MMUSIC                                                      J. Rosenberg
Internet-Draft                                                     Skype
Intended status: Standards Track                              A. Keranen
Expires: May 14, 2011                                           Ericsson
                                                             B. Lowekamp
                                                                A. Roach
                                                       November 10, 2010

    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 transport
   protocols.  This specification extends ICE to TCP-based media,
   including the ability to offer a mix of TCP and UDP-based candidates
   for a single stream.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 14, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
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   Without obtaining an adequate license from the person(s) controlling
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   it for publication as an RFC or to translate it into languages other
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  5
   4.  Sending the Initial Offer  . . . . . . . . . . . . . . . . . .  7
     4.1.  Gathering Candidates . . . . . . . . . . . . . . . . . . .  7
     4.2.  Prioritization . . . . . . . . . . . . . . . . . . . . . .  9
     4.3.  Choosing Default Candidates  . . . . . . . . . . . . . . . 10
     4.4.  Encoding the SDP . . . . . . . . . . . . . . . . . . . . . 11
   5.  Candidate Collection Techniques  . . . . . . . . . . . . . . . 11
     5.1.  Host Candidates  . . . . . . . . . . . . . . . . . . . . . 12
     5.2.  Server Reflexive Candidates  . . . . . . . . . . . . . . . 13
     5.3.  NAT-Assisted Candidates  . . . . . . . . . . . . . . . . . 13
     5.4.  UDP-Tunneled Candidates  . . . . . . . . . . . . . . . . . 13
     5.5.  Relayed Candidates . . . . . . . . . . . . . . . . . . . . 14
   6.  Receiving the Initial Offer  . . . . . . . . . . . . . . . . . 14
     6.1.  Verifying ICE Support  . . . . . . . . . . . . . . . . . . 14
     6.2.  Forming the Check Lists  . . . . . . . . . . . . . . . . . 15
   7.  Connectivity Checks  . . . . . . . . . . . . . . . . . . . . . 15
     7.1.  STUN Client Procedures . . . . . . . . . . . . . . . . . . 15
     7.2.  STUN Server Procedures . . . . . . . . . . . . . . . . . . 16
   8.  Concluding ICE Processing  . . . . . . . . . . . . . . . . . . 16
   9.  Subsequent Offer/Answer Exchanges  . . . . . . . . . . . . . . 17
     9.1.  ICE Restarts . . . . . . . . . . . . . . . . . . . . . . . 17
   10. Media Handling . . . . . . . . . . . . . . . . . . . . . . . . 17
     10.1. Sending Media  . . . . . . . . . . . . . . . . . . . . . . 17
     10.2. Receiving Media  . . . . . . . . . . . . . . . . . . . . . 18
   11. Connection Management  . . . . . . . . . . . . . . . . . . . . 18
     11.1. Connections Formed During Connectivity Checks  . . . . . . 18
     11.2. Connections Formed for Gathering Candidates  . . . . . . . 19
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 21
     15.2. Informative References . . . . . . . . . . . . . . . . . . 21
   Appendix A.  Limitations of ICE TCP  . . . . . . . . . . . . . . . 23
   Appendix B.  Implementation Considerations for BSD Sockets . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24

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1.  Introduction

   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
   transport protocols.

   There are many reasons why ICE support for TCP is important.
   Firstly, 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 answerer 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 works" in as a
   broad a set of network deployments as possible, this use case is
   particularly important.

   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

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   calling out the additions and changes necessary in each section of
   ICE to support TCP candidates.

   It should be noted that since TCP NAT traversal is more complicated
   than with UDP, ICE TCP is not as efficient as UDP-based ICE.
   Discussion about this topic can be found in Appendix A.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   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.  The main
   area of specification is around how and when connections are opened,
   and how those connections relate to candidate pairs.

   When the agents perform address allocations to gather TCP-based
   candidates, three types of candidates can be obtained.  These are
   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.  S-O candidate is one for which the agent will attempt to
   open a connection simultaneously with its peer.

   Unlike for UDP, there are no lite implementation defined for TCP.
   Instead, an implementation that meets the criteria for a lite
   implementation as discussed in Appendix A of [RFC5245] can just use
   the mechanisms defined in [RFC4145], with constraints defined here on
   selection of attribute values (see Section 4).

   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.
   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

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   the candidate is signaled.  In the case of active candidates, an IP
   address and port is present, but it is meaningless, as it 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, work
   identically for TCP as they do for UDP.

   ICE requires an agent to demultiplex STUN and application layer
   traffic, since they appear on the same port.  This demultiplexing is
   described by ICE, 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 determine
   which is which.  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 TLS is in use (for non-RTP traffic) or DTLS (for RTP traffic),
   it runs over the RFC 4571 framing shim, so that STUN runs outside of
   the (D)TLS connection.  Pictorially:

                                     |          |
                                     |    App   |
                          |          |          |
                          |   STUN   |  (D)TLS  |
                          |                     |
                          |      RFC 4571       |
                          |                     |
                          |         TCP         |
                          |                     |
                          |         IP          |

                          Figure 1: ICE TCP Stack

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   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.

   When an updated offer is generated by the controlling endpoint, 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.

4.  Sending the Initial Offer

   If an offerer meets the criteria for lite as defined in Appendix A of
   [RFC5245], it omits any ICE attributes for its TCP-based media
   streams.  Instead, the offerer follows the procedures defined in
   [RFC4145] for constructing the offer.  However, the offerer MUST use
   a setup attribute of "actpass" for those streams.

   For offerers making use of ICE for TCP streams, the procedures below
   are used.

4.1.  Gathering Candidates

   Providers of real-time communications services may decide that it is
   preferable to have no media at all than it is to have media over TCP.
   To allow for choice, it is RECOMMENDED that agents be configurable
   with whether they obtain TCP candidates for real time media.

      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.  Furthermore, 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 they won't pick up their 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

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      addresses, so that at least one of them is almost certainly going
      to work in any situation.  Implementors should consider very
      carefully any assumptions that they make 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 which 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

   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 have to yet actually allocate a port for these
   candidates, but they are used for the creation of the check lists.

   Next, the 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.

   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, below.

   It is highly recommended that a host obtains at least one set of host
   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 seconds.  This pacing refers strictly to STUN

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   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.

4.2.  Prioritization

   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.

   To accomplish this, the local preference SHOULD be defined as:

   local-preference = (2^12)*(transport-pref) +
                      (2^7)*(class-pref) +

   Transport-pref is the relative preference for candidates with this
   particular transport protocol (UDP or TCP), and class-pref is the
   preference for candidates with this particular establishment
   directionality and class (active, passive, or S-O with different
   class of NAT traversal techniques).  Other-pref is used as a
   differentiator when two candidates would otherwise have identical
   local preferences.

   Transport-pref MUST be between 0 and 15, with 15 being the most
   preferred.  Class-pref MUST be between 0 and 31, with 31 being the
   most preferred.  Other-pref MUST be between 0 and 127, with 127 being
   the most preferred.  For RTP-based media streams, it is RECOMMENDED
   that UDP have a transport-pref of 12 and TCP of 6.  It is RECOMMENDED
   that, for all connection-oriented media, candidates have a class-pref
   assigned as follows:

                   29  Host active candidate
                   28  Host passive candidate
                   27  Host S-O candidate
                   23  NAT-assisted S-O candidate
                   22  NAT-assisted active candidate
                   21  NAT-assisted passive candidate
                   17  Server reflexive S-O candidate
                   16  Server reflexive active candidate
                   15  Server reflexive passive candidate
                   11  UDP-tunneled active candidate
                   10  UDP-tunneled passive candidate
                   9   UDP-tunneled S-O candidate
                   5   Relayed active candidate

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                   4   Relayed passive candidate
                   3   Relayed S-O candidate

   If it is more important to use certain kind (NAT-assisted, server
   reflexive, etc.) of candidates rather than certain transport
   protocol, it is RECOMMENDED that the type preference for NAT-assisted
   candidates be set higher than that for server-reflexive candidates
   and that the type preference for UDP-tunneled candidates be set lower
   than that for server-reflexive candidates.  The RECOMMENDED values
   are 105 for NAT-assisted candidates and 75 for UDP-tunneled
   candidates.  However, if the transport protocol is more important,
   NAT-assisted and UDP-tunneled candidates MAY use the same type
   preference as the server-reflexive candidates.

   The class-pref priorities listed above are simply recommendations
   that try to strike a balance between success probability and
   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 act-pass
   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 act-pass candidates
   should work with them at least as well as the S-O candidates.

   If any two candidates have the same type-preference, transport-pref,
   and class-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 a preference amongst interfaces.

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, the agent MUST select the
   active candidate as default.  This ensures proper directionality of
   connection establishment for NAT traversal with non-ICE

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4.4.  Encoding the SDP

   TCP-based candidates are encoded into a=candidate lines identically
   to the UDP encoding 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-so" for TCP simultaneous-open
   candidates, "tcp-act" for TCP active candidates, and "tcp-pass" for
   TCP passive candidates.  The addr and port encoded into the candidate
   attribute for active candidates MUST be set to IP address that will
   be used for the attempt, but the port MUST be set to 9 (i.e.,
   Discard).  For active relayed candidates, the value for addr 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, the agent MUST include the a=setup
   and a=connection attributes from RFC 4145 [RFC4145], following the
   procedures defined there as if ICE was 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.  Since an
   ICE ICE offerer always uses the active candidate as default, an ICE
   ICE answerer will always use the passive attribute as default and
   include the a=setup:passive attribute in the answer.

   If an agent is utilizing SRTP [RFC3711], it MAY include a mix of UDP
   and TCP candidates.  If ICE selects a TCP candidate pair, the agent
   MUST still utilize SRTP, but run it over the connection established
   by ICE.  The alternative, RTP over TLS, MUST NOT be used.  This
   allows for the higher layer protocols (the security handshakes and
   media transport) to be independent of the underlying transport
   protocol.  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 are 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 are determined by the ICE attributes and procedures
   defined here.

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

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   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
   (e.g., STUN).

   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., 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 amount of candidates results in a large
   checklist and potentially long lasting connectivity checks.  For
   example, using multiple NAT-assisted techniques with the same NAT
   usually results only in redundant candidates and similarly out of
   multiple different UDP tunneling or relaying techniques with similar
   features using just one is often enough.

5.1.  Host Candidates

   Host candidates are the most simple candidates since they only
   require opening TCP sockets on one 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, which can either be 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 candidates.

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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 whether the NATs and hosts support TCP
   simultaneous-open technique.

   A widely used protocol for obtaining server reflexive candidates is
   STUN, whose TCP specific behavior is described in [RFC5389] Section

5.3.  NAT-Assisted Candidates

   NAT-assisted techniques communicate with the NATs directly and this
   way discover the address NAT has given to the host and 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 that the NATs allow the users to modify their

   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 UPnP forum's Internet Gateway Device (IGD) protocol
   [UPnP-IGD] and the NAT Port Mapping Protocol (PMP)
   [I-D.cheshire-nat-pmp] are widely supported NAT-assisted techniques.
   Other known protocols include SOCKS [RFC1928], Realm Specific IP
   (RSIP) [RFC3103], and SIMCO [RFC4540].  Also, MIDCOM MIB [RFC5190]
   defines an SNMP-based mechanism for controlling NATs.

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

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   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.

   For example, the Teredo protocol [RFC4380] provides automatic UDP
   tunneling and IPv6 interworking.  The Teredo UDP tunnel is visible to
   the host application as an IPv6 address and 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 that 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 techniques.

   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, using also active and S-O
   candidates will increase success probability.

   Relaying protocols capable of relaying TCP connections include TURN
   TCP [I-D.ietf-behave-turn-tcp] and SOCKS [RFC1928] (which can also be
   used for IPv4-IPv6 gatewaying [RFC3089]).  It is also possible to
   use, e.g., an SSH [RFC4250] tunnel as a relayed candidate if a
   suitable server is available and the server permits this.

6.  Receiving the Initial Offer

6.1.  Verifying ICE Support

   Since this specification does not define a lite mode for ICE TCP, a
   lite implementation will include candidate attributes for its UDP
   streams, but no such attributes for its TCP streams.  An agent

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   receiving such an offer MUST proceed with ICE in this case.  ICE will
   be used for the UDP streams, and [RFC4145] procedures will be used
   for the TCP streams.  However, if the offer indicates a setup
   direction of actpass, the answerer MUST utilize a=setup:active in the
   answer.  This is required to ensure proper directionality of
   connection establishment to work through NAT.

   Similarly, if an agent is lite, and receives an offer that includes
   streams with TCP candidates, it will omit candidates from the answer
   for those streams.  This will cause [RFC4145] procedures to be used
   for those streams.  In this case, the offer will indicate a direction
   of active, and the agent will use passive in its answer.

6.2.  Forming the Check Lists

   When forming candidate pairs, the following types of candidates can
   be paired with each other:

   Local             Remote
   Candidate         Candidate
   tcp-so            tcp-so
   tcp-act           tcp-pass
   tcp-pass          tcp-act

   When the agent prunes the check list, it MUST also remove any pair
   for which the local candidate is tcp-pass.  Also NAT-assisted
   candidates MUST be pruned from the check list like server reflexive
   candidates when the same address is used also as an active host

   The remainder of check list processing works like in the UDP case.

7.  Connectivity Checks

7.1.  STUN Client Procedures

7.1.1.  Sending the Request

   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-act, the
   agent MUST open a connection from the interface associated with that
   local candidate.  This connection MUST be opened from an unallocated
   port.  For host candidates, this is readily done by connecting from

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   the candidates interface.  For relayed candidates, the agent uses
   procedures specific to the relaying 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.

   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.

   Once the connection is established, client procedures are identical
   to those for UDP candidates.  Note that STUN responses received on an
   active TCP candidate will typically produce a remote peer reflexive

7.2.  STUN Server Procedures

   An agent MUST be prepared to receive incoming TCP connection requests
   on any host, relayed, or UDP-tunneled 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 a regular selection algorithm.

   When ICE processing for a media stream completes, each agent SHOULD
   close all TCP connections except the ones between the candidate pairs

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   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 was 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.  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.  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

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   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 SHOULD 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 which 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 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 [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
   application layer protocol.

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

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   o  An ICE restart takes place, resulting in the selection of a
      different candidate pair.

   In these cases, the agent SHOULD close the connection when that event
   occurs.  This applies to both agents in a session, in which case
   usually one of the agents will end up closing the connection first.

   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, either on purpose or due to some
   error, then:

   o  If the agent's local candidate is tcp-act 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-pass, 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, 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 an 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 restart).

   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

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   has completed.  This happens irregardless 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 type of relaying

12.  Security Considerations

   The main threat in ICE is hijacking of connections for the purposes
   of directing media streams to DoS targets or to malicious users.  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 sips 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 the agent.

   A more detailed analysis of this attack and the various ways ICE
   prevents it are described in [RFC5245].  Those considerations apply
   to this specification.

13.  IANA Considerations

   There are no IANA considerations associated with this specification.

14.  Acknowledgements

   The authors would like to thank Tim Moore, Saikat Guha, Francois
   Audet, Roni Even, Simon Perreault, and Alfred Heggestad for the
   reviews and input on this document.

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15.  References

15.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [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,
              September 2005.

   [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.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

   [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, May 2010.

15.2.  Informative References

   [RFC1928]  Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
              L. Jones, "SOCKS Protocol Version 5", RFC 1928,
              March 1996.

   [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,
              October 2001.

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   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC4250]  Lehtinen, S. and C. Lonvick, "The Secure Shell (SSH)
              Protocol Assigned Numbers", RFC 4250, January 2006.

   [RFC4380]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through
              Network Address Translations (NATs)", RFC 4380,
              February 2006.

   [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.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [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, April 2010.

              Perreault, S. and J. Rosenberg, "Traversal Using Relays
              around NAT (TURN) Extensions for TCP Allocations",
              draft-ietf-behave-turn-tcp-07 (work in progress),
              July 2010.

              Cheshire, S., "NAT Port Mapping Protocol (NAT-PMP)",
              draft-cheshire-nat-pmp-03 (work in progress), April 2008.

              Warrier, U., Iyer, P., Pennerath, F., Marynissen, G.,
              Schmitz, M., Siddiqi, W., and M. Blaszczak, "Internet

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              Gateway Device (IGD) Standardized Device Control Protocol
              V 1.0", November 2001.

   [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 Measurement,

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 hand shake packets seen in 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 SYNACK is not processed properly.

   It has been reported in [IMC05] that with the population of NATs
   deployed at the time of the measurements (2005), simultaneous-open
   technique worked in roughly 45% of the cases.  Also, all operating
   systems do not implement TCP simultaneous-open properly and thus are
   not able to use such candidates.

   Alternatively, using unidirectional opens (where one side is active
   and the other is passive) is more reliable, but will commonly require
   a relay if both sides are behind different NATs.  Therefore, in the
   spirit of the ICE philosophy, both simultaneous-open and
   unidirectional candidates are tried.  Simultaneous-opens are
   preferred since, if it does work, it will not require a relay even
   when both sides are behind a different NAT.  Also, if/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.

   Finally, 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

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   implementation of which in traditional BSD socket APIs is non-

   In particular, ICE requires an agent to obtain a local TCP candidate,
   bound to a local IP and port, and then from that local port, initiate
   a TCP connection (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()
      set(sock_i, SO_REUSEADDR)
      bind(sock_i, local)

   connect(sock_1, stun)
   connect(sock_2, remote_a)
   connect(sock_3, remote_b)

   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.

Authors' Addresses

   Jonathan Rosenberg


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   Ari Keranen
   Hirsalantie 11
   02420 Jorvas


   Bruce B. Lowekamp


   Adam Roach
   17210 Campbell Rd.
   Suite 250
   Dallas, TX 75252


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