Internet Engineering Task Force (IETF) J. Rosenberg
Request for Comments: 6544 jdrosen.net
Category: Standards Track A. Keranen
ISSN: 2070-1721 Ericsson
B. B. Lowekamp
A. B. Roach
March 2012 TCP Candidates with Interactive Connectivity Establishment (ICE)
Interactive Connectivity Establishment (ICE) defines a mechanism for
NAT traversal for multimedia communication protocols based on the
offer/answer model of session negotiation. ICE works by providing a
set of candidate transport addresses for each media stream, which are
then validated with peer-to-peer connectivity checks based on Session
Traversal Utilities for NAT (STUN). ICE provides a general framework
for describing candidates but only defines UDP-based media streams.
This specification extends ICE to TCP-based media, including the
ability to offer a mix of TCP and UDP-based candidates for a single
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
Interactive Connectivity Establishment (ICE) [RFC5245] defines a
mechanism for NAT traversal for multimedia communication protocols
based on the offer/answer model [RFC3264] of session negotiation.
ICE works by providing a set of candidate transport addresses for
each media stream, which are then validated with peer-to-peer
connectivity checks based on Session Traversal Utilities for NAT
(STUN) [RFC5389]. However, ICE only defines procedures for UDP-based
There are many reasons why ICE support for TCP is important. First,
there are media protocols that only run over TCP. Such protocols are
used, for example, for screen sharing and instant messaging
[RFC4975]. For these protocols to work in the presence of NAT,
unless they define their own NAT traversal mechanisms, ICE support
for TCP is needed. In addition, RTP can also run over TCP [RFC4571].
Typically, it is preferable to run RTP over UDP, and not TCP.
However, in a variety of network environments, overly restrictive NAT
and firewall devices prevent UDP-based communications altogether, but
general TCP-based communications are permitted. In such
environments, sending RTP over TCP, and thus establishing the media
session, may be preferable to having it fail altogether. With this
specification, agents can gather UDP and TCP candidates for a media
stream, list the UDP ones with higher priority, and then only use the
TCP-based ones if the UDP ones fail. This provides a fallback
mechanism that allows multimedia communications to be highly
The usage of RTP over TCP is particularly useful when combined with
Traversal Using Relays around NAT (TURN) [RFC5766]. In this case,
one of the agents would connect to its TURN server using TCP and
obtain a TCP-based relayed candidate. It would offer this to its
peer agent as a candidate. The other agent would initiate a TCP
connection towards the TURN server. When that connection is
established, media can flow over the connections, through the TURN
server. The benefit of this usage is that it only requires the
agents to make outbound TCP connections to a server on the public
network. This kind of operation is broadly interoperable through NAT
and firewall devices. Since it is a goal of ICE and this extension
to provide highly reliable communications that "just work" in as
broad a set of network deployments as possible, this use case is
This specification extends ICE by defining its usage with TCP
candidates. It also defines how ICE can be used with RTP and Secure
RTP (SRTP) to provide both TCP and UDP candidates. This
specification does so by following the outline of ICE itself and
calling out the additions and changes to support TCP candidates in
ICE. The base behavior of ICE [RFC5245] remains unchanged except for
the extensions in this document that define the usage of ICE with TCP
It should be noted that since TCP NAT traversal is more complicated
than with UDP, ICE TCP is not generally as efficient as UDP-based
ICE. Discussion about this topic can be found in Appendix A.
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
This document uses the same terminology as ICE (see Section 3 of
3. Overview of Operation
The usage of ICE with TCP is relatively straightforward. This
specification mainly deals with how and when connections are opened
and how those connections relate to candidate pairs.
When agents perform address allocations to gather TCP-based
candidates, three types of candidates can be obtained: active
candidates, passive candidates, and simultaneous-open (S-O)
candidates. An active candidate is one for which the agent will
attempt to open an outbound connection but will not receive incoming
connection requests. A passive candidate is one for which the agent
will receive incoming connection attempts but not attempt a
connection. An S-O candidate is one for which the agent will attempt
to open a connection simultaneously with its peer.
When gathering candidates from a host interface, the agent typically
obtains active, passive, and S-O candidates. Similarly, one can use
different techniques for obtaining, e.g., server reflexive, NAT-
assisted, tunneled, or relayed candidates of these three types (see
Section 5). Connections to servers used for relayed and server
reflexive candidates are kept open during ICE processing.
When encoding these candidates into offers and answers, the type of
the candidate is signaled. In the case of active candidates, both IP
address and port are present, but the port is meaningless (it is
there only for making encoding of active candidates consistent with
the other candidate types and is ignored by the peer). As a
consequence, active candidates do not need to be physically allocated
at the time of address gathering. Rather, the physical allocations,
which occur as a consequence of a connection attempt, occur at the
time of the connectivity checks.
When the candidates are paired together, active candidates are always
paired with passive, and S-O candidates with each other. When a
connectivity check is to be made on a candidate pair, each agent
determines whether it is to make a connection attempt for this pair.
The actual process of generating connectivity checks, managing the
state of the check list, and updating the Valid list works
identically for TCP as it does for UDP.
ICE requires an agent to demultiplex STUN and application-layer
traffic, since they appear on the same port. This demultiplexing is
described in [RFC5245] and is done using the magic cookie and other
fields of the message. Stream-oriented transports introduce another
wrinkle, since they require a way to frame the connection so that the
application and STUN packets can be extracted in order to
differentiate STUN packets from application-layer traffic. For this
reason, TCP media streams utilizing ICE use the basic framing
provided in RFC 4571 [RFC4571], even if the application layer
protocol is not RTP.
When Transport Layer Security (TLS) or Datagram Transport Layer
Security (DTLS) is used, they are also run over the RFC 4571 framing
shim, while STUN runs outside of the (D)TLS connection. The
resulting ICE TCP protocol stack is shown in Figure 1, with (D)TLS on
the left side and without it on the right side.
| App |
| | | | | |
| STUN | (D)TLS | | STUN | App |
| | | |
| RFC 4571 | | RFC 4571 |
| | | |
| TCP | | TCP |
| | | |
| IP | | IP |
Figure 1: ICE TCP Stack with and without (D)TLS
The implication of this is that, for any media stream protected by
(D)TLS, the agent will first run ICE procedures, exchanging STUN
messages. Then, once ICE completes, (D)TLS procedures begin. ICE
and (D)TLS are thus "peers" in the protocol stack. The STUN messages
are not sent over the (D)TLS connection, even ones sent for the
purposes of keepalive in the middle of the media session.
4. Sending the Initial Offer
For offerers making use of ICE for TCP streams, the procedures below
are used. The main differences compared to UDP candidates are the
new methods for gathering candidates, how TCP candidates are
prioritized, and how they are encoded in the Session Description
Protocol (SDP) offer and answer.
4.1. Gathering Candidates
Providers of real-time communications services may decide that it is
preferable to have no media at all rather than to have media over
TCP. To allow for choice, it is RECOMMENDED that it be possible to
configure agents to either obtain or not obtain TCP candidates for
Having it be configurable, and then configuring it to be off, is far
better than not having the capability at all. An important goal of
this specification is to provide a single mechanism that can be used
across all types of endpoints. As such, it is preferable to account
for provider and network variation through configuration instead of
hard-coded limitations in an implementation. Besides, network
characteristics and connectivity assumptions can, and will, change
over time. Just because an agent is communicating with a server on
the public network today doesn't mean that it won't need to
communicate with one behind a NAT tomorrow. Just because an agent is
behind a NAT with endpoint-independent mapping today doesn't mean
that tomorrow it won't pick up its agent and take it to a public
network access point where there is a NAT with address- and port-
dependent mapping properties or one that only allows outbound TCP.
The way to handle these cases and build a reliable system is for
agents to implement a diverse set of techniques for allocating
addresses, so that at least one of them is almost certainly going to
work in any situation. Implementors should consider very carefully
any assumptions made about deployments before electing not to
implement one of the mechanisms for address allocation. In
particular, implementors should consider whether the elements in the
system may be mobile and connect through different networks with
different connectivity. They should also consider whether endpoints
that are under their control, in terms of location and network
connectivity, would always be under their control. In environments
where mobility and user control are possible, a multiplicity of
techniques is essential for reliability.
First, agents SHOULD obtain host candidates as described in
Section 5.1. Then, each agent SHOULD "obtain" (allocate a
placeholder for) an active host candidate for each component of each
TCP-capable media stream on each interface that the host has. The
agent does not yet have to actually allocate a port for these
candidates, but they are used for the creation of the check lists.
The agent SHOULD then obtain server reflexive, NAT-assisted, and/or
UDP-tunneled candidates (see Section 5.2, Section 5.3, and
Section 5.4). The mechanisms for establishing these candidates and
the number of candidates to collect vary from technique to technique.
These considerations are discussed in the relevant sections.
Next, agents SHOULD obtain passive (and possibly S-O) relayed
candidates for each component as described in Section 5.5. Each
agent SHOULD also allocate a placeholder for an active relayed
candidate for each component of each TCP-capable media stream.
It is highly RECOMMENDED that a host obtains at least one set of host
candidates and one set of relayed candidates. Obtaining additional
candidates will increase the chance of successfully creating a direct
Once the candidates have been obtained, the agent MUST keep the TCP
connections open until ICE processing has completed. See Appendix B
for important implementation guidelines.
If a media stream is UDP-based (such as RTP), an agent MAY use an
additional host TCP candidate to request a UDP-based candidate from a
TURN server (or some other relay with similar functionality). Usage
of such UDP candidates follows the procedures defined in ICE for UDP
Like its UDP counterparts, TCP-based STUN transactions are paced out
at one every Ta milliseconds (see Section 16 of [RFC5245]). This
pacing refers strictly to STUN transactions (both Binding and
Allocate requests). If performance of the transaction requires
establishment of a TCP connection, then the connection gets opened
when the transaction is performed.
The transport protocol itself is a criteria for choosing one
candidate over another. If a particular media stream can run over
UDP or TCP, the UDP candidates might be preferred over the TCP
candidates. This allows ICE to use the lower latency UDP
connectivity if it exists but fallback to TCP if UDP doesn't work.
In Section 126.96.36.199 of [RFC5245], a recommended formula for UDP ICE
candidate prioritization is defined. For TCP candidates, the same
formula and candidate type preferences SHOULD be used, and the
RECOMMENDED type preferences for the new candidate types defined in
this document (see Section 5) are 105 for NAT-assisted candidates and
75 for UDP-tunneled candidates.
When both UDP and TCP candidates are offered for the same media
stream, and one transport protocol should be preferred over the
other, the type preferences for the preferred transport protocol
candidates SHOULD be increased and/or the type preferences for the
other transport protocol candidates SHOULD be decreased. How much
the values should be increased or decreased depends on whether it is
more important to choose a certain transport protocol or a certain
candidate type. If the candidate type is more important (e.g., even
if UDP is preferred, TCP host candidates are preferred over UDP
server reflexive candidates) changing type preference values by one
for the other transport protocol candidates is enough. On the other
hand, if the transport protocol is more important (e.g., any UDP
candidate is preferred over any TCP candidate), all the preferred
transport protocol candidates SHOULD have type preference higher than
the other transport protocol candidates. However, it is RECOMMENDED
that the relayed candidates are still preferred lower than the other
candidate types. For RTP-based media streams, it is RECOMMENDED that
UDP candidates are preferred over TCP candidates.
With TCP candidates, the local preference part of the recommended
priority formula is updated to also include the directionality
(active, passive, or simultaneous-open) of the TCP connection. The
RECOMMENDED local preference is then defined as:
local preference = (2^13) * direction-pref + other-pref
The direction-pref MUST be between 0 and 7 (both inclusive), with 7
being the most preferred. The other-pref MUST be between 0 and 8191
(both inclusive), with 8191 being the most preferred. It is
RECOMMENDED that the host, UDP-tunneled, and relayed TCP candidates
have the direction-pref assigned as follows: 6 for active, 4 for
passive, and 2 for S-O. For the NAT-assisted and server reflexive
candidates, the RECOMMENDED values are: 6 for S-O, 4 for active, and
2 for passive.
The preference priorities listed here are simply recommendations that
try to strike a balance between success probability and the resulting
path's efficiency. Depending on the scenario where ICE TCP is used,
different values may be appropriate. For example, if the overhead of
a UDP tunnel is not an issue, those candidates should be prioritized
higher since they are likely to have a high success probability.
Also, simultaneous-open is prioritized higher than active and passive
candidates for NAT-assisted and server reflexive candidates since if
TCP S-O is supported by the operating systems of both endpoints, it
should work at least as well as the active-passive approach. If an
implementation is uncertain whether S-O candidates are supported, it
may be reasonable to prioritize them lower. For host, UDP-tunneled,
and relayed candidates, the S-O candidates are prioritized lower than
active and passive since active-passive candidates should work with
them at least as well as the S-O candidates.
If any two candidates have the same type-preference and direction-
pref, they MUST have a unique other-pref. With this specification,
this usually only happens with multi-homed hosts, in which case
other-pref is the preference for the particular IP address from which
the candidate was obtained. When there is only a single IP address,
this value SHOULD be set to the maximum allowed value (8191).
4.3. Choosing Default Candidates
The default candidate is chosen primarily based on the likelihood of
it working with a non-ICE peer. When media streams supporting mixed
modes (both TCP and UDP) are used with ICE, it is RECOMMENDED that,
for real-time streams (such as RTP), the default candidates be UDP-
based. However, the default SHOULD NOT be a simultaneous-open
If a media stream is inherently TCP-based, it is RECOMMENDED for an
offering full agent to select an active candidate as the default
candidate and use [RFC4145] "setup" attribute value "active". This
increases the chances for a successful NAT traversal even without ICE
support if the agent is behind a NAT and the peer is not. For the
same reason, for a lite agent, it is RECOMMENDED to use a passive
candidate and "setup" attribute value "passive" in the offer.
4.4. Lite Implementation Requirements
If an offerer meets the criteria for the lite mode as described in
Appendix A of [RFC5245] (i.e., it will always have a public, globally
unique IP address), it MAY use the lite mode of ICE for TCP
candidates. In the lite mode, for TCP candidates, only passive host
candidates are gathered, unless active candidates are needed as the
default candidates. Otherwise, the procedures described for lite
mode in [RFC5245] also apply to TCP candidates. If UDP and TCP
candidates are mixed in a media stream, the mode (lite or full)
applies to both UDP and TCP candidates.
4.5. Encoding the SDP
TCP-based candidates are encoded into a=candidate lines like the UDP
candidates described in [RFC5245]. However, the transport protocol
(i.e., value of the transport-extension token defined in [RFC5245],
Section 15.1) is set to "TCP" and the connection type (active,
passive, or S-O) is encoded using a new extension attribute. With
TCP candidates, the candidate-attribute syntax with Augmented BNF
[RFC5234] is then:
candidate-attribute = "candidate" ":" foundation SP component-id SP
*(SP extension-att-name SP
tcp-type-ext = "tcptype" SP tcp-type
tcp-type = "active" / "passive" / "so"
The connection-address encoded into the candidate-attribute for
active candidates MUST be set to the IP address that will be used for
the attempt, but the port(s) MUST be set to 9 (i.e., Discard). For
active relayed candidates, the value for connection-address MUST be
identical to the IP address of a passive or simultaneous-open
candidate from the same relay server.
If the default candidate is TCP-based, the agent MUST include the
a=setup and a=connection attributes from RFC 4145 [RFC4145],
following the procedures defined there as if ICE were not in use. In
particular, if an agent is the answerer, the a=setup attribute MUST
meet the constraints in RFC 4145 based on the value in the offer.
If an agent is utilizing SRTP [RFC3711], it MAY include a mix of UDP
and TCP candidates. If ICE selects a TCP candidate pair, it is
RECOMMENDED that the agent still utilizes SRTP but runs it over the
connection established by ICE. The alternative, RTP over TLS, breaks
RTP header compression and on-path RTP analysis tools and hence
SHOULD be avoided. In the case of DTLS-SRTP [RFC5764], the
directionality attributes (a=setup) are utilized strictly to
determine the direction of the DTLS handshake. Directionality of the
TCP connection establishment is determined by the ICE attributes and
procedures defined here.
If an agent is securing non-RTP media over TCP/TLS, the SDP MUST be
constructed as described in RFC 4572 [RFC4572]. The directionality
attributes (a=setup) are utilized strictly to determine the direction
of the TLS handshake. Directionality of the TCP connection
establishment is determined by the ICE attributes and procedures
Examples of SDP offers and answers with ICE TCP extensions are shown
in Appendix C.
5. Candidate Collection Techniques
The following sections discuss a number of techniques that can be
used to obtain candidates for use with ICE TCP. It is important to
note that this list is not intended to be exhaustive, nor is
implementation of any specific technique beyond host candidates
(Section 5.1) considered mandatory.
Implementors are encouraged to implement as many of the following
techniques from the following list as is practical, as well as to
explore additional NAT-traversal techniques beyond those discussed in
this document. However, to get a reasonable success ratio, one
SHOULD implement at least one relayed technique (e.g., TURN) and one
technique for discovering the address given for the host by a NAT
To increase the success probability with the techniques described
below and to aid with transition to IPv6, implementors SHOULD take
particular care to include both IPv4 and IPv6 candidates as part of
the process of gathering candidates. If the local network or host
does not support IPv6 addressing, then clients SHOULD make use of
other techniques, e.g., TURN-IPv6 [RFC6156], Teredo [RFC4380], or
SOCKS IPv4-IPv6 gatewaying [RFC3089], for obtaining IPv6 candidates.
While implementations SHOULD support as many techniques as feasible,
they SHOULD also consider which of them to use if multiple options
are available. Since different candidates are paired with each
other, offering a large number of candidates results in a large check
list and potentially long-lasting connectivity checks. For example,
using multiple NAT-assisted techniques with the same NAT usually
results only in redundant candidates. Similarly, using just one of
the multiple UDP tunneling or relaying techniques is often enough.
5.1. Host Candidates
Host candidates are the most simple candidates since they only
require opening TCP sockets on the host's interfaces and sending/
receiving connectivity checks from them. However, if the hosts are
behind different NATs, host candidates usually fail to work. On the
other hand, if there are no NATs between the hosts, host candidates
are the most efficient method since they require no additional NAT
traversal protocols or techniques.
For each TCP-capable media stream the agent wishes to use (including
ones like RTP that can be either UDP or TCP), the agent SHOULD obtain
two host candidates (each on a different port) for each component of
the media stream on each interface that the host has -- one for the
simultaneous-open and one for the passive candidate. If an agent is
not capable of acting in one of these modes, it would omit those
5.2. Server Reflexive Candidates
Server reflexive techniques aim to discover the address a NAT has
given for the host by asking that from a server on the other side of
the NAT and then creating proper bindings (unless such already exist)
on the NATs with connectivity checks sent between the hosts. Success
of these techniques depends on the NATs' mapping and filtering
behavior [RFC5382] and also on whether the NATs and hosts support the
TCP simultaneous-open technique.
Obtaining server reflexive passive candidates may require initiating
connections from host's passive candidates; see Appendix B for
implementation details on this. Server reflexive active candidates
can be derived from passive or S-O candidates by using the same IP
addresses and interfaces as those candidates. It is useful to obtain
both server reflexive passive and S-O candidates since which one
actually works better depends on the hosts and NATs. Furthermore,
some techniques (e.g., TURN relaying) require knowing the IP address
of the peer's active candidates beforehand, so active server
reflexive candidates are needed for such techniques to function
A widely used protocol for obtaining server reflexive candidates is
STUN. Its TCP-specific behavior is described in [RFC5389], Section
5.3. NAT-Assisted Candidates
NAT-assisted techniques communicate with the NATs directly and, in
this way, discover the address that the NAT has given to the host.
NAT-assisted techniques also create proper bindings on the NATs. The
benefit of these techniques over the server reflexive techniques is
that the NATs can adjust their mapping and filtering behavior so that
connections can be successfully created. A downside of NAT-assisted
techniques is that they commonly allow communicating only with a NAT
that is in the same subnet as the host and thus often fail in
scenarios with multiple layers of NATs. These techniques also rely
on NATs supporting the specific protocols and allowing the users to
modify their behavior.
These candidates are encoded in the ICE offer and answer like the
server reflexive candidates, but they (commonly) use a higher
priority (as described in Section 4.2) and hence are tested before
the server reflexive candidates.
Currently, the Universal Plug and Play (UPnP) forum's Internet
Gateway Device (IGD) protocol [UPnP-IGD] and the NAT Port Mapping
Protocol (PMP) [NAT-PMP] are widely supported NAT-assisted
techniques. Other known protocols include Port Control Protocol
(PCP) [PCP-BASE], SOCKS [RFC1928], Realm Specific IP (RSIP)
[RFC3103], and Simple Middlebox Configuration (SIMCO) [RFC4540].
Also, the Middlebox Communications (MIDCOM) MIB [RFC5190] defines a
mechanism based on the Simple Network Management Protocol (SNMP) for
5.4. UDP-Tunneled Candidates
UDP-tunneled NAT traversal techniques utilize the fact that UDP NAT
traversal is simpler and more efficient than TCP NAT traversal. With
these techniques, the TCP packets (or possibly complete IP packets)
are encapsulated in UDP packets. Because of the encapsulation, these
techniques increase the overhead for the connection and may require
support from both of the endpoints, but on the other hand, UDP
tunneling commonly results in reliable and fairly simple TCP NAT
UDP-tunneled candidates can be encoded in the ICE offer and answer
either as relayed or server reflexive candidates, depending on
whether the tunneling protocol utilizes a relay between the hosts.
The UDP-tunneled candidates may appear to applications as host
candidates from a local pseudo-interface. Treating these candidates
as host candidates results in incorrect prioritization and possibly
non-optimal candidate selection. Implementations may attempt to
detect pseudo-interfaces, e.g., from the address prefix of the
interface, but detection details vary from technique to technique.
For example, the Teredo protocol [RFC4380] [RFC6081] provides
automatic UDP tunneling and IPv6 interworking. The Teredo UDP tunnel
is visible to the host application as an IPv6 address; thus, Teredo
candidates are encoded as IPv6 addresses.
5.5. Relayed Candidates
Relaying packets through a relay server is often the NAT traversal
technique that has the highest success probability: communicating via
a relay that is in the public Internet looks like normal client-
server communication for the NATs and is supported in practice by all
existing NATs, regardless of their filtering and mapping behavior.
However, using a relay has several drawbacks, e.g., it usually
results in a sub-optimal path for the packets, the relay needs to
exist and it needs to be discovered, the relay is a possible single
point of failure, relaying consumes potentially a lot of resources of
the relay server, etc. Therefore, relaying is often used as the last
resort when no direct path can be created with other NAT traversal
With relayed candidates, the host commonly needs to obtain only a
passive candidate since any of the peer's server reflexive (and NAT-
assisted if the peer can communicate with the outermost NAT) active
candidates should work with the passive relayed candidate. However,
if the relay is behind a NAT or a firewall, also using active and S-O
candidates will increase success probability.
Relaying protocols capable of relaying TCP connections include TURN
TCP [RFC6062] and SOCKS [RFC1928] (which can also be used for IPv4-
IPv6 gatewaying [RFC3089]). It is also possible to use a Secure
SHell (SSH) [RFC4251] tunnel as a relayed candidate if a suitable
server is available and the server permits this.
6. Receiving the Initial Offer and Answer
Handling an ICE offer with TCP candidates works in a similar way as
with UDP candidates. First, ICE support is verified (including the
check for ice-mismatch described in Section 5.1 of [RFC5245]) and
agent roles are determined. Candidates are gathered using the
techniques described in Section 5 and prioritized as described in
Section 4.2. Default candidates are selected taking into account the
considerations in Section 4.3. The SDP answer is encoded as in
Section 4.3 of [RFC5245], with the exception of TCP candidates whose
encoding is described in Section 4.5.
When the offerer receives the initial answer, it also verifies ICE
support and determines its role. If both of the agents use lite
implementations, the offerer takes the controlling role and uses the
procedures defined in [RFC4145] to select the most preferred
candidate pair with a new offer.
6.1. Considerations with Two Lite Agents
If both agents are using the lite mode and if the offerer uses the
a=setup:active attribute [RFC4145] in the new offer, the offerer MAY
initiate the TCP connection on the selected pair in parallel with the
new offer to speed up the connection establishment. Consequently,
the answerer MUST still accept incoming TCP connections to any of the
passive candidates it listed in the answer, from any of the IP
addresses the offerer listed in the initial offer.
If the answerer receives the new offer matching the candidate pair
where a connection was already created in parallel with the new
offer, it MUST accept the offer and respond to it while keeping the
already-created connection. If the connection that was created in
parallel with the new offer does not match the candidate pair in the
new offer, the connection MUST be closed, and ICE restart SHOULD be
Since the connection endpoints are not authenticated using the
connectivity checks in the scenario where both agents use the lite
mode, unless media-level security (e.g., TLS) is used, it is
RECOMMENDED to use the full mode instead. For more lite versus full
implementation considerations, see Appendix A of [RFC5245].
6.2. Forming the Check Lists
As with UDP, check lists are formed only by full ICE implementations.
When forming candidate pairs, the following types of TCP candidates
can be paired with each other:
When the agent prunes the check list, it MUST also remove any pair
for which the local candidate is a passive TCP candidate. With
pruning, the NAT-assisted candidates are treated like server
reflexive candidates if the base is also used as a host candidate.
The remainder of check list processing works in the same way as the
7. Connectivity Checks
The TCP connectivity checks, like with UDP, are generated only by
full implementations. The TCP candidate pairs are in the same check
list with the UDP candidate pairs, and they are scheduled for
connectivity checks, as described in Section 5.8 of [RFC5245], based
on the priority order.
7.1. STUN Client Procedures
When an agent wants to send a TCP-based connectivity check, it first
opens a TCP connection, if none yet exists, for the 5-tuple defined
by the candidate pair for which the check is to be sent. This
connection is opened from the local candidate of the pair to the
remote candidate of the pair. If the local candidate is tcp-active,
the agent MUST open a connection from the interface associated with
that local candidate. This connection SHOULD be opened from an
unallocated port. For host candidates, this is readily done by
connecting from the local candidate's interface. For relayed, NAT-
assisted, and UDP-tunneled candidates, the agent may need to use
additional procedures specific to the protocol.
Once the connection is established, the agent MUST utilize the shim
defined in RFC 4571 [RFC4571] for the duration this connection
remains open. The STUN Binding requests and responses are sent on
top of this shim, so that the length field defined in RFC 4571
precedes each STUN message. If TLS or DTLS-SRTP is to be utilized
for the media session, the TLS or DTLS-SRTP handshakes will take
place on top of this shim as well. However, they only start once ICE
processing has completed. In essence, the TLS or DTLS-SRTP
handshakes are considered a part of the media protocol. STUN is
never run within the TLS or DTLS-SRTP session as part of the ICE
If the TCP connection cannot be established, the check is considered
to have failed, and a full-mode agent MUST update the pair state to
Failed in the check list. See Section 7.2.2 of [RFC5389] for more
details on STUN over TCP.
Once the connection is established, client procedures are identical
to those for UDP candidates. However, retransmissions of the STUN
connectivity check messages are not needed, since TCP takes care of
reliable delivery of the messages. Note also that STUN responses
received on an active TCP candidate will typically produce a peer
reflexive candidate. If the response to the first connectivity check
on the established TCP connection is something other than a STUN
message, the remote candidate address apparently was not one of the
peer's addresses, and the agent SHOULD close the connection and
consider all pairs with that remote candidate as failed.
7.2. STUN Server Procedures
An ICE TCP agent, full or lite, MUST be prepared to receive incoming
TCP connection requests on the base of any TCP candidate that is
simultaneous-open or passive. When the connection request is
received, the agent MUST accept it. The agent MUST utilize the
framing defined in RFC 4571 [RFC4571] for the lifetime of this
connection. Due to this framing, the agent will receive data in
discrete frames. Each frame could be media (such as RTP or SRTP),
TLS, DTLS, or STUN packets. The STUN packets are extracted as
described in Section 10.2.
Once the connection is established, STUN server procedures are
identical to those for UDP candidates. Note that STUN requests
received on a passive TCP candidate will typically produce a remote
peer reflexive candidate.
8. Concluding ICE Processing
If there are TCP candidates for a media stream, a controlling agent
MUST use the regular selection algorithm.
When ICE processing for a media stream completes, each agent SHOULD
close all TCP connections (that were opened due to this ICE session)
except the ones between the candidate pairs selected by ICE.
These two rules are related; the closure of connection on completion
of ICE implies that a regular selection algorithm has to be used.
This is because aggressive selection might cause transient pairs to
be selected. Once such a pair is selected, the agents would close
the other connections, one of which may be about to be selected as a
better choice. This race condition may result in TCP connections
being accidentally closed for the pair that ICE selects.
9. Subsequent Offer/Answer Exchanges
9.1. Updated Offer
When an updated offer is generated by the controlling endpoint after
the connectivity checks have succeeded, the SDP extensions for
connection-oriented media [RFC4145] are used to signal that an
existing connection should be used, rather than opening a new one.
9.2. ICE Restarts
If an ICE restart occurs for a media stream with TCP candidate pairs
that have been selected by ICE, the agents MUST NOT close the
connections after the restart. In the offer or answer that causes
the restart, an agent MAY include a simultaneous-open candidate whose
transport address matches the previously selected candidate. If both
agents do this, the result will be a simultaneous-open candidate pair
matching an existing TCP connection. In this case, the agents MUST
NOT attempt to open a new connection (or start new TLS or DTLS-SRTP
procedures). Instead, that existing connection is reused, and STUN
checks are performed.
Once the restart completes, if the selected pair does not match the
previously selected pair, the TCP connection for the previously
selected pair SHOULD be closed by the agent.
10. Media Handling
10.1. Sending Media
When sending media, if the selected candidate pair matches an
existing TCP connection, that connection MUST be used for sending
The framing defined in RFC 4571 MUST be used when sending media. For
media streams that are not RTP-based and do not normally use RFC
4571, the agent treats the media stream as a byte stream and assumes
that it has its own framing of some sort, if needed. It then takes
an arbitrary number of bytes from the byte stream and places that as
a payload in the RFC 4571 frames, including the length. Next, the
sender checks to see if the resulting set of bytes would be viewed as
a STUN packet based on the rules in Sections 6 and 8 of [RFC5389].
This includes a check on the most significant two bits, the magic
cookie, the length, and the fingerprint. If, based on those rules,
the bytes would be viewed as a STUN message, the sender MUST utilize
a different number of bytes so that the length checks will fail.
Though it is normally highly unlikely that an arbitrary number of
bytes from a byte stream would resemble a STUN packet based on all of
the checks, it can happen if the content of the application stream
happens to contain a STUN message (for example, a file transfer of
logs from a client that includes STUN messages).
If TLS or DTLS-SRTP procedures are being utilized to protect the
media stream, those procedures start at the point that media is
permitted to flow, as defined in the ICE specification [RFC5245].
The TLS or DTLS-SRTP handshakes occur on top of the RFC 4571 shim and
are considered part of the media stream for the purposes of this
10.2. Receiving Media
The framing defined in RFC 4571 MUST be used when receiving media.
For media streams that are not RTP-based and do not normally use RFC
4571, the agent extracts the payload of each RFC 4571 frame and
determines if it is a STUN or an application-layer data based on the
procedures in ICE [RFC5245]. If media is being protected with DTLS-
SRTP, the DTLS, RTP, and STUN packets are demultiplexed as described
in Section 5.1.2 of [RFC5764].
For non-STUN data, the agent appends this to the ongoing byte stream
collected from the frames. It then parses the byte stream as if it
had been directly received over the TCP connection. This allows for
ICE TCP to work without regard to the framing mechanism used by the
11. Connection Management
11.1. Connections Formed during Connectivity Checks
Once a TCP or TCP/TLS connection is opened by ICE for the purpose of
connectivity checks, its life cycle depends on how it is used. If
that candidate pair is selected by ICE for usage for media, an agent
SHOULD keep the connection open until:
o the session terminates,
o the media stream is removed, or
o an ICE restart takes place, resulting in the selection of a
different candidate pair.
In any of these cases, the agent SHOULD close the connection when
that event occurs. This applies to both agents in a session, in
which case one of the agents will usually end up closing the
If a connection has been selected by ICE, an agent MAY close it
anyway. As described in the next paragraph, this will cause it to be
reopened almost immediately, and in the interim, media cannot be
sent. Consequently, such closures have a negative effect and are NOT
RECOMMENDED. However, there may be cases where an agent needs to
close a connection for some reason.
If an agent needs to send media on the selected candidate pair, and
its TCP connection has closed, then:
o If the agent's local candidate is tcp-active or tcp-so, it MUST
reopen a connection to the remote candidate of the selected pair.
o If the agent's local candidate is tcp-passive, the agent MUST
await an incoming connection request and, consequently, will not
be able to send media until it has been opened.
If the TCP connection is established, the framing of RFC 4571 is
utilized. If the agent opened the connection and is a full agent, it
MUST send a STUN connectivity check. An agent MUST be prepared to
receive a connectivity check over a connection it opened or accepted
(note that this is true in general; ICE requires that an agent be
prepared to receive a connectivity check at any time, even after ICE
processing completes). If a full agent receives a connectivity check
after re-establishment of the connection, it MUST generate a
triggered check over that connection in response if it has not
already sent a check. Once an agent has sent a check and received a
successful response, the connection is considered Valid, and media
can be sent (which includes a TLS or DTLS-SRTP session resumption or
If the TCP connection cannot be established, the controlling agent
SHOULD restart ICE for this media stream. This will happen in cases
where one of the agents is behind a NAT with connection-dependent
mapping properties [RFC5382].
11.2. Connections Formed for Gathering Candidates
If the agent opened a connection to a STUN server, or another similar
server, for the purposes of gathering a server reflexive candidate,
that connection SHOULD be closed by the client once ICE processing
has completed. This happens regardless of whether the candidate
learned from the server was selected by ICE.
If the agent opened a connection to a TURN server for the purposes of
gathering a relayed candidate, that connection MUST be kept open by
the client for the duration of the media session if a relayed
candidate from the TURN server was selected by ICE. Otherwise, the
connection to the TURN server SHOULD be closed once ICE processing
If, despite efforts of the client, a TCP connection to a TURN server
fails during the lifetime of the media session utilizing a transport
address allocated by that server, the client SHOULD reconnect to the
TURN server, obtain a new allocation, and restart ICE for that media
stream. Similar measures SHOULD apply also to other types of
12. Security Considerations
The main threat in ICE is hijacking of connections for the purposes
of directing media streams to denial-of-service (DoS) targets or to
malicious users. When full implementations are used, ICE TCP
prevents that by only using TCP connections that have been validated.
Validation requires a STUN transaction to take place over the
connection. This transaction cannot complete without both
participants knowing a shared secret exchanged in the rendezvous
protocol used with ICE, such as SIP [RFC3261]. This shared secret,
in turn, is protected by that protocol exchange. In the case of SIP,
the usage of the SIP Secure (SIPS) [RFC3261] mechanism is
RECOMMENDED. When this is done, an attacker, even if it knows or can
guess the port on which an agent is listening for incoming TCP
connections, will not be able to open a connection and send media to
If the rendezvous protocol exchange is compromised, the shared secret
can be learned by an attacker, and the attacker may be able to fake
the connectivity check validation and open a TCP connection to the
target. Hence, using additional security mechanisms (e.g.,
application-layer security) that mitigate these risks is RECOMMENDED.
A STUN amplification attack is described in Section 18.5.2 of
[RFC5245]. The same considerations apply to TCP, but the
amplification effect with TCP is larger due to need for establishing
a TCP connection before any checks are performed. Therefore, an ICE
agent SHOULD NOT have more than 5 outstanding TCP connection attempts
with the same peer to the same IP address.
If both agents use the lite mode, no connectivity checks are sent,
and additional procedures (e.g., media-level security) are needed to
validate the connection. The lack of connectivity checks is
especially problematic if one of the hosts is behind a NAT and has an
address from a private address space: the peer may accidentally
connect to a host in a different subnet that uses the same private
address space. This is one of the reasons why the lite mode is not
appropriate for an ICE agent located behind a NAT.
A more detailed analysis of different attacks and the various ways
ICE prevents them are described in [RFC5245]. Those considerations
apply to this specification.
13. IANA Considerations
IANA has created a new sub-registry "ICE Transport Protocols" in the
"Interactive Connectivity Establishment (ICE)" registry for ICE
candidate-attribute transport extensions. Initial values are given
below; future assignments are to be made through IETF Review or IESG
Approval [RFC5226]. Assignments consist of an extension token (as
defined in Section 15.1 of [RFC5245]) and a reference to the document
defining the extension.
UDP RFC 5245, Section 15.1
TCP RFC 6544, Section 4.514. Acknowledgements
The authors would like to thank Tim Moore, Saikat Guha, Francois
Audet, Roni Even, Simon Perreault, Alfred Heggestad, Hadriel Kaplan,
Jonathan Lennox, Flemming Andreasen, Dan Wing, and Vijay Gurbani for
the reviews and input on this document. Special thanks to Marc
Petit-Huguenin for providing the SDP examples.
15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol
(SRTP)", RFC 3711, March 2004.
[RFC4145] Yon, D. and G. Camarillo, "TCP-Based Media Transport in
the Session Description Protocol (SDP)", RFC 4145,
[RFC4571] Lazzaro, J., "Framing Real-time Transport Protocol (RTP)
and RTP Control Protocol (RTCP) Packets over Connection-
Oriented Transport", RFC 4571, July 2006.
[RFC4572] Lennox, J., "Connection-Oriented Media Transport over the
Transport Layer Security (TLS) Protocol in the Session
Description Protocol (SDP)", RFC 4572, July 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the
Secure Real-time Transport Protocol (SRTP)", RFC 5764,
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal
Using Relays around NAT (TURN): Relay Extensions to
Session Traversal Utilities for NAT (STUN)", RFC 5766,
15.2. Informative References
[IMC05] Guha, S. and P. Francis, "Characterization and
Measurement of TCP Traversal through NATs and Firewalls",
Proceedings of the 5th ACM SIGCOMM Conference on Internet
[NAT-PMP] Cheshire, S., Krochmal, M., and K. Sekar, "NAT Port
Mapping Protocol (NAT-PMP)", Work in Progress,
[PCP-BASE] Wing, D., Cheshire, S., Boucadair, M., Penno, R., and P.
Selkirk, "Port Control Protocol (PCP)", Work in Progress,
[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
L. Jones, "SOCKS Protocol Version 5", RFC 1928,
[RFC3089] Kitamura, H., "A SOCKS-based IPv6/IPv4 Gateway
Mechanism", RFC 3089, April 2001.
[RFC3103] Borella, M., Grabelsky, D., Lo, J., and K. Taniguchi,
"Realm Specific IP: Protocol Specification", RFC 3103,
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
[RFC4540] Stiemerling, M., Quittek, J., and C. Cadar, "NEC's Simple
Middlebox Configuration (SIMCO) Protocol Version 3.0",
RFC 4540, May 2006.
[RFC4975] Campbell, B., Mahy, R., and C. Jennings, "The Message
Session Relay Protocol (MSRP)", RFC 4975, September 2007.
[RFC5190] Quittek, J., Stiemerling, M., and P. Srisuresh,
"Definitions of Managed Objects for Middlebox
Communication", RFC 5190, March 2008.
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP",
BCP 142, RFC 5382, October 2008.
[RFC6062] Perreault, S. and J. Rosenberg, "Traversal Using Relays
around NAT (TURN) Extensions for TCP Allocations",
RFC 6062, November 2010.
[RFC6081] Thaler, D., "Teredo Extensions", RFC 6081, January 2011.
[RFC6156] Camarillo, G., Novo, O., and S. Perreault, "Traversal
Using Relays around NAT (TURN) Extension for IPv6",
RFC 6156, April 2011.
[UPnP-IGD] Warrier, U., Iyer, P., Pennerath, F., Marynissen, G.,
Schmitz, M., Siddiqi, W., and M. Blaszczak, "Internet
Gateway Device (IGD) Standardized Device Control Protocol
V 1.0", November 2001.
Appendix A. Limitations of ICE TCP
Compared to UDP-based ICE, ICE TCP has, in general, lower success
probability for enabling connectivity without a relay if both of the
hosts are behind a NAT. This happens because many of the currently
deployed NATs have endpoint-dependent mapping behavior, or they do
not support the flow of TCP handshake packets seen in the case of TCP
simultaneous-open, e.g., some NATs do not allow incoming TCP SYN
packets from an address where a SYN packet has been sent to recently
or the subsequent SYN-ACK is not processed properly.
It has been reported in [IMC05] that with the population of NATs
deployed at the time of the measurements (2005), one of the NAT
traversal techniques described here, TCP simultaneous-open, worked in
roughly 45% of the cases. Also, not all operating systems implement
TCP simultaneous-open properly and thus are not able to use such
candidates. However, when more NATs comply with the requirements set
by [RFC5382] and operating system TCP stacks are fixed, the success
probability of simultaneous-open is likely to increase. Also, it is
important to implement additional techniques with higher success
ratios, such as Teredo, whose success in different scenarios is
described in Figure 1 of [RFC6081].
Finally, it should be noted that implementing various techniques
listed in Section 5 should increase the success probability, but many
of these techniques require support from the endpoints and/or from
some network elements (e.g., from the NATs). Without comprehensive
experimental data on how well different techniques are supported, the
actual increase of success probability is hard to evaluate.
Appendix B. Implementation Considerations for BSD Sockets
This specification requires unusual handling of TCP connections, the
implementation of which is non-trivial in traditional BSD socket
In particular, ICE requires an agent to obtain a local TCP candidate,
bound to a local IP and port, then initiate a TCP connection from
that local port (e.g., to the STUN server in order to obtain server
reflexive candidates, to the TURN server to obtain a relayed
candidate, or to the peer as part of a connectivity check), and be
prepared to receive incoming TCP connections (for passive and
simultaneous-open candidates). A "typical" BSD socket is used either
for initiating or receiving connections, and not for both. The code
required to allow incoming and outgoing connections on the same local
IP and port is non-obvious. The following pseudocode, contributed by
Saikat Guha, has been found to work on many platforms:
for i in 0 to MAX
sock_i = socket()
The key here is that, prior to the listen() call, the full set of
sockets that need to be utilized for outgoing connections must be
allocated and bound to the local IP address and port. This number,
MAX, represents the maximum number of TCP connections to different
destinations that might need to be established from the same local
candidate. This number can be potentially large for simultaneous-
open candidates. If a request forks, ICE procedures may take place
with multiple peers. Furthermore, for each peer, connections would
need to be established to each passive or simultaneous-open candidate
for the same component. If we assume a worst case of 5 forked
branches, and for each peer, five simultaneous-open candidates, that
results in MAX=25.
Appendix C. SDP Examples
This section shows two examples of SDP offer and answer when the ICE
TCP extension is used. Both examples are based on the simplified
topology of Figure 8 in [RFC5245], with the same IP addresses. The
examples shown here should be considered strictly informative.
In the first example, the offer contains only TCP candidates (lines
are folded in examples to satisfy RFC formatting rules):
o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
c=IN IP4 192.0.2.3
m=audio 45664 TCP/RTP/AVP 0
a=candidate:1 1 TCP 2128609279 10.0.1.1 9 typ host tcptype active
a=candidate:2 1 TCP 2124414975 10.0.1.1 8998 typ host tcptype passive
a=candidate:3 1 TCP 2120220671 10.0.1.1 8999 typ host tcptype so
a=candidate:4 1 TCP 1688207359 192.0.2.3 9 typ srflx raddr 10.0.1.1
rport 9 tcptype active
a=candidate:5 1 TCP 1684013055 192.0.2.3 45664 typ srflx raddr
10.0.1.1 rport 8998 tcptype passive
a=candidate:6 1 TCP 1692401663 192.0.2.3 45687 typ srflx raddr
10.0.1.1 rport 8999 tcptype so
The answer to that offer could look like this:
o=bob 2808844564 2808844564 IN IP4 192.0.2.1
c=IN IP4 192.0.2.1
m=audio 3478 TCP/RTP/AVP 0
a=candidate:1 1 TCP 2128609279 192.0.2.1 9 typ host tcptype active
a=candidate:2 1 TCP 2124414975 192.0.2.1 3478 typ host tcptype passive
a=candidate:3 1 TCP 2120220671 192.0.2.1 3482 typ host tcptype so
In the second example, UDP and TCP media streams are mixed, but S-O
candidates are omitted due to hosts not supporting TCP simultaneous-
open, and UDP candidates are preferred (but preference order for
candidate types is kept the same) by decreasing the TCP candidate type
preferences by one (i.e., using type preference 125 for the host
candidates and 99 for the server reflexive candidates):
o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
c=IN IP4 192.0.2.3
m=audio 45664 RTP/AVP 0
a=candidate:1 1 TCP 2111832063 10.0.1.1 9 typ host tcptype active
a=candidate:2 1 TCP 2107637759 10.0.1.1 9012 typ host tcptype passive
a=candidate:3 1 TCP 1671430143 192.0.2.3 9 typ srflx raddr 10.0.1.1
rport 9 tcptype active
a=candidate:4 1 TCP 1667235839 192.0.2.3 44642 typ srflx raddr
10.0.1.1 rport 9012 tcptype passive
a=candidate:5 1 UDP 2130706431 10.0.1.1 8998 typ host
a=candidate:6 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr
10.0.1.1 rport 8998
Bruce B. Lowekamp
17210 Campbell Rd., Suite 250
Dallas, TX 75252