draft-ietf-mmusic-rtsp-nat-evaluation-14.txt   draft-ietf-mmusic-rtsp-nat-evaluation-15.txt 
Network Working Group M. Westerlund Network Working Group M. Westerlund
Internet-Draft Ericsson Internet-Draft Ericsson
Intended status: Informational T. Zeng Intended status: Informational T. Zeng
Expires: November 29, 2014 Expires: October 22, 2015 April 20, 2015
May 28, 2014
The Evaluation of Different Network Address Translator (NAT) Traversal The Comparison of Different Network Address Translator (NAT) Traversal
Techniques for Media Controlled by Real-time Streaming Protocol (RTSP) Techniques for Media Controlled by Real-time Streaming Protocol (RTSP)
draft-ietf-mmusic-rtsp-nat-evaluation-14 draft-ietf-mmusic-rtsp-nat-evaluation-15
Abstract Abstract
This document describes several Network Address Translator (NAT) This document describes several Network Address Translator (NAT)
traversal techniques that were considered to be used for establishing traversal techniques that were considered to be used for establishing
the RTP media flows controlled by the Real-time Streaming Protocol the RTP media flows controlled by the Real-time Streaming Protocol
(RTSP). Each technique includes a description of how it would be (RTSP). Each technique includes a description of how it would be
used, the security implications of using it and any other deployment used, the security implications of using it and any other deployment
considerations it has. There are also discussions on how NAT considerations it has. There are also discussions on how NAT
traversal techniques relate to firewalls and how each technique can traversal techniques relate to firewalls and how each technique can
skipping to change at page 1, line 41 skipping to change at page 1, line 40
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This Internet-Draft will expire on November 29, 2014. This Internet-Draft will expire on October 22, 2015.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Network Address Translators . . . . . . . . . . . . . . . 4 1.1. Network Address Translators . . . . . . . . . . . . . . . 5
1.2. Firewalls . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2. Firewalls . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Glossary . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3. Glossary . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Detecting the loss of NAT mappings . . . . . . . . . . . . . 7 2. Detecting the loss of NAT mappings . . . . . . . . . . . . . 7
3. Requirements on Solutions . . . . . . . . . . . . . . . . . . 8 3. Requirements on Solutions . . . . . . . . . . . . . . . . . . 8
4. NAT Traversal Techniques . . . . . . . . . . . . . . . . . . 9 4. NAT Traversal Techniques . . . . . . . . . . . . . . . . . . 10
4.1. Stand-Alone STUN . . . . . . . . . . . . . . . . . . . . 10 4.1. Stand-Alone STUN . . . . . . . . . . . . . . . . . . . . 10
4.1.1. Introduction . . . . . . . . . . . . . . . . . . . . 10 4.1.1. Introduction . . . . . . . . . . . . . . . . . . . . 10
4.1.2. Using STUN to traverse NAT without server 4.1.2. Using STUN to traverse NAT without server
modifications . . . . . . . . . . . . . . . . . . . . 10 modifications . . . . . . . . . . . . . . . . . . . . 11
4.1.3. ALG considerations . . . . . . . . . . . . . . . . . 12 4.1.3. ALG considerations . . . . . . . . . . . . . . . . . 13
4.1.4. Deployment Considerations . . . . . . . . . . . . . . 13 4.1.4. Deployment Considerations . . . . . . . . . . . . . . 14
4.1.5. Security Considerations . . . . . . . . . . . . . . . 14 4.1.5. Security Considerations . . . . . . . . . . . . . . . 15
4.2. Server Embedded STUN . . . . . . . . . . . . . . . . . . 14 4.2. Server Embedded STUN . . . . . . . . . . . . . . . . . . 15
4.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 15 4.2.1. Introduction . . . . . . . . . . . . . . . . . . . . 15
4.2.2. Embedding STUN in RTSP . . . . . . . . . . . . . . . 15 4.2.2. Embedding STUN in RTSP . . . . . . . . . . . . . . . 15
4.2.3. Discussion On Co-located STUN Server . . . . . . . . 16 4.2.3. Discussion On Co-located STUN Server . . . . . . . . 17
4.2.4. ALG considerations . . . . . . . . . . . . . . . . . 16 4.2.4. ALG considerations . . . . . . . . . . . . . . . . . 17
4.2.5. Deployment Considerations . . . . . . . . . . . . . . 16 4.2.5. Deployment Considerations . . . . . . . . . . . . . . 17
4.2.6. Security Considerations . . . . . . . . . . . . . . . 18 4.2.6. Security Considerations . . . . . . . . . . . . . . . 18
4.3. ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.3. ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.3.1. Introduction . . . . . . . . . . . . . . . . . . . . 18 4.3.1. Introduction . . . . . . . . . . . . . . . . . . . . 18
4.3.2. Using ICE in RTSP . . . . . . . . . . . . . . . . . . 18 4.3.2. Using ICE in RTSP . . . . . . . . . . . . . . . . . . 19
4.3.3. Implementation burden of ICE . . . . . . . . . . . . 20 4.3.3. Implementation burden of ICE . . . . . . . . . . . . 21
4.3.4. Deployment Considerations . . . . . . . . . . . . . . 21 4.3.4. ALG Considerations . . . . . . . . . . . . . . . . . 21
4.3.5. Security Consideration . . . . . . . . . . . . . . . 21 4.3.5. Deployment Considerations . . . . . . . . . . . . . . 22
4.4. Latching . . . . . . . . . . . . . . . . . . . . . . . . 21 4.3.6. Security Consideration . . . . . . . . . . . . . . . 22
4.4.1. Introduction . . . . . . . . . . . . . . . . . . . . 21 4.4. Latching . . . . . . . . . . . . . . . . . . . . . . . . 22
4.4.2. Necessary RTSP extensions . . . . . . . . . . . . . . 22 4.4.1. Introduction . . . . . . . . . . . . . . . . . . . . 22
4.4.3. Deployment Considerations . . . . . . . . . . . . . . 23 4.4.2. Necessary RTSP extensions . . . . . . . . . . . . . . 23
4.4.4. Security Consideration . . . . . . . . . . . . . . . 23 4.4.3. ALG Considerations . . . . . . . . . . . . . . . . . 24
4.5. A Variation to Latching . . . . . . . . . . . . . . . . . 25 4.4.4. Deployment Considerations . . . . . . . . . . . . . . 24
4.5.1. Introduction . . . . . . . . . . . . . . . . . . . . 25 4.4.5. Security Consideration . . . . . . . . . . . . . . . 25
4.5.2. Necessary RTSP extensions . . . . . . . . . . . . . . 25 4.5. A Variation to Latching . . . . . . . . . . . . . . . . . 26
4.5.3. Deployment Considerations . . . . . . . . . . . . . . 26 4.5.1. Introduction . . . . . . . . . . . . . . . . . . . . 26
4.5.4. Security Considerations . . . . . . . . . . . . . . . 26 4.5.2. Necessary RTSP extensions . . . . . . . . . . . . . . 27
4.6. Three Way Latching . . . . . . . . . . . . . . . . . . . 27 4.5.3. ALG Considerations . . . . . . . . . . . . . . . . . 27
4.6.1. Introduction . . . . . . . . . . . . . . . . . . . . 27 4.5.4. Deployment Considerations . . . . . . . . . . . . . . 27
4.6.2. Necessary RTSP extensions . . . . . . . . . . . . . . 27 4.5.5. Security Considerations . . . . . . . . . . . . . . . 28
4.6.3. Deployment Considerations . . . . . . . . . . . . . . 27 4.6. Three Way Latching . . . . . . . . . . . . . . . . . . . 28
4.7. Application Level Gateways . . . . . . . . . . . . . . . 28 4.6.1. Introduction . . . . . . . . . . . . . . . . . . . . 28
4.7.1. Introduction . . . . . . . . . . . . . . . . . . . . 28 4.6.2. Necessary RTSP extensions . . . . . . . . . . . . . . 28
4.7.2. Outline On how ALGs for RTSP work . . . . . . . . . . 28 4.6.3. ALG Considerations . . . . . . . . . . . . . . . . . 29
4.7.3. Deployment Considerations . . . . . . . . . . . . . . 29 4.6.4. Deployment Considerations . . . . . . . . . . . . . . 29
4.7.4. Security Considerations . . . . . . . . . . . . . . . 30 4.6.5. Security Considerations . . . . . . . . . . . . . . . 29
4.8. TCP Tunneling . . . . . . . . . . . . . . . . . . . . . . 30 4.7. Application Level Gateways . . . . . . . . . . . . . . . 30
4.8.1. Introduction . . . . . . . . . . . . . . . . . . . . 30 4.7.1. Introduction . . . . . . . . . . . . . . . . . . . . 30
4.8.2. Usage of TCP tunneling in RTSP . . . . . . . . . . . 31 4.7.2. Outline On how ALGs for RTSP work . . . . . . . . . . 30
4.8.3. Deployment Considerations . . . . . . . . . . . . . . 31 4.7.3. Deployment Considerations . . . . . . . . . . . . . . 31
4.8.4. Security Considerations . . . . . . . . . . . . . . . 31 4.7.4. Security Considerations . . . . . . . . . . . . . . . 32
4.9. TURN (Traversal Using Relay NAT) . . . . . . . . . . . . 31 4.8. TCP Tunneling . . . . . . . . . . . . . . . . . . . . . . 32
4.9.1. Introduction . . . . . . . . . . . . . . . . . . . . 32 4.8.1. Introduction . . . . . . . . . . . . . . . . . . . . 33
4.9.2. Usage of TURN with RTSP . . . . . . . . . . . . . . . 32 4.8.2. Usage of TCP tunneling in RTSP . . . . . . . . . . . 33
4.9.3. Deployment Considerations . . . . . . . . . . . . . . 33 4.8.3. ALG Considerations . . . . . . . . . . . . . . . . . 33
4.9.4. Security Considerations . . . . . . . . . . . . . . . 34 4.8.4. Deployment Considerations . . . . . . . . . . . . . . 33
5. Firewalls . . . . . . . . . . . . . . . . . . . . . . . . . . 34 4.8.5. Security Considerations . . . . . . . . . . . . . . . 34
6. Comparison of NAT traversal techniques . . . . . . . . . . . 35 4.9. TURN (Traversal Using Relay NAT) . . . . . . . . . . . . 34
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37 4.9.1. Introduction . . . . . . . . . . . . . . . . . . . . 34
8. Security Considerations . . . . . . . . . . . . . . . . . . . 37 4.9.2. Usage of TURN with RTSP . . . . . . . . . . . . . . . 35
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 38 4.9.3. ALG Considerations . . . . . . . . . . . . . . . . . 36
10. Informative References . . . . . . . . . . . . . . . . . . . 38 4.9.4. Deployment Considerations . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40 4.9.5. Security Considerations . . . . . . . . . . . . . . . 37
5. Firewalls . . . . . . . . . . . . . . . . . . . . . . . . . . 37
6. Comparison of NAT traversal techniques . . . . . . . . . . . 38
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40
8. Security Considerations . . . . . . . . . . . . . . . . . . . 40
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 41
10. Informative References . . . . . . . . . . . . . . . . . . . 41
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 44
1. Introduction 1. Introduction
Today there is a proliferate deployment of different flavors of Today there is a proliferating deployment of different types of
Network Address Translator (NAT) boxes that in many cases only Network Address Translator (NAT) boxes that in many cases only
loosely follow standards loosely follow standards
[RFC3022][RFC2663][RFC3424][RFC4787][RFC5382]. NATs cause [RFC3022][RFC2663][RFC3424][RFC4787][RFC5382]. NATs cause
discontinuity in address realms [RFC3424], therefore an application discontinuity in address realms [RFC3424], therefore an application
protocol, such as Real-time Streaming Protocol (RTSP) protocol, such as Real-time Streaming Protocol (RTSP)
[RFC2326][I-D.ietf-mmusic-rfc2326bis], needs to deal with such [RFC2326][I-D.ietf-mmusic-rfc2326bis], needs to deal with such
discontinuities caused by NATs. The problem is that, being a media discontinuities caused by NATs. The problem is that, being a media
control protocol managing one or more media streams, RTSP carries control protocol managing one or more media streams, RTSP carries
network address and port information within its protocol messages. network address and port information within its protocol messages.
Because of this, even if RTSP itself, when carried over Transmission Because of this, even if RTSP itself, when carried over Transmission
Control Protocol (TCP) [RFC0793] for example, is not blocked by NATs, Control Protocol (TCP) [RFC0793] for example, is not blocked by NATs,
its media streams may be blocked by NATs. This will occur unless its media streams may be blocked by NATs. This will occur unless
special protocol provisions are added to support NAT-traversal. special protocol provisions are added to support NAT-traversal.
Like NATs, firewalls are also middle boxes that need to be Like NATs, firewalls are also middle boxes that need to be
considered. Firewalls help prevent unwanted traffic from getting in considered. Firewalls help prevent unwanted traffic from getting in
or out of the protected network. RTSP is designed such that a or out of the protected network. RTSP is designed such that a
firewall can be configured to let RTSP controlled media streams go firewall can be configured to let RTSP controlled media streams go
through with minimal implementation effort. The minimal effort is to through with limited implementation effort. The effort needed is to
implement an Application Level Gateway (ALG) to interpret RTSP implement an Application Level Gateway (ALG) to interpret RTSP
parameters. There is also a large class of firewalls, commonly home parameters. There is also a large class of firewalls, commonly home
firewalls, that uses a similar filtering behavior to what NAT has. firewalls, that uses a filtering behavior that appear the same to
This type of firewalls can be handled using the same solution as what NATs have. This type of firewall will be successfully traversed
employed for NAT traversal instead of relying on ALGs. using the same solution as employed for NAT traversal, instead of
relying on a RTSP ALG. Therefore firewalls will also be discussed
and some important differences highlighted.
This document describes several NAT-traversal mechanisms for RTSP This document describes several NAT-traversal mechanisms for RTSP
controlled media streaming. Many of these NAT solutions fall into controlled media streaming. Many of these NAT solutions fall into
the category of "UNilateral Self-Address Fixing (UNSAF)" as defined the category of "UNilateral Self-Address Fixing (UNSAF)" as defined
in [RFC3424] and quoted below: in [RFC3424] and quoted below:
"UNSAF is a process whereby some originating process attempts to "UNSAF is a process whereby some originating process attempts to
determine or fix the address (and port) by which it is known - e.g. 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 to be able to use address data in the protocol exchange, or to
advertise a public address from which it will receive connections." advertise a public address from which it will receive connections."
Following the guidelines spelled out in RFC 3424, we describe the Following the guidelines spelled out in RFC 3424, we describe the
required RTSP protocol extensions for each method, transition required RTSP protocol extensions for each method, transition
strategies, and security concerns. strategies, and security concerns. The transition strategies are a
discussion of how and if the method encourage a move towards not
having any NATs on the path.
This document is capturing the evaluation done in the process to This document is capturing the evaluation done in the process to
recommend firewall/NAT traversal methods for RTSP streaming servers recommend firewall/NAT traversal methods for RTSP streaming servers
based on RFC 2326 [RFC2326] as well as the RTSP 2.0 core spec 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 [I-D.ietf-mmusic-rfc2326bis]. The evaluation is focused on NAT
traversal for the media streams carried over User Datagram Protocol traversal for the media streams carried over User Datagram Protocol
(UDP) [RFC0768] with Real-time Transport Protocol (RTP) [RFC3550] (UDP) [RFC0768] with Real-time Transport Protocol (RTP) [RFC3550]
over UDP being the main case for such usage. The findings should be over UDP being the main case for such usage. The findings should be
applicable to other protocols as long as they have similar applicable to other protocols as long as they have similar
properties. properties.
At the time when the bulk of work on this document was done, a single 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 level level of NAT was the dominant deployment for NATs, and multiple level
of NATs, including Carrier Grade NATs (CGNs) has been only partially of NATs, including Carrier Grade NATs (CGNs) was not considered.
considered. Thus, any characterizations or findings may not be applicable in such
scenarios, unless CGN or multiple level of NATs are explicitly noted.
The resulting ICE-based RTSP NAT traversal mechanism is specified in An ICE-based RTSP NAT traversal mechanism is specified in "A Network
"A Network Address Translator (NAT) Traversal mechanism for media Address Translator (NAT) Traversal mechanism for media controlled by
controlled by Real-Time Streaming Protocol (RTSP)" Real-Time Streaming Protocol (RTSP)" [I-D.ietf-mmusic-rtsp-nat].
[I-D.ietf-mmusic-rtsp-nat].
1.1. Network Address Translators 1.1. Network Address Translators
We begin by reviewing two quotes from Section 3 in "Network Address We begin by reviewing two quotes from Section 3 in "Network Address
Translation (NAT) Behavioral Requirements for Unicast UDP" [RFC4787] Translation (NAT) Behavioral Requirements for Unicast UDP" [RFC4787]
concering NATs and their terminology: concerning NATs and their terminology:
"Readers are urged to refer to [RFC2663] for information on NAT "Readers are urged to refer to [RFC2663] for information on NAT
taxonomy and terminology. Traditional NAT is the most common type of 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 [RFC3022] for detailed
information on traditional NAT. Traditional NAT has two main information on traditional NAT. Traditional NAT has two main
varieties -- Basic NAT and Network Address/Port Translator (NAPT). varieties -- Basic NAT and Network Address/Port Translator (NAPT).
NAPT is by far the most commonly deployed NAT device. NAPT allows NAPT is by far the most commonly deployed NAT device. NAPT allows
multiple internal hosts to share a single public IP address multiple internal hosts to share a single public IP address
simultaneously. When an internal host opens an outgoing TCP or UDP simultaneously. When an internal host opens an outgoing TCP or UDP
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sessions from a single (private IP, private port) endpoint to sessions from a single (private IP, private port) endpoint to
multiple distinct endpoints on the external network. In this multiple distinct endpoints on the external network. In this
specification, the term "NAT" refers to both "Basic NAT" and "Network specification, the term "NAT" refers to both "Basic NAT" and "Network
Address/Port Translator (NAPT)"." Address/Port Translator (NAPT)"."
"This document uses the term "address and port mapping" as the "This document uses the term "address and port mapping" as the
translation between an external address and port and an internal translation between an external address and port and an internal
address and port. Note that this is not the same as an "address address and port. Note that this is not the same as an "address
binding" as defined in RFC 2663." binding" as defined in RFC 2663."
Note: In the above it would be more correct to use external Note: In the above it would be more correct to use external IP
instead of public in the above text. The external IP address is address instead of public IP address in the above text. The
commonly a public one, but might be of other type if the NAT's external IP address is commonly a public one, but might be of
external side is in a private address domain. other 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 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 port mapping behaviors described in more detail in Section 4.1 of
"Network Address Translation (NAT) Behavioral Requirements for "Network Address Translation (NAT) Behavioral Requirements for
Unicast UDP" [RFC4787] that are highly relevant to the discussion in Unicast UDP" [RFC4787] that are highly relevant to the discussion in
this document. this document.
NATs also have a filtering behavior on traffic arriving on the NATs also have a filtering behavior on traffic arriving on the
external side. Such behavior affects how well different methods for external side. Such behavior affects how well different methods for
NAT traversal works through these NATs. See Section 5 of "Network NAT traversal works through these NATs. See Section 5 of "Network
Address Translation (NAT) Behavioral Requirements for Unicast UDP" Address Translation (NAT) Behavioral Requirements for Unicast UDP"
[RFC4787] for more information on the different types of filtering [RFC4787] for more information on the different types of filtering
that have been identified. that have been identified.
1.2. Firewalls 1.2. Firewalls
A firewall is a security gateway that enforces certain access control A firewall is a security gateway that enforces certain access control
policies between two network administrative domains: a private domain policies between two network administrative domains: a private domain
(intranet) and a external domain, e.g. Internet. Many organizations (intranet) and a external domain, e.g. Internet. Many organizations
use firewalls to prevent privacy intrusions and malicious attacks to use firewalls to prevent intrusions and an malicious attacks on
corporate computing resources in the private intranet [RFC2588]. computing resources in the private intranet [RFC2588].
A comparison between NAT and firewall is given below: A comparison between NAT and firewall is given below:
1. A firewall sits at security enforcement/protection points, while 1. A firewall sits at security enforcement/protection points, while
NAT sits at borders between two address domains. NAT sits at borders between two address domains.
2. NAT does not in itself provide security, although some access 2. NAT does not in itself provide security, although some access
control policies can be implemented using address translation control policies can be implemented using address translation
schemes. The inherent filtering behaviours are commonly mistaken schemes. The inherent filtering behaviours are commonly mistaken
for real security policies. for real security policies.
It should be noted that many NAT devices intended for Residential or It should be noted that many NAT devices intended for Residential or
small office/home office (SOHO) use include both NATs and firewall small office/home office (SOHO) use include both NATs and firewall
functionality. functionality.
In the rest of this memo we use the phrase "NAT traversal"
interchangeably with "firewall traversal", and "NAT/firewall
traversal".
1.3. Glossary 1.3. Glossary
Address-Dependent Mapping: The NAT reuses the port mapping for Address-Dependent Mapping: The NAT reuses the port mapping for
subsequent packets sent from the same internal IP address and subsequent packets sent from the same internal IP address and
port to the same external IP address, regardless of the port to the same external IP address, regardless of the
external port. See [RFC4787]. external port. See [RFC4787].
Address and Port-Dependent Mapping: The NAT reuses the port mapping Address and Port-Dependent Mapping: The NAT reuses the port mapping
for subsequent packets sent from the same internal IP address for subsequent packets sent from the same internal IP address
and port to the same external IP address and port while the and port to the same external IP address and port while the
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port translations, "keep-alive" and periodic re-polling may be port translations, "keep-alive" and periodic re-polling may be
required according to RFC 3424. Secondly, it is possible to detect required according to RFC 3424. Secondly, it is possible to detect
and recover from the situation where the mapping has been changed or and recover from the situation where the mapping has been changed or
removed. The loss of a mapping can be detected when no traffic removed. The loss of a mapping can be detected when no traffic
arrives for a while. Below we will give some recommendation on how arrives for a while. Below we will give some recommendation on how
to detect loss of NAT mappings when using RTP/RTCP under RTSP to detect loss of NAT mappings when using RTP/RTCP under RTSP
control. control.
A RTP session normally has both RTP and RTCP streams. The loss of a A RTP session normally has both RTP and RTCP streams. The loss of a
RTP mapping can only be detected when expected traffic does not RTP mapping can only be detected when expected traffic does not
arrive. If a client does not receive data within a few seconds after arrive. If a client does not receive media data within a few seconds
having received the "200 OK" response to a PLAY request, it may be after having received the "200 OK" response to a RTSP PLAY request
the result of a middlebox blocking the traffic. However, for a which starts the media delivery, it may be the result of a middlebox
receiver to be more certain to detect the case where no RTP traffic blocking the traffic. However, for a receiver to be more certain to
was delivered due to NAT trouble, one should monitor the RTCP Sender detect the case where no RTP traffic was delivered due to NAT
reports if they are received and not also blocked. The sender report trouble, one should monitor the RTCP Sender reports if they are
carries a field telling how many packets the server has sent. If received and not also blocked. The sender report carries a field
that has increased and no RTP packets has arrived for a few seconds telling how many packets the server has sent. If that has increased
it is likely the RTP mapping has been removed. and no RTP packets has 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 The loss of mapping for RTCP is simpler to detect. RTCP is normally
sent periodically in each direction, even during the RTSP ready sent periodically in each direction, even during the RTSP ready
state. If RTCP packets are missing for several RTCP intervals, the state. If RTCP packets are missing for several RTCP intervals, the
mapping is likely lost. Note that if neither RTCP packets nor RTSP mapping is likely lost. Note that if neither RTCP packets nor RTSP
messages are received by the RTSP server for a while, the RTSP server messages are received by the RTSP server for a while (default 60
has the option to delete the corresponding RTP session, SSRC and RTSP seconds), the RTSP server has the option to delete the corresponding
session ID, because either the client can not get through a middle RTP session, SSRC and RTSP session ID, because either the client can
box NAT/firewall, or the client is mal-functioning. not get through a middle box NAT/firewall, or the client is mal-
functioning.
3. Requirements on Solutions 3. Requirements on Solutions
This section considers the set of requirements for the evaluation of This section considers the set of requirements for the evaluation of
RTSP NAT traversal solutions. RTSP NAT traversal solutions.
RTSP is a client-server protocol. Typically service providers deploy RTSP is a client-server protocol. Typically service providers deploy
RTSP servers on the Internet or otherwise reachable address realm. RTSP servers on the Internet or otherwise reachable address realm.
However, there are use cases where the reverse is true: RTSP clients However, there are use cases where the reverse is true: RTSP clients
are connecting from any address realm to RTSP servers behind NATs, 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 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 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 users in the public address realm through a home NAT. In terms of
requirements, the primary requirement should be to solve the RTSP NAT requirements, the primary issue to solve is the RTSP NAT traversal
traversal problem for RTSP servers deployed in a network where the problem for RTSP servers deployed in a network where the server is on
server is on the external side of any NAT, i.e. server is not behind the external side of any NAT, i.e. server is not behind a NAT. The
a NAT. server behind a NAT is desirable, but of much lower priority.
An important consideration for any NAT traversal technique is whether
any protocol modification needs occur, where the implementation
burden occur, server, client or middlebox. If the incitement to get
RTSP to work over a NAT is sufficient to motivate the owner of the
server, client or middlebox to update or configure or otherwise
perform changes to the device and its software to support the NAT
traversal. Thus, the question of who this burden falls on and how
big it is is highly relevant.
The list of feature requirements for RTSP NAT solutions are given The list of feature requirements for RTSP NAT solutions are given
below: below:
1. Must work for all flavors of NATs, including NATs with Address 1. Must work for all flavors of NATs, including NATs with Address
and Port-Dependent Filtering. and Port-Dependent Filtering.
2. Must work for firewalls (subject to pertinent firewall 2. Must work for firewalls (subject to pertinent firewall
administrative policies), including those with ALGs. administrative policies), including those with ALGs.
3. Should have minimal impact on clients not behind NATs and which 3. Should have minimal impact on clients not behind NATs and which
are not dual-hosted. RTSP dual-hosting means that the RTSP are not dual-hosted. RTSP dual-hosting means that the RTSP
signalling protocol and the media protocol (e.g. RTP) are signalling protocol and the media protocol (e.g. RTP) are
implemented on different computers with different IP addresses. implemented on different computers with different IP addresses.
* For instance, no extra delay from RTSP connection till arrival * For instance, no extra protocol RTT before arrival of media.
of media
4. Should be simple to use/implement/administer so people actually 4. Should be simple to use/implement/administer so people actually
turn them on turn them on
* Otherwise people will resort to TCP tunneling through NATs
* Discovery of the address(es) assigned by NAT should happen * Discovery of the address(es) assigned by NAT should happen
automatically, if possible automatically, if possible
5. Should authenticate dual-hosted client transport handler to 5. Should authenticate dual-hosted client's media transport receiver
prevent DDoS attacks. to prevent usage of RTSP servers for DDoS attacks.
The last requirement addresses the Distributed Denial-of-Service The last requirement addresses the Distributed Denial-of-Service
(DDoS) threat, which relates to NAT traversal as explained below. (DDoS) threat, which relates to NAT traversal as explained below.
During NAT traversal, when the RTSP server determines the media During NAT traversal, when the RTSP server determines the media
destination (address and port) for the client, the result may be that 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 the IP address of the RTP receiver host is different than the IP
address of the RTSP client host. This posts a DDoS threat that has address of the RTSP client host. This posts a DDoS threat that has
significant amplification potentials because the RTP media streams in significant amplification potentials because the RTP media streams in
general consist of large number of IP packets. DDoS attacks occur if general consist of large number of IP packets. DDoS attacks can
the attacker fakes the messages in the NAT traversal mechanism to occur if the attacker can fake the messages in the NAT traversal
trick the RTSP server into believing that the client's RTP receiver mechanism to trick the RTSP server into believing that the client's
is located on a separate host. For example, user A may use his RTSP RTP receiver is located on a host to be attacked. For example, user
client to direct the RTSP server to send video RTP streams to A may use his RTSP client to direct the RTSP server to send video RTP
target.example.com in order to degrade the services provided by streams to target.example.com in order to degrade the services
target.example.com. Note a simple preventative measure commonly provided by target.example.com.
deployed is for the RTSP server to disallow the cases where the
client's RTP receiver has a different IP address than that of the Note a simple mitigation is for the RTSP server to disallow the cases
RTSP client. With the increased deployment of NAT middleboxes by where the client's RTP receiver has a different IP address than that
operators, i.e. carrier grade NAT (CGN), the reuse of an IP address of the RTSP client. This is recommended behavior in RTSP 2.0 unless
on the NAT's external side by many customers reduces the protection other solutions to prevent this attack is present, See 21.2.1 in
provided. Also in some applications (e.g., centralized [I-D.ietf-mmusic-rfc2326bis]. With the increased deployment of NAT
conferencing), dual-hosted RTSP/RTP clients have valid use cases. middleboxes by operators, i.e. carrier grade NAT (CGN), the reuse of
The key is how to authenticate the messages exchanged during the NAT an IP address on the NAT's external side by many customers reduces
traversal process. 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 4. NAT Traversal Techniques
There exists a number of potential NAT traversal techniques that can There exists a number of potential NAT traversal techniques that can
be used to allow RTSP to traverse NATs. They have different features be used to allow RTSP to traverse NATs. They have different features
and are applicable to different topologies; their costs are also and are applicable to different topologies; their costs are also
different. They also vary in security levels. In the following different. They also vary in security levels. In the following
sections, each technique is outlined with discussions on the sections, each technique is outlined with discussions on the
corresponding advantages and disadvantages. corresponding advantages and disadvantages.
The main evaluation was done prior to 2007 and is based on what was The survey of traversal techniques was done prior to 2007 and is
available then. This section includes NAT traversal techniques that based on what was available then. This section includes NAT
have not been formally specified anywhere else. The overview section traversal techniques that have not been formally specified anywhere
of this document may be the only publicly available specification of else. This document may be the only publicly available specification
some of the NAT traversal techniques. However that is not a real of some of the NAT traversal techniques. However that is not a real
barrier against doing an evaluation of the NAT traversal techniques. barrier against doing an evaluation of the NAT traversal techniques.
Some other techniques have been recommended against or are no longer Some techniques used as part of some of the traversal solutions have
possible due to standardization works' outcome or their failure to been recommended against or are no longer possible due to
progress within IETF after the initial evaluation in this document, standardization works' outcome or their failure to progress within
e.g. RTP No-Op [I-D.ietf-avt-rtp-no-op]. IETF after the initial evaluation in this document. For example RTP
No-Op [I-D.ietf-avt-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. Stand-Alone STUN
4.1.1. Introduction 4.1.1. Introduction
Session Traversal Utilities for NAT (STUN) [RFC5389] is a Session Traversal Utilities for NAT (STUN) [RFC5389] is a
standardized protocol that allows a client to use secure means to standardized protocol that allows a client to use secure means to
discover the presence of a NAT between itself and the STUN server. discover the presence of a NAT between itself and the STUN server.
The client uses the STUN server to discover the address mappings The client uses the STUN server to discover the address mappings
assigned by the NAT. STUN is a client-server protocol. The STUN assigned by the NAT. Then using the knowledge of these NAT mappings
client sends a request to a STUN server and the server returns a use the external addresses to directly connect to the independent
response. There are two types of STUN messages - Binding Requests RTSP server. However, this is only possible if the NAT mapping
and Indications. Binding requests are used when determining a behavior is such that the STUN server and RTSP server will see the
client's external address and solicits a response from the STUN same external address and port for the same internal address and
server with the seen address. 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
solicits 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 The first version of STUN [RFC3489] included categorization and
parameterization of NATs. This was abandoned in the updated version parameterization of NATs. This was abandoned in the updated version
[RFC5389] due to it being unreliable and brittle. Some of the below [RFC5389] due to it being unreliable and brittle. This particular
discussed methods are based on RFC3489 functionality which will be traversal method uses the removed RFC3489 functionality to detect the
called out and the downside of that will be part of the NAT type to give an early failure indication when the NAT is showing
characterization. the behavior that this method can't support. This method also
suggest using the RTP NO-OP payload format [I-D.ietf-avt-rtp-no-op]
for key-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 4.1.2. Using STUN to traverse NAT without server modifications
This section describes how a client can use STUN to traverse NATs to This section describes how a client can use STUN to traverse NATs to
RTSP servers without requiring server modifications. Note that this RTSP servers without requiring server modifications. Note that this
method has limited applicability and requires the server to be method has limited applicability and requires the server to be
available in the external/public address realm in regards to the available in the external/public address realm in regards to the
client located behind a NAT(s). client located behind a NAT(s).
Limitations: Limitations:
o The server must be located in either a public address realm or the o The server must be located in either a public address realm or the
next hop external address realm in regards to the client. next hop external address realm in regards to the client.
o The client may only be located behind NATs that perform "Endpoint- o The client may only be located behind NATs that perform "Endpoint-
Independent" or "Address-Dependent" Mappings. Clients behind NATs Independent" or "Address-Dependent" Mappings (STUN server and RTSP
that do "Address and Port-Dependent" Mappings cannot use this server on same IP address). Clients behind NATs that do "Address
method. See [RFC4787] for full definition of these terms. and Port-Dependent" Mappings cannot use this method. See
[RFC4787] for full definition of these terms.
o Based on the discontinued middlebox classification of the replaced o Based on the discontinued middlebox classification of the replaced
STUN specification [RFC3489]. Thus brittle and unreliable. STUN specification [RFC3489]. Thus brittle and unreliable.
Method: Method:
A RTSP client using RTP transport over UDP can use STUN to traverse a A RTSP client using RTP transport over UDP can use STUN to traverse a
NAT(s) in the following way: NAT(s) in the following way:
1. Use STUN to try to discover the type of NAT, and the timeout 1. Use STUN to try to discover the type of NAT, and the timeout
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STUN server. A successful mapping looks like: client's local STUN server. A successful mapping looks like: client's local
address/port <-> public address/port. address/port <-> public address/port.
4. Perform the RTSP SETUP for each media. In the transport header 4. Perform the RTSP SETUP for each media. In the transport header
the following parameter should be included with the given values: the following parameter should be included with the given values:
"dest_addr" [I-D.ietf-mmusic-rfc2326bis] or "destination" + "dest_addr" [I-D.ietf-mmusic-rfc2326bis] or "destination" +
"client_port" [RFC2326] with the public/external IP address and "client_port" [RFC2326] with the public/external IP address and
port pair for both RTP and RTCP. To be certain that this works port pair for both RTP and RTCP. To be certain that this works
servers must allow a client to setup the RTP stream on any port, servers must allow a client to setup the RTP stream on any port,
not only even ports and with non-contiguous port numbers for RTP not only even ports and with non-contiguous port numbers for RTP
and RTCP. This requires the new feature provided in the update and RTCP. This requires the new feature provided in RTSP 2.0
to RFC2326 [I-D.ietf-mmusic-rfc2326bis]. The server should [I-D.ietf-mmusic-rfc2326bis]. The server should respond with a
respond with a transport header containing an "src_addr" or transport header containing an "src_addr" or "source" +
"source" + "server_port" parameters with the RTP and RTCP source "server_port" parameters with the RTP and RTCP source IP address
IP address and port of the media stream. and port of the media stream.
5. To keep the mappings alive, the client should periodically send 5. To keep the mappings alive, the client should periodically send
UDP traffic over all mappings needed for the session. For the UDP traffic over all mappings needed for the session. For the
mapping carrying RTCP traffic the periodic RTCP traffic are mapping carrying RTCP traffic the periodic RTCP traffic are
likely enough. For mappings carrying RTP traffic and for likely enough. For mappings carrying RTP traffic and for
mappings carrying RTCP packets at too low a frequency, keep-alive mappings carrying RTCP packets at too low a frequency, keep-alive
messages should be sent. As keep alive messages, one could use messages should be sent.
the RTP No-Op packet [I-D.ietf-avt-rtp-no-op] to the streaming
server's discard port (port number 9). The drawback of using RTP
No-Op is 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 If a UDP mapping is lost, the above discovery process must be
repeated. The media stream also needs to be SETUP again to change repeated. The media stream also needs to be SETUP again to change
the transport parameters to the new ones. This will cause a glitch the transport parameters to the new ones. This will cause a glitch
in media playback. in media playback.
To allow UDP packets to arrive from the server to a client behind a To allow UDP packets to arrive from the server to a client behind a
"Address Dependent" filtering NAT, the client must first send a UDP "Address Dependent" or "Address and Port Dependent" filtering NAT,
packet to establish filtering state in the NAT. The client, before the client must first send a UDP packet to establish filtering state
sending a RTSP PLAY request, must send a so called hole-punching in the NAT. The client, before sending a RTSP PLAY request, must
packet (such as a RTP No-Op packet) on each mapping, to the IP send a so called hole-punching packet on each mapping, to the IP
address given as the servers source address. To create minimum address and port given as the server's source address and port. For
problems for the server these UDP packets should be sent to the a NAT that only is "Address Dependent" filtering, the hole-punching
server's discard port (port number 9). Since UDP packets are packet could be sent to the server's discard port (port number 9).
inherently unreliable, to ensure that at least one UDP message passes For "Address and Port Dependent" filtering NATs the hole-punching
the NAT, hole-punching packets should be retransmitted a reasonable packet must go to the port used for sending UDP packets to the
number of times. client. To be able to do that the server need 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.
For an "Address and Port Dependent" filtering NAT the client must As hole-punching and keep-alive messages, one could have used the RTP
send messages to the exact ports used by the server to send UDP No-Op packet [I-D.ietf-avt-rtp-no-op] had they been defined. That
packets before sending a RTSP PLAY request. This makes it possible would have ensured that the traffic would look like RTP and thus
to use the above described process with the following additional likely have the least risk of being dropped by any firewall. The
restrictions: for each port mapping, hole-punching packets need to be drawback of using RTP No-Op is that the payload type number must be
sent first to the server's source address/port. To minimize dynamically assigned through RTSP first. Other options are STUN, a
potential effects on the server from these messages the following RTP packet without any payload, or an UDP packet without any payload.
type of hole punching packets must be sent. RTP: an empty or less For RTCP it is most suitable to use correctly generated RTCP packets.
than 12 bytes UDP packet. RTCP: A correctly formatted RTCP RR or SR In general sending unsolicited traffic to the RTSP server may trigger
message. The above described adaptations for restricted NATs will security functions resulting in blocking of the keep-alive messages
not work unless the server includes the "src_addr" in the "Transport" or termination of the RTSP session itself.
header (which is the "source" transport parameter in RFC2326).
This method is brittle because it assumes one can use STUN to This method is further brittle as it doesn't support address and port
classify the NAT behavior, which was found to be problematic dependent mappings. Thus, it proposes to use the old STUN methods to
[RFC5389]. If the NAT changes the properties of the existing mapping classify the NAT behavior, thus enabling early error indication.
and filtering state for example due to load, then the methods will This is strictly not required but will lead to failures during setup
fail. when the NAT has the wrong behavior. This failure can 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 4.1.3. ALG considerations
If a NAT supports RTSP ALG (Application Level Gateway) and is not If a NAT supports RTSP ALG (Application Level Gateway) and is not
aware of the STUN traversal option, service failure may happen, aware of the STUN traversal option, service failure may happen,
because a client discovers its NAT external IP address and port because a client discovers its NAT external IP address and port
numbers, and inserts them in its SETUP requests. When the RTSP ALG numbers, and inserts them in its SETUP requests. When the RTSP ALG
processes the SETUP request it may change the destination and port processes the SETUP request it may change the destination and port
number, resulting in unpredictable behavior. An ALG should not number, resulting in unpredictable behavior. An ALG should not
update address fields which contains addresses other than the NATs update address fields which contains addresses other than the NATs
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o Only works with NATs that perform endpoint independent and address o Only works with NATs that perform endpoint independent and address
dependent mappings. Address and Port-Dependent filtering NATs dependent mappings. Address and Port-Dependent filtering NATs
create some issues. create some issues.
o Brittle to NATs changing the properties of the NAT mapping and o Brittle to NATs changing the properties of the NAT mapping and
filtering. filtering.
o Does not work with port and address dependent mapping NATs without o Does not work with port and address dependent mapping NATs without
server modifications. server modifications.
o Will mostly not work if a NAT uses multiple IP addresses, since o Will not work if a NAT uses multiple IP addresses, since RTSP
RTSP servers generally require all media streams to use the same servers generally require all media streams to use the same IP as
IP as used in the RTSP connection to prevent becoming a DDoS tool. used in the RTSP connection to prevent becoming a DDoS tool.
o Interaction problems exist when a RTSP-aware ALG interferes with o Interaction problems exist when a RTSP-aware ALG interferes with
the use of STUN for NAT traversal unless TLS secured RTSP message the use of STUN for NAT traversal unless TLS secured RTSP message
exchange is used. exchange is used.
o Using STUN requires that RTSP servers and clients support the o Using STUN requires that RTSP servers and clients support the
updated RTSP specification [I-D.ietf-mmusic-rfc2326bis], because updated RTSP specification [I-D.ietf-mmusic-rfc2326bis], because
it is no longer possible to guarantee that RTP and RTCP ports are it is no longer possible to guarantee that RTP and RTCP ports are
adjacent to each other, as required by the "client_port" and adjacent to each other, as required by the "client_port" and
"server_port" parameters in RFC2326. "server_port" parameters in RFC2326.
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In order to use STUN to traverse "address and port dependent" In order to use STUN to traverse "address and port dependent"
filtering or mapping NATs the STUN server needs to be co-located with filtering or mapping NATs the STUN server needs to be co-located with
the streaming server media output ports. This creates a de- the streaming server media output ports. This creates a de-
multiplexing problem: we must be able to differentiate a STUN packet multiplexing problem: we must be able to differentiate a STUN packet
from a media packet. This will be done based on heuristics. The from a media packet. This will be done based on heuristics. The
existing STUN heuristics is the first byte in the packet and the existing STUN heuristics is the first byte in the packet and the
Magic Cookie field (added in RFC5389), which works fine between STUN Magic Cookie field (added in RFC5389), which works fine between STUN
and RTP or RTCP where the first byte happens to be different. Thanks 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 to the magic cookie field it is unlikely that other protocols would
be mistaken for a STUN packet, but not assured. be mistaken for a STUN packet, but not assured. For more discussion
of this, please see Section 5.1.2 of [RFC5764].
4.2.4. ALG considerations 4.2.4. ALG considerations
The same ALG traversal considerations as for Stand-Alone STUN applies The same ALG traversal considerations as for Stand-Alone STUN applies
(Section 4.1.3). (Section 4.1.3).
4.2.5. Deployment Considerations 4.2.5. Deployment Considerations
For the "Embedded STUN" method the following applies: For the "Embedded STUN" method the following applies:
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in-depth security discussion. in-depth security discussion.
o This solution works as long as there is only one RTSP endpoint in 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 the private address realm, regardless of the NAT's type. There
may even be multiple NATs (see Figure 1 in [RFC5389]). may even be multiple NATs (see Figure 1 in [RFC5389]).
o Compared to other UDP based NAT traversal methods in this o Compared to other UDP based NAT traversal methods in this
document, STUN requires little new protocol development (since document, STUN requires little new protocol development (since
STUN is already a IETF standard), and most likely less STUN is already a IETF standard), and most likely less
implementation effort, since open source STUN server and client implementation effort, since open source STUN server and client
implementations are available [STUN-IMPL][PJNATH]. There is the implementations are available [STUN-IMPL][PJNATH].
need to embed STUN in RTSP server and client, which require a de-
multiplexer between STUN packets and RTP/RTCP packets. There is
also a need to register the proper feature tags.
Disadvantages: Disadvantages:
o Some extensions to the RTSP core protocol, likely signaled by RTSP o Some extensions to the RTSP core protocol, likely signaled by RTSP
feature tags, must be introduced. feature tags, must be introduced.
o Requires an embedded STUN server to be co-located on each of the 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), RTSP server's media protocol's ports (e.g. RTP and RTCP ports),
which means more processing is required to de-multiplex STUN which means more processing is required to de-multiplex STUN
packets from media packets. For example, the de-multiplexer must packets from media packets. For example, the de-multiplexer must
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2. The client gathers addresses and puts together its candidates for 2. The client gathers addresses and puts together its candidates for
each media stream indicated in the session description. each media stream indicated in the session description.
3. In each SETUP request the client includes its candidates in an 3. In each SETUP request the client includes its candidates in an
ICE specific transport specification. This indicates for the ICE specific transport specification. This indicates for the
server the ICE support by the client. One candidate is the most server the ICE support by the client. One candidate is the most
prioritized candidate and here the prioritization for this prioritized candidate and here the prioritization for this
address should be somewhat different compared to SIP. High address should be somewhat different compared to SIP. High
performance candidates is recommended rather than candidates with performance candidates is recommended rather than candidates with
the highest likelly hood of success, as it is more likely that a the highest likellihood of success, as it is more likely that a
server is not behind a NAT compared to a SIP user-agent. server is not behind a NAT compared to a SIP user-agent.
4. The server responds to the SETUP (200 OK) for each media stream 4. The server responds to the SETUP (200 OK) for each media stream
with its candidates. A server not behind a NAT usually only with its candidates. A server not behind a NAT usually only
provides a single ICE candidate. Also here one candidate is the provides a single ICE candidate. Also here one candidate is the
server primary address. server primary address.
5. The connectivity checks are performed. For the server the 5. The connectivity checks are performed. For the server the
connectivity checks from the server to the clients have an connectivity checks from the server to the clients have an
additional usage. They verify that there is someone willing to additional usage. They verify that there is someone willing to
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To keep media paths alive the client needs to periodically send data 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 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 client should be able to keep RTCP open but STUN will also be used
based on the same motivations as for ICE for SIP. based on the same motivations as for ICE for SIP.
4.3.3. Implementation burden of ICE 4.3.3. Implementation burden of ICE
The usage of ICE will require that a number of new protocols and new 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 RTSP/SDP features be implemented. This makes ICE the solution that
has the largest impact on client and server implementations amongst has the largest impact on client and server implementations among all
all the NAT/firewall traversal methods in this document. the NAT/firewall traversal methods in this document.
RTSP server implementation requirements are: RTSP server implementation requirements are:
o STUN server features o STUN server features
o Limited STUN client features o Limited STUN client features
o SDP generation with more parameters. o SDP generation with more parameters.
o RTSP error code for ICE extension o RTSP error code for ICE extension
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o SDP generation with more parameters. o SDP generation with more parameters.
o RTSP error code for ICE extension o RTSP error code for ICE extension
RTSP client implementation requirements are: RTSP client implementation requirements are:
o Limited STUN server features o Limited STUN server features
o Limited STUN client features o Limited STUN client features
o RTSP error code and ICE extension o RTSP error code and ICE extension
4.3.4. Deployment Considerations 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 manifest itself in the RTSP message primarily as
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 signalling
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 filtering
state that would allow incoming STUN messages prior to any outgoing
hole-punching packets, and in that way speed up the connectivity
checks and reduce the risk of failures.
4.3.5. Deployment Considerations
Advantages: Advantages:
o Solves NAT connectivity discovery for basically all cases as long o Solves NAT connectivity discovery for basically all cases as long
as a TCP connection between the client and server can be as a TCP connection between the client and server can be
established. This includes servers behind NATs. (Note that a established. This includes servers behind NATs. (Note that a
proxy between address domains may be required to get TCP through). proxy between address domains may be required to get TCP through).
o Improves defenses against DDoS attacks, since a media receiving o Improves defenses against DDoS attacks, since a media receiving
client requires authentications, via STUN on its media reception client requires authentications, via STUN on its media reception
ports. ports.
Disadvantages: Disadvantages:
o Increases the setup delay with at least the amount of time it o Increases the setup delay with at least the amount of time it
takes for the server to perform its STUN requests. takes for the server to perform its STUN requests.
o Assumes that it is possible to de-multiplex between the packets of o Assumes that it is possible to de-multiplex between the packets of
the media protocol and STUN packets. the media protocol and STUN packets. This is possible for RTP as
discussed for example in Section 5.1.2 of [RFC5764].
o Has fairly high implementation burden put on both RTSP server and o Has fairly high implementation burden put on both RTSP server and
client. However, several Open Source ICE implementations do client. However, several Open Source ICE implementations do
exist, such as [NICE][PJNATH]. exist, such as [NICE][PJNATH].
4.3.5. Security Consideration 4.3.6. Security Consideration
One should review the security consideration section of ICE and STUN One should review the security consideration section of ICE and STUN
to understand that ICE contains some potential issues. However these to understand that ICE contains some potential issues. However these
can be avoided by correctly using ICE in RTSP. An important factor can be avoided by correctly using ICE in RTSP. An important factor
is to secure the signalling, i.e. use TLS between RTSP client and is to secure the signalling, i.e. use TLS between RTSP client and
server. In fact ICE does help avoid the DDoS attack issue with RTSP server. In fact ICE does help avoid the DDoS attack issue with RTSP
substantially as it reduces the possibility for a DDoS using RTSP substantially as it reduces the possibility for a DDoS using RTSP
servers to attackers that are on-path between the RTSP server and the servers to attackers that are on-path between the RTSP server and the
target and capable of intercepting the STUN connectivity check target and capable of intercepting the STUN connectivity check
packets and correctly send a response to the server. packets and correctly send a response to the server.
4.4. Latching 4.4. Latching
4.4.1. Introduction 4.4.1. Introduction
Latching [I-D.ietf-mmusic-latching] is a NAT traversal solution that Latching is a NAT traversal solution that is based on requiring RTSP
is based on requiring RTSP clients to send UDP packets to the clients to send UDP packets to the server's media output ports.
server's media output ports. Conventionally, RTSP servers send RTP Conventionally, RTSP servers send RTP packets in one direction: from
packets in one direction: from server to client. Latching is similar server to client. Latching is similar to connection-oriented
to connection-oriented traffic, where one side (e.g., the RTSP traffic, where one side (e.g., the RTSP client) first "connects" by
client) first "connects" by sending a RTP packet to the other side's sending a RTP packet to the other side's RTP port, the recipient then
RTP port, the recipient then replies to the originating IP and port. replies to the originating IP and port. This method is also referred
This method is also referred to as "Late binding". It requires that to as "Late binding". It requires that all RTP/RTCP transport is
all RTP/RTCP transport is done symmetrical, i.e. Symmetric RTP done symmetrical, i.e. Symmetric RTP [RFC4961]. There exist a
[RFC4961]. description for latching of SIP negotiated media streams in Session
Border Controllers [RFC7362].
Specifically, when the RTSP server receives the latching packet 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 (a.k.a. hole-punching packet, since it is used to punch a hole in the
firewall/NAT and to aid the server for port binding and address firewall/NAT and to aid the server for port binding and address
mapping) from its client, it copies the source IP and Port number and mapping) from its client, it copies the source IP and Port number and
uses them as delivery address for media packets. By having the uses them as delivery address for media packets. By having the
server send media traffic back the same way as the client's packet server send media traffic back the same way as the client's packet
are sent to the server, address mappings will be honored. Therefore are sent to the server, address mappings will be honored. Therefore
this technique works for all types of NATs, given that the server is this technique works for all types of NATs, given that the server is
not behind a NAT. However, it does require server modifications. not behind a NAT. However, it does require server modifications.
Unless there is built-in protection mechanism, latching is very The format of the latching packet will have to be defined.
vulnerable to both hijacking and becoming a tool in Distributed
Denail of Service (DDoS) attacks (See Security Considerations of Latching is very vulnerable to both hijacking and becoming a tool in
[I-D.ietf-mmusic-latching]), because attackers can simply forge the Distributed Denial of Service (DDoS) attacks (See Security
Considerations of [RFC7362]), because attackers can simply forge the
source IP & Port of the latching packet. Using the rule for source IP & Port of the latching packet. Using the rule for
restricting IP address to the one of the signaling connection will restricting IP address to the one of the signaling connection will
need to be applied here also. However, that does not protect against need to be applied here also. However, that does not protect against
hijacking from another client behind the same NAT. This can become a hijacking from another client behind the same NAT. This can become a
serious issue in deployments with CGNs. serious issue in deployments with CGNs.
4.4.2. Necessary RTSP extensions 4.4.2. Necessary RTSP extensions
To support Latching, the RTSP signaling must be extended to allow the To support Latching, the RTSP signaling must be extended to allow the
RTSP client to indicate that it will use Latching. The client also 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 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 request. The RTP SSRC is used to establish some basic level of
security against hijacking attacks or simply avoid mis-association security against hijacking attacks or simply avoid mis-association
when multiple clients are behind the same NAT. Care must be taken in 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 choosing clients' RTP SSRC. First, it must be unique within all the
RTP sessions belonging to the same RTSP session. Secondly, if the RTP sessions belonging to the same RTSP session. Secondly, if the
RTSP server is sending out media packets to multiple clients from the RTSP server is sending out media packets to multiple clients from the
same send port, the RTP SSRC needs to be unique amongst those same send port, the RTP SSRC needs to be unique among those clients'
clients' RTP sessions. Recognizing that there is a potential that RTP sessions. Recognizing that there is a potential that RTP SSRC
RTP SSRC collisions may occur, the RTSP server must be able to signal collisions may occur, the RTSP server must be able to signal to a
to a client that a collision has occurred and that it wants the client that a collision has occurred and that it wants the client to
client to use a different RTP SSRC carried in the SETUP response or use a different RTP SSRC carried in the SETUP response or use unique
use unique ports per RTSP session. Using unique ports limits an RTSP ports per RTSP session. Using unique ports limits an RTSP server in
server in the number of sessions it can simultaneously handle per the number of sessions it can simultaneously handle per interface IP
interface IP addresses. addresses.
4.4.3. Deployment Considerations The latching packet as discussed above should have field which can
contain an 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 have been a benefit to use a well defined RTP
payload format for this purpose as the No-Op payload format proposed
[I-D.ietf-avt-rtp-no-op]. However, in the absence of such a
specification an RTP packet without a payload could be used. Using
SSRC has the benefit that RTP and RTCP both would work as is.
However, also other packet formats could be used that carry the
necessary identification of the context, and such a solution is
discussed in Section 4.5.
4.4.3. ALG Considerations
An RTSP ALG not supporting this method could interfer with the
methods used to indicate that latching is to be done, as well as the
SSRC signalling. Thus preventing the method from working. However,
if the RTSP ALG instead opens the corresponding pinholes and create
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 support this traversal method can for basic
functionality simply pass the related signalling parameters
transparently. Due to the security considerations for latching it
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: Advantages:
o Works for all types of client-facing NATs. (Requirement 1 in o Works for all types of client-facing NATs. (Requirement 1 in
Section 3). Section 3).
o Has no interaction problems with any RTSP ALG changing the o Has little interaction problems with any RTSP ALG changing the
client's information in the transport header. client's information in the transport header.
Disadvantages: Disadvantages:
o Requires modifications to both RTSP server and client. o Requires modifications to both RTSP server and client.
o Limited to work with servers that are not behind a NAT. o Limited to work with servers that are not behind a NAT.
o The format of the RTP packet for "connection setup" (a.k.a o The format of the packet for "connection setup" (a.k.a Latching
Latching packet) is not defined. One possibility considered was packet) is not defined.
to use RTP No-Op packet format in [I-D.ietf-avt-rtp-no-op], a
proposal which has been abandoned.
o SSRC management if RTP is used for latching due to risk for mis- 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 association of clients to RTSP sessions at the server if SSRC
collision occurs. collision occurs.
o Has significant security considerations (See Section 4.4.4), due o Has significant security considerations (See Section 4.4.5), due
to lack of a strong authentication mechanism and will need to use to lack of a strong authentication mechanism and will need to use
address restrictions. address restrictions.
4.4.4. Security Consideration 4.4.5. Security Consideration
Latching's major security issue is that RTP streams can be hijacked Latching's major security issue is that RTP streams can be hijacked
and directed towards any target that the attacker desires unless and directed towards any target that the attacker desires unless
address restrictions are used. In the case of NATs with multiple address restrictions are used. In the case of NATs with multiple
clients on the inside of them, hijacking can still occur. This clients on the inside of them, hijacking can still occur. This
becomes a significant threat in the context of carrier grade NATs becomes a significant threat in the context of carrier grade NATs
(CGN). (CGN).
The most serious security problem is the deliberate attack with the The most serious security problem is the deliberate attack with the
use of a RTSP client and Latching. The attacker uses RTSP to setup a use of a RTSP client and Latching. The attacker uses RTSP to setup a
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attack is based on the ability to read the RTSP signaling packets in 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 order to learn the address and port the server will send from and
also the SSRC the client will use. Having this information the also the SSRC the client will use. Having this information the
attacker can send its own Latching packets containing the correct RTP attacker can send its own Latching packets containing the correct RTP
SSRC to the correct address and port on the server. The RTSP server 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 will then use the source IP and port from the Latching packet as the
destination for the media packets it sends. destination for the media packets it sends.
Another variation of this attack is for a man in the middle to modify 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 the RTP latching packet being sent by a client to the server by
simply changing the source IP to the target one desires to attack. 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 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 RTSP signaling transport. However, if the attacker is a man in
the middle modifying latching packets, the attack is impossible to the middle modifying latching packets, the attack is impossible to
defend against other than through address restrictions. As a NAT re- defend against other than through address restrictions. As a NAT re-
writes the source IP and (possibly) port this cannot be writes the source IP and (possibly) port this cannot be
authenticated, but authentication is required in order to protect authenticated, but authentication is required in order to protect
against this type of DoS attack. against this type of DoS attack.
Yet another issue is that these attacks also can be used to deny the 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 client the service it desires from the RTSP server completely. The
attacker modifies or originates its own latching packets with another attacker modifies or originates its own latching packets with another
port than what the legit latching packets uses, which results in that port than what the legit latching packets uses, which results in that
the media server sends the RTP/RTCP traffic to ports the client isn't the media server sends the RTP/RTCP traffic to ports the client isn't
listening for RTP/RTCP on. listening for RTP/RTCP on.
The amount of random non-guessable material in the latching packet The amount of random non-guessable material in the latching packet
determines how well Latching can fend off stream-hijacking performed determines how well Latching can fend off stream-hijacking performed
by parties that are off the client to server network path, i.e. lacks by parties that are off the client to server network path, i.e. lacks
the capability to see the client's latching packets. This proposal the capability to see the client's latching packets. The proposal
uses the 32-bit RTP SSRC field to this effect. Therefore it is 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 important that this field is derived with a non-predictable random
number generator. It should not be possible by knowing the algorithm number generator. It should not be possible by knowing the algorithm
used and a couple of basic facts, to derive what random number a used and a couple of basic facts, to derive what random number a
certain client will use. certain client will use.
An attacker not knowing the SSRC but aware of which port numbers that 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 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. testing a lot of different SSRCs until it finds a matching one.
Therefore a server could implement functionality that blocks packets Therefore a server could implement functionality that blocks packets
to ports or from sources that receive or send multiple Latching to ports or from sources that receive or send multiple Latching
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RTSP signaling can be added to do the following: RTSP signaling can be added to do the following:
1. Enable or disable such Latching message exchanges. When the 1. Enable or disable such Latching message exchanges. When the
firewall/NAT has an RTSP-aware ALG, it is possible to disable firewall/NAT has an RTSP-aware ALG, it is possible to disable
Latching message exchange and let the ALG work out the address Latching message exchange and let the ALG work out the address
and port mappings. and port mappings.
2. Configure the number of re-tries and the re-try interval of the 2. Configure the number of re-tries and the re-try interval of the
Latching message exchanges. Latching message exchanges.
4.5.3. Deployment Considerations 4.5.3. ALG Considerations
See Latching ALG consideration Section 4.4.3.
4.5.4. Deployment Considerations
This approach has the following advantages when compared with the This approach has the following advantages when compared with the
Latching approach (Section 4.4): Latching approach (Section 4.4):
1. There is no need to define RTP payload format for firewall 1. There is no need to define RTP payload format for firewall
traversal, therefore it is simple to use, implement and traversal, therefore it is simple to use, implement and
administer (Requirement 4 in Section 3), instead a Latching administer (Requirement 4 in Section 3), instead a Latching
protocol must be defined. protocol must be defined.
2. When properly defined, this kind of Latching packet exchange can 2. When properly defined, this kind of Latching packet exchange can
also authenticate RTP receivers, to prevent hijacking attacks. also authenticate RTP receivers, to prevent hijacking attacks.
This approach has the following disadvantages when compared with the This approach has the following disadvantages when compared with the
Latching approach: Latching approach:
1. RTP traffic is normally accompanied by RTCP traffic. This 1. The server's sender SSRC for the RTP stream or other session
approach needs to rely on RTCP RRs and SRs to enable NAT
traversal for RTCP endpoints, use RTP/RTCP Multiplexing
[RFC5761], or use the same type of Latching packets also for RTCP
endpoints.
2. The server's sender SSRC for the RTP stream or other session
Identity information must be signaled in RTSP's SETUP response, Identity information must be signaled in RTSP's SETUP response,
in the Transport header of the RTSP SETUP response. in the Transport header of the RTSP SETUP response.
4.5.4. Security Considerations 4.5.5. Security Considerations
Compared to the security properties of Latching this variant is Compared to the security properties of Latching this variant is
slightly improved. First of all it allows for a larger random field 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 in the Latching packets which makes it more unlikely for an off-path
attacker to succeed in a hi-jack attack. Secondly the confirmation attacker to succeed in a hi-jack attack. Secondly the confirmation
allows the client to know when Latching works and when it didn't and allows the client to know when Latching works and when it didn't and
thus restart the Latching process by updating the SSRC. Thirdly if thus restart the Latching process by updating the SSRC.
an authentication mechanism is included in the latching packet
hijacking attacks can be prevented.
Still the main security issue remain that the RTSP server can't know Still the main security issue remain that the RTSP server can't know
that the source address in the latching packet was coming from a RTSP that the source address in the latching packet was coming from a RTSP
client wanting to receive media and not one that likes to direct the client wanting to receive media and not one that likes to direct the
media traffic to an DoS target. media traffic to an DoS target.
4.6. Three Way Latching 4.6. Three Way Latching
4.6.1. Introduction 4.6.1. Introduction
The three way Latching is an attempt to try to resolve the most The three way latching is an attempt to try to resolve the most
significant security issues for both previously discussed variants of significant security issues for both previously discussed variants of
Latching. By adding a server request response exchange directly Latching. By adding a server request response exchange directly
after the initial latching the server can verify that the target after the initial latching the server can verify that the target
address present in the latching packet is an active listener and address present in the latching packet is an active listener and
confirm its desire to establish a media flow. confirm its desire to establish a media flow.
4.6.2. Necessary RTSP extensions 4.6.2. Necessary RTSP extensions
Uses the same RTSP extensions as the alternative latching method Uses the same RTSP extensions as the alternative latching method
(Section 4.5) uses. The extensions for this variant are only in the (Section 4.5) uses. The extensions for this variant are only in the
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Latching packet with a Latching confirmation, it includes a random Latching packet with a Latching confirmation, it includes a random
value (Nonce) of its own in addition to echoing back the one the 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 sent. Then a third message is added to the exchange. The
client acknowledges the reception of the Latching confirmation client acknowledges the reception of the Latching confirmation
message and echoes back the server's nonce. Thus confirming that the message and echoes back the server's nonce. Thus confirming that the
Latched address goes to a RTSP client that initiated the latching and Latched address goes to a RTSP client that initiated the latching and
is actually present at that address. The RTSP server will refuse to is actually present at that address. The RTSP server will refuse to
send any media until the Latching Acknowledgement has been received send any media until the Latching Acknowledgement has been received
with a valid nonce. with a valid nonce.
4.6.3. Deployment Considerations 4.6.3. ALG Considerations
See Latching ALG consideration Section 4.4.3.
4.6.4. Deployment Considerations
A solution with a 3-way handshake and its own Latching packets can be A solution with a 3-way handshake and its own Latching packets can be
compared with the ICE-based solution (Section 4.3) and have the compared with the ICE-based solution (Section 4.3) and have the
following differences: following differences:
o Only works for servers that are not behind a NAT. o Only works for servers that are not behind a NAT.
o May be simpler to implement due to the avoidance of the ICE o May be simpler to implement due to the avoidance of the ICE
prioritization and check-board mechanisms. prioritization and check-board mechanisms.
However, a 3-way Latching protocol is very similar to using STUN in However, a 3-way Latching protocol is very similar to using STUN in
both directions as Latching and verification protocol. Using STUN both directions as Latching and verification protocol. Using STUN
would remove the need for implementing a new protocol. would remove the need for implementing a new protocol.
4.6.5. Security Considerations
The three way latching is significantly securer than its simpler
versions discussed above. The client to server nonce which is
included in signalling 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 an denial of service attack by
diverting the media.
The client to server nonce and its echoing back does not protect
against on-patch attacker, 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.
Three way latching is really only vulnerable to an on-path attacker
that is quite capable. First the attacker can either learn the
client to server nonce, by intercepting the signalling, or modifying
the source information (target destination) of a client's latching
packet. Secondly, it is also on-path between the server and target
destination and can generate a response using the server's nonce. An
adversary that has these capabilities are commonly capable of causing
significantly worse damage than this using other methods.
Three-way latching do results in that the server to client packet is
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 do 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
response it will generate before considering the latching to be
failed.
4.7. Application Level Gateways 4.7. Application Level Gateways
4.7.1. Introduction 4.7.1. Introduction
An Application Level Gateway (ALG) reads the application level An Application Level Gateway (ALG) reads the application level
messages and performs necessary changes to allow the protocol to work messages and performs necessary changes to allow the protocol to work
through the middle box. However this behavior has some problems in through the middle box. However this behavior has some problems in
regards to RTSP: regards to RTSP:
1. It does not work when the RTSP protocol is used with end-to-end 1. It does not work when the RTSP protocol is used with end-to-end
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functionality. functionality.
o When end-to-end security is used, the ALG functionality will fail. o When end-to-end security is used, the ALG functionality will fail.
o Can interfere with other types of traversal mechanisms, such as o Can interfere with other types of traversal mechanisms, such as
STUN. STUN.
Transition: Transition:
An RTSP ALG will not be phased out in any automatic way. It must be An RTSP ALG will not be phased out in any automatic way. It must be
removed, probably through the removal of the NAT it is associated removed, probably through the removal or update of the NAT it is
with. associated with.
4.7.4. Security Considerations 4.7.4. Security Considerations
An ALG will not work with deployment of end-to-end RTSP signaling An ALG will not work with deployment of end-to-end RTSP signaling
security. Therefore deployment of ALG will likely result in clients security, however it will work with the hop-by-hop security method
located behind NATs not using end-to-end security, or more likely defined in Section 19.3 of RTSP 2.0 [I-D.ietf-mmusic-rfc2326bis].
select a NAT traversal solution that allow for security. Therefore deployment of ALG may result in clients located behind NATs
not using end-to-end security, or more likely the selection a NAT
traversal solution that allow for security.
The creation of an UDP mapping based on the signalling message has The creation of an UDP mapping based on the signalling message has
some potential security implications. First of all if the RTSP some potential security implications. First of all if the RTSP
client releases its ports and another application are assigned these client releases its ports and another application are assigned these
instead it could receive RTP media as long as the mappings exist and instead it could receive RTP media as long as the mappings exist and
the RTSP server has failed to be signalled or notice the lack of the RTSP server has failed to be signalled or notice the lack of
client response. client response.
A NAT with RTSP ALG that assigns mappings based on SETUP requests A NAT with RTSP ALG that assigns mappings based on SETUP requests
could potentially become victim of a resource exhaustion attack. If could potentially become victim of a resource exhaustion attack. If
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connection opened from the private domain ensures that the server can connection opened from the private domain ensures that the server can
send data back to the client. To send data originally intended to be send data back to the client. To send data originally intended to be
transported over UDP requires the TCP connection to support some type transported over UDP requires the TCP connection to support some type
of framing of the media data packets. Using TCP also results in the of framing of the media data packets. Using TCP also results in the
client having to accept that real-time performance can be impacted. client having to accept that real-time performance can be impacted.
TCP's problem of ensuring timely delivery was one of the reasons why 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 RTP was developed. Problems that arise with TCP are: head-of-line
blocking, delay introduced by retransmissions, highly varying rate blocking, delay introduced by retransmissions, highly varying rate
due to the congestion control algorithm. If sufficient amount of due to the congestion control algorithm. If sufficient amount of
buffering (several seconds) in the receiving client can be tolerated buffering (several seconds) in the receiving client can be tolerated
then TCP clearly can work. then TCP clearly work.
4.8.2. Usage of TCP tunneling in RTSP 4.8.2. Usage of TCP tunneling in RTSP
The RTSP core specification [I-D.ietf-mmusic-rfc2326bis] supports The RTSP core specification [I-D.ietf-mmusic-rfc2326bis] supports
interleaving of media data on the TCP connection that carries RTSP interleaving of media data on the TCP connection that carries RTSP
signaling. See section 14 in [I-D.ietf-mmusic-rfc2326bis] for how to signaling. See section 14 in [I-D.ietf-mmusic-rfc2326bis] for how to
perform this type of TCP tunneling. There also exists another way of perform this type of TCP tunneling. There also exists another way of
transporting RTP over TCP defined in Appendix C.2 in transporting RTP over TCP defined in Appendix C.2 in
[I-D.ietf-mmusic-rfc2326bis]. For signaling and rules on how to [I-D.ietf-mmusic-rfc2326bis]. For signaling and rules on how to
establish the TCP connection in lieu of UDP, see appendix C.2 in 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 [I-D.ietf-mmusic-rfc2326bis]. This is based on the framing of RTP
over the TCP connection as described in RFC 4571 [RFC4571]. over the TCP connection as described in RFC 4571 [RFC4571].
4.8.3. Deployment Considerations 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 will flow 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 much resource and thus creating
performance issues.
The RTSP ALG can also effect the transport specifications that
indicate that TCP tunneling can be done and its priortization,
including removing the transport specification, thus preventing TCP
tunneling.
4.8.4. Deployment Considerations
Advantage: Advantage:
o Works through all types of NATs where the RTSP server in not NATed o Works through all types of NATs where the RTSP server in not NATed
or at least reachable like it was not. or at least reachable like it was not.
Disadvantage: Disadvantage:
o Functionality needs to be implemented on both server and client. o Functionality needs to be implemented on both server and client.
o Will not always meet multimedia stream's real-time requirements. o Will not always meet multimedia stream's real-time requirements.
Transition: Transition:
The tunneling over RTSP's TCP connection is not planned to be phased- 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 out. It is intended to be a fallback mechanism and for usage when
total media reliability is desired, even at the potential price of total media reliability is desired, even at the potential price of
loss of real-time properties. loss of real-time properties.
4.8.4. Security Considerations 4.8.5. Security Considerations
The TCP tunneling of RTP has no known security problems besides those The TCP tunneling of RTP has no known significant security problems
already presented in the RTSP specification. It is not possible to besides those already presented in the RTSP specification. It is
get any amplification effect for denial of service attacks due to difficult to get any amplification effect for denial of service
TCP's flow control. A possible security consideration, when session attacks due to TCP's flow control. The RTSP server TCP socket,
media data is interleaved with RTSP, would be the performance independently if used for media tunneling or only RTSP messages can
bottleneck when RTSP encryption is applied, since all session media be used for a redirected syn attack. By spoofing the source address
data also needs to be encrypted. 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. TURN (Traversal Using Relay NAT) 4.9. TURN (Traversal Using Relay NAT)
4.9.1. Introduction 4.9.1. Introduction
Traversal Using Relay NAT (TURN) [RFC5766] is a protocol for setting Traversal Using Relay NAT (TURN) [RFC5766] is a protocol for setting
up traffic relays that allow clients behind NATs and firewalls to up traffic relays that allow clients behind NATs and firewalls to
receive incoming traffic for both UDP and TCP. These relays are receive incoming traffic for both UDP and TCP. These relays are
controlled and have limited resources. They need to be allocated controlled and have limited resources. They need to be allocated
before usage. TURN allows a client to temporarily bind an address/ before usage. TURN allows a client to temporarily bind an address/
port pair on the relay (TURN server) to its local source address/port 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 pair, which is used to contact the TURN server. The TURN server will
then forward packets between the two sides of the relay. then forward packets between the two sides of the relay.
skipping to change at page 33, line 9 skipping to change at page 35, line 40
receive media packets. TURN supports requesting bindings of even receive media packets. TURN supports requesting bindings of even
port numbers and contiguous ranges. port numbers and contiguous ranges.
3. The RTSP client uses the acquired address and port allocations in 3. The RTSP client uses the acquired address and port allocations in
the RTSP SETUP request using the destination header. the RTSP SETUP request using the destination header.
4. The RTSP Server sends the SETUP reply, which must include the 4. The RTSP Server sends the SETUP reply, which must include the
transport headers src_addr parameter (source and port in RTSP transport headers src_addr parameter (source and port in RTSP
1.0). Note that the server is required to have a mechanism to 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 verify that it is allowed to send media traffic to the given
address. address unless TCP relaying of the RTSP messages also is
performed.
5. The RTSP Client uses the RTSP Server's response to create TURN 5. The RTSP Client uses the RTSP Server's response to create TURN
permissions for the server's media traffic. permissions for the server's media traffic.
6. The client requests that the server starts playing. The server 6. The client requests that the server starts playing. The server
starts sending media packets to the given destination address and starts sending media packets to the given destination address and
ports. ports.
7. Media packets arrive at the TURN server on the external port; If 7. Media packets arrive at the TURN server on the external port; If
the packets match an established permission, the TURN server the packets match an established permission, the TURN server
forwards the media packets to the RTSP client. forwards the media packets to the RTSP client.
8. If the client pauses and media is not sent for about 75% of the 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 mapping timeout the client should use TURN to refresh the
bindings. bindings.
4.9.3. Deployment Considerations 4.9.3. ALG Considerations
As the RTSP client inserts the address information of the TURN
relay's external allocations in the SETUP messages, and ALG that
replaces the address, without considering that the address do not
belong to the internal address realm of the NAT, will prevent this
mechanism from working. This can be prevented by securing the RTSP
signalling.
4.9.4. Deployment Considerations
Advantages: Advantages:
o Does not require any server modifications given that the server o Does not require any server modifications given that the server
includes the src_addr header in the SETUP response. includes the src_addr header in the SETUP response.
o Works for any type of NAT as long as the RTSP server has reachable o Works for any type of NAT as long as the RTSP server has reachable
IP address that is not behind a NAT. IP address that is not behind a NAT.
Disadvantage: Disadvantage:
o Requires another network element, namely the TURN server. o Requires another network element, namely the TURN server.
o A TURN server for RTSP may not scale since the number of sessions 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 it must forward is proportional to the number of client media
sessions. sessions.
o TURN server becomes a single point of failure. o The TURN server becomes a single point of failure.
o Since TURN forwards media packets, it necessarily introduces o Since TURN forwards media packets, it necessarily introduces
delay. delay.
o An RTSP ALG may change the necessary destinations parameter. This o An RTSP ALG may change the necessary destinations parameter. This
will cause the media traffic to be sent to the wrong address. will cause the media traffic to be sent to the wrong address.
Transition: Transition:
TURN is not intended to be phased-out completely, see Section 19 of TURN is not intended to be phased-out completely, see Section 19 of
[RFC5766]. However the usage of TURN could be reduced when the [RFC5766]. However the usage of TURN could be reduced when the
demand for having NAT traversal is reduced. demand for having NAT traversal is reduced.
4.9.4. Security Considerations 4.9.5. Security Considerations
The TURN server can become part of a denial of service attack towards The TURN server can become part of a denial of service attack towards
any victim. To perform this attack the attacker must be able to any victim. To perform this attack the attacker must be able to
eavesdrop on the packets from the TURN server towards a target for eavesdrop on the packets from the TURN server towards a target for
the DoS attack. The attacker uses the TURN server to setup a RTSP the DoS attack. The attacker uses the TURN server to setup a RTSP
session with media flows going through the TURN server. The attacker session with media flows going through the TURN server. The attacker
is in fact creating TURN mappings towards a target by spoofing the is in fact creating TURN mappings towards a target by spoofing the
source address of TURN requests. As the attacker will need the source address of TURN requests. As the attacker will need the
address of these mappings he must be able to eavesdrop or intercept address of these mappings he must be able to eavesdrop or intercept
the TURN responses going from the TURN server to the target. Having the TURN responses going from the TURN server to the target. Having
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ALG that reads RTSP SETUP and TEARDOWN messages. By reading the ALG that reads RTSP SETUP and TEARDOWN messages. By reading the
SETUP message the firewall can determine what type of transport and SETUP message the firewall can determine what type of transport and
from where, the media stream packets will be sent. Commonly there 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 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 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 minimized to the least necessary. The opening in the
firewall can be closed after a TEARDOWN message for that session or firewall can be closed after a TEARDOWN message for that session or
the session itself times out. the session itself times out.
The above possibilities for firewalls to inspect and respond to the The above possibilities for firewalls to inspect and respond to the
signalling are prevented if confidentiality protection is used for signalling are prevented if end-to-end confidentiality protection is
the RTSP signalling, e.g. using the specified RTSP over TLS. This used for the RTSP signalling, e.g. using the specified RTSP over TLS.
results in that firewalls can't be actively opening pinholes for the This results in that firewalls can't be actively opening pinholes for
media streams based on the signalling. Instead other methods have to the media streams based on the signalling. To enable an RTSP ALG in
be used to enable the transport flows for the media. firewall to correctly function the hop-by-hop signalling security
(See Section 19.3) in RTSP 2.0 [I-D.ietf-mmusic-rfc2326bis] can be
used. 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 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 has sent packets to the target. Depending on the security level this
can have the same behavior as a NAT. The only difference is that no can have the same behavior as a NAT. The only difference is that no
address translation is done. To use such a firewall a client would address translation is done. To use such a firewall a client would
need to implement one of the above described NAT traversal methods need to implement one of the above described NAT traversal methods
that include sending packets to the server to open up the mappings. that include sending packets to the server to open up the mappings.
6. Comparison of NAT traversal techniques 6. Comparison of NAT traversal techniques
This section evaluates the techniques described above against the This section evaluates the techniques described above against the
requirements listed in Section 3. requirements listed in Section 3.
In the following table, the columns correspond to the numbered In the following table, the columns correspond to the numbered
requirements. For instance, the column under R1 corresponds to the requirements. For instance, the column under R1 corresponds to the
first requirement in Section 3: must work for all flavors of NATs. first requirement in Section 3: must work for all flavors of NATs.
The rows represent the different NAT/firewall traversal techniques. The rows represent the different NAT/firewall traversal techniques.
Latch is short for Latching, "V. Latch" is short for "variation of Latch is short for Latching, "V. Latch" is short for "variation of
Latching" as described in Section 4.5. "3-W Latch" is short for the Latching" as described in Section 4.5. "3-W Latch" is short for the
Three Way Latching described in Section 4.6. Three Way Latching described in Section 4.6.
A Summary of the requirements are: A Summary of the requirements are:
R1: Work for all flavors of NATs R1: Work for all flavors of NATs
R2: Must work with firewalls, including those with ALGs R2: Must work with firewalls, including those with ALGs
R3: Should have minimal impact on clients not behind NATs, counted R3: Should have minimal impact on clients not behind NATs, counted
in minimal number of additional RTTs in minimal number of additional RTTs
skipping to change at page 37, line 25 skipping to change at page 40, line 25
different NAT/firewall traversal methods for RTSP discussed here. In different NAT/firewall traversal methods for RTSP discussed here. In
summary, the presence of NAT(s) is a security risk, as a client summary, the presence of NAT(s) is a security risk, as a client
cannot perform source authentication of its IP address. This cannot perform source authentication of its IP address. This
prevents the deployment of any future RTSP extensions providing prevents the deployment of any future RTSP extensions providing
security against hijacking of sessions by a man-in-the-middle. security against hijacking of sessions by a man-in-the-middle.
Each of the proposed solutions has security implications. Using STUN Each of the proposed solutions has security implications. Using STUN
will provide the same level of security as RTSP without transport will provide the same level of security as RTSP without transport
level security and source authentications, as long as the server does 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 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 client request was sent from.
of session hijacking or denial of service than normal RTSP. The
reason is that there exists a probability that an attacker is able to Using Latching will have a higher risk of session hijacking or denial
guess the random bits that the client uses to prove its identity when of service than normal RTSP. The reason is that there exists a
creating the address bindings. This can be solved in the variation probability that an attacker is able to guess the random bits that
of Latching (Section 4.5) with authentication features. Still both the client uses to prove its identity when creating the address
those variants of Latching are vulnerable against deliberate attack bindings. This can be solved in the variation of Latching
from the RTSP client to redirect the media stream requested to any (Section 4.5) with authentication features. Still both those
target assuming it can spoof the source address. This security variants of Latching are vulnerable against 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 vulnerability is solved by performing a Three-way Latching procedure
as discussed in Section 4.6. ICE resolves the binding vulnerability as discussed in Section 4.6.
of latching by using signed STUN messages, as well as requiring that
both sides perform connectivity checks to verify that the target IP ICE resolves the binding vulnerability of latching by using signed
address in the candidate pair is both reachable and willing to STUN messages, as well as requiring that both sides perform
respond. ICE can however create a significant amount of traffic if connectivity checks to verify that the target IP address in the
the number of candidate pairs are large. Thus pacing is required and 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 implementations should attempt to limit their number of candidates to
reduce the number of packets. If the signalling between the ICE reduce the number of packets.
peers (RTSP client and Server) is not confidentiality and integrity
protected ICE is vulnerable to attacks where the candidate list is If the signalling between the ICE peers (RTSP client and Server) is
manipulated. Lack of signalling security will also simplify spoofing not confidentiality and integrity protected ICE is vulnerable to
of STUN binding messages by revealing the secret used in signing. attacks where the candidate list is manipulated. Lack of signalling
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 The usage of an RTSP ALG does not in itself increase the risk for
session hijacking. However the deployment of ALGs as the sole session hijacking. However the deployment of ALGs as the sole
mechanism for RTSP NAT traversal will prevent deployment of end-to- mechanism for RTSP NAT traversal will prevent deployment of end-to-
end encrypted RTSP signaling. The usage of TCP tunneling has no end encrypted RTSP signaling.
known security problems. However, it might provide a bottleneck when
it comes to end-to-end RTSP signaling security if TCP tunneling is The usage of TCP tunneling has no known security problems. However,
used on an interleaved RTSP signaling connection. The usage of TURN it might provide a bottleneck when it comes to end-to-end RTSP
has severe risk of denial of service attacks against a client. The signaling security if TCP tunneling is used on an interleaved RTSP
TURN server can also be used as a redirect point in a DDoS attack signaling connection.
unless the server has strict enough rules for who may create
bindings. The usage of TURN has severe risk of denial of service attacks
against a client. The TURN server can also be used as a redirect
point in a DDoS attack unless the server has strict enough rules for
who may create bindings.
The latching and variant of latching have so big security issues that
they should not be used at all. The three way latching as well as
ICE mitigates these security issues and performs the important
return-routability check that prevents spoofed source addresses, and
should be recommended for that reason. RTP ALG's is a security risk
as they can create an incitement against using secure RTSP
signalling. That can be avoided as ALGs requires 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.
[I-D.ietf-mmusic-rfc2326bis]. The remaining methods can be
considered safe enough, assuming that the appropriate security
mechanisms are used and not ignored.
9. Acknowledgements 9. Acknowledgements
The author would also like to thank all persons on the MMUSIC working The author would also like to thank all persons on the MMUSIC working
group's mailing list that has commented on this document. Persons group's mailing list that has commented on this document. Persons
having contributed in such way in no special order to this protocol having contributed in such way in no special order to this protocol
are: Jonathan Rosenberg, Philippe Gentric, Tom Marshall, David Yon, are: Jonathan Rosenberg, Philippe Gentric, Tom Marshall, David Yon,
Amir Wolf, Anders Klemets, Flemming Andreasen, Ari Keranen, Bill Amir Wolf, Anders Klemets, Flemming Andreasen, Ari Keranen, Bill
Atwood, and Colin Perkins. Thomas Zeng would also like to give Atwood, Alissa Cooper, Colin Perkins, Sarah Banks and David Black.
special thanks to Greg Sherwood of PacketVideo for his input into Thomas Zeng would also like to give special thanks to Greg Sherwood
this memo. of PacketVideo for his input into this memo.
Section 1.1 contains text originally written for RFC 4787 by Francois Section 1.1 contains text originally written for RFC 4787 by Francois
Audet and Cullen Jennings. Audet and Cullen Jennings.
10. Informative References 10. Informative References
[I-D.ietf-avt-rtp-no-op] [I-D.ietf-avt-rtp-no-op]
Andreasen, F., "A No-Op Payload Format for RTP", draft- Andreasen, F., "A No-Op Payload Format for RTP", draft-
ietf-avt-rtp-no-op-04 (work in progress), May 2007. ietf-avt-rtp-no-op-04 (work in progress), May 2007.
[I-D.ietf-mmusic-latching]
Ivov, E., Kaplan, H., and D. Wing, "Latching: Hosted NAT
Traversal (HNT) for Media in Real-Time Communication",
draft-ietf-mmusic-latching-05 (work in progress), May
2014.
[I-D.ietf-mmusic-rfc2326bis] [I-D.ietf-mmusic-rfc2326bis]
Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M., Schulzrinne, H., Rao, A., Lanphier, R., Westerlund, M.,
and M. Stiemerling, "Real Time Streaming Protocol 2.0 and M. Stiemerling, "Real Time Streaming Protocol 2.0
(RTSP)", draft-ietf-mmusic-rfc2326bis-40 (work in (RTSP)", draft-ietf-mmusic-rfc2326bis-40 (work in
progress), February 2014. progress), February 2014.
[I-D.ietf-mmusic-rtsp-nat] [I-D.ietf-mmusic-rtsp-nat]
Goldberg, J., Westerlund, M., and T. Zeng, "A Network Goldberg, J., Westerlund, M., and T. Zeng, "A Network
Address Translator (NAT) Traversal Mechanism for Media Address Translator (NAT) Traversal Mechanism for Media
Controlled by Real-Time Streaming Protocol (RTSP)", draft- Controlled by Real-Time Streaming Protocol (RTSP)", draft-
ietf-mmusic-rtsp-nat-20 (work in progress), February 2014. ietf-mmusic-rtsp-nat-22 (work in progress), July 2014.
[NICE] "Libnice - The GLib ICE implementation, [NICE] "Libnice - The GLib ICE implementation,
http://nice.freedesktop.org/wiki/", May 2013. http://nice.freedesktop.org/wiki/", May 2013.
[PJNATH] "PJNATH - Open Source ICE, STUN, and TURN Library, [PJNATH] "PJNATH - Open Source ICE, STUN, and TURN Library,
http://www.pjsip.org/pjnath/docs/html/", May 2013. http://www.pjsip.org/pjnath/docs/html/", May 2013.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980. August 1980.
skipping to change at page 40, line 25 skipping to change at page 43, line 41
2010. 2010.
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P. [RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142, Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, October 2008. RFC 5382, October 2008.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389, "Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008. October 2008.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and [RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Control Packets on a Single Port", RFC 5761, April 2010. Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using [RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010. Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC6062] Perreault, S. and J. Rosenberg, "Traversal Using Relays [RFC6062] Perreault, S. and J. Rosenberg, "Traversal Using Relays
around NAT (TURN) Extensions for TCP Allocations", RFC around NAT (TURN) Extensions for TCP Allocations", RFC
6062, November 2010. 6062, November 2010.
[RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for [RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for
Keeping Alive the NAT Mappings Associated with RTP / RTP Keeping Alive the NAT Mappings Associated with RTP / RTP
Control Protocol (RTCP) Flows", RFC 6263, June 2011. Control Protocol (RTCP) Flows", RFC 6263, June 2011.
[RFC7362] Ivov, E., Kaplan, H., and D. Wing, "Latching: Hosted NAT
Traversal (HNT) for Media in Real-Time Communication", RFC
7362, September 2014.
[STUN-IMPL] [STUN-IMPL]
"Open Source STUN Server and Client, "Open Source STUN Server and Client,
http://sourceforge.net/projects/stun/", May 2013. http://sourceforge.net/projects/stun/", May 2013.
Authors' Addresses Authors' Addresses
Magnus Westerlund Magnus Westerlund
Ericsson Ericsson
Farogatan 6 Farogatan 6
Stockholm SE-164 80 Stockholm SE-164 80
Sweden Sweden
Phone: +46 8 719 0000 Phone: +46 8 719 0000
Email: magnus.westerlund@ericsson.com Email: magnus.westerlund@ericsson.com
Thomas Zeng Thomas Zeng
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