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diff --git a/doc/rfc/rfc7604.txt b/doc/rfc/rfc7604.txt new file mode 100644 index 0000000..08a2a53 --- /dev/null +++ b/doc/rfc/rfc7604.txt @@ -0,0 +1,2579 @@ + + + + + + +Internet Engineering Task Force (IETF) M. Westerlund +Request for Comments: 7604 Ericsson +Category: Informational T. Zeng +ISSN: 2070-1721 PacketVideo Corp + September 2015 + + + Comparison of Different NAT Traversal Techniques + for Media Controlled by the Real-Time Streaming Protocol (RTSP) + +Abstract + + This document describes several Network Address Translator (NAT) + traversal techniques that were considered to be used for establishing + the RTP media flows controlled by the Real-Time Streaming Protocol + (RTSP). Each technique includes a description of how it would be + used, the security implications of using it, and any other deployment + considerations it has. There are also discussions on how NAT + traversal techniques relate to firewalls and how each technique can + be applied in different use cases. These findings were used when + selecting the NAT traversal for RTSP 2.0, which is specified in a + separate document. + +Status of This Memo + + This document is not an Internet Standards Track specification; it is + published for informational purposes. + + This document is a product of the Internet Engineering Task Force + (IETF). It represents the consensus of the IETF community. It has + received public review and has been approved for publication by the + Internet Engineering Steering Group (IESG). Not all documents + approved by the IESG are a candidate for any level of Internet + Standard; see Section 2 of RFC 5741. + + Information about the current status of this document, any errata, + and how to provide feedback on it may be obtained at + http://www.rfc-editor.org/info/rfc7604. + + + + + + + + + + + + + +Westerlund & Zeng Informational [Page 1] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + +Copyright Notice + + Copyright (c) 2015 IETF Trust and the persons identified as the + document authors. All rights reserved. + + This document is subject to BCP 78 and the IETF Trust's Legal + Provisions Relating to IETF Documents + (http://trustee.ietf.org/license-info) in effect on the date of + publication of this document. Please review these documents + carefully, as they describe your rights and restrictions with respect + to this document. Code Components extracted from this document must + include Simplified BSD License text as described in Section 4.e of + the Trust Legal Provisions and are provided without warranty as + described in the Simplified BSD License. + +Table of Contents + + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 + 1.1. Network Address Translators . . . . . . . . . . . . . . . 5 + 1.2. Firewalls . . . . . . . . . . . . . . . . . . . . . . . . 6 + 1.3. Glossary . . . . . . . . . . . . . . . . . . . . . . . . 7 + 2. Detecting the Loss of NAT Mappings . . . . . . . . . . . . . 8 + 3. Requirements on Solutions . . . . . . . . . . . . . . . . . . 9 + 4. NAT-Traversal Techniques . . . . . . . . . . . . . . . . . . 10 + 4.1. Stand-Alone STUN . . . . . . . . . . . . . . . . . . . . 11 + 4.1.1. Introduction . . . . . . . . . . . . . . . . . . . . 11 + 4.1.2. Using STUN to Traverse NAT without Server + Modifications . . . . . . . . . . . . . . . . . . . . 11 + 4.1.3. ALG Considerations . . . . . . . . . . . . . . . . . 14 + 4.1.4. Deployment Considerations . . . . . . . . . . . . . . 14 + 4.1.5. Security Considerations . . . . . . . . . . . . . . . 15 + 4.2. Server Embedded STUN . . . . . . . . . . . . . . . . . . 16 + 4.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 16 + 4.2.2. Embedding STUN in RTSP . . . . . . . . . . . . . . . 16 + 4.2.3. Discussion on Co-located STUN Server . . . . . . . . 17 + 4.2.4. ALG Considerations . . . . . . . . . . . . . . . . . 17 + 4.2.5. Deployment Considerations . . . . . . . . . . . . . . 18 + 4.2.6. Security Considerations . . . . . . . . . . . . . . . 19 + 4.3. ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 + 4.3.1. Introduction . . . . . . . . . . . . . . . . . . . . 19 + 4.3.2. Using ICE in RTSP . . . . . . . . . . . . . . . . . . 20 + 4.3.3. Implementation Burden of ICE . . . . . . . . . . . . 21 + 4.3.4. ALG Considerations . . . . . . . . . . . . . . . . . 22 + 4.3.5. Deployment Considerations . . . . . . . . . . . . . . 22 + 4.3.6. Security Considerations . . . . . . . . . . . . . . . 23 + + + + + + +Westerlund & Zeng Informational [Page 2] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + 4.4. Latching . . . . . . . . . . . . . . . . . . . . . . . . 23 + 4.4.1. Introduction . . . . . . . . . . . . . . . . . . . . 23 + 4.4.2. Necessary RTSP Extensions . . . . . . . . . . . . . . 24 + 4.4.3. ALG Considerations . . . . . . . . . . . . . . . . . 25 + 4.4.4. Deployment Considerations . . . . . . . . . . . . . . 25 + 4.4.5. Security Considerations . . . . . . . . . . . . . . . 26 + 4.5. A Variation to Latching . . . . . . . . . . . . . . . . . 27 + 4.5.1. Introduction . . . . . . . . . . . . . . . . . . . . 27 + 4.5.2. Necessary RTSP Extensions . . . . . . . . . . . . . . 28 + 4.5.3. ALG Considerations . . . . . . . . . . . . . . . . . 28 + 4.5.4. Deployment Considerations . . . . . . . . . . . . . . 28 + 4.5.5. Security Considerations . . . . . . . . . . . . . . . 29 + 4.6. Three-Way Latching . . . . . . . . . . . . . . . . . . . 29 + 4.6.1. Introduction . . . . . . . . . . . . . . . . . . . . 29 + 4.6.2. Necessary RTSP Extensions . . . . . . . . . . . . . . 29 + 4.6.3. ALG Considerations . . . . . . . . . . . . . . . . . 30 + 4.6.4. Deployment Considerations . . . . . . . . . . . . . . 30 + 4.6.5. Security Considerations . . . . . . . . . . . . . . . 30 + 4.7. Application Level Gateways . . . . . . . . . . . . . . . 31 + 4.7.1. Introduction . . . . . . . . . . . . . . . . . . . . 31 + 4.7.2. Outline on How ALGs for RTSP Work . . . . . . . . . . 31 + 4.7.3. Deployment Considerations . . . . . . . . . . . . . . 32 + 4.7.4. Security Considerations . . . . . . . . . . . . . . . 33 + 4.8. TCP Tunneling . . . . . . . . . . . . . . . . . . . . . . 33 + 4.8.1. Introduction . . . . . . . . . . . . . . . . . . . . 33 + 4.8.2. Usage of TCP Tunneling in RTSP . . . . . . . . . . . 34 + 4.8.3. ALG Considerations . . . . . . . . . . . . . . . . . 34 + 4.8.4. Deployment Considerations . . . . . . . . . . . . . . 34 + 4.8.5. Security Considerations . . . . . . . . . . . . . . . 35 + 4.9. Traversal Using Relays around NAT (TURN) . . . . . . . . 35 + 4.9.1. Introduction . . . . . . . . . . . . . . . . . . . . 35 + 4.9.2. Usage of TURN with RTSP . . . . . . . . . . . . . . . 36 + 4.9.3. ALG Considerations . . . . . . . . . . . . . . . . . 37 + 4.9.4. Deployment Considerations . . . . . . . . . . . . . . 37 + 4.9.5. Security Considerations . . . . . . . . . . . . . . . 37 + 5. Firewalls . . . . . . . . . . . . . . . . . . . . . . . . . . 38 + 6. Comparison of NAT Traversal Techniques . . . . . . . . . . . 39 + 7. Security Considerations . . . . . . . . . . . . . . . . . . . 41 + 8. Informative References . . . . . . . . . . . . . . . . . . . 42 + Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 45 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 46 + + + + + + + + + + +Westerlund & Zeng Informational [Page 3] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + +1. Introduction + + Today there is a proliferating deployment of different types of + Network Address Translator (NAT) boxes that in many cases only + loosely follow standards [RFC3022] [RFC2663] [RFC3424] [RFC4787] + [RFC5382]. NATs cause discontinuity in address realms [RFC3424]; + therefore, an application protocol, such as the Real-Time Streaming + Protocol (RTSP) [RFC2326] [RTSP], needs to deal with such + discontinuities caused by NATs. The problem is that, being a media + control protocol managing one or more media streams, RTSP carries + network address and port information within its protocol messages. + Because of this, even if RTSP itself, when carried over the + Transmission Control Protocol (TCP) [RFC793], for example, is not + blocked by NATs, its media streams may be blocked by NATs. This will + occur unless special protocol provisions are added to support NAT + traversal. + + Like NATs, firewalls are also middleboxes that need to be considered. + Firewalls help prevent unwanted traffic from getting in or out of the + protected network. RTSP is designed such that a firewall can be + configured to let RTSP-controlled media streams go through with + limited implementation effort. The effort needed is to implement an + Application Level Gateway (ALG) to interpret RTSP parameters. There + is also a large class of firewalls, commonly home firewalls, that use + a filtering behavior that appears to be the same as what NATs have. + This type of firewall will be successfully traversed using the same + solution as employed for NAT traversal, instead of relying on an RTSP + ALG. Therefore, firewalls will also be discussed and some important + differences highlighted. + + This document describes several NAT traversal mechanisms for RTSP- + controlled media streaming. Many of these NAT solutions fall into + the category of "UNilateral Self-Address Fixing (UNSAF)" as defined + in [RFC3424] 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 RFC 3424, we describe the + required RTSP extensions for each method, transition strategies, and + security concerns. The transition strategies are a discussion of how + and if the method encourages a move towards not having any NATs on + the path. + + + + + +Westerlund & Zeng Informational [Page 4] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + This document is capturing the evaluation done in the process to + recommend firewall/NAT traversal methods for RTSP streaming servers + based on [RFC2326] as well as the RTSP 2.0 core specification [RTSP]. + The evaluation is focused on NAT traversal for the media streams + carried over the User Datagram Protocol (UDP) [RFC768] with RTP + [RFC3550] over UDP being the main case for such usage. The findings + should be applicable to other protocols as long as they have similar + properties. + + At the time when the bulk of work on this document was done, a single + level of NAT was the dominant deployment for NATs, and multiple + levels of NATs, including Carrier-Grade NATs (CGNs), were not + considered. Thus, any characterizations or findings may not be + applicable in such scenarios, unless CGN or multiple levels of NATs + are explicitly noted. + + An RTSP NAT traversal mechanism based on Interactive Connectivity + Establishment (ICE) is specified in "A Network Address Translator + (NAT) Traversal Mechanism for Media Controlled by Real-Time Streaming + Protocol (RTSP)" [RTSP-NAT]. + +1.1. Network Address Translators + + We begin by reviewing two quotes from Section 3 in "Network Address + Translation (NAT) Behavioral Requirements for Unicast UDP" [RFC4787] + concerning NATs and their terminology: + + Readers are urged to refer to [RFC2663] for information on NAT + taxonomy and terminology. Traditional NAT is the most common type + of NAT device deployed. Readers may refer to [RFC3022] for + detailed information on traditional NAT. Traditional NAT has two + main varieties -- Basic NAT and Network Address/Port Translator + (NAPT). + + NAPT is by far the most commonly deployed NAT device. NAPT allows + multiple internal hosts to share a single public IP address + simultaneously. When an internal host opens an outgoing TCP or + UDP session through a NAPT, the NAPT assigns the session a public + IP address and port number, so that subsequent response packets + from the external endpoint can be received by the NAPT, + translated, and forwarded to the internal host. The effect is + that the NAPT establishes a NAT session to translate the (private + IP address, private port number) tuple to a (public IP address, + public port number) tuple, and vice versa, for the duration of the + session. An issue of relevance to peer-to-peer applications is + how the NAT behaves when an internal host initiates multiple + simultaneous sessions from a single (private IP, private port) + endpoint to multiple distinct endpoints on the external network. + + + +Westerlund & Zeng Informational [Page 5] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + In this specification, the term "NAT" refers to both "Basic NAT" + and "Network Address/Port Translator (NAPT)". + + This document uses the term "Address and Port Mapping" as the + translation between an external address and port and an internal + address and port. Note that this is not the same as an "address + binding" as defined in RFC 2663. + + Note: In the above text, it would be more correct to use an + external IP address instead of a public IP address. The external + IP address is commonly a public one, but it might be of another + type if the NAT's external side is in a private address domain. + + In addition to the above quote, there exists a number of address and + port mapping behaviors described in more detail in Section 4.1 of + [RFC4787] that are highly relevant to the discussion in this + document. + + NATs also have a filtering behavior on traffic arriving on the + external side. Such behavior affects how well different methods for + NAT traversal works through these NATs. See Section 5 of [RFC4787] + for more information on the different types of filtering that have + been identified. + +1.2. Firewalls + + A firewall is a security gateway that enforces certain access control + policies between two network administrative domains: a private domain + (intranet) and an external domain, e.g., the Internet. Many + organizations use firewalls to prevent intrusions and malicious + attacks on computing resources in the private intranet [RFC2588]. + + A comparison between NAT and a firewall is given below: + + 1. A firewall sits at security enforcement/protection points, while + NAT sits at borders between two address domains. + + 2. NAT does not in itself provide security, although some access + control policies can be implemented using address translation + schemes. The inherent filtering behaviors are commonly mistaken + for real security policies. + + It should be noted that many NAT devices intended for Residential or + Small Office, Home Office (SOHO) use include both NATs and firewall + functionality. + + + + + + +Westerlund & Zeng Informational [Page 6] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + +1.3. Glossary + + Address-Dependent Mapping: The NAT reuses the port mapping for + subsequent packets sent from the same internal IP address and + port to the same external IP address, regardless of the + external port; see [RFC4787]. + + Address and Port-Dependent Mapping: The NAT reuses the port mapping + for subsequent packets sent from the same internal IP address + and port to the same external IP address and port while the + mapping is still active; see [RFC4787]. + + ALG: Application Level Gateway is 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. + + Endpoint-Independent Mapping: The NAT reuses the port mapping for + subsequent packets sent from the same internal IP address and + port to any external IP address and port; see [RFC4787]. + + ICE: Interactive Connectivity Establishment; see [RFC5245]. + + DNS: Domain Name Service + + DoS: Denial of Service + + DDoS: Distributed Denial of Service + + NAT: Network Address Translator; see [RFC3022]. + + NAPT: Network Address/Port Translator; see [RFC3022]. + + RTP: Real-Time Transport Protocol; see [RFC3550]. + + RTSP: Real-Time Streaming Protocol; see [RFC2326] and [RTSP]. + + RTT: Round Trip Times + + SDP: Session Description Protocol; see [RFC4566]. + + SSRC: Synchronization source in RTP; see [RFC3550]. + + + + + + + + + +Westerlund & Zeng Informational [Page 7] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + +2. Detecting the Loss of NAT Mappings + + Several 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 [RFC3424] 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 RFC 3424. + + First, since NATs may also dynamically reclaim or readjust address/ + port translations, "keep-alive" and periodic repolling may be + required according to RFC 3424. Second, it is possible to detect and + recover from the situation where the mapping has been changed or + removed. The loss of a mapping can be detected when no traffic + arrives for a while. Below we will give some recommendations on how + to detect the loss of NAT mappings when using RTP/RTCP under RTSP + control. + + An RTP session normally has both RTP and RTCP streams. The loss of + an RTP mapping can only be detected when expected traffic does not + arrive. If a client does not receive media data within a few seconds + after having received the "200 OK" response to an RTSP PLAY request + that starts the media delivery, it may be the result of a middlebox + blocking the traffic. However, for a receiver to be more certain to + detect the case where no RTP traffic was delivered due to NAT + trouble, one should monitor the RTCP Sender reports if they are + received and not also blocked. The sender report carries a field + telling how many packets the server has sent. If that has increased + and no RTP packets have arrived for a few seconds, it is likely the + mapping for the RTP stream has been removed. + + The loss of mapping for RTCP is simpler to detect. 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 lost. Note that if neither RTCP packets nor RTSP + messages are received by the RTSP server for a while (default 60 + seconds), the RTSP server has the option to delete the corresponding + RTP session, SSRC and RTSP session ID, because either the client can + not get through a middlebox NAT/firewall, or the client is + malfunctioning. + + + + + + + + + +Westerlund & Zeng Informational [Page 8] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + +3. Requirements on Solutions + + This section considers the set of requirements for the evaluation of + RTSP NAT traversal solutions. + + RTSP is a client-server protocol. Typically, service providers + deploy RTSP servers on the Internet or otherwise reachable address + realm. However, there are use cases where the reverse is true: RTSP + clients are connecting from any address realm to RTSP servers behind + NATs, e.g., in a home. 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. + In terms of requirements, the primary issue to solve is the RTSP NAT + traversal problem for RTSP servers deployed in a network where the + server is on the external side of any NAT, i.e., the server is not + behind a NAT. The server behind a NAT is desirable but of much lower + priority. + + Important considerations for any NAT traversal technique are whether + any protocol modifications are needed and where the implementation + burden resides (e.g., server, client, or middlebox). If the + incentive to get RTSP to work over a NAT is sufficient, it will + motivate the owner of the server, client, or middlebox to update, + configure, or otherwise perform changes to the device and its + software in order to support NAT traversal. Thus, the questions of + who this burden falls on and how big it is are highly relevant. + + The list of feature requirements for RTSP NAT solutions are given + below: + + 1. Must work for all flavors of NATs, including NATs with Address + and Port-Dependent Filtering. + + 2. Must work for firewalls (subject to pertinent firewall + administrative policies), including those with ALGs. + + 3. Should have minimal impact on clients not behind NATs and that + are not dual hosted. RTSP dual hosting means that the RTSP + signaling protocol and the media protocol (e.g., RTP) are + implemented on different computers with different IP addresses. + + * For instance, no extra protocol RTT before arrival of media. + + 4. Should be simple to use/implement/administer so people actually + turn them on. + + * Discovery of the address(es) assigned by NAT should happen + automatically, if possible. + + + +Westerlund & Zeng Informational [Page 9] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + 5. Should authenticate dual-hosted client's media transport receiver + to prevent usage of RTSP servers for 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 determines the media + destination (address and port) for the client, the result may be that + the IP address of the RTP receiver host is different than the IP + address of the RTSP client host. This poses a DDoS threat that has + significant amplification potentials because the RTP media streams in + general consist of a large number of IP packets. DDoS attacks can + occur if the attacker can fake the messages in the NAT traversal + mechanism to trick the RTSP server into believing that the client's + RTP receiver is located on a host to be attacked. For example, user + A may use his RTSP client to direct the RTSP server to send video RTP + streams to target.example.com in order to degrade the services + provided by target.example.com. + + Note that a simple mitigation is for the RTSP server to disallow the + cases where the client's RTP receiver has a different IP address than + that of the RTSP client. This is recommended behavior in RTSP 2.0 + unless other solutions to prevent this attack are present; see + Section 21.2.1 in [RTSP]. With the increased deployment of NAT + middleboxes by operators, i.e., CGN, the reuse of an IP address on + the NAT's external side by many customers reduces the protection + provided. Also in some applications (e.g., centralized + conferencing), dual-hosted RTSP/RTP clients have valid use cases. + The key is how to authenticate the messages exchanged during the NAT + traversal process. + +4. NAT-Traversal Techniques + + There exists 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 costs are also + different. They also vary in security levels. In the following + sections, each technique is outlined with discussions on the + corresponding advantages and disadvantages. + + The survey of traversal techniques was done prior to 2007 and is + based on what was available then. This section includes NAT + traversal techniques that have not been formally specified anywhere + else. This document may be the only publicly available specification + of some of the NAT traversal techniques. However, that is not a real + barrier against doing an evaluation of the NAT traversal techniques. + Some techniques used as part of some of the traversal solutions have + been recommended against or are no longer possible due to the outcome + + + +Westerlund & Zeng Informational [Page 10] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + of standardization work or their failure to progress within IETF + after the initial evaluation in this document. For example, RTP + No-Op [RTP-NO-OP] was a proposed RTP payload format that failed to be + specified; thus, it is not available for use today. In each such + case, the missing parts will be noted and some basic reasons will be + given. + +4.1. Stand-Alone STUN + +4.1.1. Introduction + + Session Traversal Utilities for NAT (STUN) [RFC5389] is a + standardized protocol that allows a client to use secure means to + discover the presence of a NAT between itself and the STUN server. + The client uses the STUN server to discover the address and port + mappings assigned by the NAT. Then using the knowledge of these NAT + mappings, it uses the external addresses to directly connect to the + independent RTSP server. However, this is only possible if the NAT + address and port mapping behavior is such that the STUN server and + RTSP server will see the same external address and port for the same + internal address and port. + + STUN is a client-server protocol. The STUN client sends a request to + a STUN server and the server returns a response. There are two types + of STUN messages -- Binding Requests and Indications. Binding + Requests are used when determining a client's external address and + soliciting a response from the STUN server with the seen address. + Indications are used by the client for keep-alive messages towards + the server and requires no response from the server. + + The first version of STUN [RFC3489] included categorization and + parameterization of NATs. This was abandoned in the updated version + [RFC5389] due to it being unreliable and brittle. This particular + traversal method uses the removed functionality described in RFC 3489 + to detect the NAT type to give an early failure indication when the + NAT is showing the behavior that this method can't support. This + method also suggests using the RTP No-Op payload format [RTP-NO-OP] + for keep-alives of the RTP traffic in the client-to-server direction. + This can be replaced with another form of UDP packet as will be + further discussed below. + +4.1.2. 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. Note that 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). + + + +Westerlund & Zeng Informational [Page 11] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + Limitations: + + o The server must be located in either a public address realm or the + next-hop external address realm in regards to the client. + + o The client may only be located behind NATs that perform Endpoint- + Independent or Address-Dependent Mappings (the STUN server and + RTSP server on the same IP address). Clients behind NATs that do + Address and Port-Dependent Mappings cannot use this method. See + [RFC4787] for the full definition of these terms. + + o Based on the discontinued middlebox classification of the replaced + STUN specification [RFC3489]; thus, it is brittle and unreliable. + + Method: + + An RTSP client using RTP transport over UDP can use STUN to traverse + a NAT(s) in the following way: + + 1. Use STUN to try 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 multimedia presentation. This information is available in + the media description protocol, e.g., SDP [RFC4566]. 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: client's 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" [RTSP] or "destination" + "client_port" [RFC2326] + with the public/external IP address and port pair for both RTP + and RTCP. To be certain that this works, servers must allow a + client to set up the RTP stream on any port, not only even ports + and with non-contiguous port numbers for RTP and RTCP. This + requires the new feature provided in RTSP 2.0 [RTSP]. The server + should respond with a Transport header containing an "src_addr" + + + + +Westerlund & Zeng Informational [Page 12] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + or "source" + "server_port" parameters 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. For the + mapping carrying RTCP traffic, the periodic RTCP traffic is + likely enough. For mappings carrying RTP traffic and for + mappings carrying RTCP packets at too low of a frequency, keep- + alive messages should be sent. + + If a UDP mapping is lost, 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 cause a glitch + in media playback. + + To allow UDP packets to arrive from the server to a client behind an + Address-Dependent or Address and Port-Dependent Filtering NAT, the + client must first send a UDP packet to establish the filtering state + in the NAT. The client, before sending an RTSP PLAY request, must + send a so-called hole-punching packet on each mapping to the IP + address and port given as the server's source address and port. For + a NAT that only is Address-Dependent Filtering, the hole-punching + packet could be sent to the server's discard port (port number 9). + For Address and Port-Dependent Filtering NATs, the hole-punching + packet must go to the port used for sending UDP packets to the + client. To be able to do that, the server needs to include the + "src_addr" in the Transport header (which is the "source" transport + parameter in RFC2326). Since UDP packets are inherently unreliable, + to ensure that at least one UDP message passes the NAT, hole-punching + packets should be retransmitted a reasonable number of times. + + One could have used RTP No-Op packets [RTP-NO-OP] as hole-punching + and keep-alive messages had they been defined. That would have + ensured that the traffic would look like RTP and thus would likely + have the least risk of being dropped by any firewall. The drawback + of using RTP No-Op is that the payload type number must be + dynamically assigned through RTSP first. Other options are STUN, an + RTP packet without any payload, or a UDP packet without any payload. + For RTCP it is most suitable to use correctly generated RTCP packets. + In general, sending unsolicited traffic to the RTSP server may + trigger security functions resulting in the blocking of the keep- + alive messages or termination of the RTSP session itself. + + This method is further brittle as it doesn't support Address and + Port-Dependent Mappings. Thus, it proposes to use the old STUN + methods to classify the NAT behavior, thus enabling early error + indication. This is strictly not required but will lead to failures + during setup when the NAT has the wrong behavior. This failure can + + + +Westerlund & Zeng Informational [Page 13] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + also occur if the NAT changes the properties of the existing mapping + and filtering state or between the classification message exchange + and the actual RTSP session setup, for example, due to load. + +4.1.3. 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 NAT external 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. An ALG should not + update address fields that contain addresses other than the NAT's + internal address domain. In cases where the ALG modifies fields + unnecessarily, two alternatives exist: + + 1. Use Transport Layer Security (TLS) to encrypt the data over the + RTSP TCP connection to prevent the ALG from reading and modifying + the RTSP messages. + + 2. Turn off the STUN-based NAT traversal mechanism. + + As it may be difficult to determine why the failure occurs, the usage + of TLS-protected RTSP message exchange at all times would avoid this + issue. + +4.1.4. Deployment Considerations + + For the stand-alone usage of STUN, the following applies: + + Advantages: + + o STUN is a solution first used by applications based on SIP + [RFC3261] (see Sections 1 and 2 of [RFC5389]). As shown above, + with little or no changes, the RTSP application can reuse STUN as + a NAT traversal solution, avoiding the pitfall of solving a + problem twice. + + o Using STUN does not require RTSP server modifications, assuming it + is a server that is compliant with RTSP 2.0; it only affects the + client implementation. + + Disadvantages: + + o Requires a STUN server deployed in the same address domain as the + server. + + + + + +Westerlund & Zeng Informational [Page 14] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + o Only works with NATs that perform Endpoint-Independent and + Address-Dependent Mappings. Address and Port-Dependent Filtering + NATs create some issues. + + o Brittle to NATs changing the properties of the NAT mapping and + filtering. + + o Does not work with Address and Port-Dependent Mapping NATs without + server modifications. + + o Will not work if a NAT uses multiple IP addresses, since RTSP + servers generally require all media streams to use the same IP as + used in the RTSP connection to prevent becoming a DDoS tool. + + o Interaction problems exist when an RTSP-aware ALG interferes with + the use of STUN for NAT traversal unless TLS-secured RTSP message + exchange is used. + + o Using STUN requires that RTSP servers and clients support the + updated RTSP specification [RTSP], 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 RFC 2326. + + Transition: + + 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. + +4.1.5. Security Considerations + + To prevent the RTSP server from being used as Denial-of-Service (DoS) + attack tools, the RTSP Transport header parameters "destination" and + "dest_addr" are generally not allowed to point to any IP address + other than the one the RTSP message originates from. The RTSP server + is only prepared to make an exception to 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 a restriction means + that STUN in general does not work for use cases where RTSP and media + transport go to different addresses. + + STUN combined with RTSP that is restricted by destination address has + the same security properties as the core RTSP. It is protected from + being used as a DoS attack tool unless the attacker has the ability + to spoof the TCP connection carrying RTSP messages. + + + +Westerlund & Zeng Informational [Page 15] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + Using STUN's support for message authentication and the secure + transport of RTSP messages, attackers cannot modify STUN responses or + RTSP messages (TLS) to change the media destination. This protects + against hijacking; however, as a client can be the initiator of an + attack, these mechanisms cannot securely prevent RTSP servers from + being used as DoS attack tools. + +4.2. Server Embedded STUN + +4.2.1. Introduction + + This section describes an alternative to the stand-alone STUN usage + in the previous section that has quite significantly different + behavior. + +4.2.2. 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 Address and Port-Dependent Mapping NATs. This would + require RTSP-level protocol changes. + + This NAT traversal solution has limitations: + + 1. It does not work if both the 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 that is different from the IP in the client RTSP + requests. + + Deviations from STUN as defined in RFC 5389: + + 1. The RTSP application must provision the client with an identity + and shared secret to use in the STUN authentication; + + 2. We require the STUN server to be co-located on the RTSP server's + media source ports. + + If the STUN server is co-located with the RTSP server's media source + port, an RTSP client using RTP transport over UDP can use STUN to + traverse ALL types of NATs. In the case of Address and Port- + Dependent Mapping NATs, the party on the inside of the NAT must + initiate UDP traffic. The STUN Binding 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 (however, only one of them can be behind a NAT). + + + +Westerlund & Zeng Informational [Page 16] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + Likewise, if an RTSP server is behind a NAT, then an embedded STUN + server must be co-located on the RTSP client's RTCP port. Also, it + will become the client that needs to disclose his destination address + rather than the server, so the server can correctly determine its NAT + external source address for the media streams. In this case, we + assume that the client has some means of establishing a TCP + connection to the RTSP server behind NAT so as to exchange RTSP + messages with the RTSP server, potentially using a proxy or static + rules. + + To minimize delay, we require that the RTSP server supporting this + option must inform the client about the RTP and RTCP ports from where + the server will send out RTP and RTCP packets, respectively. This + can be done by using the "server_port" parameter in RFC 2326 and the + "src_addr" parameter in [RTSP]. Both are in the RTSP Transport + header. But in general, this strategy will require that one first + does one SETUP request per media to learn the server ports, then + perform the STUN checks, followed by a subsequent SETUP to change the + client port and destination address to what was learned during the + STUN checks. + + To be certain that RTCP works correctly, the RTSP endpoint (server or + client) will be required to send and receive RTCP packets from the + same port. + +4.2.3. Discussion on Co-located STUN Server + + In order to use STUN to traverse Address and Port-Dependent Filtering + or Mapping NATs, the STUN server needs to be co-located with the + streaming server media output ports. This creates a demultiplexing + problem: we must be able to differentiate a STUN packet from a media + packet. This will be done based on heuristics. The existing STUN + heuristics is the first byte in the packet and the Magic Cookie field + (added in RFC 5389), which works fine between STUN and RTP or RTCP + where the first byte happens to be different. Thanks to the Magic + Cookie field, it is unlikely that other protocols would be mistaken + for a STUN packet, but this is not assured. For more discussion of + this, please see Section 5.1.2 of [RFC5764]. + +4.2.4. ALG Considerations + + The same ALG traversal considerations as for stand-alone STUN applies + (Section 4.1.3). + + + + + + + + +Westerlund & Zeng Informational [Page 17] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + +4.2.5. Deployment Considerations + + For the "Embedded STUN" method the following applies: + + Advantages: + + o STUN is a solution first used by SIP applications. As shown + above, with little or no changes, the RTSP application can reuse + STUN as a NAT traversal solution, avoiding the pitfall of solving + a problem twice. + + o STUN has built-in message authentication features, which makes it + more secure against hijacking attacks. See the next section for + an in-depth security discussion. + + o This solution works as long as there is only one RTSP endpoint in + the private address realm, regardless of the NAT's type. There + may even be multiple NATs (see Figure 1 in [RFC5389]). + + o Compared to other UDP-based NAT traversal methods in this + document, STUN requires little new protocol development (since + STUN is already an IETF standard), and most likely less + implementation effort, since open source STUN server and client + implementations are available [STUN-IMPL] [PJNATH]. + + Disadvantages: + + o Some extensions to the RTSP core protocol, likely signaled by RTSP + feature tags, must be introduced. + + o Requires an embedded STUN server to be co-located on each of the + RTSP server's media protocol's ports (e.g., RTP and RTCP ports), + which means more processing is required to demultiplex STUN + packets from media packets. For example, the demultiplexer must + be able to differentiate an RTCP RR packet from a STUN packet and + forward the former to the streaming server and the latter to the + STUN server. + + o Does not support use cases that require the RTSP connection and + the media reception to happen at different addresses, unless the + server's security policy is relaxed. + + o Interaction problems exist when an RTSP ALG is not aware of STUN + unless TLS is used to protect the RTSP messages. + + o Using STUN requires that RTSP servers and clients support the + updated RTSP specification [RTSP], and they both agree to support + the NAT traversal feature. + + + +Westerlund & Zeng Informational [Page 18] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + o Increases the setup delay with at least the amount of time it + takes to perform STUN message exchanges. Most likely an extra + SETUP sequence will be required. + + Transition: + + The usage of STUN can be phased out gradually as the first step of a + STUN-capable machine can be used 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, i.e., no NATs or firewalls. + +4.2.6. Security Considerations + + See Stand-Alone STUN (Section 4.1.5). + +4.3. ICE + +4.3.1. Introduction + + Interactive Connectivity Establishment (ICE) [RFC5245] is a + methodology for NAT traversal that has been developed for SIP using + SDP offer/answer. The basic idea is to try, in a staggered parallel + fashion, all possible connection addresses in which an endpoint may + be reached. This allows the endpoint to use the best available UDP + "connection" (meaning two UDP endpoints capable of reaching each + other). The methodology has very nice properties in that basically + all NAT topologies are possible to traverse. + + Here is how ICE works at a high level. Endpoint A collects all + possible addresses that can be used, including local IP addresses, + STUN-derived addresses, Traversal Using Relay NAT (TURN) addresses, + etc. On each local port that any of these address and port pairs + lead to, a STUN server is installed. This STUN server only accepts + STUN requests using the correct authentication through the use of a + username and password. + + Endpoint A then sends a request to establish connectivity with + endpoint B, which includes all possible "destinations" [RFC5245] to + get the media through to A. Note that each of A's local address/port + pairs (host candidates and server reflexive base) has a co-located + STUN server. B in turn provides A with all its possible destinations + for the different media streams. A and B then uses a STUN client to + try to reach all the address and port pairs specified by A from its + corresponding destination ports. The destinations for which the STUN + requests successfully complete are then indicated and one is + selected. + + + + +Westerlund & Zeng Informational [Page 19] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + 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; therefore, requests are + retransmitted multiple times. The STUN requests may also result in + more connectivity alternatives (destinations) being discovered and + conveyed in the STUN responses. + +4.3.2. 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 steps could provide ICE + support for RTSP. + + 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 be opened, or proxies are deployed, etc. + + The negotiation of ICE in RTSP of necessity will work different than + in SIP with SDP offer/answer. The protocol interactions are + different, and thus the possibilities for transfer of states are also + somewhat different. The goal is also to avoid introducing extra + delay in the setup process at least for when the server is not behind + a NAT in regards to the client, and the client is either having an + address in the same address domain or is behind the NAT(s), which can + address the address domain of the server. This process is only + intended to support PLAY mode, i.e., media traffic flows from server + to client. + + 1. ICE usage begins in the SDP. The SDP for the service indicates + that ICE is supported at the server. No candidates can be given + here as that would not work with on demand, DNS load balancing, + etc., which have the SDP indicate a resource on a server park + rather than a specific machine. + + 2. The client gathers addresses and puts together its candidates for + each media stream indicated in the session description. + + 3. In each SETUP request, the client includes its candidates in an + ICE-specific transport specification. For the server, this + indicates the ICE support by the client. One candidate is the + most prioritized candidate and here the prioritization for this + address should be somewhat different compared to SIP. High- + performance candidates are recommended rather than candidates + with the highest likelihood of success, as it is more likely that + a server is not behind a NAT compared to a SIP user agent. + + + + +Westerlund & Zeng Informational [Page 20] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + 4. The server responds to the SETUP (200 OK) for each media stream + with its candidates. A server not behind a NAT usually only + provides a single ICE candidate. Also, here one candidate is the + server primary address. + + 5. The connectivity checks are performed. For the server, the + connectivity checks from the server to the clients have an + additional usage. They verify that there is someone willing to + receive the media, thus preventing the server from unknowingly + performing a DoS attack. + + 6. Connectivity checks from the client promoting a candidate pair + were successful. Thus, no further SETUP requests are necessary + and processing can proceed with step 7. If an address other than + the primary has been verified by the client to work, that address + may then be promoted for usage in a SETUP request (go to step 7). + If the checks for the available candidates failed and if further + candidates have been derived during the connectivity checks, then + those can be signaled in new candidate lines in a SETUP request + updating the list (go to step 5). + + 7. Client issues the PLAY request. If the server also has completed + its connectivity checks for the promoted candidate pair (based on + the username as it may be derived addresses if the client was + behind NAT), then it can directly answer 200 OK (go to step 8). + If the connectivity check has not yet completed, it responds with + a 1xx code to indicate that it is verifying the connectivity. If + that fails within the set timeout, an error is reported back. + The client needs to go back to step 6. + + 8. Process completed and media can be delivered. ICE candidates not + used may be released. + + To keep media paths alive, the client needs to periodically send data + to the server. This will be realized with STUN. RTCP sent by the + client should be able to keep RTCP open, but STUN will also be used + for SIP based on the same motivations as for ICE. + +4.3.3. 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 + has the largest impact on client and server implementations among all + the NAT/firewall traversal methods in this document. + + + + + + + +Westerlund & Zeng Informational [Page 21] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + RTSP server implementation requirements are: + + o STUN server features + + o Limited STUN client features + + o SDP generation with more parameters + + o RTSP error code for ICE extension + + RTSP client implementation requirements are: + + o Limited STUN server features + + o Limited STUN client features + + o RTSP error code and ICE extension + +4.3.4. ALG Considerations + + If there is an RTSP ALG that doesn't support the NAT traversal + method, it may interfere with the NAT traversal. As the usage of ICE + for the traversal manifests itself in the RTSP message primarily as a + new transport specification, an ALG that passes through unknown will + not prevent the traversal. An ALG that discards unknown + specifications will, however, prevent the NAT traversal. These + issues can be avoided by preventing the ALG to interfere with the + signaling by using TLS for the RTSP message transport. + + An ALG that supports this traversal method can, on the most basic + level, just pass the transport specifications through. ALGs in NATs + and firewalls could use the ICE candidates to establish a filtering + state that would allow incoming STUN messages prior to any outgoing + hole-punching packets, and in that way it could speed up the + connectivity checks and reduce the risk of failures. + +4.3.5. Deployment Considerations + + Advantages: + + o Solves NAT connectivity discovery for basically all cases as long + as a TCP connection between the client and server can be + established. This includes servers behind NATs. (Note that a + proxy between address domains may be required to get TCP through.) + + o Improves defenses against DDoS attacks, since a media-receiving + client requires authentications via STUN on its media reception + ports. + + + +Westerlund & Zeng Informational [Page 22] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + Disadvantages: + + o Increases the setup delay with at least the amount of time it + takes for the server to perform its STUN requests. + + o Assumes that it is possible to demultiplex between the packets of + the media protocol and STUN packets. This is possible for RTP as + discussed, for example, in Section 5.1.2 of [RFC5764]. + + o Has a fairly high implementation burden put on both the RTSP + server and client. However, several open source ICE + implementations do exist, such as [NICE] and [PJNATH]. + +4.3.6. Security Considerations + + One should review the Security Considerations section of ICE and STUN + to understand that ICE contains some potential issues. However, + these can be avoided by correctly using ICE in RTSP. An important + factor is to secure the signaling, i.e., use TLS between the RTSP + client and server. In fact ICE does help avoid the DDoS attack issue + with RTSP substantially as it reduces the possibility for a DDoS + using RTSP servers on attackers that are on path between the RTSP + server and the target and capable of intercepting the STUN + connectivity check packets and correctly sending a response to the + server. The ICE connectivity checks with their random transaction + IDs from the server to the client serves as a return-routability + check and prevents off-path attackers to succeed with address + spoofing. This is similar to Mobile IPv6's return routability + procedure (Section 5.2.5 of [RFC6275]). + +4.4. Latching + +4.4.1. Introduction + + Latching is a NAT traversal solution that is based on requiring RTSP + clients to send UDP packets to the server's media output ports. + Conventionally, RTSP servers send RTP packets in one direction: from + server to client. Latching is similar to connection-oriented + traffic, where one side (e.g., the RTSP client) first "connects" by + sending an RTP packet to the other side's RTP port; the recipient + then replies to the originating IP and Port. This method is also + referred to as "late binding". It requires that all RTP/RTCP + transport be done symmetrically. This in effect requires Symmetric + RTP [RFC4961]. Refer to [RFC7362] for a description of the Latching + of SIP-negotiated media streams in Session Border Controllers. + + Specifically, when the RTSP server receives the Latching packet + (a.k.a. hole-punching packet, since it is used to punch a hole in the + + + +Westerlund & Zeng Informational [Page 23] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + firewall/NAT) from its client, it copies the source IP and Port + number and uses them as the delivery address for media packets. By + having the server send media traffic back the same way as the + client's packets are sent to the server, address and port mappings + will be honored. Therefore, this technique works for all types of + NATs, given that the server is not behind a NAT. However, it does + require server modifications. The format of the Latching packet will + have to be defined. + + Latching is very vulnerable to both hijacking and becoming a tool in + DDoS attacks (see Security Considerations in [RFC7362]) because + attackers can simply forge the source IP and Port of the Latching + packet. The rule for restricting IP addresses to one of the + signaling connections will need to be applied here also. However, + that does not protect against hijacking from another client behind + the same NAT. This can become a serious issue in deployments with + CGNs. + +4.4.2. Necessary RTSP Extensions + + To support Latching, RTSP signaling must be extended to allow the + RTSP client to indicate that it will use Latching. 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 or simply to avoid mis-association + when multiple clients are behind the same NAT. Care must be taken in + choosing clients' RTP SSRC. First, it must be unique within all the + RTP sessions belonging to the same RTSP session. Second, if the RTSP + server is sending out media packets to multiple clients from the same + send port, the RTP SSRC needs to be unique among those clients' RTP + sessions. Recognizing that there is a potential that RTP SSRC + collisions may occur, the RTSP server must be able to signal to a + client that a collision has occurred and that it wants the client to + use a different RTP SSRC carried in the SETUP response or use unique + ports per RTSP session. Using unique ports limits an RTSP server in + the number of sessions it can simultaneously handle per interface IP + addresses. + + The Latching packet as discussed above should have a field that can + contain a client and RTP session identifier to correctly associate + the Latching packet with the correct context. If an RTP packet is to + be used, there would be a benefit to using a well-defined RTP payload + format for this purpose as the No-Op payload format proposed + [RTP-NO-OP]. However, in the absence of such a specification, an RTP + packet without a payload could be used. Using SSRC is beneficial + because RTP and RTCP both would work as is. However, other packet + formats could be used that carry the necessary identification of the + context, and such a solution is discussed in Section 4.5. + + + +Westerlund & Zeng Informational [Page 24] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + +4.4.3. ALG Considerations + + An RTSP ALG not supporting this method could interfere with the + methods used to indicate that Latching is to be done, as well as the + SSRC signaling, thus preventing the method from working. However, if + the RTSP ALG instead opens the corresponding pinholes and creates the + necessary mapping in the NAT, traversal will still work. Securing + the RTSP message transport using TLS will avoid this issue. + + An RTSP ALG that supports this traversal method can for basic + functionality simply pass the related signaling parameters + transparently. Due to the security considerations for Latching, + there might exist a benefit for an RTSP ALG that will enable NAT + traversal to negotiate with the path and turn off the Latching + procedures when the ALG handles this. However, this opens up to + failure modes when there are multiple levels of NAT and only one + supports an RTSP ALG. + +4.4.4. Deployment Considerations + + Advantages: + + o Works for all types of client-facing NATs (requirement 1 in + Section 3). + + o Has little interaction problems with any RTSP ALG changing the + client's information in the Transport header. + + Disadvantages: + + o Requires modifications to both the RTSP server and client. + + o Limited to working with servers that are not behind a NAT. + + o The format of the packet for "connection setup" (a.k.a Latching + packet) is not defined. + + o SSRC management if RTP is used for Latching due to risk for mis- + association of clients to RTSP sessions at the server if SSRC + collision occurs. + + o Has significant security considerations (See Section 4.4.5), due + to the lack of a strong authentication mechanism and will need to + use address restrictions. + + + + + + + +Westerlund & Zeng Informational [Page 25] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + +4.4.5. Security Considerations + + Latching's major security issue is that RTP streams can be hijacked + and directed towards any target that the attacker desires unless + address restrictions are used. In the case of NATs with multiple + clients on the inside of them, hijacking can still occur. This + becomes a significant threat in the context of CGNs. + + The most serious security problem is the deliberate attack with the + use of an RTSP client and Latching. The attacker uses RTSP to set up + a media session. Then it uses Latching with a spoofed source address + of the intended target of the attack. There is no defense against + this attack other than restricting the possible address a Latching + packet can come from to the same address as the RTSP TCP connection + is from. This prevents Latching to be used in use cases that require + different addresses for media destination and signaling. Even + allowing only a limited address range containing the signaling + address from where Latching is allowed opens up a significant + vulnerability as it is difficult to determine the address usage for + the network the client connects from. + + A hijack attack can also 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 Latching packets containing the correct RTP + SSRC to the correct address and port on the server. The RTSP server + will then use the source IP and Port from the Latching packet as the + destination for the media packets it sends. + + Another variation of this attack is for a man in the middle to modify + the RTP Latching packet being sent by a client to the server by + simply changing the source IP and Port to the target one desires to + attack. + + One can fend off the snooping-based attack by applying encryption to + the RTSP signaling transport. However, if the attacker is a man in + the middle modifying Latching packets, the attack is impossible to + defend against other than through address restrictions. As a NAT + rewrites the source IP and (possibly) port, this cannot be + authenticated, but authentication is required in order to protect + against this type of DoS attack. + + Yet another issue is that these attacks also can be used to deny the + client the service it desires from the RTSP server completely. The + attacker modifies or originates its own Latching packets with a port + + + + + +Westerlund & Zeng Informational [Page 26] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + other than what the legit Latching packets use, which results in the + media server sending the RTP/RTCP traffic to ports the client isn't + listening for RTP/RTCP on. + + The amount of random non-guessable material in the Latching packet + determines how well Latching can fend off stream hijacking performed + by parties that are off the client-to-server network path, i.e., it + lacks the capability to see the client's Latching packets. The + proposal above uses the 32-bit RTP SSRC field to this effect. + Therefore, it is important that this field is derived with a non- + predictable random number generator. 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 could implement functionality that blocks packets + to ports or from sources that receive or send multiple Latching + packets with different invalid SSRCs, especially when they are coming + from the same IP and Port. Note that this mitigation in itself opens + up a new venue for DoS attacks against legit users trying to latch. + + To improve the security against attackers, the amount of random + material could be increased. To achieve a longer random tag while + still using RTP and RTCP, it will be necessary to develop RTP and + RTCP payload formats for carrying the random material. + +4.5. A Variation to Latching + +4.5.1. Introduction + + Latching as described above requires the usage of a valid RTP format + as the Latching packet, i.e., the first packet that the client sends + to the server to establish a bidirectional transport flow for RTP + streams. There is currently no appropriate RTP packet format for + this purpose, although the RTP No-Op format was a proposal to fix the + problem [RTP-NO-OP]; however, that work was abandoned. [RFC6263] + discusses the implication of different types of packets as keep- + alives for RTP, and its findings are very relevant to the format of + the Latching packet. + + Meanwhile, there have been NAT/firewall traversal techniques deployed + in the wireless streaming market place that use non-RTP messages as + Latching packets. This section describes a variant based on a subset + of those solutions that alters the previously described Latching + solution. + + + + +Westerlund & Zeng Informational [Page 27] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + +4.5.2. Necessary RTSP Extensions + + In this variation of Latching, the Latching packet is a small UDP + packet that does not contain an RTP header. In response to the + client's Latching packet, the RTSP server sends back a similar + Latching packet as a confirmation so the client can stop the so- + called "connection phase" of this NAT traversal technique. + Afterwards, the client only has to periodically send Latching 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 the Latching packet. This is valid since + an RTSP server is not expecting RTP traffic from the RTSP client. + Then, it can correlate the Latching packet with the RTSP client's + session ID or the client's SSRC and record the NAT bindings + accordingly. The server then sends a Latching packet as the response + to the client. + + The Latching packet can contain the SSRC to identify the RTP stream, + and care must be taken if the packet is bigger than 12 bytes, + ensuring that it is distinctively different from RTP packets, whose + header size is 12 bytes. + + RTSP signaling can be added to do the following: + + 1. Enable or disable such Latching message exchanges. When the + firewall/NAT has an RTSP-aware ALG, it is possible to disable + Latching message exchange and let the ALG work out the address + and port mappings. + + 2. Configure the number of retries and the retry interval of the + Latching message exchanges. + +4.5.3. ALG Considerations + + See Latching ALG considerations in Section 4.4.3. + +4.5.4. Deployment Considerations + + This approach has the following advantages when compared with the + Latching approach (Section 4.4): + + 1. There is no need to define an RTP payload format for firewall + traversal; therefore, it is more simple to use, implement, and + administer (requirement 4 in Section 3) than a Latching protocol, + which must be defined. + + + + + +Westerlund & Zeng Informational [Page 28] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + 2. When properly defined, this kind of Latching packet exchange can + also authenticate RTP receivers, to prevent hijacking attacks. + + This approach has the following disadvantage when compared with the + Latching approach: + + 1. The server's sender SSRC for the RTP stream or other session + Identity information must be signaled in the RTSP's SETUP + response, in the Transport header of the RTSP SETUP response. + +4.5.5. Security Considerations + + Compared to the security properties of Latching, this variant is + slightly improved. First of all it allows for a larger random field + in the Latching packets, which makes it more unlikely for an off-path + attacker to succeed in a hijack attack. Second, the confirmation + allows the client to know when Latching works and when it doesn't and + thus when to restart the Latching process by updating the SSRC. + + Still, the main security issue remaining is that the RTSP server + can't know that the source address in the Latching packet was coming + from an RTSP client wanting to receive media and not from one that + likes to direct the media traffic to a DoS target. + +4.6. Three-Way Latching + +4.6.1. Introduction + + Three-Way Latching is an attempt to try to resolve the most + significant security issues for both previously discussed variants of + Latching. By adding a server request response exchange directly + after the initial Latching, the server can verify that the target + address present in the Latching packet is an active listener and + confirm its desire to establish a media flow. + +4.6.2. Necessary RTSP Extensions + + Uses the same RTSP extensions as the Alternative Latching method + (Section 4.5) uses. The extensions for this variant are only in the + format and transmission of the Latching packets. + + The client-to-server Latching packet is similar to the Alternative + Latching (Section 4.5), i.e., a UDP packet with some session + identifiers and a random value. When the server responds to the + Latching packet with a Latching confirmation, it includes a random + value (nonce) of its own in addition to echoing back the one the + client sent. Then a third message is added to the exchange. The + client acknowledges the reception of the Latching confirmation + + + +Westerlund & Zeng Informational [Page 29] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + message and echoes back the server's nonce, thus confirming that the + Latched address goes to an RTSP client that initiated the Latching + and is actually present at that address. The RTSP server will refuse + to send any media until the Latching Acknowledgement has been + received with a valid nonce. + +4.6.3. ALG Considerations + + See Latching ALG considerations in Section 4.4.3. + +4.6.4. Deployment Considerations + + A solution with a three-way handshake and its own Latching packets + can be compared with the ICE-based solution (Section 4.3) and have + the following differences: + + o Only works for servers that are not behind a NAT. + + o May be simpler to implement due to the avoidance of the ICE + prioritization and check-board mechanisms. + + However, a Three-Way Latching protocol is very similar to using STUN + in both directions as a Latching and verification protocol. Using + STUN would remove the need for implementing a new protocol. + +4.6.5. Security Considerations + + Three-Way Latching is significantly more secure than its simpler + versions discussed above. The client-to-server nonce, which is + included in signaling and also can be bigger than the 32 bits of + random data that the SSRC field supports, makes it very difficult for + an off-path attacker to perform a DoS attack by diverting the media. + + The client-to-server nonce and its echoing back does not protect + against on-path attackers, including malicious clients. However, the + server-to-client nonce and its echoing back prevents malicious + clients to divert the media stream by spoofing the source address and + port, as it can't echo back the nonce in these cases. This is + similar to the Mobile IPv6 return routability procedure + (Section 5.2.5 of [RFC6275]). + + Three-Way Latching is really only vulnerable to an on-path attacker + that is quite capable. First, the attacker can learn the client- + to-server nonce either by intercepting the signaling or by modifying + the source information (target destination) of a client's Latching + packet. Second, it is also on-path between the server and target + destination and can generate a response using the server's nonce. An + + + + +Westerlund & Zeng Informational [Page 30] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + adversary that has these capabilities is commonly capable of causing + significantly worse damage than this using other methods. + + Three-Way Latching results in the server-to-client packet being + bigger than the client-to-server packet, due to the inclusion of the + server-to-client nonce in addition to the client-to-server nonce. + Thus, an amplification effect does exist; however, to achieve this + amplification effect, the attacker has to create a session state on + the RTSP server. The RTSP server can also limit the number of + responses it will generate before considering the Latching to be + failed. + +4.7. Application Level Gateways + +4.7.1. Introduction + + An ALG reads the application level messages and performs necessary + changes to allow the protocol to work through the middlebox. + However, this behavior has some problems in regards to RTSP: + + 1. It does not work when RTSP 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 middlebox. + + 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 SOHO environments. + +4.7.2. Outline on How ALGs for RTSP Work + + In this section, we provide a step-by-step outline on how one could + 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, + then the ALG should not change the address. Ways to detect that + these methods are used are: + + + + + + +Westerlund & Zeng Informational [Page 31] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + * For embedded STUN, it would be to watch for a feature tag, + like "nat.stun", and to see if any of those exist in the + "supported", "proxy-require", or "require" headers of the RTSP + exchange. + + * For stand-alone STUN and TURN-based solutions: This can 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, then such a solution is in + use. However, if multiple NATs are used, this detection may + fail. Remapping should only be done for addresses belonging + to the NAT's own private address space. + + Otherwise, continue to the next step. + + 3. Create UDP mappings (client given IP and Port <-> external IP and + Port) where needed for all possible transport specifications in + the Transport header of the request found in (step 1). Enter the + external address and port(s) of these mappings in the Transport + header. Mappings shall be created with consecutive external port + numbers 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 the SETUP response settles on RTP over TCP or RTP over RTSP as + lower transport, do nothing: let TCP tunneling take care of NAT + traversal. Otherwise, go to the next step. + + 6. The ALG should keep the UDP mappings belonging to the RTSP + session as long as: an RTSP message with the session's ID has + been sent in the last timeout interval, or a UDP message has been + sent on any of the UDP mappings during the last timeout interval. + + 7. The ALG may remove a mapping as soon as a TEARDOWN response has + been received for that media stream. + +4.7.3. Deployment Considerations + + Advantages: + + o No impact on either client or server. + + o Can work for any type of NATs. + + + +Westerlund & Zeng Informational [Page 32] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + Disadvantages: + + o When deployed, they are hard to update to reflect protocol + modifications and extensions. If not updated, they will break the + functionality. + + o When end-to-end security is used, the ALG functionality will fail. + + o Can interfere with other types of traversal mechanisms, such as + STUN. + + Transition: + + An RTSP ALG will not be phased out in any automatic way. It must be + removed, probably through the removal or update of the NAT it is + associated with. + +4.7.4. Security Considerations + + An ALG will not work with deployment of end-to-end RTSP signaling + security; however, it will work with the hop-by-hop security method + defined in Section 19.3 of RTSP 2.0 [RTSP]. Therefore, deployment of + ALG may result in clients located behind NATs not using end-to-end + security, or more likely the selection of a NAT traversal solution + that allows for security. + + The creation of a UDP mapping based on the signaling message has some + potential security implications. First of all, if the RTSP client + releases its ports and another application is assigned these instead, + it could receive RTP media as long as the mappings exist and the RTSP + server has failed to be signaled or notice the lack of client + response. + + A NAT with RTSP ALG that assigns mappings based on SETUP requests + could potentially become the victim of a resource exhaustion attack. + If an attacker creates a lot of RTSP sessions, even without starting + media transmission, this could exhaust the pool of available UDP + ports on the NAT. Thus, only a limited number of UDP mappings should + be allowed to be created by the RTSP ALG. + +4.8. TCP Tunneling + +4.8.1. 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 + + + +Westerlund & Zeng Informational [Page 33] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + transported over UDP requires the TCP connection to support some type + of framing of the media data packets. Using TCP also results in the + client having to accept that real-time performance can be impacted. + TCP's problem of ensuring timely delivery was one of the reasons why + RTP was developed. Problems that arise with TCP are: head-of-line + blocking, delay introduced by retransmissions, and a highly varying + rate due to the congestion control algorithm. If a sufficient amount + of buffering (several seconds) in the receiving client can be + tolerated, then TCP will clearly work. + +4.8.2. Usage of TCP Tunneling in RTSP + + The RTSP core specification [RTSP] supports interleaving of media + data on the TCP connection that carries RTSP signaling. See + Section 14 in [RTSP] for how to perform this type of TCP tunneling. + There also exists another way of transporting RTP over TCP, which is + defined in Appendix C.2 in [RTSP]. For signaling and rules on how to + establish the TCP connection in lieu of UDP, see Appendix C.2 in + [RTSP]. This is based on the framing of RTP over the TCP connection + as described in [RFC4571]. + +4.8.3. ALG Considerations + + An RTSP ALG will face a different issue with TCP tunneling, at least + the interleaved version. Now the full data stream can end up flowing + through the ALG implementation. Thus, it is important that the ALG + is efficient in dealing with the interleaved media data frames to + avoid consuming to many resources and thus creating performance + issues. + + The RTSP ALG can also affect the transport specifications that + indicate that TCP tunneling can be done and its prioritization, + including removing the transport specification, thus preventing TCP + tunneling. + +4.8.4. Deployment Considerations + + Advantage: + + o Works through all types of NATs where the RTSP server is not NATed + or is at least reachable like it was not. + + Disadvantages: + + o Functionality needs to be implemented on both server and client. + + o Will not always meet multimedia stream's real-time requirements. + + + + +Westerlund & Zeng Informational [Page 34] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + 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 potential price of + loss of real-time properties. + +4.8.5. Security Considerations + + The TCP tunneling of RTP has no known significant security problems + besides those already presented in the RTSP specification. It is + difficult to get any amplification effect for DoS attacks due to + TCP's flow control. The RTSP server's TCP socket, if independently + used for media tunneling or only RTSP messages, can be used for a + redirected syn attack. By spoofing the source address of any TCP + init packets, the TCP SYNs from the server can be directed towards a + target. + + 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. + +4.9. Traversal Using Relays around NAT (TURN) + +4.9.1. Introduction + + TURN [RFC5766] is a protocol for setting up traffic relays that allow + clients behind NATs and firewalls to receive incoming traffic for + both UDP and TCP. These relays are 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 allocation. On the external + side, it is limited to the source address/port pair that have been + given permission by the TURN client creating the allocation. Packets + from any other source on this address will be discarded. + + Using a TURN server makes it possible for an 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 the RTSP message arrives from. + This means that TURN could only be used if the server knows and + + + +Westerlund & Zeng Informational [Page 35] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + accepts that the IP belongs to a TURN server, and the TURN server + can't be targeted at an unknown address. Alternatively, both the + RTSP TCP connection as well as the RTP media is relayed through the + same TURN server. + +4.9.2. 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 the RTSP server. The client + retrieves the session description to determine the number of + media streams. To avoid the issue of having the RTSP connection + and media traffic from different addresses, the TCP connection + must also be done through the same TURN server as the one in the + next step. This will require the usage of TURN for TCP + [RFC6062]. + + 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 contiguous ranges. + + 3. The RTSP client uses the acquired address and port allocations in + the RTSP SETUP request using the destination header. + + 4. The RTSP server sends the SETUP reply, which must include the + Transport header's "src_addr" parameter (source and port in RTSP + 1.0). Note that the server is required to have a mechanism to + verify that it is allowed to send media traffic to the given + address unless TCP relaying of the RTSP messages also is + performed. + + 5. The RTSP client uses the RTSP server's response to create TURN + permissions for the server's media traffic. + + 6. The client requests that the server starts playing. The server + starts sending media packets to the given destination address and + ports. + + 7. Media packets arrive at the TURN server on the external port; if + the packets match an established permission, the TURN server + forwards the media packets to the RTSP client. + + 8. If the client pauses and media is not sent for about 75% of the + mapping timeout, the client should use TURN to refresh the + bindings. + + + + +Westerlund & Zeng Informational [Page 36] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + +4.9.3. ALG Considerations + + As the RTSP client inserts the address information of the TURN + relay's external allocations in the SETUP messages, the ALG that + replaces the address, without considering that the address does not + belong to the internal address realm of the NAT, will prevent this + mechanism from working. This can be prevented by securing the RTSP + signaling. + +4.9.4. Deployment Considerations + + Advantages: + + o Does not require any server modifications given that the server + includes the "src_addr" header in the SETUP response. + + o Works for any type of NAT as long as the RTSP server has a + reachable IP address that is not behind a NAT. + + Disadvantages: + + o Requires another network element, namely the TURN server. + + o A TURN server for RTSP may not scale since the number of sessions + it must forward is proportional to the number of client media + sessions. + + o The TURN server becomes a single point of failure. + + o Since TURN forwards media packets, as a necessity it introduces + delay. + + o 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 phased out completely; see Section 19 of + [RFC5766]. However, the usage of TURN could be reduced when the + demand for having NAT traversal is reduced. + +4.9.5. Security Considerations + + The TURN server can become part of a DoS 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 set up an RTSP session with + media flows going through the TURN server. The attacker is in fact + + + +Westerlund & Zeng Informational [Page 37] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + 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 an RTSP session and start 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. + + This attack requires that the attacker has access to a user account + on the TURN server to be able to set up the TURN mappings. To + prevent this attack, the RTSP server needs to verify that the + ultimate target destination accepts this media stream, which would + require something like ICE's connectivity checks being run between + the RTSP server and the RTSP client. + +5. 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 an + RTSP session to pass through it. Therefore, the firewall will need + an 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 stream packets will be sent. 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. + + The above possibilities for firewalls to inspect and respond to the + signaling are prevented if end-to-end confidentiality protection is + used for the RTSP signaling, e.g., using the specified RTSP over TLS. + As a result, firewalls can't be actively opening pinholes for the + media streams based on the signaling. To enable an RTSP ALG in the + firewall to correctly function, the hop-by-hop signaling security in + RTSP 2.0 can be used (see Section 19.3 of [RTSP]). If not, other + methods have to be used to enable the transport flows for the media. + + 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 NAT. The only difference is + + + +Westerlund & Zeng Informational [Page 38] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + that no address translation is done. 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 + create the necessary filtering state. + +6. Comparison of NAT Traversal Techniques + + This section evaluates the techniques described above against the + requirements listed in Section 3. + + In the following table, the columns correspond to the numbered + requirements. For instance, the column under R1 corresponds to the + first requirement in Section 3: must work for all flavors of NATs. + The rows represent the different NAT/firewall traversal techniques. + Latch is short for Latching, "V. Latch" is short for "variation of + Latching" as described in Section 4.5, and "3-W Latch" is short for + the Three-Way Latching described in Section 4.6. + + A summary of the requirements are: + + R1: Work for all flavors of NATs + + R2: Must work with firewalls, including those with ALGs + + R3: Should have minimal impact on clients not behind NATs, counted in + minimal number of additional RTTs + + R4: Should be simple to use, implement, and administer + + R5: Should provide mitigation against DDoS attacks + + The following considerations are also added to the requirements: + + C1: Will the solution support both clients and servers behind NAT? + + C2: Is the solution robust as NAT behaviors change? + + + + + + + + + + + + + + + +Westerlund & Zeng Informational [Page 39] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + ------------+------+------+------+------+------+------+------+ + | R1 | R2 | R3 | R4 | R5 | C1 | C2 | + ------------+------+------+------+------+------+------+------+ + STUN | No | Yes | 1 | Maybe| No | No | No | + ------------+------+------+------+------+------+------+------+ + Emb. STUN | Yes | Yes | 2 | Maybe| No | No | Yes | + ------------+------+------+------+------+------+------+------+ + ICE | Yes | Yes | 2.5 | No | Yes | Yes | Yes | + ------------+------+------+------+------+------+------+------+ + Latch | Yes | Yes | 1 | Maybe| No | No | Yes | + ------------+------+------+------+------+------+------+------+ + V. Latch | Yes | Yes | 1 | Yes | No | No | Yes | + ------------+------+------+------+------+------+------+------+ + 3-W Latch | Yes | Yes | 1.5 | Maybe| Yes | No | Yes | + ------------+------+------+------+------+------+------+------+ + ALG |(Yes) | Yes | 0 | No | Yes | No | Yes | + ------------+------+------+------+------+------+------+------+ + TCP Tunnel | Yes | Yes | 1.5 | Yes | Yes | No | Yes | + ------------+------+------+------+------+------+------+------+ + TURN | Yes | Yes | 1 | No | Yes |(Yes) | Yes | + ------------+------+------+------+------+------+------+------+ + + Figure 1: Comparison of Fulfillment of Requirements + + Looking at Figure 1, one would draw the conclusion that using TCP + Tunneling or Three-Way Latching are the solutions that best fulfill + the requirements. The different techniques were discussed in the + MMUSIC WG. It was established that the WG would pursue an ICE-based + solution due to its generality and capability of also handling + servers delivering media from behind NATs. TCP Tunneling is likely + to be available as an alternative, due to its specification in the + main RTSP specification. Thus, it can be used if desired, and the + potential downsides of using TCP is acceptable in particular + deployments. When it comes to Three-Way Latching, it is a very + competitive technique given that you don't need support for RTSP + servers behind NATs. There was some discussion in the WG about if + the increased implementation burden of ICE is sufficiently motivated + compared to a the Three-Way Latching solution for this generality. + In the end, the authors believed that the reuse of ICE, greater + flexibility, and any way needed to deploy a new solution were the + decisive factors. + + The ICE-based RTSP NAT traversal solution is specified in "A Network + Address Translator (NAT) Traversal mechanism for media controlled by + Real-Time Streaming Protocol (RTSP)" [RTSP-NAT]. + + + + + + +Westerlund & Zeng Informational [Page 40] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + +7. Security Considerations + + In the preceding sections, we have discussed security merits of the + different NAT/firewall 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 + the 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 without transport- + level security and source authentications, as long as the server does + not allow media to be sent to a different IP address than the RTSP + client request was sent from. + + Using Latching will have a higher risk of session hijacking or DoS + than normal RTSP. The reason is that there exists a probability that + an attacker is able to guess the random bits that the client uses to + prove its identity when creating the address bindings. This can be + solved in the variation of Latching (Section 4.5) with authentication + features. Still, both those variants of Latching are vulnerable + against a deliberate attack from the RTSP client to redirect the + media stream requested to any target assuming it can spoof the source + address. This security vulnerability is solved by performing a + Three-way Latching procedure as discussed in Section 4.6. + + ICE resolves the binding vulnerability of Latching by using signed + STUN messages, as well as requiring that both sides perform + connectivity checks to verify that the target IP address in the + candidate pair is both reachable and willing to respond. ICE can, + however, create a significant amount of traffic if the number of + candidate pairs are large. Thus, pacing is required and + implementations should attempt to limit their number of candidates to + reduce the number of packets. + + If the signaling between the ICE peers (RTSP client and server) is + not confidentiality and integrity protected, ICE is vulnerable to + attacks where the candidate list is manipulated. The lack of + signaling security will also simplify spoofing of STUN binding + messages by revealing the secret used in signing. + + The usage of an RTSP ALG does not in itself increase the risk for + session hijacking. However, the deployment of ALGs as the sole + mechanism for RTSP NAT traversal will prevent deployment of end- + to-end encrypted RTSP signaling. + + + + + + +Westerlund & Zeng Informational [Page 41] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + 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 severe risk of DoS 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. + + Since Latching and the variants of Latching have such big security + issues, they should not be used at all. Three-Way Latching as well + as ICE mitigates these security issues and performs the important + return-routability checks that prevent spoofed source addresses, and + they should be recommended for that reason. RTP ALGs are a security + risk as they can create an incitement against using secure RTSP + signaling. That can be avoided as ALGs require trust in the + middlebox, and that trust becomes explicit if one uses the hop-by-hop + security solution as specified in Section 19.3 of RTSP 2.0. [RTSP]. + The remaining methods can be considered safe enough, assuming that + the appropriate security mechanisms are used and not ignored. + +8. Informative References + + [NICE] Libnice, "The GLib ICE implementation", June 2015, + <http://nice.freedesktop.org/wiki/>. + + [PJNATH] "PJNATH - Open Source ICE, STUN, and TURN Library", May + 2013, <http://www.pjsip.org/pjnath/docs/html/>. + + [RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, + DOI 10.17487/RFC0768, August 1980, + <http://www.rfc-editor.org/info/rfc768>. + + [RFC793] Postel, J., "Transmission Control Protocol", STD 7, + RFC 793, DOI 10.17487/RFC0793, September 1981, + <http://www.rfc-editor.org/info/rfc793>. + + [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time + Streaming Protocol (RTSP)", RFC 2326, + DOI 10.17487/RFC2326, April 1998, + <http://www.rfc-editor.org/info/rfc2326>. + + [RFC2588] Finlayson, R., "IP Multicast and Firewalls", RFC 2588, + DOI 10.17487/RFC2588, May 1999, + <http://www.rfc-editor.org/info/rfc2588>. + + + + + +Westerlund & Zeng Informational [Page 42] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address + Translator (NAT) Terminology and Considerations", + RFC 2663, DOI 10.17487/RFC2663, August 1999, + <http://www.rfc-editor.org/info/rfc2663>. + + [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network + Address Translator (Traditional NAT)", RFC 3022, + DOI 10.17487/RFC3022, January 2001, + <http://www.rfc-editor.org/info/rfc3022>. + + [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, + A., Peterson, J., Sparks, R., Handley, M., and + E. Schooler, "SIP: Session Initiation Protocol", + RFC 3261, DOI 10.17487/RFC3261, June 2002, + <http://www.rfc-editor.org/info/rfc3261>. + + [RFC3424] Daigle, L., Ed. and IAB, "IAB Considerations for + UNilateral Self-Address Fixing (UNSAF) Across Network + Address Translation", RFC 3424, DOI 10.17487/RFC3424, + November 2002, <http://www.rfc-editor.org/info/rfc3424>. + + [RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, + "STUN - Simple Traversal of User Datagram Protocol (UDP) + Through Network Address Translators (NATs)", RFC 3489, + DOI 10.17487/RFC3489, March 2003, + <http://www.rfc-editor.org/info/rfc3489>. + + [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. + Jacobson, "RTP: A Transport Protocol for Real-Time + Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, + July 2003, <http://www.rfc-editor.org/info/rfc3550>. + + [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session + Description Protocol", RFC 4566, DOI 10.17487/RFC4566, + July 2006, <http://www.rfc-editor.org/info/rfc4566>. + + [RFC4571] Lazzaro, J., "Framing Real-time Transport Protocol (RTP) + and RTP Control Protocol (RTCP) Packets over Connection- + Oriented Transport", RFC 4571, DOI 10.17487/RFC4571, July + 2006, <http://www.rfc-editor.org/info/rfc4571>. + + [RFC4787] Audet, F., Ed. and C. Jennings, "Network Address + Translation (NAT) Behavioral Requirements for Unicast + UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January + 2007, <http://www.rfc-editor.org/info/rfc4787>. + + + + + + +Westerlund & Zeng Informational [Page 43] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + [RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)", + BCP 131, RFC 4961, DOI 10.17487/RFC4961, July 2007, + <http://www.rfc-editor.org/info/rfc4961>. + + [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment + (ICE): A Protocol for Network Address Translator (NAT) + Traversal for Offer/Answer Protocols", RFC 5245, + DOI 10.17487/RFC5245, April 2010, + <http://www.rfc-editor.org/info/rfc5245>. + + [RFC5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and + P. Srisuresh, "NAT Behavioral Requirements for TCP", + BCP 142, RFC 5382, DOI 10.17487/RFC5382, October 2008, + <http://www.rfc-editor.org/info/rfc5382>. + + [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, + "Session Traversal Utilities for NAT (STUN)", RFC 5389, + DOI 10.17487/RFC5389, October 2008, + <http://www.rfc-editor.org/info/rfc5389>. + + [RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer + Security (DTLS) Extension to Establish Keys for the + Secure Real-time Transport Protocol (SRTP)", RFC 5764, + DOI 10.17487/RFC5764, May 2010, + <http://www.rfc-editor.org/info/rfc5764>. + + [RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal + Using Relays around NAT (TURN): Relay Extensions to + Session Traversal Utilities for NAT (STUN)", RFC 5766, + DOI 10.17487/RFC5766, April 2010, + <http://www.rfc-editor.org/info/rfc5766>. + + [RFC6062] Perreault, S., Ed. and J. Rosenberg, "Traversal Using + Relays around NAT (TURN) Extensions for TCP Allocations", + RFC 6062, DOI 10.17487/RFC6062, November 2010, + <http://www.rfc-editor.org/info/rfc6062>. + + [RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for + Keeping Alive the NAT Mappings Associated with RTP / RTP + Control Protocol (RTCP) Flows", RFC 6263, + DOI 10.17487/RFC6263, June 2011, + <http://www.rfc-editor.org/info/rfc6263>. + + [RFC6275] Perkins, C., Ed., Johnson, D., and J. Arkko, "Mobility + Support in IPv6", RFC 6275, DOI 10.17487/RFC6275, July + 2011, <http://www.rfc-editor.org/info/rfc6275>. + + + + + +Westerlund & Zeng Informational [Page 44] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + + [RFC7362] Ivov, E., Kaplan, H., and D. Wing, "Latching: Hosted NAT + Traversal (HNT) for Media in Real-Time Communication", + RFC 7362, DOI 10.17487/RFC7362, September 2014, + <http://www.rfc-editor.org/info/rfc7362>. + + [RTP-NO-OP] Andreasen, F., "A No-Op Payload Format for RTP", Work in + Progress, draft-ietf-avt-rtp-no-op-04, May 2007. + + [RTSP] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., + and M. Stiemerling, "Real Time Streaming Protocol 2.0 + (RTSP)", Work in Progress, + draft-ietf-mmusic-rfc2326bis-40, February 2014. + + [RTSP-NAT] Goldberg, J., Westerlund, M., and T. Zeng, "A Network + Address Translator (NAT) Traversal Mechanism for Media + Controlled by Real-Time Streaming Protocol (RTSP)", Work + in Progress, draft-ietf-mmusic-rtsp-nat-22, July 2014. + + [STUN-IMPL] "Open Source STUN Client and Server", May 2013, + <http://sourceforge.net/projects/stun/>. + +Acknowledgements + + The authors would also like to thank all persons on the MMUSIC + working group's mailing list that have commented on this document. + Persons having contributed to this protocol, in no special order, + are: Jonathan Rosenberg, Philippe Gentric, Tom Marshall, David Yon, + Amir Wolf, Anders Klemets, Flemming Andreasen, Ari Keranen, Bill + Atwood, Alissa Cooper, Colin Perkins, Sarah Banks, David Black, and + Alvaro Retana. Thomas Zeng would also like to give special thanks to + Greg Sherwood of PacketVideo for his input into this memo. + + Section 1.1 contains text originally written for RFC 4787 by Francois + Audet and Cullen Jennings. + + + + + + + + + + + + + + + + + +Westerlund & Zeng Informational [Page 45] + +RFC 7604 Evaluation of NAT Traversal for RTSP September 2015 + + +Authors' Addresses + + Magnus Westerlund + Ericsson + Farogatan 6 + Stockholm SE-164 80 + Sweden + + Phone: +46 8 719 0000 + Email: magnus.westerlund@ericsson.com + + + Thomas Zeng + PacketVideo Corp + + Email: thomas.zeng@gmail.com + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Westerlund & Zeng Informational [Page 46] + |