Network Working Group Magnus Westerlund INTERNET-DRAFT Ericsson Expires: Jan 17 2005 Thomas Zeng PacketVideo Network Solutions July 18, 2004 How to Enable Real-Time Streaming Protocol (RTSP) Traverse Network Address Translators (NAT) and Interact with Firewalls. Status of this memo By submitting this Internet-Draft, I (we) certify that any applicable patent or other IPR claims of which I am (we are) aware have been disclosed, and any of which I (we) become aware will be disclosed, in accordance with RFC 3668 (BCP 79). By submitting this Internet-Draft, I (we) accept the provisions of Section 3 of RFC 3667 (BCP 78). 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 obsolete by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This document is an individual submission to the IETF. Comments should be directed to the authors. Abstract This document describes several types of NAT traversal techniques that can be used by RTSP. For each technique a description on how it shall be used, what security implications it has and other Westerlund, Zeng Standards Track [Page 1] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 deployment considerations are given. Further a description on how RTSP relates to firewalls is given. TABLE OF CONTENTS 1. Definitions.........................................................4 1.1. Glossary........................................................4 1.2. Terminology.....................................................4 2. Changes.............................................................4 3. Introduction........................................................5 3.1. NATs............................................................5 3.2. Firewalls.......................................................6 4. Requirements........................................................7 5. Detecting the loss of NAT mappings..................................8 6. NAT Traversal Techniques............................................9 6.1. STUN............................................................9 6.1.1. Introduction.................................................9 6.1.2. Using STUN to traverse NAT without server modifications.....10 6.1.3. Embedding STUN in RTSP......................................11 6.1.4. Discussion On Co-located STUN Server........................13 6.1.5. ALG considerations..........................................13 6.1.6. Deployment Considerations...................................13 6.1.7. Security Considerations.....................................15 6.2. ICE............................................................15 6.2.1. Introduction................................................15 6.2.2. Using ICE in RTSP...........................................16 6.2.3. Implementation burden of ICE................................17 6.2.4. Deployment Considerations...................................17 6.3. Symmetric RTP..................................................17 6.3.1. Introduction................................................17 6.3.2. Necessary RTSP extensions...................................18 6.3.3. Deployment Considerations...................................18 6.3.4. Security Consideration......................................19 6.3.5. A Variation to Symmetric RTP................................20 6.4. Application Level Gateways.....................................21 6.4.1. Introduction................................................21 6.4.2. Guidelines On Writing ALGs for RTSP.........................22 6.4.3. Deployment Considerations...................................24 6.4.4. Security Considerations.....................................24 6.5. TCP Tunneling..................................................24 6.5.1. Introduction................................................24 6.5.2. Usage of TCP tunneling in RTSP..............................25 6.5.3. Deployment Considerations...................................25 6.5.4. Security Considerations.....................................25 6.6. TURN (Traversal Using Relay NAT)...............................25 Westerlund, Zeng Standards Track [Page 2] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 6.6.1. Introduction................................................25 6.6.2. Usage of TURN with RTSP.....................................26 6.6.3. Deployment Considerations...................................27 6.6.4. Security Considerations.....................................27 7. Firewalls..........................................................28 8. Comparison of Different NAT Traversal Techniques...................29 9. Open Issues........................................................29 10. Security Consideration............................................30 11. IANA Consideration................................................30 12. Acknowledgments...................................................31 13. Author's Addresses................................................31 14. References........................................................32 15. IPR Notice........................................................34 16. Copyright Notice..................................................34 Westerlund, Zeng Standards Track [Page 3] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 1. Definitions 1.1. Glossary ALG - Application Level Gateway, an entity that can be embedded in a NAT or other middlebox to perform the application layer functions required for a particular protocol to traverse the NAT/middlebox [6] ICE - Interactive Connectivity Establishment, see [9]. DNS - Domain Name Service DDOS - Distributed Denial Of Service attacks MID - Media Identifier from Grouping of media lines in SDP, see [10]. NAT - Network Address Translator, see [12]. NAT-PT - Network Address Translator Protocol Translator, see [13] RTP - Real-time Transport Protocol, see [5]. RTSP - Real-Time Streaming Protocol, see [1] and [7]. SDP - Session Description Protocol, see [2]. SSRC - Synchronization source in RTP, see [5]. TBD - To Be Decided 1.2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [4]. 2. Changes The following changes have been done since draft-ietf-mmusic-rtsp- nat-02.txt: - Added reference to [RTP_NULL] draft. - Added overview of a variation to symmetric RTP. - Added a chapter on the comparisons of different NAT traversal techniques. - Condensed wording on STUN, ICE and Symmetric RTP, in an effort to make this draft a little shorter. - Removed the requirement that "we must use RFC2326bis". Westerlund, Zeng Standards Track [Page 4] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 3. Introduction Today there is a proliferate deployment of different flavors of Network Address Translator (NAT) boxes that in practice follow standards rather loosely [12][24][18]. NATs cause discontinuity in address realms [18], therefore a protocol, such as RTSP, needs to try to make sure that it can deal with such discontinuities caused by NATs. The problem with RTSP is that, being a media control protocol that manages one or more media streams; RTSP carries information about network addresses and ports inside itself. Because of this, even if RTSP itself, when carried over TCP for example, is not blocked by NATs, its media streams may be blocked by NATs, unless special provisions are added to support NAT-traversal. Like NATs, firewalls (FWs) are also middle boxes that need to be considered. They are deployed to prevent unwanted traffic to be able to get in or out of the protected network. RTSP is designed such that a firewall can be configured to let RTSP controlled media streams to go through with minimal implementation problems. However there is a need for more detailed information on how FWs should be configured to work with RTSP. This document describes several NAT-traversal mechanisms for RTSP based streaming. These NAT solutions fall into the category of ""UNilateral Self-Address Fixing (UNSAF)" as defined in [18] and quoted below: "UNSAF is a process whereby some originating process attempts to determine or fix the address (and port) by which it is known - e.g. to be able to use address data in the protocol exchange, or to advertise a public address from which it will receive connections." Following the guidelines spelled out in [18], we describe the required RTSP protocol extensions for each method, transition strategies, and security concerns. This document intends to recommend FW/NAT traversal methods for RTSP streaming servers based on RFC 2326 [1] as well as the updated RTSP core spec [7]. This document is intended to be updated to stay consistent with the RTSP core protocol [7]. 3.1. NATs Today there exist a number of different NAT types and usage areas. The different NAT types are cited here from STUN [6]: Full Cone: A full cone NAT is one where all requests from the same internal IP address and port are mapped to the same external IP address and port. Furthermore, any external host can send a packet Westerlund, Zeng Standards Track [Page 5] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 to the internal host, by sending a packet to the mapped external address. Restricted Cone: A restricted cone NAT is one where all requests from the same internal IP address and port are mapped to the same external IP address and port. Unlike a full cone NAT, an external host (with IP address X) can send a packet to the internal host only if the internal host had previously sent a packet to IP address X. Port Restricted Cone: A port restricted cone NAT is like a restricted cone NAT, but the restriction includes port numbers. Specifically, an external host can send a packet, with source IP address X and source port P, to the internal host only if the internal host had previously sent a packet to IP address X and port P. Symmetric: A symmetric NAT is one where all requests from the same internal IP address and port, to a specific destination IP address and port, are mapped to the same external IP address and port. If the same host sends a packet with the same source address and port, but to a different destination, a different mapping is used. Furthermore, only the external host that receives a packet can send a UDP packet back to the internal host. NATs are used on both small and large scales. The normal small-scale user is home user that has a NAT to allow multiple computers share the single IP address given by their Internet Service Provider (ISP). The large-scale users are the ISP's themselves that give their users private addresses. This is done both for control and for lack of IP addresses. Native Address Translation and Protocol Translation (NAT-PT) [13] is a mechanism used for IPv4 to IPv6 transition. This device is used to allow devices only having connectivity using one of the IP versions to communicate with the other address domain. If the other address domain is addressable through the use of domain names, then a DNS ALG assigns temporary IP addresses in the requestor's domain. The NAT-PT device translates this temporary address to the receiversİ true IP address and at the same time modifies all necessary IP header fields so they are correct in the receiver's address domain. 3.2. Firewalls A firewall (FW) is a security gateway that enforces certain access control policies between two network administrative domains: a private domain (intranet) and a public domain (public internet). Many organizations use firewalls to prevent privacy intrusions and malicious attacks to corporate computing resources in the private intranet [19]. A comparison between NAT and FW are given below: Westerlund, Zeng Standards Track [Page 6] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 1. FW must be a gateway between two network administrative domains, while NAT does not have to sit between two domains. In fact, in many corporations there are many NAT boxes within the intranet, in which case the NAT boxes sit between subnets. 2. NAT does not in itself provide security, although some access control policies can be implemented using address translation schemes. 3. NAT and FWs are similar in that they can both be configured to allow multiple network hosts to share a single public IP address. In other words, a host behind a NAT or FW can have a private IP address and a public one, so for NAT and FW there is the issue of address mapping which is important in order for RTSP protocol to work properly when there are NATs and FWs between the RTSP server and its clients. In the rest of this memo we use the phrase "NAT traversal" interchangeably with "FW traversal", "NAT/FW traversal" and "NAT/Firewall traversal". 4. Requirements This section considers the set of requirements when designing or evaluating RTSP NAT traversal solutions. RTSP is a client/server protocol, and as such the targeted applications in general deploy RTSP servers in the public address realm. However, there are use cases where the reverse is true: RTSP clients are connecting from public address realm to RTSP servers behind home NATs. This is the case for instance when home surveillance cameras running as RTSP servers intend to stream video to cell phone users in the public address realm through a home NAT. The first priority should be to solve the RTSP NAT traversal problem for RTSP servers deployed in the open. The list of feature requirements for RTSP NAT solutions are given below: 1. MUST work for all flavors of NATs, including symmetric NATs. 2. MUST work for firewalls (subject to pertinent firewall administrative policies), including those with ALGs. 3. SHOULD have minimal impact on clients in the open and not dual- hosted: o For instance, no extra delay from RTSP connection till arrival of media. 4. SHOULD be simple to use/implement/administer that people actually turn them on o Otherwise people will resort to TCP tunneling through NATs o Address discovery for NAT traversal should take place behind the scene, if possible Westerlund, Zeng Standards Track [Page 7] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 5. SHOULD authenticate dual-hosted client transport handler to prevent DDOS attacks. The last requirement addresses the Distributed Denial-Of-Service (DDOS) threat, which relates to NAT traversal as explained below. During NAT traversal, when the RTSP server performs address translation on a client, the result may be that the public IP address of the RTP receiver host is different than the public IP address of the RTSP client host. This posts a DDOS threat that has significant amplification potentials because the RTP media streams in general consist of large number of IP packets. DDOS attacks occur if the attacker fakes the messages in the NAT traversal mechanism to trick the RTSP server into believing that the clientİs RTP receiver is located in a separate host. For example, user A may use his RTSP client to direct the RTSP server to send video RTP streams to www.foo.com in order to degrade the services provided by www.foo.com. Note a simple preventative measure is for the RTSP server to disallow the cases where the clientİs RTP receiver has a different public IP address than that of the RTSP client. However, in some applications (e.g., XCON), dual-hosted RTSP/RTP clients have valid use cases. The key is how to authenticate the messages exchanged during the NAT traversal process. Message authentication is a big challenge in the current wired and wireless networking environment. It may be necessary in the immediate future to deploy NAT traversal solutions that do not have full message authentication, but provide upgrade path to add authentication features in the future. 5. Detecting the loss of NAT mappings Several of the NAT traversal techniques in the next chapter make use of the fact that the NAT UDP mapping's external address and port can be discovered. This information is then utilized to traverse the NAT box. However any such information is only good while the mapping is still valid. As the IAB's UNSAF document [18] points out, the mapping can either timeout or change its properties. It is therefore important for the NAT traversal solutions to handle the loss or change of NAT mappings, according to [18]. First, since NATs may also dynamically reclaim or readjust address/port translations, "keep-alive" and periodic re-polling may be required [18]. Secondly, it is possible to detect and recover from the situation where the mapping has been changed or removed. The possibility to detect a lost mapping is based on the fact that no traffic will arrive. Below we will give some recommendation on how to detect loss of NAT mappings when using RTP/RTCP under RTSP control. Westerlund, Zeng Standards Track [Page 8] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 For RTP session there is normally a need to have both RTP and RTCP functioning. The loss of a RTP mapping can only be detected when expected traffic does not arrive. If no data arrives after having received the 200 response to a PLAY request, one can normally expect to receive RTP packets within a few seconds. However, for a receiver to be certain to detect the case where no RTP traffic was delivered due to NAT trouble, one should monitor the RTCP Sender reports. The sender report carries a field telling how many packets the server has sent. If that has increased and no RTP packets has arrived for a few seconds it is likely the RTP mapping has been removed. The loss of mapping for RTCP is simpler to detect. As RTCP is normally sent periodically in each direction, even during the RTSP ready state, if RTCP packets are missing for several RTCP intervals, the mapping is likely to be lost. Note that if no RTCP packets are received by the RTSP server and nor RTSP messages for a while, the RTSP server has the option to delete the corresponding SSRC and RTSP session ID, because either the client can not get through a middle box NAT/FW, or that the client is mal-functioning. 6. NAT Traversal Techniques There exist a number of potential NAT traversal techniques that can be used to allow RTSP to traverse NATs. They have different features and are applicable to different topologies; their cost is also different. They also vary in security levels. In the following sections, each technique is outlined in details with discussions on the corresponding advantages and disadvantages. Not all of the techniques are yet described in the full details, because the intention is to refer to other documents, or some appendix to this document, for the full specification of a specific NAT traversal solution. Note that some of the solutions make use of protocols (e.g., RTP-NOOP, TURN and ICE) in early stage of standardization. 6.1. STUN 6.1.1. Introduction STUN "Simple Traversal of UDP Through Network Address Translators" [6][25] is a standardized protocol developed by the MIDCOM WG that allows a client to use secure means to discover the presence of a NAT between himself and the STUN server and the type of that NAT. The client then uses the STUN server to discover the address bindings assigned by the NAT. STUN is a client-server protocol. STUN client sends a request to a STUN server and the server returns a response. There are two types Westerlund, Zeng Standards Track [Page 9] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 of STUN requests Binding Requests, sent over UDP, and Shared Secret Requests, sent over TLS over TCP. 6.1.2. Using STUN to traverse NAT without server modifications This section describes how a client can use STUN to traverse NATs to RTSP servers without requiring server modifications. However this method has limited applicability and requires the server to be available in the external/public address realm in regards to the client located behind a NAT(s). Limitations: - The server must be located in either a public address realm or the next hop external address realm in regards to the client. - The client may only be located behind NATs that are of the full cone, address restricted, or port restricted type. Clients behind symmetric NATs cannot use this method. Method: A RTSP client using RTP transport over UDP can use STUN to traverse a full cone NAT(s) in the following way: 1. Use STUN to discover the type of NAT, and the timeout period for any UDP mapping on the NAT. This is RECOMMENDED to be performed in the background as soon as IP connectivity is established. If this is performed prior to establishing a streaming session the delays in the session establishment will be reduced. If no NAT is detected, normal SETUP SHOULD be used. 2. The RTSP client determines the number of UDP ports needed by counting the number of needed media transport protocols sessions in the multi-media presentation. This information is available in the media description protocol, e.g. SDP. For example, each RTP session will in general require two UDP ports, one for RTP, and one for RTCP. 3. For each UDP port required, establish a mapping and discover the public/external IP address and port number with the help of the STUN server. A successful mapping looks like below: clientİs local address/port <-> public address/port. 4. Perform the RTSP SETUP for each media. In the transport header the following parameter SHOULD be included with the given values: "dest_addr" [7] with the public/external IP address and port pair for both RTP and RTCP. To allow this to work servers MUST allow a client to setup the RTP stream on any port, not only even ports. This requires the new feature provided in the update to RFC2326 ([7]). The server SHOULD respond with a transport header Westerlund, Zeng Standards Track [Page 10] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 containing an "src_addr" parameter with the RTP and RTCP source IP address and port of the media stream. 5. To keep the mappings alive, the client SHOULD periodically send UDP traffic over all mappings needed for the session. STUN MAY be used to determine the timeout period of the NAT(s) UDP mappings. For the mapping carrying RTCP traffic the periodic RTCP traffic may be enough. For mappings carrying RTP traffic and for mappings carrying RTCP packets at too low a frequency, keep-alive messages SHOULD be sent. As keep alive messages, one could use the RTP NOOP packet ([23]) to the streaming serverİs discard port (port number 9). The drawback of using RTP NOOP is that the payload type number must be dynamically assigned through RTSP first. If a UDP mapping is lost then the above discovery process must be repeated. The media stream also needs to be SETUP again to change the transport parameters to the new ones. This will likely cause a glitch in media playback. To allow UDP packets to arrive from the server to a client behind a restricted NAT, the client must send the very first UDP packet to pinch a hole in the NAT. The client, before sending a RTSP PLAY request, must send a so called FW packet (such as a RTP NOOP packet) on each mapping, to the IP address given as the servers source address. To create minimum problems for the server these UDP packets SHOULD be sent to the server's discard port (port number 9). Since UDP packets are inherently unreliable, to ensure that at least one UDP message passes the NAT, FW packets should be retransmitted in short intervals. For a port restricted NAT the client must send messages to the exact ports used by the server to send UDP packets before sending a RTSP PLAY request. This makes it possible to use the above described process with the following additional restrictions: for each port mapping, FW packets need to be sent first to the server's source address/port. To minimize potential effects on the server from these messages the following type of FW packets MUST be sent. RTP: an empty or less than 12 bytes UDP packet. RTCP: A correctly formatted RTCP RR or SR message. The above described adaptations for restricted NATs will not work unless the server includes the "src_addr" in the "Transport" header (which is the "source" transport parameter in RFC2326). 6.1.3. Embedding STUN in RTSP This section outlines the adaptation and embedding of STUN within RTSP. This enables STUN to be used to traverse any type of NAT, including symmetric NATs. Protocol changes are beyond the scope of this memo and are instead defined in TBD internet draft. Westerlund, Zeng Standards Track [Page 11] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 Limitations: This NAT traversal solution has limitations: 1. It does not work if both RTSP client and RTSP server are behind separate NATs. 2. The RTSP server may, for security reasons, refuse to send media streams to an IP different from the IP in the client RTSP requests. Therefore, if the client is behind a NAT that has multiple public addresses, and the clientİs RTSP port and UDP port are mapped to different IP addresses, RTSP SETUP may fail. Deviations from STUN as defined in RFC 3489 Specifically, we differ from RFC3489 in two aspects: 1. We allow RTSP applications to have the option to perform STUN "Shared Secret Request" through RTSP, via extension to RTSP; 2. We require STUN server to be co-located on RTSP serverİs media output ports. In order to allow binding discovery without authentication, the STUN server embedded in RTSP application must ignore authentication tag, and the STUN client embedded in RTSP application must use dummy authentication tag. If STUN server is co-located with RTSP serverİs media output port, an RTSP client using RTP transport over UDP can use STUN to traverse ALL types of NATs that have been defined in section 3.1. In the case of symmetric NAT, the party inside the NAT must initiate UDP traffic. The STUN Bind Request, being a UDP packet itself, can serve as the traffic initiating packet. Subsequently, both the STUN Binding Response packets and the RTP/RTCP packets can traverse the NAT, regardless of whether the RTSP server or the RTSP client is behind NAT. Likewise, if a RTSP server is behind a NAT, then an embedded STUN server must co-locate on the RTSP clientİs RTCP port. In this case, we assume that the client has some means of establishing TCP connection to the RTSP server behind NAT so as to exchange RTSP messages with the RTSP server. To minimize delay, we require that the RTSP server supporting this option must inform its client the RTP and RTCP ports from where the server intend to send out RTP and RTCP packets, respectively. This can be done by using the "server_port" parameter in RFC2326, and the "src_addr" parameter in [7]. Both are in RTSP Transport header. To minimize the keep-alive traffic for address mapping, we also require that the RTSP end-point (server or client) sends and receives RTCP packets from the same port. Westerlund, Zeng Standards Track [Page 12] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 6.1.4. Discussion On Co-located STUN Server In order to use STUN to traverse symmetric NATs the STUN server needs to be co-located with the streaming server media output ports. This creates a de-multiplexing problem: we must be able to differentiate a STUN packet from a media packet. This will be done based on heuristics. This works fine between STUN and RTP or RTCP where the first byte happens to be different, but may not work with other media transport protocols. 6.1.5. ALG considerations If a NAT supports RTSP ALG (Application Level Gateway) and is not aware of the STUN traversal option, service failure may happen, because a client discovers its public IP address and port numbers, and inserts them in its SETUP requests, when the RTSP ALG processes the SETUP request it may change the destination and port number, resulting in unpredictable behavior. In such cases a convenient way should be provided to turn off STUN-based NAT traversal. 6.1.6. Deployment Considerations For the non-embedded usage of STUN the following applies: Advantages: - Using STUN does not require RTSP server modifications; it only affects the client implementation. Disadvantages: - Requires a STUN server deployed in the public address space. - Only works with Cone NATs. Restricted Cone NATs create some issues. - Does not work with symmetric NATs without server modifications. - Will mostly not work if a NAT uses multiple IP addresses, since RTSP server generally requires all media streams to use the same IP as used in the RTSP connection. - Interaction problems exist when a RTSP-aware ALG interferes with the use of STUN for NAT traversal. - Using STUN requires that RTSP servers and clients support the updated RTSP specification, because it is no longer possible to guarantee that RTP and RTCP ports are adjacent to each other, as required by the "client_port" and "server_port" parameters in RFC2326. "" Transition: Westerlund, Zeng Standards Track [Page 13] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 The usage of STUN can be phased out gradually as the first step of a STUN capable server or client should be to check the presence of NATs. The removal of STUN capability in the client implementations will have to wait until there is absolutely no need to use STUN. For the "Embedded STUN" method the following applies: Advantages: - STUN is a solution first used by SIP applications. As shown above, with little or no changes, RTSP application can re-use STUN as a NAT traversal solution, avoiding the pit-fall of solving a problem twice. - STUN has built-in message authentication features, which makes it more secure. See next section for an in-depth security discussion. - This solution works as long as there is only one RTSP end point in the private address realm, regardless of the NATİs type. There may even be multiple NATs (see figure 1 in [6]). - Compares to other UDP based NAT traversal methods in this document, STUN requires little new protocol development (since STUN is already a IETF standard), and most likely less implementation effort, since open source STUN server and client have become available [21]. There is the need to embed STUN in RTSP server and client, which require a de-multiplexer between STUN packets and RTP/RTCP packets. There is also a need to register the proper feature tags. Disadvantages: - Some extensions to the RTSP core protocol, signaled by RTSP feature tags, must be introduced. - Requires an embedded STUN server to co-locate on each of RTSP serverİs media protocol's ports (e.g. RTP and RTCP ports), which means more processing is required to de-multiplex STUN packets from media packets. For example, the de-multiplexer must be able to differentiate a RTCP RR packet from a STUN packet, and forward the former to the streaming server, the later to STUN server. - Even if the RTSP server is in the open, and the client is behind a multi-addressed NAT, it may still break if the RTSP server does not allow RTP packets to be sent to an IP differs from the IP of the clientİs RTSP request. - Interaction problems exist when a RTSP ALG is not aware of STUN. - Using STUN requires that RTSP servers and clients support the updated RTSP specification, and they both agree to support the proper feature tag. - Increases the setup delay with at least the amount of time it takes to perform STUN message exchanges. Westerlund, Zeng Standards Track [Page 14] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 Transition: The usage of STUN can be phased out gradually as the first step of a STUN capable machine can be to check the presence of NATs for the presently used network connection. The removal of STUN capability in the client implementations will have to wait until there is absolutely no need to use STUN. 6.1.7. Security Considerations To prevent RTSP server being used as Denial of Service (DoS) attack tools the RTSP Transport header parameter "destination" and "dest_addr" are generally not allowed to point to any IP address other than the one that RTSP message originates from. The RTSP server is only prepared to make an exception of this rule when the client is trusted (e.g., through the use of a secure authentication process, or through some secure method of challenging the destination to verify its willingness to accept the RTP traffic). Such restriction means that STUN does not work for NATs that would assign different IP addresses to different UDP flows on its public side. Therefore the multi-addressed NATs will at times have trouble with STUN-based RTSP NAT traversals. In terms of security property, STUN combined with destination address restricted RTSP has the same security properties as the core RTSP. It is protected from being used as a DoS attack tool unless the attacker has ability the to spoof the TCP connection carrying RTSP messages. Using STUN's support for message authentication and secure transport of RTSP messages, attackers cannot modify STUN responses or RTSP messages to change media destination. This protects against hijacking, however as a client can be the initiator of an attack, these mechanisms cannot securely prevent RTSP servers being used as DoS attack tools. 6.2. ICE 6.2.1. Introduction ICE (Interactive Connectivity Establishment) [9] is a methodology for NAT traversal that is under development for SIP. The basic idea is to try, in a parallel fashion, all possible connection addresses that an end point may have. This allows the end-point to use the best available UDP "connection" (meaning two UDP end-points capable of reaching each other). The methodology has very nice properties in that basically all NAT topologies are possible to traverse. Westerlund, Zeng Standards Track [Page 15] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 Here is how ICE works. End point A collects all possible address that can be used, including local IP addresses, STUN derived addresses, TURN addresses. On each local port that any of these address and port pairs leads to, a STUN server is installed. This STUN server only accepts STUN requests using the correct authentication through the use of username and password. End-point A then sends a request to establish connectivity with end- point B, which includes all possible ways to get the media through to A. Note that each of Aİs published address/port pairs has a STUN server co-located. B, before responding to A, uses a STUN client to try to reach all the address and port pairs specified by A. The destinations for which the STUN requests have successfully completed are then indicated. If bi-directional communication is intended the end-point B must then in its turn offer A all its reachable address and port pairs, which then are tested by A. If B fails to get any STUN response from A, all hope is not lost. Certain NAT topologies require multiple tries from both ends before successful connectivity is accomplished. The STUN requests may also result in that more connectivity alternatives are discovered and conveyed in the STUN responses. This chapter is not yet a full technical solution. It is mostly a feasibility study on how ICE could be applied to RTSP and what properties it would have. One nice thing about ICE for RTSP is that it does make it possible to deploy RTSP server behind NAT/FIRWALL, a desirable option to some RTSP applications. 6.2.2. Using ICE in RTSP The usage of ICE for RTSP requires that both client and server be updated to include the ICE functionality. If both parties implement the necessary functionality the following step-by-step algorithm could be used to accomplish connectivity for the UDP traffic. This assumes that it is possible to establish a TCP connection for the RTSP messages between the client and the server. This is not trivial in scenarios where the server is located behind a NAT, and may require some TCP ports been opened, or the deployment of proxies, etc. Refer to [22] for the mapping of ICE to RTSP. 6.2.3. Implementation burden of ICE The usage of ICE will require that a number of new protocols and new RTSP/SDP features be implemented. This makes ICE the solution that Westerlund, Zeng Standards Track [Page 16] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 has the largest impact on client and server implementations amongst all the NAT/FW traversal methods in this document. Some RTSP server implementation requirements are: - Full STUN server features - limited STUN client features - Dynamic SDP generation with more parameters. - RTSP error code for ICE extension Some client implantation requirements are: - Limited STUN server features - Limited STUN client features - RTSP error code and ICE extension 6.2.4. Deployment Considerations Advantages: - Solves NAT connectivity discovery for basically all cases as long as a TCP connection between them can be established. This includes servers behind NATs. (Note that a proxy between address domains may be required to get TCP through). - Improves defenses against DDOS attacks, as media receiving client requires authentications, via STUN on its media reception ports. See [22] for more details. Disadvantages: - Increases the setup delay with at least the amount of time it takes for the server to perform its STUN requests. - Assumes that it is possible to de-multiplex between media packets and STUN packets. - Has fairly high implementation burden put on both RTSP server and client. The precise implantation complexity needs to be assessed once ICE is fully defined as a standard. Currently ICE is still a protocol under development. 6.3. Symmetric RTP 6.3.1. Introduction Symmetric RTP is a NAT traversal solution that is based on requiring RTSP clients to send UDP packets to the serverİs media output ports. Conventionally, RTSP servers send RTP packets in one direction: from server to client. Symmetric RTP is similar to connection-oriented traffic, where one side (e.g., the RTSP client) first "connects" by sending a RTP packet to the other sideİs RTP port, the recipient then replies to the originating IP and port. Specifically, when the RTSP server receives the "connect" RTP packet (a.k.a. FW packet, since it is used to pinch a hole in the FW/NAT and to aid the server for port binding and address mapping) from its client, it copies the source IP and Port number and uses them as Westerlund, Zeng Standards Track [Page 17] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 delivery address for media packets. By having the server send media traffic back the same way as the client's packet are sent to the server, address mappings will be honored. Therefore this technique works for all types of NATs. However, it does require server modifications. Unless there is built-in protection mechanism, symmetric RTP is rather vulnerable to DDOS attacks, because attackers can simply forge the source IP & Port of the binding packet. 6.3.2. Necessary RTSP extensions To support symmetric RTP the RTSP signaling must be extended to allow the RTSP client to indicate that it will use symmetric RTP. The client also needs to be able to signal its RTP SSRC to the server in its SETUP request. The RTP SSRC is used to establish some basic level of security against hijacking attacks. Care must be taken in choosing clientİs RTP SSRC. First, it must be unique within all the RTP sessions belonging to the same RTSP session. Secondly, if the RTSP server is sending out media packets to multiple clients from the same send port, the RTP SSRC needs to be unique amongst those clientsİ RTP sessions. Recognizing that there is a potential that RTP SSRC collision may occur, the RTSP server must be able to signal to client that a collision has occurred and that it wants the client to use a different RTP SSRC carried in the SETUP response. Details of the RTSP extension are beyond the scope of this draft and will be defined in a TBD RTSP extension draft. 6.3.3. Deployment Considerations Advantages: - Works for all types of NATs, including those using multiple IP addresses. (Requirement 1 in section 4). - Have no interaction problems with any RTSP ALG changing the client's information in the transport header. Disadvantages: - Requires modifications to both RTSP server and client. - The format of the RTP packet for "connection setup" (a.k.a FW packet) is yet to be defined. One possibility is to use RTP NOOP packet format in [23]. - Has somewhat worse security situation than STUN when using address restrictions. - Still requires STUN to discover the timeout of NAT bindings. 6.3.4. Security Consideration Westerlund, Zeng Standards Track [Page 18] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 Symmetric RTP's major security issue is that RTP streams can be hijacked and directed towards any target that the attacker desires. The most serious security problem is the deliberate attack with the use of a RTSP client and symmetric RTP. The attacker uses RTSP to setup a media session. Then it uses symmetric RTP with a spoofed source address of the intended target of the attack. There is no defense against this attack other than restricting the possible bind address to be the same as the RTSP connection arrived on. This prevents symmetric RTP to be used with multi-address NATs. The hijack attack can be performed in various ways. The basic attack is based on the ability to read the RTSP signaling packets in order to learn the address and port the server will send from and also the SSRC the client will use. Having this information the attacker can send its own NAT-traversal RTP packets containing the correct RTP SSRC to the correct address and port on the server. The destination of the packets is set as the source IP and port in these RTP packets. Another variation of this attack is to modify the RTP binding packet being sent to the server by simply changing the source IP to the target one desires to attack. One can fend off the first attack by applying encryption to the RTSP signaling transport. However, the second variation is impossible to defend against. As a NAT re-writes the source IP and port this cannot be authenticated, but authentication is required in order to protect against this type of DOS attack. The random SSRC tag in the binding packet determines how well symmetric RTP can fend off stream-hijacking performed by parties that are not "man-in-the-middle". This proposal uses the 32-bit RTP SSRC field to this effect. Therefore it is important that this field is derived with a non- predictable randomizer. It should not be possible by knowing the algorithm used and a couple of basic facts, to derive what random number a certain client will use. An attacker not knowing the SSRC but aware of which port numbers that a server sends from can deploy a brute force attack on the server by testing a lot of different SSRCs until it finds a matching one. Therefore a server SHOULD implement functionality that blocks ports that receive multiple FW packets (i.e. the packet that is sent to the server for FW traversal) with different invalid SSRCs, especially when they are coming from the same IP/Port. To improve the security against attackers the random tagİs length could be increased. To achieve a longer random tag while still using Westerlund, Zeng Standards Track [Page 19] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 RTP and RTCP, it will be necessary to develop RTP and RTCP payload formats for carrying the random tag. 6.3.5. A Variation to Symmetric RTP Symmetric RTP requires a valid RTP format in the FW packet, which is the first packet that the client sends to the server to set up virtual RTP connection. There is currently no appropriate RTP packet format for this purpose, although the NOOP format is a proposal to fix the problem [23]. Meanwhile, there has been FW traversal techniques deployed in the wireless streaming market place that use non-RTP messages as FW packets. This section attempts to summarize a subset of those solutions that happens to use a variation to the standard symmetric RTP solution. In this variation of symmetric RTP, the FW packet is a small UDP packet that does not contain RTP header. Hence the solution can no longer be called symmetric RTP, yet it employs the same technique for FW traversal. In response to clientİs FW packet, RTSP server sends back a similar FW packet as a confirmation so that the client can stop the so called "connection phase" of this NAT traversal technique. Afterwards, the client only has to periodically send FW packets as keep-alive messages for the NAT mappings. The server listens on its RTP-media output port, and tries to decode any received UDP packet as FW packet. This is valid since an RTSP server is not expecting RTP traffic from the RTSP client. Then, it can correlate the FW packet with the RTSP clientİs session ID or the serverİs SSRC, and record the NAT bindings accordingly. The server then sends a FW packet as the response to the client. The FW packet normally contains the SSRC used to identify the RTP stream, and can be made no bigger than 12 bytes, making it distinctively different from RTP packets, whose header size is 12 bytes. RTSP signaling can be added to do the following: 1. Enables or disables such FW message exchanges. When the FW/NAT has an RTSP-aware ALG, it is better to disable FW message exchange and let ALG works out the address and port mappings. 2. Configures the number of re-tries and the re-try interval of the FW message exchanges. Such FW packets may also contain digital signatures to support three-way handshake based receiver authentications, so as to prevent DDoS attacks described before. Westerlund, Zeng Standards Track [Page 20] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 This approach has the following advantages when compared with the symmetric RTP approach: 1. There is no need to define RTP payload format for FW traversal, therefore it is simple to use, implement and administer (Requirement 4 in section 4), although a binding protocol must be defined (which is out side of the scope of this memo). 2. When properly defined, this kind of FW message exchange can also authenticate RTP receivers, so as to prevent DDoS attacks for dual-hosted RTSP client. By dual-hosted RTSP client we mean the kind that uses one "perceived" IP address for RTSP message exchange, and a different "perceived" IP address for RTP reception. (Requirement 5 in section 4). This approach has the following disadvantages when compared with the symmetric RTP approach: 1. RTP traffic is normally accompanied by RTCP traffic. This approach still needs to rely on RTCP RRs and SRs to enable NAT traversal for RTCP endpoints, or use the same type of FW messages for RTCP endpoints. 2. The serverİs sender SSRC for the RTP stream must be signaled in RTSPİs SETUP response, in the Transport header of the RTSP SETUP response. 6.4. Application Level Gateways 6.4.1. Introduction An Application Level Gateway (ALG) reads the application level messages and performs necessary changes to allow the protocol to work through the middle box. However this behavior has some problems in regards to RTSP: 1. It does not work when the RTSP protocol is used with end-to-end security. As the ALG can't inspect and change the application level messages the protocol will fail due to the middle box. 2. ALGs need to be updated if extensions to the protocol are added. Due to deployment issues with changing ALGs this may also break the end-to-end functionality of RTSP. Due to the above reasons it is NOT RECOMMENDED to use an RTSP ALG in NATs. This is especially important for NATs targeted to home users and small office environments, since it is very hard to upgrade NATs deployed in home or SOHO (small office/home office) environment. 6.4.2. Guidelines On Writing ALGs for RTSP Westerlund, Zeng Standards Track [Page 21] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 In this section, we provide a step-by-step guideline on how one should go about writing an ALG to enable RTSP to traverse a NAT. 1. Detect any SETUP request. 2. Try to detect the usage of any of the NAT traversal methods that replace the address and port of the Transport header parameters "destination" or "dest_addr". If any of these methods are used, the ALG SHOULD NOT change the address. Ways to detect that these methods are used are: - For embedded STUN, watch for the feature tag "nat.stun". If any of those exists in the "supported", "proxy-require", or "require" headers of the RTSP exchange. - For non-embedded STUN and TURN based solutions: This can in some case be detected by inspecting the "destination" or "dest_addr" parameter. If it contains either one of the NAT's external IP addresses or a public IP address. However if multiple NATs are used this detection may fail. Otherwise continue to the next step. 3. Create UDP mappings (client given IP/port <-> external IP/port) where needed for all possible transport specification in the transport header of the request found in (1). Enter the public address and port(s) of these mappings in transport header. Mappings SHALL be created with consecutive public port number starting on an even number for RTP for each media stream. Mappings SHOULD also be given a long timeout period, at least 5 minutes. 4. When the SETUP response is received from the server the ALG MAY remove the unused UDP mappings, i.e. the ones not present in the transport header. The session ID SHOULD also be bound to the UDP mappings part of that session. 5. If SETUP response settles on RTP over TCP or RTP over RTSP as lower transport, do nothing: let TCP tunneling to take care of NAT traversal. Otherwise go to next step. 6. The ALG SHOULD keep alive the UDP mappings belonging to the an RTSP session as long as: RTSP messages with the session's ID has been sent in the last timeout interval, or UDP messages are sent on any of the UDP mappings during the last timeout interval. 7. The ALG MAY remove a mapping as soon a TEARDOWN response has been received for that media stream. Westerlund, Zeng Standards Track [Page 22] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 6.4.3. Deployment Considerations Advantage: - No impact on either client or server - Can work for any type of NATs Disadvantage: - When deployed they are hard to update to reflect protocol modifications and extensions. If not updated they will break the functionality. - When end-to-end security is used the ALG functionality will fail. - Can interfere with other type of traversal mechanisms, such as STUN. Transition: An RTSP ALG will not be phased out in any automatically way. It must be removed, probably through the removal of the NAT it is associated with. 6.4.4. Security Considerations An ALG will not work when deployment of end-to-end RTSP signaling security. Therefore deployment of ALG will result in that clients located behind NATs will not use end-to-end security. 6.5. TCP Tunneling 6.5.1. Introduction Using a TCP connection that is established from the client to the server ensures that the server can send data to the client. The connection opened from the private domain ensures that the server can send data back to the client. To send data originally intended to be transported over UDP requires the TCP connection to support some type of framing of the RTP packets. Using TCP also results in that the client has to accept that real- time performance may no longer be possible. TCP's problem of ensuring timely deliver was the reasons why RTP was developed. Problems that arise with TCP are: head-of-line blocking, delay introduced by retransmissions, highly varying congestion control. Westerlund, Zeng Standards Track [Page 23] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 6.5.2. Usage of TCP tunneling in RTSP The RTSP core specification [7] supports interleaving of media data on the TCP connection that carries RTSP signaling. See section 10.13 in [7] for how to perform this type of TCP tunneling. There is currently new work on one more way of transporting RTP over TCP in AVT and MMUSIC. For signaling and rules on how to establish the TCP connection in lieu of UDP, see [16]. Another draft describes how to frame RTP over the TCP connection is described in [17]. 6.5.3. Deployment Considerations Advantage: - Works through all types of NATs where server is in the open. Disadvantage: - Functionality needs to be implemented on both server and client. - Will not always meet multimedia streamİs real-time requirements. Transition: The tunneling over RTSP's TCP connection is not planned to be phased -out. It is intended to be a fallback mechanism and for usage when total media reliability is desired, even at the price of loss of real-time properties. 6.5.4. Security Considerations The TCP tunneling of RTP has no known security problem besides those already present in RTSP. It is not possible to get any amplification effect that is desired for denial of service attacks due to TCP's flow control. A possible security consideration, when session media data is interleaved with RTSP, would be the performance bottleneck when RTSP encryption is applied, since all session media data also needs to be encrypted. 6.6. TURN (Traversal Using Relay NAT) 6.6.1. Introduction Traversal Using Relay NAT (TURN) [8] is a protocol for setting up traffic relays that allows clients behind NATs and firewalls to receive incoming traffic for both UDP and TCP. These relays are Westerlund, Zeng Standards Track [Page 24] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 controlled and have limited resources. They need to be allocated before usage. TURN allows a client to temporarily bind an address/port pair on the relay (TURN server) to its local source address/port pair, which is used to contact the TURN server. The TURN server will then forward packets between the two sides of the relay. To prevent DOS attacks on either recipient, the packets forwarded are restricted to the specific source address. On the client side it is restricted to the source setting up the mapping. On the external side this is limited to the source address/port pair of the first packet arriving on the binding. After the first packet has arrived the mapping is "locked down" to that address. Packets from any other source on this address will be discarded. Using a TURN server makes it possible for a RTSP client to receive media streams from even an unmodified RTSP server. However the problem is those RTSP servers most likely restrict media destinations to no other IP address than the one RTSP message arrives. This means that TURN could only be used if the server knows and accepts that the IP belongs to a TURN server and the TURN server can't be targeted at an unknown address. Unfortunately TURN servers can be targeted at any host that has a public IP address by spoofing the source IP of TURN Allocation requests. 6.6.2. Usage of TURN with RTSP To use a TURN server for NAT traversal, the following steps should be performed. 1. The RTSP client connects with RTSP server. The client retrieves the session description to determine the number of media streams. 2. The client establishes the necessary bindings on the TURN server. It must choose the local RTP and RTCP ports that it desires to receive media packets. TURN supports requesting bindings of even port numbers and continuous ranges. 3. The RTSP client uses the acquired address and port mappings in the RTSP SETUP request using the destination header. Note that the server is required to have a mechanism to verify that it is allowed to send media traffic to the given address. The server SHOULD include its RTP SSRC in the SETUP response. 4. Client requests that the Server starts playing. The server starts sending media packet to the given destination address and ports. Westerlund, Zeng Standards Track [Page 25] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 5. The first media packet to arrive at the TURN server on the external port causes "lock down"; then TURN server forwards the media packets to the RTSP client. 6. When media arrives at the client, the client should try to verify that the media packets are from the correct RTSP server, by matching the RTP SSRC of the packet. Source IP address of this packet will be that of the TURN server and can therefore not be used to verify that the correct source has caused lock down. 7. If the client notices that some other source has caused lock down on the TURN server, the client should create new bindings and change the session transport parameters to reflect the new bindings. 8. If the client pauses and media are not sent for about 75% of the mapping timeout the client should use TURN to refresh the bindings. 6.6.3. Deployment Considerations Advantages: - Does not require any server modifications. - Works for any types of NAT as long as the server has public reachable IP address. Disadvantage - TURN is not yet a standard. - Requires another network element, namely the TURN server. - Such a TURN server for RTSP is not scalable since the number of sessions it must forward is proportional to the number of client media sessions. - TURN server becomes a single point of failure. - Since TURN forwards media packets, it necessarily introduces delay. - Requires that the server can verify that the given destination address is valid to be used by the client. - An RTSP ALG MAY change the necessary destinations parameter. This will cause the media traffic to be sent to the wrong address. Transition: TURN is not intended to be phase-out completely, see chapter 11.2 of [8]. However the usage of TURN could be reduced when the demand for having NAT traversal is reduced. 6.6.4. Security Considerations Westerlund, Zeng Standards Track [Page 26] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 An eavesdropper of RTSP messages between the RTSP client and RTSP server will be able to do a simple denial of service attack on the media streams by sending messages to the destination address and port present in the RTSP SETUP messages. If the attackerİs message can reach the TURN server before the RTSP server's message, the lock down can be accomplished towards some other address. This will result in that the TURN server will drop all the media server's packets when they arrive. This can be accomplished with little risk for the attacker of being caught, as it can be performed with a spoofed source IP. The client may detect this attack when it receives the lock down packet sent by the attacker as being mal- formatted and not corresponding to the expected context. It will also notice the lack of incoming packets. See bullet 7 in section 6.6.2. The TURN server can also become part of a denial of service attack towards any victim. To perform this attack the attacker must be able to eavesdrop on the packets from the TURN server towards a target for the DOS attack. The attacker uses the TURN server to setup a RTSP session with media flows going through the TURN server. The attacker is in fact creating TURN mappings towards a target by spoofing the source address of TURN requests. As the attacker will need the address of these mappings he must be able to eavesdrop or intercept the TURN responses going from the TURN server to the target. Having these addresses, he can set up a RTSP session and starts delivery of the media. The attacker must be able to create these mappings. The attacker in this case may be traced by the TURN username in the mapping requests. The first attack can be made very hard by applying transport security for the RTSP messages, which will hide the TURN servers address and port numbers from any eavesdropper. The second attack requires that the attacker have access to a user account on the TURN server to be able set up the TURN mappings. To prevent this attack the server shall verify that the target destination accept this media stream. 7. Firewalls Firewalls exist for the purpose of protecting a network from traffic not desired by the firewall owner. Therefore it is a policy decision if a firewall will let RTSP and its media streams through or not. RTSP is designed to be firewall friendly in that it should be easy to design firewall policies to permit passage of RTSP traffic and its media streams. The firewall will need to allow the media streams associated with a RTSP session pass through it. Therefore the firewall will need an Westerlund, Zeng Standards Track [Page 27] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 ALG that reads RTSP SETUP and TEARDOWN messages. By reading the SETUP message the firewall can determine what type of transport and from where the media streams will use. Commonly there will be the need to open UDP ports for RTP/RTCP. By looking at the source and destination addresses and ports the opening in the firewall can be minimized to the least necessary. The opening in the firewall can be closed after a teardown message for that session or the session itself times out. Simpler firewalls do allow a client to receive media as long as it has sent packets to the target. Depending on the security level this can have the same behavior as a full cone NAT or a Symmetric NAT. The only difference is that no address translation is done. To be able to use such a firewall a client would need to implement one of the above described NAT traversal methods that include sending packets to the server to open up the mappings. 8. Comparison of Different NAT Traversal Techniques This section evaluates the techniques described above against the requirements listed in section 4. In the following table, the columns correspond to the numbered requirements. For instance, the column under R1 corresponds to the first requirement in section 4: MUST work for all flavors of NATs. The rows represent the different FW traversal techniques. SymRTP is short for symmetric RTP, "V.SymRTP" is short for "variation of symmetric RTP" as described in section 6.3.5. -----------------------------------------------+ | R1 | R2 | R3 | R4 | R5 | ------------+------+------+------+------+------+ STUN | Yes | Yes | No | Maybe| No | ------------+------+------+------+------+------+ ICE | Yes | Yes | No | No | Yes | ------------+------+------+------+------+------+ SymRTP | Yes | Yes | Yes |Maybe | No | ------------+------+------+------+------+------+ V. SymRTP | Yes | Yes | Yes | Yes |future| ------------+------+------+------+------+------+ TURN | Yes | Yes | No | No | Yes | -----------------------------------------------+ 9. Open Issues Some open issues with this draft: Westerlund, Zeng Standards Track [Page 28] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 - At some point we need to recommend one RTSP NAT solution so as to ensure implementations can inter-operate. This decision will require that requirements, security and desired goals be evaluated against implementation cost and the probability to get the final solution deployed. - The ALG recommendations need to be improved and clarified. - The firewall RTSP ALG recommendations need to be written as they are different from the NAT ALG in some perspectives. 10. Security Consideration In preceding sessions we have discussed security merits of each and every NAT/FW traversal methods for RTSP. In summary, the presence of NAT(s) is a security risk, as a client cannot perform source authentication of its IP address. This prevents the deployment of any future RTSP extensions providing security against hijacking of sessions by a man-in-the-middle. Each of the proposed solutions has security implications. Using STUN will provide the same level of security as RTSP with out transport level security and source authentications; as long as the server does not grant a client request to send media to different IP addresses. Using symmetric RTP will have a slightly higher risk of session hijacking than normal RTSP. The reason is that there exists a probability that an attacker is able to guess the random tag that the client uses to prove its identity when creating the address bindings. This can be solved in the variation of symmetric RTP (section 6.3.5) with authentication features. The usage of an RTSP ALG does not increase in itself the risk for session hijacking. However the deployment of ALGs as sole mechanism for RTSP NAT traversal will prevent deployment of encrypted end-to- end RTSP signaling. The usage of TCP tunneling has no known security problems. However it might provide a bottleneck when it comes to end-to-end RTSP signaling security if TCP tunneling is used on an interleaved RTSP signaling connection. The usage of TURN has high risk of denial of service attacks against a client. The TURN server can also be used as a redirect point in a DDOS attack unless the server has strict enough rules for who may create bindings. 11. IANA Consideration Westerlund, Zeng Standards Track [Page 29] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 This specification does not define any protocol extensions hence no IANA action is requested. 12. Acknowledgments The author would also like to thank all persons on the MMUSIC working group's mailing list that has commented on this specification. Persons having contributed in such way in no special order to this protocol are: Jonathan Rosenberg, Philippe Gentric, Tom Marshall, David Yon, Amir Wolf, Anders Klemets, and Colin Perkins. Thomas Zeng would also like to give special thanks to Greg Sherwood of PacketVideo for his input into this memo. 13. Author's Addresses Magnus Westerlund Tel: +46 8 4048287 Ericsson Research Email: Magnus.Westerlund@ericsson.com Ericsson AB Torshamnsgatan 23 SE-164 80 Stockholm, SWEDEN Thomas Zeng Tel: 1-858-320-3125 PacketVideo Network Solutions Email: zeng@pvnetsolutions.com 9605 Scranton Rd., Suite 400 San Diego, CA92121 Westerlund, Zeng Standards Track [Page 30] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 14. References 14.1. Normative references [1] H. Schulzrinne, et. al., "Real Time Streaming Protocol (RTSP)", IETF RFC 2326, April 1998. [2] M. Handley, V. Jacobson, "Session Description Protocol (SDP)", IETF RFC 2327, April 1998. [3] D. Crocker and P. Overell, "Augmented BNF for syntax specifications: ABNF," RFC 2234, Internet Engineering Task Force, Nov. 1997. [4] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997. [5] H. Schulzrinne, et. al., "RTP: A Transport Protocol for Real- Time Applications", STD 64, RFC 3550, IETF, July 2003. [6] J. Rosenberg, et. Al., " STUN - Simple Traversal of UDP Through Network Address Translators", IETF RFC 3489, March 2003 [7] H. Schulzrinne, et. al., "Real Time Streaming Protocol (RTSP)", draft-ietf-mmusic-rfc2326bis-06.txt, IETF draft, Feb 2004, work in progress. [8] J. Rosenberg, et. Al., "Traversal Using Relay NAT (TURN)", draft-rosenberg-midcom-turn-04.txt, IETF draft, Feb 2004, work in progress. [9] J. Rosenberg, "Interactive Connectivity Establishment (ICE): A Methodology for Network Address Translator (NAT) Traversal for the Session Initiation Protocol (SIP)," draft-ietf-mmusic-ice- 01, IETF draft, February 2004, work in progress. [10] G. Camarillo, et. al., "Grouping of Media Lines in the Session Description Protocol (SDP)," IETF RFC 3388, December 2002. [11] G. Camarillo, J. Rosenberg, "The Alternative Network Address Types Semantics (ANAT) for the Session Description Protocol (SDP) Grouping Framework," draft-camarillo-mmusic-anat-01.txt, IETF draft, June 2004, work in progress. 14.2. Informative References [12] P. Srisuresh, K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)," RFC 3022, Internet Engineering Task Force, January 2001. [13] Tsirtsis, G. and Srisuresh, P., "Network Address Translation - Protocol Translation (NAT-PT)", RFC 2766, Internet Engineering Task Force, February 2000. [14] S. Deering and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, Internet Engineering Task Force, December 1998. Westerlund, Zeng Standards Track [Page 31] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 [15] J. Postel, "internet protocol", RFC 791, Internet Engineering Task Force, September 1981. [16] D. Yon, G. Camarillo, "Connection-Oriented Media Transport in the Session Description Protocol (SDP)", IETF draft, draft- ietf-mmusic-sdp-comedia-07.txt, June 2004. [17] John Lazzaro, "Framing RTP and RTCP Packets over Connection- Oriented Transport", IETF Draft, draft-ietf-avt-rtp-framing- contrans-03.txt, July 2004. [18] D. Daigle, "IAB Considerations for UNilateral Self-Address Fixing (UNSAF) Across Network Address Translation", RFC 3424, Internet Engineering Task Force, Nov. 2002 [19] R. Finlayason, "IP Multicast and Firewalls", RFC 2588, Internet Engineering Task Force, May 1999 [20] Krawczyk, H., Bellare, M., and Canetti, R.: "HMAC: Keyed- hashing for message authentication". IETF RFC 2104, February 1997 [21] Open Source STUN Server and Client, http://www.vovida.org/applications/downloads/stun/index.html [22] Zeng, T.M.: "Mapping ICE (Interactive Connectivity Establishment) to RTSP", IETF draft, draft-zeng-mmusic-map-ice- rtsp-00.txt, Feb 2004 [23] Dan Wing, et.al. "RTP No-Op Payload Format", draft-wing-avt- rtp-noop-00.txt, March 2004 [24] P. Srisuresh and M.Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC2663, Internet Engineering Task Force, Aug. 1999 [25] J. Rosenberg, C. Huitema and R. Mahy, "STUN - Simple Traversal of UDP Through Network Address Translators", draft-rosenberg- rfc3489bis-00.txt, July 2004 Westerlund, Zeng Standards Track [Page 32] INTERNET-DRAFT How to make RTSP traverse NAT & FW Jul. 18, 2004 15. 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This Internet-Draft expires in January 2005. Westerlund, Zeng Standards Track [Page 33]