Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) TraversalEricssonHirsalantie 1102420 JorvasFinlandari.keranen@ericsson.comEricssonHirsalantie 1102420 JorvasFinlandchrister.holmberg@ericsson.comjdrosen.netMonmouthNJUSjdrosen@jdrosen.nethttp://www.jdrosen.net
ART
ICENATThis document describes a protocol for Network Address
Translator (NAT) traversal for UDP-based multimedia. This
protocol is called Interactive Connectivity Establishment
(ICE). ICE makes use of the Session Traversal Utilities
for NAT (STUN) protocol and its extension, Traversal Using
Relay NAT (TURN). This document obsoletes RFC 5245. Protocols establishing multimedia sessions between peers typically
involve exchanging IP addresses and ports for the media sources and
sinks. However this poses challenges when operated through Network Address
Translators (NATs) . These protocols also seek to
create a media flow directly between participants, so that there is
no application layer intermediary between them. This is done to
reduce media latency, decrease packet loss, and reduce the
operational costs of deploying the application. However, this is
difficult to accomplish through NAT. A full treatment of the reasons
for this is beyond the scope of this specification. Numerous solutions have been defined for allowing these protocols
to operate through NAT. These include Application Layer Gateways
(ALGs), the Middlebox Control Protocol,
the original Simple Traversal of UDP Through
NAT (STUN) specification, and Realm
Specific IP along with session
description extensions needed to make them work, such as the Session
Description Protocol (SDP) attribute for the
Real Time Control Protocol (RTCP) . Unfortunately, these techniques all have pros and
cons which, make each one optimal in some network topologies, but a
poor choice in others. The result is that administrators and
implementors are making assumptions about the topologies of the
networks in which their solutions will be deployed. This introduces
complexity and brittleness into the system. What is needed is a single
solution that is flexible enough to work well in all situations. This specification defines Interactive Connectivity Establishment
(ICE) as a technique for NAT traversal for UDP-based media streams
(though ICE has been extended to handle other transport protocols,
such as TCP ). ICE works by exchanging a
multiplicity of IP addresses and ports which are then tested for
connectivity by peer-to-peer connectivity checks. The IP addresses and
ports are exchanged via mechanisms (for example, including in a
offer/answer exchange) and the connectivity checks are performed using
Session Traversal Utilities for NAT (STUN) specification . ICE also makes use of Traversal Using Relays
around NAT (TURN) , an extension to STUN.
Because ICE exchanges a multiplicity of IP addresses and ports for
each media stream, it also allows for address selection for multihomed
and dual-stack hosts, and for this reason it deprecates and .
In a typical ICE deployment, we have two endpoints (known as ICE
AGENTS) that want to communicate. They are able to communicate
indirectly via some signaling protocol (such as SIP), by which they
can exchange ICE candidates. Note that ICE is not intended for NAT
traversal for the signaling protocol, which is assumed to be provided
via another mechanism. At the beginning of the ICE process, the agents
are ignorant of their own topologies. In particular, they might or
might not be behind a NAT (or multiple tiers of NATs). ICE allows the
agents to discover enough information about their topologies to
potentially find one or more paths by which they can communicate.
shows a typical environment for ICE
deployment. The two endpoints are labelled L and R (for left and
right, which helps visualize call flows). Both L and R are behind
their own respective NATs though they may not be aware of it. The type
of NAT and its properties are also unknown. Agents L and R are capable
of engaging in an candidate exchange process, whose purpose is to set
up a media session between L and R. Typically, this exchange will
occur through a signaling (e.g., SIP) server.
In addition to the agents, a signaling server and NATs, ICE is
typically used in concert with STUN or TURN servers in the
network. Each agent can have its own STUN or TURN server, or they can
be the same.
The basic idea behind ICE is as follows: each agent has a variety
of candidate TRANSPORT ADDRESSES (combination of IP address and port
for a particular transport protocol, which is always UDP in this
specification) it could use to communicate with the other agent. These
might include:
A transport address on a directly attached network interfaceA translated transport address on the public side of a NAT (a "server
reflexive" address)A transport address allocated from a TURN server (a "relayed
address")
Potentially, any of L's candidate transport addresses can be used to
communicate with any of R's candidate transport addresses. In
practice, however, many combinations will not work. For instance, if L
and R are both behind NATs, their directly attached interface
addresses are unlikely to be able to communicate directly (this is why
ICE is needed, after all!). The purpose of ICE is to discover which
pairs of addresses will work. The way that ICE does this is to
systematically try all possible pairs (in a carefully sorted order)
until it finds one or more that work.
In order to execute ICE, an agent has to identify all of its address
candidates. A CANDIDATE is a transport address -- a combination of IP
address and port for a particular transport protocol (with only UDP
specified here). This document defines three types of candidates, some
derived from physical or logical network interfaces, others
discoverable via STUN and TURN. Naturally, one viable candidate is a
transport address obtained directly from a local interface. Such a
candidate is called a HOST CANDIDATE. The local interface could be
Ethernet or WiFi, or it could be one that is obtained through a tunnel
mechanism, such as a Virtual Private Network (VPN) or Mobile IP
(MIP). In all cases, such a network interface appears to the agent as
a local interface from which ports (and thus candidates) can be
allocated.
If an agent is multihomed, it obtains a candidate from each IP
address. Depending on the location of the PEER (the other agent in the
session) on the IP network relative to the agent, the agent may be
reachable by the peer through one or more of those IP
addresses. Consider, for example, an agent that has a local IP address
on a private net 10 network (I1), and a second connected to the public
Internet (I2). A candidate from I1 will be directly reachable when
communicating with a peer on the same private net 10 network, while a
candidate from I2 will be directly reachable when communicating with a
peer on the public Internet. Rather than trying to guess which IP
address will work, the initiating sends both the candidates to its
peer.
Next, the agent uses STUN or TURN to obtain additional
candidates. These come in two flavors: translated addresses on the
public side of a NAT (SERVER REFLEXIVE CANDIDATES) and addresses on
TURN servers (RELAYED CANDIDATES). When TURN servers are utilized,
both types of candidates are obtained from the TURN server. If only
STUN servers are utilized, only server reflexive candidates are
obtained from them. The relationship of these candidates to the host
candidate is shown in . In this
figure, both types of candidates are discovered using TURN. In the
figure, the notation X:x means IP address X and UDP port x.
When the agent sends the TURN Allocate request from IP address and
port X:x, the NAT (assuming there is one) will create a binding
X1':x1', mapping this server reflexive candidate to the host candidate
X:x. Outgoing packets sent from the host candidate will be translated
by the NAT to the server reflexive candidate. Incoming packets sent
to the server reflexive candidate will be translated by the NAT to the
host candidate and forwarded to the agent. We call the host candidate
associated with a given server reflexive candidate the BASE.
Note: "Base" refers to the address an agent sends from for a particular
candidate. Thus, as a degenerate case host candidates also have a base,
but it's the same as the host candidate.
When there are multiple NATs between the agent and the TURN server,
the TURN request will create a binding on each NAT, but only the
outermost server reflexive candidate (the one nearest the TURN server)
will be discovered by the agent. If the agent is not behind a NAT,
then the base candidate will be the same as the server reflexive
candidate and the server reflexive candidate is redundant and will be
eliminated.
The Allocate request then arrives at the TURN server. The TURN server
allocates a port y from its local IP address Y, and generates an
Allocate response, informing the agent of this relayed candidate. The
TURN server also informs the agent of the server reflexive candidate,
X1':x1' by copying the source transport address of the Allocate
request into the Allocate response. The TURN server acts as a packet
relay, forwarding traffic between L and R. In order to send traffic to
L, R sends traffic to the TURN server at Y:y, and the TURN server
forwards that to X1':x1', which passes through the NAT where it is
mapped to X:x and delivered to L.
When only STUN servers are utilized, the agent sends a STUN Binding
request to its STUN server. The STUN server
will inform the agent of the server reflexive candidate X1':x1' by
copying the source transport address of the Binding request into the
Binding response.
Once L has gathered all of its candidates, it orders them in highest
to lowest-priority and sends them to R over the signaling
channel. When R receives the candidates from L, it performs the same
gathering process and responds with its own list of candidates. At the
end of this process, each agent has a complete list of both its
candidates and its peer's candidates. It pairs them up, resulting in
CANDIDATE PAIRS. To see which pairs work, each agent schedules a
series of CHECKS. Each check is a STUN request/response transaction
that the client will perform on a particular candidate pair by sending
a STUN request from the local candidate to the remote candidate.
The basic principle of the connectivity checks is simple:
Sort the candidate pairs in priority order.Send checks on each candidate pair in priority order.Acknowledge checks received from the other agent.
With both agents performing a check on a candidate pair, the result is
a 4-way handshake:
It is important to note that the STUN requests are sent to and from
the exact same IP addresses and ports that will be used for media
(e.g., RTP and RTCP). Consequently, agents demultiplex STUN and
RTP/RTCP using contents of the packets, rather than the port on which
they are received. Fortunately, this demultiplexing is easy to do,
especially for RTP and RTCP.
Because a STUN Binding request is used for the connectivity check, the
STUN Binding response will contain the agent's translated transport
address on the public side of any NATs between the agent and its
peer. If this transport address is different from other candidates the
agent already learned, it represents a new candidate, called a PEER
REFLEXIVE CANDIDATE, which then gets tested by ICE just the same as
any other candidate.
As an optimization, as soon as R gets L's check message, R schedules a
connectivity check message to be sent to L on the same candidate
pair. This accelerates the process of finding a valid candidate, and
is called a TRIGGERED CHECK.
At the end of this handshake, both L and R know that they can
send (and receive) messages end-to-end in both directions.
Because the algorithm above searches all candidate pairs, if a working
pair exists it will eventually find it no matter what order the
candidates are tried in. In order to produce faster (and better)
results, the candidates are sorted in a specified order. The resulting
list of sorted candidate pairs is called the CHECK LIST. The algorithm
is described in but follows two
general principles:
Each agent gives its candidates a numeric priority, which is sent
along with the candidate to the peer.The local and remote priorities are combined so that each
agent has the same ordering for the candidate pairs.
The second property is important for getting ICE to work when there
are NATs in front of L and R. Frequently, NATs will not allow packets
in from a host until the agent behind the NAT has sent a packet
towards that host. Consequently, ICE checks in each direction will not
succeed until both sides have sent a check through their respective
NATs.
The agent works through this check list by sending a STUN request for
the next candidate pair on the list periodically. These are called
ORDINARY CHECKS.
In general, the priority algorithm is designed so that candidates of
similar type get similar priorities and so that more direct routes
(that is, through fewer media relays and through fewer NATs) are
preferred over indirect ones (ones with more media relays and more
NATs). Within those guidelines, however, agents have a fair amount of
discretion about how to tune their algorithms.
The previous description only addresses the case where the agents
wish to establish a media session with one COMPONENT (a piece of a
media stream requiring a single transport address; a media stream may
require multiple components, each of which has to work for the media
stream as a whole to be work). Sometimes (e.g., with RTP and RTCP in
separate components), the agents actually need to establish
connectivity for more than one flow.
The network properties are likely to be very similar for each
component (especially because RTP and RTCP are sent and received
from the same IP address). It is usually possible to leverage
information from one media component in order to determine the best
candidates for another. ICE does this with a mechanism called "frozen
candidates".
Each candidate is associated with a property called its
FOUNDATION. Two candidates have the same foundation when they are
"similar" -- of the same type and obtained from the same host
candidate and STUN/TURN server using the same protocol. Otherwise,
their foundation is different. A candidate pair has a foundation too,
which is just the concatenation of the foundations of its two
candidates. Initially, only the candidate pairs with unique
foundations are tested. The other candidate pairs are marked
"frozen". When the connectivity checks for a candidate pair succeed,
the other candidate pairs with the same foundation are unfrozen. This
avoids repeated checking of components that are superficially more
attractive but in fact are likely to fail.
While we've described "frozen" here as a separate mechanism for
expository purposes, in fact it is an integral part of ICE and the ICE
prioritization algorithm automatically ensures that the right
candidates are unfrozen and checked in the right order. However, if
the ICE usage does not utilize multiple components or media streams,
it does not need to implement this algorithm.
Because ICE is used to discover which addresses can be used to send
media between two agents, it is important to ensure that the process
cannot be hijacked to send media to the wrong location. Each STUN
connectivity check is covered by a message authentication code (MAC)
computed using a key exchanged in the signaling channel. This MAC
provides message integrity and data origin authentication, thus
stopping an attacker from forging or modifying connectivity check
messages. Furthermore, if for example a SIP
caller is using ICE, and their call forks, the ICE exchanges happen
independently with each forked recipient. In such a case, the keys
exchanged in the signaling help associate each ICE exchange with each
forked recipient.
ICE checks are performed in a specific sequence, so that high-priority
candidate pairs are checked first, followed by lower-priority
ones. One way to conclude ICE is to declare victory as soon as a check
for each component of each media stream completes
successfully. Indeed, this is a reasonable algorithm, and details for
it are provided below. However, it is possible that a packet loss will
cause a higher-priority check to take longer to complete. In that
case, allowing ICE to run a little longer might produce better
results. More fundamentally, however, the prioritization defined by
this specification may not yield "optimal" results. As an example, if
the aim is to select low-latency media paths, usage of a relay is a
hint that latencies may be higher, but it is nothing more than a
hint. An actual round-trip time (RTT) measurement could be made, and
it might demonstrate that a pair with lower priority is actually
better than one with higher priority.
Consequently, ICE assigns one of the agents in the role of the
CONTROLLING AGENT, and the other of the CONTROLLED AGENT. The
controlling agent gets to nominate which candidate pairs will get used
for media amongst the ones that are valid.
When nominating, the controlling agent lets the checks continue
until at least one valid candidate pair for each media stream is
found. Then, it picks amongst those that are valid, and sends a second
STUN request on its NOMINATED candidate pair, but this time with a
flag set to tell the peer that this pair has been nominated for use.
This is shown in .
Once the STUN transaction with the flag completes, both sides cancel
any future checks for that media stream. ICE will now send media using
this pair. The pair an ICE agent is using for media is called the
SELECTED PAIR.
Once ICE is concluded, it can be restarted at any time for one or all
of the media streams by either agent. This is done by sending an updated
candidate information indicating a restart.
In order for ICE to be used in a call, both agents need to support it.
However, certain agents will always be connected to the public
Internet and have a public IP address at which it can receive packets
from any correspondent. To make it easier for these devices to support
ICE, ICE defines a special type of implementation called LITE (in
contrast to the normal FULL implementation). A lite implementation
doesn't gather candidates; it includes only host candidates for any
media stream. Lite agents do not generate connectivity checks or run
the state machines, though they need to be able to respond to
connectivity checks. When a lite implementation connects with a full
implementation, the full agent takes the role of the controlling
agent, and the lite agent takes on the controlled role. When two lite
implementations connect, no checks are sent.
For guidance on when a
lite implementation is appropriate, see the discussion in .
It is important to note that the lite implementation was added to this
specification to provide a stepping stone to full implementation. Even
for devices that are always connected to the public Internet, a full
implementation is preferable if achievable.
This document specifies generic use of ICE with protocols that
provide means to exchange candidate information between the ICE Peers.
The specific details of (i.e how to encode candidate information and
the actual candidate exchange process) for different protocols using
ICE are described in separate usage documents. One possible way the
agents can exchange the candidate information is to use based Offer/Answer semantics as part of the SIP
protocol .
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY",
and "OPTIONAL" in this document are to be interpreted as described in
RFC 2119.
Readers should be familiar with the terminology defined in the STUN , and NAT Behavioral requirements
for UDP .
This specification makes use of the following additional terminology:
An agent is the protocol implementation involved in the
ICE candidate exchange. There are two agents involved in a typical
candidate exchange.
An initiating agent is the protocol implementation involved in the ICE
candidate exchange that initiates the ICE candidate exchange
process.
A receiving agent is the protocol implementation involved in the ICE
candidate exchange that receives
and responds to the candidate exchange process initiated by the
Initiator.
The process where the ICE agents exchange information (e.g.,
candidates and passwords) that is needed to perform ICE. Offer/Answer with SDP encoding is one example of a
protocol that can be used for exchanging the candidate
information.
From the perspective of one of the agents in a session, its peer is
the other agent. Specifically, from the perspective of the initiating
agent, the peer is the responding agent. From the perspective of the
responding agent, the peer is the initiating agent. The combination of an IP address and
transport protocol (such as UDP or TCP) port. When ICE is used to
setup multimedia sessions, the media is usually transported over RTP,
and a media stream composes of a stream of RTP packets. When ICE is
used with other than multimedia sessions, the terms "media", "media
stream", and "media session" are still used in this specification to
refer to the IP data packets that are exchanged between the peers on
the path created and tested with ICE. A transport address
that is a potential point of contact for receipt of media. Candidates
also have properties -- their type (server reflexive, relayed, or
host), priority,foundation, and base.
A component is a piece of a media stream
requiring a single transport address; a media stream may require
multiple components, each of which has to work for the media stream as
a whole to work. For media streams based on RTP, unless RTP and RTCP
are multiplexed in the same port, there are two components per media
stream -- one for RTP, and one for RTCP. A candidate obtained by binding to a
specific port from an IP address on the host. This includes IP
addresses on physical interfaces and logical ones, such as ones
obtained through Virtual Private Networks (VPNs) and Realm Specific IP
(RSIP) (which lives at the operating system
level).
A candidate whose IP
address and port are a binding allocated by a NAT for an agent when it
sent a packet through the NAT to a server. Server reflexive candidates
can be learned by STUN servers using the Binding request, or TURN
servers, which provides both a relayed and server reflexive candidate.
A candidate whose IP
address and port are a binding allocated by a NAT for an agent when it
sent a STUN Binding request through the NAT to its peer.
A candidate obtained by sending a
TURN Allocate request from a host candidate to a TURN server. The
relayed candidate is resident on the TURN server, and the TURN server
relays packets back towards the agent.
The base of a server reflexive candidate is the
host candidate from which it was derived. A host candidate is also
said to have a base, equal to that candidate itself. Similarly, the
base of a relayed candidate is that candidate itself.
An arbitrary string that is the same for
two candidates that have the same type, base IP address, protocol
(UDP, TCP, etc.), and STUN or TURN server. If any of these are
different, then the foundation will be different. Two candidate pairs
with the same foundation pairs are likely to have similar network
characteristics. Foundations are used in the frozen algorithm.
A candidate that an agent has obtained
and shared with the peer.
A candidate that an agent received
from its peer.
The default destination
for a component of a media stream is the transport address that would
be used by an agent that is not ICE aware. A default candidate for a
component is one whose transport address matches the default
destination for that component.
A pairing containing a local candidate
and a remote candidate.
A STUN Binding
request transaction for the purposes of verifying connectivity. A
check is sent from the local candidate to the remote candidate of a
candidate pair.
An ordered set of candidate pairs that an
agent will use to generate checks.
A connectivity check generated by an
agent as a consequence of a timer that fires periodically, instructing
it to send a check.
A connectivity check generated as a
consequence of the receipt of a connectivity check from the peer.
An ordered set of candidate pairs for a
media stream that have been validated by a successful STUN
transaction.
An ICE implementation that performs the complete
set of functionality defined by this specification.
An ICE implementation that omits certain
functions, implementing only as much as is necessary for a peer
implementation that is full to gain the benefits of ICE. Lite
implementations do not maintain any of the state machines and do not
generate connectivity checks.
The ICE agent that is responsible
for selecting the final choice of candidate pairs and signaling them
through STUN. In any session, one agent is always controlling. The
other is the controlled agent.
An ICE agent that waits for the
controlling agent to select the final choice of candidate pairs.
The process of picking a valid
candidate pair for media traffic by validating the pair with one
STUN request, and then picking it by sending a second STUN request
with a flag indicating its nomination.
If a valid candidate pair has its nominated
flag set, it means that it may be selected by ICE for sending and
receiving media.
The candidate pair
selected by ICE for sending and receiving media is called the selected
pair, and each of its candidates is called the selected candidate.
The protocol that uses ICE
for NAT traversal. A usage specification defines the protocol specific
details on how the procedures defined here are applied to that
protocol.
As part of ICE processing, both the initiating and responding agents
exchange encoded candidate information as defined by the Usage
Protocol (ICE Usage). Specifics of encoding mechanism and the
semantics of candidate information exchange is out of scope of this
specification.
However at a higher level, the below diagram captures ICE processing
sequence in the agents (initiator and responder) for exchange of
their respective candidate(s) information.
As shown, the agents involved in the candidate exchange perform (1)
candidate gathering, (2) candidate prioritization, (3) eliminating
redundant candidates, (4) (possibly) choose default candidates, and
then (5) formulate and send the candidates to the Peer ICE agent. All
but the last of these five steps differ for full and lite
implementations.
An agent gathers candidates when it believes that communication is
imminent. An initiating agent 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.
The process for gathering candidates at the responding agent is
identical to the process for the initiating agent. It is RECOMMENDED
that the responding agent begins this process immediately on receipt
of the candidate information, prior to alerting the user. Such
gathering MAY begin when an agent starts.
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, with the exceptions listed below. The agent
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, unless both RTP and RTCP are multiplexed
in the same UDP port (RTP/RTCP multiplexing), the RTP itself has a
component ID of 1, and RTCP a component ID of 2. In case of RTP/RTCP
multiplexing, a component ID of 1 is used for both RTP and RTCP.When candidates are obtained, unless the agent knows for sure that
RTP/RTCP multiplexing will be used (i.e. the agent knows that the
other agent also supports, and is willing to use, RTP/RTCP
multiplexing), or unless the agent only supports RTP/RTCP
multiplexing, the agent MUST obtain a separate candidate for RTCP. If
an agent has obtained a candidate for RTCP, and ends up using RTP/RTCP
multiplexing, the agent does not need to perform connectivity checks
on the RTCP candidate.If an agent is using separate candidates for RTP and RTCP, it will
end up with 2*K host candidates if an agent has K IP addresses.Note that the responding agent, when obtaining its candidates, will
typically know if the other agent supports RTP/RTCP multiplexing, in
which case it will not need to obtain a separate candidate for
RTCP. However, absence of a component ID 2 as such does not imply use
of RTCP/RTP multiplexing, as it could also mean that RTCP is not
used. For other than RTP-based streams, use of multiple components is
discouraged since using them increases the complexity of ICE
processing. If multiple components are needed, the component IDs
SHOULD start with 1 and increase by 1 for each component.
The base for each host candidate is set to the candidate itself.
The host candidates are gathered from all IP addresses with the
following exceptions:
Addresses from a loopback interface MUST NOT be included in
the candidate addresses. Deprecated IPv4-compatible IPv6 addresses and IPv6 site-local unicast addresses MUST NOT be included in the address
candidates. IPv4-mapped IPv6 addresses SHOULD NOT be included in the
offered candidates unless the application using ICE does not
support IPv4 (i.e., is an IPv6-only application ). If one or more host candidates corresponding to an IPv6
address generated using a mechanism that prevents location
tracking are
gathered, host candidates corresponding to IPv6 addresses that do
allow location tracking, that are configured on the same
interface, and are part of the same network prefix MUST NOT be
gathered; and host candidates corresponding to IPv6 link-local
addresses MUST NOT be gathered.
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 (using
SRV records with the "stun" service) be used to discover the STUN
server, and the DNS procedures in (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 . 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 for guidance on how to set Ta and the STUN
retransmit timer, RTO.
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.
If an IPv6-only agent is in a network that utilizes NAT64 and DNS64 technologies, it
may gather also IPv4 server reflexive and/or relayed candidates from
IPv4-only STUN or TURN servers. IPv6-only agents SHOULD also utilize
IPv6 prefix discovery to discover the IPv6
prefix used by NAT64 (if any) and generate server reflexive candidates
for each IPv6-only interface accordingly. The NAT64 server reflexive
candidates are prioritized like IPv4 server reflexive candidates.
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:
they are of the same type (host, relayed,
server reflexive, or peer reflexive)their bases have the
same IP address (the ports can be different)for reflexive and relayed candidates, the STUN or TURN servers used
to obtain them have the same IP address
they were obtained using the same transport protocol (TCP, UDP,
etc.)
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.
Once server reflexive and relayed candidates are allocated, they MUST
be kept alive until ICE processing has completed, as described in
. 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 . The Refresh requests will also refresh the server
reflexive candidate.
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 and choose its parameters using the guidelines
in . 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.
The process for prioritizing candidates is common across the
initiating and the responding agent.
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:
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
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. 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 or if an agent is using multiple TURN servers. If a
host is multihomed because it is dual-stack, the local preference
should be set according to the current best practice described in .The
component ID is the component ID for the candidate, and MUST be
between 1 and 256 inclusive.
One criterion for selection of the type and local preference values
is the use of a media intermediary, such as a TURN server, a tunnel
service such as 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 have a positive or negative
effect on the latency between transmission and reception. It may or
may not 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 must be carefully chosen. 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 recommendation in should be followed. If multiple
TURN servers are used, local priorities for the candidates obtained
from the TURN servers are chosen in a similar fashion as for
multihomed local candidates: the local preference value is used to
indicate a preference among different servers but the preference MUST
be unique for each one.Another criterion for selection of preferences is IP address
family. ICE works with both IPv4 and IPv6. It 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. Implementation should follow the guidelines from to avoid excessively delays
in the connectivity check phase if broken paths exist.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.Another criterion for selecting preferences might be security or
privacy. 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 or similar tunnel 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 address.
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.
This process is common across the initiating and responding agents.
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, unless RTCP is multiplexed in the same
port with RTP, the RTP itself has a component ID of 1, and RTCP a
component ID of 2. If an agent is using RTCP without multiplexing, it
MUST obtain candidates for it. However, absence of a component ID 2
as such does not imply use of RTCP/RTP multiplexing, as it could also
mean that RTCP is not used.
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:
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 6724 .
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 RECOMMENDED.
The procedures in this section is common across the initiating and
responding agents.
Regardless of the agent being an Initiator or Responder Agent,
the following parameters and their data types needs to be conveyed
as part of the candidate exchange process. The specifics of syntax
for encoding the candidate information is out of scope of this
specification.
There will be one or more of these
for each "media stream". Each candidate is composed of:
The IP address and transport
protocol port of the candidate. An indicator of the transport protocol
for this candidate. This need not be present if the using protocol
will only ever run over a single transport protocol. If it runs
over more than one, or if others are anticipated to be used in the
future, this should be present. A sequence of up to 32 characters. This would be present only if the using
protocol were utilizing the concept of components. If it is, it
would be a positive integer that indicates the component ID for
which this is a candidate. An encoding of the 32-bit priority
value. The candidate type, as defined in
ICE. The related IP address and
port for this candidate, as defined by ICE. These MAY be omitted or
set to invalid values if the agent does not want to reveal them,
e.g., for privacy reasons.
The using protocol should
define some means for adding new per-candidate ICE parameters in
the future. If ICE lite is used by the using protocol,
it needs to convey a boolean parameter which indicates whether the
implementation is lite or not. If an agent wants to
use other than the default pacing values for the connectivity checks,
it MUST indicate this in the ICE exchange. The using protocol has
to convey a username fragment and password. The username fragment MUST
contain at least 24 bits of randomness, and the password MUST contain
at least 128 bits of randomness. In addition to the per-candidate
extensions above, the using protocol should allow for new
media-stream or session-level attributes (ice-options).
If the using protocol is using the ICE mismatch feature, a way is
needed to convey this parameter in answers. It is a boolean flag.
The exchange of parameters is symmetric; both agents need to send the
same set of attributes as defined above.
The using protocol may (or may not) need to deal with backwards
compatibility with older implementations that do not support ICE. If
the fallback mechanism is being used, then presumably the using
protocol provides a way of conveying the default candidate (its IP
address and port) in addition to the ICE parameters.
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. 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 between
the peers. In addition to providing security, the username provides
disambiguation and correlation of checks to media streams. See
Appendix B.4 for motivation.
If the initiating agent is a lite implementation, it MUST indicate
this when sending its candidates .
ICE provides for extensibility by allowing an agent to include a
series of tokens that identify ICE extensions as part
of the candidate exchange process.
Once an agent has sent its candidate information, that agent MUST be
prepared to receive both STUN and media packets on each candidate. As
discussed in , media packets can be
sent to a candidate prior to its appearance as the default destination
for media.
Once an agent has candidates from it's peer, it will check if the peer
supports ICE, determine its own role, exchanges candidates () and for full implementations, forms
the check lists and begins connectivity checks as explained in this
section.
Certain middleboxes, such as ALGs, may alter the ICE candidate
information that breaks ICE. If the using protocol is vulnerable to
this kind of changes, called ICE mismatch, the responding agent needs
to detect this and signal this back to the initiating agent. The
details on whether this is needed and how it is done is defined by the
usage specifications. One exception to the above is that an initiating
agent would never indicate ICE mismatch.
For each session, each agent (Initiating and Responding) 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 updating the peer with the ICE's selection, when needed. The
controlled agent is told which candidate pairs to use for each media
stream, and does not require updating the peer 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
follows:
The Initiating Agent 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 to nominate pairs that will be selected
by ICE, and then both agents end ICE as described in .
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 to nominate pairs that
will be selected by ICE, and use the logic in 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 . 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.
The Initiating Agent 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 candidates are exchanged, each agent performs
the processing described in 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 enabling the candidate exchange. The state of ICE processing
for each media stream is considered to be Running, and the state of
ICE overall is Running.
Once the roles are determined for a session, they persist througout
the lifetime of the session. The roles can be re-determined as part
of an ICE restart (), but an ICE agent
MUST NOT re-determine the role as part of an ICE restart unless one
or more of the following criteria is fulfilled:
If the controlling agent is full, and switches to lite, the roles MUST be
re-determined if the peer agent is also full.
If the ICE restart causes a role conflict, the roles might be
re-determined due to the role conflict procedures in .
NOTE: There are certain 3PCC scenarios where an ICE restart might cause
a role conflict.
NOTE: The ICE agents needs to inform each other whether they are full
or lite before the roles are determined. The mechanism for that is
signalling protocol specific, and outside the scope of the document.
An ICE agent MUST be prepared that the peer might re-determine the roles
as part of any ICE restart, even if the criteria for doing so are not
fulfilled. This can happen if the peer is compliant with an older version
of this specification.
There is one check list per in-use media stream resulting from the
candidate 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.
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 , agents MAY limit the number of candidates
they'll accept in an candidate exchange process. 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
initiating agent can multiplex RTP and RTCP on the same port . However, since the initiating agent doesn't know
if the peer agent can perform such multiplexing, it includes
candidates for RTP and RTCP on separate ports. If the peer agent 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.
With IPv6 it is common for a host to have multiple host candidates
for each interface. To keep the amount of resulting candidate pairs
reasonable and to avoid candidate pairs that are highly unlikely to
work, IPv6 link-local addresses MUST NOT be
paired with other than link-local addresses.
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, 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.
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.
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 , 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.
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:
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.
A check has been sent for this pair, but
the transaction is in progress.
A check for this pair was already done and
produced a successful result.
A check for this pair was already done and
failed, either
never producing any response or producing an unrecoverable failure
response.
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 .
The initial states for each pair in a check list are computed by
performing the following sequence of steps:
The agent sets all of the pairs in each check list to the Frozen
state.
The agent examines the check list for the first media stream. For
that media stream:
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 used.
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: In this state, ICE checks are still in
progress for this media stream.
In this state, ICE checks have produced
nominated pairs for each component of the media stream.
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
candidate 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.
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 sent.
Once the agent has computed the check lists as described in , 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 . 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 candidate 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:
Find the highest-priority pair in that check list that is in the
Waiting state.
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 .
Set the state of the candidate pair to In-Progress.
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 candidate
information obtained from its peer. The local username fragment is
known directly by the agent for its own candidate.
The Initiator performs the ordinary checks on receiving the candidate
information from the Peer (responder) and having formed the checklists.
On the other hand the responding agent either performs the triggered or
ordinary checks as described above.
Lite implementations skips most of the steps in except for verifying the peer's ICE
support and determining its role in the ICE processing.
On determining the role for a lite implementation being the
controlling agent means selecting a candidate pair based on the ones
in the candidate exchange (for IPv4, there is only ever one pair),
and then updating the peer with the new candidate information
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 no further candidate updates are
needed to signal this information.
This section describes how connectivity checks are performed. All ICE
implementations are required to be compliant to , as opposed to the older . 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.
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.
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
. 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.
A connectivity check is generated by sending a Binding request from
a local candidate to a remote candidate.
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 connectivity checks.
ICE extends STUN by defining several new attributes, including
PRIORITY, USE-CANDIDATE, ICE-CONTROLLED, and ICE-CONTROLLING. These
new attributes are formally defined in
, and their usage is
described in the subsections below. These STUN extensions are
applicable only to connectivity checks used for ICE.
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 ,
to a peer reflexive candidate, should one be learned as a consequence
of this check (see 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 candidate types.
The controlling agent includes the USE-CANDIDATE attribute in order
to nominate a candidate pair .
The controlled agent MUST NOT include the USE-CANDIDATE attribute in its
Binding request.
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 are used as tie-breaker values
when an ICE role conflict occurs .
The ICE-CONTROLLED and ICE-CONTROLLING attributes are defined
in .
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
initiating , agent and agent R is the responding agent. 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
response).
If the agent is using Diffserv Codepoint markings in its media packets, it SHOULD apply those same
markings to its connectivity checks.
When a Binding response is received, it is correlated to its Binding
request using the transaction ID, as defined in , 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.
If the STUN transaction generates a 487 (Role Conflict) error
response, the agent checks whether it included an ICE-CONTROLLED or
ICE-CONTROLLING attribute in the associated Binding request. If the request
contained an ICE-CONTROLLED attribute, the agent MUST switch to the
controlling role. If the request contained an ICE-CONTROLLING attribute,
the agent MUST switch to the controlled role.
Once the agent has switched its role, 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. The agent
MUST NOT change the tie-breaker value.
A change in roles will require an agent to recompute pair priorities
(), 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 candidate information for sharing 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 ) 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.
A check is considered to be a success if all of the following are
true:
The STUN transaction generated a success response.The source IP address and port of the response equals the
destination IP address and port to which the Binding request was sent.The destination IP address and port of the response match the
source IP address and port from which the Binding request was
sent.
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:
Its type is equal to peer reflexive.Its base is set equal to the local candidate of the candidate pair
from which the STUN check was sent.Its priority is set equal to the value of the PRIORITY attribute in
the Binding request.Its foundation is selected as
described in .
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 . 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
update the peer with the candidate information that includes the peer
reflexive candidate. This will cause it to be paired with all other
remote candidates.
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 . 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 candidate
exchange. 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
candidate information 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 LIST.
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 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:
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. 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
candidate exchange differs from initiating to responding agent), 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 .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, andfor 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 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, unless
the sending agent detects that the candidiate pair does not work.
This concludes the ICE processing for this media stream or all
media streams; see .
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
, and may now result in setting the
nominated flag for the pair learned from the original request.
An agent MUST NOT select a candidate pair until it has sent a
Binding request, and received the corresponding Binding response,
associated with the candidiate pair.
Regardless of whether the check was successful or failed, the
completion of the transaction may require updating of check list and
timer states.
If all of the pairs in the check
list are now either in the Failed or Succeeded state:
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. For each frozen check list,
the agent
groups together all of the pairs with the same foundation, andfor 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 .
An agent MUST be prepared to receive a Binding request on the base of
each candidate it included in its most recent candidate exchange. This
requirement holds even if the peer is a lite implementation.
The agent MUST use the short-term credential mechanism (i.e., the
MESSAGE-INTEGRITY attribute) to authenticate the request and perform a
message integrity check. Likewise, the short-term credential mechanism
MUST be used for the response. 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 candidate exchange for a session in-progress. It is possible
(and in fact very likely) that the initiating agent will receive a
Binding request prior to receiving the candidates from its peer. If
this happens, the agent MUST immediately generate a response
(including computation of the mapped address as described in ). 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, , , and
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 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.
This subsection defines the additional server procedures applicable
to full implementations.
Normally, the rules for selection of a role in
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. These procedures apply only to usages of ICE that
require conflict resolution. The usage document MUST specify whether this
mechanism is needed.
An agent MUST examine the Binding request for either the
ICE-CONTROLLING or ICE-CONTROLLED attribute. It MUST follow these
procedures:
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
detected.
If the agent is in the controlling role, and the ICE-CONTROLLING
attribute is present in the request:
If the agent's tie-breaker value 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 value is less than the contents of the
ICE-CONTROLLING attribute, the agent switches to the controlled
role.
If the agent is in the controlled role, and the ICE-CONTROLLED
attribute is present in the request:
If the agent's tie-breaker value 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 value 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.
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
(), 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 initiating exchange with updated candidate information
upon conclusion of ICE.
The remaining sections in are
followed if the server generated a successful response to the Binding
request, even if the agent changed roles.
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.
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:
The priority of the candidate is set to the PRIORITY attribute from
the request.The type of the candidate is set to peer reflexive. The foundation of the candidate is set to an arbitrary value,
different from the foundation for all other remote candidates. If any
subsequent candidate exchanges contain this peer reflexive
candidate, it will signal the actual foundation for the
candidate.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.
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 several
outcomes:
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 already present.
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
done.
These steps are done to facilitate rapid completion of ICE when both
agents are behind NAT.
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 . 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 candidates 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.
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 :
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 ). The agent now sets
the nominated flag in the valid pair to true. This may end ICE
processing for this media stream; see .
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 .
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 component.
This section describes how an agent completes ICE.
Concluding ICE involves nominating pairs by the controlling agent and
updating of state machinery.
When nominating, the controlling 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 enqueueing 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.
The controlling 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.
The controlled agent SHOULD select the nominated candidate pair if the agent
is receiving Binding responses associated with that candidiate pair.
Before the agent has received Binding responses associated with the candidiate
pair, the agent can send media on any candidiate for which it has received
Binding responses. If more than one candidate pair is nominated by the
controlling agent, the controlled agent SHOULD select the candidate pair with
the highest priority.
NOTE: A controlling agent that does not support this speification (i.e. it
is implemented according to RFC 5245) might nominate more than one candidiate pair.
This was referred to as aggressive nomination in RFC 5245. The usage of the
'ice2' ice option by endpoints supporting this specifcation should prevent
such controlling agents from using aggressive nomination.
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 .
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 .
Once the state of each check list is Completed:
The agent sets the state of ICE processing overall to Completed.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 while sending updated candidate list to its peer. If none of the check lists for other media streams are
Completed, but at least one is Running, the agent SHOULD let ICE
continue.
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 .
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 as part of subsequent candidate exchange process.
Once the candidate 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 6724 to select a single
pair. The agent adds the selected pair for each component to the valid
list. As described in , 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 updated
candidate list which will include the remote-candidates attribute.
The agent MUST NOT update the state of ICE processing until
after the candidate exchange completes. Then 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 procedures in 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.
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.
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.
An agent MAY restart ICE processing for an existing media stream.
An ICE restart will cause all previous states, excluding the roles
of the agents, of ICE processing to be flushed and checks to start anew.
The only difference between an ICE restart and a brand new media session
is that during the restart media can continue to be sent to the
previously validated pair, and that a new media session always requires
the roles to be determined.
An agent MUST restart ICE for a media stream if:
The candidate(s) is being generated for the purposes of changing the
target of the media stream. In other words, if an agent wants to
generate an updated candidate information that, had ICE not been in
use, would result in a new value for the destination of a media
component.
An agent is changing its implementation level. This typically
only happens in third party call control use cases, where the entity
performing the signaling is not the entity receiving the media, and it
has changed the target of media mid-session to another entity that has
a different ICE implementation.
To restart ICE, an agent MUST change both the password and the
user name fragment for the media stream when exchanging the
candidates. The new candidate set MAY include some, none, or all of
the previous candidates for that stream and MAY include a totally new
set of candidates.
As described in , ICE agents MUST NOT
re-determine the roles as part as an ICE restart, unless certain
criteria that require the roles to be re-determined is fulfilled.
This section defines a new ICE option, 'ice2'. The ICE option
indicates that the ICE agent that includes it (in an ice-options
attribute) is compliant to this specification. For example, the
ICE agent will not use the aggressive nomination procedure
defined in .
An ICE agent compliant to this specification MUST inform the peer
about the compliance using the 'ice2' ICE option.
NOTE: The encoding of the 'ice2' ICE option, and the message(s)
used to carry it to the peer, are protocol specific. The encoding
for the Session Description Protocol (SDP)
is defined in .
All endpoints MUST send keepalives for each media session. These
keepalives serve the purpose of keeping NAT bindings alive for the
media session. The keepalives SHOULD be sent using a
format that is supported by its peer. ICE endpoints allow for
STUN-based keepalives for UDP streams, and as such, STUN keepalives
MUST be used when an agent is a full ICE implementation and is
communicating with a peer that supports ICE (lite or full).
If there has been no packet sent on the candidate pair ICE is using
for a media component for Tr seconds (where packets include those
defined for the component (RTP or RTCP) and previous keepalives), an
agent MUST generate a keepalive on that pair. ICE endpoints SHOULD use
a Tr value of 15 seconds, but MAY use another value, e.g. based on
configuration or network/NAT characteristics. For example, if
an agent has a dynamic way to discover the binding lifetimes of the
intervening NATs, it can use that value to determine Tr. Administrators
deploying ICE in more controlled networking environments SHOULD set Tr
to the longest duration possible in their environment. ICE endpoints
MUST NOT use a Tr value smaller than 15 seconds.
When STUN is being used for keepalives, a STUN Binding Indication is
used . The Indication MUST NOT utilize any
authentication mechanism. It SHOULD contain the FINGERPRINT attribute
to aid in demultiplexing, but SHOULD NOT contain any other
attributes. It is used solely to keep the NAT bindings alive. The
Binding Indication is sent using the same local and remote candidates
that are being used for media. Though Binding Indications are used for
keepalives, an agent MUST be prepared to receive a connectivity check
as well. If a connectivity check is received, a response is generated
as discussed in , but there is no impact on
ICE processing otherwise.
An agent MUST begin the keepalive processing once ICE has selected
candidates for usage with media, or media begins to flow, whichever
happens first. Keepalives end once the session terminates or the media
stream is removed.
Procedures for sending media differ for full and lite implementations.
Agents always send media using a candidate pair, called the selected
candidate pair. An agent will send media to the remote candidate in
the selected pair (setting the destination address and port of the
packet equal to that remote candidate), and will send it from the
local candidate of the selected pair. When the local candidate is
server or peer reflexive, media is originated from the base. Media
sent from a relayed candidate is sent from the base through that TURN
server, using procedures defined in .
If the local candidate is a relayed candidate, it is RECOMMENDED that
an agent create a channel on the TURN server towards the remote
candidate. This is done using the procedures for channel creation as
defined in Section 11 of .
The selected pair for a component of a media stream is:
empty if the state of the check list for that media stream is Running,
and there is no previous selected pair for that component due to an
ICE restart
equal to the previous selected pair for a component of a media stream
if the state of the check list for that media stream is Running, and
there was a previous selected pair for that component due to an ICE
restart
equal to the highest-priority nominated pair for that component in the
valid list if the state of the check list is Completed
If the selected pair for at least one component of a media stream is
empty, an agent MUST NOT send media for any component of that media
stream. If the selected pair for each component of a media stream has
a value, an agent MAY send media for all components of that media
stream.
A lite implementation MUST NOT send media until it has a Valid list
that contains a candidate pair for each component of that media
stream. Once that happens, the agent MAY begin sending media
packets. To do that, it sends media to the remote candidate in the
pair (setting the destination address and port of the packet equal to
that remote candidate), and will send it from the local candidate.
ICE has interactions with jitter buffer adaptation mechanisms. An
RTP stream can begin using one candidate, and switch to another one,
though this happens rarely with ICE. The newer candidate may result in
RTP packets taking a different path through the network -- one with
different delay characteristics. As discussed below, agents are
encouraged to re-adjust jitter buffers when there are changes in
source or destination address of media packets. Furthermore, many
audio codecs use the marker bit to signal the beginning of a
talkspurt, for the purposes of jitter buffer adaptation. For such
codecs, it is RECOMMENDED that the sender set the marker bit when an agent switches transmission of media from
one candidate pair to another.
ICE implementations MUST be prepared to receive media on each
component on any candidates provided for that component in the most
recent candidate exchange (in the case of RTP, this would include
both RTP and RTCP if candidates were provided for both).
It is
RECOMMENDED that, when an agent receives an RTP packet with a new
source or destination IP address for a particular media stream, that
the agent re-adjust its jitter buffers.
RFC 3550 describes an algorithm in Section
8.2 for detecting synchronization source (SSRC) collisions and
loops. These algorithms are based, in part, on seeing different source
transport addresses with the same SSRC. However, when ICE is used,
such changes will sometimes occur as the media streams switch between
candidates. An agent will be able to determine that a media stream is
from the same peer as a consequence of the STUN exchange that proceeds
media transmission. Thus, if there is a change in source transport
address, but the media packets come from the same peer agent, this
SHOULD NOT be treated as an SSRC collision.
This specification makes very specific choices about how both agents
in a session coordinate to arrive at the set of candidate pairs that
are selected for media. It is anticipated that future specifications
will want to alter these algorithms, whether they are simple changes
like timer tweaks or larger changes like a revamp of the priority
algorithm. When such a change is made, providing interoperability
between the two agents in a session is critical.
First, ICE provides the ice-options attribute. Each extension or
change to ICE is associated with a token. When an agent supporting
such an extension or change triggers candidate exchange, it MUST
include the token for that extension in this attribute. This allows
each side to know what the other side is doing. This attribute MUST
NOT be present if the agent doesn't support any ICE extensions or
changes.
One of the complications in achieving interoperability is that ICE
relies on a distributed algorithm running on both agents to converge
on an agreed set of candidate pairs. If the two agents run different
algorithms, it can be difficult to guarantee convergence on the same
candidate pairs. The regular nomination procedure described in eliminates some of the tight coordination by
delegating the selection algorithm completely to the controlling
agent. Consequently, when a controlling agent is communicating with a
peer that supports options it doesn't know about, the agent MUST run a
regular nomination algorithm. When regular nomination is used, ICE
will converge perfectly even when both agents use different pair
prioritization algorithms. One of the keys to such convergence is
triggered checks, which ensure that the nominated pair is validated by
both agents. Consequently, any future ICE enhancements MUST preserve
triggered checks.
ICE is also extensible to other media streams beyond RTP, and for
transport protocols beyond UDP. Extensions to ICE for non-RTP media
streams need to specify how many components they utilize, and assign
component IDs to them, starting at 1 for the most important component
ID. Specifications for new transport protocols must define how, if at
all, various steps in the ICE processing differ from UDP.
During the ICE gathering phase () and
while ICE is performing connectivity checks (),
an agent triggers STUN and TURN transactions. These transactions are paced at
a rate indicated by Ta, and the retransmission interval for each transaction
is calculated based on the the retransmission timer for the STUN transactions
(RTO) .
This section describes how the Ta and RTO values are computed during the ICE
gathering phase and while ICE is performing connectivity checks.
NOTE: Previously, in RFC 5245, different formulas were defined for
computing Ta and RTO, depending on whether ICE was used for a real-time
media stream (e.g. RTP) or not.
The formulas below result in a behavior whereby an agent will send its first
packet for every single connectivity check before performing a
retransmit. This can be seen in the formulas for the RTO (which
represents the retransmit interval). Those formulas scale with N, the
number of checks to be performed. As a result of this, ICE maintains a
nicely constant rate, but becomes more sensitive to packet loss. The
loss of the first single packet for any connectivity check is likely
to cause that pair to take a long time to be validated, and instead, a
lower-priority check (but one for which there was no packet loss) is
much more likely to complete first. This results in ICE performing
sub-optimally, choosing lower-priority pairs over higher-priority
pairs. Implementors should be aware of this consequence, but still
should utilize the timer values described here.
ICE agents SHOULD use the default Ta value, 50 ms, but MAY use another
value based on the characteristics of the associated media. ICE agents
MUST NOT use a Ta value smaller than 5 ms.
If an ICE agent wants to use another Ta value than the default value, the
agent MUST indicate the proposed value to its peer during the ICE
exchange. Both agents MUST use the higher value of the proposed values.
If an agent does not propose a value, the default value is used for that
agent when comparing which value is higher.
NOTE: shows examples of required bandwidth,
using different Ta values.
During the ICE gathering phase, ICE agents SHOULD calculate the
RTO value using the following formula:
For connectivity checks, ICE agents SHOULD calculate the RTO value
using the following formula:
ICE agents MAY calculate the RTO value using other mechanisms than those
described above. ICE agents MUST NOT use a RTO value smaller than 500 ms.
The example is based on the simplified topology of .
Two agents, L and R, are using ICE. Both are full-mode ICE
implementations and use aggressive nomination when they are
controlling. Both agents have a single IPv4 address. For agent L, it
is 10.0.1.1 in private address space , and for
agent R, 192.0.2.1 on the public Internet. Both are configured with
the same STUN server (shown in this example for simplicity, although
in practice the agents do not need to use the same STUN server), which
is listening for STUN Binding requests at an IP address of 192.0.2.2
and port 3478. TURN servers are not used in this example. Agent L is
behind a NAT, and agent R is on the public Internet. The NAT has an
endpoint independent mapping property and an address dependent
filtering property. The public side of the NAT has an IP address of
192.0.2.3.
To facilitate understanding, transport addresses are listed using
variables that have mnemonic names. The format of the name is
entity-type-seqno, where entity refers to the entity whose IP address
the transport address is on, and is one of "L", "R", "STUN", or
"NAT". The type is either "PUB" for transport addresses that are
public, and "PRIV" for transport addresses that are private. Finally,
seq-no is a sequence number that is different for each transport
address of the same type on a particular entity. Each variable has an
IP address and port, denoted by varname.IP and varname.PORT,
respectively, where varname is the name of the variable. The STUN server has advertised transport address STUN-PUB-1 (which
is 192.0.2.2:3478).
In the call flow itself, STUN messages are annotated with several
attributes. The "S=" attribute indicates the source transport address
of the message. The "D=" attribute indicates the destination transport
address of the message. The "MA=" attribute is used in STUN Binding
response messages and refers to the mapped address. "USE-CAND" implies
the presence of the USE-CANDIDATE attribute.
The call flow examples omit STUN authentication operations and RTCP,
and focus on RTP for a single media stream between two full
implementations.
First, agent L obtains a host candidate from its local IP address
(not shown), and from that, sends a STUN Binding request to the STUN
server to get a server reflexive candidate (messages 1-4). Recall that
the NAT has the address and port independent mapping property. Here,
it creates a binding of NAT-PUB-1 for this UDP request, and this
becomes the server reflexive candidate for RTP.
Agent L sets a type preference of 126 for the host candidate and 100
for the server reflexive. The local preference is 65535. Based on
this, the priority of the host candidate is 2130706431 and for the
server reflexive candidate is 1694498815. The host candidate is
assigned a foundation of 1, and the server reflexive, a foundation of
2. These are sent to the peer.
This candidate information is received at agent R. Agent R will obtain
a host candidate, and from it, obtain a server reflexive candidate
(messages 6-7). Since R is not behind a NAT, this candidate is
identical to its host candidate, and they share the same base. It
therefore discards this redundant candidate and ends up with a single
host candidate. With identical type and local preferences as L, the
priority for this candidate is 2130706431. It chooses a foundation of
1 for its single candidate. Then R's candidates are then sent to
L.
Since neither side indicated that it is lite, the initiating agent
that began ICE processing (agent L) becomes the controlling agent.
Agents L and R both pair up the candidates. They both initially
have two pairs. However, agent L will prune the pair containing its
server reflexive candidate, resulting in just one. At agent L, this
pair has a local candidate of $L_PRIV_1 and remote candidate of
$R_PUB_1, and has a candidate pair priority of 4.57566E+18 (note that
an implementation would represent this as a 64-bit integer so as not
to lose precision). At agent R, there are two pairs. The highest
priority has a local candidate of $R_PUB_1 and remote candidate of
$L_PRIV_1 and has a priority of 4.57566E+18, and the second has a
local candidate of $R_PUB_1 and remote candidate of $NAT_PUB_1 and
priority 3.63891E+18.
Agent R begins its connectivity check (message 9) for the first
pair (between the two host candidates). Since R is the controlled
agent for this session, the check omits the USE-CANDIDATE
attribute. The host candidate from agent L is private and behind a
NAT, and thus this check won't be successful, because the packet
cannot be routed from R to L.
When agent L gets the R's candidates, it performs its one and only
connectivity check (messages 10-13). It implements the aggressive
nomination algorithm, and thus includes a USE-CANDIDATE attribute in
this check. Since the check succeeds, agent L creates a new pair,
whose local candidate is from the mapped address in the Binding
response (NAT-PUB-1 from message 13) and whose remote candidate is the
destination of the request (R-PUB-1 from message 10). This is added to
the valid list. In addition, it is marked as selected since the
Binding request contained the USE-CANDIDATE attribute. Since there is
a selected candidate in the Valid list for the one component of this
media stream, ICE processing for this stream moves into the Completed
state. Agent L can now send media if it so chooses.
Soon after receipt of the STUN Binding request from agent L (message
11), agent R will generate its triggered check. This check happens to
match the next one on its check list -- from its host candidate to
agent L's server reflexive candidate. This check (messages 14-17) will
succeed. Consequently, agent R constructs a new candidate pair using
the mapped address from the response as the local candidate (R-PUB-1)
and the destination of the request (NAT-PUB-1) as the remote
candidate. This pair is added to the Valid list for that media
stream. Since the check was generated in the reverse direction of a
check that contained the USE-CANDIDATE attribute, the candidate pair
is marked as selected. Consequently, processing for this stream moves
into the Completed state, and agent R can also send media.
There are several types of attacks possible in an ICE system. This
section considers these attacks and their countermeasures. These
countermeasures include:
Using ICE in conjunction with secure signaling techniques, such as
SIPS.Limiting the total number of connectivity checks to 100, and
optionally limiting the number of candidates they'll accept in an
candidate exchange.
An attacker might attempt to disrupt the STUN connectivity
checks. Ultimately, all of these attacks fool an agent into thinking
something incorrect about the results of the connectivity checks. The
possible false conclusions an attacker can try and cause are:
An attacker can fool a pair of agents
into thinking a candidate pair is invalid, when it isn't. This can be
used to cause an agent to prefer a different candidate (such as one
injected by the attacker) or to disrupt a call by forcing all
candidates to fail.
An attacker can fool a pair of agents into
thinking a candidate pair is valid, when it isn't. This can cause an
agent to proceed with a session, but then not be able to receive any
media.
An attacker can cause an
agent to discover a new peer reflexive candidate, when it shouldn't
have. This can be used to redirect media streams to a
Denial-of-Service (DoS) target or to the attacker, for eavesdropping
or other purposes.
An attacker has already
convinced an agent that there is a candidate with an address that
doesn't actually route to that agent (for example, by injecting a
false peer reflexive candidate or false server reflexive
candidate). It must then launch an attack that forces the agents to
believe that this candidate is valid.
If an attacker can cause a false peer reflexive candidate or false
valid on a false candidate, it can launch any of the attacks described
in .
To force the false invalid result, the attacker has to wait for
the connectivity check from one of the agents to be sent. When it is,
the attacker needs to inject a fake response with an unrecoverable
error response, such as a 400. However, since the candidate is, in
fact, valid, the original request may reach the peer agent, and result
in a success response. The attacker needs to force this packet or its
response to be dropped, through a DoS attack, layer 2 network
disruption, or other technique. If it doesn't do this, the success
response will also reach the originator, alerting it to a possible
attack. Fortunately, this attack is mitigated completely through the
STUN short-term credential mechanism. The attacker needs to inject a
fake response, and in order for this response to be processed, the
attacker needs the password. If the candidate exchange signaling is
secured, the attacker will not have the password and its response will
be discarded.
Forcing the fake valid result works in a similar way. The agent
needs to wait for the Binding request from each agent, and inject a
fake success response. The attacker won't need to worry about
disrupting the actual response since, if the candidate is not valid,
it presumably wouldn't be received anyway. However, like the fake
invalid attack, this attack is mitigated by the STUN short-term
credential mechanism in conjunction with a secure candidate exchange.
Forcing the false peer reflexive candidate result can be done
either with fake requests or responses, or with replays. We consider
the fake requests and responses case first. It requires the attacker
to send a Binding request to one agent with a source IP address and
port for the false candidate. In addition, the attacker must wait for
a Binding request from the other agent, and generate a fake response
with a XOR-MAPPED-ADDRESS attribute containing the false
candidate. Like the other attacks described here, this attack is
mitigated by the STUN message integrity mechanisms and secure
candidate exchanges.
Forcing the false peer reflexive candidate result with packet
replays is different. The attacker waits until one of the agents sends
a check. It intercepts this request, and replays it towards the other
agent with a faked source IP address. It must also prevent the
original request from reaching the remote agent, either by launching a
DoS attack to cause the packet to be dropped, or forcing it to be
dropped using layer 2 mechanisms. The replayed packet is received at
the other agent, and accepted, since the integrity check passes (the
integrity check cannot and does not cover the source IP address and
port). It is then responded to. This response will contain a
XOR-MAPPED-ADDRESS with the false candidate, and will be sent to that
false candidate. The attacker must then receive it and relay it
towards the originator.
The other agent will then initiate a connectivity check towards
that false candidate. This validation needs to succeed. This requires
the attacker to force a false valid on a false candidate. Injecting of
fake requests or responses to achieve this goal is prevented using the
integrity mechanisms of STUN and the candidate exchange. Thus, this
attack can only be launched through replays. To do that, the attacker
must intercept the check towards this false candidate, and replay it
towards the other agent. Then, it must intercept the response and
replay that back as well.
This attack is very hard to launch unless the attacker is identified
by the fake candidate. This is because it requires the attacker to
intercept and replay packets sent by two different hosts. If both
agents are on different networks (for example, across the public
Internet), this attack can be hard to coordinate, since it needs to
occur against two different endpoints on different parts of the
network at the same time.
If the attacker itself is identified by the fake candidate, the attack
is easier to coordinate. However, if the media path is secured (e.g.,
using SRTP ), the attacker will not be able to
play the media packets, but will only be able to discard them,
effectively disabling the media stream for the call. However, this
attack requires the agent to disrupt packets in order to block the
connectivity check from reaching the target. In that case, if the goal
is to disrupt the media stream, it's much easier to just disrupt it
with the same mechanism, rather than attack ICE.
ICE endpoints make use of STUN Binding requests for gathering server
reflexive candidates from a STUN server. These requests are not
authenticated in any way. As a consequence, there are numerous
techniques an attacker can employ to provide the client with a false
server reflexive candidate:
An attacker can compromise the DNS, causing DNS queries to return a
rogue STUN server address. That server can provide the client with
fake server reflexive candidates. This attack is mitigated by DNS
security, though DNS-SEC is not required to address it.
An attacker that can observe STUN messages (such as an attacker on a
shared network segment, like WiFi) can inject a fake response that
is valid and will be accepted by the client.
An attacker can compromise a STUN server by means of a virus, and
cause it to send responses with incorrect mapped addresses.
A false mapped address learned by these attacks will be used as a
server reflexive candidate in the ICE exchange. For this candidate to
actually be used for media, the attacker must also attack the
connectivity checks, and in particular, force a false valid on a false
candidate. This attack is very hard to launch if the false address
identifies a fourth party (neither the initiator, responder, nor
attacker), since it requires attacking the checks generated by each
agent in the session, and is prevented by SRTP if it identifies the
attacker themself.
If the attacker elects not to attack the connectivity checks, the
worst it can do is prevent the server reflexive candidate from being
used. However, if the peer agent has at least one candidate that is
reachable by the agent under attack, the STUN connectivity checks
themselves will provide a peer reflexive candidate that can be used
for the exchange of media. Peer reflexive candidates are generally
preferred over server reflexive candidates. As such, an attack solely
on the STUN address gathering will normally have no impact on a session
at all.
An attacker might attempt to disrupt the gathering of relayed
candidates, forcing the client to believe it has a false relayed
candidate. Exchanges with the TURN server are authenticated using a
long-term credential. Consequently, injection of fake responses or
requests will not work. In addition, unlike Binding requests, Allocate
requests are not susceptible to replay attacks with modified source IP
addresses and ports, since the source IP address and port are not
utilized to provide the client with its relayed candidate.
However, TURN servers are susceptible to DNS attacks, or to viruses
aimed at the TURN server, for purposes of turning it into a zombie or
rogue server. These attacks can be mitigated by DNS-SEC and through
good box and software security on TURN servers.
Even if an attacker has caused the client to believe in a false
relayed candidate, the connectivity checks cause such a candidate to
be used only if they succeed. Thus, an attacker must launch a false
valid on a false candidate, per above, which is a very difficult
attack to coordinate.
In addition to attacks where the attacker is a third party trying to
insert fake candidate information or stun messages, there are attacks
possible with ICE when the attacker is an authenticated and valid
participant in the ICE exchange.
The STUN amplification attack is similar to the voice hammer. However,
instead of voice packets being directed to the target, STUN
connectivity checks are directed to the target. The attacker sends an
a large number of candidates, say, 50. The responding agent receives
the candidate information, and starts its checks, which are directed
at the target, and consequently, never generate a response. The
answerer will start a new connectivity check every Ta ms (say,
Ta=20ms). However, the retransmission timers are set to a large number
due to the large number of candidates. As a consequence, packets will
be sent at an interval of one every Ta milliseconds, and then with
increasing intervals after that. Thus, STUN will not send packets at a
rate faster than media would be sent, and the STUN packets persist
only briefly, until ICE fails for the session. Nonetheless, this is an
amplification mechanism.
It is impossible to eliminate the amplification, but the volume can be
reduced through a variety of heuristics. Agents SHOULD limit the total
number of connectivity checks they perform to 100. Additionally,
agents MAY limit the number of candidates they'll accept.
Frequently, protocols that wish to avoid these kinds of attacks force
the initiator to wait for a response prior to sending the next
message. However, in the case of ICE, this is not possible. It is not
possible to differentiate the following two cases:
There was no response because the initiator is being used to launch
a DoS attack against an unsuspecting target that will not respond.
There was no response because the IP address and port are not
reachable by the initiator.
In the second case, another check should be sent at the next
opportunity, while in the former case, no further checks should be
sent.
This specification defines four new attributes, PRIORITY,
USE-CANDIDATE, ICE-CONTROLLED, and ICE-CONTROLLING.
The PRIORITY attribute indicates the priority that is to be
associated with a peer reflexive candidate, should one be discovered
by this check. It is a 32-bit unsigned integer, and has an attribute
value of 0x0024.
The USE-CANDIDATE attribute indicates that the candidate pair
resulting from this check should be used for transmission of
media. The attribute has no content (the Length field of the attribute
is zero); it serves as a flag. It has an attribute value of 0x0025.
The ICE-CONTROLLED attribute is present in a Binding request and
indicates that the client believes it is currently in the controlled
role. The content of the attribute is a 64-bit unsigned integer in
network byte order, which contains a random number. The number is used
for solving role conflicts, when it is referred to as the tie-breaker
value. An ICE agent MUST use the same number for all Binding requests,
for all streams, within an ICE session. The ICE agent MAY change
the number when an ICE restart occurs.
The ICE-CONTROLLING attribute is present in a Binding request and
indicates that the client believes it is currently in the controlling
role. The content of the attribute is a 64-bit unsigned integer in
network byte order, which contains a random number. The number is used
for solving role conflicts, when it is referred to as the tie-breaker
value. An ICE agent MUST use the same number for all Binding requests,
for all streams, within an ICE session. The ICE agent MAY change
the number when an ICE restart occurs.
This specification defines a single error response code: The Binding request contained
either the ICE-CONTROLLING or ICE-CONTROLLED attribute, indicating an
ICE role that conflicted with the server. The server compared the
tie-breaker values of the client and the server and determined that
the client needs to switch roles.
This section discusses issues relevant to network operators looking to
deploy ICE.
ICE was designed to work with existing NAT and firewall
equipment. Consequently, it is not necessary to replace or reconfigure
existing firewall and NAT equipment in order to facilitate deployment
of ICE. Indeed, ICE was developed to be deployed in environments where
the Voice over IP (VoIP) operator has no control over the IP network
infrastructure, including firewalls and NAT.
That said, ICE works best in environments where the NAT devices are
"behave" compliant, meeting the recommendations defined in and . In networks with
behave-compliant NAT, ICE will work without the need for a TURN
server, thus improving voice quality, decreasing call setup times, and
reducing the bandwidth demands on the network operator.
Deployment of ICE can have several interactions with available network
capacity that operators should take into consideration.
First and foremost, ICE makes use of TURN and STUN servers, which
would typically be located in the network operator's data centers. The
STUN servers require relatively little bandwidth. For each component
of each media stream, there will be one or more STUN transactions from
each client to the STUN server. In a basic voice-only IPv4 VoIP
deployment, there will be four transactions per call (one for RTP and
one for RTCP, for both caller and callee). Each transaction
is a single request and a single response, the former being 20 bytes
long, and the latter, 28. Consequently, if a system has N users, and
each makes four calls in a busy hour, this would require N*1.7bps. For
one million users, this is 1.7 Mbps, a very small number (relatively
speaking).
TURN traffic is more substantial. The TURN server will see traffic
volume equal to the STUN volume (indeed, if TURN servers are deployed,
there is no need for a separate STUN server), in addition to the
traffic for the actual media traffic. The amount of calls requiring
TURN for media relay is highly dependent on network topologies, and
can and will vary over time. In a network with 100% behave-compliant
NAT, it is exactly zero. At time of writing, large-scale consumer
deployments were seeing between 5 and 10 percent of calls requiring
TURN servers. Considering a voice-only deployment using G.711 (so 80
kbps in each direction), with .2 erlangs during the busy hour, this is
N*3.2 kbps. For a population of one million users, this is 3.2 Gbps,
assuming a 10% usage of TURN servers.
The process of gathering of candidates and performing of connectivity
checks can be bandwidth intensive. ICE has been designed to pace both
of these processes. The gathering phase and the connectivity check
phase are meant to generate traffic at roughly the same bandwidth as
the media traffic itself. This was done to ensure that, if a network
is designed to support multimedia traffic of a certain type (voice,
video, or just text), it will have sufficient capacity to support the
ICE checks for that media. Of course, the ICE checks will cause a
marginal increase in the total utilization; however, this will
typically be an extremely small increase.
Congestion due to the gathering and check phases has proven to be a
problem in deployments that did not utilize pacing. Typically, access
links became congested as the endpoints flooded the network with
checks as fast as they can send them. Consequently, network operators
should make sure that their ICE implementations support the pacing
feature. Though this pacing does increase call setup times, it makes
ICE network friendly and easier to deploy.
STUN keepalives (in the form of STUN Binding Indications) are sent in
the middle of a media session. However, they are sent only in the
absence of actual media traffic. In deployments that are not utilizing
Voice Activity Detection (VAD), the keepalives are never used and
there is no increase in bandwidth usage. When VAD is being used,
keepalives will be sent during silence periods. This involves a single
packet every 15-20 seconds, far less than the packet every 20-30 ms
that is sent when there is voice. Therefore, keepalives don't have any
real impact on capacity planning.
Deployments utilizing a mix of ICE and ICE-lite interoperate
perfectly. They have been explicitly designed to do so, without loss
of function.
However, ICE-lite can only be deployed in limited use cases. Those
cases, and the caveats involved in doing so, are documented in
.
ICE utilizes end-to-end connectivity checks, and places much of the
processing in the endpoints. This introduces a challenge to the
network operator -- how can they troubleshoot ICE deployments? How can
they know how ICE is performing?
ICE has built-in features to help deal with these problems. SIP
servers on the signaling path, typically deployed in the data centers
of the network operator, will see the contents of the candidate
exchanges that convey the ICE parameters. These parameters include the
type of each candidate (host, server reflexive, or relayed), along
with their related addresses. Once ICE processing has completed, an
updated candidate exchange takes place, signaling the selected
address (and its type). This updated re-INVITE is performed exactly
for the purposes of educating network equipment (such as a diagnostic
tool attached to a SIP server) about the results of ICE processing.
As a consequence, through the logs generated by the SIP server, a
network operator can observe what types of candidates are being used
for each call, and what address was selected by ICE. This is the
primary information that helps evaluate how ICE is performing.
ICE relies on several pieces of data being configured into the
endpoints. This configuration data includes timers, credentials for
TURN servers, and hostnames for STUN and TURN servers. ICE itself does
not provide a mechanism for this configuration. Instead, it is assumed
that this information is attached to whatever mechanism is used to
configure all of the other parameters in the endpoint. For SIP phones,
standard solutions such as the configuration framework
have been defined.
The original ICE specification registered four new STUN attributes,
and one new STUN error response. The STUN attributes and error
response are reproduced here. In addition, this specification
registers a new ICE option.
IANA has registered four STUN attributes:
IANA has registered following STUN error response code:
IANA is requested to register the following ICE option in the "ICE Options"
sub-registry of the "Interactive Connectivity Establishment (ICE) registry",
following the procedures defined in .
The IAB has studied the problem of "Unilateral Self-Address Fixing",
which is the general process by which a agent attempts to determine
its address in another realm on the other side of a NAT through a
collaborative protocol reflection mechanism .
ICE is an example of a protocol that performs this type of
function. Interestingly, the process for ICE is not unilateral, but
bilateral, and the difference has a significant impact on the issues
raised by IAB. Indeed, ICE can be considered a B-SAF (Bilateral
Self-Address Fixing) protocol, rather than an UNSAF
protocol. Regardless, the IAB has mandated that any protocols
developed for this purpose document a specific set of
considerations. This section meets those requirements.
>From RFC 3424, any UNSAF proposal must provide:
Precise definition of a specific, limited-scope problem that is to be
solved with the UNSAF proposal. A short-term fix should not be
generalized to solve other problems; this is why "short-term fixes
usually aren't".
The specific problems being solved by ICE are:
Provide a means for two peers to determine the set of transport
addresses that can be used for communication.
Provide a means for a agent to determine an address that is
reachable by another peer with which it wishes to communicate.
>From RFC 3424, any UNSAF proposal must provide:
Description of an exit strategy/transition plan. The better
short-term fixes are the ones that will naturally see less and
less use as the appropriate technology is deployed.
ICE itself doesn't easily get phased out. However, it is useful
even in a globally connected Internet, to serve as a means for
detecting whether a router failure has temporarily disrupted
connectivity, for example. ICE also helps prevent certain security
attacks that have nothing to do with NAT. However, what ICE does is
help phase out other UNSAF mechanisms. ICE effectively selects amongst
those mechanisms, prioritizing ones that are better, and
deprioritizing ones that are worse. Local IPv6 addresses can be
preferred. As NATs begin to dissipate as IPv6 is introduced, server
reflexive and relayed candidates (both forms of UNSAF addresses)
simply never get used, because higher-priority connectivity exists to
the native host candidates. Therefore, the servers get used less and
less, and can eventually be remove when their usage goes to zero.
Indeed, ICE can assist in the transition from IPv4 to IPv6. It can be
used to determine whether to use IPv6 or IPv4 when two dual-stack
hosts communicate with SIP (IPv6 gets used). It can also allow a
network with both 6to4 and native v6 connectivity to determine which
address to use when communicating with a peer.
>From RFC 3424, any UNSAF proposal must provide:
Discussion of specific issues that may render systems more
"brittle". For example, approaches that involve using data at
multiple network layers create more dependencies, increase
debugging challenges, and make it harder to transition.
ICE actually removes brittleness from existing UNSAF
mechanisms. In particular, classic STUN (as described in RFC 3489
) has several points of brittleness. One of
them is the discovery process that requires an agent to try to
classify the type of NAT it is behind. This process is
error-prone. With ICE, that discovery process is simply not
used. Rather than unilaterally assessing the validity of the address,
its validity is dynamically determined by measuring connectivity to a
peer. The process of determining connectivity is very robust.
Another point of brittleness in classic STUN and any other unilateral
mechanism is its absolute reliance on an additional server. ICE makes
use of a server for allocating unilateral addresses, but allows
agents to directly connect if possible. Therefore, in some cases, the
failure of a STUN server would still allow for a call to
progress when ICE is used.
Another point of brittleness in classic STUN is that it assumes
that the STUN server is on the public Internet. Interestingly, with
ICE, that is not necessary. There can be a multitude of STUN servers
in a variety of address realms. ICE will discover the one that has
provided a usable address.
The most troubling point of brittleness in classic STUN is that it
doesn't work in all network topologies. In cases where there is a
shared NAT between each agent and the STUN server, traditional STUN
may not work. With ICE, that restriction is removed.
Classic STUN also introduces some security
considerations. Fortunately, those security considerations are also
mitigated by ICE.
Consequently, ICE serves to repair the brittleness introduced in
classic STUN, and does not introduce any additional brittleness
into the system.
The penalty of these improvements is that ICE increases session
establishment times.
From RFC 3424, any UNSAF proposal must provide:
... requirements for longer term, sound technical solutions
-- contribute to the process of finding the right longer term
solution.
Our conclusions from RFC 3489 remain unchanged. However, we feel ICE
actually helps because we believe it can be part of the long-term
solution.
From RFC 3424, any UNSAF proposal must provide:
Discussion of the impact of the noted practical issues with
existing, deployed NA[P]Ts and experience reports.
A number of NAT boxes are now being deployed into the market that try
to provide "generic" ALG functionality. These generic ALGs hunt for IP
addresses, either in text or binary form within a packet, and rewrite
them if they match a binding. This interferes with classic
STUN. However, the update to STUN uses an
encoding that hides these binary addresses from generic ALGs.
Existing NAPT boxes have non-deterministic and typically short
expiration times for UDP-based bindings. This requires implementations
to send periodic keepalives to maintain those bindings. ICE uses a
default of 15 s, which is a very conservative estimate. Eventually,
over time, as NAT boxes become compliant to behave , this minimum keepalive will become deterministic
and well-known, and the ICE timers can be adjusted. Having a way to
discover and control the minimum keepalive interval would be far
better still.
Following is the list of changes from RFC 5245
The specification was generalized to be more usable with any
protocol and the parts that are specific to SIP and SDP were moved to
a SIP/SDP usage document . Default candidates, multiple components, ICE mismatch detection,
subsequent offer/answer, and role conflict resolution were made
optional since they are not needed with every protocol using ICE.
With IPv6, the precedence rules of RFC 6724 are used instead of the
obsoleted RFC 3483 and using address preferences provided by the host
operating system is recommended. Candidate gathering rules regarding loopback addresses and IPv6
addresses were clarified. Most of the text in this document comes from the original ICE
specification, RFC 5245. The authors would like to thank everyone who
has contributed to that document. For additional contributions to this
revision of the specification we would like to thank Emil Ivov, Paul
Kyzivat, Pal-Erik Martinsen, Simon Perrault, Eric Rescorla, Thomas
Stach, Peter Thatcher, Martin Thomson, Justin Uberti, and Suhas
Nandakumar.
ICE allows for two types of implementations. A full implementation
supports the controlling and controlled roles in a session, and can
also perform address gathering. In contrast, a lite implementation is
a minimalist implementation that does little but respond to STUN
checks.
Because ICE requires both endpoints to support it in order to bring
benefits to either endpoint, incremental deployment of ICE in a
network is more complicated. Many sessions involve an endpoint that
is, by itself, not behind a NAT and not one that would worry about NAT
traversal. A very common case is to have one endpoint that requires
NAT traversal (such as a VoIP hard phone or soft phone) make a call to
one of these devices. Even if the phone supports a full ICE
implementation, ICE won't be used at all if the other device doesn't
support it. The lite implementation allows for a low-cost entry point
for these devices. Once they support the lite implementation, full
implementations can connect to them and get the full benefits of ICE.
Consequently, a lite implementation is only appropriate for devices
that will *always* be connected to the public Internet and have a
public IP address at which it can receive packets from any
correspondent. ICE will not function when a lite implementation is
placed behind a NAT.
ICE allows a lite implementation to have a single IPv4 host candidate
and several IPv6 addresses. In that case, candidate pairs are selected
by the controlling agent using a static algorithm, such as the one in
RFC 6724, which is recommended by this specification. However, static
mechanisms for address selection are always prone to error, since they
cannot ever reflect the actual topology and can never provide actual
guarantees on connectivity. They are always heuristics. Consequently,
if an agent is implementing ICE just to select between its IPv4 and
IPv6 addresses, and none of its IP addresses are behind NAT, usage
of full ICE is still RECOMMENDED in order to provide the most robust
form of address selection possible.
It is important to note that the lite implementation was added to this
specification to provide a stepping stone to full implementation. Even
for devices that are always connected to the public Internet with just
a single IPv4 address, a full implementation is preferable if
achievable. Full implementations also obtain
the security benefits of ICE unrelated to NAT traversal; in
particular, the voice hammer attack described in is prevented only for full implementations,
not lite. Finally, it is often the case that a device that finds
itself with a public address today will be placed in a network
tomorrow where it will be behind a NAT. It is difficult to
definitively know, over the lifetime of a device or product, that it
will always be used on the public Internet. Full implementation
provides assurance that communications will always work.
ICE contains a number of normative behaviors that may themselves be
simple, but derive from complicated or non-obvious thinking or use
cases that merit further discussion. Since these design motivations
are not necessary to understand for purposes of implementation, they
are discussed here in an appendix to the specification. This section
is non-normative.
STUN transactions used to gather candidates and to verify connectivity
are paced out at an approximate rate of one new transaction every Ta
milliseconds. Each transaction, in turn, has a retransmission timer
RTO that is a function of Ta as well. Why are these transactions
paced, and why are these formulas used?
Sending of these STUN requests will often have the effect of
creating bindings on NAT devices between the client and the STUN
servers. Experience has shown that many NAT devices have upper limits
on the rate at which they will create new bindings. Experiments have
shown that once every 20 ms is well supported, but not much lower than
that. This is why Ta has a lower bound of 20 ms. Furthermore,
transmission of these packets on the network makes use of bandwidth
and needs to be rate limited by the agent. Deployments based on
earlier draft versions of tended to overload
rate-constrained access links and perform poorly overall, in addition
to negatively impacting the network. As a consequence, the pacing
ensures that the NAT device does not get overloaded and that traffic
is kept at a reasonable rate.
The definition of a "reasonable" rate is that STUN should not use more
bandwidth than the RTP itself will use, once media starts flowing. The
formula for Ta is designed so that, if a STUN packet were sent every
Ta seconds, it would consume the same amount of bandwidth as RTP
packets, summed across all media streams. Of course, STUN has
retransmits, and the desire is to pace those as well. For this reason,
RTO is set such that the first retransmit on the first transaction
happens just as the first STUN request on the last transaction
occurs. Pictorially:
In this picture, there are three transactions that will be sent (for
example, in the case of candidate gathering, there are three host
candidate/STUN server pairs). These are transactions A, B, and C. The
retransmit timer is set so that the first retransmission on the first
transaction (packet A2) is sent at time 3Ta.
Subsequent retransmits after the first will occur even less frequently
than Ta milliseconds apart, since STUN uses an exponential back-off on
its retransmissions.
talks about eliminating candidates that
have the same transport address and base. However, candidates with the
same transport addresses but different bases are not redundant. When
can an agent have two candidates that have the same IP address and
port, but different bases? Consider the topology of :
In this case, the initiating agent is multihomed. It has one IP
address, 10.0.1.100, on network C, which is a net 10 private
network. The responding agent is on this same network. The initiating
agent is also connected to network A, which is 192.168/16 and has an
IP address of 192.168.1.100 on this network. There is a NAT on this
network, natting into network B, which is another net 10 private
network, but not connected to network C. There is a STUN server on
network B.
The initiating agent obtains a host candidate on its IP address on
network C (10.0.1.100:2498) and a host candidate on its IP address on
network A (192.168.1.100:3344). It performs a STUN query to its
configured STUN server from 192.168.1.100:3344. This query passes
through the NAT, which happens to assign the binding
10.0.1.100:2498. The STUN server reflects this in the STUN Binding
response. Now, the initiating agent has obtained a server reflexive
candidate with a transport address that is identical to a host
candidate (10.0.1.100:2498). However, the server reflexive candidate
has a base of 192.168.1.100:3344, and the host candidate has a base of
10.0.1.100:2498.
The candidate attribute contains two values that are not used at all
by ICE itself -- related address and related port. Why are they
present?
There are two motivations for its inclusion. The first is
diagnostic. It is very useful to know the relationship between the
different types of candidates. By including it, an agent can know
which relayed candidate is associated with which reflexive candidate,
which in turn is associated with a specific host candidate. When
checks for one candidate succeed and not for others, this provides
useful diagnostics on what is going on in the network.
The second reason has to do with off-path Quality of Service (QoS)
mechanisms. When ICE is used in environments such as PacketCable 2.0,
proxies will, in addition to performing normal SIP operations, inspect
the SDP in SIP messages, and extract the IP address and port for media
traffic. They can then interact, through policy servers, with access
routers in the network, to establish guaranteed QoS for the media
flows. This QoS is provided by classifying the RTP traffic based on
5-tuple, and then providing it a guaranteed rate, or marking its
Diffserv codepoints appropriately. When a residential NAT is present,
and a relayed candidate gets selected for media, this relayed
candidate will be a transport address on an actual TURN server. That
address says nothing about the actual transport address in the access
router that would be used to classify packets for QoS
treatment. Rather, the server reflexive candidate towards the TURN
server is needed. By carrying the translation in the SDP, the proxy
can use that transport address to request QoS from the access router.
ICE requires the usage of message integrity with STUN using its
short-term credential functionality. The actual short-term credential
is formed by exchanging username fragments in the candidate
exchange. The need for this mechanism goes beyond just security; it is
actually required for correct operation of ICE in the first place.
Consider agents L, R, and Z. L and R are within private enterprise
1, which is using 10.0.0.0/8. Z is within private enterprise 2, which
is also using 10.0.0.0/8. As it turns out, R and Z both have IP
address 10.0.1.1. L sends candidates to Z. Z, in responds L with its
host candidates. In this case, those candidates are 10.0.1.1:8866 and
10.0.1.1:8877. As it turns out, R is in a session at that same time,
and is also using 10.0.1.1:8866 and 10.0.1.1:8877 as host
candidates. This means that R is prepared to accept STUN messages on
those ports, just as Z is. L will send a STUN request to 10.0.1.1:8866
and another to 10.0.1.1:8877. However, these do not go to Z as
expected. Instead, they go to R! If R just replied to them, L would
believe it has connectivity to Z, when in fact it has connectivity to
a completely different user, R. To fix this, the STUN short-term
credential mechanisms are used. The username fragments are
sufficiently random that it is highly unlikely that R would be using
the same values as Z. Consequently, R would reject the STUN request
since the credentials were invalid. In essence, the STUN username
fragments provide a form of transient host identifiers, bound to a
particular session established as part of the candidate exchange.
An unfortunate consequence of the non-uniqueness of IP addresses is
that, in the above example, R might not even be an ICE agent. It could
be any host, and the port to which the STUN packet is directed could
be any ephemeral port on that host. If there is an application
listening on this socket for packets, and it is not prepared to handle
malformed packets for whatever protocol is in use, the operation of
that application could be affected. Fortunately, since the ports
exchanged are ephemeral and usually drawn from the dynamic or
registered range, the odds are good that the port is not used to run a
server on host R, but rather is the agent side of some protocol. This
decreases the probability of hitting an allocated port, due to the
transient nature of port usage in this range. However, the possibility
of a problem does exist, and network deployers should be prepared for
it. Note that this is not a problem specific to ICE; stray packets can
arrive at a port at any time for any type of protocol, especially ones
on the public Internet. As such, this requirement is just restating a
general design guideline for Internet applications -- be prepared for
unknown packets on any port.
The priority for a candidate pair has an odd form. It is:
pair priority = 2^32*MIN(G,D) + 2*MAX(G,D) + (G>D?1:0)
Why is this? When the candidate pairs are sorted based on this value,
the resulting sorting has the MAX/MIN property. This means that the
pairs are first sorted based on decreasing value of the minimum of the
two priorities. For pairs that have the same value of the minimum
priority, the maximum priority is used to sort amongst them. If the
max and the min priorities are the same, the controlling agent's
priority is used as the tie-breaker in the last part of the
expression. The factor of 2*32 is used since the priority of a single
candidate is always less than 2*32, resulting in the pair priority
being a "concatenation" of the two component priorities. This creates
the MAX/MIN sorting. MAX/MIN ensures that, for a particular agent, a
lower-priority candidate is never used until all higher-priority
candidates have been tried.
Once media begins flowing on a candidate pair, it is still necessary
to keep the bindings alive at intermediate NATs for the duration of
the session. Normally, the media stream packets themselves (e.g., RTP)
meet this objective. However, several cases merit further
discussion. Firstly, in some RTP usages, such as SIP, the media
streams can be "put on hold". This is accomplished by using the SDP
"sendonly" or "inactive" attributes, as defined in RFC 3264 . RFC 3264 directs implementations to cease
transmission of media in these cases. However, doing so may cause NAT
bindings to timeout, and media won't be able to come off hold.
Secondly, some RTP payload formats, such as the payload format for
text conversation , may send
packets so infrequently that the interval exceeds the NAT binding
timeouts.
Thirdly, if silence suppression is in use, long periods of silence may
cause media transmission to cease sufficiently long for NAT bindings
to time out.
For these reasons, the media packets themselves cannot be relied
upon. ICE defines a simple periodic keepalive utilizing STUN Binding
indications. This makes its bandwidth requirements highly
predictable, and thus amenable to QoS reservations.
describes procedures for computing
the priority of candidate based on its type and local
preferences. That section requires that the type preference for peer
reflexive candidates always be higher than server reflexive. Why is
that? The reason has to do with the security considerations in . It is much easier for an attacker to cause an
agent to use a false server reflexive candidate than it is for an
attacker to cause an agent to use a false peer reflexive
candidate. Consequently, attacks against address gathering with
Binding requests are thwarted by ICE by preferring the peer reflexive
candidates.
Media keepalives are described in . These keepalives make use of STUN when both
endpoints are ICE capable. However, rather than using a Binding
request transaction (which generates a response), the keepalives use
an Indication. Why is that?
The primary reason has to do with network QoS mechanisms. Once media
begins flowing, network elements will assume that the media stream has
a fairly regular structure, making use of periodic packets at fixed
intervals, with the possibility of jitter. If an agent is sending
media packets, and then receives a Binding request, it would need to
generate a response packet along with its media packets. This will
increase the actual bandwidth requirements for the 5-tuple carrying
the media packets, and introduce jitter in the delivery of those
packets. Analysis has shown that this is a concern in certain layer 2
access networks that use fairly tight packet schedulers for media.
Additionally, using a Binding Indication allows integrity to be
disabled, allowing for better performance. This is useful for
large-scale endpoints, such as PSTN gateways and SBCs.
The tables below show, for IPv4 and IPv6, the bandwidth required for
performing connectivity checks, using different Ta values (given in ms)
and different ufrag sizes (given in bytes).
The results were provided by Jusin Uberti (Google) 11th April 2016.