MMUSIC                                                      J. Rosenberg
Internet-Draft                                               dynamicsoft
Expires: January 17, 2005                                  July 19, 2004

    Interactive Connectivity Establishment (ICE): A Methodology for
   Network Address Translator (NAT) Traversal for Multimedia Session
                        Establishment Protocols

Status of this Memo

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   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance with
   RFC 3668.

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   This document describes a methodology for Network Address Translator
   (NAT) traversal for multimedia session signaling protocols, such as
   the Session Initiation Protocol (SIP).  This methodology is called
   Interactive Connectivity Establishment (ICE).  ICE makes use of
   existing protocols, such as Simple Traversal of UDP Through NAT
   (STUN) and Traversal Using Relay NAT (TURN).  ICE makes use of STUN
   in peer-to-peer cooperative fashion, allowing participants to
   discover, create and verify mutual connectivity.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Multimedia Signaling Protocol Abstraction  . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Overview of ICE  . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Detailed ICE Algorithm . . . . . . . . . . . . . . . . . . . . 10
     5.1   Initiator Processing . . . . . . . . . . . . . . . . . . . 10
       5.1.1   Sending the Initiate Message . . . . . . . . . . . . . 10
       5.1.2   Processing the Accept  . . . . . . . . . . . . . . . . 10
     5.2   Responder Processing . . . . . . . . . . . . . . . . . . . 11
       5.2.1   Processing the Initiate Message  . . . . . . . . . . . 11
     5.3   Common Procedures  . . . . . . . . . . . . . . . . . . . . 12
       5.3.1   Gathering Transport Addresses  . . . . . . . . . . . . 12
       5.3.2   Enabling STUN on Each Local Transport Address  . . . . 13
       5.3.3   Prioritizing the Transport Addresses and Choosing
               a Default  . . . . . . . . . . . . . . . . . . . . . . 14
       5.3.4   Sending STUN Connectivity Checks . . . . . . . . . . . 16
       5.3.5   Receiving STUN Requests  . . . . . . . . . . . . . . . 21
   6.  Running STUN on Derived Transport Addresses  . . . . . . . . . 23
     6.1   STUN on a TURN Derived Transport Address . . . . . . . . . 23
     6.2   STUN on a STUN Derived Transport Address . . . . . . . . . 24
   7.  XML Schema for ICE Messages  . . . . . . . . . . . . . . . . . 26
   8.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     8.1   Port Restricted  . . . . . . . . . . . . . . . . . . . . . 29
   9.  Mapping ICE into SIP . . . . . . . . . . . . . . . . . . . . . 32
   10.   Security Considerations  . . . . . . . . . . . . . . . . . . 34
   11.   IANA Considerations  . . . . . . . . . . . . . . . . . . . . 35
   12.   IAB Considerations . . . . . . . . . . . . . . . . . . . . . 36
     12.1  Problem Definition . . . . . . . . . . . . . . . . . . . . 36
     12.2  Exit Strategy  . . . . . . . . . . . . . . . . . . . . . . 36
     12.3  Brittleness Introduced by ICE  . . . . . . . . . . . . . . 37
     12.4  Requirements for a Long Term Solution  . . . . . . . . . . 38
     12.5  Issues with Existing NAPT Boxes  . . . . . . . . . . . . . 38
   13.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 39
   14.   References . . . . . . . . . . . . . . . . . . . . . . . . . 40
   14.1  Normative References . . . . . . . . . . . . . . . . . . . . 40
   14.2  Informative References . . . . . . . . . . . . . . . . . . . 40
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 41
       Intellectual Property and Copyright Statements . . . . . . . . 42

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1.  Introduction

   A multimedia session signaling protocol is a protocol that exchanges
   control messages between a pair of agents for the purposes of
   establishing the flow of media traffic between them.  This media flow
   is distinct from the flow of control messages, and may take a
   different path through the network.  Examples of such protocols are
   the Session Initiation Protocol (SIP) [3], the Real Time Streaming
   Protocol (RTSP) [5] and the International Telecommunications Union
   (ITU) H.323.

   These protocols, by nature of their design, are difficult to operate
   through Network Address Translators (NAT).  Because their purpose in
   life is to establish a flow of packets, they tend to carry IP
   addresses within their messages, which is known to be problematic
   through NAT [6].  The protocols also seek to create a media flow
   directly between participants, so that there is no application layer
   intermediary between them.  This is done to reduce media latency,
   decrease packet loss, and reduce the operational costs of deploying
   the application.  However, this is difficult to accomplish through
   NAT.  A full treatment of the reasons for this is beyond the scope of
   this specification.

   Numerous solutions have been proposed for allowing these protocols to
   operate through NAT.  These include Application Layer Gateways
   (ALGs), the Middlebox Control Protocol [7], Simple Traversal of UDP
   through NAT (STUN) [1], Traversal Using Relay NAT [16], Realm
   Specific IP [8][9], symmetric RTP [10], along with session
   description extensions needed to make them work, such as [2].
   Unfortunately, these techniques all have pros and cons which make
   each one optimal in some network topologies, but a poor choice in
   others.  The result is that administrators and implementors are
   making assumptions about the topologies of the networks in which
   their solutions will be deployed.  This introduces a lot of
   complexity and brittleness into the system.  What is needed is a
   single solution which is flexible enough to work well in all

   This specification provides that solution.  It is called Interactive
   Connectivity Establishment, or ICE.  ICE makes use of many of the
   protocols above, but uses them in a specific methodology which avoids
   many of the pitfalls of using any one alone.  ICE uses STUN and TURN
   without extension, and allows for other similar protocols to be used
   as well.  However, it does require additional signaling capabilities
   to be introduced into the multimedia session signaling protocols.
   For those protocols which make use of the Session Description
   Protocol (SDP), this specification defines the necessary extensions
   to it.  Other protocols will need to define their own mechanisms.

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2.  Multimedia Signaling Protocol Abstraction

   This specification defines a general methodology that allows the
   media streams of multimedia signaling protocols to successfully
   traverse NAT.  This methodology is independent of any particular
   signaling protocol.  In order to discuss the methodology, we need to
   to define an abstraction of a multimedia signaling system, and define
   terms that can be used throughout this specification.  Figure 1 shows
   the abstraction.

                               |           |
                               |           |
                             > | Signaling |\
                            /  | Relay     | \
                           /   |           |  \
                Initiate  /    |           |   \   Initiate
               Message   /   / +-----------+    \  Message
                        /   /                <   \
                       /   /                  \   \
                      /   /                    \   \
                     /   / Accept       Accept  \   \
                    /   /   Message     Message  \   >
                   /   /                          \
       +-----------+  /                            \   +-----------+
       |           | <                                 |           |
       |           |             Media Stream          |           |
       |  Session  | ................................  | Session   |
       | Initiator |                                   | Responder |
       |           |             Media Stream          |           |
       |           | ................................  |           |
       +-----------+                                   +-----------+

                                Figure 1

   Communications occur between two clients - the session initiator and
   the session responder, also referred to as the initiator and
   responder.  The initiator is the one that decides to engage in
   communications.  To do so, it sends an initiate message.  The
   initiate message contains parameters that describe the capabilities
   and configuration of media streams for the initiator.  This message
   may travel through signaling intermediaries, called a signaling
   relay, before finally arriving at the session responder.  Assuming
   the session responder wishes to communicate, it generates an accept
   message, which is relayed back to the initiator.  This message
   contains capabilities and configuration of media streams for the

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   responder.  As a result, media streams are established between the
   initiator and responder.  The signaling protocol may also support an
   operation that allows for termination of the communications session.
   We refer to this signaling message as a terminate message.

   This abstraction is readily mapped to SIP, RTSP, and H.323, amongst
   others.  For SIP, the initiator is the User Agent Client (UAC), the
   responder is the User Agent Server (UAS), the initiate message is a
   SIP message containing an SDP offer (for example, an INVITE), the
   accept message is a SIP message containing an SDP answer (for
   example, a 200 OK), and the terminate message is a BYE.  For RTSP,
   the initiator is the RTSP client, the responder is the RTSP server,
   the initiate message is a SETUP message, and the accept message is a
   SETUP response.

   This specification defines parameters that need to be included in
   these various signaling messages in order to implement the
   functionality described by ICE.  Those parameters are represented in
   XML for convenience.  Any multimedia signaling protocol that uses ICE
   will need to define how to map those parameters into its own protocol
   messages.  Section 9 provides such a mapping for SIP.

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3.  Terminology

   Several new terms are introduced in this specification:
   Session Initiator: A software or hardware entity that, at the request
      of a user, tries to establish communications with another entity,
      called the session responder.  A session initiator is also called
      an initiator.
   Initiator: Another term for a session initiator.
   Session Responder: A software or hardware entity that receives a
      request for establishment of communications from the session
      initiator, and either accepts or declines the request.  A session
      responder is also called a responder.
   Responder: Another term for a session responder.
   Client: Either the initiator or responder.
   Peer: From the perspective of one of the clients in a session, its
      peer is the other client.  Specifically, from the perspective of
      the initiator, the peer is the responder.  From the perspective of
      the responder, the peer is the initiator.
   Signaling Relay: An intermediary of signaling messages.  Examples are
      SIP proxies and H.323 Gatekeepers.
   Initiate Message: The signaling message used by an initiator to
      establish communications.  It contains capabilities and other
      information needed by the responder to send media to the
   Accept Message: The signaling message used by a responder to agree to
      communications.  It contains capabilities and other information
      needed by the initiator to send media to the responder.
   Terminate Message The signaling message used by a client to terminate
      the session and associated media streams.
   Transport Address: The combination of an IP address and port.
   Local Transport Address: A local transport address is a transport
      address that has been allocated from the operating system on the
      host.  This includes transport addresses obtained through Virtual
      Private Networks (VPNs) and transport addresses obtained through
      Realm Specific IP (RSIP) [8] (which lives at the operating system
      level).  Transport addresses are typically obtained by binding to
      an interface.
   Derived Transport Address: A derived transport address is a transport
      address which is associated with, but different from, a local
      transport address.  The derived transport address is associated
      with the local transport address in that packets sent to the
      derived transport address are received on the socket bound to that
      local transport address.  Derived addresses are obtained using
      protocols like STUN and TURN, and more generally, any UNSAF
      protocol [11].

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   Peer Derived Transport Address: A peer derived transport address is a
      derived transport address learned from a STUN server running
      within a peer in a media session.
   TURN Derived Transport Address: A derived transport address obtained
      from a TURN server.
   STUN Derived Transport Address: A derived transport address obtained
      from a STUN server whose address has been provisioned into the UA.
      This, by definition, excludes Peer Derived Transport Addresses.
   Unilateral Allocations: Queries made to a network server which
      provides an UNSAF service.
   Bilateral Allocations: Addresses obtained by using an UNSAF service
      that actually runs on the peer of the communications session.
      Peer derived transport addresses are synonymous with bilateral

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4.  Overview of ICE

   ICE makes the fundamental assumption that clients exist in a network
   of segmented connectivity.  This segmentation is the result of a
   number of addressing realms in which a client can simultaneously be
   connected.  We use "realms" here in the broadest sense.  A realm is
   defined purely by connectivity.  Two clients are in the same realm
   if, when they exchange the addresses each has in that realm, they are
   able to send packets to each other.  This includes IPv6 and IPv4
   realms, which actually use different address spaces, in addition to
   private networks connected to the public Internet through NAT.

   The key assumption in ICE is that a client cannot know, apriori,
   which address realms it shares with any peer it may wish to
   communicate with.  Therefore, in order to communicate, it has to try
   connecting to addresses in all of the realms.

   Before the initiator establishes a session, it obtains as many IP
   address and port combinations in as many address realms as it can.
   These adresses all represent potential points at which the initiator
   will receive a specific media stream.  Any protocol that provides a
   client with an IP address and port on which it can receive traffic
   can be used.  These include STUN, TURN, RSIP, and even a VPN.  The
   client also uses any local interface addresses.  A dual-stack v4/v6
   client will obtain both a v6 and a v4 address/port.  The only
   requirement is that, across all of these addresses, the initiator can
   be certain that at least one of them will work for any responder it
   might communicate with.  Unfortunately, if the initiator communicates
   with a peer that doesn't support ICE, only one address can be
   provided to that peer.  As such, the client will need to choose one
   default address, which will be used by non-ICE clients.  This would
   typically be a TURN derived transport address, as it is most likely
   to work with unknown non-ICE peers.

   The initiator then runs a STUN server on each of the local transport
   addresses it has obtained.  The initiator will need to be able to
   demultiplex STUN messages and media messages received on that IP
   address and port, and process them appropriately.  All of these
   addresses are placed into the initiate message, and they are ordered
   in terms of preference.  Preference is a matter of local policy, but
   typically, lowest preference would be given to transport addresses
   learned from a TURN server (i.e., TURN derived transport addresses).
   The initiate message also conveys the STUN username and password
   which are required to gain access to the STUN server on each address/
   port combination.

   The initiate message is sent to the responder.  This specification
   does not address the issue of how the signaling messages themselves

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   traverse NAT.  It is assumed that signaling protocol specific
   mechanisms are used for that purpose.  The responder follows a
   similar process as the initiator followed; it obtains addresses from
   local interfaces, STUN servers, TURN servers, etc., and it places all
   of them into the accept message.

   Once the responder receives the initiate message, it has a set of
   potential addresses it can use to communicate with the initiator.
   The initiator will be running a STUN server at each address.  The
   responder sends a STUN request to each address, in parallel.  When
   the initiator receives these, it sends a STUN response.  If the
   responder receives the STUN response, it knows that it can reach its
   peer at that address.  It can then begin to send media to that
   address.  As additional STUN responses arrive, the responder will
   learn about additional transport addresses which work.  If one of
   those has a higher priority than the one currently in use, it starts
   sending media to that one instead.  No additional control messages
   (i.e., SIP signaling) occur for this change.

   The STUN messages described above happen while the accept message is
   being sent to the intitiator.  Once the intitiator receives the
   accept message, it too will have a set of potential addresses with
   which it can communicate to the responder.  It follows exactly the
   same process described above.

   Furthermore, when a either the initiator or responder receives a STUN
   request, it takes note of the source IP address and port of that
   request.  It compares that transport address to the existing set of
   potential addresses.  If it's not amongst them, it gets added as
   another potential address.  The incoming STUN message provides the
   client with enough context to associate that transport address with a
   STUN username, STUN password, and priority, just as if it had been
   sent in an initiate or accept message.  As such, the client begins
   sending STUN messages to it as well, and if those succeed, the
   address can be used if it has a higher priority.

   After a successful STUN transaction, the client will re-perform the
   STUN query periodically to revalidate connectivity.  This allows for
   recovery from NAT failures, or from route flaps which may cause
   packets to suddenly traverse a different NAT.  As such, the address
   used as the destination for media is the highest priority address to
   which connectivity currently exists.

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5.  Detailed ICE Algorithm

   This section describes the detailed processing needed for ICE.

5.1  Initiator Processing

5.1.1  Sending the Initiate Message

   When the initiator wishes to begin communications, it starts by
   gathering transport addresses, as described in Section 5.3.1, and
   starting a STUN server on each local transport address, as described
   in Section 5.3.2.  This process can actually happen at any time
   before sending an initiate message.  A client can pre-gather
   transport addresses, using a user interface cue (such as picking up
   the phone, or entry into an address book) as a hint that
   communications is imminent.

   When it comes time to initiate communications, it determines a
   priority for each one and identifies one as a default, as described
   in Section 5.3.3.

   The next step is to construct the initiate message.  Section 7
   provides the XML schema for the initiate message.  The message
   consists of a series of media streams.  For each media stream, there
   is a default address and a list of alternates.  The default address
   is the one that will be used by responders that don't understand ICE
   (for SIP, this is accomplished by mapping the default address into
   the m and c line in the SDP).  The alternates represent addresses
   that the responder should also try.  In SIP, these are conveyed with
   the new SDP alt parameter.

   The client then encodes all of its available transport addresses
   (including the default) as a series of alternate elements.  Each
   alternate element conveys a transport address for RTP, one for RTCP,
   a STUN username fragment and STUN password.  The client MUST assign
   each alternate a unique identifier.  These identifiers MUST be unique
   across all alternates used within the session.  This identifier is
   encoded in the "id" attribute of the alternate element.  The priority
   for the transport address, as computed above, is included as an
   attribute as well.

   Once the initiate message is constructed, it is sent.

5.1.2  Processing the Accept

   There are two possible cases for processing of the Accept message.
   If the recipient of the Initiate message did not support ICE, the
   Accept message will only contain the default address information.  As

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   a result, the initiator knows that it cannot perform its connectivity
   checks.  In this case, it SHOULD just send to the transport address
   listed.  However, if local configuration information tells the
   initiator to try connectivity checks by sending them through the TURN
   server, this means that packets sent directly to responder may be
   dropped by a local firewall.  To deal with this, the initiator SHOULD
   issue a SEND command using this new transport address.  The SEND
   command contains the media packet to send to the responder.  Once
   this command has been accepted, the initiator SHOULD send all media
   packets to the TURN server, which will then forward them towards the

   If the Accept message contains alternates, it implies that the
   responder supported ICE.  In that case, the initiator takes each
   transport address, STUN username, STUN password and priority, and
   places them into a list, called the candidate list.  It then begins
   processing the candidate list as described in Section 5.3.4.  That
   processing associates a state with each transport address.  As
   described there, once a successful STUN query is made to the STUN
   server at an address, the initiator can begin sending media to that

5.2  Responder Processing

5.2.1  Processing the Initiate Message

   Upon receipt of the initiate message, the client starts gathering
   transport addresses, as described in Section 5.3.1, and starts a STUN
   server on each local transport address, as described in Section
   5.3.2.  This processing is done immediately on receipt of the
   request, to prepare for the case where the user should accept the
   call, or early media needs to be generated.

   At some point, the responder will decide to accept or reject the
   communications.  A rejection terminates ICE processing, of course.
   In the case of acceptance, the accept message is constructed as

   The client first determines a priority for each transport address it
   has gathered, and identifies one as a default, as described in
   Section 5.3.3.

   Constructing the accept proceeds identically to the way in which the
   initiate message is constructed (Section 5.1.1).

   The accept is then sent.

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5.3  Common Procedures

   This section discusses procedures that are common between initiator
   and responder.

5.3.1  Gathering Transport Addresses

   A client gathers addresses when it believes that communications is
   imminent.  For initiators, this occurs before sending an initiate
   message (Section 5.1.1).  For responders, it occurs before sending a
   accept message (Section 5.2.1).

   There are two types of addresses a client can gather - local
   transport addresses, and derived transport addresses.  Local
   transport addresses are obtained by binding to an ephemeral port on
   an interface (physical or virtual) on the host.  A multi-homed host
   SHOULD attempt to bind on all interfaces for all media streams it
   wishes to receive.  For media streams carried using the Real Time
   Transport Protocol (RTP) [12], the client will need to bind to an
   ephemeral port for both RTP and RTCP.

   The result will be a set of local transport addresses.  The client
   may also have access to servers that provide unilateral self-address
   fixing (UNSAF) [11].  Examples of such protocols include STUN, TURN,
   and TEREDO [15].  All ICE implementations MUST implement STUN and
   TURN, but MAY, through configuration, disable the use of STUN or TURN
   for unilateral address allocation (STUN is mandatory for the
   connectivity checks described below).  When disabled, it MUST be
   possible through user or administrator operation to re-enable.  This
   allows all implementations to have the breadth of protocol support
   needed to work in all situations, with the flexibility to turn if off
   if its not needed.

   These protocols work by having the client send, from a specific local
   transport address, some kind of message to a server.  The server
   provides to the client, in some kind of response, an additional
   transport address, called a derived transport address.  This derived
   transport address is derived from the local transport address.  Here,
   derivation means that a request sent to the derived transport address
   might (under good network conditions) reach the client on its local
   transport address.

   For each of these protocols, the client may have access to a
   multiplicity of servers.  For example, a user connected to a natted
   cable access network might have access to a STUN server in the
   private cable network and in the public Internet.  For each local
   transport address, the client SHOULD obtain an address from every
   server for each protocol it supports.  The result of this will be a

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   set of derived transport addresses, with each derived address
   associated with the local transport address it is derived from.

5.3.2  Enabling STUN on Each Local Transport Address

   Once the client has obtained a set of transport addresses, it starts
   a STUN server on each local transport address (including ones used
   for RTCP).  This, by definition, means that the STUN service will be
   reached for requests sent to the derived addresses.

   However, the client does not need to provide STUN service on any
   other IP address or port, unlike the STUN usage described in [1].
   The need to run the service on multiple ports is to support the
   change flags.  However, those flags are not needed with ICE, and the
   server SHOULD reject, with a 400 response, any STUN requests with
   these flags set.

   Furthermore, there is no need to support TLS or to be prepared to
   receive SharedSecret request messages.  Those messages are used to
   obtain shared secrets to be used with BindingRequests.  However, with
   ICE, usernames and passwords are exchanged in the signaling protocol.

   The client will receive both STUN requests and media packets on each
   local transport address.  The client MUST be able to disambiguate
   them.  In the case of RTP/RTCP, this disambiguation is easy.  RTP and
   RTCP packets start with the bits 0b10 (v=2).  The first two bits in
   STUN are always 0b00.  This disambiguation also works for packets
   sent using Secure RTP [13], since the RTP header is in the clear.
   Disambiguating STUN with other media stream protocols may be more
   complicated.  However, it can always be possible with arbitrarily
   high probabilities by selecting an appropriately random username (see

   The need to run STUN on the same transport address as the media
   stream represents the "ugliest" piece of ICE.  However, it is an
   essential part of the story.  By sending STUN requests to the very
   same place media is sent, any bindings learned through STUN will be
   useful even when communicating through symmetric NATs.  This results
   in a substantial increase in the scope of applicability of STUN.

   For each local transport address where a STUN server is running, the
   client MUST choose a username fragment and a password.  The username
   fragment created by the client will be concatenated with the fragment
   created by its peer.  The result will serve as the username provided
   by its peer in STUN requests.  By creating the username as a
   combination of information from each side of a call, it allows a
   client to correlate the source of the request with a candidate
   transport address.  This is discussed further below.

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   The username fragment MUST be globally unique, so that no other host
   will select a username with the same value.  This username fragment
   and password will be passed to its peer in an initiate or accept
   message.  As such, the process described in this section will
   associate, with each local transport address, a username fragment and
   password.  The client also associates this same username fragment and
   password with any transport addresses derived from the local
   transport address.

   The global uniqueness requirement stems from the lack of uniquenes
   afforded by IP addresses.  Consider clients A, B, and C.  A and B are
   within private enterprise 1, which is using  C is within
   private enterprise 2, which is also using  As it turns
   out, B and C both have IP address  A initiates
   communications to C.  C, in its accept message, provides A with its
   transport addresses.  In this case, thats and 8877.  As
   it turns out, B is in a session at that same time, and is also using and 8877.  This means that B has a STUN server running
   on those ports, just as C does.  A will send a STUN request to and 8877.  However, these do not go to C as expected.
   Instead, they go to B.  If B just replied to them, A would believe it
   has connectivity to C, when in fact it has connectivity to a
   completely different user, B.  To fix this, the STUN username takes
   on the role of a unique identifier.  C provides A with a unique
   username.  A uses this username in its STUN query to
   This STUN query arrives at B.  However, the username is unknown to B,
   and so the request is rejected.  A treats the rejected STUN request
   as if there were no connectivity to C (which is actually true).
   Therefore, the error is avoided.

   Once the STUN server is started, it MUST run continuously until the
   session is completed.  While the server is running, it MUST act as a
   normal STUN server, but MUST only accept STUN requests from clients
   that authenticate, as discussed below in Section 5.3.5

5.3.3  Prioritizing the Transport Addresses and Choosing a Default

   The prioritization process takes a list of transport addresses, and
   associates each with a priority.  This priority reflects the desire
   that the UA has to receive media on that address, and is assigned as
   a value from 0 to 1 (1 being most preferred).  Priorities are
   ordinal, so that their significance is only relative to other
   transport address priorities in the same list.

   This specification makes no normative recommendations on how the
   prioritization is done.  However, some useful guidelines are
   suggested on how such a prioritization can be determined.

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   One criteria for choosing one transport address over another is
   whether or not that transport address involves the use of a relay.
   That is, if media is sent to that transport address, will the media
   first transit a relay before being received.  TURN derived transport
   addresses make use of relays (the TURN server), as to any local
   transport addresses associated with a VPN server.  When media is
   transited through a relay, it can increase the latency between
   transmission and reception.  It can increase the packet losses,
   because of the additional router hops that may be taken.  It may
   increase the cost of providing service, since media will be routed in
   and right back out of a relay run by the provider.  If these concerns
   are important, transport addresses with this property can be listed
   with lower priority.

   Another criteria for choosing one address over another is IP address
   family.  ICE works with both IPv4 and IPv6.  It therefore provides a
   transition mechanism that allows dual-stack hosts to prefer
   connectivity over IPv6, but to fall back to IPv4 in case the v6
   networks are disconnected (due, for example, to a failure in a 6to4
   relay) [14].  It can also help with hosts that have both a native
   IPv6 address and a 6to4 address.  In such a case, higher priority
   could be afforded to the native v6 address, followed by the 6to4
   address, followed by a native v4 address.  This allows a site to
   obtain and begin using native v6 addresss immediately, yet still
   fallback to 6to4 addresses when communicating with clients in other
   sites that do not yet have native v6 connectivity.

   Another criteria for choosing one address over another is security.
   If a user is a telecommuter, and therefore connected to their
   corporate network and a local home network, they may prefer their
   voice traffic to be routed over the VPN in order to keep it on the
   local network when communicating within the enterprise, but use the
   local network when communicating with users outside of the

   Another criteria for choosing one address over another is topological
   awareness.  This is most useful for transport addresses which make
   use of relays (including TURN and VPN).  In those cases, if a client
   has preconfigured or dynamically discovered knowledge of the
   topological proximity of the relays to itself, it can use that to
   select closer relays with higher priority.

   Once the transport addresses have been prioritized, one is selected
   as the default.  This is the address that will be used by a peer that
   doesn't understand ICE.  The default has no relevance when
   communicating with an ICE capable peer.  As such, it is RECOMMENDED
   that the default be chosen based on the likelihood of that address
   being useful when communicating with a peer that doesn't support ICE.

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   This will frequently be a TURN derived transport address from a TURN
   server providing public IP addresses.

5.3.4  Sending STUN Connectivity Checks

   Once a responder has received an initiate message, or an initiator
   has received an accept message, the list of transport addresses is
   extracted from the message.  These transport addresses, called the
   remote transport addresses, along with the username fragment,
   password, and priority from the message are placed into a table,
   called the candidate table.  There is a candidate table for RTP for
   each media stream, and for RTCP for each media stream.  So, if a
   session is established with audio and video, there would be four
   tables - audio RTP, audio RTCP, video RTP and video RTCP.

   The client then takes its own gathered addresses, and creates a
   subset called the sourceable addresses.  This subset is the set of
   local transport addresses (including VPN and RSIP) and TURN derived
   transport addresses.  Thus, it excludes STUN derived transport
   addresses.  The formal definition of this subset is defined below.

   Each row in this table is then replicated once for each sourceable
   transport address.  The table has a column for the sourceable
   transport address value, and this is populated upon replication.
   That table also has a column called "my username fragment", which is
   the username fragment that the client created for sourceable
   transport address in that row.  Each row in this table is called a

   Each candidate is associated with a state.  The state represents the
   current understanding of connectivity to that remote transport
   address when packets are sent from that sourceable address.  There
   are five possible states.  These states are:
   INIT: No STUN transaction has been completed towards this remote
      transport address from this sourceable address.
   HANDSHAKING: One or more STUN transactions have failed, but
      insufficient time has passed since leaving the INIT state to be
      certain that the remote transport address is unreachable from this
      sourceable address.  This state is important for connectivity
      checks made to STUN derived transport addresses through port
      restricted NAT or a TURN derived transport address.
   BAD: All STUN transactions to this remote transport address from this
      sourceable address have either timed out, or failed with a 600
      response, and a sufficient amount of time has elapsed since the
      INIT state to have high confidence that the remote transport
      address cannot be reached from this sourceable address.

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   GOOD: The last STUN transaction to this remote transport address from
      this sourceable address was successful.  However, it is not the
      highest priority candidate, and therefore, is not in use for

   When the client first populates the tables from the initiate or
   accept message, all of the transport addresses are set to the INIT

   Consider the the following example.  An initiator sends an initiate
   message with one media stream (audio), with two transport addresses,
   A and B.  A is a local transport address, and B is a STUN derived
   transport address (although that fact is not signaled in the
   message).  Both of these will have the same username fragment and
   password, but different priorities.  The initiate message is sent to
   the responder.  The responder has a local transport address, a STUN
   derived transport address, and a TURN derived transport address.
   Call these X, Y and Z respectively.  Thus, it has two sourceable
   addresses, X and Z.  The table created by the responder would have
   four rows.  Each of the two transport addresses in the initiate
   message is present twice, once with the responder's local transport
   address, and once with its TURN derived address.  Such a table might
   look like this:

   Remote   Srcable  User Frag    Passwd   My-Usr-Frag  Priority  State
   A        X        asd9f8f8==   siprulz  x-frag       0.4       INIT
   A        Z        asd9f8f8==   siprulz  z-frag       0.4       INIT
   B        X        asd9f8f8==   siprulz  x-frag       0.2       INIT
   B        Z        asd9f8f8==   siprulz  z-frag       0.2       INIT

   The client begins a STUN BindingRequest transaction for each
   candidate.  This STUN transaction is sent to the IP address and port
   from the Remote column.  It sends the request from the IP address and
   port in the sourceable column.  For local transport addresses, that
   means sending from the locally bound socket.  For VPN addresses, that
   means sending from the socket bound to the VPN interface.  For TURN
   derived transport addresses, this means using the TURN Send message
   to send a request through the TURN server.  This provides the
   definition of the sourceable flag: they represent distinct transport
   addresses that a client can send from.  A STUN derived transport
   address is not distinct from a local transport address, since a
   client cannot send a packet to a particular IP address and port with
   different source IP addresses and ports as seen by that recipient

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   The STUN USERNAME attribute MUST be present.  It is set to the
   concatenation of the user fragment from the table, with the "My User
   Fragment" from the candidate.  Thus, for the candidate with remote
   transport address A and sourceable address X, the USERNAME would be
   set to "asd9f8f8==x-frag".  The BindingRequest SHOULD contain a
   MESSAGE-INTEGRITY attribute, computed using the username in the
   USERNAME attribute, and the password from the password field in the
   row.  The BindingRequest MUST NOT contain the CHANGE-REQUEST or
   RESPONSE-ADDRESS attribute.

   Each of these STUN transactions will generate either a timeout, or a
   response.  If the response is an error, but recoverable as described
   in RFC 3489, the client SHOULD try again using the procedures
   discussed there.  Either initialy, or after retry, the STUN
   transaction will produce a timeout result, a success result, or a
   non-recoverable failure result (error codes 400, 431, or 600).  These
   correspond to "timeout", "success", and "error" events, respectively.

   These events are fed into the state machine described in Figure 3.
   This figure shows the transitions between states that occur on the
   completion of the STUN BindingRequest transaction.  After the
   completion of each transaction, the client sets a timer that
   determines when it will do another transaction for that candidate.
   The result of that next transaction drives the next transition in the
   state machine, and so on.  Since timers are set at the entry to each
   state, STUN BindingRequest tranasactions will be tried continuously
   throughout a call.  This is necessary to detect a variety of failure
   cases, as discussed below.

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                                                   .        . timeout/
                                                   .        .  Set Rapid
           +---------+                       +---------+    .  Retry Timer
           |         |                       |         |    .
           |         |                       |         |<....
           |  INIT   |......................>|  HAND   |
           |         | timeout/              | SHAKING |
           |         |  Set Rapid            |         |
           +---------+  Retry Timer,  error/ +---------+
             .  .       Giveup Timer   Set     .    .
             .  .                     Retry    .    .
      error/ .  .                     Timer    .    .
       Set   .  .  .............................    . success/
      Retry  .  .  .                                .  Set Refresh
      Timer  .  ...C..............................  .  Timer
             .     .           success/          .  .
             .     .            Set Refresh      .  .
             V     V            Timer            V  V
           +---------+                       +---------+
           |         |                       |         |
           |         |                       |         |
           |  BAD    |......................>|  GOOD   |
       ...>|         |        success/       |         |.......
       .   |         |         Set Refresh   |         |      .
       .   +---------+         Timer         +---------+      .
       .     .   ^                              .    ^        .
       .     .   .                              .    .        .
       .......   .                              .    ..........
     timeout or  ................................        success/
     error/                   timeout or                  Set Refresh
      Set                     error/                      Timer
     Retry                     Set
     Timer                    Retry

                                Figure 3

   Starting in the INIT state, if the transaction is successful, the
   client has verified connectivity to that remote transport address
   when sending from that sourceable transport address.  This means that
   media packets sent in exactly the same way will get through.  As
   such, the FSM transitions to the GOOD state, and the client sets the
   Refresh Timer.  This timer is used to continually check that a good
   candidate remains good.  It is possible for a candidate to cease
   being good if a NAT should fail and recover, resulting in loss of any
   bindings it holds, or if an IP route should flap, causing those

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   packets to be delivered through a new NAT that allocates new
   bindings, or a firewall with different policies.  The Retry Timer
   value SHOULD be configurable.  In order to rapidly recover from
   failures, it is RECOMMENDED that it default to five seconds.  [[TODO:
   Need to work this number as a function of codec rates as well,
   perhaps apply the RTCP algorithm for its computation.]]

   If, from the INIT state, the STUN transaction times out, the FSM
   enters the HANDSHAKE state.  At this point, there are two reasons
   that the STUN request might have timed out.  One reason is that the
   candidate is simply unreachable.  The other reason is that the peer
   is behind a port restricted NAT, and so STUN requests from the client
   cannot get through until its peer creates a permission by generating
   its own STUN request.  It may take some time to generate that STUN
   request, as it may depend on a response message getting delivered.
   As such, the HANDSHAKE state allows for rapid retry of the STUN
   transaction until enough time has passed to be certain that the
   remote transport address is actually unreachable.  Thus, upon
   entering the HANDSHAKE state, two timers are set.  The first, called
   the Rapid Retry timer, determines how long until the next attempt.
   This timer SHOULD be configurable.  It is RECOMMENDED that it default
   to 1 second.  The second timer, called the Giveup Timer, determines
   how long the client will keep trying until it decides that the remote
   transport address is unreachable.  This timer SHOULD be configurable.
   It is RECOMMENDED that it default to 50 seconds.  This is a
   reasonable approximation of the maximum SIP transaction duration.

   If, from the INIT state, the STUN transaction generates an error, the
   FSM moves into the BAD state.  The retry timer is set.  This retry
   timer is used to periodically retry, and see if the candidate may now
   be reachable.  The value of this timer SHOULD be configurable.  It is
   RECOMMENDED that it default to 1 minute.

   If, while in the HANDSHAKE state, the Giveup timer fires, or the STUN
   transaction results in an error, the client moves into the BAD state,
   and sets the retry timer.  The default durations for ths timer are
   identical for all entries into the BAD state, and thus it defaults to
   1 minute here as well.  If, while in the HANDSHAKE state, the Rapid
   Retry timer fires, the timer is reset and the client remains in the
   HANDSHAKE state.

   If, while in the BAD state, the retried transaction is executed and
   fails or results in a timeout, the client resets the timer and
   remains in the BAD state.  If the STUN transaction succeeds, it moves
   into the GOOD state and sets the refresh timer.  The default
   durations for this timer are the same for all entries into the GOOD
   state, and thus it defaults to 1 second.

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   If while in the GOOD state, the transaction resulting from the
   refresh timer times out or fails, the client moves into the BAD state
   and sets the retry timer.  If, however, that transaction succeeds,
   the client stays in the GOOD state and resets the refresh timer.

   As the FSM operates throughout the call, candidates will move their
   states around.  At any point in time, the client sends media packets
   (including RTCP) using one of the candidates in the GOOD state.  It
   is RECOMMENDED that the one with highest priority be used.  It
   another candidate should change state such that it moves into the
   GOOD state, and it has a higher priority, the client SHOULD switch to
   that candidate, but SHOULD do so after waiting a small period of time
   (10 seconds is RECOMMENDED) to prevent against flapping of candidates
   during periods of route flaps in the network.

   To send media to a candidate, the client sends media packets (whether
   they are RTP or RTCP or something else) to the remote transport
   address, from the sourceable transport address.

   If, for some reason, there was at least one candidate in the GOOD
   state, and due to an FSM transition, none of the candidates are in
   the GOOD state, the client SHOULD forcefully transition all of the
   candidates into the HANDSHAKE state in an attempt to rapidly
   reconnect.  If none of them succeed, and all of the candidates enter
   the BAD state, the client SHOULD terminate the call and alert the
   user to the failure [[TODO: Need to work in some good congestion
   control here; in cases where timeouts happen due to network
   congestion this is probably too agressive]].

5.3.5  Receiving STUN Requests

   When a client receives a STUN request (presumably after
   disambiguating it from a media packet), it follows the logic
   described in this section.

   The client MUST follow the procedures defined in RFC 3489 and verify
   that the USERNAME attribute is known to the server.  Here, this is
   done by taking the USERNAME attribute, and doing a prefix match
   against the "my user fragment" column in the candidate table.  If it
   doesn't match any rows, the client generates a 432 response.  If it
   matches multiple rows, the client checks the suffix of the username
   against the "user fragment" column.  If it doesn't match any rows,
   the client generates a 432 response.  If it does match rows, it will
   match those rows corresponding to the transport addresses that the
   peer could have sent this STUN request from.

   Assuming the USERNAME is valid, the client MUST generate a STUN
   response per RFC 3489.

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   Once the response is sent, the client examines the source IP and port
   where the request came from.  It matches those against the remote
   transport addresses in the candidate table.  If there is no match,
   this source address is itself another possible candidate.  As with
   other candidates, it must be associated with a STUN username
   fragment, password and priority, all normally provided by the peer,
   along with sourceable transport addresses and their username

   How does the client obtain this other information? The suffix of the
   USERNAME is the key (literally).  That suffix was already provided to
   the client in an initiate or accept message, and was used to populate
   the current candidate table.  If it matches an existing value in the
   table, it means that the STUN request came from the same transport
   address as a previously advertised candidate; however, when it showed
   up at the client, its source IP address was different than the peer
   thought it would be.  This will happen when a symmetric NAT exists
   between the clients.  In this case, the source IP address and port of
   the STUN packet now become a viable candidate, since the client
   should be able to send messages back to it and reach its peer.

   However, this connectivity, like all other connectivity, needs to be
   verified.  So, the client needs to find out the user fragment and
   password to use in STUN requests.  To do that, it takes the suffix of
   the USERNAME in the STUN request, and looks it up in the "user frag"
   column of the table.  If its a match, that is the user fragment
   needed as part of the candidate.  The password is the value from that
   row.  The sourceable transport address is also the value from that
   row.  The priority is also copied from that row.

   This new candidate can then be verified by sending STUN requests to
   it, as described in Section 5.3.4.

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6.  Running STUN on Derived Transport Addresses

   One of the seemingly bizarre operations done during the ICE
   processing is the transmission of a STUN request to a transport
   address which is obtained through TURN or STUN itself.  This actually
   does work, and in fact, has extremely useful properties.  The
   subsections below go through the detailed operations that would occur
   at each point to demonstrate correctness and the properties derived
   from it.

6.1  STUN on a TURN Derived Transport Address

   Consider a client A that is behind a NAT.  It connects to a TURN
   server on the public side of the NAT.  To do that, A binds to a local
   transport address, say, and then sends a TURN request
   to the TURN server.  The NAT translates the net-10 address to  Assume that the TURN server is running on
   and listening for TURN traffic on port 7764.  The TURN server
   allocates a derived transport address to the client,
   and returns it in the TURN response.  Remember that all traffic from
   the TURN server to the client is sent from to

   Now, the client runs a STUN server on, and advertises
   that its server actually runs on  Another client, B,
   sends a STUN request to this server.  It sends it from a local
   transport address,  When it arrives at, the TURN server "locks down" outgoing traffic, so
   that data packets received from A are sent to  The
   STUN request is then forwarded to the client, sent with a source
   address of and a destination address of  This passes through the NAT, which rewrites the
   source address to  This arrives at A's STUN server.
   The server observes the source address of, and
   generates a STUN response containing this value in the MAPPED-ADDRESS
   attribute.  The STUN response is sent with a source address fo, and a destination of  This arrives at
   the TURN server, which, because of the lock-down, sends the STUN
   response with a source address of and destination of, which is B's STUN client.

   Now, as far as B is concerned, it has obtained a new STUN derived
   transport address of  And indeed, it has! STUN
   derived transport addresses are scoped to the session, so they can
   only be used by the peer in the session.  Furthermore, that peer has
   to send requests from the socket on which the STUN server was
   running.  In this case, A is the peer, and its STUN server was on  If it sends to, the packet goes to the

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   TURN server, and due to lock-down, is forwarded to B, and
   specifically, is forwarded to the transport address B sent the STUN
   request from.  Therefore, the address is indeed a valid STUN derived
   transport address.

   The benefit of this is that it allows two clients to share the same
   TURN server for media traffic in both directions.  With "normal" TURN
   usage, both clients would obtain a derived address from their own
   TURN servers.  The result is that, for a single call, there are two
   bindings allocated by each side from their respective servers, and
   all four are used.  With ICE, that drops to two bindings allocated
   from a single server.  Of course, all four bindings are allocated
   initially.  However, once one of the clients begins receiving media
   on its STUN derived address, it can deallocate its TURN resources.

   [[TODO: Include a diagram that shows this pictorially.]]

6.2  STUN on a STUN Derived Transport Address

   Consider a client A that is behind a NAT.  It connects to a STUN
   server on the public side of the NAT.  To do that, A binds to a local
   transport address, say, and then sends a STUN request
   to the STUN server.  The NAT translates the net-10 address to  Assume that the STUN server is running on
   and listening for STUN traffic on port 3478, the default STUN port.
   The STUN server sees a source IP address of, and
   returns that to the client in the STUN response.  The NAT forwards
   the response to the client.

   Now, the client runs a STUN server on, and advertises
   that its server actually runs on  Another client, B,
   sends a STUN request to this address.  It sends it from a local
   transport address,  When it arrives at (on the NAT), the NAT rewrites the source address to, assuming that it is of the full-cone variety [1], or
   is restricted, and the permission for is open.  This
   arrives at A's STUN server.  The server observes the source address
   of, and generates a STUN response containing this
   value in the MAPPED-ADDRESS attribute.  The STUN response is sent
   with a source address of, and a destination of  This arrives at B's STUN client.

   Now, as far as B is concerned, it has obtained a new STUN derived
   transport address of  Of course, this is the same
   address as the local transport address, and therefore this derived
   address is not used.  However, had there been additonal NATs between
   B and A's NAT, B would end up seeing the binding allocated by that
   outermost NAT.  The net result is that STUN requests sent to a STUN

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   derived address behave as normal STUN would.  However, these STUN
   requests have the side-effect of creating permissions in the NATs
   which see those requests in the public to private direction.  This
   turns out to be very useful for traversing restricted NATs.

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7.  XML Schema for ICE Messages

   This section contains the XML schema used to define the initiate,
   accept, and modify messages.  Any protocol that uses ICE needs to map
   the parameters defined here into its own messages.

   Note that STUN allows both the username and password to contain the
   space character.  However, usernames and passwords used with ICE
   cannot contain the space.

   <?xml version="1.0" encoding="UTF-8"?>
   <xs:schema targetNamespace="urn:ietf:params:xml:ns:ice"
    elementFormDefault="qualified" attributeFormDefault="unqualified">
    <xs:import namespace=""
    <xs:element name="message" type="tns:message"/>
    <xs:complexType name="message">
      <xs:documentation>This is the root element, which holds a
                media-streams elements.</xs:documentation>
      <xs:element name="media-streams" type="tns:media-streams"/>
     <xs:attribute name="type" type="tns:msg-type" use="required"/>
    <xs:complexType name="media-streams">
      <xs:element name="media-stream" minOccurs="0" maxOccurs="unbounded">
        <xs:documentation>There are zero or more media stream
                   elements. Each defines attributes for a specific media
         <xs:element name="default-address">
           <xs:documentation>The default address is used for
                            sending media before connectivity has been
            <xs:extension base="tns:rtp-info"/>

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          <xs:element name="alternate" minOccurs="0" maxOccurs="unbounded">
            <xs:documentation>Each alternate is a
                              possible point of contact.
             <xs:extension base="tns:transport-data">
              <xs:attribute name="preference" type="xs:double" use="required"/>
              <xs:attribute name="id" type="xs:string" use="required"/>
    <xs:simpleType name="msg-type">
     <xs:restriction base="xs:string">
      <xs:enumeration value="initiate"/>
      <xs:enumeration value="accept"/>
      <xs:enumeration value="modify"/>
    <xs:complexType name="transport-data">
      <xs:element name="stun-user-fragment" type="xs:string"/>
      <xs:element name="stun-password" type="xs:string"/>
      <xs:element name="rtp-address" type="tns:transport-address"/>
      <xs:element name="rtcp-address" type="tns:transport-address"/>
    <xs:complexType name="transport-address">
      <xs:element name="ip-address" type="xs:string"/>
      <xs:element name="port">
        <xs:restriction base="xs:integer">
         <xs:minInclusive value="1"/>
         <xs:maxInclusive value="65535"/>

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    <xs:complexType name="rtp-info">
      <xs:element name="rtp-address" type="tns:transport-address"/>
      <xs:element name="rtcp-address" type="tns:transport-address"/>

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8.  Examples

   In the examples that follow, messages are labeled with "message name
   A,B" to mean a message from transport address A to B.  For STUN
   Requests, this is followed by curly brackets enclosing the username
   and password.  For STUN responses, this is followed by square
   brackets and the value of MAPPED ADDRESS.

8.1  Port Restricted

   This section shows a flow of two clients behind port restricted NAT
   talking to each other.

             A        P.R. NAT     STUN+TURN    P.R. NAT         B
             |(1) STUN Req P1,S+T      |            |            |
             |----------->|            |            |            |
             |            |(2) STUN Req U, S+T      |            |
             |            |----------->|            |            |
             |            |(3) STUN Res S+T,U [U]   |            |
             |            |<-----------|            |            |
             |(4) STUN Res S+T,P1 [U]  |            |            |
             |<-----------|            |            |            |
             |(5) Intitiate {P1,unameA,passA,q=0.4} |            |
             |{U,unameA,passA,q=0.3}   |            |            |
             |            |            |            |(6) STUN Req P2,S+T
             |            |            |            |<-----------|
             |            |            |(7) STUN Req V, S+T      |
             |            |            |<-----------|            |
             |            |            |(8) STUN Res S+T,V [V]   |
             |            |            |----------->|            |
             |            |            |            |(9) STUN Res S+T,P2 [V]
             |            |            |            |----------->|
             |(10) Accept {P2,unameB,passB,q=0.4}   |            |
             |{V,unameB,passB,q=0.3}   |            |            |
             |(11) STUN Req P1,P2      |            |            |
             |(unameBunameA,passB)     |            |            |
             |----------->|            |            |            |
             |            |Timeout     |            |            |
             |(12) STUN Req P1,V       |            |            |
             |(unameBunameA,passB)     |            |            |
             |----------->|            |            |            |
             |            |(13) STUN Req U,V        |            |
             |            |(unameBunameA,passB)     |            |
             |            |------------------------>|            |
             |            |Permission open V->U     |            |

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             |            |            |            |No success, Retries continue
             |            |            |            |(14) STUN Req P2,P1
             |            |            |            |(unameAunameB,passA)
             |            |            |            |<-----------|
             |            |            |            |Timeout     |
             |            |            |            |(15) STUN Req P2,U
             |            |            |            |(unameAunameB,passA)
             |            |            |            |<-----------|
             |            |(16) STUN Req V,U        |            |
             |            |(unameAunameB,passA)     |            |
             |            |<------------------------|            |
             |            |            |            |Permission open U->V
             |            |Passes NAT! |            |            |
             |(17) STUN Req V,P1       |            |            |
             |(unameAunameB,passA)     |            |            |
             |<-----------|            |            |            |
             |(18) STUN Res P1,V [V]   |            |            |
             |----------->|            |            |            |
             |            |(19) STUN Res U,V [V]    |            |
             |            |------------------------>|            |
             |            |            |            |(20) STUN Res U,P2 [V]
             |            |            |            |----------->|
             |            |Retries continue         |            |
             |(21) STUN Req P1,V       |            |            |
             |(unameBunameA,passB)     |            |            |
             |----------->|            |            |            |
             |            |(22) STUN Req U,V        |            |
             |            |(unameBunameA,passB)     |            |
             |            |------------------------>|            |
             |            |            |            |Passes NAT! |
             |            |            |            |(23) STUN Req U,P2
             |            |            |            |(unameBunameA,passB)
             |            |            |            |----------->|
             |            |            |            |(24) STUN Res P2,U [U]
             |            |            |            |<-----------|
             |            |(25) STUN Res V,U [U]    |            |
             |            |<------------------------|            |
             |(26) STUN Res V,P1 [U]   |            |            |
             |<-----------|            |            |            |
             |(27) RTP P1,V            |            |            |
             |----------->|            |            |            |
             |            |(28) RTP U,V|            |            |
             |            |------------------------>|            |
             |            |            |            |Passes NAT! |
             |            |            |            |(29) RTP U,P2
             |            |            |            |----------->|
             |            |            |            |(30) RTP P2,U
             |            |            |            |<-----------|

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             |            |(31) RTP V,U|            |            |
             |            |<------------------------|            |
             |            |Passes NAT! |            |            |
             |(32) RTP V,P1            |            |            |
             |<-----------|            |            |            |

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9.  Mapping ICE into SIP

   In this section, we show how to map ICE into SIP.  This requires
   extensions to SDP.

   A new SDP attribute is defined to support ICE.  It is called "alt".
   The alt attribute MUST be present within a media block of the SDP.
   It contains an alternative IP address and port (or pair of IP
   addresses and ports in the case of RTP) that the recipient of the SDP
   can use instead of the ones indicated in the m and c lines.  There
   MAY be multiple alt attributes in a media block.  In that case, each
   of them MUST contain a different IP address and port (or a differing
   pair of IP address and ports in the case of RTP).

   The syntax of this attribute is:

   alt-attribute = "alt" ":" id SP qvalue SP
                     username SP password SP
                     unicast-address SP port [unicast-address SP port]
                     ;qvalue from RFC 3261
                     ;unicast-address, port from RFC 2327
   username      = non-ws-string
   password      = non-ws-string
   id            = token
   derived-from  = ":" / id

   With the addition of the alt attribute, the mapping of the ICE
   messages to SIP/SDP is straightforward.  The ICE initiate message
   corresponds to a SIP message with an SDP offer.  The ICE accept
   message corresponds to a SIP message with a SDP answer.  The ICE
   modify message corresponds to a SIP INVITE or UPDATE with an offer,
   and the ICE modify accept message corresponds to an INVITE or UPDATE
   response with an answer.

   Each media stream element in an ICE message maps to a media block in
   the SDP.  The default address maps to the m and c lines in the SDP.
   If the ICE message indicates an RTCP address and port that are not
   one higher than that of the RTP, the SDP RTCP attribute [2] MUST be
   used to convey them.

   Each alternate element in an ICE message maps either to an alt
   attribute in the SDP, or a new media block, depending on the IP
   version of the alternate.  For the highest priority IPv6 alternate,
   it is mapped into a separate media block, using the ANAT grouping
   [4].  Any additional IPv6 addresses are placed as alternates within
   this media block.  For alternates that are IPv4 addresses, the alt
   attribute is used.  The rtp-address element maps to the first

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   unicast-address and port components of the alt attribute.  The
   rtcp-address element maps to the second unicast-address and port
   components of the alt attribute.  Note that, if the RTCP address is
   identical to the RTP address, and the port is one higher, the second
   unicast-address and port MAY be omitted.  The preference value from
   the alternate element is mapped to the q-value component of the alt
   attribute.  The STUN user fragment and password elements map to the
   user fragment and password components of the alt attribute.

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10.  Security Considerations

   ICE conveys the STUN username and password within its messages.  If
   an eavesdropper should see the username and password, the worst they
   can do is send STUN requests to the host.  Since STUN is a stateless
   protocol, the attacker can not alter the processing of the call or
   otherwise disrupt it.  They could flood the server with
   BindingRequest packets.  However, this would be no worse than if the
   attacker simply floods the host with any kind of packet.

   However, integrity protection of the username and password are more
   important.  If an attacker is capable of intercepting the message and
   modifying the username or password, they could prevent connectivity
   from being established between peers, and therefore disrupt the call.
   Of course, if the attacker can intercept the message, there are many
   other ways in which they could do that, such as simply discarding the
   message.  Injecting fake messages with incorect usernames and
   passwords can also disrupt a call, and does not require the
   compromise of an intermediate server.  A similar attack is possible
   by modifying most of the ICE message attributes.  To prevent these
   kinds of attacks, it is RECOMMENDED that the actual protocols the ICE
   maps to make use of security mechanisms that provide message
   integrity protection.

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11.  IANA Considerations

   This specification defines one new media attribute: alt.  Its syntax
   is defined in Section 9.

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12.  IAB Considerations

   The IAB has studied the problem of "Unilateral Self Address Fixing",
   which is the general process by which a client attempts to determine
   its address in another realm on the other side of a NAT through a
   collaborative protocol reflection mechanism [11].  ICE is an example
   of a protocol that performs this type of function.  Interestingly,
   the process for ICE is not unilateral, but bilateral, and the
   difference has a signficant impact on the issues raised by IAB.  The
   IAB has mandated that any protocols developed for this purpose
   document a specific set of considerations.  This section meets those

12.1  Problem Definition

   From RFC 3424 any UNSAF proposal must provide:
      Precise definition of a specific, limited-scope problem that is to
      be solved with the UNSAF proposal.  A short term fix should not be
      generalized to solve other problems; this is why  "short term
      fixes usually aren't".

   The specific problems being solved by ICE are:
      Provide a means for two peers to determine the set of transport
      addresses which can be used for communication.
      Provide a means for resolving many of the limitations of other
      UNSAF mechanisms by wrapping them in an additional layer of
      processing (the ICE methodology).
      Provide a means for a client to determine an address that is
      reachable by another peer with which it wishes to communicate.

12.2  Exit Strategy

   From RFC 3424, any UNSAF proposal must provide:
      Description of an exit strategy/transition plan.  The better short
      term fixes are the ones that will naturally see less and less use
      as the appropriate technology is deployed.

   ICE itself doesn't easily get phased out.  However, it is useful even
   in a globally connected Internet, to serve as a means for detecting
   whether a router failure has temporarily disrupted connectivity, for
   example.  However, what ICE does is help phase out other UNSAF
   mechanisms.  ICE effectively selects amongst those mechanisms,
   prioritizing ones that are better, and deprioritizing ones that are
   worse.  Local IPv6 addresses are always the most preferred.  As NATs
   begin to dissipate as IPv6 is introduced, derived transport addresses
   from other UNSAF mechanisms simply never get used, because higher
   priority connectivity exists.  Therefore, the servers get used less
   and less, and can eventually be remove when their usage goes to zero.

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   Indeed, ICE can assist in the transition from IPv4 to IPv6.  It can
   be used to determine whether to use IPv6 or IPv4 when two dual-stack
   hosts communicate with SIP (IPv6 gets used).  It can also allow a
   client in a v6 island to communicate with a v4 host on the other side
   of a 6to4 NAT, by allowing the v6 host to address-fix against the v4
   host, and in the process, obtain a v4 address which can be handed to
   the v4 client.

12.3  Brittleness Introduced by ICE

   From RFC3424, any UNSAF proposal must provide:
      Discussion of specific issues that may render systems more
      "brittle".  For example, approaches that involve using data at
      multiple network layers create more dependencies, increase
      debugging challenges, and make it harder to transition.

   ICE actually removes brittleness from existing UNSAF mechanisms.  In
   particular, traditional STUN (the usage described in RFC 3489) has
   several points of brittleness.  One of them is the discovery process
   which requires a client to try and classify the type of NAT it is
   behind.  This process is error-prone.  With ICE, that discovery
   process is simply not used.  Rather than unilaterally assessing the
   validity of the address, its validity is dynamically determined by
   measuring connectivity to a peer.  The process of determining
   connectivity is very robust.  The only potential problem is that
   bilaterally fixed addresses through STUN can expire if traffic does
   not keep them alive.  However, that is substantially less brittleness
   than the STUN discovery mechanisms.

   Another point of brittleness in STUN, TURN, and any other unilateral
   mechanism is its absolute reliance on an additional server.  ICE
   makes use of a server for allocating unilateral addresses, but allows
   clients to directly connect if possible.  Therefore, in some cases,
   the failure of a STUN or TURN server would still allow for a call to
   progress when ICE is used.

   Another point of brittleness in traditional STUN is that it assumes
   that the STUN server is on the public Internet.  Interestingly, with
   ICE, that is not necessary.  There can be a multitude of STUN servers
   in a variety of address realms.  ICE will discover the one that has
   provided a usable address.

   The most troubling point of brittleness in traditional STUN is that
   it doesn't work in all network topologies.  In cases where there is a
   shared NAT between each client and the STUN server, traditional STUN
   may not work.  With ICE, that restriction can be lifted.

   Traditional STUN also introduces some security considerations.

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   Unfortunately, since ICE still uses network resident STUN servers,
   those security considerations still exist.

12.4  Requirements for a Long Term Solution

   From RFC 3424, any UNSAF proposal must provide:
      Identify requirements for longer term, sound technical solutions
      -- contribute to the process of finding the right longer term

   Our conclusions from STUN remain unchanged.  However, we feel ICE
   actually helps because we believe it can be part of the long term

12.5  Issues with Existing NAPT Boxes

   From RFC 3424, any UNSAF proposal must provide:
      Discussion of the impact of the noted practical issues with
      existing, deployed NA[P]Ts and experience reports.

   A number of NAT boxes are now being deployed into the market which
   try and provide "generic" ALG functionality.  These generic ALGs hunt
   for IP addresses,  either in text or binary form within a packet, and
   rewrite them if they match a binding.  This will interfere with
   proper operation of any UNSAF mechanism, including ICE.

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13.  Acknowledgements

   The authors would like to thank Douglas Otis and Francois Audet for
   their comments and input.

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14.  References

14.1  Normative References

   [1]  Rosenberg, J., Weinberger, J., Huitema, C. and R. Mahy, "STUN -
        Simple Traversal of User Datagram Protocol (UDP) Through Network
        Address Translators (NATs)", RFC 3489, March 2003.

   [2]  Huitema, C., "Real Time Control Protocol (RTCP) attribute in
        Session Description Protocol (SDP)", RFC 3605, October 2003.

   [3]  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.

   [4]  Camarillo, G., "The Alternative Network Address Types Semantics
        for the Session Description  Protocol Grouping Framework",
        draft-ietf-mmusic-anat-01 (work in progress), June 2004.

14.2  Informative References

   [5]   Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
         Protocol (RTSP)", RFC 2326, April 1998.

   [6]   Senie, D., "Network Address Translator (NAT)-Friendly
         Application Design Guidelines", RFC 3235, January 2002.

   [7]   Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A. and A.
         Rayhan, "Middlebox communication architecture and framework",
         RFC 3303, August 2002.

   [8]   Borella, M., Lo, J., Grabelsky, D. and G. Montenegro, "Realm
         Specific IP: Framework", RFC 3102, October 2001.

   [9]   Borella, M., Grabelsky, D., Lo, J. and K. Taniguchi, "Realm
         Specific IP: Protocol Specification", RFC 3103, October 2001.

   [10]  Yon, D., "Connection-Oriented Media Transport in the Session
         Description Protocol  (SDP)", draft-ietf-mmusic-sdp-comedia-07
         (work in progress), June 2004.

   [11]  Daigle, L. and IAB, "IAB Considerations for UNilateral
         Self-Address Fixing (UNSAF) Across Network Address
         Translation", RFC 3424, November 2002.

   [12]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", RFC
         3550, July 2003.

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   [13]  Baugher, M., McGrew, D., Naslund, M., Carrara, E. and K.
         Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC
         3711, March 2004.

   [14]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains via
         IPv4 Clouds", RFC 3056, February 2001.

   [15]  Huitema, C., "Teredo: Tunneling IPv6 over UDP through NATs",
         draft-huitema-v6ops-teredo-02 (work in progress), June 2004.

   [16]  Rosenberg, J., "Traversal Using Relay NAT (TURN)",
         draft-rosenberg-midcom-turn-04 (work in progress), February

Author's Address

   Jonathan Rosenberg
   600 Lanidex Plaza
   Parsippany, NJ  07054

   Phone: +1 973 952-5000

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