--- 1/draft-ietf-mmusic-rtsp-nat-evaluation-02.txt 2011-03-14 14:14:57.000000000 +0100 +++ 2/draft-ietf-mmusic-rtsp-nat-evaluation-03.txt 2011-03-14 14:14:57.000000000 +0100 @@ -1,286 +1,283 @@ Network Working Group M. Westerlund Internet-Draft Ericsson Intended status: Informational T. Zeng -Expires: July 24, 2010 January 20, 2010 +Expires: September 15, 2011 March 14, 2011 - The evaluation of different NAT traversal Techniques for media - controlled by Real-time Streaming Protocol (RTSP) - draft-ietf-mmusic-rtsp-nat-evaluation-02 + The Evaluation of Different Network Addres Translator (NAT) Traversal + Techniques for Media Controlled by Real-time Streaming Protocol (RTSP) + draft-ietf-mmusic-rtsp-nat-evaluation-03 Abstract - This document describes several NAT traversal techniques that could - be used by RTSP. Each technique includes a description on how it - would be used, the security implications of using it and any other - deployment considerations it has. There are also disussions on how - NAT traversal techniques relates to firewalls and how each technique - can be applied in different use cases. These findings where used - when selecting the NAT traversal for RTSP solution to standardize in - the MMUSIC WG. - -Requirements Language - - The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", - "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this - document are to be interpreted as described in RFC 2119 [RFC2119]. + This document describes several Network Address Translator (NAT) + traversal techniques that was considered to be used by Real-time + Streaming Protocol (RTSP). Each technique includes a description on + how it would be used, the security implications of using it and any + other deployment considerations it has. There are also disussions on + how NAT traversal techniques relates to firewalls and how each + technique can be applied in different use cases. These findings + where used when selecting the NAT traversal for RTSP 2.0 standardized + in the MMUSIC WG. Status of this Memo - This Internet-Draft is submitted to IETF in full conformance with the + This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering - Task Force (IETF), its areas, and its working groups. Note that - other groups may also distribute working documents as Internet- - Drafts. + Task Force (IETF). Note that other groups may also distribute + working documents as Internet-Drafts. The list of current Internet- + Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - The list of current Internet-Drafts can be accessed at - http://www.ietf.org/ietf/1id-abstracts.txt. - - The list of Internet-Draft Shadow Directories can be accessed at - http://www.ietf.org/shadow.html. - - This Internet-Draft will expire on July 24, 2010. + This Internet-Draft will expire on September 15, 2011. Copyright Notice - Copyright (c) 2010 IETF Trust and the persons identified as the + Copyright (c) 2011 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 BSD License. + described in the Simplified BSD License. This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English. Table of Contents - 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 - 1.1. Network Address Translators . . . . . . . . . . . . . . . 5 - 1.2. Firewalls . . . . . . . . . . . . . . . . . . . . . . . . 5 - 1.3. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . 6 - 2. Detecting the loss of NAT mappings . . . . . . . . . . . . . . 7 - 3. Requirements on NAT-Traversal . . . . . . . . . . . . . . . . 7 - 4. NAT Traversal Techniques . . . . . . . . . . . . . . . . . . . 9 - 4.1. STUN . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 - 4.1.1. Introduction . . . . . . . . . . . . . . . . . . . . . 9 + 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 + 1.1. Network Address Translators . . . . . . . . . . . . . . . 6 + 1.2. Firewalls . . . . . . . . . . . . . . . . . . . . . . . . 7 + 1.3. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . 7 + 1.4. Definitions . . . . . . . . . . . . . . . . . . . . . . . 8 + 2. Detecting the loss of NAT mappings . . . . . . . . . . . . . . 8 + 3. Requirements on NAT-Traversal . . . . . . . . . . . . . . . . 9 + 4. NAT Traversal Techniques . . . . . . . . . . . . . . . . . . . 10 + 4.1. STUN . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 + 4.1.1. Introduction . . . . . . . . . . . . . . . . . . . . . 11 4.1.2. Using STUN to traverse NAT without server - modifications . . . . . . . . . . . . . . . . . . . . 10 - 4.1.3. Embedding STUN in RTSP . . . . . . . . . . . . . . . . 12 - 4.1.4. Discussion On Co-located STUN Server . . . . . . . . . 13 - 4.1.5. ALG considerations . . . . . . . . . . . . . . . . . . 13 - 4.1.6. Deployment Considerations . . . . . . . . . . . . . . 14 - 4.1.7. Security Considerations . . . . . . . . . . . . . . . 16 - 4.2. ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 - 4.2.1. Introduction . . . . . . . . . . . . . . . . . . . . . 17 - 4.2.2. Using ICE in RTSP . . . . . . . . . . . . . . . . . . 17 - 4.2.3. Implementation burden of ICE . . . . . . . . . . . . . 19 - 4.2.4. Deployment Considerations . . . . . . . . . . . . . . 19 - 4.2.5. Security Consideration . . . . . . . . . . . . . . . . 20 - 4.3. Symmetric RTP . . . . . . . . . . . . . . . . . . . . . . 20 - 4.3.1. Introduction . . . . . . . . . . . . . . . . . . . . . 20 - 4.3.2. Necessary RTSP extensions . . . . . . . . . . . . . . 21 - 4.3.3. Deployment Considerations . . . . . . . . . . . . . . 21 - 4.3.4. Security Consideration . . . . . . . . . . . . . . . . 22 - 4.3.5. A Variation to Symmetric RTP . . . . . . . . . . . . . 23 - 4.4. Application Level Gateways . . . . . . . . . . . . . . . . 25 - 4.4.1. Introduction . . . . . . . . . . . . . . . . . . . . . 25 - 4.4.2. Outline On how ALGs for RTSP work . . . . . . . . . . 25 - 4.4.3. Deployment Considerations . . . . . . . . . . . . . . 26 - 4.4.4. Security Considerations . . . . . . . . . . . . . . . 27 - 4.5. TCP Tunneling . . . . . . . . . . . . . . . . . . . . . . 27 - 4.5.1. Introduction . . . . . . . . . . . . . . . . . . . . . 27 - 4.5.2. Usage of TCP tunneling in RTSP . . . . . . . . . . . . 27 - 4.5.3. Deployment Considerations . . . . . . . . . . . . . . 27 - 4.5.4. Security Considerations . . . . . . . . . . . . . . . 28 - 4.6. TURN (Traversal Using Relay NAT) . . . . . . . . . . . . . 28 - 4.6.1. Introduction . . . . . . . . . . . . . . . . . . . . . 28 - 4.6.2. Usage of TURN with RTSP . . . . . . . . . . . . . . . 29 - 4.6.3. Deployment Considerations . . . . . . . . . . . . . . 30 - 4.6.4. Security Considerations . . . . . . . . . . . . . . . 30 - 5. Firewalls . . . . . . . . . . . . . . . . . . . . . . . . . . 31 - 6. Comparision of NAT traversal techniques . . . . . . . . . . . 32 - 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33 - 8. Security Considerations . . . . . . . . . . . . . . . . . . . 33 - 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34 - 10. Informative References . . . . . . . . . . . . . . . . . . . . 34 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 36 + modifications . . . . . . . . . . . . . . . . . . . . 11 + 4.1.3. Embedding STUN in RTSP . . . . . . . . . . . . . . . . 13 + 4.1.4. Discussion On Co-located STUN Server . . . . . . . . . 14 + 4.1.5. ALG considerations . . . . . . . . . . . . . . . . . . 15 + 4.1.6. Deployment Considerations . . . . . . . . . . . . . . 15 + 4.1.7. Security Considerations . . . . . . . . . . . . . . . 17 + 4.2. ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 + 4.2.1. Introduction . . . . . . . . . . . . . . . . . . . . . 18 + 4.2.2. Using ICE in RTSP . . . . . . . . . . . . . . . . . . 19 + 4.2.3. Implementation burden of ICE . . . . . . . . . . . . . 20 + 4.2.4. Deployment Considerations . . . . . . . . . . . . . . 21 + 4.2.5. Security Consideration . . . . . . . . . . . . . . . . 21 + 4.3. Symmetric RTP . . . . . . . . . . . . . . . . . . . . . . 21 + 4.3.1. Introduction . . . . . . . . . . . . . . . . . . . . . 22 + 4.3.2. Necessary RTSP extensions . . . . . . . . . . . . . . 22 + 4.3.3. Deployment Considerations . . . . . . . . . . . . . . 22 + 4.3.4. Security Consideration . . . . . . . . . . . . . . . . 23 + 4.3.5. A Variation to Symmetric RTP . . . . . . . . . . . . . 24 + 4.4. Application Level Gateways . . . . . . . . . . . . . . . . 26 + 4.4.1. Introduction . . . . . . . . . . . . . . . . . . . . . 26 + 4.4.2. Outline On how ALGs for RTSP work . . . . . . . . . . 27 + 4.4.3. Deployment Considerations . . . . . . . . . . . . . . 28 + 4.4.4. Security Considerations . . . . . . . . . . . . . . . 28 + 4.5. TCP Tunneling . . . . . . . . . . . . . . . . . . . . . . 28 + 4.5.1. Introduction . . . . . . . . . . . . . . . . . . . . . 28 + 4.5.2. Usage of TCP tunneling in RTSP . . . . . . . . . . . . 29 + 4.5.3. Deployment Considerations . . . . . . . . . . . . . . 29 + 4.5.4. Security Considerations . . . . . . . . . . . . . . . 29 + 4.6. TURN (Traversal Using Relay NAT) . . . . . . . . . . . . . 30 + 4.6.1. Introduction . . . . . . . . . . . . . . . . . . . . . 30 + 4.6.2. Usage of TURN with RTSP . . . . . . . . . . . . . . . 30 + 4.6.3. Deployment Considerations . . . . . . . . . . . . . . 31 + 4.6.4. Security Considerations . . . . . . . . . . . . . . . 32 + 5. Firewalls . . . . . . . . . . . . . . . . . . . . . . . . . . 33 + 6. Comparision of NAT traversal techniques . . . . . . . . . . . 33 + 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34 + 8. Security Considerations . . . . . . . . . . . . . . . . . . . 34 + 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35 + 10. Informative References . . . . . . . . . . . . . . . . . . . . 35 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37 1. Introduction Today there is a proliferate deployment of different flavors of Network Address Translator (NAT) boxes that in many cases only loosely follows standards[RFC3022][RFC2663][RFC3424]]. NATs cause discontinuity in address realms [RFC3424], therefore an application - protocol, such as 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 TCP for example, may not be blocked - by NATs, its media streams may be blocked by NATs. This will occur + protocol, such as Real-time Streaming Protocol (RTSP) + [RFC2326][I-D.ietf-mmusic-rfc2326bis], 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 Transmission + Control Protocol (TCP) [RFC0793] for example, may not be 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 (FWs) are also middle boxes that need to be considered. Firewalls helps 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 to go through with minimal implementation effort. The minimal effort is to - implement an ALG (Application Level Gateway) to interpret RTSP + implement an Application Level Gateway (ALG) to interpret RTSP parameters. There is also a large class of firewalls, commonly home - FWs, that uses a similar filtering behavior to what NAT has. This - type of firewalls can be handled using the same solution as employed - for NAT traversal instead of relying on ALGs. + firewalls, that uses a similar filtering behavior to what NAT has. + This type of firewalls can be handled using the same solution as + employed for NAT traversal instead of relying on ALGs. This document describes several NAT-traversal mechanisms for RTSP controlled media streaming. These NAT solutions fall into the - category of ""UNilateral Self-Address Fixing (UNSAF)" as defined in + 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 protocol extensions for each method, transition strategies, and security concerns. This document is capturing the evaluation done in the process to recommend FW/NAT traversal methods for RTSP streaming servers based on RFC 2326 [RFC2326] as well as the RTSP 2.0 core spec [I-D.ietf-mmusic-rfc2326bis]. The evaluation is focused on NAT - traversal for the media streams carried over UDP [RFC0768]. Where - 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. + traversal for the media streams carried over User Datagram Protocol + (UDP) [RFC0768]. Where Real-time Transport Protocol (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. 1.1. Network Address Translators - Readers are urged to refer to [RFC2663] for information on NAT + Readers are urged to refer to "IP Network Address Translator (NAT) + Terminology and Considerations" [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 + NAT device deployed. Readers may refer to "Traditional IP Network + Address Translator (Traditional NAT)" [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, + establishes a NAT mapping 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. 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. There exist a number of address and port mapping behaviors described in more detail in Section 4.1 of - [RFC4787]. + "Network Address Translation (NAT) Behavioral Requirements for + Unicast UDP" [RFC4787]. NATs also have a filtering behavior on traffic arriving on the external side. Such behavior effects 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. + NAT traversal works through these NATs. See Section 5 of "Network + Address Translation (NAT) Behavioral Requirements for Unicast UDP" + [RFC4787] for more information on the different types of filtering + that have been identified. 1.2. Firewalls A firewall (FW) is a security gateway that enforces certain access control policies between two network administrative domains: a - private domain (intranet) and a public domain (public Internet). - Many organizations use firewalls to prevent privacy intrusions and - malicious attacks to corporate computing resources in the private - intranet [RFC2588]. + private domain (intranet) and a external domain, e.g. public + Internet. Many organizations use firewalls to prevent privacy + intrusions and malicious attacks to corporate computing resources in + the private intranet [RFC2588]. A comparison between NAT and FW is given below: 1. A firewall must sit between two network administrative domains, - while NAT does not have to sit between two domains. In fact, - there exist cases when in corporations there are many NAT boxes - within the intranet, in which case the NAT boxes sit between - subnets. + while NAT does not have to sit between two domains. 2. NAT does not in itself provide security, although some access control policies can be implemented using address translation schemes. The inherent filtering behaviours are commonly mistaken for real security policies. It should be noted that many NAT devices intended for small office/ home office (SOHO) include both NATs and firewall functionality. In the rest of this memo we use the phrase "NAT traversal" interchangeably with "FW traversal", "NAT/FW traversal" and "NAT/ Firewall traversal". 1.3. Glossary ALG: Application Level Gateway, an entity that can be embedded in a NAT or other middlebox to perform the application layer functions required for a particular protocol to traverse the NAT/middlebox. - ICE: Interactive Connectivity Establishment, see - [I-D.ietf-mmusic-ice]. + ICE: Interactive Connectivity Establishment, see [RFC5245]. DNS: Domain Name Service DDOS: Distributed Denial Of Service attacks - MID: Media Identifier from Grouping of media lines in SDP, see - [RFC3388]. - 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 [I-D.ietf-mmusic-rfc2326bis]. SDP: Session Description Protocol, see [RFC4566]. SSRC: Synchronization source in RTP, see [RFC3550]. +1.4. Definitions + + The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", + "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and + "OPTIONAL" in this document are to be interpreted as described in RFC + 2119 [RFC2119]. + 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 RFC3424. @@ -304,37 +301,37 @@ reports. The sender report carries a field telling how many packets the server has sent. If that has increased and no RTP packets has arrived for a few seconds it is likely the RTP mapping has been removed. The loss of mapping for RTCP is simpler to detect. RTCP is normally sent periodically in each direction, even during the RTSP ready state. If RTCP packets are missing for several RTCP intervals, the mapping is likely to be lost. Note that if neither RTCP packets nor RTSP messages are received by the RTSP server for a while, the RTSP - server has the option to delete the corresponding SSRC and RTSP - session ID, because either the client can not get through a middle - box NAT/FW, or that the client is mal-functioning. + server has the option to delete the corresponding RTP session, SSRC + and RTSP session ID, because either the client can not get through a + middle box NAT/FW, or that the client is mal-functioning. 3. Requirements on NAT-Traversal This section considers the set of requirements for the evaulation of RTSP NAT traversal solutions. RTSP is a client-server protocol. Typically services providers deploy RTSP servers in the public address realm. However, there are use cases where the reverse is true: RTSP clients are connecting from public address realm to RTSP servers behind home NATs. This is the case for instance when home surveillance cameras running as RTSP servers intend to stream video to cell phone users in the public address realm through a home NAT. In terms of requirements, the - first requirement shoulb be to solve the RTSP NAT traversal problem + first requirement should be to solve the RTSP NAT traversal problem for RTSP servers deployed in a public network, i.e. no NAT at the server side. 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 restricted filtering. 2. MUST work for firewalls (subject to pertinent firewall @@ -345,90 +342,87 @@ protocol (e.g. RTP) are implemented on different computers with different IP addresses. * For instance, no extra delay from RTSP connection till arrival of media 4. SHOULD be simple to use/implement/administer that people actually turn them on * Otherwise people will resort to TCP tunneling through NATs - * Address discovery for NAT traversal should take place behind the scene, if possible 5. SHOULD authenticate dual-hosted client transport handler to prevent DDOS attacks. The last requirement addresses the Distributed Denial-Of-Service (DDOS) threat, which relates to NAT traversal as explained below. - During NAT traversal, when the RTSP server performs address - translation on a client, the result may be that the public IP address - of the RTP receiver host is different than the public IP address of - the RTSP client host. This posts a DDOS threat that has significant - amplification potentials because the RTP media streams in general - consist of large number of IP packets. DDOS attacks occur if the - attacker fakes the messages in the NAT traversal mechanism to trick - the RTSP server into believing that the client's RTP receiver is - located in a separate host. For example, user A may use his RTSP - client to direct the RTSP server to send video RTP streams to - target.example.com in order to degrade the services provided by - target.example.com. Note a simple preventative measure is for the - RTSP server to disallow the cases where the client's RTP receiver has - a different public IP address than that of the RTSP client. However, - 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. - Message authentication is a big challenge in the current wired and - wireless networking environment. It may be necessary in the - immediate future to deploy NAT traversal solutions that do not have - full message authentication, but provide upgrade path to add - authentication features in the future. + During NAT traversal, when the RTSP server determines the media + destination (Address and port) for client, the result may be that the + public IP address of the RTP receiver host is different than the + public IP address of the RTSP client host. This posts a DDOS threat + that has significant amplification potentials because the RTP media + streams in general consist of large number of IP packets. DDOS + attacks occur if the attacker fakes the messages in the NAT traversal + mechanism to trick the RTSP server into believing that the client's + RTP receiver is located in a separate host. For example, user A may + use his RTSP client to direct the RTSP server to send video RTP + streams to target.example.com in order to degrade the services + provided by target.example.com. Note a simple preventative measure + is for the RTSP server to disallow the cases where the client's RTP + receiver has a different public IP address than that of the RTSP + client. However, 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. Message authentication is a big challenge in the + current wired and wireless networking environment. It may be + necessary in the immediate future to deploy NAT traversal solutions + that do not have full message authentication, but provide upgrade + path to add authentication features in the future. 4. NAT Traversal Techniques There exist a number of potential NAT traversal techniques that can be used to allow RTSP to traverse NATs. They have different features and are applicable to different topologies; their cost is also different. They also vary in security levels. In the following sections, each technique is outlined in details with discussions on the corresponding advantages and disadvantages. This section includes NAT traversal techniques that have not been formally specified anywhere else. The overview section of this document may be the only publicly available specification of some of the NAT traversal techniques. However that is no real barrier against doing an evaluation of the NAT traversal technique. Some other techniques are currently (at the time of writing) in a state of flux due to ongoing standardization work on these techniques, e.g. - ICE [I-D.ietf-mmusic-ice], STUN [RFC5389] and RTP No-Op - [I-D.ietf-avt-rtp-no-op]. + RTP No-Op [I-D.ietf-avt-rtp-no-op]. 4.1. STUN 4.1.1. Introduction STUN - "Simple Traversal of UDP Through Network Address Translators" [RFC3489][RFC5389] is a standardized protocol that allows a client to use secure means to discover the presence of a NAT between himself - and the STUN server and the type of that NAT. The client then uses - the STUN server to discover the address bindings assigned by the NAT. - - STUN is a client-server protocol. STUN client sends a request to a - STUN server and the server returns a response. There are two types - of STUN requests - Binding Requests, sent over UDP, and Shared Secret - Requests, sent over TLS over TCP. + and the STUN server. The client uses the STUN server to discover the + address mappings assigned by the NAT. STUN is a client-server + protocol. STUN client sends a request to a STUN server and the + server returns a response. There are two types of STUN requests - + Binding Requests, sent over UDP, and Shared Secret Requests, sent + over TLS over TCP. - STUN is in the process of being updated by the BEHAVE WG to address - issues found during usage. The BEHAVE WG intends to integrate it - better with TURN [I-D.ietf-behave-turn]. + The first version of STUN [RFC3489] included categorization and + parameterization of NATs. This was abandoned in the updated version + due to it being unreliable. 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). Limitations: @@ -489,22 +483,22 @@ that the payload type number must be dynamically assigned through RTSP first. Otherwise STUN could be used for the keep-alive as well as empty UDP packets. 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 a - "Address Dependent" filtering NAT, the client must send the very - first UDP packet to punch a hole in the NAT. The client, before + "Address Dependent" filtering NAT, the client must first send a UDP + packet to establish filtering state in the NAT. The client, before sending a RTSP PLAY request, must send a so called FW packet (such as a RTP No-Op packet) on each mapping, to the IP address given as the servers source address. To create minimum problems for the server these UDP packets SHOULD be sent to the server's discard port (port number 9). Since UDP packets are inherently unreliable, to ensure that at least one UDP message passes the NAT, FW packets should be retransmitted a reasonable number of times. For a "Address and Port Dependent" filtering NAT the client must send messages to the exact ports used by the server to send UDP packets @@ -512,40 +506,40 @@ the above described process with the following additional restrictions: for each port mapping, FW packets need to be sent first to the server's source address/port. To minimize potential effects on the server from these messages the following type of FW packets MUST be sent. RTP: an empty or less than 12 bytes UDP packet. RTCP: A correctly formatted RTCP RR or SR message. The above described adaptations for restricted NATs will not work unless the server includes the "src_addr" in the "Transport" header (which is the "source" transport parameter in RFC2326). + This method is also brittle because it relies on that one can use + STUN to classify the NAT behavior. If the NAT changes the properties + of the existing mapping and filtering state for example due to load, + then the methods will fail. + 4.1.3. Embedding STUN in RTSP This section outlines the adaptation and embedding of STUN within RTSP. This enables STUN to be used to traverse any type of NAT, - including symmetric NATs. This would require protocol changes which - are beyond the scope of this memo. + including symmetric NATs. This would require protocol changes. This NAT traversal solution has limitations: 1. It does not work if both RTSP client and RTSP server are behind separate NATs. 2. The RTSP server may, for security reasons, refuse to send media streams to an IP different from the IP in the client RTSP requests. - Therefore, if the client is behind a NAT that has multiple public - addresses, and the client's RTSP port and UDP port are mapped to - different IP addresses, RTSP SETUP may fail. - Deviations from STUN as defined in RFC 3489: 1. We allow RTSP applications to have the option to perform STUN "Shared Secret Request" through RTSP, via extension to RTSP; 2. We require STUN server to be co-located on RTSP server's media output ports. In order to allow binding discovery without authentication, the STUN server embedded in RTSP application must ignore authentication tag, @@ -585,61 +579,71 @@ client) will be required to send and receive RTCP packets from the same port. 4.1.4. 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 de- multiplexing problem: we must be able to differentiate a STUN packet from a media packet. This will be done based on heuristics. A - common heuristics is the frist byte in the packet, which works fine + common heuristics is the first byte in the packet, which works fine between STUN and RTP or RTCP where the first byte happens to be different, but may not work as well with other media transport protocols. 4.1.5. ALG considerations If a NAT supports RTSP ALG (Application Level Gateway) and is not aware of the STUN traversal option, service failure may happen, because a client discovers its public IP address and port numbers, and inserts them in its SETUP requests, when the RTSP ALG processes the SETUP request it may change the destination and port number, - resulting in unpredictable behavior. In such cases two alternatives - exist: + resulting in unpredictable behavior. An ALG should not update + address fields which contains addresses other than the NATs internal + address domain. In cases where the ALG modifies fields unnecessary + two alternatives exist: 1. The usage of TLS to encrypt the RTSP TCP connection to prevent the ALG from reading and modifying the RTSP messages. - 2. To turn of the STUN based NAT traversal mechanism + 2. To 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. 4.1.6. Deployment Considerations For the non-embedded usage of STUN the following applies: Advantages: + o STUN is a solution first used by SIP applications. As shown + above, with little or no changes, RTSP application can re-use STUN + as a NAT traversal solution, avoiding the pit-fall of solving a + problem twice. + o Using STUN does not require RTSP server modifications; it only affects the client implementation. Disadvantages: o Requires a STUN server deployed in the public address space. o Only works with NATs that perform endpoint independent and address dependent mappings. Port and address dependent filtering NATs create some issues. + o Brittle to NATs changing the properties of the NAT mapping and + filtering. + o Does not work with port and address dependent mapping NATs without server modifications. o Will mostly not work if a NAT uses multiple IP addresses, since RTSP server generally requires all media streams to use the same IP as used in the RTSP connection to prevent becoming a DDOS tool. o Interaction problems exist when a RTSP-aware ALG interferes with the use of STUN for NAT traversal unless TLS secured RTSP message exchange is used. @@ -688,24 +692,23 @@ o Some extensions to the RTSP core protocol, signaled by RTSP feature tags, must be introduced. o Requires an embedded STUN server to co-locate on each of RTSP server's media protocol's ports (e.g. RTP and RTCP ports), which means more processing is required to de-multiplex STUN packets from media packets. For example, the de-multiplexer must be able to differentiate a RTCP RR packet from a STUN packet, and forward the former to the streaming server, the later to STUN server. - o Even if the RTSP server is in the open, and the client is behind a - multi-addressed NAT, it may still break if the RTSP server does - not allow RTP packets to be sent to an IP differs from the IP of - the client's RTSP request. + o Does not support use cases that requires the RTSP connection and + the media reception to happen at different addresses, unless the + servers sequrity policy is relaxed. o Interaction problems exist when a 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, and they both agree to support the NAT traversal feature. o Increases the setup delay with at least the amount of time it takes to perform STUN message exchanges. Most likely an extra @@ -722,44 +725,42 @@ 4.1.7. Security Considerations To prevent RTSP server being used as Denial of Service (DoS) attack tools the RTSP Transport header parameter "destination" and "dest_addr" are generally not allowed to point to any IP address other than the one that RTSP message originates from. The RTSP server is only prepared to make an exception of this rule when the client is trusted (e.g., through the use of a secure authentication process, or through some secure method of challenging the destination to verify its willingness to accept the RTP traffic). Such - restriction means that STUN does not work for NATs that would assign - different IP addresses to different UDP flows on its public side. - Therefore the multi-addressed NATs will at times have trouble with - STUN-based RTSP NAT traversals. + restriction means that STUN does not work for use cases where RTSP + and media transport goes to different address. In terms of security property, STUN combined with destination address restricted RTSP has the same security properties as the core RTSP. It is protected from being used as a DoS attack tool unless the attacker has ability the to spoof the TCP connection carrying RTSP messages. Using STUN's support for message authentication and secure transport of RTSP messages, attackers cannot modify STUN responses or RTSP messages to change media destination. This protects against hijacking, however as a client can be the initiator of an attack, these mechanisms cannot securely prevent RTSP servers being used as DoS attack tools. 4.2. ICE 4.2.1. Introduction - ICE (Interactive Connectivity Establishment) [I-D.ietf-mmusic-ice] is - a methodology for NAT traversal that has been developed for SIP using + ICE (Interactive Connectivity Establishment) [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 parallel fashion, all possible connection addresses that an end point may have. This allows the end-point to use the best available UDP "connection" (meaning two UDP end-points capable of reaching each other). The methodology has very nice properties in that basically all NAT topologies are possible to traverse. Here is how ICE works on a high level. End point A collects all possible address that can be used, including local IP addresses, STUN derived addresses, TURN addresses, etc. On each local port that any @@ -834,58 +835,58 @@ receive the media, thus protecting itself from performing unknowingly an DoS attack. 6. Connectivity checks from the client's primary to the server's primary was successful. Thus no further SETUP requests are necessary and processing can proceed with step 7. If another address than the primary has been verified by the client to work, that address may then be promoted for usage in a SETUP request (Goto 7). If the checks for the availble candidates failed and If further candidates have been derived during the connectivity - checks, then those can be promoted in new candidate lines in + checks, then those can be signalled in new candidate lines in SETUP request updating the list (Goto 5). 7. Client issues PLAY request. If the server also has completed its connectivity checks for this primary addresses (based on username as it may be derived addresses if the client was behind NAT) then it can directly answer 200 OK (Goto 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. Client needs to go back to 6. 8. Process completed media can be delivered. ICE testing ports may be released. - To keep media paths alive client must likely periodically send data + To keep media paths alive the client needs to periodically send data to the server. This could be realized with either STUN or RTP No-op [I-D.ietf-avt-rtp-no-op] packets. RTCP sent by client should be able to keep RTCP open. 4.2.3. Implementation burden of ICE The usage of ICE will require that a number of new protocols and new RTSP/SDP features be implemented. This makes ICE the solution that has the largest impact on client and server implementations amongst all the NAT/FW traversal methods in this document. - Some RTSP server implementation requirements are: + 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 - Some client implantation requirements are: + RTSP client implantation requirements are: o Limited STUN server features o Limited STUN client features o RTSP error code and ICE extension 4.2.4. Deployment Considerations Advantages: @@ -935,21 +935,23 @@ Specifically, when the RTSP server receives the "connect" RTP packet (a.k.a. FW packet, since it is used to punch a hole in the FW/NAT and to aid the server for port binding and address mapping) from its client, it copies the source IP and Port number and uses them as delivery address for media packets. By having the server send media traffic back the same way as the client's packet are sent to the server, address mappings will be honored. Therefore this technique works for all types of NATs. However, it does require server modifications. Unless there is built-in protection mechanism, symmetric RTP is very vulnerable to DDOS attacks, because attackers - can simply forge the source IP & Port of the binding packet. + can simply forge the source IP & Port of the binding packet. Using + the rule for restriciting IP address to that one of the signalling + connection will need to be applied here also. 4.3.2. Necessary RTSP extensions To support symmetric RTP the RTSP signaling must be extended to allow the RTSP client to indicate that it will use symmetric RTP. The client also needs to be able to signal its RTP SSRC to the server in its SETUP request. The RTP SSRC is used to establish some basic level of security against hijacking attacks. Care must be taken in choosing client's RTP SSRC. First, it must be unique within all the RTP sessions belonging to the same RTSP session. Secondly, if the @@ -976,35 +978,37 @@ Disadvantages: o Requires modifications to both RTSP server and client. o Limited to work with servers that have an public IP address. o The format of the RTP packet for "connection setup" (a.k.a FW packet) is yet to be defined. One possibility is to use RTP No-Op packet format in [I-D.ietf-avt-rtp-no-op]. - o Has worse security situation than STUN when using address - restrictions. + o Has the same security situation as STUN and will need to use + address restrictions. 4.3.4. Security Consideration Symmetric RTP's major security issue is that RTP streams can be - hijacked and directed towards any target that the attacker desires. + hijacked and directed towards any target that the attacker desires + unless address restricitons are used. The most serious security problem is the deliberate attack with the use of a RTSP client and symmetric RTP. The attacker uses RTSP to setup a media session. Then it uses symmetric RTP with a spoofed source address of the intended target of the attack. There is no defense against this attack other than restricting the possible bind address to be the same as the RTSP connection arrived on. This - prevents symmetric RTP to be used with multi-address NATs. + prevents symmetric RTP to be used in use cases that require differet + addresses for media destination and signalling. 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 NAT-traversal RTP packets containing the correct RTP SSRC to the correct address and port on the server. The destination of the packets is set as the source IP and port in these RTP packets. @@ -1017,21 +1021,21 @@ defend against. As a NAT re-writes the source IP and port this cannot be authenticated, but authentication is required in order to protect against this type of DOS attack. Yet another issues is that these attacks also can be used to deny the client the service he desire from the RTSP server completely. For a man in the middle capable of reading the signalling traffic or intercepting the binding packets can completely deny the client service by modifying or originating binding packets of itself. - The random SSRC tag in the binding packet determines how well + The random nonce used in the binding packet determines how well symmetric RTP can fend off stream-hijacking performed by parties that are not "man-in-the-middle". This proposal uses the 32-bit RTP SSRC field to this effect. Therefore it is important that this field is derived with a non-predictable randomizer. It should not be possible by knowing the algorithm used and a couple of basic facts, to derive what random number a certain client will use. An attacker not knowing the SSRC but aware of which port numbers that a server sends from can deploy a brute force attack on the server by testing a lot of different SSRCs until it finds a matching one. @@ -1256,94 +1260,93 @@ why RTP was developed. Problems that arise with TCP are: head-of- line blocking, delay introduced by retransmissions, highly varying rate due to the congestion control algorithm. 4.5.2. Usage of TCP tunneling in RTSP The RTSP core specification [I-D.ietf-mmusic-rfc2326bis] supports interleaving of media data on the TCP connection that carries RTSP signaling. See section 14 in [I-D.ietf-mmusic-rfc2326bis] for how to perform this type of TCP tunneling. There also exist another way of - transporting RTP over TCP defined in Appendix . For signaling and + transporting RTP over TCP defined in Appendix C.2. For signaling and rules on how to establish the TCP connection in lieu of UDP, see appendix C.2 in [I-D.ietf-mmusic-rfc2326bis]. This is based on the framing of RTP over the TCP connection as described in RFC 4571 [RFC4571]. 4.5.3. Deployment Considerations Advantage: o Works through all types of NATs where server is in the open. Disadvantage: o Functionality needs to be implemented on both server and client. o Will not always meet multimedia stream's real-time requirements. Transition: - The tunneling over RTSP's TCP connection is not planned to be phased - -out. It is intended to be a fallback mechanism and for usage when + The tunneling over RTSP's TCP connection is not planned to be phased- + out. It is intended to be a fallback mechanism and for usage when total media reliability is desired, even at the price of loss of real-time properties. 4.5.4. Security Considerations The TCP tunneling of RTP has no known security problem besides those already presented in the RTSP specification. It is not possible to get any amplification effect that is desired for denial of service attacks due to TCP's flow control. A possible security consideration, when session media data is interleaved with RTSP, would be the performance bottleneck when RTSP encryption is applied, since all session media data also needs to be encrypted. 4.6. TURN (Traversal Using Relay NAT) 4.6.1. Introduction - Traversal Using Relay NAT (TURN) [I-D.ietf-behave-turn] is a protocol - for setting up traffic relays that allows clients behind NATs and - firewalls to receive incoming traffic for both UDP and TCP. These - relays are controlled and have limited resources. They need to be - allocated before usage. TURN allows a client to temporarily bind an - address/port pair on the relay (TURN server) to its local source - address/port pair, which is used to contact the TURN server. The - TURN server will then forward packets between the two sides of the - relay. To prevent DOS attacks on either recipient, the packets - forwarded are restricted to the specific source address. On the - client side it is restricted to the source setting up the mapping. - On the external side this is limited to the source address/port pair - of the first packet arriving on the binding. After the first packet - has arrived the mapping is "locked down" to that address. Packets - from any other source on this address will be discarded. Using a - TURN server makes it possible for a RTSP client to receive media - streams from even an unmodified RTSP server. However the problem is - those RTSP servers most likely restrict media destinations to no - other IP address than the one RTSP message arrives. This means that - TURN could only be used if the server knows and accepts that the IP - belongs to a TURN server and the TURN server can't be targeted at an - unknown address. Unfortunately TURN servers can be targeted at any - host that has a public IP address by spoofing the source IP of TURN - Allocation requests. + Traversal Using Relay NAT (TURN) [RFC5766] is a protocol for setting + up traffic relays that allows clients behind NATs and firewalls to + receive incoming traffic for both UDP and TCP. These relays are + controlled and have limited resources. They need to be allocated + before usage. TURN allows a client to temporarily bind an address/ + port pair on the relay (TURN server) to its local source address/port + pair, which is used to contact the TURN server. The TURN server will + then forward packets between the two sides of the relay. To prevent + DOS attacks on either recipient, the packets forwarded are restricted + to the specific source address. On the client side it is restricted + to the source setting up the mapping. On the external side this is + limited to the source address/port pair of the first packet arriving + on the binding. After the first packet has arrived the mapping is + "locked down" to that address. Packets from any other source on this + address will be discarded. Using a TURN server makes it possible for + a RTSP client to receive media streams from even an unmodified RTSP + server. However the problem is those RTSP servers most likely + restrict media destinations to no other IP address than the one RTSP + message arrives. This means that TURN could only be used if the + server knows and accepts that the IP belongs to a TURN server and the + TURN server can't be targeted at an unknown address or also the RTSP + connection is relayed through the same TURN server. 4.6.2. Usage of TURN with RTSP To use a TURN server for NAT traversal, the following steps should be performed. 1. The RTSP client connects with RTSP server. The client retrieves the session description to determine the number of media streams. To avoid the issue with having RTSP connection and media traffic from different addresses also the TCP connection must be done - through the same TURN server as the one in the next step. + 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 continuous ranges. 3. The RTSP client uses the acquired address and port mappings in the RTSP SETUP request using the destination header. Note that the server is required to have a mechanism to verify that it is allowed to send media traffic to the given address. The server @@ -1387,31 +1390,28 @@ o A TURN server for RTSP is may not scale since the number of sessions it must forward is proportional to the number of client media sessions. o TURN server becomes a single point of failure. o Since TURN forwards media packets, it necessarily introduces delay. - o Requires that the server can verify that the given destination - address is valid to be used by the client. - 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 phase-out completely, see chapter 11.2 of - [I-D.ietf-behave-turn]. However the usage of TURN could be reduced - when the demand for having NAT traversal is reduced. + [RFC5766]. However the usage of TURN could be reduced when the + demand for having NAT traversal is reduced. 4.6.4. Security Considerations An eavesdropper of RTSP messages between the RTSP client and RTSP server will be able to do a simple denial of service attack on the media streams by sending messages to the destination address and port present in the RTSP SETUP messages. If the attacker's message can reach the TURN server before the RTSP server's message, the lock down can be accomplished towards some other address. This will result in that the TURN server will drop all the media server's packets when @@ -1481,32 +1481,32 @@ In the following table, the columns correspond to the numbered requirements. For instance, the column under R1 corresponds to the first requirement in section Section 3: MUST work for all flavors of NATs. The rows represent the different FW traversal techniques. SymRTP is short for symmetric RTP, "V.SymRTP" is short for "variation of symmetric RTP" as described in section Section 4.3.5. A Summary of the requirements are: R1 Work for all flavors of NATs - R2 Most work with Firewalls, including them with ALGs R3 Should have minimal impact on clients not behind NATs R4 Should be simple to use, Implement and administrate. R5 Should provide a mitigation against DDoS attacks + -----------------------------------------------+ | R1 | R2 | R3 | R4 | R5 | ------------+------+------+------+------+------+ - STU | Yes | Yes | No | Maybe| No | + STUN | Yes | Yes | No | Maybe| No | ------------+------+------+------+------+------+ ICE | Yes | Yes | No | No | Yes | ------------+------+------+------+------+------+ SymRTP | Yes | Yes | Yes |Maybe | No | ------------+------+------+------+------+------+ V. SymRTP | Yes | Yes | Yes | Yes |future| ------------+------+------+------+------+------+ 3-W SymRTP | Yes | Yes | Yes | Maybe| Yes | ------------+------+------+------+------+------+ TURN | Yes | Yes | No | No | Yes | @@ -1565,71 +1566,58 @@ like to give special thanks to Greg Sherwood of PacketVideo for his input into this memo. Section Section 1.1 contains text originally written for RFC 4787 by Francois Audet and Cullen Jennings. 10. Informative References [I-D.ietf-avt-app-rtp-keepalive] Marjou, X. and A. Sollaud, "Application Mechanism for - maintaining alive the Network Address Translator (NAT) - mappings associated to RTP flows.", - draft-ietf-avt-app-rtp-keepalive-07 (work in progress), - December 2009. + keeping alive the Network Address Translator (NAT) + mappings associated to RTP/RTCP flows.", + draft-ietf-avt-app-rtp-keepalive-10 (work in progress), + March 2011. [I-D.ietf-avt-rtp-no-op] Andreasen, F., "A No-Op Payload Format for RTP", draft-ietf-avt-rtp-no-op-04 (work in progress), May 2007. - [I-D.ietf-behave-turn] - Rosenberg, J., Mahy, R., and P. Matthews, "Traversal Using - Relays around NAT (TURN): Relay Extensions to Session - Traversal Utilities for NAT (STUN)", - draft-ietf-behave-turn-16 (work in progress), July 2009. - - [I-D.ietf-mmusic-ice] - Rosenberg, J., "Interactive Connectivity Establishment - (ICE): A Protocol for Network Address Translator (NAT) - Traversal for Offer/Answer Protocols", - draft-ietf-mmusic-ice-19 (work in progress), October 2007. - [I-D.ietf-mmusic-rfc2326bis] Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., and M. Stiemerling, "Real Time Streaming Protocol 2.0 - (RTSP)", draft-ietf-mmusic-rfc2326bis-22 (work in - progress), July 2009. + (RTSP)", draft-ietf-mmusic-rfc2326bis-27 (work in + progress), March 2011. [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. + [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, + RFC 793, September 1981. + [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming Protocol (RTSP)", RFC 2326, April 1998. [RFC2588] Finlayson, R., "IP Multicast and Firewalls", RFC 2588, May 1999. [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address Translator (NAT) Terminology and Considerations", RFC 2663, August 1999. [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. - [RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H. - Schulzrinne, "Grouping of Media Lines in the Session - Description Protocol (SDP)", RFC 3388, December 2002. - [RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-Address Fixing (UNSAF) Across Network Address Translation", RFC 3424, November 2002. [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, March 2003. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. @@ -1640,24 +1628,37 @@ Description Protocol", RFC 4566, July 2006. [RFC4571] Lazzaro, J., "Framing Real-time Transport Protocol (RTP) and RTP Control Protocol (RTCP) Packets over Connection- Oriented Transport", RFC 4571, July 2006. [RFC4787] Audet, F. and C. Jennings, "Network Address Translation (NAT) Behavioral Requirements for Unicast UDP", BCP 127, RFC 4787, January 2007. + [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment + (ICE): A Protocol for Network Address Translator (NAT) + Traversal for Offer/Answer Protocols", RFC 5245, + April 2010. + [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, "Session Traversal Utilities for NAT (STUN)", RFC 5389, October 2008. + [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, April 2010. + + [RFC6062] Perreault, S. and J. Rosenberg, "Traversal Using Relays + around NAT (TURN) Extensions for TCP Allocations", + RFC 6062, November 2010. + [STUN-IMPL] "Open Source STUN Server and Client, http:// www.vovida.org/applications/downloads/stun/index.html", June 2007. Authors' Addresses Magnus Westerlund Ericsson Farogatan 6