--- 1/draft-ietf-mmusic-ice-04.txt 2006-02-05 00:26:13.000000000 +0100 +++ 2/draft-ietf-mmusic-ice-05.txt 2006-02-05 00:26:13.000000000 +0100 @@ -1,1750 +1,1881 @@ MMUSIC J. Rosenberg Internet-Draft Cisco Systems -Expires: August 22, 2005 February 21, 2005 +Expires: January 18, 2006 July 17, 2005 - Interactive Connectivity Establishment (ICE): A Methodology for - Network Address Translator (NAT) Traversal for Multimedia Session - Establishment Protocols - draft-ietf-mmusic-ice-04 +Interactive Connectivity Establishment (ICE): A Methodology for Network + Address Translator (NAT) Traversal for Offer/Answer Protocols + draft-ietf-mmusic-ice-05 Status of this Memo - This document is an Internet-Draft and is subject to all provisions - of section 3 of RFC 3667. By submitting this Internet-Draft, each - author represents that any applicable patent or other IPR claims of - which he or she is aware have been or will be disclosed, and any of - which he or she become aware will be disclosed, in accordance with - RFC 3668. + By submitting this Internet-Draft, each author represents that any + applicable patent or other IPR claims of which he or she is aware + have been or will be disclosed, and any of which he or she becomes + aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that - other groups may also distribute working documents as - Internet-Drafts. + other groups may also distribute working documents as Internet- + Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. - This Internet-Draft will expire on August 22, 2005. + This Internet-Draft will expire on January 18, 2006. Copyright Notice Copyright (C) The Internet Society (2005). Abstract 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 - 2. Multimedia Signaling Protocol Abstraction . . . . . . . . . . 5 - 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 - 4. Overview of ICE . . . . . . . . . . . . . . . . . . . . . . . 8 - 5. Detailed ICE Algorithm . . . . . . . . . . . . . . . . . . . . 10 - 5.1 Initiator Processing . . . . . . . . . . . . . . . . . . . 11 - 5.1.1 Sending the Initiate Message . . . . . . . . . . . . . 11 - 5.1.2 Processing the Accept . . . . . . . . . . . . . . . . 12 - 5.2 Responder Processing . . . . . . . . . . . . . . . . . . . 12 - 5.2.1 Processing the Initiate Message . . . . . . . . . . . 12 - 5.3 Common Procedures . . . . . . . . . . . . . . . . . . . . 13 - 5.3.1 Gathering Transport Addresses . . . . . . . . . . . . 13 - 5.3.2 Enabling STUN on Each Local Transport Address . . . . 15 - 5.3.3 Prioritizing the Transport Addresses and Choosing - a Default . . . . . . . . . . . . . . . . . . . . . . 17 - 5.3.4 Sending STUN Connectivity Checks . . . . . . . . . . . 19 - 5.3.5 Receiving STUN Requests . . . . . . . . . . . . . . . 24 - 5.3.6 Management of Resources . . . . . . . . . . . . . . . 25 - 5.3.7 Binding Keepalives . . . . . . . . . . . . . . . . . . 25 - 6. Running STUN on Derived Transport Addresses . . . . . . . . . 26 - 6.1 STUN on a TURN Derived Transport Address . . . . . . . . . 27 - 6.2 STUN on a STUN Derived Transport Address . . . . . . . . . 29 - 7. XML Schema for ICE Messages . . . . . . . . . . . . . . . . . 30 - 8. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 - 9. Mapping ICE into SIP . . . . . . . . . . . . . . . . . . . . . 35 - 9.1 Message Mapping . . . . . . . . . . . . . . . . . . . . . 35 - 9.2 SIP and SDP Specific Security Considerations . . . . . . . 37 - 9.3 Updates in the Offer/Answer Model . . . . . . . . . . . . 37 - 10. Security Considerations . . . . . . . . . . . . . . . . . . 37 - 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . 38 - 11.1 SDP Attribute Name . . . . . . . . . . . . . . . . . . . . 38 - 11.2 URN Sub-Namespace Registration . . . . . . . . . . . . . . 39 - 11.3 XML Schema Registration . . . . . . . . . . . . . . . . . 40 - 12. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 40 - 12.1 Problem Definition . . . . . . . . . . . . . . . . . . . . 41 - 12.2 Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 41 - 12.3 Brittleness Introduced by ICE . . . . . . . . . . . . . . 42 - 12.4 Requirements for a Long Term Solution . . . . . . . . . . 42 - 12.5 Issues with Existing NAPT Boxes . . . . . . . . . . . . . 43 - 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 43 - 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 43 - 14.1 Normative References . . . . . . . . . . . . . . . . . . . . 43 - 14.2 Informative References . . . . . . . . . . . . . . . . . . . 44 - Author's Address . . . . . . . . . . . . . . . . . . . . . . . 45 - Intellectual Property and Copyright Statements . . . . . . . . 46 + 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 + 3. Overview of ICE . . . . . . . . . . . . . . . . . . . . . . . 6 + 4. Sending the Initial Offer . . . . . . . . . . . . . . . . . . 8 + 5. Receipt of the Offer and Generation of the Answer . . . . . . 9 + 6. Processing the Answer . . . . . . . . . . . . . . . . . . . . 9 + 7. Common Procedures . . . . . . . . . . . . . . . . . . . . . . 10 + 7.1 Gathering Candidates . . . . . . . . . . . . . . . . . . . 10 + 7.2 Encoding Candidates into SDP . . . . . . . . . . . . . . . 13 + 7.3 Prioritizing the Transport Addresses and Choosing an + Active One . . . . . . . . . . . . . . . . . . . . . . . . 15 + 7.4 Connectivity Checks . . . . . . . . . . . . . . . . . . . 17 + 7.4.1 UDP Connectivity Checks . . . . . . . . . . . . . . . 19 + 7.4.1.1 Send Validation . . . . . . . . . . . . . . . . . 19 + 7.4.1.2 Receive Validation . . . . . . . . . . . . . . . . 20 + 7.4.1.3 Learning New Candidates from Connectivity + Checks . . . . . . . . . . . . . . . . . . . . . . 22 + 7.4.1.3.1 On Receipt of a Binding Request . . . . . . . 23 + 7.4.1.3.2 On Receipt of a Binding Response . . . . . . . 26 + 7.4.2 TCP Connectivity Checks . . . . . . . . . . . . . . . 26 + 7.4.2.1 Connection Establishment . . . . . . . . . . . . . 26 + 7.4.2.2 Sending STUN Binding Requests . . . . . . . . . . 27 + 7.4.2.3 Receiving STUN Requests . . . . . . . . . . . . . 29 + 7.5 Promoting a Valid Candidate to Active . . . . . . . . . . 30 + 7.5.1 Minimum Requirements . . . . . . . . . . . . . . . . . 30 + 7.5.2 Suggested Algorithm . . . . . . . . . . . . . . . . . 31 + 7.6 Subsequent Offer/Answer Exchanges . . . . . . . . . . . . 33 + 7.6.1 Sending of an Offer . . . . . . . . . . . . . . . . . 33 + 7.6.2 Receiving the Offer and Sending an Answer . . . . . . 34 + 7.6.3 Receiving the Answer . . . . . . . . . . . . . . . . . 36 + 7.7 Binding Keepalives . . . . . . . . . . . . . . . . . . . . 37 + 7.8 Sending Media . . . . . . . . . . . . . . . . . . . . . . 38 + 8. Interactions with Forking . . . . . . . . . . . . . . . . . . 38 + 9. Interactions with Preconditions . . . . . . . . . . . . . . . 38 + 10. Example . . . . . . . . . . . . . . . . . . . . . . . . . . 39 + 11. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . 39 + 12. Security Considerations . . . . . . . . . . . . . . . . . . 40 + 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . 42 + 14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 42 + 14.1 Problem Definition . . . . . . . . . . . . . . . . . . . . 42 + 14.2 Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 43 + 14.3 Brittleness Introduced by ICE . . . . . . . . . . . . . . 43 + 14.4 Requirements for a Long Term Solution . . . . . . . . . . 44 + 14.5 Issues with Existing NAPT Boxes . . . . . . . . . . . . . 45 + 15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 45 + 16. References . . . . . . . . . . . . . . . . . . . . . . . . . 45 + 16.1 Normative References . . . . . . . . . . . . . . . . . . . 45 + 16.2 Informative References . . . . . . . . . . . . . . . . . . 46 + Author's Address . . . . . . . . . . . . . . . . . . . . . . . 47 + Intellectual Property and Copyright Statements . . . . . . . . 48 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) [9] and the International Telecommunications Union + Protocol (RTSP) [16] 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 [10]. The protocols also seek to create a media flow + through NAT [17]. 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 [11], Simple Traversal of UDP - through NAT (STUN) [1], Traversal Using Relay NAT [8], and Realm - Specific IP [12][13] along with session description extensions needed - to make them work, such as the SDP attribute for RTCP [2]. + (ALGs), the Middlebox Control Protocol [18], Simple Traversal of UDP + through NAT (STUN) [1], Traversal Using Relay NAT [14], and Realm + Specific IP [19] [20] along with session description extensions + needed to make them work, such as the Session Description Protocol + (SDP) [7] attribute for the Real Time Control Protocol (RTCP) [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 - situations. - - 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. - -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 - 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 the user agent that generates - an SDP offer [4], the responder is a SIP user agent that generates an - SDP answer to the offer, 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. + their solutions will be deployed. This introduces complexity and + brittleness into the system. What is needed is a single solution + which is flexible enough to work well in all situations. - The initiate and accept messages need to contain parameters, defined - by this specification, for the protocol to operate. The initiate and - accept mesages are therefore defined by this specification as XML - documents containing the relevant information. Of course, multimedia - signaling protocols will not use these XML documents directly. - Rather, those protocols will need to define extensions as needed to - show how the initiate, accept and terminate messages map to messages - in the actual protocol, and how every element and attribute in the - XML document for those messages maps into parameters of the actual - protocol. Section 9 provides such a mapping for SIP. + This specification provides that solution for protocols based on the + offer-answer model, RFC 3264 [4]. It is called Interactive + Connectivity Establishment, or ICE. ICE makes use of STUN and TURN, + but uses them in a specific methodology which avoids many of the + pitfalls of using any one alone. -3. Terminology +2. 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 - initiator. For SIP, this is any SIP message that contains an - offer. Usually, this is the initial INVITE. - - 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. For SIP, - this is any SIP message that contains an answer. Usually, this is - a 200 OK. - - Terminate Message The signaling message used by a client to terminate - the session and associated media streams. + Peer: From the perspective of one of the agents in a session, its + peer is the other agent. Specifically, from the perspective of + the offerer, the peer is the answerer. From the perspective of + the answerer, the peer is the offeror. Transport Address: The combination of an IP address and port. Local Transport Address: A local transport address 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) [12] (which lives at the operating system + Realm Specific IP (RSIP) [19] (which lives at the operating system level). Transport addresses are typically obtained by binding to an interface. - Usable Local Transport Address: A local transport address created for - the purposes of advertisement to ICE peers. - - Associated Local Transport Address: An associated transport address - is a local transport address used solely to obtain a derived - transport address. Associated local transport addresses are never - advertised in ICE messages. However, packets are received on them - when sent to the derived transport address. + m/c line: The media and connection lines in the SDP, which together + hold the transport address used for the receipt of media. Derived Transport Address: A derived transport address is a transport - address which is derived from an associated local transport - address. The derived transport address is related to the - associated local transport address in that packets sent to the - derived transport address are received on the socket bound to its - associated local transport address. Derived addresses are - obtained using protocols like STUN and TURN, and more generally, - any UNSAF protocol [14]. + address which is derived from a local transport address. The + derived transport address is related to the associated local + transport address in that packets sent to the derived transport + address are received on the socket bound to its associated local + transport address. Derived addresses are obtained using protocols + like STUN and TURN, and more generally, any UNSAF protocol [21]. - Advertised Transport Addresses: The union of the usable local - transport addresses and the derived transport addresses. These - are the ones used in ICE messages. + Candidate Transport Address: A transport address advertised by a + agent in an offer or answer. A candidate transport address can + either by a local transport address or a derived transport + address. 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. + Candidate: A sequence of candidate transport addresses that form an + atomic set for usage with a particular media stream. In the case + of RTP, there are two candidate transport addresses per candidate: + one for RTP, and another for RTCP. Connectivity is verified to + all of the candidate transport addresses within a candidate before + that candidate is used. The transport addresses that compose a + candidate are all of the same type - local, STUN derived, TURN + derived or peer derived. - 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 - allocations. + Local Candidate: A candidate whose transport addresses are local + transport addresses. -4. Overview of ICE + STUN Candidate: A candidate whose transport addresses are STUN + derived transport addresses. + + TURN Candidate: A candidate whose transport addresses are TURN + derived transport addresses. + + Peer Candidate: A candidate whose transport addresses are peer + derived transport addresses. + + Active Candidate: The candidate that is in use for exchange of media. + This is the one that an agent places in the m/c line of an offer + or answer. + +3. 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. - Initiator TURN,STUN Servers Responder + Agent A TURN,STUN Servers Agent B |(1) Gather Addresses | | |-------------------->| | - |(2) Initiate Msg. | | + |(2) Offer | | |------------------------------------------>| | |(3) Gather Addresses | | |<--------------------| - |(4) Accept Msg. | | + |(4) Answer | | |<------------------------------------------| - |(5) STUN Checks | | + |(5) Media | | |<------------------------------------------| - |(6) STUN Checks | | + |(6) Media | | |------------------------------------------>| - |(7) Media | | + |(7) STUN Checks | | |<------------------------------------------| - |(8) Media | | + |(8) STUN Checks | | + |------------------------------------------>| + |(9) Offer | | + |------------------------------------------>| + |(10) Answer | | + |<------------------------------------------| + |(11) Media | | + |<------------------------------------------| + |(12) Media | | |------------------------------------------>| - Figure 2 - - The basic flow of operation for ICE is shown in Figure 2. 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 the local transport - addresses it has obtained. These include ones that will be - advertised directly through ICE, and so-called associated local - transport addresses, which are not directly advertised; rather, the - transport address derived from them is advertised. 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 one half of the - STUN username and the 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 - 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, along with the other half of the STUN username and its - password, 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. - -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, both usable - and associated, 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. Doing so eliminates any - additional perceivable call setup delays due to address gathering. - - 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. + Figure 1 - 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 an IPv4 and/or an IPv6 default address, and a list of candidates. - Each candidate has information for RTP and optionally RTCP. RTCP - information is optional since, unfortunately, many systems don't - support it. If ICE did not indicate that RTCP was not supported, - connectivity checks would be made to the RTCP ports and fail, - confusing operation and adding unneccesary overhead. + The basic flow of operation for ICE is shown in Figure 1. Before the + offeror establishes a session, it obtains local transport addresses + from its operating system on as many interfaces as it has access to. + These interfaces can include IPv4 and IPv6 interfaces, in addition to + Virtual Private Network (VPN) interfaces or ones associated with + RSIP. For media protocols that support both UDP and TCP (such as the + Real Time Transport Protocol (RTP) [22], which can run over either), + it obtains both TCP and UDP transport addresses. In addition, the + agent obtains derived transport addresses from each local transport + address using protocols such as STUN and TURN. Each local and + derived transport address becomes a candidate for receipt of media + traffic. - 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 candidates - represent addresses that the responder should try using the - mechanisms of this specification. The list of candidates includes - the defaults. In SIP, the candidates are conveyed with the new SDP - candidate parameter. + The agent will choose one of its candidate transport addresses as its + initial media transport address for inclusion in the connection and + media lines in the offer. This transport address will be utilized + for media traffic while connectivity is verified to all of the + candidates. Since these checks may take time to execute, media + clipping will occur if the media transport address is not reachable + by the peer. To minimize the probability of clipping, the transport + address that is most likely to work is chosen. This is normally a + TURN-derived tranport address, but others can be utilized based on + local policy. - The client then encodes its usable local transport addresses and - derived transport addresses (including the one set as the default) as - a series of candidate elements. Each candidate element conveys a - transport address for RTP, a transport address for RTCP, a STUN - username fragment and STUN password for RTP, and one for RTCP. The - client MUST assign each candidate a unique identifier. These - identifiers MUST be unique across all candidates used within the - session. Though they are not used in this specification, they serve - as a convenient and short handle for each candidate within the - document. Experience has shown that explicit identifiers for - elements in SDP is a good idea. This identifier is encoded in the - "id" attribute of the element. The priority for the - transport address, as computed above, is included as an attribute as - well. + Each candidate transport address (including the one being used as the + media transport address) is listed in an a=candidate attribute in the + offer. Each candidate is given a 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). Each candidate is also assigned a distinct ID, + called a transport ID (tid). - Once the initiate message is constructed, it is sent. + The offer is then sent to the answerer. This specification does not + address the issue of how the signaling messages themselves traverse + NAT. It is assumed that signaling protocol specific mechanisms are + used for that purpose. The answerer follows a similar process as the + offeror followed; it obtains addresses from local interfaces, obtains + derived transport addresses from those, and the combination becomes + its set of candidate transport addresses. It picks one as its + initial media transport address and places it into the m/c line in + the answer, and then lists all of them in the a=candidate attributes + in the answer, along with a preference and tid. -5.1.2 Processing the Accept + Once the offer/answer exchange has completed, each agent sends media + from its media transport address to the media transport address of + its peer. This media stream may or may not work, depending on + whether or not the media transport address is reachable. In parallel + with the transmission of media, a connectivity check begins. This + check makes use of STUN messages sent from each candidate to each + other candidate. These checks will allow each agent to determine + whether it can send packets from a particular candidate to a + candidate from its peer, and whether packets can be sent back. If, + after a certain period of time, an agent determines that a pair of + candidates works, and has a higher priority than the transport + addresses currently in use for media (perhaps because the ones in use + don't work), it sends a new offer that "promotes" its candidate into + the m/c line. This causes the media traffic to switch to this new + transport address. - 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 - 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 as the - destination. 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 through the TURN server, which will - then forward them towards the responder. +4. Sending the Initial Offer - If the Accept message contains candidates, it implies that the - responder supported ICE. In that case, the initiator takes each - candidate transport address, STUN username fragment, 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 address. + When an agent wishes to begin a session by sending an initial offer, + it starts by gathering transport addresses, as described in + Section 7.1. This will produce a set of candidates, including local + ones, STUN-derived ones, and TURN-derived ones. -5.2 Responder Processing + This process of gathering candidates can actually happen at any time + before sending the initial offer. A agent 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. Doing so eliminates any additional perceivable call setup + delays due to address gathering. -5.2.1 Processing the Initiate Message + When it comes time to offer communications, it determines a priority + for each candidate and identifies the active candidate that will be + used for receipt of media, as described in Section 7.3. - 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. By gathering addresses - while the user is being alerted to the request for communications, - session establishment delays due to that gathering can be eliminated. + The next step is to construct the offer message. For each media + stream, it places its candidates into a=candidate attributes in the + offer and puts its active candidate into the m/c line. The process + for doing this is described in Section 7.2. The offer is then sent. - 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 - follows. +5. Receipt of the Offer and Generation of the Answer - The client first determines a priority for each usable local - transport address and derived transport address it has gathered, and - identifies one as a default, as described in Section 5.3.3. + Upon receipt of the offer message, the agent checks if the offer + contains any a=candidate attributes. If it does, the offeror + supports ICE. In that case, it starts gathering candidates, as + described in Section 7.1, and prioritizes them Section 7.3. This + processing is done immediately on receipt of the offer, to prepare + for the case where the user should accept the call, or early media + needs to be generated. By gathering candidates while the user is + being alerted to the request for communications, session + establishment delays due to that gathering can be eliminated. - Constructing the accept message proceeds identically to the way in - which the initiate message is constructed (Section 5.1.1). + At some point, the answerer will decide to accept or reject the + communications. A rejection terminates ICE processing. In the case + of acceptance, the answer is constructed, and if the offeror + supported ICE, the candidates are encoded into the SDP as described + in Section 7.2. The answer is then sent. If the offeror supported + ICE, the answerer begins its connectivity checks as described in + Section 7.4. - The accept message is then sent. + In addition, and regardless if the offeror supported ICE, the + answerer can begin sending media packets as it normally would. It + sends media according to the procedures in Section 7.8. -5.3 Common Procedures +6. Processing the Answer - This section discusses procedures that are common between initiator - and responder. + There are two possible cases for processing of the answer. If the + answerer did not support ICE, the answer will not contain any + a=candidate attributes. As a result, the offeror knows that it + cannot perform its connectivity checks. In this case, it proceeds + with normal media processing as if ICE was not in use. The + procedures for sending media, described in Section 7.8, MUST be + followed however. -5.3.1 Gathering Transport Addresses + If the answer contains candidates, it implies that the answerer + supported ICE. In that case, the offeror begins connectivity checks + as described in Section 7.4. It also starts sending media, using the + candidate in the m/c line, based on the procedures described in + Section 7.8. - 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). +7. Common Procedures - There are two types of addresses a client can gather - usable local - transport addresses and derived transport addresses. Usable 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) [15], the client will need to bind to an - ephemeral port for both RTP and RTCP. + This section discusses procedures that are common between offeror and + answerer. - The result will be a set of usable local transport addresses. The - client may also have access to servers that provide unilateral - self-address fixing (UNSAF) [14]. Examples of such protocols include - STUN, TURN, and TEREDO [18]. UNSAF protocols work by having the - client send, from a specific associated 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 associated 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 associated local - transport address. +7.1 Gathering Candidates - All ICE implementations SHOULD implement and use STUN and TURN for - unilateral allocation. STUN is an integral part of this - specification for connectivity checks and will always be present for - that purpose. The usage of TURN and STUN for unilateral allocations - is at SHOULD strength, and not MUST, since there are many network - environments, and there will be deployments for which one of these - will never be used and will impose needless cost. However, one of - the key ideas behind ICE is that network conditions and connectivity - assumptions can, and will change. Just because a client 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 a client is behind a full cone NAT today, doesn't mean - that tomorrow they won't pick up their client and take it to a public - network access point where there is a symmetric NAT. The way to - handle these cases and build a reliable system is for clients to - implement a diverse set of techniques for allocating addresses, so - that at least one of them is almost certainly going to work in any - situation. The combination of TURN, STUN and local address - allocations provide sufficient coverage to handle nearly any NAT - configuration. Implementors should consider very carefully any - assumptions that they make about deployments before electing not to - implement one of these 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. Only in cases - where implementors truly believe that these cases will not require - either TURN or STUN allocations, should those techniques not be - implemented. + An agent gathers candidates when it believes that communications is + imminent. For offerors, this occurs before sending an offer + (Section 4). For answerers, it occurs before sending an answer + (Section 5). - For each UNSAF protocol, 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 server for each UNSAF - protocol, the client MUST bind to a new local transport address, and - uses it to obtain a single derived transport address for it. This - local IP address and port is called an associated transport address. - These addresses are not advertised to peers in ICE messages; their - derived transport addresses are. As a result of using a different - local transport address for each derived transport address, every - transport address advertised in an ICE message is either a unique - local transport address, or else is derived from a unique local - transport address. + Each candidate is composed of a series of transport addresses of the + same type. In the case of RTP, the candidate is composed of either + one or two transport addresses. Normally there are two - one for + RTP, and one for RTCP. However, if RTCP is not in use, a candidate + will only contain a single transport address. - If a derived transport address is equal to the associated local - transport address from which it was derived, the local transport - address SHOULD be promoted to a usable local transport address. It - is preferable to do this than to use a new local transport address; - the UNSAF protocol may have caused pinholes to open in intervening - firewalls. + The first step is to gather local candidates. Local candidates are + obtained by binding to ephemeral ports on an interface (physical or + virtual, including VPN interfaces) on the host. Specifically, for + each UDP-only media stream the agent wishes to use, the agent SHOULD + obtain a set of candidates (one for each interface) by binding to N + ephemeral UDP ports on each interface, where N is the number of + transport addresses needed for the candidate. For RTP, N is + typically two. For each TCP-only media stream the agent wishes to + use, the agent SHOULD obtain a set of candidates by binding to N + ephemeral TCP ports on each interface, where N is the number of + transport addresses needed for the candidate. For media streams that + can support either UDP or TCP, the agent SHOULD obtain a set of + candidates by binding to N ephemeral UDP and N ephemeral TCP ports on + each interface, where N is the number of transport addresses needed + for the candidate. - Implementations MAY use other protocols that provide derived - transport addresses, as long as those techniques meet the following - conditions: + If a host has K local interfaces, this will result in K candidates + for each UDP stream (requiring K*N transport addresses), K candidates + for each TCP stream (requiring K*N transport addresses), and 2K + candidates for streams that support UDP and TCP (requiring 2*K*N + transport addresses). - 1. The technique does not require its peer to know about, or - understand the technique in order to interoperate. + Media streams carried using the Real Time Transport Protocol (RTP) + [22] can run over TCP [27]. As such, it is RECOMMENDED that both UDP + and TCP candidates be obtained. Transmission of real time media over + UDP is generally preferred to TCP. However, many network + environments, for better or for worse, permit only TCP traffic. + Obtaining a TCP candidate, and then using it in conjunction with a + TURN relay as described below, allows for ICE to make use of the TCP + media only when UDP connectivity is non-existent, as it may be in + these restricted environments. However, 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. - 2. The technique can provide the client with an IP address and port - that may be reachable by some peers. + 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 a 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 a agent is behind a full + cone NAT 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 symmetric NAT or one that only allows outbound TCP. + The way to handle these cases and build a reliable system is for + agents to implement a diverse set of techniques for allocating + addresses, so that at least one of them is almost certainly going + to work in any situation. Implementors should consider very + carefully any assumptions 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. Only in cases where there isn't now, and never will be, + endpoint mobility or nomadicity of any sort, should a technique be + omitted. - 3. The technique allows the client to receive STUN connectivity - checks in addition to media packets on the same IP address and - port. + Once the agent has obtained local candidates, it obtains candidates + with derived transport addresses. Agents which serve end users + directly, such as softphones, hardphones, terminal adaptors and so + on, MUST implement STUN and SHOULD use it to obtain STUN candidates. + These devices SHOULD implement and SHOULD use TURN to obtain TURN + candidates. They MAY implement and MAY use other protocols that + provide derived transport addresses, such as TEREDO [25]. As with + TCP, usage of STUN and TURN is at SHOULD strength to allow for + provider variation. If it is not to be used, it is also RECOMMENDED + that it be implemented and just disabled through configuration, so + that it can re-enabled through configuration if conditions change in + the future. - 4. The technique allows the client to send packets to a peer, so - that the peer will see the derived transport address as the - source IP address and port of the packet. + Agents which represent network servers under the control of a service + provider, such as gateways to the telephone network, media servers, + or conferencing servers that are targeted at deployment only in + networks with public IP addresses MAY use STUN, TURN or other similar + protocols to obtain candidates. -5.3.2 Enabling STUN on Each Local Transport Address + Why would these types of endpoints even bother to implement ICE? + The answer is that such an implementation greatly facilitates NAT + traversal for endpoints that connect to it. The ability to + process STUN connectivity checks allows for the network server to + obtain peer-derived transport addresses that can be used to + provide relay-free traversal of symmetric NAT for endpoints that + connect to it. Furthermore, implementation of the STUN + connectivity checks allows for NAT bindings along the way to be + kept open. ICE also provides numerous security properties that + are independent of NAT traversal, and would benefit any multimedia + endpoint. See Section 12 for a discussion on these benefits. - Once the client has obtained a set of transport addresses, it starts - a STUN server on each local transport address, including both - associated local transport addresses and usable transport addresses. - These include ones used for both RTP and RTCP. This, by definition, - means that the STUN service will be reached for requests sent to the - derived addresses. + To obtain STUN candidates (which are always UDP), the client takes a + local UDP candidate, and for each configured STUN server, produces a + STUN candidate. It is anticipated that clients may have a + multiplicity of STUN servers configured in network environments where + there are multiple layers of NAT, and that layering is known to the + provider of the client. To produce the STUN candidate from the local + candidate, it follows the procedures of Section 9 of RFC 3489 for + each local transport address in the local candidate. It obtains a + shared secret from the STUN server and then initiates a Binding + Request transaction from the local transport address to that server. + The Binding Response will provide the client with its STUN derived + transport address in the MAPPED-ADDRESS attribute. If the client had + K local candidates, this will produce S*K STUN candidates, where S is + the number of configured STUN servers. - 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. The CHANGED-ADDRESS attribute in a BindingResponse - is set to the transport address on which the server is running. + To obtain UDP TURN candidates, the client takes a local UDP + candidate, and for each configured TURN server, produces a TURN + candidate. It is anticipated that clients may have a multiplicity of + TURN servers configured in network environments where there are + multiple layers of NAT, and that layering is known to the provider of + the client. To produce the TURN candidate from the local candidate, + it follows the procedures of Section 8 of [14] for each local + transport address in the local candidate. It initiates an Allocate + Request transaction from the local transport address to that server. - 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 Allocate Response will provide the client with its TURN derived + transport address in the MAPPED-ADDRESS attribute. If the client had + K local candidates, this will produce S*K UDP TURN candidates, where + S is the number of configured TURN servers. - 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 [16], 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 - below). + To obtain a TURN-derived TCP candidates, the client takes a local TCP + candidate, and for each configured TURN server, produces a TCP TURN + candidate. It is anticipated that clients may have a multiplicity of + TURN servers configured in network environments where there are + multiple layers of NAT, and that layering is known to the provider of + the client. To produce the TURN candidate from the local candidate, + it iterates through the local transport addresses in the local + candidate, and for for each one, initiates a TCP connection from the + same interface the local transport address to the TURN server. It is + not neccesary to initiate the connection from the actual port in the + local transport address. Following the procedures of Section 8 of + [14], it initiates an Allocate Request transaction over the + connection. The Allocate Response will provide the client with its + TCP TURN derived transport address in the MAPPED-ADDRESS attribute. + If the client had K local TCP candidates, this will produce S*K TCP + TURN candidates, where S is the number of configured TURN servers. - 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. +7.2 Encoding Candidates into SDP - For each transport address advertised in the initiate message, the - client MUST choose a username fragment and a password. The username - fragment created by the client (called the local username fragment) - is concatenated with the fragment created by its peer (called the - remote username fragment) to create the actual username used for - access to the STUN server that will receive packets sent to that - transport address. This username will be present in STUN requests - sent by its peer. 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. + For each candidate to be placed into the SDP, the agent includes a + series of a=candidate attributes as media-level attributes, one for + each transport address in the candidate. Each of the transport + addresses for the same candidate MUST have the same value of the + candidate-id attribute. The a=candidate attributes for different + candidates MUST be unique within that media stream. Using a simple + sequence number, incrementing by one for each candidate for a media + stream, meets these requirements. The transport, unicast-address and + port of the attribute are set to those for the candidate. The qvalue + is set to the priority of this candidate (note that, for RTP, the RTP + and RTCP transport addresses MUST have equal priority values). The + tid MUST be chosen randomly with 128 bits of randomness. The tid is + chosen only when the transport address is placed into the SDP for the + first time; subsequent offers or answers within the same session + containing that same transport address would use the same tid used + previously. - The username fragment MUST be globally unique with high probability, - and different for each advertised transport address. It SHOULD be - persistently used over time for that particular transport address. A - value computed as the 128 bit hash of the transport address - concatenated with a 128 bit random number selected to identify the - host will meet these requirements. This results in two properties. - First - each transport address can be uniquely identified. Secondly, - no other host will select a username with the same value. The - password MUST be random with at least 128 bits of randomness and is - selected separately for each transport address advertised as part of - a distinct session. This means that RTP and RTCP, which run on - different transport addresses, will get different usernames and - passwords. The password will remain constant during a session with a - peer, but will otherwise vary across sessions. The username fragment - and password will be passed to its peer in an initiate or accept - message. Because the password is conveyed through these signaling - protocols, those protocols MUST provide facilities for encryption, - authentication and message integrity, and those facilities SHOULD be - used when ICE is employed. 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 tid serves as a unique identifier for each transport address. It + also gets combined, through concatenation, with the tid of a peer + candidate to form the username and password that is placed in the + STUN checks between the peers. This allows the STUN message to + uniquely identify the pairing whose connectivity it is checking. The + tid is needed as a unique identifier because the IP address within + the candidate fails to provide that uniqueness as a consequence of + NAT. - 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 10.0.0.0/8. C is within - private enterprise 2, which is also using 10.0.0.0/8. As it turns - out, B and C both have IP address 10.0.1.1. A initiates - communications to C. C, in its accept message, provides A with its - transport addresses. In this case, thats 10.0.1.1:8866 and 8877. As - it turns out, B is in a session at that same time, and is also using - 10.0.1.1:8866 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 - 10.0.1.1:8866 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 - fragment takes on the role of a unique identifier. C provides A with - a unique username fragment, and A provides one to C. A uses these - two fragments to construct the username in its STUN query to - 10.0.1.1:8866. 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. + Consider agents A, B, and C. A and B are within private enterprise 1, + which is using 10.0.0.0/8. C is within private enterprise 2, which + is also using 10.0.0.0/8. As it turns out, B and C both have IP + address 10.0.1.1. A sends an offer to C. C, in its answer, provides + A with its transport addresses. In this case, thats 10.0.1.1:8866 + and 8877. As it turns out, B is in a session at that same time, and + is also using 10.0.1.1:8866 and 8877. This means that B is prepared + to accept STUN messages on those ports, just as C is. A will send a + STUN request to 10.0.1.1:8866 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, tid + takes on the role of a unique identifier. C provides A with an + identifier for its transport address, and A provides one to C. A + concatenates these two identifiers and uses the result as the + username and password in its STUN query to 10.0.1.1:8866. 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. An unfortunate consequence of the non-uniqueness of IP addresses is - that, in the above example, B might not even be an ICE client. It + that, in the above example, B might not even be an ICE agent. It could be any host, and the port to which the STUN packet is directed could be any ephemeral port on that host. If there is an application listening on this socket for packets, and it is not prepared to handle malformed packets for whatever protocol is in use, the operation of that application could be effected. Fortunately, since the ports exchanged in SDP are ephemeral and ususally drawn from the dynamic or registered range, the odds are good that the port is not - used to run a server on host B, but rather is the client side of some + used to run a server on host B, but rather is the agent side of some protocol. This decreases the probability of hitting a port in-use, due to the transient nature of port usage in this range. However, the possibility of a problem does exist, and network deployers should be prepared for it. - Termination of the local STUN servers is discussed in Section 5.3.6. + Note that, because there are separate transport addresses for RTP and + RTCP, each will have a distinct tid. -5.3.3 Prioritizing the Transport Addresses and Choosing a Default + The active candidate is placed into the m/c lines of the SDP. For + RTP streams, this is done by placing the RTP address and port into + the c and m lines in the SDP respectively. If the agent it utilizing + RTCP, it MUST encode its address and port using the a=rtcp attribute + as defined in RFC 3605 [2]. If RTCP is not in use, the agent MUST + signal that using b=RS:0 and b=RR:0 as defined in RFC 3556 [8]. - The prioritization process takes the list of the advertised 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. + For media streams that are inherently TCP-based (as opposed to ones + where TCP is a fallback and would be listed as a candidate but not + the initial active address), the connections MUST be signaled using + comedia [13], and those connections MUST be in "holdconn" mode. This + has the effect of suspending connection attempts via the comedia + mechanisms, allowing ICE to open the connections instead. These + connections then get removed from holdconn mode when the ICE + procedures complete and an updated offer/answer exchange takes place + that promotes one of the existing ICE-established connections to + active. Note that this has the result of increasing the post-dial- + delay for TCP-oriented media, but brings with it substantial security + and NAT traversal properties. + +7.3 Prioritizing the Transport Addresses and Choosing an Active One + + The prioritization process takes the set of candidates and associates + each with a priority. This priority reflects the desire that the + agent 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 meaningful relative to other + candidates for a particular media stream. 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. - 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 do 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. + One criteria for choosing one candidate over another is whether or + not that candidate involves the use of a relay. That is, if media is + sent to that candidate, will the media first transit a relay before + being received. TURN candidates make use of relays (the TURN + server), as do any local candidates 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, candidates 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) [17]. 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 + Another criteria for choosing one candidate 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) [24]. 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 + fallback to 6to4 addresses when communicating with agents in other sites that do not yet have native v6 connectivity. - Another criteria for choosing one address over another is security. + Another criteria for choosing one candidate 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 corporate network when communicating within the enterprise, but use the local network when communicating with users outside of the enterprise. 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. + awareness. This is most useful for candidates which make use of + relays (including TURN and VPN). In those cases, if a agent 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. - Unfortunately, it is difficult to ascertain which address that might - be. As an example, consider a user within an enterprise. To reach - non-ICE capable clients within the enterprise, a local transport - address has to be used, since the enterprise policies may prevent + Finally, 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. + + Once the candidates have been prioritized, one is selected as the + active one. This is the candidate that will be used for actual + exchange of media, until replaced by an updated offer or answer. + Since the ICE connectivity checks can take a few seconds to execute, + media clipping can occur is this candidate doesn't work. The active + candidate will also be used to receive media from ICE-unaware peers. + As such, it is RECOMMENDED that one be chosen based on the likelihood + of that candidate to work with the peer that is being contacted. + Unfortunately, it is difficult to ascertain which candidate that + might be. As an example, consider a user within an enterprise. To + reach non-ICE capable agents within the enterprise, a local candidate + has to be used, since the enterprise policies may prevent communication between elements using a relay on the public network. However, when communicating to peers outside of the enterprise, a - TURN-based public address is needed. + TURN-based candidate from a publically accessible TURN server is + needed. Indeed, the difficulty in picking just one address that will work is the whole problem that motivated the development of this specification in the first place. As such, it is RECOMMENDED that - the default address be a TURN derived transport address from a TURN - server providing public IP addresses. Furthermore, ICE is only truly - effective when it is supported on both sides of the session. It is - therefore most prudent to deploy it to close-knit communities as a - whole, rather than piecemeal. In the example above, this would mean - that ICE would ideally be deployed completely within the enterprise, - rather than just to parts of it. + the default address be a TURN candidate from a TURN server providing + public IP addresses. Furthermore, ICE is only truly effective when + it is supported on both sides of the session. It is therefore most + prudent to deploy it to close-knit communities as a whole, rather + than piecemeal. In the example above, this would mean that ICE would + ideally be deployed completely within the enterprise, rather than + just to parts of it. -5.3.4 Sending STUN Connectivity Checks +7.4 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 from the - peer (called the remote username fragment), the password from the - peer (called the remote password), and priority from the peer (called - the remote priority) 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. An example of a candidate table for RTP - audio is shown below. + Once the offer/answer exchange has completed, both agents will have a + set of candidates for each media stream. Each agent forms a set of + pairings for each media stream by combining each of its UDP + candidates with each of the UDP candidates of its peer, and by + combining each of its TCP candidates with each of the TCP candidates + of its peer. If candidates for other transport protocols were + signaled through the offer/answer exchange, a pairing is performed + between each of those as well. If an offer/answer exchange took + place for a session comprised of an audio and a video stream, and + each stream had two UDP and two TCP candidates from each agent, there + would be 16 pairings, 8 for audio and 8 for video. Each of those + eight would be comprised of four UDP and four TCP. Note that there + is no requirement that the number of candidates from each peer be the + same. One agent can offer two UDP candidates for a media stream, and + the answer can contain three UDP candidates for the same media + stream. In that case, there would be six UDP pairings. - Remote Remote Remote Remote - Transport Username Password Priority - Address Fragment - -------------------------------------------------------------------- - 10.0.1.1:38746 asd9f8f8== 9asfhfvva9==affahnz 0.4 - 192.0.2.77:44634 xcyca87sbb f99fhaz0ftrafdgl99d 0.2 + Each candidate has a number of transport addresses. In the case of + RTP, there are either one or two. Within the pairing, the transport + addresses of each candidate are linked together one-to-one to form a + transport address pair. In the case of RTP, the result will either + be one or two transport address pairs - one for RTP, and possibly + another for RTCP. The relationship between a candidate, transport + address, pairing and transport address pair are shown in Figure 2. + This figure shows the pairing as seen by the agent that owns the + candidate {A,B}. The candidate owned by that agent is called the + native candidate, and the one owned by its peer is the remote + candidate. As the figure shows, there is one pairing between two + candidates, and two transport address pairs ({A,C} and {B,D}). If + one of the candidates only had one transport address (in the case + where RTCP was not being used by one agent), there would only be one + transport address pair, {A,C}. Each transport address is associated + with a tid. Furthermore, each transport address pair is associated + with an ID, the transport address pair ID. This ID is equal to the + concatenation of the tid of the native transport address with the tid + of the remote transport address. This means that the identifiers are + different for each agent. For the agent that owns {A,B}, the + transport address pair ID is WY for the first transport address pair, + and XZ for the second. For the agent that owns {C,D}, it would be + reversed - YW for the first transport address pair, and ZX for the + second. - Figure 3 - The client then creates a new table, called the connection table. - There is a row in this table for each gathered address and remote - transport address pair. This table has a column for the local - transport address, which is equal to the gathered address if it was a - usable local transport address, else equal to the associated local - transport address if the gathered address was a derived address. - There is also a column for the remote transport address, the local - username fragment, the remote username fragment, the remote password - and the state. Each row in this table is called a connection, and it - provides information on the connectivity when sending packets from - the local transport address to the remote transport address. + ........................................... + . . + .......... . . .......... + . . . ............. ............. . . . + . . . . . . . . . . + . -- . . . -- . . -- . . . -- . + . | A|<<<<<<<<<<| A|--------------------| C|>>>>>>>>>>>>| K| . + . -- . . . -- . Transport . -- . . . -- . + . . . . Transport . Address . Transport . . . . + . . . . Address . Pair . Address . . . . + . . . . tid=W . ID=WY . tid=Y . . . . + . . . . . . . . . . + . . . . . . . . . . + . . . . . . . . . . + . -- . . . -- . . -- . . . -- . + . | J|<<<<<<<<<<| B|--------------------| D|>>>>>>>>>>>>| D| . + . -- . . . -- . Transport . -- . . . -- . + .......... . . Transport . Address . Transport . . .......... + Associated . . Address . Pair . Address . . Associated + Local . . tid=X . ID=XZ . tid=Z . . Local + Transport . . . . . . Transport + Addresses . ............. ............. . Addresses + . Native Remote . + . Candidate Candidate . + . and and . + . Transport Addresses Transport Addresses . + . . + ........................................... - There are four possible states for each connection. These states - are: + Pairing - INIT: No STUN transaction has been completed towards this remote - transport address from this local transport address. + Figure 2 - 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 - local transport address. This state is important for connectivity - checks made to STUN derived transport addresses through port - restricted NAT. + The figure also shows that each transport address has an associated + local transport address. The associated local transport address is + the local transport address at which the agent will receive packets + sent to the transport address. For a local transport address, its + associated local transport address is the same. That is the case of + transport address A and D in the diagram. For STUN derived and TURN + derived transport addresses, however, they are not the same. The + associated local transport address is the one from which the STUN or + TURN transport was derived. - BAD: All STUN transactions to this remote transport address from this - local transport 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 local transport address. + Next, each agent begins sending connectivity checks for each + transport address pair. The procedure differs for UDP and TCP. - GOOD: A STUN transaction to this remote transport address from this - local transport address was successful. +7.4.1 UDP Connectivity Checks - When the client first populates the tables from the initiate or - accept message, all of the connections are set to the INIT state. + An agent considers a UDP pairing validated when all of its transport + address pairs have been validated. Each transport address pair is + validated if an agent successfully completed a STUN Binding Request + transaction from its native transport address to the corresponding + remote transport address, and when it has received a STUN Binding + Request transaction on its native transport address, sent from the + remote transport address. This ensures that packets can flow in each + direction. - Consider the the following example. An initiator sends an initiate - message with one media stream (audio), with two RTP transport - addresses, 10.0.1.1:38746 (which we denote "A" for shorthand) and - 192.0.2.77:44634 (which we denote "B" for shorthand). A is a usable - local transport address, and B is a STUN derived transport address - (although that fact is not signaled in the message). The usernames - and passwords for these transport addresses are shown in Figure 3. - The initiate message is sent to the responder. The responder has a - local transport address (10.0.1.76:43443), and a a STUN derived - transport address (192.0.2.64:54766) derived from (10.0.1.76:43444). - Call these two local transport addresses X and Y respectively. The - connection table created by the responder would have four rows (two - local transport addresses times two remote transport addresses). - Such a table might look like this: + Because validation of a transport address pair involves a STUN + transaction in each direction, a pair can be in one of five states - + unknown, invalid, send-valid, receive-valid and valid. Each + transport address pair starts in the unknown state. - Remote Local Remote Local Remote Remote - Trans. Trans. Username Username Password Priority - Address Address Fragment Fragment State - ------------------------------------------------------------------------ - A X asd9f8f8== 8asd77fa9 9asfhfvva9==affahnz 0.4 INIT - A Y asd9f8f8== zhff8dga^ 9asfhfvva9==affahnz 0.4 INIT - B X xcyca87sbb 8asd77fa9 f99fhaz0ftrafdgl99d 0.2 INIT - B Y xcyca87sbb zhff8dga^ f99fhaz0ftrafdgl99d 0.2 INIT +7.4.1.1 Send Validation - The client begins a STUN BindingRequest transaction for each - connection. This STUN transaction is sent to the IP address and port - from the Remote Transport Address column. It sends the request from - the IP address and port in the Local Transport Address column. The - STUN USERNAME attribute MUST be present. It is set to the - concatenation of the remote user fragment with the local user - fragment from the table. Thus, for the candidate with remote - transport address A and local transport address X, the USERNAME would - be set to "asd9f8f8==8asd77fa9". 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. + To validate a transport address pair in the send direction, an agent + needs to complete a successful STUN Binding Request transaction. + This means it needs to send a Binding Request from its native + transport address to the remote transport address, and receive a + successful Binding Response back. + + For UDP-based transport addresses, an agent initiates a STUN Binding + Request transaction by sending from its native transport address, and + sends it to the remote transport address. The meaning of "sending + from its native transport address" is clear in the case of a local + transport address - the request is sent such that the source IP + address and port of the packet is equal to that local transport + address. However, the meaning is different for STUN and TURN derived + transport addresses. For STUN derived transport address, it is sent + by sending from the local transport address used to derive that STUN + address. For TURN derived transport addresses, it is sent by using + TURN mechanisms to send the request through the TURN server (using + the SEND primitive). Sending the request through the TURN server + neccesarily requires that the request be sent from the client, using + the local transport address used to derive the TURN transport + address. + + The Binding Request sent by the agent MUST contain the USERNAME + attribute. This attribute MUST be set to the transport address pair + ID of the corresponding transport address pair as seen by its peer. + Thus, for the first transport address pair in the example above, if + the agent on the left sends the STUN Binding Request, the USERNAME + will have the value YW. The request MAY contain the MESSAGE- + INTEGRITY attribute, computed according to RFC 3489 procedures. The + MESSAGE-INTEGRITY The Binding Request MUST NOT contain the CHANGE- + REQUEST or ANSWER-ADDRESS attribute. Each of these STUN transactions will generate either a timeout, or a - response. If the response is a 420, 500, or 401, the client should + response. If the response is a 420, 500, or 401, the agent should try again as described in RFC 3489. Either initially, or after such - a retry, the STUN transaction will produce a timeout result, a - success result, a fundamentally non-recoverable failure result (error - codes 400, 431, or 600) or a failure result inapplicable to this - usage of STUN and thus unrecoverable (432, 433), or a 430 error. - These correspond to the "timeout", "success", "error" and - "race-failure" events, respectively. The 430 response code, as - described below, is generated when the server doesn't recognize the - STUN username, presumably because the BindingRequest was sent to the - initiator prior to receipt of the ICE Accept message by the - initiator. It ocurrence is thus a result of a failed race between - the BindingRequest and Accept message. As the state machine below - discusses, the client will retry in this case. + a retry, the STUN transaction might produce a non-recoverable failure + response (error codes 400, 431, or 600) or a failure result + inapplicable to this usage of STUN and thus unrecoverable (432, 433). + If this happens the transport address pair and its corresponding + candidate is considered invalid. If the STUN transaction produces a + 430 error or times out, the client SHOULD retry with a new STUN + Binding Request transaction. The 430 response code, as described + below, is generated when the server doesn't recognize the STUN + username because the BindingRequest was sent received prior to the + receipt of the answer. Its ocurrence is a result of a failed race + between the BindingRequest and the answer. This is remedied by + retrying, which allows the "slower" answer to be received. These + retry transactions carry the same USERNAME value as the original + Binding Request, and differ only in their STUN transaction ID. If + these retries have not produced a success response after Tg seconds, + the transport address pair is considered invalid. Tg SHOULD be + configurable. It is RECOMMENDED that it default to 50 seconds. This + is a reasonable approximation of the maximum SIP transaction + duration. - These events are fed into the finite state machine (FSM) described in - Figure 5. This figure shows the transitions between states that - occur on the completion of the STUN BindingRequest transaction or - upon the expiration of timers set by the FSM. + If the STUN transaction succeeds for a UDP transport address pair + (producing a success response), and the pair was previously in the + receive-valid state, it is considered valid. If the pair was + previously in the unknown state, it is considered send-valid. - race-failure,.......... - timeout/ . . .......... - Set . . . . Retry Fires/ - Retry Timer,. V . . Retry - +---------+ . +---------+ . - | | . | | . - | | .......| |<.... - | INIT |......................>| HAND | - | | race-failure, | SHAKING | - | | timeout/ | | - +---------+ Set +---------+ - . . Retry Timer, error, . . - . . Giveup Timer Giveup . . - error . . Fires . . - . . ............................. . success - . . . . - . ...C.............................. . - . . success . . + If a transport address pair is send-valid or valid, an agent MUST + generate a new STUN Binding Request transaction every Tr seconds. + This transaction ensures that NAT bindings for the transport address + pair remain open while the candidate is under consideration. They + can also be used to keep the bindings alive when the candidate is + promoted to active, as described in Section 7.7. Tr SHOULD be + configurable, and SHOULD default to 15 seconds. Each new Binding + Request transaction is processed according to the procedures in this + Section. It is possible for a previously valid candidate to later be + invalidated by a subsequent STUN transaction. This happens in cases + where the NAT bindings expire. + +7.4.1.2 Receive Validation + + As a result of providing a list of candidates in its offer or answer, + an ICE implementation will receive STUN Binding Request messages. An + agent MUST be prepared to receive STUN Binding Requests on each local + transport address from the moment it sends an offer or answer that + contains a candidate with that local transport address. Similarly, + it MUST be prepared to receive STUN Binding Requests on a local + transport address the moment it sends an offer or answer that + contains a STUN or TURN candidate derived from a local candidate + containing that local transport address. It can cease listening for + STUN messages on that local transport address after reliably sending + an updated offer or answer which does not include any candidates + equal to or derived from that local transport address. Here, + "reliably" means that the agent knows that the offer or answer was + received by its peer. This knowledge is based on the protocol + carrying the offer/answer exchanges. In the case of SIP, if the + offer is in an INVITE, the agent knows this was received by its peer + when a 200 OK or reliable provisional response [9] is received with + the answer. If the offer is in a reliable provisional response, the + agent knows it was reliably received when the PRACK arrives. If an + answer is in a 200 OK response, the agent knows this was received + when the ACK is received. + + The agent 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 answer, any STUN requests with these flags set. + The CHANGED-ADDRESS attribute in a BindingAnswer is set to the + transport address on which the server is running. + + 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, a shared secret is not needed. The tid's that are exchanged and + used to form the STUN USERNAME attribute do not actually require the + security properties associated with a shared secret in order for ICE + to operate securely; this is because ICE security is bootstrapped off + of the protocol carrying the offer/answer exchanges. + + One of the candidates will be in use as the active candidate. For + the transport addresses comprising that candidate, the agent will + receive both STUN requests and media packets on its associated local + transport addresses. The agent 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 [23], 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 + below). + + The STUN Binding Request can only be usefully processed once an + offer/answer exchange has completed. As a result, if an offeror + receives a STUN Binding Request message prior to the receipt of an + answer to its offer, it MUST reject the request with a 430 response. + This will cause the answerer to retry, and give time for the answer + (which is in transit) to arrive at the offerer. + + If the offer/answer exchange has completed, the agent 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 comparing it against the transport address pair + identifiers for each transport address pair as seen by that agent. + If there is no match, the STUN Binding Request generates a 400. If + there is a match, the resulting transport address pair is called the + matching transport address pair. The user agent proceeds with the + processing of the request and generation of a response as per RFC + 3489. In addition, the if the state of that transport address pair + was previously unknown, it changes to receive-valid. If the state + was previously send-valid, it moves to valid. + + An agent will continue to receive periodic STUN transactions as long + as it had listed its transport address in an a=candidate attribute. + It MUST process those transactions according to this section. It is + possible that a transport address pair that was previously valid may + become invalidated as a result of a subsequent failed STUN + transaction. + +7.4.1.3 Learning New Candidates from Connectivity Checks + + ICE makes use of candidate addresses learned through protocols like + STUN, as described in Section 7.1. These addresses are learned when + STUN requests are sent to configured STUN servers. However, the + peer-to-peer STUN connectivity checks can themselves provide + additional candidates that ICE can make use of. This happens when + two agents are separated by a symmetric NAT. When the agent behind + the symmetric NAT sends a Binding Request to the other agent (which + can have a public address or be behind any type of NAT except for + symmetric), the symmetric NAT will create a new NAT binding for this + Binding Request. Because of the properties of symmetric NAT, that + binding can be used be the agent on the public side of the symmetric + NAT to send packets back to the agent behind the symmetric NAT. + + To do this, ICE agents dynamically learn new candidates by examining + the source IP addresses and MAPPED-ADDRESS attributes in STUN Binding + Requests and Responses respectively. If they don't match any + existing candidates, a new candidate is added. This candidate + corresponds to the new IP address and port created by the symmetric + NAT, and is a new point of contact for the agent behind the symmetric + NAT. Since that candidate is only reachable from the very specific + IP address and port where the STUN request was sent to, the new + candidate is paired up with that transport address on the other + agent. Since all candidates need to have properties, such as tids, + priorities and candidate IDs, these are all computed algorithmically, + so that they can be determined by both agents just from the STUN + message. + + The specific procedures on receipt of a Binding Request and Response + for accomplishing this are described here. + +7.4.1.3.1 On Receipt of a Binding Request + + When a STUN Binding Request is received which generates a success + response, the source IP address and port of that request is compared + all existing remote transport addresses. If there is no match, the + agent creates a new remote candidate, and adds a transport address to + it. It sets the IP address and port of this new remote transport + address to the IP address and port that was present in the incoming + Binding Request. Since this is a new candidate transport address, it + requires a new tid. The agent creates one algorithmically, by + concatenating the tid of the remote transport address in the matching + transport address pair (recall that the matching transport address + pair is the one whose transport address pair ID matched the username + of the incoming Binding Request) with the string representation of + the source IP address and port from the incoming Binding Request. + This string representation is defined using the grammar for + "hostport" from RFC 3261 [3], which defines the familiar notation of + the IP address and port separated by a colon. + + The priority of the new candidate MUST be set to the priority of the + remote candidate in the matching transport address pair. There is no + need to compute the candidate ID for this new candidate. + + Though this is a valid transport address, the agent does not pair it + up with each of its own transport addresses. Rather, it pairs it up + only with the native transport address from the matching transport + address pair. This creates a new transport address pair. Since + connectivity has been verified in the receive direction, the agent + sets its state to receive-valid. As with all other transport address + pairs, the agent will attempt to validate send capabilities by + sending a STUN Binding Request according to the procedures in + Section 7.4.1.1. + + It is important to note that this process creates a new remote + transport address, not a whole new remote candidate. For a whole + remote candidate to come into existence, all of its component + transport addresses must come into existence, and all must have been + obtained as a result of a STUN Binding Requests between transport + address pairs in the same pairing. As an example, consider the + pairing in Figure 2. If the peer is behind a symmetric NAT, the + Binding Request sent from C to A might produce a new remote transport + address for RTP. To create a full candidate, a STUN Binding Request + from D to B has to also create a new remote transport address, to be + used for RTCP. If this were to happen, the resulting set of + relationships is shown in Figure 3. To simplify the diagram, + associated local transport address relationships have been omitted. + Notice how the tids of the new remote candidate have been constructed + by concatenating the tids of the original remote candidate with the + newly discovered transport addresses, here, {R,S}. + + ............. ............. . . . . - V V V V - +---------+ +---------+ - | | | | - | | | | - | BAD |. | GOOD | - | | | | - | | | | - +---------+ +---------+ + . -- . . -- . + . | A|---------------------------------------| C| . + . -- -----------+ Transport . -- . + . Transport . | Address . Transport . + . Address . | Pair . Address . + . tid=W . | ID=WY . tid=Y . + . . | . . + . . | . . + . . | . . + . -- . | . -- . + . | B|-----------C---------------------------| D| . + . -- ---------+ | Transport . -- . + . Transport . | | Address . Transport . + . Address . | | Pair . Address . + . tid=X . | | ID=XZ . tid=Z . + . . | | . . + ............. | | ............. + | | remote + native | | candidate + candidate | | + | | ............. + | | . . + | | . -- . + | +---------------------------| R| . + | Transport . -- . + | Address . Transport . + | Pair . Address . + | ID=WYR . tid=YR . + | . . + | . . + | . . + | . -- . + +-----------------------------| S| . + Transport . -- . + Address . Transport . + Pair . Address . + ID=XZS . tid=ZS . + . . + ............. + peer-derived + remote candidate - Figure 5 + 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 local 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. If, from the INIT - state, the STUN transaction times out, the FSM enters the HANDSHAKING - state. At this point, there are two likely reasons that the STUN - transaction 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. It is also - possible that the STUN transaction timed out due to a persistent - network failure, in which case, a retry is in order. As such, the - HANDSHAKING 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 HANDSHAKING - 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 50ms. Note that - this timer does not mean that a STUN request is repeated every 50ms. - It means that a new STUN transaction begins 50ms after the completion - of the previous one. STUN transactions themselves employ - exponentially back off retransmit timers. 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. +7.4.1.3.2 On Receipt of a Binding Response - If, from the INIT state the STUN transaction generates a race-failure - event, it means that the peer has not yet completed the - initiate/accept exchange, and thus the username has not been - allocated. Another BindingRequest transaction needs to take place to - try again. Thus, the same retry and giveup timers as in the timeout - event are started. + When an agent receives a successful Binding Response, it examines the + MAPPED-ADDRESS attribute in that response. If the MAPPED-ADDRESS + does match any of the existing candidate transport addresses, this + represents a new peer-derived transport address. - If, from the INIT state, the STUN transaction generates an error, the - FSM moves into the BAD state. This state means that the connection - is definitively unreachable, and it will not be used subsequently in - the session. + The agent creates a new local candidate, and adds a transport address + to it. It sets the IP address and port of this new native transport + address to the IP address and port that was present in the MAPPED- + ADDRESS attribute of the Binding Response. Since this is a new + candidate transport address, it requires a new tid. The agent + creates one algorithmically, by concatenating the tid of the native + transport address in the transport address pair that was being + validated by the Binding Request with the string representation of + the source IP address and port from the MAPPED-ADDRESS attribute. + This string representation is defined using the grammar for + "hostport" from RFC 3261 [3], which defines the familiar notation of + the IP address and port separated by a colon. - If, while in the HANDSHAKING state, the Giveup timer fires, or the - STUN transaction results in an error, the client moves into the BAD - state. If, while in the HANDSHAKING state, the Rapid Retry timer - fires, a new STUN transaction is started. The output of that - transaction will be subsequently fed into the FSM, but upon - initiation of the retry attempt there is no change in state. If the - pending BindingRequest transaction succeeds, the FSM moves into the - GOOD state. This transport connection is viable for communications. + The priority of the new candidate MUST be set to the priority of the + native candidate that was being validated by the Binding Request. + The agent SHOULD assign a new candidate ID to this candidate. - Once one of the connections in the connection table enters the GOOD - state, the client SHOULD begin using it for communications. It - SHOULD cease any ongoing transactions and terminate FSMs for - connections of lower priority. If, another connection of higher - priority should subsequently enter the GOOD state, the client SHOULD - switch to that one, and once more cease all ongoing transactions and - terminate FSMs for connections of lower priority. It SHOULD perform - this switch after waiting a small period of time (2 seconds is - RECOMMENDED) to prevent against quick changes in transport address as - each of the ongoing connectivity checks completes. If there are - multiple GOOD connections whose priorities are equal and higher than - any other GOOD connection, the client SHOULD pick one randomly and - use that. It SHOULD NOT change to another one of equal priority - later on. Each change in address is likely to cause a change in - transport characteristics, and manifest itself as a "glitch" to the - user. + Though this is a valid transport address, the agent does not pair it + up with each of the remote transport addresses. Rather, it pairs it + up only with the remote transport address from the transport address + pair that was being validated. This creates a new transport address + pair. Since connectivity has been verified in the send direction, + the agent sets its state to send-valid. As with all other transport + address pairs, the agent will attempt to validate receive + capabilities by waiting for a a STUN Binding Request according to the + procedures in Section 7.4.1.2. - To send media on a connection, the client sends media packets - (whether they are RTP or RTCP or something else) to the remote - transport address, from the local transport address. + It is important to note that this process creates a new native + transport address, not a whole new candidate. For a whole native + candidate to come into existence, all of its component transport + addresses must come into existence, and all must have been obtained + as a result of a STUN Binding Requests between transport address + pairs in the same pairing. -5.3.5 Receiving STUN Requests +7.4.2 TCP Connectivity Checks - When a client receives a STUN request (presumably after - disambiguating it from a media packet), it follows the logic - described in this section. +7.4.2.1 Connection Establishment - 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 "local username fragment" column in the connection table. - If it doesn't match any rows, the client generates a 400 response. - If it matches one or more rows, the client checks the suffix of the - username against the "remote username fragment" column in those - matching rows. If the final result doesn't match any rows, the - client generates a 430 response. If the final result matches a - single row, that row identifies the connection on which the STUN - request was received. The client then proceeds with the processing - of the request and generation of a response as per RFC 3489. + Because of the connection-oriented nature of TCP, the connectivity + checks work differently. After the offer/answer exchange completes, + each agent will have a set of TCP candidates at which it is waiting + to receive a connection on, and it will have a similar set from its + peer. Thus, a pairing of TCP candidates allows for the possibility + of TCP connections in each direction. Unlike the UDP checks, where + the STUN packets are sent from the native transport addresses to the + remote ones, the TCP connections are not opened from the native TCP + transport addresses to the remote ones. This would represent a + simultaneous open, and represent an unusual condition that would + either fail, or at best result in a single TCP connection. Rather, + ICE desires to attempt two connections, one in each direction, and + use one of them if both happen to succeed. - 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 of the matching connection from the previous - paragraph. If they don't match, and that remote transport address is - not elsewhere in the table, this source transport address is itself - another possible candidate. As with other candidates, it must be - associated with a STUN remote username fragment, remote password and - remote priority. These are obtained from the values of these columns - for the matching connection in the table. This candidate is then - paired with each local transport address, and the resulting set of - connections are added to the connection table and verified using STUN - connectivity checks as per Section 5.3.4. + To accomplish this, each agent will attempt to open a connection to + each remote transport address in the transport address pair, and do + so "from" its native transport address. Here, however, "from" means + something different than the UDP case. If the native transport + address is a local transport address, the agent opens the TCP + connection from the same IP interface used to obtain the local + transport address, but from a different and ephemeral port. Indeed, + that port MUST NOT be the same as the port in the local transport + address. If the native transport address is a TURN-derived TCP + transport address, no attempt is made to open a connection at all. + TURN-derived TCP transport addresses can only be used in passive + mode. - When will the source transport address of the BindingRequest not - match an existing candidate remote transport address? This happens - when there is a NAT between the peers which is not on the path - between each peer and the UNSAF servers. + As such, for each TCP transport address pair, there will be either + zero, one, or two connection attempts. If the transport address + pairs are both TURN-derived, there will be zero (both sides passive). + If one of the transport addresses is local, and the other TURN + derived, there will be one connection attempt. The agent owning the + local transport address will be in active mode, and the agent owning + the TURN-derived one will be in passive mode. If both are local + transport address, there will be two attempts, and each agent will + act in active mode. -5.3.6 Management of Resources + Because a transport address pair can produce multiple connections, + validity becomes a property of the TCP connection itself. A + transport address pair is considered valid if at least one valid + connection has been established within it. An entire pairing is + valid if all transport address pairs are valid. - The beginning of a multimedia session results in the creation of - several resources to support ICE. These include gathered addresses, - both local and derived, along with the local STUN servers that run on - the local addresses. These resources must be maintained and - eventually freed. +7.4.2.2 Sending STUN Binding Requests - It is RECOMMENDED that all gathered addresses be retained for the - duration of the session. Even if they are not used initially, this - allows them to be used later in the session should conditions change, - requiring a signaling operation to update the set of candidate - addresses. Maintaining these resources depends on the type of - resource. For a local transport address, nothing is required. The - socket is maintained until freed by the ICE application. For STUN - derived transport addresses, the bindings in the NAT for that address - need to be maintained. If the derived transport address is used by - the peer for media, the media itself serves to keep the bindings - alive (see Section 5.3.7). A client can determine that a STUN - derived transport address was used for media when the RTP packet - arrives at the associated local transport address. For the other - STUN derived transport addresses, the client SHOULD periodically - generate STUN transactions to the STUN server. Every 20 seconds is - RECOMMENDED. + Once the connection is established, the agent which opened the + connection (that is, acted in active mode) sends a STUN Binding + Request over that connection. STUN Binding Requests as described in + RFC 3489 are not normally sent over UDP, but when used in conjunction + with ICE for connectivity checks, they are sent over TCP. - For TURN derived transport addresses, the bindings in the NAT along - with the mappings in the TURN server need to be maintained. Media - traffic itself can accomplish that. The client will know that its - TURN derived transport address is in use when an RTP packet arrives - at the associated local transport address. For other TURN derived - transport addresses, the TURN keepalive mechanisms SHOULD be used. + This unusual operation requires some explanation. At first glance, a + successful TCP connection ought to be sufficient. Clearly, + connectivity is established, as TCP packets were exchanged in both + directions via the TCP handshake. While that is true, the STUN + Binding Requests serve many purposes, only one of which is to + literally test connectivity. The STUN requests also serve as a + correlation vehicle, allowing the agent to match the source of a + connection attempt with the offer/answer signaling driving the entire + mechanism. For example, in the case of a forked SIP INVITE carrying + an offer, the UAC may receive two connection attempts to each of its + passive TCP addresses, one from each branch of the fork. These are + readily disambiguated by the STUN Binding Request which will follow, + as the tid in the USERNAME tells the UAC which branch has initiated + the connection. - Once the STUN servers are started on the local transport addresses, - they MUST run until a valid media packet is detected on that - transport address. Once a media packet is received, it signals that - the peer has completed its connectivity checks and has decided to use - that transport address (or the derived transport address, as the case - may be) for media communications. 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 + More importantly, however, the STUN Binding Request is an essential + part of the security properties of ICE. Without it, an entity + eavesdropping the signaling messages would be able to deny service or + hijack media connections, and such attacks would require encryption + of the offer/answer exchanges (using a mechanism like SIPS [3]) to + prevent. However, when a STUN Binding Request exchange is added, + these attacks are completely foiled without the need for SIPS, + raising the overall security of ICE substantially with minimal cost. + These properties of ICE are discussed thoroughly in Section 12. -5.3.7 Binding Keepalives + As such, once an agent has actively opened a TCP connection to the + remote agent, it sends a STUN Binding Request over that connection. + Recall that STUN messages include length indicators, allowing them to + be framed over a connection-oriented transport protocol. The Binding + Request MUST contain the USERNAME attribute. This attribute MUST be + set to the transport address pair ID of the corresponding transport + address pair as seen by its peer. Thus, for the first transport + address pair in Figure 2, if the agent on the left sends the STUN + Binding Request, the USERNAME will have the value YW. The request + MAY contain the MESSAGE-INTEGRITY attribute, computed according to + RFC 3489 procedures. The MESSAGE-INTEGRITY The Binding Request MUST + NOT contain the CHANGE-REQUEST or ANSWER-ADDRESS attribute. The STUN + BindingRequest message SHOULD NOT be retransmitted over the + connection. - Once the STUN connectivity checks complete, STUN packets are no - longer used. However, bindings in intermediate NATs need to be kept - alive so that the media can continue to flow. Doing so is the - responsibility of the media protocol. + The STUN will generate either a timeout, or a response. If the + response is a 420, 500, or 401, the agent should try again as + described in RFC 3489. Either initially, or after such a retry, the + STUN transaction might produce a non-recoverable failure response + (error codes 400, 431, or 600) or a failure result inapplicable to + this usage of STUN and thus unrecoverable (432, 433). If this + happens the connection is considered invalid. If the STUN + transaction produces a 430 error or times out, the client SHOULD + retry with a new STUN Binding Request transaction. The 430 response + code is a result of a failed race between the BindingRequest and the + answer. This is remedied by retrying, which allows the "slower" + answer to be received. These retry transactions carry the same + USERNAME value as the original Binding Request, and differ only in + their STUN transaction ID. If these retries have not produced a + success response after Tg seconds, the connection is considered + invalid. Tg SHOULD be configurable. It is RECOMMENDED that it + default to 50 seconds. This is a reasonable approximation of the + maximum SIP transaction duration. - In the case of RTP, the RTP packets themselves normally come - sufficiently quickly to keep the bindings alive. However, several - cases merit further discussion. Firstly, in some RTP usages, such as - SIP, the media streams can be "put on hold". This is accomplished by - using the SDP "sendonly" or "inactive" attributes, as defined in RFC - 3264 [4]. RFC 3264 directs implementations to cease transmission of - media in these cases. However, doing so may cause NAT bindings to - timeout, and media won't be able to come off hold. + If the STUN Binding Request generates a successful response, the + connection over which it was sent is considered valid. Furthermore, + the agent stores the IP address and port from the MAPPED-ADDRESS + response in the STUN Binding Response. This is called the "apparent" + native transport address for the active side of the connection. It + will be used later if this connection is used for media transport. - As such, clients SHOULD instead send a media packet periodically, - independent of whether the stream is "sendonly", "recvonly" or - "inactive". At least once every 20 seconds is RECOMMENDED. These - packets can be sent using any of the payload formats listed by the - peer in its SDP. For audio streams, It is RECOMMENDED that - implementations support the RTP payload format for comfort noise [5], - which makes a good choice. For video codecs, a minimally coded frame - is a good choice. + Once a connection is valid, the agent which initiated the connection + MUST generate a new STUN Binding Request transaction every Tr + seconds. This transaction ensures that NAT bindings for the + connection remain open while the connection is under consideration as + a candidate. Tr SHOULD be configurable, and SHOULD default to 15 + seconds. Each new Binding Request transaction is processed according + to the procedures in this section. It is possible for a previously + valid candidate to later be invalidated by a subsequent STUN + transaction. This happens in cases where the NAT bindings expire. + Note that, unlike the UDP case, STUN is sent only while a connection + is is not active for media. If the connection is used as the active + connection for media, STUN MUST NOT be sent. - Secondly, some RTP payload formats, such as the payload format for - text conversation [19], may send packets so infrequently that the - interval exceeds the NAT binding timeouts. In such cases, the - implementation should send some any kind of content, if possible. If - the payload type doesn't allow anything meaningful to be sent, even a - malformed RTP packet is superior to nothing at all; the malformed - packet would be rejected by the peer, and have the side effect of - keeping the NAT bindings open. +7.4.2.3 Receiving STUN Requests -6. Running STUN on Derived Transport Addresses + When an agent acted as the passive side of a TCP connection, it will + receive a STUN Binding Request over that connection. - 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. They are tutorial in nature. + One of the candidates will be in use as the active candidate. For + the transport addresses comprising that candidate, the agent will + receive both STUN requests and media packets on its associated local + transport addresses. The agent 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 [23], 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 + below). -6.1 STUN on a TURN Derived Transport Address + The STUN Binding Request can only be usefully processed once an + offer/answer exchange has completed. As a result, if an offeror + receives a STUN Binding Request message prior to the receipt of an + answer to its offer, it MUST reject the request with a 430 response. + This will cause the answerer to retry, and give time for the answer + (which is in transit) to arrive at the offerer. - +----------+ - | |192.0.2.1:26524 - | TURN X - | Server | - | | - | | - +----------+ - 192.0.2.1:7764. ^192.0.2.1:7764 - . . - . .192.0.2.88:5063 - +----------+ - | NAT | - +----------+ - TURN . . - Response . . TURN Request - . . - 10.0.1.1:8866 V .10.0.1.1:8866 - +----------+ +----------+ - | | | | - | Client | | Client | - | | | | - | A | | B | - | | | | - +----------+ +----------+ + If the offer/answer exchange has completed, the agent 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 comparing it against the transport address pair + identifiers for each transport address pair as seen by that agent. + If there is no match, the STUN Binding Request generates a 400. If + there is a match, the resulting transport address pair is called the + matching transport address pair. The user agent proceeds with the + processing of the request and generation of a response as per RFC + 3489. In addition, the agent stores the source IP address and port + of the Binding Request, and associates it with the connection. This + address is called the "apparent" remote transport address for this + connection. - Figure 6 + An agent will continue to receive periodic STUN transactions as long + as it had listed its transport address in an a=candidate attribute. + It MUST process those transactions according to this section. It is + possible that a transport address pair that was previously valid may + become invalidated as a result of a subsequent failed STUN + transaction. - Consider a client A that is behind a NAT, shown in Figure 6. It - connects to a TURN server on the public side of the NAT. To do that, - A binds to a local transport address, say 10.0.1.1:8866, and then - sends a TURN request to the TURN server. The NAT translates the - net-10 address to 192.0.2.88:5063. Assume that the TURN server is - running on 192.0.2.1 and listening for TURN traffic on port 7764. - The TURN server allocates a derived transport address 192.0.2.1:26524 - to the client (shown as the X on the TURN server in the diagram), and - returns it in the TURN response. Remember that all traffic from the - TURN server to the client is sent from 192.0.2.1:7764 to - 10.0.1.1:8866, including the TURN response. + Note that, unlike the UDP case, there will never be simultaneous + transmission of media and STUN packets over TCP connections. This is + because the connection is listed as on hold according to comedia + procedures, and no media will be transmitted. ICE will establish the + connections as described here. Once established, an updated offer/ + answer exchange can promote those connections to active usage through + the comedia "exist" mechanism, as described below. The additional + offer/answer exchange provides a barrier synchronization point at + which a TCP connection switches from ICE control to control by the + media source and sinks. Once it is active, STUN packets will no + longer be sent on the connection. - Now, the client runs a STUN server on 10.0.1.1:8866, and advertises - that its server actually runs on 192.0.2.1:26524. Another client, B, - sends a STUN request to this server. It sends it from a local - transport address, 192.0.2.77:1296. When it arrives at - 192.0.2.1:26524, it is discarded since client A has not sent a packet - to 192.0.2.77:1296. Once client A gets client B's accept message, it - will learn about B's candidate address, and generate a STUN request - towards it. This results in a permission being installed in the TURN - server, so that packets from 192.0.2.77:1296 will now be accepted. - The next STUN request from client B will therefore succeed. This is - the normal mode of operations for port restricted NAT; as described - in TURN, the server turns a symmetric NAT into a port restricted one - [8]. +7.5 Promoting a Valid Candidate to Active - +----------+ - | |192.0.2.1:26524 STUN Request - | TURN X<............................... - | Server | STUN Response . - | |......................... . - | |192.0.2.1:26524 . . - +----------+ . . - 192.0.2.1:7764 . ^ 192.0.2.1:7764 . . - . . . . - 192.0.2.88:5063 V . 192.0.2.88:5063 . . - +----------+ . . - | NAT | . . - +----------+ . . - 192.0.2.1:7764 . ^ 192.0.2.1:7764 . . - . . 192.0.2.77:1296 . - . . . . - 10.0.1.1:8866 V . 10.0.1.1:8866 V .192.0.2.77:1296 - +----------+ +----------+ - | | | | - | Client | | Client | - | | | | - | A | | B | - | | | | - +----------+ +----------+ +7.5.1 Minimum Requirements - Figure 7 + As the STUN connectivity checks run, they will result in the + validation of pairings. Once validated, a pairing can be used by + promoting it to active. This promotion occurs by placing the + transport addresses for the native candidate of the pairing into the + m/c line and sending an updated offer. It MAY promote a candidate + associated with any validated pairing at any time, as long as the + candidate had been provided in series of a=candidate attributes in + the most recent offer (in other words, an agent can't validate a + candidate, omit that candidate from the a=candidate attribute of an + offer, and then later on, generate a new offer that promotes the + candidate to active). The procedures for doing so are described + here. - As shown in Figure 7, client B will retry, sending it STUN request - from 192.0.2.77:1296 to 192.0.2.1:26524. This successful STUN - request is forwarded to the client, sent with a source address of - 192.0.2.1:7764 and a destination address of 192.0.2.88:5063. This - passes through the NAT, which rewrites the destination address to - 10.0.1.1:8866. This arrives at A's STUN server. The server observes - the source address of 192.0.2.1:7764, and generates a STUN response - containing this value in the MAPPED-ADDRESS attribute. The STUN - response is sent with a source address of 10.0.1.1:8866, and a - destination of 192.0.2.1:7764. This arrives at the TURN server, - which, because of current destination is 192.0.2.1:7764, sends the - STUN response with a source address of 192.0.2.1:26524 and - destination of 192.0.2.77:1296, which is B's STUN client. + Any candidates which the agent would like to retain as valid + candidates are also included in a=candidate lines in the offer. It + SHOULD include any candidates learned from the peer-to-peer discovery + processing of Section 7.4.1.3, and SHOULD include any candidates of + higher priority than the one just promoted to active. It SHOULD omit + candidates of lower priority than the one being promoted to active. + It SHOULD omit any for whom all pairings that include that candidate + have become invalid. - Now, as far as A is concerned, it has obtained a new candidate - transport address of 192.0.2.1:7764. 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 - 10.0.1.1:8866. If it sends to 192.0.2.1:7764, the packet goes to the - TURN server, and since the destination address is set to - 192.0.2.77:1296, 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 candidate transport address. Its priority - is derived from the priority of client B's public IP address. + If a candidate is omitted, and that candidate was a TURN-derived + transport address, the agent SHOULD de-allocate the address from the + TURN server. If a local candidate was omitted, along with all of its + derived transport addresses, local operating system resources for + that candidate SHOULD be de-allocated. - 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. + Once it has decided on the set of candidates to provide in the + updated offer, the agent constructs the offer and follows the + procedures in Section 7.6 which defines general subsequent offer/ + answer processing. -6.2 STUN on a STUN Derived Transport Address +7.5.2 Suggested Algorithm - 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 10.0.1.1:8866, and then sends a STUN request - to the STUN server. The NAT translates the net-10 address to - 192.0.2.88:5063. Assume that the STUN server is running on 192.0.2.1 - and listening for STUN traffic on port 3478, the default STUN port. - The STUN server sees a source IP address of 192.0.2.88:5063, and - returns that to the client in the STUN response. The NAT forwards - the response to the client. + ICE leaves substantial variability to implementors around when an + agent decides to generate a new offer. However, there are good ways + to do this, and bad ways. Perhaps the worst algorithm possible would + be to generate a new offer every time a candidate with higher + priority than the active one becomes valid. This algorithm will + likely result in a large number of offer/answer exchanges in rapid + succession, many of which will produce "glare" as each agent will + independently initiate an exchange. This will consume CPU and + network resources for little benefit. Rather, the ideal algorithm + strikes a balance between usage of network resources and the desire + to use the ideal pair of candidates. - Now, the client runs a STUN server on 10.0.1.1:8866, and advertises - that its transport address is 192.0.2.88:5063. Another client, B, - sends a STUN request to this address. It sends it from a local - transport address, 192.0.2.77:1296. When it arrives at - 192.0.2.88:5063 (on the NAT), the NAT rewrites the source address to - 10.0.1.1:8866, assuming that it is of the full-cone or restricted - variety [1], and the permission for 192.0.2.77:1296 is open. This - arrives at A's local STUN server. The server observes the source - address of 192.0.2.77:1296, and generates a STUN response containing - this value in the MAPPED-ADDRESS attribute. The STUN response is - sent with a source address of 10.0.1.1:8866, and a destination of - 192.0.2.77:1296. This arrives at B's STUN client. + The following algorithm provides a good tradeoff, and usage of this + algorithm is RECOMMENDED. The algorithm results in a bounded number + of additional offer/answer exchanges after the initial one - never + more than two, and frequently one or zero. The algorithm almost + never produces a glare condition. - Now, as far as A is concerned, the STUN request had a source - transport address which was already known to A, presumably from an - ICE exchange. As far as B is concerned, the check succeeded, and the - address is viable. + Once the initial offer/answer exchange completes, media flow will + happen, though not optimally (where optimal is defined by the + policies used to set the priorities of the candidates), as long as + the candidate that is active has been validated. Thus, the objective + of the algorithm is to quickly make sure that there is a valid path + for media (to avoid clipping), and then do a single offer/answer + exchange to use the highest priority pairing that was validated. -7. XML Schema for ICE Messages + After the initial offer/answer exchange, each agent sets a timer Tu. + This timer SHOULD have a configurable baseline value, which SHOULD + default to 3 seconds. The actual timer is set to this baseline, plus + a time value chosen uniformly beween -1 and 1 seconds. This causes + the actual timer to be randomized so that the timer doesnt fire + simultaneously at each agent. In addition, each agent monitors the + status of the active pairing. If the active media stream is UDP- + based, the status of the active candidates is equal to the status of + the pairing with matching transport addresses. In the case of TCP- + based media, the active media stream is never active initially, since + it always begins with the "holdconn" state. - This section contains the XML schema used to define the initiate and - accept messages. Any protocol that uses ICE needs to map the - parameters defined here into its own messages. + If, when Tu fires, the active pairing has not been validated, and + there exists at least one pairing that has been validated, the agent + generates a new offer. This offer promotes its highest priority + candidate with a validated pairing to the active candidate. If there + are no pairings that have been validated when the timer fires, the + agent waits until one is validated, and once that happens, sets a + timer to fire randomly between 0 and 2 seconds. When the timer + fires, a new offer is generated that promotes the candidate from this + validating pairing to active. If the active pairing is validated + when the timer fires, the agent does nothing at this time. - 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. + If new offer is to be sent, the agent includes the new active + candidate in the a=candidate attribute list. It also includes all + candidates with higher priority than the one that is active, + including ones it learned from the connectivity checks themselves. - - - - - - - This is the root element, which holds a - media-streams elements. - - - - - - - - - - - There are zero or more media stream - elements. Each defines attributes for a specific media - stream. - - - - - - - - - - - - - - - Each candidate is a possible point - of media reception. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + At this point, media is flowing successfully, since a valid candidate + is active. However, it may not be optimal. So, the next stage of + the algorithm is to let the connectivity checks continue. If those + checks indicate that a pairing between the two highest priority + candidates from both agents has been validated, each agent sets a + timer whose value is randomly set between 0 and 2 seconds. When the + timer fires, a new offer is generated that promotes the candidate + from this validating pairing to active. Otherwise, when the + connectivity checks have all concluded, such that no pairing exists + in the invalid state, each agent sets a timer whose value is randomly + set between 0 and 2 seconds. When the timer fires, a new offer is + generated that promotes the candidate from the valid pairing with the + highest priority to active. -8. Example +7.6 Subsequent Offer/Answer Exchanges - In the example that follows, 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. The example shows a flow - of two clients where one is behind a full cone NAT, and the other is - on the public Internet. + An offer/answer exchange within a session can occur at any time, + whether it is the result of the algorithm described in Section 7.5.2, + or because one of the agents wishes to add or remove a media stream, + or add a codec, and so on. - A NAT STUN B - |(1) STUN Req P1,STUN-PUBLIC | | - |---------------->| | | - | |(2) STUN Req U, STUN-PUBLIC | - | |---------------->| | - | |(3) STUN Res STUN-PUBLIC, U [U] | - | |<----------------| | - |(4) STUN Res STUN-PUBLIC, P1 [U] | | - |<----------------| | | - |(5) Intitiate {P2,ufrag1A,pass1A,q=0.4} | - |{U,ufrag2A,pass2A,q=0.4} | | - |---------------------------------------------------->| - | | |(6) STUN Req P3,STUN-PUBLIC - | | |<----------------| - | | |(7) STUN Res STUN-PUBLIC,P3 [P3] - | | |---------------->| - |(8) Accept {P3,ufrag1B,pass1B,q=0.4} | - |<----------------------------------------------------| - | |(9) STUN Req P3,P2 | - | |(ufrag1Aufrag1B,pass1A) | - | |<----------------------------------| - | |Timeout | | - | |(10) STUN Req P3,U | - | |(ufrag2Aufrag1B,pass2A) | - | |<----------------------------------| - |(11) STUN Req P3,P1 | | - |(ufrag2Aufrag1B,pass2A) | | - |<----------------| | | - |(12) STUN Res P1,P3 [P3] | | - |---------------->| | | - | |(13) STUN Res U,P3 [P3] | - | |---------------------------------->| - |(14) STUN Req P2,P3 | | - |(ufrag1Bufrag1A,pass1B) | | - |---------------->| | | - | |(15) STUN Req W,P3 | - | |(ufrag1Bufrag1A,pass1B) | - | |---------------------------------->| - | |(16) STUN Res P3,W [W] | - | |<----------------------------------| - |(17) STUN Res P3,P2 [W] | | - |<----------------| | | - |(18) STUN Req P1,P3 | | - |(ufrag1Bufrag2A,pass1B) | | - |---------------->| | | - | |(19) STUN Req U,P3 | - | |(ufrag1Bufrag2A,pass1B) | - | |---------------------------------->| - | |(20) STUN Res P3,U [U] | - | |<----------------------------------| - |(21) STUN Res P3,P1 [U] | | - |<----------------| | | +7.6.1 Sending of an Offer - The initiator, client A, binds to a local transport address P1, which - will be used as an associated local transport address. As such, it - sends a STUN request to its STUN server (message 1). This passes - through a NAT, and the NAT maps private address P1 to public address - U (message 2). The STUN server mirrors this public address in the - MAPPED-ADDRESS of the STUN response (message 3), and it is forwarded - to the initiator (message 4). Now, client A has a STUN derived - transport address of U. It also binds to a second local transport - address, P2, which will be a usable local transport address. It - starts STUN servers on both local transport addresses P1 and P2. It - then generates an Initiate request to client B (message 5) which - contains both of the gathered transport addresses P2 and U, along - with username fragments and passwords. + The meaning of a=candidate attributes within a subsequent offer have + the same meaning they do in an initial offer. They are a request for + the peer to attempt (or continue to attempt if the candidate was + provided previously) a connectivity check using STUN from each of its + own candidates. As such, an a=candidate attribute is included in + subsequent offers when (1) connectivity checks haven't concluded yet + to that candidate, or (2) the checks have concluded, and the + candidate is currently active. In that case, STUN is used to keep + the bindings active. - Client B is not behind a NAT. It binds to a local transport address - P3, and sends a STUN request to its STUN server (message 6). This is - responded to by the STUN server (message 7). The client observes - that this address is identical to its local transport address, and - therefore that local transport address is, which was targeted for an - associated local transport address, is promoted to a usable local - transport address. It then sends an Accept message to client A, - including this transport address and its username fragment and - password (message 8). + If an agent sends an offer which omits candidates it had sent to its + peer previously, it MUST cease connectivity checks from that + candidate. Any pairings that include the absent native candidate are + discarded. Any STUN transactions in progress from that candidate are + immediately terminated - no further retransmissions take place, and + no further transactions from that candidate will be made. If a TCP + connection was opened to or from that candidate, and that connection + is not listed as the active one in the offer, the connection is torn + down. - Once the Accept message is sent, the client can perform its STUN - connectivity checks. B has a single local transport address (P3), - which it matches up with A's two remote transport addresses (P2 and - U). B tries P2 (message 9). This request fails since P2 is a - private address. In parallel, B tries U (message 10). Since A's NAT - is full cone, this packet is accepted and is passed to client A - (message 11). Client A generates a response (message 12) which is - forwarded to client B (message 13). The source transport address in - the STUN packet, P3, is already known to client A, and thus no new - candidates are learned. Client B learns that client A is reachable - at transport address U, but not P3. Thus, it can begin sending media - to U from local transport address P3. + The offer MAY contain a new active candidate in the m/c line. If the + new active transprot address is UDP, candidate is encoded into an + update offer as described in Section 7.2. The transport addresses + constituting the candidate SHOULD also be listed in a=candidate + attributes, so that STUN can be used as an ongoing keepalive. - Once the Accept message arrives at client A, it can begin its - connectivity checks. It has two local transport addresses P1 and P2, - which it combines with client Bs single transport address P3. It - tries to send a STUN packet from P2 to P3 (message 14). Since the - NAT has not seen source address P2 yet, it maps it to a new public - transport address W, and the STUN request is forwarded to client B - (message 15). Client B generates a STUN response (message 16), which - is forwarded back to client A (message 17). Based on this, client A - learns that it can reach P3 from P2. Client B learns a new remote - transport address, W. However, the priority of this address is the - same as P2, which is 0.4, and equal to the priority of address U, to - which client B has already connected. Thus, it does not bother to - perform the check (such a check would have succeeded if it had been - done). + If the new active transport address is TCP, it is more complicated. + Recall that each TCP connection is opened from one of the agents to + the other, such that, for each connection, one agent has the active + role, and the other, the passive. The ICE mechanisms allow the + active agent to actually choose a specific connection for use in an + offer, so long as the agent has used a different ephemeral port for + each connection it initiated (which is almost always the case). If, + however, an agent was in the passive role, it cannot choose a + specific connection. Rather, it can choose a specific native + transport address which may have been used to receive multiple + connections. This assymetric behavior brings with it some important + security properties, which are discussed in Section 12. - While the P2->P3 check is taking place, client A also sends a STUN - request from P1 to P3 (message 18). This passes through the NAT, - which maps the source transport address to the same public address it - allocated previously, U. This STUN request arrives at client B - (message 19). It generates a response (message 20), which is - forwarded to client A (message 21). Based on this check, client A - learns that P3 is also reachable from P1. Client B did not learn a - new candidate transport address, since U was already known. Now, - client A can send media to P3 from either P1 or P2. + If the agent was the active one and established the connection, it + includes its apparent native transport address in the m/c line of the + SDP (recall that this address was discovered via the STUN exchange + over the connection). Note that this is instead of the SHOULD- + strength recommendation in comedia, which recommends that the port + number sent by the entity which initiated the connection should be + '9'. The actual port number is present to facilitate identification + of the connection. The a=setup attribute MUST be present and MUST + contain the value "active". The a=connection attribute MUST be + present and MUST have the value of "existing". -9. Mapping ICE into SIP + If the agent was the passive one and was the recipient of the + connection, it includes its transport address in the m/c line of the + SDP. In this case, that address will be the same as the one it had + placed into the a=candidate line of the SDP. The a=setup attribute + MUST be present and MUST contain the value of "passive". The + a=connection attribute MUST be present and MUST have the value of + "existing". - In this section, we show how to map ICE into SIP. This mapping - involves three parts. The first is the actual mapping of the ICE - message into SIP and SDP messages, which requires extensions to SDP - documented here. The second are security considerations specific to - SIP. The third is handling of updates in the offer/answer model. +7.6.2 Receiving the Offer and Sending an Answer -9.1 Message Mapping + If an agent receives an updated offer with a=candidate attributes, it + checks to see if it already knows about the listed candidates. This + is done by comparing the tid with the candidates it had received in + the previous offer or answer from the peer. If the tid is already + known, processing for that candidate continues as if no offer had + been made. Any connectivity checks in progress continue, and any + ongoing STUN keepalives continue. - A new SDP attribute is defined to support ICE. It is called - "candidate". The candidate attribute MUST be present within a media - block of the SDP. It contains a candidate IP address and port (or - pair of IP addresses and ports in the case of RTP) that the recipient - of the SDP can use. There MAY be multiple candidate 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). + If a candidate which had been listed previously is no longer present + in the offer, this tells the answerer to cease connectivity checks. + Any pairings that include the absent remote candidate are discarded. + Any STUN transactions in progress to that candidate are immediately + terminated - no further retransmissions take place, and no further + transactions to that candidate will be made. If a TCP connection was + opened to or from that candidate, and that connection is not listed + as the active one in the offer, the connection is torn down. - The syntax of this attribute is: + The agent then sends its answer. Like the offerer, it can add or + remove candidates from its answer. If it removed candidates from its + answer, it ceases STUN connectivity checks from those candidates, and + any pairings that include those candidates are discarded. Any STUN + transactions in progress to that candidate are immediately terminated + - no further retransmissions take place, and no further transactions + to that candidate will be made. If a TCP connection was opened to or + from that candidate, and that connection is not listed as the active + one in the answer, the connection is torn down. - candidate-attribute = "candidate" ":" id SP qvalue SP - rtp-user-frag SP rtp-password SP - rtp-unicast-address SP rtp-port [SP rtcp-user-frag - SP rtcp-password [SP rtcp-unicast-address SP - rtcp-port]] - ;qvalue from RFC 3261 - rtp-port = port - rtcp-port = port - rtp-unicast-address = unicast-address - rtcp-unicast-address = unicast-address - ;unicast-address, port from RFC 2327 - rtp-user-frag = non-ws-string - rtp-password = non-ws-string - rtcp-user-frag = non-ws-string - rtcp-password = non-ws-string - id = token + After transmission of the answer, there may be a set of candidates + which were new in the offer, and a set that were new in the answer. + The agent begins connectivity checks as described in Section 7.4, + pairing each new candidate in its answer with all candidates in the + offer, and each new candidate in the offer with all of its candidates + in the answer. - With the addition of the candidate 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 m/c line may have also changed, indicating a new active + candidate. If the m/c line contains a UDP stream, the agent begins + sending media to the transport addresses listed there. In addition, + it checks to see if those transport addresses correspond to a remote + candidate in a valid pairing. So long as the remote agent has + offered up a candidate that has been validated by ICE, it should be + the case. Indeed, there may be a multitude of valid pairings + containing the transport addresses in the m/c line as the remote + candidate. In that case, the agent MUST choose the pairing whose + native candidate has the highest priority. It MUST place this + candidate in the m/c line. Transmission of media occurs as defined + in Section 7.8. - Each media stream element in an ICE message maps to either one or two - media blocks in the SDP. If the ICE message has only an IPv4 default - address or an IPv6 default address, but not both, one media block is - used. If both defaults are present, two media blocks are used. Each - default address maps to the m and c lines in the SDP media block. In - particular, the from the element maps into - the SDP c line. The from the maps into the port - in the SDP m line. If the ICE message indicates a default RTCP - address whose IP address is not identical to the default RTP address, - and whose port is not one higher than that of the RTP, the SDP RTCP - attribute [2] MUST be used to convey the RTCP transport address. + If the m/c line has changed, and now indicates a new TCP candidate, + the agent examines it. The comedia "a=connection" attribute will + normally be present and normally contain the value of "existing". If + not present, or if present but with a value of "new", comedia process + is followed, as apparently the peer has abandoned ICE operation for + this media stream. Assuming it contains a value of "existing", the + agent looks at whether the a=setup attribute is present. If its + value is "active", it means that a connection that was initiated by + the remote agent is to be used. The agent examines the transport + address in the m/c line. It looks for a matching value in the + apparent remote transport addresses of existing connections. If it + matches multiple connections (though it should normally match just + one), one of those connections is chosen. The native transport + address of that connection is then placed into the m/c line of the + answer. If no existing connections where matched, an error has + occured. The agent SHOULD respond with "holdconn", and then generate + its own offer with a connection to the peer which it believes is + valid. - Each element in an ICE message maps to a candidate - attribute in the SDP. If the IP version of the is IPv4, - it MUST be mapped into the media block containing the default IPv4 - address. If the IP version of the is IPv6, it MUST be - mapped into the media block containing the default IPv6 address. - Mapping of each individual candidate is simple. The - element of the element maps to - the rtp-user-frag component of the candidate attribute. The - element of the element maps to the - rtp-password component of the candidate attribute. The - element maps to the first unicast-address and port components of the - candidate attribute. + If the a=setup attribute had a value of "passive", it means that a + connection that was initiated by the agent itself is to be used. The + agent examines the transport address in the m/c line. It looks for a + matching value amongst the remote transport addresses in valid + pairings. If multiple pairings match, it MUST choose the one whose + native transport address has the highest priority. The apparent + native transport address associated with an active connection + initiated by the agent is then placed into the m/c line, and that TCP + connection is used to send and receive media. If no pairings match, + an error has occured. The agent SHOULD respond with "holdconn", and + then generate its own offer with a connection to the peer which it + believes is valid. - If the element is present, it means that RTCP is in - use. The rtcp-user-frag and rtcp-password components of the - candidate attribute MUST be present, and MUST be set to the - and elements of the - element, respectively. If the element is also - present, its IP address and port information is copied into the - rtcp-unicast-address and rtcp-port components of the candidate - attribute. +7.6.3 Receiving the Answer - The preference attribute from the element is mapped to - the q-value component of the candidate attribute. The id attribute - from the element is mapped into the id component of the - candidate attribute. + If an agent receives an answer with a=candidate attributes, it checks + to see if it already knows about the listed candidates. This is done + by comparing the tid with the candidates it had received in the + previous offer or answer from the peer. If the tid is already known, + processing for that candidate continues as if no offer had been made. + Any connectivity checks in progress continue, and any ongoing STUN + keepalives continue. - If the mapping process produced both an IPv6 media block (that is, a - media block with an IPv6 address in the c line, and with all IPv6 - addresses in the candidate attributes within that block) and an IPv4 - media block, these two blocks MUST be grouped using the ANAT grouping - [7]. + If a candidate which had been listed previously is no longer present + in the answer, this tells the offerer to cease connectivity checks. + Any pairings that include the absent remote candidate are discarded. + Any STUN transactions in progress to that candidate are immediately + terminated - no further retransmissions take place, and no further + transactions to that candidate will be made. If a TCP connection was + opened to or from that candidate, and that connection is not listed + as the active one in the answer, the connection is torn down. -9.2 SIP and SDP Specific Security Considerations + Furthermore, there may be a set of candidates which were new in the + offer, and a set that were new in the answer. The agent begins + connectivity checks as described in Section 7.4, pairing each new + candidate in its offer with all candidates in the answer, and each + new candidate in the answer with all of its candidates in the offer. - The SDP messages described here contain usernames and passwords. If - those passwords are transmitted in the clear, it introduces - significant security vulnerabilities, discussed in detail below. In - summary, those vulnerabilities would allow an eavesdropper that can - inject packets, to "steal" the media streams for a call unless secure - media transport (such as SRTP) is used. Even if SRTP is used, an - attacker could disrupt a call and prevent media from flowing. These - attacks, fortunately, can be obviated by providing secure transport - of the SDP. SIP-based implementations of ICE SHOULD use the sips URI - scheme when transporting SDP with ICE information, and MAY use S/MIME - [3]. + The m/c line may have also changed, indicating a new active + candidate. If the m/c line contains a UDP stream, the agent begins + sending media to the transport addresses listed there as defined in + Section 7.8. It will send from the m/c line it had signaled in the + offer. -9.3 Updates in the Offer/Answer Model + If the m/c line has changed, and now indicates a new TCP candidate, + the agent examines it. If the agent had, in its offer, indicated the + desire to use a specific connection that it had initiated, it would + have used the a=connection attribute with the value of "existing", + and the a=setup attribute with the value of "active", and have placed + its apparent native transport address in the m/c line. In that case, + the m/c line in the answer will normally have the a=connection + attribute with the value "existing", which means that the remote + agent agrees with the usage of that connection. The transport + addresses in the m/c line should correspond to the remote transport + addresses that the agent had initiated its connection to. If so, + that connection is used. - ICE itself only considers an initial exchange of messages. However, - the offer/answer model [4] allows for the session to be modified with - subsequent exchanges. How is an updated offer with SDP alternate - attributes to be treated? + If the agent had, in its offer, indicated the desire to use any + connection that had been established to a specific native transport + address, it would have, in its offer, used the a=connection attribute + with the value of "existing" and the a=setup attribute with the value + of "passive", and placed that address in the m/c line. In that case, + the m/c line in the answer will normally have the a=connection + attribute with the value of "existing" and the a=setup attribute with + the value of "active". The transport address in the m/c line will + correspond to the apparent remote transport address. The agent MUST + scan its existing connections to the native transport address it had + advertised in the offer, and find the one whose apparent remote + transport address matches the m/c line in the answer. If there is a + match, that connection is used for sending media. If there is no + match, an error has occurred. - If a user agent receives an updated offer with candidate attributes, - it checks to see if it already knows about those candidates. This is - done by comparing the transport address and username fragment with - existing values. If the combination is already known, no additional - action is taken. In particular, if STUN connectivity checks had - already been made, no new ones are performed. However, if a - candidate contains a new transport address or new username fragment, - it is treated as a totally new candidate, and STUN connectivity - checks are performed per Section 5.3.4. If a candidate formerly sent - by the peer no longer appears, that candidate is considered BAD, and - if it was in use previously, it ceases being used, and the next - highest priority connection in the GOOD state is used. +7.7 Binding Keepalives - The inclusion of the username fragment in the determination of - whether a candidate is known provides a hook that allows a peer to - request a new set of connectivity checks on an existing transport - address. It can update the username fragment and generate an updated - offer, without changing the transport address. + Once the candidates are promoted to active, and media begins flowing, + it is still necessary to keep the bindings alive at intermediate NATs + for the duration of the session. Normally, the RTP packets + themselves meet this objective. However, several cases merit further + discussion. Firstly, in some RTP usages, such as SIP, the media + streams can be "put on hold". This is accomplished by using the SDP + "sendonly" or "inactive" attributes, as defined in RFC 3264 [4]. RFC + 3264 directs implementations to cease transmission of media in these + cases. However, doing so may cause NAT bindings to timeout, and + media won't be able to come off hold. -10. Security Considerations + Secondly, some RTP payload formats, such as the payload format for + text conversation [28], may send packets so infrequently that the + interval exceeds the NAT binding timeouts. - STUN itself introduces many security considerations. In particular, - there are attacks whereby an eavesdropper replays STUN packets with a - modified source address. These modified packets can cause service - disruptions and denial-of-service attacks, which are only partially - mitigated by the heuristics described in STUN [1]. + Thirdly, if silence suppression is in use, long periods of silence + may cause media transmission to cease sufficiently long for NAT + bindings to time out. - Interestingly, when STUN is used within ICE, these security - weaknesses are mitigated completely, without the need for the - heuristics defined in RFC 3489. + To prevent these problems, ICE implementations MUST continue to list + their active transport addresses as candidates in a=candidate lines. + As a consequence of this, STUN packets will be transmitted + periodically independently of the transmission (or lack thereof) of + media packets. This provides a media independent, RTP independent, + and codec independent solution for keeping the NAT bindings alive. + + If an ICE implementation is communciating with one that does not + support ICE, keepalives MUST still be sent. In that case, it is + RECOMMENDED that an agent support the RTP No-Op payload format [15], + and send it at least once every 20 seconds if media is not otherwise + being sent. This No-Op MUST be sent even if the media stream is + inactive or recvonly. + +7.8 Sending Media + + When an agent sends media packets, it MUST send them from the same IP + address and port it has advertised in the m/c-line. This provides a + property known as symmetry, which is an essential facet of NAT + travresal. + + In the case of a STUN-derived transport address, this means that the + RTP packets are sent from the local transport address used to obtain + the STUN address. In the case of a TURN-derived transport address, + this means that media packets are sent through the TURN server (using + the TURN SEND primitive). For local transport addresses, media is + sent from that local transport address. + + This symmetric behavior MUST be followed by an agent even if its peer + in the session doesn't support ICE. + +8. Interactions with Forking + + SIP allows INVITE requests carrying offers to fork, which means that + they are delivered to multiple user agents. Each of those user + agents then provides an answer to the offer in the INVITE. The + result is that a single offer generated by the UAC produces multiple + answers. + + ICE interacts very well with forking. Indeed, ICE fixes some of the + problems associated with forking. Once the offer/answer exchange has + completed, the UAC will have an answer from each UAS that received + the INVITE. The ICE connectivity checks that ensue will carry tids + that correlate each of those checks (and thus their corresponding + source IP address and port or TCP connection) with a specific remote + user agent. As these checks happen before any media is transmitted, + ICE allows a UAC to disambiguate subsequent media traffic, and + corelate that traffic with a particular remote UA. When SIP is used + without ICE, the incoming media traffic cannot be disambiguated + without an additional offer/answer exchange. + +9. Interactions with Preconditions + + Because ICE involves multiple addresses and pre-session activities, + its interactions with preconditions [10] merits further discussion. + + Quality of Service (QoS) preconditions, which are defined in RFC + 3312, apply only to the IP addresses and ports listed in the m/c + lines in an offer/answer. If ICE changes the address and port where + media is received, this change is reflected in the m/c lines of a new + offer/answer. As such, it appears like any other re-INVITE would, + and is fully treated in RFC 3312, which applies without regard to the + fact that the m/c lines are changing due to ICE negotiations ocurring + "in the background". + + ICE also has (purposeful) interactions with connectivity + preconditions [12]. As described there, the precondition is + satisfied once ICE has verified that there exists a valid path of + connectivity for each media stream to which the precondition applies. + More specifically, it is satisfied when there is at least one valid + UDP transport address pairing or TCP connection for such a media + stream. Furthermore, when a subsequent offer is made to promote one + of those valid transport address pairings or connections into the + m/c-line, the preconditions is marked as met in that same offer/ + answer exchange. + +10. Example + + In the example that follows, 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 + (which is also the password). For STUN answers, this is followed by + square brackets containing the value of MAPPED ADDRESS. The example + shows a flow of two agents where one is behind a full cone NAT, and + the other is behind a symmetric NAT. + + TODO: Fill in. This is a big complicated flow! + +11. Grammar + + This specification defines a new SDP attribute. It is called + "candidate". The candidate attribute MUST be present within a media + block of the SDP. It contains a transport address for a candidate + that can be used for connectivity checks. There MAY be multiple + candidate attributes in a media block. + + The syntax of this attribute is: + + candidate-attribute = "candidate" ":" candidate-id SP tid SP + transport SP + qvalue SP ;qvalue from RFC 3261 + addr SP + port SP + ;addr, port from RFC 2327 + transport = "UDP" / "TCP" / transport-extension + transport-extension = token + candidate-id = 1*DIGIT + id = non-ws-string + + The candidate-id is used to group together the transport addresses + for a particular candidate. It MUST be a positive integer whose + value is less than (2^31 -1). It MUST have the same value for all + transport addresses within the same candidate. It MUST have a + different value for transport addresses within different candidates + for the same media stream. The tid production contains an + identifier, chosen with 128 bits of randomness, that identifies the + transport address. The tid of a pair of transport addresses is + combined to for the username and password of a STUN request from one + transport address to another. The transport production indicates the + transport protocol for the candidate. This can be either UDP or TCP. + Extensibility is provided to allow for future transport protocols to + be used with ICE, such as the Datagram Congestion Control Protocol + (DCCP) [26]. The unicast-address production is from RFC 2327, and + contains the IPv4 or IPv6 address of the candidate. The port + production contains its port. + +12. Security Considerations + + There are numerous threats in a system using ICE. This section + overviews these threats and discusses how they are mitigated. + + STUN itself introduces many security considerations, which receive an + extensive treatment in RFC 3489. STUN is used within ICE in two ways + - one, as a technique for address gathering, and two, as a peer-to- + peer connectivity check. All of the security considerations of RFC + 3489 apply directly to the former usage. However, the latter usage, + as a peer-to-peer connectivity check, is sufficiently different that + a discussion of its security considerations is appropriate. + + It remains the case that many attacks are rooted in a single + primitive - an attacker attempts to inject a STUN response with an + invalid MAPPED-ADDRESS attribute. In the usages of STUN described in + RFC 3489, this injection can occur as a result of compromises of STUN + servers, attacks on the DNS, rogue NATs, injection of faked responses + coupled with a dos attack, and replaying modified requests. With + peer-to-peer STUN, compromises of STUN servers are not much of a + concern, since the STUN servers are embedded in endpoints and + distributed throughout the network. Thus, compromising the STUN + server is equivalent to comprimising the endpoint, and if that + happens, far more problematic attacks are possible than those against + ICE. Similarly, DNS attacks are irrelevant since STUN servers are + not discovered via DNS, they are signaled via SIP. Rogue NATs, + injection of fake responses and relaying modified requests all can be + handled in ICE with the countermeasures discussed below. Consider an attacker that intercepts a STUN packet used for - connectivity checks, and replays it using a faked source address. If + connectivity checks, and replays it using its own source address. If successful, this would fool an endpoint into thinking that this faked source address was a valid destination for media (recall that the source transport address of received STUN packets is used as a potential candidate address). However, the recipient of the replayed packet will not just send media to that candidate. It will verify it with a STUN connectivity check. This check will be sent to that - faked source address, and if there is no response, the address will - not be used. The attacker cannot answer the STUN request without - access to the username and password, which are exchanged as part of - the signaling. Thus, if the signaling is protected as recommended - above, the attacker cannot obtain the username or password. + faked source address, and if there is no answer, the address will not + be used. The attacker cannot answer the STUN request without access + to the username and password, which are exchanged as part of the + signaling. Thus, if the signaling is protected as recommended above, + the attacker cannot obtain the username or password. If an attacker instead intercepts and replays STUN packets used for the purposes of unilateral allocation, a similar result occurs. The target of the attack will be fooled into thinking it has a STUN derived transport address that it does not. Its peer will perform a connectivity check to this address, which will fail. The attacker cannot force this check to succeed without access to the username and password, which are protected. Thus, this address will not be used. In the worst case, an attacker can generate enough traffic so that none of the valid STUN checks or unilateral allocations succeed. This would result in a service disruption. However, this attack is no worse than any pure packet flood disruption attack launched against any other protocol. These attacks cannot be prevented by any protocol means. - If an attacker could intercept and modify the contents of the - Initiate or Accept messages, they could disrupt the session, divert - the media, and otherwise take control over the session. This attack - is prevented by encryption, authentication and message integrity of - the signaling channel used for ICE. + If an attacker could intercept and modify the contents of the Offer + or Accept messages, they could disrupt the session, divert the media, + and otherwise take control over the session. This attack is + prevented by encryption, authentication and message integrity of the + signaling channel used for ICE. -11. IANA Considerations + SIP-based implementations of ICE SHOULD use the sips URI scheme when + transporting SDP with ICE information, and MAY use S/MIME [3]. -11.1 SDP Attribute Name +13. IANA Considerations This specification defines one new SDP attribute per the procedures of Appendix B of RFC 2327. The required information for the registration is: Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net. Attribute Name: candidate Long Form: candidiate @@ -1753,101 +1884,58 @@ Charset Considerations: The attribute is not subject the the charset attribute. Purpose: This attribute is used with Interactive Connectivity Establishment (ICE), and provides one of many possible candidate addresses for communication. These addresses are validated with an end-to-end connectivity check using Simple Traversal of UDP with NAT (STUN). - Appropriate Values: See Section 9 of RFC XXXX [Note to RFC-ed: please - replace XXXX with the RFC number of this specification]. - -11.2 URN Sub-Namespace Registration - - This section registers a new XML namespace, per the guidelines in [6] - - URI: The URI for this namespace is urn:ietf:params:xml:ns:ice. - - Registrant Contact: IETF, MMUSIC working group, (mmusic@ietf.org), - Jonathan Rosenberg (jdrosen@jdrosen.net). - - XML: - - BEGIN - - - - - - ICE Namespace - - -

Namespace for ICE Documents

-

urn:ietf:params:xml:ns:ice

-

See RFCXXXX. [Note to RFC-ed: please replace XXXX with the RFC - number of this specification.]

- - - END - -11.3 XML Schema Registration - - This section registers an XML schema per the procedures in [6]. - - URI: urn:ietf:params:xml:schema:ice - - Registrant Contact: IETF, MMUSIC working group, (mmusic@ietf.org), - Jonathan Rosenberg (jdrosen@jdrosen.net). - - The XML for this schema can be found as the sole content of - Section 7. + Appropriate Values: See Section 11 of RFC XXXX [Note to RFC-ed: + please replace XXXX with the RFC number of this specification]. -12. IAB Considerations +14. 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 + which is the general process by which a agent attempts to determine its address in another realm on the other side of a NAT through a - collaborative protocol reflection mechanism [14]. ICE is an example + collaborative protocol reflection mechanism [21]. 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 requirements. -12.1 Problem Definition +14.1 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 + Provide a means for a agent to determine an address that is reachable by another peer with which it wishes to communicate. -12.2 Exit Strategy +14.2 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 @@ -1859,174 +1947,209 @@ 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. Indeed, ICE can assist in the transition from IPv4 to IPv6. It can be used to determine whether to use IPv6 or IPv4 when two dual-stack hosts communicate with SIP (IPv6 gets used). It can also allow a network with both 6to4 and native v6 connectivity to determine which address to use when communicating with a peer. -12.3 Brittleness Introduced by ICE +14.3 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 + which requires a agent 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, + agents 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 + shared NAT between each agent and the STUN server, traditional STUN may not work. With ICE, that restriction can be lifted. Traditional STUN also introduces some security considerations. Fortunately, those security considerations are also mitigated by ICE. -12.4 Requirements for a Long Term Solution +14.4 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 solution. Our conclusions from STUN remain unchanged. However, we feel ICE actually helps because we believe it can be part of the long term solution. -12.5 Issues with Existing NAPT Boxes +14.5 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. -13. Acknowledgements +15. Acknowledgements The authors would like to thank Douglas Otis, Francois Audet and Magnus Westerland for their comments and input. -14. References +16. References -14.1 Normative References +16.1 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. + [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: + Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. [4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [5] Zopf, R., "Real-time Transport Protocol (RTP) Payload for Comfort Noise (CN)", RFC 3389, September 2002. - [6] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, January - 2004. + [6] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, + January 2004. - [7] Camarillo, G., "The Alternative Network Address Types Semantics + [7] Handley, M. and V. Jacobson, "SDP: Session Description + Protocol", RFC 2327, April 1998. + + [8] Casner, S., "Session Description Protocol (SDP) Bandwidth + Modifiers for RTP Control Protocol (RTCP) Bandwidth", RFC 3556, + July 2003. + + [9] Rosenberg, J. and H. Schulzrinne, "Reliability of Provisional + Responses in Session Initiation Protocol (SIP)", RFC 3262, + June 2002. + + [10] Camarillo, G., Marshall, W., and J. Rosenberg, "Integration of + Resource Management and Session Initiation Protocol (SIP)", + RFC 3312, October 2002. + + [11] Camarillo, G., "The Alternative Network Address Types Semantics (ANAT) for theSession Description Protocol (SDP) Grouping Framework", draft-ietf-mmusic-anat-02 (work in progress), October 2004. - [8] Rosenberg, J., "Traversal Using Relay NAT (TURN)", - draft-rosenberg-midcom-turn-06 (work in progress), October 2004. + [12] Andreasen, F., "Connectivity Preconditions for Session + Description Protocol Media Streams", + draft-ietf-mmusic-connectivity-precon-00 (work in progress), + May 2005. -14.2 Informative References + [13] Yon, D., "Connection-Oriented Media Transport in the Session + Description Protocol (SDP)", draft-ietf-mmusic-sdp-comedia-10 + (work in progress), November 2004. - [9] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming + [14] Rosenberg, J., "Traversal Using Relay NAT (TURN)", + draft-rosenberg-midcom-turn-07 (work in progress), + February 2005. + + [15] Andreasen, F., "A No-Op Payload Format for RTP", + draft-ietf-avt-rtp-no-op-00 (work in progress), May 2005. + +16.2 Informative References + + [16] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming Protocol (RTSP)", RFC 2326, April 1998. - [10] Senie, D., "Network Address Translator (NAT)-Friendly + [17] Senie, D., "Network Address Translator (NAT)-Friendly Application Design Guidelines", RFC 3235, January 2002. - [11] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A. and A. + [18] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and A. Rayhan, "Middlebox communication architecture and framework", RFC 3303, August 2002. - [12] Borella, M., Lo, J., Grabelsky, D. and G. Montenegro, "Realm + [19] Borella, M., Lo, J., Grabelsky, D., and G. Montenegro, "Realm Specific IP: Framework", RFC 3102, October 2001. - [13] Borella, M., Grabelsky, D., Lo, J. and K. Taniguchi, "Realm + [20] Borella, M., Grabelsky, D., Lo, J., and K. Taniguchi, "Realm Specific IP: Protocol Specification", RFC 3103, October 2001. - [14] Daigle, L. and IAB, "IAB Considerations for UNilateral - Self-Address Fixing (UNSAF) Across Network Address - Translation", RFC 3424, November 2002. + [21] Daigle, L. and IAB, "IAB Considerations for UNilateral Self- + Address Fixing (UNSAF) Across Network Address Translation", + RFC 3424, November 2002. - [15] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, - "RTP: A Transport Protocol for Real-Time Applications", RFC - 3550, July 2003. + [22] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, + "RTP: A Transport Protocol for Real-Time Applications", + RFC 3550, July 2003. - [16] Baugher, M., McGrew, D., Naslund, M., Carrara, E. and K. - Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC - 3711, March 2004. + [23] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. + Norrman, "The Secure Real-time Transport Protocol (SRTP)", + RFC 3711, March 2004. - [17] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via + [24] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4 Clouds", RFC 3056, February 2001. - [18] Huitema, C., "Teredo: Tunneling IPv6 over UDP through NATs", - draft-huitema-v6ops-teredo-04 (work in progress), January 2005. + [25] Huitema, C., "Teredo: Tunneling IPv6 over UDP through NATs", + draft-huitema-v6ops-teredo-05 (work in progress), April 2005. - [19] Hellstrom, G., "RTP Payload for Text Conversation", + [26] Kohler, E., "Datagram Congestion Control Protocol (DCCP)", + draft-ietf-dccp-spec-11 (work in progress), March 2005. + + [27] Lazzaro, J., "Framing RTP and RTCP Packets over Connection- + Oriented Transport", draft-ietf-avt-rtp-framing-contrans-05 + (work in progress), January 2005. + + [28] Hellstrom, G., "RTP Payload for Text Conversation", draft-ietf-avt-rfc2793bis-09 (work in progress), August 2004. Author's Address Jonathan Rosenberg Cisco Systems 600 Lanidex Plaza Parsippany, NJ 07054 US Phone: +1 973 952-5000 - EMail: jdrosen@cisco.com + Email: jdrosen@cisco.com URI: http://www.jdrosen.net Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information