidate. Use of
TURN servers is expensive, and when ICE is being used, they will only
be utilized when both endpoints are behind NATs that perform address
and port dependent mapping. Consequently, some deployments might
consider this use case to be marginal, and elect not to use TURN
servers. If an agent does not gather server reflexive or relayed
candidates, it is RECOMMENDED that the functionality be implemented
and just disabled through configuration, so that it can be re-enabled
through configuration if conditions change in the future.
If an agent is gathering both relayed and server reflexive
candidates, it uses a TURN server. If it is gathering just server
reflexive candidates, it uses a STUN server.
The agent next pairs each host candidate with the STUN or TURN server
with which it is configured or has discovered by some means. If a
STUN or TURN server is configured, it is RECOMMENDED that a domain
name be configured, and the DNS procedures in [RFC5389] (using SRV
records with the "stun" service) be used to discover the STUN server,
and the DNS procedures in [RFC5766] (using SRV records with the
"turn" service) be used to discover the TURN server.
This specification only considers usage of a single STUN or TURN
server. When there are multiple choices for that single STUN or TURN
server (when, for example, they are learned through DNS records and
multiple results are returned), an agent SHOULD use a single STUN or
TURN server (based on its IP address) for all candidates for a
particular session. This improves the performance of ICE. The
result is a set of pairs of host candidates with STUN or TURN
servers. The agent then chooses one pair, and sends a Binding or
Allocate request to the server from that host candidate. Binding
requests to a STUN server are not authenticated, and any ALTERNATE-
SERVER attribute in a response is ignored. Agents MUST support the
backwards compatibility mode for the Binding request defined in
[RFC5389]. Allocate requests SHOULD be authenticated using a long-
term credential obtained by the client through some other means.
Every Ta milliseconds thereafter, the agent can generate another new
STUN or TURN transaction. This transaction can either be a retry of
a previous transaction that failed with a recoverable error (such as
authentication failure), or a transaction for a new host candidate
and STUN or TURN server pair. The agent SHOULD NOT generate
transactions more frequently than one every Ta milliseconds. See
Section 16 for guidance on how to set Ta and the STUN retransmit
The agent will receive a Binding or Allocate response. A successful
Allocate response will provide the agent with a server reflexive
candidate (obtained from the mapped address) and a relayed candidate
in the XOR-RELAYED-ADDRESS attribute. If the Allocate request is
rejected because the server lacks resources to fulfill it, the agent
SHOULD instead send a Binding request to obtain a server reflexive
candidate. A Binding response will provide the agent with only a
server reflexive candidate (also obtained from the mapped address).
The base of the server reflexive candidate is the host candidate from
which the Allocate or Binding request was sent. The base of a
relayed candidate is that candidate itself. If a relayed candidate
is identical to a host candidate (which can happen in rare cases),
the relayed candidate MUST be discarded.
126.96.36.199. Computing Foundations
Finally, the agent assigns each candidate a foundation. The
foundation is an identifier, scoped within a session. Two candidates
MUST have the same foundation ID when all of the following are true:
o they are of the same type (host, relayed, server reflexive, or
o their bases have the same IP address (the ports can be different).
o for reflexive and relayed candidates, the STUN or TURN servers
used to obtain them have the same IP address.
o they were obtained using the same transport protocol (TCP, UDP,
Similarly, two candidates MUST have different foundations if their
types are different, their bases have different IP addresses, the
STUN or TURN servers used to obtain them have different IP addresses,
or their transport protocols are different.
188.8.131.52. Keeping Candidates Alive
Once server reflexive and relayed candidates are allocated, they MUST
be kept alive until ICE processing has completed, as described in
Section 8.3. For server reflexive candidates learned through a
Binding request, the bindings MUST be kept alive by additional
Binding requests to the server. Refreshes for allocations are done
using the Refresh transaction, as described in [RFC5766]. The
Refresh requests will also refresh the server reflexive candidate.
4.1.2. Prioritizing Candidates
The prioritization process results in the assignment of a priority to
each candidate. Each candidate for a media stream MUST have a unique
priority that MUST be a positive integer between 1 and (2**31 - 1).
This priority will be used by ICE to determine the order of the
connectivity checks and the relative preference for candidates.
An agent SHOULD compute this priority using the formula in
Section 184.108.40.206 and choose its parameters using the guidelines in
Section 220.127.116.11. If an agent elects to use a different formula, ICE
will take longer to converge since both agents will not be
coordinated in their checks.
18.104.22.168. Recommended Formula
When using the formula, an agent computes the priority by determining
a preference for each type of candidate (server reflexive, peer
reflexive, relayed, and host), and, when the agent is multihomed,
choosing a preference for its IP addresses. These two preferences
are then combined to compute the priority for a candidate. That
priority is computed using the following formula:
priority = (2^24)*(type preference) +
(2^8)*(local preference) +
(2^0)*(256 - component ID)
The type preference MUST be an integer from 0 to 126 inclusive, and
represents the preference for the type of the candidate (where the
types are local, server reflexive, peer reflexive, and relayed). A
126 is the highest preference, and a 0 is the lowest. Setting the
value to a 0 means that candidates of this type will only be used as
a last resort. The type preference MUST be identical for all
candidates of the same type and MUST be different for candidates of
different types. The type preference for peer reflexive candidates
MUST be higher than that of server reflexive candidates. Note that
candidates gathered based on the procedures of Section 4.1.1 will
never be peer reflexive candidates; candidates of these type are
learned from the connectivity checks performed by ICE.
The local preference MUST be an integer from 0 to 65535 inclusive.
It represents a preference for the particular IP address from which
the candidate was obtained, in cases where an agent is multihomed.
65535 represents the highest preference, and a zero, the lowest.
When there is only a single IP address, this value SHOULD be set to
65535. More generally, if there are multiple candidates for a
particular component for a particular media stream that have the same
type, the local preference MUST be unique for each one. In this
specification, this only happens for multihomed hosts. If a host is
multihomed because it is dual stack, the local preference SHOULD be
set equal to the precedence value for IP addresses described in RFC
The component ID is the component ID for the candidate, and MUST be
between 1 and 256 inclusive.
22.214.171.124. Guidelines for Choosing Type and Local Preferences
One criterion for selection of the type and local preference values
is the use of a media intermediary, such as a TURN server, VPN
server, or NAT. With a media intermediary, if media is sent to that
candidate, it will first transit the media intermediary before being
received. Relayed candidates are one type of candidate that involves
a media intermediary. Another are host candidates obtained from a
VPN interface. When media is transited through a media intermediary,
it can increase the latency between transmission and reception. It
can increase the packet losses, because of the additional router hops
that may be taken. It may increase the cost of providing service,
since media will be routed in and right back out of a media
intermediary run by a provider. If these concerns are important, the
type preference for relayed candidates SHOULD be lower than host
candidates. The RECOMMENDED values are 126 for host candidates, 100
for server reflexive candidates, 110 for peer reflexive candidates,
and 0 for relayed candidates. Furthermore, if an agent is multihomed
and has multiple IP addresses, the local preference for host
candidates from a VPN interface SHOULD have a priority of 0.
Another criterion for selection of preferences is IP address family.
ICE works with both IPv4 and IPv6. It therefore provides a
transition mechanism that allows dual-stack hosts to prefer
connectivity over IPv6, but to fall back to IPv4 in case the v6
networks are disconnected (due, for example, to a failure in a 6to4
relay) [RFC3056]. It can also help with hosts that have both a
native IPv6 address and a 6to4 address. In such a case, higher local
preferences could be assigned to the v6 addresses, followed by the
6to4 addresses, followed by the v4 addresses. This allows a site to
obtain and begin using native v6 addresses immediately, yet still
fall back to 6to4 addresses when communicating with agents in other
sites that do not yet have native v6 connectivity.
Another criterion for selecting preferences is security. If a user
is a telecommuter, and therefore connected to a corporate network and
a local home network, the user may prefer their voice traffic to be
routed over the VPN in order to keep it on the corporate network when
communicating within the enterprise, but use the local network when
communicating with users outside of the enterprise. In such a case,
a VPN address would have a higher local preference than any other
Another criterion for selecting preferences is topological awareness.
This is most useful for candidates that make use of intermediaries.
In those cases, if an agent has preconfigured or dynamically
discovered knowledge of the topological proximity of the
intermediaries to itself, it can use that to assign higher local
preferences to candidates obtained from closer intermediaries.
4.1.3. Eliminating Redundant Candidates
Next, the agent eliminates redundant candidates. A candidate is
redundant if its transport address equals another candidate, and its
base equals the base of that other candidate. Note that two
candidates can have the same transport address yet have different
bases, and these would not be considered redundant. Frequently, a
server reflexive candidate and a host candidate will be redundant
when the agent is not behind a NAT. The agent SHOULD eliminate the
redundant candidate with the lower priority.
4.1.4. Choosing Default Candidates
A candidate is said to be default if it would be the target of media
from a non-ICE peer; that target is called the DEFAULT DESTINATION.
If the default candidates are not selected by the ICE algorithm when
communicating with an ICE-aware peer, an updated offer/answer will be
required after ICE processing completes in order to "fix up" the SDP
so that the default destination for media matches the candidates
selected by ICE. If ICE happens to select the default candidates, no
updated offer/answer is required.
An agent MUST choose a set of candidates, one for each component of
each in-use media stream, to be default. A media stream is in-use if
it does not have a port of zero (which is used in RFC 3264 to reject
a media stream). Consequently, a media stream is in-use even if it
is marked as a=inactive [RFC4566] or has a bandwidth value of zero.
It is RECOMMENDED that default candidates be chosen based on the
likelihood of those candidates to work with the peer that is being
contacted. It is RECOMMENDED that the default candidates are the
relayed candidates (if relayed candidates are available), server
reflexive candidates (if server reflexive candidates are available),
and finally host candidates.
4.2. Lite Implementation Requirements
Lite implementations only utilize host candidates. A lite
implementation MUST, for each component of each media stream,
allocate zero or one IPv4 candidates. It MAY allocate zero or more
IPv6 candidates, but no more than one per each IPv6 address utilized
by the host. Since there can be no more than one IPv4 candidate per
component of each media stream, if an agent has multiple IPv4
addresses, it MUST choose one for allocating the candidate. If a
host is dual stack, it is RECOMMENDED that it allocate one IPv4
candidate and one global IPv6 address. With the lite implementation,
ICE cannot be used to dynamically choose amongst candidates.
Therefore, including more than one candidate from a particular scope
is NOT RECOMMENDED, since only a connectivity check can truly
determine whether to use one address or the other.
Each component has an ID assigned to it, called the component ID.
For RTP-based media streams, the RTP itself has a component ID of 1,
and RTCP a component ID of 2. If an agent is using RTCP, it MUST
obtain candidates for it.
Each candidate is assigned a foundation. The foundation MUST be
different for two candidates allocated from different IP addresses,
and MUST be the same otherwise. A simple integer that increments for
each IP address will suffice. In addition, each candidate MUST be
assigned a unique priority amongst all candidates for the same media
stream. This priority SHOULD be equal to:
priority = (2^24)*(126) +
(2^8)*(IP precedence) +
(2^0)*(256 - component ID)
If a host is v4-only, it SHOULD set the IP precedence to 65535. If a
host is v6 or dual stack, the IP precedence SHOULD be the precedence
value for IP addresses described in RFC 3484 [RFC3484].
Next, an agent chooses a default candidate for each component of each
media stream. If a host is IPv4 only, there would only be one
candidate for each component of each media stream, and therefore that
candidate is the default. If a host is IPv6 or dual stack, the
selection of default is a matter of local policy. This default
SHOULD be chosen such that it is the candidate most likely to be used
with a peer. For IPv6-only hosts, this would typically be a globally
scoped IPv6 address. For dual-stack hosts, the IPv4 address is
4.3. Encoding the SDP
The process of encoding the SDP is identical between full and lite
The agent will include an m line for each media stream it wishes to
use. The ordering of media streams in the SDP is relevant for ICE.
ICE will perform its connectivity checks for the first m line first,
and consequently media will be able to flow for that stream first.
Agents SHOULD place their most important media stream, if there is
one, first in the SDP.
There will be a candidate attribute for each candidate for a
particular media stream. Section 15 provides detailed rules for
constructing this attribute. The attribute carries the IP address,
port, and transport protocol for the candidate, in addition to its
properties that need to be signaled to the peer for ICE to work: the
priority, foundation, and component ID. The candidate attribute also
carries information about the candidate that is useful for
diagnostics and other functions: its type and related transport
STUN connectivity checks between agents are authenticated using the
short-term credential mechanism defined for STUN [RFC5389]. This
mechanism relies on a username and password that are exchanged
through protocol machinery between the client and server. With ICE,
the offer/answer exchange is used to exchange them. The username
part of this credential is formed by concatenating a username
fragment from each agent, separated by a colon. Each agent also
provides a password, used to compute the message integrity for
requests it receives. The username fragment and password are
exchanged in the ice-ufrag and ice-pwd attributes, respectively. In
addition to providing security, the username provides disambiguation
and correlation of checks to media streams. See Appendix B.4 for
If an agent is a lite implementation, it MUST include an "a=ice-lite"
session-level attribute in its SDP. If an agent is a full
implementation, it MUST NOT include this attribute.
The default candidates are added to the SDP as the default
destination for media. For streams based on RTP, this is done by
placing the IP address and port of the RTP candidate into the c and m
lines, respectively. If the agent is utilizing RTCP, it MUST encode
the RTCP candidate using the a=rtcp attribute as defined in RFC 3605
[RFC3605]. If RTCP is not in use, the agent MUST signal that using
b=RS:0 and b=RR:0 as defined in RFC 3556 [RFC3556].
The transport addresses that will be the default destination for
media when communicating with non-ICE peers MUST also be present as
candidates in one or more a=candidate lines.
ICE provides for extensibility by allowing an offer or answer to
contain a series of tokens that identify the ICE extensions used by
that agent. If an agent supports an ICE extension, it MUST include
the token defined for that extension in the ice-options attribute.
The following is an example SDP message that includes ICE attributes
(lines folded for readability):
o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
c=IN IP4 192.0.2.3
m=audio 45664 RTP/AVP 0
a=candidate:1 1 UDP 2130706431 10.0.1.1 8998 typ host
a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr
10.0.1.1 rport 8998
Once an agent has sent its offer or its answer, that agent MUST be
prepared to receive both STUN and media packets on each candidate.
As discussed in Section 11.1, media packets can be sent to a
candidate prior to its appearance as the default destination for
media in an offer or answer.
5. Receiving the Initial Offer
When an agent receives an initial offer, it will check if the offerer
supports ICE, determine its own role, gather candidates, prioritize
them, choose default candidates, encode and send an answer, and for
full implementations, form the check lists and begin connectivity
5.1. Verifying ICE Support
The agent will proceed with the ICE procedures defined in this
specification if, for each media stream in the SDP it received, the
default destination for each component of that media stream appears
in a candidate attribute. For example, in the case of RTP, the IP
address and port in the c and m lines, respectively, appear in a
candidate attribute and the value in the rtcp attribute appears in a
If this condition is not met, the agent MUST process the SDP based on
normal RFC 3264 procedures, without using any of the ICE mechanisms
described in the remainder of this specification with the following
1. The agent MUST follow the rules of Section 10, which describe
keepalive procedures for all agents.
2. If the agent is not proceeding with ICE because there were
a=candidate attributes, but none that matched the default
destination of the media stream, the agent MUST include an a=ice-
mismatch attribute in its answer.
3. If the default candidates were relayed candidates learned through
a TURN server, the agent MUST create permissions in the TURN
server for the IP addresses learned from its peer in the SDP it
just received. If this is not done, initial packets in the media
stream from the peer may be lost.
5.2. Determining Role
For each session, each agent takes on a role. There are two roles --
controlling and controlled. The controlling agent is responsible for
the choice of the final candidate pairs used for communications. For
a full agent, this means nominating the candidate pairs that can be
used by ICE for each media stream, and for generating the updated
offer based on ICE's selection, when needed. For a lite
implementation, being the controlling agent means selecting a
candidate pair based on the ones in the offer and answer (for IPv4,
there is only ever one pair), and then generating an updated offer
reflecting that selection, when needed (it is never needed for an
IPv4-only host). The controlled agent is told which candidate pairs
to use for each media stream, and does not generate an updated offer
to signal this information. The sections below describe in detail
the actual procedures followed by controlling and controlled nodes.
The rules for determining the role and the impact on behavior are as
Both agents are full: The agent that generated the offer which
started the ICE processing MUST take the controlling role, and the
other MUST take the controlled role. Both agents will form check
lists, run the ICE state machines, and generate connectivity
checks. The controlling agent will execute the logic in
Section 8.1 to nominate pairs that will be selected by ICE, and
then both agents end ICE as described in Section 8.1.2. In
unusual cases, described in Appendix B.11, it is possible for both
agents to mistakenly believe they are controlled or controlling.
To resolve this, each agent MUST select a random number, called
the tie-breaker, uniformly distributed between 0 and (2**64) - 1
(that is, a 64-bit positive integer). This number is used in
connectivity checks to detect and repair this case, as described
in Section 126.96.36.199.
One agent full, one lite: The full agent MUST take the controlling
role, and the lite agent MUST take the controlled role. The full
agent will form check lists, run the ICE state machines, and
generate connectivity checks. That agent will execute the logic
in Section 8.1 to nominate pairs that will be selected by ICE, and
use the logic in Section 8.1.2 to end ICE. The lite
implementation will just listen for connectivity checks, receive
them and respond to them, and then conclude ICE as described in
Section 8.2. For the lite implementation, the state of ICE
processing for each media stream is considered to be Running, and
the state of ICE overall is Running.
Both lite: The agent that generated the offer which started the ICE
processing MUST take the controlling role, and the other MUST take
the controlled role. In this case, no connectivity checks are
ever sent. Rather, once the offer/answer exchange completes, each
agent performs the processing described in Section 8 without
connectivity checks. It is possible that both agents will believe
they are controlled or controlling. In the latter case, the
conflict is resolved through glare detection capabilities in the
signaling protocol carrying the offer/answer exchange. The state
of ICE processing for each media stream is considered to be
Running, and the state of ICE overall is Running.
Once roles are determined for a session, they persist unless ICE is
restarted. An ICE restart (Section 9.1) causes a new selection of
roles and tie-breakers.
5.3. Gathering Candidates
The process for gathering candidates at the answerer is identical to
the process for the offerer as described in Section 4.1.1 for full
implementations and Section 4.2 for lite implementations. It is
RECOMMENDED that this process begin immediately on receipt of the
offer, prior to alerting the user. Such gathering MAY begin when an
5.4. Prioritizing Candidates
The process for prioritizing candidates at the answerer is identical
to the process followed by the offerer, as described in Section 4.1.2
for full implementations and Section 4.2 for lite implementations.
5.5. Choosing Default Candidates
The process for selecting default candidates at the answerer is
identical to the process followed by the offerer, as described in
Section 4.1.4 for full implementations and Section 4.2 for lite
5.6. 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 4.3.
5.7. Forming the Check Lists
Forming check lists is done only by full implementations. Lite
implementations MUST skip the steps defined in this section.
There is one check list per in-use media stream resulting from the
offer/answer exchange. To form the check list for a media stream,
the agent forms candidate pairs, computes a candidate pair priority,
orders the pairs by priority, prunes them, and sets their states.
These steps are described in this section.
5.7.1. Forming Candidate Pairs
First, the agent takes each of its candidates for a media stream
(called LOCAL CANDIDATES) and pairs them with the candidates it
received from its peer (called REMOTE CANDIDATES) for that media
stream. In order to prevent the attacks described in Section 18.5.2,
agents MAY limit the number of candidates they'll accept in an offer
or answer. A local candidate is paired with a remote candidate if
and only if the two candidates have the same component ID and have
the same IP address version. It is possible that some of the local
candidates won't get paired with remote candidates, and some of the
remote candidates won't get paired with local candidates. This can
happen if one agent doesn't include candidates for the all of the
components for a media stream. If this happens, the number of
components for that media stream is effectively reduced, and
considered to be equal to the minimum across both agents of the
maximum component ID provided by each agent across all components for
the media stream.
In the case of RTP, this would happen when one agent provides
candidates for RTCP, and the other does not. As another example, the
offerer can multiplex RTP and RTCP on the same port and signals that
it can do that in the SDP through an SDP attribute [RFC5761].
However, since the offerer doesn't know if the answerer can perform
such multiplexing, the offerer includes candidates for RTP and RTCP
on separate ports, so that the offer has two components per media
stream. If the answerer can perform such multiplexing, it would
include just a single component for each candidate - for the combined
RTP/RTCP mux. ICE would end up acting as if there was just a single
component for this candidate.
The candidate pairs whose local and remote candidates are both the
default candidates for a particular component is called,
unsurprisingly, the default candidate pair for that component. This
is the pair that would be used to transmit media if both agents had
not been ICE aware.
In order to aid understanding, Figure 6 shows the relationships
between several key concepts -- transport addresses, candidates,
candidate pairs, and check lists, in addition to indicating the main
properties of candidates and candidate pairs.
5.7.2. Computing Pair Priority and Ordering Pairs
Once the pairs are formed, a candidate pair priority is computed.
Let G be the priority for the candidate provided by the controlling
agent. Let D be the priority for the candidate provided by the
controlled agent. The priority for a pair is computed as:
pair priority = 2^32*MIN(G,D) + 2*MAX(G,D) + (G>D?1:0)
Where G>D?1:0 is an expression whose value is 1 if G is greater than
D, and 0 otherwise. Once the priority is assigned, the agent sorts
the candidate pairs in decreasing order of priority. If two pairs
have identical priority, the ordering amongst them is arbitrary.
5.7.3. Pruning the Pairs
This sorted list of candidate pairs is used to determine a sequence
of connectivity checks that will be performed. Each check involves
sending a request from a local candidate to a remote candidate.
Since an agent cannot send requests directly from a reflexive
candidate, but only from its base, the agent next goes through the
sorted list of candidate pairs. For each pair where the local
candidate is server reflexive, the server reflexive candidate MUST be
replaced by its base. Once this has been done, the agent MUST prune
the list. This is done by removing a pair if its local and remote
candidates are identical to the local and remote candidates of a pair
higher up on the priority list. The result is a sequence of ordered
candidate pairs, called the check list for that media stream.
In addition, in order to limit the attacks described in
Section 18.5.2, an agent MUST limit the total number of connectivity
checks the agent performs across all check lists to a specific value,
and this value MUST be configurable. A default of 100 is
RECOMMENDED. This limit is enforced by discarding the lower-priority
candidate pairs until there are less than 100. It is RECOMMENDED
that a lower value be utilized when possible, set to the maximum
number of plausible checks that might be seen in an actual deployment
configuration. The requirement for configuration is meant to provide
a tool for fixing this value in the field if, once deployed, it is
found to be problematic.
5.7.4. Computing States
Each candidate pair in the check list has a foundation and a state.
The foundation is the combination of the foundations of the local and
remote candidates in the pair. The state is assigned once the check
list for each media stream has been computed. There are five
potential values that the state can have:
Waiting: A check has not been performed for this pair, and can be
performed as soon as it is the highest-priority Waiting pair on
the check list.
In-Progress: A check has been sent for this pair, but the
transaction is in progress.
Succeeded: A check for this pair was already done and produced a
Failed: A check for this pair was already done and failed, either
never producing any response or producing an unrecoverable failure
Frozen: A check for this pair hasn't been performed, and it can't
yet be performed until some other check succeeds, allowing this
pair to unfreeze and move into the Waiting state.
As ICE runs, the pairs will move between states as shown in Figure 7.
2. The agent examines the check list for the first media stream (a
media stream is the first media stream when it is described by
the first m line in the SDP offer and answer). For that media
* For all pairs with the same foundation, it sets the state of
the pair with the lowest component ID to Waiting. If there is
more than one such pair, the one with the highest priority is
One of the check lists will have some number of pairs in the Waiting
state, and the other check lists will have all of their pairs in the
Frozen state. A check list with at least one pair that is Waiting is
called an active check list, and a check list with all pairs Frozen
is called a frozen check list.
The check list itself is associated with a state, which captures the
state of ICE checks for that media stream. There are three states:
Running: In this state, ICE checks are still in progress for this
Completed: In this state, ICE checks have produced nominated pairs
for each component of the media stream. Consequently, ICE has
succeeded and media can be sent.
Failed: In this state, the ICE checks have not completed
successfully for this media stream.
When a check list is first constructed as the consequence of an
offer/answer exchange, it is placed in the Running state.
ICE processing across all media streams also has a state associated
with it. This state is equal to Running while ICE processing is
under way. The state is Completed when ICE processing is complete
and Failed if it failed without success. Rules for transitioning
between states are described below.
5.8. Scheduling Checks
Checks are generated only by full implementations. Lite
implementations MUST skip the steps described in this section.
An agent performs ordinary checks and triggered checks. The
generation of both checks is governed by a timer that fires
periodically for each media stream. The agent maintains a FIFO
queue, called the triggered check queue, which contains candidate
pairs for which checks are to be sent at the next available
opportunity. When the timer fires, the agent removes the top pair
from the triggered check queue, performs a connectivity check on that
pair, and sets the state of the candidate pair to In-Progress. If
there are no pairs in the triggered check queue, an ordinary check is
Once the agent has computed the check lists as described in
Section 5.7, it sets a timer for each active check list. The timer
fires every Ta*N seconds, where N is the number of active check lists
(initially, there is only one active check list). Implementations
MAY set the timer to fire less frequently than this. Implementations
SHOULD take care to spread out these timers so that they do not fire
at the same time for each media stream. Ta and the retransmit timer
RTO are computed as described in Section 16. Multiplying by N allows
this aggregate check throughput to be split between all active check
lists. The first timer fires immediately, so that the agent performs
a connectivity check the moment the offer/answer exchange has been
done, followed by the next check Ta seconds later (since there is
only one active check list).
When the timer fires and there is no triggered check to be sent, the
agent MUST choose an ordinary check as follows:
o Find the highest-priority pair in that check list that is in the
o If there is such a pair:
* Send a STUN check from the local candidate of that pair to the
remote candidate of that pair. The procedures for forming the
STUN request for this purpose are described in Section 7.1.2.
* Set the state of the candidate pair to In-Progress.
o If there is no such pair:
* Find the highest-priority pair in that check list that is in
the Frozen state.
* If there is such a pair:
+ Unfreeze the pair.
+ Perform a check for that pair, causing its state to
transition to In-Progress.
* If there is no such pair:
+ Terminate the timer for that check list.
To compute the message integrity for the check, the agent uses the
remote username fragment and password learned from the SDP from its
peer. The local username fragment is known directly by the agent for
its own candidate.
6. Receipt of the Initial Answer
This section describes the procedures that an agent follows when it
receives the answer from the peer. It verifies that its peer
supports ICE, determines its role, and for full implementations,
forms the check list and begins performing ordinary checks.
When ICE is used with SIP, forking may result in a single offer
generating a multiplicity of answers. In that case, ICE proceeds
completely in parallel and independently for each answer, treating
the combination of its offer and each answer as an independent offer/
answer exchange, with its own set of pairs, check lists, states, and
so on. The only case in which processing of one pair impacts another
is freeing of candidates, discussed below in Section 188.8.131.52. Verifying ICE Support
The logic at the offerer is identical to that of the answerer as
described in Section 5.1, with the exception that an offerer would
not ever generate a=ice-mismatch attributes in an SDP.
In some cases, the answer may omit a=candidate attributes for the
media streams, and instead include an a=ice-mismatch attribute for
one or more of the media streams in the SDP. This signals to the
offerer that the answerer supports ICE, but that ICE processing was
not used for the session because a signaling intermediary modified
the default destination for media components without modifying the
corresponding candidate attributes. See Section 18 for a discussion
of cases where this can happen. This specification provides no
guidance on how an agent should proceed in such a failure case.
6.2. Determining Role
The offerer follows the same procedures described for the answerer in
6.3. Forming the Check List
Formation of check lists is performed only by full implementations.
The offerer follows the same procedures described for the answerer in
6.4. Performing Ordinary Checks
Ordinary checks are performed only by full implementations. The
offerer follows the same procedures described for the answerer in
7. Performing Connectivity Checks
This section describes how connectivity checks are performed. All
ICE implementations are required to be compliant to [RFC5389], as
opposed to the older [RFC3489]. However, whereas a full
implementation will both generate checks (acting as a STUN client)
and receive them (acting as a STUN server), a lite implementation
will only receive checks, and thus will only act as a STUN server.
7.1. STUN Client Procedures
These procedures define how an agent sends a connectivity check,
whether it is an ordinary or a triggered check. These procedures are
only applicable to full implementations.
7.1.1. Creating Permissions for Relayed Candidates
If the connectivity check is being sent using a relayed local
candidate, the client MUST create a permission first if it has not
already created one previously. It would have created one previously
if it had told the TURN server to create a permission for the given
relayed candidate towards the IP address of the remote candidate. To
create the permission, the agent follows the procedures defined in
[RFC5766]. The permission MUST be created towards the IP address of
the remote candidate. It is RECOMMENDED that the agent defer
creation of a TURN channel until ICE completes, in which case
permissions for connectivity checks are normally created using a
CreatePermission request. Once established, the agent MUST keep the
permission active until ICE concludes.
7.1.2. Sending the Request
The check is generated by sending a Binding request from a local
candidate to a remote candidate. [RFC5389] describes how Binding
requests are constructed and generated. A connectivity check MUST
utilize the STUN short-term credential mechanism. Support for
backwards compatibility with RFC 3489 MUST NOT be used or assumed
with connectivity checks. The FINGERPRINT mechanism MUST be used for
ICE extends STUN by defining several new attributes, including
PRIORITY, USE-CANDIDATE, ICE-CONTROLLED, and ICE-CONTROLLING. These
new attributes are formally defined in Section 19.1, and their usage
is described in the subsections below. These STUN extensions are
applicable only to connectivity checks used for ICE.
184.108.40.206. PRIORITY and USE-CANDIDATE
An agent MUST include the PRIORITY attribute in its Binding request.
The attribute MUST be set equal to the priority that would be
assigned, based on the algorithm in Section 4.1.2, to a peer
reflexive candidate, should one be learned as a consequence of this
check (see Section 220.127.116.11.1 for how peer reflexive candidates are
learned). This priority value will be computed identically to how
the priority for the local candidate of the pair was computed, except
that the type preference is set to the value for peer reflexive
The controlling agent MAY include the USE-CANDIDATE attribute in the
Binding request. The controlled agent MUST NOT include it in its
Binding request. This attribute signals that the controlling agent
wishes to cease checks for this component, and use the candidate pair
resulting from the check for this component. Section 8.1.1 provides
guidance on determining when to include it.
18.104.22.168. ICE-CONTROLLED and ICE-CONTROLLING
The agent MUST include the ICE-CONTROLLED attribute in the request if
it is in the controlled role, and MUST include the ICE-CONTROLLING
attribute in the request if it is in the controlling role. The
content of either attribute MUST be the tie-breaker that was
determined in Section 5.2. These attributes are defined fully in
22.214.171.124. Forming Credentials
A Binding request serving as a connectivity check MUST utilize the
STUN short-term credential mechanism. The username for the
credential is formed by concatenating the username fragment provided
by the peer with the username fragment of the agent sending the
request, separated by a colon (":"). The password is equal to the
password provided by the peer. For example, consider the case where
agent L is the offerer, and agent R is the answerer. Agent L
included a username fragment of LFRAG for its candidates and a
password of LPASS. Agent R provided a username fragment of RFRAG and
a password of RPASS. A connectivity check from L to R utilizes the
username RFRAG:LFRAG and a password of RPASS. A connectivity check
from R to L utilizes the username LFRAG:RFRAG and a password of
LPASS. The responses utilize the same usernames and passwords as the
requests (note that the USERNAME attribute is not present in the
126.96.36.199. DiffServ Treatment
If the agent is using Diffserv Codepoint markings [RFC2475] in its
media packets, it SHOULD apply those same markings to its
7.1.3. Processing the Response
When a Binding response is received, it is correlated to its Binding
request using the transaction ID, as defined in [RFC5389], which then
ties it to the candidate pair for which the Binding request was sent.
This section defines additional procedures for processing Binding
responses specific to this usage of STUN.
188.8.131.52. Failure Cases
If the STUN transaction generates a 487 (Role Conflict) error
response, the agent checks whether it included the ICE-CONTROLLED or
ICE-CONTROLLING attribute in the Binding request. If the request
contained the ICE-CONTROLLED attribute, the agent MUST switch to the
controlling role if it has not already done so. If the request
contained the ICE-CONTROLLING attribute, the agent MUST switch to the
controlled role if it has not already done so. Once it has switched,
the agent MUST enqueue the candidate pair whose check generated the
487 into the triggered check queue. The state of that pair is set to
Waiting. When the triggered check is sent, it will contain an ICE-
CONTROLLING or ICE-CONTROLLED attribute reflecting its new role.
Note, however, that the tie-breaker value MUST NOT be reselected.
A change in roles will require an agent to recompute pair priorities
(Section 5.7.2), since those priorities are a function of controlling
and controlled roles. The change in role will also impact whether
the agent is responsible for selecting nominated pairs and generating
updated offers upon conclusion of ICE.
Agents MAY support receipt of ICMP errors for connectivity checks.
If the STUN transaction generates an ICMP error, the agent sets the
state of the pair to Failed. If the STUN transaction generates a
STUN error response that is unrecoverable (as defined in [RFC5389])
or times out, the agent sets the state of the pair to Failed.
The agent MUST check that the source IP address and port of the
response equal the destination IP address and port to which the
Binding request was sent, and that the destination IP address and
port of the response match the source IP address and port from which
the Binding request was sent. In other words, the source and
destination transport addresses in the request and responses are
symmetric. If they are not symmetric, the agent sets the state of
the pair to Failed.
184.108.40.206. Success Cases
A check is considered to be a success if all of the following are
o The STUN transaction generated a success response.
o The source IP address and port of the response equals the
destination IP address and port to which the Binding request was
o The destination IP address and port of the response match the
source IP address and port from which the Binding request was
220.127.116.11.1. Discovering Peer Reflexive Candidates
The agent checks the mapped address from the STUN response. If the
transport address does not match any of the local candidates that the
agent knows about, the mapped address represents a new candidate -- a
peer reflexive candidate. Like other candidates, it has a type,
base, priority, and foundation. They are computed as follows:
o Its type is equal to peer reflexive.
o Its base is set equal to the local candidate of the candidate pair
from which the STUN check was sent.
o Its priority is set equal to the value of the PRIORITY attribute
in the Binding request.
o Its foundation is selected as described in Section 18.104.22.168.
This peer reflexive candidate is then added to the list of local
candidates for the media stream. Its username fragment and password
are the same as all other local candidates for that media stream.
However, the peer reflexive candidate is not paired with other remote
candidates. This is not necessary; a valid pair will be generated
from it momentarily based on the procedures in Section 22.214.171.124.2. If
an agent wishes to pair the peer reflexive candidate with other
remote candidates besides the one in the valid pair that will be
generated, the agent MAY generate an updated offer which includes the
peer reflexive candidate. This will cause it to be paired with all
other remote candidates.
126.96.36.199.2. Constructing a Valid Pair
The agent constructs a candidate pair whose local candidate equals
the mapped address of the response, and whose remote candidate equals
the destination address to which the request was sent. This is
called a valid pair, since it has been validated by a STUN
connectivity check. The valid pair may equal the pair that generated
the check, may equal a different pair in the check list, or may be a
pair not currently on any check list. If the pair equals the pair
that generated the check or is on a check list currently, it is also
added to the VALID LIST, which is maintained by the agent for each
media stream. This list is empty at the start of ICE processing, and
fills as checks are performed, resulting in valid candidate pairs.
It will be very common that the pair will not be on any check list.
Recall that the check list has pairs whose local candidates are never
server reflexive; those pairs had their local candidates converted to
the base of the server reflexive candidates, and then pruned if they
were redundant. When the response to the STUN check arrives, the
mapped address will be reflexive if there is a NAT between the two.
In that case, the valid pair will have a local candidate that doesn't
match any of the pairs in the check list.
If the pair is not on any check list, the agent computes the priority
for the pair based on the priority of each candidate, using the
algorithm in Section 5.7. The priority of the local candidate
depends on its type. If it is not peer reflexive, it is equal to the
priority signaled for that candidate in the SDP. If it is peer
reflexive, it is equal to the PRIORITY attribute the agent placed in
the Binding request that just completed. The priority of the remote
candidate is taken from the SDP of the peer. If the candidate does
not appear there, then the check must have been a triggered check to
a new remote candidate. In that case, the priority is taken as the
value of the PRIORITY attribute in the Binding request that triggered
the check that just completed. The pair is then added to the VALID
188.8.131.52.3. Updating Pair States
The agent sets the state of the pair that *generated* the check to
Succeeded. Note that, the pair which *generated* the check may be
different than the valid pair constructed in Section 184.108.40.206.2 as a
consequence of the response. The success of this check might also
cause the state of other checks to change as well. The agent MUST
perform the following two steps:
1. The agent changes the states for all other Frozen pairs for the
same media stream and same foundation to Waiting. Typically, but
not always, these other pairs will have different component IDs.
2. If there is a pair in the valid list for every component of this
media stream (where this is the actual number of components being
used, in cases where the number of components signaled in the SDP
differs from offerer to answerer), the success of this check may
unfreeze checks for other media streams. Note that this step is
followed not just the first time the valid list under
consideration has a pair for every component, but every
subsequent time a check succeeds and adds yet another pair to
that valid list. The agent examines the check list for each
other media stream in turn:
* If the check list is active, the agent changes the state of
all Frozen pairs in that check list whose foundation matches a
pair in the valid list under consideration to Waiting.
* If the check list is frozen, and there is at least one pair in
the check list whose foundation matches a pair in the valid
list under consideration, the state of all pairs in the check
list whose foundation matches a pair in the valid list under
consideration is set to Waiting. This will cause the check
list to become active, and ordinary checks will begin for it,
as described in Section 5.8.
* If the check list is frozen, and there are no pairs in the
check list whose foundation matches a pair in the valid list
under consideration, the agent
+ groups together all of the pairs with the same foundation,
+ for each group, sets the state of the pair with the lowest
component ID to Waiting. If there is more than one such
pair, the one with the highest priority is used.
220.127.116.11.4. Updating the Nominated Flag
If the agent was a controlling agent, and it had included a USE-
CANDIDATE attribute in the Binding request, the valid pair generated
from that check has its nominated flag set to true. This flag
indicates that this valid pair should be used for media if it is the
highest-priority one amongst those whose nominated flag is set. This
may conclude ICE processing for this media stream or all media
streams; see Section 8.
If the agent is the controlled agent, the response may be the result
of a triggered check that was sent in response to a request that
itself had the USE-CANDIDATE attribute. This case is described in
Section 18.104.22.168, and may now result in setting the nominated flag for
the pair learned from the original request.
22.214.171.124. Check List and Timer State Updates
Regardless of whether the check was successful or failed, the
completion of the transaction may require updating of check list and
If all of the pairs in the check list are now either in the Failed or
o 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.
o For each frozen check list, the agent
* groups together all of the pairs with the same foundation, and
* for each group, sets the state of the pair with the lowest
component ID to Waiting. If there is more than one such pair,
the one with the highest priority is used.
If none of the pairs in the check list are in the Waiting or Frozen
state, the check list is no longer considered active, and will not
count towards the value of N in the computation of timers for
ordinary checks as described in Section 5.8.
7.2. STUN Server Procedures
An agent MUST be prepared to receive a Binding request on the base of
each candidate it included in its most recent offer or answer. This
requirement holds even if the peer is a lite implementation.
The agent MUST use a short-term credential to authenticate the
request and perform a message integrity check. The agent MUST
consider the username to be valid if it consists of two values
separated by a colon, where the first value is equal to the username
fragment generated by the agent in an offer or answer for a session
in-progress. It is possible (and in fact very likely) that an
offerer will receive a Binding request prior to receiving the answer
from its peer. If this happens, the agent MUST immediately generate
a response (including computation of the mapped address as described
in Section 126.96.36.199). The agent has sufficient information at this
point to generate the response; the password from the peer is not
required. Once the answer is received, it MUST proceed with the
remaining steps required, namely, 188.8.131.52, 184.108.40.206, and 220.127.116.11 for
full implementations. In cases where multiple STUN requests are
received before the answer, this may cause several pairs to be queued
up in the triggered check queue.
An agent MUST NOT utilize the ALTERNATE-SERVER mechanism, and MUST
NOT support the backwards-compatibility mechanisms to RFC 3489. It
MUST utilize the FINGERPRINT mechanism.
If the agent is using Diffserv Codepoint markings [RFC2475] in its
media packets, it SHOULD apply those same markings to its responses
to Binding requests. The same would apply to any layer 2 markings
the endpoint might be applying to media packets.
7.2.1. Additional Procedures for Full Implementations
This subsection defines the additional server procedures applicable
to full implementations.
18.104.22.168. Detecting and Repairing Role Conflicts
Normally, the rules for selection of a role in Section 5.2 will
result in each agent selecting a different role -- one controlling
and one controlled. However, in unusual call flows, typically
utilizing third party call control, it is possible for both agents to
select the same role. This section describes procedures for checking
for this case and repairing it.
An agent MUST examine the Binding request for either the ICE-
CONTROLLING or ICE-CONTROLLED attribute. It MUST follow these
o If neither ICE-CONTROLLING nor ICE-CONTROLLED is present in the
request, the peer agent may have implemented a previous version of
this specification. There may be a conflict, but it cannot be
o If the agent is in the controlling role, and the ICE-CONTROLLING
attribute is present in the request:
* If the agent's tie-breaker is larger than or equal to the
contents of the ICE-CONTROLLING attribute, the agent generates
a Binding error response and includes an ERROR-CODE attribute
with a value of 487 (Role Conflict) but retains its role.
* If the agent's tie-breaker is less than the contents of the
ICE-CONTROLLING attribute, the agent switches to the controlled
o If the agent is in the controlled role, and the ICE-CONTROLLED
attribute is present in the request:
* If the agent's tie-breaker is larger than or equal to the
contents of the ICE-CONTROLLED attribute, the agent switches to
the controlling role.
* If the agent's tie-breaker is less than the contents of the
ICE-CONTROLLED attribute, the agent generates a Binding error
response and includes an ERROR-CODE attribute with a value of
487 (Role Conflict) but retains its role.
o If the agent is in the controlled role and the ICE-CONTROLLING
attribute was present in the request, or the agent was in the
controlling role and the ICE-CONTROLLED attribute was present in
the request, there is no conflict.
A change in roles will require an agent to recompute pair priorities
(Section 5.7.2), since those priorities are a function of controlling
and controlled roles. The change in role will also impact whether
the agent is responsible for selecting nominated pairs and generated
updated offers upon conclusion of ICE.
The remaining sections in Section 7.2.1 are followed if the server
generated a successful response to the Binding request, even if the
agent changed roles.
22.214.171.124. Computing Mapped Address
For requests being received on a relayed candidate, the source
transport address used for STUN processing (namely, generation of the
XOR-MAPPED-ADDRESS attribute) is the transport address as seen by the
TURN server. That source transport address will be present in the
XOR-PEER-ADDRESS attribute of a Data Indication message, if the
Binding request was delivered through a Data Indication. If the
Binding request was delivered through a ChannelData message, the
source transport address is the one that was bound to the channel.
126.96.36.199. Learning Peer Reflexive Candidates
If the source transport address of the request does not match any
existing remote candidates, it represents a new peer reflexive remote
candidate. This candidate is constructed as follows:
o The priority of the candidate is set to the PRIORITY attribute
from the request.
o The type of the candidate is set to peer reflexive.
o The foundation of the candidate is set to an arbitrary value,
different from the foundation for all other remote candidates. If
any subsequent offer/answer exchanges contain this peer reflexive
candidate in the SDP, it will signal the actual foundation for the
o The component ID of this candidate is set to the component ID for
the local candidate to which the request was sent.
This candidate is added to the list of remote candidates. However,
the agent does not pair this candidate with any local candidates.
188.8.131.52. Triggered Checks
Next, the agent constructs a pair whose local candidate is equal to
the transport address on which the STUN request was received, and a
remote candidate equal to the source transport address where the
request came from (which may be the peer reflexive remote candidate
that was just learned). The local candidate will either be a host
candidate (for cases where the request was not received through a
relay) or a relayed candidate (for cases where it is received through
a relay). The local candidate can never be a server reflexive
candidate. Since both candidates are known to the agent, it can
obtain their priorities and compute the candidate pair priority.
This pair is then looked up in the check list. There can be one of
o If the pair is already on the check list:
* If the state of that pair is Waiting or Frozen, a check for
that pair is enqueued into the triggered check queue if not
* If the state of that pair is In-Progress, the agent cancels the
in-progress transaction. Cancellation means that the agent
will not retransmit the request, will not treat the lack of
response to be a failure, but will wait the duration of the
transaction timeout for a response. In addition, the agent
MUST create a new connectivity check for that pair
(representing a new STUN Binding request transaction) by
enqueueing the pair in the triggered check queue. The state of
the pair is then changed to Waiting.
* If the state of the pair is Failed, it is changed to Waiting
and the agent MUST create a new connectivity check for that
pair (representing a new STUN Binding request transaction), by
enqueueing the pair in the triggered check queue.
* If the state of that pair is Succeeded, nothing further is
These steps are done to facilitate rapid completion of ICE when
both agents are behind NAT.
o If the pair is not already on the check list:
* The pair is inserted into the check list based on its priority.
* Its state is set to Waiting.
* The pair is enqueued into the triggered check queue.
When a triggered check is to be sent, it is constructed and processed
as described in Section 7.1.2. These procedures require the agent to
know the transport address, username fragment, and password for the
peer. The username fragment for the remote candidate is equal to the
part after the colon of the USERNAME in the Binding request that was
just received. Using that username fragment, the agent can check the
SDP messages received from its peer (there may be more than one in
cases of forking), and find this username fragment. The
corresponding password is then selected.
184.108.40.206. Updating the Nominated Flag
If the Binding request received by the agent had the USE-CANDIDATE
attribute set, and the agent is in the controlled role, the agent
looks at the state of the pair computed in Section 220.127.116.11:
o If the state of this pair is Succeeded, it means that the check
generated by this pair produced a successful response. This would
have caused the agent to construct a valid pair when that success
response was received (see Section 18.104.22.168.2). The agent now sets
the nominated flag in the valid pair to true. This may end ICE
processing for this media stream; see Section 8.
o If the state of this pair is In-Progress, if its check produces a
successful result, the resulting valid pair has its nominated flag
set when the response arrives. This may end ICE processing for
this media stream when it arrives; see Section 8.
7.2.2. Additional Procedures for Lite Implementations
If the check that was just received contained a USE-CANDIDATE
attribute, the agent constructs a candidate pair whose local
candidate is equal to the transport address on which the request was
received, and whose remote candidate is equal to the source transport
address of the request that was received. This candidate pair is
assigned an arbitrary priority, and placed into a list of valid
candidates called the valid list. The agent sets the nominated flag
for that pair to true. ICE processing is considered complete for a
media stream if the valid list contains a candidate pair for each