SIPPING Working Group C. Boulton, Ed. Internet-Draft Ubiquity Software Corporation Expires: January 10, 2008 J. Rosenberg Cisco Systems G. Camarillo Ericsson July 9, 2007 Best Current Practices for NAT Traversal for SIP draft-ietf-sipping-nat-scenarios-07 Status of this Memo 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. 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 January 10, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract Traversal of the Session Initiation Protocol (SIP) and the sessions it establishes through Network Address Translators (NAT) is a complex problem. Currently there are many deployment scenarios and traversal mechanisms for media traffic. This document aims to provide concrete recommendations and a unified method for NAT traversal as well as Boulton, Ed., et al. Expires January 10, 2008 [Page 1] Internet-Draft NAT Scenarios July 2007 documenting corresponding call flows. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3 3. Solution Technology Outline Description . . . . . . . . . . . 6 3.1. SIP Signaling . . . . . . . . . . . . . . . . . . . . . . 7 3.1.1. Symmetric Response . . . . . . . . . . . . . . . . . . 7 3.1.2. Connection Re-use . . . . . . . . . . . . . . . . . . 8 3.2. Media Traversal . . . . . . . . . . . . . . . . . . . . . 8 3.2.1. Symmetric RTP . . . . . . . . . . . . . . . . . . . . 8 3.2.2. STUN . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2.3. TURN . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2.4. ICE . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2.5. Solution Profiles . . . . . . . . . . . . . . . . . . 10 4. NAT Traversal Scenarios . . . . . . . . . . . . . . . . . . . 11 4.1. Basic NAT SIP Signaling Traversal . . . . . . . . . . . . 11 4.1.1. Registration (Registrar/Proxy Co-Located) . . . . . . 11 4.1.2. Registration(Registrar/Proxy not Co-Located) . . . . . 15 4.1.3. Initiating a Session . . . . . . . . . . . . . . . . . 18 4.1.4. Receiving an Invitation to a Session . . . . . . . . . 20 4.2. Basic NAT Media Traversal . . . . . . . . . . . . . . . . 23 4.2.1. Endpoint Independent NAT . . . . . . . . . . . . . . . 24 4.2.2. Address and Port Dependant NAT . . . . . . . . . . . . 43 5. IPv4-IPv6 Transition . . . . . . . . . . . . . . . . . . . . . 51 5.1. IPv4-IPv6 Transition for SIP Signaling . . . . . . . . . . 51 5.2. IPv4-IPv6 Transition for Media . . . . . . . . . . . . . . 51 6. Next Steps . . . . . . . . . . . . . . . . . . . . . . . . . . 54 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 54 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 54 8.1. Normative References . . . . . . . . . . . . . . . . . . . 54 8.2. Informative References . . . . . . . . . . . . . . . . . . 56 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 56 Intellectual Property and Copyright Statements . . . . . . . . . . 58 Boulton, Ed., et al. Expires January 10, 2008 [Page 2] Internet-Draft NAT Scenarios July 2007 1. Introduction NAT (Network Address Translators) traversal has long been identified as a large problem when considered in the context of the Session Initiation Protocol (SIP)[1] and it's associated media such as Real Time Protocol (RTP)[2]. The problem is further confused by the variety of NATs that are available in the market place today and the large number of potential deployment scenarios. Detail of different NAT behaviors can be found in 'NAT Behavioral Requirements for Unicast UDP' [11]. The IETF has produced many specifications for the traversal of NAT, including STUN, ICE, rport, symmetric RTP, TURN, SIP Outbound, SDP attribute for RTCP, and others. These each represent a part of the solution, but none of them gives the overall context for how the NAT traversal problem is decomposed and solved through this collection of specifications. This document serves to meet that need. This document attempts to provide a definitive set of 'Best Common Practices' to demonstrate the traversal of SIP and its associated media through NAT devices. The document does not propose any new functionality but does draw on existing solutions for both core SIP signaling and media traversal (as defined in Section 3). The draft will be split into distinct sections as follows: 1. A clear definition of the problem statement 2. Description of proposed solutions for both SIP protocol signaling and media signaling 3. A set of basic and advanced call flow scenarios 2. Problem Statement The traversal of SIP through NAT can be split into two categories that both require attention - The core SIP signaling and associated media traversal. The core SIP signaling has a number of issues when traversing through NATs. Firstly, the default operation for SIP response generation using unreliable protocols such as the Unicast Datagram Protocol (UDP) results in responses generated at the User Agent Server (UAS) being sent to the source address, as specified in either the SIP 'Via' header or the 'received' parameter (as defined in RFC 3261 [1]). The port is extracted from the SIP 'Via' header to complete the IP address/port combination for returning the SIP response. While the destination for the response is correct, the port contained in the Boulton, Ed., et al. Expires January 10, 2008 [Page 3] Internet-Draft NAT Scenarios July 2007 SIP 'Via' header represents the listening port of the originating client and not the port representing the open pin hole on the NAT. This results in responses being sent back to the NAT but to a port that is likely not open for SIP traffic. The SIP response will then be dropped at the NAT. This is illustrated in Figure 1 which depicts a SIP response being returned to port 5060. Private NAT Public Network | Network | | -------- SIP Request |open port 5650 -------- | |-------------------->--->-----------------------| | | | | | | | Client | |port 5060 SIP Response | Proxy | | | x<------------------------| | | | | | | -------- | -------- | | | Figure 1 Secondly, when using a reliable, connection orientated transport protocol such as TCP, SIP has an inherent mechanism that results in SIP responses reusing the connection that was created/used for the corresponding transactional request. The SIP protocol does not provide a mechanism that allows new requests generated in the reverse direction of the originating client to use the existing TCP connection created between the client and the server during registration. This results in the registered contact address not being bound to the "connection" in the case of TCP. Requests are then blocked at the NAT, as illustrated in Figure 2. This problem also exists for unreliable transport protocols such as UDP where external NAT mappings need to be re-used to reach a SIP entity on the private side of the network. Boulton, Ed., et al. Expires January 10, 2008 [Page 4] Internet-Draft NAT Scenarios July 2007 Private NAT Public Network | Network | | -------- (UAC 8023) REGISTER/Response (UAS 5060) -------- | |-------------------->---<-----------------------| | | | | | | | Client | |5060 INVITE (UAC 8015)| Proxy | | | x<------------------------| | | | | | | -------- | -------- | | | Figure 2 In Figure 2 the original REGISTER request is sent from the client on port 8023 and received on port 5060, establishing a reliable connection and opening a pin-hole in the NAT. The generation of a new request from the proxy results in a request destined for the registered entity (Contact IP address) which is not reachable from the public network. This results in the new SIP request attempting to create a connection to a private network address. This problem would be solved if the original connection was re-used. While this problem has been discussed in the context of connection orientated protocols such as TCP, the problem exists for SIP signaling using any transport protocol. The solution proposed for this problem in section 3 of this document is relevant for all SIP signaling, regardless of the transport protocol. NAT policy can dictate that connections should be closed after a period of inactivity. This period of inactivity can range drastically from a number seconds to hours. Pure SIP signaling can not be relied upon to keep alive connections for a number of reasons. Firstly, SIP entities can sometimes have no signaling traffic for long periods of time which has the potential to exceed the inactivity timer, and this can lead to problems where endpoints are not available to receive incoming requests as the connection has been closed. Secondly, if a low inactivity timer is specified, SIP signaling is not appropriate as a keep-alive mechanism as it has the potential to add a large amount of traffic to the network which uses up valuable resource and also requires processing at a SIP stack, which is also a waste of processing resources. Media associated with SIP calls also has problems traversing NAT. RTP [2]] is one of the most common media transport types used in SIP signaling. Negotiation of RTP occurs with a SIP session Boulton, Ed., et al. Expires January 10, 2008 [Page 5] Internet-Draft NAT Scenarios July 2007 establishment using the Session Description Protocol(SDP) [3] and a SIP offer/answer exchange[5]. During a SIP offer/answer exchange an IP address and port combination are specified by each client in a session as a means of receiving media such as RTP. The problem arises when a client advertises its address to receive media and it exists in a private network that is not accessible from outside the NAT. Figure 3 illustrates this problem. NAT Public Network NAT | | | | | | -------- | SIP Signaling Session | -------- | |----------------------->---<--------------------| | | | | | | | | Client | | | | Client | | A |>=====>RTP>==Unknown Address==>X | | B | | | | X<==Unknown Address==>---------<<->>-----port 5060| Client | | A | | | B | | | | | | -------- | -------- | | | Figure 4 The exact functionality for this method of response traversal is Boulton, Ed., et al. Expires January 10, 2008 [Page 7] Internet-Draft NAT Scenarios July 2007 called 'Symmetric Response' and the details are documented in RFC 3581 [6]. Additional requirements are imposed on SIP entities in this specification such as listening and sending SIP requests/ responses from the same port. 3.1.2. Connection Re-use The second problem with sip signaling, as defined in Section 2 and illustrated in Figure 2, is to allow incoming requests to be properly routed. Guidelines for devices such as User Agents that can only generate outbound connections through a NAT are documented in 'SIP Conventions for UAs with Outbound Only Connections'[13]. The document provides techniques that use a unique User Agent instance identifier (instance-id) in association with a flow identifier (Reg-id). The combination of the two identifiers provides a key to a particular connections (both UDP and TCP) that are stored in association with registration bindings. On receiving an incoming request to a SIP Address-Of-Record (AOR), a proxy routes to the associated flow created by the registration and thus a route through a NAT. It also provides a keepalive mechanism for clients to keep NAT bindings alive. This is achieved using peer-to-peer STUN multiplexed over the SIP signaling connection. Usage of this specification is RECOMMENDED. This mechanism is not transport specific and should be used for any transport protocol. Even if the SIP Outbound draft is not used, clients generating SIP requests SHOULD use the same IP address and port (i.e., socket) for both transmission and receipt of SIP messages. Doing so allows for the vast majority of industry provided solutions to properly function. 3.2. Media Traversal This document has already provided guidelines that recommend using extensions to the core SIP protocol to enable traversal of NATs. While ultimately not desirable, the additions are relatively straight forward and provide a simple, universal solution for varying types of NAT deployment. The issues of media traversal through NATs is not straight forward and requires the combination of a number of traversal methodologies. The technologies outlined in the remainder of this section provide the required solution set. 3.2.1. Symmetric RTP The primary problem identified in section 2 of this document is that internal IP address/port combinations can not be reached from the Boulton, Ed., et al. Expires January 10, 2008 [Page 8] Internet-Draft NAT Scenarios July 2007 public side of a NAT. In the case of media such as RTP, this will result in no audio traversing a NAT(as illustrated in Figure 3). To overcome this problem, a technique called 'Symmetric' RTP can be used. This involves an SIP endpoint both sending and receiving RTP traffic from the same IP Address/Port combination. This technique also requires intelligence by a client on the public internet as it identifies that incoming media for a particular session does not match the information that was conveyed in the SDP. In this case the client will ignore the SDP address/port combination and return RTP to the IP address/port combination identified as the source of the incoming media. This technique is known as 'Symmetric RTP' and is documented in [15]. 'Symmetric RTP' SHOULD only be used for traversal of RTP through NAT when one of the participants in a media session definitively knows that it is on the public network. 3.2.1.1. RTCP Attribute Normal practice when selecting a port for defining Real Time Control Protocol(RTCP) [2] is for consecutive order numbering (i.e select an incremented port for RTCP from that used for RTP). This assumption causes RTCP traffic to break when traversing many NATs due to blocked ports. To combat this problem a specific address and port need to be specified in the SDP rather than relying on such assumptions. RFC 3605 [6] defines an SDP attribute that is included to explicitly specify transport connection information for RTCP. The address details can be obtained using any appropriate method including those detailed previously in this section (e.g. STUN, TURN). 3.2.2. STUN Simple Traversal of Underneath Network Address Translators(NAT) or STUN is defined in RFC 3489bis [10]. STUN is a lightweight tool kit and protocol that provides details of the external IP address/port combination used by the NAT device to represent the internal entity on the public facing side of a NAT. On learning of such an external representation, a client can use it accordingly as the connection address in SDP to provide NAT traversal. Using terminology defined in the draft 'NAT Behavioral Requirements for Unicast UDP' [11], STUN does work with 'Endpoint Independent Mapping' but does not work with either 'Address Dependent Mapping' or 'Address and Port Dependent Mapping' type NATs. Using STUN with either of the previous two NAT mappings to probe for the external IP address/port representation will provide a different result to that required for traversal by an alternative SIP entity. The IP address/port combination deduced for the STUN server would be blocked for incoming packets from an alterative SIP entity. As mentioned in Section 3.1.2, STUN is also used as a peer-to-peer Boulton, Ed., et al. Expires January 10, 2008 [Page 9] Internet-Draft NAT Scenarios July 2007 keep-alive mechanism. 3.2.3. TURN As described in the previous section, the STUN protocol does not work for UDP traversal through certain identified NAT mappings. 'Obtaining Relay Addresses from Simple Traversal of UDP Through NAT (known as TURN)' is a usage of the STUN protocol for deriving (from a STUN server) an address that will be used to relay packet towards a client. TURN provides an external address (globally routable) at a STUN server that will act as a media relay which guarantees traffic will reach the associated internal address. The full details of the TURN specification are defined in [12]. A TURN service will almost always provide media traffic to a SIP entity but it is RECOMMENDED that this method only be used as a last resort and not as a general mechanism for NAT traversal. This is because using TURN has high performance costs when relaying media traffic and can lead to unwanted latency. 3.2.4. ICE Interactive Connectivity Establishment (ICE) is the RECOMMENDED method for traversal of existing NAT if Symmetric RTP is not appropriate. ICE is a methodology for using existing technologies such as STUN, TURN and any other UNSAF[9] compliant protocol to provide a unified solution. This is achieved by obtaining as many representative IP address/port combinations as possible using technologies such as STUN/TURN etc. Once the addresses are accumulated, they are all included in the SDP exchange in a new media attribute called 'candidate'. Each 'candidate' SDP attribute entry has detailed connection information including a media addresses (including optional RTCP information), priority, username, password and a unique session ID. The appropriate IP address/port combinations are used in the correct order depending on the specified priority. A client compliant to the ICE specification will then locally run instances of STUN servers on all addresses being advertised using ICE. Each instance will undertake connectivity checks to ensure that a client can successfully receive media on the advertised address. Only connections that pass the relevant connectivity checks are used for media exchange. The full details of the ICE methodology are contained in [16]. 3.2.5. Solution Profiles This draft has documented a number of technology solutions for the traversal of media through differing NAT deployments. A number of 'profiles' will now be defined that categorize varying levels of support for the technologies described. Boulton, Ed., et al. Expires January 10, 2008 [Page 10] Internet-Draft NAT Scenarios July 2007 3.2.5.1. Primary Profile A client falling into the 'Primary' profile supports ICE in conjunction with STUN, TURN usage and RFC 3605 [6] for RTCP. ICE is used in all cases and falls back to standard operation when dealing with non-ICE clients. A client which falls into the 'Primary' profile will be maximally interoperable and function in a rich variety of environments including enterprise, consumer and behind all varieties of NAT. 3.2.5.2. Consumer Profile A client falling into the 'Consumer' profile supports STUN and RFC 3605 [6] for RTCP. It uses STUN to allocate bindings, and can also detect when it is in the unfortunate situation of being behind a 'Symmetric' NAT, although it simply cannot function in this case. These clients will only work in deployment situations where the access is sufficiently controlled to know definitively that there won't be Symmetric NAT. This is hard to guarantee as users can always pick up their client and connect via a different access network. 3.2.5.3. Minimal Profile A client falling into the 'Minimal' profile will send/receive RTP from the same IP/port combination. This client requires proprietary network based solutions to function in any NAT traversal scenario. All clients SHOULD support the 'Primary Profile', MUST support the 'Minimal Profile' and MAY support the 'Consumer Profile'. 4. NAT Traversal Scenarios This section of the document includes detailed NAT traversal scenarios for both SIP signaling and the associated media. 4.1. Basic NAT SIP Signaling Traversal The following sub-sections concentrate on SIP signaling traversal of NAT. The scenarios include traversal for both reliable and un- reliable transport protocols. 4.1.1. Registration (Registrar/Proxy Co-Located) The set of scenarios in this section document basic signaling traversal of a SIP REGISTER method through a NAT. Boulton, Ed., et al. Expires January 10, 2008 [Page 11] Internet-Draft NAT Scenarios July 2007 4.1.1.1. UDP Client NAT Proxy | | | |(1) REGISTER | | |----------------->| | | | | | |(1) REGISTER | | |----------------->| | | | | |(2) 401 Unauth | | |<-----------------| | | | |(2) 401 Unauth | | |<-----------------| | | | | |(3) REGISTER | | |----------------->| | | | | | |(3) REGISTER | | |----------------->| | | | |*************************************| | Create Connection Re-use Tuple | |*************************************| | | | | |(4) 200 OK | | |<-----------------| | | | |(4) 200 OK | | |<-----------------| | | | | Figure 5 In this example the client sends a SIP REGISTER request through a NAT which is challenged using the Digest authentication scheme. The client will include an 'rport' parameter as described in section 3.1.1 of this document for allowing traversal of UDP responses. The original request as illustrated in (1) in Figure 5 is a standard REGISTER message: Boulton, Ed., et al. Expires January 10, 2008 [Page 12] Internet-Draft NAT Scenarios July 2007 REGISTER sip:proxy.example.com SIP/2.0 Via: SIP/2.0/UDP client.example.com:5060;rport;branch=z9hG4bK Max-Forwards: 70 Supported: path,gruu From: Client ;tag=djks8732 To: Client Call-ID: 763hdc73y7dkb37@example.com CSeq: 1 REGISTER Contact: ;reg-id=1 ;+sip.instance="" Content-Length: 0 This proxy now generates a SIP 401 response to challenge for authentication, as depicted in (2) from Figure 5: SIP/2.0 401 Unauthorized Via: SIP/2.0/UDP client.example.com:5060 ;rport=8050;branch=z9hG4bK;received=192.0.1.2 From: Client ;tag=djks8732 To: Client ;tag=876877 Call-ID: 763hdc73y7dkb37@example.com CSeq: 1 REGISTER WWW-Authenticate: [not shown] Content-Length: 0 The response will be sent to the address appearing in the 'received' parameter of the SIP 'Via' header (address 192.0.1.2). The response will not be sent to the port deduced from the SIP 'Via' header, as per standard SIP operation but will be sent to the value that has been stamped in the 'rport' parameter of the SIP 'Via' header (port 8050). For the response to successfully traverse the NAT, all of the conventions defined in RFC 3581 [6] MUST be obeyed. Make note of the both the 'connectionID' and 'sip.instance' contact header parameters. They are used to establish a connection re-use tuple as defined in [13]. The connection tuple creation is clearly shown in Figure 5. This ensures that any inbound request that causes a registration lookup will result in the re-use of the connection path established by the registration. This exonerates the need to manipulate contact header URI's to represent a globally routable address as perceived on the public side of a NAT. The subsequent messages defined in (3) and (4) from Figure 5 use the same mechanics for NAT traversal. [Editors note: Will provide more details on heartbeat mechanism in next revision] [Editors note: Can complete full flows if required on heartbeat inclusion] Boulton, Ed., et al. Expires January 10, 2008 [Page 13] Internet-Draft NAT Scenarios July 2007 4.1.1.2. Reliable Transport Client NAT Registrar | | | |(1) REGISTER | | |----------------->| | | | | | |(1) REGISTER | | |----------------->| | | | | |(2) 401 Unauth | | |<-----------------| | | | |(2) 401 Unauth | | |<-----------------| | | | | |(3) REGISTER | | |----------------->| | | | | | |(3) REGISTER | | |----------------->| | | | |*************************************| | Create Connection Re-use Tuple | |*************************************| | | | | |(4) 200 OK | | |<-----------------| | | | |(4) 200 OK | | |<-----------------| | | | | Figure 6. Traversal of SIP REGISTER requests/responses using a reliable, connection orientated protocol such as TCP does not require any additional core SIP signaling extensions. SIP responses will re-use the connection created for the initial REGISTER request, (1) from Figure 6: Boulton, Ed., et al. Expires January 10, 2008 [Page 14] Internet-Draft NAT Scenarios July 2007 REGISTER sip:proxy.example.com SIP/2.0 Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKyilassjdshfu Max-Forwards: 70 Supported: path,gruu From: Client ;tag=djks809834 To: Client Call-ID: 763hdc783hcnam73@example.com CSeq: 1 REGISTER Contact: ;reg-id=1 ;+sip.instance="" Content-Length: 0 This example was included to show the inclusion of the connection re- use Contact header parameters as defined in the Connection Re-use draft [13]. This creates an association tuple as described in the previous example for future inbound requests directed at the newly created registration binding with the only difference that the association is with a TCP connection, not a UDP pin hole binding. [Editors note: Will provide more details on heartbeat mechanism in next revision] [Editors note: Can complete full flows on inclusion of heartbeat mechanism] 4.1.2. Registration(Registrar/Proxy not Co-Located) This section demonstrates traversal mechanisms when the Registrar component is not co-located with the edge proxy element. The procedures described in this section are identical, regardless of transport protocol and so only one example will be documented in the form of TCP. Boulton, Ed., et al. Expires January 10, 2008 [Page 15] Internet-Draft NAT Scenarios July 2007 Client NAT Proxy Registrar | | | | |(1) REGISTER | | | |----------------->| | | | | | | | |(1) REGISTER | | | |----------------->| | | | | | | | |(2) REGISTER | | | |----------------->| | | | | | | |(3) 401 Unauth | | | |<-----------------| | | | | | |(4) 401 Unauth | | | |<-----------------| | | | | | |(4)401 Unauth | | | |<-----------------| | | | | | | |(5)REGISTER | | | |----------------->| | | | | | | | |(5)REGISTER | | | |----------------->| | | | | | | | |(6)REGISTER | | | |----------------->| | | | | | | |(7)200 OK | | | |<-----------------| | | | | |********************************************************| | Create Connection Re-use Tuple | |********************************************************| | | | | | |(8)200 OK | | | |<-----------------| | | | | | |(8)200 OK | | | |<-----------------| | | | | | | Figure 7. This scenario builds on the previous example contained in Section 4.1.1.2. The primary difference being that the REGISTER request is routed onwards from a Proxy Server to a separated Boulton, Ed., et al. Expires January 10, 2008 [Page 16] Internet-Draft NAT Scenarios July 2007 Registrar. The important message to note is (6) in Figure 7. The Edge proxy, on receiving a REGISTER request that contains a 'sip.instance' media feature tag, forms a unique flow identifier token as discussed in [13] . At this point, the proxy server routes the SIP REGISTER message to the Registrar. The proxy will create the connection tuple as described in SIP Outbound at the same moment as the co-located example, but for subsequent messages to arrive at the Proxy, the element needs to request to remain in the signaling path. To achieve this the proxy inserts to REGISTER message (5) a SIP PATH extension header, as defined in RFC 3327 [7]. The previously created flow token is inserted in a position within the Path header where it can easily be retrieved at a later point when receiving messages to be routed to the registration binding. REGISTER message (5) would look as follows: REGISTER sip:registrar.example.com SIP/2.0 Via: SIP/2.0/TCP proxy.example.com:5060;branch=z9hG4njkca8398hadjaa Via: SIP/2.0/TCP client.example.com:5060;branch=z9hG4bKyilassjdshfu Max-Forwards: 70 Supported: path,gruu From: Client ;tag=djks809834 To: Client Call-ID: 763hdc783hcnam73@example.com CSeq: 1 REGISTER Path: Contact: ; ;+sip.instance="";reg-id=1 Content-Length: 0 This REGISTER request results in the Path header being stored along with the AOR and it's associated binding at the Registrar. The URI contained in the Path header will be inserted as a pre-loaded SIP 'Route' header into any request that arrives at the Registrar and is directed towards the associated binding. This guarantees that all requests for the new Registration will be forwarded to the Edge Proxy. In our example, the user part of the SIP 'Path' header URI that was inserted by the Edge Proxy contains the unique token identifying the flow to the client. On receiving subsequent requests, the edge proxy will examine the user part of the pre-loaded SIP 'route' header and extract the unique flow token for use in its connection tuple comparison, as defined in the SIP Outbound specification [13]. An example which builds on this scenario (showing an inbound request to the AOR) is detailed in section 4.1.4.2 of this document. Boulton, Ed., et al. Expires January 10, 2008 [Page 17] Internet-Draft NAT Scenarios July 2007 4.1.3. Initiating a Session This section covers basic SIP signaling when initiating a call from behind a NAT. 4.1.3.1. UDP Initiating a call using UDP. Client NAT Proxy [..] | | | |(1) INVITE | | | |----------------->| | | | | | | | |(1) INVITE | | | |----------------->| | | | | | | |(2) 407 Unauth | | | |<-----------------| | | | | | |(2) 407 Unauth | | | |<-----------------| | | | | | | |(3) INVITE | | | |----------------->| | | | | | | | |(3) INVITE | | | |----------------->| | | | | | | | |(4) INVITE | | | |---------------->| | | | | | | |(5)180 RINGING | | | |<----------------| | | | | | |(6)180 RINGING | | | |<-----------------| | | | | | |(6)180 RINGING | | | |<-----------------| | | | | | | | | |(7)200 OK | | | |<----------------| | | | | | |(8)200 OK | | | |<-----------------| | Boulton, Ed., et al. Expires January 10, 2008 [Page 18] Internet-Draft NAT Scenarios July 2007 | | | | |(8)200 OK | | | |<-----------------| | | | | | | |(9)ACK | | | |----------------->| | | | | | | | |(9)ACK | | | |----------------->| | | | | | | | |(10) ACK | | | |---------------->| | | | | Figure 8. The initiating client generates an INVITE request that is to be sent through the NAT to a Proxy server. The INVITE message is represented in Figure 8 by (1) and is as follows: INVITE sip:clientB@example.com SIP/2.0 Via: SIP/2.0/UDP client.example.com:5060;rport;branch=z9hG4bK74husdHG Max-Forwards: 70 Route: From: clientA ;tag=7skjdf38l To: clientB Call-ID: 8327468763423@example.com CSeq: 1 INVITE Contact: Content-Type: application/sdp Content-Length: .. [SDP not shown] There are a number of points to note with this message: 1. Firstly, as with the registration example in Section 4.1.1.1, responses to this request will not automatically pass back through a NAT and so the SIP 'Via' header 'rport' is included as described in the 'Symmetric response' Section 3.1.1 and defined in RFC 3581 [6]. 2. Secondly, the contact inserted contains the GRUU previously obtained from the SIP 200 OK response to the registration. Use of the GRUU ensures that any SIP requests within the dialog that in the opposite direction will be able to traverse the NAT. This occurs using the mechanisms defined in the SIP Outbound Boulton, Ed., et al. Expires January 10, 2008 [Page 19] Internet-Draft NAT Scenarios July 2007 specification[13]. A request arriving at the entity which resolves to the GRUU is then able to determine a previously registered connection that will allow the request to traverse the NAT and reach the intended endpoint. 4.1.3.2. Reliable Transport When using a reliable transport such as TCP the call flow and procedures for traversing a NAT are almost identical to those described in Section 4.1.3.1. The primary difference when using reliable transport protocols is that Symmetric response[6] are not required for SIP responses to traverse a NAT. RFC 3261[1] defines procedures for SIP response messages to be sent back on the same connection on which the request arrived. 4.1.4. Receiving an Invitation to a Session This section details scenarios where a client behind a NAT receives an inbound request through a NAT. These scenarios build on the previous registration scenario from Section 4.1.1 and Section 4.1.2 in this document. 4.1.4.1. Registrar/Proxy Co-located The core SIP signaling associated with this call flow is not impacted directly by the transport protocol and so only one example scenario is necessary. The example uses UDP and follows on from the registration installed in the example from Section 4.1.1.1. Boulton, Ed., et al. Expires January 10, 2008 [Page 20] Internet-Draft NAT Scenarios July 2007 Client NAT Registrar/Proxy SIP Entity | | | | |*******************************************************| | Registration Binding Installed in | | section 4.1.1.1 | |*******************************************************| | | | | | | |(1)INVITE | | | |<----------------| | | | | | |(2)INVITE | | | |<-----------------| | | | | | |(2)INVITE | | | |<-----------------| | | | | | | | | | | Figure 9. An INVITE request arrives at the Registrar with a destination pointing to the AOR of that inserted in section 4.1.1.1. The message is illustrated by (1) in Figure 9 and looks as follows: INVITE sip:client@example.com SIP/2.0 Via: SIP/2.0/UDP external.example.com;branch=z9hG4bK74huHJ37d Max-Forwards: 70 From: External ;tag=7893hd To: client Call-ID: 8793478934897@external.example.com CSeq: 1 INVITE Contact: Content-Type: application/sdp Content-Length: .. [SDP not shown] The INVITE request matches the registration binding previously installed at the Registrar and the INVITE request-URI is re-written to the selected onward address. The proxy then examines the request URI of the INVITE and compares with its list of current open flows. It uses the incoming AOR to commence the check for associated open connections/mappings. Once matched, the proxy checks to see if the unique instance identifier (+sip.instance) associated with the binding equals the same instance identifier associated with the flow. The request is then dispatched on the appropriate flow. This is Boulton, Ed., et al. Expires January 10, 2008 [Page 21] Internet-Draft NAT Scenarios July 2007 message (2) from Figure 9 and is as follows: INVITE sip:sip:client@client.example.com SIP/2.0 Via: SIP/2.0/UDP proxy.example.com;branch=z9hG4kmlds893jhsd Via: SIP/2.0/UDP external.example.com;branch=z9hG4bK74huHJ37d Max-Forwards: 70 From: External ;tag=7893hd To: client Call-ID: 8793478934897@external.example.com CSeq: 1 INVITE Contact: Content-Type: application/sdp Content-Length: .. [SDP not shown] It is a standard SIP INVITE request with no additional functionality. The major difference being that this request will not follow the address specified in the Request-URI, as standard SIP rules would enforce but will be sent on the flow associated with the registration binding (look-up procedures in RFC 3263 [6] are overridden). This then allows the original connection/mapping from the initial registration process to be re-used. 4.1.4.2. Registrar/Proxy Not Co-located Boulton, Ed., et al. Expires January 10, 2008 [Page 22] Internet-Draft NAT Scenarios July 2007 Client NAT Proxy Registrar SIP Entity | | | | | |***********************************************************| | Registration Binding Installed in | | section 4.1.2 | |***********************************************************| | | | | | | | | |(1)INVITE | | | | |<-------------| | | | | | | | |(2)INVITE | | | | |<-------------| | | | | | | | |(3)INVITE | | | | |<-------------| | | | | | | | |(3)INVITE | | | | |<-------------| | | | | | | | | | | | | | Figure 9. 4.2. Basic NAT Media Traversal This section provides example scenarios to demonstrate basic media traversal using the techniques outlined earlier in this document. In the flow diagrams STUN messages have been annotated for simplicity as follows: o The "Src" attribute represents the source transport address of the message. o The "Dest" attribute represents the destination transport address of the message. o The "Map" attribute represents the server reflexive (XOR-MAPPED- ADDRESS STUN attribute) transport address. o The "Rel" attribute represents the relayed (RELAY-ADDRESS STUN attribute) transport address. The meaning of each STUN attribute is extensively explained in the core STUN[10] and TURN usage[12] drafts. A number of ICE SDP attributes have also been included in some of the examples. Detailed information on individual attributes can be obtained from the core ICE specification. The examples also contain a mechanism for representing transport Boulton, Ed., et al. Expires January 10, 2008 [Page 23] Internet-Draft NAT Scenarios July 2007 addresses. It would be confusing to include representations of network addresses in the call flows and make them hard to follow. For this reason network addresses will be represented using the following annotation. The first component will contain the representation of the client responsible for the address. For example in the majority of the examples "L" (left client), "R" (right client), NAT-PUB" (NAT public), PRIV (Private), and "STUN-PUB" (STUN Public) are used. To allow for multiple addresses from the same network element, each representation can also be followed by a number. These can also be used in combination. For example "L-NAT- PUB-1" would represent a public network address on the left hand side NAT while "R-NAT-PUB-1" would represent a public network address in the right hand side of the NAT. "L-PRIV-1" would represent a private network address on the left hand side of the NAT while "R-PRIV-1" represents a private address on the right hand side of the NAT. It should also be noted that during the examples it might be appropriate to signify an explicit part of a transport address. This is achieved by adding either the '.address' or '.port' tag on the end of the representation. For example, 'L-PRIV-1.address' and 'L-PRIV- 1.port'. 4.2.1. Endpoint Independent NAT This section demonstrates an example of a client both initiating and receiving calls behind an 'Endpoint independent' NAT. An example is included for both STUN and ICE with ICE being the RECOMMENDED mechanism for media traversal. 4.2.1.1. STUN Solution It is possible to traverse media through an 'Endpoint Independent NAT using STUN. The remainder of this section provides a simplified examples of the 'Binding Discovery' STUN usage as defined in [10]. The STUN messages have been simplified and do not include 'Shared Secret' requests used to obtain the temporary username and password. [Editors Note: Expand to show full flow including Auth?.] 4.2.1.1.1. Initiating Session The following example demonstrates media traversal through a NAT with 'Address Independent' properties using the STUN 'Binding Discovery' usage. It is assumed in this example that the STUN client and SIP Client are co-located on the same physical machine. Note that some SIP signaling messages have been left out for simplicity. Boulton, Ed., et al. Expires January 10, 2008 [Page 24] Internet-Draft NAT Scenarios July 2007 Client NAT STUN [..] Server | | | | |(1) BIND Req | | | |Src=L-PRIV-1 | | | |Dest=STUN-PUB | | | |----------------->| | | | | | | | |(2) BIND Req | | | |Src=NAT-PUB-1 | | | |Dest=STUN-PUB | | | |----------------->| | | | | | | |(3) BIND Resp | | | |<-----------------| | | |Src=STUN-PUB | | | |Dest=NAT-PUB-1 | | | |Map=NAT-PUB-1 | | | | | | |(4) BIND Resp | | | |<-----------------| | | |Src=STUN-PUB | | | |Dest=L-PRIV-1 | | | |XOR=NAT-PUB-1 | | | | | | | |(5) BIND Req | | | |Src=L-PRIV-2 | | | |Dest=STUN-PUB | | | |----------------->| | | | | | | | |(6) BIND Req | | | |Src=NAT-PUB-2 | | | |Dest=STUN-PUB | | | |----------------->| | | | | | | |(7) BIND Resp | | | |<-----------------| | | |Src=STUN-PUB | | | |Dest=NAT-PUB-2 | | | |Map=NAT-PUB-2 | | | | | | |(8) BIND Resp | | | |<-----------------| | | |Src=STUN-PUB | | | |Dest=L-PRIV-2 | | | |Map=NAT-PUB-2 | | | | | | | |(9)SIP INVITE | | | Boulton, Ed., et al. Expires January 10, 2008 [Page 25] Internet-Draft NAT Scenarios July 2007 |----------------->| | | | | | | | |(10)SIP INVITE | | | |------------------------------------>| | | | | | | |(11)SIP 200 OK | | |<------------------------------------| | | | | |(12)SIP 200 OK | | | |<-----------------| | | | | | | |========================================================| |>>>>>>>>>>>>Outgoing Media sent from L-PRIV-1>>>>>>>>>>>| |========================================================| | | |========================================================| |<<<<<<<<<<<>>>>>>>>>>>Outgoing RTCP sent from L-PRIV-2>>>>>>>>>>>>| |========================================================| | | |========================================================| |<<<<<<<<<<<| | | | | | | | |(14) SIP ACK | | | |------------------------------------>| | | | | Figure 18: Endpoint Independent NAT - Initiating o On deciding to initiate a SIP voice session the client starts a local STUN client on the interface and port that is to be used for media (send/receive). The STUN client generates a standard 'Binding Discovery' request as indicated in (1) from Figure 18 which also highlights the source address and port for which the client device wishes to obtain a mapping. The 'Binding Discovery' request is sent through the NAT towards the public internet and STUN server. o Message (2) traverses the NAT and breaks out onto the public internet towards the public STUN server. Note that the source address of the 'Binding Discovery' request now represents the Boulton, Ed., et al. Expires January 10, 2008 [Page 26] Internet-Draft NAT Scenarios July 2007 public address and port from the public side of the NAT. o The STUN server receives the request and processes it appropriately. This results in a successful 'Binding Discovery' response being generated and returned (3). The message contains details of the XOR mapped public address (contained in the STUN XOR-MAPPED-ADDRESS attribute) which is to be used by the originating client to receive media (see 'Map=NAT-PUB-1' from (3)). o The 'Binding Discovery' response traverses back through the NAT using the path created by the 'Binding Discovery' request and presents the new XOR mapped address to the client (4). At this point the process is repeated to obtain a second XOR-mapped address (as shown in (5)-(8)) for an alternative local address (Address has changed from "L-PRIV-1" to "L-PRIV-2"). o The client now constructs a SIP INVITE message(9). Note that traversal of SIP is not covered in this example and is discussed in earlier sections of the document. The INVITE request will use the addresses it has obtained in the previous STUN transactions to populate the SDP of the SIP INVITE as shown below: v=0 o=test 2890844526 2890842807 IN IP4 $L-PRIV-1.address c=IN IP4 $NAT-PUB-1.address t=0 0 m=audio $NAT-PUB-1.port RTP/AVP 0 a=rtcp:$NAT-PUB-2.port o Note that the XOR-mapped address obtained from the 'Binding Discovery' transactions are inserted as the connection address for the SDP (c=NAT-PUB-1.address). The Primary port for RTP is also inserted in the SDP (m=audio NAT-PUB-1.port RTP/AVP 0). Finally, the port gained from the additional 'Binding Discovery' is placed in the RTCP attribute (as discussed in Section 3.2.1.1) for traversal of RTCP (a=rtcp:NAT-PUB-2.port). o The SIP signaling then traverses the NAT and sets up the SIP session (10-12). Note that the client transmits media as soon as the 200 OK to the INVITE arrives at the client (12). Up until this point the incoming media and RTCP will not pass through the NAT as no outbound association has been created with the far end client. Two way media communication has now been established. 4.2.1.1.2. Receiving Session Invitation Receiving a session for an 'Endpoint Independent' NAT using the STUN 'Binding Discovery' usage is very similar to the example outlined in Section 4.2.1.1.1. Figure 20 illustrates the associated flow of messages. Boulton, Ed., et al. Expires January 10, 2008 [Page 27] Internet-Draft NAT Scenarios July 2007 Client NAT STUN [..] Server | | | (1)SIP INVITE | | |<-----------------|------------------| | | | | |(2) SIP INVITE | | | |<-----------------| | | | | | | |(3) BIND Req | | | |Src=L-PRIV-1 | | | |Dest=STUN-PUB | | | |----------------->| | | | | | | | |(4) BIND Req | | | |Src=NAT-PUB-1 | | | |Dest=STUN-PUB | | | |----------------->| | | | | | | |(5) BIND Resp | | | |<-----------------| | | |Src=STUN-PUB | | | |Dest=NAT-PUB-1 | | | |Map=NAT-PUB-1 | | | | | | |(6) BIND Resp | | | |<-----------------| | | |Src=STUN-PUB | | | |Dest=L-PRIV-1 | | | |Map=NAT-PUB-1 | | | | | | | |(7) BIND Req | | | |Src=L-PRIV-2 | | | |Dest=STUN-PUB | | | |----------------->| | | | | | | | |(8) BIND Req | | | |Src=NAT-PUB-2 | | | |Dest=STUN-PUB | | | |----------------->| | | | | | | |(9) BIND Resp | | | |<-----------------| | | |Src=STUN-PUB | | | |Dest=NAT-PUB-2 | | | |Map=NAT-PUB-2 | | | | | | |(10) BIND Resp | | | |<-----------------| | | Boulton, Ed., et al. Expires January 10, 2008 [Page 28] Internet-Draft NAT Scenarios July 2007 |Src=STUN-PUB | | | |Dest=L-PRIV-2 | | | |Map=NAT-PUB-2 | | | | | | | |(11)SIP 200 OK | | | |----------------->| | | | |(12)SIP 200 OK | | | |------------------------------------>| | | | | |========================================================| |>>>>>>>>>>>>Outgoing Media sent from L-PRIV-1>>>>>>>>>>>| |========================================================| | | | | |========================================================| |<<<<<<<<<<<<>>>>>>>>>>>Outgoing RTCP sent from L-PRIV-2>>>>>>>>>>>>| |========================================================| | | | | |========================================================| |<<<<<<<<<<<<| | | | | | | | | | |(2) Alloc Req | | | | |Src=L-NAT-PUB-1 | | | | |Des=STUN-PUB-1 | | | | |--------------->| | | | | | | | | |(3) Alloc Resp | | | | |<---------------| | | | |Src=STUN-PUB-1 | | | | |Dest=L-NAT-PUB-1| | | | |Map=L-NAT-PUB-1 | | | | |Rel=STUN-PUB-2 | | | | | | | | |(4) Alloc Resp | | | | |<---------------| | | | |Src=STUN-PUB-1 | | | | |Dest=L-PRIV-1 | | | | |Map=L-NAT-PUB-1 | | | | |Rel=STUN-PUB-2 | | | | | | | | | |(5) STUN Req | | | | |Src=L-PRIV-2 | | | | |Dest=STUN-PUB-1 | | | | |--------------->| | | | | | | | | | |(6) Alloc Req | | | | |Src=L-NAT-PUB-2 | | | | |Dest=STUN-PUB-1 | | | | |--------------->| | | | | | | | | |(7) Alloc Resp | | | Boulton, Ed., et al. Expires January 10, 2008 [Page 31] Internet-Draft NAT Scenarios July 2007 | |<---------------| | | | |Src=STUN-PUB-1 | | | | |Dest=NAT-PUB-2 | | | | |Map=NAT-PUB-2 | | | | |Rel=STUN-PUB-3 | | | | | | | | |(8) Alloc Resp | | | | |<---------------| | | | |Src=STUN-PUB-1 | | | | |Dest=L-PRIV-2 | | | | |Map=L-NAT-PUB-2 | | | | |Rel=STUN-PUB-3 | | | | | | | | | |(9) SIP INVITE | | | | |------------------------------------------------->| | | | | | | | | | |(10) SIP INVITE | | | | |--------------->| | | | | | | | | |(11) Alloc Req | | | | |<---------------| | | | |Src=R-PRIV-1 | | | | |Dest=STUN-PUB-1 | | | | | | | | |(12) Alloc Req | | | | |<---------------| | | | |Src=R-NAT-PUB-1 | | | | |Dest=STUN-PUB-1 | | | | | | | | | |(13) Alloc Res | | | | |--------------->| | | | |Src=STUN-PUB-1 | | | | |Dest=R-NAT-PUB-1| | | | |Map=R-NAT-PUB-1 | | | | |Rel=STUN-PUB-4 | | | | | | | | | | |(14) Alloc Res | | | | |--------------->| | | | |Src=STUN-PUB-1 | | | | |Dest=R-PRIV-1 | | | | |Map=R-NAT-PUB-1 | | | | |Rel=STUN-PUB-4 | | | | | | | | | |(15) Alloc Req | | | | |<---------------| | | | |Src=R-PRIV-2 | | | | |Dest=STUN-PUB-1 | | | | | | Boulton, Ed., et al. Expires January 10, 2008 [Page 32] Internet-Draft NAT Scenarios July 2007 | | |(16) Alloc Req | | | | |<---------------| | | | |Src=R-NAT-PUB-2 | | | | |Dest=STUN-PUB-1 | | | | | | | | | |(17) Alloc Res | | | | |--------------->| | | | |Src=STUN-PUB-1 | | | | |Dest=R-NAT-PUB-2| | | | |Map=R-NAT-PUB-2 | | | | |Rel=STUN-PUB-5 | | | | | | | | | | |(18) Alloc Res | | | | |--------------->| | | | |Src=STUN-PUB-1 | | | | |Dest=R-PRIV-2 | | | | |Map=R-NAT-PUB-2 | | | | |Rel=STUN-PUB-5 | | | | | | | | | |(19) SIP 200 OK | | |<-------------------------------------------------| | | | | | |(20) SIP 200 OK | | | | |<---------------| | | | | | | | | |(21) SIP ACK | | | | |------------------------------------------------->| | | | | | | | | | |(22) SIP ACK | | | | |--------------->| | | | | | |(23) Bind Req | | | | |------------------------>x | | | |Src=L-PRIV-1 | | | | |Dest=R-PRIV-1 | | | | | | | | | |(24) Bind Req | | | | |--------------->| | | | |Src=L-PRIV-1 | | | | |Dest=R-NAT-PUB-1| | | | | | | | | | |(25) Bind Req | | | | |-------------------------------->| | | |Src=L-NAT-PUB-1 | | | | |Dest=R-NAT-PUB-1| | | | | | | | | | | |(26) Bind Req | | | | |--------------->| Boulton, Ed., et al. Expires January 10, 2008 [Page 33] Internet-Draft NAT Scenarios July 2007 | | | |Src=L-NAT-PUB-1 | | | | |Dest=R-PRIV-1 | | | | | | | | | |(27) Bind Res | | | | |<---------------| | | | |Src=R-PRIV-1 | | | | |Dest=L-NAT-PUB-1| | | | |Map=L-NAT-PUB-1 | | | | | | | | |(28) Bind Res | | | |<--------------------------------| | | | |Src=R-NAT-PUB-1 | | | | |Dest=L-NAT-PUB-1| | | | |Map=L-NAT-PUB-1 | | | | | | | |(29) Bind Res | | | | |<---------------| | | | |Src=R-NAT-PUB-1 | | | | |Dest=L-PRIV-1 | | | | |Map=L-NAT-PUB-1 | | | | | | | | | |===================================================================| |>>>>>>>>>>>>>>>>>>>>>Outgoing RTP sent from >>>>>>>>>>>>>>>>>>>>>>>| |===================================================================| | | | | | | | | |(30) Bind Req | | | | x<-----------------------| | | | |Src=R-PRIV-1 | | | | |Dest=L-PRIV-1 | | | | | | | | | |(31) Bind Req | | | | |<---------------| | | | |Src=R-PRIV-1 | | | | |Dest=L-NAT-PUB-1| | | | | | | | |(32) Bind Req | | | |<--------------------------------| | | | |Src=R-NAT-PUB-1 | | | | |Dest=L-NAT-PUB-1| | | | | | | |(33) Bind Req | | | | |<---------------| | | | |Src=R-NAT-PUB-1 | | | | |Dest=L-PRIV-1 | | | | | | | | | |(34) Bind Res | | | | |--------------->| | | | |Src=L-PRIV-1 | | | | Boulton, Ed., et al. Expires January 10, 2008 [Page 34] Internet-Draft NAT Scenarios July 2007 |Dest=R-NAT-PUB-1| | | | |Map=R-NAT-PUB-1 | | | | | | | | | | |(35) Bind Res | | | | |-------------------------------->| | | |Src=L-NAT-PUB-1 | | | | |Dest=R-NAT-PUB-1| | | | |Map=R-NAT-PUB-1 | | | | | | | | | | | |(36) Bind Res | | | | |--------------->| | | | |Src=L-NAT-PUB-1 | | | | |Dest=R-PRIV-1 | | | | |Map=R-NAT-PUB-1 | | | | | | |===================================================================| |<<<<<<<<<<<<<<<<<<<<| | | | |Src=L-PRIV-1 | | | | |Dest=R-NAT-PUB-1| | | | |USE-CANDIDATE | | | | | | | | | | |(38) Bind Req | | | | |-------------------------------->| | | |Src=L-NAT-PUB-1 | | | | |Dest=R-NAT-PUB-1| | | | |USE-CANDIDATE | | | | | | | | | | | |(39) Bind Req | | | | |--------------->| | | | |Src=L-NAT-PUB-1 | | | | |Dest=R-PRIV-1 | | | | |USE-CANDIDATE | | | | | | | | | |(40) Bind Res | | | | |<---------------| | | | |Src=R-PRIV-1 | | | | |Dest=L-NAT-PUB-1| | | | |Map=L-NAT-PUB-1 | | | | | | | | |(41) Bind Res | | | |<--------------------------------| | | | |Src=R-NAT-PUB-1 | | | | |Dest=L-NAT-PUB-1| | | | |Map=L-NAT-PUB-1 | | | | | | | Boulton, Ed., et al. Expires January 10, 2008 [Page 35] Internet-Draft NAT Scenarios July 2007 |(42) Bind Res | | | | |<---------------| | | | |Src=R-NAT-PUB-1 | | | | |Dest=L-PRIV-1 | | | | |Map=L-NAT-PUB-1 | | | | | | | | | |(43) Bind Req | | | | |--------------->| | | | |Src=L-PRIV-2 | | | | |Dest=R-NAT-PUB-2| | | | | | | | | | |(44) Bind Req | | | | |-------------------------------->| | | |Src=L-NAT-PUB-2 | | | | |Dest=R-NAT-PUB-2| | | | | | | | | | | |(45) Bind Req | | | | |--------------->| | | | |Src=L-NAT-PUB-2 | | | | |Dest=R-PRIV-2 | | | | | | | | | |(46) Bind Res | | | | |<---------------| | | | |Src=R-PRIV-2 | | | | |Dest=L-NAT-PUB-2| | | | |Map=L-NAT-PUB-2 | | | | | | | | |(47) Bind Res | | | |<--------------------------------| | | | |Src=R-NAT-PUB-2 | | | | |Dest=L-NAT-PUB-2| | | | |Map=L-NAT-PUB-2 | | | | | | | |(48) Bind Res | | | | |<---------------| | | | |Src=R-NAT-PUB-2 | | | | |Dest=L-PRIV-2 | | | | |Map=L-NAT-PUB-2 | | | | | | | | | |===================================================================| |>>>>>>>>>>>>>>>>>>>>>Outgoing RTCP sent from >>>>>>>>>>>>>>>>>>>>>>| |===================================================================| | | | | | | | | |(49) Bind Req | | | | |<---------------| | | | |Src=R-PRIV-2 | | | | |Dest=L-NAT-PUB-2| | | | | | Boulton, Ed., et al. Expires January 10, 2008 [Page 36] Internet-Draft NAT Scenarios July 2007 | | |(50) Bind Req | | | |<--------------------------------| | | | |Src=R-NAT-PUB-2 | | | | |Dest=L-NAT-PUB-2| | | | | | | |(51) Bind Req | | | | |<---------------| | | | |Src=R-NAT-PUB-2 | | | | |Dest=L-PRIV-2 | | | | | | | | | |(52) Bind Res | | | | |--------------->| | | | |Src=L-PRIV-2 | | | | |Dest=R-NAT-PUB-2| | | | |Map=R-NAT-PUB-2 | | | | | | | | | | |(53) Bind Res | | | | |-------------------------------->| | | |Src=L-NAT-PUB-2 | | | | |Dest=R-NAT-PUB-2| | | | |Map=R-NAT-PUB-2 | | | | | | | | | | | |(54) Bind Res | | | | |--------------->| | | | |Src=L-NAT-PUB-2 | | | | |Dest=R-PRIV-2 | | | | |Map=R-NAT-PUB-2 | | | | | | |===================================================================| |<<<<<<<<<<<<<<<<<<<<| | | | |Src=L-PRIV-2 | | | | |Dest=R-NAT-PUB-2| | | | |USE-CANDIDATE | | | | | | | | | | |(56) Bind Req | | | | |-------------------------------->| | | |Src=L-NAT-PUB-2 | | | | |Dest=R-NAT-PUB-2| | | | |USE-CANDIDATE | | | | | | | | | | | |(57) Bind Req | | | | |--------------->| | | | |Src=L-NAT-PUB-2 | | | | |Dest=R-PRIV-2 | | | | |USE-CANDIDATE | Boulton, Ed., et al. Expires January 10, 2008 [Page 37] Internet-Draft NAT Scenarios July 2007 | | | | | | | | |(58) Bind Res | | | | |<---------------| | | | |Src=R-PRIV-2 | | | | |Dest=L-NAT-PUB-2| | | | |Map=L-NAT-PUB-2 | | | | | | | | |(59) Bind Res | | | |<--------------------------------| | | | |Src=R-NAT-PUB-2 | | | | |Dest=L-NAT-PUB-2| | | | |Map=L-NAT-PUB-2 | | | | | | | |(60) Bind Res | | | | |<---------------| | | | |Src=R-NAT-PUB-2 | | | | |Dest=L-PRIV-2 | | | | |Map=L-NAT-PUB-2 | | | | | | | | | | | | | | |(61) SIP INVITE | | | | |------------------------------------------------->| | | | | | | | | | |(62) SIP INVITE | | | | |--------------->| | | | | | | | | |(63) SIP 200 OK | | |<-------------------------------------------------| | | | | | |(64) SIP 200 OK | | | | |<---------------| | | | | | | | | |(65) SIP ACK | | | | |------------------------------------------------->| | | | | | | | | | |(66) SIP ACK | | | | |--------------->| | | | | | Figure 22: Endpoint Independent NAT with ICE o On deciding to initiate a SIP voice session the SIP client 'L' starts a local STUN client. The STUN client generates a standard Allocate request as indicated in (1) from Figure 22 which also highlights the source address and port combination for which the client device wishes to obtain a mapping. The Allocate request is sent through the NAT towards the public internet. Boulton, Ed., et al. Expires January 10, 2008 [Page 38] Internet-Draft NAT Scenarios July 2007 o The Allocate message (2) traverses the NAT and breaks out onto the public internet towards the public STUN server. Note that the source address of the Allocate request now represents the public address and port from the public side of the NAT (L-NAT-PUB-1). o The STUN server receives the Allocate request and processes appropriately. This results in a successful Allocate response being generated and returned (3). The message contains details of the server reflexive address which is to be used by the originating client to receive media (see 'Map=L-NAT-PUB-1') from (3)). It also contains an appropriate relayed address that can be used at the STUN server (see 'Rel=STUN-PUB-2'). o The Allocate response traverses back through the NAT using the binding created by the initial Allocate request and presents the new mapped address to the client (4). The process is repeated and a second STUN derived set of address' are obtained, as illustrated in (5)-(8) in Figure 22. At this point the User Agent behind the NAT has pairs of derived external server reflexive and relayed representations. The client would be free to gather any number of external representations using any UNSAF[9] compliant protocol. o The client now constructs a SIP INVITE message (9). The INVITE request will use the addresses it has obtained in the previous STUN/TURN interactions to populate the SDP of the SIP INVITE. This should be carried out in accordance with the semantics defined in the ICE specification[16], as shown below in Figure 23 (*note - /* signifies line continuation): v=0 o=test 2890844526 2890842807 IN IP4 $L-PRIV-1 c=IN IP4 $L-PRIV-1.address t=0 0 a=ice-pwd:$LPASS a=ice-ufrag:$LUNAME m=audio $L-PRIV-1.port RTP/AVP 0 a=rtpmap:0 PCMU/8000 a=rtcp:$L-PRIV-2.port a=candidate:$L1 1 UDP 3102938476 $L-PRIV-1.address $L-PRIV-1.port typ local a=candidate:$L1 2 UDP 3010948635 $L-PRIV-2.address $L-PRIV-2.port typ local a=candidate:$L2 1 UDP 2203948363 $L-NAT-PUB-1.address $L-NAT-PUB-1.port typ srflx /* raddr $L-PRIV-1.address rport $L-PRIV-1.port a=candidate:$L2 2 UDP 2172635342 $L-NAT-PUB-2.address $L-NAT-PUB-2.port typ srflx /* raddr $L-PRIV-1.address rport $L-PRIV-2.port a=candidate:$L3 1 UDP 1763546473 $STUN-PUB-2.address $STUN-PUB-2.port typ relay /* raddr $L-PRIV-1.address rport $L-PRIV-1.port a=candidate:$L3 2 UDP 1109384746 $STUN-PUB-3.address $STUN-PUB-3.port typ relay /* raddr $L-PRIV-1.address rport $L-PRIV-2.port Figure 23: ICE SDP Offer Boulton, Ed., et al. Expires January 10, 2008 [Page 39] Internet-Draft NAT Scenarios July 2007 o The SDP has been constructed to include all the available candidates that have been assembled. The first set of candidates (as identified by Foundation $L1) contain two local addresses that have the highest priority. They are also encoded into the connection (c=) and media (m=) lines of the SDP. The second set of candidates, as identified by Foundation $L2, contains the two server reflexive addresses obtained from the STUN server for both RTP and RTCP traffic (identified by candidate-id $L2). This entry has been given a priority lower than the pair $L1 by the client. The third and final set of candidates represents the relayed addresses (as identified by $L3) obtained from the STUN server. This pair has the lowest priority and will be used as a last resort if both $L1 or $L2 fail. o The SIP signaling then traverses the NAT and sets up the SIP session (9)-(10). On advertising a candidate address, the client should have a local STUN server running on each advertised candidate address. This is for the purpose of responding to incoming STUN connectivity checks. o On receiving the SIP INVITE request (10) client 'R' also starts local STUN servers on appropriate address/port combinations and gathers potential candidate addresses to be encoded into the SDP (as the originating client did). Steps (11-18) involve client 'R' carrying out the same steps as client 'L'. This involves obtaining local, server reflexive and relayed addresses. Client 'R' is now ready to generate an appropriate answer in the SIP 200 OK message (19). The example answer follows in Figure 23 (*note - /* signifies line continuation): v=0 o=test 3890844516 3890842803 IN IP4 $R-PRIV-1 c=IN IP4 $R-PRIV-1.address t=0 0 a=ice-pwd:$RPASS m=audio $R-PRIV-1.port RTP/AVP 0 a=rtpmap:0 PCMU/8000 a=rtcp:$R-PRIV-2.port a=candidate:$L1 1 UDP 3303948573 $R-PRIV-1.address $R-PRIV-1.port typ local a=candidate:$L1 2 UDP 3184756473 $R-PRIV-2.address $R-PRIV-2.port typ local a=candidate:$L2 1 UDP 2984756463 $R-NAT-PUB-1.address $R-NAT-PUB-1.port typ srflx /* raddr $R-PRIV-1.address rport $R-PRIV-1.port a=candidate:$L2 2 UDP 2605968473 $R-NAT-PUB-2.address $R-NAT-PUB-2.port typ srflx /* raddr $R-PRIV-1.address rport $R-PRIV-1.port a=candidate:$L3 1 UDP 1453647488 $STUN-PUB-2.address $STUN-PUB-4.port typ relay /* raddr $R-PRIV-1.address rport $R-PRIV-1.port a=candidate:$L3 2 UDP 1183948473 $STUN-PUB-3.address $STUN-PUB-5.port typ relay /* raddr $R-PRIV-1.address rport $R-PRIV-1.port Boulton, Ed., et al. Expires January 10, 2008 [Page 40] Internet-Draft NAT Scenarios July 2007 Figure 24: ICE SDP Answer o The two clients have now exchanged SDP using offer/answer and can now continue with the ICE processing - User Agent 'L' assuming the role controlling agent, as specified by ICE. The clients are now required to form their Candidate check lists to determine which will be used for the media streams. In this example User Agent 'L's 'Foundation 1' is paired with User Agent 'R's 'Foundation 1', User Agent 'L's 'Foundation 2' is paired with User Agent 'R's 'Foundation 2', and finally User Agent 'L's 'Foundation 3' is paired with User Agent 'R's 'Foundation 3'. User Agents 'L' and 'R' now have a complete candidate check list. Both clients now use the algorithm provided in ICE to determine candidate pair priorities and sort into a list of decreasing priorities. In this example, both User Agent 'L' and 'R' will have lists that firstly specifies the host address (Foundation $L1), then the server reflexive address (Foundation $L2) and lastly the relayed address (Foundation $L3). All candidate pairs have an associate state as specified in ICE. At this stage, all of the candidate pairs for User Agents 'L' and 'R' are initialized to the 'Frozen' state. The User Agents then scan the list and move the candidates to the 'Waiting' state. At this point both clients will periodically, starting with the highest candidate pair priority, work there way down the list issuing STUN checks from the local candidate to the remote candidate (of the candidate pair). As a STUN Check is attempted from each local candidate in the list, the candidate pair state transitions to 'In-Progress'. As illustrated in (23), client 'L' constructs a STUN connectivity check in an attempt to validate the remote candidate address received in the SDP of the 200 OK (20) for the highest priority in the check list. As a private address was specified in the active address in the SDP, the STUN connectivity check fails to reach its destination causing a STUN failure. Client 'L' transitions the state for this candidate pair to 'Failed'. In the mean time, Client 'L' is attempting a STUN connectivity check for the second candidate pair in the returned SDP with the second highest priority (24). As can be seen from messages (24) to (29), the STUN Bind request is successful and returns a positive outcome for the connectivity check. Client 'L' is now free to steam media to the candidate pair. Client 'R' also carries out the same set of binding requests. It firstly (in parallel) tries to contact the active address contained in the SDP (30) which results in failure. o In the mean time, a successful response to a STUN connectivity check by User Agent 'R' (27) results in a tentative check in the reverse direction - this is illustrated by messages (31) to (36). Once this check has succeeded, User Agent 'R' can transition the state of the appropriate candidate to 'Succeeded', and media can be sent (RTP). The previously described check confirm on both Boulton, Ed., et al. Expires January 10, 2008 [Page 41] Internet-Draft NAT Scenarios July 2007 sides (User Agent 'L' and 'R') that connectivity can be achieved using the appropriate candidate pair. User Agent 'L', as the controlling client now sends another connectivity check for the candidate pair, this time including the 'USE-CANDIDATE' attribute as specified in ICE to signal the chosen candidate. This exchange is illustrated in messages (37) to (42). o As part of the process in this example, both 'L' and 'R' will now complete the same connectivity checks for part 2 of the component named for the favored 'Foundation' selected for use with RTCP. The connectivity checks for part '2' of the candidate component are shown in 'L'(43-48) and 'R'(49-54). Once this has succeeded, User Agent 'L' as the controlling client sends another connectivity check for the candidate pair. This time the 'USE- CANDIDATE' attribute is again specified to signal the chosen candidate for component '2'. o The candidates have now been fully verified (and selected) and as they are the highest priority, an updated offer (61-62) is now sent from the offerer (client 'L') to the answerer (client 'R') representing the new active candidates. The new offer would look as follows: v=0 o=test 2890844526 2890842808 IN IP4 $L-PRIV-1 c=IN IP4 $L-NAT-PUB-1.address t=0 0 a=ice-pwd:$LPASS a=ice-ufrag:$LUNAME m=audio $L-NAT-PUB-1.port RTP/AVP 0 a=rtpmap:0 PCMU/8000 a=rtcp:$L-NAT-PUB-2.port a=candidate:$L2 1 UDP 2203948363 $L-NAT-PUB-1.address $L-NAT-PUB-1.port typ srflx /* raddr $L-PRIV-1.address rport $L-PRIV-1.port a=candidate:$L2 2 UDP 2172635342 $L-NAT-PUB-2.address $L-NAT-PUB-2.port typ srflx /* raddr $L-PRIV-1.address rport $L-PRIV-2.port Figure 25: ICE SDP Updated Offer o The resulting answer (63-64) for 'R' would look as follows: Boulton, Ed., et al. Expires January 10, 2008 [Page 42] Internet-Draft NAT Scenarios July 2007 v=0 o=test 3890844516 3890842804 IN IP4 $R-PRIV-1 c=IN IP4 $R-PRIV-1.address t=0 0 a=ice-pwd:$RPASS a=ice-ufrag:$RUNAME m=audio $R-PRIV-1.port RTP/AVP 0 a=rtpmap:0 PCMU/8000 a=rtcp:$R-PRIV-2.port a=candidate:$L2 1 UDP 2984756463 $R-NAT-PUB-1.address $R-NAT-PUB-1.port typ srflx /* raddr $R-PRIV-1.address rport $R-PRIV-1.port a=candidate:$L2 2 UDP 2605968473 $R-NAT-PUB-2.address $R-NAT-PUB-2.port typ srflx /* raddr $R-PRIV-1.address rport $R-PRIV-2.port Figure 26: ICE SDP Updated Answer 4.2.2. Address and Port Dependant NAT 4.2.2.1. STUN Failure This section highlights that while STUN is the preferred mechanism for traversal of NAT, it does not solve every case. The use of basic STUN on its own will not guarantee traversal through every NAT type, hence the recommendation that ICE is the preferred option. Boulton, Ed., et al. Expires January 10, 2008 [Page 43] Internet-Draft NAT Scenarios July 2007 Client PORT/ADDRESS Dependant STUN [..] NAT Server | | | | |(1) BIND Req | | | |Src=L-PRIV-1 | | | |Dest=STUN-PUB | | | |----------------->| | | | | | | | |(2) BIND Req | | | |Src=NAT-PUB-1 | | | |Dest=STUN-PUB | | | |----------------->| | | | | | | |(3) BIND Resp | | | |<-----------------| | | |Src=STUN-PUB | | | |Dest=NAT-PUB-1 | | | |Map=NAT-PUB-1 | | | | | | |(4) BIND Resp | | | |<-----------------| | | |Src=STUN-PUB | | | |Dest=L-PRIV-1 | | | |Map=NAT-PUB-1 | | | | | | | |(5)SIP INVITE | | | |------------------------------------------------------->| | | | | | | |(6)SIP 200 OK | | |<------------------------------------| | | | | |(7)SIP 200 OK | | | |<-----------------| | | | | | | |========================================================| |>>>>>>>>>>>>>>Outgoing Media sent from L-PRIV-1>>>>>>>>>| |========================================================| | | | | | x=====================================| | xIncoming Media sent to L-PRIV-1<<<<<<| | x=====================================| | | | | |(8)SIP ACK | | | |----------------->| | | | |(9) SIP ACK | | | |------------------------------------>| | | | | Boulton, Ed., et al. Expires January 10, 2008 [Page 44] Internet-Draft NAT Scenarios July 2007 Figure 27: Port/Address Dependant NAT with STUN - Failure The example in Figure 27 is conveyed in the context of a client behind the 'Port/Address Dependant' NAT initiating a call. It should be noted that the same problem applies when a client receives a SIP invitation and is behind a Port/Address Dependant NAT. o In Figure 27 the client behind the NAT obtains a server reflexive representation using standard STUN mechanisms (1)-(4) that have been used in previous examples in this document (e.g Section 4.2.1.1.1). o The external mapped address (server reflexive) obtained is also used in the outgoing SDP contained in the SIP INVITE request(5). o In this example the client is still able to send media to the external client. The problem occurs when the client outside the NAT tries to use the reflexive address supplied in the outgoing INVITE request to traverse media back through the 'Port/Address Dependent' NAT. o A 'Port/Address Dependant' NAT has differing rules from the 'Endpoint Independent' type of NAT (as defined in [11]). For any internal IP address and port combination, data sent to a different external destination does not provide the same public mapping at the NAT. In Figure 27 the STUN query produced a valid external mapping for receiving media. This mapping, however, can only be used in the context of the original STUN request that was sent to the STUN server. Any packets that attempt to use the mapped address, that do not originate from the STUN server IP address and optionally port, will be dropped at the NAT. Figure 27 shows the media being dropped at the NAT after (7) and before (8). This then leads to one way audio. 4.2.2.2. TURN Usage Solution As identified in Section 4.2.2.1, STUN provides a useful tool kit for the traversal of the majority of NATs but fails with Port/Address Dependant NAT. This led to the development of the TURN usage solution [12] which uses the STUN toolkit. It allows a client to request a relayed address at the STUN server rather than a reflexive representation. This then introduces a media relay in the path for NAT traversal (as described in Section 3.2.3). The following example explains how the TURN usage solves the previous failure when using STUN to traverse a 'Port/Address Dependant' type NAT. L Port/Address Dependant STUN [..] NAT Server | | | | |(1) Alloc Req | | | Boulton, Ed., et al. Expires January 10, 2008 [Page 45] Internet-Draft NAT Scenarios July 2007 |Src=L-PRIV-1 | | | |Dest=STUN-PUB-1 | | | |----------------->| | | | | | | | |(2) Alloc Req | | | |Src=NAT-PUB-1 | | | |Dest=STUN-PUB-1 | | | |----------------->| | | | | | | |(3) Alloc Resp | | | |<-----------------| | | |Src=STUN-PUB-1 | | | |Dest=NAT-PUB-1 | | | |Map=NAT-PUB-1 | | | |Rel=STUN-PUB-2 | | | | | | |(4) Alloc Resp | | | |<-----------------| | | |Src=STUN-PUB-1 | | | |Dest=L-PRIV-1 | | | |Map=NAT-PUB-1 | | | |Rel=STUN-PUB-2 | | | | | | | |(5) Alloc Req | | | |Src=L-PRIV-2 | | | |Dest=STUN-PUB-1 | | | |----------------->| | | | | | | | |(6) Alloc Req | | | |Src=NAT-PUB-2 | | | |Dest=STUN-PUB-1 | | | |----------------->| | | | | | | |(7) Alloc Resp | | | |<-----------------| | | |Src=STUN-PUB-1 | | | |Dest=NAT-PUB-2 | | | |Map=NAT-PUB-2 | | | |Rel=STUN-PUB-3 | | | | | | |(8) Alloc Resp | | | |<-----------------| | | |Src=STUN-PUB-1 | | | |Dest=L-PRIV-2 | | | |Map=NAT-PUB-2 | | | |Rel=STUN-PUB-3 | | | | | | | |(9)SIP INVITE | | | Boulton, Ed., et al. Expires January 10, 2008 [Page 46] Internet-Draft NAT Scenarios July 2007 |----------------->| | | | | | | | |(10)SIP INVITE | | | |------------------------------------>| | | | | | | |(11)SIP 200 OK | | |<------------------------------------| | | | | |(12)SIP 200 OK | | | |<-----------------| | | | | | | |========================================================| |>>>>>>>>>>>>>Outgoing Media sent from L-PRIV-1>>>>>>>>>>| |========================================================| | | | | | | |==================| | | |<<| | | |<<<| | | | | | | | |(14) SIP ACK | | | |------------------------------------>| | | | | Figure 28: Port/Address Dependant NAT with TURN Usage - Success o In this example, client 'L' issues a TURN allocate request(1) to obtained a relay address at the STUN server. The request traverses through the 'Port/Address Dependant' NAT and reaches the STUN server (2). The STUN server generates an Allocate response (3) that contains both a server reflexive address (Map=NAT-PUB-1) Boulton, Ed., et al. Expires January 10, 2008 [Page 47] Internet-Draft NAT Scenarios July 2007 of the client and also a relayed address (Rel=STUN-PUB-2). The relayed address maps to an address mapping on the STUN server which is bound to the public pin hole that has been opened on the NAT by the Allocate request. This results in any traffic sent to the STUN server relayed address (Rel=STUN-PUB-2) being forwarded to the external representation of the pin hole created by the Allocate request(NAT-PUB-1). o The TURN derived address (STUN-PUB-2) arrives back at the originating client(4) in an Allocate response. This address can then be used in the SDP for the outgoing SIP INVITE request as shown in the following example (note that the example also includes client 'L' obtaining a second relay address for use in the RTCP attribute (5-8)): o v=0 o=test 2890844342 2890842164 IN IP4 $L-PRIV-1 c=IN IP4 $STUN-PUB-2.address t=0 0 m=audio $STUN-PUB-2.port RTP/AVP 0 a=rtcp:$STUN-PUB-3.port o On receiving the INVITE request, the UAS is able to stream media and RTCP to the relay address (STUN-PUB-2 and STUN-PUB-3) at the STUN server. As shown in Figure 28 (between messages (12) and (13), the media from the UAS is directed to the relayed address at the STUN server. The STUN server then forwards the traffic to the open pin holes in the Port/Address Dependant NAT (NAT-PUB-1 and NAT-PUB-2). The media traffic is then able to traverse the 'Port/ Address Dependant' NAT and arrives back at client 'L'. o The TURN usage of STUN on its own will work for 'Port/Address Dependent' and other types of NAT mentioned in this specification but should only be used as a last resort. The relaying of media through an external entity is not an efficient mechanism for NAT traversal and comes at a high processing cost. 4.2.2.3. ICE Solution The previous two examples have highlighted the problem with using core STUN usage for all forms of NAT traversal and a solution using TURN usage for the Address/Port Dependant NAT case. As mentioned previously in this document, the RECOMMENDED mechanism for traversing all varieties of NAT is using ICE, as detailed in Section 3.2.4. ICE makes use of core STUN, TURN usage and any other UNSAF[9] compliant protocol to provide a list of prioritized addresses that can be used for media traffic. Detailed examples of ICE can be found in Section 4.2.1.2.1. These examples are associated with an 'Endpoint Boulton, Ed., et al. Expires January 10, 2008 [Page 48] Internet-Draft NAT Scenarios July 2007 Independent' type NAT but can be applied to any NAT type variation, including 'Address/Port Dependant' type NAT. The ICE procedures procedures carried out are the same. For a list of candidate addresses, a client will choose where to send media dependant on the results of the STUN connectivity checks and associated priority (highest priority wins). It should be noted that the inclusion of a NAT displaying Address/Port Dependent properties does not automatically result in relayed media. In fact, ICE processing will avoid use of media with the exception of two clients which both happen to be behind a NAT using Address/Port Dependent characteristics. The connectivity checks and associated selection algorithm enable traversal in this case. Figure 30 and following description provide a guide as to how this is achieved using the ICE connectivity checks. This is an abbreviated example that assumes successful SIP offer/answer exchange and illustrates the connectivity check flow. L Port/Address Dependent Address Independent R NAT NAT |========================================================| | SIP OFFER/ANSWER EXCHANGE | |========================================================| | | | | | | |(1)Bind Req | | | |<-----------------| | | |Src=R=PRIV-1 | | | |Dest=L-NAT-PUB-1 | | | | | | |(2)Bind Req | | | x<-----------------| | | |Src=R-NAT-PUB-1 | | | |Dest=L-NAT-PUB-1 | | | | | | |(3)Bind Req | | | |----------------->| | | |Src=L-PRIV-1 | | | |Dest=R-NAT-PUB-1 | | | | | | | | |(4)Bind Req | | | |----------------->| | | |Src=L-NAT-PUB-1 | | | |Dest=R-NAT-PUB-1 | | | | | | | | |(5)Bind Req | | | |----------------->| | | |Src=R-NAT-PUB-1 | Boulton, Ed., et al. Expires January 10, 2008 [Page 49] Internet-Draft NAT Scenarios July 2007 | | |Dest=R-PRIV-1 | | | | | | | |(6)Bind Resp | | | |<-----------------| | | |Src=R-PRIV-1 | | | |Dest=L-NAT-PUB-1 | | | | | | |(7)Bind Resp | | | |<-----------------| | | |Src=R-NAT-PUB-1 | | | |Dest=L-NAT-PUB-1 | | | | | | |(8)Bind Resp | | | |<-----------------| | | |Src=L-NAT-PUB-1 | | | |Dest=L-PRIV-1 | | | | | | | | | |(9)Bind Req | | | |<-----------------| | | |Src=R-Priv-1 | | | |Dest=L-NAT-PUB-1 | | |(10)Bind Req | | | |<-----------------| | | |Src=R-NAT-PUB-1 | | | |Dest=L-NAT-PUB-1 | | | | | | |(11)Bind Req | | | |<-----------------| | | |Src=L-NAT-PUB-1 | | | | | | | |(12)Bind Resp | | | |----------------->| | | |Src=L-PRIV-1 | | | |Dest=L-NAT-PUB-1 | | | | | | | | |(13)Bind Resp | | | |----------------->| | | |Src=L-NAT-PUB-1 | | | |Dest=R-NAT-PUB-1 | | | | | | | | |(14)Bind Resp | | | |----------------->| | | |Src=R-NAT-PUB-1 | | | |Dest=R-PRIV-1 | | | | | | Boulton, Ed., et al. Expires January 10, 2008 [Page 50] Internet-Draft NAT Scenarios July 2007 Figure 30: Single Port/Address Dependant NAT - Success In this abbreviated example, Client R has already received a SIP INVITE request and is starting its connectivity checks with Client L. Client R generates a connectivity check (1) and sends to client L's information as presented in the SDP offer. The request arrives at client L's Port/Address dependent NAT and fails to traverse as there is no security association. This would then move the connectivity check to a failed state. In the mean time client L has received the SDP answer in the SIP request and will also commence connectivity checks. A check is dispatched (3) to Client R. The check is able to traverse the NAT due to the association set up in the previously failed check(1). The full Bind request/response is shown in steps (3)-(8). As part of a candidate pair, Client R will now successfully be able to complete the checks, as illustrated in steps (9)-(14). The result is a successful pair of candidates that can be used without the need to relay any media. Using the example described in Section 4.2.1.2.1, lets change the example so that both clients 'L' and 'R' are both behind NATs that have Address/Port Dependant behavior. Editors Note: TODO: Expand this to a full example. 5. IPv4-IPv6 Transition This section describes how IPv6-only SIP user agents can communicate with IPv4-only SIP user agents. 5.1. IPv4-IPv6 Transition for SIP Signaling IPv4-IPv6 translations at the SIP level usually take place at dual- stack proxies that have both IPv4 and IPv6 DNS entries. Since this translations do not involve NATs that are placed in the middle of two SIP entities, they fall outside the scope of this document. A detailed description of this type of translation can be found in [19] 5.2. IPv4-IPv6 Transition for Media Figure 31 shows a network of IPv6 SIP user agents that has a relay with a pool of public IPv4 addresses. The IPv6 SIP user agents of this IPv6 network need to communicate with users on the IPv4 Internet. To do so, the IPv6 SIP user agents use the TURN usage to obtain a public IPv4 address from the relay. The mechanism that an IPv6 SIP user agent follows to obtain a public IPv4 address from a relay using the TURN usage is the same as the one followed by a user agent with a private IPv4 address to obtain a public IPv4 address. Boulton, Ed., et al. Expires January 10, 2008 [Page 51] Internet-Draft NAT Scenarios July 2007 The example in Figure 32 explains how to use the TURN usage to obtain an IPv4 address and how to use the ANAT semantics [17] of the SDP grouping framework [8] to provide both IPv4 and IPv6 addresses for a particular media stream. +----------+ | / \ | /SIP \ /Phone \ / \ ------------ IPv4 Network 192.0.2.0/8 +---------+ | | ----------------------| NAT |-------------------------- | | +---------+ IPv6 Network ++ || +-----++ | IPv6 | | SIP | | user | | agent| +------+ Figure 31: IPv6-IPv4 transition scenario Boulton, Ed., et al. Expires January 10, 2008 [Page 52] Internet-Draft NAT Scenarios July 2007 IPv6 SIP TURN IPv4 SIP User Agent Server User Agent | | | | (1) STUN Allocate | | | src=[2001:DB8::1]:30000 | | |------------------------------------>| | | (2) TURN Resp | | | rel=192.0.2.2:25000 | | | dest=[2001:DB8::1]:30000 | | |<------------------------------------| | | (3) SIP INVITE | | |------------------------------------------------------->| | (4) SIP 200 OK | | |<-------------------------------------------------------| | | | |=====================================| | |>>>>>>>>>> Outgoing Media >>>>>>>>>>>| | |=====================================| | | |==================| | |>>>>>> Media >>>>>| | |==================| | | | | |==================| | |<<<<<< Media <<<<<| | |==================| |=====================================| | |<<<<<<<<<< Outgoing Media <<<<<<<<<<<| | |=====================================| | | | | | (5) SIP ACK | | |------------------------------------------------------->| | | | Figure 32: IPv6-IPv4 translation with TURN o The IPv6 SIP user agent obtains a TURN-derived IPv4 address by issuing a STUN Allocate request (1). The STUN server generates a response that contains a relayed IPv4 address using the TURN usage. This IPv4 address maps to the IPv6 source address of the STUN Allocate request, which the IPv6 address of the SIP user agent. This results in any traffic being sent to the IPv4 address provided by STUN server (192.0.2.2:25000) will be redirected to the IPv6 address of the SIP user agent ([2001:DB8::1]:30000). o The TURN-derived address (192.0.2.2:25000) arrives back at the originating user agent (2). This address can then be used in the SDP for the outgoing SIP INVITE request. The user agent builds two media lines, one with its IPv6 address and the other with the Boulton, Ed., et al. Expires January 10, 2008 [Page 53] Internet-Draft NAT Scenarios July 2007 IPv4 address that was just obtained. The user agent groups both media lines using the ANAT semantics as shown below (note that the RTCP attribute in the IPv4 media line would have been obtained by another TURN-derived address which is not shown in the call flow for simplicity). v=0 o=test 2890844342 2890842164 IN IP6 2001:DB8::1 t=0 0 a=group:ANAT 1 2 m=audio 20000 RTP/AVP 0 c=IN IP6 2001:DB8::1 a=mid:1 m=audio 25000 RTP/AVP 0 c=IN IP4 192.0.2.2 a=rtcp:25001 a=mid:2 o On receiving the INVITE request, the user agent server rejects the IPv6 media line by setting its port to zero in the answer and starts sending media to the IPv4 address in the offer. The IPv6 user agent sends media through the relay as well, as shown in Figure 32. 6. Next Steps What exactly is needed in this draft?. WG input is quickly required now that ICE has become stable. A comprehensive list is needed that will satisfy a wide cross section of the community. o 7. Acknowledgments The authors would like to thank the members of the IETF SIPPING WG for their comments and suggestions. Detailed comments were provided by Francois Audet, kaiduan xie and Hans Persson. 8. References 8.1. Normative References [1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002. Boulton, Ed., et al. Expires January 10, 2008 [Page 54] Internet-Draft NAT Scenarios July 2007 [2] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [3] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. [4] Tsirtsis, G. and P. Srisuresh, "Network Address Translation - Protocol Translation (NAT-PT)", RFC 2766, February 2000. [5] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002. [6] Rosenberg, J. and H. Schulzrinne, "An Extension to the Session Initiation Protocol (SIP) for Symmetric Response Routing", RFC 3581, August 2003. [7] Willis, D. and B. Hoeneisen, "Session Initiation Protocol (SIP) Extension Header Field for Registering Non-Adjacent Contacts", RFC 3327, December 2002. [8] Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne, "Grouping of Media Lines in the Session Description Protocol (SDP)", RFC 3388, December 2002. [9] Daigle, L. and IAB, "IAB Considerations for UNilateral Self- Address Fixing (UNSAF) Across Network Address Translation", RFC 3424, November 2002. [10] Rosenberg, J., Huitema, C., Mahy, R., Matthews, P., and D. Wing, "Session Traversal Utilities for (NAT) (STUN)", draft-ietf-behave-rfc3489bis-07 (work in progress), July 2007. [11] Audet, F. and C. Jennings, "NAT Behavioral Requirements for Unicast UDP", draft-ietf-behave-nat-udp-08 (work in progress), October 2006. [12] Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal Underneath NAT (STUN)", draft-ietf-behave-turn-03 (work in progress), March 2007. [13] Jennings, C. and A. Hawrylyshen, "SIP Conventions for UAs with Outbound Only Connections", draft-jennings-sipping-outbound-01 (work in progress), February 2005. [14] Rosenberg, J., "Obtaining and Using Globally Routable User Agent (UA) URIs (GRUU) in the Session Initiation Protocol (SIP)", draft-ietf-sip-gruu-14 (work in progress), June 2007. Boulton, Ed., et al. Expires January 10, 2008 [Page 55] Internet-Draft NAT Scenarios July 2007 [15] Wing, D., "Symmetric RTP and RTCP Considered Helpful", draft-wing-mmusic-symmetric-rtprtcp-01 (work in progress), October 2004. [16] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", draft-ietf-mmusic-ice-16 (work in progress), June 2007. [17] 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. [18] Rosenberg, J., "TCP Candidates with Interactive Connectivity Establishment (ICE", draft-ietf-mmusic-ice-tcp-03 (work in progress), March 2007. 8.2. Informative References [19] Camarillo, G., "IPv6 Transcition in the Session Initiation Protocol (SIP)", draft-camarillo-sipping-v6-transition-00 (work in progress), February 2005. Authors' Addresses Chris Boulton Ubiquity Software Corporation Eastern Business Park St Mellons Cardiff, South Wales CF3 5EA Email: cboulton@ubiquitysoftware.com Jonathan Rosenberg Cisco Systems 600 Lanidex Plaza Parsippany, NJ 07054 Email: jdrosen@cisco.com Boulton, Ed., et al. Expires January 10, 2008 [Page 56] Internet-Draft NAT Scenarios July 2007 Gonzalo Camarillo Ericsson Hirsalantie 11 Jorvas 02420 Finland Email: Gonzalo.Camarillo@ericsson.com Boulton, Ed., et al. Expires January 10, 2008 [Page 57] Internet-Draft NAT Scenarios July 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Boulton, Ed., et al. Expires January 10, 2008 [Page 58]