draft-ietf-mmusic-rtsp-nat-evaluation-02.txt   draft-ietf-mmusic-rtsp-nat-evaluation-03.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: July 24, 2010 January 20, 2010 Expires: September 15, 2011 March 14, 2011
The evaluation of different NAT traversal Techniques for media The Evaluation of Different Network Addres Translator (NAT) Traversal
controlled by Real-time Streaming Protocol (RTSP) Techniques for Media Controlled by Real-time Streaming Protocol (RTSP)
draft-ietf-mmusic-rtsp-nat-evaluation-02 draft-ietf-mmusic-rtsp-nat-evaluation-03
Abstract Abstract
This document describes several NAT traversal techniques that could This document describes several Network Address Translator (NAT)
be used by RTSP. Each technique includes a description on how it traversal techniques that was considered to be used by Real-time
would be used, the security implications of using it and any other Streaming Protocol (RTSP). Each technique includes a description on
deployment considerations it has. There are also disussions on how how it would be used, the security implications of using it and any
NAT traversal techniques relates to firewalls and how each technique other deployment considerations it has. There are also disussions on
can be applied in different use cases. These findings where used how NAT traversal techniques relates to firewalls and how each
when selecting the NAT traversal for RTSP solution to standardize in technique can be applied in different use cases. These findings
the MMUSIC WG. where used when selecting the NAT traversal for RTSP 2.0 standardized
in the MMUSIC WG.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo Status of this Memo
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provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Network Address Translators . . . . . . . . . . . . . . . 5 1.1. Network Address Translators . . . . . . . . . . . . . . . 6
1.2. Firewalls . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2. Firewalls . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3. Glossary . . . . . . . . . . . . . . . . . . . . . . . . . 7
2. Detecting the loss of NAT mappings . . . . . . . . . . . . . . 7 1.4. Definitions . . . . . . . . . . . . . . . . . . . . . . . 8
3. Requirements on NAT-Traversal . . . . . . . . . . . . . . . . 7 2. Detecting the loss of NAT mappings . . . . . . . . . . . . . . 8
4. NAT Traversal Techniques . . . . . . . . . . . . . . . . . . . 9 3. Requirements on NAT-Traversal . . . . . . . . . . . . . . . . 9
4.1. STUN . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4. NAT Traversal Techniques . . . . . . . . . . . . . . . . . . . 10
4.1.1. Introduction . . . . . . . . . . . . . . . . . . . . . 9 4.1. STUN . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1.1. Introduction . . . . . . . . . . . . . . . . . . . . . 11
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. Embedding STUN in RTSP . . . . . . . . . . . . . . . . 12 4.1.3. Embedding STUN in RTSP . . . . . . . . . . . . . . . . 13
4.1.4. Discussion On Co-located STUN Server . . . . . . . . . 13 4.1.4. Discussion On Co-located STUN Server . . . . . . . . . 14
4.1.5. ALG considerations . . . . . . . . . . . . . . . . . . 13 4.1.5. ALG considerations . . . . . . . . . . . . . . . . . . 15
4.1.6. Deployment Considerations . . . . . . . . . . . . . . 14 4.1.6. Deployment Considerations . . . . . . . . . . . . . . 15
4.1.7. Security Considerations . . . . . . . . . . . . . . . 16 4.1.7. Security Considerations . . . . . . . . . . . . . . . 17
4.2. ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4.2. ICE . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2.1. Introduction . . . . . . . . . . . . . . . . . . . . . 17 4.2.1. Introduction . . . . . . . . . . . . . . . . . . . . . 18
4.2.2. Using ICE in RTSP . . . . . . . . . . . . . . . . . . 17 4.2.2. Using ICE in RTSP . . . . . . . . . . . . . . . . . . 19
4.2.3. Implementation burden of ICE . . . . . . . . . . . . . 19 4.2.3. Implementation burden of ICE . . . . . . . . . . . . . 20
4.2.4. Deployment Considerations . . . . . . . . . . . . . . 19 4.2.4. Deployment Considerations . . . . . . . . . . . . . . 21
4.2.5. Security Consideration . . . . . . . . . . . . . . . . 20 4.2.5. Security Consideration . . . . . . . . . . . . . . . . 21
4.3. Symmetric RTP . . . . . . . . . . . . . . . . . . . . . . 20 4.3. Symmetric RTP . . . . . . . . . . . . . . . . . . . . . . 21
4.3.1. Introduction . . . . . . . . . . . . . . . . . . . . . 20 4.3.1. Introduction . . . . . . . . . . . . . . . . . . . . . 22
4.3.2. Necessary RTSP extensions . . . . . . . . . . . . . . 21 4.3.2. Necessary RTSP extensions . . . . . . . . . . . . . . 22
4.3.3. Deployment Considerations . . . . . . . . . . . . . . 21 4.3.3. Deployment Considerations . . . . . . . . . . . . . . 22
4.3.4. Security Consideration . . . . . . . . . . . . . . . . 22 4.3.4. Security Consideration . . . . . . . . . . . . . . . . 23
4.3.5. A Variation to Symmetric RTP . . . . . . . . . . . . . 23 4.3.5. A Variation to Symmetric RTP . . . . . . . . . . . . . 24
4.4. Application Level Gateways . . . . . . . . . . . . . . . . 25 4.4. Application Level Gateways . . . . . . . . . . . . . . . . 26
4.4.1. Introduction . . . . . . . . . . . . . . . . . . . . . 25 4.4.1. Introduction . . . . . . . . . . . . . . . . . . . . . 26
4.4.2. Outline On how ALGs for RTSP work . . . . . . . . . . 25 4.4.2. Outline On how ALGs for RTSP work . . . . . . . . . . 27
4.4.3. Deployment Considerations . . . . . . . . . . . . . . 26 4.4.3. Deployment Considerations . . . . . . . . . . . . . . 28
4.4.4. Security Considerations . . . . . . . . . . . . . . . 27 4.4.4. Security Considerations . . . . . . . . . . . . . . . 28
4.5. TCP Tunneling . . . . . . . . . . . . . . . . . . . . . . 27 4.5. TCP Tunneling . . . . . . . . . . . . . . . . . . . . . . 28
4.5.1. Introduction . . . . . . . . . . . . . . . . . . . . . 27 4.5.1. Introduction . . . . . . . . . . . . . . . . . . . . . 28
4.5.2. Usage of TCP tunneling in RTSP . . . . . . . . . . . . 27 4.5.2. Usage of TCP tunneling in RTSP . . . . . . . . . . . . 29
4.5.3. Deployment Considerations . . . . . . . . . . . . . . 27 4.5.3. Deployment Considerations . . . . . . . . . . . . . . 29
4.5.4. Security Considerations . . . . . . . . . . . . . . . 28 4.5.4. Security Considerations . . . . . . . . . . . . . . . 29
4.6. TURN (Traversal Using Relay NAT) . . . . . . . . . . . . . 28 4.6. TURN (Traversal Using Relay NAT) . . . . . . . . . . . . . 30
4.6.1. Introduction . . . . . . . . . . . . . . . . . . . . . 28 4.6.1. Introduction . . . . . . . . . . . . . . . . . . . . . 30
4.6.2. Usage of TURN with RTSP . . . . . . . . . . . . . . . 29 4.6.2. Usage of TURN with RTSP . . . . . . . . . . . . . . . 30
4.6.3. Deployment Considerations . . . . . . . . . . . . . . 30 4.6.3. Deployment Considerations . . . . . . . . . . . . . . 31
4.6.4. Security Considerations . . . . . . . . . . . . . . . 30 4.6.4. Security Considerations . . . . . . . . . . . . . . . 32
5. Firewalls . . . . . . . . . . . . . . . . . . . . . . . . . . 31 5. Firewalls . . . . . . . . . . . . . . . . . . . . . . . . . . 33
6. Comparision of NAT traversal techniques . . . . . . . . . . . 32 6. Comparision of NAT traversal techniques . . . . . . . . . . . 33
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
8. Security Considerations . . . . . . . . . . . . . . . . . . . 33 8. Security Considerations . . . . . . . . . . . . . . . . . . . 34
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 34 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
10. Informative References . . . . . . . . . . . . . . . . . . . . 34 10. Informative References . . . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 36 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37
1. Introduction 1. Introduction
Today there is a proliferate deployment of different flavors of Today there is a proliferate deployment of different flavors of
Network Address Translator (NAT) boxes that in many cases only Network Address Translator (NAT) boxes that in many cases only
loosely follows standards[RFC3022][RFC2663][RFC3424]]. NATs cause loosely follows standards [RFC3022][RFC2663][RFC3424]]. NATs cause
discontinuity in address realms [RFC3424], therefore an application discontinuity in address realms [RFC3424], therefore an application
protocol, such as RTSP, needs to deal with such discontinuities protocol, such as Real-time Streaming Protocol (RTSP)
caused by NATs. The problem is that, being a media control protocol [RFC2326][I-D.ietf-mmusic-rfc2326bis], needs to deal with such
managing one or more media streams, RTSP carries network address and discontinuities caused by NATs. The problem is that, being a media
port information within its protocol messages. Because of this, even control protocol managing one or more media streams, RTSP carries
if RTSP itself, when carried over TCP for example, may not be blocked network address and port information within its protocol messages.
by NATs, its media streams may be blocked by NATs. This will occur Because of this, even if RTSP itself, when carried over Transmission
Control Protocol (TCP) [RFC0793] for example, may not be blocked by
NATs, its media streams may be blocked by NATs. This will occur
unless special protocol provisions are added to support NAT- unless special protocol provisions are added to support NAT-
traversal. traversal.
Like NATs, firewalls (FWs) are also middle boxes that need to be Like NATs, firewalls (FWs) are also middle boxes that need to be
considered. Firewalls helps prevent unwanted traffic from getting in considered. Firewalls helps 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 to go firewall can be configured to let RTSP controlled media streams to go
through with minimal implementation effort. The minimal effort is to through with minimal implementation effort. The minimal effort is to
implement an ALG (Application Level Gateway) to interpret RTSP implement an Application Level Gateway (ALG) to interpret RTSP
parameters. There is also a large class of firewalls, commonly home parameters. There is also a large class of firewalls, commonly home
FWs, that uses a similar filtering behavior to what NAT has. This firewalls, that uses a similar filtering behavior to what NAT has.
type of firewalls can be handled using the same solution as employed This type of firewalls can be handled using the same solution as
for NAT traversal instead of relying on ALGs. employed for NAT traversal instead of relying on ALGs.
This document describes several NAT-traversal mechanisms for RTSP This document describes several NAT-traversal mechanisms for RTSP
controlled media streaming. These NAT solutions fall into the controlled media streaming. These NAT solutions fall into the
category of ""UNilateral Self-Address Fixing (UNSAF)" as defined in category of "UNilateral Self-Address Fixing (UNSAF)" as defined in
[RFC3424] and quoted below: [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.
This document is capturing the evaluation done in the process to This document is capturing the evaluation done in the process to
recommend FW/NAT traversal methods for RTSP streaming servers based recommend FW/NAT traversal methods for RTSP streaming servers based
on RFC 2326 [RFC2326] as well as the RTSP 2.0 core spec 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 UDP [RFC0768]. Where traversal for the media streams carried over User Datagram Protocol
RTP [RFC3550] over UDP being the main case for such usage. The (UDP) [RFC0768]. Where Real-time Transport Protocol (RTP) [RFC3550]
findings should be applicable to other protocols as long as they have over UDP being the main case for such usage. The findings should be
similar properties. applicable to other protocols as long as they have similar
properties.
1.1. Network Address Translators 1.1. Network Address Translators
Readers are urged to refer to [RFC2663] for information on NAT Readers are urged to refer to "IP Network Address Translator (NAT)
Terminology and Considerations" [RFC2663] for information on NAT
taxonomy and terminology. Traditional NAT is the most common type of taxonomy and terminology. Traditional NAT is the most common type of
NAT device deployed. Readers may refer to [RFC3022] for detailed NAT device deployed. Readers may refer to "Traditional IP Network
Address Translator (Traditional NAT)" [RFC3022] for detailed
information on traditional NAT. Traditional NAT has two main 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
session through a NAPT, the NAPT assigns the session a public IP session through a NAPT, the NAPT assigns the session a public IP
address and port number, so that subsequent response packets from the address and port number, so that subsequent response packets from the
external endpoint can be received by the NAPT, translated, and external endpoint can be received by the NAPT, translated, and
forwarded to the internal host. The effect is that the NAPT forwarded to the internal host. The effect is that the NAPT
establishes a NAT session to translate the (private IP address, establishes a NAT mapping to translate the (private IP address,
private port number) tuple to a (public IP address, public port private port number) tuple to a (public IP address, public port
number) tuple, and vice versa, for the duration of the session. An number) tuple, and vice versa, for the duration of the session. An
issue of relevance to peer-to-peer applications is how the NAT issue of relevance to peer-to-peer applications is how the NAT
behaves when an internal host initiates multiple simultaneous behaves when an internal host initiates multiple simultaneous
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. There exist a number of address and binding" as defined in RFC 2663. There exist a number of address and
port mapping behaviors described in more detail in Section 4.1 of port mapping behaviors described in more detail in Section 4.1 of
[RFC4787]. "Network Address Translation (NAT) Behavioral Requirements for
Unicast UDP" [RFC4787].
NATs also have a filtering behavior on traffic arriving on the NATs also have a filtering behavior on traffic arriving on the
external side. Such behavior effects how well different methods for external side. Such behavior effects how well different methods for
NAT traversal works through these NATs. See Section 5 of [RFC4787] NAT traversal works through these NATs. See Section 5 of "Network
for more information on the different types of filtering that have Address Translation (NAT) Behavioral Requirements for Unicast UDP"
been identified. [RFC4787] for more information on the different types of filtering
that have been identified.
1.2. Firewalls 1.2. Firewalls
A firewall (FW) is a security gateway that enforces certain access A firewall (FW) is a security gateway that enforces certain access
control policies between two network administrative domains: a control policies between two network administrative domains: a
private domain (intranet) and a public domain (public Internet). private domain (intranet) and a external domain, e.g. public
Many organizations use firewalls to prevent privacy intrusions and Internet. Many organizations use firewalls to prevent privacy
malicious attacks to corporate computing resources in the private intrusions and malicious attacks to corporate computing resources in
intranet [RFC2588]. the private intranet [RFC2588].
A comparison between NAT and FW is given below: A comparison between NAT and FW is given below:
1. A firewall must sit between two network administrative domains, 1. A firewall must sit between two network administrative domains,
while NAT does not have to sit between two domains. In fact, while NAT does not have to sit between two domains.
there exist cases when in corporations there are many NAT boxes
within the intranet, in which case the NAT boxes sit between
subnets.
2. NAT does not in itself provide security, although some access 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 small office/ It should be noted that many NAT devices intended for small office/
home office (SOHO) include both NATs and firewall functionality. home office (SOHO) include both NATs and firewall functionality.
In the rest of this memo we use the phrase "NAT traversal" In the rest of this memo we use the phrase "NAT traversal"
interchangeably with "FW traversal", "NAT/FW traversal" and "NAT/ interchangeably with "FW traversal", "NAT/FW traversal" and "NAT/
Firewall traversal". Firewall traversal".
1.3. Glossary 1.3. Glossary
ALG: Application Level Gateway, an entity that can be embedded in a ALG: Application Level Gateway, an entity that can be embedded in a
NAT or other middlebox to perform the application layer NAT or other middlebox to perform the application layer
functions required for a particular protocol to traverse the functions required for a particular protocol to traverse the
NAT/middlebox. NAT/middlebox.
ICE: Interactive Connectivity Establishment, see ICE: Interactive Connectivity Establishment, see [RFC5245].
[I-D.ietf-mmusic-ice].
DNS: Domain Name Service DNS: Domain Name Service
DDOS: Distributed Denial Of Service attacks DDOS: Distributed Denial Of Service attacks
MID: Media Identifier from Grouping of media lines in SDP, see
[RFC3388].
NAT: Network Address Translator, see [RFC3022]. NAT: Network Address Translator, see [RFC3022].
NAPT: Network Address/Port Translator, see [RFC3022]. NAPT: Network Address/Port Translator, see [RFC3022].
RTP: Real-time Transport Protocol, see [RFC3550]. RTP: Real-time Transport Protocol, see [RFC3550].
RTSP: Real-Time Streaming Protocol, see [RFC2326] and RTSP: Real-Time Streaming Protocol, see [RFC2326] and
[I-D.ietf-mmusic-rfc2326bis]. [I-D.ietf-mmusic-rfc2326bis].
SDP: Session Description Protocol, see [RFC4566]. SDP: Session Description Protocol, see [RFC4566].
SSRC: Synchronization source in RTP, see [RFC3550]. SSRC: Synchronization source in RTP, see [RFC3550].
1.4. Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
2. Detecting the loss of NAT mappings 2. Detecting the loss of NAT mappings
Several NAT traversal techniques in the next chapter make use of the Several NAT traversal techniques in the next chapter make use of the
fact that the NAT UDP mapping's external address and port can be fact that the NAT UDP mapping's external address and port can be
discovered. This information is then utilized to traverse the NAT discovered. This information is then utilized to traverse the NAT
box. However any such information is only good while the mapping is box. However any such information is only good while the mapping is
still valid. As the IAB's UNSAF document [RFC3424] points out, the still valid. As the IAB's UNSAF document [RFC3424] points out, the
mapping can either timeout or change its properties. It is therefore mapping can either timeout or change its properties. It is therefore
important for the NAT traversal solutions to handle the loss or important for the NAT traversal solutions to handle the loss or
change of NAT mappings, according to RFC3424. change of NAT mappings, according to RFC3424.
skipping to change at page 8, line 44 skipping to change at page 9, line 8
reports. The sender report carries a field telling how many packets reports. The sender report carries a field telling how many packets
the server has sent. If that has increased and no RTP packets has the server has sent. If that has increased and no RTP packets has
arrived for a few seconds it is likely the RTP mapping has been arrived for a few seconds it is likely the RTP mapping has been
removed. 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 to be lost. Note that if neither RTCP packets nor mapping is likely to be lost. Note that if neither RTCP packets nor
RTSP messages are received by the RTSP server for a while, the RTSP RTSP messages are received by the RTSP server for a while, the RTSP
server has the option to delete the corresponding SSRC and RTSP server has the option to delete the corresponding RTP session, SSRC
session ID, because either the client can not get through a middle and RTSP session ID, because either the client can not get through a
box NAT/FW, or that the client is mal-functioning. middle box NAT/FW, or that the client is mal-functioning.
3. Requirements on NAT-Traversal 3. Requirements on NAT-Traversal
This section considers the set of requirements for the evaulation of This section considers the set of requirements for the evaulation of
RTSP NAT traversal solutions. RTSP NAT traversal solutions.
RTSP is a client-server protocol. Typically services providers RTSP is a client-server protocol. Typically services providers
deploy RTSP servers in the public address realm. However, there are deploy RTSP servers in the public address realm. However, there are
use cases where the reverse is true: RTSP clients are connecting from use cases where the reverse is true: RTSP clients are connecting from
public address realm to RTSP servers behind home NATs. This is the public address realm to RTSP servers behind home NATs. This is the
case for instance when home surveillance cameras running as RTSP case for instance when home surveillance cameras running as RTSP
servers intend to stream video to cell phone users in the public servers intend to stream video to cell phone users in the public
address realm through a home NAT. In terms of requirements, the address realm through a home NAT. In terms of requirements, the
first requirement shoulb be to solve the RTSP NAT traversal problem first requirement should be to solve the RTSP NAT traversal problem
for RTSP servers deployed in a public network, i.e. no NAT at the for RTSP servers deployed in a public network, i.e. no NAT at the
server side. server side.
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 restricted filtering. and port restricted filtering.
2. MUST work for firewalls (subject to pertinent firewall 2. MUST work for firewalls (subject to pertinent firewall
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protocol (e.g. RTP) are implemented on different computers with protocol (e.g. RTP) are implemented on different computers with
different IP addresses. different IP addresses.
* For instance, no extra delay from RTSP connection till arrival * For instance, no extra delay from RTSP connection till arrival
of media of media
4. SHOULD be simple to use/implement/administer that people actually 4. SHOULD be simple to use/implement/administer that people actually
turn them on turn them on
* Otherwise people will resort to TCP tunneling through NATs * Otherwise people will resort to TCP tunneling through NATs
* Address discovery for NAT traversal should take place behind * Address discovery for NAT traversal should take place behind
the scene, if possible the scene, if possible
5. SHOULD authenticate dual-hosted client transport handler to 5. SHOULD authenticate dual-hosted client transport handler to
prevent DDOS attacks. prevent 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 performs address During NAT traversal, when the RTSP server determines the media
translation on a client, the result may be that the public IP address destination (Address and port) for client, the result may be that the
of the RTP receiver host is different than the public IP address of public IP address of the RTP receiver host is different than the
the RTSP client host. This posts a DDOS threat that has significant public IP address of the RTSP client host. This posts a DDOS threat
amplification potentials because the RTP media streams in general that has significant amplification potentials because the RTP media
consist of large number of IP packets. DDOS attacks occur if the streams in general consist of large number of IP packets. DDOS
attacker fakes the messages in the NAT traversal mechanism to trick attacks occur if the attacker fakes the messages in the NAT traversal
the RTSP server into believing that the client's RTP receiver is mechanism to trick the RTSP server into believing that the client's
located in a separate host. For example, user A may use his RTSP RTP receiver is located in a separate host. For example, user A may
client to direct the RTSP server to send video RTP streams to 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 is for the provided by target.example.com. Note a simple preventative measure
RTSP server to disallow the cases where the client's RTP receiver has is for the RTSP server to disallow the cases where the client's RTP
a different public IP address than that of the RTSP client. However, receiver has a different public IP address than that of the RTSP
in some applications (e.g., centralized conferencing), dual-hosted client. However, in some applications (e.g., centralized
RTSP/RTP clients have valid use cases. The key is how to conferencing), dual-hosted RTSP/RTP clients have valid use cases.
authenticate the messages exchanged during the NAT traversal process. The key is how to authenticate the messages exchanged during the NAT
Message authentication is a big challenge in the current wired and traversal process. Message authentication is a big challenge in the
wireless networking environment. It may be necessary in the current wired and wireless networking environment. It may be
immediate future to deploy NAT traversal solutions that do not have necessary in the immediate future to deploy NAT traversal solutions
full message authentication, but provide upgrade path to add that do not have full message authentication, but provide upgrade
authentication features in the future. path to add authentication features in the future.
4. NAT Traversal Techniques 4. NAT Traversal Techniques
There exist a number of potential NAT traversal techniques that can There exist 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 cost is also and are applicable to different topologies; their cost is 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 in details with discussions on sections, each technique is outlined in details with discussions on
the corresponding advantages and disadvantages. the corresponding advantages and disadvantages.
This section includes NAT traversal techniques that have not been This section includes NAT traversal techniques that have not been
formally specified anywhere else. The overview section of this formally specified anywhere else. The overview section of this
document may be the only publicly available specification of some of document may be the only publicly available specification of some of
the NAT traversal techniques. However that is no real barrier the NAT traversal techniques. However that is no real barrier
against doing an evaluation of the NAT traversal technique. Some against doing an evaluation of the NAT traversal technique. Some
other techniques are currently (at the time of writing) in a state of other techniques are currently (at the time of writing) in a state of
flux due to ongoing standardization work on these techniques, e.g. flux due to ongoing standardization work on these techniques, e.g.
ICE [I-D.ietf-mmusic-ice], STUN [RFC5389] and RTP No-Op RTP No-Op [I-D.ietf-avt-rtp-no-op].
[I-D.ietf-avt-rtp-no-op].
4.1. STUN 4.1. STUN
4.1.1. Introduction 4.1.1. Introduction
STUN - "Simple Traversal of UDP Through Network Address Translators" STUN - "Simple Traversal of UDP Through Network Address Translators"
[RFC3489][RFC5389] is a standardized protocol that allows a client to [RFC3489][RFC5389] is a standardized protocol that allows a client to
use secure means to discover the presence of a NAT between himself use secure means to discover the presence of a NAT between himself
and the STUN server and the type of that NAT. The client then uses and the STUN server. The client uses the STUN server to discover the
the STUN server to discover the address bindings assigned by the NAT. address mappings assigned by the NAT. STUN is a client-server
protocol. STUN client sends a request to a STUN server and the
STUN is a client-server protocol. STUN client sends a request to a server returns a response. There are two types of STUN requests -
STUN server and the server returns a response. There are two types Binding Requests, sent over UDP, and Shared Secret Requests, sent
of STUN requests - Binding Requests, sent over UDP, and Shared Secret over TLS over TCP.
Requests, sent over TLS over TCP.
STUN is in the process of being updated by the BEHAVE WG to address The first version of STUN [RFC3489] included categorization and
issues found during usage. The BEHAVE WG intends to integrate it parameterization of NATs. This was abandoned in the updated version
better with TURN [I-D.ietf-behave-turn]. due to it being unreliable.
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:
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that the payload type number must be dynamically assigned through that the payload type number must be dynamically assigned through
RTSP first. Otherwise STUN could be used for the keep-alive as RTSP first. Otherwise STUN could be used for the keep-alive as
well as empty UDP packets. 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 send the very "Address Dependent" filtering NAT, the client must first send a UDP
first UDP packet to punch a hole in the NAT. The client, before packet to establish filtering state in the NAT. The client, before
sending a RTSP PLAY request, must send a so called FW packet (such as sending a RTSP PLAY request, must send a so called FW packet (such as
a RTP No-Op packet) on each mapping, to the IP address given as the a RTP No-Op packet) on each mapping, to the IP address given as the
servers source address. To create minimum problems for the server servers source address. To create minimum problems for the server
these UDP packets SHOULD be sent to the server's discard port (port these UDP packets SHOULD be sent to the server's discard port (port
number 9). Since UDP packets are inherently unreliable, to ensure number 9). Since UDP packets are inherently unreliable, to ensure
that at least one UDP message passes the NAT, FW packets should be that at least one UDP message passes the NAT, FW packets should be
retransmitted a reasonable number of times. retransmitted a reasonable number of times.
For a "Address and Port Dependent" filtering NAT the client must send For a "Address and Port Dependent" filtering NAT the client must send
messages to the exact ports used by the server to send UDP packets messages to the exact ports used by the server to send UDP packets
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the above described process with the following additional the above described process with the following additional
restrictions: for each port mapping, FW packets need to be sent first restrictions: for each port mapping, FW packets need to be sent first
to the server's source address/port. To minimize potential effects to the server's source address/port. To minimize potential effects
on the server from these messages the following type of FW packets on the server from these messages the following type of FW packets
MUST be sent. RTP: an empty or less than 12 bytes UDP packet. RTCP: MUST be sent. RTP: an empty or less than 12 bytes UDP packet. RTCP:
A correctly formatted RTCP RR or SR message. The above described A correctly formatted RTCP RR or SR message. The above described
adaptations for restricted NATs will not work unless the server adaptations for restricted NATs will not work unless the server
includes the "src_addr" in the "Transport" header (which is the includes the "src_addr" in the "Transport" header (which is the
"source" transport parameter in RFC2326). "source" transport parameter in RFC2326).
This method is also brittle because it relies on that one can use
STUN to classify the NAT behavior. If the NAT changes the properties
of the existing mapping and filtering state for example due to load,
then the methods will fail.
4.1.3. Embedding STUN in RTSP 4.1.3. Embedding STUN in RTSP
This section outlines the adaptation and embedding of STUN within This section outlines the adaptation and embedding of STUN within
RTSP. This enables STUN to be used to traverse any type of NAT, RTSP. This enables STUN to be used to traverse any type of NAT,
including symmetric NATs. This would require protocol changes which including symmetric NATs. This would require protocol changes.
are beyond the scope of this memo.
This NAT traversal solution has limitations: This NAT traversal solution has limitations:
1. It does not work if both RTSP client and RTSP server are behind 1. It does not work if both RTSP client and RTSP server are behind
separate NATs. separate NATs.
2. The RTSP server may, for security reasons, refuse to send media 2. The RTSP server may, for security reasons, refuse to send media
streams to an IP different from the IP in the client RTSP streams to an IP different from the IP in the client RTSP
requests. requests.
Therefore, if the client is behind a NAT that has multiple public
addresses, and the client's RTSP port and UDP port are mapped to
different IP addresses, RTSP SETUP may fail.
Deviations from STUN as defined in RFC 3489: Deviations from STUN as defined in RFC 3489:
1. We allow RTSP applications to have the option to perform STUN 1. We allow RTSP applications to have the option to perform STUN
"Shared Secret Request" through RTSP, via extension to RTSP; "Shared Secret Request" through RTSP, via extension to RTSP;
2. We require STUN server to be co-located on RTSP server's media 2. We require STUN server to be co-located on RTSP server's media
output ports. output ports.
In order to allow binding discovery without authentication, the STUN In order to allow binding discovery without authentication, the STUN
server embedded in RTSP application must ignore authentication tag, server embedded in RTSP application must ignore authentication tag,
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client) will be required to send and receive RTCP packets from the client) will be required to send and receive RTCP packets from the
same port. same port.
4.1.4. Discussion On Co-located STUN Server 4.1.4. Discussion On Co-located STUN Server
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. A from a media packet. This will be done based on heuristics. A
common heuristics is the frist byte in the packet, which works fine common heuristics is the first byte in the packet, which works fine
between STUN and RTP or RTCP where the first byte happens to be between STUN and RTP or RTCP where the first byte happens to be
different, but may not work as well with other media transport different, but may not work as well with other media transport
protocols. protocols.
4.1.5. ALG considerations 4.1.5. 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 public IP address and port numbers, because a client discovers its public IP address and port numbers,
and inserts them in its SETUP requests, when the RTSP ALG processes and inserts them in its SETUP requests, when the RTSP ALG processes
the SETUP request it may change the destination and port number, the SETUP request it may change the destination and port number,
resulting in unpredictable behavior. In such cases two alternatives resulting in unpredictable behavior. An ALG should not update
exist: address fields which contains addresses other than the NATs internal
address domain. In cases where the ALG modifies fields unnecessary
two alternatives exist:
1. The usage of TLS to encrypt the RTSP TCP connection to prevent 1. The usage of TLS to encrypt the RTSP TCP connection to prevent
the ALG from reading and modifying the RTSP messages. the ALG from reading and modifying the RTSP messages.
2. To turn of the STUN based NAT traversal mechanism 2. To turn off the STUN based NAT traversal mechanism
As it may be difficult to determine why the failure occurs, the usage As it may be difficult to determine why the failure occurs, the usage
of TLS protected RTSP message exchange at all times would avoid this of TLS protected RTSP message exchange at all times would avoid this
issue. issue.
4.1.6. Deployment Considerations 4.1.6. Deployment Considerations
For the non-embedded usage of STUN the following applies: For the non-embedded usage of STUN the following applies:
Advantages: Advantages:
o STUN is a solution first used by SIP applications. As shown
above, with little or no changes, RTSP application can re-use STUN
as a NAT traversal solution, avoiding the pit-fall of solving a
problem twice.
o Using STUN does not require RTSP server modifications; it only o Using STUN does not require RTSP server modifications; it only
affects the client implementation. affects the client implementation.
Disadvantages: Disadvantages:
o Requires a STUN server deployed in the public address space. o Requires a STUN server deployed in the public address space.
o Only works with NATs that perform endpoint independent and address o Only works with NATs that perform endpoint independent and address
dependent mappings. Port and address dependent filtering NATs dependent mappings. Port and address dependent filtering NATs
create some issues. create some issues.
o Brittle to NATs changing the properties of the NAT mapping and
filtering.
o Does not work with port and address dependent mapping NATs without 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 mostly not work if a NAT uses multiple IP addresses, since
RTSP server generally requires all media streams to use the same RTSP server generally requires all media streams to use the same
IP as used in the RTSP connection to prevent becoming a DDOS tool. IP as 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.
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o Some extensions to the RTSP core protocol, signaled by RTSP o Some extensions to the RTSP core protocol, signaled by RTSP
feature tags, must be introduced. feature tags, must be introduced.
o Requires an embedded STUN server to co-locate on each of RTSP o Requires an embedded STUN server to co-locate on each of RTSP
server's media protocol's ports (e.g. RTP and RTCP ports), which server's media protocol's ports (e.g. RTP and RTCP ports), which
means more processing is required to de-multiplex STUN packets means more processing is required to de-multiplex STUN packets
from media packets. For example, the de-multiplexer must be able from media packets. For example, the de-multiplexer must be able
to differentiate a RTCP RR packet from a STUN packet, and forward to differentiate a RTCP RR packet from a STUN packet, and forward
the former to the streaming server, the later to STUN server. the former to the streaming server, the later to STUN server.
o Even if the RTSP server is in the open, and the client is behind a o Does not support use cases that requires the RTSP connection and
multi-addressed NAT, it may still break if the RTSP server does the media reception to happen at different addresses, unless the
not allow RTP packets to be sent to an IP differs from the IP of servers sequrity policy is relaxed.
the client's RTSP request.
o Interaction problems exist when a RTSP ALG is not aware of STUN o Interaction problems exist when a RTSP ALG is not aware of STUN
unless TLS is used to protect the RTSP messages. unless TLS is used to protect the RTSP messages.
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, and they both agree to support the NAT updated RTSP specification, and they both agree to support the NAT
traversal feature. traversal feature.
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 to perform STUN message exchanges. Most likely an extra takes to perform STUN message exchanges. Most likely an extra
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4.1.7. Security Considerations 4.1.7. Security Considerations
To prevent RTSP server being used as Denial of Service (DoS) attack To prevent RTSP server being used as Denial of Service (DoS) attack
tools the RTSP Transport header parameter "destination" and tools the RTSP Transport header parameter "destination" and
"dest_addr" are generally not allowed to point to any IP address "dest_addr" are generally not allowed to point to any IP address
other than the one that RTSP message originates from. The RTSP other than the one that RTSP message originates from. The RTSP
server is only prepared to make an exception of this rule when the server is only prepared to make an exception of this rule when the
client is trusted (e.g., through the use of a secure authentication client is trusted (e.g., through the use of a secure authentication
process, or through some secure method of challenging the destination process, or through some secure method of challenging the destination
to verify its willingness to accept the RTP traffic). Such to verify its willingness to accept the RTP traffic). Such
restriction means that STUN does not work for NATs that would assign restriction means that STUN does not work for use cases where RTSP
different IP addresses to different UDP flows on its public side. and media transport goes to different address.
Therefore the multi-addressed NATs will at times have trouble with
STUN-based RTSP NAT traversals.
In terms of security property, STUN combined with destination address In terms of security property, STUN combined with destination address
restricted RTSP has the same security properties as the core RTSP. restricted RTSP has the same security properties as the core RTSP.
It is protected from being used as a DoS attack tool unless the It is protected from being used as a DoS attack tool unless the
attacker has ability the to spoof the TCP connection carrying RTSP attacker has ability the to spoof the TCP connection carrying RTSP
messages. messages.
Using STUN's support for message authentication and secure transport Using STUN's support for message authentication and secure transport
of RTSP messages, attackers cannot modify STUN responses or RTSP of RTSP messages, attackers cannot modify STUN responses or RTSP
messages to change media destination. This protects against messages to change media destination. This protects against
hijacking, however as a client can be the initiator of an attack, hijacking, however as a client can be the initiator of an attack,
these mechanisms cannot securely prevent RTSP servers being used as these mechanisms cannot securely prevent RTSP servers being used as
DoS attack tools. DoS attack tools.
4.2. ICE 4.2. ICE
4.2.1. Introduction 4.2.1. Introduction
ICE (Interactive Connectivity Establishment) [I-D.ietf-mmusic-ice] is ICE (Interactive Connectivity Establishment) [RFC5245] is a
a methodology for NAT traversal that has been developed for SIP using methodology for NAT traversal that has been developed for SIP using
SDP offer/answer. The basic idea is to try, in a parallel fashion, SDP offer/answer. The basic idea is to try, in a parallel fashion,
all possible connection addresses that an end point may have. This all possible connection addresses that an end point may have. This
allows the end-point to use the best available UDP "connection" allows the end-point to use the best available UDP "connection"
(meaning two UDP end-points capable of reaching each other). The (meaning two UDP end-points capable of reaching each other). The
methodology has very nice properties in that basically all NAT methodology has very nice properties in that basically all NAT
topologies are possible to traverse. topologies are possible to traverse.
Here is how ICE works on a high level. End point A collects all Here is how ICE works on a high level. End point A collects all
possible address that can be used, including local IP addresses, STUN possible address that can be used, including local IP addresses, STUN
derived addresses, TURN addresses, etc. On each local port that any derived addresses, TURN addresses, etc. On each local port that any
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receive the media, thus protecting itself from performing receive the media, thus protecting itself from performing
unknowingly an DoS attack. unknowingly an DoS attack.
6. Connectivity checks from the client's primary to the server's 6. Connectivity checks from the client's primary to the server's
primary was successful. Thus no further SETUP requests are primary was successful. Thus no further SETUP requests are
necessary and processing can proceed with step 7. If another necessary and processing can proceed with step 7. If another
address than the primary has been verified by the client to work, address than the primary has been verified by the client to work,
that address may then be promoted for usage in a SETUP request that address may then be promoted for usage in a SETUP request
(Goto 7). If the checks for the availble candidates failed and (Goto 7). If the checks for the availble candidates failed and
If further candidates have been derived during the connectivity If further candidates have been derived during the connectivity
checks, then those can be promoted in new candidate lines in checks, then those can be signalled in new candidate lines in
SETUP request updating the list (Goto 5). SETUP request updating the list (Goto 5).
7. Client issues PLAY request. If the server also has completed its 7. Client issues PLAY request. If the server also has completed its
connectivity checks for this primary addresses (based on username connectivity checks for this primary addresses (based on username
as it may be derived addresses if the client was behind NAT) then as it may be derived addresses if the client was behind NAT) then
it can directly answer 200 OK (Goto 8). If the connectivity it can directly answer 200 OK (Goto 8). If the connectivity
check has not yet completed it responds with a 1xx code to check has not yet completed it responds with a 1xx code to
indicate that it is verifying the connectivity. If that fails indicate that it is verifying the connectivity. If that fails
within the set timeout an error is reported back. Client needs within the set timeout an error is reported back. Client needs
to go back to 6. to go back to 6.
8. Process completed media can be delivered. ICE testing ports may 8. Process completed media can be delivered. ICE testing ports may
be released. be released.
To keep media paths alive client must likely periodically send data To keep media paths alive the client needs to periodically send data
to the server. This could be realized with either STUN or RTP No-op to the server. This could be realized with either STUN or RTP No-op
[I-D.ietf-avt-rtp-no-op] packets. RTCP sent by client should be able [I-D.ietf-avt-rtp-no-op] packets. RTCP sent by client should be able
to keep RTCP open. to keep RTCP open.
4.2.3. Implementation burden of ICE 4.2.3. 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 amongst
all the NAT/FW traversal methods in this document. all the NAT/FW traversal methods in this document.
Some RTSP server implementation requirements are: RTSP server implementation requirements are:
o STUN server features o 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
Some client implantation requirements are: RTSP client implantation 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
4.2.4. Deployment Considerations 4.2.4. Deployment Considerations
Advantages: Advantages:
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Specifically, when the RTSP server receives the "connect" RTP packet Specifically, when the RTSP server receives the "connect" RTP packet
(a.k.a. FW packet, since it is used to punch a hole in the FW/NAT (a.k.a. FW packet, since it is used to punch a hole in the FW/NAT
and to aid the server for port binding and address mapping) from its and to aid the server for port binding and address mapping) from its
client, it copies the source IP and Port number and uses them as client, it copies the source IP and Port number and uses them as
delivery address for media packets. By having the server send media delivery address for media packets. By having the server send media
traffic back the same way as the client's packet are sent to the traffic back the same way as the client's packet are sent to the
server, address mappings will be honored. Therefore this technique server, address mappings will be honored. Therefore this technique
works for all types of NATs. However, it does require server works for all types of NATs. However, it does require server
modifications. Unless there is built-in protection mechanism, modifications. Unless there is built-in protection mechanism,
symmetric RTP is very vulnerable to DDOS attacks, because attackers symmetric RTP is very vulnerable to DDOS attacks, because attackers
can simply forge the source IP & Port of the binding packet. can simply forge the source IP & Port of the binding packet. Using
the rule for restriciting IP address to that one of the signalling
connection will need to be applied here also.
4.3.2. Necessary RTSP extensions 4.3.2. Necessary RTSP extensions
To support symmetric RTP the RTSP signaling must be extended to allow To support symmetric RTP the RTSP signaling must be extended to allow
the RTSP client to indicate that it will use symmetric RTP. The the RTSP client to indicate that it will use symmetric RTP. The
client also needs to be able to signal its RTP SSRC to the server in client also needs to be able to signal its RTP SSRC to the server in
its SETUP request. The RTP SSRC is used to establish some basic its SETUP request. The RTP SSRC is used to establish some basic
level of security against hijacking attacks. Care must be taken in level of security against hijacking attacks. Care must be taken in
choosing client's RTP SSRC. First, it must be unique within all the choosing client's 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
skipping to change at page 22, line 49 skipping to change at page 23, line 21
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 have an public IP address. o Limited to work with servers that have an public IP address.
o The format of the RTP packet for "connection setup" (a.k.a FW o The format of the RTP packet for "connection setup" (a.k.a FW
packet) is yet to be defined. One possibility is to use RTP No-Op packet) is yet to be defined. One possibility is to use RTP No-Op
packet format in [I-D.ietf-avt-rtp-no-op]. packet format in [I-D.ietf-avt-rtp-no-op].
o Has worse security situation than STUN when using address o Has the same security situation as STUN and will need to use
restrictions. address restrictions.
4.3.4. Security Consideration 4.3.4. Security Consideration
Symmetric RTP's major security issue is that RTP streams can be Symmetric RTP's major security issue is that RTP streams can be
hijacked and directed towards any target that the attacker desires. hijacked and directed towards any target that the attacker desires
unless address restricitons are used.
The most serious security problem is the deliberate attack with the The most serious security problem is the deliberate attack with the
use of a RTSP client and symmetric RTP. The attacker uses RTSP to use of a RTSP client and symmetric RTP. The attacker uses RTSP to
setup a media session. Then it uses symmetric RTP with a spoofed setup a media session. Then it uses symmetric RTP with a spoofed
source address of the intended target of the attack. There is no source address of the intended target of the attack. There is no
defense against this attack other than restricting the possible bind defense against this attack other than restricting the possible bind
address to be the same as the RTSP connection arrived on. This address to be the same as the RTSP connection arrived on. This
prevents symmetric RTP to be used with multi-address NATs. prevents symmetric RTP to be used in use cases that require differet
addresses for media destination and signalling.
A hijack attack can also be performed in various ways. The basic A hijack attack can also be performed in various ways. The basic
attack is based on the ability to read the RTSP signaling packets in 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 NAT-traversal RTP packets containing the attacker can send its own NAT-traversal RTP packets containing the
correct RTP SSRC to the correct address and port on the server. The correct RTP SSRC to the correct address and port on the server. The
destination of the packets is set as the source IP and port in these destination of the packets is set as the source IP and port in these
RTP packets. RTP packets.
skipping to change at page 23, line 43 skipping to change at page 24, line 15
defend against. As a NAT re-writes the source IP and port this defend against. As a NAT re-writes the source IP and port this
cannot be authenticated, but authentication is required in order to cannot be authenticated, but authentication is required in order to
protect against this type of DOS attack. protect against this type of DOS attack.
Yet another issues is that these attacks also can be used to deny the Yet another issues is that these attacks also can be used to deny the
client the service he desire from the RTSP server completely. For a client the service he desire from the RTSP server completely. For a
man in the middle capable of reading the signalling traffic or man in the middle capable of reading the signalling traffic or
intercepting the binding packets can completely deny the client intercepting the binding packets can completely deny the client
service by modifying or originating binding packets of itself. service by modifying or originating binding packets of itself.
The random SSRC tag in the binding packet determines how well The random nonce used in the binding packet determines how well
symmetric RTP can fend off stream-hijacking performed by parties that symmetric RTP can fend off stream-hijacking performed by parties that
are not "man-in-the-middle". This proposal uses the 32-bit RTP SSRC are not "man-in-the-middle". This proposal uses the 32-bit RTP SSRC
field to this effect. Therefore it is important that this field is field to this effect. Therefore it is important that this field is
derived with a non-predictable randomizer. It should not be possible derived with a non-predictable randomizer. It should not be possible
by knowing the algorithm used and a couple of basic facts, to derive by knowing the algorithm used and a couple of basic facts, to derive
what random number a certain client will use. what random number a 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.
skipping to change at page 28, line 42 skipping to change at page 29, line 14
why RTP was developed. Problems that arise with TCP are: head-of- why RTP was developed. Problems that arise with TCP are: head-of-
line blocking, delay introduced by retransmissions, highly varying line blocking, delay introduced by retransmissions, highly varying
rate due to the congestion control algorithm. rate due to the congestion control algorithm.
4.5.2. Usage of TCP tunneling in RTSP 4.5.2. 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 exist another way of perform this type of TCP tunneling. There also exist another way of
transporting RTP over TCP defined in Appendix . For signaling and transporting RTP over TCP defined in Appendix C.2. For signaling and
rules on how to establish the TCP connection in lieu of UDP, see rules on how to establish the TCP connection in lieu of UDP, see
appendix C.2 in [I-D.ietf-mmusic-rfc2326bis]. This is based on the appendix C.2 in [I-D.ietf-mmusic-rfc2326bis]. This is based on the
framing of RTP over the TCP connection as described in RFC 4571 framing of RTP over the TCP connection as described in RFC 4571
[RFC4571]. [RFC4571].
4.5.3. Deployment Considerations 4.5.3. Deployment Considerations
Advantage: Advantage:
o Works through all types of NATs where server is in the open. o Works through all types of NATs where server is in the open.
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 price of loss of total media reliability is desired, even at the price of loss of
real-time properties. real-time properties.
4.5.4. Security Considerations 4.5.4. Security Considerations
The TCP tunneling of RTP has no known security problem besides those The TCP tunneling of RTP has no known security problem besides those
already presented in the RTSP specification. It is not possible to already presented in the RTSP specification. It is not possible to
get any amplification effect that is desired for denial of service get any amplification effect that is desired for denial of service
attacks due to TCP's flow control. A possible security attacks due to TCP's flow control. A possible security
consideration, when session media data is interleaved with RTSP, consideration, when session media data is interleaved with RTSP,
would be the performance bottleneck when RTSP encryption is applied, would be the performance bottleneck when RTSP encryption is applied,
since all session media data also needs to be encrypted. since all session media data also needs to be encrypted.
4.6. TURN (Traversal Using Relay NAT) 4.6. TURN (Traversal Using Relay NAT)
4.6.1. Introduction 4.6.1. Introduction
Traversal Using Relay NAT (TURN) [I-D.ietf-behave-turn] is a protocol Traversal Using Relay NAT (TURN) [RFC5766] is a protocol for setting
for setting up traffic relays that allows clients behind NATs and up traffic relays that allows clients behind NATs and firewalls to
firewalls to receive incoming traffic for both UDP and TCP. These receive incoming traffic for both UDP and TCP. These relays are
relays are controlled and have limited resources. They need to be controlled and have limited resources. They need to be allocated
allocated before usage. TURN allows a client to temporarily bind an before usage. TURN allows a client to temporarily bind an address/
address/port pair on the relay (TURN server) to its local source port pair on the relay (TURN server) to its local source address/port
address/port pair, which is used to contact the TURN server. The pair, which is used to contact the TURN server. The TURN server will
TURN server will then forward packets between the two sides of the then forward packets between the two sides of the relay. To prevent
relay. To prevent DOS attacks on either recipient, the packets DOS attacks on either recipient, the packets forwarded are restricted
forwarded are restricted to the specific source address. On the to the specific source address. On the client side it is restricted
client side it is restricted to the source setting up the mapping. to the source setting up the mapping. On the external side this is
On the external side this is limited to the source address/port pair limited to the source address/port pair of the first packet arriving
of the first packet arriving on the binding. After the first packet on the binding. After the first packet has arrived the mapping is
has arrived the mapping is "locked down" to that address. Packets "locked down" to that address. Packets from any other source on this
from any other source on this address will be discarded. Using a address will be discarded. Using a TURN server makes it possible for
TURN server makes it possible for a RTSP client to receive media a RTSP client to receive media streams from even an unmodified RTSP
streams from even an unmodified RTSP server. However the problem is server. However the problem is those RTSP servers most likely
those RTSP servers most likely restrict media destinations to no restrict media destinations to no other IP address than the one RTSP
other IP address than the one RTSP message arrives. This means that message arrives. This means that TURN could only be used if the
TURN could only be used if the server knows and accepts that the IP server knows and accepts that the IP belongs to a TURN server and the
belongs to a TURN server and the TURN server can't be targeted at an TURN server can't be targeted at an unknown address or also the RTSP
unknown address. Unfortunately TURN servers can be targeted at any connection is relayed through the same TURN server.
host that has a public IP address by spoofing the source IP of TURN
Allocation requests.
4.6.2. Usage of TURN with RTSP 4.6.2. Usage of TURN with RTSP
To use a TURN server for NAT traversal, the following steps should be To use a TURN server for NAT traversal, the following steps should be
performed. performed.
1. The RTSP client connects with RTSP server. The client retrieves 1. The RTSP client connects with RTSP server. The client retrieves
the session description to determine the number of media streams. the session description to determine the number of media streams.
To avoid the issue with having RTSP connection and media traffic To avoid the issue with having RTSP connection and media traffic
from different addresses also the TCP connection must be done from different addresses also the TCP connection must be done
through the same TURN server as the one in the next step. through the same TURN server as the one in the next step. This
will require the usage of TURN for TCP [RFC6062].
2. The client establishes the necessary bindings on the TURN server. 2. The client establishes the necessary bindings on the TURN server.
It must choose the local RTP and RTCP ports that it desires to It must choose the local RTP and RTCP ports that it desires to
receive media packets. TURN supports requesting bindings of even receive media packets. TURN supports requesting bindings of even
port numbers and continuous ranges. port numbers and continuous ranges.
3. The RTSP client uses the acquired address and port mappings in 3. The RTSP client uses the acquired address and port mappings in
the RTSP SETUP request using the destination header. Note that the RTSP SETUP request using the destination header. Note that
the server is required to have a mechanism to verify that it is the server is required to have a mechanism to verify that it is
allowed to send media traffic to the given address. The server allowed to send media traffic to the given address. The server
skipping to change at page 31, line 31 skipping to change at page 32, line 5
o A TURN server for RTSP is may not scale since the number of o A TURN server for RTSP is may not scale since the number of
sessions it must forward is proportional to the number of client sessions it must forward is proportional to the number of client
media sessions. media sessions.
o TURN server becomes a single point of failure. o 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 Requires that the server can verify that the given destination
address is valid to be used by the client.
o An RTSP ALG MAY change the necessary destinations parameter. This 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 phase-out completely, see chapter 11.2 of TURN is not intended to be phase-out completely, see chapter 11.2 of
[I-D.ietf-behave-turn]. However the usage of TURN could be reduced [RFC5766]. However the usage of TURN could be reduced when the
when the demand for having NAT traversal is reduced. demand for having NAT traversal is reduced.
4.6.4. Security Considerations 4.6.4. Security Considerations
An eavesdropper of RTSP messages between the RTSP client and RTSP An eavesdropper of RTSP messages between the RTSP client and RTSP
server will be able to do a simple denial of service attack on the server will be able to do a simple denial of service attack on the
media streams by sending messages to the destination address and port media streams by sending messages to the destination address and port
present in the RTSP SETUP messages. If the attacker's message can present in the RTSP SETUP messages. If the attacker's message can
reach the TURN server before the RTSP server's message, the lock down reach the TURN server before the RTSP server's message, the lock down
can be accomplished towards some other address. This will result in can be accomplished towards some other address. This will result in
that the TURN server will drop all the media server's packets when that the TURN server will drop all the media server's packets when
skipping to change at page 33, line 29 skipping to change at page 34, line 4
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 Section 3: MUST work for all flavors of first requirement in section Section 3: MUST work for all flavors of
NATs. The rows represent the different FW traversal techniques. NATs. The rows represent the different FW traversal techniques.
SymRTP is short for symmetric RTP, "V.SymRTP" is short for "variation SymRTP is short for symmetric RTP, "V.SymRTP" is short for "variation
of symmetric RTP" as described in section Section 4.3.5. of symmetric RTP" as described in section Section 4.3.5.
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 Most work with Firewalls, including them with ALGs R2 Most work with Firewalls, including them with ALGs
R3 Should have minimal impact on clients not behind NATs R3 Should have minimal impact on clients not behind NATs
R4 Should be simple to use, Implement and administrate. R4 Should be simple to use, Implement and administrate.
R5 Should provide a mitigation against DDoS attacks R5 Should provide a mitigation against DDoS attacks
-----------------------------------------------+ -----------------------------------------------+
| R1 | R2 | R3 | R4 | R5 | | R1 | R2 | R3 | R4 | R5 |
------------+------+------+------+------+------+ ------------+------+------+------+------+------+
STU | Yes | Yes | No | Maybe| No | STUN | Yes | Yes | No | Maybe| No |
------------+------+------+------+------+------+ ------------+------+------+------+------+------+
ICE | Yes | Yes | No | No | Yes | ICE | Yes | Yes | No | No | Yes |
------------+------+------+------+------+------+ ------------+------+------+------+------+------+
SymRTP | Yes | Yes | Yes |Maybe | No | SymRTP | Yes | Yes | Yes |Maybe | No |
------------+------+------+------+------+------+ ------------+------+------+------+------+------+
V. SymRTP | Yes | Yes | Yes | Yes |future| V. SymRTP | Yes | Yes | Yes | Yes |future|
------------+------+------+------+------+------+ ------------+------+------+------+------+------+
3-W SymRTP | Yes | Yes | Yes | Maybe| Yes | 3-W SymRTP | Yes | Yes | Yes | Maybe| Yes |
------------+------+------+------+------+------+ ------------+------+------+------+------+------+
TURN | Yes | Yes | No | No | Yes | TURN | Yes | Yes | No | No | Yes |
skipping to change at page 35, line 34 skipping to change at page 35, line 43
like to give special thanks to Greg Sherwood of PacketVideo for his like to give special thanks to Greg Sherwood of PacketVideo for his
input into this memo. input into this memo.
Section Section 1.1 contains text originally written for RFC 4787 by Section Section 1.1 contains text originally written for RFC 4787 by
Francois Audet and Cullen Jennings. Francois Audet and Cullen Jennings.
10. Informative References 10. Informative References
[I-D.ietf-avt-app-rtp-keepalive] [I-D.ietf-avt-app-rtp-keepalive]
Marjou, X. and A. Sollaud, "Application Mechanism for Marjou, X. and A. Sollaud, "Application Mechanism for
maintaining alive the Network Address Translator (NAT) keeping alive the Network Address Translator (NAT)
mappings associated to RTP flows.", mappings associated to RTP/RTCP flows.",
draft-ietf-avt-app-rtp-keepalive-07 (work in progress), draft-ietf-avt-app-rtp-keepalive-10 (work in progress),
December 2009. March 2011.
[I-D.ietf-avt-rtp-no-op] [I-D.ietf-avt-rtp-no-op]
Andreasen, F., "A No-Op Payload Format for RTP", Andreasen, F., "A No-Op Payload Format for RTP",
draft-ietf-avt-rtp-no-op-04 (work in progress), May 2007. draft-ietf-avt-rtp-no-op-04 (work in progress), May 2007.
[I-D.ietf-behave-turn]
Rosenberg, J., Mahy, R., and P. Matthews, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)",
draft-ietf-behave-turn-16 (work in progress), July 2009.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-19 (work in progress), October 2007.
[I-D.ietf-mmusic-rfc2326bis] [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-22 (work in (RTSP)", draft-ietf-mmusic-rfc2326bis-27 (work in
progress), July 2009. progress), March 2011.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980. August 1980.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time [RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998. Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC2588] Finlayson, R., "IP Multicast and Firewalls", RFC 2588, [RFC2588] Finlayson, R., "IP Multicast and Firewalls", RFC 2588,
May 1999. May 1999.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address [RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", Translator (NAT) Terminology and Considerations",
RFC 2663, August 1999. RFC 2663, August 1999.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, Address Translator (Traditional NAT)", RFC 3022,
January 2001. January 2001.
[RFC3388] Camarillo, G., Eriksson, G., Holler, J., and H.
Schulzrinne, "Grouping of Media Lines in the Session
Description Protocol (SDP)", RFC 3388, December 2002.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral [RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002. Translation", RFC 3424, November 2002.
[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, [RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP) "STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489, Through Network Address Translators (NATs)", RFC 3489,
March 2003. March 2003.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
skipping to change at page 37, line 16 skipping to change at page 37, line 9
Description Protocol", RFC 4566, July 2006. Description Protocol", RFC 4566, July 2006.
[RFC4571] Lazzaro, J., "Framing Real-time Transport Protocol (RTP) [RFC4571] Lazzaro, J., "Framing Real-time Transport Protocol (RTP)
and RTP Control Protocol (RTCP) Packets over Connection- and RTP Control Protocol (RTCP) Packets over Connection-
Oriented Transport", RFC 4571, July 2006. Oriented Transport", RFC 4571, July 2006.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation [RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127, (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007. RFC 4787, January 2007.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, [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.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC6062] Perreault, S. and J. Rosenberg, "Traversal Using Relays
around NAT (TURN) Extensions for TCP Allocations",
RFC 6062, November 2010.
[STUN-IMPL] [STUN-IMPL]
"Open Source STUN Server and Client, http:// "Open Source STUN Server and Client, http://
www.vovida.org/applications/downloads/stun/index.html", www.vovida.org/applications/downloads/stun/index.html",
June 2007. June 2007.
Authors' Addresses Authors' Addresses
Magnus Westerlund Magnus Westerlund
Ericsson Ericsson
Farogatan 6 Farogatan 6
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