Internet Engineering Task Force (IETF) A. Keranen
Request for Comments: 8445 C. Holmberg
Obsoletes: 5245 Ericsson
Category: Standards Track J. Rosenberg
ISSN: 2070-1721 jdrosen.net
July 2018 Interactive Connectivity Establishment (ICE):
A Protocol for Network Address Translator (NAT) Traversal
This document describes a protocol for Network Address Translator
(NAT) traversal for UDP-based communication. 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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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Protocols establishing communication sessions between peers typically
involve exchanging IP addresses and ports for the data sources and
sinks. However, this poses challenges when operated through Network
Address Translators (NATs) [RFC3235]. These protocols also seek to
create a data flow directly between participants, so that there is no
application-layer intermediary between them. This is done to reduce
data latency, decrease packet loss, and reduce the operational costs
of deploying the application. However, this is difficult to
accomplish through NATs. 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 NATs. These include Application Layer Gateways
(ALGs), the Middlebox Control Protocol [RFC3303], the original Simple
Traversal of UDP Through NAT (STUN) specification [RFC3489] (note
that RFC 3489 has been obsoleted by RFC 5389), and Realm Specific IP
[RFC3102] [RFC3103] along with session description extensions needed
to make them work, such as the Session Description Protocol (SDP)
attribute [RFC4566] for the Real-Time Control Protocol (RTCP)
[RFC3605]. Unfortunately, these techniques all have pros and cons
that make each one optimal in some network topologies, but a poor
choice in others. The result is that administrators and implementers
are making assumptions about the topologies of the networks in which
their solutions will be deployed. This introduces complexity and
brittleness into the system.
This specification defines Interactive Connectivity Establishment
(ICE) as a technique for NAT traversal for UDP-based data streams
(though ICE has been extended to handle other transport protocols,
such as TCP [RFC6544]). 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 using ICE-usage-specific mechanisms (e.g., in an Offer/
Answer exchange), and the connectivity checks are performed using
STUN [RFC5389]. ICE also makes use of Traversal Using Relay around
NAT (TURN) [RFC5766], 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.
For this reason, RFC 5245 [RFC5245] deprecated the solutions
previously defined in RFC 4091 [RFC4091] and RFC 4092 [RFC4092].
Appendix B provides background information and motivations regarding
the design decisions that were made when designing ICE.
2. Overview of ICE
In a typical ICE deployment, there are two endpoints (ICE agents)
that want to communicate. Note that ICE is not intended for NAT
traversal for the signaling protocol, which is assumed to be provided
via another mechanism. ICE assumes that the agents are able to
establish a signaling connection between each other.
Initially, the agents are ignorant of their own topologies. In
particular, the agents may or may not be behind NATs (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 establish a data session.
Figure 1 shows a typical ICE deployment. The agents are labeled L
and R. 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. L and R are capable of engaging in a candidate
exchange process, whose purpose is to set up a data session between L
and R. Typically, this exchange will occur through a signaling
server (e.g., a SIP proxy).
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
+--------+ |Signaling| +--------+
| STUN | |Server | | STUN |
| Server | +---------+ | Server |
+--------+ / \ +--------+
/ <- Signaling -> \
| NAT | | NAT |
| Agent | | Agent |
| L | | R |
Figure 1: ICE Deployment Scenario
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:
o A transport address on a directly attached network interface
o A translated transport address on the public side of a NAT (a
o A transport address allocated from a TURN server (a "relayed
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.
2.1. Gathering Candidates
In order to execute ICE, an ICE agent identifies and gathers one or
more address candidates. A candidate has a transport address -- a
combination of IP address and port for a particular transport
protocol (with only UDP specified here). There are different types
of candidates; some are derived from physical or logical network
interfaces, and others are discoverable via STUN and TURN.
The first category of candidates are those with a transport address
obtained directly from a local interface. Such a candidate is called
a "host candidate". The local interface could be Ethernet or Wi-Fi,
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.
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 Figure 2. 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.
| /------------ Relayed
Y:y | / Address
| TURN |
| Server |
| /------------ Server
| NAT |
| /------------ Local
X:x |/ Address
| Agent |
Figure 2: Candidate Relationships
When the agent sends a 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. The host candidate
associated with a given server-reflexive candidate is 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 [RFC5389] 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
2.2. Connectivity Checks
Once L has gathered all of its candidates, it orders them by 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 ICE 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 connectivity 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
The basic principle of the connectivity checks is simple:
1. Sort the candidate pairs in priority order.
2. Send checks on each candidate pair in priority order.
3. Acknowledge checks received from the other agent.
With both agents performing a check on a candidate pair, the result
is a 4-way handshake:
STUN request -> \ L's
<- STUN response / check
<- STUN request \ R's
STUN response -> / check
Figure 3: Basic Connectivity Check
It is important to note that STUN requests are sent to and from the
exact same IP addresses and ports that will be used for data (e.g.,
RTP, RTCP, or other protocols). Consequently, agents demultiplex
STUN and data using the contents of the packets rather than the port
on which they are received.
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 that of
other candidates the agent already learned, it represents a new
candidate (peer-reflexive candidate), which then gets tested by ICE
just the same as any other candidate.
Because the algorithm above searches all candidate pairs, if a
working pair exists, the algorithm 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
The agent works through the checklist by sending a STUN request for
the next candidate pair on the list periodically. These are called
"ordinary checks". When a STUN transaction succeeds, one or more
candidate pairs will become so-called "valid pairs" and will be added
to a candidate-pair list called the "valid list".
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 is called a "triggered check", and it accelerates the
process of finding valid pairs.
At the end of this handshake, both L and R know that they can send
(and receive) messages end to end in both directions.
In general, the priority algorithm is designed so that candidates of
a similar type get similar priorities so that more direct routes
(that is, routes without data relays or NATs) are preferred over
indirect routes (routes with data relays or NATs). Within those
guidelines, however, agents have a fair amount of discretion about
how to tune their algorithms.
A data stream might consist of multiple components (pieces of a data
stream that require their own set of candidates, e.g., RTP and RTCP).
2.3. Nominating Candidate Pairs and Concluding ICE
ICE assigns one of the ICE agents in the role of the controlling
agent, and the other in the role of the controlled agent. For each
component of a data stream, the controlling agent nominates a valid
pair (from the valid list) to be used for data. The exact timing of
the nomination is based on local policy.
When nominating, the controlling agent lets the checks continue until
at least one valid pair for each component of a data stream is found,
and then it picks a valid pair and sends a STUN request on that pair,
using an attribute to indicate to the controlled peer that it has
been nominated. This is shown in Figure 4.
STUN request -> \ L's
<- STUN response / check
<- STUN request \ R's
STUN response -> / check
STUN request + attribute -> \ L's
<- STUN response / check
Figure 4: Nomination
Once the controlled agent receives the STUN request with the
attribute, it will check (unless the check has already been done) the
same pair. If the transactions above succeed, the agents will set
the nominated flag for the pairs and will cancel any future checks
for that component of the data stream. Once an agent has set the
nominated flag for each component of a data stream, the pairs become
the selected pairs. After that, only the selected pairs will be used
for sending and receiving data associated with that data stream.
2.4. ICE Restart
Once ICE is concluded, it can be restarted at any time for one or all
of the data streams by either ICE agent. This is done by sending
updated candidate information indicating a restart.
2.5. Lite Implementations
Certain ICE 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). Lite agents only use
host candidates and do not generate connectivity checks or run state
machines, though they need to be able to respond to connectivity
3. ICE Usage
This document specifies generic use of ICE with protocols that
provide means to exchange candidate information between ICE agents.
The specific details (i.e., how to encode candidate information and
the actual candidate exchange process) for different protocols using
ICE (referred to as "using protocol") are described in separate usage
One mechanism that allows agents to exchange candidate information is
the utilization of Offer/Answer semantics (which are based on
[RFC3264]) as part of the SIP protocol [RFC3261] [ICE-SIP-SDP].
[RFC7825] defines an ICE usage for the Real-Time Streaming Protocol
(RTSP). Note, however, that the ICE usage is based on RFC 5245.
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
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Readers need to be familiar with the terminology defined in [RFC5389]
and NAT Behavioral requirements for UDP [RFC4787].
This specification makes use of the following additional terminology:
ICE Session: An ICE session consists of all ICE-related actions
starting with the candidate gathering, followed by the
interactions (candidate exchange, connectivity checks,
nominations, and keepalives) between the ICE agents until all the
candidates are released or an ICE restart is triggered.
ICE Agent, Agent: An ICE agent (sometimes simply referred to as an
"agent") is the protocol implementation involved in the ICE
candidate exchange. There are two agents involved in a typical
Initiating Peer, Initiating Agent, Initiator: An initiating agent is
an ICE agent that initiates the ICE candidate exchange process.
Responding Peer, Responding Agent, Responder: A responding agent is
an ICE agent that receives and responds to the candidate exchange
process initiated by the initiating agent.
ICE Candidate Exchange, Candidate Exchange: The process where ICE
agents exchange information (e.g., candidates and passwords) that
is needed to perform ICE. Offer/Answer with SDP encoding
[RFC3264] is one example of a protocol that can be used for
exchanging the candidate information.
Peer: From the perspective of one of the ICE 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
Transport Address: The combination of an IP address and the
transport protocol (such as UDP or TCP) port.
Data, Data Stream, Data Session: When ICE is used to set up data
sessions, the data is transported using some protocol. Media is
usually transported over RTP, composed of a stream of RTP packets.
Data session refers to data packets that are exchanged between the
peer on the path created and tested with ICE.
Candidate, Candidate Information: A transport address that is a
potential point of contact for receipt of data. Candidates also
have properties -- their type (server reflexive, relayed, or
host), priority, foundation, and base.
Component: A component is a piece of a data stream. A data stream
may require multiple components, each of which has to work in
order for the data stream as a whole to work. For RTP/RTCP data
streams, unless RTP and RTCP are multiplexed in the same port,
there are two components per data stream -- one for RTP, and one
for RTCP. A component has a candidate pair, which cannot be used
by other components.
Host Candidate: 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
Server-Reflexive Candidate: A candidate whose IP address and port
are a binding allocated by a NAT for an ICE agent after it sends a
packet through the NAT to a server, such as a STUN server.
Peer-Reflexive Candidate: A candidate whose IP address and port are
a binding allocated by a NAT for an ICE agent after it sends a
packet through the NAT to its peer.
Relayed Candidate: A candidate obtained from a relay server, such as
a TURN server.
Base: The transport address that an ICE agent sends from for a
particular candidate. For host, server-reflexive, and peer-
reflexive candidates, the base is the same as the host candidate.
For relayed candidates, the base is the same as the relayed
candidate (i.e., the transport address used by the TURN server to
Related Address and Port: A transport address related to a
candidate, which is useful for diagnostics and other purposes. If
a candidate is server or peer reflexive, the related address and
port is equal to the base for that server or peer-reflexive
candidate. If the candidate is relayed, the related address and
port are equal to the mapped address in the Allocate response that
provided the client with that relayed candidate. If the candidate
is a host candidate, the related address and port is identical to
the host candidate.
Foundation: An arbitrary string used in the freezing algorithm to
group similar candidates. It 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.
Local Candidate: A candidate that an ICE agent has obtained and may
send to its peer.
Remote Candidate: A candidate that an ICE agent received from its
Default Destination/Candidate: The default destination for a
component of a data stream is the transport address that would be
used by an ICE agent that is not ICE aware. A default candidate
for a component is one whose transport address matches the default
destination for that component.
Candidate Pair: A pair containing a local candidate and a remote
Check, Connectivity Check, STUN Check: A STUN Binding request for
the purpose of verifying connectivity. A check is sent from the
base of the local candidate to the remote candidate of a candidate
Checklist: An ordered set of candidate pairs that an ICE agent will
use to generate checks.
Ordinary Check: A connectivity check generated by an ICE agent as a
consequence of a timer that fires periodically, instructing it to
send a check.
Triggered Check: A connectivity check generated as a consequence of
the receipt of a connectivity check from the peer.
Valid Pair: A candidate pair whose local candidate equals the mapped
address of a successful connectivity-check response and whose
remote candidate equals the destination address to which the
connectivity-check request was sent.
Valid List: An ordered set of candidate pairs for a data stream that
have been validated by a successful STUN transaction.
Checklist Set: The ordered list of all checklists. The order is
determined by each ICE usage.
Full Implementation: An ICE implementation that performs the
complete set of functionality defined by this specification.
Lite Implementation: An ICE implementation that omits certain
functions, implementing only as much as is necessary for a peer
that is not a lite implementation to gain the benefits of ICE.
Lite implementations do not maintain any of the state machines and
do not generate connectivity checks.
Controlling Agent: The ICE agent that nominates a candidate pair.
In any session, there is always one controlling agent and one
Controlled Agent: The ICE agent that waits for the controlling agent
to nominate a candidate pair.
Nomination: The process of the controlling agent indicating to the
controlled agent which candidate pair the ICE agents will use for
sending and receiving data. The nomination process defined in
this specification was referred to as "regular nomination" in RFC
5245. The nomination process that was referred to as "aggressive
nomination" in RFC 5245 has been deprecated in this specification.
Nominated, Nominated Flag: Once the nomination of a candidate pair
has succeeded, the candidate pair has become nominated, and the
value of its nominated flag is set to true.
Selected Pair, Selected Candidate Pair: The candidate pair used for
sending and receiving data for a component of a data stream is
referred to as the "selected pair". Before selected pairs have
been produced for a data stream, any valid pair associated with a
component of a data stream can be used for sending and receiving
data for the component. Once there are nominated pairs for each
component of a data stream, the nominated pairs become the
selected pairs for the data stream. The candidates associated
with the selected pairs are referred to as "selected candidates".
Using Protocol, ICE Usage: 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
Timer Ta: The timer for generating new STUN or TURN transactions.
Timer RTO (Retransmission Timeout): The retransmission timer for a
given STUN or TURN transaction.